Antigen processing in the endoplasmic reticulum for MHC class I molecules

CD8 T cell development and immune responsiveness is dependent on Major Histocompatibility Complex class I molecules (MHC I) to present a diverse spectrum of peptides derived from intracellular proteins. Antigenic precursors are degraded in the cytoplasm and MHC I are loaded with peptides in the endoplasmic reticulum (ER). In this compartment, peptides can be trimmed at the N-terminus by the ER aminopeptidase associated with antigen processing (ERAAP). However, the extent to which the peptide pool requires trimming and how ERAAP shapes the peptide repertoire displayed by MHC I (pMHC I) is not known. To address these questions we generated ERAAP deficient mice by homologous recombination. We found that in the absence of ERAAP, peptides are not trimmed in the ER and based upon customization by ERAAP, peptides can be divided into three subsets. Some peptides are generated independently of ERAAP, while others enter the ER with N-terminal extensions and require ERAAP for processing. Another subset of peptides are sensitive to ERAAP and are dramatically upregulated in its absence.; Dearth of N-terminal trimming in the ER does not starve MHC I of peptide supply. Instead, a surprisingly large volume of untrimmed peptides are presented as unique pMHC I complexes at the cell surface of ERAAP deficient cells. These pMHC I are unstable but immunologically and structurally unique and elicit potent T cell and B cell responses in wild-type mice. Thus, ERAAP is a quintessential editor of the MHC I peptide repertoire and paradoxically its absence enhances immunogenicity.