PI3K accelerates, but is not required for, neutrophil chemotaxis to fMLP

MLK3 regulates fMLP-stimulated neutrophil motility

INTRODUCTION

Mixed lineage kinase 3 (MLK3) is part of the intracellular regulatory system that connects extracellular cytokine or mitogen signals received through G-protein coupled receptors to changes in gene expression. MLK3 activation stimulates motility of epithelial cells and epithelial-derived tumor cells, but its role in mediating the migration of other cell types remains unknown. Since neutrophils play a crucial role in innate immunity and contribute to the pathogenesis of several diseases, we therefore examined whether MLK3 might regulate the motility of mouse neutrophils responding to a chemotactic stimulus, the model bacterial chemoattractant fMLP.

METHODS

The expression of Mlk3 in mouse neutrophils was determined by immunocytochemistry and by RT-PCR. In vitro chemotaxis in a gradient of fMLP, fMLP-stimulated random motility, fMLP-stimulated F-actin formation were measured by direct microscopic observation using neutrophils pre-treated with a novel small molecule inhibitor of MLK3 (URMC099) or neutrophils obtained from Mlk3-/- mice. In vivo effects of MLK3 inhibition were measured by counting the fMLP-induced accumulation of neutrophils in the peritoneum following pre-treatment with URMC099 in wild-type C57Bl/6 or mutant Mlk3-/- mice.

RESULTS

The expression of Mlk3 mRNA and protein was observed in neutrophils purified from wild-type C57Bl/6 mice but not in neutrophils from mutant Mlk3-/- mice. Chemotaxis by wild-type neutrophils induced by a gradient of fMLP was reduced by pre-treatment with URMC099. Neutrophils from C57Bl/6 mice pretreated with URMC099 and neutrophils from Mlk3-/- mice moved far less upon fMLP-stimulation and did not form F-actin as readily as untreated neutrophils from C57Bl/6 controls. In vivo recruitment of neutrophils into the peritoneum by fMLP was significantly reduced in wild-type mice treated with URMC099, as well as in untreated Mlk3-/- mice-thereby confirming the role of MLK3 in neutrophil migration.

CONCLUSIONS

Mlk3 mRNA is expressed in murine neutrophils. Genetic or pharmacologic inhibition of MLK3 blocks fMLP-mediated motility of neutrophils both in vitro and in vivo, suggesting that MLK3 may be a therapeutic target in human diseases characterized by exuberant neutrophil migration.

TSC2 epigenetic defect in primary LAM cells. Evidence of an anchorage-independent survival

Tuberous sclerosis complex (TSC) is caused by mutations in TSC1 or TSC2 genes. Lymphangioleiomyomatosis (LAM) can be sporadic or associated with TSC and is characterized by widespread pulmonary proliferation of abnormal α-smooth muscle (ASM)-like cells. We investigated the features of ASM cells isolated from chylous thorax of a patient affected by LAM associated with TSC, named LAM/TSC cells, bearing a germline TSC2 mutation and an epigenetic defect causing the absence of tuberin. Proliferation of LAM/TSC cells is epidermal growth factor (EGF)-dependent and blockade of EGF receptor causes cell death as we previously showed in cells lacking tuberin. LAM/TSC cells spontaneously detach probably for the inactivation of the focal adhesion kinase (FAK)/Akt/mTOR pathway and display the ability to survive independently from adhesion. Non-adherent LAM/TSC cells show an extremely low proliferation rate consistent with tumour stem-cell characteristics. Moreover, LAM/TSC cells bear characteristics of stemness and secrete high amount of interleukin (IL)-6 and IL-8. Anti-EGF receptor antibodies and rapamycin affect proliferation and viability of non-adherent cells. In conclusion, the understanding of LAM/TSC cell features is important in the assessment of cell invasiveness in LAM and TSC and should provide a useful model to test therapeutic approaches aimed at controlling their migratory ability.

On-demand, competing gradient arrays for neutrophil chemotaxis

Neutrophils are the most abundant type of white blood cells in the circulation, protecting the body against pathogens and responding early to inflammation. Although we understand how neutrophils respond to individual stimuli, we know less about how they prioritize between competing signals or respond to combinational signals. This situation is due in part to the lack of adequate experimental systems to provide signals in controlled spatial and temporal fashion. To address these limitations, we designed a platform for generating on-demand, competing chemical gradients and for monitoring neutrophil migration. On this platform, we implemented forty-eight assays generating independent gradients and employed synchronized valves to control the timing of these gradients. We observed faster activation of neutrophils in response to fMLP than to LTB4 and unveiled for the first time a potentiating effect for fMLP during migration towards LTB4. Our observations, enabled by the new tools, challenge the current paradigm of inhibitory competition between distinct chemoattractant gradients and suggest that human neutrophils are capable of complex integration of chemical signals in their environment.

The multiple faces of leukocyte interstitial migration

Spatiotemporal control of leukocyte dynamics within tissues is critical for successful innate and adaptive immune responses. Homeostatic trafficking and coordinated infiltration into and within sites of inflammation and infection rely on signaling in response to extracellular cues that in turn controls a variety of intracellular protein networks regulating leukocyte motility, migration, chemotaxis, positioning, and cell-cell interaction. In contrast to mesenchymal cells, leukocytes migrate in an amoeboid fashion by rapid cycles of actin polymerization and actomyosin contraction, and their migration in tissues is generally referred to as low adhesive and nonproteolytic. The interplay of actin network expansion, contraction, and adhesion shapes the exact mode of amoeboid migration, and in this review, we explore how leukocyte subsets potentially harness the same basic biomechanical mechanisms in a cell-type-specific manner. Most of our detailed understanding of these processes derives from in vitro migration studies in three-dimensional gels and confined spaces that mimic geometrical aspects of physiological tissues. We summarize these in vitro results and then critically compare them to data from intravital imaging of leukocyte interstitial migration in mouse tissues. We outline the technical challenges of obtaining conclusive mechanistic results from intravital studies, discuss leukocyte migration strategies in vivo, and present examples of mode switching during physiological interstitial migration. These findings are also placed in the context of leukocyte migration defects in primary immunodeficiencies. This overview of both in vitro and in vivo studies highlights recent progress in understanding the molecular and biophysical mechanisms that shape robust leukocyte migration responses in physiologically complex and heterogeneous environments.

Autoantibodies to angiotensin and endothelin receptors in systemic sclerosis induce cellular and systemic events associated with disease pathogenesis

INTRODUCTION

Vasculopathy, inflammatory fibrosis and functional autoantibodies (Abs) are major manifestations of systemic sclerosis (SSc). Abs directed against the angiotensin II type 1 receptor (AT₁R) and endothelin-1 type A receptor (ETAR) are associated with characteristic disease features including vascular, inflammatory, and fibrotic complications indicating their role in SSc pathogenesis. Therefore, the impact of anti-AT₁R and anti-ETAR Abs on initiation of inflammation and fibrosis was analyzed.

METHODS

Anti-AT₁R and anti-ETAR Ab-positive immunoglobulin G (IgG) from SSc patients (SSc-IgG) was used for experiments. Healthy donor IgG served as a normal control, and AT₁R and ETAR activation was inhibited by antagonists. Protein expression was measured with ELISA, mRNA expression with real time-PCR, endothelial repair with a scratch assay, and collagen expression with immunocytochemistry. Transendothelial neutrophil migration was measured with a culture insert system, and neutrophil ROS activation with immunofluorescence. Neutrophils in bronchoalveolar lavage fluids (BALFs) were analyzed microscopically after passive transfer of SSc-IgG or NC-IgG into naïve C57BL/6J mice. KC plasma levels were quantified by a suspension array system. Histologic analyses were performed by using light microscopy.

RESULTS

Anti-AT₁R and anti-ETAR Ab-positive SSc-IgG induced activation of human microvascular endothelial cells (HMEC-1). Elevated protein and mRNA levels of the proinflammatory chemokine interleukin-8 (IL-8, CXCL8) and elevated mRNA levels of the vascular cell adhesion molecule-1 (VCAM-1) were induced in HMEC-1. Furthermore, activation of HMEC-1 with SSc-IgG increased neutrophil migration through an endothelial cell layer and activation of reactive oxygen species (ROS). SSc-IgG decreased HMEC-1 wound repair and induced type I collagen production in healthy donor skin fibroblasts. Effects of migration, wound repair, and collagen expression were dependent on the Ab-levels. Passive transfer of anti-AT1R and anti-ETAR Ab-positive SSc-IgG into naïve C57BL/6J mice increased neutrophil BALF counts. In parallel, increased levels of the murine functional IL-8 homologue, chemokine KC, were found in the plasma of SSc-IgG-treated mice as well as structural alterations of the lungs.

CONCLUSIONS

We conclude that angiotensin and endothelin-receptor activation via anti-AT₁R and anti-ETAR Abs mediate pathogenic effects, indicating their contribution to pathogenesis of SSc. Therefore, anti-AT₁R and anti-ETAR Abs could provide novel targets for therapeutic intervention in the treatment of SSc.

Oscillatory behavior of neutrophils under opposing chemoattractant gradients supports a winner-take-all mechanism

Neutrophils constitute the largest class of white blood cells and are the first responders in the innate immune response. They are able to sense and migrate up concentration gradients of chemoattractants in search of primary sites of infection and inflammation through a process known as chemotaxis. These chemoattractants include formylated peptides and various chemokines. While much is known about chemotaxis to individual chemoattractants, far less is known about chemotaxis towards many. Previous studies have shown that in opposing gradients of intermediate chemoattractants (interleukin-8 and leukotriene B₄), neutrophils preferentially migrate toward the more distant source. In this work, we investigated neutrophil chemotaxis in opposing gradients of chemoattractants using a microfluidic platform. We found that primary neutrophils exhibit oscillatory motion in opposing gradients of intermediate chemoattractants. To understand this behavior, we constructed a mathematical model of neutrophil chemotaxis. Our results suggest that sensory adaptation alone cannot explain the observed oscillatory motion. Rather, our model suggests that neutrophils employ a winner-take-all mechanism that enables them to transiently lock onto sensed targets and continuously switch between the intermediate attractant sources as they are encountered. These findings uncover a previously unseen behavior of neutrophils in opposing gradients of chemoattractants that will further aid in our understanding of neutrophil chemotaxis and the innate immune response. In addition, we propose a winner-take-all mechanism allows the cells to avoid stagnation near local chemical maxima when migrating through a network of chemoattractant sources.

Store-operated Ca²⁺ entry mediated regulation of polarization in differentiated human neutrophil-like HL-60 cells under hypoxia

The regulation of neutrophil polarization by calcium entry is critical for maintaining an effective host response. Hypoxia has a major effect on the apoptosis of neutrophils, however the role of store-operated Ca2+ entry (SOCE) in neutrophil polarization under hypoxia remains to be elucidated. In the present study, we examined the polarization of differentiated human neutrophil-like HL-60 (dHL-60) cells exposed to hypoxia (3% O2) and the results demonstrated that the percentage of polarized cells following exposure to an N-formyl-Met-Leu-Phe (fMLP) gradient in the Zigmond chamber was increased. We examined stromal interaction molecule 1 (STIM1) and Orai1 expression in dHL-60 cells during hypoxia, and it was observed that the expression of STIM1 and Orai1 was significantly reduced at day 2. However, no apparent change was observed on the first day, indicating that this effect is dependent on stimulation time. Fluo-4/acetoxymethyl (AM) ester imaging also demonstrated that SOCE was decreased in dHL-60 cells. The plasmid overexpression assay demonstrated that the response of polarization was returned to the control level. We demonstrated the inhibitory role of SOCE on the polarization of dHL-60 cells under hypoxic conditions, which may be the mechanism for the adaptation of neutrophils to hypoxia. SOCE is also suggested to be a key modulator of immune deficiency under hypoxic conditions and is potentially a therapeutic target.

Phosphoinositide 3-kinase inhibition restores neutrophil accuracy in the elderly: toward targeted treatments for immunosenescence

Immunosenescence is the functional deterioration of the immune system during natural aging. Despite increased susceptibility to bacterial infections in older adults, age-associated changes to neutrophil responses are only partially understood, and neutrophil migration has not been characterized in detail. Here we describe reduced chemotaxis but preserved chemokinesis toward a range of inflammatory stimuli in migrating neutrophils isolated from healthy older subjects. Cross-sectional data indicate that migratory behavior changes in the sixth decade of life. Crucially, aberrant migration may increase "bystander" tissue damage and heighten inflammation as a result of excess proteinase release during inaccurate chemotaxis, as well as reducing pathogen clearance. We show evidence of increased neutrophil proteinase activity in older adults, namely, raised levels of neutrophil proteinase substrate-derived peptides and evidence of primary granule release, associated with increased systemic inflammation. Inaccurate migration was causally associated with increased constitutive phosphoinositide 3-kinase (PI3K) signaling; untreated neutrophils from old donors demonstrated significant PI3K activation compared with cells from young donors. PI3K-blocking strategies, specifically inhibition of PI3Kγ or PI3Kδ, restored neutrophil migratory accuracy, whereas SHIP1 inhibition worsened migratory flaws. Targeting PI3K signaling may therefore offer a new strategy in improving neutrophil functions during infections and reduce inappropriate inflammation in older patients.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Reprint of Neutrophil cell surface receptors and their intracellular signal transduction pathways

Neutrophils play a critical role in the host defense against bacterial and fungal infections, but their inappropriate activation also contributes to tissue damage during autoimmune and inflammatory diseases. Neutrophils express a large number of cell surface receptors for the recognition of pathogen invasion and the inflammatory environment. Those include G-protein-coupled chemokine and chemoattractant receptors, Fc-receptors, adhesion receptors such as selectins/selectin ligands and integrins, various cytokine receptors, as well as innate immune receptors such as Toll-like receptors and C-type lectins. The various cell surface receptors trigger very diverse signal transduction pathways including activation of heterotrimeric and monomeric G-proteins, receptor-induced and store-operated Ca(2+) signals, protein and lipid kinases, adapter proteins and cytoskeletal rearrangement. Here we provide an overview of the receptors involved in neutrophil activation and the intracellular signal transduction processes they trigger. This knowledge is crucial for understanding how neutrophils participate in antimicrobial host defense and inflammatory tissue damage and may also point to possible future targets of the pharmacological therapy of neutrophil-mediated autoimmune or inflammatory diseases.

Mammalian target of rapamycin and Rictor control neutrophil chemotaxis by regulating Rac/Cdc42 activity and the actin cytoskeleton

Chemotaxis allows neutrophils to seek out sites of infection and inflammation. The asymmetric accumulation of filamentous actin (F-actin) at the leading edge provides the driving force for protrusion and is essential for the development and maintenance of neutrophil polarity. The mechanism that governs actin cytoskeleton dynamics and assembly in neutrophils has been extensively explored and is still not fully understood. By using neutrophil-like HL-60 cells, we describe a pivotal role for Rictor, a component of mammalian target of rapamycin complex 2 (mTORC2), in regulating assembly of the actin cytoskeleton during neutrophil chemotaxis. Depletion of mTOR and Rictor, but not Raptor, impairs actin polymerization, leading-edge establishment, and directional migration in neutrophils stimulated with chemoattractants. Of interest, depletion of mSin1, an integral component of mTORC2, causes no detectable defects in neutrophil polarity and chemotaxis. In addition, experiments with chemical inhibition and kinase-dead mutants indicate that mTOR kinase activity and AKT phosphorylation are dispensable for chemotaxis. Instead, our results suggest that the small Rho GTPases Rac and Cdc42 serve as downstream effectors of Rictor to regulate actin assembly and organization in neutrophils. Together our findings reveal an mTORC2- and mTOR kinase-independent function and mechanism of Rictor in the regulation of neutrophil chemotaxis.

Neutrophil cell surface receptors and their intracellular signal transduction pathways

Neutrophils play a critical role in the host defense against bacterial and fungal infections, but their inappropriate activation also contributes to tissue damage during autoimmune and inflammatory diseases. Neutrophils express a large number of cell surface receptors for the recognition of pathogen invasion and the inflammatory environment. Those include G-protein-coupled chemokine and chemoattractant receptors, Fc-receptors, adhesion receptors such as selectins/selectin ligands and integrins, various cytokine receptors, as well as innate immune receptors such as Toll-like receptors and C-type lectins. The various cell surface receptors trigger very diverse signal transduction pathways including activation of heterotrimeric and monomeric G-proteins, receptor-induced and store-operated Ca^2 + signals, protein and lipid kinases, adapter proteins and cytoskeletal rearrangement. Here we provide an overview of the receptors involved in neutrophil activation and the intracellular signal transduction processes they trigger. This knowledge is crucial for understanding how neutrophils participate in antimicrobial host defense and inflammatory tissue damage and may also point to possible future targets of the pharmacological therapy of neutrophil-mediated autoimmune or inflammatory diseases.

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Neutrophils are crucial players in innate and adaptive immunity.

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Neutrophils also participate in autoimmune and inflammatory diseases.

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Various neutrophil receptors recognize pathogens and the inflammatory environment.

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The various cell surface receptors trigger diverse intracellular signaling.

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Neutrophil receptors and signaling are potential targets in inflammatory diseases.

Treponema denticola major outer sheath protein impairs the cellular phosphoinositide balance that regulates neutrophil chemotaxis

The major outer sheath protein (Msp) of Treponema denticola inhibits neutrophil polarization and directed chemotaxis together with actin dynamics in vitro in response to the chemoattractant N-formyl-methionine-leucine-phenylanine (fMLP). Msp disorients chemotaxis through inhibition of a Rac1-dependent signaling pathway, but the upstream mechanisms are unknown. We challenged murine bone marrow neutrophils with enriched native Msp to determine the role of phospholipid modifying enzymes in chemotaxis and actin assembly downstream of fMLP-stimulation. Msp modulated cellular phosphoinositide levels through inhibition of phosphatidylinositol 3-kinase (PI3-kinase) together with activation of the lipid phosphatase, phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Impaired phosphatidylinositol[(3,4,5)]-triphosphate (PIP3) levels prevented recruitment and activation of the downstream mediator Akt. Release of the actin capping proteins gelsolin and CapZ in response to fMLP was also inhibited by Msp exposure. Chemical inhibition of PTEN restored PIP3 signaling, as measured by Akt activation, Rac1 activation, actin uncapping, neutrophil polarization and chemotaxis in response to fMLP-stimulation, even in the presence of Msp. Transduction with active Rac1 also restored fMLP-mediated actin uncapping, suggesting that Msp acts at the level of PIP3 in the hierarchical feedback loop of PIP3 and Rac1 activation. Taken together, Msp alters the phosphoinositide balance in neutrophils, impairing the cell “compass”, which leads to inhibition of downstream chemotactic events.

TRPC6 regulates CXCR2-mediated chemotaxis of murine neutrophils

Unraveling the mechanisms involved in chemotactic navigation of immune cells is of particular interest for the development of new immunoregulatory therapies. It is generally agreed upon that members of the classical transient receptor potential channel family (TRPC) are involved in chemotaxis. However, the regulatory role of TRPC channels in chemoattractant receptor-mediated signaling has not yet been clarified in detail. In this study, we demonstrate that the TRPC6 channels play a pronounced role in CXCR2-mediated intermediary chemotaxis, whereas N-formyl-methionine-leucine-phenylalanine receptor-mediated end-target chemotaxis is TRPC6 independent. The knockout of TRPC6 channels in murine neutrophils led to a strongly impaired intermediary chemotaxis after CXCR2 activation which is not further reinforced by CXCR2, PI3K, or p38 MAPK inhibition. Furthermore, CXCR2-mediated Ca(2+) influx but not Ca(2+) store release was attenuated in TRPC6(-/-) neutrophils. We demonstrate that the TRPC6 deficiency affected phosphorylation of AKT and MAPK downstream of CXCR2 receptor activation and led to altered remodeling of actin. The relevance of this TRPC6-depending defect in neutrophil chemotaxis is underscored by our in vivo findings. A nonseptic peritoneal inflammation revealed an attenuated recruitment of neutrophils in the peritoneal cavity of TRPC6(-/-) mice. In summary, this paper defines a specific role of TRPC6 channels in CXCR2-induced intermediary chemotaxis. In particular, TRPC6-mediated supply of calcium appears to be critical for activation of downstream signaling components.

Loss of phosphoinositide 3-kinase P110γ is protective in the acute phase but detrimental in the resolution phase of hapten-induced colitis

BACKGROUND

Pharmacologic inhibition or genetic ablation of phosphoinositide 3-kinase gamma (PI3Kγ) has been shown to be protective against experimental colitis. However, the role of PI3Kγ in the resolution phase of colitis remains unexplored. In this study, we assess the effects of genetic knockout of PI3Kγ on the induction and resolution of colitis induced by the hapten trinitrobenzene sulfonic acid (TNBS).

METHODS

Colitis was induced in wild-type C57/Bl6 or PI3Kγ-/- mice by intrarectal administration of 2.5 mg of TNBS in 50% ethanol. Body weights were monitored daily, and colon tissues were collected at days 3, 7, or 14 after treatment, and colitis was assessed using disease activity and histologic damage scores, measurement of tissue myeloperoxidase and neutrophil infiltration, and local cytokine production.

RESULTS

Mice lacking PI3Kγ were significantly protected from disease during the acute phase (day 3) of TNBS colitis. However, PI3Kγ-/- mice have difficulty resolving acute inflammation because they failed to restore lost weight and had significantly elevated histologic damage scores and tissue myeloperoxidase levels at days 7 and 14 after TNBS administration compared with wild-type controls. This phenomenon was dependent on presensitization with TNBS and seems to involve an inability to clear invading bacteria, resulting in the generation of a persistent inflammatory cytokine response.

CONCLUSIONS

This study confirms that PI3Kγ plays a role in the induction of colitis. However, PI3Kγ is also required for the resolution of intestinal damage following acute inflammation. This must be taken into consideration before the inhibition of PI3Kγ can be used as a treatment for disorders such as inflammatory bowel disease.

Requirement for both receptor-operated and store-operated calcium entry in N-formyl-methionine-leucine-phenylalanine-induced neutrophil polarization

Tissue penetration of neutrophils is a key process in many inflammatory diseases. In response to inflammatory stimuli such as N-formyl-methionine-leucine-phenylalanine (fMLP), neutrophils polarize and migrate towards the chemotactic gradient of the stimulus. Elevated intracellular Ca(2+) concentration is known to play a critical role in neutrophil polarization and migration; however, the exact mechanism remains elusive. Here, we demonstrated that fMLP stimulation caused not only store-operated calcium entry (SOCE), but also receptor-operated calcium entry (ROCE) in neutrophils by using both pharmacological and neutralizing monoclonal antibody approaches. We also investigated neither Rac2 nor Cdc42 activation could take place if either SOCE or ROCE was inhibited. This study thus provides the first evidence for coordination of Ca(2+) influx by SOCE and ROCE to regulate neutrophil polarization.

P38 and JNK have opposing effects on persistence of in vivo leukocyte migration in zebrafish

The recruitment and migration of macrophages and neutrophils is an important process during the early stages of the innate immune system in response to acute injury. Transgenic pu.1:EGFP zebrafish permit the acquisition of leukocyte migration trajectories during inflammation. Currently, these high-quality live-imaging data are mainly analysed using general statistics, for example, cell velocity. Here, we present a spatio-temporal analysis of the cell dynamics using transition matrices, which provide information of the type of cell migration. We find evidence that leukocytes exhibit types of migratory behaviour, which differ from previously described random walk processes. Dimethyl sulfoxide treatment decreased the level of persistence at early time points after wounding and ablated temporal dependencies observed in untreated embryos. We then use pharmacological inhibition of p38 and c-Jun N-terminal kinase mitogen-activated protein kinases to determine their effects on in vivo leukocyte migration patterns and discuss how they modify the characteristics of the cell migration process. In particular, we find that their respective inhibition leads to decreased and increased levels of persistent motion in leukocytes following wounding. This example shows the high level of information content, which can be gained from live-imaging data if appropriate statistical tools are used.

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.

PI3Ks and small GTPases in neutrophil migration: two sides of the same coin

Cell migration is a key event in physiological processes such as embryonic development, tissue repair, angiogenesis and immune responses. Alteration of the migration program is an important component in multiple pathologies, including chronic inflammation, autoimmunity and tumor metastasis. Understanding of the precise mechanisms at the basis of cellular migration may lead to the identification of novel therapeutic approach for these diseases. Recent evidences show that the interplay between the lipid kinases phosphatidylinositol 3-kinase (PI3Ks) and small GTPases play a critical role in driving cell migration. In this review we will describe the role of these molecules and the interaction between their signal cascades in leukocyte polarization and amoeboid migration.

Regulation of PTEN activity by p38δ-PKD1 signaling in neutrophils confers inflammatory responses in the lung

Deletion of p38 MAP kinase p38 d results in decreased alveolar neutrophil accumulation and attenuation of acute lung injury through activation of protein kinase D1 and PTEN.

Despite their role in resolving inflammatory insults, neutrophils trigger inflammation-induced acute lung injury (ALI), culminating in acute respiratory distress syndrome (ARDS), a frequent complication with high mortality in humans. Molecular mechanisms underlying recruitment of neutrophils to sites of inflammation remain poorly understood. Here, we show that p38 MAP kinase p38δ is required for recruitment of neutrophils into inflammatory sites. Global and myeloid-restricted deletion of p38δ in mice results in decreased alveolar neutrophil accumulation and attenuation of ALI. p38δ counteracts the activity of its downstream target protein kinase D1 (PKD1) in neutrophils and myeloid-restricted inactivation of PKD1 leads to exacerbated lung inflammation. Importantly, p38δ and PKD1 conversely regulate PTEN activity in neutrophils, thereby controlling their extravasation and chemotaxis. PKD1 phosphorylates p85α to enhance its interaction with PTEN, leading to polarized PTEN activity, thereby regulating neutrophil migration. Thus, aberrant p38δ–PKD1 signaling in neutrophils may underlie development of ALI and life-threatening ARDS in humans.

A versatile assay for monitoring in vivo-like transendothelial migration of neutrophils

Spatiotemporal analysis of the inflammatory response has been limited by the difficulties of in vivo imaging and reconstitution of inflammation in vitro. Here, we present a novel method for establishing in vivo-like inflammatory models in a microfluidic device and quantitatively measuring the three-dimensional transmigration of neutrophils during the inflammatory process. This enabled us to concurrently characterize transendothelial migration behaviors of neutrophils under the influence of various inflammatory stimuli.

Shear-induced resistance to neutrophil activation via the formyl peptide receptor

The application of fluid shear stress on leukocytes is critical for physiological functions including initial adhesion to the endothelium, the formation of pseudopods, and migration into tissues. The formyl peptide receptor (FPR) on neutrophils, which binds to formyl-methionyl-leucyl-phenylalanine (fMLP) and plays a role in neutrophil chemotaxis, has been implicated as a fluid shear stress sensor that controls pseudopod formation. The role of shear forces on earlier indicators of neutrophil activation, such as L-selectin shedding and α(M)β(2) integrin activation, remains unclear. Here, human neutrophils exposed to uniform shear stress (0.1-4.0 dyn/cm(2)) in a cone-and-plate viscometer for 1-120 min showed a significant reduction in both α(M)β(2) integrin activation and L-selectin shedding after stimulation with 0.5 nM of fMLP. Neutrophil resistance to activation was directly linked to fluid shear stress, as the response increased in a shear stress force- and time-dependent manner. Significant shear-induced loss of FPR surface expression on neutrophils was observed, and high-resolution confocal microscopy revealed FPR internalized within neutrophils. These results suggest that physiological shear forces alter neutrophil activation via FPR by reducing L-selectin shedding and α(M)β(2) integrin activation in the presence of soluble ligand.

AKT-mediated regulation of polarization in differentiated human neutrophil-like HL-60 cells

OBJECTIVES

Neutrophil polarization is critical for the inflammatory response. AKT is a serine/threonine protein kinase and has been implicated in cell migration. However, it is not completely clear whether AKT affects neutrophil polarization. In this study, we tested the hypothesis that AKT regulates the polarization of neutrophil-like differentiated HL-60 cells (dHL-60) in response to fMLP.

METHODS

HL-60 cells were differentiated into dHL-60 by incubation in medium containing 1.3 % DMSO for up to 6 days. Polarization of dHL-60 cells and primary human neutrophils were measured by Zigmond chamber. Phospho-Akt was analyzed by immunofluorescence and Western blot analysis. F-actin polymerization was detected by Rhodamine-Phalloidine staining. Rac2 activation was evaluated using GST Pull-down assay.

RESULTS

We found that changes in the rate of cell polarization were consistent with the changes in AKT phosphorylation levels during HL-60 cell differentiation in response to fMLP. Moreover, cell polarization and AKT phosphorylation were reduced in fMLP-stimulated dHL-60 cells pretreated with the PI3 kinase inhibitors or the AKT inhibitors, which was confirmed in the primary human neutrophils. The AKT inhibitors altered fMLP-induced F-actin polymerization. Rac2 GTPases was also decreased by the AKT inhibitors in fMLP-stimulated dHL-60 cells.

CONCLUSION

This study demonstrates that AKT activation plays a crucial role in dHL-60 cell polarization.

The 3BP2 adapter protein is required for chemoattractant-mediated neutrophil activation

3BP2 is a pleckstrin homology and Src homology 2 domain-containing adapter protein mutated in cherubism, a rare autosomal-dominant human bone disorder. Previously, we have demonstrated a functional role for 3BP2 in peripheral B cell development and in peritoneal B1 and splenic marginal zone B cell-mediated Ab responses. In this study, we show that 3BP2 is required for G protein-coupled receptor-mediated neutrophil functions. Neutrophils derived from 3BP2-deficient (Sh3bp2-/-) mice failed to polarize their actin cytoskeleton or migrate in response to a gradient of chemotactic peptide, fMLF. Sh3bp2-/- neutrophils failed to adhere, crawl, and emigrate out of the vasculature in response to fMLF superfusion. 3BP2 is required for optimal activation of Src family kinases, small GTPase Rac2, neutrophil superoxide anion production, and for Listeria monocytogenes bacterial clearance in vivo. The functional defects observed in Sh3bp2-/- neutrophils may partially be explained by the failure to fully activate Vav1 guanine nucleotide exchange factor and properly localize P-Rex1 guanine nucleotide exchange factor at the leading edge of migrating cells. Our results reveal an obligate requirement for the adapter protein 3BP2 in G protein-coupled receptor-mediated neutrophil function.

Neutrophil chemotaxis within a competing gradient of chemoattractants

The dynamics of neutrophil chemotaxis under competing chemoattractant gradients was studied using a microfluidic platform. This microfluidic platform, which establishes a stable and dynamic gradient of chemoattractants across a cell culture chamber, enabled the investigation of human neutrophil migration patterns in the presences of four different chemoattractants (leukotriene B(4), chemokine C-X-C motif ligands 2 and 8, and fMLP) and competing gradients of all pairwise combinations. The migration patterns for individual cells were tracked and quantitatively analyzed, and the results suggest a hierarchy among these chemoattractants of fMLP > CXCL8 > CXCL2 > leukotriene B(4). In all conditions, over 60% of neutrophils exposed to a competing gradient move toward the stronger signal though the weaker chemoattractant still influences neutrophil motility. These results yield insight about how each chemoattractant contributes to overall neutrophil chemotaxis within complex physiological environments.

Actin polymerization stabilizes α4β1 integrin anchors that mediate monocyte adhesion

Rapid signaling and structural adaptations to the actin cytoskeleton enable leukocytes to stabilize α4 integrin–mediated adhesion and resist detachment from inflamed endothelium.

Leukocytes arrested on inflamed endothelium via integrins are subjected to force imparted by flowing blood. How leukocytes respond to this force and resist detachment is poorly understood. Live-cell imaging with Lifeact-transfected U937 cells revealed that force triggers actin polymerization at upstream α4β1 integrin adhesion sites and the adjacent cortical cytoskeleton. Scanning electron microscopy revealed that this culminates in the formation of structures that anchor monocyte adhesion. Inhibition of actin polymerization resulted in cell deformation, displacement, and detachment. Transfection of dominant-negative constructs and inhibition of function or expression revealed key signaling steps required for upstream actin polymerization and adhesion stabilization. These included activation of Rap1, phosphoinositide 3-kinase γ isoform, and Rac but not Cdc42. Thus, rapid signaling and structural adaptations enable leukocytes to stabilize adhesion and resist detachment forces.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

LTB4 is a signal-relay molecule during neutrophil chemotaxis

Neutrophil recruitment to inflammation sites purportedly depends on sequential waves of chemoattractants. Current models propose that leukotriene B(4) (LTB(4)), a secondary chemoattractant secreted by neutrophils in response to primary chemoattractants such as formyl peptides, is important in initiating the inflammation process. In this study we demonstrate that LTB(4) plays a central role in neutrophil activation and migration to formyl peptides. We show that LTB(4) production dramatically amplifies formyl peptide-mediated neutrophil polarization and chemotaxis by regulating specific signaling pathways acting upstream of actin polymerization and MyoII phosphorylation. Importantly, by analyzing the migration of neutrophils isolated from wild-type mice and mice lacking the formyl peptide receptor 1, we demonstrate that LTB(4) acts as a signal to relay information from cell to cell over long distances. Together, our findings imply that LTB(4) is a signal-relay molecule that exquisitely regulates neutrophil chemotaxis to formyl peptides, which are produced at the core of inflammation sites.

The role of neutrophils in autoimmune diseases

Though chronic autoimmune disorders such as rheumatoid arthritis or systemic lupus erythematosus affect a significant percentage of the human population and strongly diminish the quality of life and life expectancy in Western societies, the molecular pathomechanisms of those diseases are still poorly understood, hindering the development of novel treatment strategies. Autoimmune diseases are thought to be caused by disturbed recognition of foreign and self antigens, leading to the emergence of autoreactive T-cells (so-called immunization phase). Those autoreactive T-cells then trigger the second (so-called effector) phase of the disease which is characterized by immune-mediated damage to host tissues. For a long time, neutrophils have mainly been neglected as potential players of the development of autoimmune diseases. However, a significant amount of new experimental data now indicates that neutrophils likely play an important role in both the immunization and the effector phase of autoimmune diseases. Here we review the current literature on the role of neutrophils in autoimmune diseases with special emphasis on rheumatoid arthritis, systemic lupus erythematosus, autoimmune vasculitides and blistering skin diseases. We also discuss the role of neutrophil cell surface receptors (e.g. integrins, Fc-receptors or chemokine receptors) and intracellular signal transduction pathways (e.g. Syk and other tyrosine kinases) in the pathogenesis of autoimmune inflammation. Though many of the results discussed in this review were obtained using animal models, additional data indicate that those mechanisms likely also contribute to human pathology. Taken together, neutrophils should be considered as one of the important cell types in autoimmune disease pathogenesis and they may also prove to be suitable targets of the pharmacological control of those diseases in the future.

TLR signaling paralyzes monocyte chemotaxis through synergized effects of p38 MAPK and global Rap-1 activation

Toll-like receptors (TLRs) that recognize pathogen associated molecular patterns and chemoattractant receptors (CKRs) that orchestrate leukocyte migration to infected tissue are two arms of host innate immunity. Although TLR signaling induces synthesis and secretion of proinflammatory cytokines and chemokines, which recruit leukocytes, many studies have reported the paradoxical observation that TLR stimulation inhibits leukocyte chemotaxis in vitro and impairs their recruitment to tissues during sepsis. There is consensus that physical loss of chemokine receptor (CKR) at the RNA or protein level or receptor usage switching are the mechanisms underlying this effect. We show here that a brief (<15 min) stimulation with LPS (lipopolysaccharide) at ~0.2 ng/ml inhibited chemotactic response from CCR2, CXCR4 and FPR receptors in monocytes without downmodulation of receptors. A 3 min LPS pre-treatment abolished the polarized accumulation of F-actin, integrins and PIP(3) (phosphatidylinositol-3,4,5-trisphosphate) in response to chemokines in monocytes, but not in polymorphonuclear neutrophils (PMNs). If chemoattractants were added before or simultaneously with LPS, chemotactic polarization was preserved. LPS did not alter the initial G-protein signaling, or endocytosis kinetics of agonist-occupied chemoattractant receptors (CKRs). The chemotaxis arrest did not result from downmodulation of receptors or from inordinate increase in adhesion. LPS induced rapid p38 MAPK activation, global redistribution of activated Rap1 (Ras-proximate-1 or Ras-related protein 1) GTPase and Rap1GEF (guanylate exchange factor) Epac1 (exchange proteins activated by cyclic AMP) and disruption of intracellular gradient. Co-inhibition of p38 MAPK and Rap1 GTPase reversed the LPS induced breakdown of chemotaxis suggesting that LPS effect requires the combined function of p38 MAPK and Rap1 GTPase.

Rac regulates PtdInsP₃ signaling and the chemotactic compass through a redox-mediated feedback loop

Directional cell migration is an essential requirement for efficient neutrophil translocation to sites of infection and requires the establishment of a polarized cell characterized by an actin-rich leading edge facing the chemoattractant gradient. The asymmetrical accumulation of phosphatidylinositol(3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] in the up-gradient leading edge is a hallmark of polarization and regulates the recruitment and localization of various effector proteins at the leading-edge plasma membrane. How shallow gradients of chemoattractants trigger and maintain a much steeper intracellular gradient of PtdIns(3,4,5)P(3) is a critical question in the study of leukocyte chemotaxis. Our data demonstrate that the migration of neutrophils toward the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine depends on the generation of reactive oxygen species by the phagocytic NADPH oxidase (NOX2) and subsequent oxidation and inhibition of phosphatase and tensin homolog. Moreover, we show that events downstream of PtdIns(3,4,5)P(3), including phosphorylation of AKT, Rac activation, uncapping of actin filaments, and directional migration, can be attenuated by ROS scavengers or genetic ablation of NOX2. Using Rac mutants that are defective in their ability to activate NOX2, we show that Rac regulates a redox-mediated feedback loop that mediates directional migration of neutrophils.

Image-based analysis of primary human neutrophil chemotaxis in an automated direct-viewing assay

Multi-well assays based on the Boyden chamber have enabled highly parallel studies of chemotaxis-the directional migration of cells in response to molecular gradients-while direct-viewing approaches have allowed more detailed questions to be asked at low throughput. Boyden-based plates provide a count of cells that pass through a membrane, but no information about cell appearance. In contrast, direct-viewing devices enable the observation of cells during chemotaxis, which allows measurement of many parameters including area, shape, and location. Here we show automated chemotaxis and cell morphology assays in a 96-unit direct-viewing plate. Using only 12000 primary human neutrophils per datum, we measured dose-dependent stimulation and inhibition of chemotaxis and quantified the effects of inhibitors on cell area and elongation. With 60 parallel conditions we demonstrated 5-fold increase in throughput compared to previously reported direct-viewing approaches.

PTEN negatively regulates engulfment of apoptotic cells by modulating activation of Rac GTPase

Efficient clearance of apoptotic cells by phagocytes (efferocytosis) is critical for normal tissue homeostasis and regulation of the immune system. Apoptotic cells are recognized by a vast repertoire of receptors on macrophage that lead to transient formation of phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] and subsequent cytoskeletal reorganization necessary for engulfment. Certain PI3K isoforms are required for engulfment of apoptotic cells, but relatively little is known about the role of lipid phosphatases in this process. In this study, we report that the activity of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a phosphatidylinositol 3-phosphatase, is elevated upon efferocytosis. Depletion of PTEN in macrophage results in elevated PtdIns(3,4,5)P(3) production and enhanced phagocytic ability both in vivo and in vitro, whereas overexpression of wild-type PTEN abrogates this process. Loss of PTEN in macrophage leads to activation of the pleckstrin homology domain-containing guanine-nucleotide exchange factor Vav1 and subsequent activation of Rac1 GTPase, resulting in increased amounts of F-actin upon engulfment of apoptotic cells. PTEN disruption also leads to increased production of anti-inflammatory cytokine IL-10 and decreased production of proinflammatory IL-6 and TNF-α upon engulfment of apoptotic cells. These data suggest that PTEN exerts control over efferocytosis potentially by regulating PtdIns(3,4,5)P(3) levels that modulate Rac GTPase and F-actin reorganization through Vav1 exchange factor and enhancing apoptotic cell-induced anti-inflammatory response.

Tracking neutrophil intraluminal crawling, transendothelial migration and chemotaxis in tissue by intravital video microscopy

The recruitment of circulating leukocytes from blood stream to the inflamed tissue is a crucial and complex process of inflammation(1,2). In the postcapillary venules of inflamed tissue, leukocytes initially tether and roll on the luminal surface of venular wall. Rolling leukocytes arrest on endothelium and undergo firm adhesion in response to chemokine or other chemoattractants on the venular surface. Many adherent leukocytes relocate from the initial site of adhesion to the junctional extravasation site in endothelium, a process termed intraluminal crawling(3). Following crawling, leukocytes move across endothelium (transmigration) and migrate in extravascular tissue toward the source of chemoattractant (chemotaxis)(4). Intravital microscopy is a powerful tool for visualizing leukocyte-endothelial cell interactions in vivo and revealing cellular and molecular mechanisms of leukocyte recruitment(2,5). In this report, we provide a comprehensive description of using brightfield intravital microscopy to visualize and determine the detailed processes of neutrophil recruitment in mouse cremaster muscle in response to the gradient of a neutrophil chemoattractant. To induce neutrophil recruitment, a small piece of agarose gel (~1-mm(3) size) containing neutrophil chemoattractant MIP-2 (CXCL2, a CXC chemokine) or WKYMVm (Trp-Lys-Tyr-Val-D-Met, a synthetic analog of bacterial peptide) is placed on the muscle tissue adjacent to the observed postcapillary venule. With time-lapsed video photography and computer software ImageJ, neutrophil intraluminal crawling on endothelium, neutrophil transendothelial migration and the migration and chemotaxis in tissue are visualized and tracked. This protocol allows reliable and quantitative analysis of many neutrophil recruitment parameters such as intraluminal crawling velocity, transmigration time, detachment time, migration velocity, chemotaxis velocity and chemotaxis index in tissue. We demonstrate that using this protocol, these neutrophil recruitment parameters can be stably determined and the single cell locomotion conveniently tracked in vivo.

Lectin-based isolation and culture of mouse embryonic motoneurons

Spinal motoneurons develop towards postmitotic stages through early embryonic nervous system development and subsequently grow out dendrites and axons. Neuroepithelial cells of the neural tube that express Nkx6.1 are the unique precursor cells for spinal motoneurons¹. Though postmitotic motoneurons move towards their final position and organize themselves into columns along the spinal tract^2,3. More than 90% of all these differentiated and positioned motoneurons express the transcription factors Islet 1/2. They innervate the muscles of the limbs as well as those of the body and the inner organs. Among others, motoneurons typically express the high affinity receptors for brain derived neurotrophic factor (BDNF) and Neurotrophin-3 (NT-3), the tropomyosin-related kinase B and C (TrkB, TrkC). They do not express the tropomyosin-related kinase A (TrkA)⁴. Beside the two high affinity receptors, motoneurons do express the low affinity neurotrophin receptor p75^NTR. The p75^NTR can bind all neurotrophins with similar but lower affinity to all neurotrophins than the high affinity receptors would bind the mature neurotrophins. Within the embryonic spinal cord, the p75^NTR is exclusively expressed by the spinal motoneurons⁵. This has been used to develop motoneuron isolation techniques to purify the cells from the vast majority of surrounding cells⁶. Isolating motoneurons with the help of specific antibodies (panning) against the extracellular domains of p75^NTR has turned out to be an expensive method as the amount of antibody used for a single experiment is high due to the size of the plate used for panning. A much more economical alternative is the use of lectin. Lectin has been shown to specifically bind to p75^NTR as well⁷. The following method describes an alternative technique using wheat germ agglutinin for a preplating procedure instead of the p75^NTR antibody. The lectin is an extremely inexpensive alternative to the p75^NTR antibody and the purification grades using lectin are comparable to that of the p75^NTR antibody. Motoneurons from the embryonic spinal cord can be isolated by this method, survive and grow out neurites.

Gene expression pattern in swine neutrophils after lipopolysaccharide exposure: a time course comparison

Background

Experimental exposure of swine neutrophils to bacterial lipopolysaccharide (LPS) represents a model to study the innate immune response during bacterial infection. Neutrophils can effectively limit the infection by secreting lipid mediators, antimicrobial molecules and a combination of reactive oxygen species (ROS) without new synthesis of proteins. However, it is known that neutrophils can modify the gene expression after LPS exposure. We performed microarray gene expression analysis in order to elucidate the less known transcriptional response of neutrophils during infection.

Methods

Blood samples were collected from four healthy Iberian pigs and neutrophils were isolated and incubated during 6, 9 and 18 hrs in presence or absence of lipopolysaccharide (LPS) from Salmonella enterica serovar Typhimurium. RNA was isolated and hybridized to Affymetrix Porcine GeneChip^®. Microarray data were normalized using Robust Microarray Analysis (RMA) and then, differential expression was obtained by an analysis of variance (ANOVA).

Results

ANOVA data analysis showed that the number of differentially expressed genes (DEG) after LPS treatment vary with time. The highest transcriptional response occurred at 9 hr post LPS stimulation with 1494 DEG whereas at 6 and 18 hr showed 125 and 108 DEG, respectively. Three different gene expression tendencies were observed: genes in cluster 1 showed a tendency toward up-regulation; cluster 2 genes showing a tendency for down-regulation at 9 hr; and cluster 3 genes were up-regulated at 9 hr post LPS stimulation. Ingenuity Pathway Analysis revealed a delay of neutrophil apoptosis at 9 hr. Many genes controlling biological functions were altered with time including those controlling metabolism and cell organization, ubiquitination, adhesion, movement or inflammatory response.

Conclusions

LPS stimulation alters the transcriptional pattern in neutrophils and the present results show that the robust transcriptional potential of neutrophils under infection conditions, indicating that active regulation of gene expression plays a major role in the neutrophil-mediated- innate immune response.

PI3K and chemotaxis: a priming issue?

Although the spatiotemporal activation of phosphoinositide 3-kinases (PI3Ks) at the leading edge of chemotaxing cells represents a key marker of polarity, both Dictyostelium discoideum and neutrophils lacking measurable PI3K activity can still migrate directionally under certain conditions. Evidence from various papers suggests that the differentiation state of cells or their priming status can consolidate otherwise contradictory findings.

In chemotaxing fibroblasts, both high-fidelity and weakly biased cell movements track the localization of PI3K signaling

Cell movement biased by a chemical gradient, or chemotaxis, coordinates the recruitment of cells and collective migration of cell populations. During wound healing, chemotaxis of fibroblasts is stimulated by platelet-derived growth factor (PDGF) and certain other chemoattractants. Whereas the immediate PDGF gradient sensing response has been characterized previously at the level of phosphoinositide 3-kinase (PI3K) signaling, the sensitivity of the response at the level of cell migration bias has not yet been studied quantitatively. In this work, we used live-cell total internal reflection fluorescence microscopy to monitor PI3K signaling dynamics and cell movements for extended periods. We show that persistent and properly aligned (i.e., high-fidelity) fibroblast migration does indeed correlate with polarized PI3K signaling; accordingly, this behavior is seen only under conditions of high gradient steepness (>10% across a typical cell length of 50 μm) and a certain range of PDGF concentrations. Under suboptimal conditions, cells execute a random or biased random walk, but nonetheless move in a predictable fashion according to the changing pattern of PI3K signaling. Inhibition of PI3K during chemotaxis is accompanied by loss of both cell-substratum contact and morphological polarity, but after a recovery period, PI3K-inhibited fibroblasts often regain the ability to orient toward the PDGF gradient.

Quercetin reduces neutrophil recruitment induced by CXCL8, LTB4, and fMLP: inhibition of actin polymerization

Recent in vitro data have suggested that the flavonoid quercetin (1) does not affect the functioning of neutrophils. Therefore, we evaluated in vivo and in vitro whether or not 1 affects neutrophil function, focusing on recruitment. The in vivo treatment with 1 inhibited in a dose-dependent manner the recruitment of neutrophils to the peritoneal cavity of mice induced by known chemotatic factors such as CXCL1, CXCL5, LTB(4), and fMLP. Furthermore, 1 also inhibited in a concentration-dependent manner the chemoattraction of human neutrophils induced by CXCL8, LTB(4), and fMLP in a Boyden chamber. In vitro treatment with 1 did not affect human neutrophil surface expression of CXCR1, CXCR2, BLT1, or FLPR1, but rather reduced actin polymerization. These results suggest that 1 inhibits actin polymerization, hence, explaining the inhibition of neutrophil recruitment in vivo and in vitro and highlighting its possible usefulness to diminish excessive neutrophil migration during inflammation.

Differential splenic cytokine responses to dietary immune modulation by diverse chicken lines

Nutritional modulation of the immune system is an often exploited but poorly characterized process. In chickens and other food production animals, dietary enhancement of the immune response is an attractive alternative to antimicrobial use. A yeast cell wall component, beta-1,3/1,6-glucan, augments the response to disease in poultry and other species; however, the mechanism of action is not clear. Ascorbic acid and corticosterone are better characterized immunomodulators. In chickens, the spleen acts both as reservoir and activation site for leukocytes and, therefore, splenic gene expression reflects systemic immune function. To determine effects of genetic line and dietary immunomodulators, chickens of outbred broiler and inbred Leghorn and Fayoumi lines were fed either a basal diet or an experimental diet containing beta-glucans, ascorbic acid, or corticosterone from 56 to 77 d of age. Spleens were harvested, mRNA was isolated, and expression of interleukin (IL)-4, IL-6, IL-18, macrophage inflammatory protein-1beta, interferon-gamma, and phosphoinositide 3-kinase p110gamma transcripts was measured by quantitative reverse transcription PCR. Effects of diet, genetic line, sex, and diet x genetic line interaction on weight gain and gene expression were analyzed. At 1, 2, and 3 wk after starting the diet treatments, birds fed the corticosterone diet had gained less weight compared with birds fed the other diets (P < 0.001). Sex affected expression of IL-18 (P = 0.010), with higher levels in males. There was a significant interaction between genetic line and diet on expression of IL-4, IL-6, and IL-18 (P = 0.021, 0.006, and 0.026, respectively). Broiler line gene expression did not change in response to the experimental diet. Splenic expression of IL-6 was higher in Leghorns fed the basal or ascorbic acid diets, rather than the beta-glucan or corticosterone diets, whereas the opposite relationship was observed in the Fayoumi line. Expression of IL-4 and IL-18 responded to diet only within the Fayoumi line. The differential splenic expression of birds from diverse genetic lines in response to nutritional immunomodulation emphasizes the need for further study of this process.

Slit2 regulates attractive eosinophil and repulsive neutrophil chemotaxis through differential srGAP1 expression during lung inflammation

Directional migration of leukocytes is an essential step in leukocyte trafficking during inflammatory responses. However, the molecular mechanisms governing directional chemotaxis of leukocytes remain poorly understood. The Slit family of guidance cues has been implicated for inhibition of leuocyte migration. We report that Clara cells in the bronchial epithelium secreted Slit2, whereas eosinophils and neutrophils expressed its cell-surface receptor, Robo1. Compared to neutrophils, eosinophils exhibited a significantly lower level of Slit-Robo GTPase-activating protein 1 (srGAP1), leading to activation of Cdc42, recruitment of PI3K to Robo1, enhancment of eotaxin-induced eosinophil chemotaxis, and exaggeration of allergic airway inflammation. Notably, OVA sensitization elicited a Slit2 gradient at so-called bronchus-alveoli axis, with a higher level of Slit2 in the bronchial epithelium and a lower level in the alveolar tissue. Aerosol administration of rSlit2 accelerated eosinophil infiltration, whereas i.v. administered Slit2 reduced eosinophil deposition. In contrast, Slit2 inactivated Cdc42 and suppressed stromal cell-derived factor-1α-induced chemotaxis of neutrophils for inhibiting endotoxin-induced lung inflammation, which were reversed by blockade of srGAP1 binding to Robo1. These results indicate that the newly identified Slit2 gradient at the bronchus-alveoli axis induces attractive PI3K signaling in eosinophils and repulsive srGAP1 signaling in neutrophils through differential srGAP1 expression during lung inflammation.

Understanding eukaryotic chemotaxis: a pseudopod-centred view

Current descriptions of eukaryotic chemotaxis and cell movement focus on how extracellular signals (chemoattractants) cause new pseudopods to form. This 'signal-centred' approach is widely accepted but is derived mostly from special cases, particularly steep chemoattractant gradients. I propose a 'pseudopod-centred' explanation, whereby most pseudopods form themselves, without needing exogenous signals, and chemoattractants only bias internal pseudopod dynamics. This reinterpretation of recent data suggests that future research should focus on pseudopod mechanics, not signal processing.

Hematopoietic Pyk2 regulates migration of differentiated HL-60 cells

Background

Pyk2 is a non-receptor cytoplasmic tyrosine kinase that belongs to the focal adhesion kinase family and has been implicated in neutrophil spreading and respiratory burst activity caused by TNF-α. However, the role of Pyk2 in neutrophil migration is incompletely defined. In this study, we tested the hypothesis that Pyk2 regulates the migration of neutrophil-like differentiated HL-60 cells subsequent to β2-integrin mediated cell adhesion.

Methods

HL-60 cells were induced to differentiate into neutrophil-like cells (dHL60) by incubation in medium containing 1.25% DMSO for up to 4 days. Pyk2 expression and tyrosine phosphorylation was measured by Western blot analysis. Adhesion of dHL60 cells to plated fibrinogen was measured by residual myeloperoxidase activity. dHL60 cell migration was evaluated using a 96-well chemoTx chamber.

Results

Western blot analysis demonstrated that hematopoietic Pyk2 was predominantly expressed after HL60 cell differentiation. Pyk2 was tyrosine phosphorylated upon adhesion of dHL60 cells to plated fibrinogen in the presence of fMLP. By contrast, tyrosine phosphorylation of Pyk2 was insignificant in dHL60 cells treated in suspension with fMLP. Antibodies against CD18 blocked both phosphorylation of Pyk2 and adhesion of dHL60 cells to fibrinogen, demonstrating that phosphorylation of Pyk2 was β₂-integrin dependent. TAT-Pyk2-CT, a dominant negative fusion protein in which the TAT protein transduction domain was fused to the c-terminal Pyk2, attenuated fMLP-stimulated spreading, migration and phosphorylation of endogenous Pyk2 without blocking adhesion of dHL-60 cells to fibrinogen. Similarly, silencing of Pyk2 expression by siRNA in dHL60 cells also attenuated dHL60 cell migration caused by fMLP. Phospho-Pyk2 was evenly distributed around cell membrane circumferentially in unstimulated dHL-60 cells adherent to plated fibrinogen. In dHL60 cells treated with fMLP to cause cell spreading and polarization, Pyk2 was concentrated at the leading edge of pseudopods or at the trailing edge of uropods during migration of neutrophilic dHL-60 cells.

Conclusions

We conclude that Pyk2 is activated by β2-integrin adhesion. The activated concentration of Pyk2 and colocalization with F-actin in pseudopodia suggests that Pyk2 may regulate cell spreading and migration in dHL60 cells.

Induction of neutrophil degranulation by S100A9 via a MAPK-dependent mechanism

S100A9 is a proinflammatory protein, expressed abundantly in the cytosol of neutrophils and monocytes. High extracellular S100A9 concentrations have been correlated with chronic inflammatory diseases such as rheumatoid arthritis and Crohn's disease, as well as with phagocyte extravasation. This study tested the hypothesis that S100A9 induces degranulation in human neutrophils. S100A9 was found to up-regulate the surface expression of CD35 and CD66b, proteins contained in secretory vesicles and specific/gelatinase granules, respectively. In addition, gelatinase and albumin, stored, respectively, in specific/gelatinase granules and secretory vesicles, were detected in the supernatants of neutrophils stimulated with S100A9. In contrast, stimulation with S100A9 had no effect on CD63 expression or MPO secretion, two proteins contained in azurophilic granules. S100A9 induced the phosphorylation of the MAPKs, ERK1/2, p38, and JNK. Inhibition of p38 and JNK but not ERK1/2, with specific inhibitors (SB203580, JNKII, and PD98059, respectively), blocked neutrophil degranulation induced by S100A9. Taken together, these results support the hypothesis and clearly indicate that S100A9 induces the degranulation of secretory and specific/gelatinase granules but not of azurophilic granules in a process involving p38 and JNK and further support its classification as a DAMP.

PI3Kgamma protects from myocardial ischemia and reperfusion injury through a kinase-independent pathway

Background

PI3Kγ functions in the immune compartment to promote inflammation in response to G-protein-coupled receptor (GPCR) agonists and PI3Kγ also acts within the heart itself both as a negative regulator of cardiac contractility and as a pro-survival factor. Thus, PI3Kγ has the potential to both promote and limit M I/R injury.

Methodology/Principal Findings

Complete PI3Kγ^−/− mutant mice, catalytically inactive PI3Kγ^KD/KD (KD) knock-in mice, and control wild type (WT) mice were subjected to in vivo myocardial ischemia and reperfusion (M I/R) injury. Additionally, bone-marrow chimeric mice were constructed to elucidate the contribution of the inflammatory response to cardiac damage. PI3Kγ^−/− mice exhibited a significantly increased infarction size following reperfusion. Mechanistically, PI3Kγ is required for activation of the Reperfusion Injury Salvage Kinase (RISK) pathway (AKT/ERK1/2) and regulates phospholamban phosphorylation in the acute injury response. Using bone marrow chimeras, the cardioprotective role of PI3Kγ was mapped to non-haematopoietic cells. Importantly, this massive increase in M I/R injury in PI3Kγ^−/− mice was rescued in PI3Kγ kinase-dead (PI3Kγ^KD/KD) knock-in mice. However, PI3Kγ^KD/KD mice exhibited a cardiac injury similar to wild type animals, suggesting that specific blockade of PI3Kγ catalytic activity has no beneficial effects.

Conclusions/Significance

Our data show that PI3Kγ is cardioprotective during M I/R injury independent of its catalytic kinase activity and that loss of PI3Kγ function in the hematopoietic compartment does not affect disease outcome. Thus, clinical development of specific PI3Kγ blockers should proceed with caution.

Molecular regulators of leucocyte chemotaxis during inflammation

A fundamental feature of any immune response is the movement of leucocytes from one site in the body to another to provide effector functions. Therefore, elucidating the molecular mechanisms underlying the migration of leucocytes from the blood to tissues is critical to our understanding of immune function during inflammation. The classic steps of leucocyte trafficking involve leucocyte tethering and rolling on vessel walls of the vasculature, followed by firm adhesion to the endothelium. Recent evidence suggests that upon adhering, leucocytes crawl within the vessels before transmigrating across vessel walls and crawling into targeted tissues. The directed nature of the crawling events is orchestrated by a complex array of soluble factors and molecular regulators in combination with the local intravascular and extracellular environment. In fact, this process is known as chemotaxis and orientates cell movement in relation to the ligand gradient. Several signalling pathways have been proposed to be involved in this gradient-sensing and amplification process, but the best studied, discussed in detail here, is the phosphatidylinositol 3-kinase pathway. Substantial progress has been made in understanding how cells roll and adhere in blood vessels; however, how cells crawl in blood vessels, emigrate, and then crawl in tissues has received much less attention. Therefore, the focus of this review is to provide recent insights into molecular mechanisms and cellular processes that mediate leucocyte crawling in blood vessels and tissues during the inflammatory response.