Autoantibodies interacting with purified native thyrotropin receptor

Longitudinal Characterization of Autoantibodies to the Thyrotropin Receptor (TRAb) During Alemtuzumab Therapy: Evidence that TRAb May Precede Thyroid Dysfunction by Many Years

BACKGROUND

Thyroid autoimmunity, especially Graves' disease or hypothyroidism with positive autoantibodies (TRAb) to the thyrotropin receptor (TSHR), occurs in 30-40% of patients with relapsing multiple sclerosis following treatment with alemtuzumab (ALTZ). ALTZ therapy therefore provides a unique opportunity to study the evolution of TRAb prior to clinical presentation. TRAb can stimulate (TSAb), block (TBAb), or not affect ("neutral") the TSHR function, causing hyperthyroidism, hypothyroidism, or euthyroidism, respectively.

METHODS

A longitudinal retrospective analysis was conducted of TRAb bioactivity over a period of nine years in 45 multiple sclerosis patients receiving ALTZ using available stored serum. Of these 45 patients, 31 developed thyroid dysfunction (TD) and 14 remained euthyroid despite being followed for a minimum of five years (NO-TD). The presence of TRAb was evaluated at standardized time points: (i) before ALTZ, (ii) latest time available following ALTZ and before TD onset, and (iii) following ALTZ during/after TD onset. Serum TRAb were detected by published in-house assays (ihTRAb): flow cytometry detecting any TSHR-binding TRAb, and luciferase bioassays detecting TSAb/TBAb bioactivity. Purified immunoglobulin G was used to verify TSAb/TBAb in selected hypothyroid cases. Standard clinical automated measurements of TRAb, antithyroid peroxidase autoantibodies (TPOAb), thyrotropin, free thyroxine, and free triiodothyronine were also collected.

RESULTS

Before ALTZ, combined ihTRAb (positive with flow cytometry and/or luciferase bioassay) but not automated TRAb were present in 5/16 (31.2%) TD versus 0/14 (0%) NO-TD (p = 0.017). Detectable ihTRAb preceded TD development in 9/28 (32.1%) and by a median of 1.2 years (range 28 days-7.3 years). Combination testing of ihTRAb and TPOAb at baseline predicted 20% of subsequent cases of hyperthyroidism and 83% of hypothyroidism.

CONCLUSIONS

Evidence is presented that TRAb measured with custom-made assays can be detected prior to any change in thyroid function in up to a third of cases of ALTZ-related TD. Furthermore, the presence of ihTRAb prior to ALTZ treatment was strongly predictive of subsequent TD. The findings suggest that a period of affinity maturation of TRAb may precede clinical disease onset in some cases. Combined testing of TPOAb and ihTRAb may increase the ability to predict those who will develop TD following ALTZ.

The role of thyrotrophin receptor antibody assays in graves' disease

Thyrotrophin receptor antibodies (TRAb) exist as stimulating or blocking antibodies in the serum (neutral TRAb have been identified recently). The clinical features of GD occur when stimulating TRAb predominate. But the relationship of TRAb to clinical phenotype and outcome is not clear when current assay methods are used. Therefore no consensus exists about its utility in diagnosing and predicting outcome in GD. The most commonly used TRAb assays, measure thyroid binding inhibiting immunoglobulins (TBII or “receptor assays”) and don't differentiate between stimulating and blocking antibodies. However, the more expensive, technically demanding and less freely available “biological assays” differentiate between them by their ability to stimulate cyclic AMP or failure to do so. Failure to differentiate between TRAb types and its heterogeneous molecular and functional properties has limited TBII use to GD diagnosis and differentiating from other forms of thyrotoxicosis. The current 2nd-3rd generation receptor assays are highly sensitive and specific when used for this purpose. TRAb assays should also be done in appropriate pregnant women. Current data do not support its use in outcome prediction as there is a significant variability of assay methodology, population characteristics and study design in published data, resulting in a lack of consensus.

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.

TSH receptor autoantibodies

Thyrotropin receptor autoantibodies (TSHR-Abs) of the stimulating variety are the hallmark of Graves' disease. The presence of immune defects leading to synthesis of TSHR-Abs causes hyperthyroidism and is associated with other extrathyroidal manifestations. Further characterization of these antibodies has now been made possible by the generation of monoclonal antibodies with this unique stimulating capacity as well as similar TSHR-Abs not associated with hyperthyroidism. Their present classification divides TSHR-Abs into stimulating, blocking (competing with TSH binding) and neutral (no signaling). Recent studies using monoclonal TSHR-Abs has revealed that stimulating and blocking antibodies bind to the receptor using mostly conformational epitopes, whilst neutral antibodies utilize exclusively linear peptides. Subtle differences in epitopes for stimulating and blocking antibodies account for the diversity of their biological actions. Recently non-classical signaling elicited by neutral antibodies has also been described, raising the need for a new classification of TSHR-Abs.

A DNA vaccine for multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a disease in which safety is of paramount importance when developing a potential therapeutic. Antigen-specific treatments provide a method for achieving efficacy while maintaining safety. DNA vaccines are one such form of treatment that have been tested in clinical trials

OBJECTIVE

To determine if a DNA vaccine is a viable method of antigen-specific treatment of MS.

RESULTS/CONCLUSION

Phase I and II trials of BHT-3009, a DNA vaccine encoding myelin basic protein, demonstrated that it was safe, well-tolerated, and caused antigen-specific immune tolerance. BHT-3009 showed efficacy in reducing brain lesion activity as well as clinical relapses in patients that were immunologically active at baseline. BHT-3009 is a promising therapy in development for MS, and may prove to be one of the first antigen-specific treatments for this disease.

Technical evaluation of the first fully automated assay for the detection of TSH receptor autoantibodies

BACKGROUND

Graves' disease (GD) is mediated by autoantibodies which bind to the TSH receptor (TRAb). The aim of the present study was to evaluate the technical performance of the first fully automated immunoassay for TRAb detection.

METHODS

The Elecsys Anti-TSHR immunoassay utilizes a porcine TSH receptor (TSHR) and the human thyroid stimulating monoclonal TSHR autoantibody M22.

RESULTS

Intraassay and total imprecision CV were determined between 1.4%-14.9%, and 2.4%-28.8%, respectively. Using the 20% CV criteria the functional sensitivity was found at 0.73 IU/L. The median CV at the cut-off (1.75 IU/L) was found to be 11%. Comparison studies with five TRAb immunoassays yielded slopes and intercepts between 1.02-1.48, and -0.74-0.56, respectively. Correlation coefficients were determined between 0.895 and 0.978. ROC plot analysis of patients with GD, patients with other thyroid disorders and healthy controls revealed an AUC of 0.99 resulting in a sensitivity of 97% and a specificity of 99% at a TRAb level of 1.75 IU/L.

CONCLUSION

The evaluation of the TRAb immunoassay generated homogeneous performance data and demonstrated a high degree of comparability to established TRAb assays. The automated TRAb assay represents a major improvement of thyroid testing in clinical practice.

Estimation of serum TSH receptor autoantibody concentration and affinity

We have used the human monoclonal TSH receptor (TSHR) autoantibody (M22) as a labeled ligand in competition with individual patient TSHR autoantibodies (TRAb) to estimate their serum concentrations and affinities. TSHR coated tubes, (125)I-labeled M22 IgG and Fab, and patient sera IgG and Fab were used in these studies. In 15 patients with Graves' disease, TRAb concentrations ranged from 50 to 500 ng/mL of serum (5- 60 parts per million of total serum IgG) and TRAb IgG affinities from 3.0 +/- 1.0-6.7 +/- 1.54-10(10) L/mol (mean +/- SD; n=3). Fab fragment affinities were similar to those of intact IgG. Serum TRAb with blocking (TSH antagonist; 4 patients) activity had similar affinities (3.0 +/- 0.25-7.2 +/- 2.2-10(10) L/mol) to TRAb IgG from patients with Graves' disease, but blocking TRAb concentrations were higher (1.7 - 27 mg/mL of serum). The concentrations of TRAb that we observed in the sera of the 15 Graves' patient (0.33 - 3.3 nmol/L) can be compared with that of circulating TSH. In particular, a serum TSH concentration of 100mU/L (0.7 nmol/L) is in the same range as the concentrations of TRAb we observed. Such a TSH concentration (similar to that observed after injection of 0.9 mg of recombinant human TSH) would be expected to cause a similar degree of thyrotoxicosis as seen in Graves' disease. Consequently, the thyroid-stimulating potencies (i.e., activity per mol) of patient serum TRAb and human TSH appear to be of a similar magnitude in vivo as well as in vitro. Overall, our results indicate that serum TRAb affinities are high and show only limited variations between different sera whereas concentrations of the autoantibodies vary widely.

Monoclonal antibodies to the thyrotropin receptor

The thyrotropin receptor (TSHR) is a seven transmembrane G-protein linked glycoprotein expressed on the thyroid cell surface and which, under the regulation of TSH, controls the production and secretion of thyroid hormone from the thyroid gland. This membrane protein is also a major target antigen in the autoimmune thyroid diseases. In Graves' disease, autoantibodies to the TSHR (TSHR-Abs) stimulate the TSHR to produce thyroid hormone excessively. In autoimmune thyroid failure, some patients exhibit TSHR-Abs which block TSH action on the receptor. There have been many attempts to generate human stimulating TSHR-mAbs, but to date, only one pathologically relevant human stimulating TSHR-mAb has been isolated. Most mAbs to the TSHR have been derived from rodents immunized with TSHR antigen from bacteria or insect cells. These antigens lacked the native conformation of the TSHR and the resulting mAbs were exclusively blocking or neutral TSHR-mAbs. However, mAbs raised against intact native TSHR antigen have included stimulating mAbs. One such stimulating mAb has demonstrated a number of differences in its regulation of TSHR post-translational processing. These differences are likely to be reflective of TSHR-Abs seen in Graves' disease.

Thyrotropin receptor trafficking relies on the hScrib-betaPIX-GIT1-ARF6 pathway

G protein-coupled receptors are regulated by ligand stimulation, endocytosis, degradation of recycling to the cell surface. Little information is available on the molecular mechanisms underlying G protein-coupled receptors recycling. We have investigated recycling of the G protein-coupled thyroid stimulating hormone receptor (TSHR) and found that it relies on hScrib, a membrane-associated PDZ protein. hScrib directly binds to TSHR, inhibits basal receptor endocytosis and promotes recycling, and thus TSHR signalling, at the cell membrane. We previously demonstrated that hScrib is associated with a betaPIX-GIT1 complex comprised of a guanine nucleotide exchange factor and a GTPase-activating protein for ADP ribosylation factors that is involved in vesicle trafficking. We used dominant-negative constructs and small interfering RNA to show that TSHR recycling is regulated by the interaction between hScrib and betaPIX, and by the activity of GIT1. In addition, ARF6, a major target for GIT1, is activated during TSH stimulation of HEK293 and FRTL-5 thyroid cells, and plays a key role in TSHR recycling. Thus, we have uncovered an hScrib-betaPIX-GIT1-ARF6 pathway devoted to TSHR trafficking and function.

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.

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.

Antibody-mediated gastrointestinal dysmotility in scleroderma

BACKGROUND & AIMS

Defects in enteric excitatory neurotransmission have been proposed to underlie the gastrointestinal dysmotility associated with scleroderma (systemic sclerosis). This study investigated whether patients with scleroderma produce antibodies that inhibit M3-muscarinic or neurokinin receptor-mediated intestinal contractions, either directly or via an effect on L-type voltage-gated calcium channels (VGCCs).

METHODS

Responses of mouse colon longitudinal muscle to stimulation by the muscarinic agonist carbachol (1-300 micromol/L) and neurokinin-1 and -2 receptor agonists were measured in the absence and presence of serum (2%) or immunoglobulin G (IgG) (0.3-1.0 mg/mL) from patients with scleroderma, those with other autoimmune disorders, and healthy controls. The role of L-type VGCCs in carbachol- and tachykinin-evoked contractions was assessed using nicardipine.

RESULTS

M3-muscarinic receptor-mediated contractions were inhibited by Ig fractions from 7 of 9 patients with scleroderma (limited and diffuse forms), 4 of 4 patients with primary Sjögren's syndrome, and 3 of 3 patients with secondary Sjögren's syndrome. Ig fractions from healthy controls did not inhibit the M3-muscarinic receptor-mediated contractions. Inhibition by Ig was concentration-dependent; a maximum inhibition of approximately 40% occurred at 0.6 mg/mL IgG. Both M3-muscarinic and neurokinin receptor-mediated contractions were L-type VGCC dependent. Patient sera had no effect on responses to neurokinin receptor stimulation, demonstrating the lack of antibodies inhibiting L-type VGCCs.

CONCLUSIONS

Functional antibodies specifically inhibiting M3-muscarinic receptor-mediated enteric cholinergic neurotransmission may provide a pathogenic mechanism for the gastrointestinal dysfunction seen in patients with scleroderma.

TSH receptor interaction with the extracellular matrix: role on constitutive activity and sensitivity to hormonal stimulation

Using immunocytochemistry, we have observed that the TSH receptor (TSHR) is concentrated at the leading edge of lamellipodia in both cultured human thyroid cells and in various transfected cells. This segregation of the receptor is due to its interaction with extracellular matrix (ECM) and specially with fibronectin. The TSHR, which interacts with the ECM, is known to undergo cleavage by a matrix metalloprotease. The homologous LH receptor, which does not interact with ECM, is not cleaved. The attachment to the ECM modifies the functional properties of the receptor: it increases adenylate cyclase stimulation by hormone, whereas PLC stimulation is not modified. Furthermore, the constitutive activity of the TSHR is only observed in attached cells, suggesting that it is dependent on TSHR interaction with the ECM. Thus, aside from its classical properties of hormone binding and signalization through G proteins, the TSHR is also involved in cell-matrix interactions, which modulate its functional properties.

Demonstration of immunoglobulin G, A, and E autoantibodies to the human thyrotropin receptor using flow cytometry

Human TSH receptor (TSHR) autoantibodies with biological activity result in thyroid dysfunction, but antibodies that simply bind do not. We have applied flow cytometry to the measurements of IgG, IgA, and IgE immunoreactivity to the TSHR in patients with Graves' disease (GD) and thyroid eye disease (TED) and in normal controls. CHO cells stably expressing the extracellular domain of the TSHR with a glycophosphatidylinositol anchor were produced and found to express approximately 4 times as many receptors, but of similar affinity, as JP09 in TSH binding studies. Substantial increases in median fluorescence and peak channel fluorescence were obtained by flow cytometry using TSHR monoclonal antibodies on the glycophosphatidylinositol cells. IgG autoantibodies were demonstrated in 55 of 65 untreated GD patients, 3 of 25 normal subjects, and 4 of 8 atypical TED sera (negative for TSHR autoantibodies with biological activity) by flow cytometry and correlated poorly with thyroid-stimulating antibodies. IgA antibodies were present in 1 of 12 normal, 1 of 7 treated GD with TED, and 3 of 8 atypical TED sera. IgE binding was observed in 1 of 12 normal, 2 of 8 treated GD without TED, 1 of 6 treated GD with TED, and 0 of 8 atypical TED sera. In conclusion, we have demonstrated autoantibodies that bind directly to the TSHR in the majority of GD patients and in 50% of patients with atypical TED and a small number of normal controls lacking TSHR antibodies that affect function. Although predominantly IgG lambda, TSHR autoantibodies of the IgA and IgE isotypes are also detectable.

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.

Reactivity of thyrotropin receptor autoantibodies with the thyrotropin receptor on western blots

Affinity purified recombinant human thyrotropin receptor (TSHR) was run on sodium dodecyl sulfate (SDS) gels and subjected to a renaturing and blotting procedure. Twenty sera from thyrotropin receptor autoantibodies (TRAb)-positive patients with a history of hyperthyroidism and 20 sera with high levels of TSH blocking activity were analyzed. Four of 20 sera with blocking-type of TRAb (i.e., TSH antagonist activity) were able to recognize the mature, fully glycosylated 120-kd form of the receptor on blots of gels run under reducing conditions. No sera recognized the 100-kd high mannose precursor form of the TSHR. Three of the four recognized a 74-kd band and 2 of the 4 recognized a 50-kd band. These bands are probably proteolytic cleavage fragments of the mature 120-kd TSHR. In the absence of reducing agent the same 4 of 20 sera described above together with a further serum sample (i.e., 5/20 in total) reacted with the 120-kd form of the receptor. No specific reaction with the TSHR was observed on Western blots with the remaining 15 sera with TSH blocking activity, nor with 20 sera from patients with a history of hyperthyroidism, nor with sera from 10 healthy blood donors, 10 Hashimoto sera (negative for TRAb) and 10 systemic lupus erythematosus sera. No clear differences were observed in the TRAb positive sera that were reactive and nonreactive on Western blots in terms of their ability to inhibit TSH binding or to immunoprecipitate 125I-labeled TSHR. Overall, our results indicate that the mature 120-kd form of the TSHR that is principally responsible for binding TSH is also responsible for binding TRAb (when this binding can be detected). These observations together with immunoprecipitation and TSH binding inhibition studies, emphasize the close relationship between the receptor's binding sites for TSH and TRAb.