ABSTRACT Breast cancer is the second most common cause of cancer-related death and most common cancer occurring in females in the U.S. Numerous large-scale clinical trials have demonstrated that use of exogenous progestins (synthetic progesterone [P4]) increases the risk of invasive breast cancer in females. Although it has been clearly established in the literature that P4 plays a role in the development of human breast cancer, the mechanism by which P4 promotes breast cancer tumorigenesis remains unknown. Recently, we reported a link between progesterone receptor (PR) and immune signaling pathways. Our recent publications have demonstrated that P4/PR can repress type I interferon signaling pathways in human breast cancer cells. Given these findings, we sought to investigate whether P4/PR drive immunomodulation in the mammary gland to promote formation of mammary gland tumors. Therefore, the central hypothesis of this proposal is that P4 and PR decrease recruitment and function of DCs in the mammary gland, which promotes immunosuppression and subsequent development and growth of mammary gland tumors. We will address this hypothesis through the following specific aims: 1) Define the mechanism by which P4 treatment and PR expression affect DC recruitment to the murine mammary gland and PR+ mammary gland tumors; 2) Characterize the maturation and activity state of DCs isolated from P4-treated PR+ mammary gland tumors; and 3) Determine the contribution of P4-mediated DC suppression to mammary gland tumor growth. For aim 1, we will utilize single cell RNA sequencing (scRNA-seq) to determine if P4 affects the release of cytokines involved in DC recruitment to mammary gland tumor cells. In aim 2, we will determine how P4 treatment affects the activity and function of tumor-infiltrating DCs using syngeneic tumor models, in which markers will be measured via scRNA-seq, flow cytometry, and T cell proliferation assays. Aim 3 will utilize transgenic mouse models to determine whether depletion of DCs impacts P4’s growth-promoting effects on mammary gland tumors. Successful completion of the proposed work will provide novel insight into immune tolerance mechanisms promoting the development and growth of breast cancers. Additionally, it will provide a rationale to target P4/PR signaling with anti-progestins to promote DC function and enhance immune-mediated elimination of tumor cells.