ABSTRACT Certain mouse and human cancers stimulate profound expansion of a type of highly immunosuppressive immune cell called polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs). These cells potently inhibit antitumor immunity and thus remain a significant barrier to the efficacy of immunotherapy-based treatment modalities for cancers that induce PMN-MDSCs. Mouse and human breast cancer including the triple negative subset (TNBC) is one such cancer that efficiently expands PMN-MDSCs, which negatively correlates with clinical outcomes in FDA-approved chemo-immunotherapy (CTx-I) protocols for a particular TNBC subset that expresses PD-L1. Unfortunately, efforts to improve CTx-I strategies for PMN-MDSC inducing cancers by therapeutically targeting these cells have proven unsuccessful. To this end, we found that our clinical-stage immunotherapy drug called entolimod stimulates antitumor immunity against metastatic 4T1, a well-recognized pre-clinical PD-L1+ TNBC tumor model that expands PMN-MDSCs. In fact, we found that entolimod stimulates antitumor immunity against 4T1 in part by diminishing the immunosuppressive activity of PMN-MDSCs. Moreover, mirroring the CTx-I protocols used to treat patients diagnosed with PD-L1+ TNBC, we found that entolimod boosts antitumor efficacy of CTx-I against 4T1, albeit, through unresolved mechanisms. Despite these findings in pre-clinical PD-L1+ TNBC, to what extent these findings translate to human PMN-MDSCs in breast cancer patients and implications of these findings for boosting CTx-I in cancer patients remain unclear. Thus, this proposal seeks to uncover the mechanistic underpinnings by which entolimod dampens PMN-MDSC activity in both mice and humans in order to fuel the design of improved CTx-I therapies for those cancers in which efficacy is hindered by PMN-MDSC expansion including PD-L1+ TNBC. And in this regard, entolimod has successfully completed Phase I safety trials cumulatively involving nearly 200 subjects including healthy volunteers and cancer patients thus facilitating the clinical translation of entolimod with CTx-I protocols.