Molecular mechanisms of transmembrane signaling in B lymphocytes

CD5 is associated with the human B cell antigen receptor complex

On human B cells the antigen receptor complex is composed of the membrane form of the immunoglobulin molecule and the non-covalently associated Ig alpha/beta heterodimer. A small subpopulation of normal B cells and chronic lymphocytic leukemia B cells express (analogous to T cells) the transmembrane molecule CD5, a counterstructure of B cell-specific CD72. Numbers of CD5+ B cells are increased in several physiological and pathological conditions. Moreover, CD5+ B cells are being held responsible for the production of autoreactive antibodies and seem to have signaling characteristics distinct from conventional B cells. On T cells, CD5 associates with the T cell receptor CD3 complex and ligation of CD5 leads to the generation of co-stimulatory signals, that act on T cell activation. We here demonstrate that CD5 is associated with the B cell receptor (BCR) complex and serves as substrate for BCR-induced tyrosine kinase activity. Hence, CD5+ B cells have a unique potential to modulate BCR signals.

The antigen-specific immunoglobulin G receptor is more sensitive to stimulation than the IgM receptor in transfected B cells

A murine lymphoma cell line (M12.4) was transfected with immunoglobulin (Ig) genes encoding a T15+ (idiotype) IgM antibody or an idiotypically identical IgG antibody. Three transfectant clones of each class which showed similar (albeit distinguishable) levels of membrane expression of the transfected genes were used in the study. The response of each cell population to stimulation with anti-T15 antibodies was followed by measurement of the change in the intracellular Ca++ concentration. The IgG transfectants were found to be significantly more responsive to such stimulation than the IgM cells. In contrast, there was no difference in their response to a nonspecific reagent, the calcium ionophore A23187.

Activation of phosphatidylinositol-3' kinase by Src-family kinase SH3 binding to the p85 subunit

Engagement of antigen receptor complexes induces rapid activation of Src-family kinases and association with phosphatidylinositol-3' kinase (PI-3 kinase). Here it was found that the Src homology 3 (SH3) domain of Lyn and Fyn bound to a proline-rich region (residues 84 to 99) within the 85-kilodalton subunit (p85) of PI-3 kinase. The binding of SH3 to the purified kinase led to a five- to sevenfold increase in the specific activity of PI-3 kinase. Ligand-induced receptor stimulation activated PI-3 kinase, and this activation was blocked by a peptide containing residues 84 to 99 of p85. These data demonstrate a mechanism for PI-3 kinase activation and show that binding of SH3 domains to proline-rich target sequences can regulate enzymatic activity.

A 13-amino-acid motif in the cytoplasmic domain of Fc gamma RIIB modulates B-cell receptor signalling

The Fc receptor on B lymphocytes, Fc gamma RIIB (beta 1 isoform), helps to modulate B-cell activation triggered by the surface immunoglobulin complex. Crosslinking of membrane immunoglobulin by antigen or anti-Ig F(ab')2 antibody induces a transient increase in cytosolic free Ca2+, a rise in inositol-3-phosphate, activation of protein kinase C, and enhanced protein tyrosine phosphorylation. Crosslinking Fc gamma RIIB with the surface immunoglobulin complex confers a dominant signal that prevents or aborts lymphocyte activation triggered through the ARH-1 motifs of the signal transduction subunits Ig-alpha and Ig-beta. Here we show that Fc gamma RIIB modulates membrane immunoglobulin-induced Ca2+ mobilization by inhibiting Ca2+ influx, without changing the pattern of tyrosine phosphorylation. A 13-amino-acid motif in the cytoplasmic domain of Fc gamma RIIB is both necessary and sufficient for this effect. Tyrosine at residue 309 in this motif is phosphorylated upon co-crosslinking with surface immunoglobulin; mutation of this residue aborts the inhibitory effect of Fc gamma RIIB. This inhibition is directly coupled to signalling mediated through Ig-alpha and Ig-beta as evidenced by chimaeric IgM/alpha and IgM/beta molecules. The 13-residue motif in Fc gamma RIIB controls lymphocyte activation by inhibiting a Ca2+ signalling pathway triggered through ARH-1 motifs as a result of recruitment of novel SH2-containing proteins that interact with this Fc gamma RIIB cytoplasmic motif.

Induction of phosphatidylinositol turnover and EGR-1 mRNA expression by crosslinking of surface IgM and IgD in the human B cell line B104

We have previously shown that a human B lymphoma cell line, B104, expressed surface IgM (sIgM) and surface IgD (sIgD), and that crosslinking of sIgM and sIgD by anti-IgM antibody (Ab) and anti-IgD Ab, respectively, induced Ca2+ influx to almost the same degree, whereas only sIgM-crosslinking caused B104 cell death. Here, we investigated the accumulation of cyclic AMP (cAMP), the hydrolysis of inositol phosphates, protein kinase C (PKC) activity and the induction of Egr-1 and c-fos mRNA expression by sIgM- and sIgD-crosslinking to examine differences in the signals mediated through sIgM and sIgD in B104 cells. Both sIgM- and sIgD-crosslinking with antibodies induced elevation of cAMP levels, phosphatidylinositol turnover, PKC activation and expression of Egr-1 and c-fos mRNA, although sIgM-crosslinking was more effective than sIgD-crosslinking, presumably due to the higher expression of sIgM than of sIgD. Egr-1 mRNA expression induced by sIgM- and sIgD-crosslinking was inhibited by H7, erbstatin and genistein, but not by HA1004. Erbstatin and genistein inhibited the sIg-crosslinking-induced Egr-1 mRNA expression in a dose-dependent manner parallel to that observed in the inhibition of sIg-crosslinking-induced protein tyrosine phosphorylation. Phorbol myristate acetate induced Egr-1 mRNA expression but forskolin and dibutyryl cyclic AMP did not. These findings suggest that the Egr-1 mRNA activating signals through sIgM and sIgD are protein tyrosine kinase- and PKC-dependent, but protein kinase A-independent. Cyclosporin A (CsA) and FK506 rescued B104 cells from death induced by anti-IgM Ab, but did not affect the expression of Egr-1 and c-fos mRNA, showing that CsA and FK506 affect signal transducers differently from or downstream to these molecules. The difference in signals transduced through sIgM and sIgD in B104 cells is discussed.

Pathogenicity of molecularly cloned bovine leukemia virus

To delineate the mechanisms of bovine leukemia virus (BLV) pathogenesis, four full-length BLV clones, 1, 8, 9, and 13, derived from the transformed cell line FLK-BLV and a clone construct, pBLV913, were introduced into bovine spleen cells by microinjection. Microinjected cells exhibited cytopathic effects and produced BLV p24 and gp51 antigens and infectious virus. The construct, pBLV913, was selected for infection of two sheep by inoculation of microinjected cells. After 15 months, peripheral blood mononuclear cells from these sheep served as inocula for the transfer of infection to four additional sheep. All six infected sheep seroconverted to BLV and had detectable BLV DNA in peripheral blood mononuclear cells after amplification by polymerase chain reaction. Four of the six sheep developed altered B/T-lymphocyte ratios between 33 and 53 months postinfection. One sheep died of unrelated causes, and one remained hematologically normal. Two of the affected sheep developed B lymphocytosis comparable to that observed in animals inoculated with peripheral blood mononuclear cells from BLV-infected cattle. This expanded B-lymphocyte population was characterized by elevated expression of B-cell surface markers, spontaneous blastogenesis, virus expression in vitro, and increased, polyclonally integrated provirus. One of these two sheep developed lymphocytic leukemia-lymphoma at 57 months postinfection. Leukemic cells had the same phenotype and harbored a single, monoclonally integrated provirus but produced no virus after in vitro cultivation. The range in clinical response to in vivo infection with cloned BLV suggests an important role for host immune response in the progression of virus replication and pathogenesis.

Follicular dendritic cells help resting B cells to become effective antigen-presenting cells: induction of B7/BB1 and upregulation of major histocompatibility complex class II molecules

This study was designed to investigate whether follicular dendritic cells (FDC) can activate B cells to a state in which they can function as effective antigen-presenting cells (APC). High buoyant density (i.e., resting) B cells specific for 2,4-dinitro-fluorobenzene (DNP) were incubated with DNP-ovalbumin (OVA) bearing FDC, after which their capacity to process and present to an OVA-specific T cell clone was assessed. The efficacies of alternative sources of antigen and activation signals in the induction of B cell APC function were compared with those provided by FDC. Only FDC and Sepharose beads coated with anti-immunoglobulin (Ig)kappa monoclonal antibody provided the necessary stimulus. FDC carrying inappropriate antigens also induced B cell APC function in the presence of exogenous DNP-OVA. However, in circumstances where soluble DNP-OVA was limiting, FDC bearing complexes containing DNP, which could crosslink B cell Ig receptors, induced the most potent APC function. Analysis by flow cytometry revealed that within 24 h of coculture with FDC, a significant percentage of B cells increased in size and expressed higher levels of major histocompatibility complex class II. By 48 h, an upregulation of the costimulatory molecule, B7/BB1, occurred, but only when exposed to the FDC bearing DNP. Taken together, the results demonstrate that FDC have the capacity to activate resting B cells to a state in which they can function as APC for T cells. The stimuli that FDC provide may include: (a) an antigen-dependent signal that influences the upregulation of B7/BB1; and (b) possibly a signal independent of crosslinking mIg that results in Ig internalization. The relevance of these findings to the formation of germinal centers and maintenance of the humoral response is discussed.

Formation and hydrolysis of cyclic ADP-ribose catalyzed by lymphocyte antigen CD38

CD38 is a 42-kilodalton glycoprotein expressed extensively on B and T lymphocytes. CD38 exhibits a structural homology to Aplysia adenosine diphosphate (ADP)-ribosyl cyclase. This enzyme catalyzes the synthesis of cyclic ADP-ribose (cADPR), a metabolite of nicotinamide adenine dinucleotide (NAD+) with calcium-mobilizing activity. A complementary DNA encoding the extracellular domain of murine CD38 was constructed and expressed, and the resultant recombinant soluble CD38 was purified to homogeneity. Soluble CD38 catalyzed the formation and hydrolysis of cADPR when added to NAD+. Purified cADPR augmented the proliferative response of activated murine B cells, potentially implicating the enzymatic activity of CD38 in lymphocyte function.

Myelin-associated glycoprotein is phosphorylated by protein kinase C

The myelin-associated glycoprotein (MAG) is a neural recognition molecule involved in heterophilic interactions between myelin-forming cells and neurons. To characterize the molecular mechanisms underlying post-translational modifications which may be instrumental in signal transduction following the recognition event, we have studied the stimuli leading to modification of 32P-orthophosphate incorporation into MAG in cultures of oligodendrocytes or transformed differentiated Schwann cells. Here we show that in oligodendrocytes both the 67 and 72 kD isoforms of MAG were phosphorylated exclusively on serine, while in the transformed Schwann cells only the 67 kD isoform was found to be present and phosphorylated. The phorbol ester phorbol-12-myristoyl-13-acetate (PMA) did not affect biosynthesis of the protein backbone, but enhanced incorporation of phosphate by a factor of 2-3, indicating the involvement of protein kinase C. Exclusive phosphorylation of serine residues was also observed, when purified MAG was incubated with protein kinase C in the presence of [gamma-32P]ATP. In searching for the physiological stimuli which may trigger phosphorylation of MAG, cultures of oligodendrocytes were exposed to extracellular signals, such as coculture with dorsal root ganglion and spinal cord neurons carrying the MAG receptor, to membrane fractions of these neurons, monoclonal MAG antibody 513 binding to the recognition site of MAG, or platelet-derived growth factor. None of these additives modified the phosphorylation of MAG. These observations point to the possibility that phosphorylation of MAG is controlled by yet unknown intracellular cues rather than by extracellular signals interacting with cell surface receptors of oligodendrocytes.

Elevation of cytosolic calcium is sufficient to induce growth inhibition in a B cell lymphoma

Recently, we have described that anti-IgM antibodies profoundly inhibited the growth of BKS-2, an immature B cell lymphoma. In this report, we demonstrated that ionomycin alone at very low concentrations (20 nM) inhibited the growth of BKS-2 cells completely. The levels of intracellular Ca2+ induced by the inhibitory concentrations of ionomycin were comparable to those in anti-IgM-treated cells. The growth inhibition caused by ionomycin was reversed by phorbol 12-myristate 13-acetate and lipopolysaccharide. In addition, the immunosuppressants, cyclosporin A and FK506 conferred significant protection from the negative signal induced by ionomycin. However, either cyclosporin A, FK506 or lipopolysaccharide was not found to have direct effect on ionomycin-induced Ca2+ mobilization in BKS-2 cells. Also, ionomycin augmented the anti-IgM-induced growth arrest in these cells. Furthermore, BKS-2 cells that were exposed to anti-IgM or ionomycin underwent apoptosis as characterized by DNA fragmentation. Thus, the characteristics of growth inhibition induced by ionomycin and anti-IgM appeared to be similar in that phorbol 12-myristate 13-acetate, lipopolysaccharide, cyclosporin A and FK506 caused significant reversal from such negative signals and both ionomycin and anti-IgM induced apoptosis in these cells. Altogether, these results showed that the elevation of intracellular Ca2+ alone was sufficient to inhibit the growth of some B lymphoma cells.

Immune stimulation by an antisense oligomer complementary to the rev gene of HIV-1

Mice developed massive splenomegaly and polyclonal hypergammaglobulinemia within 2 days after intravenous injection of a phosphorothioate oligomer that is antisense to a portion of the rev region of the HIV-1 genome. Histologic examination of spleens from injected animals showed marked expansion of a uniform-appearing population of small lymphocytes and many mitoses. Spleen mononuclear cells (SMNCs) from injected animals showed approximately a 10-fold-increased uptake of [3H]thymidine and production of IgM and IgG. Flow cytometry analysis indicated that the responding cells were predominantly B-lymphocytes. The anti-rev oligomer also was mitogenic in vitro and stimulated immunoglobulin production by normal mouse SMNCs and human peripheral blood mononuclear cells. Similar immunologic effects were observed with an anti-rev 21-mer phosphorothioate, truncated at the 3' end, but not with a 20-mer human p53 antisense phosphorothioate or a 28-mer anti-rev phosphodiester. These observations are consistent with the possibility that DNA sequences homologous to the rev gene participate in the regulation of mammalian lymphocyte activation, proliferation and maturation.

The structure of the mu/pseudo light chain complex on human pre-B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre-B cells can express mu heavy (H) chains at the cell surface in association with pseudo (psi) L chains. This complex may be essential for B cell development. We have investigated the composition of the mu/psi L chain complex of a human pre-B cell line, in view of its potential role in transmembrane signal transduction. The mu/lambda receptor of a mature B cell line was analyzed in comparison. The mu/psi L chain complex is associated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the mu/lambda receptor, the lyn and lck Tyr kinases could clearly be identified. The mb-1 and B29 proteins in both mu/lambda and mu/psi L chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase also phosphorylates the src-related kinases in the mu/lambda receptor and a 43-kDa mu-associated protein that is present in both complexes. The 43-kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.

T cell-dependent induction of NF-kappa B in B cells

In comparison to B cell stimulation mediated by surface immunoglobulin (Ig) antigen receptor ligation, little is known about the intracellular events associated with T cell-dependent B cell responses. A model for the efferent phase of T cell-B cell interaction was used to examine the capacity of activated T cells to trigger nuclear expression of the trans-acting transcription factor, NF-kappa B, in B cells. Fixed, activated, but not fixed, resting Th2 cells were found to induce increased binding activity for a kappa B site-containing oligonucleotide in a time-dependent manner. This induction of NF-kappa B was eliminated by an antibody directed against a 39-kD cell interaction protein on activated T cells as well as by a soluble form of B cell CD40. Of particular relevance to intracellular signaling, NF-kappa B induction was not diminished by prior depletion of B cell protein kinase C (PKC) with phorbol myristate acetate. These results strongly suggest that T cell-dependent B cell stimulation is associated with NF-kappa B induction via p39-CD40 interaction and that this is brought about by non-PKC dependent signaling, in marked contrast to the previously documented requirement for PKC in sIg receptor-mediated stimulation. This suggest that NF-kappa B responds to more than one receptor-mediated intracellular signaling pathway in B cells and may be part of a "final common pathway" for B cell stimulation.

Lipopolysaccharide nonresponder cells: the C3H/HeJ defect

Since its initial discovery as endotoxin resistant, the C3H/HeJ mouse has been extensively studied and used as a comparative model to help reveal the mechanism under genetic control which governs host responses to endotoxin. Most of the research has focused on the B lymphocyte and macrophage of this strain which fail to be activated by LPS. Recently, specific LPS binding proteins have been isolated on lymphocytes and other cells; however a receptor which transduces an activation signal has not been isolated as yet from responder cells which is missing or altered on C3H/HeJ nonresponder cells. Investigations into the signal transduction pathways used by C3H/HeJ B cells when they are activated by a protein mitogen have been found to be similar to those used by LPS responder cells when activated by LPS. Protein kinase C and tyrosine kinase, which phosphorylate signal proteins in cells have been found to be operative in C3H/HeJ and C3H/OuJ B cells. In both cases, DNA synthesis is shut off by either PKC or PTK blockade; however, PTK inhibition will also block activation of PKC stimulated DNA synthesis, indicating tyrosine kinase initiated phosphorylation may regulate the PKC signal pathway. Further analysis of the proteins that are phosphorylated in LPS responder and LPS nonresponder B cells is needed before conclusions can be drawn as to whether the defect in C3H/HeJ cells resides in the signal pathway leading to gene activation and proliferation. Nevertheless, the notion of a missing or defective signal receptor still remains as a working hypothesis to explain C3H/HeJ cell hyporesponsiveness to LPS. Isolation of the Lpsn gene and its product will provide the evidence needed for a clearer understanding of how LPS reacts with cells.

Receptor-mediated calcium signal playing a nuclear third messenger in the activation of antigen-specific B cells

We have studied receptor-mediated calcium signals in antigen-specific B cells (trinitrophenol-specific B cell clone, TP67.21) using a confocal fluorescence microscope with an argon ion laser (488 nm) and a He-Cd laser (325 nm). Confocal fluorescence images of fluo-3 loaded B cells, excited by an argon ion laser, became much brighter and more nonhomogeneous than those before antigen stimulation. Time-dependent fluorescence changes in intensities were abrupt and quite similar to the patterns of the intracellular calcium ion concentration [Ca2+]i observed by a conventional fluorescence microscope using fura-2. From the morphological patterns of the calcium images, the parts of the bright fluorescence seemed to belong to the nucleus in B cells. To confirm the above events we measured the confocal fluorescence images of the nucleus. From the fluorescence images of co-loaded Hoechst 33342 (a DNA-specific fluorescent probe), which excited by a He-Cd laser, the brighter parts of the fluo-3 fluorescence intensities were identified to the nucleus in B cells. This suggested the possibility that the increased intranuclear calcium ions may play a nuclear third messenger in B cells.

Immunostimulatory potential of smokeless tobacco extract in in vitro cultures of murine lymphoid tissues

Despite numerous studies on the general toxicologic effects of smokeless tobacco (ST) little immunotoxicologic information is available. As a first step in assessing the potential activity of ST on the immune system, the effects of an aqueous extract of ST was studied in in vitro cultures of mouse lymphoid cells. There was a significant increase in the proliferation of spleen cells cultured with different concentrations of ST extract. The polyclonal IgM antibody responses as determined by protein A plaque assay were also elevated in ST stimulated spleen cell cultures. Similar immunostimulatory results were seen in the mesenteric lymph node cell cultures also. ST extract was able to stimulate the spleen cells of the immune defective CBA/N mice. The mitogenic ability of ST extract may not be due to lipopolysaccharide (LPS) contamination as determined by its response in the LPS resistant C3H/HeJ mice spleen cells. ST extract was mitogenic not only to B cells but also to T cells. However the magnitude of response was less in T cells than in B cells. The proliferation of T cells was not accompanied by secretion of IL-2 or expression of IL-2 receptors on T cells. However there was an increase of IL-1 activity in spleen cells cultured with ST extract. Finally, activation of B or T lymphocytes by ST did not result in the elevation of intracellular calcium levels. Since ST is consumed orally, the chronic immunostimulation by ST in oral mucosal lymphoid tissues may be associated with the increased incidence of gingivitis, leukoplakia and oral cancer seen in human ST users.

Multiple forms of the G protein-related beta subunit in Daudi lymphoblastoid cells

We have explored the forms of the G protein-related beta subunit which are present in Daudi lymphoblastoid cells. Northern blotting with labeled beta-1 and beta-2 probes indicates that two messages of 3.3 kb and 1.7 kb are present for both beta-1 and beta-2, implying that multiple forms of the beta subunit are present. Antibodies were raised against two peptides of the beta subunit (residues 1-23 and 127-145). Both antibodies detected subunits at 35 kDa and 31 kDa, of which the 35-kDa form predominates in the membrane fraction and the 31-kDa one in the cell cytosol. Crosslinking of the membrane fraction with the cleavable crosslinker (DTSSP) caused a simultaneous diminution in the 31-kDa form while increasing the amount of the 35-kDa form--a pattern which was reversed upon the reduction of these crosslinks with DTT. Studies of the soluble form indicate that this is truly a soluble protein since centrifugation at 200,000 x g for 2 h did not diminish the levels of the protein in the soluble fraction. Sedimentation analysis indicates that the soluble beta-homologue is found in fractions which overlap with those which contain the mu chain of immunoglobulin at a position clearly distinct from the expected positions of free mu or free beta. Our results suggest that at least two forms of a subunit which is closely related to, or identical with, the beta subunit of G proteins are present in Daudi cells.

Structural and functional properties of ribosomal protein L7 from humans and rodents

By subtractive screening of a library made from mRNA of lipopolysaccharide (LPS)-stimulated mouse B lymphocytes we isolated cDNA-clones encoding the ribosomal protein L7. Human L7 mRNA was cloned from activated T-lymphocytes. Although no specific function of L7 in the translation apparatus is known as yet, it should be a critical one as indicated by its high degree of structural conservation during evolution and its regulated expression in lymphoid cells. Human and rodent L7 proteins carry sequences similar to the basic-region-leucine-zipper(BZIP)-motif of DNA-binding eucaryotic transcription factors. We show here that the region of L7 carrying the latter motif mediates L7-dimerization and stable binding to DNA and RNA. A preferential binding to RNA-structures is demonstrated.

Biochemistry of B lymphocyte activation

The activation of B lymphocytes from resting cells proceeds from the events of early activation to clonal proliferation to final differentiation into either an antibody-secreting plasma cell or a memory B cell. This is a complex activation process marked by several alternative pathways, depending on the nature of the initial antigenic stimulus. Over the past 5-10 years, there has been an explosion of studies examining the biochemical nature of various steps in these pathways. Some of that progress is reviewed here. In particular, we have described in detail what is known about the structure and function of the AgR, as this molecule plays a pivotal role in B cell responses of various types. We have also reviewed recent progress in understanding the mechanism of action of contact-dependent T cell help and of the cytokine receptors, particularly the receptors for IL-2, IL-4, and IL-6. Clearly, all of these areas represent active areas of investigation and great progress can be anticipated in the next few years.

A sense phosphorothioate oligonucleotide directed to the initiation codon of transcription factor NF-kappa B p65 causes sequence-specific immune stimulation

Antisense oligonucleotides have proved effective in achieving targeted inhibition of gene expression. In such experiments, sense oligonucleotides have frequently been used as a control for nonspecific effects, but the results have been variable, raising questions about the reliability of sense oligomers as a control. It is possible that some of the effects of sense oligonucleotides may be specific. We have shown that phosphorothioate antisense oligonucleotides to the p65 subunit of NF-kappa B, a transcription factor, cause a block in cell adhesion. In our efforts to test the efficacy of NF-kappa B p65 oligonucleotides in vivo, we unexpectedly observed that the control p65-sense, but not the p65-antisense, oligonucleotides caused massive splenomegaly in mice. In the current study we demonstrate a sequence-specific stimulation of splenic cell proliferation, both in vivo and in vitro, by treatment with p65-sense oligonucleotides. Cells expanded by this treatment are primarily B-220+, sIg+ B cells. The secretion of immunoglobulins by the p65-sense oligonucleotide-treated splenocytes is also enhanced. In addition, the p65-sense-treated splenocytes, but not several other cell lines, showed an upregulation of NF-kappa B-like activity in the nuclear extracts, an effect not dependent on new protein or RNA synthesis. These results demonstrate that phosphorothioate oligonucleotides can exert sequence-specific effects in vivo, irrespective of sense or antisense orientation.

Dual antigen recognition by B cells

Foreign antigens opsonized by complement degradation products may be bound by both the B-cell antigen receptor (BCR) and CR2-CD19 complexes. Under these circumstances, the extensive cytoplasmic tail of CD19 endows the BCR with additional tyrosine kinase activity and with potential docking sites for molecules involved in cell signalling. Here, Carel van Noesel and colleagues argue that cooperation between BCR and CR2-CD19 at both the extra- and intracellular level provide for optimal recognition of antigen and amplification of ensuing intracellular signals.

Tyrosine phosphorylation and the mechanism of signal transduction by the B-lymphocyte antigen receptor

Lymphocytes provide a powerful defense against infectious agents with their exquisite ability to distinguish between macromolecules of the host and macromolecules of foreign invaders. This ability derives from the antigen receptors, which are created from precursor minigenes by a series of genetic-recombination reactions [1, 2] and from cellular mechanisms that inactivate lymphocytes expressing self-reactive antigen receptors [3, 4]. Central to the problem of distinguishing self from non-self is the means by which these antigen receptors recognize antigen and transmit the information of that recognition to the interior of the cell. This information ultimately leads to lymphocyte activation or inactivation, depending upon the context. In this review, I shall summarize recent advances in understanding the structural elements of the antigen receptor complex of B lymphocytes and in understanding the signal-transduction events initiated by this receptor.

Inhibition of calcium-dependent pathways of B-cell activation by DMBA

The purpose of the experiments described in these studies was to determine the effects of 7,12-dimethylbenz[a]anthracene (DMBA) on B-cell activation produced by anti-IgD antibodies and interleukin-4 (IL-4). B and T cells are known to share many of the same biochemical pathways for cell activation by mitogen and antigen receptors. Previous studies in this laboratory have shown that DMBA inhibits mitogen-induced Ca2+ mobilization in murine and human T cells and produces an increase in intracellular Ca2+ in resting cells. The results of the present studies demonstrate that DMBA increases Ca2+ in resting B cells and inhibits B cell activation produced by anti-IgD antibodies, as measured by mobilization of free intracellular Ca2+ and [3H]thymidine incorporation. The proliferative response of B cells to insolubilized anti-IgD was suppressed only when cells were preexposed to DMBA. In contrast, IL-4 pathways of B-cell activation were insensitive to inhibition by DMBA, even when cells were preexposed. The induction of Class II MHC antigen (Ia) antigens on B cells by IL-4 was also found to be insensitive to DMBA treatment. These results suggest that DMBA suppresses only Ca(2+)-dependent pathways of B cell activation and indicate that altered Ca2+ homeostasis may be responsible for immunosuppression induced by this agent.

Membrane region of surface IgM is not sufficient for transducing growth inhibitory signals in an immature B cell line WEHI-231

The murine B lymphoma line WEHI-231 is representative of immature B cells. Like normal immature B cells, WEHI-231 is susceptible to growth arrest following cross-linking of surface IgM (sIgM). Previously, we have shown using a WEHI-231 immunoglobulin (Ig) delta-transfectant that sIgD cross-linking failed to initiate growth arrest, in contrast to sIgM. In this report, we extend our research to investigate the structural requirement of Ig mu chain for regulating growth inhibition. Recombinant, chimeric Ig molecules delta/mu m and mu/delta m consisting of exons encoding extracellular delta and mu domains and membrane regions of different isotypes were constructed and introduced into WEHI-231 cells. A similar approach was used for sIgG2b-expressing transfectants. Our findings indicate that the mu m region is not sufficient for regulation of growth inhibition in WEHI-231 cells and suggest that additional extracellular region(s) of mu chain may be required for this response.

Selective activation of p42 mitogen-activated protein (MAP) kinase in murine B lymphoma cell lines by membrane immunoglobulin cross-linking. Evidence for protein kinase C-independent and -dependent mechanisms of activation

Cross-linking of membrane immunoglobulin (mIg), the B lymphocyte antigen receptor, with anti-receptor antibodies stimulates tyrosine phosphorylation of a number of proteins, including one of 42 kDa. Proteins with a similar molecular mass are tyrosine-phosphorylated in response to receptor stimulation in other cell types and have been identified as serine/threonine kinases, termed mitogen-activated protein (MAP) kinases or extracellular signal-regulated kinases (ERKs). The MAP kinases constitute a family of related kinases, at least three of which have molecular masses of 40-45 kDa. In this paper we show that mIg cross-linking stimulated the myelin basic protein phosphotransferase activity characteristic of MAP kinase in both mature and immature murine B cell lines. This enzyme activity co-purified on three different columns with a 42 kDa protein that was tyrosine-phosphorylated (pp42) in response to mIg cross-linking and which reacted with a panel of anti-(MAP kinase) antibodies. Although immunoblotting with the anti-(MAP kinase) antibodies showed that these B cell lines expressed both 42 kDa and 44 kDa forms of MAP kinase, only the 42 kDa form was activated and tyrosine-phosphorylated to a significant extent. Activation of protein kinase C (PKC) with phorbol esters also resulted in selective tyrosine phosphorylation and activation of the 42 kDa MAP kinase. This suggested that mIg-induced MAP kinase activation could be due to stimulation of PKC by mIg. However, mIg-stimulated MAP kinase activation and pp42 tyrosine phosphorylation was only partially blocked by a PKC inhibitor, the staurosporine analogue Compound 3. In contrast, Compound 3 completely blocked the ability of phorbol esters to stimulate MAP kinase activity and induce tyrosine phosphorylation of pp42. Thus mIg may activate MAP kinase by both PKC-dependent and -independent mechanisms.

Differential structure-function requirements of the transmembranal domain of the B cell antigen receptor

By generating phosphorylcholine (PC)-specific, wild-type (mu), and chimeric (mu-I-A alpha) antigen receptor transfectants of mature B cells, we have shown that the COOH terminus of the mu heavy chain is essential for three major functions: immediate signal transduction (measured as changes in intracellular Ca2+), antigen presentation, and induction of immunoglobulin M secretion. A more detailed analysis of structural requirements of the COOH-terminal domains contributing to these functions was achieved by systematically replacing the spacer, cytoplasmic, and transmembranal domains of the mu-I-A alpha chimeric chain with those of mu. Using this rescue approach, we show that the carboxyl two-thirds of the transmembranal domain (proximal to the cytoplasmic domain) is required for induction of intracellular Ca2+, whereas the complete transmembranal domain is required for the function of antigen presentation but is dispensable for induction of antibody secretion.

Immunoglobulin M and D antigen receptors are both capable of mediating B lymphocyte activation, deletion, or anergy after interaction with specific antigen

A series of immunoglobulin (Ig)-transgenic mice were generated to study the functional capabilities of the IgM and IgD classes of B lymphocyte antigen receptor in regulating both cellular development and responses to specific antigen. B cells from Ig-transgenic mice expressing either hen-egg lysozyme (HEL)-specific IgM or IgD alone were compared with B cells from mice that coexpressed IgM and IgD of the same anti-HEL specificity. In all three types of Ig-transgenic mice, conventional B cells specific for HEL exhibited exclusion of endogenous Ig expression and matured to populate the usual microenvironments in peripheral lymphoid tissues. These peripheral B cells could be stimulated by HEL through either IgM or IgD antigen receptors to generate T cell dependent antibody production in vivo or to enhance T cell independent proliferative responses to lipopolysaccharide in vitro. Conversely, when HEL was encountered in vivo as a self-antigen, B cells expressing HEL-specific IgM or IgD alone were both rendered tolerant. In each case this occurred by clonal anergy in response to soluble autologous HEL, and clonal deletion when HEL was recognized as a membrane-bound self-antigen. Taken together, these findings indicate that IgM and IgD antigen receptors expressed alone on conventional B cells can support normal differentiation, antigen-dependent activation, and induction of self-tolerance, the only overt difference lying in a greater degree of receptor downregulation for IgM relative to IgD after induction of clonal anergy by soluble HEL.

Heterogeneity of immunoglobulin-associated molecules on human B cells identified by monoclonal antibodies

Two covalently linked transmembrane molecules, encoded in mice by the mb-1 and B29 genes, have been defined as integral components of the antibody receptor units expressed on B cells. We have produced monoclonal antibodies against an exposed extracellular epitope on the putative human equivalent of the mouse B29 product. These antibodies, CB3-1 and -2, were used to show that cytoplasmic expression of this molecule begins in human pro-B cells (terminal deoxynucleotidyltransferase-positive, mu chain-negative), whereas surface expression coincides strictly with surface immunoglobulin expression of all isotypes. Immunochemical analysis of the human immunoglobulin-associated molecules revealed greater molecular heterogeneity than has been noted for the murine analogues. This molecular heterogeneity of immunoglobulin-associated molecules varied as a function of differentiation stage and the immunoglobulin isotypes expressed by B-lineage cells. Our data support the hypothesis that biochemical heterogeneity of the surface immunoglobulin-associated molecules may contribute to the variability in biological effects of antigen receptor crosslinkage on B cells of different maturational stages. Because the CB3 antibodies are capable of down-modulating the antigen receptors on all B cells, they may prove therapeutically useful as universal B-cell suppressants.

Characterization of molecular components associated with surface immunoglobulin M in human B lymphocytes: presence of tyrosine and serine/threonine protein kinases

To characterize the signal transduction through the antigen receptor (AgR) on human B lymphocytes, we analyzed its association with other molecular components. The surface IgM (sIgM) complex isolated in digitonin contains two surface expressed polypeptides--the previously described Ig alpha and Ig beta proteins--covalently linked to each other in a 48/39-kDa heterodimer. We show herein that the human sIgM complex isolated from the Burkitt's lymphoma cell line, Ramos, or from dense tonsillar B cells contains additional molecules--160 kDa and 75 kDa in size--and enzymatic activities able to phosphorylate on tyrosine as well as serine/threonine residues the 39-, 48-, 75- and 160-kDa polypeptides. By specific immunoprecipitation with antibodies to src-family kinases, we consistently detected p56lyn in the sIgM complex. In the Ramos cell line, both p56lck and p59fyn activity were also observed, although to a much lesser extent than p56lyn. These kinases are associated with sIgM before cell stimulation. As shown by two-dimensional electrophoresis, they interact in a tight complex with multimeric forms of the Ig alpha and Ig beta components. The kinases are active in vitro but must be highly regulated in vivo: Western blotting with anti-phosphotyrosine antibodies revealed that stimulation of the AgR on viable B cells increased detectable phosphotyrosine residues on the components present in the sIgM complex. Based on these phosphorylation changes, the 39-, 48-, 75- and 160-kDa molecules are likely to be functionally active elements in an IgM complex crucial for the transduction of the antigenic signal.

Effect of priming with a thymus-independent antigen on susceptibility to B-cell tolerance

The effects of priming on the susceptibility of B-cell subsets to tolerance induction have been tested in a model system in which anti-immunoglobulin (anti-Ig) has been employed as a surrogate for tolerogen. T-cell-depleted B cells were primed in vitro with fluorescein or trinitrophenylated Ficoll (a thymus-independent (TI) antigen) and then exposed overnight to anti-Ig to attempt to induce B-cell anergy. Primed cells were relatively resistant to this tolerance protocol and resistance was hapten specific. The dose response and kinetics suggested that this process was not due to receptor blockade or modulation, but was an active process. Moreover, this priming for resistance to tolerance was reproduced in vivo upon intraperitoneal treatment with haptenated Ficoll. Such in vivo priming for tolerance resistance was long-lasting and did not occur with a thymus-dependent priming protocol with fluoresceinated hemocyanin. These results are discussed in terms of TI priming to drive B cells into cycle and express novel functional and phenotypic properties.

Transient down-regulation of PKC-zeta RNA following crosslinking of membrane IgM on WEHI-231 B lymphoma cells

Regulation of protein kinase C (PKC) isoform mRNAs has been studied in the immature, murine B lymphoma WEHI-231 by the MAPPing protocol and by slot blot analysis of unamplified mRNA. This membrane IgM (mIgM)-positive cell line has been previously used as a model to study signal transduction by mIgM in immature B lymphocytes and the role of those signals in the induction of immune tolerance in the B cell compartment. Stimulation of the cells by anti-mu antibodies, phorbol ester, or Ca2+ ionophore caused growth arrest and death of the cells. IL 4 and IL 5 slowed the growth of the cells. Of these stimuli, only anti-mu stimulation affected PKC mRNA levels. Anti-mu treatment caused a transient decrease in the amount of PKC-zeta isoform mRNA within 3 hr. Within 24 hr levels returned toward normal. Anti-mu had little or no effect on the expression of mRNA for the alpha, beta, delta, or epsilon isoforms of PKC. WEHI-231 cells do not express PKC-gamma. Although anti-mu treatment blocked progression of the cells from the G0/G1 stage into the S phase of cell cycle, viable sort selected cells in either the G0/G1 or the S/G2/M phases showed no clear difference in the expression of PKC-zeta message. Thus, there is not preferential regulation of expression of PKC-zeta during stages of the cell cycle. The results show that mIgM on WEHI-231 cells can transduce a signal that is not mediated by PKC or Ca2+ mobilization alone. The signal causes transient, selective down-regulation of mRNA encoding the zeta PKC isoform.

Examination of B lymphoid cell lines for membrane immunoglobulin-stimulated tyrosine phosphorylation and src-family tyrosine kinase mRNA expression

Crosslinking of membrane immunoglobulin (mIg) on B cells induces two signal transduction pathways: protein tyrosine phosphorylation and phosphoinositide turnover. A panel of murine and human B cell-lines, representing different stages of B cell development, was examined for the presence of anti-immunoglobulin-induced protein tyrosine phosphorylation. Of 10 B cell lines examined, only one, the human Raji cell line, had no detectably induced protein tyrosine phosphorylation. The pattern of proteins that were phosphorylated on tyrosine in response to mIg crosslinking differed somewhat in cell lines representing different stages of B cell development. Differences in the levels of constitutive phosphorylation of proteins were also observed between the cell lines. The identity of the tyrosine kinase(s) activated by membrane immunoglobulin ligation is not known. However, members of the src family of intracellular tyrosine kinases have been implicated as signal transduction molecules. As the tyrosine phosphorylation of proteins is a general phenomenon of signal transduction by membrane immunoglobulin, the tyrosine kinase(s) activated by it might be expected to be present in all cell lines in which the tyrosine phosphorylation signalling occurs. Therefore we examined these B cells for expression of mRNAs encoding the eight known src-like tyrosine kinases. Surprisingly, all eight kinase mRNAs were expressed in at least some of the B cell lines examined. The expression pattern of the fyn, hck, and lck genes suggests that expression of these kinases may be developmentally regulated in the B cell lineage. Three of the kinases, p55blk, p53/p56lyn and p60src, were detected in all 10 B cell lines. Whereas the src gene shows a ubiquitous pattern of expression, the expression of the blk and lyn genes is mostly restricted to cells of hematopoietic origin, and more especially B lymphoid cells. Thus, p55blk and p53/p56lyn may be particularly good candidates for the membrane immunoglobulin-activated tyrosine kinase.

Inhibition of protein kinase C results in decreased expression of bovine leukemia virus

The in vitro expression of bovine leukemia virus (BLV) in short-term cultured bovine peripheral blood mononuclear cells (PBMC) is associated with increased spontaneous lymphocyte blastogenesis. The purpose of this study was to determine whether intracellular pathways responsible for antigen- or mitogen-induced lymphocyte blastogenesis were also responsible for induction of BLV expression. The protein kinase C (PKC) inhibitor 1-(5-isoquinolinylsulfonyl)-3-methylpiperazine dihydrochloride (3-methyl H7) decreased blastogenesis in a dose-dependent manner, as measured by [3H]thymidine incorporation, in unstimulated, lipopolysaccharide-stimulated and phorbol ester (PMA)-stimulated BLV-infected PBMC. Similarly, 3-methyl H7 decreased BLV expression, as measured by production of gp51 envelope antigen or p24gag antigen, in BLV-infected PBMC under the same conditions. Using an RNase protection assay, the inhibition of BLV expression by 3-methyl H7 was shown to be due to decreased transcriptional activity. The cyclic GMP-dependent protein kinase and cyclic AMP-dependent protein kinase inhibitor N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA1004) did not inhibit either BLV expression or blastogenesis of BLV-infected bovine PBMC. Additional evidence for the PKC-dependent expression of BLV was obtained by using a persistently BLV-infected B-lymphocyte cell line, NBC-13. Activation of PKC by PMA in NBC-13 cells increased BLV expression. 3-methyl H7 decreased the PMA-induced expression of BLV in NBC-13 cells in a dose-dependent manner, whereas HA1004 did not inhibit this expression. These results identify a mechanism for the induction of BLV expression through PKC activation and therefore indicate that latency and replication of BLV is controlled by normal B-lymphocyte intracellular signaling pathways.

Cytoplasmic domain heterogeneity and functions of IgG Fc receptors in B lymphocytes

B lymphocytes and macrophages express closely related immunoglobulin G (IgG) Fc receptors (Fc gamma RII) that differ only in the structures of their cytoplasmic domains. Because of cell type-specific alternative messenger RNA splicing, B-cell Fc gamma RII contains an insertion of 47 amino acids that participates in determining receptor function in these cells. Transfection of an Fc gamma RII-negative B-cell line with complementary DNA's encoding the two splice products and various receptor mutants indicated that the insertion was responsible for preventing both Fc gamma RII-mediated endocytosis and Fc gamma RII-mediated antigen presentation. The insertion was not required for Fc gamma RII to modulate surface immunoglobulin-triggered B-cell activation. Instead, regulation of activation involved a region of the cytoplasmic domain common to both the lymphocyte and macrophage receptor isoforms. In contrast, the insertion did contribute to the formation of caps in response to receptor cross-linking, consistent with suggestions that the lymphocyte but not macrophage form of the receptor can associate with the detergent-insoluble cytoskeleton.

Relationships between the degree of cross-linking of surface immunoglobulin and the associated inositol 1,4,5-trisphosphate and Ca2+ signals in human B cells

Cross-linking of surface immunoglobulin (Ig) receptors on human B cells leads to the activation of a tyrosine kinase. The activated tyrosine kinase subsequently phosphorylates a number of substrates, including phospholipase C-gamma. This enzyme breaks down phosphoinositol bisphosphate to form two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate, leading to the activation of protein kinase C and the release of intracellular Ca2+ respectively. We have used h.p.l.c. and flow cytometry to measure accurately the inositol phosphate turnover and Ca2+ release in anti-Ig-stimulated human B cells. In particular, we have examined the effect of dose of the cross-linking antibody on the two responses. The identity of putative messenger inositol phosphates has been verified by structural analysis, and the amounts of both inositol phosphates and Ca2+ present have been quantified. In the Ramos Burkitt lymphoma, which is very sensitive to stimulus through its Ig receptors, both inositol phosphate production and Ca2+ release were found to be related to the dose of anti-Ig antibody applied. This suggests that phospholipase C-mediated signal transduction in human B cells converts the degree of cross-linking of the immunoglobulin receptor quantitatively into intracellular signals.

The origin and fate of beta 2m-free MHC class I molecules induced on activated T cells

We report here that the expression of major histocompatibility complex (MHC) class I heavy chains not associated with beta 2-microglobulin is induced on resting human T cells by a variety of stimuli. These beta 2m-free class I heavy chains are not transported as such from the endoplasmic reticulum but originate from surface beta 2m-associated MHC class I molecules. beta 2m-free class I heavy chains are spontaneously released from the surface of activated cells. Cross-linking of beta 2m-free class I heavy chains with specific monoclonal antibodies results in the rapid down-regulation and internalization of these molecules. In contrast, beta 2m-associated MHC class I molecules display a different pattern of modulation. Previously, we reported that beta 2m-free class I heavy chains interact with CD8 molecules expressed on the same activated T cells. We propose that interactions between these molecules are involved in a mechanism regulating the function of activated T cells.

Evidence for clonal heterogeneity of the expression of six protein kinase C isoforms in murine B and T lymphocytes

Protein kinase C (PKC), which plays a pivotal role in lymphocyte activation, represents a homologous family of at least nine proteins. Seven genes that encode PKC proteins have been identified. Since the regulatory properties and substrate specificities of the isoforms are not identical in vitro, it is possible that each isoform plays a unique role in cell activation. Toward an understanding of the role of PKC isoforms in lymphocyte activation we have studied the expression of mRNA encoding six of the isoforms (alpha, beta, gamma, delta, epsilon, and zeta) in T cell clones and B cell lines. PKC isoform phenotyping was done by MAPPing using isoform-specific primers and slot-blot analyses of mRNA were performed using specific probes. T cell clones and B cell lines were determined to express levels of the delta, epsilon, and zeta isoforms of PKC that were detectable by MAPPing. Plasmacytomas did not express PKC-beta message detectable by MAPPing. Slot blot analyses and Western blot analyses with peptide-specific antibody confirmed that B cell plasmacytomas did not express PKC-beta mRNA or protein. T cell clones and B cell lines were similar in that none expressed PKC-gamma. In cells that expressed PKC isoforms that were detectable by the MAPPing protocol, there was heterogeneity in the relative abundance of isoform mRNA (PKC-delta and -beta) and protein (PKC-beta and -epsilon). Such diversity of isoform expression could be responsible for the differential responsiveness of lymphocyte clones to activating stimuli.

Association between B-lymphocyte membrane immunoglobulin and multiple members of the Src family of protein tyrosine kinases

Treatment of B lymphocytes with antibodies to membrane immunoglobulin (Ig) stimulates protein tyrosine phosphorylation. We have examined the phosphorylation in vitro of proteins associated with membrane Ig. The Src family protein tyrosine kinases p53/56lyn, p59fyn, and p56lck are associated with membrane Ig in spleen B cells and B-cell lines and undergo phosphorylation in vitro. The pattern of expression of Src family protein tyrosine kinases in B cells varied. Our studies suggest that multiple kinases can potentially interact with membrane Ig and that within any one B-cell type, all of the Src family kinases expressed can be found in association with membrane Ig. We also observed that the Ig-associated Ig alpha protein, multiple forms of Ig beta, and proteins of 100 and 25 kDa were tyrosine phosphorylated in vitro. The 100- and 25-kDa proteins remain unidentified.

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.

Egr-1 mRNA expression is independent of regulatory proliferative responses in the immature B cell line WEHI-231

We have recently reported cellular growth arrest induced following crosslinking of surface IgM (sIgM) but not surface IgD (sIgD) in the WEHI-231 cell line, representative of the immature B cell stage, and its delta heavy chain (delta) transfectant. An initial report has indicated WEHI-231.7, a subclone of WEHI-231, failed to express Egr-1 mRNA following sIgM crosslinking, in contrast to significant up-regulation found in mature B lymphocytes. The implication for linkage between selective surface immunoglobulin (sIg) signal transduction, expression of immediate/early genes and control of cellular growth imposes an attractive model for induction of immature B cell tolerance. Our investigations examined the relationships between Egr-1 mRNA expression and growth regulation in WEHI-231, WEHI-231.7 and their respective delta-transfectants (WEHI-delta, WEHI-delta 7). We report sIgM and sIgD crosslinking leads to a rapid increase of Egr-1 mRNA expression in WEHI-231 and WEHI-delta but not in the subclone WEHI-231.7 and WEHI-delta 7. Nevertheless, both WEHI-231, WEHI-231.7 and their delta-transfectants demonstrate the ability to induce growth arrest following sIgM but not sIgD crosslinking. Furthermore, we found Egr-1 expression could be achieved by direct activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) circumventing the classical sIg activated phosphatidylinositol signal transduction pathway. Our results suggest Egr-1 expression does not directly participate in growth regulation of immature B cell clones but rather is a consequence of signal transduction through sIg.

Inhibitory and stimulatory signaling via immunoglobulin receptors: dichotomous responses elicited in clonal B cell populations

The paradox that cross-linking of IgM antigen receptors on B cells by native or surrogate antigen can inhibit, as well as stimulate, B cell functions has previously been attributed to changes during maturation and activation, programming either a negative or a positive response at defined developmental stages. In contrast to this concept, we show here that some B cells possess the potential for both types of response at the same stage. In three clonal malignant human B cell populations, bivalent soluble monoclonal antibodies to IgM or idiotype, but not IgD completely inhibited spontaneous DNA synthesis, but significantly induced [3H]thymidine uptake when coupled to insoluble compounds. In co-incubation experiments mitogenic stimuli were dominant over inhibitory ones and were still effective after prolonged pretreatment of the B cells with inhibitory reagents. Ionomycin, known to increase intracellular calcium levels, and low doses of phorbol ester, described to activate protein kinase C, also suppressed DNA synthesis. High doses of phorbol ester alone or in combination with ionomycin, however, induced DNA synthesis in two of the lymphomas. We conclude that some B cells may respond to cross-linking of surface IgM in a dose-dependent manner so that all signals increase DNA synthesis once a threshold has been reached. These dose-dependent effects may in part involve signaling via breakdown of membrane inositol phosphates.