Investigating how murine cytomegalovirus downregulates ERAAP and modulates non-classical T cell responses

The endoplasmic reticulum aminopeptidase associated with antigen presentation (ERAAP) plays a crucial role in shaping the peptide-MHC I repertoire and maintaining proper immune surveillance. While murine cytomegalovirus (MCMV) has multiple strategies for manipulating the antigen processing pathway to evade immune responses, the host has also developed ways to combat viral immune evasion. In this report, we describe how MCMV modulates ERAAP and induces a novel non-classical Qa-1b restricted anti-viral T cell response. Specifically, ERAAP downregulation during infection led to presentation of a self-peptide FL9 on non-classical Qa-1b, thereby eliciting non-classical QFL T cells to proliferate in infected mice and control infection. This report also investigates the mechanism of ERAAP downregulation during MCMV infection. By testing several MCMV mutants and screening all MCMV genes for ERAAP downregulation, we identified several candidate viral genes that are potentially involved in targeting ERAAP. In addition, through a CRISPR-Cas9 screen, we found and confirmed a host protein, ERp44, that is essential for ERAAP retention in the ER and showed that this interaction may be disrupted during viral infection. Specifically, we found that MCMV reduces the intracellular oxidation state of cells, which may disrupt disulfide bond formation between ERp44-ERAAP, leading to ERAAP secretion. Altogether, our study investigates how MCMV infection impacts ERAAP function and highlights the consequences of ERAAP downregulation on non-classical T cell responses during viral infection. These findings could be applied to studies of HCMV, which also downregulates ERAAP, and provide ideal targets for use in immunotherapies and vaccines against CMV infection