Murine pluripotent hematopoietic progenitors constitutively expressing a normal erythropoietin receptor proliferate in response to erythropoietin without preferential erythroid cell differentiation

Correction of murine β-thalassemia after minimal lentiviral gene transfer and homeostatic in vivo erythroid expansion

A challenge for gene therapy of genetic diseases is to maintain corrected cell populations in subjects undergoing transplantation in cases in which the corrected cells do not have intrinsic selective advantage over nontransduced cells. For inherited hematopoietic disorders, limitations include inefficient transduction of stem cell pools, the requirement for toxic myelosuppression, and a lack of optimal methods for cell selection after transduction. Here, we have designed a lentiviral vector that encodes human β-globin and a truncated erythropoietin receptor, both under erythroid-specific transcriptional control. This truncated receptor confers enhanced sensitivity to erythropoietin and a benign course in human carriers. Transplantation of marrow transduced with the vector into syngenic thalassemic mice, which have elevated plasma erythropoietin levels, resulted in long-term correction of the disease even at low ratios of transduced/untransduced cells. Amplification of the red over the white blood cell lineages was self-controlled and averaged ∼ 100-fold instead of ∼ 5-fold for β-globin expression alone. There was no detectable amplification of white blood cells or alteration of hematopoietic homeostasis. Notwithstanding legitimate safety concerns in the context of randomly integrating vectors, this approach may prove especially valuable in combination with targeted integration or in situ homologous recombination/repair and may lower the required level of pretransplantation myelosuppression.

NKT cells: from totipotency to regenerative medicine

The recent discovery that natural killer T (NKT) cell nuclei are totipotent opens a novel avenue for further understanding NKT cell function in normal and diseased states. The progeny of a cloned mouse harboring the in-frame rearranged Valpha14-Jalpha18 T cell receptor in one allele showed a significant increase in NKT cell number compared with wild-type or littermate control mice that possessed a different TCR. Importantly, NKT cells from such progeny produced both interferon-gamma and interleukin-4, a hallmark of NKT cells. In these progeny, NKT cell development appeared to be instructively, rather than permissively, determined. Using embryonic stem cells prepared via the somatic cell nuclear transfer of NKT nuclei, relatively mature NKT cells were induced under conditions permissible for T cell induction. Furthermore, these NKT cells matured autonomously upon injection into mice, resulting in an antigen-specific adjuvant effect.

Retroviral transduction of IL-7Rα into IL-7Rα−/− bone marrow progenitors: correction of lymphoid deficiency and induction of neutrophilia

Defects in the gene for the IL-7 receptor (R) alpha chain are one cause of severe combined immunodeficiency disease (SCID) based on a strict requirement for IL-7 in T lymphoid development and survival. We tested the feasibility and potentially undesirable consequences of IL-7Ralpha gene transfer as a therapy for this genetic defect. The murine IL-7Ralpha gene was introduced into IL-7Ralpha(-/-) bone marrow progenitors using retrovirus and transplanted into Rag(-/-) recipient mice. Both alphabeta and gammadelta T cells were reconstituted in thymus and spleen showing proof of principle. B-cell development was also restored in some mice, but their numbers were much lower than in the T-cell compartment. Splenomegaly was observed due to an increase in neutrophils. We showed that hematopoietic progenitors, after transfection with IL-7Ralpha, could respond to IL-7 in vitro by a striking production of neutrophils and other myeloid cells. These data indicate that although IL-7 is a critical lymphopoietin, ectopic expression of its receptor on multipotential progenitors can also induce production of myeloid cells, presumably through survival and proliferation signals that are not restricted to lymphoid cells. This supports the stochastic model of progenitor differentiation, in which cytokines give permissive and not instructive signals.

Zyxin is up-regulated during megakaryocytic differentiation of human UT-7/c-mpl cells

To characterize genes involved in megakaryocytic commitment, we compared expression profiles of bipotent cells (UT-7/c-mpl) with those of the same cells induced to differentiate towards megakaryopoiesis in the presence of TPO. Using cDNA arrays, we showed that 12 out of 2260 genes changed their expression level after 6h of TPO stimulation. One of these genes encodes for zyxin, a cytoskeleton protein component. Zyxin is up-regulated at the mRNA and protein levels in UT-7/c-mpl cells in response to TPO confirming the reliability of the cDNA array technology. Similarly, when CD34 positive cells were induced to differentiate into megakaryocytes, zyxin mRNA was accumulated. Furthermore, when megakaryocytes were allowed to spread on fibrinogen, formation of stress fibers and lamellipodia was induced and zyxin was localized at the picks of actin stress fibers. These results suggest an important role for zyxin during megakaryocytic differentiation and more precisely in the regulation of the integrin mediated adhesion process in megakaryocytes.

Receptor specificity in the self-renewal and differentiation of primary multipotential hemopoietic cells

To determine whether cytokine-induced signals generate unique responses in multipotential hemopoietic progenitor cells, the signaling domains of 3 different growth factor receptors (Mpl, granulocyte-colony-stimulating factor [G-CSF] receptor, and Flt-3) were inserted into mouse primary bone marrow cells. To circumvent the activation of endogenous receptors, each signaling domain was incorporated into an FK506 binding protein (FKBP) fusion to allow for its specific activation using synthetic FKBP ligands. Each signaling domain supported the growth of Ba/F3 cells; however, only Mpl supported the sustained growth of transduced marrow cells, with a dramatic expansion of multipotential progenitors and megakaryocytes. These findings demonstrate that the self-renewal and differentiation of multipotential progenitor cells can be influenced through distinct, receptor-initiated signaling pathways.

Megakaryocytic Maturation is Regulated by Maintaining a Balance Against Cytokine Induced-cell Proliferation: Steel Factor Retards Thrombopoietin-induced Megakaryocytic Differentiation While Synergistically Stimulating Mitogenesis; Hematopoiesis

Using a factor-dependent cell line MO7ER, which contains a stably transduced human erythropoietin (EPO) receptor gene in human megakaryoblastic cell line MO7e and which resulted in concomitant expression of EPO receptor, c-Mpl and c-Kit, we investigated the biological effects of these cytokines in terms of cell growth and differentiation. Thrombopoietin (TPO), EPO and Steel factor (SLF) all stimulated MO7ER cell proliferation in a dose-dependent manner. Combined stimulation of cells with SLF plus either TPO or EPO resulted in striking synergistic enhancement of MO7ER cell growth as compared with each cytokine alone, whereas combination of TPO plus EPO showed only an additive effect on cell proliferation. With regards to cell differentiation, either TPO or EPO treatment induced enhancement of platelet glycoprotein (GP) IIb/IIIa and GPIb expression. SLF induced GPIIb/IIIa and GPIb expression, but the effect was much weaker than that of EPO or TPO. However, addition of SLF to either TPO- or EPO- containing cultures (which induced potent mitogenesis in MO7ER cells) resulted in suppression of these megakaryocyte specific antigens. Addition of low-dose cytosine arabinoside (Ara-C)(1 to 10 ng/ml) enhanced TPO- or EPO- induced megakaryocytic differentiation in MO7ER cells while mildly suppressing cell growth. Treatment the cells with low-dose Ara-C plus TPO plus SLF overrode the proliferative enhancing effects of SLF and induced GPIIb/IIIa and GPIb expression as efficient as TPO alone. Retardation of TPO-induced megakaryocytic maturation was also observed in normal murine bone marrow cells by combined stimulation with TPO and SLF as assessed by the numbers of acetylcholinesterase staining-positive cells and megakaryocyte nuclear polyploidy. These results suggest that megakaryocytic maturation is, at least in part, regulated by countering cytokine-induced cell proliferation.

The granulocyte colony-stimulating factor receptor supports erythroid differentiation in the absence of the erythropoietin receptor or Stat5

To evaluate the functional conservation of signal transduction mechanisms between haematopoietic receptors and to characterize the molecules activated in this phenomenon, we introduced granulocyte colony-stimulating factor receptor (G-CSFR) cDNA into mouse fetal liver cells using a retroviral vector. In semi-solid medium assays, G-CSFR-infected cells gave rise to all types of colonies [granulocyte-macrophage (GM), megakaryocyte (MK) and mixed lineage (GEMM) colony-forming units (CFU) and erythroid burst-forming units (BFU-E)] in the presence of G-CSF alone. The direct effect of G-CSF on erythroid differentiation of G-CSFR-transduced erythroid progenitors was demonstrated by the development of erythroid colonies using G-CSFR-expressing Lin- cells cloned at one cell per well in liquid culture in the presence of G-CSF. Interestingly, while Stat5, but not Stat3, was activated in erythroid cells in response to erythropoietin (EPO), both were activated in erythroid and granulocytic cells stimulated by G-CSF. Furthermore, G-CSF induced the growth of erythroid colonies from G-CSFR-expressing fetal liver cells from EPO receptor-/- (EPO-R-/-) or Stat5a-/- Stat5b-/- mice, demonstrating that erythroid differentiation can occur in the absence of EPO-R or Stat5. These data show that forced expression of G-CSFR allows G-CSF-dependent multilineage proliferation and differentiation of haematopoietic progenitors and rescues EPO-R-/- erythroid cells. While G-CSF induces Stat5 activation in G-CSFR-expressing erythroid cells, this activation is not necessary for the terminal erythroid differentiation induced by G-CSF.

Receptor for macrophage colony-stimulating factor transduces a signal decreasing erythroid potential in the multipotent hematopoietic EML cell line

OBJECTIVE

To test the hypothesis that hematopoietic growth factors may influence lineage choice in pluripotent progenitor cells, we investigated the effects of macrophage colony-stimulating factor (M-CSF) on erythroid and myeloid potentials of multipotent EML cells ectopically expressing M-CSF receptor (M-CSFR).

METHODS

EML cells are stem cell factor (SCF)-dependent murine cells that give rise spontaneously to pre-B cells, burst-forming unit erythroid (BFU-E), and colony-forming unit granulocyte macrophage (CFU-GM). We determined BFU-E and CFU-GM frequencies among EML cells transduced with murine M-CSFR, human M-CSFR, or chimeric receptors, and cultivated in the presence of SCF, M-CSF, or both growth factors. Effects of specific inhibitors of signaling molecules were investigated.

RESULTS

EML cells transduced with murine M-CSFR proliferated in response to M-CSF but also exhibited a sharp and rapid decrease in BFU-E frequency associated with an increase in CFU-GM frequency. In contrast, EML cells expressing human M-CSFR proliferated in response to M-CSF without any changes in erythroid or myeloid potential. Using chimeric receptors between human and murine M-CSFR, we showed that the effects of M-CSF on EML cell differentiation potential are mediated by a large region in the intracellular domain of murine M-CSFR. Furthermore, phospholipase C (PLC) inhibitor U73122 interfered with the negative effects of ligand-activated murine M-CSFR on EML cell erythroid potential.

CONCLUSION

We propose that signaling pathways activated by tyrosine kinase receptors may regulate erythroid potential and commitment decisions in multipotent progenitor cells and that PLC may play a key role in this process.

Combination of granulocyte colony-stimulating factor and low-dose cytosine arabinoside further enhances myeloid differentiation in leukemia cells in vitro

We examined the differentiation-inducing effect on freshly isolated myeloid leukemia cells in liquid suspension culture by combined treatment with granulocyte colony-stimulating factor (G-CSF) plus low-dose cytosine arabinoside (Ara-C; 5-10 ng/ml) in 25 patients with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) in leukemic transformation. Culture with G-CSF alone showed leukemic cell growth stimulation in 15 out of the 25 cases (60%) and induction of cell differentiation in 19 out of the 25 cases (76%), respectively. In 23 cases (92%), either growth stimulation and/or differentiation induction of leukemia cells was observed in response to G-CSF. This suggests that most myeloid leukemia cells are able to respond to G-CSF stimulation. In addition, treatment of cells with low-dose Ara-C alone resulted in the enhancement of myeloid specific antigens expression in 16 cases (64%). Treatment of leukemia cells with higher concentrations of Ara-C (over 50 ng/ml) alone resulted in cytocidal effects but not in the induction of differentiation. Furthermore, 15 cases (60%) showed pronounced myeloid differentiation of leukemia cells after combined exposure to G-CSF plus low-dose Ara-C as compared with cells treated with either G-CSF or Ara-C alone. The enhanced effect of differentiation induction by combining G-CSF plus low-dose Ara-C was also observed in a murine myeloid leukemia cell line WEHI-3B in vitro. These data suggest that treatment with G-CSF plus low-dose Ara-C is capable of inducing differentiation of leukemic cells in vitro, and also appears to be useful for the differentiation-based therapy of patients with AML and MDS.

Hematopoietic stem cells with controllable tEpoR transgenes have a competitive advantage in bone marrow transplantation

In a previous study, it was found that a truncated erythropoietin receptor transgene (tEpoR tg) enables multilineage hematopoietic progenitor amplification after treatment with erythropoietin (epo) in vitro and in vivo. This study used competitive bone marrow (BM) repopulation to show that tEpoR tg facilitates transplantation by hematopoietic stem cells (HSC). Individual multilineage colonies, committed myeloid progenitor colonies, and lymphoid colonies (pre-B colony-forming units) were grown from the marrow of animals 6 months after they received a 50/50 mixture of transgene and wild-type BM cells. In epo-treated recipients, the transgene-bearing cells significantly outcompeted the wild-type cells (84%-100% versus 16%-0%, respectively). In recipients treated with phosphate-buffered saline, the repopulation was minimally different from the donor mixture (49%-64% transgene versus 51%-36% wild-type). The epo-induced repopulation advantage is maintained in secondary transplants. In addition, neither accelerated HSC depletion nor uncontrollable proliferation occurred during epo-stimulated serial transplants of transgene-containing BM. Thus, the tEpoR tg functions in a benign fashion in HSC and allows for a significant and controllable repopulation advantage in vivo without excessive HSC depletion relative to wild-type BM.

Hematopoietic stem cells with controllable tEpoR transgenes have a competitive advantage in bone marrow transplantation

In a previous study, it was found that a truncated erythropoietin receptor transgene (tEpoR tg) enables multilineage hematopoietic progenitor amplification after treatment with erythropoietin (epo) in vitro and in vivo. This study used competitive bone marrow (BM) repopulation to show that tEpoR tg facilitates transplantation by hematopoietic stem cells (HSC). Individual multilineage colonies, committed myeloid progenitor colonies, and lymphoid colonies (pre-B colony-forming units) were grown from the marrow of animals 6 months after they received a 50/50 mixture of transgene and wild-type BM cells. In epo-treated recipients, the transgene-bearing cells significantly outcompeted the wild-type cells (84%-100% versus 16%-0%, respectively). In recipients treated with phosphate-buffered saline, the repopulation was minimally different from the donor mixture (49%-64% transgene versus 51%-36% wild-type). The epo-induced repopulation advantage is maintained in secondary transplants. In addition, neither accelerated HSC depletion nor uncontrollable proliferation occurred during epo-stimulated serial transplants of transgene-containing BM. Thus, the tEpoR tg functions in a benign fashion in HSC and allows for a significant and controllable repopulation advantage in vivo without excessive HSC depletion relative to wild-type BM.

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.

Specificity of signaling by hematopoietic cytokine receptors: instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-dimensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system. The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine receptor superfamily and signal by activating cytoplasmic tyrosine kinases of the Janus kinase (Jak) family and other downstream signaling pathways. The relevance of each of these pathways for eliciting a specific cellular response remains to be determined. This review will focus on cytokine receptors which play a role in the regulation of hematopoiesis and summarize data the address the question how specificity of signaling is achieved.

Human Granulocyte Colony-Stimulating Factor (G-CSF) Stimulates the In Vitro and In Vivo Development But Not Commitment of Primitive Multipotential Progenitors From Transgenic Mice Expressing the Human G-CSF Receptor

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation and restricted differentiation of hematopoietic progenitors into neutrophils. To clarify the effects of G-CSF on hematopoietic progenitors, we generated transgenic (Tg) mice that had ubiquitous expression of the human G-CSF receptor (hG-CSFR). In clonal cultures of bone marrow and spleen cells obtained from these mice, hG-CSF supported the growth of myelocytic as well as megakaryocytic, mast cell, mixed, and blast cell colonies. Single-cell cultures of lineage-negative (Lin−)c-Kit+Sca-1+ or Sca-1− cells obtained from the Tg mice confirmed the direct effects of hG-CSF on the proliferation and differentiation of various progenitors. hG-CSF also had stimulatory effects on the formation of blast cell colonies in cultures using 5-fluorouracil–resistant hematopoietic progenitors and clone-sorted Lin−c-Kit+Sca-1+ primitive hematopoietic cells. These colonies contained different progenitors in proportions similar to those obtained when mouse interleukin-3 was used in place of hG-CSF. Administration of hG-CSF to Tg mice led to significant increases in spleen colony-forming and mixed/blast cell colony-forming cells in bone marrow and spleen, but did not alter the proportion of myeloid progenitors in total clonogenic cells. These results show that, when functional G-CSFR is present on the cell surface, hG-CSF stimulates the development of primitive multipotential progenitors both in vitro and in vivo, but does not induce exclusive commitment to the myeloid lineage.

Human Granulocyte Colony-Stimulating Factor (G-CSF) Stimulates the In Vitro and In Vivo Development But Not Commitment of Primitive Multipotential Progenitors From Transgenic Mice Expressing the Human G-CSF Receptor

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation and restricted differentiation of hematopoietic progenitors into neutrophils. To clarify the effects of G-CSF on hematopoietic progenitors, we generated transgenic (Tg) mice that had ubiquitous expression of the human G-CSF receptor (hG-CSFR). In clonal cultures of bone marrow and spleen cells obtained from these mice, hG-CSF supported the growth of myelocytic as well as megakaryocytic, mast cell, mixed, and blast cell colonies. Single-cell cultures of lineage-negative (Lin−)c-Kit+Sca-1+ or Sca-1− cells obtained from the Tg mice confirmed the direct effects of hG-CSF on the proliferation and differentiation of various progenitors. hG-CSF also had stimulatory effects on the formation of blast cell colonies in cultures using 5-fluorouracil–resistant hematopoietic progenitors and clone-sorted Lin−c-Kit+Sca-1+ primitive hematopoietic cells. These colonies contained different progenitors in proportions similar to those obtained when mouse interleukin-3 was used in place of hG-CSF. Administration of hG-CSF to Tg mice led to significant increases in spleen colony-forming and mixed/blast cell colony-forming cells in bone marrow and spleen, but did not alter the proportion of myeloid progenitors in total clonogenic cells. These results show that, when functional G-CSFR is present on the cell surface, hG-CSF stimulates the development of primitive multipotential progenitors both in vitro and in vivo, but does not induce exclusive commitment to the myeloid lineage.

Specific Signals Generated by the Cytoplasmic Domain of the Granulocyte Colony-Stimulating Factor (G-CSF) Receptor Are Not Required for G-CSF–Dependent Granulocytic Differentiation

Granulocyte colony-stimulating factor (G-CSF) is the principal growth factor regulating the production of neutrophils, yet its role in lineage commitment and terminal differentiation of hematopoietic progenitor cells is controversial. In this study, we describe a system to study the role of G-CSF receptor (G-CSFR) signals in granulocytic differentiation using retroviral transduction of G-CSFR–deficient, primary hematopoietic progenitor cells. We show that ectopic expression of wild-type G-CSFR in hematopoietic progenitor cells supports G-CSF–dependent differentiation of these cells into mature granulocytes, macrophages, megakaryocytes, and erythroid cells. Furthermore, we show that two mutant G-CSFR proteins, a truncation mutant that deletes the carboxy-terminal 96 amino acids and a chimeric receptor containing the extracellular and transmembrane domains of the G-CSFR fused to the cytoplasmic domain of the erythropoietin receptor, are able to support the production of morphologically mature, chloroacetate esterase-positive, Gr-1/Mac-1–positive neutrophils in response to G-CSF. These results demonstrate that ectopic expression of the G-CSFR in hematopoietic progenitor cells allows for multilineage differentiation and suggest that unique signals generated by the cytoplasmic domain of the G-CSFR are not required for G-CSF–dependent granulocytic differentiation.

Specific Signals Generated by the Cytoplasmic Domain of the Granulocyte Colony-Stimulating Factor (G-CSF) Receptor Are Not Required for G-CSF–Dependent Granulocytic Differentiation

Granulocyte colony-stimulating factor (G-CSF) is the principal growth factor regulating the production of neutrophils, yet its role in lineage commitment and terminal differentiation of hematopoietic progenitor cells is controversial. In this study, we describe a system to study the role of G-CSF receptor (G-CSFR) signals in granulocytic differentiation using retroviral transduction of G-CSFR–deficient, primary hematopoietic progenitor cells. We show that ectopic expression of wild-type G-CSFR in hematopoietic progenitor cells supports G-CSF–dependent differentiation of these cells into mature granulocytes, macrophages, megakaryocytes, and erythroid cells. Furthermore, we show that two mutant G-CSFR proteins, a truncation mutant that deletes the carboxy-terminal 96 amino acids and a chimeric receptor containing the extracellular and transmembrane domains of the G-CSFR fused to the cytoplasmic domain of the erythropoietin receptor, are able to support the production of morphologically mature, chloroacetate esterase-positive, Gr-1/Mac-1–positive neutrophils in response to G-CSF. These results demonstrate that ectopic expression of the G-CSFR in hematopoietic progenitor cells allows for multilineage differentiation and suggest that unique signals generated by the cytoplasmic domain of the G-CSFR are not required for G-CSF–dependent granulocytic differentiation.

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF)-dependent in vitro and in vivo proliferation and differentiation of all hematopoietic progenitor cells in hGM-CSF receptor transgenic mice

To examine the relation between receptor expression and differentiation of hematopoietic cells, we produced transgenic mice that constitutively expressed the human granulocyte-macrophage colony stimulating factor (hGM-CSF) receptor at almost all stages of hematopoietic cell development. The high-affinity GM-CSF receptor is species specific, allowing analysis of the specific effects of hGM-CSF in our mouse model. Proliferation and differentiation of hematopoietic progenitor cells from transgenic mice were analyzed by means of methylcellulose colony-forming assay and in vivo treatment with hGM-CSF, respectively. We found that hGM-CSF supported various types of colonies, including granulocyte-macrophage, mast cell, megakaryocyte, blast cell, and mixed hematopoietic colonies, whereas mouse GM-CSF supported only granulocyte-macrophage colonies. In addition, hGM-CSF generated erythrocyte colonies in the absence of erythropoietin. Furthermore, in vivo administration of hGM-CSF to transgenic mice resulted in a dose-dependent increase in reticulocytes and white blood cells in the peripheral blood. The spleens of the mice showed gross enlargement, mainly caused by an increase of erythroid cells and their progenitors. Taken together, these results indicate that hGM-CSF receptor-mediated signals can support the growth of cells of all hematopoietic cell lineages if this receptor is present on the cell surface. This implies that the differentiation of hematopoietic progenitor cells is not determined by exogenous cytokine stimulation (instruction model) but by an intrinsic cell program in which cytokines simply select cells that express the appropriate receptor (stochastic model).

Hematopoietic and Lymphopoietic Responses in Human Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF ) Receptor Transgenic Mice Injected With Human GM-CSF

Using a clonal assay of bone marrow (BM) cells from transgenic mice (Tg-mice) expressing the human granulocyte-macrophage colony-stimulating factor receptor (hGM-CSFR), we found in earlier studies that hGM-CSF alone supported the development not only of granulocyte-macrophage colonies, but also of erythrocytes, megakaryocytes, mast cells, blast cells, and mixed hematopoietic colonies. In this report, we evaluated the in vivo effects of hGM-CSF on hematopoietic and lymphopoietic responses in the hGM-CSFR Tg-mice. Administration of this factor to Tg-mice resulted in dose-dependent increases in numbers of reticulocytes and white blood cells (WBCs) in the peripheral blood. Morphological analysis of WBCs showed that the numbers of all types of the cell, including neutrophils, eosinophils, monocytes, and lymphocytes increased; the most remarkable being in lymphocytes that contained a number of large granular lymphocytes (LGLs) in addition to mature T and B cells. However, total cellularity of the BM of the Tg-mice decreased in a dose-dependent manner when hGM-CSF was injected. In sharp contrast to the BM, spleens of the Tg-mice were grossly enlarged. Although all types of blood cells and hematopoietic progenitors increased in the spleen, erythroid cells and their progenitors showed the most significant increase. Increased numbers of megakaryocytes and LGLs were also observed in spleen and liver of the treated Tg-mice. Flow cytometric analysis showed that LGLs expanded in Tg-mice expressed Mac-1+CD3−NK1.1+. The thymus of Tg-mice treated with hGM-CSF exhibited a dose-dependent shrinkage and a remarkable decrease in CD4+CD8+ cells. Thus, hGM-CSF stimulated not only myelopoiesis but also erythropoiesis and megakaryopoiesis of hGM-CSFR Tg-mice in vivo, in accordance with our reported in vitro findings. In addition, hGM-CSF affected the development of lymphoid cells, including natural killer cells of these Tg-mice.

Hematopoietic and Lymphopoietic Responses in Human Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF ) Receptor Transgenic Mice Injected With Human GM-CSF

Using a clonal assay of bone marrow (BM) cells from transgenic mice (Tg-mice) expressing the human granulocyte-macrophage colony-stimulating factor receptor (hGM-CSFR), we found in earlier studies that hGM-CSF alone supported the development not only of granulocyte-macrophage colonies, but also of erythrocytes, megakaryocytes, mast cells, blast cells, and mixed hematopoietic colonies. In this report, we evaluated the in vivo effects of hGM-CSF on hematopoietic and lymphopoietic responses in the hGM-CSFR Tg-mice. Administration of this factor to Tg-mice resulted in dose-dependent increases in numbers of reticulocytes and white blood cells (WBCs) in the peripheral blood. Morphological analysis of WBCs showed that the numbers of all types of the cell, including neutrophils, eosinophils, monocytes, and lymphocytes increased; the most remarkable being in lymphocytes that contained a number of large granular lymphocytes (LGLs) in addition to mature T and B cells. However, total cellularity of the BM of the Tg-mice decreased in a dose-dependent manner when hGM-CSF was injected. In sharp contrast to the BM, spleens of the Tg-mice were grossly enlarged. Although all types of blood cells and hematopoietic progenitors increased in the spleen, erythroid cells and their progenitors showed the most significant increase. Increased numbers of megakaryocytes and LGLs were also observed in spleen and liver of the treated Tg-mice. Flow cytometric analysis showed that LGLs expanded in Tg-mice expressed Mac-1+CD3−NK1.1+. The thymus of Tg-mice treated with hGM-CSF exhibited a dose-dependent shrinkage and a remarkable decrease in CD4+CD8+ cells. Thus, hGM-CSF stimulated not only myelopoiesis but also erythropoiesis and megakaryopoiesis of hGM-CSFR Tg-mice in vivo, in accordance with our reported in vitro findings. In addition, hGM-CSF affected the development of lymphoid cells, including natural killer cells of these Tg-mice.

Thrombopoietin Does Not Induce Lineage-Restricted Commitment of Mpl-R Expressing Pluripotent Progenitors But Permits Their Complete Erythroid and Megakaryocytic Differentiation

In this study, we examined the in vitro and in vivo effects of forced expression of Mpl-R (the thrombopoietin receptor) on the progeny of murine hematopoietic stem cells. Bone marrow cells from 5-FU–treated mice were transduced with retroviral vectors containing the human Mpl-R cDNA, or the neomycine gene as a control. After 7 days cocultivation on virus-producer cells, GpE86-Mpl-R or Gp86-Neo, the types of hematopoietic progenitor cells responding to thrombopoietin (TPO) were studied by clonogenic assays. Mpl-R–infected cells gave rise to CFU-GEMM, BFU-E, CFU-MK, but not CFU-GM while Neo-infected cells produced only megakaryocytic colonies. In addition, when nonadherent cells from GpE86-Mpl-R cocultures were grown with TPO as the only stimulus for 7 days, a marked expansion of CFU-GEMM, BFU-E, and CFU-MK was observed, while no change in CFU-GM number was seen. Erythroid and megakaryocytic maturation occurred in the presence of TPO while a block in granulocytic differentiation was observed at the myeloblast stage. The direct effects of TPO on Mpl-R–transduced progenitor cells were demonstrated by single cell cloning experiments. To analyze the effects of the constitutive expression of Mpl-R on the determination of multipotent progenitors (CFU-S) and long-term repopulating stem cells, Mpl-R– or Neo-infected cells were injected into lethally irradiated recipient mice. No difference was seen in (1) the number of committed progenitor cells contained in individual CFU-S12 whether colonies arose from noninfected or Mpl-R–infected CFU-S; (2) the mean numbers of progenitor cells per leg or spleen of mice reconstituted with Mpl-R– or Neo-infected cells, 1 or 7 months after the graft; and (3) the blood parameters of the two groups of animals, with the exception of a 50% reduction in circulating platelet counts after 7 months in mice repopulated with Mpl-R–infected bone marrow cells. These results indicate that retrovirus-mediated expression of Mpl-R in murine stem cells does not modify their ability to reconstitute all myeloid lineages of differentiation and does not result in a preferential commitment toward the megakaryocytic lineage.

Thrombopoietin Does Not Induce Lineage-Restricted Commitment of Mpl-R Expressing Pluripotent Progenitors But Permits Their Complete Erythroid and Megakaryocytic Differentiation

In this study, we examined the in vitro and in vivo effects of forced expression of Mpl-R (the thrombopoietin receptor) on the progeny of murine hematopoietic stem cells. Bone marrow cells from 5-FU–treated mice were transduced with retroviral vectors containing the human Mpl-R cDNA, or the neomycine gene as a control. After 7 days cocultivation on virus-producer cells, GpE86-Mpl-R or Gp86-Neo, the types of hematopoietic progenitor cells responding to thrombopoietin (TPO) were studied by clonogenic assays. Mpl-R–infected cells gave rise to CFU-GEMM, BFU-E, CFU-MK, but not CFU-GM while Neo-infected cells produced only megakaryocytic colonies. In addition, when nonadherent cells from GpE86-Mpl-R cocultures were grown with TPO as the only stimulus for 7 days, a marked expansion of CFU-GEMM, BFU-E, and CFU-MK was observed, while no change in CFU-GM number was seen. Erythroid and megakaryocytic maturation occurred in the presence of TPO while a block in granulocytic differentiation was observed at the myeloblast stage. The direct effects of TPO on Mpl-R–transduced progenitor cells were demonstrated by single cell cloning experiments. To analyze the effects of the constitutive expression of Mpl-R on the determination of multipotent progenitors (CFU-S) and long-term repopulating stem cells, Mpl-R– or Neo-infected cells were injected into lethally irradiated recipient mice. No difference was seen in (1) the number of committed progenitor cells contained in individual CFU-S12 whether colonies arose from noninfected or Mpl-R–infected CFU-S; (2) the mean numbers of progenitor cells per leg or spleen of mice reconstituted with Mpl-R– or Neo-infected cells, 1 or 7 months after the graft; and (3) the blood parameters of the two groups of animals, with the exception of a 50% reduction in circulating platelet counts after 7 months in mice repopulated with Mpl-R–infected bone marrow cells. These results indicate that retrovirus-mediated expression of Mpl-R in murine stem cells does not modify their ability to reconstitute all myeloid lineages of differentiation and does not result in a preferential commitment toward the megakaryocytic lineage.

Receptors That Induce Erythroid Differentiation of Ba/F3 Cells: Structural Requirements and Effect on STAT5 Binding

Ectopic expression of the erythropoietin receptor (EpoR) in the interleukin-3 (IL-3)–dependent cell line Ba/F3 results in growth and partial erythroid differentiation in Epo. In contrast, introduction and activation of the interleukin-5 receptor (IL-5R) or of the granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) results in proliferation only. As this effect is specific to the EpoR, the role of its extracellular or cytoplasmic domain in differentiation was tested after construction of two chimeric receptors. One receptor contained the extracellular domain of EpoR fused to the endodomain of IL-3R β-chain (E/β), while the other contained the EpoR cytoplasmic region fused to the extracellular domain of GM-CSFR α-chain (GMER). Surprisingly, both receptors induced differentiation ruling out a strict specificity of the extracellular or cytoplasmic region of EpoR in this process. Instead the ability to signal differentiation correlated with structural features shared by the EpoR, GMER, and E/β receptors. Dimerization of all three receptors results in the pairing of two signal transducing chains in the cytoplasm, in contrast to the mitogenic receptors IL-3R, IL-5R, GM-CSFR, which assemble as αβ heterodimers. Two new chimeric receptors that fulfilled the structural requirement exemplified by EpoR, but lacked any part of EpoR, were designed to consolidate this model. They consisted of the ectodomains of the GMR-α and IL-5Rα, respectively, fused to the endodomain of IL-3R β-chain. Both receptors were as effective as EpoR in signaling differentiation in response to their cognate ligand. Another property of receptors fulfilling these structural requirements is that they cause a marked delay in signal transducers and activators of transcription 5 (STAT5) activation on ligand stimulation. Taken together our studies show that structural assembly of receptors dictates their potential to signal erythroid differentiation in Ba/F3 cells, that differentiation can take place in the absence of Epo and that a delay in STAT5 activation is highly predictive of this process.

Ectopic Expression of the Erythropoietin Receptor in a Murine Interleukin-6–Dependent Plasmacytoma Cell Line (TEPC-2027) Confers Proliferative Responsiveness to Erythropoietin

To compare the signal transduction pathways used by erythropoietin (Epo) and interleukin-6 (IL-6), the cDNA for the murine Epo receptor (Epo-R) was introduced into an IL-6–responsive plasmacytoma cell line (TEPC-2027) by retrovirally mediated gene transfer. G418-resistant clones were amplified in IL-6 and studied for their ability to grow and differentiate in response to Epo. Epo-R synthesized from the viral gene showed the same affinity for Epo as did the receptor on erythroid cells; however, the numbers of Epo receptors expressed on the cell membrane varied among clones. After a delay of 3 to 5 days in the presence of Epo, all the clones studied proliferated as well in response to Epo as in response to IL-6. In response to IL-6, Stat3 was activated and JunB mRNA was accumulated, whereas in response to Epo, Jak2 and Stat5 were activated and JunB mRNA was not accumulated in Epo-R–expressing TEPC (Epo-R/TEPC) cells. These results suggest that Epo and IL-6 transduced their proliferative signals through different pathways. Further studies showed that, in Epo-R/TEPC cells, Epo neither induces the synthesis of erythroid-specific mRNA nor modifies the synthesis of γ1 Ig heavy chain, suggesting that ectopic expression of the Epo-R in plasmacytoma cells does not modify their differentiative potential. The data show that Epo induces a proliferative response without differentiation providing a new cellular model for evaluating molecular events specific for proliferation.

Erythropoietin-induced erythroid differentiation of the human erythroleukemia cell line TF-1 correlates with impaired STAT5 activation

The TF-1 cell line has been established from a patient with erythroleukemia. While various cytokines induce TF-1 cell proliferation, erythropoietin (Epo) only sustains the short-term growth of these cells and induces their differentiation along the erythroid lineage. A truncated Epo receptor (EpoR) is overexpressed in these cells. The truncation removed the 96 C-terminal amino acids, including seven tyrosine residues. An additional single mutation at position +3 of Tyr344 led to the replacement of leucine 347 by proline. Stimulation by Epo induced an impaired activation of the STAT5 transcription factor in these cells. The same defect in STAT5 activation was found in the murine FDCP-1 cell line transfected with a chimeric EpoR containing the abnormal TF-1 EpoR cytoplasmic domain. Infection of TF-1 cells with a retrovirus containing a normal murine EpoR was able to restore both Epo-induced STAT5 activity and cellular proliferation. In contrast, Epo-induced differentiation was reduced strongly in infected TF-1ER cells. These results suggest that Epo-induced differentiation correlates with impaired Epo-induced STAT5 activation.