Abstract 5224: Systemic microenvironment determines the disease outcome in murine models of breast cancer

Abstract The systemic immune microenvironment plays a critical role in determining disease outcomes in breast cancer, yet its molecular and functional characteristics remain largely unknown. Using single-cell RNA sequencing (scRNAseq), CyTOF, and functional assays, we investigated immune dynamics in metastatic (4T1) and non-invasive (EMT6) murine models of triple negative breast cancer (TNBC). While primary tumors in both models exhibited similar infiltration by macrophages and myeloid-derived suppressor cells (MDSCs), profoundly distinct immune profiles in target tissues of metastatic mice. In the metastatic 4T1 model, metastatic tissues showed an expansion of neutrophil progenitors and immature neutrophils (CD11b+CD33+Ly6G+CD115-Ly6Clow) at the expense of natural killer (NK) cells, T cells, and B cells. These immature neutrophils mediated systemic immune suppression, contributing to the establishment of pre-metastatic niches where they regulate cancer stem cell (CSC) proliferation. Recombinant S100A8/A9 (calprotectin) proteins induced a similar expansion of these immunosuppressive neutrophils in EMT6 models, driving metastasis and immune evasion. In contrast, S100A9 knockout mice showed reduced metastatic burden and enhanced anti-tumor immune responses. Depletion of Ly6G+ immature neutrophils using anti-Ly6G antibody significantly restored systemic anti-tumor immunity and eliminated metastasis, while having no effect on primary tumor growth. Conversely, depletion of CD115+ macrophages selectively inhibited EMT6 tumor growth without altering systemic immune profiles. Our findings highlight the critical influence of systemic immune dynamics in shaping metastatic progression and therapeutic resistance. The identification of neutrophil progenitors and inflammatory S100A8/A9 proteins as key mediators of immune suppression offers novel insights into the interplay between systemic immunity and tumor biology. Targeting these components may provide a promising approach to mitigating metastasis and improving therapeutic outcomes in aggressive breast cancer subtypes. Citation Format: Fulya Koksalar Alkan, Ahmet Burak Caglayan, Hilmi Kaan Alkan, Cordarryl Jones, Adriana Langer Gramer, Roni Bollag, Eric Sebzda, Kristen S. Purrington, Max Wicha, Huidong Shi, Hasan Korkaya. Systemic microenvironment determines the disease outcome in murine models of breast cancer [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 5224.