Stem cell factor (SCF) synergizes with megakaryocyte colony stimulating activity in post-irradiated aplastic plasma in stimulating human megakaryocytopoiesis

Plasma obtained from lethally irradiated animals contains a megakaryocyte (MK) growth factor which has recently been identified as the ligand for the c-mpl receptor and has been named thrombopoietin (TPO). We demonstrate that post-irradiation aplastic canine plasma (PICS-J) and plasma from a human subject (ML) who was accidentally exposed to lethal irradiation, contain high levels of this activity, which support both MK proliferation and maturation in a dose-dependent manner. These plasma were far more active in stimulating human MK colony formation than other types of thrombocytopenic plasma or a number of exogenously added human recombinant cytokines and their combinations. The addition of stem cell factor (SCF), which alone has a minimal stimulatory affect, to post lethal-irradiation plasma provided a synergistic stimulation of megakaryocytopoiesis both in colony assays and liquid cultures. In colony assays, the combination of SCF with PICS-J or ML almost doubled the number of burst forming units (BFU-MK) and provided a 1.5- fold increase in colony forming units (CFU-MK). A 1.6-fold increase in the number of CD34⁺ BM cell-derived MK colonies was also elicited. In liquid cultures, the presence of both SCF and PICS-J or ML induced the appearance of a high proportion of CD34⁺ (6.56% vs 0.6% control) and CD41⁺ (3.5% vs 1.2% control) cells after 3 days in culture. By day 10, 66.8 x 10⁴ CD41⁺ cells and 29.8 x 10⁺ CD34⁺ cells were derived from 2 x 10⁶ BMMC originally seeded. We propose that these unique plasma, which do not contain elevated levels of IL-6, IL-3, GM-CSF, IL-1β, erythropoietin or SCF, probably contain high levels of TPO. The addition of SCF to the postirradiation plasma provides a synergistic stimulation of megakaryocytopoiesis which may become relevant for future clinical application.