PRESENTATION OF ENDOGENOUS PEPTIDE MHC CLASS-I COMPLEXES IS PROFOUNDLY INFLUENCED BY SPECIFIC C-TERMINAL FLANKING RESIDUES

Processing of intracellular proteins yields 8-11 residue peptides that are displayed on the APC surface as peptide/MHC class I complexes. Th e remarkably precise excision of antigenic peptides from their precurs or polypeptides raises the question of whether specific flanking resid ues influence presentation efficiency. Here we addressed this question by analyzing the generation of OVA/K-b or influenza nucleoprotein/D-b complexes in APC expressing precursors with varying N- or C-terminal flanking residues. We find that T cell responses were not significantl y affected by varying the N-terminal flanking residue in the precursor s. In contrast, presentation of peptide/MHC complexes was inhibited wi th the addition of a single C-terminal flanking residue. The most dram atic inhibition was observed with isoleucine, leucine, cysteine, and p roline as the C-terminal flanking residues. These residue-specific var iations in presentation activity could not be accounted for by differe nces in the stimulatory activity of corresponding synthetic peptides b ut were; proportional to the relative amounts of naturally processed p eptides recovered in the cell extracts. These findings suggest differe nces in the susceptibility of N- vs C-terminal flanking residues to pr oteolytic cleavage during Ag processing, The strong influence of speci fic C-terminal flanking residue(s) could be an important factor affect ing the choice of peptides presented to T cells on the APC surface.