Abstract PS3-12-03: Targeting ERβ signaling as a novel immunotherapeutic strategy in TNBC

Abstract Purpose: To assess the potential of an estrogen receptor beta (ERβ) agonist as an immunotherapy for triple negative breast cancer (TNBC). Background: Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by the absence of ERα, PR, and HER2 expression with limited treatment options. Although immune checkpoint inhibitors (ICIs) have shown some benefit, response rates remain low in advanced-stage TNBC. This highlights a critical gap in our understanding of oncogenic mechanisms, especially those leading to immunosuppressive tumor microenvironment (TME), and underscores the urgent need for innovative therapeutic targets to improve patient outcomes. Estrogen receptor beta (ERβ), expressed in approximately 30% of TNBCs, is a subtype of estrogen receptor with known anti-tumor properties. While prior studies have established ERβ’s tumor-suppressive role, its effect on the TME remains poorly defined. Despite recently observed clinical associations of ERβ with improved prognosis and immune-related markers, the mechanisms by which ERβ impacts immune cell recruitment, phenotype, and response to immunotherapy have not been thoroughly studied. Here, we aim to define how ERβ regulates immune activity in TNBC and to evaluate its utility as an immunotherapeutic target and biomarker. Methods: To test whether tumor-specific ERβ activity contributes to an immunostimulatory tumor microenvironment, we generated unique syngeneic models and patient-derived xenografts (PDXs) of TNBC with altered ERβ expression and investigated differences in tumor growth and immunophenotype using flow cytometry and immunohistochemistry. Single-cell and spatial genomics were used to study how ERβ alters signaling in tumor cells, affecting effector cells to reduce immunosuppression. To determine whether activating ERβ with agonists further stimulates tumor immunity and inhibits tumor progression, we administered the ERβ agonist LY500307, which is clinically tested in other diseases. ERβ targets and downstream signaling were validated through transcriptional and functional studies in isolated immune cells and cocultures. Lastly, we correlated ERβ’s expression and function with immune signatures and clinical outcome by deconvoluting sequencing data from TCGA-profiled breast cancers and through spatial proteomics analysis of a tissue microarray with TNBC samples. Result: We observed that activated tumor-specific ERβ leads to slowed tumor growth and a less immunosuppressive TME, which is characterized by a significant reduction in tumor-associated M2-like macrophages and increased cytotoxic T cell activity. RNA-seq and cytokine profiling of LY500307-treated ERβ-expressing breast cancer cells identified key target cytokines as downstream mediators, particularly CXCL2, CCL5, CSF2, and IL4 that regulate macrophage function and antigen presentation during tumor adaptive immunity. Preliminary clinical analysis revealed an inverse correlation between ERβ expression and the frequency of myeloid cells that predicted a worse patient outcome, reinforcing the prognostic significance of the receptor. Conclusions: By integrating spatial and single-cell genomics, and immune-functional assays, this study uncovers important functional and mechanistic features of the immunomodulatory role of ERβ. Our findings aim to transform our understanding of ERβ as a tumor suppressor in the immunosuppressive TME of TNBC and could help identify predictive biomarkers and therapy targets to improve patient outcomes. Citation Format: K. A. Zambo, F. Nikolos, H. Nagandla, K. Cap, A. Phillips, W. Qian, J. Qian, K. Chan, J. Chang, C. Thomas. Targeting ERβ signaling as a novel immunotherapeutic strategy in TNBC abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-12-03.