小鼠p27Kip1基因的克隆及与外周B细胞凋亡和生存的关系

目的：克隆小鼠的p27^kip1基因，并以此为探针分析p27^kip1在外周血B细胞中的表达，以及其在抗原受体和CD40介导的抑制和促进增殖信号中的作用。方法：从培养的B细胞中提取总RNA，用RT-PCR扩增并克隆p27^kip1基因，用流式细胞仪检测B细胞凋亡，用Northern blot观察p27^kip1mRNA的表达水平。结果：成功地克隆了小鼠的p27^kip1基因，发现抗原受体交联在引起外周血B细胞凋亡之前，出现p27^kip1表达上调，若同时激活CD40则使p27^kip1表达降低，B细胞也免于凋亡用针对p27^kip1的反义寡核苷酸处理，可阻断抗原受体交联引起的外周血B细胞凋亡。结论：p27^kip1可能在抗原受体信号介导的B细胞凋亡和CD40信号介导的B细胞生存中发挥一定的作用。     

Co-stimulation by anti-immunoglobulin is required for B cell activation by CD40Llow T cells

During cognate B : T interactions, B cells encounter antigen (Ag) through surface immuno-globulin (sIg) and present antigenic peptides to T helper (Th) cells. However, most in vitro systems used to study contact events involved in the delivery of T help for B cells circumvent the requirement for T cell Ag specificity by using anti-CD3/T cell receptor (TcR) monoclonal antibodies (mAb) to activate T cells. To study the role of sIg engagement in the responsiveness of B cells to T help, we pre-treated small resting B cells with soluble anti-χ mAb prior to contact with an activated Th1 clone. By reducing the concentration of anti-TcR mAb we obtained low levels of CD40 ligand (CD40L^low) on Th cells, comparable to those expressed by lymph node T cells activated in vitro (ex vivo T cells). In contrast to untreated B cells, which did not respond to CD40L^low Th, anti-Ig-treated B cells responded strongly. Low buoyant density B cells also responded to CD40L^low Th cells. There was no B cell response to resting Th cells. mAb against CD54/intercellular adhesion molecule-1 or major histocompatibility complex (MHC) class II completely inhibited B cell responses to CD40L^low Th1 cells, equivalent to the effects of blocking CD40 interactions. This contrasts with mAb blocking responses to CD40L^highTh, where CD40 effects predominate. Our data show that sIg engagement is necessary for the induction of B cell response to CD40L^low Th cells. Anti-CD3-activated ex vivo T cells that were also CD40L^low did not provide help to small resting B cells, but did induce responses from sIg-stimulated B cells. Thus, our data support a requirement for sIg signaling in physiological B cell activation, and further confirm previous work showing CD40 ligation to be necessary but not sufficient for delivery of T help to B cells.     

Engagement of CD40 lowers the threshold for activation of resting B cells via antigen receptor

Cross-linking of surface Ig (sIg) on resting B cells can generate intracellular signals; however, for T-dependent antigens to promote growth and differentiation additional surface receptors must be engaged. Ligation of CD40 can stimulate B cell proliferation in the presence of interleukin-4. A recently identified counterstructure for CD40 is found on T helper cells and is believed to represent the cognate ligand for B cell activation. This study investigates the role of CD40 as an accessory molecule in sIg-dependent B cell activation. Simultaneous ligation of sIg and CD40 by monoclonal antibodies (mAb) in the presence of mouse L cells which express human Fey receptor type II (FcγRII-L cells) results in potent stimulation of small resting B cells. When CD40 is co-ligated, picomolar concentrations of mouse IgG1 anti-μ, and anti-δ mAb can stimulate B cell proliferation. This requires interaction of the anti-Ig mAb with the FcγRII-L cells: a mouse IgG2a anti-μ, mAb which is not recognized by FcγRII, was ≥ 1000-fold less effective. These findings suggest a mechanism for B cell activation whereby engagement of T cells via CD40 and its cognate ligand provides potent enhancement of signals delivered through sIg. Based on these observations, models for the activation of B cells by T-dependent antigens are presented.     

CD40 expression in HCV-associated chronic liver diseases

CD40 is expressed primarily on B cells and plays an important role in antigen presentation, B cell proliferation, and T cell activation. It has been reported that the CD40 signal modulates apoptosis and has an anti-viral effect in certain cells. Therefore, we investigated the expression and the function of CD40 in HCV-associated chronic liver disease. The expression of CD40 on liver tissues was determined through immunohistochemistry on 50 liver specimens obtained from HCV-positive patients. The effect of CD40 signaling on apoptosis of HepG2 cells was assessed using the MTT assay. The effect of CD40 stimulation on NF-kappaB activation was determined in NF-kappaB reporter gene-transfected HepG2 cells with the Luciferase assay. CD40 positive hepatocytes were observed in both periportal and lobular areas, accompanied by inflammation. In both areas, CD40 staining intensity became significantly stronger, correlating with the histological grading. Similarly, it became stronger with the progression of the histological staging in F1, F2 and F3 cases; however, the expression level decreased in F4 cases. CD40 ligation induced apoptosis in HepG2 cells in the presence of 500 ng/ml of actinomycin D, while CD40 ligation alone could not. Anti-CD40 monoclonal antibody caused NF-kappaB activation in HepG2 cells in a dose-dependent manner. These results suggest that hepatocyte over-expression of CD40 might play an important role in regulating hepatocyte survival and death in HCV-associated chronic liver diseases.     

Antigen-specific suppression of a primed immune response by dendritic cells mediated by regulatory T cells secreting interleukin-10

Antigen-specific suppression of a previously primed immune response is a major challenge for immunotherapy of autoimmune disease. RelB activation is required for myeloid DC differentiation. Here, we show that antigen-exposed DCs in which RelB function is inhibited lack cell surface CD40, prevent priming of immunity, and suppress previously primed immune responses. DCs generated from CD40-deficient mice similarly confer suppression. Regulatory CD4+ T cells induced by the DCs transfer antigen-specific "infectious" tolerance to primed recipients in an interleukin-10-dependent fashion. Thus CD40, regulated by RelB activity, determines the consequences of antigen presentation by myeloid DCs. These observations have significance for autoimmune immunotherapy and suggest a mechanism by which peripheral tolerance might be constitutively maintained by RelB(-) CD40(-) DCs.     

Potential role of CARMA1 in CD40-induced splenic B cell proliferation and marginal zone B cell maturation

NF-kappaB activation through B cell receptor (BCR) ligation is critical for B cell development, survival and antigen-mediated activation of B cells. CARD domain and MAGUK-domain containing protein-1 (CARMA1), recently identified adaptor molecule, has been shown to play an essential role in BCR-induced NF-kappaB activation. CARMA1-deficient B cells fail to proliferate upon BCR stimulation, leading to defective humoral responses. Surprisingly, CARMA1-deficient B cells are also defective in CD40-induced proliferation. The mechanisms responsible for CD40-induced proliferation defect have not yet been characterized. In this study, we show that signaling cascades activated by CD40 stimulation are largely unaffected in CARMA1-deficient B cells. Instead, we have found that the defective proliferation of CARMA1-deficient B cells is due to two events. First, CARMA1-deficient B cells show defective cell-cycle progression. Secondly, the numbers of marginal zone (MZ) B cells, which are the main responders upon CD40 stimulation, are greatly diminished in CARMA1-deficient mice. Since B cell maturation requires basal signaling through BCR and NF-kappaB activation, we propose that impaired BCR signaling in CARMA1-deficient mice leads to defective maturation of MZ B cell population, which in turn, contributes to impaired proliferation upon CD40 stimulation.     

The CD40-induced protection against CD95-mediated apoptosis is associated with a rapid upregulation of anti-apoptotic c-FLIP

CD95/Fas and CD40 receptors are important regulators of cell survival during germinal center reaction. In this study we used a human follicular lymphoma cell line, HF1A3, to study molecular mechanisms of CD95-mediated apoptosis and CD40-induced rescue from apoptosis. CD95 stimulation induced activation of caspase-8 and -3, collapse of mitochondrial membrane potential (DeltaPsim), release of cytochrome c and fragmentation of nuclear DNA. All these apoptotic events were abrogated, when cells were pretreated with CD40 antibodies before CD95 stimulation. CD40 induced a rapid up regulation of both short and long isoforms of c-FLIP, as these proteins were detectable 4h after receptor stimulation. The induction of c-FLIP as well as the anti-apoptotic function of CD40 was completely abolished when NF-kappaB activity was inhibited by a selective inhibitor PDTC. We conclude that the anti-apoptotic signaling of CD40 involves NF-kappaB-mediated induction of c-FLIP proteins which can interfere with caspase-8 activation. However, it remains to be seen whether c-FLIP proteins are the only one ones involved in CD40-mediated protection.     

Expression of functional activating and inhibitory Fcgamma receptors on human B cells

BACKGROUND: The CD32 (FcgammaII) receptor is involved in the regulation of the B cell response to antigen. The sole Fc receptor demonstrated in mice is the inhibitory FcgammaIIB receptor. Crosslinking this receptor does not lead to downstream signaling or cell activation. Instead, when immune complexes bind to Fcy on murine B cells, cell activation through the B cell antigen receptor is attenuated. The objective of this study was to evaluate the expression of the FcgammaII receptor and the response to immune complex stimulation in human B cells. METHODS: Human lymphoblastoid, peripheral and tonsillar B cells were stained with anti-CD32 antibodies IV.3 and 8.26 to determine the relative expression of the activating (FcgammaIIA) and inhibitory (FcgammaIIB) isoforms of CD32. Tetanus immune complexes were added to B cells and the activation of c-Jun amino-terminal kinase was assayed. RESULTS: Unlike murine cells, human B cells express high levels of the activating form of the Fcgamma receptor IIA. Addition of immune complexes to peripheral B cells resulted in signaling of Jun kinase, an important downstream kinase involved in the regulation of B cell function. The level of expression of FcgammaIIA on human B cells was not uniform, but depended on activation status. Peripheral blood B cells expressed high levels of FcgammaIIA, while tonsillar B cells predominantly expressed FcgammaIIB. Furthermore, when peripheral B cells were activated, the expression of FcgammaIIA relative to FcgammaIIB decreased. CONCLUSION: The response of human B cells to binding of immune complexes depends on the relative expression of activating (FcgammaIIA) versus inhibitory (FcgammaIIB) receptors.     

Changes in the Expression of Ig-Associated Proteins on B Lymphocytes Activated by Anti-IgM Antibodies

Binding of antigen to receptor complexes on B cells elicits a cascade of intracellular signalling events leading to proliferation and. together with T-cell help, Ig secretion. Components of the antigen receptor (AgR) complex have been demonstrated to be either covalenlly bound or associated with surface Ig (sIg) molecules. The function of these proteins is still unknown. In order to address this question, we have stimulated B cells with anti-μ antibodies and have studied possible changes in the expression of AgR complexes. After anti-μ stimulation, the IgM molecules disappeared rapidly from the cell surface together with the covalently bound proteins. The IgM molecules were internalized and probably degraded. The IgM-associated heterodimer Ig-α/Ig-β was also removed from the cells, leaving the IgD-associated heterodimer unaffected. Two proteins showed an enhanced association with slg after 15 min and then were gradually removed from the cell surface. Two other proteins became increasingly attached to sIg. This association remained stable for the rest of the culture period (up to 4 h). Further studies are underway to characterize these proteins more closely and to examine possible interactions with downstream members of the signalling cascade.     

Differential modulation of cyclin-dependent kinase inhibitor p27Kip1 by negative signaling via the antigen receptor of B cells and positive signaling via CD40

The cross-linking of surface immunoglobulins (sIg) of B cells can transmit a negative signal, resulting in cell cycle arrest, apoptosis or both. Signaling via the B cell antigen CD40 reverses the sIg-mediated negative signaling and induces activation and proliferation of B cells. We investigated the molecular mechanisms for cell cycle regulation by negative and positive signaling via sIg and CD40, respectively, by using the B cell line WEHI-231. Cross-linking of sIg almost completely reduced the activity of cyclin-dependent kinase (Cdk) 2, essential for cell cycle progression in the late G1 phase, although the level of Cdk2 was not reduced. Among the factors that regulate Cdk2 activation, the activity of the Cdk-activating kinase (CAK) appeared intact and cyclin E was reduced only partially in sIg-cross-linked WEHI-231. In contrast, sIg cross-linking induced a significant Cdk inhibitor (CKI) activity. Since a 27-kDa protein was co-precipitated with Cdk2 in anti-Ig-treated, but not untreated WEHI-231, and the CKI activity in anti-Ig-treated WEHI-231 was neutralized by anti-p27^Kip1, antibodies, it is most likely that p27^Kip1 is responsible for the CKI activity induced by sIg cross-linking. p27^Kip1 may thus play a role in growth inhibition of B cells by negative signaling via sIg. In contrast, CD40 signaling enhanced Cdk2 activity and reduced the p27^Kip1 level in anti-Ig-treated WEHI-231, suggesting that the reduction of p27^Kip1 plays an important role in the abrogation of sIg-mediated growth arrest by CD40 signaling. Taken together, p27^Kip1 is likely to be a crucial target molecule of the negative signaling via sIg and the positive signaling via CD40 essential for T cell-dependent immune responses.     

Impaired B cell maturation in mice lacking Bruton's tyrosine kinase (Btk) and CD40

Mutations in Bruton's tyrosine kinase (Btk) gene, in mice, result in reduced numbers and responses of peripheral B cells. Surface Ig- mediated signaling is defective in Btk mutant B cells as they do not proliferate upon slg cross-linking and lack thymus-independent (TI) type II responses. Signals through sIg and CD40 play a critical role in B cell maturation. To investigate the consequences of the lack of both Btk and CD40 on B cell development and function, mice were generated that were homozygous for targeted mutations in the Btk and the CD40 genes (BtkMCD40M). The CD40 mutation (CD40M) had a synergistic effect on the BtkM defects. In BtkMCD40M mice the number of B cells was reduced 3- to 4-fold compared to BtkM mice and mature B cells (IgMlow/IgDhigh) were virtually absent; serum levels of all Ig isotypes were diminished; and antibody responses to TI-I TI-II and thymus- dependent antigens were impaired. Furthermore, although wild-type BtkM and CD40M mice produced germinal centers in response to TI-I antigen, the BtkMCD40M mice did not. Maturational and functional B cell defects in BtkMCD40M mice may result from a combination of intrinsic B cell defects, lack of CD40L-dependent T cell help and microenvironmental defects. These data suggest that signals through Btk and CD40 are necessary for the production and maintenance of the mature B cell.      

Distinct regulation of CD40-mediated interleukin-6 and interleukin-12 productions via mitogen-activated protein kinase and nuclear factor kappaB-inducing kinase in mature dendritic cells

The role of mitogen-activated protein kinase (MAPK) and nuclear factor kappaB (NF-kappaB) pathways, especially NF-kappaB-inducing kinase (NIK)-mediated alternative pathway, in CD40-mediated interleukin (IL)-6 and IL-12 productions by immature or mature dendritic cells (DCs) was investigated. Murine myeloid DCs were matured by treatment with lipopolysaccharide. CD40 ligation induced modest or vigorous cytokine productions in immature or mature DCs, respectively. After CD40 ligation, p38 MAPK was significantly activated in either immature or mature DCs. SB203580, a p38 MAPK inhibitor, markedly decreased CD40-mediated IL-6 and IL-12 productions in immature DCs. In mature DCs, SB203580 significantly decreased CD40-mediated IL-6 but not IL-12 production. On the other hand, CD40 ligation induced vigorous activation of the NF-kappaB alternative pathway including p100 phosphorylation and subsequent nuclear translocations of p52, a processed form of p100, and RelB in mature but not immature DCs. The CD40-mediated phosphorylation of p100 was completely abolished in NIK-mutated mature DCs. The NIK mutation markedly reduced CD40-mediated IL-12 but not IL-6 production by mature DCs. Taken together, we concluded that IL-6 and IL-12 productions in response to CD40 ligation were controlled by p38 MAPK and NIK mediated alternative pathway, respectively, in mature DCs.     

Antigen receptor-induced apoptosis of human germinal center B cells is targeted to a centrocytic subset

The outcome of the signals transduced through the B cell antigen receptor (BCR) depends both on their maturational stage and on the extent of receptor cross-linking. It is established that the BCR-mediated apoptosis of immature B cells represents an important mechanism for tolerance induction in the pre-immune B cell compartment. We show here that mature germinal center (GC) B cells can re-acquire sensitivity to BCR-induced cell death following CD40 ligation. In contrast, neither virgin nor memory B cells become susceptible to antigen receptor-triggered apoptosis upon CD40 stimulation, suggesting that this phenomenon may play a role in the shaping of the mature B cell repertoire in GC. Our data reveal that the death signal evoked through the BCR does not involve the Fcγ receptors, does not operate through the Fas/Fas ligand system, and can be blocked by interleukin-4. Finally, we found that the acquisition of sensitivity to the death-promoting effect of anti-Ig antibodies in CD40-stimulated GC B cell cultures correlates with the induction of a centrocytic phenotype. We propose that negative regulatory signals via the BCR may delete somatically mutated centrocytes with self-reactivity.     

Expression of CD40 and CD43 during Activation of Human B Lymphocytes

CD40 and CD43 are two cell-surface glycoproteins that appear to be functionally involved in the growth stimulation of human B cells. Whereas CD40 is structurally similar to the NGF receptor and is present on all resting B cells, CD43 displays no homology to other known proteins and is expressed only on a subpopulation of these cells. To further understand the extra- and intracellular signals regulating these molecules and in which stage of activation they may play a role, we used various activation strategies and studied their expression on tonsillar B cells. As expected, activation of protein kinase C by TPA increased both CD40 and CD43. In contrast, a rise in intracellular Ca2+, e.g. by ionomycin, did not influence the expression of these antigens. However, in the presence of TPA, ionomycin further up-regulated CD43 but not CD40. Anti-IgM behaved similarly to ionomycin suggesting that the effect of this reagent was due primarily to its ability to increase intracellular Ca2+. Of three interleukins (IL-2, IL-4 and IL-6) only IL-4 had a significant effect when used alone in that it up-regulated CD40 but not CD43. However, in the presence of anti-IgM, both IL-2 and IL-4 synergistically up-regulated the two antigens. Complementation of antigen receptor stimulation with TPA or IL-4 increased CD40 during the first 24 h, whereas up-regulation of CD43 did not occur until 24 to 48 h after stimulation. With regard both to up-regulation in response to different stimuli and to kinetics, CD40 expression paralleled that of the early activation antigen CD23, whereas CD43 was induced in parallel with the transferrin receptor (CD71). Taken together, our results suggests that the expression of CD40 and CD43 is regulated by different intracellular signals and that CD40 may be important during early activation, whereas CD43 may have its major function during later stages of B-cell differentiation. These assumptions are in line with the observations that CD40 antibodies can directly activate resting B cells and that CD43 are retained on plasma cells.