Fundamentally different roles for LFA-1, Mac-1 and α4-integrin in neutrophil chemotaxis

Modeling cell gradient sensing and migration in competing chemoattractant fields

Directed cell migration mediates physiological and pathological processes. In particular, immune cell trafficking in tissues is crucial for inducing immune responses and is coordinated by multiple environmental cues such as chemoattractant gradients. Although the chemotaxis mechanism has been extensively studied, how cells integrate multiple chemotactic signals for effective trafficking and positioning in tissues is not clearly defined. Results from previous neutrophil chemotaxis experiments and modeling studies suggested that ligand-induced homologous receptor desensitization may provide an important mechanism for cell migration in competing chemoattractant gradients. However, the previous mathematical model is oversimplified to cell gradient sensing in one-dimensional (1-D) environment. To better understand the receptor desensitization mechanism for chemotactic navigation, we further developed the model to test the role of homologous receptor desensitization in regulating both cell gradient sensing and migration in different configurations of chemoattractant fields in two-dimension (2-D). Our results show that cells expressing normal desensitizable receptors preferentially orient and migrate toward the distant gradient in the presence of a second local competing gradient, which are consistent with the experimentally observed preferential migration of cells toward the distant attractant source and confirm the requirement of receptor desensitization for such migratory behaviors. Furthermore, our results are in qualitative agreement with the experimentally observed cell migration patterns in different configurations of competing chemoattractant fields.

Cytohesin-1 regulates human blood neutrophil adhesion to endothelial cells through β2 integrin activation

Cytohesin-1 is a guanine nucleotide exchange factor for ADP ribosylation factor 6 (Arf6) in human blood neutrophils and differentiated PLB-985 neutrophil-like cells. Cytohesin-1 regulates adhesion and the transendothelial migration of monocytes, dendritic cells and T lymphocytes through activation of the β2 integrin LFA-1. In this study we investigated the role of cytohesin-1 in neutrophil and neutrophil-like cell adhesion to HUVECs, immobilized ICAM-1, and the α4β1 and α5β1 integrin extracellular matrix ligand fibronectin. We show that cytohesin-1 knockdown or inhibition with secinH3 inhibits fMLF-mediated cell adhesion to HUVECs and immobilized ICAM-1, whereas cytohesin-1 over-expression has the opposing effect. Binding of PLB-985 cells to HUVECs correlated with expression of the high-affinity β2 integrin epitope recognized by mAb24. Adhesion to HUVECs was inhibited by soluble ICAM-1, anti-ICAM-1, anti-CD11a and anti-CD18, but not anti-CD11b, blocking antibodies. We also demonstrate that cytohesin-1 knockdown promotes fMLF-mediated cell adhesion to fibronectin whereas cytohesin-1 over-expression has the opposing effect. Crosstalk between β1 and β2 integrins also exists since inhibition of β1 integrin functions with blocking antibodies enhanced adhesion of PLB-985 over-expressing cytohesin-1 to ICAM-1. We suggest that cytohesin-1 is a key regulator of neutrophil adhesion to endothelial cells and to components of extracellular matrix, which may influence cell emigration through its dual opposing effect on β2 and β1 integrin activation.

An obligate role for membrane-associated neutral sphingomyelinase activity in orienting chemotactic migration of human neutrophils

For polymorphonuclear neutrophils (PMNs) to orient migration to chemotactic gradients, weak external asymmetries must be amplified into larger internal signaling gradients. Lipid mediators, associated with the plasma membrane and within the cell, participate in generating these gradients. This study examined the role in PMN chemotaxis of neutral sphingomyelinase (N-SMase), a plasma membrane-associated enzyme that converts sphingomyelin to ceramide. A noncompetitive N-SMase inhibitor, GW4869 (5 mM, 5 minutes), did not inhibit PMN motility (as percentage of motile cells, or mean cell velocity), but it abrogated any orientation of movement toward the source of the chemotaxin, formylmethionylleucylphenylanaline (FMLP) (net displacement along the gradient axis in micrometers, or as percentage of total migration distance). This defect could be completely reversed by treatment with lignoceric ceramide (5 μg/ml, 15 minutes). Immunolocalization studies demonstrated that N-SMase (1) distributes preferentially toward the leading edge of some elongated cells, (2) is associated with the plasma membrane, (3) is more than 99.5% localized to the cytofacial aspect of the plasma membrane, (4) is excluded from pseudopodial extensions, and (5) increases rapidly in response to FMLP. Morphologically, the inhibition of N-SMase limited cellular spreading and the extension of sheet-like pseudopods. Elongated PMNs demonstrated a polarized distribution of GTPases, with Rac 1/2 accumulated at, and RhoA excluded from, the front of the cell. This polarity was negated by N-SMase inhibition and restored by lignoceric ceramide. We conclude that N-SMase at the cytofacial plasma membrane is an essential element for the proper orientation of PMNs in FMLP gradients, at least in part by polarizing the distribution of Rac 1/2 and RhoA GTPases.

Cytohesin-1 regulates fMLF-mediated activation and functions of the β2 integrin Mac-1 in human neutrophils

The nucleotide exchange factor cytohesin-1 was previously reported to interact with the cytoplasmic domains of the integrin β-chain common to all β(2) integrins such as LFA-1 and Mac-1. We show here that cytohesin-1, which contributes to fMLF-induced functional responses in PMNs through activation of Arf6, restrains the activation of the β(2) integrin Mac-1 (αMβ(2)) in PMNs or dcAMP-differentiated PLB-985 cells. We found that the cytohesin-1 inhibitor SecinH3 or siRNA increased cell adhesion to immobilized fibrinogen and fMLF-mediated conformational changes of Mac-1, monitored using mAb CBRM1/5, specific for the activation epitope of the αM subunit. In contrast, PLB-985 cells overexpressing cytohesin-1 showed little adhesion to fibrinogen. The use of SecinH3 and siRNA also revealed that interference with cytohesin-1 signaling also enhanced phagocytosis of zymosan particles and chemotaxis toward fMLF in transwell migration assays. These increments of phagocytosis and chemotaxis in cells treated with SecinH3 and cytohesin-1 siRNA were reversed by a blocking mAb to the integrin-αM subunit. We provide evidence for increased polymerized cortical actin in cells treated with SecinH3 and that altered signaling through cytohesin-1 increased cell surface expression of FPRL-1 and impairs the late calcium mobilization response elicited by fMLF. The data provide evidence that stimulation with fMLF initiates a signaling cascade that restrains Mac-1 activation in PMNs. Such crosstalk between FPRL-1 and Mac-1 involves cytohesin-1. We suggest that cytohesin-1 may coordinate activation of the β(2) integrins to regulate PMN adhesion, phagocytosis, and chemotaxis.

Abacavir and didanosine induce the interaction between human leukocytes and endothelial cells through Mac-1 upregulation

OBJECTIVE

Abacavir and didanosine are nucleoside reverse transcriptase inhibitors (NRTI) widely used in therapy for HIV-infection but which have been linked to cardiovascular complications. The objective of this study was to analyze the effects of clinically relevant doses of abacavir and didanosine on human leukocyte-endothelium interactions and to compare them with those of other NRTIs.

DESIGN AND METHODS

The interactions between human leukocytes - specifically peripheral blood polymorphonuclear (PMN) or mononuclear (PBMC) cells - and human umbilical vein endothelial cells were evaluated in a flow chamber system that reproduces conditions in vivo. The expression of adhesion molecules was analyzed by flow cytometry.

RESULTS

Abacavir induced a dose-dependent increase in PMN and PBMC rolling and adhesion. This was reproduced by didanosine but not by lamivudine or zidovudine. Both abacavir and didanosine increased Mac-1 expression in neutrophils and monocytes, but produced no effects on either lymphocytes or the expression of endothelial adhesion molecules. The PMN/PBMC rolling and adhesion induced by abacavir or didanosine did not occur when antibodies against Mac-1 or its ligand ICAM-1 were blocked.

CONCLUSION

Abacavir induces significant human leukocyte accumulation through the activation of Mac-1, which in turn interacts with its endothelial ligand ICAM-1. The fact that didanosine exhibits similar effects and that lamivudine and zidovudine do not points to a relationship between the chemical structure of NRTIs and the induction of leukocyte/endothelial cell interactions. This mechanism may be especially relevant to the progression of the vascular damage associated with atherosclerosis and myocardial infarction in abacavir and didanosine-treated patients.

Neutrophil migration during liver injury is under nucleotide-binding oligomerization domain 1 control

BACKGROUND & AIMS

A more complete understanding of the mechanisms involved in pathogen-associated molecular pattern signaling is crucial in the setting of liver injury. In intestinal diseases, nucleotide-binding oligomerization domain 1 (NOD1), a receptor for bacteria, appears to regulate cross-talk between innate and adaptive immunity, involving polymorphonuclear neutrophils (PMNs). Our aim was to explore the role of NOD1 in PMN-induced liver injury.

METHODS

Nod1(+/+) and Nod1(-/-) mice were challenged with carbon tetrachloride (CCl(4)). Migration and phagocytosis of Nod1(+/+) and Nod1(-/-) PMN were studied in vivo and ex vivo. We evaluated main inflammatory pathways in PMNs by Western blot and CD11b expression using fluorescence-activated cell sorting. Mice were submitted to liver ischemia/reperfusion.

RESULTS

After CCl(4) exposure, livers of Nod1(-/-) mice had more than 50% less PMN infiltration within necrotic areas than those of Nod1(+/+). PMNs isolated from Nod1(-/-) mice displayed a 90% decrease in migration capacity compared with Nod1(+/+) PMNs, whereas FK 565, a potent NOD1 ligand, increased PMN migration. Upon FK 565 stimulation, mitogen-activated protein kinase and nuclear factor kappaB were activated in Nod1(+/+) PMNs, but less so in Nod1(-/-) PMNs. Expression of CD11b on the Nod1(-/-) PMN was decreased compared with Nod1(+/+). The phagocytic capacity of Nod1(-/-) PMNs was decreased by more than 50% compared with Nod1(+/+). In an ischemia/reperfusion model of PMN-induced liver injury, FK 565 increased lesions, whereas Nod1(-/-) mice were protected.

CONCLUSIONS

The identification of NOD1 as a modulator of PMN function and migration in the liver suggests that this receptor may represent a new therapeutic target in PMN-dependent liver diseases.

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.

Independent roles of macrophage migration inhibitory factor and endogenous, but not exogenous glucocorticoids in regulating leukocyte trafficking

OBJECTIVES

Macrophage migration inhibitory factor (MIF) promotes leukocyte recruitment and antagonizes the anti-inflammatory effects of glucocorticoids (GC). The aim of this study was to examine whether interaction between MIF and GC underlies the ability of MIF to promote leukocyte-endothelial cell (EC) interactions.

METHODS

Intravital microscopy was used to assess leukocyte-EC interactions in wild-type and MIF(-/-) mice following treatment with lipopolysaccharide (LPS), the GC dexamethasone, and inhibition of endogenous GC, using the GC-receptor antagonist, RU486.

RESULTS

Dexamethasone reduced LPS-induced leukocyte interactions in wild-type mice to levels similar to those observed in MIF(-/-) mice not treated with dexamethasone, whereas in MIF(-/-) mice, leukocyte interactions were not further inhibited by dexamethasone. RU486 increased LPS-induced leukocyte adhesion and emigration to a similar extent in both wild-type and MIF(-/-) mice, indicating that endogenous GC exert a similar inhibitory effect on leukocyte trafficking in wild-type and MIF(-/-) mice. Both MIF deficiency and RU486 treatment reduced VCAM-1 expression, while neither treatment modulated expression of ICAM-1 or chemokines CCL2, KC, and MIP-2.

CONCLUSIONS

These results suggest that endogenous MIF and GC regulate leukocyte-EC interactions in vivo reciprocally but through predominantly independent mechanisms, and that the anti-inflammatory effect of MIF deficiency is comparable to that of exogenous GC.

Signaling and Dynamics of Activation of LFA-1 and Mac-1 by Immobilized IL-8

The dynamic response of neutrophils to interleukin-8 (IL-8) is of central interest in inflammation. Chemokine -induced β(2) integrin dependent adhesion can take several minutes after initial contact with IL-8 as evidenced by increased cell adhesion to intracellular adhesion molecule 1 (ICAM-1). The goal of this study is to identify signaling events that are critical for this response. We demonstrate that neither the PI3K inhibitor wortmannin, nor the PKC inhibitor bisindolymaleimide had any effect on IL-8 induced adhesion to ICAM-1. However, inhibition of PLC with U73122 or stopping the release of intracellular calcium by its downstream effector IP3 with caffeine or 2-aminoethoxydiphenyl borate completely blocked the adhesive response. Chelation of intracellular calcium with BAPTA or extracellular calcium with EGTA completely abrogated neutrophil adhesion to ICAM-1. This adhesion is mediated by LFA-1 (α(L)β(2)) within first 300 seconds after chemokine stimulation, followed by Mac-1 (α(M)β(2)) mediated adhesion, beginning 350 seconds after stimulus. Inhibition of p38MAP kinase results in a time course similar to Mac-1 inhibition, consistent with published evidence that Mac-1 mediated adhesion is p38MAP kinase dependent. These findings confirm a PLC dependent, PKC independent pathway from chemokine stimulus to integrin activation previously identified in other cell types, and demonstrate distinct dynamics and different requirements for LFA-1 vs. Mac-1 activation in primary human neutrophils.

The obesity and inflammatory marker haptoglobin attracts monocytes via interaction with chemokine (C-C motif) receptor 2 (CCR2)

Background

Obesity is a chronic low inflammatory state. In the obesity condition the white adipose tissue (WAT) is massively infiltrated with monocytes/macrophages, and the nature of the signals recruiting these inflammatory cells has yet to be fully elucidated. Haptoglobin (Hp) is an inflammatory marker and its expression is induced in the WAT of obese subjects. In an effort to elucidate the biological significance of Hp presence in the WAT and of its upregulation in obesity we formulated the hypothesis that Hp may serve as a macrophage chemoattractant.

Results

We demonstrated by chemotaxis assay that Hp is able to attract chemokine (C-C motif) receptor 2 (CCR2)-transfected pre-B lymphocytes and monocytes in a dose-dependent manner. Moreover, Hp-mediated migration of monocytes is impaired by CCR2-specific inhibition or previous cell exposure to monocyte chemoattractant protein 1 (MCP1) (also known as CCR2 ligand or chemokine (C-C motif) ligand 2 (CCL2)). Downstream effects of Hp/CCR2 interaction were also investigated: flow cytometry proved that monocytes treated with Hp show reduced CCR2 expression on their surface; Hp interaction induces calcium release that is reduced upon pretreatment with CCR2 antagonist; extracellular signal-regulated kinase (ERK)1/2, a signal transducer activated by CCR2, is phosphorylated following Hp treatment and this phosphorylation is reduced when cells are pretreated with a specific CCR2 inhibitor. Consistently, blocking the ERK1/2 pathway with U0126, the selective inhibitor of the ERK upstream mitogen-activated protein (MAP)-ERK kinase (MEK), results in a dramatic reduction (by almost 100%) of the capability of Hp to induce monocyte migration.

Conclusions

Our data show that Hp is a novel monocyte chemoattractant and that its chemotactic potential is mediated, at least in part. by its interaction with CCR2.

Migrating human neutrophils exhibit dynamic spatiotemporal variation in membrane lipid organization

Highly ordered sphingolipid-enriched lipid raft microdomains (LRMs) within plasma membranes purportedly function as specialized signaling platforms. Leukocyte migration is believed to entail LRM redistribution, but progress in studying LRMs in situ during cell movement has been limited. By using an improved method for imaging the spectral shift of the environmentally sensitive probe, laurdan (expressed as a generalized polarization function), the plasma membrane order (i.e., tight packing of membrane bilayer lipids) of human polymorphonuclear neutrophils (PMNs) was mapped in real time during migration. Morphologically polarized PMNs exhibited prominent LRM clusters at the uropod, where in every instance membrane order was found to oscillate with mean periodicities of 37.0 ± 1.46 and 149.9 ± 9.0 seconds (P < 0.01). LRM aggregates were also demonstrated in punctate and clustered distributions of nonpolarized cells and transiently at the lamellipodia of polarized PMNs. Cellular polarization was not accompanied by an overall increase in membrane order. LRM disorganization with methyl-β-cyclodextrin had small negative effects on cell velocity, but it abrogated directionally biased migration toward chemotactic gradients of FMLP or leukotriene B(4). LRMs disruption also caused redistribution of Rac 1/2 GTPase and GM3 ganglioside away from the lamellipodium, as well as extension of multiple pseudopods simultaneously or in rapid succession, rather than formation of a defined leading edge. Thus, we demonstrate that the plasma membrane order of migrating PMNs changes dynamically, with prominent oscillations consistently seen at the uropod. These findings solidify the existence of rapidly reorganizing LRMs in situ and support a role for LRMs in chemotaxin responsiveness.

G protein-coupled receptor 43 is essential for neutrophil recruitment during intestinal inflammation

Molecular danger signals attract neutrophilic granulocytes (polymorphonuclear leukocytes (PMNs)) to sites of infection. The G protein-coupled receptor (GPR) 43 recognizes propionate and butyrate and is abundantly expressed on PMNs. The functional role of GPR43 activation for in vivo orchestration of immune response is unclear. We examined dextrane sodium sulfate (DSS)-induced acute and chronic intestinal inflammatory response in wild-type and Gpr43-deficient mice. The severity of colonic inflammation was assessed by clinical signs, histological scoring, and cytokine production. Chemotaxis of wild-type and Gpr43-deficient PMNs was assessed through transwell cell chemotactic assay. A reduced invasion of PMNs and increased mortality due to septic complications were observed in acute DSS colitis. In chronic DSS colitis, Gpr43(-/-) animals showed diminished PMN intestinal migration, but protection against inflammatory tissue destruction. No significant difference in PMN migration and cytokine secretion was detected in a sterile inflammatory model. Ex vivo experiments show that GPR43-induced migration is dependent on activation of the protein kinase p38alpha, and that this signal acts in cooperation with the chemotactic cytokine keratinocyte chemoattractant. Interestingly, shedding of L-selectin in response to propionate and butyrate was compromised in Gpr43(-/-) mice. These results indicate a critical role for GPR43-mediated recruitment of PMNs in containing intestinal bacterial translocation, yet also emphasize the bipotential role of PMNs in mediating tissue destruction in chronic intestinal inflammation.

Vav1 is essential for mechanotactic crawling and migration of neutrophils out of the inflamed microvasculature

Mac-1-dependent crawling is a new step in the leukocyte recruitment cascade that follows LFA-1-dependent adhesion and precedes emigration. Neutrophil adhesion via LFA-1 has been shown to induce cytoskeletal reorganization through Vav1-dependent signaling, and the current study investigates the role of Vav1 in the leukocyte recruitment process in vivo with particular attention to the events immediately downstream of LFA-1-dependent adhesion. Intravital and spinning-disk-confocal microscopy was used to investigate intravascular crawling in relation to endothelial junctions in vivo in wild-type and Vav1(-/-) mice. Adherent wild-type neutrophils almost immediately began crawling perpendicular to blood flow via Mac-1 until they reached an endothelial junction where they often changed direction. This pattern of perpendicular, mechanotactic crawling was recapitulated in vitro when shear was applied. In sharp contrast, the movement of Vav1(-/-) neutrophils was always in the direction of flow and appeared more passive as if the cells were dragged in the direction of flow in vivo and in vitro. More than 80% of Vav1(-/-) neutrophils moved independent of Mac-1 and could be detached with LFA-1 Abs. An inability to release the uropod was frequently noted for Vav1(-/-) neutrophils, leading to greatly elongated tails. The Vav1(-/-) neutrophils failed to stop or follow junctions and ultimately detached, leading to fewer emigrated neutrophils. The Vav1(-/-) phenotype resulted in fewer neutrophils recruited in a relevant model of infectious peritonitis. Clearly, Vav1 is critical for the complex interplay between LFA-1 and Mac-1 that underlies the programmed intravascular crawling of neutrophils.

The ins and outs of leukocyte integrin signaling

Integrins are the principal cell adhesion receptors that mediate leukocyte migration and activation in the immune system. These receptors signal bidirectionally through the plasma membrane in pathways referred to as inside-out and outside-in signaling. Each of these pathways is mediated by conformational changes in the integrin structure. Such changes allow high-affinity binding of the receptor with counter-adhesion molecules on the vascular endothelium or extracellular matrix and lead to association of the cytoplasmic tails of the integrins with intracellular signaling molecules. Leukocyte functional responses resulting from outside-in signaling include migration, proliferation, cytokine secretion, and degranulation. Here, we review the key signaling events that occur in the inside-out versus outside-in pathways, highlighting recent advances in our understanding of how integrins are activated by a variety of stimuli and how they mediate a diverse array of cellular responses.

Antimyeloperoxidase antibodies rapidly induce alpha-4-integrin-dependent glomerular neutrophil adhesion

Patients with antineutrophil cytoplasmic antibodies (ANCAs) frequently develop severe vasculitis and glomerulonephritis. Although ANCAs, particularly antimyeloperoxidase (anti-MPO), have been shown to promote leukocyte adhesion in postcapillary venules, their ability to promote adhesion in the glomerular vasculature is less clear. We used intravital microscopy to examine glomerular leukocyte adhesion induced by anti-MPO. In mice pretreated with LPS, 50 microg anti-MPO induced LFA-1-dependent adhesion in glomeruli. In concert with this finding, in mice pretreated with LPS, more than 80% of circulating neutrophils bound anti-MPO within 5 minutes of intravenous administration. However, even in the absence of LPS, more than 40% of circulating neutrophils bound anti-MPO in vivo, a response not seen in MPO(-/-) mice. In addition, a higher dose of anti-MPO (200 microg) induced robust glomerular leukocyte adhesion in the absence of LPS. The latter response was beta2-integrin independent, instead requiring the alpha4-integrin, which was up-regulated on neutrophils in response to anti-MPO. These data indicate that anti-MPO antibodies bind to circulating neutrophils, and can induce glomerular leukocyte adhesion via multiple pathways. Lower doses induce adhesion only after an infection-related stimulus, whereas higher doses are capable of inducing responses in the absence of an additional inflammatory stimulus, via alternative adhesion mechanisms.

Leukocyte integrins and their ligand interactions

Although critical for cell adhesion and migration during normal immune-mediated reactions, leukocyte integrins are also involved in the pathogenesis of diverse clinical conditions including autoimmune diseases and chronic inflammation. Leukocyte integrins therefore have been targets for anti-adhesive therapies to treat the inflammatory disorders. Recently, the therapeutic potential of integrin antagonists has been demonstrated in psoriasis and multiple sclerosis. However, current therapeutics broadly affect integrin functions and, thus, yield unfavorable side effects. This review discusses the major leukocyte integrins and the anti-adhesion strategies for treating immune diseases.

Modeling the role of homologous receptor desensitization in cell gradient sensing

G-protein-coupled chemoattractant receptors signal transiently upon ligand binding to effect cell orientation and motility but then are rapidly desensitized. The importance of desensitization has been unclear, because mutated nondesensitizable receptors mediate efficient chemotaxis. We hypothesized that homologous receptor desensitization is required for cellular navigation in fields of competing attractants. Modeling of receptor-mediated orientation shows that desensitization allows integration of attractant signals. Cells expressing normal receptors are predicted to 1) orient preferentially to distant gradients; 2) seek an intermediate position between balanced agonist sources; 3) and can be repositioned between chemoattractant-defined microenvironmental domains by modest changes in receptor number. In contrast, in the absence of desensitization, orientation is dominated by local agonist sources, precluding continued navigation. Furthermore, cell orientation in competing ligand gradients depends on the relative kinetic rates of receptor desensitization and recycling. We propose that homologous receptor desensitization is critical for cellular navigation in complex chemoattractant fields.

Oxidative stress alters syndecan-1 distribution in lungs with pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is an interstitial lung disease characterized by severe, progressive fibrosis. Roles for inflammation and oxidative stress have recently been demonstrated, but despite advances in understanding the pathogenesis, there are still no effective therapies for IPF. This study investigates how extracellular superoxide dismutase (EC-SOD), a syndecan-binding antioxidant enzyme, inhibits inflammation and lung fibrosis. We hypothesize that EC-SOD protects the lung from oxidant damage by preventing syndecan fragmentation/shedding. Wild-type or EC-SOD-null mice were exposed to an intratracheal instillation of asbestos or bleomycin. Western blot was used to detect syndecans in the bronchoalveolar lavage fluid and lung. Human lung samples (normal and IPF) were also analyzed. Immunohistochemistry for syndecan-1 and EC-SOD was performed on human and mouse lungs. In vitro, alveolar epithelial cells were exposed to oxidative stress and EC-SOD. Cell supernatants were analyzed for shed syndecan-1 by Western blot. Syndecan-1 ectodomain was assessed in wound healing and neutrophil chemotaxis. Increases in human syndecan-1 are detected in lung homogenates and lavage fluid of IPF lungs. Syndecan-1 is also significantly elevated in the lavage fluid of EC-SOD-null mice after asbestos and bleomycin exposure. On IHC, syndecan-1 staining increases within fibrotic areas of human and mouse lungs. In vitro, EC-SOD inhibits oxidant-induced loss of syndecan-1 from A549 cells. Shed and exogenous syndecan-1 ectodomain induce neutrophil chemotaxis, inhibit alveolar epithelial wound healing, and promote fibrogenesis. Oxidative shedding of syndecan-1 is an underlying cause of neutrophil chemotaxis and aberrant wound healing that may contribute to pulmonary fibrosis.

Neutrophil-induced skeletal muscle damage: a calculated and controlled response following hindlimb unloading and reloading

Neutrophils phagocyte necrotic debris and release cytokines, enzymes, and oxidative factors. In the present study, we investigated the contribution of neutrophils to muscle injury, dysfunction, and recovery using an unloading and reloading model. Mice were submitted to 10 days of hindlimb unloading and were transiently depleted in neutrophils with anti-Ly6G/Ly6C antibody prior to reloading. Leukocyte accumulation and muscle function were assessed immunohistologically and functionally in vitro. In addition, soleus muscles submitted to unloading and reloading were incubated in vitro with LPS (100 microg/ml) to determine whether exogenous stimulus would activate neutrophil response and produce extensive muscle damage. Contractile properties were recorded every hour for 6 h, and muscles were subsequently incubated in procion orange to assess muscle damage. Neutrophil depletion affected neither the loss in muscle force nor the time of recovery in atrophied and reloaded soleus muscles. However, atrophied and reloaded soleus muscles that contained high concentration of neutrophils experienced a 20% greater loss in force than atrophied and reloaded soleus muscles depleted in neutrophils following in vitro incubation with LPS. Procion orange dye also confirmed that neutrophils induced a 2.5-fold increase in muscle membrane damage in the presence of LPS. These results show that neutrophil infiltration during modified mechanical loading is highly regulated and efficiently eliminated, with no significant muscle fiber injury unless the activation state of neutrophils is modified by the presence of LPS.

Moving towards a better understanding of chemotaxis

Eukaryotic cells are thought to move across supporting surfaces through a combination of coordinated processes: polarisation; extension of dynamic protrusions from a leading edge; adhesion-associated stabilisation of some protrusions; centripetal pulling against those leading adhesions; and de-adhesion at the rear. Gradients of extracellular ligands can be detected by cells and then used to guide them either towards the source (in the case of a chemoattractant) or away from the source (in the case of a chemorepellent)--such migration is termed chemotaxis. Recent work suggests that chemotaxis probably emerges from the ability of cells to spatially encode extracellular gradients of ligands, a process for which phosphoinositide 3'-kinase (PI3K) signals alone are insufficient, and to use that vectorial information to bias movement by enhancing the survival, and not the formation, of the protrusions that experience the greatest stimulation.

Interaction of osteopontin with neutrophil alpha(4)beta(1) and alpha(9)beta(1) integrins in a rodent model of alcoholic liver disease

Previous studies from our laboratory have reported that osteopontin (OPN) mediated higher hepatic neutrophil infiltration makes female rats more susceptible to alcoholic steatohepatitis (ASH) than their male counterparts. The objective of the current work was to investigate the patho-mechanism by which OPN attracts the hepatic neutrophils in ASH. We hypothesized that OPN-mediated hepatic neutrophil infiltration is a result of signaling by N-terminal integrin binding motif (SLAYGLR) of OPN through its receptor alpha(9)beta(1) (VLA9) and alpha(4)beta(1) (VLA4) integrins on neutrophils. Compared to the males, females in the ASH group exhibited higher expression of alpha(4)beta(1) and alpha(9)beta(1) protein and mRNA and a significant decrease in the expression of these integrins was observed in rats treated with neutralizing OPN antibody. Immunoprecipitation experiments suggested the binding of OPN to alpha(4)beta(1) and alpha(9)beta(1) integrins. OPN-mediated neutrophil infiltration was also confirmed using Boyden chamber assays, and antibodies directed against alpha(4) and beta(1) integrins was found to significantly inhibit neutrophilic migration in vitro. In conclusion, these data suggest that SLAYGLR-mediated alpha(4)beta(1) and alpha(9)beta(1) integrin signaling may be responsible for higher hepatic neutrophil infiltration and higher liver injury in the rat ASH model.

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

PI3K activity, resulting in the accumulation of PIP(3) along the leading edge of a chemotaxing cell, has been proposed to be an indispensable signaling event that is required for cells to undergo chemotaxis to endogenous and exogenous chemoattractants. Some studies have suggested that this might be the case for chemoattractants such as IL8, whereas chemotaxis to other stimuli, such as the bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP), might occur normally in the absence of PI3K activity. Herein, we systematically analyze the role of PI3K in mediating chemotaxis to fMLP, both in vitro and in vivo. Using short- and long-term in vitro assays, as well as an in vivo chemotaxis assay, we investigated the importance of PI3K in response to the prototypic chemoattractant fMLP. Exposure of neutrophils to fMLP induced an immediate polarization, which resulted in directional migration towards fMLP within 2-3 minutes. PI3K-inhibited cells also polarized and migrated in a directional fashion towards fMLP; however, this process was delayed by approximately 15 minutes, demonstrating that PI3K accelerates the initial response to fMLP, but an alternative pathway replaces PI3K over time. By contrast, p38-MAPK-inhibited cells, or cells lacking MK2, were unable to polarize in response to fMLP. Long-term chemotaxis assays using a pan-PI3K inhibitor, a PI3Kdelta-specific inhibitor or PI3Kgamma-knockout neutrophils, demonstrated no role for PI3K in mediating chemotaxis to fMLP, regardless of the steepness of the fMLP gradient. Similar results were observed in vivo, with PI3Kgamma(-/-) cells displaying a delayed, but otherwise normal, chemotactic response to gradients of fMLP. Together, these data demonstrate that, although PI3K can enhance early responses to the bacterial chemoattractant fMLP, it is not required for migration towards this chemoattractant.

Convergence of immunoreceptor and integrin signaling

A common signaling pathway is known to operate downstream of immunoreceptors, such as the T-cell, B-cell, or Fc receptors, following engagement by their respective ligands. This pathway involves Src family kinase-mediated tyrosine phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) that recruit and activate spleen tyrosine kinase (Syk) or Zap70 (zeta-associated protein of 70 kDa) kinases, which in turn activate a variety of downstream signals. Evidence has been building from a variety of sources, particularly mouse models, that molecules involved in the immunoreceptor signaling pathway are also required for signals initiated by integrins. Integrins are the major cell surface receptors that mediate adhesion of leukocytes to a variety of extracellular matrix proteins and counter-receptors expressed on endothelial cells. Integrin ligation is a critical step in the activation of leukocyte effector functions (such as neutrophil degranulation or lymphocyte proliferation). Integrin signaling through pathways common to those utilized by immunoreceptors provides a mechanism by which leukocyte adhesion can regulate activation of cellular responses. In animal models, integrin-mediated signal transduction plays a critical role in inflammatory disease. In this review, we discuss the convergence of immunoreceptor and integrin signaling, focusing on how these pathways modulate leukocyte activation.

Distinction between bacterial and viral infections

PURPOSE OF REVIEW

To commence proper treatment as rapidly as possible and to reduce unnecessary antibiotic treatments, timely knowledge of whether the infection is bacterial or viral in origin would be beneficial for the clinician. As a reliable prediction of the causative agent of bacterial infection is not possible based on clinical features, there is an ongoing need for sensitive and specific markers of bacterial infection.

RECENT FINDINGS

The most common differential diagnosis methods are reviewed here. It is also demonstrated that the measurement of the expression of complement receptors, particularly CR1 (CD35), on neutrophils can be a useful preliminary test to differentiate between bacterial and viral infections. In addition, a novel marker of local and systemic bacterial infections designated 'clinical infection score (CIS) point', which incorporates quantitative analysis of complement receptors on neutrophils and standard clinical laboratory data and displays 98% sensitivity and 97% specificity in distinguishing between bacterial and viral infections, is presented.

SUMMARY

We conclude that the diagnostic yield of measured individual variables in distinguishing between bacterial and viral infections increases upon combination.

Leukocyte PI3Kgamma and PI3Kdelta have temporally distinct roles for leukocyte recruitment in vivo

Phosphoinositide 3-kinases (PI3Ks) have been considered important in leukocyte motility. PI3Kgamma, the class I(B) PI3K, expressed prominently in leukocytes and also in endothelial cells, mediates leukocyte functional responses induced by chemoattractants. To reveal its role in leukocyte recruitment, we used intravital microscopy to directly visualize leukocyte rolling, adhesion, and emigration in postcapillary venules in PI3Kgamma-deficient (PI3Kgamma(-/-)) mice. We report here that PI3Kgamma deficiency had no significant effects on leukocyte rolling flux or rolling velocity and minor effects on adhesion (30% to 35%) in response to CXC chemokine MIP-2 (CXCL2) or KC (CXCL1). However, leukocyte emigration was severely impaired in PI3Kgamma(-/-) mice in an early (first 90 minutes) response to MIP-2 or KC. Chimeric mice receiving bone marrow transplants revealed that this early response was entirely dependent upon PI3Kgamma in neutrophils but not parenchymal cells (endothelium and others). Identical responses were observed when endogenous chemokine production was induced by TNFalpha; leukocyte emigration was reduced in PI3Kgamma(-/-) mice. More prolonged responses to MIP-2 (for 4 to 5 hours) or TNFalpha (6 to 8 hours) were almost entirely PI3Kgamma independent and largely dependent on PI3Kdelta. Our results reveal that leukocyte emigration response to CXC chemokines is entirely dependent upon PI3Kgamma or PI3Kdelta, but these are nonoverlapping, temporally distinct events in inflamed tissues in vivo.

Fibronectin-alpha4beta1 integrin interactions regulate metalloproteinase-9 expression in steatotic liver ischemia and reperfusion injury

Ischemia/reperfusion injury is a major cause of the highly dysfunctional rate observed in marginal steatotic orthotopic liver transplantation. In this study, we document that the interactions between fibronectin, a key extracellular matrix protein, and its integrin receptor alpha4beta1, expressed on leukocytes, specifically up-regulated the expression and activation of metalloproteinase-9 (MMP-9, gelatinase B) in a well-established steatotic rat liver model of ex vivo ice-cold ischemia followed by isotransplantation. The presence of the active form of MMP-9 was accompanied by massive intragraft leukocyte infiltration, high levels of proinflammatory cytokines, such as interleukin-1beta and tumor necrosis factor-alpha, and impaired liver function. Interestingly, MMP-9 activity in steatotic liver grafts was, to a certain extent, independent of the expression of its natural inhibitor, the tissue inhibitor of metalloproteinases-1. Moreover, the blockade of fibronectin-alpha4beta1-integrin interactions inhibited the expression/activation of MMP-9 in steatotic orthotopic liver transplantations without significantly affecting the expression of metalloproteinase-2 (MMP-2, gelatinase A). Finally, we identified T lymphocytes and monocytes/macrophages as major sources of MMP-9 in steatotic liver grafts. Hence, these findings reveal a novel aspect of the function of fibronectin-alpha4beta1 integrin interactions that holds significance for the successful use of marginal steatotic livers in transplantation.

Macrophage migration inhibitory factor induces macrophage recruitment via CC chemokine ligand 2

Macrophage migration inhibitory factor (MIF) was originally identified for its ability to inhibit the random migration of macrophages in vitro. MIF is now recognized as an important mediator in a range of inflammatory disorders. We recently observed that the absence of MIF is associated with a reduction in leukocyte-endothelial cell interactions induced by a range of inflammatory mediators, suggesting that one mechanism whereby MIF acts during inflammatory responses is by promoting leukocyte recruitment. However, it is unknown whether MIF is capable of inducing leukocyte recruitment independently of additional inflammatory stimuli. In this study, we report that MIF is capable of inducing leukocyte adhesion and transmigration in postcapillary venules in vivo. Moreover, leukocytes recruited in response to MIF were predominantly CD68(+) cells of the monocyte/macrophage lineage. Abs against the monocyte-selective chemokine CCL2 (JE/MCP-1) and its receptor CCR2, but not CCL3 and CXCL2, significantly inhibited MIF-induced monocyte adhesion and transmigration. CCL2(-/-) mice displayed a similar reduction in MIF-induced recruitment indicating a critical role of CCL2 in the MIF-induced response. This hypothesis was supported by findings that MIF induced CCL2 release from primary microvascular endothelial cells. These data demonstrate a previously unrecognized function of this pleiotropic cytokine: induction of monocyte migration into tissues. This function may be critical to the ability of MIF to promote diseases such as atherosclerosis and rheumatoid arthritis, in which macrophages are key participants.

Induction of matrix metalloproteinase gene expression in an endothelial cell line by direct interaction with malignant cells

Mouse endothelial TKD2 cells in monolayers were cocultured with various human cell lines for 24 h, and the expression of several secreted matrix metalloproteinases (MMP) and cell adhesion molecules was examined by real-time reverse transcription-polymerase chain reaction using mouse-specific primers. Coculture with normal fibroblasts did not elicit the expression of these molecules, but coculture with cancer cells induced the expression of MMP-3, MMP-9 and MMP-10 mRNA in endothelial cells, and in normal mouse embryonic fibroblasts. The induction of MMP mRNA was dependent on direct cell adhesion, as separate culture of A549 cells in Boyden chambers did not induce MMP mRNA, and neutralizing antibody against VLA-4 abolished the induction. An inhibitor of phosphatidylinositol-3-phosphate kinase strongly suppressed the induction of MMP-3, MMP-9 and MMP-10 mRNA, and expression of the dominant-negative mutant of phosphatidylinositol-3-phosphate kinase also decreased the induction. It was suggested that intracellular reactive oxygen species (ROS) levels were increased in TKD2 cells following adhesion to cancer cells. ROS scavengers decreased the levels of MMP induction, and roterone, an inhibitor of mitochondrial complex I, strongly suppressed the induction of MMP-3, MMP-9 and MMP-10. The depletion of mitochondria in TKD2 cells decreased the induction of MMP-9, but the induction of MMP-3 and MMP-10 was not affected. These results indicate that the adhesion of cancer cells to endothelial cells activates several distinct signaling pathways to induce MMP gene expression, and the pathways for MMP-3, MMP-9 and MMP-10 are partly different. For the induction of MMP-9, mitochondria participate in induction, possibly through the production of ROS.

HIV and other lentiviral infections cause defects in neutrophil chemotaxis, recruitment, and cell structure: immunorestorative effects of granulocyte-macrophage colony-stimulating factor

Patients with HIV infection exhibit deficits in bacterial and fungal clearance, and possibly depressed innate immunity. In this study, we observed that neutrophils from HIV-infected patients have a profound defect in chemotaxis in response to endogenous (IL-8) and bacterial (fMLP) chemoattractants, which was directly correlated with peripheral CD4(+) lymphocyte levels but not plasma viral load. A similar chemotactic defect was observed in the feline immunodeficiency virus (FIV) model of HIV infection. Intravital microscopy of FIV-infected animals revealed marked impairment in the in vivo recruitment of leukocytes; specifically integrin-dependent neutrophil adhesion and emigration induced by bacterial products. Treatment of FIV-infected animals with GM-CSF re-established both neutrophil recruitment (rolling, adhesion, and emigration) and in vitro chemotaxis to the levels seen in uninfected animals. This restoration of neutrophil responses was not due to GM-CSF-mediated priming. Rather, HIV and FIV infections resulted in defective neutrophil development, with an ensuing reduction in neutrophil granularity and chemotactic receptor expression. GM-CSF therapy restored neutrophil granularity, implying restoration of normal neutrophil development. Together, our findings underscore the fundamental defects in innate immunity caused by lentivirus infections, while also indicating that GM-CSF may be a potential immunorestorative therapy for HIV-infected patients.

Transendothelial migration enhances integrin-dependent human neutrophil chemokinesis

Transendothelial migration of neutrophils induces phenotypic changes that influence the interactions of neutrophils with extravascular tissue components. To assess the influence of transmigration on neutrophil chemokinetic motility, we used polyethylene glycol hydrogels covalently modified with specific peptide sequences relevant to extracellular matrix proteins. We evaluated fMLP-stimulated human neutrophil motility on peptides Arg-Gly-Asp-Ser (RGDS) and TMKIIPFNRTLIGG (P2), alone and in combination. RGDS is a bioactive sequence found in a number of proteins, and P2 is a membrane-activated complex-1 (Mac-1) ligand located in the gamma-chain of the fibrinogen protein. We evaluated, via video microscopy, cell motility by measuring cell displacement from origin and total accumulated distance traveled and then calculated average velocity. Results indicate that although adhesion and shape change were supported by hydrogels containing RGD alone, motility was not. Mac-1-dependent motility was supported on hydrogels containing P2 alone. Motility was enhanced through combined presentation of RGD and P2, engaging Mac-1, alpha(V)beta(3), and beta(1) integrins. Naïve neutrophil motility on combined peptide substrates was dependent on Mac-1, and alpha(4)beta(1) while alpha(6)beta(1) contributed to speed and linear movement. Transmigrated neutrophil motility was dependent on alpha(v)beta(3) and alpha(5)beta(1), and alpha(4)beta(1), alpha(6)beta(1), and Mac-1 contributed to speed and linear motion. Together, the data demonstrate that efficient neutrophil migration, dependent on multi-integrin interaction, is enhanced after transendothelial migration.

Antibodies to Complement Receptor 3 Treat Established Inflammation in Murine Models of Colitis and a Novel Model of Psoriasiform Dermatitis

Prior studies indicated the ability of Abs to complement receptor 3 (CR3, CD11b/CD18) to suppress the production of IL-12 from immune cells. Therefore, we tested the ability of an anti-CR3 Ab (clone M1/70) to treat established IL-12-dependent Th1-mediated inflammation in murine models. Systemic administration of anti-CR3 significantly ameliorated established intestinal inflammation following the intrarectal administration of trinitrobenzene sulfonic acid (TNBS-colitis), as well as colitis and skin inflammation in C57BL/10 RAG-2(-/-) mice reconstituted with CD4+CD45RBhigh T cells. The hyperproliferative skin inflammation in this novel murine model demonstrated many characteristics of human psoriasis, and was prevented by the adoptive transfer of CD45RBlow T cells. In vitro and in vivo studies suggest that anti-CR3 treatment may act, at least in part, by directly inhibiting IL-12 production by APCs. Administration of anti-CR3 may be a useful therapeutic approach to consider for the treatment of inflammatory bowel disease and psoriasis in humans.

T cell chemotaxis in a simple microfluidic device

This paper describes the use of a simple microfluidic device for studying T cell chemotaxis. The microfluidic device is fabricated in poly(dimethylsiloxane) (PDMS) using soft-lithography and consists of a "Y" type fluidic channel. Solutions are infused into the device by syringe pumps and generate a concentration gradient in the channel by diffusion. We show that the experimentally measured gradient profiles agree nicely with theoretical predictions and the gradient is stable in the observation region for cell migration. Using this device, we demonstrate robust chemotaxis of human T cells in response to single and competing gradients of chemokine CCL19 and CXCL12. Because of the simplicity of the device, it can flexibly control gradient generation in space and time, and would allow generation of multiple gradient conditions in a single chip for highly parallel chemotaxis experimentation. Visualization of T cell chemotaxis has previously been limited to studies in 3D matrices or under agarose assays, which do not allow precise control or variation in conditions. Acknowledging the importance of lymphocyte homing in the adaptive immune response, the ability to study T cell chemotaxis in microfluidic devices offers a new approach for investigating lymphocyte migration and chemotaxis in vitro.