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

The 5-transmembrane AC133 is expressed on a subpopulation of human hematopoietic cells that includes the CD34^bright cells. We evaluated the development 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 non-cycling stem/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 stem/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.