Mouse B cell activation is inhibited by CD44 cross-linking

Lymphocyte activation and trafficking are indispensable to the immune system. CD44 is an adhesion molecule with known importance in T cell activation, lymphocyte trafficking, and tumor metastasis. Although CD44 has been shown to participate in the activation, rolling and adhesion, and homing of T cells, the role of CD44 on B cells is relatively unknown. The effects of CD44 cross-linking on murine B cell activation via CD40L was explored using the anti-CD44 mAbs RK3G9 and IM7. When immobilized on a plate, both RK3G9 and IM7 were found to strongly inhibit B cell proliferation and Ig production, especially at lower cell input concentrations. IgE inhibition was especially prominent. In contrast, soluble RK3G9 added to the B cell cultures had no effect. The inhibitory effect of anti-CD44 on B cell activation was not influenced by the addition of the anti-FcγRII, indicating that Fc cross-linking did not play a role in this inhibition. As Ig production requires several days for both B cell proliferation and differentiation to occur, the effects of delayed addition of immobilized anti-CD44 mAbs were studied, and the results indicated no inhibition after 96 hrs of culture. Finally, B cells were activated by either LPS or anti-IgM F(ab')₂. While LPS-induced B cell activation was inhibited by immobilized anti-CD44 mAbs, anti-IgM activation was refractory. Interestingly, addition of both anti-IgM and CD40L or LPS resulted in some modulation of the inhibitory activity. These results suggest that CD44 cross-linking could control polyclonal B cell activation by CD40L, but allow sIgM/CD40L activation to continue.     

CD2-CD48 interaction prevents apoptosis in murine B lymphocytes by up-regulating bcl-2 expression

Antigen receptor engagement initiates clonal expansion and antibody secretion in B lymphocytes in response to foreign antigens. However, binding of self antigen to antigen receptors targets self-reactive B cell clones for elimination or inactivation. The antigen-triggered biochemical events and the eventual response of the cells are dependent on the simultaneous occupancy of co-stimulatory receptors. CD2 is an intercellular adhesion molecule implicated in cell activation and expressed in human T and natural killer cells as well as in mouse B lymphocytes. Mouse B cells specific for allogeneic major histocompatibility complex (MHC) class I initiate a suicide program that leads to DNA fragmentation and cell death when confronted with soluble MHC class I while undergoing clonal expansion when the antigen is present on mitomycin C-treated cells. Here we show that occupancy of CD2 in mouse B cells by the presence of either monoclonal antibody (mAb) specific for CD2, or soluble recombinant mouse CD48, its natural ligand in mouse, prevents the induction of apoptosis. Furthermore, the in vitro activation by mitomycin C-treated allogeneic cells, is abrogated in the presence of anti-CD48 mAb (OX78). These results indicate that a CD2-CD48 interaction is involved in the control of B cell activation.     

反义c-myc寡核苷酸诱导骨肉瘤细胞MG-63凋亡的实验研究

目的： 研究反义c-myc寡核苷酸诱导人骨肉瘤MG-63细胞凋亡的作用。方法： 设计反义c-myc寡核苷酸片段,转染入人骨肉瘤MG-63细胞，通过MTT法、流式细胞仪、HE染色及透射电镜方法，观察和分析其对人骨肉瘤MG-63细胞作用的效果。结果： MTT法示反义c-myc寡核苷酸可抑制人骨肉瘤MG-63细胞的增殖，10.0 μmol/L、作用48 h效果最明显；流式细胞仪检测证实反义c-myc寡核苷酸（终浓度10.0 μmol/L）诱导瘤细胞的凋亡率达37.92%，出现明显的凋亡峰，并可抑制c-myc基因蛋白的表达；细胞呈典型凋亡特征。结论： 反义c-myc寡核苷酸能有效诱导人骨肉瘤MG-63细胞的凋亡。     

T cell contact-mediated activation of respiratory burst in human polymorphonuclear leukocytes is inhibited by high-density lipoproteins and involves CD18

Polymorphonuclear neutrophils (PMN) are recruited to sites of inflammation, where they are in close vicinity with other immune cell types. The present study demonstrates that direct cell-cell contact with stimulated T cells activates PMN respiratory burst. To discard interferences with soluble products, membranes isolated from human T lymphocytes (msT) or the monocytic cell line HUT-78 (msHUT) were used to mimic cellular contact. msT and msHUT induced a dose-dependent production of radical oxygen species (ROS) in PMN, as detected by chemiluminescence. Similar results were obtained with fixed, stimulated T cells, confirming that ROS production was a result of cell-surface molecules and not to soluble products of T cells. ROS production was mainly intracellular, suggesting that ROS may take part in intracellular processes. High-density lipoproteins (HDL), which had previously been shown to inhibit T cell contact-induced cytokine production in monocyte-macrophages, potently reduced ROS production induced in PMN upon contact with stimulated T cells. This supports the emerging role of HDL as immunomodulators in inflammatory diseases. Furthermore, monoclonal antibodies to CD18 inhibited 60% of the PMN respiratory burst induced by msT, suggesting that CD18 contributed to PMN activation. The present results emphasize the importance of direct cell-cell contact with stimulated T cells in inflammatory processes.     

Induction of B cell activities by interleukin 4 is inhibited by a receptor-specific monoclonal antibody in vitro

The effects of interleukin (IL) 4 on B cell growth and differentiation are mediated through binding of IL 4 to a specific cell surface receptor. The murine T cell IL 4 receptor (IL 4R) has recently been cloned and monoclonal antibodies (mAb) which bind specifically to the IL 4R have been developed. The ability of two of these anti-IL 4R mAb (M1 and M2) to inhibit IL 4-induced B cell functions in vitro was examined. The M1 mAb inhibited the ability of IL 4 to induce B cell proliferation in a dose-related fashion. The inhibition was specific for proliferation induced by IL 4 in that the antibody did not affect induction of proliferation by IL 1. Similarly, M1 inhibited IL 4-dependent B cell differentiation as measured by induction of IgG1 and IgE secretion, decreased IgG3 secretion, increased Ia expression, and increased Fc epsilon R (CD23) expression. In contrast, the anti-IL 4R-specific mAb M2 had no effect upon any of these activities. The ability of M1 but not M2 to inhibit IL 4-induced B cell growth and differentiation correlated with the inhibition of binding of radiolabeled IL 4 by M1. These reagents should be valuable tools with which to analyze the involvement of IL 4 in immune responses.     

Modulation of malignant B cell activation and apoptosis by bcl-2 antisense ODN and immunostimulatory CpG ODN

Inhibition of bcl-2 expression by antisense oligodeoxynucleotides (ODN) might render bcl-2 overexpressing malignant B cells more susceptible to chemotherapy. ODN containing unmethylated CG dinucleotides (CpG) are known to activate B cells. We studied the effects of two bcl-2 antisense ODN, with (G3139) or without CG dinucleotides (NOV 2009) within the sequence, and the effects of a nonantisense, CpG-containing ODN (ODN 2006) on activation and apoptosis of malignant B cell lines and primary B-CLL cells. Without cationic lipids, no antisense-mediated inhibition of bcl-2 synthesis was achieved with G3139 and NOV 2009. Instead, G3139, but not NOV 2009, induced similar changes as ODN 2006 in proliferation, expression of costimulatory and antigen-presenting molecules, as well as in bcl-2 and bcl-xL levels of primary B-CLL cells. G3139 and ODN 2006 inhibited in vitro, spontaneous apoptosis in B-CLL cells of patients with high serum thymidine kinase activity (s-TK, marker for proliferative activity of malignant B cells), whereas in patients with low s-TK activity, apoptosis was induced. In conclusion, our results suggest that modulation of malignant B cell apoptosis by G3139 depends on its immunostimulatory properties rather than on antisense-mediated reduction of bcl-2 expression. Immunostimulatory CpG ODN may have a therapeutic potential in patients with B-CLL, especially those with low s-TK activity.     

Prevention of B cell antigen receptor-induced apoptosis by ligation of CD40 occurs downstream of cell cycle regulation

Cross-linking of the B cell antigen receptor (BCR) on germinal center B cells can induce growth arrest and apoptosis, thereby eliminating potentially autoreactive B cells. Using the Burkitt lymphoma cell line Ramos as a model, we studied the commitment to apoptosis following growth arrest, as well as how triggering of CD40 or addition of tumor necrosis factor (TNF)-alpha can interfere to block cell death. Both BCR triggering and direct induction of growth arrest by sodium butyrate (n-But) caused hypophosphorylation of the retinoblastoma protein (pRb), followed by apoptosis. Interestingly, although CD40 ligation or TNF-alpha efficiently prevented BCR-induced and n-But-induced apoptosis, these co-stimuli did not inhibit, but rather augmented, growth arrest. Analysis of cell cycle regulators showed that each apoptotic and T(h) stimulus distinctly affected cyclins or cyclin-dependent kinase inhibitors, indicating that growth arrest can be uncoupled from apoptosis. BCR ligation and growth arrest activated the intrinsic or mitochondrial route of apoptosis. CD40 ligation and TNF-alpha prevented release of cytochrome c and activation of caspase-3, which could not be explained by effects on the expression of Bcl-2, Bcl-x(L) or Bax. Finally, the onset of BCR-induced apoptosis occurred after 10-12 h and addition of CD40 mAb or TNF-alpha at that point still prevented further execution of apoptosis. We conclude that in mature B cells apoptosis is not an obligatory event following growth arrest. Instead, commitment to apoptosis can be rapidly controlled by T cells via CD40 ligand and TNF-alpha, downstream of the pRb-regulated restriction point of the cell cycle, but prior to mitochondrial cytochrome c release.     

反义bcl-2硫代磷酸寡脱氧核苷酸对小细胞肺癌细胞株NCI-H446的抑制作用及诱导细胞凋亡的研究

目的:观察反义bcl-2硫代磷酸寡脱氧核苷酸(AS-PS-ODN)对小细胞肺癌细胞株NCI-H446mRNA、蛋白以及增殖、活力和凋亡的影响。方法: 合成bcl-2AS-PS-ODN作用于小细胞肺癌细胞株NCI-H446, 半定量RT-PCR检测bcl-2mRNA表达;免疫细胞化学染色和流式细胞仪检测Bcl-2蛋白表达;通过克隆形成率、细胞计数、流式细胞仪DNA倍体分析、TUNEL等指标观察bcl-2 AS-PS-ODN对细胞增殖、活力和凋亡的影响。结果:①bcl-2 AS-PS-ODN能特异性地降低NCI-H446细胞bcl-2mRNA和Bcl-2蛋白的表达。1μmol/LAS-PS-ODN作用24h后bcl-2mRNA表达量下降69.5%, 48h后Bcl-2蛋白下降62.7%。②bcl-2 AS-PS-ODN能够抑制细胞增殖和活力, 诱导细胞凋亡, 1μmol/LAS-PS-ODN作用24h后, 细胞凋亡率约为22.3%-32.7%。结论:bcl-2 AS-PS-ODN能够特异性降低bcl-2mRNA、蛋白表达, 抑制NCI-H446细胞的增殖和活力, 并诱导细胞凋亡。     

乳腺癌中凋亡与细胞增殖及Rb、bcl-2、c-myc蛋白表达的关系

目的：了解人乳腺癌中凋亡与细胞增殖的关系,以及与相关基因蛋白表达的关系及其预后意义。方法：用免疫组化ＬＳＡＢ法检测了９０例乳腺标本（包括１３例良性乳腺病变和７７例乳腺癌）中Ｒｂ、ｂｃｌ２和ｃｍｙｃ的蛋白表达;并计数了癌组织中的凋亡指数（ＡＩ,ＴＵＮＥＬ法）和有丝分裂指数（ＭＩ）。结果：ＡＩ与ＭＩ呈显著的正相关（ｒ＝０８１,Ｐ＜００１）;ＡＩ、ＭＩ和Ｒｂ蛋白表达与肿瘤大小和组织学分级有关,ＡＩ、ＭＩ低和ｂｃｌ２的高表达与５年生存率有关,ｃｍｙｃ的表达仅与组织学等级有关（Ｐ＜００５）;Ｒｂ表达与ＡＩ、ＭＩ均有关（Ｐ＜００１）,而ｂｃｌ２的表达仅与ＡＩ有关（Ｐ＜００５）。结论：乳腺癌中凋亡与细胞增殖和ｂｃｌ２表达有关,提示凋亡在具有不同生长潜能的亚群的克隆性选择中发挥重要作用。Ｒｂ与凋亡的关系提示细胞凋亡与细胞周期有关。     

慢性缺氧对大鼠肺内细胞增殖、凋亡及相关基因表达的影响

目的：探讨慢性缺氧性肺动脉高压肺血管结构改建（ｐｕｌｍｏｎａｒｙｖａｓｃｕｌａｒｓｔｒｕｃｔｕｒａｌｒｅｍｏｄｅｌｉｎｇ，ＰＶＳＲ）的细胞及分子机制。方法：大鼠慢性缺氧模型复制，免疫组化及Ｎｏｒｔｈｅｒｎ杂交检测大鼠肺内ｃ－ｍｙｃ及ｂｃｌ－２两基因蛋白及ｍＲＮＡ的表达，免疫组化及原位缺口末端标记法检测大鼠肺内增殖相关抗原Ｋｉ－６７及凋亡细胞。结果：正常及慢性缺氧大鼠肺内均检出一定比率的增殖或凋亡细胞，缺氧１、２周大鼠肺内细胞增殖／凋亡比值比对照组分别增高约３、３５倍。与细胞增殖、凋亡密切相关的ｃ－ｍｙｃ、ｂｃｌ－２基因表达在缺氧１周大鼠肺内显著增高，在缺氧２周更显著。结论：细胞增殖、凋亡机制共同参与了慢性缺氧性肺动脉高压时ＰＶＳＲ的调节。     

Functional interaction between B cell subpopulations defined by CD23 expression

Using the CD23 monoclonal antibody (mAb) MHM6 and sheep anti-mouse Ig bound to magnetic beads we have obtained highly purified populations of MHM6+ and MHM6- tonsil B cells. We have found that the increased expression of MHM6 reactivity seen on B cells after activation results from up-regulation of antigen on cells already weakly positive and not from expression of new antigen on the previously negative population. The strong proliferative responses of MHM6+ cells seen in the presence of anti-IgM (alpha mu) and interleukin 4 (IL4) or the CDw40 mAb G28-5, and with Staphylococcus aureus Cowan I (SAC), and to a lesser extent with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), resemble that seen among unfractionated B cells. In contrast, the MHM6- population cultured alone responds only weakly to alpha mu + G28-5 or SAC and exhibits virtually no response to alpha mu + IL4 or TPA. With all these mitogenic stimuli, tritiated thymidine uptake by the MHM6- population is augmented three- to sixfold by the addition of mitomycin C (MC)-treated MHM6+ cells. Pretreatment of cells with anti-leukocyte functional antigen 1 mAb has little effect on the subsequent proliferation of the MHM6- population but shows cell contact to be critical for the proliferation of MHM6+ cells. Such pretreatment has revealed that the functional interaction observed between MHM6+ and MHM6- cells is dependent on both cell contact and the presence of an MHM6+ cell-derived soluble component. We have found that addition of soluble CD23, purified from Epstein-Barr virus-transformed lymphoblastoid cell line supernatant, increases the proliferative response of MHM6- tonsil B cells to mitogenic stimuli in the presence of inactivated MHM6+ cells but has no effect on proliferation when MHM6+ cells are absent. By way of contrast to normal B lymphocytes, we have examined functional responses of prolymphocytic leukemia (PLL) B cells. Although these cells, when freshly isolated, show comparable levels of CD23 expression to normal B cells, this expression is not increased upon activation. In addition, in contrast to normal B cells, the PLL MHM6- population cultured alone shows a strong proliferative response to various mitogenic stimuli, comparable to that of MHM6+ or unfractionated cells, and this response is not augmented by the addition of MC-treated MHM6+ cells. Thus, a novel functional interaction is described between normal, but not leukemic, B cell populations defined by their expression of CD23.(ABSTRACT TRUNCATED AT 400 WORDS)     

Expression of CD23 antigen is not necessary for human 12-kDa B cell growth factor-mediated B cell proliferation

In an attempt to understand the relationship between the CD23 antigen and the human 12-kDa B cell growth factor (BCGF) receptor, we have undertaken studies to define the biological efficacy of 12-kDa-BCGF on CD23+ (Raji) and CD23- (P3HR1) human Burkitt B cell lines. Our results show that recombinant 12-kDa-BCGF can induce efficient [3H]thymidine incorporation and proliferation in cells of either phenotype. These results strongly suggest that a functional receptor for the 12-kDa-BCGF may exist independently of the CD23 molecule.     

Occipital foramina development involves localised regulation of mesenchyme proliferation and is independent of apoptosis

Cranial foramina are holes within the skull, formed during development, allowing entry and exit of blood vessels and nerves. Once formed they must remain open, due to the vital structures they contain, i.e. optic nerves, jugular vein, carotid artery, and other cranial nerves and blood vessels. Understanding cranial foramina development is essential as cranial malformations lead to the stenosis or complete closure of these structures, resulting in blindness, deafness, facial paralysis, raised intracranial pressure and lethality. Here we focus on describing early events in the formation of the jugular, carotid and hypoglossal cranial foramina that form in the mesoderm‐derived, endochondral occipital bones at the base of the embryonic chick skull. Whole‐mount skeletal staining of skulls indicates the appearance of these foramina from HH32/D7.5 onwards. Haematoxylin & eosin staining of sections shows that the intimately associated mesenchyme, neighbouring the contents of these cranial foramina, is initially very dense and gradually becomes sparser as development proceeds. Histological examination also revealed that these foramina initially contain relatively large‐diameter nerves, which later become refined, and are closely associated with the blood vessel, which they also innervate within the confines of the foramina. Interestingly cranial foramina in the base of the skull contain blood vessels lacking smooth muscle actin, which suggests these blood vessels belong to glomus body structures within the foramina. The blood vessel shape also appears to dictate the overall shape of the resulting foramina. We initially hypothesised that cranial foramina development could involve targeted proliferation and local apoptosis to cause ‘mesenchymal clearing’ and the creation of cavities in a mechanism similar to joint cavitation. We find that this is not the case, and propose that a mechanism reliant upon local nerve/blood vessel‐derived restriction of ossification may contribute to foramina formation during cranial development.     

Induction of cytolytic T lymphocytes by mitogenic lectins is specifically inhibited by anti-Lyt-2 antibodies

The effects of anti-Lyt-2 antibodies at different levels of lectin-induced cytotoxic T lymphocyte (CTL) responses have been analyzed. Induction of interleukin 2 (T cell growth factor) reactivity (“step 1”) by three different lectins was profoundly inhibited by anti-Lyt-2 antibodies. Also lectin-dependent cytotoxicity of polyclonally induced CTL was significantly inhibited by anti-Lyt-2 antibodies, whereas the interjacent IL2-dependent proliferation of “step 1”-induced CTL precursors was unaffected by these antibodies. The anti-Lyt-2-mediated inhibition was selective and specific to the target cell determinants and limited to the events in CTL responses which require interactions with the lectins. These results parallel those obtained in antigen-induced responses and indicate that antigens and lectins induce CTL responses by interacting with the same type of membrane receptor complex containing Lyt-2 molecules and antigen-specific receptors. It follows that T cell mitogenic lectins do not appear to deliver triggering signals through “mitogen” receptors.     

CD23/CD21 interaction is required for presentation of soluble protein antigen by lymphoblastoid B cell lines to specific CD4+ T cell clones

Previous studies have documented a role for membrane-bound CD23 (the low affinity FcεRII) in presentation of alloantigens by B cells. The aim of the present study was to examine the involvement of cell surface CD23 in presentation of more conventional soluble protein antigens to T cells. We show that antibodies to CD23 and to its lymphocyte-associated second ligand, CD21, inhibit presentation of the cow's milk allergen casein, by autologous CD23⁺CD21⁺ B-EBV cell lines to casein-specific HLA-DP-restricted CD4⁺ T cell clones obtained from patients with either reaginic or enterophatic forms of cow's milk protein intolerance. Maximal inhibition was achieved when the antibodies were added at the initiation of the culture. The absence of specific inhibition by an anti-DRα monoclonal antibody (mAb) argues against a steric hindrance phenomenon impeding access of the T cell receptor to major histocompatibility complex class II molecules. Rather, anti-CD23 and anti-CD21 mAb-induced inhibition of antigen presentation seems to affect at least partly, heterotypic conjugate formation through CD23/CD21 interaction. Double immunofluorescence labeling of the T cell clones and antibody inhibition of T/B conjugate formation shows that functional CD23 and CD21 molecules are induced on T cells following contact with B-EBV cell lines. Taken together, these data indicate that CD23/CD21 interactions between T and B cells are required for presentation of soluble protein antigens by B-EBV cell lines to specific CD4⁺ T cells. The potential implications of these findings for allergen-specific T cell activation are discussed.     

Induction of B cell costimulatory function by recombinant murine CD40 ligand

T cell-dependent regulation of B cell growth and differentiation involves an interaction between CD40, a B cell surface molecule, and the CD40 ligand (CD40L) which is expressed on activated CD4⁺ T cells. In the current study, we show that recombinant membrane-bound murine CD40L induces B cells to express costimulatory function for the proliferation of CD4⁺ Tcells. CD40L- or lipopolysaccharide (LPS)-activated, but not control-cultured B cells were strong costimulators of anti-CD3 or alloantigen-dependent T cell responses. The molecular interactions responsible for the increased costimulatory functions were examined by analyzing the activated B cells for changes in the expression of two costimulatory molecules, B7 and heat-stable antigen (HSA), as well as by the use of antagonists of B7 and HSA (CTLA4.Fc and 20C9, respectively). The expression of both B7 and HSA was enhanced on B cells activated with LPS. As observed in previous studies, the costimulatory activity of the LPS-activated B cells was dependent on both B7 and HSA and was completely inhibited in the presence of a combination of CTLA4.Fc and 20C9. In contrast, activation of B cells with CD40L induced the expression of B7 but did not enhance the expression of HSA. In addition the costimulatory activity of the CD40L-activated B cells was partially, but not completely, inhibited by the combination of CTLA4.Fc and 20C9. These results demonstrate that CD40L regulates costimulatory function of B cells in part by inducing the expression of B7 and suggest that CD40L-activated B cells express an additional costimulatory activity that is not associated with LPS-activated B cells.     

Role of c-myc and CD45 in spontaneous and anti-receptor-induced apoptosis in adult murine B cells

Although adult murine B cells can be stimulated to proliferateby IgM receptor cross-linking, we and others have shown thatthese cells will undergo apoptosis in vitro in a dose-, time-and temperature-dependent manner with polyclonal but not monoclonalantl-lgM. To test the role of c-myc and cell cycle progressionin B cell apoptosis, we examined normal, Sp6 antl-TNP Ig andEµ-myc transgenic splenocytes for receptor-mediated apoptosisin vitro. In normal mice, both spontaneous and anti-lgM-lnducedprogrammed cell death were specifically blocked by antisenseoligodeoxynucleotides for the c-myc proto-oncogene, whereasnonsense myc oligonucleotides and irrelevant oligonucleotideshad only a minor effect. Similarly, TNP-dextran-induced apoptosisin Sp6 antl-TNP transgenics was inhibited by antisense c-myc.This effect was not due to the mitogenic effects of unmethylatedCpG-contalnlng sequences because ones lacking this motif, aswell as methylated oligonucleotides containing this motif, preventedapoptosis, and mitogenic doses of lipopolysaccharlde failedto Inhibit anti-lgM-driven cell death. Importantly, antisensec-myc also prevented the anti-lgM-lnduced increase in myc proteinspecies. Moreover, spontaneous apoptosis in vitro was exaggeratedin Eµ-myc transgenic B cells. To examine the role of CD45in anti-lgM-induced apoptosis, we treated spleen cells fromCD45 knockout mice, which do not proliferate with anti-IgM,and found that B cells from these underwent apoptosis normallydespite the lack of entry into S. These data suggest that anti-IgMdriven apoptosis does not require CD45. Rather, apoptosis maybe due to an overexpression of myc protein in the absence ofsignals which can drive B cells productively Into S, but thefailure to proliferate normally is insufficient for apoptosisto occur.     

The expression of murine B cell CD23, in vivo, is regulated by its ligand, IgE

CD23, the low-affinity IgE receptor, is believed to participate in immune responses by mediating antigen capture for presentation by B cells and by shedding fragments with immunomodulatory properties. The number of CD23 molecules on B cells is increased during allergic responses and infection with helminths. This can be attributed in part to regulation of CD23 expression by cytokines, including IL-4. In addition, there is evidence that CD23 can be induced on cultured B cells by its ligand, IgE. In the current study we use IgE-deficient (IgE-/-) mice to establish the effects of IgE on CD23 expression by B cells in vivo, in the absence of allergic or parasitic stimuli. The spleens of IgE-/- and wild-type mice contained similar proportions of CD23+ B lymphocytes. However, cells from IgE-/- mice were found to have nearly 3-fold less CD23 on their surface. The mutant B cells had a corresponding defect in their ability to bind IgE. CD23 could be normally induced on IgE-/- B cells after culture with IL-4 or CD40 ligand, indicating that these cells had no inherent defect in CD23 biosynthesis. CD23 expression and IgE-binding capacity were both restored when splenocytes from IgE-/- mice were cultured in the presence of IgE. IgE-induced up-regulation of CD23 could be elicited in vivo as well. In IgE-/- mice, i.v. infusion of IgE corrected CD23 expression to wild-type levels. Our results demonstrate that IgE directly participates in CD23 regulation in vivo. This positive feedback loop may constitute a mechanism for the amplification of ongoing allergic responses.