Effect of interleukin 6 on in vitro human megakaryocytopoiesis: Its interaction with other cytokines

The effects of human recombinant interleukin 6 (rIL-6) on in vitro human megakaryocytopoiesis were studied utilizing a serum-depleted culture system. Recombinant IL-6 increased both the number of megakaryocyte (MK) colonies formed and the number of cells comprising individual MK colonies cloned from normal low-density bone marrow (LDBM) cells. This stimulation of MK colony number and size was significantly less than that observed following the addition of recombinant interleukin 3 (rIL-3) or granulocyte-macrophage colony-stimulating factor (rGM-CSF). The addition of either rIL-3 or rGM-CSF, but not rIL-6 promoted MK colony formation by nonadherent, low-density, T-cell-depleted (NALDT^-) marrow cells. Recombinant interleukin 1α (rIL-1α) and interleukin 4 (rIL-4) failed either to promote LDBM MK colony formation when added alone or to significantly increase rIL-6-promoted MK colony formation. MK colony formation promoted by optimal doses of rIL-6 was, in fact, significantly inhibited by rIL-1α at all concentrations tested. Addition of either recombinant erythropoietin (rEpo) or purified thrombocytopoiesis-stimulating factor (TSF) to assays containing rIL-6 also resulted in significant inhibition of MK colony formation. The effect of suboptimal concentrations of rIL-6 on MK colony formation was additive to that of rIL-3 but not rGM-CSF. The addition of transforming growth factor β (TGF-β) resulted in a 58% reduction of rIL-6-promoted MK colony formation by LDBM. These data suggest that rIL-6 can promote in vitro megakaryocytopoiesis and that this effect can be either augmented or inhibited by the addition of several other cytokines. Recombinant IL-6, however, might affect the MK colony-forming unit (CFU-MK) by acting through bone marrow accessory cells or requiring the presence of as yet unidentified additional cytokines.