Shear and time-dependent changes in Mac-1, LFA-1, and ICAM-3 binding regulate neutrophil homotypic adhesion

Machine learning/molecular dynamic protein structure prediction approach to investigate the protein conformational ensemble

Proteins exist in several different conformations. These structural changes are often associated with fluctuations at the residue level. Recent findings show that co-evolutionary analysis coupled with machine-learning techniques improves the precision by providing quantitative distance predictions between pairs of residues. The predicted statistical distance distribution from Multi Sequence Analysis reveals the presence of different local maxima suggesting the flexibility of key residue pairs. Here we investigate the ability of the residue-residue distance prediction to provide insights into the protein conformational ensemble. We combine deep learning approaches with mechanistic modeling to a set of proteins that experimentally showed conformational changes. The predicted protein models were filtered based on energy scores, RMSD clustering, and the centroids selected as the lowest energy structure per cluster. These models were compared to the experimental-Molecular Dynamics (MD) relaxed structure by analyzing the backbone residue torsional distribution and the sidechain orientations. Our pipeline allows to retrieve the experimental structural dynamics experimentally represented by different X-ray conformations for the same sequence as well the conformational space observed with the MD simulations. We show the potential correlation between the experimental structure dynamics and the predicted model ensemble demonstrating the susceptibility of the current state-of-the-art methods in protein folding and dynamics prediction and pointing out the areas of improvement.

Brothers in arms: platelets and neutrophils in ischemic stroke

PURPOSE OF REVIEW

In this review, we will describe how the combined ability of platelets and neutrophils to interact with each other drives ischemic stroke brain injury.

RECENT FINDINGS

Neutrophils are one of the first cells to respond during ischemic stroke. Although animals stroke models have indicated targeting neutrophils improves outcomes, clinical trials have failed to yield successful strategies. Platelets play a critical role in recruiting neutrophils to sites of injury by acting as a bridge to the injured endothelium. After initial platelet adhesion, neutrophils can rapidly bind platelets through P-selectin and glycoprotein Ibα. In addition, recent data implicated platelet phosphatidylserine as a novel key regulator of platelet-neutrophil interactions in the setting of ischemic stroke. Inhibition of procoagulant platelets decreases circulating platelet-neutrophil aggregates and thereby reduces infarct size. Platelet binding alters neutrophil function, which contributes to the injury associated with ischemic stroke. This includes inducing the release of neutrophil extracellular traps, which are neurotoxic and pro-thrombotic, leading to impaired stroke outcomes.

SUMMARY

Platelet-neutrophil interactions significantly contribute to the pathophysiology of ischemic stroke brain injury. Better understanding the mechanisms behind their formation and the downstream consequences of their interactions will lead to improved therapies for stroke patients.

Trovafloxacin-Induced Liver Injury: Lack in Regulation of Inflammation by Inhibition of Nucleotide Release and Neutrophil Movement

The fluoroquinolone trovafloxacin (TVX) is associated with a high risk of drug-induced liver injury (DILI). Although part of the liver damage by TVX+TNF relies on neutrophils, we have recently demonstrated that liver recruitment of monocytes and neutrophils is delayed by TVX. Here we show that the delayed leukocyte recruitment is caused by a combination of effects which are linked to the capacity of TVX to block the hemichannel pannexin 1. TVX inhibited find-me signal release in apoptotic HepG2 hepatocytes, decelerated freshly isolated human neutrophils toward IL-8 and f-MLF, and decreased the liver expression of ICAM-1. In blood of TVX+TNF-treated mice, we observed an accumulation of activated neutrophils despite an increased MIP-2 release by the liver. Depletion of monocytes and neutrophils caused increased serum concentrations of TNF, IL-6, and MIP-2 in TVX-treated mice as well as in mice treated with the fluoroquinolone levofloxacin, known to have a lower DILI-inducing profile. This supports the idea that early leukocyte recruitment regulates inflammation. In conclusion, disrupted regulation by leukocytes appears to constitute a fundamental step in the onset of TVX-induced liver injury, acting in concert with the capability of TVX to induce hepatocyte cell death. Interference of leukocyte-mediated regulation of inflammation represents a novel mechanism to explain the onset of DILI.

Recombinant thrombomodulin ameliorates autoimmune vasculitis via immune response regulation and tissue injury protection

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is characterized by necrotizing vasculitis with the presence of pathogenic ANCA. ANCA can potentially cause neutrophil activation and induce neutrophil extracellular traps (NETs), resulting in endothelial damage as well as activation of autoreactive B cells and alternative complement pathway. Recombinant thrombomodulin (rTM) protects the endothelium from vascular injury during disseminated intravascular coagulation, thus we hypothesized that rTM ameliorates necrotizing vasculitis in AAV. In this study, rTM was administered in an experimental AAV rat model. Treatment of experimental AAV rats with rTM improved pulmonary hemorrhage and glomerulonephritis, with a suppression of ANCA production and NETs formation. In addition, in vitro experiments showed that rTM bound to neutrophils via Mac-1 (macrophage-1 antigen) and inhibited ANCA-induced NETs formation accompanied by a suppression of histone citrullination, leading to a protection of the endothelium from NETs toxicity. Additionally, rTM affected lymphocytes leading to the inhibition of pro-inflammatory cytokine/chemokin in PBMC during the antibody production process, which might indirectly be involved in the reduction of pathogenic ANCA. Our data revealed that the rTM could ameliorate autoimmune vasculitis through a combination of different biological mechanisms.

Integrins as A New Target for Cancer Treatment

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Despite the great progress in the development of targeted therapies for different types of cancer utilizing monoclonal antibodies (e.g., cetuximab for colorectal cancer and head and neck cancer therapy), kinase inhibitors (e.g., sorafenib for kidney cancer and gastrointestinal stromal tumours therapy), and immunomodulatory treatments (e.g., nivolumab and pembrolizumab for melanoma therapy and lung cancer therapy), there is still a need to search for new, more effective treatments.

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Integrins are responsible for intercellular adhesion and interaction with the cellular matrix. The function of integrins is related to the transduction of intracellular signals associated with adhesion, migration, cell proliferation, differentiation, and apoptosis. Molecules targeting integrins that lead to cancer cell death have been developed. The most advanced molecules studied in clinical trials are abituzumab, intetumumab and cilengitide. There are different groups of anti-integrin drugs: monoclonal antibodies (e.g., abituzumab) and other such as cilengitide, E7820 and MK-0429. These drugs have been evaluated in various cancer types. However, they have shown modest efficacy, and none of them have yet been approved for cancer treatment. Studies have shown that patient selection using biomarkers might improve the efficacy of anti-integrin cancer treatment. Many preclinical models have demonstrated promising results using integrin visualization for cancer detection and treatment efficacy monitoring; however, these strategies require further evaluation in humans.

CD169+ macrophages orchestrate innate immune responses by regulating bacterial localization in the spleen

The spleen is an important site for generating protective immune responses against pathogens. After infection, immune cells undergo rapid reorganization to initiate and maintain localized inflammatory responses; however, the mechanisms governing this spatial and temporal cellular reorganization remain unclear. We show that the strategic position of splenic marginal zone CD169⁺ macrophages is vital for rapid initiation of antibacterial responses. In addition to controlling initial bacterial growth, CD169⁺ macrophages orchestrate a second phase of innate protection by mediating the transport of bacteria to splenic T cell zones. This compartmentalization of bacteria within the spleen was essential for driving the reorganization of innate immune cells into hierarchical clusters and for local interferon-γ production near sites of bacterial replication foci. Our results show that both phases of the antimicrobial innate immune response were dependent on CD169⁺ macrophages, and, in their absence, the series of events needed for pathogen clearance and subsequent survival of the host was disrupted. Our study provides insight into how lymphoid organ structure and function are related at a fundamental level.

Complement Factor H Inhibits Anti-Neutrophil Cytoplasmic Autoantibody-Induced Neutrophil Activation by Interacting With Neutrophils

Our previous study demonstrated that plasma levels of complement factor H (FH) were inversely associated with the disease activity of patients with anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV). In addition to serving as an inhibitor of the alternative complement pathway, there is increasing evidence demonstrating direct regulatory roles of FH on several cell types. Here, we investigated the role of FH in the process of ANCA-mediated activation of neutrophils and neutrophil–endothelium interaction. We demonstrated that FH bound to neutrophils by immunostaining and flow cytometry. Interestingly, ANCA-induced activation of neutrophils, including respiratory burst and degranulation, was inhibited by FH. Although FH enhanced neutrophils adhesion and migration toward human glomerular endothelial cells (hGEnCs), it inhibited ANCA-induced activation of neutrophils in the coculture system of hGEnCs and neutrophils. Moreover, the activation and injury of hGEnCs, reflected by the level of endothelin-1 in the supernatant of cocultures, was markedly reduced by FH. However, we found that FH from patients with active AAV exhibited a deficient ability in binding neutrophils and inhibiting ANCA-induced neutrophil activation in fluid phase and on endothelial cells, as compared with that from healthy controls. Therefore, our findings indicate a novel role of FH in inhibiting ANCA-induced neutrophil activation and protecting against glomerular endothelial injury. However, FH from patients with active AAV are deficient in their ability to bind neutrophils and inhibit neutrophil activation by ANCA. It further extends the current understanding of the pathogenesis of AAV, thus providing potential clues for intervention strategies.

Neutrophil recruitment limited by high-affinity bent β2 integrin binding ligand in cis

Neutrophils are essential for innate immunity and inflammation and many neutrophil functions are β2 integrin-dependent. Integrins can extend (E(+)) and acquire a high-affinity conformation with an 'open' headpiece (H(+)). The canonical switchblade model of integrin activation proposes that the E(+) conformation precedes H(+), and the two are believed to be structurally linked. Here we show, using high-resolution quantitative dynamic footprinting (qDF) microscopy combined with a homogenous conformation-reporter binding assay in a microfluidic device, that a substantial fraction of β2 integrins on human neutrophils acquire an unexpected E(-)H(+) conformation. E(-)H(+) β2 integrins bind intercellular adhesion molecules (ICAMs) in cis, which inhibits leukocyte adhesion in vitro and in vivo. This endogenous anti-inflammatory mechanism inhibits neutrophil aggregation, accumulation and inflammation.

Effects of cellular viscoelasticity in lifetime extraction of single receptor-ligand bonds

Single-molecule force spectroscopy is widely used to determine kinetic parameters of dissociation by analyzing bond rupture data obtained via applying mechanical force to cells, capsules, and beads that are attached to an intermolecular bond. The bond rupture data are obtained in experiments either at a constant force or at a constant loading rate. We explore the effect of cellular viscoelasticity in constant-force experiments. Specifically, we perform Monte Carlo simulations of bond rupture at a given constant force to obtain the bond lifetime as a function of force in the absence and in the presence of bond force modulation due to cellular viscoelasticity, to explore its effect on the bond lifetime.

Choline Transporter-Like Protein-2: New von Willebrand Factor-Binding Partner Involved in Antibody-Mediated Neutrophil Activation and Transfusion-Related Acute Lung Injury

OBJECTIVE

In contrast to other antibodies involved in transfusion-related acute lung injury, anti-HNA-3a antibodies are incapable of inducing direct neutrophil activation and seem to interact with endothelial cells (ECs) primarily. In animal studies, anti-HNA-3a-mediated transfusion-related acute lung injury could be precipitated in the absence of neutrophils, but was stronger when neutrophils were present. In a different context the target protein of these antibodies, choline transporter-like protein-2 (CTL-2), was reported to interact with a protein of the inner ear carrying 2 von Willebrand factor (VWF) A-domains. These observations prompted us to investigate whether VWF might be involved in anti-HNA-3a-mediated neutrophil activation, and whether signaling via CD11b/CD18 is involved, as in various other experimental settings.

APPROACH AND RESULTS

Cell adhesion demonstrated specific binding of CTL-2 to VWF. Immunoprecipitation analysis of CTL-2/CD11b/CD18 coexpressing cells indicated that anti-HNA-3a colocalizes CTL-2 and CD11b/CD18 when VWF is present. Functional studies revealed that anti-HNA-3a-mediated neutrophil agglutination is an active, protein kinase C-dependent and partially Fc-dependent process. Agglutination and the production of reactive oxygen species seem to require the formation of a trimolecular complex between the target antigen (CTL-2), CD11b/CD18 and VWF. In line with these observations, anti-HNA-3a induced less severe transfusion-related acute lung injury and less neutrophil recruitment to the alveolar space in VWF knockout mice.

CONCLUSIONS

We introduce CTL-2 as a new binding partner for VWF. Interaction of neutrophils with VWF via CTL-2 allows anti-HNA-3a to induce signal transduction via CD11b/CD18, which leads to neutrophil activation and agglutination. In transfusion-related acute lung injury, this mechanism may further aggravate endothelial leakage.

Effects of cellular viscoelasticity in multiple-bond force spectroscopy

Receptor-ligand bonds are often subjected to forces that regulate their detachment via modulating off-rates. Though the dynamics of detachment is primarily controlled by the physical chemistry of adhesion molecules cellular features such as cell deformability and microvillus viscoelasticity have been shown to have an effect on it as well. In this work, Monte Carlo simulation of the rupture of multiple receptor-ligand bonds between substrate and a polymorphonuclear leukocyte (PMN) cell suspended in a Newtonian fluid is performed. It is demonstrated via various micromechanical models of the PMN cell adhered to the substrate by multiple receptor-ligand bonds that viscous drag caused by relative motion of cell suspended in a Newtonian fluid and cellular viscoelasticity modulate transmission of an applied external load to receptor-ligand bonds. It is demonstrated that due to cellular viscoelasticity the instantaneous intermolecular bond force is lower than the instantaneous applied force. It is also demonstrated that due to cellular viscoelasticity, the mean intermolecular bond rupture forces are lowered while the mean bond lifetime increases.

Rupture of single receptor-ligand bonds: a new insight into probability distribution function

Single molecule force spectroscopy is widely used to determine kinetic parameters of dissociation by analyzing bond rupture data obtained via applying mechanical force to cells, capsules, and beads that are attached to an intermolecular bond. The current analysis assumes that the intermolecular bond force is equal to the externally applied mechanical force. We confirm that viscous drag alone or in combination with cellular deformation resulting in viscoelasticity modulates bond force so that the instantaneous intermolecular bond force is not equivalent to the applied force. The bond force modulation leads to bond rupture time and force histograms that differ from those predicted by probability distribution function (PDF) using the current approach. A new methodology that accounts for bond force modulation in obtaining PDF is presented. The predicted histograms from the new methodology are in excellent agreement with the respective histograms obtained from Monte Carlo simulation.

Effect of viscoelasticity on the analysis of single-molecule force spectroscopy on live cells

Single-molecule force spectroscopy is used to probe the kinetics of receptor-ligand bonds by applying mechanical forces to an intermediate media on which the molecules reside. When this intermediate media is a live cell, the viscoelastic properties can affect the calculation of rate constants. We theoretically investigate the effect of media viscoelasticity on the common assumption that the bond force is equal to the instantaneous applied force. Dynamic force spectroscopy is simulated between two cells of varying micromechanical properties adhered by a single bond with a constant kinetic off-rate. We show that cell and microvilli deformation, and hydrodynamic drag contribute to bond forces that can be 28-90% lower than the applied force for loading rates of 10(3)-10(7) pN/s, resulting in longer bond lifetimes. These longer bond lifetimes are not caused by changes in bond kinetics; rather, they are due to the mechanical response of the intermediate media on which the bonds reside. Under the assumption that the instantaneous bond force is equal to the applied force--thereby ignoring viscoelasticity--leads to 14-39% error in the determination of off-rates. We present an approach that incorporates viscoelastic properties in calculating the instantaneous bond force and kinetic dissociation parameter of the intermolecular bond.

Endothelium-neutrophil interactions in ANCA-associated diseases

The two salient features of ANCA-associated vasculitis (AAV) are the restricted microvessel localization and the mechanism of inflammatory damage, independent of vascular immune deposits. The microvessel localization of the disease is due to the ANCA antigen accessibility, which is restricted to the membrane of neutrophils engaged in β2-integrin-mediated adhesion, while these antigens are cytoplasmic and inaccessible in resting neutrophils. The inflammatory vascular damage is the consequence of maximal proinflammatory responses of neutrophils, which face cumulative stimulations by TNF-α, β2-integrin engagement, C5a, and ANCA by the FcγRII receptor. This results in the premature intravascular explosive release by adherent neutrophils of all of their available weapons, normally designed to kill IgG-opsonized bacteria after migration in infected tissues.

Roles of Mac-1 and glycoprotein IIb/IIIa integrins in leukocyte-platelet aggregate formation: stabilization by Mac-1 and inhibition by GpIIb/IIIa blockers

Circulating platelet-leukocyte hetero-aggregates play an important role in acute cardiovascular events and hypersensitivity reactions. The association involves the receptor families of selectins and integrin. The objective of this study was to investigate the role of CD11b/CD18 integrin (Mac-1) in hetero-aggregate formation and search for a counter-receptor on platelets ready to interact with Mac-1. As a model of leukocytes, Mac-1 presenting Chinese hamster ovary (CHO) cells were used to evaluate the role of Mac-1 in hetero-aggregate formation. The amount of CHO cell-bound active and inactive platelets was measured by flow cytometry, while the counter-receptors on platelets were identified via using blocking antibodies. We observed significant platelet adhesion on Mac-1-bearing cells when platelet-rich plasma or activated platelets were present. Inactive platelets did not adhere to Mac-1-bearing cells. Addition of fibrinogen, a ligand of Mac-1 significantly increased platelet binding. CD40L was demonstrated to act similarly on Mac-1. Inhibition of platelet GpIIb/IIIa completely abolished CHO cell-platelet aggregation. In our study, we have shown for the first time that Mac-1 mediates the formation of hetero-aggregates without selectin tethering when Mac-1 ligands such as fibrinogen or CD40L are present and blockers of platelet GpIIb/IIIa are able to diminish this interaction.

Structural basis and kinetics of force-induced conformational changes of an αA domain-containing integrin

BACKGROUND

Integrin α(L)β₂ (lymphocyte function-associated antigen, LFA-1) bears force upon binding to its ligand intercellular adhesion molecule 1 (ICAM-1) when a leukocyte adheres to vascular endothelium or an antigen presenting cell (APC) during immune responses. The ligand binding propensity of LFA-1 is related to its conformations, which can be regulated by force. Three conformations of the LFA-1 αA domain, determined by the position of its α₇-helix, have been suggested to correspond to three different affinity states for ligand binding.

METHODOLOGY/PRINCIPAL FINDINGS

The kinetics of the force-driven transitions between these conformations has not been defined and dynamically coupled to the force-dependent dissociation from ligand. Here we show, by steered molecular dynamics (SMD) simulations, that the αA domain was successively transitioned through three distinct conformations upon pulling the C-terminus of its α₇-helix. Based on these sequential transitions, we have constructed a mathematical model to describe the coupling between the αA domain conformational changes of LFA-1 and its dissociation from ICAM-1 under force. Using this model to analyze the published data on the force-induced dissociation of single LFA-1/ICAM-1 bonds, we estimated the force-dependent kinetic rates of interstate transition from the short-lived to intermediate-lived and from intermediate-lived to long-lived states. Interestingly, force increased these transition rates; hence activation of LFA-1 was accelerated by pulling it via an engaged ICAM-1.

CONCLUSIONS/SIGNIFICANCE

Our study defines the structural basis for mechanical regulation of the kinetics of LFA-1 αA domain conformational changes and relates these simulation results to experimental data of force-induced dissociation of single LFA-1/ICAM-1 bonds by a new mathematical model, thus provided detailed structural and kinetic characterizations for force-stabilization of LFA-1/ICAM-1 interaction.

A miniature Couette to generate shear for flow cytometry: studying real-time modulation of intracellular calcium in monocytic cells

Extracellular hydrodynamic forces may be transmitted to the interior of cells through the alteration of integrin conformation and affinity. Integrin activation regulates leukocyte recruitment, cell activation, and transmigration. The cellular and molecular mechanisms for integrin activation are not precisely known, although intracellular calcium signaling is involved. Flow cytometry offers a versatile way to study intracellular calcium signaling in real-time. We report a novel method to generate defined shear by using a miniature Couette. Testing involved measuring shear-induced intracellular calcium signals of human monoblastoid U937 cells in suspension. The Couette was connected externally to a flow cytometer and pressurized at 6 PSI (4.1 N/m(2) ). Cells were subjected to a well-defined shear between 0 and 1,000 s(-1) and delivered continuously within 10 s to a FACScan at 1 μl/s. Intracellular calcium levels and the percentage of cells activated increased as shear increased in duration and intensity.

Leukocyte transepithelial migration in lung induced by DMSA functionalized magnetic nanoparticles

Magnetic nanoparticles surface-covered with meso-2,3-dimercaptosuccinic acid (MNPs-DMSA) constitute a promising approach for tissue- and cell-targeted delivery of therapeutic drugs in the lung. However, they can also induce a transient transendothelial migration of leukocytes in the organ as a side effect after endovenous administration of MNPs-DMSA. We demonstrated that monocytes/macrophages constitute the main subpopulation of leukocytes involved in this process. Our recent research found that MNPs-DMSA up-regulated the mRNA expression of E-, L- and P-selectin and macrophage-1 antigen, and increased concentration of tumor necrosis factor-α in lung, in a time dependent manner. The critical relevance of the β2 integrin-dependent pathway in leukocyte transmigration elicited by MNPs-DMSA was demonstrated by use of knockout mice. Our work characterizes mechanisms of the pro-inflammatory effects of MNPs-DMSA in the lung, and identifies β2 integrin-targeted interventions as promising strategies to reduce pulmonary side effects of MNPs-DMSA during biomedical applications. In addition, MNPs-DMSA could be used as modulators of lung immune response.

Disulfide trapping of protein complexes on the yeast surface

Protein complexes are common in nature and play important roles in biology, but studying the quaternary structure formation in vitro is challenging since it involves lengthy and expensive biochemical steps. There are frequent technical difficulties as well with the sensitivity and resolution of the assays. In this regard, a technique that can analyze protein-protein interactions in high throughput would be a useful experimental tool. Here, we introduce a combination of yeast display and disulfide trapping that we refer to as stabilization of transient and unstable complexes by engineered disulfide (STUCKED) that can be used to detect the formation of a broad spectrum of protein complexes on the yeast surface using fluorescence labeling. The technique uses an engineered intersubunit disulfide to covalently crosslink the subunits of a complex, so that the disulfide-trapped complex can be displayed on the yeast surface for detection and analysis. Transient protein complexes are difficult to display on the yeast surface, since they may dissociate before they can be detected due to a long induction period in yeast. To this end, we show that three different quaternary structures with the subunit dissociation constant K(d) approximately 0.5-20 microM, the antibody variable domain (Fv), the IL-8 dimer, and the p53-MDM2 complex, cannot be displayed on the yeast surface as a noncovalent complex. However, when we introduce an interchain disulfide between the subunits, all three systems are efficiently displayed on the yeast surface, showing that disulfide trapping can help display protein complexes that cannot be displayed otherwise. We also demonstrate that a disulfide forms only between the subunits that interact specifically, the displayed complexes exhibit functional characteristics that are expected of wt proteins, the mutations that decrease the affinity of subunit interaction also reduce the display efficiency, and most of the disulfide stabilized complexes are formed within the secretory pathway during export to the surface. Disulfide crosslinking is therefore a convenient way to study weak protein association in the context of yeast display.

Multi-scale simulation of L-selectin-PSGL-1-dependent homotypic leukocyte binding and rupture

L-selectin-PSGL-1-mediated polymorphonuclear (PMN) leukocyte homotypic interactions potentiate the extent of PMN recruitment to endothelial sites of inflammation. Cell-cell adhesion is a complex phenomenon involving the interplay of bond kinetics and hydrodynamics. As a first step, a 3-D computational model based on the Immersed Boundary Method is developed to simulate adhesion-detachment of two PMN cells in quiescent conditions. Our simulations predict that the total number of bonds formed is dictated by the number of available receptors (PSGL-1) when ligands (L-selectin) are in excess, while the excess amount of ligands influences the rate of bond formation. Increasing equilibrium bond length results in a higher number of receptor-ligand bonds due to an increased intercellular contact area. On-rate constants determine the rate of bond formation, while off-rates control the average number of bonds by modulating bond lifetimes. Application of an external pulling force leads to time-dependent on- and off-rates and causes bond rupture. Moreover, the time required for bond rupture in response to an external force is inversely proportional to the applied load and decreases with increasing off-rate.

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.

Immune complexes formed following the binding of anti-platelet factor 4 (CXCL4) antibodies to CXCL4 stimulate human neutrophil activation and cell adhesion

We tested the possibility that immune complexes formed following platelet factor 4 (PF4/CXCL4) binding to anti-PF4 antibody can stimulate neutrophil activation, similar to previous reports with platelets. Monoclonal Abs against PF4 and IgG from a heparin-induced thrombocytopenia (HIT) patient were applied. We observed that although PF4 or anti-PF4 antibody alone did not alter neutrophil function, costimulation with both reagents resulted in approximately 3-fold increase in cell surface Mac-1 expression, enhanced cell adhesion via L-selectin and CD18 integrins, and degranulation of secondary and tertiary granules. The level of Mac-1 up-regulation peaked at an intermediate PF4 dose, suggesting that functional response varies with antigen-antibody stoichiometry. PF4 binding to neutrophils was blocked by chondroitinase ABC. Cell activation was inhibited by both chondroitinase ABC and anti-CD32/FcgammaRII blocking mAb, IV.3. Confocal microscopy demonstrated that immune complexes colocalize with CD32a. Studies with HIT IgG demonstrated that neutrophils could be activated in the absence of exogenous heparin. These data, together, show that leukocyte surface chondroitin sulfates promote neutrophil activation by enhancing immune-complex binding to CD32a. Studies with recombinant PF4 suggest a role for arginine 49 in stabilizing PF4-chondroitin binding. Neutrophils activated via this mechanism may contribute to thrombosis and inflammation in patients mounting an immune response to PF4-heparin.

Differential regulation of neutrophil CD18 integrin function by di- and tri-valent cations: manganese vs. gadolinium

Affinity regulation of integrin function plays an important role during both leukocyte-endothelial and leukocyte-leukocyte interactions. We compared the roles of Mn(2+) (Manganese) and Gd(3+) (Gadolinium) in regulating leukocyte CD18-integrin function. We observed that: (i) Both cations prolonged neutrophil homotypic aggregation following chemoattractant IL-8 stimulation, with Gd(3+) being effective at doses two orders of magnitude (10 microM range) lower that Mn(2+). (ii) While both Gd(3+) and Mn(2+) mediate homotypic cell aggregation via L: -selectin and CD18 integrins, their effects on the integrin subunits, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18), was different. Gd(3+) altered both LFA-1 and Mac-1 function, while the dominant effect of Mn(2+) was on Mac-1. This effect of Gd(3+) on LFA-1 function was confirmed in cell-free studies that measured the binding of recombinant ICAM-1 to LFA-1 immobilized on beads. (iii) Both ions augmented the binding of 327C, an antibody that recognizes active CD18 on human neutrophils, both in the presence and absence of exogenous IL-8. The effects of Mn(2+) was more pronounced since it caused 3-4-fold increase in mAb 327C binding to neutrophils compared to Gd(3+) which increased antibody binding by only approximately 80%. 327C binding was partially reduced by Ca(2+). Further, 327C binding induced by Mn(2+) did not correlate tightly with cell adhesion function. (iv) In studies that monitored intracellular Ca(2+) ([Ca(2+)](i)), the addition of Mn(2+) but not Gd(3+) to neutrophils altered [Ca(2+)](i) levels. Overall, while both Gd(3+) and Mn(2+) stabilize high affinity CD18 mediated cell adhesion, Gd(3+) affects integrin conformation while Mn(2+) may also trigger other effects.

Molecular mechanics and dynamics of leukocyte recruitment during inflammation

Discovery of new genes and proteins directly supporting leukocyte adhesion is waning, whereas there is heightened interest in the cell mechanics and receptor dynamics that lead from transient tethering via selectins to affinity shifts and adhesion strengthening through integrins. New optical tools enable real-time imaging of leukocyte rolling and arrest in parallel plate flow channels (PPFCs), and detection of single-molecule force spectroscopy provides an inner view of the intercellular adhesive contact region. Leukocyte recruitment during acute inflammation is triggered by ligation of G protein-coupled chemotactic receptors (GPCRs) and clustering of selectins. This, in turn, activates beta(2)-integrin (CD18), which facilitates cell capture and arrest in shear flow. This review provides a conceptual model for the molecular events supporting leukocyte recruitment.

Preferential binding of platelets to monocytes over neutrophils under flow

This study was undertaken to systematically investigate the binding kinetics of platelet recruitment by monocytes relative to neutrophils in bulk suspensions subjected to shear as well as the molecular requirements of leukocyte-platelet binding. Hydrodynamic shear-induced collisions augment the proportion of monocytes with adherent platelets more drastically than that of neutrophils with bound platelets. These heterotypic interactions are further potentiated by platelet activation with thrombin or to a lesser extent by monocyte stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP). Monocyte-platelet heteroaggregation increases with increasing shear rate and shear exposure time. Platelet P-selectin binding to monocyte P-selectin-glycoprotein-ligand-1 is solely responsible for maximal platelet adhesion to unstimulated monocytes in shear flow. However, the enhanced platelet binding to fMLP-treated monocytes involves a sequential two-step process, wherein P-selectin-PSGL-1 interactions are stabilized by CD18-integrin involvement. Blocking platelet alpha(IIb)beta(3) or monocyte beta(1)-integrin function had no effect. This study underscores the preferential recruitment of platelets by monocytes relative to neutrophils in shear flow, and demonstrates that the shear environment of the vasculature coupled to the state of cell activation modulates the dynamics and molecular constituents mediating monocyte-platelet adhesion.

Transport features, reaction kinetics and receptor biomechanics controlling selectin and integrin mediated cell adhesion

The distinct and overlapping roles of adhesion molecules belonging to the selectin and integrin families control the rate of leukocyte adhesion to stimulated vascular endothelial cells under hydrodynamic shear flow. Crystal structures have appeared for some of these interactions which complement molecular biology experiments, and clarify the molecular mechanism of the receptor-ligand binding interactions. Binding affinity data have also appeared using surface plasmon resonance and single-molecule biophysics experiments. These studies confirm and extend the predictions of previous experiments carried out in parallel-plate flow chambers, and cone and plate viscometers. This review discusses the current state of understanding on how molecular bond formation rates coupled with cellular and hydrodynamic features regulate leukocyte binding to endothelial cells.

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

In human PMN (polymorphonuclear cells), challenged by P-selectin, the beta2-integrin Mac-1 (macrophage antigen-1) promoted the activation of the SRC (cellular homologue of Rous sarcoma virus oncogenic protein) family members HCK (haematopoietic cell kinase) and LYN (an SRC family protein tyrosine kinase) and phosphorylation of a P-110 (110 kDa protein). SRC kinase activity in turn was necessary for macrophage antigen-1-mediated adhesion [Piccardoni, Sideri, Manarini, Piccoli, Martelli, de Gaetano, Cerletti and Evangelista (2001) Blood 98, 108-116]. This suggested that an SRC-dependent outside-in signalling strengthens the beta2-integrin interaction with the ligand. To support this hypothesis further, in the present study, we used the monoclonal antibody KIM127 or manganese to lock beta2 integrins in a high-affinity state, and homotypic PMN adhesion was analysed to monitor beta2-integrin adhesive function. KIM127 or manganese induced PMN homotypic adhesion and P-110 phosphorylation. Both these processes were abolished by blocking antibodies against the common beta2 chain, by a combination of antibodies against alphaL and alphaM or by inhibitors of SRC activity. Confocal microscopy showed that activation epitopes were expressed by beta2 integrins co-localized with patches of F-actin at the adhesion sites. Blockade of SRC kinases or of actin polymerization prevented clustering of activated integrins as well as F-actin accumulation. FACS analysis showed that SRC inhibitors modified neither basal nor manganese-induced KIM127 binding. An SRC-dependent outside-in signalling initiated by beta2 integrins was also required for adhesion triggered by interleukin-8. These results confirm the hypothesis that an SRC-dependent outside-in signalling triggered by high affinity and ligand binding is necessary to stabilize beta2-integrin-mediated adhesion. Allowing clustering of activated integrins, SRC might link the high-affinity with the high-avidity state. Proline-rich tyrosine kinase-2 appears to be involved in this process.

Aspects of hydrodynamic shear regulating shear-induced platelet activation and self-association of von Willebrand factor in suspension

The binding of plasma von Willebrand factor (VWF) to platelet receptor GpIb under high hydrodynamic shear leads to platelet activation and subsequent shear-induced platelet aggregation (SIPA). We quantitatively examined the aspects of fluid flow that regulate platelet activation by subjecting human blood and isolated platelets to well-defined shear conditions in a cone-plate viscometer. We made the following observations. First, Annexin V binding to phosphatidyl serine expressed on activated cells was detectable within 10 seconds of shear application. Second, fluid shear stress rather than shear rate controls platelet activation, and a threshold shear stress of approximately 80 dyn/cm(2) is necessary to induce significant activation. Under these conditions, individual domains of soluble VWF and platelet GpIb are subjected to similar magnitudes of fluid forces on the order of 0.1 pN, whereas GpIb with bound VWF is subjected to 1 pN. Third, cell-cell collisions and time-varying stresses are not essential for platelet activation. Fourth, the mechanism of platelet activation can be resolved in 2 steps based on the contribution of VWF and fluid forces. Fluid shear and VWF are required during the first step, when GpIb-VWF binding likely occurs. Subsequently, high shear forces alone in the absence of VWF in suspension can induce platelet activation. In other experiments, purified VWF was subjected to shear in the viscometer, and VWF morphology was assessed using light scattering. These studies demonstrate, for the first time, the ability of hydrodynamic forces to induce VWF aggregation in suspension. This VWF self-association may be an additional feature involved in controlling cell adhesion rates in circulation.

Fluid shear- and time-dependent modulation of molecular interactions between PMNs and colon carcinomas

This study compares the effects of fluid shear on the kinetics, adhesion efficiency, stability, and molecular requirements of polymorphonuclear leukocyte (PMN) binding to two colon adenocarcinoma cell-lines, the CD54-negative/sLe(x)-bearing LS174T cells and the CD54-expressing/sLe(x)-low HCT-8 cells. The efficiency of PMN-colon carcinoma heteroaggregation decreases with increasing shear, with PMNs binding HCT-8 more efficiently than LS174T cells at low shear (50-200 s(-1)). In the low shear regime, CD11b is sufficient to mediate PMN binding to LS174T cells. In contrast, both CD11a and CD11b contribute to PMN-HCT-8 heteroaggregation, with CD54 on HCT-8 cells acting as a CD11a ligand at early time points. At high shear, only PMN-LS174T heteroaggregation occurs, which is initiated by PMN L-selectin binding to a sialylated, O-linked, protease-sensitive ligand on LS174T cells. PMN-LS174T heteroaggregation is primarily dependent on the intercellular contact duration (or shear rate), whereas PMN-HCT-8 binding is a function of both the intercellular contact duration and the applied force (or shear stress). Cumulatively, these studies suggest that fluid shear modulates the kinetics and molecular mechanisms of PMN-colon carcinoma cell aggregation.

Flow cytometric analysis of ligand-receptor interactions and molecular assemblies

Flow cytometers make homogeneous real-time measurements of ligand-receptor interactions and, simultaneously, the physiological responses of cells. Their multiparameter capabilities are also useful in resolving multicomponent assemblies or in developing multiplexed assays. Recent advances suggest that these approaches can be extended in several important ways. Sample delivery in the millisecond time domain is applicable to the analysis of complex binding kinetics and reaction mechanisms. The homogeneous discrimination of free components and particle-based assemblies can be extended into the micromolar concentration range. Measurements can be made of molecular assemblies among proteins, DNA, RNA, lipids, and carbohydrates on beads. The topography and assembly of components within cells can be evaluated with resonance energy transfer. Temperature dependence can be evaluated with Peltier temperature control. Many assembly endpoints can be assessed through new tools for high-throughput flow cytometry using plate-based assay formats and small volume samples.

Hydrodynamic shear regulates the kinetics and receptor specificity of polymorphonuclear leukocyte-colon carcinoma cell adhesive interactions

The ability of tumor cells to metastasize hematogenously is regulated by their interactions with polymorphonuclear leukocytes (PMNs). However, the mechanisms mediating PMN binding to tumor cells under physiological shear forces remain largely unknown. This study was designed to characterize the molecular interactions between PMNs and tumor cells as a function of the dynamic shear environment, using two human colon adenocarcinoma cell lines (LS174T and HCT-8) as models. PMN and colon carcinoma cell suspensions, labeled with distinct fluorophores, were sheared in a cone-and-plate rheometer in the presence of the PMN activator fMLP. The size distribution and cellular composition of formed aggregates were determined by flow cytometry. PMN binding to LS174T cells was maximal at 100 s(-1) and decreased with increasing shear. At low shear (100 s(-1)) PMN CD11b alone mediates PMN-LS174T heteroaggregation. However, L-selectin, CD11a, and CD11b are all required for PMN binding to sialyl Lewis(x)-bearing LS174T cells at high shear (800 s(-1)). In contrast, sialyl Lewis(x)-low HCT-8 cells fail to aggregate with PMNs at high shear conditions, despite extensive adhesive interactions at low shear. Taken together, our data suggest that PMN L-selectin initiates LS174T cell tethering at high shear by binding to sialylated moieties on the carcinoma cell surface, whereas the subsequent involvement of CD11a and CD11b converts these transient tethers into stable adhesion. This study demonstrates that the shear environment of the vasculature modulates the dynamics and molecular constituents mediating PMN-tumor cell adhesion.

Dynamics of neutrophil aggregation in couette flow revealed by videomicroscopy: effect of shear rate on two-body collision efficiency and doublet lifetime

During inflammation, neutrophil capture by vascular endothelial cells is dependent on L-selectin and beta(2)-integrin adhesion receptors. One of us (S.I.S.) previously demonstrated that homotypic neutrophil aggregation is analogous to this process in that it is also mediated by these receptors, thus providing a model for studying the dynamics of neutrophil adhesion. In the present work, we set out to confirm the hypothesis that cell-cell adhesion via selectins serves to increase the lifetimes of neutrophil doublets formed through shear-induced two-body collisions. In turn, this would facilitate the engagement of more stable beta(2)-integrin bonds and thus increase the two-body collision efficiency (fraction of collisions resulting in the formation of nonseparating doublets). To this end, suspensions of unstimulated neutrophils were subjected to a uniform shear field in a transparent counter-rotating cone and plate rheoscope, and the formation of doublets and growth of aggregates recorded using high-speed videomicroscopy. The dependence of neutrophil doublet lifetime and two-body collision-capture efficiency on shear rate, G, from 14 to 220 s(-1) was investigated. Bond formation during a two-body collision was indicated by doublets rotating well past the orientation predicted for break-up of doublets of inert spheres. A striking dependence of doublet lifetime on shear rate was observed. At low shear (G = 14 s(-1)), no collision capture occurred, and doublet lifetimes were no different from those of neutrophils pretreated with a blocking antibody to L-selectin, or in Ca(++)-depleted EDTA buffers. At G > or = 66 s(-1), doublet lifetimes increased, with increasing G reaching values twice those for the L-selectin-blocked controls. This correlated with capture efficiencies in excess of 20%, and, at G > or = 110 s(-1), led to the rapid formation of large aggregates, and this in the absence of exogenous chemotactic stimuli. Moreover, the aggregates almost completely broke up when the shear rate was reduced below 66 s(-1). Partial inhibition of aggregate formation was achieved by blocking beta(2)-integrin receptors with antibody. By direct observation of the shear-induced interactions between neutrophils, these data reveal that steady application of a threshold level of shear rate is sufficient to support homotypic neutrophil aggregation.

Nonlinear flow affects hydrodynamic forces and neutrophil adhesion rates in cone-plate viscometers

We present a theoretical and experimental analysis of the effects of nonlinear flow in a cone-plate viscometer. The analysis predicts that flow in the viscometer is a function of two parameters, the Reynolds number and the cone angle. Nonlinear flow occurs at high shear rates and causes spatial variations in wall shear stress, collision frequency, interparticle forces and attachment times within the viscometer. We examined the effect of these features on cellular adhesion kinetics. Based on recent data (Taylor, A. D., S. Neelamegham, J. D. Hellums, et al. 1996. Biophys. J. 71:3488-3500), we modeled neutrophil homotypic aggregation as a process that is integrin-limited at low shear and selectin-limited at high shear. Our calculations suggest that selectin and integrin on-rates lie in the order of 10(-2)-10(-4)/s. They also indicate that secondary flow causes positional variations in adhesion efficiency in the viscometer, and that the overall efficiency is dependent not only on the shear rate, but also the sample volume and the cone angle. Experiments performed with isolated neutrophils confirmed these predictions. In these experiments, enhancing secondary flow by increasing the sample volume from 100 to 1000 microl at 1500/s for a 2 degrees cone caused up to an approximately 45% drop in adhesion efficiency. Our results suggest that secondary flow may significantly influence cellular aggregation, platelet activation, and endothelial cell mechanotransduction measurements made in the viscometer over the range of conditions applied in typical biological studies.