Combination Of Stem Cell Factor Research Articles

Tumor necrosis factor-alpha inhibits stem cell factor-induced proliferation of human bone marrow progenitor cells in vitro. Role of p55 and p75 tumor necrosis factor receptors.

Stem cell factor (SCF), a key regulator of hematopoiesis, potently synergizes with a number of hematopoietic growth factors. However, little is known about growth factors capable of inhibiting the actions of SCF. TNF-alpha has been shown to act as a bidirectional regulator of myeloid cell proliferation and differentiation. This study was designed to examine interactions between TNF-alpha and SCF. Here, we demonstrate that TNF-alpha potently and directly inhibits SCF-stimulated proliferation of CD34+ hematopoietic progenitor cells. Furthermore, TNF-alpha blocked all colony formation stimulated by SCF in combination with granulocyte colony-stimulating factor (CSF) or CSF-1. The synergistic effect of SCF observed in combination with GM-CSF or IL-3 was also inhibited by TNF-alpha, resulting in colony numbers similar to those obtained in the absence of SCF. These effects of TNF-alpha were mediated through the p55 TNF receptor, whereas little or no inhibition was signaled through the p75 TNF receptor. Finally, TNF-alpha downregulated c-kit cell-surface expression on CD34+ bone marrow cells, and this was predominantly a p55 TNF receptor-mediated event as well.

Protection from Lethal Irradiation by the Combination of Stem Cell Factor and Tempol

Cytokines that stimulate growth and differentiation of hematopoietic precursor cells have been used as protectors in vivo against ionizing radiation. Recently, we have shown that the nitroxide tempol is also an effective radiation protector in vivo. The purpose of the present study was to determine if the combination of tempol with stem cell factor (SCF, c-kit ligand) would provide enhanced radiation protection in C57 mice compared with the protection afforded by either agent alone. Mice were exposed to whole-body irradiation and assessed for survival at 30 days after irradiation. No control mice survived doses of more than 9 Gy. Treatment of mice before and after radiation with SCF alone (100 micrograms/kg at -20 h, -4 h and +4 h) protected mice from radiation at doses of as high as 10 Gy (76% survival). Tempol (350 mg/kg) given 10 min prior to radiation was a radioprotector at 9 Gy (55% survival). The combination of SCF and tempol increased the survival of mice exposed to radiation doses up to 11 Gy (32% survival for the combination vs 4% for SCF alone and 0% for tempol alone; P < 0.001 for the combination vs either agent alone). Lower doses of SCF alone (1 microgram/kg) or tempol alone (275 mg/kg) did not protect mice from radiation. However, the combination of these reduced doses of SCF and tempol protected mice from lethal irradiation at 10 Gy. Stem cell factor and tempol given either singly or together were well tolerated by the animals. These data show that SCF and tempol are radiation protectors and that their radioprotective effects are more than additive when the agents are given together.

The in vitro effects of stem cell factor, interleukin 3 and granulocyte-macrophage colony stimulating factor on haemopoietic progenitor cells from premature infants.

Circulating haemopoietic progenitor cells from premature infants were assessed for their ability to respond to interleukin 3, granulocyte-macrophage colony stimulating factor and stem cell factor (SCF) in vitro. All three cytokines increased the number of colonies derived from burst forming units erythroid (BFU-E), colony forming units granulocyte-macrophage (CFU-GM) and multi-lineage progenitors (CFU-Mix) grown in the presence of erythropoietin (Epo). The size and haemoglobin content of BFU-E derived colonies also increased in the presence of the cytokines. Of those tested, SCF was found to be the most potent additive to Epo for the enhanced growth of BFU-E and CFU-Mix. In short-term liquid cultures without Epo, SCF alone induced globin synthesizing cells. Progenitors from premature infants were at least as responsive to all three cytokines as those from healthy adults. The use of SCF in combination with Epo in the prevention or treatment of anaemia in premature infants warrants further investigation.

Stem cell factor and basic fibroblast growth factor are synergistic in augmenting committed myeloid progenitor cell growth

Stem cell factor (SCF) and basic fibroblast growth factor (bFGF) are hematopoietic cytokines produced by bone marrow stromal cells. It is known that, although SCF and bFGF have limited clonogenic activity on their own, they can augment colony-stimulating factor (CSF)-mediated progenitor cell growth. Because these factors are both sequestered by stromal cells, we examined their interaction on progenitor cell growth in conjunction with granulocyte-macrophage-CSF (GM-CSF). In this study, we show that clonogenic growth derived from low-density bone marrow cells stimulated by GM-CSF is significantly augmented (P &lt; .001) in the presence of maximal (100 ng/mL) concentrations of SCF in combination with 100 ng/mL of bFGF. When CD34+ cells are used, the synergistic effect of bFGF and SCF for GM-CSF-mediated progenitor cell growth is further increased, resulting in as much as a sevenfold increase in detectable colony-forming units granulocyte-macrophage (P &lt; .001). These data suggest that the synergistic activity of bFGF and SCF is mediated directly on hematopoietic precursors. These observations suggest that bFGF and SCF, concentrated locally on stromal cell surfaces, might interact in concert with other hematopoietic cytokines to regulate stem cell proliferation and differentiation in hematopoietic niches in the bone marrow.

Stem Cell Factor and Basic Fibroblast Growth Factor Are Synergistic in Augmenting Committed Myeloid Progenitor Cell Growth

Stem cell factor (SCF) and basic fibroblast growth factor (bFGF) are hematopoietic cytokines produced by bone marrow stromal cells. It is known that, although SCF and bFGF have limited clonogenic activity on their own, they can augment colony-stimulating factor (CSF)-mediated progenitor cell growth. Because these factors are both sequestered by stromal cells, we examined their interaction on progenitor cell growth in conjunction with granulocyte-macrophage-CSF (GM-CSF). In this study, we show that clonogenic growth derived from low-density bone marrow cells stimulated by GM-CSF is significantly augmented (P < .001) in the presence of maximal (100 ng/mL) concentrations of SCF in combination with 100 ng/mL of bFGF. When CD34+ cells are used, the synergistic effect of bFGF and SCF for GM-CSF-mediated progenitor cell growth is further increased, resulting in as much as a sevenfold increase in detectable colony-forming units granulocyte-macrophage (P < .001). These data suggest that the synergistic activity of bFGF and SCF is mediated directly on hematopoietic precursors. These observations suggest that bFGF and SCF, concentrated locally on stromal cell surfaces, might interact in concert with other hematopoietic cytokines to regulate stem cell proliferation and differentiation in hematopoietic niches in the bone marrow.

Growth factors and the molecular control of haematopoiesis.

In the absence of appropriate growth factors, for example interleukin-3 or GM-CSF, cultured bone marrow stem cells die by a process known as apoptosis or programmed cell death. Apoptosis may occur in vivo when concentrations of specific growth factors are limiting and may be a means of regulating cell numbers. Growth factors are also essential for proliferation of bone marrow stem cells but differentiation can occur, provided there is a survival stimulus in the absence of growth factors. Combinations of growth factors may be synergistic in stimulating the survival and proliferation of multipotent stem cells. Although neither stem cell factor, nor GM-CSF alone can significantly induce the proliferation of stem cells, the combination induces the proliferation of these cells. Committed progenitor cells such as granulocyte-macrophage colony-forming cells, however, are stimulated to proliferate by GM-CSF alone, while stem cell factor in combination with GM-CSF results in only a slight additive effect. To date, most research has concentrated on the growth stimulatory factors. GM-CSF has an important role in the reversal of chemotherapy-induced myelosuppression in cancer patients and in other bone marrow disorders. A number of growth inhibitory molecules have now been identified, such as macrophage inhibitory protein-1 alpha. In the future, it is possible that improvements in cure rates may be achieved in cancer patients by combining the growth inhibitory factors with the stimulatory factors. Inhibitory factors may be given before chemotherapy to prevent toxicity and stimulatory factors may be given afterwards to treat neutropenic patients.

Effects of stem cell factor and granulocyte colony-stimulating factor on granulocyte recovery and Candida albicans infection in granulocytopenic mice.

The combined action of stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) on granulocyte recovery and infection with Candida albicans was studied. In mice rendered granulocytopenic with 200 mg/kg cyclophosphamide, daily coinjection of SCF (100 micrograms/kg) and G-CSF (10 micrograms/kg) for 5 days synergistically stimulated granulocyte recovery compared with either factor alone. Growth of C. albicans in the kidneys of granulocytopenic mice with experimental C. albicans infection was reduced by the combined regimen to 25% of that in vehicle-treated controls (P = .018), whereas either factor administered alone in the same doses had no effect. The combined regimen also significantly protected against lethal infection with C. albicans (P < .01). Either factor administered alone in the same doses had no effect on lethality compared with vehicle-treated controls. The combination of SCF and G-CSF may be useful in the treatment of infections with C. albicans in the granulocytopenic host.

Severe Congenital Neutropenia: Abnormal Growth and Differentiation of Myeloid Progenitors to Granulocyte Colony-Stimulating Factor (G-CSF) But Normal Response to G-CSF Plus Stem Cell Factor

Several mechanisms have been proposed to explain the pathogenesis of severe congenital neutropenia (SCN); however, the mechanism(s) still remains unknown. In particular, clinical observations suggest that abnormal responsiveness of myeloid progenitors to hematopoietic growth factors (HGFs) is a possible mechanism. Therefore, to better define the status of hematopoietic progenitors in the bone marrow (BM) of patients with SCN, the responsiveness of myeloid progenitors to HGFs from two SCN patients was compared with the responsiveness of progenitors from healthy individuals. BM cells (BMCs) from the first SCN patient required higher (10- to 100-fold) concentrations of granulocyte colony-stimulating factor (G-CSF) to achieve maximal and half-maximal colony growth in vitro compared with BMCs from controls. In contrast, the dose-response of interleukin-3 (IL-3) and granulocyte-macrophage-CSF (GM-CSF) in colony formation was normal. Interestingly, IL-3, GM-CSF, and G-CSF at optimal doses showed reduced ability to induce neutrophil differentiation of BMCs from a SCN patient compared with BMCs from controls. Despite an abnormal responsiveness of mature myeloid progenitors to G-CSF in this SCN patient, myeloid progenitors responsive to the combination of stem cell factor (SCF) and G-CSF showed normal dose-response. In contrast to G-CSF alone, the combination of G-CSF and SCF induced the formation of neutrophils almost to the same extent compared with cultures of normal BMCs. Furthermore, also on BM progenitor cells obtained from the second patient with SCN, SCF highly synergized with G-CSF to promote neutrophil progenitor cell growth and differentiation in vitro. Thus, these results indicate that one mechanism of the pathogenesis in SCN patients is reduced responsiveness of neutrophil progenitor cells to G-CSF and that SCF can enhance the responsiveness of these cells to G-CSF.

Severe congenital neutropenia: abnormal growth and differentiation of myeloid progenitors to granulocyte colony-stimulating factor (G-CSF) but normal response to G-CSF plus stem cell factor

Several mechanisms have been proposed to explain the pathogenesis of severe congenital neutropenia (SCN); however, the mechanism(s) still remains unknown. In particular, clinical observations suggest that abnormal responsiveness of myeloid progenitors to hematopoietic growth factors (HGFs) is a possible mechanism. Therefore, to better define the status of hematopoietic progenitors in the bone marrow (BM) of patients with SCN, the responsiveness of myeloid progenitors to HGFs from two SCN patients was compared with the responsiveness of progenitors from healthy individuals. BM cells (BMCs) from the first SCN patient required higher (10- to 100-fold) concentrations of granulocyte colony- stimulating factor (G-CSF) to achieve maximal and half-maximal colony growth in vitro compared with BMCs from controls. In contrast, the dose- response of interleukin-3 (IL-3) and granulocyte-macrophage-CSF (GM- CSF) in colony formation was normal. Interestingly, IL-3, GM-CSF, and G- CSF at optimal doses showed reduced ability to induce neutrophil differentiation of BMCs from a SCN patient compared with BMCs from controls. Despite an abnormal responsiveness of mature myeloid progenitors to G-CSF in this SCN patient, myeloid progenitors responsive to the combination of stem cell factor (SCF) and G-CSF showed normal dose-response. In contrast to G-CSF alone, the combination of G-CSF and SCF induced the formation of neutrophils almost to the same extent compared with cultures of normal BMCs. Furthermore, also on BM progenitor cells obtained from the second patient with SCN, SCF highly synergized with G-CSF to promote neutrophil progenitor cell growth and differentiation in vitro. Thus, these results indicate that one mechanism of the pathogenesis in SCN patients is reduced responsiveness of neutrophil progenitor cells to G- CSF and that SCF can enhance the responsiveness of these cells to G-CSF.

The role of stem cell factor in mobilization of peripheral blood progenitor cells: synergy with G-CSF.

The use of cytokine mobilized peripheral blood progenitor cells (PBPC) in transplantation following chemotherapy has led to enhanced engraftment. Granulocyte-colony stimulating factor (G-CSF) has been shown in a number of clinical studies to be an effective mobilizer of PBPC. Preclinical data in mice and primates have demonstrated a potential role for the use of stem cell factor (SCF) in mobilization of PBPC. In the studies presented here, low doses of SCF are shown to synergize with optimal doses of G-CSF to enhance the number and quality of PBPC compared to G-CSF alone. Phase I studies using r-metHuSCF demonstrated mast cell-related dose limiting effects. The data presented here have led to Phase I/II studies to evaluate the potential use of low doses of SCF in combination with G-CSF for mobilization of PBPC.

Activity of stem cell factor and IL-7 in combination on normal bone marrow B lineage cells.

The production of B cells is regulated by soluble and cell contact signals presumably provided by bone marrow stromal cells. Among these is IL-7, a well characterized proliferative stimulus for a subset of pre-B cells. Stem cell factor (SCF), a stromal cell-derived cytokine with broad hemopoietic effects, has been reported to synergize with IL-7 to drive the proliferation and differentiation of B220- bone marrow cells into B220+ B cell precursors in long term culture. A subsequent report has cast doubt on this result by showing that SCF and IL-7 were incapable of producing mu+ pre-B cells after short term culture. Here, using the cell sorter to assure discrete separation of B220+ and B220- cells followed by soft agar culture to prevent interaction with accessory cells, we demonstrate that the combination of SCF and IL-7 does not stimulate the expansion or differentiation of B220- lymphoid precursors but can act synergistically in the clonal proliferation of B220+ cells.

Functional expression of c-kit by acute myelogenous leukemia blasts is enhanced by tumor necrosis factor-alpha through posttranscriptional mRNA stabilization by a labile protein

The c-kit proto-oncogene encodes a transmembrane glycoprotein identical to the receptor for the recently cloned stem cell factor (SCF). The present study examines constitutive synthesis of transcripts in primary acute myelogenous leukemia (AML) blasts and the effects of recombinant human tumor necrosis factor (TNF)-alpha on c-kit mRNA expression in these cells. The c-kit transcripts were detectable at low levels in 10 of 10 different AML samples investigated. TNF treatment of AML cells was associated with enhanced c-kit mRNA expression in all specimens. Nuclear run-on transcription assays indicated that the c-kit gene was transcriptionally active in all leukemias examined and the rate of transcription was unaffected by exposure to TNF, suggesting posttranscriptional control mechanisms of c-kit mRNA accumulation. In the absence of TNF, the half-life of c-kit transcripts was 2 to 3 hours, while in TNF-treated AML cells, c-kit half-life was found to be 5 to 9 hours. Inhibition of protein synthesis reduced TNF-induced c-kit mRNA expression by Northern blot analysis, but did not affect the rate of c-kit gene transcription. In the presence of inhibition of protein synthesis, the half-life of c-kit transcripts in TNF-induced leukemia cells decreased to 2 to 4 hours. These findings indicate that levels of c-kit mRNA are controlled by a labile protein that is involved in TNF- mediated stabilization of c-kit transcripts. The effects of TNF-alpha also extended to the protein level in that TNF-alpha treatment of primary AMLs was associated with enhanced surface expression of the SCF receptor by some of these cells. While exogenous SCF induced clonogenic growth of all primary AML samples investigated, TNF-alpha failed to stimulate leukemic cells to proliferate. However, the combination of SCF and TNF-alpha resulted in synergistic growth stimulation in seven of nine different AML specimens investigated. The finding of transmodulation of the SCF receptor through posttranscriptional modifications might further contribute to our understanding of the synergistic interplay of TNF-alpha and SCF.

Functional Expression of c-kit by Acute Myelogenous Leukemia Blasts Is Enhanced by Tumor Necrosis Factor-α Through Posttranscriptional mRNA Stabilization by a Labile Protein

The c-kit proto-oncogene encodes a transmembrane glycoprotein identical to the receptor for the recently cloned stem cell factor (SCF). The present study examines constitutive synthesis of transcripts in primary acute myelogenous leukemia (AML) blasts and the effects of recombinant human tumor necrosis factor (TNF)-alpha on c-kit mRNA expression in these cells. The c-kit transcripts were detectable at low levels in 10 of 10 different AML samples investigated. TNF treatment of AML cells was associated with enhanced c-kit mRNA expression in all specimens. Nuclear run-on transcription assays indicated that the c-kit gene was transcriptionally active in all leukemias examined and the rate of transcription was unaffected by exposure to TNF, suggesting posttranscriptional control mechanisms of c-kit mRNA accumulation. In the absence of TNF, the half-life of c-kit transcripts was 2 to 3 hours, while in TNF-treated AML cells, c-kit half-life was found to be 5 to 9 hours. Inhibition of protein synthesis reduced TNF-induced c-kit mRNA expression by Northern blot analysis, but did not affect the rate of c-kit gene transcription. In the presence of inhibition of protein synthesis, the half-life of c-kit transcripts in TNF-induced leukemia cells decreased to 2 to 4 hours. These findings indicate that levels of c-kit mRNA are controlled by a labile protein that is involved in TNF-mediated stabilization of c-kit transcripts. The effects of TNF-alpha also extended to the protein level in that TNF-alpha treatment of primary AMLs was associated with enhanced surface expression of the SCF receptor by some of these cells. While exogenous SCF induced clonogenic growth of all primary AML samples investigated, TNF-alpha failed to stimulate leukemic cells to proliferate. However, the combination of SCF and TNF-alpha resulted in synergistic growth stimulation in seven of nine different AML specimens investigated. The finding of transmodulation of the SCF receptor through posttranscriptional modifications might further contribute to our understanding of the synergistic interplay of TNF-alpha and SCF.

Stem cell factor increases colony-forming unit-spleen number in vitro in synergy with interleukin-6, and in vivo in Sl/Sld mice as a single factor

Hematopoiesis is thought to be modulated by interactions of progenitor cells with hematopoietic growth factors. We have shown that colony- forming units-spleen (CFU-S) and repopulating stem cells require interleukin-3 (IL-3) to survive in vitro, and that CFU-S number and long-term repopulating ability can be increased by culture in the combination of IL-3 and IL-6. In this report, we describe the effects of stem cell factor (SCF) on CFU-S and repopulating stem cells. Injection of SCF into anemic Sl/Sld mice caused a twofold and 20-fold increase in CFU-S number in the bone marrow and spleen of treated animals, respectively. After 6 days in suspension culture, CFU-S number increased threefold in cultures supplemented with SCF and IL-6, or SCF, IL-3, and IL-6 relative to the number at day 0. The long-term repopulating ability of cells cultured in SCF, IL-3, and IL-6 was approximately sevenfold better than that of cells cultured in IL-3 or SCF. Similar experiments were performed on populations of bone marrow cells enriched for, or depleted of, CFU-S by elutriation and lineage subtraction. The combination of SCF and IL-6 increased CFU-S number approximately fourfold to eightfold in the CFU-S-enriched fraction, but had no effect on the CFU-S-depleted cells. These results show that SCF alone can increase CFU-S number in vivo, and in combination with other growth factors increases CFU-S numbers in vitro.

Stem Cell Factor Increases Colony-Forming Unit-Spleen Number In Vitro in Synergy With Interleukin-6, and In Vivo in Sl/Sld Mice as a Single Factor

Hematopoiesis is thought to be modulated by interactions of progenitor cells with hematopoietic growth factors. We have shown that colony-forming units-spleen (CFU-S) and repopulating stem cells require interleukin-3 (IL-3) to survive in vitro, and that CFU-S number and long-term repopulating ability can be increased by culture in the combination of IL-3 and IL-6. In this report, we describe the effects of stem cell factor (SCF) on CFU-S and repopulating stem cells. Injection of SCF into anemic Sl/Sld mice caused a twofold and 20-fold increase in CFU-S number in the bone marrow and spleen of treated animals, respectively. After 6 days in suspension culture, CFU-S number increased threefold in cultures supplemented with SCF and IL-6, or SCF, IL-3, and IL-6 relative to the number at day 0. The long-term repopulating ability of cells cultured in SCF, IL-3, and IL-6 was approximately sevenfold better than that of cells cultured in IL-3 or SCF. Similar experiments were performed on populations of bone marrow cells enriched for, or depleted of, CFU-S by elutriation and lineage subtraction. The combination of SCF and IL-6 increased CFU-S number approximately fourfold to eightfold in the CFU-S-enriched fraction, but had no effect on the CFU-S-depleted cells. These results show that SCF alone can increase CFU-S number in vivo, and in combination with other growth factors increases CFU-S numbers in vitro.

Stem Cell Factor in Combination With Granulocyte Colony-Stimulating Factor (CSF) or Granulocyte-Macrophage CSF Synergistically Increases Granulonoiesis In Vivo

Recombinant rat stem cell factor (rrSCF) and recombinant human granulocyte colony-stimulating factor (G-CSF) coinjected for 1 week in rats cause a synergistic increase in mature marrow neutrophils accompanied by a striking decrease in erythroid and lymphoid marrow elements. The spleens of the same rats show increased granulopoiesis as well as increased erythropoiesis as compared with the spleens of rats treated with either growth factor alone. Splenic extramedullary erythropoiesis may act to compensate for the decrease in marrow erythropoiesis. The coinjection of rrSCF and G-CSF causes an increase in marrow mast cells at the end of 1 week, but the increase is much less than in rrSCF-alone-treated rats. The combination of rrSCF and G-CSF increases the rate of release of marrow neutrophils into the circulation and causes a dramatic synergistic peripheral neutrophilia, beginning especially after 4 days of treatment. Colony-forming assays of all experimental groups showed a synergistic increase in colony-forming unit granulocyte-macrophage (CFU-GM) in the marrow, but not in peripheral blood, after coincubation with SCF plus granulocyte-macrophage CSF (GM-CSF) as opposed to GM-CSF alone, showing anatomic compartmentalization between a more primitive marrow CFU-GM subset and a more mature peripheral blood CFU-GM subset. In vivo daily administration of SCF plus GM-CSF results in a synergistic increase in marrow neutrophils, but not the striking synergistic increase in circulating neutrophils that is observed with SCF plus G-CSF.

Stem cell factor in combination with granulocyte colony-stimulating factor (CSF) or granulocyte-macrophage CSF synergistically increases granulopoiesis in vivo

Recombinant rat stem cell factor (rrSCF) and recombinant human granulocyte colony-stimulating factor (G-CSF) coinjected for 1 week in rats cause a synergistic increase in mature marrow neutrophils accompanied by a striking decrease in erythroid and lymphoid marrow elements. The spleens of the same rats show increased granulopoiesis as well as increased erythropoiesis as compared with the spleens of rats treated with either growth factor alone. Splenic extramedullary erythropoiesis may act to compensate for the decrease in marrow erythropoiesis. The coinjection of rrSCF and G-CSF causes an increase in marrow mast cells at the end of 1 week, but the increase is much less than in rrSCF-alone-treated rats. The combination of rrSCF and G- CSF increases the rate of release of marrow neutrophils into the circulation and causes a dramatic synergistic peripheral neutrophilia, beginning especially after 4 days of treatment. Colony-forming assays of all experimental groups showed a synergistic increase in colony- forming unit granulocyte-macrophage (CFU-GM) in the marrow, but not in peripheral blood, after coincubation with SCF plus granulocyte- macrophage CSF (GM-CSF) as opposed to GM-CSF alone, showing anatomic compartmentalization between a more primitive marrow CFU-GM subset and a more mature peripheral blood CFU-GM subset. In vivo daily administration of SCF plus GM-CSF results in a synergistic increase in marrow neutrophils, but not the striking synergistic increase in circulating neutrophils that is observed with SCF plus G-CSF.

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

Stem cell factor (SCF), the ligand for the c-kit protooncogene product, was able to stimulate blast cell and granulocytic colony formation by precursors from normal murine bone marrow. The blast cell colonies contained a high content of progenitor cells able to form macrophage and/or granulocyte colonies. Clone transfer studies, the secondary culture of colony cells, and the culture of populations freed of accessory cells all indicated a direct proliferative action of SCF. SCF receptors were present in high numbers on blast cells and in lower numbers on immature granulocytic, monocytic, and eosinophilic cells. Combination of SCF with granulocyte, granulocyte-macrophage, or multipotential colony-stimulating factors, but not macrophage colony-stimulating factor, resulted in enhancement of colony size. Granulocyte colony-stimulating factor enhanced cell proliferation initiated by SCF, but not vice-versa, and resulted in a 10-fold increase in colony cell numbers and a 7-fold increase in progenitor cells in blast colonies. No evidence was obtained that SCF, alone or in combination with granulocyte colony-stimulating factor, could stimulate self-generation by blast colony-forming cells.