Synergistic and Inhibitory Interactions in the In Vitro Control of Murine Megakaryocyte Colony Formation

The formation of megakaryocyte colonies in agar cultures of murine bone marrow or spleen cells can be stimulated by the addition of interleukin‐3 (IL‐3), erythropoietin (EPO), thrombopoietin (TPO), or IL‐6. However, greater numbers of colonies developed if combinations of two or more of these stimuli were used, particularly combinations including stem cell factor, with maximal numbers of colonies developing with the combination of stem cell factor plus IL‐3 plus EPO. The data indicate that most committed progenitor cells in the megakaryocyte lineage were unusual in that they required stimulation by two or more hematopoietic growth factors. In tests using a range of growth factors, G‐CSF was exceptional in that it consistently specifically inhibited megakaryocyte colony formation stimulated by EPO, TPO, or IL‐6 but not that stimulated by IL‐3. The mechanisms involved in this inhibitory action of G‐CSF are unknown, but the inhibitory action could be of relevance for the dose‐dependent lowering of platelet levels observed in some subjects injected with G‐CSF.