Soluble CD21 induces activation and differentiation of human monocytes through binding to membrane CD23

Interactions between CD23, the low-affinity receptor for IgE, and CD21, the C3d/EBV receptor, modulate several intracellular events in lymphocytes. A soluble form of CD21 (sCD21) corresponding to the extracellular domain of the receptor circulates in normal plasma. We now demonstrate that purified sCD21 acts as a functional ligand for CD23-expressing monocytes. Soluble CD21 induced an increase in intracellular cGMP levels and the production of IL-6 and TNF-α in IL-4-pretreated monocytes induced to express CD23 but not in unstimulated CD23⁻ monocytes. The accumulation of cGMP and the production of TNF-α were inhibited by N^G-monomethyl-L -arginine (L -NMMA), indicating that sCD21 activates the L -arginine pathway of NO production. We demonstrated that sCD21 activates NO synthase (NOS) since it was found to enhance the conversion of L -arginine into L -citrulline and induce the intracellular expression of inducible NOS in CD23⁺ monocytes. In addition, sCD21 was shown to up-regulate the expression of HLA-DR and CD40 and decrease that of CD14 on cultured CD23⁺ monocytes. Thus, in a fashion similar to IgE complexes, sCD21 is able to efficiently trigger CD23 signaling pathways, inducing the release of pro-inflammatory mediators by human monocytes. Soluble CD21 up-regulates the expression of molecules involved in antigen presentation, further suggesting a potential immunoregulatory function for the soluble molecule.     

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 concensus 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^TM 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.     

Soluble CD23 in allergic diseases.

CD23, a differentiation marker of B cells is identified with the low-affinity receptor for IgE--FcepsilonRII. The CD23 molecule is continuously cleaved by autoproteolysis into soluble fragments called sCD23, considered as a multifunctional cytokine. sCD23 is supposed to play an important role in IgE synthesis. IgE is a hallmark of atopy and its overproduction is a characteristic feature of allergic diseases. The aim of this study was to determine sCD23 (25 kDA) serum levels in patients with inhalant allergy and hymenoptera venom-induced allergy with relevance to IgE system. The trial consisted of 18 patients with pollinosis, 25 with house dust mite allergy and 12 with hymenoptera venom-induced allergy. Eighteen healthy volunteers without signs of atopy served as a control group. Serum levels of sCD23 (25 kDa), total IgE and allergen specific IgE were measured as well. The results were presented as median value, 25-75% range and a total value range. Nonparametric tests (the U Mann-Whitney test, Kruskal and Wallis test and Spearman's correlation rang test) were used. In patients with allergic disorders serum levels of sCD23 were significantly higher than in the control group (p<0.05). No correlation between IgE levels and sCD23 was detected in all the investigated groups. sCD23 does not appear to be a hallmark of allergic diseases, however serum level of that molecule is significantly elevated in patients suffering from allergic disorders. No correlation between sCD23 and IgE has been observed. sCD23 serum level has no relevance to the types of allergic diseases.     

Purification and characterization of soluble CD21 from human plasma by affinity chromatography and density gradient centrifugation

Complement receptor II (CD21) is the receptor for C3d fragments on immune complexes. It also serves as a receptor for Epstein-Barr virus (EBV) and on B-lymphocytes CD21 amplifies signalling through the B cell receptor. CD21 is shed from the surface of the cell and is found circulating in plasma. There, soluble CD21 (sCD21) binds to CD23 and complement fragments, thereby modulating the immune response. sCD21 activates monocytes through binding to membrane CD23. The clinical significance of sCD21 is shown by the increased levels found in the sera of patients with B lymphomas, EBV infections and other lymphoblastoid tumors. In this paper, we report the isolation of soluble CD21 from human plasma using affinity chromatography and density gradient centrifugation. sCD21 was found to be a single 126 kDa molecular species. By determining the sedimentation coefficient, we have calculated the partial specific volume, diffusion coefficient and frictional coefficient of the protein. These values show that the sCD21 isolated from human plasma is an elongated rod-shaped molecule.     

Expression of sCD23 in atopic and nonatopic blood donors: correlation with age, total serum IgE, and allergic symptoms

Although structure, biologic activities, and expression of the low-affinity IgE receptor (Fc_eRII, CD23) have been investigated, the diagnostic value for allergies of this molecule and its soluble circulating fragment (sCD23) remains unclear. Therefore, serum sCD23 levels were measured in 203 blood donors. They were divided into atopic and nonatopic subjects by allergy history, physical findings of allergic symptoms, and corresponding specific circulating IgE antibodies. The group consisting of nonatopic subjects was divided into four age categories in order to exclude age-dependent variations in the expression of the low-affinity IgE receptor. In our study population, sCD23 serum levels were not influenced by age. Furthermore, no significant differences, especially no decrease in serum sCD23 levels, between the four nonatopic age groups were detected. There was no significant increase of sCD23 serum levels in atopic subjects in comparison with nonatopic blood donors. In addition, no correlation between total IgE levels and sCD23 serum levels could be detected, in either the group of atopic donors or the group of nonatopics. Our data suggest that the circulating low-affinity IgE receptor does not appear to be an additional general marker for the diagnosis of allergies, as previously suggested.     

Soluble CD23 (sCD23) serum levels and lymphocyte subpopulations in peripheral blood in rhinitis and extrinsic and intrinsic asthma

To determine serum levels of IgE and sCD23 and lymphocyte subpopulations, we studied 37 control subjects and 84 patients (27 with allergic rhinitis, 27 with extrinsic asthma, and 30 with intrinsic asthma). A rise in surface CD23 on B and monocyte cells and sCD23 serum levels was exhibited by patients with rhinitis and extrinsic asthma. Unexpectedly, in intrinsic asthmatic patients, high CD23 expression on monocytes and high sCD23 levels were seen that did not result in IgE production. It appears that CD23, in its soluble form, could be a good disease marker, especially in asthma. Atopic patients yielded a significantly lower proportion of CD4⁺ T cells than intrinsic asthmatic patients and normal persons. Otherwise, CD4⁺ CD29⁺ CD45RA - and CD4⁺ CD29 – CD45RA ⁺ T-cell subsets were significantly decreased in all patient groups.     

Expression,regulation and function of human FcεRII (CD23) antigen

CD23, also known as the low affinity receptor for IgE (Fc epsilon RII), belongs to a novel superfamily of type-II integral membrane proteins. Fc epsilon RII expression was originally described on B cells but subsequent studies showed that CD23 is expressed on a variety of hematopoietic cells and is regulated by several cytokines (i.e., interleukin-4, interferons) in a tissue-specific manner. In some pathological conditions such as B-chronic lymphocytic leukemia, the CD23 gene is abnormally regulated resulting in CD23 overexpression. CD23 is not only an IgE receptor but also a membrane-bound precursor of soluble molecules that still bind IgE (sCD23 or IgE-binding factors). The functions of membrane CD23 are IgE-dependent and vary according to the cell types on which it is expressed. In contrast, sCD23, in addition to being an IgE regulatory molecule, displays multiple cytokine activities that are IgE-independent.     

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.     

Origin and properties of soluble CD21 (CR2) in human blood

By analysis with a panel of CD21 MoAbs it is shown that a large part of the soluble CD21 in human blood plasma is of the long isoform (CD21L), as judged by comparison with antigen produced by mouse L cells transfected with CD21L-cDNA and reactivity with the restricted CD21 MoAb R4/23. This is compatible with the hypothesis that soluble CD21 in the blood is mainly derived from follicular dendritic cells (FDC). Cells from a human keratinocyte cell line transfected with cDNA from the Burkitt lymphoma cell line Raji also produced soluble CD21L (sCD21L), whereas the short form of sCD21 (sCD21S) was the major component of sCD21 produced by the B lymphoblastoid cell line LICR-LON-HMy and the T cell line Jurkat. Confocal studies of FDC isolated from human tonsil revealed that CD21 was present in the cytoplasm. On gel filtration sCD21 from untreated serum has an apparent size considerably greater than the 130 kD found by SDS–PAGE analysis. This may be partly accounted for by the non-globular shape of the molecule, but may also indicate, as reported by others, that in its native state sCD21 is complexed with other proteins. However, no evidence of complexing with sCD23 or C3d could be found.     

CD23/Fc��RII: 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.     

Increased plasma levels of soluble CD23 in haemorrhagic fever with renal syndrome; relation to virus-specific IgE

In 15 consecutive patients hospitalized with nephropathia epidemica, a European form of haemorrhagic fever with renal syndrome (HFRS) caused by Puumala virus, plasma concentrations of soluble CD23 (sCD23) and Puumala virus-specific IgE were determined. In the acute phase of illness, 11/15 patients had increased sCD23 levels (> 91 U/ml), whereas in convalescence, values of 8/10 patients were normalized. Maximal sCD23 values were correlated to maximal concentrations of Puumala virus-specific serum IgE (r= 0.597; P= 0.025). The results are compatible with a known ability of sCD23 to augment IgE production.     

Circulating elevated levels of soluble CD23, interleukin-4, and CD20+CD23+ lymphocytes in atopic subjects with elevated serum IgE concentrations.

Circulating IgE protein levels, leukocyte counts, lymphocyte subsets, IL-4, and soluble CD23 levels were quantitated in 43 atopic and 19 nonatopic subjects. Mean values of IgE protein levels, total eosinophil counts, CD20+CD23+ cells (B cells with low-affinity IgE receptor), IL-4 and sCD23 levels were elevated in atopic patients compared with nonatopic controls. The results suggest that sCD23, IL-4, and CD20+CD23+ lymphocytes may play a role in the increased production of IgE in atopic subjects in a manner similar to that observed by other investigators in prior in vitro studies.     

Serum levels of sCD23 and sCD25 in children with asthma and in healthy controls*

To investigate whether markers of lymphocyte activation are useful markers of disease activity in childhood asthma, we studied serum levels of soluble CD25 (receptor for IL-2) and soluble CD23 (low-affinity receptor for IgE) in 178 children (aged 2-18 years) suffering from mild to moderate asthma (mean asthma severity score: 2, range: 1–4), and in 175 healthy age-matched controls. Levels of sCD23 and sCD25 were invesely related to age. sCD23 was lower in patients with asthma (means per age group: 4.93–2.29 ug/1; controls: 6.92–4.11 ug/1, P <0.05), while sCD25 tended to be higher (1601–597 kU/ml, controls: 1350–-661 kU/ml, P = NS). sCD25 correlated significantly with asthma severity score (r=0.41; P <0.01) and MEF₂₅ (maximum experatory flow at 25% of vital capacity, r= -O.43; P<0.05) in children <10 years, while sCD23 correlated with asthma severity (r=O.28; P <0.05) in children > 10 years. On follow-up, levels of sCD25 normalized with clinical improvement. In children with nonatopic asthma, levels of sCD25 were significantly higher than in atopic patients. Our observations provide further evidence of the role of T-cell activation in asthma. Monitoring of lymphocyte activation markers, particularly levels of sCD25, may be useful in the follow-up of asthmatic children.     

Native and recombinant soluble CD23 fragments with IgE suppressive activity.

CD23-bearing cells are known to release 37,000 33,000 and 25,000 MW soluble CD23 (sCD23) fragments that were reported to display multiple biological activities, including the potentiation of IgE synthesis. We previously reported that tunicamycin treatment of RPMI-8866 cells switched the biological activity of the sCD23 released by these cells from IgE potentiation to IgE suppression. In this study we show that tunicamycin-treated cells release small CD23 fragments with a MW of 16,000. These fragments are formed by truncation of the N-terminal 160 amino acids and truncation of the carboxy-terminal end of CD23. Two observations indicate that the cleavage of surface CD23 into 16,000 MW fragments is not caused by tunicamycin-mediated inhibition of the N-glycosylation of CD23 but rather by the deletion of the carboxy terminal end of the molecule: (1) Chinese hamster ovary (CHO) transfectants expressing a CD23 mutant lacking the N-glycosylation site release 37,000-33,000 MW sCD23 unless they are treated with tunicamycin; (2) transfectants expressing a CD23 deletion mutant lacking the last 33 carboxy-terminal amino acids release 16,000 MW sCD23. Highly purified native and recombinant 16,000 MW sCD23 bind to IgE and down-regulate the ongoing and the interleukin-4 (IL-4)-stimulated synthesis of IgE.     

Urinary excretion of CD23 antigen in normal individuals and patients with chronic lymphocytic leukaemia (CLL)

A soluble form of CD23 (sCD23) was found in the urine from 12 normal individuals but was not present in 20 normal sera, suggesting that sCD23 produced by cells in tissues is eliminated in the urine. The sCD23 from urine differed in physicochemical properties from the sCD23 found in supernates from B-lymphoblastoid cell lines (B-LCL) and in the sera of patients with B type chronic lymphocytic leukaemia (B-CLL). On SDS-PAGE analysis under reducing conditions urinary sCD23 showed two bands corresponding to molecular weights of 45-60 kD and 28-35 kD indicating that sCD23 may be excreted in combination with another molecule. When subjected to gel filtration in its native state, sCD23 from urine showed a major peak at approximately 150 kD and a minor peak (probably a breakdown product) at 21 kD. Urinary sCD23 was more strongly held by DEAE-cellulose and required 0.5 M buffer pH 8.0 for elution, suggesting that it is more anionic than sCD23 from culture supernates. Five MoAbs recognizing different epitopes on sCD23 from B-LCL supernates were tested on urinary sCD23. Four of the MoAbs were reactive but one (EBVCS-1) was not. Urinary sCD23 did not bind to IgE. The level of sCD23 found in normal urine (approximately 0.02-0.05 micrograms/ml) was exceeded in 17 of 24 cases of B-CLL. In one case with a high cell count and a serum concentration of 10 micrograms/ml, the urine contained 80 micrograms/ml sCD23. In another case a high serum sCD23 was not matched by a high urinary level. In this case the gel filtration pattern was closer to that found with urine sCD23 rather than the B-LCL pattern found with sera of other B-CLL patients.     

Serum soluble CD23 in patients with hypogammaglobulinaemia.

Serum levels of the soluble form of the low-affinity receptor for IgE (FcERII, CD23) (sCD23) are elevated in autoimmune conditions associated with hypergammaglobulinaemia and B cell hyperactivity. Very high levels of sCD23 are found in patients with B-chronic lymphatic leukaemia (B-CLL) who are, however, frequently hypogammaglobulinaemic. We therefore compared the serum levels of sCD23 in healthy controls (n = 33) with three conditions associated with hypogammaglobulinaemia (HGG) and varying B cell numbers: X-linked agammaglobulinaemia (XLA, n = 12), common variable immunodeficiency (CVI, n = 20) and B-chronic lymphatic leukaemia (n = 33). Serum levels of sCD23 showed a significant correlation with the CD19+ B cell count in both normals and patients with CVI (r = 0.65, P < 0.0001). Amongst the different clinical groups, serum levels of sCD23 were increased in the order XLA < CVI < normals < CLL (medians 2.5, 7.7, 11.1 and 540, respectively; P < 0.001 for all comparisons except CVI versus normals P < 0.03 in a one-tailed test). In the CVI group, serum sCD23 was lowest amongst four patients with low B cell numbers. There was no overlap in sCD23 between patients with XLA and this subgroup of CVI patients. Serum sCD23 is, therefore, derived predominantly from B cells, and is significantly related to the peripheral blood B cell count.     

Cutaneous CD30+ cells in children with atopic dermatitis

BACKGROUND: CD30 expression can be considered a marker of Th2 cells. We investigated the presence of CD30+ cells in the lesional skin of children with atopic dermatitis (AD). We also analyzed the possible relationship between CD30+ cells and serum soluble CD 30 (sCD30) levels, and IgE, soluble interleukin-2 (IL-2) receptor (sIL-2R) or soluble CD23 (sCD23) levels in the blood, and clinical score. METHODS: Ten eczematous children (4 males, 6 females; median age: 4 years and 5 months; range: 11 months to 14 years), 9 sex- and age-matched control children and an adult control group were studied. A clinical score (SCORAD index), was given to eczematous lesions. Blood was taken for the determination of IgE, sCD30, sIL-2R and sCD23 levels. Punch biopsies of lesional skin were stained with hematoxylin and eosin or incubated with anti-CD30 monoclonal antibodies. Skin prick tests (SPTs) were also performed. RESULTS: In the biopsy specimens, CD30 expression was observed in high proportions of infiltrating cells. In children with AD, total serum IgE, sCD30, sIL-2R, sCD23 and eosinophils were significantly elevated compared to controls. CD30+ cells were not associated with serum IgE, sCD30, sIL-2R, sCD23, or SPT results, score of inflammatory cells in lesional skin or clinical score. Children with AD who had high total IgE and specific IgE antibodies did not differ from those with normal total IgE and negative specific IgE in respect of age, clinical score, number of CD30+ cells, sCD30, sIL-2R and sCD23 levels, score of inflammatory cells in skin or clinical score. CONCLUSION: Our results showed remarkable numbers of CD30+ cells in the lesional skin and high sCD30 in the serum of children with AD. CD30+ cells did not correlate with systemic markers of IgE reaction.     

Inhibition of CD23 processing correlates with inhibition of IL-4-stimulated IgE production in human PBL and hu-PBL-reconstituted SCID mice

BACKGROUND: CD23, the low affinity serum immunoglobulin E (IgE) receptor, is upregulated on B cells following interleukin (IL)-4 stimulation and is concomitantly cleaved to generate soluble CD23 (sCD23) fragments with cytokine-like activity. OBJECTIVE: Compounds that selectively inhibit the proteolytic release of CD23 to generate sCD23 were assessed for their ability to inhibit IgE production in order to evaluate the contribution of sCD23 in the production of human IgE as well as the ability of such compounds to block IgE production. METHODS: IgE production was measured in IL-4-stimulated human peripheral blood lymphocytes (PBL) and PBL-reconstituted SCID mice in the presence of a broad-spectrum matrix metalloprotease (MMP) inhibitor, a compound selective for inhibition of CD23 processing over MMPs and an anti-CD23 mAb, MHM6. RESULTS: The two compounds were equipotent in inhibiting IgE production without inhibition of IgG production by IL-4/anti-CD40-stimulated PBL. Soluble CD23 release was also shown to precede IgE accumulation in the cell-free medium. Addition of compound at later times other than day 0 in the 14 day assay resulted in progressively less inhibition of both IgE and sCD23, and exactly paralleled the effect of an anti-CD23 mAb, MHM6 on IgE levels. Both compounds also inhibited the release of CD23 from human RPMI 8866 cells adoptively transferred i. p. to mice. Doses required for inhibition of CD23 correlated well with the doses required for inhibition of IgE production in IL-4-challenged hu-PBL-SCID mice. IgE was selectively inhibited over total IgG in the SCID mice as well. CONCLUSIONS: Inhibition of CD23 processing alone is sufficient to inhibit IL-4-stimulated IgE production both in vitro and in vivo.     

Epitope‐dependent synergism and antagonism between CD40 antibodies and soluble CD40 ligand for the regulation of CD23 expression and IgE synthesis in human B cells

BACKGROUND: The induction of IgE synthesis in naive B cells requires two T-cell-derived signals: one delivered through CD40 and the other via interleukin-4 (IL-4). The natural counterstructure to CD40 is the CD40 ligand (CD40L). We have asked about the interplay between CD40L and CD40 mAb that recognize distinct epitopes in delivering signals for regulating IL-4-dependent IgE synthesis and the expression of CD23, the low-affinity IgE receptor, in resting B cells. METHODS: After culture of purified human tonsillar B cells with CD40 agonists and IL-4, surface CD23 was determined by flow cytometric analysis. CD23 levels in cell lysates and supernatants were quantified by ELISA, as were those of secreted IgE. RESULTS: With regard to both induction of CD23 and IgE production, soluble CD40L trimer (sCD40LT) showed synergistic interaction with two mAb to CD40 which bind to epitopes located outside the ligand binding site (EA5 and 5C3), but not with a mAb (G28-5) which effectively competes for CD40L binding to CD40. Each of the two noncompeting mAb to CD40 was able to cooperate strongly with sCD40LT in promoting high-level induction of CD23 even in the absence of IL-4, an effect mirrored in the promotion of strong homotypic clustering and high-rate DNA synthesis. G28-5, uniquely, induced a down-regulation in IL-4-induced CD23 expression with time, a change that was accompanied by an increase in the amount of soluble CD23 detected. While the two noncompeting mAb consistently synergized with sCD40LT for the promotion of IL-4-dependent IgE synthesis, sCD40LT and G28-5 (which, by itself, was the most potent of the CD40 mAb at inducing IL-4-dependent IgE production) exhibited mutual antagonism in this regard, the level of which could be quite profound. CONCLUSIONS: This study demonstrates that appropriate targeting of CD40 can modulate IgE synthesis either positively or negatively.     

Serum measurements of soluble CD23 in HIV infection.

A gradual reduction in cell-mediated immunity is thought to occur with the progression of human immunodeficiency virus (HIV) infection. This suggests a selective attrition of the Th1 subset. The regulation of the soluble form of the low-affinity receptor for IgE (sCD23) by the opposing actions of interleukin-4 (IL-4) and interferon-gamma (IFN-gamma) allows the assessment of the overall balance of Th1 to Th2 activity in a given disease. In order to investigate this further we employed an enhanced chemiluminescent ELISA to analyse serum levels of sCD23 in male subjects with and without HIV infection. Serum levels of sCD23 were similar in 34 HIV seronegative homosexuals, 39 homosexuals with asymptomatic HIV infection, 27 homosexuals with acquired immune deficiency syndrome (AIDS) and 20 healthy controls. This suggests that HIV has no predilection for either the Th1 or Th2 subsets of CD4 T cells.