The large size of poxvirus genomes has stymied attempts to identify determinants recognized by CD8⁺ T cells and greatly impeded development of mouse smallpox vaccination models. Here, we use a vaccinia virus (VACV) expression library containing each of the predicted 258 open reading frames to identify five peptide determinants that account for approximately half of the VACV-specific CD8⁺ T cell response in C57BL/6 mice. We show that the primary immunodominance hierarchy is greatly affected by the route of VACV infection and the poxvirus strain used. Modified vaccinia virus ankara (MVA), a candidate replacement smallpox vaccine, failed to induce responses to two of the defined determinants. This could not be predicted by genomic comparison of viruses and is not due strictly to limited MVA replication in mice. Several determinants are immunogenic in cowpox and ectromelia (mousepox) virus infections, and immunization with the immunodominant determinant provided significant protection against lethal mousepox. These findings have important implications for understanding poxvirus immunity in animal models and bench-marking immune responses to poxvirus vaccines in humans.

“Cross-priming” describes the activation of naïve CD8 ⁺ T cells by professional antigen-presenting cells that have acquired viral or tumor antigens from “donor” cells. Antigen transfer is believed to be mediated by donor cell–derived molecular chaperones bearing short peptide ligands generated by proteasome degradation of protein antigens. We show here that cross-priming is based on the transfer of proteasome substrates rather than peptides. These findings are potentially important for the rational design of vaccines that elicit CD8 ⁺ T cell responses.

The role of CD8+ T cells in resistance to herpes simplex virus (HSV) was examined. After cutaneous inoculation, HSV spreads to the peripheral nervous system (PNS) where it replicates in ganglionic neurons. In normal mice, replication of virus in the PNS was rapidly terminated and evidence of neuronal destruction, assessed by a quantitative histological assay, was sparse. Clearance of infectious virus was impaired, and a strikingly high proportion of ganglionic neurons was killed, in mice treated with an antibody that depleted them of CD8+ T cells. These results suggest that CD8+ T cells play an important role in maintaining the integrity of the sensory nervous system during primary infection with HSV. Therefore, viral epitopes recognized by CD8+ T cells and restricting class I major histocompatibility complex genes are, in principle, implicated as interacting genetic determinants of neurovirulence.