Viral immune evasions differentially impact cell surface presentation of MHC I allomorphs already at the peptide-loading complex

The transporter associated with antigen processing (TAP) translocates peptides from the cytosol into the endoplasmic reticulum (ER) where they are loaded onto major histocompatibility class I (MHC I) molecules. Stable peptide-MHC I complexes travel to the cell surface and present their antigenic cargo to cytotoxic T lymphocytes. As central part of the peptide-loading complex (PLC), TAP is targeted by several viral proteins, which inhibit peptide translocation and thereby impact MHC I-mediated immune responses. However, it is still poorly understood whether the MHC I allotypes are differently affected by TAP inhibition. Here, we show that conditional expression of herpes simplex virus (HSV) ICP47 suppresses surface presentation of the MHC I allotypes HLA-A and HLA-C, but not of HLA-B, while expression of cytomegalovirus (CMV) US6 reduces surface levels of all allotypes. This difference is directly reflected in a specific enrichment of HLA-B molecules at US6 arrested PLC. Since ICP47 and US6 inhibit TAP in different transport-incompetent conformations, our data imply that MHC I allomorphs are preferentially recruited or trapped at the PLC leading to selective downregulation of MHC I surface presentation. Our findings suggest that viral immune evasions not only inhibit peptide supply to the ER by TAP, but also modulate the assembly and chaperone activity of the PLC.