Tumor-Intrinsic RIG-I Signaling Promotes Anti-CTLA-4 Checkpoint Inhibitor-Mediated Anticancer Immunity

Strong inter-individual variation in clinical response to checkpoint inhibitors such as anti-CTLA-4 remains a major challenge. In a retrospective analysis, expression of viral defense genes in human melanomas including the cytosolic RNA receptor RIG-I (DDx58) has been associated with clinical benefit to CTLA-4 blockade. However, possible interconnections of these two distinct pathways remain unknown. Using CRISPR/Cas9 technology to generate B16 melanoma cells lines that lack nucleic acid receptors or downstream signaling molecules (RIG-I, STING, IRF3/7) together with available genetically deficient mouse models, we addressed the importance of nucleic acid receptor signaling in both tumor and host cells for the efficacy of anti-CTLA-4 immunotherapy. We here provide experimental proof that anti-CTLA-4 immunotherapy relies on tumor cell-intrinsic RIG-I but not STING signaling. Following anti-CTLA-4 treatment, tumor-intrinsic RIG-I signaling critically impacts on cross-presentation of tumor-associated antigen by CD103⁺ dendritic cells, the expansion of tumor antigen-specific CD8⁺ T cells and ultimately the accumulation of CD8⁺ T cells within tumor tissue. Consistently, therapeutic targeting of RIG-I with 5'-phosphorylated-RNA in both tumor and non-malignant cells potently augmented the efficacy of CTLA-4 checkpoint blockade. These processes were additionally dependent on host STING, MAVS and type I interferon signaling and were closely linked to RIG-I-mediated tumor cell death. In summary, our study identifies both tumor- and host-intrinsic RIG-I signaling as fundamental requirements for anti-CTLA-4-mediated antitumor immunity. Our data predict that targeting RIG-I may serve as a basis for the development of new combination strategies to increase the response rate of checkpoint inhibitor-based immunotherapy of malignancy including lymphoma, particularly in individuals that do not have a sufficient spontaneous antitumor T-cell immune response.