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

Cloning and characterization of canine thyroglobulin complementary DNA

Canine thyroglobulin (cTg) is one of the thyroid autoantigens associated with hypothyroidism caused by autoimmune thyroiditis in dog. To identify canine-specific areas in cTg, we cloned, by reverse transcriptase PCR, and sequenced the complete cDNA of cTg. It revealed an open reading frame of 8289 nucleotides, which encode a polypeptide of 2762 amino acids that is 78.9 and 78.1% identical to bovine and human thyroglobulin, respectively. This complete cTg sequence may be useful to promote the understanding of the primary structure of cTg and, it will be informative data in the further search about antigenic epitopes associated with autoimmune thyroiditis and pathogenesis of cTg-associated thyroid diseases in dog.

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.

Iodination of human thyroglobulin (Tg) alters its immunoreactivity. II. Fine specificity of a monoclonal antibody that recognizes iodinated Tg

In a previous investigation, we found that murine MoAb 42C3, raised against human Tg, recognized Tg differently depending upon its level of iodination of Tg. A possible explanation for this finding is that iodine is directly involved with the specific epitope recognized by MoAb 42C3. In the present study, we report that the binding of MoAb 42C3 to iodinated Tg is inhibited by T4, T3, reverse T3 (rT3), triiodothyroacetic acid (triac), diiodothyronine (T2), diiodotyrosine (DIT), but not by thyronine (TO) or tyrosine. The order of inhibition of these iodinated compounds is T4 > T3 > rT3 > triac > T2 > DIT. The MoAb 42C3 does not have the same specificity as the T3, T4-receptor since the order of binding of these iodinated compounds on the receptor differed from the order of their inhibition of this MoAb. Monoclonal antibody 42C3 also recognized non-iodinated Tg that was subsequently iodinated in vitro. It failed to recognize another protein, bovine serum albumin, that was iodinated in vitro by the same method. These results suggest that iodinated tyrosines and thyronines determine the binding specificity of MoAb 42C3. The inhibitory effects of these compounds on MoAb 42C3 depend on their iodine content as well as location of iodine in the aromatic ring.

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.

Molecular analysis of the antibody response to thyroglobulin and thyroid peroxidase

In this review, we discuss the latest results concerning the molecular analysis of antibodies (Ab) directed toward thyroid autoantigens. In particular, we attempt to define patterns within the Ab repertoire that correlate best to their activities. Whilst a considerable amount is now known concerning the Ab response to thyroid peroxidase (TPO), there is still much we do not understand. We review evidence for the site of interaction of TPO-reactive Ab with native TPO. The Ab responses to thyroglobulin (Tg) and, in particular, the thyroid-stimulating hormone receptor (TSH-R), are much less well characterised. In this review, we focus on the molecular analysis of the Ab response to Tg and TPO, assessing the repertoire as it is currently known. In addition, we have tried to link this information with the analysis of the epitopes recognised by the various Ab. Finally, we discuss one of the more unusual features of the thyroid Ab repertoire, the use of D-D fusion at heavy chain junctions, and questions raised by our current state of knowledge, such as the role of Ab using germline V regions in antigen recognition.