Non-classical monocytes (NCMs) are circulating phagocytes with similarities to tissue-resident macrophages in their roles in immunosurveillance, scavenging and tissue repair. The macrophage niche concept posits that specialized tissue niches support the survival and functions of macrophages. Thierry, Baudon et al. now investigate the homeostasis of NCMs, showing that the whole vascular tree functions as a nurturing niche.

Previous work had shown that bone marrow endothelial cells support NCMs by providing the growth factor CSF1. Here, the authors generated mice with constitutive or inducible deletion of Csf1 in all endothelial cells, showing that NCMs were markedly reduced in bone marrow and blood. Adoptive transfer experiments showed that endothelial CSF1 was required for the survival of NCMs, rather than for their differentiation from classical monocytes.