Infi ltration of human melanomas with cytotoxic immune cells correlates with spontaneous type I IFN activation and a favorable prognosis. Therapeutic blockade of immune-inhibitory receptors in patients with preexisting lymphocytic infi ltrates prolongs survival, but new complementary strategies are needed to activate cellular antitumor immunity in immune cell-poor melanomas. Here, we show that primary melanomas in Hgf-Cdk4 R24C mice, which imitate human immune cell-poor melanomas with a poor outcome, escape IFN-induced immune surveillance and editing. Peritumoral injections of immunostimulatory RNA initiated a cytotoxic infl ammatory response in the tumor microenvironment and signifi cantly impaired tumor growth. This critically required the coordinated induction of type I IFN responses by dendritic, myeloid, natural killer, and T cells. Importantly, antibodymediated blockade of the IFN-induced immune-inhibitory interaction between PD-L1 and PD-1 receptors further prolonged the survival. These results highlight important interconnections between type I IFNs and immune-inhibitory receptors in melanoma pathogenesis, which serve as targets for combination immunotherapies. SIGNIFICANCE:Using a genetically engineered mouse melanoma model, we demonstrate that targeted activation of the type I IFN system with immunostimulatory RNA in combination with blockade of immune-inhibitory receptors is a rational strategy to expose immune cell-poor tumors to cellular immune surveillance. Cancer Discov; 4(6);

Enhanced green fluorescent protein (EGFP) is a novel marker gene product, which is readily detectable using techniques of fluorescence microscopy, flow cytometry, or macroscopic imaging. In the present studies, we have examined the immunogenicity of EGFP in murine models. A stable transfectant of the transplantable CMS4 sarcoma of BALB/c origin expressing EGFP, CMS4-EGFP-Zeo, was generated. Splenocytes harvested from mice immunized with a recombinant adenovirus expressing EGFP (Ad-EGFP) were restimulated in vitro with CMS4-EGFP-Zeo. Effector lymphocytes displayed strong cytotoxicity against CMS4-EGFPZeo, but not against mock-transfected CMS4-Zeo tumor