Abstract PS2-11-02: Chemotherapy-educated macrophages promote dormant tumor cell reactivation by shaping a pro-fibrotic niche

Abstract Recurrence of metastatic breast cancer (BC) after neoadjuvant chemotherapy remains a major clinical challenge and is often driven by dormant disseminated tumor cells (DTCs) that escape initial treatment and later reawaken in distant organs. While cytotoxic therapies target proliferating tumor cells, they may also alter the tumor microenvironment in ways that promote metastatic progression. However, the mechanisms linking chemotherapy to DTCs reactivation remain poorly understood. We hypothesized that macrophages engulfing chemotherapy-treated tumor cells are reprogrammed into a pro-metastatic phenotype, secreting factors that contribute to the formation of a fibrotic niche permissive to DTCs outgrowth. To test this, we performed RNA sequencing on macrophages that had phagocytosed Her2+ breast cancer cells (D2A1 cells) pre-treated with either doxorubicin (DOX) or paclitaxel (PTX). Transcriptional profiling revealed distinct signatures: DOX-exposed macrophages (Mφ-DOX) upregulated immune defense genes, whereas PTX-exposed macrophages (Mφ-PTX) showed a strong pro-fibrotic signature. Both macrophage populations shared enrichment in pathways associated with extracellular matrix remodeling, angiogenesis, and wound healing, hallmarks of metastatic niche activation. Secreted mediators of Mφ-DOX and Mφ-PTX promoted the escape of dormant DTCs from quiescence in a 3D dormancy model and in vivo. Previously, we demonstrated that a fibrotic niche supports DTCs outgrowth. Likewise, our results suggest that secreted mediators of these chemo-educated macrophages contribute to fibrotic niche formation, accelerating DTCs reactivation via beta1 integrin. Significance: These findings uncover a novel mechanism by which chemotherapy may inadvertently promote recurrence by educating macrophages into pro-metastatic effectors. Identifying the pro-metastatic mediators secreted by these chemo-educated macrophages may uncover novel therapeutic targets. This approach holds promise for developing complementary treatments to prevent BC recurrence and improve long-term patient outcomes. Citation Format: V. Zubenko, S. Michaeli-Ashkenasi, K. Weidenfeld-Barenboim, C. Ezra, D. Barkan. Chemotherapy-educated macrophages promote dormant tumor cell reactivation by shaping a pro-fibrotic niche 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 PS2-11-02.