Preferential signaling and induction of allergy-promoting lymphokines upon weak stimulation of the high affinity IgE receptor on mast cells

FcεRI/PLC axis promotes anandamide synthesis and the formation of CB2-GPR55 heteromers, modulating cytokine production in mast cells

Mast cells (MC) are crucial effectors in immediate allergic reactions. Monomeric IgE sensitizes MC and triggers various signaling responses. FcεRI/IgE/antigen crosslinking induces the release of several mediators, including bioactive lipids, but little is known about endocannabinoids (eCBs) secretion. Here, we studied the effects of IgE-induced sensitization and FcεRI crosslinking on anandamide (AEA) synthesis and release in bone marrow-derived mast cells (BMMC). Our results showed that mIgE induced AEA secretion through phospholipase C activation. Secreted AEA contributed to p38 phosphorylation induced by mIgE sensitization. Prolonged mIgE sensitization promoted the formation of long-lasting CB2-GPR55 heteromers. FcεRI crosslinking also caused AEA production. Notably, CB2 deficiency increased IL-2 and IL-3 cytokine expression in response to FcɛRI crosslinking. CB2 and GPR55 agonists reduced IL-2 and IL-3 mRNA expression caused by FcεRI activation. Our findings suggest that a) IgE binding to FcɛRI and its antigen-dependent activation leads to an AEA-dependent autocrine regulatory loop that contributes to intracellular signaling in MC and that b) CB2 and GPR55 receptors play a critical role in modulating the effector phase of MC activation by specifically regulating cytokine expression.

Anti-microbial cetylpyridinium chloride suppresses mast cell function by targeting tyrosine phosphorylation of Syk kinase

Cetylpyridinium chloride (CPC) is a quaternary ammonium antimicrobial used in numerous personal care products, human food, cosmetic products, and cleaning solutions. Yet, there is minimal published data on CPC effects on eukaryotes, immune signaling, and human health. Previously, it was shown that low-micromolar CPC inhibits rat mast cell function by inhibiting antigen (Ag)-stimulated Ca2+ mobilization, microtubule polymerization, and degranulation. In the current study, these findings are extended to human mast cells (LAD2); this paper presents data indicating that a mechanism of action for CPC might center on its positively-charged quaternary nitrogen in its pyridinium headgroup. The inhibitory effect of CPC was independent of signaling platform receptor architecture. Tyrosine phosphorylation events are a trigger of Ca2+ mobilization necessary for degranulation. CPC inhibits global tyrosine phosphorylation in Ag-stimulated mast cells. Specifically, CPC inhibits tyrosine phosphorylation of specific key players Syk kinase and LAT, a substrate of Syk. In contrast, CPC did not affect Lyn kinase phosphorylation. Thus, a root mechanism for CPC effect might be electrostatic disruption of particular tyrosine phosphorylation events essential for signaling. This work presented here outlines biochemical mechanisms underlying the effects of CPC on immune signaling.

Antimicrobial cetylpyridinium chloride suppresses mast cell function by targeting tyrosine phosphorylation of Syk kinase

Cetylpyridinium chloride (CPC) is a quaternary ammonium antimicrobial used in numerous personal care products, human food, cosmetic products, and cleaning solutions. Yet, there is minimal published data on CPC effects on eukaryotes, immune signaling, and human health. Previously, we showed that low-micromolar CPC inhibits rat mast cell function by inhibiting antigen (Ag)-stimulated Ca 2+ mobilization, microtubule polymerization, and degranulation. In this study, we extend the findings to human mast cells (LAD2) and present data indicating that CPC's mechanism of action centers on its positively-charged quaternary nitrogen in its pyridinium headgroup. CPC's inhibitory effect is independent of signaling platform receptor architecture. Tyrosine phosphorylation events are a trigger of Ca 2+ mobilization necessary for degranulation. CPC inhibits global tyrosine phosphorylation in Ag-stimulated mast cells. Specifically, CPC inhibits tyrosine phosphorylation of specific key players Syk kinase and LAT, a substrate of Syk. In contrast, CPC does not affect Lyn kinase phosphorylation. Thus, CPC's root mechanism is electrostatic disruption of particular tyrosine phosphorylation events essential for signaling. This work outlines the biochemical mechanisms underlying the effects of CPC on immune signaling and allows the prediction of CPC effects on cell types, like T cells, that share similar signaling elements.

VAMP7 knockdown in secretory granules impairs CCL2 secretion in mast cells

Mast cells (MCs) possess numerous potent inflammatory mediators and undergo differential regulation in response to antigen (Ag) stimulation. Among the regulatory systems governing secretory responses, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) play a pivotal role in facilitating granule-plasma membrane fusion and subsequent secretion. Our previous investigation documented the involvement of vesicle-associated membrane protein 3 (VAMP3) in regulating cytokine secretions in RBL-2H3 cells, a model for MC IgE-mediated responses. In addition to VAMP3, VAMP7 is expressed in MCs, but its functional role remains elusive. The present study seeks to explore VAMP7-specific regulatory mechanisms in MCs, shedding light on one of the mechanisms governing heterogeneous secretory responses in these cells. Murine bone marrow-derived mast cells (BMMCs) were examined to analyze the subcellular distribution of inflammatory mediators, specifically TNFα, CCL2, and histamine, and VAMPs (i.e., VAMP3, VAMP7, and VAMP8). Immunocytochemistry and the transient expression of fluorescent protein-conjugated target proteins were used to discern the distribution of various inflammatory mediators and VAMP7 through confocal laser scanning microscopy. Each inflammatory mediator (TNFα, CCL2, and histamine) was found in secretory granules of different sizes within BMMCs. VAMP7 exhibited a distinct distribution compared to VAMP3 in these granules. Notably, an overlapping distribution was observed between VAMP7 and CCL2, but not between VAMP7 and TNFα or VAMP7 and histamine. This suggests that CCL2 resides within VAMP7-expressing granules and is subject to VAMP7-dependent secretory regulation. Consistently, BMMCs with VAMP7 knockdown showed markedly reduced CCL2 secretion after Ag stimulation. These observations underscore the heterogeneity of MC secretory responses and unveil a novel VAMP7-dependent CCL2 secretion mechanism within MCs. This discovery might pave the way for the development of more precise therapeutic strategies to modulate MC secretion in allergic conditions.

Mast cell tissue heterogeneity and specificity of immune cell recruitment

Mast cells occupy a unique niche within tissues as long lived perpetrators of IgE mediated hypersensitivity and anaphylaxis, as well as other immune responses. However, mast cells are not identical in different tissues and the impact of this tissue heterogeneity on the interaction with other immune cells and on defined immune responses is still unclear. In this review, we synthesize the characteristics of mast cell heterogeneity in the gut and the skin. Furthermore, we attempt to connect mast cell heterogeneity with functional diversity by exploring differences in mast cell-induced immune cell recruitment in these two model organs. The differential expression of certain receptors on mast cells of different tissues, notably tissue-specific expression patterns of integrins, complement receptors and MRGPRX2, could indicate that tissue environment-dependent factors skew mast cell-immune cell interactions, for example by regulating the expression of these receptors.

Cyclic Hypoxia Induces Transcriptomic Changes in Mast Cells Leading to a Hyperresponsive Phenotype after FcεRI Cross-Linking

Mast cells (MCs) play important roles in tumor development, executing pro- or antitumoral functions depending on tumor type and tumor microenvironment (TME) conditions. Cyclic hypoxia (cyH) is a common feature of TME since tumor blood vessels fail to provide a continuous supply of oxygen to the tumor mass. Here, we hypothesized that the localization of MCs in cyH regions within solid tumors could modify their transcriptional profile and activation parameters. Using confocal microscopy, we found an important number of MCs in cyH zones of murine melanoma B16-F1 tumors. Applying microarray analysis to examine the transcriptome of murine bone-marrow-derived MCs (BMMCs) exposed to interleaved cycles of hypoxia and re-oxygenation, we identified altered expression of 2512 genes. Functional enrichment analysis revealed that the transcriptional signature of MCs exposed to cyH is associated with oxidative phosphorylation and the FcεRI signaling pathway. Interestingly, FcεRI-dependent degranulation, calcium mobilization, and PLC-γ activity, as well as Tnf-α, Il-4, and Il-2 gene expression after IgE/antigen challenge were increased in BMMCs exposed to cyH compared with those maintained in normoxia. Taken together, our findings indicate that cyH causes an important phenotypic change in MCs that should be considered in the design of inflammation-targeted therapies to control tumor growth.

Mast Cells and Acupuncture Analgesia

Mast cells are widely distributed in various parts of the human body and play a vital role in the progression of many diseases. Recently, the close relationship between mast cells and acupoints was elucidated, and the role of mast cells in acupuncture analgesia has attracted the attention of researchers worldwide. Using mast cells, acupuncture analgesia and acupoint as key words to search CNKI, PubMed, Web of Science and other databases, combining the representative articles in these databases with the published research papers of our group, we summarized: The enrichment of mast cells and the dense arrangement of collagen fibers, microvessels, and nerves form the basis for acupoints as the reaction sites of acupuncture; acupuncture can cause the deformation of collagen fibers and activate TRPV channels on mast cells membrane, so as to stimulate mast cells to release bioactive substances and activate nerve receptors to generate analgesic effect; system biology models are set up to explain the quantitative process of information initiation and transmission at acupuncture points, and indicate that the acupuncture effect depends on the local mast cells density. In a conclusion, this review will give a scientific explanation of acupuncture analgesia from the material basis of acupoints, the local initiation, and afferent biological mechanism.

FcεRI: A Master Regulator of Mast Cell Functions

Mast cells (MCs) perform multiple functions thought to underlie different manifestations of allergies. Various aspects of antigens (Ags) and their interactions with immunoglobulin E (IgE) cause diverse responses in MCs. FcεRI, a high-affinity IgE receptor, deciphers the Ag–IgE interaction and drives allergic responses. FcεRI clustering is essential for signal transduction and, therefore, determines the quality of MC responses. Ag properties precisely regulate FcεRI dynamics, which consequently initiates differential outcomes by switching the intracellular-signaling pathway, suggesting that Ag properties can control MC responses, both qualitatively and quantitatively. Thus, the therapeutic benefits of FcεRI-targeting strategies have long been examined. Disrupting IgE–FcεRI interactions is a potential therapeutic strategy because the binding affinity between IgE and FcεRI is extremely high. Specifically, FcεRI desensitization, due to internalization, is also a potential therapeutic target that is involved in the mechanisms of allergen-specific immunotherapy. Several recent findings have suggested that silent internalization is strongly associated with FcεRI dynamics. A comprehensive understanding of the role of FcεRI may lead to the development of novel therapies for allergies. Here, we review the qualitatively diverse responses of MCs that impact the attenuation/development of allergies with a focus on the role of FcεRI toward Ag exposure.

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.

Tuning IgE: IgE-Associating Molecules and Their Effects on IgE-Dependent Mast Cell Reactions

The recent emergence of anti-immunoglobulin E (IgE) drugs and their candidates for humans has endorsed the significance of IgE-dependent pathways in allergic disorders. IgE is distributed locally in the tissues or systemically to confer a sensory mechanism in a domain of adaptive immunity to the otherwise innate type of effector cells, namely, mast cells and basophils. Bound on the high-affinity IgE receptor FcεRI, IgE enables fast memory responses against revisiting threats of venoms, parasites, and bacteria. However, the dysregulation of IgE-dependent reactions leads to potentially life-threatening allergic diseases, such as asthma and anaphylaxis. Therefore, reactivity of the IgE sensor is fine-tuned by various IgE-associating molecules. In this review, we discuss the mechanistic basis for how IgE-dependent mast cell activation is regulated by the IgE-associating molecules, including the newly developed therapeutic candidates.

The high affinity IgE receptor: a signaling update

Here we update receptor proximal and distant signaling events of the mast cell high affinity IgE receptor (FcεRI) launching immediate type I hypersensitivity and an inflammatory cytokine-chemokine cascade. Different physiologic antigen concentrations, their affinity, and valency for the IgE ligand produce distinct intracellular signaling events with different outcomes. Investigating mast cell degranulation has revealed a complex molecular machinery that relays proximal signaling to cytoskeletal reorganization, granule transport and membrane fusion. Several new phosphorylation- and calcium-responsive effectors have been described. FcεRI signaling also promotes de novo gene transcription. Recent progress has identified enhancers at genes that are upregulated in mast cells after stimulation through FcεRI using next generation sequencing methods. Enhancers at genes that respond to antigenic stimulation in human mast cells revealed Ca²⁺-dependency. Stimulation-responsive super enhancers in mouse mast cells have also been identified. Mast cell lineage-determining transcription factor GATA2 primes these enhancers to respond to antigenic stimulation.

Chemokines during anaphylaxis: the importance of CCL2 and CCL2-dependent chemotactic activity for basophils

Background

The role of chemokines in anaphylaxis is unclear.

Methods

We prospectively recruited 49 patients presenting to the emergency department with an acute episode of anaphylaxis and 28 healthy subjects. We measured serum levels of the chemokines CCL2, CCL5, CCL7, CCL8, CCL11, CCL13, CCL17, CCL21, CCL22, CCL24, and CCL26, tryptase, the absolute number of circulating basophils, monocytes, lymphocytes, and PMNs, and whole blood FCER1A, CPA3 and HDC gene expression at two time points: during the anaphylactic episode and in convalescent samples collected approximately 3 months later. We then investigated the in vitro chemotactic activity of chemokines induced during anaphylaxis for the in vitro migration of the corresponding cells.

Results

Only CCL2 chemokine levels were significantly increased in anaphylaxis samples (median 514 pg/ml) compared to convalescent samples (284 pg/ml, P < 0.0001) and healthy subjects (279 pg/ml, P < 0.0001); there was no significant difference in any of the other chemokines. There was a significant positive correlation between the rates of increase of serum CCL2 (median [range]: 106.0% [− 44.7% to 557.4%]) and tryptase (133.8% [− 6.6% to 893.4%]; r = 0.68, P < 0.0001) and between the acute concentration of serum CCL2 and the acute concentration of serum tryptase (r = 0.77, P < 0.0001). The number of circulating basophils, but not other blood cells, significantly decreased during anaphylaxis (median 5.0 vs. 19.1 cells/µl in convalescent samples; P < 0.0001); a decrease in whole-blood gene expression of basophil markers (P ≤ 0.0018) confirmed these changes. Anaphylactic serum enhances the in vitro migration of basophils via CCL2-dependent chemotactic activity; in contrast, no CCL2-dependent chemotactic activity was observed for convalescent samples.

Conclusions

Our findings imply an important and specific role for CCL2-mediated chemotactic activity in the pathophysiology of human anaphylaxis.

Signal Transduction Pathways Activated by Innate Immunity in Mast Cells: Translating Sensing of Changes into Specific Responses

Mast cells (MCs) constitute an essential cell lineage that participates in innate and adaptive immune responses and whose phenotype and function are influenced by tissue-specific conditions. Their mechanisms of activation in type I hypersensitivity reactions have been the subject of multiple studies, but the signaling pathways behind their activation by innate immunity stimuli are not so well described. Here, we review the recent evidence regarding the main molecular elements and signaling pathways connecting the innate immune receptors and hypoxic microenvironment to cytokine synthesis and the secretion of soluble or exosome-contained mediators in this cell type. When known, the positive and negative control mechanisms of those pathways are presented, together with their possible implications for the understanding of mast cell-driven chronic inflammation. Finally, we discuss the relevance of the knowledge about signaling in this cell type in the recognition of MCs as central elements on innate immunity, whose remarkable plasticity converts them in sensors of micro-environmental discontinuities and controllers of tissue homeostasis.

Critical Signaling Events in the Mechanoactivation of Human Mast Cells through p.C492Y-ADGRE2

A role for the adhesion G-protein coupled receptor ADGRE2 or EMR2 in mechanosensing was revealed by the finding of a missense substitution (p.C492Y) associated with familial vibratory urticaria. In these patients, friction of the skin induces mast cell hyper-degranulation through p.C492Y-ADGRE2, causing localized hives, flushing, and hypotension. We have now characterized the responses and intracellular signals elicited by mechanical activation in human mast cells expressing p.C492Y-ADGRE2 and attached to dermatan sulfate, a ligand for ADGRE2. The presence of p.C492Y-ADGRE2 reduced the threshold to activation and increased the extent of degranulation along with the percentage of mast cells responding. Vibration caused phospholipase C activation, transient increases in cytosolic calcium, and downstream activation of phosphoinositide 3-kinase and extracellular signal-regulated kinases 1 and 2 by Gβγ, Gαq/11, and Gαi/o-independent mechanisms. Degranulation induced by vibration was dependent on phospholipase C pathways, including calcium, protein kinase C, and phosphoinositide 3-kinase but not extracellular signal-regulated kinases 1/2 pathways, along with pertussis toxin-sensitive signals. In addition, mechanoactivation of mast cells stimulated the synthesis and release of prostaglandin D2, to our knowledge a previously unreported mediator in vibratory urticaria, and extracellular signal-regulated kinases 1/2 activation was required for this response together with calcium, protein kinase C, and to some extent, phosphoinositide 3-kinase. Our studies thus identified critical molecular events initiated by mechanical forces and potential therapeutic targets for patients with vibratory urticaria.

Tetraspanins in the regulation of mast cell function

Mast cells (MCs) are long-living immune cells highly specialized in the storage and release of different biologically active compounds and are involved in the regulation of innate and adaptive immunity. MC degranulation and replacement of MC granules are accompanied by active membrane remodelling. Tetraspanins represent an evolutionary conserved family of transmembrane proteins. By interacting with lipids and other membrane and intracellular proteins, they are involved in organisation of membrane protein complexes and act as "molecular facilitators" connecting extracellular and cytoplasmic signaling elements. MCs express different tetraspanins and MC degranulation is accompanied by changes in membrane organisation. Therefore, tetraspanins are very likely involved in the regulation of MC exocytosis and membrane reorganisation after degranulation. Antiviral response and production of exosomes are further aspects of MC function characterized by dynamic changes of membrane organization. In this review, we pay a particular attention to tetraspanin gene expression in different human and murine MC populations, discuss tetraspanin involvement in regulation of key MC signaling complexes, and analyze the potential contribution of tetraspanins to MC antiviral response and exosome production. In-depth knowledge of tetraspanin-mediated molecular mechanisms involved in different aspects of the regulation of MC response will be beneficial for patients with allergies, characterized by overwhelming MC reactions.

Identification of 5-Hydroxymethylfurfural (5-HMF) as an Active Component Citrus Jabara That Suppresses FcεRI-Mediated Mast Cell Activation

Jabara (Citrus jabara Hort. ex Y. Tanaka) is a type of citrus fruit known for its beneficial effect against seasonal allergies. Jabara is rich in the antioxidant narirutin whose anti-allergy effect has been demonstrated. One of the disadvantages in consuming Jabara is its bitter flavor. Therefore, we fermented the fruit to reduce the bitterness and make Jabara easy to consume. Here, we examined whether fermentation alters the anti-allergic property of Jabara. Suppression of degranulation and cytokine production was observed in mast cells treated with fermented Jabara and the effect was dependent on the length of fermentation. We also showed that 5-hydroxymethylfurfural (5-HMF) increases as fermentation progresses and was identified as an active component of fermented Jabara, which inhibited mast cell degranulation. Mast cells treated with 5-HMF also exhibited reduced degranulation and cytokine production. In addition, we showed that the expression levels of phospho-PLCγ1 and phospho-ERK1/2 were markedly reduced upon FcεRI stimulation. These results indicate that 5-HMF is one of the active components of fermented Jabara that is involved in the inhibition of mast cell activation.

Mutant Huntingtin affects toll-like receptor 4 intracellular trafficking and cytokine production in mast cells

BACKGROUND

Huntington's disease (HD) is caused by the expression of a mutated variant of Huntingtin (mHtt), which results in the complex pathology characterized by a defective function of the nervous system and altered inflammatory responses. While the neuronal effects of mHtt expression have been extensively studied, its effects on the physiology of immune cells have not been fully described. Mast cells (MCs) are unique tissue-resident immune cells whose activation has been linked to protective responses against parasites and bacteria, but also to deleterious inflammatory allergic reactions and, recently, to neurodegenerative diseases.

METHODS

Bone marrow-derived mast cells (BMMCs) were obtained from wild-type (WT-) and mHtt-expressing (R6/1) mice to evaluate the main activation parameters triggered by the high-affinity IgE receptor (FcεRI) and the Toll-like receptor (TLR) 4. Degranulation was assessed by measuring the secretion of β-hexosaminidase, MAP kinase activation was detected by Western blot, and cytokine production was determined by RT-PCR and ELISA. TLR-4 receptor and Htt vesicular trafficking was analyzed by confocal microscopy. In vivo, MC-deficient mice (c-KitWsh/Wsh) were intraperitonally reconstituted with WT or R6/1 BMMCs and the TLR4-induced production of the tumor necrosis factor (TNF) was determined by ELISA. A survival curve of mice treated with a sub-lethal dose of bacterial lipopolysaccharide (LPS) was constructed.

RESULTS

R6/1 BMMCs showed normal β-hexosaminidase release levels in response to FcεRI, but lower cytokine production upon LPS stimulus. Impaired TLR4-induced TNF production was associated to the lack of intracellular dynamin-dependent TLR-4 receptor trafficking to perinuclear regions in BMMCs, a diminished ERK1/2 and ELK-1 phosphorylation, and a decrease in c-fos and TNF mRNA accumulation. R6/1 BMMCs also failed to produce TLR4-induced anti-inflammatory cytokines (like IL-10 and TGF-β). The detected defects were also observed in vivo, in a MCs-dependent model of endotoxemia. R6/1 and c-KitWsh/Wsh mice reconstituted with R6/1 BMMCs showed a decreased TLR4-induced TNF production and lower survival rates to LPS challenge than WT mice.

CONCLUSIONS

Our data show that mHtt expression causes an impaired production of pro- and anti-inflammatory mediators triggered by TLR-4 receptor in MCs in vitro and in vivo, which could contribute to the aberrant immunophenotype observed in HD.

The lectin ArtinM activates RBL-2H3 mast cells without inducing degranulation

Mast cells are connective tissue resident cells with morphological and functional characteristics that contribute to their role in allergic and inflammatory processes, host defense and maintenance of tissue homeostasis. Mast cell activation results in the release of pro-inflammatory mediators which are largely responsible for the physiological functions of mast cells. The lectin ArtinM, extracted from Artocarpus heterophyllus (jackfruit), binds to D-manose, thus inducing degranulation of mast cells. ArtinM has several immunomodulatory properties including acceleration of wound healing, and induction of cytokine release. The aim of the present study was to investigate the role of ArtinM in the activation and proliferation of mast cells. The rat mast cell line RBL-2H3 was used throughout this study. At a low concentration (0.25μg/mL), ArtinM induced mast cell activation and the release of IL-6 without stimulating the release of pre-formed or newly formed mediators. Additionally, when the cells were activated by ArtinM protein tyrosine phosphorylation was stimulated. The low concentration of ArtinM also activated the transcription factor NFkB, but not NFAT. ArtinM also affected the cell cycle and stimulated cell proliferation. Therefore, ArtinM may have therapeutic applications by modulating immune responses due to its ability to activate mast cells and promote the release of newly synthesized mediators. Additionally, ArtinM could have beneficial effects at low concentrations without degranulating mast cells and inducing allergic reactions.

Mast Cells Localize in Hypoxic Zones of Tumors and Secrete CCL-2 under Hypoxia through Activation of L-Type Calcium Channels

Hypoxia is a condition that together with low pH, high amounts of reactive oxygen species (ROS), and increased adenosine levels characterize tumor microenvironment. Mast cells (MCs) are part of tumor microenvironment, but the effect of hypoxia on the production of MC-derived cytokines has not been fully described. Using the hypoxia marker pimonidazole in vivo, we found that MCs were largely located in the low-oxygen areas within B16-F1 mice melanoma tumors. In vitro, hypoxia promoted ROS production, a ROS-dependent increase of intracellular calcium, and the production of MCP 1 (CCL-2) in murine bone marrow-derived MCs. Hypoxia-induced CCL-2 production was sensitive to the antioxidant trolox and to nifedipine, a blocker of L-type voltage-dependent Ca²⁺ channels (LVDCCs). Simultaneously with CCL-2 production, hypoxia caused the ROS-dependent glutathionylation and membrane translocation of the α1c subunit of Cav1.2 LVDCCs. Relationship between ROS production, calcium rise, and CCL-2 synthesis was also observed when cells were treated with H₂O₂ In vivo, high CCL-2 production was detected on hypoxic zones of melanoma tumors (where tryptase-positive MCs were also found). Pimonidazole and CCL-2 positive staining diminished when B16-F1 cell-inoculated animals were treated with trolox, nifedipine, or the adenosine receptor 2A antagonist KW6002. Our results show that MCs are located preferentially in hypoxic zones of melanoma tumors, hypoxia-induced CCL-2 production in MCs requires calcium rise mediated by glutathionylation and membrane translocation of LVDCCs, and this mechanism of CCL-2 synthesis seems to operate in other cells inside melanoma tumors, with the participation of the adenosine receptor 2A.

Regulation of the pleiotropic effects of tissue-resident mast cells

Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca²⁺- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease.

Intravenous immunoglobulin induces IL-4 in human basophils by signaling through surface-bound IgE

BACKGROUND

Therapeutic normal IgG or intravenous immunoglobulin (IVIG) exerts anti-inflammatory effects through several mutually nonexclusive mechanisms. Recent data in mouse models of autoimmune disease suggest that IVIG induces IL-4 in basophils by enhancing IL-33 in SIGN-related 1-positive innate cells. However, translational insight on these data is lacking.

OBJECTIVE

We sought to investigate the effect of IVIG on human basophil functions.

METHODS

Isolated circulating basophils from healthy donors were cultured in the presence of IL-3, IL-33, GM-CSF, thymic stromal lymphopoietin, or IL-25. The effect of IVIG and F(ab')₂ and Fc IVIG fragments was examined based on expression of various surface molecules, phosphorylation of spleen tyrosine kinase, induction of cytokines, and histamine release. Basophil phenotypes were also analyzed from IVIG-treated patients with myopathy. Approaches, such as depletion of anti-IgE reactivity from IVIG, blocking antibodies, or inhibitors, were used to investigate the mechanisms.

RESULTS

We report that IVIG directly induces activation of IL-3-primed human basophils, but IL-33 and other cytokines were dispensable for this effect. Activation of basophils by IVIG led to enhanced expression of CD69 and secretion of IL-4, IL-6, and IL-8. IVIG-treated patients with myopathy displayed enhanced expression of CD69 on basophils. The spleen tyrosine kinase pathway is implicated in these functions of IVIG and were mediated by F(ab')₂ fragments. Mechanistically, IVIG induced IL-4 in human basophils by interacting with basophil surface-bound IgE but independent of FcγRII, type II Fc receptors, C-type lectin receptors, and sialic acid-binding immunoglobulin-like lectins.

CONCLUSION

These results uncovered a pathway of promoting the T_H2 response by IVIG through direct interaction of IgG with human basophils.

Anandamide inhibits FcεRI-dependent degranulation and cytokine synthesis in mast cells through CB2 and GPR55 receptor activation. Possible involvement of CB2-GPR55 heteromers

Activation of high affinity receptor for IgE (FcεRI) by IgE/antigen complexes in mast cells (MCs) leads to the release of preformed pro-inflammatory mediators stored in granules by a Ca²⁺-dependent process known as anaphylactic degranulation. Degranulation inhibition has been proposed as a strategy to control allergies and chronic inflammation conditions. Cannabinoids are important inhibitors of inflammatory reactions but their effects on IgE/Ag-mediated MCs responses are not well described. In this study, we analyzed the effect of the endocannabinoid anandamide (AEA), the selective CB₂ receptor agonist HU308, and the GPR55 receptor agonist lysophosphatidylinositol (LPI) on FcεRI-induced activation in murine bone marrow-derived mast cells (BMMCs). Our results show that AEA, HU380 and LPI inhibited FcεRI-induced degranulation in a concentration-dependent manner. This effect was mediated by CB₂ and GPR55 receptor activation through a mechanism insensitive to pertussis toxin. Degranulation inhibition was prevented by CB₂ and GPR55 antagonism, but not by CB₁ receptor blockage. AEA also inhibited calcium-dependent cytokine mRNA synthesis induced by FcεRI crosslinking, without affecting early phosphorylation events. In addition, AEA, HU308 and LPI inhibited intracellular Ca²⁺ rise in response to IgE/Ag. CB₂ and GPR55 receptor antagonism could not prevent the inhibition produced by AEA and HU308, but partially blocked the one caused by LPI. These results indicate that AEA inhibits IgE/Ag-induced degranulation through a mechanism that includes the participation of CB₂ and GPR55 receptors acting in close crosstalk, and show that CB₂-GPR55 heteromers are important negative regulators of FcεRI-induced responses in MCs.

The Emerging Picture of Mast Cell Activation: The Complex Regulatory Network of High-Affinity Receptor for Immunoglobulin E Signaling

It is now well known that immunoglobulin E (IgE) and mast cells (MCs) are important participants in allergic diseases. MCs contain electron-dense secretory granules which are filled with inflammatory mediators. The interaction of an allergen (antigen) with an antigen-specific IgE-bound high-affinity receptor for IgE (FcεRI) is an essential step in MC activation as well as subsequent downstream signaling events. What we know is that IgE and FcεRI activate a complex regulatory network (i.e., signaling molecules and messengers) that governs both the type of MC activation and the symptoms of allergic disease. This review focuses on recent discoveries that shed new light on FcεRI signaling networks, holding promise for the development of new therapeutic solutions in the treatment of allergic diseases.

Licarin A is a candidate compound for the treatment of immediate hypersensitivity via inhibition of rat mast cell line RBL-2H3 cells

OBJECTIVES

We previously demonstrated that some phenylpropanoids are capable of inhibiting activated mast cells. This study evaluated the anti-allergic effects of licarin A, a neolignan isolated from various plants, on antigen-stimulated rat mast cell line.

METHODS

The inhibitory effects of licarin A on histamine release, tumour necrosis factor-α (TNF-α) and prostaglandin D2 (PGD2) production, and cyclooxygenase-2 (COX-2) expression in dinitrophenyl-human serum albumin (DNP-HSA) rat basophilic leukemia cells (DNP-HSA-stimulated RBL-2H3 cells), were investigated by spectrofluorometry, ELISA and immunoblotting.

KEY FINDINGS

Licarin A significantly and dose-dependently reduced TNF-α production (IC50 12.6 ± 0.3 μm) in DNP-HSA-stimulated RBL-2H3 cells. Furthermore, the levels of PGD2 secretion in DNP-HSA-stimulated cells pretreated with licarin A were lower than those stimulated with DNP-HSA alone (positive control). Treatment with licarin A at 20 μm produced slight suppression of DNP-HSA-induced increases in COX-2 mRNA and protein levels. We identified several signalling pathways that mediated these pharmacological effects. Licarin A treatment tended to reduce phosphorylated protein kinase C alpha/beta II (PKCα/βII) and p38 mitogen-activated protein kinase (MAPK) protein levels.

CONCLUSIONS

Our results demonstrate that licarin A reduces TNF-α and PGD2 secretion via the inhibition of PKCα/βII and p38 MAPK pathways; this compound may be useful for attenuating immediate hypersensitivity.

Treatment of murine mast cells with IgEκ and protein L enhances apoptotic cell death induced by IL-3 withdrawal

Engagement of the high-affinity IgE receptor (FcεRI) can be either protective or non-protective against apoptotic cell death (ACD) in bone marrow-derived murine mast cells (BMMCs) after IL-3 withdrawal, depending on the avidity between IgE and its antigen. We recently reported that protein L (PpL), a bacterial Igκ-binding soluble protein, is able to stimulate intracellular signaling to induce activation of BMMCs by interacting with the IgEκ-FcεRI complex. However, it is unclear if cross-linking of FcεRI with IgEκ and PpL prevents or enhances IL-3-dependent ACD in BMMCs. In the present study, we found that IL-3-dependent ACD of BMMCs is accelerated by loading soluble PpL in the presence of IgEκ-occupied FcεRIα. For this purpose, soluble PpL was incorporated into the BMMCs. Unlike soluble PpL, immobilized PpL failed to enhance ACD, although both forms of PpL induced IL-6 production equally in BMMCs. In addition, we observed that DNS5-BSA protected anti-DNS IgE-sensitized BMMCs from IL-3 depletion-mediated ACD by inducing the production of autocrine IL-3. In contrast, DNS5-PpL enhanced IL-3 withdrawal-induced ACD of anti-DNS IgE-sensitized BMMCs and reduced the production of autocrine IL-3. These findings suggest that PpL increases IL-3 withdrawal-induced ACD of IgEκ-sensitized BMMCs by incorporating PpL into the BMMCs and that this internalized PpL may interfere with survival signals via FcεRI.

New insights on the signaling and function of the high-affinity receptor for IgE

Clustering of the high-affinity receptor for immunoglobulin E (FcεRI) through the interaction of receptor-bound immunoglobulin E (IgE) antibodies with their cognate antigen is required to couple IgE antibody production to cellular responses and physiological consequences. IgE-induced responses through FcεRI are well known to defend the host against certain infectious agents and to lead to unwanted allergic responses to normally innocuous substances. However, the cellular and/or physiological response of individuals that produce IgE antibodies may be markedly different and such antibodies (even to the same antigenic epitope) can differ in their antigen-binding affinity. How affinity variation in the interaction of FcεRI-bound IgE antibodies with antigen is interpreted into cellular responses and how the local environment may influence these responses is of interest. In this chapter, we focus on recent advances that begin to unravel how FcεRI distinguishes differences in the affinity of IgE-antigen interactions and how such discrimination along with surrounding environmental stimuli can shape the (patho) physiological response.

Vesicular trafficking and signaling for cytokine and chemokine secretion in mast cells

Upon activation mast cells (MCs) secrete numerous inflammatory compounds stored in their cytoplasmic secretory granules by a process called anaphylactic degranulation, which is responsible for type I hypersensitivity responses. Prestored mediators include histamine and MC proteases but also some cytokines and growth factors making them available within minutes for a maximal biological effect. Degranulation is followed by the de novo synthesis of lipid mediators such as prostaglandins and leukotrienes as well as a vast array of cytokines, chemokines, and growth factors, which are responsible for late phase inflammatory responses. While lipid mediators diffuse freely out of the cell through lipid bilayers, both anaphylactic degranulation and secretion of cytokines, chemokines, and growth factors depends on highly regulated vesicular trafficking steps that occur along the secretory pathway starting with the translocation of proteins to the endoplasmic reticulum. Vesicular trafficking in MCs also intersects with endocytic routes, notably to form specialized cytoplasmic granules called secretory lysosomes. Some of the mediators like histamine reach granules via specific vesicular monoamine transporters directly from the cytoplasm. In this review, we try to summarize the available data on granule biogenesis and signaling events that coordinate the complex steps that lead to the release of the inflammatory mediators from the various vesicular carriers in MCs.

Rethinking the role of immunoglobulin E and its high-affinity receptor: new insights into allergy and beyond

Immunoglobulin E (IgE) and its high-affinity receptor (FcεRI) are well-known participants in the allergic response. The interaction of allergens with FcεRI-bound IgE antibodies is an essential step in mast cell/basophil activation and the subsequent release of allergic mediators. It is known that the affinity of the interaction between an IgE antibody and an allergen may differ, raising the question of whether FcεRI can decipher these differences. If so, do the cellular and physiological outcomes vary? Are the molecular mechanisms initiated by FcεRI similarly under low- or high-affinity interactions? Could the resulting inflammatory response differ? Recent discoveries summarized herein are beginning to shed new light on these important questions. What we have learned from them is that IgE and FcεRI form a complex regulatory network influencing the inflammatory response in allergy and beyond.

Immunoglobulin E induces VEGF production in mast cells and potentiates their pro-tumorigenic actions through a Fyn kinase-dependent mechanism

Background

High concentrations of plasmatic IgE have been related to distinct systemic inflammatory conditions that frequently predispose individuals to hypersensitivity reactions. Although effects of IgE have been suggested to relay on the low-intensity activation of distinct effector elements of the immune system, such as mast cells (MC), experimental evidence on the role of IgE-induced production of inflammatory mediators on specific pathologies is scarce. MC are an important component in tumor microenvironment where they seem to secrete a number of immunomodulatory and angiogenic mediators, such as the Vascular Endothelial Growth Factor (VEGF) by not well-described mechanisms. In this work, we investigated the effect of monomeric IgE (in the absence of antigen) on the production of VEGF in MC, analyzed if monomeric IgE could exacerbate the pro-tumorigenic properties of that cell type and characterized some of the molecular mechanisms behind the effects of IgE on VEGF production and tumor growth.

Methods

For in vitro studies, murine bone marrow-derived mast cells (BMMCs) were used. Pharmacological inhibitors and phosphorylation of key elements controlling VEGF secretion and protein translation were used to characterize the mechanism of VEGF production triggered by IgE.

In vivo, the effect of a single i.v. administration of monomeric IgE on B16 melanoma tumor weight, intratumoral blood vessel formation and tumor-associated MC was assessed in four groups of mice: MC-proficient (WT), MC-deficient (Wsh), Wsh reconstituted with MC derived from WT mice (Wsh Rec WT) and Wsh reconstituted with MC derived from Fyn −/− mice (Wsh Rec Fyn −/−).

Results

Monomeric IgE induced VEGF secretion through a Fyn kinase-dependent mechanism and modulated de novo protein synthesis modifying the activity of the translational regulator 4E-BP1 in BMMCs. In vivo, monomeric IgE increased melanoma tumor growth, peritumoral MC and blood vessel numbers in WT but not in Wsh mice. The positive effects of IgE on melanoma tumor growth were reproduced after reconstitution of Wsh mice with WT but not with Fyn −/− BMMCs.

Conclusion

Our data suggest that monomeric IgE, in the absence of antigen, induces VEGF production in MC and in vivo contributes to melanoma tumor growth through a Fyn kinase-dependent mechanism.

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.

Single-cell measurements of IgE-mediated FcεRI signaling using an integrated microfluidic platform

Heterogeneity in responses of cells to a stimulus, such as a pathogen or allergen, can potentially play an important role in deciding the fate of the responding cell population and the overall systemic response. Measuring heterogeneous responses requires tools capable of interrogating individual cells. Cell signaling studies commonly do not have single-cell resolution because of the limitations of techniques used such as Westerns, ELISAs, mass spectrometry, and DNA microarrays. Microfluidics devices are increasingly being used to overcome these limitations. Here, we report on a microfluidic platform for cell signaling analysis that combines two orthogonal single-cell measurement technologies: on-chip flow cytometry and optical imaging. The device seamlessly integrates cell culture, stimulation, and preparation with downstream measurements permitting hands-free, automated analysis to minimize experimental variability. The platform was used to interrogate IgE receptor (FcεRI) signaling, which is responsible for triggering allergic reactions, in RBL-2H3 cells. Following on-chip crosslinking of IgE-FcεRI complexes by multivalent antigen, we monitored signaling events including protein phosphorylation, calcium mobilization and the release of inflammatory mediators. The results demonstrate the ability of our platform to produce quantitative measurements on a cell-by-cell basis from just a few hundred cells. Model-based analysis of the Syk phosphorylation data suggests that heterogeneity in Syk phosphorylation can be attributed to protein copy number variations, with the level of Syk phosphorylation being particularly sensitive to the copy number of Lyn.

Knockout of the Trpc1 gene reveals that TRPC1 can promote recovery from anaphylaxis by negatively regulating mast cell TNF-α production

Antigen-mediated mast cell (MC) degranulation is the critical early event in the induction of allergic reactions. Transient receptor potential channels (TRPC), particularly TRPC1, are thought to contribute to such MC activation. To explore the contribution of TRPC1 in MC-driven allergic reactions, we examined antigen-mediated anaphylaxis in Trpc1⁻/⁻ and WT mice, and TRPC1 involvement in the activation of MCs derived from the bone marrow (BMMCs) of these mice. In vivo, we observed a similar induction of passive systemic anaphylaxis in the Trpc1⁻/⁻ mice compared to WT controls. Nevertheless, there was delayed recovery from this response in Trpc1⁻/⁻ mice. Furthermore, contrary to expectations, Trpc1⁻/⁻ BMMCs responded to antigen with enhanced calcium signaling but with little defect in degranulation or associated signaling. In contrast, antigen-mediated production of TNF-α, and other cytokines, was enhanced in the Trpc1⁻/⁻ BMMCs, as were calcium-dependent events required for these responses. Additionally, circulating levels of TNF-α in response to antigen were preferentially elevated in the Trpc1⁻/⁻ mice, and administration of an anti-TNF-α antibody blocked the delay in recovery from anaphylaxis in these mice. These data thus provide evidence that, in this model, TRPC1 promotes recovery from the anaphylactic response by repressing antigen-mediated TNF-α release from MCs.

Cross-talk between tetraspanin CD9 and transmembrane adaptor protein non-T cell activation linker (NTAL) in mast cell activation and chemotaxis

Chemotaxis, a process leading to movement of cells toward increasing concentrations of chemoattractants, is essential, among others, for recruitment of mast cells within target tissues where they play an important role in innate and adaptive immunity. Chemotaxis is driven by chemoattractants, produced by various cell types, as well as by intrinsic cellular regulators, which are poorly understood. In this study we prepared a new mAb specific for the tetraspanin CD9. Binding of the antibody to bone marrow-derived mast cells triggered activation events that included cell degranulation, Ca(2+) response, dephosphorylation of ezrin/radixin/moesin (ERM) family proteins, and potent tyrosine phosphorylation of the non-T cell activation linker (NTAL) but only weak phosphorylation of the linker for activation of T cells (LAT). Phosphorylation of the NTAL was observed with whole antibody but not with its F(ab)(2) or Fab fragments. This indicated involvement of the Fcγ receptors. As documented by electron microscopy of isolated plasma membrane sheets, CD9 colocalized with the high-affinity IgE receptor (FcεRI) and NTAL but not with LAT. Further tests showed that both anti-CD9 antibody and its F(ab)(2) fragment inhibited mast cell chemotaxis toward antigen. Experiments with bone marrow-derived mast cells deficient in NTAL and/or LAT revealed different roles of these two adaptors in antigen-driven chemotaxis. The combined data indicate that chemotaxis toward antigen is controlled in mast cells by a cross-talk among FcεRI, tetraspanin CD9, transmembrane adaptor proteins NTAL and LAT, and cytoskeleton-regulatory proteins of the ERM family.

Activation/Inhibition of mast cells by supra-optimal antigen concentrations

Mast cells (MCs) are tissue resident cells of hemopoietic origin and are critically involved in allergic diseases. MCs bind IgE by means of their high-affinity receptor for IgE (FcεRI). The FcεRI belongs to a family of multi-chain immune recognition receptors and is activated by cross-linking in response to multivalent antigens (Ags)/allergens. Activation of the FcεRI results in immediate release of preformed granular substances (e.g. histamine, heparin, and proteases), generation of arachidonic acid metabolites, and production of pro-inflammatory cytokines. The FcεRI shows a remarkable, bell-shaped dose-response behavior with weak induction of effector responses at both low and high (so-called supra-optimal) Ag concentrations. This is significantly different from many other receptors, which reach a plateau phase in response to high ligand concentrations. To explain this unusual dose-response behavior of the FcεRI, scientists in the past have drawn parallels to so-called precipitin curves resulting from titration of Ag against a fixed concentration of antibody (Ab) in solution (a.k.a. Heidelberger curves). Thus, for high, supra-optimal Ag concentrations one could assume that every IgE-bound FcεRI formed a monovalent complex with “its own Ag”, thus resulting in marginal induction of effector functions due to absence of receptor cross-linking. However, this was never proven to be the case. More recently, careful studies of FcεRI activation and signaling events in MCs in response to supra-optimal Ag concentrations have suggested a molecular explanation for the descending part of this bell-shaped curve. It is obvious now that extensive FcεRI/IgE/Ag clusters are formed and inhibitory molecules and signalosomes are engaged in response to supra-optimal cross-linking (amongst them the Src family kinase Lyn and the inositol-5′-phosphatase SHIP1) and they actively down-regulate MC effector responses. Thus, the analysis of MC signaling triggered by supra-optimal crosslinking holds great potential for identifying novel targets for pharmacologic therapeutic intervention to benefit patients with acute and chronic allergic diseases.

Pretreatment with glycomacropeptide reduces allergen sensitization, alleviates immediate cutaneous hypersensitivity and protects from anaphylaxis

Allergic disorders are characterized by the involvement of allergen-specific immunoglobulin (Ig)E antibodies and T helper type 2 (Th2) cells. The search for new therapies for allergic diseases has been the primary focus of interest for many investigators in recent years. Glycomacropeptide (GMP) is a biologically active component of milk that exhibits a range of immunomodulatory functions. We examined whether oral administration of GMP could affect the development of allergic sensitization and the severity of immediate cutaneous hypersensitivity reactions and of anaphylaxis. Rats treated with or without GMP were ovalbumin (OVA)-sensitized and several indicators of allergy were evaluated. Pretreatment with GMP resulted in reduction of antigen-specific IgE titre in rats when sensitized with OVA. GMP administration also markedly suppressed the proliferative response of splenocytes to antigen and the production of interleukin (IL)-13 by splenocytes of sensitized animals. In addition, GMP pretreatment attenuated the intensity of the immediate cutaneous reaction induced by antigen and protected the sensitized rats from severe anaphylaxis. These data demonstrate, for the first time, that the administration of GMP prevents allergen sensitization and reduces the severity of the early-phase reaction induced by antigen in cutaneous hypersensitivity and in anaphylaxis. GMP may be used as a novel prophylactic agent for the control of allergic diseases.

Evidence that formation of vimentin mitogen-activated protein kinase (MAPK) complex mediates mast cell activation following FcεRI/CC chemokine receptor 1 cross-talk

Accumulating evidence points to cross-talk between FcεRI and CC chemokine receptor (CCR)-mediated signaling pathways in mast cells. Here, we propose that vimentin, a protein comprising type III intermediate filament, participates in such cross-talk for CCL2/monocyte chemotactic protein 1 (MCP-1) production in mast cells, which is a mechanism for allergic inflammation. Co-stimulation via FcεRI, using IgE/antigen, and CCR1, using recombinant CCL3/macrophage inflammatory protein-1α (MIP-1α), increased expression of phosphorylated, disassembled, and soluble vimentin in rat basophilic leukemia (RBL)-2H3 cells expressing human CCR1 (RBL-CCR1 cells) and bone marrow-derived murine mast cells, both models of mucosal type mast cells. Furthermore, co-stimulation enhanced production of CCL2 as well as phosphorylation of MAPK. Treating the cells with p38 MAPK inhibitor SB203580, but not with MEK inhibitor PD98058, reduced CCL2 production, suggesting that p38 MAPK, but not ERK1/2, plays a critical role in the chemokine production. Immunoprecipitation analysis showed that vimentin interacts with phosphorylated ERK1/2 and p38 MAPKs in the co-simulated cells. Preventing disassembly of the vimentin by aggregating vimentin filaments using β,β'-iminodipropionitrile reduced the interaction of vimentin with phosphorylated MAPKs as well as CCL2 production in the cells. Taken together, disassembled vimentin interacting with phosphorylated p38 MAPK could mediate CCL2 production in mast cells upon FcεRI and CCR1 activation.

Modulation of FcεRI-dependent mast cell response by OX40L via Fyn, PI3K, and RhoA

BACKGROUND

The interaction of mast cells (MCs) with regulatory T cells through the OX40 ligand (OX40L):OX40 axis downregulates FcεRI-dependent immediate hypersensitivity responses both in vitro and in vivo. Little is known on OX40L-mediated intracellular signaling or on the mechanism by which OX40L engagement suppresses MC degranulation.

OBJECTIVE

We explored the role of OX40L engagement on IgE/antigen-triggered MCs both in vitro and in vivo.

METHODS

The soluble form of OX40 molecule was used to selectively trigger OX40L on MCs in vitro and was used to dissect OX40L contribution in an in vivo model of systemic anaphylaxis.

RESULTS

OX40L:OX40 interaction led to the recruitment of C-terminal src kinase into lipid rafts, causing a preferential suppression of Fyn kinase activity and subsequent reduction in the phosphorylation of Gab2, the phosphatidylinositol 3-OH kinase regulatory subunit p85, and Akt, without affecting the Lyn pathway. Dampening of Fyn kinase activity also inhibited RhoA activation and microtubule nucleation, key regulators of MC degranulation. The in vivo administration of a blocking antibody to OX40L in wild-type mice caused enhanced immediate hypersensitivity, whereas the administration of soluble OX40 to regulatory T-cell-depleted or OX40-deficient mice reduced MC degranulation.

CONCLUSIONS

The engagement of OX40L selectively suppresses Fyn-initiated signals required for MC degranulation and serves to limit immediate hypersensitivity. Our data suggest that soluble OX40 can restore the aberrant or absent regulatory T-cell activity, revealing a previously unappreciated homeostatic role for OX40L in setting the basal threshold of MC response.

The impact of bacterial infection on mast cell degranulation

In developed countries, the prevalence of allergy is on the rise. Although the causes are unknown, it seems that (1) the disappearance of microbiota may play a role in the increase of allergies and (2) exposure to bacterial infections during childhood decreases the incidence of allergies. Although several cell types are involved in the development of allergy, mast cells play a major role in orchestrating inflammation. Upon activation, mast cell secretory granules fuse with the plasma membrane, resulting in the release of a number of inflammatory mediators. In addition to allergy, mast cells contribute to the innate immune response against a variety of bacteria. This is accomplished through the secretion of cytokines and other soluble mediators. Interestingly, there is growing evidence that mast cells exposed to bacteria down-regulate degranulation in response to IgE/Allergen stimulation. This inhibitory effect seems to require direct contact between bacteria and mast cells, but the intracellular mechanism by which bacterial contact suppresses allergic responses is unknown. Here, we review different aspects of mast cell physiology and discuss hypotheses as to how bacteria may influence mast cell degranulation.

Sequential class switching is required for the generation of high affinity IgE antibodies

Generation of anaphylaxis-inducing high affinity IgE requires sequential class switching.

IgE antibodies with high affinity for their antigens can be stably cross-linked at low concentrations by trace amounts of antigen, whereas IgE antibodies with low affinity bind their antigens weakly. In this study, we find that there are two distinct pathways to generate high and low affinity IgE. High affinity IgE is generated through sequential class switching (μ→γ→ε) in which an intermediary IgG phase is necessary for the affinity maturation of the IgE response, where the IgE inherits somatic hypermutations and high affinity from the IgG1 phase. In contrast, low affinity IgE is generated through direct class switching (μ→ε) and is much less mutated. Mice deficient in IgG1 production cannot produce high affinity IgE, even after repeated immunizations. We demonstrate that a small amount of high affinity IgE can cause anaphylaxis and is pathogenic. Low affinity IgE competes with high affinity IgE for binding to Fcε receptors and prevents anaphylaxis and is thus beneficial.

Mast cells and inflammation

Mast cells are well known for their role in allergic and anaphylactic reactions, as well as their involvement in acquired and innate immunity. Increasing evidence now implicates mast cells in inflammatory diseases where they are activated by non-allergic triggers, such as neuropeptides and cytokines, often exerting synergistic effects as in the case of IL-33 and neurotensin. Mast cells can also release pro-inflammatory mediators selectively without degranulation. In particular, IL-1 induces selective release of IL-6, while corticotropin-releasing hormone secreted under stress induces the release of vascular endothelial growth factor. Many inflammatory diseases involve mast cells in cross-talk with T cells, such as atopic dermatitis, psoriasis and multiple sclerosis, which all worsen by stress. How mast cell differential responses are regulated is still unresolved. Preliminary evidence suggests that mitochondrial function and dynamics control mast cell degranulation, but not selective release. Recent findings also indicate that mast cells have immunomodulatory properties. Understanding selective release of mediators could explain how mast cells participate in numerous diverse biologic processes, and how they exert both immunostimulatory and immunosuppressive actions. Unraveling selective mast cell secretion could also help develop unique mast cell inhibitors with novel therapeutic applications. This article is part of a Special Issue entitled: Mast cells in inflammation.

The lectin ArtinM binds to mast cells inducing cell activation and mediator release

Mast cells are inflammatory cells that respond to signals of innate and adaptive immunity with immediate and delayed release of mediators. ArtinM, a lectin from Artocarpus integrifolia with immunomodulatory activities, is able to induce mast cell activation, but the mechanisms remain unknown. This study sought to further investigate the effects of the lectin on mast cells. We showed that ArtinM binds to mast cells, possibly to the high affinity receptor for immunoglobulin E (IgE) - FcεRI - and/or to IgE bound to FcεRI. Binding of the lectin resulted in protein tyrosine phosphorylation and release of the pre- and newly-formed mediators, β-hexosaminidase and LTB(4) by mast cells, activities that were potentiated in the presence of IgE. ArtinM also induced the activation of the transcription factors NFκB and NFAT, resulting in expression of some of their target genes such as IL-4 and TNF-α. In view of the established significance of mast cells in many immunological and inflammatory reactions, a better understanding of the mechanisms involved in mast cell activation by ArtinM is crucial to the pharmacological application of the lectin.

Mast cell interleukin-2 production contributes to suppression of chronic allergic dermatitis

The incidence of chronic allergic dermatitis is rapidly increasing. Regulatory control of this disease has not been adequately explored. Here we report that mast cell-derived interleukin-2 (IL-2) contributes to the suppression of chronic allergic dermatitis. Mice deficient in IL-2 production, or deficient in mast cells (Kit(W-sh/W-sh)), showed exacerbated dermatitis upon repeated oxazolone challenge when compared to their wild-type counterparts. Adoptive transfer of wild-type, but not Il2(-/-), mast cells into Kit(W-sh/W-sh) mice dampened the inflammatory response. During the course of disease, mast cell expansion occurred at the site of inflammation and also in the spleen, where production of IL-2 by mast cells was markedly enhanced. In the absence of mast cell IL-2 production, the ratio of activated to regulatory T cells at the site of inflammation was increased. Thus, MC-derived IL-2 contributes to the maintenance of suppression in chronic allergic skin inflammation.

Mast cells are activated by Leishmania mexicana LPG and regulate the disease outcome depending on the genetic background of the host

The regulatory effect of mast cells on the pathogenesis of leishmaniasis is unclear. We report a comparative analysis of TLR2 membrane expression, TNF-α, IL-10 and MIP-1α production, and granule release of bone marrow-derived mast cells (BMMCs) from susceptible BALB/c and resistant C57BL/6 mice, stimulated in vitro with Leishmania mexicana lipophosphoglycan (LPG). We studied the kinetics of mast cell degranulation and parasite numbers in lesions of both mouse strains infected with L. mexicana. We found that BMMCs of C57BL/6 mice expressed more TLR2 and produced higher levels of both cytokines and MIP-1α, whereas BALB/c BMMCs significantly augmented their granule release. Lesions of BALB/c mice showed higher levels of degranulated mast cells at 3 h of infection, whereas after 3 days of infection, the number of degranulated mast cells in C57BL/6 was higher than in BALB/c lesions. Throughout infection, BALB/c mice harboured more parasites. The regulatory effect of mast cells seems to depend on the genetic background of the host: mast cells of BALB/c mice facilitate disease progression due to an augmented inflammatory response early in the infection, whereas mast cells of C57BL/6 mice produce cytokines that regulate inflammation and maintain an elevated number of immune cells in the lesions, promoting disease control.