Myeloid and endothelial cells cooperate to promote hematopoietic stem cells expansion in the fetal niche.

During embryonic development, very few hematopoietic stem cells (HSCs) are produced from the hemogenic endothelium, that will be expanded in a very specific niche. This fetal HSC niche comprises a complex and dynamic molecular network of interactions between multiple cell types, including endothelial cells (ECs) and mesenchymal stromal cells. It is known that functional changes in the hematopoietic niche, such as aging, vascular cell remodelling or inflammation can directly affect HSCs. Among all these inflammatory regulators, the eicosanoid prostaglandin E (PGE2) has been shown to be very important during embryonic life. However, the precise source of PGE2 in the embryo is still elusive. Here we show that all the genes involved in PGE2 synthesis and transport are expressed by distinct cells of the caudal hematopoietic tissue (CHT) in the zebrafish embryo and in the mouse fetal liver, suggesting that each cell type acts sequentially and collaboratively with the others to produce PGE2 and ultimately expand HSCs. Among these cells, we found myeloid cells (both neutrophils and macrophages) to be absolutely necessary, as they concur to the production of PGH2, the precursor of PGE2. To measure the impact of myeloid cells, we generated a genetic model of myeloid ablation, which caused a loss of HSCs in the CHT, that could be rescued by supplementing zebrafish embryos with PGE2 or PGH2. ECs expressed the slco2b1 transporter to import PGH2, and ptges3, the necessary enzyme to convert this latter into PGE2. Taken altogether, our data show that the triad composed of neutrophils, macrophages and ECs concurs to HSC expansion in the CHT.