Direct and reversible inhibition of platelet factor 4 on megakaryocyte development from CD34+ cord blood cells: comparative studies with transforming growth factor β1

Mechanisms of the action of platelet factor 4 (PF4) on the growth of megakaryocyte (MK) progenitor cells in CD34⁺ cord blood (CB) cells were studied in comparison with transforming growth factor β1 (TGFβ1). Development of MK from CD34⁺ CB cells in both plasma clot culture and liquid culture was significantly inhibited by either purified human PF4 and by recombinant human TGFβ1. Inhibition of MK colony formation by PF4 was reversible, because CD34⁺ cells preincubated with PF4 could regenerate colonies after washing and replating into secondary cultures. In contrast, TGFβ1-preincubated CD34⁺ cells gave rise to few colonies following replating. Moreover, incubation of CD34⁺ cells with PF4 in liquid culture caused the increased number of both stem cell factor (SCF)-binding cells and CD34 antigen-bearing cells. In addition, PF4-preincubated CD34⁺ cells exibited a higher potential in MK colony formation in the presence of 5-fluorouracil (5FU). These results demonstrate that both PF4 and TGFβ1 inhibit MK development from CD34⁺ CB cells by different mechanisms, and suggest that PF4, unlike TGFβ1, exerts its inhibitory effect on the growth of the target cells in a reversible manner which results in a preservation of a more immature and 5FU-resistant cell population.