Single-cell transcriptomics of the myeloid milieu reveals an angiogenic niche in triple-negative breast cancer

Intratumoral myeloid cells are highly heterogeneous in terms of development and function and are pivotal for forming and regulating the tumor microenvironment. However, the myeloid milieu in triple-negative breast cancer (TNBC) remains poorly understood. Here, to elucidate this myeloid milieu, we integrated in-house and public single-cell RNA sequencing data. We detected diverse neutrophil and mononuclear-phagocyte subtypes and delineated their developmental trajectories and functions. Of particular interest were the VEGFA^hi neutrophil and SPP1^hi macrophage subtypes, which displayed protumoral functions, including angiogenesis. Spatial transcriptomics revealed that they colocalized with epithelial cancer cells and APLN^hi endothelial tip cells in a hypoxic region forming an angiogenic niche. Moreover, patients with SPP1^hi macrophage-enriched TNBC showed poor prognosis, which worsened in patients who also displayed abundant VEGFA^hi neutrophils. These subtypes were also conserved in multiple murine TNBC models. This comprehensive analysis of the myeloid population in TNBC thus reveals a previously undercharacterized interaction between VEGFA^hi neutrophils and SPP1^hi macrophages, elucidating their contributions in the formation of an angiogenic niche.