The role of reactive oxygen species and nitric oxide in mast cell-dependent inflammatory processes

Summary:  Reactive oxygen species (ROS) and reactive nitrogen oxide species (RNOS), including nitric oxide, are produced in cells by a variety of enzymatic and non-enzymatic mechanisms. At high levels, both types of oxidants are used to kill ingested organisms within phagocytes. At low levels, RNOS may diffuse outside cells where they impact the vasculature and nervous system. Recent evidence suggests that low levels of ROS produced within cells are involved in cell signaling. Along with these physiological roles, many pathological conditions exist where detrimental high-level ROS and RNOS are produced. Many situations in which ROS/RNOS are associated also involve mast cell activation. In innate immunity, such mast cells are involved in the immune response toward pathogens. In acquired immunity, activation of mast cells by cross-linking of receptor-bound immunoglobulin E causes the release of mediators involved in the allergic inflammatory response. In this review, we describe the principle pathways for ROS and RNOS generation by cells and discuss the existence of such pathways in mast cells. In addition, we examine the evidence for a functional role for ROS and RNOS in mast cell secretory responses and discuss evidence for a direct relationship between ROS, RNOS, and mast cells in mast cell-dependent inflammatory conditions.     

Role of mast cells in airway remodeling

The extent of airway remodeling correlates with severity of asthma. Persistent airway hyperresponsiveness (AHR) is associated with airway remodeling, but not with inflammation. The increase in ASM mass is recognized as one of the most important factors related to AHR and to the severity of asthma. The infiltration of ASM by mast cells (MCs) is associated with the disordered airway function. The mediators such as tryptase and cytokines from MCs can modulate ASM cell function and induce goblet cell hyperplasia. MCs were found to contribute to the development of multiple features of chronic asthma in MC-deficient mice. Therefore, MCs play an important role not only in immediate hypersensitivity and late phase inflammation but also in tissue remodeling in the airway.     

The controversial role of mast cells in fibrosis

Fibrosis is a medical condition characterized by an excessive deposition of extracellular matrix compounds such as collagen in tissues. Fibrotic lesions are present in many diseases and can affect all organs. The excessive extracellular matrix accumulation in these conditions can often have serious consequences and in many cases be life-threatening. A typical event seen in many fibrotic conditions is a profound accumulation of mast cells (MCs), suggesting that these cells can contribute to the pathology. Indeed, there is now substantialv evidence pointing to an important role of MCs in fibrotic disease. However, investigations from various clinical settings and different animal models have arrived at partly contradictory conclusions as to how MCs affect fibrosis, with many studies suggesting a detrimental role of MCs whereas others suggest that MCs can be protective. Here, we review the current knowledge of how MCs can affect fibrosis.     

Mast cells and inflammation

Mast cells have long been recognized as potent producers of a large panel of biologically highly active mediators such as biogenic amines, arachidonic acid metabolites, cytokines and chemokines, but most of their biological functions have been elusive and speculative. By taking advantage of mast cell-deficient mice, the role of mast cells in a variety of experimental settings can now be studied in detail and such approaches have dramatically altered and enlarged our knowledge about mast cell biology and function. Herein we will focus on the role of mast cells in inflammatory reactions of diverse origin, such as delayed type hypersensitivity, atopy, immune complex-mediated inflammation and innate immune responses. From the current standpoint, there is no doubt that the most outstanding and beneficial feature of mast cells is their recently discovered ability to induce a life-saving inflammatory response rapidly upon encountering microbes and microbial constituents. Nevertheless, the picture is also emerging that mast cells are deeply involved in the induction and maintenance of a variety of severe allergic and autoimmune diseases. However, a deeper understanding of their activation and immune-modulatory capacity might open a new window for the development of curative strategies.     

MAST CELL DISTRIBUTION,ACTIVATION, AND PHENOTYPE IN ATHEROSCLEROTIC LESIONS OF HUMAN CAROTID ARTERIES

Immunohistochemical staining for mast cell tryptase and chymase was used to examine the distribution, activation, and tryptase/chymase phenotype of mast cells (MCs) in 250 samples of atherosclerotic lesions (type I to VI) of human carotid arteries. Dual immunolocalization and histochemical techniques were used to identify the associations of MCs with macrophages, smooth muscle cells, and extracellular matrix components. Whereas normal carotid arteries contained very few MCs within the intima, atherosclerotic lesions showed increased MC numbers with variable focal accumulations. MCs were identifiable from the earliest stages of atherosclerosis, and especially at the shoulder regions of the fully formed atheroma. They were observed in close association with macrophages (HAM56 positive) and extracellular lipid, as well as at sites of foam cell formation. MCs and diffuse tryptase staining were also evident within sites of new calcification and around small calcified deposits. Extensive MC activation/degranulation, as judged by diffuse extracellular tryptase staining, was a common feature of the advanced atherosclerotic plaques complicated by fissure, haemorrhage, and thrombus formation. Moreover, such sites of extracellular MC tryptase were often associated with localized oedema and disruption of the stromal matrix. MCs which contained both tryptase and chymase (the MC_TC phenotype) represented approximately 80–95 per cent of all MCs. These studies are the first to demonstrate significant numbers and focal accumulations of MCs in all developmental stages of atherosclerotic carotid arteries. Since MCs contain or express a variety of potent mediators, their release could profoundly influence the development and pathological complications of atherosclerotic plaques. © 1997 John Wiley & Sons, Ltd.     

The role of mast cells and basophils in inflammation

Mast cells are positioned in the asthmatic airways so that they are able to respond to the inhaled environment. During active disease, the cells are primed to secrete an array of preformed and newly generated inflammatory mediators including histamine, neutral proteases and heparin sulphate, prostaglandins and cysteinyl leukotrienes as well as an array of cytokines and chemokines that are involved in leucocyte recruitment and activation. These cells are a potent source of mediators in both allergen- and exercise-induced asthma and possibly in asthma provoked by other stimuli such as adenosine and inhaled air pollutants. The important role played by mast cells in maintaining airway dysfunction in asthma is underpinned by the efficacy of mediator inhibitors, such as those interfering with the release or action of the leukotrienes, agents that inhibit mast cell activation such as sodium cromoglycate and the recently studied E-20 humanized monoclonal antibody that binds to and removes IgE. The recent discovery of novel inhibitory pathways involving inhibitory motifs (ITIMS) on critical cell surface signalling molecules has opened up new possibilities for preventing mast cell activation. Future research will focus on more effective ways for inhibiting the mast cell's contribution to asthma and understanding what role this unique cell has in the pathogenesis of airway wall remodelling.     

Mast cells crosstalk with B cells in the gut and sustain IgA response in the inflamed intestine

B lymphocytes are among the cell types whose effector functions are modulated by mast cells (MCs). The B/MC crosstalk emerged in several pathological settings, notably the colon of inflammatory bowel disease (IBD) patients is a privileged site in which MCs and IgA⁺ cells physically interact. Herein, by inducing conditional depletion of MCs in red MC and basophil (RMB) mice, we show that MCs control B cell distribution in the gut and IgA serum levels. Moreover, in dextran sulfate sodium (DSS)-treated RMB mice, the presence of MCs is fundamental for the enlargement of the IgA⁺ population in the bowel and the increase of systemic IgA production. Since both conventional B-2 and peritoneal-derived B cells populate the intestine and communicate with MCs in physiological conditions and during inflammation, we further explored this interplay through the use of co-cultures. We show that MCs finely regulate different aspects of splenic B cell biology while peritoneal B cells are unresponsive to the supporting effects provided by MCs. Interestingly, peritoneal B cells induce a pro-inflammatory skewing in MCs, characterized by increased ST2 and TNF-α expression. Altogether, this study uncovers the versatility of the B/MC liaison and highlights key aspects for the resolution of intestinal inflammation.     

Biomarkers for evaluation of mast cell and basophil activation

Mast cells and basophils play a pathogenetic role in allergic, inflammatory, and autoimmune disorders. These cells have different development, anatomical location and life span but share many similarities in mechanisms of activation and type of mediators. Mediators secreted by mast cells and basophils correlate with clinical severity in asthma, chronic urticaria, anaphylaxis, and other diseases. Therefore, effective biomarkers to measure mast cell and basophil activation in vivo could potentially have high diagnostic and prognostic values. An ideal biomarker should be specific for mast cells or basophils, easily and reproducibly detectable in blood or biological fluids and should be metabolically stable. Markers of mast cell and basophil include molecules secreted by stimulated cells and surface molecules expressed upon activation. Some markers, such as histamine and lipid mediators are common to mast cells and basophils whereas others, such as tryptase and other proteases, are relatively specific for mast cells. The best surface markers of activation expressed on mast cells and basophils are CD63 and CD203. While these mediators and surface molecules have been associated to a variety of diseases, none of them fulfills requirements for an optimal biomarker and search for better indicators of mast cell/basophil activation in vivo is ongoing.     

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 dynamic and complex role of mast cells in allergic disease

Mast cells (MCs) are found widely distributed in tissues and contribute to regulation of inflammatory responses and ongoing modulation of the tissues. Although MCs are important in a variety of processes, including innate immunity, their role in allergic disease has received increasing attention in the past decade. MCs are located throughout the human body and, upon allergen exposure, they are stimulated via the immunoglobulin E (IgE) receptor (Fc(epsilon)RI) to release several pro-inflammatory mediators such as tumor necrosis factor (TNF), reactive oxygen species such as nitric oxide (NO), proteases, and lipid-derived mediators. However, we now recognize that MCs can be activated by a variety of mechanisms and that mediator release is a consequence of several intra- and extracellular signals. Some of these mechanisms, such as Fc receptor aggregation and proteinase-activated receptor (PAR)-mediated activation facilitate and augment local inflammatory responses. Other mechanisms, such as interferon gamma (IFN-gamma) induction of NO, may inhibit MC function and downregulate inflammatory responses. Increased understanding of these complex pathways has encouraged the development of therapies for allergic inflammation that target specific MC functions and mediators. Some novel strategies include oligonucleotides that induce or inhibit the production of specific mediators. Such approaches may yield useful therapies for allergic individuals in the near future.     

The inflammatory theory of asthma

In summary, these observations suggest a model for asthma which is summarized in Table III. Initially, mast cells and possibly other bronchial cells, e.g., alveolar macrophages, are activated either in an IgE-dependent or, in intrinsic asthma, in an IgE-independent fashion. These cells release two sets of mediators which may be either preformed or newly synthesized. One set of mediators is responsible for the immediate bronchospastic response. This bronchospasm is transient, readily reversible, and not associated with either airway inflammation or bronchial hyperreactivity. The second set of mediators, however, promote chemotaxis and activation of neutrophils and eosinophils. The subsequent bronchial inflammation causes damage and desquamation of the respiratory epithelium. The increased exposure of irritant receptors results in hyperreactive airways. In addition, these inflammatory cells induce mast cell degranulation and recurrent bronchospasm. Thus, after the initial exposure to allergen, a vicious cycle of inflammation, hyperreactivity and recurrent mast cell degranulation develops, ultimately leading to the pathological picture of chronic asthma.     

The ingenious mast cell: Contemporary insights into mast cell behavior and function

Mast cells are (in)famous for their role in allergic diseases, but the physiological and pathophysiological roles of this ingenious cell are still not fully understood. Mast cells are important for homeostasis and surveillance of the human system, recognizing both endogenous and exogenous agents, which induce release of a variety of mediators acting on both immune and non-immune cells, including nerve cells, fibroblasts, endothelial cells, smooth muscle cells, and epithelial cells. During recent years, clinical and experimental studies on human mast cells, as well as experiments using animal models, have resulted in many discoveries that help decipher the function of mast cells in health and disease. In this review, we focus particularly on new insights into mast cell biology, with a focus on mast cell development, recruitment, heterogeneity, and reactivity. We also highlight the development in our understanding of mast cell-driven diseases and discuss the development of novel strategies to treat such conditions.     

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.     

Regulation of mast cell-mediated innate immunity during early response to bacterial infection

Summary Although the area of research on the role of MCs in innate immunity is relatively new, a number of studies that are reviewed here provide substantial evidence that MCs play a critical role in host immune defense against gram-negative bacteria. The studies show that mast cells have the ability to recognize and engulf bacteria and they release a number of inflammatory mediators including interleukin (IL)-4, IL-6, IL-10, TNFα, and leukotrienes in response to bacterial challenge. MC-derived TNFα and leukotrienes are shown to be important for bacterial clearance and early recruitment of phagocytic help at the site of infection. Studies directed at elucidating the molecular mechanisms associated with mast cell recognition of bacteria and subsequent events leading to mast cell mediator release revealed that GPI anchored CD48 molecule present on the cell surface of mast cells serves as a receptor for the bacterial adhesion molecule, FimH. The ligation of CD48 receptor by FimH-expresing bacteria results in bacterial uptake into caveolar chambers. This distinct mechanism of bacterial uptake promot es bacteral survival inside the cytosol of the mast cells. Although the exact mechanism(s) of how MC-dependent inflammatory responses are regulated is currently not known, recent studies have shown that complement, CD11β/CD18 (Mac-1) and protein tyrosine kinase JAK3, and TLR4 are important for the full expression of MC-dependent innate immunity in mice.     

Mast cell mediators in allergic inflammation and mastocytosis

Mast cells possess an array of potent inflammatory mediators capable of inducing acute symptoms after cell activation, including urticaria, angioedema, bronchoconstriction, diarrhea, vomiting, hypotension, cardiovascular collapse, and death in few minutes. In contrast, mast cells can provide an array of beneficial mediators in the setting of acute infections, cardiovascular diseases, and cancer. The balance between the detrimental and beneficial roles of mast cells is not completely understood. Although the symptoms of acute mast cell mediator release can be reversed with epinephrine, adrenergic agonists, and mediator blockers, the continued release of histamine, proteases, prostaglandins, leukotrienes, cytokines, and chemokines leads to chronic and debilitating disease, such as mastocytosis. Identification of the molecular factors and mechanisms that control the synthesis and release of mast cell mediators should benefit all patients with mast cell activation syndromes and mastocytosis.     

The role of the mast cell in asthma: induction of airway hyperresponsiveness by interaction with smooth muscle?

In a recent study, the difference between asthma and eosinophilic bronchitis (a condition characterized by cough but not airway hyperresponsiveness or airflow obstruction) was infiltration of airway smooth muscle (ASM) by mast cells. Mast cells produce a variety of lipid mediators, chemokines, cytokines, and enzymes that may interact with ASM cells to cause hyperreactivity to constrictive stimuli and proliferation, and activated ASM can produce stem cell factor and other chemokines, cytokines, and growth factors that may act in recruitment, differentiation, and retention of mast cells. Mast cell infiltration of the airways in asthma is T-cell-dependent, and TH2 cytokines from T cells and other sources act in mast cell expansion from circulating and tissue precursors. The recent data on interactions of mast cells and ASM suggest that this could be an important contributor to airway hyperresponsiveness in asthma. Why this occurs in asthma and how it is sustained remain to be established.     

Mast cell activation: A complex interplay of positive and negative signaling pathways

Mast cells regulate the immunological responses causing allergy and autoimmunity, and contribute to the tumor microenvironment through generation and secretion of a broad array of preformed, granule‐stored and de novo synthesized bioactive compounds. The release and production of mast cell mediators is the result of a coordinated signaling machinery, followed by the FcεRI and FcγR antigen ligation. In this review, we present the latest understanding of FcεRI and FcγR signaling, required for the canonical mast cell activation during allergic responses and anaphylaxis. We then describe the cooperation between the signaling of FcR and other recently characterized membrane‐bound receptors (i.e., IL‐33R and thymic stromal lymphopoietin receptor) and their role in the chronic settings, where mast cell activation is crucial for the development and the sustainment of chronic diseases, such as asthma or airway inflammation. Finally, we report how the FcR activation could be used as a therapeutic approach to treat allergic and atopic diseases by mast cell inactivation. Understanding the magnitude and the complexity of mast cell signaling is necessary to identify the mechanisms underlying the potential effector and regulatory roles of mast cells in the biology and pathology of those disease settings in which mast cells are activated.     

Mast cell contribution to angiogenesis related to tumour progression

The current wisdom is that tumours are endowed with an angiogenic capability and that their growth, invasion and metastasis are angiogenesis dependent. It is now well documented that neoplastic cells are influenced by their microenvironment and vice versa. The specific organ microenvironment determines the extent of cancer cell proliferation, angiogenesis, invasion and survival. Tumour cells are surrounded by an infiltrate of inflammatory cells, namely lymphocytes, neutrophils, macrophages and mast cells (MCs), which communicate via a complex network of intercellular signalling pathways, mediated by surface adhesion molecules, cytokines and their receptors. This review article summarizes: (i) the MC mediators involved in angiogenesis; (ii) the experimental evidence concerning the role played by MCs in angiogenesis; (iii) the list of solid and haematological tumours in which a close relationship between angiogenesis, tumour progression and MCs has been demonstrated; (iv) the circumstances in which MCs are a critical source of angiogenic factors in vivo, and in such cases, the signals that regulate their production and secretion that need to be determined as a prelude to the elaboration of new therapeutic strategies associated with MC presence and activation.     

A comparative study of the spatial distribution of mast cells and microvessels in the foetal, adult human thymus and thymoma

Mast cells (MCs) are widely distributed in human and animal tissues and have been shown to play an important role in angiogenesis in normal and pathological conditions. Few data are available about the relationship between MCs and blood vessels in the normal human thymus, and there are virtually no data about their distribution and significance in thymoma. The aim of this study was to analyse the spatial distribution of MCs and microvessels in the normal foetal and adult thymus and thymoma. Twenty biopsy specimens of human thymus, including foetal and adult normal thymus and thymoma were analysed. Double staining with CD34 and mast cell tryptase was used to count both mast cells and microvessels in the same fields. Computer-assisted image analysis was performed to characterize the spatial distribution of MCs and blood vessels in selected specimens. Results demonstrated that MCs were localized exclusively to the medulla. Their number was significantly higher in thymoma specimens as compared with adult and foetal normal specimens respectively. In contrast the microvessel area was unchanged. The analysis of the spatial distribution and relationship between MCs and microvessels revealed that only in the thymoma specimens was there a significant spatial association between MCs and microvessels. Overall, these data suggest that MCs do not contribute significantly to the development of the vascular network in foetal and adult thymus, whereas in thymoma they show a close relationship to blood vessels. This could be an expression of their involvement not only in endothelial cells but also in tumour cell proliferation.     

The allergic cascade: review of the most important molecules in the asthmatic lung

Asthma is the most common chronic inflammatory disorder of the airways among children. It is a complex clinical disease characterized by airway obstruction, airway inflammation and airway hyperresponsiveness to a variety of stimuli. The development of allergic asthma exists of three phases, namely the induction phase, the early-phase asthmatic reaction (EAR) and the late-phase asthmatic reaction (LAR). Each phase is characterized by the production and interplay of various cell-derived mediators. In the induction phase, T helper cytokines are important in the development of asthma. Most important mediators in the EAR are preformed mediators, newly synthesized lipid mediators and cytokines that are produced by mast cells. During the LAR, inflammatory molecules are produced by various cell types, such as eosinophils, neutrophils, T cells, macrophages, dendritic cells, and structural cells. Chronical inflammation leads to structural changes of the airway architecture. In this review, the most important mediators involved in the induction phase, the early-phase and late-phase asthmatic reaction are discussed.