Partial purification and characterization of human megakaryocyte colony-stimulating factor (Meg-CSF)

High Concentrations of Thrombopoietin Activate Platelets In Vitro

Thrombopoietin (TPO), the ligand for the proto- oncogene c-mpl, has been cloned and expressed from both human and murine sources. Thrombopoietin increases platelet counts when given in vivo and acts on progenitor cells to in crease their proliferation and maturation into megakaryocytes. The effects of TPO on mature platelets were investigated by evaluating platelet aggregation and platelet activation- dependent antigen expression. Platelet aggregation score, a quantitative representation of aggregation, showed potentiation of response to ADP-induced aggregation but no direct agonist response to TPO alone. Soluble c-mpl blocked the effect of TPO on the platelet aggregation score. Flow cytometry showed that TPO at concentrations >250 U/ml (50 ng/ml) caused a minority population of platelets to express the activation mark ers CD62, CD63 and activated glycoprotein IIb/IIIa. While stem cell factor and interleukins-3 and -6 did not affect platelet activation antigen expression, interleukin-11 increased CD62 expression on platelets in vitro. The effects of TPO on antigenic expression and aggregability were partially inhibited in vitro by preincubation with aspirin. We conclude that high concentra tions of TPO promote platelet activation antigen expression on a proportion of platelets and potentiate platelet aggregability to ADP in vitro by a process that is partially inhibited by aspirin.

The role of interleukin 6 in megakaryocyte formation, megakaryocyte development and platelet production

Megakaryocytopoiesis is the cellular amplification and differentiation of precursors into immature megakaryocytes, and the cytoplasmic maturation of these megakaryocytes, a process terminating in the release of platelets into the circulation. Interleukin 6 (IL-6) stimulates megakaryocytopoiesis in the bone marrow, increasing platelet numbers in the circulation. IL-6 alone is poorly active on the growth of stem cell populations, but acts in synergy with stem cell factor (c-kit ligand) to expand the committed myeloid progenitor compartments but not the megakaryocyte progenitors. IL-6 has a direct action on megakaryocyte progenitors but only in synergy with low doses of interleukin 3 (IL-3), increasing the number of immature megakaryocytes and enhancing the processes of development into mature megakaryocytes. IL-6 is about 10 times more active on megakaryocytes than on megakaryocyte progenitors in cell culture. It is active alone and will stimulate increases in cell size and DNA content. IL-6 does not appear to stimulate the process of platelet release. IL-6 is found in bone marrow, in both macrophage subsets and megakaryocytes, indicating that it may be an important physiological regulator of both paracrinal (microenvironmental) and autocrinal mechanisms controlling megakaryocyte development in bone marrow.

The physiologic role and therapeutic potential of the Mpl-ligand in thrombopoiesis

The constant and appropriate production of megakaryocytes, and subsequently platelets, is critical for maintenance of hemostasis. Inadequate megakaryopoiesis and/or thrombopoiesis can lead to serious bleeding disorders. The humoral factors regulating these processes have been the subject of study for several decades. Although many cytokines have been shown to influence megakaryocyte development and platelet production, none appeared to do so in a lineage-dominant fashion analogous to the situation with erythrocyte and neutrophil production. More recently, a ligand for the hematopoietic cytokine receptor encoded by the c-mpl gene (Mpl ligand) has been shown to have profound effects on megakaryocyte growth and development. These effects appear to include the expansion of megakaryocyte progenitors (i.e. megakaryocyte-colony stimulating activity), and induction of megakaryocyte maturation to the point of platelet production (i.e. thrombopoietin). Administration of recombinant Mpl-ligand to rodents or primates treated with myelosuppressive agents abrogates or alleviates the severity and the duration of the resultant thrombocytopenias. The in vitro and in vivo data to date indicate that this new cytokine holds tremendous promise as a therapeutic agent for the treatment of thrombocytopenia associated with cancer therapies.

Megakaryocyte growth and development factor. Analyses of in vitro effects on human megakaryopoiesis and endogenous serum levels during chemotherapy-induced thrombocytopenia

The present study shows that recombinant human megakaryocyte growth and development factor (r-HuMGDF) behaves both as a megakaryocyte colony stimulating factor and as a differentiation factor in human progenitor cell cultures. Megakaryocyte colony formation induced with r-HuMGDF is synergistically affected by stem cell factor but not by interleukin 3. Megakaryocytes stimulated with r-HuMGDF demonstrate progressive cytoplasmic and nuclear maturation. Measurable levels of megakaryocyte growth and development factor in serum from patients undergoing myeloablative therapy and transplantation are shown to be elaborated in response to thrombocytopenic stress. These data support the concept that megakaryocyte growth and development factor is a physiologically regulated cytokine that is capable of supporting several aspects of megakaryopoiesis.

Recombinant human megakaryocyte growth and development factor (rHuMGDF), a ligand for c-Mpl, produces functional human platelets in vitro

Platelet formation, occurring from bone marrow or lung megakaryocytes, has been difficult to study mechanistically. Recombinant human megakaryocyte growth and development factor (rHuMGDF), a recently described cytokine, has now been used to establish an in vitro system in which this important and little understood process occurs. CD34+ cells cultured with rHuMGDF develop into megakaryocytes which form long cytoplasmic extensions (proplatelets) that fragment into platelet-sized particles (in vitro platelets). Morphologically, in vitro and human plasma-derived platelets (control platelets) are virtually identical with respect to size, dense granule distribution and ultrastructural features. Functionally, in vitro and control platelets have similar aggregation and activation responses, and similarly incorporate mepacrine into dense granules. These findings suggest that rHuMGDF is sufficient to generate platelet-synthesizing megakaryocytes from CD34+ cells and provide an experimental setting in which the study of human platelet formation can be adequately performed.

Cloning and characterization of the human megakaryocyte growth and development factor (MGDF) gene

The megakaryocyte growth and development factor (MGDF) is a cytokine that regulates megakaryocyte development and is a ligand for the MPL receptor. In this study, we describe the genomic structure of the human MGDF gene. The MGDF gene was found to consist of seven exons and six introns spanning 8 kilobases. The protein is encoded by exons 3 through 7. The human MGDF gene has been mapped to chromosome 3q26.3. In addition to the previously described full-length cDNA, two cDNA variants were isolated from human fetal liver. Comparison of these two cDNA sequences with the genomic sequence indicates that they arise by differential splicing.

Megakaryocytopoiesis and platelet production: does stem cell factor play a role?

Megakaryocytopoiesis, resulting in the production and release of platelets, is a multistage procession of cellular differentiation and maturation which is regulated by a constellation of cytokines. Since thrombocytopenia is a frequent dose-limiting toxicity of chemotherapy, newly-identified cytokines have been actively investigated for their potential megakaryocyte/platelet-promoting properties. Stem cell factor (SCF, also known as mast cell growth factor, Steel factor or Kit ligand) has been found to synergize with GM-CSF, IL-6, IL-3, IL-11 or Epo to increase the numbers of megakaryocyte-containing colonies (i.e., CFU-Meg, BFU-Meg, CFU-GMM, CFU-GEMM). On the other hand, SCF increased the number of megakaryocytes per colony in the presence of IL-3, GM-CSF or IL-6. SCF also stimulated the proliferation of specific megakaryocytic cell lines (i.e., CMK, M-07e). SCF did not, however, alter megakaryocyte markers or increase cell ploidy. Thus, SCF appears to expand the committed myeloid progenitor compartments, rather than increase the rate of megakaryocyte maturation or the number of platelets released. We describe studies in which SCF stimulated murine CFU-Meg alone and in the presence of IL-3. However, a decrease in cultured cell plating density resulted in ablation of this SCF-stimulation of CFU-Meg colonies. CFU-Meg colony stimulation by SCF was dose dependent, even under serum-free conditions. The effects of SCF in other in vitro and in vivo animal model systems are reviewed.

Megakaryocyte colony-stimulating factor (Meg-CSF) is a unique cytokine specific for the megakaryocyte lineage

The regulation of megakaryocytopoiesis and platelet production has not yet been clearly elucidated. Several cytokines have been shown to be capable of producing megakaryocyte colonies from bone marrow [i.e. Interleukin (IL)-3, granulocyte-macrophage (GM)-colony-stimulating factor (CSF), erythropoietin (Epo)]. In addition, other activities have been reported to stimulate megakaryocyte precursors, yet a megakaryocyte-CSF (Meg-CSF) has not been purified to homogeneity and IL-3, GM-CSF and/or Epo often contaminate purification attempts which could account for the activities. A Meg-CSF has been isolated from the urine of patients with aplastic anaemia and purified by sequential ultrafiltration, cation exchange, G-50 chromatography, preparative PAGE, chromatofocusing and cation exchange HPLC. The activity of this material is 2-4 x 10(4) CFU-Meg/mg as measured in a murine marrow, serum-containing assay. This activity also stimulates CFU-Meg in the absence of adherent accessory cells and in serum-free cultures, indicative of the direct stimulation on CFU-Meg. Immunoassays, colony forming assays, and proliferation assays demonstrate that purified Meg-CSF has no GM-CSF, IL-3, M-CSF, G-CSF or IL-1 alpha, -3, -6, -9 and -11. In confirmation of these results, neutralizing antibody to IL-6 also did not abrogate Meg-CSF activity. Therefore the previously-reported megakaryocyte colony-stimulating activity in purified aplastic anaemia patient urine is due to a unique cytokine: Meg-CSF.