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 VEGFAhi neutrophil and SPP1hi macrophage subtypes, which displayed protumoral functions, including angiogenesis. Spatial transcriptomics revealed that they colocalized with epithelial cancer cells and APLNhi endothelial tip cells in a hypoxic region forming an angiogenic niche. Moreover, patients with SPP1hi macrophage-enriched TNBC showed poor prognosis, which worsened in patients who also displayed abundant VEGFAhi 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 VEGFAhi neutrophils and SPP1hi macrophages, elucidating their contributions in the formation of an angiogenic niche.