Summary. Triple negative breast cancer (TNBC) is an aggressive form of breast cancer that is treated with neo-adjuvant therapy that targets both the primary tumor and systemic disease, with subsequent immunotherapy in the adjuvant setting. Even with the most cutting-edge therapeutic approach, approximately 20% of these patients with a pathologic complete response (pCR) will recur. Our long-term goal is to develop strategies that target the innate immune system and aim to prevent the establishment of or destabilize sites in which the tumor cells are protected from destruction by the immune system. Immunotherapies such as CAR-T cells or immune checkpoint blockade have emerged as promising therapies targeting adaptive immune cells; however, the efficacy of these treatments remains limited due to the immune protection offered at the metastatic site by innate cells. We propose a nanoparticle (NP) strategy for targeting monocytes and neutrophils in circulation, prior to their arrival at a metastatic niche (MN), which we propose can destabilize the immune suppressive environment that is needed to protect tumor cells from the immune system. The scientific premise of this application is to investigate the design of NPs to program circulating monocytes and neutrophils that would normally travel to a MN, with programming indicating the ability to redirect immune cell trafficking and alter immune cell polarization. Specific Aim 1 will investigate the NP design for polarization of monocytes and neutrophils that programs an anti-tumor phenotype that leads to tumor cell clearance from metastatic sites. Properties such as size influence the biodistribution, with the chemical composition influencing cell interactions and polarization. We propose to initially assess internlization, the biodistribution, safety, and persistence of NP-associated cells in the MN, the cell types that associate with NPs and the phenotypic programming, and the impact of NPs on systemic inflammation and tumor cell numbers. We also propose to analyze the mechanisms of NP function through assessment of i) TC recruitment and persistence in vivo, ii) immune cell trafficking following adoptive transfer to determine the direct and indirect effects of NPs, and iii) modulation of neutrophils numbers or phenotypes. Specific Aim 2 will investigate NP design to relieve immune suppression at the MN and enhance/enable adaptive immune responses. We will investigate the composition and phenotype of T cells at the MN with and without NP administration in the neoadjuvant and adjuvant setting. Additionally, we will investigate the mechanisms by which innate cells direct T cell responses, using studies such as depletion of T cell subsets, and performing studies to investigate T cell trafficking and persistence at the MN. The research team includes breast cancer biologist and clinician (Jeruss), an engineer with a successful history of translating NPs to the clinic (Shea) and am immunologist with expertise i...