De Novo Hematopoiesis from the Fetal LUNG

Hemogenic endothelial cells (HECs) are specialized cells that undergo endothelial to hematopoietic transition (EHT) to give rise to the earliest precursors of hematopoietic progenitors that will eventually sustain hematopoiesis throughout the lifetime of an organism. Although HECs are thought to be primarily limited to the aorta gonad mesonephros (AGM) during early development, EHT has been described in various other hematopoietic organs and embryonic vessels. Though not traditionally seen as a hematopoietic organ, the lung houses many resident hematopoietic cells, aids in platelet biogenesis and is a reservoir for hematopoietic stem and progenitor cells (HSPCs). However, lung HECs have never been described. Here we demonstrate that the fetal lung is a novel source of HECs that have the functional capacity to undergo EHT to produce de novo HSPCs and their resultant progeny. Explant cultures of murine and human fetal lungs display adherent endothelial cells transitioning into floating hematopoietic cells accompanied by the gradual loss of an endothelial signature. Flow cytometric and functional assessment of fetal lung explants showed the production of multipotent HSPCs that expressed the EHT and pre-HSPC markers EPCR, CD41, CD43 and CD44. scRNA-Seq and small molecule modulation demonstrated that fetal lung HECs rely on canonical signaling pathways to undergo EHT, including TGFβ/BMP, Notch and YAP. Immunofluorescent stains of fixed frozen fetal lung samples showed discrete regions of clustered hematopoietic cells expressing VE-cadherin, CD41 and CD45 adjacent to fetal lung vasculature. Notably, these clusters were oriented away from the vascular lumen, suggesting they may be fated to remain within the lung interstitium. Collectively, these data strongly support the concept that post-AGM development, functional HECs are present in the fetal lung establishing this location as a potential extramedullary site of de-novo hematopoiesis.

No relevant conflicts of interest to declare.