Thyrotropin (TSH) interacts with multiple discrete regions of the TSH receptor: polyclonal rabbit antibodies to one or more of these regions can inhibit TSH binding and function

The hinge region: an important receptor component for GPHR function

Glycoprotein hormone receptors (GPHRs) are members of the seven-transmembrane-spanning receptor family characterized by a large ectodomain. The hinge region belongs to a part of the GPHR ectodomain for which the three-dimensional structure has not yet been deciphered, leaving important questions unanswered concerning ligand binding and GPHR activation. Recent publications indicate that specific residues of the hinge region mediate hormone binding, receptor activation and/or intramolecular signaling for the three GPHRs, emphasizing the importance of this region. Based on these findings, the hinge region is involved at least in part in hormone binding and receptor activation. This review summarizes functional data regarding the hinge region, demonstrating that this receptor portion represents a link between ligand binding and subsequent GPHR activation.

The superagonistic activity of bovine thyroid-stimulating hormone (TSH) and the human TR1401 TSH analog is determined by specific amino acids in the hinge region of the human TSH receptor

Bovine TSH (bTSH) has a higher affinity to the human TSHR (hTSHR) and a higher signaling activity than human TSH (hTSH). The molecular reasons for these phenomena are unknown. Distinct negatively charged residues (Glu297, Glu303, and Asp382) in the hinge region of the hTSHR are known to be important for bTSH binding and signaling. To investigate the potential relevance of these positions for differences between bTSH and hTSH in the interaction to the hTSHR, we determined bTSH- and hTSH-mediated cAMP production of several substitutions at these three hinge residues. To examine specific variations of hTSH, we also investigated the superagonistic hTSH analog TR1401 (TR1401), whose sequence differs from hTSH by four additional positively charged amino acids that are also present in bTSH. To characterize possible interactions between the acidic hTSHR positions Glu297, Glu303, or Asp382 and the additional basic residues of TR1401, we investigated TR1401 binding and signaling properties. Our data reveal increased cAMP signaling of the hTSHR using TR1401 and bTSH compared with hTSH. Whereas Asp382 seems to be important for bTSH- and TR1401-mediated but not for hTSH-mediated signaling, the substitution E297K exhibits a decreased signaling for all three TSH variants. Interestingly, bTSH and TR1401 showed only a slightly different binding pattern. These observations imply that specific residues of the hinge region are mediators of the superagonistic activity of bTSH and TR1401 in contrast to hTSH. Moreover, the simultaneous localization of binding components in the glycoprotein hormone molecule and the receptor hinge region permits important reevaluation of interacting hormone receptor domains.

Extended hormone binding site of the human thyroid stimulating hormone receptor: distinctive acidic residues in the hinge region are involved in bovine thyroid stimulating hormone binding and receptor activation

The human thyroid stimulating hormone receptor (hTSHR) belongs to the glycoprotein hormone receptors that bind the hormones at their large extracellular domain. The extracellular hinge region of the TSHR connects the N-terminal leucine-rich repeat domain with the membrane-spanning serpentine domain. From previous studies we reasoned that apart from hormone binding at the leucine-rich repeat domain, additional multiple hormone contacts might exist at the hinge region of the TSHR by complementary charge-charge recognition. Here we investigated highly conserved charged residues in the hinge region of the TSHR by site-directed mutagenesis to identify amino acids interacting with bovine TSH (bTSH). Indeed, the residues Glu-297, Glu-303, and Asp-382 in the TSHR hinge region are essential for bTSH binding and partially for signal transduction. Side chain substitutions showed that the negative charge of Glu-297 and Asp-382 is necessary for recognition of bTSH by the hTSHR. Multiple combinations of alanine mutants of the identified positions revealed an increased negative effect on hormone binding. An assembled model suggests that the deciphered acidic residues form negatively charged patches at the hinge region resulting in an extended binding mode for bTSH on the hTSHR. Our data indicate that certain positively charged residues of bTSH might be involved in interaction with the identified negatively charged amino acids of the hTSHR hinge region. We demonstrate that the hinge region represents an extracellular intermediate connector for both hormone binding and signal transduction of the hTSHR.

Residues 34-39 in the thyrotropin receptor are not the target of autoantibodies from sera of patients with Graves' disease

The thyrotropin receptor (TSHR) and alphal-antytripsin contain a fragment of sequence composed of 6 amino acids in which 5 residues are identical. Previously, we have suggested that this region of similarity [residues 34-39: (EEDFRV) in TSHR] is not the target for Graves' disease patients' autoantibodies. To verify this suggestion, we studied the reaction of patients' sera with alphal-antitrypsin. Two methods were used: TRAK assay, normally designed to estimate anti-TSHR autoantibodies in patients' sera, and immunoblotting. A modified version of the former assay was also used to study the influence of the synthetic peptide, corresponding to the region of similarity in TSHR, on Graves' patients sera or on thyrotropin (TSH) binding, and to study the influence of this peptide antiserum on TSH binding to the receptor. The TSHR stimulating and blocking activity of antisera to this peptide was studied in transfected Chinese hamster ovary cells. No influence of alphal-antitrypsin on the binding of patients' antibodies to the receptor were noticed nor were there reactions of autoantibodies with alphal-antitrypsin. We found that patients with anti-TSHR autoantibodies had a normal concentration of alphal-antitrypsin. A peptide corresponding to residues 34-39 did not influence Graves' patients sera and TSH binding and antiserum to this peptide did not influence TSH binding and adenylate cyclase activity. In summary, the results indicated that the sequence EEDFRV is not the target for patients autoantibodies.

A method for identification of the peptides that bind to a clone of thyroid-stimulating antibodies in the serum of Graves' disease patients

A method was developed for identification of the peptide sequences that bind to thyroid-stimulating antibody (TSAb) clones from phage-displayed peptide library. Immunoglobulin G (IgG) was purified from the serum of a Graves' disease patient that stimulates the synthesis of cAMP in the cells that express TSH receptor (TSHR). The IgG that binds to TSHR was purified by an affinity column packed with the resin cross-linked with the extracellular domain of human TSHR. The receptor-binding IgG was then mixed with phages that display linear or cyclic peptides at the end of tail protein pIII. The bound phages were eluted with acidic glycine after extensive washing. From sequencing of the pIII gene of the bound phages, one can deduce the sequences of the peptides that bind to the receptor-binding IgG. Each peptide sequence was then tested for inhibition of the synthesis of cAMP from thyroid cells induced by the serum of a Graves' patient. In this way, one can obtain the peptides that bind to a clone of TSAb. We obtained a peptide sequence that inhibits the action of TSAb at an extremely low concentration (<10(-14) M). Such a peptide will be useful for various studies on TSAb.

Adjuvant effects of cholera toxin b subunit on immune response to recombinant thyrotropin receptor in mice

We had previously shown that BALB/c mice immunized with the extracellular domain of human thyrotropin receptor (ETSHR) developed moderate hyperthyroxinemia. The antibody responses in these mice were predominantly of the IgG1 subclass. Since cholera toxin B subunit (CT-B) has direct effects on the thyroid, and is known to activate B lymphocytes and cause enhanced IgG1 production, we tested the ability of CT-B to modulate the antibody response to ETSHR. CT-B is unique in that it not only elicits a strong immune response to itself, but more importantly, when given with other antigens acts as a potent adjuvant. In the present study, BALB/c mice given ETSHR with CFA or CT-B via ip route showed higher titers of antibodies to ETSHR when compared to mice similarly immunized with ETSHR alone, or with IFA. Antibodies in ETSHR+CT-B immunized mice were mostly of the IgG1 subclass and reacted predominantly with ETSHR peptides 1 (aa 22-41), 21 (aa 322-341), and 23 (352-371). In contrast, animals immunized with ETSHR+CFA showed IgG1, IgG2a and IgG2b responses and reacted with peptides 1 and 21. Furthermore, mice immunized with ETSHR along with CT-B showed significantly higher levels of thyrotropin (TSH) binding inhibitory immunoglobulins (TBII) compared to those that did not receive CT-B. None of the mice immunized with a control antigen showed antibody response to ETSHR. These results suggested that CT-B could enhance and modulate immune response to ETSHR.

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

Recently, we showed that the glycosylated ectodomain of the human thyrotropin receptor (hET-gp) reacts with autoantibodies from autoimmune thyroid disease (AITD) patients' sera. To better understand the effects of glycosylation of thyrotropin receptor (TSHR) in antibody induction, we immunized rabbits with hET-gp protein. The rabbits developed relatively high titers of antibodies with highly potent TSH binding inhibitory immunoglobulin (TBII) and thyroid stimulatory blocking antibody (TSBAb) activities. Both the hET-gp and a nonglycosylated ectodomain of the human TSHR (hETSHR) protein significantly reversed the TBII as well as TSBAb activity. Based on the ability of synthetic peptides to significantly reverse the functional activity of these rabbit antisera, we identified three discrete regions of the TSH R, represented by amino acids 202-221, 292-311 and 367-386, as TBII epitopes and four regions represented by amino acids 352-371, 367-386, 382-401 and 392-415 as TSBAb epitopes. These data demonstrate that rabbit antibodies that bind to amino acids 367-386 mediate their TSBAb activity by inhibiting the binding of TSH to TSHR; whereas, antibodies to regions 352-415, excluding aa 367-386, exert their TSBAb activity by affecting a step subsequent to TSH binding. Coincident with the elevation of TBII and TSBAb activity, serum total T4 levels declined and thus suggested that the antibodies exerted functional effects on thyroid in vivo. Together, these data demonstrate that glycosylated hET-gp protein is a more potent immunogen and it can induce a broader antibody response directed against multiple TBII and TSBAb epitopes.

Testes exhibit elevated expression of calcitonin gene-related peptide receptor component protein

Calcitonin gene-related peptide (CGRP) receptor component protein (RCP) is a novel protein that modulates CGRP responsiveness in a variety of cell types. Using probes based on the isolation of CGRP-RCP complementary DNA (cDNA) from a guinea pig organ of Corti cDNA library, we cloned human (h) and mouse (m) CGRP-RCP cDNAs, both of which encode 148-residue proteins that at the amino acid levels are approximately 88% identical to each other and to the 146-residue guinea pig CGRP-RCP. Northern blot analysis confirmed the presence of CGRP-RCP messenger RNA in all of the human and mouse tissues tested. In these human tissues, hCGRP-RCP messenger RNA (major band at approximately 3.1 kb, minor band at approximately 7.5 kb) was most prevalent in the testis. In the mouse, the highest abundance of CGRP-RCP RNA was clearly in the testis (major band at approximately 1.6 kb, minor band at approximately 1.1 kb). Based on this tissue distribution of RNA, we sought to identify the cells in the murine testis that contained CGRP-RCP protein. Numerous antisera generated against hCGRP-RCP, including one to recombinant hCGRP-RCP, exhibited strong immunoreactivity localized to the head region of spermatozoa. No CGRP-RCP immunoreactivity was observed in other cells at less mature stages of sperm maturation, in Sertoli or interstitial (Leydig) cells, or in human spermatozoa. Murine epididymal (mature) spermatozoa exhibited CGRP-RCP immunoreactivity identical to that of testicular spermatozoa. Spermatozoa that underwent an experimentally induced acrosome reaction (acrosomal discharge) lost their CGRP-RCP immunoreactivity. Therefore, it appears that CGRP-RCP is associated with the acrosome, suggesting that it may play an important role in reproduction.

Effect of solubilization of porcine thyrotropin (TSH) receptor on TSH binding and on radio-receptor assay for anti-TSH receptor antibodies

The effect of solubilization of porcine thyrotropin receptor (TSHR) on TSH binding and on the radio-receptor assay for anti-TSHR antibodies was examined. After TSHR solubilization with 1% dodecylpolyethy-leneglycoether, TSH binding affinity was increased, from Kd = 1.15 nM to 0.45 nM, and TSH binding capacity was slightly increased, from 0.15 nM to 0.19 nM. With a particulate membrane suspension from thyroid cells, blocking of TSH binding to the membrane suspension by anti-thyrotropin receptor antibody was observed only for thyroid stimulation blocking antibody (TSBAb), not for thyroid-stimulating antibody (TSAb). After the solubilization of TSHR, both TSBAb and TSAb blocked TSH-binding to the solubilized TSHR. We speculate that TSAb interacts with the TSHR in the native conformation without interfering with TSH binding, and that after the solubilization, any anti-TSHR antibody interferes with TSH-binding due to the conformational change in TSHR. With these particulate thyroid cell membrane preparations, we can detect only TSBAb by the radio-receptor assay.

Induction of thyroiditis in mice with thyrotropin receptor lacking serologically dominant regions

Grave's disease (GD) is characterized by pathogenic autoantibodies to the human thyrotropin receptor (hTSH-R), and is frequently associated with a lymphocytic infiltrate of the thyroid gland. In attempts to establish a murine model of GD, we and others have previously shown that immunization of mice with recombinant preparations of the hTSH-R ectodomain induces high titres of specific antibodies, which, however, are not pathogenic, nor is the response accompanied by the development of thyroiditis. Since earlier reports identified the serological immunodominant determinants within the N- and C-terminal regions of hTSH-R ectodomain, we reasoned that immunization of mice with truncated fragments of ectodomain lacking these dominant regions might result in skewing of the response to other determinants of the molecule, with consequent induction of immunopathological features present in GD. We show here that multiple challenge of BALB/c mice with an amino acid fragment of residues 43-282 generates antibodies directed at hTSH-R peptides 37-56, 157-176, 217-236 and 232-251. This reactivity pattern is distinct from that induced previously with the whole ectodomain of hTSH-R in BALB/c animals. Thyroid function remained unaffected in these mice, suggesting that pathogenic antibodies were not being induced. Interestingly, some animals developed lymphocytic infiltration of the thyroid gland, clearly indicating the presence of pathogenic T cell determinants within the 43-282 fragment. Challenge with the related fragment 43-316 produced the same pattern of serological response to the synthetic peptides as fragment 43 282, but was not accompanied by thyroiditis. The results demonstrate: (i) the presence of thyroiditogenic determinants within hTSH-R, and (ii) that these pathogenic determinants are likely to be cryptic, as their effect is exhibited only when the hierarchy of immunodominance within hTSH-R is drastically altered.

Characterisation of the antibody response to the extracellular region of recombinant thyrotropin receptor

Autoantibodies to the human thyrotropin receptor (TSH-R) are pathogenic in a number of autoimmune thyroid diseases including Graves' disease. We have characterised polyclonal antisera to TSH-R for antibodies which may mimic those present in autoimmune thyroid disease. For immunisations, recombinant extracellular region of human TSH-R which does not interact with its ligand TSH was used. The induced antibodies react with the full length membrane receptor in transfected mammalian cells by flow cytometry showing the presence of antibody capable of recognising the native functional receptor. The properties of the generated antibodies have been compared after two injections or following a multiple immunisation protocol with the receptor in adjuvant. High titre antisera were readily generated after the short injection protocol and further immunisations did not lead to any change in antibody titers. Analysis of the epitopes recognised using synthetic peptides confirmed previous observations that the immunodominant determinants localise to the amino and the carboxyl terminal part of the extracellular region of the receptor. Antisera from both rabbits contain TSH blocking antibody as assessed by inhibition of TSH mediated cAMP stimulation. There was an increase in TSH binding inhibitory immunoglobulin (TBII) activity with multiple injections. Furthermore, the increase in TBII activity was not related to spreading of the antibody response to new determinants on TSH-R. Our results support previous observations on the difficulties in reproducing, by adjuvant immunisation with recombinant TSH-R preparations, the fine specificity of antibodies to TSH-R present in autoimmune disorders such as Graves' disease or primary myxoedema.

Changes in epitopes for thyroid-stimulating antibodies in Graves' disease sera during treatment of hyperthyroidism: therapeutic implications

To determine whether there are changes in epitope recognition by stimulating TSH receptor antibodies (TSHRAbs) during treatment of hyperthyroidism and to evaluate the clinical relevance of such changes, we serially measured the activity of IgG preparations from 39 patients with Graves' disease over an 8-month period. To measure epitope changes of the stimulating TSHRAbs, we used Chinese hamster ovary (CHO) cells transfected with wild-type human TSHR (hTSHR) or TSHR chimeras with residues 90-165 (Mc2) substituted by equivalent residues of the rat LH/CG receptor. When initially examined, 37 of the 39 patients had significant stimulating TSHRAb activity measured with wild-type CHO-hTSHR cells. Serial measurements of stimulating TSHRAb activity in Mc2 chimera-transfected cells divided the 39 patients into three distinct groups. Thus, 10 patients (heterogeneous epitope group) exhibited low but significant activity in Mc2 chimera assays at the start of the study; 10 patients who were initially negative in Mc2 chimera assays remained negative (persistently homogeneous epitope group); and 19 patients who were initially negative in Mc2 chimera assays became transiently or persistently positive during treatment, despite a simultaneous decrease in TSHRAb activity measured with wild-type TSHR (changing epitope group). The functional stimulating TSHRAb epitope thus changed from residues 90-165 to residues outside this region in the last group, which comprises nearly two-thirds of the initially Mc2-negative patients (19 of 29) and one-half of all patients (19 of 39). Patients in the changing epitope group responded more quickly and to lower doses of methimazole than patients in the persistently homogeneous epitope group, behaving in this respect exactly as the patients in the heterogeneous epitope group. Additionally, although the decrease in stimulating TSHRAb activities during the 8-month treatment period was similar in the two groups, the thyrotropin binding inhibitor immunoglobulin (TBII) activities decreased more rapidly in patients in the persistently homogeneous epitope group than in patients in the changing epitope group (P < 0.05). There were no differences in initial stimulating TSHRAb or TBII activities, degree of hyperthyroidism, goiter size, or prior duration of symptoms between the persistently homogeneous epitope group and changing epitope group. In summation, we show that the epitopes of stimulating TSHRAbs in Graves' disease patients may change during their clinical course or treatment period, and that the change is from antibodies recognizing N-terminal TSHR residues 90-165 to antibodies recognizing other regions of the TSHR. We also show that the development of stimulating TSHRAbs with this heterogeneous epitope or their presence at the initial screening for disease activity seems to be associated with increased responsiveness to antithyroid drug therapy. We suggest, therefore, that Mc2 chimera assays may be useful to predict the response of patients to antithyroid drug therapy.

Subtypes of anti-TSH receptor antibodies classified by various assays using CHO cells expressing wild-type or chimeric human TSH receptor

To analyze the heterogeneity of anti-TSH receptor antibodies (TSHRAb), we measured serum TSH-binding inhibitory immunoglobulin (TBII), thyroid-stimulating antibody (TSAb), and thyroid stimulation blocking antibody (TSBAb) activities in 31 patients with positive TSHRAb, using CHO cells expressing wild-type TSHR (WT) or TSHR chimera (Mc2) wherein residues 90-165 were substituted by the LH/CG receptor. Using membranes from WT cells, we detected TBII activity in all 31 patients; 10 (32%), all with TSAb activity only, completely lost TBII activity using Mc2 membranes. TSAb activity was found in 26 sera using WT cells; 20 (77%) completely lost TSAb activity in Mc2 cells. Comparisons of TBII and TSAb activity in WT cells did not exhibit a strong positive correlation (r = 0.52). Of the 20 sera that completely lost TSAb activity in Mc2 cells, 10 retained some TBII activity in Mc2 cells. In each of the sera with retained TBII activity, TSAb activity was recovered in Mc2 cells using the conversion assay, which measures the conversion of a nonstimulating TSHRAb to a TSAb by the action of an anti-human IgG. Additionally, the TBII and conversion assay values in Mc2 cells exhibited a strong positive correlation (r = 0.86). Of the 31 sera, TSBAb was found in 7 samples, with no difference in WT and Mc2 cells. TBII activity was detected in all 7 sera with WT cells; TSAb activity in only 2. In the 5 sera with TSBAb but no TSAb activity, and with only a minimal or no decrease in TBII activity in Mc2 cell membranes, the in vitro conversion assay uncovered TSAb activity. Analyzing these data, we classify the sera into 5 groups containing multiple, different TSHR autoantibodies, including two different TSAbs, three different TBIIs, and one nonfunctional antibody. The heterogeneity of TBIIs as well as TSAbs provides a basis to explain the lack of correlation between TBII and TSAb activities in some past studies of Graves' sera.

Influence of adjuvants on the induction of autoantibodies to the thyrotropin receptor

To determine the influence of adjuvant on the induction of antibodies to thyrotropin receptor (TSHR), we immunized BALB/c mice with a extracellular domain of the TSHR (ETSHR) protein in complete Freund's adjuvant (CFA), Titer Max (TM) and Gerbu. Similarly, control groups of mice were immunized with bovine serum albumin (BSA) in each of the different adjuvants. As determined by ELISA, ETSHR given along with CFA elicited high titers of antibodies to ETSHR which were mainly restricted to the IgG1 subclass. Mice immunized with ETSHR in TM also developed high titers of anti-ETSHR antibodies but had higher levels of both IgG1 and IgG2a. However, immunization with ETSHR in Gerbu resulted in low titers of antibodies, restricted to IgG1 subclass. Immunization of mice with BSA in each of the three adjuvants induced higher antibody titers to BSA. The subclass of antibodies in mice immunized with BSA in CFA and TM were predominantly IgG1 and IgG2a with lower levels of IgG2b, whereas in Gerbu treated group, antibody to BSA was restricted to IgG1 subclass. Analysis of specificity of antibodies against ETSHR, in mice immunized with ETSHR, revealed that irrespective of the adjuvant used, the dominant reactivity was against peptide 1 (AA 22-41) with weaker reactivity against several other. peptides. The only exception was in mice immunized with ETSHR in TM which also showed significant reactivity against peptide 23 (AA 352-371). Mice immunized with the ETSHR in CFA or in TM showed elevated levels of serum TSH binding inhibitory immunoglobulins (TBII). However, mice immunized with ETSHR in Gerbu, which had lower titers of antibodies to ETSHR, showed normal TBII levels. These studies showed that adjuvant composition could influence the titer, subclass and fine specificity of antibodies to ETSHR which in turn could affect the development of TBII activity.

Differential roles of exoloop 1 of the human follicle-stimulating hormone receptor in hormone binding and receptor activation

The follicle-stimulating hormone (FSH) receptor is a member of the glycoprotein hormone receptor subfamily of the seven-transmembrane receptor superfamily. These receptors have an extracellular N-terminal half of approximately 350 amino acids and a membrane-associated C-terminal half of approximately 350 amino acids. The N-terminal halves have the high affinity hormone binding site. On the other hand, the C-terminal half of the lutropin/choriogonadotropin receptor has the receptor activation site. However, little is known about the activation site and mechanism of the FSH receptor, although the existing evidence indicates crucial differences in the activation of the FSH receptor and the lutropin/choriogonadotropin receptor. As a first step to resolve this issue, we examined the upstream juxtamembrane five amino acids, Asp405-Ile406-His407-Thr408-Lys409, of the exoloop 1. Ala scan and multi-substitutions show that the five amino acid sequence is important for both hormone binding and receptor activation to induce cAMP synthesis, despite its short length. Specifically, His407 is important for high affinity hormone binding, whereas Asp405, Thr408, and Lys409 are crucial for receptor activation. The data suggest that the five amino acids may form a turn of helix that is an extension of the transmembrane helix 2. In this helical arrangement, Asp405, Thr408, and Lys409 are grouped to form a hydrophilic face of the helix, suggesting a correlation between this arrangement and receptor activation. In addition, the diverse and differential roles of the five amino acids indicate that high affinity hormone binding and receptor activation are discernible functions. These novel observations will be helpful for understanding the activation mechanism of the FSH receptor.

Unaltered thyroid function in mice responding to a highly immunogenic thyrotropin receptor: implications for the establishment of a mouse model for Graves' disease

Grave's disease (GD) is a common disorder characterized by the presence of autoantibodies to the thyrotropin receptor. In the past, the exceedingly low expression of the thyrotropin receptor on thyrocytes has not allowed its purification in quantities sufficient to investigate the establishment of an animal model for this disease. In this study, we have purified the 398-amino acid, extracellular region of the human thyrotropin receptor (TSH-R.E) from insect cells using recombinant baculovirus, and explored its immunopathogenic properties in H-2b,d,q,k,s strains of mice. The receptor preparation was highly immunogenic since it elicited strong specific proliferative T cell responses as well as IgG responses in all strains tested. In addition, hyperimmunization with TSH-R.E induced (i) serum antibodies that blocked the binding of 125I-TSH to its receptor, a common feature of GD autoantibodies; and (ii) IgG that reacted with a synthetic peptide (residues 32-54) from the N-terminus of the receptor, a region implicated in the binding of thyroid stimulating antibodies. In SJL animals only, a weak antibody response to two other thyroid antigens, thyroglobulin and thyroid peroxidase, was also observed. The presence of these antibodies, however, was not accompanied by a detectable alteration in thyroid function as assessed by the measurement of serum TSH, T4 and iodine levels. Also mononuclear infiltration of the thyroid gland or morphological changes compatible with an activation state of thyrocytes were not apparent in TSH-R-challenged mice. In contrast, mice treated with the anti-oxidant aminotriazole showed a dramatic increase in serum TSH levels and an activated follicular epithelium. These data demonstrate that a highly immunogenic TSH-R.E in mice does not necessarily provide a proper stimulus for the induction of a hyper- or hypothyroid status as defined by hormonal or histological criteria. Main reasons for the inability to induce receptor-specific antibodies that affect thyroid function such as those generated in GD are likely to be the inappropriate folding of the recombinant extracellular domain of the receptor, or the xenogeneic nature of the autoantigen.

Induction of hyperthyroxinemia in BALB/C but not in several other strains of mice

We recently expressed the extracellular domain of the human TSHR (ETSHR) protein using a baculovirus expression system and purified it to homogeneity. The ETSHR specifically binds both TSH and antibodies to TSHR. In the present study, C57BL/6J, SJL/J, BALB/cJ and B10BR.SgSnJ mice were immunized with the recombinant ETSHR or an equivalent amount of control antigen. All strains of mice produced high titers of antibody against the TSHR protein which were capable of blocking the binding of TSH to native TSHR. However, only BALB/cJ mice showed significantly elevated levels of thyroxine in their sera compared to the control mice. Similarly, BALB/cJ mice primed with ETSHR and then challenged with thyroid membranes showed significantly elevated levels of thyroxine. In addition, histopathological examination of thyroid glands from affected mice showed morphological changes characterized by hydropic and subnuclear vacuolar changes and focal scalloping, with no apparent inflammation or glandular destruction. Moreover, mice with elevated thyroxine levels showed increased in vivo thyroidal uptake of 131Iodine. Together, these data suggest that BALB/cJ mice are susceptible to the induction of hyperthyroxinemia.