Mouse thyroglobulin: conservation of sequence homology in C-terminal immunogenic regions of thyroglobulin

Antigenicity and immunogenicity of the C-terminal peptide of human thyroglobulin

Thyroglobulin (Tg) is cleaved into several peptides during thyroid hormone synthesis, an oxidative process. P40, an iodinated C-terminal peptide from human Tg, has a molecular weight of about 40 kDa and contains two hormonogenic sites. P40 is the smallest peptide that is still recognized by monoclonal antibodies from mice immunized with human Tg directed against its immunodominant region. Since P40 also contains several T-cell epitopes, it is a good candidate for studying the primary events involved in the process of hormone synthesis leading to thyroid autoimmunity. The present results show that P40 is recognized by Tg antibodies from patients with thyroid disorders and induces Tg antibodies in CBA mice. P40 may therefore be involved in the autoimmune process, thus providing a useful tool for diagnostic and therapeutic purposes.

Pathogenic thyroglobulin peptides as model antigens: insights on the induction and maintenance of autoimmune thyroiditis

In recent years, the discovery of pathogenic thyroglobulin (Tg) peptides has given a new impetus to study, at the basic level, mechanisms of induction and immunoregulation of autoimmune thyroiditis. The genetic control of the immune response against defined Tg epitopes and the diversity of the T-cell receptor repertoire recruited for their recognition were among the first issues examined. Some of these epitopes contained hormonogenic sites, i.e. thyroxine residues, and thus offered an excellent opportunity to study how post-translational modifications such as iodination, can influence induction of thyroiditogenic cells. The delineation of pathogenic Tg determinants also enabled the search for "molecular mimics" i.e. peptides of microbial origin that may be involved in the pathogenesis of the disease. In addition, factors promoting the generation of pathogenic epitopes during Tg processing in antigen presenting cells could now be systematically investigated. This review summarizes recent findings in these areas.

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.

Involvement of Epitope Mimicry in Potentiation But Not Initiation of Autoimmune Disease

We have examined whether the peptide (368–381) from the murine adenovirus type 1 E1B sequence, exhibiting a high degree of homology with the known pathogenic thyroglobulin (Tg) T cell epitope (2695–2706), can induce experimental autoimmune thyroiditis (EAT) in SJL/J mice. The viral peptide was a poor immunogen at the T or B cell level and did not elicit EAT either directly or by adoptive transfer assays. Surprisingly, however, the viral peptide was highly antigenic in vitro, activating a Tg2695–2706-specific T cell clone and reacting with serum IgG from mice primed with the Tg homologue. The viral peptide also induced strong recall responses in Tg2695–2706-primed lymph node cells, and subsequent adoptive transfer of these cells into naive mice led to development of highly significant EAT. These data demonstrate that nonimmunogenic viral peptides can act as agonists for preactivated autoreactive T cells and suggest that epitope mimicry may at times play a potentiating rather than a precipitating role in the pathogenesis of autoimmune disease.

Thyroglobulin peptides of specific primary hormonogenic sites can generate cytotoxic T cells and serve as target autoantigens in experimental autoimmune thyroiditis

Previously we demonstrated that thyroxine (T4)-containing, 12-mer peptides from positions 5 (1-12) and 2553 (2549-2560), as well as thyronine (T0)-substituted 2553 peptide, derived from human (H) thyroglobulin (Tg) are capable of activating T cells that infiltrate the thyroid (thyroiditogenic). In contrast, peptides T4(2567) and T0(2567) (2559-2570) are not. To determine if these thyroiditogenic peptides, T4(5), T4(2553), and T0(2553), activated cytotoxic T cells (Tc) and served as target autoantigens when loaded onto indicator cells (BW5147 lymphoma, H2k), lymph node cells from CBA mice immunized with mouse (M) Tg were cultured in vitro with MTg, HTg, or Tg peptide. After MTg or HTg activation, Tc were detected for both MTg- and HTg-loaded target cells in an 18-h, 51Cr-release assay at an effector:target cell ratio of 50:1. These Tc also killed target cells labeled with T4(5), T4(2553), or T0(2553), but not the control peptide T4(2567). When MTg-primed lymphocytes were cultured with T4(5), T4(2553), or T0(2553), specific Tc were also generated against target cells labeled with the respective peptide. The data suggest that one of the thyroiditogenic properties of these peptides previously shown by adoptive transfer of thyroiditis is related to the generation of Tc. In addition, these conserved autoepitopes of Tg also serve as target antigens for Tc.

A thyroxine-containing thyroglobulin peptide induces both lymphocytic and granulomatous forms of experimental autoimmune thyroiditis

Mouse thyroglobulin (MTg)-sensitized spleen cells activated in vitro with MTg can induce two histologically distinct forms of experimental autoimmune thyroiditis (EAT). MTg-sensitized cells activated with MTg alone induce a mild chronic form of EAT in which the thyroid infiltrate consists primarily of lymphocytes and other mononuclear cells (lymphocytic EAT). The same donor cells activated with MTg and anti-IL2R mAb induce a more severe and acute form of EAT with a thyroid inflammatory lesion having granulomatous histopathological features. A thyroxine-containing (T4) peptide, corresponding to positions 2549-2560 of human Tg, was shown by others to activate spleen cells of mouse thyroglobulin (MTg)-sensitized CBA/J mice to induce lymphocytic EAT. To determine if the CD4+ effector T cells that induce granulomatous EAT can respond to the same T-cell epitope, the present study was undertaken to determine if both forms of EAT could be induced by the 2549-2560 thyroxine (T4)-containing peptide. This peptide was very effective for activation of T cells from MTg-primed CBA/J donors to induce granulomatous EAT but, in contrast to MTg, did not activate T cells from AKR/J or DBA/1 mice to induce granulomatous EAT. The T4 peptide did not apparently activate peptide-specific B cells in vivo but did activate MTg-primed B cells in vitro to produce anti-MTg autoantibody in recipient mice. These results demonstrate that a single 12-amino-acid thyroxine-containing peptide can activate T cells from CBA/J mice to induce both lymphocytic and granulomatous EAT. However, this peptide does not activate T cells from some other EAT-susceptible strains of mice, suggesting that MTg contains multiple epitopes able to activate T cells to induce granulomatous EAT.

Primary hormonogenic sites as conserved autoepitopes on thyroglobulin in murine autoimmune thyroiditis: role of MHC class II

A few synthetic peptides corresponding to amino acid sequences on human thyroglobulin (Tg) have been reported to induce moderate thyroiditis or activate mouse Tg (MTg)-primed T cells to transfer thyroiditis in mice susceptible to experimental autoimmune thyroiditis. Using three pairs of 12-mer peptides (1-12, 2549-2560, 2559-2570), with thyroxine (T4) or noniodinated thyronine (T0) at the conserved, hormonogenic site 5, 2553, or 2567 respectively, we reported that iodination was not required for a Tg hormonogenic site to be a thyroiditogenic autoepitope. To determine the relative importance of MHC class II and T cell receptor (TCR) repertoire, we compared two EAT-susceptible k and s (CBA and A.SW) haplotypes and their respective MHC-identical strain (C57BR and SJL) with approximately 50% genomic deletion of TCR Vbeta genes. Whereas k and s strains develop MTg-induced EAT, vigorous immunization with peptides containing T4 or T0 at either 5 or 2553, but not at 2567, led to mild (10-20%) thyroiditis only in some mice of either k strain. TCR Vbeta gene differences played a minor role. T cell responses to all peptide pairs were quite similar in CBA and C57BR mice, and both hT0(2553) and hT4(2553) reciprocally primed and stimulated their T cells. In adoptive transfer, SJL mice were somewhat more responsive to peptide activation than A.SW but much weaker than k strains. By comparing T4- and T0-containing peptides in different haplotypes, we show further that antigenicity of conserved hormonogenic sites is intrinsic, dependent more on amino acid sequence and binding to appropriate class II molecules and less on TCR repertoire or iodination of T0.

Cloning and characterization of murine thyroglobulin cDNA

Thyroglobulin is used to induce in mice experimental autoimmune thyroiditis (EAT), a model for Hashimoto thyroiditis. Because murine thyroglobulin is a more potent inducer of EAT than heterologous thyroglobulins, it has been hypothesized that it contains unique pathogenic epitopes. The validation of this hypothesis has been hampered by the lack of the murine thyroglobulin sequence. To identify murine-specific areas in thyroglobulin, we cloned, by reverse transcriptase PCR, and sequenced the complete murine thyroglobulin cDNA. This encodes a polypeptide of 2748 amino acids that is 73.5 and 71.8% identical to bovine and human thyroglobulin, respectively. Six regions are unique to each species. We also analyzed through EpiMer the sequences able to bind to the I-Ek major histocompatibility allele and, therefore, function as T cell epitopes. EpiMer analysis showed seven murine-specific T cell epitopes in thyroglobulin. The availability of the complete murine thyroglobulin sequence should promote the understanding of the pathogenesis and immunoregulation of EAT.

Recruitment of multiple V beta genes in the TCR repertoire against a single pathogenic thyroglobulin epitope

In autoimmune thyroid disease, the question whether thyroid-infiltrating, autoreactive T cells are derived from a polyclonal or oligoclonal subset has been the subject of considerable debate. In this report, we have examined the T-cell receptor (TCR) V beta profile of mouse clonal T cells responding to a single thyroiditogenic epitope, the As-restricted, 9mer mouse thyroglobulin (MTg) peptide (2496-04). In vitro recall assays based on lymph node cell (LNC) proliferation and cytokine release demonstrated that this peptide is a minimal T-cell epitope inducing a T-helper 1 (Th1) type of response in SJL hosts. A panel of cloned, interleukin-2 (IL-2)-secreting hybridomas was generated from this Th1 subset and their TCR-V beta gene utilization was assessed by reverse transcription-polymerase chain reaction (RT-PCR). Ten clones derived from two independent fusions were found to utilize three V beta gene families (V beta 2, 4, and 17). To the extent that Tg or other thyroid autoantigens encompass multiple pathogenic epitopes it appears unlikely from these data that a restricted TCR-V beta chain usage will be a general characteristic of thyroiditogenic T cells.

Searching for pathogenic epitopes in thyroglobulin: parameters and caveats

Last year marked the 40th anniversary of the discovery that thyroglobulin (Tg) is a major autoantigen in autoimmune thyroiditis. The Tg molecule presents unique challenges for epitope mapping owing to its large size and extensive iodination. Consequently, pathogenic determinants have only recently been identified. Here, George Carayanniotis and Varada Rao summarize the approaches used to determine pathogenic Tg T-cell epitopes and discuss caveats in this unusual quest.