A major human thyroglobulin epitope defined with monoclonal antibodies is mainly recognized by human autoantibodies

Thyroid Autoantibodies Display both "Original Antigenic Sin" and Epitope Spreading

Evidence for original antigenic sin in spontaneous thyroid autoimmunity is revealed by autoantibody interactions with immunodominant regions on thyroid autoantigens, thyroglobulin (Tg), thyroid peroxidase (TPO), and the thyrotropin receptor (TSHR) A-subunit. In contrast, antibodies induced by immunization of rabbits or mice recognize diverse epitopes. Recognition of immunodominant regions persists despite fluctuations in autoantibody levels following treatment or over time. The enhancement of spontaneously arising pathogenic TSHR antibodies in transgenic human thyrotropin receptor/NOD.H2^h4 mice by injecting a non-pathogenic form of TSHR A-subunit protein also provides evidence for original antigenic sin. From other studies, antigen presentation by B cells, not dendritic cells, is likely responsible for original antigenic sin. Recognition of restricted epitopes on the large glycosylated thyroid autoantigens (60-kDa A-subunit, 100-kDa TPO, and 600-kDa Tg) facilitates exploring the amino acid locations in the immunodominant regions. Epitope spreading has also been revealed by autoantibodies in thyroid autoimmunity. In humans, and in mice that spontaneously develop autoimmunity to all three thyroid autoantigens, autoantibodies develop first to Tg and later to TPO and the TSHR A-subunit. The pattern of intermolecular epitope spreading is related in part to the thyroidal content of Tg, TPO and TSHR A-subunit and to the molecular sizes of these proteins. Importantly, the epitope spreading pattern provides a rationale for future antigen-specific manipulation to block the development of all thyroid autoantibodies by inducing tolerance to Tg, first in the autoantigen cascade. Because of its abundance, Tg may be the autoantigen of choice to explore antigen-specific treatment, preventing the development of pathogenic TSHR antibodies.

Autoimmune thyroid diseases: genetic susceptibility of thyroid-specific genes and thyroid autoantigens contributions

Autoimmune thyroid diseases are common polygenic multifactorial disorders with the environment contributing importantly to the emergence of the disease phenotype. Some of the disease manifestations, such as severe thyroid-associated ophthalmopathy, pretibial myxedema and thyroid antigen/antibody immune complex nephritis are unusual to rare. The spectrum of autoimmune thyroid diseases includes: Graves' disease (GD), Hashimoto's thyroiditis (HT), atrophic autoimmune thyroiditis, postpartum thyroiditis, painless thyroiditis unrelated to pregnancy and thyroid-associated ophthalmopathy. This spectrum present contrasts in terms of thyroid function, disease duration and spread to other anatomic location. The genetic basis of autoimmune thyroid disease (AITD) is complex and likely to be due to genes of both large and small effects. In GD the autoimmune process results in the production of thyroid-stimulating antibodies and lead to hyperthyroidism, whereas in HT the end result is destruction of thyroid cells and hypothyroidism. Recent studies in the field of autoimmune thyroid diseases have largely focused on (i) the genes involved in immune response and/or thyroid physiology with could influence susceptibility to disease, (ii) the delineation of B-cell autoepitopes recognized by the main autoantigens, thyroglobulin, thyroperoxidase and TSH receptor, to improve our understanding of how these molecules are seen by the immune system and (iii) the regulatory network controlling the synthesis of thyroid hormones and its dysfunction in AITD. The aim of the present review is to summarize the current knowledge regarding the relation existing between some susceptibility genes, autoantigens and dysfunction of thyroid function during AITD.

Molecular advances in thyroglobulin disorders

Synthesis of tri-iodothyronine (T(3)) and thyroxine (T(4)) follows a metabolic pathway that depends on the integrity of the thyroglobulin structure. This large glycoprotein is a homodimer of 660 kDa synthesized and secreted by the thyroid cells into the lumen of thyroid follicle. In humans it is coded by a single copy gene, 270 kb long, that maps on chromosome 8q24 and contains an 8.5 kb coding sequence divided into 48 exons. The preprotein monomer is composed of a 19-amino acid signal peptide followed by a 2749-amino acid polypeptide. In the last decade, several mutations in the thyroglobulin gene were reported. In animals, four of them have been observed in Afrikander cattle (p.R697X), Dutch goats (p.Y296X), cog/cog mouse (p.L2263P) and rdw rats (p.G2300R). Mutations in the human thyroglobulin gene are associated with congenital goiter or endemic and nonendemic simple goiter. Thirty-five inactivating mutations have been identified and characterized in the human thyroglobulin gene: 20 missense mutations (p.C175G, p.Q310P, p.Q851H, p.S971I, p.R989C, p.P993L, p.C1058R, p.C1245R, p.S1447N, p.C1588F, p.C1878Y, p.I1912V, p.C1977S, p.C1987Y, p.C2135Y, p.R2223H, p.G2300D, p.R2317Q, p.G2355V, p.G2356R), 8 splice site mutations (g.IVS3-3C>G, g.IVS5+1G>A, g.IVS10-1G>A, g.IVS24+1G>C, g.IVS30+1G>T, g.IVS30+1G>A, g.IVS34-1G>C, g.IVS45+2T>A) 5 nonsense mutations (p.R277X, p.Q692X, p.W1418X, p.R1511X, p.Q2638X) and 2 single nucleotide deletions (p.G362fsX382, p.D1494fsX1547). The thyroglobulin gene has been also identified as the major susceptibility gene for familial autoimmune thyroid diseases (AITD) by linkage analysis using highly informative polymorphic markers. In conclusion the identification of mutations in the thyrogobulin gene has provided important insights into structure-function relationships.

Iodination of murine thyroglobulin enhances autoimmune reactivity in the NOD.H2 mouse

Autoimmune thyroiditis in humans has been linked to excess iodine intake. A causative relationship between dietary iodine and thyroiditis has been clearly established in animal models of thyroiditis, including the NOD.H2(h4) mouse strain, which develops enhanced thyroiditis spontaneously after supplementation of drinking water with sodium iodide. To assess the mechanisms by which iodine may contribute to disease pathogenesis, we have purified hypoiodinated thyroglobulin (Lo-I Tg) from the thyroids of mice fed methimazole and potassium perchlorate. This preparation contained only a trace of iodine and was poorly reactive to monoclonal antibody 42C3, which has been shown previously to distinguish hypoiodinated from normal Tg. A cloned T cell line 2D11 from a diseased NOD.H2(h4) mouse proliferated in response to normal Tg, but not to Lo-I Tg. Serum antibodies from NOD.H2(h4) mice with thyroiditis were poorly reactive to Lo-I Tg. To determine that these changes were due specifically to iodine content, Lo-I Tg was reiodinated in vitro. Reiodination of Lo-I Tg partially re-established the reactivity of NOD.H2(h4) serum antibodies. The data demonstrate that the reactivity of thyroglobulin-specific antibodies and certain T cells are dependent on the iodine content of thyroglobulin. These findings suggest that iodine contributes to autoimmune thyroiditis in the NOD.H2(h4) mouse by directly enhancing the antigenicity of thyroglobulin.

Thyroglobulin as an autoantigen: what can we learn about immunopathogenicity from the correlation of antigenic properties with protein structure?

Autoantibodies against human thyroglobulin are a hallmark of autoimmune thyroid disease in humans, and are often found in normal subjects. Their pathogenic significance is debated. Several B-cell epitope-bearing peptides have been identified in thyroglobulin. They are generally located away from the cysteine-rich regions of tandem sequence repetition. It is possible that our current epitopic map is incomplete because of the difficulty that proteolytic and recombinant approaches have in restituting conformational epitopes based upon proper pairing between numerous cysteinyl residues. Furthermore, the homology of cysteine-rich repeats with a motif occurring in several proteins, endowed with antiprotease activity, suggests that these regions may normally escape processing and presentation to the immune system, and brings attention to the mechanisms, such as oxidative cleavage, by which such cryptic epitopes may be exposed. A number of T-cell epitope-bearing peptides, endowed with thyroiditogenic power in susceptible mice, were also identified. None of them was dominant, as none was able to prime in vivo lymph node cells that would proliferate or transfer autoimmune thyroiditis to syngeneic hosts, upon stimulation with intact thyroglobulin in vitro. More than half of them are located within the acetylcholinesterase-homologous domain of thyroglobulin, and overlap B-cell epitopes associated with autoimmune thyroid disease, while the others are located within cysteine-rich repeats. The immunopathogenic, non-dominant character of these epitopes also favours the view that the development of autoimmune thyroid disease may involve the unmasking of cryptic epitopes, whose exposure may cause the breaking of peripheral tolerance to thyroglobulin. Further research in this direction seems warranted.

Evidence for intramolecular B-cell epitope spreading during experimental immunization with an immunogenic thyroglobulin peptide

Thyroglobulin (Tg) is a target autoantigen in autoimmune thyroid diseases, such as Graves' disease (GD) and Hashimoto's thyroiditis. In a previous study we identified three 20mer Tg peptides bearing epitopes of autoantibodies associated with GD (TgP15, TgP26 and TgP41: sequences 2339-2358, 2471-2490 and 2651-2670 of human Tg, respectively). In the present study, we investigated the antigenicity of the above peptides in experimental immunization with Tg, the immunogenicity of antigenic peptides and the possibility of intramolecular B-cell epitope spreading during peptide immunization. For this purpose, two rabbits were injected with human Tg in CFA six times, every three weeks. Two control animals were injected only with CFA. Testing of antisera and of affinity-purified antibodies, by ELISA against the three peptides, revealed reactivity only to TgP41. This synthetic peptide was subsequently administered to two rabbits, in its free form (100 micro g in CFA six times, every two weeks). A strong serological response was developed not only against TgP41, but also to intact human and rabbit Tg. Immunization with TgP41 induced intramolecular B-cell epitope spreading, i.e. production of antibodies to sites on Tg other than that corresponding to TgP41, as revealed by immunoadsorption and competitive ELISA. Histopathological studies did not reveal any infiltration in thyroid glands. We conclude that peptide TgP41 encompasses not only an epitope of disease-associated autoantibodies, but also a dominant immunogenic epitope of experimentally induced Tg-specific antibodies, able to drive B-cell epitope spreading.

Peptides of Human Thyroglobulin Reactive with Sera of Patients with Autoimmune Thyroid Disease

Autoantibodies to thyroglobulin (Tg) are a prominent feature of the two autoimmune thyroid diseases, chronic lymphocytic (Hashimoto’s) thyroiditis and Graves’ disease. Similar autoantibodies are found in the serum of many normal individuals without evidence of thyroid disease. Previous studies have indicated that patients with autoimmune thyroid disease recognize epitopes of Tg which are not usually recognized by normal individuals. The goal of this investigation was to identify peptide fragments of Tg bearing these disease-associated epitopes. For this purpose, we utilized a panel of mAbs that bind to different epitopes of the Tg molecule. One of these mAbs (137C1) reacted with an epitope that was also recognized by the sera of patients with autoimmune thyroiditis. In the present study, we show that two peptides (15 and 23 kDa) that reacted with mAb 137C1 are located in different parts of the Tg molecule. Each peptide inhibited the binding of mAb 137C1 to the other peptide and to the intact Tg, indicating that the same epitope was represented on the two peptides. Loops and helices of the secondary structure of the two peptides might be involved in the conformational epitope recognized by mAb 137C1. A striking finding of this study is that two apparently unrelated fragments of the Tg molecule bind to the same mAb. These findings may have important ramifications with regard to epitope spread and the progression of the autoimmune response to disease.

Enhancing or Suppressive Effects of Antibodies on Processing of a Pathogenic T Cell Epitope in Thyroglobulin

Thyroglobulin (Tg)-specific Abs occur commonly in thyroid disease, but it is not clear to what extent they affect Tg processing and presentation to T cells. Here we show that generation of the nondominant pathogenic Tg epitope (2549–2560), containing thyroxine (T4) at position 2553 (T4(2553)), is augmented by Tg-specific IgG mAbs that facilitate FcR-mediated internalization of Tg. However, other mAbs of the same (IgG1) subclass enhanced Tg uptake by APC but had no effect on the generation of this peptide. Treatment of APC with chloroquine or glutaraldehyde abrogated enhanced generation of T4(2553). The boosting effect was selective, since the enhancing mAbs did not facilitate generation of the neighboring cryptic (2495–2511) peptide, which is also pathogenic in mice. When Tg was simultaneously complexed to a mAb reactive with T4(2553) and to a mixture of boosting mAbs, the presentation of this epitope was totally suppressed. These results suggest that Tg-specific Abs alter Tg processing and may boost or suppress the presentation of nondominant pathogenic determinants during the course of disease.

Reactivity of anti-thyroid antibodies to thyroglobulin tryptic fragments: comparison of autoimmune and non-autoimmune thyroid diseases

Studies concerning the antigenicity of thyroglobulin fragments allow the characterization of the epitopes but do not consider the role of heavier antigenic fragments that could result in vivo from the action of endoproteases. Here we assess the relative importance of the fragments obtained from thyroglobulin by limited proteolysis with trypsin and compare by immunoblotting their reactivity to serum from patients with autoimmune (Graves' disease and Hashimoto's thyroiditis) and non-autoimmune (subacute thyroiditis) disease. The results showed no difference in frequency of recognition of any peptide by sera from patients with autoimmune thyroiditis. In contrast, sera from patients with subacute thyroiditis reacted more frequently with a peptide of 80 kDa. These results suggest the presence of antibody subpopulations directed at fragments produced in vivo by enzymatic cleavage of thyroglobulin. This fragment and antibodies to it may represent markers for subacute thyroiditis.

Iodination of human thyroglobulin (Tg) alters its immunoreactivity. I. Iodination alters multiple epitopes of human Tg

Human Tg, the site of synthesis of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), is one of the major autoantigens in autoimmune thyroiditis. The degree of iodination of Tg may have a major impact on its immunological properties by changing its antigenicity with respect to antibody binding. We have previously prepared a panel of MoAbs that bind to different epitopes of the Tg molecule. In the present study, we show that iodination alters the conformation of Tg molecule in such a way that it is recognized differently by different MoAbs. Monoclonal antibody 137C1 recognizes Tg regardless of its iodine content. Monoclonal antibody 42C3 recognizes Tg only if the Tg is iodinated either in vitro or in vivo. Monoclonal antibody 133B1 recognizes both in vivo iodinated Tg and non-iodinated Tg, but this MoAb did not recognize Tg following in vitro iodination. Monoclonal antibody 41A5 recognizes intact Tg and tryptic peptides of normal (in vivo) iodinated and non-iodinated Tg, but did not bind the tryptic peptides of artificially (in vitro) iodinated Tg. From the results of these experiments, we conclude that iodination of Tg by either in vivo or in vitro methods changes its conformation in such a way that some natural epitopes are 'lost' and some 'new' epitopes are generated. The generation of new epitopes may be important in the generation of autoimmune responses leading to autoimmune disease.

Thyroid iodide transporter: local sequence homologies with thyroid autoantigens

Here we show the existence of local amino acid (aa) sequence homologies between rat thyroid iodide transporter (Na+/l- symporter or NIS), whose gene was recently cloned, and known human thyroid autoantigens [thyroglobulin (Tg), thyroid peroxidase (TPO) and thyrotropin receptor (TSHR)] NIS sequences corresponding to the fourth (aa 264-282) and fifth extracellular loop (aa 386-414) are 15 to 40% identical and 30 to 60% similar to sequences corresponding to known or putative epitopes of Tg, TPO and TSHR. The sixth extracellular loop (aa 465-485) beared homology (44% identity, 52% similarity) only to a region of Tg which flanks one of its immunodominant domains. Sequences of thyroid autoantigens other than NIS shared homology, especially Tg and TPO. We conclude that in all likelihood NIS is an additional thyroid antigen, which shares common epitopes with the other thyroid autoantigens. Addendum: A study in abstract form appeared after submission of our paper finds experimental evidence for the antigenicity of two extracellular segments (aa 262-280 and 468-487) and of a portion of the intracellular C-terminus (aa 560-579).

Antigenic mapping of human thyroglobulin--topographic relationship between antigenic regions and functional domains

We characterized 26 mAb to human thyroglobulin to obtain a topographic map of the thyroglobulin antigenic surface. Among these mAb, three bind thyroglobulin peptides that are located in the primary sequence of thyroglobulin at either the N terminus or in the middle part of the molecule, three bind thyroglobulin via epitopes comprising the thyroid-hormone moiety, and three bind thyroglobulin through epitopes involved in the recognition of the molecule by its receptor. The 18 remaining mAb bind thyroglobulin through undetermined epitopes; most of these epitopes are resistant to trypsinization. We used two methods to map the antigenic regions of thyroglobulin: all 26 mAb were grouped, by means of cross-inhibition experiments, in 11 clusters corresponding to 11 antigenic regions of the thyroglobulin surface; by means of thyroglobulin peptides of decreasing size, obtained by time-controlled tryptic digestion, we analyzed the relative distance between pairs of epitopes in sandwich immunoassays. By combining these two methods, we organized most of the 11 antigenic regions on a topographic representation of the thyroglobulin surface. This new topographic map of thyroglobulin led us to some unexpected features of the thyroglobulin structure. First, antigenic region 8 located far from the N-terminal region is in close contact with two remote N-terminal antigenic regions (1 and 4), both involved in hormone formation. This antigenic region is likely to play a role in the correct positioning of hormonogenic tyrosines so as to optimize iodination-coupling reactions. Secondly, the domain involved in the binding of thyroglobulin to its receptor, probed by three mAb, is shared by two distinct mid-molecule antigenic regions, one being the main autoantigenic region of thyroglobulin.

Analysis of the individual contributions of immunoglobulin heavy and light chains to the binding of antigen using cell transfection and plasmon resonance analysis

We have cloned the Tg10 murine monoclonal antibody, which is specific for a human thyroglobulin (hTg) epitope targeted by autoantibodies in several thyroid pathologies. Transfection of COS-7 cells with plasmids expressing Tg10H and -kappa chains combined with surface plasmon resonance analysis (BIAcore) of culture supernatants showed that the entire cloned Tg10 antibody displays an affinity comparable to that of the parental antibody. This approach also permitted determination of the probable role of each chain to the recognition of the cognate epitope due to the ability of COS-7 cells to secrete independently each of the two constituting immunoglobulin chains. Tg10 heavy chain recognizes hTg in the absence of the light chain, but with a ten-fold lower affinity mainly due to an increase in kappaoff. In contrast, the light chain is unable to bind hTg on its own. This suggests that the latter is probably involved in stabilization rather than in initiating the formation of the antibody/antigen complex and that the specificity of Tg10 is mostly, if not exclusively, carried by the heavy chain. The potential applications of combined cell transfection and surface plasmon resonance to our understanding of antigen/antibody interactions are discussed.

Idiotypic restriction of murine monoclonal antibodies to a defined antigenic region of human thyroglobulin

In previous studies, we demonstrated that anti-human thyroglobulin (hTg) autoantibodies in patients with thyroid disorders exhibit a restricted epitopic specificity towards antigenic region II defined by its reactivity with four murine monoclonal antibodies (mAb 3, 6, 10, 15). To analyze the relationships between epitopic specificity and idiotypic expression of these mAb, two polyclonal anti-idiotypic sera were generated in rabbits by immunization with F(ab')2 fragments of mAb 3 and mAb 10. These anti-idiotypic preparations (AI 3 and AI 10) were tested against a panel of hTg-mAb produced in different strains of mice (HR BIOZZI and BALB/c). The idiotypic analysis showed that AI 3 and AI 10 specifically recognized framework-associated idiotopes as well as paratope-associated idiotopes shared by region II mAb. These results demonstrate that specificity for region II was strongly associated with a restricted idiotype suggesting a high sequence homology between V regions. In addition, naïve BALB/c mice immunized with AI 3 or AI 10 produced anti-hTg (Ab3) antibodies that recognize region II epitopes. These latter findings reveal that anti-Id contain a population of Ab2 beta carrying the internal image of region II epitopes.

Amino acid sequence of a tryptic peptide of human thyroglobulin reactive with sera of patients with thyroid diseases

Autoantibodies to human thyroglobulin (hTg) are found in the sera of many patients with thyroid diseases. To localize epitopes recognized by these autoantibodies, hTg was incubated with tryspin for 4 hours at 37 degrees C under non-reducing conditions. Releasing peptides from hTg in their natural conformation. These peptides were then analyzed by western immunoblot using either autoantibodies from patients with autoimmune thyroiditis or murine monoclonal antibodies (mAb) produced against hTg. The autoantibodies reacted primarily with two low molecular weight peptides with apparent molecular weights (MWap) of 15 and 20 kDa. The pattern of tryptic peptides recognized by these autoantibodies resembled that of one of the mAbs (137C1), as shown by immunoblots in either one or two dimensional SDS-PAGE. To characterize these peptides further, they were separated by a high performance liquid chromatography (HPLC). The column separated the 4-hour tryptic digest of hTg into multiple peptide peaks. Further analysis by SDS-PAGE showed that one of these peaks contained the 15 kDa peptide. The 15 amino acid sequence at the amino-terminus of this peptide was determined. This amino acid sequence (KVPTFATPWPDFVPR) corresponds to a unique sequence near the carboxyl-terminal end of hTg, starting with amino acid 2657. The size of the peptide indicates that it extends to the carboxyl-terminal end of hTg. This fragment contains one of the antigenic sites of hTg that binds autoantibodies from patients with autoimmune thyroid disease.

Tryptic peptides of human thyroglobulin: I. Immunoreactivity with murine monoclonal antibodies

Human thyroglobulin (Tg) was treated with trypsin at different concentrations of trypsin/Tg for various incubation times at 37 degrees C using non-reducing conditions. A ratio of trypsin to Tg of 1:100 (w/w) was optimal to release small peptides that were reactive to murine MoAbs to human Tg. Most peptides were released after only 1 h incubation with trypsin, but these peptides were further degraded at longer incubation times. However, a few small peptides, the largest of which with an apparent molecular weight (MWap) of 40 kD, resisted tryptic digestion up to at least 12 h of incubation. These resistant peptides were further degraded by trypsin at 18-24 h of incubation. Tryptic peptides of Tg, released at 1 h and 4 h of incubation, were analysed for their immunoreactivity to 16 well characterized anti-Tg MoAbs by Western immunoblot. Patterns of peptide recognition of these MoAbs were generally unique. Eight MoAbs reacted with peptides of MWap of 10-25 kD and above. Four other MoAbs reacted with peptides of MWap of 25-43 kD and above, and the remaining four reacted with peptides of MWap > 43 kD. Nine of these MoAbs failed to recognize peptides after reduction, suggesting that the MoAbs bind conformation-dependent epitopes. The above information will promote the development of models relating the structure of Tg to the autoimmune process, and may provide an understanding of those regions of Tg responsible for the induction of autoimmune thyroiditis.

Tryptic peptides of human thyroglobulin: II. Immunoreactivity with sera from patients with thyroid diseases

Tryptic peptides of human thyroglobulin (Tg) were analysed by Western immunoblot for their reactivity to circulating autoantibodies from patients with Hashimoto's thyroiditis (HT), Graves' disease (GD) and thyroid carcinoma, and from normal human controls. Low molecular weight peptides were released after 4 h incubation of Tg with trypsin. The sera of thyroid disease patients reacted with several peptides, but predominantly bound three peptides with apparent molecular weights (MWap) of 25 kD, 20 kD, and 15 kD; the sera of normal individuals did not bind these fragments of Tg. The pattern of tryptic peptides recognized by the majority of sera from GD patients differed from that recognized by sera from most patients with HT. Autoantibodies from both groups of patients recognized a 15-kD peptide with a high frequency, but the sera from 26/43 (60%) GD patients also recognized a peptide with MWap of 25 kD, whereas the sera from 22/35 (63%) of HT patients recognized a 20-kD peptide. A few sera from patients with thyroid carcinoma reacted with peptides with MWap of 15 and 20-kD, and none bound the 25-kD peptide. The immunoreactivity of autoantibodies in HT sera to the 20-kD peptide paralleled the competitive inhibition of the MoAb 137C1 by these sera. In addition, MoAb 137C1 and Hashimoto's sera showed the same Western immunoblot-binding pattern to Tg tryptic peptides, suggesting that a Hashimoto-associated epitope and the 137C1-binding site are found on the same peptide. These findings suggest that distinct peptides are recognized by Tg autoantibodies from patients with different thyroid diseases.

Selection of the expressed B cell repertoire by infusion of normal immunoglobulin G in a patient with autoimmune thyroiditis

In the present study we have analyzed the changes in the expressed antibody repertoire and in temporal fluctuations of antibody levels in serum that followed infusion of normal IgG (IVIg) in a patient with autoimmune thyroiditis. Administration of IVIg resulted in the stimulation of IgM production, in alterations of expressed antibody activity in serum that could not merely be accounted for by the passive transfer of antibody specificities contained in IVIg, in transient down-regulation of B cells clones expressing a specific disease-related idiotype and in the increase in serum in recipient's autoantibodies specifically reactive with F(ab')2 fragments of IVIg. In addition, infusion of IVIg shifted the pattern of spontaneous fluctuations of autoantibody activities in the patient's serum from a pattern indicative of disconnected events in the immune network to a pattern similar to that which is consistently observed in healthy controls. These results suggest that normal IgG may modulate autoreactivity by selecting expressed antibody repertoire through V region-dependent interactions with antibodies.

Thyroid autoimmunity

Antigenic structure remains a major focus in thyroid immunology. The genes for three major thyroid antigens--thyroglobulin, thyroid peroxidase and the thyrotropin receptor--were sequenced in the late 1980's, and epitopes for antibody and T cells have been reported within the last year. In addition, new evidence for selective use of T-cell receptor V gene segments in human thyroid infiltrates may point the way to specific immunotherapy.

Tyrosine iodination and iodotyrosyl coupling of the N-terminal thyroid hormone forming site of human thyroglobulin modulate its binding to auto- and monoclonal antibodies

The present work was aimed at studying the interaction of autoantibodies (aAb) and monoclonal antibodies (mAb) with the N-terminal thyroid hormone forming site of human thyroglobulin (TG). Obtained by CNBr treatment of TG, the peptide (22 kDa) containing the complete major hormonogenic site of human TG was purified in three forms according to the degree of iodination and iodotyrosine coupling: the native, poorly iodinated form (n-22K), the iodinated form containing iodotyrosine but not hormone residues (i-22K) and the form containing thyroid hormone (t-22K). We report that aAb from some patients with autoimmune thyroid diseases showed significant binding to both iodinated 22 kDa forms. Furthermore, a detailed study using mAb evidenced that iodination and coupling induced changes in the antigenicity of the molecule, some occurring without direct implication of iodine or thyroid hormones. The 22 kDa peptide appears as an interesting model to study the antigenic changes induced by the structural modifications in the course of thyroid hormone synthesis. This observation could be relevant to the etiopathogenic process of thyroid autoimmune diseases.