Monoclonal pathogenic antibodies to the thyroid-stimulating hormone receptor in Graves' disease with potent thyroid-stimulating activity but differential blocking activity activate multiple signaling pathways

Bulbar Conjunctival Microvascular Alterations in Thyroid-Associated Ophthalmopathy Patients with Different Activities

Purpose: To evaluate the morphologic and hemodynamic changes of bulbar conjunctival vessels in thyroid-associated ophthalmopathy (TAO) patients and the correlations with the activity.Methods: Patients diagnosed as TAO with different clinical activity scores (CAS) and healthy participants were recruited. All subjects underwent a complete ophthalmic examination and functional slit-lamp biomicroscope. Vascular variables including the vessel density, vessel complexity, average diameter, blood flow velocity and blood flow rate in microvascular networks were measured. The correlations among microvascular parameters, CAS and exophthalmos were analyzed. Areas under the receiver operating characteristic curves (AUROCs) were applied to evaluate the diagnostic accuracy of microvascular alterations for active TAO.Results: A total of 46 eyes were enrolled in our study. The vessel complexity and blood flow velocity increased in the active TAO group significantly compared with the inactive group and healthy controls (P < .05). Meanwhile, the vessel complexity and blood flow rate were positively correlated with CAS (r = 0.641 and r = 0.526). Bulbar conjunctival microvascular parameters performed a good ability in distinguishing the active stage of TAO (AUROC = 0.793).Conclusions: Increasing bulbar conjunctival vessel complexity and blood flow were evident in TAO with severe inflammation. The measurements of bulbar conjunctival microvasculature could be a reference to evaluate activity in TAO.

Antinuclear Antibodies With a Nucleolar Pattern Are Associated With a Significant Reduction in the Overall Survival of Patients With Leukemia: A Retrospective Cohort Study

OBJECTIVE

Antinuclear antibodies (ANAs) have been reported to be associated with cancers. However, the role of different ANA patterns in cancers is poorly understood, especially in leukemia. This study aimed to investigate the association between ANA patterns and the outcome of leukemia in a retrospective cohort.

METHODS

A total of 429 adult patients initially diagnosed with leukemia at Henan Provincial People's Hospital from January 2014 to December 2018 were included in this study, including information on patients without positive ANAs at the time of initial diagnosis, preexisting autoimmune diseases, infectious diseases, etc. The data were retrieved up to December 2020. The final sample included 196 adult patients. The risk of death outcome according to ANA patterns was estimated using multivariable Cox proportional hazards models and the overall survival for ANA patterns was analyzed using Kaplan-Meier curve.

RESULTS

ANAs with a nucleolar pattern versus negative ANA were associated with a two-fold increased risk of death outcome in leukemia, independent of sex, age, leukemia immunophenotype, cytogenetic abnormality, treatment, and blood transfusion. Further analysis revealed that the association was more significant in elder patients (≥60 years) and patients treated with tyrosine kinase inhibitor or chemotherapy (P for interaction = 0.042 and 0.010). Notably, the patients with a nucleolar pattern had shorter survival than the patients with a non-nucleolar pattern or without ANA (p < 0.001).

CONCLUSION

ANAs with a nucleolar pattern are a significant predictor of poor prognosis, providing clues for prognostic assessment in patients with leukemia.

Is There Evidence for IGF1R-Stimulating Abs in Graves' Orbitopathy Pathogenesis?

In this review, we summarize the evidence against direct stimulation of insulin-like growth factor 1 receptors (IGF1Rs) by autoantibodies in Graves' orbitopathy (GO) pathogenesis. We describe a model of thyroid-stimulating hormone (TSH) receptor (TSHR)/IGF1R crosstalk and present evidence that observations indicating IGF1R's role in GO could be explained by this mechanism. We evaluate the evidence for and against IGF1R as a direct target of stimulating IGF1R antibodies (IGF1RAbs) and conclude that GO pathogenesis does not involve directly stimulating IGF1RAbs. We further conclude that the preponderance of evidence supports TSHR as the direct and only target of stimulating autoantibodies in GO and maintain that the TSHR should remain a major target for further development of a medical therapy for GO in concert with drugs that target TSHR/IGF1R crosstalk.

Exploring the Role of SRC in Extraocular Muscle Fibrosis of the Graves' Ophthalmopathy

The Graves’ disease is an autoimmune disease highly associated with thyroid cancer. The Graves’ ophthalmopathy (GO) is a special Graves’ disease with inflammatory ophthalmopathy being a typical extrathymic complication. GO is caused by the formation of orbital fat and extraocular muscle fibrosis due to the inflammation of orbital connective tissues. Thus, controlling extraocular muscle fibrosis is critical for the prognosis of GO. The objective of this study is to identify and experimentally validate key genes associated with GO and explore their potential function mechanisms especially on extraocular muscle fibrosis. Specifically, we first created a GO mouse model, and performed RNA sequencing on the extraocular muscles of fibrotic GO mice and controls. SRC was identified as the most significant unstudied differentially expressed gene between GO mice and controls. Thus, we conducted a few in vitro analyses to explore the roles and functions of SRC in GO, for which we selected primary cultured orbital fibroblast (OF) as the in vitro cell line model. It is known that myofibroblast (MFB), which expresses α-SMA, is an important target cell in the process of fibrosis. Our experiment suggests that TGF-β can induce the transformation from OF to MFB, however, the transformation was inhibited by silencing the SRC gene in OF. In addition, we also inhibited TGF-β/Smad, NF-κB, and PI3K/Akt signaling pathways to analyze the interaction between these pathways and SRC. In conclusion, the silence of SRC in OF can inhibit the transformation from OF to MFB, which might be associated with the interaction between SRC and a few pathways such as TGF-β/Smad, NF-κB, and PI3K/Akt.

TSH Receptor Homodimerization in Regulation of cAMP Production in Human Thyrocytes in vitro

Thyrotropin hormone (TSH) was reported to exhibit biphasic regulation of cAMP production in human thyroid slices; specifically, upregulation at low TSH doses transitioning to inhibition at high doses. We observed this phenomenon in HEK293 cells overexpressing TSH receptors (TSHRs) but in only 25% of human thyrocytes (hThyros) in vitro. Because TSHR expression in hThyros in vitro was low, we tested the hypothesis that high, in situ levels of TSHRs were needed for biphasic cAMP regulation. We increased expression of TSHRs by infecting hThyros with adenoviruses expressing human TSHR (AdhTSHR), measured TSH-stimulated cAMP production and TSHR homodimerization. TSHR mRNA levels in hThyros in vitro were 100-fold lower than in human thyroid tissue. AdhTSHR infection increased TSHR mRNA expression to levels found in thyroid tissue and flow cytometry showed that cell-surface TSHRs increased more than 15-fold. Most uninfected hThyro preparations exhibited monotonic cAMP production. In contrast, most hThyro preparations infected with AdhTSHR expressing TSHR at in vivo levels exhibited biphasic TSH dose responses. Treatment of AdhTSHR-infected hThyros with pertussis toxin resulted in monotonic dose response curves demonstrating that lower levels of cAMP production at high TSH doses were mediated by Gi/Go proteins. Proximity ligation assays confirmed that AdhTSHR infection markedly increased the number of TSHR homodimers. We conclude that in situ levels of TSHRs as homodimers are needed for hThyros to exhibit biphasic TSH regulation of cAMP production.

Antibodies specific for disease-associated antigens (DAA) expressed in non-malignant diseases reveal potential new tumor-associated antigens (TAA) for immunotherapy or immunoprevention

Immune responses to a large number of mutated and non-mutated tumor antigens have been studied in an attempt to unravel the highly complex immune response to cancer. Better understanding of both the effectors and the targets of successful immunosurveillance can inform various immunotherapeutic approaches, which can strengthen or replace natural immunosurveillance that a tumor has managed to escape. In this review we highlight targets of antibodies generated in the context of diseases other than cancer, such as asthma, allergies, autoimmune disorders, inflammation and infections, where the antibody presence correlates either with an increased or a reduced lifetime risk of cancer. We focus on their target antigens, self-molecules abnormally expressed on diseased cells or cross-reactive with exogenous antigens and found on cancer cells as tumor associated antigens (TAA). We refer to them as disease-associated antigens (DAA). We review 4 distinct categories of antibodies according to their target DAA, their origin and their reported impact on cancer risk: natural antibodies, autoantibodies, long-term memory antibodies and allergy-associated antibodies. Increased understanding and focus on their specific targets could enable a more rational choice of antigens for both therapeutic and preventative cancer vaccines and other more effective and less toxic cancer immunotherapies.

Thyrotropin, but Not Thyroid-Stimulating Antibodies, Induces Biphasic Regulation of Gene Expression in Human Thyrocytes

Background: Thyrotropin (TSH) and thyroid-stimulating antibodies (TSAbs) activate TSH receptor (TSHR) signaling by binding to its extracellular domain. TSHR signaling has been studied extensively in animal thyrocytes and in engineered cell lines, and differences in signaling have been observed in different cell systems. We, therefore, decided to characterize and compare TSHR signaling mediated by TSH and monoclonal TSAbs in human thyrocytes in primary culture. Methods: We used quantitative reverse transcription-polymerase chain reaction to measure mRNA levels of thyroid-specific genes thyroglobulin (TG), thyroperoxidase (TPO), iodothyronine deiodinase type 2 (DIO2), sodium-iodide symporter (NIS), and TSHR after stimulation by TSH or two monoclonal TSAbs, KSAb1 and M22. We also compared secreted TG protein after TSHR activation by TSH and TSAbs using an enzyme-linked immunosorbent assay. TSHR cell surface expression was determined using fluorescence activated cell sorting (FACS). Results: We found that TSH at low doses increases and at high doses (>1 mU/mL) decreases levels of gene expression for TSHR, TG, TPO, NIS, and DIO2. The biphasic effect of TSH on signaling was not caused by downregulation of cell surface TSHRs. This bell-shaped biphasic dose-response curve has been termed an inverted U-shaped dose-response curve (IUDRC). An IUDRC was also found for TSH-induced regulation of TG secretion. In contrast, KSAb1- and M22-induced regulation of TSHR, TG, TPO, NIS, and DIO2 gene expression, and secreted TG followed a monotonic dose-response curve that plateaus at high doses of activating antibody. Conclusions: Our data demonstrate that the physiological activation of TSHRs by TSH in primary cultures of human thyrocytes is characterized by a regulatory mechanism that may inhibit thyrocyte overstimulation. In contrast, TSAbs do not exhibit biphasic regulation. Although KSAb1 and M22 may not be representative of all TSAbs found in patients with Graves' disease, we suggest that persistent robust stimulation of TSHRs by TSAbs, unrelieved by a decrease at high TSAb levels, fosters chronic stimulation of thyrocytes in Graves' hyperthyroidism.

Human placenta-derived mesenchymal stem cells ameliorate orbital adipogenesis in female mice models of Graves' ophthalmopathy

Background

Graves’ ophthalmopathy (GO) is a complication of Graves’ disease (GD), in which orbital connective tissues become inflamed and increase in volume and orbital fibroblasts within the orbital fat and extraocular muscles differentiate into adipocytes in vitro when stimulated by hormones, several cytokines, and growth factors including TSH, IGF-1, IL-1, interferon γ, and platelet-derived growth factor. Human placental mesenchymal stem cells (hPMSCs) have immunomodulatory effects in disease pathogenesis. Although a number of studies have reported that hPMSCs can elicit therapeutic effects, these are not sufficient. Therefore, we constructed a GO animal model in order to find out the hPMSCs recovery effect.

Methods

We investigated their anti-adipogenic effects in in vitro cultures of orbital fibroblasts established from GO patients. Primary orbital fibroblasts were exposed to differentiation medium for 10 days. After being co-cultured with hPMSCs, the characteristics of orbital fibroblast were determined by Oil Red O stain and real-time PCR. Then, we explored the in vivo regulatory effects of hPMSCs in an experimental mouse model of GO. We developed the GO mouse model using immunization by leg muscle electroporation of pTriEx1.1Neo-hTSHR A-subunit plasmid. Human PMSC injection was performed into the left orbit. We also analyzed the effects of hPMSCs in the GO animal model.

Result

We found that hPMSCs inhibited a lipid accumulation and activated factors, such as ADIPONECTIN, PPARγ, C/EBPα, and TGFβ2 genes in adipogenesis-induced primary orbital fibroblasts from GO patients. Moreover, hPMSCs were highly effective at ameliorating adipogenesis in the orbital tissue of the model.

Conclusion

These data indicate that hPMSCs recover pathogenic activation of orbital fibroblasts in animals undergoing experimental GO and confirm the feasibility of applying hPMSCs as a novel treatment for GO patients.

Arrestin-β-1 Physically Scaffolds TSH and IGF1 Receptors to Enable Crosstalk

Endogenously expressed TSH receptors (TSHRs) on orbital fibroblasts of patients with Graves ophthalmopathy (GO) use crosstalk with IGF1 receptors (IGF1R) to synergistically stimulate secretion of hyaluronan (HA), a major component of GO pathology. We previously showed crosstalk occurred upstream of mitogen-activated protein kinase (ERK) phosphorylation. Because other G protein-coupled receptors engage arrestin-β-1 (ARRB1) and ERK, we tested whether ARRB1 was a necessary component of TSHR/IGF1R crosstalk. HA secretion was stimulated by the TSHR-stimulating monoclonal antibodies M22 and KSAb1, or immunoglobulins from patients with GO (GO-Igs). Treatment with M22, as previously shown, resulted in biphasic dose-response stimulation of HA secretion. The high-potency phase was IGF1R dependent, and the low-potency phase was partly IGF1R independent. KSAb1 produced a monophasic dose-response stimulation of HA secretion, whose potency was lowered >20-fold after IGF1R knockdown. ARRB1 knockdown abolished M22's high-potency phase and lowered KSAb1's potency and efficacy. ARRB1 knockdown inhibited GO-Ig stimulation of HA secretion and of ERK phosphorylation. Last, ARRB1 was shown to be necessary for TSHR/IGF1R proximity. In contrast, ARRB2 knockdowns did not show these effects. Thus, TSHR must neighbor IGF1R for crosstalk in GO fibroblasts to occur, and this depends on ARRB1 acting as a scaffold. Similar scaffolding of TSHR and IGF1R by ARRB1 was found in human osteoblast-like cells and human thyrocytes. These findings support a model of TSHR/IGF1R crosstalk that may be a general mechanism for G-protein-coupled receptor/receptor tyrosine kinase crosstalk dependent on ARRB1.

The role of thyroglobulin in thyroid hormonogenesis

In humans, the thyroid hormones T₃ and T₄ are synthesized in the thyroid gland in a process that crucially involves the iodoglycoprotein thyroglobulin. The overall structure of thyroglobulin is conserved in all vertebrates. Upon thyroglobulin delivery from thyrocytes to the follicular lumen of the thyroid gland via the secretory pathway, multiple tyrosine residues can become iodinated to form mono-iodotyrosine (MIT) and/or di-iodotyrosine (DIT); however, selective tyrosine residues lead to preferential formation of T₄ and T₃ at distinct sites. T₄ formation involves oxidative coupling between two DIT side chains, and de novo T₃ formation involves coupling between an MIT donor and a DIT acceptor. Thyroid hormone synthesis is stimulated by TSH activating its receptor (TSHR), which upregulates the activity of many thyroid gene products involved in hormonogenesis. Additionally, TSH regulates post-translational changes in thyroglobulin that selectively enhance its capacity for T₃ formation - this process is important in iodide deficiency and in Graves disease. 167 different mutations, many of which are newly discovered, are now known to exist in TG (encoding human thyroglobulin) that can lead to defective thyroid hormone synthesis, resulting in congenital hypothyroidism.

Nanoparticles Bearing TSH Receptor Protein and a Tolerogenic Molecule Do Not Induce Immune Tolerance but Exacerbate Thyroid Autoimmunity in hTSHR/NOD.H2h4 Mice

Transgenic NOD.H2h4 mice that express the human (h) TSHR A-subunit in the thyroid gland spontaneously develop pathogenic TSHR autoantibodies resembling those in patients with Graves disease. Nanoparticles coupled to recombinant hTSHR A-subunit protein and a tolerogenic molecule (ligand for the endogenous aryl-hydrocarbon receptor; ITE) were injected i.p. four times at weekly intervals into hTSHR/NOD.H2h4 mice with the goal of blocking TSHR Ab development. Unexpectedly, in transgenic mice, injecting TSHR A-subunit-ITE nanoparticles (not ITE-nanoparticles or buffer) accelerated and enhanced the development of pathogenic TSHR Abs measured by inhibition of TSH binding to the TSHR. Nonpathogenic TSHR Abs (ELISA) were enhanced in transgenics and induced in wild-type littermates. Serendipitously, these findings have important implications for disease pathogenesis: development of Graves TSHR Abs is limited by the availability of A-subunit protein, which is shed from membrane bound TSHR, expressed at low levels in the thyroid. The enhanced TSHR Ab response following injected TSHR A-subunit protein-nanoparticles is reminiscent of the transient increase in pathogenic TSHR Abs following the release of thyroid autoantigens after radio-iodine therapy in Graves patients. However, in the hTSHR/NOD.H2h4 model, enhancement is specific for TSHR Abs, with Abs to thyroglobulin and thyroid peroxidase remaining unchanged. In conclusion, despite the inclusion of a tolerogenic molecule, injected nanoparticles coated with TSHR A-subunit protein enhanced and accelerated development of pathogenic TSHR Abs in hTSHR/NOD. NOD.H2h4 These findings emphasize the need for sufficient TSHR A-subunit protein to activate the immune system and the generation of stimulatory TSHR Abs in genetically predisposed individuals.

A New Highly Thyrotropin Receptor-Selective Small-Molecule Antagonist with Potential for the Treatment of Graves' Orbitopathy

BACKGROUND

The thyrotropin receptor (TSHR) is the target for autoimmune thyroid stimulating antibodies (TSAb) triggering hyperthyroidism. Whereas elevated thyroid hormone synthesis by the thyroid in Graves' disease can be treated by antithyroid agents, for the pathogenic activation of TSHR in retro-orbital fibroblasts of the eye, leading to Graves' orbitopathy (GO), no causal TSHR directed therapy is available.

METHODS

Due to the therapeutic gap for severe GO, TSHR inhibitors were identified by high-throughput screening in Chinese hamster ovary cells expressing the TSHR. Stereo-selective synthesis of the screening hits led to the molecule S37, which contains seven chiral centers. Enantiomeric separation of the molecule S37 resulted in the enantiopure molecule S37a-a micro-molar antagonist of thyrotropin-induced cyclic adenosine monophosphate accumulation in HEK 293 cells expressing the TSHR.

RESULTS

The unique rigid bent shape of molecule S37a may mediate the observed high TSHR selectivity. Most importantly, the closely related follitropin and lutropin receptors were not affected by this compound. S37a not only inhibits the TSHR activation by thyrotropin itself but also activation by monoclonal TSAb M22 (human), KSAb1 (murine), and the allosteric small-molecule agonist C2. Disease-related ex vivo studies in HEK 293 cells expressing the TSHR showed that S37a also inhibits cyclic adenosine monophosphate formation by oligoclonal TSAb, which are highly enriched in GO patients' sera. Initial in vivo pharmacokinetic studies revealed no toxicity of S37a and a remarkable 53% oral bioavailability in mice.

CONCLUSION

In summary, a novel highly selective inhibitor for the TSHR is presented, which has promising potential for further development for the treatment of GO.

De novo triiodothyronine formation from thyrocytes activated by thyroid-stimulating hormone

The thyroid gland secretes primarily tetraiodothyronine (T₄), and some triiodothyronine (T₃). Under normal physiological circumstances, only one-fifth of circulating T₃ is directly released by the thyroid, but in states of hyperactivation of thyroid-stimulating hormone receptors (TSHRs), patients develop a syndrome of relative T₃ toxicosis. Thyroidal T₄ production results from iodination of thyroglobulin (TG) at residues Tyr⁵ and Tyr¹³⁰, whereas thyroidal T₃ production may originate in several different ways. In this study, the data demonstrate that within the carboxyl-terminal portion of mouse TG, T₃ is formed de novo independently of deiodination from T₄ We found that upon iodination in vitro, de novo T₃ formation in TG was decreased in mice lacking TSHRs. Conversely, de novo T₃ that can be formed upon iodination of TG secreted from PCCL3 (rat thyrocyte) cells was augmented from cells previously exposed to increased TSH, a TSHR agonist, a cAMP analog, or a TSHR-stimulating antibody. We present data suggesting that TSH-stimulated TG phosphorylation contributes to enhanced de novo T₃ formation. These effects were reversed within a few days after removal of the hyperstimulating conditions. Indeed, direct exposure of PCCL3 cells to human serum from two patients with Graves' disease, but not control sera, led to secretion of TG with an increased intrinsic ability to form T₃ upon in vitro iodination. Furthermore, TG secreted from human thyrocyte cultures hyperstimulated with TSH also showed an increased intrinsic ability to form T₃ Our data support the hypothesis that TG processing in the secretory pathway of TSHR-hyperstimulated thyrocytes alters the structure of the iodination substrate in a way that enhances de novo T₃ formation, contributing to the relative T₃ toxicosis of Graves' disease.

Mechanisms of Autoantibody-Induced Pathology

Autoantibodies are frequently observed in healthy individuals. In a minority of these individuals, they lead to manifestation of autoimmune diseases, such as rheumatoid arthritis or Graves' disease. Overall, more than 2.5% of the population is affected by autoantibody-driven autoimmune disease. Pathways leading to autoantibody-induced pathology greatly differ among different diseases, and autoantibodies directed against the same antigen, depending on the targeted epitope, can have diverse effects. To foster knowledge in autoantibody-induced pathology and to encourage development of urgently needed novel therapeutic strategies, we here categorized autoantibodies according to their effects. According to our algorithm, autoantibodies can be classified into the following categories: (1) mimic receptor stimulation, (2) blocking of neural transmission, (3) induction of altered signaling, triggering uncontrolled (4) microthrombosis, (5) cell lysis, (6) neutrophil activation, and (7) induction of inflammation. These mechanisms in relation to disease, as well as principles of autoantibody generation and detection, are reviewed herein.

The Emerging Role of Epigenetics in Autoimmune Thyroid Diseases

Autoimmune thyroid diseases (AITD) are a group of both B cell- and T cell-mediated organ-specific autoimmune diseases. Graves’ disease and Hashimoto thyroiditis are the two main clinical presentations of AITD. Both genetic and environmental factors have important roles in the development of AITD. Epigenetics have been considered to exert key roles in integrating those genetic and environmental factors, and epigenetic modifications caused by environmental factors may drive genetically susceptibility individuals to develop AITD. Recent studies on the epigenetics of AITD have provided some novel insights into the pathogenesis of AITD. The aim of this review is to provide an overview of recent advances in the epigenetic mechanisms of AITD, such as DNA methylation, histone modifications, and non-coding RNAs. This review highlights the key roles of epigenetics in the pathogenesis of AITD and potential clinical utility. However, the epigenetic roles in AITD are still not fully elucidated, and more researches are needed to provide further deeper insights into the roles of epigenetics in AITD and to uncover new therapeutic targets. Although there are many studies assessing the epigenetic modifications in AITD patients, the clinical utility of epigenetics in AITD remains poorly defined. More studies are needed to identify the underlying epigenetic modifications that can contribute to accurate diagnosis of AITD, adequate choice of treatment approach, and precise prediction of treatment outcomes.

High-level intrathymic thyrotrophin receptor expression in thyroiditis-prone mice protects against the spontaneous generation of pathogenic thyrotrophin receptor autoantibodies

The thyrotrophin receptor (TSHR) A-subunit is the autoantigen targeted by pathogenic autoantibodies that cause Graves' hyperthyroidism, a common autoimmune disease in humans. Previously, we reported that pathogenic TSHR antibodies develop spontaneously in thyroiditis-susceptible non-obese diabetic (NOD).H2^h4 mice bearing a human TSHR A-subunit transgene, which is expressed at low levels in both the thyroid and thymus (Lo-expressor transgene). The present study tested recent evidence that high intrathymic TSHR expression protects against the development of pathogenic TSHR antibodies in humans. By successive back-crossing, we transferred to the NOD.H2^h4 background a human TSHR A-subunit transgene expressed at high levels in the thyroid and thymus (Hi-expressor transgene). In the sixth back-cross generation (> 98% NOD.H2^h4 genome), only transgenic offspring produced spontaneously immunoglobulin (Ig)G class non-pathogenic human TSHR A-subunit antibodies. In contrast, both transgenic and non-transgenic offspring developed antibodies to thyroglobulin and thyroid peroxidase. However, non-pathogenic human TSHR antibody levels in Hi-expressor offspring were lower than in Lo-expressor transgenic mice. Moreover, pathogenic TSHR antibodies, detected by inhibition of TSH binding to the TSHR, only developed in back-cross offspring bearing the Lo-expressor, but not the Hi-expressor, transgene. High versus low expression human TSHR A-subunit in the NOD.H2^h4 thymus was not explained by the transgene locations, namely chromosome 2 (127-147 Mb; Hi-expressor) and chromosome 1 (22.9-39.3 Mb; low expressor). Nevertheless, using thyroiditis-prone NOD.H2^h4 mice and two transgenic lines, our data support the association from human studies that low intrathymic TSHR expression is associated with susceptibility to developing pathogenic TSHR antibodies, while high intrathymic TSHR expression is protective.

Comparative Assessment of Female Mouse Model of Graves' Orbitopathy Under Different Environments, Accompanied by Proinflammatory Cytokine and T-Cell Responses to Thyrotropin Hormone Receptor Antigen

We recently described a preclinical model of Graves' orbitopathy (GO), induced by genetic immunization of eukaryotic expression plasmid encoding human TSH receptor (TSHR) A-subunit by muscle electroporation in female BALB/c mice. The onset of orbital pathology is characterized by muscle inflammation, adipogenesis, and fibrosis. Animal models of autoimmunity are influenced by their environmental exposures. This follow-up study was undertaken to investigate the development of experimental GO in 2 different locations, run in parallel under comparable housing conditions. Functional antibodies to TSHR were induced in TSHR A-subunit plasmid-immunized animals, and antibodies to IGF-1 receptor α-subunit were also present, whereas control animals were negative in both locations. Splenic T cells from TSHR A-subunit primed animals undergoing GO in both locations showed proliferative responses to purified TSHR antigen and secreted interferon-γ, IL-10, IL-6, and TNF-α cytokines. Histopathological evaluation showed orbital tissue damage in mice undergoing GO, manifest by adipogenesis, fibrosis, and muscle damage with classic signs of myopathy. Although no inflammatory infiltrate was observed in orbital tissue in either location, the appearances were consistent with a "hit-and-run" immune-mediated inflammatory event. A statistically significant increase of cumulative incidence of orbital pathology when compared with control animals was shown for both locations, confirming onset of orbital dysimmune myopathy. Our findings confirm expansion of the model in different environments, accompanied with increased prevalence of T cell-derived proinflammatory cytokines, with relevance for pathogenesis. Wider availability of the model makes it suitable for mechanistic studies into pathogenesis and undertaking of novel therapeutic approaches.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Crystal structure of a TSH receptor monoclonal antibody: insight into Graves' disease pathogenesis

The TSH receptor (TSHR) A-subunit is more effective than the holoreceptor in inducing thyroid-stimulating antibodies (TSAb) that cause Graves' disease. A puzzling phenomenon is that 2 recombinant, eukaryotic forms of A-subunits (residues 22-289), termed active and inactive, are recognized mutually exclusively by pathogenic TSAb and mouse monoclonal antibody 3BD10, respectively. Understanding the structural difference between these TSHR A-subunit forms could provide insight into Graves' disease pathogenesis. The 3-dimensional structure of the active A-subunit (in complex with a human TSAb Fab, M22) is known, but the structural difference with inactive A-subunits is unknown. We solved the 3BD10 Fab 3-dimensional crystal structure. Guided by prior knowledge of a portion of its epitope, 3BD10 docked in silico with the known active TSHR-289 monomeric structure. Because both TSAb and 3BD10 recognize the active TSHR A-subunit monomer, this form of the molecule can be excluded as the basis for the active-inactive dichotomy, suggesting, instead a role for A-subunit quaternary structure. Indeed, in silico analysis revealed that M22, but not 3BD10, bound to a TSHR-289 trimer. In contrast, 3BD10, but not M22, bound to a TSHR-289 dimer. The validity of these models is supported experimentally by the temperature-dependent balance between active and inactive TSHR-289. In summary, we provide evidence for a structural basis to explain the conformational heterogeneity of TSHR A-subunits (TSHR-289). The pathophysiologic importance of these findings is that affinity maturation of pathogenic TSAb in Graves' disease is likely to involve a trimer of the shed TSHR A-subunit.

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

Modeling Graves' Orbitopathy in Experimental Graves' Disease

Graves' orbitopathy (GO), also known as thyroid eye disease is an inflammatory disease of the orbital tissue of the eye that arises as a consequence of autoimmune thyroid disease. The central feature of the disease is the production of antibodies to the thyrotropin hormone receptor (TSHR) that modulate the function of the receptor leading to autoimmune hyperthyroidism and GO. Over the years, all viable preclinical models of Graves' disease have been incomplete and singularly failed to progress in the treatment of orbital complications. A new mouse model of GO based upon immunogenic presentation of human TSHR A-subunit plasmid by close field electroporation is shown to lead to induction of prolonged functional antibodies to TSHR resulting in chronic disease with subsequent progression to GO. The stable preclinical GO model exhibited pathologies reminiscent of human disease characterized by orbital remodeling by inflammation and adipogenesis. Inflammatory lesions characterized by CD3+ T cells and macrophages were localized in the orbital muscle tissue. This was accompanied by extensive adipogenesis of orbital fat in some immune animals. Surprisingly, other signs of orbital involvement were reminiscent of eyelid inflammation involving chemosis, with dilated and congested orbital blood vessels. More recently, the model is replicated in the author's independent laboratories. The pre-clinical model will provide the basis to study the pathogenic and regulatory roles of immune T and B cells and their subpopulations to understand the initiation, pathophysiology, and progression of GO.

Mechanisms of Action of TSHR Autoantibodies

The availability of human monoclonal antibodies (MAbs) to the TSHR has enabled major advances in our understanding of how TSHR autoantibodies interact with the receptor. These advances include determination of the crystal structures of the TSHR LRD in complex with a stimulating autoantibody (M22) and with a blocking type autoantibody (K1-70). The high affinity of MAbs for the TSHR makes them particularly suitable for use as ligands in assays for patient serum TSHR autoantibodies. Also, M22 and K1-70 are effective at low concentrations in vivo as TSHR agonists and antagonists respectively. K1-70 has important potential in the treatment of the hyperthyroidism of Graves' disease and Graves' ophthalmopathy. Small molecule TSHR antagonists described to date do not appear to have the potency and/or specificity shown by K1-70. New models of the TSHR ECD in complex with various ligands have been built. These models suggest that initial binding of TSH to the TSHR causes a conformational change in the hormone. This opens a positively charged pocket in receptor-bound TSH which attracts the negatively charged sulphated tyrosine 385 on the hinge region of the receptor. The ensuing movement of the receptor's hinge region may then cause activation. Similar activation mechanisms seem to take place in the case of FSH and the FSHR and LH and the LHR. However, stimulating TSHR autoantibodies do not appear to activate the TSHR in the same way as TSH.

Evidence that TSH Receptor A-Subunit Multimers, Not Monomers, Drive Antibody Affinity Maturation in Graves' Disease

CONTEXT

The TSH receptor (TSHR) A-subunit shed from the cell surface contributes to the induction and/or affinity maturation of pathogenic TSHR autoantibodies in Graves' disease.

OBJECTIVE

This study aimed to determine whether the quaternary structure (multimerization) of shed A-subunits influences pathogenic TSHR autoantibody generation.

DESIGN

The isolated TSHR A-subunit generated by transfected mammalian cells exists in two forms; one (active) is recognized only by Graves' TSHR autoantibodies, the second (inactive) is recognized only by mouse monoclonal antibody (mAb) 3BD10. Recent evidence suggests that both Graves' TSHR autoantibodies and mAb 3BD10 recognize the A-subunit monomer. Therefore, if the A-subunit monomer is an immunogen, Graves' sera should have antibodies to both active and inactive A-subunits. Conversely, restriction of TSHR autoantibodies to active A-subunits would be evidence of a role for shed A-subunit multimers, not monomers, in the pathogenesis of Graves' disease. Therefore, we tested a panel of Graves' sera for their relative recognition of active and inactive A-subunits.

RESULTS

Of 34 sera from unselected Graves' patients, 28 were unequivocally positive in a clinical TSH binding inhibition assay. None of the latter sera, as well as 8/9 sera from control individuals, recognized inactive A-subunits on ELISA. In contrast to Graves' sera, antibodies induced in mice, not by shedding from the TSHR holoreceptor, but by immunization with adenovirus expressing the free human A-subunit, were directed to both the active and inactive A-subunit forms.

CONCLUSIONS

The present study supports the concept that pathogenic TSHR autoantibody affinity maturation in Graves' disease is driven by A-subunit multimers, not monomers.

A unique mouse strain that develops spontaneous, iodine-accelerated, pathogenic antibodies to the human thyrotrophin receptor

Abs that stimulate the thyrotropin receptor (TSHR), the cause of Graves' hyperthyroidism, only develop in humans. TSHR Abs can be induced in mice by immunization, but studying pathogenesis and therapeutic intervention requires a model without immunization. Spontaneous, iodine-accelerated, thyroid autoimmunity develops in NOD.H2(h4) mice associated with thyroglobulin and thyroid-peroxidase, but not TSHR, Abs. We hypothesized that transferring the human TSHR A-subunit to NOD.H2(h4) mice would result in loss of tolerance to this protein. BALB/c human TSHR A-subunit mice were bred to NOD.H2(h4) mice, and transgenic offspring were repeatedly backcrossed to NOD.H2(h4) mice. All offspring developed Abs to thyroglobulin and thyroid-peroxidase. However, only TSHR-transgenic NOD.H2(h4) mice (TSHR/NOD.H2(h4)) developed pathogenic TSHR Abs as detected using clinical Graves' disease assays. As in humans, TSHR/NOD.H2(h4) female mice were more prone than male mice to developing pathogenic TSHR Abs. Fortunately, in view of the confounding effect of excess thyroid hormone on immune responses, spontaneously arising pathogenic human TSHR Abs cross-react poorly with the mouse TSHR and do not cause thyrotoxicosis. In summary, the TSHR/NOD.H2(h4) mouse strain develops spontaneous, iodine-accelerated, pathogenic TSHR Abs in female mice, providing a unique model to investigate disease pathogenesis and test novel TSHR Ag-specific immunotherapies aimed at curing Graves' disease in humans.

Breaking tolerance to thyroid antigens: changing concepts in thyroid autoimmunity

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

Cutting edge: retrobulbar inflammation, adipogenesis, and acute orbital congestion in a preclinical female mouse model of Graves' orbitopathy induced by thyrotropin receptor plasmid-in vivo electroporation

Graves' orbitopathy (GO) is a complication in Graves' disease (GD) but mechanistic insights into pathogenesis remain unresolved, hampered by lack of animal model. The TSH receptor (TSHR) and perhaps IGF-1 receptor (IGF-1R) are considered relevant antigens. We show that genetic immunization of human TSHR (hTSHR) A-subunit plasmid leads to extensive remodeling of orbital tissue, recapitulating GO. Female BALB/c mice immunized with hTSHR A-subunit or control plasmids by in vivo muscle electroporation were evaluated for orbital remodeling by histopathology and magnetic resonance imaging (MRI). Antibodies to TSHR and IGF-1R were present in animals challenged with hTSHR A-subunit plasmid, with predominantly TSH blocking antibodies and were profoundly hypothyroid. Orbital pathology was characterized by interstitial inflammation of extraocular muscles with CD3+ T cells, F4/80+ macrophages, and mast cells, accompanied by glycosaminoglycan deposition with resultant separation of individual muscle fibers. Some animals showed heterogeneity in orbital pathology with 1) large infiltrate surrounding the optic nerve or 2) extensive adipogenesis with expansion of retrobulbar adipose tissue. A striking finding that underpins the new model were the in vivo MRI scans of mouse orbital region that provided clear and quantifiable evidence of orbital muscle hypertrophy with protrusion (proptosis) of the eye. Additionally, eyelid manifestations of chemosis, including dilated and congested orbital blood vessels, were visually apparent. Immunization with control plasmids failed to show any orbital pathology. Overall, these findings support TSHR as the pathogenic antigen in GO. Development of a new preclinical model will facilitate molecular investigations on GO and evaluation of new therapeutic interventions.

Yersinia enterocolitica provides the link between thyroid-stimulating antibodies and their germline counterparts in Graves' disease

Graves' disease results from thyroid-stimulating Abs (TSAbs) activating the thyrotropin receptor (TSHR). How TSAbs arise from early precursor B cells has not been established. Genetic and environmental factors may contribute to pathogenesis, including the bacterium Yersinia enterocolitica. We developed two pathogenic monoclonal TSAbs from a single experimental mouse undergoing Graves' disease, which shared the same H and L chain germline gene rearrangements and then diversified by numerous somatic hypermutations. To address the Ag specificity of the shared germline precursor of the monoclonal TSAbs, we prepared rFab germline, which showed negligible binding to TSHR, indicating importance of somatic hypermutation in acquiring TSAb activity. Using rFab chimeras, we demonstrate the dominant role of the H chain V region in TSHR recognition. The role of microbial Ags was tested with Y. enterocolitica proteins. The monoclonal TSAbs recognize 37-kDa envelope proteins, also recognized by rFab germline. MALDI-TOF identified the proteins as outer membrane porin (Omp) A and OmpC. Using recombinant OmpA, OmpC, and related OmpF, we demonstrate cross-reactivity of monoclonal TSAbs with the heterogeneous porins. Importantly, rFab germline binds recombinant OmpA, OmpC, and OmpF confirming reactivity with Y. enterocolitica. A human monoclonal TSAb, M22 with similar properties to murine TSAbs, also binds recombinant porins, showing cross-reactivity of a spontaneously arising pathogenic Ab with Y. enterocolitica. The data provide a mechanistic framework for molecular mimicry in Graves' disease, where early precursor B cells are expanded by Y. enterocolitica porins to undergo somatic hypermutation to acquire a cross-reactive pathogenic response to TSHR.

Blocking type TSH receptor antibodies

TSH receptor (TSHR) autoantibodies (TRAbs) play a key role in the pathogenesis of Graves' disease. In the majority of patients, TRAbs stimulate thyroid hormone synthesis via activation of the TSHR (stimulating TRAbs, TSHR agonists). In some patients, TRAbs bind to the receptor but do not cause activation (blocking TRAbs, TSHR antagonists). Isolation of human TSHR monoclonal antibodies (MAbs) with either stimulating (M22 and K1-18) or blocking activities (5C9 and K1-70) has been a major advance in studies on the TSHR. The binding characteristics of the blocking MAbs, their interaction with the TSHR and their effect on TSHR constitutive activity are summarised in this review. In addition, the binding arrangement in the crystal structures of the TSHR in complex with the blocking MAb K1-70 and with the stimulating MAb M22 (2.55 Å and 1.9 Å resolution, respectively) are compared. The stimulating effect of M22 and the inhibiting effect of K1-70 on thyroid hormone secretion in vivo is discussed. Furthermore the ability of K1-70 to inhibit the thyroid stimulating activity of M22 in vivo is shown. Human MAbs which act as TSHR antagonists are potentially important new therapeutics. For example, in Graves' disease, K1-70 may well be effective in controlling hyperthyroidism and the eye signs caused by stimulating TRAb. In addition, hyperthyroidism caused by autonomous TSH secretion should be treatable by K1-70, and 5C9 has the potential to control hyperthyroidism associated with TSHR activating mutations. Furthermore, K1-70 has potential applications in thyroid imaging as well as targeted drug delivery to TSHR expressing tissues.

TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis

This paper provides the reader with an overview of our current knowledge of hypothalamic-pituitary-thyroid feedback from a cybernetic standpoint. Over the past decades we have gained a plethora of information from biochemical, clinical, and epidemiological investigation, especially on the role of TSH and other thyrotropic agonists as critical components of this complex relationship. Integrating these data into a systems perspective delivers new insights into static and dynamic behaviour of thyroid homeostasis. Explicit usage of this information with mathematical methods promises to deliver a better understanding of thyrotropic feedback control and new options for personalised diagnosis of thyroid dysfunction and targeted therapy, also by permitting a new perspective on the conundrum of the TSH reference range.

Growth stimulating antibody, as another predisposing factor of Graves' disease (GD): analysis using monoclonal TSH receptor antibodies derived from patients with GD

TSH receptor antibody (TRAb) is clinically classified into thyroid stimulating antibody (TSAb) and thyroid-stimulation blocking antibody (TSBAb). Although the former is considered to cause Graves' disease (GD), its activity does not necessarily reflect hormone production and goiter size. Moreover, uptake of 99mTcO4(-), the best indicator for GD, is correlated with activity of TSH binding inhibitor immunoglobulin better than activity of TSAb. Because uptake of 99mTcO4(-) reflects thyroid volume, these observations suggest that there exist TRAb with thyrocyte growth stimulating activity (GSA) other than TSAb. In this study, we analyzed GSA of monoclonal TRAb established from patients with GD or idiopathic myxedema (IME). GSA was measured as the degree of FRTL-5 cell growth stimulated by each TRAb. The signaling pathways of the cell growth were pharmacologically analyzed. The cell growth stimulated by TSH was strongly suppressed by protein kinase A (PKA) inhibitor, but was not affected by extracellular signal regulated kinase kinase (MEK) inhibitor. Although TSAb from GD stimulated the cell growth, both inhibitors suppressed it. Surprisingly, the cell growth was also induced by TSBAb from GD and was only suppressed by MEK inhibitor. TSBAb from IME did not have GSA and attenuated the cell growth stimulated by TSH. We concluded that 1; in GD, not only TSAb but some TSBAb could stimulate thyrocyte growth. 2; TSBAb might be classified with respect to their effects on thyrocyte growth; i.e., thyrocyte growth stimulating antibody and thyrocyte growth-stimulation blocking antibody.

In vivo effects of a human thyroid-stimulating monoclonal autoantibody (M22) and a human thyroid-blocking autoantibody (K1-70)

PURPOSE

To study in vivo effects of the human monoclonal TSH receptor (TSHR) autoantibodies M22 (stimulating type) and K1-70 (blocking type) on thyroid hormone levels in rats.

METHODS

Serum levels of total T4, free T4, M22 and K1-70 were measured following intramuscular injection of M22 IgG (2-4 μg/animal), K1-70 IgG (10-200 μg/animal) or both into rats. Thyroid pathology was assessed in M22-injected rats.

RESULTS

Serum levels of total T4 and free T4 increased in a dose-dependent manner following injection of M22 IgG. Thyroid follicular cell hypertrophy was dependent on the dose of M22 IgG. K1-70 IgG caused a dose dependent decrease of total T4 and free T4 levels in rats receiving K1-70 only. The stimulating effects of M22 IgG on T4 levels in rats were completely inhibited by K1-70 IgG.

CONCLUSION

M22 is a potent stimulator of thyroid hormone secretion in vivo. In contrast, K1-70 inhibits thyroid hormone secretion in vivo. Furthermore, K1-70 has the ability to inhibit the stimulating activity of M22 in vivo and as such has potential as a new drug to block TSHR stimulation by autoantibodies in Graves' disease.

Orbital fibrosis in a mouse model of Graves' disease induced by genetic immunization of thyrotropin receptor cDNA

The TSH receptor (TSHR) is the critical target for antibody production in Graves' disease (GD). Insulin-like growth factor 1 receptor (IGF1R) has been proposed as a second autoantigen in complications of GD such as orbitopathy. We attempted to induce orbital tissue remodeling in mice undergoing immunizations with plasmids encoding TSHR and IGF1R delivered by in vivo skeletal muscle electroporation, a procedure known to give a sustained, long-term antibody response. Female BALB/c mice were challenged with TSHR A-subunit or IGF1Rα subunit plasmid by injection and electroporation. Mice challenged with TSHR A-subunit plasmid resulted in high frequency (75%) of hyperthyroidism and thyroid-stimulating antibodies. But strikingly, immunization with TSHR A-subunit plasmid also elicited antibody to IGF1Rα subunit. Mice challenged in the same manner with IGF1Rα subunit plasmid produced strong antibody responses to IGF1R, but did not undergo any changes in phenotype. Simultaneous challenge by double antigen immunization with the two plasmids in distant anatomical sites reduced the incidence of hyperthyroidism, potentially as a consequence of antigenic competition. Thyroid glands from the TSHR A-subunit plasmid-challenged group were enlarged with patchy microscopic infiltrates. Histological analysis of the orbital tissues demonstrated moderate connective tissue fibrosis and deposition of Masson's trichrome staining material. Our findings imply that immunization with TSHR A-subunit plasmid leads to generation of IGF1R antibodies, which together with thyroid-stimulating antibodies may precipitate remodeling of orbital tissue, raising our understanding of its close association with GD.

Neutral antibodies to the TSH receptor are present in Graves' disease and regulate selective signaling cascades

TSH receptor (TSHR) antibodies (Abs) may be stimulating, blocking, or neutral in their functional influences and are found in patients with autoimmune thyroid disease, especially Graves' disease (GD). Stimulators are known to activate the thyroid epithelial cells via both Gs- and Gq-coupled signaling pathways, whereas blockers inhibit the action of TSH and may act as weak agonists. However, TSHR neutral Abs do not block TSH binding and are unable to induce cAMP via Gsα. The importance of such neutral Abs in GD remains unclear because their functional consequence has been assumed to be zero. We hypothesized that: 1) neutral TSHR Abs are more common to GD than generally recognized; 2) they may induce distinct signaling imprints at the TSHR not seen with TSH itself; and 3) these signaling events may alter cellular function. To evaluate these hypotheses, we first confirmed the presence of neutral TSHR Abs in sera from patients with GD and then, using mouse and hamster neutral TSHR monoclonal Abs (N-mAbs) performed detailed signaling studies, including a proteomic Ab array, with rat thyrocytes (FRTL-5) as targets. This allowed us to examine a battery of signaling cascades and their downstream effectors. Neutral TSHR Abs were indeed frequently present in sera from patients with GD. Sixteen of 27 patients (59%) had detectable neutral TSHR Abs by competition assay with N-mAbs. On examining signaling cascades, we found that N-mAbs induced signal transduction, primarily via the protein kinase A II cascade. In addition to the activation of phosphatidylinositol 3K/Akt, N-mAbs, unlike TSH, had the ability to exclusively activate the mammalian target of rapamycin/p70 S6K, nuclear factor-κB, and MAPK-ERK1/2/p38α signaling cascades and their downstream effectors p90 ribosomal kinase/MAPK-interacting kinase-1/mitogen and stress-activated kinase-1 and N-mAbs activated all forms of protein kinase C isozymes. To define the downstream effector mechanisms produced by these signaling cascades, cytokine production, proliferation, and apoptosis in thyrocytes were investigated. Although N-mAbs produced less cytokines and proliferation compared with TSH, they had the distinction of inducing thyroid cell apoptosis under the experimental conditions used. When dissecting out possible mechanisms of apoptosis, we found that activation of multiple oxidative stress markers was the primary mechanism orchestrating the death signals. Therefore, using oxidative stress-induced apoptosis, N-mAbs may be capable of exacerbating the autoimmune response in GD via apoptotic cells inducing antigen-driven mechanisms. This may help explain the inflammatory nature of this common disorder.

TSH receptor monoclonal antibodies with agonist, antagonist, and inverse agonist activities

Autoantibodies in autoimmune thyroid disease (AITD) bind to the TSH receptor (TSHR) and can act as either agonists, mimicking the biological activity of TSH, or as antagonists inhibiting the action of TSH. Furthermore, some antibodies with antagonist activity can also inhibit the constitutive activity of the TSHR, that is, act as inverse agonists. The production of animal TSHR monoclonal antibodies (MAbs) with the characteristics of patient autoantibodies and the isolation of human autoantibodies from patients with AITD has allowed us to analyze the interactions of these antibodies with the TSHR at the molecular level. In the case of animal MAbs, advances such as DNA immunization allowed the production of the first MAbs which showed the characteristics of human TSHR autoantibodies (TRAbs). Mouse MAbs (TSMAbs 1-3) and a hamster MAb (MS-1) were obtained that acted as TSHR agonists with the ability to stimulate cyclic AMP production in CHO cells expressing the TSHR. In addition, a mouse TSHR MAb (MAb-B2) that had the ability to act as an antagonist of TRAbs and TSH was isolated and characterized. Also, a mouse TSHR MAb that showed TSH antagonist and TSHR inverse agonist activity (CS-17) was described. Furthermore, a panel of human TRAbs has been obtained from the peripheral blood lymphocytes of patients with AITD and extensively characterized. These MAbs have all the characteristics of TRAbs and are active at ng/mL levels. To date, two human MAbs with TSHR agonist activity (M22 and K1-18), one human MAb with TSHR antagonist activity (K1-70) and one human MAb (5C9) with both TSHR antagonist and TSHR inverse agonist activity have been isolated. Early experiments showed that the binding sites for TSH and for TRAbs with thyroid stimulating or blocking activities were located on the extracellular domain of the TSHR. Extensive studies using TSHRs with single amino acid mutations identified TSHR residues that were important for binding and biological activity of TSHR MAbs (human and animal) and TSH. The structures of several TSHR MAb Fab fragments were solved by X-ray crystallography and provided details of the topography of the antigen binding sites of antibodies with either agonist or antagonist activity. Furthermore stable complexes of the leucine-rich repeat domain (LRD) of the TSHR with a human MAb (M22) with agonist activity and with a human MAb (K1-70) with antagonist activity have been produced and their structures solved by X-ray crystallography at 2.55 and 1.9Å resolution, respectively. Together these experiments have given detailed insights into the interactions of antibodies with different biological activities (agonist, antagonist, and inverse agonist) with the TSHR. Although the nature of ligand binding to the TSHR is now understood in some detail, it is far from clear how these initial interactions lead to functional effects on activation or inactivation of the receptor.

Human TSH receptor ligands as pharmacological probes with potential clinical application

The biologic role of thyroid-stimulating hormone (TSH; thyrotropin) as an activator (agonist) of the TSH receptor (TSHR) in the hypothalamic-pituitary-thyroid axis is well known and activation of TSHR by recombinant human TSH is used clinically in patients with thyroid cancer. TSHR ligands other than TSH could be used to probe TSHR biology in thyroidal and extrathyroidal tissues, and potentially be employed in patients. A number of different TSHR ligands have been reported, including TSH analogs, antibodies and small-molecule, drug-like compounds. In this review, we will provide an update on all these classes of TSHR agonists and antagonists but place emphasis on small-molecule ligands.

Clonal relationships between thyroid-stimulating hormone receptor-stimulating antibodies illustrate the effect of hypermutation on antibody function

Graves' disease is characterized by production of agonist antibodies to the thyroid-stimulating hormone receptor (TSHR), but knowledge of the genetic and somatic events leading to their aberrant production is limited. We describe the genetic analysis of two monoclonal antibodies (mAbs) with thyroid-stimulating activity (TSAb) obtained from a single mouse with experimental Graves' disease. The mAbs were class switched, but used the same rearrangement of immunoglobulin heavy chain, variable region (IGHV) and immunoglobulin light chain, variable region (IGLV) germline genes, implying a clonal relationship and derivation from a single precursor B-cell clone. The IGHV-region genes of the two mAbs underwent high degrees of somatic hypermutation by sharing numerous mutations before diverging, while the IGLV genes evolved separately. Interestingly, the mutations were present in both the complementarity-determining regions (CDRs) and the framework regions. The cloned IGHV and IGLV genes were confirmed to have TSAb properties in experiments in which they were expressed as recombinant Fabs (rFabs). In other experiments, we swapped the IGLV genes with IGHV genes by constructing chimeric rFabs and showed that the chimeras retained TSAb activities, confirming the close functional relatedness of the V-region genes. Importantly, the IGLV genes in chimeric rFabs had a dominant stimulatory effect at low concentrations, while the IGHV genes had a dominant effect at higher concentrations. Our findings demonstrate that, in experimentally immunized mice, multiple pathogenic antibodies to TSHR can arise from a single clone by a series of somatic mutations in the V-region genes and may give an insight into how such antibodies develop spontaneously in autoimmune Graves' disease.

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

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

The thyroid-stimulating hormone receptor: impact of thyroid-stimulating hormone and thyroid-stimulating hormone receptor antibodies on multimerization, cleavage, and signaling

The thyroid-stimulating hormone receptor (TSHR) has a central role in thyrocyte function and is also one of the major autoantigens for the autoimmune thyroid diseases. We review the post-translational processing, multimerization, and intramolecular cleavage of TSHR, all of which may modulate its signal transduction. The recent characterization of monoclonal antibodies to the TSHR, including stimulating, blocking, and neutral antibodies, have also revealed unique biologic insights into receptor activation and the variety of these TSHR antibodies may help explain the multiple clinical phenotypes seen in autoimmune thyroid diseases. Knowledge of the structure/function relationship of the TSHR is beginning to provide a greater understanding of thyroid physiology and thyroid autoimmunity.

Toward better models of hyperthyroid Graves' disease

Graves' disease affects only humans. Although it is a treatable illness, medical therapy with antithyroid drugs is imperfect, showing high rates of recurrence. Furthermore, the etiology and treatment of the associated ophthalmopathy still represent problematic issues. Animal models could contribute to the solution of such problems by providing a better understanding of the underlying pathogenesis and could be used for evaluating novel therapeutic strategies. This article discusses the pursuit of a better experimental model for hyperthyroid Graves' disease and outlines how this research has clarified the immunology of the disease.

Evidence that shed thyrotropin receptor A subunits drive affinity maturation of autoantibodies causing Graves' disease

CONTEXT

In Graves' disease, thyroid-stimulating antibodies (TSAb) activate the TSH receptor (TSHR) causing hyperthyroidism. Serum polyclonal TSAb are difficult to study because of their extremely low serum levels.

OBJECTIVE

Our objective was to determine whether monoclonal TSAb possess characteristics previously reported for polyclonal autoantibodies in Graves' sera.

DESIGN

We studied monoclonal TSAb from three laboratories: six generated from mice with induced hyperthyroidism; and one, M22, a human autoantibody obtained from Graves' B cells.

RESULTS

All TSAb with one exception were potent activators of TSHR-mediated cAMP generation, with relatively similar half-maximal stimulatory concentrations. Like polyclonal autoantibodies, monoclonal TSAb were largely neutralized by conformationally "active" (but not "inactive") recombinant TSHR A subunits (the N-terminal cleavage product of the TSHR). Chimeric substitutions of TSHR amino acids 25-30 (the extreme N terminus after removal of the 21 residue signal peptide) abrogated the binding and function of all monoclonal TSAb but with one antibody (TSAb4) revealing a nonidentical epitope. Remarkably, these residues are uninvolved in the M22 epitope determined by x-ray analysis. Finally, flow-cytometric dose-response analyses, not previously possible with polyclonal TSAb, revealed that all monoclonal TSAb, human and murine, bound with lower affinity to their in vivo target, the TSH-holoreceptor, than to the isolated TSHR ectodomain.

CONCLUSIONS

TSAb function does not require antibodies with identical epitopes, and human autoantibody M22 may, therefore, not represent the full epitopic repertoire of polyclonal TSAb in Graves' disease. Most important, we provide strong evidence that the shed ectodomain (primarily the A subunit) is the primary antigen driving affinity maturation of TSAb producing B cells.

Characterization of thyrotropin receptor antibody-induced signaling cascades

The TSH receptor (TSHR) is constitutively active and is further enhanced by TSH ligand binding or by stimulating TSHR antibodies (TSHR-Abs) as seen in Graves' disease. TSH is known to activate the thyroid epithelial cell via both Galphas-cAMP/protein kinase A/ERK and Galphaq-Akt/protein kinase C coupled signaling networks. The recent development of monoclonal antibodies to the TSHR has enabled us to investigate the hypothesis that different TSHR-Abs may have unique signaling imprints that differ from TSH ligand itself. We have, therefore, performed sequential studies, using rat thyrocytes (FRTL-5, passages 5-20) as targets, to examine the signaling pathways activated by a series of monoclonal TSHR-Abs in comparison with TSH itself. Activation of key signaling molecules was estimated by specific immunoblots and/or enzyme immunoassays. Continuing constitutive TSHR activity in thyroid cells, deprived of TSH and serum for 48 h, was demonstrated by pathway-specific chemical inhibition. Under our experimental conditions, TSH ligand and TSHR-stimulating antibodies activated both Galphas and Galphaq effectors. Importantly, some TSHR-blocking and TSHR-neutral antibodies were also able to generate signals, influencing primarily the Galphaq effectors and induced cell proliferation. Most strikingly, antibodies that used the Galphaq cascades used c-Raf-ERK-p90RSK as a unique signaling cascade not activated by TSH. Our study demonstrated that individual TSHR-Abs had unique molecular signatures which resulted in sequential preferences. Because downstream thyroid cell signaling by the TSHR is both ligand dependent and independent, this may explain why TSHR-Abs are able to have variable influences on thyroid cell biology.

Implications of new monoclonal antibodies and the crystal structure of the TSH receptor for the treatment and management of thyroid diseases

Autoantibodies to the thyroid-stimulating hormone receptor (TSHR) cause the hyperthyroidism of Graves’ disease and contribute to Graves’ eye signs. Human monoclonal TSHR autoantibodies prepared from patients’ lymphocytes have important clinical applications in terms of their ability to stimulate TSHR-containing tissues. Also, TSHR monoclonal antibodies that act as antagonists may well be useful in treating Graves’ eye disease. Recently, the high-resolution (2.55 Å) crystal structure of the TSHR in complex with a monoclonal thyroid-stimulating autoantibody has been determined, and this provides key insights into how the autoantibodies interact with the receptor. Furthermore, the structure can be used in the rational design of small molecules that will disrupt receptor binding by thyroid-stimulating autoantibodies, thus providing new strategies to control TSHR activation in addition to monoclonal antibodies.

Application of new therapies in Graves' disease and thyroid-associated ophthalmopathy: animal models and translation to human clinical trials

Most current approaches for treating Graves' disease are based essentially upon regimes developed nearly 50 years ago. Moreover, therapeutic approaches for complications such as thyroid-associated ophthalmopathy (TAO) and dermopathy are singularly dependent on conventional approaches of nonspecific immunosuppression. The recent development of an induced model of experimental Graves' disease, although incomplete as it lacks the extrathyroidal manifestations, provided opportunities to investigate immune intervention strategies, including influence upon the autoreactive B and T cell players in the autoimmune process. These major advances are generating new possibilities for therapeutic interventions for patients with Graves' disease and TAO.

Identification of key amino acid residues in a thyrotropin receptor monoclonal antibody epitope provides insight into its inverse agonist and antagonist properties

CS-17 is a murine monoclonal antibody to the human TSH receptor (TSHR) with both inverse agonist and antagonist properties. Thus, in the absence of ligand, CS-17 reduces constitutive TSHR cAMP generation and also competes for TSH binding to the receptor. The present data indicate that for both of these functions, the monovalent CS-17 Fab (50 kDa) behaves identically to the intact, divalent IgG molecule (150 kDa). The surprising observation that CS-17 competes for TSH binding to the human but not porcine TSHR enabled identification of a number of amino acids in its epitope. Replacement of only three human TSHR residues (Y195, Q235, and S243) with the homologous porcine TSHR residues totally abolishes CS-17 binding as detected by flow cytometry. TSH binding is unaffected. Of these residues, Y195 is most important, with Q235 and S243 contributing to CS-17 binding to a much lesser degree. The functional effects of CS-17 IgG and Fab on constitutive cAMP generation by porcinized human TSHR confirm the CS-17 binding data. The location of TSHR amino acid residues Y195, Q235, and S243 deduced from the crystal structure of the FSH receptor leucine-rich domain provides valuable insight into the CS-17 and TSH binding sites. Whereas hormone ligands bind primarily to the concave surface of the leucine-rich domains, a major portion of the CS-17 epitope lies on the opposite convex surface with a minor component in close proximity to known TSH binding residues.

The link between Graves' disease and Hashimoto's thyroiditis: a role for regulatory T cells

Hyperthyroidism in Graves' disease is caused by thyroid-stimulating autoantibodies to the TSH receptor (TSHR), whereas hypothyroidism in Hashimoto's thyroiditis is associated with thyroid peroxidase and thyroglobulin autoantibodies. In some Graves' patients, thyroiditis becomes sufficiently extensive to cure the hyperthyroidism with resultant hypothyroidism. Factors determining the balance between these two diseases, the commonest organ-specific autoimmune diseases affecting humans, are unknown. Serendipitous findings in transgenic BALB/c mice, with the human TSHR A-subunit targeted to the thyroid, shed light on this relationship. Of three transgenic lines, two expressed high levels and one expressed low intrathyroidal A-subunit levels (Hi- and Lo-transgenics, respectively). Transgenics and wild-type littermates were depleted of T regulatory cells (Treg) using antibodies to CD25 (CD4(+) T cells) or CD122 (CD8(+) T cells) before TSHR-adenovirus immunization. Regardless of Treg depletion, high-expressor transgenics remained tolerant to A-subunit-adenovirus immunization (no TSHR antibodies and no hyperthyroidism). Tolerance was broken in low-transgenics, although TSHR antibody levels were lower than in wild-type littermates and no mice became hyperthyroid. Treg depletion before immunization did not significantly alter the TSHR antibody response. However, Treg depletion (particularly CD25) induced thyroid lymphocytic infiltrates in Lo-transgenics with transient or permanent hypothyroidism (low T(4), elevated TSH). Neither thyroid lymphocytic infiltration nor hypothyroidism developed in similarly treated wild-type littermates. Remarkably, lymphocytic infiltration was associated with intermolecular spreading of the TSHR antibody response to other self thyroid antigens, murine thyroid peroxidase and thyroglobulin. These data suggest a role for Treg in the natural progression of hyperthyroid Graves' disease to Hashimoto's thyroiditis and hypothyroidism in humans.

The thyrotropin receptor in Graves' disease

The application of molecular biology to the study of the thyrotropin receptor (TSHR) has led to major advances in our understanding of its structure, function, and relationship to the pathogenesis of Graves' disease. This review summarizes many of these features and also provides a personal perspective, questioning some assumptions and general concepts, as well as describing remaining challenges. Among the issues raised are the limits in our understanding of the spatial orientation of the structural domains of the TSHR, including the enigmatic hinge region. We review the phenomenon of TSHR intramolecular cleavage, the shedding of the A-subunit component of the ectodomain, and the importance of the latter in generating thyroid-stimulating antibodies. The epitopes of thyroid-stimulating and -blocking autoantibodies have been a confusing and controversial subject that requires review and evaluation of available data. Finally, we address the potential physiological or pathophysiological significance of TSHR multimerization in TSHR. Taken together, this review will, hopefully, convey the fascination and excitement that molecular biology has contributed to the study of the TSHR, especially as it relates to the pathogenesis of Graves' disease.

Chronic exposure in vivo to thyrotropin receptor stimulating monoclonal antibodies sustains high thyroxine levels and thyroid hyperplasia in thyroid autoimmunity-prone HLA-DRB1*0301 transgenic mice

We have examined the induction of autoimmunity and the maintenance of sustained hyperthyroidism in autoimmunity-prone human leucocyte antigen (HLA) DR3 transgenic non-obese diabetic (NOD) mice following chronic stimulation of the thyrotropin receptor (TSHR) by monoclonal thyroid-stimulating autoantibodies (TSAbs). Animals received weekly injections over the course of 9 weeks of monoclonal antibodies (mAbs) with strong thyroid-stimulating properties. Administration of the mAbs KSAb1 (IgG2b) or KSAb2 (IgG2a), which have similar stimulating properties but different TSH-binding blocking activity, resulted in significantly elevated serum thyroxine (T(4)) levels and thyroid hyperplasia. After the first injection, an initial surge then fall in serum T(4) levels was followed by sustained elevated levels with subsequent injections for at least 63 days. Examination of KSAb1 and KSAb2 serum bioactivity showed that the accumulation of the TSAbs in serum was related to their subclass half-lives. The thyroid glands were enlarged and histological examination showed hyperplastic follicles, with minimal accompanying thyroid inflammation. Our results show that chronic in vivo administration of mAbs with strong thyroid-stimulating activity resulted in elevated T(4) levels, suggesting persistent stimulation without receptor desensitization, giving a potential explanation for the sustained hyperthyroid status in patients with Graves' disease. Moreover, despite the presence of HLA disease susceptibility alleles and the autoimmune prone NOD background genes, chronic stimulation of the thyroid gland did not lead to immune cell-mediated follicular destruction, suggesting the persistence of immunoregulatory influences to suppress autoimmunity.

TSH receptor antibodies

The discovery of thyroid-stimulating autoantibodies by Adams and Purves 50 years ago was one of the most important observations in the history of thyroidology. Since that time, the thyroid-stimulating hormone receptor (TSHR) has been shown to be the antigen recognized by these autoantibodies (1974) and the receptor cloned (1989). More recently, different mouse monoclonal antibodies (MAbs) to the TSHR have been produced, culminating in 2002 in the preparation of mouse and hamster MAbs with strong thyroid-stimulating activity. Further, in 2003 a human MAb to the TSHR (M22) with the characteristics of patient thyroid-stimulating autoantibodies was described. M22 has been particularly useful in advancing our knowledge of the TSHR and TSHR autoimmunity, including the development of new assays for TSHR autoantibodies (2004) and determination of a high-resolution (2.55 A) crystal structure of the TSHR leucine-rich domain in combination with M22 (2007). The structure shows that M22 positions itself on the TSHR in an almost identical way to the native hormone TSH but the evolutionary forces that have resulted in production of a common autoantibody that mimics the actions of TSH so well are far from clear at this time. Very recently, a human MAb (5C9) with the characteristics of blocking-type patient serum TSHR autoantibodies has been isolated (2007). Studies on how 5C9 interacts with the TSHR at the molecular level are planned and should provide key insights as to the differences between TSHR autoantibodies with blocking and with stimulating activities. Also, 5C9 and similar MAbs have considerable potential as drugs to inhibit TSHR stimulation by autoantibodies. Further, now the M22-TSHR structure is known at the atomic level, rational design of specific low-molecular-weight inhibitors of the TSHR-TSHR autoantibody interaction is feasible.

Suppression of thyrotropin receptor constitutive activity by a monoclonal antibody with inverse agonist activity

TSH binding to the TSH receptor (TSHR) induces thyrocyte growth and proliferation primarily by activating the adenylyl cyclase signaling pathway. Relative to the other glycoprotein hormone receptors, the TSHR has considerable ligand-independent (constitutive) activity. We describe a TSHR monoclonal antibody (CS-17) with the previously unrecognized property of being an inverse agonist for TSHR constitutive activity. This property is retained, even when constitutive activity is extremely high consequent to diverse TSHR extracellular region mutations. A similar effect on an activating mutation at the base of the sixth transmembrane helix (not accessible to direct CS-17 contact) indicates that CS-17 is acting allosterically. Administered to mice in vivo, CS-17 reduces serum T(4) levels. The CS-17 epitope is conformational and a significant portion lies in the C-terminal region of the TSHR leucine-rich domain (residues 260-289). By interacting with the large TSHR extracellular domain, CS-17 is, to our knowledge, the first antibody reported to be an inverse agonist for a member of the G protein receptor superfamily. After humanization of its murine constant region, CS-17 has the potential to be an adjunctive therapeutic agent in athyreotic patients with residual well-differentiated thyroid carcinoma as well as pending definitive treatment in some selected hyperthyroidism states.