PROJECT SUMMARY/ABSTRACT Colorectal cancer (CRC) is a devastating disease with no effective therapeutic options for patients suffering from advanced metastasis. Immune checkpoint inhibitors (ICIs) offer promising treatment potential for a small subset of patients with microsatellite instable (MSI)/mismatch repair deficient (dMMR) tumors. However, most CRC patients have microsatellite stable (MSS)/mismatch repair proficient (pMMR) tumors that respond poorly to ICIs. There is therefore an urgent need to identify novel targets and biomarkers of response to immunotherapy in MSS tumors, which constitute over 90% of metastatic CRC. ZFP36L2, an RNA-binding protein mutated in 5-10% of CRC, is a member of the TIS11 gene family which is known to suppress inflammation by blocking the expression of proinflammatory chemokine and cytokine mRNAs in immune cells. Preliminary data from our lab suggests that loss of ZFP36L2 function in human CRC leads to increased T cell recruitment into the normally immunologically “cold” tumor microenvironment (TME) of MSS/dMMR TME. These data suggest that ZFP36L2 may also suppress the translation of cytokines and chemokines in CRC as a mechanism of immune evasion, and further that ZFP36L2 mutations may give rise to increased immune cell infiltration into CRC tumors, which may enhance responses to immunotherapy. To address this, we have developed a novel syngeneic mouse ZFP36L2 knockout CRC organoid model and a ZFP36L2 knockout genetically engineered mouse CRC model. These tools will allow us to delineate the ability of ZFP36L2 to regulate the inflammatory secretome of CRC (Aim1) and assess how ZFP36L2 mutations impact the composition and functionality of the TME in CRC (Aim 2). This research will elucidate the functional consequences of mutations in ZFP36L2, whose function in human cancer is poorly understood. More broadly, our work will increase our understanding of how tumors shape their local immune environment to support disease progression. Our interrogation of the role of ZFP36L2 in CRC will address critical unmet needs to develop novel treatment strategies for MSS CRC and identify clinically actionable biomarkers of response to immunotherapy. This work has the potential to significantly improve the survival and quality of life for CRC patients.