Phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P3)/Tec kinase-dependent calcium signaling pathway: a target for SHIP-mediated inhibitory signals

SHIP-1 increases early oxidative burst and regulates phagosome maturation in macrophages

Although the inositol phosphatase SHIP-1 is generally thought to inhibit signaling for Fc receptor-mediated phagocytosis, the product of its activity, phosphatidylinositol 3,4 bisphosphate (PI(3,4)P(2)), has been implicated in activation of the NADPH oxidase. This suggests that SHIP-1 positively regulates the generation of reactive oxygen species after phagocytosis. To examine how SHIP-1 activity contributes to Fc receptor-mediated phagocytosis, we measured and compared phospholipid dynamics, membrane trafficking, and the oxidative burst in macrophages from SHIP-1-deficient and wild-type mice. SHIP-1-deficient macrophages showed significantly elevated ratios of PI(3,4,5)P(3) to PI(3,4)P(2) on phagosomal membranes. Imaging reactive oxygen intermediate activities in phagosomes revealed decreased early NADPH oxidase activity in SHIP-1-deficient macrophages. SHIP-1 deficiency also altered later stages of phagosome maturation, as indicated by the persistent elevation of PI(3)P and the early localization of Rab5a to phagosomes. These direct measurements of individual organelles indicate that phagosomal SHIP-1 enhances the early oxidative burst through localized alteration of the membrane 3'-phosphoinositide composition.

Deletion of PI3K-p85alpha gene impairs lineage commitment, terminal maturation, cytokine generation and cytotoxicity of NK cells

Class IA phosphotidylinositol-3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that are important in regulating signaling events in B and T cells. However, their role in natural killer (NK) cells is not understood. Here, using mice that lack the regulatory p85alpha subunit and its alternatively spliced variants p55alpha/p50alpha (collectively termed as p85alpha(-/-)), we defined the role of PI3K in NK cell development and function. p85alpha(-/-) mice had impaired lineage commitment leading to reduced NK cellularity in the bone marrow and liver. p85alpha(-/-) NK cells showed a defective Ly49 subset specification and a decreased expression of CD43. Lack of p85alpha severely reduced the NK-mediated cytotoxicity against tumor cells representing 'induced-self' and 'missing-self'. More importantly, NKG2D and NK1.1 receptor-mediated cytokine and chemokine generation was significantly compromised in p85alpha(-/-) NK cells. These results reveal a previously unrecognized role of p85alpha in the development, terminal maturation, cytokine/chemokine generation and tumor clearance of NK cells.

A novel druglike spleen tyrosine kinase binder prevents anaphylactic shock when administered orally

BACKGROUND

The spleen tyrosine kinase (Syk) is recognized as a potential pharmaceutical target for the treatment of type I hypersensitivity reactions including allergic rhinitis, urticaria, asthma, and anaphylaxis because of its critical position upstream of immunoreceptor signaling complexes that regulate inflammatory responses in leukocytes.

OBJECTIVE

Our aim was to improve the selectivity of anti-Syk therapies by impeding the interaction of Syk with its cellular partners, instead of targeting its catalytic site.

METHODS

We have previously studied the inhibitory effects of the anti-Syk intracellular antibody G4G11 on Fc epsilonRI-induced release of allergic mediators. A compound collection was screened by using an antibody displacement assay to identify functional mimics of G4G11 that act as potential inhibitors of the allergic response. The effects of the selected druglike compounds on mast cell activation were evaluated in vitro and in vivo.

RESULTS

We discovered compound 13, a small molecule that inhibits Fc epsilonRI-induced mast cell degranulation in vitro and anaphylactic shock in vivo. Importantly, compound 13 was efficient when administered orally to mice. Structural analysis, docking, and site-directed mutagenesis allowed us to identify the binding cavity of this compound, located at the interface between the 2 Src homology 2 domains and the interdomain A of Syk.

CONCLUSION

We have isolated a new class of druglike compounds that modulate the interaction of Syk with some of its macromolecular substrates implicated in the degranulation pathway in mast cells.

Phospholipase C-gamma2 and Vav cooperate within signaling microclusters to propagate B cell spreading in response to membrane-bound antigen

B cell receptor (BCR) recognition of membrane-bound antigen initiates a spreading and contraction response, the extent of which is controlled through the formation of signaling-active BCR-antigen microclusters and ultimately affects the outcome of B cell activation. We followed a genetic approach to define the molecular requirements of BCR-induced spreading and microcluster formation. We identify a key role for phospholipase C-γ2 (PLCγ2), Vav, B cell linker, and Bruton's tyrosine kinase in the formation of highly coordinated “microsignalosomes,” the efficient assembly of which is absolutely dependent on Lyn and Syk. Using total internal reflection fluorescence microscopy, we examine at high resolution the recruitment of PLCγ2 and Vav to microsignalosomes, establishing a novel synergistic relationship between the two. Thus, we demonstrate the importance of cooperation between components of the microsignalosome in the amplification of signaling and propagation of B cell spreading, which is critical for appropriate B cell activation.

FcgammaRIIB as a modulator of autoimmune disease susceptibility

Antibodies are secreted to recognize and in some cases directly neutralize pathogens. Another important means by which they are essential components of the immune system is through binding to Fc receptors. Effector responses triggered by antibody binding of Fc receptors affect a host of important cellular responses such as phagocytosis, inflammatory cytokine release, antigen presentation, and regulation of humoral responses. A crucial check on this antibody-mediated signal is through the inhibitory receptor, FcgammaRIIB. In this review we discuss how dysregulation of FcgammaRIIB can result in a lowered threshold for autoimmunity in mice and humans. We close with a discussion of the potential for applying these findings to immunotherapy.

PI(3,4,5)P3 and PI(3,4)P2 levels correlate with PKB/akt phosphorylation at Thr308 and Ser473, respectively; PI(3,4)P2 levels determine PKB activity

The PI3K-PKB pathway is an important and widely studied pathway in cell signaling. The enzyme activity of PI3K produces D-3 phosphoinositides, including the lipid second messengers PI(3,4,5)P3 and PI(3,4)P2. PI(3,4,5)P3 has been deemed to be the most important second messenger for triggering PKB phosphorylation. PKB has two regulatory phosphorylation sites, Thr308 and Ser473, both of which contribute to its full activity. The direct relationship between PI3K lipid products and PKB phosphorylation is still not entirely clear. Our previous study showed that PI(3,4)P2 has a specific role in contributing to PKB phosphorylation on Ser473 sites in mast cells. In this study, we used two strategies to further elucidate this question in a well-established B cell system. First, by SHIP overexpression, we examined PKB activation under conditions where PI(3,4,5)P3 accumulation is largely suppressed. Second, we used dose response of different forms of B-cell receptor ligands to manipulate the relative levels of PI(3,4,5)P3 and PI(3,4)P2. Our results demonstrate a close relationship between PI(3,4,5)P3 levels and Thr308 phosphorylation levels, and PI(3,4)P2 levels and Ser473 phosphorylation levels, respectively. Furthermore, overall PKB activity, primarily consisting of cytosolic enzyme, was dependent upon levels of PI(3,4)P2, while only membrane-associated PKB activity was dependent upon PI(3,4,5)P3 levels. We conclude that PI(3,4,5)P3 and PI(3,4)P2 have distinct roles in determining PKB phosphorylation and activity. Thus, when investigating PI3K-PKB pathways, the importance of both lipids must be considered.

Calcium signalling and cell-fate choice in B cells

Alterations in the cytosolic concentration of calcium ions (Ca2+) transmit information that is crucial for the development and function of B cells. Cytosolic Ca2+ concentration is determined by a balance of active transport and gradient-driven Ca2+ fluxes, both of which are subject to the influence of multiple receptors and environmental sensing pathways. Recent advances in genomics have allowed for the compilation of an increasingly comprehensive list of Ca2+ transporters and channels expressed by B cells. The increasing understanding of the function and regulation of these proteins has begun to shift the frontier of Ca2+ physiology in B cells from molecular analysis to determining how diverse inputs to cytosolic Ca2+ concentration are integrated in specific immunological contexts.

The role of SHIP1 in T-lymphocyte life and death

SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase-1], an inositol 5-phosphatase expressed in haemopoietic cells, acts by hydrolysing the 5-phosphates from PtdIns(3,4,5)P(3) and Ins(1,3,4,5)P(4), thereby negatively regulating the PI3K (phosphoinositide 3-kinase) pathway. SHIP1 plays a major role in inhibiting proliferation of myeloid cells. As a result, SHIP1(-/-) mice have an increased number of neutrophils and monocytes/macrophages due to enhanced survival and proliferation of their progenitors. Although SHIP1 contributes to PtdIns(3,4,5)P(3) metabolism in T-lymphocytes, its exact role in this cell type is much less explored. Jurkat cells have recently emerged as an interesting tool to study SHIP1 function in T-cells because they do not express SHIP1 at the protein level, thereby allowing reintroduction experiments in a relatively easy-to-use system. Data obtained from SHIP1 reintroduction have revealed that SHIP1 not only acts as a negative player in T-cell lines proliferation, but also regulates critical pathways, such as NF-kappaB (nuclear factor kappaB) activation, and also appears to remarkably inhibit T-cell apoptosis. On the other hand, experiments using primary T-cells from SHIP1(-/-) mice have highlighted a new role for SHIP1 in regulatory T-cell development, but also emphasize that this protein is not required for T-cell proliferation. In support of these results, SHIP1(-/-) mice are lymphopenic, suggesting that SHIP1 function in T-cells differs from its role in the myeloid lineage.

The inositol 5'-phosphatase SHIP-2 negatively regulates IgE-induced mast cell degranulation and cytokine production

Aggregation of the high-affinity IgE receptor (FcepsilonRI) on mast cells initiates signaling pathways leading to degranulation and cytokine release. It has been reported that SHIP-1 negatively regulates FcepsilonRI-triggered pathways but it is unknown whether its homologous protein SHIP-2 has the same function. We have used a lentiviral-based RNA interference technique to obtain SHIP-2 knockdown bone marrow-derived mast cells (BMMCs) and have found that elimination of SHIP-2 results in both increased mast cell degranulation and cytokine (IL-4 and IL-13) gene expression upon FcepsilonRI stimulation. Elimination of SHIP-2 from BMMCs has no effect on FcepsilonRI-triggered calcium flux, tyrosine phosphorylation of MAPKs or in actin depolymerization following activation. Rather, we observe that absence of SHIP-2 results in increased activation of the small GTPase Rac-1 and in enhanced microtubule polymerization upon FcepsilonRI engagement. Coimmunoprecipitation experiments in rat basophilic leukemia (RBL 2H3) cells show that SHIP-2 interacts with the FcepsilonRI beta-chain, Gab2 and Lyn and that unlike SHIP-1, it does not associate with SHC in mast cells. Our results report a negative regulatory role of SHIP-2 on mast cell activation that is calcium independent and distinct from the regulation by SHIP-1.

Inhibition of pathologic inflammation by leukocyte Ig-like receptor B4 and related inhibitory receptors

Leukocyte immunoglobulin (Ig)-like receptor B4 (LILRB4)(previously termed gp49B1) is a member of the Ig superfamily expressed constitutively on the surface of mast cells, neutrophils, and macrophages. LILRB4 inhibits IgE-dependent activation of mast cells in vitro through its two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that recruit the src homology domain type-2-containing tyrosine phosphatase 1 into the cell membrane. Accordingly, Lilrb4(-/-) mice exhibit greater incidence and severity of IgE- and mast cell-dependent anaphylactic inflammation compared with mice that express LILRB4. In addition, mast cell-dependent inflammation induced by the interaction of stem cell factor (SCF) with its receptor Kit is also more severe in Lilrb4(-/-) mice, indicating that the counterregulatory function of LILRB4 extends beyond inflammation induced by Fc receptors, which signal through ITIMs, to responses initiated through a receptor tyrosine kinase. Indeed, pathologic inflammatory responses induced by activation of neutrophils with lipopolysaccharide (LPS) alone or with tissue-specific autoantibodies are greatly exacerbated in Lilrb4(-/-) mice. The rapid upregulation of LILRB4 expression on neutrophils in Lilrb4(+/+) mice in response to LPS suggests it is an innate counterregulatory response designed to reduce pathologic inflammation. Nevertheless, LILRB4 also serves a similar purpose for inflammation induced by the humoral adaptive immune response that is manifested through effector cells bearing Fc receptors.

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

T cell-specific deletion of the inositol phosphatase SHIP reveals its role in regulating Th1/Th2 and cytotoxic responses

The 5'-phosphoinositol phosphatase SHIP negatively regulates signaling pathways triggered by antigen, cytokine and Fc receptors in both lymphocytes and myeloid cells. Mice with germ-line (null) deletion of SHIP develop a myeloproliferative-like syndrome that causes early lethality. Lymphocyte anomalies have been observed in SHIP-null mice, but it is unclear whether they are due to an intrinsic requirement of SHIP in these cells or a consequence of the severe myeloid pathology. To precisely address the function of SHIP in T cells, we have generated mice with T cell-specific deletion of SHIP. In the absence of SHIP, we found no differences in thymic selection or in the activation state and numbers of regulatory T cells in the periphery. In contrast, SHIP-deficient T cells do not skew efficiently to Th2 in vitro. Mice with T cell-specific deletion of SHIP show poor antibody responses on Alum/NP-CGG immunization and diminished Th2 cytokine production when challenged with Schistosoma mansoni eggs. The failure to skew to Th2 responses may be the consequence of increased basal levels of the Th1-associated transcriptional factor T-bet, resulting from enhanced sensitivity to cytokine-mediated T-bet induction. SHIP-deficient CD8(+) cells show enhanced cytotoxic responses, consistent with elevated T-bet levels in these cells. Overall our experiments indicate that in T cells SHIP negatively regulates cytokine-mediated activation in a way that allows effective Th2 responses and limits T cell cytotoxicity.

The mast cell IgG receptors and their roles in tissue inflammation

Mast cells are effector cells of the innate immune system, but because they express Fc receptors (FcRs), they can be engaged in adaptive immunity by antibodies. Mast cell FcRs include immunoglobulin E (IgE) and IgG receptors and, among these, activating and inhibitory receptors. The engagement of mast cell IgG receptors by immune complexes may or may not trigger cell activation, depending on the type of mast cell. The coengagement of IgG and IgE receptors results in inhibition of mast cell activation. The Src homology-2 domain-containing inositol 5-phosphatase-1 is a major effector of negative regulation. Biological responses of mast cells depend on the balance between positive and negative signals that are generated in FcR complexes. The contribution of human mast cell IgG receptors in allergies remains to be clarified. Increasing evidence indicates that mast cells play critical roles in IgG-dependent tissue-specific autoimmune diseases. Convincing evidence was obtained in murine models of multiple sclerosis, rheumatoid arthritis, bullous pemphigoid, and glomerulonephritis. In these models, the intensity of lesions depended on the relative engagement of activating and inhibitory IgG receptors. In vitro models of mature tissue-specific murine mast cells are needed to investigate the roles of mast cells in these diseases. One such model unraveled unique differentiation/maturation-dependent biological responses of serosal-type mast cells.

Btk and phospholipase C gamma 2 can function independently during B cell development

The pre-BCR and the BCR regulate B cell development via a signalosome nucleated by the adaptor protein B cell linker protein (BLNK). Formation of this complex facilitates activation of phospholipase C (PLC) gamma2 by Bruton's tyrosine kinase (Btk). To determine whether Btk and PLCgamma2 also have separate functions, we generated Btk(-/-)PLCgamma2(-/-) mice. They demonstrated a block in development at the pre-B stage and increased pre-BCR surface expression. This phenotype was more severe than that of Btk(-/-) or PLCgamma2(-/-) mice. Although both Btk and PLCgamma2 were required for proliferation of splenic B cells in response to BCR cross-linking, they contributed differently to anti-IgM-induced phosphorylation of ERK. Btk(-/-) and PLCgamma2(-/-) mice each had a reduced frequency of Iglambda-expressing B cells and impaired migration of pre-B cells towards stromal cell-derived factor 1. However, the increase in pre-B cell malignancy that occurs in BLNK(-/-) mice in the absence of Btk was not observed in the absence of PLCgamma2. Thus, Btk and PLCgamma2 act both in concert and independently throughout B cell development.

The IREM-1 (CD300f) inhibitory receptor associates with the p85alpha subunit of phosphoinositide 3-kinase

The immune receptor expressed by myeloid cell 1 (IREM-1) (CD300f) inhibitory receptor displays five cytoplasmic tyrosine residues, two of them (Y205 and Y249) fit with ITIMs, whereas Y236 and Y263 constitute putative binding sites for PI3K. In the present study, immunoprecipitation analysis revealed that both the p85alpha subunit of PI3K and Src homology region 2 domain-containing phosphatase-1 could be recruited by IREM-1 in transfected cells as well as in the U937 monocytic leukemia cells, which constitutively express the receptor. By assaying the ability of different IREM-1 mutants to regulate the secretion of beta-hexosaminidase induced via FcRepsilonI in rat basophilic leukemia cells, both Y205 and Y249 appeared crucial for IREM-1-mediated inhibition. Remarkably, engagement of an IREM-1 mutant (Y(205,249,284)F), which did not recruit Src homology region 2 domain-containing phosphatase-1 and lost its inhibitory function, induced rat basophilic leukemia cell degranulation. This effect was dependent on the recruitment of PI3K, requiring the integrity of Y236 and Y263, and was blocked by PI3K inhibitors (i.e., wortmannin and LY-294002). Altogether, these data reveal a putative functional duality of the IREM-1 myeloid cell receptor.

An essential role for RasGRP1 in mast cell function and IgE-mediated allergic response

Cross-linking of the FcepsilonRI activates the phosphatidyl inositol 3 kinase (PI3K) and mitogen-activated protein kinase pathways. Previous studies demonstrate that Ras guanyl nucleotide-releasing protein (RasGRP)1 is essential in T cell receptor-mediated Ras-Erk activation. Here, we report that RasGRP1 plays an important role in FcepsilonRI-mediated PI3K activation and mast cell function. RasGRP1-deficient mice failed to mount anaphylactic allergic reactions. RasGRP1-/- mast cells had markedly reduced degranulation and cytokine production. Although FcepsilonRI-mediated Erk activation was normal, PI3K activation was diminished. Consequently, activation of Akt, PIP3-dependent kinase, and protein kinase C delta was defective. Expression of a constitutively active form of N-Ras could rescue the degranulation defect and Akt activation. We further demonstrated that RasGRP1-/- mast cells were defective in granule translocation, microtubule formation, and RhoA activation. Our results identified RasGRP1 as an essential regulator of mast cell function.

Fcgamma receptor signaling in phagocytes

Fcgamma receptors are among the best-studied phagocytic receptors. The key features of Fcgamma receptor-mediated phagocytosis include phagocytic cup formation by extensive actin cytoskeletal rearrangements, particle engulfment, and the release of proinflammatory mediators such as cytokines and reactive oxygen species. These events are elegantly regulated by the simultaneous engagement of activating and inhibitory Fcgamma receptors and by intracellular signaling molecules. Extensive studies in the past several years have defined the molecular mechanisms of the phagocytic process. The purpose of this review is to revisit some of the well-established signaling pathways as well as to summarize the new findings in this field.

Fcε- and Fcγ-receptor signaling in diseases

It has become increasingly clear that receptors for the immunoglobulin Fc region play pivotal roles in immune homeostasis and disease. This review describes the fine regulation of the high-affinity IgE-receptor (FcepsilonRI) signaling, especially focusing on the early events that are coordinately regulated by Src family protein tyrosine kinases (PTKs), FcepsilonRI beta-subunit, and membrane lipid rafts. Because allergen-mediated FcepsilonRI cross-linking leads to the synthesis and release of a variety of proinflammatory mediators and cytokines, the duration and amplitude of the signal need to be strictly controlled, and the counterbalancing signaling is provided by specialized inhibitory receptors and molecules. However, recent work have revealed that Src family PTKs and FcepsilonRI beta-subunit transduce both positive and negative signaling with unexpectedly complex mechanisms. FcgammaRIIB exerts a unique inhibitory function on cell activation processes after the engagement of Fcgamma, FcepsilonRI and B cell receptors. Recent work has shown that FcgammaRIIB polymorphisms are associated with systemic lupus erythematosus, and that a transmembrane polymorphism in FcgammaRIIB results in an impaired distribution to lipid rafts and a reduced inhibitory function. Studies addressing the functions of disease-associated polymorphisms in the FcepsilonRI beta-subunit and low-affinity FcgammaRs are also considered.

B cell receptor signaling in human systemic lupus erythematosus

PURPOSE OF REVIEW

The purpose of this review is to inform the scientific community of the most recent findings surrounding B cell receptor signaling function in human systemic lupus erythematosus and how altered B cell signaling may explain the characteristic hyperactivity of B cells in active disease and contribute to its pathogenesis.

RECENT FINDINGS

B cell receptor signaling is abnormal in patients with active systemic lupus erythematosus as demonstrated by increased calcium flux and global B cell hyperactivity. Altered signaling has been explained by a variety of factors such as defective FcgammaRIIB signaling, decreased expression of the protein tyrosine kinase Lyn, and increased serum levels of B lymphocyte stimulator.

SUMMARY

The studies reviewed suggest that B cells from systemic lupus erythematosus patients display molecular signaling defects that most likely contribute to pathogenesis of the disease and explain the characteristic hyperactivity of B cells in active disease.

Distinct gene expression signature in Btk-defective T1 B-cells

Bruton's tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important for B-lymphocyte maturation. Mutations in Btk give rise to the primary immunodeficiency disease X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mice. Recent studies have subdivided the mouse immature, or transitional, B-cells into two distinct subsets according to their respective surface markers. Transitional type 1 (T1) and transitional type 2 (T2) cells are also located in distinct anatomic locations. Based on a limited number of markers it has previously been reported that the earliest phenotypic sign of Btk deficiency is manifested at the T2 stage in mice. Here, we report on distinct genome-wide transcriptomic signature differences found in T1 B-lymphocytes from Btk-defective compared to normal mice and demonstrate that Btk deficiency is visible already at this stage.

The adaptor protein Gab1 couples the stimulation of vascular endothelial growth factor receptor-2 to the activation of phosphoinositide 3-kinase

Phosphoinositide 3-kinase (PI3K) mediates essential functions of vascular endothelial growth factor (VEGF), including the stimulation of endothelial cell proliferation and migration. Nevertheless, the mechanisms coupling the receptor VEGFR-2 to PI3K remain obscure. We observed that the Grb2-bound adapter Gab1 is tyrosine-phosphorylated and relocated to membrane fractions upon VEGF stimulation of endothelial cells. We could detect the PI3K regulatory subunit p85 in immunoprecipitates of endogenous Gab1, and vice versa, and measure a Gab1-associated lipid kinase activity upon VEGF stimulation. Furthermore, transfection of the Gab1-YF3 mutant lacking all p85-binding sites strongly repressed PI3K activation measured in vitro. Moreover, Gab1-YF3 severely decreased the cellular amount of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) generated in response to VEGF. Furthermore, adenoviral expression of Gab1-YF3 suppressed both Akt phosphorylation and recovery of wounded human umbilical vein endothelial cell monolayers, a VEGF-dependent process involving cell migration and proliferation under PI3K control. Transfection of other Gab1 mutants, lacking Grb2-binding sites or the pleckstrin homology (PH) domain, also prevented Akt activation, further demonstrating Gab1 involvement in PI3K activation. These mutants were also used to show that interactions with both Grb2 and PtdIns(3,4,5)P3 mediate Gab1 recruitment by VEGFR-2. Importantly, Gab1 mobilization was impaired by (i) PI3K inhibitors, (ii) deletion of Gab1 PH domain, (iii) PTEN (phosphatase and tensin homolog deleted on chromosome 10) overexpression to repress PtdIns(3,4,5)P3 production, and (iv) overexpression of a competitor PH domain for PtdIns(3,4,5)P3 binding, which altogether demonstrated that PI3K is also an upstream regulator of Gab1. Gab1 thus appears as a primary actor in coupling VEGFR-2 to PI3K/Akt, recruited through an amplification loop involving PtdIns(3,4,5)P3 and its PH domain.

The SH2 domain-containing inositol 5-phosphatase SHIP1 is recruited to the intracytoplasmic domain of human FcγRIIB and is mandatory for negative regulation of B cell activation

Murine FcgammaRIIB were demonstrated to recruit SH2 domain-containing inositol 5-phosphatases (SHIP1/2), when their ITIM is tyrosyl-phosphorylated upon co-aggregation with BCR, and SHIP1 to account for FcgammaRIIB-dependent negative regulation of murine B cell activation. Although human FcgammaRIIB share the same ITIM as murine FcgammaRIIB and similarly inhibit human B cell activation, which among the four known SH2 domain-containing (tyrosine or inositol) phosphatases is/are recruited by human FcgammaRIIB is unclear. Our recent finding that, besides the ITIM, a second tyrosine-based motif is mandatory for murine FcgammaRIIB to recruit SHIP1 challenged the possibility that human FcgammaRIIB recruit this phosphatase. Human FcgammaRIIB indeed lack this motif. Using an experimental model which enabled us to compare human FcgammaRIIB and murine FcgammaRIIB under strictly controlled conditions, we show that SHIP1 is recruited to the intracytoplasmic domain of human FcgammaRIIB and inhibits the same biological responses and intracellular signals as when recruited by murine FcgammaRIIB. Identical results were observed in murine and in human B cells. We demonstrate that SHIP is necessary for human FcgammaRIIB to inhibit BCR signaling, and cannot be replaced by SHP-1 or SHP-2. Although it contains no tyrosine, the C-terminal segment of human FcgammaRIIB was as mandatory as the tyrosine-containing C-terminal segment of murine FcgammaRIIB for SHIP1 to be recruited to the ITIM. This segment, however, did not recruit the adapters Grb2/Grap which were demonstrated to stabilize the recruitment of SHIP1 to the ITIM in murine FcgammaRIIB.

Downstream of Tyrosine Kinases-1 and Src Homology 2-Containing Inositol 5′-Phosphatase Are Required for Regulation of CD4+CD25+ T Cell Development

The adaptor protein, downstream of tyrosine kinases-1 (Dok-1), and the phosphatase SHIP are both tyrosine phosphorylated in response to T cell stimulation. However, a function for these molecules in T cell development has not been defined. To clarify the role of Dok-1 and SHIP in T cell development in vivo, we compared the T cell phenotype of wild-type, Dok-1 knockout (KO), SHIP KO, and Dok-1/SHIP double-knockout (DKO) mice. Dok-1/SHIP DKO mice were runted and had a shorter life span compared with either Dok-1 KO or SHIP KO mice. Thymocyte numbers from Dok-1/SHIP DKO mice were reduced by 90%. Surface expression of both CD25 and CD69 was elevated on freshly isolated splenic CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO, suggesting these cells were constitutively activated. However, these T cells did not proliferate or produce IL-2 after stimulation. Interestingly, the CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO mice produced higher levels of TGF-beta, expressed Foxp3, and inhibited IL-2 production by CD3-stimulated CD4(+)CD25(-) T cells in vitro. These findings suggest Dok-1 and SHIP function in pathways that influence regulatory T cell development.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Cellular calcium mobilization in response to phosphoinositide delivery

Intracellular calcium [Ca(2+)](i) is mobilized in many cell types in response to activation of phosphoinositide (PIP(n)) signaling pathways involving PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3). To further explore the relationship between increases in intracellular PIP(n) concentrations and mobilization of [Ca(2+)](i), each of the seven phosphorylated phosphoinositides (PIP(n)s) were delivered into cells and the metabolism and physiological effects of the exogenously administered PIP(n)s were determined. The efficient cellular delivery of fluorophore-tagged and native PIP(n)s was accomplished using histone protein, neomycin, and dendrimeric polyamines. PtdIns(4,5)P(2) fluorophore-tagged analogs with short- and long-acyl chains were substrates for cellular enzymes in vitro and for phospholipases in stimulated fibroblasts. PtdIns(4)P, PtdIns(3,4)P(2) and PtdIns(4,5)P(2), each induced calcium mobilization rapidly after exogenous addition to fibroblasts. PtdIns(3,4,5)P(3) induced a significant, but smaller increase in intracellular calcium. These observations suggest that PIP(n)s other than PtdIns(4,5)P(2) or PtdIns(3,4,5)P(3) may have direct roles in signaling involving [Ca(2+)](i).

Fc receptors and their role in immune regulation and autoimmunity

The activation threshold of cells in the immune system is often tuned by cell surface molecules. The Fc receptors expressed on various hematopoietic cells constitute critical elements for activating or downmodulating immune responses and combines humoral and cell-mediated immunity. Thus, Fc receptors are the intelligent sensors of the immune status in the individual. However, impaired regulation by Fc receptors will lead to unresponsiveness or hyperreactivity to foreign as well as self-antigens. Murine models for autoimmune disease indicate the indispensable roles of the inhibitory Fc receptor in the suppression of such disorders, whereas activating-type FcRs are crucial for the onset and exacerbation of the disease. The development of many autoimmune diseases in humans may be caused by impairment of the human Fc receptor regulatory system. This review is aimed at providing a current overview of the mechanism of Fc receptor-based immune regulation and the possible scenario of how autoimmune disease might result from their dysfunction.

Failed up-regulation of the inhibitory IgG Fc receptor Fc gamma RIIB on germinal center B cells in autoimmune-prone mice is not associated with deletion polymorphisms in the promoter region of the Fc gamma RIIB gene

FcgammaRIIB, a low-affinity FcR for IgG, inhibits BCR-mediated activation when these two receptors are co-cross-linked by Ags and IgG-containing immune complexes. Although a role for FcgammaRIIB in the germinal center (GC) reaction has been proposed, conflicting results have been published regarding the levels of FcgammaRIIB expressed on GC B cells in normal and autoimmune-prone mice and humans. In the present study, we investigate this issue in detail in mice by using multiple GC B cell markers, two different antigenic systems, primary and secondary GC responses, and by excluding the influence of splenic influx of immature B cells and passive acquisition of FcgammaRIIB from follicular dendritic cells. Our results are in concordance with previous data indicating that FcgammaRIIB expression is up-regulated on GC B cells in normal mice. In contrast, we observe comparable levels of FcgammaRIIB on GC and non-GC B cells in New Zealand White, New Zealand Black, and B6.Sle1 autoimmune-prone strains. Therefore, we suggest that these strains exhibit failed up-regulation of FcgammaRIIB on GC B cells, rather than down-regulation, as previously suggested. Also, in contrast to previous indications, this perturbed regulation is not uniquely associated with deletion polymorphisms in the promoter region of the FcgammaRIIB gene but does appear to be independent of genetic background. Finally, we present evidence indicating that FcgammaRIII, a low-affinity activating IgG FcR, is expressed on the GC B cells of normal but not autoimmune-prone mice.

Phospholipase C-gamma 2 is a critical signaling mediator for murine NK cell activating receptors

Phospholipase C-gamma (PLCgamma) is a key regulator of intracellular Ca(2+) mobilization. Two isoforms of PLCgamma have been identified, PLCgamma1 and PLCgamma2. Previously, in vitro studies indicated that activating NK cell receptors signal through both isoforms. However, PLCgamma2 deficiency alone was sufficient to induce a substantial impairment of NK cell-mediated cytotoxicity in vitro. Why PLCgamma2 is more important than PLCgamma1 for NK cell activation and whether PLCgamma2 is also critical for NK cell development, secretion of IFN-gamma, and clearance of viral infections in vivo is not known. In this study, we report that PLCgamma2 is the predominant isoform expressed in murine NK cells. PLCgamma2 deficiency did not affect NK cell numbers in bone marrow and spleen, but acquisition of Ly49 receptors by NK cells was partially impaired. PLCgamma2-deficient NK cells exhibited a dramatic impairment of cytolytic function and IFN-gamma production upon ligation of activating receptors, whereas they did secrete IFN-gamma in response to cytokines. Consequently, mice lacking PLCgamma2 controlled murine CMV infection substantially less effectively than did wild-type animals, and this defect was most evident in the spleen, where viral clearance mostly depends on NK cell lytic function. These results demonstrate that PLCgamma2 is crucial for development of the NK cell receptor repertoire and signaling of activating NK cell receptors, mediating optimal NK cell function in vivo.

FcγRIIB Ile232Thr transmembrane polymorphism associated with human systemic lupus erythematosus decreases affinity to lipid rafts and attenuates inhibitory effects on B cell receptor signaling

The B cell inhibitory receptor FcgammaRIIB plays crucial roles in the maintenance of self-tolerance. We have identified a polymorphism FCGR2B c.695T>C that results in the non-conservative replacement of 232Ile at the transmembrane helix to Thr and demonstrated the association of the polymorphism with susceptibility to systemic lupus erythematosus (SLE) in Asians. In this study, we examined the impact of FCGR2B c.695T>C on the functional properties of FcgammaRIIB by expressing each allele product in a human B cell line ST486 lacking endogenous FcgammaRIIB. FcgammaRIIB 232Thr was found to be significantly less potent than wild-type 232Ile in inhibiting B cell receptor (BCR)-mediated phosphatidylinositol-3,4,5-trisphosphate accumulation, Akt and PLCgamma2 activation and calcium mobilization, and to display decreased levels of tyrosine phosphorylation and SH2-containing 5'-inositolphosphate phosphatase recruitment compared with 232Ile after IgG Fc-mediated coligation with BCR. Notably, a quantitative analysis of the subcellular distribution of FcgammaRIIB using 125I-labeled anti-FcgammaRIIB revealed that FcgammaRIIB 232Thr is less effectively distributed to detergent-insoluble lipid rafts than 232Ile, findings in accordance with the importance of the transmembrane amino acid residues, in particular large hydrophobic amino acids including Ile, in the association of membrane proteins with lipid rafts. Given the crucial roles of lipid rafts in integrating BCR signaling, decreased association of FcgammaRIIB 232Thr could contribute to its impaired inhibitory potential. Collectively, the present findings indicate that the Ile232Thr substitution affects the localization and function of FcgammaRIIB and that the molecular mechanism may link the polymorphism and susceptibility to SLE.

B cell signaling and tumorigenesis

The proliferation and differentiation of lymphocytes are regulated by receptors localized on the cell surface. Engagement of these receptors induces the activation of intracellular signaling proteins that transmit the receptor signals to distinct targets and control the cellular responses. The first signaling proteins to be discovered in higher organisms were the products of oncogenes. For example, the kinases Src and Abelson (Abl) were originally identified as oncogenes and were later characterized as important proteins for signal transduction in various cell types, including lymphocytes. Now, as many cellular signaling molecules have been discovered and ordered into certain pathways, we can better understand why particular signaling proteins are associated with tumorigenesis. In this review, we discuss recent progress in unraveling the molecular mechanisms of signaling pathways that control the proliferation and differentiation of early B cells. We point out the concepts of auto-inhibition and subcellular localization as crucial aspects in the regulation of B cell signaling.

Regulation of highly cytokinergic IgE-induced mast cell adhesion by Src, Syk, Tec, and protein kinase C family kinases

Mast cells play a critical role in IgE-dependent immediate hypersensitivity. Recent studies have shown that, contrary to the traditional view, binding of monomeric IgE to Fc epsilon RI results in a number of biological outcomes in mast cells, including survival. However, IgE molecules display heterogeneity in inducing cytokine production; highly cytokinergic (HC) IgEs cause extensive Fc epsilon RI aggregation, which leads to potent enhancement of survival and other activation events, whereas poorly cytokinergic (PC) IgEs can do so inefficiently. The present study demonstrates that HC, but not PC, IgEs can efficiently induce adhesion and spreading of mouse mast cells on fibronectin-coated plates in slow and sustained kinetics. HC IgE-induced adhesion through beta1 and beta7 integrins promotes survival, IL-6 production, and DNA synthesis. Importantly, we have identified Lyn and Syk as requisite tyrosine kinases and Hck, Btk, and protein kinase C theta as contributory kinases in HC IgE-induced adhesion and spreading, whereas protein kinase C epsilon plays a negative role. Consistent with these results, Lyn, Syk, and Btk are activated in HC IgE-stimulated cells in a slower but more sustained manner, compared with cells stimulated with IgE and Ag. Thus, binding of HC IgEs to Fc epsilon RI induces adhesion of mast cells to fibronectin by modulating cellular activation signals in a unique fashion.

The Pro335 --> Leu polymorphism of type 3 inositol 1,4,5-trisphosphate receptor found in mouse inbred lines results in functional change

Inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ channel involved in various cellular signaling. Type 3 IP3R (IP3R3) retains ligand-gated Ca2+ channel properties differing from other subtypes in terms of IP3-binding affinity and regulation of its channel activity by effector molecules. In this study, we found the natural Pro335 --> Leu polymorphism of mouse IP3R3 between BALB/c and C57BL/6J. We investigated the functional differences between Pro335IP3R3 and Leu335IP3R3 with purified receptors reconstituted into proteoliposomes as well as with soluble ligand binding domains. Pro335IP3R3 exhibited significantly higher IP3-binding affinity and IP3-induced Ca2+ release than those of Leu335IP3R3 in both forms of the receptor. Moreover, the polymorphic change caused differences in the effect of external Ca2+ on IP3-induced Ca2+ release. The Pro335 --> Leu substitution alters the conformation of soluble ligand binding domain as revealed by intrinsic fluorescence and circular dichroism spectra with or without Ca2+. The results indicate that the polymorphism of IP3R3 causes changes in receptor function, presumably affecting intracellular Ca2+ signaling.

Phosphatidylinositol 3,4,5-trisphosphate modulation in SHIP2-deficient mouse embryonic fibroblasts

SHIP2, the ubiquitous SH2 domain containing inositol 5-phosphatase, includes a series of protein interacting domains and has the ability to dephosphorylate phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)]in vitro. The present study, which was undertaken to evaluate the impact of SHIP2 on PtdIns(3,4,5)P(3) levels, was performed in a mouse embryonic fibroblast (MEF) model using SHIP2 deficient (-/-) MEF cells derived from knockout mice. PtdIns(3,4,5)P(3) was upregulated in serum stimulated -/- MEF cells as compared to +/+ MEF cells. Although the absence of SHIP2 had no effect on basal PtdIns(3,4,5)P(3) levels, we show here that this lipid was significantly upregulated in SHIP2 -/- cells but only after short-term (i.e. 5-10 min) incubation with serum. The difference in PtdIns(3,4,5)P(3) levels in heterozygous fibroblast cells was intermediate between the +/+ and the -/- cells. In our model, insulin-like growth factor-1 stimulation did not show this upregulation. Serum stimulated phosphoinositide 3-kinase (PI 3-kinase) activity appeared to be comparable between +/+ and -/- cells. Moreover, protein kinase B, but not mitogen activated protein kinase activity, was also potentiated in SHIP2 deficient cells stimulated by serum. The upregulation of protein kinase B activity in serum stimulated cells was totally reversed in the presence of the PI 3-kinase inhibitor LY-294002, in both +/+ and -/- cells. Altogether, these data establish a link between SHIP2 and the acute control of PtdIns(3,4,5)P(3) levels in intact cells.

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

Interleukin-2-inducible T cell kinase regulates mast cell degranulation and acute allergic responses

Bruton's tyrosine kinase (Btk) is thought to positively regulate mast cell activation, implying a role in allergic responses. We have compared acute and late phase allergic airway reactions in mice lacking either Btk or interleukin-2-inducible T cell kinase (Itk), another Tec kinase expressed in mast cells. Btk(-/-) mice showed minor protection against allergic symptoms when challenged with allergen via the airways. In sharp contrast, both acute and late phase inflammatory allergic responses were markedly reduced in Itk(-/-) mice. Notably, airway mast cell degranulation in Itk(-/-) mice was severely impaired, despite wild-type levels of allergen-specific IgE and IgG1. The degranulation defect was confirmed in DNP-conjugated human serum albumin-challenged mice passively sensitized with anti-DNP IgE antibodies, and was also observed after direct G-protein stimulation with the mast cell secretagogue c48/80. Moreover, late phase inflammatory changes, including eosinophilia, lymphocyte infiltration, and Th2 cytokine production in the lungs, was eliminated in Itk(-/-) mice. Collectively, our data suggest a critical role of Itk in airway mast cell degranulation in vivo that together with an impaired T cell response prevents the development of both acute and late phase inflammatory allergic reactions.

Bruton's tyrosine kinase: cell biology, sequence conservation, mutation spectrum, siRNA modifications, and expression profiling

Bruton's tyrosine kinase (Btk) is encoded by the gene that when mutated causes the primary immunodeficiency disease X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Btk is a member of the Tec family of protein tyrosine kinases (PTKs) and plays a vital, but diverse, modulatory role in many cellular processes. Mutations affecting Btk block B-lymphocyte development. Btk is conserved among species, and in this review, we present the sequence of the full-length rat Btk and find it to be analogous to the mouse Btk sequence. We have also analyzed the wealth of information compiled in the mutation database for XLA (BTKbase), representing 554 unique molecular events in 823 families and demonstrate that only selected amino acids are sensitive to replacement (P < 0.001). Although genotype-phenotype correlations have not been established in XLA, based on these findings, we hypothesize that this relationship indeed exists. Using short interfering-RNA technology, we have previously generated active constructs downregulating Btk expression. However, application of recently established guidelines to enhance or decrease the activity was not successful, demonstrating the importance of the primary sequence. We also review the outcome of expression profiling, comparing B lymphocytes from XLA-, Xid-, and Btk-knockout (KO) donors to healthy controls. Finally, in spite of a few genes differing in expression between Xid- and Btk-KO mice, in vivo competition between cells expressing either mutation shows that there is no selective survival advantage of cells carrying one genetic defect over the other. We conclusively demonstrate that for the R28C-missense mutant (Xid), there is no biologically relevant residual activity or any dominant negative effect versus other proteins.

Anti-CD63 antibodies suppress IgE-dependent allergic reactions in vitro and in vivo

High-affinity IgE receptor (FcɛRI) cross-linking on mast cells (MCs) induces secretion of preformed allergy mediators (degranulation) and synthesis of lipid mediators and cytokines. Degranulation produces many symptoms of immediate-type allergic reactions and is modulated by adhesion to surfaces coated with specific extracellular matrix (ECM) proteins. The signals involved in this modulation are mostly unknown and their contribution to allergic reactions in vivo is unclear. Here we report the generation of monoclonal antibodies that potently suppress FcɛRI-induced degranulation, but not leukotriene synthesis. We identified the antibody target as the tetraspanin CD63. Tetraspanins are membrane molecules that form multimolecular complexes with a broad array of molecules including ECM protein-binding β integrins. We found that anti-CD63 inhibits MC adhesion to fibronectin and vitronectin. Furthermore, anti-CD63 inhibits FcɛRI-mediated degranulation in cells adherent to those ECM proteins but not in nonadherent cells. Thus the inhibition of degranulation by anti-CD63 correlates with its effect on adhesion. In support of a mechanistic linkage between the two types of inhibition, anti-CD63 had no effect on FcɛRI-induced global tyrosine phosphorylation and calcium mobilization but impaired the Gab2–PI3K pathway that is known to be essential for both degranulation and adhesion. Finally, we showed that these antibodies inhibited FcɛRI-mediated allergic reactions in vivo. These properties raise the possibility that anti-CD63 could be used as therapeutic agents in MC-dependent diseases.

Phosphoinositide 3-kinase and its targets in B-cell and T-cell signaling

Phosphoinositide 3-kinase (PI3K) activation is essential for lymphocyte proliferation driven by receptors for antigen, costimulatory ligands and cytokines. The lipid products of PI3K contribute to the assembly of membrane-associated signaling complexes by promoting recruitment of selected proteins from the cytoplasm. Many proteins possess domains that are able to bind selectively to PI3K products. Different 'PI3K effector' proteins are coupled to distinct biological responses, depending on cell type and on the receptor that is engaged. In B cells and T cells, Tec-family tyrosine kinases and Akt serine/threonine kinases are emerging as crucial mediators of proliferation and survival signals downstream of PI3K. Of particular interest is recent evidence that PI3K signaling controls increases in lymphocyte size and metabolic activity that accompany cell cycle progression.

CD22: A Multifunctional Receptor That Regulates B Lymphocyte Survival and Signal Transduction

Recent advances in the study of CD22 indicate a complex role for this transmembrane glycoprotein member of the immunoglobulin superfamily in the regulation of B lymphocyte survival and proliferation. CD22 has been previously recognized as a potential lectin-like adhesion molecule that binds alpha2,6-linked sialic acid-bearing ligands and as an important regulator of B-cell antigen receptor (BCR) signaling. However, genetic studies in mice reveal that some CD22 functions are regulated by ligand binding, whereas other functions are ligand-independent and may only require expression of an intact CD22 cytoplasmic domain at the B-cell surface. Until recently, most of the functional activity of CD22 has been widely attributed to CD22's ability to recruit potent intracellular phosphatases and limit the intensity of BCR-generated signals. However, a more complex role for CD22 has recently emerged, including a central role in a novel regulatory loop controlling the CD19/CD21-Src-family protein tyrosine kinase (PTK) amplification pathway that regulates basal signaling thresholds and intensifies Src-family kinase activation after BCR ligation. CD22 is also central to the regulation of peripheral B-cell homeostasis and survival, the promotion of BCR-induced cell cycle progression, and is a potent regulator of CD40 signaling. Herein we discuss our current understanding of how CD22 governs these complex and overlapping processes, how alterations in these tightly controlled regulatory activities may influence autoimmune disease, and the current and future applications of CD22-directed therapies in oncology and autoimmunity.

Commentary: New insights into the complexity of phosphatidylinositol lipid signaling in B lymphocytes

Stimulation of B cells through the B cell antigen receptor (BCR) leads to the activation of numerous intracellular signaling pathways including phosphoinositide (PI) 3-kinases (PI3K). These generate the 3-PI lipids which in turn act as critical second messengers. Gene-targeting experiments had shown that B cells deficient in p85alpha, an adaptor protein required for PI3K function, were defective in their ability to proliferate in response to BCR stimulation. In this issue of the European Journal of Immunology, Hess et al. report analysis of intracellular signaling pathways in p85alpha-deficient B cells and show that in the absence of p85alpha there is a partial impairment in BCR-induced calcium flux, and a reduction in activation of the transcription factor NF-kappaB. Unexpectedly, they show that while the BCR-induced phosphorylation of the PI3K-dependent kinase Akt is reduced in p85alpha-deficient cells, the phosphorylation of two downstream targets of Akt -- FOXO1 and ribosomal protein S6 -- is largely unaffected. Furthermore, they show that treatment of wild-type B cells with PI3K inhibitors had a more profound effect than disruption of the p85alpha gene. Taken together, these results indicate that in the absence of p85alpha, there is still significant residual PI3K activity. These results highlight the need for careful measurement of PI3K activity in gene-targeted mice and cells, by directly measuring levels of the 3-PI lipids.

SHIP Down-Regulates FcεR1-Induced Degranulation at Supraoptimal IgE or Antigen Levels

Cross-linking of the IgE-loaded high-affinity IgE receptor (FcepsilonR1) by multivalent Ags results in mast cell activation and subsequent release of multiple proinflammatory mediators. The dose-response curve for FcepsilonR1-mediated degranulation is bell-shaped, regardless of whether the IgE or the Ag concentration is varied. Although overall calcium influx follows this bell-shaped curve, intracellular calcium release continues to increase at supraoptimal IgE or Ag concentrations. As well, overall calcium mobilization adopts more transient kinetics when stimulations are conducted with supraoptimal instead of optimal Ag concentrations. Moreover, certain early signaling events continue to increase whereas degranulation drops under supraoptimal conditions. We identified SHIP, possibly in association with the FcepsilonR1 beta-chain, as a critical negative regulator acting within the inhibitory (supraoptimal) region of the dose-response curve that shifts the kinetics of calcium mobilization from a sustained to a transient response. Consistent with this, we found that degranulation of SHIP-deficient murine bone marrow-derived mast cells was not significantly reduced at supraoptimal Ag levels. A potential mediator of SHIP action, Bruton's tyrosine kinase, did not seem to play a role within the supraoptimal suppression of degranulation. Interestingly, SHIP was found to colocalize with the actin cytoskeleton (which has been shown previously to mediate the inhibition of degranulation at supraoptimal Ag doses). These results suggest that SHIP, together with other negative regulators, restrains bone marrow-derived mast cell activation at supraoptimal IgE or Ag concentrations in concert with the actin cytoskeleton.

Mechanism of B-cell receptor-induced phosphorylation and activation of phospholipase C-gamma2

Phospholipase C-gamma2 (PLC-gamma2) plays an important role in B-cell signaling. Phosphorylation of various tyrosine residues of PLC-gamma2 has been implicated in regulation of its lipase activity. With the use of antibodies specific for each of the putative phosphorylation sites, we have now shown that PLC-gamma2 is phosphorylated on Y753, Y759, and Y1217 in response to engagement of the B-cell receptor in Ramos cells, as well as in murine splenic B cells. In cells stimulated maximally via this receptor, the extent of phosphorylation of Y1217 was three times that of Y753 or of Y759. Stimulation of Jurkat T cells or platelets via their immunoreceptors also elicited phosphorylation of Y753 and Y759 but not that of Y1217. A basal level of phosphorylation of Y753 was apparent in unstimulated lymphocytes. The extent of phosphorylation of Y753 and Y759, but not that of Y1217, correlated with the lipase activity of PLC-gamma2. Examination of the effects of various pharmacological inhibitors and of RNA interference in Ramos cells suggested that Btk is largely, but not completely, responsible for phosphorylation of Y753 and Y759, whereas phosphorylation of Y1217 is independent of Btk. Finally, phosphorylation of Y1217 and that of Y753 and Y759 occurred on different PLC-gamma2 molecules.

SHIP Family Inositol Phosphatases Interact with and Negatively Regulate the Tec Tyrosine Kinase

The Tec family of protein-tyrosine kinases (PTKs), that includes Tec, Itk, Btk, Bmx, and Txk, plays an essential role in phospholipase Cgamma (PLCgamma) activation following antigen receptor stimulation. This function requires activation of phosphatidylinositol 3-kinase (PI 3-kinase), which promotes Tec membrane localization through phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) generation. The mechanism of negative regulation of Tec family PTKs is poorly understood. In this study, we show that the inositol 5' phosphatases SHIP1 and SHIP2 interact preferentially with Tec, compared with other Tec family members. Four lines of evidence suggest that SHIP phosphatases are negative regulators of Tec. First, SHIP1 and SHIP2 are potent inhibitors of Tec activity. Second, inactivation of the Tec SH3 domain, which is necessary and sufficient for SHIP binding, generates a hyperactive form of Tec. Third, SHIP1 inhibits Tec membrane localization. Finally, constitutively targeting Tec to the membrane relieves SHIP1-mediated inhibition. These data suggest that SHIP phosphatases can interact with and functionally inactivate Tec by de-phosphorylation of local PtdIns 3,4,5-P(3) and inhibition of Tec membrane localization.

Two Distinct Tyrosine-based Motifs Enable the Inhibitory Receptor FcγRIIB to Cooperatively Recruit the Inositol Phosphatases SHIP1/2 and the Adapters Grb2/Grap

FcgammaRIIB are low-affinity receptors for IgG that contain an immunoreceptor tyrosine-based inhibition motif (ITIM) and inhibit immunoreceptor tyrosine-based activation motif (ITAM)-dependent cell activation. When coaggregated with ITAM-bearing receptors, FcgammaRIIB become tyrosyl-phosphorylated and recruit the Src homology 2 (SH2) domain-containing inositol 5'-phosphatases SHIP1 and SHIP2, which mediate inhibition. The FcgammaRIIB ITIM was proposed to be necessary and sufficient for recruiting SHIP1/2. We show here that a second tyrosine-containing motif in the intracytoplasmic domain of FcgammaRIIB is required for SHIP1/2 to be coprecipitated with the receptor. This motif functions as a docking site for the SH2 domain-containing adapters Grb2 and Grap. These adapters interact via their C-terminal SH3 domain with SHIP1/2 to form a stable receptor-phosphatase-adapter trimolecular complex. Both Grb2 and Grap are required for an optimal coprecipitation of SHIP with FcgammaRIIB, but one adapter is sufficient for the phosphatase to coprecipitate in a detectable manner with the receptors. In addition to facilitating the recruitment of SHIPs, the second tyrosine-based motif may confer upon FcgammaRIIB the properties of scaffold proteins capable of altering the composition and stability of the signaling complexes generated following receptor engagement.

Recruitment of C-terminal Src kinase by the leukocyte inhibitory receptor CD85j

The CD85j inhibitory receptor (also termed ILT2 or LIR-1) is a type-I transmembrane protein that belongs to the Ig superfamily and is expressed by different leukocyte lineages. The extracellular region of CD85j binds HLA class I molecules and its cytoplasmic domain displays four immunoreceptor tyrosine-based inhibition motifs (ITIM). Upon tyrosine phosphorylation CD85j recruits the SHP-1 tyrosine phosphatase, involved in negative signaling. In order to identify other molecules to which CD85j might interact with in a phosphotyrosine-dependent manner, a cDNA B-cell library was screened in a three-hybrid system in yeast using the CD85j cytoplasmic tail as bait in the presence of the Src-kinase c-fyn420, 531Y-F, 176R-Q mutant. In this system, the C-terminal Src kinase (Csk) was shown to interact with CD85j. Phosphorylation-dependent recruitment of Csk to the CD85j cytoplasmic tail was confirmed in CD85j-transfected mammalian cells by immunoprecipitation and Western blot analysis. Mutational analyses and phospho-peptide mapping suggested that the SH2 domain of Csk may preferentially bind to ITIM Y562 of CD85j; yet, mutation to phenylalanine of Y533, Y614, and Y644 also significantly reduced Csk recruitment by CD85j. Even though CD85j was detected in both anti-SHP1 and CSK immunoprecipitates, these two molecules did not co-precipitate together with CD85j. Our data support the possibility that Csk regulates the function of CD85j.

A role of FcgammaRIIB in the development of collagen-induced arthritis

Immune inhibitory receptors play an important role in the maintenance of adequate activation threshold of various cells in our immune system. The inhibitory Fc receptor, type IIB Fc receptor for IgG (FcgammaRIIB), is one of the critical molecules for the regulation of immune responses through antibodies. Analysis of murine models indicates that FcgammaRIIB plays an essential role in the suppression of various autoimmune disorders. Recent studies reveal the novel regulatory role of FcgammaR in the development of collagen-induced arthritis (CIA), an animal model relevant to human rheumatoid arthritis (RA). This review provides an overview of FcgammaRIIB-mediated immune regulation, highlighting the implication of FcgammaRIIB in the selection of peripheral B cell development during the CIA course.