Stromal-conditioned medium synergizes with thrombopoietin in stimulating megakaryocytopoiesis

We previously demonstrated that stromal cells release a soluble factor(s) that enhances thrombopoietin (TPO)-induced megakaryocyte (MK) production. We have now further characterized this enhancing activity of human bone marrow stroma and have also analyzed the role of direct contact of stromal cells with human bone marrow CD34⁺ cells on MK production. The three- to sixfold stromal-enhancing activity of conditioned media is constitutive and is not influenced by the presence of TPO. The addition of interleukins 3, 6 (IL-3, IL-6), stem cell factor (SCF), and TPO at concentrations found in stroma to a stroma-free system only enhanced by 58% the TPO-stimulated MK production. No influence of heparan sulfates on MK production was observed. In co-cultures of CD34⁺ cells in direct contact with stromal cells, nonadherent and adherent MK, and proplatelet structures were observed, either in the absence or presence of TPO. Direct contact did not further improve the enhancing activity of stromal cells on MK production induced by TPO. We conclude that the stromal enhancement of MK production is neither accounted for by stromal TPO, IL-3, IL-6, and SCF activities nor modified by physical contact.