Hematopoietic differentiation of umbilical cord blood-derived very small embryonic epiblast-like stem cells

A population of CD133 + Lin − CD45 − very small embryonic/epiblast-like stem cells (VSELs) has been purified by multiparameter sorting from umbilical cord blood (UCB). To speed up isolation of these cells, we employed anti-CD133-conjugated paramagnetic beads followed by staining with Aldefluor to detect aldehyde dehydrogenase (ALDH) activity; we subsequently sorted CD45 − /GlyA − /CD133 + /ALDH high and CD45 − /GlyA − /CD133 + /ALDH low cells, which are enriched for VSELs, and CD45 + /GlyA /CD133 + /ALDH high and CD45 + /GlyA − /CD133 + /ALDH low cells, which are enriched for hematopoietic stem/progenitor cells (HSPCs). Although freshly isolated CD45 − VSELs did not grow hematopoietic colonies, the same cells, when activated/expanded over OP9 stromal support, acquired hematopoietic potential and grew colonies composed of CD45 + hematopoietic cells in methylcellulose cultures. We also observed that CD45 − /GlyA − /CD133 + /ALDH high VSELs grew colonies earlier than CD45 − /GlyA − /CD133 + /ALDH low VSELs, which suggests that the latter cells need more time to acquire hematopoietic commitment. In support of this possibility, real-time polymerase chain reaction analysis confirmed that, whereas freshly isolated CD45 − /GlyA − /CD133 + /ALDH high VSELs express more hematopoietic transcripts (for example, c-myb), CD45 − /GlyA − /CD133 + /ALDH low VSELs exhibit higher levels of pluripotent stem cell markers (for example, Oct-4). More importantly, hematopoietic cells derived from VSELs that were co-cultured over OP9 support were able to establish human lympho-hematopoietic chimerism in lethally irradiated non-obese diabetic/severe combined immunodeficiency mice 4–6 weeks after transplantation. Overall, our data suggest that UCB-VSELs correspond to the most primitive population of HSPCs in UCB.