The development of human megakaryocytes: III. Development of mature megakaryocytes from highly purified committed progenitors in synthetic culture media and inhibition of thrombopoietin-induced polyploidization by interleukin-3.

Megakaryocyte (MK) progenitors, CD34+CD41+ cells, were isolated from human bone marrow with a purity greater than 98% and a viability of 95%, using affinity techniques with magnetic beads followed by fluorescence-activated cell sorting. These cells were incubated in synthetic media containing the cytokines thrombopoietin (TPO), interleukin-3 (IL-3), stem cell factor (SCF), and IL-6, obviating the confounding effects of serum growth factors or cytokine secretions of non-MK cells on MK maturation. MK number, MK colony-forming units (CFU-MK), and MK ploidy and phenotype were examined during 7 days in culture. TPO in serum-free cultures without any other exogenously added cytokine supported MK growth and maturation. SCF synergized with TPO to augment MK production and maturation and could partially replace it under some conditions. Both TPO and IL-3 alone increased MK number (12- and 5-fold, respectively) and CFU-MK (approximately 15-fold each). SCF alone had no effect on MK proliferation in the absence of TPO, but increased both MK number and CFU-MK by 1.5- to 2.0-fold in the presence of TPO. When combined with IL-3, SCF increased both MK number and CFU-MK by 15- to 20-fold in the absence of TPO. In the presence of TPO, the combination of IL-3 and SCF produced only modest increases (1.5- to 2.0-fold) in both MK number and CFU-MK. The proportion of polyploid MK increased greater than fivefold in the presence of TPO. SCF had little effect on MK ploidy in the presence of TPO, but enhanced ploidy twofold to threefold in the absence of TPO. IL-3 alone never increased the level of polyploidization. Rather, it consistently inhibited TPO- and SCF-induced polyploidization of MK. This inhibition was observed in cultures with or without SCF or IL-6. Although IL-3 also supported the proliferation of CD41+ cells and CFU-MK production, the cells that developed under the influence of IL-3 were phenotypically unusual (CD41dim, CD42dim) and of relatively low ploidy. Mature MK were not produced. When added with TPO, IL-3 suppressed polyploidization. Therefore, TPO stimulates MK growth and maturation, whereas IL-3 stimulates growth without maturation and may serve to conserve the immature MK compartment.