Study of the pathogenic mechanisms of experimental autoimmune thyroiditis

Experimental autoimmune thyroiditis (EAT) develops in mice following administration of thyroglobulin (Tg), the main autoantigen of the thyroid gland, emulsified in adjuvant. EAT exhibits many of the characteristics of Hashimoto‟s thyroiditis in humans: mononuclear cell infiltration and destruction of the thyroid follicles, autoantibody production and T-cell proliferative response to Tg. Tg is the largest autoantigen known (660 KDa) and that is why many studies have focused on the mapping of Tg T-cell epitopes [8mer to 9mer Tg peptides presented by major histocompatibility complex molecules (MHC) class II and recognized by T lymphocytes]. The outcome of these studies was the mapping of 15 Tg peptides (from the 32 examined) able to induce EAT only in susceptible mouse strains, that bear the H2k and Η2s MHC molecules and develop severe EAT following immunization with the intact molecule of Tg. None of the Tg peptide tested was able to cause thyroiditis in resistant mouse strains, that bear the H2b and Η2d MHC molecules and develop EAT of low severity. The lack of knowledge on pathogenic Tg T-cell epitopes in resistant mouse strains impairs the ability to study pathogenic and immunoregulatory mechanisms in these mice.The aim of the present study was to investigate the thyroiditogenic potential of the 20mer peptide of human Tg p2340 (2340-2359, QVAALTWVQTHIRGFGGDPR) in susceptible and resistant mouse strains. This peptide was already known from a previous study of the Immunology laboratory of the Hellenic Pasteur Institute to be able to induce EAT in AKR/J (H2k). Moreover, the study of its aminoacid sequence with specialized algorithms had shown its potential binding to MHC class II molecules in resistant mouse strains.In the present study, p2340 was indeed found to induce the activation of T lymphocytes of CBA/J (H2k), SJLJ (H2s), BALB/c (H2d) and C57BL/6 (H2b) mice, following administration of the peptide with adjuvant (in vitro proliferation assay with radioactive thymidine). P2340 was also found to be able to induce the activation of B lymphocytes leading to the production of specific antibodies in all mouse strains of the study (study of anti-p2340 reactivity of antisera from mice immunized with p2340 performed by ELISA). The most important finding was that p2340-primed lymphocytes infiltrated the thyroid gland of both susceptible and resistant mice (after histological study of thyroid gland sections).As this was the first time that a Tg peptide was pathogenic in both susceptible and resistant mice, we proceeded to the mapping of the minimal T-cell epitope(s) within the p2340 sequence in order to investigate whether the immuopathogenic properties of this sequence are due to recognition of one or more distinct epitopes presented in the context of susceptible or resistant MHC class II genes. Use of 14 overlapping nonamer peptides showed that p2340 bears two distinct T-cell epitopes (p2344: LTWVQTHIR and p2350: HIRGFGGDP). Although, p2344 was recognized by p2340-primed T cells from both susceptible and resistant strains, it was able to induce EAT only in255susceptible mice. T-cell epitope p2350 was antigenic only in H2s hosts but not immunogenic. The B-cell epitope mapping of p2340 also resulted in identification of two B-cell epitopes (p2344: LTWVQTHIR θαη p2351: IRGFGGDPR).In conclusion, p2340 is the first Tg peptide found to be pathogenic in resistant as well as susceptible mouse strains and this is attributed to one common minimal T-cell epitope. This common T-cell epitope is able to bind to the four different MHC molecules of mice of the study and to be recognized by their T lymphocytes. The p2340-based animal model generated in this study is of significant importance as it will allow us to investigate mechanisms of EAT induction in genetically resistant hosts and on the other hand to study immunoregulatory mechanisms responsible for their resistant phenotype in comparison to those of susceptible mouse strains