Ligation of membrane immunoglobulin leads to inactivation of the signal-transducing ability of membrane immunoglobulin, CD19, CD21, and B-cell gp95

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

Revisiting the Coreceptor Function of Complement Receptor Type 2 (CR2, CD21); Coengagement With the B-Cell Receptor Inhibits the Activation, Proliferation, and Antibody Production of Human B Cells

The positive coreceptor function of complement receptor type 2 [CR2 (CD21)] on B cells is generally accepted, although its role in the enhancement of antibody production had only been proven in mice. The importance of this phenomenon prompted reinvestigation of the functional consequences of coclustering CD21 and the B cell receptor (BCR) on primary human cells. We found that, at non-stimulatory concentrations of anti-IgG/A/M, coclustering the BCR and CR2 enhanced the Ca²⁺ response, while activation marker expression, cytokine production, proliferation, and antibody production were all inhibited upon the coengagement of CR2 and BCR on human B cells. Thus, the “textbook dogma” claiming that C3d acts as an adjuvant to enhance humoral immunity is relevant only to mice and not to humans.

BCR and chemokine responses upon anti-IgM and anti-IgD stimulation in chronic lymphocytic leukaemia

Dysregulation of B cell receptor (BCR) signalling is a hallmark of chronic lymphocytic leukaemia (CLL) pathology, and targeting BCR pathway kinases has brought great therapeutic advances. Activation of the BCR in lymphoid organs has been associated with CLL cell proliferation and survival, leading to progressive disease. While these responses are mediated predominantly by IgM, the role of IgD is less clear. Seeking to uncover downstream consequences of individual and combined stimulation of the two BCR isotypes, we found an amplification of IgD expression and IgD-mediated calcium signalling by previous stimulation of IgM in CLL. Furthermore, no heterologous downmodulation of the isotypes, as observed in healthy donors, was present. Only marginal downregulation of the expression of various chemokine receptors by α-IgM and α-IgD stimulation was found as compared to normal B cells. Consistently, calcium responses of CLL cells to different chemokines were only weakly affected by preceding BCR activation. In contrast, migration towards the two homeostatic chemokines CXCL12 and CCL21 was differentially regulated by IgM and IgD. While IgM activation reduced migration of CLL cells towards CXCL12, but not CCL21, IgD activation predominantly impacted on CCL21 but not CXCL12-mediated chemotaxis. This indicates that the preference for one chemokine over the other may depend on the functional presence of the two isotypes in CLL. Inhibitors against the kinases Syk, Lyn, and Btk antagonised both BCR- and chemokine-induced calcium signals.

Manipulating mIgD-expressing B cells with anti-migis-δ monoclonal antibodies

Surface IgD and IgM doubly positive cells comprise the major population of B cells in the human immune system. The heavy chain of membrane-bound IgD (mδ) differs from that of IgD (δ) in that mδ contains a C-terminal membrane-anchor peptide. Our group previously proposed that the N-terminal extracellular segment of 27 aa residues of the membrane-anchor peptide of mδ, referred to as the mIg isotype-specific-δ (migis-δ) segment, may provide a unique antigenic site for isotype-specific targeting of mIgD(+) B cells. Here we report the preparation of mouse mAbs specific for human migis-δ. The mAbs bound to human migis-δ-containing recombinant proteins in an ELISA and to mIgD-expressing transfectants of a CHO cell line as analyzed by flow cytometry. MAb 20E6, which binds to an epitope toward the N-terminal of human migis-δ, could stain human B cell line MC116, which expressed mIgD and mIgM. MC116 cells could be induced to undergo apoptosis by treatment with 20E6 in the presence of a second crosslinking antibody. Chimeric 20E6 caused antibody-dependent cellular cytotoxicity of MC116 cells in the presence of human PBMCs as the source of effector cells. In cultures of PBMCs, 20E6 down-regulated the population of mIgD(+) B cells. The production of human IgM by transplanted MC116 cells in NOD-SCID (NOD.CB17-Prkdc(scid)/IcrCrlBltw) mice could be suppressed by 20E6. These results encourage further investigation of the potential of anti-migis-δ mAbs to control mIgD(+) B cells, when such a manipulation may alleviate a disease state.

Anti-CD180 (RP105) activates B cells to rapidly produce polyclonal Ig via a T cell and MyD88-independent pathway

CD180 is homologous to TLR4 and regulates TLR4 signaling, yet its function is unclear. We report that injection of anti-CD180 mAb into mice induced rapid Ig production of all classes and subclasses, with the exception of IgA and IgG2b, with up to 50-fold increases in serum IgG1 and IgG3. IgG production after anti-CD180 injection was not due to reactivation of memory B cells and was retained in T cell-deficient (TCR knockout [KO]), CD40 KO, IL-4 KO, and MyD88 KO mice. Anti-CD180 rapidly increased both transitional and mature B cells, with especially robust increases in transitional B cell number, marginal zone B cell proliferation, and CD86, but not CD80, expression. In contrast, anti-CD40 induced primarily follicular B cell and myeloid expansion, with increases in expression of CD80 and CD95 but not CD86. The expansion of splenic B cells was due, in part, to proliferation and occurred in wild-type and TCR KO mice, whereas T cell expansion occurred in wild-type, but not in B cell-deficient, mice, indicating a direct role for B cells in CD180 stimulation in vivo. Combination of anti-CD180 with various MyD88-dependent TLR ligands biased B cell fate because coinjection diminished Ig production, but purified B cells exhibited synergistic proliferation. Anti-CD180 had no effect on cytokine production from B cells, but it increased IL-6, IL-10, and TNF-α production in combination with LPS or CpG. Thus, CD180 stimulation induces intrinsic B cell proliferation and differentiation, causing rapid increases in IgG, and integrates MyD88-dependent TLR signals to regulate proliferation, cytokine production, and differentiation.

Regulation of B lymphocyte responses to foreign and self-antigens by the CD19/CD21 complex

The membrane protein complex CD19/CD21 couples the innate immune recognition of microbial antigens by the complement system to the activation of B cells. CD21 binds the C3d fragment of activated C3 that becomes covalently attached to targets of complement activation, and CD19 co-stimulates signaling through the antigen receptor, membrane immunoglobulin. CD21 is also expressed by follicular dendritic cells and mediates the long-term retention of antigen that is required for the maintenance of memory B cells. Understanding of the biology of this receptor complex has been enriched by analyses of genetically modified mice; these analyses have uncovered roles not only in positive responses to foreign antigens, but also in the development of tolerance to self-antigens. Studies of signal transduction have begun to determine the basis for the coreceptor activities of CD19. The integration of innate and adaptive immune recognition at this molecular site on the B cell guides the appropriate selection of antigen by adaptive immunity and emphasizes the importance of this coreceptor complex.

CD19 is a central response regulator of B lymphocyte signaling thresholds governing autoimmunity

The CD19/CD21 complex is categorized among the 'response regulator' class of receptors which determine the magnitude and outcomes of B cell receptor signals. Small changes in CD19 expression have dramatic effects on signaling thresholds within B cells, which in turn has considerable impact on the balance between humoral immune responses and tolerance induction. B cell signaling thresholds lowered by increased CD19 expression may significantly augment host susceptibility to the development of autoimmunity. Signals generated by C3d-antigen complex binding to CD21 may also be involved in the development of autoimmunity by regulating CD19 function. Since CD19 serves as a central regulator of signaling thresholds in B cells, the CD19/CD21 complex may be an appropriate target for suppressing the development of autoimmunity.

Cross-Linking CD21/CD35 or CD19 Increases Both B7-1 and B7-2 Expression on Murine Splenic B Cells

Activation of the complement cascade and ligation of complement C3 receptors on B cells represent an important bridge between innate and Ag-specific acquired immunity. We show here that cross-linking of mouse CD21 (complement receptor type 2, CR2, C3d receptor) and CD35 (complement receptor type 1, CR1, C3b/C4b receptor) or co-cross-linking of CD21/CD35 and surface IgM rapidly up-regulates both B7-1 and B7-2 expression on murine resting splenic B cells. CD21/CD35-mediated up-regulation of both B7-1 and B7-2 expression is observed within 14 h, while other stimuli up-regulate only B7-2 but not B7-1 at this early time point. Consistent with the increase in B7 levels, BALB/c B cells on which surface IgM and CD21/CD35 have been co-cross-linked stimulate C57BL/6 T cells more effectively than controls. This CD21/CD35-enhanced allogeneic MLR is blocked nearly completely by anti-B7-2 mAbs and partially by anti-B7-1 mAbs. In addition, cross-linking of CD19, which is physically associated with CD21/CD35, leads to increased B7-1 and B7-2 expression. These data suggest that CD21/CD35 ligation results in enhanced B cell Ag presentation using costimulatory mechanisms shared with other activators and thus works cooperatively in this process. Rapid up-regulation of B7-1 expression, a unique response to CD21/CD35 and CD19 cross-linking, may be a particularly important effect of C3-containing ligands. We propose that CD21/CD35- and CD19-mediated B7-1 and B7-2 up-regulation is an important mechanism by which complement activation links innate and acquired immunity.

Qualitative regulation of B cell antigen receptor signaling by CD19: selective requirement for PI3-kinase activation, inositol-1,4,5-trisphosphate production and Ca2+ mobilization

Genetic ablation of the B cell surface glycoprotein CD19 severely impairs the humoral immune response. This requirement is thought to reflect a critical role of CD19 in signal transduction that occurs upon antigen C3dg coligation of antigen receptors with CD19 containing type 2 complement receptors (CR2). Here we show that CD19 plays a key accessory role in B cell antigen receptor signaling independent of CR2 coligation and define molecular circuitry by which this function is mediated. While CD19 is not required for antigen-mediated activation of receptor proximal tyrosines kinases, it is critical for activation of phosphatidylinositol 3-kinase (PI3-kinase). PI3-Kinase activation is dependent on phosphorylation of CD19 Y484 and Y515. Antigen-induced CD19-dependent PI3-kinase activation is required for normal phosphoinositide hydrolysis and Ca2+ mobilization responses. Thus, CD19 functions as a B cell antigen receptor accessory molecule that modifies antigen receptor signaling in a qualitative manner.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Protein kinase C mu (PKC mu) associates with the B cell antigen receptor complex and regulates lymphocyte signaling

We have identified a Ser/Thr kinase associated with the B cell receptor (BCR) complex as protein kinase C mu (PKC mu). PKC mu activity is up-regulated after cross-linking the BCR and CD19 on B cells, and PKC mu co-precipitates with Syk and phospholipase C-gamma 1/2 (PLC gamma 1/2). In vitro phosphorylation of fusion proteins showed that both Syk and PLC gamma 1 are potential substrates of PKC mu in vivo. Analysis of mutants of the chicken B cell line DT40 deficient in either Syk, Lyn, Btk, or PLC gamma 2 revealed that BCR-induced activation of PKC mu, like activation of PLC gamma 2, requires Syk and is partially regulated by Btk, but is Lyn independent. PKC mu can down-regulate the ability of Syk to phosphorylate PLC gamma 1 in vitro. Thus, PKC mu may function in a negative feedback loop regulating BCR-initiated signaling cascades.

The CD19 signal transduction molecule is a response regulator of B-lymphocyte differentiation

The phenotypes of CD19-deficient (CD19-/-) mice, and human CD19-transgenic (hCD19TG) mice that overexpress CD19 indicate that CD19 is a response regulator of B-lymphocyte surface receptor signaling. To further characterize the function of CD19 during B-cell differentiation, humoral immune responses to a T-cell-independent type 1 [trinitrophenyl-lipopolysaccharide (TNP-LPS)], a T-cell-independent type 2 [dinitrophenyl (DNP)-Ficoll], and a T-cell-dependent [DNP-keyhole limpet hemocyanin (KLH)] antigen were assessed in CD19-/- and hCD19TG mice. B cells from CD19-/- mice differentiated and underwent immunoglobulin isotype switching in vitro in response to mitogens and cytokines. In vivo, CD19-/- mice generated humoral responses to TNP-LPS and DNP-KLH that were dramatically lower than those of wild-type littermates. Surprisingly, the humoral response to DNP-Ficoll was significantly greater in CD19-/- mice. In contrast, hCD19TG mice were hyperresponsive to TNP-LPS and DNP-KLH immunization but were hyporesponsive to DNP-Ficoll. These results demonstrate that CD19 is not required for B-cell differentiation and isotype switching but serves as a response regulator which modulates B-cell differentiation. Since humoral responses to both T-cell-dependent and T-cell-independent antigens were similarly affected by alterations in CD19 expression, these differences are most likely to result from intrinsic changes in B-cell function rather than from the selective disruption of B-cell interactions with T cells.

Co-ligation of ICAM-1 (CD54) and membrane IgM negatively affects B cell receptor signaling

A possible role of intercellular adhesion molecule 1 (ICAM-1, CD54) in transmembrane signaling was investigated in B cells from the Burkitt lymphoma cell line MTLM4. Cross-linking of membrane IgM (mIgM) induced an increase in intracellular free Ca2+ as a result of the release from intracellular stores and an influx of extracellular Ca2+. When the B cells were incubated with limiting concentrations of anti-IgM, co-ligation of mIgM and CD54, but not CD19, resulted in an inhibition of the Ca2+ response. Separate cross-linking of mIgM and CD54 under these conditions, using isotype mismatched monoclonal antibodies (mAb), did not affect the mobilization of Ca2+. The CD54-mediated inhibition of the Ca2+ response was also observed in the absence of extracellular Ca2+. All CD54 mAb tested (F10.2, F10.3 and F7.11) interfered with mIgM signaling. The results presented in this report imply that CD54 is linked to intracellular signaling pathways and, via co-ligation with mIgM, interferes in the release of Ca2+ from intracellular stores.

Tissue-specific expression of the human CD19 gene in transgenic mice inhibits antigen-independent B-lymphocyte development

CD19 is a B-cell-specific member of the immunoglobulin superfamily expressed from early pre-B-cell development until plasma cell differentiation. In vitro studies demonstrate that the CD19 signal transduction molecule can serve as a costimulatory molecule for activation through other B-lymphocyte cell surface molecules. However, much remains to be known regarding how CD19 functions in vivo and whether CD19 has different roles at particular stages of B-cell differentiation. Therefore, transgenic mice overexpressing the human CD19 (hCD19) gene were generated to determine whether this transgene would be expressed in a B-lineage-specific fashion and to dissect the in vivo role of CD19 in B-cell development and activation. Expression of the human transgene product was specifically restricted to all B-lineage cells and appeared early in development as occurs with hCD19. In addition, expression of hCD19 severely impaired the development of immature B cells in the bone marrow, with dramatically fewer B cells found in the spleen, peripheral circulation, and peritoneal cavity. The level of hCD19 expressed on the cell surface correlated directly with the severity of the defect in different transgenic lines. These results demonstrate that the hCD19 gene is expressed in a lineage-specific fashion in mice, indicating that the hCD19 gene may be useful for mediating B-lineage-specific expression of other transgene products. In addition, these results indicate an important role for the lineage-specific CD19 molecule during early B-cell development before antigen-dependent activation.

Tissue-specific expression of the human CD19 gene in transgenic mice inhibits antigen-independent B-lymphocyte development

CD19 is a B-cell-specific member of the immunoglobulin superfamily expressed from early pre-B-cell development until plasma cell differentiation. In vitro studies demonstrate that the CD19 signal transduction molecule can serve as a costimulatory molecule for activation through other B-lymphocyte cell surface molecules. However, much remains to be known regarding how CD19 functions in vivo and whether CD19 has different roles at particular stages of B-cell differentiation. Therefore, transgenic mice overexpressing the human CD19 (hCD19) gene were generated to determine whether this transgene would be expressed in a B-lineage-specific fashion and to dissect the in vivo role of CD19 in B-cell development and activation. Expression of the human transgene product was specifically restricted to all B-lineage cells and appeared early in development as occurs with hCD19. In addition, expression of hCD19 severely impaired the development of immature B cells in the bone marrow, with dramatically fewer B cells found in the spleen, peripheral circulation, and peritoneal cavity. The level of hCD19 expressed on the cell surface correlated directly with the severity of the defect in different transgenic lines. These results demonstrate that the hCD19 gene is expressed in a lineage-specific fashion in mice, indicating that the hCD19 gene may be useful for mediating B-lineage-specific expression of other transgene products. In addition, these results indicate an important role for the lineage-specific CD19 molecule during early B-cell development before antigen-dependent activation.

Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells

The specificity of antibody (Ab) responses depends on focusing helper T (Th) lymphocyte signals to suitable B lymphocytes capable of binding foreign antigens (Ags), and away from nonspecific or self-reactive B cells. To investigate the molecular mechanisms that prevent the activation of self-reactive B lymphocytes, the activation requirements of B cells specific for the Ag hen egg lysozyme (HEL) obtained from immunoglobulin (Ig)-transgenic mice were compared with those of functionally tolerant B cells isolated from Ig-transgenic mice which also express soluble HEL. To eliminate the need for surface (s)Ig-mediated Ag uptake and presentation and allow the effects of sIg signaling to be studied in isolation, we assessed the ability of allogeneic T cells from bm12 strain mice to provide in vivo help to C57BL/6 strain-transgenic B cells. Interestingly, non-tolerant Ig-transgenic B cells required both allogeneic Th cells and binding of soluble HEL for efficient activation and Ab production. By contrast, tolerant self-reactive B cells from Ig/HEL double transgenic mice responded poorly to the same combination of allogeneic T cells and soluble HEL. The tolerant B cells were nevertheless normally responsive to stimulation with interleukin 4 and anti-CD40 Abs in vitro, suggesting that they retained the capacity to respond to mediators of T cell help. However, the tolerant B cells exhibited a proximal block in the sIg signaling pathway which prevented activation of receptor-associated tyrosine kinases in response to the binding of soluble HEL. The functional significance of this sIg signaling defect was confirmed by using a more potent membrane-bound form of HEL capable of triggering sIg signaling in tolerant B cells, which markedly restored their ability to collaborate with allogeneic Th cells and produce Ab. These findings indicate that Ag-specific B cells require two signals for mounting a T cell-dependent Ab response and identify regulation of sIg signaling as a mechanism for controlling self-reactive B cells.

Human CD38 is associated to distinct molecules which mediate transmembrane signaling in different lineages

The CD38 antigen displays restricted functional associations with surface molecules involved in immune system and complement. Capping of the CD38 molecule in normal or neoplastic T cells is followed by rapid and specific co-modulation of the CD3-T cell receptor (TcR) complex. In normal and tumor cells of B lineage, CD38 was found to be also associated with surface Ig (sIg) and with the complement receptor 2 (CR2)/CD19 complex. The CD38 molecule expressed by purified natural killer cells displayed an association with the low affinity IgG Fc receptor (Fc gamma RIII) CD16. These observations suggest that specialized areas in the plasma membrane contain co-modulating structures, including different receptors involved in the transduction of extracellular signals. We propose a model whereby TcR, CR2 and CD16 are ligand binding structures in their respective lineages, while CD38 is a molecule involved in the intracellular transduction of the signals.

Cellular signalling mechanisms in B lymphocytes

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Ethanol inhibits ligand-activated Ca2+ channels in human B lymphocytes

Ethanol reportedly is immunosuppressive, interfering with lymphocyte proliferation. To investigate the basis for this immunosuppression, the effects of acute treatment with ethanol were studied on Ca2+ mobilization in tonsillar B lymphocytes and the human lymphoblastoid B-cell line, Ramos. The level of intracellular Ca2+ was monitored in cells loaded with the fluorescent dye indo-1 following stimulation with either anti-IgM antibody or platelet activating factor. The effect of ethanol was also examined on the induction of the early proto-oncogene c-fos in these cells. Ethanol inhibited ligand-activated Ca2+ mobilization due to transmembrane influx but not intracellular store release, in a dose- and time-dependent manner. This inhibition was not due to the inability of anti-IgM to bind to its surface receptor nor to membrane depolarization induced by ethanol. Ethanol also inhibited the Ca2(+)-dependent induction by anti-IgM of c-fos in these cells. The inhibitory effects of ethanol on ligand-activated Ca2+ channels and subsequent induction of c-fos may provide the basis for its immunosuppressive action.

Platelet-activating factor-mediated transmembrane signaling in human B lymphocytes is regulated through a pertussis- and cholera toxin-sensitive pathway

Platelet-activating factor (PAF) stimulates human B cells, resulting in elevation of intracellular calcium and the release of inositol phosphates. This signaling pathway is inhibited in the presence of pertussis (PT) or cholera toxin (CT). Preincubation of human B cells with either toxin, but not their inactive subunits, for 3 h blocked these PAF-induced responses in two B-lymphoblastoid cell lines. This effect was time dependent, with some inhibition noted at 30 min, but only after preincubation for 2-3 h was maximum inhibition achieved. This inhibitory activity was also dose dependent. The toxins blocked both PAF-induced transmembrane uptake of Ca2+ as well as release of Ca2+ from internal stores, and were selective in that activation events after cross-linking of surface IgM were not affected. Further, the toxins did not appear to act through elevation of intracellular levels of cAMP. These data, coupled with previous observations on the absence of heterologous desensitization between PAF and sIgM receptors, may delineate distinct signaling pathways in human B cells. This may reflect different roles for GTP-binding proteins in the activation of human B cells.

An alternative pathway of B cell activation: stilbene disulfonates interact with a Cl- binding motif on AEn-related proteins to stimulate mitogenesis

Stilbene disulfonates are known to competitively inhibit Cl-/HCO3- flux through Band 3-related anion exchange (AE) proteins. To study the role of AE in lymphocyte activation, stilbene disulfonates were added to cultures of rat splenocytes (SPL). Four different stilbene derivatives were tested and each directly stimulated mitogenic proliferative responses of SPL. The mitogenic activity of these analogs paralleled their known patterns of interaction with Band 3-related AE proteins, as measured by; (a) their effective mitogenic concentrations, (b) their rank order of mitogenic potency [DIDS greater than SITS greater than DNDS congruent to DAzDS], (c) their patterns of nonreversible binding to the mitogenic receptor [DIDS much greater than SITS, DNDS], and (d) the specific, noncompetitive inhibition of their activity by the antagonist niflumic acid. Stilbene disulfonates directly activated purified B cell populations but not isolated T cells and furthermore, acted in synergy with anti-IgM to stimulate proliferation of SPL. These findings show that stilbene disulfonates represent a novel class of mitogens that interact with AEn-related proteins to stimulate an alternative activation pathway in B cells. These studies also indicate that immunomodulating activities of nonsteroidal anti-inflammatory drugs such as niflumic acid may be mediated, in part, by their interactions with AEn-related proteins.