CD23/FcεRII: 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.

SDF-1 and PDGF enhance alphavbeta5-mediated ERK activation and adhesion-independent growth of human pre-B cell lines.

CD23 acts through the alphavbeta5 integrin to promote growth of human pre-B cell lines in an adhesion-independent manner. alphavbeta5 is expressed on normal B-cell precursors in the bone marrow. Soluble CD23 (sCD23), short CD23-derived peptides containing the arg-lys-cys (RKC) motif recognized by alphavbeta5 and anti-alphavbeta5 monoclonal antibodies (MAbs) all sustain growth of pre-B cell lines. The chemokine stromal cell-derived factor-1 (SDF-1) regulates key processes during B-cell development. SDF-1 enhanced the growth-sustaining effect driven by ligation of alphavbeta5 with anti-alphavbeta5 MAb 15F-11, sCD23 or CD23-derived RKC-containing peptides. This effect was restricted to B-cell precursors and was specific to SDF-1. The enhancement in growth was associated with the activation of extracellular signal-regulated kinase (ERK) and both these responses were attenuated by the MEK inhibitor U0126. Finally, platelet-derived growth factor also enhanced both alphavbeta5-mediated cell growth and ERK activation. The data suggest that adhesion-independent growth-promoting signals delivered to B-cell precursors through the alphavbeta5 integrin can be modulated by cross-talk with receptors linked to both G-protein and tyrosine kinase-coupled signalling pathways.

The antimitotic effect of the neem terpenoid azadirachtin on cultured insect cells.

When cultured insect cells (Sf9) were grown in the presence of 5 x 10(-6) M azadirachtin, there was a rapid increase in the mitotic index, with the appearance of many aberrant mitotic figures. Flow cytometry established that cells accumulated in the G2/M phase of the cell cycle, and that the effect was concentration-dependent. At 10(-8) M a period of 20 h was necessary to raise the proportion in G2/M to 42% above the control values, but at 5 x 10(-6) M more than 90% of the cells were in this phase. Azadirachtin had the same effect on C6/36 mosquito cells, but failed to affect L929 murine fibroblast cells even at a concentration of 10(-4) M over 72 h. Experiments with colchcine and taxol showed similarities of action between azadirachtin and colchicine, and azadirachtin was apparently able to displace colchicine-fluorescein from binding-sites in living insect cells. Another similarity between azdirachtin and colchicine was that both phytochemicals prevented the polymerisatrion in vitro of mammalian tubulin, although the azadirachtin was much less effective.

The CD23a and CD23b proximal promoters display different sensitivities to exogenous stimuli in B lymphocytes.

The single human CD23 gene encodes two protein products differing by six or seven amino acids in the extreme N-terminal cytoplasmic domain. The patterns of expression of CD23a and CD23b transcripts differs as a function of cell type and cell stimulation, with expression of CD23a being largely restricted to B cells and CD23b synthesis being inducible in a variety of haematopoietic cells by a range of exogenous stimuli. In this study, short defined sequences of the CD23a and CD23b proximal promoter regions were used to drive expression of exogenous reporter genes in transiently-transfected B cells exposed to a range of cellular stimuli. The CD23a promoter was activated only by IL-4, whereas the CD23b promoter was stimulated not only by IL-4, but also by stimulation with anti-mu, and anti-CD40. Deletion mutant analysis illustrated that of the two putative STAT6 binding sites present in the CD23a proximal promoter, deletion of the first site abrogated IL-4-driven transcriptional activation. Conversely, deletion of both STAT6 binding sites in the CD23b promoter was required before IL-4 sensitivity was lost. When the same CD23b promoter mutants were studied in the context of anti-CD40 and anti-mu stimulation of transfected cells, deletion of the NF-kappaB site abrogated anti-CD40-driven transcriptional activation, but not anti-mu-mediated effects which required additional deletion of putative AP1 sites lying close to the CD23b initiator methionine codon. The data of this report are consistent with the interpretation that the upstream regions of the CD23a and CD23b isoform coding sequences show distinct sensitivities to agents which induce CD23 protein expression at the plasma membrane, and that transcriptional activation by discrete stimuli reflects activation of particular transcriptional regulatory factors.

A functional role for interleukin (IL)-4-driven cyclic amp accumulation in human b lymphocytes.

Interleukin-4 (IL-4) regulates the expression of the 55-kDa alpha-subunit (CD25) of the IL-2 receptor complex in human B lymphocytes. This report suggests that the cAMP/protein kinase A (PKA) component of the IL-4 receptor signalling programme in human tonsillar B cells has a functionally important role in regulating expression of the CD25 gene by attenuating activity of a protein binding to a potent negative regulatory element (NRE) in the CD25 promoter; this effect can be mimicked by agents that elevate cAMP and blocked by inhibitors of PKA but not protein kinase C (PKC). In a B-cell line that fails to elevate cAMP, attenuate NRE-binding protein (NRE-BP) activity or express CD25 following IL-4 treatment, stimulation of cAMP accumulation by forskolin facilitates IL-4-mediated induction of both the endogenous gene and an exogenous reporter gene under the control of a minimal promoter/enhancer fragment of the CD25 gene.

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.

Studies on the site and mechanism of attachment of phosphorylcholine to a filarial nematode secreted glycoprotein.

We have recently shown that the immunomodulatory substance phosphorylcholine (PC) is covalently attached to ES-62, a major secreted protein of the filarial nematode parasite Acanthocheilonema viteae, via an N-linked glycan. Linkage of PC to N-glycans is previously unreported, and hence we have investigated the biochemical events underlying it. PC addition was found by pulse-chase experiments to be a fairly early event during intracellular transport, occurring within 40-60 min of protein synthesis. Biosynthetic labeling/immunoprecipitation experiments revealed that addition of PC to ES-62 was blocked by (i) brefeldin A, an inhibitor of trafficking of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi, (ii) 1-deoxynorijirimycin, an inhibitor of glucosidase activity in the ER, and (iii) 1-deoxymannojirimycin, an inhibitor of mannosidase I in the cis Golgi. Swainsonine, an inhibitor of mannosidase II in the medial Golgi, did not affect PC addition. Taken together these data indicate that PC attachment is a post-ER event which is dependent on generation of an appropriate substrate during oligosaccharide processing. Furthermore, they strongly suggest that PC addition takes place in the medial Golgi and that the substrate for addition is the 3-linked branch of Man5GlcNAc3 or Man3GLcNAc3.

Negative regulation of the interleukin (IL)-2 receptor alpha chain (CD25) promoter region in human B lymphocytes.

Studies of the CD25 gene promoter region in T cells have revealed the presence of a 31-bp region, including an 11-bp negative regulatory element (NRE), which profoundly suppresses CD25 expression. This report illustrates that the same region acts as a negative regulator of CD25 expression in human B lymphocytes. Human B cells contain DNA-binding protein activities which bind specifically to an oligonucleotide equivalent to the 11-bp core region of the NRE, and stimulation of tonsillar B cells or Daudi Burkitt's lymphoma B cells with interleukin 4 (IL-4) results in loss of binding activity for oligonucleotides containing the NRE; in contrast, IL-4 enhanced binding activity for the NRE in the Jurkat T cell line. Transient transfection analyses using deletion mutants lacking both the 11-bp core NRE and both the NRE and an adjacent putative retinoic acid response element (RARE) motif illustrated that the NRE element is a functional suppressor of CD25 transcription in B cells. Thus, deletion of the NRE element increased the basal level of CD25 promoter activity and also conferred IL-4 inducibility on reporter gene expression in transiently transfected tonsillar B lymphocytes.

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.

Induction of CD25 expression in human B lymphocytes by pharmacological activators of cellular signalling pathways.

IL-4 promotes simultaneous expression of both the CD23 and CD25 antigens in resting human B lymphocytes in a dose-dependent manner. Simultaneous three-colour flow cytometric analysis revealed that CD19+/CD23+/CD25+ triple-positive cells were derived from a CD19+/CD23-/CD25- pool, and that induction of CD23 required lower doses of IL-4 than did induction of CD25. Although the concentrations of IL-4 required for half-maximal up-regulation of CD23 (35 pM) and CD25 (150 pM) expression were different, the capacity of IL-4 to promote expression of the two markers could be mimicked by the same combination of pharmacological agents. Thus, maximal expression of CD23 and CD25 was obtained with a 30 (or 120) second pulse with phorbol ester and/or ionomycin followed by a sustained (20 minute) treatment with forskolin. Use of BAPTA to chelate intracellular calcium suggested that IL-4 driven CD25 expression required mobilization of intracellular Ca2+. Finally, down-regulation of cellular protein kinase C by chronic treatment of resting B lymphocytes with phorbol ester abolished the ability of IL-4 to elevate CD23 and CD25 expression; phorbol ester treatment similarly abrogated the ability of anti-CD40 and anti-Ig reagents to promote expression of CD25. The data are consistent with the proposal that IL-4 influences CD23 and CD25 expression via a similar signal transduction pathway which involves both protein kinase C activation and elevation of intracellular cAMP levels.

Anti-immunoglobulin and anti-CD40 stimulation induces CD25 expression by resting human tonsillar B lymphocytes.

In this report, the effect of ligation of a number of B-cell surface molecules upon expression of CD25, the 55-kDa inducible component of the IL-2 receptor complex found on T and B lymphocytes, is reported. IL-4 is the only cytokine apparently capable of promoting CD25 expression in human high-density quiescent tonsillar B cells; neither IL-10 nor IL-13 could induce CD25 expression. Cross-linking of the antigen receptors or CD40 with antibody elicited CD25 expression in a dose-dependent manner. Stimulation with anti-CD40 promoted CD25 expression in approximately 25% of B cells, while anti-Ig caused 80% or more of cells to become CD25+. In experiments where the stimuli were used in combination, some additive effects upon CD25 expression were noted, but no obvious synergistic effects could be detected.

Expression of platelet-derived growth factor and its receptors by two pre-B acute lymphocytic leukemia cell lines.

Platelet-derived growth factors (PDGF) are potent regulators of cell proliferation. The three isoforms of PDGF AA, AB, and BB are encoded by two genes: PDGF A and PDGF B. The v-sis oncogene is homologous to the PDGF-B gene. v-sis can transform cells that express the appropriate PDGF receptors. Two different types of receptors, PDGF-alpha and PDGF-beta, also encoded by two genes, have been identified. We show that two cell lines. SMS-SB and NALM-6, both derived from pre-B-cell acute lymphocytic leukemias, express the PDGF-A chain gene, and one of them, SMS-SB, releases PDGF-A chains into the media. The SMS-SB cells also express the PDGF-beta receptor, whereas NALM-6 cells express the PDGF-alpha receptor and bind PDGF. This extends the possible targets for PDGF to the B-cell lineage lymphocytes.

Mitogenesis by v-Src: a need for active oncoprotein both in leaving G0 and in completing G1 phases of the cell cycle.

Activation of the tyrosine kinase of a temperature sensitive v-Src mutant of Rous sarcoma virus in quiescent Rat-1 cells leads to passage through the cell cycle. This is accompanied by a transient increase of the DNA binding activity of the transcription factor AP-1 which is not sufficient for the v-Src mediated cell cycle traverse. There is another need for v-Src later in the G1 phase of the cycle, and after completion of that event, cells are able to progress through DNA synthesis and division in the absence of either v-Src or other growth factors. When cells are exposed to v-Src activity for periods insufficient for it to behave as a complete mitogen, it can act as either a competence or progression factor in conjunction with appropriate purified growth factors.

Aneuploid subpopulations in tumour-invaded lymph nodes from breast cancer patients.

Fresh, paired primary tumours and lymph node metastases from breast cancer patients were compared by DNA flow cytometry. Although 65% of primary tumours were aneuploid, the detection of aneuploid peaks in corresponding nodal metastases was rare (only 6 cases out of 25) in single-parameter DNA analysis. Detection of aneuploid subpopulations in lymph nodes was greatly improved in dual-parameter DNA analysis using an anticytokeratin (CK) antibody which allowed ploidy determination on CK+ epithelial cells alone. Examination of 12 lymph nodes for CK+ cells revealed the presence of both diploid and aneuploid tumour cells in tumour invaded nodes. In patients with multiploid primary tumours, a subpopulation of the primary aneuploid cells was dominant in the nodal metastases. This suggests that aneuploidy is an integral property of metastatic cells and that within a primary tumour a subpopulation may have a higher metastatic potential.

Flow cytometric studies of IL-4-stimulated expression of the CD25 antigen by quiescent human B lymphocyte subpopulations.

The membrane immunoglobulin (mIg) phenotype of high-density human tonsillar B lymphocytes which display elevated levels of the CD25 marker in response to treatment with interleukin-4 (IL-4) has been investigated. IL-4 promotes elevation of CD25 levels in B lymphocytes expressing membrane IgM (mIgM), mIgD, mIgG and mIgA. Three-colour flow cytometric analyses indicate that 25-35% of high-density tonsillar B cells are simultaneously positive for mIgM, mIgD and CD25 following IL-4 stimulation. The hypothesis that B cells could respond to IL-4 by up-regulation of either CD23 or CD25, but not both simultaneously, was evaluated. Three-colour flow cytometric studies indicated that approximately one-third of the B-lymphocyte population was simultaneously positive for the CD19, CD23 and CD25 antigens following stimulation with IL-4. These data are consistent with the proposal that IL-4 can promote expression of CD23 and CD25 antigens in the same B lymphocyte.

Flow cytometric analysis of cell surface carbohydrates in metastatic human breast cancer.

Helix pomatia agglutinin (HPA)- and Concanavalin A (Con A)-binding carbohydrate expression were studied on 32 tumour samples from primary adenocarcinoma of the breast and 12 samples from lymph node metastases. Live cells were spilled from each of the fresh samples and the extent of fluorescent-labelled HPA and Con A-binding was assessed by flow cytometry. The extent of brightness was expressed in a defined quantitative fashion and the percentage of positive cells was accurately determined from a sample of 10,000 cells per tumour. Correlation of binding with clinicopathological features showed that HPA but not Con A related to lymph node involvement (P = 0.001) in tumours of higher grade (II and III). Spilled tumour cells (non-lymphocytes) were selected from the lymph nodes and the presence of HPA binding cells in the involved lymph nodes was found to relate to positive HPA binding in autologous primary tumours (P = 0.002). Dual-label analysis of HPA and Con A binding showed characteristic features for each tumour. The study demonstrates the use of flow cytometry as a simple and effective technique in detecting differences in lectin binding in live spilled cells from fresh breast cancer tissues. This method may prove to be particularly useful if performed preoperatively on cells in fine-needle aspirates.

Recombinant interleukin-4 promotes expression of the CD25 (Tac) antigen at the plasma membrane of high-density human tonsillar B lymphocytes.

High-density human B lymphocytes were prepared from tonsillar mononuclear cells by depletion of adherent cells and T lymphocytes, followed by discontinuous density gradient centrifugation. The B cells were analysed for the levels of expression of the CD25 (Tac) antigen marker by flow cytometry following culture with a variety of cytokines. IL-4 could induce elevated levels of CD25 on high-density, putatively resting B lymphocytes in a dose-dependent fashion. Expression of CD25 at the B-cell surface could not be promoted by interleukin-2 (IL-2), interferon-gamma (IFN-gamma) or by a crude preparation of B-cell growth factor 2 (BCGF-2). Mitogenic challenge of the B cells with pokeweed mitogen (PWM) and a combination of phorbol ester and calcium ionophore were similarly ineffective, although a small increase in CD25 expression could be detected when the B cells were cultured with phytohaemagglutinin A (PHA). The ability of IL-4 to promote CD25 expression was abolished by the presence of IFN-gamma in the culture. Titration experiments suggested that the amount of IL-4 required to produce a half-maximal increase in CD25 expression was approximately 40 U/ml; this is considerably greater than the 8-10 U/ml required to produce the equivalent effect on CD23 expression. The ability of IL-4 to promote CD25 expression in the high-density B-lymphocyte population was apparently independent of proliferation of the cells. IL-4 could not promote Tac expression on high-density T cells prepared from the same tissue source.

Phosphorylation of class I but not class II MHC molecules by membrane-localized protein kinase C.

Membranes were isolated from B cells stimulated with phorbol 12-myristate 13-acetate (PMA) for a time sufficient to allow maximal redistribution and activation of protein kinase C (PKC). Exposure of such membranes to a short incubation with [gamma-32P]ATP resulted in the detection of at least nine unique or hyperphosphorylated membrane proteins by SDS-PAGE and autoradiography. The appearance of these phosphoproteins was blocked by pretreatment of the membranes with H-7 or sangivamycin, two selective inhibitors of PKC. In addition, membranes purified from B cells treated with an inactive phorbol ester or stimulated with dibutyryl cAMP failed to exhibit a pattern of new phosphoproteins. These results are consistent with the involvement of PKC in the phosphorylation of the proteins. These phosphoproteins are also candidates for proteins whose functions are modified as a consequence of early signal delivery to resting B cells following membrane immunoglobulin occupancy. This system was utilized to identify the heavy chain of MHC class I molecules as one of the membrane proteins phosphorylated by PKC. The MHC class II molecules were not phosphorylated in membranes isolated from PMA-treated normal B cells or from PMA-treated B cells which had previously been exposed to IL-4. These results indicate that class I, but not class II, MHC molecules are phosphorylated by PKC. It is possible that such a modification of cell surface class I molecules may be involved during the process of signal transduction leading to B cell activation.

Appearance of a novel plasma membrane phosphoprotein following culture of resting murine B lymphocytes with recombinant interleukin 4.

Culture of high density resting murine B cells with recombinant IL-4 caused changes in the patterns of phosphoproteins associated with the plasma membranes isolated from such cells. The principal difference was the appearance of an alkali-resistant phosphoprotein of Mr = 75,000-80,000. Culture with LPS also gave rise to a characteristic alteration in phosphoprotein profiles, but this was distinct from the cytokine-induced changes and did not include induction of a 75 kDa signal. Culture in medium plus serum only failed to produce an alteration of the phosphoprotein profile of membranes isolated from B cells cultured in this manner. Culture of high density murine splenic B cells with IL-4 in the presence of the 11B11 anti-IL-4 monoclonal antibody prevented the appearance of the 75 kDa phosphoprotein, indicating that IL-4 binding was necessary for induction of the 75 kDa phosphoprotein signal. The possible molecular identity of the 75 kDa phosphoprotein and its role in control of B lymphocyte differentiation is discussed.