CD21 is a ligand for CD23 and regulates IgE production

An intranasal glucocorticoid inhibits the increase of specific IgE initiated during birch pollen season

BACKGROUND

Recent in vitro findings show that glucocorticoids in combination with IL-4 can induce the synthesis of IgE, indicating that glucocorticoids may promote allergy.

OBJECTIVE

A double-blind, placebo-controlled study was performed to evaluate the effect of an intranasal glucocorticoid on the levels of birch pollen-specific IgE antibodies in serum from patients with allergic rhinitis.

METHODS

Eighteen patients with allergic rhinitis received treatment with an intranasal glucocorticoid (beclomethasone dipropionate, 400 microg/day) or placebo for 5 weeks, starting from the beginning of the birch pollen season. Blood samples for anti-birch IgE evaluation were taken before treatment was initiated and at 2 and 5 weeks after the beginning of the study.

RESULTS

The beclomethasone group (n = 9) had significantly lower symptom scores when compared with the placebo group (n = 9) (0.86 +/- 0.26 vs 2.79 +/- 0.76, p value = 0.01). Both the treatment group and the placebo group showed a trend of an increase in anti-birch IgE levels 2 weeks after the beginning of the treatment (from 33.1 +/- 13.1 kU/L to 44.9 +/- 20.9 kU/L in the beclomethasone group and from 53.2 +/- 18.9 kU/L to 64.1 +/- 22.1 kU/L in the placebo group). Treatment with beclomethasone returned anti-birch IgE levels to baseline by the end of the study, whereas in the placebo group the anti-birch IgE levels continued to increase (final values, 33.1 +/- 11.9 kU/L vs 72.6 +/- 23.2 kU/L, respectively). The change in IgE antibody levels in the placebo group was significantly higher than that in the beclomethasone group. No statistically significant changes in total IgE or soluble CD23 levels were detected.

CONCLUSION

We conclude that treatment with an intranasal glucocorticoid initiated at the beginning of the pollen season inhibits the induced increase in specific IgE.

The mast cell function-associated antigen exhibits saccharide binding capacity

The mast cell function-associated antigen (MAFA) is a membrane glycoprotein first identified on rat mucosal type mast cells (line RBL-2H3) and known to inhibit the Fc epsilon RI-mediated secretory response. In its extracellular domain, an amino acid stretch homologous to the carbohydrate binding domain of calcium-dependent animal lectins has been found. To investigate its carbohydrate binding capacity, the MAFA has been expressed in the Spodoptera frugiperda insect cell line (Sf9) using the baculovirus expression system. Analysis by flow cytometry and surface labeling with 125I showed that the recombinant MAFA (rMAFA) was expressed as a monomeric and disulfide-linked homodimeric glycoprotein in the membrane of the insect cells, and both forms exhibited the same epitopes as the protein isolated from RBL-2H3 cells. Immunoaffinity-purified rMAFA was then employed for studies of its saccharide binding capacity by using different neoglycans and glycoproteins. The rMAFA was found to bind specifically terminal mannose residues in a Ca(2+)-dependent manner. These results support the notion that the extracellular domain of the MAFA is indeed able to bind ligands, which may be modulatory for the mast cell response.

Mouse CD23 regulates monocyte activation through an interaction with the adhesion molecule CD11b/CD18

CD23 is expressed on a variety of hemopoietic cells. Recently, we have reported that blocking CD23 interactions in a murine model of arthritis resulted in a marked improvement of disease severity. Here, we demonstrate that CD11b, the alpha chain of the beta 2 integrin adhesion molecule complex CD11b/CD18 expressed on monocytes interacts with CD23. Using a recombinant fusion protein (ZZ-CD23), murine CD23 was shown to bind to peritoneal macrophages and peripheral blood cells isolated from mice as well as the murine macrophage cell line, RAW. The interactions between mouse ZZ-CD23 and CD11b/CD18-expressing cells were significantly inhibited by anti-CD11b monoclonal antibodies. A functional consequence was then demonstrated by inducing an up-regulation of interleukin-6 (IL-6) production following ZZ-CD23 incubation with monocytes. The addition of Fab fragments generated from the monoclonal antibody CD11b impaired this cytokine production by 50%. Interestingly, a positive autocrine loop was identified as IL-6 was shown to increase CD23 binding to macrophages. These results demonstrate that similar to findings using human cells, murine CD23 binds to the surface adhesion molecule, CD11b, and these interactions regulate biological activities of murine myeloid cells.

B cell activation and Ig, especially IgE, production is inhibited by high CD23 levels in vivo and in vitro

The capacity of CD23 to regulate IgE production was evaluated in both an in vitro and an in vivo system. The decreased IgE response seen in CD23 transgenic mice was confirmed and observed to occur at all antigen doses used. In addition, purified B cells from the Tg animals in general exhibited lower IgE production when stimulated with CD40L and IL-4. To examine this down-regulating activity of CD23 an in vitro model system was developed. CHO cells were transfected with CD23, ICAM-1, or both CD23 and ICAM-1. ICAM-1 was chosen to enhance B cell-B cell interaction. Purified resting B cells were placed into culture with the mitomycin C-treated transfected or control CHO cells and activated with IL-4, IL-5, and CD40L-CHO. A dose-dependent decrease in IgE production was observed with increasing cell numbers of the CHO transfectants that expressed CD23. The effect lasted up to Day 3 of culture. B cell proliferation was also inhibited in a dose-dependent fashion by increasing numbers of CD23-expressing cells suggesting a potential effect of CD23 on B cell apoptosis. In contrast, ICAM-1-transfected or CHO control cells had minimal effects on either Ig production or B cell proliferation. While IgE production was inhibited up to 95% by high numbers of CD23-transfected CHO cells, some inhibition of IgG and IgM production was also seen. Finally, the mechanism of CD23-mediated inhibition of IgE production was compared with the inhibition in IgE production seen when B cell were coactivated with multivalent anti-IgD in conjunction with CD40L plus optimal IL-4. To this end we used RT-PCR to compare the relative levels of epsilon-germline transcripts in control cultures and cultures coactivated by anti-IgD, CD40L, and IL-5 or activated in the presence of high levels of CD23-expressing cells. CD22 was used as an internal standard since levels change little with B cell activation. Coactivation strongly inhibited epsilon-germline transcript levels but the presence of CD23-expressing cells did not. Thus, coactivation potentially operates prior to isotype switching, while high CD23 coculture blocks either recombination or more likely B cell differentiation to high Ig producers stage. Our data support the hypothesis that IL-4 induces both IgE and a controlling agent for IgE, namely, CD23.

Characterization of a constitutive type III nitric oxide synthase in human U937 monocytic cells: stimulation by soluble CD23

The soluble cleavage fragment of the low-affinity immunoglobulin E (IgE) receptor/CD23 (sCD23 25000 MW) and antibodies directed against their receptors on monocytes, CD11b and CD11c, stimulate the production of nitric oxide (NO) by these cells and we have suggested that the enzyme involved could be related to the endothelial constitutive type III nitric oxide synthase (ecNOS). In the present work, we have analysed the characteristic properties of this NOS isoform in the model of the human promonocytic cells U937 By reverse-transcription polymerase chain reaction (RT-PCR), the presence of an mRNA coding for type III NOS was found in U937 cells and the corresponding protein was detected by immunofluorescence in permeabilized cells with a specific anti-ecNOS monoclonal antibody (mAb). Membrane extracts displayed a NOS activity dependent on the presence of calcium and calmodulin in the reaction medium and that was abrogated in the presence of EGTA. Recombinant soluble CD23 (25000 MW) was found to trigger an NO-dependent cGMP accumulation in these cells, which was abrogated by calcium chelators and inhibitors of the calcium/calmodulin complex. Moreover, sCD23 elicited a transient augmentation of intracytoplasmic free calcium concentration [Ca2+]i that was dependent on the presence of calcium in the external buffer and was prevented in the presence of EGTA, indicating that it was due to a calcium influx. In conclusion, human promonocytic cells such as U937 exhibit a functional type III NOS that can be stimulated by calcium-raising agents, such as sCD23.

Inhibition of Apoptosis in a Human Pre-B–Cell Line by CD23 Is Mediated Via a Novel Receptor

Human CD23 is a 45-kD type II membrane glycoprotein, which functions as a low-affinity receptor for IgE and as a ligand for the CD21 and CD11b/CD11c differentiation antigens. CD23 is released from the surface of cells as soluble fragments, and a 25-kD species of soluble CD23 (sCD23) appears to act as a multifunctional cytokine. In this report, sCD23 is shown to sustain the growth of low cell density cultures of a human pre-B–acute lymphocytic leukemia cell line, SMS-SB: no other cytokine tested was able to induce this effect. Flow cytometric analysis indicates that sCD23 acts to prevent apoptosis of SMS-SB cells. SMS-SB cells cultured at low cell density possess low levels of bcl-2 protein. Addition of sCD23 to cells at low cell density maintained bcl-2 expression at levels equivalent to those observed in SMS-SB cells cultured at higher cell densities. No CD23 mRNA was found in SMS-SB cells, ruling out an autocrine function for CD23 in this cell line model. Although SMS-SB cells do not express the known receptors for CD23, namely CD21, CD11b-CD18, or CD11c-CD18, the cells specifically bind CD23-containing liposomes, but not glycophorin-containing liposomes. Binding of CD23-containing liposomes is inhibited by anti-CD23 but not by anti-CD21 or anti-CD11b/c monoclonal antibodies. The data show that sCD23 prevents apoptosis of the SMS-SB cell line by acting through a novel receptor.

Inhibition of Apoptosis in a Human Pre-B–Cell Line by CD23 Is Mediated Via a Novel Receptor

Human CD23 is a 45-kD type II membrane glycoprotein, which functions as a low-affinity receptor for IgE and as a ligand for the CD21 and CD11b/CD11c differentiation antigens. CD23 is released from the surface of cells as soluble fragments, and a 25-kD species of soluble CD23 (sCD23) appears to act as a multifunctional cytokine. In this report, sCD23 is shown to sustain the growth of low cell density cultures of a human pre-B–acute lymphocytic leukemia cell line, SMS-SB: no other cytokine tested was able to induce this effect. Flow cytometric analysis indicates that sCD23 acts to prevent apoptosis of SMS-SB cells. SMS-SB cells cultured at low cell density possess low levels of bcl-2 protein. Addition of sCD23 to cells at low cell density maintained bcl-2 expression at levels equivalent to those observed in SMS-SB cells cultured at higher cell densities. No CD23 mRNA was found in SMS-SB cells, ruling out an autocrine function for CD23 in this cell line model. Although SMS-SB cells do not express the known receptors for CD23, namely CD21, CD11b-CD18, or CD11c-CD18, the cells specifically bind CD23-containing liposomes, but not glycophorin-containing liposomes. Binding of CD23-containing liposomes is inhibited by anti-CD23 but not by anti-CD21 or anti-CD11b/c monoclonal antibodies. The data show that sCD23 prevents apoptosis of the SMS-SB cell line by acting through a novel receptor.

Dendritic cells generated from human blood in granulocyte macrophage-colony stimulating factor and interleukin-7

Dendritic cells (DC), with potentially important clinical applications, have been generated from human peripheral blood monocytes in the presence of GM-CSF and IL-4 (G4 DC). In the present report we show that DC with a novel phenotype can be generated from blood adherent mononuclear cells in the presence of GM-CSF and IL-7 (G7 DC). Adherent cells from PBMC, cultured in GM-CSF (600 U/ml) and IL-7 (6 U/ml), were transformed over 7 days into cells with DC morphology, at a yield of 1.2-1.6 x 10(6) per 10(7) PBMC. G7 DC not only expressed class I and class II MHC, CD1a, CD11c, CD23, CD40, CD54, CD58, CD80, CD86 and CD95, like G4 DC, but also CD21, which is the complement receptor type 2, a ligand for CD23 and a receptor for EBV and IFN-alpha. G7 DC were at least one log more effective in the autologous MLR and at least two logs more effective in the allogeneic MLR, than PBMC. They elicited proliferative responses of CD4 T cells to tetanus toxoid and CD8 T cells to an EBV peptide, and stronger T-cell cytotoxicity to EBV peptide than G4 DC. Expression of CD21 by G7 DC suggests that IL-7 delivers a distinct signal to DC precursors and that G7 DC may be functionally distinct.

CD86 (B7-2) on human B cells. A functional role in proliferation and selective differentiation into IgE- and IgG4-producing cells

Immunoglobulin (Ig) E production by B cells requires two primary signals provided by T cells, interleukin (IL)-4 or IL-13 and CD40 ligand (CD40L). In addition, costimulatory signals, such as CD23-CD21 interaction, contribute further ensuring a selective control over this production. Recently, CD28, expressed on T cells, has been reported to be involved in this process. The CD28 ligands, CD80 (B7-1) and CD86 (B7-2), are expressed on human tonsillar B cells, and their expression is up-regulated by IL-4, IL-13, and/or an anti-CD40 monoclonal antibody (mAb). We have investigated whether signaling via the B7 molecules affects IgE synthesis. Human B cells were stimulated by IL-4 plus anti-CD40 mAb in the presence of different anti-B7 mAbs. Cross-linking of CD86 with IT2.2 potentiated IgE and IgG4 production and epsilon transcripts expression. The production of the other isotypes was not modulated. Conversely, the anti-CD80 and the other anti-CD86 mAbs tested had no effect. The increase of IgE and IgG4 production induced by IT2.2 was accompanied by an increase in proliferation, in cell surface density of CD23, and in CD23 binding to CD21-expressing B cells. In contrast, the expression of other B cell surface molecules such as CD11a, CD30, and CD58 remained unaffected. Since IT2.2 favors CD23-CD21 pairing, we tested whether blocking this interaction affected IT2.2-increased IgE production. The neutralizing anti-CD23 mAb, Mab 25, caused a dose-dependent inhibition of the effect of IT2.2 on IgE synthesis. Finally, IT2.2 potentiation on B cell proliferation and IgE production required the two primary signals, IL-4 and anti-CD40 mAb, since IT2.2 alone or in combination with only one of these stimuli did not show any effect on B cells. This study is the first demonstration of a signaling role for CD86. Together with IL-4 or IL-13 and CD40L, CD86 favors CD23-CD21 pairing and consequently functions as a selective and potent costimulus for human IgE and IgG4 synthesis.

Expression of CD23 in the germinal center of thymus from myasthenia gravis patients

In order to investigate a pathogenic role of germinal centers which appear in the hyperplastic thymus of myasthenia gravis (MG) patients, we performed an immunohistochemical study using various monoclonal antibodies including CD23. In contrast with tonsilar germinal centers from non-MG individuals, CD23 was strongly and diffusely expressed in the whole area of germinal centers of MG thymi, including the outer zone. In addition, we measured the serum level of soluble CD23 (sCD23) in MG patients at various clinical stages. The high serum sCD23 levels, which were noted in the unthymectomized patients, fell to within normal range over 5 years after thymectomy, and the decline of serum sCD23 correlated well with clinical improvement. CD23 is thought to be responsible for preventing unselected germinal center B cells from entering apoptosis and, in turn, leads to the survival of auto-reactive B cell clones.

Correlation between Epstein Barr virus antibodies, serum IgE and atopic disease

It is currently accepted that viral infections may influence the development of atopy. In the present study we evaluated serum IgE levels as well as the prevalence of symptoms indicative of atopic disease and EBV antibodies in 353 children aged from 1 month to 19 years. Antibodies against EBV were detected by immunofluorescence. IgE levels in serum were measured by enzyme immunoassay. Dividing the study population according to EBV seropositivity and age, we noted that the prevalence of high IgE levels (> 2 s.d.) was, in total, more frequent in the EBV negative (32.9%) than in the positive subjects (27.6%). Interestingly, this higher prevalence was found only in the groups aged under six, especially in the 7 to 29 month group, where it was statistically significant (p=0.037), whereas in the 6-19 year group the situation was reversed. Furthermore, selecting only the atopic children younger than 3 years of age with high IgE levels and clinical symptoms of atopy (wheezing and/or dermatitis) it was possible to demonstrate lower EBV seropositivity compared with the normal IgE controls for each group, even though these differences were not statistically significant. In conclusion, the results of our study suggest that, in our selected population, EBV infection in the first years of life is associated with a lower prevalence of high IgE levels.

Pathways controlling the expression of surface CD21 (CR2) and CD23 (Fc(epsilon)IIR) proteins in human malignant B cells

The aim of the present study was to analyze the pathways regulating the expression of CD21 and CD23 B-cell differentiation antigens on human malignant B cells. Exposure of Farage cells, derived from a human B-cell lymphoma, to phorbol 12-myristate 13-acetate (PMA) down-regulated CD21 and CD23 expression, while interleukin 4 (IL4) inhibited the expression of CD21 but augmented CD23 expression. When Farage cells were stained with either anti-CD21 or anti-CD23 monoclonal antibodies (mAb), subsequent exposure to IL4 failed to change the staining of the cells, indicating that IL4 did not affect the turnover of CD21 and CD23 molecules. Inhibition of protein synthesis with cycloheximide (CXM) had no effect on the expression of CD21 molecules, but abrogated their down-regulation by IL4, suggesting that IL4 induced the synthesis of proteins which modify the processing of CD21 molecules. The inhibitory effect of IL4 on the expression of CD21 and its augmentary effect on the expression of CD23 was abrogated by H7 (1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine), an inhibitor of serine protein kinase. Staurosporine, an additional inhibitor of serine kinases also abrogated the effect of IL4 on CD23 expression. H8 (N-(2-[Methylamino]ethyl)-5-isoquinolinesulfonamide), a preferential inhibitor of protein kinases A and G, and genistein, an inhibitor of tyrosine kinases had no effect on IL4-induced modulation of CD21 and CD23 in Farage cells. The exposure of B-chronic lymphocytic leukemia (CLL) cells to PMA reduced the expression of CD21, but increased the expression of CD23. IL4 had no effect on the expression of CD21 on CLL-cells but strongly enhanced the level of CD23. H7, H8 and genistein each abrogated to a different extent the effect of IL4 on the expression of CD23 by CLL-cells. These data indicate that activation of serine/threonine kinases in malignant B cells inhibited the production of CD21 proteins, while different protein kinases appeared to be involved in up- and down-regulation of CD23 in different B lymphocytes.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

Phenotypic analysis of receptor-ligand pairs on B-cells in B-chronic lymphocytic leukemia

Whole-blood three-color immunofluorescence analysis was used to investigate the role of CD5/CD72 and CD21/CD23 receptor-ligand pair formation on B-chronic lymphocytic leukemia (B-CLL) cells as well as sCD23 and bcl-2 oncoprotein expression in disease progression and activity and total tumor mass in B-cell chronic leukemia (B-CLL) patients. Thirty-four patients with B-CLL and 19 controls were included in the study. The majority of B-cells in B-CLL patients coexpressed CD5 and CD72 as well as the CD23 antigen. Unlike B-cells in B-CLL patients, B-cells in all healthy controls tested had high expression of CD21 antigen. We identified two groups of B-CLL patients according to high (n = 20) or low levels (n = 14) of CD21 expression on CD19+CD23+ B-cells. Only in the patients with high CD21 expression, were sCD23 levels positively correlated with factors known to have prognostic significance in B-CLL (Rai stage and TTM) and could, therefore, be used as a prognostic parameter for these B-CLL patients. Bcl-2 oncoprotein expression did not differ between these patient groups. We presumed that in patients with a lower expression of CD21 antigen, the contribution of the CD21 molecule to homotypic adhesion was lacking. Further studies are necessary to determine the possible association of higher expression of the CD21 antigen with disease progression and the aggressive character of the B-CLL.

Cleavage of the low-affinity receptor for human IgE (CD23) by a mite cysteine protease: nature of the cleaved fragment in relation to the structure and function of CD23

Der p I, a cysteine protease representing a major allergen of the house dust mite Dermatophagoides pteronyssinus, has recently been shown to cleave CD23 from the surface of cultured human B cells (RPMI 8866 B cell line). We have now undertaken a detailed investigation of CD23 cleavage by Der p I. We demonstrate that Der p I cleaves CD23 at two sites (Ser155-Ser156 and Glu298-Ser299) to produce a 17-kDa fragment containing the lectin domain and only part of the C-terminal tail. No such effect was demonstrable with mouse CD23, a finding which was anticipated based on its lack of the cleavage sites identified on human CD23. Based on the cleavage pattern and the model of CD23, we propose a sequence of events leading to the liberation of the 17-kDa soluble CD23 fragment. The biological significance of such cleavage is underlined by the demonstration that Der p I-treated B lymphocytes lose their ability to bind IgE, and that the 17-kDa fragment (amino acids 156-298) contains the minimum structural requirement (amino acids 156-288) for binding to both IgE and CD21.

Novel genetic regulation of T helper 1 (Th1)/Th2 cytokine production and encephalitogenicity in inbred mouse strains

Development of T helper cell (Th)1 or Th2 cytokine responses is essential for effector and regulatory functions of T helper cells. We have compared cytokine profiles of myelin basic protein (MBP) Ac1-16 peptide-specific T helper cells from inbred mouse strains expressing identical k haplotype-derived MHC class II molecules B10.A and B10.BR, B10.BR T cell lines (TCL) produced Th1 cytokines (including high levels of TNF-alpha) and induced experimental autoimmune encephalomyelitis after adoptive transfer. In contrast, B10.A TCL produced Th2 cytokines (including low levels of TNF-alpha) and were poorly encephalitogenic. The contributions of the genetic origin of the T cells and the APC were explored. Serial restimulations of the B10.BR TCL with B10.A or (B10.A x B10.BR) F1 splenic antigen presenting cells (APC) during the establishment of TCL markedly reduced both Th1 cytokine production and encephalitogenicity. In addition, a single restimulation with B10. A splenic APC reduced IFN-gamma and TNF-alpha production by established Th1 MBP-specific Ak-restricted B10.BR TCL and by a Th1 KLH-specific, Ek-restricted B10.BR T cell clone. These studies suggest that B10.A and B10.BR APC differ in their ability to stimulate IFN-gamma and TNF-alpha production by mature Th1 cells and also influence their Th1/Th2 commitment in vivo. The nature of the downregulatory activity of B10.A APC on IFN-gamma and TNF-alpha production was explored. 2-hour supernatants from antigen-activated B10.A APC/TCL cultures or from B10.A APC activated by LPS had the same inhibitory effects on IFN-gamma and TNF-alpha production by B10.BR TCL. The downregulatory effects of B10.A APC are independent of TNF-alpha, IL-4, IL-10, IL-12p40, IFN-gamma, IL-13, TGF-beta, and PGE2. Thus, genetic difference(s) between B10.A and B10.BR APC appear(s) to control the production or activity of a novel soluble cytokine regulatory factor that influences Th1/Th2 commitment and controls production of IFN-gamma and TNF-alpha by mature Th1 cells.

Function of CD23 in the response of human B cells to antigen

Co-ligation of antigen receptor and complement receptor 2 (CD21) in the B cell membrane is important in the immune response to T-dependent antigens. Four CD21 ligands have so far been identified, but only the activated products of the third component of complement (C3) are known to augment the immune response to specific antigens. The most recently discovered ligand for CD21 is CD23. We have generated a CD32+ CD23+ fibroblast cell line which presents a surrogate antigen (anti-IgM) to human tonsil B cells in vitro. Incubation with these cells causes a 10- to 100-fold reduction in the threshold concentration of anti-IgM required for B cell proliferation. Anti-CD19 further enhances the response to antigen and induces proliferation in the absence of anti-IgM. Addition of soluble CD21 totally inhibits the effect of CD23, suggesting that CD21 mediates synergistic signaling by CD23.

Follicular dendritic cells specifically express the long CR2/CD21 isoform

This paper describes an antibody (mAb 7D6) that specifically recognizes human follicular dendritic cells (FDCs). By expression cloning, a cDNA clone encoding for the long human CR2/ CD21 isoform (CD21L) that contains an additional exon (10a) was isolated. We demonstrated that FDCs selectively express CD21L, while B cells selectively express the short CR2/CD21 lacking exon 10a (CD21S). By screening mouse Ltk- cells transfected with the CD21L cDNA, we further showed that the other two anti-human FDC mAbs DRC-1 and KiM4 also recognize CD21L. Thus, CD21L represents the first characterized human FDC-specific molecule, which may confer unique functions of FDCs in germinal center development.

CD21 augments antigen presentation in immune individuals

CD21 (complement receptor 2, CR2) binds the C3 degradation products, iC3b and C3d, which are covalently linked to antigen or immune complexes in the process of complement activation. The ability of antigen-nonspecific B cells to present immune complexes containing high titers of acquired antibodies was tested. Influenza virus was incubated with serum from immune donors to create complement-containing complexes. These bound specifically to CD21 on transfected fibroblasts and B cell lines, as measured by microcytofluorimetry. Binding of immune complexes was ablated by inactivation of serum complement. In addition, the immunoglobulin in immune human serum blocked influenza binding to cells in the absence of complement, implying a minimal role for immunoglobulin-Fc receptor interactions in this system. Significant immune complex binding required a threshold level of CD21 expression, suggesting that only those cells with the highest levels of CD21 are likely to participate in the processing of macromolecular antigens. B cells pulsed with complement-influenza complexes elicited an augmented response from a panel of influenza-specific, class II-restricted T cell clones, as compared with those which had bound immunoglobulin-influenza complexes lacking complement. This enhanced response did not require CD35. In addition, B cell lines expressing higher levels of CD21 were more efficient in processing antigen than those with lower levels. These findings suggest that presentation of antigen by B cells in immune individuals is dependent on the binding of complement-antigen immune complexes to CD21.

Lectin-carbohydrate interaction in the immune system

The immune system consists of various types of cells and molecules that specifically interact with each other to initiate the host defense mechanism. Recent studies have shown that carbohydrates and lectins (carbohydrate-binding proteins) play an essential role in mediating such interactions. Both lectins and carbohydrates are widely distributed in the mammalian tissues as well as in microorganisms. Carbohydrates, due to their chemical nature, can potentially form structures that are more variable than proteins and nucleic acids. Lectins can exist in either soluble or cell-associated form, and although overall structures vary, invariably possess carbohydrate-recognition domains (CRD) with various specificities. The interaction between lectins and carbohydrates have been shown to be involved in such activities as opsonization of microorganisms, phagocytosis, cell adhesion and migration, cell activation and differentiation, and apoptosis. The number of lectins identified in the immune system is increasing at a rapid pace. The development in this area has opened a new aspect in studying the immune system, and at the same time, provided new therapeutic routes for the treatment and prevention of disease.

IL-10 suppresses cell surface CD23/Fc epsilon RII expression, not by enhancing soluble CD23 release, but by reducing CD23 mRNA expression in human monocytes

To examine a possible involvement of interleukin-10 (IL-10) in CD23/Fc epsilon RII expression in human monocytes, effects of IL-10 on the cell surface CD23 expression, soluble CD23 (sCD23) release, and CD23 type b mRNA expression were investigated. IL-10 suppressed IL-4-induced surface CD23 expression on monocytes in a dose-dependent manner, and this effect was completely neutralized by anti-IL-10 antibody. The suppressive effect of IL-10 on surface CD23 expression was not due to enhancement of sCD23 release from the cell surface because no increase in sCD23 in culture supernatant was detected after incubation with IL-10. Instead, the effect of IL-10 seemed to be exerted at the transcriptional level since IL-4-induced expression of CD23 type b mRNA was significantly reduced when IL-10 was present. Although IL-4 induced surface CD23 expression on both monocytes and B cells, the suppressive effect of IL-10 was observed only on monocytes, which underscores different regulatory mechanisms for CD23 expression between the two cell types.

Triggering through CD40 promotes interleukin-4-induced CD23 production and enhanced soluble CD23 release in atopic disease

The pathogenesis of atopic disease is closely linked to the overproduction of IgE. CD23 and CD40 are two cellular receptors involved in the regulation of IgE production and both receptors are elevated in atopic disease. We have examined the role of CD40 in the regulation of CD23 and soluble CD23 production in healthy and atopic donors. Triggering of the B cell CD40 receptor directly enhances interleukin (IL)-4-mediated up-regulation of CD23 at both the protein and the mRNA level. When atopic donors were studied, the synergistic effect of CD40 triggering on the IL-4-induced up-regulation of CD23 and soluble CD23 (sCD23) was enhanced and there was a relative skewing toward production of sCD23. These studies implicate the CD40 receptor in the hyperproduction of CD23 and sCD23 in atopic disease and suggest that abnormalities may exist in the cellular pathways leading to sCD23 production.

Development of immune functions related to allergic mechanisms in young children

The newborn immune system differs quantitatively and functionally from that of adults. Development of the immune system has important implications for childhood diseases. The immaturity of the immune system in the first years of life may contribute to failure of tolerance induction and in the development of allergic disease. T cell function is diminished, especially the capacity to produce cytokines; production of interferon (IFN)-gamma, and IL-4 is strongly reduced. IFN-gamma has been found to be even lower in cord blood of newborns with a family history of atopy. Differences in other cell types (natural killer cells, antigen-presenting cells, and B cells) could also play a role in the development of allergic disease. Current data suggest that irregularities in IgE synthesis, helper T cell subsets (Th1, Th2, CD45RA, and CD45RO), cytokines (IL-4, IFN-gamma), and possibly other cell types may play a role in the development of allergy in childhood. Moreover, the role of cell surface molecules, like co-stimulatory molecules (CD28, CD40L), activation markers (CD25), and adhesion molecules (LFA-1/ICAM-1, VLA-4/ VCAM-1) is also discussed. These variables are modulated by genetic (relevant loci are identified on chromosome 5q, 11q, and 14) and environmental forces (allergen exposure, viral infections, and smoke). The low sensitivity of current predictive factors for the development of allergic diseases, such as cord blood IgE levels, improves in combination with family history and by measurement of in vitro responses of lymphocytes and skin reactivity to allergens. New therapeutic approaches are being considered on the basis of our current understanding of the immunopathology of allergic disease, for instance cytokine therapy and vaccination with tolerizing doses of allergen or peptides.

Human lymphocytes shed a soluble form of CD21 (the C3dg/Epstein-Barr virus receptor, CR2) that binds iC3b and CD23

We report on a soluble (s) form of CD21 (the C3dg/Epstein-Barr virus receptor, CR2) that is spontaneously released by B and T lymphocytes. Immunoprecipitation with anti-CD21 mAb of culture supernatants of surface and biosynthetically labeled B and T cell lines revealed a single band with an apparent molecular mass of 135 kDa. The molecule exhibited a molecular mass 10 kDa lower than that of membrane CD21. The release of soluble CD21 (sCD21) was time dependent and correlated with a parallel decrease in the expression of the membrane-associated molecule. The protein was also found in culture supernatants of tonsillar B cells and normal human thymocytes. Epitopic analysis using combinations of anti-CD21 monoclonal antibodies (mAb) indicated that sCD21 and membrane CD21 were similarly recognized by mAb directed against short consensus repeats (SCR) 1-2, SCR 4-5 and SCR 9-11. Affinity-purified sCD21 was capable of binding to purified human iC3b and to human recombinant CD23, as assessed by enzyme-linked immunosorbent assay and by using the BIAcore technology. In addition, normal human serum was found to contain a soluble form of CD21 that exhibited a similar molecular mass to that of the molecule shed by B and T cells in culture. The serum form of CD21 was recognized by all anti-CD21 mAb that we tested and showed a high reactivity with mAb directed against SCR 1-2. Our observations suggest that B and T cells shed the extracellular portion of CD21 and release a soluble molecule that retains the ligand-binding properties of CD21, thus having a potential role in immunoregulation.

Compartmentalization of B-cell antigen receptor functions

Receptor tyrosine kinases (RTK), like the PDGF-receptor, translate information from the extracellular environment into cytoplasmic signals that regulate a spectrum of cellular functions. RTK molecules consist of ligand binding extracellular domains, cytoplasmic kinase domains and tyrosine phosphorylation sites [Ullrich and Schlessinger, 1990 (Cell 61, 203-212); Heldin, 1992 (EMBO J. 11, 4251-4259)]. Upon ligand-induced RTK oligomerization, the kinase domains will become activated and induce auto(trans)phosphorylation of a number of cytoplasmic tyrosine residues. These phosphorylated tyrosine residues are incorporated in distinct sequence motifs and act as specific docking sites for SH2 domain-containing proteins [Songyang et al., 1993 (Cell 72, 767-778)]. In contrast to single- or oligo-chain RTK, immunological receptors such as antigen receptors, FcR and cytokine receptors are multi-chain complexes in which distinct receptor functions appear to be compartmentalized in distinct polypeptides. Here, we summarize current knowledge on the structural and functional characteristics of the B-cell antigen receptor complex (BCR) and address the specific ability of accessory molecules to recruit intracellular signaling intermediates towards the activated receptor complex.

CD23-mediated nitric oxide synthase pathway induction in human keratinocytes is inhibited by retinoic acid derivatives

Retinoids exert various functions including anti-proliferative and anti-inflammatory effects on many cell types including keratinocytes and are widely used in skin diseases, such as psoriasis and acne. We have previously shown that human keratinocytes express low affinity immunoglobulin E receptor (FcepsilonRII/CD23) when stimulated with interleukin-4. Immunoglobulin E ligates CD23 and induces the production of nitrites (reflecting the mobilization of the nitric oxide [NO]-pathway) and tumor necrosis factor-alpha by human keratinocytes. Here, 13-cis and all-trans retinoic acid (RA) were shown to reduce the production of nitrites by immunoglobulin E-activated keratinocytes by 80% in a time- and concentration-dependent fashion. As a consequence, RA derivatives also reduced the production of tumor necrosis factor alpha by these cells by 70%. The level of inducible NO synthase activity in activated human keratinocytes was significantly decreased upon treatment of the cells with RA derivatives (inhibition by 60% of the mean inducible NO synthase activity with 13-cis RA, 2 microM). Treatment for 24 h with RA derivatives almost completely abolished transcription of inducible NO synthase-specific mRNA in activated keratinocytes. Therefore, RA derivatives downregulate tumor necrosis factor-alpha release and the NO-transduction pathway through the inhibition of inducible NO synthase transcription. Together, our data provide evidence for inhibition of the NO-pathway by 13-cis and all-trans retinoic acid on CD23-activated human keratinocytes. These data may clarify the mechanism of the anti-inflammatory activity of RA derivatives in skin diseases.

Soluble CD40 ligand induces expression of CD25 and CD23 in resting human tonsillar B lymphocytes

In this report, we describe the dose-dependent increase in both CD25 and CD23 levels on resting human B cells in response to CD40 ligation, as mediated by soluble CD40 ligand (sCD40L) or anti-CD40 antibody. In combination with interleukin (IL)-4, sCD40L had limited additive effects on CD25 expression, but significantly enhanced CD23 expression on tonsillar B cells. Interferon-gamma (IFN-gamma) exerted no inhibitory effect upon increases in CD25 or CD23 driven by CD40 ligation with sCD40L or anti-CD40 antibody. These data suggest that the induction of CD25 and CD23 genes by IL-4 is mediated, at least in part, by an IFN-gamma-sensitive component, whereas gene activation driven via CD40 ligation involves signaling pathways which are not sensitive to IFN-gamma.

The Epstein-Barr virus-binding site on CD21 is involved in CD23 binding and interleukin-4-induced IgE and IgG4 production by human B cells

Human CD21 has previously been described as a receptor for the C3d,g and iC3b proteins of complement, as a receptor for the gp350/220 envelope glycoprotein of the Epstein-Barr virus (EBV) and also as a receptor for inerferon-alpha (IFN-alpha). Structurally, CD21 consists of 15 to 16 short consensus repeats (SCR) of 60 to 75 amino acids followed by a transmembrane domain and an intracytoplasmic region. We reported that CD23, a low-affinity receptor for IgE (Fc epsilon R2), is a new functional ligand for CD21. We recently found that the sites of interaction of CD23 on CD21 are on SCR 5 to 8 and 1-2. The first site is a lectin-sugar type of interaction and the second site is a protein-protein interaction. We report here that amongst the other ligands for CD21 (EBV, C3d,g and IFN-alpha), only EBV is able to inhibit the binding of CD23 to CD21. Furthermore, even a peptide from gp350/220 of EBV known to bind to CD21 is able to decrease CD23 binding to CD21. Since CD23/CD21 pairing is important in the control of IgE production, we tested the effect of the EBV-derived peptide on immunoglobulin production from peripheral blood mononuclear cells and purified tonsillar B cells. Interestingly, the EBV-peptide inhibited IgE and IgG4 production induced by interleukin-4, in a dose-dependent manner. The same results were obtained using either peripheral blood mononuclear cells or purified tonsillar B cells. Another CD21 ligand, C3, did not affect binding of CD23 to CD21 nor the production of IgE and IgG4. This study indicates that blocking CD23 binding to CD21 SCR 2 on human B cells selectively modulates immunoglobulin production.

Markedly impaired humoral immune response in mice deficient in complement receptors 1 and 2

Complement receptor 1 (CR1, CD35) and complement receptor 2 (CR2, CD21) have been implicated as regulators of B-cell activation. We explored the role of these receptors in the development of humoral immunity by generating CR1- and CR2-deficient mice using gene-targeting techniques. These mice have normal basal levels of IgM and of IgG isotypes. B- and T-cell development are overtly normal. Nevertheless, B-cell responses to low and high doses of a T-cell-dependent antigen are impaired with decreased titers of antigen-specific IgM and IgG isotypes. This defect is not complete because there is still partial activation of B lymphocytes during the primary immune response, with generation of splenic germinal centers and a detectable, although reduced, secondary antibody response. These data suggest that certain T-dependent antigens manifest an absolute dependence on complement receptors for the initiation of a normally robust immune response.

Antigen presentation in allergic sensitization

IgE antibodies, when cross-linked by allergen on the surface of effector cells such as mast cells and basophils, are known to be directly responsible for immediate type hypersensitivity reactions. In addition, IgE may be involved in other, indirect, mechanisms, fundamental to the pathogenesis of allergic diseases, such as enhancement of the antigen capturing capacity of antigen presenting cells. IgE mediated antigen presentation could lead to a continuous activation of the immune system by very low concentrations of allergen. As a result, Th2 cell populations may expand and may induce more B cells to switch to IgE production. Subsequently, the overproduction of IgE and Th2 cells in a patient may explain the clinical observation that certain allergic patients deteriorate from sensitivity to a single group of allergens to sensitivity to multiple groups of allergens. Therefore, control of IgE production is not only important for the treatment of allergic symptoms, but may also regulate deterioration of allergy via the mechanism of CD23/IgE mediated allergen presentation by naive B cells. The role that monocytes, which have recently been found to express Fc epsilon RI, play in the pathogenesis of allergy, remains speculative. We hypothesize that their role may be to remove IgE from the circulation and re-direct the immune response from naive B cells. IgG antibodies which cannot be used for antigen uptake by B cells also direct the immune response to monocytes.

Antibody response to a T-dependent antigen requires B cell expression of complement receptors

Several lines of evidence indicate that antibody responses to T-dependent antigens require complement receptors expressed on either B lymphocytes or follicular dendritic cells. We have used RAG-2 deficient blastocyst complementation to create mice specifically lacking B cell complement receptors. Despite normal expression of complement receptor 1 (CR1[CD35]) and CR2 (CD21) on follicular dendritic cells, these mice have a profound defect in their capacity to mount a T-dependent antibody response. This is the first direct demonstration in vivo that B cell expression of complement receptors is required for a humoral immune response. This is the first direct demonstration in vivo that B cell expression of complement receptors is required for a humoral immune response. This suggests that CD21 and/or CD35 on B lymphocytes may be required for cellular activation, adsorptive endocytosis of antigen, recruitment to germinal centers, and/or protection from apoptosis during the humoral response to T-dependent antigens.

CD40 antibodies defining distinct epitopes display qualitative differences in their induction of B-cell differentiation

IgE production can be obtained in vitro by stimulating B lymphocytes with CD40 antibodies and interleukin-4 (IL-4). This stimulation also results in homotypic aggregation and cell proliferation. We have shown previously that IgE synthesis may be dependent on additional signals provided by the close cellular contact. Thus inhibition of the aggregation by lymphocyte function-associated antigen-1 (LFA-1) antibodies leads to a decrease in IgE production. In the present study we show that the inhibitory effect of LFA-1 antibodies is critically dependent on the CD40 antibody used for stimulation. Thus, while previously using the monoclonal antibody (mAb) S2C6, IgE production induced by the CD40 antibody mAb89 was generally higher and could be enhanced more than fivefold in the presence of LFA-1 antibodies. Similarly, the addition of the CD23 mAb MHM6, which blocked aggregation to a similar degree as the LFA-1 antibodies, inhibited S2C6-induced IgE production but enhanced that induced by mAb89. In contrast to these opposing effects on IgE synthesis, proliferation induced by the two CD40 antibodies was affected similarly by the blocking antibodies. As the interaction between CD23 and CD21 has been suggested to involve recognition of carbohydrate structures on CD21 by the lectin-like domain on CD23, we also tested the effect of some different sugars on IgE synthesis and proliferation. Addition of fucose-1-phosphate to anti-CD40 and IL-4-stimulated B cells completely inhibited IgE synthesis and proliferation. Inhibition was also seen with mannose-6-phosphate but not with glucose-1-phosphate. In contrast to the MHM6 antibody, the effect of the sugars was similar irrespective of the CD40 antibody used for stimulation. The study shows that different antibodies to CD40 may give rise to qualitatively distinct signals depending on the epitope recognized.

Induction of LFA-1 independent human B cell proliferation and differentiation by binding of CD40 with its ligand

Engagement of CD40 on resting B cells in the presence of IL-4 triggers B cell proliferation, differentiation and homotypic adhesion. This study was designed to investigate the role of LFA-1/ICAM-1 interactions in homotypic adhesion and proliferation of CD40-activated human B lymphocytes. Freshly isolated B cells were cultured in vitro in the presence of IL-4 and of L cells expressing CD40L, the CD40 ligand. The addition to the culture medium of LFA-1 and ICAM-1 antibodies inhibited homotypic B lymphocyte adhesion. However, these antibodies failed to affect B lymphocyte proliferation and antibody production. These results indicate that aggregation and proliferation are independent events although both induced by CD40 activation.

Decreased levels of circulating IFN-alpha and increased sCD23 in patients with acute infectious mononucleosis

Epstein-Barr virus (EBV) is the etiological agent for acute infectious mononucleosis (AIM). It is also associated with certain malignant disorders in individuals with immunodeficiencies such as B cell lymphoproliferative disorder (BLPD). Our previous study with BLPD patients had demonstrated significantly higher serum IL-4 and IgE levels and significantly decreased IFN-alpha levels. These observations were consistent with the model of regulation of B cell growth by T cell-derived cytokines, in which IL-4 promotes B cell growth and switch to IgE synthesis whereas IFN-alpha and IFN-gamma inhibit these IL-4 mediated effects. Since AIM is also EBV associated, this study was designed to examine IL-4, IFN-alpha, IFN-gamma, IgE, and soluble CD23 (sCD23) serum levels in AIM patients. In this study we report for the first time that in contrast to BLPD patients, AIM patients did not exhibit increased levels of IL-4 and IgE; however AIM patients to exhibit decreased IFN-alpha levels and, additionally, also exhibit significantly higher sCD23 levels. This could result in B cell activation and have implications for the survival of the virus in B cells.

Circulating levels of soluble CD23 reflect clinical and biological features of leukemic B-cell chronic lymphoproliferative disorders

One hundred and twenty-four sera from patients with various leukemic B-cell chronic lymphoproliferative diseases were investigated at diagnosis by ELISA for their soluble CD23 content. Immunophenotyping was carried out in all patients, and in a selected subset the mean number of membrane-bound CD23 molecules per cell was also investigated. Seventy-three patients had typical B chronic lymphocytic leukemia, 41 leukemic B-cell disorders with atypical morphological and/or immunophenotypic features, 5 had low-grade follicular cell lymphoma in the leukemic phase, and 5 had splenic lymphoma with villous lymphocytes. Soluble CD23 levels were significantly higher than in normal sera (mean +/- SD: typical B chronic lymphocytic leukemia 3,650 +/- 4,654 U/ml, atypical B chronic lymphocytic leukemia 3,440 +/- 4,671 U/ml, follicular cell lymphoma 3,200 +/- 1,511 U/ml, splenic lymphoma with villous lymphocytes 8,236 +/- 7,294 U/ml, controls 137 +/- 128 U/ml; P < 0.001). More advanced Rai's stages were related to higher soluble CD23 levels (P < 0.01), both in typical and atypical B chronic lymphocytic leukemias, the highest levels and the best correlation with the absolute number of circulating CD19+ cells (r = 0.50) being observed in the typical form. The number of membrane-bound CD23 molecules per cell was significantly higher in typical than in atypical B chronic lymphocytic leukemias (mean number 156,727 +/- 94,668 vs. 12,010 +/- 10,643, P < 0.001). Our data suggest that soluble CD23 levels correlate with the clinical and biological features of leukemic B-cell lymphoproliferative disorders.

Thiols decrease human interleukin (IL) 4 production and IL-4-induced immunoglobulin synthesis

N-Acetyl-L-cysteine (NAC) is an antioxidant precursor of intracellular glutathione (GSH), usually given in human as a mucolytic agent. In vitro, NAC and GSH have been shown to act on T cells by increasing interleukin (IL) 2 production, synthesis and turnover of IL-2 receptors, proliferation, cytotoxic properties, and resistance to apoptosis. We report here that NAC and GSH decrease in a dose-dependent manner human IL-4 production by stimulated peripheral blood T cells and by T helper (Th) 0- and Th2-like T cell clones. This effect was associated with a decrease in IL-4 messenger RNA transcription. In contrast, NAC and GSH had no effect on interferon gamma and increased IL-2 production and T cell proliferation. A functional consequence was the capacity of NAC and GSH to selectively decrease in a dose-dependent manner IL-4-induced immunoglobulin (Ig) E and IgG4 production by human peripheral blood mononuclear cells. Interestingly, NAC and GSH also acted directly on purified tonsillar B cells by decreasing the mature epsilon messenger RNA, hence decreasing IgE production. In contrast, IgA and IgM production were not affected. At the same time, B cell proliferation was increased in a dose-dependent manner. Not all antioxidants tested but only SH-bearing molecules mimicked these properties. Finally, when given orally to mice, NAC decreased both IgE and IgG1 antibody responses to ovalbumin. These results demonstrate that NAC, GSH, and other thiols may control the production of both the Th2-derived cytokine IL-4 and IL-4-induced Ig in vitro and in vivo.

The Fc epsilon RII/CD23 gene is actively transcribed during all stages of murine B-lymphocyte development

It is generally accepted that the expression of Fc epsilon RII/CD23 on the surface of the B-lineage cells is restricted to the stage of the resting, mature (sIgM+/sIgD+) B-lymphocyte. However, it is unknown whether activation of the Fc epsilon RII/CD23 gene is also restricted to the stage of the mature B-lymphocyte. To address this question we investigated a panel of B-lineage cell lines for the presence of transcripts encoding Fc epsilon RII/CD23. We detected transcripts in 16 of 26 B-lineage cell lines representing the entire spectrum of B-cell development. In most cases (13 of 16) active transcription of the murine Fc epsilon RII/CD23 gene was not coupled with the expression of cell surface Fc epsilon RII/CD23 expression did not hold for all murine B-cell lines. One post-switch B-cell line (sIgM-/sIgG+) expressed Fc epsilon RII/CD23 on the cell surface and another could be induced with IL-4 and LPS to express surface Fc epsilon RII/CD23. Transcription of the murine CD23 gene in the absence of cell surface expression of Fc epsilon RII/CD23 does not appear to simply be an aberrant feature of transformed B-cells since we found transcripts, but not surface expression, in some normal splenic and peritoneal B-lymphocytes. Our findings suggest that the potential for expression of Fc epsilon RII/CD23 may occur over a much broader development window of the B-lineage than previously suspected. Transcription of the Fc epsilon RII/CD23 gene, in the absence of detectable cell surface protein expression in B-lineage cell lines, and in sort-purified B-lymphocyte subpopulations, implies that in addition to regulatory mechanisms already known, murine CD23 is also regulated through post-transcriptional mechanisms that have not yet been characterized.

A major house dust mite allergen disrupts the immunoglobulin E network by selectively cleaving CD23: innate protection by antiproteases

Asthma is a chronic life-threatening disease of worldwide importance. Although allergic asthma and related atopic conditions correlate strongly with immune sensitization to house dust mites, it is unclear why antigens from mites provoke such powerful allergic immune responses. We have characterized the protease activity of Der p I, the group I protease allergen of the house dust mite Dermatophagoides pteronyssinus, and here report that it cleaves the low-affinity immunoglobulin (Ig) E Fc receptor (CD23) from the surface of human B lymphocytes. Der p I selectively cleaves CD23 and has no effect on the expression of any other B cell surface molecules tested. We speculate that this loss of cell surface CD23 from IgE-secreting B cells may promote and enhance IgE immune responses by ablating an important feedback inhibitory mechanism that normally limits IgE synthesis. Furthermore, since soluble CD23 is reported to promote IgE production, fragments of CD23 released by Der p I may directly enhance the synthesis of IgE. alpha 1-Antiprotease, a pulmonary antiprotease, is also shown to inhibit the cleavage of CD23 by Der p I. This may be significant in the etiopathogenesis of asthma, because other indoor pollutants associated with asthma are known to potently inhibit this antiprotease. These data suggest that the proteolytic activity of Der p I, the group I allergen of the house dust mite D. pteronyssinus, is mechanistically linked to the potent allergenicity of house dust mites. Furthermore, inhibition of Der p I by alpha 1-antiprotease suggests a mechanism by which confounding factors, such as tobacco smoke, may act as a risk factor for allergic asthma.