Class I-restricted cytotoxic T cell recognition of split peptide ligands

We developed a novel approach to probe the molecular basis of TCR recognition of the MHC class I-peptide complex and to determine how constraints placed on peptide binding by the class I molecule influence T cell 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 Ld peptide ligands, tum- (QNHRALDL) and p2Ca (LSPFPFDL), were tested for the ability to sensitize targets for recognition by peptide-specific cytotoxic T lymphocytes (CTL). The tum-derived tetramer QNHR, containing both primary TCR contact residues (H17 and R18), is recognized only when used in combination with ALDL which contains the primary binding residues (A19, D21 and L22). This suggests that both peptides of the pair contribute to positioning of the TCR contacts. Remarkably, CTL clone P24 recognized target cells sensitized with a trimer (QNH) combined with a pentamer (RALDL), demonstrating that TCR recognition can occur when the TCR contacts are separated (placed on separate peptide subunits). For the p2Ca peptide LSPFPFDL, the C-terminal tetramer PFDL, which contains both the primary TCR contact residue (P) and the dominant binding residue (L), is sufficient for recognition. In addition, PFDL was able to bind effectively to Ld and to activate naive antigen-specific T cells. These data suggest that peptide subunits and complementary peptide pairs composed of trimeric, tetrameric or pentameric peptides can bind independently to the Ld molecule in the same register and orientation as they do when contained within the parent peptide.