Localization of two cytotoxic T lymphocyte epitopes and three anchoring residues on a single nonameric peptide that binds to H‐2Ld and is recognized by cytotoxic T lymphocytes against mouse tumor P815

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Abstract
Mouse mastocytoma P815 expresses tumor antigens P815A and P815B encoded by a single gene called P1A and carried by a single peptide named P1A 35–43 (NH2-Leu-Pro-Tyr-Leu-Gly-Trp-Leu-Val-Phe-COOH). P1A 35–43 is presented to anti-P815A and anti-P815B cytotoxic T lymphocytes (CTL) by major histo-compatibility complex (MHC) H-2Ld molecules. In order to determine the individual role played by each amino acid residue of P1A 35-43 in binding to H-2Ld and in recognition by anti-A and anti-B T cell receptors (TcR), a series of P1A 35-43 peptides substituted by alanine at single positions was synthesized and tested for binding to H-2Ld and for CTL recognition. Binding to H-2Ld was estimated by measuring the ability of the peptide to up-regulate cell surface expression of H-2Ld. We found that three residues were important for interaction of P1A 35-43 with H-2Ld. Two of them, Pro at position 2 and Phe at position 9 were consistent with the described H-2Ld binding motif. A third residue, Trp at position 6, was also required for effective MHC binding of the tumor antigen. CTL sensitization assays showed that alanine substitution at position 7 (Leu) or at position 8 (Val) dramatically affected peptide recognition by anti-A CTL while positions 3 (Tyr) and 4 (Leu) were critical for recognition by anti-B CTL. We conclude that Pro2, Trp6 and Phe9 constitute the anchor residues of P1A 35-43 to H-2Ld, whereas the dipeptidyl sequences Tyr3-Leu4 and Leu7-Val8 form the core epitopes recognized by the TcR of anti-P815B and anti-P815A CTL, respectively.