Differential Role for CD23 Splice Forms in Apical to Basolateral Transcytosis of IgE/Allergen Complexes

Inflammatory and Microbiota-Related Regulation of the Intestinal Epithelial Barrier

The intestinal epithelial barrier (IEB) is one of the largest interfaces between the environment and the internal milieu of the body. It is essential to limit the passage of harmful antigens and microorganisms and, on the other side, to assure the absorption of nutrients and water. The maintenance of this delicate equilibrium is tightly regulated as it is essential for human homeostasis. Luminal solutes and ions can pass across the IEB via two main routes: the transcellular pathway or the paracellular pathway. Tight junctions (TJs) are a multi-protein complex responsible for the regulation of paracellular permeability. TJs control the passage of antigens through the IEB and have a key role in maintaining barrier integrity. Several factors, including cytokines, gut microbiota, and dietary components are known to regulate intestinal TJs. Gut microbiota participates in several human functions including the modulation of epithelial cells and immune system through the release of several metabolites, such as short-chain fatty acids (SCFAs). Mediators released by immune cells can induce epithelial cell damage and TJs dysfunction. The subsequent disruption of the IEB allows the passage of antigens into the mucosa leading to further inflammation. Growing evidence indicates that dysbiosis, immune activation, and IEB dysfunction have a role in several diseases, including irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and gluten-related conditions. Here we summarize the interplay between the IEB and gut microbiota and mucosal immune system and their involvement in IBS, IBD, and gluten-related disorders.

Enterocytes in Food Hypersensitivity Reactions

Simple Summary

Hypersensitivity to food, affecting both animals and humans, is increasing. Until a decade ago, it was thought that enterocytes, the most abundant constituent of the intestinal surface mucosa layer, served only to absorb digested food and prevent foreign and non-digested substances from passing below the intestinal layer. Growing evidence supports the involvement of enterocytes in immunological responses. Here, we present a comprehensive review of the new roles of enterocytes in food hypersensitivity conducted in animal models in order to better understand complicated immune pathological conditions. In addition, resources for further work in this area are suggested, along with a literature overview of the specific roles of enterocytes in maintaining oral tolerance. Lastly, it will be beneficial to investigate the various animal models involved in food hypersensitivity to reach the needed momentum necessary for the complete and profound understanding of the mechanisms of the ever-growing number of food allergies in animal and human populations.

Abstract

Food hypersensitivity reactions are adverse reactions to harmless dietary substances, whose causes are hidden within derangements of the complex immune machinery of humans and mammals. Until recently, enterocytes were considered as solely absorptive cells providing a physical barrier for unwanted lumen constituents. This review focuses on the enterocytes, which are the hub for innate and adaptive immune reactions. Furthermore, the ambiguous nature of enterocytes is also reflected in the fact that enterocytes can be considered as antigen-presenting cells since they constitutively express major histocompatibility complex (MHC) class II molecules. Taken together, it becomes clear that enterocytes have an immense role in maintaining oral tolerance to foreign antigens. In general, the immune system and its mechanisms underlying food hypersensitivity are still unknown and the involvement of components belonging to other anatomical systems, such as enterocytes, in these mechanisms make their elucidation even more difficult. The findings from studies with animal models provide us with valuable information about allergic mechanisms in the animal world, while on the other hand, these models are used to extrapolate results to the pathological conditions occurring in humans. There is a constant need for studies that deal with this topic and can overcome the glitches related to ethics in working with animals.

Establishment of a Mouse Anti-ovalbumin IgE Monoclonal Antibody That Induces FcɛRII (CD23)-dependent Activation Without FcɛRI-Dependent Activation

IgE mainly activates cells via two receptors, FcɛRI and FcɛRII. Blocking antibodies against and animals genetically targeted for these receptors have been successfully used to distinguish between these two activating pathways. In the present study, we investigated whether our newly established anti-ovalbumin (OVA) monoclonal IgE OE-2 induced FcɛRII-dependent activation, but not FcɛRI-dependent activation in vivo and in vitro, in contrast to the previously established anti-OVA IgE OE-1, which stimulated FcɛRI and FcɛRII. The FcɛRI-mediated degranulation of RBL2H3 cells and passive systemic anaphylaxis in mice were induced by OE-1 but not OE-2. On the other hand, the production of nitric oxide by rat peritoneal macrophages and the primary antibody response in mice against co-injected OVA, which were mediated through FcɛRII, were induced and enhanced by OE-1 and OE-2. Differences in the epitopes recognized by OE-1 and OE-2 may partially explain why OE-1, but not OE-2, triggered FcɛRI-dependent activation. OE-1 bridged FcɛRI through effective aggregation with OVA, whereas OE-2 crosslinked the receptor strongly and only moderately upon the addition of an anti-kappa antibody and polymerized OVA, namely, an OVA-conjugated resin, respectively, resulting in degranulation. Our results offer a novel approach for determining the relative importance of FcɛRI and FcɛRII in various IgE-dependent responses by using OE-1 and OE-2.

In vivo intranasal anti-CD23 treatment inhibits allergic responses in a murine model of allergic rhinitis

Although CD23-dependent transcytosis of IgE and IgE-derived immune complexes across respiratory epithelial cells is likely to play a pivotal role in the initiation and development of airway allergic inflammation, there is currently a lack of physiological support for this phenomena to suggest that the targeting of CD23 could be used as a means of therapeutic intervention. The present study was designed to detect the CD23 expression in the nasal mucosa of allergic rhinitis (AR) murine model by immunohistochemistry and western blotting, and to investigate whether intranasal anti-CD23 treatment could inhibit allergen-induced upper airway inflammation in the AR model. This is the first report to show that CD23 was constitutively expressed in murine nasal epithelial cells, and its expression was significantly up-regulated in the AR murine model. In vivo, the up-regulation of CD23 expression was correlated with increased serum IL-4 levels. Following intranasal anti-CD23 treatment, nasal symptoms were alleviated and histopathologic examination showed a significant decrease in eosinophilic infiltration. Meanwhile, ELISA analysis showed levels of serum leukotriene C4 (LTC4), eosinophil cation protein (ECP), ovalbumin (OVA)-specific IgE and IL-4 also significantly decreased, as were LTC4 and OVA-specific IgE in the nasal lavage fluid. Furthermore, Western blotting analysis showed that ECP expression in the nasal mucosa was down-regulated. Finally, flow cytometric analysis revealed anti-CD23 treatment inhibited Th2 cell responses. These results indicate that intranasal anti-CD23 treatment can reduce allergic responses in a murine model of allergic rhinitis.

Oral administration of fermented red ginseng suppressed ovalbumin-induced allergic responses in female BALB/c mice

Anti-allergic efficacy of red ginseng (RG) and fermented red ginseng (FRG) was evaluated. RG or FRG were administered to ovalbumin (OVA)-sensitized mice for 8 weeks. Immunoglobulins (Igs), Th1/Th2 type cytokines, and β-lactoglobulin (BLG) in serum, and intestinal barrier-related molecules in jejunum were measured using enzyme-linked immunosorbent assay or reverse transcription-polymerase chain reaction. Mice sensitized with OVA increased serum IgG₁, IgE, OVA-IgG₁, and OVA-IgE. Both RG and FRG decreased serum IgE, OVA-IgE, and pro-inflammatory cytokines. Serum BLG, a marker of gut permeability, was significantly higher in sensitized animals and was decreased in mice fed RG or FRG. In addition, intestinal barrier-related markers such as MMCP-1, IL-4, TNF-α, COX-2, and iNOS mRNA expressions were decreased by RG or FRG. Our results suggest in vivo anti-allergic activities of RG or FRG, which are associated with the regulation of Th1/Th2 balance, intestinal inflammation and subsequent the suppression of IgE.

The high affinity IgE receptor (FcεRI) expression and function in airway smooth muscle

The airway smooth muscle (ASM) is no longer considered as merely a contractile apparatus and passive recipient of growth factors, neurotransmitters and inflammatory mediators signal but a critical player in the perpetuation and modulation of airway inflammation and remodeling. In recent years, a molecular link between ASM and IgE has been established through Fc epsilon receptors (FcεRs) in modulating the phenotype and function of these cells. Particularly, the expression of high affinity IgE receptor (FcεRI) has been noted in primary human ASM cells in vitro and in vivo within bronchial biopsies of allergic asthmatic subjects. The activation of FcεRI on ASM cells suggests a critical yet almost completely ignored network which may modulate ASM cell function in allergic asthma. This review is intended to provide a historical perspective of IgE effects on ASM and highlights the recent updates in the expression and function of FcεRI, and to present future perspectives of activation of this pathway in ASM cells.

Serum and tissue CD23, IL-15, and FasL in cow's-milk protein-sensitive enteropathy and in coeliac disease

OBJECTIVES

The aim of the study was to explore pathogenesis and find new serum markers for cow's-milk-sensitive enteropathy (CMSE) and coeliac disease (CD). We assessed the intestinal expression and serum concentration of CD23, IL-15, and FasL. We hypothesised that the serum levels of CD23, a protein expressed in the lymphoid follicles, would be associated with lymphonodular hyperplasia (LNH), a feature characteristic of CMSE. We also presumed that interleukin (IL)-15 and FasL, functionally connected with proliferation and apoptosis of the intraepithelial lymphocytes (IELs), would relate with the increased numbers of IELs present in both CMSE and CD.

METHODS

Twenty-three children with CMSE, 20 with untreated CD, and 14 controls were studied for CD3, α/β- and γ/δ-expressing IELs, and for duodenal and ileal expression of CD23, FasL, and IL-15 by immunohistochemistry, and for serum concentration of sCD23, sFasL, and sIL-15 by enzyme-linked immunosorbent assay.

RESULTS

There was a trend for increase in sCD23 serum levels in untreated CMSE and in CD (P = 0.074; P = 0.077). CD23 was expressed in the mucosal germinal centres, but sCD23 was not related to presence of LNH. In CMSE, there was a trend for increase in serum sFasL (P = 0.07) and high levels associated with LNH (P = 0.025) and correlated with the IEL numbers (P < 0.05). Mucosal high endothelial venules adjacent to lymphoid follicles showed an intensive FasL expression.

CONCLUSIONS

Serum sCD23 shows a trend of increment in CMSE and CD, and in the latter, sCD23 level may provide information about the severity of villous atrophy. In CMSE, high serum sFasL indicates both LNH and an increase of IELs, suggesting importance of FasL-mediated mechanisms in the pathogenesis of these features characteristic of CMSE. Further studies are necessary to evaluate whether intensive FasL expression in mucosal high endothelial venules presents a regulatory element in mucosal immunity.

Intestinal epithelial barrier dysfunction in food hypersensitivity

Intestinal epithelial barrier plays a critical role in the maintenance of gut homeostasis by limiting the penetration of luminal bacteria and dietary allergens, yet allowing antigen sampling for the generation of tolerance. Undigested proteins normally do not gain access to the lamina propria due to physical exclusion by tight junctions at the cell-cell contact sites and intracellular degradation by lysosomal enzymes in enterocytes. An intriguing question then arises: how do macromolecular food antigens cross the epithelial barrier? This review discusses the epithelial barrier dysfunction in sensitized intestine with special emphasis on the molecular mechanism of the enhanced transcytotic rates of allergens. The sensitization phase of allergy is characterized by antigen-induced cross-linking of IgE bound to high affinity FcεRI on mast cell surface, leading to anaphylactic responses. Recent studies have demonstrated that prior to mast cell activation, food allergens are transported in large quantity across the epithelium and are protected from lysosomal degradation by binding to cell surface IgE and low-affinity receptor CD23/FcεRII. Improved immunotherapies are currently under study including anti-IgE and anti-CD23 antibodies for the management of atopic disorders.

Immunoglobulin responses at the mucosal interface

Mucosal surfaces are colonized by large communities of commensal bacteria and represent the primary site of entry for pathogenic agents. To prevent microbial intrusion, mucosal B cells release large amounts of immunoglobulin (Ig) molecules through multiple follicular and extrafollicular pathways. IgA is the most abundant antibody isotype in mucosal secretions and owes its success in frontline immunity to its ability to undergo transcytosis across epithelial cells. In addition to translocating IgA onto the mucosal surface, epithelial cells educate the mucosal immune system as to the composition of the local microbiota and instruct B cells to initiate IgA responses that generate immune protection while preserving immune homeostasis. Here we review recent advances in our understanding of the cellular interactions and signaling pathways governing IgA production at mucosal surfaces and discuss new findings on the regulation and function of mucosal IgD, the most enigmatic isotype of our mucosal antibody repertoire.

CD23-dependent transcytosis of IgE and immune complex across the polarized human respiratory epithelial cells

IgE-mediated allergic inflammation occurs when allergens cross-link IgE on the surface of immune cells, thereby triggering the release of inflammatory mediators as well as enhancing Ag presentations. IgE is frequently present in airway secretions, and its level can be enhanced in human patients with allergic rhinitis and bronchial asthma. However, it remains completely unknown how IgE appears in the airway secretions. In this study, we show that CD23 (FcεRII) is constitutively expressed in established or primary human airway epithelial cells, and its expression is significantly upregulated when airway epithelial cells were subjected to IL-4 stimulation. In a transcytosis assay, human IgE or IgE-derived immune complex (IC) was transported across a polarized Calu-3 monolayer. Exposure of the Calu-3 monolayer to IL-4 stimulation also enhanced the transcytosis of either human IgE or the IC. A CD23-specific Ab or soluble CD23 significantly reduced the efficiency of IgE or IC transcytosis, suggesting a specific receptor-mediated transport by CD23. Transcytosis of both IgE and the IC was further verified in primary human airway epithelial cell monolayers. Furthermore, the transcytosed Ag-IgE complexes were competent in inducing degranulation of the cultured human mast cells. Because airway epithelial cells are the first cell layer to come into contact with inhaled allergens, our study implies CD23-mediated IgE transcytosis in human airway epithelial cells may play a critical role in initiating and contributing to the perpetuation of airway allergic inflammation.

Gut permeability and food allergies

Intestinal permeability is a critical feature of the gastrointestinal epithelium as it must allow an efficient passage of nutrients and restrict the entry of larger molecules, such as protein antigen, in order to facilitate appropriate immune responses towards food antigens. The proper regulation of the epithelial barrier relies on multiple, intricate physiological and immunologic mechanisms, in terms of which recent progresses regarding the cellular and molecular components have been unravelled. In genetically predisposed individuals, breakdown of oral tolerance can occur, leading to the inadequate production of allergen-specific IgE and the recruitment of mast cells in the gastrointestinal mucosa. Under such conditions, the intestinal permeability towards allergen is altered via different mechanisms, with IgE-CD23-mediated transport across the mucosa playing an important amplification role. Additionally, during the effector phase of the allergic reaction, when mast cells degranulate, a series of inflammatory mediators, such as proteases and cytokines, are released and further affects intestinal permeability. This leads to an increase in the passage of allergens and hence contributes to perpetuate the inflammatory reaction. In this review, we describe the importance of properly balanced intestinal permeability in oral tolerance induction and address the processes involved in damaging the intestinal barrier in the sensitized epithelium and during allergic reactions. We conclude by speculating on the effect of increased intestinal permeability on the onset of sensitization towards dietary antigens.

CD23/FcεRII: molecular multi-tasking

CD23 is the low-affinity receptor for immunoglobulin (Ig)E and plays important roles in the regulation of IgE responses. CD23 can be cleaved from cell surfaces to yield a range of soluble CD23 (sCD23) proteins that have pleiotropic cytokine-like activities. The regions of CD23 responsible for interaction with many of its known ligands, including IgE, CD21, major histocompatibility complex (MHC) class II and integrins, have been identified and help to explain the structure-function relationships within the CD23 protein. Translational studies of CD23 underline its credibility as a target for therapeutic intervention strategies and illustrate its involvement in mediating therapeutic effects of antibodies directed at other targets.

Multiple facets of intestinal permeability and epithelial handling of dietary antigens

The intestinal epithelium, the largest interface between the host and environment, regulates fluxes of ions and nutrients and limits host contact with the massive load of luminal antigens. Local protective and tolerogenic immune responses toward luminal content depend on antigen sampling by the gut epithelial layer. Whether, and how exaggerated, the entrance of antigenic macromolecules across the gut epithelium might initiate and/or perpetuate chronic inflammation as well as the respective contribution of paracellular and transcellular permeability remains a matter of debate. To this extent, experimental studies involving the in vivo assessment of intestinal permeability using small inert molecules do not necessarily correlate with the uptake of larger dietary antigens. This review analyzes both the structural and functional aspects of intestinal permeability with special emphasis on antigen handling in healthy and diseased states and consequences on local immune responses to food antigens.

Quercetin and kaempferol suppress immunoglobulin E-mediated allergic inflammation in RBL-2H3 and Caco-2 cells

OBJECTIVE

We investigated the inhibitory effects of quercetin and kaempferol treatment on the suppression of immunoglobulin E (IgE)-mediated allergic responses in relation to intestinal epithelium barrier function in RBL-2H3 and Caco-2 cells.

METHODS

RBL-2H3 cells as a model of intestinal mucosa mast cells were treated with flavonols followed by IgE-anti-dinitrophenyl sensitization. The extent of degranulation and the release of pro-inflammatory cytokines were measured. Caco-2 cells were stimulated with interleukin (IL)-4 or IgE-allergen with or without flavonol pretreatment and changes in the expression of CD23 mRNA and mitogen-activated protein kinase (MAPK), and chemokine release were determined.

RESULTS

Flavonols inhibited the secretion of allergic mediators in RBL-2H3 cells and suppressed the CD23 mRNA expression and p38 MAPK activation in IL-4 stimulated Caco-2 cells. Flavonols also suppressed IgE-OVA induced extra signal-regulated protein kinase (ERK) activation and chemokine release.

CONCLUSIONS

Quercetin and kaempferol effectively suppressed the development of IgE-mediated allergic inflammation of intestinal cell models.

Selective transport of IgE into ovine mammary secretions

Immunoglobulin (Ig) E is actively transported into ovine colostrum. Here we examine the degree of selectivity and the mechanism by which this transfer occurs in sheep. Results indicate that during colostrogenesis in sheep, transfer of immunoglobulins was most selective for IgG1 and IgA followed by IgE, IgM and IgG2. In milk, selectivity was greatest for IgA, followed by IgM, IgE, IgG1 and IgG2. The neonatal Fc receptor (FcRn) and poly immunoglobulin receptor (pIgR) mediate the transport of IgG1 and IgA across the ovine mammary epithelium respectively. In primates and rodents, the low-affinity IgE receptor, Fc epsilonRII, functions to transport IgE across the intestinal epithelium. We therefore investigated the expression of the low-affinity IgE receptor (CD23), pIgR and FcRn transcripts in the ovine mammary gland. The expression profiles of FcRn, pIgR and CD23 mRNA reflected concentrations of their Ig ligands in mammary secretions. These findings suggest a role for CD23 in transport of IgE across the mammary epithelium of sheep.

The epithelial gatekeeper against food allergy

The rapid rise of allergic disorders in developed countries has been attributed to the hygiene hypothesis, implicating that increased environmental sanitation in early childhood may be associated with higher incidence of hypersensitivity. Intestinal epithelial barriers play a crucial role in the maintenance of gut homeostasis by limiting penetration of luminal bacteria and dietary allergens, yet allowing antigen sampling via the follicle-associated epithelium for generation of tolerance. However, this intricate balance is upset in allergic intestines, whereby luminal proteins with antigenic properties gain access to the subepithelial compartment and stimulate mast cell degranulation. Recent studies demonstrated that food allergens were protected from lysosomal degradation, and were transported in large quantities across the epithelium by binding to cell surface IgE/CD23 (FcepsilonRII) that prevented the antigenic protein from lysosomal degradation in enterocytes. IL-4 (a Th2-type cytokine) not only increased production of IgE from B cells, but also upregulated the expression of CD23 on intestinal epithelial cells. Further studies indicated that CD23 was responsible for the bidirectional transport of IgE across epithelium. The presence of IgE/CD23 opens a gate for intact dietary allergens to transcytose across the epithelial cells, and thus foments the mast cell-dependent anaphylactic responses. The understanding of the molecular mechanism responsible for epithelial barrier defects may be helpful in designing novel therapies to treat food allergy and other allergic diseases.

Antibody-mediated antigen sampling across intestinal epithelial barriers

The epithelium of the gastrointestinal tract is the interface between luminal contents and the mucosal immune system. It must function as a selective barrier to limit penetration of antigens yet keep the mucosal immune system "informed" for the purpose of generating oral tolerance responses to food antigens or commensal organisms and host defense responses against pathogens. Alterations in epithelial barrier function have been proposed to play a significant role in gastrointestinal disease. In this review, we will discuss mechanisms of regulation of epithelial barrier function, and we will focus on the emerging understanding of how secreted immunoglobulins play a role in antigen-specific antigen sampling across the gastrointestinal epithelium.

Structural changes in the lectin domain of CD23, the low-affinity IgE receptor, upon calcium binding

CD23, the low-affinity receptor for IgE (Fc epsilonRII), regulates IgE synthesis and also mediates IgE-dependent antigen transport and processing. CD23 is a unique Fc receptor belonging to the C-type lectin-like domain superfamily and binds IgE in an unusual, non-lectin-like manner, requiring calcium but not carbohydrate. We have solved the high-resolution crystal structures of the human CD23 lectin domain in the presence and absence of Ca2+. The crystal structures differ significantly from a previously determined NMR structure and show that calcium binding occurs at the principal binding site, but not at an auxiliary site that appears to be absent in human CD23. Conformational differences between the apo and Ca2+ bound structures suggest how IgE-Fc binding can be both calcium-dependent and carbohydrate-independent.

Transcytosis of IgE-antigen complexes by CD23a in human intestinal epithelial cells and its role in food allergy

BACKGROUND & AIMS

Secreted immunoglobulins play an integral role in host defense at mucosal surfaces, and recent evidence shows that IgG can participate in antigen sampling from the intestinal lumen. We examined whether IgE also could facilitate transepithelial antigen sampling.

METHODS

Stool samples from food-allergic patients undergoing oral food challenge were analyzed for CD23 and food-specific IgE. CD23 isoform expression on primary human intestinal epithelial cells (IEC) was analyzed by polymerase chain reaction. The role of CD23 isoforms in transcytosis of antigen and IgE-antigen complexes was assessed using polarized human T84 cells retrovirally transfected with CD23a or CD23b.

RESULTS

CD23 was expressed constitutively on IECs, and food-allergic patients had increased levels of soluble CD23 and food-specific IgE in the stool after challenge. CD23a, but not CD23b, was expressed by primary human IECs. We show in transcytosis assays that CD23a, but not CD23b, acts as a bidirectional transporter of IgE. In addition, specific IgE facilitated the uptake of antigen from the apical surface of an epithelial monolayer by diverting antigen from delivery to lysosomes. Finally, delivery of antigen-IgE complexes across the epithelial barrier could induce the degranulation of rat basophil leukemia cells transfected with the human high-affinity IgE receptor.

CONCLUSIONS

These studies show that CD23a is expressed normally on human IECs, and in the presence of IgE can function as an antigen-sampling mechanism capable of activating subepithelial mast cells. IgE may serve as a secretory immunoglobulin that in concert with CD23 participates in food-induced pathophysiology of the gastrointestinal tract.

The high-affinity IgE receptor (FcepsilonRI): a critical regulator of airway smooth muscle cells?

The airway smooth muscle (ASM) has been typically described as a contractile tissue, responding to neurotransmitters and inflammatory mediators. However, it has recently been recognized that ASM cells can also secrete cytokines and chemokines and express cell adhesion molecules that are important for the perpetuation and modulation of airway inflammation. Recent progress has revealed the importance of IgE Fc receptors in stimulating and modulating the function of these cells. In particular, the high-affinity receptor for IgE (FcepsilonRI) has been identified in primary human ASM cells in vitro and in vivo within bronchial biopsies of atopic asthmatic individuals. Moreover, activation of this receptor has been found to induce marked increases in the intracellular calcium concentrations and T helper 2 cytokines and chemokines release. This and other evidence discussed in this review provide an emerging view of FcepsilonR/IgE network as a critical modulator of ASM cell function in allergic asthma.

How antibodies act as natural adjuvants

Antibodies can act like adjuvants. They can potently enhance the antibody response, and in the case of IgG and IgE also the T cell response, to the very antigen they are specific for. In this review we will discuss the recent advances made in our understanding of the underlying mechanisms of antibody-mediated feedback enhancement. The immuno-stimulatory properties of IgM, IgG1, IgG2a, IgG2b, IgG3 and IgE will be reviewed in relationship to the complement system and Fc receptors and the physiological relevance will be discussed.

Gut barrier dysfunction in food allergy

Intestinal permeability tests used in the diagnosis of allergic diseases in response to oral food challenge have led to the conclusion that constitutive defects of the intestinal barrier are not the primary cause of allergic diseases. However, perturbation of environmental factors (infection, stress), by increasing intestinal permeability and enhancing danger signals, may favour food allergy in susceptible individuals. The mechanisms of enhanced permeability to specific and bystander antigens have been delineated as well as the molecular events involved in the sequential phases of allergic reactions. Intestinal absorption of food antigens and immune responses are mutually dependent, and luminal (environmental) and serosal (intrinsic) factors synergize to maintain a self-perpetuating cycle in which antigens penetrate the mucosa and induce allergic inflammation.

Regulation of allergy by Fc receptors

The aggregation of high-affinity IgE receptors (FcepsilonRI) on mast cells and basophils has long been known as the critical event that initiates allergic reactions. Monomeric IgE was recently found to induce a variety of effects when binding to FcepsilonRI. Upregulation of FcepsilonRI only requires binding, whereas other responses require FcepsilonRI aggregation. Interestingly, FcepsilonRI aggregation has recently been understood to generate a mixture of positive and negative intracellular signals. Mast cells and basophils also express low-affinity and, under specific conditions, high-affinity IgG receptors. When co-engaging these receptors with FcepsilonRI, IgG antibodies can amplify or dampen IgE-induced mast cell activation. On the basis of these findings, it has been proposed that FcRs can be used as targets and/or tools for new therapeutic approaches to allergies.

Production of biologically active recombinant human soluble CD23 and its effect on PBMCs isolated from hyper-IgE blood

A recombinant form of human soluble CD23 (sCD23), the low affinity receptor for IgE (FcepsilonRII), was produced by PCR cloning the lectin-binding domain sequence into a bacterial expression vector. After renaturation and purification, the sCD23 bound IgE and divalent metal ions, indicating its activity. The recombinant human sCD23 exhibited similar proinflammatory properties as the native protein. Although interleukin-1beta, tumour necrosis factor-alpha, and nuclear factor-kappaB appeared not to be enhanced significantly in unstimulated RPMI 8866 B-lymphoblastoid and U937 promonocytic cell lines with 24 h incubation of recombinant sCD23, they were produced in both healthy and hyper-IgE-derived peripheral blood mononuclear cells, especially tumour necrosis factor-alpha. This study concludes that while recombinant and chimeric sCD23 may be useful in blocking IgE binding to immune cells and decreasing IgE synthesis by B-lymphocytes, the production of proinflammatory cytokines, particularly tumour necrosis factor-alpha will enhance immune responses in cases of asthma, allergy, and hyper-IgE syndrome.

Intracellular Trafficking of CD23: Differential Regulation in Humans and Mice by Both Extracellular and Intracellular Exons

In mouse models of food allergy, we recently characterized a new CD23b-derived splice form lacking extracellular exon 5, bDelta5, which undergoes constitutive internalization and mediates the transepithelial transport of free IgE, whereas classical CD23b is more efficient in transporting IgE/allergen complexes. These data suggested that regulation of endocytosis plays a central role in CD23 functions and drove us to systematically compare the intracellular trafficking properties of human and murine CD23 splice forms. We found that CD23 species show similar endocytic behaviors in both species; CD23a undergoes constitutive clathrin-dependent internalization, whereas CD23b is stable at the plasma membrane. However, the mechanisms controlling these similar behaviors appeared to be different. In mice, a positive internalization signal was localized in the cytoplasmic region shared by all CD23 splice forms. This positive signal was negatively regulated by the intracellular CD23b-specific exon. In addition, the fact that alternative splice forms lacking exons of the extracellular region (5, 6, 7, and/or 8) were all constitutively internalized suggested that endocytosis of murine CD23 is regulated by a process similar to the outside-in signaling of integrins. In humans, the internalization signal was mapped in the CD23a-specific intracellular exon. Interestingly, this signal also behaved as a basolateral targeting signal in polarized Madin-Darby canine kidney cells. The latter result and the fact that human intestinal cell lines were found to coexpress both CD23a and CD23b provide a molecular explanation for the initial observations that CD23 was found at the basolateral membrane of intestinal epithelial cells from allergic patients.