Human Lung Mast Cells Research Articles

Characterization of syk expression in human lung mast cells: relationship with function

Previous studies indicate that the protein tyrosine kinase, syk, is critical in transducing FcɛRI-mediated signals. In human basophils, 'releasability' has been linked to the extent of syk expression. Human lung mast cells, like basophils, are also found to be variably responsive to IgE-dependent activation. The aim of the present study was to determine whether the wide variability in human lung mast cell responses, following IgE-dependent activation, has a relationship with syk expression. Mast cells were isolated from human lung tissue and 'releasability' was determined by activating the cells with a maximal releasing concentration of anti-IgE. Syk levels in mast cells were determined by immunoblotting and flow cytometry. Histamine release from mast cells, challenged with a maximal releasing concentration of anti-IgE, ranged from 0% to 69% (mean±SEM, 24±2%, n=53). A proportion of these preparations (nine out of 53) released very low levels of histamine (5%) in response to anti-IgE. Flow cytometry of a subset of preparations indicated that a weak response to anti-IgE was not related to a lack of surface IgE. Immunoblotting and flow cytometry studies demonstrated that, compared with mononuclear cells, human lung mast cells express low and variable levels of syk. However, there was no correlation between syk expression and mast cell releasability. Nonetheless, a number of putative inhibitors of syk including NVP-QAB205 (EC₅₀, 0.2 μm) effectively attenuated the IgE-dependent release of histamine from mast cells. These studies indicate that although syk may play an important role in mediating degranulation, the relative level of syk expression does not govern human lung mast cell releasability. Identification of the mechanisms that govern IgE-dependent activation of human lung mast cells is likely to be of wider clinical significance, given the central role that mast cells play in the development of allergic asthma.

Adenosine Potentiates Human Lung Mast Cell Tissue Plasminogen Activator Activity

We investigated whether adenosine, a potent contributor to the regulation of pulmonary function, can modulate human lung mast cell (HLMC) fibrinolytic activity. Tissue plasminogen activator (tPA) activity and tPA transcript expression levels from a human mast cell line (HMC-1) and HLMC were monitored following adenosine application. Adenosine potentiated mast cell tPA activity and tPA gene expression in a dose-dependent manner. Adenosine effects were abolished in the presence of adenosine deaminase. HMC-1 cells and HLMC predominantly expressed adenosine A(2A) and A(2B) receptor transcripts (A(2B) ≈ A(2A) > A(3) >> A(1)). Pharmacological and signaling studies suggest that the A(2A) receptor is the major subtype accounting for adenosine-induced mast cell tPA activity. Finally, the supernatant from HMC-1 cells and HLMC treated with adenosine (for 24 h) significantly increased fibrin clot lysis, whereas ZM241385, an A(2A) receptor antagonist, abolished this effect. To our knowledge, this study provides the first data to demonstrate the potentiating effect of adenosine on mast cell tPA activity and fibrin clot lysis.

S31 Characterisation of cell adhesion molecule-1 in lung mast cells

Introduction and ObjectivesCell adhesion molecule 1 (CADM1) is implicated in several diseases and a prognostic factor for lung cancer. In human lung mast cells (HLMC) it contributes to cell-cell adhesion...

S43 The expression of CRACM ion channels in human lung mast cells

<h3>Introduction and objectives</h3> The influx of extracellular Ca²⁺ into human lung mast cells (HLMC) is essential for the IgE-dependent release of preformed granule derived mediators and newly synthesised autacoids and cytokines. However the identity of the ion channels underlying the influx of Ca²⁺ remains unknown. Members of the recently discovered CRACM (also known as Orai) ion channel family are candidates for this function. The channels carry Ca²⁺ selective currents (I_CRAC) that are activated when endoplasmic reticulum Ca²⁺ stores are emptied. Here we investigate whether CRACM channels are expressed in HLMC and their possible contribution to Ca²⁺ influx in these cells. <h3>Methods</h3> HLMC were purified from lung tissue by immuno-magnetic affinity selection. Total RNA purified from cells was used in quantitative RT-PCR using TaqMan probes and primers (Applied Biosystems). For electrophysiology the whole cell method of patch clamp was used. <h3>Results</h3> CRACM1, CRACM2 and CRACM3 mRNA transcripts were all readily detectable in HLMC. CRACM1 transcripts (4.50±0.51 copies/10³ beta-actin transcripts, n=7 HLMC donors) were more abundant than CRACM2 transcripts (1.79±0.67 copies/10³ beta-actin transcripts, n=5) and CRACM3 transcripts (3.33±0.95 copies/10³ beta-actin transcripts, n=5). Consistent with the presence of CRACM transcripts, an I_CRAC-like current was detected in HLMC following IgE-dependent activation and also in cells dialysed with 30 μM inositol triphosphate to empty endoplasmic reticulum Ca²⁺ stores. The Ca²⁺-selective current obtained under both conditions was blocked by the addition of 10 μM La³⁺, a known blocker of CRACM channels. Thus following addition of anti-IgE to 11 HLMC, current at −120 mV increased from −12.1±1.1 pA at baseline to −34.2±2.5 pA (p&lt;0.0001) and following addition of 10 mM La³⁺ this was reduced to −17.4±2.1 pA. (p=0.0003). When HLMC were dialysed with IP₃ (n=12), current increased from −15.3±1.2 pA to −40.2±3.2 pA (p&lt;0.0001), and decreased to −17.7±1.5 pA (p&lt;0.0001) by La³⁺. <h3>Conclusions</h3> The presence of an I_CRAC-like current and CRACM1, -2 and -3 mRNA transcripts in HLMC supports a role for CRACM channels in the influx of extracellular Ca²⁺ into these cells following activation. Such a role would make CRACM channels a potential therapeutic target in the treatment of asthma.

S33 Suppression of constitutive and stimulated secretion of histamine from human lung mast cells by a secreted factor from lung epithelial cells

IntroductionConstitutive and IgE-dependent secretion of histamine from human lung mast cells (HLMC) is suppressed by direct contact with the BEAS-2B bronchial epithelial cell line, but not conditioned epithelial cell media....

P182 Adenosine potentiates human mast cell fibrinolytic activity

We investigated whether adenosine, a potent contributor to the regulation of pulmonary function, can modulate human lung mast cell (HLMC) fibrinolytic activity. Tissue plasminogen activator (tPA) activity and tPA transcript...

Mast Cell Fibroblastoid Differentiation Mediated by Airway Smooth Muscle in Asthma

Mast cell microlocalization to the airway smooth muscle (ASM) bundle is a key feature of asthma, but whether these mast cells have an altered phenotype is uncertain. In this paper, we report that in vivo, mast cells within the ASM bundle, in contrast to mast cells in the bronchial submucosa, commonly expressed fibroblast markers and the number of these cells was closely related to the degree of airway hyperresponsiveness. In vitro human lung mast cells and mast cell lines cultured with fibronectin or with primary human ASM cells acquired typical fibroblastic markers and morphology. This differentiation toward a fibroblastoid phenotype was mediated by ASM-derived extracellular matrix proteins, independent of cell adhesion molecule-1, and was attenuated by α5β1 blockade. Fibroblastoid mast cells demonstrated increased chymase expression and activation with exaggerated spontaneous histamine release. Together these data indicate that in asthma, ASM-derived extracellular matrix proteins mediate human mast cell transition to a fibroblastoid phenotype, suggesting that this may be pivotal in the development of airway dysfunction in asthma.

Mast cells have a protumorigenic role in human thyroid cancer

In different human carcinoma types, mast cell infiltrate increases with respect to normal tissue and mast cell density correlates with a bad prognosis. To assess the role of mast cells in human thyroid cancer, we compared the density of tryptase-positive mast cells in 96 papillary thyroid carcinomas (PTCs) versus normal thyroid tissue from 14 healthy individuals. Mast cell density was higher in 95% of PTCs (n=91) than in control tissue. Mast cell infiltrate correlated with extrathyroidal extension (P=0.0005) of PTCs. We show that thyroid cancer cell-line-derived soluble factors induce mast cell activation and chemoattraction in vitro. Different mast cell lines (HMC-1 and LAD2) and primary human lung mast cells induced thyroid cancer cell invasive ability, survival and DNA synthesis in vitro. The latter effect was mainly mediated by three mast-cell-derived mediators: histamine, and chemokines CXCL1/GROα and CXCL10/IP10. We show that xenografts of thyroid carcinoma cells (8505-C) could recruit mast cells injected into the tail vein of mice. Co-injection of human mast cells accelerated the growth of thyroid cancer cell (8505-C) xenografts in athymic mice. This effect was mediated by increased tumor vascularization and proliferation, and was reverted by treating mice with sodium cromoglycate (Cromolyn), a specific mast cell inhibitor. In conclusion, our study data suggest that mast cells are recruited into thyroid carcinomas and promote proliferation, survival and invasive ability of cancer cells, thereby contributing to thyroid carcinoma growth and invasiveness.

Angiogenesis and lymphangiogenesis in bronchial asthma

Neovascularization plays a prominent role in inflammation and tissue remodeling in several chronic inflammatory disorders. Vessel number and size, vascular surface area and vascular leakage are all increased in biopsies from patients with asthma. High levels of VEGF and other angiogenic factors have been detected in tissues and biological samples of patients with asthma and correlate with disease activity and inversely with airway hyper-responsiveness. Inflammation in the lung stimulates the growth of new blood vessels and these contribute to the airway obstruction or airway hyper-responsiveness, or both. Effector cells of inflammation (human lung mast cells, basophils, eosinophils, macrophages, etc.) are major sources of a vast array of angiogenic and lymphangiogenic factors. Inhaled corticosteroids reduce vascularity and growth factor expression and might modulate bronchial vascular remodeling in asthma. Specific antagonists to VEGF and other angiogenic factors and their receptors might help to control chronic airway inflammation and vascular remodeling and offer a novel approach for the treatment of chronic inflammatory lung disorders.

Mast cell adhesion to bronchial smooth muscle in asthma specifically depends on CD51 and CD44 variant 6

Mast cells infiltrate the bronchial smooth muscle (BSM) in asthmatic patients, but the mechanism of mast cell adhesion is still unknown. The adhesion molecules CD44 (i.e. hyaluronate receptor) and CD51 (i.e. vitronectin receptor) are widely expressed and bind to many extracellular matrix (ECM) proteins. The aims of the study are (i) to identify the role of ECM in mast cell adhesion to BSM and (ii) to examine the role of CD51 and CD44 in this adhesion. Human lung mast cells, human mast cell line (HMC-1), and BSM cells from control donors or asthmatic patients were cultured in the presence/absence of various cytokines. Mast cell-BSM interaction was assessed using (3)H-thymidine-pulsed mast cells, confocal immunofluorescence, or electron microscopy. Adhesion molecules expression and collagen production on both cell types were evaluated by quantitative RT-PCR, western blot, and flow cytometry. Mast cell adhesion to BSM cells mostly involved type I collagen of the ECM. Such an adhesion was increased in normal BSM cells under inflammatory condition, whereas it was maximal in asthmatic BSM cells. Blockade of either CD51 or CD44 significantly decreased mast cell adhesion to BSM. At the molecular level, protein and the transcriptional expression of type I collagen, CD51 or CD44 remained unchanged in asthmatic BSM cells or in mast cells/BSM cells under inflammatory conditions, whereas that of CD44 variant isoform 6 (v6) was increased. Mast cell-BSM cell adhesion involved collagen, CD44, and CD51, particularly under inflammatory conditions. CD44v6 expression is increased in asthmatic BSM cells.

A novel FcεRIβ-chain truncation regulates human mast cell proliferation and survival.

Mast cells contribute to allergy through IgE-dependent activation via the high-affinity IgE receptor FcεRI. The role of the FcεRIβ chain (MS4A2) in mast cell function is not understood fully, although it serves to amplify FcεRI-dependent signaling. We demonstrate the expression of a novel MS4A2 truncation lacking exon 3 in human mast cells termed MS4A2(trunc). MS4A2(trunc) gene expression was regulated negatively by the mast cell growth factor stem cell factor (SCF), and its expression was not detected in the SCF receptor gain-of-function human mast cell line HMC-1. Unlike MS4A2, MS4A2(trunc) did not traffic to the cytoplasmic membrane but instead was associated with the nuclear membrane. Overexpression of MS4A2(trunc) induced human lung mast cell death and profoundly inhibited HMC-1 cell proliferation by inducing G(2)-phase cell cycle arrest and apoptosis. Thus, we have identified a novel splice variant of MS4A2 that might be important in the regulation of human mast cell proliferation and survival. This finding demonstrates that the MS4A2 gene has multiple roles, extending beyond the regulation of acute allergic responses. By understanding the mechanisms regulating its function, it might be possible to induce its expression in mast cells in vivo, which could lead to better treatments for diseases such as mastocytosis and asthma.

Human Lung Mast Cells Mediate Pneumococcal Cell Death in Response to Activation by Pneumolysin

Mast cells are emerging as contributors to innate immunity. Mouse mast cells have a pivotal role in protection against bacterial infection, and human cord blood-derived mast cells reduce bacterial viability in culture. The objectives of this study were to determine whether human lung mast cells (HLMCs) might be protective against pneumococcal lung infection through direct antimicrobial activity. Tissue-derived HLMCs and the human mast cell lines HMC-1 and LAD2 were cocultured with wild-type and mutant pneumococci, and viability and functional assays were performed. Mast cells were also stimulated with purified pneumolysin. HLMCs killed wild-type serotype-2 (D39) pneumococci in coculture but had no effect on an isogenic pneumolysin-deficient (PLN-A) pneumococcus. D39 wild-type, but not PLN-A pneumococci, induced the release of leukotriene C4 from human mast cells in a dose-dependent manner, which was not accompanied by histamine release. Stimulation of mast cells with sublytic concentrations of purified pneumolysin replicated this effect. Furthermore, pneumolysin induced the release of the cathelicidin LL-37 from HLMCs, purified LL-37 reduced pneumococcal viability, and neutralizing Ab to LL-37 attenuated mast cell-dependent pneumococcal killing. In addition, at high concentrations, all pneumococcal strains tested reduced HLMC viability through a combination of pneumolysin and H2O2-dependent mechanisms. HLMCs exhibit direct antimicrobial activity to pneumococci through their activation by pneumolysin. This antimicrobial activity is mediated, in part, by the release of LL-37 from HLMCs. This suggests that mast cells provide an early warning system and potentially limit pneumococcal dissemination early in the course of invasive pulmonary pneumococcal disease.

Activation of human mast cells through the platelet-activating factor receptor

In human subjects platelet-activating factor (PAF) concentrations are markedly increased in the plasma after anaphylactic reactions, and these correlate strongly with the severity of the response. The mechanism for the systemic spread of mast cell (MC) activation in anaphylaxis is often assumed to relate to the hematogenous spread of allergen, but this is implausible, and amplification mechanisms need to be considered. We have investigated the ability of PAF to induce human MC degranulation using skin, lung, and peripheral blood (PB)-derived cultured MCs and the signaling pathways activated in PB-derived MCs in response to PAF. The expression of PAF receptor was investigated by means of RT-PCR and Western blot analysis. Cell-signaling pathways in PB-derived MCs in response to PAF were investigated by analyzing the effect of various inhibitors and the silencing of phospholipase C (PLC) mRNA on PAF-mediated histamine release. We show for the first time that PAF induces histamine release from human lung MCs and PB-derived MCs but not skin MCs. Activation of PAF receptor-coupled G(alphai) leads to degranulation through PLCgamma1 and PLCbeta2 activation in human MCs. PAF-induced degranulation was rapid, being maximal at 5 seconds, and was partially dependent on extracellular Ca(2+). Our findings provide a mechanism whereby PAF mediates an amplification loop for MC activation in the generation of anaphylaxis.

Adenosine receptor A3AR is responsible for augmenting FcϵRI-induced degranulation of human lung mast cells by extracellular adenosine. (86.17)

Abstract Inhaled adenosine induces a bronchoconstrictive response in asthmatics that is attributed to enhanced activation of mast cells in the airways. In this study, we demonstrate that extracellular adenosine augments FcϵRI-mediated degranulation of human mast cells from lung via the adenosine A3 receptor (A3AR). We found that low concentrations (10-7 - 10-6 M) of adenosine and its stable analog NECA enhanced degranulation of mast cells from lung, but not skin. Quantitative real time PCR analysis revealed that lung and skin mast cells expressed equal amounts of A2aAR and A2bAR (A1AR was not detectable), but that lung mast cells expressed a 5-fold greater amount of A3AR mRNA transcripts compared with skin mast cells. This suggested that A3AR was responsible for augmenting degranulation of lung mast cells. Indeed, targeted stimulation with the A3AR-specific agonist HEMADO significantly enhanced mediator release from lung mast cells following FcϵRI crosslinking with an anti-FcϵRIα MAb, but had little effect on skin mast cells. In addition, the enhancing effect of adenosine on lung mast cells could be completely inhibited by inactivating Gi-coupled A3ARs by ADP ribosylation with pertussis toxin. These data provide direct evidence that extracellular adenosine enhances IgE-induced degranulation of human lung mast cells through an A3AR-mediated mechanism; thus supporting a role for A3AR in the bronchoconstrictive response to inhaled adenosine in asthmatics.

Human Lung Mast Cell Cultivation in Interleukin-4 but not Interleukin-13 Regulates Histamine Release and Proliferation Responses

Human Lung Mast Cell Cultivation in Interleukin-4 but not Interleukin-13 Regulates Histamine Release and Proliferation Responses

Airway smooth muscle proliferation and survival is not modulated by mast cells

BackgroundAirway smooth muscle (ASM) hyperplasia and mast cell localization within the ASM bundle are important features of asthma. The cause of this increased ASM mass is uncertain and whether it is a consequence of ASM–mast cell interactions is unknown.ObjectiveWe sought to investigate ASM proliferation and survival in asthma and the effects of co-culture with mast cells.MethodsPrimary ASM cultures were derived from 11 subjects with asthma and 12 non-asthmatic controls. ASM cells were cultured for up to 10 days in the presence or absence of serum either alone or in co-culture with the human mast cell line-1, unstimulated human lung mast cells (HLMC) or IgE/anti-IgE-activated HLMC. Proliferation was assessed by cell counts, CFSE assay and thymidine incorporation. Apoptosis and necrosis were analysed by Annexin V/propidium iodide staining using flow cytometry and by assessment of nuclear morphology using immunofluorescence. Mast cell activation was confirmed by the measurement of histamine release.ResultsUsing a number of techniques, we found that ASM proliferation and survival was not significantly different between cells derived from subjects with or without asthma. Co-culture with mast cells did not affect the rate of proliferation or survival of ASM cells.ConclusionOur findings do not support a role for increased airway smooth proliferation and survival as the major mechanism driving ASM hyperplasia in asthma.Cite this as: D. Kaur, F. Hollins, R. Saunders, L. Woodman, A. Sutcliffe, G. Cruse, P. Bradding and C. Brightling, Clinical & Experimental Allergy, 2010 (40) 279– 288.

Expression and functionality of proteinase-activated receptors (PARs) on primary human lung mast cells

Introduction: Mast cell (MC) activation is known to promote anaphylactic reactions as well as the development of allergic and non-allergic inflammatory diseases. Apart from reacting to cross-linking of IgE-receptors by allergen, MC can be activated through a variety of alternative mechanisms. There is initial evidence that also proteinase-activated receptors (PARs) could have a role. PARs belong to the G protein-coupled receptor family and comprise four members (PAR-1 to 4). While the expression of functional PAR-2 on human skin MC as well as mRNA expression for PAR-1, PAR-3 and PAR-4 has been reported recently, so far nothing is known about the expression and the potential function(s) of PARs on primary human lung MC. We, therefore, isolated primary MC from human lung tissue and examined the expression and functionality of PARs. Methods: Human primary lung MC were isolated by a three-step procedure including enzymatic lung tissue digestion, density gradient centrifugation, and immunoaffinity purification by using anti-CD117 and anti-CD203c antibodies. Results: Using &gt;95% pure MC we could show mRNA expression for PAR-1, PAR-2, PAR-3 and PAR-4. PAR-specific antibodies revealed the expression of PAR-1 on the cell membrane and of PAR-2 being located intracellularly, whereas PAR-3 and PAR-4 were not detected by the antibodies used. PAR-activating peptides (PAR-APs) selective for PAR-1, PAR-2 and PAR-4 were able to induce elevation of intracellular free Ca2+-ion concentration ([Ca2+]i) in these cells. Moreover, PAR-1-AP and PAR-4-AP, but not PAR-2-AP stimulated MC degranulation as determined by the release of β-hexosaminidase. Stimulation with PAR-1-selective ligands led to downregulation of PAR-1. Looking for the effect of natural PAR-ligands, we tested proteinases from blood (thrombin, factor Xa, plasmin), from leukocytes (elastase, cathepsin G, proteinase 3), trypsin, and also MC-specific proteinases (chymase, tryptase). Out of these, only thrombin (PAR-1, and -4-ligand) and trypsin (PAR-2-ligand) induced Ca2+-signalling whereas none induced degranulation. Discussion: Our results demonstrate the functional expression of PAR-1, PAR-2 and PAR-4 in primary human lung MC. However, the induction of Ca2+-signalling in the absence of a degranulation response to thrombin and trypsin suggests, that natural ligands for these receptors may act differently from synthetic APs and moreover are likely to stimulate alternative functions in human primary lung MC.

Induction of microvascular leakage and histamine release by promutoxin, an Arg49 phospholipase A 2

It has been recognized that phospholipase A 2 (PLA 2) is a crucial factor of snake venom induced inflammation. Recently, promutoxin, a novel member of minor subgroup of snake venom PLA 2 (R49 PLA 2) has been characterized in our laboratory, but its roles in induction of inflammation remain uninvestigated. Using highly purified promutoxin, we found this enzymatically inactive PLA 2 provoked a dose-dependent increase in microvascular leakage in the skin of rats. Pretreatment of rats with compound 48/80 diminished promutoxin-induced skin reaction and reduced mast cell numbers in rats. Cyproheptadine, terfenadine, Ginkgolide B and heparin inhibited promutoxin elicited microvascular leakage when they were co-injected with the stimulus to rat skin. Moreover, promutoxin was found to induce histamine release from human colon, lung and tonsil mast cells, and both metabolic inhibitors and pertussis toxin were capable of inhibiting promutoxin elicited histamine release. Provocation of microvascular leakage and mast cell activation by promutoxin suggests further that snake venom induced inflammation is related to mast cell activation and certain anti-inflammatory drugs could be therapeutic effective in treating snake wound.

Alterations in Lung Mast Cell Populations in Patients with Chronic Obstructive Pulmonary Disease

Mast cells have important roles in innate immunity and tissue remodeling but have remained poorly studied in inflammatory airway diseases like chronic obstructive pulmonary disease (COPD). To perform a detailed histological characterization of human lung mast cell populations at different severities of COPD, comparing with smoking and never-smoking control subjects. Mast cells were analyzed in lung tissues from patients with mild to very severe COPD, GOLD I-IV (n = 25, 10 of whom were treated with corticosteroids). Never-smokers and smokers served as controls. The density, morphology, and molecular characteristics of mucosal and connective tissue mast cells (MC(T) and MC(TC), respectively) were analyzed in several lung regions. In all compartments of COPD lungs, especially at severe stages, the MC(TC) population increased in density, whereas the MC(T) population decreased. The net result was a reduction in total mast cell density. This phenomenon was paralleled by increased numbers of luminal mast cells, whereas the numbers of terminal transferase dUTP nick end labeling (TUNEL)(+) apoptotic mast cells remained unchanged. In COPD lungs, the MC(T) and MC(TC) populations showed alterations in morphology and expression of CD88 (C5a-R), transforming growth factor (TGF)-beta, and renin. Statistically significant correlations were found between several COPD-related mast cell alterations and lung function parameters. As COPD progresses to its severe stages, the mast cell populations in the lung undergo changes in density, distribution, and molecular expression. In COPD lungs, these novel histopathological features were found to be correlated to lung function and they may thus have clinical consequences.

Counterregulation of β2-adrenoceptor function in human mast cells by stem cell factor

Mast cells contribute to the pathophysiology of asthma with the sustained release of both preformed and newly generated mediators in response to allergens and other diverse stimuli. Stem cell factor (SCF) is the key human mast cell growth factor, but also primes mast cells for mediator release. SCF expression is markedly increased in asthmatic airways. Short-acting beta(2)-adrenoceptor drugs such as albuterol inhibit human lung mast cell (HLMC) degranulation in vitro in the absence of SCF, but their effect in the presence of SCF is not known. The aim of this study was to elucidate the effects of albuterol on HLMC function in the presence of SCF. Mediator release and K(Ca)3.1 ion channel activity were analyzed in purified HLMC. Intracellular signalling and beta(2)-adrenoceptor phosphorylation and internalization were analyzed in the HMC-1 human mast cell line. beta(2)-Adrenoceptor agonist-dependent inhibition of K(Ca)3.1 ion channels and HLMC mediator release was markedly attenuated in the presence of SCF. Remarkably, albuterol actually potentiated IgE-induced histamine release in a dose-dependent manner when both SCF and IgE were present. These effects were related to the SCF-dependent phosphorylation of Tyr350 on the beta(2)-adrenoceptor with immediate uncoupling of the receptor followed by receptor internalization. The potentially beneficial effects of beta(2)-adrenoceptor agonists in asthmatic airways may be blunted as a result of the high concentrations of SCF present.