Purification of cytochrome b5 from pig testis microsomes by isoelectric focusing in an immobiline pH gradient

To evaluate the diagnostic value of thrombopoietin (TPO, c-mpl ligand) measurements, and clarify the regulatory mechanisms of TPO in normal and in thrombocytopenic conditions, the plasma TPO concentration was determined in normal individuals (n = 20), umbilical cord blood (n = 40), chronic idiopathic thrombocytopenic purpura (ITP; n = 16), in severe aplastic anaemia (SAA; n = 3), chemotherapy-induced bone marrow hypoplasia (n = 10), myelodysplastic syndrome (MDS; n = 11), and sequentially during peripheral blood progenitor cell transplantation (n = 7). A commercially available ELISA and EDTA-plasma samples were used for the analysis. The plasma TPO concentration in the normals and umbilical cord blood were 52 +/- 12 pg/ml and 66 +/- 12 pg/ml, respectively. The corresponding values in patients with SAA and chemotherapy-induced bone marrow hypoplasia were 1514 +/- 336 pg/ml and 1950 +/- 1684 pg/ml, respectively, and the TPO concentration, measured sequentially after myeloablative chemotherapy and peripheral blood progenitor cell transplantation, was inversely related to the platelet count. In contrast, the plasma TPO recorded in patients with ITP (64 +/- 20 pg/ml) and MDS (68 +/- 23 pg/ml) were only slightly higher than normal levels. In conclusion, TPO levels were significantly elevated in patients in which bone marrow megakaryocytes and platelets in circulation were markedly reduced, whereas TPO levels were normal in ITP patients, and only slightly increased in the MDS patients. These latter patients displayed a preserved number of megakaryocytes in bone marrow biopsies. Our data support the suggestion that megakaryocyte mass affects the plasma TPO concentration. In thrombocytopenic patients a substantially increased plasma TPO implies deficient megakaryocyte numbers. However, TPO measurements do not distinguish between ITP and thrombocytopenia due to dysmegakaryopoiesis, as seen in MDS patients.     

Ineffective platelet production in thrombocytopenic human immunodeficiency virus-infected patients

Thrombocytopenia has been characterized in six patients infected with human immunodeficiency virus (HIV) with respect to the delivery of viable platelets into the peripheral circulation (peripheral platelet mass turnover), marrow megakaryocyte mass (product of megakaryocyte number and volume), megakaryocyte progenitor cells, circulating levels of endogenous thrombopoietin (TPO) and platelet TPO receptor number, and serum antiplatelet glycoprotein (GP) IIIa49-66 antibody (GPIIIa49-66Ab), an antibody associated with thrombocytopenia in HIV-infected patients. Peripheral platelet counts in these patients averaged 46 +/- 43 x 10(3)/microL (P = . 0001 compared to normal controls of 250 +/- 40x 10(3)/microL), and the mean platelet volume (MPV) was 10.5 +/- 2.0 fL (P > 0.3 compared with normal control of 9.5 +/- 1.7 fL). The mean life span of autologous 111In-platelets was 87 +/- 39 hours (P = .0001 compared with 232 +/- 38 hours in 20 normal controls), and immediate mean recovery of 111In-platelets injected into the systemic circulation was 33% +/- 16% (P = .0001 compared with 65% +/- 5% in 20 normal controls). The resultant mean peripheral platelet mass turnover was 3.8 +/- 1.5 x 10(5) fL/microL/d versus 3.8 +/- 0.4 x 10(5) fL/microL/d in 20 normal controls (P > .5). The mean endogenous TPO level was 596 +/- 471 pg/mL (P = .0001 compared with 95 +/- 6 pg/mL in 98 normal control subjects), and mean platelet TPO receptor number was 461 +/- 259 receptors/platelet (P = .05 compared with 207 +/- 99 receptors/platelet in nine normal controls). Antiplatelet GPIIIa49-66Ab levels in sera were uniformly increased in HIV thrombocytopenic patients (P < .001). In this cohort of thrombocytopenic HIV patients, marrow megakaryocyte number was increased to 30 +/- 15 x 10(6)/kg (P = .02 compared with 11 +/- 2.1 x 10(6)/kg in 20 normal controls), and marrow megakaryocyte volume was 32 +/- 0.9 x 10(3) fL (P = .05 compared with 28 +/- 4.5 x 10(3) fL in normal controls). Marrow megakaryocyte mass was expanded to 93 +/- 47 x 10(10) fL/kg (P = .007 compared with normal control of 31 +/- 5.3 x 10(10) fL/kg). Marrow megakaryocyte progenitor cells averaged 3.3 (range, 0.4 to 7.3) CFU-Meg/1,000 CD34(+) cells compared with 27 (range, 0.1 to 84) CFU-Meg/1,000 CD34(+) cells in seven normal subjects (P = .02). Thus, thrombocytopenia in these HIV patients was caused by a combination of shortening of platelet life span by two thirds and doubling of splenic platelet sequestration, coupled with ineffective delivery of viable platelets to the peripheral blood, despite a threefold TPO-driven expansion in marrow megakaryocyte mass. We postulate that this disparity between circulating platelet product and marrow platelet substrate results from direct impairment in platelet formation by HIV-infected marrow megakaryocytes.     

The diagnostic value of thrombopoietin level measurements in thrombocytopenia

It has been reported that blood trombopoietin (TPO) levels can discriminate between thrombocytopenia due to increased platelet destruction and decreased platelet production. With our TPO ELISA and a glycocalicin ELISA we analysed a large group of patients in detail and could confirm and amplify the above notion in detail. TPO levels were determined in plasma from 178 clinically and serologically well-defined thrombocytopenic patients: 72 patients with idiopathic autoimmune thrombocytopenia (AITP), 29 patients with secondary AITP, 5 patients with amegakaryocytic thrombocytopenia and 72 patients who suffered from various diseases (46 in whom megakaryocyte deficiency was not and 26 in whom it was expected). In addition, we measured the level of glycocalicin as a marker of total body mass of platelets. In all patients with primary AITP and secondary AITP, TPO levels were within the normal range or in some (n = 7) cases only slightly increased. The level of glycocalicin was not significantly different from that of the controls (n = 95). The patients with amegakaryocytic thrombocytopenia had strongly elevated TPO levels and significantly decreased glycocalicin levels. Similarly, among the 72 thrombocytopenic patients with various disorders, elevated TPO levels were only found in patients in whom platelet production was depressed. The mean level of glycocalicin in these patients was decreased compared to that in controls and patients with AITP, but was not as low as in patients with amegakaryocytic thrombocytopenia. In conclusion, all patients with depressed platelet production had elevated levels of circulating TPO, whereas the TPO levels in patients with an immune-mediated thrombocytopenia were mostly within the normal range. Therefore, measurement of plasma TPO levels provides valuable diagnostic information for the analysis of thrombocytopenia in general. Moreover, treatment with TPO may be an option in AITP.     

Nursing research--taking an active interest

Transient myeloproliferative disorder (TMD) in neonates with Down syndrome is characterized by increased megakaryoblastic cells in the peripheral blood. Despite their spontaneous regression in weeks, prognosis is not always favorable because of fatal hepatic fibrosis. In this study, blood thrombopoietin (TPO) levels were measured by ELISA in six TMD patients and the expression of c-Mpl, a ligand for TPO, was examined on the blast cells from four patients by flow cytometer. At the onset, TPO level was undetectable in one patient and significantly lower in five patients than six neonatal controls (mean 0.52 fmol/ml, range 0.30-0.93 vs. 3.70, 1.38-8.33, P < 0.001), although platelet counts were similar (mean 321 x 10(9)/l, range 42-1,040 vs. 253 x 10(9)/l, 124-381). Two patients died of hepatic failure. TPO levels were measured in five patients after regression of the blast cells. With regression of blast cells, TPO levels were remarkably increased in four survived patients. In one patient with hepatic failure, TPO level was poorly elevated and relatively lower compared to the others. TPO levels were inversely correlated with blast numbers (r = -0.85, P < 0.001), but not with platelet counts (r = 0.426). Blast cells from four patients were all positive for c-Mpl. Our findings suggest that megakaryocyte mass is a major regulator of TPO levels and hepatic failure may affect the TPO level because liver is a major source of TPO production.     

Blunted thrombopoietin response to interferon alfa-induced thrombocytopenia during treatment for hepatitis C

Thrombocytopenia is common in advanced-stage liver disease and is partly caused by inadequate thrombopoietin (TPO) production in the failing liver. Treatment of chronic hepatitis C with interferon alfa (IFN-) often induces thrombocytopenia, sometimes even leading to discontinuation of treatment. TPO regulation in response to IFN--induced thrombocytopenia was studied in patients with chronic hepatitis C with and without cirrhosis (Child A). An in vitro culture system with HepG2 cells was used to demonstrate any direct effects of IFN- on TPO mRNA expression, TPO synthesis, or TPO secretion from liver cells. Thrombocyte count was lower (U test: P < .05) in patients with hepatitis C cirrhosis compared with patients with chronic hepatitis C without cirrhosis before IFN therapy, and decreased in both patient groups (Wilcoxon matched-pairs test: P < . 05) on IFN therapy, the median decrease in both groups being comparable (noncirrhotic patients, 35%; cirrhotic patients, 32%; U test: P = .57). TPO levels rose in noncirrhotic patients (Wilcoxon matched-pairs test: P < .05), but not in patients with cirrhosis (noncirrhotic patients' median increase: 43% vs. cirrhotic patients' median decrease: 5%; U test: P < .001). Even in patients without cirrhosis, the increase in TPO levels was relatively small for the decrease in platelet count. No effect of IFN- could be demonstrated on TPO mRNA expression in vitro, but TPO secretion from liver cells was significantly reduced. Lower platelet counts but similar TPO levels in patients with chronic hepatitis C and cirrhosis compared with noncirrhotic patients and a moderate increase in TPO levels in noncirrhotic patients with a missing increase in cirrhotic patients during IFN--induced thrombocytopenia provide further evidence for an impairment of TPO production in patients with cirrhosis and during IFN therapy. Recombinant human TPO could be of value in patients developing severe thrombocytopenia under IFN- therapy.     

Purification of recombinant human rhinovirus 14 3C protease expressed in Escherichia coli

A physiologically relevant thrombopoietin (TPO) must be a humoral regulator with lineage specificity for megakaryocytes and their precursors. It should be capable of stimulating platelet production in normal animals, and elevated levels of TPO should be detectable in the plasma following acute, severe thrombocytopenia. Acute thrombocytopenia provides a model system that is likely to predict the effects of TPO, since many of the effects on megakaryocytes and platelets observed after induction of acute thrombocytopenia would be mediated by TPO. Important questions remain to be answered. Do the currently available data for the c-Mpl ligand explain previously published data that describe elevated levels of Meg-CSF in the circulation following production of bone marrow aplasia? Does the c-Mpl ligand account for all of the megakaryocyte stimulatory factors that have been described? Is there another factor that accounts for at least some of the acute alterations in megakaryocytopoiesis that occur immediately following a decrease in platelet levels?     

Expression of interleukin-6 and interleukin-6 receptors in human granulosa lutein cells

Prolonged isolated thrombocytopenia, defined as recovery of other cell counts with continuous dependence on platelet transfusions for greater than 90 days after hematopoietic stem cell transplantation (HSCT), develops in approximately 5% of patients who undergo HSCT. Although the clinical conditions associated with prolonged isolated thrombocytopenia have been studied, a systematic review of bone marrow biopsies has not been performed and the pathophysiologic basis has not been defined. We reviewed all HSCT at one center from 1990 to 1995 (n = 454) and found 12 cases that met criteria for prolonged isolated thrombocytopenia (incidence = 12/454 or 3%). Bone marrow core biopsies from 12 patients with prolonged isolated thrombocytopenia were reviewed to determine cellularity, numbers of megakaryocytes, the presence of atypical forms, and clusters of megakaryocytes. These marrow megakaryocyte counts were compared to age and disease matched controls, and 11 normal donors. Patients (aged 1-56 years, mean 32 years) who underwent HSCT (four sibling HLA-identical, five autologous bone marrow, three autologous peripheral stem cell) with prolonged isolated thrombocytopenia had a statistically significant lower absolute megakaryocyte count in bone marrow biopsies performed before transplantation and more than 30 days after transplantation compared to control patients (aged 4 months to 50 years, mean 31 years) who underwent HSCT (four sibling HLA-identical, four autologous bone marrow, four autologous peripheral stem cell) for similar conditions. No apparent differences were seen in size of megakaryocytes, nuclear-cytoplasmic ratios, or clustering of megakaryocytes. Overall marrow cellularities were similar in the three groups. These findings suggest that decreased differentiation of megakaryocytes from stem cells, rather than ineffective platelet production or peripheral destruction of platelets, causes prolonged isolated thrombocytopenia in HSCT patients. Low megakaryocyte counts prior to HSCT may be a useful prognostic indicator, as this feature was associated with the development of prolonged isolated thrombocytopenia.     

Plasma thrombopoietin (TPO) levels and expression of TPO receptor on platelets in patients with myelodysplastic syndromes

Data on endogenous thrombopoietin (TPO) levels and their regulation in myelodysplastic syndromes (MDS) are sparse. We examined the plasma TPO level of 85 MDS patients by a sensitive enzyme immunoassay and the platelet expression of TPO receptor (TPO-R) protein, which metabolizes endogenous TPO, in 19 MDS patients with an equilibrium binding assay using 125I-TPO. The MDS patients had higher plasma TPO levels (7.0 +/- 9.3 fmol/ml) than 52 normal subjects (P < 0.0001). Refractory anaemia (RA) patients (n = 39) had higher plasma TPO levels than patients (n = 28) with RA with excess blasts (RAEB) or RAEB in transformation (RAEB-t) (P = 0.0002), irrespective of similar platelet counts in these groups. The plasma TPO level correlated inversely with the platelet count in RA patients (P = 0.0027) but not in RAEB and RAEB-t patients (P = 0.7865). These data suggest that the physiological pathway for TPO production and metabolism is conserved, at least partially, in RA, but deranged in RAEB/RAEB-t. The number of TPO-R per platelet was significantly smaller in 19 MDS patients (17.5 +/- 13.3) than in normals (P = 0.0014), but similar between RA patients and patients with RAEB and RAEB-t. Further, the bone marrow megakaryocyte count, determined in 31 MDS patients, was quite similar between RA patients and patients with RAEB or RAEB-t. Thus, in addition to thrombocytopenia, a reduced platelet TPO-R number may contribute to elevated plasma TPO levels in MDS, and a regulatory pathway for circulating TPO other than platelet TPO-R and marrow megakaryocytes, such as blasts expressing TPO-R, may operate in RAEB/RAEB-t.     

Treatment of thrombocytopenia in chimpanzees infected with human immunodeficiency virus by pegylated recombinant human megakaryocyte growth and development factor

Three chimpanzees experimentally infected with human immunodeficiency virus (HIV) developed significant chronic thrombocytopenia after 5, 4, and 2 years, with peripheral platelet counts averaging 64 +/- 19 x 10(3)/microL (P = .004 compared with 228 +/- 92 x 10(3)/microL in 44 normal control animals), mean platelet volumes of 11.2 +/- 1.8 fL (P > .5 compared with 10.9 +/- 0. 7 fL in normal controls), endogenous thrombopoietin (TPO) levels of 926 +/- 364 pg/mL (P < .001 compared with 324 +/- 256 pg/mL in normal controls), uniformly elevated platelet anti-glycoprotein (GP) IIIa49-66 antibodies, and corresponding viral loads of 534, 260, and 15 x 10(3) RNA viral copies/mL. Pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) was administered subcutaneously (25 microg/kg twice weekly for 3 doses) to determine the effects of stimulating platelet production on peripheral platelet concentrations in this cohort of thrombocytopenic HIV-infected chimpanzees. PEG-rHuMGDF therapy increased (1) peripheral platelet counts 10-fold (from 64 +/- 19 to 599 +/- 260 x 10(3) platelets/microL; P = .02); (2) marrow megakaryocyte numbers 30-fold (from 11.7 +/- 6.5 x 10(6)/kg to 353 +/- 255 x 10(6)/kg; P = .04); (3) marrow megakaryocyte progenitor cells fourfold (from a mean of 3.6 +/- 0.6 to 14.1 x 10(3) CFU-Meg/1, 000 CD34(+) marrow cells); and (4) serum levels of Mpl ligand from 926 +/- 364 pg/mL (endogenous TPO) to predosing trough levels of 1, 840 +/- 353 pg/mL PEG-rHuMGDF (P = .02). The peripheral neutrophil counts were also transiently increased from 5.2 +/- 2.6 x 10(3)/microL to 9.9 +/- 5.0 x 10(3)/microL (P = .01), but neither the erythrocyte counts nor the reticulocyte counts were altered significantly (P > .1). The serum levels of antiplatelet GPIIIa49-66 antibodies exhibited reciprocal reductions during periods of thrombocytosis (P < .07). PEG-rHuMGDF therapy did not increase viral loads significantly (395, 189, and 53 x 10(3) RNA viral copies/mL; P > .5 compared with baseline values). The striking increase in peripheral platelet counts produced by PEG-rHuMGDF therapy implies that thrombocytopenia in HIV-infected chimpanzees is attributable to insufficient compensatory expansion in platelet production resulting from HIV-impaired delivery of platelets despite stimulated megakaryocytopoiesis. These data suggest that PEG-rHuMGDF therapy may similarly correct peripheral platelet counts in thrombocytopenic HIV-infected patients.     

Thrombopoietin stimulates myelodysplastic syndrome granulocyte-macrophage and erythroid progenitor proliferation

Thrombopoietin (TPO) has been successfully used to stimulate megakaryocyte progenitor proliferation and platelet production both in vitro and in vivo. We and other investigators have found that TPO also stimulates normal marrow colony-forming unit granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-E) growth. In contrast to its effect on normal marrow precursors, TPO stimulates acute myelogenous leukemia (AML) progenitor proliferation in only 25% of the cases. Because the hematopoietic cells in Myelodysplastic syndrome (MDS) originate from both the normal and leukemic clones, we hypothesized that TPO may be a useful therapeutic agent for MDS. To test this hypothesis, we used fresh marrow samples taken from 14 MDS patients. We found that in the presence of fetal calf serum (FCS) and erythropoietin (EPO) TPO (5 to 40 ng/ml) MDS CFU-GM and BFU-E colony-forming cell proliferation were stimulated in a dose-dependent fashion by up to 103% and 93% respectively. This effect was similar to the stimulation obtained with optimal concentrations of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or interleukin-3 (IL-3). Furthermore, TPO increased the colony-stimulatory effects of G-CSF, GM-CSF, IL-3, and stem cell factor (SCF) on MDS marrow cells. However, depletion of either T lymphocytes or adherent cells abrogated the effect of TPO, suggesting that the effect is not a direct one but is mediated through interaction with cytokines produced by accessory cells. Taken together, our data suggest that the therapeutic role of TPO in the management of MDS warrants further investigation.     

Increased serum levels of thrombopoietin in patients with thrombotic thrombocytopenic purpura, idiopathic thrombocytopenic purpura, or disseminated intravascular coagulation

The serum levels of thrombopoietin (TPO) were measured in 16 patients with thrombotic thrombocytopenic purpura (TTP), 12 with hemolytic uremic syndrome (HUS), 10 with aplastic anemia (AA), 10 with disseminated intravascular coagulation (DIC), and 71 with idiopathic thrombocytopenic purpura (ITP). The serum TPO levels were measured with a sensitive sandwich enzyme-linked immunosorbent assay. The serum TPO level in the ITP group (1.68 +/- 0.85 fmol/ml) were not significantly increased compared with those of the normal subjects. The TPO levels in the TTP (2.77 +/- 1.38 fmol/ml) and HUS groups (5.77 +/- 4.41 fmol/ml) were higher than those of the normal subjects. The patients with AA (12.7 +/- 8.0 fmol/ml) and those with DIC (13.3 +/- 5.7 mol/ml) had significantly higher serum TPO levels than did the normal subjects and ITP patients. The TPO levels were well correlated with the platelet counts in the TTP patients, and were negatively correlated with the platelet counts in the ITP patients. These results suggest that the serum TPO levels in some thrombocytopenic diseases are regulated not only by the platelet count and the megakaryocyte mass, but also by other factors.     

Serum thrombopoietin levels in patients with aplastic anaemia

Endogenous serum thrombopoietin (TPO) levels were measured in 31 patients with aplastic anaemia (AA) using an enzyme immunoassay with a sensitivity of 20 pg/ ml. The median platelet count for all AA patients was 30 +/- 29 x 10(9)/l (range 5-102) compared with a median of 284 +/- 59 x 10(9)/l (range 148-538) for normal controls. Serum TPO levels were significantly elevated in all patients compared with normals (1706 +/- 1114.2, range 375-5000 v 78 +/- 54, range 16.5-312.9, P < 0.0001). There was no correlation between serum TPO levels and the degree of thrombocytopenia in AA patients, but TPO levels were significantly higher in patients who were platelet transfusion dependent than in patients who were transfusion independent (P < 0.01). There was a trend for higher TPO levels in patients with severe AA compared with non-severe AA patients. Clinical trials of TPO and a related truncated, pegylated molecule, megakaryocyte growth and development factor (PEG-rHuMGDF), are awaited to determine whether treatment with these drugs will result in increased platelet counts in patients with AA.     

Lack of efficacy of thrombopoietin and granulocyte colony-stimulating factor after high dose total-body irradiation and autologous stem cell or bone marrow transplantation in rhesus monkeys

The efficacy of recombinant human thrombopoietin (TPO) and recombinant human granulocyte colony stimulating factor (G-CSF) in stimulating platelet and neutrophil recovery was evaluated in a placebo-controlled study involving transplantation of limited numbers (1-3 x 10(4)/kg) of highly purified autologous stem cells (CD34++/RhLA-DR[dull]) into rhesus monkeys after the animals were subjected to 8 Gy of total body irradiation (TBI) (x-rays). The grafts shortened profound TBI-induced pancytopenia from 5 to 6 weeks to 3 weeks. Daily subcutaneous (sc) injection of TPO (10 microg/kg/day, days 1-21 after TBI) did not stimulate platelet regeneration after transplantation either alone or in combination with G-CSF (5 microg/kg/day sc, days 1-21 after TBI). G-CSF treatment failed to prevent neutropenia in the monkeys and did not stimulate recovery to normal neutrophil levels. Simultaneous administration of TPO and G-CSF did not influence the observed recovery patterns. To test the hypothesis that the limited number of cells transplanted or the subset chosen was responsible for the lack of effectiveness of TPO, three additional monkeys were transplanted with 10(7)/kg unfractionated autologous bone marrow cells. Two of these animals received TPO and the other served as a control. In this setting, as well, TPO treatment did not prevent thrombocytopenia. This study demonstrates that treatment with TPO does not accelerate platelet reconstitution from transplanted stem cells after high-dose TBI. These findings contrast with the rapid TPO-stimulated platelet recovery in myelosuppression induced by 5 Gy of TBI in rhesus monkeys; we conclude from this that the clinical effectiveness of the TPO response depends on the availability of TPO target cells in the first week after TBI, that is, before endogenous TPO levels reach the saturation point. In addition, protracted isolated thrombocytopenia was observed in two G-CSF-treated monkeys, one of which also received TPO. Furthermore, TPO treatment for 7 days in the 6th week after TBI during severe thrombocytopenia in one monkey produced prompt clinical improvement and an increase in platelet counts.     

Thrombopoietin enhances the production of myeloid cells, but not megakaryocytes, in juvenile chronic myelogenous leukemia

We previously reported the aberrant growth of granulocyte-macrophage (GM) progenitors induced by a combination of stem cell factor (SCF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) in juvenile chronic myelogenous leukemia (JCML). We examined here the effects of thrombopoietin (TPO) on the proliferation and differentiation of hematopoietic progenitors in JCML. In serum-deprived single-cell cultures of normal bone marrow (BM) CD34+CD38high cells, the addition of TPO to the culture containing SCF + GM-CSF resulted in an increase in the number and size of GM colonies. In the JCML cultures, in contrast, the number of SCF + GM-CSF-dependent GM colonies was not increased by the addition of TPO. However, the TPO addition caused an enlargement of GM colonies in cultures from the JCML patients to a significantly greater extent compared with the normal controls. There was no difference in the type of the constituent cells of GM colonies with or without TPO grown by JCML BM cells. A flow cytometric analysis showed that the c-Mpl expression was found on CD13+ myeloid cells generated by CD34+CD38high BM cells from JCML patients, but was at an undetectable level in normal controls. The addition of TPO to the culture containing SCF or SCF + GM-CSF caused a significant increase in the production of GM colony-forming cells by JCML CD34+CD38neg/low population, indicating the stimulatory effects of TPO on JCML primitive hematopoietic progenitors. Normal BM cells yielded a significant number of megakaryocytes as well as myeloid cells in response to a combination of SCF, GM-CSF, and/or TPO. In contrast, megakaryocytic cells were barely produced by the JCML progenitors. Our results may provide a fundamental insight that the administration of TPO enhances the aberrant growth of GM progenitors rather than the recovery of megakaryocytopoiesis.     

An agonist murine monoclonal antibody to the human c-Mpl receptor stimulates megakaryocytopoiesis

Thrombopoietin (TPO) is a hematopoietic growth factor that stimulates megakaryocytopoiesis and platelet production in vivo and promotes the development of identifiable megakaryocytes in vitro. We have developed a murine monoclonal antibody, BAH-1, raised against human megakaryocytic cells, which specifically recognizes the c-Mpl receptor and shows agonist activity by stimulating megakaryocytopoiesis in vitro. BAH-1 antibody specifically binds to platelets and to recombinant c-Mpl with high affinity. Similar to TPO, BAH-1 alone supported the formation of colony-forming unit-megakaryocyte (CFU-MK) colonies. The combination of BAH-1 plus interleukin-3 or of BAH-1 plus human TPO significantly increased the number of human CFU-MK colonies. In addition, BAH-1 monoclonal antibody stimulated the proliferation and maturation of primary bone marrow megakaryocytes in a dynamic heterogeneous liquid culture system. Individual large megakaryocytes as well as small megakaryocytic cells were observed in cultures of CD34(+) CD41(+) cells in the presence of BAH-1 antibodies. Similar to TPO, BAH-1 antibody induced a significant response of murine immature megakaryocytes as observed by an increase in the detectable numbers of acetylcholinesterase-positive megakaryocytes. No effects of BAH-1 antibody were observed on colony-forming unit-granulocyte-macrophage, burst-forming unit-erythroid, or colony-forming unit-erythroid colonies. In vivo studies showed that BAH-1, alone or in combination with TPO, expands the numbers of megakaryocytic progenitor cells in myelosuppressed mice. This antibody should prove useful in understanding the structure-function aspects of the c-Mpl receptor as well as in evaluating the effects of the sustained activation of this receptor in preclinical models of severe thrombocytopenia.     

Physiological regulation of early and late stages of megakaryocytopoiesis by thrombopoietin

Thrombopoietin (TPO) has recently been cloned and shown to regulate megakaryocyte and platelet production by activating the cytokine receptor c-mpl. To determine whether TPO is the only ligand for c-mpl and the major regulator of megakaryocytopoiesis, TPO deficient mice were generated by gene targeting. TPO-/- mice have a >80% decrease in their platelets and megakaryocytes but have normal levels of all the other hematopoietic cell types. A gene dosage effect observed in heterozygous mice suggests that the TPO gene is constitutively expressed and that the circulating TPO level is directly regulated by the platelet mass. Bone marrow from TPO-/- mice have decreased numbers of megakaryocyte-committed progenitors as well as lower ploidy in the megakaryocytes that are present. These results demonstrate that TPO alone is the major physiological regulator of both proliferation and differentiation of hematopoietic progenitor cells into mature megakaryocytes but that TPO is not critical to the final step of platelet production.     

Synergistic effects of thrombopoietin and granulocyte colony-stimulating factor on neutrophil recovery in myelosuppressed mice

Severe suppression of the hematopoietic system is a major factor in limiting chemotherapy dose escalation. To determine whether a combination of human recombinant granulocyte colony-stimulating factor (G-CSF) and thrombopoietin (TPO) would alter recovery of platelets, red blood cells (RBCs), or neutrophils after myeloablative therapy, myelosuppressed mice were treated with sc injections of TPO (90 micrograms/kg), G-CSF (250 micrograms/kg). TPO plus G-CSF or vehicle and complete blood counts were measured. Marrow and spleen cells were obtained at various times and assayed for erythroid, myeloid, and megakaryocytic progenitors. The prolonged neutropenia in vehicle controls (14 days) was significantly shortened in mice treated with G-CSF or TPO for 14 days. The combination of TPO plus G-CSF further reduced the duration of neutropenia. TPO and TPO plus G-CSF treatments also significantly shortened thrombocytopenia compared to vehicle. Recovery of RBCs was also enhanced in mice treated with either G-CSF or TPO, or the combination. Furthermore, treatment with G-CSF and/or TPO hastened myeloid, erythroid, and megakaryocyte progenitor recovery compared to vehicle controls. These results show that the combination of TPO plus G-CSF acts synergistically to accelerate neutrophil recovery in myelosuppressed mice and does not compromise the platelet or RBC response to TPO therapy.     

Juvenile psoriatic arthritis: followup and evaluation of diagnostic criteria

Platelet counts in newborns are similar to those of adults and children. However, newborn infants admitted to intensive care nurseries have a high prevalence of thrombocytopenia. The mechanisms responsible for the increased susceptibility to thrombocytopenia are not known. In addition, some studies have documented functional abnormalities in newborn platelets. In an effort to understand differences between platelets in newborns and in adults, we examined megakaryocyte ploidy in bone marrow from fetuses and compared it with bone marrow from adults, using a modified Feulgen stain to measure DNA of individual megakaryocytes. Faced with small fixed tissue samples, we developed a technique for use on bone marrow biopsies to estimate megakaryocyte ploidy and compared the results obtained with this method to those obtained from bone marrow aspirates. This study demonstrated that the overall mean ploidy of fetal megakaryocytes is decreased compared with adults. Additionally, fetal megakaryocyte ploidy increases as megakaryocyte maturation increases, but not to the same extent that adult megakaryocyte ploidy increases with megakaryocyte maturation. Over the gestational period studied, there was no relationship between gestational age and mean ploidy. The small size, shift to a less mature population, and decreased ploidy of fetal megakaryocytes indicate that there are differences in the post mitotic phase of megakaryocyte development in the fetus. Such differences may be related to quantitative and qualitative platelet abnormalities in the newborn. Understanding the physiology and regulation of megakaryocytopoiesis in the fetus and newborn will be valuable in determining the pathophysiologic basis of platelet dysfunction in the newborn.     

Mpl ligand (thrombopoietin) and platelet regulation

After 35 years of research, the physiological regulator of platelet production has been isolated and its gene cloned. This discovery originates from studies performed with the myeloproliferative leukemia virus (MPLV), a murine retrovirus which induces an acute myeloproliferative syndrome in adult mice. MPLV carries in its genome the v-mpl oncogene which corresponds to a truncated form the c-mpl proto-oncogene. c-mpl encodes a cytokine receptor (Mpl-R) belonging to the hematopoietin receptor superfamily. Among the hematopoietic cell lineages, Mpl-R is preferentially expressed on late megakaryocyte progenitors, megakaryocytes and platelets. The ligand for Mpl-R, called Mpl-L or TPO or MGDF or megapoietin, is a glycosylated hormone of 352 amino acids in human which comprises two domains: the N-terminus domain shares 50% similarity with erythropoietin and is responsible for the biological activity; the C-terminus part is required for secretion. Notwithstanding its major action on megakaryocytopoiesis and thrombocytopoiesis, Mpl-L also potentiates the action of other cytokines on several hematopoietic lineages. Mpl-L/TPO/MGDF, the homeostatic regulator of platelet production, might be a useful therapeutical cytokine to treat thrombocytopenia induced in patients by chemotherapy.