In Vivo Competition Between Self Peptides and Foreign Antigens in T-cell Activation

Cytotoxic and helper T lymphocytes recognize foreign antigen in the form of short peptides associated with class I and class II major histocompatibility complex (MHC) molecules, respectively. A recent study of the three-dimensional structure of a class I MHC molecule revealed a cleft formed by the amino-terminal half of the protein, which could serve as the binding site for these peptides. Because an individual possesses only a limited set of different MHC molecules, each molecule of this set must have the ability to bind a large number of different peptides in order to ensure full immunocompetence. Thus, it can be anticipated that peptides with unrelated sequences compete for binding to the same MHC molecule, and, indeed, this has been shown to occur in vitro. We therefore decided to see whether such competition could also regulate the cell responses in vivo. We have found that a synthetic peptide corresponding to residues 46-62 of mouse lysozyme, although not immunogenic itself, effectively inhibits the priming for T-cell responses when injected into mice together with foreign protein or peptide antigens. The inhibition observed strictly correlates with the capacity of the competitor to bind to the particular MHC molecule presenting the foreign antigen, and its extent depends on the molar ratio between antigen and competitor.