[Updated human hematopoietic stem cell findings: purification of human hematopoietic stem cells and elucidation of their hierarchy].

Hematopoietic stem cell (HSC) biology is a current topic of interest having significant implications for clinical HSC transplantation and basic HSC research. It was long believed that the most primitive HSCs in mammals, including those in mice and humans, were CD34 antigen-positive (CD34+) cells. However, Nakauchi et al. reported that murine long-term lymphohematopoietic reconstituting HSCs were lineage marker-negative (Lin-) c-kit+Sca-1+CD34-low/negative (CD34low/-), known as CD34low/-KSL cells. We have previously identified very primitive human cord blood (CB) -derived CD34-negative (CD34-) severe combined immunodeficiency (SCID) -repopulating cells (SRCs) utilizing an intra-bone marrow injection method and have proposed a new concept that CD34-SRCs (HSCs) reside at the apex of human HSC hierarchy. Recently, we developed an ultra-high-resolution purification method using the two positive/enrichment markers CD133 and GPI-80 and succeeded in purifying CD34+/-HSCs on a single-cell level. On the basis of these data, we propose a revised roadmap for the commitment of human CD34-HSCs. This review updates the concept of the stem cell nature of human CB-derived primitive CD34+/-HSCs.