| Vaccinia virus (VV) is the prototypic member of the Poxviridae family and serves as the live vaccine for the causative agent of smallpox, variola virus, which has been eradicated due to one of the most successful worldwide vaccination programs to date. Interest in poxvirus pathogenesis has increased due to recent bioterrorism threats, the fact that a large proportion of our society remains unvaccinated to variola virus, and the desire to minimize adverse reactions to the vaccine in susceptible individuals. Moreover, VV has considerable potential as a vaccine vector. There remains a great deal to be learned about poxvirus immunity since variola virus was eradicated before significant advances in immunology and virology were made. Therefore, I have investigated the effect of VV infection on several aspects of cellular immunity, namely the initiation of a CD8+ T cell response by VV-infected dendritic cells (DC) and the effect of VV dose on the ensuing cellular immune response in vivo. These studies show that, similar to the human system, VV does not induce the maturation of mouse DC, as shown by lack of costimulatory molecule expression and inflammatory cytokine secretion. The significance of this finding is illustrated by the fact that VV-infected DC cannot activate naive CD8+ T cells in the presence of limiting concentrations of exogenous antigen. Furthermore, DC previously matured by LPS can subsequently become infected with VV and can activate naive CD8+ T cells, providing a possible mechanism for direct presentation of VV antigens by DC in vivo. I propose a model whereby uninfected immature DC in vivo are matured by necrotic cells or inflammatory mediators secreted from neighboring infected cells and are subsequently infected with VV. These mature VV-infected DC then have all signals necessary to induce effective activation of naive CD8+ T cells.; The second project presented herein investigates the effects of VV dose on the ensuing cellular immune response as a follow-up to previous studies in our lab showing a dose-dependent decrease in the number of IFNgamma-secreting CD8+ T cells in the spleen after intranasal (i.n.) infection. My results show that CD8+ T cells undergo active caspase 3-mediated apoptosis after high dose infection. Moreover, this phenomenon occurs with naive and activated CD4+ T cells as well as B cells in the spleen and mediastinal lymph node (MLN). Additionally, lymphocyte apoptosis arising from high dose infection is not a result of VV cytopathic effects, immunosuppressive glucocorticoids, IFNgamma, or Fas. Importantly, low dose VV infection results in a robust VV-specific CD8+ T cell response with little to no virus replication in infected mice, while high dose infection results in high viral loads in the lungs that are eventually cleared. Collectively these data indicate that a high virus dose can result in systemic lymphopenia while a low virus dose, even without detectable virus, can induce a robust cellular immune response.; This thesis provides significant insight into the many ways that VV can modulate the cellular immune response to infection. Not only do these findings contribute to our knowledge and understanding of poxvirus pathogenesis, but they also have implications for the use of VV as a vaccine vector. |
