Introduction: Targeting inhibitory T-cell receptors such as CTLA-4 has shown great promise in cancer therapy. However, strong inter-individual variation in clinical response to checkpoint inhibitors remains a major challenge. Expression of viral defense genes in human melanomas including the cytosolic RNA receptor RIG-I (DDx58) has recently been associated with clinical benefit to CTLA-4 blockade.

Methods: 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 address the importance of nucleic acid receptor signaling for the efficacy of anti-CTLA-4 immunotherapy.

Results: We provide experimental proof that anti-CTLA-4 immunotherapy indeed relies on tumor cell-intrinsic RIG-I/MAVS but not cGAS/STING signaling. Consistently, therapeutic targeting of RIG-I with the specific ligand 5'‑triphosphate-RNA (3pRNA) potently augments the efficacy of CTLA-4 checkpoint blockade. In situ vaccination by intratumoral 3pRNA injection enhanced cross-presentation of tumor-associated antigens by CD103+ migratory DCs in local draining lymph nodes. Subsequent expansion of tumor-specific CD8+ T cells resulted in control of local and distant tumors, otherwise resistant to anti-CTLA‑4 monotherapy. These processes were critically dependent on tumor cell-intrinsic RIG-I-mediated immunogenic cell death (ICD), but not tumor-derived type I IFN release. In such, the synergistic therapeutic effect of RIG-I activation and anti-CTLA-4 was restricted to tumor models that showed susceptibility to RIG‑I-mediated ICD. Furthermore, systemic antitumor immunity following anti-CTLA-4 +/- 3pRNA treatment required RIG-I signaling in both tumor and non-malignant host cells.

Conclusion: Our study identifies a hitherto unrecognized role of RIG-I signaling in tumors and their microenvironment as a crucial component for checkpoint inhibitor-mediated immunotherapy of cancer. Therapeutic targeting of RIG-I using specific agonists strongly augments the efficacy of anti-CTLA-4 blockade and may thus serve as the basis for new combinatorial approaches.

Citation Format: Simon Heidegger, Alexander Wintges, Diana Kreppel, Sarah Bek, Michael Bscheider, Martina Schmickl, Marcel R.M. van den Brink, Christian Peschel, Tobias Haas, Hendrik Poeck. Tumor- and host-intrinsic RIG-I signaling promote anticancer immunity by CTLA-4 blockade [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A005.