Prospective Isolation Of Human Erythroid Lineage-Committed Progenitors

Anemia is one of the findings most frequently seen in patients with myelodysplastic syndrome (MDS); however, in our hands, significant loss of megakaryocyte/erythrocyte progenitors (MEP) is not observed among these patients (PNAS, 2013, 110(8):3011-6). Unipotent erythroid-committed progenitors (EPs), estimated to exist downstream of bipotent MEPs, may be involved in the pathogenesis of refractory anemia among MDS patients. Previously, we identified mouse (m) EPs (mEPs) in bone marrow. The expression of endoglin (CD105) was a key marker to isolate mEPs: mEPs were the Lineage(lin)-Sca-1-c-kit+CD16/32-CD150+CD105+CD41- population in murine bone marrow (Cell Stem Cell, 2007, 1(4):428-42). Here we show that the human (h) counterpart (hEPs) is prospectively isolatable in human bone marrow. We analyzed the expression of hCD105 in addition to hCD71 (known to be an early erythroid marker) in human stem and progenitor populations, and found a fraction (32.9±13.6%, n = 5) of common myeloid progenitors (CMPs; lin-CD34+CD38+CD45RA-IL-3Ra+) and a major part (85.1±5.5%) of MEPs (lin-CD34+CD38+CD45RA-IL-3Ra-) expressed hCD71. A portion of CD71+ MEPs co-expressed CD105 (CD105+MEPs; 32.3±6.5% of MEPs). Neither hCD105 nor hCD71 was detectable in hematopoietic stem cells (HSCs; lin-CD34+CD38-CD45RA-), common lymphoid progenitors (CLPs; lin-CD34+CD38+CD10+), or granulocyte/monocyte progenitors (GMPs; lin-CD34+CD38+CD45RA+IL-3Ra+) by FACS. In vitro, CD71+ cells within the CMP fraction showed differentiation potential skewed toward MegE lineage, representing the transitional stage to MEPs. CD71+MEPs mostly generated BFU-E, although they still retained some MegK potential, while output of CD105+MEPs was completely restricted to erythroid lineage, and colonies from this fraction contained a small number of mature (enucleated) erythrocytes (CFU-E type). Thus we termed CD71+MEPs and CD105+MEPs as human erythroid-biased MEPs (E-MEPs) and hEPs, respectively. In short-term liquid culture, hE-MEPs gave rise to hEPs in the presence of SCF, TPO and EPO, whereas hEPs could not generate hE-MEPs, suggesting a hierarchical progression from E-MEPs to EPs. These newly classified populations might be a very useful tool for understanding the molecular mechanisms of human erythroid development, and should be analyzed in patients with erythroid-related disorders (e.g., MDS, polycythemia vera, or aplastic anemia).