Insights into immunoglobulin E receptor signaling from structurally defined ligands

Antigen Geometry Tunes Mast Cell Signaling Through Distinct FcεRI Aggregation and Structural Changes

Immunoreceptor tyrosine-based activation motif (ITAM)-containing Fc receptors are critical components of the innate and adaptive immune systems. FcεRI mediates the allergic response via crosslinking of IgE-bound receptors by multivalent antigens. Yet, the underlying molecular mechanisms that govern the response of FcεRI to specific antigens remain poorly understood. We compared responses induced by two antigens with distinct geometries, high valency DNP-BSA and trivalent DF3, and found unique secretion and receptor phosphorylation profiles that are due to differential recruitment of Lyn and SHIP1. To understand how these two antigens can cause such markedly different outcomes, we used direct stochastic optical reconstruction microscopy (dSTORM) super-resolution imaging combined with Bayesian Grouping of Localizations (BaGoL) analysis to compare the nanoscale characteristics of FcεRI aggregates. DF3 aggregates were found to be smaller and more densely packed than DNP-BSA aggregates. Using lifetime-based Förster resonance energy transfer (FRET) measurements, we discovered that FcεRI subunits undergo structural rearrangements upon crosslinking with either antigen, and in response to interaction with monovalent antigen presented on a supported lipid bilayer. The extent of conformational change is positively correlated with signaling efficiency. Finally, we provide evidence for forces in optimizing FcεRI signaling, such that immobilizing DF3 on a rigid surface promoted degranulation while increasing DNP-BSA flexibility lowered degranulation. These results provide a link between the physical attributes of allergens, including size, shape, valency, and flexibility, and FcεRI signaling strength. Thus, the antigen modulates mast cell outcomes by creating unique aggregate geometries that tune FcεRI conformation, phosphorylation and signaling partner recruitment.

Hapten-Decorated DNA Nanostructures Decipher the Antigen-Mediated Spatial Organization of Antibodies Involved in Mast Cell Activation

The immunological response of mast cells is controlled by the multivalent binding of antigens to immunoglobulin E (IgE) antibodies bound to the high-affinity receptor FcεRI on the cell membrane surface. However, the spatial organization of antigen-antibody-receptor complexes at the nanometer scale and the structural constraints involved in the initial events at the cell surface are not yet fully understood. For example, it is unclear what influence the affinity and nanoscale distance between the binding partners involved have on the activation of mast cells to degranulate inflammatory mediators from storage granules. We report the use of DNA origami nanostructures (DON) functionalized with different arrangements of the haptenic 2,4-dinitrophenyl (DNP) ligand to generate multivalent artificial antigens with full control over valency and nanoscale ligand architecture. To investigate the spatial requirements for mast cell activation, the DNP-DON complexes were initially used in surface plasmon resonance (SPR) analysis to study the binding kinetics of isolated IgE under physiological conditions. The most stable binding was observed in a narrow window of approximately 16 nm spacing between haptens. In contrast, affinity studies with FcεRI-linked IgE antibodies on the surface of rat basophilic leukemia cells (RBL-2H3) indicated virtually no distance-dependent variations in the binding of the differently structured DNP-DON complexes but suggested a supramolecular oligovalent nature of the interaction. Finally, the use of DNP-DON complexes for mast cell activation revealed that antigen-directed tight assembly of antibody-receptor complexes is the critical factor for triggering degranulation, even more critical than ligand valence. Our study emphasizes the significance of DNA nanostructures for the study of fundamental biological processes.

Transbilayer Coupling of Lipids in Cells Investigated by Imaging Fluorescence Correlation Spectroscopy

Plasma membranes host numerous receptors, sensors, and ion channels involved in cellular signaling. Phase separation within the plasma membrane has emerged as a key biophysical regulator of signaling reactions in multiple physiological and pathological contexts. There is much evidence that plasma membrane composition supports the coexistence of liquid-ordered (Lo) and liquid-disordered (Ld) phases or domains at physiological conditions. However, this phase/domain separation is nanoscopic and transient in live cells. It has been recently proposed that transbilayer coupling between the inner and outer leaflets of the plasma membrane is driven by their asymmetric lipid distribution and by dynamic cytoskeleton-lipid composites that contribute to the formation and transience of Lo/Ld phase separation in live cells. In this Perspective, we highlight new approaches to investigate how transbilayer coupling may influence phase separation. For quantitative evaluation of the impact of these interactions, we introduce an experimental strategy centered around Imaging Fluorescence Correlation Spectroscopy (ImFCS), which measures membrane diffusion with very high precision. To demonstrate this strategy, we choose two well-established model systems for transbilayer interactions: cross-linking by multivalent antigen of immunoglobulin E bound to receptor FcεRI and cross-linking by cholera toxin B of GM1 gangliosides. We discuss emerging methods to systematically perturb membrane lipid composition, particularly exchange of outer leaflet lipids with exogenous lipids using methyl alpha cyclodextrin. These selective perturbations may be quantitatively evaluated with ImFCS and other high-resolution biophysical tools to discover novel principles of lipid-mediated phase separation in live cells in the context of their pathophysiological relevance.

The role of allergen-specific IgE, IgG and IgA in allergic disease

Immunoglobulin E (IgE)‐mediated allergy is the most common hypersensitivity disease affecting more than 30% of the population. Exposure to even minute quantities of allergens can lead to the production of IgE antibodies in atopic individuals. This is termed allergic sensitization, which occurs mainly in early childhood. Allergen‐specific IgE then binds to the high (FcεRI) and low‐affinity receptors (FcεRII, also called CD23) for IgE on effector cells and antigen‐presenting cells. Subsequent and repeated allergen exposure increases allergen‐specific IgE levels and, by receptor cross‐linking, triggers immediate release of inflammatory mediators from mast cells and basophils whereas IgE‐facilitated allergen presentation perpetuates T cell–mediated allergic inflammation. Due to engagement of receptors which are highly selective for IgE, even tiny amounts of allergens can induce massive inflammation. Naturally occurring allergen‐specific IgG and IgA antibodies usually recognize different epitopes on allergens compared with IgE and do not efficiently interfere with allergen‐induced inflammation. However, IgG and IgA antibodies to these important IgE epitopes can be induced by allergen‐specific immunotherapy or by passive immunization. These will lead to competition with IgE for binding with the allergen and prevent allergic responses. Similarly, anti‐IgE treatment does the same by preventing IgE from binding to its receptor on mast cells and basophils. Here, we review the complex interplay of allergen‐specific IgE, IgG and IgA and the corresponding cell receptors in allergic diseases and its relevance for diagnosis, treatment and prevention of allergy.

Reviving lost binding sites: Exploring calcium-binding site transitions between human and murine CD23

Immunoglobulin E (IgE) is a central regulatory and triggering molecule of allergic immune responses. IgE's interaction with CD23 modulates both IgE production and functional activities.CD23 is a noncanonical immunoglobulin receptor, unrelated to receptors of other antibody isotypes. Human CD23 is a calcium-dependent (C-type) lectin-like domain that has apparently lost its carbohydrate-binding capability. The calcium-binding site classically required for carbohydrate binding in C-type lectins is absent in human CD23 but is present in the murine molecule. To determine whether the absence of this calcium-binding site affects the structure and function of human CD23, CD23 mutant proteins with increasingly "murine-like" sequences were generated. Restoration of the calcium-binding site was confirmed by NMR spectroscopy, and structures of mutant human CD23 proteins were determined by X-ray crystallography, although no electron density for calcium was observed. This study offers insights into the evolutionary differences between murine and human CD23 and some of the functional differences between CD23 in different species.

Transcription Factors in the Development and Pro-Allergic Function of Mast Cells

Mast cells (MCs) are innate immune cells of hematopoietic origin localized in the mucosal tissues of the body and are broadly implicated in the pathogenesis of allergic inflammation. Transcription factors have a pivotal role in the development and differentiation of mast cells in response to various microenvironmental signals encountered in the resident tissues. Understanding the regulation of mast cells by transcription factors is therefore vital for mechanistic insights into allergic diseases. In this review we summarize advances in defining the transcription factors that impact the development of mast cells throughout the body and in specific tissues, and factors that are involved in responding to the extracellular milieu. We will further describe the complex networks of transcription factors that impact mast cell physiology and expansion during allergic inflammation and functions from degranulation to cytokine secretion. As our understanding of the heterogeneity of mast cells becomes more detailed, the contribution of specific transcription factors in mast cell-dependent functions will potentially offer new pathways for therapeutic targeting.

Fenebrutinib in H1 antihistamine-refractory chronic spontaneous urticaria: a randomized phase 2 trial

Bruton's tyrosine kinase (BTK) is crucial for FcεRI-mediated mast cell activation and essential for autoantibody production by B cells in chronic spontaneous urticaria (CSU). Fenebrutinib, an orally administered, potent, highly selective, reversible BTK inhibitor, may be effective in CSU. This double-blind, placebo-controlled, phase 2 trial (EudraCT ID 2016-004624-35 ) randomized 93 adults with antihistamine-refractory CSU to 50 mg daily, 150 mg daily and 200 mg twice daily of fenebrutinib or placebo for 8 weeks. The primary end point was change from baseline in urticaria activity score over 7 d (UAS7) at week 8. Secondary end points were the change from baseline in UAS7 at week 4 and the proportion of patients well-controlled (UAS7 ≤ 6) at week 8. Fenebrutinib efficacy in patients with type IIb autoimmunity and effects on IgG-anti-FcεRI were exploratory end points. Safety was also evaluated. The primary end point was met, with dose-dependent improvements in UAS7 at week 8 occurring at 200 mg twice daily and 150 mg daily, but not at 50 mg daily of fenebrutinib versus placebo. Asymptomatic, reversible grade 2 and 3 liver transaminase elevations occurred in the fenebrutinib 150 mg daily and 200 mg twice daily groups (2 patients each). Fenebrutinib diminished disease activity in patients with antihistamine-refractory CSU, including more patients with refractory type IIb autoimmunity. These results support the potential use of BTK inhibition in antihistamine-refractory CSU.

Interplay between Affinity and Valency in Effector Cell Degranulation: A Model System with Polcalcin Allergens and Human Patient-Derived IgE Antibodies

An allergic reaction is rapidly generated when allergens bind and cross-link IgE bound to its receptor FcεRI on effector cells, resulting in cell degranulation and release of proinflammatory mediators. The extent of effector cell activation is linked to allergen affinity, oligomeric state, valency, and spacing of IgE-binding epitopes on the allergen. Whereas most of these observations come from studies using synthetic allergens, in this study we have used Timothy grass pollen allergen Phl p 7 and birch pollen allergen Bet v 4 to study these effects. Despite the high homology of these polcalcin family allergens, Phl p 7 and Bet v 4 display different binding characteristics toward two human patient-derived polcalcin-specific IgE Abs. We have used native polcalcin dimers and engineered multimeric allergens to test the effects of affinity and oligomeric state on IgE binding and effector cell activation. Our results indicate that polcalcin multimers are required to stimulate high levels of effector cell degranulation when using the humanized RBL-SX38 cell model and that multivalency can overcome the need for high-affinity interactions.

Controlling Mast Cell Activation and Homeostasis: Work Influenced by Bill Paul That Continues Today

Mast cells are tissue resident, innate immune cells with heterogenous phenotypes tuned by cytokines and other microenvironmental stimuli. Playing a protective role in parasitic, bacterial, and viral infections, mast cells are also known for their role in the pathogenesis of allergy, asthma, and autoimmune diseases. Here, we review factors controlling mast cell activation, with a focus on receptor signaling and potential therapies for allergic disease. Specifically, we will discuss our work with FcεRI and FγR signaling, IL-4, IL-10, and TGF-β1 treatment, and Stat5. We conclude with potential therapeutics for allergic disease. Much of these efforts have been influenced by the work of Bill Paul. With many mechanistic targets for mast cell activation and different classes of therapeutics being studied, there is reason to be hopeful for continued clinical progress in this area.

Omalizumab for severe asthma: toward personalized treatment based on biomarker profile and clinical history

Asthma is a heterogeneous syndrome with numerous underlining molecular and inflammatory mechanisms contributing to the wide spectrum of clinical phenotypes. Multiple therapies targeting severe asthma with type 2 (T2) high inflammation are or soon will be available. T2 high inflammation is defined as inflammation associated with atopy or eosinophilia or an increase in cytokines associated with T-helper 2 lymphocytes. Omalizumab is a humanized anti-IgE monoclonal antibody and the first biologic therapy approved for moderate-severe allergic asthma. Despite the specificity of biologic therapies like omalizumab, clinical response is variable, with approximately 50% of treated patients achieving the primary outcome. A prior identification of the ideal candidate for therapy would improve patient outcomes and optimize the use of health care resources. As the number of biologic therapies for asthma increases, the goal is identification of biomarkers or clinical phenotypes likely to respond to a specific therapy. This review focuses on potential biomarkers and clinical history that may identify responders to omalizumab therapy for asthma.

Biological characteristics of intestinal IgE and gut diseases

Immunoglobulin E (IgE), a crucial protective substance for the intestinal tract, plays an important role in gut immunity. IgE is secreted by plasma cells in the submucosal lamina propria upon antigenic invasion and, together with certain cytokines and immune cells, is involved in the regulation of gastrointestinal immunity in normal or abnormal conditions via the high affinity IgE receptor (FcεR I) and low affinity IgE receptor (CD23⁺). In this paper, we review the structure, synthetic transport, secretory regulation, receptor classification, and function of intestinal IgE as well as the related gut diseases.

Targeting Intramembrane Protein-Protein Interactions: Novel Therapeutic Strategy of Millions Years Old

Intramembrane protein-protein interactions (PPIs) are involved in transmembrane signal transduction mediated by cell surface receptors and play an important role in health and disease. Recently, receptor-specific modulatory peptides rationally designed using a general platform of transmembrane signaling, the signaling chain homooligomerization (SCHOOL) model, have been proposed to therapeutically target these interactions in a variety of serious diseases with unmet needs including cancer, sepsis, arthritis, retinopathy, and thrombosis. These peptide drug candidates use ligand-independent mechanisms of action (SCHOOL mechanisms) and demonstrate potent efficacy in vitro and in vivo. Recent studies surprisingly revealed that in order to modify and/or escape the host immune response, human viruses use similar mechanisms and modulate cell surface receptors by targeting intramembrane PPIs in a ligand-independent manner. Here, I review these intriguing mechanistic similarities and discuss how the viral strategies optimized over a billion years of the coevolution of viruses and their hosts can help to revolutionize drug discovery science and develop new, disruptive therapies. Examples are given.

Applying Pattern Recognition to High-Resolution Images to Determine Cellular Signaling Status

Two frequently used tools to acquire high- resolution images of cells are scanning electron microscopy (SEM) and atomic force microscopy (AFM). The former provides a nanometer resolution view of cellular features rapidly and with high throughput, while the latter enables visualizing hydrated and living cells. In current practice, these images are viewed by eye to determine cellular status, e.g., activated versus resting. Automatic and quantitative data analysis is lacking. This paper develops an algorithm of pattern recognition that works very effectively for AFM and SEM images. Using rat basophilic leukemia cells, our approach creates a support vector machine to automatically classify resting and activated cells. Ten-fold cross-validation with cells that are known to be activated or resting gives a good estimate of the generalized classification results. The pattern recognition of AFM images achieves 100% accuracy, while SEM reaches 95.4% for our images as well as images published in prior literature. This outcome suggests that our methodology could become an important and frequently used tool for researchers utilizing AFM and SEM for structural characterization as well as determining cellular signaling status and function.

Differential Ca2+ mobilization and mast cell degranulation by FcεRI- and GPCR-mediated signaling

Mast cells play a primary role in allergic diseases. During an allergic reaction, mast cell activation is initiated by cross-linking IgE-FcεRI complex by multivalent antigen resulting in degranulation. Additionally, G protein-coupled receptors also induce degranulation upon activation. However, the spatio-temporal relationship between Ca²⁺ mobilization and mast cell degranulation is not well understood. We investigated the relationship between oscillations in Ca²⁺ level and mast cell degranulation upon stimulation in rat RBL-2H3 cells. Nile red and Fluo-4 were used as probes for monitoring histamine and intracellular Ca²⁺ levels, respectively. Histamine release and Ca²⁺ oscillations in real-time were monitored using total internal reflection fluorescence microscopy (TIRFM). Mast cell degranulation followed immediately after FcεRI and GPCR-mediated Ca²⁺ increase. FcεRI-induced Ca²⁺ increase was higher and more sustained than that induced by GPCRs. However, no significant difference in mast cell degranulation rates was observed. Although intracellular Ca²⁺ release was both necessary and sufficient for mast cell degranulation, extracellular Ca²⁺ influx enhanced the process. Furthermore, cytosolic Ca²⁺ levels and mast cell degranulation were significantly decreased by downregulation of store-operated Ca²⁺ entry (SOCE) via Orai1 knockdown, 2-aminoethyl diphenylborinate (2-APB) or tubastatin A (TSA) treatment. Collectively, this study has demonstrated the role of Ca²⁺ signaling in regulating histamine degranulation.

Molecular Mechanisms of Phagosome Formation

Phagocytosis refers to the active process that allows cells to take up large particulate material upon binding to surface receptors. The discovery of phagocytosis in 1883 by Elie Metchnikoff, leading to the concept that specialized cells are implicated in the defense against microbes, was one of the starting points of the field of immunology. After more than a century of research, phagocytosis is now appreciated to be a widely used process that enables the cellular uptake of a remarkable variety of particles, including bacteria, fungi, parasites, viruses, dead cells, and assorted debris and solid materials. Uptake of foreign particles is performed almost exclusively by specialized myeloid cells, commonly termed "professional phagocytes": neutrophils, monocytes, macrophages, and dendritic cells. Phagocytosis of microbes not only stops or at least restricts the spread of infection but also plays an important role in regulating the innate and adaptive immune responses. Activation of the myeloid cells upon phagocytosis leads to the secretion of cytokines and chemokines that convey signals to a variety of immune cells. Moreover, foreign antigens generated by the degradation of microbes following phagocytosis are loaded onto the major histocompatibility complex for presentation to specific T lymphocytes. However, phagocytosis is not restricted to professional myeloid phagocytes; an expanding diversity of cell types appear capable of engulfing apoptotic bodies and debris, playing a critical role in tissue remodeling and in the clearance of billions of effete cells every day.

The endocytic pathway in microglia during health, aging and Alzheimer's disease

Microglia, the main phagocytes of the central nervous system (CNS), are involved in the surveillance and maintenance of nervous tissue. During normal tissue homeostasis, microglia migrates within the CNS, phagocytose dead cells and tissue debris, and modulate synapse pruning and spine formation via controlled phagocytosis. In the event of an invasion by a foreign body, microglia are able to phagocytose the invading pathogen and process it proteolytically for antigen presentation. Internalized substrates are incorporated and sorted within the endocytic pathway and thereafter transported via complex vesicular routes. When targeted for degradation, substrates are delivered to acidic late endosomes and lysosomes. In these, the enzymatic degradation relies on pH and enzyme content. Endocytosis, sorting, transport, compartment acidification and degradation are regulated by complex signaling mechanisms, and these may be altered during aging and pathology. In this review, we discuss the endocytic pathway in microglia, with insight into the mechanisms controlling lysosomal biogenesis and pH regulation. We also discuss microglial lysosome function associated with Alzheimer's disease (AD) and the mechanisms of amyloid-beta (Aβ) internalization and degradation. Finally, we explore some therapies currently being investigated to treat AD and their effects on microglial response to Aβ, with insight in those involving enhancement of lysosomal function.

Functional nanoscale coupling of Lyn kinase with IgE-FcεRI is restricted by the actin cytoskeleton in early antigen-stimulated signaling

Spatial targeting of signaling components to activated receptors on the plasma membrane is key for initiating signal transduction. The actin cytoskeleton restricts antigen-stimulated colocalization of IgE-FcεRI with membrane-anchored signaling partner Lyn kinase, and this regulation is mediated by organization of plasma membrane lipids.

The allergic response is initiated on the plasma membrane of mast cells by phosphorylation of the receptor for immunoglobulin E (IgE), FcεRI, by Lyn kinase after IgE-FcεRI complexes are cross-linked by multivalent antigen. Signal transduction requires reorganization of receptors and membrane signaling proteins, but this spatial regulation is not well defined. We used fluorescence localization microscopy (FLM) and pair-correlation analysis to measure the codistribution of IgE-FcεRI and Lyn on the plasma membrane of fixed cells with 20- to 25-nm resolution. We directly visualized Lyn recruitment to IgE-FcεRI within 1 min of antigen stimulation. Parallel FLM experiments captured stimulation-induced FcεRI phosphorylation and colocalization of a saturated lipid-anchor probe derived from Lyn’s membrane anchorage. We used cytochalasin and latrunculin to investigate participation of the actin cytoskeleton in regulating functional interactions of FcεRI. Inhibition of actin polymerization by these agents enhanced colocalization of IgE-FcεRI with Lyn and its saturated lipid anchor at early stimulation times, accompanied by augmented phosphorylation within FcεRI clusters. Ising model simulations provide a simplified model consistent with our results. These findings extend previous evidence that IgE-FcεRI signaling is initiated by colocalization with Lyn in ordered lipid regions and that the actin cytoskeleton regulates this functional interaction by influencing the organization of membrane lipids.

Anti-IgE treatment, airway inflammation and remodelling in severe allergic asthma: current knowledge and future perspectives

Asthma is a disorder of the airways involving various inflammatory cells and mediators and characterised by bronchial hyperresponsiveness, chronic inflammation and structural alterations in the airways, also known as remodelling. IgE is an important mediator of allergic reactions and has a central role in allergic asthma pathophysiology, as it is implicated in both the early and late phase allergic response. Moreover, clinical and mechanistic evidence has lately emerged, implicating IgE in the development of airway remodelling. The use of monoclonal antibodies targeting IgE, such as omalizumab, has proven very effective in improving respiratory symptoms and quality of life, while reducing asthma exacerbations, emergency room visits and the use of systemic corticosteroids in allergic severe asthma. These effects are believed to be mainly mediated by omalizumab's inhibitory effect on the initiation and further propagation of the allergic inflammation cascade. However, there is evidence to suggest that anti-IgE treatment remains effective long after it has been discontinued. In part, these findings could be attributed to the possible ameliorating effects of anti-IgE treatment on airway remodelling. In this review, we discuss recent findings supporting the notion that anti-IgE treatment modulates the complex immune responses that manifest clinically as asthma and ameliorates airway remodelling changes often observed in allergic severe asthma phenotypes.

The multiple facets of FcRn in immunity

The neonatal Fc receptor, FcRn, is best known for its role in transporting IgG in various tissues, providing newborns with humoral immunity, and for prolonging the half-life of IgG. Recent findings implicate the involvement of FcRn in a far wider range of biological and immunological processes, as FcRn has been found to bind and extend the half-life of albumin; to be involved in IgG transport and antigen sampling at mucosal surfaces; and to be crucial for efficient IgG-mediated phagocytosis. Herein, the function of FcRn will be reviewed, with emphasis on its recently documented significance for IgG polymorphisms affecting the half-life and biodistribution of IgG3, on its role in phagocyte biology, and the subsequent role for the presentation of antigens to lymphocytes.

A novel real time imaging platform to quantify macrophage phagocytosis

Phagocytosis of pathogens, apoptotic cells and debris is a key feature of macrophage function in host defense and tissue homeostasis. Quantification of macrophage phagocytosis in vitro has traditionally been technically challenging. Here we report the optimization and validation of the IncuCyte ZOOM® real time imaging platform for macrophage phagocytosis based on pHrodo® pathogen bioparticles, which only fluoresce when localized in the acidic environment of the phagolysosome. Image analysis and fluorescence quantification were performed with the automated IncuCyte™ Basic Software. Titration of the bioparticle number showed that the system is more sensitive than a spectrofluorometer, as it can detect phagocytosis when using 20× less E. coli bioparticles. We exemplified the power of this real time imaging platform by studying phagocytosis of murine alveolar, bone marrow and peritoneal macrophages. We further demonstrate the ability of this platform to study modulation of the phagocytic process, as pharmacological inhibitors of phagocytosis suppressed bioparticle uptake in a concentration-dependent manner, whereas opsonins augmented phagocytosis. We also investigated the effects of macrophage polarization on E. coli phagocytosis. Bone marrow-derived macrophage (BMDM) priming with M2 stimuli, such as IL-4 and IL-10 resulted in higher engulfment of bioparticles in comparison with M1 polarization. Moreover, we demonstrated that tolerization of BMDMs with lipopolysaccharide (LPS) results in impaired E. coli bioparticle phagocytosis. This novel real time assay will enable researchers to quantify macrophage phagocytosis with a higher degree of accuracy and sensitivity and will allow investigation of limited populations of primary phagocytes in vitro.

Biochemical and Functional Insights into the Integrated Regulation of Innate Immune Cell Responses by Teleost Leukocyte Immune-Type Receptors

Across vertebrates, innate immunity consists of a complex assortment of highly specialized cells capable of unleashing potent effector responses designed to destroy or mitigate foreign pathogens. The execution of various innate cellular behaviors such as phagocytosis, degranulation, or cell-mediated cytotoxicity are functionally indistinguishable when being performed by immune cells isolated from humans or teleost fishes; vertebrates that diverged from one another more than 450 million years ago. This suggests that vital components of the vertebrate innate defense machinery are conserved and investigating such processes in a range of model systems provides an important opportunity to identify fundamental features of vertebrate immunity. One characteristic that is highly conserved across vertebrate systems is that cellular immune responses are dependent on specialized immunoregulatory receptors that sense environmental stimuli and initiate intracellular cascades that can elicit appropriate effector responses. A wide variety of immunoregulatory receptor families have been extensively studied in mammals, and many have been identified as cell- and function-specific regulators of a range of innate responses. Although much less is known in fish, the growing database of genomic information has recently allowed for the identification of several immunoregulatory receptor gene families in teleosts. Many of these putative immunoregulatory receptors have yet to be assigned any specific role(s), and much of what is known has been based solely on structural and/or phylogenetic relationships with mammalian receptor families. As an attempt to address some of these shortcomings, this review will focus on our growing understanding of the functional roles played by specific members of the channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs), which appear to be important regulators of several innate cellular responses via classical as well as unique biochemical signaling networks.

Potential allergenicity response to structural modification of irradiated bovine α-lactalbumin

Bovine α-lactalbumin (α-La) is a major food allergen found in milk and is characterized by high conformational stability because of its four disulfide bridges and being calcium bound.

Anti-IgE treatment, airway inflammation and remodelling in severe allergic asthma: current knowledge and future perspectives

Asthma is a disorder of the airways involving various inflammatory cells and mediators and characterised by bronchial hyperresponsiveness, chronic inflammation and structural alterations in the airways, also known as remodelling. IgE is an important mediator of allergic reactions and has a central role in allergic asthma pathophysiology, as it is implicated in both the early and late phase allergic response. Moreover, clinical and mechanistic evidence has lately emerged, implicating IgE in the development of airway remodelling. The use of monoclonal antibodies targeting IgE, such as omalizumab, has proven very effective in improving respiratory symptoms and quality of life, while reducing asthma exacerbations, emergency room visits and the use of systemic corticosteroids in allergic severe asthma. These effects are believed to be mainly mediated by omalizumab's inhibitory effect on the initiation and further propagation of the allergic inflammation cascade. However, there is evidence to suggest that anti-IgE treatment remains effective long after it has been discontinued. In part, these findings could be attributed to the possible ameliorating effects of anti-IgE treatment on airway remodelling. In this review, we discuss recent findings supporting the notion that anti-IgE treatment modulates the complex immune responses that manifest clinically as asthma and ameliorates airway remodelling changes often observed in allergic severe asthma phenotypes.

Engineered Nanostructures of Haptens Lead to Unexpected Formation of Membrane Nanotubes Connecting Rat Basophilic Leukemia Cells

A recent finding reports that co-stimulation of the high-affinity immunoglobulin E (IgE) receptor (FcεRI) and the chemokine receptor 1 (CCR1) triggered formation of membrane nanotubes among bone-marrow-derived mast cells. The co-stimulation was attained using corresponding ligands: IgE binding antigen and macrophage inflammatory protein 1α (MIP1 α), respectively. However, this approach failed to trigger formation of nanotubes among rat basophilic leukemia (RBL) cells due to the lack of CCR1 on the cell surface (Int. Immunol. 2010, 22 (2), 113-128). RBL cells are frequently used as a model for mast cells and are best known for antibody-mediated activation via FcεRI. This work reports the successful formation of membrane nanotubes among RBLs using only one stimulus, a hapten of 2,4-dinitrophenyl (DNP) molecules, which are presented as nanostructures with our designed spatial arrangements. This observation underlines the significance of the local presentation of ligands in the context of impacting the cellular signaling cascades. In the case of RBL, certain DNP nanostructures suppress antigen-induced degranulation and facilitate the rearrangement of the cytoskeleton to form nanotubes. These results demonstrate an important scientific concept; engineered nanostructures enable cellular signaling cascades, where current technologies encounter great difficulties. More importantly, nanotechnology offers a new platform to selectively activate and/or inhibit desired cellular signaling cascades.

A Crystallin Fold in the Interleukin-4-inducing Principle of Schistosoma mansoni Eggs (IPSE/α-1) Mediates IgE Binding for Antigen-independent Basophil Activation

The IL-4-inducing principle from Schistosoma mansoni eggs (IPSE/α-1), the major secretory product of eggs from the parasitic worm S. mansoni, efficiently triggers basophils to release the immunomodulatory key cytokine interleukin-4. Activation by IPSE/α-1 requires the presence of IgE on the basophils, but the detailed molecular mechanism underlying activation is unknown. NMR and crystallographic analysis of IPSEΔNLS, a monomeric IPSE/α-1 mutant, revealed that IPSE/α-1 is a new member of the βγ-crystallin superfamily. We demonstrate that this molecule is a general immunoglobulin-binding factor with highest affinity for IgE. NMR binding studies of IPSEΔNLS with the 180-kDa molecule IgE identified a large positively charged binding surface that includes a flexible loop, which is unique to the IPSE/α-1 crystallin fold. Mutational analysis of amino acids in the binding interface showed that residues contributing to IgE binding are important for IgE-dependent activation of basophils. As IPSE/α-1 is unable to cross-link IgE, we propose that this molecule, by taking advantage of its unique IgE-binding crystallin fold, activates basophils by a novel, cross-linking-independent mechanism.

Probing heterobivalent binding to the endocytic AP-2 adaptor complex by DNA-based spatial screening

The DNA-programmed peptide display in brain extract revealed a co-operation between the binding sites on the AP-2 alpha-appendage domain.

IgE immunotherapy against cancer

The success of antibody therapy in cancer is consistent with the ability of these molecules to activate immune responses against tumors. Experience in clinical applications, antibody design, and advancement in technology have enabled antibodies to be engineered with enhanced efficacy against cancer cells. This allows re-evaluation of current antibody approaches dominated by antibodies of the IgG class with a new light. Antibodies of the IgE class play a central role in allergic reactions and have many properties that may be advantageous for cancer therapy. IgE-based active and passive immunotherapeutic approaches have been shown to be effective in both in vitro and in vivo models of cancer, suggesting the potential use of these approaches in humans. Further studies on the anticancer efficacy and safety profile of these IgE-based approaches are warranted in preparation for translation toward clinical application.

Actin cytoskeleton reorganization by Syk regulates Fcγ receptor responsiveness by increasing its lateral mobility and clustering

Clustering of immunoreceptors upon association with multivalent ligands triggers important responses including phagocytosis, secretion of cytokines, and production of immunoglobulins. We applied single-molecule detection and tracking methods to study the factors that control the mobility and clustering of phagocytic Fcγ receptors (FcγR). While the receptors exist as monomers in resting macrophages, two distinct populations were discernible based on their mobility: some diffuse by apparent free motion, while others are confined within submicron boundaries that reduce the frequency of spontaneous collisions. Src-family and Syk kinases determine the structure of the actin cytoskeleton, which is fenestrated, accounting for the heterogeneous diffusion of the FcγR. Stimulation of these kinases during phagocytosis induces reorganization of the cytoskeleton both locally and distally in a manner that alters receptor mobility and clustering, generating a feedback loop that facilitates engagement of FcγR at the tip of pseudopods, directing the progression of phagocytosis.

Optimal aggregation of FcεRI with a structurally defined trivalent ligand overrides negative regulation driven by phosphatases

To investigate why responses of mast cells to antigen-induced IgE receptor (FcεRI) aggregation depend nonlinearly on antigen dose, we characterized a new artificial ligand, DF3, through complementary modeling and experimentation. This ligand is a stable trimer of peptides derived from bacteriophage T4 fibritin, each conjugated to a hapten (DNP). We found low and high doses of DF3 at which degranulation of mast cells sensitized with DNP-specific IgE is minimal, but ligand-induced receptor aggregation is comparable to aggregation at an intermediate dose, optimal for degranulation. This finding makes DF3 an ideal reagent for studying the balance of negative and positive signaling in the FcεRI pathway. We find that the lipid phosphatase SHIP and the protein tyrosine phosphatase SHP-1 negatively regulate mast cell degranulation over all doses considered. In contrast, SHP-2 promotes degranulation. With high DF3 doses, relatively rapid recruitment of SHIP to the plasma membrane may explain the reduced degranulation response. Our results demonstrate that optimal secretory responses of mast cells depend on the formation of receptor aggregates that promote sufficient positive signaling by Syk to override phosphatase-mediated negative regulatory signals.

An Interaction Library for the FcεRI Signaling Network

Antigen receptors play a central role in adaptive immune responses. Although the molecular networks associated with these receptors have been extensively studied, we currently lack a systems-level understanding of how combinations of non-covalent interactions and post-translational modifications are regulated during signaling to impact cellular decision-making. To fill this knowledge gap, it will be necessary to formalize and piece together information about individual molecular mechanisms to form large-scale computational models of signaling networks. To this end, we have developed an interaction library for signaling by the high-affinity IgE receptor, FcεRI. The library consists of executable rules for protein–protein and protein–lipid interactions. This library extends earlier models for FcεRI signaling and introduces new interactions that have not previously been considered in a model. Thus, this interaction library is a toolkit with which existing models can be expanded and from which new models can be built. As an example, we present models of branching pathways from the adaptor protein Lat, which influence production of the phospholipid PIP₃ at the plasma membrane and the soluble second messenger IP₃. We find that inclusion of a positive feedback loop gives rise to a bistable switch, which may ensure robust responses to stimulation above a threshold level. In addition, the library is visualized to facilitate understanding of network circuitry and identification of network motifs.

Fyn kinase regulates type II PtdIns 4-kinases in RBL 2H3 cells

Type II phosphatidylinositol 4-kinases are implicated in FcεRI-mediated signaling cascades leading to release of inflammatory molecules. Cross-linking of FcεRI on RBL 2H3 cells results in protein tyrosine phosphorylation and activation of type II PtdIns 4-kinase activity. Protein tyrosine kinase(s) that phosphorylate type II PtdIns 4-kinase(s) in RBL 2H3 cells remains elusive and is being addressed in this manuscript. Anti-Fyn kinase antibodies co-immunoprecipitated type II PtdIns 4-kinase activity from FcεRI cross-linked RBL 2H3 cells. In reciprocal assays, His-tagged types II PtdIns 4-kinases were shown to pull down Fyn kinase. Further, anti-Fyn immunoprecipitates were shown to phosphorylate type II PtdIns 4-kinase α and β in in vitro assays. Pull down studies with GST-Fyn-SH2 and GST-Fyn-SH3 domains showed that type II PtdIns 4-kinases associate with Fyn-SH2 domain. Knockdown of Fyn kinase in RBL 2H3 cells abrogated activation of type II PtdIns 4-kinase activity in response to FcεRI cross-linking and type II PtdIns 4-kinase activity in anti-phosphotyrosine immunoprecipitates. Knockdown of Fyn kinase was also strongly correlated with a reduction in β-hexosaminidase release in response to FcεRI cross-linking. These results suggest that type II PtdIns 4-kinases act downstream of Fyn kinase in FcεRI signaling cascades and are regulated by Fyn kinase.

The role of Cε2, Cε3, and Cε4 domains in human and canine IgE and their contribution to FcεRIα interaction

The Cε2 and Cε4 domains are considered as scaffolds, allowing Cε3 domains to assume an appropriate orientation to interact with FcεRI (Wurzburg and Jardetzky, 2002; Hunter et al., 2008). Human/canine IgE chimeric antibodies were expressed to assess the nature of the contribution of Cε2 and Cε4 domains to bind to and induce target cell degranulation via FcεRIα. Our results indicate that for (1) Cε3 domains in IgE of canine and human origin are the only necessary region for binding to FcεRIα. (2) The interaction of canine IgE with human sFcεRIα is significantly enhanced by contributions from both Cε2 and Cε4 domains of dog origin. (3) The canine/human IgE chimeric antibody construct rapidly dissociates from its the receptor when the canine Cε2 and Cε4 domains are replaced by the homologous human Fc domains which do not confer a conformation on the Cε3 domain to facilitate stable interaction with canine FcRIα. Kinetic constants for the binding of this chimera to the soluble extracellular domain of the receptor indicate an approximate 120-fold decrease in the affinity for canine sFcεRIα (ka=5.30 × 10(2)M(-1)s(-1)) and a 330-fold increase in the dissociation from canine sFcεRIα (KD=6.9 × 10(-6)M(-1)), compared to the wild type IgE kinetic constants (Ka=6.30 × 10(4)M(-1)s(-1); KD=2.1 × 10(-8)M(-1)). Although canine IgE does engage human FcεRIα, canine Cε2 and Cε4 do not contribute to the high-affinity of interaction with human FcεRIα. Upon replacement of human Cε2 and Cε4 domain by the canine homologues, human IgE Cε3 only retains a low affinity for the human receptor, which shows that Cε2 and Cε4 domains in human IgE Fc contribute significantly to the interaction with its cognate receptor.

In vitro modelling of rat mucosal mast cell function in Trichinella spiralis infection

Intestinal infection with the parasitic nematode, Trichinella spiralis, provides a robust context for the study of mucosal mast cell function. In rats, mucosal mast cells are exposed to parasites during the earliest stage of infection, affording an opportunity for mast cells to contribute to an innate response to infection. During secondary infection, degranulation of rat mucosal mast cells coincides with expulsion of challenge larvae from the intestine. The goal of this study was to evaluate the rat bone marrow-derived mast cells (BMMC) and the rat basophilic leukaemia cell line (RBL-2H3) as models for mucosal mast cells, using parasite glycoproteins and antibody reagents that have been tested extensively in rats in vivo. We found that BMMC displayed a more robust mucosal phenotype. Although T. spiralis glycoproteins bound to mast cell surfaces in the absence of antibodies, they did not stimulate degranulation, nor did they inhibit degranulation triggered by immune complexes. Parasite glycoproteins complexed with specific monoclonal IgGs provoked release of rat mast cell protease II (RMCPII) and β-hexosaminidase from both cell types in a manner that replicated results observed previously in passively immunized rats. Our results document that RBL-2H3 cells and BMMC model rat mucosal mast cells in the contexts of innate and adaptive responses to T. spiralis.

Metabolic regulation of the squid nerve Na(+)/Ca (2+) exchanger: recent developments

In squid nerves, MgATP modulation of the Na(+)/Ca(2+) exchanger requires the presence of a cytosolic protein which becomes phosphorylated during the process. This factor has been recently identified. Mass spectroscopy and Western blot analysis established that it is a member of the lipocalin superfamily of lipid-binding proteins (LBP or FABP) of 132 amino acids. We called it regulatory protein of squid nerve sodium/calcium exchanger (ReP1-NCXSQ, access to GenBank EU981897).ReP1-NCXSQ was cloned, expressed, and purified. Circular dichroism, far-UV, and infrared spectroscopy suggest a secondary structure, predominantly of beta-sheets. The tertiary structure prediction provides ten beta-sheets and two alpha-helices, characteristic of most of LPB. Functional experiments showed that, to be active, ReP1-NCXSQ must be phosphorylated by MgATP, through the action of a kinase present in the plasma membrane. Moreover, PO4-ReP1-NCXSQ can stimulate the exchanger in the absence of ATP. An additional crucial observation was that, in proteoliposomes containing only the purified Na(+)/Ca(2+) exchanger, PO4-ReP1-NCXSQ promotes activation; therefore, this upregulation has no other requirement than a lipid membrane and the incorporated exchanger protein.Recently, we solved the crystal structure of ReP1-NCXSQ which was as predicted: a "barrel" consisting of ten beta-sheets and two alpha-helices. Inside the barrel is the fatty acid coordinated by hydrogen bonds with Arg126 and Tyr128. Point mutations showed that neither Tyr20Ala, Arg58Val, Ser99Ala, nor Arg126Val is necessary for protein phosphorylation or activity. On the other hand, Tyr128 is essential for activity but not for phosphorylation. We can conclude that (1) for the first time, a role of an LBP is demonstrated in the metabolic regulation of an ion exchanger; (2) phosphorylation of this LBP can be separated from the activation capacity; and (3) Tyr128, a candidate to coordinate lipid binding inside the barrel, is essential for activity.

Design of a heterotetravalent synthetic allergen that reflects epitope heterogeneity and IgE antibody variability to study mast cell degranulation

The present paper describes the design of a HtTA (heterotetravalent allergen) as a multi-component experimental system that enables an integrative approach to study mast cell degranulation. The HtTA design allows presentation of two distinct haptens, each with a valency of 2, thereby better reflecting the complexity of natural allergens by displaying epitope heterogeneity and IgE antibody variability. Using the HtTA design, synthetic allergens HtTA-1 and HtTA-2 were synthesized to model a combination of epitope/IgE affinities. HtTA-1 presented DNP (2,4-dinitrophenyl) and dansyl haptens (Kd=22 and 54 nM for IgEDNP and IgEdansyl respectively) and HtTA-2 presented dansyl and the weak-affinity DNP-Pro (DNP-proline) haptens (Kd=550 nM for IgEDNP). Both HtTAs effectively induced degranulation when mast cells were primed with both IgEDNP and IgEdansyl antibodies. Interestingly tetravalent DNP-Pro or bivalent dansyl were insufficient in stimulating a degranulation response, illustrating the significance of valency, affinity and synergy in allergen-IgE interactions. Importantly, maximum degranulation with both HtTA-1 and HtTA-2 was observed when only 50% of the mast cell-bound IgEs were hapten-specific (25% IgEdansyl and 25% IgEDNP). Taken together, results of the present study establish the HtTA system as a physiologically relevant experimental model and demonstrates its utility in elucidating critical mechanisms of mast cell degranulation.

Inhibitors of PI(4,5)P2 synthesis reveal dynamic regulation of IgE receptor signaling by phosphoinositides in RBL mast cells

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a versatile phospholipid that participates in many membrane-associated signaling processes. PI(4,5)P2 production at the plasma membrane (PM) depends on levels of its precursor, phosphatidylinositol 4-phosphate (PI4P), synthesized principally by two intracellular enzymes, PI4-kinases IIIα and IIIb; the former is preferentially inhibited by phenylarsine oxide (PAO). We found that PAO and quercetin, another lipid kinase inhibitor, rapidly inhibit Ca(2+) responses to antigen in IgE-sensitized rat basophilic leukemia mast cells. Quercetin also rapidly inhibits store-operated Ca(2+) influx stimulated by thapsigargin. In addition, quercetin and PAO effectively inhibit antigen-stimulated ruffling and spreading in these cells, and they inhibit endocytosis of crosslinked IgE receptor complexes, evidently by inhibiting pinching off of endocytic vesicles containing the clustered IgE receptors. A minimal model to account for these diverse effects is inhibition of PI(4,5)P2 synthesis by PAO and quercetin. To characterize the direct effects of these agents on PI(4,5)P2 synthesis, we monitored the reappearance of the PI(4,5)P2-specific PH domain PH-phospholipase C δ-EGFP at the PM after Ca(2+) ionophore (A23187)-induced PI(4,5)P2 hydrolysis, followed by Ca(2+) chelation with excess EGTA. Resynthesized PI(4,5)P2 initially appears as micron-sized patches near the PM. Addition of quercetin subsequent to A23187-induced PI(4,5)P2 hydrolysis reduces PI(4,5)P2 resynthesis in PM-associated patches, and PAO reduces PI(4,5)P2 at the PM while enhancing PI(4,5)P2 accumulation at the Golgi complex. Taken together, these results provide evidence that PI4P generated by PI4-kinase IIIα is dynamically coupled to PI(4,5)P2 pools at the PM that are important for downstream signaling processes activated by IgE receptors.

Nanostructures of designed geometry and functionality enable regulation of cellular signaling processes

Extracellular matrices (ECM) triggered cellular signaling processes often begin with the clustering of the cellular receptors such as integrin and FcεRI. The sizes of these initial protein complexes or clusters are tens to 100 nm in dimension; therefore, engineered nanostructures could provide effective mimics of ECM for investigation and control of the initial and downstream specific signaling processes. This current topic discusses recent advances in nanotechnology in the context of design and production of matching chemical functionality and geometry for control of specific cellular signaling processes. Two investigations are reported to demonstrate this concept: (a) how the presentation of antigen at the nanometer scale would influence the aggregation of FcεRI, which would impact the formation of activation complexes, leading to the rearrangement of actin in cytoskeleton and degranulation or activation of mast cells; (b) how the engineered nanostructure could guide the initial integrin clustering, which would impact the formation of focal adhesion and downstream cell signaling cascades, leading to polarization, migration, and morphological changes. Complementary to engineered ECMs using synthetic ligands or peptides, or topographic control at the micrometer scale, nanostructures of designed geometry and chemical functionality provide new and effective biochemical cues for regulation of cellular signaling processes and downstream behaviors.

Antigen binding characteristics of immunoglobulin free light chains: crosslinking by antigen is essential to induce allergic inflammation

Beside the production of complete immunoglobulins IgG, IgE, IgA, IgM and IgD, consisting of tetrameric heterodimers of immunoglobulin heavy and light chains, B cells also secrete immunoglobulin free light chains (Ig-fLC). Previous studies showed that Ig-fLCs are able to induce immediate hypersensitivity reactions. It is apparent that recognition and binding of antigen are crucial steps in the onset of these inflammatory responses. In this study, the binding characteristics of Ig-fLC to antigen were further investigated using various biochemical approaches. In addition, we investigated whether antigen-mediated crosslinking of Ig-fLC is required to initiate allergic skin inflammation in vivo. Our study shows that binding of Ig-fLCs to antigen can be measured with different experimental setups. Surface plasmon resonance analysis showed real-time antigen binding characteristics. Specific antigen binding by Ig-fLCs was further detected using immunoblotting and ELISA. Using the ELISA-based assay, a binding affinity of 76.9±3.8 nM was determined for TNP-specific Ig-fLC. Antigen-induced ear swelling in mice passively sensitized with trinitrophenol-specific Ig-fLC was inhibited when multivalent antigen was combined with excess of monovalent antigen during challenge. We conclude that Ig-fLCs are able to interact with antigen, a prerequisite for antigen-specific cellular activation. In analogy to antigen-specific Fc receptor-induced mast cell activation, crosslinking of Ig-fLCs is necessary to initiate a local allergic response.

The 2S albumin allergens of Arachis hypogaea, Ara h 2 and Ara h 6, are the major elicitors of anaphylaxis and can effectively desensitize peanut-allergic mice

BACKGROUND

Ara h 2 and Ara h 6, co-purified together in a 13-25 kD fraction (Ara h 2/6; 20 kD fraction) on gel filtration chromatography, account for the majority of effector activity in a crude peanut extract (CPE) when assayed with RBL SX-38 cells sensitized with IgE from human peanut allergic sera.

OBJECTIVES

To determine if Ara h 2/6 are the primary peanut allergens responsible for allergic reactions in vivo and to determine if Ara h 2/6 would be sufficient to prevent allergic reactions to a complete CPE.

METHODS

An oral sensitization mouse model of peanut allergy was used to assess the activity of Ara h 2/6 (20 kD) and CPE without the 20 kD fraction (CPE w/o 20 kD) for allergic provocation challenge and immunotherapy. The activity of these preparations was also tested in an assay of histamine release from human basophils in whole blood.

RESULTS

Compared with mice challenged with control CPE, mice challenged with CPE w/o 20 kD experienced reduced symptoms (P < 0.05) and a smaller decrease in body temperature (P < 0.01). Results with the basophil histamine release assay corroborated these findings (P < 0.01). The mouse model was also used to administer Ara h 2/6 (20 kD) in an immunotherapy protocol, in which peanut-allergic mice treated with the 20 kD fraction experienced significantly reduced symptoms, changes in body temperature, and mast cell protease (MMCP-1) release compared with placebo (P < 0.01 for all parameters). Importantly, immunotherapy with the 20 kD fraction was just as effective as treatment with CPE, whereas CPE w/o 20 kD was significantly less effective for higher dose peanut challenges.

CONCLUSIONS AND CLINICAL RELEVANCE

Ara h 2/6 are the most potent peanut allergens in vivo and can be used to desensitize peanut-allergic mice. These results have potential implications for clinical research in the areas of diagnosis and immunotherapy for peanut allergy.

Quantitative nanoscale analysis of IgE-FcεRI clustering and coupling to early signaling proteins

Antigen-mediated cross-linking of IgE bound to its receptor, FcεRI, initiates a transmembrane signaling cascade that results in mast cell activation in the allergic response. Using immunogold labeling of intact RBL mast cells and scanning electron microscopy (SEM), we visualize molecular reorganization of IgE-FcεRI and early signaling proteins on both leaflets of the plasma membrane, without the need for ripped off membrane sheets. As quantified by pair correlation analysis, we observe dramatic changes in the nanoscale distribution of IgE-FcεRI after binding of multivalent antigen to stimulate transmembrane signaling, and this is accompanied by similar clustering of Lyn and Syk tyrosine kinases, and adaptor protein LAT. We find that Lyn co-redistributes with IgE-FcεRI into clusters that cross-correlate throughout 20 min of stimulation. Inhibition of tyrosine kinase activity reduces the numbers of both IgE-FcεRI and Lyn in stimulated clusters. Coupling of these proteins is also decreased when membrane cholesterol is reduced either before or after antigen addition. These results provide evidence for involvement of FcεRI phosphorylation and cholesterol-dependent membrane structure in the interactions that accompany IgE-mediated activation of RBL mast cells. More generally, this SEM view of intact cell surfaces provides new insights into the nanoscale organization of receptor-mediated signaling complexes in the plasma membrane.

Examination of the stimulatory signaling potential of a channel catfish leukocyte immune-type receptor and associated adaptor

Expressed by various subsets of myeloid and lymphoid immune cells, channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are predicted to play a key role in the initiation and termination of teleost cellular effector responses. These type I transmembrane proteins belong to the immunoglobulin superfamily and display features of immunoregulatory receptors with inhibitory and/or stimulatory signaling potential. Expanding on our previous work, which demonstrated that putative stimulatory IpLITR-types associated with the catfish adaptor proteins IpFcRγ and FcRγ-L, this study focuses on the functional significance of this immune receptor-adaptor signaling complex. Specifically, we generated an epitope-tagged chimeric receptor construct by fusing the extracellular domain of IpLITR 2.6b with the transmembrane region and cytoplasmic tail of IpFcRγ-L. This chimera was stably expressed in a rat basophilic leukemia (RBL) cell line, RBL-2H3, and following cross-linking of the surface receptor with an anti-hemagglutinin monoclonal antibody or opsonized microspheres, the chimeric teleost receptor induced cellular degranulation and phagocytic responses, respectively. Site-directed mutagenesis of the immunoreceptor tyrosine-based activation motif encoded within the cytoplasmic tail of the chimera confirmed that these functional responses were dependent on the phosphorylated tyrosines within this motif. Using a combination of phospho-specific antibodies and pharmacological inhibitors, we also demonstrate that the IpLITR/IpFcRγ-L-induced degranulation response requires the activity of Src homology 2 domain containing protein tyrosine phosphatases, phosphatidylinositol 3-kinase, protein kinase C, and mitogen-activated protein kinases but appears independent of the c-Jun N-terminal kinase and p38 MAP kinase pathways. In addition to this first look at stimulatory IpLITR-mediated signaling and its influence on cellular effector responses, the advantage of generating RBL-2H3 cells stably expressing a functional IpLITR-adaptor chimera will be discussed.

Engineered nanostructures of antigen provide an effective means for regulating mast cell activation

Nanostructures containing 2,4-dinitrophenyl (DNP) as antigen were designed and produced to investigate antibody-mediated activation of mast cells. The design consists of nanogrids of DNP termini inlaid in alkanethiol self-assembled monolayers (SAMs). Using scanning probe-based nanografting, nanometer precision was attained for designed geometry, size, and periodicity. Rat basophilic leukemia (RBL) cells exhibited high sensitivity to the geometry and local environment of DNP presented on these nanostructures. The impact included cellular adherence, spreading, membrane morphology, cytoskeleton structure, and activation. The highest level of spreading and activation was induced by nanogrids of 17 nm line width and 40 nm periodicity, with DNP haptens 1.4 nm above the surroundings. The high efficacy is attributed to two main factors. First, DNP sites in the nanostructure are highly accessible by anti-DNP IgE during recognition. Second, the arrangement or geometry of DNP termini in nanostructures promotes clustering of FcεRI receptors that are prelinked to IgE. The clustering effectively initiates Lyn-mediated signaling cascades, ultimately leading to the degranulation of RBL cells. This work demonstrates an important concept: that nanostructures of ligands provide new and effective cues for directing cellular signaling processes.

Metabolic regulation of the squid nerve Na⁺/Ca²⁺ exchanger: recent kinetic, biochemical and structural developments

The Na⁺/Ca²⁺ exchangers are structural membrane proteins, essential for the extrusion of Ca²⁺ from most animal cells. Apart from the transport sites, they have several interacting ionic and metabolic sites located at the intracellular loop of the exchanger protein. One of these, the intracellular Ca²⁺ regulatory sites, are essential and must be occupied by Ca²⁺ to allow any type of ion (Na⁺ or Ca²⁺) translocation. Intracellular protons and Na⁺ are inhibitory by reducing the affinity of the regulatory sites for Ca²⁺; MgATP stimulates by antagonizing H⁺ and Na⁺. We have proposed a kinetic scheme to explain all ionic and metabolic regulation of the squid nerve Na⁺/Ca²⁺ exchanger. This model uniquely accounts for most of the new kinetic data provided here; however, none of the existing models can explain the trans effects of the Ca(i)²⁺-regulatory sites on external cation transport sites; i.e. all models are incomplete. MgATP up-regulation of the squid Na⁺/Ca²⁺ exchanger requires a cytosolic protein, which has been recently identified as a member of the lipocalin super family of Lipid Binding Proteins (LBP or FABP) of 132 amino acids (ReP1-NCXSQ, access to GenBank EU981897). This protein was cloned, expressed and purified. To be active, ReP1-NCXSQ must be phosphorylated from MgATP by a kinase present in the plasma membrane. Phosphorylated ReP1-NCXSQ can stimulate the exchanger in the absence of ATP. Experiments with proteoliposomes proved that this up-regulation can take place just with the lipid membrane and the exchanger protein. The structure of ReP1-NCXSQ predicted from the amino acid sequence has been confirmed by X-ray crystal analysis; it has a "barrel" formed by ten beta sheets and two alpha helices, with a lipid coordinated by hydrogen bonds with Arg 126 and Tyr 128.