Potent cytolytic response by a CD8+ CTL clone to multiple peptides from the same protein in association with an allogeneic class I MHC molecule.

CTL clone 2C recognizes the allogeneic class I MHC molecule L(d) in association with peptides derived from alpha-ketoglutarate dehydrogenase (oxoglutarate dehydrogenase (OGDH)), a ubiquitous intracellular protein. One of these peptides, QLSPFPFDL (QL9), elicits more vigorous cytolytic responses than two previously identified naturally processed peptides with overlapping sequences, LSPFPFDL (p2Ca) and VAITRIEQLSPFPFDL (p2Cb), from OGDH. In this study, we show that QL9 forms a more stable complex with cell surface L(d) than does p2Ca or p2Cb and is processed from the longer, naturally occurring peptide p2Cb by 20S proteosomes in vitro. The N-terminal cyclized pyroglutaminyl QL9 (pyroQL9), a form of QL9 to which it is converted at the low pH used for peptide isolation from tissue extracts, is even more active than QL9 in cytotoxicity assays with 2C CTL. Overall, the results indicate that along with the abundant natural peptides p2Ca and p2Cb, the QL9 and other OGDH peptides of various lengths, sharing a conserved C-terminal sequence, are also processed and presented with L(d) as allogeneic ligands for T cells expressing 2C TCR. All these peptides, each available in a low amount, could act in concert at the cell surface, resulting in a high density of cognate ligands that accounts for the exceptionally potent cytolytic response by 2C CTL.