PROJECT SUMMARY/ABSTRACT Immune checkpoint inhibitors (ICIs) have significantly improved long-term survival across diverse cancer types including melanoma, non-small cell lung cancer, triple negative breast cancer, and others. However, ICI efficacy relies on multiple cancer, host, and environmental variables, and only a small fraction of patients will respond to these antibody drugs. Methods to improve ICI responsiveness are therefore a highly desirable, unmet clinical need. Human-associated microbes are critical regulators of host health and disease including cancer treatment. Clinical studies have shown that specific gut bacterial species correlate with improved patient outcomes of ICI therapy, and colonization by these active microbes can directly elicit antitumor activity in preclinical animal models. These observations raise a fundamental question: what are the microbial mechanisms that dictate ICI efficacy? My previous work has demonstrated that a secreted bacterial peptidoglycan hydrolase is sufficient to broadly improve ICI therapy in murine models of cancer. Moreover, this phenotype could be recapitulated simply by coadministration of a synthetic fragment that mimics the product of the peptidoglycan hydrolase. These findings raise the exciting hypothesis that the production of microbial metabolites can directly improve ICI efficacy. The main objective of my proposal is to examine enzymatic mobilization of bacterial PG metabolites as a general mechanism of immune modulation during cancer ICI therapy. Aim 1 will explore host enzymes as new factors that determine ICI efficacy. Aim 2 will produce chemical probes to discover ICI-activating bacterial enzymes. Aim 3 will examine PG mobilization as a broad-spectrum strategy to potentiate ICI response in new indications and against new checkpoint targets. To accomplish these goals, I have built a broad and interdisciplinary skill set from my graduate work in chemical tool development with Dr. Linda Hsieh-Wilson at Caltech and my postdoctoral work in host-microbial communication and cancer immunology with Dr. Howard Hang at Scripps Research. To complement these strengths, I have established collaborations with leaders in the fields of cancer immunotherapy and host-microbial interactions to provide training in new cancer model systems and access to critical human-derived isolates, which will greatly aid in my efforts to establish the generality and human relevance of PG mobilization during ICI treatment. In addition, I have proposed a comprehensive career development plan to address any residual gaps in my abilities to effectively manage a laboratory, disseminate our findings, and obtain independent funding. The acquisition of these skills during the K22 period will fuel progress towards the completion of my proposal, providing key preliminary data needed for my first NCI R01 grant application. My scientific and career development enabled by the K22 award will help me to achieve my long-term career goal to become a successful independent investigator at the intersection of host-microbial communication and cancer immunotherapy. Moreover, these efforts may yield mechanistic insights and translational avenues to understand and augment differential ICI responses in the clinic.

PROJECT NARRATIVE Cancer immunotherapies including checkpoint inhibitors have revolutionized the treatment of multiple cancer types, yet these drugs are only effective in a small subset of patients. Although specific members of the gut microbiota have been positively correlated with patient outcomes, the mechanisms underlying these clinical observations remain largely unknown. Using a combination of chemical tools, biochemical characterization, and in vivo model systems, this proposal will examine enzymatic mobilization of microbial glycans from the gut as a common mechanism to prime host immune responses during checkpoint blockade, with the goal of augmenting the efficacy of current and next-generation checkpoint immunotherapies.