Identification of a thyroiditogenic sequence within the thyroglobulin molecule.

Thyroglobulin (Tg)-specific T cells are important in the induction of experimental autoimmune thyroiditis (EAT), but the nature and the number of the Tg T cell epitopes involved in the disease process are unknown. Through the use of computerized algorithms that search for putative T cell epitopes, a 17-mer peptide (TgP1) was identified within the known portion of the rat Tg sequence (corresponding to amino acids 2495 to 2511 of the human Tg sequence) that induced strong mononuclear cell infiltration of the thyroid in classic EAT-susceptible murine strains such as SJL, C3H, and B10.BR and low or undetectable infiltration in EAT-resistant strains such as BALB/c and B10. TgP1 appears to be phylogenetically conserved since it is completely homologous to its bovine counterpart and differs at a single amino acid position from its human analogue. After priming with TgP1 in vivo, significant proliferative T cell responses to TgP1 in vitro were observed only with lymphocytes from susceptible (high responder) strains, thus correlating proliferative capacity with EAT induction. TgP1-primed T cells did not respond to intact mouse Tg (MTg) or rat Tg in vitro and, conversely, T cells primed in vivo with MTg or rat Tg did not respond to TgP1 in culture, suggesting that TgP1 is comprised of non-immunodominant T cell determinants. TgP1 was defined as a serologically nonimmunodominant epitope as well, since in vivo priming of all strains with MTg led to strong MTg-specific IgG responses but no TgP1-specific responses in ELISA assays. This was not due to lack of immunogenic B cell determinants on TgP1, however, because peptide challenge of EAT-susceptible strains elicited TgP1-specific IgG that also cross-reacted with MTg and rat, human, bovine, and porcine Tg. The data demonstrate that TgP1 delineates nonimmunodominant but highly immunogenic determinants at both the T and B cell level, which may play an important role in the development of autoimmune thyroiditis.