PROJECT SUMMARY/ABSTRACT Immune checkpoint inhibitors (ICIs) have revolutionized cancer immunotherapy but remain effective in only a subset of patients and tumor types. The successful treatment of tumors that lack T cell infiltration, known as immunologically cold tumors, remains a major roadblock in cancer immunotherapy. Tumor mutational burden has been correlated with ICI efficacy due to the abundance of neoantigens that are recognizable by the immune system after ICI. This proposal aims to enhance the visibility of multiple tumor types with low mutational burden by inducing the expression of neoantigens through RNA editing approaches. Our preliminary data in a highly aggressive, immunologically cold model of murine melanoma show that the combination of RNA editing and anti-PD-1 ICI results in a significant survival benefit over anti-PD-1 alone. We hypothesize that the induction of neoantigens enhances T cell infiltration into the tumor and promotes epitope spreading such that the immune system recognizes endogenous tumor antigens in addition to induced neoantigens. We will test this hypothesis with two specific aims. Aim 1 will elucidate the mechanism of action of this novel cancer immunotherapy through transcriptomic profiling of isolated tumor cells, analysis of tumor- infiltrating immune cells, and evaluation of the abscopal effect whereby growth of a distant tumor is suppressed following localized treatment of the primary tumor. Aim 2 will determine the generalizability of this approach to additional immunologically cold murine cancer models as well as its translation to human cancers through evaluation in multiple patient-derived tumor organoids. The successful completion of these aims would provide preclinical validation and further support the advancement of this innovative approach. This strategy has the potential to diversify the neoantigen repertoire and expand ICIs as frontline therapies in many tumor types, improving clinical outcomes for cancer patients.