T-CELL RESPONSES AFFECTED BY AMINOPEPTIDASE-N (CD13)-MEDIATED TRIMMING OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II-BOUND PEPTIDES

Endocytosed protein antigens are believed to be fragmented in what app ears to be a balance between proteolysis and MHC-mediated epitope prot ection, and the resulting peptide-MHC complexes are transported to the surface of the antigen-presenting cells (APC) and presented to T cell s. The events that lead to antigenic peptide generation and the compar tments where antigen processing takes place remains somewhat enigmatic . The importance of intracellular antigen processing has been well est ablished; however, it is unclear whether additional processing occurs at the APC surface. To follow antigen processing, we have identified a pair of T cell hybridomas that recognize. a long vs. a short version of the same epitope. We have used prefixed APC and various protease in hibitors to demonstate that the APC surface has a considerable potenti al for antigen processing. Specific antibodies further identified the exopeptidase Aminopeptidase N (APN, CD13) as one of the enzymes involv ed in the observed cell-surface antigen processing. The NH2-terminal e nd of the longer peptide could, even while bound to major histocompati bility complex (MHC) class II molecules, be digested by APN with drama tic consequences for T cell antigen recognition. This could be demonst rated both in cell-free systems using purified reagents and in cellula r systems. Thus, MHC class II and APN may act in concert to generate t he final T cell epitopes.