Project Summary/Abstract Triple-negative breast cancer (TNBC) is characterized by the lack of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2, all of which are important therapeutic targets. TNBC is the most difficult-to-treat subgroup of breast cancers and is resistant to many current cancer therapies. The present situation of poor prognosis with limited therapy options in TNBC emphasizes an urgent need for more effective therapeutics. The ability to escape from the surveillance by the immune system is regarded as one of the essential hallmarks of cancer cells. Recent exciting discoveries have identified many important signals and mechanisms mediating cancer cell immune evasion. Immunotherapies have been developed to target these signals, revolutionizing the treatment of a variety of human cancers. Tumor-associated macrophages (TAMs) represent the major components of the tumor microenvironment in TNBC. Recent studies demonstrate that the blockade of a “don’t eat me” signal CD47 leads to direct phagocytosis of living cancer cells by macrophages, and significantly inhibits the engraftment of various malignant hematopoietic and solid tumor cells in mice that lack T, B, and NK cells, indicating a critical role of macrophages in cancer immunosurveillance. Targeting TAMs in the tumor microenvironment represents a new class of promising cancer immunotherapy. While inducing anticancer functions of TAMs holds considerable promise for cancer treatment, there are several barriers that need to be overcome to achieve desired efficacy for treating TNBC. In preliminary studies, we found that TAMs can be reprogrammed by small molecule antineoplastic compounds to induce their phagocytic ability against TNBC cells. However, the underlying molecular mechanisms regulating the reprogramming of macrophages remains unclear. The overall objective of the proposed study is to understand the underlying mechanisms of macrophage-mediated immunosurveillance in TNBC and to develop strategies to effectively treat TNBC by exploiting tumoricidal roles of TAMs, with a combination of in vitro and in vivo preclinical TNBC models. In Aim1, we will assess the efficacy of reprogramming macrophages in TNBC treatment by using metastatic TNBC models and chemotherapy-resistant patient-derived xenograft models. In Aim2, we will study the molecular mechanisms by which macrophages are reprogrammed by dissecting the functions and roles of Pattern Recognition Receptor signaling pathways in macrophage reprogramming and characterizing TAM subgroups in TNBC tumors. In Aim3, we will determine the effects of targeting macrophage cell surface molecular machinery on activating TAMs for TNBC treatment. Successful completion of the proposed studies should shed light on the basic principles of cancer cell immune evasion and inspire the development of novel therapeutics for TNBC treatment.