Effect of thrombopoietin (c-Mpl ligand) alone and in combination with other hematopoietic growth factors on human megakaryocytopoiesis in serum-free cultures.

The effect of human recombinant (hr) thrombopoietin (TPO) on human megakaryocytopoiesis was studied in a serum-free system. hrTPO induced megakaryocyte colony formation by purified CD 34-positive cells and polyploidization of megakaryocytes by purified CD41a-positive cells. hrTPO gave rise to much smaller colonies which appeared at an earlier time compared to the use of human recombinant interleukin-3 (hrIL-3), suggesting that hrTPO predominantly affects the population of megakaryocyte progenitor cells in the late stage. hrIL-3 additively increased the hrTPO-induced megakaryocyte colony formation by CD34-positive cells. The hrTPO-induced megkaryocyte colony formation was also increased by the presence of hrIL-6, hrIL-11, human recombinant erythropoietin (hrEpo) or human recombinant stem cell factor (hrSCF), none of which stimulated megakaryocyte colony growth when added alone. The combined addition of hrTPO, hrIL-3 and hrSCF to CD34-positive cells markedly stimulated megakaryocyte colony formation and produced large numbers of megakaryocytes. hrTPO stimulated the polyploidization of CD34-positive cell-derived megakaryocytes in liquid culture. However, the addition of hrIL-6, hrIL-11 or hrEpo to hrTPO did not further enhance the hrTPO-induced polyploidization. These findings indicate that at the megakaryocyte progenitor cell level, the effect of hrTPO can be promoted by the presence of various hematopoietic growth factors involved in human megakaryocytopoiesis.