Negative regulation of mast cell proliferation by FcgammaRIIB

Regulation of Trafficking and Signaling of the High Affinity IgE Receptor by FcεRIβ and the Potential Impact of FcεRIβ Splicing in Allergic Inflammation

Mast cells are tissue-resident immune cells that function in both innate and adaptive immunity through the release of both preformed granule-stored mediators, and newly generated proinflammatory mediators that contribute to the generation of both the early and late phases of the allergic inflammatory response. Although mast cells can be activated by a vast array of mediators to contribute to homeostasis and pathophysiology in diverse settings and contexts, in this review, we will focus on the canonical setting of IgE-mediated activation and allergic inflammation. IgE-dependent activation of mast cells occurs through the high affinity IgE receptor, FcεRI, which is a multimeric receptor complex that, once crosslinked by antigen, triggers a cascade of signaling to generate a robust response in mast cells. Here, we discuss FcεRI structure and function, and describe established and emerging roles of the β subunit of FcεRI (FcεRIβ) in regulating mast cell function and FcεRI trafficking and signaling. We discuss current approaches to target IgE and FcεRI signaling and emerging approaches that could target FcεRIβ specifically. We examine how alternative splicing of FcεRIβ alters protein function and how manipulation of splicing could be employed as a therapeutic approach. Targeting FcεRI directly and/or IgE binding to FcεRI are promising approaches to therapeutics for allergic inflammation. The characteristic role of FcεRIβ in both trafficking and signaling of the FcεRI receptor complex, the specificity to IgE-mediated activation pathways, and the preferential expression in mast cells and basophils, makes FcεRIβ an excellent, but challenging, candidate for therapeutic strategies in allergy and asthma, if targeting can be realized.

Protein 4.1R negatively regulates P815 cells proliferation by inhibiting C-Kit-mediated signal transduction

The proliferation of mast cells (MCs) plays a crucial role in either physiological or pathological progression of human physical. C-Kit-mediated signaling pathway has been confirmed to play a key role in MCs proliferation, and the regulatory mechanisms of C-Kit-mediated MCs proliferation need to be further explored. Our previous study found that protein 4.1R could negatively regulate T cell receptor (TCR) mediated signal pathways in CD4+ T cells. Little is known about the function of 4.1R in C-Kit-mediated proliferation of MCs. In this study, P815-4.1R-/- cells were constructed by using CRISPR/Cas9 technique. Lack of 4.1R significantly enhanced P815 cells proliferation by accelerating the progression of cell cycle. 4.1R could also significantly alleviate the clinical symptoms of systemic mastocytosis (SM) and improve the overall survival of SM mice. Further study showed that 4.1R could interact directly with C-Kit to inhibit the activation of C-Kit-mediated Ras-Raf-MAPKs and PI3K-AKT signal pathways. Taken together, our findings demonstrate that protein 4.1R, a novel negative regulator, negatively regulates MCs proliferation by inhibiting C-Kit-mediated signal transduction, which maybe provide a potential target to the prevention and treatment of abnormal MCs proliferation-related diseases.

IgE and mast cells: The endogenous adjuvant

Mast cells and IgE are most familiar as the effectors of type I hypersensitivity reactions including anaphylaxis. It is becoming clear however that this pair has important immunomodulatory effects on innate and adaptive cells of the immune system. In this purview, they act as endogenous adjuvants to ignite evolving immune responses, promote the transition of allergic disease into chronic illness and disrupt the development of active mechanisms of tolerance to ingested foods. Suppression of IgE-mediated mast cell activation can be exerted by molecules targeting IgE, FcɛRI or signaling kinases including Syk, or by IgG antibodies acting via inhibitory Fcγ receptors. In 2015 we reviewed the evidence for the adjuvant functions of mast cells. This update includes the original text, incorporates some important developments in the field over the past five years and discusses how interventions targeting these pathways might have promise in the development of strategies to treat allergic disease.

Human eosinophils and mast cells: Birds of a feather flock together

While the origin of the phrase "birds of a feather flock together" is unclear, it has been in use for centuries and is typically employed to describe the phenomenon that people with similar tastes or interests tend to seek each other out and congregate together. In this review, we have co-opted this phrase to compare innate immune cells of related origin, the eosinophil and mast cell, because they very often accumulate together in tissue sites under both homeostatic and inflammatory conditions. To highlight overlapping yet distinct features, their hematopoietic development, cell surface phenotype, mediator release profiles and roles in diseases have been compared and contrasted. What emerges is a sense that these two cell types often interact with each other and their tissue environment to provide synergistic contributions to a variety of normal and pathologic immune responses.

The role of basophils as innate immune regulatory cells in allergy and immunotherapy

Basophils are circulating cells that are associated quite exclusively with allergy response and hypersensitivity reactions but their role in the immune network might be much more intriguing and complex than previously expected. The feasibility of testing their biology in vitro for allergy research and diagnosis, due fundamentally to their quite easy availability in the peripheral blood, made them the major source for assessing allergy in the laboratory assay, when yet many further cells such as mast cells and eosinophils are much more involved as effector cells in allergy than circulating basophils. Interestingly, basophil numbers change rarely in peripheral blood during an atopic response, while we might yet observe an increase in eosinophils and modification in the biology of mast cells in the tissue during an hypersensitivity response. Furthermore, the fact that basophils are very scanty in numbers suggests that they should mainly serve as regulatory cells in immunity, rather than effector leukocytes, as still believed by the majority of physicians. In this review we will try to describe and elucidate the possible role of these cells, known as "innate IL4-producing cells" in the immune regulation of allergy and their function in allergen immunotherapy.

IVIG activates FcγRIIB-SHIP1-PIP3 Pathway to stabilize mast cells and suppress inflammation after ICH in mice

Following intracerebral hemorrhage (ICH), the activation of mast cell contributes to brain inflammation and brain injury. The mast cell activation is negatively regulated by an inhibitory IgG-receptor. It's signals are mediated by SHIP (Src homology 2-containing inositol 5' phosphatase), in particular SHIP1, which activation leads to hydrolyzation of PIP3 (Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃, leading to the inhibition of calcium mobilization and to the attenuation of mast cell activation. Intravenous immunoglobulin (IVIG) is a FDA-approved drug containing IgG. We hypothesized that IVIG will attenuate the ICH-induced mast cell activation via FcγRIIB/SHIP1 pathway, resulting in a decrease of brain inflammation, protection of the blood-brain-barrier, and improvement of neurological functions after ICH. To prove this hypothesis we employed the ICH collagenase mouse model. We demonstrated that while ICH induced mast cell activation/degranulation, IVIG attenuated post-ICH mast cell activation. Mast cell deactivation resulted in reduced inflammation, consequently attenuating brain edema and improving of neurological functions after ICH. Furthermore using siRNA-induced in vivo knockdown approach we demonstrated that beneficial effects of IVIG were mediated, at least partly, via SHIP1/PIP3 pathway. We conclude that IVIG treatment represents a promising therapeutic approach potentially able to decrease mortality and morbidity after ICH in experimental models.

Regulation of IgE-mediated signalling in human basophils by CD32b and its role in Syk down-regulation: basic mechanisms in allergic disease

BACKGROUND

CD32b has been previously demonstrated to modulate IgE-mediated secretion from human basophils. However, exploration of the implications of this regulation has been limited. One unstudied area is whether regulation of signalling by CD32 also alters some of the phenotypic changes induced by IgE-mediated activation. The reported character of CD32-mediated signal transduction is not clear for human basophils and the two primary mechanisms considered important in this reaction predict different long-term outcomes, notably predicting different outcomes for down-regulation of syk expression.

OBJECTIVE

Syk expression was considered a unique point of phenotypic control in human basophils and the role of CD32b in its regulation is explored in this study. However, initial pilot studies discovered that IL-3 could markedly up-regulate CD32 expression and first describing the consequences of this up-regulation became an additional focus of this study.

METHODS

Human basophils were examined for the changes in IgE-mediated signalling during simultaneous engagement of CD32b.

RESULTS

Preliminary experiments noted that CD32b could be up-regulated by IL-3 (3- to 12-fold). Both natural variation and induced up-regulation of CD32b modulated the efficacy of this receptor to inhibit IgE-mediated release. Signalling induced by engagement of CD32b (lyn, syk, SHP-1, or SHIP1 phosphorylation) was more consistent with a mode of action involving SHIP1 rather than SHP-1. IgE-mediated down-regulation of syk expression was not altered by co-engagement of CD32b, a result also consistent with a SHIP1-dependent mechanism of inhibition.

CONCLUSIONS

Taken together these results suggest that the combined action of IgE and IgG could generate a natural mechanism, whereby the significant variation in syk expression in allergic subjects occurs without necessarily also inducing mediator release.

Bridging autoantibodies and arthritis: the role of Fc receptors

Autoantibodies represent a hallmark of Rheumatoid arthritis (RA), which is a chronic inflammatory autoimmune disease characterized by inflammation and damage in the joints. Anti-Citrullinated Protein Antibodies (ACPA) are the most prominent autoantibodies present in RA patients. These autoantibodies have been intensively investigated during the last 20 years due to their diagnostic and predictive value. Furthermore, they are believed to be involved in mediating the damage associated with RA. Antibodies of the IgG isotype interact with the immune system via Fcγ receptors expressed on immune cells as well as nonimmune cells. These receptors, therefore, form the bridge between Fcγ receptor-positive cells and antibodies complexed to antigen allowing the modulation and activation of cellular immune responses that are involved in immune defense against invading microorganisms. However, in case triggered by antibodies against self-antigens, they can also play a pivotal role in the induction and perpetuation of autoimmune diseases such as RA. Mouse models have been indispensably important for understanding the role of Fcγ receptors in the development of arthritis. Here we discuss the contribution of autoantibodies to the pathogenesis of arthritis in preclinical animal models, as well as RA, in relation to their interaction with the different (immune inhibitory and activating) Fcγ receptors.

Functional Fcgamma receptor polymorphisms are associated with human allergy

Objective

IgG Fc receptors (FcγRs) play important roles in immune responses. It is not clear whether FcγR receptors play a role in human asthma and allergy. The aim of current study was to investigate whether functional single nucleotide polymorphisms (SNPs) of FcγR genes (FCGR) are associated with human asthma and allergy.

Methods

Functional SNPs of FCGR2A (FcγRIIA-131His>Arg, rs1801274), FCGR2B (FcγRIIB-187Ile>Thr, rs1050501), FCGR2C (FcγRIIC-13Gln>Stop, rs10917661), FCGR3A (FcγRIIIA-158Val>Phe, rs396991), and FCGR3B variants (FcγRIIIB NA1 and NA2) were genotyped in an asthma family cohort including 370 atopy positive, 239 atopy negative, and 169 asthma positive subjects. The genotype and phenotype data (asthma, bronchial hyper-responsiveness, and atopy) of subjects were analyzed using family-based association tests (FBAT) and logistic regression adjusted for age and sex.

Result

The FcγRIIA-131His>Arg SNP is significantly associated with atopy in a family-based association test (P = 0.00287) and in a logistic regression analysis (P = 0.0269, OR 0.732, 95% CI: 0.555–0.965). The FcγRIIA-131His (or rs1801274-A) allele capable of binding human IgG2 has a protective role against atopy. In addition, the rare FcγRIIB-187Thr (or rs1050501-C) allele defective for the receptor-mediated inhibitory signals is a risk factor for atopy (P = 0.0031, OR 1.758, 95% CI: 1.209–2.556) and IgE production (P<0.001). However, variants of activating FcγRIIIA (rs396991), and FcγRIIIB (NA1 and NA2), and FcγRIIC (rs10917661) are not associated with asthma, BHR, and atopy (P>0.05).

Conclusions

FcγRIIA and FcγRIIB functional polymorphisms may have a role in the pathogenesis of allergy.

Fc gamma receptors in respiratory syncytial virus infections: implications for innate immunity

RSV infections are a major burden in infants less than 3 months of age. Newborns and infants express a distinct immune system that is largely dependent on innate immunity and passive immunity from maternal antibodies. Antibodies can regulate immune responses against viruses through interaction with Fc gamma receptors leading to enhancement or neutralization of viral infections. The mechanisms underlying the immunomodulatory effect of Fc gamma receptors on viral infections have yet to be elucidated in infants. Herein, we will discuss current knowledge of the effects of antibodies and Fc gamma receptors on infant innate immunity to RSV. A better understanding of the pathogenesis of RSV infections in young infants may provide insight into novel therapeutic strategies such as vaccination.

Suppression of mast cell degranulation through a dual-targeting tandem IgE-IgG Fc domain biologic engineered to bind with high affinity to FcγRIIb

Mast cells and basophils play a central role in allergy, asthma, and anaphylaxis, as well as in non-allergic inflammatory, neurological and autoimmune diseases. Allergen-mediated cross-linking of IgE bound to FcεRI leads to cellular activation, and the low-affinity Fc receptor FcγRIIb is a key inhibitor of subsequent degranulation. FcγRIIb, when coengaged with FcεRI via allergen bound to IgE, stimulates ITIM domain-mediated inhibitory signaling that efficiently suppresses mast cell and basophil activation. To assess the therapeutic potential of directed coengagement of FcεRI and FcγRIIb in the absence of FcεRI crosslinking, we developed a fusion protein comprising the coupled Fc domains of murine IgE and human IgG1. As a key functional component of this tandem Fcε-Fcγ biologic, we engineered its IgG1 Fc domain to bind to human FcγRIIb with 100-fold enhanced affinity relative to native IgG1 Fc. Using mast cells from mice transgenic for human FcγRIIb, we show that this tandem Fc binds with high affinity to murine FcεRI and human FcγRIIb on mast cells, triggers phosphorylation of FcγRIIb, and inhibits FcεRI-dependent calcium mobilization. Control tandem Fc biologics containing a native IgG1 Fc domain or lacking binding to Fcγ receptors were markedly less active, demonstrating that the affinity-optimized tandem Fc can inhibit degranulation through stimulation of FcγRIIb signaling as well as through competition with allergen-IgE immune complex for FcεRI binding. We propose that in the context of a fully human tandem Fc biologic, high-affinity coengagement of FcεRI and FcγRIIb has potential as a novel therapy for allergy and other mast cell and basophil-mediated pathologies.

Generation and epitope analysis of human monoclonal antibody isotypes with specificity for the Timothy grass major allergen Phl p 5a

The scarcity of monoclonal human IgE antibodies with specificity for defined allergens is a bottleneck for the molecular characterisation of allergens and their epitopes. Insights into the characteristics of such antibodies may allow for analyses of the molecular basis underlying allergenicity and cross-reactivity, standardisation of allergens as well as improvement of allergy diagnostics and therapeutics. Here we report the generation and application of the first set of authentic human IgG, IgE and IgA antibodies. On the basis of a Phl p 5a specific antibody fragment, a lambda light chain and the IgG1, IgG4, IgE, IgA1, and IgA2 heavy chains, the corresponding human immunoglobulins were constructed and produced in mammalian cells. In parallel, a murine hybridoma line with specificity for Phl p 5a was established, recloned and produced as human chimeric IgE. After purification, immunoreactivity of the antibodies with the allergen was assessed. Applicability in allergy diagnostics was confirmed by establishment of artificial human sera. Functionality of both antibodies was further demonstrated in receptor binding studies and mediator release assays using humanised rat basophil leukaemia cells (RBL-SX38) suggesting the presence of spatially separate epitopes. By using Phl p 5 fusion proteins and recombinant IgE in immunoblotting and mediator release assays we assigned the epitope of the authentic IgE to a looped stretch exclusively present in Phl p 5a. In summary, the Phl p 5-specific antibodies are the first full set of allergy-related antibody isotypes of their kind and represent valuable tools for studies of fundamental mechanisms and structure/function relationships in allergy.

Down-regulation of mast cell responses through ITIM containing inhibitory receptors

The multiple cell types that comprise the immune system provide an efficient defense system against invading pathogens and micro-organisms. In general, immune cells are activated for disparate functions, such as proliferation, production and release of mediators and chemotaxis, as a result of interactions between ligands and their matching immunoreceptors. This in turn leads to the recruitment and activation of a cascade of second messengers, via their regulators/adaptors, that determine the net effect of the initial response. However, activation of cells of the immune system must be tightly regulated by a finely tuned interplay between activation and inhibition to avoid excessive or inappropriate responsiveness and to maintain homeostasis. Loss of inhibitory signals may disrupt this balance, leading to various pathological processes such as allergic and auto-immune diseases. In this chapter, we will discuss down-regulating mechanisms of mast cells focusing on immunoreceptor tyrosine-based inhibition motifs (ITIM)-containing inhibitory receptors (IR).

Tyrosine phosphorylation of SHIP promotes its proteasomal degradation

OBJECTIVE

The activity of the SH2-containing-phosphatidylinositol-5'-phosphatase (SHIP, also known as SHIP1), a critical hematopoietic-restricted negative regulator of the PI3 kinase (PI3K) pathway, is regulated in large part via its protein levels. We sought to determine the mechanism(s) involved in its downregulation by BCR-ABL and by interleukin (IL)-4.

MATERIALS AND METHODS

We used Ba/F3(p210-tetOFF) cells to study the downregulation of SHIP by BCR-ABL and bone marrow-derived macrophages to study SHIP's downregulation by IL-4.

RESULTS

We show herein that BCR-ABL downregulates SHIP, but not SHIP2 or PTEN, and this can be blocked with the Src kinase inhibitor PP2, which inhibits the tyrosine phosphorylation of SHIP, or with the proteasomal inhibitor MG-132. We also show, using anti-SHIP immunoprecipitates, that c-Cbl and Cbl-b are associated with SHIP and that BCR-ABL induces SHIP's polyubiquitination. This ubiquitination can be blocked with PP2, consistent with the tyrosine phosphorylation of SHIP acting as a signal for its ubiquitination. In bone marrow-derived macrophages, IL-4 also leads to the proteasomal degradation of SHIP but, unlike in Ba/F3(p210-tetOFF) cells, SHIP2 is also proteasomally degraded and the degradation of both inositol phosphatases can be prevented with PP2 or MG-132.

CONCLUSION

Our results suggest that SHIP protein levels can be reduced via BCR-ABL and/or Src family member-induced tyrosine phosphorylation of SHIP because this triggers its polyubiquitination and degradation within the proteasome.

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.

The Src family kinase, Lyn, suppresses osteoclastogenesis in vitro and in vivo

c-Src kinase is a rate-limiting activator of osteoclast (OC) function and Src inhibitors are therefore candidate antiosteoporosis drugs. By affecting alphavbeta3 and macrophage-colony stimulating factor (M-CSF)-induced signaling, c-Src is central to osteoclast activity, but not differentiation. We find Lyn, another member of Src family kinases (SFK) is, in contrast, a negative regulator of osteoclastic bone resorption. The absence of Lyn enhances receptor activator of NF-kappaB ligand (RANKL)-mediated differentiation of osteoclast precursors without affecting proliferation and survival, while its overexpression decreases osteoclast formation. In further contrast to c-Src, Lyn deficiency does not impact the activity of the mature cell. Reflecting increased osteoclast development in vitro, Lyn-/- mice undergo accelerated osteoclastogenesis and bone loss, in vivo, in response to RANKL. Mechanistically, Lyn forms a complex with receptor activator of NF-kappaB (RANK), the tyrosine phosphatase, SHP-1, and the adapter protein, Grb2-associated binder 2 (Gab2). Upon RANKL exposure, Gab2 phosphorylation, JNK, and NF-kappaB activation are enhanced in Lyn-/- osteoclasts, all critical events in osteoclast development. We therefore establish that Lyn regulates osteoclast formation and does it in a manner antithetical to that of c-Src. The most pragmatic aspect of our findings is that successful therapeutic inhibition of c-Src, in the context of the osteoclast, will require its stringent targeting.

Inducing an anergic state in mast cells and basophils without secretion

BACKGROUND

IgE-mediated secretion from mast cells or basophils depends on the activity of both spleen tyrosine kinase (syk) and phosphatidyl inositol 3' kinase (PI3K), but several specific downregulatory pathways (eg, loss of syk expression) do not.

OBJECTIVE

We tested whether stimulation with antigen in the presence of a syk inhibitor (NVP-QAB205) would ablate secretion while simultaneously allowing anergy.

METHODS

The anergic or desensitized state in human basophils, cultured-derived mast cells, and in situ stimulated airway mast cells (in organ baths) was assessed after stimulation with antigen in the presence of syk inhibitor.

RESULTS

Antigen caused 35 +/- 7% and 62 +/- 10% histamine release from basophils and mast cells, respectively, and it caused an 87 +/- 5% histamine/leukotriene D(4)-dependent contraction of human isolated bronchi. All of these responses were blocked >95% by the syk inhibitor. Rechallenging the preparations with antigen, after first washing out the syk inhibitor and antigen, revealed that near complete anergy (92% to 100%) occurred in each case. A similar result was found when using a PI3K inhibitor, LY294002, in studies of basophils.

CONCLUSION

Although the syk inhibitor nearly abolished the antigen-induced secretion from mast cells and basophils, it had little effect on the pathways involved in anergy. These results suggest that syk and PI3K are not involved in downregulation leading to anergy.

Inhibition of message for FcepsilonRI alpha chain blocks mast cell IL-4 production induced by co-culture with Mycoplasma pneumoniae

We have previously described the activation of RBL-2H3 mast cells for IL-4 production by Mycoplasma pneumoniae but the mechanism remains unclear. M. pneumoniae binds eukaryotic cells primarily through sialoglycoproteins on the target cell surface. This study was undertaken to determine whether the sialated FcepsilonRI alpha chain on RBL cells is important for M. pneumoniae-induced IL-4 production. We found that IgE-mediated IL-4 release by a series of RBL sublines correlated with the release induced by M. pneumoniae. Further, aggregation of FcgammaRII (CD32) in RBL cells using a monoclonal antibody inhibited both IgE-mediated and mycoplasma-induced IL-4 production, providing further evidence for an Fc receptor-mediated mechanism of activation. To examine the role of FcepsilonRI in mycoplasma-induced IL-4 release, we created stably transfected RBL sublines using a vector expressing a short hairpin sequence designed to inhibit message for the FcepsilonRI alpha chain. IgE-induced IL-4 production by the transfected sublines was reduced in similar proportion to the degree of message suppression. M. pneumoniae-induced IL-4 production in the four transfected sublines was completely blocked in contrast to results with the controls or parent RBL cells. We conclude that the heavily glycosylated FcepsilonRI alpha chain is required for activation of mast cells for IL-4 production by M. pneumoniae.

Affinity of IgE and IgG against cross-reactive carbohydrate determinants on plant and insect glycoproteins

BACKGROUND

Cross-reactive carbohydrate determinants (CCDs) are probably the most widely occurring IgE epitopes. Approximately one fifth of patients with allergy develop IgE antibodies against such glycans. However, they appear to be of low clinical significance.

OBJECTIVE

We wanted to elucidate the reasons for this lack of clinical symptoms on contact with CCD allergens by determination of the binding affinities of patients' IgE and IgG antibodies.

METHODS

IgE and IgG against CCDs were affinity-purified from sera of selected patients. The binding affinity to defined glyco-epitopes was measured by surface plasmon resonance.

RESULTS

From a pool of CCD-positive sera, we isolated 0.1 and 25 microg CCD-specific IgE and IgG, respectively. The binding affinity of purified IgE antibodies to core alpha1,3-fucosylated glycans was in the 10(-10) mol/L range. The affinity was highest when both fucose and xylose were present, whereas xylosylation alone did not cause IgE binding. CCD-specific IgG exhibited a dissociation constant of approximately 10(-8) mol/L. IgG(4) amounted to only 20% of the CCD-specific IgG (as well as total IgG).

CONCLUSION

Low binding affinity of anti-CCD IgE cannot be the reason for the observed clinical insignificance of IgE against plant/insect glycan epitopes. Notably, the affinity of IgG to CCDs is higher than that to protein allergens, and it may therefore function as blocking antibody.

Generation of Human Monoclonal Allergen-Specific IgE and IgG Antibodies from Synthetic Antibody Libraries

Background: Allergen-specific IgE and IgG antibodies play pivotal roles in the induction and progression of allergic hypersensitivity reactions. Consequently, monoclonal human IgE and IgG4 antibodies with defined specificity for allergens should be useful in allergy research and diagnostic tests. We used combinatorial antibody libraries and subsequent recombinant production to make and assess IgE, IgG1, and IgG4 allergen-specific antibodies.

Methods: We used phage display to select a synthetic single-chain antibody fragment (scFv) library against 3 different allergens, from bee venom, bovine milk, and apple. The scFv obtained were converted into IgG1, IgG4, and IgE antibody formats and assessed for their biochemical properties by ELISA, immunoblotting, and fluorescence-activated cell sorting.

Results: Two different antibody formats for each IgG1, IgG4, and IgE antibody were produced in mammalian cells as disulfide-linked and glycosylated Ig, which were usable in allergen-specific ELISA assays and immunoblots. In addition, the recombinant IgE antibodies mediated the binding of allergens to HEK-293 cells transfected with the high-affinity IgE receptor, and this binding was blocked by corresponding IgG antibodies.

Conclusions: The use of synthetic libraries for the generation of allergen-specific recombinant IgE and IgG antibodies should have broad applications in allergological research and diagnosis.

Role of mast cells in allergic and non-allergic immune responses: comparison of human and murine data

The versatile role of mast cells in allergy, in innate immune responses and in the regulation of tissue homeostasis is well recognized. However, it is often not made clear that most mast-cell data derive solely from experiments in mice or rats, species that obviously never suffer from allergic and most other mast-cell-associated human diseases. Data on human mast cells are limited, and the mast-cell source and species from which findings derive are frequently not indicated in the titles and summaries of research publications. This Review summarizes recent data on human mast cells, discusses differences with murine mast cells, and describes new tools to study this increasingly meaningful cell type in humans.

[IgE and respiratory disease]

INTRODUCTION

IgE is known to provide the biological basis for allergy and immediate hypersensitivity. However, recent data provide some evidence that IgE responses are involved in other inflammatory processes apart from allergy, including several respiratory diseases.

STATE OF THE ART

IgE binds to mast cells and basophils but also to other inflammatory cells, which are involved in non-allergic processes. IgE has a role in antigen presentation and is implicated in a number of other immune mechanisms. In the airways, IgE plays an important role in bronchial hyperactivity, even in the absence of an allergen. Epidemiological studies have demonstrated that IgE response is related not only to allergy but also to asthma symptoms, in the presence or absence of atopy, as well as exposure to cigarette smoke. IgE response is altered in several respiratory diseases including extrinsic and intrinsic asthma and allergic bronchopulmonary aspergillosis.

CONCLUSION AND PERSPECTIVES

Since anti-IgE monoclonal antibodies are now available for administration to humans, a better understanding of the IgE response may allow the identification of novel therapeutic targets in the field of respiratory disease.

Fc receptors as determinants of allergic reactions

Activation of the high-affinity receptor for IgE (FcepsilonRI) on allergic effector cells induces a multitude of positive signals via immunoreceptor tyrosine-based activation motifs, which leads to the rapid manifestation of allergic inflammatory reactions. As a counterbalance, the coaggregation of the IgG receptor FcgammaRIIB mediates inhibitory signals via immunoreceptor tyrosine-based inhibition motifs. Advances in the positive and negative regulation of Fc receptor expression and signaling have shed light on the role of Fc receptors in our immune system, indicating them to be bifunctional, inhibitory and activating structures. Based on these findings, exciting new therapeutic strategies have been developed, such as the use of chimeric fusion proteins, which concomitantly activate FcepsilonRI and FcgammaRIIB. These new approaches successfully take advantage of the bivalent character of Fc receptors and pave the way for innovative strategies to modulate allergic immune reactions.

The Inhibitory Receptor IRp60 (CD300a) Is Expressed and Functional on Human Mast Cells

Mast cell-mediated responses are likely to be regulated by the cross talk between activatory and inhibitory signals. We have screened human cord blood mast cells for recently characterized inhibitory receptors expressed on NK cells. We found that IRp60, an Ig superfamily member, is expressed on human mast cells. On NK cells, IRp60 cross-linking leads to the inhibition of cytotoxic activity vs target cells in vitro. IRp60 is constitutively expressed on mast cells but is down-regulated in vitro by the eosinophil proteins major basic protein and eosinophil-derived neurotoxin. An immune complex-mediated cross-linking of IRp60 led to inhibition of IgE-induced degranulation and stem cell factor-mediated survival via a mechanism involving tyrosine phosphorylation, phosphatase recruitment, and termination of cellular calcium influx. To evaluate the role of IRp60 in regulation of allergic responses in vivo, a murine model of allergic peritonitis was used in which the murine homolog of IRp60, LMIR1, was neutralized in BALB/c mice by mAbs. This neutralization led to a significantly augmented release of inflammatory mediators and eosinophilic infiltration. These data demonstrate a novel pathway for the regulation of human mast cell function and allergic responses, indicating IRp60 as a candidate target for future treatment of allergic and mast cell-associated diseases.

Identification of tyrosine residues crucial for CD200R-mediated inhibition of mast cell activation

CD200 and its receptor CD200R are type-1 membrane glycoproteins, which contain two immunoglobulin-like domains. Engagement of CD200R by CD200 inhibits activation of myeloid cells. Unlike the majority of immune inhibitory receptors, CD200R does not contain an immunoreceptor tyrosine-based inhibitory motif but contains three tyrosine residues (Y286, Y289, and Y297) in the cytoplasmic domain. Y297 is located in an NPxY motif. Previously, we have shown that engagement of CD200R in mouse mast cells induces its tyrosine phosphorylation and recruitment of inhibitory adaptor proteins Dok1 and Dok2, leading to the inhibition of Ras/mitogen-activated protein kinase activation. In the present study, we examined the roles of these three tyrosines in CD200R-mediated inhibition by site-directed mutagenesis in mouse mast cells. Our data show that Y286 and Y297 are the major phosphorylation sites and are critical for CD200R-mediated inhibition of mast cell activation, and Y289 is dispensable. Our data also suggest that the Src family kinase may mediate the phosphorylation of CD200R and Dok.

Affinity determinations of purified IgE and IgG antibodies against the major pollen allergens Phl p 5a and Bet v 1a: discrepancy between IgE and IgG binding strength

Allergen-specific IgE and IgG antibodies coexist in allergic individuals, but only IgE has anaphylactogenic capacity. This study aimed to determine the association, dissociation and equilibrium constants for the interaction of allergen-specific IgE and IgG with the major grass and birch pollen allergens Phl p 5a and Bet v 1a. We isolated specific IgE and IgG antibodies from pollen allergic patients' sera by a two-step affinity chromatography protocol and controlled the high purity in a recombinant allergen chip microarray. Surface plasmon resonance measurements of polyclonal IgE and IgG species revealed that their affinities diverge widely, being in the range of 10(-10) and 10(-11) M for IgE, but only 10(-6)-10(-7) M for IgG. Moreover, murine monoclonal IgG1 antibodies against the allergens showed affinities of 10(-7)-10(-8) M. Thus, we conclude from our data that even stringently affinity matured IgG cannot score the superior affinity of IgE antibodies to allergens.

The CD200 receptor is a novel and potent regulator of murine and human mast cell function

CD200R is a member of the Ig supergene family that is primarily expressed on myeloid cells. Recent in vivo studies have suggested that CD200R is an inhibitory receptor capable of regulating the activation threshold of inflammatory immune responses. Here we provide definitive evidence that CD200R is expressed on mouse and human mast cells and that engagement of CD200R by agonist Abs or ligand results in a potent inhibition of mast cell degranulation and cytokine secretion responses. CD200R-mediated inhibition of FcepsilonRI activation was observed both in vitro and in vivo and did not require the coligation of CD200R to FcepsilonRI. Unlike the majority of myeloid inhibitory receptors, CD200R does not contain a phosphatase recruiting inhibitory motif (ITIM); therefore, we conclude that CD200R represents a novel and potent inhibitory receptor that can be targeted in vivo to regulate mast cell-dependent pathologies.

Molecular Mechanisms of CD200 Inhibition of Mast Cell Activation

CD200 and its receptor CD200R are both type I membrane glycoproteins that contain two Ig-like domains. Engagement of CD200R by CD200 inhibits activation of myeloid cells. Unlike the majority of immune inhibitory receptors, CD200R lacks an ITIM in the cytoplasmic domain. The molecular mechanism of CD200R inhibition of myeloid cell activation is unknown. In this study, we examined the CD200R signaling pathways that control degranulation of mouse bone marrow-derived mast cells. We found that upon ligand binding, CD200R is phosphorylated on tyrosine and subsequently binds to adapter proteins Dok1 and Dok2. Upon phosphorylation, Dok1 binds to SHIP and both Dok1 and Dok2 recruit RasGAP, which mediates the inhibition of the Ras/MAPK pathways. Activation of ERK, JNK, and p38 MAPK are all inhibited by CD200R engagement. The reduced activation of these MAPKs is responsible for the observed inhibition of mast cell degranulation and cytokine production. Similar signaling events were also observed upon CD200R engagement in mouse peritoneal cells. These data define a novel inhibitory pathway used by CD200R in modulating mast cell function and help to explain how engagement of this receptor in vivo regulates myeloid cell function.

Systematic identification of immunoreceptor tyrosine-based inhibitory motifs in the human proteome

Immunoreceptor tyrosine-based inhibitory motifs (ITIMs) are short sequences of the consensus (ILV)-x-x-Y-x-(LV) in the cytoplasmic tail of immune receptors. The phosphorylation of tyrosines in ITIMs is known to be an important signalling mechanism regulating the activation of immune cells. The shortness of the motif makes it difficult to predict ITIMs in large protein databases. Simple pattern searches find ITIMs in approximately 30% of the protein sequences in the RefSeq database. The majority are false positive predictions. We propose a new database search strategy for ITIM-bearing transmembrane receptors based on the use of sequence context, i.e. the predictions of signal peptides, transmembrane helices (TMs) and protein domains. Our new protocol allowed us to narrow down the number of potential human ITIM receptors to 109 proteins (0.7% of RefPep). Of these, 36 have been described as ITIM receptors in the literature before. Many ITIMs are conserved between orthologous human and mouse proteins which represent novel ITIM receptor candidates. Publicly available DNA array expression data revealed that ITIM receptors are not exclusively expressed in blood cells. We hypothesise that ITIM signalling is not restricted to immune cells, but also functions in diverse solid organs of mouse and man.