Abstract 3981: Immune targeting progesterone receptor in hormone receptor positive breast cancer

Abstract Background: Hormone receptor positive (HR⁺) breast cancer (BC) is a subtype of BC that is characterized by expression of estrogen receptor (ER) and progesterone receptor (PR). A cornerstone treatment for this subtype of BC is endocrine inhibition therapy (EIT), in which estrogen and ER are suppressed using antagonists to suppress tumor growth. However, patients eventually obtain resistance to EIT. Metastatic HR⁺BC is the deadliest form of BC, with no cure. Current therapies currently only utilize the ER-axis, with no PR-targeted therapies. Little is known about the role of PR in these BCs, but we have found evidence for an immunosuppressive role for PR in HR⁺BC. Yet, it is unclear how PR mediates immunosuppression and if this axis can be therapeutically targeted. We hypothesized that PR confers local immunosuppression through the stimulation of immunosuppressive transcriptional activity and that we can immunologically target this axis using a PR vaccine. Experimental methods: To assess the immune impact of PR, we expressed the isoforms of PR (A, B and A+B) in mouse mammary cancer cells and assessed their capacity to elicit PR signaling. We assessed the impact of PR expression on T cell activity in targeting PR⁺ mammary tumors that express GFP, along with using GFP-specific JEDI CD8⁺ T cells. We explored the ability of PR to protect against GFP xenoantigen immune responses by engrafting PR⁺ mammary cells into immune competent and deficient mice. Lastly, we engineered 1st generation adenoviral vectors encoding mouse and human PR and utilized two antiprogestins (Mifepristone and Onapristone) in vivo. Using our vectors, we vaccinated BALB/c in the footpad and assessed PR-specific immunity using IFNγ ELISPOT. Results: Using PR signaling reporters, we found canonical PR signaling activity in the absence of progesterone (P4) by PR-A and in the presence of P4 by PR-B, but the most profound activation of PR signaling was seen through combined expression of PR-A and B. While expression of these genes did not confer striking cytotoxic T cell resistance in vitro, combination of PR-A+PR-B in 67NR cells permitted successful tumor engraftment in immune competent mice, despite xenoantigen expression. GFP expression did not alter tumor growth in immunodeficient mice, suggesting a key role in modulating local anti-tumor immunity. The use of either Mifepristone or Onapristone significantly suppressed tumor engraftment in immune competent mice in vivo. We found that an Ad-PR vaccine could elicit T cell responses against human PR-specific epitopes, a potential step towards immune targeting these BCs. Conclusions: We found that murine PR-A and PR-B can confer robust P4 signaling that is linked to an immunosuppressive phenotype in vivo. We developed a PR vaccine that elicited PR-specific T cell responses in vivo. We are currently examining the mechanisms that underlie PR-mediated immunosuppression and exploring the potential of PR vaccines in HR⁺BC. Citation Format: Andrea E. Wilson, Junping Wei, Gangjun Lei, Xiao-Yi Yang, Cong-Xiao Liu, Melissa Gajda, Tao Wang, Christy Hagan, Zachary C. Hartman. Immune targeting progesterone receptor in hormone receptor positive breast cancer [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(6_Suppl):Abstract nr 3981.