Recruitment and phosphorylation of SH2-containing inositol phosphatase and Shc to the B-cell Fc gamma immunoreceptor tyrosine-based inhibition motif peptide motif

Activated Src kinases downstream of BCR-ABL and Flt3 induces proteasomal degradation of SHIP1 by phosphorylation of tyrosine 1021

Within the various subtypes of ALL, patients with a BCR-ABL-positive background as well as with a genetic change in the KMT2A gene have by far the worst survival probabilities. Interestingly, both subtypes are characterized by highly activated tyrosine kinases. SHIP1 serves as an important negative regulator of the PI3K/AKT signaling pathway, which is often constitutively activated in ALL. The protein expression of SHIP1 is decreased in most T-ALL and in some subgroups of B-ALL. In this study, we analyzed the expression of SHIP1 protein in detail in the context of groups with aberrant activated tyrosine kinases, namely BCR-ABL (Ph+) and Flt3 (KMT2A translocations). We demonstrate that constitutively activated Src kinases downstream of BCR-ABL and receptor tyrosine kinases reduce the SHIP1 expression in a SHIP1-Y1021 phosphorylated-dependent manner with subsequent ubiquitin marked proteasomal degradation. Inhibition of BCR-ABL (Imatinib), Flt3 (Quizartinib) or Src-Kinase-Family (Saracatinib) leads to significant reconstitution of SHIP1 protein expression. These results further support a functional role of SHIP1 as tumor suppressor protein and could be the basis for the establishment of a targeted therapy form.

Increased levels of a pro-inflammatory IgG receptor in the midbrain of people with schizophrenia

BACKGROUND

There is growing evidence that neuroinflammation may contribute to schizophrenia neuropathology. Elevated pro-inflammatory cytokines are evident in the midbrain from schizophrenia subjects, findings that are driven by a subgroup of patients, characterised as a "high inflammation" biotype. Cytokines trigger the release of antibodies, of which immunoglobulin G (IgG) is the most common. The level and function of IgG is regulated by its transporter (FcGRT) and by pro-inflammatory IgG receptors (including FcGR3A) in balance with the anti-inflammatory IgG receptor FcGR2B. Testing whether abnormalities in IgG activity contribute to the neuroinflammatory abnormalities schizophrenia patients, particularly those with elevated cytokines, may help identify novel treatment targets.

METHODS

Post-mortem midbrain tissue from healthy controls and schizophrenia cases (n = 58 total) was used to determine the localisation and abundance of IgG and IgG transporters and receptors in the midbrain of healthy controls and schizophrenia patients. Protein levels of IgG and FcGRT were quantified using western blot, and gene transcript levels of FcGRT, FcGR3A and FcGR2B were assessed using qPCR. The distribution of IgG in the midbrain was assessed using immunohistochemistry and immunofluorescence. Results were compared between diagnostic (schizophrenia vs control) and inflammatory (high vs low inflammation) groups.

RESULTS

We found that IgG and FcGRT protein abundance (relative to β-actin) was unchanged in people with schizophrenia compared with controls irrespective of inflammatory subtype. In contrast, FcGRT and FcGR3A mRNA levels were elevated in the midbrain from "high inflammation" schizophrenia cases (FcGRT; p = 0.02, FcGR3A; p < 0.0001) in comparison to low-inflammation patients and healthy controls, while FcGR2B mRNA levels were unchanged. IgG immunoreactivity was evident in the midbrain, and approximately 24% of all individuals (control subjects and schizophrenia cases) showed diffusion of IgG from blood vessels into the brain. However, the intensity and distribution of IgG was comparable across schizophrenia cases and control subjects.

CONCLUSION

These findings suggest that an increase in the pro-inflammatory Fcγ receptor FcGR3A, rather than an overall increase in IgG levels, contribute to midbrain neuroinflammation in schizophrenia patients. However, more precise information about IgG-Fcγ receptor interactions is needed to determine their potential role in schizophrenia neuropathology.

Beyond the Cell Surface: Targeting Intracellular Negative Regulators to Enhance T cell Anti-Tumor Activity

It is well established that extracellular proteins that negatively regulate T cell function, such as Cytotoxic T-Lymphocyte-Associated protein 4 (CTLA-4) and Programmed Cell Death protein 1 (PD-1), can be effectively targeted to enhance cancer immunotherapies and Chimeric Antigen Receptor T cells (CAR-T cells). Intracellular proteins that inhibit T cell receptor (TCR) signal transduction, though less well studied, are also potentially useful therapeutic targets to enhance T cell activity against tumor. Four major classes of enzymes that attenuate TCR signaling include E3 ubiquitin kinases such as the Casitas B-lineage lymphoma proteins (Cbl-b and c-Cbl), and Itchy (Itch), inhibitory tyrosine phosphatases, such as Src homology region 2 domain-containing phosphatases (SHP-1 and SHP-2), inhibitory protein kinases, such as C-terminal Src kinase (Csk), and inhibitory lipid kinases such as Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase (SHIP) and Diacylglycerol kinases (DGKs). This review describes the mechanism of action of eighteen intracellular inhibitory regulatory proteins in T cells within these four classes, and assesses their potential value as clinical targets to enhance the anti-tumor activity of endogenous T cells and CAR-T cells.

Regulation of hematopoietic cell signaling by SHIP-1 inositol phosphatase: growth factors and beyond

SHIP-1 is a hematopoietic-specific inositol phosphatase activated downstream of a multitude of receptors including those for growth factors, cytokines, antigen, immunoglobulin and toll-like receptor agonists where it exerts inhibitory control. While it is constitutively expressed in all immune cells, SHIP-1 expression is negatively regulated by the inflammatory and oncogenic micro-RNA miR-155. Knockout mouse studies have shown the importance of SHIP-1 in various immune cell subsets and have revealed a range of immune-mediated pathologies that are engendered due to loss of SHIP-1's regulatory activity, impelling investigations into the role of SHIP-1 in human disease. In this review, we provide an overview of the literature relating to the role of SHIP-1 in hematopoietic cell signaling and function, we summarize recent reports that highlight the dysregulation of the SHIP-1 pathway in cancers, autoimmune disorders and inflammatory diseases, and lastly we discuss the importance of SHIP-1 in restraining myeloid growth factor signaling.

FcγRIIB-Independent Mechanisms Controlling Membrane Localization of the Inhibitory Phosphatase SHIP in Human B Cells

SHIP is an important regulator of immune cell signaling that functions to dephosphorylate the phosphoinositide phosphatidylinositol 3,4,5-trisphosphate at the plasma membrane and mediate protein-protein interactions. One established paradigm for SHIP activation involves its recruitment to the phospho-ITIM motif of the inhibitory receptor FcγRIIB. Although SHIP is essential for the inhibitory function of FcγRIIB, it also has critical modulating functions in signaling initiated from activating immunoreceptors such as B cell Ag receptor. In this study, we found that SHIP is indistinguishably recruited to the plasma membrane after BCR stimulation with or without FcγRIIB coligation in human cell lines and primary cells. Interestingly, fluorescence recovery after photobleaching analysis reveals differential mobility of SHIP-enhanced GFP depending on the mode of stimulation, suggesting that although BCR and FcγRIIB can both recruit SHIP, this occurs via distinct molecular complexes. Mutagenesis of a SHIP-enhanced GFP fusion protein reveals that the SHIP-Src homology 2 domain is essential in both cases whereas the C terminus is required for recruitment via BCR stimulation, but is less important with FcγRIIB coligation. Experiments with pharmacological inhibitors reveal that Syk activity is required for optimal stimulation-induced membrane localization of SHIP, whereas neither PI3K or Src kinase activity is essential. BCR-induced association of SHIP with binding partner Shc1 is dependent on Syk, as is tyrosine phosphorylation of both partners. Our results indicate that FcγRIIB is not uniquely able to promote membrane recruitment of SHIP, but rather modulates its function via formation of distinct signaling complexes. Membrane recruitment of SHIP via Syk-dependent mechanisms may be an important factor modulating immunoreceptor signaling.

PSTPIP2, a Protein Associated with Autoinflammatory Disease, Interacts with Inhibitory Enzymes SHIP1 and Csk

Mutations in the adaptor protein PSTPIP2 are the cause of the autoinflammatory disease chronic multifocal osteomyelitis in mice. This disease closely resembles the human disorder chronic recurrent multifocal osteomyelitis, characterized by sterile inflammation of the bones and often associated with inflammation in other organs, such as the skin. The most critical process in the disease's development is the enhanced production of IL-1β. This excessive IL-1β is likely produced by neutrophils. In addition, the increased activity of macrophages, osteoclasts, and megakaryocytes has also been described. However, the molecular mechanism of how PSTPIP2 deficiency results in this phenotype is poorly understood. Part of the PSTPIP2 inhibitory function is mediated by protein tyrosine phosphatases from the proline-, glutamic acid-, serine- and threonine-rich (PEST) family, which are known to interact with the central part of this protein, but other regions of PSTPIP2 not required for PEST-family phosphatase binding were also shown to be indispensable for PSTPIP2 function. In this article, we show that PSTPIP2 binds the inhibitory enzymes Csk and SHIP1. The interaction with SHIP1 is of particular importance because it binds to the critical tyrosine residues at the C terminus of PSTPIP2, which is known to be crucial for its PEST-phosphatase-independent inhibitory effects in different cellular systems. We demonstrate that in neutrophils this region is important for the PSTPIP2-mediated suppression of IL-1β processing and that SHIP1 inhibition results in the enhancement of this processing. We also describe deregulated neutrophil response to multiple activators, including silica, Ab aggregates, and LPS, which is suggestive of a rather generalized hypersensitivity of these cells to various external stimulants.

A physical interaction between the adaptor proteins DOK3 and DAP12 is required to inhibit lipopolysaccharide signaling in macrophages

DNAX-activating protein of 12 kD (DAP12) is an immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor protein found in myeloid cells and natural killer cells, and it couples to various receptors that mediate either cellular activation or inhibition. DAP12 inhibits Toll-like receptor (TLR) signaling, such as that of TLR4 in response to its ligand lipopolysaccharide (LPS), as well as cytokine responses by coupling to TREM2 (triggering receptor expressed on myeloid cells 2) at the plasma membrane. Understanding the mechanisms that inhibit inflammatory responses in macrophages is important for the development of therapies to treat inflammatory diseases. We show that inhibition of LPS responses by DAP12 is mediated by the adaptor protein DOK3 (downstream of kinase 3). DOK3 physically associated with the ITAM of DAP12 through its phosphotyrosine-binding domain. In response to LPS, DOK3 was phosphorylated in a DAP12- and Src-dependent manner, which led to translocation of phosphorylated DOK3 to the plasma membrane. DOK3-deficient cells exhibited increased production of proinflammatory cytokines and activation of extracellular signal-regulated kinase (ERK). Compared to wild-type mice, DOK3-deficient mice had increased susceptibility to challenge with a sublethal dose of LPS and produced increased serum concentrations of the inflammatory cytokine tumor necrosis factor-α (TNF-α). Together, these data suggest the mechanism by which DAP12 and TREM2 inhibit LPS signaling in macrophages to prevent inflammation.

The phosphoinositide 3-kinase signaling pathway in normal and malignant B cells: activation mechanisms, regulation and impact on cellular functions

The phosphoinositide 3-kinase (PI3K) pathway is a central signal transduction axis controlling normal B cell homeostasis and activation in humoral immunity. The p110δ PI3K catalytic subunit has emerged as a critical mediator of multiple B cell functions. The activity of this pathway is regulated at multiple levels, with inositol phosphatases PTEN and SHIP both playing critical roles. When deregulated, the PI3K pathway can contribute to B cell malignancies and autoantibody production. This review summarizes current knowledge on key mechanisms that activate and regulate the PI3K pathway and influence normal B cell functional responses including the development of B cell subsets, antigen presentation, immunoglobulin isotype switch, germinal center responses, and maintenance of B cell anergy. We also discuss PI3K pathway alterations reported in select B cell malignancies and highlight studies indicating the functional significance of this pathway in malignant B cell survival and growth within tissue microenvironments. Finally, we comment on early clinical trial results, which support PI3K inhibition as a promising treatment of chronic lymphocytic leukemia.

Suppression of mast cell degranulation through a dual-targeting tandem IgE-IgG Fc domain biologic engineered to bind with high affinity to FcγRIIb

Mast cells and basophils play a central role in allergy, asthma, and anaphylaxis, as well as in non-allergic inflammatory, neurological and autoimmune diseases. Allergen-mediated cross-linking of IgE bound to FcεRI leads to cellular activation, and the low-affinity Fc receptor FcγRIIb is a key inhibitor of subsequent degranulation. FcγRIIb, when coengaged with FcεRI via allergen bound to IgE, stimulates ITIM domain-mediated inhibitory signaling that efficiently suppresses mast cell and basophil activation. To assess the therapeutic potential of directed coengagement of FcεRI and FcγRIIb in the absence of FcεRI crosslinking, we developed a fusion protein comprising the coupled Fc domains of murine IgE and human IgG1. As a key functional component of this tandem Fcε-Fcγ biologic, we engineered its IgG1 Fc domain to bind to human FcγRIIb with 100-fold enhanced affinity relative to native IgG1 Fc. Using mast cells from mice transgenic for human FcγRIIb, we show that this tandem Fc binds with high affinity to murine FcεRI and human FcγRIIb on mast cells, triggers phosphorylation of FcγRIIb, and inhibits FcεRI-dependent calcium mobilization. Control tandem Fc biologics containing a native IgG1 Fc domain or lacking binding to Fcγ receptors were markedly less active, demonstrating that the affinity-optimized tandem Fc can inhibit degranulation through stimulation of FcγRIIb signaling as well as through competition with allergen-IgE immune complex for FcεRI binding. We propose that in the context of a fully human tandem Fc biologic, high-affinity coengagement of FcεRI and FcγRIIb has potential as a novel therapy for allergy and other mast cell and basophil-mediated pathologies.

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

Positive and negative regulation of antigen receptor signaling by the Shc family of protein adapters

The Shc adapter family includes four members that are expressed as multiple isoforms and participate in signaling by a variety of cell-surface receptors. The biological relevance of Shc proteins as well as their variegated function, which relies on their highly conserved modular structure, is underscored by the distinct and dramatic phenotypic alterations resulting from deletion of individual Shc isoforms both in the mouse and in two model organisms, Drosophila melanogaster and Caenorhabditis elegans. The p52 isoform of ShcA couples antigen and cytokine receptors to Ras activation in both lymphoid and myeloid cells. However, the recognition of the spectrum of activities of p52ShcA in the immune system has been steadily expanding in recent years to other fundamental processes both at the cell and organism levels. Two other Shc family members, p66ShcA and p52ShcC/Rai, have been identified recently in T and B lymphocytes, where they antagonize survival and attenuate antigen receptor signaling. These developments reveal an unexpected and complex interplay of multiple Shc proteins in lymphocytes.

TREM2- and DAP12-dependent activation of PI3K requires DAP10 and is inhibited by SHIP1

The activation and fusion of macrophages and of osteoclasts require the adaptor molecule DNAX-activating protein of 12 kD (DAP12), which contains immunoreceptor tyrosine-based activation motifs (ITAMs). TREM2 (triggering receptor expressed on myeloid cells-2) is the main DAP12-associated receptor in osteoclasts and, similar to DAP12 deficiency, loss of TREM2 in humans leads to Nasu-Hakola disease, which is characterized by bone cysts and dementia. Furthermore, in vitro experiments have shown that deficiency in DAP12 or TREM2 leads to impaired osteoclast development and the formation of mononuclear osteoclasts. Here, we demonstrate that the ligation of TREM2 activated phosphatidylinositol 3-kinase (PI3K), extracellular signal-regulated kinase 1 (ERK1) and ERK2, and the guanine nucleotide exchange factor Vav3; induced the mobilization of intracellular calcium (Ca(2+)) and the reorganization of actin; and prevented apoptosis. The signaling adaptor molecule DAP10 played a key role in the TREM2- and DAP12-dependent recruitment of PI3K to the signaling complex. Src homology 2 (SH2) domain-containing inositol phosphatase-1 (SHIP1) inhibited TREM2- and DAP12-induced signaling by binding to DAP12 in an SH2 domain-dependent manner and preventing the recruitment of PI3K to DAP12. These results demonstrate a previously uncharacterized interaction of SHIP1 with DAP12 that functionally limits TREM2- and DAP12-dependent signaling and identify a mechanism through which SHIP1 regulates key ITAM-containing receptors by directly blocking the binding and activation of PI3K.

Fc receptor-targeted mucosal vaccination as a novel strategy for the generation of enhanced immunity against mucosal and non-mucosal pathogens

Numerous studies have demonstrated that targeting immunogens to Fcgamma receptors (FcgammaR) on antigen (Ag)-presenting cells (APC) can enhance humoral and cellular immunity in vitro and in vivo. FcgammaR are classified based on their molecular weight, IgG-Fc binding affinities, IgG subclass binding specificity, and cellular distribution and they consist of activating and inhibitory receptors. However, despite the potential advantages of targeting Ag to FcR at mucosal sites, very little is known regarding the role of FcR in mucosal immunity or the efficacy of FcR-targeted mucosal vaccines. In addition, recent work has suggested that FcRn is present in the lungs of adult mice and humans and can transport FcRn-targeted Ag to FcgammaR-bearing APC within mucosal lymphoid tissue. In this review we will discuss the need for new vaccine strategies, the potential for FcR-targeted vaccines to fill this need, the impact of activating versus inhibitory FcgammaR on FcR-targeted vaccination, the significance of focusing on mucosal immunity, as well as caveats that could impact the use of FcR targeting as a mucosal vaccine strategy.

Differential roles for the inositol phosphatase SHIP in the regulation of macrophages and lymphocytes

The SH2 domain-containing inositol 5'-phosphatase (SHIP) negatively regulates antigen, cytokine, and Fc receptor signaling pathways in immune cells. Our knowledge of the function of SHIP largely derives from in vitro studies that utilized SHIP-deficient cell lines and immune cells isolated from SHIP null mice. To avoid the pleiotropic effects observed in mice with germline deletion of SHIP, we have used the Cre-lox system to generate SHIP conditional knockout mice with deletion in specific immune cell populations. In this review we summarize our observations from mice with deletion of SHIP in lymphocyte and macrophage lineages and contrast them with earlier data gathered by the analysis of SHIP null mice.

Regulation of hematopoietic cell function by inhibitory immunoglobulin G receptors and their inositol lipid phosphatase effectors

Numerous autoimmune and inflammatory disorders stem from the dysregulation of hematopoietic cell activation. The activity of inositol lipid and protein tyrosine phosphatases, and the receptors that recruit them, is critical for prevention of these disorders. Balanced signaling by inhibitory and activating receptors is now recognized to be an important factor in tuning cell function and inflammatory potential. In this review, we provide an overview of current knowledge of membrane proximal events in signaling by inhibitory/regulatory receptors focusing on structural and functional characteristics of receptors and their effectors Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 and SH2 domain-containing inositol 5-phosphatase-1. We review use of new strategies to identify novel regulatory receptors and effectors. Finally, we discuss complementary actions of paired inhibitory and activating receptors, using Fc gammaRIIA and Fc gammaRIIB regulation human basophil activation as a prototype.

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.

Differential association of phosphatidylinositol 3-kinase, SHIP-1, and PTEN with forming phagosomes

In macrophages, enzymes that synthesize or hydrolyze phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)] regulate Fcgamma receptor-mediated phagocytosis. Inhibition of phosphatidylinositol 3-kinase (PI3K) or overexpression of the lipid phosphatases phosphatase and tensin homologue (PTEN) and Src homology 2 domain-containing inositol phosphatase (SHIP-1), which hydrolyze PI(3,4,5)P(3) to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4-bisphosphate [PI(3,4)P(2)], respectively, inhibit phagocytosis in macrophages. To examine how these enzymes regulate phagosome formation, the distributions of yellow fluorescent protein (YFP) chimeras of enzymes and pleckstrin homology (PH) domains specific for their substrates and products were analyzed quantitatively. PTEN-YFP did not localize to phagosomes, suggesting that PTEN regulates phagocytosis globally within the macrophage. SHIP1-YFP and p85-YFP were recruited to forming phagosomes. SHIP1-YFP sequestered to the leading edge and dissociated from phagocytic cups earlier than did p85-cyan fluorescent protein, indicating that SHIP-1 inhibitory activities are restricted to the early stages of phagocytosis. PH domain chimeras indicated that early during phagocytosis, PI(3,4,5)P(3) was slightly more abundant than PI(3,4)P(2) at the leading edge of the forming cup. These results support a model in which phagosomal PI3K generates PI(3,4,5)P(3) necessary for later stages of phagocytosis, PTEN determines whether those late stages can occur, and SHIP-1 regulates when and where they occur by transiently suppressing PI(3,4,5)P(3)-dependent activities necessary for completion of phagocytosis.

Effects of SHIP-1 on MMP2 Secretion and Invasion of SR3Y1 Cells

SHIP-1 is an SH2 domain containing inositol-5-phosphatase that appears to be a negative regulator of hematopoiesis. To the potential effects of SHIP-1 on MMP2 secretion and migration of cancer cells, three murine SHIP-1 mutants were made: DeltaSH2-SHIP-1, DeltaPtase-SHIP-1, DeltaCter-SHIP-1. These mutant forms were subcloned as well as the wild type (WT) of murine SHIP-1 cDNA were subcloned into pcDNA3 expression vector, then transfected into and overexpressed SHIP-1 and its mutants in a Src-transformed 3Y1 cell line (SR3Y1). The results showed that overexpression of wild type of SHIP-1 does not affect the MMP2 secretion in both SR3Y1 and 3Y1 cells, but can induce MMP9 secretion, while either WT SHIP-1, the SH2 domain, phosphatase domain, or C terminus deletion mutants could significantly block the MMP2 and MMP9 secretion in SR3Y1 cells and suppress cell invasion ability. The results confirmed SHIP-1 as a negative regulator for cell migration and invasion in transformed cells, and implied that it may function through each of its three domains.

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.

The Src Homology 2-Containing Inositol 5-Phosphatase 1 (SHIP1) is involved in CD32a signaling in human neutrophils

Phosphatidylinositol(3,4,5)triphosphate (PtdIns(3,4,5)P(3)) plays important signaling roles in immune cells, particularly in the control of activating pathways and of survival. It is formed by a family of phosphatidylinositol 3'-kinases (PI3Ks) which phosphorylate PtdIns(4,5)P(2) in vivo. In human neutrophils, the levels of PtdIns(3,4,5)P(3) increase rapidly at the leading edge of locomoting cells and at the base of the phagocytic cup during FcgammaR-mediated particle ingestion. Even though these, and other, data indicate that PtdIns(3,4,5)P(3) is involved in the control of chemotaxis and phagocytosis in human neutrophils, the mechanisms that regulate its levels have yet to be fully elucidated in these cells. We evaluated the potential implication of SHIP1 and PTEN, two lipid phosphatases that utilize PtdIns(3,4,5)P(3) as substrate, in the signaling pathways called upon in response to CD32a cross-linking. We observed that the cross-linking of CD32a resulted in a transient accumulation of PtdIns(3,4,5)P(3). CD32a cross-linking also induced the tyrosine phosphorylation of SHIP1, its translocation to the plasma membrane and its co-immunoprecipitation with CD32a. CD32a cross-linking had no effect on the level of serine/threonine phosphorylation of PTEN and did not stimulate its translocation to the plasma membrane. PP2, a Src kinase inhibitor, inhibited the tyrosine phosphorylation of SHIP1 as well as its translocation to the plasma membrane. Wortmannin, a PI3K inhibitor, had no effect on either of these two indices of activation of SHIP1. Our results indicate that SHIP1 is involved, in a Src kinase-dependent manner, in the early signaling events observed upon the cross-linking of CD32a in human neutrophils.

Macrophage pro-inflammatory response to Francisella novicida infection is regulated by SHIP

Francisella tularensis, a Gram-negative facultative intracellular pathogen infecting principally macrophages and monocytes, is the etiological agent of tularemia. Macrophage responses to F. tularensis infection include the production of pro-inflammatory cytokines such as interleukin (IL)-12, which is critical for immunity against infection. Molecular mechanisms regulating production of these inflammatory mediators are poorly understood. Herein we report that the SH2 domain-containing inositol phosphatase (SHIP) is phosphorylated upon infection of primary murine macrophages with the genetically related F. novicida, and negatively regulates F. novicida-induced cytokine production. Analyses of the molecular details revealed that in addition to activating the MAP kinases, F. novicida infection also activated the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in these cells. Interestingly, SHIP-deficient macrophages displayed enhanced Akt activation upon F. novicida infection, suggesting elevated PI3K-dependent activation pathways in absence of SHIP. Inhibition of PI3K/Akt resulted in suppression of F. novicida-induced cytokine production through the inhibition of NFkappaB. Consistently, macrophages lacking SHIP displayed enhanced NFkappaB-driven gene transcription, whereas overexpression of SHIP led to decreased NFkappaB activation. Thus, we propose that SHIP negatively regulates F. novicida-induced inflammatory cytokine response by antagonizing the PI3K/Akt pathway and suppressing NFkappaB-mediated gene transcription. A detailed analysis of phosphoinositide signaling may provide valuable clues for better understanding the pathogenesis of tularemia.

Src homology 2 (SH2)-containing 5'-inositol phosphatase localizes to podosomes, and the SH2 domain is implicated in the attenuation of bone resorption in osteoclasts

c-Src plays an important role in bone resorption by osteoclasts. Here, we show using wild-type and ship(-/-) osteoclasts that Src homology 2 (SH2)-containing 5'-inositol phosphatase (SHIP) appeared to negatively regulate bone resorption activated by c-Src. SHIP was found to localize to podosomes under the influence of c-Src, and the presence of either the amino-terminal region comprising the SH2 domain or the carboxyl-terminal region was sufficient for its localization. Although SHIP lacking a functional SH2 domain was still found in podosomes, it could not rescue the hyper-bone resorbing activity and hypersensitivity to receptor activator of nuclear factor-kappaB ligand in ship(-/-) osteoclasts, suggesting that the localization of SHIP to podosomes per se was not sufficient and the SH2 domain was indispensable for its function. Cas and c-Cbl, known to function in podosomes of osteoclasts, were identified as novel proteins binding to the SHIP SH2 domain by mass spectrometric analysis, and this interaction appeared to be dependent on the Src kinase activity. These results demonstrate that c-Src enhances the translocation of SHIP to podosomes and regulates its function there through the SH2 domain, leading to an attenuation of bone resorption.

FcgammaR-induced production of superoxide and inflammatory cytokines is differentially regulated by SHIP through its influence on PI3K and/or Ras/Erk pathways

Phagocytosis of IgG-coated particles via FcgammaR is accompanied by the generation of superoxide and inflammatory cytokines, which can cause collateral tissue damage in the absence of regulation. Molecular mechanisms regulating these phagocytosis-associated events are not known. SHIP is an inositol phosphatase that downregulates PI3K-mediated activation events. Here, we have examined the role of SHIP in FcgammaR-induced production of superoxide and inflammatory cytokines. We report that primary SHIP-deficient bone marrow macrophages produce elevated levels of superoxide upon FcgammaR clustering. Analysis of the molecular mechanism revealed that SHIP regulates upstream Rac-GTP binding, an obligatory event for superoxide production. Likewise, SHIP-deficient macrophages displayed enhanced IL-1beta and IL-6 production in response to FcgammaR clustering. Interestingly, whereas IL-6 production required activation of both PI3K and Ras/Erk pathways, IL-1beta production was dependent only on Ras/Erk activation, suggesting that SHIP may also regulate the Ras/Erk pathway in macrophages. Consistently, SHIP-deficient macrophages displayed enhanced activation of Erk upon FcgammaR clustering. Inhibition of Ras/Erk or PI3K suppressed the enhanced production of IL-6 in SHIP-deficient macrophages. In contrast, inhibition of Ras/Erk, but not PI3K, suppressed IL-1beta production in these cells. Together, these data demonstrate that SHIP regulates phagocytosis-associated events through the inhibition of PI3K and Ras/Erk pathways.

B cell antigen receptor and CD40 differentially regulate CD22 tyrosine phosphorylation

Cell surface molecules on lymphocytes positively or negatively modulate the Ag receptor signaling, and thus regulate the fate of the cell. CD22 is a B cell-specific cell surface protein that contains multiple ITIMs in the cytoplasmic tail, and critically regulates B cell activation and survival. CD22 regulation on B cell signaling is complex because CD22 can have both positive and negative roles in various contexts. We generated phosphospecific polyclonal Abs reacting four major CD22 tyrosine motifs (Y762, Y807, Y822, and Y842) and analyzed the pattern and intensity of phosphorylation of these tyrosine residues. The tyrosine motifs, Y762, Y822, and Y842, are considered as ITIM, whereas the other, Y807, is suggested to be important for Grb2 recruitment. Approximately 10% of the four tyrosine residues were constitutively phosphorylated. Upon anti-IgM ligation, CD22 Y762 underwent most rapid phosphorylation, whereas all four tyrosine residues were eventually phosphorylated equally at approximately 35% of all CD22 molecules in the cell. By contrast, anti-CD40 stimulation specifically up-regulated anti-IgM-induced phosphorylation of tyrosines within two ITIM motifs, Y762 and Y842, which was consistent with in vivo finding of the negative role of CD22 in CD40 signaling. Thus, CD22 phosphorylation is not only quantitatively but also qualitatively regulated by different stimulations, which may determine the outcome of B cell signaling.

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.

Molecular analysis of expression and function of hFcgammaRIIbl and b2 isoforms in myeloid cells

The inhibitory receptor FcgammaRIIb becomes tyrosine phosphorylated and associates with the inositol phosphatase SHIP to downregulate phagocytosis. The two splice variants of FcgammaRIIb, b1 and b2, are differentially expressed in hematopoetic cells. Both isoforms of FcgammaRIIb are expressed in human myeloid cells although FcgammaRIIb2 predominates. In murine B cells FcgammaRIIb2 associates with clathrin-coated pits and undergoes endocytosis, whereas FcgammaRIIbl is excluded from the coated pits, indicating that the two isoforms serve partially differing functions. In humans, there are conflicting reports with regard to the ability of FcgammaRIIb2 to become tyrosine phosphorylated, and the functional capacities of the two isoforms are poorly understood. We, and others, have previously reported that the expression of FcgammaRIIb is upregulated in human monocytes by the anti-inflammatory cytokine IL-4. Here, we extend these findings to demonstrate that the IL-4-induced upregulation of FcgammaRIIb is synergistically enhanced by the addition of IL-10, both at the protein and the mRNA level. The upregulated receptors are functional as assessed by their ability to become tyrosine phosphorylated and to downregulate phagocytosis. Interestingly, both b1 and b2 isoforms are upregulated by anti-inflammatory cytokines. Transfection experiments expressing human FcgammaRIIbl or b2 in Raw 264.7 murine macrophage cells revealed that both isoforms are tyrosine phosphorylated and promote SHIP phosphorylation. Finally, both b1 and b2 isoforms of FcgammaRIIb downregulate phagocytosis to a similar extent. Thus we conclude that FcgammaRIIbl and b2 are both functional inhibitory receptors in the phagocytic process.

Vav activation and function as a rac guanine nucleotide exchange factor in macrophage colony-stimulating factor-induced macrophage chemotaxis

Signal transduction mediated by phosphatidylinositol 3-kinase (PI 3-kinase) is regulated by hydrolysis of its products, a function performed by the 145-kDa SH2 domain-containing inositol phosphatase (SHIP). Here, we show that bone marrow macrophages of SHIP(-/-) animals have elevated levels of phosphatidylinositol 3,4,5-trisphosphate [PI (3,4,5)P(3)] and displayed higher and more prolonged chemotactic responses to macrophage colony-stimulating factor (M-CSF) and elevated levels of F-actin relative to wild-type macrophages. We also found that the small GTPase Rac was constitutively active and its upstream activator Vav was constitutively phosphorylated in SHIP(-/-) macrophages. Furthermore, we show that Vav in wild-type macrophages is recruited to the membrane in a PI 3-kinase-dependent manner through the Vav pleckstrin homology domain upon M-CSF stimulation. Dominant inhibitory mutants of both Rac and Vav blocked chemotaxis. We conclude that Vav acts as a PI 3-kinase-dependent activator for Rac activation in macrophages stimulated with M-CSF and that SHIP regulates macrophage M-CSF-triggered chemotaxis by hydrolysis of PI (3,4,5)P(3).

Cytokine-mediated regulation of activating and inhibitory Fcγ receptors in human monocytes

Fc gamma receptors (Fc gammaR) trigger inflammatory reactions in response to immunoglobulin-opsonized pathogens and antigen-antibody complexes. The coordinate expression of activating and inhibitory Fc gammaR ensures the homeostasis of immune complex-driven inflammatory responses. In this study, we used antibodies with preferential binding for activating Fc gammaRIIa and inhibitory Fc gammaRIIb receptors to investigate the expression and regulation of Fc gammaRII isoforms in human monocytes. Cross-linking of Fc gammaRIIa triggered phagocytosis and cytokine production. Cross-linking of Fc gammaRIIb was associated with phosphorylation of the immunoreceptor tyrosine-based inhibitory motif and with a marked reduction in monocyte effector functions. Our study revealed that tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-10, and IL-13 altered the transcriptional activity of the Fc gammaRIIB promoter in transfected cell lines and skewed the balance of activating versus inhibitory Fc gammaR in human monocytes. TNF-alpha decreased the expression of inhibitory Fc gammaRIIb. IL-10 up-regulated all classes of Fc gammaR and induced alternative activation in monocytes, an effect that was synergistic with that of TNF-alpha. In contrast, IL-4 and IL-13, in combination with TNF-alpha, decreased the expression of activating Fc gammaR and markedly down-regulated Fc gammaR-mediated function. Our findings suggest that the cytokine milieu can induce changes in the relative expression of Fc gammaR with opposing function and thus, may regulate the amplitude of Fc gammaR-mediated uptake and inflammation.

Loss of SHIP and CIS recruitment to the granulocyte colony-stimulating factor receptor contribute to hyperproliferative responses in severe congenital neutropenia/acute myelogenous leukemia

Mutations in the G-CSF receptor (G-CSFR) in patients with severe congenital neutropenia (SCN) are postulated to contribute to transformation to acute myelogenous leukemia (AML). These mutations result in defective receptor internalization and sustained cellular activation, suggesting a loss of negative signaling by the G-CSFR. In this paper we investigated the roles of SHIP and cytokine-inducible Src homology 2 protein (CIS) in down-modulating G-CSFR signals and demonstrate that loss of their recruitment as a consequence of receptor mutations leads to aberrant signaling. We show that SHIP binds to phosphopeptides corresponding to Tyr744 and Tyr764 in the G-CSFR and that Tyr764 is required for in vivo phosphorylation of SHIP and the formation of SHIP/Shc complexes. Cells expressing a G-CSFR form lacking Tyr764 exhibited hypersensitivity to G-CSF and enhanced proliferation, but to a lesser degree than observed with the most common mutant G-CSFR form in patients with SCN/AML, prompting us to investigate whether suppressor of cytokine signaling proteins also down-modulate G-CSFR signals. G-CSF was found to induce the expression of CIS and of CIS bound to phosphopeptides corresponding to Tyr729 and Tyr744 of the G-CSFR. The expression of CIS was prolonged in cells with the SCN/AML mutant G-CSFR lacking Tyr729 and Tyr744, which also correlated with increased G-CSFR expression. These findings suggest that SHIP and CIS interact with distal phosphotyrosine residues in the G-CSFR to negatively regulate G-CSFR signaling by limiting proliferation and modulating surface expression of the G-CSFR, respectively. Novel therapeutic approaches targeting inhibitory pathways that limit G-CSFR signaling may have promise in the treatment of patients with SCN/AML.

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.

Lipopolysaccharide-Induced Macrophage Inflammatory Response Is Regulated by SHIP

LPS stimulates monocytes/macrophages through TLR4, resulting in the activation of a series of signaling events that potentiate the production of inflammatory mediators. Recent reports indicated that the inflammatory response to LPS is diminished by PI3K, through the activation of the serine/threonine kinase Akt. SHIP is an inositol phosphatase that can reverse the activation events initiated by PI3K, including the activation of Akt. However, it is not known whether SHIP is involved in TLR4 signaling. In this study, we demonstrate that LPS stimulation of Raw 264.7 mouse macrophage cells induces the association of SHIP with lipid rafts, along with IL-1R-associated kinase. In addition, SHIP is tyrosine phosphorylated upon LPS stimulation. Transient transfection experiments analyzing the function of SHIP indicated that overexpression of a wild-type SHIP, but not the SHIP Src homology 2 domain-lacking catalytic activity, up-regulates NF-kappaB-dependent gene transcription in response to LPS stimulation. These results suggest that SHIP positively regulates LPS-induced activation of Raw 264.7 cells. To test the validity of these observations in primary macrophages, LPS-induced events were compared in bone marrow macrophages derived from SHIP(+/+) and SHIP(-/-) mice. Results indicated that LPS-induced MAPK phosphorylation is enhanced in SHIP(+/+) cells, whereas Akt phosphorylation is enhanced in SHIP(-/-) cells compared with SHIP(+/+) cells. Finally, LPS-induced TNF-alpha and IL-6 production was significantly lower in SHIP(-/-) bone marrow-derived macrophages. These results are the first to demonstrate a role for SHIP in TLR4 signaling, and propose that SHIP is a positive regulator of LPS-induced inflammation.

The role of SHIP in cytokine-induced signaling

The phosphatidylinositol (PI)-3 kinase (PI3K) pathway plays a central role in regulating many biological processes via the generation of the key second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P3). This membrane-associated phospholipid, which is rapidly, albeit transiently, synthesized from PI-4,5-P2 by PI3K in response to a diverse array of extracellular stimuli, attracts pleckstrin homology (PH) domain-containing proteins to membranes to mediate its many effects. To ensure that the activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed tumor suppressor PTEN hydrolyzes PI-3,4,5-P3 back to PI-4,5-P2 while the 145-kDa hemopoietic-restricted SH2-containing inositol 5'- phosphatase, SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP (sSHIP) and the more widely expressed 150-kDa SHIP2 hydrolyze PI-3,4,5-P3 to PI-3,4-P2. In this review we will concentrate on the properties of the three SHIPs, with special emphasis being placed on the role that SHIP plays in cytokine-induced signaling.

SHIP, SHIP2, and PTEN activities are regulated in vivo by modulation of their protein levels: SHIP is up-regulated in macrophages and mast cells by lipopolysaccharide

The phosphatidylinositol-3 kinase (PI3K) pathway plays a central role in regulating numerous biologic processes, including survival, adhesion, migration, metabolic activity, proliferation, differentiation, and end cell activation through the generation of the potent second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P(3)). To ensure that activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed 54-kDa tumor suppressor PTEN hydrolyzes PI-3,4,5-P(3) to PI-4,5-P(2), whereas the 145-kDa hematopoietic-restricted SH2-containing inositol 5'-phosphatase SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP sSHIP, and the more widely expressed 150-kDa SHIP2 break it down to PI-3,4-P(2). In this review, we focus on the properties of these phospholipid phosphatases and summarize recent data showing that the activities of these negative regulators often are modulated by simply altering their protein levels. We also highlight the critical role that SHIP plays in lipopolysaccharide-induced macrophage activation and in endotoxin tolerance.

Src homology 2-containing 5-inositol phosphatase (SHIP) suppresses an early stage of lymphoid cell development through elevated interleukin-6 production by myeloid cells in bone marrow

The Src homology (SH)2–containing inositol 5-phosphatase (SHIP) negatively regulates a variety of immune responses through inhibitory immune receptors. In SHIP^−/− animals, we found that the number of early lymphoid progenitors in the bone marrow was significantly reduced and accompanied by expansion of myeloid cells. We exploited an in vitro system using hematopoietic progenitors that reproduced the in vivo phenotype of SHIP^−/− mice. Lineage-negative marrow (Lin⁻) cells isolated from wild-type mice failed to differentiate into B cells when cocultured with those of SHIP^−/− mice. Furthermore, culture supernatants of SHIP^−/− Lin⁻ cells suppressed the B lineage expansion of wild-type lineage-negative cells, suggesting the presence of a suppressive cytokine. SHIP^−/− Lin⁻ cells contained more IL-6 transcripts than wild-type Lin⁻ cells, and neutralizing anti–IL-6 antibody rescued the B lineage expansion suppressed by the supernatants of SHIP^−/− Lin⁻ cells. Finally, we found that addition of recombinant IL-6 to cultures of wild-type Lin⁻ bone marrow cells reproduced the phenotype of SHIP^−/− bone marrow cultures: suppression of B cell development and expansion of myeloid cells. The results identify IL-6 as an important regulatory cytokine that can suppress B lineage differentiation and drive excessive myeloid development in bone marrow.

The inositol 5'-phosphatase SHIP-1 and the Src kinase Lyn negatively regulate macrophage colony-stimulating factor-induced Akt activity

Upon encountering macrophage colony-stimulating factor (M-CSF), human monocytes undergo a series of cellular signaling events leading to an increase in Akt activity. However, the regulation of these events is not completely understood. Because the inositol 5'-phosphatase SHIP-1 is an important regulator of intracellular levels of phosphatidylinositol 3,4,5-trisphosphate, an important second messenger necessary for Akt activation, we hypothesized that SHIP-1 was involved in the regulation of M-CSF receptor (M-CSF-R)-induced Akt activation. In the human monocytic cell line, THP-1, SHIP-1 became tyrosine-phosphorylated following M-CSF activation in a Src family kinase-dependent manner. Transfection of 3T3-Fms cells, which express the human M-CSF-R, with wild-type SHIP-1 showed that SHIP-1 was necessary for the negative regulation of M-CSF-induced Akt activation. In THP-1 cells, SHIP-1 bound Lyn, independent of the kinase activity of Lyn, following M-CSF activation. Utilizing a glutathione S-transferase fusion protein, we found that SHIP-1 bound to Lyn via the SHIP-1 Src homology 2 domain. Furthermore, transfection of THP-1 cells with a wild-type SHIP-1 construct reduced NF-kappaB-dependent transcriptional activation of a reporter gene, whereas a SHIP-1 Src homology 2 domain construct resulted in an increase in NF-kappaB activation. Additionally, in 3T3-Fms cells, Lyn enhanced the ability of SHIP-1 to regulate Akt activation by stabilizing SHIP-1 at the cellular membrane. Finally, macrophages isolated from both SHIP-1- and Lyn-deficient mice exhibited enhanced Akt phosphorylation following M-CSF stimulation. These data provide the first evidence of the involvement of both SHIP-1 and Lyn in the negative regulation of M-CSF-R-induced Akt activation.

How do inhibitory phosphatases work?

We present a hypothesis regarding the mode of induction of the inhibitory phosphatases SHP-1 and SHIP in hematopoietic cells. One mode is a general one in which the phosphatase regulates but does not abort signal transduction and biology. Regulator phosphatases are induced by directly or indirectly engaging the amino acid motifs present in the activating receptor, and act to control the biochemical and biological output. The other mode of induction is a specific one, which critically involves paired co-clustering of activating and inhibitory receptors. Phosphatases working in this way act only under conditions of paired co-clustering of activating and inhibitory receptors, and directly bind amino acid motifs present in the inhibitory receptor. However, this mode of induction is apparently more efficient, as cellular activation is completely aborted. This review presents several examples of each mode of inhibition and speculates on their mechanisms.

The Src homology 2 domain-containing inositol 5-phosphatase negatively regulates Fcgamma receptor-mediated phagocytosis through immunoreceptor tyrosine-based activation motif-bearing phagocytic receptors

Molecular mechanisms by which the Src homology 2 domain-containing inositol 5-phosphatase (SHIP) negatively regulates phagocytosis in macrophages are unclear. We addressed the issue using bone marrow-derived macrophages from FcgammaR- or SHIP-deficient mice. Phagocytic activities of macrophages from FcgammaRII(b)(-/-) and SHIP(-/-) mice were enhanced to a similar extent, relative to those from wild type. However, calcium influx was only marginally affected in FcgammaRII(b)(-/-), but greatly enhanced in SHIP(-/-) macrophages. Furthermore, SHIP was phosphorylated on tyrosine residues upon FcgammaR aggregation even in macrophages from FcgammaRII(b)(-/-) mice or upon clustering of a chimeric receptor containing CD8 and the immunoreceptor tyrosine-based activation motif (ITAM)-bearing gamma-chain or human-restricted FcgammaRIIa. These findings indicate that, unlike B cells, SHIP is efficiently phosphorylated in the absence of an immunoreceptor tyrosine-based inhibition motif (ITIM)-bearing receptor. We further demonstrate that SHIP directly bound to phosphorylated peptides derived from FcgammaRIIa with a high affinity, comparable to that of FcgammaRII(b). Lastly, FcgammaRIIa-mediated phagocytosis was significantly enhanced in THP-1 cells overexpressing dominant-negative form of SHIP in the absence of FcgammaRII(b). These results indicate that SHIP negatively regulates FcgammaR-mediated phagocytosis through all ITAM-containing IgG receptors using a molecular mechanism distinct from that in B cells.

Src homology 2 domain-containing inositol polyphosphate phosphatase regulates NF-kappa B-mediated gene transcription by phagocytic Fc gamma Rs in human myeloid cells

FcgammaR-mediated phagocytosis is accompanied by the generation of tissue-damaging products such as inflammatory cytokines and reactive oxygen species. Hence, the phagocytic response must be a tightly regulated process. Recent studies have established that clustering FcgammaR on human myeloid cells causes tyrosine phosphorylation of Src homology 2 domain-containing inositol polyphosphate phosphatase (SHIP). However, it is not known how these immunoreceptor tyrosine-based activation motif (ITAM)-bearing phagocytic FcgammaR activate SHIP, or whether the activation of SHIP by ITAMs has any functional relevance. Experiments addressing the mechanism of SHIP association with ITAMs have been done in in vitro systems using phosphopeptides. In this study we undertook to dissect the molecular mechanism by which SHIP associates with the native ITAM-FcgammaR and becomes phosphorylated. In this report we provide evidence that first, SHIP is indeed phosphorylated by ITAM-FcgammaR, using cell systems that lack FcgammaRIIb expression; second, coimmunoprecipitation experiments demonstrate that SHIP associates with native ITAM-bearing FcgammaRIIa in vivo; and third, phosphorylation of SHIP by FcgammaRIIa is inhibited by overexpressing either the SHIP Src homology 2 domain or a dominant negative mutant of Shc. In contrast, SHIP phosphorylation was not inhibited by a dominant negative mutant of Grb2. We extend these observations to show that SHIP activation by ITAM-FcgammaR down-regulates NF-kappaB-induced gene transcription. These findings both provide a molecular mechanism for SHIP association with native ITAM-bearing receptors and demonstrate that SHIP association with ITAM-FcgammaR serves to regulate gene expression during the phagocytic process.

Regulation of mast-cell and basophil function and survival by IgE

Mast cells and basophils are important effector cells in T helper 2 (T(H)2)-cell-dependent, immunoglobulin-E-associated allergic disorders and immune responses to parasites. The crosslinking of IgE that is bound to the high-affinity receptor Fc epsilon RI with multivalent antigen results in the aggregation of Fc epsilon RI and the secretion of products that can have effector, immunoregulatory or autocrine effects. This response can be enhanced markedly in cells that have been exposed to high levels of IgE, which results in the increased surface expression of Fc epsilon RI. Moreover, recent work indicates that monomeric IgE (in the absence of crosslinking) can render mast cells resistant to apoptosis induced by growth-factor deprivation in vitro and, under certain circumstances, can induce the release of cytokines. So, the binding of IgE to Fc epsilon RI might influence mast-cell and basophil survival directly or indirectly, and can also regulate cellular function.

SHIP represses mast cell activation and reveals that IgE alone triggers signaling pathways which enhance normal mast cell survival

The hemopoietic specific, Src homology 2-containing inositol 5' phosphatase (SHIP) hydrolyzes the phosphatidylinositol (PI)-3-kinase generated second messenger, PI-3,4,5-trisphosphate (PIP(3)), to PI-3,4-bisphosphate (PI-3,4-P(2)) in normal bone marrow derived mast cells (BMMCs). As a consequence, SHIP negatively regulates IgE+antigen (Ag)-induced degranulation as well as leukotriene and inflammatory cytokine production. Interestingly, in the absence of SHIP, BMMCs degranulate extensively with IgE alone, i.e. without Ag, suggesting that IgE alone is capable of stimulating signaling in normal BMMCs and that SHIP prevents this signaling from progressing to degranulation. To test this, we compared signaling events triggered by monomeric IgE versus IgE+Ag in normal BMMCs and found that multiple pathways are triggered by monomeric IgE alone and, while they are in general weaker than those stimulated by IgE+Ag, they are more prolonged. Moreover, while SHIP prevents this IgE-induced signalling from progressing to degranulation or leukotriene production it allows sufficient production of autocrine acting cytokines, in part by activation of NFkappaB, to enhance BMMC survival. Interestingly, the activation of NFkappaB and the level of cytokines produced are far higher with IgE than with IgE+Ag. Moreover, IgE alone maintains Bcl-X(L) levels and enhances the adhesion of BMMCs to fibronectin and this likely enhances their survival still further.

CD19, CD21, and CD22: multifaceted response regulators of B lymphocyte signal transduction

B lymphocyte development and function depend upon the activity of intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted, amplified, fine-tuned, or suppressed through the precise actions of specialized cell surface coreceptors, or "response regulators," that inform B cells of their extracellular environment. Important cell surface response regulators include the CD19/CD21 complex, CD22, and CD72. CD19 establishes a novel Src-family protein tyrosine kinase (PTK) amplification loop that regulates basal signaling thresholds and intensifies Src-family PTK activation following BCR ligation. In turn, CD22 limits the intensity of CD19-dependent, BCR-generated signals through the recruitment of potent phosphotyrosine and phosphoinositide phosphatases. Herein we discuss our current understanding of how CD19/CD21 and CD22 govern the emergence and intensity of BCR-mediated signals, and how alterations in these tightly controlled regulatory activities contribute to autoimmunity in mice and humans.

The role of SHIP in mast cell degranulation and IgE-induced mast cell survival

Atopic disorders are on the increase in the Western world and are due, at least in part, to an overactive mast cell response. A better understanding of the intracellular signalling pathways that regulate both mast cell degranulation and the secretion of arachidonic acid metabolites and inflammatory cytokines could help in the treatment of these disorders. The src homology 2-containing inositol-polyphosphate 5'-phosphatase, SHIP, has been shown to be a key 'gatekeeper' of mast cell degranulation. SHIP prevents degranulation from occuring when IgE alone binds to the high-affinity receptor for IgE (FcvarepsilonR1), SHIP restrains it when IgE-bound FcvarepsilonR1 are engaged by multivalent allergens, and SHIP inhibits it when an IgG against the same allergen co-clusters the inhibitory low-affinity receptor for IgG (FcgammaRIIB) with the IgE receptor. SHIP acts as a negative regulator of degranulation by hydrolyzing phosphatidylinositol-3,4,5-trisphosphate, a second messenger generated in activated cells by phosphatidylinositol 3-kinase. Our finding that binding of only IgE to the FcvarepsilonR1 of SHIP-deficient mast cells results in massive degranulation, led us to investigate the signalling pathways that are triggered in normal murine bone marrow-derived mast cells by monomeric IgE. We report here that monomeric IgE activates signalling pathways resulting in mast cell survival, without stimulating degranulation or proliferation. These studies demonstrate that mast cell sensitization by IgE is an active rather than a passive process.

Downstream of kinase, p62(dok), is a mediator of Fc gamma IIB inhibition of Fc epsilon RI signaling

The low-affinity receptor for IgG, Fc gamma RIIB, is expressed widely in the immune system and functions to attenuate Ag-induced immune responses. In mast cells, coaggregation of Fc gamma RIIB with the high-affinity IgE receptor, Fc epsilon RI, leads to inhibition of Ag-induced degranulation and cytokine production. Fc gamma RIIB inhibitory activity requires a conserved motif within the Fc gamma RIIB cytoplasmic domain termed the immunoreceptor tyrosine-based inhibition motif. When coaggregated with an activating receptor (e.g., Fc epsilon RI, B cell Ag receptor), Fc gamma RIIB is rapidly phosphorylated on tyrosine and recruits the SH2 domain-containing inositol 5-phosphatase (SHIP). However, the mechanisms by which SHIP mediates Fc gamma RIIB inhibitory function in mast cells remain poorly defined. In this report we demonstrate that Fc gamma RIIB coaggregation with Fc epsilon RI stimulates enhanced SHIP tyrosine phosphorylation and association with Shc and p62(dok). Concurrently, enhanced p62(dok) tyrosine phosphorylation and association with RasGAP are observed, suggesting that SHIP may mediate Fc gamma RIIB inhibitory function in mast cells via recruitment of p62(dok) and RasGAP. Supporting this hypothesis, recruitment of p62(dok) to Fc epsilon RI is sufficient to inhibit Fc epsilon RI-induced calcium mobilization and extracellular signal-regulated kinase 1/2 activation. Interestingly, both the amino-terminal pleckstrin homology and phosphotyrosine binding domains and the carboxyl-terminal proline/tyrosine-rich region of p62(dok) can mediate inhibition, suggesting activation of parallel downstream signaling pathways that converge at extracellular signal-regulated kinase 1/2 activation. Finally, studies using gene-ablated mice indicate that p62(dok) is dispensable for Fc gamma RIIB inhibitory signaling in mast cells. Taken together, these data suggest a role for p62(dok) as a mediator of Fc gamma RIIB inhibition of Fc epsilon RI signal transduction in mast cells.

Activation of SHIP by NADPH oxidase-stimulated Lyn leads to enhanced apoptosis in neutrophils

Neutrophils undergo rapid spontaneous apoptosis. Multiple antiapoptotic stimuli can inhibit this process via activation of the Akt pathway. However, despite no such effect singly, combined anti- and proapoptotic stimuli inhibit Akt activity, leaving the cells susceptible to accelerated apoptosis. The blockade of Akt activation depended on reduced phosphoinositide 3,4,5-trisphosphate levels but not decreased phosphatidylinositol 3-kinase activity, thus implicating the involvement of an inositol phosphatase. Evidence for SHIP involvement was provided by SHIP localization to membrane receptors and subsequent activation along with the observed inability of SHIP -/- neutrophils to exhibit enhanced apoptosis with the stimulus combination. Activation of SHIP was found to depend on Lyn activation, and this, in turn, required NADPH oxidase. Neutrophils from chronic granulomatous disease patients and Lyn -/- mice no longer responded to the combined stimuli. Thus, we propose a role for oxidants and Lyn in SHIP regulation and suggest a novel mechanism for regulating neutrophil apoptosis.