CD 19 function in central and peripheral B-cell development

Although the B-cell antigen receptor (BCR) factors most prominently in the maintenance and differentiation of mature B cells, it is now appreciated that co-receptor molecules can positively or negatively modulate signals through the BCR. Co-receptors are functionally defined as modifiers of BCR engagement and signal transuction, and are distinct from other accessory molecules that act independently to regulate B-cell growth. The co-receptor CD19 functions to augment signals by the pre-BCR/BCR and in doing so can modulate B-cell fate decisions at multiple stages of development. In mature B cells, CD 19 also associates with complement receptor 2 (CR2/CD21) and is privotal for transducing signals inducdd by co-recognition of complement C3d-fixed antigens by the BCR and CD21. In this article, we focus on recent progress in the understanding of CD 19 function through the characterization of mouse models that relate in vivo function to biochemical properties of CD 19.     

CD40L: a multi-functional ligand

B cells can be stimulated to proliferate and differentiate in response to cell-contact dependent signals provided by activated, but not resting, T cells. In the human system, antibodies specific for the surface antigen CD40 induce similar B cell responses. The cloning of a ligand for CD40, and the generation of reagents which can block the interaction of this ligand with its receptor, have demonstrated that the major component of the contact-dependent signal leading to B cell activation is CD40 ligand. Studies of individuals lacking functional CD40 ligand have indicated that signaling through CD40 is essential for immunoglobulin (Ig) heavy chain switching and the production of all isotypes other than IgM. In addition to its activities on B cells, CD40 ligand is stimulatory for cells of monocyte and T lineages suggesting a pleiotropic role for CD40 ligand in vivo.     

Helper effector function of human T cells stimulated by anti-CD3 mAb can be enhanced by co-stimulatory signals and is partially dependent on CD40-CD40 ligand interaction

In this study we have investigated whether anti-CD3-induced human T cell help for immunoglobulin production could be enhanced by co-stimulation of the T cells via other T cell surface molecules, and the contribution of CD40-CD40 ligand interaction to the execution of T helper effector function induced by these different stimulatory signals. In a system in which irradiated tonsillar T cells were stimulated with immobilized anti-CD3 monoclonal antibody (mAb), it was found that ligation of CD2 with a mitogenic pair of mAb considerably enhanced anti-CD3-induced T cell help for immunoglobulin production. Likewise, ligation of CD28 with mAb enhanced T helper activity, although to a lesser extent. Upon addition of anti-CD28 and anti-CD2 mAb together, an even higher immunoglobulin production was observed. This combination resulted in a four- to fivefold increase in immunoglobulin production as compared to cultures in which T cells were stimulated with anti-CD3 mAb alone. The effect of ligation with B7, the natural ligand of CD28, was studied in a system which utilizes the presentation of anti-CD3 mAb on human FcγRII-expressing mouse fibroblasts which were co-transfected with human B7. It appeared that B7 could stimulate help for immunoglobulin production much more efficiently than ligation of CD28 with mAb did. Physical separation of B cells from T cells led to complete abrogation of immunoglobulin production. Blocking of CD40 with specific mAb, which have no intrinsic B cell stimulatory properties, or the CD40 ligand with a soluble CD40-human IgM fusion protein, resulted in dose-dependent, but only partial, inhibition of T cell-dependent immunoglobulin production with all modes of T cell activation tested. A clear correlation was found between the induction of CD40 ligand expression on the T cells by the different modes of co-stimulation and subsequent immunoglobulin production by the B cells. It is concluded that ligation of CD28 and/or CTLA-4, and of CD2 can generate co-stimulatory signals for T cell help for immunoglobulin production, which was found to be only partially dependent on the CD40-CD40 ligand interaction.     

B-cell-activating factor inhibits CD20-mediated and B-cell receptor-mediated apoptosis in human B cells

B-cell-activating factor (BAFF) is a survival and maturation factor for B cells belonging to the tumour necrosis factor superfamily. Among three identified functional receptors, the BAFF receptor (BAFF-R) is thought to be responsible for the effect of BAFF on B cells though details of how remain unclear. We determined that a hairy-cell leukaemia line, MLMA, expressed a relatively high level of BAFF-R and was susceptible to apoptosis mediated by either CD20 or B-cell antigen receptor (BCR). Using MLMA cells as an in vitro model of mature B cells, we found that treatment with BAFF could inhibit apoptosis mediated by both CD20 and BCR. We also observed, using immunoblot analysis and microarray analysis, that BAFF treatment induced activation of nuclear factor-kappaB2 following elevation of the expression level of Bcl-2, which may be involved in the molecular mechanism of BAFF-mediated inhibition of apoptosis. Interestingly, BAFF treatment was also found to induce the expression of a series of genes, such as that for CD40, related to cell survival, suggesting the involvement of a multiple mechanism in the BAFF-mediated anti-apoptotic effect. MLMA cells should provide a model for investigating the molecular basis of the effect of BAFF on B cells in vitro and will help to elucidate how B cells survive in the immune system in which BAFF-mediated signalling is involved.     

Distinct effect of CD40 and TNF-signaling on the chemokine/chemokine receptor expression and function of the human monocyte-derived dendritic cells

A key and limiting step in the process of human monocyte-derived dendritic cells （mDCs） for clinical use is their in vitro maturation and in vivo migration. We previously observed that CD40 signal facilitated human mDC growth and maturation. To further explore this process, mDCs generated with GM-CSF and IL-4 were co-cultured with apoptotic tumor cells for 24 hours, followed by incubating with anti-CD40 monoclonal antibody or TNF-a for 48 hours to generate mature DCs. The chemokine/chemokine receptor expression and functions of mature DCs upon various stimuli were determined. The expression of costimulatory molecules on apoptotic tumor cell-loaded mature DCs co-cultured with either anti-CD40 antibody （anti-CD40-DCs） or TNF-a （TNF-DCs） were up-regulated compared to immature DCs, consistent with the abilities of these cytokine to drive DC maturation in vitro. The mRNA levels of chemokines such as stromal cell-derived factor-1a （SDF-1a）, EBV-induced molecule 1 ligand chemokine （ELC）, and IFN inducible protein-10 （IP-10） in anti-CD40 activated DCs were increased and the dendritic cell-specific chemokine 1 （DC-CK1） was moderately up-regulated as compared with other mature DCs. The corresponding chemokine receptors CXCR4 and CCR7 of anti-CD40-DCs were significantly expressed. The CXCR3 expression on activated T cells stimulated by anti-CD40-DCs was also increased. Moreover, the anti-CD40-DCs had a stronger ability to stimulate T cell proliferation than any other DCs. The NF-xB activity was much higher in anti-CD40-DCs than that of TNF-DCs. These results offer further evidence of the importance of the CD40 signal in developing efficient human DC vaccines for cancer immune therapy. Cellular ＆ Molecular Immunology.     

B-cell stage and context-dependent requirements for survival signals from BAFF and the B-cell receptor

Summary: One remarkable feature of the immune system is its capacity to maintain constant numbers of resting immune cells despite the complex nature of signals needed throughout development and maturation. For many years, B-cell survival was thought to rely solely on B-cell receptor (BCR) tonic signals that would trigger necessary basal survival pathways. The discovery of the tumor necrosis factor (TNF)-like ligand BAFF(B-cell activating factor belonging to the TNF family)/BLyS (B-lymphocyte stimulator) changed these views entirely, as BAFF-deficient mice lack most mature B cells, and treatment with BAFF inhibitors leads to their loss, establishing BAFF as an unappreciated key B-cell survival factor. BAFF-mediated survival signals have been mapped and signaling crosstalk with the BCR has been identified, explaining the need for both BCR- and BAFF-mediated signals for B-cell survival. However, this crosstalk only explains how BCR and BAFF signals cooperate to produce survival proteins and yet, inactivating pro-apoptotic factors such as FOXO proteins, which may be managed separately by BAFF and the BCR, has emerged as an equally important step for survival. In this review, we present new views on B-cell survival, at all stages of B-cell life, and suggest that, in most cases, survival results from the production of appropriate survival factors balanced with the adequate and timely degradation of pro-apoptotic proteins.     

CD22 and Siglec-G: B-cell inhibitory receptors with distinct functions

Summary:  Siglecs (sialic acid-binding immunoglobulin-like lectins) are sialic acid-binding proteins, which are expressed on many cell types of the immune system. B cells express two members of the Siglec family, CD22 (Siglec-2) and Siglec-G, both of which have been shown to inhibit B-cell signaling. CD22 recruits the tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) to immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and inhibits B-cell receptor (BCR)-induced Ca²⁺ signaling on normal B cells. CD22 interacts specifically with ligands carrying α2–6-linked sialic acids. Interaction with these ligands in cis regulates the association of CD22 with the BCR and thereby modulates the inhibitory function of CD22. Interaction of CD22 to ligands in trans can regulate both B-cell migration as well as the BCR signaling threshold. Siglec-G is a recently identified protein with an inhibitory function restricted to a B-cell subset, the B1 cells. Siglec-G inhibits Ca²⁺ signaling specifically in these cells. In addition, it controls the cellular expansion and antibody secretion of B1 cells. Thus, both Siglecs modulate BCR signaling on different B-cell populations in a mutually exclusive fashion.     

Is the activated T helper stimulus able to induce Ig isotype switching in cultured human B cells?

It Is confirmed that large amounts of IgM, IgG, and IgA areproduced when human B cells are cultured with T cells activatedby immobilized CD3 antibody (CD3 system). IL-2 was essential;lowerlevels of Ig production with different isotype ratios were obtainedif IL-4 or IL-6 replaced IL-2. Depletion of sIgG⁺ or sIgA⁺ cellsfrom the B population to be cultured markedly reduced productionof IgG or IgA. Culturesof B cells selected with the pan-B markersCD19, CD72, or CD21 contained similar levels of Ig of all threeisotypes, whereas B cells selected for sIgM or sIgD expressionproduced IgM but very little IgG or IgA indicating that littleisotype switching was occurring. Production of IgG or IgA fromcells expressing these isotypes was more efficient than productionof IgM from IgM⁺ IgD⁺ cells. These results are considered inthe light of the demonstration by others of the production ofmultiple isotypes from single sIgM⁺-selected B cells. Clonedhuman T cells from a single donor induced production of allthree isotypes, but the proportions varied indicating that thepotent T-B cell interactions inducing B cell activation mayoverride and conceal the operation of isotype specific cellinteractions. Some T clones used at an optimal dose were aseffective untreated as X-irradiated, whereas with other clonesmaximumIg production was not achieved without irradiation.     

B7.2-Ig fusion proteins enhance IL-4-dependent differentiation of tumor-sensitized CD8+ cytotoxic T lymphocyte precursors

The B7/CD28 co-stimulatory pathway plays a critical role in T cell activation and differentiation. Our previous study demonstrated that administration of B7.2-Ig fusion proteins to tumor-bearing mice elicits IL-4-dependent, CD8+ T cell-mediated tumor regression. Here, we investigated whether B7.2-Ig stimulation of tumor-sensitized CD8+ CTL precursors during in vitro antigen re-sensitization actually results in their differentiation into mature CTLs and if so, whether such a process depends on IL-4 signals. Splenocytes from tumor-sensitized (tumor-bearing or tumor-immunized) mice exhibited low levels of anti-tumor CTL responses upon culturing alone, but induced strikingly enhanced CTL responses when stimulated in vitro with B7.2-Ig fusion proteins. Because CTLs were not generated from normal splenocytes even by B7.2-Ig stimulation, the expression of the B7.2-Ig effect required the in vivo tumor sensitization of CD8+ CTL precursors. Administration of anti-CD4 or anti-CD40 ligand (CD40L) to mice before tumor sensitization resulted in almost complete inhibition of CTL responses generated in the subsequent culture containing B7.2-Ig. In contrast, anti-IL-4 did not influence in vivo tumor sensitization required for CTL induction. However, B7.2-Ig stimulation of tumor-sensitized splenocytes enhanced IL-4 production and neutralization of this IL-4 with anti-IL-4 potently down-regulated CTL responses. These results indicate that B7.2-Ig enhances IL-4-dependent differentiation of anti-tumor CD8+ CTL precursors that can be sensitized in vivo depending on collaboration with CD4+ T cells involving CD40L function.     

Synergistic antibody induction by antigen-CD40 ligand fusion protein as improved immunogen

Full scale B-cell activation requires not only B-cell receptor (BCR) engagement with antigen, but also costimulatory signals provided by T helper cells through the CD40-CD40 ligand (CD40L) interaction. It is hypothesized that a fusion protein of an antigen and soluble CD40L (CD40LT) would selectively target the costimulation to antigen-specific B cells, leading to synergy in the antibody response. This hypothesis was investigated by fusing green fluorescence protein (GFP), a generic antigen, with mouse CD40LT. Studies revealed that immunization in mice with the plasmid encoding GFP-CD40LT fusion protein led to synergistic induction of GFP-specific antibodies, while control plasmid(s) for GFP, CD40LT, or GFP plus CD40LT did not. Immunization with a single dose of the fusion protein also provoked a vigorous GFP-specific immunoglobulin G1 antibody response, but not other antibody isotypes. These results suggest that GFP-CD40LT fusion protein induces a GFP-specific B-cell activation and antibody response in an antigen-guided fashion. The potential application of this novel strategy in vaccine development is discussed.     

Humoral immune responses in humanized BLT mice immunized with West Nile virus and HIV-1 envelope proteins are largely mediated via human CD5+ B cells

BLT mice, constructed by surgical implantation of human fetal thymus-liver tissues and intravenous delivery of autologous CD34+ haematopoietic stem cells into adult non-obese diabetic/severe combined immunodeficiency mice, were evaluated for vaccine-induced humoral immune responses. Following engraftment, these mice developed a human lymphoid system; however, the majority of the peripheral human B lymphocytes displayed an immature phenotype as evidenced by surface CD10 expression. Over 50% of the human B cells in the periphery but not in the bone marrow also expressed the CD5 antigen, which is found only infrequently on mature follicular B cells in humans. A single intramuscular immunization with recombinant viral envelope antigens, e.g., HIVgp140 and West Nile Virus envelope proteins, together with the immune stimulatory KLK/ODN1a composition) [corrected] adjuvant resulted in seroconversion characterized by antigen-specific human antibodies predominantly of the IgM isotype. However, repeated booster immunizations did not induce secondary immune responses as evidenced by the lack of class switching and specific IgM levels remaining relatively unchanged. Interestingly, the peripheral CD19+ CD5+ but not the CD19+ CD5- human B lymphocytes displayed a late developing CD27+ IgM+ memory phenotype, suggesting that the CD5+ B-cell subset, previously implicated in 'natural antibody' production, may play a role in the vaccine-induced antibody response. Furthermore, human T lymphocytes from these mice demonstrated suboptimal proliferative responses and loss of co-stimulatory surface proteins ex vivo that could be partially reversed with human interleukin-2 and interleukin-7. Therefore, vaccine-induced immune responses in BLT mice resemble a T-cell-independent pathway that can potentially be modulated in vivo by the exogenous delivery of human cytokines/growth factors.     

BAFF promotes regulatory T-cell apoptosis and blocks cytokine production by activating B cells in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic and slowly progressive cholestatic liver disease of autoimmune etiology. A number of questions regarding its etiology are unclear. CD4+CD25+ regulatory T cells (Tregs) play a critical role in self-tolerance and, for unknown reasons, their relative number is reduced in PBC patients. B-cell-activating factor (BAFF) is a key survival factor during B-cell maturation and its concentration is increased in peripheral blood of PBC patients. It has been reported that activated B cells inhibit Treg cell proliferation and there are no BAFF receptors on Tregs. Therefore, we speculated that excessive BAFF may result in Treg reduction via B cells. To prove our hypothesis, we isolated Tregs and B cells from PBC and healthy donors. BAFF and IgM concentrations were then analyzed by ELISA and CD40, CD80, CD86, IL-10, and TGF-β expression in B cells and Tregs were measured by flow cytometry. BAFF up-regulated CD40, CD80, CD86, and IgM expression in B cells. However, BAFF had no direct effect on Treg cell apoptosis and cytokine secretion. Nonetheless, we observed that BAFF-activated B cells could induce Treg cell apoptosis and reduce IL-10 and TGF-β expression. We also showed that BAFF-activated CD4+ T cells had no effect on Treg apoptosis. Furthermore, we verified that bezafibrate, a hypolipidemic drug, can inhibit BAFF-induced Treg cell apoptosis. In conclusion, BAFF promotes Treg cell apoptosis and inhibits cytokine production by activating B cells in PBC patients. The results of this study suggest that inhibition of BAFF activation is a strategy for PBC treatment.     

Interaction of B cells with activated T cells reduces the threshold for CD40-mediated B cell activation

CD154-CD40 interactions are of central importance for the induction of antibody responses to T-dependent antigens. Since most anti-CD40 mAb are only weak B cell mitogens, it is believed that under physiological conditions, signals through CD40 synergize with those from other receptors on B cells to induce B cell activation. We show here that the interaction of either normal B cells, or those from CBA/N (xid) mice, with CD3-activated primary T cells in whole spleen cell cultures markedly reduces the threshold for B cell activation via CD40. Hence, these pre-activated cells undergo vigorous proliferation when stimulated with either optimal or suboptimal concentrations of weakly mitogenic anti-CD40 mAb, or with soluble CD40 ligand. Blocking experiments indicate that the establishment of this priming effect requires stimulation via CD40 itself, plus T cell-derived IL-2. In support of this concept, only CD3/CD28-pre-activated, but not CD3-pre-activated T cells induce this effect, unless the co-cultures of B cells with the latter T cells are supplemented with IL-2. Although B cells activated in this fashion do express higher levels of CD40 than naive cells, we believe that this is insufficient to explain the observed dramatic effects on their proliferative capacity. Rather we propose that T cell-dependent B cell activation induces fundamental changes in the signalling machinery invoked by ligation of CD40. It is likely that this amplification loop could play an important role during the initiation of antibody responses to T-dependent antigens, when activated CD4 T cells only express low levels of CD154.     

CD40 ligand is a T cell growth factor

CD40 ligand (CD40L) is a 33-kDa type II membrane glycoprotein induced on T cells upon activation. CD40L has previously been shown to induce proliferation of resting B cells, immunoglobulin (Ig) secretion from B cells cultured with cytokines and cytokine secretion and tumoricidal activity from monocytes. In this report CD40L is shown to be stimulatory for human T cells, inducing CD25 (p55 IL-2R) and CD40L expression on resting peripheral blood T cells, enhanced expression of these molecules and CD69 on CD3-activated cells and secretion of interferon-y, tumor necrosis factor-a and interleukin (IL)-2 from T cells cultured in the presence of a sub-mitogenic concentration of phytohemagglutinin A (PHA). Furthermore, stimulation with CD40L induces proliferation of CD3- or PHA-activated T cells of blood, tonsillar or thymic origin. A similar proliferative response is observed with CD4^? and CD8⁺ T cells and this effect is largely IL-2 independent. A soluble construct of the extracellular domain of the CD40L has similar activity to that of membrane-expressed ligand in the induction of T cell surface antigens and proliferation. The results presented here taken together with the various activities ascribed for CD40L on B cells and monocytes demonstrate that CD40L has pleiotropic biological activity for cells of the hemopoietic lineage.     

Maturation of CD4-8- alpha/beta TCR+ T cells induced by CD2-stimulation in vivo and in vitro.

Newly generated ('virgin') rat thymocytes of the immature CD4+8+ double positive (DP) subset were treated in suspension culture for 2 days with the stimulatory pair of anti-CD2 monoclonal antibodies OX-54 and OX-55. Approximately 50% of the recovered cells had downregulated CD4 and CD8 and upregulated the T cell antigen receptor (TCR). CD2-stimulated, but not control thymocytes proliferated in response to TCR plus IL-2 stimulation. In vivo, postnatal injection of OX-54/55 led to a dramatic and selective increase in functionally mature CD4-CD8- double negative (DN) alpha/beta--TCR(high) thymocytes and peripheral T cells. These findings show that CD2 stimulation can promote T cell differentiation and suggest that DN TCR(high) thymocytes can be generated from DP thymocytes via alternative pathways of T cell maturation.     

Defective integration of activating signals derived from the T cell receptor (TCR) and costimulatory molecules in both CD4+ and CD8+ T lymphocytes of common variable immunodeficiency (CVID) patients

CVID is characterized by hypogammaglobulinaemia and impaired antibody production. Previous studies demonstrated defects at the T cell level. In the present study the response of purified CD4⁺ and CD8⁺ T lymphocytes to stimulation with anti-TCR monoclonal antibody (the first signal) in combination with anti-CD4 or anti-CD8, anti-CD2 and anti-CD28 MoAbs (the costimulatory signals) was investigated. Both CD4⁺ and CD8⁺ T cells from the patients showed significantly reduced IL-2 release following stimulation via TCR and costimulation via CD4 or CD8 and CD2, respectively. However, normal IL-2 production following TCR plus phorbol myristate acetate (PMA) costimulation and normal expression of an early activation marker, CD69, after TCR + CD28 stimulation indicated that TCR was able to transduce a signal. Furthermore, both IL-2 and IL-4 release were impaired in CD4⁺ lymphocytes following TCR + CD28 stimulation. In addition, stimulation via TCR + CD28 resulted in significantly decreased expression of CD40 ligand in the patients. These results suggest that the integration of activating signals derived from the TCR and costimulatory molecules is defective in CVID patients; the defect is not confined to costimulation via a single molecule, or restricted to cells producing Thl-type cytokines such as IL-2, and is expressed in both CD4⁺ and CD8⁺T cell subsets.     

Effect of B-cell receptor engagement on CD40-stimulated B cells.

Activation of human B cells in vitro either by cross-linking of surface immunoglobulins (sIg) or by triggering CD40 antigen, in the presence of interleukin-10 (IL-10) and interleukin-2 (IL-2), may result in high levels of immunoglobulin secretion in vitro. We studied the combined effects of ligation of the B-cell receptor (BCR) and CD40 [with anti-CD40 monoclonal antibody (mAb)] on B-cell proliferation and production of human immunoglobulin. For this purpose highly purified splenic B cells were cultured with various combinations of anti-CD40 and IL-10/IL-2 or IL-4 in the presence of CD32-transfected L cells. Simultaneous cross-linking of the BCR was achieved by mAb held on CD32-L cells or Staphylococcus aureus (SA). We found that dual BCR and CD40 ligation with IL-10/IL-2 leads to reduced immunoglobulin G (IgG) secretion compared with B cells stimulated with either anti-CD40 and IL-10/IL-2, or compared with B cells stimulated with SA or anti-BCR mAb and IL-10/IL-2. Dual BCR and CD40 ligation with anti-immunoglobulin mAb (anti-kappa + anti-lambda light chains) but not with SA induced a similar reduction in IgM production. The reduced immunoglobulin secretion found during dual ligation is accompanied by increased proliferation. This was independent of cytokine stimulation but SA/CD40-induced proliferation was increased in the presence of IL-10/IL-2, although not with IL-4. The combination anti-kappa and anti-lambda with anti-CD40 showed a long-term suppression of IgG and IgM production (at least 14 days), while anti-kappa or anti-lambda alone, or SA, allowed a moderate recovery of immunoglobulin production by day 14. These results suggest that simultaneous B-cell antigen receptor cross-linking and CD40 engagement via CD40L on T cells induces strong initial proliferation. This may be followed later by antibody production depending on the strength of the BCR signal and the presence of the appropriate cytokines.     

CD40 cross-linking inhibits specific antibody production by human B cells

Ligation of CD40 on B cells is a co-stimulatory signal for proliferation,antibody secretion, heavy chain switching and rescue from apoptosisafter somatic mutation in the germinal centre. The importanceof these manifold responses to CD40 activation for humoral immunityis exemplified by the inability of boys with X-linked hyperIgM syndrome to make IgG, IgE or IgA due to a mutation in inthe gene coding for CD40 ligand (CD40L). In the present study,we have investigated the effect of CD40 ligation on specificantibody production by human B cells to influenza virus. Theantibody. response was T cell dependent and specific for thestrain of influenza virus used as antigen. Addition of eitherCD40 mAb or recombinant trimeric CD40L profoundly inhibitedspecific antibody production. Antibody production by unseparatedtonsillar mononuclear cells and by T-depleted B cells stimulatedwith antigen in the presence of T cell replacing factor wereequally inhibited with CD40 antibody showing that the effectwas due to ligation of CD40 on B cells rather than blockingof T cell help. The specific antibody detected in these experimentswas mostly IgG with little or no IgM and was obtained from surfaceIgM B cells consistent with activation of a secondary (memoryresponse. Co-stimulation of tonsillar B cells with CD40 antibodyand anti-IgG induced proliferation of IgG⁺ B cells. These resultssuggest that CD40 ligation can inhibit specific antibody responsesand stimulate proliferation in the same IgG⁺ (memory) B cellsubpopulation. Addition of CD40 antibody during the first 24–48h of the response was required for inhibition, suggesting thatthe effect was on early B cell activation and/or proliferationrequired for antibody production. There was no correlation,however, between the ability of CD40 mAb to stimulate proliferationand inhibit antibody production. We suggest that early activationof CD40 in the specific antibody response inhibits the formationof plasma cells and promotes instead the generation of memorycells.     

CD19 differentially regulates BCR signalling through the recruitment of PI3K

Abstract

CD19 is a co-stimulatory surface protein expressed exclusively on B cells and serves to reduce the threshold for signalling via the B-cell receptor (BCR). Co-ligation of CD19 with the BCR synergistically enhances mitogen-activated protein (MAP) kinase activity, calcium release and proliferation. We recently found that these parameters were also enhanced in CD19-null primary murine B cells following BCR ligation, suggesting a regulatory role for CD19 in BCR signalling. In this study, we demonstrate that the enhanced BCR signalling in the absence of CD19 was not dependent on the src kinase Lyn, but linked to phosphoinositide 3-kinase (PI3K) activity. Consistent with this, we detect PI3K associated with CD19 outside the lipid raft in resting B cells. Pre-ligation of CD19 to restrict its translocation with the BCR into lipid rafts attenuated BCR-induced PI3K and MAP kinase activation and subsequent B-cell proliferation. Thus, we propose that CD19 can modulate BCR signalling in both a positive and negative manner depending on the receptor/ligand interaction in vivo.