A Brief Review of Immunologic Reactions
1 Immune System Development and Function-associated Antigens
The human immune system is comprised of about 2 X 10 12 lymphocytes containing approximately equal ratios of T and B cells. B lymphocytes synthesize immunoglobulins that are first expressed on their membranes as clonally distributed antigen-specific receptors and then secreted as antibodies following antigenic stimulation. The ability of the immune system to recognize antigens is remarkable. A human being can produce more than 10 7 antibodies with different specificities. The concentration of antibodies in human serum is 15 mg/ml, which represents about 3 x 10 20 immunoglobulin molecules per person! Since each B cell has approximately 10 5 antibody molecules of identical specificity on its surface, the human humoral immune system scans the antigenic universe with about 10 17 cell bound receptors. To maximize the chances of encountering antigen, lymphocytes recirculate from blood to lymphoid tissues and back to the blood. The 10 10 lymphocytes in human blood have a mean residence time of approximately 30 minutes, thus an exchange rate of almost 50 times per day.
T lymphocytes develop from precursor stem cells in fetal liver and bone marrow and differentiate into mature cell types during residence in the thymus. Mature T lymphocytes (antigen responding, response control, and response mediating cells) are present in thymus, spleen, lymph nodes, throughout skin and other lymphatic organs, and in the bloodstream. B lymphocytes (immunoglobulin producing cells) develop from precursor cells in fetal liver and bone marrow and are found in all lymphoid organs and in the bloodstream. The ontogeny and functions of these cells have been identified in a variety of ways, including morphologic and functional criteria, and by antibodies identifying surface proteins which correlate to a varying extent with specific functions. Lymphocytes develop through stages leading to pools of cells which can be operationally defined, and be recognized by acquisition of specific antigenic determinants (1) (Fig. 7-1, Table 7-1). Human B and T cells normally express class I (HLA-A, B, C) major histocompatibility complex (MHC) antigens on their surface, and B cells express class II antigens (HLA‑DR, DP, DQ). “Activated” T cells also express class II antigens on their surface, and are then described as “DR+” (or sometimes as Ia+).
Differentiation Antigens Which Characterize Specific Lymphocyte Subsets
|CD2||LFA-2||NK Cells, T Cells||Cytoadhesion molecule cognate to LFA-3|
|CD3||T3, Leu 4||All peripheral T cells||T Cell receptor complex|
|CD4||T4, Leu 3 (L3T4 in mice)||Class II Restricted T Cells (55-70% of peripheral T cells)||CD4 binds to MHC Class II|
|CD8||T8, Leu 2 Lyt 2||Class I Restricted T Cells (25-40% of peripheral T Cells)||CD8 binds to MHC Class I|
|CD11a||LFA-1 chain||Leukocytes||LFA‑1 chain adhesion molecule, binds to ICAM-1|
|CD14||LPS Receptor||Monocytes||Marker for monocytes|
|CD16||Fc R111||NK Cells, granulocytes||Low affinity Fc receptor|
|CD20||B1||B Cells||Marker for B Cells|
|CD25||TAC, IL2||Activated T and cell growth||Complexes with Chain; T B Cells and monocytes|
|CD28||Tp44||Most T Cells||T Cell receptor for B7-1 and B7-2|
|CD29||Chain of VLA protein, 1 “integrin”||40‑45% of CD4+1 and CD8+ cells||An “integrin” type of adhesion molecule|
|CD40||-||B Cells||B Cell activation|
|CD45RO||-||25‑40% of peripheral||Expressed on naive T Cell; T Cells subsets|
|CD54||ICAM-1||T and B Cells||Cognate to LFA-1|
|CD56||NKH1||NK Cells, some T cells||Neural cell adhesion molecule; NK marker|
A.2. Lymphocyte Surface Molecules
T cells have on their surface receptors (T cell antigen receptor — TCR), which recognize an antigen/HLA complex, accessory molecules which recognize HLA determinants, and adhesion molecules which recognize their counterpart ligands on antigen presenting cells (APCs). After activation, T cells also have new receptors for cytokines, the hormone products mainly produced by macrophages, T cells and B cells, which control other T or B cells (2) (Table 7-2). The T cell antigen recognition complex, “receptor”, consists of disulfide-linked heterodimers, usually the TCR- and TCR- chains, plus five or more associated peptides making up the CD3 complex (3). A small proportion of T cells have TCR and TCR chain instead of and chains. TCR- and peptides and peptides are derived from rearranged genes coding for proteins which are unique in each cell clone. The germline TCR genes are very large, containing 40 – 100 different V (variable) segments, D (diversity) segments (in genes), many J (junctional) segments, and one or two C (constant) segments (Fig. 7-2).
|Cytokine||Cell Source||Targets||Primary Effects On Targets|
|Type 1 IFN (IFN- , )||Mononuclear phagocyte, fibroblast||All||Antiviral, antiproliferative, increased class I MHC expression|
|Tumor necrosis factor||Mononuclear phagocyte, T cell||Neutrophil Liver Muscle Hypothalamus||Inflammation, Acute phase reactants, Catabolism, Fever|
|Thymocyte Endothelial cell
|Costimulator, Inflammation, Fever, Acute phase reactants, Catabolism (cachexia)|
phagocyte, endothelial cell, T cell
|Thymocyte Mature B cell Liver||Costimulator, Growth, Acute phase reactants|
|Interleukin-2||T cells||T cell NK cell B cell||Growth; cytokine production, Growth, activation, Growth, antibody synthesis|
|Interleukin-4||CD4 + T cell, mast cell||B cell Mononuclear phagocyte T cell||Isotype switching, Inhibit activation, Growth|
|Transforming growth factor- ||T cells, mononuclear phagocyte, other||T cell Mononuclear phagocyte Other cell types||Inhibit activation, Inhibit activation Growth regulation|
|Interferon- ||T cell, NK cell||Mononuclear phagocyte Endothelial cell NK Cell All||Activation Activation Activation. Increased class I and class II MHC|
|Cytokine||Cell Source||Targets||Primary Effects On Targets|
|Lymphotoxin||T cell||Neutrophil Endothelial cell NK cell||Activation Activation Activation|
|Interleukin- 10||T cell||Mononuclear phagocyte B cell||Inhibition Activation|
|Interleukin-5||T cell||Eosinophil B cell||Activation Growth and activation|
|Interleukin- 12||Macrophages||NK cells T cells||Activation Activation|
Adapted from tables in Cellular and Molecular Immunology, Edition II by AK Abbas, AH Lichtman, and JS Pober,
WB Saunders Company, Philadelphia
During development of each T cell, segments of the germline gene are rearranged so that one TCR gene V segment becomes associated with one D (in the case of TCR- ), one J, and one C segment to produce a unique gene sequence. This random combination of different V, D, and J and C segments, and additional variations in DNA sequence introduced in the J and D region during recombination, provides the enormous diversity of specific TCRs required to recognize the entire universe of T cell antigens. This process is described as “instructive”, rather than “educational”, in the sense that antigen specific TCRs develop because of intrinsic genetic instructions, rather than in response to exposure to antigens. This process, for example, means that all individuals can (before clonal deletion) have preformed TCRs able to recognize thyroid antigens as well as thousands of other antigens.
The set of V, D, and J segments present in one individual’s inherited (“germ line”) TCR , , , and chains differs from those comprising another individual’s genes. This variation can be recognized by the process of “Southern blotting”, in which DNA is digested by restriction enzymes which cut the DNA at specific infrequently-occurring sequences. However, the technique of reverse transcription-polymerase chain reaction (RT-PCR) is now much more widely used to analyse the V gene repertoire of T cells either ex vivo or after in vitro expansion (4), in the absence of suitable monoclonal antibodies to recognize the separate receptor families.
Each individual T cell, and its progeny, have a “rearranged” gene with one unique combination of V, D, and J segments. Current technology makes it possible to clone individual T cells which respond to a specific antigen, to expand the progeny of a single cell many fold, and then to determine the DNA sequence of the V, D, and J regions which provide the unique recognition function of the TCR. Based on such studies, it is now evident that specific V segments are preferentially used in the response to certain antigens (4). We may thus infer that the availability of such a V segment in an individual’s TCR repertoire must favor an immune response to a specific antigen, including an autoantigen.
Each TCR recognizes one specific antigenic peptide sequence (5), which may consist of 8 – 9 amino acids for class I restricted T cells, and 13 – 17 amino acids for class II restricted T cells. However, some T cells respond to various portions (epitopes) of one antigen; these may represent overlapping peptide segments of the epitope. Thus the response of each individual T (and B) cell is extremely specific, but the combined effect of many T (and B) cells acting together is observed in the typical final “polyclonal” response.
T cells recognize antigen in association with (“presented by”) an MHC-molecule; CD4+ T cells (often functioning as helper cells) recognize class II molecules + antigen, and CD8+ T cells (often functioning as cytotoxic cells) recognize class I molecules plus antigen. The antigen (a small peptide, v.i.) fits within a cleft in the HLA-DR molecule (Fig. 7-3). The TCR functions to recognize the antigenic peptide on the MHC molecule (Fig. 7-3). The five associated peptides of the CD3 complex are believed to be signal-transducers and to initiate intracellular events following antigen recognition. The normal response proceeds via TCR antigen recognition, then “activation” of the T cell through the combined effect of antigen recognition and costimulatory signals, including interleukin (IL)-1 action, leading to T cell IL-2 secretion and IL-2 receptor expression, followed by proliferation of the T cell into an active clone.
Lymphocyte development is controlled by cytokines released by macrophages, lymphocytes, and other cells. Both T and B cells release a large array of cytokines which carry out their effector functions and alter the function of other cells (Fig. 7-1, Table 7-2). As lymphocytes mature in the thymus, and become activated on exposure to antigen, the types of cytokines to which they respond — and produce — become altered. In animals, and to a lesser extent in man, types of lymphocytes can be operationally defined by the cytokines produced. For example, “Th1″ T cells produce IL-2, IFN- and TNF and are predominant in “delayed hypersensitivity” type reactions, whereas “Th2″ T cells produce IL-4 and IL-5, stimulate B cells, and are involved especially in antibody-mediated reactions. Cytokines produced by Th1 cells enhance the activity of this subset but inhibit Th2 cells, and vice versa. This type of regulation may be critical in determining an immune response and in suppressor phenomena. Additional Th subsets are now recognized, including Th17 cells which secrete IL-17 amongst other cytokines and which are strongly pro-inflammatory.
As well as cytokines and their receptors, T cells express a number of receptors for chemokines, integrins and selectins which are involved in the sequential stages of cell adhesion which leads to T cell homing to tissues (7). A word of caution is necessary however in terms of translating these findings into the human situation where boundaries between the subsets are less clear. It is also increasingly recognized that the simple dichotomy of T cells into two types is over-simple (6a), with cytokines such as IL-12 being assigned to the Th1 subset although not being secreted by T cells, and production of this cytokine is stimulated by the Th2 cytokines IL-4 and IL-13, which will drive the immune response from Th2 towards Th1. The blurring of pattern that is seen in many autoimmune diseases challenges the dogma of an easy divide in the type of immune response.
Each B cell produces a unique immunoglobulin (Ig) programmed by an Ig gene which has also been “rearranged” from the germline V, D, J, and C segments (as for the TCR) (8). The TCR and Ig genes are, not surprisingly, members of one gene superfamily. Further diversity is provided by “antigen-driven” somatic mutations which occur during amplification of the progeny of a stimulated B cell, causing the production of a family of Igs with slightly different sequences. Some of these Igs may be better antibodies, and others might recognize other antigens including “self”. B cells secrete their unique Igs into surrounding fluids, and some normally remain on the B cell surface, where they can bind the antigen which is recognized by (fits the structure of) the specific Ig (Fig. 7-4). The surface Ig is therefore a B cell receptor for antigen, having a specific face or “idiotype” which fits the conformation of the antigen molecule “epitope”. The recognition process involves the shape of the epitope – i.e. it is “conformational” and for B cells probably normally involves unprocessed or “native” antigen. This implies that B cell and T cell epitopes for the same antigen are usually different segments or forms of the molecule.
B.3. Antigen Presentation On MHC Molecules
Antigen is normally recognized by the TCR complex only when presented in association with a class I or class II MHC molecule. Typically CD8+ T cells recognize antigen with class I (HLA-A, B, C) proteins, and CD4+ T cells are “restricted” in their recognition to antigen presented with class II (HLA‑DR, DP, DQ) proteins. Antigens which originate within the cell are preferentially presented by class I molecules to CD8+ cells. This indicates an orientation of CD8+ cells toward destroying cells invaded by viruses or producing abnormal antigens. Class II molecules are directed toward presentation of external or alloantigens to CD4+ helper cells.
The genes for the HLA‑A, B, C and HLA‑DR, DP, DQ molecules are on chromosome 6, and comprise some of the genes in a large immune response control complex (Fig. 7-5). Each cell surface HLA molecule is made up of 2 peptide chains; an and 2 microglobulin for class I molecules, and and chains for class II. Each individual inherits from each parent one HLA-A, B, and C, one DR and 3 DR genes, a pair each of DP and DQ and genes, and other related genes which are not expressed, including DX and DO. 2 microglobulin polypeptides are the same for all individuals (Fig. 7-5). The genes are expressed in a co-dominant manner, and (in contrast to TCR and Ig molecules) are invariant in individuals. However, the genes are all highly “polymorphic”, that is, many alleles may exist for each gene. The actual evolutionary drive for this diversity is unknown. We may note that different HLA molecules will presumably present different epitopes and thus lead to selection of a unique T cell repertoire for each HLA allele. While TCR gene rearrangement provides the T cell repertoire to respond to individual antigens, HLA diversity guarantees that different individuals will have different T cell repertoires. We may theorize that this is beneficial for survival of some members of the species when all are attacked by a pathogen – for example, the plague.
The HLA molecules play a central role in T cell clonal selection during fetal development, in normal immune responses, and in presentation of “self-antigens”. In many instances — including autoimmune thyroid disease (AITD) as detailed below — inheritance of a specific HLA gene correlates with increased susceptibility to a specific disease. In some cases this can now be related to a gene coding for a specific amino acid in the HLA molecule which is believed to control epitope selection (often called determinant selection) and thus to be associated with disease susceptibility.
Antigen can be presented to CD4 + T cells by “conventional” (or “professional”) APCs, particularly dendritic cells (9), and also by B cells and activated T cells, and less effectively by a variety of other cells (fibroblasts, glial cells, thyrocytes), when these normally HLA-DR-negative cells are altered and express HLA class II molecules on their surface. This is because non-classical APCs cannot provide the necessary costimulatory signals, including the B7-1 (CD80) and B7-2 (CD86) molecules, which bind to CD28 on the T cell and are necessary for activation of certain T cells. Recent evidence in experimental type 1 diabetes mellitus suggests a simple two-stage model for these molecules in the development of autoimmunity against the islet, with B7-2 playing a primary role in T cell priming, while, if B7-1 has a role, it is dependent on expression in the local microenvironment and is most important in the efector phase of the autommune response (10).
If B7 molecules bind instead to CD152 (CTLA-4) on the T cell, the immune response is terminated. The individual roles of CD80 and CD86 are not clearly established, although some functions appear to be distinct (eg CD80 appears to stimulate CD152) and some overlapping (eg both stimulate CD28), and the tempo of their involvement at different times of the immune response is likely to be critical to the type of response produced (11). The maturation state of the dendritic cell is another determinant of immune homeostasis. Semi-maturation, induced by proinflammatory cytokines like TNF- , allows the development of a tolerogenic stage for these cells. Full maturation, induced by signalling through toll-like receptors, complement receptors or antibody Fc receptors, induces proinflammatory cytokine production by the dendritic cell and allows them to generate T cell immunity (12).
Extracellular (usually foreign protein) antigen is endocytosed by macrophages and dendritic cells, facilitated by a variety of antigen uptake receptors (9). B cells collect the antigen by binding it to cognate surface Ig, and internalizing the Ig-antigen complex. Inside these cells the antigen molecule is broken down to peptides which are 13 – 17 amino acids long. Many of these peptides are destroyed, but some are retained to reappear on the cell surface as T cell antigen “epitopes”. These peptides, possibly in a Golgi-like organelle, become associated with HLA‑DR, DP, or DQ molecules, are transported to the cell surface, and there can be “presented” to a T cell (as in Figure 7-3). The initial response to antigen proceeds via the antigen-APC-T cell route. In autoimmunity this route may be favored by the particular differentiation pathway of dendritic cells, initiated by certain cytokines, or because of intrinsic defects in genes controlling dendritic cell function (14). Secondary T cell responses may follow the same route, or may more frequently follow the antigen-B cell-T cell route. This is because the surface Ig of the B cell allows it to collect specific antigen circulating at a very low level and concentrate it for presentation to a T cell. In addition it is increasingly recognized that 50-90% of proteins which might form autoantigens are post-translationally modified, and some of these modifications may be important in creating new self antigens. These modified proteins have unknown effects on positive and negative selection of T cells, and of course complicate the analysis of epitopes based on native protein sequences that do not encompass the modified proteins.
Proteins produced within the cell (including normal proteins or products encoded by invading viruses) are processed by a separate pathway and appear on the cell surface as peptides of 8 – 9 amino acids associated with HLA class I molecules.
C.4. T and B Cell Responses
Antigen presentation to T cells leads to a variety of responses which include proliferative or suppressive functions, development of cell cytotoxic responses, control of Ig secretion, and many more. In addition, under specific circumstances, antigen presentation may cause the T cell to become non-responsive or “anergized” (10, 15).
Presumably the APC, with its surface covered by HLA-DR-antigen complexes, is met by T cells having the correct receptor (idiotype) matching the processed antigen exposed on the surface of the HLA molecule. T cell and APC adhere by segments of HLA-DR and CD4 molecules which are, effectively, “sticky” (see Figure 7-3), fostering close contact of APC and T cells. Co-stimulation is provided by other “adhesion molecules” on the APCs which also pair with their counterparts on T cells. Specifically, LFA-1 on T cells binds to ICAM-1 on the APC, and CD2 binds to ICAM-2 and these increase intercellular adhesion as a preliminary step. Binding of B7 to T cell receptor CD28 or CTLA-4 is especially important in costimulation but other costimulators exist (16). A recent development has been the recognition that the B7/CD28 pathway is more complex than previously realized, and includes a molecule, inducible costimulator (ICOS), that binds to a B7-like protein, B7RP-1; in mice both ICOS and C28 regulate T cell expansion and deliver complementary signals necessary for optimal T cell activation (17). These reactions dramatically increase the bond between APC and T cell, ensure close approximation, and provide the additional, or “second signal” needed to activate the T cell.
Presentation of antigen and the accompanying second signal are required to activate a naive T cell and initiate an immune response; previously activated T cells are much less dependent on B7-mediated costimulation. Antigen recognition, and APC-produced IL-1 (Table 7-2), cause T cell stimulation. This activates the T cell to express IL-2 receptors and to secrete IL-2 itself. Increased T-cell secreted IL-2 induces the responding T cell, and nearby (“bystander”) T cells to proliferate. T-cell secreted IL-2, IL-6 and other cytokines and IL-4 cause B cells to be stimulated and proliferate and cell surface receptors such as CD40 on B cells and its ligand on T cells are also involved in B cell activation (18). B cells themselves secrete distinct profiles of cytokines, in response to the engagement on CD40, and these cytokines can upregulate or downgregulate an immune response in a manner which depends on whether the B cell is simultaneously stimulated by antigen (19). Intimate T-cell to B-cell contact may account for “antigen-specific help” for T cell and B cell responses, whereas the effect of T cell-secreted lymphokines on “bystander” T or B cells may account for stimulation of “non-antigen-specific” responses by these lymphocytes (20). T cell – B cell interaction usually occurs as T cells percolate through lymph nodes. The T cell receptors must in some way “scan” the exposed DR‑antigen combinations on B cells, until by chance the T cell finds a B cell presenting the antigen epitope recognized by the TCR. The beneficial effects of rituximab, a CD20 specific, B depleting monoclonal antibody, in autoimmune conditions including Graves’ disease (20a) is related to its effects on inhibiting this interaction between T and B cells.
Responding lymphocytes can be segregated into groups based on whether they are naive or memory cells, CD4 + “helper” cells, or CD8 + “cytotoxic/suppressor” cells. CD8 + T cell activation requires additional costimulatory pathways to the B7-dependent pathway largely used by CD4 + T cells, and ICAM-1 is particularly important (21, 22). Although not providing as a clear separation as in the murine system, a functional separation based on lymphokine secretion seems to provide an important categorization for CD4 + cells. Th1 cells function as “inflammatory” cells, typical of a delayed hypersensitivity type reaction, while Th2 cells are more specifically helper cells for B cell immunoglobulin synthesis. A number of factors including TCR affinity and ligand density, and non-T cell-derived cytokines such as IL-4 and IL-12, determine whether the outcome of an immune response is predominantly by Th1 or Th2 cells (23). Some CD4+ and some CD8+ cells appear to provide suppressor signals. The nature of both the recognition process in the suppressor function and the putative effector molecules is not clear. It may involve TCR recognition of T cell or B cell idiotype, secreted molecules representing part of the TCR receptor, or lymphokines but none of these has conclusive support (24). Most recent attention has focused on alternates of local secretion of TGF-β and direct cell contact which involves the binding of CTLA-4 on the T regulatory T cell (25).
A third population of T helper cells has been defined recently, based on their secretion of the pleiotropic proinflammatory cytokine IL-17, and are so called Th17 cells. The differentiation and expansion of these cells depends on the coordinate effects of IL-6, transforming growth factor beta (TGFb) and IL-23 (25a). These Th17 cells are responsible for defence against certain micro-organisms such as Klebsiella, Borrelia and fungi. Of relevance to this discussion, they also have important roles in tissue inflammation and organ-specific autoimmunity.
Although the concept of suppressor cells fell into disrepute during the late 1980s, there has been resurgence in interest with the recognition that CD4 + cells expressing high levels of the IL-2 receptor, CD25, act in a way entirely in keeping with the previously defined suppressor population. These CD4 + , CD25 + T cells have been termined regulatory or Treg cells. Such cells can prevent autoimmunity when transferred from healthy, naïve animals and their depletion results in autoimmune disease. Such cells express Foxp3 which encodes a critical transcription factor for their function: mutation of this gene in man results in the lethal immunological disorder IPEX syndrome that includes autoimmune hypothyroidism anmongst its manifestations (26). The exact mechanism by which suppression is induced is unclear but IL-2 appears to be critical to maintaining peripheral tolerance by supporting the function of these cells, whcih in turn explains some of the apparently contradictory effects of this classically agonistic cytokine (27). Similar cells have been found in man and it is likely that the next few years will see a renewed attempt to identify defects in this population in thyroid autoimmunity, especially as the fundamental questions of antigen specificity and recognition, and mechanisms of action, become clear (28).
Recently it has been established that APCs have a central role in controlling Treg cells, with resting APCs (including thymic epithelial cells) promoting their development through the induction of the transcription factor Foxp3 (29). Activation of APCs, for instance through their T cell-like receptors, has the opposite effect, and at least one component responsible for the suppression of Tregs then is the cytokine IL-6; this pathway allows effector T cells to predominate over Tregs, thereby shifting the dynamic equilibrium in favor of an immune (or autoimmune) response. Another critical molecule in the Treg cell pathway is the costimulatory signal receptor, CD28, which is required for both development and maintenance of Treg function. CD4 + , CD25 + Treg cells may be particularly suited to respond to their selecting self-peptide, which may direct their accumulation to where the self-peptide is expressed, in turn allowing them to be effective in maintaining non-self-responsiveness (30).
TGFb is yet another critical factor and recent work suggests that exposure to this cytokine induces Tregs, but when combined with IL-6, Th17 effector cells are generated (25a). The absence or presence of IL-6 is thus critical to determining whether there is a regulatory milieu or a proinflammtory response mounted by Th17 cells. It seems that both Th1 and Th17 cells are potent inducers of organ-specific autoimmunity, but their relative roles in each type of disease remain to be clarified.
It is increasingly clear that Treg are more complex a group of cells than originally clear. Currently T regulatory cells can be classified as those which arise within the thymus and express Foxp3, and a Th3-like population which probably does not express this molecule and which develops in the periphery. The glucocorticoid inducible tumor necrosis factor receptor (GITR) is expressed by both populations but CD25 –bright expression is not a requirement for regulatory T cell function. In addition, there is a CD4+, CD69+ population of regulatory T cells and a CD8+ suppressor population which recognizes self-peptides expressed by an MHC class Ib (Qa1 in mice) molecule (31). The latter seem particularly critical in the remissions of multiple sclerosis that characterize many cases of this disease. Since T cells stimulate or suppress other T or B cells, they may develop feedback circuits to limit responses to antigens.
The reciprocal relationship between Th1 and Th2 cells, exerted through secretion of cytokines, serves as another model of suppressor function and this could be an important future immunotherapeutic area. This paradigm is conceptually useful but is almost certainly too simplistic, not least because there may exist within the Th2 compartment different types of T cells, some with pathological effector function and others which act as physiological regulators of Th1 responses. Endeavors to manipulate the entire Th2 population, to deviate an immune response away from Th1 cells, may therefore lead to exacerbation of the immune response, and may explain the reciprocal relation between the prevalence of infectious disease and autoimmunity (32).
It must be noted that the complexity of T cell function is much greater than suggested by this brief outline. For example, CD4+ T cells can function as suppressor or cytotoxic cells and may do so at different times in their life cycle. In fact an important generalization is that the T cell surface molecules so far defined only rarely identify unique functional sets of cells. On the other hand, functions of T cells to help T or B responses, to suppress such responses, and to kill target cells under certain conditions, can be operationally defined, and these functions often correlate with expression of specific T cell antigens. While recognizing the inherent simplifications we have employed, it remains useful to discuss T cell subsets in relation to surface antigens and function, since so much work has been published using these concepts.
D.5. Killer (K) and Natural Killer (NK) Cells
In addition to the standard T cell function described above, other cells participate in immune responses. Macrophages may destroy cells having immune complexes on their surface through recognition of the Fc portion of bound Ig. Other cells which do not bear the CD3 marker of T cell lineage exist (K and NK cells) and have the ability to spontaneously kill other cells (especially those expressing HLA antigens). NK cells can be detected by specific monoclonal antibodies such as anti-CD16, and are recognized phenotypically as large granular lymphocytes. Like T cells, NK cells can have a type 1 or 2 pattern of cytokines release (33). Macrophages, T, K, NK, or other cells also kill cells coated with immune complexes in the process of antibody-dependent-cell-cytotoxicity (ADCC) (Fig. 7-6).
E.6. Self-Non-Self Discrimination
The immune system, which evolved to defend us from invading foreign proteins, normally “tolerates” (does not develop recognizable responses to) self‑antigens. The level of this control is variable. For example, self‑reactivity to serum albumin is not seen. However, antibodies to thyroid antigens exist in up to 36% of adult women, and their presence must be considered effectively normal. The development of tolerance is closely associated with the restriction of TCRs to recognizing an antigen only when presented by an HLA molecule. The process, which for T cells occurs in the fetal thymus, leads to elimination of some T cells, and retention of others with TCRs having desirable features. Self-antigens are believed to be presented on HLA molecules to T cells developing in the thymus. This implies that antigen must be in the thymus or in the circulation for tolerance to develop. T cells bearing TCRs which react strongly to HLA molecules not bearing antigen (“autoreactive cells”) are largely inactivated or destroyed. T cells which have the capacity to react with foreign antigens presented by self MHC molecules are somehow retained. Most T cells with TCRs which bind strongly to class I or class II molecules bearing self-antigen are also “clonally deleted” (34) (Fig. 7-7). Presumably some T cells which react with MHC molecules plus self antigen are not deleted, since otherwise an excessive number of T cells would be lost, leaving a “hole” in the available TCR repertoire which would compromise its ability to mount a future immune defense.
The best evidence that thymic deletion prevents autoimmunity in man comes from auoimmune polyglandular syndrome (APS) type 1, which is the result of a muatation in the AIRE (AutoImmune REgulator) gene. Such patients have multiple autoimmune disorders, principally Addison’s disease and hypoparathyroidism but including thyroid autoimmunity. The AIRE protein is expressed in the thyrmus by medullary epithelial cells and regulates the rather surprising expression of an array of self proteins (normally confined to extrathymic tissues) by these cells during fetal development. When such self antigens cannot be expressed to allow clonal deletion, autoimmunity ensues and this accounts for the multiple autoimmunity and early onset found in this syndrome (reviewed in 34). It is also now clear that additional factors must be involved in determinig the target –organ specificity of the consequences of AIRE deficiency, and these may include other proteins that regulate the expression of autoantigens for utoimmune disease that are rare or absent in APS type 1 (35).
This fine discrimination between perfection in clonal deletion, and repertoire maintenance, allows a limited number of autoreactive and self-antigen-reactive T cells to survive, and thus sets the stage for autoimmune disease. It is unclear how this mechanism produces tolerance to antigens not present in the fetal thymus or circulation – especially antigens which may be expressed only in the mature individual. It is presumed that a mechanism working outside the thymus — “peripheral tolerance” — completes T cell selection. Not all suppressor cells are of the CD8 + lineage. In rodents an important regulatory T cell population is CD4 + CD25 + which mediate their effects by both cell-cell contact and cytokine secretion, as discussed in Section4.
Factors which control the production of Treg include the way antigen is presented. CD28 and CTLA-4 are both expressed by these cells and can influence their function, while stimulation of T reg via B7-2 inhibits their suppressive function, whereas B7-1 enhances this function. Since the maturation state of dendritic cells influences the expression of B7-1 and B7-2, this helps to explain how the activation state of the antigen presenting cell may influence the immune response. IL-2 is critical for the development and expansion of Treg (36) and may represent an important point at which this immune system could be regulated therapeutically, as any way to enhance Treg function is clearly likely to be of benefit in autoimmune disorders. Perhaps the clearest evidence so far that these cells play an important role in maintaining self-tolerance is the demonstration that their removal in vitro allows the derivation of CD4 + cells from peripheral blood that respond to islet cell, melanocyte and testis autoantigens, while adding CD4 + , CD25 + T cells back suppresses the expansion of these autoractive helper cells. (37).
During maturation in the thymus, probably 95% or more of the lymphocytes produced are negatively selected, and die through a process described as “programmed cell death” — or apoptosis. This process involves several genes including those required for apoptosis, such as Fas. A similar process is thought to ensue whenever a T cell is stimulated by its cognate antigen but does not receive a “second signal”, and during induction of anergy by other mechanisms. Defects in Fas lead to preservation of autoreactive T cells in some models of animal autoimmune disease (38).
B cells undergo a similar selection process in fetal bone marrow or liver, except for the participation of MHC molecules. If exposed to antigen during this early stage of development, B cells are somehow permanently inactivated. As for T cells, the selection process is not perfect, and leaves some B cells having the ability to make antibodies directed to self‑antigens in the adult. However, B cells require T cell “help” in order to proliferate and differentiate into mature Ig secreting cells. In the effective absence of self‑reactive T cells to “help” such B cells, these B cells remain dormant, and expanding clones do not develop. Although such clonal ignorance may be an important pathway in preventing B cell autoreactivity, it is not the only mechanism, and physiological concentrations of autoantigen may induce anergy of B cells, even when their affinity for autoantigen is low.
Tolerance to self‑antigen can be overcome (“broken”) in animals by injecting the antigen in an unusual site on the body, especially in the presence of adjuvant compounds such as tubercule bacillus fragments and oil, or alum, or by slightly altering the antigen structure, or by altering the responding immune system (for example, by whole body irradiation, or depletion of suppressor T cells). An additional mechanism for the inflammatory component of many autoimmune disorders has recently been proposed based on the evolutionary origins of mitochondria from bacteria. Given that the prime function of the immune system is to defend the organism from microbes, it is possible that the immune system may mistake mitochondria released from damaged tissue through pattern-recognition receptors and thereby induce a ‘mistaken’ inflammatory response (38a).
It is clear that some degree of recognition of self-antigens is normal, but is also normally well controlled by suppressor circuits in the immune system. Based on current knowledge, a number of therapeutic strategies already exist through which memory T and B cells can be targeted, with increasingly impressive results in autoimmune disease (39). Factors which may lead to amplification of the normal small number of self reactive T cells, and thus produce “autoimmune” disease, are described subsequently.
The Syndromes of Thyroid Autoimmunity
The syndromes comprising autoimmune thyroid disease are three intimately related illnesses: (1) Graves’ disease with goiter, hyperthyroidism and, in many patients, associated ophthalmopathy (2) Hashimoto’s thyroiditis with goiter and euthyroidism or hypothyroidism; and (3) primary thyroid failure or myxedema. Many variations of these syndromes are also recognized, including transient thyroid dysfunction occurring independently of pregnancy and in 5 – 6% of postpartum women, neonatal hyperthyroidism, and neonatal hypothyroidism. The syndromes are bound together by their similar thyroid pathology, similar immune mechanisms, co-occurrence in family groups, and transition from one clinical picture to the other within the same individual over time. The immunological mechanisms involved in these three diseases must be closely related, while the phenotypes probably differ because of the specific type of immunological response that occurs. For example, if immunity against the TSH receptor leads to production of thyroid stimulating antibodies, Graves’ disease is produced, whereas if TSH blocking antibodies are formed or a cell destructive process occurs, the result is Hashimoto’s thyroiditis or primary myxedema.
Associated with autoimmune thyroid disease in some patients are other organ specific autoimmune syndromes including pernicious anemia, vitiligo, myasthenia gravis, primary adrenal autoimmune disease, ovarian insufficiency, rarely pituitary insufficiency, alopecia, and sometimes Sjogren’s syndrome or rheumatoid arthritis or lupus, as manifestations of non-organ specific autoimmunity. Of particular interest recently has been the description of pituitary antibodies and growth hormone deficiency in a around a third of patients with autoimmune hypothyroidism, implying the existence of a substantial reservoir of pituitary autoimmunity in these patients but further work is needed to confirm these findings and to understand the basis for the autoimmune response against the pituitary (39a).
The Antigens in AUTOIMMUNE THYROID DISEASE
The three most important antigens involved in thyroid autoimmunity are clearly defined. First to be recognized was thyroglobulin (TG), the 670 kD protein synthesized in thyroid cells and in which T 3 and T 4 are produced. Although considered to be a single unique protein, the proteins prepared from different thyroid glands, especially those with Graves’ disease and thyroid malignancy, react differently with polyvalent rabbit anti-TG antisera (40), suggesting that the fine structure of TG differs from person to person. Four to six B cell epitopes of TG are known to be involved in the human autoimmune responses and epitope recognition is similar in both Graves’ disease and Hashimoto’s thyroiditis (40a). Animal studies suggest that antigenicity of the molecule is related to iodine content, but studies on human antisera do not bear this out (40). A recent review has described these species differences in detail and discussed the role of measuring TG antibodies in thyroid disease (41). Antibodies appear to recognize conformation of large fragments of TG, whereas T cells recognize peptide segments and their primary structure. Mouse experiments suggest that, to induce autoimmunity to TG, initial tolerance to dominant epitopes must be overcome, and the immune response then spreads to cryptic epitopes that are the major inducers of thyroidal T cell infiltration (42). A particular TG T cell epitope, Tg.2098, has recently been identified which is a strong and specific binder to the MHC class II disease susceptibility HLA-DRβ1-Arg74 molecule, and stimulates T cells from both mice and humans that develop AITD (42a). This could be a major T cell epitope which might be involved in pathogenesis through initiating an immune response that then spreads to involve other autoantigens.
There is also renewed interest in the concept that TG itself may be present in orbital tissue in ophthalmopathy patients where it could act as a co-antigen in ophthalmopathy (43). The fact that it is present in fat but not muscle from the orbit may indicate its involvement in a subset of such patients, but the failure to find TG in normal orbital tissue is certainly very suggestive of a possible role.
G.2. TSH RECEPTOR
The second antigen to be identified was the TSH receptor (TSH-R), a 764 aa glycoprotein. Antibodies to TSH‑R mimic the function of TSH, and cause disease by binding to the TSH‑R and stimulating (or inhibiting) thyroid cells, as described later. The human TSH-R has been cloned and sequenced in several laboratories, and is known to be a member of a family of cell surface hormone receptors which are characterized by an extra-membraneous portion, seven transmembrane loops, and an intracellular domain which binds the G S subunit of adenyl cyclase (44, 45). It undergoes complex post-translational processing, forming a Z subunit structure comprising the A subunit. Graves’ patients’ IgGs are reported to bind to specific sequences in the extracellular domain, and much effort is currently being directed to definition of the B and T cell epitopes (46). Human TSH-R epitopes are non-linear or “conformational”, perhaps composed of several segments of the protein.
The recent description of two mouse and a hamster monoclonal antibody has significance for several reasons (47-49). Firstly, these antibodies confirm that a single antibody is sufficient to activate the receptor, rather than two or more simultaneously. Secondly, they will permit epitope mapping, already partially achieved. One antibody preferentially recognizes the free A subunit, not the holoreceptor, suggesting that free A subunit, shed from thyroid cells, may initiate or amplify the autoimmune response. The hamster monoclonal antibody, in contrast to TSH, does not enhance post-translational TSH-R cleavage, which may extend the receptor half-life and thus account for the prolonged thyroid stimulation seen following antibody binding (49). It has become clear that in the hamster model, high-affinity TBAb recognize at least 2 conformational epitopes, one of which was indistinguishable from the epitope for thyroid stimulating antibodies (50). The epitopes for the latter are more restricted but these antibodies need not have identical epitopes, although their binding does require interaction with the highly conformational N terminus of the A subunit (51). Finally, these pave the way for the development of human monoclonal antibodies which will allow a greatly improved understanding of the mechanisms involved in Graves’ disease.
A human monoclonal TSH-R stimulating antibody has been produced (52). Both the intact IgG and its Fab bound to the TSH-R with high affinity and the monoclonal had similar features in all respects to known TSAb. This observation indicates that indeed only a single species of antibody is needed to stimulate the receptor and opens the way to a more detailed analysis of receptor-antibody interaction. More conventional approaches based on different methods of expressing the TSH-R have shown that TSAb preferentially recognize the free A subunit rather than the holoreceptor, either because of steric hindrance from the plasma membrane or membrane spanning region of the receptor or because of TSH-R dimerisation (53). The epitopes for TBAb overlap with those for TSAb but are more focused on the C terminus and are able to recognize holoreceptor more efficiently. These observations have provided support from the hypothesis that shedding of free TSH-R A subunits may be critical in initiating or amplifying the autoimmune response in Graves’ disease. Further evidence comes from immunization of mice with adenoviruses expressing different structural forms of the TSH-R: goiter and hyperthyroidism occur more frequently when mice are given virus that expresses the free A subunit rather than a receptor with minimal cleavage into subunits (54).
Patients with autoimmune thyroid disease may have both stimulating and blocking antibodies in their sera, the clinical picture being the result of the relative potency of each species. Switching between one type of antibody and another in unusual patients, involving changes in concentration, potency and affinity, may be caused by a number of factors including levothyroxine treatment, antithyroid drug treatment and pregnancy, and can lead to difficulties in clinical care (54a).
The major T cell epitopes are heterogeneous and T cell reactivity against certain TSH-R epitopes has been demonstrated in high frequency in normal subjects (55). Identification of the critical epitopes has proved elusive although peptides 132-150 do appear to constitute one key epitope; there is poor correlation between binding affinity and T cell immunogenicity in experiments to attempt such localization (56). In animal studies, however there is clear evidence of epitope spreading when mice are immunized with TSHR peptide epitopes or TSHR cDNA, indicating that dominant TSHR epitopes are, at best, elusive (56a). Apparent hTSH-R mRNA transcripts and protein have been identified in retrobulbar ocular tissue, particularly the preadipocyte fibroblast, suggesting that TSH-R expression in the orbit could well be involved in the development of autoimmunity and ophthalmopathy, and similar TSH_R-expressing fibroblasts have also been found in the thyroid gland itself (57). Further support for involvement of the THS-R comes from experiments showing that activation of the TSH-R stimulates early differentiation of preadipocytes, but terminal differentiation is not induced (57a). It should be noted that an alternative pathway for fibroblast involvement in ophthalmopathy may depend on the production of insulin-like growth factor antibodies in these patients but it is difficult to reconcile these findings with the orbital specificity of the autoimmune process in thyroid eye disease (57b).
Recently TSH-R neutral antibodies have been identified which do not block TSH binding and are unable to stimulate cAMP production; these antibodies are capable of inducing thyroid cell apoptosis in vitro and therefore could conceivably play a role in pathogenesis by inducing release of thyroid autoantigens (57c).
H.3. THYROID PEROXIDASE
The third thyroid antigen was described as “microsomal antigen” was identified as thyroid peroxidase (TPO) in 1985 (58) (Fig. 7-8). For more than three decades, since antisera from humans with thyroid autoimmunity reacted with an easily denatured protein present on the surface of thyroid cells and in cell cytoplasm. DeGroot’s laboratory demonstrated that human antisera reacting to “microsomal antigen” precipitated human thyroid peroxidase (TPO) prepared from Graves’ disease thyroid tissue (59) (Fig. 7-8) and at the same time Czarnocka et al. purified human TPO and confirmed identity with the microsomal antigen (60). The cDNA was cloned and sequenced in several laboratories (61 – 63). Alternative splicing of the mRNA probably provides the explanation for the 101 and 107kD forms of the protein (64). As yet a functional difference for the two forms has not been described. The interaction of human anti-TPO antisera and monoclonal antibodies also indicate the presence of several B cell epitopes which map to two main domains, A and B (reviewed in 65). The three-dimensional structure of TPO has been modeled and the location of the B determinant has been defined (66). Recently, further experiments with monoclonal antibodies have defined individual amino acid residues that are critical for the the two immunodominant regions (67). The epitopes recognized by antibodies are stable within a patient and may be genetically determined (68). Investigation of linear epitopes of TPO recognized by T cells from patients with AITD has produced conflicting results to date but certain sequences are beginning to emerge which are shared between reports on various patients (69, 70). There is also debate as to whether patients with autoimmune hypothyroidism differ in their pattern of epitope recognition from healthy controls who are TPO antibody positive, and further work is required to analyze this in detail , as it might allow better prediction of those antibody positive individuals who will progress to overt hypothyroidism (70a)
The microsomal antigen/TPO is expressed on the thyroid cell surface as well as in the cytoplasm, and may represent the cell-surface antigen involved in complement-mediated cytotoxicity as well as antibody-dependent cell mediated cytotoxicity (71). Intracytoplasmic binding of antibodies to TPO indicates that there is access to this compartment, but the consequences in vivo are unclear. Expression of the antigen is increased by incubation of thyroid cells with TSH and lectins, and this response is augmented by interferon- (IFN- ) (72). IFN- alone does not stimulate production of the antigen. Surprisingly, TPO and TG are reported by some workers to share common epitopes (73). There are small areas of amino acid sequence homology, but it is uncertain that these are important B cell epitopes. The most recent study in this area, using monoclonal antibodies, has concluded that TG-TPO autoantibodies are polyreactive rather than bispecific (74).
It is of considerable interest that the three major antigens involved in AITD are involved in production of thyroid hormone. On the other hand, perhaps this is circuitous reasoning, since clearly it is sensible that autoimmunity to unique thyroid antigens would produce “thyroid disease.” If autoimmunity cross-reacted with a variety of other organs, the disease produced would present as a different syndrome. These antigens are unique to the thyroid gland although fat and possibly other tissues may express TSH-R (57), and cross-reactivity of antibodies with proteins in other organs is extremely limited.
I.4. OTHER ANTIGENS
Antibodies against the sodium/iodide symporter (NIS) were first shown functionally in cultured dog thyroid cells (75). With the cloning of NIS, antibodies were also demonstrated in the majority of Graves’ disease sera by immunoblotting (76), albeit using the rat sequence. We have recently found that a third of Graves’ disease sera contain antibodies capable of blocking NIS-mediated iodide uptake in cells transfected with the human NIS but the relevance of this for thyroid function is unclear (77). The same antibodies have also been detected using an immunoprecipitation assay (78). Others have found no such blocking activity using assays with cell lines displaying much higher 131 I uptake, in turn suggesting that any NIS blocking activity only occurs at limiting conditions (79). This implies that NIS autoantibodies probably have no effect in vivo. NIS expression on TECs is upregulated by TSH and downregulated by cytokines and the latter could impair thyroid function in the setting of AITD when such cytokines are synthesised in the thyroid (80). Recently pendrin, an apical protein responsible for mediating iodide efflux from thyroid cells into the follicular lumen, has been identified as an autoantigen. Autoantibodies were initially found in 81% of patients with AITD by immunoblotting (more frequently and at higher titer in Hashimoto’s than Graves’ patients) and only in 9% of controls (80a) but the frequency of these autoantibodies detected using a radioligand binding assay is rather low at around 10% of patients (80b).
Antibodies to a variety of other thyroid cell components are also occasionally present in AITD, including antibodies that react with thyroxine or triiodothyronine (81), antibodies reacting with tubulin and megalin, calmodulin and antibodies reacting to DNA or DNA associated proteins (82 – 84). More recently the insulin-like growth factor receptor has emerged as a possible autoantigen involved in ophthalmopathy, with antibodies being detected in patients with this complication, and this receptor co-localizes with the TSH-R on both fibroblasts and thyrocytes (84a).
Immune Reactions in Autoimmune thyroid disease
J.1. HUMORAL IMMUNITY
The principal autoantibodies identified in AITD and the methods for detecting them are listed in Table 7-3. Antibodies to the TSH receptor are discussed in detail in Chapter 10, but, in brief, observation of a factor in serum of patients with Graves’ disease causing long acting stimulation of thyroid hormone release from an animal’s thyroid, in contrast to the short acting stimulation produced by TSH, led directly to our knowledge of anti-TSH-R antibodies. We summarize here a huge amount of clinical and laboratory research. The antibodies directed to the TSH-R are currently separated into three types. Some antibodies bind to an important epitope in TSH-R and activate the receptor, producing the same effects as TSH, in particular causing generation of cyclic AMP. These antibodies may be referred to as TSI or TSAb — thyroid stimulating immunoglobulins or thyroid stimulating antibodies. Other antibodies bind to different, or the same epitopes and interfere with radiolabelled TSH binding in certain assays — thus they are known as thyrotrophin binding inhibitory immunoglobulins or TBII. Still others bind and prevent the action of TSH — thus blocking antibodies. These may either interfere directly with TSH binding or have less well characterised inhibitory effects. Numerous other names are used.
Antibodies Reacting with Thyroid Autoantigens in AITD and Techniques for Detection
|Antigen||Test Used To Identify Antibody|
Immunofluorescence on fixed sections of thyroid tissue: colloid localization
Hemolytic plaque assay
|Colloid component other than TG||Immunofluorescence on fixed sections: colloid localization|
|Microsomal antigen/ TPO||Complement fixation
Immunofluorescence on unfixed sections; thyroid tissue cell localization
Cytotoxic effect on cultured thyroid cells
TPO activity inhibition
|TSH-R||Bioassay in mice
cAMP production by thyroid cells, TSH-R transfected cells or membranes
Iodide uptake by thyroid cells
Thymidine incorporation by thyroid cells
Inhibition of TSH action on thyroid cells
Inhibition of TSH binding to cells or membranes
|Sodium/iodide symporter||Western blotting
Bioassay using cultured thyroid cells or cells transfected with the symporter
|Nuclear Component||Immunofluorescence on unfixed sections of tissue: nuclear localization|
TSI cause non-TSH dependent (often called “autonomous”) stimulation of thyroid function, which, if of sufficient intensity, is hyperthyroidism. TBII comprise the mixture of TSI and TSH blocking antibodies, and therefore function cannot be predicted from the TBII level. Predominance of TSI characterizes Graves’ disease, and TSH blocking antibodies are often present in Hashimoto’s disease and primary myxedema. Probably a combination is present in most patients with AITD. Recent work indicates that both types of TSH-R antibody are present in Graves’ sera at low concentration with high affinity and similar (but nonetheless subtly distinct) binding epitopes (84a). TSI directly cause thyroid overactivity, their level correlates roughly with disease intensity, and a drop in levels correlates loosely with disease remission. The intact TSH-R epitopes recognized by B cells remain uncertain, in large part because the epitopes are probably conformational, and made up of discontinuous but continguous portions of the extracellular domain of the receptor (85). Unlike TG and TPO antibodies which are polyclonal and not restricted by immunoglobulin subclass (reviewed in 66, 86), there is evidence that some TSH-R are restricted to particular heavy and light chain subclasses, which may indicate an oligoclonal origin (87, 88), and TSH-R stimulating antibodies are present at much lower concentration than TG and TPO antibodies (85). Recent work has shown that normal subjects can have TSH-R antibodies that bind to but do not activate the TSH-R and that generally have low affinity (88a). These natural autoantibodies may be the precursors of the TSI that cause Graves’ disease and it is possible that affinity maturation, with class switching of immunoglobulin isotype, is critical in determining the clinical consequences of TSH-R antibody production. Conversely, using the most sensitive binding assays, there are still a very small number of patients with Graves’ disease who are apparently negative for these antibodies when their serum is tested; it is likely that the explanation lies in either assay sensitivity or exclusively intrathyroidal production of these antibodies (88b).
Precipitating antibodies to TG were first detected by mixing antibody and antigen in equivalent concentrations, or by agar gel diffusion, as in the Ouchterlony plate technique. Subsequently, much more sensitive methods were developed, such as solid phase ELISA (89) and RIA (90), and the tanned red cell hemagglutination test (TGHA) (91). In the latter, TG is adsorbed on the surface of red cells that have been treated with dilute tannic acid. Agglutination of these treated cells occurs in the presence of antibody to TG. By this method, antibody can be detected at a concentration that is 1/40,000th of that required for a positive precipitin reaction. Current RIAs for TGAb are as, or more sensitive, than TGHA. Immunoradiometric assays (IRMA) used currently involve binding of serum Ab to solid phase antigen, and secondary quantitation of antibody by binding labelled monoclonal anti-human Ig antibody. These tests are also reported to be very sensitive and specific. Hemagglutination titers of up to 1 in 5 million have been obtained with sera from patients with chronic thyroiditis. A high anti-TG titer (1/1,250 or more) is strong evidence of AITD and helps to distinguish it from multinodular goiter and thyroid carcinoma. In some instances, sufficient TG is released into the circulation to form circulating antigen-antibody complexes that prevent the detection of free antibodies, unless a special technique is used (90).
Antibodies directed against TG are rarely present in children without evidence of thyroid disease. The prevalence in “well” persons increases with age, and low levels are frequently present in normal adults (91). The greatest frequency occurs in women aged 40‑60 years. The frequency of antibodies in well persons correlates with the incidence of lymphocytic infiltration found on microscopic examination of “normal” thyroids (92), and antibody levels correlate well with the presence of lymphocyte infiltration in the thyroid (93). Over 90% of patients with Hashimoto’s thyroiditis have these antibodies. Low to moderate titers (< 1/2500) are found in half of patients with Graves’ disease. High antibody levels in this disease are often found in patients who become hypothyroid after thyroidectomy (94) and 131 I treatment (95). Almost all patients with idiopathic hypothyroidism have high titers. Antibody levels are either absent or low in patients with subacute (De Quervain’s) thyroiditis, who may present clinically like patients with Hashimoto’s thyroiditis. In general human TG and its autoantibody bind complement weakly due to the widely scattered epitopes which are unable to allow antibody cross-linking (66).
The second important antigen-antibody system was originally recognized by antibodies which, by immunofluorescence, were observed to bind to non-denatured thyroid cytoplasm, to fix complement in the presence of human thyroid membranes (“microsomes”), or to bind to microsome‑coated red cells (the MCHA assay). We now know this antigen is TPO (see previous Section 3), but will in discussions refer to it both as microsomal antigen, since this conforms to many original reports, and as TPO, the designation used in recent studies (Fig. 7-8). Almost all patients with Hashimoto’s thyroiditis have TPO/microsomal antibodies. They also occur in the normal population in the absence of clinically significant thyroid disease: in a recent survey of a population followed for 20 years, 26% of adult women and 9% of adult men had microsomal and/or TG levels (91). However, the presence of such antibodies was shown to be associated with an increased risk of future hypothyroidism, especially if the TSH was also raised (subclinical hypothyroidism). Few sera from AITD contain TG antibodies in the absence of microsomal antibodies, but the converse is not true, so it has been proposed that screening for AITD could be undertaken initially with assays for microsomal/TPO antibodies (96). This is particularly the case if the hemagglutination assay is used for TG antibodies; sensitive RIAs may detect a very high frequency of TG antibodies in individuals with autoimmune thyroid disease, even more than TPO antibodies (97). Current assay techniques include MCHA, RIA, ELISA, and IRMA, and use of purified TPO (98) or recombinant human TPO. Using modern types of assay, TG antibodies occurring in isolation from TPO antibodies are more commonly found, and thus measurement of both antibodies might have clinical utility in certain situations, for instance in diagnosing possible causes for impaired fertility in women (98a).
Although all antibodies reacting with “microsomes” may not be directed to TPO, it is likely that TPO antibodies constitute the very major portion of “antimicrosomal” activity (99). These antibodies also bind to and inhibit the enzymatic function of TPO, as shown by Okamoto et al (100). This effect is probably limited in vivo by inability of the antibodies to penetrate the thyroid and reach TPO on the surface of the cells facing the colloid space.
Antibodies detected by these techniques are believed to be similar to antibodies that fix complement in the presence of extracts from a thyrotoxic gland (101), and to a cytotoxic antibody found in patients with Hashimoto’s thyroiditis (71, 102). Sera from patients with Hashimoto’s thyroiditis usually have high cytotoxic activity (103). Complement-mediated sublethal injury probably occurs in vivo since complement containing complexes have been identified in thyroid tissue of patients with GD and HT (104). Thyroid cell expression of membrane proteins, especially CD59, helps prevent complement-mediated lysis (105), and this protein is upregulated by IL-1 and IFN- .
With the sera of certain patients with Hashimoto’s thyroiditis investigated by the fluorescent antibody technique, a reaction is localized to the nuclei of the thyroid slice (106). This is a coincidental antinuclear factor (antibody) and DNA antibodies are found in some AITD sera but have unknown significance (83, 107).
In early studies, TG antibodies were demonstrated by the passive cutaneous anaphylaxis technique and by a skin test in which an extract of human thyroid gland was injected intradermally (108). Positive skin reactions were found in patients with Hashimoto’s thyroiditis and primary myxedema, and these conditions were closely correlated with the presence of circulating thyroid precipitins. The tissue change at the reaction site was that of an Arthus response. This type of study obviously carries a risk of viral transmission, and thus is not now acceptable (or necessary). The frequency of autoantibodies detected by the TGHA and MCHA tests combined in Hashimoto’s thyroiditis was 95%: the figures in diffuse toxic goiter (Graves’ disease) and nontoxic nodular goiter were 87% and 25%, respectively (109). The highest antibody titers are found in Hashimoto’s thyroiditis and diffuse toxic goiter.
Sera from some patients can contain antibodies that have T 4 and T 3 binding activity (81,110). This activity represents another antibody response to the TG antigen. The antibodies do not alter thyroid function significantly, but can cause confusion in diagnosis due to artifacts in the T 4 and T 3 RIA.
The cytotoxicity of circulating antibodies has also been explored using systems to detect antibody-dependent cell-mediated cytotoxicity (ADCC) in which nonimmunized lymphocytes (NK cells) or macrophages act as effector cells and kill antigen-coated target cells, following incubation of the targets with antibody (71, 111, 112). This reaction does not require complement, instead depending on the interaction of antibody on the target cell with Fc receptors on the effector cells. The exact role of ADCC in the pathogenesis of autoimmune thyroid disease is unclear, as it has been investigated only as an in vitro phenomenon. Antibodies capable of mediating ADCC on target cells include those against TG and TPO, but other antigens may also be targets, and sera from patients with Hashimoto’s thyroiditis, primary myxedema and Graves’ disease cause ADCC, although the frequency is lower in Graves’ disease (86, 113). A further possible role for TPO antibodies has been suggested by the finding that these bind to cultured astrocytes and it is therefore possible that the controversial entity of Hashimoto’s encephalopathy is the result of some autoimmune cross-reaction between thyroid and central nervous system (113a).
Antibody titers for all types of autoantibody obviously increase during the process of development of AITD, but this is not clearly documented. It is possible that one critical step in the production of TG autoimmune responsiveness is the generation of immunoreactive C-terminal fragments during hormone synthesis (which results in oxidative stress); these fragments may also lead to preferential presentation of TG epitopes by thyroid cells (114). After first observation, they tend to be stable over months. Recent studies have shown that so-called natural autoantibodies against TG may be more important in pathogenesis than previously thought. These low affinity, mainly IgM antibodies, which are frequent in healthy individuals, can complex TG with complement and such opsonized complexes can be taken up by B cells and presented to CD4 + T cells, most likely in some regulatory fashion (115).
Radioactive iodine therapy leads to a rise in thyroid antibody levels in general in Graves’ disease (116), and acute viral infections, or exposure to high levels of IL-2 (117) or IFN- (118) also does so. With treatment of Graves’ disease, or replacement therapy in Hashimoto’s thyroiditis or myxedema, there is characteristically a gradual reduction in antibody levels over months or years, and some patients with total destruction of thyroid tissue eventually lose detectable antibody titers.
Information is beginning to emerge on the specific B cell epitopes for TG and TPO. There are two major conformational epitopes on the TG molecule that are recognized differentially by sera from healthy subjects and those with AITD; linear epitopes are recognized by polyclonal antibodies from healthy individuals (119 – 121). Similar studies on TPO have indicated at least eight major domains for human autoantibodies which are probably conformational epitopes. Using recombinant proteins and synthetic peptides, human anti-TPO antibodies are found to recognize apparently linear epitopes in the area of amino acids 590-622 and 710-722 (122) but, again, the important epitopes are most likely to be conformational (57, 66). A full 3-dimensional model will be required to identify TPO epitopes fully (123).
Peripheral blood mononuclear cells (PBMC) and thyroid lymphocytes from patients with AITD have among them activated cells that spontaneously secrete anti-TG and anti-McAg/TPO antibodies (124, 125). B cell production of antibodies to McAg/TPO and TG is most easily shown using cells incubated with mitogens. Specific antibody secretion in response to PBMC stimulation by TG or purified TPO is more difficult to demonstrate (125, 126). In patients with AITD, approximately 50 B cells secreting anti-TG antibodies are found per 10 6 PBMC (~2% of total Ig secreting cells) by using plaque-forming assays after stimulation of PBMC with pokeweed mitogen. B cells from AITD patients synthesize antibodies in response to insolubilized TG bound to Sepharose (127), which appears to function as an especially good antigen. There are reports of production of anti-TSH-R antibodies in vitro, but in general this response has been difficult to observe.
During initiation of AITD, thyroid autoantibody formation presumably occurs in lymph nodes draining the thyroid. In fully developed AITD, the thyroid is clearly an important source of autoantibody. In fact, since there are relatively few circulating specific autoantibody-secreting B cells (128), it has been suggested that autoantibody formation occurs mainly or uniquely in the thyroid, where spontaneous autoantibody secretion by B cells are more easily demonstrated (129). This is supported by the histopathological features, including the demonstration of thyroid antigen-specific B cells and the occurrence of secondary immunoglobulin gene rearrangement in intrathyroidal lymphoid follicles, together with a congruent pattern of adhesion molecule and chemokine expression (130). However, lymph nodes, bone marrow and possibly other organs also contribute to autoantibody production (131) and this explains why patients with apparently destroyed thyroid tissue, or those with resected thyroids, continue to have circulating thyroid auto-antibodies
K.2. Cell-mediated Immunity in AUTOIMMUNE THYROID DISEASE
Techniques for identification of T lymphocyte reactivity to foreign or autologous antigens depend on culturing mixed peripheral leukocytes or semipurified thyroid or blood lymphocytes with an antigen to which the cells may have been pre-sensitized. Upon re-exposure to antigen, the sensitized cells change to a blast-like immature form, synthesize new protein, RNA, and DNA, and directly or through liberated effector molecules alter the function of target cells. Different endpoints characterize the various assays, including measurement of [ 3 H]-thymidine uptake, assay of migration inhibition factor (MIF), or leukocyte migration inhibition (LMI) (132), assessment of the mobility of lymphocytes, and cytokine assay, all after stimulation with antigen in culture.
Numerous reports have shown that T cell immunity can be detected in Graves’ disease, Hashimoto’s thyroiditis, and primary myxedema, although responsivity of T cells to thyroid antigens is much less than to exogenous antigens such as tetanus toxoid or tuberculin. PBMC, or thyroid T cells plus B cells and APCs, respond to TG and microsomal antigen with production of antibodies, although the response is usually weak and not present in many patients (127, 128). Peripheral blood T cells respond to incubation with TG or microsomal antigen by thymidine incorporation, the so-called proliferation assay (133, 134). Responses by separated lymphocytes are generally weak; better responses are seen by adding IL-2 to thyroid antigen-stimulated cultures of diluted whole blood (135). T cells secrete lymphotoxin in response to incubation with thyroid particulate membrane material, which presumably includes microsomal antigen (136). Thyroid T cells responding to TG are of the CD4+ T helper type (137), or occasionally CD8+ cells (138). T cells also respond to crude thyroid antigen, possibly “microsomal antigen”, in LMI (132, 139). T cell lines and short term T cell clones (CD4+) are stimulated during coculture with TECs to incorporate [ 3 H]-thymidine; DR+ TECs are especially effective stimulators (140 – 142). The identity of the antigen recognized on TECs is unknown but may well be TPO and/or TG.
The specific peptide epitope fragments of TPO recognized by lymphocytes of patients with HT were noted previously. T cell epitopes present within the extracellular domain of the TSH-R are also heterogeneous with peptides bearing sequences of aa 158-176, 237-252, and 248-263 and 343-362 being especially important (143) but other epitopes (aa 57-71, 142-161, 202-221, 247-266) have been identified by others using different assay parameters (144). HLA-DR3 molecules bind TSH-R peptides with high affinity, which may explain the genetic association of this HLA specificity with Graves’ disease (145).
T cell responses to an antigenic stimulus may use a wide variety of variable (V) TCR gene segments, or the response may involve (be “restricted” to) a few V segments. Restriction of autoreactive T cells to use of one or more V gene segments has been found in some experimental autoimmune models (4). Restricted V and V usage has been reported by Davies and co-workers (146, 147) but not found by others in the whole intrathyroidal lymphocyte population (148, 149). However, CD8+ do display a degree of restriction although their autoreactive potential is at present not known (150). Presumably at an early stage of disease, the T cell response is clonally restricted, but as it advances, spreading of the immune response occurs, involving many more epitopes, leading to an unrestricted response as demonstrated in an animal model of AITD (151). Recent evidence has emerged of a combined TG and TPO epitope-specific cellular immunity, with CD8+ T cells reacting against these epitopes rising to 9% in the peripheral blood of patients with long-standing Hashimoto’s thyroiditis (151a).
While T cell immunization is conventionally recognized by a stimulatory effect of antigen, direct T cell cytotoxicity of thyroid cells has been recognized in a few studies. For example, Davies and co‑workers developed a CD8+ T cell clone which was cytotoxic to autologous TEC and was appropriately class I restricted (152). An interesting potential consequence of T lymphocytic adherence to thyroid cells, via ICAM-1/LFA-3 interaction, is the stimulation of thyroid cell proliferation, which could lead to goiter formation (153).
L.3. Immune Complexes
In addition to the antibody and T cell responses, circulating immune complexes are found in patients with autoimmune thyroid disease as would be anticipated], although their pathogenic importance appears minimal. In a certain sense this is most fortuitous. Since many individuals have circulating TG antibodies and antigen, if the immune complexes caused serious disease, it would be a plague. Fortunately the immune complexes of TG and its antibody do not bind complement weakly and do not cause serious illness such as immune-complex nephritis, except in rare instances (154, 155). Immune complexes, including complement terminal components, can be recognized around the basement membrane of thyroid follicular cells (104) and may cause sublethal effects including release of proinflammatory mediators by TECs (156). Release of TG into the circulation can cause formation of immune complexes which are rapidly removed from the circulation, and the process could lead to depletion of circulating antibody levels. It is possible that this antigen-dependent antibody depletion contributes to the lower levels of anti-TG antibody found in Graves’ disease, compared to Hashimoto’s thyroiditis, since the thyroid of Graves’ disease releases more TG than that of Hashimoto’s thyroiditis.
M.4. K and NK Cell Responses
Many studies have been reported on natural killer (NK) cell activity and antibody dependent cell-mediated cytotoxicity (ADCC); their conclusions vary. Endo et al (162) found NK cells were decreased in Graves’ disease and Hashimoto’s thyroiditis, and presented evidence that this was due to saturation of their Fc receptors by immune complexes. Normal NK effector function was found in Hashimoto’s thyroiditis PBMC (158) in one study, although by phenotyping, decreased NK cells in Graves’ disease, and increased NK cells in Hashimoto’s thyroiditis were reported in another (159). ADCC of thyroid cells, mediated by normal PBMC, was induced by anti-McAg/TPO antibody positive sera (160) but other, unknown antibody-antigen systems also contribute (113). Effector cell activity in ADCC was increased in Hashimoto’s thyroiditis and in post-partum thyroiditis, and thought to be related to thyroid cell destruction (161). Other data have indicted that ADCC may be more important in primary myxedema than Hashimoto’s thyroiditis explaining the difference in clinical presentation (162), but this has not been confirmed in two other studies showing equal ADCC activity in sera from both diseases (113, 163).
Cytokines lie at the heart of the autoimmune response and can have a number of direct and indirect effects (Fig. 7-9). For example, IFN- is produced in the thyroid by infiltrating lymphocytes and causes HLA class I expression on the surface of TECs to increase and initiates class II expression. It also has a direct inhibitory function on TEC iodination and TG synthesis (164, 165). These effects are mediated by multiple, temporally distinct mechanisms, at least in part acting via effects on cAMP response element-regulated gene expression (166). IFN- is not essential for the development of AITD in mice but exacerbates disease activity (167). IL-2 can activate lymphocytes to produce IFN- , and activate NK cells. TNF is produced by infiltrating macrophages and is potentially cytotoxic to TEC. TEC can produce several cytokines, including IL-1, which may activate T cells, IL-6, which stimulates T and B cells and IL-8, a chemokine which attracts inflammatory cells (reviewed in 165). Dendritic cells are important sources of IL-1 and IL-6 in the thyroid and can inhibit thyroid follicular cell growth (168). Plasmacytoid dendritic cell numbers are decreased in the blood in AITD, together with an alteration in their phenotype, but these cells increase in the thyroid gland, also suggesting that this cell type may be important in pathogenesis (168a)
In addition, IL-1 causes dissociation of junctional complexes between thyroid cells which could expose hidden autoantigens (169). An ever wider array of factors besides the classical cytokines has been implicated in the pathogenesis of AITD , including the finding that thyroid cells can release angiopoietin-1 and -2 (169a). These ligands serve as a chemoattractant for monocytes and the angiopoietin receptor, Tie-2, is increased in monocytes form AITD patients, suggesting a role for monocytes in thyroid damage. Vascular endothelial growth factor expression is increased in AITD and is important in angiogenesis in autoimmune goiters (170). Cytokines also seem to play a major role in the pathogenesis of thyroid-associated ophthalmopathy through their stimulatory actions on orbital fibroblasts (167, 171). Exogenous cytokines given therapeutically can also precipitate autoimmune thyroid disease, probably in predisposed individuals. The best described such reaction is α interferon (118). Destructive thyroiditis accounts for the majority of thyroid dysfunction after treatment with this cytokine, and risks are highest in white women, whereas smoking is protective (118a).
To summarize, augmented pools of activated and resting T and B cells reactive to thyroid antigen exist in patients with AITD. The time course of development of these reactive cells, before clinical disease is apparent, has not been established. The cells respond to biochemically normal antigen, and some reactive cells exist in otherwise healthy individuals. Immune complex formation appears to be of limited importance in the disease process. K and NK activity may be reduced in Graves’ disease and increased in Hashimoto’s thyroiditis and may contribute to the course of the disease – proliferative in Graves’ disease and destructive in Hashimoto’s thyroiditis. Cytokines have multiple actions in the thyroid in AITD and are likely to determine clinical manifestations such as ophthalmopathy. The role of the TEC in the autoimmune response is not simply passive and, as discussed below, the interaction between TECs and cells of the classical immune system may be critical in determining the outcome of an initially mild thyroiditis.
EXPERIMENTAL THYROIDITIS IN ANIMALS
Chronic thyroiditis histologically identical to that in Hashimoto’s thyroiditis occurs spontaneously in Obese strain (OS) chickens (172), beagles (173), mice, and rats. It can be induced in dogs (174), mice, rats, hamsters, guinea pigs, rabbits, monkeys (175), and baboons (176) by immunization with autologous or allogenic thyroid homogenate mixed with adjuvants, or by using heterologous TG , or TG that has been arsenylated or otherwise chemically modified (1). The need for modification of TG or adjuvant to break tolerance can also be overcome by immunization with cDNA (177). An important thyroiditogenic epitope includes a thyroxine residue (aa 2553) in human TG (178, 179) but the role of iodination at this site is unclear and may depend on the type of T cell assay system used, as well as other parameters (180). Mice have been the most frequently used model and have provided key insights into genetic susceptibility, pathogenesis and the development of T reg and autoreactive T cell repertoires (180a).
Induced thyroiditis leads to formation of humoral antibodies and T cell- mediated immunity. Usually the histologic pattern conforms to that of T cell-mediated immunity (181). The role of TG antibodies is unclear but minor. An idiotype-anti-idiotype network exists for TG antibodies in mice but the induction of those antibodies does not lead to thyroiditis (182). Furthermore, the intensity of the thyroiditis correlates better with T cell mediated immunity than with antibody levels, and can be transferred by T cells but not antibodies, and both CD4+ and CD8+ T cells are usually needed for transfer (183). In normal mice, thyroiditis can be produced by immunization with mouse TG in adjuvant, and transferred to isogenic animals by sensitized Ly-1+ T cells. The same cells, given before immunization, vaccinate against the development of thyroiditis during subsequent immunization (184).
However, a subpopulation of CD4+ T cells has an important regulatory role in tolerance to murine TG, keeping in check those TG-reactive T cells which escape thymic deletion and peripheral anergy-inducing mechanisms (185). Amelioration of thyroiditis by oral administration of TG (186) operates through enhancing the activity of these regulatory T cells although other mechanisms are possible. Recent studies have emphasised the importance of regulatory T cells in suppression of thyroiditis in animals immunised with TG. In particular, semi-mature dendritic cells, which can be induced with granulocyte-macrophage colony stimulating factor, can induce the function of TG-specific CD4 + ,CD25 + T cells which can suppress thyroiditis through the production of IL-10 (187, 188).
Another recent model has used homologous (murine) TPO in an immunization protocol and this method established thyroiditis and TPO antibody production although none of the immunized mice developed hypothyroidism (190). Another unique model is the creation of HLA-DRB1*0301 (DR3) transgenic mice which are susceptible to thyroiditis induced by TG immunization, unlike DR2 transgenics, thus confirming that HLA-DRB1 polymorphism determines susceptibility to autoimmune thyroiditis, and his model has been extended to study of the immune response to TSH-R, with results again showing the importance of the DR3 specificity (191). However, when the modeling has attempted to reproduce Graves’ disease by immunization of mice with adenovirus expressing the TSH-R, it is non-MHC genes which play a major role in controlling the development of hyperthyroidism (192). This concurs with the polygenic susceptibility and rather weak effect of HLA-DR3 in Graves’ disease. This model has also been used recently to show that dietary iodide enhances the development of thyroid disease and depletion of CD4+, CD25+ Tregs exacerbates this iodide-induced thyroiditis (193a)
Spontaneous thyroiditis in OS chickens more closely resembles Hashimoto’s thyroiditis than the immunization models just discussed, particularly as the birds develop hypothyroidism as a consequence of the autoimmune process. Some evidence suggests that the thyroid of the newly hatched chick is intrinsically abnormal, since its function is partially nonsuppressible by thyroid hormone and this constitutes an important element of the genetic susceptibility of these birds, together with genes controlling T cell responses and possibly glucocorticoid tonus. The MHC conversely has only a limited effect. Iodine plays a critical role in the induction of thyroid injury in OS chickens, most likely through the generation of reactive oxygen metabolites, and this injury is an early event, preceding lymphocytic infiltration (193). Iodination of TG is a second path by which iodine influences disease in OS chickens, as autoreactive T cells respond to the antigen only if it is iodinated (194).
Lymphocytic thyroiditis occurs spontaneously in the Buffalo and BB/W rat strains and the NOD mouse line (195). In both species, there are associated abnormalities in the animals’ immune system. As in the OS chicken, administration of excess iodine augments the incidence of rat thyroiditis and iodine depletion reduces it (196). Iodine also enhances the susceptibility of NOD mice to thyroiditis, and further exploration of this model has demonstrated a key role for Th17 cells which accumulate within the thyroid (196a). IL-17-deficient mice have a markedly reduced frequency of Tg autoantibodies and thyroid lesions. The susceptibility of NOD mice has also been exploited in a recent model in which the CCR7 gene was knocked out in this strain: such mice do not develop diabetes but do develop severe inflammation elsewhere including a severe thyroiditis with TG autoantibody formation and hypothyroidism (196b). CCR7 is a chemokine receptor which is expressed by T regs; the CC7-deficient mice had lower numbers of these cells. As well as this effect, it is possible that CCR7 deficiency impaired negative selection of thyroid reactive T cells. Another intriguing aspect of this model comes from long-term observations in NOD.H-2h4 mice which have shown that TG antibodies occur initially and much later TPO antibodies appear, suggesting that tolerance at the B cell and presumably T cell level is broken first for TG and then by spreading (see above) for TPO (196c). These results suggest a more important role for TG as an autoantigen in AITD than it is currently assigned. When engineered through CD28 knockout to have a deficiency of Treg cells, NOD mice develop more severe thyroiditis than control animals, with thyroid fibrosis and hypothyroidism. Transferring healthy Treg cells reduces thyroiditis without increasing the total number of Treg cells, suggesting that endogenous Tregs in these mice are functionally defective (196d).
A third kind of model is produced by manipulation of T cells. The original description of thyroiditis in genetically susceptible rats by sublethal irradiation and thymectomy (197) has been followed by a number of more refined models in which T cell subsets can be perturbed more or less specifically to induce disease (181). For instance CD7/CD28 double-deficient mice have impaired Treg function and such animals develop spontaneous thyroiditis after 1 year of age (198). These experiments clearly demonstrate the recurrence of autoreactive T and B cells in normal animals and show that any of a number of factors which can perturb the regulation of these could result in autoimmune thyroiditis (Fig. 7-10). The most elegant model resulting from T cell manipulation is the generation of transgenic mice expressing a human T cell receptor specific for a TPO epitope, which resulted in a spontaneous destructive hypothyroidism and hypothyroidism (199). The CD8 T cells recognizing the epitope in these animals unconventionally were MHC class II rather than class I restricted and it is unclear whether this atypical behavior is significant to the creation of the model, nor is it yet clear what the mechanism is for thyroid cell destruction.
Another intriguing model is the recent description that necrotic thyroid cells can induce maturation of dendritic cells in vitro, and when injected back into autologous mice EAT is induced, with a lymphocytic thyroiditis and TG-specific IgG (200). It is not clear whether this protocol yields cryptic TG epitopes which can break tolerance. It is possible that such work could be in a sense be reversed to allow attenuation of EAT by pulsing tolerogenic dendritic cells.
Establishing an animal of Graves’ disease has been surprisingly difficult despite the cloning of the TSH-R. Spontaneous models are not obvious, suggesting that critical differences in the TSH-R receptor between man and other mammals (such as glycosylation) may be necessary to break tolerance (201). However, immunization of AKR/N mice (but not other strains sharing the same MHC haplotype) with murine fibroblasts doubly transfected with the human TSH-R and haploidentical MHC class II genes results in a syndrome similar to Graves’ disease except that thyroid lymphocytic infiltration was not induced (202), whereas thyroiditis is a feature of immunization with the TSH-R (203). This is a promising model although its exact physiological parallel remains unclear, particularly as fibroblasts may behave differently to TECs in terms of antigen presentation. This is because the fibroblasts used express the critical costimulatory molecule B7-1 and also because the procedure causes generalized in vivo immune activation. This model is therefore not evidence that thyroid follicular cells (which do not normally express B7) could initiate thyroid autoimmunity. Recent models include the use of transgenic mice expressing the A-subunit of the TSH-R, which develop lymphocytic infiltration of the thyroid, hypothyroidism and autoantibodies against TG and TPO as well as TSH-R following immunization with the TSH-R expressed in adenovirus and regulatory T cell depletion (203a). Although obviously a contrived system, this model does clearly show that spreading of the immune response can occur to include the normal array of antibodies found in patients, and that this can result in a severe thyroiditis. Some of the difficulties in producing reliable animal models of Graves’ disease are seen in the disparity between hyperthyroidism in the animal and the presence of TSH-R antibodies detected by bioassays using human TSH-R. This may be the result of loci in the immunoglobulin heavy chain variable region contributing in a strain-specific manner to the development of antibodies specific for the human or the mouse TSH-R (203b). This novel finding of a role for immunoglobulin heavy chain variable region genes in TSAb specificity indicates a possible role for them genetic susceptibility to human Graves’ disease.
One unexpected finding has been the observation that mice with a TSH-R knockout do not differ in their response to immunization with TSH-R when compared to healthy animals, whereas the expectation was that such animals would have no tolerance to this autoantigen (as it had been absent throughout development) and therefore a greater immune response would be predicted (204). This suggests that thymic (central) tolerance is not a critical step in self tolerance to this autoantigen. Similar conclusions have been drawn from the finding of similar intrathymic transcript levels of thyroid autoantigens (TPO and TSHR) in mice which are genetically susceptible or resistant to the development of EAT (204a). However the situation may be more complex than originally imagined, as the same group have identified a role of the Aire gene in the response to TSHR and in Aire -deficient mice, intrathymic transcripts of TSHR and Tg are reduced while the expression of TPO is nearly abolished. These results are compatible with the finding of an increase in AITD in autoimmune polyglandular syndrome type 1, but at a much lower frequency than the classical disorders of Addison’s disease and hypoparathyroidism. It is also intriguing that TPO transcripts are so much more affected in the Aire -deficient murine thymus, perhaps explaining (via more rigorous tolerance) the rather weak response to this autoantigen, compared to TG, in the mouse.
Balb/c strain mice appeared to develop orbital changes suggestive of ophthalmopathy when given TSH-R primed T cells derived from donor mice immunized with TSH-R protein or cDNA but this model has not proved reproducible by the original authors, for reasons which are not yet clear, although complex histological artefacts may be part of the answer (205). A much more convincing model of ophthalmopathy has been described recently in which deep injection of plasmid containing the TSH-R A subunit into the leg muscles of BALB/c mice followed by electroporation resulted in a variety of histological orbital changes and obvious eye signs (205a). However the animals developed TSH-R blocking rather than stimulating antibodies and thyroiditis was absent. Nonetheless these finding strongly support a pathogenic role for the TSH-R in the pathogenesis of thyroid eye disease.
Another novel Graves’ disease model has used the hamster rather than mouse-immunization with TSH-R-transfected CHO cells, co-expressing MHC class II molecules, produced mainly blocking antibodies, whereas MHC class II negative cells induced, rarely, TSAb together with a focal lymphocytic infiltrate (206).
One general concept derived from all of these studies is that a genetically controlled balance of helper and suppressor T cell function is needed to prevent autoimmunity, and that a variety of perturbations leads to onset of the disease.
Relation of the Immune Response to THE Thyroid Cell: Stimulation and Destruction
For certain we know that the autoantibodies can stimulate the thyroid and cause overactivity in Graves’ disease, and can in select circumstances inhibit thyroid function and cause hypothyroidism in neonates and some adults. Whether anti-TG or anti-McAg/TPO antibodies are primary cytotoxic agents in AITD remains an unsettled issue. TG antibodies are probably not normally cytotoxic, but TPO antibodies can certainly mediate complement dependent thyroid cell cytotoxicity and ADCC. However, the frequently reproduced natural experiment of transplacental antibody passage from a mother with AITD to her fetus, without evidence of thyroid damage, clearly shows that antibodies alone are not destructive to the thyroid.
Cell-mediated immunity is thought to be important in thyroid cell destruction, and T cells have been shown to be reactive to TECs. T cell lines or clones have been shown to react to TECs (140 – 142), but the nature of the antigen recognized is unknown. One CD8+ T cell clone in man has been shown to be cytotoxic specifically to autologous TECs (152), suggesting that cell-mediated TEC destruction is an important process, and similar activity has been reported in CD8+ T cell lines and clones derived from mice with experimental autoimmune thyroiditis (207). A second type of T cell-mediated cytotoxicity is that mediated by TCR-bearing T cells and specific recognition of TECs by such cells has been reported in Graves’ disease, but the exact autoantigen involved is unknown (208). In animals it is clearly shown that there can be a marked dissociation between the extent of histologic thyroiditis and the levels of antibodies, again suggesting that T cells rather than antibodies mediate cell destruction. However, it must be admitted that the hard evidence for direct T cell-mediated cytotoxicity in thyroid autoimmunity in man is meagre at present.
There are 3 mechanisms by which T cells might mediate TEC destruction and evidence for all 3 operating in AITD has accrued. Firstly, cell lysis might be effected via T cell-derived perforin, which leads to pore formation in target cell surfaces, and certainly the thyroid lymphocytic infiltrate contains perforin-expressing T cells in AITD (209). Secondly, T cells expressing Fas ligand, especially the CD8+ subset, can induce apoptosis in TECs expressing Fas (210). Fas is induced by IL-1 on TECs, whereas TSH-R stimulation inhibits Fas expression (211), and this may lead to the involvement of this pathway in Hashimoto’s thyroiditis but not Graves’ disease, as TSI would act like TSH in the latter to diminish Fas expression (and other regulatory molecules) (209). It has been suggested that T cells may not be necessary, as Hashimoto TEC may express Fas ligand, and autocrine/paracrine interaction with Fas may lead to TEC death (212). The mechanisms for this are unclear and as yet there is no consensus on the role this may have in AITD. The picture is complicated by the upregulation of molecules which protect against apoptosis such as Bcl-2. The pattern of expression of this molecule is different in Graves’ and Hashimoto’s diseases, suggesting that TECs are protected in the former and more sensitive to destruction in the latter (213). Whether these differences depend on cytokines, genetics or other factors is at present unknown (214). Finally, T cell-derived cytokines can injure the TECs directly, leading to functional impairment (164, 165), and by triggering other phlogistic pathways such as nitric oxide synthesis (215).
Possible Explanations for Autoimmunity
Many reasons for the development of autoimmunity have been advanced, and several of these are briefly catalogued below. Some of these have been examined in relation to AITD and are discussed more extensively in the following sections. Currently, cross-reacting epitopes, aberrant T or B cell regulatory mechanisms, inheritance of specific immune response-related genes, and aberrant HLA-DR expression on TECs are considered important for development and progression of thyroid autoimmunity.
1. Abnormal presentation of antigen could occur due to cell destruction, or viral invasion, so that large amounts of antigen or cell fragments are liberated locally into the lymphatics. Excessive levels of antigen are produced, thereby overwhelming the usual low dose tolerance mechanism.
2. Abnormal antigen could be produced by a malignancy, or damage to the cell by viral attack, or other means. This antigen could be a partially degraded or denatured normal antigen, for example.
3. Cross-reacting bacterial or viral epitopes e.g. Yersinia enterocolitica (216) could induce immune responses that happen to cross-react with a self-antigen having identical conformation. An extension of this concept is that the normal anti-idiotypic control response happens to produce an Ig or T cell that cross-reacts with self-antigen. For example, experimentally produced anti-idiotypic monoclonal antibodies directed to TSH antibodies bind to and stimulate the TSH-R (217).
4. Somatic mutation of a TCR gene could lead to a clone of self-reactive cells. However, somatic mutation of TCR genes is believed to occur very rarely if at all, and such monoclonal or oligoclonal activation has not been documented in autoimmune disease. Somatic mutation of B cell Ig genes is, as described above, a normal phenomenon during an antigen‑driven proliferative response. Such an event could occur by chance during response to any antigen and this does not effectively introduce any new variable, since B cells capable of producing Igs that can react with self-antigens are already normally present. However, TSI seem to be clonally restricted and, until the V gene usage of these antibodies is documented, it remains possible that Graves’ disease is due to the inheritance of a unique, etiologically critical V gene encoding TSI.
5. Inheritance of specific HLA, TCR, or other genes that code for proteins having especially effective ability to process or present antigen.
6. T cell or B cell feedback control mechanisms could be aberrant due to hereditary or environmental factors.
7. Failure of clonal deletion could leave self-reactive T cells present in the adult. In fact this is clearly normal, as described above.
8. Failure of normal maturation of immune system could allow fetal T and B cells that are autoreactive and of wide specificity to persist.
9. Polyclonal activation of T or B cells, by some unknown stimulus, could lead to B cells producing self-reactive Ig, in the apparent absence of antigenic stimulus. This theory is in a sense impossible to disprove but would need to co-exist with other abnormalities to explain disease remission, genetic associations, associated diseases, etc. Polyclonal activation is not typical of peripheral lymphocytes of patients with AITD (218).
10. TECs could express MHC class II molecules as a primary event and then could function as APCs, including antigens on their cell surface.
11. Environmental factors could distort normal control. For example, stress or steroids may alter immunoregulation, and the potential role of dietary iodine has been mentioned above.
P.Abnormal Exposure to Thyroid Antigens and THE EFFECTS OF PREGNANCY
Damage to the thyroid might release normally sequestered antigens, inducing an immune response. Damage to thyroid cells does indeed occur in viral thyroiditis, such as in association with mumps or in subacute thyroiditis of unknown cause, but autoantibodies appear only transiently at low titer, and progressive lesions of the thyroid do not usually occur (reviewed in 219, 220). External irradiation to the thyroid, including that from nuclear fallout, can also lead to an increase in Graves’ disease or thyroid antibody production (221, 222), but it is unclear if this is caused by autoantigen release or an effect on the lymphocytes which are radio-sensitive. Even occupational exposure to ionizing radiation appears to be a risk factor for the development of autoimmune thyroiditis (223). Another possible example where exposure to thyroid antigens released by gland injury leads to autoimmunity is the precipitation of Graves’ disease and ophthalmopathy after ethanol injection of thyroid nodules (223a).
A powerful argument against the hidden antigen hypothesis is that TG is a normal component of circulating plasma (224). One might turn the first argument around and suggest that thyroiditis results from a lack of exposure to TG at some period, an exposure that is necessary to depress continuously an otherwise inevitable immune response. This suggestion has no clinical or experimental support, and the available evidence indicates that TG is present in the plasma of patients with active immunity. It remains to be seen how sequestered TPO and TSH-R are, but the appearance of T cells capable of proliferating in response to these antigens, in apparently healthy individuals, also argues against any sequestration (54, 225). What is clear is that availability of the thyroid autoantigen is essential to maintain the autoimmune response: complete removal of thyroid antigens following thyroidectomy and remnant ablation with radioiodine leads to disappearance of antibodies to Tg, TPO and TSH-R (226). Although this is not surprising, it does suggest that extrathyroidal sources TSH-R are insufficient normally to maintain an autoimmune response.
A new variant on this theme is that of microchimerism, the persistence of fetal cells in maternal tissues. Studies have found evidence of microchimerism in thyroid tissue from patients with and without AITD (227). Could such sequestered fetal material make the thyroid prone to an alloautoimmune response, and be responsible for the exacerbation of AITD seen in the postpartum period? If so, this phenomenon would help to explain the high frequency of AITD in women. Twins from opposite sex pairs should have an increased risk of thyroid autoimmunity compared to monozygotic twins if microchimerism has a role, and indeed such twins have been found to have more frequent thyroid autoantibodies (228). However, although parity is associated with an 11% increase in the risk of all female-associated autoimmune disorders, there is no increase with multiple pregnancies, which rather argues against a microchimerism mechanism (229). During and after pregnancy, major changes in Treg function occur and direct effects on the cytokines produced by T cells can also be demonstrated (229a). It is these alterations are most probably the ultimate cause of the increase in autoimmunity after pregnancy.
It seems likely that sex steroids play a role in determining the autoimmune response. Another hypothetical reason for the unequal sex ratio is that skewed X chromosome inactivation, which has recently been demonstrated in scleroderma and in autoimmune thyroid disease, could contribute through the failure of some autoantigens expressed on one X chromosome to be expressed at a critical point in the disease pathway (230).
An abnormal antigen might also serve to produce an immune reaction. The protein abnormality could be either congenital or acquired by an injury such as a virus infection. To date there is no evidence which indicates that TG, TPO, or other proteins of the thyroid of a patient with autoimmunity are abnormal. Minor allelic differences apparently do occur but attempts to associate thyroid disease with polymorphisms of the TPO and TSH-R genes have been unsuccessful.
The theory that immune reactivity to an environmental antigen could lead to antibodies that cross‑react with thyroid antigens has been bolstered by studies which show a relationship between Graves’ disease and antibodies to the common enteropathogen Yersinia enterocolitica . An increased incidence of antibodies to Yersinia is found by some, but not all authors, in patients who have Graves’ disease (219), and there are saturable binding sites for TSH on Yersinia proteins (231). After infection by Yersinia , human sera contain Igs that bind to TEC cytoplasm (216), and IgGs which appear to compete with TSH for binding to thyroid membrane TSH receptors (232). The antigens involved may in fact include proteins encoded by plasmids present in the Yersinia , rather than intrinsic Yersinia proteins, but that does not alter the general concept (233). Arguing strongly against a role for Yersinia is the fact that there is no unique pattern of serological immunoreactivity to Yersinia antigens in patients with AITD (234), and most patients with this infection do not develop Graves’ disease. Moreover, there was no association between Yersinia infection and autoimmune thyroid disease in a large prospective study (234a).
Heat shock proteins (HSPs) are produced by prokaryotic and eukaryotic cells in response to heat and other forms of cellular injury. Because their structure is highly conserved in all cells studied, the potential cross-reactivity between HSPs from pathogens and human is great. A number of pathogenetic organisms present HSPs as antigens in animal and human models. It is therefore possible that immunization against HSPs from a pathogen may lead to cross-reactivity with autologous HSPs released from damaged tissues. The HLA region contains genes for the major HSP70, and polymorphisms in the HSP70 gene are associated with Graves’ disease (235). This finding could provide another link between HLA and susceptibility to infection and autoimmunity. HSPs are expressed at a high level in thyroid cells from patients with Graves’ disease, and in fibroblasts from patients with exophthalmos (236). These changes may be secondary to cytokine stimulation, but could be involved in a secondary immune response.
Antibodies to TG sometimes also recognize TPO (237). The exact reason for this is uncertain, but may — or may not — relate to short stretches of shared peptide sequence and hence shared B cell epitope. In theory an initial response to one antigen might proceed by reacting to the other antigen, and thereby spread and augment the autoimmune process. In the context of T cell autoreactivity there is much greater scope for molecular mimicry whereby a response to an exogenous epitope leads to a cross-reactive response to an endogenous autoantigenic epitope. Simple sequence homology is insufficient to predict this, as shown elegantly by the cross-reactivity of two TPO epitopes showing a similar surface but not amino acid sequence (238). This makes the prediction and study of molecular mimicry much more difficult than is generally appreciated (239). For these reasons, it may be naïve to believe that the putative orbital antigen responsible for ophthalmopathy will be the identical protein (eg. TSH-R) to that expressed in the thyroid.
Virus infection has for years been speculated to be an etiological factor in most autoimmune diseases, by causing cell destruction and liberating antigens, by forming altered antigens or causing molecular mimicry, by inducing DR expression, or by inducing CD8+ T cell responses to viral antigens expressed on the cell surface. Antithyroid antibodies are elevated transiently after subacute thyroiditis, which is thought to be a virus-associated syndrome, but no clear evidence of virus-induced autoimmune thyroiditis in humans has been presented. In this regard it is of great interest that persistent, apparently benign virus infection of the thyroid can be induced in mice (240), and that infection of neonatal mice with Reo virus induces a polyendocrine autoimmunity (Fig. 7-11). These agents could work by liberating thyroid antigens. Virus infection might also augment autoimmunity by causing non-specific secretion of IL-2, or by inducing MHC class II expression on TEC. Despite many attempts to implicate retroviruses in AITD, results to date remain inconclusive (219, 241), although a recent study has detected elevated levels of reverse transcriptase in well conducted experiments with Graves’ thyroid tissue, reviving this concept (242). Human T lymphotrophic virus-1 has been repeatedly associated with various autoimmune disorders, including Hashimoto’s thyroiditis; presumably the virus alters immunoregulatory pathways allowing autoimmunity to emerge (243).
S.Lymphocyte Mutation and Oligoclonality
Apart from the evidence that some TSI may have an oligoclonal origin (88, 244), there is no evidence to support a clonal B cell abnormality in AITD. V gene usage by TSI will need to be analysed to determine whether Graves’ disease has a unique pathogenesis determined by germ-line immunoglobulin genes. Thyroid-reactive T cells are present in healthy animals and man, as noted above, and therefore a defect at the clonal T cell level is less likely as a primary event in etiology than previously thought. A few autoreactive T cells can be expected to escape tolerance normally, particularly if the autoantigen in question is not available to delete T cells in thymus during fetal development. Stochastic events later in life affecting such undeleted T cells could readily explain the lack of complete concordance for AITD in genetically identical twins (245), and this lack of such concordance argues against an inherited pathogenic TCR as a primary event in AITD.
A role of heredity in AITD is clearly demonstrated by family studies (246, 247). The role of heredity in AITD is clear, since there is an increased frequency of AITD among family members, first degree relatives, and twins of patients with the illness (248). Indeed a recent careful analysis of concordance in Danish twins with Graves’ disease came up with the estimate that 79% of the liability for this disorder was attributable to genetic factors (249). A recent study from the USA has found similar concordance rates for Graves’ disease as in this Danish study, while among the unaffected monozygotic twins of the patients with Graves’ disease, 17% had chronic thyroiditis, while 10% had pernicious anemia or other autoimmune disorders (250). In an investigation of the relatives of a group of propositi with high circulating antibody levels and clinical thyroid disease, approximately half of the siblings and parents (first‑order relatives) were found to have significant titers of thyroid antibodies, many being without clinical thyroid disease (251) but the transmission of thyroid autoantibodies is a more complex trait than the dominant inheritance originally thought (252, 253).
Together, such observations suggest that these diseases have a common genetic defect, although other genes are likely to be disease-specific in their effects, as reviewed extensively elsewhere (254). The most important susceptibility factor so far recognized is the inheritance of certain MHC class II genes. Inheritance of HLA-DR3 causes a 2 to 6-fold increased risk for the occurrence of Graves’ disease or autoimmune thyroiditis in Caucasians, and inheritance of HLA-DR4 and DR5 has been found in some studies to increase the incidence of goitrous hypothyroidism (255). In post-partum painless thyroiditis an association is found particularly with HLA-DR5 (256). Recent studies have identified HLA-DQA1*0501, which is often linked to DR3, as having an even more pronounced predisposing effect in Caucasians with Graves’ disease (257). HLA-DRB1*07 may be protective (258). One intriguing observation which needs further work is the observation that familial clustering of juvenile thyroid autoimmunity has a higher risk when the children’s fathers, not mothers, are HLA-DR3-positive, especially if the fathers also have TPO antibodies (259). This may imply an X-linked gene interaction with the HLA haplotype.
The HLA linkages found in Caucasians are not found in American Blacks, and different HLA associations are found in other ethnic groups such as Koreans, Chinese, and Japanese (255). This tends to suggest that such HLA associations do not depend on critical binding between selected class II molecules and epitopes from thyroid antigens (so-called determinant selection). Instead, the frequent association of HLA-DR3 with many autoimmune diseases may reflect a non-specific enhanced immunoresponsiveness encoded by DR3-linked haplotypes. It is noteworthy also that the relative risks conferred by HLA alleles is rather modest, borne out by the relatively low concordance for Graves’ disease in HLA-identical siblings of patients with Graves’ disease (259). This suggests the operation of other genetic susceptibility loci, also emphasised by the weak lod scores for linkage with the HLA region in family studies of AITD (260, 261).
The nature of these other loci is unclear and their identification is likely to require an extensive analysis involving several hundred families in studies using modern molecular techniques coupled to either genome screening or the transmission disequilibrium test. Association studies have been the method of choice to date, investigating various candidate genes, but with mixed success. It is now clear that to detect common, low-risk variants with reliability, huge sample sizes are essential facilitated by the haplotypic data available from the HapMap project, which means that genome wide variability can be detected using half a million single nucleotide polymorphisms (261a). These studies present considerable logistical challenges, and many older studies of genetic associations in AITD have produced conflicting results as because of lack of power or population stratification issues.
Inconclusive results have been reported for associations of AITD with TCR polymorphisms, immunoglobulin allotype and TSH-R polymorphisms. The most consistent non-HLA association is between polymorphisms in the CTLA-4 gene and both Graves’ disease and Hashimoto’s thyroiditis (262, 263). Despite claims to the contrary, there appears to be no additional risk conferred by CTLA-4 (or HLA) polymorphisms in Graves’ patients with clinical evidence of ophthalmopathy (264), but these CTLA-4 polymorphisms may partially determine outcome after antithyroid drug (265, 265a). Given the most important role of the interaction between CTLA-4 on T cells and the B7 family of molecules on APCs, it is possible that this association represents a genetic effect on immunoregulation, although, as with HLA-DR3, this is not specific for thyroid autoimmunity; the same polymorphism is also associated with type I diabetes mellitus and several other autoimmune disorders. Recent fine mapping of the CTLA-4 region has confirmed that it is indeed this gene, rather than those in linkage disequilibrium, which is responsible for the associations, and the polymorphisms may exert their effects by causing variation in levels of soluble CTLA-4, which in turn may after T cell activation, especially in Treg cells (266).
Another recent possible candidate is the association of a polymorphism of the vitamin D receptor with Graves’ disease, an association which has some biological plausibility as vitamin D has immunological effects (267). However a very large survey comprising 768 patients with Graves’ disease from the UK, compared to 864 controls, found no evidence of an association (268) and there is not yet any good evidence yet for vitamin D deficiency being associated with AITD (268a). Polymorphisms in genes encoding molecules involved the NFkB inhibitor pathway modulating B cell function (FCRL3 amd MAP3K7IP2) may also be involved in susceptibility to Graves’ disease (269, 269a). The most likely candidate for a third genetic susceptibility locus in Graves’ disease, besides HLA and CTLA-4, is polymorphism in the lymphoid tyrosine phosphatase gene, which has been associated with functional changes in T cell receptor signaling. A recent study of 549 patients and 429 controls found that a codon 620 tryptophan allele conferred an odds ratio of 1.88 (270), although it should be noted that similar effects have been seen in many other autoimmune diseases. This result has recently been confirmed (271a) and a fourth likely locus is the IL-2 receptor alpha (CD25) gene region, another locus associated with other autoimmune diseases like type diabetes (271b). Finally there seems to be conclusive proof from both linkage disequilibrium and association studies, that polymorphisms in the TSH-R confer susceptibility to Graves’ disease but not autoimmune hypothyroidism (271, 271c). This is one of the few susceptibility factors that segregates with one rather than both types of thyroid autoimmunity.
A different approach has been genome scanning although huge effort is required to undertake such studies. Based largely on this approach, other loci which may be important have been identified on chromosomes 14q31, 20q11 and Xq21 (261, 272), and the importance of a gene on the X chromosome is supported by the increased frequency of AITD in women with Turners syndrome, especially those with an isochromosome-X karyotype (273). However in a large genome scan, involving 1119 relative pairs, there was no replication of these findings (274). An even more impressive genome wide scan of thousands of individuals with Graves’ disease confirmed susceptibility loci in the major histocompatibility complex, TSHR, CTLA4 and FCRL3 and identified two new loci; the RNASET2-FGFR1OP-CCR6 region at 6q27 and an intergenic region at 4p14 (275). Seven new loci for AITD, including MMEL1, LPP, BACH2, FGFR1OP and PRICKLE1, have been uncovered by using a custom made SNP array across 186 susceptibility loci known for immune-mediated diseases (275a). In another study of almost 10000 Chinese pateints with Graves’ disease, five additional novel loci were identified and polymorphism in the TG gene was also confirmed to be associated with Graves’ disease (275b). Thus the genetic factors involved in AITD are increasingly more complex and their interactions with each other and with environmental factors in disease pathogenesis will be a major task to uncover. Further developments in genetic analysis will no doubt bring even greater complexity to this area, albeit with the prospect of better defining patient subsets (275c).
As an aside, it should be noted that low birth weight, a known risk factor for several chronic disorders, has not associated with clinically overt thyroid disease or with the production of thyroid autoantibodies in one study (276) but others have come to an opposite conclusion, with prematurity irrespective of birth size being another risk factor (276a, 276b).
U.Co-occurrence of Autoimmune Diseases
The co-existence of AITD and other diseases possibly of autoimmune cause has often been reported, and suggests some intrinsic abnormality in immune regulation. An extensive review of these associations has been published (277) and extensive population data bases have clarified the strength of the various associations (277a). A striking association is with pernicious anemia. Perhaps 45% of patients with autoimmune thyroiditis have circulating antigastric antibodies (278), and the reverse association is almost as strong (279). Up to 14% of patients with pernicious anemia have primary myxedema, and pernicious anemia is increased in prevalence in patients with hypothyroidism (280). Another strong association is with celiac disease, which is found 3 times more commonly in patients with AITD. Intriguingly the autoantibodies which are the hallmark of celiac disease, directed against transglutaminase, can bind to thyroid cells and thus could be implicated directly in thyroid disease pathogenesis (280a). The association of Sjogren’s syndrome and thyroiditis is not uncommon and both systemic lupus erythematosus (SLE) and rheumatoid arthritis are also significantly associated with AITD (281, 282). A high frequency of antibodies to nucleus, smooth muscle, and single-stranded DNA (26-36%) is found in AITD (283). Although multiple sclerosis has stood out as a putative autoimmune disease which is not obviously associated with AITD, meta-analysis has revealed there is an odds ratio of 1.7 for AITD in these patients (283a).
Autoimmune Addison’s disease and/or type I diabetes mellitus and AITD occasionally co-exist and this forms the autoimmune polyglandular syndrome (APS) type 2 (284). This is an autosomal dominant disorder with incomplete penetrance and is often associated with other disorders, such as vitiligo, celiac disease, myasthenia gravis, premature ovarian failure and chronic active hepatitis (285, 286). AITD is an infrequent feature of the much rarer APS type I (287) and there is no association between mutations in the AIRE gene, which causes APS type I, and sporadic AITD (288).
Together these data provide convincing proof of an association of other autoimmune phenomena with AITD. Most typically, this immunity is organ specific, but in one subset of patients, antithyroid immunity develops in association with the non-organic-specific collagen diseases. A syndrome, or running together, of course, does not prove a causal association. Nevertheless, the plethora of associations and their familial occurrence indicates that a defect in the immune system may be more likely than primary defects in each organ. This in turn suggests a shared immunoregulatory defect, which is at least partly genetically determined, as these diseases often share similar genetic associations, including HLA, CTLA-4, PTPN22 and CD25 gene polymorphisms. It is also clear however that there is a difference in the kind of clustering of other autoimmune disease in Hashimoto’s thyroiditis and Graves’ disease, presumably related to differences between these two types of thyroid disease in genetic predisposition (288a).
Recently, analysis of HLA molecules has shown a pocket amino acid signature, DRβ-Tyr-26, DRβ-Leu-67, DRβ-Lys-71, and DRβ-Arg-74, that was strongly associated with type 1 diabetes and AITD (288b). This could confer joint susceptibility to these diseases in the same individual by causing significant structural changes in the MHC II peptide binding pocket and influencing peptide binding and presentation.
V.IMMUNORegulation: Phenomena and Mechanisms
Possible abnormalities in immunoregulation have been addressed in hundreds of studies. The basic hypothesis of this work is that a deficiency of functional T suppressor cells — either antigen-specific or nonspecific — may allow uncontrolled T and B cell immune responses to thyroid (or other) antigens. As noted above, this concept is a major theme in experimentally induced or naturally occurring thyroiditis in animal models. Immune regulation is extremely complex and still only partially understood. The studies described below have been reported over more than two decades, during which time our understanding of lymphocyte function, terminology, and methods of analysis have continually changed. Most of these studies define immunoregulatory responses in relation to in vitro assays done in unique conditions, or a group of cells bearing a specific surface antigen (e.g. CD4, CD8, etc.). As we have previously noted, T cell antigen expression and function can vary depending on stimulating event, culture conditions, etc. Further, whether a unique group of “suppressor cells” actually exists is uncertain. Thus we present these observations as reported (by us and others), and in the terms used by the authors.
Sridama and DeGroot found decreased suppressor cells, defined as CD8+ peripheral blood T cells in patients with Graves’ disease (289, 290). These results have been challenged, and some investigators have reported depression of CD4+ cells in AITD (291). However, overall, there is now agreement that, in thyrotoxic patients with Graves’ disease, a decrease in CD8+ T cell number (292, 293) is characteristically present, and that a similar abnormality exists in the thyroid. CD8+ cells return gradually toward normal during therapy, and are usually but not always normal at the end of therapy (292) (Fig. 7-12). The phenomenon is present but less evident in Hashimoto’s thyroiditis patients. It has been attributed by some workers to increased thyroid hormone levels (294), although this issue is clouded, since there are reports disproving the idea that hyperthyroidism per se induces suppressor cell abnormalities in humans, and reduced suppressor T cells (Ts) are found present long after thyrotoxicosis is cured (295). Our interpretation is that the abnormality is not due specifically to excess T 4 in blood, but is a manifestation of ongoing active autoimmunity, for reasons which are unclear. Reduced nonspecific “suppressor” T cell function may be in part an inherited abnormality, and is probably also a manifestation of the augmented immune reactivity ongoing in toxic Graves’ disease patients. It may be largely a secondary phenomenon, but one which augments and continues the immunological disease. The mechanism causing reduced Ts number and function is unclear. Ts could be reduced during an active immune response by binding immune complexes on their surface, which could inactivate the lymphocytes or cause them to be removed from circulation. Ts could also be removed by cytotoxic antibodies. These findings need to be related to recent developments in understanding Treg function, studies which have yet to be undertaken.
T cells from patients with Graves’ disease were unable to suppress Ig synthesis when mixed with B cells, in comparison to T cells from normal individuals (290). Okita et al. (296) suggested that this is due to a low number of histamine H2 receptor-positive CD8+ cells in Graves’ disease patients. Another possibly related phenomenon is the decreased pokeweed mitogen (PWM) responsivity of PBMC found in Graves’ disease patients during illness (297) and also when cured of disease (298).
An alteration in helper T (Th)/Ts ratio may also predispose to the occurrence of postpartum transient thyrotoxicosis. We have shown a decrease in CD4+ cells in normal pregnancy (299), possibly representing one of the factors causing the diminished immunoreactivity typically found in pregnancy. A rebound increase in CD4+ cells, which occurs during the first two or three months following delivery, may lead to a recrudescence of immuno-reactivity, including antibody levels and the occurrence of postpartum transient thyroiditis in some women (300).
Thyrotoxic Graves’ disease patients and those with active Hashimoto’s thyroiditis have a high proportion of DR+ T cells in their peripheral circulation (292, 301), which indicates the presence of activated T cells. It is unlikely that these cells (> 20% of circulating T cells) are all responsive related to thyroid antigens, so they must include DR+ T cells with TCRs for many other antigens. There is also a marked increase in circulating soluble IL-2 receptors in thyrotoxic Graves’ disease, but this appears to be typical of thyrotoxicosis per se, and not specifically Graves’ disease (302). Nevertheless, there is no evidence for a generalized ongoing immune hyper-responsiveness in thyrotoxic patients. Perhaps these T cells (for many different specificities) are stimulated by IL-2, but in the absence of the required “second signal” provided by antigen exposure, do not induce B cell proliferation or cytotoxic responses. In keeping with the importance of the Th17 subset in inflammatory autoimmune diseases discussed earlier, there is an increased differentiation of circulating Th17 lymphocytes and an enhanced synthesis of Th17 cytokines in AITD, mainly in those patients with Hashimoto thyroiditis (302a). Nonetheless a recent study has found an increase in both Th22 and Th17 cells and the levels of plasma IL-22 and IL-17 in patients with Graves’ disease; the magnitude of these increases correlated TSH-R antibody levels (302b). Another newly recognized T cell subset involved in the regulation of antibody production, follicular helper T cells, are increased in the circulation of patients with AITD and correlate with autoantibody levels (302c).
Diminished, non-specific suppressor cell function is also observed in many autoimmune diseases including lupus, and multiple sclerosis and the results in AITD are equally non-specific. Functional assays attempting to show a deficiency in antigen-specific suppressor cells have been reported by several groups (303 – 308) in particular using the MIF assay or measuring effects on antibody synthesis in vitro. The pathophysiological relevance of some of these systems is questionable, results have not always been reproducible and there continues to be controversy over the nature of antigen-specific suppressor activity (309-311). The most likely explanation for many “suppressor” phenomena is the reciprocal inhibition of Th1 and Th2 cells by their cytokine products, and powerful evidence shows how important this regulatory mechanism is in exacerbating or inhibiting autoimmune disease, at least in animal models. However regulatory phenomena utilizing cytokines are much more complex, and include both Th17 cells and invariant NKT (iNKT) cells. The latter share receptors with T and NK cells, with the α chain of the T cell receptor being invariant gene segment-encoded, and are notable for releasing cytokines when stimulated by antigen, thus endowing them with regulatory properties which may be either stimulatory or inhibitory. Recently iNKT cell lines have been identified that can be stimulated with TG to induce EAT (311a).
In addition, work is now needed to review the potential role of CD4 + CD25 + Treg cells, discussed earlier, in AITD. One recent study has found that despite increased numbers of CD4+ T cells bearing the T regulatory cell markers CD25, Foxp3, GITR and CD69, in both thyroid and PBMC of patients with AITD, there is a non-specific defect in regulatory function in vitro, which in turn must explain somehow why the increased number of regulatory T cells are so patently ineffective (312). The existence of a functional rather than numerical deficiency in regulatory T cells has also been suggested in a study of AITD patients, in which the defect was found to be detectable only when optimal in vitro conditions were achieved (312a). Analysis in the earliest phases of disease may of course yield different results and unlocking how T regulatory cells can be activated seems an obvious but at present unrealizable therapeutic strategy. The finding that many thyroid infiltrating lymphocytes, early on in the disease process, are in fact recent thymic emigrants does suggest that there is a problem with central tolerance that allows autoreactive T cells to accumulate in the gland where the strength of local immunoregulation could be critical in determining whether disease progresses (312b).
Many studies have examined T cell subsets in thyroid tissue of patients with active AITD (255). For example, Margolick et al (313) found increased CD8+ cytotoxic/suppressor cells and also increased CD4+ T helper cells, and a normal Th/Ts ratio. Canonica et al (314) found increased proportions of cytotoxic/suppressor T cells in thyroids of Hashimoto’s thyroiditis patients. Infiltrating cytotoxic/suppressor cells in Hashimoto’s thyroiditis were found usually to be activated and to express DR antigen, whereas this response was not so obvious in Graves’ disease (315). Canonica et al (314) reported an increased proportion of activated T helper/inducer cells in both Graves’ disease and Hashimoto’s thyroiditis, and increased cells thought to represent cytotoxic T cells in Hashimoto’s thyroiditis. Chemokine expression within the thyroid is likely to be an important determinant of this infiltration (316).
Increased CD8+CD11B- cells, presumed to be cytotoxic cells, were found in Graves’ disease thyroids (in comparison to PBMC of Graves’ disease or normal subjects), whereas “dull” CD8+CD11B+ natural killer cells were diminished (317). Other studies have suggested a reduction in NK cells in Graves’ disease and an increase in Hashimoto’s thyroiditis. Tezuka et al found decreased NK cells in Graves’ disease thyroid tissue, no differences in the NK activity of PBMC between Graves’ and normal patients, and that the NK cells in Graves’ disease did not kill autologous thyroid epithelial cells (318). We have already indicated other reports of normal NK and ADCC in Hashimoto’s PBMC, and of increased ADCC in Hashimoto’s thyroiditis. Most studies that have looked at Graves’ disease tissues also indicate an increased proportion of B cells compared to peripheral blood subsets.
Cell cloning has also been used to examine thyroid and peripheral blood lymphocyte subsets. Bagnasco et al (319) found a predominance of cytolytic clones, releasing IFN- , in Hashimoto’s thyroiditis but not in Graves’ disease. Del Prete et al (320) found a high proportion of cytolytic cells with the CD8+ phenotype in clones from thyroid tissue, and felt these results may relate to autoimmune destruction of TEC but the non-specific methods used to derive such cytotoxic T cells raises questions about any pathophysiological relevance. There is no clear predominance of Th1 or Th2 cytokines in the thyroid of patients with Graves’ disease or Hashimoto’s thyroiditis (165, 321), although Th1 clones seem to predominate in the retrobulbar tissues in ophthalmopathy (322). It might simplistically be thought that Graves’ disease represents a Th2 response, but the fact that some patients end up with hypothyroidism itself indicates the likely presence of a Th1 response too. This is supported by evidence from an animal model of Graves’ disease: immune deviation away from a Th1 response, in -IFN knockout mice, did not enhance the response to TSH-R cDNA vaccination (323).
One situation in which it is likely that perturbation the cytokine milieu is responsible for the emergence of Graves’ disease is during reconstitution of the immune system following lymphopenia induced by alemtuzumab treatment for multiple sclerosis, bone marrow or stem cell transplantation or after highly active antiretroviral therapy for HIV infection (323a,b). In these situations there is an initial increase in the Th1 response flowed by a Th2 response at the time when Graves’ disease becomes apparent. Defects in T regulatory cells may also contribute.
A general summary of these data is difficult. The results probably at least indicate there are increased B cells, increased DR+ T cells, increased CD4+DR+ T helper cells, decreased CD8+DR+ T suppressor/cytotoxic cells, and possibly lower NK cells in Graves’ disease AITD tissue and in blood than among normal subjects’ PBMCs. The intrathyroidal T cells are a mix of Th1 and Th2. Such studies have been performed primarily on patients with well developed and often treated disease, and do not bear directly on early stages of the disease, nor on whether the changes represent primary or secondary phenomena.
To date there has been no certain indication that a non-specific or specific suppressor cell defect exists in patients who are genetically predisposed to have AITD, or in most patients who have recovered from the illness. Rather, the data suggest, at least for the better studied changes in antigen nonspecific T cell subsets, that the changes may be a secondary, although possibly still important part of disease pathology for augmentation and continuation, rather than initiation.
Whereas anti-idiotypic antibodies are thought to play a physiological role in immunoregulation, there is little evidence for participation in, or abnormality of, this function in AITD. Immunoglobulins from some patients with Graves’ disease bind TSH (324). This suggests that anti-idiotypes to TSH antibodies are present and might theoretically function as thyroid stimulating immunoglobulins; or conversely that anti-idiotypes to thyroid stimulating antibody exist and can bind TSH. Either possibility remains to be confirmed. TG antibodies can be induced in experimental autoimmune thyroiditis by idiotype-anti-idiotype manipulation (182) and Sikorska (325) demonstrated the presence of antibodies in sera of AITD patients which inhibit binding of TG to monoclonal anti-TG antibodies, and interpreted these as anti-idiotypes. We have looked very carefully for anti-TG anti-idiotypes in patients with autoimmune thyroid disease and failed to find them (326 – 327). On the other hand, weak anti-idiotypes of the IgM class have been found which bind to TPO antibodies and are present in pooled normal immunoglobulins as well as certain patient sera (327). Although one could postulate that a failure to produce anti-idiotype antibodies could be a feature of AITD, a more likely hypothesis is that anti-idiotypic antibodies are simply rarely produced at a detectable level. Since anti-idiotype antibodies raised in animals will suppress in vitro anti-TG antibody production, the theory that lack of anti-idiotype control is causal in AITD remains attractive, but data to support it are scant.
X.DE NOVO Expression of Class II Antigens on Thyroid Cells
De novo expression of HLA-DR on thyroid epithelial cells, from patients with Graves’ disease, was first reported by Hanafusa et al (328) and was proposed as the cause of autoimmunity by Bottazzo et al. (329) who suggested that de novo expression of MHC class II molecules on these cells, which are normally negative, allows them to function as APCs. Lymphocyte-produced IFN- augments the expression of HLA-DR (also DP and DQ) on thyroid epithelial cells, and that TNF- further increases the induction caused by IFN- (330, 331). HLA-DR+ TECs definitely can stimulate T cells (332, 333) but this is critically dependent on the requirements of the T cell for a costimulatory signal, as Graves’ TECs do not express B7-1 or B7-2 (334, 335). In contrast B7.1 expression on Hashimoto TEC has been recorded, but how this is differentially regulated, compared to Graves’ disease, is unknown (336). We have shown that TECs can present antigen to T cell clones which no longer require costimulation through B7, yet not only fail to stimulate B7-dependent T cells but also induce anergy in these cells by at least two mechanisms, one of which is Fas-dependent (337, 338). Perhaps the most conclusive proof that class II expression by thyroid cells cannot induce thyroiditis comes from the creation of transgenic mice expressing such molecules on TECs – such animals have no thyroiditis and have normal thyroid functionm (338a). For reasons which remain unclear, thyroid follicular and papillary cancers may express B7.1 and B7.2, and B7.2 expression is associated with an unfavourable prognosis (339).
HLA-DR is also expressed on TECs in multinodular goiter and in many benign and malignant thyroid tumors, and this does not appear to induce thyroid autoimmunity (340). Aberrant DR expression has not been shown to develop before autoimmunity. Normal animal thyroids not expressing class II molecules can become the focus of induced thyroiditis, and then express class II molecules (341). Furthermore, HLA-DR expression on Graves’ disease thyroid tissue is lost when tissue is transplanted to nude mice (342). Thus a consensus position is that class II expression could be important, but is a secondary phenomenon in AITD, dependent on the T cell-derived cytokine, -IFN, and only allowing TECs to become APCs for T cells which have already received B7-dependent costimulation elsewhere. This could clearly exacerbate AITD once initiated, but teleologically the role of class II expression seems to be as a peripheral tolerance mechanism, allowing the induction of anergy in potentially autoreactive but still naive (ie. B7-dependent) T cells (Fig. 7-13). The recent description of hyperinducibility of HLA class II expression by TECs from Graves’ disease suggests that such patients may be genetically predisposed to display a more vigorous local class II response and this would increase the likelihood of disease progression (343). The genes controlling this response are therefore worthwhile candidates for future studies of genetic susceptibility.
Environmental factors include viral and other infections, discussed above. Strong evidence for an important role for environmental factors is provided by the incomplete concordance seen in the monozygotic twins or other siblings of individuals with AITD. Also, there are temporal changes in disease incidence that can only be the result of environmental influences, such as the rise in Graves’ disease in children in Hong Kong, the steady rise in autoimmune thyroid disease in Calabria, Italy, the more than two-fold increase in lymphocytic thyroiditis over 31 years in Austria, and the changes in the rates of histologically diagnosed Hashimoto’s thyroiditis over a 124 year period (344, 344a, 344b, 344c). Such studies also show that environmental factors may change rapidly, making their ascertainment difficult and challenging. Epidemiological studies have also shown that there is a higher prevalence of thyroid autoimmunity in children raised in environments that have higher prosperity and standards of hygiene (344d). This falls in line with the so-called hygiene hypothesis, that is, the idea that early exposure to infections may skew the immune system away from Th2 responses like allergy and also away from autoimmunity. IL-2 administration for treatment of cancer leads to the production of antithyroid antibodies, and hypothyroidism (and possibly a better tumor response) (345). IFN- administration and other cytokines (118), as well as highly active antiretroviral therapy for HIV infection (346), have a similar effect, although interferon- 1b treatment has no significant adverse effect on AITD (347). Recently however long-term follow up studies have shown that around a quarter of multiple sclerosis patients treated with this latter cytokine may develop autoimmune thyroid disease within the first year of treatment (348). It remains unclear how relevant any lessons from these observations are for AITD pathogenesis, as of course the doses of cytokines and drugs used therapeutically are vast. However, it has been reported that thyrotoxicosis tends to recur following attacks of allergic rhinitis (349). Possibly this is due to a rise in endogenous cytokines and the recent association of raised IgE levels with newly diagnosed Graves’ disease indicates that this may be mediated by preferential Th2 activation (350).
Cigarette smoking is associated with Graves’ disease, and with ophthalmopathy (reviewed in 351) although it seems to be that smoking is associated with a lower risk of autoimmune hypothyroidism (351a). The mechanisms behind these complex changes uncertain and is doubtless more complex than a local irritative effect. Environmental tobacco smoke induces allergic sensitization in mice, associated with increased production of Th2 cytokines, but a reduction in Th1 cytokines, by the respiratory tract (352). It is therefore possible that modulation of cytokines contributes to the worsening of ophthalmopathy with smoking. On the other hand, as noted above, the opposite effect prevails in hypothyroidism and smoking exposure was associated with a lower prevalence of thyroid autoantibodies in a large population survey of over 15000 US citizens (353) and smoking cessation is known to induce a transient rise in AITD (353a). To explain this, investigations have been undertaken on anatabine, an alkaloid found in tobacco; this compound ameliorates EAT and reduces TG antibody levels in human subjects with Hashimoto’s thyroiditis(353b).
More general environmental pollutants have not been thoroughly explored for their possible effects (although there is some evidence from older experiments that methylcholanthracene can induce thyroiditis) but a recent study has demonstrated that polychlorinated biphenyls can induce the formation of TPO antibodies and lymphocytic thyroiditis in rats (353c). A cross-sectional survey in Brazil has fond that Hashimoto’s thyroiditis and thyroid autoantibodies are more frequent in individuals living near to a petrochemical complex than in controls (353d). In addition pesticide use, especially of the fungicides benomyl and maneb/mancozeb, has been associated with an increased odds of developing thyroid dysfunction although the mechanism of action is unclear (353e). it clear that this aspect of the environment warrants further study in human thyroid disease.
The role of dietary iodine is clearly established in animal models of AITD and circumstantial evidence exists for a similar role in man (196, 255, 354, 355, 355a). The response is complex and recently it has been shown that iodide may exacerbate thyroiditis in NOD mice but not affect the production of TSH-R antibodies in the same strain (356). Such findings are intriguing as they raise the possibility that the thyroiditis which accompanies Graves’ disease may not be due to the immune response to the TSH-R. Iodine may affect several aspects of the autoimmune response, as detailed in the section on experimental thyroiditis above. In addition, iodide stimulates thyroid follicular cells to produce the chemokines CCL2, CXCL8, and CXCL14 (356a). These observations suggest that iodide at high concentrations could induce AITD through chemokine upregulation thus attracting lymphocytes into thyroid gland.
Dietary selenium may also be important as high dose selenium supplementation reduced TPO antibody levels in women with AITD (356b). Vitamin D may be important in autoimmunity and many other disorders, as it is now recognized that individuals living in northerly latitudes may have suboptimal levels based on a fresh understanding of what normal levels of this vitamin should be. A significant inverse correlation has been observed between 25(OH)D levels and TPO antibody levels in Indian subjects, although the overall impact of this effect in terms of causality was low (356c).
A variety of lifestyle factors that are difficult to investigate may also be involved. It is otherwise difficult to account for the increase in AITD seen in same-sex marriages (356d). Stress is likely to be important in the etiology of Graves’ disease, although studies to date have had to rely on retrospective measures of this (reviewed in 357). Moreover stress does not appear to be asociated with the development of TPO antibodies in euthyroid women (358). Presumably stress acts on the immune system via pertubations in the neuroendocrine network, including alterations in glucocorticoids, but the complex interaction between the nervous, endocrine and immune systems includes the actions of neurotransmitters, CRF, leptin and melanocyte stimulating hormone as well and so unravelling the pathways whereby stress may alter the course of autoimmunity is difficult in the extreme (359). Indirect support for such a mechanism, mediated through norepinephrine, comes from experiments showing dramatic enhancement of delayed-type hypersensitivity by acute stress, the result of sympathetic nervous system activation on the migration of dendritic cells and subsequent enhanced T cell stimulation (360). Moderate consumption of alcohol appears to have a protective effect with regard to AIITD (360a,b). Given the diversity of these environmental factors, presumably operating on different genetic backgrounds, it will be difficult (if not impossible with current tools) to establish the relative importance of each in AITD.
“Normal” people express antithyroid immunity, as previously described, and this must be important in understanding the overall mechanism of AITD. Antibodies to TG and TPO are present in both Graves’ disease and Hashimoto’s thyroiditis up to 7 years prior to diagnosis, increasing over time in the former and consistently elevated in the alter (360c). Many people with low levels of antibodies but without clinical disease can be shown to have lymphocyte infiltrates in the thyroid at autopsy. B cells from normal individuals can be induced to secrete anti‑TG antibody in vitro. These observations clearly show that incomplete deletion of clonal self‑reactive T cells is indeed the normal (and indeed perhaps necessary) circumstance, and provide strong support for the idea that disordered control of this low level immunity may be important in the etiology of AITD (115).
AA.Effect of Antithyroid Drugs on the Immune Response
Antithyroid drugs are used in Graves’ disease to decrease production of thyroid hormone, and also lead to diminution in TSI and other antibody levels. Clinical studies show that antithyroid drug administration also leads to a diminution in antibody production in thyroxine replaced Hashimoto’s thyroiditis patients (361), proving that their effect is not simply due to control of hyperthyroidism in Graves’ disease. Surprisingly (362), administration of KClO 4 to patients with Graves’ disease leads to diminished serum antibodies, suggesting that the effect of treatment is not specific for thionamide drugs, but could be mimicked by this compound. Antithyroid drugs inhibit macrophage function, interfering with oxygen metabolite production (362).
Following antithyroid drug treatment of active Graves’ disease, there is a prompt short‑term increase of DR+CD8+ T cells in the bloodstream as described above. Antithyroid drugs inhibit the production of cytokines, reactive oxygen metabolites and prostaglandin E 2 by TECs and the reduction in these inflammatory mediators may explain the site-specificity of the immunomodulation produced by antithyroid drugs (156). Another pathway for an immunomodulatory action of these drugs is via the upregulation of Fas ligand expression, which may then attenuate the autoimmune response of Fas-expressing T cells (363). Only approximately 50% of patients enter remission after treatment with antithyroid drugs, a fact which must be accommodated in any hypothesis concerning an immunomodulatory action of these agents. Those patients with Graves’ disease who have the highest IgE and IL-13 levels in the circulation are the most likely to relapse (364). In turn, this suggests that antithyroid drugs only effect remission in individuals who do not have a strong Th2 response; those with the strongest such responses seem unlikely to be affected by the relatively weak action of such drugs.
AITD As A Consequence of a Multifactoral Process (Table 4) (Fig. 7-14)
Development of Autoimmune Thyroid Disease
|Stage 1 ‑‑ Basal State
Normal exposure to antigen such as TG and normal low levels of
Inherited susceptibility via HLA‑DR, DQ, or other genes
|Stage 2a ‑‑ Initial Thyroid Damage and Low Level Immune Response
Viral or other damage with release of normal or altered TG, TPO,
Increased antibody levels in genetically susceptible host with
high efficiency HLA‑DR, DQ, TCR molecules
Infection induced elevation of IL‑2 or IFN-
IL‑2 stimulation of antigen specific or nonspecific Th
IFN- stimulation of DR expression and NK activation
Glucocorticoid‑induced alterations in lymphocyte function during
|Stage 2b ‑‑ Spontaneous Regression of Immune Response
Diminished antigen exposure
Antigen specific Ts induction
|Stage 3 ‑‑ Antigen Driven Thyroid Cell Damage (or Stimulation)
Complement dependent antibody mediated cytotoxicity
Fc receptor+ cell ADCC by T, NK, or macrophage cells
NK cell attack
Direct CD4+ or CD8+ T cell cytotoxicity
Antibody‑mediated thyroid cell stimulation
|Stage 4 ‑‑ Secondary Disease Augmenting Factors
Thyroid cell DR, DQ expression ‑‑ (?) APC function
Other molecules (cytokines, CD40, adhesion molecules) expressed by thyroid cell
Immune complex binding and removal of Ts
|Stage 5 ‑‑ Antigen Independent Disease Progression
Recruitment of nonspecific Th or autoreactive Th
Autoreactive Th bind DR+ TEC or B cells
IL‑2 activation of bystander Th
|Stage 6 ‑‑ Clonal Expansion with Development of Associated Diseases
Antigen release and new Th and B recruitment
(?) Cross reactivity with orbital antigen
IL‑2, IFN- augmentation of normal immune response to intrinsic factor, acetylcholine receptor, DNA, melanocytes, hair follicles, etc.
Thus one is led to the uncomfortable position that AITD is probably not caused by a single factor, but rather due to many factors which interact. In terms of genetic and environmental factors, as well as factors that may be termed existential (such as age, being female and parity), these may all have to coincide in a favorable way for AITD to occur, in keeping with the Swiss-cheese model for accidents (Fig 7-15). We have divided the roles of these potential disease activity factors into a series of stages, emphasizing the predisposing events, antigen driven responses, and then the secondary and nonspecific amplification which ensues.
Stage 1 — In the basal state, Stage 1, immune reactivity to autologous antigen occurs as a normal process. This probably exists at a physiologically insignificant level, since not all T or B cells reacting with TSH-R, TPO or TG are clonally deleted, and Ag is normally present in the circulation. If assays become sensitive enough, we probably will find some level of antibodies to TSH-R, TPO and TG present in some or all healthy persons, increasing in prevalence and concentration with age, and especially in women, since “femaleness” somehow augments antithyroid immunity manyfold. Patients who have inherited certain susceptibility genes, such as those encoding HLA-DR3 or DR5 specificities, will be especially prone to develop AITD because their T and B cell repertoire includes cells recognizing self-antigen, or their immunocytes are especially good at collecting, presenting, and responding to antigen, or are unable to effectively clear immune complexes from their circulation.
Stage 2 — Possibly viral infection, or other causes of cell damage, or cross‑reacting antibodies present after Yersinia (or other) infection, leads to release of increased amounts of (or possibly modified) thyroid antigens which, in genetically prone individuals, leads to an increased but still a low level immune response. Nonspecific production of TNF- and IFN- , in response to any infection or immune response, may augment MHC class expression on TECs, allow these cells to function as APCs, and increase production of the already established, normally occurring low levels of antibodies. The process may be affected by stress, although the mechanism remains quite uncertain. The process may go on over years, and wax and wane, as it has been shown that thyroiditis can be clinically apparent and then disappear. Factors involved in temporary or permanent suppression of the autoimmune response may include diminished thyroidal release of antigen, B cell anti-idiotype feedback, or the normal auto-regulatory induction of T cells with a suppressor function, including those engendered by the mutual regulation of Th1 and Th2 subsets. In some individuals, thyroid cells may be less able to express DR, or may secrete TGF- and suppress immune responses. Glucocorticoid administration and other immunosuppressives can also temporarily prevent the expression of nascent autoimmunity.
Stage 3 — If suppressive factors do not control the developing immune response, the disease progresses to a new intensity, now driven by specific antigens, inducing cell hyperfunction (TSI), or hypofunction (TSH blocking or NIS antibodies), or cell destruction. Direct T cell cytolysis and apoptosis, ADCC, and K or NK cell attack presumably play an important role at this stage, and now the disease becomes clinically evident.
Stage 4 — As the disease develops, a variety of secondary factors come into play, and augment antithyroid immunoreactivity. Any stimulus which causes increased DR expression on thyroid cells, such as T cell release of IFN- , combined with increased TSH stimulation, may allow TECs to function as APCs. Although perhaps poor in this function, they are large in number and localized in one area. The TECs may also participate in the autoimmune process by several other pathways, including the expression of adhesion molecules, Fas, Fas ligand, CD40 and complement regulatory proteins, and the production of a number of inflammatory mediators such as cytokines, reactive oxygen metabolites, nitric oxide and prostaglandins. These events are, like class II expression, dependent on cytokines and other signals generated by the intrathyroidal lymphocytic infiltrate. Some patients may inherit diminished antigen-specific suppressor cell function. Development of hyperthyroidism, or more likely the ongoing immune reaction itself, may lead to nonspecific suppressor cell dysfunction, further augmenting immunoreactivity. Antigen “non-specific”, and antigen specific suppressor T cells, may be reduced by binding immune complexes.
Stage 5 — T cell derived cytokines may non-specifically induce bystander antigen specific T and B cells to be activated and produce antibody. Autoreactive cells will now accumulate in thyroid tissue because of the many strongly DR+ positive lymphocytes and TECs, and augment the developing response by lymphokine secretion or cytolysis, in a manner independent of thyroid antigens. At this stage in the disease, non-specific autoreactive immune processes may dominate a disease process which no longer depends upon antigen for its continuation.
Stage 6 — As the concentration of activated T and B cells builds in thyroid tissue, and autoreactive and antigen nonspecific T cells become progressively involved, cell destruction may lead to release of new antigens. Cross-reacting epitopes, and nonspecific stimulation of T cells in genetically prone individuals, may cause the addition of new immunologic syndromes (exophthalmos, pretibial myxedema, atrophic gastritis) typical of older patients with more long standing and florid disease.
THYROIDITIS, MYXEDEMA, AND GRAVES’ DISEASE AS AUTOIMMUNE DISEASES
BB.Hashimoto’s Thyroiditis (Fig. 7-16)
How well do the changes of Hashimoto’s thyroiditis fulfill the criteria of an immunologic reaction? Neither the presence of autoantibodies in the serum of patients with Hashimoto’s thyroiditis nor the demonstration in vitro of cytotoxicity of the serum constitutes definitive evidence that autoimmunity is the cause of the disease. Rarely, if ever, is there a well-defined initial immunizing event, and accordingly a shortened latent period after a secondary stimulus has not been observed. Further, experimental passive transfer of the immune state in normal recipients has not yet been attempted and has failed when human sera have been transfused into monkeys and other animals. This experiment is conducted by nature during pregnancy, since maternal antibodies cross the placenta. Transplacental passage of thyroid stimulating antibodies can produce neonatal thyrotoxicosis, and TSH blocking antibodies can produce transient neonatal hypothyroidism. Passage of TG-antibody or TPO-antibody has no detectable cytotoxic effect. The lack of response to passive transfer of this type of antibody is not surprising, since living syngeneic cells must usually be transferred for the development of experimental thyroiditis. The continuing improvements in assays for T cell reactivity in man, supplemented by data from animal models, provide compelling evidence of the autoimmune basis for Hashimoto’s thyroiditis, but this does not exclude an amplifying role for TG and TPO antibodies via ADCC, and, for TPO antibodies, via complement fixation. It may well be that T cell-mediated damage is required initially for all of these antibody-mediated events to take place, as this could be necessary for such access. Another striking feature of Hashimoto’s thyroiditis is the development of Hurthle cells, with granular eosinophilic cytoplasm,. This appears to be the result of a chronic inflammatory milieu, resulting in overexpression of immunoproteasomes (364a).
The evidence is now overwhelming that an immune reaction mediated by T lymphocytes is involved in the development of experimental thyroiditis in animals and several mechanisms may operate singly or together in man to injure TECs. Lymphocytes presensitized to antigens of the thyroid are present in the circulation of most if not all patients and are believed to localise to the thyroid itself. Since T cell mediated immunity is frequently lethal to cells, it is logical to assume that the T cell mediated immune response in thyroiditis could cause first a goiter, with lymphocyte infiltration and compensatory thyroid cell hyperplasia, and then gradual cell death and gland atrophy. The circulating antibodies may also be a functional part of this reaction. We can probably accept the idea that T cell mediated immunity is the major pathogenic factor in thyroiditis.
Even before the present era of immunologic study, the basic unity of Hashimoto’s thyroiditis and myxedema was realized. To quote from Crile, writing in 1954 (365): “Struma lymphomatosa is responsible not only for large lymphadenoid goiters, but also for fibrosis and atrophy of the thyroid. The clinical spectrum of struma lymphomatosa extends from spontaneous myxedema with no palpable thyroid tissue to a rapidly growing goiter associated with no clinical evidence of thyroid failure.”
Hubble (366) also drew attention to the occurrence of syndromes intermediate between those of myxedema and Hashimoto’s thyroiditis, in which a small, firm thyroid gland can be felt on careful palpation. The histologic studies of Bastenie (367) and Douglass and Jacobson (368) revealed a close similarity in appearance of the thyroid remnant in myxedema and the Hashimoto gland. The immunologic studies of Owen and Smart (369), and the experience in most thyroid laboratories, indicate a similar incidence and titer of antibodies in myxedema and Hashimoto’s thyroiditis. The familial association of myxedema and thyroiditis was described earlier and so far no clear genetic susceptibility difference has been reported in the two diseases. Attempts to ascribe atrophy of the thyroid gland in myxedema to particular antibodies, such as those inhibiting growth or TSH, or which mediate ADCC (370) have not been confirmed by other studies (reviewed in 255).
Thus, idiopathic myxedema is probably the end result of Hashimoto’s thyroiditis, in which the phase of thyroid enlargement was minimal or was overlooked. We may assume that in idiopathic myxedema the cell‑destructive T cell-mediated immune response is an important pathogenic factor in the illness, and that cytotoxic antibodies and TSH blocking antibodies contribute to the development of hypothyroidism, but perhaps in only a proportion.
Graves’ disease is associated with a similar type of thyroid autoimmunity, since most hyperthyroid patients have circulating TG and TPO antibodies. High antibody levels are found in a small group of hyperthyroid patients, and histologic examination of their glands show changes of both cell stimulation and focal thyroiditis (371). Some patients with clinical Graves’ disease have tissue changes in the thyroid that are typical of thyroiditis (372). This type of patient with Graves’ disease most often becomes hypothyroid after operation (373), after 131 I therapy (374), or possibly spontaneously (375). It is also well known that some patients fluctuate from hyper- to hypothyroidism over a period of months and others behave in the converse fashion, and of course the familial association of Graves’ disease with autoimmune hypothyroidism is well established.
The humoral response in Graves’ disease leads to production of TG and microsomal TPO antibodies, but most importantly, as described in Chapter 10, B cells produce TSI, TBII and, in some, TSH blocking antibodies (376, 377). TSI stimulate thyroid release of hormone primarily via cyclic AMP, although other pathways may also be activated by TSI in a proportion of patients (378, 379). TSI are true cell stimulators and can even induce experimental goiter. However, the clinical picture in Graves’ disease will be a balance between the stimulation produced by TSI and the opposing effects of any TSH blocking antibodies which may be present. An example of this has recently been reported during pregnancy in Graves’ disease patients; the ratio of stimulatory to blocking antibodies decreases and this may explain the remissions usually seen in the last trimester (380).
Evidence also supports a role for T cell mediated immunity to thyroid antigens in Graves’ disease, and against orbital antigens in patients with associated ophthalmopathy. We speculate that Graves’ disease may be a condition representing a semistable balance between stimulatory, blocking, and cell‑lethal immune responses. Thus, TSI could cause thyroid hyperplasia and produce hyperthyroidism. Other antibodies might block the action of TSI either directly or, as in the case of NIS antibodies, indirectly, and prevent this hyperplastic response in some patients. Cytotoxic T cells will also gradually destroy cells and produce hypothyroidism either spontaneously or after therapy. It must be admitted that the etiology of ophthalmopathy remains obscure, although the key role of cytokines in pathogenesis, causing fibroblast activation, seems firmly established.
RELATION TO OTHER DISEASES
Thyroid Cancer Thyroid antibodies are present in increased prevalence (up to 32%) in patients with carcinoma of the thyroid, and usually are at low titer. Histologic evidence of thyroiditis is found in 26% of tumors (382, 383). Histologic changes range from diffuse thyroiditis to focal collections of lymphocytes around the tumor or reactive lymphoid hyperplasia. Possibly release of antigens leads to increased thyroid autoimmunity. Some evidence suggests that patients who have thyroid antibodies have a better prognosis than antibody negative patients. Lymphoma and lymphosarcoma of the thyroid are associated with Hashimoto’s thyroiditis (383-385), and there is compelling evidence that thyroiditis precedes development of the tumor. An increased frequency of carcinoma, especially of the papillary type, has been suggested in Hashimoto’s thyroiditis (386). Our experience does not indicate an association greater than that dictated by chance. Woolner et al (383), in a study of 600 cases, reached the same conclusion. It is also possible that focal thyroiditis in thyroid cancer represents a secondary immune response to the tumor.
Adolescent Goiter Enlargement of the thyroid during the second decade, accompanied by normal results of function tests, usually is labeled adolescent goiter. If the examination includes needle biopsy, an appreciable incidence of Hashimoto’s thyroiditis is found (387) – up to 65%. Eighty percent of these children with thyroiditis have a positive thyroid antibody test result. The parents of many of them have either overt thyroid disease or circulating thyroid antibodies. Hyperplasia, in response to an increased demand for thyroid hormone, and colloid involution are at the root of some of these goiters, but Hashimoto’s thyroiditis is the most frequent explanation of adolescent goiter in iodine sufficient areas.
Transient Thyrotoxicosis, Painless Thyroiditis, Postpartum Thyroiditis, and Related Syndromes These illnesses, all similar, involve an acute exacerbation of thyroid autoimmunity occurring independent of, or following pregnancy in women, and in men. They are characterized by sequential inflammation-induced T 4 and TG release, transient hypothyroidism, usually return to euthyroidism, and are discussed in Chapters 8 and 14. A useful review of these various types of thyroiditis has also appeared recently (388). They are considered subtypes of Hashimoto’s thyroiditis, and in the postpartum period, appear to result from release of the immunoregulatory effects of normal pregnancy (229a).
Focal Thyroiditis Focal lymphocytic infiltrations are frequently seen in Graves’ disease, nodular goiter, nontoxic or colloid goiter, and thyroid carcinoma. The significance of these changes is not precisely known, but they correlate with positive antibody titers and may represent variations that do not differ qualitatively from thyroiditis.
RIEDEL’S THyroiditis This rare thyroid disorder is associated with both Hashimoto’s thyroiditis and Graves’ disease and in addition many patients have evidence of fibrosis elsewhere, such as the retroperitoneum, lung, biliary tract and orbit (388a). In one large series, 12 of 15 patients had positive TPO antibodies (389) It is now recognized that some of these patients with multifocal fibrosclerosis have IgG4-related sclerosing disease in which lymphocytes and IgG4-positive plasma cells infiltrate the affected tissues, especially the lacrimal gland, biliary tree and pancreas but the exact relationship of this entity to Reidels’ thyroiditis is unclear at present. There is a predominance of IgA rather than IgG4 in Reidel’s thyroiditis, but the effects of steroids may obscure the few analyses that have been undertaken (390, 391). However it does appear that Hashimoto’s thyroiditis can be divided into two discrete entities based on whether IgG4 plasma cells predominate in the thyroid infiltrate: in those individuals with IgG4 predominance, there is a greater male frequency, more rapid progression to hypothyroidism and more intense gland fibrosis (392).
OTHER Problems An association between the occurrence of maternal antithyroid antibodies and recurrent abortion has been reported (393) and although this association has been disputed, a recent study showed clear evidence that the presence of TPO antibodies was associated with a 3-4-fold increased risk of miscarriage in women having in vitro fertilization (394). There is also an association between breast cancer and thyroid autoimmunity (395, 396) and between depression in middle-aged women and the presence of TPO antibodies (397). The nature of these associations is unclear; does thyroid autoimmunity predispose to such adverse events, or is the presence of thyroid autoimmunity simply a marker of a non-specific disturbance in the immune system due to whatever has caused miscarriage, cancer or depression? Having thyroid autoimmunity is not all bad news. Community-dwelling older women who have TG and TPO antibodies are less likely to be frail than those who are antibody-negative (398). Again the reason for this unexpected finding is unclear but it certainly warrants follow-up.
While the preceding construction cannot yet be supported in each detail by direct observations, it may be of value in helping to direct future studies on the pathogenesis of thyroid autoimmunity. It stresses the normal occurrence of immune self-reactivity, the genetic and environmental forces that may amplify such responses, the role of the antigen-driven immune attack, secondary disease-enhancing factors, and the important contributory role of antigen-independent immune reactivity. Least understood is the last area, that of clonal expansion involved in development of the associate immunological syndromes. Research on thyroid autoimmunity has benefited greatly by knowledge of the specific target antigens and easy access to blood cells and involved target tissue. As research moves now into the realm of molecular immunology and genetics, we may look for rapid progress in understanding and controlling these common illnesses.
1. Which of the following statements concerning T cells is true?
- All T cells express a T cell receptor composed of an and chain
- T cell receptor genes undergo somatic mutation to increase the diversity of antigens recognized
- T cell receptors recognize antigenic peptides of around 25 amino acids in size
- Specific V gene segments encoding the receptor rearrange with D and J segments to provide the unique recognition function of the receptor (True)
- All T cells require costimulatory signals from B7.1 or B7.2 to undergo activation
2. Which cytokine typifies a Th2 T cells?
- IL-4 (True)
3. Which of the following statements concerning thyroglobulin as an autoantigen is true?
- It is a 670 kD molecule comprised of 4 identical subunits
- There are 4-6 major B cell epitopes (True)
- Thyroglobulin antibodies fix complement
- Iodide content does not affect the antigenicity of thyroglobulin in animal models of thyroiditis
- There is sequence homology with the TSH-receptor
4. Which of the following is not the target of the autoimmune response in patients with thyroid autoimmunity?
- Na + /I - symporter
- Acetylcholine receptor (True)
- Thyroid peroxidase
5. Antibodies to the TSH receptor can display all but one of the following characteristics
- Stimulate the receptor
- Block the receptor
- Displace 125 I-TSH binding
- Bind to conformational epitopes
- Fix complement (correct answer)
6. Experimental autoimmune thyroiditis does not occur in which of the following?
- NOD mouse
- BB/W rat
- Immunization of mice with fibroblasts transfected with TSH-receptor (True)
- OS chicken
- Genetic immunization of mice with TSH-receptor cDNA
7. Antibodies to which of the following thyroid autoantigens causes neonatal disease after transplacental transfer?
- Na + /I - symporter
- Thyroid peroxidase
- TSH receptor (True)
8. Which of the following has not been associated with autoimmune thyroid disease in Caucasians?
- HLA-DRB1*07 (True)
- HLA-DR3-linked haplotypes
9. Which of the following is not associated with autoimmune thyroid disease?
- Sjögren’s syndrome
- Celiac disease
- Premature ovarian failure
- Autoimmune polyglandular syndrome type 1
- Psoriasis (True)
10. Which of the following statements concerning T cells in Graves’ disease is not true?
- A. HLA-DR-positive T cells increase in the circulation after methimazole treatment (correct)
- B. CD4 + T cells infiltrate the thyroid
- C. Changes in circulating T cell subsets are similar to those in other autoimmune diseases
- D. CD8 + T cells infiltrate the thyroid
- E. There is no clear Th2 predominance
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