Abstract
The cell surface marker CD133 has been used to describe a revised model of adult human hematopoiesis, with hematopoietic stem cells and multipotent progenitors (HSCs/MPPs: CD133(+)CD45RA(-)CD34(+)) giving rise to lymphomyeloid-primed progenitors (LMPPs: CD133(+)CD45RA(+)CD34(+)) and erythromyeloid progenitors (EMPs: CD133(low)CD45RA(-)CD34(+)). Because adult and fetal hematopoietic stem and progenitor cells (HSPCs) differ in their gene expression profile, differentiation capabilities, and cell surface marker expression, we were interested in whether the reported segregation of lineage potentials in adult human hematopoiesis would also apply to human fetal liver. CD133 expression was easily detected in human fetal liver cells, and the defined hematopoietic subpopulations were similar to those found for adult HSPCs. Fetal HSPCs were enriched for EMPs and HSCs/MPPs, which were primed toward erythromyeloid differentiation. However, the frequency of multipotent CD133(+)CD45RA(-)CD34(+) HSPCs was much lower than previously reported and comparable to that of umbilical cord blood. We noted that engraftment in NSG (NOD scid gamma [NOD.Cg-Prkdc(scid) Il2rg(tm1Wjl)/SzJ]) mice was driven mostly by LMPPs, confirming recent findings that repopulation in mice is not a unique feature of multipotent HSCs/MPPs. Thus, our data challenge the general assumption that human fetal liver contains a greater percentage of multipotent HSCs/MPPs than any adult HSC source, and the mouse model may have to be re-evaluated with respect to the type of readout it provides.