Novel monoclonal antibody reactive with thrombin-sensitive 74-kDa glycoproteins present on platelets and megakaryocytes both from mouse and rat

An expandable, inducible hemangioblast state regulated by fibroblast growth factor

During development, the hematopoietic and vascular lineages are thought to descend from common mesodermal progenitors called hemangioblasts. Here we identify six transcription factors, Gata2, Lmo2, Mycn, Pitx2, Sox17, and Tal1, that “trap” murine cells in a proliferative state and endow them with a hemangioblast potential. These “expandable” hemangioblasts (eHBs) are capable, once released from the control of the ectopic factors, to give rise to functional endothelial cells, multilineage hematopoietic cells, and smooth muscle cells. The eHBs can be derived from embryonic stem cells, from fetal liver cells, or poorly from fibroblasts. The eHBs reveal a central role for fibroblast growth factor, which not only promotes their expansion, but also facilitates their ability to give rise to endothelial cells and leukocytes, but not erythrocytes. This study serves as a demonstration that ephemeral progenitor states can be harnessed in vitro, enabling the creation of tractable progenitor cell lines.

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Gata2, Lmo2, Mycn, Pitx2, Sox17, and Tal1 induce and maintain a hemangioblast state

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FGF2 promotes the expansion of these progenitors and impacts their potency

Multipotential differentiation ability of GATA-1-null erythroid-committed cells

GATA-1, a zinc finger transcription factor, has been believed to be indispensable for the survival of proerythroblasts. However, we found that GATA-1-null proerythroblasts could survive and proliferate on OP9 stroma cells in the presence of erythropoietin. Furthermore, myeloid and mast cells were induced from the GATA-1-null proerythroblasts by the stimulation of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3), respectively, but lymphoid differentiation was not achieved by in vivo transfer. Thus, without activity of the transcription factor required for terminal differentiation, even relatively mature and committed cells proliferate continuously with the differentiation capacity to other lineages. Our data suggest that GATA-1 is a critical transcription factor to fix erythroid progenitors to the erythroid lineage.

Characterization of new monoclonal antibodies against porcine lymphocytes: molecular characterization of clone 7G3, an antibody reactive with the constant region of the T-cell receptor δ-chains

A battery of mouse monoclonal antibodies (mAbs) reactive with porcine peripheral blood (PB) leukocytes was generated. Among the mAbs, 6F10 was found to react probably with cluster of differentiation (CD)8 alpha-chain, while 7G3 and 3E12 were found to recognize gammadelta T-cells, as revealed by two-color flow cytometric and immunoprecipitation studies. 7G3 was shown to react with the constant (C) region of the T-cell receptor (TCR) delta-chain by the following facts: (1) 7G3 immunoprecipitated full-length TCR delta-chain protein fused with glutathione S-transferase (GST) produced by Esherichia coli and (2) 7G3 reacted with TCR delta-chain expressing Cos-7 cells transfected with either full-length or N-terminal deleted mutant cDNA, but did not react with Cos-7 cells transfected with C-terminal deleted mutant TCR delta-chain cDNA. All three mAbs produced high-quality immunostaining results on frozen sections, revealing a distinct distribution of gammadelta T-cells and CD8(+) cells. This report precisely characterizes mAbs against porcine TCR for the first time, facilitating molecular biological investigations of the porcine immune system.

Granulocyte colony-stimulating factor directly affects human monocytes and modulates cytokine secretion

OBJECTIVE

Recent reports have indicated that monocytes express receptors for the granulocyte colony-stimulating factor (G-CSF). The direct effects of G-CSF on cytokine secretion in monocytes were examined.

MATERIALS AND METHODS

A monocytic cell line NOMO-1 that secretes multiple cytokines upon stimulation with lipopolysaccharide (LPS) was used. Normal human monocytes were purified by negative selection using magnetic beads. Cells pretreated with or without G-CSF were stimulated with LPS, and the subsequent concentrations of cytokines and chemokines in supernatants were determined by sandwich enzyme-linked immunosorbent assay.

RESULTS

NOMO-1 cells were found to express receptors for G-CSF. Although G-CSF stimulation did not induce cytokine secretion, pretreatment with G-CSF significantly attenuated LPS-stimulated secretion of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin (IL)-12 in NOMO-1 cells. Simultaneously, however, G-CSF pretreatment apparently enhanced LPS-induced secretion of IL-10 and monocyte chemoattractant protein-1, whereas secretions of IL-1beta, IL-6, and IL-8 were unaffected. When normal human monocytes from healthy volunteers were similarly examined, marked individual variations in LPS-induced secretion of cytokines were observed. Although some exceptions exist, a similar tendency as to the effects of G-CSF treatment on cytokine secretions as that in NOMO-1 cells was observed in human monocytes.

CONCLUSIONS

Our data suggest that G-CSF directly affects monocytes and modulates their cytokine secretion. NOMO-1 cells can provide an alternate model for in vitro culture of monocytes to investigate the effects of G-CSF on cytokine secretion by these cells.

GATA-2 and GATA-2/ER display opposing activities in the development and differentiation of blood progenitors

GATA-2 is a zinc finger transcription factor essential for the development of hematopoiesis. While GATA-2 is generally considered to play an important role in the biology of hematopoietic stem and progenitor cells, its function within these compartments is not well understood. Here we have employed both conditional expression of GATA-2 and conditional activation of a GATA-2/estrogen receptor (ER) chimera to examine the effect of enforced GATA-2 expression in the development and differentiation of hematopoietic progenitors from murine embryonic stem cells. Consistent with the phenotype of GATA-2 null animals, conditional expression of GATA-2 from a tetracycline-inducible promoter enhanced the production of hematopoietic progenitors. Conditional activation of a GATA-2/ER chimera produced essentially opposite effects to those observed with conditional GATA-2 expression. GATA-2 and GATA-2/ER differ in their binding activities and transcriptional interactions from other hematopoietic-associated transcription factors such as c-Myb and PU.1. While we have exploited these differences in activity to explore the transcriptional networks underlying hematopoietic cell fate determination, our results suggest that care should be taken in interpreting results obtained using only chimeric proteins.

Characterization of monoclonal antibodies against mouse and rat platelet glycoprotein V (CD42d)

The mouse- and rat-platelet-specific hamster monoclonal antibody (MAb) 1C2, previously found to react with a thrombin-sensitive 74-kD glycoprotein, was now shown to recognize platelet glycoprotein V (GPV, CD42d). 1C2 reacted with NIH-3T3 cells in which recombinant mouse or rat GPV was expressed. Both 1C2 and 4A5, another mouse-platelet-specific rat MAb, immunoprecipitated GVP, although they recognized different epitopes. Side-by-side comparison confirmed that 1C2 as well as RPM.9, a MAb against rat GPV, recognized the same rat platelet molecule. In a mouse bone marrow culture, 1C2+ megakaryocytes emerged from CD41 (GPIIb)+1C2- megakaryocytes. Because 1C2+ megakaryocytes exhibited higher DNA ploidy distribution than CD41+ cells, GPV likely appears in the late stage of megakaryocyte maturation. This study established 1C2 as a MAb against mouse and rat GPV, namely CD42d, and as useful tool to study rodent megakaryopoiesis.

Functional conservation of platelet glycoprotein V promoter between mouse and human megakaryocytes

OBJECTIVE

In an attempt to clarify the megakaryo-specific regulatory mechanism of GPV gene transcription, we characterized the 5'-flanking region of the mouse GPV gene.

MATERIALS AND METHODS

The promotor activity of a -481/+22 5'-fragment of the mouse GPV gene was examined in normal mouse bone marrow cells (BMC) and various human cell lines using two distinct reporter gene assay systems, luciferase and green fluorescence protein (GFP).

RESULTS

When a DNA construct consisting of this fragment and a GFP reporter gene were transiently expressed in thrombopoietin-supported mouse BMC culture, GFP was identified only in megakaryocytes. The same construct expressed high levels of GFP in the human megakaryocytic Dami line. When assessed by dual luciferase assay, the full -481/+22 fragment could drive variable promoter activity in human as well as mouse megakaryocytic lines but did not work in non-megakaryocytic cells. Sufficient transcriptional activation of this fragment was restricted to the cells expressing apparent GPV mRNA. A deletion and point mutation study indicated that GATA and Ets motifs, typical cis-acting elements for platelet-specific genes, located of -75 and -46, respectively, were essential for promoter function.

CONCLUSION

The GPV promoter has the general characteristics found in platelet-specific genes, and the mechanism for megakaryocyte-specific, maturation-dependent regulation of GPV gene transcription is highly conserved between mouse and human. Analysis of GPV transcription mechanism utilizing human lines as well as BMC should provide new information on the final maturational process of megakaryocytes.

Stromal cell-derived factor-1 (SDF-1) acts together with thrombopoietin to enhance the development of megakaryocytic progenitor cells (CFU-MK)

Stromal cell-derived factor-1 (SDF-1) is a CXC chemokine that acts as a stimulator of pre-B lymphocyte cell growth and as a chemoattractant for T cells, monocytes, and hematopoietic stem cells. More recent studies also suggest that megakaryocytes migrate in response to SDF-1. Because genetic elimination of SDF-1 or its receptor lead to marrow aplasia, we investigated the effect of SDF-1 on megakaryocyte progenitors (colony-forming units-megakaryocyte [CFU-MK]). We report that SDF-1 augments the growth of CFU-MK from whole murine bone marrow cells when combined with thrombopoietin (TPO). The addition of SDF-1 to interleukin-3 (IL-3) or stem cell factor (SCF) had no effect. Specific antagonists for CXCR4 (the sole receptor for SDF-1), T22, and 1-9 (P2G) SDF-1 reduced megakaryocyte colony growth induced by TPO alone, suggesting that many culture systems contain endogenous levels of the chemokine that contributes to the TPO effect. To examine whether SDF-1 has direct effects on CFU-MK, we developed a new protocol to purify megakaryocyte progenitors. CFU-MK were highly enriched in CD41high c-kithigh cells generated from lineage-depleted TPO-primed marrow cells. Because the growth-promoting effects of SDF-1 were also observed when highly purified populations of CFU-MK were tested in serum-free cultures, these results suggest that SDF-1 directly promotes the proliferation of megakaryocytic progenitors in the presence of TPO, and in this way contributes to the favorable effects of the bone marrow microenvironment on megakaryocyte development.

Differential Effects of Human Granulocyte Colony-Stimulating Factor (hG-CSF) and Thrombopoietin on Megakaryopoiesis and Platelet Function in hG-CSF Receptor-Transgenic Mice

Granulocyte-colony stimulating factor (G-CSF) has been found to act on the neutrophilic lineage. We recently showed that human G-CSF (hG-CSF) has effects similar to early-acting cytokines such as interleukin-3 (IL-3) in the development of multipotential hematopoietic progenitors in transgenic (Tg) mice expressing receptors (R) for hG-CSF. In the present study, we examined the effects of hG-CSF on more mature hematopoietic cells committed to megakaryocytic lineage in these Tg mice. The administration of hG-CSF to the Tg mice increased the numbers of both platelets in peripheral blood and megakaryocytes in the spleen, indicating that hG-CSF stimulates megakaryopoiesis in the Tg mice in vivo. The stimulatory effect of hG-CSF was also supported by the results of studies in vitro. hG-CSF supported megakaryocyte colony formation in a dose-dependent fashion in clonal cultures of bone marrow cells derived from the Tg mice. Direct effects of hG-CSF on megakaryocytic progenitors in the Tg mice were confirmed by culture of single-cell sorted from bone marrow cells. hG-CSF showed a stronger effect on maturation of megakaryocytes in the Tg mice than that of IL-3 alone, but weaker than that of TPO alone. In addition, hG-CSF induced phosphorylation of STAT3 but not Jak2 or STAT5, while TPO induced phosphorylation of both. In contrast to TPO, hG-CSF did not enhance ADP-induced aggregation. Thus, hG-CSF has a wide variety of functions in megakaryopoiesis of hG-CSFR-Tg mice, as compared with other megakaryopoietic cytokines, but the activity of hG-CSF in megakaryocytes and platelets does not stand up to a comparison with that of TPO. Specific signals may be required for the full maturation and activation of platelets.

Collagen Mediates Changes in Intracellular Calcium in Primary Mouse Megakaryocytes Through syk-Dependent and -Independent Pathways

We have characterized changes in [Ca2+]iin primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin 2β1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59fyn and p72syk. This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin 2β1. Collagen elicited a sustained increase in [Ca2+]i composed primarily of influx of extracellular Ca2+ with some Ca2+release from internal stores. In contrast to CRP, this response was only partially (∼30%) inhibited by the src-family kinase inhibitor PP1 (10 μmol/L) or by microinjection of the tandem SH2 domains of p72syk. Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59fyn or p72syk, although the response was reduced by approximately 40% in both cases: Cross-linking of the 2 integrin increased [Ca2+]iin these cells exclusively via Ca2+ influx. This response was reduced by approximately 50% after PP1 pretreatment, but was significantly increased in fyn-deficient megakaryocytes. Collagen therefore increases [Ca2+]i in mouse megakaryocytes via multiple receptors, including GPVI, which causes Ca2+ mobilization, and 2β1, which stimulates a substantial influx of extracellular Ca2+.

Collagen Mediates Changes in Intracellular Calcium in Primary Mouse Megakaryocytes Through syk-Dependent and -Independent Pathways

We have characterized changes in [Ca2+]iin primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin 2β1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59fyn and p72syk. This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin 2β1. Collagen elicited a sustained increase in [Ca2+]i composed primarily of influx of extracellular Ca2+ with some Ca2+release from internal stores. In contrast to CRP, this response was only partially (∼30%) inhibited by the src-family kinase inhibitor PP1 (10 μmol/L) or by microinjection of the tandem SH2 domains of p72syk. Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59fyn or p72syk, although the response was reduced by approximately 40% in both cases: Cross-linking of the 2 integrin increased [Ca2+]iin these cells exclusively via Ca2+ influx. This response was reduced by approximately 50% after PP1 pretreatment, but was significantly increased in fyn-deficient megakaryocytes. Collagen therefore increases [Ca2+]i in mouse megakaryocytes via multiple receptors, including GPVI, which causes Ca2+ mobilization, and 2β1, which stimulates a substantial influx of extracellular Ca2+.

Expression and functional analysis of a hemopoietic progenitor antigen, NJ-1 (114/A10), in the megakaryocytic lineage

To analyze the functions of molecules expressed in hemopoietic progenitor cells, we obtained several monoclonal antibodies by immunizing Wistar rats with antigens from a murine immature leukemic cell line, DA-1. Here, we characterize one antibody designated NJ-1, which recognizes a 145-kd molecule, and identify the cDNA encoding the NJ-1 antigen by retrovirus-mediated expression cloning. Sequence analysis of the cDNA reveals that it is identical to a previously reported cDNA encoding a surface molecule of 573 amino acids recognized by monoclonal antibody 114/A10. Our studies show that expression of NJ-1 antigen is upregulated in a murine megakaryoblastic cell line, L8057, when it differentiates into a megakaryocytic lineage in response to 12-O-tetradecanoyl phorbol-13-acetate. Overexpression of NJ-1 antigen in L8057 cells inhibits cell adhesion to fibronectin, suggesting that it may act as a negative regulator of cell adhesion in the megakaryocytic lineage.

A Single Intravenous Dose of Murine Megakaryocyte Growth and Development Factor Potently Stimulates Platelet Production, Challenging the Necessity for Daily Administration

The thrombopoietic efficacy of recombinant forms of c-mplligand is being actively investigated in preclinical studies using daily dosing schedules. However, a comprehensive kinetic study of the thrombopoietic response to a single injection of recombinant c-mpl ligand has not been performed. Here, we present the results of a detailed kinetic analysis of the platelet response to a single intravenous administration of pegylated recombinant murine megakaryocyte growth and development factor (PEG-rmMGDF) in mice. In addition, we compare the efficacy of single versus daily dosing in stimulating platelet production. A single intravenous injection of PEG-rmMGDF produced a marked and dose-dependent elevation in platelet number and a moderate increase in mean platelet volume (MPV). After administration of 25 or 250 μg/kg of PEG-rmMGDF, platelet number was first increased on day 3 and peaked at 2.7-fold (25 μg/kg) and 5.7-fold of normal (250 μg/kg) on day 5. Thereafter, platelet number declined and returned to baseline by days 9 and 14, with the 25 and 250 μg/kg doses, respectively. MPV began to increase on day 2 after PEG-rmMGDF, reaching maximum values of 1.2-fold (25 μg/kg) and 1.5-fold of normal (250 μg/kg) on day 4. Subsequently, MPV declined and was downregulated on days 6 to 7 (25 μg/kg) and day 8 (250 μg/kg). Based on these results, we evaluated the platelet response to PEG-rmMGDF administered intravenously as a single dose versus daily for 5 days. A single administration of 100 μg/kg produced a higher platelet number on day 5 than daily administration of 100 or 20 μg/kg for 5 days. However, the thrombocytosis was less sustained after single versus daily dosing. The smaller platelet number increase on day 5 after daily dosing reflected the production of larger platelets, rather than suppression of thrombopoiesis. Our results indicate that PEG-rmMGDF administered as a single intravenous dose potently stimulates platelet production in mice, challenging the need for its daily administration. Adoption of an intermittent administration schedule of this cytokine could be more efficacious and is merited in future clinical trials.

A Single Intravenous Dose of Murine Megakaryocyte Growth and Development Factor Potently Stimulates Platelet Production, Challenging the Necessity for Daily Administration

The thrombopoietic efficacy of recombinant forms of c-mplligand is being actively investigated in preclinical studies using daily dosing schedules. However, a comprehensive kinetic study of the thrombopoietic response to a single injection of recombinant c-mpl ligand has not been performed. Here, we present the results of a detailed kinetic analysis of the platelet response to a single intravenous administration of pegylated recombinant murine megakaryocyte growth and development factor (PEG-rmMGDF) in mice. In addition, we compare the efficacy of single versus daily dosing in stimulating platelet production. A single intravenous injection of PEG-rmMGDF produced a marked and dose-dependent elevation in platelet number and a moderate increase in mean platelet volume (MPV). After administration of 25 or 250 μg/kg of PEG-rmMGDF, platelet number was first increased on day 3 and peaked at 2.7-fold (25 μg/kg) and 5.7-fold of normal (250 μg/kg) on day 5. Thereafter, platelet number declined and returned to baseline by days 9 and 14, with the 25 and 250 μg/kg doses, respectively. MPV began to increase on day 2 after PEG-rmMGDF, reaching maximum values of 1.2-fold (25 μg/kg) and 1.5-fold of normal (250 μg/kg) on day 4. Subsequently, MPV declined and was downregulated on days 6 to 7 (25 μg/kg) and day 8 (250 μg/kg). Based on these results, we evaluated the platelet response to PEG-rmMGDF administered intravenously as a single dose versus daily for 5 days. A single administration of 100 μg/kg produced a higher platelet number on day 5 than daily administration of 100 or 20 μg/kg for 5 days. However, the thrombocytosis was less sustained after single versus daily dosing. The smaller platelet number increase on day 5 after daily dosing reflected the production of larger platelets, rather than suppression of thrombopoiesis. Our results indicate that PEG-rmMGDF administered as a single intravenous dose potently stimulates platelet production in mice, challenging the need for its daily administration. Adoption of an intermittent administration schedule of this cytokine could be more efficacious and is merited in future clinical trials.

A Single Injection of Pegylated Murine Megakaryocyte Growth and Development Factor (MGDF ) Into Mice Is Sufficient to Produce a Profound Stimulation of Megakaryocyte Frequency, Size, and Ploidization

Despite numerous studies investigating the action of c-mpl ligand, no reports have defined the in vivo changes in megakaryocytopoiesis in response to a single injection of this cytokine. Here we compare the kinetics of the megakaryocytopoietic response in C57Bl/6J mice administered 25 μg/kg or 250 μg/kg of pegylated (PEG) murine megakaryocyte growth and development factor (MGDF ) as a single intravenous injection. Megakaryocytes of mice treated with MGDF had normal ultrastructure, showing a typical distribution of the demarcation membrane system, α-granules, and other cytoplasmic organelles. Megakaryocyte ploidy, size, and frequency were markedly increased with both MGDF doses. Megakaryocyte ploidy was maximally increased from a modal value of 16N to 64N on day 3, with both doses of MGDF. Similarly, a comparable increase in megakaryocyte size occurred in the two MGDF groups. Increased megakaryocyte size was coupled to the increase in megakaryocyte ploidy, and no evidence for independent regulation of megakaryocyte size within individual ploidy classes was apparent. In contrast to megakaryocyte ploidy and size, the increase in megakaryocyte frequency was markedly different with the two doses of MGDF. The proportion of 2N and 4N cells was increased from a baseline of 0.035% to 0.430% by day 4 in mice treated with the higher dose of MGDF, but only to 0.175% in mice administered 25 μg/kg of MGDF. The marked increase in the pool of these immature megakaryocytes translated to a sustained elevation in the frequency of polyploid megakaryocytes (8N cells and greater). In contrast to the sustained increase in the frequency of polyploid cells, the level of polyploidization was downregulated on days 6 to 10, but normalized by day 14. We conclude that a single injection of MGDF is able to expand the megakaryocytic pool in a dose-dependent manner, which, with subsequent maturation, should lead to an increased rate of platelet production.

Overexpression of granulocyte colony-stimulating factor in vivo decreases the level of polyploidization of mouse bone marrow megakaryocytes

The in vivo effect of G-CSF on the maturation of mouse bone marrow megakaryocytes was studied by monitoring the DNA contents. Megakaryocytes were first identified by a specific 1C2 monoclonal antibody against mouse platelets and megakaryocytes and DNA contents of these cells were measured by propidium iodine. Megakaryocytes of mice transgenic for human G-CSF had a modal DNA class of 8N, showing a striking contrast to the previous reports that normal mouse megakaryocytes from most strains have 16N DNA content as a modal class. Daily 10 micrograms administration of G-CSF to mice for three to five days affected the DNA distribution pattern of bone marrow megakaryocytes, with a higher proportion of cells having 8N DNA contents. This G-CSF treatment, however, did not influence the peripheral blood platelet count or bone marrow megakaryocyte number. Administration of G-CSF along with thrombopoietin (TPO) reduced the proportion of megakaryocytes, with 32N DNA, the DNA class that was increased by TPO. Finally, the presence of mRNA for the mouse G-CSF receptor was demonstrated in two megakaryoblastic cell lines by reverse transcriptase polymerase chain reaction. These results indicated that G-CSF may have a suppressive effect on the maturation of mouse bone marrow megakaryocytes when monitored by the DNA polyploidy. Although further study is clearly necessary, the presence of mRNA for the G-CSF receptor in megakaryocytic lineage strongly suggests the direct action of G-CSF on this cell lineage.