Direct proliferative actions of stem cell factor on murine bone marrow cells in vitro: effects of combination with colony-stimulating factors

Hematopoietic Stem Cells and Regeneration

Hematopoietic stem cell (HSC) regeneration is the remarkable process by which extremely rare, normally inactive cells of the bone marrow can replace an entire organ if called to do so by injury or harnessed by transplantation. HSC research is arguably the first quantitative single-cell science and the foundation of adult stem cell biology. Bone marrow transplant is the oldest and most refined technique of regenerative medicine. Here we review the intertwined history of the discovery of HSCs and bone marrow transplant, the molecular and cellular mechanisms of HSC self-renewal, and the use of HSCs and their derivatives for cell therapy.

Unbiased phenotypic identification of functionally distinct hematopoietic progenitors

Background

Hematopoiesis is a model-system for studying cellular development and differentiation. Phenotypic and functional characterization of hematopoietic progenitors has significantly aided our understanding of the mechanisms that govern fate choice, lineage specification and maturity. Methods for progenitor isolation have historically relied on complex flow-cytometric strategies based on nested, arbitrary gates within defined panels of immunophenotypic markers. The resulted populations are then functionally assessed, although functional homogeneity or absolute linkage between function and phenotype is not always achieved, thus distorting our view on progenitor biology.

Method

In this study, we present a protocol for unbiased phenotypic identification and functional characterization which combines index sorting and clonogenic assessment of individual progenitor cells. Single-cells are plated into custom media allowing multiple hematopoietic fates to emerge and are allowed to give rise to unilineage colonies or mixed. After colony identification, lineage potential is assigned to each progenitor and finally the indexed phenotype of the initial cell is recalled and a phenotype is assigned to each functional output.

Conclusions

Our approach overcomes the limitations of the current protocols expanding beyond the established cell-surface marker panels and abolishing the need for nested gating. Using this method we were able to resolve the relationships of myeloid progenitors according to the revised model of hematopoiesis, as well as identify a novel marker for erythroid progenitors. Finally, this protocol can be applied to the characterization of any progenitor cell with measurable function.

Electronic supplementary material

The online version of this article (10.1186/s40709-019-0097-7) contains supplementary material, which is available to authorized users.

Impaired haematopoietic stem cell differentiation and enhanced skewing towards myeloid progenitors in aged caspase-2-deficient mice

The apoptotic cysteine protease caspase-2 has been shown to suppress tumourigenesis in mice and its reduced expression correlates with poor prognosis in some human malignancies. Caspase-2-deficient mice develop normally but show ageing-related traits and, when challenged by oncogenic stimuli or certain stress, show enhanced tumour development, often accompanied by extensive aneuploidy. As stem cells are susceptible to acquiring age-related functional defects because of their self-renewal and proliferative capacity, we examined whether loss of caspase-2 promotes such defects with age. Using young and aged Casp2^−/− mice, we demonstrate that deficiency of caspase-2 results in enhanced aneuploidy and DNA damage in bone marrow (BM) cells with ageing. Furthermore, we demonstrate for the first time that caspase-2 loss results in significant increase in immunophenotypically defined short-term haematopoietic stem cells (HSCs) and multipotent progenitors fractions in BM with a skewed differentiation towards myeloid progenitors with ageing. Caspase-2 deficiency leads to enhanced granulocyte macrophage and erythroid progenitors in aged mice. Colony-forming assays and long-term culture-initiating assay further recapitulated these results. Our results provide the first evidence of caspase-2 in regulating HSC and progenitor differentiation, as well as aneuploidy, in vivo.

Exploring erythropoiesis of common carp (Cyprinus carpio) using an in vitro colony assay in the presence of recombinant carp kit ligand A and erythropoietin

The use of in vitro colony assays in mammals has contributed to identification of erythroid progenitor cells such as burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) progenitors, and serves to examine functions of erythropoietic growth factors like Erythropoietin (Epo) and Kit ligand. Here, we established an in vitro colony-forming assay capable of investigating erythropoiesis in carp (Cyprinus carpio), cloned and functionally characterized recombinant homologous molecules Epo and Kit ligand A (Kitla), and identified three distinct erythroid progenitor cells in carp. Recombinant carp Epo induced the formation of CFU-E-like and BFU-E-like erythroid colonies, expressing erythroid marker genes, β-globin, epor and gata1. Recombinant carp Kitla alone induced limited colony formation, whereas a combination of Kitla and Epo dramatically enhanced erythroid colony formation and colony cell growth, as well as stimulated the formation of thrombocytic/erythroid colonies expressing not only erythroid markers but also thrombocytic markers, cd41 and c-mpl. Utilizing this colony assay to examine the distribution of distinct erythroid progenitor cells in carp, we demonstrated that carp head and trunk kidney play a primary role in erythropoiesis, while the spleen plays a secondary. Furthermore, we showed that presumably bi-potent thrombocytic/erythroid progenitor cells localize principally in the trunk kidney. Our results indicate that teleost fish possess mechanisms of Epo- and Kitla-dependent erythropoiesis similar to those in other vertebrates, and also help to demonstrate the diversity of erythropoietic sites among vertebrates.

c-Kit and stem cell factor regulate PANC-1 cell differentiation into insulin- and glucagon-producing cells

Recent evidence has shown that stem cell factor (SCF) and its receptor, c-Kit, have an important role in pancreatic islet development by promoting islet cell differentiation and proliferation. In this study, we examined the role of c-Kit and SCF in the differentiation and proliferation of insulin- and glucagon-producing cells using a human pancreatic duct cell line (PANC-1). Our study showed that increased expression of endocrine cell markers (such as insulin and glucagon) and transcription factors (such as PDX-1 and PAX-6) coincided with a decrease in CK19(+) and c-Kit(+) cells (P<0.001) during PANC-1 cell differentiation, determined by immunofluorescence and qRT-PCR. Cells cultured with exogenous SCF showed an increase in insulin(+) (26%) and glucagon(+) (35%) cell differentiation (P<0.01), an increase in cell proliferation (P<0.05) and a decrease in cell apoptosis (P<0.01). siRNA knockdown of c-Kit resulted in a decrease in endocrine cell differentiation with a reduction in PDX-1 and insulin mRNA, as well as the number of cells immunostaining for PDX-1 and insulin. Taken together, these results show that c-Kit/SCF interactions are involved in mediating islet-like cluster formation and islet-like cell differentiation in a human pancreatic duct cell line.

The colony-stimulating factors and cancer

The four colony-stimulating factors (CSFs) are glycoproteins that regulate the generation and some functions of infection-protective granulocytes and macrophages. Recombinant granulocyte-CSF (G-CSF) and granulocyte-macrophage-CSF (GM-CSF) have now been used to increase dangerously low white blood cell levels in many millions of cancer patients following chemotherapy. These CSFs also release haematopoietic stem cells to the peripheral blood, and these cells have now largely replaced bone marrow as more effective populations for transplantation to cancer patients who have treatment-induced bone marrow damage.

beta-Catenin expression in the bone marrow microenvironment is required for long-term maintenance of primitive hematopoietic cells

Hematopoiesis is dependent upon the bone marrow microenvironment, which is comprised of multiple mesenchymal cell types, including fibroblasts, endothelial cells, osteoblasts, and stroma progenitors. The canonical Wnt signaling pathway, which relies on the beta-catenin protein to mediate its signal, is necessary for the normal development of mesenchymal tissue. We hypothesized that canonical Wnt signaling regulates the cellular composition and function of the bone marrow microenvironment. We observed that a beta-catenin-deficient bone marrow microenvironment maintained hematopoietic stem cells but exhibited a decreased capacity to support primitive hematopoietic cells. These results correlated with decreased numbers of osteoblasts and with decreased production of basic fibroblast growth factor, stem cell factor, and vascular cell adhesion molecule-1. From these data, we propose a model in which beta-catenin in the microenvironment is required noncell autonomously for long-term maintenance of hematopoietic progenitors.

Socs3 maintains the specificity of biological responses to cytokine signals during granulocyte and macrophage differentiation

Granulocyte colony-stimulating factor (G-CSF) and interleukin-6 (IL-6) play key roles in regulating emergency granulopoiesis and inflammation, and are both negatively regulated by the inducible intracellular protein suppressor of cytokine signaling-3 (Socs3). Mice with Socs3 deleted specifically in hematopoietic cells succumb to severe neutrophil and macrophage-driven inflammation by 1 year of age, and responses to G-CSF are grossly exacerbated. In order to determine which elements of cellular responses to cytokines require Socs3, we have examined the differentiative and proliferative capacity of hematopoietic progenitor cells stimulated by G-CSF and IL-6. The differentiation of Socs3-deficient progenitor cells is skewed toward macrophage production in response to G-CSF or IL-6, whereas wild-type progenitor cells produce mainly neutrophils. The proliferative capacity of Socs3-deficient progenitor cells is greatly enhanced in response to G-CSF at all concentrations, but only at low concentrations for IL-6. Strikingly, synergistic responses to costimulation with stem cell factor and IL-6 (but not G-CSF) are lost at higher concentrations in Socs3-deficient progenitor cells. Cytokine-induced expression of transcriptional regulators including cebpb, Ets2, Bcl3, c-Myc, Jun, and Fosl2 are differentially regulated in Socs3-deficient cells. The tight regulation by Socs3 of signal transducer and activator of transcription 3 phosphorylation and gene transcription after cytokine receptor ligation significantly influences the fate of myeloid progenitor cells.

Lhx2 expression promotes self-renewal of a distinct multipotential hematopoietic progenitor cell in embryonic stem cell-derived embryoid bodies

The molecular mechanisms regulating the expansion of the hematopoietic system including hematopoietic stem cells (HSCs) in the fetal liver during embryonic development are largely unknown. The LIM-homeobox gene Lhx2 is a candidate regulator of fetal hematopoiesis since it is expressed in the fetal liver and Lhx2^−/− mice die in utero due to severe anemia. Moreover, expression of Lhx2 in embryonic stem (ES) cell-derived embryoid bodies (EBs) can lead to the generation of HSC-like cell lines. To further define the role of this transcription factor in hematopoietic regulation, we generated ES cell lines that enabled tet-inducible expression of Lhx2. Using this approach we observed that Lhx2 expression synergises with specific signalling pathways, resulting in increased frequency of colony forming cells in developing EB cells. The increase in growth factor-responsive progenitor cells directly correlates to the efficiency in generating HSC-like cell lines, suggesting that Lhx2 expression induce self-renewal of a distinct multipotential hematopoietic progenitor cell in EBs. Signalling via the c-kit tyrosine kinase receptor and the gp130 signal transducer by IL-6 is necessary and sufficient for the Lhx2 induced self-renewal. While inducing self-renewal of multipotential progenitor cells, expression of Lhx2 inhibited proliferation of primitive erythroid precursor cells and interfered with early ES cell commitment, indicating striking lineage specificity of this effect.

Stem cell c-KIT and HOXB4 genes: critical roles and mechanisms in self-renewal, proliferation, and differentiation

Hematopoietic stem cells (HSCs) possess a distinct ability to perpetuate through self-renewal and to generate progeny that differentiate into mature cells of myeloid and lymphoid lineages. A better understanding of the molecular mechanisms by which HSCs replicate and differentiate from the perspective of developing new approaches for HSC transplantation is necessary for further advances. The interaction of the receptor tyrosine kinase--c-KIT--with its ligand stem cell factor plays a key role in HSC survival, mitogenesis, proliferation, differentiation, adhesion, homing, migration, and functional activation. Evidence that activating site-directed point mutations in the c-KIT gene contributes to its ligand-independent constitutive activation, which induces enhanced proliferation of HSCs, is accumulating. Similarly, and equally important, self-renewal is a process by which HSCs generate daughter cells via division. Self-renewal is necessary for retaining the HSC pool. Therefore, elucidating the molecular machinery that governs self-renewal is of key importance. The transcription factor, HOXB4 is a key molecule that has been reported to induce the in vitro expansion of HSCs via self-renewal. However, critical downstream effector molecules of HOXB4 remain to be determined. This concisely reviewed information on c-KIT and HOXB4 helps us to update our understanding of their function and mechanism of action in self-renewal, proliferation, and differentiation of HSCs, particularly modulation by c-KIT mutant interactions, and HOXB4 overexpression showing certain therapeutic implications.

Expression of a novel recombinant stem cell factor/macrophage colony-stimulating factor fusion protein in baculovirus-infected insect cells

A novel human stem cell factor (SCF)/macrophage colony-stimulating factor (M-CSF) fusion protein gene was constructed, in which the coding regions of human SCF cDNA (1-165aa) and the truncated M-CSF cDNA (1-149aa) were connected by a linker sequence encoding a short peptide GGGGSGGGGSGG. The SCF/M-CSF gene was cloned into baculovirus transfer vector pVL1392 under the control of polyhedrin promoter and expressed in the Sf9 cells (Spodoptera frugiperda). SDS-PAGE and Western blot analysis showed that the purified fusion protein was a homodimer with a molecular weight about 84kDa under non-reducing conditions or a monomer about 42kDa under reducing conditions. The specific activity of rhSCF/M-CSF was 17 times as high as that of monomeric rhSCF to stimulate the proliferation of TF-1 cell. The results of macrophages colony-forming (CFU-M) assay performed with human bone marrow mononuclear cells demonstrated that rhSCF/M-CSF was more potent in promoting CFU-M than the equimolar of SCF, M-CSF or that of two cytokines mixture.

Design of Recombinant Stem Cell Factor–macrophage Colony Stimulating Factor Fusion Proteins and their Biological Activity In Vitro

Stem cell factor (SCF) and macrophage colony stimulating factor (M-CSF) can act in synergistic way to promote the growth of mononuclear phagocytes. SCF-M-CSF fusion proteins were designed on the computer using the Homology and Biopolymer modules of the software packages InsightII. Several existing crystal structures were used as templates to generate models of the complexes of receptor with fusion protein. The structure rationality of the fusion protein incorporated a series of flexible linker peptide was analyzed on InsightII system. Then, a suitable peptide GGGGSGGGGSGG was chosen for the fusion protein. Two recombinant SCF-M-CSF fusion proteins were generated by construction of a plasmid in which the coding regions of human SCF (1-165aa) and M-CSF (1-149aa) cDNA were connected by this linker peptide coding sequence followed by subsequent expression in insect cell. The results of Western blot and activity analysis showed that these two recombinant fusion proteins existed as a dimer with a molecular weight of approximately 84 KD under non-reducing conditions and a monomer of approximately 42 KD at reducing condition. The results of cell proliferation assays showed that each fusion protein induced a dose-dependent proliferative response. At equimolar concentration, SCF/M-CSF was about 20 times more potent than the standard monomeric SCF in stimulating TF-1 cell line growth, while M-CSF/SCF was 10 times of monomeric SCF. No activity difference of M-CSF/SCF or SCF/M-CSF to M-CSF (at same molar) was found in stimulating the HL-60 cell linear growth. The synergistic effect of SCF and M-CSF moieties in the fusion proteins was demonstrated by the result of clonogenic assay performed with human bone mononuclear, in which both SCF/M-CSF and M-CSF/SCF induced much higher number of CFU-M than equimolar amount of SCF or M-CSF or that of two cytokines mixture.

Circulation and chemotaxis of fetal hematopoietic stem cells

The major site of hematopoiesis transitions from the fetal liver to the spleen and bone marrow late in fetal development. To date, experiments have not been performed to evaluate functionally the migration and seeding of hematopoietic stem cells (HSCs) during this period in ontogeny. It has been proposed that developmentally timed waves of HSCs enter the bloodstream only during distinct windows to seed the newly forming hematopoietic organs. Using competitive reconstitution assays to measure HSC activity, we determined the localization of HSCs in the mid-to-late gestation fetus. We found that multilineage reconstituting HSCs are present at low numbers in the blood at all timepoints measured. Seeding of fetal bone marrow and spleen occurred over several days, possibly while stem cell niches formed. In addition, using dual-chamber migration assays, we determined that like bone marrow HSCs, fetal liver HSCs migrate in response to stromal cell-derived factor-1α (SDF-1α); however, unlike bone marrow HSCs, the migratory response of fetal liver HSCs to SDF-1α is greatly increased in the presence of Steel factor (SLF), suggesting an important role for SLF in HSC homing to and seeding of the fetal hematopoietic tissues. Together, these data demonstrate that seeding of fetal organs by fetal liver HSCs does not require large fluxes of HSCs entering the fetal bloodstream, and that HSCs constitutively circulate at low levels during the gestational period from 12 to 17 days postconception. Newly forming hematopoietic tissues are seeded gradually by HSCs, suggesting initial seeding is occurring as hematopoietic niches in the spleen and bone marrow form and become capable of supporting HSC self-renewal. We demonstrate that fetal and adult HSCs exhibit specific differences in chemotactic behavior. While both migrate in response to SDF-1α, fetal HSCs also respond significantly to the cytokine SLF. In addition, the combination of SDF-1α and SLF results in substantially enhanced migration of fetal HSCs, leading to migration of nearly all fetal HSCs in this assay. This finding indicates the importance of the combined effects of SLF and SDF-1α in the migration of fetal HSCs, and is, to our knowledge, the first demonstration of a synergistic effect of two chemoattractive agents on HSCs.

New results on the migratory behavior of blood cell precursors in the early embryo might be relevant to bone marrow transplants and other clinical therapies

Radiation protective effect of an extract from Chelidonium majus

We earlier reported that CM-AIa isolated from Chelidonium majus had mitogenic activity, generated lymphokine-activated killer cells, and increased the number of granulocyte-macrophage colony-forming cells (GM-CFC). In an extended effort to search for other immunostimulatory effects, we evaluated the protective effects of in vivo injected CM-AIa against irradiation. CM-AIa was found to increase the number of bone marrow cells, spleen cells, GM-CFC, and platelets in irradiated mice. In addition, this agent induced endogenous production of cytokines such as interleukin 1 and tumor necrosis factor alpha, which are required for hematopoietic recovery. We also demonstrated that CM-AIa treatment 24 hours before irradiation protected mice with 80% survival at lethal dose 100/15. These findings indicate that CM-AIa may be a useful agent for reducing the time needed for reconstitution of hematopoietic cells after irradiation treatment.

Radioprotective effects of ginsan, an immunomodulator

We previously reported that ginsan, a purified polysaccharide isolated from Panax ginseng, had a mitogenic activity, induced LAK cells, and increased levels of several cytokines. In an effort to identify other immunostimulatory effects, we evaluated the protective effects of ginsan injected in vivo against radiation by measuring its effects on the CFU-S bone marrow cells and spleen cells. Ginsan was found to significantly increase the number of bone marrow cells, spleen cells, granulocyte-macrophage colony-forming cells (GM-CFC), and circulating neutrophils, lymphocytes and platelets in irradiated mice. In addition, ginsan induced the endogenous production of cytokines such as Il1, Il6, Ifng and Il12, which are required for hematopoietic recovery, and was able to enhance Th1 function while interfering with the Th2 response in irradiated mice. We demonstrated that pretreatment with ginsan protected mice from the lethal effects of ionizing radiation more effectively than when it was given immediately after or at various times after irradiation. A significant increase in the LD(50/30) from 7.54 Gy for PBS injection to 10.93 Gy for mice pretreated with 100 mg/kg ginsan was observed. These findings indicate that ginsan may be a useful agent to reduce the time necessary for reconstituting hematopoietic cells after irradiation.

Stem cell factor can stimulate the formation of eosinophils by two types of murine eosinophil progenitor cells

There are only three known stimuli for eosinophil formation-GM-CSF, interleukin-5 (IL-5), and IL-3. Because mice with inactivation of the gene encoding the common beta receptor chain for GM-CSF, IL-5, and IL-3 (betac(-/-) mice) cannot respond to GM-CSF or IL-5 and do not produce IL-3, they should lack eosinophils. However, they produce reduced numbers of eosinophils, indicating the existence of at least one additional stimulatory factor. Use of betac(-/-) mouse marrow cells failed to detect a novel eosinophil-stimulating factor in cell line- or organ-conditioned media, but stem cell factor (SCF) was found to have a previously unrecognized ability to stimulate the formation of eosinophil colonies or mixed neutrophil-eosinophil colonies. This action of SCF was also observable with marrow cells from other mouse strains and was enhanced by the addition of G-CSF but no other factor tested. Recloning of SCF-stimulated blast colonies showed that progenitors forming pure eosinophil or mixed neutrophil-eosinophil colonies can have a common ancestor but many appear to arise independently from different preprogenitor cells. The observed activity of SCF in marrow cultures was relatively weak, but its action may be stronger in vivo, and SCF needs to be added to GM-CSF, IL-5, and IL-3 in the list of cytokines able to stimulate eosinophil formation.

Hematopoietic progenitor/stem cells immortalized by Lhx2 generate functional hematopoietic cells in vivo

Hematopoietic stem cells (HSCs) are unique in their capacity to maintain blood formation following transplantation into immunocompromised hosts. Expansion of HSCs in vitro is therefore important for many clinical applications but has met with limited success because the mechanisms regulating the self-renewal process are poorly defined. We have previously shown that expression of the LIM-homeobox gene Lhx2 in hematopoietic progenitor cells derived from embryonic stem cells differentiated in vitro generates immortalized multipotent hematopoietic progenitor cell lines. However, HSCs of early embryonic origin, including those derived from differentiated embryonic stem cells, are inefficient in engrafting adult recipients upon transplantation. To address whether Lhx2 can immortalize hematopoietic progenitor/stem cells that can engraft adult recipients, we expressed Lhx2 in hematopoietic progenitor/stem cells derived from adult bone marrow. This approach allowed for the generation of immortalized growth factor-dependent hematopoietic progenitor/stem cell lines that can generate erythroid, myeloid, and lymphoid cells upon transplantation into lethally irradiated mice. When transplanted into stem cell-deficient mice, these cell lines can generate a significant proportion of circulating erythrocytes in primary, secondary, and tertiary recipients for at least 18 months. Thus, Lhx2 immortalizes multipotent hematopoietic progenitor/stem cells that can generate functional progeny following transplantation into lethally irradiated hosts and can long-term repopulate stem cell-deficient hosts.

Cyclic AMP in ovarian cancer cells both inhibits proliferation and increases c-KIT expression

C-KIT encodes a tyrosine kinase receptor (KIT) that, when activated by its ligand (KL), stimulates proliferation, differentiation, migration, and survival. Greater than 70% of epithelial ovarian cancers coexpress c-KIT and KL. C-KIT and KL expression levels have been shown to be up-regulated by cAMP in some cell types. Additionally, cAMP is well-recognized for its anti-proliferative effects in cancer cells. The goal of these experiments was to investigate these seemingly contradictory consequences of cAMP treatment by: (1) confirming the growth inhibitory actions of cAMP on ovarian cancer cells; (2) investigating the ability of cAMP to affect c-KIT and KL expression in these cells; and (3) examining the possible role of endogenous and/or cAMP-regulated c-KIT and KL expression in ovarian cancer cell proliferation. HEY cells, an ovarian cancer cell line which expresses c-KIT and KL, were treated with dibutyryl cyclic AMP (dbcAMP), 8-bromo-cAMP, and cholera toxin over a range of concentrations. With all treatments, stimulation of cAMP signaling caused a dose-dependent inhibition of HEY cell proliferation by up to 40, 62, and 38%, respectively. This inhibition of proliferation correlated with a dose-dependent increase in c-KIT mRNA expression, yielding 4- to 7-fold elevations in transcript abundance; there were no changes in steady-state levels of KL transcripts. In order to determine whether KIT expression/activity was responsible for the observed decrease in proliferation, dbcAMP-treated HEY cells were exposed either to anti-KIT neutralizing antibodies or to the KIT inhibitor STI571. These experiments demonstrated that KIT inhibition did not alter the growth rate of cells or reverse the dbcAMP-induced inhibition of proliferation. These results suggest that cAMP signaling pathways regulate both cell proliferation and c-KIT expression in ovarian cancer cells; however, KIT is not assuming its well-established role as a growth factor.

Increased granulopoiesis through interleukin-17 and granulocyte colony-stimulating factor in leukocyte adhesion molecule-deficient mice

Many mutant mice deficient in leukocyte adhesion molecules display altered hematopoiesis and neutrophilia. This study investigated whether peripheral blood neutrophil concentrations in these mice are elevated as a result of accumulation of neutrophils in the circulation or altered hematopoiesis mediated by a disrupted regulatory feedback loop. Chimeric mice were generated by transplanting various ratios of CD18(+/+) and CD18(-/-) unfractionated bone marrow cells into lethally irradiated wild-type mice, resulting in approximately 0%, 10%, 50%, 90%, or 100% CD18 null neutrophils in the blood. The presence of only 10% CD18(+/+) neutrophils was sufficient to prevent the severe neutrophilia seen in mice reconstituted with CD18(-/-) bone marrow cells. These data show that the neutrophilia in CD18(-/-) mice is not caused by enhanced neutrophil survival or the inability of neutrophils to leave the vascular compartment. In CD18(-/-), CD18(-/-)E(-/-), CD18(-/-)P(-/-), EP(-/-), and EPI(-/-) mice, levels of granulocyte colony-stimulating factor (G-CSF) and interleukin-17 (IL-17) were elevated in proportion to the neutrophilia seen in these mice, regardless of the underlying mutation. Antibiotic treatment or the propensity to develop skin lesions did not correlate with neutrophil counts. Blocking IL-17 or G-CSF function in vivo significantly reduced neutrophil counts in severely neutrophilic mice by approximately 50% (P <.05) or 70% (P <.01), respectively. These data show that peripheral blood neutrophil numbers are regulated by a feedback loop involving G-CSF and IL-17 and that this feedback loop is disrupted when neutrophils cannot migrate into peripheral tissues.

Multipotent hematopoietic progenitor cells immortalized by Lhx2 self-renew by a cell nonautonomous mechanism

OBJECTIVE

Direct molecular and cellular studies of hematopoietic stem cells (HSCs) are hampered by the low levels of HSCs in hematopoietic tissues. To address these issues, we generated immortalized multipotent hematopoietic precursor cell (HPC) lines by expressing the LIM-homeobox gene Lhx2 (previously LH2) in hematopoietic progenitors derived from embryonic stem cells differentiated in vitro.

MATERIALS AND METHODS

To validate further the relevance of the HPC lines as a model for normal HSCs, we analyzed in detail the growth requirements of HPC lines in vitro.

RESULTS

Lhx2 immortalized the HPC lines by a putatively novel and cell nonautonomous mechanism. Self-renewal of the HPC lines is dependent on functional Lhx2 expression. Most early-acting hematopoiesis-related growth factors show synergistic effects on the HPC lines, whereas late-acting factors do not induce differentiation by themselves. Transforming growth factor-beta(1) is a potent inhibitor of proliferation of the HPC lines. HPC lines form cobblestone areas with high efficiency when seeded onto stromal cell lines, and the cobblestone area-forming cell can be maintained in these cultures for several months.

CONCLUSIONS

Our data show that, in many respects, HPC lines are similar to normal hematopoietic progenitor/stem cells on the cellular level, in contrast to most previously described multipotent hematopoietic cell lines. The cell nonautonomous mechanism for immortalization of the HPC lines suggests that Lhx2 regulates, directly or indirectly, soluble mediators involved in self-renewal of the HPC lines.

Expression and distribution of the Kit receptor in bovine bone marrow cells

OBJECTIVE

To characterize the expression and distribution of the Kit receptor in bovine bone marrow cells (BMC) and to define the function of its ligand, stem cell factor (SCF).

ANIMALS

Six 7- to 70-day-old healthy male Holstein-Friesian calves.

PROCEDURES

Expression and distribution of the Kit receptor were assessed by use of flow cytometry with monoclonal antibodies (mAb) against the bovine Kit protein. Using Giemsa-stained centrifuged preparations, the histologic appearance of Kit receptor positive (Kit+) BMC were evaluated. Semisolid cultures supplemented with granulocyte colony-stimulating factor (G-CSF) and SCF were used to measure the colony formation capacity of Kit+ BMC.

RESULTS

The Kit receptor was expressed on approximately 18% of total BMC. Most of Kit+ BMC did not coexpress lineage markers, but a small subset of this population did coexpress CD3. The Kit+CD3- BMC were a heterogeneous cell population comprising blast-like cells such as myeloblasts, promyelocytes, rubriblasts, and prorubricytes. Conversely, Kit+CD3+ BMC had a lymphocyte-like appearance. Kit+ BMC formed colonies in semisolid culture with G-CSF, whereas Kit- BMC failed to grow. Addition of SCF to G-CSF resulted in superadditive enhancement in colony numbers and size.

CONCLUSIONS

The Kit receptor is expressed primarily on immature blood cells in bovine bone marrow, and Kit+ BMC contain hematopoietic progenitor cells that are reactive to G-CSF. In addition, SCF synergizes with G-CSF to stimulate colony formation by these cells. Our results suggest that the Kit receptor and its ligand, SCF, are involved in early stages of granulopoiesis in calves.

Cytokine-augmented culture of haematopoietic progenitor cells in a novel three-dimensional cell growth matrix

Studies aimed at the in vitro expansion of haematopoietic progenitor cells (HPCs) have suffered from the conflict of increasing cell numbers while maintaining long-term repopulating ability. We have developed a long-term bone marrow bioreactor culture system resembling the marrow-microenvironment that cultures HPCs in an inert, three-dimensional, porous biomatrix termed Cellfoam. Previous studies have shown that the short-term culture of CD34(+)cells in Cellfoam improved the maintenance and multipotency of haematopoietic stem cells compared to cells cultured on plastic dishes. In this study, we examined the effects of low concentrations of cytokines including stem cell factor (SCF), IL-3, and Flk-2/Flt-3 ligand, on the maintenance, preservation and multipotency of CD34(+) cells cultured for 3 or 6 weeks in Cellfoam. Analysis of cell yields using flow cytometry showed that in SCF and Flk-2/Flt-3 ligand-supplemented cultures as well as cytokine-free cultures, a higher number of CD45(+)34(+) and CD45(+)34(+)38(-) cells is observed in Cellfoam cultures as compared to plastic cultures. The function of cultured cells was evaluated in colony-forming assays. The data demonstrate that Cellfoam cultures supplemented with SCF and Flk-2/Flt-3 ligand resulted in a higher output of colony activity compared to plastic cultures. Analysis of CAFC (29 days) activity also demonstrated that primitive progenitors were maintained to a greater extent in Cellfoam cultures containing either no cytokines or low concentrations of early-acting cytokines. These data suggest that culture of HPCs in three-dimensional bioreactors such as Cellfoam for extended periods may benefit from the addition of low levels of early-acting cytokines, including SCF and Flk-2/Flt-3 ligand, resulting in high yields of cells that are enriched for multipotent haematopoietic progenitors. These findings demonstrate that a three-dimensional matrix promotes the survival of primitive HPCs in culture and may modulate the in vitro effects of cytokines.

Signal through gp130 activated by soluble interleukin (IL)-6 receptor (R) and IL-6 or IL-6R/IL-6 fusion protein enhances ex vivo expansion of human peripheral blood-derived hematopoietic progenitors

This study was designed to investigate the effects of a combination of soluble interleukin (sIL)-6 receptor (R) and IL-6 on the ex vivo expansion of human peripheral blood (PB)-derived hematopoietic progenitor cells in a short-term serum-free liquid suspension culture system, using PB-derived CD34(+)IL-6R(+/-) cells as a target. In combination with stem cell factor (SCF), IL-3, and sIL-6R/IL-6, the expansion efficiency (EE) for granulocyte/macrophage colony-forming unit (CFU-GM) reached a peak level on day 10 of incubation. On the other hand, the EE for erythroid burst (BFU-E) and mixed colony-forming unit (CFU-Mix) reached a peak level on day 7 of incubation. Among the cytokine combinations tested, SCF + IL-3 + sIL-6R/IL-6 + flt3 ligand (FL) most effectively expanded CFU-GM and CFU-Mix. The maximum EEs for CFU-GM and CFU-Mix were 208-fold and 42-fold, respectively. While the EE for BFU-E was 70-90-fold in the presence of SCF + IL-3 + sIL-6R/IL-6, FL significantly augmented the EE for CFU-GM and CFU-Mix. In contrast, thrombopoietin (TPO) significantly augmented the EE for CFU-Mix. Interestingly, in combination with IL-3 and SCF, newly generated IL-6R/IL-6 fusion protein (FP) expanded PB-derived BFU-E and CFU-Mix twice more effectively than a combination of sIL-6R and IL-6. These results demonstrated that human PB-derived committed progenitors were effectively expanded in vitro using sIL-6R/IL-6 or FP, in combination with IL-3, SCF and/or FL or TPO, and that FP may transduce a stronger intracellular signal than a combination of sIL-6R and IL-6.

The molecular control of hematopoiesis: progress and problems with gene manipulation

The in vitro-based discovery and characterization of hematopoietic regulators were of great value in identifying many of the agents active in controlling hematopoiesis. Subsequent in vivo studies have validated most of the information obtained from the in vitro studies, although the in vitro studies proved to be somewhat misleading in predicting which agents would exhibit the greatest quantitative effects in vivo. Establishing more clearly the actual situation in vivo has required a return to more complex, and often less satisfactory, studies on genetically-manipulated whole animals. Of the two possible general approaches, gene inactivation models have proved more informative than transgenic, overexpression models. Each model has raised multiple questions in need of further resolution and the deletion studies have also indicated that other regulators must exist for various lineages, but have yet to be discovered. Of particular interest is the finding from gene inactivation studies that both G-CSF and thrombopoietin are necessary for the maintenance of normal numbers of progenitor cells in multiple lineages, suggesting that each of these lineage-dominant regulators may have broader actions when operating on cells in the stem cell and progenitor cell compartments.

Use of granulocyte colony-stimulating factor after high-dose chemotherapy and autologous peripheral blood stem cell transplantation: what is the optimal timing?

Administration of granulocyte colony-stimulating factor to patients undergoing high-dose chemotherapy and autologous peripheral blood stem cell transplantation accelerates neutrophil recovery and decreases hospitalization time. The optimal timing for granulocyte colony-stimulating factor infusion remains unknown. In this retrospective, case-controlled, two-armed study, we reviewed our experience at Hahnemann University Hospital to determine whether initiating granulocyte colony-stimulating factor infusions on posttransplant day 0 versus day 8 affects neutrophil recovery time, posttransplant discharge date, total hospital days after high-dose chemotherapy, and autologous peripheral blood stem cell transplantation. All patients hospitalized between 1994 and 1998 at Hahnemann University Hospital, Bone Marrow Transplantation Unit with breast cancer or non-Hodgkin's lymphoma, who underwent high-dose chemotherapy followed by autologous peripheral blood stem cell transplantation and received granulocyte colony-stimulating factor either on posttransplant day 0 (16 patients) or day 8 (16 patients). The day 0 and day 8 groups had no statistically significant differences in age, sex, weight, height, body surface area, disease characteristics, pretransplant harvesting or conditioning regimens, or transplant CD34+ cell counts. Our main outcome measure was the mean time to reach absolute neutrophil count greater than or equal to 0.5 x 10(9)/l, the number of hospital days after transplant, and the total hospital days. The mean days to neutrophil recovery (10.56 versus 9.68, p = 0.48), posttransplant hospital days (13.62 versus 12.81, p = 0.39), and total hospital days (20.25 versus 20.25, p = 1.00) were not significantly different between day 8 and day 0 groups, respectively. No significant effects on neutrophil recovery time, posttransplant hospital days, or total hospital days were observed with the initial granulocyte colony-stimulating factor infusion on day 0 versus day 8 after transplant. Delayed administration may allow substantial cost savings (US$200 x 8 approximately equal to US $1,600 per patient) without affecting clinical outcome. More studies are needed to determine whether greater delay is feasible.

Regulatory mechanisms controlling hematopoiesis: principles and problems

Hematopoiesis is regulated by the combined action of specialized stromal cells and a consortium of hematopoietic regulatory factors. The multiplicity of these regulatory controls does result in overlapping regulator action, but multiple regulators are required to stimulate stem cell proliferation and are more efficient than single regulators when stimulation of progenitor cells is required. Gene inactivation studies have indicated that despite overlapping actions each hematopoietic regulator does have unique functions. Delayed elevations of stem and progenitor cells in the blood are a feature of enhanced hematopoiesis induced by the injection of regulators. These cells are not a random sample of marrow cells in such situations and may well be selected to rapidly amplify hematopoiesis by seeding previously inactive hematopoietic regions.

Differentiation commitment and regulator-specific granulocyte-macrophage maturation in a novel pro-B murine leukemic cell line

The cloned pro-B-lymphocyte murine leukemic cell line GB2, was established from a leukemic Max41 x Emu-myc double transgenic mouse. Its Igh alleles are rearranged and its surface markers are primarily B-lymphoid, but a small proportion of the cells also express surface Gr-1 and some cells develop the morphology of maturing granulocytes. The cell line grows continuously in suspension culture without the addition of growth factors, but expresses mRNA for M-CSF, TPO and Flt-3-ligand. When stimulated in agar cultures by GM-CSF, G-CSF, M-CSF, IL-3, SCF, IL-6, leukemia inhibitory factor (LIF), IL-5 or IFNgamma, GB2 cells generated blast colonies or colonies of maturing granulocytes and macrophages. There was a striking similarity in colony types, relative colony numbers and maturation of colony cells to those formed by normal bone marrow cells in response to the same stimuli. GB2 blast colony-forming cells exhibited self-renewal as well as an ability to form granulocyte-macrophage colony-forming progeny, with evidence that a hierarchical sequence of clonogenic cells is generated in the cell line even after subcloning. Factor-specific maturation was clearly initiated by the action of the added growth factors. In contrast, FACS-sorting experiments showed that commitment to various types of colony-forming cell occurs in maintenance suspension cultures in the apparent absence of potentially relevant growth factors.

Transforming growth factor-beta regulates Kit ligand expression in rat ovarian surface epithelial cells

In preparation for ovulation, paracrine communication between the preovulatory follicle and overlying theca/stromal cells and ovarian surface epithelium (OSE) must take place to facilitate the degradative and apoptotic events associated with ovulation. Kit tyrosine kinase receptors and their ligand, kit ligand (KL) are expressed within ovarian follicles, and ligand-induced receptor activation appears to account for some of the cell - cell interactions important for oocyte development. We investigated the expression of Kit receptors and KL in OSE cells and the possibility that modulation of their expression could affect OSE cell activity. KL mRNA and protein were detected in the OSE cell layer of rat ovaries, and primary cultures of rat OSE as well as the immortalized rat OSE cell line, ROSE 199, expressed KL, but not Kit receptors. Both primary and immortalized OSE cells preferentially expressed KL-1, rather than KL-2, transcripts, suggesting that these cells produce predominantly the soluble form of KL. Activation of the cAMP signalling pathway using dibutyryl cAMP decreased proliferation of ROSE 199 cells and elicited a threefold increase in KL expression. TGF-beta similarly inhibited ROSE 199 cell proliferation, but strongly inhibited dibutyryl cAMP-induced KL expression, indicating that changes in KL expression were not directly associated with OSE cell proliferation. The expression of mostly soluble KL in the surface epithelium suggests that this cytokine may be acting in a paracrine fashion, perhaps interacting with nearby Kit receptor-bearing theca cells.

Stem cells, pre-progenitor cells and lineage-committed cells: are our dogmas correct?

Recent developments warrant careful reexamination of several of the central dogmas of hematopoiesis. The bioassays previously used may have predetermined which subsets of hematopoietic stem cells are regarded as having long-term repopulating activity and thus have produced misleading data. Lineage commitment in multipotential cells has been regarded as an immutable stochastic process but may be a process that can be modified by extrinsic signaling. Finally, loss of self-renewal activity has been regarded as progressive and irreversible but this response to signaling can be blocked by cytokine-inducible modulating proteins.

Optimal Proliferation of a Hematopoietic Progenitor Cell Line Requires Either Costimulation With Stem Cell Factor or Increase of Receptor Expression That Can Be Replaced by Overexpression of Bcl-2

In vitro proliferation of hematopoietic stem cells requires costimulation by multiple regulatory factors whereas expansion of lineage-committed progenitor cells generated by stem cells usually requires only a single factor. The distinct requirement of factors for proliferation coincides with the differential temporal expression of the subunits of cytokine receptors during early stem cell differentiation. In this study, we explored the underlying mechanism of the requirement of costimulation in a hematopoietic progenitor cell line TF-1. We found that granulocyte-macrophage colony-stimulating factor (GM-CSF) optimally activated proliferation of TF-1 cells regardless of the presence or absence of stem cell factor (SCF). However, interleukin-5 (IL-5) alone sustained survival of TF-1 cells and required costimulation of SCF for optimal proliferation. The synergistic effect of SCF was partly due to its anti-apoptosis activity. Overexpression of the IL-5 receptor  subunit (IL5R) in TF-1 cells by genetic selection or retroviral infection also resumed optimal proliferation due to correction of the defect in apoptosis suppression. Exogenous expression of an oncogenic anti-apoptosis protein, Bcl-2, conferred on TF-1 cells an IL-5–dependent phenotype. In summary, our data suggested SCF costimulation is only necessary when the expression level of IL5R is low and apoptosis suppression is defective in the signal transduction of IL-5. Expression of Bcl-2 proteins released the growth restriction of the progenitor cells and may be implicated in leukemia formation.

Optimal Proliferation of a Hematopoietic Progenitor Cell Line Requires Either Costimulation With Stem Cell Factor or Increase of Receptor Expression That Can Be Replaced by Overexpression of Bcl-2

In vitro proliferation of hematopoietic stem cells requires costimulation by multiple regulatory factors whereas expansion of lineage-committed progenitor cells generated by stem cells usually requires only a single factor. The distinct requirement of factors for proliferation coincides with the differential temporal expression of the subunits of cytokine receptors during early stem cell differentiation. In this study, we explored the underlying mechanism of the requirement of costimulation in a hematopoietic progenitor cell line TF-1. We found that granulocyte-macrophage colony-stimulating factor (GM-CSF) optimally activated proliferation of TF-1 cells regardless of the presence or absence of stem cell factor (SCF). However, interleukin-5 (IL-5) alone sustained survival of TF-1 cells and required costimulation of SCF for optimal proliferation. The synergistic effect of SCF was partly due to its anti-apoptosis activity. Overexpression of the IL-5 receptor  subunit (IL5R) in TF-1 cells by genetic selection or retroviral infection also resumed optimal proliferation due to correction of the defect in apoptosis suppression. Exogenous expression of an oncogenic anti-apoptosis protein, Bcl-2, conferred on TF-1 cells an IL-5–dependent phenotype. In summary, our data suggested SCF costimulation is only necessary when the expression level of IL5R is low and apoptosis suppression is defective in the signal transduction of IL-5. Expression of Bcl-2 proteins released the growth restriction of the progenitor cells and may be implicated in leukemia formation.

Flt3 ligand can promote survival and macrophage development without proliferation in myeloid progenitor cells

Flt3 ligand elicits a variety of effects on early hemopoietic progenitors by occupying its cognate receptor, Flt3, a member of the type III tyrosine kinase receptor family. The cytokines macrophage colony-stimulating factor (M-CSF) and stem cell factor (SCF) bind to related members of this tyrosine kinase receptors family, c-fms and c-kit, respectively. The relative effects of the cytokines M-CSF, SCF, and Flt3L on the proliferation and development of the late myeloid progenitors granulocyte-macrophage colony-forming cells (GM-CFC) were investigated. Distinct biologic responses were stimulated by ligand binding to these different tyrosine kinase receptors in enriched GM-CFC. M-CSF stimulated GM-CFC to proliferate and develop into macrophages. SCF, on the other hand, stimulated GM-CFC to develop into neutrophils. Flt3 ligand had a relatively small proliferative effect on enriched GM-CFC compared to SCF and M-CSF and had no ability to either stimulate colony formation or synergize with these two cytokines in promoting DNA synthesis, colony formation, or expansion in liquid culture. Flt3 ligand, however, was capable of maintaining the clonogenic potential of GM-CFC and acted as an anti-apoptotic agent as assessed using the Annexin-V apoptosis assay. GM-CFC cultured in Flt3 ligand eventually formed macrophages and neutrophils in liquid culture. Labeling with the membrane-associated cell tracker dye PKH26 indicated that the majority of the enriched GM-CFC responded to Flt3 ligand by undergoing limited proliferation and macrophage development, whereas other cells survived but did not proliferate and differentiate into macrophages. Thus, Flt3 ligand promoted survival and stimulated development without proliferation in primary-enriched myeloid progenitor cells.

Circulating cytokines response and the level of erythropoiesis in sickle cell anemia

A hemoglobin F (HbF) level between eight and nine percent divides sickle cell anemia (SS) patients into two populations, according to the kinetics of circulating burst forming units-erythroid (BFU-E), long term culture-initialing cells (LTC-IC), and cytokine plasma concentrations. The SS patients with HbF levels lower than 8-9% are more anemic (LFSS patients) than those with HbF levels higher than 8-9% who have less severe anemia (HFSS patients). We report here that the level of erythropoiesis [evaluated by the levels of soluble transferin receptors (sTfR)] is not identical in these two patient populations, supporting the idea that a different set of regulatory mechanisms might be required to maintain the two levels of increased hematopoiesis. The plasma sTfR concentration was increased in all SS samples compared with controls (P < 0.002) and sTfR levels were negatively correlated with peripheral HbF%. (r = -0.574, P < 0.002). Furthermore, sTfR levels were higher in LFSS than in HFSS patients. Erythropoietin (Epo) levels were increased in the plasma of LFSS individuals (range = 34-215 ml U/ml), while the values in HFSS patients were in the normal range (3-20 ml U/ml). Furthermore, we identify here stem cell factor (SCF) and transforming growth factor-beta (TGF-beta) as regulatory factors specifically affected by the presence of SS genotype and its level of severity. The plasma concentrations of SCF and TGF-beta were increased compared with normal controls and high levels of SCF (up to 7,000 pg/ml) were detected in LFSS patients. The latter also showed increased proportion of SCF+ CD34 enriched circulating cells (49%). Low SCF in HFSS patients is associated with elevated TGF-beta, suggesting a regulatory role of the latter on either SCF release or c-kit expression in progenitor cells. Occasional elevation of granulocyte macrophage-colony stimulating factor (G-CSF), interleukin (IL)-7, and macrophage inflammatory protein (MIP)-1alpha in plasma of SS patients is not specific because no relation to HbF could be demonstrated. All plasma tested for leukemia inhibitory factor (LIF) were negative. Data presented here, complementing previously published information, supports a model in which HFSS patients achieve a balance between inhibitory (TGF-beta) and stimulatory (SCF, IL-3) factors, resulting in moderate erythropoietic response. In contrast, in LFSS patients, low levels of TGF-beta and the increased release of GM-CSF and SCF maintain the intense erythropoiesis in response to higher erythropoietic stress, in these more severe patients.

Self-renewal, maturation, and differentiation of the rat myelomonocytic hematopoietic stem cell

Hematopoiesis is viewed as a differentiating system emanating from a pluripotent hematopoietic stem cell capable of both self-renewal and differentiation. By identifying and characterizing a novel and highly specific in vitro mitogenic response to the N-acetyl glucosamyl/sialic acid specific, stem cell-binding lectin wheat germ agglutinin (WGA), we demonstrate the existance of a rare (0.1%), plastic adherent precursor in rat bone marrow capable of proliferation (two to seven divisions) in response to WGA. Stimulated cells possess a lineage (lin)low/- immunophenotype and immature blastoid morphology (WGA blasts). A subsequent proliferative response to stem cell factor (SCF), the ligand for the proto-oncogene receptor tyrosine kinase c-kit, is characterized by an initial maturation in immunophenotype and subsequent self-renewal of cells (SCF blasts) without differentiation for at least 50 generations. Although granulocyte colony-stimulating factor (G-CSF), interleukin (IL) -6, IL-7, and IL-11 synergize with SCF to increase blast colony formation, cytokines such as granulocyte-macrophage CSF or IL-3 are without significant effect. At all time points in culture, however, cells rapidly differentiate to mature neutrophils with dexamethasone or to mainly monocytes/macrophages in the presence of 1alpha,25-dihydroxyvitamin D3, characterized by cell morphology and cytochemistry. Removal of SCF during blast maturation, self-renewal, or induction of differentiation phases results in apoptotic cell death. Data indicate a pivotal role for SCF/c-kit interaction during antigenic maturation, self-renewal, and apoptotic protection of these lineage-restricted progenitors during non-CSF-mediated induction of differentiation. This approach provides a source of many normal, proliferating myelomonocytic precursor cells, and introduces possible clinical applications of ex vivo expanded myeloid stem cells.

The molecular control of hematopoiesis: progress and problems with gene manipulation

The in vitro-based discovery and characterization of hematopoietic regulators were of great value in identifying many of the agents active in controlling hematopoiesis. Subsequent in vivo studies have validated most of the information obtained from the in vitro studies, although the in vitro studies proved to be somewhat misleading in predicting which agents would exhibit the greatest quantitative effects in vivo. Establishing more clearly the actual situation in vivo has required a return to more complex, and often less satisfactory, studies on genetically manipulated whole animals. Of the two possible general approaches, gene inactivation models have proved more informative than transgenic, overexpression models. Each model has raised multiple questions in need of further resolution and the deletion studies have also indicated that other regulators must exist for various lineages, but have yet to be discovered. Of particular interest is the finding from gene inactivation studies that both G-CSF and thrombopoietin are necessary for the maintenance of normal numbers of progenitor cells in multiple lineages, suggesting that each of these lineage-dominant regulators may have broader actions when operating on cells in the stem cell and progenitor cell compartments.

Expression of the LIM-homeobox gene LH2 generates immortalized steel factor-dependent multipotent hematopoietic precursors

The genes controlling self-renewal and differentiation in the hematopoietic system are largely unknown. The LIM-homeobox genes are known to be important for asymmetric cell divisions and differentiation of specific cell types and organs. One member of this family, LH2, is expressed in fetal liver at the time of active hematopoiesis. Therefore, we have assessed the function of LH2 during the formation and initial expansion of the hematopoietic system by differentiating LH2-transduced embryonic stem (ES) cells in vitro. This procedure generated multipotent hematopoietic precursor cell (HPC) lines that required Steel factor for growth. HPC lines have been maintained in an undifferentiated state in culture for >7 months. Other growth factors tested efficiently induce terminal differentiation of HPCs into various mature myeloid lineages. Steel factor is also required and acts synergistically with the other growth factors to generate multilineage colonies from the HPCs. These HPC lines express transcription factors that are consistent with an immature progenitor, and the pattern of cell surface marker expression is similar to that of early fetal multipotent hematopoietic progenitors. Collectively, these data suggest that the HPC lines represent an early fetal multipotent hematopoietic progenitor, and suggest a role for LH2 in the control of cell fate decision and/or proliferation in the hematopoietic system.

Cell-cell signalling in the regulation of blood cell formation and function

Blood cell formation is continuous in adult life and requires complex regulatory control to achieve the necessary cell proliferation, differentiation commitment and maturation. Much of this control is achieved by specific glycoprotein haemopoietic regulators, more than 20 of which have now been identified and produced in recombinant form. For the granulocyte-macrophage lineage, major regulators are the colony stimulating factors (CSF) that are produced by many cell types dispersed throughout the body. The production of CSF is rapidly increased in response to induction signals, most usually of microbial origin, representing a demand-generated signalling system designed to activate and generate cells to rapidly eliminate the micro-organisms. The CSF not only control cell division but certain aspects of differentiation commitment, maturation initiation and functional activation. These multiple responses are initiated by signals arising from different regions of the cytoplasmic domains of the CSF membrane receptors. Less well understood are the signalling systems controlling the special gene activation events required to initiate the formation of the first haemopoietic cells and the specialized microenvironmental cells needed to support haemopoietic cells as they respond to haemopoietic regulators.

Stromal Cell-Derived Factor-1α and Stem Cell Factor/kit Ligand Share Signaling Pathways in Hemopoietic Progenitors: A Potential Mechanism for Cooperative Induction of Chemotaxis

Stromal cell-derived factor (SDF-1α), the ligand for CXCR4, is a chemokine that acts as a potent chemoattractant for hemopoietic progenitor cells. Stem cell factor/kit ligand (SCF/KL), an early acting cytokine, has recently been reported to enhance the chemotaxis induced by SDF-1α. However, very little is known about downstream signaling events following these receptor-ligand interactions. To investigate these events, we utilized a model progenitor cell line, CTS, which expresses both the CXCR4 and c-kit receptors. We observed strong Ca2+ mobilization and enhancement of chemotaxis following treatment with SDF-1α or SCF/KL. A combination of these factors enhanced this chemotaxis in CTS cells as well as in CD34+ bone marrow cells. Prior treatment of CTS cells with pertussis toxin inhibited the SDF-1α-induced chemotaxis, suggesting that SDF-1α signaling involves a pertussis-sensitive Gi-coupled protein. SDF-1α treatment resulted in a rapid phosphorylation of the focal adhesion molecules RAFTK (related adhesion focal tyrosine kinase), paxillin, and p130cas, which then declined within minutes. SCF/KL alone or in combination with SDF-1α induced a rapid and sustained effect on phosphorylation of these substrates. SDF-1α treatment resulted in a rapid and robust activation of p44/42 mitogen-activated protein kinase compared with the relatively weak and delayed effect of SCF/KL treatment. Interestingly, a delayed but sustained activation of mitogen-activated protein kinase activation was observed when the factors were used in combination. Such cooperativity in downstream signaling pathways may explain the enhanced chemotaxis of progenitors observed with SDF-1α in combination with SCF/KL.

High proliferative potential-quiescent cells: a working model to study primitive quiescent hematopoietic cells

Human adult hematopoietic stem cells are mostly quiescent or slow cycling. We have previously demonstrated that blocking of transforming growth factor-beta1 (TGF-beta1) is able to activate, in the presence of cytokines, primitive quiescent hematopoietic multipotent progenitors which could not grow in a two week semi-solid culture assay (short term culture). We have also shown that anti-TGF-beta1 can up-modulate c-KIT, the receptor of the stem cell factor (steel factor). To elucidate whether TGF-beta1 plays a central role in controlling the quiescence of hematopoietic primitive cells, it was necessary to demonstrate, as detailed in this study, that: (1) whatever the cytokine combination tested, addition of anti-TGF-beta1 releases from quiescence multipotent progenitors with a significantly higher hematopoietic potential than those activated by cytokines alone. (2) Other important cytokine receptors controlling the most primitive hematopoietic cells such as FLT3 and the IL6 receptor (IL6-R) are down-modulated by TGF-beta1 but rapidly up-modulated by anti-TGF-beta1. (3) Anti-TGF-beta1-sensitive multipotent and high proliferative potential progenitors express these cytokine receptors at a low level (FLT3(low) and IL6-Rlow). According to these results, we propose the working model of ‘High Proliferative Potential-Quiescent cells’ to refer to these primitive hematopoietic multipotent progenitors that are highly sensitive to the growth inhibitory effect of TGF-beta1. This model could be valid not only to study the human hematopoietic quiescent progenitors but also for other somatic stem cell systems.

Stem cell factor induces proliferation and differentiation of fetal progenitor cells in the mouse

We have investigated the kinetics of the amplification of the progenitor cell compartments (CFC) in haemopoietic organs during murine ontogenesis and compared the growth requirements of fetal and adult CFC. Two haemopoietic phases were recognized in the fetal liver (FL): an exponential growth phase, from 11.5 to 15.5 d post conception (p.c.), during which the mean number of nucleated cells and of CFC in the FL increased from 4.9 x 10(5) to 7.0 x 10(7) and from 4.5 x 10(3) to 2.7 x 10(5), respectively, and a recessive phase after 15.5 d p.c., during which the CFC number in the FL gradually decreased, although some CFC were still detectable in the liver after birth. In serum-deprived cultures, FL and adult marrow (AM) CFC had similar responses to GM-CSF, and did not respond to G-CSF or IL-3. In contrast, FL, but not AM, erythroid colonies grew Epo-independently whereas SCF alone induced formation of maximal numbers of erythroid bursts from FL, but not from AM cells. The proliferative and differentiative effect of SCF alone on fetal cells was confirmed in serum-deprived cultures of purified early progenitor cells isolated by cell sorting on the basis of multiple parameters from FL and AM light-density cells. In culture of purified FL cells, SCF alone induced a similar amplification of total cells (maximal amplification at day 12: 800-300-fold) and total CFC (11-38-fold of maximal amplification at day 6) to the combination of SCF plus IL-3 (1300-800-fold amplification of total cells and 31-88-fold amplification of CFC). In contrast, SCF alone allowed only survival of purified AM early progenitor cells. Therefore FL early progenitor cells have an intrinsic higher potential than their adult counterpart to respond to SCF, confirming the potent role of this growth factor in the development of the murine haemopoietic system.

Lineage commitment in the progeny of murine hematopoietic preprogenitor cells: influence of thrombopoietin and interleukin 5

Normal mouse marrow cells were stimulated by stem cell factor (SCF) to form dispersed or multicentric blast colonies containing progenitor cells committed to various hematopoietic lineages. Combination of the eosinophil-specific regulator interleukin 5 with SCF increased the frequency of colonies containing eosinophil-committed progenitor cells with multicentric but not dispersed blast colonies. Combination of thrombopoietin with SCF increased the frequency of colonies containing megakaryocyte-committed progenitor cells with both types of blast colony. Neither interleukin 5 nor thrombopoietin significantly altered the number or total cell content of blast colonies or progenitor cell numbers in blast colonies from those stimulated by SCF alone. No correlation was observed between total progenitor cell content and the presence or absence of either eosinophil or megakaryocyte progenitors in either type of blast colony. The data argue against a random process as being responsible for the formation of particular committed progenitor cells or the possibility that lineage-specific regulators merely enhance survival of such committed progenitor cells formed in developing blast colonies.

Advances in hematopoietic stem cell culture

Recent advances in our understanding of the earliest stages of hematopoietic cell differentiation, and how these may be manipulated under defined conditions in vitro, have set the stage for the development of robust bioprocess technology applicable to hematopoietic cells. Sensitive and specific assays now exist for measuring the frequency of hematopoietic stem cells with long-term in vivo repopulating activity from human as well as murine sources. The production of natural or engineered ligands through recombinant DNA and/or combinatorial chemistry strategies is providing new reagents for enhancing the productivity of hematopoietic cell cultures. Multifactorial and dose-response analyses have yielded new insight into the different types and concentrations of factors required to optimize the rate and the extent of amplification of specific subpopulations of primitive hematopoietic cells. In addition, the rate of cytokine depletion from the medium has also been found to be dependent on the types of cell present. The discovery of these cell-type-specific parameters affecting cytokine concentrations and responses has introduced a new level of complexity into the design of optimized hematopoietic bioprocess systems.

An activated protein kinase C alpha gives a differentiation signal for hematopoietic progenitor cells and mimicks macrophage colony-stimulating factor-stimulated signaling events

Highly enriched, bipotent, hematopoietic granulocyte macrophage colony-forming cells (GM-CFC) require cytokines for their survival, proliferation, and development. GM-CFC will form neutrophils in the presence of the cytokines stem cell factor and granulocyte colony-stimulating factor, whereas macrophage colony-stimulating factor leads to macrophage formation. Previously, we have shown that the commitment to the macrophage lineage is associated with lipid hydrolysis and translocation of protein kinase C alpha (PKCalpha) to the nucleus. Here we have transfected freshly prepared GM-CFC with a constitutively activated form of PKCalpha, namely PKAC, in which the regulatory domain has been truncated. Greater than 95% of the transfected cells showed over a twofold increase in PKCalpha expression with the protein being located primarily within the nucleus. The expression of PKAC caused macrophage development even in the presence of stimuli that normally promote only neutrophilic development. Thus, M-CSF-stimulated translocation of PKCalpha to the nucleus is a signal associated with macrophage development in primary mammalian hematopoietic progenitor cells, and this signal can be mimicked by ectopic PKAC, which is also expressed in the nucleus.

Myb and Ets Proteins Are Candidate Regulators of c-kit Expression in Human Hematopoietic Cells

Kit is a tyrosine kinase receptor that plays an important role in human hematopoietic cell growth. The promoter elements that modulate the gene's expression have not been extensively studied. Because of c-kit's acknowledged importance in hematopoiesis, we sought to address this issue in more detail. To perform these studies we analyzed a human c-kit 5′ flanking fragment ∼1 kilobase in length. Deletion constructs showed a region ∼139 nucleotides upstream from the translation initiation site that was critical for promoter activity. A region containing a potential silencing element was also identified. Sequence analysis indicated several potential Myb- and Ets-binding sites. The functional significance of these sites was explored by showing that both wild-type Myb and Ets-2 protein, but not a DNA binding-deficient Myb mutant protein, bound to distinct 5′ flanking fragments that included these sites. Furthermore, binding of recombinant Myb and Ets-2 protein to these fragments could be competed with an excess of double stranded oligodeoxynucleotides containing canonical, but not mutated,Myb- or Ets-binding sites. We also showed that the 5′ flanking region of c-kit exhibited promoter activity in nonhematopoietic cells only when the cells were transfected with c-myb or ets-2 expression vectors. Moreover,Myb and Ets-2 coexpression in such cells augmented transactivation of c-kit promoter constructs in comparison to that observed in cells transfected with either construct alone. Promoter constructs lacking various Myb and Ets sites deleted were much less effective in this same system. Finally,Myb and Ets-2 mRNA expression was detected in CD34+, Kitlow as well as CD34+, Kitbright cells. In aggregate, these data further define the human c-kit promoter's functional anatomy and suggest that Myb and Etsproteins play an important, perhaps cooperative, role in regulating expression of this critical hematopoietic cell receptor.

Myb and Ets Proteins Are Candidate Regulators of c-kit Expression in Human Hematopoietic Cells

Kit is a tyrosine kinase receptor that plays an important role in human hematopoietic cell growth. The promoter elements that modulate the gene's expression have not been extensively studied. Because of c-kit's acknowledged importance in hematopoiesis, we sought to address this issue in more detail. To perform these studies we analyzed a human c-kit 5′ flanking fragment ∼1 kilobase in length. Deletion constructs showed a region ∼139 nucleotides upstream from the translation initiation site that was critical for promoter activity. A region containing a potential silencing element was also identified. Sequence analysis indicated several potential Myb- and Ets-binding sites. The functional significance of these sites was explored by showing that both wild-type Myb and Ets-2 protein, but not a DNA binding-deficient Myb mutant protein, bound to distinct 5′ flanking fragments that included these sites. Furthermore, binding of recombinant Myb and Ets-2 protein to these fragments could be competed with an excess of double stranded oligodeoxynucleotides containing canonical, but not mutated,Myb- or Ets-binding sites. We also showed that the 5′ flanking region of c-kit exhibited promoter activity in nonhematopoietic cells only when the cells were transfected with c-myb or ets-2 expression vectors. Moreover,Myb and Ets-2 coexpression in such cells augmented transactivation of c-kit promoter constructs in comparison to that observed in cells transfected with either construct alone. Promoter constructs lacking various Myb and Ets sites deleted were much less effective in this same system. Finally,Myb and Ets-2 mRNA expression was detected in CD34+, Kitlow as well as CD34+, Kitbright cells. In aggregate, these data further define the human c-kit promoter's functional anatomy and suggest that Myb and Etsproteins play an important, perhaps cooperative, role in regulating expression of this critical hematopoietic cell receptor.