B lymphocyte receptors and polyphosphoinositide degradation

Maintenance of quiescent immune memory in the bone marrow

The adaptive immune system has the important ability to generate and maintain a memory for antigens once encountered. Recent progress in understanding the organization of immunological memory has challenged the established paradigm of maintenance of memory by restless, circulating, and "homeostatically" proliferating lymphocytes. Among other tissues, the bone marrow has emerged as a preferred resting place for memory lymphocytes providing both local and systemic long-term protection. Why the bone marrow? There, mesenchymal stromal cells provide a privileged environment for quiescent memory B and T lymphocytes, the protagonists of secondary immune reactions, and for memory plasma cells providing persistent humoral immunity. In this review, we discuss the dedicated role of the bone marrow for the maintenance of memory lymphocytes and its implications for immunological memory.

Multilayer Control of B Cell Activation by the B Cell Antigen Receptor: Following Themes Initiated With Bill Paul

This article describes the work I did in Bill Paul's lab as a postdoctoral fellow between 1979 and 1983, and to a lesser extent puts that work in the context of other work on B cell activation and antibody responses that was going on in Bill's lab at that time and shortly beforehand, including the discovery of interleukin 4. In addition, this work describes the subsequent and continuing work in my own lab following-up on themes I began during my time working directly with Bill. A particular emphasis was on understanding the biochemical mechanisms of signaling by the B cell antigen receptor (BCR) to the interior of the B cell. Some of the studies from my lab related to the regulation of BCR signaling by Lyn are described in relationship to the lymphocyte tuning hypothesis put forth by Grossman and Paul in 1992 and subsequently.

Molecular pathology via IR and Raman spectral imaging

During the last 15 years, vibrational spectroscopic methods have been developed that can be viewed as molecular pathology methods that depend on sampling the entire genome, proteome and metabolome of cells and tissues, rather than probing for the presence of selected markers. First, this review introduces the background and fundamentals of the spectroscopies underlying the new methodologies, namely infrared and Raman spectroscopy. Then, results are presented in the context of spectral histopathology of tissues for detection of metastases in lymph nodes, squamous cell carcinoma, adenocarcinomas, brain tumors and brain metastases. Results from spectral cytopathology of cells are discussed for screening of oral and cervical mucosa, and circulating tumor cells. It is concluded that infrared and Raman spectroscopy can complement histopathology and reveal information that is available in classical methods only by costly and time-consuming steps such as immunohistochemistry, polymerase chain reaction or gene arrays. Due to the inherent sensitivity toward changes in the bio-molecular composition of different cell and tissue types, vibrational spectroscopy can even provide information that is in some cases superior to that of any one of the conventional techniques.

Modulation of in vitro murine B-lymphocyte response by curcumin

Curcumin is a phenolic natural product isolated from the rhizome of Curcuma longa (tumeric). It was previously described that curcumin had a potent anti-inflammatory effect and inhibited the proliferation of a variety of tumor cells. In the present study, we investigated the inhibitory effects of curcumin on the response of normal murine splenic B cells. Curcumin inhibited the proliferative response of purified splenic B cells from BALB/c mice stimulated with the Toll-like receptor ligands LPS and CpG oligodeoxynucleotides. LPS-induced IgM secretion was also inhibited by curcumin. The proliferative response induced by either the T-independent type 2 stimuli anti-delta-dextran or anti-IgM antibodies was relatively resistant to the effect of curcumin. We investigated the intracellular signaling events involved in the inhibitory effects of curcumin on murine B cells. Curcumin did not inhibit the increase in calcium levels induced by anti-IgM antibody. Western blotting analysis showed that curcumin inhibited TLR ligands and anti-IgM-induced phosphorylation of ERK, IkappaB and p38. Curcumin also decreased the nuclear levels of NFkappaB. Our results suggested that curcumin is an important inhibitor of signaling pathways activated upon B cell stimulation by TLR ligands. These data indicate that curcumin could be a potent pharmacological inhibitor of B cell activation.

Analysis of inositol phospholipid turnover during lymphocyte activation

Receptor-mediated activation of phospholipase C (PLC) leads to the hydrolysis of membrane inositol phospholipids, generating diacylglycerol (DAG) and water-soluble inositol phosphates. This signaling mechanism is used by antigen receptors on T and B cells that have been implicated as mediators of receptor-induced influx of extracellular Ca(2+). This unit provides protocols that describe the resolution of InsP by Dowex anion-exchange chromatography. This technique provides a reliable means of separating inositol monophosphate, inositol bisphosphate, and inositol trisphosphate, but does not resolve isomers of these. An Alternate Protocol describes the separation of inositol phosphates by anion-exchange HPLC. A protocol for resolution of inositol phospholipids by thin-layer chromatography (TLC) is also provided.

Micro-scale flow cytometry-based and biochemical analysis of lipid signaling in primary B cell subpopulations

B cell subpopulations in the spleen have been extensively characterized phenotypically; however, biochemical properties of these cell populations following B cell antigen receptor engagement have not been fully determined due to technical difficulties and limiting cell numbers. We therefore employed mini-scale protocols to assess lipid signaling, particularly that of diacylglycerol and inositol trisphosphate, with as few as 0.5x10(6) purified early (T1) and late (T2) transitional B cells. Additionally, utilizing flow cytometric techniques, we determined levels of phosphatidylinositol bisphosphate and calcium mobilization in T1 and T2 cells, as well as mature follicular and marginal zone B cells using less than 1x10(6 )primary B cells. Thus, these biochemical and flow cytometric methodologies can be used to analyse signal-induced changes in phosphatidylinositol bisphosphate levels, diacylglycerol and inositol triphosphate production and calcium in each B cell population.

Role of protein kinase C in calcium-mediated signal transduction

Information from certain extracellular signals, including a group of peptide hormones and some neurotransmitters, appears to flow from the cell surface into the cell interior through two pathways, protein kinase C activation and Ca2+ mobilization, both of which become available by a single ligand-receptor interaction. Under normal conditions protein kinase C is activated by association with membrane phospholipids in the presence of 1,2-diacylglycerol. This diacylglycerol may arise in the membrane only transiently from the receptor-mediated hydrolysis of inositol phospholipids. By using a synthetic permeable diacylglycerol or tumour-promoting phorbol ester (as a substitute for active diacylglycerol) it has been shown that signal passage through this protein kinase pathway is an essential prerequisite, often synergistic to that via the Ca2+ pathway, for full physiological responses, such as transmitter release and exocytosis, to be obtained. Presumably, such a role of protein kinase C may be extrapolated to the activation of many other cellular processes, including membrane conductance, gene expression and some metabolic reactions, as well as to the modulation of other receptor-mediated signal pathways. Some morphological findings with monoclonal antibodies raised against protein kinase C are presented.

Transitional B cell fate is associated with developmental stage-specific regulation of diacylglycerol and calcium signaling upon B cell receptor engagement

Functional peripheral mature follicular B (FoB) lymphocytes are thought to develop from immature transitional cells in a BCR-dependent manner. We have previously shown that BCR cross-linking in vitro results in death of early transitional (T1) B cells, whereas late transitional (T2) B cells survive and display phenotypic characteristics of mature FoB cells. We now demonstrate that diacylglycerol (DAG), a lipid second messenger implicated in cell survival and differentiation, is produced preferentially in T2 compared with T1 B cells upon BCR cross-linking. Consistently, inositol 1,4,5-triphosphate is also produced preferentially in T2 compared with T1 B cells. Unexpectedly, the initial calcium peak appears similar in both T1 and T2 B cells, whereas sustained calcium levels are higher in T1 B cells. Pretreatment with 2-aminoethoxydiphenylborate, an inhibitor of inositol 1,4,5-triphosphate receptor-mediated calcium release, and verapamil, an inhibitor of L-type calcium channels, preferentially affects T1 B cells, suggesting that distinct mechanisms regulate calcium mobilization in each of the two transitional B cell subsets. Finally, BCR-mediated DAG production is dependent upon Bruton's tyrosine kinase and phospholipase C-gamma2, enzymes required for the development of FoB from T2 B cells. These results suggest that calcium signaling in the absence of DAG-mediated signals may lead to T1 B cell tolerance, whereas the combined action of DAG and calcium signaling is necessary for survival and differentiation of T2 into mature FoB lymphocytes.

A G protein-associated ERK pathway is involved in LPS-induced proliferation and a PTK-associated p38 MAPK pathway is involved in LPS-induced differentiation in resting B cells

We, previously, showed that PKC-dependent ERK/p38 MAPK activation was inhibited by treating the resting B cell line 38B9 with an anti-MHC class II antibody. Further studies in this work demonstrated that PKA was involved in lipopolysaccharide (LPS)-induced proliferation of the cells, such that the PKC inhibitor activated PKA with concomitant LPS-induced proliferation but not IgG secretion. Consequently, the PKA inhibitor down-regulated ERK and p38 MAPK, and decreased cell proliferation. In addition, the treatment of LPS-stimulated 38B9 cells with PTK inhibitor reduced PKC- and PKA-dependent p38 MAPK activation and reduced the level of IgG secretion rather than the level of proliferation. However, the treatment of LPS-stimulated 38B9 cells with pertussis toxin (PTX), an inhibitor for the G protein-coupled receptor, inhibited the activation of both PKC- and PKA-dependent ERK and significantly reduced LPS-induced proliferation but not IgG secretion. Furthermore, ERK and p38 MAPK inhibitors reduced LPS-induced proliferation and differentiation, respectively, in 38B9 cells in a dose-dependent manner. These results suggest that LPS-induced proliferation of resting B cells is mainly mediated through a G protein-associated PKA/PKC-dependent ERK pathway and that a PTK-associated PKC/PKA-dependent p38 MAPK pathway is mostly involved in LPS-induced differentiation of the resting B cells.

New phenotypic, functional and electrophysiological characteristics of KG-1 cells

Myeloid dendritic cells (DC) are representatives of a rare and phenotypically diverse population of professional antigen presenting cells possessing high functional heterogeneity and flexibility. Here we studied the phenotypic, functional and electrophysiological characteristics of KG-1 cells, an erythroleukemia model cell line, which shares morphological and physiological similarities with immature and mature myeloid DC. We compared the expression of internalizing receptors and other cell surface molecules, antigen uptake and migration of unstimulated and activated KG-1 cells with the characteristics of immature and mature DC. Unstimulated KG-1 cells were less potent in capturing extracellular materials than immature DC. In contrast to monocyte-derived DC KG-1 cells stimulated by PMA and ionomycin ceased to migrate along the MIP-3beta chemokine gradient despite their high expression of CCR7 chemokine receptor and MDR, a transporter implicated in DC migration. Moreover, we determined the ion channel repertoire of KG-1 cells before and after treatment with PMA and ionomycin by using the patch-clamp technique. We found that both unstimulated and activated KG-1 cells expressed time- and voltage-independent, ChTx sensitive intracellular Ca(2+)-gated potassium conductance suggesting the presence of K(Ca) channels in their membranes. Based on our results we propose that KG-1 cells resemble myeloid DC but also possess unique phenotypic, functional and electrophysiological characteristics.

Proliferation and survival of activated B cells requires sustained antigen receptor engagement and phosphoinositide 3-kinase activation

In this study, we investigate the extracellular and intracellular signals that drive cell cycle progression of activated B cells in the absence of T cell help. We find that brief engagement of the B cell receptor is sufficient to induce a single cell division in a fraction of cells, but that survival during successive cell divisions requires sustained receptor stimulation. In contrast, T cells have been shown previously to commit to multiple cell divisions following brief TCR engagement. Both early and late B cell receptor signals are blocked by inhibitors of phosphoinositide 3-kinase and mammalian target of rapamycin and are associated with S6 kinase activation and increased cell size. The requirement for ongoing Ag receptor signaling can be overcome by engagement of CD40 but only partially by IL-4. Proliferation driven by LPS also requires sustained exposure to the stimulus. These findings reveal checkpoints that may limit T-independent B cell responses when Ag exposure is transient.

Antigen-receptor cross-linking and lipopolysaccharide trigger distinct phosphoinositide 3-kinase-dependent pathways to NF-kappa B activation in primary B cells

The NF-kappaB/Rel transcription factors play an important role in the expression of genes involved in B cell development, differentiation and function. Nuclear NF-kappaB is induced in B cells by engagement of either the BCR or CD40 or by stimulation with lipopolysaccharide (LPS). Despite the importance of NF-kappaB to B cell function, little is known about the signaling pathways leading to NF-kappaB activation. In this report we address the role of phosphoinositide 3'-kinase (PI 3-kinase) in BCR- and LPS-induced NF-kappaB activation using populations of primary murine resting B cells. Using the specific pharmacological inhibitors of PI 3-kinase, Wortmannin and LY294002, we demonstrate that PI 3-kinase activity is vital for BCR-induced NF-kappaB DNA-binding activity. Furthermore, we show that this is achieved via protein kinase C-dependent degradation of IkappaBalpha. Similar analyses reveal that PI 3-kinase is also critical in triggering NF-kappaB DNA-binding activity and IkappaBalpha degradation following LPS stimulation. Interestingly, a PKC inhibitor which blocked the BCR-induced IkappaBalpha degradation had no effect on the degradation of IkappaBalpha after LPS stimulation. Taken together, our results indicate the involvement of PI 3-kinase in at least two distinct signaling pathways leading to activation of NF-kappaB in B cells.

Signal transduction through Vav-2 participates in humoral immune responses and B cell maturation

B and T lymphocytes develop normally in mice lacking the guanine nucleotide exchange factor Vav-2. However, the immune responses to type II thymus-independent antigen as well as the primary response to thymus-dependent (TD) antigen are defective. Vav-2-deficient mice are also defective in their ability to switch immunoglobulin class, form germinal centers and generate secondary immune responses to TD antigens. Mice lacking both Vav-1 and Vav-2 contain reduced numbers of B lymphocytes and display a maturational block in the development of mature B cells. B cells from Vav-1(-/-)Vav-2(-/-) mice respond poorly to antigen receptor triggering, both in terms of proliferation and calcium release. These studies show the importance of Vav-2 in humoral immune responses and B cell maturation.

The CGM1a (CEACAM3/CD66d)-mediated phagocytic pathway of Neisseria gonorrhoeae expressing opacity proteins is also the pathway to cell death

Phagocytosis of Opa+ Neisseria gonorrhoeae (gonococcus, GC) by neutrophils is in part dependent on the interaction of Opa proteins with CGM1a (CEACAM3/CD66d) antigens, a neutrophil-specific receptor. However, the signaling pathways leading to phagocytosis have not been characterized. Here we show that interaction of OpaI bacteria with neutrophils or CGM1a-transfected DT40 cells induces calcium flux, which correlates with phagocytosis of bacteria. We identified an immunoreceptor tyrosine-based activation motif (ITAM) in CGM1a, and showed that the ability of CGM1a to transduce signals and mediate phagocytosis was abolished by mutation of the ITAM tyrosines. We also demonstrated that CGM1a-ITAM-mediated bacterial phagocytosis is dependent on Syk and phospholipase C activity in DT40 cells. Unexpectedly, the activation of the CGM1a-ITAM phagocytic pathway by Opa+ GC results in induction of cell death.

Differential effects of fluoxetine on murine B-cell proliferation depending on the biochemical pathways triggered by distinct mitogens

The effect of fluoxetine on mitogen-induced B-cell proliferation was studied. In particular, we analyzed the influence of fluoxetine on the signal transduction pathways triggered after stimulation with lipopolysaccharide (LPS) and anti-immunoglobulin M antibodies (anti-IgM). We showed that fluoxetine had a dual effect on anti-IgM-stimulated B-cell proliferation: at optimal anti-IgM concentration, fluoxetine inhibited proliferation, whereas at suboptimal anti-IgM concentration, the drug enhanced proliferation. Fluoxetine exerted only an inhibitory effect on LPS-induced B-cell proliferation. Calcium influx seemed to be involved in these effects.

A microfilament formation inhibitor, cytochalasin strongly enhances the low-affinity Fc epsilon receptor II (CD23) expression on the human monocyte-like cell line, U937

Enhancement of the low-affinity Fc epsilon receptor (CD23) expression by cytochalasin was analyzed on the human monocytelike cell line, U937. The CD23 expression on the U937 cells was enhanced at 24 hr after culture with cytochalasin B, D, or E, especially cytochalasin E having the most remarkable effect on it at the low concentration. This enhanced expression was found to be associated with a concomitant increase of a CD23 (about 45-kDa) protein on the U937 cells as assessed by Western blotting analysis. On the other hand, CD11a, CD18, CD31, CD49d, or CD54 was not markedly enhanced on the U937 cells by culture with cytochalasin E, although the mean fluorescence intensities (MFIs) of CD11a, CD18, and CD54 on U937 was partially up-regulated. Cell growth of U937 cultured with cytochalasin E was completely suppressed for 72 hr, but cell viability was sufficiently maintained (more than 95%). Soluble-formed CD23 (sCD23) also was released from the U937 cells at 24 to 72 hr after culture with cytochalasin E. In addition, the protein tyrosine kinase activity was detected in the U937 cells cultured with cytochalasin E for 24 hr using the enzyme immunoassay. Enhancement of the CD23 expression on the U937 cells at 24 to 72 hr cultured with cytochalasin E was sufficiently blocked by protein tyrosine kinase inhibitors herbimycin A and genistein, and a protein synthesis inhibitor, cychloheximide. On the other hand, protein kinase C inhibitors such as H-7 and H-8 had no effect on this CD23 expression. These results suggest that a mechanism underlying enhancement of the CD23 expression on the U937 cells cultured with cytochalasin E is mediated through tyrosine phosphorylation and protein synthesis.

Antigen receptor signalling in apoptosis-resistant mutants of WEHI 231 cells

Ligation of membrane immunoglobulin M (mIgM) induces cell cycle arrest and apoptosis in the WEHI 231 B-lymphoma cell line. The molecular mechanisms which link receptor ligation and the nuclear events that underlie this response, have yet to be fully elucidated. Here we have examined the signals induced following mIgM cross-linking in variants of WEHI 231 that no longer undergo apoptosis in response to this stimulus. Tyrosine phosphorylation of cellular substrates in two of the variants is identical to that seen in wild-type cells but in one of the mutants, VS2.12, a restricted set of substrates becomes tyrosine phosphorylated. In a second variant (E8), mIgM cross-linking does not induce elevation of intracellular Ca2+, although tyrosine phosphorylation of PLCgamma2 is induced to an equivalent extent to that seen in WEHI 231 cells. A third variant, 2E10.F9, is resistant to apoptosis despite the fact that all signals analysed appear to be similar to those induced in wild-type cells. Our findings show that resistance to apoptosis can arise as a result of mutations affecting discrete stages of the mIgM signalling pathway. The mutant lines reported here show defects that have not yet been identified in previous studies and are likely to be useful tools in dissecting the signalling of cell death in B lymphocytes.

Reconstitution of killer cell inhibitory receptor-mediated signal transduction machinery in a cell-free model system

Recognition of class I MHC molecules on target cells by killer cell inhibitory receptors (KIRs) blocks natural cytotoxicity and antibody-dependent cell cytotoxicity of NK cells and CD3/TCR dependent cytotoxicity of T cells. The inhibitory effect of KIR ligation requires phosphorylation of the cytoplasmic tail of KIR and subsequent recruitment of an SH2-containing protein tyrosine phosphatase, SHP-1. To better understand the molecular mechanism of the KIR-mediated inhibitory signal transduction, we developed an in vitro assay system using a purified His-tag fusion protein of KIR cytoplasmic tail (His-CytKIR) and Jurkat T cell lysates. We identified a target molecule of SHP-1 by comparing the phosphorylation of major cellular substrates following in vitro phosphorylation of Jurkat cell lysates in the presence and absence of the His-CytKIR in this cell-free model system. The His-CytKIR was tyrosine phosphorylated by Lck in vitro, and the phosphorylated His-CytKIR recruited SHP-1. Interestingly, we observed that among major substrates phosphorylated in vitro, PLC-gamma exhibited a dramatic decrease in phosphorylation when the His-CytKIR was mixed with Jurkat T cell lysates. However, PLC-gamma exhibited no decrease in phosphorylation when SHP-1 or Lck was depleted or deficient in this reaction mixture, suggesting that the SHP-1 recruited by the phosphorylated His-CytKIR directly mediate the dephosphorylation of PLC-gamma. The cell-free model system could be used to reveal the detailed molecular interactions in the KIR-mediated signal transduction.

Antagonistic Roles for Phospholipase D Activities in B Cell Signaling: While the Antigen Receptors Transduce Mitogenic Signals Via a Novel Phospholipase D Activity, Phosphatidylcholine-Phospholipase D Mediates Antiproliferative Signals

Cross-linking of the Ag receptors on B cells induces DNA synthesis and proliferation. Butanol trap experiments suggest that one or more phospholipase D activities play a key role in this process. Although phosphatidylcholine-phospholipase D has been shown to play a central role in the transduction of proliferative responses for a wide variety of calcium-mobilizing receptors, we show that the Ag receptors are not coupled to this phospholipase. In addition, phosphatidylcholine-phospholipase D is not stimulated under conditions that mimic T cell-dependent B cell activation. In contrast, ATP, which inhibits surface Ig (sIg)-mediated DNA synthesis in murine B cells via P2-purinoceptors, activates phosphatidylcholine-phospholipase D. Phosphatidylcholine-phospholipase D is therefore associated with antiproliferative signal transduction in mature B cells, but it does not transduce early signals associated with sIg-mediated growth arrest or apoptosis in immature B cells. Mitogenic stimulation of sIg is, however, coupled to a novel nonphosphatidylcholine-hydrolyzing phospholipase D activity. The resultant sIg-generated phosphatidic acid, unlike the phosphatidylcholine-derived phosphatidic acid generated via the purinoceptors, is converted to diacylglycerol. These data provide the first evidence that while the novel sIg-coupled phospholipase D and resultant diacylglycerol generation may play a role in B cell survival and proliferation, phosphatidylcholine-phospholipase D may transduce, via phosphatidic acid, negative immunomodulatory signals in mature B lymphocytes.

Role of SHIP in FcgammaRIIb-mediated inhibition of Ras activation in B cells

Previous studies by our lab and others established that co-crosslinking sIg and IgG receptor FcgammaRIIb in B cells in a feedback suppression model (negative signaling) promoted tyrosine phosphorylation of the inositol 5-phosphatase SHIP and its interaction with Shc and that these events were associated with inhibition of the Ras pathway. We therefore hypothesized a competition model in which the SH2 domain of SHIP competes with that of Grb2 for binding to phospho-Shc to inhibit the Ras pathway. Here, we provide evidence consistent with this hypothesis. First, FcgammaRIIb-deficient B cells, which do not undergo SHIP tyrosine phosphorylation nor interaction with Shc, displayed an active Ras pathway under negative signaling conditions; reconstitution of FcgammaRIIb expression restored the block in Ras. Second, under conditions of negative signaling leading to SHIP-Shc interaction in wild-type B cells, we observed a profound reduction in the activation-induced association of Grb2 to Sos. Experiments reported here and elsewhere revealed the Grb2-Sos interaction required the engagement of the Grb2 SH2 domain by phospho-Shc. Third, we demonstrated that phospho-Shc cannot concomitantly bind Grb2 and SHIP, indicating that the two proteins competed for the same phospho-tyrosine residue on Shc. These data are consistent with the proposed competition model, and further indicate that the activation induced Grb2-Sos association is rate limiting for Ras activation.

Mitogenic response of murine B lymphocytes to Salmonella typhimurium lipopolysaccharide requires protein kinase C-dependent late tyrosine phosphorylations

Unlike the cross-linking of membrane immunoglobulins, the activation of B cells by lipopolysaccharide (LPS) does not involve the phosphoinositol turnover and the initial activation of tyrosine kinases. However, LPS-induced B-cell proliferation was inhibited by the tyrosine kinase inhibitors genistein and herbimycin A even when added 48 h after the beginning of the culture. Tyrosyl-phosphorylated proteins were detected by Western blotting after 24 h of culture with LPS, reaching a maximum concentration after 72 h. Late tyrosine phosphorylations were also detected in B cells activated for 72 h with anti-immunoglobulin M antibody and were abrogated by the protein synthesis inhibitor cycloheximide, the tyrosine kinase inhibitors genistein and herbimycin A, and the protein kinase C inhibitor chelerythrine. The role of protein kinase C in late tyrosine kinase activation is independent of Ca2+ mobilization and was confirmed by detection of a comparable but restricted pattern of tyrosine-phosphorylated substrates in B cells treated with phorbol myristate acetate alone or in association with ionomycin. Tyrosine kinase activation was dependent on de novo protein synthesis. However, culture supernatants of LPS-activated B cells were devoid of mitogenic activity and induced a phosphorylation pattern more restricted than that achieved by LPS. Altogether these data indicate that proliferation signals induced by LPS or by the cross-linking of membrane immunoglobulins are controlled by late tyrosine phosphorylations occurring throughout the first 3 days of culture, controlled in part by protein kinase C activation, and dependent on the synthesis of an intermediate protein(s) either not secreted in the culture supernatant or present but biologically inactive in naive B cells.

SHIP modulates immune receptor responses by regulating membrane association of Btk

Membrane recruitment of SHIP is responsible for the inhibitory signal generated by FcgammaRIIB coligation to the BCR. By reducing the level of PIP3, SHIP regulates the association of the tyrosine kinase Btk with the membrane through PH domain-phosphoinositol lipid interactions. Inhibition of BCR signaling by either FcgammaRIIB coligation, membrane expression of SHIP, or inhibition of P13K, conditions which result in decreased levels of PIP3, is suppressed by the expression of Btk as a membrane-associated chimera. Conversely, increasing PIP3 levels by deletion of SHIP results in increased Btk association with the membrane and hyperresponsive BCR signaling. These results suggest a central role for PIP3 in regulating the B cell stimulatory state by modulating Btk localization and thereby calcium fluxes.

Involvement of dihydropyridine-sensitive calcium channels in human dendritic cell function. Competition by HIV-1 Tat

The entry of extracellular calcium in leukocytes mediates several cellular processes; however, unlike in excitable tissues, the underlying molecular mechanisms are poorly defined. In this paper we provide phenotypical and biochemical evidence that peripheral blood-derived human dendritic cells express dihydropyridine-sensitive calcium channels. Exposure to the dihydropyridine drug nifedipine, which binds L-type calcium channels blocking calcium influx, prevents two dendritic cell functions that are dependent on extracellular calcium entry: apoptotic body engulfment and interleukin-12 production induced by cross-linking of the surface lectin NKRP1A. It is known that exogenous human immunodeficiency virus, type 1 Tat affects several Ca2+-dependent immune cell responses. Here we demonstrate that Tat inhibits apoptotic body engulfment and interleukin-12 production by blocking extracellular calcium influx. This inhibition is prevented by the calcium channel agonist dihydropyridine derivative Bay K 8644, suggesting the involvement of L-type calcium channels. This hypothesis is further supported by the observation that Tat and dihydropyridine drugs compete for binding to dendritic cells. Taken together, these findings indicate that exogenous Tat exerts its inhibitory effects on dendritic cells by blocking dihydropyridine-sensitive L-type calcium channels.

Negative signaling in B cells: SHIP Grbs Shc

Negative signaling in B cells is initiated by co-crosslinking of the antigen receptor and the Fcy receptor, resulting in cessation of B-cell signaling events and, in turn, inhibiting B-cell proliferation and antibody secretion. Here, a competitive role is proposed for SHIP in blocking the interaction of Shc with the Grb2-Sos complex of proteins that lead to Ras activation in B cells.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

Protein kinase C plays a key role in the cross-talk between intracellular signalings via prostanoid receptors in a megakaryoblastic cell line, MEG-01s

In a previous study, we characterized prostanoid and thrombin receptors expressed on a megakaryoblastic cell line, MEG-01s (Blood 78, 2328-2336, 1991). In this study, we examines the mechanism of cross-talk between intracellular Ca2+ ([Ca2+]i) and cAMP signalings through prostanoid and thrombin receptors. Addition of a thromboxane (TX)A2 mimetic (U46619 or STA2) or thrombin stimulated the formation of inositol phosphates and dose-dependently augmented a prostaglandin (PG)I2 mimetic (iloprost)- or forskolin-induced cAMP formation. 12-O-tetradecanoylphorbol-13-acetate (TPA) and ionomycin, to lesser extent, also augmented iloprost-induced cAMP formation. The enhancing effect of U46619 or TPA on cAMP formation was inhibited by prolonged pretreatment of the cells with TPA (2.5 microM, 24 h), but not with calmodulin-antagonists; W-7, W-5, or KN-62. The elevation of [Ca2+]i induced by thrombin, STA2 or PGE2 was significantly suppressed by pretreatment of the cells with TPA (100 nM) as well as cAMP mimetics such as dibutyryl cAMP (5 mM), forskolin (5 microM) and iloprost (1 microM). These results suggest the key role of PKC on the cross-talk between [Ca2+]i and cAMP signalings through prostanoid and thrombin receptors; PKC, which is activated with TXA2 or thrombin, concomitantly suppress further [Ca2+]i elevation and enhances the PGI2 receptor-mediated cAMP formation, which, in turn, suppress [Ca2+]i elevation.

Positive and negative regulation of cell proliferation through prostaglandin receptors in NIH-3T3 cells

Among major eicosanoids and their analogs, prostaglandin (PG) F2 alpha > PGD2 > PGE1 > or = PGE2 > iloprost, a stable agonist of PGI2, dose-dependently stimulated DNA synthesis in quiescent NIH-3T3 cells. PGF2 alpha, PGD2, and PGE2, in that order, formed inositol phosphates and elevated intracellular Ca2+ ([Ca2+]i) but did not form cAMP nor inhibit forskolin-induced cAMP formation. Iloprost, PGI2, and PGE1 induced cAMP formation dose dependently with an ED50 of around 10(-7) M, and PGE2 at more than 10(-6) M did it. [3H]PGF2 alpha and [3H]PGD2 bindings membranes from NIH-3T3 cells were displaced in the order of PGF2 alpha > PGD2 > or = PGE2, while [3H]PGE2 binding was displaced by PGE2 > PGD2 > or = PGF2 alpha. Expression of mRNA encoding EP1 and EP4 (EP2) subtypes could be detected by reverse transcription- polymerase chain reaction using primers specific for EP1 and EP4 (EP2) cDNAs, but not that of EP3 subtype mRNA. The dose dependence of cAMP formation on iloprost and PGI2 and that of [Ca2+]i elevation on PGF2 alpha, D2, and E2 were similar to that of [3H]thymidine incorporation on the corresponding agonists. Fluprostenol (1 microM), a PGF2 alpha receptor agonist > 17-phenyl-trinor-PGE2 (1 microM), an EP1 receptor agonist stimulated [3H]thymidine incorporation, but an EP3 receptor agonist, ONO-AP-324 nor an EP4 (EP2) receptor agonist, 11-deoxy-PGE1 (1 microM) did not. Iloprost, dibutyryl cAMP, forskolin, or cholera toxin, when applied alone, enhanced [3H]thymidine incorporation, while they inhibited [3H]thymidine incorporation induced by submaximal concentrations of PGF2 alpha or epidermal growth factor (EGF), when applied within 12 hr after agonist stimulation. These results suggest that the proliferation of NIH-3T3 cells is stimulated by PGs via the PGF2 alpha receptor, EP1 subtype of PGE receptor, and the PGI2/PGE1 receptor through [Ca2+]i- and cAMP-dependent pathways, and that cAMP pathway negatively cross-talks with [Ca2+]i-or receptor tyrosine kinase-mediated DNA synthesis in a cell cycle-dependent manner.

Induction of cell cycle regulatory proteins in anti-immunoglobulin-stimulated mature B lymphocytes

Progression through the cell cycle is a tightly controlled process that integrates signals generated at the plasma membrane with the proteins that form the cell cycle machinery. The current study chronicles the induction of cyclins, cyclin-dependent kinases (cdk), and cdk inhibitors in low density primary mouse B lymphocytes after anti-immunoglobulin plus interleukin 4 (IgM + IL-4) stimulation. In this system, > 85% of cells remain in the G0/G1 phase of cell cycle for an initial 24-h period, followed by entry of up to 50% of the cells into S phase, commencing around 30 h and peaking at 48 h. Extensive time course analyses of these anti-IgM + IL-4-stimulated B cells revealed that the G1-associated D-type cyclins D2 and D3 were induced by 3 h after stimulation, and that cyclins E, A, and B were subsequently induced sequentially, beginning at mid-G1, G1/S transition, and S phase, respectively. The G1-associated cyclin D1 was not expressed at any stage of the anti-Ig + IL-4-induced B cell cycle. cdk2, cdk4, and cdk6 were induced during G1, whereas cell division cycle-2 (cdc2) was induced concomitantly with S phase. Irrespective of their expression, the kinases cdk2 and cdc2 were only active from S phase onwards, suggesting that productive cyclin/kinase complex formation did not occur until that time. Cell cycle inhibitors p21 and p19 were induced by anti-Ig + IL-4, peaking in expression at mid-G1 and S phase, respectively. Stimulation of low density B cells with anti-Ig + IL-4 caused rapid down regulation of the p27 inhibitor, however this protein was reexpressed at 54-96 h after stimulation. In contrast, B cells stimulated with anti-CD40, a stimulus which induces long-term B cell proliferation, permanently down regulated p27. These findings are consistent with the concept that p27 reexpression contributes to the G1 arrest that follows antigen receptor crosslinking. Low density B cells cultured in the viability-enhancing cytokine IL-4 alone also showed induction of D2 and D3 cyclin expression. However, the D2 expression was transient, and the D3 expression was substantially lower than that observed in B cells induced to proliferate by anti-Ig + IL-4. This partial induction of D2 and D3 expression may explain IL-4's ability to promote B cell entry into G1 but not S phase of cell cycle, and furthermore, its ability to truncate G1 progression when B cells are subsequently stimulated with anti-Ig.

Ligation of CD40 with soluble CD40 ligand reverses anti-immunoglobulin-mediated negative signalling in murine B lymphoma cell lines but not in immature B cells from neonatal mice

Ligation of surface immunoglobulin (sIg) on certain murine B-lymphoma lines has been shown to initiate a programme leading to growth arrest and death of the cells by apoptosis. The cell lines WEHI 231 and CH33 which respond in this way to receptor cross-linking have phenotypic characteristics resembling those of immature normal B cells, and their responses have been taken to model those responsible for clonal deletion or anergy. Cross-linking of sIg on normal neonatal B cells has also been shown to inhibit their responsiveness to polyclonal activators. We have examined the ability of various co-stimuli to modify the response of growth-inhibitable B lymphoma lines to sIg cross-linking. Our findings indicate that cell-cell contact between cells of the WEHI 231 or CH33 lines and activated T cells rescues these cells from growth arrest and apoptosis. Cell-free supernatants from some T-cell lines were also protective although recombinant IL-4 had no effect. Analysis of the most effective signals and timing for inducing this protection suggested that it might, in part, be mediated by CD40 ligand (CD40L) expressed on or secreted by activated T cells. Using a soluble recombinant CD40L-CD8 fusion protein we have now shown that co-ligation of CD40 is sufficient to rescue WEHI231 and CH33 cells from anti-Ig-induced apoptosis. In contrast, the inhibitory effect of anti-Ig antibodies on the lipopolysaccharide (LPS)-driven proliferation of neonatal B cells was not relieved by co-ligation of CD40 with CD40L. These findings bring into question the usefulness of 'immature' B-cell lines as models for tolerance induction.

Anti-Ig-induced calcium influx in rat B lymphocytes mediated by cGMP through a dihydropyridine-sensitive channel

In contrast to excitable tissues where calcium channels are well characterized, the nature of the B lymphocyte calcium channel is unresolved. Here, we demonstrate by single cell analysis of freshly isolated rat B cells that the anti-immunoglobulin (Ig)-induced calcium influx takes place through a channel which shares pharmacologic and serologic properties with the L-type calcium channel found in excitable tissues. It is sensitive to the dihydropyridines nicardipine and Bay K 8644, to calciseptine, and to an anti-peptide antibody raised against the alpha1 subunit of the L-type calcium channel, but is voltage-insensitive. Anti-alpha1 and anti-alpha2 antibodies stain B but not T lymphocytes. Application of a cGMP agonist, measurement of cGMP levels in anti-Ig-stimulated B cells, and examining the effect of a guanylyl cyclase inhibitor on the anti-Ig response show that cGMP mediates the influx. This possibly involves a cGMP-dependent protein kinase. The anti-Ig-induced response is not abolished by prior treatment of B cells with a high dose of thapsigargin. These findings undermine the widely held belief of a categorical divide between excitable and non-excitable tissue calcium channels, demonstrate the limitations of the capacitative calcium influx theory, and point to a distinction between the calcium response mechanisms utilized by B and T lymphocytes.

The CREB, ATF-1, and ATF-2 transcription factors from bovine leukemia virus-infected B lymphocytes activate viral expression

Efficient transcription and replication of the bovine leukemia virus (BLV) genome require both the viral long terminal repeat (LTR) and the virus-coded transcriptional activator Tax, which functions through a 21-bp sequence (Tax-responsive element [TxRE]) which is repeated three times within the LTR. Since Tax does not bind directly to DNA, host cell transcription factors play a central role in BLV expression. Electrophoretic mobility shift assays with nuclear extracts prepared with infected bovine B lymphocytes revealed five TxRE-specific complexes (C1, C2, C3, C4, and C5). Here, by using a UV-induced indirect labeling technique (UV cross-linking) in conjunction with mobility shift assays, eight major polypeptides of 31, 33, 42, 46, 51, 57, 87, and 119 kDa were identified within these five complexes. Immunoprecipitation experiments identified the 57- and 119-kDa proteins as cyclic AMP response element-binding (CREB) proteins, the 46- and 51-kDa proteins as activating transcription factor-1 (ATF-1), and the 87-kDa as protein ATF-2. All of these proteins (except the ATF-1 protein of 51 kDa) belong to the complex C1, which is the major complex identified in freshly isolated BLV-infected lymphocytes from cattle with persistent lymphocytosis. In transient-cotransfection experiments, these three transcription factors were able to activate LTR-directed gene expression in the presence of protein kinase A or Ca2+/calmodulin-dependent protein kinase IV. CREB protein, ATF-1, and ATF-2 thus appear to be the major transcription factors involved in the early stages of viral expression.

Regulation of B cell function by the immunosuppressive agent leflunomide

Leflunomide is an immunosuppressive drug capable of inhibiting cellular and humoral mediated responses in vivo. The mechanism responsible for suppression of B cell antibody responses in vivo has not been identified. In this study we demonstrate that leflunomide functions to inhibit murine B cell antibody production by directly acting on the B cell. Experiments performed in vivo showed that both T cell-dependent as well as T cell-independent antigen responses were suppressed by leflunomide. Initial in vitro experiments demonstrated that leflunomide inhibited B cell antibody production by decreasing B cell proliferation. The suppression of B cell proliferation induced by a variety of stimuli that use different signal cascade components suggested that leflunomide was acting on a common component required for B cell proliferation. Kinetic studies with LPS activated B cells revealed that leflunomide retained its inhibitory activity when added as late as 24 hr after stimulation in an 88-hr assay. By analyzing the cell cycle of LPS-stimulated B cells we observed that leflunomide targets two different stages in cell cycle transition: (1) from G1 to S phase and (2) from S phase to G2/M phase. Analysis of one of the cyclin-dependent kinases, Cdk2 protein, by Western blot revealed that Cdk2 levels were decreased, in the presence of leflunomide, 48 hr after stimulation. These data further confirmed that leflunomide inhibited B cell progression through the S phase. We also present evidence that the addition of exogenous uridine reversed the antiproliferative activity of leflunomide. This indicated that leflunomide acted as a pyrimidine synthesis inhibitor, thereby inhibiting B cell proliferation and cell cycle progression.

Selective activation of c-Jun kinase mitogen-activated protein kinase by CD40 on human B cells

The B cell surface antigen receptor, surface IgM (sIgM), is involved in B cell activation and proliferation. CD40 is involved in regulating IgE production and B cell survival. Cross-linking of B cell sIgM activates the Ras/Raf/p42erk2 pathway. In contrast, ligation of CD40 by antibody or soluble gp39 (CD40 ligand) leads to activation of the c-Jun kinase (JNK)/stress-activated protein kinase pathway. JNK/stress-activated protein kinase activation correlated with the stimulation of MEK kinase activity. CD40 does not activate the p42erk2 pathway, and sIgM fails to regulate the JNK/stress-activated protein kinase pathway in B cells. Thus, two important cell surface receptors involved in controlling specific B cell response differentially regulate sequential protein kinase pathways involving different members of the mitogen-activated protein kinase family. Anti-CD40 also rescued B cell apoptosis induced by anti-IgM. CD40 ligation did not affect the sIgM stimulation of p42erk2 activity. Conversely, sIgM ligation did not influence CD40 stimulation of JNK/stress-activated protein kinase. These results suggest that independent, parallel protein kinase response pathways are involved in the integration of sIgM and CD40 control of B cell phenotype and function.

Altered calcium signal transduction in B-1 malignant cells

Lymphocyte proliferation is guided by receptor-mediated signal transduction pathways that dictate the immunological response/clonality of that cell. We have previously reported that NZB-derived malignant B-1 cells, which serve as a murine model for human chronic lymphocytic leukaemia, demonstrate altered expression of surface IgM and CD45 signalling molecules, and a failure to proliferate following membrane IgM stimulation. To examine receptor-mediated cytosolic calcium (Cai) signalling in B cell leukaemia, we studied IgM-induced Cai responses in malignant B-1 cells and B cells from non-leukaemic mice. Basal Cai was slightly lower in malignant B-1 cells than in non-leukaemic cells. Anti-IgM stimulation induced a sustained increase in Cai to levels 1.3-fold greater than basal Cai in conventional B cells. In contrast, leukaemic B-1 cells demonstrated a sharp but transient rise in Cai followed by a gradual increase to levels 2.3-fold greater than basal [Ca]i Ca influx from extracellular sources contributed to the early and late Cai signal in both sets of cells. Pre-incubation (2-30 min) with anti-CD45 had no effect on basal Cai or the anti-IgM Cai signal in B cells, but reduced the Cai transient in malignant B-1 cells. Additional experiments characterized the effects of phosphorylation/dephosphorylation events on the Cai profile following anti-IgM stimulation. Protein tyrosine kinase inhibitors decreased the anti-IgM-induced Cai transient in malignant B-1 cells by 80%, but only moderately affected (40%) of the Cai response in non-leukaemic B cells. Protein tyrosine phosphatase inhibitors and protein kinase C (PKC) activators attenuated the Cai response to the same degree in normal and leukaemic B cells. These results show that Cai signalling differs widely between non-malignant B cells and malignant B-1 cells, and that tyrosine phosphorylation and CD45 modulation of IgM signalling are involved in the altered Cai responses in malignant B-1 cells.

Impaired proliferation of peripheral B cells and indication of autoimmune disease in lyn-deficient mice

The Src family protein-tyrosine kinase Lyn associates physically with the BCR and has been suggested to play an important role in BCR-mediated signaling. Studies with lyn-/- mice showed that the number of B cells decreased by half in their peripheral tissues. In addition, these B cells do not respond normally to a number of stimuli, including BCR cross-linking and CD40 ligand. Induction of tyrosine phosphorylation on a variety of cellular proteins, such as Vav, Cbl, and HS1, upon BCR cross-linking was also abolished in these B cells. Despite the impaired BCR-mediated signaling, concentrations of IgM and IgA in sera were remarkably elevated, and production of autoantibodies was detected in lyn-/- mice. Histological study showed splenomegaly and enlargement of lymph nodes that became evident with age in the mutant mice. The spleen contained significant number of plasma cells as well as unusual lymphoblast-like cells carrying Mac1 antigen and cytoplasmic IgM. These cells spontaneously secreted a large amount of IgM in vitro. Finally, significant number of lyn-/- mice show glomerulonephritis, an indication of autoimmune disease. From these data, we conclude that Lyn plays a role in signal transduction for not only clonal expansion and terminal differentiation of peripheral B cells but also elimination of autoreactive B cells.

Interferon-gamma and lipopolysaccharide reduce cAMP responses in cultured glial cells: reversal by a type IV phosphodiesterase inhibitor

The aim of the present study was to determine whether two classical macrophage activators, bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) could affect the accumulation of the second messenger cAMP in cultured rat microglia and astrocytes. Purified microglia and astrocyte secondary cultures obtained from the neonatal rat were grown for 3 days in basal medium Eagle (BME) + 10% fetal calf serum (FCS). Exposure of microglia to LPS resulted into a dose- and time-dependent decrease in the accumulation of cAMP induced by receptor-mediated (isoproterenol or prostaglandin E2) or direct (forskolin) activation of adenylate cyclase. The inhibitory effect of LPS was rapid (a 10 min preincubation was sufficient to approach a maximal effect), occurred at low doses (IC50 = 1.2 ng/ml), and was not abrogated by pertussis toxin. A selective inhibitor of type IV phosphodiesterase (rolipram, 100 nM) prevented the effect of LPS on cAMP accumulation, while inhibitors of other forms of phosphodiesterase were unable to do so. IFN-gamma (100 u/ml) also caused a depression of the evoked cAMP accumulation in microglia after a 10 min preincubation, and its effect was prevented by rolipram, as in the case of LPS. Astrocytes differed from microglia in that LPS (1-100 ng/ml) did not inhibit the accumulation of cAMP induced by either isoproterenol or forskolin; on the other hand, IFN-gamma did have an inhibitory effect (though less pronounced than in microglia) that could be prevented by rolipram.(ABSTRACT TRUNCATED AT 250 WORDS)

BTS 71 412: in vitro profile of a novel pyrazolinone immunosuppressant

The effect of BTS 71 412, 4-acetyl-1-(4-chlorophenyl)-3-(2-methylthiophenyl)- 3-pyrazolin-5-one, has been determined on a variety of immune reactions in vitro in order to gain a further insight into the mechanisms whereby this novel immunosuppressive drug suppresses cell and antibody mediated immune responses in vivo. BTS 71 412 markedly inhibited [3H]thymidine incorporation by mouse splenocytes activated with concanavalin-A (IC50 = 20.1 microM), phytohaemagglutinin (IC50 = 4.6 microM), lipopolysaccharide (LPS) (IC50 = 3.2 microM), anti-IgM (mu-chain specific) (IC50 = 2.6 microM), or mixed lymphocyte culture (IC50 = 8.4 microM). Activity of BTS 71 412 was not associated with a reduction in cell viability. BTS 71 412 also prevented [3H]thymidine incorporation by the murine HT-2 helper T-cell clone when cultured with IL-2 (IC50 = 7.6 microM) or IL-4 (IC50 = 7.3 microM), enriched Thy-1+ T-lymphocytes stimulated with phorbol 12-myristate 13-acetate plus ionomycin (IC50 = 3.6 microM), and enriched B220- B-lymphocytes stimulated with LPS (IC50 = 3.0 microM). Splenocytes cultured with BTS 71 412 produced lower levels of interleukin (IL)-2, IL-10 and interferon-gamma when stimulated with concanavalin-A (IC50 values 42 microM, 22 microM and 60 microM, respectively). The compound suppressed in vitro antibody responses to keyhole limpet haemocyanin (IC50 = 2.3 microM), but did not reproducibly inhibit IL-6 or tumour necrosis factor alpha production by adherent peritoneal macrophages stimulated with LPS in vitro. These data indicate that BTS 71 412 specifically inhibits both B- and T-lymphocyte activation and proliferation but does not affect macrophage activation in vitro.

Prostaglandin F2 alpha stimulates formation of p21ras-GTP complex and mitogen-activated protein kinase in NIH-3T3 cells via Gq-protein-coupled pathway

Prostaglandin (PG) F2 alpha activated mitogen-activated protein (MAP) kinase and MAP kinase kinase in NIH-3T3 cells by a mechanism that was completely inhibited by protein kinase inhibitors, staurosporine (20 nM) or H-7 (20 microM), but was insensitive to pretreatment with islet-activating protein (100 ng/ml; 24 h) or 12-O-tetradecanoylphorbol 13-acetate (2.5 microM; 24 h). PGF2 alpha stimulation also led to a significant increase in Ras.GTP complex. Transfection of a cDNA encoding a constitutively active mutant of Gq alpha-subunit (Q209L) mimicked PGF2 alpha-induced MAP kinase activation, increase in Ras.GTP complex, and DNA synthesis in these cells, suggesting that activation of Gq mediates the PGF2 alpha-activation of Ras-MAP kinase pathway and mitogenesis in NIH-3T3 cells. These data provide a new insight into regulatory mechanisms of Ras-MAP kinase pathway through heterotrimeric G-protein-mediated pathways.

Cell-cycle kinetics of proliferating mouse B lymphocytes in vitro

Resting mouse B lymphocytes were stimulated in vitro with lipopolysaccharide, Sepharose-coupled anti-kappa antibodies or a combination of the two. B lymphocytes stimulated with anti-kappa entered the cell-cycle with more rapid kinetics and at a higher frequency than did the corresponding cell population stimulated with lipopolysaccharide. Using cell cycle analysis after DNA staining combined with an M phase block, the cell-cycle kinetics of in vitro cultured B-lymphocytes was studied. The labelling index of lipopolysaccharide stimulated B lymphocytes was 60% while that for anti-kappa Sepharose stimulated cells was 85%. The generation time of the actively cycling population from both types of cultures was constant and was of the order of 18 h. Thus, the fraction of B lymphocytes induced to proliferate in vitro varies depending on the stimulus, while the growth kinetics of the actively proliferating populations are remarkably constant.

Stimulation of human lymphocytes by cathepsin G

We investigated the effect of cathepsin G, a serine protease in polymorphonuclear granulocytes, on the function of human lymphocytes. Cathepsin G increased the [3H]thymidine incorporation into human lymphocytes. This mitogenic activity was due to the proteolytic activity of cathepsin G. Both B and T cells showed increased [3H]thymidine incorporation, and this effect was more remarkable for T cells than for B cells. Among the T cell subsets, CD4+ T cells showed the increase in DNA synthesis, but CD8+ T cells did not. When human lymphocytes were stimulated with cathepsin G, intracellular free Ca2+ concentration ([Ca2+]i) increased in B and T cells, including CD4+ T cells and CD8+ T cells. The change in intracellular Ca2+ was due to Ca2+ influx and release of intracellular stores. Cathepsin G also induced the production of inositol 1,4,5-trisphosphate (IP3) in B cells, CD4+ T cells, and CD8+ T cells, leading to the release of Ca2+ from intracellular stores. Moreover, the stimulation with cathepsin G resulted in alkalization of the cytosol of B cells, CD4+ T cells, and CD8+ T cells as the result of Na+/H+ antiport activation. The change in intracellular Ca2+, production of IP3, and cytoplasmic alkalization in lymphocytes were due to its proteolytic activity. Cathepsin G released from granulocytes is considered to act on human lymphocytes in vivo and lead to the increase in DNA synthesis of B cells and CD4+ T cells through IP3 production, an increase in [Ca2+]i, and alkalization. However, these second messengers do not lead to the increase in DNA synthesis of CD8+ T cells.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

Functional analysis of Csk in signal transduction through the B-cell antigen receptor

In B cells, two classes of protein tyrosine kinases (PTKs), the Src family of PTKs (Lyn, Fyn, Lck, and Blk) and non-Src family of PTKs (Syk), are known to be involved in signal transduction induced by the stimulation of the B-cell antigen receptor (BCR). Previous studies using Lyn-negative chicken B-cell clones revealed that Lyn is necessary for transduction of signals through the BCR. The kinase activity of the Src family of PTKs is negatively regulated by phosphorylation at the C-terminal tyrosine residue, and the PTK Csk has been demonstrated to phosphorylate this C-terminal residue of the Src family of PTKs. To investigate the role of Csk in BCR signaling, Csk-negative chicken B-cell clones were generated. In these Csk-negative cells, Lyn became constitutively active and highly phosphorylated at the autophosphorylation site, indicating that Csk is necessary to sustain Lyn in an inactive state. Since the C-terminal tyrosine phosphorylation of Lyn is barely detectable in the unstimulated, wild-type B cells, our data suggest that the activities of Csk and a certain protein tyrosine phosphatase(s) are balanced to maintain Lyn at a hypophosphorylated and inactive state. Moreover, we show that the kinase activity of Syk was also constitutively activated in Csk-negative cells. The degree of activation of both the Lyn and Syk kinases in Csk-negative cells was comparable to that observed in wild-type cells after BCR stimulation. However, BCR stimulation was still necessary in Csk-negative cells to elicit tyrosine phosphorylation of cellular proteins, as well as calcium mobilization and inositol 1,4,5-trisphosphate generation. These results suggest that not only activation of the Lyn and Syk kinases but also additional signals induced by the cross-linking of the BCR are required for full transduction of BCR signaling.

Functional analysis of Csk in signal transduction through the B-cell antigen receptor

In B cells, two classes of protein tyrosine kinases (PTKs), the Src family of PTKs (Lyn, Fyn, Lck, and Blk) and non-Src family of PTKs (Syk), are known to be involved in signal transduction induced by the stimulation of the B-cell antigen receptor (BCR). Previous studies using Lyn-negative chicken B-cell clones revealed that Lyn is necessary for transduction of signals through the BCR. The kinase activity of the Src family of PTKs is negatively regulated by phosphorylation at the C-terminal tyrosine residue, and the PTK Csk has been demonstrated to phosphorylate this C-terminal residue of the Src family of PTKs. To investigate the role of Csk in BCR signaling, Csk-negative chicken B-cell clones were generated. In these Csk-negative cells, Lyn became constitutively active and highly phosphorylated at the autophosphorylation site, indicating that Csk is necessary to sustain Lyn in an inactive state. Since the C-terminal tyrosine phosphorylation of Lyn is barely detectable in the unstimulated, wild-type B cells, our data suggest that the activities of Csk and a certain protein tyrosine phosphatase(s) are balanced to maintain Lyn at a hypophosphorylated and inactive state. Moreover, we show that the kinase activity of Syk was also constitutively activated in Csk-negative cells. The degree of activation of both the Lyn and Syk kinases in Csk-negative cells was comparable to that observed in wild-type cells after BCR stimulation. However, BCR stimulation was still necessary in Csk-negative cells to elicit tyrosine phosphorylation of cellular proteins, as well as calcium mobilization and inositol 1,4,5-trisphosphate generation. These results suggest that not only activation of the Lyn and Syk kinases but also additional signals induced by the cross-linking of the BCR are required for full transduction of BCR signaling.