Molecular mechanisms of transmembrane signaling in B lymphocytes

Lymphoma models for B cell activation and tolerance. VIII. Cross-desensitization by sIgM and sIgD and its effects on growth regulation by anti-isotype antibodies

ECH408-1 is a murine B cell lymphoma expressing idiotypically and allotypically distinguishable transfected and endogenous IgD. Previously, we demonstrated that this cell line was not growth inhibited by antibodies directed at membrane IgD, but could be inhibited by antibodies which crosslink membrane IgM. Herein, we demonstrate that both anti-mu and anti-delta will cause calcium mobilization in this transfected cell line; this is followed by a period during which antibodies against the alternative isotype are unable to induce significant increases in intracellular calcium concentrations. This phenomenon, called "desensitization," is short-lived, lasting 20 min. We further demonstrate that acute desensitization of these cells by anti-delta has no effect on immediate growth inhibition which is elicited by anti-mu. These data confirm our earlier proposal that the rapid, initial calcium response seen in these lymphomas is not required for the negative signal for growth. Moreover, we also demonstrate that pretreatment of these lymphoma cells with phorbol myristate acetate (PMA) also renders these lymphoma cells temporarily incapable of manifesting a significant calcium signal. Nonetheless, PMA-pretreated B lymphoma cells are not altered in their subsequent sensitivity to anti-mu growth inhibition, nor are they affected in their resistance to inhibition by anti-delta. Our data confirm the proposal that neither the calcium signal nor protein kinase-C activation is involved in the modulation of B lymphoma growth.

Cross-linking of the surface immunoglobulin on lymphocytes from the bursa of Fabricius results in second messenger generation

The bursa of Fabricius represents the major site of B lymphocyte development in avian species. Although the vast majority of bursal lymphocytes express cell surface immunoglobulin (sIg), it is generally considered that the bursa does not represent a significant site of antigen-induced B cell maturation to Ig secretion. However, the question as to whether antigen, either exogenous or self, can induce positive or negative selection of bursal lymphocytes in such a way as to modify the peripheral B cell repertoire remains open. Clearly, such intrabursal selection would require that bursal lymphocyte sIg have the molecular machinery to transduce signals into the cell as a consequence of its interaction with antigen. In this report we demonstrate that exposure of bursal lymphocytes to antibodies directed against sIg induced a rapid increase in cytosolic free calcium ion concentrations [Ca2+]i. Furthermore, such antibodies also induced a rapid increase in intracellular phosphatidic acid concentrations followed by a rise in intracellular phosphatidylinositol. Increased [Ca2+]i, phosphatidic acid and phosphatidylinositol levels required the cross-linking of sIg and were not induced by antibodies to other bursal cell surface antigens. Thus, cross-linking of the sIg on bursal lymphocytes results in second messenger generation, demonstrating that bursal sIg is a functional signal transduction element.

Elevated expression of Pgp-1 (Ly-24) by murine peritoneal B lymphocytes

B cell expression of the surface glycoprotein, Pgp-1 (Ly-24), was evaluated using flow cytometric analysis. Pgp-1 expression on naive, conventional (splenic) B cells was low but could be increased by mitogenic stimulation. Pgp-1 expression on naive peritoneal B cells was higher than expression by unmanipulated conventional B cells, suggesting the possibility that peritoneal B cells have been activated in vivo. However, the elevated expression of Pgp-1 by peritoneal B cells was not accompanied by increased expression of surface Ia but was correlated with Ly-1 status. Further, Pgp-1 expression by B cells from germ-free mice did not differ from that of normal animals. The elevated expression of Pgp-1 by peritoneal B cells is superficially similar to that provoked by mitogenic stimulation of conventional B cells; however, several characteristics suggest that the elevated expression of Pgp-1 by peritoneal B cells does not reflect prior activation of these cells.

Two DNA-binding proteins discriminate between the promoters of different members of the major histocompatibility complex class II multigene family

The regulation of major histocompatibility complex (MHC) class II gene expression is a key feature of the control of normal and abnormal immune responses. In humans, class II alpha - and beta-chain genes are organized in a multigene family with three distinct subregions, HLA-DR, -DQ, and -DP. The regulation of these genes is generally coordinated, and their promoters contain highly conserved motifs, in particular the X and Y boxes. We have identified five distinct proteins that bind to specific DNA sequences within the first 145 base pairs of the HLA-DR promoter, a segment known to be functionally essential for class II gene regulation. Among these, RF-X is of special interest, since mutants affected in the regulation of MHC class II gene expression have a specific defect in RF-X binding. Unexpectedly, RF-X displays a characteristic gradient of binding affinities for the X boxes of three alpha-chain genes (DRA greater than DPA much greater than DQA). The same observation was made with recombinant RF-X. We also describe a novel factor, NF-S, which bound to the spacer region between the X and Y boxes of class II promoters. NF-S exhibited a reverse gradient of affinity compared with RF-X (DQA greater than DPA much greater than DRA). As expected, RF-X bound well to the mouse IE alpha promoter, while NF-S bound well to IA alpha. The drastic differences in the binding of RF-X and NF-S to different MHC class II promoters contrasts with the coordinate regulation of HLA-DR, -DQ, and -DP genes.

Signal transduction mechanisms of Ia induction in B cells by interleukin 4 and immunoglobulin receptors

Molecular mechanisms of signal transduction through receptors for interleukin 4 (IL-4) are still largely unknown. To elucidate the second messenger(s) of IL-4 action in mature B cells, we performed blocking experiments with inhibitors of various aspects of cellular responses, using Ia-inducing activity of IL-4 as a readout system. In the event, only agents that are shown to inhibit calcium ion (Ca2+) release from intracellular stores, such as 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8) and ryanodine, could block the IL-4 action in a dose-dependent fashion. These results suggest that the process leading to the final expression of IL-4 action may be mediated, at some point, by the release of Ca2+ from intracellular stores. In the parallel experiments with antiimmunoglobulin (Ig) antibody, we found that amiloride, an inhibitor of Na+/H+ pump, blocks the Ia induction by anti-IgM antibody. Thus the Na+/H+ exchange system activated by anti-Ig antibody may be present in mature B cells.

Two DNA-binding proteins discriminate between the promoters of different members of the major histocompatibility complex class II multigene family

The regulation of major histocompatibility complex (MHC) class II gene expression is a key feature of the control of normal and abnormal immune responses. In humans, class II alpha - and beta-chain genes are organized in a multigene family with three distinct subregions, HLA-DR, -DQ, and -DP. The regulation of these genes is generally coordinated, and their promoters contain highly conserved motifs, in particular the X and Y boxes. We have identified five distinct proteins that bind to specific DNA sequences within the first 145 base pairs of the HLA-DR promoter, a segment known to be functionally essential for class II gene regulation. Among these, RF-X is of special interest, since mutants affected in the regulation of MHC class II gene expression have a specific defect in RF-X binding. Unexpectedly, RF-X displays a characteristic gradient of binding affinities for the X boxes of three alpha-chain genes (DRA greater than DPA much greater than DQA). The same observation was made with recombinant RF-X. We also describe a novel factor, NF-S, which bound to the spacer region between the X and Y boxes of class II promoters. NF-S exhibited a reverse gradient of affinity compared with RF-X (DQA greater than DPA much greater than DRA). As expected, RF-X bound well to the mouse IE alpha promoter, while NF-S bound well to IA alpha. The drastic differences in the binding of RF-X and NF-S to different MHC class II promoters contrasts with the coordinate regulation of HLA-DR, -DQ, and -DP genes.

Muramyl dipeptide (MDP) synergizes with interleukin 2 and interleukin 4 to stimulate, respectively, the differentiation and proliferation of B cells

The synthetic immunomodulator muramyldipeptide (MDP) can stimulate B cells. MDP, when used alone, was apparently unable to induce the differentiation or proliferation of resting B cells. In contrast, MDP appeared to synergize with a single recombinant interleukin (IL) to stimulate either their differentiation or proliferation. We used single interleukins to avoid synergistic and antagonistic effects inherent in the use of several factors. IL-2 was found to be sufficient to restore the specific immune response of resting B cells to sheep erythrocytes; MDP greatly increased the number of plaque-forming cells of such IL-2-stimulated B cells. In contrast, IL-4 and interferon-gamma (IFN-gamma), either alone or in the presence of MDP, had no effect in this differentiation assay. MDP was also able to stimulate polyclonally activated B cells. IL-4 increased the proliferation of anti-IgM-stimulated B cells, leading to enlargement and driving more cells into the cell cycle; these effects were further enhanced by MDP, more cells being induced to proliferate, to enlarge, and to progress into the cycle with a higher frequency of cells in the G1B, S, and G2/M compartments. Intracellular free calcium levels were not increased by IL-4 and/or MDP, and the two compounds did not modify the anti-IgM-induced calcium mobilization. Therefore, MDP appears to amplify cytokine effects in B cell activation, by a mechanism which does not appear to involve free calcium mobilization.

Molecular components of the B-cell antigen receptor complex of the IgM class

The antigen receptors on mature B lymphocytes are membrane-bound immunoglobulins of the IgM and IgD classes whose cross-linking by polyvalent antigens results in B-cell proliferation and differentiation. How these membrane-bound immunoglobulin chains, which lack a cytoplasmic tail, generate a cell activation signal is not at present known. We now show that the IgM molecule is non-covalently associated in the membrane of B cells with two proteins of relative molecular mass 34,000 (Mr 34 K; IgM-alpha) and 39 K (Ig-beta) which form a disulphide-linked heterodimer. Surface expression of IgM seems to require the formation of an appropriate complex between IgM and the heterodimer. A transfection experiment indicates that IgM-alpha is the product of mb-1, a B-cell specific gene encoding a transmembrane protein with sequence homology to proteins of the T-cell antigen receptor-CD3 complex.

Synergistic effect of phorbol myristate acetate on bacterial lipopolysaccharide induced rat splenocyte proliferation

1. Bacterial lipopolysaccharide (LPS) stimulated [3H]TdR incorporation of rat splenocytes in a concentration dependent manner. 2. Phorbol 12-myristate 13-acetate (PMA) alone has little effect on rat splenocyte proliferation but it exerted a marked synergistic effect on LPS-induced [3H]TdR incorporation when added at the first few hours of incubation with LPS. Minimal synergistic effect of PMA was observed if it was added later than 4 hr after LPS application. 3. Both LPS-stimulated and PMA synergized incorporation of [3H]TdR in rat splenocytes were inhibited by H-7, a protein kinase C inhibitor. 4. The results support the notion that the activation of protein kinase C is a necessary but insufficient cellular signal in the initiation of proliferative response of rat splenocytes by LPS.

In vitro transformation by Epstein-Barr virus induces a switch in growth factor and anti-IgM responsiveness in a human leukemic B cell clone

By in vitro transformation with Epstein-Barr virus (EBV), we have previously established EBV+ lymphoblastoid cell lines (LCL) from a patient with leukemic centrocytic B cell lymphoma. EBV-transformed LCL and EBV genome-negative leukemic B cells showed identical chromosome aberrations and IgH gene rearrangements. In the present study we have analyzed the effect of exogenous cytokines [interleukin (IL) 1, 2, 3, 4, 6, tumor necrosis factor, lymphotoxin, transforming growth factor beta, (TGF-beta)] and anti-IgM antibodies on the in vitro proliferation of EBV- leukemic B cells and EBV-converted LCL. In contrast to conventional chronic lymphocytic leukemia, B cells of the patient DUL spontaneously proliferated for up to two weeks in the absence of exogenous lymphokines. The spontaneous proliferative capacity of clonal DUL B cells was not modulated by IL 1, IL 3, IL 6, TNF or LT. In vitro growth of DUL B cells was increased, however, by exogenous recombinant (r)IL 2, and was abrogated by TGF-beta, rIL 4 and anti-IgM. rIL 4 not only inhibited spontaneous B cell proliferation but also neutralized the enhancing effect of rIL 2. In contrast, growth of the EBV-transformed DUL LCL was not affected by any of these factors. These data demonstrate that in vitro infection and transformation of a clonal B cell population by EBV induces a switch in responsiveness to rIL 4, TGF-beta and anti-IgM. In addition, this report is the first to demonstrate an inhibitory effect of rIL 4 on a spontaneously proliferating human leukemic B cell clone.

Differential sensitivity of anti-IgM-induced and NaF-induced inositol phospholipid metabolism to serine protease inhibitors in BAL17 B lymphoma cells

A BAL17 B lymphoma cell line bearing mu and delta chains on its surface behaves in a similar manner to normal mature B cells in terms of initial biochemical transmembrane signalling [Mizuguchi, Beaven, Ohara & Paul (1986) J. Immunol. 137, 2162-2167; Mizuguchi, Yong-Yong, Nakabayashi, Huang, Beaven, Chused & Paul (1987) J. Immunol. 139, 1054-1059]. Therefore the effects of protease inhibitors on increases in inositol phospholipid metabolism and intracellular free calcium concentration ([Ca2+]i) were examined. We show that the serine protease inhibitors Tos-Phe-CH2Cl (1-chloro-4-phenyl-3-L-tosylamidobutan-2-one-, TPCK) and Tos-Lys-CH2Cl (7-amino-1-chloro-3-L-tosylamidoheptan-2-one; TLCK) inhibit anti-IgM-mediated accumulation of inositol phosphates in a dose-dependent manner. InsP3 production induced by anti-IgM is also inhibited by pretreatment with Tos-Lys-CH2Cl or Tos-Phe-CH2Cl. Tos-Lys-CH2Cl- Tos-Phe-CH2Cl-mediated inhibition is not overcome by high concentrations of anti-IgM. Moreover, anti-IgM-mediated increases in [Ca2+]i are inhibited by pretreatment of the cells with these inhibitors. However, increases in inositol phospholipid metabolism caused by NaF, an activator of guanine-nucleotide-binding proteins (G-proteins), are approx. 10-fold more resistant to Tos-Lys-CH2Cl and Tos-Phe-CH2Cl inhibition compared with anti-IgM-induced changes. Furthermore, NaF-induced increases in [Ca2+]i are not inhibited by Tos-Lys-CH2Cl or Tos-Phe-CH2Cl pretreatment, suggesting that the inhibitors act at a step proximal to phospholipase C activation. The Tos-Lys-CH2Cl or Tos-Phe-CH2Cl treatment does not change the membrane IgM density as measured by flow cytometry, indicating that the active site of the inhibitors is distal to the membrane IgM molecule. These results indicate that serine proteases may be involved in coupling the receptor cross-linkage to G-protein.

A major myristylated substrate of protein kinase C and protein kinase C itself are differentially regulated during murine B- and T-lymphocyte development and activation

The regulation and expression of protein kinase C (PKC) and phosphomyristin C (PMC) (a principal substrate of PKC which is the major myristylated protein in lymphocyte and glioma lines that express it) in murine B and T lymphocytes were investigated. Both PMC and PKC are differentially regulated during T-cell development. The level of PMC expression is highest in CD4-8-, intermediate in CD4+8+, and lowest in J11d-, CD4, or CD8 single-positive thymocytes. PKC is equally expressed by all three thymic populations. In striking contrast to thymocytes, resting peripheral lymph node T cells and T-cell clones express little if any PMC and reduced levels of PKC. Neither PKC nor PMC is significantly induced upon the activation of lymph node T cells: treatment with anti-CD3 antibodies or anti-CD3 and interleukin-2 fails to induce PKC, whereas PMC is not induced by anti-CD3 alone and is only slightly induced by anti-CD3 and interleukin-2. In contrast to the situation with T cells, PMC and PKC are constitutively expressed at moderate levels in mature B cells. PMC is greatly increased in B-cell blasts generated by cross-linking the antigen receptor with anti-immunoglobulin. These results demonstrate that PMC and PKC are differentially regulated during the development and activation of B and T cells, suggesting that cellular events that rely upon PKC and PMC may differ during ontogeny and activation of different lymphocyte subsets.

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.

Differential lymphocyte growth-modifying effects of oxidants: changes in cytosolic Ca+2

An increase in the concentration of cytosolic Ca+2 ([Ca-2]i) is among the earliest changes seen in mitogen-stimulated lymphocytes and is a consequence of signal transduction which usually results in the initiation of cell cycle progression. However, increased [Ca+2]i has also been correlated with cytotoxicity. We have determined whether modulations of [Ca+2]i are involved in the functional inactivation of cells observed with sublethal concentrations of oxidants. Specifically, [Ca+2]i was measured in mouse splenic lymphocytes that were treated with different oxidants in order to determine if oxidative stress interferes with mitogen-stimulated increases in [Ca+2]i, if oxidants themselves modulated [Ca+2]i, and, if so, whether such Ca+2 modulations by oxidants had stimulatory or inhibitory effects on the response of lymphocytes to mitogens. The oxidants employed were copper phenanthroline (CuP; surface thiol oxidizer), N-ethyl maleimide (NEM; permeant thiol alkylator), hydrogen peroxide (H2O2; generates hydroxyl radical within the cell), and radiation (Cs137; generates hydroxyl radical by radiolysis). Growth of all treated cells was equally inhibited upon stimulation with Con A or PMA/A23187, suggesting that all the oxidants inhibited cell functions required distal in activation to the transduction pathway utilized by Con A but bypassed by PMA/A23187. Doses of CuP, NEM, and radiation which fully inhibited Con A-stimulated proliferation had little effect on resting or mitogen-stimulated changes of [Ca+2]i, but H2O2 doses which fully inhibited proliferation increased [Ca+2]i in unstimulated cells and prevented the increase normally caused by Con A. Both intra- and extracellular Ca+2 contributed to the increased [Ca+2]i seen in unstimulated cells. An elevated [Ca+2]i was sufficient to reduce responsiveness, since pharmacologically increasing the [Ca+2]i with the ionophore A23187 rendered lymphocytes less responsive to Con A. Unlike A23187, H2O2 was unable to synergize with PMA, suggesting that the H2O2-induced increase of [Ca+2]i delivered predominantly negative signals to the cell. The results also suggest that [Ca+2]i utilization by Con A versus PMA-activated lymphocytes must be different. When cells were treated with H2O2 under conditions where intracellular and extracellular Ca+2 were chelated with BAPTA and EGTA, respectively, the response to Con A was restored. Under these conditions, higher concentrations of H2O2 were required to inhibit the response to Con A. Our results indicate that signal transduction may be compromised in cells treated with H2O2, but not in cells treated with CuP, NEM, or radiation.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

A major myristylated substrate of protein kinase C and protein kinase C itself are differentially regulated during murine B- and T-lymphocyte development and activation

The regulation and expression of protein kinase C (PKC) and phosphomyristin C (PMC) (a principal substrate of PKC which is the major myristylated protein in lymphocyte and glioma lines that express it) in murine B and T lymphocytes were investigated. Both PMC and PKC are differentially regulated during T-cell development. The level of PMC expression is highest in CD4-8-, intermediate in CD4+8+, and lowest in J11d-, CD4, or CD8 single-positive thymocytes. PKC is equally expressed by all three thymic populations. In striking contrast to thymocytes, resting peripheral lymph node T cells and T-cell clones express little if any PMC and reduced levels of PKC. Neither PKC nor PMC is significantly induced upon the activation of lymph node T cells: treatment with anti-CD3 antibodies or anti-CD3 and interleukin-2 fails to induce PKC, whereas PMC is not induced by anti-CD3 alone and is only slightly induced by anti-CD3 and interleukin-2. In contrast to the situation with T cells, PMC and PKC are constitutively expressed at moderate levels in mature B cells. PMC is greatly increased in B-cell blasts generated by cross-linking the antigen receptor with anti-immunoglobulin. These results demonstrate that PMC and PKC are differentially regulated during the development and activation of B and T cells, suggesting that cellular events that rely upon PKC and PMC may differ during ontogeny and activation of different lymphocyte subsets.

The influence of C3-coated homologous erythrocytes on pokeweed-mitogen-induced polyclonal differentiation of human B cells

The present study was undertaken in order to test the hypothesis that homologous erythrocytes (E) coated in vivo with C3d could modulate the immunoglobulin (Ig) synthesis of human peripheral blood mononuclear cells (PBMC), stimulated with pokeweed mitogen (PWM) in vitro. E from healthy individuals were found to enhance markedly the Ig synthesis of PMBC cultures stimulated with suboptimal doses (0.01 microgram/ml) of PWM. E coated in vivo with increasing amounts of C3d (1.4-6.3 times the amounts on normal E), obtained from patients with systemic lupus erythematosus, failed to induce any significant increase in Ig synthesis of PBMC cultures stimulated with suboptimal PWM doses, compared with cultures co-stimulated in parallel with normal E. In contrast, an increase in IgM and IgG synthesis was observed in about 50% of PBMC cultures from different donors when stimulated with PWM in the presence of E coated with C3b in vivo (from a patient with congenital factor I deficiency), compared with the Ig synthesis in cultures co-stimulated in parallel with normal E. In contrast to the inability of C3d-coated E to modulate B-cell proliferation, the monoclonal anti-CR2 antibody OKB7 was found to be mitogenic for unstimulated peripheral B cells.

Fc gamma RII expression in resting and activated B lymphocytes

In this report, we analyze the expression of the type II receptor for the Fc region of IgG (Fc gamma RII) in resting and lipopolysaccharide (LPS)-activated murine B lymphocytes. Fc gamma RII is encoded by two genes, alpha and beta. The beta gene encodes two mRNA, beta 1 and beta 2, which are generated by alternative splicing. Using an S1 nuclease protection assay, we found that resting and activated B lymphocytes express predominantly the beta 1 transcript. Very low levels of the beta 2 mRNA were detected in this assay, while no expression of the alpha transcript could be detected. Quantitative Northern blot analysis showed that the amount of Fc gamma RII beta mRNA was increased 9-fold in LPS-activated B lymphocytes. The expression of Fc gamma RII during the various phases of B cell activation was then studied by immunofluorescence using the monoclonal antibody 2.4G2. LPS stimulation induced an increase of the Fc gamma RII cellular pool as well as of its expression at the surface of B lymphocytes. The rise in Fc gamma RII surface expression occurred after the induction of class II antigens (Ia) and before transferrin receptor induction. Fc gamma RII expression was found to be enhanced during the G1 phase of the cell cycle since (a) only large cells (i.e. those that had entered the G1 phase) expressed an increased amount of Fc gamma RII and (b) blocking the entry of activated cells into the S phase (with the ion channel blocker quinine) did not affect the Fc gamma RII induction by LPS. Furthermore, only B cell activators that induced cells to enter into G1 [LPS and F(ab')2 anti-IgM antibodies, but not interleukin 4] caused an increase in the expression of Fc gamma RII. These results show that the increase in the membrane expression of Fc gamma RII occurs during the early G1 phase, establishing it as a marker for the entry of B lymphocytes into the cell cycle.

Production of lymphotoxin by isolated human tonsillar B lymphocytes and B lymphocyte cell lines

The expression of lymphotoxin (LT) mRNA and cytokine in human tonsillar B cells and B cell lines was examined by Northern blots and cytotoxicity assays, respectively. In tonsillar B cells, phorbol myristate acetate (PMA) or Staphylococcus aureus Cowan l (SAC) alone induced low levels of LT mRNA accumulation. However, SAC and anti-mu were strongly synergistic with PMA in this induction. Peak LT mRNA expression in tonsillar B cells stimulated by PMA plus SAC occurred between 48 and 72 h and was approximately half as much as that in PMA plus anti-CD3-stimulated T cells. Cyclosporine A was not effective in inhibiting LT mRNA accumulation by stimulated tonsillar B cells. A number of B cell lines could also be stimulated by PMA to express LT mRNA. Peak accumulation of LT mRNA in the cell line RPMI 1788 stimulated with PMA peaked about 8 h. A23187 in combination with PMA caused this accumulation to increase slightly and to peak earlier. The cytotoxic effects in the supernatants of stimulated B cells were contributed mostly by LT. The results indicate that tonsillar B cells are important in LT production and that there are important differences in the stimulation requirements for LT production and in LT mRNA expression kinetics between tonsillar B cells and B cell lines.

Lack of correlation between translocation and biological effects mediated by protein kinase C: an appraisal

Protein kinase C is involved in the mechanism of action of hormones, growth factors, mitogens and tumour promoters. The correlation between the extent of the biological effects mediated by protein kinase C and its fractional activation shows cell type specific patterns of behaviour. The discrepancy between enzyme activity and biological effects elicited by protein kinase C is particularly relevant to lymphoblastic cells. In B cells, the full expression of some biological responses mediated by protein kinase C may be achieved by activation of less than 5% of the enzyme activity.

Transcriptional activation of bovine leukemia virus in blood cells from experimentally infected, asymptomatic sheep with latent infections

Infection by bovine leukemia virus (BLV) is characterized by a long latency period, after which some individuals develop B-cell tumors. The behavior of BLV and related retroviruses during the latency period between initial infection and subsequent tumorigenesis is poorly understood. We used in situ hybridization to detect BLV transcripts in individual peripheral blood mononuclear cells from experimentally infected, asymptomatic sheep with latent infections. Viral RNA was not found in most peripheral blood cells that had been isolated as rapidly as possible from circulating blood, but it was present in rare cells. BLV RNA transcripts increased in a biphasic manner within a few hours after the blood cells were placed in culture. Exposure to fetal bovine serum was identified as the principal cause of this transcriptional activation, which occurred in fewer than 1 in 1,000 cells. Agents known to activate immune cells polyclonally caused a further increase in the number of cells containing BLV RNA within 8 h. In some cases, the numbers of viral transcripts within individual cells also increased. Thus, BLV is not detectably expressed in most resting lymphocytes circulating in the blood, but its transcription is activated by components of fetal bovine serum and can be augmented by molecules that mimic activation of immune cells. This activation, which might occur in lymphoid tissue during an immune response, may lead to the synthesis of viral regulatory proteins that are thought to initiate tumorigenesis through host cell genes.

Characterization of Ca2+ and K+ currents in the human Jurkat T cell line: effects of phytohaemagglutinin

1. Inward and outward currents were recorded in the human Jurkat T cell line using the whole-cell configuration of the patch-clamp technique. 2. The transient outward current was activated at membrane potentials positive to -60 mV. The activation time constant-voltage relationship decreased from 17 ms to 2 ms for membrane potentials ranging from -40 to +40 mV. The inactivation phase could be fitted by a single-exponential function and the inactivation time constant decreased from 250 ms to 150 ms for membrane potentials ranging from -20 to +100 mV. 3. The steady-state inactivation-voltage relationship showed a mid-point potential of -32 +/- 2.6 mV, and the slope factor was 10.8 +/- 1.8 mv (n = 3). 4. The calcium ionophore A23187 provoked a decrease in the amplitude of the outward current, suggesting a dependence of this current on the cytosolic concentration of Ca2+. 5. The K+ outward current was blocked by tetraethylammonium (TEA, Michaelis-Menten constant (Km), 6 mM) and by the calcium channel blockers Ni2+, Co2+, Mn2+ and Cd2+. 6. Forty per cent (n = 120) of the patched Jurkat cells displayed an inward current. In a physiological medium containing Ca2+ (2.2 mM), the inward current threshold voltage was -60 mV, the maximum current was observed at -40 mV and the zero current voltage was positive to +20 mV. At negative membrane potentials, the time required to reach 50% of the maximum amplitude was 60 ms and grew shorter with increasing depolarization, reaching a value of 5 ms at -5 mV. The inactivation of the inward current was very slow and the time constant varied from 1200 ms at -35 mV to approximately 250 ms for potentials positive to -10 mV. 7. The current availability had a value of one for potentials negative to -50 mV and zero for potentials positive to -15 mV. The mid-point potential was -31 +/- 3.4 mV and the slope factor was 3.3 +/- 0.2 mV (n = 3). 8. The inward channels were permeable to Sr2+, but were blocked by classical Ca2+ channel inhibitors such as Co2+, Mn2+ and Ni2+. 9. Phaseolus vulgaris phytohaemagglutinin (PHA), an inducer of interleukin-2 production in Jurkat cells, increased the inward current amplitude by 32 +/- 20% (n = 4). This increase was concomitant with a decrease (45 +/- 12%) in the amplitude of the outward current, but only when the current was carried by Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth inhibition of Epstein-Barr virus-transformed B cells by anti-HLA-DR antibody L243: possible relationship to L243-induced down-regulation of CD23 antigen expression

Among five anti-HLA class II monoclonal antibodies (mab's) tested, the anti-HLA-DR mab L243 selectively inhibited the in vitro proliferation of Epstein-Barr virus-transformed lymphoblastoid cell lines (LCL). Saturating amounts of L243 mab induced 50% suppression of LCL growth as revealed by measuring [3H]thymidine incorporation or counting cell numbers. Preincubation for 20 hr at 37 degrees C in the presence of L243 mab dramatically reduced the expression of certain B cell-specific antigens on LCL, notably CD23 (BLAST-2, low affinity Fc epsilon receptor). In view of the known function of the CD23 antigen as a B cell growth factor receptor, we discuss the possibility that the suppressive effect of L243 mab on LCL proliferation is due to down-regulation of CD23 antigen expression.

A specific defect in the proliferative capacity of B cells from old mice stimulated with autoreactive T cells

B lymphocytes from aged mice were found to be defective in their ability to proliferate in response to stimulation with an autoreactive T cell clone D1.4. The differentiative response leading to antibody secretion was also impaired in the auto D1.4 T cell-stimulated B cells from old mice in comparison to similarly stimulated B cells from young mice. The B cells from old mice were competent in activating the autoreactive T cells such that the T cells were induced to proliferate. The B cell defect appears to be restricted to a certain phase of B cell activation, since old mouse B cells responded to the auto D1.4 T cells by increasing cell surface Ia as well as size, but failed to incorporate tritiated thymidine. The responsiveness to interleukin-4 was found to be similar between B cells from young and old mice. It appeared that the B cells from old mice are specifically defective in progressing from the G0 phase of cell cycle into the S phase when stimulated with the auto D1.4 T cells.

Induction of human IgE synthesis requires interleukin 4 and T/B cell interactions involving the T cell receptor/CD3 complex and MHC class II antigens

The induction of IgE synthesis by IL-4 requires T cells and monocytes, as well as T cell- and monocyte-derived cytokines. Optimal cytokine combinations, however, fail to induce highly purified B cells to secrete IgE, indicating that additional signals are required. We show herein that the induction of human IgE synthesis by rIL-4 requires cognate interaction between the T cell receptor/CD3 complex on T cells and MHC class II antigens on B cells: mAbs directed against these molecules completely blocked IL-4-dependent IgE induction. mAbs against cell adhesion molecules (CD2, CD4, LFA-1) also inhibited IgE synthesis induced by IL-4, confirming that cell-cell contact is necessary for IgE induction. The requirement for cognate T/B cell interaction was further shown by comparing the IgE-inducing ability of two human IL-4-producing alloreactive T cell clones: F6, which recognizes MHC class II antigens on both B cells and monocytes, and A1, which recognizes an HLA-DP-associated epitope expressed on monocytes, but not on B cells. When incubated with B cells and monocytes from a normal donor bearing the appropriate alloantigen, clone F6, but not clone A1, induced vigorous IgE synthesis, although both clones proliferated and secreted IL-4. Taken together, our results suggest that at least two, possibly synergizing, signals are required for the T cell-dependent induction of IgE synthesis by B cells: one signal is delivered by cognate T/B cell interaction, the other by T cell-derived IL-4.

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.

Immunoglobulin receptor signaling depends on the carboxyl terminus but not the heavy-chain class

To examine the isotypic and structural requirements involved in signaling through the immunoglobulin (Ig) receptor on B lymphocytes, we established a panel of T15 idiotype-positive transfectants that expressed wild-type IgM, wild-type IgD, or hybrid IgM molecules. Growth inhibition of the transfected lymphoma cells in response to anti-idiotype antibodies was used to measure signaling. Hybrid IgM molecules were constructed so that the membrane-spanning region of the mu heavy chain was replaced by that of delta, gamma 2b, or alpha heavy chains or that of the I-Ab class II (Ia) alpha chain. All transfectants that expressed IgM or hybrid IgM molecules with membrane-spanning regions from another Ig isotype underwent signaling in response to anti-idiotype antibodies, whereas the IgM-Ia hybrid transfectants did not. Transfectants that expressed wild-type IgD molecules also underwent signaling, although this response was particularly sensitive to serum concentrations. These results imply that signaling occurs in a similar manner through heavy-chain receptors of any isotype and suggest that conserved amino acid sequences in the transmembrane regions are important in this process.

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.

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.

Antigen-presenting function of B lymphocytes

Here we review our current results studying B cells as APC and the mechanisms by which processed antigen is transported to and held on the cell surface for recognition by the specific T cell along with the MHC class II molecules. These studies were carried out using the globular protein cytochrome c as antigen for which the T-cell antigenic determinant was localized to a C-terminal 10-amino acid peptide fragment. For certain analyses, native cytochrome c or antigenic peptide fragments were covalently coupled to antibodies directed toward B-cell surface structures, allowing the targeting of antigen to the APC surface. Our findings indicate that all B cells function as APC and that the APC function is not differentially regulated in defined B-cell subpopulations. Using cytochrome c-antibody conjugates, it was shown that the surface Ig plays two significant roles in augmenting the B-cell APC function following antigen binding: signalling for enhanced APC function and concentrating antigen for subsequent internalization and processing. Both IgM and IgD appear to function identically in facilitating antigen processing in both immune and nonimmune B-cell populations. Furthermore, the surface Ig does not appear to be specially differentiated to function in concentrating antigen, as antigen artificially bound to other B-cell surface structures including MHC class I and class II molecules is also effectively presented. Lastly, evidence is presented that a previously described B-cell activating factor activity is strongly associated with the membranes of activated but not unactivated helper T cells, providing a mechanism by which the T-cell helper function can be focused on the specific antigen-presenting B cell. Concerning the mechanism by which processed antigen is presented at the B-cell surface, evidence is presented suggesting a role of peptide-binding chaperone proteins which may function to transport peptide to the APC surface and facilitate its association with the appropriate Ia. One candidate protein, PBP72/74, is described which binds peptides but not native antigens, is a member of the hsp70 family and appears to play a role in antigen presentation by the ability of antisera raised against it to block APC functions. Peptide-antibody conjugates were used to explore the spacial restrictions on MHC-restricted peptide presentation and it was shown that peptides covalently coupled to antibodies specific for Ig, class I or class II molecules are effective antigens in vitro even in the absence of processing.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Activation of the phosphatidylinositol metabolic pathway by low molecular weight B cell growth factor

The possible role of phosphatidylinositol breakdown in the induction of proliferation of human activated B cells by low molecular weight B cell growth factor (LMW-BCGF) was examined. LMW-BCGF was found to induce a rapid rise in the concentration of inositol trisphosphate (InsP3) in [3H]inositol-loaded B cell blasts, obtained by prior anti-mu antibody activation. A concomitant decrease in the concentration of phosphatidylinositol 4,5-bisphosphate could be detected at the same time. Maximum generation of InsP3 occurred within 15-30 s after the addition of the LMW-BCGF ligand to the activated B cells, then was followed by a slow decrease and return to control values. The amount of InsP3 generated by phosphatidylinositol hydrolysis was dependent on the concentration of LMW-BCGF. This effect was only detected in B cells already preactivated by a first signal such as anti-mu antibody and not in resting unstimulated B cells. In contrast, under similar conditions, interleukin 2, another B cell growth-promoting lymphokine, did not alter the rate of formation of the various phosphatidylinositol breakdown products. An augmentation of the [Ca2+]i concentration was also detected in activated B cells upon addition of LMW-BCGF and this increase could be blocked by TMB-8, a specific inhibitor of endoplasmic reticulum calcium release. Hydrolysis of phosphoinositides thus represents an essential component in the mechanism of transduction of the signal provided by LMW-BCGF.

Stepwise intraclonal maturation of antibody affinity through somatic hypermutation

Using recombinant DNA techniques, we reconstructed a genealogical tree (Sablitzky, F., Wildner, G. & Rajewsky, K. (1985) EMBO J. 4, 345-350) that connects three clonally related B cells producing somatically mutated antibodies to a progenitor cell expressing a germ line-encoded antibody. The somatic mutants had been isolated from an in vivo immune response. The germ line-encoded progenitor antibody bound the antigen with high affinity. Intraclonal affinity maturation occurred stepwise over a 15-fold range.

Elucidation of cellular targets responsible for tetrachlorodibenzo-p-dioxin (TCDD)-induced suppression of antibody responses: I. The role of the B lymphocyte

The objective of these studies was to identify the primary cellular target(s) responsible for TCDD (tetrachlorodibenzo-p-dioxin)-induced suppression of antibody production. Responses to T-independent and T-dependent antigens (administered in vivo or directly to splenic culture) were suppressed in a dose-related fashion in female B6C3F1 mice dosed for 5 consecutive days with 0.1, 1.0 and 10.0 micrograms/kg of 2,3,7,8-TCDD. When nonadherent (B and T) and adherent (macrophage) cells from vehicle- and TCDD-treated mice were combined in various combinations and immunized with either DNP-Ficoll (T-independent) or sRBCs (T-dependent), it was demonstrated that nonadherent cells, but not adherent cells, were functionally affected by TCDD. Similarly, as various combinations of B + macrophage and T + macrophage populations were immunized with sRBCs, the B cell was shown to be the primary target. Using LPS as the stimulus, an inhibition of the antibody response with no effect on the mitogenic response further indicated that the primary target of the TCDD-induced suppression of IgM antibody production is the B lymphocyte at the level of cell differentiation.

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.

Immunoglobulins D and M mediate signals that are qualitatively different in B cells with an immature phenotype

The CH family of murine B-cell lymphomas includes several members that are sensitive to growth inhibition when their membrane IgM (mIgM) receptors are cross-linked by anti-mu chain, anti-kappa chain, or anti-idiotypic antibodies. These lymphomas are IgM+, Ia+, and IgD +/- and resemble neonatal B cells in terms of their exquisite sensitivity to anti-IgM-mediated negative signaling as a model for tolerance induction. In this report, we describe the properties of one of these lymphomas, CH33, which had been transfected with a construct containing an allotypically different delta chain constant region and the heavy chain variable region fragment from S107 (T15 idiotype positive). This transfected cell line allowed us to investigate the possibility that membrane IgD (mIgD) and mIgM can mediate different signals. Our results show that the transfected cells retained their exquisite sensitivity to anti-IgM-mediated growth inhibition; however, crosslinking of IgD with anti-delta chain antibody did not inhibit their growth. Furthermore, even prolonged pretreatment with anti-IgD antibodies did not affect cell growth nor did it modulate the inhibitory effects of anti-IgM antibody. Moreover, identical results were obtained with clones of CH33 that express significant amounts of endogenous IgD. Thus, the failure of mIgD to deliver a negative signal does not reflect a defect in the transfected IgD but appears to be a general property of IgD in these cells. The mIgD was shown to mediate transmembrane signals because anti-delta chain treatment resulted in Ca2+ mobilization in transfected CH33 cells and capping of those receptors. We conclude that mIgD can mediate qualitatively different signals than mIgM can and that mIgD expression per se is not sufficient to change the functional phenotype of immature B cells.