Project Summary/Abstract Breast cancer brain metastasis (BCBM) has emerged as a dominant clinical problem as treatments for peripheral disease improve and patients live longer. The blood brain barrier creates a unique challenge as it restricts access of most standard drugs to brain lesions, so median survival rates have remained stagnant at <2 years. Although the immune system is a critical controller of metastatic spread in the periphery and the basis for many effective new therapeutics, little is known about the immune response to BCBM. We have utilized single-cell RNA- sequencing to investigate the response of microglia, the predominant immune effector in the central nervous system (CNS), to BCBM. We find that microglia display a dramatic pro-inflammatory response to BCBM, characterized by increased expression of a diverse array of chemokines and machinery for antigen presentation through MHCI and MHCII, and that pharmacological depletion of microglia increases metastatic progression. We will test the hypothesis that microglia play critical roles in suppressing BCBM through 1) chemokine-mediated peripheral immune recruitment to initiate an inflammatory cascade, and 2) local antigen presentation to sustain T cell activation in the CNS. This work will establish the basic mechanisms by which microglia control metastatic outgrowth, how CNS immunity to BCBM develops and ultimately fails, and provide new insights for how to therapeutically exploit the immune system to control BCBM in patients.