Differential cytokine effects on primitive (CD34+CD38(-)) human hematopoietic cells: Novel responses to Flt3-ligand and thrombopoietin

A high proportion of the CD34+CD38(-) cells in normal human marrow are defined as longterm culture-initiating cells (LTC-IC) because they can proliferate and differentiate when cocultured with cytokine-producing stromal feeder layers. In contrast, very few CD34(+)CD38(-) cells will divide in cytokine-containing methylcellulose and thus are not classifiable as direct colony-forming cells (CFC), although most can proliferate in serum-free liquid cultures containing certain soluble cytokines. Analysis of the effects of 16 cytokines on CD34(+)CD38(-) cells in the latter type of culture showed that Flt3-ligand (FL), Steel factor (SF), and interleukin (IL)-3 were both necessary and sufficient to obtain an similar to 30-fold amplification of the input LTC-IC population within 10 d. As single factors, only FL and thrombopoietin (TPO) stimulated a net increase in LTC-IC within 10 d. Interestingly, a significantly increased proportion of the CFC produced from the TPO-amplified LTC-IC were erythroid. Increases in the number of directly detectable CFC of >500-fold were also obtainable within 10 d in serum-free cultures of CD34(+)CD38(-) cells. However, this required the presence of IL-6 and/or granulocyte/colony-stimulating factor and/or nerve growth factor beta in addition to FL, SF, and IL-3. Also, for this response, the most potent single-acting factor tested was IL-3, not FL. Identification of cytokine combinations that differentially stimulate primitive human hematopoietic cell self-renewal and lineage determination should facilitate analysis of the intracellular pathways that regulate these decisions as well as the development of improved ex vivo expansion and gene transfer protocols.