PROJECT SUMMARY Microbiota are associated with remarkable effects on host health and disease. Notably, discrete species of commensal bacteria have been correlated with improved patient responses to cancer immunotherapy. However, the molecular mechanisms underlying the functions of these beneficial bacteria remain poorly understood. In particular, specific strains of Enterococci have been linked with improved response to anti-PD-1/PD-L1 treatment in patients with metastatic melanoma, lung, and kidney cancers, but their mechanism of action has not been elucidated nor employed to improve cancer immunotherapy. Recent work from the Hang laboratory has demonstrated that these beneficial strains of Enterococci have unique peptidoglycan composition and remodeling enzymes. Based on these studies, this project hypothesizes that specific strains of Enterococci may prime innate immune signaling pathways and enhance anti-PD-1/PD-L1 immunotherapy against metastatic cancers. To evaluate the activity and mechanism(s) of Enterococci during immunotherapy as well as co-opt their protective factors for cancer immunotherapy, this proposal will examine how specific Enterococci strains alter cancer growth, immune cell populations, and microbiota composition in mouse models of cancer immunotherapy. In addition, the Hang laboratory will identify Enterococci protective factors and engineer them into existing human probiotics to translate our basic microbiota-cancer immunotherapy findings into novel therapeutic agents. Finally, the Hang laboratory will also synthesize novel immunomodulatory small molecules that activate host pathways used by Enterococci to enhance cancer immunotherapy. These studies will reveal fundamental microbiota- cancer immunotherapy mechanisms and develop new therapeutic strategies and agents to enhance cancer immunotherapy.