IDENTIFICATION OF NATURALLY PROCESSED T-CELL EPITOPES FROM GLUTAMIC-ACID DECARBOXYLASE PRESENTED IN THE CONTEXT OF HLA-DR ALLELES BY T-LYMPHOCYTES OF RECENT-ONSET IDDM PATIENTS

Glutamic acid decarboxylase (GAD) has been defined as a major target a ntigen in insulin-dependent diabetes mellitus (IDDM). To identify the molecular ligands triggering a T cell response to GAD, a panel of huma n GAD65-specific T lymphocyte lines was generated from peripheral bloo d of three recent onset IDDM patients. All lines derived from a patien t expressing the high-risk-conferring HLA-DR0301/*0401 haplotypes rec ognized a single epitope localized between amino acid positions 270 an d 283 of GAD65, a stretch that is located in close proximity to the ho mology region shared with Coxsackie virus P2-C protein. All lines with this specificity were restricted to the DRA, B10401 product of the D R4 haplotype. Analysis of the GAD-specific T cell response in a second patient homozygous for DR4 haplotypes demonstrated that the same DRA, B10401 allele selected T cells specific for a different determinant. The T cell response profile in a third patient showed that DR1501/*1 601-encoding haplotypes could present at least three different epitope s to GAD65-specific T lymphocytes. One of these epitopes was presented by a DR allele associated with the resistance-conferring DRB11501 ha plotype. GAD-specific T cell lines could not be isolated from HLA clas s II-matched normal individuals. Our data reveal that (a) the T cell r esponse to GAD65 is quite heterogenous in recent on-set IDDM patients; (b) HLA-DR, not DQ, seems to be the principal restriction element use d by T cells present at the onset of the disease; and (c) T cells resp onding to epitopes containing identical sequences to Coxsackie virus P 2-C protein were not detected.