The kit ligand, stem cell factor

Stem cell factor and interleukin-4 induce murine bone marrow cells to develop into mast cells with connective tissue type characteristics in vitro

In this study, we have developed a method to obtain mast cells with connective tissue type mast cell (CTMC) characteristics directly from mouse bone marrow (BM) cells. BM cells were grown for 3 weeks in presence of interleukin-4 (IL-4) plus stem cell factor (SCF). SCF alone poorly supported growth and development of mast cells. IL-4 dose-dependently enhanced the expression of c-kit and high-affinity receptor for IgE (Fc(epsilon)RI) on the cell surface of SCF-cultured BM cells. Furthermore, cytoplasmic granulation and histamine synthesis of BM-derived mast cells were increased in presence of IL-4 and SCF. Histochemical staining demonstrated that granules were safranin positive. BM-derived mast cells could be activated for granule exocytosis (beta-hexosaminidase release) and lipid mediator generation (LTC4 production) via Fc(epsilon)RI after sensitization with IgE and subsequent crosslinking with multivalent antigen. In addition, mast cells derived from BM cells cultured with SCF plus IL-4 could be activated by substance P, a nonimmunologic stimulus, to release beta-hexosaminidase. The results presented indicate that IL-4 and SCF both have a prominent role in the development of mast cells from murine BM cells in vitro. Mast cells can directly be derived from BM cells in presence of SCF and IL-4 and the cultured cells show typical hallmarks of CTMC, indicating that precursor cells for CTMC may be present in BM. The described culture procedure may be useful to investigate the molecular aspects of the development of committed mast cell lineages.

The role of cytokine receptor signaling in lymphocyte development

Cytokine receptor signaling plays an essential role in the early stages of lymphocyte development. Signals through various cytokine receptors - such as c-kit, flt3/flk2, CXCR4, the IL-7 receptor and the IL-15 receptor - are known to promote the expansion and survival of uncommitted progenitor cells as well as their migration to the appropriate microenvironment and subsequent differentiation into B, T or natural killer cells. The recent generation of mice deficient in one or more of these signaling pathways has revealed which cytokines play unique and/or redundant roles in each of these lymphocyte lineages during this developmental process.

Spontaneous canine mast cell tumors express tandem duplications in the proto-oncogene c-kit

Spontaneous mast cell tumors (MCT) are the most common malignant neoplasm in the dog, representing between 7% and 21% of all canine tumors, an incidence much higher than that found in humans. These tumors often behave in an aggressive manner, metastasizing to local lymph nodes, liver, spleen, and bone marrow. The proto-oncogene c-kit is known to play a critical role in the development and function of mast cells. Point mutations in the kinase domain of c-kit leading to tyrosine phosphorylation in the absence of ligand binding have been identified in three mastocytoma lines, (P815, RBL, and HMC-1), and some human patients with various forms of mastocytosis. We now demonstrate that although c-kit derived from canine MCT did not contain the previously described activating point mutations, 5 of the 11 tumors analyzed possessed novel mutations consisting of tandem duplications involving exons 11 and 12. We also show that one such duplication, detected in a canine mastocytoma cell line, was associated with constitutive phosphorylation of c-kit protein (KIT), suggesting that these mutations may contribute to the development or progression of canine MCT.

Stem cell factor-induced airway hyperreactivity in allergic and normal mice

The induction of airway hyperreactivity during allergic responses involves multiple ill-defined mechanisms. Recently a role for stem cell factor (SCF) in the development of allergic eosinophilic airway inflammation has been identified. In the present study we demonstrate that SCF has a role in both the inflammatory response and airway hyperreactivity. Neutralization of SCF or examination of SCF-mutant mice, which were deficient in SCF and pulmonary mast cells, demonstrated significant alterations in the allergen-induced airway hyperreactive responses. The reduced hyperreactivity response was accompanied by a significant reduction in eosinophil accumulation. To examine the direct role of SCF on airway hyperreactivity, we administered SCF into the airways of normal mice via intratracheal injections and demonstrated a dose dependent increase in airway hyperreactivity at 4 hours that was maintained at 24 hours after administration. Instillation of SCF into SCF-deficient (mast cell deficient) mice demonstrated significantly lower increases in airway hyperreactivity compared with the littermate controls with normal mast cell numbers. These studies demonstrate that locally expressed SCF can induce changes in airway physiology via mast cell activation, verifying the role of SCF in allergic airway inflammation and hyperreactivity.

Mast cells as sentinels of innate immunity

Mast cells are widely regarded as important effector cells in immune responses associated with Th2 cells and IgE. Recent work shows that they can also contribute significantly to the expression of innate immunity; furthermore, survival in a model of acute bacterial infection that is dependent on complement and mast cells can be greatly enhanced by long-term treatment of mice with the kit ligand (stem cell factor) at least in part because of the effects of such treatment on mast cell numbers and/or function. These findings not only indicate that mast cells can represent a critical component of host defense in natural immunity but also suggest that mast cell function in this setting can be manipulated for therapeutic ends.

Synergistic action of stem-cell factor and interleukin-7 in a human immature T-cell line

The thymus provides the microenvironment that is optimal for T-cell differentiation. The most immature cells in the human thymus express the stem-cell marker CD34 and they respond to cytokines, including stem-cell factor (SCF) and interleukin-7 (IL-7). For the normal progression of T-cell development these two cytokines appear to be vital. We have established and characterized a human pre-T-cell line, PER-487, which mirrors this requirement. This study shows that the simultaneous presence of IL-7 and SCF produces a proliferative response far exceeding additive effects. Furthermore, providing these signals in succession did not achieve the effect observed when they were provided simultaneously. This finding suggests that the effect was not mediated via secretion of molecules or modulation of surface expression. The convergence of the signal transduction pathways of the two cytokines is not known, thus cell line PER-487 provides a unique model for studying the synergistic interaction of IL-7 and SCF.

Lack of C-KIT+ mast cells and the development of idiopathic gastric perforation in neonates

BACKGROUND/PURPOSE

The proto-oncogene c-kit encodes a receptor tyrosine kinase C-KIT. W/Wv mice, which are devoid of C-KIT+ mast cells as a result of mutations in the c-kit gene, develop spontaneous gastric ulceration or perforation after day 7 of life at a high frequency, whereas normal litter mates do not. The authors hypothesized that a lack of C-KIT+ mast cells may be implicated in the development of idiopathic gastric perforation (GP) in neonates.

METHODS

Postmortem gastric wall specimens were taken from neonates who died of GP (idiopathic, n = 6; secondary, n = 4), and other causes (controls, n = 6). Specimens were taken at random from various sites in the stomach and labeled with antibody to C-KIT. The number of C-KIT+ mast cells from five random fields per specimen were compared under light microscopy (200x).

RESULTS

Overall, the number of C-KIT+ mast cells was significantly lower in gastric wall specimens from cases of idiopathic GP when compared with controls or cases of secondary GP irrespective of the sites of sampling (P<.01, analysis of variance test) with the distribution of cells being uniform and unique for each stomach.

CONCLUSION

A lack of C-KIT+ mast cells may underlie the development of idiopathic GP in neonates.

Distinct actions of interleukin-9 and interleukin-4 on a hematopoietic stem cell line, EMLC1

EMLC1 is a hematopoietic stem cell line that depends on stem cell factor (SCF) for growth and generates lymphoid, erythroid and myeloid progenitors in the presence of different cytokines. We have studied signaling events leading to cell proliferation and differentiation of EMLC1 mediated by interleukin (IL)-4 and IL-9. It was found that IL-9 enhances SCF-induced cell proliferation and promotes erythropoietin (EPO)-dependent erythroid differentiation of EMLC1 cells. However, IL-9 alone cannot support the growth of this cell line. In contrast, IL-4 by itself is sufficient to promote the growth of EMLC1 cells, even in the absence of SCF. Antiphosphotyrosine immunoblots of total cell lysates demonstrated that IL-4 and IL-9 induce tyrosine phosphorylation of different cellular substrates. Both IL-4 and IL-9 stimulated tyrosine phosphorylation of SHP-2, whereas the 90-kD tyrosine phosphorylated protein induced by IL-9 stimulation is Stat3. We have also shown that IL-4 is much more potent than IL-9 in inducing the expression of primary response gene c-myc. It was further determined that c-myc antisense oligodeoxynucleotide blocked IL-4 supported cell growth. Taken together, these results indicate that IL-4 may serve as a growth-promoting factor for hematopoietic stem cells, and IL-9 enhances both growth and erythroid differentiation of primitive hematopoietic progenitors. The results also suggest that differences in tyrosine phosphorylation induced by IL-4 and IL-9 may in part determine their distinct biological functions.

The c-kit ligand, stem cell factor, can enhance innate immunity through effects on mast cells

Mast cells are thought to contribute significantly to the pathology and mortality associated with anaphylaxis and other allergic disorders. However, studies using genetically mast cell-deficient WBB6F1-KitW/KitW-v and congenic wild-type (WBB6F1-+/+) mice indicate that mast cells can also promote health, by participating in natural immune responses to bacterial infection. We previously reported that repetitive administration of the c-kit ligand, stem cell factor (SCF), can increase mast cell numbers in normal mice in vivo. In vitro studies have indicated that SCF can also modulate mast cell effector function. We now report that treatment with SCF can significantly improve the survival of normal C57BL/6 mice in a model of acute bacterial peritonitis, cecal ligation and puncture (CLP). Experiments in mast cell-reconstituted WBB6F1-KitW/KitW-v mice indicate that this effect of SCF treatment reflects, at least in part, the actions of SCF on mast cells. Repetitive administration of SCF also can enhance survival in mice that genetically lack tumor necrosis factor (TNF)-alpha, demonstrating that the ability of SCF treatment to improve survival after CLP does not solely reflect effects of SCF on mast cell- dependent (or -independent) production of TNF-alpha. These findings identify c-kit and mast cells as potential therapeutic targets for enhancing innate immune responses.

Cutting Edge: Impaired Mast Cell Development and Innate Immunity in Mac-1 (CD11b/CD18, CR3)-Deficient Mice

Mac-1 (CD11b/CD18, CR3), a β2 integrin expressed on leukocytes, is important in leukocyte migration. We demonstrate that Mac-1 is also expressed on peritoneal mast cells and LPS stimulated bone marrow-derived cultured mast cells, and that Mac-1-deficient mice, which lack this receptor, have significant reductions in the numbers of mast cells resident in the peritoneal cavity, peritoneal wall, and dorsal skin. The reduced numbers of mast cells in Mac-1-deficient mice may have important functional consequences, in that Mac-1-deficient mice exhibit significantly increased mortality after cecal ligation and puncture, a model of acute septic peritonitis in which host resistance has been shown to be dependent on both mast cells and complement. These findings demonstrate that Mac-1 is required for the expression of normal levels of mast cells in the peritoneal cavity, peritoneal wall, and certain areas of the skin, as well as for maintaining adequate mast cell-dependent host defense against bacterial infection.

Commitment and differentiation of stem cells to the osteoclast lineage

Osteoclasts are hematopoietic cells which play important roles in bone remodeling and resorption. They have phenotypic characteristics of the monocyte/macrophage lineages. In this review we first describe the phylogeny of osteoclasts. Osteoclast generation is closely linked to the presence of bone tissues. The formation of bone cavities in aquatic animals is underdeveloped, even though they have cells which have the potential to differentiate into osteoclasts. Next we describe recent advances in our understanding of osteoclastogenesis that have resulted from the identification of critical molecules and mutated genes of osteopetrotic mice. Reports that transcriptional factors PU.1 and c-Fos are essential for commitment and (or) differentiation into the osteoclast lineage and novel culture systems, which have clarified some characteristics of osteoclast precursors, are also described. We are now able to induce mature osteoclasts from hematopoietic stem cells and even from totipotent embryonic stem cells. Cell lines that differentiate into osteoclasts are also available. Using these culture systems and cell lines, the interactions of osteoclasts with osteoblastic stromal cells, which produce critical molecules for osteoclastogenesis, have been studied. Very recently, one of these critical molecules, osteoclast differentiation factor / osteoprotegerin-ligand, was cloned. The presence of this factor and macrophage-colony-stimulating factor is sufficient to induce osteoclast development in cultures inoculated only with an osteoclast precursor cell line. We review the present status and the remaining questions in osteoclast biology.Key words: osteoclast, stem cell, osteopetrosis, M-CSF, ODF/OPGL, hematopoiesis.

The IL-1 Receptor-Related T1 Antigen Is Expressed on Immature and Mature Mast Cells and on Fetal Blood Mast Cell Progenitors

Expression of the T1 gene, also known as ST2, DER4, and Fit-1, has been shown to be associated with cell proliferation. It gives rise to two different mRNAs that encode a receptor-like protein and a soluble molecule representing the ectodomain of the receptor form. Although T1 is a member of the IL-1R family, its biologic function is currently unknown. In this study, we have analyzed the expression of the T1 surface Ag in murine hemopoietic organs. Mast cells (MCs) were shown to be the only identifiable cell lineage that expressed T1 at high levels. T1 expression was found on cultured bone marrow-derived immature MCs. Similarly, freshly isolated connective tissue-type MCs from the i.p. cavity were also shown to express high levels of T1. Interestingly, the earliest detectable committed MC precursor isolated from fetal blood (FB) at day 15.5 of gestation, but not circulating hemopoietic stem cells in FB, also expresses high level of T1. Since FB promastocytes lack expression of the high affinity IgE receptor (FcεRI), T1 expression precedes expression of FcεRI in MC ontogeny. The finding that the T1 Ag is selectively expressed at several stages during development of the MC lineage suggests that this cell surface molecule, in combination with the well-established markers c-Kit and FcεRI, should be valuable for studying the MC lineage.

Brain mast cells lack the c-kit receptor: immunocytochemical evidence

Mast cells are reported to differ from other cells of the hematopoietic lineage in that as mature cells, they retain the c-kit receptor, and are thus capable of responding to the stem cell factor (SCF) ligand. SCF is important for development and survival of mast cells. In this study, c-kit expression was examined immunocytochemically in the brains of mice, rats and doves. The results indicate that brain mast cells lack the c-kit receptor; those of the leptomeninges and other connective tissues are a mixed population of c-kit positive and negative cells. The mechanisms whereby brain mast cells might survive in the absence of SCF-c-kit signaling are discussed.

Mast cells can secrete vascular permeability factor/ vascular endothelial cell growth factor and exhibit enhanced release after immunoglobulin E-dependent upregulation of fc epsilon receptor I expression

Vascular permeability factor/vascular endothelial cell growth factor (VPF/VEGF) can both potently enhance vascular permeability and induce proliferation of vascular endothelial cells. We report here that mouse or human mast cells can produce and secrete VPF/VEGF. Mouse mast cells release VPF/VEGF upon stimulation through Fcepsilon receptor I (FcepsilonRI) or c-kit, or after challenge with the protein kinase C activator, phorbol myristate acetate, or the calcium ionophore, A23187; such mast cells can rapidly release VPF/VEGF, apparently from a preformed pool, and can then sustain release by secreting newly synthesized protein. Notably, the Fc epsilonRI-dependent secretion of VPF/VEGF by either mouse or human mast cells can be significantly increased in cells which have undergone upregulation of Fc epsilonRI surface expression by a 4-d preincubation with immunoglobulin E. These findings establish that at least one cell type, the mast cell, can be stimulated to secrete VPF/VEGF upon immunologically specific activation via a member of the multichain immune recognition receptor family. Our observations also identify a new mechanism by which mast cells can contribute to enhanced vascular permeability and/or angiogenesis, in both allergic diseases and other settings.

Transgene expression of steel factor in the basal layer of epidermis promotes survival, proliferation, differentiation and migration of melanocyte precursors

Mutations at the murine dominant white spotting (KitW) and steel (MgfSl) loci, encoding c-Kit receptor kinase and its ligand respectively, exert developmental defects on hematopoietic cells, melanocytes, germ cells and interstitial cells of Cajal. The expression patterns of steel factor (SLF) observed in the skin and gonads suggest that SLF mediates a migratory or a chemotactic signal for c-Kit-expressing stem cells (melanocyte precursors and primordial germ cells). By targeting expression of SLF to epidermal keratinocytes in mice, we observed extended distribution of melanocytes in a number of sites including oral epithelium and footpads where neither melanocytes nor their precursors are normally detected. In addition, enlarged pigmented spots of KitW and other spotting mutant mice were observed in the presence of the SLF transgene. These results provide direct evidence that SLF stimulates migration of melanocytes in vivo. We also present data suggesting that SLF does not simply support survival and proliferation of melanocytes but also promotes differentiation of these cells. Unexpectedly, melanocyte stem cells independent of the c-Kit signal were maintained in the skin of the SLF transgenic mice. After the elimination of c-Kit-dependent melanoblasts by function-blocking anti-c-Kit antibody, these stem cells continued to proliferate and differentiate into mature melanocytes. These melanoblasts are able to migrate to cover most of the epidermis after several months. The SLF transgenic mice described in this report will be useful in the study of melanocyte biology.

Metaplastic transformation of urinary bladder epithelium: effect on mast cell recruitment, distribution, and phenotype expression

Mucosal mast cells (MCs) are normally found in the connective tissue stroma but are redistributed into the epithelium in conditions associated with immunoglobulin E responses, such as allergic inflammation and nematode infections, as well as in interstitial cystitis, a condition of unknown etiology. The potential role of epithelium-derived factors in this response prompted this inquiry into growth and differentiation signaling in normal tissue as well as in tissues from five different metaplastic conditions of the urothelium (cystitic cystica, cystitis glandularis, colonic metaplasia, squamous cell metaplasia, and nephrogenic metaplasia). Expression of the two major human MC growth factors, stem cell factor (or kit ligand) and interleukin 6, was detected using immunohistochemistry. In the case of interleukin 6, its mRNA expression was also detected using in situ reverse transcription-polymerase chain reaction. Among the different metaplastic lesions, nephrogenic metaplasia was the only one associated with an abundance of MCs, which were distributed within or in close relationship to the epithelium. Unlike in the other types of metaplasia, the epithelium strongly co-expressed interleukin 6 and stem cell factor. The MCs expressed the stem cell factor receptor CD117 and exhibited a variable tryptase immunoreactivity, but lacked chymase. They also displayed a relative deficiency of granular glycosaminoglycan, as indicated by a lack of metachromasia, and were sensitive to strong aldehyde fixation. The findings suggest that the MC response in nephrogenic metaplasia may be the result of local epithelial stem cell factor/interleukin 6 expression.

Novel Role of Transmembrane SCF for Mast Cell Activation and Eotaxin Production in Mast Cell-Fibroblast Interactions

Mast cell activation can be induced by multiple mechanisms, including IgE-, complement-, and stem cell factor (SCF)-mediated pathways. In addition, the interaction of mast cells with particular cell populations, such as fibroblasts, have also demonstrated increased mast cell reactivity. In these studies, we have investigated the role of fibroblast-mast cell interaction for induction of histamine release and chemokine production and the specific role of SCF during this interaction. Primary pulmonary fibroblast cell lines were grown in culture and used throughout these studies. Mast cells were grown in parallel with fibroblasts by incubation of bone marrow cells with SCF and IL-3. During mast cell-fibroblast coculture, increased histamine release could be attenuated either by separation of the cell populations using a Trans-Well setup, which did not allow cellular contact, or by specific anti-SCF Ab. In addition, a significant increase in eotaxin, a potent eosinophil-specific C-C chemokine, was also observed during fibroblast-mast cell interaction. The production of eotaxin was cell contact dependent and could be inhibited using an anti-SCF Ab or specific antisense therapy. SCF was constitutively produced from fibroblasts in its transmembrane form and could be induced by TNF. SCF-coated plates induced significant mast cell-derived eotaxin production, whereas soluble SCF induced little or no eotaxin, suggesting a necessity for receptor cross-linking for activation. These studies indicate that fibroblast-mast cell contact plays a role in exacerbation of histamine release and eotaxin production.

Morphological alterations in rat peritoneal mast cells by stem cell factor

Stem cell factor (SCF) stimulates mast cell adhesion and, because SCF is produced normally in tissues, it may be a major factor responsible for the adhesion of mast cells to connective tissue matrix. We found that the morphology of rat peritoneal mast cells (RPMC) altered after the addition of recombinant murine SCF (rmSCF) in vitro. The ability of rmSCF to enhance morphological alteration was dose dependent and completely abolished by anti-c-kit ACK2 monoclonal antibody. Exposure of RPMC to transforming growth factor-beta 1, wortmannin, genistein, herbimycin A, staurosporine, indomethacin and cytochalasin D before the addition of rmSCF antagonized rmSCF-induced morphological alteration. However, nordihydroguiaretic acid had no effect. Many RPMC appeared to respond also to nerve growth factor (NGF) but the total number of cells with altered morphology was much greater when the culture was stimulated by rmSCF than by NGF. We suggest that morphological alterations of mast cells by rmSCF is an important step for the participation in adhesion to tissue under resident physiological conditions.

An elevation of stem cell factor in patients with hyperthyroid Graves' disease

Graves' disease is an autoimmune disorder characterized by the presence of antibodies against thyrotropin receptor (TRAb). Stem cell factor (SCF), derived from bone marrow, is known to promote lymphohematopoiesis. To investigate the relation between the alteration in plasma levels of SCF, thyroid hormone status, and TRAb measured by thyrotropin binding inhibition (TBI), 13 untreated, 21 treated, and 4 relapsed hyperthyroid Graves' disease patients, 21 patients with Hashimoto's thyroiditis, 6 patients with subacute thyroiditis, and 11 control subjects were examined. In untreated hyperthyroid Graves' disease patients, serum levels of thyroxine (T4) and triiodothyronine decreased rapidly by methimazole treatment, and TBI decreased progressively, but variably. Simultaneously, the elevated plasma levels of SCF decreased gradually and progressively. The plasma levels of SCF correlated curvilinearly with the serum levels of T4. In 4 patients with relapsed hyperthyroid Graves' disease, TBI was marginally positive in 3 patients and negative in 1, but plasma levels of SCF were elevated significantly in all 4 patients. In patients with subacute thyroiditis and Hashimoto's thyroiditis with or without T4 replacement, plasma levels of SCF did not differ from that of controls. These findings indicate that the elevation of plasma levels of SCF relates to the longstanding thyrotoxic state and that short-term thyrotoxicosis does not significantly affect plasma levels of SCF. It remains to be determined whether the elevation in plasma levels of SCF is induced by excess thyroid hormone, reflecting the hypermetabolic state, or whether the elevation of plasma levels of SCF contributes to stimulation of lymphocytes producing TRAb.

Antigen-receptor junctional diversity in growth-factor-receptor mutant mice

Precursor lymphocytes undergo expansion prior to immunoglobulin (Ig) or T cell receptor (TCR) rearrangements. Development of thymocytes, but not B cells, is entirely blocked in mice lacking both the receptor-tyrosine-kinase c-kit and the common cytokine receptor gamma chain (gamma c). In c-kit-gamma c-mice, TCR beta rearrangements are limited to mono- or oligoclonal DJ junctions. Here, effects of lack of c-kit or gamma c, or both, on the junctional diversity of TCR gamma and delta, and Ig VH(DH)JH loci were analyzed. All rearrangements were present in wildtype and mutant mice. However, sequencing of the junctions revealed monoclonal TCR gamma (V gamma 2 J gamma 1) and TCR delta (V delta 1(D delta)J delta 2) joints in c-kit-gamma c-, but not c-kit+ gamma c- or wildtype thymocytes. In contrast to TCR beta, gamma and delta loci, VHDHJH junctions were more diverse in c-kit-gamma c-mice. Thus, the two analyzed growth factor receptors mediate signaling pathways required for progenitor expansion and generation of junctional diversity at TCR loci, but have less influence on the diversity of IgH junctions.

Intraepithelial Infiltration by Mast Cells with Both Connective Tissue-Type and Mucosal-Type Characteristics in Gut, Trachea, and Kidneys of IL-9 Transgenic Mice

IL-9 transgenic mice were analyzed for the presence of mast cells in different tissues. In these mice, increased mast cell infiltration was found in the gastric and intestinal epithelium as well as in the upper airways and kidney epithelium, but not in other organs, such as skin. IL-9 transgenic mast cells do not show signs of massive degranulation such as that found in IL-4 transgenic mice and are not involved in spontaneous pathologic changes. Gastric mast cells showed a phenotype related to connective-type mast cells, since they were stained by safranin, and strong expression of mouse mast cell protease-4 and -5 was found in this organ. However, they also expressed proteases related to the mucosal cell type, such as mouse mast cell protease-1 and -2. In vitro, although IL-9 by itself did not induce mast cell development from bone marrow progenitors, it strongly synergized with stem cell factor for the growth and differentiation of mast cells expressing the same protease pattern as that observed in IL-9 transgenic mice. Since constitutive stem cell factor expression was observed in vivo, and anti-c-Kit Abs inhibited IL-9 transgenic mastocytosis in the gut, this synergistic combination of factors is likely to be responsible for the mastocytosis observed in IL-9 transgenic mice. Taken together, these data demonstrate that IL-9 induces the in vivo amplification of a nonclassical mast cell subset with a mucosal localization but expressing proteases characteristic of both connective tissue-type and mucosal mast cells.

In vivo roles of receptor tyrosine kinases and cytokine receptors in early thymocyte development

The early phases of T-cell development require both cell-cell interactions and soluble factors provided by stromal cells within the thymic microenvironment. Still, the precise nature of the signals delivered in vivo by cytokines (resulting in survival, proliferation or differentiation) remains unclear. Recent studies using mice deficient in cytokines or in their receptors have helped to identify essential signaling pathways required for the development of intrathymic precursors to mature alpha beta and gamma delta T cells. In addition, cytokine requirements for the development of natural killer cells were revealed in such mutants. The results obtained demonstrate that the development of all classes of lymphocytes (natural killer, gamma delta T cells and alpha beta T cells) is cytokine dependent, but the specific requirements differ for each lineage.

Growth factors and hematopoietic stem cells

The hematopoietic system is mediated in part by cell-to-cell interactions and soluble mediators or growth factors (cytokines). A large number of cytokines directly and potently control hematopoietic stem and precursor cell proliferation and differentiation. This review focuses on the recent studies devoted to the role of cytokines in the ex vivo expansion and differentiation of hematopoietic stem and precursor cells.

The road less travelled: c-kit and stem cell factor

Autocrine stimulation of growth factor receptors by autonomously produced ligands regulates different aspects of cellular transformation and progression. In several tumors, including gliomas, multiple autocrine systems are activated and may exert different functions in the malignant transformation process. The c-kit proto-oncogene is widely expressed in human gliomas, and it may be activated by its co-expressed ligand, stem cell factor (SCF). Studies in glioma cell lines as well as different tumor types suggest the possibility of intracellular interactions of c-kit with SCF. Although c-kit and SCF may not play a primary and causal role in the initiation and progression of glial tumors they may still be contributing factors in glioma biology. It can be hypothesized that the parallel activation of several autocrine systems including some of which have found less attention in gliomas, such as c-kit/SCF, could compromise the efficacy of therapies targeting different autocrine loops. A better understanding of the multiplicity and mechanisms of autocrine stimulation has implications for the development of new therapies interfering with autocrine tumor cell growth.

Stem cell factor, the JAK-STAT pathway and signal transduction

Recent work has demonstrated the importance of Janus family kinases (JAKs) and signal transducers and activators of transcription (STATs) in the stimulus-response coupling of receptors lacking intrinsic tyrosine kinase activity. In particular, the JAK-STAT pathway appears critical in signal transduction by interferon as well as numerous hematopoietic growth factors interacting with members of the hemapoietin receptor superfamily. Although ligands that interact with receptor tyrosine kinases (RTK), such as epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and colony stimulating factor-1 (CSF-1), have been shown to induce increases in phosphorylation of both JAKs and STATs, little is known about activation of this pathway by stem cell factor (SCF). This review will summarize what is known about the JAK/STAT pathway in relation to SCF signal transduction.

Bovine stem cell factor: production of a biologically active protein and mRNA analysis in cattle infected with Trypanosoma congolense

The cDNA coding for the soluble form of bovine stem cell factor (boSCFAla165) was cloned and recombinant protein was produced in bacteria as a histidine tagged-protein. The protein was purified from the inclusion bodies in one step by metal chelation chromatography under denaturing conditions. Recombinant bovine SCF was shown to act synergistically with interleukin 3 (IL-3) and erythropoietin (EPO) in stimulating the growth of bone marrow progenitor cells such as colony forming units-granulocyte macrophage (CFU-GM) and burst forming units-erythroid (BFU-E). Analysis of SCF mRNA expression by reverse transcription-polymerase chain reaction (RT-PCR) revealed that the transcripts were detectable in bone marrow, lymph nodes and spleen of cattle, and that the level of transcription was upregulated in lymph nodes of cattle infected with Trypanosoma congolense. Two isoforms of SCF mRNA were amplified by RT-PCR. The availability of recombinant bovine SCF provides a valuable tool for studying the role of SCF in the development, growth and differentiation of bovine hematopoietic cells.

Th2-mediated host protective immunity to intestinal nematode infections

Despite many years of study, relatively little is known about the effector mechanisms that operate against intestine-dwelling nematodes. Most of the current understanding comes from studies of laboratory model systems in rodents. It is clear that when an intestinal helminth infection takes place the immune system generates a strong Th2-mediated response, which regulates a variety of responses characteristic of helminth infections such as eosinophilia, intestinal mastocytosis and elevated IgE production. The ability to modulate the host's immune response in vivo with cytokine-specific monoclonal antibodies and recombinant cytokines, together with the use of animals with disruption of key genes involved in the immune response, have provided powerful tools with which to dissect the potential effector mechanisms operating. In the absence of a T-cell compartment the host is unable to expel the parasite. If a Th1-dominated response is generated, protective immunity is almost universally compromised. Thus, it it would appear that some aspect of a Th2-mediated response controls effector mechanisms. Although it is clear that for some infections the mast cell appears to be involved in protection, probably through the generation of a non-specific inflammatory response, how these cells become activated remains unclear. Data from infections in transgenic animals suggest that activation is not through the high-affinity receptor for IgE. Such studies also call into doubt the importance of conventional interactions between effector leucocytes and antibody. There is little evidence to support a protective role for eosinophilia in any system. New data also imply that, although interleukin 4 (IL-4) is generally important (and can exert effects independent of an adaptive immune response), it is not always sufficient to mediate protection; other Th2 cytokines (e.g. IL-13) may warrant closer investigation. It is apparent that a number of potential Th2-controlled effector mechanisms (some of which may be particularly important at mucosal surfaces) remain to be explored. Overall, it is likely that worm expulsion is the result of a combination of multiple mechanisms, some of which are more critical to some species of parasite than to others.

Stem cell factor in nasal polyposis and allergic rhinitis: increased expression by structural cells is suppressed by in vivo topical corticosteroids

BACKGROUND

Mast cells are increased in nasal polyp (Np) and allergic rhinitis (AR) tissue and are suppressed by topical corticosteroid treatment. Stem cell factor (SCF), a mast cell growth and survival factor, may explain these phenomena.

OBJECTIVE

We investigated structural cell gene expression and production of SCF in nasal tissues in patients who had received and who had not received in vivo intranasal corticosteroid therapy.

METHODS

Northern blot analyses for messenger RNA and ELISA for biologically active SCF protein from cultured Np epithelial cells and fibroblasts were performed. Immunostaining for SCF in cultured and tissue nasal structural cells in the presence or absence of steroid treatment was also performed.

RESULTS

We detected significant expression of SCF mRNA and protein by cultured Np epithelial cells and Np fibroblasts; Np fibroblast SCF supported the differentiation of mast cells in vitro. There were more immunoreactive SCF-positive Np epithelial cells in patients with AR than in control subjects (97.2 +/- 2.8 vs 45.6 +/- 22.0%; p < 0.0001). SCF that could be immunostained was significantly diminished overall in Np structural cells in the group given in vivo steroid treatment, with a modest (trend to significant) effect on any given cell type analyzed. In vitro treatment with budesonide of SCF-producing fibroblasts demonstrated inhibition of unstimulated, primary Np fibroblasts but not of IL-1-stimulated fibroblasts or transformed cell lines.

CONCLUSIONS

Human Np and AR tissue structural cells express and produce increased SCF. Our in vitro studies suggest that intranasal steroids blunt SCF expression in Nps, an effect that may be responsible for a decrease in mast cells and symptoms.

Mast cells distribution in human liver disease and experimental rat liver fibrosis. Indications for mast cell participation in development of liver fibrosis

BACKGROUND/AIMS

The development of liver fibrosis due to chronic liver diseases is thought to be mediated by inflammatory cells releasing fibrogenic mediators that activate fat-storing cells (Ito-cells). Recently, the involvement of mast cells in fibrogenesis has been suggested. We studied the distribution of these cells in normal human liver and human nonfibrotic and fibrotic liver disease as well as in normal rat liver and acutely and chronically injured rat liver (CCl4 model).

METHODS

Mast cells were identified by histochemical and immunohistochemical methods. The immunoreactivity of liver and comparatively of rat peritoneal mast cells to the serpins alpha1-antitrypsin, alpha1-antichymotrypsin and antithrombin III was also studied.

RESULTS

In normal human and rat liver, mast cells were rarely found in portal tracts, and there was no change in cell numbers in nonfibrotic human or acutely injured rat livers. In contrast, cirrhotic human and rat livers contained numerous mast cells in the portal tracts and the fibrous septa. They exhibited strong immunoreactivity to the serpins, as did rat peritoneal mast cells.

CONCLUSIONS

The results indicate that in the late stages of liver fibrogenesis, mast cells may be involved by displaying protease inhibitory activity in the fibrotic septa.

Autocrine regulation of neural crest cell development by steel factor

Steel factor (SLF) and its cognate receptor, c-kit, have been implicated in the generation of melanocytes from migrating neural crest (NC) cells during early vertebrate embryogenesis. However, the source of SLF in the early avian embryo and its precise role in melanogenesis are unclear. We report here that NC cells themselves express and release SLF protein, which in turn acts as an autocrine factor to induce melanogenesis in nearby NC cells. These results indicate that NC cell subpopulations play an active role in the determination of their cell fate and suggest a different developmental role for the embryonic microenvironment than what has been previously proposed.

Stepwise requirement of c-kit tyrosine kinase in mouse ovarian follicle development

Ovarian follicle development is controlled by the cycling variation of gonadotrophins derived from the central nervous system. Intragonadal signals are also required, especially in the autonomous development of small follicles. Receptor tyrosine kinase c-kit and its ligand SLF (Steel factor) are expressed on the surface of specific populations of follicle-forming cells in a contiguous manner and are thought to have important roles in follicular development. We blocked the interaction of c-kit and its ligand by administering the function-blocking antibody ACK2 to developing mice at various times after birth and monitored ovarian follicle development. A blockade of c-kit function disturbed the onset of primordial follicle development, primary follicle growth, follicular fluid formation of preantral follicles, and penultimate-stage ovarian follicle maturation before ovulation. Ovarian follicle growth was dependent on c-kit during the first 5 days after birth when the functional FSH receptor is not yet expressed in mouse ovary. In contrast, primordial follicle formation and survival, small preantral or antral follicle development, ovulation, and luteinization of the ovulated follicle were not affected by this antibody. These findings indicate the stepwise requirement of c-kit and its ligand interaction system in the developing ovarian follicle and that c-kit with its ligand supports the autonomous development of ovarian follicle independent of gonadotrophins.

C-kit receptors in childhood malignant lymphoblastic cells

The product of the protooncogene c-kit is the receptor for the hematopoietic cytokine stem cell factor (SCF). C-kit is expressed on leukemic cells of the erythroid, myeloid, and mast-cell lineage and SCF has a proliferative effect on some of these cells. The role of SCF and c-kit in lymphoid malignancies is much less clear. Here we review the role of c-kit in normal lymphopoiesis and summarize its role in lymphoid malignancies. C-kit is expressed in normal lymphopoiesis and its ligand SCF synergizes with IL-7 to enhance the proliferation of B- and T-cell progenitors. In malignant lymphopoiesis, c-kit can also be expressed in B and T-lymphoblastic cells from children with non Hodgkin's lymphoma (NHL) or acute lymphoblastic leukemia (ALL) when analyzed by the highly sensitive reverse transcriptase polymerase chain reaction (RT-PCR). While c-kit receptors were detected by flow cytometric (FCM) analysis on about 40% of fresh T-lymphoblastic biopsy tumor cell preparations or T-lymphoblastic cell lines, no receptors were detected on B-lymphoblastic fresh cells or cell lines from children with B-ALL or Burkitt's lymphoma (BL). Almost all of the lymphoblastic cells expressing c-kit protein responded to recombinant human (rh)SCF with a downregulation of c-kit receptors. A proliferative response was detected only in a minority of these cells. B-ALL or BL cell lines showed no response to rhSCF. Upregulation of c-kit in T-lymphoblastic cells could be demonstrated by the addition of IL-1 beta, TNF-alpha, TGF-beta or A23187, and downregulation by rhSCF or phorbol myristate acetate (PMA). Despite upregulation of c-kit mRNA, protein remained undetectable on B-ALL or BL cells in the presence of A23187. The metabolic state of the cells seemed to influence c-kit expression, since c-kit was upregulated in T-lymphoblastic cells by the addition of new medium. C-kit appears to play a role in the growth of some malignant T-lymphoblastic but not B-lymphoblastic cells.

Systemic anaphylaxis in the mouse can be mediated largely through IgG1 and Fc gammaRIII. Assessment of the cardiopulmonary changes, mast cell degranulation, and death associated with active or IgE- or IgG1-dependent passive anaphylaxis

We attempted to elicit active anaphylaxis to ovalbumin, or passive IgE- or IgG1-dependent anaphylaxis, in mice lacking either the Fc epsilonRI alpha chain or the FcR gamma chain common to Fc epsilonRI and Fc gammaRI/III, or in mice lacking mast cells (KitW/ KitW-v mice), and compared the responses to those in the corresponding wild-type mice. We found that the FcR gamma chain is required for the death, as well as for most of the pathophysiological changes, associated with active anaphylaxis or IgE- or IgG1-dependent passive anaphylaxis. Moreover, some of the physiological changes associated with either active, or IgG1-dependent passive, anaphylactic responses were significantly greater in Fc epsilonRI alpha chain -/- mice than in the corresponding normal mice. Finally, while both KitW/KitW-v and congenic +/+ mice exhibited fatal active anaphylaxis, mast cell-deficient mice exhibited weaker physiological responses than the corresponding wild-type mice in both active and IgG1-dependent passive systemic anaphylaxis. Our findings strongly suggest that while IgE antibodies and Fc epsilonRI may influence the intensity and/or kinetics of some of the pathophysiological changes associated with active anaphylaxis in the mouse, the mortality associated with this response can be mediated largely by IgG1 antibodies and Fc gammaRIII.

Pro-thymocyte expansion by c-kit and the common cytokine receptor gamma chain is essential for repertoire formation

Growth factors have been implicated in thymocyte development, but mutants lacking cytokines, or their receptors, have failed to reveal essential roles for growth/differentiation factors in the thymus. Mutations in the receptor tyrosine kinase c-kit and the common cytokine receptor gamma chain (gamma c) reduce cellularity, but are permissive for thymocyte development. We now report that thymocyte development is completely abrogated in mice lacking both c-kit and gamma c (c-kit-gamma c-). Thymic hypocellularity is so severe that the T cell receptor repertoire fails to form except for monoclonal or oligoclonal beta chain DJ rearrangements. B lymphopoiesis is only mildly reduced in c-kit-gamma c- as compared with c-kit+gamma c- mice, and hematological values are identical comparing c-kit-deficient and c-kit-gamma c- mice. These experiments reveal essential, overlapping, and synergistic functions for two distinct signaling pathways, one utilizing c-kit and the other cytokine receptor gamma c complexes coupling to Janus kinases and signal transducers and activators of transcription.

Absence of Fc epsilonRI alpha chain results in upregulation of Fc gammaRIII-dependent mast cell degranulation and anaphylaxis. Evidence of competition between Fc epsilonRI and Fc gammaRIII for limiting amounts of FcR beta and gamma chains

In mouse mast cells, both Fc epsilonRI and Fc gammaRIII are alpha beta gamma2 tetrameric complexes in which different alpha chains confer IgE or IgG ligand recognition while the signaling FcR beta and gamma chains are identical. We used primarily noninvasive techniques (changes in body temperature, dye extravasation) to assess systemic anaphylactic responses in nonanesthetized wild-type, Fc epsilonRI alpha chain -/- and FcR gamma chain -/- mice. We confirm that systemic anaphylaxis in mice can be mediated largely through IgG1 and Fc gammaRIII and we provide direct evidence that these responses reflect activation of Fc gammaRIII rather than Fc gammaRI. Furthermore, we show that Fc gammaRIII-dependent responses are more intense in normal than in congenic mast cell-deficient KitW/KitW-v mice, indicating that Fc gammaRIII responses have mast cell-dependent and -independent components. Finally, we demonstrate that the upregulation of cell surface expression of Fc gammaRIII seen in Fc epsilonRI alpha chain -/- mice corresponds to an increased association of Fc gammaRIII alpha chains with FcR beta and gamma chains and is associated with enhanced Fc gammaRIII-dependent mast cell degranulation and systemic anaphylactic responses. Therefore, the phenotype of the Fc epsilonRI alpha chain -/- mice suggests that expression of Fc epsilonRI and Fc gammaRIII is limited by availability of the FcR beta and gamma chains and that, in normal mice, changes in the expression of one receptor (Fc epsilonRI) may influence the expression of functional responses dependent on the other (Fc gammaRIII).

p62(dok): a constitutively tyrosine-phosphorylated, GAP-associated protein in chronic myelogenous leukemia progenitor cells

Characteristic of chronic myelogenous leukemia (CML) is the presence of the chimeric p210(bcr-abl) protein possessing elevated protein tyrosine kinase activity relative to normal c-abl tyrosine kinase. Hematopoietic progenitors isolated from CML patients in the chronic phase contain a constitutively tyrosine-phosphorylated protein that migrates at 62 kDa by SDS-PAGE and associates with the p120 ras GTPase-activating protein (GAP). We have purified p62(dok) from a hematopoietic cell line expressing p210(bcr-abl). p62(dok) is a novel protein with features of a signaling molecule. Association of p62(dok) with GAP correlates with its tyrosine phosphorylation. p62(dok) is rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases.

Specificity and functional effects of antibodies to human stem cell factor

Three monoclonal antibodies (Mabs), 7H6, 4B10 and Genzyme Mab, and a commercially-available polyclonal antiserum (Genzyme) to human Stem Cell Factor (SCF) were compared for their ability to detect native and recombinant SCF in a variety of assays, and for blocking of SCF function. All antibodies were found to bind to the membrane bound isoform as well as soluble SCF and to bind to both glycosylated (yeast MGF) and unglycosylated (E. coli SCF) recombinant factor. Mabs 7H6 and 4B10, as well as the polyclonal antiserum could immunoprecipitate membrane-associated SCF and all the antibodies could detect recombinant soluble SCF on western blots, although the binding of all except 7H6 was partially sensitive to reduction. Titration of the antibodies on CHO cells expressing membrane-associated human SCF showed similar dose-dependence for all Mabs with 70% of maximum binding seen at 3, 5 and 8 micrograms/ml for 7H6, 4B10 and Genzyme Mab respectively, however the maximum binding seen with 7H6 was approximately 2-fold greater than with 4B10 and 7-fold greater than Genzyme Mab. Competitive binding experiments of the Mabs on cells expressing membrane SCF gave non-reciprocal blocking in all cases with 7H6 completely blocking 4B10 and Genzyme Mab binding. All antibodies except the Genzyme Mab effectively blocked SCF binding to c-Kit-expressing cells, and were strongly inhibitory in an assay of in vitro haemopoiesis which is believed to depend on adhesive interactions, as well as the "classical' cytokine-receptor interaction, mediated by SCF binding to c-Kit.

Promotion of mouse fibroblast proliferation by IgE-dependent activation of mouse mast cells: role for mast cell tumor necrosis factor-alpha and transforming growth factor-beta 1

Pathologic fibroblast proliferation or tissue fibrosis develops in certain chronic allergic diseases and in a wide array of other inflammatory disorders in which mast cell activation is also a prominent feature. In this study we investigated a number of potential mechanisms by which IgE-dependent activation of mouse mast cells might influence the proliferation of mouse fibroblasts in vitro. We found that supernatants from in vitro-derived mast cells that had been activated by IgE and specific antigen (but not those from quiescent mast cells) promoted the proliferation of mouse embryonic skin or 3T3 fibroblasts, and we showed that this effect was detectable in the absence of fetal calf serum. We analyzed the kinetics with which the fibroblast-proliferative activity was secreted from bone marrow-derived cultured mast cells and found that it was released both rapidly (i.e., in 30 minutes or less) and for a more prolonged period (i.e., for more than 2 hours) after IgE-dependent mast cell activation. We then measured the levels at which the mast cells produce a number of cytokines that are known to affect fibroblasts (IL-1, IL-6, transforming growth factor-beta 1 [TGF-beta 1], and tumor necrosis factor-alpha [TNF-alpha]) and assessed their relative effects, as recombinant cytokines, on fibroblast proliferation. Our mast cells secreted high levels of TGF-beta 1 and TNF-alpha, intermediate amounts of IL-6, and low levels of IL-1. We titrated the fibroproliferative effects of each of these cytokines and determined that at a dose of 50 pg/ml their rank order of activity was TGF-beta 1 > TNF-alpha > IL-1 > IL-6, with all but IL-6 having significant effects. The ability of supernatants from activated bone marrow-derived cultured mast cells to promote fibroblast proliferation was partially diminished by absorption with neutralizing antibodies against either TNF-alpha or TGF-beta 1, and absorption of the supernatants with a combination of antibodies against TNF-alpha and TGF-beta 1 reduced their ability to induce fibroblast proliferation by approximately 50% (p < or = 0.001, n = 5). These findings show that IgE-dependent activation of mouse mast cells can result in the release of mediators that promote fibroblast proliferation in the absence of any other cell type and suggest that mast cell-derived TNF-alpha and TGF-beta 1 contribute substantially to this effect. They also suggest that these cytokines exert their effects through synergistic interactions with other mast cell mediators.

Cross-linking of the high-affinity IgE receptor induces the expression of cyclo-oxygenase 2 and attendant prostaglandin generation requiring interleukin 10 and interleukin 1 beta in mouse cultured mast cells

When mouse bone marrow-derived mast cells (BMMC) developed in interleukin (IL)-3 were activated with IgE and antigen (IgE/antigen) in the presence of both IL-10 and IL-1 beta, two sequential phases of prostaglandin (PG)D2 generation were elicited, in which the first phase occurred by 1 h and the second phase from 2 to 10 h. The delayed phase of PGD2 generation was accompanied by a marked induction of cyclo-oxygenase (COX)-2 mRNA, which reached a peak at 1-2 h, followed by that of its protein from 2-10 h, with a peak at 5 h. The immediate phase of PGD2 generation was completely abrogated by the irreversible inhibition of pre-existing COX-1 by aspirin pretreatment, whereas the delayed phase of PGD2 generation was almost undetectable in the presence of the COX-2 inhibitor NS-398. A detailed analysis of the individual effects of IgE/antigen, IL-10 and IL-1 beta on COX-2 expression revealed that IgE/antigen and IL-10 each initiated and stabilized COX-2 mRNA expression, leading to an increase in the expression of its protein. Conversely, IL-1 beta stabilized the COX-2 protein without affecting its mRNA level. The induction of COX-2 by IgE/antigen with IL-10 and IL-1 beta preceded the induction of transcripts for endogenous cytokines such as IL-6, IL-1 beta and IL-10. The inhibition of PGD2 generation by indomethacin did not affect the induction of COX-2 or these cytokines. Thus the two major delayed-phase responses of BMMC after IgE-dependent activation, namely COX-2-dependent PGD2 generation and cytokine production, are regulated independently.

Mucosal mast cells and the allergic response against nematode parasites

IgE-mediated Type-I allergic reactions at nematode-infected mucosal surfaces are considered to have a direct protective function. The contribution of mucosal mast cells (MMC) to these mucosal allergic responses is reviewed. In addition to the T helper 2 cell-mediated regulation of MMC hyperplasia during nematode infection the kit ligand, stem cell factor (SCF), plays a key role in the early development of the MMC response. Studies in the mouse suggest that MMC protect against certain nematodes which enter the mucosa but not against lumen dwelling nematodes. The protective roles of MMC in other species, including sheep, are less certain and there is some evidence that MMC might enhance parasite fecundity. The measurement of MMC-specific granule chymases released systemically, and into the gut lumen, permits precise monitoring of mast cell activation and suggests that the secreted chymases may target epithelial junctional complex proteins, thereby causing increased mucosal permeability. The abundant intraepithelial MMC found in parasitised mucosa may, therefore, serve as epithelial gatekeepers permitting the translocation of plasma proteins onto the mucosal surface.

Expression of stem cell factor receptor (c-kit) by the malignant mast cells from spontaneous canine mast cell tumours

Stem cell factor receptor (SCFR, c-kit), normally expressed on haematopoietic and mast cells, plays a regulatory role in cellular growth and differentiation. Dysregulated expression of SCFR may contribute to neoplastic transformation. We investigated expression of SCFR on malignant canine mast cells obtained directly from spontaneous canine mast cell neoplasms, in an attempt to determine whether these undifferentiated cells maintained expression of this growth-promoting cytokine receptor. Malignant mast cells (histological grade 2) from skin tumours or lymph node metastases were collected from canine patients, and SCFRs were detected by flow cytometric analysis of these cells. All of the tumours bound mouse and canine recombinant stem cell factor (SCF), indicating that the cells not only expressed SCFRs, but that the receptors possessed the functional property of ligand binding. Immunoglobulin Fc receptors for canine IgE were identified on these cells by flow cytometry, a further indication that the cells analysed were mast cells and retained some differentiated features. Immunohistochemical analysis of formalin-fixed, paraffin wax-embedded mast cell tumour biopsies confirmed expression of SCFRs by malignant cells from each tumour. The relative binding of SCF to suspensions of tumour cells, as assessed by flow cytometry, correlated with the intensity of immunolabelling for SCFR in sections of the same tumours, suggesting variability in SCFR expression between tumours. Agarose gel electrophoresis of the products of SCFR reverse transcription-polymerase chain reaction derived from each tumour had the molecular weight predicted for canine SCFR cDNA on the basis of the mouse and human counterparts. This further confirmed SCFR expression by malignant canine mast cells. Taken together, these results show that a membrane receptor capable of triggering cell growth is expressed by malignant canine mast cells, suggesting a role for this receptor in the aetiology of canine mast cell cancer. This relatively common malignancy of the dog would seem to present an opportunity for the investigation of the potential role of the SCF/SCFR pathway in the development of spontaneous malignancies of mast cells.

Characterization of whole-cell currents in mucosal and connective tissue rat mast cells using amphotericin-B-perforated patches and temperature control

Rat mucosal type mast cells are thought to possess only a K+-selective inwardly rectifying (IRK) current in the resting state. We used rat-bone-marrow-derived mast cells (BMMCs) as a model of mucosal mast cells and recorded whole-cell membrane currents from cells perforated with amphotericin B. Under these conditions, both inwardly rectifying (IR) and outwardly rectifying (OR) currents were observed. The reversal potential and conductance of the IR current depended on the extracellular K+ concentration, indicating that the channel was K+ selective. The OR current was not affected by changes in extracellular K+ concentration, but lowering extracellular Cl- concentration reduced the conductance and shifted the reversal potential in a positive direction. The OR current was not affected by K+ channel blockers, but was reversibly blocked by the chloride channel blocker 4,4'-diisothiocyanato-2,2'-stilbenedisulphonate (DIDS), again indicating a Cl- conductance. The IRK current was also detected in the majority of cells using the conventional whole-cell recording configuration at room temperature. In contrast, the ORCl current was only observed in 7% of recordings made at room temperature with the conventional whole-cell voltage-clamp mode, but was detected in 66% of cells if the bath temperature was increased and the integrity of the cell's cytoplasm was preserved by using the perforated-patch technique. Under similar conditions, the ORCl current was also present in rat peritoneal mast cells, a connective tissue phenotype previously thought to have no whole-cell currents in the resting state. The role of this current and factors affecting its activation are discussed.

Stem cell factor potentiates histamine secretion by multiple mechanisms, but does not affect tumour necrosis factor-alpha release from rat mast cells

The effect of stem cell factor (SCF) on histamine and tumour necrosis factor-alpha (TNF-alpha) release from rat peritoneal mast cells (PMC) was determined and the intracellular pathways involved in the potentiation of histamine secretion were investigated. The effects of SCF (2-100 ng/ml) were examined following both short-term (0 and 20 min) and long-term (up to 24hr) preincubations with SCF. Pretreatment of PMC with SCF for 0 min (concurrent) or 20 min did not induce histamine secretion directly, but significantly increased antigen (Ag)-induced histamine secretion. SCF potentiated Ag-induced intracellular Ca2+ increase and calcium ionophore A23187-induced histamine secretion. Pertussis toxin (PT) inhibited SCF-induced potentiation of IgE-dependent histamine secretion, indicating that PT-sensitive G-proteins are involved in the immediate effects of SCF. In long-term incubation experiments, SCF pretreatment for 18-24 hr significantly enhanced Ag-induced histamine secretion, but did not affect Ag-induced intracellular Ca2+ levels. The effects of long-term incubation with SCF, but not the short-term effects, were blocked by cycloheximide. Interestingly, spontaneous and Ag-induced TNF-alpha release from rat PMC were not affected by pretreatment with SCF (2-500 ng/ml) for 1 to 24 hr. Thus, through immediate and delayed mechanisms, SCF potentiates histamine release from PMC, but has not effect on TNF-alpha release. The regulation of MC by SCF may be important in allergic and other inflammatory diseases.

Identification of novel lymphoid tissues in murine intestinal mucosa where clusters of c-kit+ IL-7R+ Thy1+ lympho-hemopoietic progenitors develop

We have revealed that about one and a half thousand tiny clusters, filled with one thousand closely packed lymphocytes, can be found throughout the murine small and large intestinal mucosa. They are located in crypt lamina propria (cryptopatches; CP) and can be first detected at 14-17 d after birth. A large fraction of lymphocytes in CP expresses c-kit, IL-7R, Thy1 and a lymphocyte function-associated antigen, LFA-1, whereas most of them remain CD3-, TCR alpha beta-, TCR gamma delta-, sIgM-, and B220-. The population size of IL-2R alpha+, HSA+ and Pgp-1+ subsets is variable (20-50%) and the composition of CD8+, Ly-1+, and CD4+ subsets is smaller but also variable (3-20%). In the small intestine, CP do not contain cells undergoing apoptosis nor cells bearing RAG-1 molecules, but do contain dendritic stromal cells bearing CD11c/CD18 molecules. The frequency of DNA replicating cells in CP is higher than that in Peyer's patches (PP), is lower than that in the thymic cortex and is almost comparable with that in the thymic medulla. The numbers of CP remain the same in aged mice (> 114 wk) but double after estrogen treatment even though the thymi are attenuated sharply in both conditions. Thus, with respect to histogenesis, lymphocyte composition and tissue level of cellular behavior, neither PP, isolated lymphoid follicles, peripheral LNs, nor thymus are identical with CP. Finally, CP are virtually absent in lamina propria of IL-7R-deficient mice that display a profound reduction in thymic and peripheral lymphoid cellularity. By contrast, CP are present in germ-free mice and in athymic (nu/nu), SCID, TCR beta x delta-/-, RAG-2-/-, PP-deficient (aly/aly), stem cell factor (Sl/Sld) and c-kit (W/Wv) mutant mice. Taking all of these results together, CP are the first identification of gut-associated murine lymphoid tissues where the generation of IL-7-dependent lympho-hematopoietic progenitors for T and/or B cell descendants may start to take place at the age of commencement of weaning.