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

Regulation of megakaryocytopoiesis by thrombopoietin

It is clear that thrombopoietin is a major hormonal regulator of megakaryocytopoiesis both in vitro and in vivo, and, thus, blood platelet production. Existing data show that the action, chemical nature, and immunologic properties of thrombopoietin from HEK cell culture medium and either endogenously produced or exogenously administered thrombopoietin from animal sources are similar, if not identical. Absolute identity, however, will require comparisons of amino acid sequences of the two preparations. Recent studies have shown that not only does TSF potentiate the action of meg-CSF, but it also has a direct effect on precursor cells to increase the number of megakaryocytic colonies. Other in vitro work showed that TSF stimulates directly the SAChE+ precursor cells to become mature megakaryocytes and causes FMLC to differentiate into megakaryocytic colonies. In vivo, TSF increases megakaryocyte size and number, it causes an elevation in the number of the SAChE+ precursor cells in mouse marrow and increases the maturation of megakaryocytes. Moreover, TSF increases the endomitosis of megakaryocytes in the marrow of mice, along with elevating the number of megakaryocytic colonies in spleens of lethally irradiated bone marrow reconstituted mice. Platelet production is also stimulated in mice by TSF as evidenced by elevated isotopic incorporation into platelets; it increases platelet sizes, and when administered in high doses TSF elevates platelet counts. Full development of colonies of megakaryocytes may depend on two growth factors. It has been hypothesized that one factor, meg-CSF, is effective in clonal expansion whereas a second factor is predominately involved in the endomitotic phase of megakaryocyte development. Multifactoral regulation has been observed for the other cell lineages, and a general proposal for hematopoietic development has been outlined by Iscove. In this scheme, specificity of erythropoietin to erythroid cell lineage is indicated. Previous work, however, shows that recombinant erythropoietin can act as a meg-CSF stimulus, indicating that much is yet to be learned about the action of hematopoietic regulatory factors. Although the present study showed that TSF may in some circumstances stimulate an early cell in the megakaryocytic series, its major effect is probably on the more differentiated population, leading to maturation of megakaryocytes and platelet production.(ABSTRACT TRUNCATED AT 400 WORDS)

Human megakaryocytic progenitor cells

Megakaryocytopoiesis represents one of several differentiation pathways that hematopoietic stem cells may enter. Cells representing intermediate stages of differentiation between pluripotent stem cells and maturing megakaryocytes are called megakaryocytic progenitor cells. They are identified in human bone marrow and peripheral blood by their ability to proliferate in culture (colony forming unit-megakaryocyte, CFU-M); at some point they lose the capacity for cell division and acquire the ability for endoreduplication of DNA, a phenomenon that is unique to the megakaryocyte lineage. This review summarizes current understanding of the biology of human megakaryocytic progenitor cells, including characterization of their proliferation potentials, their antigenic determinants, and the mechanisms that govern their proliferation and maturation. Finally the involvement of CFU-M in various disorders of thrombopoiesis is discussed.

Megakaryocytopoiesis in vitro of patients with essential thrombocythaemia: effect of plasma and serum on megakaryocytic colony formation

To clarify the mechanism of increased numbers of megakaryocytes in patients with essential thrombocythaemia (ET), we studied in vitro megakaryocytopoiesis in ET and other myeloproliferative disorders, using a megakaryocytic colony assay in methylcellulose containing plasma or serum and medium conditioned by phytohaemagglutinin (PHA) stimulated leucocytes (PHA-LCM). Megakaryocytic colony formation was supported well by heparinized or citrated plasma and citrated serum which was harvested after clot formation of citrated plasma. Whole serum was inhibitory for megakaryocytic colony growth. The addition of platelet releasates and partially purified platelet derived growth factor (PDGF) resulted in a decrease in the number of megakaryocytic colonies. These findings suggested that platelet-derived factor(s) in serum was inhibitory to megakaryocytic colony formation. ET plasma supported the megakaryocytic colony formation by normal or ET bone marrow cells better than normal plasma. Moreover, in ET bone marrow cells, spontaneous megakaryocytic colonies were formed in the absence of PHA-LCM. Increased megakaryocytopoiesis in ET may be ascribed to (i) increased megakaryocyte-colony stimulating activity (Meg-CSA) in plasma and (ii) increased sensitivity to Meg-CSA or autonomous proliferation of megakaryocytic progenitor cells.

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

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

Thrombopoietic activity in preterm newborns and infants

Platelet count and thrombopoietic activity were investigated in preterm infants at birth and during their first four months of life. Thrombopoiesis stimulating factor activity in cord serum was significantly lower than that of adults and of the respective mothers. No difference was noted between thrombopoietic activity in cord serum in the various gestational ages studied--that is, 24 through 39 weeks. Preterm infants followed during their first four months of life showed a mean platelet count significantly higher than that observed in term infants at the respective age. Furthermore, thrombocytosis was found in premature thriving infants at the age of 1 and 2 months. It is suggested that this thrombocytosis is responsible for the low thrombopoietic activity observed in these infants during their first four months of life.

Human urinary megakaryocyte colony-stimulating factor in thrombopoietic disorders

The level of urinary Meg-CSF activity in patients with various thrombopoietic disorders was studied. Five out of eight patients with idiopathic thrombocytopenic purpura had megakaryocytic hyperplasia in the marrow and increased Meg-CSF activity in the urine. Urinary Meg-CSF activity in patients with polycythaemia vera and essential thrombocythaemia was normal. There was a significant inverse correlation between urinary Meg-CSF activity and peripheral blood platelet count but not bone marrow megakaryocyte mass. There was a significant increase of urinary Meg-CSF activity during the period of thrombocytopenia after chemotherapy in patients with acute leukaemia who were in complete remission. The timing of maximal Meg-CSF levels corresponded to the nadirs of platelet counts. These results support the concept that Meg-CSF may play a significant role in the regulation of megakaryopoiesis and/or thrombopoiesis in vivo.

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

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

Characterization of human megakaryocytic colony formation in human plasma

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

Myeloproliferative disorders

Myeloproliferative disorders are uncommon in the dog and may be classified as chronic or acute. Excessive proliferation of mature cells leads to an overproduction of terminally differentiated blood cells (chronic MPD). Inability of cells to mature results in the accumulation of poorly differentiated blast cells in the peripheral blood and bone marrow (acute MPD). Because the lesion appears to be at the level of the hematopoietic stem cell, all cell lines in the bone marrow may be affected. Diagnosis depends upon the accurate identification of neoplastic cells and the absence of other diseases associated with bone marrow hyperplasia. The prognosis for chronic MPD is guarded, whereas for acute MPD it is grave. Accurate identification of these disorders in animals is important. Investigation and greater understanding of the pathophysiologic mechanisms may lead to more lasting therapeutic successes in the future.

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

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

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

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

Current considerations of the etiology of aplastic anemia

Aplastic anemia is a disorder characterized by marrow aplasia and pancytopenia. The pathogenetic mechanisms that lead to bone marrow aplasia have been intensively studied. Data obtained from these studies suggest that aplastic anemia is a heterogeneous disorder with regards to pathogenesis. Bone marrow aplasia may result from a number of abnormalities including qualitative or quantitative abnormalities of hematopoietic stem cells, abnormal interaction between bone marrow accessory cells (lymphocytes and macrophages) and hematopoietic stem cells, cytotoxic humoral inhibitors of hematopoiesis, and abnormalities of the bone marrow microenvironment. A number of new therapeutic options have improved the survival of patients with aplastic anemia. Allogeneic bone marrow transplantation has actually resulted in the cure of patients. Unfortunately, only a minority of patients have a suitable bone marrow donor and alternate modes of therapy have been sought. Encouraging results have been reported from several centers concerning the use of antilymphocyte serum in patients with aplastic anemia. Certainty of the ultimate long-term benefit of this type of immunosuppressive therapy is not possible until careful, randomized, prospective studies of its use are completed.

Immunological study of in vitro maturation of human megakaryocytes

Human megakarocyte colonies were grown from the bone marrow in plasma clot or methyl cellulose cultures. Maturation of the megakaryocytic cells was sequentially studied from day 5 to day 16 of culture by fluorescent labelling with a panel of monoclonal and polyclonal antibodies against different platelet glycoproteins (Gp), P1 A1 antigen, factor VIII RAg platelet factor 4 (PF 4), fibrinogen and platelet-derived growth factor (PDGF). Expression of Gp Ib was also studied by immunogold technique at electron microscopy. The first cells identifiable by these antibodies were found at day 5 of culture. They had the size of a lymphocyte. These small megakaryocyte precursors already expressed all the platelet antigens, HLA-DR and transferrin receptors and were devoid of erythroid or myeloid markers. Among the platelet antigens, Gp IIIa was the most sensitive marker for the identification of these precursors. However, double-fluorescent labelling demonstrated that the different platelet markers were coexpressed in a large majority of cells. Interestingly, cytoplasmic markers demonstrated that these small megakaryocyte precursors were themselves heterogenous by morphological criteria. During maturation, expression of Gps, particularly of Gp Ib, increased while the labelling pattern of anti factor VIII RAg and anti PF 4 antibodies switched from diffuse to granular staining. PDGF could also be detected in the megakaryocytes grown in culture.

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

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

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

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

The growth of large megakaryocyte colonies from human bone marrow

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