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Abstract Breast Cancer is the most common cancer diagnosed in women in the United States. Second only to lung cancer, breast cancer results in the greatest number of cancer deaths for women. Hormone receptor positive (HR+) breast cancers - cancers expressing the nuclear receptors for steroid hormones estrogen and progesterone, termed the estrogen receptor (ER) and progesterone receptor (PR) - make up the largest proportion of diagnosed breast cancers by a significant margin. So, although the HR+ subtype is less aggressive compared to others, it continues to be the largest contributor to breast cancer mortality. Treatment consists of anti-endocrine therapies, largely targeting ER/estrogen, the established drivers of breast cancer growth. Most women respond well to these therapies, but for women who do not, there are few alternative options. With the advent of immunotherapy and its successes in treating and controlling certain tumor types, many were hopeful that it could be employed for breast cancer as well. However, breast cancers, especially HR+ breast cancers, are considered “immune cold. ” They lack high proportions of tumor-infiltrating lymphocytes (TILs), have low neoantigen production, and high percentages of immunosuppressive cells. A contributing factor could be the hormones present in the tumor microenvironment. The accepted paradigm is that lifetime exposure to estrogens drives the development of HR+ breast cancer, but both estrogen and progesterone are part of the normal menstrual cycle. Prior work from our lab has uncovered a novel immune-modulatory role for progesterone, pivotal to promoting the growth of PR-positive mammary tumors. Recent preliminary data from our lab show that progesterone treatment conveys immune protection against T cell killing in a co-culture with E0771 mammary gland tumor cells. However, when activated in monoculture, T cells exposed to increasing concentrations of progesterone have no differing expression of cell surface activation or exhaustion markers, implying that progesterone is not eliciting this effect by interfering with the T cell activation cascade nor is it inducing premature exhaustion. Interestingly, progesterone treatment instead leads to a concentration-dependent increase in T cell death compared to the untreated control. In vivo, these results are supported as progesterone treatment decreases the amount of total CD3+ T cells, and specifically the cytotoxic CD8+ subset, in the mammary glands of mice. These results suggest that progesterone could modulate the breast and tumor microenvironment, providing tumor cells with protection from immunosurveillance and contributing to HR+ breast cancer development. These data also argue for the use of anti-progestins to be added to the HR+ breast cancer treatment regimen to boost anti-tumor immune activity. Citation Format: Amanda Heard, Lauryn R. Werner, Eilidh I. Chowanec, Harmony Saunders, Julio Tinoco, Zachary C. Hartman, Christy R. Hagan. Progesterone promotes immunosuppression in the murine mammary gland by causing T cell death abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6877.
Heard et al. (Fri,) studied this question.