Prevalence of the primitive megakaryocyte progenitors (BFU-meg) in adult human peripheral blood.

The in vitro growth of early (megakaryocyte burst-forming units, BFU-meg) and late (megakaryocyte colony-forming units, CFU-meg) megakaryocyte (meg) progenitors has been evaluated in normal adult human peripheral blood (PB). All the experiments were carried out using CD34+ cells, which were assayed in a serum-free fibrinclot assay. PB BFU-meg were morphologically characterized as plurifocal aggregates containing greater than 50 cells/colony, distinct from unifocal CFU-meg, in a limiting dilution assay. At variance with PB CFU-meg, PB BFU-meg were unaffected by the complement-mediated cytotoxicity with anti-HLA-DR. The optimal source of colony-stimulating activity for PB BFU-meg growth was recombinant human interleukin 3 (rhIL-3; 100 U/ml), which supported a significantly higher number of BFU-meg in comparison with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; 200 U/ml, p = 0.043). Combinations of rhIL-3 (100 U/ml) plus rhGM-CSF (200 U/ml), rhIL-3 plus recombinant human interleukin 6 (rhIL-6; 100 U plus 100 U/ml) or rhIL-3 plus rhGM-CSF plus rhIL-6 (100 U plus 200 U/ml plus 100 U/ml) failed to further increase the number of PB BFU-meg with respect to rhIL-3 (100 U/ml) alone. Both PB BFU-meg and CFU-meg were markedly inhibited, in a dose-dependent fashion, by increasing doses of human purified transforming growth factor-beta 1 (TGF-beta 1) (from 0.001 to 10 ng/ml). Finally, the CFU-meg/BFU-meg ratio in PB (0.52) was significantly different from that of normal bone marrow (2.3), clearly indicating that adult human peripheral blood predominantly carries primitive megakaryocytic progenitors.