CD133-enriched CD34^- (CD33/CD38/CD71)^- cord blood cells acquire CD34 prior to cell division and hematopoietic activity is exclusively associated with CD34 expression

We compared the cell division behavior of CD34^- and CD34⁺ (CD33/CD38/CD71)-negative (Lin^-) CD133⁺ cord blood cells stimulated with the cytokines Flt3-ligand, stem cell factor, and thrombopoietin. Within a 4-day time frame, Lin^-CD34^- CD133⁺ (CD34^-) cells underwent more cell divisions in serum-free culture than their Lin^-CD34⁺ CD133⁺ (CD34⁺) counterparts. The majority of CD34^- cells acquired expression of CD34 in vitro, including most undivided cells. Moreover, hematopoietic activity from both CD34^- and CD34⁺ cells was exclusively retained within the cell fraction expressing CD34 after 4 days in culture. Most strikingly, in cultures from Lin^-CD34^- cells hematopoietic activity was associated with the fraction of divided cells, whereas in cultures of CD34⁺ cells, hematopoietic activity associated with the undivided cell fraction. Therefore, clonogenic CD34⁺ cells either do not divide or lose their clonogenic capacity upon cell division in vitro, while CD34^- cells divide and retain this capacity under the same specific conditions. In conclusion, we demonstrate that CD133-enriched Lin^-CD34^- cord blood cells acquire CD34 prior to cell division and that long-term hematopoietic activity is associated exclusively with expression of CD34.