Integrin signaling in neutrophils and macrophages uses adaptors containing immunoreceptor tyrosine-based activation motifs

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.

DAP12 and CD11b contribute to the microglial-induced death of dopaminergic neurons in vitro but not in vivo in the MPTP mouse model of Parkinson's disease

Background

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms orchestrated by microglial cell-mediated inflammatory processes. Although the mechanisms and molecular network underlying this deleterious cross-talk between DN and microglial cells remain largely unknown, previous work indicates that, upon DN injury, activation of the β2 integrin subunit CD11b is required for microglia-mediated DN cell death. Interestingly, during brain development, the CD11b integrin is also involved in microglial induction of neuronal apoptosis and has been shown to act in concert with the DAP12 immunoreceptor. Whether such a developmental CD11b/DAP12 pathway could be reactivated in a pathological context such as PD and play a role in microglia-induced DN cell death is a tantalizing hypothesis that we wished to test in this study.

Methods

To test the possibility that DAP12 could be involved in microglia-associated DN injury, we used both in vitro and in vivo toxin-based experimental models of PD recapitulating microglial-mediated non-cell autonomous mechanisms of DN cell death. In vitro, enriched mesencephalic neuronal/microglial co-cultures were exposed to the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) whereas in vivo, mice were administrated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) according to acute or subchronic mode. Mice deficient for DAP12 or CD11b were used to determine the pathological function of the CD11b/DAP12 pathway in our disease models.

Results

Our results show that DAP12 and CD11b partially contribute to microglia-induced DN cell death in vitro. Yet, in vivo, mice deficient for either of these factors develop similar neuropathological alterations as their wild-type counterparts in two different MPTP mouse models of PD.

Conclusion

Overall, our data suggest that DAP12 and CD11b contribute to microglial-induced DN cell death in vitro but not in vivo in the MPTP mouse model of PD. Therefore, the CD11b/DAP12 pathway may not be considered as a promising therapeutic target for PD.

Distinct roles for neutrophils and dendritic cells in inflammation and autoimmunity in motheaten mice

The motheaten mouse has long served as a paradigm for complex autoimmune and inflammatory disease. Null mutations in Ptpn6, which encodes the nonreceptor protein-tyrosine phosphatase Shp1, cause the motheaten phenotype. However, Shp1 regulates multiple signaling pathways in different hematopoietic cell types, so the cellular and molecular mechanism of autoimmunity and inflammation in the motheaten mouse has remained unclear. By using floxed Ptpn6 mice, we dissected the contribution of innate immune cells to the motheaten phenotype. Ptpn6 deletion in neutrophils resulted in cutaneous inflammation, but not autoimmunity, providing an animal model of human neutrophilic dermatoses. By contrast, dendritic cell deletion caused severe autoimmunity, without inflammation. Genetic and biochemical analysis showed that inflammation was caused by enhanced neutrophil integrin signaling through Src-family and Syk kinases, whereas autoimmunity resulted from exaggerated MyD88-dependent signaling in dendritic cells. Our data demonstrate that disruption of distinct Shp1-regulated pathways in different cell types combine to cause motheaten disease.

Modulation by Syk of Bcl-2, calcium and the calpain-calpastatin proteolytic system in human breast cancer cells

Syk is a 72kDa non-receptor tyrosine kinase that is best characterized in hematopoietic cells. While Syk is pro-tumorigenic in some cancer cell types, it also has been reported as a negative regulator of metastatic cell growth in others. An examination of the RelA (p65) subunit of NF-κB expressed in MCF7 breast cancer cells indicated that either treatment with pervanadate or stable expression of Syk protected RelA from calpain-mediated proteolysis. Similar results were observed with the tyrosine phosphatase, PTP1B, another sensitive calpain substrate. The activity of calpain in MCF7 cell lysates was inhibited by both treatment with hydrogen peroxide and expression of Syk, the former due to oxidative inactivation of calpain and the latter to enhanced expression of calpastatin (CAST), the endogenous calpain inhibitor. The level of CAST was elevated in the cytosolic fraction of Syk-positive breast cancer cells resulting in more CAST present in complex with calpain in cell lysates. The high levels of CAST coincided with elevated basal levels of calcium-and of intracellular calpain activity-in Syk-expressing cells resulting from decreased levels of Bcl-2, an inhibitor of IP3-receptor-mediated calcium release. The inhibition of cellular calpain stimulated the Syk-mediated enhancement of NF-κB induced by TNF-α, enhanced tyrosine phosphorylation resulting from integrin crosslinking, and increased the localization of Syk to the plasma membrane.

Collagen VI in cancer and its biological mechanisms

Collagen VI is a widely distributed extracellular matrix protein highly expressed in a variety of cancers that favors tumor growth and progression. A growing number of studies indicate that collagen VI directly affects malignant cells by acting on the Akt-GSK-3β-β-catenin-TCF/LEF axis, enhancing the production of protumorigenic factors and inducing epithelial-mesenchymal transition. Moreover, it affects the tumor microenvironment by increasing the recruitment of macrophages and endothelial cells, thus promoting tumor inflammation and angiogenesis. Furthermore, collagen VI promotes chemotherapy resistance and can be regarded as a potential biomarker for cancer diagnosis. Collectively, these findings strongly support a role for collagen VI as an important regulator in tumors and provide new targets for cancer therapies.

The response of human macrophages to β-glucans depends on the inflammatory milieu

Background

β-glucans are fungal cell wall components that bind to the C-type lectin-like receptor dectin-1. Polymorphisms of dectin-1 gene are associated with susceptibility to invasive fungal infection and medically refractory ulcerative colitis. The purpose of this study has been addressing the response of human macrophages to β-glucans under different conditions mimicking the composition of the inflammatory milieu in view of the wide plasticity and large range of phenotypical changes showed by these cells, and the relevant role of dectin-1 in several pathophysiological conditions.

Principal Findings

Serum-differentiated macrophages stimulated with β-glucans showed a low production of TNFα and IL-1β, a high production of IL-6 and IL-23, and a delayed induction of cyclooxygenase-2 and PGE₂ biosynthesis that resembled the responses elicited by crystals and those produced when phagosomal degradation of the phagocytic cargo increases ligand access to intracellular pattern recognition receptors. Priming with a low concentration of LPS produced a rapid induction of cyclooxygenase-2 and a synergistic release of PGE₂. When the differentiation of the macrophages was carried out in the presence of M-CSF, an increased expression of dectin-1 B isoform was observed. In addition, this treatment made the cells capable to release arachidonic acid in response to β-glucan.

Conclusions

These results indicate that the macrophage response to fungal β-glucans is strongly influenced by cytokines and microbial-derived factors that are usual components of the inflammatory milieu. These responses can be sorted into three main patterns i) an elementary response dependent on phagosomal processing of pathogen-associated molecular patterns and/or receptor-independent, direct membrane binding linked to the immunoreceptor tyrosine-based activation motif-bearing transmembrane adaptor DNAX-activating protein 12, ii) a response primed by TLR4-dependent signals, and iii) a response dependent on M-CSF and dectin-1 B isoform expression that mainly signals through the dectin-1 B/spleen tyrosine kinase/cytosolic phospholipase A₂ route.

β2 integrin-derived signals induce cell survival and proliferation of AML blasts by activating a Syk/STAT signaling axis

Spleen tyrosine kinase (Syk) induces cell survival and proliferation in a high proportion of acute myeloid leukemia (AML) blasts, but the underlying molecular events of Syk signaling have not been investigated. Proteomic techniques have allowed us to identify the multiprotein complex that is nucleated by constitutively active Syk in AML cells. This complex differs from the B-lymphoid Syk interactome with respect to several proteins, especially the integrin receptor Mac-1, the Fc-γ receptor I (FcγRI), and the transcription factors STAT3 and STAT5. We show in several AML cell line models that tonic signals derived from the Fc-γ chain lead to Syk-dependent activation of STAT3 and STAT5, which in turn induces cell survival and proliferation. Moreover, stimulation of Mac-1 or FcγRI intensifies the constitutive Syk-mediated STAT3/5 activation in AML cells, a scenario likely to take place in the bone marrow niche. In accordance with these findings, we observed that β2 integrins, including Mac-1, trigger proliferation of AML cells in an AML cell/stroma coculture model. Taken together, we identified an oncogenic integrin/Syk/STAT3/5 signaling axis that might serve as a therapeutic target of AML in the future.

Targeting pathological B cell receptor signalling in lymphoid malignancies

Signalling through the B cell receptor (BCR) is central to the development and maintenance of B cells. In light of the numerous proliferative and survival pathways activated downstream of the BCR, it comes as no surprise that malignant B cells would co-opt this receptor to promote their own growth and survival. However, direct evidence for BCR signalling in human lymphoma has only come to light recently. Roles for antigen-dependent and antigen-independent, or tonic, BCR signalling have now been described for several different lymphoma subtypes. Furthermore, correlative data implicate antigen-dependent BCR signalling in many other forms of lymphoma. A host of therapeutic agents targeting effectors of the BCR signalling pathway are now in clinical trials and have shown initial success against multiple forms of lymphoma.

A role of kindlin-3 in integrin αMβ2 outside-in signaling and the Syk-Vav1-Rac1/Cdc42 signaling axis

Integrins mediate cell-cell and cell-extracellular matrix attachments. Integrins are signaling receptors because their cytoplasmic tails are docking sites for cytoskeletal and signaling proteins. Kindlins are a family of band 4.1-ezrin-radixin-moesin-containing intracellular proteins. Apart from regulating integrin ligand-binding affinity, recent evidence suggests that kindlins are involved in integrin outside-in signaling. Kindlin-3 is expressed in platelets, hematopoietic cells and endothelial cells. In humans, loss of kindlin-3 expression accounts for the rare autosomal disease leukocyte adhesion deficiency (LAD) type III that is characterized by bleeding disorders and defective recruitment of leukocytes into sites of infection. Studies have shown that the loss of kindlin-3 expression leads to poor ligand-binding properties of β1, β2 and β3 integrin subfamilies. The leukocyte-restricted β2 integrin subfamily comprises four members, namely αLβ2, αMβ2, αXβ2 and αDβ2. Integrin αMβ2 mediates leukocyte adhesion, phagocytosis, degranulation and it is involved in the maintenance of immune tolerance. Here we provide further evidence that kindlin-3 is required for integrin αMβ2-mediated cell adhesion and spreading using transfected K562 cells that expressed endogenous kindlin-3 but not β2 integrins. K562 stable cell line expressing si-RNA targeting kindlin-3, but not control-si-RNA, and transfected with constitutively activated integrin αMβ2N329S adhered and spread poorly on iC3b. We also show that kindlin-3 is required for the integrin αMβ2-Syk-Vav1 signaling axis that regulates Rac1 and Cdc42 activities. These findings reinforce a role for kindlin-3 in integrin outside-in signaling.

β(2) integrins inhibit TLR responses by regulating NF-κB pathway and p38 MAPK activation

Outside-in signals from β(2) integrins require immunoreceptor tyrosine-based activation motif adapters in myeloid cells that are known to dampen TLR responses. However, the relationship between β(2) integrins and TLR regulation is unclear. Here we show that deficiency in β(2) integrins (Itgb2(-/-) ) causes hyperresponsiveness to TLR stimulation, demonstrating that β(2) integrins inhibit signals downstream of TLR ligation. Itgb2(-/-) macrophages and dendritic cells produced more IL-12 and IL-6 than WT cells when stimulated with TLR agonists and Itgb2(-/-) mice produced more inflammatory cytokines than WT mice when injected with LPS. TLR hypersensitivity was not the result of insufficient ABIN-3, A20, Hes-1, or IRAK-M expression, nor to changes in IL-10 production or sensitivity, though Itgb2(-/-) macrophages had reduced p38 MAPK phosphorylation after LPS treatment. Furthermore, a Cbl-b-MyD88 regulatory axis is not required for TLR inhibition in macrophages. Instead, Itgb2(-/-) macrophages presented with enhanced IκBα degradation, leading to changes in NF-κB recruitment to target promoters and elevated cytokine, chemokine, and anti-apoptotic gene transcription. Thus, β(2) integrins limit TLR signaling by inhibiting NF-κB pathway activation and promoting p38 MAPK activation, thereby fine-tuning TLR-induced inflammatory responses.

Targeting the SYK-BTK axis for the treatment of immunological and hematological disorders: recent progress and therapeutic perspectives

Spleen Tyrosine Kinase (SYK) and Bruton's Tyrosine Kinase (BTK) are non-receptor cytoplasmic tyrosine kinases that are primarily expressed in cells of hematopoietic lineage. Both are key mediators in coupling activated immunoreceptors to downstream signaling events that affect diverse biological functions, from cellular proliferation, differentiation and adhesion to innate and adaptive immune responses. As such, pharmacological inhibitors of SYK or BTK are being actively pursued as potential immunomodulatory agents for the treatment of autoimmune and inflammatory disorders. Deregulation of SYK or BTK activity has also been implicated in certain hematological malignancies. To date, from a clinical perspective, pharmacological inhibition of SYK activity has demonstrated encouraging efficacy in patients with rheumatoid arthritis (RA), while patients with relapsed or refractory chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) have benefited from covalent inhibitors of BTK in early clinical studies. Here, we review and discuss recent insights into the emerging role of the SYK-BTK axis in innate immune cell function as well as in the maintenance of survival and homing signals for tumor cell progression. The current progress on the clinical development of SYK and BTK inhibitors is also highlighted.

Multimolecular signaling complexes enable Syk-mediated signaling of CD36 internalization

CD36 is a versatile receptor known to play a central role in the development of atherosclerosis, the pathogenesis of malaria, and the removal of apoptotic cells. Remarkably, the short cytosolically exposed regions of CD36 lack identifiable motifs, which has hampered elucidation of its mode of signaling. Using a combination of phosphoprotein isolation, mass spectrometry, superresolution imaging, and gene silencing, we have determined that the receptor induces ligand internalization through a heteromeric complex consisting of CD36, β1 and/or β2 integrins, and the tetraspanins CD9 and/or CD81. This receptor complex serves to link CD36 to the adaptor FcRγ, which bears an immunoreceptor tyrosine activation motif. By coupling to FcRγ, CD36 is able to engage Src-family kinases and Syk, which in turn drives the internalization of CD36 and its bound ligands.

Involvement of Toso in activation of monocytes, macrophages, and granulocytes

Rapid activation of immune responses is necessary for antibacterial defense, but excessive immune activation can result in life-threatening septic shock. Understanding how these processes are balanced may provide novel therapeutic potential in treating inflammatory disease. Fc receptors are crucial for innate immune activation. However, the role of the putative Fc receptor for IgM, known as Toso/Faim3, has to this point been unclear. In this study, we generated Toso-deficient mice and used them to uncover a critical regulatory function of Toso in innate immune activation. Development of innate immune cells was intact in the absence of Toso, but Toso-deficient neutrophils exhibited more reactive oxygen species production and reduced phagocytosis of pathogens compared with controls. Cytokine production was also decreased in Toso(-/-) mice compared with WT animals, rendering them resistant to septic shock induced by lipopolysaccharide. However, Toso(-/-) mice also displayed limited cytokine production after infection with the bacterium Listeria monocytogenes that was correlated with elevated presence of Listeria throughout the body. Accordingly, Toso(-/-) mice succumbed to infections of L. monocytogenes, whereas WT mice successfully eliminated the infection. Taken together, our data reveal Toso to be a unique regulator of innate immune responses during bacterial infection and septic shock.

Siglec-E is a negative regulator of acute pulmonary neutrophil inflammation and suppresses CD11b β2-integrin-dependent signaling

Neutrophil entry into the lung tissues is a key step in host defense to bacterial and yeast infections, but if uncontrolled can lead to severe tissue damage. Here, we demonstrate for the first time that sialic acid binding Ig-like lectin E (siglec-E) functions to selectively regulate early neutrophil recruitment into the lung. In a model of acute lung inflammation induced by aerosolized lipopolysaccharide, siglec-E-deficient mice exhibited exaggerated neutrophil recruitment that was reversed by blockade of the β2 integrin, CD11b. Siglec-E suppressed CD11b "outside-in" signaling, because siglec-E-deficient neutrophils plated on the CD11b ligand fibrinogen showed exaggerated phosphorylation of Syk and p38 mitogen-activated protein kinase. Sialidase treatment of fibrinogen reversed the suppressive effect of siglec-E on CD11b signaling, suggesting that sialic acid recognition by siglec-E is required for its inhibitory function. Siglec-E in neutrophils was constitutively associated with the tyrosine phosphatase SHP-1 and may therefore function to constitutively dampen inflammatory responses of neutrophils. These data reveal that siglec-E is an important negative regulator of neutrophil recruitment to the lung and β2 integrin-dependent signaling. Our findings have implications for the human functional ortholog, siglec-9, and its potential role in regulating inflammatory lung disease.

Controlling natural killer cell responses: integration of signals for activation and inhibition

Understanding how signals are integrated to control natural killer (NK) cell responsiveness in the absence of antigen-specific receptors has been a challenge, but recent work has revealed some underlying principles that govern NK cell responses. NK cells use an array of innate receptors to sense their environment and respond to alterations caused by infections, cellular stress, and transformation. No single activation receptor dominates; instead, synergistic signals from combinations of receptors are integrated to activate natural cytotoxicity and cytokine production. Inhibitory receptors for major histocompatibility complex class I (MHC-I) have a critical role in controlling NK cell responses and, paradoxically, in maintaining NK cells in a state of responsiveness to subsequent activation events, a process referred to as licensing. MHC-I-specific inhibitory receptors both block activation signals and trigger signals to phosphorylate and inactivate the small adaptor Crk. These different facets of inhibitory signaling are incorporated into a revocable license model for the reversible tuning of NK cell responsiveness.

Cooperative integrin/ITAM signaling in platelets enhances thrombus formation in vitro and in vivo

The integrin family is composed of a series of 24 αβ heterodimer transmembrane adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. Adaptor molecules bearing immunoreceptor tyrosine-based activation motifs (ITAMs) have recently been shown to cooperate with specific integrins to increase the efficiency of transmitting ligand-binding-induced signals into cells. In human platelets, Fc receptor γ-chain IIa (FcγRIIa) has been identified as an ITAM-bearing transmembrane receptor responsible for mediating "outside-in" signaling through αIIbβ3, the major adhesion receptor on the platelet surface. To explore the importance of FcγRIIa in thrombosis and hemostasis, we subjected FcγRIIa-negative and FcγRIIa-positive murine platelets to a number of well-accepted models of platelet function. Compared with their FcγRIIa-negative counterparts, FcγRIIa-positive platelets exhibited increased tyrosine phosphorylation of Syk and phospholipase Cγ2 and increased spreading upon interaction with immobilized fibrinogen, retracted a fibrin clot faster, and showed markedly enhanced thrombus formation when perfused over a collagen-coated flow chamber under conditions of arterial and venous shear. They also displayed increased thrombus formation and fibrin deposition in in vivo models of vascular injury. Taken together, these data establish FcγRIIa as a physiologically important functional conduit for αIIbβ3-mediated outside-in signaling, and suggest that modulating the activity of this novel integrin/ITAM pair might be effective in controlling thrombosis.

Phosphoinositide 3-OH kinase regulates integrin-dependent processes in neutrophils by signaling through its effector ARAP3

ARAP3, a GTPase activating protein for Rho and Arf family GTPases, is one of many phosphoinositide 3-OH kinase (PI3K) effectors. In this study, we investigate the regulatory input of PI3K upstream of ARAP3 by analyzing neutrophils from an ARAP3 pleckstrin homology (PH) domain point mutation knock-in mouse (R302, 303A), in which ARAP3 is uncoupled from activation by PI3K. ARAP3 PH domain point mutant neutrophils are characterized by disturbed responses linked to stimulation by either integrin ligands or immobilized immune complexes. These cells exhibit increased β2 integrin inside-out signaling (binding affinity and avidity), and our work suggests the disturbed responses to immobilized immune complexes are secondary to this. In vitro, neutrophil chemotaxis is affected in the mutant. In vivo, ARAP3 PH domain point mutant bone marrow chimeras exhibit reduced neutrophil recruitment to the peritoneum on induction of sterile peritonitis and also reduced inflammation in a model for rheumatoid arthritis. The current work suggests a dramatic regulatory input of PI3K into the regulation of β2 integrin activity, and processes dependent on this, by signaling through its effector ARAP3.

Molecular mechanisms of spontaneous and directed mast cell motility

Migration is a fundamental function of immune cells, and a role for Ca(2+) in immune cell migration has been an interest of scientific investigations for many decades. Mast cells are the major effector cells in IgE-mediated immune responses, and cross-linking of IgE-FcεRI complexes at the mast cell surface by antigen activates a signaling cascade that causes mast cell activation, resulting in Ca(2+) mobilization and granule exocytosis. These cells are known to accumulate at sites of inflammation in response to parasite and bacterial infections. Using real-time imaging, we monitored chemotactic migration of RBL and rat BMMCs in response to a gradient of soluble multivalent antigen. Here, we show that Ca(2+) influx via Orai1 plays an important role in regulating spontaneous motility and directional migration of mast cells toward antigen via IgER complexes. Inhibition of Ca(2+) influx or knockdown of the Ca(2+) entry channel protein Orai1 by shRNA causes inhibition of both of these processes. In addition, a mutant Syk- shows impaired spontaneous motility and chemotaxis toward antigen that is rescued by expression of Syk. Our findings identify a novel Ca(2+) influx-mediated, Orai1-dependent mechanism for mast cell migration.

Human lupus serum induces neutrophil-mediated organ damage in mice that is enabled by Mac-1 deficiency

Systemic lupus erythematosus (SLE) is a chronic, multiorgan inflammatory autoimmune disorder associated with high levels of circulating autoantibodies and immune complexes. We report that passive transfer of human SLE sera into mice expressing the uniquely human FcγRIIA and FcγRIIIB on neutrophils induces lupus nephritis and in some cases arthritis only when the mice additionally lack the CD18 integrin, Mac-1. The prevailing view is that Mac-1 on macrophages is responsible for immune complex clearance. However, disease permitted by the absence of Mac-1 is not related to enhanced renal immune complex deposition or in situ C1q/C3 complement activation and proceeds even in the absence of macrophages. Instead, disease is associated with increased FcγRIIA-induced neutrophil accumulation that is enabled by Mac-1 deficiency. Intravital microscopy in the cremasteric vasculature reveals that Mac-1 mitigates FcγRIIA-dependent neutrophil recruitment in response to deposited immune complexes. Our results provide direct evidence that human SLE immune complexes are pathogenic, demonstrate that neutrophils are primary mediators of end organ damage in a novel humanized lupus mouse model, and identify Mac-1 regulation of FcγRIIA-mediated neutrophil recruitment as a key step in development of target organ damage.

Src kinase and Syk activation initiate PI3K signaling by a chimeric latent membrane protein 1 in Epstein-Barr virus (EBV)+ B cell lymphomas

The B lymphotrophic γ-herpesvirus EBV is associated with a variety of lymphoid- and epithelial-derived malignancies, including B cell lymphomas in immunocompromised and immunosuppressed individuals. The primary oncogene of EBV, latent membrane protein 1 (LMP1), activates the PI3K/Akt pathway to induce the autocrine growth factor, IL-10, in EBV-infected B cells, but the mechanisms underlying PI3K activation remain incompletely understood. Using small molecule inhibition and siRNA strategies in human B cell lines expressing a chimeric, signaling-inducible LMP1 protein, nerve growth factor receptor (NGFR)-LMP1, we show that NGFR-LMP1 utilizes Syk to activate PI3K/Akt signaling and induce IL-10 production. NGFR-LMP1 signaling induces phosphorylation of BLNK, a marker of Syk activation. Whereas Src kinases are often required for Syk activation, we show here that PI3K/Akt activation and autocrine IL-10 production by NGFR-LMP1 involves the Src family kinase Fyn. Finally, we demonstrate that NGFR-LMP1 induces phosphorylation of c-Cbl in a Syk- and Fyn-dependent fashion. Our results indicate that the EBV protein LMP1, which lacks the canonical ITAM required for Syk activation, can nevertheless activate Syk, and the Src kinase Fyn, resulting in downstream c-Cbl and PI3K/Akt activation. Fyn, Syk, and PI3K/Akt antagonists thus may present potential new therapeutic strategies that target the oncogene LMP1 for treatment of EBV+ B cell lymphomas.

Polyethylene particles stimulate expression of ITAM-related molecules in peri-implant tissues and when stimulating osteoclastogenesis in vitro

Wear particle-induced orthopaedic prosthesis loosening is associated with elevated osteoclast activity. The immunoreceptor tyrosine-based activation motif (ITAM)-related molecules OSCAR, FcRγ, TREM2 and DAP12 are important for osteoclast formation. The aim of this study was to determine if these molecules are involved in peri-implant loosening by investigating their expression in peri-implant tissues obtained at revision of joint replacement components containing polyethylene (PE) wear particles, and in osteoclasts formed in vitro in the presence of PE particles. The results showed that there was a marked and statistically significant increase in protein levels of the ITAM-related molecules in the revision tissues. The levels of OSCAR, FcRγ, TREM2 and DAP12 mRNA in the revision tissues were also increased. In vitro PE particles stimulated osteoclast resorption in the presence of 50 ng ml(-1) receptor activator NFκB (RANKL) and significantly elevated the expression of OSCAR, FcRγ, TREM2 and DAP12 during osteoclast formation. These findings suggest that the ITAM signalling molecules and their co-receptors have a role in pathogenic bone loss associated with implant PE wear.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Complementary phosphorylation sites in the adaptor protein SLP-76 promote synergistic activation of natural killer cells

The cytotoxic effects of natural killer (NK) cells and their ability to secrete cytokines require synergistic signals from specific pairs of co-activation receptors, such as CD314 (also known as NKG2D) and CD244 (2B4), which bind to distinct ligands present on target cells. These signals are required to overcome inhibition mediated by the E3 ubiquitin ligase c-Cbl of the guanine nucleotide exchange factor Vav1, which promotes activation of NK cells. Here, we showed that the adaptor protein SLP-76 (Src homology 2 domain-containing leukocyte phosphoprotein of 76 kilodaltons) was required for this synergy and that distinct tyrosine residues in SLP-76 were phosphorylated by each member of a pair of synergistic receptors. Selective phosphorylation of tyrosine 113 or tyrosine 128 in SLP-76 enabled binding of SLP-76 to Vav1. Selective phosphorylation of SLP-76 at these residues was restricted to receptors that stimulated ligand-dependent target cell killing; antibody-dependent stimulation of the Fc receptor CD16 promoted phosphorylation at both sites. Knockdown and reconstitution experiments with SLP-76 mutant proteins showed the distinct role of each tyrosine in the synergistic mobilization of Ca2+, revealing an unexpected degree of selectivity in the phosphorylation of SLP-76 by NK cell co-activation receptors. Together, these data suggest that combined phosphorylation of separate tyrosine residues in SLP-76 forms the basis of synergistic NK cell activation.

Dasatinib inhibits proinflammatory functions of mature human neutrophils

Dasatinib is a tyrosine kinase inhibitor used to treat imatinib-resistant chronic myeloid leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia. At present, little is known about how dasatinib influences nonmalignant cells. In the present study, we tested the effect of dasatinib on functional responses of normal mature human neutrophils. Dasatinib completely blocked integrin- and Fc-receptor-mediated neutrophil functions, with the lowest IC(50) values below 10nM under serum-free conditions. Dasatinib caused a partial inhibition of neutrophil responses triggered by G-protein-coupled receptors and had a moderate effect on neutrophil responses triggered by microbial compounds. Whereas dasatinib inhibited neutrophil chemotaxis under static conditions in 2 dimensions, it did not affect migration under flow conditions or in 3-dimensional environments. Dasatinib did not have any major effect on phagocytosis or killing of bacteria by neutrophils. Adhesion of human neutrophils in the presence of whole serum was significantly inhibited by 50-100nM dasatinib, which corresponds to the reported serum concentrations in dasatinib-treated patients. Finally, ex vivo adhesion of mouse peripheral blood neutrophils was strongly reduced after oral administration of 5 mg/kg of dasatinib. Those results suggest that dasatinib treatment may affect the proinflammatory functions of mature neutrophils and raise the possibility that dasatinib-related compounds may provide clinical benefit in neutrophil-mediated inflammatory diseases.

Integrin signalling and function in immune cells

Integrins not only mediate cell-cell and cell-extracellular matrix adhesion, but also affect the multitude of signal transduction cascades in control of cell survival, proliferation, differentiation and organ development. Mutations in integrins or the major effectors of integrin signalling pathways cause defective organ development, immunodeficiency, cancer or autoimmune disease. Understanding of the signalling events that drive integrin activation and signalling is therefore crucial to uncover the molecular mechanisms of these diseases. This review discusses the key signalling complexes regulating integrin activation and function in both 'inside-out' and 'outside-in' pathways in T lymphocytes, including kinases, SLP-76, VAV1, ADAP, SKAP-55, RapL, RIAM, Rap1, Talin and Kindlin.

Neutrophils Promote Mycobacterial Trehalose Dimycolate-Induced Lung Inflammation via the Mincle Pathway

Trehalose 6,6'-dimycolate (TDM), a cord factor of Mycobacterium tuberculosis (Mtb), is an important regulator of immune responses during Mtb infections. Macrophages recognize TDM through the Mincle receptor and initiate TDM-induced inflammatory responses, leading to lung granuloma formation. Although various immune cells are recruited to lung granulomas, the roles of other immune cells, especially during the initial process of TDM-induced inflammation, are not clear. In this study, Mincle signaling on neutrophils played an important role in TDM-induced lung inflammation by promoting adhesion and innate immune responses. Neutrophils were recruited during the early stage of lung inflammation following TDM-induced granuloma formation. Mincle expression on neutrophils was required for infiltration of TDM-challenged sites in a granuloma model induced by TDM-coated-beads. TDM-induced Mincle signaling on neutrophils increased cell adherence by enhancing F-actin polymerization and CD11b/CD18 surface expression. The TDM-induced effects were dependent on Src, Syk, and MAPK/ERK kinases (MEK). Moreover, coactivation of the Mincle and TLR2 pathways by TDM and Pam3CSK4 treatment synergistically induced CD11b/CD18 surface expression, reactive oxygen species, and TNFα production by neutrophils. These synergistically-enhanced immune responses correlated with the degree of Mincle expression on neutrophil surfaces. The physiological relevance of the Mincle-mediated anti-TDM immune response was confirmed by defective immune responses in Mincle⁻/⁻ mice upon aerosol infections with Mtb. Mincle-mutant mice had higher inflammation levels and mycobacterial loads than WT mice. Neutrophil depletion with anti-Ly6G antibody caused a reduction in IL-6 and monocyte chemotactic protein-1 expression upon TDM treatment, and reduced levels of immune cell recruitment during the initial stage of infection. These findings suggest a new role of Mincle signaling on neutrophils during anti-mycobacterial responses.

Peptide LSARLAF induces integrin β3 dependent outside-in signaling in platelets

INTRODUCTION

Peptide LSARLAF (LSA) can bind and activate integrin αIIbβ3 in the absence of 'inside-out' signal. The active αIIbβ3 mediates 'outside-in' signaling that elicits platelet aggregation, granule secretion and TxA2 production. Here we identify the membrane glycoproteins which mediate LSA-induced platelet activation other than αIIbβ3, and determine the roles of Src, PLCγ2, FcRγ-chain, and SLP-76 in LSA-induced platelet activation.

METHOD

Ligand-receptor binding assay was performed to study the effect of peptide LSA or its control peptide FRALASL (FRA) on integrins binding to their ligands. Spreading of CHO cells expressing αIIbβ3 or αVβ3 on immobilized fibrinogen was measured in the presence of LSA or FRA. Washed β3, Src, FcRγ-chain, LAT and SLP-76 deficient platelets aggregation and secretion were tested in response to LSA.

RESULTS

Ligand-receptor binding assay indicated that LSA promoted the binding of multiple ligands to αIIbβ3 or αVβ3. LSA also enhanced CHO cells with αIIbβ3 or αVβ3 expression spreading on immobilized fibrinogen. β3 deficient platelets failed to aggregate and secrete in response to LSA. The phosphorylation of PLCγ2 and Syk was also β3 dependent. Src, FcRγ-chain, LAT and SLP-76 deficient platelets did not aggregate, secrete ATP or produce TxA2 in response to LSA.

CONCLUSION

LSA-induced platelet activation is β3 dependent, and signaling molecules Src, FcRγ-chain, SLP-76 and LAT play crucial roles in LSA-induced β3 mediated signaling.

Intravenous immunoglobulins modulate neutrophil activation and vascular injury through FcγRIII and SHP-1

RATIONALE

Intravascular neutrophil recruitment and activation are key pathogenic factors that contribute to vascular injury. Intravenous immunoglobulin (IVIG) has been shown to have a beneficial effect in systemic inflammatory disorders; however, the mechanisms underlying IVIG's inhibitory effect on neutrophil recruitment and activation are not understood.

OBJECTIVE

We studied the mechanisms by which IVIG exerts protection from neutrophil-mediated acute vascular injury.

METHODS AND RESULTS

We examined neutrophil behavior in response to IVIG in vivo, using real-time intravital microscopy. We found that an antibody that blocks both FcγRIII and its inhibitory receptor counterpart, FcγRIIB, abrogated the inhibitory effect of IVIG on leukocyte recruitment and heterotypic red blood cell (RBC) interactions with adherent leukocytes in wild-type mice. In the context of sickle cell disease, the blockade of both FcγRIIB and III abrogated the protective effect of IVIG on acute vaso-occlusive crisis caused by neutrophil recruitment and activation. Analysis of FcγRIIB- and FcγRIII-deficient mice revealed the predominant expression of FcγRIII on circulating neutrophils. FcγRIII mediated IVIG-triggered inhibition of leukocyte recruitment, circulating RBC capture, and enhanced Mac-1 activity, whereas FcγRIIB was dispensable. In addition, FcγRIII-induced IVIG anti-inflammatory activity in neutrophils was mediated by recruitment of Src homology 2 (SH2)-containing tyrosine phosphatase-1 (SHP-1). Indeed, the protective effect of IVIG on leukocyte recruitment and activation was abrogated in SHP-1-mutant mice.

CONCLUSIONS

FcγRIII, a classic activating receptor, has an unexpected inhibitory role on neutrophil adhesion and activation via recruitment of SHP-1 in response to IVIG. Our results identify SHP-1 as a therapeutic target in neutrophil-mediated vascular injury.

Protein kinase C-θ is required for murine neutrophil recruitment and adhesion strengthening under flow

Protein kinase C (PKC)-θ is involved in T cell activation via regulating the avidity of the β(2) integrin LFA-1 in the immunological synapse. LFA-1 also mediates leukocyte adhesion. To investigate the role of PKC-θ in neutrophil adhesion, we performed intravital microscopy in cremaster venules of mice reconstituted with bone marrow from LysM-GFP(+) (wild-type [WT]) and PKC-θ gene-deficient (Prkcq(-/-)) mice. Following stimulation with CXCL1, both WT and Prkcq(-/-) cells became adherent. Although most WT neutrophils remained adherent for at least 180 s, 50% of Prkcq(-/-) neutrophils were detached after 105 s and most by 180 s. Upon CXCL1 injection, rolling of all WT neutrophils stopped for 90 s, but rolling of Prkcq(-/-) neutrophils started 30 s after CXCL1 stimulation. A similar neutrophil adhesion defect was seen in vitro, and spreading of Prkcq(-/-) neutrophils was delayed. Prkcq(-/-) neutrophil recruitment was impaired in fMLP-induced transmigration into the cremaster muscle, thioglycollate-induced peritonitis, and LPS-induced lung injury. We conclude that PKC-θ mediates integrin-dependent neutrophil functions and is required to sustain neutrophil adhesion in postcapillary venules in vivo. These findings suggest that the role of PKC-θ in outside-in signaling following engagement of neutrophil integrins is relevant for inflammation in vivo.

Immune complexes and late complement proteins trigger activation of Syk tyrosine kinase in human CD4(+) T cells

In systemic lupus erythematosus (SLE), the autoantibodies that form immune complexes (ICs) trigger activation of the complement system. This results in the formation of membrane attack complex (MAC) on cell membrane and the soluble terminal complement complex (TCC). Hyperactive T cell responses are hallmark of SLE pathogenesis. How complement activation influences the T cell responses in SLE is not fully understood. We observed that aggregated human γ-globulin (AHG) bound to a subset of CD4(+) T cells in peripheral blood mononuclear cells and this population increased in the SLE patients. Human naive CD4(+) T cells, when treated with purified ICs and TCC, triggered recruitment of the FcRγ chain with the membrane receptor and co-localized with phosphorylated Syk. These events were also associated with aggregation of membrane rafts. Thus, results presented suggest a role for ICs and complement in the activation of Syk in CD4(+) T cells. Thus, we propose that the shift in signalling from ζ-chain-ZAP70 to FcRγ chain-Syk observed in T cells of SLE patients is triggered by ICs and complement. These results demonstrate a link among ICs, complement activation and phosphorylation of Syk in CD4(+) T cells.

ITAM-coupled receptors inhibit IFNAR signaling and alter macrophage responses to TLR4 and Listeria monocytogenes

ITAM-coupled receptors play an essential role in regulating macrophage activation and function by cross-regulating signaling from heterologous receptors. We investigated mechanisms by which ITAM-associated receptors inhibit type I IFN (IFN-α/β) signaling in primary human macrophages and tested the effects of simultaneous ligation of ITAM-associated receptors and TLR4 on TLR4-induced Jak-STAT signaling that is mediated by autocrine IFN-β. Preligation of ITAM-coupled β2 integrins and FcγRs inhibited proximal signaling by the type I IFN receptor IFNAR. Cross-inhibition of IFNAR signaling by β2 integrins resulted in decreased Jak1 activation and was mediated by partial downregulation of the IFNAR1 subunit and MAPK-dependent induction of USP18, which blocks the association of Jak1 with IFNAR2. Simultaneous engagement of ITAM-coupled β2 integrins or Dectin-1 with TLR4 did not affect TLR4-induced direct activation of inflammatory target genes such as TNF or IL6 but abrogated subsequent induction of IFN response genes that is mediated by autocrine IFN-β signaling. Type I IFNs promote macrophage death postinfection by Listeria monocytogenes. Consequently, attenuation of IFN responses by β2 integrins protected primary human macrophages from L. monocytogenes-induced apoptosis. These results provide a mechanism for cross-inhibition of type I IFN signaling by ITAM-coupled β2 integrins and demonstrate that ITAM signaling qualitatively modulates macrophage responses to pathogen-associated molecular patterns and pathogens by selectively suppressing IFN responses.

Crucial role of SLP-76 and ADAP for neutrophil recruitment in mouse kidney ischemia-reperfusion injury

Leukocyte recruitment to the kidney during acute injury is mediated by E-selectin–mediated rolling and requires SLP-76 and the adaptor protein ADAP.

Neutrophils trigger inflammation-induced acute kidney injury (AKI), a frequent and potentially lethal occurrence in humans. Molecular mechanisms underlying neutrophil recruitment to sites of inflammation have proved elusive. In this study, we demonstrate that SLP-76 (SH2 domain–containing leukocyte phosphoprotein of 76 kD) and ADAP (adhesion and degranulation promoting adaptor protein) are involved in E-selectin–mediated integrin activation and slow leukocyte rolling, which promotes ischemia-reperfusion–induced AKI in mice. By using genetically engineered mice and transduced Slp76^−/− primary leukocytes, we demonstrate that ADAP as well as two N-terminal–located tyrosines and the SH2 domain of SLP-76 are required for downstream signaling and slow leukocyte rolling. The Tec family kinase Bruton tyrosine kinase is downstream of SLP-76 and, together with ADAP, regulates PI3Kγ (phosphoinositide 3-kinase–γ)- and PLCγ2 (phospholipase Cγ2)-dependent pathways. Blocking both pathways completely abolishes integrin affinity and avidity regulation. Thus, SLP-76 and ADAP are involved in E-selectin–mediated integrin activation and neutrophil recruitment to inflamed kidneys, which may underlie the development of life-threatening ischemia-reperfusion–induced AKI in humans.

Role of ITAM signaling module in signal integration

Diverse cell types use a small number of evolutionarily conserved signaling modules to integrate external cues and elicit distinct functions. A question thus arises as to how does a receptor, which contains a single signaling module, produce distinct outcomes to diverse signals, particularly if such module is shared amongst a family of receptors? Emerging data suggest that many immunoreceptors, all of which use a conserved ITAM-module for their signaling, can couple with members of additional classes of membrane receptors to deliver unique signal(s) to the cell. We discuss the possible biological purposes and mechanisms behind these interactions at the plasma membrane. We offer a conceptual framework to understand information processing within the immune system and discuss the new biology of old receptors involving their structural and functional collaborations that evolved to deliver unique signal(s) to the cell using a limited set of conserved signaling modules.

Overview of integrin signaling in the immune system

It has been well established that integrins mediate cell-cell and cell-matrix adhesion and play crucial roles in the immune system such as leukocyte-endothelium interactions, immune synapse formation, and effector functions. Since the discovery that integrins undergo dynamic changes of adhesive activities in response to external stimuli, intensive studies have been conducted to elucidate the signaling events that control the activation of integrins (inside-out signaling) and signaling events from the induced integrin-dependent adhesion (outside-in signaling). The molecular characterization of these signaling pathways highlights the importance of integrins as bidirectional signaling receptors. The characteristics of integrin signaling are best exemplified in the immune system. This chapter highlights the recent studies of intracellular signaling pathways that regulate integrins in immunological contexts.

Phospholipase Cγ2 is required for basal but not oestrogen deficiency-induced bone resorption

BACKGROUND

Osteoclasts play a critical role in bone resorption under basal conditions, but they also contribute to pathological bone loss during diseases including postmenopausal osteoporosis. Phospholipase Cγ2 (PLCγ2) is an important signalling molecule in diverse haematopoietic lineages. Here, we tested the role of PLCγ2 in basal and ovariectomy-induced bone resorption, as well as in in vitro osteoclast cultures using PLCγ2-deficient (PLCγ2(-/-) ) mice.

MATERIALS AND METHODS

The trabecular architecture of long bone metaphyses was tested by micro-CT and histomorphometric analyses. Postmenopausal osteoporosis was modelled by surgical ovariectomy. Osteoclast development and function, gene expression and PLCγ2 phosphorylation were tested on in vitro osteoclast and macrophage cultures.

RESULTS

  PLCγ2(-/-) mice had significantly higher trabecular bone mass under basal conditions than wild-type mice. PLCγ2 was required for in vitro development and resorptive function of osteoclasts, but not for upregulation of osteoclast-specific gene expression. PLCγ2 was phosphorylated in a Src-family-dependent manner upon macrophage adhesion but not upon stimulation by M-CSF or RANKL. Surprisingly, ovariectomy-induced bone resorption in PLCγ2(-/-) mice was similar to, or even more robust than, that in wild-type animals.

CONCLUSIONS

Our results indicate that PLCγ2 participates in bone resorption under basal conditions, likely because of its role in adhesion receptor signalling during osteoclast development. In contrast, PLCγ2 does not appear to play a major role in ovariectomy-induced bone loss. These results suggest that basal and oestrogen deficiency-induced bone resorption utilizes different signalling pathways and that PLCγ2 may not be a suitable therapeutic target in postmenopausal osteoporosis.

Spleen tyrosine kinase promotes acute neutrophil-mediated glomerular injury via activation of JNK and p38 MAPK in rat nephrotoxic serum nephritis

Glomerular antibody deposition induces acute neutrophil-mediated glomerular injury via activation of c-Jun amino terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). However, the link between antibody deposition and activation of JNK/p38 MAPK signalling is unclear. This study tested the postulate that spleen tyrosine kinase (Syk), which is activated via Fcγ-receptor ligation, is required for activation of JNK and p38 signalling and acute neutrophil-mediated glomerular injury. We used a Syk inhibitor (SYKi) in rat nephrotoxic serum nephritis (NTN) in which neutrophil-mediated glomerular injury is dependent upon JNK and p38 signalling. SYKi or vehicle treatment of Sprague-Dawley rats began 30 min before administration of anti-GBM serum with rats killed 3 or 24 h later. Immunostaining identified de novo glomerular Syk activation (p-Tyr 525/526) in untreated NTN, being most prominent in neutrophils. Vehicle and untreated NTN exhibited heavy proteinuria and glomerular thrombosis at 24 h with P-selectin and fibrin immunostaining within capillaries, glomerular macrophage and T cell infiltration, activation of JNK and p38 MAPK signalling, and upregulation of glomerular mRNA levels of pro-inflammatory molecules (TNF-α, NOS2, MMP-12 and CCL2). In contrast, SYKi treatment provided complete protection from proteinuria, with a profound reduction in glomerular thrombosis and immunostaining for P-selectin and fibrin, and a substantial reduction in glomerular mRNA levels of pro-inflammatory molecules. SYKi treatment also reduced the acute glomerular neutrophil influx and pro-inflammatory response at 3 h in NTN. These protective effects were associated with a significant reduction in glomerular JNK and p38 MAPK activation. In addition, activation of Syk, JNK and p38 was identified in human biopsy samples of acute crescentic glomerulonephritis. In conclusion, this study demonstrates that Syk signalling is required for JNK and p38 MAPK signalling and acute neutrophil-dependent glomerular injury in rat NTN. These findings identify Syk as a potential therapeutic target in antibody-dependent kidney disease.

The oral spleen tyrosine kinase inhibitor fostamatinib attenuates inflammation and atherogenesis in low-density lipoprotein receptor-deficient mice

OBJECTIVE

Spleen tyrosine kinase (SYK) has come into focus as a potential therapeutic target in chronic inflammatory diseases, such as rheumatoid arthritis and asthma, as well as in B-cell lymphomas. SYK has also been involved in the signaling of immunoreceptors, cytokine receptors, and integrins. We therefore hypothesized that inhibition of SYK attenuates the inflammatory process underlying atherosclerosis and reduces plaque development.

METHODS AND RESULTS

Low-density lipoprotein receptor-deficient mice consuming a high-cholesterol diet supplemented with 2 doses of the orally available SYK inhibitor fostamatinib for 16 weeks showed a dose-dependent reduction in atherosclerotic lesion size by up to 59±6% compared with the respective controls. Lesions of fostamatinib-treated animals contained fewer macrophages but more smooth muscle cells and collagen-characteristics associated with more stable plaques in humans. Mechanistically, fostamatinib attenuated adhesion and migration of inflammatory cells and limited macrophage survival. Furthermore, fostamatinib normalized high-cholesterol diet -induced monocytosis and inflammatory gene expression.

CONCLUSIONS

We present the novel finding that the SYK inhibitor fostamatinib attenuates atherogenesis in mice. Our data identify SYK inhibition as a potentially fruitful antiinflammatory therapeutic strategy in atherosclerosis.

The GTPase-activating protein ARAP3 regulates chemotaxis and adhesion-dependent processes in neutrophils

Neutrophils form a vital part of the innate immune response, but at the same time their inappropriate activation contributes to autoimmune diseases. Many molecular components are involved in fine-tuning neutrophil function. We report here the first characterization of the role of ARAP3, a PI3K and Rap-regulated GTPase-activating protein for RhoA and Arf6 in murine neutrophils. We show that neutrophils lacking ARAP3 are preactivated in vitro and in vivo, exhibiting increased β2 integrin affinity and avidity. ARAP3-deficient neutrophils are hyperresponsive in several adhesion-dependent situations in vitro, including the formation of reactive oxygen species, adhesion, spreading, and granule release. ARAP3-deficient cells adhere more firmly under flow conditions in vitro and to the vessel wall in vivo. Finally, loss of ARAP3 interferes with integrin-dependent neutrophil chemotaxis. The results of the present study suggest an important function of ARAP3 downstream of Rap. By modulating β2 integrin activity, ARAP3 guards neutrophils in their quiescent state unless activated.

PI3Kβ plays a critical role in neutrophil activation by immune complexes

Neutrophils are activated by immunoglobulin G (IgG)-containing immune complexes through receptors that recognize the Fc portion of IgG (FcγRs). Here, we used genetic and pharmacological approaches to define a selective role for the β isoform of phosphoinositide 3-kinase (PI3Kβ) in FcγR-dependent activation of mouse neutrophils by immune complexes of IgG and antigen immobilized on a plate surface. At low concentrations of immune complexes, loss of PI3Kβ alone substantially inhibited the production of reactive oxygen species (ROS) by neutrophils, whereas at higher doses, similar suppression of ROS production was achieved only by targeting both PI3Kβ and PI3Kδ, suggesting that this pathway displays stimulus strength-dependent redundancy. Activation of PI3Kβ by immune complexes involved cooperation between FcγRs and BLT1, the receptor for the endogenous proinflammatory lipid leukotriene B₄. Coincident activation by a tyrosine kinase-coupled receptor (FcγR) and a heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor (BLT1) may provide a rationale for the preferential activation of the β isoform of PI3K. PI3Kβ-deficient mice were highly protected in an FcγR-dependent model of autoantibody-induced skin blistering and were partially protected in an FcγR-dependent model of inflammatory arthritis, whereas combined deficiency of PI3Kβ and PI3Kδ resulted in near-complete protection in the latter case. These results define PI3Kβ as a potential therapeutic target in inflammatory disease.

The integrins Mac-1 and alpha4beta1 perform crucial roles in neutrophil and T cell recruitment to lungs during Streptococcus pneumoniae infection

Neutrophils and T cells play an important role in host protection against pulmonary infection caused by Streptococcus pneumoniae. However, the role of the integrins in recruitment of these cells to infected lungs is not well understood. In this study we used the twin approaches of mAb blockade and gene-deficient mice to investigate the relative impact of specific integrins on cellular recruitment and bacterial loads following pneumococcal infection. We find that both Mac-1 (CD11b/CD18) and α(4)β(1) (CD49d/CD29) integrins, but surprisingly not LFA-1 (CD11a/CD18), contribute to two aspects of the response. In terms of recruitment from the circulation into lungs, neutrophils depend on Mac-1 and α(4)β(1), whereas the T cells are entirely dependent on α(4)β(1). Second, immunohistochemistry results indicate that adhesion also plays a role within infected lung tissue itself. There is widespread expression of ICAM-1 within lung tissue. Use of ICAM-1(-/-) mice revealed that neutrophils make use of this Mac-1 ligand, not for lung entry or for migration within lung tissue, but for combating the pneumococcal infection. In contrast to ICAM-1, there is restricted and constitutive expression of the α(4)β(1) ligand, VCAM-1, on the bronchioles, allowing direct access of the leukocytes to the airways via this integrin at an early stage of pneumococcal infection. Therefore, integrins Mac-1 and α(4)β(1) have a pivotal role in prevention of pneumococcal outgrowth during disease both in regulating neutrophil and T cell recruitment into infected lungs and by influencing their behavior within the lung tissue itself.

2B4 utilizes ITAM-containing receptor complexes to initiate intracellular signaling and cytolysis

2B4 is a member of the SLAM receptor family capable of activating NK cell cytotoxicity in the context of EBV infection. SAP (SLAM Associated Protein) deficiency causes defective signaling downstream of SLAM family receptors and high susceptibility to EBV. 2B4 costimulates natural cytotoxicity receptor (NCR) and TCR initiated signals to induce cellular cytotoxicity and cytokine release. The 2B4-SAP signal transduction pathway is not predicted to overlap with the TCR-ITAM pathway, although SAP is required for some TCR-induced signals. We therefore examined the functional relationship between SLAM family receptor 2B4 and ITAM-containing adaptor complexes. Removal of FcɛRIγ or CD3ζ-containing complexes, using genetically manipulated cell lines or siRNA specific suppression, significantly reduces 2B4-initiated functions in NK and T cells, respectively. Consistent with this relationship, Syk and ZAP-70 are capable of transducing 2B4 signals for calcium mobilization and cytolysis. Furthermore, ITAM-containing molecules constitutively associate with SAP. These results suggest a potential physical association between 2B4 and the ITAM receptor complexes that is required for 2B4-initiated signaling and cell-mediated killing.

Src-family and Syk kinases in activating and inhibitory pathways in innate immune cells: signaling cross talk

The response of innate immune cells to growth factors, immune complexes, extracellular matrix proteins, cytokines, pathogens, cellular damage, and many other stimuli is regulated by a complex net of intracellular signal transduction pathways. The majority of these pathways are either initiated or modulated by Src-family or Syk tyrosine kinases present in innate cells. The Src-family kinases modulate the broadest range of signaling responses, including regulating immunoreceptors, C-type lectins, integrins, G-protein-coupled receptors, and many others. Src-family kinases also modulate the activity of other kinases, including the Tec-family members as well as FAK and Pyk2. Syk kinase is required for initiation of signaling involving receptors that utilize immunoreceptor tyrosine activation (ITAM) domains. This article reviews the major activating and inhibitory signaling pathways regulated by these cytoplasmic tyrosine kinases, illuminating the many examples of signaling cross talk between pathways.

Immune inhibitory receptors: essential regulators of phagocyte function

Phagocytes, including neutrophils, monocytes, and macrophages, play a crucial role in host defense by recognition and elimination of invading pathogens. Phagocytic cells produce reactive oxygen species (ROS), inflammatory cytokines, and chemokines, leading to bacterial killing and to recruitment and activation of additional immune cells. However, inflammatory mediators are potentially harmful for the host and their production is therefore tightly controlled by multiple regulatory mechanisms. One such mechanism is immune suppression by immune inhibitory receptors, which are increasingly acknowledged as potent regulators of the immune response. So far, research has focused on the role of these receptors in the regulation of NK cells, B cells, and T cells. Importantly, an accumulating number of inhibitory receptors have been identified on phagocytes. Here, we review the role of inhibitory receptors in the regulation of phagocyte cytokine production, migration, apoptosis, ROS production, and phagocytosis. Furthermore, we discuss the intracellular mechanisms utilized by distinct inhibitory receptors to regulate specific phagocyte functions. We demonstrate that inhibitory receptors are important regulators of the immune response, which bacteria can use to their advantage.