Abstract 167: Preclinic study of src3-ko regulatory T cells in mouse breast cancer model.

Abstract Introduction: Regulatory T cells (Tregs) play a critical role in balancing immune responses to prevent autoimmune disease through their immunosuppressive activities. However, the suppressive activity of Tregs can be coopted by cancers to evade immune surveillance, leading to tumor progression. Steroid receptor coactivator 3 (SRC-3) is highly expressed in Tregs and we have discovered that genentically engineered mice with SRC-3 specifically knocked out in Tregs are resistant to breast cancer growth, suggesting SRC-3-KO Tregs are a potential therapy for breast cancer patients. In this study, we explored the therapeutic effect of SRC-3 KO Tregs in a mouse model of triple negative breast cancer (TNBC). Method: A mouse syngeneic, immune-intact breast cancer model was produced by injecting luciferase expressing E0771 TNBC cells into the fourth mammary pad. To closely mimic a clinical scenario, Treg cells were isolated from the spleens of tumor-bearing C57BL/6J donor mice. After disrupting the SRC-3 gene by CRISPR/Cas9 targeting, Tregs were allowed to recover and proliferate in vitro for 4 days before collecting them for adoptive cell therapy (ACT) into C57BL/6J recipient mice with established breast cancer. Wild type Treg cells without gene editing were used as control. Tumor growth was monitored by bioluminescence imaging twice a week for 3 weeks. By the end of experiment, mice were sacrificed and spleen, blood, and tumors were collected for flow cytometry analysis as well as immunofluorescence staining. Results: ACT of SRC-3 KO Tregs significantly reduced tumor growth or cleared tumosr in all treated mice. SRC-3KO Treg treatment also prolonged mice overall survival. Both ACTed WT and SRC-3KO Tregs were detected in the spleen, blood, and tumors of recipient mice. Moreover, many more cytotoxic T cells, including CD8 and granzyme B+ cells were found in SRC-3-KO Treg treated tumor-bearing mice, compared to WT Treg treated controls. Conclusion: Deleting the SRC-3 gene in Tregs derived from tumor-bearing donors possess potent anti-tumor effects in an immune-intact mouse breast cancer model. These SRC-3 KO Tregs exert their effects by enhancing the cytotoxicity of effector CD8+ T cells in the tumor microenvironment. Our results reinforce the clinical potential of using SRC-3 KO Treg cells as an effective immunotherapy for TNBCs. Citation Format: Yan Xia, Yosef Gilad, Davis Graham, Bert W. O'Malley, David M. Lonard. Preclinic study of src3-ko regulatory T cells in mouse breast cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 167.