Experimental autoimmune thyroiditis induced in mice by thyroglobulin T-cell determinants

Varada, Panduranga Rao (1997) Experimental autoimmune thyroiditis induced in mice by thyroglobulin T-cell determinants. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Experimental autoimmune thyroiditis (EAT) induced in mice following challenge with thyroglobulin (Tg) in adjuvant, serves as a model for Hashimoto's thyroiditis (HT). Earlier work in our laboratory established that EAT can be induced in mice following challenge with a 17mer MTg(2495-11) or an 18mer MTg(2695-13) peptide, in complete Freunds adjuvant: (2495-11) peptide induced thyroiditis in H-2k,s haplotypes while (2695-13) peptide induced disease only in H-2S haplotype. In the present study, truncation analysis using a panel of peptide-specific TCR αβ⁺ CD4⁺ CD8⁻ T-cell hybridomas from EAT-susceptible C3H mice, identified two overlapping 9mer minimal determinants (2496-04) and (2499-07) within MTg(2495-11). The Ek-restricted (2496-04) determinant was immunogenic and pathogenic not only in C3H but also in SJL mice. Further, the determinants within MTg(2495-11) recognized by As-restricted SJL T cells were mapped identical to those restricted by the Ek and Ak molecules. MTg(2496-04)-primed lymph node cells (LNC) secreted IL-2, IFN-γ but not IL-4 upon specific activation in vitro, suggesting that induction of Th1 cells follows priming with the minimal Tg peptide in SJL mice. In addition, TCR-Vβ 2, 4 and 17 genes were utilized by a panel of ten MTg(2496-04)-specific IL-2-secreting hybrid T-cell clones generated from this Th1 subset, thus providing the first evidence of multiple TCR-Vβ gene usage in EAT induced with a minimal Tg epitope. In parallel, work involving the 18mer MTg(2695-13) peptide established a panel of TCR αβ⁺ CD4⁺ CD8⁻ IL-2 secreting T-cell hybridomas from SJL mice. Using two As-restricted (2695-13)-specific T-cell hybrids and three overlapping 12mer peptides spanning the core, N- and C- terminal regions of MTg(2695-13), two distinct T-cell determinants were localized within the 18mer peptide. The N-terminal 12mer (2695-06) was highly immunogenic and induced severe EAT following adoptive transfer of peptide-specific Th1 cells. A human homologue of MTg(2695-06) carrying two Ser substitutions of GLn2703 and Thr2704, on the other hand, showed contrasting immunopathogenic properties: it failed to activate Th1 cells; it did not cross-react with MTg(2695-06)-specific T cells and induced only mild thyroiditis. These findings highlight caution against extrapolating the epitope mapping data across heterologous Tgs despite their high homology. Finally, a computer search of SWISS-PROT data bank to identify sequences highly homologous with pathogenic Tg determinants, has led to identification of a 14mer adenoviral E1B(368-81) peptide (AVP). The viral peptide cross-reacted significantly with MTg(2696-04) at B- and T- cell level. AVP, however, failed to induce specific B- or T- cell responses. Despite its poor immunogenicity, AVP, when used as an Ag in vitro, conferred the EAT-inducing ability on MTg(2696-04)- primed LNC in adoptive transfer experiments. These findings suggest that viral peptides, such as AVP, when generated during infection, can amplify autoreactive T cells (via molecular mimicry) present in the host and thus, precipitate autoimmune thyroid disease.

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