Assay of an activity in the serum of patients with disorders of thrombopoiesis that stimulates formation of megakaryocytic colonies

Idiopathic thrombocytopenic purpura and dysmegakaryocytopoiesis

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder characterized with thrombocytopenia, primarily caused by platelet destruction. However, the studies of platelet kinetics show platelet turn over are normal or decreased, suggesting that reduced platelet production may lead to severity of ITP. We review recent research progress on abnormal cell events involved in megakaryocytopoiesis contributing to thrombocytopenia.

The Effect of Antiplatelet Autoantibodies on Megakaryocytopoiesis

Immune thrombocytopenic purpura (ITP) is a disorder manifested by isolated thrombocytopenia. In vivo infusion studies in the 1950s and 1960s provided evidence that the thrombocytopenia was due to autoantibody-induced platelet destruction. However, there is mounting evidence that platelet production in this disorder may also be suppressed by antibodies. Early morphologic studies showed megakaryocytic damage in ITP, and these results have been confirmed by ultrastructural studies. Autologous platelet turnover studies in the 1980s showed that most ITP patients have either normal or reduced platelet turnover rather than increased turnover, as would be expected if platelet destruction were the only pathogenetic mechanism. More recently, in vitro culture studies of both adult and pediatric ITP have shown that some ITP plasmas suppress both megakaryocytopoiesis and thrombopoiesis. In view of these findings, both platelet destruction and suppression of platelet production seem likely to be involved in the pathogenesis of ITP.

Suppression of in vitro megakaryocyte production by antiplatelet autoantibodies from adult patients with chronic ITP

Chronic immune thrombocytopenic purpura (ITP) is manifested by autoantibody-induced platelet destruction. Platelet turnover studies suggest that autoantibody may also affect platelet production. To evaluate this, we studied the effect of plasma from adult patients with chronic ITP on in vitro megakaryocyte production. CD34(+) cells, obtained from healthy donors, were cultured in medium containing PEG-rHuMGDF and 10% plasma from either ITP patients or healthy subjects. Cultures containing plasma from 12 of 18 ITP patients showed a significant decrease (26%-95%) in megakaryocyte production when compared with control cultures. Positive ITP plasmas not only reduced the total number of megakaryocytes produced during the culture period but also inhibited megakaryocyte maturation, resulting in fewer 4N, 8N, and 16N cells. The role of antibody in this suppression is supported by 2 factors: (1) immunoglobulin G (IgG) from ITP patients inhibited megakaryocyte production when compared with control IgG; and (2) adsorption of autoantibody, using immobilized antigen, resulted in significantly less inhibition of megakaryocyte production when compared with unadsorbed plasma. These results show that plasma autoantibody from some adult patients with ITP inhibits in vitro megakaryocyte production, suggesting that a similar effect may occur in vivo.

Immune thrombocytopenic purpura (ITP) plasma and purified ITP monoclonal autoantibodies inhibit megakaryocytopoiesis in vitro

To determine if megakaryocytes are targeted by immune thrombocytopenic purpura (ITP) autoantibodies, as are platelets, we have studied the effects of ITP plasma on in vitro megakaryocytopoiesis. Umbilical cord blood mononuclear cells were incubated in the presence of thrombopoietin and 10% plasma from either ITP patients (n = 53) or healthy donors. The yield of megakaryocytic cells, as determined by flow cytometry, was significantly reduced in the presence of ITP plasma containing antiplatelet glycoprotein Ib (GPIb) autoantibodies (P <.001) as compared with both the control and patient plasma with no detectable anti-GPIIb/IIIa or anti-GPIb autoantibodies. Platelet absorption of anti-GPIb autoantibodies in ITP plasmas resulted in double the megakaryocyte production of the same plasmas without absorption, whereas platelet absorption of control plasma had no effect on megakaryocyte yield. Furthermore, 2 human monoclonal autoantibodies isolated from ITP patients, 2E7, specific for human platelet glycoprotein IIb heavy chain, and 5E5, specific for a neoantigen on glycoprotein IIIa expressed on activated platelets, had significant inhibitory effects on in vitro megakaryocytopoiesis (P <.001). Taken together, these data indicate that autoantibodies against either platelet GPIb or platelet GPIIb/IIIa in ITP plasma not only are involved in platelet destruction, but may also contribute to the inhibition of platelet production.

Circulating thrombopoietin in clonal versus reactive thrombocytosis

INTRODUCTION

The aim of this study is to assess circulating thrombopoietin concentrations in patients with both clonal and reactive thrombocytosis (RT), which are two distinct categories of extreme platelet production circumstances. Investigation of the thrombopoietin levels in clonal versus reactive thrombocytosis may help us to understand the interactions of this key regulatory cytokine and the conditions in which abnormally increased platelet formation exist.

MATERIALS AND METHODS

Thrombopoietin levels were measured in patients with platelet counts greater than 500 x1 0(3) microl(-1). The study population consisted of 21 patients with RT (13 with iron deficiency anemia, and 8 with rheumatoid arthritis), 24 patients with clonal thrombocytosis (six with essential thrombocytosis, three with myelofibrosis, eight with chronic myelogenous leukemia, and seven with polycythemia vera (PV)) and 16 healthy subjects were used as controls.

RESULTS

The median plasma thrombopoietin concentration was 100.5 pg ml(-1) in patients with RT, 467 pg ml(-1) in patients with clonal thrombocytosis and 62.65 pg ml(-1) in the control group. The thrombopoietin concentration was found to be higher in the patients with primary thrombocytosis when compared to the control group (p=0.001), as well as in patients with RT (p=0.002). However, there was no statistically significant difference between the patients with RT and the control group (p=0.14). There was no correlation between thrombopoietin levels and the platelet counts in patients with clonal thrombocytosis, including essential thrombocythemia (ET). CONCLUSIION: Increased levels of thrombopoietin were found in patients with clonal thrombocytosis versus patients with RT and control subjects as well. Defective clearance of thrombopoietin by megakaryocytes and platelets due to a reduced number of thrombopoietin receptors may be the causative mechanism behind this. These results indicate that plasma thrombopoietin levels may be helpful in distinguishing between clonal and reactive thrombocytosis.

Colony-forming unit-megakaryocyte (CFR-meg) numbers and serum thrombopoietin concentrations in thrombocytopenic disorders: an inverse correlation in myelodysplastic syndromes

We studied both serum-free colony-forming unit-megakaryocyte (CFU-meg) numbers and serum thrombopoietin (TPO) levels in 14 patients with aplastic anemia (AA), 37 patients with myelodysplastic syndromes (MDS) and 23 patients with idiopathic thrombocytopenic purpura (ITP) to assess thrombopoiesis in these thrombocytopenic disorders. The mean CFU-meg numbers were lower in AA and MDS patients (10.7 +/- 11.4 and 42.3 +/- 58.5/10(5) BMLD cells) than in healthy controls (103.1 +/- 57.3/10(5) BMLD cells) (P < 0.0001 and P= 0.0053, respectively), although they were distributed variably in MDS. ITP patients showed higher CFU-meg numbers (223.2 +/- 143.5/10(5) BMLD cells) (P= 0.017). The mean TPO concentrations were higher in both AA (986.8 +/- 500.8 pg/ml) and MDS patients (838.2 +/- 639.1 pg/ml) than in healthy controls (80.7 +/- 38.8 pg/ml) (P < 0.0001), although they were distributed from high to low in MDS. ITP patients showed a slight elevation of TPO (123.1 +/- 55.3 pg/ml) P = 0.0106). The TPO levels was inversely correlated to both platelet counts and CFU-meg numbers (correlative coefficient (CC): -0.719 and -0.682, P < 0.0001) in AA, but not in ITP. In MDS, the inverse correlation to TPO was stronger in CFU-meg (CC: -0.678, P < 0.0001) than in platelet counts (CC: -0.538, P = 0.0014), suggesting that CFU-meg plays an important role in regulating TPO production in this heterogenous disorder. CFU-meg and TPO may provide useful information for understanding thrombopoiesis of MDS, especially for application of TPO.

Hematopoietic deficiencies in c-mpl and TPO knockout mice

Thrombopoietin (TPO) is the primary regulatory of megakaryocyte (Meg) and platelet production. Its receptor, c-mpl, is a member of the cytokine receptor superfamily. Major insight into the physiological role of this receptor/ligand pair came from the study of mice carrying disrupted alleles of these two genes. Both TPO and c-mpl knockout mice are viable, but have a 90% reduction in platelet counts. Their thrombocytopenia is caused by a reduction in progenitor cell numbers and a decrease in Meg ploidy. However, the Megs and platelets produced in the absence of TPO or c-mpl appear morphologically and functionally normal indicating that, in vivo, the main role of TPO is to control their numbers, rather than their maturation. In addition to its effect on the Meg lineage, TPO also affects hematopoietic stem cells as measured by a reduction of the repopulating capacity of bone marrow cells from c-mpl-deficient mice. Finally, analysis of these gene targeted mice provided substantial evidence to a model where the circulating TPO level is directly regulated by the platelet mass through binding to c-mpl receptors present at the platelet surface. This elegant feedback mechanism allows a tight regulation of the amount of TPO available to stimulate megakaryocytopoiesis.

Adenovector-Mediated Expression of Human Thrombopoietin cDNA in Immune-Compromised Mice: Insights into the Pathophysiology of Osteomyelofibrosis

Thrombopoietin (TPO) cDNA can be effectively delivered in vivo by adenovectors. Immune normal mice (BALB/c) and syngeneic mice with variable degrees of immune dysfunction nu, SCID, and NOD-SCID) were treated with an adenovirus vector expressing the human TPO cDNA (AdTPO). Platelet peaks were significantly higher in SCID and NOD-SCID mice compared with BALB/c and nu mice. Human plasma TPO concentration correlated with the platelet counts. SCID and NOD-SCID mice exhibited also granulocytosis and increased numbers of hemopoietic progenitors in bone marrow. Following platelet peak, BALB/c mice developed autoantibodies against murine TPO leading to thrombocytopenia and depletion of megakaryocytes and hemopoietic progenitors in bone marrow. AdTPO-treated SCID mice developed osteomyelofibrosis and extramedullary/extrasplenal hemopoiesis. In contrast, NOD-SCID mice with a similar magnitude of TPO overexpression did not show fibrotic changes in bone marrow. We conclude, first, that a chronic high level of TPO overexpression stimulates megakaryocytopoiesis and myelopoiesis leading to thrombocytosis and granulocytosis. Second, increased megakaryocytopoiesis is not sufficient for development of secondary osteomyelofibrosis. The functionally deficient monocytes and macrophages of NOD-SCID mice probably prevented fibrotic marrow changes. Third, immune deficiency enhances expression of adenovirally mediated transgenes, and fourth, xenogeneic transgene delivered by adenovector to a host with normal immune functions may induce loss of immune tolerance and autoimmune phenomenon.

Clinical aspects of aplastic anemia

Severe aplastic anemia is a disorder characterized by peripheral pancytopenia and marrow hypoplasia. Although its pathophysiology is understood poorly, the majority of patients appear to have some immunologic destruction or suppression of hematopoietic cells. The only curative therapy to date is allogeneic stem cell transplantation, although the success of palliative immunosuppressive therapies has improved over the last two decades. Making the best therapy choice is complex and often requires balancing very divergent toxicity profiles, both acute and long-term.

Regulation of the Serum Concentration of Thrombopoietin in Thrombocytopenic NF-E2 Knockout Mice

The mechanisms that regulate circulating levels of thrombopoietin (Tpo) are incompletely understood. According to one favored model, the rate of Tpo synthesis is constant, whereas the serum concentration of free Tpo is modulated through binding to c-Mpl receptor expressed on blood platelets. Additionally, a role for c-Mpl expressed on megakaryocytes is suggested, particularly by the observation that serum Tpo levels are not elevated in human immune thrombocytopenic purpura. Whereas direct binding of Tpo to platelets has been demonstrated in vitro and in vivo, the role of megakaryocytes in modulating serum Tpo levels has not been addressed experimentally. The profoundly thrombocytopenic mice lacking transcription factor p45 NF-E2 do not show the predicted increase in serum Tpo concentration. To evaluate the fate of the ligand in these animals, we injected 125I-Tpo intravenously into mutant and control mice. In contrast to normal littermates, NF-E2 knockout mice show negligible association of radioactivity with blood cellular components, consistent with an absence of platelets. There is no corresponding increase in plasma-associated radioactivity to suggest persistence in the circulation. However, a greater fraction of the radioligand is bound to hematopoietic tissues. In the bone marrow this is detected virtually exclusively in association with megakaryocytes, whereas in the spleen it is associated with megakaryocytes and small, abnormal, platelet-like particles or megakaryocyte fragments that are found within or in close contact with macrophages. These findings implicate the combination of megakaryocytes and the latter particles as a sink for circulating Tpo in NF-E2 knockout mice, and provide an explanation for the lack of elevated serum Tpo levels in this unique animal model of thrombocytopenia.

Megakaryocytes and platelets in myeloproliferative disorders

Increased megakaryocyte (MK) proliferation in bone marrow is a feature common to the three Ph-negative myeloproliferative disorders (MPDs), i.e. essential thrombocythaemia (ET), polycythaemia vera (PV), and myelofibrosis with splenic myeloid metaplasia (MMM), and to chronic myelocytic leukaemia (CML). Enlarged MKs with multilobulated nuclei and cell clustering in close proximity are the hallmark of all the Ph negative MPDs. Clonality of haematopoietic cells, based on X chromosome inactivation, can now be studied in a majority of female patients in all nucleated cell fractions as well as in platelets. Cytofluorometric studies have demonstrated a shift towards higher ploidy classes in PV and ET MKs which may be useful in discriminating between both primary and reactive thrombocytosis and CML patients which show a significant shift to lower MK ploidy values. The role of MK proliferation on the evolution of myelofibrosis common to MPDs has been firmly established. Implication of platelet-derived growth factor (PDGF) in myelofibrosis has already been demonstrated. More recently transforming growth factor beta (TGF beta) synthesized and secreted by MK has been implicated in fibroblasts stimulation. A significant increase in circulating colony-forming units of MKs (CFU-MK) has been repeatedly observed in MPDs as well as a spontaneous MK colony formation in a majority of ET patients. Hypersensitivity to thrombopoietin (TPO) in relation to a functional defect of the TPO-MPL pathway may play a major role in spontaneous MK growth. There is no currently available test of platelet functions able to predict the risk of occurrence of thrombotic or haemorrhagic complications in MPD patients. However, the role of platelet activation in the pathogenesis of ischaemic erythromelalgia has been established and a correlation between presenting haemorrhagic manifestations and platelet counts in excess of 1000 x 10(9)/l has been found.

Clonality assays and megakaryocyte culture techniques in essential thrombocythemia

The development of techniques permitting in vitro growth of human megakaryocytes progenitors and more recently identification of the proto oncogene c-mpl (Mpl-R) and its ligand (Mpl-L) have created new opportunities for studying pathophysiology of E.T. Plasma or serum of E.T. patients was unable to overestimulate MK colony formation by normal bone marrow cells. Significant increases in circulating CFU MK in E.T. patients have been repeatedly observed while in E.T. marrow, due to inappropriate sampling, colony number was not significantly different from normal. Spontaneous colony formation is observed in approximately 100% bone marrow and 85% blood from E.T. patients. Spontaneous colony formation persisted in plasma clot assay without added plasma or serum and in serum free agar cultures but only at a slightly lower rate than in plasma clot. Spontaneous colony formation in culture condition without plasma and serum were never observed with normal bone marrow and blood. Spontaneous MK growth was observed in a higher proportion of E.T. patients than erythroid colony formation but both phenomenon can occur in about 50% of the patients. CFU MK colony formation disappeared in serum free cultures using highly purified CD 34 cells. MK development is not completely independent of regular control. An hypersensitivity of E.T. MK progenitors to growth factors known to stimulate normal hematopoiesis (IL3.IL6, GM CSF, has been shown as well as a decreased sensitivity to negative regulators (TGF beta), has been suggested. The number of spontaneous MK colonies was not significantly decreased by added anti IL3, IL6 or anti GM CSF, antibodies in culture medium. Pre incubation of blood non adherent mononuclear cells of E.T. patients with antisense oligonucleotides to c-mpl significantly decreased the cloning efficiency of spontaneous megakaryocyte growth as compared to the introduction of scrambled oligomers. Finally m RNA expression of the Mpl-L (TPO) was not formed in MK spontaneously grown in serum free liquid cultures after 12 days. These results suggest that human c-mpl proto oncogene may be implicated in the pathway of spontaneous megakaryocytopoiesis in MPD but an absence of autocrine-stimulation by TPO of spontaneous growth in MPD. Analysis of peripheral blood cell clonality was performed in 55 E.T. patients using either the DNA methylation pattern of the androgen receptor (AR) gene or mRNA transcripts of G6PD or IDS genes. 51 out of 55 patients were informative. Non random X inactivation was found on unfractioned blood in 73% as compared with 23% in normal females (skewed Lyonisation). In 12 patients monoclonality of hematopoiesis was definitely confirmed by recording polyclonality of the mononuclear fraction or of T lymphocytes. In 4 patients monoclonal hematopoiesis was limited to platelets, 7 patients remained polyclonal in whole blood and all cellular fractions studied. MK colony formation (provided that the serum free agar culture system is clearly standardised) and clonality studies on whole blood or granulocyte, T lymphocyte and platelet fractions may be proposed as positive criteria for diagnosis of E.T.

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.

Spontaneous cytokine overproduction by peripheral blood mononuclear cells from patients with myelodysplastic syndromes and aplastic anemia

We studied spontaneous cytokine production by peripheral blood mononuclear cells (PBMC) obtained from 14 patients with aplastic anemia (AA) and 28 various myelodysplastic syndromes (MDS). The levels of interleukin-6, interleukin-1 beta, and tumor necrosis factor-alpha in cultured PBMC were measured by ELISA. The average levels of these cytokines were higher in AA or in refractory anemia (RA) than in RA with excess of blasts (RAEB) or in RAEB in transformation (RAEB-T). Marked cytokine overproduction was observed in RA as well as in AA. High cytokine levels were observed in hypocellularity and low blast cell counts in the bone marrow. These results may suggest that the increase of cytokines may be a reactive response in hypocellular bone marrow.

Elevated plasma thrombopoietic activity in patients with metastatic cancer-related thrombocytosis

BACKGROUND AND PURPOSE

High platelet counts are occasionally seen in patients suffering from progressive malignant disorders. While granulocyte colony-stimulating factor (G-CSF) has been implicated in paraneoplastic leukemoid reactions, the stimulus for thrombocytosis is unknown. Our purpose in this study was to determine if plasma from cancer patients with thrombocytosis contains a factor or factors with thrombopoietic activity.

METHODS

We tested the effects of plasma obtained from 5 individuals with advanced tumors and high platelet counts and from 4 patients with advanced cancer and normal platelet counts on megakaryocytic differentiation of two megakaryoblastic cell lines (Dami and HEL). Differentiation was evaluated by assessing the expression of the platelet-specific cell-surface antigens CD41 (HUPL-mI) and glycoprotein IIb-IIIa using an immunocytochemical staining score. In addition, plasma samples from 7 of the 9 patients and from 5 additional cancer patients with thrombocytosis were assayed for the levels of interleukin (IL)-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, and IL-1 beta protein using an enzyme-linked immunosorbent assay (ELISA).

RESULTS

Expression of platelet-specific cell-surface antigen was increased in HEL cells after exposure to plasma from all 5 of the cancer patients with thrombocytosis, and in Dami cells after exposure to plasma from 4 of the 5. Similar, but less significant, results were found when these cells were incubated with control combinations of recombinant GM-CSF plus IL-6 or of IL-3 plus IL-6. Platelet-specific cell-surface-antigen expression was not increased in HEL or Dami cells after exposure to the plasma from the 4 cancer patients with normal platelet counts or to normal control plasma. ELISA revealed elevated levels of IL-6 in the plasma from 4 patients with thrombocytosis (38, 40, 63, and 99 pg/mL). In addition, GM-CSF concentration was high in 3 of these 4 patients (33, 47, and 127 pg/mL), and the G-CSF level was elevated in 1 (543 pg/mL). IL-1 beta and IL-3 levels were undetectable.

CONCLUSIONS

Our data suggest that the thrombocytosis observed in individuals with advanced malignant disease is mediated by a humoral mechanism. Levels of IL-6, GM-CSF, and G-CSF are elevated in some of these patients, but the plasma concentrations are generally lower than those required for in vitro induction of megakaryocytic differentiation. Plasma from patients with paraneoplastic thrombocytosis may therefore contain thrombopoietins that have not yet been identified, and which might have clinical usefulness.

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.

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.

Megakaryocyte potentiating activity of IL-1, IL-6 and GM-CSF as evaluated by their action on in vitro human megakaryocytic colonies

We examined whether recombinant cytokines enhance the in vitro platelet production of interleukin-3 (IL-3)-induced human megakaryocytic colonies (Meg-colony). We classified Meg-colonies into four categories based on platelet production during in situ observation on day 14: type 0, absence of cytoplasmic processes in a colony; type 1, one to three processes in at least one megakaryocyte in a colony; type 2, four to eight processes; type 3, more than nine processes or division of cytoplasm. Type 3 colonies were considered to be platelet-producing. In control cultures, type 1 Meg-colonies were dominant, followed by type 2, type 3 and type 0. Of the cytokines added at the initiation of culture, interleukin-1 alpha (IL-1 alpha), interleukin-6 (IL-6), and granulocyte/macrophage colony stimulating factor (GM-CSF) significantly increased the number of colonies. Furthermore, these three cytokines significantly elevated the proportion of type 3 colonies. Interleukin-4 (IL-4), granulocyte-CSF, macrophage-CSF and erythropoietin did not affect the colony count or distribution of colony type. IL-1 alpha, IL-6 and GM-CSF also significantly elevated the proportion of type 3 colonies, even when added to the culture on days 8 or 11. These results indicate that IL-1 alpha, IL-6 and GM-CSF promote platelet production of in vitro Meg-colonies.

Studies of in vitro megakaryocytopoiesis in adult immune thrombocytopenic purpura (ITP)

In vitro megakaryocytopoiesis was studied in 8 patients with chronic immune thrombocytopenia (ITP). A significant increase of megakaryocyte (MK) colony formation was observed in 5/5 patients studied. Furthermore, the serum of these 8 patients was able to enhance MK colony formation by normal marrow cells. This effect was neither due to a decrease of inhibitors of megakaryocytopoiesis such as betathromboglobulin (beta TG) nor to the IgG fraction of patients' serum. In addition, the level of interleukin 6, which is above all a stimulus for MK maturation, was found within the normal range in 8/8 patients tested. These data suggest that in chronic ITP there is an increase of MK progenitor cell number which may be due to an increased level of MK colony-stimulating activity.

Megakaryocyte progenitors in immune thrombocytopenic purpura (ITP)

Megakaryocyte colony formation was increased in 13 patients with ITP. Out of splenectomized ITP patients Colony Forming Unit-Megakaryocyte (CFU-Mk) number normalized in four but was lower than normal in the remaining two. The proportion of immature megakaryocytes was higher in ITP than in normals or in ITP patients after splenectomy. In conclusion, in ITP accelerated platelet destruction leads to stimulation of megakaryocytopoiesis, but some features of its ineffectiveness can be observed.

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.

Humoral regulation of megakaryocytopoiesis

If compared to erythroid and granulomacrophage lineages, the knowledge of the regulation of megakaryocytopoiesis has progressed slowly, and only the recent advent of specific clonogenic methods has permitted studies aimed at investigating this aspect of hematopoiesis. The analysis of Mk differentiation and platelet production is still difficult, because methods such as the 75SeM or 35S incorporation are time consuming and their sensitivity is relatively low. A number of laboratories have been able to purify, partially or to homogeneity, fractions stimulating the proliferation and differentiation of megakaryocytes. The biochemical identity between IL-3 and the active fractions found in the C.M. of some cell lines stands for a role of this hemopoietin in the regulation of megakaryocytopoiesis. However, the function of Epo and, above all, of GM-CSF cannot be ruled out, on the basis of experimental works, although only in some clinical trials GM-CSF seems to have been able to modify the platelet number. Hopefully, data on the therapeutic use of rhIL-3, and the sequentiation and identification of a molecule capable of action on the maturative compartment will shed new light on the regulation of megakaryocytopoiesis and the possibility to correct its disorders.

Molecular regulation of human megakaryocyte development

Extracellular regulators of human megakaryocyte development are becoming better defined. How these regulators function at the subcellular and, in particular, the molecular levels remains almost completely unknown. The recent development of molecular micromethodologies such as in situ hybridization, the polymerase chain reaction, and the use of antisense oligodeoxynucleotides now make such studies possible in normal cells. We therefore examined the effect of several recombinant human hematopoietic growth factors and the maturation agonist phorbol myristate acetate on the expression of selected growth-regulated and maturation/function-related genes. We also examined the role of the c-myb proto-oncogene in regulating megakaryocyte proliferative activity and ploidy development. Our results demonstrate that growth factors have complex time and concentration effects on gene expression in morphologically recognizable human megakaryocytes. They also suggest that a more complete understanding of normal megakaryocyte development at the molecular level will soon be possible.

Thrombocytopenia after bone marrow transplantation for leukaemia: changes in megakaryocyte growth and growth-promoting activity

Megakaryocyte growth-promoting activity (MK-GPA) was scored on a scale of 0-3 in the serum of 23 patients up to 120 d following bone marrow transplantation (BMT) for leukaemia. Nine of 19 allografts and two of four autografts had thrombocytopenia requiring platelet transfusion more than 30 d after BMT. There was a close correlation between MK-GPA and platelet count. MK-GPA reached a maximum before day 30 after BMT but remained elevated in patients with persisting thrombocytopenia secondary to poor engraftment, graft-versus-host disease (GVHD) or relapse. Recent platelet transfusion did not suppress serum MK-GPA. Two of four patients undergoing autologous BMT for acute myeloid leukaemia (AML) showed delayed platelet recovery and persistence of MK-GPA in the serum. Seven further AML remission marrows were tested for megakaryocyte production before or after autologous BMT, using pooled sera with known MK-GPA activity. Megakaryocyte generation was reduced before BMT and absent in post transplant samples. This failure of MK production was not corrected by T-cell depletion or by the presence of adherent cells from normal marrow. We conclude that thrombocytopenia after BMT is associated with an appropriate increase in MK-GPA levels in response to a reduction in the megakaryocyte pool rather than the platelet pool, and that persisting thrombocytopenia after autologous BMT is due to decreased numbers of available megakaryocyte precursors.

Anti-thoracic duct lymphocyte globulin stimulates human megakaryocytopoiesis in vitro

Anti-thoracic duct lymphocyte globulin (ALG) therapy is effective in patients with aplastic anemia. We examined the effect of ALG on human megakaryocyte progenitor cells (colony-forming unit-megakaryocyte, CFU-Meg) in vitro. Normal human bone marrow mononuclear cells (MNC) were cultured in plasma clots with varying concentrations of ALG or non-immunized horse IgG. After 12 days of culture, significant megakaryocyte colony formation was observed in cultures containing ALG but not in cultures containing non-immunized horse IgG. The peak stimulatory effect seemed to occur with 10-25 micrograms/ml of ALG. When marrow MNC, depleted of adherent and T cells, were cultured in plasma clots with ALG, its stimulatory effect on megakaryocytopoiesis decreased markedly. Finally, it was demonstrated that ALG stimulated marrow MNC to produce a factor stimulatory for CFU-Meg. The in vitro megakaryocytopoietic stimulatory effect of ALG may be related to its clinical efficacy in some patients with aplastic anemia.

Megakaryocyte precursors (pro- and megakaryoblasts) in bone marrow tissue from patients with reactive thrombocytosis, polycythemia vera and primary (essential) thrombocythemia. An immunomorphometric study

To determine the number of megakaryocyte precursors (pro- and megakaryoblasts), an immunomorphometric study was performed on paraffin-embedded trephine biopsies of the bone marrow using a monoclonal antibody against platelet glycoprotein IIIa. Eighteen control specimens from patients with no evidence of any hematological disorder and a normal platelet count were selected and assessed together with the same number of specimens from patients with reactive thrombocytosis, polycythemia vera rubra (P. vera) or primary (essential) thrombocythemia (PTH). A strikingly proportionate increase in early megakaryocytes occurred in all patients enrolled in this study, compared with the controls. Moreover, there were no significant correlations between counts for precursors or total megakaryocytes per square millimeter of bone marrow with the corresponding values for platelets. This indicates that despite an orderly increase in immature forms in the bone marrow, the number of platelets circulating in the blood is influenced by other additional factors, such as the expanded platelet pool in the enlarged spleen. The non-disproportionate expansion of megakaryocyte precursors extends previous findings on progenitor cells of this lineage in vitro, particularly in PTH. Histological evaluation of the bone marrow of patients with P. vera and PTH indicated that megakaryopoiesis proceeded to the production of appropriate mature forms with no obvious excess of very small or blastic elements.

Hematological complications of alcoholism

Numerous clinical observations support the notion that ethanol has multiple pathologic effects on hematopoietic tissue. The effects of alcohol on blood are diverse. The long-term ingestion of large quantities of ethanol has been shown to alter a substantial number of physiologic and biochemical variables. Abnormalities involving leukocytes, platelets, and erythrocytes may occur singly or in various combinations. Due to the frequent concomitant presence of alcohol-related hepatic disease, nutritional deficiencies, infection, and other chronic diseases, it is often difficult to distinguish the specific hematologic toxicities of alcohol ingestion from the hematologic toxicities of associated morbid conditions. Depressed hematopoietic cell formation (Table 2), increased destruction, and alterations in morphology and function of hematopoietic cells have been described.

The regulation of megakaryocyte and platelet production

Thrombopoietin, a hormone that regulates blood platelet production, is now recognized to be an important in vivo hematopoietic stimulator. In this concise review, the background on thrombopoietin, development of assays, and identification of sources of the hormone are summarized, along with brief descriptions of other controlling factors, sites of thrombopoietin production, results of producing antibodies against the factor, sites of action of thrombopoietin both in vitro and in vivo, and its effect on blood platelet production. Suitable assays and stable sources of thrombopoietin have now been identified and their development will permit production of recombinant material. Once the gene is cloned, it is expected that recombinant thrombopoietin will be invaluable for treating patients with several platelet production problems.

Megakaryocytopoiesis in spontaneously hypertensive rats (SHR)

Bone marrow megakaryocytes and their progenitors were studied in SHR in order to obtain more information about megakaryocytopoiesis in hypertension since it is known that various anomalies of platelet function occur in hypertension. Megakaryocytopoiesis under steady state conditions and following stimulated erythropoiesis and thrombocytopenia was not found to be significantly different in SHR from that in normotensive Wistar controls.

Serum from patients with various thrombopoietic disorders alters terminal cytoplasmic maturation of human megakaryocytes in vitro

Human bone marrow was depleted of progenitors (CFU-MK), but enriched for recognizable megakaryocytes (MK), and placed in cultures with serum from either normal donors (NABS) or patients with primary (PTS) or secondary (STS) thrombocytosis, autoimmune thrombocytopenia (ATS) or aplastic anemia (AAS). Mean MK diameters shifted during the 3-4 days of incubation. Endomitotic figure were visible and mean ploidy increased slightly during cytoplasmic maturation, where decreases in immature cells (stages 1 and 2) were accompanied by increases in the mature MK (stages 3 and 4). Cytoplasmic maturation was faster in AAS, ATS and STS than PTS or NABS; mean size and ploidy were similar in all cultures. Recognizable MK were not forced to undergo additional endoreduplication in response to stimulation. Only AAS augmented MK colony formation, which indicated that at least two humoral factors can regulate megakaryocytopoiesis at separate levels, the progenitors and morphologically recognizable MK.

Stimulation of CFU-Mk colony growth by normal plasma and plasma from myelodysplastic patients

The ability of plasma from myelodysplastic patients to support the clonal growth of normal megakaryocyte progenitors (CFU-Mk) was compared with that of plasma from normal subjects. The resultant megakaryocyte colonies were expressed as a plasma factor index megakaryocyte (PFI-Mk). All cultures included PHA-LCM and medium conditioned by the human bladder carcinoma cell line 5637, and some of them had EPO. PFI-Mk (MDS) was significantly lower than PFI-Mk (normal), both with and without EPO. A positive correlation was found between megakaryocyte and platelet count in normal subjects, but was not present in MDS patients. There was no correlation between platelet count and PFI-Mk in neither group. In MDS there was a negative correlation between megakaryocyte number and PFI-Mk, both with and without EPO. Although, the mean megakaryocyte number in MDS and in normal bone marrow was similar, the proportion of immature megakaryocytes was much higher in MDS. Previous work indicates an abnormal clonal origin of megakaryocytes in MDS. The present study suggests that abnormal plasma factors affects megakaryocytopoiesis in this condition.

Growth factors in haemopoiesis

Haemopoietic growth factors have for over two decades allowed experimentalists to grow haemopoietic bone marrow cells in vitro. With refinements in technique and the discovery of novel growth factors, all of the known haemopoietic lineages can now be grown in vitro. This has allowed a much greater understanding of the complex process of haemopoiesis from the haemopoietic stem cell to the mature, functioning end-cell. The in vivo action of these growth factors has been harder to investigate. Although recombinant technology has afforded us the much greater quantities necessary for in vivo work, problems remain with administration because of effects on other tissues. Interpretation of results is difficult because of the complex inter-relationships which exist between factors. Some of these have been defined in vitro and it appears likely that they also operate in vivo. Erythropoietin is a physiological regulator of erythropoiesis. It has been detected in vivo with levels responding appropriately to stress (i.e. elevated in anaemia) and, when administered in pharmacological doses, has been shown to correct anaemia. Granulocyte/macrophage colony-stimulating factor (GM-CSF) has been detected in vivo and may influence the production and function of granulocytes and macrophages, although how it is regulated is unknown. Granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor are ore lineage-specific. Interleukin 3 (IL-3), although it has not been detected in vivo, may act on a primitive marrow precursor by expanding the population and making these cells more susceptible to other growth factors, such as GM-CSF. Interleukin 1 (IL-1) has been detected in vivo, does not appear to have any isolated action on bone marrow (except possibly radioprotection) but probably acts synergistically with other growth factors, such as G-CSF. Interleukins 2, 4, 5 and 6 have not been detected in vivo. All have effects on B-cells. In addition IL-2 is an essential factor for the in vitro growth of T-cells and may have antitumour effects in vivo. IL-5 is an eosinophil growth factor in vitro. Megakaryocytopoiesis is also affected by humoral factors. Factors, alone or in combination, may be useful to restore functional granulopoiesis when used therapeutically. Some can be used as anticancer agents, although there may be a risk of induction of haematological malignancy. Increased understanding of their physiological roles will allow a more rational use.

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.

Human platelet alpha granules contain a nonspecific inhibitor of megakaryocyte colony formation: its relationship to type beta transforming growth factor (TGF-beta)

Whole blood serum (WBS) and platelet-poor plasma-derived serum (PDS) from the same normal subject were compared for their abilities to support human megakaryocyte (MK) colony formation. In all cases, PDS promoted the growth of a higher number (20-50%) of MK colonies than did WBS. Increasing amounts of WBS decreased the number of colonies, whereas increasing concentration of PDS had no marked effects. Crude platelet extracts or platelet secretory products from thrombin-activated platelets also elicited an inhibition of MK colony formation in a dose-dependent manner. A complete inhibition was found for a dose equivalent to 1.10(9) platelets/ml and a 50% inhibition in a range of 1.10(7)-1.10(8) platelets/ml. These platelet products were also inhibitory for erythroid progenitor growth. Platelets from two patients with gray platelet syndrome elicited only a minor inhibition of MK growth, suggesting that the platelet alpha granule is the origin of this inhibition. When platelet extracts were acid-treated, the biological activity of the inhibitor on CFU-MK and CFU-E growth was 20-50-fold higher. In addition, a potent stimulatory activity on the growth of day 7 CFU-GM was observed. The enhancement of biological activities by acid treatment suggests that type beta transforming growth factor (TGF-beta) could be involved in this platelet inhibitory activity. The homogeneous native TGF-beta (from 1 pg to 1 ng/ml) produced the same effects previously induced by platelet products. It totally inhibited CFU-MK growth (at a 500 pg/ml), it inhibited CFU-E growth, and it stimulated growth of day 7 CFU-GM in the presence of a colony-stimulating factor. The inhibition of CFU-MK growth was also observed on purified progenitors. In conclusion, these results suggest that TGF-beta may be implicated in negative autocrine regulation of megakaryopoiesis. However, since this molecule has ubiquitous biological activities, its physiologic relevance as a normal regulator of megakaryopoiesis requires further investigation.

Significance of polyploidy in megakaryocytes and other cells in health and tumor disease

Polyploidy--the doubling of chromosome sets of cells caused by a stop of mitosis at different levels of the mitotic cycle--is a phenomenon widely observed in plants, protozoa, metazoa, and animals. In man obligate polyploid tissues are found in liver parenchyma, heart muscle cells, and bone marrow megakaryocytes. Polyploidy occurs mostly in stable and highly differentiated cells and tissues. Besides age, stimulation of proliferation and increased metabolic function lead to polyploidization in these organs. Aneuploidy, however, is exclusively found in tumor cells. Megakaryocyte differentiation and polyploidy are controlled by thrombopoietin-like activities, of which the loci of production are still unknown. Megakaryocytes are unique among polyploid mammal cells. On the precursor level they maintain their proliferative activity independently of the mammal's age. Once having entered the incomplete mitotic cycle they stop cytokinesis and develop into highly polyploid cells. Polyploidization of megakaryocytes is the basic requirement for establishing highly effective hemostasis in mammals, which exhibit blood circulation based on high blood pressures. Every polyploidization results in increased production of membrane materials with which the platelet becomes endowed. By shedding cytoplasmic fragments approximately 3000 platelets are set free from a 32c megakaryocyte, compared with only 16 nucleated thrombocytes by mitotic division. There is further evidence that the heterogeneity of platelets mostly depends on the different polyploidy classes of the megakaryocytes from which they are derived. Changes in the polyploidy pattern of megakaryocytes could therefore have consequences for hemostatic disorders in several human diseases, particularly in malignancy.

Megakaryocyte colony growth-supporting activities in human plasma: modification by platelets and platelet membranes

Human megakaryocyte colonies are grown in methylcellulose with platelet-poor plasma and medium conditioned by phytohemagglutinin-stimulated leukocytes (PHA-LCM) as a source of megakaryocyte colony stimulating factor (MEG-CSF). The megakaryocyte colony growth-supporting activity in human plasma can be absorbed by intact platelets or degranulated platelet membranes. It was possible to recover the activity by solubilizing platelet membranes with cholic acid. Filtration of the solubilized platelet membrane preparations through a Sephadex G-100 column yielded at least two activity peaks. The molecular weight of these two activities differs from that of the growth-promoting activity in PHA-LCM.