Direct action of type I IFN on NK cells is required for their activation in response to vaccinia viral infection in vivo.

Type I IFN plays an important role in the activation of NK cells. However, the mechanism underlying type I IFN-dependent NK cell activation remains largely unknown. A recent report suggested that type I IFN acted on accessory dendritic cells, leading to IL-15 production, and that subsequent trans-presentation of IL-15 was required for NK cell activation upon stimulation with synthetic TLR ligands. It is not clear how type I IFN regulates NK cell activation in response to live pathogens. Using a murine model of infection with vaccinia virus (VV), we previously demonstrated a critical role for type I IFN in the innate immune control of VV infection. In this study, we first showed that type I IFN did not directly protect L929 cells from VV infection in vitro and that type I IFN-dependent innate immune control of VV infection in vivo was mediated by activated NK cells. We further demonstrated that direct action of type I IFN on NK cells, but not on dendritic cells, is required for the activation of NK cells in response to VV infection both in vitro and in vivo, leading to efficient VV clearance. Our findings may help design effective strategies for the control of poxviral infections in vivo.