Diversity of epitopes recognized by cytotoxic T lymphocytes that are specific for rejection antigen peptide pRL1a presented on BALB/c leukemia RL♂1

Cytotoxic T lymphocytes (CTL) generated in (BALB/c x C57BL/6)F₁ (CB6F₁) and BALB/c spleen cells stimulated with BALB/c radiation-leukemia RL♂1 cells or pRL1a (IPGLPLSL) peptide itself recognized pRL1a on RL♂1 in association with L(d). We first studied pRL1a peptide residues used for binding to the L(d) molecule by examining the inhibition by variant peptides with single Ala substitutions at each position (P) of recognition of P815 target cells sensitized with L(d)-binding p2Ca (LSPFPFDL) peptide for BALB/c anti-p2Ca CTL. The results showed that Leu at P8 is predominantly involved in the binding and Pro at P2 is partially involved. Substitution of Gly to Ala at P3 increased binding. We then investigated the epitope residues recognized by four pRL1a-specific CTL clones by examining their cytotoxicity against the P815 target sensitized with variant pRL1a peptides. Recognition by clone Y-15 involved predominantly Leu at P4 and P6, and also Pro at P5 and Ser at P7, and partially Ile at P1. Recognition by clone U-41 involved predominantly Ile at P1 and Leu at P6, and partially Gly at P3, Leu at P4, Pro at P5 and Ser at P7. Recognition by clone P-2 involved predominantly Leu at P4 and P6, and Ser at P7, with no partial involvement of other substitutions being observed. Finally, recognition by clone B-24 predominantly involved all residues, except Gly at P3, which was partially involved, TCR V(β) genes utilized by those CTL clones were different. The findings show that tumor antigen peptide pRL1a generates a wide repertoire of CTL clones that differ in TCR V(β) usage and in the intrapeptide epitope residues they recognize.