A revised road map for the commitment of human cord blood CD34-negative hematopoietic stem cells

We previously identified CD34-negative (CD34^-) severe combined immunodeficiency (SCID)-repopulating cells as primitive hematopoietic stem cells (HSCs) in human cord blood. In this study, we develop a prospective ultra-high-resolution purification method by applying two positive markers, CD133 and GPI-80. Using this method, we succeed in purifying single long-term repopulating CD34^- HSCs with self-renewing capability residing at the apex of the human HSC hierarchy from cord blood, as evidenced by a single-cell-initiated serial transplantation analysis. The gene expression profiles of individual CD34⁺ and CD34^- HSCs and a global gene expression analysis demonstrate the unique molecular signature of CD34^- HSCs. We find that the purified CD34^- HSCs show a potent megakaryocyte/erythrocyte differentiation potential in vitro and in vivo. Megakaryocyte/erythrocyte progenitors may thus be generated directly via a bypass route from the CD34^- HSCs. Based on these data, we propose a revised road map for the commitment of human CD34^- HSCs in cord blood.