Hematopoietic Reprogramming In Vitro Informs In Vivo Identification of Hemogenic Precursors to Definitive Hematopoietic Stem Cells

Definitive hematopoiesis emerges via an endothelial-to-hematopoietic transition in the embryo and placenta; however, the precursor cells to hemogenic endothelium are not defined phenotypically. We previously demonstrated that the induction of hematopoietic progenitors from fibroblasts progresses through hemogenic precursors that are Prom1⁺Sca1⁺CD34⁺CD45^- (PS34CD45^-). Guided by these studies, we analyzed mouse placentas and identified a population with this phenotype. These cells express endothelial markers, are heterogeneous for early hematopoietic markers, and localize to the vascular labyrinth. Remarkably, global gene expression profiles of PS34CD45^- cells correlate with reprogrammed precursors and establish a hemogenic precursor cell molecular signature. PS34CD45^- cells are also present in intra-embryonic hemogenic sites. After stromal co-culture, PS34CD45^- cells give rise to all blood lineages and engraft primary and secondary immunodeficient mice. In summary, we show that reprogramming reveals a phenotype for in vivo precursors to hemogenic endothelium, establishing that direct in vitro conversion informs developmental processes in vivo.