Amplification mechanisms for the enhancement of antigen-mediated mast cell activation

A Puzzling Mast Cell Trilogy: Anaphylaxis, MCAS, and Mastocytosis

Our knowledge of biology and mast cell (MC) function, as well as disorders associated with the pathologic activation of MCs, has evolved over the last few decades. Anaphylaxis, mast cell activation syndrome (MCAS), and mastocytosis are interrelated yet distinct conditions within the spectrum of mast cell activation disorders. Nevertheless, all three conditions can co-exist in one and the same patient, as pathologic MC activation is the key finding in all three. When mediator release is excessive and involves multiple systems, anaphylaxis and MCAS may occur. Furthermore, mastocytosis is a clonal disorder of MCs and often presents with anaphylaxis and MCAS. Nevertheless, in some cases, even the proliferative and accumulative features of MCs in mastocytosis can account for symptoms and disease progression. In each case, diagnosis can be only made when the diagnostic consensus criteria are fulfilled. The current article aims to provide a concise clinical update and pinpoint the main difficulties in diagnosing these puzzling disorders of MCs in medical practice.

Research Advances in Mast Cell Biology and Their Translation Into Novel Therapies for Anaphylaxis

Anaphylaxis is an acute, potentially life-threatening systemic allergic reaction for which there are no known reliable preventative therapies. Its primary cell mediator, the mast cell, has several pathophysiologic roles and functions in IgE-mediated reactions that continue to be poorly understood. Recent advances in the understanding of allergic mechanisms have identified novel targets for inhibiting mast cell function and activation. The prevention of anaphylaxis is within reach with new drugs that could modulate immune tolerance, mast cell proliferation and differentiation, and IgE regulation and production. Several US Food and Drug Administration-approved drugs for chronic urticaria, mastocytosis, and cancer are also being repurposed to prevent anaphylaxis. New therapeutics have not only shown promise in potential efficacy for preventing IgE-mediated reactions, but in some cases, they are able to inform us about mast cell mechanisms in vivo. This review summarizes the most recent advances in the treatment of anaphylaxis that have arisen from new pharmacologic tools and our current understanding of mast cell biology.

NJK14047 Suppression of the p38 MAPK Ameliorates OVA-Induced Allergic Asthma during Sensitization and Challenge Periods

p38 MAPK has been implicated in the pathogenesis of asthma as well as pro-allergic Th2 cytokines, orosomucoid-like protein isoform 3 (ORMDL3), regulation of sphingolipid biosynthesis, and regulatory T cell-derived IL-35. To elucidate the role of p38 MAPK in the pathogenesis of asthma, we examined the effect of NJK14047, an inhibitor of p38 MAPK, against ovalbumin (OVA)-induced allergic asthma; we administrated NJK14047 before OVA sensitization or challenge in BALB/c mice. As ORMDL3 regulation of sphingolipid biosynthesis has been implicated in childhood asthma, ORMDL3 expression and sphingolipids contents were also analyzed. NJK14047 inhibited antigen-induced degranulation of RBL-2H3 mast cells. NJK14047 administration both before OVA sensitization and challenge strongly inhibited the increase in eosinophil and lymphocyte counts in the bronchoalveolar lavage fluid. In addition, NJK14047 administration inhibited the increase in the levels of Th2 cytokines. Moreover, NJK14047 reduced the inflammatory score and the number of periodic acid-Schiff-stained cells in the lungs. Further, OVA-induced increase in the levels of C16:0 and C24:1 ceramides was not altered by NJK14047. These results suggest that p38 MAPK plays crucial roles in activation of dendritic and mast cells during sensitization and challenge periods, but not in ORMDL3 and sphingolipid biosynthesis.

6-O-angeloylplenolin inhibits anti-IgE-stimulated human mast cell activation via suppressing calcium influx and ERK phosphorylation

Objectives

Mast cells are important immune cells that primarily localize in the interface between the host and external environment, and protect us from pathogen infection. However, they are also involved in the pathology of allergic diseases such as asthma and atopic dermatitis. A novel S phase kinase-associated protein 1 (SKP1) inhibitor 6-O-angeloylplenolin (6-OAP), was studied with its potential ability to alleviate the anti-IgE-induced inflammatory responses of primary human cultured mast cells (HCMCs) and LAD2 cell line.

Materials and Methods

We isolated the HCMCs from the buffy coat of voluntary blood donors. The effects of 6-OAP on mast cell activation were evaluated by measuring degranulation, cytokine release, migration, calcium influx, and ERK phosphorylation using spectro-fluorescence assay, multiplex cytometric bead assay/ELISA, migration assay, Fluo-4 calcium flux assay, and western blot, respectively.

Results

It was found that 6-OAP exerted anti-inflammatory effects on human mast cells by dose-dependently suppressing the anti-IgE-mediated degranulation and release of cytokines such as proinflammatory cytokines (IL-8 and TNF-α), growth factors (GM-CSF, VEGF, and FGF), and chemokines (CCL2 and CCL3) in HCMC and LAD2 cells. It also suppressed the migration of immature HCMCs induced by CXCL12. Moreover, the process of calcium influx and ERK phosphorylation in activated HCMC cells were inhibited by 6-OAP administration.

Conclusion

Our results showed that 6-OAP inhibited anti-IgE-induced inflammatory responses of human mast cells via suppressing calcium influx and ERK phosphorylation.

Anaphylaxis and Mast Cell Disorders

There is strong evidence of an association between severe anaphylaxis, especially hymenoptera venom induced, and mast cell (MC) disorders. It has been thought that intrinsic abnormalities in MCs, including the presence of the activating KIT D816V mutation in mastocytosis or of genetic trait, hereditary alpha-tryptasemia, may influence susceptibility to severe anaphylaxis. This article evaluates the potential mechanisms leading to severe MC activation, as well as the differential diagnosis of and range of symptoms attributable to MC mediator release. Also, we offer a global classification for disorders related to MC activation.

Suppressive Effect of CYM50358 S1P4 Antagonist on Mast Cell Degranulation and Allergic Asthma in Mice

Levels of sphingosine 1-phosphate (S1P), an intercellular signaling molecule, reportedly increase in the bronchoalveolar lavage fluids of patients with asthma. Although the type 4 S1P receptor, S1P₄ has been detected in mast cells, its functions have been poorly investigated in an allergic asthma model in vivo. S1P₄ functions were evaluated following treatment of CYM50358, a selective antagonist of S1P₄, in an ovalbumin-induced allergic asthma model, and antigen-induced degranulation of mast cells. CYM50358 inhibited antigen-induced degranulation in RBL-2H3 mast cells. Eosinophil accumulation and an increase of Th2 cytokine levels were measured in the bronchoalveolar lavage fluid and via the inflammation of the lungs in ovalbumin-induced allergic asthma mice. CYM50358 administration before ovalbumin sensitization and before the antigen challenge strongly inhibited the increase of eosinophils and lymphocytes in the bronchoalveolar lavage fluid. CYM50358 administration inhibited the increase of IL-4 cytokines and serum IgE levels. Histological studies revealed that CYM50358 reduced inflammatory scores and PAS (periodic acid–Schiff)-stained cells in the lungs. The pro-allergic functions of S1P₄ were elucidated using in vitro mast cells and in vivo ovalbumin-induced allergic asthma model experiments. These results suggest that S1P₄ antagonist CYM50358 may have therapeutic potential in the treatment of allergic asthma.

Early development and functional properties of tryptase/chymase double-positive mast cells from human pluripotent stem cells

Mast cells (MCs) play a pivotal role in the hypersensitivity reaction by regulating the innate and adaptive immune responses. Humans have two types of MCs. The first type, termed MC_TC, is found in the skin and other connective tissues and expresses both tryptase and chymase, while the second, termed MC_T, which only expresses tryptase, is found primarily in the mucosa. MCs induced from human adult-type CD34⁺ cells are reported to be of the MC_T type, but the development of MCs during embryonic/fetal stages is largely unknown. Using an efficient coculture system, we identified that a CD34⁺c-kit⁺ cell population, which appeared prior to the emergence of CD34⁺CD45⁺ hematopoietic stem and progenitor cells (HSPCs), stimulated robust production of pure Tryptase⁺Chymase⁺ MCs (MC_TCs). Single-cell analysis revealed dual development directions of CD34⁺c-kit⁺ progenitors, with one lineage developing into erythro-myeloid progenitors (EMP) and the other lineage developing into HSPC. Interestingly, MC_TCs derived from early CD34⁺c-kit⁺ cells exhibited strong histamine release and immune response functions. Particularly, robust release of IL-17 suggested that these early developing tissue-type MC_TCs could play a central role in tumor immunity. These findings could help elucidate the mechanisms controlling early development of MC_TCs and have significant therapeutic implications.

Environmental and Endogenous Acids Can Trigger Allergic-Type Airway Reactions

Inflammatory allergic and nonallergic respiratory disorders are spreading worldwide and often coexist. The root cause is not clear. This review demonstrates that, from a biochemical point of view, it is ascribable to protons (H+) released into cells by exogenous and endogenous acids. The hypothesis of acids as the common cause stems from two considerations: (a) it has long been known that exogenous acids present in air pollutants can induce the irritation of epithelial surfaces, particularly the airways, inflammation, and bronchospasm; (b) according to recent articles, endogenous acids, generated in cells by phospholipases, play a key role in the biochemical mechanisms of initiation and progression of allergic-type reactions. Therefore, the intracellular acidification and consequent Ca2+ increase, induced by protons generated by either acid pollutants or endogenous phospholipases, may constitute the basic mechanism of the multimorbidity of these disorders, and environmental acidity may contribute to their spread.

Secretion of Mast Cell Inflammatory Mediators Is Enhanced by CADM1-Dependent Adhesion to Sensory Neurons

Neuroimmune interactions are important in the pathophysiology of many chronic inflammatory diseases, particularly those associated with alterations in sensory processing and pain. Mast cells and sensory neuron nerve endings are found in areas of the body exposed to the external environment, both are specialized to sense potential damage by injury or pathogens and signal to the immune system and nervous system, respectively, to elicit protective responses. Cell adhesion molecule 1 (CADM1), also known as SynCAM1, has previously been identified as an adhesion molecule which may couple mast cells to sensory neurons however, whether this molecule exerts a functional as well as structural role in neuroimmune cross-talk is unknown. Here we show, using a newly developed in vitro co-culture system consisting of murine bone marrow derived mast cells (BMMC) and adult sensory neurons isolated from dorsal root ganglions (DRG), that CADM1 is expressed in mast cells and adult sensory neurons and mediates strong adhesion between the two cell types. Non-neuronal cells in the DRG cultures did not express CADM1, and mast cells did not adhere to them. The interaction of BMMCs with sensory neurons was found to induce mast cell degranulation and IL-6 secretion and to enhance responses to antigen stimulation and activation of FcεRI receptors. Secretion of TNFα in contrast was not affected, nor was secretion evoked by compound 48/80. Co-cultures of BMMCs with HEK 293 cells, which also express CADM1, while also leading to adhesion did not replicate the effects of sensory neurons on mast cells, indicative of a neuron-specific interaction. Application of a CADM1 blocking peptide or knockdown of CADM1 in BMMCs significantly decreased BMMC attachment to sensory neurites and abolished the enhanced secretory responses of mast cells. In conclusion, CADM1 is necessary and sufficient to drive mast cell-sensory neuron adhesion and promote the development of a microenvironment in which neurons enhance mast cell responsiveness to antigen, this interaction could explain why the incidence of painful neuroinflammatory disorders such as irritable bowel syndrome (IBS) are increased in atopic patients.

Mastocytosis - pathogenesis, clinical manifestation and treatment

The term mastocytosis designates a group of rare disorders characterized by typical skin lesions, frequently associated episodes of anaphylaxis, and clinical symptoms related to the release of various mediators. Dermatologists/allergists are frequently the first to establish the diagnosis. The condition is based on clonal mast cell proliferation, usually in the skin or bone marrow and only rarely in the gastrointestinal tract or other tissues. In general, mastocytosis has a good prognosis in terms of life expectancy. Rare variants - including mast cell leukemia, aggressive mastocytosis, and the exceedingly rare mast cell sarcoma - require cytoreductive therapy. In cases associated with hematological neoplasms, the prognosis depends on the underlying hematologic disorder.

S1P₄ Regulates Passive Systemic Anaphylaxis in Mice but Is Dispensable for Canonical IgE-Mediated Responses in Mast Cells

Mast cells are key players in the development of inflammatory allergic reactions. Cross-linking of the high-affinity receptor for IgE (FcεRI) on mast cells leads to the generation and secretion of the sphingolipid mediator, sphingosine-1-phosphate (S1P) which is able, in turn, to transactivate its receptors on mast cells. Previous reports have identified the expression of two of the five receptors for S1P on mast cells, S1P₁ and S1P₂, with functions in FcεRI-mediated chemotaxis and degranulation, respectively. Here, we show that cultured mouse mast cells also express abundant message for S1P₄. Genetic deletion of S1pr4 did not affect the differentiation of bone marrow progenitors into mast cells or the proliferation of mast cells in culture. A comprehensive characterization of IgE-mediated responses in S1P₄-deficient bone marrow-derived and peritoneal mouse mast cells indicated that this receptor is dispensable for mast cell degranulation, cytokine/chemokine production and FcεRI-mediated chemotaxis in vitro. However, interleukin-33 (IL-33)-mediated enhancement of IgE-induced degranulation was reduced in S1P₄-deficient peritoneal mast cells, revealing a potential negative regulatory role for S1P₄ in an IL-33-rich environment. Surprisingly, genetic deletion of S1pr4 resulted in exacerbation of passive systemic anaphylaxis to IgE/anti-IgE in mice, a phenotype likely related to mast cell-extrinsic influences, such as the high circulating levels of IgE in these mice which increases FcεRI expression and consequently the extent of the response to FcεRI engagement. Thus, we provide evidence that S1P₄ modulates anaphylaxis in an unexpected manner that does not involve regulation of mast cell responsiveness to IgE stimulation.

Pathogenesis and Pathology of Mastocytosis

Systemic mastocytosis is a clonal disorder of mast cells that may variably present with characteristic skin lesions, episodes of mast cell mediator release, and disturbances of hematopoiesis. No curative therapy presently exists. Conventional management has relied on agents that antagonize mediators released by mast cells, inhibit mediator secretion, or modulate mast cell proliferation. Recent advances in the molecular understanding of the pathophysiology of systemic mastocytosis have provided new therapeutic considerations, including new and novel tyrosine kinase inhibitors.

Frontline Science: Corticotropin-releasing factor receptor subtype 1 is a critical modulator of mast cell degranulation and stress-induced pathophysiology

Life stress is a major risk factor in the onset and exacerbation of mast cell-associated diseases, including allergy/anaphylaxis, asthma, and irritable bowel syndrome. Although it is known that mast cells are highly activated upon stressful events, the mechanisms by which stress modulates mast cell function and disease pathophysiology remains poorly understood. Here, we investigated the role of corticotropin-releasing factor receptor subtype 1 (CRF₁) in mast cell degranulation and associated disease pathophysiology. In a mast cell-dependent model of IgE-mediated passive systemic anaphylaxis (PSA), prophylactic administration of the CRF₁-antagonist antalarmin attenuated mast cell degranulation and hypothermia. Mast cell-deficient Kit^W-sh/W-sh mice engrafted with CRF₁^-/- bone marrow-derived mast cells (BMMCs) exhibited attenuated PSA-induced serum histamine, hypothermia, and clinical scores compared with wild-type BMMC-engrafted Kit^W-sh/W-sh mice. Kit^W-sh/W-sh mice engrafted with CRF₁^-/- BMMCs also exhibited suppressed in vivo mast cell degranulation and intestinal permeability in response to acute restraint stress. Genetic and pharmacologic experiments with murine BMMCs, rat RBL-2H3, and human LAD2 mast cells demonstrated that although CRF₁ activation did not directly induce MC degranulation, CRF₁ signaling potentiated the degranulation responses triggered by diverse mast cell stimuli and was associated with enhanced release of Ca²⁺ from intracellular stores. Taken together, our results revealed a prominent role for CRF₁ signaling in mast cells as a positive modulator of stimuli-induced degranulation and in vivo pathophysiologic responses to immunologic and psychologic stress.

Decoupling the Functional Pleiotropy of Stem Cell Factor by Tuning c-Kit Signaling

Most secreted growth factors and cytokines are functionally pleiotropic because their receptors are expressed on diverse cell types. While important for normal mammalian physiology, pleiotropy limits the efficacy of cytokines and growth factors as therapeutics. Stem cell factor (SCF) is a growth factor that acts through the c-Kit receptor tyrosine kinase to elicit hematopoietic progenitor expansion but can be toxic when administered in vivo because it concurrently activates mast cells. We engineered a mechanism-based SCF partial agonist that impaired c-Kit dimerization, truncating downstream signaling amplitude. This SCF variant elicited biased activation of hematopoietic progenitors over mast cells in vitro and in vivo. Mouse models of SCF-mediated anaphylaxis, radioprotection, and hematopoietic expansion revealed that this SCF partial agonist retained therapeutic efficacy while exhibiting virtually no anaphylactic off-target effects. The approach of biasing cell activation by tuning signaling thresholds and outputs has applications to many dimeric receptor-ligand systems.

Trimellitic anhydride induces low-grade mast cell degranulation without specific IgE

Objectives

Low-molecular-weight (LMW) substances are known to be causative agents of occupational asthma (OA) and occupational rhinitis (OR). Although most LMW substances are irritants or allergens, some can cause immediate type immunoglobulin E (IgE)-mediated allergic reactions. Trimellitic anhydride (TMA) is one such LMW substance, which is known as an immunological sensitizer. However, the exact molecular biological details of the effects of TMA remain unclear.

Methods

We measured the β-hexosaminidase release from mast cells after directly exposing the cells to various LMW substances. The tyrosine phosphorylation of whole cellular molecules and the phosphorylation of extracellular signal-regulated kinase (ERK) were assessed by immunoblot assay.

Results

Among the LMW substances tested, only TMA induced β-hexosaminidase release. However, the mast cell degranulation induced by TMA was lower than that induced by an antigen or a calcium ionophore. Moreover, the pattern of tyrosine phosphorylation of whole cellular molecules was quite different between IgE-mediated antigen stimulation and TMA exposure. The TMA effect on mast cells was independent of not only IgE but also Ca²⁺ influx. ERK phosphorylation was not detected in mast cells exposed to TMA.

Conclusions

TMA induced mild degranulation of mast cells without IgE, even though the phosphorylation of ERK was not detected. This reaction suggests that TMA affects humans even upon first exposure. Therefore, it is imperative to avoid human exposure to high concentrations of TMA. In order to stop the development of severe asthma in individuals with OR, we need to be able to identify cases of OR caused by TMA as soon as possible.

Go is required for the release of IL-8 and TNF-α, but not degranulation in human mast cells

Mast cells activated by IgE-dependent and -independent mechanisms play important roles in innate and acquired immune responses. Activation of pertussis toxin (PTX)-sensitive Gi/o proteins is the key step in mast cell degranulation and release of de novo synthesized inflammatory mediators through IgE-independent mechanism. However, the roles of Gi and Go proteins in mast cells activation have not yet been differentiated. In the current study, the functional roles of Go proteins in the activities of LAD2 cells, a human mast cell line, are identified. Knockdown of Gαo expression significantly inhibited the synthesis of IL-8 and TNF-α from substance P activated LAD2 cells but demonstrated no effect on degranulation. This effect was associated with the activation of Erk and JNK/MAPKs signaling, whereas PI3K-Akt, calcium mobilization and NFAT translocation remained unchanged. These results suggest that Gi and Go proteins differentially regulate human mast cells activities through activating distinct signaling cascades.

Tim-3 enhances FcεRI-proximal signaling to modulate mast cell activation

Phong et al. show that depending on the expression of p-Lyn, mast cell activation by antigen can result in dichotomous effects on mast cell function and signaling that can be accentuated by Tim-3 ligation.

T cell (or transmembrane) immunoglobulin and mucin domain protein 3 (Tim-3) has attracted significant attention as a novel immune checkpoint receptor (ICR) on chronically stimulated, often dysfunctional, T cells. Antibodies to Tim-3 can enhance antiviral and antitumor immune responses. Tim-3 is also constitutively expressed by mast cells, NK cells and specific subsets of macrophages and dendritic cells. There is ample evidence for a positive role for Tim-3 in these latter cell types, which is at odds with the model of Tim-3 as an inhibitory molecule on T cells. At this point, little is known about the molecular mechanisms by which Tim-3 regulates the function of T cells or other cell types. We have focused on defining the effects of Tim-3 ligation on mast cell activation, as these cells constitutively express Tim-3 and are activated through an ITAM-containing receptor for IgE (FcεRI), using signaling pathways analogous to those in T cells. Using a variety of gain- and loss-of-function approaches, we find that Tim-3 acts at a receptor-proximal point to enhance Lyn kinase-dependent signaling pathways that modulate both immediate-phase degranulation and late-phase cytokine production downstream of FcεRI ligation.

Mast cell function: a new vision of an old cell

Since first described by Paul Ehrlich in 1878, mast cells have been mostly viewed as effectors of allergy. It has been only in the past two decades that mast cells have gained recognition for their involvement in other physiological and pathological processes. Mast cells have a widespread distribution and are found predominantly at the interface between the host and the external environment. Mast cell maturation, phenotype and function are a direct consequence of the local microenvironment and have a marked influence on their ability to specifically recognize and respond to various stimuli through the release of an array of biologically active mediators. These features enable mast cells to act as both first responders in harmful situations as well as to respond to changes in their environment by communicating with a variety of other cells implicated in physiological and immunological responses. Therefore, the critical role of mast cells in both innate and adaptive immunity, including immune tolerance, has gained increased prominence. Conversely, mast cell dysfunction has pointed to these cells as the main offenders in several chronic allergic/inflammatory disorders, cancer and autoimmune diseases. This review summarizes the current knowledge of mast cell function in both normal and pathological conditions with regards to their regulation, phenotype and role.

Role of basophils in the pathogenesis of minimal change nephrotic syndrome: A literature review

A number of studies have verified that minimal change nephrotic syndrome (MCNS) may result from the dysfunction of T cells and B cells, although the precise mechanisms are yet to be elucidated. It is widely recognized that MCNS is a T helper (Th)2-dominant glomerular disease caused by an imbalanced Th1/Th2 immune response. Increased levels of the Th2 cytokines, interleukin (IL)-4 and IL-13, have been demonstrated to be closely associated with disease activity. In addition, basophils can affect the Th1/Th2 balance by enhancing the Th2 response and impairing the Th1 response, which are then involved in the development of numerous diseases. However, whether basophils are vital in the pathogenesis of MCNS remains unknown. Frequent positivity of the human basophil degranulation test in patients with MCNS has been observed. Thus, basophils should be analyzed in order to determine their role in the pathogenesis of MCNS.

Characterization of mast cell secretory granules and their cell biology

Exocytosis and secretion of secretory granule (SG) contained inflammatory mediators is the primary mechanism by which mast cells exert their protective immune responses in host defense, as well as their pathological functions in allergic reactions and anaphylaxis. Despite their central role in mast cell function, the molecular mechanisms underlying the biogenesis and secretion of mast cell SGs remain largely unresolved. Early studies have established the lysosomal nature of the mast cell SGs and implicated SG homotypic fusion as an important step occurring during both their biogenesis and compound secretion. However, the molecular mechanisms that account for key features of this process largely remain to be defined. A novel high-resolution imaging based methodology allowed us to screen Rab GTPases for their phenotypic and functional impact and identify Rab networks that regulate mast cell secretion. This screen has identified Rab5 as a novel regulator of homotypic fusion of the mast cell SGs that thereby regulates their size and cargo composition.

The extra-cellular signal regulated kinases ERK1 and ERK2 segregate displaying distinct spatiotemporal characteristics in activated mast cells

ERK1 and ERK2 are highly homologous isoforms that often play redundant roles in regulating cellular functions. We analyzed the spatiotemporal patterns of ERK1 and ERK2 in resting and activated mast cells. Strikingly, we identified distinct pathways for these kinases. ERK1 localized to the cytosol and translocated to the nucleus upon cell activation and kinase phosphorylation. In contrast, ERK2 distributed between the cytosol and near the microtubule organizing center (MTOC) in resting cells and accumulated further at a pericentrosomal region upon cell trigger. Pericentrosomal accumulation of ERK2 was phosphorylation independent, required an intact microtubule network and was significantly enhanced by the overexpression of Neuronal Calcium Sensor-1 (NCS-1). We also identified γ-tubulin and phosphatidylinositol 4 kinaseβ (PI4Kβ), a downstream effector of NCS-1, as novel partner proteins of ERK2. Taken together, our results imply non-redundant functions of ERK1 and ERK2 in mast cells and implicate NCS-1 and PΙ4Κβ as regulators of ERK2 trafficking.

Interrogation of sphingosine-1-phosphate receptor 2 function in vivo reveals a prominent role in the recovery from IgE and IgG-mediated anaphylaxis with minimal effect on its onset

Autocrine stimulation of S1PR2, a receptor for the lipid mediator sphingosine-1-phosphate (S1P), has been implicated in mast cell degranulation to IgE/antigen (Ag) although, paradoxically, its ligand cannot trigger substantial degranulation. Additionally, the in vivo role of S1PR2 in the overall allergic responses is unclear since S1PR2 was reported to be required for the onset of systemic anaphylaxis by IgE/Ag but, in apparent contradiction, also for the recovery from histamine-induced anaphylaxis in a mast cell independent manner. Here, we sought to clarify the role of S1PR2 in mast cell degranulation and in IgE and IgG-mediated anaphylaxis. Lack of S1PR2 reduced IgE/Ag-induced degranulation in in vitro experiments with mucosal mast cells, but had no effect on connective tissue type mast cells. This latter response correlated with a lack of involvement of S1PR2 in the onset of non-lethal IgE/Ag-mediated systemic and cutaneous anaphylaxis. However, S1pr2(-/-) mice were slow to recover (or did not recover) from FcɛRI-mediated anaphylaxis, an outcome that mirrored their known impairment in histamine clearance due to defective vascular tone. A minor role for S1PR2 in mast cell degranulation was uncovered upon engagement of low affinity receptors for IgG and in the onset of IgG-mediated anaphylaxis. Our findings show that S1PR2 is dispensable for initiating IgE/Ag-mediated connective tissue mast cell degranulation and anaphylaxis, but it is required for normal recovery from anaphylaxis.

Scratching the surface of allergic transfusion reactions

Allergic transfusion reactions (ATRs) are a spectrum of hypersensitivity reactions that are the most common adverse reaction to platelets and plasma, occurring in up to 2% of transfusions. Despite the ubiquity of these reactions, little is known about their mechanism. In a small subset of severe reactions, specific antibody has been implicated as causal, although this mechanism does not explain all ATRs. Evidence suggests that donor, product, and recipient factors are involved, and it is possible that many ATRs are multifactorial. Further understanding of the mechanisms of ATRs is necessary so that rationally designed and cost-effective prevention measures can be developed.

Mast cell adenosine receptors function: a focus on the a3 adenosine receptor and inflammation

Adenosine is a metabolite, which has long been implicated in a variety of inflammatory processes. Inhaled adenosine provokes bronchoconstriction in asthmatics or chronic obstructive pulmonary disease patients, but not in non-asthmatics. This hyper responsiveness to adenosine appears to be mediated by mast cell activation. These observations have marked the receptor that mediates the bronchoconstrictor effect of adenosine on mast cells (MCs), as an attractive drug candidate. Four subtypes (A1, A2a, A2b, and A3) of adenosine receptors have been cloned and shown to display distinct tissue distributions and functions. Animal models have firmly established the ultimate role of the A3 adenosine receptor (A3R) in mediating hyper responsiveness to adenosine in MCs, although the influence of the A2b adenosine receptor was confirmed as well. In contrast, studies of the A3R in humans have been controversial. In this review, we summarize data on the role of different adenosine receptors in mast cell regulation of inflammation and pathology, with a focus on the common and distinct functions of the A3R in rodent and human MCs. The relevance of mouse studies to the human is discussed.

Pathogenesis, classification and treatment of mastocytosis: state of the art in 2010 and future perspectives

Mastocytosis is a myeloid neoplasm characterized by abnormal accumulation and frequent activation of mast cells (MCs) in various organs. Organ systems typically involved are the bone marrow, skin, liver and gastrointestinal tract. In most adult patients, the systemic form of mastocytosis (SM) is diagnosed, which includes an indolent subvariant, an aggressive subvariant and a leukemic subvariant, also termed MC leukemia. Whereas in pediatric mastocytosis, which is usually confined to the skin, a number of different KIT mutations and other defects may be detected, the KIT mutation D816V is detectable in most (adult) patients with SM. In a subset of these patients, additional oncogenic factors may lead to enhanced survival and growth of MCs and, thus, to advanced SM. Other factors may lead to MC activation, with consecutive anaphylactic reactions that can be severe or even fatal. Treatment of SM usually focuses on symptom relief by histamine receptor antagonists and other supportive therapy. However, in aggressive and leukemic variants, cytoreductive and targeted drugs must be applied. Unfortunately, the prognosis in these patients remains poor, even when treated with novel KIT-targeting agents, polychemotherapy or stem cell transplantation. This article provides a summary of our knowledge on the pathogenesis and on treatment options in SM.

TRPV1-lineage neurons are required for thermal sensation

The ion-channel TRPV1 is believed to be a major sensor of noxious heat, but surprisingly animals lacking TRPV1 still display marked responses to elevated temperature. In this study, we explored the role of TRPV1-expressing neurons in somatosensation by generating mice wherein this lineage of cells was selectively labelled or ablated. Our data show that TRPV1 is an embryonic marker of many nociceptors including all TRPV1- and TRPM8-neurons as well as many Mrg-expressing neurons. Mutant mice lacking these cells are completely insensitive to hot or cold but in marked contrast retain normal touch and mechanical pain sensation. These animals also exhibit defective body temperature control and lose both itch and pain reactions to potent chemical mediators. Together with previous cell ablation studies, our results define and delimit the roles of TRPV1- and TRPM8-neurons in thermosensation, thermoregulation and nociception, thus significantly extending the concept of labelled lines in somatosensory coding.

P2Y(13) receptor is responsible for ADP-mediated degranulation in RBL-2H3 rat mast cells

Extracellular ADP is known to play many important physiological roles. In this study, we identified the P2Y(13) receptor in a rat mast cell line (RBL-2H3) and explored the functional role of ADP, its endogenous agonist. ADP induced both intracellular calcium mobilization and release of hexosaminidase (Hex). In an assay of intracellular calcium, ADP was 100-fold less potent than and equally efficacious as the P2Y(1) receptor-selective agonist MRS2365. However, ADP was more potent and efficacious than MRS2365 in inducing Hex release and in enhancing antigen-induced Hex release. ADP-induced intracellular calcium mobilization was blocked by phospholipase C inhibitor U73122 and by P2Y(1) receptor-selective antagonist MRS2500, but not by pertussis toxin (PTX), suggesting a mechanism mediated by the G(q)-coupled P2Y(1) receptor, but not P2Y(13) (G(i)-coupled) or P2X receptors. ADP-induced Hex release was blocked by PTX and a selective P2Y(13) receptor antagonist MRS2211, but not by MRS2500 or P2Y(1) receptor-specific siRNA, suggesting a G(i)-coupled P2Y(13) receptor-related mechanism. Measurement of gene expression confirmed high expression of both P2Y(1) and P2Y(13) receptors (in comparison to a previously reported P2Y(14) receptor) in RBL-2H3 cells. Thus, we demonstrated that ADP-mediated intracellular calcium mobilization and Hex release in RBL-2H3 cells are via P2Y(1) and P2Y(13) receptors, respectively. Selective antagonists of the P2Y(13) receptor might be novel therapeutic agents for various allergic conditions.

LAB/NTAL/Lat2: a force to be reckoned with in all leukocytes?

LAB/NTAL/Lat2 is a transmembrane adaptor protein closely related to LAT. It is expressed in various myeloid and lymphoid cells, many of which also express LAT. Phosphorylation of LAB occurs following engagement of various ITAM- and non-ITAM-linked receptors and can play positive and negative roles following receptor engagement. LAT binds PLCγ directly, resulting in efficient Ca²+ flux and degranulation. However, LAB does not contain a PLCγ-binding motif and only binds PLCγ indirectly, possibly via Grb2, thereby resulting in suboptimal signaling. As LAT can signal more efficiently than LAB, competition between the 2 for space/substrates in the lipid rafts can attenuate signaling. This competition model requires coexpression of LAT; however, LAB is repressive, even in cells lacking substantial LAT expression such as macrophages and mature B cells. The reported interaction between LAB and the ubiquitin E3-ligase c-Cbl suggests 1 possible mechanism for LAT-independent inhibition by LAB, but such a model requires further investigation. Given the wide-reaching expression pattern of LAB, LAB has the ability to modulate signaling in virtually every type of leukocyte. Regardless of its ultimate mode of action, the potent regulatory capability of LAB proves this protein to be a complex adaptor that warrants continued, substantial scrutiny by biochemists and immunologists alike.

Mastocytosis

It is known that patients with mastocytosis have an increased risk of anaphylaxis. This also appears to be the case with patients with evidence of a clonal mast cell disorder resulting in the monoclonal mast cell activation syndrome (MMAS) who do not express the full mastocytosis phenotype. Most patients with mastocytosis are recognized by their characteristic skin lesions. An increased level of baseline serum mast cell tryptase is also an indicator for a possible clonal mast cell disorder including mastocytosis. Other markers for mast cell clonality and for mastocytosis include abnormal immunostaining of mast cells with CD25 and CD2, clustering of mast cells in tissues, abnormal mast cell morphology, and the presence of a mutation in the proto-oncogene c-kit encoding for the mast cell growth receptor KIT. As recognition depends on an understanding of mastocytosis, and this disease should be considered in patients with recurrent anaphylaxis, we describe the features of mast cell clonality, MMAS and mastocytosis, and review recent findings.

CD72 negatively regulates KIT-mediated responses in human mast cells

KIT activation, through binding of its ligand, stem cell factor, is crucial for normal mast cell growth, differentiation, and survival. Furthermore, KIT may also contribute to mast cell homing and cytokine generation. Activating mutations in KIT lead to the dysregulated mast cell growth associated with the myeloproliferative disorder, mastocytosis. We investigated the potential of downregulating such responses through mast cell inhibitory receptor activation. In this study, we report that the B cell-associated ITIM-containing inhibitory receptor, CD72, is expressed in human mast cells. Ligation of CD72 with the agonistic Ab, BU40, or with recombinant human CD100 (rCD100), its natural ligand, induced the phosphorylation of CD72 with a resulting increase in its association with the tyrosine phosphatase SH2 domain-containing phosphatase-1. This, in turn, resulted in an inhibition of KIT-induced phosphorylation of Src family kinases and extracellular-regulated kinases (ERK1/2). As a consequence of these effects, KIT-mediated mast cell proliferation, chemotaxis, and chemokine production were significantly reduced by BU40 and rCD100. Furthermore, BU40 and rCD100 also downregulated the growth of the HMC1.2 human mast cell line. Thus, targeting CD72 may provide a novel approach to the suppression of mast cell disease such as mastocytosis.

UDP-glucose acting at P2Y14 receptors is a mediator of mast cell degranulation

UDP-glucose (UDPG), a glycosyl donor in the biosynthesis of carbohydrates, is an endogenous agonist of the G protein-coupled P2Y(14) receptor. RBL-2H3 mast cells endogenously express a P2Y(14) receptor at which UDPG mediates degranulation as indicated by beta-hexosaminidase (HEX) release. Both UDPG and a more potent, selective 2-thio-modified UDPG analog, MRS2690 (diphosphoric acid 1-alpha-d-glucopyranosyl ester 2-[(2-thio)uridin-5''-yl] ester), caused a substantial calcium transient in RBL-2H3 cells, which was blocked by pertussis toxin, indicating the presence of the G(i)-coupled P2Y(14) receptor, supported also by quantitative detection of abundant mRNA. Expression of the closely related P2Y(6) receptor was over 100 times lower than the P2Y(14) receptor, and the P2Y(6) agonist 3-phenacyl-UDP was inactive in RBL-2H3 cells. P2Y(14) receptor agonists also induced [(35)S]GTPgammaS binding to RBL-2H3 cell membranes, and phosphorylation of ERK1/2, P38 and JNK. UDPG and MRS2690 concentration-dependently enhanced HEX release with EC(50) values of 1150+/-320 and 103+/-18nM, respectively. The enhancement was completely blocked by pertussis toxin and significantly diminished by P2Y(14) receptor-specific siRNA. Thus, mast cells express an endogenous P2Y(14) receptor, which mediates G(i)-dependent degranulation and is therefore a potential novel therapeutic target for allergic conditions.

Molecular regulation of mast cell development and maturation

Mast cells play a crucial role in the pathogenesis of allergic diseases. In recent years, tremendous progresses have been made in studies of mast cell origination, migration, proliferation, maturation and survival, and the cytokines regulating these activities. These advances have significantly improved our understandings to mast cell biology and to the molecular mechanisms of mast cells in the pathogenesis of allergic diseases.