Identification of MHC class II-restricted peptide ligands, including a glutamic acid decarboxylase 65 sequence, that stimulate diabetogenic T cells from transgenic BDC2.5 nonobese diabetic mice. 
Nonobese diabetic (NOD) mice spontaneously develop insulitis and destruction of pancreatic islet beta cells similar to type 1 diabetes mellitis in humans.  Insulitis also occurs in the BDC2.5 TCR transgenic line of NOD mice that express the rearranged TCR alpha- and beta-chain genes of a diabetogenic NOD CD4 T cell clone.  When activated with syngeneic islet cells in culture, BDC2.5 T cells adoptively transfer disease to NOD recipients, but the identity of the islet cell Ag responsible for pathogenicity is not known.  To characterize the autoantigen(s) involved, BDC2.5 T cells were used to screen a combinatorial peptide library arranged in a positional scanning format.  We identified more than 100 decapeptides that stimulate these T cells at nanomolar concentrations; they are then capable of transferring disease to NOD-scid mice.  Surprisingly, some of the peptides include sequences similar (8 of 10 residues) to those found within the 528-539 fragment of glutamic acid decarboxylase 65.  Although this 12-mer glutamic acid decarboxylase 65 fragment is only slightly stimulatory for BDC2.5 T cells (EC(50) > 100 microM), a larger 16-mer fragment, 526-541, shows activity in the low micromolar range (EC(50) = 2.3 microM).  Finally, T cells from prediabetic NOD mice respond spontaneously to these peptide analogs in culture; this finding validates them as being related to a critical autoantigen involved in the etiology of spontaneous diabetes and indicates that their further characterization is important for a better understanding of underlying disease mechanisms.