CD34− Cells at the Apex of the Human Hematopoietic Stem Cell Hierarchy Have Distinctive Cellular and Molecular Signatures

In addition to well-characterized CD34+ hematopoietic stem and progenitor cells (HSPCs), the human hematopoietic stem cell (HSC) hierarchy contains a rare CD34− population with severe combined immunodeficiency-repopulating capacity. However, little is known about the molecular characteristics of these CD34− cells or their relationship to the CD34+ populations. Here, we show that the self-renewing Lin−CD34−CD38−CD93hi population contains cells that not only function as HSCs, but can also be placed above the CD34+ populations in the hematopoietic hierarchy. These cells have an active Notch pathway, in which signaling through Delta4 is crucial for maintenance of the primitive state, and combined signals from Jagged1 and TGF-β are important in controlling its quiescence. They are also refractory to proliferative signals and show a repressed canonical Wnt pathway, in part regulated by Notch. Overall, therefore, CD34− cells represent an immature and quiescent human HSC population maintained through a distinctive network of cellular signaling interactions. [Display omitted] •Human CD34− HSCs have self-renewal and repopulating capacities•Cells in this population are immature and quiescent•Distinctive cellular and molecular features include active Notch and TGF-β pathways•Repressive canonical Wnt signaling is in part regulated by Notch Cellular and molecular analysis of human CD34− HSCs puts them at the top of the pile in the hematopoietic hierarchy, above the previously characterized CD34+ populations.