The kit ligand, stem cell factor

Neural and skin cell-specific expression pattern conferred by steel factor regulatory sequence in transgenic mice

We have produced transgenic mice expressing a lacZ reporter gene under the control of a fragment of a Steel factor (SLF). The function of this gene is essential for the development of hematopoietic cells, germ cells, melanocytes and pacemaker cells of the intestine. The expression of the transgene, containing 2 kb DNA 5' regulatory sequence, was restricted to neural and skin tissues in appropriate spatial and temporal pattern compared with endogenous SLF mRNA expression. This indicates that the regulatory elements necessary for the neural and skin specific expression are present in this 2 kb DNA sequence, although strong position-dependence of transgene expression was observed. As we could not detect transgene expression in hematopoietic tissues and germ cells after extending 10 kb upstream, elements important for these organs must reside in other regions. Our results indicate that neural crest derived enteric ganglion cells provide SLF to the neighboring pace maker cells expressing c-kit, the receptor for SLF. Cells expressing the transgene in the intestine are ganglion cells derived from neural crest since homozygosity for the lethal spotting (Is) mutation results in loss of such ganglion cells in transgenic mice. We have also shown that the dermal papillae of the hair follicle expresses the transgene, suggesting its roles to support the c-kit dependent growth and development of melanocytes in the hair follicle.

Genetic analysis reveals cell type-specific regulation of receptor tyrosine kinase c-Kit by the protein tyrosine phosphatase SHP1

Receptor protein tyrosine kinases (RTKs) transmit downstream signals via interactions with secondary signaling molecules containing SH2 domains. Although many SH2-phosphotyrosyl interactions have been defined in vitro, little is known about the physiological significance of specific RTK/SH2 interactions in vivo. Also, little is known about the mechanisms by which specific RTKs interact with and/or are regulated by specific protein tyrosine phosphatases (PTPs). To address such issue, we carried out a genetic analysis of the previously reported biochemical interaction between the RTK c-Kit, encoded at the W locus, and the SH2-containing non-transmembrane PTP SHP1, encoded at the motheaten (me) locus (1). Mice carrying a kinase-defective allele of c-Kit (Wv/+) were crossed with me/+ mice, which carry one effectively null allele of SHP1, and then backcrossed to generate all possible allelic combinations. Our results indicate strong intergenic complementation between these loci in hematopoietic progenitor cells. Compared to progenitors purified from normal mice, bone marrow progenitor cells (lin-) from me/me mice markedly hyper-proliferated in response to Kit ligand (KL). stimulation. Superimposition of the me/me genotype increased the number of one marrow-derived CFU-E from Wv/+ mice. Conversely, the presence of one or two copies of Wv decreased the number of macrophages and granulocytes in me/me lung, skin, peripheral blood and bone marrow, thereby decreasing the severity of the me/me phenotype. The decrease in dermal mast cells in Wv/Wv mice was rescued to levels found in Wv/+mice by superimposition of the me/me genotype. Surprisingly, however, the presence or absence of SHP1 had no effect on the proliferative response of bone marrow-derived cultured mast cells to KL or IL3 ex vivo. Nevertheless, the immediate-early response to KL stimulation, as measured by KL-induced tyrosyl phosphorylation, was substantially increased in mast cells from Wv/+:me/me compared to Wv/ +:+/+ mice, strongly suggesting that SHP1 directly dephosphorylates and regulates c-Kit. Taken together, our results establish that SHP1 negatively regulates signaling from c-Kit in vivo, but in a cell type-specific manner.

Human thymocyte responsiveness to stem cell factor: synergy with interleukin-2 for the generation of NK/LAK cytotoxicity

Human recombinant stem cell factor (SCF) increases the viability and cell size of a subset of thymocytes in vitro, but does not independently induce phenotypic changes on thymocytes indicative of T cell differentiation. The SCF-responsive thymocytes have characteristics of large granular cells, that do not express T, B or NK cell-related antigens, and are primarily found in immature thymocyte subsets. These large granular thymocytes do not display cytotoxic activity. However, SCF acts synergistically with IL-2 in the generation of cytotoxic effector cells from thymocyte precursors. Synergy in cytotoxicity is observed to both NK-sensitive and NK-resistant targets. Studies of the SCF receptors on thymocytes show that receptors are expressed on mature 'bright' CD3+ cells, immature 'dim' CD3+ cells as well as CD3- cells. IL-2 increases the frequency of SCF receptor-positive cells in cultured thymocytes, which may explain its synergy with SCF in the generation of NK/LAK cytotoxicity. These data show that SCF enhances the functional development of thymic NK/LAK cells in vitro.

A role for stem cell factor (SCF): c-kit interaction(s) in the intestinal tract response to Salmonella typhimurium infection

Cholera toxin (CT) has been shown to induce stem cell factor (SCF) production in mouse ligated intestinal loops. Further, SCF interaction(s) with its receptor (c-kit) was shown to be important for the intestinal tract secretory response after CT exposure. In this study, we have investigated whether SCF production is induced in the intestinal tract after exposure to Salmonella typhimurium and whether this production could be an important intestinal tract response to Salmonella infection. Using a mouse ligated intestinal loop model, increased levels of SCF mRNA were detected at 2-4 h post-Salmonella challenge. Intestinal fluid obtained from Salmonella-challenged loops contained high levels of SCF by ELISA. Human and murine intestinal epithelial cell lines were also shown to have increased levels of SCF mRNA after exposure to Salmonella. Inhibition of Salmonella invasion of epithelial cells was shown to be one potentially important role for SCF:c-kit interactions in host defense to Salmonella infection. Pretreatment of human or murine intestinal cell lines with SCF resulted in a cellular state that was resistant to Salmonella invasion. Finally, mice having mutations in the white spotting (W) locus, which encodes the SCF-receptor (c-kit), were significantly more susceptible to oral Salmonella challenge than their control littermates. Taken together, the above results suggest that an important intestinal tract response to Salmonella infection is an enhanced production of SCF and its subsequent interactions with c-kit.

Regulatory roles of the ligand for Flk2/Flt3 tyrosine kinase receptor on human hematopoiesis

The biological activities of the ligand for the Flk2/Flt3 receptor tyrosine kinase (FL) on human hematopoietic cells are reviewed. In in vitro studies, FL shows relatively few effects by itself on the proliferation and differentiation of hematopoietic cells, but exhibits a potent costimulatory activity in enhancing the proliferation of progenitor cells of multiple lineages. FL promotes the growth of clonogenic myeloid progenitor cells in the presence of other cytokines known to be active on myeloid progenitors, including GM-CSF, interleukin 3 (IL-3), kit ligand (KL), M-CSF and G-CSF. In addition, FL synergizes with IL-7 in inducing the proliferation of pro-B cells, whereas FL has little effect on the growth of clonogenic erythroid progenitors. Furthermore, FL induces the in vitro expansion of the high proliferative potential colony-forming cells (HPP-CFC) and stimulates the proliferation of long-term culture-initiating cells (LTC-IC), suggesting an activity on the proliferation of putative stem cells. Thus, FL plays important roles in regulating the proliferation of hematopoietic progenitor cells and, therefore, may have therapeutic applications.

Rat bone marrow-derived mast cells co-cultured with 3T3 fibroblasts in the absence of T-cell derived cytokines require stem cell factor for their survival and maintain their mucosal mast cell-like phenotype

When cultured without fibroblasts, rat bone marrow-derived mast cells (BMMC) contain abundant rat mast cell proteinase type II (RMCP-II), and exhibit survival and proliferation when maintained in mesenteric lymph node conditioned medium (CM). When BMMC were co-cultured with 3T3 fibroblasts in the absence of CM, BMMC numbers increased for 7 days and the BMMC survived for up to 23 days. There was a gradual loss of stored RMCP-II in BMMC that were co-cultured with 3T3 cells, but the fibroblast microenvironment did not induce a detectable increase in the low levels of the connective tissue mast cell (CTMC)-associated proteinase, RMCP-I, in the BMMC. Nor did 3T3 cell co-culture induce significant heparin synthesis in BMMC as judged by the cells' reactivity with the fluorescent heparin-binding dye, berberine sulphate. These results suggest that rat BMMC, unlike murine BMMC, do not have the potential to develop multiple CTMC-like characteristics upon co-culture with 3T3 cells. However, when BMMC and fibroblast co-cultures were treated with an antibody to recombinant rat stem cell factor (rrSCF), mast cell survival was completely abrogated. This result suggests that endogenous, fibroblast-derived SCF is essential for the maintenance of rat BMMC viability in the absence of CM. On the other hand, prior treatment of the fibroblasts with the anti-rrSCF antibody did not affect the adherence of BMMC to the monolayer, implying that (an) other molecule(s) is(are) involved in the attachment process. The demonstration that rat BMMC survival on fibroblasts in vitro is dependent upon SCF may indicate an important mechanism by which tissue mucosal cells can be maintained in vivo in the absence of T-cell derived factors.

Recombinant human stem cell factor (kit ligand) promotes human mast cell and melanocyte hyperplasia and functional activation in vivo

Stem cell factor (SCF), also known as mast cell growth factor, kit ligand, and steel factor, is the ligand for the tyrosine kinase receptor (SCFR) that is encoded by the c-kit proto-oncogene. We analyzed the effects of recombinant human SCF (r-hSCF, 5-50 micrograms/kg/day, injected subcutaneously) on mast cells and melanocytes in a phase I study of 10 patients with advanced breast carcinoma. A wheal and flare reaction developed at each r-hSCF injection site; by electron microscopy, most dermal mast cells at these sites exhibited extensive, anaphylactic-type degranulation. A 14-d course of r-hSCF significantly increased dermal mast cell density at sites distant to those injected with the cytokine and also increased both urinary levels of the major histamine metabolite, methyl-histamine, and serum levels of mast cell alpha-tryptase. Five subjects developed areas of persistent hyperpigmentation at r-hSCF injection sites; by light microscopy, these sites exhibited markedly increased epidermal melanization and increased numbers of melanocytes. The demonstration that r-hSCF can promote both the hyperplasia and the functional activation of human mast cells and melanocytes in vivo has implications for our understanding of the role of endogenous SCF in health and disease. These findings also indicate that the interaction between SCF and its receptor represents a potential therapeutic target for regulating the numbers and functional activity of both mast cells and cutaneous melanocytes.

Stem cell factor enhances immunoglobulin E-dependent mediator release from cultured rat bone marrow-derived mast cells: activation of previously unresponsive cells demonstrated by a novel ELISPOT assay

Mucosal mast cells (MMC) are important effector cells in the immune response against gastrointestinal nematodes. We used cultured rat bone marrow-derived mast cells (BMMC) as an in vitro model of MMC to study the effects of the multifunctional cytokine stem cell factor (SCF) on immunoglobulin E (IgE)-dependent secretion of granule mediators. SCF (< or = 1000 ng/ml) was not a direct secretagogue for these cells, but it significantly enhanced IgE-mediated secretion of the granule constituents rat mast cell protease-II (RMCP-II) and beta-hexosaminidase from mature BMMC in a dose-dependent manner (> 10 ng/ml). Maximum up-regulation of secretion occurred after cells were pretreated with SCF (50 ng/ml) for 5 minutes before challenge with anti-IgE, but the effect then declined and was absent in cells incubated with the cytokine for 3 to 24 h. In a novel ELISPOT assay developed to identify individual BMMC secreting RMCP-II, the proportion of mature BMMC responding to anti-IgE was significantly increased by treatment with SCF. To investigate this effect further, the percentage release of RMCP-II and beta-hexosaminidase from populations of mature BMMC was directly compared to the proportion of individual cells releasing RMCP-II as detected by ELISPOT. The release of both mediators was enhanced by SCF, and the increased percentage release reflected both an increased proportion of secreting cells, and enhanced mediator release from individual cells. These results suggest that SCF can enhance IgE-dependent mediator release from BMMC not only by augmenting the secretory response from individual cells, but also by activating previously unresponsive cells.

The cloning and sequencing of cDNAs encoding two isoforms of feline stem cell factor

cDNA clones encoding two isoforms of feline stem cell factor (fSCF) have been isolated using RT-PCR and their sequences determined. The cDNAs encode a predicted full length fSCF protein of 274 amino-acids and a shorter isoform of 246 amino acids. Feline SCF shows a high degree of homology to the SCFs of other species at both the nucleic acid and protein level.

A role for stem cell factor and c-kit in the murine intestinal tract secretory response to cholera toxin

The role of stem cell factor (SCF) and its receptor (c-kit) in the intestinal secretory response to cholera toxin (CT) was investigated using a ligated intestinal loop model in mice having mutations in the dominant white spotting (W) locus and the steel (Sl) locus. W/Wv mice, which express an aberrant form of the c-kit protein, failed to give an intestinal secretory response after luminal CT challenge. In contrast, W/Wv mice and their control littermates had equivalent intestinal secretory responses to Escherichia coli heat-stable enterotoxin (STa). Sl/Sld mice, which express only a soluble truncated form of SCF, also gave a significantly reduced intestinal secretory response to CT when compared to the secretory response of their littermate controls. The unresponsiveness of W/Wv mice to CT was restricted to the intestinal tract since these mice had foot pad swelling responses to CT challenge that were equivalent to their littermate controls. Restoration of mast cells in W/Wv mice by bone marrow transplantation of control littermate bone marrow did not reverse the CT-unresponsiveness of the intestinal tract. Histological evaluation of the gastrointestinal tract from W/Wv mice showed a normal distribution of enterochromaffin cells (ECC). CT challenge of either ligated intestinal loops from C57B1/6 mice or a mouse intestinal epithelial cell line (MODE-K) resulted in elevated levels of mRNA for SCF. MODE-K cells exposed to CT also had enhanced expression of c-kit. Finally, fluid obtained from CT-challenged ligated intestinal loops from C57B1/6 mice contained significant levels of SCF. Taken together, the above results suggest that CT-induced intestinal secretory responses are dependent upon SCF-c-kit interactions. These interactions appear to be induced as a consequence of CT stimulation of the intestinal tract and may also play a role in the development or functionality of the enteric nervous system.

Intrathymically expressed c-kit ligand (stem cell factor) is a major factor driving expansion of very immature thymocytes in vivo

To investigate the role of the receptor-type tyrosine kinase, c-kit and its ligand, stem cell factor (SCF) in T cell development, we analyzed c-kit (W/W) and SCF (SI/SI) deficient mice. We also engrafted wild-type or SCF-deficient fetal thymi onto wild-type recipient mice and analyzed the rate of proliferation by in vivo bromodeoxyuridine labeling. The results show that the most immature thymocyte compartment defined as CD3-CD4-CD8- is significantly reduced in SI/SI grafts and W/W thymi compared with wild-type counterparts. Also, the expansion rate of these immature thymocytes in SI/SI graft is reduced by -50%. These experiments provide direct evidence for an important role for c-kit-SCF interactions in expansion of very early thymocytes.

The immediate phase of c-kit ligand stimulation of mouse bone marrow-derived mast cells elicits rapid leukotriene C4 generation through posttranslational activation of cytosolic phospholipase A2 and 5-lipoxygenase

c-kit ligand (KL) activated mouse bone marrow-derived mast cells (BMMC) for the dose- and time-dependent release of arachidonic acid from cell membrane phospholipids, with generation of leukotriene (LT) C4 in preference to prostaglandin (PG)D2. KL at concentrations of 10 ng/ml elicited half-maximal eicosanoid generation and at concentrations of > 50 ng/ml elicited a maximal generation of approximately 15 ng LTC4 and 1 ng PGD2 per 10(6) cells, with 20% net beta-hexosaminidase release 10 min after stimulation. Of the other cytokines tested, none, either alone or in combination with KL, elicited or modulated the immediate phase of mediator release by BMMC, indicating strict specificity for KL. Activation of BMMC in response to KL was accompanied by transient phosphorylation of cytosolic phospholipase A2 and reversible translocation of 5-lipoxygenase to a cell membrane fraction 2-5 min after stimulation, when the rate of arachidonic acid release and LTC4 production were maximal. BMMC continuously exposed to KL in the presence of IL-10 and IL-1 beta generated LTC4 in marked preference to PGD2 over the first 10 min followed by delayed generation of PGD2 with no LTC4 over several hours. Pharmacologic studies revealed that PGD2 generation in the immediate phase depended on prostaglandin endoperoxide synthase (PGHS)-1 and in the delayed phase on PGHS-2. Thus, KL provided a nonallergic stimulus for biphasic eicosanoid generation by mast cells. The immediate phase is dominated by LTC4 generation with kinetics and postreceptor biosynthetic events similar to those observed after cell activation through the high affinity IgE receptor, whereas the delayed phase of slow and selective PGD2 production is mediated by induction of PGHS-2.

Interleukin 4 suppresses c-kit ligand-induced expression of cytosolic phospholipase A2 and prostaglandin endoperoxide synthase 2 and their roles in separate pathways of eicosanoid synthesis in mouse bone marrow-derived mast cells

Mouse bone marrow-derived mast cells (BMMCs) developed with interleukin 3 (IL-3) can be stimulated by c-kit ligand (KL) and accessory cytokines over a period of hours for direct delayed prostaglandin (PG) generation or over a period of days to prime for augmented IgE-dependent PG and leukotriene (LT) production, as previously reported. We now report that IL-4 is counterregulatory for each of these distinct KL-dependent responses. BMMCs cultured for 4 days with KL + IL-3 or with KL + IL-10 produced 5- to 7-fold more PGD2 and approximately 2-fold more LTC4 in response to IgE-dependent activation than BMMCs maintained in IL-3 alone. IL-4 inhibited the priming for increased IgE-dependent PGD2 and LTC4 production to the level obtained by activation of BMMCs maintained in IL-3 alone with an IC50 of approximately 0.2 ng/ml. IL-4 inhibited the KL-induced increase in expression of cytosolic phospholipase A2 (cPLA2) but had no effect on the incremental expression of PG endoperoxide synthase 1 (PGHS-1) and hematopoietic PGD2 synthase or on the continued baseline expression of 5-lipoxygenase, 5-lipoxygenase activating protein, and LTC4 synthase. BMMCs stimulated by KL + IL-10 for 10 h exhibited a delayed phase of PGD2 generation, which was dependent on de novo induction of PGHS-2. IL-4 inhibited the induction of PGHS-2 expression and the accompanying cytokine-initiated delayed PGD2 generation with an IC50 of approximately 6 ng/ml. IL-4 had no effect on the expression of PGHS-2 and the production of PGD2 elicited by addition of IL-1 beta to the combination of KL + IL-10. IL-4 had no effect on the immediate phase of eicosanoid synthesis elicited by KL alone or by IgE and antigen in BMMCs maintained in IL-3. Thus, the counterregulatory action of IL-4 on eicosanoid generation is highly selective for the induced incremental expression of cPLA2 and the de novo expression of PGHS-2, thereby attenuating time-dependent cytokine-regulated responses to stimulation via Fc epsilon receptor I and stimulation via c-kit, respectively.

Pathways from hematopoietic stem cells to thymocytes

A long-standing debate has been whether commitment to the T cell lineage occurs exclusively following thymus colonization, or whether prethymic T lineage restricted progenitors exist. Recently, the analysis of murine fetal blood for the presence of hematopoietic progenitor cells has led to the identification of a T lineage committed precursor population (designated prothymocytes). Fetal blood prothymocytes lack multipotent progenitor potential as shown by the fact that they fail to reconstitute B lymphocyte, myeloid and erythroid lineages. In addition to prothymocytes, fetal blood also contains a phenotypically distinct, pluripotent progenitor population which can reconstitute both T and B lymphocytes as well as myeloid and erythroid lineages. The identification of a circulating, T lineage restricted precursor population, which is also found in the blood of fetal athymic mice, provides strong evidence that T lineage commitment can precede thymus colonization. The thymus is not, however, exclusively colonized by prothymocytes. Under appropriate developmental conditions, multipotent precursor activity for non-T lineages such as B lymphocytes and thymic dendritic cells can be revealed within the intrathymic precursor pool. Moreover, evidence has been accumulated for a common progenitor for T cells and natural killer cells which may be distinct from multipotent intrathymic progenitors.

The FLT3 ligand potently and directly stimulates the growth and expansion of primitive murine bone marrow progenitor cells in vitro: synergistic interactions with interleukin (IL) 11, IL-12, and other hematopoietic growth factors

The recently cloned murine flt3 ligand (FL) was studied for its ability to stimulate the growth of primitive (Lin-Sca-1+) and more committed (Lin-Sca-1-) murine bone marrow progenitor cells, alone and in combination with other hematopoietic growth factors (HGFs). Whereas FL was a weak proliferative stimulator alone, it potently synergized with a number of other HGFs, including all four colony-stimulating factor (CSF), interleukin (IL) 6, IL-11, IL-12, and stem cell factor (SCF), to promote the colony formation of Lin-Sca-1+, but not Lin-Sca-1- or erythroid progenitor cells. The synergistic activity of FL was concentration dependent, with maximum stimulation occurring at 250 ng/ml, and was observed when cells were plated at a concentration of one cell per culture, suggesting that its effects are directly mediated. 2 wk of treatment with FL in combination with IL-3 or SCF resulted in the production of a high proportion of mature myeloid cells (granulocytes and macrophages), whereas the combination of FL with G-CSF, IL-11, or IL-12 resulted predominantly in the formation of cells with an immature blast cell appearance. Accordingly, FL in combination with G-CSF or IL-11 expanded the number of progenitors more than 40-fold after 2 wk incubation. Thus, FL emerges as a potent synergistic HGF, that in combination with numerous other HGFs, can directly stimulate the proliferation, myeloid differentiation, and expansion of primitive hematopoietic progenitor cells.

Tec kinase associates with c-kit and is tyrosine phosphorylated and activated following stem cell factor binding

Stem cell factor (SCF) plays a crucial role in hematopoiesis through its interaction with the receptor tyrosine kinase c-kit. However, the signaling events that are activated by this interaction and involved in the control of growth or differentiation are not completely understood. We demonstrate here that Tec, a cytoplasmic, src-related kinase, physically associates with c-kit through a region that contains a proline-rich motif, amino terminal of the SH3 domain. Following SCF binding, Tec is tyrosine phosphorylated and its in vitro kinase activity is increased. Tyrosine phosphorylation of Tec is not detected in the response to other cytokines controlling hematopoiesis, including colony-stimulating factor-1 (CSF-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). Conversely, the cytoplasmic kinase JAK2 is activated by IL-3 but not by SCF stimulation. The activation of distinct cytoplasmic kinases may account for the synergy seen in the actions of SCF and IL-3 on hematopoietic stem cells.

Tec kinase associates with c-kit and is tyrosine phosphorylated and activated following stem cell factor binding

Stem cell factor (SCF) plays a crucial role in hematopoiesis through its interaction with the receptor tyrosine kinase c-kit. However, the signaling events that are activated by this interaction and involved in the control of growth or differentiation are not completely understood. We demonstrate here that Tec, a cytoplasmic, src-related kinase, physically associates with c-kit through a region that contains a proline-rich motif, amino terminal of the SH3 domain. Following SCF binding, Tec is tyrosine phosphorylated and its in vitro kinase activity is increased. Tyrosine phosphorylation of Tec is not detected in the response to other cytokines controlling hematopoiesis, including colony-stimulating factor-1 (CSF-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). Conversely, the cytoplasmic kinase JAK2 is activated by IL-3 but not by SCF stimulation. The activation of distinct cytoplasmic kinases may account for the synergy seen in the actions of SCF and IL-3 on hematopoietic stem cells.