T cell responses affected by aminopeptidase N (CD13)-mediated trimming of major histocompatibility complex class II-bound peptides.

TLDR

Antigen processing balances proteolysis and MHC‑mediated protection, yet the exact compartments and mechanisms—particularly at the APC surface—remain poorly understood. The study aimed to track antigen processing by employing hybridomas that recognize long versus short versions of the same epitope. Using prefixed APCs and protease inhibitors, the authors showed that the APC surface can actively process antigens. APN (CD13) trims the N‑terminus of MHC II‑bound peptides on the APC surface, altering T‑cell recognition, as demonstrated in both cell‑free and cellular assays, indicating that MHC II and APN cooperate to generate final epitopes.

Abstract

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 of 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 demonstrate 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 cell-free 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.