PROJECT ABSTRACT While immunotherapy has revolutionized the treatment of certain malignancies it has yet to yield the same results in breast cancer. This is striking, given that there is an abundance of intra-tumoral immune cells that affect disease outcomes. Evidence suggests that these leukocytes are strategically positioned to execute their functions and yet little is known about their spatial dynamics over tumor progression and regression. I aim to detail a comprehensive and mechanistic picture of these spatiotemporal dynamics using recent advances in high-dimensional in situ imaging. I hypothesize that as tumors progress, immunosuppressive cells will block immune cell organizational structures and cell-cell communications that promote antitumor immunity and that this phenotype will be reversed upon tumor regression. This will be assessed by characterizing the spatial organization, cell-cell interactions, activation/inhibition markers, and cytokine production of the main leukocyte lineages in situ over disease course. My preliminary data demonstrate that I can identify most of these major immune cell types at once within breast tumors, including subsets of macrophages, dendritic cells, and T cells. In addition, I found that two subsets of macrophages have differential localization: one which is intra-tumoral and another that is almost exclusively peritumoral. This peritumoral macrophage population closely interacts with T regulatory cells, which are important for immunosuppression in breast cancer. As such, I hypothesize that peritumoral FDMs spatially organize to interact with T regulatory cells via TCR-MHC II, thereby activating Tregs and skewing tumors toward an immunosuppressed microenvironment. To test this, I will use genetic mouse models to deplete these populations in a tumor stage-specific manner and assess the impact on tumor growth, survival, and response to clinical therapies. Overall, this proposal aims to understand the spatial dynamics of leukocytes in breast cancer, which will reveal key cell-cell interactions, organizational structures, and cellular communication networks that need to be disrupted/promoted in order to reengage antitumor immunity, facilitating the development of effective immunotherapies.