Regulation of human megakaryocytopoiesis. An in vito analysis

Ex Vivo Cultured Megakaryocytes Express Functional Glycoprotein IIb-IIIa Receptors and Are Capable of Adenovirus-Mediated Transgene Expression

Investigation of the molecular basis of megakaryocyte (MK) and platelet biology has been limited by an inadequate source of genetically manipulable cells exhibiting physiologic MK and platelet functions. We hypothesized that ex vivo cultured MKs would exhibit agonist inducible glycoprotein (GP) IIb-IIIa activation characteristic of blood platelets and that these cultured MKs would be capable of transgene expression. Microscopic and flow cytometric analyses confirmed that human hematopoietic stem cells cultured in the presence of pegylated recombinant human MK growth and development factor (PEG-rHuMGDF) differentiated into morphologic and phenotypic MKs over 2 weeks. Cultured MKs expressed functional GPIIb-IIIa receptors as assessed by agonist inducible soluble fibrinogen and PAC1 binding. The specificity and kinetics of fibrinogen binding to MK GPIIb-IIIa receptors were similar to those described for blood platelets. The reversibility and internalization of ligands bound to MK GPIIb-IIIa also shared similarities with those observed in platelets. Cultured MKs were transduced with an adenoviral vector encoding green fluorescence protein (GFP) or β-galactosidase (β-gal). Efficiency of gene transfer increased with increasing multiplicities of infection and incubation time, with 45% of MKs expressing GFP 72 hours after viral infection. Transduced MKs remained capable of agonist induced GPIIb-IIIa activation. Thus, ex vivo cultured MKs (1) express agonist responsive GPIIb-IIIa receptors, (2) are capable of expressing transgenes, and (3) may prove useful for investigation of the molecular basis of MK differentiation and GPIIb-IIIa function.

Ex Vivo Cultured Megakaryocytes Express Functional Glycoprotein IIb-IIIa Receptors and Are Capable of Adenovirus-Mediated Transgene Expression

Investigation of the molecular basis of megakaryocyte (MK) and platelet biology has been limited by an inadequate source of genetically manipulable cells exhibiting physiologic MK and platelet functions. We hypothesized that ex vivo cultured MKs would exhibit agonist inducible glycoprotein (GP) IIb-IIIa activation characteristic of blood platelets and that these cultured MKs would be capable of transgene expression. Microscopic and flow cytometric analyses confirmed that human hematopoietic stem cells cultured in the presence of pegylated recombinant human MK growth and development factor (PEG-rHuMGDF) differentiated into morphologic and phenotypic MKs over 2 weeks. Cultured MKs expressed functional GPIIb-IIIa receptors as assessed by agonist inducible soluble fibrinogen and PAC1 binding. The specificity and kinetics of fibrinogen binding to MK GPIIb-IIIa receptors were similar to those described for blood platelets. The reversibility and internalization of ligands bound to MK GPIIb-IIIa also shared similarities with those observed in platelets. Cultured MKs were transduced with an adenoviral vector encoding green fluorescence protein (GFP) or β-galactosidase (β-gal). Efficiency of gene transfer increased with increasing multiplicities of infection and incubation time, with 45% of MKs expressing GFP 72 hours after viral infection. Transduced MKs remained capable of agonist induced GPIIb-IIIa activation. Thus, ex vivo cultured MKs (1) express agonist responsive GPIIb-IIIa receptors, (2) are capable of expressing transgenes, and (3) may prove useful for investigation of the molecular basis of MK differentiation and GPIIb-IIIa function.

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 10(4) CD41+ cells and 29.8 x 10(4) CD34+ cells were derived from 2 x 10(6) BMMC originally seeded. We propose that these unique plasma, which do not contain elevated level of IL-6, IL-3, GM-CSF, IL-1 beta, erythropoietin or SCF, probably contain high levels of TPO. The addition of SCF to the post-irradiation plasma provides a synergistic stimulation of megakaryocytopoiesis which may become relevant for future clinical application.

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.

Transcription factor NF-E2 is required for platelet formation independent of the actions of thrombopoietin/MGDF in megakaryocyte development

Despite the importance of blood platelets in health and disease, the mechanisms regulating their formation within megakaryocytes are unknown. We generated mice lacking the hematopoietic subunit (p45) of the heterodimeric erythroid transcription factor NF-E2. Unexpectedly, NF-E2-/- mice lack circulating platelets and die of hemorrhage; their megakaryocytes show no cytoplasmic platelet formation. Though platelets are absent, serum levels of the growth factor thrombopoietin/MGDF are not elevated above controls. Nonetheless, NF-E2-/- megakaryocytes proliferate in vivo in response to thrombopoietin administration. Thus, as an essential factor for megakaryocyte maturation and platelet production, NF-E2 must regulate critical target genes independent of the action of thrombopoietin. These findings provide insight into the genetic analysis of megakaryocyte maturation and thrombopoiesis.

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.

Megakaryocyte colony stimulating activity in allogenic bone marrow recipients prepared with busulfan and cyclophosphamide

Increased megakaryocyte colony stimulating activity (MK-CSA) has been reported after total body irradiation (TBI) for bone marrow transplant (BMT). We studied the effect of a busulfan (Bu) and cyclophosphamide (Cy) marrow transplant conditioning regimen, without radiation, on MK-CSA production. Initial screening of MK-CSA was done on previously collected and banked sera from 14 BMT patients. MK-CSA was expressed as the ability to stimulate growth of megakaryocyte progenitors (CFU-MK) in standard plasma clot cultures. In the initial samples, MK-CSA peaked at day 7. This preliminary data led to a prospective study of MK-CSA and clinical parameters in seven allogeneic recipients. MK-CSA activity increased from day -7 pre-transplant (2.9 +/- 1.7 CFU-MK/10(5) NATD, mean +/- SD) to day 0 (10.3 +/- 4.7 CFU-MK) and peaked by day 9 post-transplant (20.6 +/- 6.4 CFU-MK). MK-CSA activity decreased in all seven patients by day 21 at which time five of seven patients studied had recovery of platelet counts to greater than 100 x 10(9)/l. MK-CSA activity rose rapidly in both groups of sera after the initiation of this non-irradiation, BMT preparative regimen. High MK-CSA levels, early after transplant, may contribute to the rapid platelet recovery in some patients.

Bone marrow changes following treatment of renal anemia with erythropoietin

In 14 severely anemic patients with end-stage renal disease and chronic hemodialysis the effect of recombinant human erythropoietin (EPO) on hemopoiesis was investigated. Bone marrow biopsies were taken before and after four and 26 months of treatment with EPO to evaluate quantitative and qualitative changes of histomorphology. EPO induced normalization of maturation and an increase in cell mass of the erythropoietic line in all patients. The number of megakaryocyte also increased significantly with EPO treatment (P less than 0.01). At the time of the third bone marrow biopsy (26 months) erythropoiesis was normal. Megakaryopoiesis remained unchanged compared to the second biopsy (4 months). No cytomorphologic abnormalities or other evidence for malignant disorder could be detected in any of the patients. Hematocrit increased from a mean of 19 to 31 percent at the second evaluation (P less than 0.001). Platelet count had risen by a mean of 30,000 at four months (P less than 0.05) and slightly decreased at 26 months. These observations suggest great safety of long-term treatment with recombinant human erythropoietin, and demonstrate efficacy in correcting reduced and immature erythropoiesis in chronically hemodialyzed patients. EPO also stimulates human megakaryopoiesis.

The clinical spectrum of thrombocytosis and thrombocythemia

Platelet production is the result of a highly ordered maturation of a developmental hierarchy of megakaryocytic progenitor cells regulated by a variety of cytokines. GM-CSF, II-3 and II-6 have a stimulatory effect and several cytokines (TGF-beta, platelet released glycoprotein, platelet factor 4 and interferons) have inhibitory effects down regulating platelet production perhaps as part of an autocrine control loop. Excess platelet production can be clinically characterized as pseudothrombocytosis, thrombocytosis or thrombocythemia; the clinical features and criteria for each are defined. The term thrombocytosis infers its reactive nature and, in the absence of arterial disease or prolonged immobility, it poses little risk regardless of platelet numbers. By contrast, in thrombocythemia, whether primary or associated with other myeloproliferative lesions, significant thrombohemorrhagic events occur. The natural history, rationale, and approach to platelet reduction and control of clinical sequela are reviewed. Clinical therapeutic options include a new agent, Anagrelide.

Cytokine-dependent long-term culture of highly enriched precursors of hematopoietic progenitor cells from human bone marrow

Human marrow cells positive for the CD34 antigen but not expressing HLA-DR, CD15, or CD71 antigens were isolated. In a liquid culture system supplemented with 48-hourly additions of recombinant interleukins IL-1 alpha, IL-3, IL-6, or granulocyte/macrophage colony-stimulating factor (GM-CSF), these cells were capable of sustaining in vitro hematopoiesis for up to eight weeks. The establishment of an adherent cell layer was never observed. Cultures containing no exogenous cytokine produced clonogenic cells for only 1 wk. IL-1 alpha and IL-6 were alone able to support hematopoiesis for 2 or 3 wk. Cells maintained with GM-CSF proliferated and contained assayable colony-forming cells for 3 or 4 wk, while maximal cellular expansion and generation of assayable progenitor cells occurred in the presence of IL-3 for 4-5 wk. When IL-3 was combined with IL-1 alpha or IL-6, hematopoiesis was sustained for 8 wks. Basophil numbers were markedly increased in the presence of IL-3. These studies indicate that marrow subpopulations can sustain hematopoiesis in vitro in the presence of repeated additions of cytokines. We conclude that a major function of marrow adherent cells in long-term cultures is that of providing cytokines which promote the proliferation and differentiation of primitive hematopoietic cells.

Synergistic regulation of human megakaryocyte development

Little information exists concerning differing levels of regulation occurring during human megakaryocyte development. We hypothesize that megakaryocytic proliferation and maturation is controlled by two, synergistic regulatory factors. One, megakaryocyte colony-stimulating activity, is an obligate requirement for colony formation and drives the development of relatively immature cells. Megakaryocyte colony-stimulating activity is a functional component of the human recombinant proteins, interleukin 3 or GM-CSF. Human recombinant growth factors, interleukin 1, interleukin 6, or crythropoietin, do not effect megakaryocyte development either alone or in combination with interleukin 3. Full maturation requires a second synergistic activity which increases megakaryocyte number, size, and cytoplasmic and antigenic content. In culture, this synergistic regulator augments maturation by increasing the number of colonies, colony cellularity, and size. In suspension cultures, this cofactor increases megakaryocyte cytoplasmic and antigenic content, and shifts the morphological distribution from immature to mature megakaryocytes. Finally, this activity also increases the number of antigen positive megakaryocytes, either by stimulating proliferation or conversion of antigen-negative to antigen-positive cells. Comparative studies of megakaryocytic regulation suggests that this in vitro regulator mimicks some of the known effects of thrombopoietin in vivo.

Defective megakaryocytopoiesis in the syndrome of thrombocytopenia with absent radii

The syndrome of thrombocytopenia with absent radii (TAR) is a hereditary condition whose pathogenesis is poorly understood. In this investigation we evaluated a female infant with TAR and her parents using in vitro haematopoietic colony forming assays and an antiserum against platelet membrane glycoproteins (PGP) to label smears of her bone marrow. Megakaryocyte colony growth in vitro was virtually absent in optimally stimulated cultures of the patient's bone marrow progenitors. In contrast, erythroid and myeloid colony growth from the TAR infant's marrow cells was preserved. Staining of the patient's bone marrow smears with PGP antiserum detected no immature, small megakaryocyte precursors. A high level of megakaryocyte colony stimulating activity was detected in serum from the TAR infant, activity comparable to that present in sera from adults with aplastic anaemia. The elevated serum activity decreased by 6 months of age at which time partial platelet recovery had occurred. Evaluation of both peripheral blood haematopoietic progenitor cells and sera from the TAR infant's parents demonstrated no significant abnormalities. We conclude that the principle haematopoietic defect in this patient with TAR syndrome is the absence or arrested development of the committed megakaryocyte progenitor cell. Humoral regulation of megakaryocytopoiesis appears intact and is responsive to the degree of megakaryocytic hypoplasia.

Effect of recombinant and purified human haematopoietic growth factors on in vitro colony formation by enriched populations of human megakaryocyte progenitor cells

Nonadherent low density T-lymphocyte depleted (NALT-) marrow cells from normal donors were sorted on a Coulter Epics 753 Dye Laser System using Texas Red labelled My10 and phycoerythrin conjugated anti HLA-DR monoclonal antibodies in order to obtain enriched populations of colony forming unit-megakaryocyte (CFU-MK). The CFU-MK cloning efficiency (CE) was 1.1 +/- 0.5% for cells expressing both high densities of My10 and low densities of HLA-DR (My10 DR+). This procedure resulted in an 18-fold increase in CE over NALT- cells. The effect of purified or recombinant human haematopoietic growth factors including erythropoietin (Epo), thrombocytopoiesis stimulating factor (TSF), interleukin 1 alpha (IL-1 alpha), granulocyte colony stimulating factor (G-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF), macrophage colony stimulating factor (M-CSF or CSF-1) and interleukin MK colony formation by My10 DR+ cells was determined utilizing a serum depleted assay system. Neither Epo, TSF, CSF-1, IL-1 alpha nor G-CSF alone augmented MK colony formation above baseline (2.5 +/- 0.8/5 x 10(3) My10 DR+ cells plated). In contrast, the addition of GM-CSF and IL-3 each increased both CFU-MK colony formation and the size of colonies with maximal stimulation occurring following the addition of 200 units/ml of IL-3 and 25 units/ml of GM-CSF. At maximal concentration, IL-3 had a greater ability to promote megakaryocyte colony formation than GM-CSF. The stimulatory effects of GM-CSF and IL-3 were also additive in that the effects of a combination of the two factors approximated the sum of colony formation in the presence of each factor alone. The CFU-MK appears, therefore, to express HPCA-1 and HLA-DR antigens. These studies also indicate that GM-CSF and IL-3 are important in vitro regulators of megakaryocytopoiesis, and that these growth factors are not dependent on the presence of large numbers of macrophages or T cells for their activity since the My10 DR+ cells are largely devoid of these accessory cells.

Recombinant gibbon interleukin-3 stimulates megakaryocyte colony growth in vitro from human peripheral blood progenitor cells

Gibbon interleukin-3 (rIL-3) has recently been cloned and found to have a high degree of homology with the human IL-3 molecule. In this investigation, we evaluated the effects of gibbon rIL-3 on normal human peripheral blood megakaryocyte progenitor cell growth in vitro. Gibbon rIL-3 exhibited substantial megakaryocyte colony stimulatory activity (Meg-CSA), supporting peak colony numbers at a concentration of 1 U/ml. Megakaryocyte colony growth induced by rIL-3 reached 58% of the maximum achieved with the active, Meg-CSA-containing protein fraction of aplastic canine serum. Increasing gibbon rIL-3 concentrations also stimulated a 4-5-fold increase in megakaryocyte colony size and resulted in a decrease in geometric mean megakaryocyte ploidy. Ploidy values fell from 8.5N +/- 1.4 (+/- SEM) at an rIL-3 concentration of 0.1 U/ml to a minimum of 2.9N +/- 0.3 at 10 U/ml. In the presence of rIL-3 at 1.0 U/ml, megakaryocyte colony growth was linear with cell plating density and the regression line passed approximately through the origin. The effects of rIL-3 on megakaryocyte colony growth were independent of the presence of T-lymphocytes in the cultures. Cross-species evaluation of murine and gibbon IL-3 indicated that its bioactivity is species restricted. Murine IL-3 did not support colony growth from human megakaryocyte progenitors and gibbon rIL-3 showed no activity in stimulating acetylcholinesterase production by murine bone marrow cells. Gibbon rIL-3 is a potent stimulator of the early events of human megakaryocyte progenitor cell development promoting predominantly mitosis and early megakaryocytic differentiation.

Do bone marrow fat cells or their precursors have a pathogenic role in idiopathic aplastic anaemia?

Idiopathic aplastic anaemia (AA), aplastic anaemia of unknown aetiology, is usually defined as marrow failure with fatty replacement of hemopoietic tissue and peripheral pancytopenia. The pathophysiology is largely unknown, though many mechanisms have been hypothesized. These include the absence of or defects in hemopoietic stem cells (HSC), abnormalities of the bone marrow (BM) microenvironment, immune system disorders and abnormalities of the regulatory factors that control hemopoiesis. The characteristic feature of AA is the replacement of hematopoietically active marrow by fat cells; however, the fat cells themselves have received little attention to date, and this apparent fatty marrow infiltration has been considered a secondary phenomenon. That the marrow fat cells in AA may be abnormal and may have a pathogenic role has never been considered. This communication, postulates that AA may result from an abnormal and excessive proliferation of marrow fat cells and the displacement of the hematopoietic tissue of the marrow; and that the resultant marrow failure could be a secondary phenomenon.

Suppression of maturation of megakaryocyte colony forming unit in vitro by a platelet-released glycoprotein

The suppressive role of platelets on the growth of human marrow megakaryocyte colony forming units (CFU-M) in vitro was investigated by the use of a plasma clot assay. An inverse correlation was established between the number of megakaryocytic colonies grown and the platelet concentration of the plasma or the resultant serum used in the culture system. The suppressive effect of platelets on megakaryocyte colony formation reached a plateau at normal human blood platelet concentration and was specific for CFU-M growth, since marrow cell erythroid burst formation (BFU-E) and granulocytic-monocytic colony formation (CFU-GM) remained unaffected. The inhibitory activity was detectable in the supernatants of platelet suspensions aggregated by thrombin or ADP, and the inhibitory activity released from ADP-stimulated platelets was blocked by pretreatment of platelets with monoclonal antibody HuPl-m1. Partial purification of this activity was achieved by diethylaminoethyl (DEAE)-ion exchange and phytohemagglutinin (PHA)-E agarose affinity chromatography. This inhibitor is a glycoprotein with a molecular weight of 12-17K daltons. This platelet released glycoprotein does not affect the early proliferative phase of CFU-M in vitro but acts on a day 6-8 CFU-M-derived cell by adversely affecting its maturation into recognizable megakaryocytes. These findings demonstrate that a glycoprotein released from platelets suppresses the maturation of CFU-M into megakaryocytes.

Effect of human recombinant erythropoietin on human marrow megakaryocyte colony formation in vitro

The effect of human recombinant erythropoietin (EP) on the growth of human marrow megakaryocyte colony forming units (CFU-M) in vitro was investigated by the use of a plasma clot assay. EP as a single stimulating factor or as an additional factor to optimal concentration of leucocyte conditioned medium (PHA-LCM) had no effect on the number of CFU-M derived colonies. However, addition of EP (0.5-1 U/ml) to cultures with suboptimal concentrations of PHA-LCM increased megakaryocytic colony formation by 50-90% but had no effect on the number of granulocytic-monocytic colonies (CFU-GM). Exposure of marrow cells to EP for 24-48 h in liquid suspension cultures, followed by removal of the hormone and assaying the cells for CFU-M in plasma clots, resulted in a 50-100% increase of megakaryocyte colony formation in vitro. The augmenting effect of EP on CFU-M growth in vitro was abolished when EP was added to the medium after the third day of culture. The presence of factors in human serum and in PHA-LCM was an absolute requirement for the hormone to exert its potentiating effect on human CFU-M growth in vitro. Recombinant EP potentiates the growth of human marrow CFU-M and this effect seems to be exerted during the early stages of CFU-M development in vitro.

Human recombinant erythropoietin promotes differentiation of murine megakaryocytes in vitro

To determine if erythropoietin affects megakaryocytopoiesis, we measured acetylcholinesterase (AchE) activity, a marker of the murine megakaryocytic lineage, after the addition of human recombinant erythropoietin to serumless murine bone marrow cultures. Erythropoietin increased AchE activity substantially. Moreover, when the hormone was added to serumless cultures of 426 isolated single megakaryocytes derived from megakaryocytic colonies, erythropoietin induced a significant increase in the diameters of these cells. From a Bayesian analysis of the likelihood that some megakaryocytes increased in DNA content during the culture period, we estimate that 61% of the cells increased in ploidy. These data indicate that the action of erythropoietin is not restricted to the erythroid lineage.

Studies of human megakaryocytopoiesis using an anti-megakaryocyte colony-stimulating factor antiserum

We produced an antiserum by immunizing rabbits with purified human megakaryocyte colony stimulating factor (Meg-CSF). With the use of an anti-Meg-CSF IgG fraction (AM-IgG), we detected immunoreactive Meg-CSF both in human aplastic anemia serum (AAS) and normal serum. Based on our immunological and biological analyses, Meg-CSF appeared to be antigenically as well as functionally distinct from human urinary erythropoietin (EPO) and thrombopoietic stimulating factor. The AM-IgG fraction was able to suppress the ability of both aplastic anemia serum and purified Meg-CSF to promote megakaryocyte colony formation. In addition, the supernatant formed after immune precipitation of the AAS with AM-IgG no longer possessed Meg-CSF-like activity. The AM-IgG did not suppress the ability of EPO, phytohemagglutinin-stimulated leukocyte conditioned medium (PHA-LCM), or PHA-LCM + EPO to promote erythroid, granulocyte-macrophage, or mixed colony formation, respectively. The use of this antibody has further defined the dependency of human megakaryocytopoiesis on Meg-CSF.

Alpha-interferon: differential suppression of colony growth from human erythroid, myeloid, and megakaryocytic hematopoietic progenitor cells

The effect of varying concentrations of interferon-alpha (IFN-alpha) on the in vitro colony growth from all single-lineage human hematopoietic colony-forming progenitor cells was evaluated. IFN-alpha was tested at concentrations of 0, 2, 20, and 200 U/ml in optimally stimulated bone marrow cultures from each of 4 volunteer donors. Substantial donor-to-donor variability and distinct, lineage-specific patterns of stem cell sensitivity to IFN-alpha were observed. In the erythroid series, the more primitive progenitor or burst-forming unit (BFU-E) was substantially more resistant to growth inhibition at low IFN-alpha concentrations than the mature colony-forming unit (CFU-E). Colony growth by the megakaryocyte progenitor cell (CFU-Meg) was decreased by all concentrations of IFN-alpha which produced a biphasic, inhibitory dose response. The response of the colony-forming unit granulocyte/macrophage (CFU-GM) was heterogeneous among the donors tested. CFU-GM growth from 2 donors was insensitive to IFN-alpha at all concentrations. Conversely, CFU-GM from the other 2 donors manifested a steep dose-response curve that was similar to that of the CFU-E. These data demonstrate a heterogeneity of progenitor cell sensitivity to growth suppression by IFN-alpha which appears to be influenced by (i) hematopoietic lineage, (ii) degree of differentiation of the progenitor cell, and (iii) individual variability.

Transitory hypomegakaryocytic thrombocytopenia: aetiological association with ethanol abuse and implications regarding regulation of human megakaryocytopoiesis

We studied a patient with a long history of ethanol abuse who presented to the hospital with profound weakness, anaemia and thrombocytopenia. Evaluation of these problems revealed the patient's bone marrow to be hypercellular but severely iron depleted and almost totally devoid of morphologically recognizable megakaryocytes. However, we were able to detect the presence of non-morphologically recognizable, immature megakaryocytes in the same sample using an immunochemical detection technique. This circumstance allowed us to study the relative importance of both megakaryocyte maturation and peripheral blood platelet count on the production of megakaryocyte colony stimulating activity (Meg-CSA), a putative regulator of the megakaryocyte colony forming unit (CFU-M). The results of our investigations disclosed a rapid decline in serum Meg-CSA levels which preceded recovery of the platelet count and appeared to coincide with the maturation of megakaryocytes into the morphologically recognizable pool. The effect of ETOH on the patient's CFU-M cloning efficiency was also studied. ETOH in amounts up to 454 mg/dl did not inhibit cloning of the patient's peripheral blood CFU-M in plasma clot cultures. Our results suggest that regulation of Meg-CSA production is a complex function which appears to be dependent on a number of factors including the level of megakaryocyte maturation in the marrow. We also speculate that ethanol associated thrombocytopenia may occasionally be brought about by a disruption in the process of megakaryocyte maturation at the level of a progenitor more mature than the CFU-M.

Characterization of human megakaryocytic colony formation in human plasma

We have analysed the contribution to megakaryocyte colony formation in methylcellulose made by human plasma, serum, media conditioned by phytohemagglutinin (PHA) stimulated leukocytes (PHA-LCM), erythropoietin (EPO) preparations, and platelets. The culture system was used as a bioassay for megakaryocyte colony stimulating activity (Meg-CSA) in plasma samples of patients with perturbed megakaryocytopoiesis. Preparations of heparinized platelet-poor plasma yielded the most consistent results. Platelet-poor plasma of normal subjects will at best facilitate the occasional growth of small megakaryocyte colonies. Colony frequency and size are reproducibly enhanced in the presence of PHA-LCM as a source of exogenous Meg-CSA. Commercially available EPO preparations may vary in their content of activities that influence megakaryocyte colony formation. Addition of these preparations to cultures that contain plasma and PHA-LCM usually does not enhance colony formation. In contrast to platelet-poor plasma, platelet rich plasma and serum are less supportive of megakaryocyte colony growth. It is suggested that this loss of activity may be related to the release of inhibitors by activated platelets or alternatively caused by absorption of activities by platelets. Plasma samples from patients with megakaryocytopoietic dysfunction may contain components that promote colony formation without addition of PHA-LCM or EPO. This phenomenon is consistently observed for patients with severe aplastic anemia and bone marrow transplant recipients after completion of their ablative preparative regimen.

Thrombopoiesis and thrombokinetics--an approach to the evaluation of thrombocytopenia

Thrombocytopenia is a common clinical disorder with a diverse group of etiologies. Traditionally, the approach to identifying the mechanism of thrombocytopenia has been empirical, primarily due to a lack of clear understanding of normal thrombopoiesis and its control. Additionally, readily available clinical measurements that reflect patterns of altered thrombopoiesis are unavailable. Recent experimental and clinical observations permit us to approach this disorder from a kinetic point of view to classify thrombocytopenia by four mechanistic categories: peripheral destruction and consumption, hypoproliferative thrombocytopenia, ineffective thrombopoiesis, and distributional causes. The application of the measurement of mean platelet size, in conjunction with a bone marrow examination, allows the clinician to more readily identify the cause of a low platelet count in a less empirical manner.

Purification and partial characterization of a megakaryocyte colony-stimulating factor from human plasma

Human plasma obtained from patients with hypomegakaryocytic thrombocytopenia contains a factor that promotes megakaryocyte colony formation by normal human marrow cells. This megakaryocyte colony-stimulating factor was purified from such a plasma specimen. A four-step purification scheme which included ammonium sulfate precipitation, diethylaminoethyl-Sepharose chromatography, affinity chromatography on wheat germ lectin-Sepharose 6MB, and reverse-phase high performance liquid chromatography resulted in a recovery of 16.6% of the initial biological activity and an increase in specific activity by 3,489-fold. The purified protein produced a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified megakaryocyte colony-stimulating factor was capable of promoting megakaryocyte colony formation at a concentration of 7.6 X 10(-8) M. Megakaryocyte colony-stimulating factor was shown to be a glycoprotein and had an apparent 46,000 mol wt. Deglycosylation of megakaryocyte colony-stimulating factor by treatment with trifluoromethane-sulfonate resulted in the loss of its ability to promote megakaryocyte colony formation. Megakaryocyte colony-stimulating factor appears to be an important regulator of in vitro human megakaryocytopoiesis at the level of the colony-forming unit megakaryocyte and may be of importance physiologically.

The relation of megakaryocyte ploidy to platelet volume

To determine how alterations of megakaryocyte proliferation will affect platelet production, we measured mean platelet volume (MPV), platelet volume heterogeneity, platelet count, and mean megakaryocyte ploidy in 42 patients. In normal subjects, mean platelet volume and megakaryocyte ploidy were related inversely but nonlinearly to platelet count, whereas mean platelet volume and platelet volume heterogeneity were related directly. In patients with immune thrombocytopenic purpura (low platelet count, MPV above normal, and increased megakaryocyte ploidy), and in those with reactive thrombocytosis (high platelet count, low MPV and megakaryocyte ploidy), the relation of MPV to megakaryocyte ploidy, platelet volume heterogeneity, and platelet count resembled or extended the relations found in normal subjects. By contrast, in patients with aplastic anemia or megaloblastic anemia, or in patients who were undergoing chemotherapy for leukemia, heterogeneity was increased abnormally at any MPV, and both MPV and megakaryocyte ploidy were substantially lower, at any platelet volume, than in normals or the above other groups. The most common ploidy class was 8N in all patients, and the mean megakaryocyte ploidy correlated directly and linearly with mean platelet volume. The data show that bone marrow with megakaryocytes of higher ploidy produces platelets that are both larger and more heterogeneous.

Human megakaryocytic progenitors (CFU-M) assayed in methylcellulose: physical characteristics and requirements for growth

The basic culture requirements and several physical characteristics were defined for megakaryocytic colony-forming cells (CFU-M) from normal human marrow growing in methylcellulose. Ficoll-hypaque separated mononuclear cells from human marrow gave rise to megakaryocytic colonies in the presence of normal human plasma and phytohemagglutinin-stimulated leukocyte-conditioned medium (PHA-LCM). Their identity as megakaryocytic colonies was confirmed by immunofluorescence staining with a monoclonal antibody to human factor VIII antigen and by electron microscopy of individually harvested colonies. Demonstration of the single-cell origin of the colonies was provided by analysis of the glucose-6-phosphate dehydrogenase (G-6-PD) enzyme type of individually harvested colonies grown from a G-6-PD heterozygote. The colonies grew best in heparinized or citrated plasma as opposed to serum. Detailed studies suggested that platelet-release products were responsible for this difference. Tritiated thymidine suicide studies showed that the percentage of CFU-M in DNA synthesis was 23 +/- 8% (n = 10). The modal velocity sedimentation rate of CFU-M was 4.9 +/- 0.6 mm/hr (n = 4) while that of concurrently studied granulocyte/macrophage colony-forming cells (CFU-GM) was 5.7 +/- 0.5 mm/hr. Examination of the PHA-LCM dose-response characteristics suggested the presence in the conditioned medium of an inhibitor to megakaryocyte colony growth which was partially removed by chromatography of the medium on Sephadex G-100. The resulting conditioned medium increased the cloning efficiency for CFU-M compared with that with crude PHA-LCM (15.3 +/- 7.0 and 8.2 +/- 5.3/10(5) marrow cells, respectively).

Apparent heterogeneity of human megakaryocyte colony- and thrombopoiesis-stimulating factors: studies on urinary extracts from patients with aplastic anaemia and idiopathic thrombocytopenic purpura

Megakaryocyte colon-stimulating factor (MEG-CSF) in the urinary extracts from patients wit aplastic anaemia (AA) revealed two distinct peaks of activity on Sephadex G-200 gel filtration with apparent molecular weights of 155,000 and 76,000. Both fractions induced significant thrombocytosis in peripheral blood and megakaryocytosis in the spleen of rats. Heterogeneity of MEG-CSF was also found in the extracts from the urine of patients with idiopathic thrombocytopenic purpura. The higher molecular weight MEG-CSF was significantly reduced when the gel filtration was performed under the dissociating conditions. Ion-exchange chromatography indicated that the higher molecular weight MEG-CSF had a different charge from the lower molecular weight MEG-CSF. These results suggest that the apparent heterogeneity of MEG-CSF is due to interaction of MEG-CSF with other proteins in the urinary extracts.

The growth of large megakaryocyte colonies from human bone marrow

The growth of large, compact megakaryocyte colonies in cultures of human bone marrow is promoted by fresh human plasma and medium conditioned by phyto-hemagglutinin stimulated leukocytes (PHA-LCM). These colonies are typically composed of large cells with translucent cytoplasma, surrounded by a highly refractile border. In addition, they may also contain smaller cells of similar morphology. Independent of their size, all cells react positively with antibodies directed against human factor VIII antigen. The frequency of megakaryocyte colonies may vary for different individuals from 1-35 colonies per 10(5) mononuclear bone marrow cells. The observed linear relationships between the number of cultured cells and the frequency of colonies suggests a single cell origin. The described culture conditions also support the development of a larger megakaryocyte component within multilineage mixed colonies, so that it will now be feasible to investigate the mechanisms involved in directing pluripotent cells towards megakaryocytopoiesis.