Orthopoxviruses are clinically important infectious agents. In their large DNA genomes, orthopoxviruses encode many immune evasion genes targeting both innate and adaptive immune effectors. Although previous studies suggested that CD8 T cells play an important role in defense against orthopoxvirus infection, evasion of CD8 T cell responses by orthopoxvirus has not been reported. This is in contrast with other DNA viruses that encode one or more genes inhibiting MHC class I antigen presentation to escape antiviral CD8 T cell responses.

Cowpox virus (CPXV), an orthopoxvirus with wild rodent reservoirs, is ideal for the study of orthopoxvirus-encoded immune evasion mechanism. We demonstrated that CPXV downregulates MHC class I molecules on the cell surface. Two CPXV-encoded proteins that mediate MHC class I downregulation, CPXV203 and CPXV12, were identified by gain-of-function and loss-of-function screening, respectively. Our data suggest that both proteins inhibit the trafficking of MHC class I molecules to the cell surface, but via distinct mechanisms. CPXV203 physically interacts with MHC class I molecules and retains them in the ER via its C-terminal KDEL-like motif, whereas CPXV12 impairs optimal peptide loading of MHC class I. Viruses deleted of both CPXV12 and CPXV203 were unable to downregulate MHC class I and exhibited attenuated virulence in murine hosts. CD8 T cells did not confer protection against CPXV if these proteins were expressed, indicating MHC class I downregulation can abrogate antiviral CD8 T cell response against CPXV.

Taken together, we report the original observation of CPXV-mediated MHC class I downregulation and demonstrate its molecular bases. Importantly, we provide in vivo evidence that CPXV escapes antiviral CD8 T cell response by evading MHC class I antigen presentation. Our study contributes to better understanding of the interaction between orthopoxviruses and their hosts.