MyD88-dependent TLR signaling oppositely regulates hematopoietic progenitor and stem cell formation in the embryo

Hemogenic endothelial (HE) cells in the dorsal aorta undergo an endothelial to hematopoietic transition (EHT) to form lympho-myeloid biased progenitors (LMPs), pre-hematopoietic stem cells (pre-HSCs) and adult-repopulating HSCs. These briefly accumulate in intra-arterial hematopoietic clusters (IAHCs) before being released into the circulation. It is generally assumed that the number of IAHC cells correlates with the number of HSCs. Here we show that changes in the number of IAHC cells, LMPs, and HSCs can be uncoupled. Mutations impairing MyD88-dependent toll-like receptor (TLR) signaling decreased the number of IAHC cells and LMPs but increased the number of HSCs in the aorta-gonad-mesonephros region of mouse embryos. TLR4 -deficient embryos generated normal numbers of HE cells but the proliferation of IAHC cells was decreased. Loss of MyD88-dependent TLR signaling in innate immune myeloid cells had no effect on IAHC cell numbers. Instead, TLR4 deletion in endothelial cells recapitulated the phenotype observed with germline deletion, demonstrating that MyD88-dependent TLR signaling in endothelial cells and/or in IAHCs regulates the balance between generating LMPs and HSCs. Summary Statement Toll-like receptor signaling in endothelial cells restricts the number of hematopoietic stem cells but increases the number of committed progenitors and intra-arterial hematopoietic cluster cells by promoting their proliferation.