Targeted delivery of murine IFN-γ using a recombinant fowlpox virus: NK cell recruitment to regional lymph nodes and priming of tumor-specific host immunity

Interferon-γ (IFN-γ) is a proinflammatory cytokine that also acts as a potent immunomodulatory agent. In this study, a replication-deficient recombinant avian (fowlpox) virus was engineered to express the murine IFN-γ gene (rF-MuIFN-γ) with the rationale of delivering concentrated levels of the cytokine to a local tissue microenvironment. Subcutaneous (s.c.) rF-MuIFN-γ administration resulted in IFN-γ production that (1) was restricted to the tissue microenvironment of the injection site and (2) was biologically active, as evidenced by a significant increase of class I MHC expression levels in s.c. growing tumors following rF-MuIFN-γ administration. Infection of a highly tumorigenic murine cell line, MC38, with rF-MuIFN-γ functioned as an effective tumor cell-based vaccine by protecting mice from the formation of primary tumors and from subsequent tumor challenge. The cell-based vaccine was completely ineffective if mice were vaccinated with MC38 cells either pretreated with rIFN-γ or infected with the wild-type fowlpox virus (FP-WT). Analysis of the regional lymph nodes draining the site of injection of the rF-MuIFN-γ- based tumor cell vaccine revealed the presence of tumor-specific cell lysis (CTL) as well as a significant amount of lysis directed at natural killer (NK)-sensitive YAC-1 cells. Flow cytometric analyses coupled with functional assays confirmed the sustained presence of NK1.1⁺ cells within those draining lymph nodes for up to 5 days after rF-MuIFN-γ injection. Mice treated with NK cell-depleting antibodies prior to the injection of the rF-MuIFN-γ- infected MC38 tumor cells were not protected from primary tumor growth; analysis of the lymph nodes draining the injection site in NK-depleted mice revealed an accompanying loss of the tumor-specific CTL activity. The findings provide evidence that NK cells, known for their contributions to host innate immunity, also provide immunoregulatory signals required for the development of an adaptive immune response, which, in turn, protected vaccinated mice against tumor growth.