PROJECT ABSTRACT Host-microbe interactions profoundly impact cancer. This is exemplified by well-documented infections that promote cancer, and the ability to prevent these cancers through vaccination or pathogen avoidance. However, humans are densely colonized with trillions of normally beneficial microbes, termed the microbiota, which also have the ability to promote cancers through the induction of inflammation or genomic instability. Further, recent seminal studies demonstrated that intestinal microbiota are also required for anti-tumor immunity in the context of therapeutic interventions, such as checkpoint blockade. Despite these advances, the specific pathways by which microbiota shape pro- versus anti-tumor immunity remain poorly defined, and the potential relevance of these findings to specific types of cancer are unknown. The fundamental focus of this proposal is to mechanistically define a novel pathway that controls host-microbiota interactions to protect from tumor progression and promote the efficacy of immunotherapies in colorectal cancer (CRC). In recently published data (Goc et al., Cell, 2021), we have determined that group 3 innate lymphoid cells (ILC3s) are fundamentally altered in CRC and contribute to tumor progression and immunotherapy responsiveness by coordinating host-microbiota interactions. These data provoke a fundamental hypothesis that intestinal ILC3s are protective in cancer, but become inherently disrupted in CRC, subsequently driving dysfunctional adaptive immunity and alterations to the microbiota that support tumor progression and immunotherapy resistance. We will mechanistically test this hypothesis by asking the following specific questions: (1) What drives dysfunction of ILC3s in CRC?; (2) What are the microbial and host pathways by which ILC3s protect from tumor progression?; And (3) What are the microbial and host pathways by which ILC3s protect from immunotherapy resistance? Finally, we will directly test a number of interventional strategies that target the microbiota to limit tumor progression and break resistance to cancer checkpoint inhibitors. Results from these experiments will pave the way for a greater understanding of host-microbiota interactions in cancer, and could provoke novel preventative, therapeutic or curative strategies in cancer by modulating host-microbiota interactions.