| Experimental autoimmune thyroiditis (EAT) is characterized by the infiltration and destruction of the thyroid gland. Susceptibility and resistance to EAT is encoded by MHC H2A genes, and influenced by regulatory T cells (Tregs). Here we studied three strains of mice expressing A b, Eb, or both Ab and Eb class II molecules. The A-E+ transgenic mouse, highly susceptible to human thyroglobulin (hTg)-induced thyroiditis, but strongly tolerant to challenge with mouse thyroglobulin (mTg), contrasts traditionally susceptible strains, wherein mTg induces stronger thyroiditis. To identify self epitopes recognized by destructive, hTg-primed T cells, we used the three hTg epitopes known to be presented by H2Eb, as the basis for synthesizing potential mTg epitopes. Peptide mTg409 (15-mer) primed T cells, elicited antibodies, and induced thyroiditis. A second pathogenic, 19-mer mTg peptide, mTg179, was subsequently discovered. These mTg autoepitopes help to explain the severe thyroiditis seen in this novel A-E + transgenic model. Traditionally resistant A+E - mice permit thyroiditis induction with mTg after depleting Tregs, supporting Ab presentation to thyroiditogenic T cells. Yet, A+E+ mice succumb to thyroiditis without Treg depletion. To explore the effect of Eb expression on mTg presentation by Ab, we focused on high-affinity peptide Ealpha52-68, which occupies ∼15% of Ab molecules in A+E + mice. Seven putative, 15-16-mer Ab-binding peptides were synthesized, and five were immunogenic for Ab-expressing mice. Ealpha52-68 competitively reduced the proliferative responses to mTg, mTg1677 and mTg2342. Moreover, mTg1677 induced thyroiditis in Ab-expressing mice. Ealpha52-68 competition with mTg-derived peptides may impede clonal deletion of mTg-specific T cells with thyroiditogenic potential. In susceptible Ak strains, tolerance to EAT induced by raised mTg levels is mediated by CD4+CD25+ Tregs. Here we used Foxp3, the most specific marker of Tregs, to show that naturally-existing Tregs mediated tolerance to endogenously released mTg in CBA mice. Moreover, these Tregs in A-E+ mice mediated tolerance induced to hTg and suppressed hTg-specific T cells. Lastly, mTg-tolerized A+E+ mice, activated with mTg during immunization, were not protected from hTg-induced thyroiditis, ruling out bystander suppression in EAT. These studies help elucidate the interplay of different MCH class II molecules and Tregs in EAT. |
