The structure of thyroid autoantigens

Clinical review about TRAb assay's history

Commercial assays to measure thyroid stimulating hormone (TSH) receptor (TSHR) autoantibodies (TRAb) have been available for the serological diagnosis of autoimmune thyroid diseases (AITD) for several years. The widespread assessment of this parameter has identified Graves' disease (GD) as a common organ-specific autoimmune disease. Within the present article we aim to review immunobiological and epidemiological aspects as well as diagnostic methods available for the detection of TRAb. Over the last decade, TRAb detection in GD became more sensitive since TRAb assays were being largely improved by named research groups. Therefore, functional assay (fas) and diagnostic sensitivity of current TRAb assays will be discussed. Within the second part of this review we will focus on clinical applications of TRAb measurement for outcome prediction of GD as well as the importance of this method to distinguish GD from other AITD.

Point-of-care assays for autoantibodies to thyroid peroxidase and to thyroglobulin

Point-of-care (POC) assays for autoantibodies to thyroid peroxidase (TPOAb) and to thyroglobulin (TgAb) are described. Both assays are based on the ability of autoantibodies in test samples (whole blood, plasma, or sera) to inhibit the binding of monoclonal antibodies to TPO or to Tg. The assays require no special equipment and give results in 10 minutes. Analysis of samples from healthy blood donors (n = 80), patients with autoimmune thyroid disease (n = 97) and nonthyroid autoimmune diseases (n = 20) showed that results with the POC tests compared well to those obtained by agglutination assay and enzyme-linked immunosorbent assay (ELISA). The reference immunoprecipitation assays (IPA) based on 125I-labeled TPO or Tg were more sensitive than the POC tests particularly in the case of TgAb measurements. However, no samples were found positive by POC test and negative by IPA emphasizing the high specificity of the POC assays. Our results suggest that POC testing for TPOAb and TgAb with assays such as those we describe could be useful in certain situations. These include prediction of postpartum thyroiditis and the development of interferon-alpha-related thyroid disease.

Thyroid autoantibodies

The characteristics of thyroid autoantibodies are reviewed and new assays for the autoantibodies described, in particular point of care (POC) tests for thyroid peroxidase (TPO) autoantibodies and for thyroglobulin (Tg) autoantibodies. These POC tests depend on the ability of the autoantibodies to inhibit gold labelled human monoclonal antibodies binding to TPO or to Tg. The POC tests show similar sensitivity and specificity to conventional ELISA for the autoantibodies. A new ELISA to measure autoantibodies to the TSH receptor (TRAb) is described, is based on TSH receptor coated onto plate wells by way of a monoclonal antibody. Comparison of porcine and human TSH receptors indicates that there is no advantage in using human TSHR in assay systems based on competition between TRAb and bovine or porcine TSH for immobilised TSHR. In terms of the origins of Graves' disease, it is speculated that this most common overt autoimmune disease in man might have occurred first when Homo sapiens sapiens migrated rapidly out of Africa about 100,000 years ago.

A new assay for thyroglobulin concentration in serum using monoclonal antibodies against synthetic peptides

The concentration of thyroglobulin (Tg) measured by radioimmunoassay (RIA) or enzyme-linked immunosorbent assay (ELISA) is greatly affected by the presence of anti-Tg autoantibodies in sera. We developed a new assay for detecting Tg in the presence of high concentrations of anti-Tg autoantibodies. A 48-kDa fragment was purified from Tg after treatment with V8 protease. This fragment did not appear to bind to two types of monoclonal antibodies (57Ab and 28D3) against a peptide in the C-terminus (amino acids 2735-2748) of Tg and intact Tg, respectively, by ELISA and Western blot analysis. In contrast, anti-Tg autoantibody or anti-Tg polyclonal antibody reacted well with this fragment. Our new ELISA used 57Ab as a solid phase antibody and 28D3 as a antibody conjugated to horseradish peroxidase. Buffer containing purified 48-kDa fragment was used to neutralize autoantibodies against Tg. With this assay, the recovery of Tg was 84.0-89.6% in normal healthy donors (n=5) in the presence of immunoglobulin G (IgG) purified from sera positive for anti-Tg autoantibody, and 76.2-104.4% in patient sera Grave's disease (n=15). Furthermore, the Tg concentrations in sera from patients with Grave's disease (n=20) ranged from 25 to 526 ng/ml, even though the Tg concentration, as measured by a commercial RIA did not exceed 55 ng/ml. There was good agreement between Tg concentrations measured by new Tg-ELISA and commercial Tg-RIA in sera that were negative for anti-Tg autoantibody. Overall, our new ELISA containing a Tg fragment to neutralize the presence of autoantibodies, showed good sensitivity and precision, and may be useful for routine use. Further investigations with the new assay should allow wider assessment of the prevalence and pattern of thyroid autoimmunity or thyroid neoplasms.

The immune response to the iodide transporter

In addition to physiologic, diagnostic, and therapeutic implications, the recently cloned and characterized sodium iodide symporter (NIS) also may play an important role in the pathogenesis of autoimmune thyroid disease. Sodium iodide symporter expression patterns characteristically are changed in autoimmune thyroid disease, including Graves' disease and Hashimoto's thyroiditis, which may be caused, in part, by the regulation of sodium iodide symporter expression of cytokines involved in the pathogenesis of autoimmune thyroid disease. Further, there is increasing evidence that NIS-directed antibodies are present in sera from patients with autoimmune thyroid disease, and these antibodies also may affect NIS functional activity.

The pathogenesis of Graves' disease

Graves' disease, one of the autoimmune thyroid diseases, is caused by the production of IgG autoantibodies directed against the thyrotropin receptor. These antibodies bind to and activate the receptor, causing the autonomous production of thyroid hormones. Despite recent improvements in our understanding of the cellular and molecular basis of autoimmunity, our currently available treatments for Graves' disease have remained largely unchanged over the last 50 years. Nevertheless, new concepts in immune system regulation hold out the prospect in the future for intervention designed to modify, and possibly cure, the underlying disease process.

Binding of immunoglobulin G from patients with autoimmune thyroid disease to rat sodium-iodide symporter peptides: evidence for the iodide transporter as an autoantigen

The recent cloning of the rat sodium-iodide symporter (rNIS) from FRTL-5 cells makes possible studies of the role of this thyroid-specific protein as an antigen in autoimmune diseases of the thyroid (AITD). We generated 21 synthetic peptides replicating the entire sequence of the extramembranous domains (ExMD) of rNIS. Each was synthesized by automated chemistry, purified by high-pressure liquid chromatography (HPLC), and characterized by mass spectroscopy. Immunoglobulins were purified using protein A from serum of 27 patients with Graves' disease (GD), 27 patients with autoimmune hypothyroidism (HT), and 20 normal controls. Binding of IgG from patients and controls to each of the rNIS peptides was measured by enzyme-linked immunosorbent assay (ELISA). Binding of patient IgG significantly greater than control was observed with six peptides: peptide 262-280 (representing ExMD 8 between transmembrane [TM] domains VII and VIII), peptide 437-444 (ExMD 11), peptides 468-487, 483-602, and 498-517 from ExMD 12, and peptides 560-579 from the proximal portion of the carboxyl terminus (ExMD 13). 63% of GD patients and 26% of HT patients immunoglobulin G (IgG) bound peptide 498-517 compared to zero controls. Similarly, 59% of GD were positive against peptide 468-487 versus zero controls. Peptide 262-280 bound IgG from 44% of GD patients, 15% of HT patients, and none of the controls. The remaining peptides showed little or no binding of patient IgG. These data indicate that patients with GD and HT possess antibodies that recognize rNIS significantly greater than do normal individuals, suggesting that the iodide transporter represents an important autoantigen in AITD. They further suggest that the incidence of the antibodies is higher in GD than HT, and that the antigenic epitopes involve ExMD 8, 11, 12, and 13.

Variability of serum thyroglobulin levels is determined by a major gene

OBJECTIVE

There are large variations in the circulating concentrations of thyroglobulin. The purpose of this study was to explore the possibility of a genetic basis for the variability of serum concentration of thyroglobulin (Tg) in euthyroid individuals.

DESIGN

The serum concentration of thyroglobulin (Tg) varies several-fold in euthyroid individuals. Other circulating proteins also show wide normal ranges of concentration and these variations have been shown to have a genetic as well as an environmental basis. To explore the possibility of a genetic basis for variability in serum Tg levels, an analysis was made of serum Tg levels in 44 pairs of identical twins and 66 nuclear families who were euthyroid and thyroid autoantibody negative (thereby eliminating subclinical autoimmune thyroid disease and Tg autoantibody interference with the Tg assay).

RESULTS

Each pair of identical twins tended to have a similar Tg level and the overall correlation was highly significant (r = 0.734, P < 0.001). There was no relation between Tg and TSH levels in the twins (r = 0.119; P = 0.366). Segregation analysis of the 66 families showed that where both parents had Tg levels above the overall median for the subjects (males, 19 micrograms/l; females, 33 micrograms/l), 73% of the offspring also had concentrations above these levels, compared with 30% of the offspring when one parent had a high Tg level and only 16% in families where neither parent had a high Tg level.

CONCLUSIONS

Complex segregation analysis using the computer program Pointer suggested that variability in Tg levels was the result of a major dominant-like gene effect (accounting for 80% of the variability) combined with a multifactorial component. Thyroglobulin, a template for thyroid hormone production, is also a major thyroid autoantigen and inherited variations in serum Tg levels may have implications for the pathogenesis of autoimmune thyroid disease.

Identification of epitopes and affinity purification of thyroid stimulating auto-antibodies using synthetic human TSH receptor peptides

We prepared a series of overlapping peptides (29 in total, 20 amino acids each) containing the sequence of the entire extracellular domain of the human TSH receptor. Three peptides (181-200, 376-394, and EC3 (629-639)) bound IgG from patients with Graves' disease in an enzyme linked immunoassay. Peptide 181-200 bound IgG from 9 of 10, EC3 from 8 of 10, and 376-394 from 6 of 10 patients respectively, compared to 0 of 9 controls. We affinity purified TSHr auto-antibodies from four Graves' patients using the three above noted peptides bound to epoxy-activated sepharose. Thyroid stimulating activity was enriched in the bound fraction from at least two of the three peptide affinity columns in each of the four patients, although the pattern of affinity enrichment differed between patients. One patient was found to possess a combination of stimulatory and inhibitory TSHr antibodies and, after affinity purification, the anti-376-394 and anti-EC3 fractions were enriched in stimulatory activity, suggesting that those regions of the receptor were epitopes for stimulatory antibodies. However, affinity purification against peptide 181-200 produced an IgG preparation that was not stimulatory, but was a potent thyroid inhibitor. Thus, we have not only partially purified TSHr auto-antibodies, but also successfully separated stimulatory and inhibitory antibodies from a single patient using combination TSHr peptide affinity.

Cloning and characterisation of TPO autoantibodies using combinatorial phage display libraries

Thyroid lymphocyte RNA from a Hashimoto patient with high serum levels of autoantibodies to thyroid peroxidase (TPO) was used to construct a phage display antibody library in the phagemid vector pComb3. The library (100,000cfu) encoded IgG1 heavy chains together with kappa light chains. Selection of the phages displaying TPO antibody on TPO-coated ELISA plates yielded a phage population enriched for surface expression of TPO antibody Fabs. 3 different Fabs specific for TPO were subsequently isolated with affinities in the region of 10(9) molar-1. 2 of the Fabs recognised the same, or closely related, epitopes on TPO whereas the third Fab recognised a different epitope. These 2 epitopes were recognised by TPO autoantibodies in the serum of the lymphocyte donor and a series of 10 patient sera. Available sequence data showed that several non-self antibodies and non-thyroid autoantibodies use the same V kappa and VH germline genes as TPO autoantibodies. There appeared to be no clear relationship between gene sequence or gene family usage by TPO autoantibodies of the same or similar epitope specificity.

Is TPO detectable in the circulation?

Recent reports have suggested that thyroid peroxidase (TPO) can be detected in the circulation of normal subjects and of patients with Graves' disease and we have attempted to confirm and extend these observations. A TPO radioimmunoassay with a sensitivity of 1 ng/mL was used to measure TPO in the sera from 20 normal subjects and 21 patients with Graves' disease. In addition, TPO was measured in serum samples from six normal subjects before and after oral TRH. We were unable to detect TPO in 46 out of the 47 sera studied (normals and autoimmune thyroid disease). In the one remaining serum (from a normal subject), low levels of TPO were apparently detected, but we demonstrated that this result was due to assay interference from TPO autoantibodies. Overall our studies suggest that (1) thyroid peroxidase is not detectable in normal subjects nor in TPO autoantibody negative patients with Graves' disease; (2) endogenous TPO autoantibodies can interfere in the TPO radioimmunoassay leading to false positive results; and (3) an acute increase of TSH in normal subjects does not result in TPO release into the circulation.

Genetic linkage analysis of thyroid autoantibodies

Segregation analysis has suggested that the inheritance of thyroid autoantibodies (to thyroglobulin and to thyroid peroxidase) is a dominant Mendelian trait. In this study we describe an attempt to find the chromosomal location(s) of gene(s) responsible for thyroid autoantibody production. We have examined a number of restriction length polymorphisms (RFLPs) and highly polymorphic markers (mini- and microsatellite) for genetic linkage with thyroid autoantibodies using a panel of 16 families with autoimmune thyroid disease. None of the markers used in this study gave evidence of linkage, however minisatellite markers (MS1, MS31, MS32, MS43a, M851, G3) for TPO antibody, minisatellite markers (MS1, MS32, MS43a, MS51, G3) for Tg antibody, and all microsatellite markers used, provided evidence for exclusion of genetic linkage.

Production and characterisation of a human monoclonal thyroid peroxidase autoantibody

A human-mouse hybridoma has been produced by fusion of Hashimoto thyroid lymphocytes with the mouse myeloma line X63-Ag8.653. The cloned hybridoma secreted 2.5 micrograms per 10(6) cells per day of an IgG kappa thyroid peroxidase (TPO) autoantibody (2G4) with high affinity (2.5 x 10(9) molar-1) and specificity for human TPO. 2G4 did not react with lactoperoxidase, horseradish peroxidase or human myeloperoxidase or with porcine TPO or with human thyroglobulin. Plastic tubes coated with 2G4 bound about 50% of 125I-labelled human TPO added and the binding was inhibited by IgGs prepared from 18/18 TPO autoantibody-positive sera. This indicated that all 18 sera contained autoantibodies which recognised the same (or closely related) epitope as 2G4. Plastic tubes coated with IgGs from different TPO autoantibody-positive patient sera also bound 125I-labelled TPO but inhibition by 2G4 in this system was not complete. This suggested that the sera contained at least 2 types of TPO autoantibodies, with only one type of autoantibody reactive with the same epitope as 2G4.

Use of recombinant epitopes to study the heterogeneous nature of the autoantibodies against thyroid peroxidase in autoimmune thyroid disease

Microsomal antigen is often recognized by the sera from patients with autoimmune thyroid disease (AITD). Human thyroid peroxidase (hTPO) is the main component of this antigen. In a previous study, we expressed hTPO cDNA as fusion proteins in prokaryotic vector; we thereby defined seven antigenic peptides by using two rabbit polyclonal anti-hTPO antibodies. In the present study we used the seven epitopes and three widened peptides to define the reactivity pattern of 61 sera from patients with AITD. Thirty-eight of them reacted against at least one of the seven hTPO-restricted epitopes; 14 were negative against the seven determinants but recognized one or two of the extended peptides. Thus, the antibody response against hTPO appeared to be highly heterogeneous in AITD patient sera. Moreover, we demonstrated that the immunodetection of the hTPO on Western blotting with deoxycholate solubilized microsomes can be perfectly correlated with the recognition of one of the epitopes in the region 554-735.

Anti-human thyroid peroxidase and anti-human thyroglobulin antibodies present no cross-reactivity on recombinant peptides

Thyroglobulin (Tg) and thyroid peroxidase (TPO) are two antigens largely recognized by the sera from patients with autoimmune thyroid disease (AITD). Recently, the complete mapping of both antigens was established with rabbit polyclonal antibodies by the use of recombinant proteins expressed in prokaryotic vector. Several investigators have argued for the existence of a cross-reactivity of some hetero- and autologous antibodies versus these two proteins. In the present study, using rabbit polyclonal antibody, mouse polyclonal antibody and autoimmune antibody (aAb), we observed no common epitope on human Tg (hTg) and human TPO (hTPO).

The microsomal/peroxidase antigen: modulation of its expression in thyroid cells

Evidence has accumulated in the last few years that the expression of the microsomal/peroxidase antigen (M/TPO-Ag) in thyroid cells is induced by TSH, through pathways which involve intracellular cAMP accumulation and protein synthesis. These data have been found true in any thyroid system studied so far, both in terms of immunologic and enzymatic activity of TPO. TSH and cAMP also increase the levels of the specific mRNA for TPO in thyroid cells from different species. Whether this phenomenon is due to a direct transcriptional regulation of the TPO gene, as shown in dog thyroid cells, or to posttranscriptional effects, as it would appear in FRTL-5 cells, remains to be clarified by future experiments. Thyroid stimulating antibody (TSAb) of Graves' disease also stimulates the expression of M/TPO-Ag. This finding gives further support to the relevance of TSAb in the pathogenesis of hyperthyroidism and explains the well known observation that the "microsomal" antigen is particularly abundant in glands of Graves' patients. The modulation of M/TPO-Ag surface expression by TSH can explain the decrease of circulating anti-MAb observed during L-thyroxine therapy in hypothyroid patients with Hashimoto's thyroiditis. Other agents, such as methimazole and sodium iodide, which influence thyroid cell function, do not directly interfere with the expression of M/TPO-Ag. Cytokines, such as gamma-interferon, interleukin-1, and interleukin-6 have been shown to inhibit the TSH-induced increase of TPO mRNA, but further investigations are required to elucidate the exact role of cytokines in the regulation of M/TPO-Ag expression.