SRC-dependent outside-in signalling is a key step in the process of autoregulation of beta2 integrins in polymorphonuclear cells

Targeting Neutrophil β2-Integrins: A Review of Relevant Resources, Tools, and Methods

Neutrophils are important innate immune cells that respond during inflammation and infection. These migratory cells utilize β₂-integrin cell surface receptors to move out of the vasculature into inflamed tissues and to perform various anti-inflammatory responses. Although critical for fighting off infection, neutrophil responses can also become dysregulated and contribute to disease pathophysiology. In order to limit neutrophil-mediated damage, investigators have focused on β₂-integrins as potential therapeutic targets, but so far these strategies have failed in clinical trials. As the field continues to move forward, a better understanding of β₂-integrin function and signaling will aid the design of future therapeutics. Here, we provide a detailed review of resources, tools, experimental methods, and in vivo models that have been and will continue to be utilized to investigate the vitally important cell surface receptors, neutrophil β₂-integrins.

The regulatory roles of glycosphingolipid-enriched lipid rafts in immune systems

Lipid rafts formed by glycosphingolipids (GSLs) on cellular membranes play important roles in innate and adaptive immunity. Lactosylceramide (LacCer) forms lipid rafts on plasma and granular membranes of human neutrophils. These LacCer-enriched lipid rafts bind directly to pathogenic components, such as pathogenic fungi-derived β-glucan and Mycobacteria-derived lipoarabinomannan via carbohydrate-carbohydrate interactions, and mediate innate immune responses to these pathogens. In contrast, a-series and o-series gangliosides form distinct rafts on CD4⁺ and CD8⁺ T cell subsets, respectively, contributing to the respective functions of these cells and stimulating adaptive immune responses through T cell receptors. These findings suggest that gangliosides play indispensable roles in T cell selection and activation. This Review introduces the involvement of GSL-enriched lipid rafts in innate and adaptive immunity.

[Lactosylceramide-enriched Lipid Raft-mediated Infection Immunity]

More than 100 years have passed since Elie Metchnikoff discovered phagocytes. As molecular biological techniques have been developed and improved, we have gained deeper knowledge about the molecular mechanisms of immunological responses to invasion. The innate immune system is the inborn defense mechanism and the first line of defense against all kinds of pathogenic organisms, including bacteria, fungi, viruses, etc. Innate immunity was originally considered to comprise non-specific reactions. However, we now know that innate immune systems develop molecular mechanisms specific to pathogenic microorganisms. In the 1970s, a neutral glycosphingolipid lactosylceramide (LacCer) was found to bind specifically to several kinds of microorganisms. LacCer is highly expressed in phagocytes and epithelial cells. LacCer forms lipid rafts on human neutrophils and is involved in neutrophil migration, phagocytosis, and superoxide generation. In contrast, mouse neutrophils express relatively little LacCer on their cell surfaces. Thus, it is difficult to observe LacCer-mediated innate immunological reactions in mice. Mycobacterium tuberculosis is a typical pathogen for humans but not mice in general. Interestingly, M. tuberculosis can escape killing by neutrophils through regulation of the LacCer-enriched lipid raft-mediated immunological reactions of these cells. These observations indicate that LacCer-enriched lipid rafts play an essential role in human innate immunity. This review describes LacCer-mediated innate immunity in humans.

The Neutrophil's Choice: Phagocytose vs Make Neutrophil Extracellular Traps

Neutrophils recognize particulate substrates of microbial or endogenous origin and react by sequestering the cargo via phagocytosis or by releasing neutrophil extracellular traps (NETs) outside the cell, thus modifying and alerting the environment and bystander leukocytes. The signals that determine the choice between phagocytosis and the generation of NETs are still poorly characterized. Neutrophils that had phagocytosed bulky particulate substrates, such as apoptotic cells and activated platelets, appear to be “poised” in an unresponsive state. Environmental conditions, the metabolic, adhesive and activation state of the phagocyte, and the size of and signals associated with the tethered phagocytic cargo influence the choice of the neutrophils, prompting either phagocytic clearance or the generation of NETs. The choice is dichotomic and apparently irreversible. Defects in phagocytosis may foster the intravascular generation of NETs, thus promoting vascular inflammation and morbidities associated with diseases characterized by defective phagocytic clearance, such as systemic lupus erythematosus. There is a strong potential for novel treatments based on new knowledge of the events determining the inflammatory and pro-thrombotic function of inflammatory leukocytes.

Gangliosides in the Immune System: Role of Glycosphingolipids and Glycosphingolipid-Enriched Lipid Rafts in Immunological Functions

Although individuals are constantly exposed to infectious agents, these agents are generally resisted by the innate and acquired immune systems. Both the innate and acquired immune systems protect against invading organisms, but they differ functionally in several ways. The innate immune system is the body's inborn defense mechanism and the first line of defense against invading organisms, such as bacteria, fungi, and viruses. Glycosphingolipids (GSLs), which are expressed on the outer leaflet of plasma membranes (Murate et al., J Cell Sci 128(8):1627-1638, 2015), are involved in both innate and acquired immunity (Inokuchi et al., Biochim Biophys Acta 1851(1):98-106, 2015; Nakayama et al., Arch Immunol Ther Exp (Warsz) 61(3):217-228, 2013; Rueda, Br J Nutr 98(Suppl 1):S68-73, 2007; Popa and Portoukalian, Pathol Biol (Paris) 51(5):253-255, 2003).Recent studies have indicated that innate immunity is not a "nonspecific" immune system. Large numbers of viruses, bacteria, and bacterial toxins have been reported to bind to host surface carbohydrates, a number of which are components of GSLs (Schengrund, Biochem Pharmacol 65(5):699-707, 2003). Binding studies have also demonstrated that some glycolipids function as receptors for microorganisms and bacterial toxins (Yates and Rampersaud, Ann N Y Acad Sci 845:57-71, 1998). These findings clearly indicate that GSLs are involved in host-pathogen interactions.GSLs are composed of hydrophobic ceramide and hydrophilic sugar moieties (Hakomori, Annu Rev Biochem 50:733-764, 1980). The ceramide moiety of sphingolipids and the cholesterol sterol-ring system are thought to interact via hydrogen bonds and hydrophobic van der Waal's forces (Mukherjee and Maxfield, Annu Rev Cell Dev Biol 20:839-866, 2004). Additional hydrophilic cis interactions among GSL headgroups have been found to promote their lateral associations with surrounding lipid and protein membrane components. These interactions result in the separation in cell membranes of lipid rafts, which are lipid domains rich in GSLs, cholesterol, glycosylphosphatidylinositol (GPI)-anchored proteins and membrane-anchored signaling molecules (Pike, J Lipid Res 47(7):1597-1598, 2006). These GSL-enriched lipid rafts play important roles in immunological functions (Inokuchi et al., Biochim Biophys Acta 1851(1):98-106, 2015; Iwabuchi et al., Mediators Inflamm 2015:120748, 2015; Anderson and Roche, Biochim Biophys Acta 1853(4):775-780, 2015; Zuidscherwoude et al., J Leukoc Biol 95(2):251-263, 2014; Dykstra et al., Annu Rev Immunol 21:457-481, 2003). This introductory chapter describes the roles of GSLs and their lipid rafts in the immune system.

Roflumilast inhibits leukocyte-platelet interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes and monocytes

ESSENTIALS: Thrombosis is a major comorbidity in patients with chronic obstructive pulmonary disease (COPD). Roflumilast is a selective phosphodiesterase type-4 (PDE4) inhibitor approved for treatment of severe COPD. PDE4 blockade by roflumilast inhibits prothrombotic functions of neutrophils and monocytes. PDE4 inhibitors may reduce thrombotic risk in COPD as well as in other vascular diseases.

BACKGROUND

Roflumilast, an oral selective phosphodiesterase type 4 inhibitor, is approved for the treatment of severe chronic obstructive pulmonary disease (COPD). A recent meta-analysis of trials on COPD revealed that treatment with roflumilast was associated with a significant reduction in the rate of major cardiovascular events. The mechanisms of this effect remain unknown.

OBJECTIVES

We tested the hypothesis that roflumilast N-oxide (RNO), the active metabolite of roflumilast, curbs the molecular mechanisms required for leukocyte-platelet (PLT) interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes (PMNs) and monocytes (MNs).

METHODS

Using well-characterized in vitro models, we analysed the effects of RNO on: (i) PMN adhesiveness; (ii) the release of neutrophil extracellular traps (NETs); and (iii) tissue factor expression in MNs. Key biochemical events underlying the inhibitory effects of RNO were defined.

RESULTS AND CONCLUSIONS

In PMNs, RNO prevented phosphoinositide 3-kinase (PI3K)-dependent phosphorylation of Akt on Ser473, and Src family kinase (SFK)-mediated Pyk2 phosphorylation on Tyr579-580, while inducing protein kinase A-mediated phosphorylation of C-terminal Src kinase, the major negative regulator of SFKs. Modulation of these signaling pathways by RNO resulted in a significant impairment of PMN adhesion to activated PLTs or human umbilical vein endothelial cells, mainly mediated by inhibition of the adhesive function of Mac-1. Moreover RNO curbed SFK/PI3K-mediated NET release by PMNs adherent on fibrinogen-coated surfaces. In MNs interacting with activated PLTs, RNO curbed PI3K-mediated expression of tissue factor. The efficacy of RNO was significantly potentiated by formoterol, a long acting β-adrenergic receptor agonist. This study reveals novel antithrombotic activities by which roflumilast may exert protective effects against cardiovascular comorbodities in COPD.

Organization and functions of glycolipid-enriched microdomains in phagocytes

Populations of glycolipids change markedly during leukocyte differentiation, suggesting that these molecules are involved in biological functions. About 70% of the glycosphingolipids in human neutrophils are lactosylceramide, a molecule also expressed on monocytes and dendritic cells, but not on lymphocytes. In contrast, phosphatidylglucoside is mainly expressed on neutrophils. STED microscopic analysis showed that phosphatidylglucoside and lactosylceramide form different domains on plasma membranes of neutrophils, with phosphatidylglucoside preferentially expressed along the neutrophil differentiation pathway. Phosphatidylglucoside was found to mediate the differentiation of HL-60 cells into the neutrophilic lineage, and to be involved in FAS-dependent neutrophil apoptosis. In contrast, lactosylceramide was only expressed on mature neutrophils. Complexes of lactosylceramide and the Src family kinase Lyn form membrane microdomains. LacCer-enriched membrane microdomains mediate neutrophil innate immune responses; e.g. chemotaxis, phagocytosis, and superoxide generation. C24 fatty acid chains of LacCer are indispensable for the formation of LacCer-Lyn complexes and for LacCer-dependent functions. Moreover, Lyn-coupled LacCer-enriched microdomains serve as signal transduction platforms for αMβ2 integrin-mediated phagocytosis. This review describes the organization and potential functions of glycolipids in phagocytes, as well as the roles of both phosphatidylglucoside and lactosylceramide in neutrophils. This article is part of a Special Issue entitled Linking transcription to physiology in lipidomics.

Phosphodiesterase type 4 blockade prevents platelet-mediated neutrophil recruitment at the site of vascular injury

OBJECTIVE

Platelet-neutrophil interactions play a key role in cardiovascular disease and inflammatory processes. Src family kinases mediate P-selectin glycoprotein ligand-1-Mac-1 cross talk necessary for firm platelet-neutrophil adhesion. Because Src family kinase activity can be regulated by cAMP-dependent pathways, in this work, we evaluated the role of phosphodiesterases in the signaling events that are required to sustain platelet-neutrophil interactions and neutrophil recruitment at the site of vascular injury.

APPROACH AND RESULTS

In neutrophils exposed to P-selectin, selective phosphodiesterase 4 (PDE4) inhibition prevented Src family kinase-mediated phosphorylation of the proline-rich tyrosine kinase 2 on Tyr579/Tyr580. The effects of PDE4 inhibition required protein kinase A, likely through protein kinase A-mediated activation of COOH-terminal Src kinase, a major negative regulator of Src family kinases. PDE4, but not other phosphodiesterase inhibitors, reduced platelet-neutrophil conjugates as well as neutrophil firm adhesion on spread platelets under flow conditions. The effect of PDE4 inhibition on neutrophil adhesion was primarily mediated by downregulation of P-selectin-induced activation of Mac-1. In a murine model of endovascular injury, selective inhibition of PDE4 significantly reduced neutrophil recruitment at the site of vascular damage.

CONCLUSIONS

This study identifies PDE4 as a central node in the signaling network that mediates platelet-neutrophil adhesion and suggests that pharmacological inhibition of PDE4 may represent a novel therapeutic avenue for the treatment of cardiovascular disease.

Discovery of platyhelminth-specific α/β-integrin families and evidence for their role in reproduction in Schistosoma mansoni

In all metazoa, the response of cells to molecular stimuli from their environment represents a fundamental principle of regulatory processes controlling cell growth and differentiation. Among the membrane-linked receptors mediating extracellular communication processes are integrin receptors. Besides managing adhesion to the extracellular matrix or to other cells, they arrange information flow into the cells by activating intracellular signaling pathways often acting synergistically through cooperation with growth factor receptors. Although a wealth of information exists on integrins in different model organisms, there is a big gap of knowledge for platyhelminths. Here we report on the in silico detection and reconstruction of α and β integrins from free-living and parasitic platyhelminths, which according to structural and phylogenetic analyses form specific clades separate from each other and from further metazoan integrins. As representative orthologs of parasitic platyhelminths we have cloned one beta-integrin (Smβ-Int1) and four alpha-integrins (Smα-Int1 - Smα-Int4) from Schistosoma mansoni; they were characterized by molecular and biochemical analyses. Evidence is provided that Smβ-Int1 interacts and co-localizes in the reproductive organs with known schistosome cellular tyrosine kinases (CTKs), of which the Syk kinase SmTK4 appeared to be the strongest interaction partner as shown by yeast two-hybrid analyses and coimmunoprecipitation experiments. By a novel RNAi approach with adult schistosomes in vitro we demonstrate for the first time multinucleated oocytes in treated females, indicating a decisive role Smβ-Int1 during oogenesis as phenotypically analyzed by confocal laser scanning microscopy (CLSM). Our findings provide a first comprehensive overview about platyhelminth integrins, of which the parasite group exhibits unique features allowing a clear distinction from the free-living groups. Furthermore, we shed first lights on the functions of integrins in a trematode model parasite, revealing the complexity of molecular processes involved in its reproductive biology, which may be representative for other platyhelminths.

Leukocyte function antigen-1, kindlin-3, and calcium flux orchestrate neutrophil recruitment during inflammation

Neutrophil arrest and migration on inflamed endothelium involves a conformational shift in CD11a/CD18 (leukocyte function antigen-1; LFA-1) to a high-affinity and clustered state that determines the strength and lifetime of bond formation with ICAM-1. Cytoskeletal adapter proteins Kindlin-3 and Talin-1 anchor clustered LFA-1 to the cytoskeleton and facilitate the transition from neutrophil rolling to arrest. We recently reported that tensile force acts on LFA-1 bonds inducing their colocalization with Orai1, the predominant membrane store operated Ca(2+) channel that cooperates with the endoplasmic reticulum to elicit cytosolic flux. Because Kindlin-3 was recently reported to initiate LFA-1 clustering in lymphocytes, we hypothesized that it cooperates with Orai1 and LFA-1 in signaling local Ca(2+) flux necessary for shear-resistant neutrophil arrest. Using microfluidic flow channels combined with total internal reflection fluorescence microscopy, we applied defined shear stress to low- or high-affinity LFA-1 and imaged the spatiotemporal regulation of bond formation with Kindlin-3 recruitment and Ca(2+) influx. Orai1 and Kindlin-3 genes were silenced in neutrophil-like HL-60 cells to assess their respective roles in this process. Kindlin-3 was enriched within focal clusters of high-affinity LFA-1, which promoted physical linkage with Orai1. This macromolecular complex functioned to amplify inside-out Ca(2+) signaling in response to IL-8 stimulation by catalyzing an increased density of Talin-1 and consolidating LFA-1 clusters within sites of contact with ICAM-1. In this manner, neutrophils use focal adhesions as mechanosensors that convert shear stress-mediated tensile force into local bursts of Ca(2+) influx that catalyze cytoskeletal engagement and an adhesion-strengthened migratory phenotype.

Chemokines, selectins and intracellular calcium flux: temporal and spatial cues for leukocyte arrest

Leukocyte trafficking to acute sites of injury or infection requires spatial and temporal cues that fine tune precise sites of firm adhesion and guide migration to endothelial junctions where they undergo diapedesis to sites of insult. Many detailed studies on the location and gradient of chemokines such as IL-8 and other CXCR ligands reveal that their recognition shortly after selectin-mediated capture and rolling exerts acute effects on integrin activation and subsequent binding to their ligands on the endothelium, which directs firm adhesion, adhesion strengthening, and downstream migration. In this process, G-protein coupled receptor (GPCR) signaling has been found to play an integral role in activating and mobilizing intracellular stores of calcium, GTPases such as Rap-1 and Rho and cytokeletal proteins such as Talin and F-actin to facilitate cell polarity and directional pseudopod formation. A critical question remaining is how intracellular Ca(2+) flux from CRAC channels such as Orai1 synergizes with cytosolic stores to mediate a rapid flux which is critical to the onset of PMN arrest and polarization. Our review will highlight a specific role for calcium as a signaling messenger in activating focal clusters of integrins bound to the cytoskeleton which allows the cell to attain a migratory phenotype. The precise interplay between chemokines, selectins, and integrins binding under the ubiquitous presence of shear stress from blood flow provides an essential cooperative signaling mechanism for effective leukocyte recruitment.

Membrane microdomains in immunity: glycosphingolipid-enriched domain-mediated innate immune responses

Over the last 30 years, many studies have indicated that glycosphingolipids (GSLs) expressed on the cell surface may act as binding sites for microorganisms. Based on their physicochemical characteristics, GSLs form membrane microdomains with cholesterol, sphingomyelin, glycosylphosphatidylinositol (GPI)-anchored proteins, and various signaling molecules, and GSL-enriched domains have been shown to be involved in these defense responses. Among the GSLs, lactosylceramide (LacCer, CDw17) can bind to various microorganisms. LacCer is expressed at high levels on the plasma membrane of human neutrophils, and forms membrane microdomains associated with the Src family tyrosine kinase Lyn. LacCer-enriched membrane microdomains mediate superoxide generation, chemotaxis, and non-opsonic phagocytosis. Therefore, LacCer-enriched membrane microdomains are thought to function as pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) expressed on microorganisms. In contrast, several pathogens have developed infection mechanisms using membrane microdomains. In addition, some pathogens have the ability to avoid degradation by escaping from the vacuolar compartment or preventing phagosome maturation, utilizing membrane microdomains, such as LacCer-enriched domains, of host cells. The detailed molecular mechanisms of these membrane microdomain-associated host-pathogen interactions remain to be elucidated.

Protein kinase C isoforms in neutrophil adhesion and activation

Neutrophils are the first line of defense against bacterial and mycotic pathogens. In order to reach the pathogens, neutrophils need to transmigrate through the vascular endothelium and migrate to the site of infection. Defense strategies against pathogens include phagocytosis, production and release of oxygen radicals through the oxidative burst, and degranulation of antimicrobial and inflammatory molecules. Protein kinase C (PKC)-δ is required for full assembly of NADPH oxidase and activation of the respiratory burst. Neutrophils also express PKC-α and -β, which may be involved in adhesion, degranulation and phagocytosis, but the evidence is not conclusive yet. This review focuses on the potential impact of protein kinase C isoforms on neutrophil adhesion and activation.

Neutrophils phagocytose activated platelets in vivo: a phosphatidylserine, P-selectin, and β2 integrin–dependent cell clearance program

Activated platelets express ligands, which are recognized by counterreceptors on neutrophils. Here, we show that the ensuing cell-to-cell interaction programs neutrophil phagocytic function, resulting in activated platelet clearance. Neutrophils that have internalized platelets circulate in the blood of patients with acute myocardial infarction, and the extent of platelet clearance correlates with expression of platelet activation, including P-selectin. Activated platelets injected intravenously in experimental animals are detectable in circulating neutrophils 60 minutes after, and within 3 hours, more than 70% circulating neutrophils have internalized platelets. Platelet clearance comprises 2 events: adhesion to neutrophils, which requires divalent cations and depends on P-selectin, on the P-selectin glycoprotein ligand-1 (PSGL-1), and on the CD11b/CD18 β2 integrin; and internalization, which is abrogated by the phosphatidylserine-binding protein annexin A5. Adhesion to platelets causes neutrophil degranulation and is blocked by antibodies specific for P-selectin and PSGL-1, either in a synthetic medium in vitro or in the whole blood, therefore in the presence of a physiologic array of plasma cofactors and opsonins. The data suggest that the interaction between circulating platelets and neutrophils influences innate immune functions, possibly contributing to regulate vascular inflammation.

Translational mini-review series on immunology of vascular disease: mechanisms of vascular inflammation and remodelling in systemic vasculitis

Vessel walls are the primary inflammatory sites in systemic vasculitides. In most cases the initiating event is unknown, and a self-sustaining circuit attracts and activates inflammatory leucocytes in the wall of vessels of various size and anatomical characteristics. Recent studies have revealed homeostatic roles of vascular inflammation and have identified the action of humoral innate immunity, in particular injury-associated signals and acute phase proteins, on the activation of circulating leucocytes, platelets and endothelial cells. These advances have provided clues to the molecular mechanisms underlying the vicious circle that maintains and amplifies vessel and tissue injury.

Transmigration of neutrophils across inflamed endothelium is signaled through LFA-1 and Src family kinase

Leukocyte capture on inflamed endothelium is facilitated by a shift in LFA-1 from low to high affinity that supports binding to ICAM-1. LFA-1 bonds help anchor polymorphonuclear leukocytes (PMN) to inflamed endothelium in shear flow, and their redistribution to the leading edge guides pseudopod formation, migration, and extravasation. These events can be disrupted at the plasma membrane by stabilizing LFA-1 in a low- or intermediate-affinity state with allosteric small molecules. We hypothesized that a minimum dimeric bond formation between high-affinity LFA-1 and ICAM-1 under shear stress is necessary to catalyze transmembrane signaling of directed cell migration. Microspheres and substrates were derivatized with monomeric or dimeric ICAM-1 to simulate the surface of inflamed endothelium under defined ligand valence. Binding to dimeric ICAM-1, and not monomeric ICAM-1, was sufficient to elicit assembly of F-actin and phosphorylation of Src family kinases that colocalized with LFA-1 on adherent PMN. Genetic deletion or small molecule inhibition of Src family kinases disrupted their association with LFA-1 that correlated with diminished polarization of arrested PMN and abrogation of transmigration on inflamed endothelium. We conclude that dimeric bond clusters of LFA-1/ICAM-1 provide a key outside-in signal for orienting cytoskeletal dynamics that direct PMN extravasation at sites of inflammation.

Role of very long fatty acid-containing glycosphingolipids in membrane organization and cell signaling: the model of lactosylceramide in neutrophils

Glycosphingolipids are highly enriched in specialized membrane microdomains ("lipid rafts", caveolar domains and glycosynapses), and they participate to the process of transduction of information across the membrane. Lactosylceramide (LacCer) is specifically coupled with the Src family kinase Lyn in plasma membrane microdomains of human neutrophils. Ligand binding to LacCer activates Lyn, resulting in neutrophil functions, such as superoxide generation and migration. The beta-Gal-(1-4)-beta-Glc disaccharide structure of LacCer is necessary, but it is not sufficient for LacCer-mediated Lyn activation. For this function, the presence of a LacCer molecular species with ceramide containing a very long fatty acid chain is also required. In this manuscript, we discuss the importance of interdigitation within the membrane, promoted by the presence of glycosphingolipid species with very long fatty acyl chains as determinants for membrane organization, instrumental to the signaling process.

Integrin CD11c contributes to monocyte adhesion with CD11b in a differential manner and requires Src family kinase activity

Integrin-mediated adhesion of human monocytes to fibrinogen regulated by CD11b/CD18 and the closely related integrin CD11c/CD18, play a key role in inflammation. Peripheral blood monocytes isolated from human donors despite expressing CD11c primarily utilized CD11b to mediate adhesion to fibrinogen upon stimulation with granulocyte macrophage-colony stimulating factor (GM-CSF) and fMLP. Blocking with anti-CD11b resulted in 90% (p<0.001, n=3) inhibition of monocyte adhesion. Monocytes cultured in human serum showed a shift in the participation of integrins, adhesion to fibrinogen involving both CD11b and CD11c. The participation of CD11c in cultured monocytes corresponded to a 3.4-fold increase in expression in CD11c. Blocking cultured monocytes with anti-CD11b or anti-CD11c alone showed no significant effect on adhesion. Treatment with both anti-CD11b and anti-CD11c resulted in inhibition of adhesion by 85% (p<0.001, n=3). Abrogation in adhesion upon treatment with PP1 or PP2 showed that Src family kinase activity was required for CD11b and CD11c mediated adhesion of cultured monocytes to fibrinogen upon stimulation with GM-CSF and fMLP. The clustering of CD11c on cultured monocytes upon adhesion to fibrinogen was diminished on inhibition with PP2 indicating a role for Src family kinase activity in regulating CD11c avidity. CD11b was critical to cytoskeletal events leading to increased spreading and formation of actin foci in cultured monocytes following adhesion to fibrinogen. Blocking cultured monocytes with anti-CD11b or anti-CD11c alone showed that the increase in spread area was diminished by 67+/-3% and 36+/-9%, respectively. The differential involvement of CD11c and CD11b in adhesion and subsequent cytoskeletal changes in monocytes exposed to different conditions indicates the importance of each integrin in distinct responses during inflammation.

Lyn, one of the Src-family tyrosine kinases expressed in phagocytes, plays an important role in beta2 integrin-signalling pathways in opsonized zymosan-activated macrophage-like U937 cells

We have investigated the contribution of Hck, Lyn and Fgr, highly expressed Src family tyrosine kinases (SFKs) in signalling pathways in opsonized zymosan (OZ)-activated phagocytes by using short interfering RNAs (siRNAs). Treatment of macrophage-like U937 cells with the siRNAs targeted to these transcripts decreased the protein content of each kinase to less than half that of untreated cells. Among these siRNAs, siRNA targeted to Lyn was the most effective in diminishing two kinds of phagocyte functions, that is oxidative burst and phagocytosis. Phosphorylation of c-Cbl, a multidomain adaptor protein in the beta2 integrin-signalling pathway, was also largely inhibited by treatment with siRNA to Lyn. Thus, the results with siRNAs highly specific for Hck, Lyn and Fgr suggested that, among these three SFKs, Lyn plays the most important role in signalling pathways downstream of beta2 integrins in OZ-stimulated phagocytes.

Src family kinases mediate neutrophil adhesion to adherent platelets

Polymorphonuclear leukocyte (PMN)-platelet interactions at sites of vascular damage contribute to local and systemic inflammation. We sought to determine the role of "outside-in" signaling by Src-family tyrosine kinases (SFKs) in the regulation of alphaMbeta2-integrin-dependent PMN recruitment by activated platelets under (patho)physiologic conditions. Activation-dependent epitopes in beta2 integrin were exposed at the contact sites between PMNs and platelets and were abolished by SFK inhibitors. PMNs from alphaMbeta2(-/-), hck(-/-)fgr(-/-), and hck(-/-)fgr(-/-)lyn(-/-) mice had an impaired capacity to adhere with activated platelets in suspension. Phosphorylation of Pyk2 accompanied PMN adhesion to platelets and was blocked by inhibition as well as by genetic deletion of alphaMbeta2 integrin and SFKs. A Pyk2 inhibitor reduced platelet-PMN adhesion, indicating that Pyk2 may be a downstream effector of SFKs. Analysis of PMN-platelet interactions under flow revealed that SFK signaling was required for alphaMbeta2-mediated shear-resistant adhesion of PMNs to adherent platelets, but was dispensable for P-selectin-PSGL-1-mediated recruitment and rolling. Finally, SFK activity was required to support PMN accumulation along adherent platelets at the site of vascular injury, in vivo. These results definitely establish a role for SFKs in PMN recruitment by activated platelets and suggest novel targets to disrupt the pathophysiologic consequences of platelet-leukocyte interactions in vascular disease.

CD18 in monogenic and polygenic inflammatory processes of the skin

The beta 2 integrin family (CD11/CD18) of leukocyte adhesion molecules plays a key role in inflammation. Absence of the common chain (CD18) leads to leukocyte adhesion deficiency-1 (LAD1) in humans. We here summarize data of two genetically defined mice models of beta 2 integrin deficiency, one with a CD18 null mutation (CD18-/-), and the other one with a hypomorphic CD18 mutation (CD18hypo). Firstly, we focus on the underlying mechanism of a severely impaired wound healing in CD18-/- mice, outlining a scenario in which a defective extravasation and phagocytosis of CD18-/- neutrophils results in delayed myofibroblast-dependent wound contraction owing to a deficient transforming growth factor-beta 1 release. Based on this, we have identified a potential therapy that fully rescued the impaired wound healing in CD18-/- mice. Secondly, we expand on a CD18hyp0 PL/J mouse model closely resembling human psoriasis. Apart from common clinical and pathophysiological features, this psoriasiform dermatitis also depends on the presence of activated CD4+ T cells. We here recapitulate the influence of a reduced CD18 gene expression on T-cell function, also with regard to CD18 gene-dose effects, and its contribution to the pathogenesis of this disease. Taken together, these unique features make this model a valuable tool for investigations into the pathogenesis of human psoriasis--including its polygenic base--and future preclinical studies.

Src-family kinases mediate an outside-in signal necessary for beta2 integrins to achieve full activation and sustain firm adhesion of polymorphonuclear leucocytes tethered on E-selectin

In cell suspensions subjected to high-shear rotatory motion, human PMN (polymorphonuclear cells) adhered to E-selectin-expressing CHO (Chinese-hamster ovary) cells (CHO-E), and formed homotypic aggregates when challenged by E-selectin-IgG fusion protein, by a mechanism that involved beta2 integrins. Both heterotypic and homotypic PMN adhesion was accompanied by tyrosine phosphorylation of a 110 kDa protein (P110). This event was prevented by blocking anti-(beta2 integrin) antibodies and by inhibitors of Src-family kinases, suggesting that it was part of an 'outside-in' signalling that was initiated by integrin engagement. Interestingly, Src-family kinase inhibitors prevented beta2-integrin-mediated (i) homotypic PMN adhesion triggered by E-selectin-IgG, (ii) heterotypic CHO-E/PMN adhesion in mixed-cell suspensions, and (iii) firm adhesion of PMN to CHO-E monolayers under physiological flow. Similarly to PMN treated with Src-family kinase inhibitors, PMN from hck-/-fgr-/- and hck-/-fgr-/-lyn-/- mice showed significant impairment of beta2-integrin-mediated adhesion to CHO-E. Moreover, the expression of beta2 integrin activation epitopes at the sites of cell-cell contact in CHO-E/PMN conjugates was abolished by Src-family kinase inhibitors. One component of P110 was identified as the FAK (focal adhesion kinase) Pyk2 (proline-rich tyrosine kinase 2), which was phosphorylated in a beta2 integrin- and Src-family-kinase-dependent manner. Thus, Src-family kinases, and perhaps Pyk2, mediate a signal necessary for beta2 integrin function in PMN tethered by E-selectin.

Integrin inhibition through Lyn-dependent cross talk from CXCR4 chemokine receptors in normal human CD34+ marrow cells

We studied the effects of Lyn ablation on CXCR4 receptor-mediated migration and adhesion of hematopoietic precursors. Down-regulation of Lyn expression with siRNA greatly reduced CXCR4-dependent hematopoietic cell movement, and increased cell adherence to stroma. This increase was associated with the up-regulated expression of activation-dependent epitopes of the beta(2) integrin LFA-1 and was prevented by antibodies that selectively block cell adhesion mediated by ICAM-1. Attachment to surfaces coated with ICAM-1 was also enhanced in Lyn-depleted hematopoietic cells, as compared with Lyn-expressing cells. Functional rescue experiments with Lyn siRNA targeting the 3' UTR indicated that the observed effects can be attributed directly to specific inhibition of Lyn. Our results show that in chemokine-stimulated hematopoietic cells Lyn kinase is a positive regulator of cell movement while negatively regulating adhesion to stromal cells by inhibiting the ICAM-1-binding activity of beta(2) integrins. These results provide a molecular mechanism for cross talk between the chemokine receptor CXCR4 and beta(2) integrins. This cross talk may allow chemokine receptors to modulate the arrest of rolling hematopoietic precursors on the surface of bone marrow stromal cells.

Integrin structure, allostery, and bidirectional signaling

Alphabeta heterodimeric integrins mediate dynamic adhesive cell-cell and cell-extracellular matrix (ECM) interactions in metazoa that are critical in growth and development, hemostasis, and host defense. A central feature of these receptors is their capacity to change rapidly and reversibly their adhesive functions by modulating their ligand-binding affinity. This is normally achieved through interactions of the short cytoplasmic integrin tails with intracellular proteins, which trigger restructuring of the ligand-binding site through long-range conformational changes in the ectodomain. Ligand binding in turn elicits conformational changes that are transmitted back to the cell to regulate diverse responses. The publication of the integrin alphaVbeta3 crystal structure has provided the context for interpreting decades-old biochemical studies. Newer NMR, crystallographic, and EM data, reviewed here, are providing a better picture of the dynamic integrin structure and the allosteric changes that guide its diverse functions.

The small GTPase Rap1b regulates the cross talk between platelet integrin alpha2beta1 and integrin alphaIIbbeta3

The involvement of the small GTPase Rap1b in platelet integrin alpha2beta1-dependent outside-in signaling was investigated. Platelet adhesion to 4 different specific ligands for integrin alpha2beta1, monomeric collagen, decorin, and collagen-derived peptides CB8(II) and CB11(II), induced a robust and rapid activation of Rap1b. This process did not require secreted ADP or thromboxane A2 production but was critically regulated by phospholipase C (PLC)-derived second messengers. Both Ca2+ and protein kinase C were found to organize independent but additive pathways for Rap1b activation downstream of integrin-alpha2beta1, which were completely blocked by inhibition of PLC with U73122. Moreover, integrin alpha2beta1 engagement failed to trigger Rap1b activation in murine platelets lacking CalDAG-GEFI, a guanine nucleotide exchange factor regulated by Ca2+ and diacylglycerol, despite normal phosphorylation and activation of PLCgamma2. In addition, CalDAG-GEFI-deficient platelets showed defective integrin alpha2beta1-dependent adhesion and spreading. We found that outside-in signaling through integrin alpha2beta1 triggered inside-out activation of integrin alphaIIbbeta3 and promoted fibrinogen binding. Similarly to Rap1b stimulation, this process occurred downstream of PLC activation and was dramatically impaired in murine platelets lacking the Rap1 exchange factor CalDAG-GEFI. These results demonstrate that Rap1b is an important element in integrin-dependent outside-in signaling during platelet adhesion and regulates the cross talk between adhesive receptors.

Human milk oligosaccharides reduce platelet-neutrophil complex formation leading to a decrease in neutrophil beta 2 integrin expression

Human milk is thought by many authorities to be preferable to formula as a source of nutrients for infants. Some of the benefits may stem from its high concentration of unbound oligosaccharides (5-10 g/L). These sugars have structural similarities to selectin ligands, known to mediate important cell-cell interactions in the immune system. Platelet-neutrophil complexes (PNC) exist in healthy individuals but have been implicated in disease states. Formation of these complexes requires selectins and as such, could be influenced by human milk oligosaccharides (HMO). Here, we investigate this possibility by examining the effect of HMO on the formation of PNC and activation of associated neutrophils. We collected blood from 10 healthy volunteers, activated platelets with adenosine 5'-diphosphate, and added HMO, oligosaccharide standards, or phosphate-buffered saline as a control. We determined the influence of HMO on PNC formation and adjacent neutrophil activation with fluorescein-activated cell sorter analysis after labeling with antibodies for the platelet marker CD42a and the neutrophil activation marker CD11b. Within physiologically achievable concentrations (6.25-125 microg/mL), an acidic HMO fraction reduced PNC formation up to 20%, which was similar to the effect seen with high concentrations of sialyl-Lewis x. Associated neutrophils showed a dose-dependent decrease in beta 2 integrin expression, up to 30%, at high but physiological concentrations. The neutral HMO fraction had no effect. These results support the hypothesis that acidic HMO serve as anti-inflammatory components of human milk and thus, contribute to the lower incidence of inflammatory diseases such as necrotizing enterocolitis in breast-fed versus formula-fed infants.