Immunization of mice with a newly identified thyroid-stimulating hormone receptor splice variant induces Graves'-like disease

A Mouse Thyrotropin Receptor A-Subunit Transgene Expressed in Thyroiditis-Prone Mice May Provide Insight into Why Graves' Disease Only Occurs in Humans

Background: Graves' disease, caused by autoantibodies that activate the thyrotropin (TSH) receptor (TSHR), has only been reported in humans. Thyroiditis-prone NOD.H2^h4 mice develop autoantibodies to thyroglobulin (Tg) and thyroid peroxidase (TPO) but not to the TSHR. Evidence supports the importance of the shed TSHR A-subunit in the initiation and/or amplification of the autoimmune response to the holoreceptor. Cells expressing the gene for the isolated A-subunit secrete A-subunit protein, a surrogate for holoreceptor A-subunit shedding. NOD.H2^h4 mice with the human TSHR A-subunit targeted to the thyroid (a "self" antigen in such transgenic (Tgic) animals), unlike their wild-type (wt) siblings, spontaneously develop pathogenic TSHR antibodies to the human-TSH holoreceptor. These autoantibodies do not recognize the endogenous mouse-TSH holoreceptor and do not cause hyperthyroidism. Methods: We have now generated NOD.H2^h4 mice with the mouse-TSHR A-subunit transgene targeted to the thyroid. Tgic mice and wt littermates were compared for intrathyroidal expression of the mouse A-subunit. Sera from six-month-old mice were tested for the presence of autoantibodies to Tg and TPO as well as for pathogenic TSHR antibodies (TSH binding inhibition, bioassay for thyroid stimulating antibodies) and nonpathogenic TSHR antibodies (ELISA). Results: Expression of the mouse TSHR A-subunit transgene in the thyroid was confirmed by real-time polymerase chain reaction in the Tgics and had no effect on the spontaneous development of autoantibodies to Tg or TPO. However, unlike the same NOD.H2^h4 strain with the human-TSHR A-subunit target to the thyroid, mice expressing intrathyroidal mouse-TSHR A subunit failed to develop either pathogenic or nonpathogenic TSHR antibodies. The mouse TSHR A-subunit differs from the human TSHR A-subunit in terms of its amino acid sequence and has one less glycosylation site than the human TSHR A-subunit. Conclusions: Multiple genetic and environmental factors contribute to the pathogenesis of Graves' disease. The present study suggests that the TSHR A-subunit structure (possibly including posttranslational modification such as glycosylation) may explain, in part, why Graves' disease only develops in humans.

A Modifying Autoantigen in Graves' Disease

The TSH receptor (TSHR) is the major autoantigen in Graves' disease (GD). Bioinformatic analyses predict the existence of several human TSHR isoforms from alternative splicing, which can lead to the coexpression of multiple receptor forms. The most abundant of these is TSHRv1.3. In silico modeling of TSHRv1.3 demonstrated the structural integrity of this truncated receptor isoform and its potential binding of TSH. Tissue profiling revealed wide expression of TSHRv1.3, with a predominant presence in thyroid, bone marrow, thymus, and adipose tissue. To gain insight into the role of this v1.3 receptor isoform in thyroid pathophysiology, we cloned the entire open reading frame into a mammalian expression vector. Immunoprecipitation studies demonstrated that both TSHR-stimulating antibody and human TSH could bind v1.3. Furthermore, TSHRv1.3 inhibited the stimulatory effect of TSH and TSHR-Ab MS-1 antibody on TSHR-induced cAMP generation in a dose-dependent manner. To confirm the antigenicity of v1.3, we used a peptide ELISA against two different epitopes. Of 13 GD samples, 11 (84.6%) were positive for a carboxy terminal peptide and 10 (76.9%) were positive with a junction region peptide. To demonstrate that intracellular v1.3 could serve as an autoantigen and modulate disease, we used double-transfected Chinese hamster ovary cells that expressed both green fluorescent protein (GFP)-tagged TSHRv1.3 and full-length TSHR. We then induced cell stress and apoptosis using a TSHR monoclonal antibody and observed the culture supernatant contained v1.3-GFP protein, demonstrating the release of the intracellular receptor variant by this mechanism.

Genetic immunization with mouse thyrotrophin hormone receptor plasmid breaks self-tolerance for a murine model of autoimmune thyroid disease and Graves' orbitopathy

Experimental models of Graves' hyperthyroid disease accompanied by Graves' orbitopathy (GO) can be induced efficiently in susceptible inbred strains of mice by immunization by electroporation of heterologous human TSH receptor (TSHR) A-subunit plasmid. In this study, we report on the development of a bona fide murine model of autoimmune Graves' disease induced with homologous mouse TSHR A-subunit plasmid. Autoimmune thyroid disease in the self-antigen model was accompanied by GO and characterized by histopathology of hyperplastic glands with large thyroid follicular cells. Examination of orbital tissues showed significant inflammation in extra-ocular muscle with accumulation of T cells and macrophages together with substantial deposition of adipose tissue. Notably, increased levels of brown adipose tissue were present in the orbital tissue of animals undergoing experimental GO. Further analysis of inflammatory loci by ¹⁹ F-magnetic resonance imaging showed inflammation to be confined to orbital muscle and optic nerve, but orbital fat showed no difference in inflammatory signs in comparison to control β-Gal-immunized animals. Pathogenic antibodies induced to mouse TSHR were specific for the self-antigen, with minimal cross-reactivity to human TSHR. Moreover, compared to other self-antigen models of murine Graves' disease induced in TSHR knock-out mice, the repertoire of autoantibodies to mouse TSHR generated following the breakdown of thymic self-tolerance is different to those that arise when tolerance is not breached immunologically, as in the knock-out models. Overall, we show that mouse TSHR A-subunit plasmid immunization by electroporation overcomes tolerance to self-antigen to provide a faithful model of Graves' disease and GO.

Thyrotropin Receptor Epitope and Human Leukocyte Antigen in Graves' Disease

Graves' disease (GD) is an organ-specific autoimmune disease, and thyrotropin (TSH) receptor (TSHR) is a major autoantigen in this condition. Since the extracellular domain of human TSHR (TSHR-ECD) is shed into the circulation, TSHR-ECD is a preferentially immunogenic portion of TSHR. Both genetic factors and environmental factors contribute to development of GD. Inheritance of human leukocyte antigen (HLA) genes, especially HLA-DR3, is associated with GD. TSHR-ECD protein is endocytosed into antigen-presenting cells (APCs), and processed to TSHR-ECD peptides. These peptide epitopes bind to HLA-class II molecules, and subsequently the complex of HLA-class II and TSHR-ECD epitope is presented to CD4+ T cells. The activated CD4+ T cells secrete cytokines/chemokines that stimulate B-cells to produce TSAb, and in turn hyperthyroidism occurs. Numerous studies have been done to identify T- and B-cell epitopes in TSHR-ECD, including (1) in silico, (2) in vitro, (3) in vivo, and (4) clinical experiments. Murine models of GD and HLA-transgenic mice have played a pivotal role in elucidating the immunological mechanisms. To date, linear or conformational epitopes of TSHR-ECD, as well as the molecular structure of the epitope-binding groove in HLA-DR, were reported to be related to the pathogenesis in GD. Dysfunction of central tolerance in the thymus, or in peripheral tolerance, such as regulatory T cells, could allow development of GD. Novel treatments using TSHR antagonists or mutated TSHR peptides have been reported to be effective. We review and update the role of immunogenic TSHR epitopes and HLA in GD, and offer perspectives on TSHR epitope specific treatments.

Animal models of Graves' disease and Graves' orbitopathy

PURPOSE OF REVIEW

The purpose of this article is to summarize the recent advances on experimental Graves' hyperthyroidism and orbitopathy as studied in two widely used mouse models, which involve repetitive genetic vaccinations using either adenovirus or in-vivo electroporation of the eukaryotic expression plasmid expressing the thyrotropin receptor (TSHR) as a vector.

RECENT FINDINGS

The models have been improved by using different types of antigens, including the holo receptor, the receptor A-subunit, an alternatively spliced form of variant receptor lacking a single leucine-rich repeat in the codomain, the receptors of human or mouse origin; different mice such as wild-type, TSHR knockout, TSHR transgenic and different inbred mice; and different immunization protocols. They are now useful for elucidating the pathogenic mechanisms of not only Graves' hyperthyroidism but also Graves' orbitopathy.

SUMMARY

This review summarizes the literature of mouse models of Graves' hyperthyroidism and orbitopathy published over the last 3 years.

Human autoimmune diseases: a comprehensive update

There have been significant advances in our understanding of human autoimmunity that have led to improvements in classification and diagnosis and, most importantly, research advances in new therapies. The importance of autoimmunity and the mechanisms that lead to clinical disease were first recognized about 50 years ago following the pioneering studies of Macfarlane Burnett and his Nobel Prize-winning hypothesis of the 'forbidden clone'. Such pioneering efforts led to a better understanding not only of autoimmunity, but also of lymphoid cell development, thymic education, apoptosis and deletion of autoreactive cells. Contemporary theories suggest that the development of an autoimmune disease requires a genetic predisposition and environmental factors that trigger the immune pathways that lead, ultimately, to tissue destruction. Despite extensive research, there are no genetic tools that can be used clinically to predict the risk of autoimmune disease. Indeed, the concordance of autoimmune disease in identical twins is 12-67%, highlighting not only a role for environmental factors, but also the potential importance of stochastic or epigenetic phenomena. On the other hand, the identification of cytokines and chemokines, and their cognate receptors, has led to novel therapies that block pathological inflammatory responses within the target organ and have greatly improved the therapeutic effect in patients with autoimmune disease, particularly rheumatoid arthritis. Further advances involving the use of multiplex platforms for diagnosis and identification of new therapeutic agents should lead to major breakthroughs within the next decade.

Breaking tolerance to thyroid antigens: changing concepts in thyroid autoimmunity

Thyroid autoimmunity involves loss of tolerance to thyroid proteins in genetically susceptible individuals in association with environmental factors. In central tolerance, intrathymic autoantigen presentation deletes immature T cells with high affinity for autoantigen-derived peptides. Regulatory T cells provide an alternative mechanism to silence autoimmune T cells in the periphery. The TSH receptor (TSHR), thyroid peroxidase (TPO), and thyroglobulin (Tg) have unusual properties ("immunogenicity") that contribute to breaking tolerance, including size, abundance, membrane association, glycosylation, and polymorphisms. Insight into loss of tolerance to thyroid proteins comes from spontaneous and induced animal models: 1) intrathymic expression controls self-tolerance to the TSHR, not TPO or Tg; 2) regulatory T cells are not involved in TSHR self-tolerance and instead control the balance between Graves' disease and thyroiditis; 3) breaking TSHR tolerance involves contributions from major histocompatibility complex molecules (humans and induced mouse models), TSHR polymorphism(s) (humans), and alternative splicing (mice); 4) loss of tolerance to Tg before TPO indicates that greater Tg immunogenicity vs TPO dominates central tolerance expectations; 5) tolerance is induced by thyroid autoantigen administration before autoimmunity is established; 6) interferon-α therapy for hepatitis C infection enhances thyroid autoimmunity in patients with intact immunity; Graves' disease developing after T-cell depletion reflects reconstitution autoimmunity; and 7) most environmental factors (including excess iodine) "reveal," but do not induce, thyroid autoimmunity. Micro-organisms likely exert their effects via bystander stimulation. Finally, no single mechanism explains the loss of tolerance to thyroid proteins. The goal of inducing self-tolerance to prevent autoimmune thyroid disease will require accurate prediction of at-risk individuals together with an antigen-specific, not blanket, therapeutic approach.

Diagnosis and classification of Graves' disease

Graves' disease (GD) is an autoimmune disorder involving the thyroid gland, typically characterized by the presence of circulating autoantibodies that bind to and stimulate the thyroid hormone receptor (TSHR), resulting in hyperthyroidism and goiter. Organs other than the thyroid can also be affected, leading to the extrathyroidal manifestations of GD, namely Graves' ophthalmopathy, which is observed in ~50% of patients, and Graves' dermopathy and acropachy, which are quite rare. Presumably, the extrathyroidal manifestations of GD are due to autoimmunity against antigens common to the thyroid and other affected organs. Although its exact etiology remains to be completely understood, GD is believed to result from a complex interaction between genetic susceptibility and environmental factors. Clinically, GD is characterized by the manifestations of thyrotoxicosis as well as by its extrathyroidal features when present, the latter making the diagnosis almost unmistakable. In the absence of ophthalmopathy, the diagnosis is generally based on the association of hyperthyroidism and usually diffuse goiter confirmed with serum anti-TSHR autoantibodies (TRAbs). Hyperthyroidism is generally treated with anti-thyroid drugs, but a common long term treatment strategy in patients relapsing after a course of anti-thyroid drugs (60-70%), implies the use of radioactive iodine or surgery.