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A population of monocytes, known as Ly6C lo monocytes, patrol blood vessels by crawling along the vascular endothelium. Here we show that endothelial cells control their origin through Notch signalling. Using combinations of conditional genetic deletion strategies and cell-fate tracking experiments we show that Notch2 regulates conversion of Ly6C hi monocytes into Ly6C lo monocytes in vivo and in vitro, thereby regulating monocyte cell fate under steady-state conditions. This process is controlled by Notch ligand delta-like 1 (Dll1) expressed by a population of endothelial cells that constitute distinct vascular niches in the bone marrow and spleen in vivo , while culture on recombinant DLL1 induces monocyte conversion in vitro . Thus, blood vessels regulate monocyte conversion, a form of committed myeloid cell fate regulation. Circulating Ly6C lo monocytes are thought to be derived from Ly6C hi subset. Here the authors show that Notch signalling is activated in Ly6C lo cells and is required for their differentiation, and that Notch ligands that initiate this signalling are provided by a subset of endothelial cells.

Nature Communications 7: Article number: 12597 (2016); Published: 31 August 2016; Updated: 3 May 2017 The authors inadvertently omitted Christine Häger, who was involved in the initial characterization of Notch mutant mice presented in this Article, from the author list and Author contributions statement.