Intra-tumoral delivery of 5'ppp-dsRNA induces a robust antitumor response via RIG-I activation and Bcl-2 gene downregulation in a murine model of prostate cancer

Abstract

Onco-immunotherapy via blocking checkpoint inhibitors has revolutionized the treatment-landscape of several malignancies, though not in the metastatic castration-resistant prostate cancer (PCa) owing to an immunosuppressive and poorly immunogenic "cold" tumor microenvironment (TME). Turning up the heat of such a cold TME via triggering innate immunity is now of increasing interest to restore immune-surveillance. Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) are cytosolic innate-sensors that can detect exogenous RNAs and induce type-I interferons and other pro-inflammatory signaling. RIG-I activation is suggested to be a valuable addition to the treatment approaches for several cancers. However, the knowledge about RIG-I signaling in PCa remains elusive. The present study evaluated the expression of two important RLRs, RIG-I and melanoma differentiation-associated protein 5 (MDA5), along with their downstream partners, mitochondrial antiviral-signaling protein (MAVS) and ERA G-protein-like 1 (ERAL1), during PCa progression in the transgenic adenocarcinoma of mouse prostate (TRAMP) model. The early stage of PCa revealed a significant increment in the expression of RLRs but not MAVS. However, the advanced stage showed downregulated RLR signaling. Further, the therapeutic implication of 5'ppp-dsRNA, a synthetic RIG-I agonist and Bcl2 gene silencer, has been investigated in vitro and in vivo. Intra-tumoral delivery of 5'ppp-dsRNA regressed tumor growth via triggering tumor cell apoptosis, immunomodulation, and inducing phagocytic "eat me" signals. These findings highlight that, for the first time, RIG-I activation and Bcl-2 silencing with 5'ppp-dsRNA can serve as a potent tumor-suppressor strategy in PCa and has a significant clinical implication in transforming a "cold" TME into an immunogenic "hot" TME of PCa.

Keywords: TRAMP; immunotherapy; innate immunity; pattern recognition receptor; tumor microenvironment.