Prevention of F‐actin assembly switches the response to SCF from chemotaxis to degranulation in human mast cells

Following antigen/IgE‐mediated aggregation of high affinity IgE‐receptors (FcεRI), mast cells (MCs) degranulate and release inflammatory mediators leading to the induction of allergic reactions including anaphylaxis. Migration of MCs to resident tissues and sites of inflammation is regulated by tissue chemotactic factors such as stem cell factor (SCF (KIT ligand)). Despite inducing similar early signaling events to antigen, chemotactic factors, including SCF, produce minimal degranulation in the absence of other stimuli. We therefore investigated whether processes regulating MC chemotaxis are rate limiting for MC mediator release. To investigate this issue, we disrupted actin polymerization, a requirement for MC chemotaxis, with latrunculin B and cytochalasin B, then examined chemotaxis and mediator release in human (hu)MCs induced by antigen or SCF. As expected, such disruption minimally affected early signaling pathways, but attenuated SCF‐induced human mast cell chemotaxis. In contrast, SCF, in the absence of other stimuli, induced substantial degranulation in a concentration‐dependent manner following actin disassembly. It also moderately enhanced antigen‐mediated human mast cell degranulation which was further enhanced in the presence of SCF. These observations suggest that processes regulating cell migration limit MC degranulation as a consequence of cytoskeletal reorganization.     

The mast cell: an antenna of the microenvironment that directs the immune response

Mast cells (MCs) have long been considered as critical effector cells during immunoglobulin (Ig)E-mediated allergic disease and immune response to parasites. Recent studies, however, suggest that this understanding of MC function is incomplete and does not consider the complex roles that MCs play in adaptive and innate immunity. The added function gives an innovative vision of regulation of immune responses and the development of autoimmune diseases. It had been assumed that the aggregation of Fc epsilon receptor I with IgE and specific antigen is the main stimulus able to induce the MC activation, degranulation, release, and generation of mediators of the allergic reaction. However, MCs exhibit an array of molecules involved in cell-cell and cell-extracellular matrix adhesion, mediating delivery of costimulatory signals that empower those cells with an ability to react to multiple nonspecific and specific stimuli. Their tissue distribution and their capability to release many cytokines after stimulation indicate MCs as potential regulatory linkers between innate and acquired immunity. In this review, we will summarize some findings on the roles of MCs in innate and acquired immunity, on the molecular mechanism and signaling pathways, and on selective signals that induce discrete MC response and its ability to polarize adaptive-immune response.     

Lipoteichoic acid downregulates FcepsilonRI expression on human mast cells through Toll-like receptor 2

BACKGROUND: FcepsilonRI on the surface of mast cells (MCs) plays a central role in allergic responses. Recent evidence shows that exposure to microbial components corresponds with a significant reduction in the risk for allergic diseases. Although many reports suggest that this is due to changes in T-cell functions, how MC functions are altered by bacterial infection remains unknown. OBJECTIVE: We sought to elucidate the effect of bacterial infection on MC function and expression of Fc receptors, such as FcepsilonRI. METHODS: Isolated human pulmonary MCs and a human MC line (LAD2) were stimulated with bacterial components, and the function and surface expression of Fc receptors were measured. RESULTS: Lipoteichoic acid (LTA) and peptidoglycan, but not LPS, flagellin, or 3CpG-oligodeoxynucleotide, reduced the expression of FcepsilonRI on LAD2 cells. An antibody to Toll-like receptor (TLR) 2 partially blocked the effect of LTA but not peptidoglycan. Both LTA and peptidoglycan reduced MC degranulation caused by an antigen-specific IgE. Furthermore, exposure of pulmonary MCs to LTA reduced both FcepsilonRI expression and IgE-induced degranulation. None of the bacterial components affected the expression of other Fc receptors, such as Fcgamma receptors or Fcalpha receptor I. CONCLUSIONS: Our results indicate that LTA reduces the surface expression of FcepsilonRI through TLR2 and suggests that TLR2 ligands could be used as a novel therapy for controlling allergic disorders. CLINICAL IMPLICATIONS: By knowing how bacterial components modulate MC function, we can expand our possibilities for therapeutic interventions of allergic diseases.     

Inhibitory receptors and allergy

Recent studies of the major cell types involved in the initiation and progression of allergic inflammation have revealed that they express an unexpectedly large number of surface receptors that inhibit the release of proinflammatory mediators from mast cells and basophils in vitro. Moreover, analyses of animals deficient in some of these receptors, for example, Fc(gamma)RIIB, gp49B1 and paired Ig-like receptor (PIR)-B, have shown that the molecules indeed suppress allergic responses driven by the adaptive immune response in vivo. These findings support the emerging concept that allergic diseases are caused not only by excessive activation of cells but also from deficiencies in receptors that suppress these activation responses.     

Rapidly changing perspectives about mast cells at mucosal surfaces

Summary: Mast cells (MCs) are major effector cells of immunoglobulin E (IgE)‐mediated allergic inflammation. However, it has become increasingly clear that they also play important roles in diverse physiological and pathological processes. Recent advances have focused on the importance of MCs in both innate and adaptive immune responses and have fostered studies of MCs beyond the myopic focus on allergic reactions. MCs possess a variety of surface receptors and may be activated by inflammatory mediators, IgE, IgG, light chains, complement fragments, proteases, hormones, neuropeptides, and microbial products. Following activation, they produce a plethora of pro‐inflammatory mediators and participate in inflammatory reactions in many organs. This review focuses on the role of MCs in inflammatory reactions in mucosal surfaces with particular emphasis on their role in respiratory and gastrointestinal inflammatory conditions.     

Activated mast cells synthesize and release soluble ST2‐a decoy receptor for IL‐33

IL‐33 released from damaged cells plays a central role in allergic inflammation by acting through its membrane‐bound receptor, ST2 receptor (ST2L). IL‐33 activity can be neutralized by the soluble spliced variant of ST2 (sST2) that has been associated with allergic inflammation but its source is not well defined. We investigated whether mast cells (MCs) are a significant source of sST2 following activation through FcεRI or ST2. We find that antigen and IL‐33 induce substantial production and release of sST2 from human and mouse MCs in culture and do so synergistically when added together or in combination with stem cell factor. Moreover, increases in circulating sST2 during anaphylaxis in mice were dependent on the presence of MCs. Human MCs activated via FcεRI failed to generate IL‐33 and IL‐33 produced by mouse bone marrow‐derived MCs was retained within the cells. Therefore, FcεRI‐mediated sST2 production is independent of MC‐derived IL‐33 acting in an autocrine manner. These results are consistent with the conclusion that both mouse and human MCs when activated are a significant inducible source of sST2 but not IL‐33 and thus have the ability to modulate the biologic impact of IL‐33 produced locally by other cell types during allergic inflammation.     

Mast cell mediator responses and their suppression by pathogenic and commensal microorganisms

Mast cells (MCs) are selectively found at the host environment interface and are capable of secreting a wide array of pharmacologically active mediators, many of which are prepackaged in granules. Over the past two decades, it has become clear that these cells have the capacity to recognize a range of infectious agents allowing them to play a key role in initiating and modulating early immune responses to infectious agents. However, a number of pathogenic and commensal microbes appear to have evolved distinct mechanisms to suppress MC mediator release to avoid elimination in the host. Understanding how these microbes suppress MC functions may have significant therapeutic value to relieve inflammatory disorders mediated by MCs.     

Novel Strategies to Target Mast Cells in Disease

Mast cells (MCs) are versatile effector cells of the immune system, characterized by a large content of secretory granules containing a variety of inflammatory mediators. They are implicated in the host protection toward various external insults, but are mostly well known for their detrimental impact on a variety of pathological conditions, including allergic disorders such as asthma and a range of additional disease settings. Based on this, there is currently a large demand for therapeutic regimens that can dampen the detrimental impact of MCs in these respective pathological conditions. This can be accomplished by several strategies, including targeting of individual mediators released by MCs, blockade of receptors for MC-released compounds, inhibition of MC activation, limiting mast cell growth or by inducing mast cell apoptosis. Here, we review the currently available and emerging regimens to interfere with harmful mast cell activities in asthma and other pathological settings and discuss the advantages and limitations of such strategies.     

Inhibition of human cord blood-derived mast cell responses by anti-Fc epsilon RI mAb 15/1 versus anti-IgE Omalizumab

Aggregation of the alpha-chain of the high affinity IgE receptor (Fc epsilon RI alpha) on mast cells or basophils after cross-linking of receptor-bound IgE by its antigen or an anti-IgE antibody results in cell activation and release of inflammatory mediators. Omalizumab (Xolair), Novartis Pharmaceuticals; Genentech Inc.) is a recombinant humanized anti-IgE mAb developed for the treatment of severe allergic asthma. It complexes with free serum IgE, which prevents its binding to Fc epsilon RI and thereby interrupts the allergic cascade. Administration of an inhibitory anti-Fc epsilon RI alpha mAb may represent an alternative strategy to neutralize IgE-mediated receptor activation. In the present report, for the first time, we have performed direct side of side comparison between the inhibitory anti-Fc epsilon RI alpha mAb designated 15/1 and Omalizumab for their effects on human cord blood-derived mast cells. We provide the first evidence that both 15/1 mAb and Omalizumab efficiently inhibit Fc epsilon RI-mediated human mast cell responses in vitro (degranulation, activation, release of IL-8 and IL-13, phosphorylation of Akt) and that mAb 15/1 is a non-anaphylactogenic antibody, which compared to Omalizumab, displays markedly higher inhibitory potency in the presence of high IgE levels.     

S100A12 provokes mast cell activation: a potential amplification pathway in asthma and innate immunity

BACKGROUND: The calcium-binding protein S100A12 might provoke inflammation and monocyte recruitment through the receptor for advanced glycation end products. OBJECTIVE: Because inflammation elicited by S100A12 in vivo had characteristics of mast cell (MC) activation, we aimed to define the mechanism. METHODS: Various MC populations were used to test S100A12 activation assessed on the basis of morphology, histamine release, leukotriene production, and cytokine induction. MC dependence of S100A12-provoked inflammation was tested in mice and on the rat microcirculation by means of intravital microscopy. Immunohistochemistry localized S100A12 in the asthmatic lung, and levels in sputum from asthmatic patients were quantitated by means of ELISA. Expression of the receptor for advanced glycation end products was evaluated by means of RT-PCR and Western blotting. RESULTS: S100A12 provoked degranulation of mucosal and tissue MCs in vitro and in vivo and amplified IgE-mediated responses. It induced a cytokine profile indicating a role in innate/T(H)1-mediated responses. S100A12-induced edema and leukocyte rolling, adhesion, and transmigration in the microcirculation were MC dependent. Eosinophils in airway tissue from asthmatic patients were S100A12 positive, and levels were increased in sputum. S100A12 responses were partially blocked by an antagonist to the receptor for advanced glycation end products, but MCs did not express mRNA or protein, suggesting an alternate receptor. CONCLUSION: This novel pathway highlights the potential importance of S100A12 in allergic responses and in infectious and chronic inflammatory diseases. CLINICAL IMPLICATIONS: MC activation by S100A12 might exacerbate allergic inflammation and asthma. S100A12 might provide a novel marker for eosinophilic asthma.     

The role of mast cells in host defense and their subversion by bacterial pathogens

Mast cells (MCs) play a prominent role in the early immune response to invading pathogenic bacteria. This newly discovered role for MCs involves the release of chemoattractants that recruit neutrophils and the direct phagocytosis and killing of opsonized bacteria. Whereas these activities are clearly beneficial to the host, certain pathogens have evolved mechanisms to evoke anomalous MC responses to the detriment of the host. These include evoking phagocytosis without killing of unopsonized bacteria and the production of toxins that corrupt the release of mediators by MCs. Elucidating how pathogens subvert the activities of MCs could provide clues to limiting the pathological activities of these cells during infectious diseases.     

H4 histamine receptor mediates optimal migration of mast cell precursors to CXCL12

BACKGROUND: CXCL12, a constitutive chemokine (ligand of CXCR4 and CXCR7), is expressed in the skin and airway epithelium and plays a significant role in allergic airway diseases. The pleiotropic effects of CXCL12 are enhanced by cofactors specific to the target cell. OBJECTIVE: We hypothesized that histamine, a major mediator of allergic reactions, could interact with CXCL12 to promote human mast cell (MC) migration. METHODS: The chemotactic effects of CXCL12 alone or in combination with histamine were evaluated on human precursor and mature MCs by using in vitro migration assays. RESULTS: CXCL12 exerts a chemotactic activity on both precursor and fully mature MCs. Histamine and supernatants from IgE-activated MCs enhanced CXCL12 chemotactic activity on the precursor MC population. The synergy between histamine and CXCL12 was not observed with mature MCs, CD4(+) T cells, and monocytes. Inhibition of histamine receptors pharmacologically or with specific small interfering RNA (siRNA) indicated that synergy between histamine and CXCL12 required the H(4) receptor. CONCLUSION: Histamine released by allergen-activated mature MCs might promote MC-rich allergic inflammation by enhancing recruitment of their precursors in tissues constitutively expressing CXCL12, including skin and airways. CLINICAL IMPLICATIONS: This work highlights a novel role of the H(4) receptor in the perpetuation of allergic responses and provides evidence for the utility of H(4) receptor antagonists in the treatment of allergic diseases.     

Potential therapeutic use of IL-37: a key suppressor of innate immunity and allergic immune responses mediated by mast cells

The host response to either exogenous or endogenous insults produces a series of changes, characterized by alterations in immunological functions and generation of mediators called cytokines which include the interleukin-1 (IL-1) family members. IL-1 acts as a hormone mediating the host responses to infection and inflammation. Blocking inflammatory IL-1 family members can be effective against inflammatory disorders, including allergies. IL-37, (formerly IL-1 family member 7), emerges as an inhibitor of innate and adaptive immunity by reducing circulating and organ cytokine levels. IL-37, mainly expressed in dendritic cells, monocytes, and plasma cells after TIR ligand activation, inhibits inflammatory cytokines and augments the level of anti-inflammatory IL-10. IL-37 is involved in allergic reaction and its expression in dendritic cells causes tollerogenicity and inhibits inflammatory response. Mast cells (MCs) are ubiquitous in the body, reside in numerous mucosal tissues, and are mediators of allergic reaction, and innate and adaptive immunity. MCs are important regulators of cytokine generation in the course of inflammatory responses and allergy, and are implicated in the pathophysiology of allergic asthma. Cysteine protease caspase-1 activation leads to the cleavage of pro-form of IL-1 into active mature IL-1 which is present in stimulated and unstimulated inflammatory MCs. Inflammatory cytokine inhibition, along with the augmentation of anti-inflammatory IL-10 by IL-37, is certainly beneficial and improves the pathogenesis of allergic disorders. However, in these studies, the exact mechanism(s) of IL-37-induced anti-inflammatory and anti-allergic activity along with its side effect(s) remain to be determined.     

Mast cell plasticity and sphingosine-1-phosphate in immunity,inflammation and cancer

Mast cells (MC) are found in all vascularized tissues at homeostasis and, until recently, were viewed only as effector cells of allergic reactions via degranulation, the canonical process through which MC release mediators, including histamine and pre-formed proteases and cytokines such as TNF. Cross-linking of IgE bound to surface high affinity receptors for IgE (Fc?RI) by a specific antigen (Ag) triggers signaling events leading to degranulation. We and others have reported the concomitant production and export of an influential multifaceted sphingolipid mediator, sphingosine-1-phosphate (S1P) transported outside of MC by ATP-binding cassettes (ABC) transporters, i.e., independently of degranulation. Indeed, the MC horizon expanded by the discovery of their unique ability to selectively release mediators depending upon the stimulus and receptors involved. Aside from degranulation and transporter usage, MC are also endowed with piecemeal degranulation, a slower process during which mediator release occurs with minor morphological changes. The broad spectrum of pro- and anti-inflammatory bioactive substances MC produce and release, their amounts and delivery pace render these cells bona fide fine-tuners of the immune response. In this viewpoint article, MC developmental, phenotypic and functional plasticity, its modulation by microRNAs and its relevance to immunity, inflammation and cancer will be discussed.     

Calcineurin Aα Contributes to IgE-Dependent Mast-Cell Mediator Secretion in Allergic Inflammation

Mast cells (MCs) are key mediators of allergic inflammation through the activation of cross-linked immunoglobulin E (IgE) bound to the high-affinity IgE receptor (FcϵRI) on the cell surface, leading to the release of biologically potent mediators, either from preformed granules or newly synthesized. Pharmacological inhibitors have been developed to target a key signaling protein phosphatase in this pathway, calcineurin, yet there is a lack of genetic and definitive evidence for the various isoforms of calcineurin subunits in FcϵRI-mediated responses. In this study, we hypothesized that deficiency in the calcineurin Aα isoform will result in a decreased allergic immune response by the MCs. In a model of passive cutaneous anaphylaxis, there was a reduction in vascular permeability in MC-deficient mouse tissues reconstituted with calcineurin subunit A (CnAα) gene-knockout (CnAα^−/−) MCs, and in vitro experiments identified a significant reduction in release of preformed mediators from granules. Furthermore, released levels of de novo synthesized cytokines were reduced upon FcϵRI activation of CnAα^−/− MCs in vitro. Characterizing the mechanisms associated with this deficit response, we found a significant impairment of nuclear factor of kappa light polypeptide gene enhancer in B cell phosphorylation and impaired nuclear factor kappa-light-chain-enhancer of activated B-cell inhibitor alpha (NF-κB) activation. Thus, we concluded that CnAα contributes to the release of preformed mediators and newly synthesized mediators from FcϵRI-mediated activation of MCs, and this regulation includes NF-κB signaling.     

Foreign body response to subcutaneous biomaterial implants in a mast cell-deficient Kitw-Sh murine model

Mast cells (MCs)_are recognized for their functional role in wound-healing and allergic and inflammatory responses – host responses that are frequently detrimental to implanted biomaterials if extended beyond acute reactivity. These tissue reactions impact especially on the performance of sensing implants such as continuous glucose monitoring (CGM) devices. Our hypothesis that effective blockade of MC activity around implants could alter the host foreign body response (FBR) and enhance the in vivo lifetime of these implantable devices motivated this study. Stem cell factor and its ligand c-KIT receptor are critically important for MC survival, differentiation and degranulation. Therefore, an MC-deficient sash mouse model was used to assess MC relationships to the in vivo performance of CGM implants. Additionally, local delivery of a tyrosine kinase inhibitor (TKI) that inhibits c-KIT activity was also used to evaluate the role of MCs in modulating the FBR. Model sensor implants comprising polyester fibers coated with a rapidly dissolving polymer coating containing drug-releasing degradable microspheres were implanted subcutaneously in sash mice for various time points, and the FBR was evaluated for chronic inflammation and fibrous capsule formation around the implants. No significant differences were observed in the foreign body capsule formation between control and drug-releasing implant groups in MC-deficient mice. However, fibrous encapsulation was significantly greater around the drug-releasing implants in sash mice compared to drug-releasing implants in wild-type (e.g. MC-competent) mice. These results provide insights into the role of MCs in the FBR, suggesting that MC deficiency provides alternative pathways for host inflammatory responses to implanted biomaterials.     

The mast cell and allergic diseases: role in pathogenesis and implications for therapy

Mast cells have long been recognized for their role in the genesis of allergic inflammation; and more recently for their participation in innate and acquired immune responses. Mast cells reside within tissues including the skin and mucosal membranes, which interface with the external environment; as well as being found within vascularized tissues next to nerves, blood vessels and glandular structures. Mast cells have the capability of reacting both within minutes and over hours to specific stimuli, with local and systemic effects. Mast cells express the high affinity IgE receptor (Fc?RI) and upon aggregation of Fc?RI by allergen‐specific IgE, mast cells release and generate biologically active preformed and newly synthesized mediators which are involved in many aspects of allergic inflammation. While mast cells have been well documented to be essential for acute allergic reactions, more recently the importance of mast cells in reacting through pattern recognition receptors in innate immune responses has become recognized. Moreover, as our molecular understanding of the mast cell has evolved, novel targets for modulation have been identified with promising therapeutic potential.     

Influence of aging on mast cell production of a nitric oxide-like factor: Comparison between normal and hypertensive rats

Rat serosal mast cells (MCs) isolated from Wistar albino rats synthesize a nitric oxide (NO)-like factor which inhibits platelet aggregation, stimulates guanylate cyclase and modulates mast cell histamine release. Aging can affect the ability of MCs to generate and release chemical mediators.

The aim of the present study was to evaluate the ability to generate a NO-like factor by MCs isolated from 2, 6 and 18 month old normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SH) rats (systolic blood pressure 180±2.5 mm Hg) by two bioassays: inhibition of platelet aggregation and stimulation of MC guanylate cyclase.

MCs from SH rats produced less NO-like activity than MCs from normotensive WKY rats of the same age. In both strains aging decreased the capacity of MCs to generate the NO-like factor. These results suggest that NO synthesis and/or release from MCs is involved in some events associated with aging and hypertension.