Glycosylated ectodomain of the human thyrotropin receptor induces antibodies capable of reacting with multiple blocking antibody epitopes

TSH signalling and cancer

Thyroid cancers are the most frequent endocrine neoplasms and mutations in the thyrotropin receptor (TSHR) are unusually frequent. Here we present the state-of-the-art concerning the role of TSHR in thyroid cancer and discuss it in light of the cancer stem cell theory or the classical view. We briefly review the gene and protein structure updating the cancer related TSHR mutations database. Intriguingly, hyperfunctioning TSHR mutants characterise differentiated cancers in contrast to undifferentiated thyroid cancers which very often bear silenced TSHR. It remains unclear whether TSHR alterations in thyroid cancers play a role in the onset or they appear as a consequence of genetic instability during evolution, but the presence of functional TSHR is exploited in therapy. We outline the signalling network build up in the thyrocyte between TSHR/PKA and other proliferative pathways such as Wnt, PI3K and MAPK. This networks integrity surely plays a role in the onset/evolution of thyroid cancer and needs further research. Lastly, future investigation of epigenetic events occurring at the TSHR and other loci may give better clues for molecular based therapy of undifferentiated thyroid carcinomas. Targeted demethylating agents, histone deacetylase inhibitors combined with retinoids and specific RNAis may help treatment in the future.

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.

Antibodies to TSH-receptor in thyroid autoimmune disease interact with monoclonal antibodies whose epitopes are broadly distributed on the receptor

The hyperthyroidism of Graves' disease (GD) is caused by TSH-receptor (TSH-R) stimulating autoantibodies (TSAb), leading to overproduction of thyroid hormones. We present evidence for TSAb interaction with three distinct regions of the TSH-R, one in immediate vicinity of the carboxy terminal serpentine. Three murine monoclonal antibodies (MoAbs 28.1, A9 and 31.7) directed to amino acids 36-40, 147-228 and 382-415 were labelled and tested for their binding to human recombinant TSH-R on solid phase. All MoAbs bound to TSH-R with a K(d) of 8-12 nm and showed no competition among themselves. We tested 114 sera from euthyroid controls, 118 TBII positive sera from patients with GD (containing TSAb confirmed by bioassays), 16 TBII positive sera from patients with autoimmune thyroid disease (AIT), who were hypothyroid and had TSH blocking antibodies (TBAb), and 20 patients with AIT, who were hypothyroid but negative for all TRAb. Mid-regional MoAb A9 tracer achieved the highest sensitivity in the GD group (72.0%), whereas C-terminal MoAb 31.7 found most sera positive in the AIT group (87.5%). Surprisingly, the N-terminal MoAb 28.1 had the lowest sensitivity in the GD (10.4%) and AIT group (43.8%). Using a mixture of all three tracer MoAbs did not increase the sensitivity in the GD or AIT group, compared to the best single MoAb alone. Median inhibition of MoAb A9 was significantly (P < 0.001) higher than inhibition of MoAbs 28.1 or 31.7 in the group of GD patients but not in other groups. Almost all patient sera with positive reactivity in the MoAb tracer assays had TBII values in the higher range. However, there were many highly TBII positive sera, which did not show a displacement of the MoAb tracers. We conclude that, contrary to some reports, the binding of TSAb and TBAb to the TSH-R is not restricted to distinct and distant epitopes. The middle part of the TSH-R seems to be more relevant for TSAb binding than the N-terminal part, while a proportion of TSAb autoantibodies also binds to a C-terminal epitope of the TSH-R. The method described here is a TSH independent competitive assay for the detection of TSH-R autoantibodies.

Thyroid-stimulating hormone receptor and its role in Graves' disease

The thyroid-stimulating hormone (TSH, or thyrotropin) receptor (TSHR) mediates the activating action of TSH to the thyroid gland, resulting in the growth and proliferation of thyrocytes and thyroid hormone production. In Graves' disease, thyroid-stimulating autoantibodies can mimic TSH action and stimulate thyroid cells. This leads to hyperthyroidism and abnormal overproduction of thyroid hormone. TSHR-antibodies-binding epitopes on the receptor molecule are well studied. Mechanism of TSHR-autoantibodies production is more or less clear but a susceptibility gene, which is linked to their production, is still unknown. Genetic studies show no linkage between the TSHR gene and Graves' disease. Among three common polymorphisms in the TSHR gene, only the D727E germline polymorphism in the cytoplasmic tail of the receptor showed an association with the disease, and this association is weak. The absence of a strong genetic effect of the TSHR polymorphisms in such a common and complex disorder as Graves' disease may be explained by a high degree of evolutionary conservation in TSHR. This can be shown by naturally existing germline and somatic mutations in the TSHR gene that cause various types of nonautoimmune and hereditary thyroid disease.

The effect of thyrotropin-receptor blocking antibodies on stimulating autoantibodies from patients with Graves' disease

The hyperthyroidism of Graves' disease (GD) is caused by thyrotropin-receptor (TSHR) stimulating autoantibodies (TSAb), which lead to overproduction of thyroid hormones. In this study we tried to block the stimulatory effect of patients' TSAb to the TSHR with monoclonal antibodies (mAbs) and sera from hypothyroid patients. Two groups of blocking mAbs raised by different methods from two independent groups were tested for their ability to inhibit TSH binding to the TSHR, and also the binding of TSAb from the serum of patients with GD. Group 1 mAbs (7E3, 3H10, 4C1, 1B1, 4E9) bind to amino acids 378-387 and group 2 mAbs (23.1 and 31.7) to amino acids 382-415 of the human TSHR. These results were compared to the TSH- and TSAb-inhibiting effect of sera from hypothyroid patients containing bona fide thyroid blocking antibodies (TBAb) without agonistic activity. All studies were done in a conventional cyclic adenosine monophosphate (cAMP) or a modified luciferase reporter gene bioassay. TSH-induced cAMP/luciferase signal was reduced (> 70% inhibition) by all 7 mAbs, verifying the blocking nature. Comparable results (82.2%-96.3% inhibition) were seen when cells were preincubated with 8 TBAb sera. These TBAb sera also inhibited cAMP/luciferase induction of TSAb-positive sera from patients with GD (median of 27 experiments 62.2% inhibition; range, 26.8%-93.9%), and maintained inhibition greater than 20% even when diluted 1:150. However, when mAbs were incubated with these sera, results were heterogeneous: 17 of 30 sera (57%) incubated with mAb 31.7 caused reduced cAMP production compared to incubation with the control antibody, as did 18 of 34 sera (53%) incubated with mAb 7E3, 17 of 33 sera (52%) incubated with mAb 3H10, and 16 of 31 (52%) with mAb 23.1. Mixing all four mAbs did not enhance the cAMP-reductive effect (16/27 sera; 59% inhibited). Inhibition was less pronounced than with TBAb sera (0%-76% of a control antibody) and only present at antibody concentrations greater than 10 microg/mL. We conclude that despite the strong TBAb activity of the mAbs, their effect on TSAb-induced TSHR activation of sera from patients with GD was weaker than that of human TBAb autoantibodies. Thus, the latter are not only strong inhibitors of TSH activity, but also block the stimulatory effect of autoantibodies from patients with GD. However, this effect could not be reproduced by experimental mAbs to the same extent, because it may be the result of a broader spectrum of antibodies present in the TBAb sera, interacting with or in the vicinity of TSAb epitopes. Also of interest, when a TBAb serum was added to a TSAb serum, the TBAb effect was predominant even at high dilutions.

Thyroid-stimulating monoclonal antibodies

Thyrotropin (TSH) receptor monoclonal antibodies (TSHR mAbs) were obtained from cDNA-immunized NMRI mice. Three mAb immunoglobulin Gs (IgGs) (TSmAbs 1-3) that had distinct V(H )and V(L) region sequences stimulated cyclic adenosine monophosphate (cAMP) production in isolated porcine thyroid cells greater than 10x basal and as little as 20 ng/mL (0.13 nmol/L) of TSmAb 1 IgG caused a 2x basal stimulation. TSmAb 1 and 2 Fab fragments were also effective stimulators and thyroid-stimulating activities of the IgGs and Fabs were confirmed using TSHR transfected Chinese hamster ovary (CHO) cells. The TSmAbs also inhibited (125)I-labeled TSH binding to TSHR-coated tubes by 50% or more at concentrations of 1 microg/mL or less and gave 15%-20% inhibition at 20-50 ng/mL. (125)I-labeled TSmAbs bound to TSHR-coated tubes with high affinity (approximately 10(10) L/mol) and this binding was inhibited by TSHR autoantibodies with both TSH agonist and antagonist activities. Inhibition of labeled TSmAb binding by Graves' sera correlated well with inhibition of TSH binding (r = 0.96; n = 18; p < 0.001 for TSmAb 2). The TSmAbs have considerable potential as (1) new probes for TSHR structure-function studies, (2) reagents for new assays for TSHR autoantibodies, and (3) alternatives to recombinant TSH in various in vivo applications.

Characterization of the thyrotropin binding pocket

A panel of monoclonal antibodies (mAbs) to the thyrotropin receptor (TSHR) was prepared using three different immunization strategies. The mAbs obtained (n = 138) reacted with linear epitopes covering most of the TSHR extracellular domain and with conformational epitopes. mAbs that bound to five different regions of the TSHR (amino acids [aa] 32-41, aa 36-42, aa 246-260, aa 277-296, and aa 381-385) were able to inhibit (125)I-labeled thyrotropin (TSH) binding to solubilized TSHR preparations. Fab and immunoglobulin G (IgG) preparations were similarly effective inhibitors for mAbs reactive with aa 246-260, aa 277-291 and aa 381-385 suggesting that these three regions of the TSHR are involved in TSH binding. In contrast mAbs reactive with aa 32-41 and aa 36-42 were not effective at inhibiting TSH binding when Fab preparations were used, suggesting that these N terminal regions of the TSHR were less critical for TSH binding. Our studies suggest that three distinct and discontinuous regions of the TSHR (aa 246-260 and 277-296 on the TSHR A subunit) and aa 381-385 (on the TSHR B subunit) fold together to form a complex TSH binding pocket. Alignment of the aa sequences of these three regions in TSHRs from different species indicates that they are highly conserved.

Antithyrotropin receptor antibody: an update

Numerous studies have reported the characteristics and significance concerning antithyrotropin receptor antibodies (TSHR-Abs), which cause Graves' disease and in some cases primary hypothyroidism. However, many unsolved questions concerning those antibodies remain. Here, recent developments in the study of TSHR-Abs are reviewed based on three aspects: mechanisms of TSHR-Ab production, antibody binding epitopes, and clinical TSHR-Ab assays. Mechanisms of TSHR-Ab production are discussed from five points of view: aberrant expression of the major histocompatibility complex, dysregulation of T cells, molecular mimicry, bystander effect, and expansion of autoreactive B cells. Regarding epitopes, unique TSHR-Abs have been reported that may explain the complicated pathophysiology of patients with TSHR-Ab diseases. Finally, recent efforts to improve TSHR-Ab measurements are introduced. Such efforts will contribute to clinical examinations and treatments for thyroid diseases as well as experimental methods of thyroidology.

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

The mouse and human thyrotropin receptors show greater than 87% homology in their amino acid sequences. However, glycosylated extracellular domains of mouse (mET-gp) and human (hET-gp) thyrotropin receptors showed differences in their ability to react with patient autoantibodies to thyrotropin receptor (TSHR). To test for potential differences in their immunogenicity, we immunized BALB/c mice with either gel pure non-glycosylated ectodomain of human TSHR (ETSHR II), or hET-gp (hET-gp III), or mET-gp (mET-gp III). Alternatively, mice were primed with gel pure hET-gp or mET-gp and subsequently immunized with insect cells expressing hET-gp (hET-gp II) or mET-gp (mET-gp II) respectively. All groups of mice immunized with TSHR developed high titers of antibodies against the respective immunogens. As shown earlier, sera obtained from mice immunized with ETSHR showed strong reactivity to peptide 1 (aa 22-41) and weak reactivity to peptides 23 (aa 352-371), 24 (aa 367-386), 25 (aa 382-401), and 26 (aa 397-415). Mice immunized with hET-gp or mET-gp showed comparable titers to peptides 1 and 23 and lower reactivity to other peptides. Mice immunized with hET-gp showed higher TBII reactivity (52.2%) compared to mice immunized with either ETSHR (20.9%) or mET-gp (34.5%). Peptides from the C-terminal region of ETSHR could neutralize the TBII activities of sera from mice immunized with ETSHR or hET-gp but not mET-gp. Compared to corresponding control mice, T4 levels in mET-gp II mice were only marginally higher. These data suggested that outcome of immunization with mouse ETSHR is comparable to that seen after immunization with human ETSHR.