Abstract 3444: Characterization of immune environment in SV40C3-tag mouse model of triple-negative breast progression

Abstract Background: Triple-Negative Breast Cancer (TNBC) is aggressive subtype and studies have reported impact of immune system on prognosis and treatment response. The role of the immune system during TNBC tumorigenesis, in the transition from precancer to cancer in unknown. We hypothesize that progression towards tumor is associated with both quantitative and functional changes to the immune microenvironment. Methodology. SV40C3-TAg female mice, a TNBC progression model that develops tumors resembling human Ductal Carcinoma In-situ (DCIS) and Invasive Ductal Carcinoma (IDC) was used. To characterize immune population, mammary glands were harvested at 6-weeks (precancer) and 22-weeks (tumor) and immune cell population was analyzed by Fluorescence Assisted Cell Sorting (FACS). Animals were also sacrificed at multiple timepoints prior to tumor development to investigate immune cell populations and cytokine mileu during tumorigenic progression. Histopathological grades were assigned to mammary tissues: 0 for simple epithelium, 1-3 for mid to atypical hyperplasia (non-malignant low grade) and 4-5 for in-situ to invasive carcinoma (high grade lesions). Average number of B220+ B cells and CD8+ T cell per high-power-field (HPF) across the mammary tissue samples was determined. To understand functional characteristics of immune population mammary tissue samples were analyzed for levels of cytokines IL-4, IL-10, INF- ɣ using ELISA and Luminex Discovery Assay. Results: The average number of B220+ B cells increased from 0.73 cells/HPF in low grade (G0-3) to 1.61 cells/HPF in high grade (G4-5) mammary lesions (p=0.0329). Similarly in flow cytometry analysis, a 2-fold increase (p=0.0051) of B cell population was seen when comparing mammary tumors in 22-week animals to mammary glands from 6-week tumor free animals. Similarly, a significant infiltration of CD8+ T cells from low (0.6 cells/HPF) to high (3.62 cells/HPF) grade lesions was seen by IHC (p<0.0015), mostly localized in peri-tumoral region, which is substantiated with 10-fold increase in CD8+ T cells (p=0.0012) in FACS analysis of mammary tumors vs mammary tissue from 6 week animals. Furthermore, when we investigated the subpopulation of CD8+ T cells, there is an 2-3-fold increase in CD8+ T FoxP3+ and CD8+T CTLA+ (p=0.0004) cells when comparing 22 week tumor bearing glands to mammary glands at the 6 week timepoint and 2-3-fold elevation in CD8+ T IL-10 (p=0.0291). In line with FACS data, we observed increasing trend of IL-10 and IL-4 along with reduction in INF- ɣ when comparing cytokine levels in high grade mammary tissues vs low grade mammary tissues. Conclusions: The influx of immune B cells and immune regulatory cells T cells indicate active response to tumor. The patterns of change in quantitative cell counts and immune mileu suggest changes from immune-stimulatory (Th1) to suppressive environment (Th2) with tumor progression. Citation Format: Janvi Sandhu, Anjana Bhardwaj, Alexander Koh, Raniv D. Rojo, Rim Ouni, Matthew Embury, Roza Nurieva, Isabelle Bedrosian. Characterization of immune environment in SV40C3-tag mouse model of triple-negative breast progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3444.