Intratumoral delivery of IL-18 naked DNA induces T-cell activation and Th1 response in a mouse hepatic cancer model

BACKGROUND: The novel cytokine, interleukin (IL)-18, is a strong interferon-γ inducer and costimulatory factor in Th1 cell activation. IL-18 triggers IFN-γ production and enhances cytolytic activity in both T and NK cells. However, the exact mechanism of antitumor action of IL-18 remains to be clarified. To determine the effects of IL-18 plasmid DNA on hepatic cancer in mice, CT26 murine colon adenocarcinoma cells were established in mouse liver. METHODS: Plasmid vectors encoding IL-18 were transferred directly into the liver 7 days after tumor injection to restrict IL-18 expression within the tumor site. The IL-18 protein level was increased in the liver 4 days after plasmid injection, and a marked antitumoral effect was observed at day 7. Antitumor effects were evaluated by measuring tumor regression, immune cell population, and IFN-γ production. RESULTS: The IL-18 plasmid controlled the growth of hepatic tumors and proliferation of splenic immune cells. Moreover, treatment of CT26 tumors with the IL-18 plasmid significantly enhanced the population of the effector T and NK cells in the spleen and peripheral blood. In spleen, the population of CD4(+)CD62(Low )cells was augmented in response to IL-18 on day 7. These results are consistent with the increase in CD4(+ )T cells secreting IFN-γ, but not CD8(+ )T cells. The marked reduction of tumor growth in tumor-bearing mice was associated with the maintenance of IFN-γ production in spleen in response to IL-18. These antitumoral effects were maintained until 14 days after plasmid injection. CONCLUSION: Our results suggest that direct plasmid DNA transfer of IL-18 with no accompanying reagents to augment transfection efficiency may be useful in tumor immunotherapy.