Direct priming of CD8+ T cells persists in the face of cowpox virus inhibitors of antigen presentation

Vaccinia virus (VACV) was the vaccine used to eradicate smallpox and is being repurposed as a vaccine vector. CD8+ T cells are key antiviral mediators but require priming to become effector or memory cells. Priming requires an interaction with dendritic cells that are either infected (direct priming) or have acquired virus proteins but remain uninfected (cross priming). To investigate CD8+ T cell priming pathways for VACV, we engineered the virus to express CPXV12 and CPXV203, two inhibitors of antigen presentation encoded by cowpox virus. These intracellular proteins would be expected to block direct but not cross priming. The inhibitors had diverse impacts on the size of anti-VACV CD8+ T cell responses across epitopes and by different infection routes in mice, superficially suggesting variable use of direct and cross priming. However, when we then tested a form of antigen that requires direct priming, we found, surprisingly, that CD8+ T cell responses were not diminished by coexpression with CPXV12 and CPXV203. We then directly quantified the impact of CPXV12 and CPXV203 on viral antigen presentation using mass spectrometry, which revealed strong but incomplete inhibition of antigen presentation by the CPXV proteins. Therefore, direct priming of CD8+ T cells by poxviruses is robust enough to withstand highly potent viral inhibitors of antigen presentation. This is a reminder of the limits of viral immune evasion and shows that viral inhibitors of antigen presentation cannot be assumed to dissect cleanly direct and cross priming of antiviral CD8+ T cells.