Cellular and Humoral Immunity against Vaccinia Virus Infection of Mice

Despite the widespread use of vaccinia virus (VV) as a vector for other Ags and as the smallpox vaccine, there is little information available about the protective components of the immune response following VV infection. In this study, protection against wild-type VV was evaluated in mice with respect to the relative contributions of CD8⁺ T cells vs that of CD4 ⁺ T cells and Ab. C57BL/6 mice primed with the Western Reserve strain of VV mount significant IgM and IgG Ab responses, specific cytotoxic T cell responses, IFN-γ responses in CD4⁺ and CD8⁺ T cells, and effectively clear the virus. This protection was abrogated by in vivo depletion of CD4⁺ T cells or B cells in IgH^-/- mice, but was not sensitive to CD8⁺ T cell depletion alone. However, a role for CD8⁺ T cells in primary protection was demonstrated in MHC class II^-/- mice, where depleting CD8⁺ T cells lead to increase severity of disease. Unlike control MHC class II^-/- mice, the group depleted of CD8⁺ T cells developed skin lesions on the tail and feet and had adrenal necrosis. Adoptive transfer experiments also show CD8⁺ T cells can mediate protective memory. These results collectively show that both CD4⁺ and CD8⁺ T cell-mediated immunity can contribute to protection against VV infection. However, CD4 ⁺ T cell-dependent anti-virus Ab production plays a more important role in clearing virus following acute infection, while in the absence of Ab, CD8⁺ T cells can contribute to protection against disease.