The thyroxine-containing thyroglobulin peptide (aa 2549-2560) is a target epitope in iodide-accelerated spontaneous autoimmune thyroiditis.

Enhanced iodide ingestion is known to accelerate the incidence and severity of spontaneous autoimmune thyroiditis [iodide-accelerated spontaneous autoimmune thyroiditis (ISAT)] in NOD.H2(h4) mice. CD4+ cells are required for the development and maintenance of ISAT, but their target epitopes remain unknown. In this study, we show that the previously identified thyroglobulin (Tg) T cell epitope p2549-2560 containing thyroxine at position 2553 (T4p2553) induces thyroiditis as well as strong specific T and B cell responses in NOD.H2(h4) mice. In ISAT, activated CD4+ T cells specific for T4p2553 are detected before the disease onset in thyroid-draining cervical lymph nodes only in mice placed on an iodide-rich diet and not in age-matched controls. In addition, selective enrichment of CD4+ IFN-γ+ T4p2553-specific cells is observed among cervical lymph node cells and intrathyroidal lymphocytes. T4p2553 was equally detectable on dendritic cells obtained ex vivo from cervical lymph node cells of NaI-fed or control mice, suggesting that the iodide-rich diet contributes to the activation of autoreactive cells rather than the generation of the autoantigenic epitope. Furthermore, spontaneous T4p2553-specific IgG are not detectable within the strong Tg-specific autoantibody response. To our knowledge, these data identify for the first time a Tg T cell epitope as a spontaneous target in ISAT.

Iodine content of thyroglobulin in Nod.H2h4 mice developing iodine-accelerated autoimmune thyroiditis.

OBJECTIVE: In NOD.H2h4 mice, high dietary iodine intake has been known to cause iodine-accelerated spontaneous autoimmune thyroiditis (ISAT) via an unknown mechanism. The aim of the study was to examine whether the NOD.H2h4 genetic background predisposes to enhanced iodine organification in thyroglobulin (Tg), a target autoantigen in ISAT. DESIGN: To avoid issues associated with an ongoing anti-Tg antibody response, we assessed Tg iodination levels in iodine-fed, B-cell deficient NOD.H2h4 mice. Additionally, we tested whether humoral or cellular immune responses of iodine-fed NOD.H2h4 mice are preferentially directed to Tg with increased iodine content (I-Tg) or known pathogenic Tg peptides that contained iodine. RESULTS: The iodine content of Tg was not significantly different between control (9.0 +/- 2.7 I atoms per monomer) and iodine-fed mice (10.9 +/- 0.3 I atoms per monomer). Furthermore, in iodine-fed NOD.H2h4 mice developing ISAT, strong but equivalent serum IgG responses were detected to both Tg or I-Tg, whereas their lymphoid cells were stimulated weakly but equally well by Tg or I-Tg in vitro and did not show reactivity against a panel of five pathogenic Tg peptides that contained iodine. CONCLUSIONS: The results suggest that development of ISAT in NOD.H2h4 mice is not associated with enhanced iodine organification or differential B- or T-cell responses to iodinated determinants in Tg.

Variable influences of iodine on the T-cell recognition of a single thyroglobulin epitope.

We have previously shown that iodotyrosyl formation within certain innocuous thyroglobulin (Tg) peptides confers on them immunopathogenic properties. In this report, we generated a panel of T-cell hybridoma clones specific for the immunogenic 16 mer Tg peptide p179 (amino acids 179-94) or its iodinated analogue (I-p179), with a view to examining the effects of a single iodine atom at the Y192 amino acid residue on T-cell recognition. We found that the peptide p179 was subdominant, and its binding to both A(k) and E(k) molecules was not significantly influenced by iodine. T-cell receptor (TCR) engagement was unaffected by the bulky iodine atom in two clones that responded to both analogues but it was sterically hindered in two other clones that recognized only p179. One clone was reactive only to I-p179, suggesting that the iodine atom is an integral part of its TCR ligand. Truncation analysis localized the determinant seen by all clones within the 11 mer peptide p184 (amino acids 184-194), suggesting that the cross-reactive clones were not activated by a minimal epitope lacking Y192 and that the negative influence of iodine was not the result of a flanking residue effect. These results demonstrate, at the clonal level, variable influences of a single iodine atom on the recognition of a single Tg peptide. Iodination of tyrosyl-containing, immunopathogenic Tg peptides may have unpredictable effects at the polyclonal level, depending on the extent of iodination at the particular site, and the relative number or effector function of autoreactive T-cell clones that are switched on or off by the neoantigenic determinant.

Thyroxine-binding antibodies inhibit T cell recognition of a pathogenic thyroglobulin epitope.

Thyroid hormone-binding (THB) Abs are frequently detected in autoimmune thyroid disorders but it is unknown whether they can exert immunoregulatory effects. We report that a THB mAb recognizing the 5' iodine atom of the outer phenolic ring of thyroxine (T4) can block T cell recognition of the pathogenic thyroglobulin (Tg) peptide (2549-2560) that contains T4 at aa position 2553 (T4(2553)). Following peptide binding to the MHC groove, the THB mAb inhibited activation of the A(k)-restricted, T4(2553)-specific, mouse T cell hybridoma clone 3.47, which does not recognize other T4-containing epitopes or noniodinated peptide analogues. Addition of the same THB mAb to T4(2553)-pulsed splenocytes largely inhibited specific activation of T4(2553)-primed lymph node cells and significantly reduced their capacity to adoptively transfer thyroiditis to naive CBA/J mice. These data demonstrate that some THB Abs can block recognition of iodine-containing Tg epitopes by autoaggressive T cells and support the view that such Abs may influence the development or maintenance of thyroid disease.