To evaluate the effect of all-trans retinoic acid (RA) on fetal haemopoiesis, we performed serum-free liquid and semisolid cultures using CD34+ cells purified from midtrimester human fetal blood samples. RA, at both physiological (10(-11) and 10(-12)M) and pharmacological (10(-6) and 10(-7)M) concentrations, significantly (P < 0.01) promoted the survival of fetal CD34+ cells in liquid cultures from day 3 onwards, by suppressing apoptosis induced by serum and growth factor deprivation. On the other hand, RA alone had no significant effect on the proliferation and differentiation of fetal haemopoietic progenitors. In the presence of optimal concentrations of recombinant interleukin-3 (IL-3), stem cell factor (SCF), granulocyte/macrophage-colony stimulating factor (GM-CSF), and erythropoietin (Epo), low and high doses of RA induced striking differential effects on CD34+ cell proliferation in liquid cultures and colony formation in semisolid assays. In fact, 10(-11)M and 10(-12)M RA were able to: (i) significantly (P < 0.05) increase 3H-thymidine uptake by fetal CD34+ cells in liquid cultures, and (ii) variably promote the growth of pluripotent (CFU-GEMM, P < 0.05), early (BFU-meg) and late (CFU-meg, P < 0.01) megakaryocyte, granulocyte/macrophage (CFU-GM, P < 0.01) and erythroid (BFU-E) progenitors in semisolid cultures. On the contrary, 10(-6) and 10(-7)M RA induced: (i) an overall inhibition (P < 0.01) of CD34+ cell growth in liquid cultures; (ii) a marked suppression of BFU-E colony formation (P < 0.01) at all Epo concentrations examined (0.002-4 IU/ml); and (iii) a significant (P < 0.01) stimulation of CFU-GM with a shift from mixed granulocyte/macrophage to pure granulocyte colonies, whereas it had little effect on the growth of CFU-GEMM, BFU-meg and CFU-meg. Our data, as a whole, demonstrate that RA has direct complex effects on the survival, growth and clonal expansion of fetal haemopoietic progenitor cells, mainly depending on the presence of recombinant cytokines, the type of progenitor and the concentrations of RA
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