ABSTRACT: Microbes in the gastrointestinal tract (GI) play a critical role in maintaining the metabolic and immunological health of their human hosts. The impact of the microbiota on local and systemic immunity is being increasingly recognized as an important modifier of the success of immunotherapies, including those directed against cancers. For instance, germ free animals colonized with microbiotas derived from cancer patients that responded (responders - R) to immune checkpoint inhibitors (ICI) displayed enhanced clearance of tumors upon ICI therapy as compared to animal colonized with microbiotas derived from patients that did not respond (non responders - NR). One of the organisms that is consistently associated with R patients is the obligate anaerobic bacterium Akkermansia muciniphila, which may contribute to dampening mucosal inflammation and regulating systemic immunity. Importantly for cancer immunotherapy, administration of A. muciniphila is sufficient to enhance the effectiveness of ICIs in mouse models of renal cell carcinomas (RCC) and non-small cell lung cancers. We propose to survey the diversity of clinical A. muciniphila isolates in two unique patient populations (one focused on cancer and one on metabolic disease), test their impact in mouse models of cancer immunotherapy, and apply new tools in Akkermansia molecular genetics to define the contribution of various A. muciniphila factor(s) on successful anti-tumor therapy. We expect that by the end of this study that we will have identified strains with the greatest potential for use as “adjuvants” of ICI and defined the genetic basis of some of the traits responsible for their enhanced activities. Given the current commercial interest on Akkermansia as a probiotic, we envision a relatively rapid path from strain identification and/or engineering to a biological that can be fast-tracked for clinical use.