Project Abstract Triple-negative breast cancer (TNBC) is an aggressive and invasive breast cancer subtype, accounting for 10-15% of breast cancer diagnoses. TNBC affects African-American (AA) women three times more than Caucasian-American (CA) women. Metastasis to distant vital organs such as bone, liver, lung, and brain is the most devastating feature of TNBC in AA women. Tumor-derived exosomes are mediators of aggressive distant metastasis by contributing to the establishment of a pre-metastatic niche. Therefore, it is critical to investigate the molecular mechanism(s) that drive tumor-derived exosome-mediated immune modulation and pre- metastatic niche formation for aggressive metastasis in TNBC. Annexin A2 (AnxA2) is an often identified exosomal protein with elevated levels in TNBC patient sera and cell lines. The higher expression of exosomal AnxA2 (exo-AnxA2) in serum samples is associated with poor overall survival and poor disease-free survival in breast cancer patients. In addition, the expression of serum exo-AnxA2 is significantly high in AA women with TNBC and promotes angiogenesis. Our data suggest a role for Exo-AnxA2 in establishing a pre- metastatic niche by immune modulation, which subsequently promotes TNBC metastasis. The goal of our research is to develop improved therapeutic options for TNBC. We hypothesize that high expression of Exo- AnxA2 plays a vital role in immune modulation at the pre-metastatic niche, thereby promoting TNBC metastasis. We will address this hypothesis by the following two specific aims: Aim 1: Determine the mechanism(s) that drive exo-AnxA2-mediated immune modulation and metastatic niche formation for aggressive metastasis in breast cancer. Aim 2: Determine the role of patient-derived exo-AnxA2 with measures of disease aggressiveness among racially distinct populations of TNBC patients using humanized mouse model system. We predict that high concentrations of exo-AnxA2 in the sera of TNBC patients will be shown to contribute to the aggressive biology of this disease, especially in AA women, by immune modulation at the pre-metastatic niche site.