An optimized retinoic acid-inducible gene I agonist M8 induces immunogenic cell death markers in human cancer cells and dendritic cell activation

RIG-I is a cytosolic RNA sensor that recognizes short 5' triphosphate RNA, commonly generated during virus Infection. Upon activation, RIG-I initiates Antiviral immunity, and in some circumstances, induces cell death. Because of this dual capacity, RIG-I has emerged as a promising target for Cancer Immunotherapy. Previously, a sequence-optimized RIG-I agonist (termed M8) was generated and shown to stimulate a robust immune response capable of blocking viral Infection and to function as an Adjuvant in vaccination strategies. Here, we investigated the potential of M8 as an anti-cancer agent by analyzing its ability to induce cell death and activate the immune response. In multiple Cancer cell lines, M8 treatment strongly activated Caspase 3-dependent Apoptosis, that relied on an intrinsic NOXA and PUMA-driven pathway that was dependent on IFN-I signaling. Additionally, cell death induced by M8 was characterized by the expression of markers of immunogenic cell death-related damage-associated molecular patterns (ICD-DAMP)-calreticulin, HMGB1 and ATP-and high levels of ICD-related cytokines CXCL10, IFNβ, CCL2 and CXCL1. Moreover, M8 increased the levels of HLA-ABC expression on the tumor cell surface, as well as up-regulation of genes involved in antigen processing and presentation. M8 induction of the RIG-I pathway in Cancer cells favored dendritic cell phagocytosis and induction of co-stimulatory molecules CD80 and CD86, together with increased expression of IL12 and CXCL10. Altogether, these results highlight the potential of M8 in Cancer Immunotherapy, with the capacity to induce ICD-DAMP on tumor cells and activate immunostimulatory signals that synergize with current therapies.