Despite recent advances in chemotherapies, breast cancer remains a major cause of death among women. Metastasis serves as a marker of cancer progression and dictates patient outcome; it is the ultimate cause of death in a majority of patients. Understanding the progression of breast cancer to metastasis and the ways in which we can alter this development are essential for improving patient outcomes. Macrophages have been identified as the major cell type responsible for metastatic progression in many cancers. Specifically, M2 macrophages exert a pro-tumor phenotype, whereas M1 macrophages are considered to be anti-tumor. Products of M2 macrophages are known to support many steps of the metastatic cascade, from invasion to the promotion of the pre-metastatic niche. Investigating factors that can alter macrophage phenotypes may reveal novel ways to diminish their pro-metastatic behavior. Recently, thymic stromal lymphopoietin (TSLP) has been identified in allergic disorders as a factor that can exacerbate M2 macrophage phenotypes. While M2 macrophages are part of the typical progression of allergic responses, these M2 products are pro-tumorigenic in cancer settings. TSLP has recently been identified in several human cancers and has been linked to cancer progression. Specifically, it has been identified in human breast cancer as a cytokine that amplifies pro-tumorigenic CD4 T cell responses. The effect of TSLP on macrophages in breast cancer remains understudied, despite data in allergy supporting this interaction. Therefore, we hypothesize that TSLP drives tumor progression to metastasis by amplifying M2-like macrophage responses. We aim to show that tumor-derived TSLP facilitates spontaneous metastasis via an M2 macrophage-dependent mechanism (Aim 1). We feel this is an essential gap in the field of macrophage biology that may expand our understanding of metastasis and lead to novel targets for therapy. Additionally, anti-TSLP therapy has newly been initiated in clinical trials in asthmatic patients. Recent data has suggested that checkpoint inhibitors, which are traditionally ineffective therapies in breast cancer, are efficacious against aggressive breast cancer when combined with therapies that target innate immunity. We aim to show that TSLP blockade enhances cancer immunotherapy efficacy (Aim 2). We plan to evaluate the efficacy of TSLP therapy both as a monotherapy, as well as in combination with checkpoint inhibitors. Given the potential importance of TSLP as a therapeutic target, we will also interrogate the prognostic merit of TSLP levels in breast cancer patients. The overall goal of this proposal is to increase our understanding of macrophage biology in breast cancer to expand potential therapeutic options for patients.