Precision use of radiation for in situ cancer immunization - PROJECT SUMMARY/ABSTRACT The successes of immune checkpoint inhibitors (ICI) have demonstrated the power of T cells to reject tumors, but responses are limited to a subset of patients. Recent data show that cancer cell-intrinsic cGAS/STING signaling is a prerequisite for effective immunotherapy, suggesting that treatments such as radiation therapy (RT) that activate this pathway should synergize with ICI. Consistently, we have previously shown that RT- induced cancer cell-intrinsic cGAS/STING signaling is required for the activation of systemic anti-tumor T cell responses in combination with ICI. Despite the substantial preclinical data that RT increases responses to ICI, clinical evidence has been inconsistent with a mixture of positive, negative and inconclusive results in clinical studies, challenging initial assumptions that RT could be broadly beneficial in unselected patient populations. These results highlight the need to identify the determinants of RT’s ability to convert a tumor into a hub for in situ immunization. Efforts have largely focused on investigating the barriers at the level of the tumor microenvironment and on improving RT use (e.g., dose and delivery schedule) to maximize its pro-immunogenic effects. The proposed studies represent a shift in focus from the search for actionable targets in the immune compartment to the role of the most frequently mutated gene in human tumors, p53, as a central regulator of the pro-immunogenic effects of RT. Recent data demonstrate that p53 modulates the IFN-I response induced by cytosolic DNA via cGAS/STING, with opposite effects of wild type p53 (p53wt), which increases the response by promoting TREX1 degradation, and mutant p53, which decreases it by hindering the interaction of TBK1 with STING and IRF3. The central hypothesis of this application is that the p53 mutational status of a tumor is a key determinant of the ability of RT to induce cancer cell-intrinsic IFN-I pathway activation and in situ immunization. To test this hypothesis, we will use several p53-isogenic mouse and human cancer cell lines and primary patient-derived tumor organoids (PDOs) that carry the most common mutations in p53 gene. Firstly, we will determine if RT-induced IFN-I is increased by p53wt and decreased by different p53 mutants in human and mouse cancer cells in vitro. Secondly, the effect of p53 status on RT-induced immune activation will be determined in syngeneic immunocompetent mouse models. Thirdly, we will evaluate RT-induced IFN-I pathway activation in a cohort of 38 PDOs (derived from breast, colorectal, and lung cancers) with different p53 mutations and 38 tumor type-matched p53 wild type control PDOs.
