Abstract 622: Progesterone promotes immunosuppression in the mammary gland through regulatory T cell activity
Authors: Chowanec Eilidh I. , Werner Lauryn R. , Hagan Christy 
Published: April 2023

Abstract
Estrogen and progesterone signaling together drive hormone-receptor positive breast cancer growth and progression. However, there have been limited studies on the independent role that progesterone and its receptor, the progesterone receptor, play in breast cancer development. Studies in our lab have shown that progesterone signaling promotes mammary tumor growth and changes the immune landscape of the mammary gland towards immunosuppression. Regulatory T-cells (Tregs) are immune cells known to promote immunosuppression in the tumor microenvironment, in part through decreasing the activity of anti-tumor CD8+ T cells. The data from our lab also show that progesterone increases the number of Tregs in the normal mammary gland. To determine if progesterone signaling increases the number of Tregs in the tumor-bearing mammary gland, we used a syngeneic orthotopic murine tumor model, E0771 cells, engineered to express mouse PR or control empty vector. Mice were treated with progesterone or placebo for 7 days before implantation of the modified E0771 cells. Results from this study show that progesterone promoted tumor growth, as well as changed the immune landscape of the tumor. Flow cytometry analysis at tumor endpoint showed that progesterone receptor-positive tumors treated with progesterone harbored more Tregs than placebo treated mice. Additionally, initial studies into activity markers harbored by Tregs in progesterone-treated mice indicate increased Treg activity following progesterone treatment. As progesterone treatment promoted tumor growth and Treg recruitment in progesterone receptor-positive tumors, these data imply that progesterone treatment may promote the release of secreted factors impacting the immune cells in the mammary microenvironment. To determine if progesterone treatment results in cytokine or chemokine changes that may affect Treg recruitment, cytokine arrays and enzyme-linked immunosorbent assays (ELISAs) were performed using cell culture supernatant from murine mammary tumor cells expressing progesterone receptor (E0771-PR) treated with progesterone (0-72 hours). E0771-PR cells treated with progesterone showed higher levels of cytokines and chemokines that have been linked to Treg recruitment and differentiation, such as CCL5. Our findings demonstrate that progesterone signaling promotes an immune suppressed microenvironment in the mammary gland through an increase in Tregs. This evidence provides rationale to investigate the use of anti-progestins as a mechanism to overcome the immune-cold tumor microenvironment seen in progesterone receptor positive breast tumors.
Citation Format: Eilidh I. Chowanec, Lauryn R. Werner, Christy Hagan. Progesterone promotes immunosuppression in the mammary gland through regulatory T cell activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 622.