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

Recent developments in ex vivo platelet production

The platelet is a component of blood that functions to initiate blood clotting. Abnormal platelet count and function can lead to a life-threatening condition caused by excessive bleeding. At present, platelet supply for transfusion can be obtained only from platelet donation. However, platelets cannot be stored for longer than 7 days, meaning that routine isolation is required to maintain platelet supply for transfusion. To mitigate for potential platelet shortages, several strategies have been proposed to generate platelets ex vivo. By employing both of natural and artificial approaches, several researchers have successfully generated biomaterials with characteristics similar to human-derived platelets. Their reports indicated that the biomaterials could mimic the aggregation of human-isolated platelets, further suggesting the possibility to substitute or complement human-isolated platelets. The current review summarizes the progress in ex vivo platelet production and gives a prospect for the possible approaches to achieving a feasible platelet factory, based on the Good Manufacturing Practice standards.

The role of interleukin 6 in megakaryocyte formation, megakaryocyte development and platelet production

Megakaryocytopoiesis is the cellular amplification and differentiation of precursors into immature megakaryocytes, and the cytoplasmic maturation of these megakaryocytes, a process terminating in the release of platelets into the circulation. Interleukin 6 (IL-6) stimulates megakaryocytopoiesis in the bone marrow, increasing platelet numbers in the circulation. IL-6 alone is poorly active on the growth of stem cell populations, but acts in synergy with stem cell factor (c-kit ligand) to expand the committed myeloid progenitor compartments but not the megakaryocyte progenitors. IL-6 has a direct action on megakaryocyte progenitors but only in synergy with low doses of interleukin 3 (IL-3), increasing the number of immature megakaryocytes and enhancing the processes of development into mature megakaryocytes. IL-6 is about 10 times more active on megakaryocytes than on megakaryocyte progenitors in cell culture. It is active alone and will stimulate increases in cell size and DNA content. IL-6 does not appear to stimulate the process of platelet release. IL-6 is found in bone marrow, in both macrophage subsets and megakaryocytes, indicating that it may be an important physiological regulator of both paracrinal (microenvironmental) and autocrinal mechanisms controlling megakaryocyte development in bone marrow.

Protein characteristics of thrombopoietin

Thrombopoietin (TPO) was purified from irradiated thrombocytopenic rat plasma. In the process of purification, some biochemical and biological characteristics were investigated. Rat plasma TPO was extremely hydrophobic and exhibited multiple peaks of activity on gel filtration. Both the low and high molecular weight fractions were separately subjected to further purification. Consequently, a rat TPO cDNA was cloned based on the amino acid sequences of purified rat plasma TPO. It revealed that each final purified rat plasma TPO was not a full-length form. In addition, rat hepatocytes and three rat hepatoma cell lines were found to produce rat TPO. Each native TPO derived from cultured cells was also partially purified, and hepatocyte-derived TPOs were shown to be heterogeneous in molecular weight. To study the structure of TPO, various recombinant TPO molecules were generated. Two disulfide bonds (Cys7-Cys151 and Cys29-Cys85) located in the N-terminal domain of TPO have an important effect on its biological activity. The human TPO muteins, sequentially deleted from the C-terminal, were expressed in COS-1 cells. TPO (1-151) was active, but TPO (1-150), which lacks Cys151, did not exhibit TPO activity. These findings indicate that the region essential for TPO activity is the N-terminal domain, which contains two disulfide bonds. Although the role(s) of the C-terminal domain is not clear at present, the potential N-glycosylation in the C-terminal domain is not directly required for exhibiting TPO activity.

PEGylation prevents the N-terminal degradation of megakaryocyte growth and development factor

PURPOSE

Determine the effect of PEGylation on in-vitro degradation for recombinant human Megakaryocyte Growth and Development Factor (rHuMGDF) in the neutral pH range.

METHODS

Degradation products were characterized by cation-exchange HPLC, N-terminal sequencing and mass spectrometry.

RESULTS

The main route of degradation was through non-enzymatic cyclization of the first two amino acids and subsequent cleavage to form a diketopiperazine and des(Ser, Pro)rHuMGDE This reaction was prevented by alkylation of the N-terminus by polyethylene glycol (PEG).

CONCLUSIONS

PEGylation of proteins is commonly performed to achieve increased in-vivo circulation half-lives. For rHuMGDF, an additional advantage of PEGylation was enhanced in-vitro shelf-life stability.

Thrombopoietin

The term "thrombopoietin" (TPO) was first coined in 1958 but, despite several cytokines having thrombopoietic effects, the major megakaryocytic growth and development factor was not cloned until 1994. Wendling and colleagues (Virol, 1986; 149: 242-246) recognised a novel murine viral oncogene, v-mpl, responsible for a myeloproliferative syndrome. The corresponding human proto-oncogene, c-mpl, codes for a member of the haemopoietic cytokine receptor family. Its ligand is TPO. The structure and function of TPO, together with its action in stimulating proliferation and maturation of megakaryocytes, are discussed in detail in this review which also details the pre-clinical usage and clinical potential of TPO as a platelet sparing agent in haemato-oncological practice.

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.

Megakaryocyte growth and development factor. Analyses of in vitro effects on human megakaryopoiesis and endogenous serum levels during chemotherapy-induced thrombocytopenia

The present study shows that recombinant human megakaryocyte growth and development factor (r-HuMGDF) behaves both as a megakaryocyte colony stimulating factor and as a differentiation factor in human progenitor cell cultures. Megakaryocyte colony formation induced with r-HuMGDF is synergistically affected by stem cell factor but not by interleukin 3. Megakaryocytes stimulated with r-HuMGDF demonstrate progressive cytoplasmic and nuclear maturation. Measurable levels of megakaryocyte growth and development factor in serum from patients undergoing myeloablative therapy and transplantation are shown to be elaborated in response to thrombocytopenic stress. These data support the concept that megakaryocyte growth and development factor is a physiologically regulated cytokine that is capable of supporting several aspects of megakaryopoiesis.

The regulation of megakaryocytopoiesis

The process of megakaryocytopoiesis begins with the commitment of a pluripotent hematopoietic stem cell to a differentiation pathway that culminates in the release of mature platelets into the circulation. A variety of megakaryocyte precursor cells have been identified after stem cell commitment has occurred and these may be recognized by their morphologic or immunophenotypic characteristics. Megakaryocytopoiesis is regulated by a number of cytokines with either stimulatory or inhibitory effects and by a variety of cell-cell interactions. Some factors potentiating platelet development promote the proliferation of megakaryocyte progenitor cells, while others result in their maturation. Thrombopoietin, a cytokine with specific megakaryocyte maturational activity recently has been identified as the c-Mpl ligand, and it will be evaluated as a therapeutic agent in the setting of thrombocytopenia due to impaired megakaryocytopoiesis.

The structure, biology and potential therapeutic applications of recombinant thrombopoietin

Platelets, an integral component of hemostasis, are produced by megakaryocytes derived from the differentiation of pluripotent stem cells in the bone marrow or spleen. After decades of study, the regulation of this process is still not well understood. However, the recent cloning and characterization of thrombopoietin, a ligand for the receptor encoded by the c-mpl proto-oncogene, provides new insights into the humoral regulation of megakaryocytopoiesis and platelet production. Consistent with the proposed role as a major physiological regulator of megakaryocytopoiesis, thrombopoietin has potent effects on megakaryocytopoiesis in vitro and in vivo. In addition to the original supposition that thrombopoietin functions as a late-acting megakaryocyte maturation factor, recombinant thrombopoietin proves also to be a potent stimulator of hematopoietic progenitor cells, inducing them to undergo proliferation and differentiation into megakaryocytic colonies. When administered to mice, thrombopoietin causes an increase in peripheral platelet numbers to previously unattainable levels within a few days. Studies of the efficacy of thrombopoietin are underway. It is envisaged that this new cytokine will have widespread applications as a therapeutic agent for the management of bleeding due to thrombocytopenias, in particular those resulting from cancer chemo- or irradiation therapy.

cMpl ligand is a humoral regulator of megakaryocytopoiesis

Megakaryocytopoiesis is the cellular developmental process that leads to platelet production. At least two humoral growth factors may be necessary for megakaryocyte proliferation and maturation. One is a megakaryocyte-colony stimulating factor (MK-CSF) which induces the proliferation and differentiation of megakaryocyte progenitors, and the second, thrombopoietin, is a megakaryocyte maturation factor. Neither of these factors has been fully characterized. The proto-oncogene c-mpl, an orphan member of the haematopoietin receptor family, is specifically involved in megakaryocyte regulation. Here we present evidence that the c-mpl-encoded receptor binds a ligand (c-Mpl ligand) which is a humoral factor implicated in platelet homeostasis. Our results suggest that c-Mpl ligand, thrombopoietin and MK-CSF might be the same molecule.

Immunocytochemical identification of murine and human megakaryocyte colonies in soft-agar cultures

Murine megakaryocyte (MK) colonies in soft-agar cultures were immunocytochemically stained with platelet antiserum and an immuno-alkaline phosphatase procedure. Subsequently, cytochemical staining for acetylcholinesterase was used to confirm the specificity of the immunolabelling technique. The correlation of numbers of megakaryocyte colonies enumerated by independent observers was excellent. A comparable platelet antiserum directed against human platelet epitopes was utilized to identify human MK colonies in soft-agar cultures of human bone marrow. Using this method, we determined that the frequency of detectable human MK colonies in our agar culture system was maximal between days 10 and 12. The immunocytochemical staining technique we have developed for identification of MK colonies in soft-agar cultures yielded good cellular morphology and produced an intensely specific label against a clear background; it therefore facilitated accurate enumeration of MK colonies. This non-fluorescent method avoids dependence upon a non-permanent marker, and allows the simultaneous enumeration of positive and negative colonies.

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.

Population heterogeneity among cells of the megakaryocyte lineage

Understanding the developmental steps in megakaryocyte differentiation requires information regarding the microenvironmental influences which direct or permit the growth and differentiation of these cells. The megakaryocyte microenvironment, like other lineages, is a complex structure comprised of the various megakaryocytic cells, the extracellular matrix (ECM) surrounding them, and the hematopoietic stromal cells which elaborate both growth factors and ECM. As a result, definition of the minimal essential requirements for megakaryocyte development is difficult. The intricacies of megakaryocyte development are further complicated by the cellular heterogeneity of both mature megakaryocytes and their precursors, as well as a differential responsiveness of these cells to hematopoietic growth factors. This review focuses on defining the various subpopulations of megakaryocytic cells and examining their functional distinctions and in vitro responsiveness to various stimuli.

Combination of cytokines: current status and future prospects

Clinical trials with individual cytokines and extensive in vitro studies have provided the basis for the in vivo use of these molecules in combination. Animal models, with haemopoietic growth factors as well as preliminary studies in humans--as shown by our studies with the sequential use of IL-3 and GM-CSF in patients receiving intensive chemotherapy--indicate that the selection of the appropriate cytokines could optimize haematological responses according to particular clinical requirements. That immunotherapy with IL-2 can induce regression of disseminated human malignancies serves as an encouraging starting point for combinations with other cytokines with the goal of improving the therapeutic efficacy and reducing toxicity. Future prospects of combination therapy will be discussed.

Differentiation of HL-60 myeloid leukaemia cells is associated with a transient block in the G2 phase of the cell cycle

The human promyelocytic leukaemia cell line HL-60 can be induced to differentiate towards mature granulocytes by treatment with dibutyryl cyclic adenosine-3',5'-monophosphate (dbcAMP). Differentiation begins within 16-24 h of treatment and is associated with a time- and dose-dependent accumulation of cells in the G0/G1 phase of the cell cycle with a concomitant decrease in the number of cells in the S and G2 + M phases. Using acridine orange staining, we found that the RNA content of the cells also decreased following differentiation. Stathmokinetic analysis of HL-60 cell populations following dbcAMP treatment showed no effect on the total number of cells in the G0/G1 or S phases, or the rate of progression of cells through these cell cycle compartments. In contrast, dbcAMP was found to induce a transient arrest of the cells in the G2 phase. We also found that differentiation induced by dbcAMP did not require progression of the cells through the cell cycle. Cells arrested in either G1/S by hydroxyurea or G2 + M by colcemid eventually expressed markers of mature granulocytes. These results demonstrate that dbcAMP modulates cell cycle progression. However, these cell cycle changes alone are insufficient to induce granulocytic differentiation of HL-60 cells.

Essential thrombocythemia: impaired regulation of megakaryocyte progenitors

In this paper, the in vitro growth of bone marrow early (megakaryocyte burst-forming units, BFU-meg) and late (megakaryocyte colony-forming units, CFU-meg) progenitors was evaluated in 18 essential thrombocythemia (ET) patients and 22 normal control subjects. BFU-meg clonality was demonstrated both in normal and ET bone marrows, cultivating these primitive progenitors at limiting dilutions in plasma clot assay: 1 to 7 BFU-meg/2.5 x 10(4) mononuclear non-adherent cells were observed, with a strong correlation in ET [r = 0.955 stimulated by recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) plus recombinant human interleukin (rhIL) 3], as well as in normal controls (r = 0.969). In order to clearly elucidate the in vitro response of ET megakaryocyte (meg) progenitors to recombinant growth factors, the interference of accessory cells (i.e., monocytes, T lymphocytes, and natural killer cells) and human serum were avoided by performing experiments on CD34+ cells in a serum-free fibrin clot assay. The number of both early and late meg progenitors in ET was significantly increased in response to rhIL-3, rhIL-3 plus rhIL-6, and rhIL-3 plus rhGM-CSF, but not in response to rhGM-CSF alone. Furthermore, both meg progenitors were investigated for their response to rh transfer growth factor (TGF)-beta 1, tested at concentrations from 0.01 to 10 ng/ml. rhTGF-beta 1 was able to inhibit CFU-meg and BFU-meg in a dose-response manner normal, whereas ET CFU-meg appeared less sensitive to the lower doses investigated (p less than 0.05) and ET BFU-meg were slightly reduced in number only at the higher concentrations of rhTGF-beta 1 (p less than 0.01). Our data suggest that the increased thrombopoiesis in ET may depend on an increased sensitivity of meg progenitors to some of the physiological growth factors and to a disrupted sensitivity to at least one negative regulator of megakaryocytopoiesis. Since these abnormalities involve both meg progenitors, this can be considered a demonstration that the neoplastic event hits the most primitive hemopoietic progenitors.

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.

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

A liquid culture technique associated with either double staining and flow cytometry or electron microscopy was used to study human megakaryocytopoiesis. During development from the embryo to the adult, a progressive increase in ploidy classes associated with an enhancement of megakaryocyte (meg) size was observed. Granulocyte-macrophage colony-stimulating factor had no effects on adult marrow cultures. In contrast, interleukin (IL) 3 induced a marked proliferation, but was unable to promote polyploidization. Furthermore, it abrogated the effects on endomitosis of aplastic plasma (AP). This negative effect on polyploidization of IL-3 could be partially dissociated from its effects on proliferation by a delayed addition in culture. AP acted on both proliferation and endoreplication, which was not due to the main hematopoietic growth factors, including IL-6. A synthesis of IL-6 was detected by in situ hybridization in cultured cells including megs which also express receptors for IL-6. These results suggest that terminal meg differentiation may be regulated by an autocrine IL-6 loop, and that megakaryocytopoiesis may be independently regulated at early and late stages of differentiation.

Partial purification and characterization of human megakaryocyte colony-stimulating factor (Meg-CSF)

Megakaryocyte colony-stimulating factor (Meg-CSF) in urinary extracts from patients with aplastic anemia was partially characterized and purified. Using Meg-CSF-enriched fractions, we established that the moiety has the following characteristics: 1) portions of the molecules having Meg-CSF activity have sialic acid, probably with a biantennary structure, and beta-galactose residues as the terminal and penultimate sugars; 2) disulfide residues are an essential chemical group of the molecule and are located on its surface; and 3) Meg-CSF activity is stable in n-propanol, but not in acetonitrile with trifluoroacetic acid. Partial purification of Meg-CSF by a four-step procedure of ethanol precipitation, CM Affi-Gel Blue chromatography, wheat germ agglutinin-sepharose chromatography, and high-resolution hydroxyapatite chromatography, yielded a concentrate with a 430- to 630-fold increase in specific activity. The partially purified Meg-CSF fractions stimulated both human and murine megakaryocytopoiesis in vitro (CFU-meg). When analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreduced conditions, Meg-CSF activity was recovered in the 29-34 kDa molecular weight fractions. We have also shown that Meg-CSF, purified from the urine of aplastic anemia patients, stimulated murine megakaryocytopoiesis and platelet production in vivo. Final purification of human urinary Meg-CSF is currently in progress.

Numerous growth factors can influence in vitro megakaryocytopoiesis

At least two classes of human megakaryocyte progenitor cells have been identified: the burst-forming unit megakaryocyte (BFU-MK) and the colony-forming unit megakaryocyte (CFU-MK). The BFU-MK is the most primitive progenitor cell committed to the megakaryocytic lineage. The CFU-MK appears to be a more differentiated megakaryocyte progenitor cell and is thought to be ultimately a descendant of the BFU-MK. A number of recombinant cytokines have recently been shown to be able to promote megakaryocyte colony formation in vitro. Recombinant GM-CSF and IL-3, in particular, have the ability to promote both CFU-MK- and BFU-MK-derived colony stimulatory formation. The activities of these two cytokines on in vitro megakaryocytopoiesis are also additive. Recent results of clinical trials in both primates and humans, in which these glycoproteins were administered in vivo, suggest that these cytokines, both alone and in combination, can enhance in vivo thrombopoiesis and therefore may be potentially useful in the treatment of thrombocytopenic disorders.

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.

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

Inhibition of human megakaryocytopoiesis in vitro by platelet factor 4 (PF4) and a synthetic COOH-terminal PF4 peptide

We report that highly purified human platelet factor 4 (PF4) inhibits human megakaryocytopoiesis in vitro. At greater than or equal to 25 micrograms/ml, PF4 inhibited megakaryocyte colony formation approximately 80% in unstimulated cultures, and approximately 58% in cultures containing recombinant human IL 3 and granulocyte-macrophage colony-stimulating factor. Because PF4 (25 micrograms/ml) had no effect on either myeloid or erythroid colony formation lineage specificity of this effect was suggested. A synthetic COOH-terminal PF4 peptide of 24, but not 13 residues, also inhibited megakaryocyte colony formation, whereas a synthetic 18-residue beta-thromboglobulin (beta-TG) peptide and native beta-TG had no such effect when assayed at similar concentrations. The mechanism of PF4-mediated inhibition was investigated. First, we enumerated total cell number, and examined cell maturation in control colonies (n = 200) and colonies (n = 100) that arose in PF4-containing cultures. Total cells per colony did not differ dramatically in the two groups (6.1 +/- 3.0 vs. 4.2 +/- 1.6, respectively), but the numbers of mature large cells per colony was significantly decreased in the presence of PF4 when compared with controls (1.6 +/- 1.5 vs. 3.9 +/- 2.3; P less than 0.001). Second, by using the human leukemia cell line HEL as a model for primitive megakaryocytic cells, we studied the effect of PF4 on cell doubling time, on the expression of both growth-regulated (H3, p53, c-myc,and c-myb), and non-growth-regulated (beta 2-microglobulin) genes. At high concentrations of native PF4 (50 micrograms/ml), no effect on cell doubling time, or H3 or p53 expression was discerned. In contrast, c-myc and c-myb were both upregulated. These results suggested the PF4 inhibited colony formation by impeding cell maturation, as opposed to cell proliferation, perhaps by inducing expression of c-myc and c-myb. The ability of PF4 to inhibit a normal cell maturation function was then tested. Megakaryocytes were incubated in synthetic PF4, or beta-TG peptides for 18 h and effect on Factor V steady-state mRNA levels was determined in 600 individual cells by in situ hybridization. beta-TG peptide had no effect on FV mRNA levels, whereas a approximately 60% decrease in expression of Factor V mRNA was found in megakaryocytes exposed to greater than or equal 100 ng/ml synthetic COOH-terminal PF4 peptide. Accordingly, PF4 modulates megakaryocyte maturation in vitro, and may function as a negative autocrine regulator of human megakaryocytopoiesis.

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.

Evidences that fibroblasts and epithelial cells produce a specific type of macrophage and granulocyte inducer, also known as colony-stimulating factor, and that monocyte-macrophages can produce another factor with proliferative inducing activity on myeloid cells and differentiative activity on macrophages

Molecules with the property to induce proliferation of bone marrow cells in liquid cultures, and with colony-stimulating activity, were found on media conditioned (MC) by lung fibroblasts and kidney epithelial cells. These factors presented an apparent mol wt of 70,000 and 22,000 d respectively. Also when MC by epithelial cells from lungs was tested for the induction of proliferation of bone marrow cells a molecule with 22,000 d was detected. These molecules are thought to be CSF because they induce colony formation, and they are also similar in mol wt to two of the already known CSF. In fact the GM-CSF obtained from endotoxic lungs with a large epithelial cell content has a mot wt of 22,000 d, and the CSF-1 produced by a fibroblast cell line had 70,000. When the MC by fibroblast was used to induce bone marrow cells to proliferate, three new molecules with colony-stimulating activity were secreted. These molecules with apparent mol wts of 45,000, 30,000 and 17,000 d were also found in the MC by bone marrow cells when induced to proliferate with MC by epithelial cells. When the 45,000-d molecules was used in induced bone marrow cells to proliferate, once again the 30,000- and the 17,000-d molecules were secreted. Evidence is also provided that the 45,000-d molecule is produced by the monocyte-macrophage cells, and that it can induce Fc receptors or resident and elicited peritoneal macrophages. The possibility that the production of CSF is cell specific is discussed together with two models to explain the way in which these molecules can participate as proliferative (MGI-1) and differentiative (MGI-2) function in normal myeloid cell differentiation. Finally, a new terminology is proposed to classify this family of molecules.

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.

Analysis of murine megakaryocyte colony size and ploidy: effects of interleukin-3

Megakaryocytes develop from diploid precursor cells that, after variable numbers of mitoses, cease cell division and then undergo synchronous nuclear endoreduplication (endomitosis). Megakaryocyte colony formation represents an in-vitro model of these processes in which the number and ploidy of colony cells reflect the activity of the mitotic and endomitotic pathways, respectively. We have analyzed the size and ploidization of murine megakaryocyte colonies grown in agar and examined the influence of interleukin-3 (IL-3) on these parameters. Colonies were identified in situ by staining for acetylcholinesterase and the ploidy of colony cells was determined by fluorescence cytophotometry. More than 98% of the megakaryocytes that developed in culture could be analyzed. In cultures of unfractionated marrow cells stimulated by either pokeweed mitogen-stimulated spleen cell-conditioned medium (PWM-SCM, a crude source of megakaryocyte colony-stimulating activity) or IL-3, the modal ploidy of day-5 colony megakaryocytes was 16N (range 2N-128N). The distribution of colony size was described by an inverse exponential relationship. Colony size and geometric mean ploidy were inversely correlated under conditions of maximal stimulation with PWM-SCM and at all concentrations of IL-3 tested. Increasing concentrations of IL-3 in cultures of either unfractionated marrow cells or nonadherent T-lymphocyte-depleted (NATD) marrow cells stimulated similar dose-dependent increases in the mean size and numbers of megakaryocyte colonies but did not significantly alter their ploidy distribution. However, the mean ploidy of colony megakaryocytes in IL-3-stimulated cultures of NATD marrow cells was significantly less (P less than 0.001) than the mean ploidy of megakaryocytes in IL-3-stimulated cultures of unfractionated marrow cells. The mean ploidy of megakaryocytes, which developed in PWM-SCM-stimulated cultures, was not affected by initial accessory cell depletion. We conclude that the size and ploidy characteristics of day-5 murine megakaryocyte colonies are structured as continua and that IL-3 stimulates an increase in mean colony size and numbers without affecting ploidization. T-lymphocytes and adherent cells elaborate an activity which promotes endomitosis in vitro; factors in PWM-SCM can substitute for this activity.

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.

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.

The production of myeloid blood cells and their regulation during health and disease

The regulation of myelopoiesis in vivo most likely entails a complex set of interactions between cell-derived biomolecules and their target cells: hematopoietic stem and progenitor cells and accessory cells. Stimulating and suppressing factors have been characterized through in vitro studies, and their mechanisms of action in vitro and in vivo have begun to be elucidated. Among those factors being studied are the hematopoietic colony-stimulating factors (CSF): interleukin-3 (multi-CSF), granulocyte-macrophage-CSF, granulocyte-CSF, and macrophage-CSF; other molecules include erythropoietin, B-cell-stimulating factor-1, interleukin-1, interleukin-2, prostaglandin E, leukotrienes, acidic ferritins, lactoferrin, transferrin, the interferons-gamma, -alpha, and -beta, and the tumor necrosis factors-alpha and -beta (lymphotoxin). These factors interact to modulate blood cell production in vitro and in vivo. The proposed review characterizes these biomolecules biochemically and functionally, including receptor-ligand interactions and the secondary messengers within the cell which mediate their functional activity. The production and action of the molecules are described under conditions of hematopoietic disorders, as well as under normal conditions. Studies in vitro are correlated with studies in vivo using animal models to give an overall view of what is known about these molecules and their relevance physiologically and pathologically.

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.

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)

Purification of a membrane-derived human erythroid growth factor

We have purified erythroid burst-promoting activity (BPA) from human lymphocyte plasma membranes by detergent extraction followed by gel-filtration, ion-exchange, and hydroxylapatite chromatography. BPA is a heat-stable integral membrane glycoprotein of Mr 28,000 by gel filtration whose activity is eluted from NaDodSO4/polyacrylamide gels as a broad band at Mr 25,000-29,000. The growth stimulator appears to be erythroid-specific, stimulating proliferation of the human erythroid burst-forming unit (BFU-E) by up to 600% of control values when tested in serum-free bone marrow culture. In contrast, it is devoid of granulocyte/macrophage colony-stimulating factor activity and has a negligible effect on the formation of human megakaryocyte and mixed hematopoietic colonies. Polyclonal anti-lymphocyte membrane IgG, which neutralizes BPA expression in culture, completely absorbs BPA from all lymphocyte-derived sources [solubilized lymphocyte plasma membranes, membrane-containing vesicles shed into lymphocyte conditioned medium (LCM) and soluble vesicle-free LCM supernatants], suggesting that soluble and membrane-derived lymphocyte BPA are antigenically related. This membrane glycoprotein may be an important mediator of proximal cellular interactions that are known to promote erythropoiesis in vitro.

Improvement of culture conditions for human megakaryocytic and pluripotent progenitor cells by low oxygen tension

The effect of low oxygen tension on the growth of human hemopoietic progenitor cells in bone marrow was investigated using the semisolid methylcellulose colony assay. The clonal growth of granulocyte-macrophage progenitors (CFU-gm), early (BFU-e) and late (CFU-e) erythroid progenitors, megakaryocyte progenitors (CFU-meg) and pluripotent progenitors (CFU-mix) improved more markedly incubation at the low oxygen tension (5%) than in conventional air (20%). The thiol compound 2-mercaptoethanol had a strong additive effect on colony growth in conventional air, but little or no effect in the low oxygen tension. These results suggest that enhancement of colony growth in the low oxygen tension may be due to a decrease in the production of oxygen intermediates.

Increased biological activity of deglycosylated recombinant human granulocyte/macrophage colony-stimulating factor produced by yeast or animal cells

Human granulocyte/macrophage colony-stimulating factor (hGM-CSF) produced by several recombinant sources including Escherichia coli, yeast, and animal cells was studied. Recombinant animal cells produced hGM-CSF in low quantities and in multiple forms of varying size. Mammalian hGM-CSF was purified 200,000-fold using immunoaffinity and lectin chromatography. Partially purified proteins produced in yeast and mammalian cells were assayed for the effects of deglycosylation. Following enzymatic deglycosylation, immunoreactivity was measured by radioimmunoassay and biological activity was measured in vitro on responsive human primary cells. Removal of N-linked oligosaccharides from both proteins increased their immunoreactivities by 4- to 8-fold. Removal of these oligosaccharides also increased their specific biological activities about 20-fold, to reach approximately the specific activity of recombinant hGM-CSF from E. coli. The E. coli produced-protein--lacking any carbohydrate--had by far the highest specific activity observed for the recombinant hGM-CSFs.

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.