Mast-cell responses to pathogens

Mast Cells in Inflammation and Disease: Recent Progress and Ongoing Concerns

Mast cells have existed long before the development of adaptive immunity, although they have been given different names. Thus, in the marine urochordate Styela plicata, they have been designated as test cells. However, based on their morphological characteristics (including prominent cytoplasmic granules) and mediator content (including heparin, histamine, and neutral proteases), test cells are thought to represent members of the lineage known in vertebrates as mast cells. So this lineage presumably had important functions that preceded the development of antibodies, including IgE. Yet mast cells are best known, in humans, as key sources of mediators responsible for acute allergic reactions, notably including anaphylaxis, a severe and potentially fatal IgE-dependent immediate hypersensitivity reaction to apparently harmless antigens, including many found in foods and medicines. In this review, we briefly describe the origins of tissue mast cells and outline evidence that these cells can have beneficial as well as detrimental functions, both innately and as participants in adaptive immune responses. We also discuss aspects of mast cell heterogeneity and comment on how the plasticity of this lineage may provide insight into its roles in health and disease. Finally, we consider some currently open questions that are yet unresolved.

Molecular Mechanisms of Mast Cell Activation by Cholesterol-Dependent Cytolysins

Mast cells are potent immune sensors of the tissue microenvironment. Within seconds of activation, they release various preformed biologically active products and initiate the process of de novo synthesis of cytokines, chemokines, and other inflammatory mediators. This process is regulated at multiple levels. Besides the extensively studied IgE and IgG receptors, toll-like receptors, MRGPR, and other protein receptor signaling pathways, there is a critical activation pathway based on cholesterol-dependent, pore-forming cytolytic exotoxins produced by Gram-positive bacterial pathogens. This pathway is initiated by binding the exotoxins to the cholesterol-rich membrane, followed by their dimerization, multimerization, pre-pore formation, and pore formation. At low sublytic concentrations, the exotoxins induce mast cell activation, including degranulation, intracellular calcium concentration changes, and transcriptional activation, resulting in production of cytokines and other inflammatory mediators. Higher toxin concentrations lead to cell death. Similar activation events are observed when mast cells are exposed to sublytic concentrations of saponins or some other compounds interfering with the membrane integrity. We review the molecular mechanisms of mast cell activation by pore-forming bacterial exotoxins, and other compounds inducing cholesterol-dependent plasma membrane perturbations. We discuss the importance of these signaling pathways in innate and acquired immunity.

Mast cells modulate early responses to Mycobacterium bovis Bacillus Calmette-Guerin by phagocytosis and formation of extracellular traps

The early interactions between the vaccine Mycobacterium bovis Bacillus Calmette Guerin (BCG) and host peripheral innate immune cells like Mast cells (MCs) may pave the way for generating appropriate protective innate and adaptive immune responses. Mice on administration of BCG by intratracheal instillation showed a massive increase in MC numbers in the infected lung. In vitro co-culture of BCG and rodent Rat Basophilic Leukaemia (RBL-2H3) MCs led to significant killing of BCG. RBL-2H3 MCs were able to phagocytose BCG, take up BCG-derived antigens by macropinocytosis, generate Reactive Oxygen Species (ROS) and degranulate. Further, a few MCs died and released MC extracellular traps (MCETs) having DNA, histones and tryptase to trap BCG. This study highlights the multi-pronged effector responses of MCs on encountering BCG. These responses or their evasion may lead to success or failure of BCG vaccine to provide long term immunity to infections.

IgE-activated mast cells enhance TLR4-mediated antigen-specific CD4+ T cell responses

Mast cells are potent mediators of allergy and asthma, yet their role in regulating adaptive immunity remains ambiguous. On the surface of mast cells, the crosslinking of IgE bound to FcεRI by a specific antigen recognized by that IgE triggers the release of immune mediators such as histamine and cytokines capable of activating other immune cells; however, little is known about the mast cell contribution to the induction of endogenous, antigen-specific CD4⁺ T cells. Here we examined the effects of specific mast cell activation in vivo on the initiation of an antigen-specific CD4⁺ T cell response. While CD4⁺ T cells were not enhanced by FcεRI stimulation alone, their activation was synergistically enhanced when FcεRI activation was combined with TLR4 stimulation. This enhanced activation was dependent on global TLR4 stimulation but appeared to be less dependent on mast cell expressed TLR4. This study provides important new evidence to support the role of mast cells as mediators of the antigen-specific adaptive immune response.

MYO1F Regulates IgE and MRGPRX2-Dependent Mast Cell Exocytosis

The activation and degranulation of mast cells is critical in the pathogenesis of allergic inflammation and modulation of inflammation. Recently, we demonstrated that the unconventional long-tailed myosin, MYO1F, localizes with cortical F-actin and mediates adhesion and migration of mast cells. In this study, we show that knockdown of MYO1F by short hairpin RNA reduces human mast cell degranulation induced by both IgE crosslinking and by stimulation of the Mas-related G protein-coupled receptor X2 (MRGPRX2), which has been associated with allergic and pseudoallergic drug reactions, respectively. Defective degranulation was accompanied by a reduced reassembly of the cortical actin ring after activation but reversed by inhibition of actin polymerization. Our data show that MYO1F is required for full Cdc42 GTPase activation, a critical step in exocytosis. Furthermore, MYO1F knockdown resulted in less granule localization in the cell membrane and fewer fissioned mitochondria along with deficient mitochondria translocation to exocytic sites. Consistent with that, AKT and DRP1 phosphorylation are diminished in MYO1F knockdown cells. Altogether, our data point to MYO1F as an important regulator of mast cell degranulation by contributing to the dynamics of the cortical actin ring and the distribution of both the secretory granules and mitochondria.

Mast Cells in the Skin: Defenders of Integrity or Offenders in Inflammation?

Mast cells (MCs) are best-known as key effector cells of immediate-type allergic reactions that may even culminate in life-threatening anaphylactic shock syndromes. However, strategically positioned at the host–environment interfaces and equipped with a plethora of receptors, MCs also play an important role in the first-line defense against pathogens. Their main characteristic, the huge amount of preformed proinflammatory mediators embedded in secretory granules, allows for a rapid response and initiation of further immune effector cell recruitment. The same mechanism, however, may account for detrimental overshooting responses. MCs are not only detrimental in MC-driven diseases but also responsible for disease exacerbation in other inflammatory disorders. Focusing on the skin as the largest immune organ, we herein review both beneficial and detrimental functions of skin MCs, from skin barrier integrity via host defense mechanisms to MC-driven inflammatory skin disorders. Moreover, we emphasize the importance of IgE-independent pathways of MC activation and their role in sustained chronic skin inflammation and disease exacerbation.

Mast Cells and Skin and Breast Cancers: A Complicated and Microenvironment-Dependent Role

Mast cells are important sentinel cells in host defense against infection and major effector cells in allergic disease. The role of these cells in cancer settings has been widely debated. The diverse range of mast cell functions in both immunity and tissue remodeling events, such as angiogenesis, provides multiple opportunities for mast cells to modify the tumor microenvironment. In this review, we consider both skin and breast cancer settings to address the controversy surrounding the importance of mast cells in the host response to tumors. We specifically address the key mediators produced by mast cells which impact tumor development. The role of environmental challenges in modifying mast cell responses and opportunities to modify mast cell responses to enhance anti-tumor immunity are also considered. While the mast cell's role in many cancer contexts is complicated and poorly understood, the activities of these tissue resident and radioresistant cells can provide important opportunities to enhance anti-cancer responses and limit cancer development.

Innate Immune Cells in the Adipose Tissue in Health and Metabolic Disease

Metabolic disorders, such as obesity, type 2 diabetes mellitus, and nonalcoholic fatty liver disease, are characterized by chronic low-grade tissue and systemic inflammation. During obesity, the adipose tissue undergoes immunometabolic and functional transformation. Adipose tissue inflammation is driven by innate and adaptive immune cells and instigates insulin resistance. Here, we discuss the role of innate immune cells, that is, macrophages, neutrophils, eosinophils, natural killer cells, innate lymphoid type 2 cells, dendritic cells, and mast cells, in the adipose tissue in the healthy (lean) and diseased (obese) state and describe how their function is shaped by the obesogenic microenvironment, and humoral, paracrine, and cellular interactions. Moreover, we particularly outline the role of hypoxia as a central regulator in adipose tissue inflammation. Finally, we discuss the long-lasting effects of adipose tissue inflammation and its potential reversibility through drugs, caloric restriction, or exercise training.

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.

Intracranial mast cells contribute to the control of social behavior in male mice

Mast cells (MCs) exist intracranially and have been reported to affect higher brain functions in rodents. However, the role of MCs in the regulation of emotionality and social behavior is unclear. In the present study, using male mice, we examined the relationship between MCs and social behavior and investigated the underlying mechanisms. Wild-type male mice intraventricularly injected with a degranulator of MCs exhibited a marked increase in a three-chamber sociability test. In addition, removal of MCs in Mast cell-specific Toxin Receptor-mediated Conditional cell Knock out (Mas-TRECK) male mice showed reduced social preference levels in a three-chamber sociability test without other behavioral changes, such as anxiety-like and depression-like behavior. Mas-TRECK male mice also had reduced serotonin content and serotonin receptor expression and increased oxytocin receptor expression in the brain. These results suggested that MCs may contribute to the regulation of social behavior in male mice. This effect may be partially mediated by serotonin derived from MCs in the brain.

A basophil-neuronal axis promotes itch

Itch is an evolutionarily conserved sensation that facilitates expulsion of pathogens and noxious stimuli from the skin. However, in organ failure, cancer, and chronic inflammatory disorders such as atopic dermatitis (AD), itch becomes chronic, intractable, and debilitating. In addition to chronic itch, patients often experience intense acute itch exacerbations. Recent discoveries have unearthed the neuroimmune circuitry of itch, leading to the development of anti-itch treatments. However, mechanisms underlying acute itch exacerbations remain overlooked. Herein, we identify that a large proportion of patients with AD harbor allergen-specific immunoglobulin E (IgE) and exhibit a propensity for acute itch flares. In mice, while allergen-provoked acute itch is mediated by the mast cell-histamine axis in steady state, AD-associated inflammation renders this pathway dispensable. Instead, a previously unrecognized basophil-leukotriene (LT) axis emerges as critical for acute itch flares. By probing fundamental itch mechanisms, our study highlights a basophil-neuronal circuit that may underlie a variety of neuroimmune processes.

Tryptase- and ghrelin positive mast cells in the interalveolar septa of rat’s lung

The mast cell mediators and distribution of lung mast cells in rats are often discussed in experimental studies on pulmonary fibrotic and allergic processes associated with changes in numbers of these cells, but information on the normal distribution of metachromatic and tryptase-positive mast cells in the interalveolar septa is scarce. There are no data on the presence of ghrelin in lung mast cells as well as the age-specific features of localisation and the number of mast cells in the interalveolar septa in rats of different ages. Therefore, the purpose of the present study was to determine the distribution of metachromatic, tryptase-, and ghrelin-positive mast cells in the interalveolar septa in 20 day-, 3 month- and 1 year-old rats. Tissue sections stained with toluidine blue had been taken from the left lung to visualise metachromasia and immunohistochemical expression of tryptase and ghrelin. The results showed that the amount of metachromatic mast cells in the interalveolar septa was significantly lower than that of tryptase- and ghrelin-positive cells. This allowed suggesting that mast cells were permanent occupants of the rat lung parenchyma and, on the other hand, the expression of ghrelin in their granules was most likely related to the synthesis of this protein. Our study showed that immunohistochemical identification by tryptase expression was more accurate than toluidine blue staining.

B regulatory cells in allergy

B cells have classically been recognized for their unique and indispensable role in the production of antibodies. Their potential as immunoregulatory cells with anti-inflammatory functions has received increasing attention during the last two decades. Herein, we highlight pioneering studies in the field of regulatory B cell (Breg) research. We will review the literature on Bregs with a particular focus on their role in the regulation of allergic inflammation.

MRGPRX2 is critical for clozapine induced pseudo-allergic reactions

BACKGROUND

Clozapine is one of the most widely used second-generation antipsychotics in clinic. However, allergy-like symptoms such as rash and angioedema have been reported frequently, and the mechanism is still not clear. Mas-related G protein-coupled receptor X2 (MRGPRX2) expressed on mast cells is a crucial receptor for drug induced pseudo-allergic reactions. Therefore, we explored whether the symptoms induced by clozapine were associated with allergic reaction through MRGPRX2.

METHODS

The effects of clozapine on pseudo-allergic reactions were evaluated by mast cells degranulation and calcium mobilization assay in vitro, and mice hindpaw swelling, serum histamine detection, avidin and H&E staining assay in vivo. The overexpressed MRGPRX2 cells membrane chromatography (MRGPRX2-HEK293/CMC), MRGPRX2-HEK293 cells calcium mobilization assay and molecular docking were applied to research the correlation between clozapine and MRGPRX2.

RESULTS

The study showed that clozapine induced the release of β-hexosaminidase, histamine and monocyte chemoattractant protein-1 (MCP-1), and trigged calcium mobilization in mast cells. In vivo, clozapine induced paw swelling, degranulation and vasodilation. Furthermore, clozapine could activate the calcium mobilization obviously in MRGPRX2-HEK293 cells, not in NC-HEK293 cells. Clozapine also had a good retention characteristic on MRGPRX2-HEK293/CMC column and the K _D value is (2.33 ± 0.21)×10^-01M.

CONCLUSIONS

Our findings demonstrated that clozapine could induce pseudo-allergic reactions and MRGPRX2 might be the critical receptor for it.

Mast Cell Functions Linking Innate Sensing to Adaptive Immunity

Although mast cells (MCs) are known as key drivers of type I allergic reactions, there is increasing evidence for their critical role in host defense. MCs not only play an important role in initiating innate immune responses, but also influence the onset, kinetics, and amplitude of the adaptive arm of immunity or fine-tune the mode of the adaptive reaction. Intriguingly, MCs have been shown to affect T-cell activation by direct interaction or indirectly, by modifying the properties of antigen-presenting cells, and can even modulate lymph node-borne adaptive responses remotely from the periphery. In this review, we provide a summary of recent findings that explain how MCs act as a link between the innate and adaptive immunity, all the way from sensing inflammatory insult to orchestrating the final outcome of the immune response.

MRGPRX2 sensing of cationic compounds-A bridge between nociception and skin diseases?

Mast cells are innate immune cells located at many barrier sites in the body and known to protect the host against environmental threats and to be involved in allergic diseases. More recently, new studies have investigated their roles in the regulation of skin inflammation and transmission of pain and itch sensations. Mast cell signalling through the Mas-related G protein-coupled receptor (MRGPR) X2 or its mouse orthologue MRGPRB2 has been reported to be one of the major mechanism by which mast cell can regulate such processes. MRGPRX2 and MRGPRB2 can induce mast cell degranulation upon binding to a broad panel of cationic molecules such as neuropeptides, bacteria-derived quorum sensing molecules, venom peptides, host defense peptides and, unfortunately, various FDA-approved drugs. Upon activation, mast cells release granule-associated proteases, lipids and multiple cytokines that can modulate vascular permeability, immune cells recruitment and activation status of tissue-projecting nociceptive sensory neurons (ie nociceptors). Here, we discuss the modality of MRGPRX2-dependent mast cell activation and its different consequences on the patterns of skin inflammation and associated diseases. We notably emphasize how MRGPRX2-dependent skin mast cell activation might trigger various pathological traits such as pruritus, pain and inflammation and therefore become a potential therapeutic target for inflammatory pain, itch, atopic dermatitis and drugs-induced injection site reactions.

Nonlethal Plasmodium yoelii Infection Drives Complex Patterns of Th2-Type Host Immunity and Mast Cell-Dependent Bacteremia

Malaria strongly predisposes to bacteremia, which is associated with sequestration of parasitized red blood cells and increased gastrointestinal permeability. The mechanisms underlying this disruption are poorly understood. Here, we evaluated the expression of factors associated with mast cell activation and malaria-associated bacteremia in a rodent model. C57BL/6J mice were infected with Plasmodium yoeliiyoelli 17XNL, and blood and tissues were collected over time to assay for circulating levels of bacterial 16S DNA, IgE, mast cell protease 1 (Mcpt-1) and Mcpt-4, Th1 and Th2 cytokines, and patterns of ileal mastocytosis and intestinal permeability.

Malaria strongly predisposes to bacteremia, which is associated with sequestration of parasitized red blood cells and increased gastrointestinal permeability. The mechanisms underlying this disruption are poorly understood. Here, we evaluated the expression of factors associated with mast cell activation and malaria-associated bacteremia in a rodent model. C57BL/6J mice were infected with Plasmodium yoeliiyoelli 17XNL, and blood and tissues were collected over time to assay for circulating levels of bacterial 16S DNA, IgE, mast cell protease 1 (Mcpt-1) and Mcpt-4, Th1 and Th2 cytokines, and patterns of ileal mastocytosis and intestinal permeability. The anti-inflammatory cytokines (interleukin-4 [IL-4], IL-6, and IL-10) and MCP-1/CCL2 were detected early after P. yoeliiyoelii 17XNL infection. This was followed by the appearance of IL-9 and IL-13, cytokines known for their roles in mast cell activation and growth-enhancing activity as well as IgE production. Later increases in circulating IgE, which can induce mast cell degranulation, as well as Mcpt-1 and Mcpt-4, were observed concurrently with bacteremia and increased intestinal permeability. These results suggest that P. yoeliiyoelii 17XNL infection induces the production of early cytokines that activate mast cells and drive IgE production, followed by elevated IgE, IL-9, and IL-13 that maintain and enhance mast cell activation while disrupting the protease/antiprotease balance in the intestine, contributing to epithelial damage and increased permeability.

Safety, Immunogenicity, and Effectiveness of Defective Viral Particles Arising in Mast Cells Against Influenza in Mice

Mast cells play pivotal roles in the pathogenesis of influenza A virus (IAV) infections. Defective viral particles (DPs) often arise during IAV replication, which can interfere with the replication of infectious viruses and stimulate the antiviral response of host cells. Therefore, DPs are expected to have immune-protective functions in clinic. However, the potent immunogenicity and effectiveness of DPs arising in mast cells during IAV replication have not been reported. In the present study, we showed that DPs generated in the human mastocytoma cell line HMC-1 following H1N1 infection were safe to mice after vaccination. Compared with lung adenocarcinoma cells, A549, DPs generated in infected mast cells had much better immunostimulatory activity, enhancing both humoral and cellular immunity of hosts. Notably, they could significantly increase the expression of immune-associated cytokines, especially the IFN-γ. Due to the robust immunogenicity, thus DPs generated in infected mast cells could stimulate the robust protective immune reaction effectively to fight against lethal IAV re-challenge after vaccination, which result in the high survival, decreased lung injury as well as inhibition of viral replication and inflammatory response in lungs. This study is the first to illustrate and explore the safety, immunogenicity, and effectiveness of DPs arising in mast cells against influenza as favorable potential vaccination. The results provide insight into the advances of new prophylactic strategies to fight influenza by focusing on DPs generated in mast cells.

Defective Viral Particles Produced in Mast Cells Can Effectively Fight Against Lethal Influenza A Virus

Mast cells play an important role in the pathogenesis of highly pathogenic H5N1 avian influenza virus (H5N1-HPAIV) infection. Defective viral particles (DPs) can interfere with the replication of infectious viruses and stimulate the innate immune response of host cells. However, DPs arising from mast cells during HPAIV replication and their potent antiviral actions has not been reported. Here, we showed that the human mastocytoma cell line, HMC-1, allowed for the productive replication of the H5N1-HPAIV. Compared with alveolar cell line A549, DPs were propagated preferentially and abundantly in mast cells following IAV infection, which can be attributed to the wide existence of Argonaute 2 (AGO2) in HMC-1 cells. In addition, DPs generated in H5N1-infected cells could provide great therapeutic protection on mice to fight against various influenza A viruses, which included not only homologous H5N1-HPAIV, but also heterologous H1N1, H3N2, H7N2, and H9N2. Importantly, DPs generated in H5N1-infected HMC-1 cells could diminish viral virulence in vivo and in vitro by triggering a robust antiviral response through type II interferon signaling pathways. This study is the first to illustrate the arising of DPs in H5N1-HPAIV infected mast cells and explore their favorable ability to protect mice from influenza A viruses infection, which provides a novel insight and valuable information for the progress of new strategies to fight influenza A viruses infection, especially highly pathogenic avian influenza virus infection by focusing on the DPs generated in mast cells.

Mast Cell Regulation and Irritable Bowel Syndrome: Effects of Food Components with Potential Nutraceutical Use

Mast cells are key actors in inflammatory reactions. Upon activation, they release histamine, heparin and nerve growth factor, among many other mediators that modulate immune response and neuron sensitization. One important feature of mast cells is that their population is usually increased in animal models and biopsies from patients with irritable bowel syndrome (IBS). Therefore, mast cells and mast cell mediators are regarded as key components in IBS pathophysiology. IBS is a common functional gastrointestinal disorder affecting the quality of life of up to 20% of the population worldwide. It is characterized by abdominal pain and altered bowel habits, with heterogeneous phenotypes ranging from constipation to diarrhea, with a mixed subtype and even an unclassified form. Nutrient intake is one of the triggering factors of IBS. In this respect, certain components of the daily food, such as fatty acids, amino acids or plant-derived substances like flavonoids, have been described to modulate mast cells' activity. In this review, we will focus on the effect of these molecules, either stimulatory or inhibitory, on mast cell degranulation, looking for a nutraceutical capable of decreasing IBS symptoms.

Endogenous sulfur dioxide is a novel inhibitor of hypoxia-induced mast cell degranulation

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Endogenous SO₂/AAT pathway exists in mast cells (MCs).

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Endogenous SO₂ is a novel MC membrane stabilizer under hypoxic circumstance.

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MC-derived SO₂ upregulates cAMP level, thereby suppressing MC degranulation.

Introduction

Mast cell (MC) degranulation is an important step in the pathogenesis of inflammatory reactions and allergies; however, the mechanism of stabilizing MC membranes to reduce their degranulation is unclear.

Methods

SO₂ content in MC culture supernatant was measured by HPLC-FD. The protein and mRNA expressions of the key enzymes aspartate aminotransferase 1 (AAT1) and AAT2 and intracellular AAT activity were detected. The cAMP level in MCs was detected by immunofluorescence and ELISA. The release rate of MC degranulation marker β-hexosaminidase was measured. The expression of AAT1 and cAMP, the MC accumulation and degranulation in lung tissues were detected.

Objectives

To exam whether an endogenous sulfur dioxide (SO₂) pathway exists in MCs and if it serves as a novel endogenous MC stabilizer.

Results

We firstly show the existence of the endogenous SO₂/AAT pathway in MCs. Moreover, when AAT1 was knocked down in MCs, MC degranulation was significantly increased, and could be rescued by a SO₂ donor. Mechanistically, AAT1 knockdown decreased the cyclic adenosine monophosphate (cAMP) content in MCs, while SO₂ prevented this reduction in a dose-independent manner. Pretreatment with the cAMP-synthesizing agonist forskolin or the cAMP degradation inhibitor IBMX significantly blocked the increase in AAT1 knockdown-induced MC degranulation. Furthermore, in hypoxia-stimulated MCs, AAT1 protein expression and SO₂ production were markedly down regulated, and MC degranulation was activated, which were blunted by AAT1 overexpression. The cAMP synthesis inhibitor SQ22536 disrupted the suppressive effect of AAT1 overexpression on hypoxia-induced MC degranulation. In a hypoxic environment, mRNA and protein expression of AAT1 was significantly reduced in lung tissues of rats. Supplementation of SO₂ elevated the cAMP level and reduced perivascular MC accumulation and degranulation in lung tissues of rats exposed to a hypoxic environment in vivo.

Conclusion

SO₂ serves as an endogenous MC stabilizer via upregulating the cAMP pathway under hypoxic circumstance.

A functional role for eicosanoid-lysophospholipids in activating monocyte signaling

Recently, eicosanoid-lysophospholipids were identified as novel metabolites generated from the direct cyclooxygenase- or lipoxygenase-catalyzed oxidation of 2-arachidonoyl-lysophospholipids produced from either phospholipase A1-mediated hydrolysis of diacyl arachidonoyl-phospholipids or through the cytochrome c-catalyzed oxidative hydrolysis of the vinyl ether linkage of arachidonoyl-plasmalogens. Although the metabolic pathways generating eicosanoid-lysophospholipids have been increasingly appreciated, the signaling functions of eicosanoid-lysophospholipids remain largely unknown. Herein, we demonstrate that 2-12(S)-HETE-lysophospholipids as well as nonesterified 12(S)-HETE are potent lipid mediators that activate THP-1 human monocytic cells to generate tumor necrosis factor α (TNFα) and interleukin 8 (IL8). Remarkably, low nanomolar concentrations of 12(S)-HETE-lysophospholipids, but not other oxidized signaling lipids examined activated THP-1 cells resulting in the production of large amounts of TNFα. Moreover, TNFα release induced by 12(S)-HETE-lysophospholipids was inhibited by the TNFα converting enzyme inhibitor TAPI-0 indicating normal processing of TNFα in THP-1 cells stimulated with these agonists. Western blotting analyses revealed that 12(S)-HETE-lysophospholipids activated the phosphorylation of NFκB p65, suggesting activation of the canonical NFκB signaling pathway. Importantly, activation of THP-1 cells to release TNFα was stereoselective with 12(S)-HETE favored over 12(R)-HETE. Furthermore, the EC50 of 2-12(S)-HETE-lysophosphatidylcholine in activating THP-1 cells was 2.1 nm, whereas the EC50 of free 12(S)-HETE was 23 nm Additionally, lipid extracts of activated platelets were separated by RP-HPLC demonstrating the coelution of 12(S)-HETE with fractions initiating TNFα release. Collectively, these results demonstrate the potent signaling properties of 2-12(S)-HETE-lysophospholipids and 12(S)-HETE by their ability to release TNFα and activate NFκB signaling thereby revealing a previously unknown role of 2-12(S)-HETE-lysophospholipids in mediating inflammatory responses.

Mast cells and angiogenesis in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease, characterized by multiple demyelination of axons in both white and gray matter in the Central Nervous System (CNS). There is increasing evidence to support the notion that angiogenesis and chronic inflammation are mutually related. Different immune cells, including monocytes-macrophages, lymphocytes, neutrophils, mast cells (MCs) and dendritic cells are able to secrete an array of angiogenic cytokines, which promote growth, migration, and activation of endothelial cells. MCs play various roles in MS pathogenesis, influencing the innate immune response in peripheral tissues and in CNS. The aim of this review article is to discuss the role of MCs in MS pathogenesis with particular reference to the involvement of these inflammatory cells in the angiogenic processes occurring during MS.

Mast cell degranulation and histamine release during A/H5N1 influenza infection in influenza-sensitized mice

Here we evaluate the role of mast cells in infection with influenza A/H5N1 virus in immunized mice. CBA mice were immunized intramuscularly with formalin-inactivated A/Vietnam/1194/2004 (H5N1)NIBRG-14 (H5N1). Serum samples were obtained on days 7, 12, 14, 21 after immunization. At day 14, the mice were infected intranasally with the A/Indonesia/5/2005 (H5N1)IDCDC-RG2 (H5N1) influenza virus with half of the animals receiving a mixture of the antihistamines. 67% of the vaccinated mice were protected from the lethality compared to 43% in the PBS-immunized group. Administration of antihistamines increased survival up to 85%–95%. Immunohistochemical examination using CD117 staining of the lungs demonstrated a larger quantity of activated mast cells after infection of immunized mice compared to mock-immunized mice. This was correlated to increased histamine level in the lungs and blood. Our experimental results suggest the involvement of mast cells and the histamine they produce in the pathogenesis of influenza infection in case of incomplete formation of the immune response to vaccination and mismatch of the vaccine and infection influenza viruses.

Mast cells exhibit intracellular microbicidal activity against Aggregatibacter actinomycetemcomitans

BACKGROUND AND OBJECTIVE

Several studies have demonstrated that mast cells are equipped with versatile tools to combat and kill bacteria. Additionally, mast cells produce and secrete a variety of mediators, which either regulate the host's immune system or directly attack bacteria. In this study, the intracellular microbicidal capacity of mast cells against Aggregatibacter actinomycetemcomitans was evaluated.

METHODS

Murine mast cells were challenged in vitro with A actinomycetemcomitans for 3, 5, 10, and 24 hours. Subsequently, the colony-forming units were counted. Additionally, the production and release of nitric oxide and hydrogen peroxide were analyzed by DAF-FM diacetate, the Griess reaction, and the Amplex Red kit, respectively. Cell death was evaluated using FITC Annexin V and propidium iodide staining.

RESULTS

Mast cells are able to efficiently eliminate periodontopathogen, with best results after 10 hours of intracellular challenge. The production/release of nitric oxide-and to a lesser extent of hydrogen peroxide-by mast cells was in agreement with its microbicidal capacity. Ninety percent of the mast cells  maintained their cellular viability even after 24 hours of bacterial challenge.

CONCLUSIONS

This is-to the best of our knowledge-the first report to describe the intracellular microbicidal activity of mast cells against A actinomycetemcomitans, concerning the production and release of potentially bactericidal substances. Further, the low number of cell deaths confirms that the decreased number of colony-forming units was due to the higher antimicrobial activity of mast cells. The results highlight the importance of these cells in the defense mechanisms of biofilm-induced periodontal disease.

[Tryptase: A practical guide for the physician]

Tryptase is the most abundant endopeptidase released by mast cells degranulation, involved in many pro and anti-inflammatory processes. Normal serum tryptase range is 0-11.4 μg/L. Tryptase is a useful diagnostic tool for anaphylaxis, systemic mastocytosis (SM) and mast cell activation syndrome (MCAS), where specific threshold values must be used. SM diagnosis criteria include evidence of dense mast cell infiltrate either in the bone marrow or the affected organ (such as skin), presence of KIT D816V mutation and elevated serum tryptase level (>20 μg/L). In SM, tryptase level is correlated with the burden of mast cells in bone marrow. MCAS should be considered in case of severe and recurrent typical clinical signs of systemic mast cell activation involving at least two organs, associated with an increase in serum tryptase level of 20% + 2 μg/L from the individual's baseline. Anaphylaxis is the most severe among hypersensitivity reactions. A clonal mast cell disorder is a central question in anaphylaxis and appropriate explorations should be conducted in these patients. Triggers for anaphylactic reactions vary significantly in the general population and in patients with MS or MCAS. Finally, physicians must be aware of the many pathological and physiological situations that affect tryptase levels.

A role for mast cells and mast cell tryptase in driving neutrophil recruitment in LPS-induced lung inflammation via protease-activated receptor 2 in mice

OBJECTIVE

This study aims to investigate the role of protease-activated receptor (PAR) 2 and mast cell (MC) tryptase in LPS-induced lung inflammation and neutrophil recruitment in the lungs of C57BL/6 mice.

METHODS

C57BL/6 mice were pretreated with the PAR2 antagonist ENMD-1068, compound 48/80 or aprotinin prior to intranasal instillation of MC tryptase or LPS. Blood leukocytes, C-X-C motif chemokine ligand (CXCL) 1 production leukocytes recovered from bronchoalveolar lavage fluid (BALF), and histopathological analysis of the lung were evaluated 4 h later. Furthermore, we performed experiments to determine intracellular calcium signaling in RAW 264.7 cells stimulated with LPS in the presence or absence of a protease inhibitor cocktail or ENMD-1068 and evaluated PAR2 expression in the lungs of LPS-treated mice.

RESULTS

Pharmacological blockade of PAR2 or inhibition of proteases reduced neutrophils recovered in BALF and LPS-induced calcium signaling. PAR2 blockade impaired LPS-induced lung inflammation, PAR2 expression in the lung and CXCL1 release in BALF, and increased circulating blood neutrophils. Intranasal instillation of MC tryptase increased the number of neutrophils recovered in BALF, and MC depletion with compound 48/80 impaired LPS-induced neutrophil migration.

CONCLUSION

Our study provides, for the first time, evidence of a pivotal role for MCs and MC tryptase in neutrophil migration, lung inflammation and macrophage activation triggered by LPS, by a mechanism dependent on PAR2 activation.

Novel Insight into the in vivo Function of Mast Cell Chymase: Lessons from Knockouts and Inhibitors

Mast cells are now recognized as key players in diverse pathologies, but the mechanisms by which they contribute in such settings are only partially understood. Mast cells are packed with secretory granules, and when they undergo degranulation in response to activation the contents of the granules are expelled to the extracellular milieu. Chymases, neutral serine proteases, are the major constituents of the mast cell granules and are hence released in large amounts upon mast cell activation. Following their release, chymases can cleave one or several of a myriad of potential substrates, and the cleavage of many of these could potentially have a profound impact on the respective pathology. Indeed, chymases have recently been implicated in several pathological contexts, in particular through studies using chymase inhibitors and by the use of chymase-deficient animals. In many cases, chymase has been shown to account for mast cell-dependent detrimental effects in the respective conditions and is therefore emerging as a promising drug target. On the other hand, chymase has been shown to have protective roles in other pathological settings. More unexpectedly, chymase has also been shown to control certain homeostatic processes. Here, these findings are reviewed.

Immune cell diversity contributes to the pathogenesis of myocarditis

Myocarditis (MCD) is a type of inflammatory disease in which inflammatory cells infiltrate the myocardium, leading to cardiac dysfunction, myocardial necrosis, and fibrosis. Although it has been reported that MCD is mediated by T cells, the immune system is complex and includes many types of immune cells that interact with one another. Through investigations of the inflammatory responses in MCD including myocardial necrosis, fibrosis, and arrhythmia, we have gained further insight into the pathogenesis of MCD. This article aims to discuss the diversity and the roles of immune cells involved in the pathogenesis of MCD. Moreover, immunotherapy for the treatment of MCD remains controversial, and further investigation is required to identify accurate immunotherapies for special cell types.

Do Mast Cells Have a Role in Tendon Healing and Inflammation?

Understanding the links between the tendon healing process, inflammatory mechanisms, and tendon homeostasis/pain after tissue damage is crucial in developing novel therapeutics for human tendon disorders. The inflammatory mechanisms that are operative in response to tendon injury are not fully understood, but it has been suggested that inflammation occurring in response to nerve signaling, i.e., neurogenic inflammation, has a pathogenic role. The mechanisms driving such neurogenic inflammation are presently not clear. However, it has recently been demonstrated that mast cells present within the injured tendon can express glutamate receptors, raising the possibility that mast cells may be sensitive to glutamate signaling and thereby modulate neurogenic inflammation following tissue injury. In this review, we discuss the role of mast cells in the communication with peripheral nerves, and their emerging role in tendon healing and inflammation after injury.

Extracellular ATP Augments Antigen-Induced Murine Mast Cell Degranulation and Allergic Responses via P2X4 Receptor Activation

Extracellular ATP released from stimulated and/or damaged cells modulates physiological responses via stimulation of various purinoceptors. We previously showed that ATP potentiated the Ag-induced mast cell (MC) degranulation via purinoceptors pharmacologically similar to the ionotropic P2X4 receptor. In this study, we investigated the role of P2X4 receptor in MC degranulation induced by stimulation of IgE-FcεRI complex with Ag, using bone marrow-derived MCs (BMMCs) prepared from wild type and P2X4 receptor-deficient (P2rx4^-/- ) mice. ATP significantly increased Ag-induced degranulation in BMMCs prepared from wild type mice. This effect of ATP was reduced in BMMCs prepared from P2rx4^-/- mice. The potentiating effect of ATP was restored by expressing P2X4 receptor in P2rx4^-/- BMMCs. The P2X4 receptor-mediated effects were maintained even after differentiating into the connective tissue-type MCs. P2X4 receptor stimulation did not affect the Ag-induced Ca²⁺ response but enhanced Ag-induced early signals, such as tyrosine phosphorylation of Syk and phospholipase C-γ. Interestingly, these effects of ATP on Syk phosphorylation were not impaired by pretreatment with Cu²⁺, an inhibitor of the P2X4 receptor channel, or removal of external Ca²⁺, suggesting that a mechanisms other than Ca²⁺ influx through ion channel activity may be involved. In vivo experiments revealed that systemic and intradermal passive anaphylaxis responses were significantly alleviated in P2rx4^-/- mice. Taken together, the present data suggest that the P2X4 receptor plays an essential role in ATP-induced upregulation of MC degranulation in response to Ag, and also contributes to the Ag-induced allergic response in vivo.

Analysis of Mrgprb2 Receptor-Evoked Ca 2+ Signaling in Bone Marrow Derived (BMMC) and Peritoneal (PMC) Mast Cells of TRPC-Deficient Mice

Mast cells are a heterogeneous group of immune cells. The simplest and commonly accepted classification divides them in two groups according to their protease content. We have compared the action of diverse secretagogues on bone marrow derived (BMMC) and peritoneal (PMC) mast cells which represent classical models of mucosal and connective tissue type mast cells in mice. Whereas, antigen stimulation of the FcεRI receptors was similarly effective in triggering elevations of free intracellular Ca²⁺ concentration ([Ca²⁺]_i) in both BMMC and PMC, robust [Ca²⁺]_i rise following Endothelin-1 stimulation was observed only in a fraction of BMMC. Leukotriene C4 activating cysteinyl leukotriene type I receptors failed to evoke [Ca²⁺]_i rise in either mast cell model. Stimulation of the recently identified target of many small-molecule drugs associated with systemic pseudo-allergic reactions, Mrgprb2, with compound 48/80, a mast cell activator with unknown receptor studied for many years, triggered Ca²⁺ oscillations in BMMC and robust [Ca²⁺]_i rise in PMCs similarly to that evoked by FcεRI stimulation. [Ca²⁺]_i rise in PMC could also be evoked by other Mrgprb2 agonists such as Tubocurarine, LL-37, and Substance P. The extent of [Ca²⁺]_i rise correlated with mast cell degranulation. Expression analysis of TRPC channels as potential candidates mediating agonist evoked Ca²⁺ entry revealed the presence of transcripts of all members of the TRPC subfamily of TRP channels in PMCs. The amplitude and AUC of compound 48/80-evoked [Ca²⁺]_i rise was reduced by ~20% in PMC from Trpc1/4/6^−/− mice compared to Trpc1/4^−/− littermatched control mice, whereas FcεRI-evoked [Ca²⁺]_i rise was unaltered. Whole-cell patch clamp recordings showed that the reduction in compound 48/80-evoked [Ca²⁺]_i rise in Trpc1/4/6^−/− PMC was accompanied by a reduced amplitude of Compound 48/80-induced cation currents which exhibited typical features of TRPC currents. Together, this study demonstrates that PMC are an appropriate mast cell model to study mechanisms of Mrgprb2 receptor-mediated mast cell activation, and it reveals that TRPC channels contribute at least partially to Mrgprb2-mediated mast cellactivation but not following FcεRI stimulation. However, the channels conducting most of the Ca²⁺ entry in mast cells triggered by Mrgprb2 receptor stimulation remains to be identified.

Coumarin alleviates ovalbumin-induced food anaphylaxis in a mouse model by affecting mast cell function

Coumarin is an important organic heterocyclic compound with a wide range of sources in nature. It plays an important role in the drug discovery process due to its existence in diverse biologically active compounds and its broad bioactivity. In this study, the anti-allergic activity of coumarin was evaluated using an ovalbumin (OVA)-induced mouse food allergy model and an immunoglobulin (Ig)E mediated mouse bone marrow-derived mast cell (BMMC) model. Coumarin could alleviate the OVA-induced allergic symptoms, decrease the diarrhea rates, and promote the rectal temperature rise in allergic mice. Moreover, coumarin had the ability to reduce the levels of histamine and mouse mast cell proteinases, inhibit OVA-specific IgE, and significantly decrease the population of mast cells in the spleen and mesenteric lymph nodes. Coumarin could also significantly suppress mast cell-dependent passive cutaneous anaphylaxis. Additionally, the number of mature BMMCs was decreased as coumarin caused the suppression of c-KIT receptors. Furthermore, coumarin up-regulated the apoptosis of OVA-activated BMMCs in a concentration-dependent manner. In conclusion, coumarin displayed effective anti-food allergy activity via the regulation of mast cell function and numbers. Coumarin and its derivatives provide a new direction for the development of anti-food allergic drug components.

Histamine-mediated autocrine signaling in mesenteric perilymphatic mast cells

Lymphatic vessels play a critical role in mounting a proper immune response by trafficking peripheral immune cells to draining lymph nodes. Mast cells (MCs) are well known for their roles in type I hypersensitivity reactions, but little is known about their secretory regulation in the lymphatic niche. MCs, as innate sensor and effector cells, reside close to mesenteric lymphatic vessels (MLVs), and their activation and ability to release histamine influences the lymphatic microenvironment in a histamine-NF-κB-dependent manner. Using an established experimental protocol involving surgical isolation of rat mesenteric tissue segments, including MLVs and surrounding perilymphatic tissues, we tested the hypothesis that perilymphatic mesenteric MCs possess histamine receptors (HRs) that bind and respond to the histamine released from these same MCs. Under various experimental conditions, including inflammatory stimulation by LPS, we measured histamine in mesenteric perilymphatic tissues, evaluated expression of histidine decarboxylase in MCs along with the degree of MC degranulation, assessed the functional status of HRs in MCs, and evaluated the ability of histamine itself to induce MC activation. Finally, we evaluated the importance of MCs and HR1 and -2 for MLV-directed trafficking of CD11b/c-positive cells during acute tissue inflammation. Our data indicate the existence of a functionally potent MC-histamine autocrine regulatory loop, the elements of which are crucially important for acute inflammation-induced trafficking of the CD11b/c-positive cells toward MLVs. This MC-histamine loop serves as a first-line cellular servo control system, playing a key role in the innate and adaptive immune response as well as NF-κB-mediated maintenance of body homeostasis.

Histological and Immunohistochemical Study of Wounds in Sheep Skin in Maggot Therapy by Using Protophormia terraenovae (Diptera: Calliphoridae) Larvae

A study was performed to evaluate the implication of Protophormia terraenovae larvae as a surgical therapy for wounded skin. Three groups of sheep (n = 25) were considered based on larval doses. Groups 1 and 2 were artificially infested with low and high concentrations of L1 stage P. terraenovae, respectively, and group 0 served as a control. Skin biopsies were taken at 4 and 14 d postinfestation (D.P.If). A histopathological study was carried out to evaluate the lesions with a score, numbers of eosinophils and mast cells, and an immunohistochemical analysis of CD3, CD79α, and CD68 as T lymphocytes, B lymphosytes, and macrophages, respectively. The results indicated that higher larval doses led to faster regeneration by 14 D.P.If. Furthermore, the higher larval doses showed a high number of the CD68 marker and eosinophils and a low number of CD3 and CD79α markers and mast cells. In addition, the number of mast cells, T lymphocytes, and macrophage markers increased when the lesion progressed; however, a low number of immunolabeled CD79α cells and eosinophils were observed. The results indicate a possible positive effect of larvae in the healing of certain wounds.

Localization of cannabinoid and cannabinoid related receptors in the cat gastrointestinal tract

A growing body of literature indicates that activation of cannabinoid receptors may exert beneficial effects on gastrointestinal inflammation and visceral hypersensitivity. The present study aimed to immunohistochemically investigate the distribution of the canonical cannabinoid receptors CB₁ (CB1R) and CB₂ (CB2R) and the putative cannabinoid receptors G protein-coupled receptor 55 (GPR55), nuclear peroxisome proliferator-activated receptor alpha (PPARα), transient receptor potential ankyrin 1 (TRPA1), and serotonin receptor 5-HT1a 5-HT1aR) in tissue samples of the gastrointestinal tract of the cat. CB1R-immunoreactivity (CB1R-IR) was observed in gastric epithelial cells, intestinal enteroendocrine cells (EECs) and goblet cells, lamina propria mast cells (MCs), and enteric neurons. CB2R-IR was expressed by EECs, enterocytes, and macrophages. GPR55-IR was expressed by EECs, macrophages, immunocytes, and MP neurons. PPARα-IR was expressed by immunocytes, smooth muscle cells, and enteroglial cells. TRPA1-IR was expressed by enteric neurons and intestinal goblet cells. 5-HT1a receptor-IR was expressed by gastrointestinal epithelial cells and gastric smooth muscle cells. Cannabinoid receptors showed a wide distribution in the feline gastrointestinal tract layers. Although not yet confirmed/supported by functional evidences, the present research might represent an anatomical substrate potentially useful to support, in feline species, the therapeutic use of cannabinoids during gastrointestinal inflammatory diseases.

Identification and Immunophenotypic Characterization of Normal and Pathological Mast Cells

Mast cells (MCs) are secretory cells that are central players in human allergic disease and immune responses. With the exception of a few pathological situations, MCs are usually present at relatively low frequencies in most tissues. Since their first description, MCs in tissues were identified mostly using their morphological characteristics and their typical coloration when stained with aniline dyes. However, increasing availability of highly specific antibodies now permits the use of fluorescence-based flow cytometry as the method of choice for the quantification, characterization, and purification of cells in suspension. This technique allows for a rapid analysis of thousands of events and for the identification of cells present at frequencies as low as one event in 10⁶ unwanted cells. This method also permits for simultaneous characterization of multiple antigens at a single cell level, which is ideal in order to study rare populations of cells like MCs. Here we describe the basis of flow cytometry-based immunophenotyping applied to the study of MC. The protocol focuses on the study of human MCs present in body fluids (mainly bone marrow) but can easily be adapted to studying MCs from other tissues and species.

Alleviation of Atopic Dermatitis Lesions by a Benzylideneacetophenone Derivative via the MAPK Signaling Pathway

This experiment was conducted to investigate the effects of a benzylideneacetophenone derivative ((2E)-3-(4-hydroxy-3-methoxyphenyl)phenylpro-2-en-l-one (JC3)) on trimellitic anhydride (TMA)-induced atopic dermatitis (AD)-like symptoms in mice. To induce AD, the dorsal skins of mice were treated with 5% TMA on day 0 and both ears were treated with 5% TMA on day 5 and with 2% TMA from day 6 to day 14. JC3 (1, 5, 10 mg/kg, i.p.) was treated once daily from day 9 to day 14 before TMA treatment. Histological analysis was performed and auricular lymph node weights, ear thicknesses, skin water contents, scratching behaviors, and serum immunoglobulin (IgE) and IFN-γ, and interleukin-4 (IL-4) levels in serum and ear tissues were determined. In addition, the anti-AD activity of JC3 was investigated on phorbol 12-myristate 13-acetate (PMA)-stimulated human mast cells (HMC-1 cells) derived from patients. Levels of TNF-α, IL-4, and mitogen-activated protein kinase (MAPK) were investigated after treating cultured cells with JC3. Treating mice with JC3 (10 mg/kg) significantly decreased ear thicknesses, lymph node weights, skin scores, skin water contents, scratching behavior, and IFN-γ, IL-4 cytokine levels, and serum IgE levels. Moreover, treatment with JC3 (10 mg/kg) significantly decreased serum and ear tissues levels of IFN-γ and IL-4 in AD mice. Furthermore, treatment with JC3 at 10 μg/ml reduced TNF-α and IL-4 levels and decreased MAPK phosphorylation in the HMC-1 cells. The results of this study provide a molecular basis for developing new therapeutics for the treatment of various inflammatory diseases, such as, eczema, asthma, and AD.

To Be or Not to Be a Pathogen: Candida albicans and Celiac Disease

Celiac disease (CD) is an immune-mediated disorder triggered by the ingestion of gluten and characterized by reversible small-bowel mucosal atrophy in genetically predisposed subjects. Although the prevalence of CD has increased, many aspects of this pathology are still unrecognized. Candida albicans, a commensal of the human gastrointestinal tract, has been linked to CD for a long time based, among others, upon the observation of similarity between the fungal wall component, hyphal wall protein 1, and CD-related gliadin T-cell epitopes. We have recently demonstrated that Candida may switch from commensal to pathogen contingent upon several players, including mast cells, key sentinels of the immune system at the interface between the environment and the host, and the pleiotropic cytokine IL-9. However, other factors are likely to play a role by altering the balance between inflammation and tolerance. In this regard, tryptophan and its metabolites are increasingly being recognized in promoting mucosal homeostasis by balancing the immune response to external cues. Based on these premises, we will discuss how the output of Candida colonization in the gut is highly contextual, being determined at the intersection of many immunological (IL-9/mast cells) and metabolic (tryptophan) pathways that ultimately dictate the Candida commensalism vs. pathogenicity in CD, thus paving the way for novel therapeutic opportunities in CD.

Mast Cells as a Double-Edged Sword in Immunity: Their Function in Health and Disease. First of Two Parts

Mast cells (MCs) have recently been re-interpreted in the context of the immune scenario in the sense that their pro-allergic role is no longer exclusive. In fact, MCs even in steady state conditions maintain homeostatic functions, producing mediators and intensively cross-talking with other immune cells. Here, emphasis will be placed on the array of receptors expressed by MCs and the variety of cytokines they produce. Then, the bulk of data discussed will provide readers with a wealth of information on the dual ability of MCs not only to defend but also to offend the host. This double attitude of MCs relies on many variables, such as their subsets, tissues of residency and type of stimuli ranging from microbes to allergens and food antigens. Finally, the relationship between MCs with basophils and eosinophils will be discussed.

Decoding communication patterns of the innate immune system by quantitative proteomics

The innate immune system is a collective network of cell types involved in cell recruitment and activation using a robust and refined communication system. Engagement of receptor-mediated intracellular signaling initiates communication cascades by conveying information about the host cell status to surrounding cells for surveillance and protection. Comprehensive profiling of innate immune cells is challenging due to low cell numbers, high dynamic range of the cellular proteome, low abundance of secreted proteins, and the release of degradative enzymes (e.g., proteases). However, recent advances in mass spectrometry-based proteomics provides the capability to overcome these limitations through profiling the dynamics of cellular processes, signaling cascades, post-translational modifications, and interaction networks. Moreover, integration of technologies and molecular datasets provide a holistic view of a complex and intricate network of communications underscoring host defense and tissue homeostasis mechanisms. In this Review, we explore the diverse applications of mass spectrometry-based proteomics in innate immunity to define communication patterns of the innate immune cells during health and disease. We also provide a technical overview of mass spectrometry-based proteomic workflows, with a focus on bottom-up approaches, and we present the emerging role of proteomics in immune-based drug discovery while providing a perspective on new applications in the future.

Colonic mast cell numbers, symptom profile, and mucosal expression of elements of the epithelial barrier in irritable bowel syndrome

BACKGROUND

This study aimed to determine whether patients with IBS displayed altered mucosal mast cell (MC) numbers and proportions of MCs co-localizing with nerves compared with healthy subjects (HS) and whether these MC characteristics correlated with IBS symptoms, elements of the epithelial barrier, or visceral sensitivity.

METHODS

Mucosal MC characteristics were determined using immunoassay. IBS symptoms, gene expression of elements of the epithelial barrier, fecal serine protease activity, and visceral sensitivity were assessed.

KEY RESULTS

The MC numbers per mm² were 2.0 (0.0-6.0) in patients with IBS (n = 43) and 3.5 (1.1-9.1) in HS (n = 20, P = .26). Of these, MCs were 0.0 (0.0-20) % vs 3.1 (0.0-18) % (P = .76) in IBS and HS, respectively, in co-localization with nerve fibers. MC characteristics were equivalent in the different IBS subtypes. Hierarchical cluster analysis identified two distinct groups among patients with IBS: MC high (higher MC numbers and proportions of MCs co-localizing with nerves) and MC low (lower MC numbers and proportions of MCs co-localizing with nerves). The MC high and MC low groups could not be discriminated with regard to IBS symptoms, parameters of visceral sensitivity, gene expression of elements of the epithelial barrier, and fecal protease activity.

CONCLUSION AND INFERENCES

There was no evidence of increased infiltration or altered localization of MCs in the colonic mucosa of patients with IBS. These MC characteristics were not linked to global IBS symptoms or mucosal expression of elements of the epithelial barrier. These findings indicate that quantity and location of mucosal MCs are factors not involved in the pathophysiology of IBS.

Mast Cells as Potential Accelerators of Human Atherosclerosis-From Early to Late Lesions

Mast cells are present in atherosclerotic lesions throughout their development. The process of atherogenesis itself is characterized by infiltration and retention of cholesterol-containing blood-derived low-density lipoprotein (LDL) particles in the intimal layer of the arterial wall, where the particles become modified and ingested by macrophages, resulting in the formation of cholesterol-filled foam cells. Provided the blood-derived high-density lipoproteins (HDL) particles are able to efficiently carry cholesterol from the foam cells back to the circulation, the early lesions may stay stable or even disappear. However, the modified LDL particles also trigger a permanent local inflammatory reaction characterized by the presence of activated macrophages, T cells, and mast cells, which drive lesion progression. Then, the HDL particles become modified and unable to remove cholesterol from the foam cells. Ultimately, the aging foam cells die and form a necrotic lipid core. In such advanced lesions, the lipid core is separated from the circulating blood by a collagenous cap, which may become thin and fragile and susceptible to rupture, so causing an acute atherothrombotic event. Regarding the potential contribution of mast cells in the initiation and progression of atherosclerotic lesions, immunohistochemical studies in autopsied human subjects and studies in cell culture systems and in atherosclerotic mouse models have collectively provided evidence that the compounds released by activated mast cells may promote atherogenesis at various steps along the path of lesion development. This review focuses on the presence of activated mast cells in human atherosclerotic lesions. Moreover, some of the molecular mechanisms potentially governing activation and effector functions of mast cells in such lesions are presented and discussed.

Future Needs in Mast Cell Biology

The pathophysiological roles of mast cells are still not fully understood, over 140 years since their description by Paul Ehrlich in 1878. Initial studies have attempted to identify distinct "subpopulations" of mast cells based on a relatively small number of biochemical characteristics. More recently, "subtypes" of mast cells have been described based on the analysis of transcriptomes of anatomically distinct mouse mast cell populations. Although mast cells can potently alter homeostasis, in certain circumstances, these cells can also contribute to the restoration of homeostasis. Both solid and hematologic tumors are associated with the accumulation of peritumoral and/or intratumoral mast cells, suggesting that these cells can help to promote and/or limit tumorigenesis. We suggest that at least two major subsets of mast cells, MC1 (meaning anti-tumorigenic) and MC2 (meaning pro-tumorigenic), and/or different mast cell mediators derived from otherwise similar cells, could play distinct or even opposite roles in tumorigenesis. Mast cells are also strategically located in the human myocardium, in atherosclerotic plaques, in close proximity to nerves and in the aortic valve. Recent studies have revealed evidence that cardiac mast cells can participate both in physiological and pathological processes in the heart. It seems likely that different subsets of mast cells, like those of cardiac macrophages, can exert distinct, even opposite, effects in different pathophysiological processes in the heart. In this chapter, we have commented on possible future needs of the ongoing efforts to identify the diverse functions of mast cells in health and disease.

Human Mast Cell Development from Hematopoietic Stem Cells in a Connective Tissue-Equivalent Model

Mast cells (MCs) play critical roles in the pathogenesis of IgE- and non-IgE-mediated immune responses, as well as host defense against parasites, bacteria, and viruses. Due to the effect of extracellular matrix components on tissue morphogenesis and cell behavior, utilizing a tissue model that mimics MC microenvironmental conditions in vivo has greater relevance for in vitro studies. For this work, MCs were developed within a connective tissue-equivalent model and cell function was examined in response to an allergen. MCs are located in proximity to fibroblasts and endothelial cells (ECs) that play a role in MC development and maturity. Accordingly, MC progenitors isolated from human peripheral blood were co-cultured with human primary fibroblasts in a 3D collagen matrix to represent the connective tissue. The matrix was coated with type IV collagen and fibronectin before seeding with primary human ECs, representing the capillary wall. The stem cell-derived cells demonstrated MC characteristics, including typical MC morphology, and the expression of cytoplasmic granules and phenotypic markers. Also, the generated cells released histamine in IgE-mediated reactions, showing typical MC functional phenotype in an immediate-type allergenic response. The created tissue model is applicable to a variety of research studies and allergy testing. Impact Statement Mast cells (MCs) are key effector and immunoregulatory cells in immune disorders; however, their role is not fully understood. Few studies have investigated human ex vivo MCs in culture, due to the difficulties in isolating large numbers. Our study demonstrates, for the first time, the generation of cells exhibiting MC phenotypic and functional characteristics from hematopoietic stem cells within a connective tissue-equivalent model with ancillary cells. Utilizing the 3D matrix-embedded cells can advance our understanding of MC biological profile and immunoregulatory roles. The tissue model can also be used for studying the mechanism of allergic diseases and other inflammatory disorders.

Mast Cell Eavesdropping on Bacterial Communications

Bacteria communicate through quorum sensing, which enables bacterial coordination of collective behaviors and environment-induced adaptations. In this issue of Cell Host & Microbe, Pundir et al. (2019) report that connective-tissue-type mast cells spy on bacterial communication by detecting the messenger molecules via their receptor Mrgprb2-MRGPRX2 to the host benefit.

Streptococcal sagA activates a proinflammatory response in mast cells by a sublytic mechanism

Mast cells are implicated in the innate proinflammatory immune defence against bacterial insult, but the mechanisms through which mast cells respond to bacterial encounter are poorly defined. Here, we addressed this issue and show that mast cells respond vividly to wild type Streptococcus equi by up‐regulating a panel of proinflammatory genes and by secreting proinflammatory cytokines. However, this response was completely abrogated when the bacteria lacked expression of sagA, whereas the lack of a range of other potential virulence genes (seeH, seeI, seeL, seeM, hasA, seM, aroB, pyrC, and recA) had no effect on the amplitude of the mast cell responses. The sagA gene encodes streptolysin S, a lytic toxin, and we next showed that the wild type strain but not a sagA‐deficient mutant induced lysis of mast cells. To investigate whether host cell membrane perturbation per se could play a role in the activation of the proinflammatory response, we evaluated the effects of detergent‐ and pneumolysin‐dependent lysis on mast cells. Indeed, exposure of mast cells to sublytic concentrations of all these agents resulted in cytokine responses of similar amplitudes as those caused by wild type streptococci. This suggests that sublytic membrane perturbation is sufficient to trigger full‐blown proinflammatory signalling in mast cells. Subsequent analysis showed that the p38 and Erk1/2 signalling pathways had central roles in the proinflammatory response of mast cells challenged by either sagA‐expressing streptococci or detergent. Altogether, these findings suggest that sagA‐dependent mast cell membrane perturbation is a mechanism capable of activating the innate immune response upon bacterial challenge.