Spontaneous and ligand-induced endocytosis of CD23 (Fc epsilon receptor II) from the surface of B lymphocytes generates a 16-kDa intracellular fragment

Allergen-specific IgE levels and the ability of IgE-allergen complexes to cross-link determine the extent of CD23-mediated T-cell activation

Background

CD23 mediates IgE-facilitated allergen presentation and subsequent allergen-specific T-cell activation in allergic patients.

Objective

We sought to investigate key factors regulating IgE-facilitated allergen presentation through CD23 and subsequent T-cell activation.

Methods

To study T-cell activation by free allergens and different types of IgE–Bet v 1 complexes, we used a molecular model based on monoclonal human Bet v 1–specific IgE, monomeric and oligomeric Bet v 1 allergen, an MHC-matched CD23-expressing B-cell line, and a T-cell line expressing a human Bet v 1–specific T-cell receptor. The ability to cross-link Fcε receptors of complexes consisting of either IgE and monomeric Bet v 1 or IgE and oligomeric Bet v 1 was studied in human FcεRI-expressing basophils. T-cell proliferation by monomeric or oligomeric Bet v 1, which cross-links Fcε receptors to a different extent, was studied in allergic patients’ PBMCs with and without CD23-expressing B cells.

Results

In our model non–cross-linking IgE–Bet v 1 monomer complexes, as well as cross-linking IgE–Bet v 1 oligomer complexes, induced T-cell activation, which was dependent on the concentration of specific IgE. However, T-cell activation by cross-linking IgE–Bet v 1 oligomer complexes was approximately 125-fold more efficient. Relevant T-cell proliferation occurred in allergic patients’ PBMCs only in the presence of B cells, and its magnitude depended on the ability of IgE–Bet v 1 complexes to cross-link CD23.

Conclusion

The extent of CD23-mediated T-cell activation depends on the concentration of allergen-specific IgE and the cross-linking ability of IgE-allergen complexes.

A soluble form of the high affinity IgE receptor, Fc-epsilon-RI, circulates in human serum

Soluble IgE receptors are potential in vivo modulators of IgE-mediated immune responses and are thus important for our basic understanding of allergic responses. We here characterize a novel soluble version of the IgE-binding alpha-chain of Fc-epsilon-RI (sFcεRI), the high affinity receptor for IgE. sFcεRI immunoprecipitates as a protein of ∼40 kDa and contains an intact IgE-binding site. In human serum, sFcεRI is found as a soluble free IgE receptor as well as a complex with IgE. Using a newly established ELISA, we show that serum sFcεRI levels correlate with serum IgE in patients with elevated IgE. We also show that serum of individuals with normal IgE levels can be found to contain high levels of sFcεRI. After IgE-antigen-mediated crosslinking of surface FcεRI, we detect sFcεRI in the exosome-depleted, soluble fraction of cell culture supernatants. We further show that sFcεRI can block binding of IgE to FcεRI expressed at the cell surface. In summary, we here describe the alpha-chain of FcεRI as a circulating soluble IgE receptor isoform in human serum.

Proteases produced by activated neutrophils are able to release soluble CD23 fragments endowed with proinflammatory effects

Polymorphonuclear neutrophils (PMNs) are the major source of proteolytic activities involved mainly in tissue injuries observed in chronic inflammatory disorders. High levels of soluble forms of CD23 (the low-affinity receptor for IgE) were found in biological fluids from these patients, and recent reports focused on a CD23-mediated regulation of inflammatory response. In this context, we show here that co-culture of activated PMN with CD23+ B cells resulted in a drastic release of soluble CD23 fragments from the cell surface. This cleavage was inhibited by serine proteases inhibitors, including a1-antitrypsin. We next demonstrated that purified human leukocyte elastase or cathepsin G efficiently cleaved membrane CD23 on B cells with a high specificity. Soluble fragments released by serine proteases-mediated CD23 proteolysis stimulated resting monocytes to produce oxidative burst and proinflammatory cytokine without any co-stimulatory signal. This work strongly supports the idea that the capacity of PMN-derived proteases to release soluble forms of CD23 participates in the inflammatory process mediated by these cells.

Endocytosis and recycling of the complex between CD23 and HLA-DR in human B cells

The presentation of extremely low doses of antigen to T cells is enhanced by immunoglobulin E (IgE)-dependent antigen focusing to CD23, the low-affinity receptor for IgE, expressed on activated B cells. CD23 contains a C-type lectin domain in its extracellular sequence and a targeting signal for coated pits, required for endocytosis, in its cytoplasmic sequence. CD23 is non-covalently associated with the major histocompatibility complex class II antigen, human leucocyte antigen HLA-DR, on the surface of human B cells, but the fate of this complex following endocytosis is unknown. To answer this question we have labelled these proteins on the surface of RPMI 8866 B cells and traced their route through the cytoplasm. Endocytosis mediated by anti-CD23 antibodies (BU38 and MHM6) led to the loss of CD23 from the cells. Endocytosis mediated by an antibody to HLA-DR (CR3/43) or an antigen-IgE complex (NP-BSA-anti-NP IgE), however, led to recycling of the HLA-DR-CD23 complex to the cell surface on a time scale (3-6 hr) consistent with the recycling of HLA-DR in antigen presentation. Along the latter pathway CD23 label was observed in cytoplasmic organelles that resembled the 'compartments for peptide loading' or 'class II vesicles' described by previous authors. Two features of the recycling process may contribute to the efficiency of antigen presentation. Peptide exchange may be facilitated by the proximity of HLA-DR and antigen in peptide loading compartments of the endosomal network. The return of CD23 with HLA-DR to the cell surface may then help to stabilize specific B-cell-T-cell interactions, contributing to T-cell activation.

Immune complex-mediated enhancement of antibody responses without induction of delayed-type hypersensitivity

Immunoglobulin (Ig)G and IgE antibodies enhance the humoral response in vivo to soluble antigens with which they form complexes. In vitro, antigen is targeted to B cells by IgE antibodies and to macrophages and dendritic cells (DCs) by IgG, thus leading to increased antigen presentation to specific T cells. Possibly these mechanisms are also responsible for antibody-mediated enhancement in vivo. We now address the question of whether IgG- and/or IgE-antigen complexes can prime for delayed-type hypersensitivity (DTH), a reaction known to require primed T helper (Th)1 cells. Mice were immunized with IgG-anti-2,4,6-trinitrophenyl (TNP)/BSA-TNP or IgE-anti-TNP/BSA-TNP. Mice given BSA-TNP alone or BSA-TNP in complete Freund's adjuvans (CFA) were used as controls. DTH and IgG-anti-BSA levels were measured after subsequent challenge with BSA. A potent BSA-specific antibody response was induced by IgE- or IgG-complexed antigen as well as by CFA/antigen but DTH-reactions were only observed in mice immunized with CFA/antigen. Both IgE and IgG enhanced the production of BSA-specific IgG1, IgG2a and IgG2b, although the most pronounced enhancement was seen in the production of IgG1. These findings suggest that Th2 cells rather than Th1 cells are involved in the immune response to IgG- and IgE-immune complexes.

Inhibitors of the p38 Mitogen-Activated Kinase Modulate IL-4 Induction of Low Affinity IgE Receptor (CD23) in Human Monocytes

CD23, the low affinity IgE receptor, is up-regulated on the surface of IL-4-treated B cells and monocytes and is immediately proteolytically processed, releasing soluble fragments of CD23. Here, we report that inhibitors of the p38 mitogen-activated kinase (p38 MAPK), SK&F 86002 or the more selective inhibitor, SB 203580, reduce the levels of soluble CD23 formed by IL-4-stimulated human monocytes or the human monocytic cell line, U937. In contrast to compounds such as the metalloprotease inhibitor batimastat ([4-(N-hydroxyamino)-2-(R)-isobutyl-3-(S)-(2-thiophenethiomethyl)succinyl]-(S)-phenylalanine-N-methylamide, sodium salt), p38 MAPK inhibitors do not directly inhibit proteolytic processing of CD23. Further, evaluation of surface intact CD23 (iCD23) by flow cytometry demonstrated that SK&F 86002 and SB 203580 reduced the surface expression of iCD23 in a concentration-dependent fashion, while batimastat increased the surface expression of iCD23. The decrease in surface iCD23 was accompanied by a decrease in total cell-associated CD23 protein levels but not CD23 mRNA. IL-4 induced a late (>4-h) increase in p38 MAPK activity and corresponding activation of its substrate MAPKAPK-2. This activation was blocked by addition of SB 203580 before IL-4 induction, in parallel with the inhibition of CD23 expression. Modulation of CD23 by antibodies has been shown to alleviate the symptoms of murine collagen-induced arthritis, implicating CD23 as an important proinflammatory agent. These data show that in addition to the known cytokine inhibitory actions of SK&F 86002 and SB 203580, they also confer an additional potential anti-inflammatory activity through modulation of CD23 expression.

Binding of anti-CD23 monoclonal antibody to the leucine zipper motif of FcepsilonRII/CD23 on B cell membrane promotes its proteolytic cleavage. Evidence for an effect on the oligomer/monomer equilibrium

In the present study we have compared the binding of two monoclonal antibodies to CD23, EBVCS1 and mAb25, which recognize the stalk and the lectin domain, respectively, on the CD23 molecule. At 4 degreesC, EBVCS1 binds to about 10% of the receptors recognized by mAb25 on the B cell surface. At 37 degreesC, whereas mAb25 reaches its maximal binding within a few seconds, EBVCS1 requires 60 min to bind to the same extent. Stabilization of the oligomeric structure of CD23 with IgE strongly affects in a dose-dependent fashion the number of binding sites seen by EBVCS1 but not the t1/2 to reach them, suggesting that EBVCS1 binds to the coiled coil region through an allosteric mechanism. EBVCS1 rapidly down-modulates the membrane CD23 expression with a coincident increase of CD23-soluble fragments in the culture medium, an effect that is inhibited by IgE. In contrast, mAb25, as well as IgE, protects CD23 from proteolytic cleavage and stimulates its endocytosis. These results suggest that EBVCS1 unravels the coiled coil structure of CD23, rendering it more susceptible to proteolytic attack. This supports the oligomeric model proposed previously (Gould, H., Sutton, B., Edmeades, R., and Beavil, A. (1991) Monogr. Allergy 29, 28-49). The biological significance of these observations is discussed.

Impaired Antibody Responses in H-2Ab Mice

In murine in vivo systems, Ags administered in physiologic solutions together with specific IgE induce a significantly higher Ab response than Ags administered alone. In vitro, IgE in complex with Ag enhances B cell-mediated presentation of the Ag to T cells. Both phenomena require an intact low affinity receptor for IgE (FcεRII/CD23), suggesting that the effect on in vivo Ab responses is caused by increased Ag presentation. We here show that mice carrying the MHC class II Ab molecule (e.g., C57BL/6 and 129/Sv) do not produce Abs to BSA when immunized with BSA-2,4,6-trinitrophenyl (TNP) in complex with monoclonal IgE anti-TNP. In contrast, strains of all other MHC haplotypes tested (H-2d, H-2k, H-2p, H-2q, and H-2s) respond vigorously to IgE/BSA-TNP complexes, with Ab responses several hundred-fold higher than the responses in H-2b mice. C57BL/6 mice were unable to produce a carrier-specific response also after immunization with IgE/OVA-TNP, IgE/diphtheria toxoid-TNP, or IgE/tetanus toxoid-TNP. Although the low responsiveness mapped to the Ab region, responsiveness was not restored in C57BL/6 mice carrying transgenic Ak, suggesting that a nonclassical A-region-encoded gene product is involved. Most importantly, our data call attention to the fact that the C57BL/6 and 129 mouse strains, which are widely used for producing transgenic animals, have defective immune responses.

Structure and functions of CD23

This review summarizes recent data on CD23, a low affinity receptor for IgE (Fc epsilon RII). CD23 is the only FcR which does not belong to the immunoglobulin gene superfamily. The CD23 molecule was discovered independently as an IgE receptor on human lymphoblastoid B cells [1], as a cell surface marker expressed on Epstein-Barr-Virus-transformed B cells (EBVCS) [2] and as a B-cell activation antigen (Blast 2) [3]. CD23 was shown to be a low affinity receptor for IgE [4,5]. Similar to most FcR, soluble forms of CD23 (sCD23) are released into extracellular fluids. The soluble fragments formed by proteolytic cleavage of surface CD23 are not only capable of binding IgE (IgE binding factors) but also exhibit multiple functions that are not IgE related. These observations together with the finding that CD23 displays significant homology with Ca(2+)-dependent (C-type) animal lectins, suggested the existence of natural ligands other than IgE. The recent finding that CD23 interacts with CD21, CD11b and CD11c indicates that CD23 should be viewed not only as a low affinity IgE receptor but also as an adhesion molecule involved in cell-cell interaction. After a brief overview of the molecular structure, there follows a discussion of the biological activities ascribed to human CD23.

CD38 Functions Are Regulated Through an Internalization Step

The endocytosis of the human CD38 molecule has been investigated in normal lymphocytes and in a number of leukemia- and lymphoma-derived cell lines. CD38 internalization was followed using radioiodinated Abs in an acidic elution endocytosis assay to monitor the effects of cross-linking on internalization processes and to quantify the ratio of the internalized molecule. Second, conventional, confocal, and electron microscopies were used to evaluate the morphologic effects induced by ligation of the molecule with Abs mimicking the natural ligand(s). The results demonstrated that internalization is a reproducible phenomenon following CD38 ligation with both agonistic and nonagonistic specific Abs and involving only a fraction of the entire amount of the surface molecule. It is independent from signal transduction as can be inferred by the observation that 1) both agonistic and non agonistic Abs are effective and 2) the dynamic of internalization is much slower than that of cellular signaling. Morphologic studies demonstrated that endocytosis induced as a result of CD38 ligation presents a very specific pathway consisting of subcellular organelles fundamental to the processing of the complex. Our data indicate that down-regulation by endocytosis may be, in parallel with shedding, a regulatory element in activation and adhesion processes mediated by CD38. However, internalization seems not to be a key step in triggering intracellular signaling; more likely, it is a negative feedback control mechanism which interrupts signal transduction or cell-cell cross-talks mediated by membrane CD38.

B cell hybridoma presents both B-cell and T-cell epitopes for stimulating antibody production via CD23 pathway

CD23+ B cell hybridoma 17A11, pulsed with IgE:TNP-KLH triggered IgA, IgG, and IgE antibody production via CD23-mediated presentation. Prior anti-CD23 treatment abrogated 95% of the humoral antibody responses. Both B and T cell epitopes were presented by 17A11 B cell epitopes as recognized by IgG but not T cell epitopes were sensitive to treatment with 0.2 M acetic acid. Efficacy of antigen presentation via CD23 on 17A11 was comparable to that mediated via surface immunoglobulins (sIg) on a CD23 negative 4.5 parental fusion partner B cell line. This is the first demonstration that IgE:TNP-KLH pulsed B cell hybridomas present both B- and T-cell epitopes in stimulating IgA, IgG, and IgE antibody production, and raise a pertinent issue whether IgE antibodies produced under pathophysiological conditions may serve as positive feedback signal for sustaining production of different classes of antibodies.

CD20 monoclonal antibodies decrease interleukin-4-stimulated expression of the low-affinity receptor for IgE (Fc epsilon RII/CD23) in human B cells by increasing the extent of its cleavage

CD20 monoclonal antibody (mAb) B1 is known to inhibit B cell proliferation. We show that B1 reduced both anti-mu + interleukin-4 (IL-4)-induced DNA synthesis and the concomitant expression of CD23 at the surface of human tonsillar B cells. B1 mAb had no effect on CD23 mRNA levels. The disappearance of CD23 molecule from the surface correlates with an increase of soluble CD23 fragments in the culture medium, indicating that CD20 mAb B1 stimulated the cleavage of the molecule. B1 also inhibits IgE production by peripheral blood mononuclear cells cultured in the presence of IL-4. Suppression of IgE synthesis and enhancement of CD23 cleavage are concomitant but appear not to be functionally related.

CD23-mediated homotypic cell adhesion: the role of proteolysis

CD23 is a multifunctional molecule expressed by cells of lymphoid, myeloid and hematopoietic lineages. As a cell surface molecule CD23 acts both as a low-affinity receptor for IgE (Fc epsilon RII) and as a cell adhesion molecule. CD23 can undergo autoproteolysis to release soluble 37-25-kDa CD23 (s-CD23) molecules with a range of cytokine activities. Here we show a causal link between the two apparently disparate functions of autoproteolysis and cell adhesion. The Epstein-Barr virus-transformed B cell line RPMI-8866 formed macroscopic cell clusters solely via CD23. Cell adhesion was inhibited by mAb to CD23 and by IgE. Cell adhesion was also dependent on serum as cells grown in serum-free media failed to form clusters. In serum-free conditions cell adhesion could be induced by the addition of not only 10% FCS but also s-CD23. As s-CD23 is reported to possess proteolytic activity we screened a range of proteases to determine whether they also could induce cell adhesion in serum-free medium. It was found that chymotrypsin and elastase induced cell:cell adhesion in RPMI-8866 cells. The same panel of proteases were screened against a range of CD23-positive (Jijoye, AF-10, T2, U937, ICH-1) and CD23-negative (RPMI-8226, U266, MOLT-4, Ramos) cell lines. It was found that chymotrypsin and elastase induce cell adhesion only in cells expressing CD23. Peptide mapping studies showed that chymotrypsin and elastase cleaved immunoprecipitated CD23 near the same site by which 37-kDa s-CD23 is released (Ala 80). Serum demonstrated no proteolytic activity towards CD23. However, it was found that cells grown in serum-free medium released 25-kDa s-CD23 without the need for prior cleavage at the 37-kDa cleavage site. To confirm the role of proteolysis in CD23-mediated cell adhesion we screened a range of protease inhibitors for their ability to antagonize this process. It was found that tosyl-lysine chloromethyl ketone inhibited CD23-mediated cell adhesion. Lactoperoxidase treatment, which inhibits CD23 cleavage, also inhibited cell adhesion. Addition of chymotrypsin and elastase to lactoperoxidase-treated cells induced cell adhesion. From these data we propose that intact CD23 has no demonstrable role in cell adhesion; instead, the portion of CD23 remaining on the cell surface following cleavage appears to mediate cell adhesion.

Antigen focusing by specific monomeric immunoglobulin E bound to CD23 on Epstein-Barr virus-transformed B cells

Monomeric IgE bound to the low-affinity receptor for IgE (FcERII-CD23) on EBV-transformed human B cells selectively enhances binding of antigen and therefore presentation to specific T-cell clones. To demonstrate the role of monomeric IgE in antigen focusing, we have made use of a system consisting of human T-cell clones specific for Der-P1 (major allergen of the Dermathophagoides pteronyssinus), Der-P1 coupled to NIP (Der-P1-NIP), and the commercially available chimeric (human-murine) monoclonal IgE antibodies with specificity for the hapten NIP. We have found that monomeric IgE binds to CD23 and remains detectable on the surface of the B cells for a period of at least 16 hours at 37 degrees C. Pulsing of these IgE-anti-NIP (1 microgram/ml) treated B cells for 1 hour at 37 degrees C with low amounts (10 ng/ml) of Der-P1-NIP antigen allows the B cells to stimulate Der-P1-specific T cells. Even with IgE concentrations as low as 20 ng/ml, which were not detectable by immunofluorescence, we were able to induce a significant T-cell response. Furthermore, ongoing specific T-cell-B-cell interactions were not inhibited by the presence of high concentrations of nonspecific IgE molecules (incubated with up to 25 micrograms/ml) on the surface of the B cells. Our data confirm the hypothesis that IgE, bound by either CD23 or the high-affinity receptor for IgE, potentiates the immune response. Therefore, IgE may be seen as the fourth general mechanism for antigen capture by (nonspecific) antigen-presenting cells.