Vaccinia virus infection of mature dendritic cells results in activation of virus-specific naïve CD8+ T cells: a potential mechanism for direct presentation.

Understanding how vaccinia virus (VV) generates immunity necessitates an appreciation for how this virus interacts with dendritic cells (DC), which are the most potent activators of naïve CD8(+) T cells. In order to optimally activate naïve CD8(+) T cells, DC must undergo maturation, during which costimulatory molecules are upregulated and cytokines are produced. In this report, we show that VV infection of immature murine bone marrow-derived DC (BMDC) failed to induce maturation. Similar results were obtained when CD8(+) DC were analyzed, a subset shown previously to be important in vivo in the generation of a vaccinia-specific response. The finding that VV infection of DC resulted in APC that were incapable of initiating T-cell activation was surprising given the previously reported role for direct presentation in the generation of anti-VV CD8(+) T-cell responses in mice. To address the potential mechanism responsible for direct presentation, we tested the hypothesis that previously matured DC were susceptible to vaccinia virus infection and could present newly synthesized VV-derived epitopes for CD8(+) T-cell activation. Our results show, that during VV infection of mature DC, threshold levels of viral protein are produced that promote T-cell activation. These results suggest that, even though VV cannot mature DC, previously matured DC exposed to VV can generate a VV-specific CD8(+) T-cell response providing a potential mechanism by which direct infection results in T-cell activation in vivo.