New insights on mast cell activation via the high affinity receptor for IgE

Janus kinase-3 dependent inflammatory responses in allergic asthma

Allergic asthma is a chronic inflammatory condition of the lung characterized by reversible airway obstruction, high serum immunoglobulin (Ig) E levels, and chronic airway inflammation. A number of cells including mast cells, T cells, macrophages and dendritic cells play a role in the pathogenesis of the disease. Janus kinase (JAK)-3, a non-receptor protein tyrosine kinase, traditionally known to mediate cytokine signaling, also regulates functional responses of these cells. In this review the role of JAK-3 in regulating various pathogenic processes in allergic asthma is discussed. We propose that targeting JAK-3 is a rationale approach to control the inflammatory responses of multiple cell types responsible for the pathogenesis of allergic asthma.

IgE, mast cells, basophils, and eosinophils

IgE, mast cells, basophils, and eosinophils are essential components of allergic inflammation. Antigen-specific IgE production, with subsequent fixation of IgE to FcepsilonRI receptors on mast cells and basophils, is central to the initiation and propagation of immediate hypersensitivity reactions. Mast cells, basophils, and eosinophils are central effector cells in allergic inflammation, as well as in innate and adaptive immunity. This review highlights what is known about these components and their roles in disease pathogenesis.

ORAI1 and STIM1 deficiency in human and mice: roles of store-operated Ca2+ entry in the immune system and beyond

Store-operated Ca2+ entry (SOCE) is a mechanism used by many cells types including lymphocytes and other immune cells to increase intracellular Ca2+ concentrations to initiate signal transduction. Activation of immunoreceptors such as the T-cell receptor, B-cell receptor, or Fc receptors results in the release of Ca2+ ions from endoplasmic reticulum (ER) Ca2+ stores and subsequent activation of plasma membrane Ca2+ channels such as the well-characterized Ca2+ release-activated Ca2+ (CRAC) channel. Two genes have been identified that are essential for SOCE: ORAI1 as the pore-forming subunit of the CRAC channel in the plasma membrane and stromal interaction molecule-1 (STIM1) sensing the ER Ca2+ concentration and activating ORAI1-CRAC channels. Intense efforts in the past several years have focused on understanding the molecular mechanism of SOCE and the role it plays for cell functions in vitro and in vivo. A number of transgenic mouse models have been generated to investigate the role of ORAI1 and STIM1 in immunity. In addition, mutations in ORAI1 and STIM1 identified in immunodeficient patients provide valuable insight into the role of both genes and SOCE. This review focuses on the role of ORAI1 and STIM1 in vivo, discussing the phenotypes of ORAI1- and STIM1-deficient human patients and mice.

Involvement of Fc receptors in disorders of the central nervous system

Immunoglobulins are proteins with a highly variable antigen-binding domain and a constant region (Fc domain) that binds to a cell surface receptor (FcR). Activation of FcRs in immune cells (lymphocytes, macrophages, and mast cells) triggers effector responses including cytokine production, phagocytosis, and degranulation. In addition to their roles in normal responses to infection or tissue injury, and in immune-related diseases, FcRs are increasingly recognized for their involvement in neurological disorders. One or more FcRs are expressed in microglia, astrocytes, oligodendrocytes, and neurons. Aberrant activation of FcRs in such neural cells may contribute to the pathogenesis of major neurodegenerative conditions including Alzheimer's disease, Parkinson's disease, ischemic stroke, and multiple sclerosis. On the other hand, FcRs may play beneficial roles in counteracting pathological processes; for e.g., FcRs may facilitate removal of amyloid peptides from the brain and so protect against Alzheimer's disease. Knowledge of the functions of FcRs in the nervous system in health and disease is leading to novel preventative and therapeutic strategies for stroke, Alzheimer's disease, and other neurological disorders.

Immunopathogenesis of bronchial asthma

Bronchial asthma is a common immune-mediated disorder characterized by reversible airway inflammation, mucus production, and variable airflow obstruction with airway hyperresponsiveness. Allergen exposure results in the activation of numerous cells of the immune system, of which dendritic cells (DCs) and Th2 lymphocytes are of paramount importance. Although the epithelium was initially considered to function solely as a physical barrier, it is now evident that it plays a central role in the Th2-cell sensitization process due to its ability to activate DCs. Cytokines are inevitable factors in driving immune responses. To the list of numerous cytokines already known to be involved in the regulation of allergic reactions, new cytokines were added, such as TSLP, IL-25, and IL-33. IgE is also a central player in the allergic response. The activity of IgE is associated with a network of proteins, especially with its high- and low-affinity Fc receptors. Understanding the cellular and molecular mechanisms of allergic reactions helps us not only to understand the mechanisms of current treatments, but is also important for the identification of new targets for biological intervention. An IgE-specific monoclonal antibody, omalizumab, has already reached the clinic and similar biological agents will surely follow.

Time to death and mast cell degranulation in fatal asthma

BACKGROUND AND OBJECTIVE

Attacks of fatal asthma have been shown to be either of short duration or long duration and associated with differing degrees of smooth muscle contraction, luminal mucus deposition and ratios of eosinophils to neutrophils in the airway wall. We hypothesized that this bimodal distribution might be related to airway mast cell degranulation.

METHODS

Airway sections from cases of fatal asthma in the second Victorian asthma mortality study and from cases coming to coronial autopsy in Perth were examined. Tryptase-positive mast cells, numbers of intact and degranulated mast cells, post-mortem blood salbutamol levels and airway dimensions were measured.

RESULTS

Although the total number of mast cells were similar, the ratio of degranulated to intact mast cells (D/I) was significantly increased in the smooth muscle (P < 0.05) and outer airway wall (P < 0.001), in short-duration cases compared with long-duration cases. Proportional muscle shortening was significantly increased (P < 0.05) in short-duration cases (17 +/- 7%) compared with long-duration cases (11 +/- 7%). Blood salbutamol levels were related to the total airway wall mast cell D/I ratio for all cases combined (r = 0.57, P = 0.01).

CONCLUSIONS

The duration of a fatal attack of asthma may be partly determined by the degranulation of mast cells.

Lipid raft-dependent FcepsilonRI ubiquitination regulates receptor endocytosis through the action of ubiquitin binding adaptors

The best characterized role for ubiquitination of membrane receptors is to negatively regulate signaling by targeting receptors for lysosomal degradation. The high affinity receptor for IgE (FcεRI) expressed on mast cells and basophils is rapidly ubiquitinated upon antigen stimulation. However, the nature and the role of this covalent modification are still largelly unknown. Here, we show that FcεRI subunits are preferentially ubiquitinated at multiple sites upon stimulation, and provide evidence for a role of ubiquitin as an internalization signal: under conditions of impaired receptor ubiquitination a decrease of receptor entry is observed by FACS analysis and fluorescence microscopy. We also used biochemical approaches combined with fluorescence microscopy, to demonstrate that receptor endocytosis requires the integrity of specific membrane domains, namely lipid rafts. Additionally, by RNA interference we demonstrate the involvement of ubiquitin-binding endocytic adaptors in FcεRI internalization and sorting. Notably, the triple depletion of Eps15, Eps15R and Epsin1 negatively affects the early steps of Ag-induced receptor endocytosis, whereas Hrs depletion retains ubiquitinated receptors into early endosomes and partially prevents their sorting into lysosomes for degradation. Our results are compatible with a scenario in which the accumulation of engaged receptor subunits into lipid rafts is required for receptor ubiquitination, a prerequisite for efficient receptor internalization, sorting and delivery to a lysosomal compartment.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.

Siglec-8 on human eosinophils and mast cells, and Siglec-F on murine eosinophils, are functionally related inhibitory receptors

Siglecs (sialic acid-binding, Ig-like lectins) are a family of single-pass transmembrane cell surface proteins found predominantly on leucocytes. Their unique structural characteristics include an N-terminal carbohydrate-binding ('lectin') domain that binds sialic acid, followed by a variable number of Ig-like domains, hence these structures are a subset of the Ig gene superfamily. Another unique feature of Siglecs is that most, but not all, possess so-called immunoreceptor tyrosine-based inhibitory motifs in their cytoplasmic domains, suggesting that these molecules function in an inhibitory capacity. Siglec-8, the eighth member identified at the time, was discovered as part of an effort initiated almost a decade ago to identify novel human eosinophil and mast cell proteins. Since that time, its selective expression on human eosinophils and mast cells has been confirmed. On eosinophils, Siglec-8 engagement results in apoptosis, whereas on mast cells, inhibition of FcepsilonRI-dependent mediator release, without apoptosis, is seen. It has subsequently been determined that the closest functional paralog in the mouse is Siglec-F, selectively expressed by eosinophils but not expressed on mast cells. Despite only modest homology, both Siglec-8 and Siglec-F preferentially recognize a sulphated glycan ligand closely related to sialyl Lewis X, a common ligand for the selectin family of adhesion molecules. Murine experiments in normal, Siglec-F-deficient mice and hypereosinophilic mice have resulted in similar conclusions that Siglec-F, like Siglec-8, plays a distinctive and important role in regulating eosinophil accumulation and survival in vivo. Given the resurgent interest in eosinophil-directed therapies for a variety of disorders, plus its unique additional ability to also target the mast cell, therapies focusing on Siglec-8 could some day prove to be a useful adjunct to our current armamentarium for the treatment of asthma, allergies and related disorders where overproduction and overactivity of eosinophils and mast cells is occurring.