Discoveries in Thyroid Autoimmunity in the Past Century

Abstract

This review on the 100th anniversary of the American Thyroid Association summarizes the remarkable progress attained during the past century regarding the pathogenesis and treatment of thyroid autoimmune diseases. Indeed, the general concept of autoimmune diseases in humans was established 70 years ago by thyroid investigators. Graves' disease is a paradigm for the rare occurrence of how autoimmunity can cause disease by stimulating rather than destroying an organ system. Therapeutic advances in the mid 20th century involving administration of thyroid hormones, thionamide drugs, and radioiodine have been hugely beneficial for human health. However, these approaches can only treat, but not cure, thyroid autoimmunity. Investigation of these diseases is facilitated by the identification of a limited number of specific autoantigens, whose molecular cloning has provided much information on their structure. This knowledge has led to highly sensitive and specific diagnostic tests, provided insight into novel aspects regarding the pathogenesis of thyroid autoimmunity, and has opened avenues for the development of new therapeutic agents. Immunotherapy for a cure as opposed to therapy of Graves' disease and Hashimoto's thyroiditis remains the holy grail for the 21st century.

Introduction

In this year, 2023, the 100th anniversary of the American Thyroid Association, we review the remarkable progress attained during this period regarding the pathogenesis and treatment of thyroid autoimmune diseases. Advances in our knowledge have been made equally by members of our society and colleagues in many countries, often involving collaborations. Limited in length and number of citations, and covering 100 years, this review cannot be comprehensive. Our goal is to distill the essence of major, clearly established conceptual advances. Consequently, we cite reviews that readers may wish to explore, understanding that the authors of these reviews are, in many instances, not those to be credited for the discoveries (for which reason we state “reviewed in”). Discoveries during the past century are not listed chronologically in the text but are listed in this order in Table 1.

Table 1.

Time Line of Selected Major Discoveries in Thyroid Autoimmunity

1927, 1953	Synthetic thyroxine and triiodothyronine and treatment of autoimmune-related hypothyroidism	^50,51
1941	Radioiodine ablation for Graves' hyperthyroidism	Reviewed in Hertz⁵⁴
1942	Thionamide drugs to treat Graves' hyperthyroidism	Reviewed in Burch and Cooper⁵²
1956	Tg, an autoantigen in Hashimoto's disease	¹
1956	Thyroiditis and anti-Tg in rabbits induced by immunization with Tg	²
1956	Long-acting thyroid stimulator	³
1959	Thyroid cytotoxic antibody in patient's sera	⁴
1960	Genetic predisposition to developing thyroid autoantibodies and thyroid autoimmune disease: HLA molecules (major histocompatibility complex class I, class II), cytotoxic T lymphocyte antigen 4, CD40, PTPN22, and CD25 (FoxP3); Tg variants, TSHR SNPs	Reviewed in Lee et al²⁹
1964	Transplacental transfer of TSAb causing neonatal hyperthyroidism	⁶
1980	Transplacental transfer of TBAb causing neonatal hypothyroidism	⁷
1979–1988	Lymphocytes in patients' thyroid glands, but not lymph nodes or peripheral blood, spontaneously produce thyroid autoantibodies	Reviewed in Rees Smith et al¹³
1982	TSHR intramolecular cleavage into A and B subunits	⁴³
1983	Aberrant human leucocyte antigen-DR isotype expression on thyroid cells in thyroid autoimmunity	¹⁹
1985	Cloning of human Tg	³⁸
1986	TSHR synergism with the insulin-like growth factor-1 receptor	⁷¹
1987	Cloning of human TPO, identity with the thyroid microsomal antigen	¹⁴; reviewed in McLachlan and Rapoport¹⁵
1989–1990	Cloning of the TSHR	Reviewed in Rapoport et al⁵
1988–1991	Cloning T cells specific for TPO and Tg from diseased thyroids	^26 –29
1993	TSHR expression in orbital fibroblasts	Reviewed in Bahn⁶³
1993–2000	Cloning repertoires of human monoclonal TPO and Tg autoantibody genes and their expression to define their epitopes	Reviewed in Rapoport and McLachlan¹²
1995–1999	NOD.H2h4 mice spontaneously develop thyroiditis and Tg antibodies, enhanced by iodide	Reviewed in Rapoport and McLachlan¹²
1996	First Graves' disease mouse model; mice injected with eukaryotic cells co-expressing the human TSHR and HLA	⁵⁵
1998, 2002	Graves' disease mouse models generated by immunization with cDNA for the TSHR or its A subunit	Reviewed in Rapoport and McLachlan¹²
2003	Human monoclonal TSAb isolated from a Graves' patient	¹¹
2004	Transgenic mouse expressing TPO-specific T cell receptor gene develops spontaneous Hashimoto's thyroiditis	²⁴
2007	Crystal structure of portion of TSHR in complex with monoclonal TSAb defines epitope of latter	⁴¹
2011	Crystal structure of portion of TSHR in complex with monoclonal TBAb defines epitope of latter	⁴²
2011, 2114	Intrathymic low-level TSHR expression is associated with particular TSHR SNPs and susceptibility to Graves' disease; dysregulation of thymic TSHR expression triggers Graves' disease	^30,31
2015	Mouse model that spontaneously develops TSAb; NOD.H2h4 with the human TSHR A subunit targeted to the thyroid	⁶⁰
2021	Thyroid autoimmunity enhanced by immune checkpoint inhibitors	Reviewed in Wright et al³⁵
2022	Treatment for GO using monoclonal blocking TSHR antibody K1-70	⁷³

GO, Graves' ophthalmopathy; HLA, human leucocyte antigen; SNP, single nucleotide polymorphism; TBAb, TSH receptor blocking autoantibodies; Tg, thyroglobulin; TPO, thyroid peroxidase; TSAb, thyroid stimulating antibody; TSH, thyrotropin; TSHR, TSH receptor.

Discovery of the Concept of Autoimmunity

Until well into the 20th century, the notion that the body was capable of attacking itself was considered extremely unlikely, hence the term “horror autotoxicus” coined by Paul Ehrlich in Germany. This concept, undermined by phenomena such as erythrocyte cold agglutinins, was finally put to rest by unequivocal evidence for thyroid autoimmunity reported in 1956. In this seminal year, sera from Hashimoto patients were shown to bind to thyroglobulin (Tg), a self, or auto, antigen.¹ Moreover, rabbits immunized with Tg developed antibodies to Tg, as expected, but also thyroid lymphocytic infiltrates as observed in patients with Hashimoto's thyroiditis.² Also in 1956, sera from Graves' patients injected into guinea pigs in vivo stimulated the release of radioactivity from thyroids prelabeled with radioiodine.

The prolonged release of radioactivity relative to thyrotropin (TSH) led to naming of the unidentified stimulator factor as “Long Acting Thyroid Stimulator,” or LATS.³ Shortly thereafter, in 1959, antibodies in some Hashimoto patients were reported to recognize a thyroid antigen different from Tg, hence the term “Thyroid Microsomal Antigen.”⁴ In 1964, LATS was identified to be an autoantibody (IgG), a thyroid stimulating antibody (TSAb) (reviewed in Rapoport et al⁵), clinching the concept that Graves' disease, like Hashimoto's, was an autoimmune disease. The observation that transplacental transfer of TSAb and TSH receptor (TSHR) blocking autoantibodies (TBAb) could cause neonatal hyperthyroidism⁶ and hypothyroidism,⁷ respectively, was an important milestone in confirming the humoral basis for Graves' disease.

Clinical Assays for Thyroid Autoantibodies

TSHR autoantibodies

Studying the action of TSH, in 1966, Pastan et al. were the first to suggest that polypeptide hormones exerted their effect by binding to a cell surface receptor.⁸ Subsequently, in 1973, radiolabeled TSH binding to thyroid plasma membranes confirmed the existence of a TSHR.⁹ Given the similar actions of TSH and LATS, it was intuitive that LATS, now called TSAb, also interacted with the TSHR, as subsequently demonstrated by Graves' IgG competing for radiolabeled TSH binding to this receptor.¹⁰

Replacement of in vivo animal bioassays for TSAb by detection of TSHR activation in cultured cells, initially thyrocytes then recombinant TSHR in non-thyroidal cells, as well as competition assays for TSAb binding to diverse TSHR preparations, represented major advances in the diagnosis and management of Graves' disease (reviewed in Rapoport et al⁵). Subsequently, over many years, improvements and refinements in these assays have increased their specificity, sensitivity, and practicality facilitating automation. The most important advance followed the isolation by Sanders et al. of a human TSHR monoclonal TSAb¹¹ that permitted replacement of TSH with a more stable nonradioactive ligand in a binding assay.

The nomenclature of TSHR antibodies varies depending on the type of assay employed, and commercial competition has attempted to claim superiority for one over another (reviewed in Rapoport and McLachlan¹²). However, all assays that are equally sensitive and specific for Graves' hyperthyroidism are measuring TSAb (the patient is the bioassay) and, for simplicity, we will use this term. Bioassay of TSHR antibodies that block TSH action (reviewed in Rees Smith et al¹³) can be of value in hypothyroid patients. Whether “neutral” autoantibodies that neither activate nor block the TSHR play a role in human thyroid autoimmune disease remains an open question (reviewed in Rapoport and McLachlan¹²).

Thyroid peroxidase autoantibodies

The demonstration in 1985 that autoantibodies to the thyroid microsomal antigen interacted with purified human thyroid peroxidase (TPO) provided strong evidence that this unidentified antigen was TPO.¹⁴ The molecular cloning of human TPO in 1987, facilitated by a report in 1986 of the nucleotide sequence of a large portion of porcine TPO, confirmed this identity (reviewed in McLachlan and Rapoport¹⁵). Because TPO is far less abundant than Tg and more difficult to purify from thyroid tissue, the subsequent generation of recombinant human TPO greatly facilitated the development of sensitive and specific assays for TPO autoantibodies (TPO-Ab) that are now in routine clinical use. An important issue is that human TPO-Ab are highly specific for human antigen, unlike the TSHR autoantibodies that interact well with porcine TSHR purified from readily available porcine thyroids.

Even before the availability of TPO-Ab assays, data obtained for antimicrosomal autoantibodies revealed the clinical importance of these autoantibodies in predicting the progression of Hashimoto's thyroiditis to subclinical, then clinical, hypothyroidism.¹⁶ This conceptually vital finding provided the basis for innumerable subsequent clinical studies evaluating the relationship between TPO-Ab and autoimmune thyroiditis.

Humoral and Cell-Mediated Immunity in Autoimmune Thyroid Diseases

The availability of thyroid autoantigens, in particular recombinant human TSHR and TPO, in the 1990s led to a great burst of information regarding the interaction of these autoantigens with autoantibodies and T cells, leading to further discoveries regarding the pathogenesis and potential immunotherapy of autoimmune thyroid diseases, as discussed below. It is seldom appreciated that these thyroid autoantigens provide a great investigative advantage relative to the majority of other autoimmune diseases, which lack a single or limited number of unequivocal, specific autoantigenic targets for the immune system.

There have a number of fundamentally important immunological discoveries regarding the pathogenesis of thyroid autoimmunity including Hashimoto's thyroiditis in the past 25–30 years, which are as follows:

1. B cells (plasma cells) infiltrating Hashimoto and Graves' thyroids, but not in thyroid-draining lymph nodes or peripheral blood, spontaneously secrete microsomal (TPO) autoantibodies, Tg autoantibodies, and TSHR autoantibodies (reviewed in Rees Smith et al¹³). This insight led to the realization that cDNA libraries from Graves' thyroids would contain cDNA for thyroid-specific autoantibodies. Consequently, heavy (H) and light (L) chain genes for anti-TPO and anti-Tg autoantibodies of IgG class were isolated from H and L chain combinatorial cDNA libraries constructed from Graves' thyroids and their expression as recombinant Fab (reviewed in McLachlan and Rapoport¹⁷). Data obtained with these TPO and Tg autoantibody Fab (reviewed in Rapoport and McLachlan¹²) revealed the following:

a. A limited number of H and L chain genes coded for the great majority of anti-TPO-Ab genes.

b. The repertoires of TPO-Ab focused on two overlapping epitopes on a restricted region of the TPO molecule, that is, an immunodominant region, consistent with the previous observation of epitopic restriction with hybridoma-derived human TPO-Ab.¹⁸ Similar observations for recognition of an immunodominant region were made for Tg autoantibodies.

c. Recombinant TPO-Ab Fab competition for TPO-Ab in patients' sera permitted “fingerprinting” of TPO epitopes in an individual patient. Tg epitopes have been fingerprinted in the same way.

d. TPO-Ab epitopic fingerprints in an individual were stable over many years despite variation of the titer and there was evidence for a familial pattern of epitopic inheritance.

2. Expression of major histocompatibility complex (MHC) class II molecules on the thyrocyte surface, most likely induced by cytokines particularly interferon-gamma. This groundbreaking discovery in the thyroid field by Bottazzo et al¹⁹ had broad application to other autoimmune diseases by indicating that an autoimmune target tissue cell can function as a (nonprofessional) antigen-presenting cell, processing and presenting its own antigens to T cells.

3. B cells as antigen-presenting cells. For many years, with the exception of a few organ-specific autoimmune diseases caused directly by autoantibodies (e.g., Graves' disease, myasthenia gravis, and pemphigus vulgaris), in the great majority of autoimmune diseases, T cells were the dominant players, whereas B cells were the “poor cousin,” present but of no significance. The early observation that B cells, not only macrophages and dendritic cells, could function as “professional” antigen-presenting cells had little impact but is now receiving much attention in autoimmunity in general. The importance of this finding is that, unlike macrophages and dendritic cells that function as low-affinity “vacuum cleaners” sweeping up all antigens in the vicinity, B cells bearing IgG molecules on their surface function as high-affinity receptors for specific proteins. Following internalization of the complex, the bound antibody can influence which antigenic peptides (possibly cryptic) are released by proteolysis and subsequently presented to T cells.^20,21 This phenomenon was first reported to be important in thyroid autoimmunity (reviewed in Quaratino et al²²). Indeed, such antibody-mediated bias in T cell epitope presentation is a likely explanation for the finding mentioned above of TPO and Tg autoantibody recognition of immunodominant epitopic regions (reviewed in Rapoport and McLachlan¹²).

4. Although autoantibodies to both Tg and TPO are present in human disease, the latter predominate in clinical autoimmune thyroid diseases.²³ Whether these autoantibodies contribute to thyrocyte damage, are markers of thyroiditis, or simply occur in some normal individuals has been debated for many years. Early evidence for thyroid autoantibody cytotoxic effects, either complement mediated or as “pathfinders” for cytotoxic T cells, is now eclipsed by unequivocal data indicating that T cells in the absence of autoantibodies can be cytotoxic to thyrocytes. In a classic report in 2004, Quaratino et al. generated transgenic mice in which mouse T cells expressed a human T cell receptor cloned from intrathyroidal T cells from a patient with autoimmune thyroid disease with specificity for a cryptic TPO peptide epitope.²⁴ These mice spontaneously developed thyroiditis with all the clinical features of human Hashimoto's thyroiditis.

5. The cloning of antigen-specific T cells and their T cell receptors from thyroid-infiltrating T cells, with identification of their peptide epitope recognition (reviewed in Feldmann et al²⁵). Much information has been obtained in humans and mice regarding Tg, TPO, and TSHR T cell epitopes and their presentation by MHC molecules (reviewed in Refs.^26

–29).

6. Central tolerance: Expression of low intrathymic levels of the TSHR, associated with particular TSHR single nucleotide polymorphisms, is associated with reduced central tolerance to the TSHR and susceptibility to developing Graves' disease.^30,31

7. Peripheral tolerance: The appreciation that self antigens may escape induction of central tolerance in the thymus led to the concept of “suppressor T cells” maintaining peripheral tolerance, thereby preventing autoimmunity, as suggested with respect to the thyroid by Robert Volpe more than 30 years ago.³² Suppressor T cells, initially met with skepticism, have now returned under a different name, “regulatory T cells,” or Treg.

There is now an expanding literature on Treg in thyroid autoimmunity, but primarily involving data on cells obtained from peripheral blood without evidence of thyroid antigen specificity. Clearly, a global abnormality in peripheral Treg could not be responsible for thyroid autoimmunity alone. Nevertheless, T cell reconstitution that follows therapeutic T cell depletion for a variety of conditions, or that occurs with HIV therapy, can lead to an autoimmune diathesis, very commonly Graves' disease, that may relate to a Treg imbalance (reviewed in McLachlan and Rapoport³³). Furthermore, Treg depletion in an induced mouse model of Graves' disease suggested an important role for these cells in the progression of Graves' disease to autoimmune thyroiditis.³⁴

8. Immune checkpoint inhibitors: Important insight into immune mechanisms contributing to thyroid autoimmunity has been derived from the recent use of drugs that remove immunological constraints for T cell-mediated immunotherapy of neoplasia in humans. The targets of these drugs include cytotoxic T lymphocyte antigen 4 (CTLA4), programmed cell death protein 1 (PD-1), and its ligand PD-L1 (reviewed in Wright et al³⁵). Remarkably, in 1995, genetic screening of Graves' patients first reported the CTLA4 gene to be associated with this disease.³⁶

Molecular Cloning of the Three Major Thyroid Autoantigens

These technical breakthroughs, although not in themselves representing conceptual advances, were critical tools for subsequent studies relating to thyroid autoimmunity (reviewed in Ludgate and Vassart).³⁷

Thyroglobulin

The cloning of the cDNA for Tg in 1985 was the first molecular characterization of a thyroid autoantigen.³⁸ However, its great abundance and ready purification from thyroid glands, as well as its extremely large size, has limited the subsequent benefits of this molecular information in the study of thyroid autoimmunity, aside from the ability to design short synthetic peptides based on its primary amino acid sequence for investigation of T cell specificity (reviewed in Refs.^26,39).

Thyroid peroxidase

As described above, the availability of recombinant human TPO following the molecular cloning of its cDNA in 1987 led to highly sensitive and specific clinical assays for TPO-Ab, as well as the molecular cloning and characterization of essentially the entire repertoire of H and L chains used in these autoantibodies, to our knowledge the first organ-specific autoimmune disease for which this has been accomplished.

TSH receptor

In 1989, the molecular cloning of the closely related luteinizing hormone receptor facilitated the design of nucleotide probes to screen thyroid cDNA libraries to isolate the cDNA for the dog⁴⁰ and the human TSHR (reviewed in Rapoport et al⁵). Molecular information on the TSHR, besides permitting improved clinical assays for TSHR autoantibodies (see section Clinical assays for thyroid autoantibodies, TSHR autoantibodies), has led to a number of conceptual advances regarding the pathogenesis of Graves' disease and studies exploring approaches to therapy of this disease. Crystallization of a portion (260 amino acids) of the 764 amino acid holoreceptor in complex with a human TSAb⁴¹ and a human TBAb,⁴² thereby defining their binding sites, was an advance of major importance.

Pathogenesis of Graves' Disease

Studies over four decades have provided conceptually fascinating clues regarding the pathogenesis of Graves' disease, the latter being unique among the organ-specific autoimmune diseases in that autoantibodies can activate its cognate receptor and cause glandular hyperfunction. Thus, although not explaining the loss of immune tolerance to the TSHR, the structure of the TSHR itself contributes to the development of TSAb leading to hyperthyroidism.

The story began in 1985 with the observation by Rees Smith and colleagues that the TSHR expressed on the cell surface undergoes intramolecular cleavage into two subunits that remain linked by disulfide bonds: an extracellular A subunit and a largely intracellular B subunit.⁴³ In this process, a polypeptide segment of ∼50 amino acids is lost, termed a “C peptide” because of similarity with the cleavage of proinsulin to insulin.⁴⁴ Linkage between the A and B subunits can be broken by “maltreating” membranes or cells by freeze-thawing⁴⁵ or hypotonic shock.⁴⁶

However, under physiological conditions, A subunit shedding was deduced because TSAb (unlike TBAb) bound to the isolated TSHR extracellular region (ectodomain), whereas binding to the holoreceptor on the cell surface was sterically hindered.⁴⁷ Proof that the shed A subunit was the effective antigen inducing or amplifying TSHR generation in Graves' disease was provided by the observation that TSAb could only be induced in mice immunized with an adenovirus vector encoding cDNA for the isolated A subunit, unlike a holo-TSHR modified to be unable to cleave into subunits.⁴⁸ Indeed, this A subunit component of the TSHR is now in use by groups throughout the world in induced mouse models of Graves' disease (discussed further below).

These findings introduced two novel concepts. First, because of its unique structure, the TSHR is the culprit as well as the victim in the pathogenesis of Graves' disease. Indeed, unlike the TSHR, the closely related gonadotropin receptors do not cleave into subunits permitting ectodomain shedding, and there is no “Graves” disease of the gonads’. Second, the foregoing data provide insight into the mechanism by which TSAb activate the TSHR, thereby leading to hyperthyroidism. Thus, only the TSH holoreceptor, not the free A-subunit, can induce a signal. By overcoming partial steric hindrance to its binding, TSAb-mediated torsion in the ectodomain will lead to a shift in the seven membrane-spanning alpha-helices causing signal transduction. This conclusion is supported by data reported for the follicle stimulating hormone receptor.⁴⁹

Clearly, additional factors are involved in the loss of immune tolerance to the TSHR. That Graves' disease commonly occurs in members of the same family (reviewed in Lee et al²⁹) has led to very many studies that have identified genes that contribute to the development of Graves' disease and Hashimoto's thyroiditis, including those for the TSHR and Tg (reviewed in Lee et al²⁹). Like many other autoimmune diseases, no single gene has been identified to be responsible for Graves' or Hashimoto's diseases. However, a number of immune modulatory genes appear to contribute to the development of disease, in addition to human leucocyte antigen molecules (MHC class I and class II), CTLA4, CD40, PTPN22, and CD25 (FoxP3) (reviewed in Lee et al²⁹).

Therapy of Hashimoto's and Graves' Diseases

The past century has witnessed dramatic advances in the therapy of these diseases.

Synthetic thyroid hormones. Although used for conditions unassociated with thyroid autoimmunity, the availability of synthetic thyroxine⁵⁰ and triiodothyronine⁵¹ has been among the most important advances in the past century for treatment of hypothyroidism in Hashimoto's thyroiditis and following thyroid ablative therapy in Graves' disease.

Thionamide antithyroid drugs for Graves' hyperthyroidism. The first clinical use of these drugs by Astwood in 1942 was a critical milestone in the therapy of this condition (reviewed in Burch and Cooper⁵²). There is evidence that these drugs have an immunoregulatory effect in decreasing autoantibody levels.⁵³

Radioiodine ablative therapy in Graves' disease, first used by Hertz in 1941, was another critical development in the history of thyroid autoimmunity (reviewed in Hertz⁵⁴).

Immunotherapy. It must be emphasized that thionamide drugs and radioiodine can treat, but not cure, Graves' hyperthyroidism. The latter almost invariably leads to the onset of a secondary disease, hypothyroidism. A major goal for the next century will be the cure of Graves' disease, an approach that will require some form of immunotherapy to re-institute immune tolerance to the TSHR. Preliminary work to this end is ongoing in many laboratories and, unavoidably, requires an animal model of Graves' disease. Unfortunately, Graves' disease appear to be a uniquely human disease, and for many decades, attempts to induce this disease in sundry animal species by immunization with a variety of thyroid tissue preparations were fruitless.

In 1996, a milestone was the first success in this endeavor when Shimojo et al. reported the induction of Graves’-like disease in mice by immunization with fibroblasts transfected to co-express the TSHR and a class II MHC molecule.⁵⁵ However, a serious limitation of this model was its application only to mice strains with the same MHC molecule. This handicap was overcome by the first application by Costagliola et al. of genetic immunization of outbred mice with the cDNA for the human TSHR.⁵⁶ Although not successfully applied in other laboratories, this approach opened the door to more reproducible results using a variety of vectors, immunization techniques and, in particular (see section Pathogenesis of Graves, disease) to immunization with the cDNA for the A subunit rather than the TSH holoreceptor (reviewed in Rapoport and McLachlan⁵⁷). In addition to the induction of hyperthyroidism, orbital changes (primarily histological) similar to those observed in human disease have been reported,⁵⁸ a potentially useful model in future studies.

In recent years, using the foregoing mouse model, there have been numerous attempts at therapy to prevent or reverse induced Graves' disease by injecting recombinant TSHR protein or TSHR-derived synthetic peptides. Although pretreatment with TSHR protein antigen can ameliorate the induction of TSAb and hyperthyroidism, injecting the antigen after disease induction is ineffective, ruling out the likelihood of success in treating human disease (reviewed in Rapoport and McLachlan¹²). Nevertheless, clinical trials are underway in humans using synthetic TSHR peptides with a presently uncertain outcome.⁵⁹

In 2015, the first mouse model that spontaneously develops TSAb was reported, a more satisfactory animal in which to test immune therapeutical approaches to cure human Graves' disease.⁶⁰ This model was generated by crossing a transgenic mouse with the human TSHR A subunit targeted to the thyroid with NOD.H2h4 mice that spontaneously develop Tg and TPO-Ab, but not TSAb. In this transgenic/NOD.H2h4 cross, TSAb do develop from loss of tolerance to the human TSHR but do not cross react with the mouse TSHR. That the mice remain euthyroid is an advantage for immune therapeutic investigation because thyroid hormones directly influence the immune system and variation in their levels would have a confounding effect in assessing experimental results. Moreover, the goal of immunotherapy in humans is to attenuate or eliminate TSAb generation, the direct cause of the disease.

Extrathyroidal Manifestations of Graves' Disease

There have been numerous conceptual and therapeutic advances regarding these distressing conditions. The advent of computerized tomography (CT) scanning in the 1970s supported earlier anatomical studies noting prominent orbital extraocular muscle thickening⁶¹ on which basis these muscles were considered to be the target of the immune system. Later, CT and magnetic resonance imaging studies focused on increased volume in orbital adipose tissue and, subsequently, adipocytes became cells of greater interest in the pathogenesis of Graves' ophthalmopathy (GO). Studies in recent years have revealed orbital fibroblasts to be the most important cells in this process. In a voluminous literature, the concept emerged that orbital fibroblasts represented a distinct subgroup, including pre-adipocytes, and that cytokines secreted by thyroid-specific T cells were the proximal cause of the pathological features of GO, including hyaluronan generation and pre-adipocyte maturation (reviewed in Taylor et al⁶²).

1. The revolutionary discovery in 1993 of TSHR expression on orbital fibroblasts, adipocytes, and marrow-derived fibrocytes, enhanced by sundry cytokines, introduced TSAb as an important pathogenetic factor in GO (reviewed in Bahn⁶³). Evidence has since accumulated that TSAb, independent of T cells (other than receiving “help” for antibody production), are themselves sufficient for the characteristic pathological changes in the orbit. For example:

a. GO and Graves' dermopathy are associated with particularly high TSAb levels.^64,65

b. Radioiodine therapy for Graves' hyperthyroidism increases TSAb levels and exacerbates GO.⁶⁶ In contrast, total thyroidectomy reduces TSAb levels⁶⁷ and ameliorates GO, with further benefit by thyroid remnant ablation by radioiodine (reviewed in Menconi et al⁶⁸). Data for this conclusion are limited because of the infrequently used radical nature of this form of therapy.

c. A TSAb monoclonal antibody increases hyaluronan production by cultured orbital fibroblasts, the former being a major component contributing to orbital edema.⁶⁹

2. Therapy of GO. Numerous therapeutic agents have been used for this purpose in recent years, the most prominent being rituximab (anti CD20 on B cells) and teprotumumab (anti insulin-like growth factor [IGF] receptor) (reviewed in Bartalena et al⁷⁰). The efficacy of the latter agent was anticipated because of the discovery in 1986 of synergy between IGF-1 and TSAb in activating the TSHR,⁷¹ a phenomenon reproduced in terms of stimulation of hyaluronan production by cultured human orbital fibroblasts (reviewed in Neumann et al⁷²). However, the use of a human monoclonal TSHR blocking antibody, presently in clinical trials,⁷³ is likely to be more specific and with fewer side effects than teprotumumab.

3. Local mechanical factors in the pathogenesis of GO and dermopathy. A paradox is that TSHR protein is expressed in human fibroblasts in many regions of the body in addition to those in the orbit and in areas of Graves' dermopathy.^74,75 Much evidence exists to provide an explanation for this paradox (reviewed in Rapoport and McLachlan¹²).

Conclusions

The past century has seen dramatic advances in knowledge regarding the pathogenesis and therapy of autoimmune thyroid diseases. Indeed, these diseases introduced the concept of autoimmunity in general. Graves' disease is a paradigm for the rare occurrence of how autoimmunity can cause disease by stimulating rather than destroying an organ system. Therapeutic advances in the mid 20th century involving administration of thyroid hormones, thionamide drugs, and radioiodine had a huge beneficial effect on human health.

More recent knowledge on autoimmune thyroid disease pathogenesis has led to new therapeutic agents being tested. However, the greater expense and risk of side effects of these approaches may lessen their competitiveness with earlier forms of therapy, with the possible exception of monoclonal antibody treatment for GO. Immunotherapy for a cure, a major goal for the 21st century, is unlikely to occur by targeting the immune system with agents that are not thyroid specific. Fortunately, investigation of thyroid autoimmune disease is facilitated by clearly defined antigenic targets.

Note added in proof: Since submission of this manuscript, three important studies have reported the cryo-electron microscopy structure of the TSH holoreceptor in complex with thyrotropin and TSAb,^76,77 as well as with a TBAb.⁷⁸

Footnotes

Authors' Contributions

S.M.M. and B.R. are equally responsible for conceptualizing and writing the article.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

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