CLASS I-RESTRICTED CYTOTOXIC T-CELL RECOGNITION OF SPLIT PEPTIDE LIGANDS

We developed a novel approach to probe the molecular basis of TCR reco gnition of the MHC class I-peptide complex and to determine how constr aints placed on peptide binding by the class I molecule influence T ce ll recognition, We synthesized peptide pairs derived from the N- and C -terminal regions of class I peptide ligands in which the TCR contacts and dominant binding residues were placed together or were separated, Complementary peptide pairs derived from two well-characterized L(d) peptide ligands, tum(-) (QNHRALDL) and p2Ca (LSPFPFDL), were tested fo r the ability to sensitize targets for recognition by peptide-specific cytotoxic T lymphocytes (CTL), The tum-derived tetramer QNHR, contain ing both primary TCR contact residues (H17 and R18), is recognized onl y when used in combination with ALDL which contains the primary bindin g residues (A19, D21 and L22). This suggests that both peptides of the pair contribute to positioning of the TCR contacts. Remarkably, CTL c lone P24 recognized target cells sensitized with a trimer (QNH) combin ed with a pentamer (RALDL), demonstrating that TCR recognition can occ ur when the TCR contacts are separated (placed on separate peptide sub units), For the p2Ca peptide LSPFPFDL, the C-terminal tetramer PFDL, w hich contains both the primary TCR contact residue (P) and the dominan t binding residue (L), is sufficient for recognition. In addition, PFD L was able to bind effectively to L(d) and to activate naive antigen-s pecific T cells, These data suggest that peptide subunits and compleme ntary peptide pairs composed of trimeric, tetrameric or pentameric pep tides can bind independently to the L(d) molecule in the same register and orientation as they do when contained within the parent peptide.