AMINO-TERMINAL TRIMMING OF PEPTIDES FOR PRESENTATION ON MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II MOLECULES

Major histocompatibility complex (MHC) class II molecules bind antigen ic peptides for display to T lymphocytes. Although the enzymes involve d remain to be identified, it is commonly believed that class II assoc iated peptides are released from intact antigens through a series of p roteolytic steps carried out inside antigen presenting cells, We have examined the effect of amino acid substitutions on proteolytic process ing of the model antigen hen-egg lysozyme (HEL). Altered HEL molecules , engineered by site-directed mutagenesis of a HEL cDNA, were expresse d as separate stable transfectants in a B cell lymphoma line. Each tra nsfectant processed a different mutant HEL protein for presentation on MHC class II. We purified the resulting class II-associated peptides and analyzed them by mass spectrometry, Our results strongly support t he hypothesis that antigen processing continues after peptide binding to the MHC class II molecule and are most consistent with a scenario i n which long peptides first bind to MHC class II and are then trimmed by exopeptidase.