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

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.

CD23 is a glycan-binding receptor in some mammalian species

CD23, the low-affinity IgE receptor found on B lymphocytes and other cells, contains a C-terminal lectin-like domain that resembles C-type carbohydrate-recognition domains (CRDs) found in many glycan-binding receptors. In most mammalian species, the CD23 residues required to form a sugar-binding site are present, although binding of CD23 to IgE does not involve sugars. Solid-phase binding competition assays, glycoprotein blotting experiments, and glycan array analysis employing the lectin-like domains of cow and mouse CD23 demonstrate that they bind to mannose, GlcNAc, glucose, and fucose and to glycoproteins that bear these sugars in nonreducing terminal positions. Crystal structures of the cow CRD in the presence of α-methyl mannoside and GlcNAcβ1-2Man reveal that a range of oligosaccharide ligands can be accommodated in an open binding site in which most interactions are with a single terminal sugar residue. Although mouse CD23 shows a pattern of monosaccharide and glycoprotein binding similar to cow CD23, the binding is weaker. In contrast, no sugar binding was observed in similar experiments with human CD23. The absence of sugar-binding activity correlates with accumulation of mutations in the gene for CD23 in the primate lineage leading to humans, resulting in loss of key sugar-binding residues. These results are consistent with a role for CD23 in many species as a receptor for potentially pathogenic microorganisms as well as IgE. However, the ability of CD23 to bind several different ligands varies between species, suggesting that it has distinct functions in different organisms.

Mucosal antibodies in the regulation of tolerance and allergy to foods

The intestinal mucosa is densely packed with antibody-secreting B cells, the majority of which produce IgA. Mucosal antibodies have traditionally been thought of as neutralizing antibodies that exclude antigens, but they also function in antigen sampling, allowing for selective transcytosis of antigens from the intestinal lumen. IgE-mediated antigen uptake can facilitate the development of allergic reactions to foods, but emerging evidence indicates that IgG-mediated antigen uptake may also play an important role in the development of immune tolerance to foods, particularly in the neonate. This review will focus on the role of intestinal immunoglobulins in the development of clinical tolerance and allergy to food 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.

CD23 Sheddase A disintegrin and metalloproteinase 10 (ADAM10) is also required for CD23 sorting into B cell-derived exosomes

The low affinity receptor for IgE, CD23, is the natural regulator of IgE synthesis, and understanding both the synthesis and the catabolism of CD23 are, thus, important issues. Membrane CD23 is cleaved by a disintegrin and metalloproteinase 10 (ADAM10) and this cleavage influences the ability of CD23 to regulate IgE. In contrast to the belief that cleavage is a cell surface event, endosomal neutralization with NH(4)Cl was found to dramatically reduce CD23 cleavage, suggesting that the majority of CD23 cleavage occurred subsequent to internalization in the endosomal pathway and not at the cell surface. In line with this, full-length CD23 was shown to be sorted in an ADAM10-dependent manner into exosomes. Greatly increased ADAM10-mediated CD23 cleavage was seen at endosomal pH. Additionally, the stalk region of CD23 was found to interact with ADAM10 and ADAM10 binding of CD23 was found to be protease independent. SPR analysis of the interaction indicated about a 10-fold increase in the R(max) at endosomal pH (pH 5.8) compared with pH 7.4, whereas the affinity of the interaction was not significantly changed. The R(max) change, combined with the increased cleavage at endosomal pH, indicates greater accessibility of the CD23 stalk region for ADAM10 at the lower pH. These results indicate a model where CD23 internalization results in ADAM10-dependent incorporation into exosomes, followed by partial cleavage of CD23 by ADAM10 prior to being released from the cell. The increased cleavage at endosomal pH also has implications for other ADAM10 substrates.

Murine B cells regulate serum IgE levels in a CD23-dependent manner

The manifestations of allergic disorders are closely tied to the biologic effects of IgE activation with Ag. In immediate hypersensitivity reactions, IgE effector function requires prior binding to innate immune cells, primarily mast cells and basophils, with the blood acting as a reservoir for unbound IgE. As the severity of allergic disease is proportional to the size of this unbound IgE pool, we hypothesized that cellular mechanisms exist to limit the size and/or enhance the clearance of free IgE molecules. We examined this in mice by engineering a reporter IgE molecule that allowed us to track the fate of IgE molecules in vivo. The absence of FcεRI-expressing cells did not affect serum IgE levels, but B cells regulated serum IgE by controlling the size of the free IgE pool. B cells captured IgE by direct binding to the low-affinity IgE receptor, CD23. These data indicate a mechanism regulating serum IgE and additionally clarify the role of CD23 in this process.

Vav and Rac activation in B cell antigen receptor endocytosis involves Vav recruitment to the adapter protein LAB

The signal transduction events supporting B cell antigen receptor (BCR) endocytosis are not well understood. We have identified a pathway supporting BCR internalization that begins with tyrosine phosphorylation of the adapter protein LAB. Phosphorylated LAB recruits a complex of Grb2-dynamin and the guanine nucleotide exchange factor Vav. Vav is required for activation of the small GTPases Rac1 and Rac2. All these proteins contribute to (and dynamin, Vav, and Rac1/2 are required for) BCR endocytosis and presentation of antigen to T cells. This is the first description of a sequential signal transduction pathway from BCR to internalization and antigen presentation.

Immunopathogenesis of bronchial asthma

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

Molecular blocking of CD23 supports its role in the pathogenesis of arthritis

Background

CD23 is a differentiation/activation antigen expressed by a variety of hematopoietic and epithelial cells. It can also be detected in soluble forms in biological fluids. Initially known as the low-affinity receptor for immunoglobulin E (FcεRII), CD23 displays various other physiologic ligands such as CD21, CD11b/c, CD47-vitronectin, and mannose-containing proteins. CD23 mediates numerous immune responses by enhancing IgE-specific antigen presentation, regulating IgE synthesis, influencing cell differentiation and growth of both B- and T-cells. CD23-crosslinking promotes the secretion of pro-inflammatory mediators from human monocytes/macrophages, eosinophils and epithelial cells. Increased CD23 expression is found in patients during allergic reactions and rheumatoid arthritis while its physiopathologic role in these diseases remains to be clarified.

Methodology/Principal Findings

We previously generated heptapeptidic countrestructures of human CD23. Based on in vitro studies on healthy and arthritic patients' cells, we showed that CD23-specific peptide addition to human macrophages greatly diminished the transcription of genes encoding inflammatory cytokines. This was also confirmed by significant reduction of mediator levels in cell supernatants. We also show that CD23 peptide decreased IgE-mediated activation of both human and rat CD23⁺ macrophages. In vivo studies in rat model of arthritis showed that CD23-blocking peptide ameliorates clinical scores and prevent bone destruction in a dose dependent manner. Ex-vivo analysis of rat macrophages further confirmed the inhibitory effect of peptides on their activation. Taken together our results support the role of CD23 activation and subsequent inflammatory response in arthritis.

Conclusion

CD23-blocking peptide (p30A) prevents the activation of monocytes/macrophages without cell toxicity. Thus, targeting CD23 by antagonistic peptide decreases inflammatory markers and may have clinical value in the treatment of human arthritis and allergic reactions involving CD23.

Immunophysiology of experimental food allergy

Animal models of food allergy have been used to identify mechanisms involved in the development of sensitization to food proteins as well as immunologic mechanisms of adverse reactions to allergen reexposure. To counteract the normal tolerant responses to antigen generated in the gastrointestinal tract, investigators have used mucosal adjuvants or manipulated the mucosal barrier, taken advantage of endogenous adjuvanticity of some food allergens, or bypassed the oral route and sensitized through the skin. Site of antigen uptake in the gastrointestinal tract is a critical factor in both sensitization and anaphylaxis, and antigen uptake can be facilitated by immunoglobulin-E (IgE)-antigen complexes binding to CD23 on the epithelial cell surface. Studies on systemic anaphylaxis or local gastrointestinal manifestations of food allergy in mice have highlighted the contribution of IgE, mast cells, and pathogenic Th2 lymphocytes in experimental food allergy.

IgE in allergy and asthma today

The spreading epidemic of allergies and asthma has heightened interest in IgE, the central player in the allergic response. The activity of IgE is associated with a network of proteins; prominent among these are its two principal receptors, FcepsilonRI (high-affinity Fc receptor for IgE) and CD23, as well as galectin-3 and several co-receptors for CD23, notably CD21 and various integrins. Here, we review recent progress in uncovering the structures of these proteins and their complexes, and in our understanding of how IgE exerts its effects and how its expression is regulated. The information that has emerged suggests new therapeutic directions for combating allergic disease.

Allergen-IgE complexes trigger CD23-dependent CCL20 release from human intestinal epithelial cells

BACKGROUND & AIMS

In food allergic individuals, exposure to food allergens by the oral route can trigger immediate (within minutes) local hypersensitivity reactions in the intestine followed by a late-phase inflammatory response. Previous work has shown that CD23 is constitutively expressed by human intestinal epithelial cells and mediates the uptake of allergen-IgE complexes. We hypothesized that allergen-IgE complexes could also signal via CD23 to trigger an inflammatory cascade in the local environment.

METHODS

Caco-2 monolayers were stimulated with human IgE-antigen (Ag) complexes. IL-8 and CCL20 mRNA and protein were determined by RT-PCR and ELISA, respectively. Signaling pathways were assessed by immunoblotting. Endogenous CD23 expression was knocked down by stable transfection with CD23 shRNA retroviral plasmid. Migration assays were performed using human monocyte-derived dendritic cells.

RESULTS

Stimulation of Caco-2 cells with IgE-Ag complexes triggered upregulation of IL-8 and CCL20 at the mRNA and protein level. Allergen complexes induced phosphorylation of ERK and JNK, but not p38 MAP kinase or NK-kappaB, and resulted in AP-1 activation. Cross-linking of CD23 replicated the findings with IgE-Ag complexes, and silencing of CD23 expression abrogated the response to allergen-IgE complexes. Supernatant from IgE-Ag-stimulated epithelial cells induced migration of dendritic cells in a CCL20-dependent manner. Finally, immunostaining of duodenal biopsies demonstrated that CCL20 was constitutively expressed by epithelial cells in vivo.

CONCLUSIONS

Signaling via epithelial CD23 may participate in the late-phase inflammatory response by the release of chemokines capable of recruiting antigen presenting cells and effector cells of allergic inflammation.

Maternal transmission of resistance to development of allergic airway disease

Parental phenotype is known to influence the inheritance of atopic diseases, such as allergic asthma, with a maternal history being a more significant risk factor for progeny than paternal history. We hypothesized that recall Th1- or Th2-type immune responses during pregnancy would result in transfer of maternal factors that would differentially impact development of immune responsiveness in offspring. Following weaning, susceptibility and severity of allergic airway disease (a murine model of human asthma) was evaluated in progeny, disease being elicited by immunization with OVA-Al(OH)(3) and challenge with aerosolized OVA. We found that progeny of mothers with Th1-biased immunity to OVA subjected to recall aerosol challenge during pregnancy had reduced levels of Ag-specific IgE and airway eosinophilia compared with progeny of mothers with Th2-biased immunity to OVA or naive mothers. Interestingly, progeny of mothers with Th1-type immunity to a heterologous albumin, BSA, were not protected from developing OVA-induced allergic airway disease. These findings demonstrated that maternal transfer of protection from development of allergic airway disease to offspring in this model of maternal Th1-type immunity was Ag specific.

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.

CD23-mediated transport of IgE/immune complexes across human intestinal epithelium: role of p38 MAPK

We previously reported that CD23/FcepsilonRII (low-affinity IgE receptor) is expressed on human intestinal epithelial cells and is responsible for transepithelial transport of IgE. In this study, we compared the transport of IgE with that of immune complexes in both the apical-to-serosal and the serosal-to-apical directions across HT29 epithelial cell layers and examined the effects of two p38 MAPK inhibitors, SKF86002 and SB203580, on the expression and function of CD23. Our study showed that both p38 MAPK inhibitors at 10 microM significantly inhibited constitutive and IL-4-upregulated CD23 protein expression in epithelial cells. Both inhibitors, in a concentration-dependent manner, also significantly reduced IgE binding and uptake into cells. Transepithelial transport of IgE and immune complexes across the epithelial barrier were similarly inhibited. IL-4 upregulated the phosphorylation and activity of p38 MAPK and the phosphorylation of the downstream substrate MAPKAPK-2 (MK-2). The inhibitors exerted effects in the pathway post the p38 MAPK; SB203580 significantly inhibited the phosphorylation of MK-2. Our results indicate that CD23 expression in these human intestinal epithelial cells is mediated through the p38 MAPK pathway and that inhibition of p38 MAPK consequently interferes with the transport of IgE and immune complexes across the intestinal epithelial barrier.

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.