IL-12 Pretreatments Enhance IFN-α-Induced Janus Kinase-STAT Signaling and Potentiate the Antitumor Effects of IFN-α in a Murine Model of Malignant Melanoma

IFN-alpha 2b (IFN-alpha) has been used to treat patients with metastatic malignant melanoma and patients rendered disease-free via surgery but at high risk for recurrence. We hypothesized that IL-12 pretreatments would result in endogenous IFN-gamma production, and that this, in turn, would up-regulate levels of Janus kinase-STAT signaling intermediates and lead to increased expression of genes regulated by IFN-alpha. Treatment of PBMCs with IL-12 stimulated a significant and dose-dependent production of IFN-gamma. Pretreatment of PBMCs and tumor cells with IFN-gamma-containing supernatants from IL-12-stimulated PBMCs led to up-regulation of STAT1, STAT2, and IFN regulatory factor 9 (IRF9) and potentiated IFN-alpha-induced STAT signaling within PBMCs and tumor cells. These effects were abrogated by neutralization of IFN-gamma in the PBMC supernatants with an anti-IFN-gamma Ab. Pretreatment of HT144 melanoma cells and PBMCs with IFN-gamma or IFN-gamma-containing supernatants enhanced the actions of IFN-alpha at the transcriptional level, as measured by real-time RT PCR analysis of the IFN-stimulated gene 15. Experiments in wild-type C57BL/6 and IFN-gamma receptor knockout (B6.129S7-Ifngr(tm1Agt)) mice demonstrated that a regimen of IL-12 pretreatment, followed by IFN-alpha, could cure mice of i.p. B16F1 melanoma tumors (p < 0.007), whereas mice treated with either agent alone or PBS succumbed to fatal tumor burden. However, this treatment regimen did not significantly prolong the survival of IFN-gamma-deficient (B6.129S7-Ifng(tm1Ts)) mice compared with mice treated with IFN-alpha alone. These results suggest that the response to IFN-alpha immunotherapy can be significantly enhanced by IL-12 pretreatment, and this effect is dependent upon endogenous IFN-gamma production and its actions on melanoma cells.