Breast cancer (BC) is the second most common non-dermatologic cancer and the second leading cause of cancer-related death of women in the United States. Tumor size, tumor grade, and lymph node status are key predictors of outcome. Patients with poor prognostic markers are given chemotherapy while patients with favorable prognostic markers receive conservative treatment. Unfortunately, 20%-30% of women with favorable prognostic markers still develop metastatic disease within 10 years. Thus, there is a critical need for (1) additional prognostic assays to accurately predict risk of disease recurrence, (2) assays that predict response to therapy, and (3) development of novel therapeutic strategies providing durable tumor suppression. The immediate benefit of these improved capabilities to women with BC will be to increase survival. While BC has not historically been linked to underlying inflammation or infection, it exhibits tumor-associated inflammation marked by infiltration of immune cells into developing tumors. In BC, macrophages are one of the most abundant immune cells present. Data from our lab demonstrate that mammary tumor-bearing mice treated with a novel drug (PLX3397) that reduces infiltration of macrophages into tumors enhances sensitivity to chemotherapy, thereby reducing the spread of cancer to the lungs and prolonging survival. In addition, antibodies targeting macrophages resulted in similar effects, confirming that targeting macrophage function is an important mechanism for increasing response to chemotherapy. PLX3397 is a novel drug that is completing early phase testing in humans. Preliminary results indicate that this drug has few side effects and shows promise in treatment of advanced staged tumors. In this proposal, we will determine the clinical benefit of this entirely novel approach for the treatment of BC. We seek to achieve the following aims: (1) expand our understanding of macrophages in human BC by isolating macrophages and examining their gene expression signatures to identify unique components to use as biomarkers for risk stratification and response to therapy;( 2) using animal models of BC, determine which subtypes of BC can be inhibited by combining macrophage-targeted therapy with chemotherapy in order to find the most effective combination regimen; and (3) test the best regimen identified in the animal experiments with BC-vaccines to identify the best combination therapy providing durable tumor suppression. Targeting tissue-associated macrophages is a completely new way of treating cancer, and laboratory data indicate that it could be highly effective in treatment of advanced stage BCs, with potential to prolong survival in advanced stage disease and minimize chemotherapy-resistant disease. Successful completion of our aims would provide patients with metastatic BC access to this treatment in the next 3 years and could allow for its use in a clinical trial for early stage patients in the next 5 years. The proposed approach will represent a significant advance in the treatment of BC and will help address the biologic basis for varying outcomes of the different molecular subtypes of BC.