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Identification of Pathogenic T Cell Epitopes Near Cathepsin Cleavage Sites in Thyroglobulin

Experimental autoimmune thyroiditis, induced in mice after challenge with thyroglobulin (Tg), is known to be under the genetic control of the H2Ak locus. Because cathepsins are known to influence proteolytic processing of Tg in vivo, we examined in this study whether putative H2Ak-binding Tg epitopes, located near cathepsin cleavage sites within mouse Tg, have immunopathogenic properties. Cathepsin L, B, and D cleavage sites in mouse Tg were predicted based on homology with known cathepsin cleavage sites in rabbit Tg. We used an algorithm-based approach to identify H2Ak-binding motifs within 20-aa residue segments adjacent to cathepsin cleavage sites, and five 12mer peptides encompassing these sequences were synthesized. Two of them, p2369 (aa 2369–2380) and p2439 (aa 2439–2450) were immunogenic, eliciting significant proliferative T cell responses using lymph node cells from peptide-primed mice and production of IL-2 and IFN-γ in recall assays in vitro. Both peptides induced experimental autoimmune thyroiditis upon direct challenge of CBA/J mice with peptide in CFA and by adoptive transfer of peptide-primed lymph node cells into naive recipient hosts, but neither peptide was characterized as dominant.