Immunoglobulins from Graves' disease patients interact with different sites on TSH receptor/LH-CG receptor chimeras than either TSH or immunoglobulins from idiopathic myxedema patients

Bioassays for thyrotropin receptor autoantibodies

Bioassays using animal models were essential tools in the discovery of thyrotropin and in enhancing our understanding of the physiology of the pituitary-thyroid axis. These same bioassays were also instrumental in the discovery of autoantibodies to the thyrotropin receptor (TSH-R-Ab) and in identifying their role in the pathophysiology of Graves' disease. The development of cell-based bioassays led to further advances in our knowledge of the functional activity of TSH-R-Ab and to the discovery that TSH-R-Ab can be either thyroid-stimulating or thyroid blocking, and that they occur in other types of autoimmune thyroid diseases (AITD) besides Graves' disease. More recently, TSH-R-Ab bioassays have been advanced from research tools to clinical laboratory tests. Whereas TSH-R-Ab can be measured with competitive-binding immunoassays, these assays do not provide information on the functional activity of TSH-R-Ab. Bioassays, in contrast, can differentiate between the stimulatory or blocking activity of TSH-R-Ab which provides clinically useful information that can inform the management of patients with AITD. The clinical use of TSH-R-Ab bioassays, however, has been limited to-date by their inherent complexity and long turn-around-time. Recent advances in biosensors have been applied to the development of TSH-R-Ab bioassays that are rapid and simple to perform. We now are entering an era in which bioassays for TSH-R-Ab can be measured routinely by virtually any clinical laboratory.

The role of laboratory medicine in the diagnosis of the hyperthyroidism

Hyperthyroidism is a clinical condition characterized by inappropriately high synthesis and secretion of thyroid hormones by the thyroid gland. It has multiple aetiologies, manifestations and potential therapies. Graves' disease is the most common form of hyperthyroidism, due to the production of autoantibodies against thyrotropin receptor, capable of over-stimulating thyroid function. A reliable diagnosis of hyperthyroidism can be established on clinical grounds, followed by the evaluation of serum thyroid function tests (thyrotropin first and then free thyroxine, adding the measurement of free triiodothyronine in selected specific situations). The recent guidelines of both the American and European Thyroid Associations have strongly recommended the measurement of thyrotropin receptor autoantibodies for the accurate diagnosis and management of Graves' disease. If autoantibody test is negative, a radioiodine uptake should be performed. Considering the most recent laboratory improvements, binding assays can be considered the best first solution for the measurement of thyrotropin receptor autoantibodies in diagnosis and management of overt cases of Graves' disease. In fact, they have a satisfactory clinical sensitivity and specificity (97.4% and 99.2%, respectively) being performed in clinical laboratories on automated platforms together with the other thyroid function tests. In this setting, the bioassays should be reserved for fine and complex diagnoses and for particular clinical conditions where it is essential to document the transition from stimulating to blocking activity or vice versa (e.g. pregnancy and post-partum, related thyroid eye disease, Hashimoto's thyroiditis with extrathyroidal manifestations, unusual cases after LT4 therapy for hypothyroidism or after antithyroid drug treatment for Graves' disease). Undoubtedly, technological advances will help improve laboratory diagnostics of hyperthyroidism. Nevertheless, despite future progress, the dialogue between clinicians and laboratory will continue to be crucial for an adequate knowledge and interpretation of the laboratory tests and, therefore, for an accurate diagnosis and correct management of the patient.

Comparison of a Novel Homogeneous Cyclic Amp Assay and a Luciferase Assay for Measuring Stimulating Thyrotropin-Receptor Autoantibodies

OBJECTIVE

Stimulating thyrotropin-receptor antibodies (TSAb) cause Graves' disease (GD). We tested a novel homogeneous fluorescent 3',5' cyclic adenine monophosphate (cAMP) assay for the detection of TSAb in a bioassay.

METHODS

Chinese hamster ovary (CHO) cell lines expressing either a chimeric (MC4) or wild-type (WT) TSH-R were incubated with the adenyl cyclase activator forskolin, a human TSAb monoclonal antibody (M22), and with sera from GD patients. Intracellular cAMP levels were measured using a Bridge-It® cAMP assay, and the results were compared with a luciferase-based bioassay.

RESULTS

Both cell lines were stimulated with forskolin concentrations (0.006-200 µM) in a dose-dependent manner. The linear range in the MC4 and WT cells was 0.8-25 and 3.1-50 µM, respectively. Levels of cAMP and luciferase in forskolin-treated MC4 and WT cells were positively correlated (r = 0.91 and 0.84, both p < 0.001). The 50% maximum stimulatory concentration of forskolin was more than 16-fold higher for the CHO-WT cells than the CHO-MC4 cells in the cAMP assay and 4-fold higher in the luciferase assay. Incubation of both cell lines with M22 (0.006-50 ng/mL) resulted in a dose-dependent increase in cAMP levels with linear ranges for the MC4 and WT cells of 0.8-12.5 and 0.2-3.125 ng/mL, respectively. Comparison of cAMP and luciferase levels in M22-treated MC4 and WT cells also showed a positive correlation (r = 0.88, p < 0.001 and 0.75, p = 0.002). A positive correlation was also noted when using patient samples (r = 0.96, p < 0.001) that were all TSH-R-Ab binding assay positive.

CONCLUSION

The novel, rapid, simple-to-perform cAMP assay provides TSAb-mediated stimulatory results comparable to a luciferase-based bioassay.

Thyroid Stimulating Hormone Receptor Antibodies in Thyroid Eye Disease-Methodology and Clinical Applications

BACKGROUND

Thyroid stimulating hormone receptor antibodies (TSHR-Ab) cause autoimmune hyperthyroidism and are prevalent in patients with related thyroid eye disease (TED).

PURPOSE

To provide a historical perspective on TSHR-Ab and to present evidence-based recommendations for clinical contemporary use.

METHODS

The authors review the recent literature pertaining to TSHR-Ab in patients with TED and describe the various immunoassays currently used for detecting TSHR-Ab and their clinical applications.

RESULTS

We provide a historical summary and description of the various methods used to detect TSHR-Ab, foremost, the functional TSHR-Ab. Increasing experimental and clinical data demonstrate the clinical usefulness of cell-based bioassays for measurements of functional TSHR-Ab in the diagnosis and management of patients with autoimmune TED and in the characterization of patients with autoimmune-induced hyperthyroidism and hypothyroidism. Thyroid stimulating hormone receptor antibodies, especially the functional stimulating antibodies, are sensitive, specific, and reproducible biomarkers for patients with autoimmune TED and correlate well with clinical disease activity and clinical severity. Unlike competitive-binding assays, bioassays have the advantage of indicating not only the presence of antibodies but also their functional activity and potency.

CONCLUSIONS

Measurement of TSHR-Ab (especially stimulating antibodies) is a clinically useful tool for the management of patients with TED.

Bioassays for TSH Receptor Autoantibodies, from FRTL-5 Cells to TSH Receptor-LH/CG Receptor Chimeras: The Contribution of Leonard D. Kohn

Since the discovery 60 years ago of the "long-acting thyroid stimulator" by Adams and Purves, great progress has been made in the detection of thyroid-stimulating hormone (TSH) receptor (TSHR) autoantibodies (TRAbs) in Graves' disease. Today, commercial assays are available that can detect TRAbs with high accuracy and provide diagnostic and prognostic evaluation of patients with Graves' disease. The present review focuses on the development of TRAbs bioassays, and particularly on the role that Leonard D. Kohn had in this. Indeed, 30 years ago, the Kohn group developed a bioassay based on the use of FRTL-5 cells that was characterized by high reproducibility, feasibility, and diagnostic accuracy. Using this FRTL-5 bioassay, Kohn and his colleagues were the first to develop monoclonal antibodies (moAbs) against the TSHR. Furthermore, they demonstrated the multifaceted functional nature of TRAbs in patients with Graves' disease, with the identification of stimulating and blocking TRAbs, and even antibodies that activated pathways other than cAMP. After the cloning of the TSHR, the Kohn laboratory constructed human TSHR-rat luteinizing hormone/chorionic gonadotropin receptor chimeras. This paved the way to a new bioassay based on the use of non-thyroid cells transfected with the Mc4 chimera. The new Mc4 bioassay is characterized by high diagnostic and prognostic accuracy, greater than for other assays. The availability of a commercial kit based on the Mc4 chimera is spreading the use of this assay worldwide, indicating its benefits for these patients with Graves' disease. This review also describes the main contributions made by other researchers in TSHR molecular biology and TRAbs assay, especially with the development of highly potent moAbs. A comparison of the diagnostic accuracies of the main TRAbs assays, as both immunoassays and bioassays, is also provided.

High cut-off value of a chimeric TSH receptor (Mc4)-based bioassay may improve prediction of relapse in Graves' disease for 12 months

There are scarce reports regarding a functional prognostic value of thyroid-stimulating autoantibody (TSAb) levels using a thyroid-stimulating hormone receptor chimera (Mc4) in Graves' disease (GD) in iodine sufficient area. The aim of this study was to investigate whether Mc4-TSAb can predict GD remission/relapse after antithyroid drug (ATD) treatment and to compare Mc4-TSAb with a binding assay using M22 monoclonal antibody (M22-TRAb) in GD patients. We retrospectively reviewed the results of M22-TRAb and Mc4-TSAb in GD patients treated with ATD for 12 months. GD patients who underwent ATD treatment for at least 12 months were included. We compared the predictive values of M22-TRAb and Mc4-TSAb for GD remission and relapse. Of the 92 patients, 60 (65.2%) achieved remission and 32 (34.8%) relapsed within 12 months. In receiver operating characteristic analysis, there were no significant differences in the area under the curves (AUCs) between Mc4-TSAb [AUC=0.79 (95% CI 0.69-0.89)] and M22-TRAb [AUC=0.69 (95% CI 0.58-0.81)]. The optimal predictive cut-off values of M22-TRAb and Mc4-TSAb were 2.23 IU/L and 230%, respectively. At a high Mc4-TSAb cut-off, the better specificity of 85.0% and positive predictive value (PPV) of 69.0% were shown compared with those at the best cut-off for M22-TRAb. In conclusion, a high cut-off for an Mc4 assay may improve the predictive value of relapse with superior specificity and PPV compared with M22-TRAb in treated GD.

Similarities and differences in interactions of thyroid stimulating and blocking autoantibodies with the TSH receptor

Binding of a new thyroid-stimulating human monoclonal autoantibody (MAb) K1-18 to the TSH receptor (TSHR) leucine-rich domain (LRD) was predicted using charge-charge interaction mapping based on unique complementarities between the TSHR in interactions with the thyroid-stimulating human MAb M22 or the thyroid-blocking human MAb K1-70. The interactions of K1-18 with the TSHR LRD were compared with the interactions in the crystal structures of the M22-TSHR LRD and K1-70-TSHR LRD complexes. Furthermore, the predicted position of K1-18 on the TSHR was validated by the effects of TSHR mutations on the stimulating activity of K1-18. A similar approach was adopted for predicting binding of a mouse thyroid-blocking MAb RSR-B2 to the TSHR. K1-18 is predicted to bind to the TSHR LRD in a similar way as TSH and M22. The binding analysis suggests that K1-18 light chain (LC) mimics binding of the TSH-α chain and the heavy chain (HC) mimics binding of the TSH-β chain. By contrast, M22 HC mimics the interactions of TSH-α while M22 LC mimics TSH-β in interactions with the TSHR. The observed interactions in the M22-TSHR LRD and K1-70-TSHR LRD complexes (crystal structures) with TSH-TSHR LRD (comparative model) and K1-18-TSHR LRD (predictive binding) suggest that K1-18 and M22 interactions with the receptor may reflect interaction of thyroid-stimulating autoantibodies in general. Furthermore, K1-70 and RSR-B2 interactions with the TSHR LRD may reflect binding of TSHR-blocking autoantibodies in general. Interactions involving the C-terminal part of the TSHR LRD may be important for receptor activation by autoantibodies.

Targeting the thyrotropin receptor in thyroid disease

INTRODUCTION

The thyrotropin receptor (TSHR) is essential for thyroid growth and for the production of thyroid hormones. It is unique among the glycoprotein hormone receptors, in that some of the TSHRs undergo cleavage and shedding of the alpha subunit.

AREAS COVERED

This review discusses the structure and function of the TSHR, followed by an evaluation of its role in thyroid disease. Possible limitations of the TSHR as a therapeutic target are also discussed.

EXPERT OPINION

The TSHR is involved in a number of hereditary and acquired disorders of the thyroid making it of potential importance as a therapeutic target in thyroid disease. Expression of the TSHR in several non-thyroidal tissues and the development of systemic manifestations of thyroid disease suggest that the TSHR is also of interest as a therapeutic target outside the thyroid.

Gestational diabetes and thyroid autoimmunity

Background. About 10% of pregnancies are complicated by previously unknown impairment of glucose metabolism, which is defined as gestational diabetes. There are little data available on prevalence of thyroid disorders in patients affected by gestational diabetes, and about their postgestational thyroid function and autoimmunity. We therefore investigated pancreatic and thyroid autoimmunity in gestational diabetic patients and in women who had had a previous gestational diabetic pregnancy. Methods. We investigated 126 pregnant women at the time of a 100-g oral glucose tolerance test: 91 were classified as gestational diabetics, and 35 were negative (controls). We also studied 69 women who had delivered a baby 18-120 months prior to this investigation and who were classified at that time gestational diabetics (38 women) or normally pregnant (31 women; controls). Results. Our data show no differences for both thyroid function and prevalence of autoimmune disorders during pregnancy; however, a significant increase in thyroid autoimmunity was seen in women previously affected by gestational diabetes. This increased prevalence of thyroid autoimmunity was not associated with the development of impaired glucose metabolism after pregnancy. Conclusions. Our data suggest that maternal hyperglycemia is a risk factor for the development of thyroid autoimmunity, a conclusion that should now be confirmed in a larger cohort of patients.

Similar clinical performance of a novel chimeric thyroid-stimulating hormone receptor bioassay and an automated thyroid-stimulating hormone receptor binding assay in Graves' disease

BACKGROUND

Graves' disease (GD) is caused by the continuous stimulation of the thyroid gland by autoantibodies directed against the thyroid-stimulating hormone receptor (TSHR). Two frequent assays for the measurement of TSHR autoantibodies (TSHRAb) were compared, one measuring stimulation of cyclic adenosine monophosphate (cAMP) production and one measuring inhibition of TSH binding, with regard to diagnostic accuracy for GD as well as whether there was an existence of their discordant results in patients with GD and painless thyroiditis (PT).

METHODS

Using 106 sera from untreated GD and 80 sera from autoimmune PT, we compared the diagnostic performance of two TSHRAb assays that have been recently developed. The first one is a bioreporter assay using chimera TSHR (Mc-4), which detects a stimulation signal of cAMP level in cultured CHO cells (Mc4-TSAb assay). The second is a binding inhibition assay using the extracelluar domain of porcine TSHR and a monoclonal antibody (M22) closely mimicking the binding to TSH (M22-TRAb assay). In addition, we compared both assays by using eight sera from eight GD subjects becoming spontaneously hypothyroid due to appearance of thyroid blocking autoantibodies (TBAb) that were measured with inhibition rates of TSH-stimulated cAMP in porcine cells.

RESULTS

The Mc4-TSAb assay and the M22-TRAb assay were positive in 94.3% and 92.5% of the GD patients, respectively, whereas they were negative in 95.0% and 98.8% of the PT subjects. However, 10 of 106 GD sera (9.4%) showed discordant results. Six of 106 cases with untreated GD (5.7%) were Mc4-TSAb positive and M22-TRAb negative. In contrast, 4 of 106 sera (3.8%) were Mc4-TSAb negative but M22-TRAb positive. Two cases of untreated GD were negative for both Mc4-TSAb and M22-TRAb. In eight GD subjects with TBAb and hypothyroidism, the binding assay was highly positive, although Mc4-TSAb was negative.

CONCLUSION

Similar and excellent performance was noted for the Mc4-TSAb and M22-TRAb assays in a large group of patients with GD. However, there was 9.4% discordance with regard to false negatives for GD and 3.8% discordance between the two tests with regard to false positives for PT. With regard to the relatively high rate of discordancy, a combination of both assays could reduce the presence of TSHRAb-seronegative GD.

Autoimmune alternating hypo- and hyperthyroidism in children

Two children presented with autoimmune alternating hypo- and hyperthyroidism related to the presence of blocking and stimulating thyroid antibodies. It was difficult to control their thyroid function adequately with an appropriate single drug regimen, and both children underwent total thyroidectomy with subsequent stable management with levothyroxine replacement therapy postsurgically. Although this phenomenon is well described in adults, this report is the first of such occurrence in children. The possible mechanism for the variation in the type of clinical presentation and options for management are discussed.

Diagnostic value of a chimeric TSH receptor (Mc4)-based bioassay for Graves' disease

BACKGROUND/AIMS

Graves' disease (GD) is caused by thyroid-stimulating hormone receptor (TSHR) and thyroid-stimulating immunoglobulin (TSI). We used a recently introduced, technically enhanced TSI bioassay to assess its diagnostic value and determine the cut-off in patients in high iodine intake area.

METHODS

In a cross-sectional setting, we collected serum from 67 patients with untreated GD, 130 with GD under treatment, 22 with GD in remission, 42 with Hashimoto's thyroiditis, 12 with subacute thyroiditis, 20 with postpartum thyroiditis, and 93 euthyroid controls. TSI was measured using the Thyretain™ bioassay, which is based on Chinese hamster ovary cells transfected with chimeric TSHR (Mc4). TSI levels are reported as a specimen-to-reference ratio percentage (SRR%).

RESULTS

The TSI levels in patients with GD (either treated or not) were significantly higher than those of the remaining patients (p < 0.05). The new bioassay showed a sensitivity of 97.0% and a specificity of 95.9% with a cut-off value of 123.0 SRR% for GD. A weak correlation was found between TSI and thyrotropin-binding inhibiting immunoglobulin (TBII) (r(s) = 0.259, p = 0.03), but no correlation was found between TSI and tri-iodothyronine or free thyroxine.

CONCLUSIONS

The Mc4-CHO bioassay showed comparable diagnostic value for GD with the conventional TBII assay. We propose a cut-off of 123.0 SRR% in areas where iodine intake is high.

A new small-molecule antagonist inhibits Graves' disease antibody activation of the TSH receptor

CONTEXT

Graves' disease (GD) is caused by persistent, unregulated stimulation of thyrocytes by thyroid-stimulating antibodies (TSAbs) that activate the TSH receptor (TSHR). We previously reported the first small-molecule antagonist of human TSHR and showed that it inhibited receptor signaling stimulated by sera from four patients with GD.

OBJECTIVE

Our objective was to develop a better TSHR antagonist and use it to determine whether inhibition of TSAb activation of TSHR is a general phenomenon.

DESIGN

We aimed to chemically modify a previously reported small-molecule TSHR ligand to develop a better antagonist and determine whether it inhibits TSHR signaling by 30 GD sera. TSHR signaling was measured in two in vitro systems: model HEK-EM293 cells stably overexpressing human TSHRs and primary cultures of human thyrocytes. TSHR signaling was measured as cAMP production and by effects on thyroid peroxidase mRNA.

RESULTS

We tested analogs of a previously reported small-molecule TSHR inverse agonist and selected the best NCGC00229600 for further study. In the model system, NCGC00229600 inhibited basal and TSH-stimulated cAMP production. NCGC00229600 inhibition of TSH signaling was competitive even though it did not compete for TSH binding; that is, NCGC00229600 is an allosteric inverse agonist. NCGC00229600 inhibited cAMP production by 39 ± 2.6% by all 30 GD sera tested. In primary cultures of human thyrocytes, NCGC00229600 inhibited TSHR-mediated basal and GD sera up-regulation of thyroperoxidase mRNA levels by 65 ± 2.0%.

CONCLUSION

NCGC00229600, a small-molecule allosteric inverse agonist of TSHR, is a general antagonist of TSH receptor activation by TSAbs in GD patient sera.

A novel bioreporter assay for thyrotropin receptor antibodies using a chimeric thyrotropin receptor (mc4) is more useful in differentiation of Graves' disease from painless thyroiditis than conventional thyrotropin-stimulating antibody assay using porcine thyroid cells

BACKGROUND

Graves' disease (GD) is caused by thyrotropin (TSH) receptor antibodies (TSHRAbs) that bind to TSHR and activate thyrocytes. The measurement of TSHRAbs therefore has been used to assist in the diagnosis and management of GD.

METHODS

In this study, we evaluated the clinical significance of a newly developed bioreporter assay for the detection of TSHRAbs (Thyretain). The Thyretain bioreporter assay utilizes a chimeric receptor (Mc4), in which residues 262-335 of TSHR are replaced with a rat lutropin-choriogonadtropin receptor segment. This bioreporter is designed to specifically detect stimulating TSHRAbs (Mc4-TSHRAbs).

RESULTS

The Mc4-TSHRAb level of sera obtained from 110 normal healthy controls, 103, 99, and 50 patients with untreated GD, painless Hashimoto's thyroiditis (PT), and subacute thyroiditis (SAT) were 27.3% +/- 11.3%, 327.8% +/- 105.9%, 48.9% +/- 48.5%, and 24.9% +/- 13.4%, respectively. Compared with the Mc4-TSHRAb levels of patients with PT and SAT, and normal healthy controls, the Mc4-TSHRAb levels of untreated GD patients were significantly higher (p < 0.01). The sensitivity and specificity of the Thyretain bioreporter assay for GD and PT were 95.1% and 96.0%, respectively, at the optimal cut-off value of 128%. Measurement of TSHRAbs with a bioassay that uses porcine thyroid cells (TSH-stimulating antibody [TSAb]) showed a positive correlation (r = 0.472, p < 0.001) with the Thyretain assay for untreated GD, and strong positive correlation (r = 0.821, p < 0.001) for the entire untreated GD, PT, and SAT population. The positive rate of Mc4-TSHRAbs for GD was significantly higher than that of TSAb (95.1% vs. 89.3%, p < 0.05) and the negative rate of PT by Mc4-TSHRAbs was also significantly higher than that of TSAb (96.0% vs. 86.9%, p < 0.01). As a result, Mc4-TSHRAbs showed statistically better (p < 0.01) diagnostic accuracy in differentiating GD from PT than TSAb.

CONCLUSIONS

These data suggest that the Thyretain bioreporter assay with a chimeric TSHR (Mc4) is more useful in the differential diagnosis of GD from PT than the bioassay with wild-type TSHR on porcine thyroid cells.

A novel thyroid stimulating immunoglobulin bioassay is a functional indicator of activity and severity of Graves' orbitopathy

CONTEXT

Immunoglobulins stimulating the TSH receptor (TSI) influence thyroid function and likely mediate extrathyroidal manifestations of Graves' disease (GD).

OBJECTIVES

The aim of this study was to assess the clinical relevance of TSI in GD patients with or without Graves' orbitopathy (GO), to correlate the TSI levels with activity/severity of GO, and to compare the sensitivity/specificity of a novel TSI bioassay with TSH receptor (TSH-R) binding methods (TRAb).

DESIGN

TSI were tested in two reporter cell lines designed to measure Igs binding the TSH-R and transmitting signals for cAMP/CREB/cAMP regulatory element complex-dependent activation of luciferase gene expression. Responsiveness to TSI of the novel chimeric (Mc4) TSH-R (amino acid residues 262-335 of human TSH-R replaced by rat LH-R) was compared with the wild-type (wt) TSH-R.

RESULTS

All hyperthyroid GD/GO patients were TSI-positive. TSI were detected in 150 of 155 (97%, Mc4) and 148 of 155 (95%, wt) GO patients, in six of 45 (13%, Mc4) and 20 of 45 (44%, wt) mostly treated GD subjects, and in 0 of 40 (Mc4) and one of 40 (wt) controls. Serum TSI titers were 3- and 8-fold higher in GO vs. GD and control, respectively. All patients with diplopia and optic neuropathy and smokers were TSI-positive. TSI strongly correlated with GO activity (r = 0.87 and r = 0.7; both P < 0.001) and severity (r = 0.87 and r = 0.72; both P < 0.001) in the Mc4 and wt bioassays, respectively. Clinical sensitivity (97 vs. 77%; P < 0.001) and specificity (89 vs. 43%; P < 0.001) of the Mc4/TSI were greater than TRAb in GO. All 11 of 200 (5.5%) TSI-positive/TRAb-negative patients had GO, whereas all seven of 200 (3.5%) TSI-negative/TRAb-positive subjects had GD only.

CONCLUSION

The novel Mc4/TSI is a functional indicator of GO activity and severity.

Targeted expression of the human thyrotropin receptor A-subunit to the mouse thyroid: insight into overcoming the lack of response to A-subunit adenovirus immunization

The thyrotropin receptor (TSHR), the major autoantigen in Graves' disease, is posttranslationally modified by intramolecular cleavage to form disulfide-linked A- and B-subunits. Because Graves' hyperthyroidism is preferentially induced in BALB/c mice using adenovirus encoding the free A-subunit rather than full-length human TSHR, the shed A-subunit appears to drive the disease-associated autoimmune response. To further investigate this phenomenon, we generated transgenic mice with the human A-subunit targeted to the thyroid. Founder transgenic mice had normal thyroid function and were backcrossed to BALB/c. The A-subunit mRNA expression was confirmed in thyroid tissue. Unlike wild-type littermates, transgenic mice immunized with low-dose A-subunit adenovirus failed to develop TSHR Abs, hyperthyroidism, or splenocyte responses to TSHR Ag. Conventional immunization with A-subunit protein and adjuvants induced TSHR Abs lacking the characteristics of human autoantibodies. Unresponsiveness was partially overcome using high-dose, full-length human TSHR adenovirus. Although of low titer, these induced Abs recognized the N terminus of the A-subunit, and splenocytes responded to A-subunit peptides. Therefore, "non-self" regions in the B-subunit did not contribute to inducing responses. Indeed, transgenic mice immunized with high-dose A-subunit adenovirus developed TSHR Abs with thyrotropin-binding inhibitory activity, although at lower titers than wild-type littermates, suggesting down-regulation in the transgenic mice. In conclusion, in mice expressing a human A-subunit transgene in the thyroid, non-self human B-subunit epitopes are not necessary to induce responses to the A-subunit. Our findings raise the possibility that autoimmunity to the TSHR in humans may not involve epitopes on a cross-reacting protein, but rather, strong adjuvant signals provided in bystander immune responses.

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.

Thyrotropin receptor antibodies: new insights into their actions and clinical relevance

The thyrotropin receptor (TSHR) is a G-protein-coupled receptor with a large ectodomain. TSH, acting via TSHR, regulates thyroid growth and thyroid hormone production and secretion. The TSHR undergoes complex post-translational processing involving dimerization, intramolecular cleavage, and shedding of its ectodomain, and each of these processes may influence the antigenicity of the TSHR. The TSHR is also the major autoantigen in Graves' disease, as well as a leading candidate autoantigen in both Graves' ophthalmopathy and pretibial myxedema. The naturally conformed TSHR is most effectively presented as an autoantigen to the immune system, causing the production of stimulating TSHR-Abs. There are also autoantibodies which block the TSHR from TSH action, and neutral TSHR-Abs which have no influence on TSH action. TSHR-Abs can be detected by competition assays of TSHR-Abs for labeled TSH, or monoclonal TSHR-Ab binding to solubilized TSHRs, or by bioassays using thyroid cells or mammalian cells expressing recombinant TSHRs.

Differences in TSH receptor binding and thyroid-stimulating properties between TSH and Graves' IgG. Slowly-acting TSH receptor antibody moieties in Graves' sera affect assay data

We analyzed TSH receptor (TSHR) effects, both binding and thyroid-stimulation, of TSH and Graves' IgG. A new TRAb assay system utilizes rhTSHR coated tubes and is comprised of two step incubation, the first incubation with patient serum followed by a second incubation with 125I-bTSH. We called TRAb measured by this method as hTRAb. 125I-bTSH binding capacity of the tube was found close to saturation at 1 hr with 200 microl of 125I-bTSH. Up to 5 hr of first incubation for hTRAb assay revealed significant increases in all hTRAb activities. hTRAb was not affected by second incubation time or dose of 125I-bTSH. When 1 step incubation with 125I-bTSH and Graves' serum was performed, hTRAb again increased significantly with time. A simple competitive equilibrium model could not be applied to these ligands. Second, Graves' IgG and bTSH were compared for in vitro thyroid-stimulation sequentially up to 24 hr, measuring cAMP generation from cultured porcine thyrocytes. While bTSH yielded peak cAMP generation by 8 hr, TSAb revealed more cAMP generation by 24 hr than at 8 hr. We concluded that individual Graves' sera contain heterogeneous TRAb of variable avidities, and that slow-acting TRAb, which may lack biological activity, can be detected by prolonged incubation.

Dissecting linear and conformational epitopes on the native thyrotropin receptor

The TSH receptor (TSHR) is the primary antigen in Graves' disease. In this condition, autoantibodies to the TSHR that have intrinsic thyroid-stimulating activity develop. We studied the epitopes on the native TSHR using polyclonal antisera and monoclonal antibodies (mAbs) derived from an Armenian hamster model of Graves' disease. Of 14 hamster mAbs analyzed, five were shown to bind to conformational epitopes including one mAb with potent thyroid-stimulating activity. Overlapping conformational epitopes were determined by cell-binding competition assays using fluorescently labeled mAbs. We identified two distinct conformational epitopes: epitope A for both stimulating and blocking mAbs and epitope B for only blocking mAbs. Examination of an additional three mouse-derived stimulating TSHR-mAbs also showed exclusive binding to epitope A. The remaining nine hamster-derived mAbs were neutral or low-affinity blocking antibodies that recognized linear epitopes within the TSHR cleaved region (residues 316-366) (epitope C). Serum from the immunized hamsters also recognized conformational epitopes A and B but, in addition, also contained high levels of TSHR-Abs interacting within the linear epitope C region. In summary, these studies indicated that the natively conformed TSHR had a restricted set of epitopes recognized by TSHR-mAbs and that the binding site for stimulating TSHR-Abs was highly conserved. However, high-affinity TSHR-blocking antibodies recognized two conformational epitopes, one of which was indistinguishable from the thyroid-stimulating epitope. Hence, TSHR-stimulating and blocking antibodies cannot be distinguished purely on the basis of their conformational epitope recognition.

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.

Thyrotropin receptor autoantibodies (TSHRAbs): epitopes, origins and clinical significance

Epitopes for > 95% stimulating thyrotropin receptor autoantibodies (TSHRAbs) causally implicated in Graves' disease (Basedow's disease or primary hyperthyroidism) have been identified on on the N-terminal portion of the TSHR extracellular domain, residues 8-165. If the stimulating TSHRAb activity is solely dependent on this region, it is termed homogeneous; if its activity is only largely related to this region, it is termed heterogeneous. The presence of a heterogeneous stimulating TSHRAb in a patient is associated with rapid responses to propylthiouracil or methimazole and may be predictive of long term remission with these oral immunosuppressives. Epitopes for two different Graves' autoantibodies that inhibit TSH binding, TSH binding inhibition immunoglobulins or TBIIs, have also been identified on this region of the TSHR. They do not increase cAMP levels, although one may activate the inositol phosphate, Ca++, arachidonate release signal system. The epitope of blocking TSHRAbs with the ability to inhibit TSH binding (TBII activity), TSH activity, and stimulating TSHRAb activity, and that are causally implicated in the primary hypothyroidism of patients with idiopathic myxedema or some patients with Hashimoto's disease have, in contrast, been largely identified largely on the C-terminal portion of the TSHR extracellular domain, residues 270-395. They have been implicated as important in pregnancy where they attenuate the signs and symptoms of Graves' hyperthyroidism. The appearance of these blocking TSHRAbs during pregnancy in Graves' patients might cause overt or occult hypothyroidism, with resultant effects on fetal development and postnatal intelligence levels. The different TSHRAbs can exist in the same patient at any moment in time, potentially making disease expression a sum of their activities. Assays taking advantage of the epitope mapping findings enable us to detect individual TSHRAbs within a single patient and to better understand their clinical significance.

Thyroid stimulation does not require antibodies with identical epitopes but does involve recognition of a critical conformation at the N terminus of the thyrotropin receptor A-subunit

Whether monoclonal antibodies with thyroid-stimulating activity [thyroid-stimulating antibody/antibodies (TSAb)] from immunized animals are identical to human autoantibodies in Graves' disease is unknown. Here, we compared properties of a monoclonal hamster TSAb (MS-1) with human autoantibodies. The epitopes of neither MS-1 nor human autoantibodies can be determined by peptide scanning, indicating their conformational nature. A property of human TSAb is that their epitope is partially obscured on the TSH holoreceptor on the cell surface relative to the TSH receptor (TSHR) ectodomain tethered to the membrane by a glycosylphosphatidyl inositol anchor. On flow cytometry, as for human autoantibodies, MS-1 preferentially recognized the glycosylphosphatidyl inositol-anchored ectodomain vs. the TSH holoreceptor on Chinese hamster ovary cells. Also, as with human autoantibodies, only A-subunits with the active (but not the inactive) conformation adsorbed MS-1 binding activity. This difference localizes antibody binding to a cysteine-rich region at the TSHR N terminus. Remarkably, active TSHR A-subunit more effectively ( approximately 40-fold) neutralized human autoantibodies than it did MS-1. Therefore, MS-1 interacts less well than autoantibodies with the free A-subunit. In summary, we provide evidence that TSAb need not have identical epitopes. However, the TSAb epitope does appear to require involvement of the highly conformational N terminus of the A-subunit.

A coated tube assay for the detection of blocking thyrotropin receptor autoantibodies

We developed a coated tube assay to discriminate TSH-receptor-stimulating autoantibodies [thyroid-stimulating antibodies (TSAb)] from those autoantibodies blocking TSH binding without intrinsic activation [thyroid-blocking antibodies (TBAb)]. The wild-type TSH receptor in the TSH binding-inhibitory assay was exchanged for a chimeric receptor where a TSAb epitope (amino acids 8-165) was replaced by comparable LH-R residues. Binding of (125)I-labeled TSH to this chimera could be inhibited by sera containing TBAb up to 95%. Sera from 316 patients with Graves' disease and 17 with autoimmune thyroid disease were grouped according to their bioassay activity. At the decision threshold, the chimera A assay had a sensitivity of 78.0% for TBAb with a specificity of 90.2%. In detail, 19 of 22 (86.4%) TBAb sera and 15 of 23 (65.2%) TSAb/TBAb sera were positive but only 32 of 216 (14.0%) TSAb sera and 5 of 72 (6.9%) bioassay negative sera. There was a weak but significant positive correlation (r = 0.46) between the chimera assay and the bioassay for TBAb. This is the first report of a coated tube assay for the determination of TBAb employing an adaptation of the TSH binding-inhibitory format, which could be a useful alternative to the bioassay.

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.

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.

Absence of IL-4, and not suppression of the Th2 response, prevents development of experimental autoimmune Graves' disease

In autoimmune Graves' disease (GD), autoantibodies bind to the thyrotropin receptor (TSHR) and cause hyperthyroidism. We studied the effects of fms-like tyrosine kinase receptor 3 ligand (Flt3-L) or GM-CSF treatment on the development of experimental autoimmune GD (EAGD) in mice, a slowly progressing Ab-mediated organ-specific autoimmune disease of the thyroid induced by immunization with syngeneic cells expressing TSHR. Flt3-L and GM-CSF treatment resulted in up-regulation of CD8a(+) and CD8a(-) dendritic cells, and skewing of cytokine and immune responses to TSHR in favor of Th1 and Th2, respectively. However, this skewing did not persist until the later stages, and thus failed to affect the course or severity of the disease. To determine whether the total absence of either IL-4 or IFN-gamma could affect the development of EAGD, we immunized wild-type, IFN-gamma(-/-) and IL-4(-/-) BALB/c mice with TSHR. Nearly 100% of the wild-type and IFN-gamma(-/-) mice developed EAGD with optimal TSHR-specific immune responses, while IL-4(-/-) mice completely resisted disease and showed delayed and suboptimal pathogenic Ab response. These data demonstrated that skewing immune responses to TSHR, using either Flt3-L or GM-CSF, in favor of Th1 or Th2, respectively, may not be sufficient to alter the course of the disease, while the complete absence of IL-4, but not IFN-gamma, can prevent the development of EAGD.

Epitope heterogeneity of thyroid-stimulating antibodies predicts long-term outcome in Graves' patients treated with antithyroid drugs

Differences in the epitopes of thyroid-stimulating antibodies (TSAbs) from patients with untreated Graves' disease were compared with long-term response to antithyroid drugs. Epitopes were measured using Chinese hamster ovary cells transfected with wild-type human TSH receptor (TSHR) and two receptor chimeras, wherein TSHR residues 9-165 or 90-165 had been substituted with comparable residues of the LH/chorionic gonadotropin receptor. Of 159 patients studied, 52 (32.7%) exhibited positive TSAb activity with one or both chimera lines (heterogeneous group), and 107 (67.3%) had no activity with either (homogeneous group). Independent of all other parameters, patients with heterogeneous epitopes responded more favorably to oral antithyroid drugs than patients with homogeneous epitopes (65.4% vs. 41.9%, P = 0.011: estimated odds ratio by logistic regression, 2.17). Although most clinical parameters were not different at presentation, significant differences in the size of goiters, total T(3) concentrations, and titers of TSH-binding inhibitory Igs were evident in the successfully treated group (n = 80) by comparison to the group of patients whose treatment failed (n = 79). Alone, these three parameters did not predict outcome; however, when either of these parameters were considered together with epitope heterogeneity, predictability of a positive therapeutic response was increased to nearly 80%. Thus, the presence of TSAbs with a heterogeneous epitope in a patient with Graves' disease is not only associated with a favorable response to antithyroid drug treatment, it may help predict the response to treatment when the patient is initially seen.

Expression of hypothalamic-pituitary-thyroid axis related genes in the human skin

The skin is commonly affected in thyroid diseases, but the mechanism for this association is still unclear. As the skin expresses numerous neuroendocrine elements, we tested the additional cutaneous expression of mediators operating in the hypothalamic-pituitary-thyroid axis. We found significant expression of the thyroid-stimulating hormone receptor mRNA in cultured keratinocytes, epidermal melanocytes, and melanoma cells. The presence of thyroid-stimulating hormone receptor was confirmed by northern analyses and the thyroid-stimulating hormone receptor was found to be functionally active in cyclic adenosine monophosphate signal assays. Thyroid-stimulating hormone receptor expressing cells also expressed the sodium iodide symporter and thyroglobulin genes. We also found expression of deiodinases 2 and 3 (mainly deiodinase 2) in whole skin biopsy specimens, and in the majority of epidermal and dermal cells by reverse transcription-polymerase chain reaction followed by sequencing of the amplified gene segments. There was selective expression of the gene for thyroid-stimulating hormone beta; detection of the thyroid-releasing hormone gene was minimal and thyroid-releasing hormone receptor mRNA was not detected in most of the samples. Expression of functional thyroid-stimulating hormone receptor in the skin may have significant physiologic and pathologic consequences, particularly in autoimmune conditions associated with production of stimulating antibodies against the thyroid-stimulating hormone receptor. We conclude that the expanding list of neuroendocrine elements expressed in the skin supports a strong role for this system in cutaneous biology.

Involvement of the protein kinase C pathway in thyrotropin-induced STAT3 activation in FRTL-5 thyroid cells

The binding of thyrotropin (TSH) to the TSH receptor (TSHR) activates two signaling pathways: the cAMP-protein kinase A (PKA) and the protein kinase C (PKC) systems. We have recently demonstrated that TSH activates the Janus kinases (JAK)/signal transducer and activator of transcription (STAT) pathway via TSHR. This study aimed to investigate whether the cAMP/PKA or the PKC system is involved in STAT3 activation in response to TSH. Treatment with TSH activated STAT3 phosphorylation in FRTL-5 thyrocytes and human TSHR-expressing Chinese hamster ovary cells. TSH-induced STAT3 activation was inhibited by a blocking antibody directed against TSHR that was isolated from patients with primary myxoedema. Increased intracellular cAMP activated STAT3 but inhibition of PKA did not affect STAT3 activation. On the other hand, the PKC stimulant PMA induced STAT3 phosphorylation and the PKC inhibitors inhibited it. Moreover, inhibition of PKC blocked STAT3 activation induced by a stimulator of cAMP. Our data suggest that TSH activates STAT3 via TSHR and cAMP- and PKC-dependent pathways, and provide evidence that PKC may be involved in the pathway downstream from cAMP.

Thyroid-stimulating autoantibodies in Graves disease preferentially recognize the free A subunit, not the thyrotropin holoreceptor

Graves disease is directly caused by thyroid-stimulating autoantibodies (TSAb's) that activate the thyrotropin receptor (TSHR). We observed upon flow cytometry using intact cells that a mouse mAb (3BD10) recognized the TSHR ectodomain with a glycosidylphosphatidylinositol (ECD-GPI) anchor approximately tenfold better than the same ectodomain on the wild-type TSHR, despite the far higher level of expression of the latter. The 3BD10 epitope contains the N-terminal cysteine cluster critical for TSAb action. Consequently, we hypothesized and confirmed that TSAb (but not thyrotropin-blocking autoantibodies [TBAb's]) also poorly recognize the wild-type TSHR relative to the ECD-GPI. Despite poor recognition by TSAb of the holoreceptor, soluble TSHR A subunits (known to be shed from surface TSHR) fully neutralized autoantibody-binding activity. These data indicate that the epitope(s) for TSAb's, but not for TBAb's, are partially sterically hindered on the holoreceptor by the plasma membrane, the serpentine region of the TSHR, or by TSHR dimerization. However, the TSAb epitope on the soluble A subunit is freely accessible. This observation, as well as other evidence, supports the concept that A subunit shedding either initiates or amplifies the autoimmune response to the TSHR, thereby causing Graves disease in genetically susceptible individuals.

Activation of the cAMP pathway by the TSH receptor involves switching of the ectodomain from a tethered inverse agonist to an agonist

Several lines of evidence indicate that constraining intramolecular interactions between transmembrane domains are required to maintain G protein-coupled receptors in an inactive conformation in the absence of agonist. For the glycoprotein hormone receptors, which harbor a long amino-terminal ectodomain responsible for hormone binding, it has been suggested that the ectodomain could contribute to these negative constraints. To test this hypothesis, we expressed at the surface of COS-7 cells mutants of the TSH receptor in which variable portions of the amino-terminal ectodomain are replaced by a 19-residue tag from bovine rhodopsin. Whereas none of the rhodopsin-tagged truncated mutants could be activated by saturating concentrations of TSH, the constructs with the shortest amino-terminal extension displayed increased constitutive activity toward the cAMP pathway, when compared with the wild-type holoreceptor. The shortest truncated construct was strongly activated by the introduction of mutations in transmembrane segment VI (D633A), or in the third intracellular loop (A623I) of the receptor. The magnitude of the stimulation was similar to that observed when the same mutations were introduced in the intact wild-type receptor. On the contrary, the shortest truncated construct was unaffected by activating mutations affecting residues of the extracellular loop region (I486F, I568T) or the top of transmembrane segment VII (del658-661). Together, our results are compatible with a model in which activation of the cAMP pathway by the TSH receptor involves switching of the ectodomain from a tethered inverse agonist to a true agonist.

A novel murine model of Graves' hyperthyroidism with intramuscular injection of adenovirus expressing the thyrotropin receptor

In this work we report a novel method to efficiently induce a murine model of Graves' hyperthyroidism. Inbred mice of different strains were immunized by i.m. injection with adenovirus expressing thyrotropin receptor (TSHR) or beta-galactosidase (1 x 10(11) particles/mouse, three times at 3-wk intervals) and followed up to 8 wk after the third immunization. Fifty-five percent of female and 33% of male BALB/c (H-2(d)) and 25% of female C57BL/6 (H-2(b)) mice developed Graves'-like hyperthyroidism with elevated serum thyroxine (T(4)) levels and positive anti-TSHR autoantibodies with thyroid-stimulating Ig (TSI) and TSH-binding inhibiting Ig (TBII) activities. In contrast, none of female CBA/J (H-2(k)), DBA/1J (H-2(q)), or SJL/J (H-2(s)) mice developed Graves' hyperthyroidism or anti-TSHR autoantibodies except SJL/J, which showed strong TBII activities. There was a significant positive correlation between TSI values and T(4) levels, but the correlations between T(4) and TBII and between TSI and TBII were very weak. TSI activities in sera from hyperthyroid mice measured with some chimeric TSH/lutropin receptors suggested that their epitope(s) on TSHR appeared similar to those in patients with Graves' disease. The thyroid glands from hyperthyroid mice displayed diffuse enlargement with hypertrophy and hypercellularity of follicular epithelia with occasional protrusion into the follicular lumen, characteristics of Graves' hyperthyroidism. Decreased amounts of colloid were also observed. However, there was no inflammatory cell infiltration. Furthermore, extraocular muscles from hyperthyroid mice were normal. Thus, the highly efficient means that we now report to induce Graves' hyperthyroidism in mice will be very useful for studying the pathogenesis of autoimmunity in Graves' disease.

Evidence for a simplified view of autoantibody interactions with the thyrotropin receptor

Recently, many exceptions have been reported that undermine the concept that the epitopes for thyroid-stimulating autoantibodies (TSAb) and thyrotropin-blocking autoantibodies (TBAb) lie within the N-terminal and C-terminal portions of the thyrotropin receptor (TSHR) ectodomain, respectively. Here we have used a new approach to examine the issue by attempting to neutralize autoantibodies with the purified, N-terminal 289 amino acids of the TSHR ectodomain (TSHR-289), essentially the A subunit. Immunoglobulin G (IgG) with TSAb activity from 10 patients with Graves' disease was preincubated with or without purified active or inactive TSHR-289. Active, but not inactive, TSHR-289 completely neutralized TSAb activity in all sera. We then tested the ability of active TSHR-289 to neutralize TBAb activity in two rare hypothyroid patients with pure TBAb activity lacking agonist activity. IgG from both patients was extremely potent in the TBAb assay. Unlike with TSAb, preincubation of the IgG with a large excess of active TSHR-289 (20 microg/mL) revealed a remarkable divergence. TBAb activity from one patient was totally neutralized whereas in the other patient TBAb activity was totally unaffected. In conclusion, using a new approach, we confirm that the major portion of the TSAb epitope in the 418 amino acid ectodomain lies upstream of residue 289 (within the A subunit). In contrast, TBAb that cause hyperthyroidism can be more heterogeneous, with epitopes that lie largely upstream (A subunit) or downstream (B subunit) of residue 289.

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.

Clinical significance of classification of Graves' disease according to the characteristics of TSH receptor antibodies

BACKGROUND

It has been widely accepted that the epitope(s) and/or functional characteristics of thyrotropin receptor antibodies (TSHRAb) from Graves' patients are heterogenous among patients. However, the clinical significance of such heterogeneity has not been systematically evaluated yet. We were to elucidate and find the clinical significance of heterogeneity for TSH receptor antibodies in Graves' disease.

METHODS

We measured stimulating TSHRAb (TSAb) activities using CHO-hTSHR cells, FRTL-5 cells and chimeric receptor expressing cells (Mc1 + 2 and Mc2), specific blocking TSHRAb (TSBAb) activities using Mc2 cells and TBII activities using porcine thyroid membrane in 136 patients with untreated hyperthyroid Graves' disease.

RESULTS

Based on various TSHRAb activities from each patient, the patients could be categorized into 7 subgroups by cluster analysis; 1) Group 1 (n = 41) was characterized by moderate TSAb activities both in CHO-hTSHR cells and in FRTL-5 cells, typical TSAb epitope, rare blocking antibodies and high TBII activities. 2) Group 2 (n = 16) was characterized by the presence of blocking TSHRAb in most patients, albeit the other characteristics were the same as those in Group 1. 3) Group 3 (n = 19) patients had low TSAb activities both in CHO-hTSHR cells and in FRTL-5 cells, seldom had blocking TSHRAb, but they had high TBII activities. 4) Group 4 (n = 30) could be categorized as 'mild disease' group, as they had low activities in all kinds of TSHRAb assay and had low antimicrosomal antibody activities. 5) Group 5 (n = 14) was characterized by moderate TSAb activities with atypical epitope(s), rare blocking TSHRAb and moderate TBII activities. 6) Group 6 (n = 10) patients had very high TSAb activities with typical epitopes, seldom blocking TSHRAb and low TBII activities. 7) Group 7 (n = 6) was characterized by very high TSAb activities with atypical epitopes and high TBII activities. Pretreatment serum thyroid hormone level was low only in group 4 patients compared to the other 6 groups (p < 0.05). The size of goiter was significantly larger in those in group 1 and group 3 (p < 0.05) compared to the other 5 groups. The prevalence of clinically significant ophthalmopathy was higher in group 2 patients than the other 6 groups (50% vs. 27.5%, p = 0.06). Among 6 kinds of TSHRAb activities, only the blocking TSHRAb activity was significantly associated with the presence of ophthalmopathy in multivariate analysis.

CONCLUSION

These results suggest that the differences in epitopes for TSAb or the presence of blocking TSHRAb is not a major factor in determining the degree of thyrotoxicosis in Graves' disease. Although the pathogenic mechanism is not clear yet, we suggest that patients with ophthalmopathy have different TSHRAb repertoire from those without ophthalmopathy in Graves' disease.

A full biological response to autoantibodies in Graves' disease requires a disulfide-bonded loop in the thyrotropin receptor N terminus homologous to a laminin epidermal growth factor-like domain

We observed amino acid homology between the cysteine-rich N terminus of the thyrotropin receptor (TSHR) ectodomain and epidermal growth factor-like repeats in the laminin gamma1 chain. Thyroid-stimulating autoantibodies (TSAb), the cause of Graves' disease, interact with this region of the TSHR in a manner critically dependent on antigen conformation. We studied the role of the cluster of four cysteine (Cys) residues in this region of the TSHR on the functional response to TSAb in Graves' patients' sera. As a benchmark we also studied TSH binding and action. Removal in various permutations of the four cysteines at TSHR positions 24, 29, 31, and 41 (signal peptide residues are 1-21) revealed Cys(41) to be the key residue for receptor expression. Forced pairing of Cys(41) with any one of the three upstream Cys residues was necessary for trafficking to the cell surface of a TSHR with high affinity TSH binding similar to the wild-type receptor. However, for a full biological response to TSAb, forced pairing of Cys(41) with Cys(29) or with Cys(31), but not with Cys(24), retained functional activity comparable with the wild-type TSHR. These data suggest that an N-terminal disulfide-bonded loop between Cys(41) and Cys(29) or its close neighbor Cys(31) comprises, in part, the highly conformational epitope for TSAb at the critical N terminus of the TSHR. Amino acid homology, as well as cysteine pairing similar to the laminin gamma1 chain epidermal growth factor-like repeat 11, suggests conformational similarity between the two molecules and raises the possibility of molecular mimicry in the pathogenesis of Graves' disease.

Reassessment of the location of the thyrotropin receptor 50 amino acid "insertion" provides evidence in favor of a second downstream cleavage site

Cleavage of thyrotropin receptors (TSHR) on the cell surface into disulfide-linked A and B subunits involves deletion of an intervening region that corresponds approximately to a 50 amino acid "insertion" in the TSHR relative to the noncleaving luteinizing hormone/choriogonadotropin receptor (LH/CGR). The location of this insertion is imprecise because of the relatively low homology between the two receptors in this region. We tested the hypothesis that the TSHR 50 amino acid insertion was further downstream than we previously concluded, a possibility that would relocate the crucial LH/CGR glycan at N291 relative to the position of the TSHR insertion, and that would mitigate against the 50 amino acid insertion playing a role in TSHR intramolecular cleavage. Thus, we transferred the LH/CGR glycan at amino acid 291 from downstream (N367) to upstream of the 50 amino acid insertion (N317) in the TSHR, leaving this insertion intact. TSHR cleavage persisted. Moreover, deletion of amino acid residues 320-366 in addition to the upstream N291 substitution (ALN317-319NET) also did not prevent cleavage. On the other hand, deletion of three contiguous downstream residues (GQE367-369) in the TSHR 50 amino acid insertion abolished receptor cleavage into subunits. In summary, the present data are consistent with our previous location of the TSHR 50 amino acid insertion and, therefore, do not undermine evidence for the involvement of this insertion in TSHR cleavage. In addition, the data regarding TSHR residues GQE367-369 (far downstream of cleavage site 1) support the controversial possibility of a secondary cleavage site downstream of the insertion.

Characterization of thyroid-stimulating blocking antibodies that appeared during transient hypothyroidism after radioactive iodine therapy

Hypothyroidism after radioactive iodine (RAI) therapy for Graves' disease can be transient or permanent. The cause for early transient hypothyroidism is unknown. We evaluated 11 patients who developed transient hypothyroidism within 6 months of RAI and 12 who remained euthyroid after RAI. Approximately equal numbers of patients in each group had thyroid-stimulating antibody (TSAb) that increased cyclic adenosine monophosphate (cAMP) levels in Chinese hamster ovary (CHO) cells transfected with the recombinant human thyrotropin receptor (TSHR) (WT cells). Approximately equal numbers of patients from both groups had an increase in TSAb activity post-RAI. All TSAbs had their dominant functional epitope on the N-terminus of the TSHR extracellular domain, requiring residues 90-165 for activity because they, but not TSH, completely lost stimulating activity in a receptor chimera, wherein TSHR residues 90-165 were substituted by equivalent residues of the lutropin/choriogonadotropin receptor (LH/CGR). Although equal numbers of patients in both groups had thyrotropin-binding inhibiting immunoglobulin activity (TBII), as measured by radioreceptor assay before RAI, patients with transient hypothyroidism had a surge in TBII activity and all except one became positive for thyroid-stimulating blocking antibodies (TSBAb), as measured by inhibition of TSH-stimulated cAMP from WT cells. When immunoglobulin G (IgGs) were epitope-mapped using TSHR/LH-CGR chimeras with different substitutions, 8 hypothyroid subjects had TSBAbs directed against residues 90-165 of the TSHR, as well as TSHR residues 261-370. Two had functional epitopes directed at residues 9-89 as well as TSHR residues 261-370. None of the euthyroid control patients developed TSBAbs and their TBII activity decreased post-RAI. When patients with transient hypothyroidism reverted to a euthyroid state, TSAb was still detectable in 5; however, TBII was present in all and TSBAb, although decreased, was still positive in 9. In summary, RAI therapy was associated with a change in thyroid antibody characteristics in most patients. Additionally, patients with a surge in TBII and the appearance of TSBAb developed transient hypothyroidism after RAI.

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.

Selective binding of thyrotropin receptor autoantibodies to recombinant extracellular domain of thyrotropin/lutropin-chorionic gonadotropin receptor chimeric proteins

The extracellular domain of the glycosylated human thyrotropin receptor (ET-gp) contains epitopes that can adsorb pathogenic antibodies from sera of patients with Graves' disease (GD). In an attempt to define the regions within the ETSHR with which autoantibodies interact, we expressed extracellular domains of eight thyrotropin receptor/chorionic gonadotropin receptor (TSHR/LH-CGR) chimeric proteins in insect cells. The levels of expression were high and chimeric proteins were glycosylated. Chimeric proteins designated as EMc2+4 and EMc2+3+4, in which amino acids (aa) 90-165 and 261-370, and aa 90-370, respectively, of TSHR were replaced with corresponding aa of LH-CGR, partially reversed the thyrotropin binding inhibitory immunoglobulin (TBII) activity of experimental anti-TSHR antisera (anti-ET-gp). The other six chimeras almost completely reversed the TBII activity of these anti-ET-GP antisera. Next, we tested the ability of these chimeric proteins to reverse the TBII activity of GD patients' sera. Similar to our earlier study, ET-gp protein reversed the TBII activity of all eight GD patients' sera tested. Chimera EMc2, in which aa 90-165 of TSHR has been replaced with corresponding aa of LH-CGR, and EMc2+4 partially reversed the TBII activity of only three of the eight GD patients' sera. However, the other six chimeric proteins failed to neutralize the TBII activity of any of GD patients' sera. These data showed the following: (1) There is considerable heterogeneity amongst autoantibodies in GD patients' sera, (2) The TBII activity of some, but not others, is dependent on aa 90-165 and 261-370, and (3) Most Graves' sera, with TBII activity, failed to react with chimeric proteins in which either N-terminal or C-terminal regions of the extra cellular domain of the TSHR were replaced with corresponding regions of LH-CGR. These results suggest that the TBII activity of GD patients' sera is dependent on conformational epitopes and replacement of certain regions of TSHR with homologous regions of LH-CGR results in sufficient alteration in the conformation of the protein leading to loss of reactivity.

Epitope heterogeneity of thyrotropin receptor-blocking antibodies in Graves' patients as detected with wild-type versus chimeric thyrotropin receptors

The stable transfectants of wild-type (W25) and mutant thyrotropin-receptor (TSH-R) allow detection of the bioactivities of TSH-R antibodies in Graves' patients. A mutant Chinese hamster ovary (CHO) cell line (Mc1+2) transfected with a chimeric construct, where residues 8 to 165 of the TSH-R are replaced with residues 10 to 166 of the lutropin/choriogonadotropin (LH/CGR) receptor, lacks the cyclic adenosine monophosphate (cAMP) response to most thyrotropin stimulating antibodies (TSAb), yet retains the response to TSH and acquires the response to LH/CG. We compared Mc1+2 cells with wild-type W25 cells for their ability to detect TSAb as well as thyrotropin-blocking antibodies (TBAb) in Graves' sera. Eighteen normal and 39 Graves' sera were tested for TSAb and TBAb levels by in vitro bioassays using W25 and Mc1+2 cells. In addition, these sera were also tested for thyrotropin-binding inhibitory activity (TBII) by a radioreceptor assay. Eighteen (47%) Graves' sera had TBAb activity measured with W25 cells but not with Mc1+2 cells. These TBAbs were, therefore, a population of antibodies with functional epitopes on the N-terminus of the extracellular domain. This TBAb activity by W25 cells exhibited a high degree of correlation with TBII levels by a radioreceptor assay (r = 0.70, p = 0.001). Ten (25.6%) Graves' sera had positive TBAb activity in both W25 and Mc1+2 cells; moreover, their activity in both assays was similar (r = 0.83, p < 0.001). The TBAb activity in these sera, however, did not correlate with TBII activity. Eleven (28%) Graves' sera had no TBAb activity. Overall, thyroid-stimulating antibodies were detected in 87% and 28% of the 39 Graves' sera by W25 and Mc1 +2 cells, respectively. Thus, using the 2 cell lines, at least 2 distinct populations of TBAbs were detected. One is detected in a similar fashion by both W25 and Mc1+2 cell lines and likely interacts with the epitopes residing in the unaltered C-terminus of the TSH-R. The other is reactive in W25 cells only, indicating the loss of TBAb epitope in the chimeric receptor located in the N-terminus of the TSH-R. Furthermore, our results indicate that the TBAb binding epitope in 8-165 residues of the native TSH-R is highly associated with TBII activity in Graves' disease. These results indicate that patients with Graves' disease harbor TBAbs with epitope heterogeneity and favor the notion that there are different sites and mechanisms by which TBAbs act in Graves' patients. It remains to be determined whether or not TBAb subtyping will have a useful predictive role in the management of patients with Graves' disease.

Two Graves' disease patients who spontaneously developed hypothyroidism after antithyroid drug treatment: characteristics of epitopes for thyrotropin receptor antibodies

Few reports have identified blocking thyrotropin receptor antibodies (TSHRAbs) as a pathogenic mechanism explaining spontaneous hypothyroidism after antithyroid drug (ATD) treatment of Graves' disease. Here we report 2 Graves' patients who showed different courses of hypothyroidism after ATD treatment. The first patient had Graves' hyperthyroidism and was treated with ATD for 1 year. After a short period of euthyroidism, she developed permanent hypothyroidism with blocking TSHRAb. The second patient became euthyroid after 1 year of ATD treatment. After 3 years, however, she presented with hypothyroidism with blocking TSHRAb activity. Her hypothyroidism was transient, and restoration of euthyroidism was followed by disappearance of blocking TSHRAb. Blocking and stimulating TSHRAbs activities of these 2 patients were serially measured using Chinese hamster ovary (CHO) cells transfected with wild-type human TSHR (CHO-hTSHR) and 2 TSHR chimeras with residues 8-165 (Mc1+2) or 90-165 (Mc2) substituted by equivalent residues of the luteinizing hormone/chorionic gonadotropin receptor (LH/CGR). During their hypothyroid phases, blocking TSHRAbs activities were positive in all 3 kinds of assays and stimulating TSHRAbs activities were negative in CHO-hTSHR or in Mc 1+2 assay. Mc2 stimulating TSHRAb activity was detected in sera of hypothyroid phase of the second patient who had transient hypothyroidism but not in the first whose hypothyroidism was permanent. In these 2 cases, we demonstrate the causative role of blocking TSHRAb in the development of hypothyroidism after ATD treatment in Graves' patients. Interestingly, the difference in the course of blocking TSHRAb-induced hypothyroidism was associated with the difference in epitope reactivities of TRAb during hypothyroid phase that developed after ATD treatment of Graves' disease.

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.