Quantifying rolling adhesion with a cell-free assay: E-selectin and its carbohydrate ligands

Nano- to microscale dynamics of P-selectin detachment from leukocyte interfaces. I. Membrane separation from the cytoskeleton

We have used a biomembrane force probe decorated with P-selectin to form point attachments with PSGL-1 receptors on a human neutrophil (PMN) in a calcium-containing medium and then to quantify the forces experienced by the attachment during retraction of the PMN at fixed speed. From first touch to final detachment, the typical force history exhibited the following sequence of events: i), an initial linear-elastic displacement of the PMN surface, ii), an abrupt crossover to viscoplastic flow that signaled membrane separation from the interior cytoskeleton and the beginning of a membrane tether, and iii), the final detachment from the probe tip by usually one precipitous step of P-selectin:PSGL-1 dissociation. In this first article I, we focus on the initial elastic response and its termination by membrane separation from the cytoskeleton, initiating tether formation. Quantifying membrane unbinding forces for rates of loading (force/time) in the elastic regime from 240 pN/s to 38,000 pN/s, we discovered that the force distributions agreed well with the theory for kinetically limited failure of a weak bond. The kinetic rate for membrane unbinding was found to increase as an exponential function of the pulling force, characterized by an e-fold scale in force of approximately 17 pN and a preexponential factor, or apparent unstressed off rate, of approximately 1/s. The rheological properties of tether growth subsequent to the membrane unbinding events are presented in a companion article II.

Nano- to microscale dynamics of P-selectin detachment from leukocyte interfaces. II. Tether flow terminated by P-selectin dissociation from PSGL-1

We have used a biomembrane force probe decorated with P-selectin to form point attachments with PSGL-1 receptors on a human neutrophil (PMN) in a calcium-containing medium and then to quantify the forces experienced by the attachment during retraction of the PMN at fixed speed. From first touch to final detachment, the typical force history exhibited the following sequence of events: i), an initial linear-elastic displacement of the PMN surface, ii), an abrupt crossover to viscoplastic flow that signaled membrane separation from the interior cytoskeleton and the beginning of a membrane tether, and iii), the final detachment from the probe tip most often by one precipitous step of P-selectin:PSGL-1 dissociation. Analyzing the initial elastic response and membrane unbinding from the cytoskeleton in our companion article I, we focus in this article on the regime of tether extrusion that nearly always occurred before release of the extracellular adhesion bond at pulling speeds > or =1 microm/s. The force during tether growth appeared to approach a plateau at long times. Examined over a large range of pulling speeds up to 150 microm/s, the plateau force exhibited a significant shear thinning as indicated by a weak power-law dependence on pulling speed, f(infinity) = 60 pN(nu(pull)/microm/s)(0.25). Using this shear-thinning response to describe the viscous element in a nonlinear Maxwell-like fluid model, we show that a weak serial-elastic component with a stiffness of approximately 0.07 pN/nm provides good agreement with the time course of the tether force approach to the plateau under constant pulling speed.

Distinct kinetic and mechanical properties govern selectin-leukocyte interactions

Leukocytes are recruited from the bloodstream to sites of inflammation by the selectin family of adhesion receptors. In vivo and in vitro studies reveal distinctive rolling velocities of polymorphonuclear leukocytes over E-, P- and L-selectin substrates. The kinetic and mechanical properties of the selectin-ligand bonds responsible for these differences at the single-molecule level are not well understood. Using single-molecule force spectroscopy, we probe in situ the rupture force, unstressed off-rate and reactive compliance of single selectin receptors to single ligands on whole human polymorphonuclear leukocytes (PMNs) under conditions that preserve the proper orientation and post-translational modifications of the selectin ligands. Single L-selectin bonds to PMNs were more labile than either E- or P-selectin in the presence of an applied force. This outcome, along with a higher unstressed off-rate and a higher reactive compliance, explain the faster L-selectin-mediated rolling. By quantifying binding frequency in the presence of a specific blocking monoclonal antibody or following enzyme treatment, we determined that P-selectin glycoprotein ligand-1 is a high-affinity ligand for E-selectin on PMNs under force. The rupture force spectra and corresponding unstressed off-rate and reactive compliance of selectin-ligand bonds provide mechanistic insights that might help to explain the variable rolling of leukocytes over different selectin substrates.

Dynamic alterations of membrane tethers stabilize leukocyte rolling on P-selectin

Leukocytes rolling on selectins extrude thin membrane tethers that might stabilize rolling velocities despite marked alterations in wall shear stress. To test this hypothesis, we used differential interference contrast videomicroscopy to visualize formation and breakage of membrane tethers as neutrophils rolled on P-selectin under flow. Neutrophils rapidly increased tether number as wall shear stress rose and decreased tether number as wall shear stress declined. Membrane tethers invariably accompanied slower, more uniform rolling steps that translated into lower mean rolling velocities and variances in velocity. Unexpectedly, neutrophils, but not fixed cells or microspheres bearing selectin ligands, rolled progressively more slowly and uniformly over time. Scanning electron microscopy revealed that neutrophils developed larger, more complex tether structures as they rolled for longer periods. These data provide evidence that neutrophils stabilize selectin-mediated rolling by rapidly adjusting tether number in response to changes in wall shear stress. Gradual remodeling of tether architecture may further reduce rolling velocities, facilitating integrin-dependent deceleration and arrest on inflamed vascular surfaces.

The molecular mechanics of P- and L-selectin lectin domains binding to PSGL-1

A laser trap was used to compare the load-dependent binding kinetics between truncated P- and L-selectin to their natural ligand, P-selectin glycoprotein ligand-1 (PSGL-1) over the predicted physiological range of loading rates. Human PSGL-1 was covalently coupled to polystyrene beads. Chimeric selectins were adsorbed to nitrocellulose-coated glass beads on a coverslip. A PSGL-1 bead was held in a laser trap and touched to a vertical surface of the glass bead, allowing a bond to form between selectin and ligand. The surface was moved away from the microsphere, applying load at a constant rate until bond rupture. Rupture force for both selectins increased with loading rate, but significant differences in rupture force between P- and L-selectin were observed only above 460 pN/s. These data are best represented as two energy barriers to unbinding, with the transition from the low to high loading rate regime at 260-290 pN/s. The data also allow the first estimate of a two-dimensional specific on-rate for binding of these two selectins to their natural ligand (1.7 microm2/s). These data suggest that P- and L-selectin lectin domains have very similar kinetics under physiological conditions.

A semianalytic model of leukocyte rolling

Rolling allows leukocytes to maintain adhesion to vascular endothelium and to molecularly coated surfaces in flow chambers. Using insights from adhesive dynamics, a computational method for simulating leukocyte rolling and firm adhesion, we have developed a semianalytic model for the steady-state rolling of a leukocyte. After formation in a force-free region of the contact zone, receptor-ligand bonds are transported into the trailing edge of the contact zone. Rolling velocity results from a balance of the convective flux of bonds and the rate of dissociation at the back edge of the contact zone. We compare the model's results to that of adhesive dynamics and to experimental data on the rolling of leukocytes, with good agreement. We calculate the dependence of rolling velocity on shear rate, intrinsic forward and reverse reaction rates, bond stiffness, and reactive compliance, and use the model to calculate a state diagram relating molecular parameters and the dynamic state of adhesion. A dimensionless form of the analytic model permits exploration of the parameters that control rolling. The chemical affinity of a receptor-ligand pair does not uniquely determine rolling velocity. We elucidate a fundamental relationship between off-rate, ligand density, and reactive compliance at the transition between firm and rolling adhesion. The model provides a rapid method for screening system parameters for the potential to mediate rolling.

Interplay between rolling and firm adhesion elucidated with a cell-free system engineered with two distinct receptor-ligand pairs

The firm arrest of leukocytes to the endothelium during inflammation is known to be mediated by endothelial intercellular adhesion molecules (ICAMs) binding to activated integrins displayed on leukocyte surface. Selectin-ligand interactions, which mediate rolling, are believed to be important for facilitating firm adhesion, either by activating integrins or by facilitating the transition to firm adhesion by making it easier for integrins to bind. Although leukocytes employ two distinct adhesion molecules that mediate different states of adhesion, the fundamental biophysical mechanisms by which two pairs of adhesion molecules facilitate cell adhesion is not well understood. In this work, we attempt to understand the interaction between two molecular systems using a cell-free system in which polystyrene microspheres functionalized with the selectin ligand, sialyl Lewis(X) (sLe(X)), and an antibody against ICAM-1, aICAM-1, are perfused over P-selectin/ICAM-1 coated surfaces in a parallel plate flow chamber. Separately, sLe(X)/P-selectin interactions support rolling and aICAM-1/ICAM-1 interactions mediate firm adhesion. Our results show that sLe(X)/aICAM-1 microspheres will firmly adhere to P-selectin/ICAM-1 coated surfaces, and that the extent of firm adhesion of microspheres is dependent on wall shear stress within the flow chamber, sLe(X)/aICAM-1 microsphere site density, and P-selectin/ICAM-1 surface density ratio. We show that P-selectin's interaction with sLe(X) mechanistically facilitates firm adhesion mediated by antibody binding to ICAM-1: the extent of firm adhesion for the same concentration of aICAM-1/ICAM-1 interaction is greater when sLe(X)/P-selectin interactions are present. aICAM-1/ICAM-1 interactions also stabilize rolling by increasing pause times and decreasing average rolling velocities. Although aICAM-1 is a surrogate for beta(2)-integrin, the kinetics of association between aICAM-1 and ICAM-1 is within a factor of 1.5 of activated integrin binding ICAM-1, suggesting the findings from this model system may be insightful to the mechanism of leukocyte firm adhesion. In particular, these experimental results show how two molecule systems can interact to produce an effect not achievable by either system alone, a fundamental mechanism that may pervade leukocyte adhesion biology.

Selectins-an emerging target for drug delivery

Selectins are multifunctional adhesion molecules that mediate the initial interactions between circulating leukocytes and cells of the endothelium. First identified over a decade ago, selectins have provided insight into areas as diverse as normal lymphocyte homing, leukocyte recruitment during inflammatory responses, carbohydrate ligand biosynthesis and adhesion-mediated signalling. Of late, selectins were introduced as targets for drug delivery in the development of new anti-inflammatory therapeutics and in anti-cancer therapy. This review will examine the selectins and their ligands with a focus on recent findings on their role in physiology and pathophysiology as well as the emerging role of selectins as targets in controlled drug delivery.

Structurally distinct requirements for binding of P-selectin glycoprotein ligand-1 and sialyl Lewis x to Anaplasma phagocytophilum and P-selectin

Colonization of neutrophils by the bacterium Anaplasma phagocytophilum causes the disease human granulocytic ehrlichiosis. The pathogen also infects mice, its natural host. Like binding of P-selectin, binding of A. phagocytophilum to human neutrophils requires expression of P-selectin glycoprotein ligand-1 (PSGL-1) and alpha1-3-fucosyltransferases that construct the glycan determinant sialyl Lewis x (sLex). Binding of A. phagocytophilum to murine neutrophils, however, requires expression of alpha1-3-fucosyltransferases but not PSGL-1. To further characterize the molecular features that A. phagocytophilum recognizes, we measured bacterial binding to microspheres bearing specific glycoconjugates or to cells expressing human PSGL-1 and particular glycosyltransferases. Like P-selectin, A. phagocytophilum bound to purified human PSGL-1 and to glycopeptides modeled after the N terminus of human PSGL-1 that presented sLex on an O-glycan. Unlike P-selectin, A. phagocytophilum bound to glycopeptides that contained sLex but lacked tyrosine sulfation or a specific core-2 orientation of sLex on the O-glycan. A. phagocytophilum bound only to glycopeptides that contained a short amino acid sequence found in the N-terminal region of human but not murine PSGL-1. Unlike P-selectin, A. phagocytophilum bound to cells expressing PSGL-1 in cooperation with sLex on both N-and O-glycans. Moreover, bacteria bound to microspheres coupled independently with glycopeptide lacking sLex and with sLex lacking peptide. These results demonstrate that, unlike P-selectin, A. phagocytophilum binds cooperatively to a nonsulfated N-terminal peptide in human PSGL-1 and to sLex expressed on PSGL-1 or other glycoproteins. Distinct bacterial adhesins may mediate these cooperative interactions.

The state diagram for cell adhesion mediated by two receptors

Leukocyte recruitment from the bloodstream to surrounding tissues is an essential component of the immune response. Capture of blood-borne leukocytes onto vascular endothelium proceeds via a two-step mechanism, with each step mediated by a distinct receptor-ligand pair. Cells first transiently adhere, or "roll" (via interactions between selectins and sialyl-Lewis-x), and then firmly adhere to the vascular wall (via interactions between integrins and ICAM-1). We have reported that a computational method called adhesive dynamics (AD) accurately reproduces the fine-scale dynamics of selectin-mediated rolling. This paper extends the use of AD simulations to model the dynamics of cell adhesion when two classes of receptors are simultaneously active: one class (selectins or selectin ligands) with weakly adhesive properties, and the other (integrins) with strongly adhesive properties. AD simulations predict synergistic functions of the two receptors in mediating adhesion. At a fixed density of surface ICAM-1, increasing selectin densities lead to greater pause times and an increased tendency toward firm adhesion; thus, selectins mechanistically facilitate firm adhesion mediated by integrins. Conversely, at a fixed density of surface selectin, increasing ICAM-1 densities lead to greater pause times and an increased tendency to firm adhesion. We present this relationship in a two-receptor state diagram, a map that relates the densities and properties of adhesion molecules to various adhesive behaviors that they code, such as rolling or firm adhesion. We also present a state diagram for neutrophil activation, which relates beta(2)-integrin density and integrin-ICAM-1 kinetic on rate to neutrophil adhesive behavior. The predictions of two-receptor adhesive dynamics are validated by the ability of the model to reproduce in vivo neutrophil rolling velocities from the literature.

Artificial polymeric cells for targeted drug delivery

Selectins are optimal biological molecules for targeted delivery of therapeutic agents because of their localized and carefully regulated expression in several human diseases, and their highly specific interactions with their counter receptors. In this study, we describe a targeted delivery system that can potentially deliver anti-inflammatory drug to sites of chronic inflammation using Poly(lactic-co-glycolic acid) (PLGA) and selectin-ligand chemistry. Biotinylated-sialyl Lewis(x) (sLe(x)), a carbohydrate that serves as a ligand to selectins, was attached to the surface of avidin-linked PLGA microspheres. These carbohydrate-coated microspheres mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow. The rolling velocity of these artificial leukocytes is similar to that displayed by leukocytes rolling on P- or E-selectin coated surfaces. We can tune rolling velocity, and hence residence time of capsules on surfaces, by changing the density of sialyl Lewis(x) on the microsphere surfaces. Therefore, we have made a targeted drug delivery vehicle that mimics the adhesive properties of leukocytes and is biodegradable.

Distinct molecular and cellular contributions to stabilizing selectin-mediated rolling under flow

Leukocytes roll on selectins at nearly constant velocities over a wide range of wall shear stresses. Ligand-coupled microspheres roll faster on selectins and detach quickly as wall shear stress is increased. To examine whether the superior performance of leukocytes reflects molecular features of native ligands or cellular properties that favor selectin-mediated rolling, we coupled structurally defined selectin ligands to microspheres or K562 cells and compared their rolling on P-selectin. Microspheres bearing soluble P-selectin glycoprotein ligand (sPSGL)-1 or 2-glycosulfopeptide (GSP)-6, a GSP modeled after the NH2-terminal P-selectin-binding region of PSGL-1, rolled equivalently but unstably on P-selectin. K562 cells displaying randomly coupled 2-GSP-6 also rolled unstably. In contrast, K562 cells bearing randomly coupled sPSGL-1 or 2-GSP-6 targeted to a membrane-distal region of the presumed glycocalyx rolled more like leukocytes: rolling steps were more uniform and shear resistant, and rolling velocities tended to plateau as wall shear stress was increased. K562 cells treated with paraformaldehyde or methyl-beta-cyclodextrin before ligand coupling were less deformable and rolled unstably like microspheres. Cells treated with cytochalasin D were more deformable, further resisted detachment, and rolled slowly despite increases in wall shear stress. Thus, stable, shear-resistant rolling requires cellular properties that optimize selectin-ligand interactions.

Characterization of biodegradable drug delivery vehicles with the adhesive properties of leukocytes

The site-specific expression of selectins (E- and P-selectin) on endothelial cells of blood vessels during inflammation provides an opportunity for the targeted delivery of anti-inflammatory drugs to inflammatory sites. Previous work in our laboratory has shown that artificial capsules with the adhesive properties of leukocytes can be made by attaching leukocyte adhesive ligands to polystyrene microspheres. In this work, we have adapted this technology to create a targeted delivery system using biodegradable, poly lactic-co-glycolic-acid (PLGA) microspheres. Biotinylated-Sialyl Lewis(x) (sLe(x)), a carbohydrate that serves as a ligand to selectins, was attached to the surface of avidin-linked PLGA microspheres. These carbohydrate-coated microspheres mimic the adhesive behavior of leukocytes on selectins in flow chambers, displaying slow rolling under flow. The rolling velocities displayed by sLe(x)-coated microspheres were similar to those displayed by leukocytes rolling on P- or E-selectin coated surfaces, and these rolling velocities, which relate to the residence time of the capsules, can be tuned by changing the density of carbohydrate residues on microsphere surfaces. We have also demonstrated that these microspheres will release model drugs on a time scale of several days. Therefore, we have made a targeted drug delivery vehicle that mimics the adhesive properties of leukocytes and is biodegradable.

Comparison of PSGL-1 microbead and neutrophil rolling: microvillus elongation stabilizes P-selectin bond clusters

A cell-scaled microbead system was used to analyze the force-dependent kinetics of P-selectin adhesive bonds independent of micromechanical properties of the neutrophil's surface microvilli, an elastic structure on which P-selectin ligand glycoprotein-1 (PSGL-1) is localized. Microvillus extension has been hypothesized in contributing to the dynamic range of leukocyte rolling observed in vivo during inflammatory processes. To evaluate PSGL-1/P-selectin bond kinetics of microbeads and neutrophils, rolling and tethering on P-selectin-coated substrates were compared in a parallel-plate flow chamber. The dissociation rates for PSGL-1 microbeads on P-selectin were briefer than those of neutrophils for any wall shear stress, and increased more rapidly with increasing flow. The microvillus length necessary to reconcile dissociation constants of PSGL-1 microbeads and neutrophils on P-selectin was 0.21 microm at 0.4 dyn/cm2, and increased to 1.58 microm at 2 dyn/cm2. The apparent elastic spring constant of the microvillus ranged from 1340 to 152 pN/microm at 0.4 and 2.0 dyn/cm2 wall shear stress. Scanning electron micrographs of neutrophils rolling on P-selectin confirmed the existence of micrometer-scaled tethers. Fixation of neutrophils to abrogate microvillus elasticity resulted in rolling behavior similar to PSGL-1 microbeads. Our results suggest that microvillus extension during transient PSGL-1/P-selectin bonding may enhance the robustness of neutrophil rolling interactions.

Tyrosine sulfation enhances but is not required for PSGL-1 rolling adhesion on P-selectin

P-selectin glycoprotein ligand-1 (PSGL-1) is a large (240 kDa) glycoprotein found on the surface of nearly all leukocytes. The mature molecule is decorated with multiple N- and O-linked glycans and displays copies of the tetrasaccharide sialyl-Lewis(x) (sLe(X)), as well as a cluster of three tyrosine sulfate (tyr-SO(3)) groups near the N-terminus of the processed protein. Previous studies have suggested that PSGL-1 needs to be tyrosine-sulfated, in addition to glycosylated with sLe(X), to successfully interact with P-selectin. To better understand how biochemical features of the PSGL-1 ligand are related to its adhesion phenotype, we have measured the dynamics of adhesion under flow of a series of well-defined PSGL-1 variants that differ in their biochemical modification, to both P- and E-selectin-coated substrates. These variants are distinct PSGL-1 peptides: one that possesses sLe(X) in conjunction with three N-terminal tyr-SO(3) groups (SGP3), one that possesses sLe(X) without tyrosine sulfation (GP1), and one that lacks sLe(X) but has three N-terminal tyr-SO(3) groups (SP3). Although all peptides expressing sLe(X), tyr-SO(3), or both supported some form of rolling adhesion on P-selectin, only peptides expressing sLe(X) groups showed rolling adhesion on E-selectin. On P-selectin, the PSGL-1 peptides demonstrated a decreasing strength of adhesion in the following order: SGP3 > GP1 > SP3. Robust, rolling adhesion on P-selectin was mediated by the GP1 peptide, despite its lack of tyrosine sulfation. However, the addition of tyrosine sulfation to glycosylated peptides (SGP3) creates a super ligand for P-selectin that supports slower rolling adhesion at all shear rates and supports rolling adhesion at much higher shear rates. Tyrosine sulfation has no similar effect on PSGL-1 rolling on E-selectin. Such functional distinctions in rolling dynamics are uniquely realized with a cell-free system, which permits precise, unambiguous identification of the functional activity of adhesive ligands. These findings are consistent with structural and functional characterizations of the interactions between these peptides and E- and P-selectin published recently.

Multiparticle adhesive dynamics. Interactions between stably rolling cells

A novel numerical simulation of adhesive particles (cells) reversibly interacting with an adhesive surface under flow is presented. Particle--particle and particle--wall hydrodynamic interactions in low Reynolds number Couette flow are calculated using a boundary element method that solves an integral representation of the Stokes equation. Molecular bonds between surfaces are modeled as linear springs and stochastically formed and broken according to postulated descriptions of force-dependent kinetics. The resulting simulation, Multiparticle Adhesive Dynamics, is applied to the problem of selectin-mediated rolling of hard spheres coated with leukocyte adhesion molecules (cell-free system). Simulation results are compared to flow chamber experiments performed with carbohydrate-coated spherical beads rolling on P-selectin. Good agreement is found between theory and experiment, with the main observation being a decrease in rolling velocity with increasing concentration of rolling cells or increasing proximity between rolling cells. Pause times are found to increase and deviation motion is found to decrease as pairs of rolling cells become closer together or align with the flow.

Molecular properties in cell adhesion: a physical and engineering perspective

The past several years have seen accelerating growth in research directed towards the understanding and control of cell adhesion processes, from a spectrum of disciplinary approaches including molecular cell biology, biochemistry, biophysics and bioengineering. Consequently, our understanding of the mechanisms involved in cell adhesion has increased substantially. Corresponding quantitative analysis and modeling of the key molecular properties governing their action in regulating dynamic cell attachment and detachment events is crucial for advancing conceptual insight along with technological applications.

Glycolipids support E-selectin-specific strong cell tethering under flow

This study provides functional evidence that glycosphingolipids constitute ligands for E-selectin but not P-selectin. Chinese hamster ovary (CHO) cells expressing E-selectin (CHO-E) or P-selectin (CHO-P) were perfused over alpha2,3-sialyl Lewis X (alpha2,3-sLe(x)) presented as the hexaosylceramide glycosphingolipid adsorbed in a monolayer containing phosphatidylcholine and cholesterol. CHO-E cells tethered extensively and formed slow, stable rolling interactions with alpha2,3-sLe(x) glycosphingolipid but not with the comparable alpha2,6-sLe(x) glycosphingolipid. Tethering/rolling varied with wall shear stress, selectin density, and ligand density. In contrast, alpha2,3-sLe(x) glycosphingolipid supported only limited, fast CHO-P cell rolling. As calculated from a stochastic model of cell rolling, the step size between successive bond releases from the alpha2,3-sLe(x) glycosphingolipid was smaller for CHO-E than CHO-P cells, whereas the opposite effect was observed for the waiting time between these events. Detachment assays revealed stronger adhesive interactions of CHO-E than CHO-P cells with alpha2,3-sLe(x) glycosphingolipid. These findings indicate that glycosphingolipids expressing an appropriate oligosaccharide mediate cell tethering/rolling via E-selectin but not P-selectin.

Cell separation mediated by differential rolling adhesion

Recently, we showed a correlation between the maturity of hematopoietic stem and progenitor cells during development and rolling efficiency on selectins. These findings motivated us to explore a novel separation that exploits differences in selectin-mediated rolling adhesion between populations of cells. We extend the use of a previously developed cell-free system to study the separation of populations of sialyl Lewis x (sLe(x))-coated microspheres designed to roll with different average velocities on L-selectin chimeric substrates under well-defined flow. Results show that a separation that exploits differences in average rolling velocities between cell or microsphere populations is attainable. Excellent recovery and purity values for the slower rolling, or more desirable, populations are obtained and can be estimated from rolling velocity measurements. We also assess the feasibility of a selectin-mediated separation of adult bone marrow cell populations using previously obtained rolling velocity and rolling flux data for CD34+ and CD34- adult bone marrow cells on L-selectin substrates. We believe that a cell separation mediated by differential rolling adhesion can be used to enrich populations of hematopoietic stem and progenitor cells from an adult bone marrow cell preparation and that this method possesses several major advantages over existing antibody-mediated cell-affinity chromatography technologies.

Particle diameter influences adhesion under flow

The diameter of circulating cells that may adhere to the vascular endothelium spans an order of magnitude from approximately 2 microm (e.g., platelets) to approximately 20 microm (e.g., a metastatic cell). Although mathematical models indicate that the adhesion exhibited by a cell will be a function of cell diameter, there have been few experimental investigations into the role of cell diameter in adhesion. Thus, in this study, we coated 5-, 10-, 15-, and 20-microm-diameter microspheres with the recombinant P-selectin glycoprotein ligand-1 construct 19.ek.Fc. We compared the adhesion of the 19.ek.Fc microspheres to P-selectin under in vitro flow conditions. We found that 1) at relatively high shear, the rate of attachment of the 19.ek.Fc microspheres decreased with increasing microsphere diameter whereas, at a lower shear, the rate of attachment was not affected by the microsphere diameter; 2) the shear stress required to set in motion a firmly adherent 19.ek.Fc microsphere decreased with increasing microsphere diameter; and 3) the rolling velocity of the 19.ek.Fc microspheres increased with increasing microsphere diameter. These results suggest that attachment, rolling, and firm adhesion are functions of particle diameter and provide experimental proof for theoretical models that indicate a role for cell diameter in adhesion.

A microcantilever device to assess the effect of force on the lifetime of selectin-carbohydrate bonds

A microcantilever technique was used to apply force to receptor-ligand molecules involved in leukocyte rolling on blood vessel walls. E-selectin was adsorbed onto 3-microm-diameter, 4-mm-long glass fibers, and the selectin ligand, sialyl Lewis(x), was coupled to latex microspheres. After binding, the microsphere and bound fiber were retracted using a computerized loading protocol that combines hydrodynamic and Hookean forces on the fiber to produce a range of force loading rates (force/time), r(f). From the distribution of forces at failure, the average force was determined and plotted as a function of ln r(f). The slope and intercept of the plot yield the unstressed reverse reaction rate, k(r)(o), and a parameter that describes the force dependence of reverse reaction rates, r(o). The ligand was titrated so adhesion occurred in approximately 30% of tests, implying that >80% of adhesive events involve single bonds. Monte Carlo simulations show that this level of multiple bonding has little effect on parameter estimation. The estimates are r(o) = 0.048 and 0.016 nm and k(r)(o) = 0.72 and 2.2 s(-1) for loading rates in the ranges 200-1000 and 1000-5000 pN s(-1), respectively. Levenberg-Marquardt fitting across all values of r(f) gives r(o) = 0.034 nm and k(r)(o) = 0.82 s(-1). The values of these parameters are in the range required for rolling, as suggested by adhesive dynamics simulations.

Local hemodynamics affect monocytic cell adhesion to a three-dimensional flow model coated with E-selectin

Monocyte adhesion to the endothelium depends on concentrations of receptors/ligands, local concentrations of chemoattractants, monocyte transport to the endothelial surface and hemodynamic forces. Monocyte adhesion to the inert surface of a three-dimensional perfusion model was shown to correlate inversely with wall shear stress, but was also affected by flow patterns which influenced the near-wall cell availability. We hypothesized that (a) under the same flow conditions, insolubilized E-selectin on the model's surface may mediate adhesive interactions at higher wall shear stresses, compared to an uncoated model, and (b) pulsatile flow may modify the adhesion profile obtained under steady flow. An axisymmetric flow model with a stenosis and a sudden expansion produced a range of wall shear stresses and a separated flow with recirculation and reattachment. Pre-activated U937 cells were perfused through the model under either steady (Re = 100, 140) or pulsatile (Remean = 107) flow. The velocity field was characterized through computational fluid dynamics and validated by inert particle tracking. Surface E-selectin greatly increased cell adhesion in all regions at Re = 100 and 140, compared to an uncoated model under the same flow conditions. In regions where the cells near the wall were abundant (taper and stenosis), adhesion to E-selectin correlated with the reciprocal of local wall shear stress when flow was steady. Pulsatile flow distributed the adherent cells more evenly throughout the coated model. Hence, characterizing both the local hemodynamics and the biological activity on the vessel wall is important in leukocyte adhesion.

P-selectin glycoprotein ligand-1 supports rolling on E- and P-selectin in vivo

Selectin-dependent rolling is the earliest observable event in the recruitment of leukocytes to inflamed tissues. Several glycoproteins decorated with sialic acid, fucose, and/or sulfate have been shown to bind the selectins. The best-characterized selectin ligand is P-selectin glycoprotein-1 (PSGL-1) that supports P-selectin– dependent rolling in vitro and in vivo. In vitro studies have suggested that PSGL-1 may also be a ligand for E- and L-selectins. To study the in vivo function of PSGL-1, without the influence of other leukocyte proteins, the authors observed the interaction of PSGL-1–coated microspheres in mouse venules stimulated to express P- and/or E-selectin. Microspheres coated with functional recombinant PSGL-1 rolled in surgically stimulated and tumor necrosis factor alpha (TNFα)-stimulated mouse venules. P-selectin deficiency or inhibition abolished microsphere rolling in surgically and TNFα-stimulated venules, whereas E-selectin deficiency or inhibition increased microsphere rolling velocity in TNFα-stimulated venules. The results suggest that P-selectin–PSGL-1 interaction alone is sufficient to mediate rolling in vivo and that E-selectin–PSGL-1 interaction supports slow rolling.

P-selectin glycoprotein ligand-1 supports rolling on E- and P-selectin in vivo

Selectin-dependent rolling is the earliest observable event in the recruitment of leukocytes to inflamed tissues. Several glycoproteins decorated with sialic acid, fucose, and/or sulfate have been shown to bind the selectins. The best-characterized selectin ligand is P-selectin glycoprotein-1 (PSGL-1) that supports P-selectin– dependent rolling in vitro and in vivo. In vitro studies have suggested that PSGL-1 may also be a ligand for E- and L-selectins. To study the in vivo function of PSGL-1, without the influence of other leukocyte proteins, the authors observed the interaction of PSGL-1–coated microspheres in mouse venules stimulated to express P- and/or E-selectin. Microspheres coated with functional recombinant PSGL-1 rolled in surgically stimulated and tumor necrosis factor alpha (TNFα)-stimulated mouse venules. P-selectin deficiency or inhibition abolished microsphere rolling in surgically and TNFα-stimulated venules, whereas E-selectin deficiency or inhibition increased microsphere rolling velocity in TNFα-stimulated venules. The results suggest that P-selectin–PSGL-1 interaction alone is sufficient to mediate rolling in vivo and that E-selectin–PSGL-1 interaction supports slow rolling.

Cell-free rolling mediated by L-selectin and sialyl Lewis(x) reveals the shear threshold effect

The selectin family of adhesion molecules mediates attachment and rolling of neutrophils to stimulated endothelial cells. This step of the inflammatory response is a prerequisite to firm attachment and extravasation. We have reported that microspheres coated with sialyl Lewis(x) (sLe(x)) interact specifically and roll over E-selectin and P-selectin substrates (Brunk et al., 1996; Rodgers et al 2000). This paper extends the use of the cell-free system to the study of the interactions between L-selectin and sLe(x) under flow. We find that sLe(x) microspheres specifically interact with and roll on L-selectin substrates. Rolling velocity increases with wall shear stress and decreases with increasing L-selectin density. Rolling velocities are fast, between 25 and 225 microm/s, typical of L-selectin interactions. The variability of rolling velocity, quantified by the variance in rolling velocity, scales linearly with rolling velocity. Rolling flux varies with both wall shear stress and L-selectin site density. At a density of L-selectin of 800 sites/microm(2), the rolling flux of sLe(x) coated microspheres goes through a clear maximum with respect to shear stress at 0.7 dyne/cm(2). This behavior, in which the maintenance and promotion of rolling interactions on selectins requires shear stress above a threshold value, is known as the shear threshold effect. We found that the magnitude of the effect is greatest at an L-selectin density of 800 sites/microm(2) and gradually diminishes with increasing L-selectin site density. Our study is the first to reveal the shear threshold effect with a cell free system and the first to show the dependence of the shear threshold effect on L-selectin site density in a reconstituted system. Our ability to recreate the shear threshold effect in a cell-free system strongly suggests the origin of the effect is in the physical chemistry of L-selectin interaction with its ligand, and largely eliminates cellular features such as deformability or topography as its cause.

The state diagram for cell adhesion under flow: leukocyte rolling and firm adhesion

Leukocyte adhesion under flow in the microvasculature is mediated by binding between cell surface receptors and complementary ligands expressed on the surface of the endothelium. Leukocytes adhere to endothelium in a two-step mechanism: rolling (primarily mediated by selectins) followed by firm adhesion (primarily mediated by integrins). Using a computational method called "Adhesive Dynamics," we have simulated the adhesion of a cell to a surface in flow, and elucidated the relationship between receptor-ligand functional properties and the dynamics of adhesion. We express this relationship in a state diagram, a one-to-one map between the biophysical properties of adhesion molecules and various adhesive behaviors. Behaviors that are observed in simulations include firm adhesion, transient adhesion (rolling), and no adhesion. We varied the dissociative properties, association rate, bond elasticity, and shear rate and found that the unstressed dissociation rate, k(r)(o), and the bond interaction length, gamma, are the most important molecular properties controlling the dynamics of adhesion. Experimental k(r)(o) and gamma values from the literature for molecules that are known to mediate rolling adhesion fall within the rolling region of the state diagram. We explain why L-selectin-mediated rolling, which has faster k(r)(o) than other selectins, is accompanied by a smaller value for gamma. We also show how changes in association rate, shear rate, and bond elasticity alter the dynamics of adhesion. The state diagram (which must be mapped for each receptor-ligand system) presents a concise and comprehensive means of understanding the relationship between bond functional properties and the dynamics of adhesion mediated by receptor-ligand bonds.

Adhesive dynamics simulations of sialyl-Lewis(x)/E-selectin-mediated rolling in a cell-free system

Selectin-mediated leukocyte rolling is crucial for the proper function of the immune response. Recently, selectin-mediated rolling was recreated in a cell-free system (Biophysical Journal 71:2902-2907 (1996)); it was shown that sialyl Lewis(x) (sLe(x))-coated microspheres roll over E-selectin-coated surfaces under hydrodynamic flow. The cell-free system removes many confounding cellular features, such as cell deformability and signaling, allowing us to focus on the role of carbohydrate/selectin physical chemistry in mediating rolling. In this paper, we use adhesive dynamics, a computational method that allows us to simulate adhesion, to analyze the experimental data produced in the cell-free system. We simulate the effects of shear rate, ligand density, and number of receptors per particle on rolling velocity and compare them with experimental results obtained with the cell-free system. If we assume the population of particles is homogeneous in receptor density, we predict that particle rolling velocity calculated in simulations is more sensitive to shear rate than found in experiments. Also, the calculated rolling velocity is more sensitive to the number of receptors on the microspheres than to the ligand density on the surface, again in contrast to experiment. We argue that heterogeneity in the distribution of receptors throughout the particle population causes these discrepancies. We improve the agreement between experiment and simulation by calculating the average rolling velocity of a population whose receptors follow a normal distribution, suggesting heterogeneity among particles significantly affects the experimental results. Further comparison between theory and experiment yields an estimate of the reactive compliance of sLe(x)/E-selectin interactions of 0.25 A, close to that reported in the literature for E-selectin and its natural ligand (0.3 A). We also provide an estimate of the value of the intrinsic association rate (between 10(4) and 10(5) s(-1)) for the formation of sLe(x)/E-selectin bonds.

Sialyl Lewis(x)-mediated, PSGL-1-independent rolling adhesion on P-selectin

Selectin-mediated cell adhesion is an essential component of the inflammatory response. In an attempt to unambiguously identify molecular features of ligands that are necessary to support rolling adhesion on P-selectin, we have used a reconstituted ("cell-free") system in which ligand-coated beads are perfused over soluble P-selectin surfaces. We find that beads coated with the saccharides sialyl Lewis(x) (sLe(x)), sialyl Lewis(a) (sLe(a)), and sulfated Lewis(x) (HSO(3)Le(x) support rolling adhesion on P-selectin surfaces. Although it has been suggested that glycosylation and sulfation of P-selectin glycoprotein ligand-1 (PSGL-1) is required for high-affinity binding and rolling on P-selectin, our findings indicate that sulfation of N-terminal tyrosine residues is not required for binding or rolling. However, beads coated with a tyrosine-sulfated, sLe(x)-modified, PSGL-1-Fc chimera support slower rolling on P-selectin than beads coated with sLe(x) alone, suggesting that sulfation improves rolling adhesion by modulating binding to P-selectin. In addition, we find it is not necessary that P-selectin carbohydrate ligands be multivalent for robust rolling to occur. Our results demonstrate that beads coated with monovalent sLe(x), exhibiting a more sparse distribution of carbohydrate than a similar amount of the multivalent form, are sufficient to yield rolling adhesion. The relative abilities of various ligands to support rolling on P-selectin are quantitatively examined among themselves and in comparison to human neutrophils. Using stop-time distributions, rolling dynamics at video frame rate resolution, and the average and variance of the rolling velocity, we find that P-selectin ligands display the following quantitative trend, in order of decreasing ability to support rolling adhesion on P-selectin: PSGL-1-Fc > sLe(a) approximately sLe(x) > HSO(3)Le(x).

An automatic braking system that stabilizes leukocyte rolling by an increase in selectin bond number with shear

Wall shear stress in postcapillary venules varies widely within and between tissues and in response to inflammation and exercise. However, the speed at which leukocytes roll in vivo has been shown to be almost constant within a wide range of wall shear stress, i.e., force on the cell. Similarly, rolling velocities on purified selectins and their ligands in vitro tend to plateau. This may be important to enable rolling leukocytes to be exposed uniformly to activating stimuli on endothelium, independent of local hemodynamic conditions. Wall shear stress increases the rate of dissociation of individual selectin-ligand tether bonds exponentially (, ) thereby destabilizing rolling. We find that this is compensated by a shear-dependent increase in the number of bonds per rolling step. We also find an increase in the number of microvillous tethers to the substrate. This explains (a) the lack of firm adhesion through selectins at low shear stress or high ligand density, and (b) the stability of rolling on selectins to wide variation in wall shear stress and ligand density, in contrast to rolling on antibodies (). Furthermore, our data successfully predict the threshold wall shear stress below which rolling does not occur. This is a special case of the more general regulation by shear of the number of bonds, in which the number of bonds falls below one.

Kinetics of adhesion of IgE-sensitized rat basophilic leukemia cells to surface-immobilized antigen in Couette flow

Antigen-antibody systems provide the flexibility of varying the kinetics and affinity of molecular interaction and studying the resulting effect on adhesion. In a parallel-plate flow chamber, we measured the extent and rate of adhesion of rat basophilic leukemia cells preincubated with anti-dinitrophenyl IgE clones SPE-7 or H1 26. 82 to dinitrophenyl-coated polyacrylamide gel substrates in a linear shear field. Both of these IgEs bind dinitrophenyl, but H1 26.82 has a 10-fold greater on rate and a 30-fold greater affinity. Adhesion was found to be binary; cells either arrested irreversibly or continued at their unencumbered hydrodynamic velocity. Under identical conditions, more adhesion was seen with the higher affinity (higher on rate) IgE clone. At some shear rates, adhesion was robust with H1 26.82, but negligible with SPE-7. Reduction in receptor number or ligand density reduced the maximum level of adhesion seen at any shear rate, but did not decrease the shear rate at which adhesion was first observed. The spatial pattern of adhesion for both IgE clones is well represented by the first-order kinetic rate constant kad, and we have determined how kad depends on ligand and receptor densities and shear rate. The rate constant kad found with H1 26.82 was approximately fivefold greater than with SPE-7. The dependence of kad on site density and shear rate for SPE-7 is complex: kad increases linearly with antigen site density at low to moderate shear rates, but is insensitive to site density at high shear. kad increases with shear rate at low site density but decreases with shear at high site density. With H1 26.82, the functional dependence of kad with shear rate was similar. Although these data are consistent with the hypothesis that we have sampled both transport and reaction-limited adhesion regimes, they point out deficiencies in current theories describing cell attachment under flow.