T Cell Responses Affected by Aminopeptidase N (CD13)-mediated Trimming of Major Histocompatibility Complex Class lI-bound Peptides

Endocytosed protein antigens are believed to be fragmented in what appears to be a balance between proteolysis and MHC-mediated epitope protection, and the resulting peptide-MHC complexes are transported to the surface of the antigen-presenting cells (APC) and presented to T cells. The events that lead to antigenic peptide generation and the compartments where antigen processing takes place remains somewhat enigmatic. The importance of intracellular antigen processing has been well established; however, it is unclear whether additional processing occurs at the APC surface. To follow antigen processing, we have identified a pair o f T cell hybridomas that recognize a long vs. a short version of the same epitope. We have used prefixed APC and various protease inhibitors to demonstate that the APC surface has a considerable potential for antigen processing. Specific antibodies further identified the exopeptidase Aminopeptidase N (APN, CD13) as one of the enzymes involved in the observed cell-surface antigen processing. The NH2-terminal end of the longer peptide could, even while bound to major histocompatibility complex (MHC) class II molecules, be digested by APN with dramatic consequences for T cell antigen recognition. This could be demonstrated both in cellfree systems using purified reagents and in cellular systems. Thus, MHC class II and APN may act in concert to generate the final T cell epitopes.