Interactions through L-selectin between leukocytes and adherent leukocytes nucleate rolling adhesions on selectins and VCAM-1 in shear flow

An Intriguing Structural Modification in Neutrophil Migration Across Blood Vessels to Inflammatory Sites: Progress in the Core Mechanisms

The role and function of neutrophils are well known, but we still have incomplete understanding of the mechanisms by which neutrophils migrate from blood vessels to inflammatory sites. Neutrophil migration is a complex process that involves several distinct steps. To resist the blood flow and maintain their rolling, neutrophils employ tether and sling formation. They also polarize and form pseudopods and uropods, guided by hierarchical chemotactic agents that enable precise directional movement. Meanwhile, chemotactic agents secreted by neutrophils, such as CXCL1, CXCL8, LTB4, and C5a, can recruit more neutrophils and amplify their response. In the context of diapedesis neutrophils traverse the endothelial cells via two pathways: the transmigratory cup and the lateral border recycling department. These structures aid in overcoming the narrow pore size of the endothelial barrier, resulting in more efficient transmembrane migration. Interestingly, neutrophils exhibit a preference for the paracellular pathway over the transcellular pathway, likely due to the former's lower resistance. In this review, we will delve into the intricate process of neutrophil migration by focusing on critical structures that underpins this process.

Where the Action Is-Leukocyte Recruitment in Atherosclerosis

Atherosclerosis is the leading cause of death worldwide and leukocyte recruitment is a key element of this phenomenon, thus allowing immune cells to enter the arterial wall. There, in concert with accumulating lipids, the invading leukocytes trigger a plethora of inflammatory responses which promote the influx of additional leukocytes and lead to the continued growth of atherosclerotic plaques. The recruitment process follows a precise scheme of tethering, rolling, firm arrest, crawling and transmigration and involves multiple cellular and subcellular players. This review aims to provide a comprehensive up-to-date insight into the process of leukocyte recruitment relevant to atherosclerosis, each from the perspective of endothelial cells, monocytes and macrophages, neutrophils, T lymphocytes and platelets. In addition, therapeutic options targeting leukocyte recruitment into atherosclerotic lesions-or potentially arising from the growing body of insights into its precise mechanisms-are highlighted.

New Pharmacological Tools to Target Leukocyte Trafficking in Lung Disease

Infection and inflammation of the lung results in the recruitment of non-resident immune cells, including neutrophils, eosinophils and monocytes. This swift response should ensure clearance of the threat and resolution of stimuli which drive inflammation. However, once the threat is subdued this influx of immune cells should be followed by clearance of recruited cells through apoptosis and subsequent efferocytosis, expectoration or retrograde migration back into the circulation. This cycle of cell recruitment, containment of threat and then clearance of immune cells and repair is held in exquisite balance to limit host damage. Advanced age is often associated with detrimental changes to the balance described above. Cellular functions are altered including a reduced ability to traffic accurately towards inflammation, a reduced ability to clear pathogens and sustained inflammation. These changes, seen with age, are heightened in lung disease, and most chronic and acute lung diseases are associated with an exaggerated influx of immune cells, such as neutrophils, to the airways as well as considerable inflammation. Indeed, across many lung diseases, pathogenesis and progression has been associated with the sustained presence of trafficking cells, with examples including chronic diseases such as Chronic Obstructive Pulmonary Disease and Idiopathic Pulmonary Fibrosis and acute infections such as Pneumonia and Pneumonitis. In these instances, there is evidence that dysfunctional and sustained recruitment of cells to the airways not only increases host damage but impairs the hosts ability to effectively respond to microbial invasion. Targeting leukocyte migration in these instances, to normalise cellular responses, has therapeutic promise. In this review we discuss the current evidence to support the trafficking cell as an immunotherapeutic target in lung disease, and which potential mechanisms or pathways have shown promise in early drug trials, with a focus on the neutrophil, as the quintessential trafficking immune cell.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Overexpression of VLA-4 in glial-restricted precursors enhances their endothelial docking and induces diapedesis in a mouse stroke model

The loss of oligodendrocytes after stroke is one of the major causes of secondary injury. Glial-restricted progenitors (GRPs) have remylenating potential after intraparenchymal cerebral transplantation. The intraarterial (IA) injection route is an attractive gateway for global brain delivery, but, after IA infusion, naive GRPs fail to bind to the cerebral vasculature. The aim of this study was to test whether overexpression of Very Late Antigen-4 (VLA-4) increases endothelial docking and cerebral homing of GRPs in a stroke model. Mouse GRPs were co-transfected with DNA plasmids encoding VLA-4 subunits (α4, β1). The adhesion capacity and migration were assessed using a microfluidic assay. In vivo imaging of the docking and homing of IA-infused cells was performed using two-photon microscopy in a mouse middle cerebral artery occlusion (MCAO) model. Compared to naïve GRPs, transfection of GRPs with VLA-4 resulted in >60% higher adhesion (p < 0.05) to both purified Vascular Cell Adhesion Molecule-11 (VCAM-11) and TNFα-induced endothelial VCAM-1. VLA-4⁺GRPs displayed a higher migration in response to a chemoattractant gradient. Following IA infusion, VLA-4⁺GRPs adhered to the vasculature at three-fold greater numbers than naïve GRPs. Multi-photon imaging confirmed that VLA-4 overexpression increases the efficiency of GRP docking and leads to diapedesis after IA transplantation. This strategy may be further exploited to increase the efficacy of cellular therapeutics.

P-, but not E- or L-, selectin-mediated rolling adhesion persistence in hemodynamic flow diverges between metastatic and leukocytic cells

The ability of leukocytic cells to engage selectins via rolling adhesion is critical to inflammation, but selectins are also implicated in mediating metastatic dissemination. Using a microfluidic- and flow-based cell adhesion chromatography experimental and analytical technique, we interrogated the cell-subtype differences in engagement and sustainment of rolling adhesion on P-, E-, and L-selectin-functionalized surfaces in physiological flow. Our results indicate that, particularly at low concentrations of P-selectin, metastatic but not leukocytic cells exhibit reduced rolling adhesion persistence, whereas both cell subtypes exhibited reduced persistence on L-selectin and high persistence on E-selectin, differences not revealed by flow cytometry analysis or reflected in the extent or velocity of rolling adhesion. Conditions under which adhesion persistence was found to be significantly reduced corresponded to those exhibiting the greatest sensitivity to a selectin-antagonist. Our results suggest that potentially therapeutically exploitable differences in metastatic and leukocytic cell subtype interactions with selectins in physiological flow are identifiable through implementation of functional assays of adhesion persistence in hemodynamic flow utilizing this integrated, flow-based cell adhesion chromatography analytical technique.

Angiopoietin-1 enhances neutrophil chemotaxis in vitro and migration in vivo through interaction with CD18 and release of CCL4

Angiopoietins are a family of growth factors that are ligands for the tyrosine kinase receptor, Tie2. Angiopoietin 1 (Ang-1) is agonistic for Tie2, plays a key role in blood vessel maturation and stability and has been shown to possess anti-inflammatory properties. However, Tie2 expression has been demonstrated on human neutrophils and the observation that neutrophils migrate in response to Ang-1 in vitro has confounded research into its exact role in inflammation as well as its potential use as a therapeutic agent. We used a mouse model of peritoneal neutrophilic inflammation to determine if Ang-1 could stimulate neutrophil migration in vivo. Tie2 expression was demonstrated on mouse neutrophils. In addition, recombinant human Ang-1 induced significant chemotaxis of isolated mouse neutrophils in a Tie2- and CD18-dependent manner. Subsequently, co-immunoprecipitation of Ang-1 and CD18 demonstrated their interaction. Intraperitoneal injection of an engineered angiopoietin-1, MAT.Ang-1, induced significant neutrophil migration into the peritoneum and a significant increase in the levels of CCL4 in peritoneal lavage fluid. Depletion of resident peritoneal macrophages prior to, or concomitant injections of an anti-CCL4 antibody with MAT.Ang-1 resulted in a significant reduction in neutrophil recruitment. These data indicate a pro-inflammatory role for Ang-1 with respect to neutrophil recruitment.

Dynamic biochemical tissue analysis detects functional L-selectin ligands on colon cancer tissues

A growing body of evidence suggests that L-selectin ligands presented on circulating tumor cells facilitate metastasis by binding L-selectin presented on leukocytes. Commonly used methods for detecting L-selectin ligands on tissues, e.g., immunostaining, are performed under static, no-flow conditions. However, such analysis does not assay for functional L-selectin ligands, specifically those ligands that promote adhesion under shear flow conditions. Recently our lab developed a method, termed dynamic biochemical tissue analysis (DBTA), to detect functional selectin ligands in situ by probing tissues with L-selectin-coated microspheres under hemodynamic flow conditions. In this investigation, DBTA was used to probe human colon tissues for L-selectin ligand activity. The detection of L-selectin ligands using DBTA was highly specific. Furthermore, DBTA reproducibly detected functional L-selectin ligands on diseased, e.g., cancerous or inflamed, tissues but not on noncancerous tissues. In addition, DBTA revealed a heterogeneous distribution of functional L-selectin ligands on colon cancer tissues. Most notably, detection of L-selectin ligands by immunostaining using HECA-452 antibody only partially correlated with functional L-selectin ligands detected by DBTA. In summation, the results of this study demonstrate that DBTA detects functional selectin ligands to provide a unique characterization of pathological tissue.

T cell immunoengineering with advanced biomaterials

Recent advances in biomaterials design offer the potential to actively control immune cell activation and behaviour. Many human diseases, such as infections, cancer, and autoimmune disorders, are partly mediated by inappropriate or insufficient activation of the immune system. T cells play a central role in the host immune response to these diseases, and so constitute a promising cell type for manipulation. In vivo, T cells are stimulated by antigen presenting cells (APC), therefore to design immunoengineering biomaterials that control T cell behaviour, artificial interfaces that mimic the natural APC-T cell interaction are required. This review draws together research in the design and fabrication of such biomaterial interfaces, and highlights efforts to elucidate key parameters in T cell activation, such as substrate mechanical properties and spatial organization of receptors, illustrating how they can be manipulated by bioengineering approaches to alter T cell function.

Leukocytes Crossing the Endothelium: A Matter of Communication

Leukocytes cross the endothelial vessel wall in a process called transendothelial migration (TEM). The purpose of leukocyte TEM is to clear the causing agents of inflammation in underlying tissues, for example, bacteria and viruses. During TEM, endothelial cells initiate signals that attract and guide leukocytes to sites of tissue damage. Leukocytes react by attaching to these sites and signal their readiness to move back to endothelial cells. Endothelial cells in turn respond by facilitating the passage of leukocytes while retaining overall integrity. In this review, we present recent findings in the field and we have endeavored to synthesize a coherent picture of the intricate interplay between endothelial cells and leukocytes during TEM.

Regulation of L-selectin-dependent hydrodynamic shear thresholding by leukocyte deformability and shear dependent bond number

BACKGROUND

During inflammation leukocyte attachment to the blood vessel wall is augmented by capture of near-wall flowing leukocytes by previously adherent leukocytes. Adhesive interactions between flowing and adherent leukocytes are mediated by L-selectin and P-selectin Glycoprotein Ligand-1 (PSGL-1) co-expressed on the leukocyte surface and ultimately regulated by hydrodynamic shear thresholding.

OBJECTIVE

We hypothesized that leukocyte deformability is a significant contributory factor in shear thresholding and secondary capture.

METHODS

Cytochalasin D (CD) was used to increase neutrophil deformability and fixation was used to reduce deformability. Neutrophil rolling on PSGL-1 coated planar surfaces and collisions with PSGL-1 coated microbeads were analyzed using high-speed videomicroscopy (250 fps).

RESULTS

Increased deformability led to an increase in neutrophil rolling flux on PSGL-1 surfaces while fixation led to a decrease in rolling flux. Abrupt drops in flow below the shear threshold resulted in extended release times from the substrate for CD-treated neutrophils, suggesting increased bond number. In a cell-microbead collision assay lower flow rates were correlated with briefer adhesion lifetimes and smaller adhesive contact patches.

CONCLUSIONS

Leukocyte deformation may control selectin bond number at the flow rates associated with hydrodynamic shear thresholding. Model analysis supported a requirement for both L-selectin catch-slip bond properties and multiple bond formation for shear thresholding.

Modulation of radiation injury response in retinal endothelial cells by quinic acid derivative KZ-41 involves p38 MAPK

Radiation-induced damage to the retina triggers leukostasis, retinal endothelial cell (REC) death, and subsequent hypoxia. Resultant ischemia leads to visual loss and compensatory retinal neovascularization (RNV). Using human RECs, we demonstrated that radiation induced leukocyte adhesion through mechanisms involving p38MAPK, p53, and ICAM-1 activation. Additional phenotypic changes included p38MAPK-dependent tyrosine phosphorylation of the focal adhesion scaffolding protein, paxillin (Tyr118). The quinic acid derivative KZ-41 lessened leukocyte adhesion and paxillin-dependent proliferation via inhibition of p38MAPK-p53-ICAM-1 signaling. Using the murine oxygen-induced retinopathy (OIR) model, we examined the effect of KZ-41 on pathologic RNV. Daily ocular application of a KZ-41-loaded nanoemulsion significantly reduced both the avascular and neovascular areas in harvested retinal flat mounts when compared to the contralateral eye receiving vehicle alone. Our data highlight the potential benefit of KZ-41 in reducing both the retinal ischemia and neovascularization provoked by genotoxic insults. Further research into how quinic acid derivatives target and mitigate inflammation is needed to fully appreciate their therapeutic potential for the treatment of inflammatory retinal vasculopathies.

Podocytes regulate neutrophil recruitment by glomerular endothelial cells via IL-6-mediated crosstalk

Stromal cells actively modulate the inflammatory process, in part by influencing the ability of neighboring endothelial cells to support the recruitment of circulating leukocytes. We hypothesized that podocytes influence the ability of glomerular endothelial cells (GEnCs) to recruit neutrophils during inflammation. To address this, human podocytes and human GEnCs were cultured on opposite sides of porous inserts and then treated with or without increasing concentrations of TNF-α prior to addition of neutrophils. The presence of podocytes significantly reduced neutrophil recruitment to GEnCs by up to 50% when cultures were treated with high-dose TNF-α (100 U/ml), when compared with GEnC monocultures. Importantly, this phenomenon was dependent on paracrine actions of soluble IL-6, predominantly released by podocytes. A similar response was absent when HUVECs were cocultured with podocytes, indicating a tissue-specific phenomenon. Suppressor of cytokine signaling 3 elicited the immunosuppressive actions of IL-6 in a process that disrupted the presentation of chemokines on GEnCs by altering the expression of the duffy Ag receptor for chemokines. Interestingly, suppressor of cytokine signaling 3 knockdown in GEnCs upregulated duffy Ag receptor for chemokines and CXCL5 expression, thereby restoring the neutrophil recruitment. In summary, these studies reveal that podocytes can negatively regulate neutrophil recruitment to inflamed GEnCs by modulating IL-6 signaling, identifying a potential novel anti-inflammatory role of IL-6 in renal glomeruli.

Inactivation of heparan sulfate 2-O-sulfotransferase accentuates neutrophil infiltration during acute inflammation in mice

Neutrophil recruitment and extravasation at sites of inflammation provide a mechanism for host defense. We showed previously that heparan sulfate, a type of sulfated glycosaminoglycan, facilitates neutrophil recruitment based on the reduction of neutrophil infiltration in mice in which the overall sulfation of the chains was reduced by selective inactivation of N-acetylglucosamine N-deacetylase-N-sulfotransferase (Ndst1) in endothelial cells. Here we show that inactivation of uronyl 2-O-sulfotransferase in endothelial cells (Hs2st), an enzyme that acts downstream from Ndst1, results in enhanced neutrophil recruitment in several models of acute inflammation. Enhanced neutrophil infiltration resulted in part from reduced rolling velocity under flow both in vivo and in vitro, which correlated with stronger binding of neutrophil L-selectin to mutant endothelial cells. Hs2st-deficient endothelial cells also displayed a striking increase in binding of IL-8 and macrophage inflammatory protein-2. The enhanced binding of these mediators of neutrophil recruitment resulted from a change in heparan sulfate structure caused by increased N-sulfation and 6-O-sulfation of glucosamine units in response to the decrease in 2-O-sulfation of uronic acid residues. This gain-of-function phenotype provides formidable evidence demonstrating the importance of endothelial heparan sulfate in inflammation and suggests a novel enzyme target for enhancing the innate immune response.

Force-dependent bond dissociation govern rolling of HL-60 cells through E-selectin

E-selectin-mediated rolling on vascular surface of circulating leukocyte on vascular surface is a key initial event during inflammatory response and lymphocyte homing. This event depends not only on the specific interactions of adhesive molecules but also on the hemodynamics of blood flow. Little is still understood about whether wall shear stress or shear rate regulates the rolling. With flow chamber techniques, we here measured the effects of transport, shear stress and cell deformation on rolling of both unfixed and fixed HL-60 cells on E-selectin either in the absence or in the presence of 3% Ficoll in medium at various wall shear stresses from 0.05 to 0.7 dyn/cm(2). The results demonstrated a triphasic force-dependent rolling, that is, as increasing of force, the rolling would be accelerated firstly, then followed a decelerating phase occurred at the initial shear threshold of about 0.1 dyn/cm(2), and lastly returned to an accelerating process starting at the optimal shear threshold of 0.35 dyn/cm(2) approximately. The catch bond regime was completely reflected to rolling behaviors, such as tether lifetime, cell stop time and rolling velocity, meaning that the dominant factor to govern rolling is force. The initial shear threshold might be the minimum level of wall shear stress to sustain a stationary rolling, and the optimal shear threshold would make rolling to the most stable and regular. These findings strongly elucidate the catch bond mechanism for flow-enhanced rolling through E-selectin since longer bond lifetimes led to slower and stabler rolling.

Intra-ophthalmic artery chemotherapy triggers vascular toxicity through endothelial cell inflammation and leukostasis

Purpose. Super-selective intra-ophthalmic artery chemotherapy (SSIOAC) is an eye-targeted drug-delivery strategy to treat retinoblastoma, the most prevalent primary ocular malignancy in children. Unfortunately, recent clinical reports associate adverse vascular toxicities with SSIOAC using melphalan, the most commonly used chemotherapeutic. Methods. To explore reasons for the unexpected vascular toxicities, we examined the effects of melphalan, as well as carboplatin (another chemotherapeutic used with retinoblastoma), in vitro using primary human retinal endothelial cells, and in vivo using a non-human primate model, which allowed us to monitor the retina in real time during SSIOAC. Results. Both melphalan and carboplatin triggered human retinal endothelial cell migration, proliferation, apoptosis, and increased expression of adhesion proteins intracellullar adhesion molecule-1 [ICAM-1] and soluble chemotactic factors (IL-8). Melphalan increased monocytic adhesion to human retinal endothelial cells. Consistent with these in vitro findings, histopathology showed vessel wall endothelial cell changes, leukostasis, and vessel occlusion. Conclusions. These results reflect a direct interaction of chemotherapeutic drugs with both the vascular endothelium and monocytes. The vascular toxicity may be related to the pH, the pulsatile delivery, or the chemotherapeutic drugs used. Our long-term goal is to determine if changes in the drug of choice and/or delivery procedures will decrease vascular toxicity and lead to better eye-targeted treatment strategies.

Leukocyte ligands for endothelial selectins: specialized glycoconjugates that mediate rolling and signaling under flow

Reversible interactions of glycoconjugates on leukocytes with P- and E-selectin on endothelial cells mediate tethering and rolling of leukocytes in inflamed vascular beds, the first step in their recruitment to sites of injury. Although selectin ligands on hematopoietic precursors have been identified, here we review evidence that PSGL-1, CD44, and ESL-1 on mature leukocytes are physiologic glycoprotein ligands for endothelial selectins. Each ligand has specialized adhesive functions during tethering and rolling. Furthermore, PSGL-1 and CD44 induce signals that activate the β2 integrin LFA-1 and promote slow rolling, whereas ESL-1 induces signals that activate the β2 integrin Mac-1 in adherent neutrophils. We also review evidence for glycolipids, CD43, L-selectin, and other glycoconjugates as potential physiologic ligands for endothelial selectins on neutrophils or lymphocytes. Although the physiologic characterization of these ligands has been obtained in mice, we also note reported similarities and differences with human selectin ligands.

Biomechanics of leukocyte rolling

Leukocyte rolling on endothelial cells and other P-selectin substrates is mediated by P-selectin binding to P-selectin glycoprotein ligand-1 expressed on the tips of leukocyte microvilli. Leukocyte rolling is a result of rapid, yet balanced formation and dissociation of selectin-ligand bonds in the presence of hydrodynamic shear forces. The hydrodynamic forces acting on the bonds may either increase (catch bonds) or decrease (slip bonds) their lifetimes. The force-dependent 'catch-slip' bond kinetics are explained using the 'two pathway model' for bond dissociation. Both the 'sliding-rebinding' and the 'allosteric' mechanisms attribute 'catch-slip' bond behavior to the force-induced conformational changes in the lectin-EGF domain hinge of selectins. Below a threshold shear stress, selectins cannot mediate rolling. This 'shear-threshold' phenomenon is a consequence of shear-enhanced tethering and catch bond-enhanced rolling. Quantitative dynamic footprinting microscopy has revealed that leukocytes rolling at venular shear stresses (>0.6 Pa) undergo cellular deformation (large footprint) and form long tethers. The hydrodynamic shear force and torque acting on the rolling cell are thought to be synergistically balanced by the forces acting on tethers and stressed microvilli, however, their relative contribution remains to be determined. Thus, improvement beyond the current understanding requires in silico models that can predict both cellular and microvillus deformation and experiments that allow measurement of forces acting on individual microvilli and tethers.

Probing dynamic cell-substrate interactions using photochemically generated surface-immobilized gradients: application to selectin-mediated leukocyte rolling

Model substrates presenting biochemical cues immobilized in a controlled and well-defined manner are of great interest for their applications in biointerface studies that elucidate the molecular basis of cell receptor-ligand interactions. Herein, we describe a direct, photochemical method to generate surface-immobilized biomolecular gradients that are applied to the study of selectin-mediated leukocyte rolling. The technique employs benzophenone-modified glass substrates, which upon controlled exposure to UV light (350-365 nm) in the presence of protein-containing solutions facilitate the generation of covalently immobilized protein gradients. Conditions were optimized to generate gradient substrates presenting P-selectin and PSGL-1 (P-selectin glycoprotein ligand-1) immobilized at site densities over a 5- to 10-fold range (from as low as ∼200 molecules μm(-2) to as high as 6000 molecules μm(-2)). The resulting substrates were quantitatively characterized via fluorescence analysis and radioimmunoassays before their use in the leukocyte rolling assays. HL-60 promyelocytes and Jurkat T lymphocytes were assessed for their ability to tether to and roll on substrates presenting immobilized P-selectin and PSGL-1 under conditions of physiologically relevant shear stress. The results of these flow assays reveal the combined effect of immobilized protein site density and applied wall shear stress on cell rolling behavior. Two-component substrates presenting P-selectin and ICAM-1 (intercellular adhesion molecule-1) were also generated to assess the interplay between these two proteins and their effect on cell rolling and adhesion. These proof-of-principle studies verify that the described gradient generation approach yields well-defined gradient substrates that present immobilized proteins over a large range of site densities that are applicable for investigation of cell-materials interactions, including multi-parameter leukocyte flow studies. Future applications of this enabling methodology may lead to new insights into the biophysical phenomena and molecular mechanism underlying complex biological processes such as leukocyte recruitment and the inflammatory response.

Biophysics of selectin-ligand interactions in inflammation and cancer

Selectins (L-, E- and P-selectin) are calcium-dependent transmembrane glycoproteins that are expressed on the surface of circulating leukocytes, activated platelets, and inflamed endothelial cells. Selectins bind predominantly to sialofucosylated glycoproteins and glycolipids (E-selectin only) present on the surface of apposing cells, and mediate transient adhesive interactions pertinent to inflammation and cancer metastasis. The rapid turnover of selectin-ligand bonds, due to their fast on- and off-rates along with their remarkably high tensile strengths, enables them to mediate cell tethering and rolling in shear flow. This paper presents the current body of knowledge regarding the role of selectins in inflammation and cancer metastasis, and discusses experimental methodologies and mathematical models used to resolve the biophysics of selectin-mediated cell adhesion. Understanding the biochemistry and biomechanics of selectin-ligand interactions pertinent to inflammatory disorders and cancer metastasis may provide insights for developing promising therapies and/or diagnostic tools to combat these disorders.

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.

Mucin AgC10 from Trypanosoma cruzi Interferes with L-selectin-mediated monocyte adhesion

The protozoan parasite Trypanosoma cruzi has evolved sophisticated systems to evade the immune response. An important requirement for a productive immune response is recruitment of the appropriate immune cells from the bloodstream to the sites of infection. Here, we show that a mucin expressed and secreted by the metacyclic infective form of T. cruzi, AgC10, is able to interfere with L-selectin-mediated monocyte adhesion. Thus, incubation of U937 monocytic cells stably expressing L-selectin (U937LAM) with AgC10 strongly reduced their adhesion on P-selectin under flow, which is dependent on L-selectin. This treatment also results in a significant inhibition by AgC10 of U937LAM and human primary monocyte adhesion to activated vascular endothelium. This effect was specific for L-selectin, because vascular cell adhesion molecule 1 (VCAM-1)-mediated adhesion was not affected by AgC10 pretreatment. This effect of AgC10 is likely due to its ability to induce L-selectin shedding from the monocyte membrane, since pharmacologic blocking of this shedding prevents AgC10 activity. This is the first description of a mechanism that prevents leukocyte adhesion to the endothelium by a parasite and represents a new potential countermeasure to evade the generation of a correct immune response.

Disruption of tissue-specific fucosyltransferase VII, an enzyme necessary for selectin ligand synthesis, suppresses atherosclerosis in mice

A hallmark feature of atherosclerosis is that circulating mononuclear cells adhere to the endothelium and migrate into the subendothelial space. This adhesion is mediated by molecules such as selectins that are expressed on the surfaces of both leukocytes and endothelial cells. In this study, we have determined the role of tissue-specific fucosyltransferase VII (FucT-VII), an enzyme necessary for selectin ligand synthesis, in the development of atherosclerosis. We adopted a scheme of transplanting either FucT-VII(-/-)GFP(+) bone marrow into lethally irradiated low-density lipoprotein receptor low density lipoprotein receptor mice or FucT-VII(+/+) GFP(+) bone marrow into FucT-VII(-/-), low density lipoprotein receptor double-mutant mice to evaluate the roles of E- and P-selectin ligands versus L-selectin ligands, respectively, in diet-induced atherosclerosis. GFP was used to track the transplanted cells. Our results indicate that, compared with controls, selective disruption of E- and P-selectin ligand synthesis resulted in a significant reduction in atherosclerosis. Selective disruption of L-selectin ligand production did not reduce atherosclerosis as robustly as disruption of E- and P-selectin ligands. In both groups, however, there was a significant reduction in the accumulation of macrophages in the lesion. These studies indicate that selectin ligands, particularly those for E- and P-selectins, play an important role in the pathogenesis of atherosclerosis by regulating macrophage accumulation in atherosclerotic lesions.

Leukocyte adhesion molecules in animal models of inflammatory bowel disease

The dysregulated recruitment of leukocytes into the intestine is required for the initiation and maintenance of inflammatory bowel disease (IBD). Several families of molecules regulate the influx of these cells into sites of inflammation. Interference with some of these molecules has already shown efficacy in the clinics and antibodies that target the molecules involved have been approved by the FDA for use in Crohn's disease (CD), multiple sclerosis (i.e., natalizumab), and psoriasis (i.e., efalizumab). Here, we discuss basic aspects of the different families of relevant molecules and compile a large body of preclinical studies that supported the targeting of specific steps of the leukocyte adhesion cascade for therapeutic purposes in colitis and in novel models of CD-like ileitis.

An L-selectin ligand distinct from P-selectin glycoprotein ligand-1 is expressed on endothelial cells and promotes neutrophil rolling in inflammation

Neutrophils recruited from the blood are key players in the innate immune response. Selectins play critical roles in neutrophil recruitment by mediating their tethering and rolling in inflamed venules. While the roles of P- and E-selectin in this process are well established, the mechanisms of L-selectin-mediated neutrophil recruitment remain elusive. One proposal is that tethering is mediated by L-selectin on flowing neutrophils interacting with P-selectin glycoprotein ligand-1 (PSGL-1) on adherent neutrophils. To clarify whether L-selectin-mediated neutrophil recruitment depends entirely on PSGL-1, we examined the impact of L-selectin deficiency in mice with a PSGL-1-deficient background. L-selectin and PSGL-1 double-knockout mice exhibited a higher increase in their peripheral blood neutrophil count and a worse defect in neutrophil recruitment into the inflamed peritoneum than PSGL-1-deficient mice. Intravital microscopy of inflamed cremaster muscle venules showed that L-selectindeficiency or antibody blockade of L-selectin reduced the residual leukocyte rolling in PSGL-1-deficient mice. Flow cytometric analyses showed that the endothelial cells from the cremaster muscle bound L-selectin in a PSGL-1-independent manner. These results provide evidence for the existence of an L-selectin ligand distinct from PSGL-1 in inflammation and indicate that such a ligand is expressed on endothelial cells, promoting neutrophil rolling in vivo.

Endothelial CD47 interaction with SIRPgamma is required for human T-cell transendothelial migration under shear flow conditions in vitro

Leukocyte transendothelial migration (TEM) is a critical event during inflammation. CD47 has been implicated in myeloid cell migration across endothelium and epithelium. CD47 binds to signal regulatory protein (SIRP), SIRPalpha and SIRPgamma. So far, little is known about the role of endothelial CD47 in T-cell TEM in vivo or under flow conditions in vitro. Fluorescence-activated cell sorting and biochemical analysis show that CD3(+) T cells express SIRPgamma but not SIRPalpha, and fluorescence microscopy showed that CD47 was enriched at endothelial junctions. These expression patterns suggested that CD47 plays a role in T-cell TEM through binding interactions with SIRPgamma. We tested, therefore, whether CD47-SIRPgamma interactions affect T-cell transmigration using blocking mAb against CD47 or SIRPgamma in an in vitro flow model. These antibodies inhibited T-cell TEM by 70% plus or minus 6% and 82% plus or minus 1%, respectively, but had no effect on adhesion. In agreement with human mAb studies, transmigration of murine wild-type T helper type 1 cells across TNF-alpha-activated murine CD47(-/-) endothelium was reduced by 75% plus or minus 2% even though murine T cells appear to lack SIRPgamma. Nonetheless, these findings suggest endothelial cell CD47 interacting with T-cell ligands, such as SIRPgamma, play an important role in T-cell transendothelial migration.

P-selectin glycoprotein ligand-1 is highly expressed on Ly-6Chi monocytes and a major determinant for Ly-6Chi monocyte recruitment to sites of atherosclerosis in mice

BACKGROUND

Ly-6C(hi) monocytes are key contributors to atherosclerosis in mice. However, the manner in which Ly-6C(hi) monocytes selectively accumulate in atherosclerotic lesions is largely unknown. Monocyte homing to sites of atherosclerosis is primarily initiated by rolling on P- and E-selectin expressed on endothelium. We hypothesize that P-selectin glycoprotein ligand-1 (PSGL-1), the common ligand of P- and E-selectin on leukocytes, contributes to the preferential homing of Ly-6C(hi) monocytes to atherosclerotic lesions.

METHODS AND RESULTS

To test this hypothesis, we examined the expression and function of PSGL-1 on Ly-6C(hi) and Ly-6C(lo) monocytes from wild-type mice, ApoE(-/-) mice, and mice lacking both ApoE and PSGL-1 genes (ApoE(-/-)/PSGL-1(-/-)). We found that Ly-6C(hi) monocytes expressed a higher level of PSGL-1 and had enhanced binding to fluid-phase P- and E-selectin compared with Ly-6C(lo) monocytes. Under in vitro flow conditions, more Ly-6C(hi) monocytes rolled on P-, E-, and L-selectin at slower velocities than Ly-6C(lo) cells. In an ex vivo perfused carotid artery model, Ly-6C(hi) monocytes interacted preferentially with atherosclerotic endothelium compared with Ly-6C(lo) monocytes in a PSGL-1-dependent manner. In vivo, ApoE(-/-) mice lacking PSGL-1 had impaired Ly-6C(hi) monocyte recruitment to atherosclerotic lesions. Moreover, ApoE(-/-)/PSGL-1(-/-) mice exhibited significantly reduced monocyte infiltration in wire injury-induced neointima and in atherosclerotic lesions. ApoE(-/-)/PSGL-1(-/-) mice also developed smaller neointima and atherosclerotic plaques.

CONCLUSIONS

These data indicate that PSGL-1 is a new marker for Ly-6C(hi) monocytes and a major determinant for Ly-6C(hi) cell recruitment to sites of atherosclerosis in mice.

No detectable endothelial- or leukocyte-derived L-selectin ligand activity on the endothelium in inflamed cremaster muscle venules

L-selectin is important in mediating leukocyte recruitment in inflammation. The role of L-selectin was for long believed to be influenced by an inducible endothelial ligand; however, L-selectin ligand activity was recently shown to be mediated by leukocytic P-selectin glycoprotein ligand 1 (PSGL-1). Still, it is unknown whether PSGL-1 is deposited on the endothelium or whether leukocyte fragments or leukocytic uropods are presented on the venular surface. Moreover, it is unclear whether ligands for L-selectin other than PSGL-1 are present in inflammation. Overall, this has complicated understanding of the mechanisms that guide recruitment of inflammatory cells. Here, I used intravital microscopy on mouse cremaster muscle venules to show that L-selectin influences leukocyte rolling in inflammation exclusively by mediating L-selectin/PSGL-1-dependent, secondary capture to rolling and adherent leukocytes. I show that leukocyte primary capture in inflammation is mediated almost entirely by P-selectin, whereas the capacity of E-selectin to mediate capture appears to be minimal. In parallel, primary capture remaining after function inhibition of P-selectin is not decreased by blockage or absence of L-selectin. Rolling along the endothelium in venules following a number of inflammatory treatments was abolished by simultaneous blockage of P-selectin, E-selectin, and VCAM-1, indicating that there is no additional adhesive pathway involving L-selectin or any other molecule that can mediate leukocyte rolling in inflamed cremaster muscle venules in response to the used stimuli. Moreover, in vivo staining failed to detect any L-selectin ligand activity on the endothelium. These data demonstrate that expression of L-selectin on leukocytes is insufficient for mediating rolling and efficient recruitment of leukocytes in inflammation.

L-selectin shear thresholding modulates leukocyte secondary capture

Transient homotypic adhesions between flowing leukocytes and those previously adherent on the vessel wall has been proposed to amplify the accumulation of leukocytes at sites of inflammation. While adhesion of leukocytes to the vessel wall (primary capture) is mediated primarily by P-selectin on the endothelium and P-selectin Glycoprotein Ligand-1 (PSGL-1) on the leukocyte, the homotypic interactions leading to downstream leukocyte adhesion (secondary capture) are mediated primarily by reciprocal interactions between PSGL-1 and L-selectin on apposing leukocytes. One consequence of leukocyte secondary capture events are the formation of strings of adherent leukocytes as each recently captured leukocyte in turn captures another one flowing over its surface. Interestingly, PSGL-1-L-selectin interactions also mediate leukocyte hydrodynamic shear thresholding, whereby leukocyte rolling on purified L-selectin ligands such as PSGL-1 is maximized at a wall shear stress of approximately 1 dyne/cm(2) and minimized at both higher and lower flow rates. Using a novel quantitative method, we analyzed leukocyte string formation in vitro and found that hydrodynamic shear thresholding precluded secondary capture at low shear stresses yet amplified it at high shear stresses. Addition of the L-selectin mAb DREG-56 strongly inhibited leukocyte string formation, suggesting adhesion contributed significantly to hydrodynamic interactions in secondary capture processes. Taken together, the data suggest that secondary capture is modulated by the shear thresholding property of L-selectin. L-selectin mediated shear thresholding may therefore play a significant role in the regulation of leukocyte secondary capture in addition to recently described hydrodynamic recruitment mechanisms.

Enhancement of L-selectin, but not P-selectin, bond formation frequency by convective flow

L-selectin-mediated leukocyte rolling has been proposed to require a high rate of bond formation compared to that of P-selectin to compensate for its much higher off-rate. To test this hypothesis, a microbead system was utilized to measure relative L-selectin and P-selectin bond formation rates on their common ligand P-selectin glycoprotein ligand-1 (PSGL-1) under shear flow. Using video microscopy, we tracked selectin-coated microbeads to detect the formation frequency of adhesive tether bonds. From velocity distributions of noninteracting and interacting microbeads, we observed that tether bond formation rates for P-selectin on PSGL-1 decreased with increasing wall shear stress, from 0.14 +/- 0.04 bonds/microm at 0.2 dyn/cm(2) to 0.014 +/- 0.003 bonds/microm at 1.0 dyn/cm(2). In contrast, L-selectin tether bond formation increased from 0.017 +/- 0.005 bonds/microm at 0.2 dyn/cm(2) to 0.031 +/- 0.005 bonds/microm at 1.0 dyn/cm(2). L-selectin tether bond formation rates appeared to be enhanced by convective transport, whereas P-selectin rates were inhibited. The transition force for the L-selectin catch-slip transition of 44 pN/bond agreed well with theoretical models (Pereverzev et al. 2005. Biophys. J. 89:1446-1454). Despite catch bond behavior, hydrodymanic shear thresholding was not detected with L-selectin beads rolling on PSGL-1. We speculate that shear flow generated compressive forces may enhance L-selectin bond formation relative to that of P-selectin and that L-selectin bonds with PSGL-1 may be tuned for the compressive forces characteristic of leukocyte-leukocyte collisions during secondary capture on the blood vessel wall. This is the first report, to our knowledge, comparing L-selectin and P-selectin bond formation frequencies in shear flow.

Endothelial-dependent mechanisms of leukocyte recruitment to the vascular wall

Inflammation is a fundamental process that protects organisms by removing or neutralizing injurious agents. A key event in the inflammatory response is the localized recruitment of various leukocyte subsets. Here we address the cellular and regulatory mechanisms of leukocyte recruitment to the vessel wall in cardiovascular disease and discuss our evolving understanding of the role of the vascular endothelium in this process. The vascular endothelium is the continuous single-cell lining of the cardiovascular system that forms a critical interface between the blood and its components on one side and the tissues and organs on the other. It is heterogeneous and has many synthetic and metabolic functions including secretion of platelet-derived growth factor, von Willebrand factor, prostacyclin, NO, endothelin-1, and chemokines and the expression of adhesion molecules. It also acts as a nonthrombogenic and selective permeable barrier. Endothelial cells also interact closely with the extracellular matrix and with adjacent cells including pericytes and smooth muscle cells within the vessel wall. A central question in vascular biology is the role of the endothelium in the initiation of inflammatory response, the extent of its "molecular conversations" with recruited leukocytes, and its influence on the extent and/or outcome of this response.

Micropatterned surfaces for controlling cell adhesion and rolling under flow

Cell adhesion and rolling on the vascular wall is critical to both inflammation and thrombosis. In this study we demonstrate the feasibility of using microfluidic patterning for controlling cell adhesion and rolling under physiological flow conditions. By controlling the width of the lines (50-1000 microm) and the spacing between them (50-100 microm) we were able to fabricate surfaces with well-defined patterns of adhesion molecules. We demonstrate the versatility of this technique by patterning surfaces with 3 different adhesion molecules (P-selectin, E-selectin, and von Willebrand Factor) and controlling the adhesion and rolling of three different cell types (neutrophils, Chinese Hamster Ovary cells, and platelets). By varying the concentration of the incubating solution we could control the surface ligand density and hence the cell rolling velocity. Finally by patterning surfaces with both P-selectin and von Willebrand Factor we could control the rolling of both leukocytes and platelets simultaneously. The technique described in this paper provides and effective and inexpensive way to fabricate patterned surfaces for use in cell rolling assays under physiologic flow conditions.

Complete identification of E-selectin ligands on neutrophils reveals distinct functions of PSGL-1, ESL-1, and CD44

The selectins and their ligands are required for leukocyte extravasation during inflammation. Several glycoproteins have been suggested to bind to E-selectin in vitro, but the complete identification of its physiological ligands has remained elusive. Here, we showed that E-selectin ligand-1 (ESL-1), P-selectin glycoprotein ligand-1 (PSGL-1), and CD44 encompassed all endothelial-selectin ligand activity on neutrophils by using gene- and RNA-targeted loss of function. PSGL-1 played a major role in the initial leukocyte capture, whereas ESL-1 was critical for converting initial tethers into steady slow rolling. CD44 controlled rolling velocity and mediated E-selectin-dependent redistribution of PSGL-1 and L-selectin to a major pole on slowly rolling leukocytes through p38 signaling. These results suggest distinct and dynamic contributions of these three glycoproteins in selectin-mediated neutrophil adhesion and signaling.

Modulating leukocyte recruitment in inflammation

Much information has been obtained regarding how white cells are recruited in the microcirculation to sites of inflammation. In this review we summarize the leukocyte recruitment cascade, highlighting the molecular mechanisms that underlie each of the major steps. Major emphasis is placed on the selectins and integrins and their role in rolling and adhesion. Intraluminal crawling and emigration are also briefly discussed. In addition, we summarize some of the data that implicate these molecules in eosinophil recruitment in animal models of asthma and in lymphocyte recruitment in skin contact sensitivity. There is a growing body of evidence to suggest that leukocyte recruitment could be used as an effective means for future therapeutics, and some of these issues are also raised.

Shear modulation of intercellular contact area between two deformable cells colliding under flow

Shear rate has been shown to critically affect the kinetics and receptor specificity of cell-cell interactions. In this study, the collision process between two modeled cells interacting in a linear shear flow is numerically investigated. The two identical biological or artificial cells are modeled as deformable capsules composed of an elastic membrane. The cell deformation and trajectories are computed using the immersed boundary method (IBM) for shear rates of 100-400s(-1). As the two cells collide under hydrodynamic shear, large local cell deformations develop. The effective contact area between the two cells is modulated by the shear rate, and reaches a maximum value at intermediate levels of shear. At relatively low shear rate, the contact area is an enclosed region. As the shear rate increases, dimples form on the membrane surface, and the contact region becomes annular. The nonmonotonic increase of the contact area with the increase of shear rate from computational results implies that there is a maximum effective receptor-ligand binding area for cell adhesion. This finding suggests the existence of possible hydrodynamic mechanism that could be used to interpret the observed maximum leukocyte aggregation in shear flow. The critical shear rate for maximum intercellular contact area is shown to vary with cell properties such as radius and membrane elastic modulus.

Adhesive dynamics simulations of the shear threshold effect for leukocytes

Many experiments have measured the effect of force on the dissociation of single selectin bonds, but it is not yet clear how the force dependence of molecular dissociation can influence the rolling of cells expressing selectin molecules. Recent experiments using constant-force atomic force microscopy or high-resolution microscopic observations of pause-time distributions of cells in a flow chamber show that for some bonds, the dissociation rate is high at low force and initially decreases with force, indicating a catch bond. As the force continues to increase, the dissociation rate increases again, like a slip bond. It has been proposed that this catch-slip bond leads to the shear threshold effect, in which a certain level of shear rate is required to achieve rolling. We have incorporated a catch-slip dissociation rate into adhesive dynamics simulations of cell rolling. Using a relatively simple model for the shear-controlled association rate for selectin bonds, we were able to recreate characteristics of the shear threshold effect seen most prominently for rolling through L-selectin. The rolling velocity as a function of shear rate showed a minimum near 100 s-1. Furthermore, cells were observed to roll at a shear rate near the threshold, but detach and move more quickly when the shear rate was dropped below the threshold. Finally, using adhesive dynamics, we were able to determine ranges of parameters necessary to see the shear threshold effect in the rolling velocity. In summary, we found through simulation that the catch-slip behavior of selectin bonds can be responsible for the shear threshold effect.

beta2 Integrins are characteristically absent in acute promyelocytic leukemia and rapidly upregulated in vivo upon differentiation with all-trans retinoic acid

Although little is known about migration of hematopoietic stem cells and their neoplastic counterparts into tissues and peripheral blood, adhesion proteins likely play an important role. We studied 339 patients with acute myelogenous leukemia (AML) to discern the relationship between adhesion protein expression, circulating blasts, and white blood cell (WBC) count. Expression levels of CD11b and CD11c strongly correlated with increased WBC count, independent of FAB subtype (p<0.0001). However, 93% (25/27) of cases of AML-M3 completely lacked beta2 integrin expression, compared to 11% (35/312) of the non-M3 cases (p<0.0001). Seven of the 27 patients with AML-M3 were followed during standard induction therapy with ATRA. Within 3 days, weak CD11c became detectable, followed by CD11b and CD11a. Our data suggest an important link between beta2 integrin expression and the level of circulating leukemic cells in AML. We demonstrate the clinical usefulness of a panel of beta2 integrins (CD11a, CD11b and CD11c) in accurate prediction of AML-M3, and recommend inclusion of this immunophenotypic analysis to identify patients who require ATRA therapy. Finally, we illustrate the rapidity at which AML-M3 blasts up-regulate beta2 integrins, and suggest a possible association between this finding and the tissue infiltration that characterizes the "ATRA syndrome".

The effects of anti-P-selectin antibody on leucocyte activity related to cigarette smoke in rats

Nicotine-induced expression of P-selectin is implicated in endothelial cell damage related to smoking. Thirty male Sprague-Dawley rats were divided into three groups: two experimental and one control. Both experimental groups were exposed to cigarette smoke for four weeks, but one group was also given anti-P-selectin antibody (100 microg IgG per 100 g body weight) intravenously. Rolling and adhesion leucocytes within the microcirculation of the cremaster muscle were measured. The urine cotinine concentrations of rats exposed to smoke were 612 ng/ml higher than those of non-smokers. Both rolling and adherent leucocytes were highest and steady in the group given anti-P-selectin antibody at about 50 and 10, respectively. There was a significant drop in both rolling and adherent leucocytes (31 to 4 and 5 to 2) after the anti-P-selectin antibody had been given. However, this effect was short-lived as both increased above those at 35 minutes by 120 minutes (p<0.001). The increase in leucocyte rolling and adherence caused by smoking can be lowered by giving anti-P-selectin antibody. The effective period in rats was two hours, with the maximal effect one hour after injection.

Association of the Phe206Leu Allele of the L-Selectin Gene with Coronary Artery Disease

BACKGROUND AND AIMS

The aim of this study was to assess the association between the L-selectin Phe206Leu polymorphism and coronary artery disease.

METHODS

A total of 322 patients (221 men and 101 women) with coronary artery disease in one or more vessels documented by angiography were studied; 157 subjects (85 men and 72 women) without atherosclerosis were included as controls. All subjects were genotyped for the L-selectin Phe206Leu gene polymorphism using polymerase chain reaction with sequence-specific primer (PCR-SSP). To assess disease severity, all patients were classified by numbers of coronary arteries with 50% stenosis.

RESULTS

A significantly increased frequency of the 206Leu mutant allele was observed in patients with coronary artery disease compared to the controls. The 206Leu allele frequency occurred in 42% of the patients with coronary artery disease compared to 30% of the controls (p<0.009). No association was found between the severity of coronary artery disease and the L-selectin Phe206Leu polymorphism.

CONCLUSION

Our findings suggest that carriage of L-selectin 206Leu mutant allele could contribute to susceptibility of Iranian individuals to contracting coronary artery disease.

The role of ICAM-1 in Plasmodium falciparum cytoadherence

Parasite sequestration at microvascular sites is a fundamental phenomenon in the manifestation of the symptoms of malaria and the progression to severe disease. Here, we review the endothelial cell-expressed intercellular adhesion molecule-1 (ICAM-1) and its role in mediating the interaction between the parasitised red blood cell (PRBC) and the vascular endothelium. We highlight the nature of the interaction between ICAM-1 and the parasite-expressed PfEMP-1 molecule at the molecular level. The review also discusses the complexity of the PRBC-endothelial cell interaction and the mechanisms that underlie parasite cytoadherence.

Leukocyte adhesion: what's the catch?

A recent study shows that the leukocyte adhesion molecules known as selectins form 'catch' bonds, the dissociation rate of which decreases with increasing applied force. The ability of selectins to switch between catch and slip bonds, where dissociation increases with force, can explain the shear threshold effect, in which leukocyte adhesion goes through a maximum with increasing shear rate.

L- and P-selectins collaborate to support leukocyte rolling in vivo when high-affinity P-selectin-P-selectin glycoprotein ligand-1 interaction is inhibited

P-selectin glycoprotein ligand-1 (PSGL-1) binding to P-selectin controls early leukocyte rolling during inflammation. Interestingly, antibodies and pharmacological inhibitors (eg, rPSGL-Ig) that target the N-terminus of PSGL-1 reduce but do not abolish P-selectin-dependent leukocyte rolling in vivo whereas PSGL-1-deficient mice have almost no P-selectin-dependent rolling. We have investigated mechanisms of P-selectin-dependent, PSGL-1-independent rolling using intravital microscopy. Initially we used fluorescent microspheres to study the potential of L-selectin and the minimal selectin ligand sialyl Lewis(x) (sLe(x)) to interact with postcapillary venules in the absence of PSGL-1. Microspheres coated with combinations of L-selectin and sLe(x) interacted with surgically stimulated cremaster venules in a P-selectin-dependent manner. Microspheres coated with either L-selectin or sLe(x) alone showed less evidence of interaction. We also investigated leukocyte rolling in the presence of PSGL-1 antibody or inhibitor (rPSGL-Ig), both of which partially inhibited P-selectin-dependent leukocyte rolling. Residual rolling was substantially inhibited by L-selectin-blocking antibody or a previously described sLe(x) mimetic (CGP69669A). Together these data suggest that leukocytes can continue to roll in the absence of optimal P-selectin/PSGL-1 interaction using an alternative mechanism that involves P-selectin-, L-selectin-, and sLe(x)-bearing ligands.

Cytoskeletal interactions regulate inducible L-selectin clustering

L-selectin (CD62L) amplifies neutrophil capture within the microvasculature at sites of inflammation. Activation by G protein-coupled stimuli or through ligation of L-selectin promotes clustering of L-selectin and serves to increase its adhesiveness, signaling, and colocalization with beta(2)-integrins. Currently, little is known about the molecular process regulating the lateral mobility of L-selectin. On neutrophil stimulation, a progressive change takes place in the organization of its plasma membrane, resulting in membrane domains that are characteristically enriched in glycosyl phosphatidylinositol (GPI)-anchored proteins and exclude the transmembrane protein CD45. Clustering of L-selectin, facilitated by E-selectin engagement or antibody cross-linking, resulted in its colocalization with GPI-anchored CD55, but not with CD45 or CD11c. Disrupting microfilaments in neutrophils or removing a conserved cationic motif in the cytoplasmic domain of L-selectin increased its mobility and membrane domain localization in the plasma membrane. In addition, the conserved element was critical for L-selectin-dependent tethering under shear flow. Our data indicate that L-selectin's lateral mobility is regulated by interactions with the actin cytoskeleton that in turn fortifies leukocyte tethering. We hypothesize that both membrane mobility and stabilization augment L-selectin's effector functions and are regulated by dynamic associations with membrane domains and the actin cytoskeleton.

Neutrophil string formation: hydrodynamic thresholding and cellular deformation during cell collisions

Neutrophils unexpectedly display flow-enhanced adhesion (hydrodynamic thresholding) to L-selectin in rolling or aggregation assays. We report that the primary collision efficiency (epsilon) of flowing neutrophils with preadhered neutrophils on intercellular adhesion molecule-1 (ICAM-1) or fibrinogen also displayed a maximum of epsilon approximately 0.4-0.45 at a wall shear rate of 100 s(-1), an example of thresholding. Primary collision lifetime with no detectable bonding decreased from 130 to 10 ms as wall shear rate increased from 30 to 300 s(-1), whereas collision lifetimes with bonding decreased from 300 to 100 ms over this shear range using preadhered neutrophils on ICAM-1, with similar results for fibrinogen. Antibodies against L-selectin, but not against CD11a, CD11b, or CD18, reduced epsilon at 100 s(-1) by >85%. High resolution imaging detected large scale deformation of the flowing neutrophil during the collision at 100 s(-1) with the apparent contact area increasing up to approximately 40 microm(2). We observed the formation of long linear string assemblies of neutrophils downstream of neutrophils preadhered to ICAM-1, but not fibrinogen, with a maximum in string formation at 100 s(-1). Secondary capture events to the ICAM-1 or fibrinogen coated surfaces after primary collisions were infrequent and short lived, typically lasting from 500 to 3500 ms. Between 5 and 20% of neutrophil interactions with ICAM-1 substrate converted to firm arrest (>3500 ms) and greatly exceeded that observed for fibrinogen, thus defining the root cause of poor string formation on fibrinogen at all shear rates. Additionally, neutrophils mobilized calcium after incorporation into strings. Static adhesion also caused calcium mobilization, as did the subsequent onset of flow. To our knowledge, this is the first report of 1). hydrodynamic thresholding in neutrophil string formation; 2). string formation on ICAM-1 but not on fibrinogen; 3). large cellular deformation due to collisions at a venous shear rate; and 4), mechanosensing through neutrophil beta(2)-integrin/adhesion. The increased contact area during deformation was likely responsible for the hydrodynamic threshold observed in the primary collision efficiency since no increase in primary collision lifetime was detected as shear forces were increased (for either surface coating).

Analysis of the Contributions of l -Selectin and CXCR3 in Mediating Leukocyte Homing to Pregnant Mouse Uterus

PROBLEM

Dynamic changes occur in endometrial immune cell populations during pregnancy but regulatory events promoting cell recruitment to the uterus are not established. Ovarian steroid hormones promote l-selectin and alpha4-integrin-mediated interactions between human peripheral natural killer (NK) cells and uterine endothelium while CXCR3, CXCR4 and their ligands are implicated in homing of human uNK cells to decidua.

METHOD OF STUDY

Mice genetically-ablated for l-selectin or CXCR3 were studied. Morphometric analyses of implantation sites and assays of cell function (in vitro adhesion; in vivo homing following adoptive cell transfer) were undertaken.

RESULTS

Leukocytes home to anti-mesometrial decidua with l-selectin making an early (<6 hr in vivo) contribution. Unexpectedly, CXCR3 deletion had no effect on homing but reduced the ability of uNK cells to modify placental spiral arterioles.

CONCLUSIONS

Redundant mechanisms underlie homing of leukocytes to the uterus and their importance can be evaluated by an in vivo approach described herein.