Comparison of immune responses to extracellular domains of mouse and human thyrotropin receptor

Role of cytokines in the pathogenesis and suppression of thyroid autoimmunity

Autoimmune thyroid diseases (AITD) are one of the most common organ-specific autoimmune disorders, of which Hashimoto's thyroiditis (HT) and Graves' disease (GD) are 2 of the most common clinical expressions. HT is characterized by hypothyroidism that results from the destruction of the thyroid by thyroglobulin-specific T cell-mediated autoimmune response. In contrast, GD is characterized by hyperthyroidism due to excessive production of thyroid hormone induced by thyrotropin receptor-specific stimulatory autoantibodies. Cytokines play a crucial role in modulating immune responses that affect the balance between maintenance of self-tolerance and initiation of autoimmunity. However, the role of cytokines is often confusing and is neither independent nor exclusive of other immune mediators. A regulatory cytokine may either favor induction of tolerance against thyroid autoimmune disease or favor activation and/or exacerbation of autoimmune responses. These apparently contradictory functions of a given cytokine are primarily influenced by the nature of co-signaling delivered by other cytokines. Consequently, a thorough understanding of the role of a particular cytokine in the context of a specific immune response is essential for the development of appropriate strategies to modulate cytokine responses to maintain or restore health. This review provides a summary of recent research pertaining to the role of cytokines in the pathogenesis of AITD with a particular emphasis on the therapeutic applications of cytokine modulation.

Studies in mice deficient for the autoimmune regulator (Aire) and transgenic for the thyrotropin receptor reveal a role for Aire in tolerance for thyroid autoantigens

The autoimmune regulator (Aire) mediates central tolerance for many autoantigens, and autoimmunity occurs spontaneously in Aire-deficient humans and mice. Using a mouse model of Graves' disease, we investigated the role of Aire in tolerance to the TSH receptor (TSHR) in Aire-deficient and wild-type mice (hyperthyroid-susceptible BALB/c background). Mice were immunized three times with TSHR A-subunit expressing adenovirus. The lack of Aire did not influence T-cell responses to TSHR protein or TSHR peptides. However, antibody levels were higher in Aire-deficient than wild-type mice after the second (but not the third) immunization. After the third immunization, hyperthyroidism persisted in a higher proportion of Aire-deficient than wild-type mice. Aire-deficient mice were crossed with transgenic strains expressing high or low-intrathyroidal levels of human TSHR A subunits. In the low-expressor transgenics, Aire deficiency had the same effect on the pattern of the TSHR antibody response to immunization as in nontransgenics, although the amplitude of the response was lower in the transgenics. High-expressor A-subunit transgenics were unresponsive to immunization. We examined intrathymic expression of murine TSHR, thyroglobulin, and thyroid peroxidase (TPO), the latter two being the dominant autoantigens in Hashimoto's thyroiditis (particularly TPO). Expression of the TSHR and thyroglobulin were reduced in the absence of Aire. Dramatically, thymic expression of TPO was nearly abolished. In contrast, the human A-subunit transgene, lacking a potential Aire-binding motif, was unaffected. Our findings provide insight into how varying intrathymic autoantigen expression may modulate thyroid autoimmunity and suggest that Aire deficiency may contribute more to developing Hashimoto's thyroiditis than Graves' disease.

Differential Susceptibility of BALB/c and BALB/cBy mice to Graves' hyperthyroidism

BALB/c mice are susceptible to the induction of Graves' hyperthyroidism. To investigate the susceptibility of BALB/c substrains of mice to the induction of hyperthyroidism, we immunized BALB/cJ and BALB/cByJ mice with an adenovirus expressing amino acid residues 1-289 of thyrotropin receptor (TSHR). The data presented in this article showed that 17 of 26 (65%) BALB/c and only 4 of 30 (13%) BALB/cBy mice developed hyperthyroidism. Hyperthyroid mice displayed characteristics of Graves' disease, such as thyroid-stimulating antibodies and enlarged thyroid glands. To explore the differences in the susceptibility of these substrains for hyperthyroidism, we examined the TSHR antibodies in three different assays. The TSHR antibodies determined in a radioreceptor assay (TSH binding inhibitory immunoglobulins) were similar in both of these BALB/c substrains. The TSHR antibody titers of total IgG, IgG1, and IgG2a were measured by an enzyme-linked immunosorbent assay and were found to be similar in these mice. There were no significant differences between these two groups of mice in the thyroid-stimulating antibody activity. However, BALB/cBy mice had significantly higher TSH-blocking antibody activity compared to BALB/c mice. TSHR-specific proliferation of splenocytes and secretion of cytokines interferon-gamma and interleukin-4 by spleen cells were comparable in both the groups. BALB/cJ and BALB/cByJ mice both belong to same MHC haplotype, H-2(d), but differ in the Qa-2 region of class Ib molecule. This report shows the importance of other genes, such as Qa-2 region of class Ib molecule in addition to MHC class II, in the susceptibility of Graves' hyperthyroidism.

Differential requirement of signal transducer and activator of transcription-4 (Stat4) and Stat6 in a thyrotropin receptor-289-adenovirus-induced model of Graves' hyperthyroidism

T helper type 1 (Th1) and Th2 cells have critical roles in the development of cell-mediated and humoral immune responses, respectively. This division of function predicts that Th1 cells mediate inflammatory diseases and Th2 cells promote antibody (Ab)-mediated autoimmunity. Our previous studies using HEK-293 cells expressing the extracellular domain of the TSH receptor (TSHR) showed that Stat4-/- mice, which lack Th1 cells, are susceptible, whereas Stat6-/- mice, which lack Th2 cells, are resistant to the induction of Graves' hyperthyroidism. To investigate the role of Stat4 and Stat6 genes in other murine models of hyperthyroidism, we injected wild-type BALB/c, Stat4-/-, and Stat6-/- mice with an adenovirus expressing amino acid residues 1-289 of TSHR (TSHR-289-ad or 289-ad). The viral system induces a much stronger immune response with much more rapid onset of disease. Our results showed that 56% of wild-type, 75% of Stat4-/-, and 39% of Stat6-/- mice developed hyperthyroidism. Hyperthyroid mice exhibited thyroid stimulatory Abs. The Stat4-/- mice developed a higher incidence and greater severity of hyperthyroidism compared with wild-type and Stat6-/- mice. BALB/c and Stat4-/- mice showed significantly higher TSHR Abs of the IgG1 subclass and IL-4 compared with Stat6-/- mice. In contrast, Stat6-/- mice had predominantly the IgG2a subclass of TSHR Ab and produced significantly higher amounts of IFN-gamma than BALB/c and Stat4-/- mice. All hyperthyroid mice showed enlarged thyroid glands with hyperactivity. These results suggest that in the TSHR-289-ad model, the Th2 cells are more efficient in mediating disease, but in the absence of Th2 cells, Th1 cells may still initiate a reduced incidence of Graves' hyperthyroidism.

Insight into Graves' hyperthyroidism from animal models

Graves' hyperthyroidism can be induced in mice or hamsters by novel approaches, namely injecting cells expressing the TSH receptor (TSHR) or vaccination with TSHR-DNA in plasmid or adenoviral vectors. These models provide unique insight into several aspects of Graves' disease: 1) manipulating immunity toward Th1 or Th2 cytokines enhances or suppresses hyperthyroidism in different models, perhaps reflecting human disease heterogeneity; 2) the role of TSHR cleavage and A subunit shedding in immunity leading to thyroid-stimulating antibodies (TSAbs); and 3) epitope spreading away from TSAbs and toward TSH-blocking antibodies in association with increased TSHR antibody titers (as in rare hypothyroid patients). Major developments from the models include the isolation of high-affinity monoclonal TSAbs and analysis of antigen presentation, T cells, and immune tolerance to the TSHR. Studies of inbred mouse strains emphasize the contribution of non-MHC vs. MHC genes, as in humans, supporting the relevance of the models to human disease. Moreover, other findings suggest that the development of Graves' disease is affected by environmental factors, including infectious pathogens, regardless of modifications in the Th1/Th2 balance. Finally, developing immunospecific forms of therapy for Graves' disease will require painstaking dissection of immune recognition and responses to the TSHR.

Induction of Experimental Autoimmune Graves’ Disease in BALB/c Mice

We immunized BALB/c mice with M12 cells (H-2d) expressing either mouse (mM12 cells) or human thyrotropin receptor (TSHR) (hM12 cells). Immunized mice developed autoantibodies to native TSHR by day 90 and, by day 180, showed considerable stimulatory Ab activity as measured by their ability to enhance cAMP production (ranging from 6.52 to 20.83 pmol/ml in different treatment groups relative to 1.83 pmol/ml for controls) by TSHR-expressing Chinese hamster ovary cells. These mice developed severe hyperthyroidism with significant elevations in both tetraiodothyronine and triiodothyronine hormones. Tetraiodothyronine levels in different experimental groups ranged from a mean of 8.66–12.4 μg/dl, relative to 4.8 μg/dl in controls. Similarly, mean triiodothyronine values ranged from 156.18 to 195.13 ng/dl, relative to 34.99 ng/dl for controls. Next, we immunized BALB/c mice with a soluble extracellular domain of human TSHR (TBP), or TBP expressed on human embryonic kidney cells (293 cells) (293-TBP cells). These mice showed severe hyperthyroidism in a manner very similar to that described above for mice immunized with the mouse TSHR or human TSHR, and exhibited significant weight loss, with average weight for treatment groups ranging from 20.6 to 21.67 g, while controls weighed 24.2 g. Early after onset of the disease, histopathological examination of thyroids showed enlargement of colloids and thinning of epithelial cells without inflammation. However, later during disease, focal necrosis and lymphocytic infiltration were apparent. Our results showed that conformationally intact ectodomain of TSHR is sufficient for disease induction. Availability of a reproducible model in which 100% of the animals develop disease should facilitate studies aimed at understanding the molecular pathogenesis of Graves’ disease.