Proliferative Responses to Interleukin-3 and Granulocyte Colony-Stimulating Factor Distinguish a Minor Subpopulation of CD34-Positive Marrow Progenitors

Human hematopoietic colony-forming cells (CFC) express the CD34 antigen (CD34+) as well as differentiation antigens such as CD33 and HLA-DR. CD34' cells that do not express these latter differentiation antigens have been shown to contain few CFC in direct culture, but generate increasing numbers of CFC when cultured over a marrow stromal cell layer in the long-term culture system. In this study we determined if CD34' cells with low or absent expression of CD33 and a novel antigen, 7B9 (CD34+CD33-7B9-), could be distinguished from CD34' cells expressing these antigens (CD34TD33'7B9') based on their proliferative responses to interleukin-3 (IL-3) and granulocyte colony-stimulating factor (G-CSF) in a short-term liquid culture system. These two populations were separated by fluorescence-activated cell sorting, cultured with IL-3 ( I O ng/mL), G-CSF (100 ng/mL), or IL-3 and G-CSF, and 3H-thymidine uptake was measured. CD34TD33-7B9cells proliferated in the presence of IL-3, but not G-CSF. However, a synergistic response to the combination of IL-3 and G-CSF was seen in most experiments. In contrast, CD34+CD33t7B9' cells proliferated in the presence of either IL-3 or G-CSF but did not display an additive or synergistic response to the combination of IL-3 and G-CSF. In colony-forming assays performed before and after liquid culture, the CD34+CD33-7B9cells in two experi-