Structural basis of MHCI antigen presentation sabotage by cowpox encoded protein CPXV203 (168.18)

Abstract

Abstract Viral dissemination can be greatly facilitated by immune evasion proteins that subvert MHCI antigen presentation, thereby conferring the ability to evade cytotoxic T-lymphocyte (CTL) detection and clearance. Surprisingly, ablation of MHCI-specific immune evasion proteins has been reported to be dispensable for both immediate and latent MCMV and RhCMV infections, while murine infection by the equivalent mutant cowpox virus (ΔCPXV012, ΔCPXV203) showed attenuated virulence in a CTL-dependent manner. Here we show that the cowpox virus encoded CPXV203 protein inhibits MHCI surface expression by directly binding mature MHCI heterodimers, thus bridging MHCI to the KDEL receptor (KDELR) rescue pathway via the viral protein’s C-terminal ER-retention sequence (KTEL). Similar to KDELR/KDEL binding, CPXV203/MHCI binding was found to be pH-dependent, insuring high-affinity association of KDELR/CPXV203/MHCI in the Golgi for retrieval to the ER. Ensuing structural studies of CPXV203/MHCI revealed that CPXV203 binds MHCI through conserved tapasin, CD8, & NKR interaction sites. Mutagenesis of the CPXV203/MHCI interface followed by cellular & biophysical studies identified the critical importance of CPXV203 recognition of the MHCI α3 domain and CPXV203 histidines that confer pH-dependent binding. These studies clarify mechanistically how CPXV203 coordinates with CPXV012 to effectively block antigen presentation through the targeting of distinct MHCI assembly states.