Regulation of human megakaryocytopoiesis: analysis of proliferation, ploidy and maturation in liquid cultures

Enrichment and characterization of peripheral blood-derived megakaryocyte progenitors that mature in short-term liquid culture

A population of peripheral blood-derived cells that mature into megakaryocytes within four to eight days of liquid culture is described. This population was enriched from normal leukapheresis units by counterflow centrifugal elutriation and CD34 selection. The cells were incubated in suspension with known megakaryocyte growth or maturation factors. Megakaryocytes were identified within the cultures with antibodies to platelet-glycoproteins (Ib and IIb) and cytologically classified as stage I-IV cells. Plasma from aplastic dogs (APK9) or human recombinant interleukin 3 (IL-3) were the only culture additives which reproducibly resulted in megakaryocyte development. The activity present in APK9 was relatively megakaryocyte-specific while IL-3 was not. The phenotype of the short-term megakaryocyte progenitor cell population was determined by FACS and found to be CD34bright but not CD34dull or CD34-. The population was further characterized as CD34+/CD38+ and CD34+/HLA-DR+. Both CD34+/CD41- and CD34+/CD41+ populations contained megakaryocyte progenitor cells, although megakaryocytes that developed from the latter population were fewer in number. In summary, we have developed an enrichment protocol that, when coupled with a liquid culture assay system, enabled us to characterize short-term megakaryocyte progenitors from peripheral blood. These cells may be important for early platelet recovery in transplantation settings.

Stimulation of megakaryocytopoiesis and thrombopoiesis by the c-Mpl ligand

Physiological platelet synthesis is thought to require the humoral activities of meg-CSF and thrombopoietin, which respectively promote proliferation and maturation of megakaryocytic cells. A meg-CSF/thrombopoietin-like protein that is present in plasma of irradiated pigs has been purified and cloned. This protein binds to and activates the c-mpl protein, a member of the cytokine receptor superfamily. The isolated Mpl ligand shares homology with erythropoietin and stimulates both megakaryocytopoiesis and thrombopoiesis.

Cellular signalling events stimulated by myeloid haemopoietic growth factors

In haemopoietic cells, proliferation, commitment to development, lineage restriction and survival via suppression of apoptosis can all be controlled by haemopoietic growth factors. The mechanisms underlying the regulation of these events can now be studied since recombinant forms of most of these haemopoietic growth factors are now available. Recent advances in cell purification techniques and the development of multipotent cell lines (see Spangrude et al, 1988; Whetton, 1990; Heyworth et al, 1988, 1990a; Jones et al, 1990) have provided suitable cell populations on which to study the cellular signalling events associated with differentiation and lineage restriction. This process has started with the elucidation of the structure and expression of many of the myeloid growth factor receptors, which should now facilitate progress in the study of the signal transduction mechanisms these growth factors employ. Another important facet of these studies will be to determine whether a single growth factor with multiple target cell types, ranging from multipotent cells to postmitotic cells (e.g. neutrophils), employs distinct signalling mechanisms depending on the target cell in question. The cellular signalling events elicited by each of these growth factors and the ways in which they can regulate the transcriptional activation of genes associated with specific developmental events are going to be key areas of haemopoietic research in the next few years.