Apoptosis induced by the antigen receptor and Fas in a variant of the immature B cell line WEHI-231 and in splenic immature B cells.

Signaling by the BCR causes proliferation and resistance to Fas-induced apoptosis in mature B cells, but growth arrest and apoptosis in immature B cells. We have identified a variant of the immature B cell line WEHI 231 that retains the apoptotic response to the BCR but has acquired susceptibility to Fas-induced apoptosis. The Fas susceptibility was associated with increased Fas expression on the cell surface and down-regulated IgD expression. These cells exhibited a distinctive functional relationship in response to signals from the BCR, Fas and CD40: BCR stimulation markedly promoted Fas-mediated apoptosis (and vice versa) and Fas-induced apoptosis was not subject to modulation by CD40 signaling. While BCR-induced apoptosis was effectively rescued by CD40, it was not affected by the expression of a dominant-negative FADD. The mechanistic distinctions between BCR- and Fas-induced apoptosis were further characterized by the differential effects of different caspase inhibitors on these two processes which imply the involvement of different subsets of caspases. For BCR-induced apoptosis, we provide evidence that the final apoptotic destruction phase can be inhibited by the pan-caspase inhibitor BOC-Asp-FMK (BD) and that, in the presence of BD, the BCR only induces growth arrest which is reversible. The striking enhancing effects of Fas on BCR-induced apoptosis seen in the variant cells prompted us to examine if a similar cooperation in induction of apoptosis occurs in the highly tolerizable immature B cells of the spleen. We found that the splenic immature B population contains a significant number of Fas-expressing cells, but neither Fas-induced apoptosis nor an enhancing effect of Fas on BCR-induced apoptosis of these cells was detected in vitro.

Inhibition of the MEK/ERK signaling pathway blocks a subset of B cell responses to antigen.

Signal transduction initiated by B cell Ag receptor (BCR) cross-linking plays an important role in the development and activation of B cells. Therefore, considerable effort has gone into determining the biochemical signaling events initiated by the BCR and delineating which events participate in specific biological responses to Ag. We used two inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) 1 and MEK2, PD98059, and U0126, to assess the role the Ras-mitogen-activated protein kinase pathway plays in several BCR-induced responses. PD98059 or U0126 treatment substantially inhibited the BCR-induced activation of the extracellular signal-regulated kinase (ERK) forms of mitogen-activated protein kinase in the immature B cell line WEHI-231, in immature splenic B cells, and in mature splenic B cells. However, MEK-ERK inhibition did not block BCR-induced growth arrest or apoptosis of WEHI-231 cells or apoptosis of immature splenic B cells, indicating that the MEK-ERK pathway is not required for these events. In contrast, PD98059 and U0126 treatment did inhibit the up-regulation of specific BCR-induced proteins, including the transcription factor Egr-1 in WEHI-231 and mature splenic B cells, and the CD44 adhesion molecule and CD69 activation marker in mature splenic B cells. Moreover, both inhibitors suppressed BCR-induced proliferation of mature splenic B cells, in the absence and in the presence of IL-4. Therefore, activation of the MEK-ERK pathway is necessary for a subset of B cell responses to Ag.

Lupus-like kidney disease in mice deficient in the Src family tyrosine kinases Lyn and Fyn.

Systemic lupus erythematosus (SLE) is an autoimmune disease whose cause is poorly understood. Mice rendered deficient in specific genes have served as useful animal models in deciphering the genetic control of the disease [1]. We [2] and others [3, 4] previously demonstrated that mice deficient in the Src family tyrosine kinase Lyn developed a mild lupus-like disease with high survival rates. During the course of investigating the functional interaction of Src family kinases, we generated a mouse strain deficient in both Lyn and Fyn. The double-mutant mice died at relatively young ages and developed a severe lupus-like kidney disease. Unlike the double-mutant mice, single mutants deficient in either Lyn or Fyn lived longer and had distinct subsets of the symptoms found in the former. Lyn deficiency led to high levels of autoantibody production and glomerulonephritis, as previously reported [2--4], whereas loss of Fyn contributed to proteinuria by a B and T lymphocyte-independent mechanism. Our data suggest that the severe kidney disease in the double-mutant mice results from a combination of immunological and kidney-intrinsic defects. This new animal model may be informative about the causes of human SLE.

CD19 regulates Src family protein tyrosine kinase activation in B lymphocytes through processive amplification.

CD19 regulates constitutive and antigen receptor-induced signaling thresholds in B lymphocytes through its unique cytoplasmic domain. Herein, we demonstrate a novel molecular mechanism where interactions between CD19 and Lyn amplify basal and antigen receptor-induced Src family kinase activation. Lyn expression was required for CD19 tyrosine phosphorylation in primary B cells. Experiments with purified proteins demonstrated that CD19-Y513 was Lyn's initial phosphorylation and binding site. This led to processive phosphorylation of CD19-Y482, which recruited a second Lyn molecule, allowing for transphosphorylation and amplification of Lyn activation. In vivo, CD19 deficiency impaired, and CD19 overexpression enhanced, Lyn kinase activity. Thus, CD19 functions as a specialized adapter protein for the amplification of Src family kinases that is crucial for intrinsic and antigen receptor-induced signal transduction.

Phosphatidylinositol 3-kinase and mTOR mediate lipopolysaccharide-stimulated nitric oxide production in macrophages via interferon-beta.

Bacterial lipopolysaccharide (LPS) elicits responses by macrophages that help the body repel infections. Recent evidence indicates that phosphatidylinositol 3-kinase (PI 3-kinase) may mediate some of these responses. Here, we show that exposing macrophages to LPS rapidly increased membrane-associated PI 3-kinase activity and also elevated p70 S6 kinase activity. Inhibitors of PI 3-kinase or the mammalian target of rapamycin (mTOR) fully blocked p70 S6 kinase activation, implying that this kinase is controlled by PI 3-kinase and mTOR. These inhibitors also substantially reduced LPS-induced nitric oxide (NO) production. This inhibition was, in part, attributable to impaired LPS-stimulated secretion of interferon-beta, an autocrine co-factor for NO production. However, the addition of exogenous interferon-beta did not fully restore NO production, indicating that the NO response was being inhibited by another mechanism as well. Together, these data suggest that PI 3-kinase, mTOR, and possibly p70 S6 kinase mediate LPS-induced NO production by regulating the secretion of interferon-beta and by a second undefined mechanism.

Fcgamma receptor-mediated phagocytosis in macrophages lacking the Src family tyrosine kinases Hck, Fgr, and Lyn.

Macrophage Fcgamma receptors (FcgammaRs) mediate the uptake and destruction of antibody-coated viruses, bacteria, and parasites. We examined FcgammaR signaling and phagocytic function in bone marrow-derived macrophages from mutant mice lacking the major Src family kinases expressed in these cells, Hck, Fgr, and Lyn. Many FcgammaR-induced functional responses and signaling events were diminished or delayed in these macrophages, including immunoglobulin (Ig)G-coated erythrocyte phagocytosis, respiratory burst, actin cup formation, and activation of Syk, phosphatidylinositol 3-kinase, and extracellular signal-regulated kinases 1 and 2. Significant reduction of IgG-dependent phagocytosis was not seen in hck(-)(/)-fgr(-)(/)- or lyn(-)(/)- cells, although the single mutant lyn(-)(/)- macrophages did manifest signaling defects. Thus, Src family kinases clearly have roles in two events leading to FcgammaR-mediated phagocytosis, one involving initiation of actin polymerization and the second involving activation of Syk and subsequent internalization. Since FcgammaR-mediated phagocytosis did occur at modest levels in a delayed fashion in triple mutant macrophages, these Src family kinases are not absolutely required for uptake of IgG-opsonized particles.

The src homology domain 2-containing inositol phosphatase SHIP forms a ternary complex with Shc and Grb2 in antigen receptor-stimulated B lymphocytes.

The inositol phosphatase SHIP has been implicated in signaling events downstream of a variety of receptors and is thought to play an inhibitory role in stimulated B cells. We and others have reported that SHIP is rapidly tyrosine phosphorylated upon B cell antigen receptor (BCR) cross-linking and forms a complex with the adapter protein Shc. Here, we report that cross-linking of the BCR induces association between Grb2 and SHIP as well as association between Shc and SHIP. We made use of a Grb2-deficient B cell line to demonstrate both in vitro and in vivo that Grb2 expression is required for the efficient association between Shc and SHIP. The results indicate that SHIP, Shc, and Grb2 form a ternary complex in stimulated B cells, with Grb2 stabilizing the interaction between Shc and SHIP. The interactions between Shc, Grb2, and SHIP are therefore analogous to the interactions between Shc, Grb2, and SOS. Shc and Grb2 may help to localize SHIP to the cell membrane, regulating SHIP's inhibitory function following BCR stimulation.

Role of ceramide in lipopolysaccharide (LPS)-induced signaling. LPS increases ceramide rather than acting as a structural homolog.

Ceramide and ceramide-activated enzymes have been implicated in responses to bacterial lipopolysaccharide (LPS) and the proinflammatory cytokines tumor necrosis factor-alpha (TNF) and interleukin-1beta (IL-1). Although TNF and IL-1 cause elevation of cellular ceramide, which is thought to act as a second messenger, LPS has been proposed to signal by virtue of structural similarity to ceramide. We have investigated the relationship between ceramide and LPS by comparing the effects of a cell-permeable ceramide analog (C2-ceramide) and LPS on murine macrophage cell lines and by measuring ceramide levels in macrophages exposed to LPS. We found that while both C2-ceramide and LPS activated c-Jun N-terminal kinase (JNK), only LPS also activated extracellular signal-regulated kinases (ERKs). C2-ceramide was also unable to activate NF-kappaB, a transcription factor important for LPS-induced gene expression. Upon measurement of cellular ceramide in macrophage lines, we observed a small but rapid rise in ceramide, similar to that seen upon IL-1 or TNF treatment, suggesting LPS induces an increase in ceramide rather than interacting directly with ceramide-responsive enzymes. We found that C2-ceramide activated JNK and induced growth arrest in macrophages cell lines from both normal mice (Lpsn) and mice genetically unresponsive to LPS (Lpsd), whereas only Lpsn macrophages made these responses to LPS. Surprisingly, LPS treatment of Lpsd macrophages induced a rise in ceramide similar to that observed in LPS-responsive cells. These results indicate that the wild type Lps allele is not required for LPS-induced ceramide generation and suggest that ceramide elevation alone is insufficent stimulus for most responses to LPS.

Positive and negative roles of the tyrosine kinase Lyn in B cell function.

The function of Lyn in B cell activation has been studied recently by examining the properties of B cells from mice in which the lyn gene has been inactivated by gene targeting. These mice show evidence of B cell hyperreactivity in vivo, as the number of B lymphoblastoid cells greatly increase with age, IgM levels increase by 10-fold or more, and auto-antibodies to double-stranded DNA and other nuclear antigens become apparent. B cells from lyn -/- mice also exhibit enhanced BCR-induced activation of MAP kinases, intracellular calcium elevation and proliferative responses in vitro. These phenomena may relate to participation of Lyn in events that serve to decrease B cell responses to antigen. Among the leading candidates for these suppressive events are the inhibition of B cell antigen receptor function by Fc gamma RIIb1 and by CD22. Although Lyn also participates positively in the initial events of B cell antigen receptor signal transduction, this function can also be supplied by other tyrosine kinases, presumably other Src-family kinases. In contrast, some aspects of inhibition by CD22 appear to be almost completely dependent upon Lyn and Fc gamma RIIb1 inhibition is also diminished in the absence of Lyn. Thus, the net effect of Lyn action is negative rather than positive for B cell activation.

Defective negative regulation of antigen receptor signaling in Lyn-deficient B lymphocytes.

BACKGROUND: To elucidate the role of the Src family kinase Lyn in B cell receptor (BCR) signaling, we and others previously generated lyn-/- mice and analyzed their B cell responses. Although the initiation of BCR signaling in lyn-/- B cells is delayed, BCR-induced ERK2 activation and proliferation are enhanced. As the co-receptors Fc gamma RIIb1 and CD22 have been shown to be negative regulators of BCR signaling, we have now examined their functional roles in lyn-/- B cells. RESULTS: B cells from lyn-/- mice have increased expression of the protein product of the early response gene egr-1, enhanced activation of Jun N-terminal kinase (JNK), and elevated calcium responses upon BCR cross-linking. Tyrosine phosphorylation of Fc gamma RIIb1 in lyn-/- B cells was reduced but negative regulation of the BCR signal by Fc gamma RIIb1 was only modestly impaired. In contrast, tyrosine phosphorylation of CD22 was greatly decreased in lyn-/- B cells, correlating with the inability of CD22 to downregulate the BCR-induced calcium response in these cells. Surprisingly, CD22 remains capable of regulating the ERK2 and JNK pathways in lyn-/- B cells, which may relate to the small residual increase in BCR-induced CD22 phosphorylation. CONCLUSIONS: BCR signal initiation and negative regulation by Fc gamma RIIb1 is not critically dependent on Lyn. In contrast, Lyn plays a particularly important role in the tyrosine phosphorylation of CD22 and in the consequent inhibition of BCR-induced calcium influx. The net result of the Lyn deficiency in B cells is hyperresponsiveness to antigen stimulation, which may explain the autoimmunity observed in lyn-/- mice.

A critical role for Syk in signal transduction and phagocytosis mediated by Fcgamma receptors on macrophages.

Receptors on macrophages for the Fc region of IgG (FcgammaR) mediate a number of responses important for host immunity. Signaling events necessary for these responses are likely initiated by the activation of Src-family and Syk-family tyrosine kinases after FcgammaR cross-linking. Macrophages derived from Syk-deficient (Syk-) mice were defective in phagocytosis of particles bound by FcgammaRs, as well as in many FcgammaR-induced signaling events, including tyrosine phosphorylation of a number of cellular substrates and activation of MAP kinases. In contrast, Syk- macrophages exhibited normal responses to another potent macrophage stimulus, lipopolysaccharide. Phagocytosis of latex beads and Escherichia coli bacteria was also not affected. Syk- macrophages exhibited formation of polymerized actin structures opposing particles bound to the cells by FcgammaRs (actin cups), but failed to proceed to internalization. Interestingly, inhibitors of phosphatidylinositol 3-kinase also blocked FcgammaR-mediated phagocytosis at this stage. Thus, PI 3-kinase may participate in a Syk-dependent signaling pathway critical for FcgammaR-mediated phagocytosis. Macrophages derived from mice deficient for the three members of the Src-family of kinases expressed in these cells, Hck, Fgr, and Lyn, exhibited poor Syk activation upon FcgammaR engagement, accompanied by a delay in FcgammaR-mediated phagocytosis. These observations demonstrate that Syk is critical for FcgammaR-mediated phagocytosis, as well as for signal transduction in macrophages. Additionally, our findings provide evidence to support a model of sequential tyrosine kinase activation by FcgammaR's analogous to models of signaling by the B and T cell antigen receptors.

Shc contains two Grb2 binding sites needed for efficient formation of complexes with SOS in B lymphocytes.

Cross-linking of the B-cell antigen receptor (BCR) induces tyrosine phosphorylation of Shc, which is believed to lead to the activation of Ras. Previous work has shown that tyrosine-phosphorylated Shc forms complexes with another adapter protein, Grb2, and the Ras guanine nucleotide exchange factor SOS. Here, we demonstrate that phosphorylation of Shc by the hematopoietic cell-specific tyrosine kinase Syk induces binding of Grb2 to Shc, suggesting that Syk phosphorylates Shc in stimulated B cells. Surprisingly, Syk-phosphorylated Shc possesses two Grb2 binding sites rather than the one site that has been previously reported. Both of these sites are required for efficient formation of Shc-Grb2-SOS complexes in vitro and in vivo. We suggest that two Grb2 proteins anchored by a single Shc protein bind simultaneously to one SOS molecule, resulting in a complex that is more stable than a complex containing only a single Grb2 protein bound to one SOS molecule. This model is consistent with our observation that BCR stimulation greatly increases the amount of SOS associated with Grb2.

Characterization of the B lymphocyte populations in Lyn-deficient mice and the role of Lyn in signal initiation and down-regulation.

Lyn-deficient mice were generated to analyze the role of Lyn in B cell antigen receptor (BCR) signaling. These mice had a reduced number of peripheral B cells with a greater proportion of immature cells and a higher than normal turnover rate. Aged lyn-/- mice developed splenomegaly, produced autoantibodies, and had an expanded population of B lymphoblasts of the B1 lineage. Splenic B cells from young lyn-/- mice initiated early BCR signaling events, although in a delayed fashion. Unexpectedly, lyn-/- B cells exhibited an enhanced MAP kinase activation and an increased proliferative response to BCR engagement. Stimulation of lyn-/- B cells with intact and F(ab')2 anti-IgM revealed defects in at least two mechanisms that negatively regulate BCR signaling, one of which involves Fc gammaRIIb1.

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.

The two-headed antigen. B-cell co-receptors.

Lymphocytes often recognize antigens using not only their antigen receptors but also 'co-receptors' that bind other molecules associated with the antigen; the co-receptors then modulate the response to antigen. This concept has been used to make chimeric antigens that are extremely potent inducers of antibody responses.

Activation of c-Jun N-terminal kinase in bacterial lipopolysaccharide-stimulated macrophages.

Activation of macrophages by bacterial lipopolysaccharide (LPS) induces transcription of genes that encode for proinflammatory regulators of the immune response. Previous work has suggested that activation of the transcription factor activator protein 1 (AP-1) is one LPS-induced event that mediates this response. Consistent with this notion, we found that LPS stimulated AP-1-mediated transcription of a transfected reporter gene in the murine macrophage cell line RAW 264.7. As AP-1 activity is regulated in part by activation of the c-Jun N-terminal kinase (JNK), which phosphorylates and subsequently increases the transcriptional activity of c-Jun, we examined whether LPS treatment of macrophages resulted in activation of this kinase. LPS treatment of RAW 264.7 cells, murine bone marrow-derived macrophages, and the human monocyte cell line THP-1 resulted in rapid activation of the p46 and p54 isoforms of JNK. Treatment with wild-type and rough mutant forms of LPS and synthetic lipid A resulted in JNK activation, while pretreatment with the tyrosine kinase inhibitor herbimycin A inhibited this response. Binding of LPS-LPS binding protein (LBP) complexes to CD14, a surface receptor that mediates many LPS responses, was found to be crucial, as pretreatment of THP-1 cells with the monoclonal antibody 60b, which blocks this binding, inhibited JNK activation. These results suggest that LPS activation of JNK in monocyte/macrophage cells is a CD14- and protein tyrosine phosphorylation-dependent event that may mediate the early activation of AP-1 in regulating LPS-triggered gene induction.