AbstractSignal transducer and activator of transcription 1 (Stat1) is considered a key transcription factor that inhibits tumorigenesis, and Stat1 activation in the host is required for interleukin-12 (IL-12)–mediated generation of CTL activity. Using syngeneic Stat1−/− C3H mice bearing SCCVII tumors in this study, we discovered opposite results. Stat1 deficiency in the host significantly enhances IL-12–mediated tumor regression, resulting in tumor eradication from 60% of SCCVII tumor–bearing mice and significant inhibition of tumor growth when compared with control treatment (P < 0.01). This effect is independent of both Stat1-activating cytokine IFN-γ and Stat1-downstream effector molecule FasL because neither neutralization of IFN-γ nor knocking out of FasL enhances or inhibits IL-12–mediated tumor regression. IL-12 induces a high intensity of tumor-specific CTL activity in Stat1-deficient mice (P < 0.01), increases the CD8 T-cell density in tumor bearing Stat1−/− mice, and induces a T-cell–dependent tumor regression. The increased CTL activity and the high-intensity infiltration of T cells into the tumors in IL-12–treated Stat1−/− mice are likely due to the longer survival than the same cells from wild-type mice. Together, the data show that inhibition of Stat1 expression in the host enhances tumor-local IL-12 gene therapy for regressing tumors. This conclusion provides a new concept for designing an effective treatment strategy. (Cancer Res 2006; 66(8): 4461-7)