Distinct Endothelial Phenotype Associates with Macrophage-Enriched Microenvironments in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive malignancy with limited therapeutic options due to the absence of targetable receptors and pronounced intra-tumoral heterogeneity. Among the various contributors to this heterogeneity, the aberrant tumor vasculature plays a critical role by restricting drug delivery and immune cell infiltration, thereby promoting therapeutic resistance. Using previously established murine TNBC models differing in macrophage abundance, we found marked differences in tumor vasculature between these models. Macrophage-enriched tumors exhibited hallmarks of vascular normalization, including increased pericyte coverage and enhanced lectin-based vessel perfusion compared to macrophage-poor tumors. Single-cell RNA sequencing revealed that endothelial cells from macrophage-enriched tumors upregulated inflammatory and macrophage–monocyte interaction genes, including interferon-γ–responsive pathways, and were enriched for venous-like states. In contrast, non-macrophage-enriched tumors displayed arterial-, lymphatic-, and homeostatic endothelial programs. Analysis of human TNBC single-cell datasets confirmed that endothelial composition and activation state vary with macrophage abundance, with macrophage-enriched tumors exhibiting more immunologically active endothelium. Collectively, these results identify conserved associations between macrophage-enriched microenvironments and vascular states, highlighting coordinated immune–vascular dynamics in TNBC tumors.