Summary MDSCs are one of the major types of immune cells that contribute to tumor-induced immune suppression and escape from immune elimination. Importantly, MDSCs have been suggested to contribute to resistance to various cancer therapies, including to anti-CTLA-4 and anti-PD-1 blockade. Hence, targeting MDSCs could be an attractive approach to modulate tumor immunity to improve current cancer immunotherapies. We have very recently reported that phenformin, a mitochondrial respiratory chain complex I inhibitor, selectively reduced proportion of polymorphonuclear MDSCs (PMN-MDSCs), but not other immune cells in spleens of tumor-bearing mice. Phenformin also inhibited proliferation of PMN-MDSCs derived from bone marrow co-cultured with tumor cells in vitro. Furthermore, PMN-MDSCs derived from mice treated with phenformin showed attenuated suppressive activities towards CD8+ T cells in T cell proliferation assays. In our unpublished metabolomics profiling studies of PMN-MDSCs, we have uncovered additional potential metabolic vulnerabilities of PMN- MDSCs. Based on these findings, we hypothesize that PMN-MDSCs possess distinguishing metabolic features. Given the important contribution of MDSCs to tumor immune escape and immunotherapy resistance, we further hypothesize that targeting the metabolic vulnerabilities of PMN-MDSCs may cooperate with immune checkpoint blockade to unleash T cell responses, leading to improved anti-tumor efficacy. In aim 1, we plan to characterize the metabolic vulnerabilities of PMN-MDSCs. We will not only elucidate metabolic features of PMN-MDSCs that underlie their sensitivity to phenformin, but also identify other metabolic targets of PMN-MDSCs, such as those involved in the glutamine metabolism. In aim 2, we will assess the therapeutic benefit of combining targeting the metabolic vulnerabilities of PMN-MDSCs with immune checkpoint blockade in mouse cancer models. We will evaluate the effects of metabolic drugs in combination with anti-PD-1 blockade, on the tumor microenvironment and analyze the contribution of PMN-MDSC modulatory activity of these metabolic drugs using the PMN-MDSC adoptive transfer approach.