A functional hierarchy among the CD34+ hematopoietic cells based on in vitro proliferative and differentiative potential of AC133+CD34bright and AC133dim/-CD34+ human cord blood cells

The 5-transmembrane receptor AC133 is expressed on a subpopulation of human hematopoietic cells that includes the CD34(bright) cells. We evaluated the developmental potential of AC133(+)CD34(bright) and AC133(dim-)CD34(+) cells isolated from 5 cord blood (CB) samples by studying the in vitro proliferative and differentiative potential of each population in both progenitor and mature cell expansion cultures. Seven-day culture of AC133(+)CD34(bright) cells with a cytokine combination favoring primitive progenitor cells causes a significant increase in CD34(+), CFU-C and noncycling stent/progenitor cells HPP-Q (High Proliferative Potential-Quiescent), whereas culture of AC133(dim/-)CD34(+) cells shows a limited increase in committed progenitor cells only. HPP-Q cells were not found in freshly isolated AC133(dim/-)CD34(+) nor in expanded CD34(+) cells derived from AC133(dim/-)CD34(+) cells. No statistically significant difference was observed between the 1-week expanded AC133(+) and the initial AC133(+)CD34(bright) cells regarding their clonogenic efficiency (CE), while expanded CD34(+) cells derived from AC133(dim/-)CD34(+) cells exhibited a decreased CE. Subexpansion of the reselected AC133+ derived from AC133(+)CD34(bright) cells reveals a further increase of stent/progenitor cells and the 14-day expanded AC133+ cells reveal an unchanged CE. Subexpansion of reselected 7-day CD34(+) cells derived from AC133(dim/-)CD34(+) cells was not possible. Culture of AC133(+)CD34(bright) cells in cytokines that favor megakaryopoiesis or erythropoiesis resulted in a significant expansion of CD41(+) and CD71(+) cells, respectively; AC133(dim/-)CD34(+), in comparison, showed a limited potential to megakaryocytic differentiation and a decreased production of erythroid cells. Our data indicate that early high proliferating stem/progenitor cells and early committed progenitors are present in AC133(+)CD34(bright) cells, but not in AC133(dim/-)CD34(+) cells; the latter represent late committed progenitors with limited proliferative potential.