Peptide binding to class I MHC on living cells and quantitation of complexes required for CTL lysis

Abstract

ANTIGENIC peptides are presented to CD8⁺T lymphocytes by class I major histocompatibility complex (MHC) molecules^1,2. Peptides specifically bind to purified class I molecules in vitro^3–7, and to class I molecules on cells at nonphysiological temperatures⁸. We report here the kinetic and equilibrium parameters for the binding of radiolabelled influenza nucleoprotein peptides (NP-Y365-380 and shorter homologues) to the murine H–2D^b7 molecule on intact, viable cells at 37 °C. In contrast to earlier reports, we show that peptide binding is rapid and reversible, with dissociation constants ranging from nanomolar to micromolar, suggestive of typical ligand-receptor interactions. Only 10% of cell-surface D^b7 molecules can bind these peptides. To address the relationship between peptide binding and T-cell recognition of the antigen-MHC complex, we determined the minimum number of complexes required to sensitize a target cell for lysis by class I-restricted cytotoxic T-lymphocytes. Our data indicate that EL4 thymoma cells (H-2^b7) can be sensitized for lysis by cytotoxic T-lymphocytes when as few as 200 class I-peptide complexes (less than 0.08% of surface D^b7 molecules) are present per cell.