B lymphocyte receptors and polyphosphoinositide degradation

Lipopolysaccharide/lipid A receptors on lymphocytes and macrophages

Significant advances have been realized during the past five years in the understanding of the mechanism(s) by which endotoxic LPS interactions with mammalian lymphoreticular cells leads to characteristic cellular responses. There is now strong experimental evidence to support the concept that specific receptors for the lipid A component of LPS do, in fact, exist and are functional on these cells. While the available data do not rule out a potential contribution of nonspecific hydrophobic interactions of lipid A with the membrane bilayer in the cellular activation process, it would appear that interaction with the LPS receptor alone is sufficient to initiate triggering. Whether there exist more than one molecular entity which might function on mammalian cell membranes as a specific receptor for LPS, or whether different cell types may manifest different LPS receptors remains as an interesting area for future research. Further, the concept that molecular complexes of LPS with mammalian host proteins, such as the acute phase LPS binding protein, might trigger additional novel pathways for cell activation is both exciting and of potential importance. The precise mechanism or mechanisms by which LPS-receptor ligand interactions translate into appropriate transmembrane signalling events is currently uncertain. Clearly there exists evidence for contribution of many of the traditional second signals, although at present, the data are incomplete and not always consistent between laboratories. Of potential concern in this respect are the sometimes rather striking differences noted between lipid A and intact polysaccharide containing S-LPS. While such differences may be significant and important, it should be remembered that S-LPS itself is a potent stimulus for many lymphoreticular cell subpopulations, and any postulated pathways must encompass S-LPS as well as lipid A. In any case, it is likely that the further molecular-biochemical characterization of LPS receptors will yield crucial information for the eventual elucidation of the precise pathways for LPS transmembrane signalling. Such information will be invaluable in the future harnessing of the immunostimulatory potential of LPS as well as the abrogation of its profound deleterious pathophysiological effects in endotoxin shock.

Synergistic action of calcium ionophore A23187 and protein kinase C activator bryostatin 1 on human B cell activation and proliferation

In this study we have examined the immunostimulatory effects of the macrocyclic lactone bryostatin 1 on various aspects of B cell activation and proliferation using human tonsillar B cells. Bryostatin 1 is an activator of protein kinase C (PKC) and its properties were compared to those of the classical PKC activator phorbol 12-myristate 13-acetate (PMA), a phorbol ester. Time-course kinetics and dose-response curves of RNA and DNA synthesis induced by bryostatin 1 or PMA were comparable, albeit the phorbol ester was significantly more potent. The responses triggered by both bryostatin 1 and PMA could be blocked by the PKC inhibitor H7. Bryostatin 1 and PMA mediated similar effects with regard to the activation parameters, increase in cell size, expression of activation-associated antigens and hyperexpression of major histocompatibility complex class II antigens. Addition of the calcium ionophore A23187 to bryostatin 1-treated cultures resulted in synergistically enhanced activation and proliferation responses, and this potentiation by A23187 could be inhibited by cyclosporin A. Bryostatin 1 antagonized the effects of PMA-triggered stimulation in a time- and dose-dependent manner. The basis for this modulation of PMA-induced effects and the reason for the difference in the abilities of the two agents to stimulate B cells is unclear; possibly, bryostatin 1 and PMA activate different isoforms of PKC and elicit different signals on intracellular biochemical pathways. Bryostatin 1 lacks the tumor-promoting activity of PMA and is a potent anti-neoplastic substance. These features together with its immunomodulatory properties qualify bryostatin 1 as a candidate for in vivo use as a biological response modifier.

Epstein-Barr virus/complement fragment C3d receptor (CR2) reacts with p53, a cellular antioncogene-encoded membrane phosphoprotein: detection by polyclonal anti-idiotypic anti-CR2 antibodies

Epstein-Barr virus and the C3d fragment of the third component of complement are specific extracellular ligands for complement receptor type 2 (CR2). However, intracellular proteins that react specifically with CR2 and are involved in post-membrane signals remain unknown. We recently prepared polyclonal anti-idiotypic anti-CR2 antibodies (Ab2) by using the highly purified CR2 molecule as original immunogen. We showed that Ab2 contained anti-idiotypic specificities that mimicked extracellular domains of CR2 and detected two distinct binding sites on CR2 for its specific extracellular ligands, Epstein-Barr virus and C3d. We postulated that Ab2 might also contain specificities that could mimic intracellular domains of CR2. Here we report that Ab2, which did not react with Raji B-lymphoma cell surface components, detected specifically, among all components solubilized from Raji cell membranes, a single intracellular membrane protein of apparent molecular mass of 53 kDa. This protein was identified as the p53 cellular antioncogene-encoded membrane phosphoprotein by analyzing its antigenic properties with Pab1801, a monoclonal anti-p53 antibody, and by comparing its biochemical properties with those of p53. Additionally, solubilized and purified CR2 bound to solubilized p53 immobilized on Pab1801-Sepharose. p53, like CR2, was localized only in purified plasma membranes and nuclei of Raji cells. These data suggest strongly that p53, a cellular antioncogene-encoded phosphoprotein, reacted specifically with CR2 in Raji membranes. This interaction may represent one of the important steps through which CR2 could be involved in human B-lymphocyte proliferation and transformation.

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

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

Biochemical basis of synergy between antigen and T-helper (Th) cell-mediated activation of resting human B cells

We have utilized CD23 expression as a marker for B cell activation in order to investigate the biochemical basis for synergy between antigen and T helper (Th) cells in the activation of resting human B cells. Our results confirm that while ligation of surface immunoglobulin (sIg) receptors by antigen analogues (e.g., F(ab')2 goat anti-human IgM) does not lead to CD23 expression, this stimulus markedly enhances CD23 expression induced during antigen specific Th-B cell interaction or by rIL-4. Utilizing a panel of monoclonal anti-human IgM antibodies, we observed a positive correlation between the capacity of a particular antibody to synergize with rIL-4 in CD23 expression and with B cell growth factor in B cell proliferation; suggesting that synergy in CD23 expression reflects the transduction of a functionally important signal via the sIg receptor. We next assayed analogues of the "second messenger" molecules, released during inositol lipid hydrolysis, for their capacity to amplify CD23 expression. These studies showed that protein kinase C (PKC) activating phorbol esters and the synthetic diacylgylcerol analogue, DiC8, synergize with either Th cells or rIL-4 in CD23 expression, while under no experimental condition does increasing B cell [Ca2+]i with ionomycin enhance CD23 expression. Taken together, these data suggest that activation of B cell PKC is the crucial biochemical event that primes antigen-activated B cells to respond more vigorously to interaction with Th cells and/or their soluble products.

Analysis of signaling via surface immunoglobulin receptors on B cells from CBA/N mice

CBA/N mice, which carry the xid immunodeficiency, lack a mature subpopulation of B cells. The residual B cells in these mice do not make antibodies to type-2 T-independent antigens, nor do they synthesize DNA in response to mitogenic forms of anti-Ig antibodies. It is therefore an attractive hypothesis that the surface immunoglobulin receptors (sIgR) on xid B cells signal abnormally following cross-linking. We show here that anti-Ig antibodies do cause inositol phospholipid hydrolysis and Ca2+ mobilization in xid B cells. However, the response of these cells are only 40%-50% of those of normal B cells. Studies with permeabilized cells demonstrated that the hyporesponsiveness is not due to ineffective coupling of sIgR to their associated G-protein. Rather it is apparently due to a quantitative and/or qualitative deficiency in the polyphosphoinositide-specific phosphodiesterase which mediates sIgR-induced inositol phospholipid hydrolysis. These observations may provide a biochemical explanation for the immunological abnormalities resulting from the xid mutation.

Capping of surface receptors and concomitant cortical tension are generated by conventional myosin

We have investigated the role of cytoskeletal contraction in the capping of surface proteins crosslinked by concanavalin A on mutant Dictyostelium cells lacking conventional myosin. Measurements of cellular deformability to indicate the development of cortical tension show that cells of the wild-type parental strain, AX4, stiffen early during capping and relax back towards the softer resting state as the process is completed. Mutant cells lacking myosin (mhcA-) have a lower resting-state stiffness, and fail to stiffen and to cap crosslinked proteins on binding concanavalin A. Hence conventional myosin is essential both for capping and for the concomitant increase in cell stiffness. Furthermore, depletion of cellular ATP by azide causes a 'rigor' contraction in AX4 cells which makes them stiffen and become spherical. By contrast, the mhcA- cells fail to respond in these ways. These measurements of cortical tension in non-muscle cells can thus be directly correlated with the presence of conventional myosin, demonstrating that contractile tension generated by myosin can drive both a change of cell shape and the capping of crosslinked surface receptors.

Involvement of a specific guanine nucleotide binding protein in receptor immunoglobulin stimulated inositol phospholipid hydrolysis

The role of a specific guanine nucleotide binding (G protein) protein in coupling murine B lymphocyte receptor immunoglobulin to inositol phospholipid hydrolysis was investigated. Using an in vitro system with isolated membranes, we have observed specific enhancement of GTP binding subsequent to ligand-induced receptor crosslinking. Induced increases were inhibited by pretreatment with pertussis toxin which catalyzed ADP-ribosylation of a 43 kDa substrate. Involvement of this G protein with receptor immunoglobulin-induced inositol phospholipid hydrolysis was evidenced by the ability of pertussis toxin to block this response. This report, then, indicates that the B lymphocyte antigen receptor belongs to a family of receptors which are linked to inositol phospholipid hydrolysis through a G protein.

T helper cell-induced CD23 (BLAST-2) expression: an activation marker for the high density fraction of human B cells

We previously reported that the coculture of cloned, allospecific human T helper (Th) cells with allogenic B cells bearing the relevant major histocompatibility complex class II antigen induces expression of the B cell activation antigen CD23 (BLAST-2) on a fraction of the B cells. To determine if Th cell-induced CD23 expression defines a distinct subset of human B cells, allospecific Th cells were cultured with B cell fractions isolated on discontinuous Percoll gradients. Our results show that the majority of high density resting B cells, those bearing surface IgD and little of the 4F2 activation antigen, express high intensity CD23 after culture with relevant allospecific Th cells. Essentially all of the low density, presumably in vivo-activated, B cell subpopulation and a fraction of the high density B cell pool remain CD23 negative after repeated culture with relevant allospecific Th cells. We utilized the CD23 induction assay to investigate a potential synergistic effect in B cell activation mediated by Th cell signaling and antigen analog-induced cross-linking of B cell surface Ig receptors. These studies show that phorbols known to result in PKC activation, one of the biochemical consequences of sIg-mediated B cell signaling, enhance both the intensity of CD23 expression and the percentage of cells expressing CD23 after allospecific Th cell or IL-4 interaction with high density, but not low density B cells. Finally, we show that while Th-induced B cell activation, as measured by CD23 expression, is a property of high density B cells, induction of Th cell proliferation is a property of the low density B cell population. These results suggest that the antigen-specific interaction between Th cells and resting B cells may serve to activate the B cell in preference to the T cell.

Regulation of surface IgM- and IgD-mediated inositol phosphate formation and Ca2+ mobilization in murine B lymphocytes

Cross-linking of surface IgM or IgD receptors on B cells initiates a signaling cascade involving the activation of an (uncharacterized) G-protein: this in turn activates a polyphosphoinositide-specific phosphodiesterase (PPI-PDE), thereby leading to the release of inositol phosphates. In order to investigate if the two isotypes of sIg share a common G-protein, we stimulated B cells sequentially with anti-mu and anti-delta antibodies. Ligation of either class of receptor for 1 h led to the activation of the PPI-PDE, which persisted for several hours. However, this was accompanied by inhibition of further stimulation of the enzyme via the heterologous receptors. This desensitization was shown to operate at the level of the coupling between G-protein and the PPI-PDE. These effects waned after 4-8 h of stimulation, when signaling via the heterologous receptors had essentially returned to normal. In addition, stimulation of B cells by anti-mu and anti-delta together did not elicit additive responses, either in terms of increases in inositol phosphate production, or in terms of increases in intracellular Ca2+ levels. Taken together, these results indicate that sIgM and IgD receptors share a common G-protein and that signaling via these receptors is under both positive and negative feedback control. The mechanisms involved are unknown, but these effects may well be due to modulation of the activities of components of the signaling cascade by protein kinase C.

B cell activation by synthetic lipopeptide analogues of bacterial lipoprotein bypassing phosphatidylinositol metabolism and proteinkinase C translocation

Synthetic analogues of bacterial lipoprotein induce proliferation of murine small resting B lymphocytes. We investigated the role of proteinkinase C (PKC) activation in lipopeptide-induced B cell stimulation. Using a standardized extraction procedure, there was no change in membrane bound and soluble PKC activity upon stimulation with lipopeptide. However, omitting Ca2+ chelators from the standard extraction medium resulted in a decrease of membrane bound PKC activity after stimulation. Lipopeptide failed to induce phosphoinositide degradation and the generation of the two second messengers cAMP and cGMP. To test whether guanosinetriphosphate-binding proteins are involved in lipopeptide-induced signal transfer we investigated the effect of LiCl, choleratoxin and pertussistoxin on B lymphocyte proliferation. LiCl and pertussistoxin did not inhibit cell activation, whereas choleratoxin reduced the proliferation rate at concentrations higher than 0.5 micrograms/ml. Similar results were observed when LPS was used as mitogen, whereas the anti-immunoglobulin-induced B cell activation was inhibited by all three compounds. Our results show, that B cell activation by bacterial lipopeptides bypasses phosphatidylinositol metabolism and PKC translocation.

Cyclic AMP can enhance mouse B cell activation by regulating progression into the late G1/S phase

This study demonstrates that several substances which raise intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in different ways were able to enhance both RNA and DNA synthesis in mouse purified B cells co-stimulated by the protein kinase C activator phorbol 12-myristate 13-acetate and the Ca2+ ionophore ionomycin, while earlier activation events were not modified. These included early changes in cell size and chromatin decondensing demonstrated by light scatter properties at narrow and 90 degrees angles, increase in Ia expression and loss of surface IgD. We concluded that cAMP can up-regulate mouse B cell activation by controlling progression into the late G1 (G1B)/S phase, but not transition from G0 to early G1 (G1A). Furthermore, because cAMP could synergize with ionomycin but not with phorbol 12-myristate 13-acetate to induce RNA and DNA synthesis, we proposed that the cAMP effects in this model may be related to the protein kinase C pathway.

Mechanisms of alcohol-induced suppression of B-cell response

Several investigators have shown that alcohol can suppress the production of antibodies by animals and humans. The studies reported here were designed to determine whether alcohol has a direct effect on the B-lymphocyte and to determine which stage of the B-cell response is inhibited by alcohol. B-lymphocyte lines specific for the antigen dinitrophenyl were used to study the effect of alcohol on the B-lymphocyte. As little as 100 mg% of alcohol inhibited the response of these pure B-cell lines to stimulation by either antigen (dinitrophenyl-Ficoll) or anti-mu antibody. Since no other cell types were present in the system the suppressive effect was on the B-cell itself. However, alcohol did not inhibit membrane depolarization induced by antigen crosslinking of immunoglobulin receptors, and it did not inhibit activation of the phosphatidyl inositol pathway by receptor crosslinking. When alcohol was added to antigen stimulated B-lymphocyte lines for varying periods during the immune response it was found that antibody production was inhibited if 150 mg% alcohol was present from 30 to 48 hr after the antigen was added to lymphocytes. Thirty-six to 48 hr is the time required for a stimulated mature B-lymphocyte to enter the proliferative phase of the immune response. These data raise the possibility that low doses of alcohol can inhibit antigen-induced B-cell proliferation.

Signalling through sIgA on a novel murine B lymphoma

A novel murine B lymphoma expressing membrane-associated IgA was isolated and used to compare mechanisms of signal transduction by sIgM and sIgA. Like other isotypes so far studied, crosslinking of sIgA by anti-immunoglobulin antibodies stimulates hydrolysis of inositol phospholipids and causes elevation of intracellular free calcium. Furthermore, signals generated through sIgA are coupled to elevation of c-fos proto-oncogene expression. Coupling appears to be through the protein kinase C rather than through the Ca2+ component of sIg signalling as phorbol diester, but the Ca2+ ionophore cannot mediate this effect. Thus these results, coupled with those from earlier studies, show that early signal transduction through surface immunoglobulin appears to be similar regardless of the particular isotype involved in binding ligand.

Membrane IgM cross-linking is not coupled to protein kinase C translocation in WEHI-231 B lymphoma cells

The early molecular events involved in the process of signal transduction via membrane immunoglobulins (mIg) include phosphatidyl inositol metabolism, intracellular Ca2+ mobilization, and protein kinase C (PKC) activation. Anti-mIg antibodies exert either stimulating or inhibitory effects depending on the activation state and/or the differentiation stage of B cells. WEHI-231 is a murine B lymphoma that becomes inactivated upon anti-mIg treatment. This lymphoma has an immature B cell phenotype and is considered as a model for tolerance induction in B lymphocytes. In this study, we have investigated the relationship between mIg triggering, Ca2+ elevation, PKC translocation, and growth inhibition in WEHI-231 cells. Monoclonal antibodies to mu and kappa chains of the mIgM receptor promoted a rapid increase in intracytoplasmic Ca2+ and were potent inhibitors of cell growth. Ca2+ elevation and PKC translocation have been previously shown to be associated in B lymphocytes. To study the subcellular distribution of PKC in WEHI-231 cells, we used enzymatic assays and immunodetection methods. Although phorbol 12-myristate 13-acetate induced a rapid and almost complete redistribution of cytosolic PKC to the membrane fraction, anti-mIg treatment failed to modify the compartmentalization of PKC. These findings extend recent observations suggesting that B cell triggering through mIg receptors may involve additional pathways independent from PKC activation. PKC activation in normal B cells is also believed to provide a regulatory signal which limits the magnitude of the early signals produced by anti-mIg. Such a regulatory control is unlikely in WEHI-231 cells, due to the dissociation between Ca2+ mobilization and PKC translocation. Our findings therefore suggest that the sensitivity of immature B cells such as WEHI-231 to the inhibitory effects of anti-mIg antibodies may result in part from alterations of the phosphoinositide signal transduction pathway.

Intracellular events associated with inhibition of B cell activation by monoclonal antibodies to HLA class II antigens

We have investigated several aspects of the inhibitory effects of monoclonal antibodies (mAb) directed against MHC class II antigens in B cell activation/proliferation, using a panel of mAb specifically reactive with antigens encoded by HLA class II loci (DP, DQ, DR). All mAb except the anti-DP mAb inhibited significantly anti-mu plus B cell growth factor-induced DNA synthesis. Only one mAb, however, which was reactive with gene products of all three class II loci (DP, DQ, DR) inhibited anti-mu-induced DNA synthesis as well as c-myc mRNA expression. In addition, the same mAb inhibited the early events induced by anti-mu stimulation alone, including phosphatidylinositol turnover and elevation of [Ca2+]i. In contrast to previous findings in the murine system, none of the anti-MHC class II mAb used in this study increased the cAMP levels.

The immunomodulatory effects of interferon-gamma on mature B-lymphocyte responses

Interferon-gamma (IFN-gamma) exerts a broad spectrum of activities which affect the responses of mature B-cells. It strongly inhibits B-cell activation, acts as a B-cell growth factor (BCGF), and also induces final differentiation to immunoglobulin (Ig) production. IFN-gamma is deeply involved in the differential control of isotype expression, as it enhances IgG2a production and suppresses both IgG1 and IgE production. Although it is now possible to draw a general scheme of the effects of IFN-gamma on B-cells, a number of paradoxical results still exist in the field. In this manuscript, different experimental systems are analyzed in an attempt to explain these apparent paradoxes.

Activation of the phosphatidylinositol cycle in spreading cells

Metabolites of the phosphatidylinositol cycle were analyzed in BHK-21 (C13) cells spreading on fibronectin-coated culture plates in comparison with attached nonspreading cells 45 min after plating. Among the water-soluble metabolites (glycerophosphoinositol, inositol, inositol monophosphate, inositol bisphosphate, inositol trisphosphate, and inositol tetrakisphosphate), significant elevations were found for inositol monophosphate, inositol bisphosphate, and inositol tetrakisphosphate. In the lipid fraction, phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate were significantly elevated. The activation of the phosphatidylinositol cycle in spreading versus nonspreading attached BHK-21 (C13) cells may be involved in the permissive effect of the extracellular matrix on cell proliferation.

Inhibition of T-cell antigen receptor-mediated transmembrane signaling by protein kinase C activation

The murine T-lymphoma cell line LBRM-33 is known to require synergistic signals delivered through the antigen receptor (Ti-CD3) complex, together with interleukin 1 (IL-1), for activation of IL-2 gene expression and IL-2 production. Although 12-O-tetradecanoylphorbol-13-acetate (TPA) was capable of replacing IL-1 as an activating stimulus under certain conditions, biologic studies indicated that TPA failed to synergize with Ti-CD3-dependent stimuli under conditions in which IL-1 was clearly active. Acute exposure to TPA and other active phorbol esters resulted in a concentration-dependent inhibition of the increases in phosphoinositide hydrolysis and intracellular free Ca2+ concentration stimulated by phytohemagglutinin or anti-Ti antibodies. TPA treatment induced no direct alteration of phospholipase C enzymatic activities in LBRM-33 cells. In contrast, both Ti-CD3 cross-linkage and TPA rapidly stimulated the phosphorylation of identical CD3 complex polypeptides, presumably via activation of protein kinase C. Exposure of LBRM-33 cells to TPA resulted in a time-dependent, partial down-regulation of surface Ti-CD3 expression. Thus, TPA treatment inhibited the responsiveness of LBRM-33 cells to Ti-CD3-dependent stimuli by inducing an early desensitization of Ti-CD3 receptors, followed by a decrease in membrane receptor expression. These studies indicate that phorbol esters deliver bidirectional signals that both inhibit Ti-CD3-dependent phosphoinositide hydrolysis and augment IL-2 production in LBRM-33 cells.

Epstein-Barr virus and a tumour-promoting phorbol ester use similar mechanisms in the stimulation of human B-cell proliferation

In marked contrast to ligands which activate B cells via their physiological receptors for antigen, transforming Epstein-Barr virus (EBV) was found to be mitogenic for human B lymphocytes without increasing inositol phospholipid hydrolysis. B-cell stimulation by EBV showed similar characteristics to those achieved by the tumour-promoting phorbol ester TPA, in terms of the temporal appearance of surface activation antigens, the induction of RNA and DNA synthesis and the lower requirement for medium Ca++ in comparison to agonists that lead to an increase in inositol phospholipid hydrolysis. The calcium- and phospholipid-dependent kinase, protein kinase C (PKC), is activated by TPA and a proteolytically cleaved fragment (PKM) results. EBV induced the appearance of a calcium- and phospholipid-independent activity that was chromatographically inseparable from PKM and this activity was capable of phosphorylating vimentin, a cell component that is thought to participate in the signal transduction cascade. These findings are discussed with special reference to the biochemical signalling pathways on which EBV might impinge to usurp growth control in B lymphocytes.

Isolation and characterization of a B lymphocyte mutant with altered signal transduction through its antigen receptor

A receptor surface Ig (sIg) signaling variant of WEHI-231 was constructed to investigate components and linkages between various signaling events associated with signal transduction through sIg. Unlike the wildtype, crosslinking of sIgM on VS2.12-cl.2 did not result in downregulation of proliferation. Similarly, receptor crosslinking was uncoupled from inositol phospholipid (PI) hydrolysis and upregulation of c-fos expression in the variant. The signaling defect in VS2.12-cl.2 appears to be proximal to phospholipase C activation as direct G protein activation by A1F4- triggers PI hydrolysis and bypassing PI hydrolysis using phorbol diester stimulation of protein kinase C restores the inhibitable phenotype and the ability to upregulate c-fos. Even more interesting, sIg-linked Ca2+ responses by VS2.12-cl.2 are equivalent to these observed in the wildtype WEHI-231. These latter results suggest that contrary to current thought, sIg-generated signals may not be coupled to Ca2+ fluxes entirely via inositol phospholipid hydrolysis. Thus, VS2.12-cl.2 is a new and powerful tool with which to analyze signaling through sIg at the molecular level.

Characterization of activation of a partially defective B cell subpopulation with immunocompetence restricted to IgM and IgA

A B-cell subpopulation (BM-A cell) responding to an antigen with the production of IgM and IgA plaque-forming cells but not of IgG plaque-forming cells was isolated from neonatally bursectomized chickens and was examined for the mode of activation by B-cell mitogens. The BM-A cells did not elevate both glucose consumption and protein synthesis with the B-cell mitogens, in striking contrast to normal B cells. The BM-A cells were also not activated by an activator of protein kinase C, phorbol myristate acetate. Both anti-Ig and a calcium ionophore, A23187, however, primed the BM-A cells to increase intracellular free calcium ion as well as normal B cells. From these results it is conceived that the lack of protein kinase C activation may be responsible for the failure of activation of BM-A cells.

Co-cross-linking of surface immunoglobulin Fc gamma receptors on B lymphocytes uncouples the antigen receptors from their associated G protein

Cross-linking of surface Ig receptors (sIg) by mitogenic forms of anti-Ig antibodies (e.g. F(ab')2 fragments of rabbit anti-Ig) causes the rapid, and prolonged breakdown of phosphatidylinositol 4,5-bisphosphate. This response involves an unidentified guanine nucleotide regulatory protein (termed Gp), which couples sIg to the polyphosphoinositide-specific phosphodiesterase. Intact (IgG) rabbit anti-Ig antibodies, which co-cross-link sIg and Fc gamma receptors on B cells, only induce short-lived inositol phospholipid breakdown and abortive B cell activation. We show here that in permeabilized B cells intact anti-Ig inhibits the reconstituted breakdown of inositol phospholipids given by a combination of F(ab')2 anti-Ig and the non-hydrolyzable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S), but not the basal stimulation of Gp induced by GTP gamma S alone. These results therefore indicate that co-cross-linkage of sIg and Fc gamma receptors on B cells uncouples the antigen receptors from the associated G protein, but does not affect coupling between Gp and the phosphodiesterase. These observations therefore provide further insight into the mechanisms whereby engaging Fc receptors on B cells, by antigen-antibody complexes for example, could modulate antigen-induced B cell activation.

Mechanism of interleukin-2 signaling: mediation of different outcomes by a single receptor and transduction pathway

The T cell lymphokine, interleukin-2 (IL-2), plays a pivotal role in an immune response by stimulating antigen-activated B lymphocytes to progress through the cell cycle and to differentiate into antibody-secreting cells. An IL-2 inducible B lymphoma line, in which the growth and differentiation responses are uncoupled, provides a model system for dissecting the signaling mechanisms operating in each response. This system was used to show that both signals are initiated by IL-2 binding to a single, unifunctional receptor complex. Moreover, both signals are transduced by a pathway that does not involve any known second messenger system and that can be blocked by a second T cell lymphokine, interleukin 4. These findings suggest that the pleiotrophic effects of IL-2 are determined by different translations of the signal in the nucleus.

Stimulation of phosphatidylinositol 4,5-bisphosphate phospholipase C activity by phosphatidic acid

Phosphatidic acid was a potent activator of the phosphatidylinositol 4,5-bisphosphate (PtdIns-P2) phospholipase C activity associated with human platelet membranes. Lysophosphatidic acid was half as active as phosphatidic acid, and shortening the fatty acid chain reduced the effectiveness of the corresponding phosphatidic acid. Compounds lacking either the phosphate group (diacylglycerol or phorbol ester) or the fatty acid (glycerol phosphate) were not activators. When the negative charge was contributed by a carboxyl group (fatty acid or phosphatidylserine), stimulation of phospholipase C was weak but detectable. Structural analogs of phosphatidic acid (lipopolysaccharide, lipid A, and 2,3-diacylglucosamine 1-phosphate) were less effective but also enhanced PtdIns-P2 hydrolysis. Phosphatidic acid potentiated the activation of phospholipase C by alpha-thrombin, chelators, and guanine nucleotides. Phosphatidylinositol 4-phosphate and PtdIns-P2 were also effective activators of PtdIns-P2 degradation. Other phospholipids were without effect. The production of inositol 1,4,5-trisphosphate and diacylglycerol via the activation of phospholipase C provides a rationale for the cellular responses evoked by phosphatidic acid and the ability of this phospholipid to potentiate and initiate hormonal responses.

Interferon (IFN)-like antiviral effect is induced by unspecific cross-linking of cell surface receptors

Treatment of human amniotic cells (UAC) with Cytodex 1 (DEAE-dextran) results in the development of an antiviral state of the cells, as proven by studying (i) the cytopathic effect and (ii) [3H]uridine incorporation into the RNA of vesicular stomatitis virus (VSV) after VSV infection. The same treatment transiently triggers the breakdown of inositol phospholipids and activates the translocation of protein kinase C (PKC). On the basis of these data it can be suggested that cross-linking of cell surface receptors by a solid carrier bearing covalently bound positive charges may result in IFN-like effects.

Correlation between ribosylation of pertussis toxin substrates and inhibition of peptidoglycan-, muramyl dipeptide- and lipopolysaccharide-induced mitogenic stimulation in B lymphocytes

Selective inhibition by pertussis toxin (PT) of mitogenic activation of mouse B lymphocytes by bacterial mitogens (peptidoglycan and lipopolysaccharide) and muramyl dipeptide (a synthetic analog of peptidoglycan fragment) was demonstrated. Mitogenic activation of B cells by protein kinase C activators and ionomycin was insensitive to PT. Also PT did not inhibit peptidoglycan- and lipopolysaccharide-induced differentiation of B cells into Ig-secreting cells, when it was added to the cultures after the proliferative stage of the response. B lymphocyte membranes contained two major PT substrates (40 and 41 kDa). The extent of PT-mediated ADP ribosylation of these substrates correlated with the degree of PT-mediated inhibition of mitogenic stimulation of B cells. B cell stimulation by all mitogens tested was not inhibited by cholera toxin at nontoxic concentrations that are known to cause maximal increase in cAMP in B cells. Since the only known substrates for PT-mediated ADP ribosylation in mammalian cells are the alpha subunits of some G proteins, our data suggest that G proteins are present in B cell membranes and that they are involved in B cell activation induced by bacterial mitogens.

Signal transduction via the B cell antigen receptor: involvement of a G protein and regulation of signaling

The antigen receptors on B lymphocytes, membrane forms of immunoglobulins, transduce signals regulating B cell growth and differentiation by activating a phosphoinositide-specific phospholipase C. In this report, we describe our recent work aimed at understanding this process in greater detail. We have shown that a GTP-binding component is a necessary cofactor in the stimulation of phospholipase C by mIgM. This component has a number of properties in common with the G protein family of receptor-effector coupling components seen in the adenylate cyclase and other signaling systems. For example, analogues of GTP that cannot be hydrolyzed stimulated mIgM-triggered phosphoinositide breakdown, and an analogue of GDP that cannot be converted to GTP inhibited the reactions. Furthermore, aluminum fluoride, which activates known G proteins, also stimulates phosphoinositide breakdown. The G protein that appears to link mIgM to phospholipase C is not one of the well characterized G proteins involved in the regulation of adenylate cyclase or cGMP phosphodiesterase (GS, Gi, and transducin), as judged by its insensitivity to two bacterial toxins that modify these G proteins, cholera toxin and pertussis toxin. Interestingly, analysis of pertussis toxin sensitivity indicates that there are at least 2 distinct G proteins that couple receptors to phospholipase C. For example, the G protein required for chemotactic peptide receptor signaling in neutrophils is sensitive to pertussis toxin, in contrast to the phosphoinositide signaling G protein in B cells. We have also begun to explore the mechanisms by which mIgM signal transduction can be modulated. Stimulation of protein kinase C with phorbol esters or synthetic DG was found to inhibit mIgM-triggered phosphoinositide breakdown. This regulation probably represents a feedback inhibition that would occur with DG produced by phosphoinositide breakdown. Alternatively, there appear to be other signaling pathways that generate DG33, and they could possibly inhibit phosphoinositide breakdown via protein kinase C. This could be an important locus of regulation during B cell activation. For example, other signals could increase or decrease the potency of this feedback inhibition, and thereby adjust the sensitivity of the B cell to antigen. Alternatively, other agents could stimulate protein kinase C directly, or could stimulate another protein kinase which can do the same thing in this regard, and thereby make the B cell insensitive to antigen by preventing antigen receptor signaling.(ABSTRACT TRUNCATED AT 400 WORDS)

The DNA synthesis of leukemic (L2C) guinea pig B lymphocytes involves a permanent activation of protein kinase C without corresponding phosphoinositide hydrolysis

L2C B lymphocytes have a constant high DNA synthesis due to their continuous proliferative state. The addition of polymyxin B (PmB), a rather selective inhibitor of protein kinase C, stopped (3H)thymidine incorporation with an IC50 of 10 microM when added 18 h before measuring DNA synthesis. Interestingly, PmB inhibition of DNA synthesis was suppressed when 4 nM 12-O-tetradecanoylphorbol-13-acetate was added along with PmB, indicating that PmB may act through inhibition of protein kinase C. In the node and spleen lymphocytes of normal guinea pigs, protein kinase C activity was entirely cytosolic and was eluted at 0.12 M NaCl when adsorbed on DEAE-cellulose. In L2C leukemic lymphocytes, total protein kinase C activity was of the same order of magnitude, but 20% of it was associated with the membrane fraction. The lipid-dependent activity, eluted at 0.12 M NaCl from cytosolic and membrane fractions, was suppressed by staurosporine with an IC50 of 10-40 nM and by polymyxin B with an IC50 of 2-6 microM. Phosphoinositide metabolism was studied in the transformed cells. Incorporation of 32Pi into polyphosphoinositides was considerable, whereas much more time was required for a tiny incorporation of inositol. We detected no release of radioactive inositol triphosphate. Taken together, these results suggest that protein kinase C function is indispensible for triggering L2C leukemic lymphocyte proliferation. The causes of this permanent activation merit further investigation.

Involvement of a pertussis toxin-sensitive G-protein-coupled phospholipase A2 in lipopolysaccharide-stimulated prostaglandin E2 synthesis in cultured rat mesangial cells

E. coli lipopolysaccharide (LPS) stimulated a dose- and time-dependent release of prostaglandin E2 (PGE2) in cultured rat glomerular mesangial cells. Pertussis toxin, an inhibitor of several GTP-binding proteins (G proteins), blocked nearly 80% of the LPS-stimulated PGE2 formation, while having virtually no effect on calcium ionophore-stimulated PGE2 production. We tested the possibility that a G protein-coupled activation of phospholipase A2 mediated the LPS-stimulated PGE2 production. Evidence for LPS activation of phospholipase A2 included a time-dependent LPS-induced generation of [32P]lysophosphatidylcholine and the inhibitory effects of a phospholipase A2 inhibitor, mepacrine, on LPS-induced PGE2 formation. Possible roles for phospholipase C-dependent activation of PGE2 synthesis by LPS seemed unlikely, as LPS did not elevate the cytosolic free calcium concentration or augment the appearance of water-soluble inositol phosphates. We conclude that LPS-induced PGE2 synthesis in rat glomerular mesangial cells is mediated through a G-protein-coupled phospholipase A2 activation. The activation of phospholipase A2 releases arachidonic acid and stimulates PGE2 synthesis preferentially, thereby improving glomerular hemodynamic events in endotoxemia.

Anti-immunoglobulin pretreatment induces a calcium-mobilization response to the chemotactic agent N-formylmethionylleucylphenylalanine in Daudi lymphoblastoid cells

Anti-immunoglobulin treatment of fura-2-loaded Daudi cells induces a calcium mobilization as judged by the increase in the fluorescence of the dye fura-2, AM. No calcium mobilization by N-fMet-Leu-Phe is observed in these cells. However, exposure of the cells to N-fMet-Leu-Phe after the first hit with anti-immunoglobulin (but not after soluble IgG) shows a rapid, dose-dependent calcium mobilization by N-fMet-Leu-Phe. The expression of the calcium-mobilizing response occurs in less than 2 min and is stable. Binding of tritiated N-fMet-Leu-Phe is increased in anti-immunoglobulin-treated but not control cells. The induction is specific for N-fMet-Leu-Phe because the chemoattractant platelet-activating factor did not induce any calcium mobilization. The N-fMet-Leu-Phe antagonist t-butoxycarbonyl-L-Phe-D-Leu-L-Phe-D-Leu-L-Phe- OH did not show any calcium mobilization on its own, either before or after anti-immunoglobulin treatment, and inhibited the calcium mobilization of N-fMet-Leu-Phe at low concentrations. Treatment of the cells with phorbol 12-myristate 13-acetate or pertussis toxin prior to anti-immunoglobulin treatment caused a dose-dependent abolition of both the anti-immunoglobulin-mediated calcium mobilization and the subsequent calcium mobilization by N-fMet-Leu-Phe. Metabolic inhibitors that act predominantly by lowering the ATP levels within the cell (iodoacetate, sodium fluoride, oligomycin, and 2-deoxyglucose) all produced a greater inhibition of the N-fMet-Leu-Phe-mediated calcium mobilization than the anti-immunoglobulin-mediated response. Lowering the temperature from 37 degrees C to 22 degrees C reduced the anti-immunoglobulin response and completely inhibited the expression of the N-fMet-Leu-Phe effect. Our results indicate that activation of the calcium-mobilization pathway in B cells by crosslinking of bound surface immunoglobulin causes an induction of N-fMet-Leu-Phe-sensitive calcium mobilization.

Lipopolysaccharide-induced murine B cell proliferation: a role of protein kinase C

Lipopolysaccharide (LPS)-induced murine B cell proliferation was blocked by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine dihydrochloride (H7), an inhibitor of protein kinase C (PKC), in a dose- and time-dependent manner. The maximum inhibition of B cell proliferation was observed when H7 was added at the initiation of cultures. H7-induced inhibition was prolonged and irreversible. Furthermore, pretreatment of B cells with phorbol myristate acetate ester, a process that degrades membrane-associated PKC, rendered them unresponsive to LPS. These data strongly suggest that the activation of PKC is one of the mechanisms of LPS-induced murine B cell proliferation.

Inhibition of T-cell antigen receptor-mediated transmembrane signaling by protein kinase C activation

The murine T-lymphoma cell line LBRM-33 is known to require synergistic signals delivered through the antigen receptor (Ti-CD3) complex, together with interleukin 1 (IL-1), for activation of IL-2 gene expression and IL-2 production. Although 12-O-tetradecanoylphorbol-13-acetate (TPA) was capable of replacing IL-1 as an activating stimulus under certain conditions, biologic studies indicated that TPA failed to synergize with Ti-CD3-dependent stimuli under conditions in which IL-1 was clearly active. Acute exposure to TPA and other active phorbol esters resulted in a concentration-dependent inhibition of the increases in phosphoinositide hydrolysis and intracellular free Ca2+ concentration stimulated by phytohemagglutinin or anti-Ti antibodies. TPA treatment induced no direct alteration of phospholipase C enzymatic activities in LBRM-33 cells. In contrast, both Ti-CD3 cross-linkage and TPA rapidly stimulated the phosphorylation of identical CD3 complex polypeptides, presumably via activation of protein kinase C. Exposure of LBRM-33 cells to TPA resulted in a time-dependent, partial down-regulation of surface Ti-CD3 expression. Thus, TPA treatment inhibited the responsiveness of LBRM-33 cells to Ti-CD3-dependent stimuli by inducing an early desensitization of Ti-CD3 receptors, followed by a decrease in membrane receptor expression. These studies indicate that phorbol esters deliver bidirectional signals that both inhibit Ti-CD3-dependent phosphoinositide hydrolysis and augment IL-2 production in LBRM-33 cells.

Protein kinase C activators inhibit the antigen receptor-coupled polyphosphoinositide phosphodiesterase in murine B lymphocytes

Protein kinase C activators (e.g. PMA) inhibit the inositol phosphate release generated by crosslinking antigen receptors (sIgM and sIgD) on murine B lymphocytes with anti-receptor antibodies. Unlike other Ca2+-mobilizing receptor systems, the antigen receptor signal transduction pathway in B cells is not interrupted by PMA at the level of receptor/G-protein or G-protein/polyphosphoinositide phosphodiesterase coupling. In these cells, PMA, presumably by activating protein kinase C, inhibits inositol phosphate release by direct effects on the polyphosphoinositide-specific phosphodiesterase.

Role of proteinkinase C and phosphatidylinositol metabolism in lipopeptide-induced leukocyte activation as signal transducing mechanism

Synthetic lipopeptides are potent B-lymphocyte and macrophage activators. The role of phosphatidylinositol metabolism and proteinkinase C in lipopeptide induced leukocyte activation were investigated. In murine B-lymphocytes and in bone marrow derived macrophages, lipopeptide failed to induce phosphatidylinositol breakdown, whereas in the macrophage cell line P388D1 formation of inositolphosphates was increased. Translocation of proteinkinase C from a cytosolic to a membrane compartment was only observed in the cell line P388D1 indicating that in the other cells tested lipopeptide acts via different signal transduction pathways.

Cross-linking of membrane IgM on B CLL cells: dissociation between intracellular free Ca2+ mobilization and cell proliferation

It has been shown previously that chronic lymphocytic leukemia (CLL) B cells are frozen at different stages of activation with unique requirements for proliferation. Although most B CLL cells express surface IgM, anti-mu antibodies are able to trigger only some of them to proliferate and/or respond to B cell growth factor (BCGF) or interleukin 2 (IL2), as normal B cells. In this report we extend these observations using three different monoclonal antibodies (mAb) to human mu chain (one mitogenic in soluble form for normal B cells, the two others mitogenic only when coupled on Sepharose 4B beads). Cells from only 3 out of 11 B CLL patients proliferated in the presence of either mitogenic anti-mu. When the early events following surface Ig cross-linking, such as calcium mobilization (by flow cytometry on indo-1-labeled cells), were studied all three mAb in soluble form were able to induce a similar increase in the intracellular free calcium concentration ([Ca2+]i); analogous to [Ca2+]i rise after the mitogenic F(ab')2 anti-mu stimulation. This response was seen only in 8 out of the 12 CLL B cells studied. All B CLL cells, however, proliferate in response to a combination of phorbol ester 12,13-dibutyrate (PBt2) and ionomycin. Therefore, three patterns of response to sIg cross-linking by anti-mu could be distinguished: cells from 4 out of 12 cases proliferate and mobilize Ca2+ upon anti-mu triggering (behaving like resting B lymphocytes); in 4 other cases anti-mu lead to Ca2+ mobilization without cell proliferation; in the last 4 cases neither Ca2+ mobilization, IP3 generation (in the one case studied) nor cell proliferation are observed although these cells do proliferate directly in response to growth factors. Moreover, anti-mu stimulation in this group leads to increased proliferation in response to BCGF and IL2 suggesting an anti-Ig signaling independent of inositol phosphate metabolism. These results are interpreted in terms of differential anti-mu signaling on B cells at different stages of activation.

Triggering of neoplastic B cells via surface IgM and the cell surface antigens CD20 and CDw40. Responses differ from normal blood B cells and are restricted to certain morphologic subsets

By raising monoclonal antibodies (MAbs) against B cells, a number of cell surface molecules have recently been identified which after binding by their specific antibody can trigger B cells, either alone or in co-operation with antibodies to surface immunoglobulin (sIg). The anti-CD20 (Bp35) MAb IF5 can deliver a strong activation signal to resting normal B cells, and the anti-CDw40 (Bp50) MAb G28-5 can promote activated G1 B cells to enter S phase. These antibodies were tested for their functional effects in vitro on suspended cells from 17 follicle-center-cell (FCC) lymphomas, 5 cases of chronic lymphatic B-cell leukemia (B-CLL) and 8 cases of various histological types. Changes in cellular volume, RNA and DNA synthesis were compared with the results obtained with a polyclonal anti-mu [F(ab')2] antiserum, a MAb to surface IgM (AF6), 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and B-cell growth factor (low-molecular-weight BCGF). Our data reveal differences in the requirements for triggering of various B-cell subsets: cells from CLL responded strongly to TPA but not to anti-mu, which is a potent stimulator not only of normal B cells but also of cells from individual cases of FCC lymphomas. Our observations suggest that the differentiation stage of B-CLL cells is distinct from that of small resting B cells from peripheral blood. Centrocytic lymphomas could not be activated by any of the reagents. CD20-mediated triggering was seen in neoplastic B cells from only 4 of 30 cases, indicating that most B-cell neoplasias were not responsive to this activation pathway. In contrast, the anti-CDw40 MAb consistently stimulated DNA synthesis together with anti-mu or TPA in cells from FCC lymphomas, but not from CLL. Together, these results suggest that activation in different neoplastic B-cell subsets depends on distinct signal transduction mechanisms.

The effect of cyclosporin A on the growth and prolactin binding to Nb-2 rat lymphoma cells

Cyclosporin A, an immunosuppressive agent, inhibited the prolactin stimulated growth of rat lymphoma Nb-2 cells. In the presence of 1 ng/ml of prolactin, 50% inhibition of growth was at 5 x 10(-6) M and the inhibition was reversible. The Kd of cyclosporin A binding to the Nb-2 cells was 10(-7) M and was independent of prolactin. The Kd of prolactin binding to the Nb-2 cells was 2 x 10(-10) M. Cyclosporin A did not influence the binding of prolactin to the cells and vice versa. The inhibitory effect of cyclosporin A on the growth of Nb-2 cells is due to some step other than the binding of prolactin to the cells.

Ligand-induced desensitization of B-cell membrane immunoglobulin-mediated Ca2+ mobilization and protein kinase C translocation

Binding of ligand to B-cell membrane immunoglobulin (mIg) can lead to activation of a number of distinct biologic responses, including altered expression of genes encoding c-fos, c-myc, and Ia, as well as proliferation and immunologic tolerance. Tolerance could reflect a functional uncoupling of receptors from systems that generate intracellular second messengers (i.e., receptor desensitization). To better understand the molecular basis of immune regulation, we examined the ability of mIg to function as a signal transducer after the cell's initial contact with mIg-binding ligand. The results show that ligand binding to as little as 2-10% of mIgM or mIgD renders the cell unresponsive to ligand binding to the reciprocal isotype as judged by Ca2+ mobilization and protein kinase C translocation responses. This heterologous receptor desensitization lasts longer than 24 hr and does not reflect loss of receptor from the cell surface. Studies with the calcium ionophore ionomycin, 1,2-dioctanoyl-sn-glycerol, and the protein kinase inhibitor staurosporine indicate that both protein kinase C-dependent and protein kinase C-independent (staurosporine-insensitive) mechanisms mediate heterologous desensitization after mIg crosslinking.

Functional differences between immunoglobulins M and D expressed on the surface of an immature B-cell line

Crosslinked IgM molecules expressed on the surface of immature B cells mediate responses that inhibit further development, in contrast to the activational and proliferative events that follow crosslinking of the mu heavy chain in mature B cells. Concomitant with this change in IgM signaling capacity is the appearance of surface IgD, which has been proposed to modulate the response elicited by the mu heavy chain. In an attempt to gain insight into the mechanism(s) by which surface IgM is able to generate such disparate responses, delta heavy chain gene transfectants of the murine B-cell lymphoma line WEHI-231 were established. WEHI-231 cells resemble phenotypically immature B cells, in addition to being highly susceptible to the growth-inhibitory effect of surface IgM cross-linking. Endogenous mu and exogenous delta heavy chains expressed on the surface of the transfectants were compared for their role in cell proliferation and on gene expression. Our results indicate that the growth-inhibitory response is associated only with the mu heavy chain and that surface IgD does not mediate such a response. Furthermore, in contrast to IgM, IgD molecules appear to have an inductive effect on the expression of Myc and the endogenous mu and exogenous delta Ig heavy chain genes but not on the expression of the housekeeping gene encoding beta 2-microglobulin. These findings suggest that IgM and IgD are functionally distinct when expressed on the surface of an immature B cell.

Cyclic AMP has the ability to influence multiple events during B cell stimulation

Negative regulators of cellular proliferation are important in maintaining a balanced growth control. In this study we have examined the effects of the diterpene forskolin on various parameters of B cell activation. Forskolin is known to elevate intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels and thereby to influence B cell stimulation. We found that forskolin exerted an inhibitory effect on early as well as late events during stimulation of resting normal human B cells. Cells were activated either by antibodies to surface immunoglobulins (anti-mu), by the monoclonal antibody 1F5 reactive with the CD20 antigen or by 12-O-tetradecanoylphorbol 13-acetate. While anti-mu stimulation induces increased phosphatidylinositol (PI) turnover and [Ca2+]i fluxes, the latter two reagents confer an activation of B cells independent of the PI/Ca2+ pathway. We found a clear inhibitory effect of forskolin on the anti-mu-induced PI turnover and [Ca2+]i fluxes as well as on later parameters of cell activation. There was also a clear inhibition of G1 entry and DNA synthesis when PI/Ca2+-independent activation was employed, indicating that cAMP interferes with B lymphocyte stimulation in several ways. Importantly, forskolin maintained its inhibitory effect when added late after anti-mu stimulation, implying an effect also at multiple stages of activation. When examining the inhibitory effect of forskolin on neoplastic B cells, we found essentially no differences from the responses in normal cells.

Phorbol 12,13-dibutyrate and mitogens increase fructose 2,6-bisphosphate in lymphocytes. Comparison of lymphocyte and rat-liver 6-phosphofructo-2-kinase

The influence of tumour promoters and growth factors on glycolysis and on fructose-2,6-bisphosphate concentration was studied in isolated mouse spleen lymphocytes and in purified B-cells. The intracellular concentration of fructose 2,6-bisphosphate and the rate of lactate release were increased 2-3-fold in spleen lymphocytes exposed to active phorbol esters, mitogenic lectins, interleukin 4 or lipopolysaccharide. The maximal effect was observed after 1 h of exposure. In these cells hexose 6-phosphates increased 2-fold and 6-phosphofructo-2-kinase activity remained unchanged after treatment with phorbol 12,13-dibutyrate or with lectins. Exposure of B-cells to phorbol 12,13-dibutyrate, interleukin 4 or lipopolysaccharide increased the glycolytic flux and the concentration of fructose 2,6-bisphosphate without relation to their mitogenic activity. Lymphocytes and rat liver 6-phosphofructo-2-kinase were partially purified using the same procedure. The lymphocyte enzyme was not inhibited by sn-glycerol 3-phosphate in contrast to the potent inhibition observed in liver. Treatment of both enzymes with the catalytic subunit of the cyclic-AMP-dependent protein kinase failed to inactivate 6-phosphofructo-2-kinase from lymphocytes. These differences suggest that lymphocytes and liver contain different forms of this enzyme.

Phosphatidylinositol metabolism and protein kinase C activation in leukocytes by lipopeptides

The synthetic lipopeptide Pam3Cys-Ser-Ser-Asn-Ala, an analogue of the N-terminal part of bacterial lipoprotein, constitutes a potent macrophage and B lymphocyte activator. In the macrophage cell-line P388D1 Pam3Cys-Ser-Ser-Asn-Ala stimulated phosphoinositol turnover, whereas in small resting lymphocytes no enhanced turnover was observed. Upon lipopeptide stimulation, a translocation of PKC from the cytosol to the plasma membrane was found in the cell line P388D1 but not in lymphocytes. Substituting lipopeptide for diacylglycerol shows that Pam3Cys-Ser-Ser-Asn-Ala leads to an activation of protein kinase C at Ca2+ concentrations of 0.5 mM. Thus, mitogenic lipopeptides constitute novel tools for investigating the molecular mechanism of transmembrane signaling in leukocyte activation.

Studies of surface immunoglobulin-dependent B cell activation

Studies from a number of laboratories have firmly established the potential of surface immunoglobulin-generated signals in B lymphocyte activation. While clearly there are multiple ways of activating B lymphocytes, some of which may not involve surface immunoglobulin, it is clear that crosslinking of surface immunoglobulin whether by antigen or antireceptor antibody can generate signals relevant to B cell activation. Although considerable insight into the mechanism of transduction of mIg-generated signals across the plasma membrane has been realized, a molecular explanation for linking inositol phospholipid hydrolysis to changes within the cytoplasm and nucleus of the B cell is still speculative. A more rigorous definition of the PKC and calcium components of the mIg signal transduction pathway are critical for a thorough understanding of the mechanism of signal transduction by this receptor. The use of tumor cell models allowing selection of mutants within the signalling pathway(s) will be invaluable to fully defining the critical molecular and biochemical events involved in B cell activation.