IgE receptors on human lymphocytes. I. Identification of the molecules binding to monoclonal anti-Fc epsilon receptor antibodies

Role of CD23 in astrocytes inflammatory reaction during HIV-1 related encephalitis

Soluble factors released by intra-cerebral activated cells are implicated in neuronal alterations during central nervous system inflammatory diseases. In this study, the role of the CD23 pathway in astrocyte activation and its participation in human immunodeficiency virus-1 (HIV-1)-induced neuropathology were evaluated. In human primary astrocytes, CD23 protein membrane expression was dose-dependently upregulated by gp120. It was also upregulated by gamma-interferon (gamma-IFN) and modulated by interleukin-1-beta (IL-1beta) whereas microglial cells in these stimulation conditions did not express CD23. Cell surface stimulation of CD23 expressed by astrocytes induced production of nitric oxide (NO) and IL-1beta which was inhibited by a specific inducible NO-synthase (iNOS) inhibitor (aminoguanidine), indicating the implication of this receptor in the astrocyte inflammatory reaction. On brain tissues from five out of five patients with HIV-1-related encephalitis, CD23 was expressed by astrocytes and by some microglial cells, whereas it was not detectable on brain tissue from five of five HIV-1-infected patients without central nervous system (CNS) disease or from two of two control subjects. In addition, CD23 antigen was co-localized with iNOS and nitrotyrosine on brain tissue from patients with HIV1-related encephalitis, suggesting that CD23 participates in iNOS activation of astrocytes in vivo. In conclusion, CD23 ligation is an alternative pathway in the induction of inflammatory product synthesis by astrocytes and participates in CNS inflammation.

FcepsilonRII/CD23 is expressed in Parkinson's disease and induces, in vitro, production of nitric oxide and tumor necrosis factor-alpha in glial cells

Oxidative stress is thought to be involved in the mechanism of nerve cell death in Parkinson's disease (PD). Among several toxic oxidative species, nitric oxide (NO) has been proposed as a key element on the basis of the increased density of glial cells expressing inducible nitric oxide synthase (iNOS) in the substantia nigra (SN) of patients with PD. However, the mechanism of iNOS induction in the CNS is poorly understood, especially under pathological conditions. Because cytokines and FcepsilonRII/CD23 antigen have been implicated in the induction of iNOS in the immune system, we investigated their role in glial cells in vitro and in the SN of patients with PD and matched control subjects. We show that, in vitro, interferon-gamma (IFN-gamma) together with interleukin-1beta (Il-1beta) and tumor necrosis factor-alpha (TNF-alpha) can induce the expression of CD23 in glial cells. Ligation of CD23 with specific antibodies resulted in the induction of iNOS and the subsequent release of NO. The activation of CD23 also led to an upregulation of TNF-alpha production, which was dependent on NO release. In the SN of PD patients, a significant increase in the density of glial cells expressing TNF-alpha, Il-1beta, and IFN-gamma was observed. Furthermore, although CD23 was not detectable in the SN of control subjects, it was found in both astroglial and microglial cells in parkinsonian patients. Altogether, these data demonstrate the existence of a cytokine/CD23-dependent activation pathway of iNOS and of proinflammatory mediators in glial cells and their involvement in the pathophysiology of PD.

CD23 (FcepsilonRII) release from cell membranes is mediated by a membrane-bound metalloprotease

CD23 (low-affinity IgE receptor, FcepsilonRII) is expressed as a Type II extracellular protein on a variety of cells such as B-cells, monocytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surface as well as the generation of soluble fragments of CD23 has been shown to be involved in the regulation of IgE synthesis. Here we report that the release of CD23 from the cell surface is mediated by a metalloprotease. An assay utilizing purified CD23 and an neo-epitope antibody specific for one of the known cleavage products is described and used to demonstrate unambiguously the cleavage of CD23 by a distinct protease. Characterization of the mechanism of CD23 processing shows that the protease exists as an integral membrane protein with a functional molecular mass of approx. 63 kDa as determined by gel-filtration chromatography. The CD23-cleaving activity found in enriched plasma membranes from RPMI 8866 cells is inhibited by the metalloprotease inhibitors 1, 10-phenanthroline and imidazole and by the matrix metalloprotease inhibitor batimastat, but not by inhibitors of cysteine proteases, serine proteases or acid proteases. The same or a similar activity that cleaves CD23 to the known 33 kDa fragment and is inhibited by batimastat is present in diverse cell types such as unstimulated fibroblasts and monocytic cell lines not expressing CD23, as well as in the Epstein-Barr virus-transformed B-cell line, RPMI 8866, which constitutively expresses CD23.

Mechanism of formation of human IgE-binding factors (soluble CD23): III. Evidence for a receptor (Fc epsilon RII)-associated proteolytic activity

There is mounting evidence that Fc epsilon RII (CD23) and its soluble fragments (IgE-binding factors [BFs] or soluble CD23) have pleiotropic activities. IgE-BFs are formed mainly by the proteolytic cleavage of surface Fc epsilon RII; they are first released as 37- and 33-kD unstable molecules that are subsequently transformed into 25-kD IgE-BFs. In this study, purified and radioiodinated 37-kD IgE-BFs as well as 45-kD Fc epsilon RII were used as substrates to identify the proteases leading to the formation of 25-kD IgE-BFs. These substrates generate 25-kD IgE-BFs when incubated with several Fc epsilon RII-bearing cells, including CHO1-7 cells (transfected with Fc epsilon RII cDNA); by contrast Fc epsilon RII- cells, including CHO control cells, have no effect. Highly purified unlabeled native 37-kD and recombinant 29-kD IgE-BFs also cleave labeled 45-kD Fc epsilon RII into 25-kD IgE-BFs. The proteolytic activity of these purified IgE-BFs is specifically removed by immunoprecipitation with an antibody against IgE-BFs. These data strongly suggest that Fc epsilon RII and some of its soluble fragments play an active role in the proteolytic mechanism generating IgE-BFs. They are supported by the observation that IgE-BFs released by CHO1-7 cells are cleaved exactly at the same sites as B cell-derived IgE-BFs. Taken collectively, the results are compatible with an autoproteolytic process.

Expression of CD23 antigen and its regulation by IL-4 in chronic lymphocytic leukemia

In our previous studies, we reported that the sera from CLL patients contain 3 to 500 times more IgE-BFs (or soluble CD23) than the sera from normal individuals (Sarfati M., Bron D., Lagneaux L., Fonteyn C., Frost H. & Delespesse G. (1988) Elevation of IgE-binding factors in serum of patients with B cell-derived chronic lymphocytic leukemia. Blood 71, 94). In the present study, we have investigated some of the mechanisms accounting for this observation. We report that the density of CD23 expression per cell is significantly higher on the B-CLLs than on the control cells, although the proportion of CD23+ B cells is comparable in both groups (70-90% of CD20+ cells express CD23 antigen). We have next examined the influence of IL-4 on the CD23 expression, the proliferation and the differentiation of B-CLLs. The results indicate that IL-4 (i) increases CD23 expression and IgE-BFs production by normal and CLL B cells; (ii) does not promote B-CLLs proliferation or differentiation, neither by itself nor in costimulation with either anti-IgM or PMA; (iii) significantly potentiates PMA-induced IgM and IgE-BFs production by B-CLLs. It is further shown that anti-CD23 Mab does not interfere with B-CLLs proliferation or differentiation. It is concluded that: (i) the excessive production of IgE-BFs in CLL patients results not only from the enlarged pool of CD23+ B cells but also from an over-expression of CD23 on B-CLLs; (ii) CD23 is not constitutively expressed on B-CLLS and it is upregulated by IL-4; and (iii) by contrast to normal B cells, CD23 on B-CLLs may not be associated with the functional LMW-BCGF receptor.

Mechanisms of formation of IgE-binding factors (soluble CD23)--I. Fc epsilon R II bearing B cells generate IgE-binding factors of different molecular weights

IgE-binding factors (soluble CD23) are generally considered to have an Mr of 25,000-27,000. The present study first indicates that IgE-BFs with an Mr of 33,000 or 37,000 may also be produced by Fc epsilon R II bearing B cells, depending upon the culture conditions and the nature of the Fc epsilon R II bearing cells. Extending our previous observations that the Mr 25,000-27,000 IgE-BFs are derived from the cleavage of soluble Mr 37,000 precursors, we show here that this cleavage is specifically inhibited by iodoacetamide but not by several other protease inhibitors. The proteolytic enzyme involved in the cleavage of Mr 33,000-37,000 precursors into Mr 25,000-27,000 IgE-BFs is cell-associated and is specifically expressed on Fc epsilon R II bearing cells. As expected, these Mr 33,000 and 37,000 fragments of Fc epsilon R II are capable of binding to IgE. The site at which these molecules are cleaved from Fc epsilon R II was located by determining their amino-terminal sequence. The Mr 37,000 IgE-BFs start at position 81 (glutamine) and the Mr 33,000 IgE-BFs start at position 102 (leucine) of the Fc epsilon R II sequence. Taken collectively, the present study not only contributes to our understanding of the mechanisms of formation of IgE-BFs, but also provides a means to prepare different molecular forms of IgE-BFs which may display different biological activity.

Human IgE-binding factors

The production of IgE antibodies is known to be regulated by isotype-specific mechanisms that are not antigen specific. During the last decade several studies have indicated that soluble factors with affinity for IgE (IgE-binding factors, IgE-BFs) may exert such a role by interacting with IgE-bearing B lymphocytes. In the human, some of these IgE-BFs appear to be identical to soluble CD23, a B-cell surface marker thought to be involved in the control of B-cell proliferation or differentiation. In this article, Guy Delespesse and colleagues summarize several new findings regarding the cellular origin, structure and function of IgE-BFs/sCD23.

Expression and functional role of CD23 on T cells

We have found that approximately 10%-15% of tonsil, but not peripheral blood, T cells express the CD23 antigen following activation with 12-O-tetradecanoylphorbol 13-acetate (TPA), phytohemagglutinin (PHA) or recombinant interleukin 4. The proliferative response of tonsil T cells is significantly increased when CD23 monoclonal antibodies (mAb) are present in the cultures. In contrast, no such proliferative augmentation is seen when peripheral blood T cells are cultured in this way. Supernatant (SN) of Epstein-Barr Virus-transformed B lymphoblastoid cell lines (EBVLCL), is found to have a similar co-stimulatory effect on the proliferation of tonsil T cells to that seen with CD23 mAb. This effect is greatly diminished by preclearing SN with CD23 mAb. Similarly, SN from a CD23+ L cell transfectant augments the proliferative response of tonsil T cells to both TPA and PHA. The CD23 molecule expressed by TPA-driven T cell blasts appears identical in size to the 45-kDa glycoprotein present on EBVLCL and activated B cells. In contrast, a 42-kDa molecule is observed when CD23 is precipitated from T cells activated with PHA. The results presented here demonstrate that CD23 is expressed on activated tonsil, but not peripheral blood T cells and plays a role, via the binding of CD23 mAb and CD23+ material, present in EBVLCL and CD23+ transfectant SN, in the regulation of T cell proliferation in response to mitogens such as PHA and TPA.

IgE-binding factors: their possible role in the regulation of IgE synthesis

B cell-derived IgE-BFs (sCD23) are cleavage fragments of surface Fc epsilon R II. Their production is increased by IL4 and suppressed by IFN-gamma and IFN-alpha. IgE-BFs are likely to play a role in the regulation of human IgE synthesis as shown by the following two observations: i. MabER specifically blocks both the spontaneous IgE by synthesis by atopic B cells and the IL4-induced IgE synthesis by normal lymphocytes, ii. purified IgE-BFs enhance the IL4-induced and the spontaneous IgE synthesis. Soluble fragments of Fc epsilon R II also display BCGF-like activity although the exact structure of these fragments is not yet identified. The cDNA coding for Fc epsilon R II has been cloned and functionally expressed. The predicted amino acid sequence reveals no homology between human and rodent IgE-BFs indicating that they are unrelated molecules.

Quantitative and qualitative analysis of the Fc receptor for IgE (Fc epsilon RII) on human eosinophils

In order to characterize the Fc receptor for IgE (Fc epsilon RII) on human eosinophils, we have compared the binding of human IgE myeloma protein to that of a monoclonal antibody (mAb BB10) directed against a common antigenic determinant of the Fc epsilon RII present on eosinophils, platelets and macrophages. Scatchard analysis of the binding to human eosinophils of the BB10 mAb revealed a linear monophasic binding curve, with a binding affinity of 1.17 x 10(7) M-1 and a number of 10(5) binding sites per cell. Biochemical analysis of the human eosinophil Fc epsilon R, performed by immunosorbent chromatography with either BB10 mAb or IgE, showed under nonreducing conditions a major component of 200 kDa. Under reducing conditions, 3 peptide fragments were obtained, with molecular masses of 45-50, 23 and 15 kDa. Finally, comparative analysis suggested that the Fc epsilon RII of human eosinophils and of a human macrophage cell line (U937) are structurally related and differ from the high-affinity Fc epsilon RI present on basophilic granulocytes.