Cleavage Of CD23 Research Articles

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.

A mite subversive: cleavage of CD23 and CD25 by Der p 1 enhances allergenicity

The dust mite Dermatophagoides pteronyssinus is a major cause of allergic disease in the Western world. Der p 1 is considered to be the most immunodominant allergen involved in the expression of IgE-mediated dust-mite hypersensitivity. Here, Farouk Shakib and colleagues suggest that it is the proteolytic effect of Der p 1 on CD23 and CD25 which makes it such a potent inducer of IgE synthesis.

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.

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.

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

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

CD23‐mediated homotypic cell adhesion: The role of proteolysis

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