Signal transduction by the B cell antigen receptor and its coreceptors

Identification of two tyrosine phosphoproteins, pp70 and pp68, which interact with phospholipase Cgamma, Grb2, and Vav after B cell antigen receptor activation

Tyrosine phosphorylation of cellular proteins mediates the assembly and localization of effector proteins through interactions facilitated by modular Src homology 2 (SH2) and phosphotyrosine binding domains. We describe here two tyrosine-phosphorylated proteins with Mr values of 70,000 and 68,000 that interact with Grb2, phospholipase C (PLCgamma1 and PLCgamma2), and Vav after B cell receptor cross-linking. The interaction of pp70 and pp68 with PLC and Vav is mediated by the carboxyl-terminal SH2 domain of PLC and the SH2 domain of Vav. In contrast, the interaction of pp70 and pp68 with Grb2 requires cooperative binding of the SH2 and SH3 domains of Grb2. Western blot analysis demonstrated that neither pp70 nor pp68 represented the recently described linker protein SLP-76, which binds Grb2, PLC, and Vav in T cells after T cell receptor activation. Moreover, SLP-76 protein was not detected in a number of B cell lines or in normal mouse B cells. Hence, we propose that pp70 and pp68 likely represent B cell homologs of SLP-76 which facilitate and coordinate B cell activation.

Phosphorylation of two regulatory tyrosine residues in the activation of Bruton's tyrosine kinase via alternative receptors

Mutation of Bruton's tyrosine kinase (Btk) impairs B cell maturation and function and results in a clinical phenotype of X-linked agammaglobulinemia. Activation of Btk correlates with an increase in the phosphorylation of two regulatory Btk tyrosine residues. Y551 (site 1) within the Src homology type 1 (SH1) domain is transphosphorylated by the Src family tyrosine kinases. Y223 (site 2) is an autophosphorylation site within the Btk SH3 domain. Polyclonal, phosphopeptide-specific antibodies were developed to evaluate the phosphorylation of Btk sites 1 and 2. Crosslinking of the B cell antigen receptor (BCR) or the mast cell Fcepsilon receptor, or interleukin 5 receptor stimulation each induced rapid phosphorylation at Btk sites 1 and 2 in a tightly coupled manner. Btk molecules were singly and doubly tyrosine-phosphorylated. Phosphorylated Btk comprised only a small fraction (</=5%) of the total pool of Btk molecules in the BCR-activated B cells. Increased dosage of Lyn in B cells augmented BCR-induced phosphorylation at both sites. Kinetic analysis supports a sequential activation mechanism in which individual Btk molecules undergo serial transphosphorylation (site 1) then autophosphorylation (site 2), followed by successive dephosphorylation of site 1 then site 2. The phosphorylation of conserved tyrosine residues within structurally related Tec family kinases is likely to regulate their activation.

Role for CD40-mediated activation of c-Rel and maintenance of c-myc RNA levels in mitigating anti-IgM-induced growth arrest

CD40 crosslinking on B cells activates NF-kappaB and stress-activated protein kinase (SAPK) pathways. Since CD40 crosslinking rescues WEHI 231 B cells from anti-IgM-induced apoptosis, those pathways were likely candidates to be involved. Indeed, both signaling cascades predominated in anti-IgM-treated WEHI 231 cells, treated concurrently with anti-CD40 to rescue them from apoptosis. Crosslinking of CD40 activated the NF-kappaB proteins c-Rel and p50, but had no influence on their cytoplasmic steady state level. However, in contrast to-and even in the presence of-anti-IgM-mediated signals, engagement of CD40 resulted in a prolonged nuclear translocation of c-Rel, thereby allowing the formation of active NF-kappaB complexes. Consistent with this, the upstream regulatory element of the c-myc promoter, known to be regulated by NF-kappaB, was differently regulated after BCR ligation vs BCR plus CD40 crosslinking. The level of c-myc RNA was rapidly downregulated after BCR engagement, but persistent in the presence of CD40 signaling.

The CD38/cyclic ADP-ribose system: a topological paradox

CD38 was first identified as a lymphocyte differentiation antigen that showed typical properties of an orphan receptor involved in many programs of cell proliferation and activation. However, CD38 proved also to be a bifunctional ectoenzyme that catalyzes the transient formation of cyclic ADP-ribose (cADPR) in a variety of cell types. This property raises many intriguing and so far unanswered questions, since cADPR is a new second messenger molecule directly involved in the control of calcium homeostasis by means of receptor-mediated release of calcium from ryanodine-sensitive intracellular stores. The relationship between receptor-like and enzymatic properties of CD38 is still unknown. The apparent topological paradox of ectocellular synthesis and intracellular activity of cADPR might be explained by: (a) influx of cADPR across the plasma membrane to reach its target stores, as suggested by experiments on cerebellar granule cells; and (b) NAD(+)-induced internalization, following membrane oligomerization, of CD38 with consequent partial import of cADPR metabolism to an intracellular compartment, as recently observed in lymphoid B cells. These two distinct mechanisms and other potential ones (e.g. binding of ectocellularly formed cADPR to cell surface receptors and initiation of signal-transducing pathways across the plasmamembrane) seem to be paradigmatic of processes affecting different types of cells. Although in some biological systems, such as Aplysia and sea urchin egg, cADPR metabolism is restricted to the intracellular environment, in mammalian cells the CD38/cADPR system provides new challenges in terms of subcellular compartmentation and qualifies as an unusual example of "ectobiochemistry" with potential, still unrecognized, properties of cellular regulation.

The immunoreceptor tyrosine-based activation motif of Epstein-Barr virus LMP2A is essential for blocking BCR-mediated signal transduction

The Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) blocks B-cell receptor (BCR) signal transduction in EBV-immortalized B lymphocytes in vitro. The cytoplasmic amino-terminal domain of LMP2A contains an immunoreceptor tyrosine activation motif (ITAM). ITAMs consist of paired tyrosine and leucine residues and play a central role in signal transduction of the BCR and the T-cell receptor (TCR). To investigate the importance of the LMP2A ITAM, two EBV recombinants were constructed, each containing a tyrosine-to-phenylalanine point mutation at amino acid 74 or 85 within the ITAM of LMP2A. Tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression were no longer blocked in the LMP2A ITAM mutant LCLs following BCR cross-linking. In addition, the Syk protein tyrosine kinase (PTK) was unable to bind LMP2A in unstimulated LCLs infected with either of the LMP2A ITAM mutants. Analysis of Syk phosphorylation before and after BCR cross-linking in the LMP2A mutant ITAM LCLs compared with wild-type EBV LCLs indicates a specific role of the LMP2A ITAM on the LMP2A-mediated negative effect on the Syk PTK. These data indicate the importance of the LMP2A ITAM motif in the LMP2A-mediated block on BCR signal transduction and position the role of the Syk PTK as being central to the function of LMP2A.

The motheaten mutation rescues B cell signaling and development in CD45-deficient mice

The cytosolic SHP-1 and transmembrane CD45 protein tyrosine phosphatases (PTP) play critical roles in regulating signal transduction via the B cell antigen receptor (BCR). These PTPs differ, however, in their effects on BCR function. For example, BCR-mediated mitogenesis is essentially ablated in mice lacking CD45 (CD45(-)), but is enhanced in SHP-1-deficient motheaten (me) and viable motheaten (mev) mice. To determine whether these PTPs act independently or coordinately in modulating the physiologic outcome of BCR engagement, we assessed B cell development and signaling in CD45-deficient mev (CD45-/SHP-1-) mice. Here we report that the CD45-/SHP-1-) cells undergo appropriate induction of protein kinase activity, mitogen-activated protein kinase activation, and proliferative responses after BCR aggregation. However, BCR-elicited increases in the tyrosine phosphorylation of several SHP-1-associated phosphoproteins, including CD19, were substantially enhanced in CD45-/SHP-1-, compared to wild-type and CD45- cells. In addition, we observed that the patterns of cell surface expression of mu, delta, and CD5, which distinguish the PTP-deficient from normal mice, are largely restored to normal levels in the double mutant animals. These findings indicate a critical role for the balance of SHP-1 and CD45 activities in determining the outcome of BCR stimulation and suggest that these PTPs act in a coordinate fashion to couple antigen receptor engagement to B cell activation and maturation.

Protein-tyrosine phosphatase SHP-1 is dispensable for FcgammaRIIB-mediated inhibition of B cell antigen receptor activation

The inhibitory Fc receptor, FcgammaRIIB, provides a signal that aborts B cell antigen receptor activation, blocking extracellular calcium influx. Because the protein-tyrosine phosphatase SHP-1 binds tyrosyl phosphorylated FcgammaRIIB and FcgammaRIIB-mediated inhibition is defective in motheaten (me/me) mice, which do not express SHP-1, it was proposed that SHP-1 mediates FcgammaRIIB signaling in B cells (D'Ambrosio, D., Hippen, K. L., Minskoff, S. A., Mellman, I., Pani, G., Siminovitch, K. A., and Cambier, J. C. (1995) Science 268, 293-297). However, SHP-1 is dispensable for FcgammaRIIB-mediated inhibition of FcepsilonRI signaling in mast cells (Ono, M., Bolland, S., Tempst, P., and Ravetch, J. V. (1996) Nature 383, 263-266), prompting us to re-examine the role of SHP-1 in FcgammaRIIB signaling in B cells. We generated immortalized sIgM+, FcgammaRIIB+ cell lines from me/me mice and normal littermates. Co-ligation of FcgammaRIIB and the sIgM antigen receptor inhibits calcium influx in both cell lines. Inhibition is reversed by preincubation with anti-FcgammaRIIB antibodies, indicating that it is mediated by FcgammaRIIB. The inositol 5' phosphatase SHIP is recruited to tyrosyl-phosphorylated FcgammaRIIB in both cell lines. FcgammaRIIB-mediated CD19 dephosphorylation also occurs in the presence or the absence of SHP-1. Our results establish that SHP-1 is dispensable for FcgammaRIIB-mediated inhibition of sIgM antigen receptor signaling.

Negative signaling pathways of the killer cell inhibitory receptor and Fc gamma RIIb1 require distinct phosphatases

Inhibition of natural killer (NK) cells by the killer cell inhibitory receptor (KIR) involves recruitment of the tyrosine phosphatase SHP-1 by KIR and is prevented by expression of a dominant negative SHP-1 mutant. Another inhibitory receptor, the low affinity Fc receptor for immunoglobulin G (IgG) (Fc gamma RIIb1), has been shown to bind SHP-1 when cocross-linked with the antigen receptor on B cells (BCR). However, coligation of Fc gamma RIIb1 with BCR and with Fc epsilon RI on mast cells leads to recruitment of the inositol 5' phosphatase SHIP and to inhibition of mast cells from SHP-1-deficient mice. In this study, we evaluated the ability of these two inhibitory receptors to block target cell lysis by NK cells, and the contribution of SHP-1 and SHIP to inhibition. Recombinant vaccinia viruses encoding chimeric receptors and dominant negative mutants of SHP-1 and SHIP were used for expression in mouse and human NK cells. When the KIR cytoplasmic tail was replaced by that of Fc gamma RIIb1, recognition of HLA class I on target cells by the extracellular domain resulted in inhibition. A dominant negative mutant of SHP-1 reverted the inhibition mediated by the KIR cytoplasmic tail but not that mediated by Fc gamma RIIb1. In contrast, a dominant negative mutant of SHIP reverted only the inhibition mediated by the Fc gamma RIIb1 tail, providing functional evidence that SHIP plays a role in the Fc gamma RIIb1-mediated negative signal. These data demonstrate that inhibition of NK cells by KIR involves primarily the tyrosine phosphatase SHP-1, whereas inhibition mediated by Fc gamma RIIb1 requires the inositol phosphatase SHIP.

Characterization of intracellular calcium responses produced by polycyclic aromatic hydrocarbons in surface marker-defined human peripheral blood mononuclear cells

Previous studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene (BaP) and 7,12-dimethybenz[a]anthracene (DMBA), and possibly 2,3,7,8-tetrachlorodibenzo(p)dioxin (TCDD), may exert their immunosuppressive effects by altering intracellular Ca2+ homeostasis in lymphocytes. In these studies, we examined the effects of two immunosuppressive PAHs (BaP and DMBA), two nonimmunosuppressive PAHs (benzo[e]pyrene (BeP) and anthracene (ANTH)), and TCDD on intracellular Ca2+ levels in surface marker-defined human peripheral blood mononuclear cells (HPBMC). BaP and DMBA, but not BeP and ANTH, were found to produce a time-dependent increase in intracellular Ca2+ with maximal effects achieved following 42- to 66-hr exposures. In a series of studies with HPBMC obtained from 10 donors exposed in vitro for 42 hr, BaP and DMBA were found to produce a significant increase in Ca2+ in CD3+ T cells, CD19+ B cells, and CD14+ monocytes. BeP and ANTH did not produce a statistically significant increase in Ca2+ in the group of donors, but occasionally produced an apparent nonspecific elevation of Ca2+ in HPBMC from individual donors. Interestingly, TCDD produced a small and statistically significant increase in Ca2+ only in B cells analyzed for the pooled 10 donors. Certain BaP metabolites, such as the 7,8-dihydrodiol and the 7,8-diol-9,10-epoxide, were more effective in elevating Ca2+ in HPBMC lymphocytes at 20 hr than was BaP. These results demonstrate in normal HPBMC that immunosuppressive PAHs alter intracellular Ca2+ homeostasis in B cells, T cells, and monocytes, and suggest that P450 metabolism may play an important role in the immunotoxicity of certain PAHs.

Antigen endocytosis and presentation mediated by human membrane IgG1 in the absence of the Ig(alpha)/Ig(beta) dimer

Membrane immunoglobulin (mIg) M and D heavy chains possess minimal (KVK) cytoplasmic tails and associate with the Ig alpha/Ig beta (CD79) dimer to achieve surface expression and antigen presentation function. In contrast, the cytoplasmic tail of mIgG is extended by 25 residues (gamma ct). We have tested the possibility that mIgG can perform antigen capture and presentation functions independently of the Ig(alpha)/beta dimer. We show that CD4/(gamma)ct chimeras are efficiently endocytosed partially dependent on a tyrosine residue in (gamma)ct. In addition, human mIgG was expressed on the surface of Ig(alpha)/Ig(beta)-negative non-lymphoid cells and mediated antigen capture and endocytosis. Antigen-specific human mIgG targeted antigen to MIIC-type vesicles in the Ig(alpha)/beta negative melanoma Mel JuSo and augmented antigen presentation 1000-fold, identical to the augmentation seen in Ig(alpha)/beta-positive B-cells expressing the same transfected mIgG. Thus, unlike mIgM, mIgG has autonomous antigen capture and presentation capacity, which may have evolved to reduce or eliminate the BCR's dependence on additional accessory molecules.

Protein tyrosine phosphatase roles in the regulation of lymphocyte signaling

Tyrosine phosphorylation-based signaling cascades represent an integral component of the signaling circuitry connecting extracellular stimuli to cell response. As the molecular elements which drive such cascades have become increasingly well-characterized, appreciation has grown for the critical roles played by protein tyrosine phosphatases (PTPs) in intracellular signal relay and for the capacity of PTPs to act not only as a counterbalance for protein kinase activities, but also as pivotal enzymes in directing and modulating signal relay and the translation of given stimuli to cell behaviour. PTP function has been particularly well studied in relation to lymphocyte antigen receptor signaling and the results of these studies have provided many novel and significant insights into the biochemical mechanisms whereby PTPs participate in the integration and interpretation of the complex transmembrane stimulatory signals driving cell function and development.

Epstein-Barr virus latent membrane protein 2 associates with and is a substrate for mitogen-activated protein kinase

The latent membrane protein 2 (LMP2) of Epstein-Barr virus interferes with B-lymphocyte signal transduction through the immunoglobulin (Ig) receptor. Two isoforms of LMP2 exist and differ only in that one isoform (LMP2a) contains an N-terminal cytoplasmic domain that the other isoform does not. LMP2a is a phosphoprotein that is phosphorylated on tyrosines and serines in the cytoplasmic domain. GST1-119, a glutathione S-transferase (GST) fusion protein containing the 119 amino acids of the cytoplasmic domain, affinity precipitated serine kinase activity from BJAB cell extracts. The affinity-precipitated kinase phosphorylated LMP2a sequences, and kinase activity was increased following induction. Probing of Western immunoblots of affinity-precipitated proteins showed that the Erk1 form of mitogen-activated protein kinase (MAPK) was present. Purified MAPK phosphorylated GST fusion proteins containing the cytoplasmic domain of LMP2a and mutational analyses were used to identify S15 and S102 as the sites of in vitro phosphorylation. A polyclonal rabbit antiserum was prepared against a maltose binding protein-LMP2a cytoplasmic domain fusion protein (MBP1-119) and used to immunoprecipitate LMP2a from the in vitro-immortalized lymphoblastoid B-cell line B95-8CR. LMP2a immunoprecipitates from B95-8CR contained MAPK as a coprecipitated protein. Cross-linking surface Ig on B95-8CR cells failed to induce MAPK activity within the cells. Treatment of B95-8CR with phorbol myristate acetate (PMA) was able to bypass the Ig receptor block and activate MAPK activity. Phosphorylation of LMP2a on serine residues increased after PMA induction. The possible role for LMP2a serine phosphorylation by MAPK in the control of latency is discussed.

The negative signaling molecule SH2 domain-containing inositol-polyphosphate 5-phosphatase (SHIP) binds to the tyrosine-phosphorylated beta subunit of the high affinity IgE receptor

The SH2 domain-containing inositol-polyphosphate 5-phosphatase, SHIP, associates with FcgammaRIIB and negatively regulates both B-cell and mast cell function. We report here that SHIP was tyrosine-phosphorylated after high affinity IgE receptor (FcepsilonRI) aggregation in rat basophilic leukemia RBL-2H3 cells. The tyrosine phosphorylation of SHIP was an early event after receptor aggregation and was present in cells deficient in the protein-tyrosine kinase Syk. Furthermore it was not secondary to the increase of intracellular calcium or the activation of protein kinase C. SHIP was precipitated by immobilized phosphorylated synthetic peptides based on the immunoreceptor tyrosine-based activation motif (ITAM) of the beta but not the gamma subunit of the high affinity IgE receptor. Tyrosine phosphorylation of SHIP and its association with the tyrosine-phosphorylated beta subunit of FcepsilonRI could play an important role in down-regulating receptor-mediated signal transduction in mast cells. Thus, whereas the activation molecule Syk associates with the gamma subunit ITAM, the beta subunit ITAM binds the negative signaling molecule SHIP. Therefore, unlike B cells where the antigen receptor and coreceptors such as FcgammaRIIB or CD22 each recruits molecules with opposite effects, the FcepsilonRI contains subunits which recruit molecules that activate and inhibit signal transduction.

A novel inhibitory receptor (ILT3) expressed on monocytes, macrophages, and dendritic cells involved in antigen processing

Immunoglobulin-like transcript (ILT) 3 is a novel cell surface molecule of the immunoglobulin superfamily, which is selectively expressed by myeloid antigen presenting cells (APCs) such as monocytes, macrophages, and dendritic cells. The cytoplasmic region of ILT3 contains putative immunoreceptor tyrosine-based inhibitory motifs that suggest an inhibitory function of ILT3. Indeed, co-ligation of ILT3 to stimulatory receptors expressed by APCs results in a dramatic blunting of the increased [Ca2+]i and tyrosine phosphorylation triggered by these receptors. Signal extinction involves SH2-containing protein tyrosine phosphatase 1, which is recruited by ILT3 upon cross-linking. ILT3 can also function in antigen capture and presentation. It is efficiently internalized upon cross-linking, and delivers its ligand to an intracellular compartment where it is processed and presented to T cells. Thus, ILT3 is a novel inhibitory receptor that can negatively regulate activation of APCs and can be used by APCs for antigen uptake.

Engagement of the B lymphocyte antigen receptor induces presentation of intrinsic immunoglobulin peptides on major histocompatibility complex class II molecules

By means of the clonotypic variable region, the immunoglobulin (Ig) is a tumor-specific antigen on B cell neoplasms. We report that engagement of the B cell antigen receptor (BcR) promotes presentation of peptides derived from the B cell's intrinsic Ig to major histocompatibility complex (MHC) class II-restricted T cells. Thus, anti-Ig endowed normal, ex vivo B lymphocytes from H-2d, Ig constant heavy chain allotype b (IgCHb) mice with the capacity to stimulate an I-Ad-restricted T cell clone which recognizes the gamma 2ab 435-451 allopeptide. The corresponding self gamma 2aa peptide is cryptic and 6000-fold less antigenic than the gamma 2ab allopeptide. Even so, the syngeneic B cell lymphoma A20 which expresses surface(s) IgG2aa, was also recognized by the T cells after BcR ligation. Thus, anti-Ig triggered the disclosure of a cryptic tumor antigen determinant. We propose that autoantigens, by engaging the BcR of self-reactive B cells, induce presentation of intrinsic Ig peptides to which the T helper cell (Th) repertoire is not tolerant. In this way, B cells with anti-self potential may be activated without Th recognition of nominal autoantigen.

Differential activation of transcription factors induced by Ca2+ response amplitude and duration

An increase in the intracellular calcium ion concentration ([Ca2+]i) controls a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Calcium signalling patterns can occur as single transients, repetitive oscillations or sustained plateaux, but it is not known whether these patterns are responsible for encoding the specificity of cellular responses. We report here that the amplitude and duration of calcium signals in B lymphocytes controls differential activation of the pro-inflammatory transcriptional regulators NF-kappaB, c-Jun N-terminal kinase (JNK) and NFAT. NF-kappaB and JNK are selectively activated by a large transient [Ca2+]i rise, whereas NFAT is activated by a low, sustained Ca2+ plateau. Differential activation results from differences in the Ca2+ sensitivities and kinetic behaviour of the three pathways. Our results show how downstream effectors can decode information contained in the amplitude and duration of Ca2+ signals, revealing a mechanism by which a multifunctional second messenger such as Ca2+ can achieve specificity in signalling to the nucleus.

Syk activation and dissociation from the B-cell antigen receptor is mediated by phosphorylation of tyrosine 130

Syk (p72(syk)) is a 72-kDa cytoplasmic protein-tyrosine kinase that serves as an essential component of the signal transduction machinery coupled to the B-cell antigen receptor. Syk is recruited to the receptor when it is cross-linked and, in response, becomes tyrosine-phosphorylated and activated before it dissociates from the receptor and appears in the cytoplasm. To begin to explore how tyrosine phosphorylation affects Syk activation and receptor binding, Tyr-130, which is localized within the Syk inter-Src homology 2 domain region, was substituted with Phe or Glu. Substitution of Tyr-130 with Phe enhanced the binding of Syk to the receptor and increased receptor-mediated protein tyrosine phosphorylation, while substitution with Glu greatly reduced this interaction. Replacement of Tyr-130 with Glu also increased the basal activity of the kinase, while replacement with Phe decreased its activity and uncoupled kinase activation from receptor engagement. These data suggest that the phosphorylation of Tyr-130 normally plays an important role in mediating both the activation of Syk and its release from the antigen receptor.

Defective signaling through the B cell antigen receptor in Epstein-Barr virus-transformed ataxia-telangiectasia cells

A characteristic series of immunological abnormalities are observed in the human genetic disorder ataxia-telangiectasia (A-T). The recent cloning of a gene mutated in this syndrome provides additional evidence for a defect in intracellular signaling in A-T. We have investigated the possibility that signaling through the B cell antigen receptor is one manifestation of the A-T defect. In response to cross-linking of the B cell receptor, several A-T cell lines were defective in their mitogenic response; in addition Ca2+ mobilization from internal stores was either absent or considerably reduced in these cell lines in response to cross-linking. The defect in signaling was not due to difference in expression of surface immunoglobulin. The defective response in A-T cells was also evident in several arms of the intracellular cascade activated by B cell cross-linking. Tyrosine phosphorylation of phospholipase Cgamma1, a key step in activation of the enzyme, was reduced or negligible in some A-T cell lines. This defect in signaling was also seen at the level of Lyn tyrosine kinase activation and its association with and activation of phosphatidylinositol 3-kinase. Our results provide evidence for a role for the ATM gene product in intracellular signaling which may account at least in part for the abnormalities in B cell function in A-T.

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Cloning of novel immunoglobulin superfamily receptors expressed on human myeloid and lymphoid cells: structural evidence for new stimulatory and inhibitory pathways

We have identified two novel human cDNA encoding transmembrane proteins of the immunoglobulin superfamily (IgSF). The two cDNA, called immunoglobulin-like transcripts 1 and 2 (ILT1 and ILT2), are expressed in myeloid and lymphoid cells and are homologous to bovine Fc gamma2R, human killer cell inhibitory receptors (KIR), human Fc alphaR, and mouse gp49. Furthermore, ILT1 and ILT2 are encoded on chromosome 19, as are Fc alphaR and KIR. While the ILT1 and ILT2 extracellular domains are homologous, the transmembrane and cytoplasmic domains differ substantially. ILT1 has an arginine within the transmembrane region, followed by a short cytoplasmic tail, similar to human Fc alphaRI and bovine Fc gamma2R. ILT2 has a long cytoplasmic tail, which contains two YxxV and two YxxL pairs similar to the immunoreceptor tyrosine-based inhibitory motifs in KIR that are known to bind the phosphotyrosine phosphatase SHP-1. These cytoplasmic features suggest that ILT1 and ILT2 may mediate novel transmembrane signals by which myeloid and lymphoid cell responses can be either activated or inhibited.

B-Cell Antigen Receptor–Induced Apoptosis Requires Both Igα and Igβ

The response of a B cell to antigen is dependent on the surface expression of a clonotypic B-cell receptor complex (BCR) consisting of membrane-bound Ig and disulfide-linked heterodimers of Igα/β. Studies of Igα or Igβ have shown that the immunoreceptor tyrosine-based activation motif (ITAM) found in each cytoplasmic tail is capable of inducing most receptor signaling events. However, Igα, Igβ, and most of the other receptor chains that contain ITAMs, including CD3ε, CD3γ, TCRζ, and FcεRIγ, are found as components of multimeric and heterogenous complexes. In such a complex it is possible that cooperativity between individual chains imparts functional capacities to the intact receptor that are not predicted from the properties of its constituents. Therefore, we developed a novel system in which we could form and then aggregate dimers, representative of partial receptor complexes, which contained either Igα alone, Igβ alone, or the two chains together and then examine their ability to induce apoptosis in the immature B-cell line, WEHI-231. Here we present evidence that heterodimers of Igα and Igβ efficiently induced apoptosis while homodimers of either chain did not. Apoptosis was associated with the inductive tyrosine phosphorylation of a very restricted set of proteins including the tyrosine kinase Syk. These findings may provide insight into the mechanisms by which the BCR, and other such multimeric receptor complexes, initiate both apoptotic and proliferative responses to antigen.

B-Cell Antigen Receptor–Induced Apoptosis Requires Both Igα and Igβ

The response of a B cell to antigen is dependent on the surface expression of a clonotypic B-cell receptor complex (BCR) consisting of membrane-bound Ig and disulfide-linked heterodimers of Igα/β. Studies of Igα or Igβ have shown that the immunoreceptor tyrosine-based activation motif (ITAM) found in each cytoplasmic tail is capable of inducing most receptor signaling events. However, Igα, Igβ, and most of the other receptor chains that contain ITAMs, including CD3ε, CD3γ, TCRζ, and FcεRIγ, are found as components of multimeric and heterogenous complexes. In such a complex it is possible that cooperativity between individual chains imparts functional capacities to the intact receptor that are not predicted from the properties of its constituents. Therefore, we developed a novel system in which we could form and then aggregate dimers, representative of partial receptor complexes, which contained either Igα alone, Igβ alone, or the two chains together and then examine their ability to induce apoptosis in the immature B-cell line, WEHI-231. Here we present evidence that heterodimers of Igα and Igβ efficiently induced apoptosis while homodimers of either chain did not. Apoptosis was associated with the inductive tyrosine phosphorylation of a very restricted set of proteins including the tyrosine kinase Syk. These findings may provide insight into the mechanisms by which the BCR, and other such multimeric receptor complexes, initiate both apoptotic and proliferative responses to antigen.

Csk overexpression reduces several monokines and nitric oxide productions but enhances prostaglandin E2 production in response to lipopolysaccharide in the macrophage cell line J774A.1

The catalytic activity of src-family protein tyrosine kinases (src-PTK) is suppressed when a C-terminal tyrosine is phosphorylated by an intracellular PTK, C-terminal Src kinase (Csk). In the present report, to study the regulatory functions of the Csk in cells of monocyte/macrophage lineage, we transfected a eukaryotic expression vector containing rat csk cDNA in a macrophage cell line, J774A.1, and examined alterations of the response to lipopolysaccharide (LPS) in the transfectants which overexpressed Csk. Csk overexpression resulted primarily in a down-regulation of Fgr activity, an src-PTK expressed in J774A.1, and hyperphosphorylation of several cellular proteins of 35, 57, 66, 97 and 120-130 kDa. Furthermore, in these Csk transfectants, production of interleukin (IL)-1alpha, IL-6, tumor necrosis factor-alpha, and nitric oxide (NO) following LPS stimulation were reduced compared with those in parental J774A.1 or J774A.1 transfected with the vector alone. The extent of reduction paralleled the amounts of Csk proteins expressed in the Csk-transfected J774A.1. The reduced NO production in these cells was associated with low levels of mRNA of inducible NO synthetase. On the other hand, an enhancement of prostaglandin E2 production was observed in the Csk-transfected J774A.1 cells upon stimulation with LPS, which appeared to result from the high level of prostaglandin-H synthetase in the transfectants. The present findings indicate that overexpression of Csk has differential effects on the regulation of production of chemical mediators and monokines, probably via modulation of signal transduction downstream of LPS-mediated signals.

Potent immunogenic short linear peptide constructs composed of B cell epitopes and Pan DR T helper epitopes (PADRE) for antibody responses in vivo

Induction of humoral immune responses against protein antigen requires that two independent signals be delivered to B cells. It is currently assumed that simple monovalent synthetic peptides would not be effective immunogens for antibody responses because they would not be anticipated to effectively generate the necessary signals unless conjugated to a complex carrier system. In this study, the immunogenicity of short linear peptide constructs comprising Plasmodium vivax B cell epitopes (PVB) and non-natural Pan-DR T helper cell epitopes (PADRE) was assessed in mice and compared to other types of antigen constructs. The 33-residue PADRE-PVB linear constructs were highly immunogenic and induced responses comparable to those obtained with the multiple antigen peptides (MAP) constructs, both in terms of absolute titers and quality of antibody responses. The anti-PVB antibody responses were of long duration, composed mostly of IgG and reactive with intact sporozoites. The PADRE-PVB constructs were immunogenic when formulated in adjuvants such as Alum and Montanide ISA 51 underlining the relevance of these findings for vaccine development.

Compartmentalized activation of the high affinity immunoglobulin E receptor within membrane domains

The earliest known step in the activation of the high affinity IgE receptor, FcepsilonRI, is the tyrosine phosphorylation of its beta and gamma subunits by the Src family tyrosine kinase, Lyn. We report here that aggregation-dependent association of FcepsilonRI with specialized regions of the plasma membrane precedes its tyrosine phosphorylation and appears necessary for this event. Tyrosine phosphorylation of beta and gamma occurs in intact cells only for FcepsilonRI that associate with these detergent-resistant membrane domains, which are enriched in active Lyn. Furthermore, efficient in vitro tyrosine phosphorylation of FcepsilonRI subunits occurs only for those associated with isolated domains. This association and in vitro phosphorylation are highly sensitive to low concentrations of detergent, suggesting that lipid-mediated interactions with Lyn are important in FcepsilonRI activation. Participation of membrane domains accounts for previously unexplained aspects of FcepsilonRI-mediated signaling and may be relevant to signaling by other multichain immune receptors.

Co-ligation of the antigen and Fc receptors gives rise to the selective modulation of intracellular signaling in B cells. Regulation of the association of phosphatidylinositol 3-kinase and inositol 5'-phosphatase with the antigen receptor complex

Cross-linking of the Fc receptor (FcR) to surface immunoglobulin (sIg) on B cells inhibits the influx of extracellular calcium and abrogates the proliferative signal. The mechanism by which this occurs is not well understood. In this report we show that co-cross-linking the FcR to the antigen receptor gives rise to very selective modulation of signal transduction in B cells. Co-cross-linking sIg and the FcR enhanced the phosphorylation of the FcR, the adapter protein, Shc, and the inositol 5'-phosphatase Ship. Furthermore, phosphorylation of the FcR induced its association with Ship. Cross-linking of the FcR and sIg decreased the tyrosine phosphorylation of CD19, which led to a reduction in the association of phosphatidylinositol 3-kinase. In addition, the phosphorylation of several other proteins of 73, 39, and 34 kDa was reduced. Activation of the cells with either F(ab')2 or intact anti-IgG induced very similar changes in levels of tyrosine phosphorylation of most other proteins, and no differences in the activation of several protein kinases were observed. These results indicate that the inhibitory signal that is transmitted through the FcR is not mediated by a global shutdown of tyrosine phosphorylation but is, rather, a selective mechanism involving localized changes in the interactions of adapter proteins and the enzymes Ship and phosphatidylinositol 3-kinase with the antigen receptor complex.

Adjuvants--a classification and review of their modes of action

Since early this century, various substances have been added to vaccines and certain formulations have been devised in an attempt to render vaccines more effective. Despite a plethora of options, only aluminium salts have gained acceptance as human vaccine adjuvants and even veterinary vaccines are largely dependent upon the use of aluminium salts. Currently, many new vaccines are under development and there is a desire to simplify vaccination schedules both by increasing the number of components per vaccine and decreasing the number of doses required for a vaccine course. New, more effective adjuvants will be required to achieve this.

The role of complement receptor type 1 (CR1, CD35) in determining the cellular distribution of opsonized immune complexes between whole blood cells: kinetic analysis of the buffering capacity of erythrocytes

Erythrocytes (E) express complement receptor, type 1 (CR1, CD35), by which they bind opsonized immune complexes (IC) in competition with leucocytes expressing higher numbers of CR1 as well as other complement- and Fc-receptors. This may prevent inappropriate activation of phagocytic cells. We examined the distribution on whole blood cells of preformed tetanus toxoid (TT)/human anti-TT IC, opsonized in situ in 80% autologous serum. Binding to E occurred rapidly and reflected the kinetics of C3-fragment incorporation into the IC. Among eight donors, expressing 180-361 CR1 per E. > 90% of the cell-bound IC were associated with E from 1 to 5 min of incubation, decreasing to 12 +/- 13% after 40 min. Upon comparison of the IC-binding to leucocytes in whole blood with that of isolated leucocytes we found that E, despite their extensive early complex uptake, only reduced the IC-deposition on polymorphonuclear leucocytes (PMN) by 61 +/- 26% after 30 seconds of incubation and 47 +/- 14% after 5 min. During the subsequent 10 min, this buffering capacity of E was essentially abolished E restricted the initial IC-binding to B cells by 73 +/- 19%, but from 3 min of incubation the presence of E promoted, in a CR1-dependent manner, a progressive uptake via CR2 by the B cells. CR1 was the dominant receptor in the early IC-uptake by B cells as well as PMN and monocytes, since CR1-blockade inhibited the initial IC-uptake by these populations in a preparation of isolated leucocytes suspended in serum by > or = 84% after 30 seconds of incubation. We conclude, that E exert a substantial buffering effect on the IC-deposition on PMN, monocytes and B cells, while CR1 is the dominant receptor in the uptake by these cells. However, this effect is short-lived and less than expected from the proportion of IC bound to E. Moreover, E are efficient processors of IC-attached C3b/iC3b fragments to C3dg as indicated by a pronounced enhancement by E of IC-uptake via CR2 on B cells. We propose that this mechanism may play a role in preventing phagocyte activation via CR3.

Capture and processing of exogenous antigens for presentation on MHC molecules

Class I and class II MHC molecules bind peptides during their biosynthetic maturation and provide a continuously updated display of intracellular and environmental protein composition, respectively, for scrutiny by T cells. Receptor-mediated endocytosis, phagocytosis, and macropinocytosis all contribute to antigen uptake by class II MHC-positive antigen-presenting cells. Capture of antigenic peptides by class II MHC molecules is facilitated because antigen catabolism and class II MHC maturation take place in the same compartments or in communicating compartments of the endosome/lysosome system. These class II MHC-rich, multivesicular endosomes receive incoming antigen and can support not only antigen processing and class II MHC peptide loading but also the export of peptide/class II MHC complexes to the cell surface. A balance between production and destruction of antigenic peptides is achieved by the activity of local proteases and may be influenced by binding of antigen to other proteins both prior to the onset of processing (e.g. antibodies) and during antigen unfolding (e.g. MHC molecules). T cell determinants that can be released for MHC binding without a substantial processing requirement may be able to utilize a distinct minor population of cell surface class II MHC molecules that become available during peripheral recycling. Although peptides derived from exogenous protein sources are usually excluded from presentation on class I MHC molecules, recent evidence shows that this embargo may be lifted in certain professional antigen-presenting cells to increase the spectrum of antigens that may be displayed on class I MHC.

The importance of efficacy and partial agonism in evaluating models of B lymphocyte activation

Immunologists have developed a range of in vitro techniques for probing the receptor mediated response of cells comprising the immune system. An important and ubiquitous method is the use of antibodies in either soluble or aggregated form to engage cell surface receptors and transmit a signal. Models of cell and molecular interactions, derived from the use of these antibodies, form the basis of our efforts to understand and explain the corresponding in vivo systems. However, interpreting in vitro experiments and distinguishing between alternative models is difficult. This complexity is illustrated here using B cell stimulation by surface immunoglobulin and CD40. The fluorescent cell labelling dye carboxyfluorescein, diacetate, succinimidyl ester (CFSE) is used to show that many anti-Ig and CD40 stimulatory agents, used to assess the role of B cells and lymphokines, are partial agonists. By modelling each step in B cell signalling, activation and division it is possible to show that small changes in signal contributed by a second receptor can generate numerous distinct dose response curves that are highly dependent on the "efficacy" of signal transmission by the primary ligand and the number of cell divisions taken in culture. Differences in dose response curves become particularly striking if the primary activating stimulus is a partial agonist. Although exemplified here with B cell stimulation the conclusions are applicable to other in vitro activation systems and suggest ways to improve both the design and interpretation of in vitro experiments.

Initiation and processing of signals from the B cell antigen receptor

Current models of signal transduction from the antigen receptors on B and T cells still resemble equations with several unknown elements. Data from recent knockout experiments in cell lines and mice contradict the assumption that Src-family kinase and tyrosine kinases of the Syk/Zap-70 family are the transducer elements that set signaling from these receptors in motion. Using a functional definition of signaling elements, we discuss the current knowledge of signaling events from the BCR and suggest the existence of an as-yet-unknown BCR transducer complex.

Neutralizing antiviral B cell responses

Neutralizing antiviral B cell responses differ in various aspects from the many usually measured B cell responses specific for protein in adjuvants. In particular, such neutralizing antiviral B cell responses are more rapidly induced, reach higher titers, are longer lived, and are efficiently generated without adjuvants. Evidence is summarized here that the repetitiveness of many viral antigens is a key factor responsible for the efficiency of these B cell responses, amplifying B cells early and rapidly for potent IgM responses and also for efficient switching to IgG. The data reviewed indicate that B cells discriminate antigen patterns via the degree of surface Ig-cross-linking and use antigen repetitiveness as a self/nonself discriminator.

Enhancement of lymphocyte responsiveness by a gain-of-function mutation of ZAP-70

The protein tyrosine kinase ZAP-70 plays an essential role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 is associated with the receptor and is phosphorylated on many tyrosine residues, including tyrosine 292 (Y-292), in the region between the C-terminal SH2 domain and the kinase domain (interdomain B). Here we show that a mutation of Y-292 (292F) or deletion of interdomain B enhanced the ability of ZAP-70 to reconstitute B-cell receptor stimulation-dependent NF-AT induction in a B-cell line deficient in Syk. In contrast, in a T-cell line, expression of 292F led to basal NF-AT induction independent of T-cell receptor stimulation. These results demonstrate that the role of Y-292 is to negatively regulate the function of ZAP-70 in lymphocytes. This appears to be a dominant function of interdomain B because deletion of most of interdomain B also resulted in a mutant of ZAP-70 with enhanced ability to reconstitute Syk-deficient DT-40 B cells. Since our biochemical studies did not reveal an effect of the 292F mutation on either the kinase activity of ZAP-70 or on the ability of ZAP-70 to bind to the receptor, we propose a model in which Y-292 interacts with an inhibitory protein to negatively regulate ZAP-70 function.

The two membrane isoforms of human IgE assemble into functionally distinct B cell antigen receptors

The human C epsilon gene expresses two membrane IgE heavy chain mRNAs which differ in the sequence that encodes their extracellular membrane-proximal domain. In the long IgE isoform (mLIgE), this domain contains a stretch of 52 amino acids which are absent in the short variant (mSIgE). We have now generated B cell transfectoma cell lines that express these two isoforms and show that both types of mIgE form functional B cell antigen receptors (BCR). Both receptors associate with the Ig-alpha/Ig-beta heterodimer, as well as with protein kinases that are capable of phosphorylating this complex. Upon their cross-linking, both receptors can activate protein tyrosine kinases that phosphorylate the same substrate proteins. Both IgE receptors also associate with two novel proteins that do not bind to mIgM. Apart from these similarities, the two IgE-BCRs show several differences of which some are analogous to the differences between the IgM- and IgD-BCRs. First, the mSIgE is transported to the cell surface at a higher rate than the mLIgE. Second, the two IgE-BCRs associate with differently glycosylated Ig-alpha proteins, the mLIgE associates with the completely glycosylated form, whereas the mSIgE associates with an Ig-alpha glycoform that is partially sensitive to endoglycosidase H. Third, the kinetics of protein tyrosine phosphorylation induced by receptor cross-linking is significantly different for the two IgE-BCRs. Finally, cross-linking of the mSIgE-BCR leads to growth inhibition of the B cell transfectoma, whereas signaling through the mLIgE-BCR does not affect the cellular proliferation. These data show that the two human membrane IgE isoforms assemble into functionally distinct antigen receptors which can induce different cellular responses.

B cells from patients with systemic lupus erythematosus display abnormal antigen receptor-mediated early signal transduction events

To understand the molecular mechanisms that are responsible for the B cell overactivity that is observed in patients with SLE, we have conducted experiments in which the surface immunoglobulin (sIg)-mediated early cell signaling events were studied. The anti-sIgM-mediated free intracytoplasmic calcium ([Ca2+]i) responses were significantly higher in SLE B cells compared with responses of normal individuals and to those of patients with other systemic autoimmune rheumatic diseases. The anti-IgD mAb induced [Ca2+]i responses were also higher in lupus B cells than in controls. The magnitude of anti-sIgM-mediated Ca2+ release from intracellular stores was also increased in B cells from SLE patients compared with normal controls. The amount of inositol phosphate metabolites produced upon crosslinking of sIgM was slightly higher in patients with lupus than in normal controls, although the difference was not statistically significant. In contrast, the degree of anti-sIgM-induced protein tyrosine phosphorylation was obviously increased in lupus patients. Our study demonstrates clearly for the first time that SLE B cells exhibit aberrant early signal transduction events, including augmented calcium responses after crosslinking of the B cell receptor and increased antigen-receptor-mediated phosphorylation of protein tyrosine residues. Because the above abnormalities did not correlate with disease activity or treatment status, we propose that they may have pathogenic significance.

Characterization of Cbl tyrosine phosphorylation and a Cbl-Syk complex in RBL-2H3 cells

Tyrosine phosphorylation of the Cbl protooncogene has been shown to occur after engagement of a number of different receptors on hematopoietic cells. However, the mechanisms by which these receptors induce Cbl tyrosine phosphorylation are poorly understood. Here we demonstrate that engagement of the high affinity IgE receptor (Fc epsilon R1) leads to the tyrosine phosphorylation of Cbl and analyze how this occurs. We show that at least part of Fc epsilon R1-induced Cbl tyrosine phosphorylation is mediated by the Syk tyrosine kinase, and that the Syk-dependent tyrosine phosphorylation of Cbl occurs mainly distal to the Cbl proline-rich region within the COOH-terminal 250 amino acids. Furthermore, we show by coprecipitation that Cbl is present in a complex with Syk before receptor engagement, that the proline-rich region of Cbl and a region of Syk comprised of the two SH2 domains and intradomain linker are required for formation of the complex, and that little or no tyrosine-phosphorylated Cbl is detected in complex with Syk. Overexpression of truncation mutants of Cbl capable of binding Syk has the effect of blocking tyrosine phosphorylation of endogenous Cbl. These results define a potentially important intramolecular interaction in mast cells and suggest a complex function for Cbl in intracellular signaling pathways.

Hyperresponsive B cells in CD22-deficient mice

CD22 is a surface glycoprotein of B lymphocytes that is rapidly phosphorylated on cytoplasmic tyrosines after antigen receptor cross-linking. Splenic B cells from mice with a disrupted CD22 gene were found to be hyperresponsive to receptor signaling: Heightened calcium fluxes and cell proliferation were obtained at lower ligand concentrations. The mice gave an augmented immune response, had an expanded peritoneal B-1 cell population, and contained increased serum titers of autoantibody. Thus, CD22 is a negative regulator of antigen receptor signaling whose onset of expression at the mature B cell stage may serve to raise the antigen concentration threshold required for B cell triggering.

Downstream signaling molecules bind to different phosphorylated immunoreceptor tyrosine-based activation motif (ITAM) peptides of the high affinity IgE receptor

The cytoplasmic tails of both the beta and gamma subunits of the high affinity IgE receptor (FcepsilonRI) contain a consensus sequence termed the immunoreceptor tyrosine-based activation motif (ITAM). This motif plays a critical role in receptor-mediated signal transduction. Synthetic peptides based on the ITAM sequences of the beta and gamma subunits of FcepsilonRI were used to investigate which proteins associate with these motifs. Tyrosine-phosphorylated beta and gamma ITAM peptides immobilized on beads precipitated Syk, Lyn, Shc, Grb2, and phospholipase C-gamma1 from lysates of rat basophilic leukemia RBL-2H3 cells. Syk was precipitated predominantly by the tyrosine-diphosphorylated gamma ITAM peptide, but much less by the diphosphorylated beta ITAM peptide or by the monophosphorylated peptides. Phospholipase C-gamma1, Shc, and Grb2 were precipitated only by the diphosphorylated beta ITAM peptide. Non-phosphorylated ITAM peptides did not precipitate these proteins. In membrane binding assays, fusion proteins containing the Src homology 2 domains of phospholipase C-gamma1, Shc, Syk, and Lyn directly bound the tyrosine-phosphorylated ITAM peptides. Although the ITAM sequences of the beta and gamma subunits of FcepsilonRI are similar, once they are tyrosine-phosphorylated they preferentially bind different downstream signaling molecules. Tyrosine phosphorylation of the ITAM of the gamma subunit recruits and activates Syk, whereas the beta subunit may be important for the Ras signaling pathway.

Role of protein kinase activity in apoptosis

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.

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

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

Differential modulation of cyclin-dependent kinase inhibitor p27Kip1 by negative signaling via the antigen receptor of B cells and positive signaling via CD40

The cross-linking of surface immunoglobulins (sIg) of B cells can transmit a negative signal, resulting in cell cycle arrest, apoptosis or both. Signaling via the B cell antigen CD40 reverses the sIg-mediated negative signaling and induces activation and proliferation of B cells. We investigated the molecular mechanism for cell cycle regulation by negative and positive signaling via sIg and CD40, respectively, by using the B cell line WEHI-231. Cross-linking of sIg almost completely reduced the activity of cyclin-dependent kinase (Cdk) 2, essential for cell cycle progression in the late G1 phase, although the level of Cdk2 was not reduced. Among the factors that regulate Cdk2 activation, the activity of the Cdk-activating kinase (CAK) appeared intact and cyclin E was reduced only partially in sIg-cross-linked WEHI-231. In contrast, sIg cross-linking induced a significant Cdk inhibitor (CKI) activity. Since a 27-kDa protein was co-precipitated with Cdk2 in anti-Ig-treated, but not untreated WEHI-231, and the CKI activity in anti-Ig-treated WEHI-231 was neutralized by anti-p27Kip1 antibodies, it is most likely that p27Kip1 is responsible for the CKI activity induced by sIg cross-linking. p27Kip1 may thus play a role in growth inhibition of B cells by negative signaling via sIg. In contrast, CD40 signaling enhanced Cdk2 activity and reduced the p27Kip1 level in anti-Ig-treated WEHI-231, suggesting that the reduction of p27Kip1 plays an important role in the abrogation of sIg-mediated growth arrest by CD40 signaling. Taken together, p27Kip1 is likely to be a crucial target molecule of the negative signaling via sIg and the positive signaling via CD40 essential for T cell-dependent immune responses.

Tyrosine kinase and cAMP-dependent protein kinase activities in CD40-activated human B lymphocytes

In vitro, human B lymphocytes undergo long-term proliferation when activated through CD40, a protein expressed on their cell surface. The nature of CD40-dependent signals in proliferating fresh human Epstein-Barr virus-negative B lymphocytes is currently unknown. In this study, a CD40-dependent B cell culture system was used to examine the role of different signal transduction elements. Protein kinase C (PKC) depletion generated by a long-term phorbol 12 myristate 13-acetate treatment had weak effects on proliferation. Rather, tyrosine phosphorylation was shown to be directly involved in mediating CD40-dependent signals. The use of the protein tyrosine kinase (PTK)-specific inhibitor herbimycin A dramatically decreased cellular proliferation without altering the activity of the human immunodeficiency virus-1 long terminal repeat (HIV-1 LTR), a promoter largely dependent on the binding of nuclear factor kappa B (NF- kappa B). In contrast, the cAMP-dependent protein kinase specific inhibitor H-89 totally inhibited HIV-1 LTR activity at a concentration as low as 100 nM without affecting cellular proliferation. Electrophoretic mobility shift assay (EMSA) and supershift assay using an NF-kappa B binding sequence from the kappa light chain as a probe, revealed that both p65 (RelA) and c-Rel were present in CD40-stimulated B cells. While PKC depletion did not alter the NF-kappa B level, treatment of B lymphocytes with H-89 or herbimycin A provoked a decrease in the NF-kappa B level. These observations establish the importance of different signal transducing pathways leading to CD40 activation of B lymphocytes.

Use of isolated immature-stage B cells to understand negative selection and tolerance induction at the molecular level

Encounter with antigen by newly developing antigen receptor-positive B cells leads to negative selection. This process positions the B cell antigen receptor (BCR) in a central role for initiating the process of negative selection and suggests developmental regulation of its signaling. The observation that immature B cells are more susceptible to negative selection than are mature B cells has been demonstrated in a number of in vitro and in vivo model systems and support the idea of developmental regulation of BCR-initiated responses. Since identical antigen receptors are expressed on immature and mature B cells, the critical fate-determining distinction between these developmental stages must lie downstream of the receptor-ligand interaction itself, in the form of different BCR-linked signaling processes or with different secondary events occurring subsequent to BCR cross-linking. To address the first possibility, our laboratory and others have sought to define the differences in BCR-mediated signal transduction in immature and mature B lymphocytes. In this review article we will discuss current in vitro systems to study this question in primary, nontransformed murine B lymphocytes. In addition, we will discuss our previously published work in order to illustrate how these model systems have been useful in beginning to unravel the molecular basis for immune B cell negative selection and tolerance.

Identification of latent membrane protein 2A (LMP2A) domains essential for the LMP2A dominant-negative effect on B-lymphocyte surface immunoglobulin signal transduction

Epstein-Barr virus (EBV) recombinants which carry three different deletion mutations in the LMP2A cytoplasmic amino-terminal domain were constructed. The presence of each mutation, LMP2A delta 21-36, LMP2A delta 21-64, and LMP2A delta 21-85, in EBV-infected transformed lymphoblastoid cell lines was confirmed by PCR analysis and Southern blot hybridization. Confirmation of mutant LMP2A protein expression was by immunofluorescence and immunoblotting with a newly identified rat monoclonal antibody that recognizes each of the LMP2A deletion mutations. Lymphoblastoid cell lines infected with recombinant EBV DNAs containing the mutations were analyzed for loss of LMP2A's dominant-negative effect on surface immunoglobulin signal transduction by monitoring induction of tyrosine phosphorylation, calcium mobilization, and activation of lytic replication following surface immunoglobulin cross-linking. Domains of LMP2A important for induction of tyrosine phosphorylation, calcium mobilization, and activation of lytic replication were identified.

Distinct tyrosine phosphorylation sites in ZAP-70 mediate activation and negative regulation of antigen receptor function

Biochemical and genetic evidence has implicated two families of protein tyrosine kinases (PTKs), the Src- and Syk-PTKs, in T- and B-cell antigen receptor signaling. ZAP-70 is a member of the Syk-PTKs that associates with the T-cell antigen receptor and undergoes tyrosine phosphorylation following receptor activation. Three tyrosine residues, Tyr-292, -492, and -493, have been identified as sites of phosphorylation following T-cell antigen receptor engagement. Utilizing ZAP-70- and Syk-deficient lymphocytes (Syk-DT40 cells), we provide biochemical and functional evidence that heterologous trans-phosphorylation of Tyr-493 by a Src-PTK is required for antigen receptor-mediated activation of both the calcium and ras pathways. In contrast, cells expressing mutations at Tyr-292 or -492 demonstrate hyperactive T- and B-cell antigen receptor phenotypes. Thus, phosphorylation of ZAP-70 mediates both activation and inactivation of antigen receptor signaling.

Negative signaling via FcgammaRIIB1 in B cells blocks phospholipase Cgamma2 tyrosine phosphorylation but not Syk or Lyn activation

Crosslinking of the B cell antigen receptor surface immunoglobulin induces tyrosine phosphorylation and activation of the Src family and Syk tyrosine protein kinases, tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) and increases in intracellular second messengers inositol phosphates and Ca2+. These activation events, in conjunction with other pathways, culminate in the induction of B cell proliferation and differentiation. In contrast, co-crosslinking surface Ig with the B cell IgG Fc receptor prevents many of these activation events, including B cell proliferation and differentiation. The precise nature of the negative signal(s) derived from Fc receptors that prevent B cell activation is not known. Here, early activation events were examined in B cells stimulated via the antigen receptor alone or under co-crosslinking conditions. The data indicated a selective block in the tyrosine phosphorylation and activation of PLCgamma2 but not in activation of the upstream kinases, Syk and Lyn, under co-crosslinking conditions. We conclude that the negative signal acts directly on PLCgamma2 and is consistent with recent studies describing an activation-induced association of a phosphotyrosine phosphatase with tyrosine-phosphorylated B cell Fc receptor.

Anti-IgM-mediated regulation of c-myc and its possible relationship to apoptosis

Anti-IgM treatment of Burkitt's lymphoma cells is followed by either growth arrest or induction of apoptosis. In this study we have explored the role of c-myc in these events. Our results in Ramos cells indicate the following. (a) The decline in c-myc mRNA occurs at about 4 h; inhibition of about 80% being observed. (b) The stability of c-myc message is involved since the half-life of c-myc mRNA is decreased from about 30 min in untreated cells to about 15 min following treatment with anti-IgM. In the presence of cycloheximide, a protein synthesis inhibitor, the half-life is increased to about 50 min and was unaltered by treatment with anti-IgM. (c) By contrast, nuclear run-on experiments indicated no change in transcription rates for c-myc message due to treatment with anti-IgM. (d) A decrease in c-myc causes apoptosis since specific repression of c-myc with antisense oligonucleotides decreases the levels of c-Myc, inhibits growth rate, decreases viability, and induces apoptosis. (e) Anti-CD40 inhibition of apoptosis occurs without alteration in anti-IgM-induced down-regulation of c-myc mRNA, suggesting that it acts distally to c-myc down-regulation. Other cell lines were also investigated. In Epstein-Barr virus (EBV)-positive cell lines (Daudi, Raji, and Namalwa), anti-IgM treatment for 24 h results in growth inhibition without induction of apoptosis. In EBV-negative cell lines (ST486 and CA46, as well as Ramos), a more heterogeneous pattern of responses to anti-IgM are observed. Ramos and ST486 cells both show growth inhibition and apoptosis upon anti-IgM treatment; CA46 cells shown only growth inhibition but not apoptosis. Anti-IgM causes a decline in c-myc mRNA levels in all of these lines, as well as in c-Myc protein level in the two lines investigated, Daudi and Ramos, regardless of apoptosis. Addition of antisense c-myc oligonucleotides to the cells reduced growth in both Daudi and Ramos cells lines, however it resulted in substantial apoptosis only in Ramos cells. These results suggest that anti-IgM destabilizes c-myc mRNA by a process that involves mRNA turnover, rather than transcription rates. However anti-IgM exerts differential effects in EBV-positive and EBV-negative cell lines. EBV-positive cells are uniformly resistant to apoptosis, while EBV-negative cell lines show a tendency to apoptosis but with exceptions. Growth inhibition can be uncoupled from apoptosis in EBV-positive cell lines, but not in those EBV-negative cell lines prone to apoptosis. Furthermore, down-regulation of c-myc message correlates with growth inhibition in these cells, but is an insufficient link to apoptosis. By contrast inhibition of apoptosis by anti-CD40 occurs even though c-myc mRNA is decreased.

Evidence for a preformed transducer complex organized by the B cell antigen receptor

The B cell antigen receptor (BCR) consists of the membrane-bound immunoglobulin (mIg) molecule and the Ig-alpha/Ig-beta heterodimer, which functions as signaling subunit of the receptor. Stimulation of the BCR activates protein tyrosine kinases (PTKs) that phosphorylate a number of substrate proteins, including the Ig-alpha/Ig-beta heterodimer of the BCR itself. How the PTKs become activated after BCR engagement is not known at present. Here, we show that BCR-negative J558L cells treated with the protein tyrosine phosphatase inhibitor pervanadate/H2O2 display only a weak substrate phosphorylation. However, in BCR-positive transfectants of J558L, treatment with pervanadate/H2O2 induces a strong phosphorylation of several substrate proteins. Treatment with pervanadate/H2O2 does not result in receptor crosslinking, yet the pattern of protein phosphorylation is similar to that observed after BCR stimulation by antigen. The response requires cellular integrity because tyrosine phosphorylation of most substrates is not visible in cell lysates. Cells that express a BCR containing an Ig-alpha subunit with a mutated immunoreceptor tyrosine-based activation motif display a delayed response. The data suggest that, once expressed on the surface, the BCR organizes protein tyrosine phosphatases, PTKs, and their substrates into a transducer complex that can be activated by pervanadate/H202 in the absence of BCR crosslinking. Assembly of this preformed complex seems to be a prerequisite for BCR-mediated signal transduction.

Activation mediated by RP105 but not CD40 makes normal B cells susceptible to anti-IgM-induced apoptosis: a role for Fc receptor coligation

Signals through the B cell antigen receptor lead to a variety of cellular events such as activation, anergy, and apoptosis. B cells select these outcomes to establish and maintain self-tolerance, and to mount adequate antibody responses. However, it is not fully understood how one and the same signal causes such different consequences. In the present study, we have studied the effect of activation signals on the outcome of responses to antigen receptor ligation. Two distinct growth-promoting signals were used to activate B cells. Ligation of either RP105, a newly discovered B cell surface molecule, or the CD40 molecule, drove B cells to proliferate. Resultant blastic cells were then exposed to anti-immunoglobulin M (IgM). Blast cells that had been stimulated with anti-RP105 ceased growing and underwent apoptosis after cross-linking of surface IgM. Coligation of the Fc gamma receptor IIB with surface IgM augmented, rather than aborted, this response. In contrast to RP105-activated B cells, blast cells that had been activated by CD40 ligation were unaltered by anti-IgM. On the other hand, CD40-activated B cells became extremely susceptible to Fas-mediated apoptosis, whereas RP105-activated B cells were much less sensitive. Anti-IgM-induced apoptosis in RP105 blasts was independent of Fas, because it was demonstrable with Fas-deficient MRL-lpr/lpr mice. These results demonstrate that the nature of an initial activation signal has a great influence on the fate of activated B cells after (re)engagement of the antigen receptor. RP105, as well as CD40, may be important in this life/death decision.

Transfection of Syk protein tyrosine kinase reconstitutes high affinity IgE receptor-mediated degranulation in a Syk-negative variant of rat basophilic leukemia RBL-2H3 cells

Aggregation of the high affinity receptor for immunoglobulin E (Fc epsilon RI) on mast cells results in rapid tyrosine phosphorylation and activation of Syk, a cytoplasmic protein tyrosine kinase. To examine the role of Syk in the Fc epsilon RI signaling pathway, we identified a variant of RBL-2H3 cells that has no detectable Syk by immunoblotting and by in vitro kinase reactions. In these Syk-deficient TB1A2 cells, aggregation of Fc epsilon RI induced no histamine release and no detectable increase in total cellular protein tyrosine phosphorylation. However, stimulation of these cells with the calcium ionophore did induce degranulation. Fc epsilon RI aggregation induced tyrosine phosphorylation of the beta and gamma subunits of the receptor, but no increase in the tyrosine phosphorylation of phospholipase C-gamma 1 and phospholipase C-gamma 2 and no detectable increase in intracellular free Ca2+ concentration. By transfection, cloned lines were established with stable expression of Syk. In these reconstituted cells, Fc epsilon RI aggregation induced tyrosine phosphorylation of phospholipase C-gamma 1 and phospholipase C-gamma 2, an increase in intracellular free Ca2+ and histamine release. These results demonstrate that Syk plays a critical role in the early Fc epsilon RI-mediated signaling events. It further demonstrates that Syk activation occurs downstream of receptor phosphorylation, but upstream of most of the Fc epsilon RI-mediated protein tyrosine phosphorylations.