Monomeric IgE stimulates NFAT translocation into the nucleus, a rise in cytosol Ca2+, degranulation, and membrane ruffling in the cultured rat basophilic leukemia-2H3 mast cell line

Targeted Single Particle Tracking with Upconverting Nanoparticles

Single particle tracking (SPT) is a powerful technique for real-time microscopic visualization of the movement of individual biomolecules within or on the surface of living cells. However, SPT often suffers from the suboptimal performance of the photon-emitting labels used to tag the biomolecules of interest. For example, fluorescent dyes have poor photostability, while quantum dots suffer from blinking that hampers track acquisition and interpretation. Upconverting nanoparticles (UCNPs) have recently emerged as a promising anti-Stokes luminescent label for SPT. In this work, we demonstrated targeted SPT using UCNPs. For this, we synthesized 30 nm diameter doped UCNPs and coated them with amphiphilic polymers decorated with polyethylene glycol chains to make them water-dispersible and minimize their nonspecific interactions with cells. Coated UCNPs highly homogeneous in brightness (as confirmed by a single particle investigation) were functionalized by immunoglobulin E (IgE) using a biotin-streptavidin strategy. Using these IgE-UCNP SPT labels, we tracked high-affinity IgE receptors (FcεRI) on the membrane of living RBL-2H3 mast cells at 37 °C in the presence and absence of antigen and obtained good agreement with the literature. Moreover, we used the FcεRI-IgE receptor-antibody system to directly compare the performance of UCNP-based SPT labels to organic dyes (AlexaFluor647) and quantum dots (QD655). Due to their photostability as well as their backgroundless and continuous luminescence, SPT trajectories obtained with UCNP labels are no longer limited by the photophysics of the label but only by the dynamics of the system and, in particular, the movement of the label out of the field of view and/or focal plane.

Atopy in chronic urticaria: an important yet overlooked issue

Chronic urticaria (CU) is one of the most common dermatological diseases and has a significant impact on the quality of life of patients. However, the pathogenesis of this disease remains unclear. Autoimmunity in chronic spontaneous urticaria (CSU) has received considerable attention and has been studied previously. Atopy is an important characteristic of CU; however, it has not been fully recognized. Atopy predisposes individuals to immune responses to allergens, leading to type 2 inflammation and immunoglobulin E (IgE) overproduction. Compared with healthy individuals, patients with CU have a higher proportion of atopy, and an atopic background is correlated with the clinical characteristics of CU. The total IgE levels in patients with CU is significantly higher than those in healthy individuals. Although its level is not higher than that in classic allergic diseases, it is closely related to CU. Exogenous allergens, auto-allergens, and specific IgEs, which are closely related to atopy, have been reported, and their roles in CU pathogenesis are also being studied. Local and systemic atopic inflammation is present in patients with CU. This review summarizes the current knowledge regarding atopy and CU, speculating that there are CU subtypes, such as atopic CSU or atopic chronic inducible urticaria (CIndU) and that atopy may be involved in the pathogenesis of CU. These findings provide a new perspective for a comprehensive understanding of the clinical features of CU and further research regarding its pathogenesis.

Biologics and airway remodeling in severe asthma

Asthma is a chronic inflammatory airway disease resulting in airflow obstruction, which in part can become irreversible to conventional therapies, defining the concept of airway remodeling. The introduction of biologics in severe asthma has led in some patients to the complete normalization of previously considered irreversible airflow obstruction. This highlights the need to distinguish a "fixed" airflow obstruction due to structural changes unresponsive to current therapies, from a "reversible" one as demonstrated by lung function normalization during biological therapies not previously obtained even with high-dose systemic glucocorticoids. The mechanisms by which exposure to environmental factors initiates the inflammatory responses that trigger airway remodeling are still incompletely understood. Alarmins represent epithelial-derived cytokines that initiate immunologic events leading to inflammatory airway remodeling. Biological therapies can improve airflow obstruction by addressing these airway inflammatory changes. In addition, biologics might prevent and possibly even revert "fixed" remodeling due to structural changes. Hence, it appears clinically important to separate the therapeutic effects (early and late) of biologics as a new paradigm to evaluate the effects of these drugs and future treatments on airway remodeling in severe asthma.

Roles of IgE and Histamine in Mast Cell Maturation

Mast cells are activated upon immunoglobulin E (IgE)-mediated antigen stimulation, and release a wide variety of mediators, including histamine to trigger inflammatory responses. The surface expression levels of Fcε receptor I (FcεRI), a high affinity receptor of IgE, were found to be positively regulated by IgE. IgE could protect murine cultured mast cells from apoptotic cell death induced by the deprivation of interleukin-3 and a certain kind of IgE could activate immature mast cells in the absence of antigens, leading to the release of pro-inflammatory cytokines and a transient increase in histamine synthesis. Histamine synthesis in mast cells was found to be required for the maturation of murine connective tissue-type mast cells, raising the possibility that IgE indirectly modulates local mast cell maturation. Although it remains controversial to what extent this concept of "monomeric IgE effects" could have relevance in the modulation of human mast cell functions, the therapeutic effects of anti-IgE antibodies might be accounted for in terms of the decreased serum IgE concentrations. Because drastic increases in serum IgE concentrations are often observed in patients with atopic dermatitis and chronic urticaria, a close investigation of the roles of IgE in mast cell maturation should contribute to development of novel therapeutic approaches for these inflammatory diseases.

IgE and IgG Antibodies as Regulators of Mast Cell and Basophil Functions in Food Allergy

Food allergy is a major health issue, affecting the lives of 8% of U.S. children and their families. There is an urgent need to identify the environmental and endogenous signals that induce and sustain allergic responses to ingested allergens. Acute reactions to foods are triggered by the activation of mast cells and basophils, both of which release inflammatory mediators that lead to a range of clinical manifestations, including gastrointestinal, cutaneous, and respiratory reactions as well as systemic anaphylaxis. Both of these innate effector cell types express the high affinity IgE receptor, FcϵRI, on their surface and are armed for adaptive antigen recognition by very-tightly bound IgE antibodies which, when cross-linked by polyvalent allergen, trigger degranulation. These cells also express inhibitory receptors, including the IgG Fc receptor, FcγRIIb, that suppress their IgE-mediated activation. Recent studies have shown that natural resolution of food allergies is associated with increasing food-specific IgG levels. Furthermore, oral immunotherapy, the sequential administration of incrementally increasing doses of food allergen, is accompanied by the strong induction of allergen-specific IgG antibodies in both human subjects and murine models. These can deliver inhibitory signals via FcγRIIb that block IgE-induced immediate food reactions. In addition to their role in mediating immediate hypersensitivity reactions, mast cells and basophils serve separate but critical functions as adjuvants for type 2 immunity in food allergy. Mast cells and basophils, activated by IgE, are key sources of IL-4 that tilts the immune balance away from tolerance and towards type 2 immunity by promoting the induction of Th2 cells along with the innate effectors of type 2 immunity, ILC2s, while suppressing the development of regulatory T cells and driving their subversion to a pathogenic pro-Th2 phenotype. This adjuvant effect of mast cells and basophils is suppressed when inhibitory signals are delivered by IgG antibodies signaling via FcγRIIb. This review summarizes current understanding of the immunoregulatory effects of mast cells and basophils and how these functions are modulated by IgE and IgG antibodies. Understanding these pathways could provide important insights into innovative strategies for preventing and/or reversing food allergy in patients.

Activation of Human Peripheral Basophils in Response to High IgE Antibody Concentrations without Antigens

Basophils and mast cells have high affinity IgE receptors (FcεRI) on their plasma membrane and play important roles in FcεRI-associated allergic diseases, such as pollen allergy, food allergy, chronic spontaneous urticarial (CSU), and atopic dermatitis (AD). To date, several reports have revealed that high IgE antibody concentrations activate mast cells—which reside in tissue—in the absence of any antigens (allergens). However, IgE antibody-induced activation of basophils—which circulate in blood—has not been reported. Here, we investigated whether IgE antibodies may regulate functions of human peripheral basophils without antigens in vitro. We successfully removed IgE antibodies bound to FcεRI on the surface of human peripheral basophils by treating with 0.1% lactic acid. We also demonstrated that high IgE antibody concentrations (>1 μM) induced histamine release, polarization, and CD203c upregulation of IgE antibody-stripped basophils. Thus, high IgE antibody concentrations directly activate basophils, which express IgE-free FcεRI on the cell surface. This mechanism may contribute to the pathogenesis of patients with AD and CSU who have higher serum IgE concentrations compared to healthy donors.

IgE Trimers Drive SPE-7 Cytokinergic Activity

Degranulation of mast cells and basophils, with release of agents of the allergic response, ensues when multivalent antigens bind to and cross-link the cells' receptor-bound IgE antibodies. A widely used commercial monoclonal IgE antibody, SPE-7 IgE from Sigma, was found to possess the radically anomalous property, termed "cytokinergic", of inducing basophil degranulation without the intervention of an antigen. We show here that the IgE monomer, freed of protein contaminants, is devoid of this activity, and that the source of the anomaly is a trace impurity, identified as a dissociation-resistant IgE trimer. Possible models for the formation of IgE trimers and the manner in which they cross-link cell surface receptors are suggested herein.

Fifty years later: Emerging functions of IgE antibodies in host defense, immune regulation, and allergic diseases

Fifty years ago, after a long search, IgE emerged as the circulating factor responsible for triggering allergic reactions. Its extremely low concentration in plasma created significant hurdles for scientists working to reveal its identity. We now know that IgE levels are invariably increased in patients affected by atopic conditions and that IgE provides the critical link between the antigen recognition role of the adaptive immune system and the effector functions of mast cells and basophils at mucosal and cutaneous sites of environmental exposure. This review discusses the established mechanisms of action of IgE in pathologic immediate hypersensitivity, as well as its multifaceted roles in protective immunity, control of mast cell homeostasis, and its more recently revealed immunomodulatory functions.

Mast cells in asthma--state of the art

Mast cells (MCs) play a central role in tissue homoeostasis, sensing the local environment through numerous innate cell surface receptors. This enables them to respond rapidly to perceived tissue insults with a view to initiating a co-ordinated programme of inflammation and repair. However, when the tissue insult is chronic, the ongoing release of multiple pro-inflammatory mediators, proteases, cytokines and chemokines leads to tissue damage and remodelling. In asthma, there is strong evidence of ongoing MC activation, and their mediators and cell-cell signals are capable of regulating many facets of asthma pathophysiology. This article reviews the evidence behind this.

Immune-inflammatory responses in atherosclerosis: Role of an adaptive immunity mainly driven by T and B cells

Adaptive immune response plays an important role in atherogenesis. In atherosclerosis, the proinflammatory immune response driven by Th1 is predominant but the anti-inflammatory response mediated mainly by regulatory T cells is also present. The role of Th2 and Th17 cells in atherogenesis is still debated. In the plaque, other T helper cells can be observed such as Th9 and Th22 but is little is known about their impact in atherosclerosis. Heterogeneity of CD4(+) T cell subsets presented in the plaque may suggest for plasticity of T cell that can switch the phenotype dependening on the local microenvironment and activating/blocking stimuli. Effector T cells are able to recognize self-antigens released by necrotic and apoptotic vascular cells and induce a humoral immune reaction. Tth cells resided in the germinal centers help B cells to switch the antibody class to the production of high-affinity antibodies. Humoral immunity is mediated by B cells that release antigen-specific antibodies. A variety of B cell subsets were found in human and murine atherosclerotic plaques. In mice, B1 cells could spontaneously produce atheroprotective natural IgM antibodies. Conventional B2 lymphocytes secrete either proatherogenic IgG, IgA, and IgE or atheroprotective IgG and IgM antibodies reactive with oxidation-specific epitopes on atherosclerosis-associated antigens. A small population of innate response activator (IRA) B cells, which is phenotypically intermediate between B1 and B2 cells, produces IgM but possesses proatherosclerotic properties. Finally, there is a minor subset of splenic regulatory B cells (Bregs) that protect against atherosclerotic inflammation through support of generation of Tregs and production of anti-inflammatory cytokines IL-10 and TGF-β and proapoptotic molecules.

Establishment of a Mouse Anti-ovalbumin IgE Monoclonal Antibody That Induces FcɛRII (CD23)-dependent Activation Without FcɛRI-Dependent Activation

IgE mainly activates cells via two receptors, FcɛRI and FcɛRII. Blocking antibodies against and animals genetically targeted for these receptors have been successfully used to distinguish between these two activating pathways. In the present study, we investigated whether our newly established anti-ovalbumin (OVA) monoclonal IgE OE-2 induced FcɛRII-dependent activation, but not FcɛRI-dependent activation in vivo and in vitro, in contrast to the previously established anti-OVA IgE OE-1, which stimulated FcɛRI and FcɛRII. The FcɛRI-mediated degranulation of RBL2H3 cells and passive systemic anaphylaxis in mice were induced by OE-1 but not OE-2. On the other hand, the production of nitric oxide by rat peritoneal macrophages and the primary antibody response in mice against co-injected OVA, which were mediated through FcɛRII, were induced and enhanced by OE-1 and OE-2. Differences in the epitopes recognized by OE-1 and OE-2 may partially explain why OE-1, but not OE-2, triggered FcɛRI-dependent activation. OE-1 bridged FcɛRI through effective aggregation with OVA, whereas OE-2 crosslinked the receptor strongly and only moderately upon the addition of an anti-kappa antibody and polymerized OVA, namely, an OVA-conjugated resin, respectively, resulting in degranulation. Our results offer a novel approach for determining the relative importance of FcɛRI and FcɛRII in various IgE-dependent responses by using OE-1 and OE-2.

Anti-allergic and anti-inflammatory effects of aqueous extract of Pogostemon cablin

Allergic disease is caused by exposure to normally innocuous substances that activate mast cells. Mast cell-mediated allergic inflammation is closely related to a number of allergic disorders, such as anaphylaxis, allergic rhinitis, asthma and atopic dermatitis. The discovery of drugs for treating allergic disease is an interesting subject and important to human health. The aim of the present study was to investigate the anti‑allergic and anti-inflammatory effects of the aqueous extract of Pogostemon cablin (Blanco) Benth (AEPC) (a member of the Labiatae family) using mast cells, and also to determine its possible mechanisms of action. An intraperitoneal injection of compound 48/80 or a serial injection of immunoglobulin E and antigen was used to induce anaphylaxis in mice. We found that AEPC inhibited compound 48/80‑induced systemic and immunoglobulin E-mediated cutaneous anaphylaxis in a dose-dependent manner. The release of histamine from mast cells was reduced by AEPC, and this suppressive effect was associated with the regulation of calcium influx. In addition, AEPC attenuated the phorbol 12-myristate 13-acetate plus calcium ionophore A23187 (PMACI)-stimulated expression of pro-inflammatory cytokines in mast cells. The inhibitory effects of AEPC on pro-inflammatory cytokines were dependent on the activation of nuclear factor (NF)-κB and p38 mitogen-activated protein kinase (MAPK). AEPC blocked the PMACI-induced translocation of NF-κB into the nucleus by hindering the degradation of IκBα and the phosphorylation of p38 MAPK. Our results thus indicate that AEPC inhibits mast cell‑mediated allergic inflammation by suppressing mast cell degranulation and the expression of pro-inflammatory cytokines caused by reduced intracellular calcium levels and the activation of NF-κB and p38 MAPK.

Mast cells in airway diseases and interstitial lung disease

Mast cells are major effector cells of inflammation and there is strong evidence that mast cells play a significant role in asthma pathophysiology. There is also a growing body of evidence that mast cells contribute to other inflammatory and fibrotic lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. This review discusses the role that mast cells play in airway diseases and highlights how mast cell microlocalisation within specific lung compartments and their cellular interactions are likely to be critical for their effector function in disease.

Mechanism of the antigen-independent cytokinergic SPE-7 IgE activation of human mast cells in vitro

Release of pro-inflammatory mediators by mast cells is a key feature of allergic disease. The ‘dogma’ is that IgE molecules merely sensitise mast cells by binding FcεRI prior to cross-linking by multivalent allergen, receptor aggregation and mast cell activation. However, certain monoclonal IgE antibodies have been shown to elicit mast cell activation in an antigen-independent cytokinergic manner, and DNP-specific murine SPE-7 IgE is the most highly cytokinergic antibody known. We show that both monovalent hapten and recombinant SPE-7 IgE Fab inhibit its cytokinergic activity as measured by mast cell degranulation and TNF-α release. Using SPE-7 IgE, a non-cytokinergic human IgE and a poorly cytokinergic murine IgE, we reveal that interaction of the Fab region of ‘free’ SPE-7 IgE with the Fab of FcεRI-bound SPE-7 IgE is the basis of its cytokinergic activity. We rule out involvement of IgE Fc, Cε1 and Cλ/κ domains, and propose that ‘free’ SPE-7 IgE binds to FcεRI-bound SPE-7 IgE by an Fv-Fv interaction. Initial formation of a tri-molecular complex (one ‘free’ IgE molecule cross-linking two receptor-bound IgE molecules) leads to capture of further ‘free’ and receptor-bound IgEs to form larger clusters that trigger mast cell activation.

Mast cell mediators: their differential release and the secretory pathways involved

Mast cells (MC) are widely distributed throughout the body and are common at mucosal surfaces, a major host-environment interface. MC are functionally and phenotypically heterogeneous depending on the microenvironment in which they mature. Although MC have been classically viewed as effector cells of IgE-mediated allergic diseases, they are also recognized as important in host defense, innate and acquired immunity, homeostatic responses, and immunoregulation. MC activation can induce release of pre-formed mediators such as histamine from their granules, as well as release of de novo synthesized lipid mediators, cytokines, and chemokines that play diverse roles, not only in allergic reactions but also in numerous physiological and pathophysiological responses. Indeed, MC release their mediators in a discriminating and chronological manner, depending upon the stimuli involved and their signaling cascades (e.g., IgE-mediated or Toll-like receptor-mediated). However, the precise mechanisms underlying differential mediator release in response to these stimuli are poorly known. This review summarizes our knowledge of MC mediators and will focus on what is known about the discriminatory release of these mediators dependent upon diverse stimuli, MC phenotypes, and species of origin, as well as on the intracellular synthesis, storage, and secretory processes involved.

Optimisation and use of humanised RBL NF-AT-GFP and NF-AT-DsRed reporter cell lines suitable for high-throughput scale detection of allergic sensitisation in array format and identification of the ECM-integrin interaction as critical factor

We have previously described a microarray platform combining live basophils with protein arrays suitable for high-throughput detection of sensitisation against allergens. During optimisation of this technique, we observed severe losses of adhering cells during the washing steps, particularly after activation. In order to preserve cell binding, we tested the cell adhesion characteristics of different extracellular matrix proteins: human collagen I, fibronectin (FN) from bovine plasma and laminin (LN). FN was more effective than LN and collagen. Cell detachment after activation was in part due to reduced surface expression of VLA-4, the main ligand for FN, which was significantly decreased within 15 min of stimulation with 1 μg/mL calcium ionophore A23187, reaching a minimum after 2 h then slowly recovering. These optimised conditions were used for testing of well-characterised sera from allergic patients using two newly developed rat basophil leukaemia stable reporter cell lines (RBL NF-AT/GFP and RBL NF-AT/DsRed), which both express the human high-affinity IgE receptor alpha chain (FcεRIα). Both cell lines were able to detect sensitisation to specific allergens showing the expected bell-shaped dose–response curve, and correlated (R² = 0.75) with the standard beta-hexosaminidase assay, which is not suitable for an array format.

B cells and humoral immunity in atherosclerosis

Insights into the important contribution of inflammation and immune functions in the development and progression of atherosclerosis have greatly improved our understanding of this disease. Although the role of T cells has been extensively studied for decades, only recently has the role of B cells gained more attention. Recent studies have identified differential effects of different B-cell subsets and helped to clarify the still poorly understood mechanisms by which these act. B1 cells have been shown to prevent lesion formation, whereas B2 cells have been suggested to promote it. Natural IgM antibodies, mainly derived from B1 cells, have been shown to mediate atheroprotective effects, but the functional role of other immunoglobulin classes, particularly IgG, still remains elusive. In this review, we will focus on recent insights on the role of B cells and various immunoglobulin classes and how these may mediate their effects in atherosclerotic lesion formation. Moreover, we will highlight potential therapeutic approaches focusing on B-cell depletion that could be used to translate experimental evidence to human disease.

Inhibitory effect of putranjivain A on allergic inflammation through suppression of mast cell activation

A great number of people are suffering from allergic inflammatory disease such as asthma, atopic dermatitis, and sinusitis. Therefore discovery of drugs for the treatment of these diseases is an important subject in human health. Putranjivain A (PJA), member of ellagitannin, is known to possess beneficial effects including anti-cancer and anti-viral activities. The aim of the present study was to elucidate whether PJA modulates the allergic inflammatory reaction and to study its possible mechanisms of action using mast cell-based in vitro and in vivo models. The study was performed in anaphylaxis mouse model and cultured mast cells. PJA inhibited the expression of pro-inflammatory cytokines in immunoglobulin E-stimulated mast cells. PJA reduced this expression by inhibiting nuclear factor (NF)-κB and nuclear factor of activated T cell. The oral administration of PJA reduced systemic and cutaneous anaphylaxis, the release of serum histamine, and the expression of the histamine H1 receptor. In addition, PJA attenuated the activation of mast cells. PJA inhibited the release of histamine from various types of mast cells by the suppression of intracellular calcium. The inhibitory activity of PJA on the allergic reaction was similar to that of disodium cromoglycate, a known anti-allergic drug. These results suggest that PJA can facilitate the prevention or treatment of allergic inflammatory diseases mediated by mast cells.

Histamine-releasing factor and immunoglobulins in asthma and allergy

Factors that can induce the release of histamine from basophils have been studied for more than 30 years. A protein termed histamine-releasing factor (HRF) was purified and molecularly cloned in 1995. HRF can stimulate histamine release and IL-4 and IL-13 production from IgE-sensitized basophils and mast cells. HRF-like activities were found in bodily fluids during the late phase of allergic reactions, implicating HRF in allergic diseases. However, definitive evidence for the role of HRF in allergic diseases has remained elusive. On the other hand, we found effects of monomeric IgE on the survival and activation of mast cells without the involvement of a specific antigen, as well as heterogeneity of IgEs in their ability to cause such effects. The latter property of IgE molecules seemed to be similar to the heterogeneity of IgEs in their ability to prime basophils in response to HRF. This similarity led to our recent finding that ~30% of IgE molecules can bind to HRF via their Fab interactions with two binding sites within the HRF molecule. The use of peptide inhibitors that block HRF-IgE interactions revealed an essential role of HRF to promote skin hypersensitivity and airway inflammation. This review summarizes this and more recent findings and provides a perspective on how they impact our understanding of allergy pathogenesis and potentially change the treatment of allergic diseases.

Rapid desensitization induces internalization of antigen-specific IgE on mouse mast cells

BACKGROUND

Rapid desensitization transiently prevents severe allergic reactions, allowing administration of life-saving therapies in previously sensitized patients. However, the mechanisms underlying successful rapid desensitization are not fully understood.

OBJECTIVES

We sought to investigate whether the mast cell (MC) is an important target of rapid desensitization in mice sensitized to exhibit IgE-dependent passive systemic anaphylaxis in vivo and to investigate the antigen specificity and underlying mechanisms of rapid desensitization in our mouse model.

METHODS

C57BL/6 mice (in vivo) or primary isolated C57BL/6 mouse peritoneal mast cells (PMCs; in vitro) were passively sensitized with antigen-specific anti-2,4-dinitrophenyl IgE, anti-ovalbumin IgE, or both. MCs were exposed over a short period of time to increasing amounts of antigen (2,4-dinitrophenyl-human serum albumin or ovalbumin) in the presence of extracellular calcium in vitro or by means of intravenous administration to sensitized mice in vivo before challenging the mice with or exposing the PMCs to optimal amounts of specific or irrelevant antigen.

RESULTS

Rapidly exposing mice or PMCs to progressively increasing amounts of specific antigen inhibited the development of antigen-induced hypothermia in sensitized mice in vivo and inhibited antigen-induced PMC degranulation and prostaglandin D2 synthesis in vitro. Such MC hyporesponsiveness was induced antigen-specifically and was associated with a significant reduction in antigen-specific IgE levels on MC surfaces.

CONCLUSIONS

Rapidly exposing MCs to progressively increasing amounts of antigen can both enhance the internalization of antigen-specific IgE on the MC surface and also desensitize these cells in an antigen-specific manner in vivo and in vitro.

Cytokinergic IgE Action in Mast Cell Activation

Some 10 years ago it emerged that at sufficiently high concentrations certain monoclonal mouse IgEs exert previously unsuspected effects on mast cells. Thus they can both promote survival and induce activation of mast cells without the requirement for antigens. This was a wake up call that appears to have been missed (or dismissed) by the majority of immunologists. The structural attributes responsible for the potency of the so-called “highly cytokinergic” or HC IgEs have not yet been determined, but the events that ensue when such IgEs bind to the high-affinity receptor, FcεRI, on mast cells have been thoroughly studied, and are strikingly similar to those engendered by antigens when they form cross-linked complexes with the receptors. We review the evidence for the cytokinergic activity of IgE, and the structural features and known properties of immunoglobulins, and of IgE in particular, most likely to be implicated in the phenomenon. We suggest that IgEs with cytokinergic activity may be generated by local germinal center reactions in the target organs of allergy. We consider also the important implications that the existence of cytokinergic IgE may have for a fuller understanding of adaptive immunity and of the action of IgE in asthma and other diseases.

Most Highly Cytokinergic IgEs Have Polyreactivity to Autoantigens

Purpose

Monomeric IgE molecules, when bound to the high-affinity receptor, exhibit a vast heterogeneity in their ability to induce survival promotion and cytokine production in mast cells. At one end of this spectrum, highly cytokinergic (HC) IgEs can induce potent survival promotion, degranulation, cytokine production, migration, etc., whereas at the other end, poorly cytokinergic (PC) IgEs can do so inefficiently. In this study, we investigated whether IgEs recognize autoantigens and whether IgEs' binding of autoantigens correlates with difference s in HC versus PC properties.

Methods

Enzyme-linked immunosorbent assays were performed to test whether IgEs bind antigens. Histamine-releasing factor in human sera was quantified by western blotting. Cultured mast cells derived from human cord blood were used to test the effects of human sera on cytokine production.

Results

Most (7/8) of mouse monoclonal HC IgEs exhibited polyreactivity to double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), β-galactosidase, thyroglobulin and/or histamine-releasing factor. By contrast, mouse PC IgEs failed to react with these antigens. A human monoclonal HC IgE also showed polyreactivity to histamine-releasing factor, dsDNA and ssDNA. Interestingly, sera from atopic dermatitis patients showed increased reactivity to ssDNA and β-galactosidase and increased levels of histamine-releasing factor. Some atopic dermatitis patients, but not healthy individuals, had substantial serum levels of HRF-reactive IgE. Sera from atopic dermatitis patients with high titers of DNA-reactive IgE could induce several fold more IL-8 secretion in human mast cells than sera from healthy individuals.

Conclusions

The results show that most HC, but not PC, IgEs exhibit polyreactivity to autoantigens, supporting the autoimmune mechanism in the pathogenesis of atopic dermatitis.

Different agonists recruit different stromal interaction molecule proteins to support cytoplasmic Ca2+ oscillations and gene expression

Stimulation of cells with physiological concentrations of calcium-mobilizing agonists often results in the generation of repetitive cytoplasmic Ca(2+) oscillations. Although oscillations arise from regenerative Ca(2+) release, they are sustained by store-operated Ca(2+) entry through Ca(2+) release-activated Ca(2+) (CRAC) channels. Here, we show that following stimulation of cysteinyl leukotriene type I receptors in rat basophilic leukemia (RBL)-1 cells, large amplitude Ca(2+) oscillations, CRAC channel activity, and downstream Ca(2+)-dependent nuclear factor of activated T cells (NFAT)-driven gene expression are all exclusively maintained by the endoplasmic reticulum Ca(2+) sensor stromal interaction molecule (STIM) 1. However, stimulation of tyrosine kinase-coupled FCεRI receptors evoked Ca(2+) oscillations and NFAT-dependent gene expression through recruitment of both STIM2 and STIM1. We conclude that different agonists activate different STIM proteins to sustain Ca(2+) signals and downstream responses.

Monomeric IgE and mast cell development, survival and function

Mast cells play a major role in allergy and anaphylaxis, as well as a protective role in immunity against bacteria and venoms (innate immunity) and T-cell activation (acquired immunity).1,2 It was long thought that two steps are essential to mast cell activation. The first step (sensitization) occurs when antigen-specific IgE binds to its high-affinity IgE receptor (FcεRI) expressed on the surface of mast cells. The second step occurs when antigen (Ag) or anti-IgE binds antigen-specific IgE antibodies bound to FcεRI present on the mast cell surface (this mode of stimulation hereafter referred to as IgE+Ag or IgE+anti-IgE stimulation, respectively).Conventional wisdom has been that monomeric IgE plays only an initial, passive role in mast cell activation. However, recent findings have shown that IgE binding to its receptor FcεRI can mediate mast cell activation events even in the absence of antigen (this mode of stimulation hereafter referred to as IgE(-Ag) stimulation). Different subtypes of monomeric IgEs act via IgE(-Ag) stimulation to elicit varied effects on mast cells function, survival and differentiation. This chapter will describe the role of monomeric IgE molecules in allergic reaction, the various effects and mechanisms of action of IgE(-Ag) stimulation on mast cells and what possible developments may arise from this knowledge in the future. Since mast cells are involved in a variety of pathologic and protective responses, understanding the role that monomeric IgE plays in mast cell function, survival and differentiation will hopefully lead to better understanding and treatment of asthma and other allergic diseases, as well as improved understanding of host response to infections.

Mast cells in lung inflammation

Mast cells play an important role in the lung in both health and disease. Their primary role is to initiate an appropriate program of inflammation and repair in response to tissue damage initiated by a variety of diverse stimuli. They are important for host immunity against bacterial infection and potentially in the host immune response to non small cell lung cancer. In situations of ongoing tissue damage, the sustained release of numerous pro-inflammatory mediators, proteases and cytokines, contributes to the pathophysiology of lung diseases such as asthma and interstitial lung disease. A key goal is the development of treatments which attenuate adverse mast cell function when administered chronically to humans in vivo. Such therapies may offer a novel approach to the treatment of many life-threatening diseases.

Immune surveillance by mast cells during dengue infection promotes natural killer (NK) and NKT-cell recruitment and viral clearance

A wealth of evidence supports the essential contributions of mast cells (MCs) to immune defense against bacteria and parasites; however, the role of MCs in viral infections has not been defined. We now report that rodent, monkey, and human MCs are able to detect dengue virus (DENV), a lymphotropic, enveloped, single-stranded, positive-sense RNA virus that results in MC activation and degranulation. We observe that the response of MCs to DENV also involves the activation of antiviral intracellular host response pathways, melanoma differentiation-associated gene 5 (MDA5) and retinoic acid inducible gene 1 (RIG-I), and the de novo transcription of cytokines, including TNF-α and IFN-α, and chemokines, such as CCL5, CXCL12, and CX3CL1. This multifaceted response of MCs to DENV is consequential to the containment of DENV in vivo because, after s.c. infection, MC-deficient mice show increased viral burden within draining lymph nodes, which are known to be targeted organs during DENV spread, compared with MC-sufficient mice. This containment of DENV is linked to the MC-driven recruitment of natural killer and natural killer T cells into the infected skin. These findings support expanding the defined role of immunosurveillance by MCs to include viral pathogens.

Fucoidan suppresses IgE production in peripheral blood mononuclear cells from patients with atopic dermatitis

We previously reported that fucoidan, a dietary fiber purified from seaweed, inhibited IgE production in B cells from mice spleen in vitro and ovalbumin-sensitized mice in vivo. In this study, we examined the effect of fucoidan on IgE production in human peripheral blood mononuclear cells (PBMC) in vitro. PBMC, obtained from healthy donors or patients with atopic dermatitis (AD) with high levels of serum IgE, were cultured with IL-4 and anti-CD40 antibody in the presence or absence of fucoidan. Fucoidan significantly reduced IgE production in PBMC without affecting cell proliferation and IFN-γ production. Fucoidan also inhibited immunoglobulin germline transcripts of B cells in PBMC, and decreased the number of IgE-secreting cells. The inhibitory effects of fucoidan were similarly observed for both PBMC from patients with AD and those with healthy donors. Our findings indicate that fucoidan suppresses IgE induction by inhibiting immunoglobulin class-switching to IgE in human B cells, even after the onset of AD.

Anti-allergic inflammatory activity of the fruit of Prunus persica: role of calcium and NF-kappaB

Mast cell-mediated allergic symptoms are involved in many diseases, such as asthma and sinusitis. In this study, we investigated the effect of ethanol extract of fruits of Prunus persica (L) Batsch (FPP) on the mast cell-mediated allergic inflammation and studied the possible mechanism of action. FPP dose-dependently inhibited compound 48/80-induced systemic anaphylaxis and immunoglobulin E-mediated local allergic reactions. Histamine releasing from mast cells was reduced by FPP, which was mediated by modulation of intracellular calcium. In addition, FPP attenuated the phorbol 12-myristate 13-acetate and calcium ionophore A23187 (PMACI)-stimulated expression and secretion of pro-inflammatory cytokines in human mast cells. The inhibitory effect of FPP on pro-inflammatory cytokines was nuclear factor (NF)-kappaB dependent. Our findings provide evidence that FPP inhibits mast cell-derived allergic inflammation and involvement of calcium and NF-kappaB in these effects.

Reversible bleb formation in mast cells stimulated with antigen is Ca2+/calmodulin-dependent and bleb size is regulated by ARF6

Mast cells stimulated with antigen undergo extensive changes in their cytoskeleton. In the present study, we assess the impact of actin-modifying drugs and report that, in the presence of cytochalasin D, mast cells stop membrane ruffling, but instead bleb. Bleb formation is reversible following washout of cytochalasin D and occurs in an actin-polymerization-dependent manner. Bleb formation is inhibited by expression of constitutively active ezrin-T567D. Blebbing is also inhibited by blebbistatin, a myosin II inhibitor, implying myosin II activation in the process. We used a selection of inhibitors and observed that myosin II activation is dependent mainly on Ca2+–calmodulin, with only a small contribution from Rho kinase. The signalling pathways stimulated by antigen include PLC (phospholipase C) and PLD (phospholipase D). Bleb formation was dependent on activation of PLC, but not PLD. Primary alcohols, used previously as a means to reduce PLD-derived phosphatidic acid, were potent inhibitors of membrane blebbing, but a more selective inhibitor of PLD, FIPI (5-fluoro-2-indolyl des-chlorohalopemide), was without effect. FIPI also did not inhibit membrane ruffling or degranulation of mast cells, indicating that inhibition by primary alcohols works through an unidentified mechanism rather than via diversion of PLD activity as assumed. We also examined the requirement for ARF6 (ADP-ribosylation factor 6) and observed that its expression led to an increase in bleb size and a further increase was observed with the dominant-active mutant, ARF6-Q67L. Since ARF6-T27N had no effect on bleb size, we conclude that ARF6 needs to be active to regulate the size of the blebs.

Polyclonal IgE induces mast cell survival and cytokine production

BACKGROUND

Ag-dependent activation of IgE-bearing mast cells is a critical first step in immediate hypersensitivity and other allergic responses. Recent studies have revealed Ag-independent effects of monoclonal mouse IgE molecules on mast cell survival and activation. However, no studies have been performed on the effects of polyclonal IgE molecules. Here, we tested whether polyclonal mouse and human IgE molecules affect survival and cytokine production in mast cells.

METHODS

Mast cells were cultured in the presence of polyclonal mouse and human IgE molecules, and cell survival and cytokine production were analyzed.

RESULTS

Polyclonal mouse IgE molecules in sera from mice with atopic dermatitis-like allergic skin inflammation, enhanced survival and cytokine production in mast cell cultures. Similar to the effects of monoclonal IgE, the polyclonal IgE effects were mediated by the high-affinity IgE receptor, FcepsilonRI. Human polyclonal IgE molecules present in sera from atopic dermatitis patients were also capable of activating mast cells, and inducing IL-8 production in human cord blood-derived mast cells.

CONCLUSIONS

These results imply that polyclonal IgE in atopic dermatitis and other atopic conditions might modulate mast cell number and function, thus amplifying the allergic response.

Peritoneal injection of fucoidan suppresses the increase of plasma IgE induced by OVA-sensitization

We previously reported that fucoidan, a dietary fiber purified from seaweed, inhibited IgE production by B cells in vitro. In this study, we examined the effect of fucoidan on IgE production in vivo. The OVA-induced increase of plasma IgE was significantly suppressed when fucoidan was intraperitoneally, but not orally, administered prior to the first immunization with OVA. The production of IL-4 and IFN-gamma in response to OVA in spleen cells isolated from OVA-sensitized mice treated with fucoidan in vivo was lower than that from mice treated without fucoidan. Moreover, the flow cytometric analysis and ELISpot assay revealed that the administration of fucoidan suppressed a number of IgE-expressing and IgE-secreting B cells, respectively. These results indicate that fucoidan inhibits the increase of plasma IgE through the suppression of IgE-producing B cell population, and the effect of fucoidan in vivo is crucially dependent on the route and timing of its administration.

IgE-induced mast cell survival requires the prolonged generation of reactive oxygen species

We show in this study that the ability of five different monomeric IgEs to enhance murine bone marrow-derived mast cell (BMMC) survival correlates with their ability to stimulate extracellular calcium (Ca(2+)) entry. However, whereas IgE+Ag more potently stimulates Ca(2+) entry, it does not enhance survival under our conditions. Exploring this further, we found that whereas all five monomeric IgEs stimulate a less robust Ca(2+) entry than IgE+Ag initially, they all trigger a more prolonged Ca(2+) influx, generation of reactive oxygen species (ROS), and ERK phosphorylation. These prolonged signaling events correlate with their survival-enhancing ability and positively feedback on each other to generate the prosurvival cytokine, IL-3. Interestingly, the prolonged ERK phosphorylation induced by IgE appears to be regulated by a MAPK phosphatase rather than MEK. IgE-induced ROS generation, unlike that triggered by IgE+Ag, is not mediated by 5-lipoxygenase. Moreover, ROS inhibitors, which block both IgE-induced ROS production and Ca(2+) influx, convert the prolonged ERK phosphorylation induced by IgE into the abbreviated phosphorylation pattern observed with IgE+Ag and prevent IL-3 generation. In support of the essential role that IgE-induced ROS plays in IgE-enhanced BMMC survival, we found the addition of H(2)O(2) to IgE+Ag-stimulated BMMCs leads to IL-3 secretion.

Contribution of natural inhibitors to the understanding of the PI3K/PDK1/PKB pathway in the insulin-mediated intracellular signaling cascade

The critical initial steps in insulin action include phosphorylation of adapter proteins and activation of phosphatidylinositol 3-kinase (PI3K). One of important components in this process is a protein called Akt/protein kinase B (PKB). The work of numerous different researchers indicates a role of PKB in regulating insulin-stimulated glucose uptake. The crucial role of lipid second messengers in PKB activation has been dissected through the use of the PI3K-specific inhibitors wortmannin and LY294002. Receptor-activated PI3K synthesizes the lipid second messenger PtdIns[3,4,5]-trisphosphate, leading to the recruitment of PKB to the membrane. Membrane attachment of PKB is mediated by its pleckstrin homology domain binding to PtdIns[3,4,5]-trisphosphate or PtdIns[3,4]-bisphosphate with high affinity. Activation of PKB alpha is then achieved at the plasma membrane by phosphorylation of Thr308 in the activation-loop of the kinase domain and Ser473 in the carboxy-terminal regulatory region, respectively. 3-Phosphoinositide-dependent protein kinase-1 (PDK1) is responsible for T308 phosphorylation. The usage of specific inhibitors and natural compound has significantly contributed to investigate the molecular mechanism of PI3K/PDK1/PKB signaling pathway, leading to the putative therapeutics benefits of patients. This review focuses on the contribution of natural inhibitor or compound in our understanding of the mechanism by which insulin induces, especially in PI3K/PDK1/PKB signaling.

Induction of nitric oxide synthases in primary human cultured mast cells by IgE and proinflammatory cytokines

The mast cells have been suggested to be a cellular source of nitric oxide (NO) which level is increased in the pathogenesis of asthma. However, isoforms of the NO generating enzyme, nitric oxide synthase (NOS), in primary human mast cells have not been defined due to the lack of a suitable model. We hence examined directly the expression of NOS mRNA and proteins in primary human cultured mast cells (HCMC). Mature HCMC were cultured from CD34+ progenitors isolated from buffy coat preparations and were subjected to IgE sensitisation, IgE receptor mediated activation and cytokines induced stimulation. While expression of NOS mRNA was detected by conventional reverse transcription-polymerase chain reaction (RT-PCR) and quantitatively analyzed with real-time RT-PCR, expression of NOS proteins was detected by immunostaining. In non-stimulated HCMC incubated in medium alone, expressions of NOS were not detected. While overnight incubation of HCMC with IgE significantly increased the expression of NOS2 and NOS3, only NOS2 expression was up-regulated after overnight incubation with a mixture of TNF-alpha, IFN-gamma and IL-1beta. Cross-linking of IgE with anti-IgE further increased NOS2 expression with a concomitant decrease in NOS3 expression. NOS1 was not detected in all treatments. In conclusion, we have shown for the first time that NOS2 and NOS3 expressions are induced in primary human mast cells following appropriate stimulations. Comparisons between the differential expressions of NOS isoforms in HCMC to the changes in NOS expressions in asthma models suggest that the mast cell is a source of NO in asthmatic airways.

IgE alone promotes human lung mast cell survival through the autocrine production of IL-6

Background

Mast cells play a key role in asthma and recent evidence indicates that their ongoing activation in this disease is mediated, in part, via IgE in the absence of antigen. In this study we have examined whether IgE alone enhances human lung mast cell (HLMC) survival.

Methods

Purified HLMC were cultured for 4 weeks and survival assays then performed over 10 days following cytokine withdrawal in the presence or absence of human myeloma IgE. Quantitative real time RT-PCR was carried out to examine IL-6 mRNA expression and IL-6 protein was measured in HLMC supernatants by ELISA.

Results

IgE alone promoted the survival of HLMC in a dose-dependent manner following cytokine withdrawal. IgE-induced survival was eliminated with the addition of neutralising anti-IL-6 antibody but not by the addition of neutralising anti-stem cell factor. IgE sensitisation initiated profound upregulation of IL-6 mRNA in HLMC, and IL-6 concentrations were also raised in the culture supernatants of IgE-exposed cells.

Conclusion

These data taken together suggest that IgE in the absence of antigen promotes HLMC survival through the autocrine production of IL-6. This provides a further mechanism through which IL-6 and IgE contribute to the pathogenesis of asthma, and through which anti-IgE therapy might achieve its therapeutic effect.

Cutaneous mast cell receptors

Two types of mast cells, MC(T) and MC(TC), exist in humans. MC(T) and MC(TC) are different in their granular neutral proteases, tissue localizations, and functions. This article describes the differences between the cutaneous mast cell receptors.

Insights into immunoglobulin E receptor signaling from structurally defined ligands

The asymmetrical structure of bent immunoglobulin E (IgE) bound to its high-affinity receptor, Fc epsilon RI, suggests a possible role for this configuration in the regulation of signaling mediated by cross-linking of Fc epsilon RI on the surface of mast cells and basophils. Indeed, the presence of bound IgE strongly influences the capacity of cross-linked Fc epsilon RI dimers to trigger mast cell degranulation, implicating orientational constraints by bound IgE. Bivalent ligands that cross-link by binding to bivalent IgE can form linear and cyclic chains of IgE/Fc epsilon RI complexes, and these exhibit only limited capacity to stimulate downstream signaling and degranulation, whereas structurally analogous trivalent ligands, which can form branched networks of cross-linked IgE/Fc epsilon RI complexes, are more effective at cell activation. Long bivalent ligands with flexible spacers can form intramolecular cross-links with IgE, and these stable 1:1 complexes are very potent inhibitors of mast cell degranulation stimulated by multivalent antigen. In contrast, trivalent ligands with rigid double-stranded DNA spacers effectively stimulate degranulation responses in a length-dependent manner, providing direct evidence for receptor transphosphorylation as a key step in the mechanism of signaling by Fc epsilon RI. Thus, studies with chemically defined oligovalent ligands show important features of IgE receptor cross-linking that regulate signaling, leading to mast cell activation.

Demonstration of an aberrant mast-cell population with clonal markers in a subset of patients with "idiopathic" anaphylaxis

Idiopathic anaphylaxis remains a perplexing disorder in which existing prophylactic therapy is inadequate. In this prospective study, we sought to determine whether patients with idiopathic anaphylaxis might have evidence for a clonal disorder of mast cells related to mastocytosis and for which novel targeted therapies might be considered. We report 12 patients with "idiopathic" anaphylaxis who did not exhibit either urticaria pigmentosa or the characteristic bone marrow biopsy finding of multifocal mast-cell aggregates observed in systemic mastocytosis. Of these 12 patients, 5 had evidence of 1 or more minor criteria for mastocytosis. C-KIT mutational analysis was positive for the 816D>V activating mutation in 3 of 3 patients in CD25(+) bone marrow cells where the analysis was performed. These results demonstrate the presence of an aberrant mast-cell population carrying clonal markers in a subset of patients diagnosed with "idiopathic" anaphylaxis, who may respond to inhibitors targeting mutated C-KIT. This intramural clinical trial was conducted in 2003 and 2004 and was registered at (http://clinicalcenter.nih.gov) with a study number 03-I-0010. Since the study is now closed, it is no longer available online.

Regulation of the mast cell response to the type 1 Fc epsilon receptor

The type I Fc epsilon receptor (Fc epsilon RI) is one of the better understood members of its class and is central to the immunological activation of mast cells and basophils, the key players in immunoglobulin E (IgE)-dependent immediate hypersensitivity. This review provides background information on several distinct regulatory mechanisms controlling this receptor's stimulus-response coupling network. First, we review the current understanding of this network's operation, and then we focus on the inhibitory regulatory mechanisms. In particular, we discuss the different known cytosolic molecules (e.g. kinases, phosphatases, and adapters) as well as cell membrane proteins involved in negatively regulating the Fc epsilon RI-induced secretory responses. Knowledge of this field is developing at a fast rate, as new proteins endowed with regulatory functions are still being discovered. Our understanding of the complex networks by which these proteins exert regulation is limited. Although the scope of this review does not include addressing several important biochemical and biophysical aspects of the regulatory mechanisms, it does provide general insights into a central field in immunology.

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

Anti-IgE antibodies for the treatment of IgE-mediated allergic diseases

The pharmacological purposes of the anti-IgE therapy are to neutralize IgE and to inhibit its production to attenuate type I hypersensitivity reactions. The therapy is based on humanized IgG1 antibodies that bind to free IgE and to membrane-bound IgE on B cells, but not to IgE bound by the high-affinity IgE.Fc receptors on basophils and mast cells or by the low-affinity IgE.Fc receptors on B cells. After nearly 20 years since inception, therapeutic anti-IgE antibodies (anti-IgE) have been studied in about 30 Phase II and III clinical trials in many allergy indications, and a lead antibody, omalizumab, has been approved for treating patients (12 years and older) with moderate-to-severe allergic asthma. Anti-IgE has confirmed the roles of IgE in the pathogenesis of asthma and helped define the concept "allergic asthma" in clinical practice. It has been shown to be safe and efficacious in treating pediatric allergic asthma and treating allergic rhinitis and is being investigated for treating peanut allergy, atopic dermatitis, latex allergy, and others. It has potential for use to combine with specific and rush immunotherapy for increased safety and efficacy. Anti-IgE thus appears to provide a prophylactic and therapeutic option for moderate to severe cases of many allergic diseases and conditions in which IgE plays a significant role. This chapter reviews the evolution of the anti-IgE concept and the clinical studies of anti-IgE on various disease indications, and presents a comprehensive analysis on the multiple intricate immunoregulatory pharmacological effects of anti-IgE. Finally, it reviews other approaches that target IgE or IgE-expressing B cells.

LAT and NTAL mediate immunoglobulin E-induced sustained extracellular signal-regulated kinase activation critical for mast cell survival

Immunoglobulin E (IgE) induces mast cell survival in the absence of antigen (Ag) through the high-affinity IgE receptor, Fcepsilon receptor I (FcepsilonRI). Although we have shown that protein tyrosine kinase Syk and sustained extracellular signal-regulated kinase (Erk) activation are required for IgE-induced mast cell survival, how Syk couples with sustained Erk activation is still unclear. Here, we report that the transmembrane adaptors LAT and NTAL are phosphorylated slowly upon IgE stimulation and that sustained but not transient Erk activation induced by IgE was inhibited in LAT(-/-) NTAL(-/-) bone marrow-derived mast cells (BMMCs). IgE-induced survival requires Ras activation, and both were impaired in LAT(-/-) NTAL(-/-) BMMCs. Sos was preferentially required for FcepsilonRI signals by IgE rather than IgE plus Ag. Survival impaired in LAT(-/-) NTAL(-/-) BMMCs was restored to levels comparable to those of the wild type by membrane-targeted Sos, which bypasses the Grb2-mediated membrane recruitment of Sos. The IgE-induced survival of BMMCs lacking Gads, an adaptor critical for the formation of the LAT-SLP-76-phospholipase Cgamma (PLCgamma) complex, was observed to be normal. IgE stimulation induced the membrane retention of Grb2-green fluorescent protein fusion proteins in wild-type but not LAT(-/-) NTAL(-/-) BMMCs. These results suggest that LAT and NTAL contribute to the maintenance of Erk activation and survival through the membrane retention of the Ras-activating complex Grb2-Sos and, further, that the LAT-Gads-SLP-76-PLCgamma and LAT/NTAL-Grb2-Sos pathways are differentially required for degranulation and survival, respectively.

Distinct roles of Ca2+ mobilization and G protein usage on regulation of Toll-like receptor function in human and murine mast cells

Toll-like receptors (TLRs) expressed in mast cells play important roles in orchestrating host defence against bacterial pathogens. Previous studies demonstrated that TLR2 agonist tripalmitoyl-S-glycero-Cys-(Lys)4 (Pam3Cys) stimulates both degranulation and cytokine production in human mast cells but only induces cytokine production in murine mast cells. To determine the molecular basis for this difference, we utilized a human mast cell line LAD 2, murine lung and bone marrow-derived mast cells (MLMC and BMMC). We found that Pam3Cys caused a sustained Ca2+ mobilization and degranulation in LAD 2 mast cells but not in MLMC or BMMC. Despite these differences, Pam3Cys stimulated equivalent chemokine CCL2 generation in all mast cell types tested. Cyclosporin A (CsA), an inhibitor of Ca2+/calcineurin-mediated nuclear factor of activated T cells (NFAT) activation, blocked chemokine production in LAD 2 but not in MLMC or BMMC. In contrast, inhibitors of nuclear factor kappa B (NF-kappaB) completely blocked CCL2 production in MLMC and BMMC but not in LAD 2 mast cells. Pertussis toxin and U0126, which, respectively, inhibit Galphai, extracellular signal-regulated kinase (ERK) phosphorylation substantially inhibited Pam3Cys-induced CCL2 generation in LAD 2 mast cells but had little or no effect on chemokine generation in MLMC and BMMC. These findings suggest that TLR2 activation in human LAD 2 mast cells and MLMC/BMMC promotes the release of different classes of mediators via distinct signalling pathways that depend on Ca2+ mobilization and G protein usage.