Static and dynamic lengths of neutrophil microvilli

An analytical model for determining two-dimensional receptor-ligand kinetics

Cell-cell adhesive interactions play a pivotal role in major pathophysiological vascular processes, such as inflammation, infection, thrombosis, and cancer metastasis, and are regulated by hemodynamic forces generated by blood flow. Cell adhesion is mediated by the binding of receptors to ligands, which are both anchored on two-dimensional (2-D) membranes of apposing cells. Biophysical assays have been developed to determine the unstressed (no-force) 2-D affinity but fail to disclose its dependence on force. Here we develop an analytical model to estimate the 2-D kinetics of diverse receptor-ligand pairs as a function of force, including antibody-antigen, vascular selectin-ligand, and bacterial adhesin-ligand interactions. The model can account for multiple bond interactions necessary to mediate adhesion and resist detachment amid high hemodynamic forces. Using this model, we provide a generalized biophysical interpretation of the counterintuitive force-induced stabilization of cell rolling observed by a select subset of receptor-ligand pairs with specific intrinsic kinetic properties. This study enables us to understand how single-molecule and multibond biophysics modulate the macroscopic cell behavior in diverse pathophysiological processes.

Determining beta2-integrin and intercellular adhesion molecule 1 binding kinetics in tumor cell adhesion to leukocytes and endothelial cells by a gas-driven micropipette assay

Interactions between polymorphonuclear neutrophils (PMNs) and tumor cells have been reported to facilitate the adhesion and subsequent extravasation of tumor cells through the endothelium under blood flow, both of which are mediated by binding β(2)-integrin to intercellular adhesion molecule 1 (ICAM-1). Here the adhesions between human WM9 metastatic melanoma cells, PMNs, and human pulmonary microvascular endothelial cells (HPMECs) were quantified by a gas-driven micropipette aspiration technique (GDMAT). Our data indicated that the cellular binding affinity of PMN-WM9 pair was 3.9-fold higher than that of the PMN-HPMEC pair. However, the effective binding affinities per molecular pair were comparable between the two cell pairs no matter whether WM9 cells or HPMECs were quiescent or cytokine-activated, indicating that the stronger adhesion between PMN-WM9 pair is mainly attributed to the high expression of ICAM-1 on WM9 cells. These results proposed an alternative mechanism, where WM9 melanoma cells adhere first with PMNs near vessel-wall regions and then bind to endothelial cells via PMNs under blood flow. In contrast, the adhesions between human MDA-MB-231 metastatic breast carcinoma cells and PMNs showed a comparable cellular binding affinity to PMN-HPMEC pair because the ICAM-1 expressions on MDA-MB-231 cells and HPMECs are similar. Furthermore, differences were observed in the intrinsic forward and reverse rates of the β(2)-integrin-ICAM-1 bond between PMN-TC and PMN-EC pairs. This GDMAT assay enables us to quantify the binding kinetics of cell adhesion molecules physiologically expressed on nucleated cells. The findings also further the understanding of leukocyte-facilitated tumor cell adhesion from the viewpoint of molecular binding kinetics.

Cell protrusions and tethers: a unified approach

Low pulling forces applied locally to cell surface membranes produce viscoelastic cell surface protrusions. As the force increases, the membrane can locally separate from the cytoskeleton and a tether forms. Tethers can grow to great lengths exceeding the cell diameter. The protrusion-to-tether transition is known as the crossover. Here we propose a unified approach to protrusions and tethers providing, to our knowledge, new insights into their biomechanics. We derive a necessary and sufficient condition for a crossover to occur, a formula for predicting the crossover time, conditions for a tether to establish a dynamic equilibrium (characterized by constant nonzero pulling force and tether extension rate), a general formula for the tether material after crossover, and a general modeling method for tether pulling experiments. We introduce two general protrusion parameters, the spring constant and effective viscosity, valid before and after crossover. Their first estimates for neutrophils are 50 pN μm(-1) and 9 pN s μm(-1), respectively. The tether elongation after crossover is described as elongation of a viscoelastic-like material with a nonlinearly decaying spring (NLDs-viscoelastic material). Our model correctly describes the results of the published protrusion and tether pulling experiments, suggesting that it is universally applicable to such experiments.

Effect of cell and microvillus mechanics on the transmission of applied loads to single bonds in dynamic force spectroscopy

Receptor-ligand interactions that mediate cellular adhesion are often subjected to forces that regulate their detachment via modulating off-rates. Although the dynamics of detachment is primarily controlled by the physical chemistry of adhesion molecules, cellular features such as cell deformability and microvillus viscoelasticity have been shown to affect the rolling velocity of leukocytes in vitro through experiments and simulation. In this work, we demonstrate via various micromechanical models of two cells adhered by a single (intramolecular) bond that cell deformability and microvillus viscoelasticity modulate transmission of an applied external load to an intramolecular bond, and thus the dynamics of detachment. Specifically, it is demonstrated that the intermolecular bond force is not equivalent to the instantaneous applied force and that the instantaneous bond force decreases with cellular and microvillus compliance. As cellular compliance increases, not only does the time lag between the applied load and the bond force increase, an initial response time is observed during which cell deformation is observed without transfer of force to the bond. It is further demonstrated that following tether formation the instantaneous intramoleular bond force increases linearly at a rate dependent on microvillus viscosity. Monte Carlo simulations with fixed kinetic parameters predict that both cell and microvillus compliance increase the average rupture time, although the average rupture force based on bond length remains nearly unchanged.

A semianalytical model to study the effect of cortical tension on cell rolling

Cell rolling on the vascular endothelium plays an important role in trafficking of leukocytes, stem cells, and cancer cells. We describe a semianalytical model of cell rolling that focuses on the microvillus as the unit of cell-substrate interaction and integrates microvillus mechanics, receptor clustering, force-dependent receptor-ligand kinetics, and cortical tension that enables incorporation of cell body deformation. Using parameters obtained from independent experiments, the model showed excellent agreement with experimental studies of neutrophil rolling on P-selectin and predicted different regimes of cell rolling, including spreading of the cells on the substrate under high shear. The cortical tension affected the cell-surface contact area and influenced the rolling velocity, and modulated the dependence of rolling velocity on microvillus stiffness. Moreover, at the same shear stress, microvilli of cells with higher cortical tension carried a greater load compared to those with lower cortical tension. We also used the model to obtain a scaling dependence of the contact radius and cell rolling velocity under different conditions of shear stress, cortical tension, and ligand density. This model advances theoretical understanding of cell rolling by incorporating cortical tension and microvillus extension into a versatile, semianalytical framework.

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.

Rolling cell adhesion

Rolling adhesion on vascular surfaces is the first step in recruiting circulating leukocytes, hematopoietic progenitors, or platelets to specific organs or to sites of infection or injury. Rolling requires the rapid yet balanced formation and dissociation of adhesive bonds in the challenging environment of blood flow. This review explores how structurally distinct adhesion receptors interact through mechanically regulated kinetics with their ligands to meet these challenges. Remarkably, increasing force applied to adhesive bonds first prolongs their lifetimes (catch bonds) and then shortens their lifetimes (slip bonds). Catch bonds mediate the counterintuitive phenomenon of flow-enhanced rolling adhesion. Force-regulated disruptions of receptor interdomain or intradomain interactions remote from the ligand-binding surface generate catch bonds. Adhesion receptor dimerization, clustering in membrane domains, and interactions with the cytoskeleton modulate the forces applied to bonds. Both inside-out and outside-in cell signals regulate these processes.

Quantitative dynamic footprinting microscopy reveals mechanisms of neutrophil rolling

We introduce quantitative dynamic footprinting microscopy to resolve neutrophil rolling on P-selectin. We show that the footprint of a rolling neutrophil is four times larger than previously thought, the P-selectin-PSGL-1 bonds are relaxed at the leading edge of the rolling cell, compressed under the cell center, and stretched at the trailing edge. Each rolling neutrophil also forms 3-4 long tethers that extend up to 16 μm behind the rolling cell.

Temperature modulation of integrin-mediated cell adhesion

In response to external stimuli, cells modulate their adhesive state by regulating the number and intrinsic affinity of receptor/ligand bonds. A number of studies have shown that cell adhesion is dramatically reduced at room or lower temperatures as compared with physiological temperature. However, the underlying mechanism that modulates adhesion is still unclear. Here, we investigated the adhesion of the monocytic cell line THP-1 to a surface coated with intercellular adhesion molecule-1 (ICAM-1) as a function of temperature. THP-1 cells express the integrin lymphocyte function-associated antigen-1 (LFA-1), a receptor for ICAM-1. Direct force measurements of cell adhesion and cell elasticity were carried out by atomic force microscopy. Force measurements revealed an increase of the work of de-adhesion with temperature that was coupled to a gradual decrease in cellular stiffness. Of interest, single-molecule measurements revealed that the rupture force of the LFA-1/ICAM-1 complex decreased with temperature. A detailed analysis of the force curves indicated that temperature-modulated cell adhesion was mainly due to the enhanced ability of cells to deform and to form a greater number of longer membrane tethers at physiological temperatures. Together, these results emphasize the importance of cell mechanics and membrane-cytoskeleton interaction on the modulation of cell adhesion.

The constitutive equation for membrane tether extraction

Membrane tethers or nanotubes play a critical role in a variety of cellular and subcellular processes such as leukocyte rolling and intercellular mass transport. The current constitutive equations that describe the relationship between the pulling force and the tether velocity during tether extraction have serious limitations. In this article, we propose a new phenomenological constitutive equation that captures all known characteristics of nanotube formation, including nonlinearity, nonzero threshold force, and possible negative tether velocity. We used tether extraction from endothelial cells as a prototype to illustrate how to obtain the material constants in the constitutive equation. With the micropipette aspiration technique, we measured tether pulling forces at both positive and negative tether velocities. We also determined the threshold force of 55 pN experimentally for the first time. This new constitutive equation unites two established ones and provides us a unified platform to better understand not only the physiological role of tether extraction during leukocyte rolling and intercellular or intracellular transport, but also the physics of membrane tether growth or retraction.

The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

Models of dynamic extraction of lipid tethers from cell membranes

When a ligand that is bound to an integral membrane receptor is pulled, the membrane and the underlying cytoskeleton can deform before either the membrane delaminates from the cytoskeleton or the ligand detaches from the receptor. If the membrane delaminates from the cytoskeleton, it may be further extruded and form a membrane tether. We develop a phenomenological model for this process by assuming that deformations obey Hooke's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. We compute the probability of tether formation and show that tethers can be extruded only within an intermediate range of force loading rates and pulling velocities. The mean tether length that arises at the moment of ligand detachment is computed as are the force loading rates and pulling velocities that yield the longest tethers.

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.

Cell surface topology creates high Ca2+ signalling microdomains

It has long been speculated that cellular microdomains are important for many cellular processes, especially those involving Ca²⁺ signalling. Measurements of cytosolic Ca²⁺ report maximum concentrations of less than few micromolar, yet several cytosolic enzymes require concentrations of more than 20 μM Ca²⁺ to be activated. In this paper, we have resolved this apparent paradox by showing that the surface topology of cells represents an important and hitherto unrecognized feature for generating microdomains of high Ca²⁺ in cells. We show that whereas the standard modeling assumption of a smooth cell surface predicts only moderate localized effects, the more realistic “wrinkled” surface topology predicts that Ca²⁺ concentrations up to 80 μM can persist within the folds of membranes for significant times. This intra-wrinkle location may account for 5% of the total cell volume. Using different geometries of wrinkles, our simulations show that high Ca²⁺ microdomains will be generated most effectively by long narrow membrane wrinkles of similar dimensions to those found experimentally. This is a new concept which has not previously been considered, but which has ramifications as the intra-wrinkle location is also a strategic location at which Ca²⁺ acts as a regulator of the cortical cytoskeleton and plasma membrane expansion.

Microbial alkaloid staurosporine induces formation of nanometer-wide membrane tubular extensions (cytonemes, membrane tethers) in human neutrophils

In the present work, we demonstrate that microbial alkaloid staurosporine (STS) and Ro 31-8220, structurally related to STS protein kinase C inhibitor, caused development of membrane tubular extensions in human neutrophils upon adhesion to fibronectin-coated substrata. STS-induced tubular extensions interconnected neutrophils in a network and bound serum-opsonized bacteria Salmonella enterica serovar Typhimurium. The diameter of STS-induced extensions varied in the range 160-200 nm. The extensions were filled with cytoplasm and covered with membrane, as they included fluorescent cytoplasmic and lipid dyes. Neither protein kinase C inhibitors H-7 and bisindolylmaleimide VII, nor tyrosine protein kinase inhibitors tyrphostin AG 82 and genistein caused such extensions formation. Supposedly, STS induces membrane tubular extension formation promoting actin cytoskeleton depolymerization or affecting NO synthesis.

Nano-motion dynamics are determined by surface-tethered selectin mechanokinetics and bond formation

The interaction of proteins at cellular interfaces is critical for many biological processes, from intercellular signaling to cell adhesion. For example, the selectin family of adhesion receptors plays a critical role in trafficking during inflammation and immunosurveillance. Quantitative measurements of binding rates between surface-constrained proteins elicit insight into how molecular structural details and post-translational modifications contribute to function. However, nano-scale transport effects can obfuscate measurements in experimental assays. We constructed a biophysical simulation of the motion of a rigid microsphere coated with biomolecular adhesion receptors in shearing flow undergoing thermal motion. The simulation enabled in silico investigation of the effects of kinetic force dependence, molecular deformation, grouping adhesion receptors into clusters, surface-constrained bond formation, and nano-scale vertical transport on outputs that directly map to observable motions. Simulations recreated the jerky, discrete stop-and-go motions observed in P-selectin/PSGL-1 microbead assays with physiologic ligand densities. Motion statistics tied detailed simulated motion data to experimentally reported quantities. New deductions about biomolecular function for P-selectin/PSGL-1 interactions were made. Distributing adhesive forces among P-selectin/PSGL-1 molecules closely grouped in clusters was necessary to achieve bond lifetimes observed in microbead assays. Initial, capturing bond formation effectively occurred across the entire molecular contour length. However, subsequent rebinding events were enhanced by the reduced separation distance following the initial capture. The result demonstrates that vertical transport can contribute to an enhancement in the apparent bond formation rate. A detailed analysis of in silico motions prompted the proposition of wobble autocorrelation as an indicator of two-dimensional function. Insight into two-dimensional bond formation gained from flow cell assays might therefore be important to understand processes involving extended cellular interactions, such as immunological synapse formation. A biologically informative in silico system was created with minimal, high-confidence inputs. Incorporating random effects in surface separation through thermal motion enabled new deductions of the effects of surface-constrained biomolecular function. Important molecular information is embedded in the patterns and statistics of motion.

Cell adhesion molecule distribution relative to neutrophil surface topography assessed by TIRFM

The positioning of adhesion molecules relative to the microtopography of the cell surface has a significant influence on the molecule's availability to form adhesive contacts. Measurements of the ratio of fluorescence intensity per unit area in epi-fluorescence images versus total internal reflection fluorescence images provides a means to assess the relative accessibility for bond formation of different fluorescently labeled molecules in cells pressed against a flat substrate. Measurements of the four principal adhesion molecules on human neutrophils reveal that L-selectin has the highest ratio of total internal reflection fluorescence/epi intensity, and that P-selectin glycoprotein ligand-1 (PSGL-1) and the integrins alphaLbeta2 (LFA-1) and alphaMbeta2 (Mac-1) have ratios similar to each other but lower than for L-selectin. All of the ratios increased with increasing impingement, indicating an alteration of surface topography with increasing surface compression. These results are consistent with model predictions for molecules concentrated near the tips of microvilli in the case of L-selectin, and sequestered away from the microvillus tips in the case of LFA-1, Mac-1, and PSGL-1. The results confirm differences among adhesion molecules in their surface distribution and reveal how the availability of specific adhesion molecules is altered by mechanical compression of the surface in live cells.

Molecular accessibility in relation to cell surface topography and compression against a flat substrate

The recruitment of cells to the vascular wall in vivo or the capture of cell subpopulations at the surface of a fabricated device requires the formation of bonds between specific molecular pairs on the cell and the substrate. The ability of a molecule to form a bond depends critically on its localization relative to the cell surface topography. In this report, we present a framework for the quantitative assessment of molecular availability that accounts for the deformability of the cell surface and the balance of forces in the interface, as well as the variability of surface protrusion lengths and the preference for molecules to reside at or away from the tips of surface projections. We also examined how molecular availability should change with increasing compression of the cell against the substrate. Finally, we convolved the distribution of molecules at the interface with a decaying evanescent excitation to predict the fluorescence intensity in total internal reflectance fluorescence microscopy, which can provide a quantitative measure of the relative availability of different molecules at a cell-substrate interface. Model predictions show good agreement with measurements of fluorescence intensity of different molecules labeled fluorescently on the surface of a human neutrophil compressed against a glass surface.

Influence of type IV pilus retraction on the architecture of the Neisseria gonorrhoeae-infected cell cortex

Early in infection, Neisseria gonorrhoeae can be observed to attach to the epithelial cell surface as microcolonies and induce dramatic changes to the host cell cortex. We tested the hypothesis that type IV pili (Tfp) retraction plays a role in the ultrastructure of both the host cell cortex and the bacterial microcolony. Using serial ultrathin sectioning, transmission electron microscopy and 3D reconstruction of serial 2D images, we have obtained what we believe to be the first 3D reconstructions of the N. gonorrhoeae-host cell interface, and determined the architecture of infected cell microvilli as well as the attached microcolony. Tfp connect both wild-type (wt) and Tfp retraction-deficient bacteria with each other, and with the host cell membrane. Tfp fibres and microvilli form a lattice in the wt microcolony and at its periphery. Wt microcolonies induce microvilli formation and increases of surface area, leading to an approximately ninefold increase in the surface area of the host cell membrane at the site of attachment. In contrast, Tfp retraction-deficient microcolonies do not affect these parameters. Wt microcolonies had a symmetrical, dome-shaped structure with a circular 'footprint', while Tfp retraction-deficient microcolonies were notably less symmetrical. These findings support a major role for Tfp retraction in microvilli and microcolony architecture. They are consistent with the biophysical attributes of Tfp and the effects of Tfp retraction on epithelial cell signalling.

Action at a distance: lengthening adhesion bonds with poly(ethylene glycol) spacers enhances mechanically stressed affinity for improved vascular targeting of microparticles

Poly(ethylene glycol) (PEG) chains were used to decorate microparticles with long adhesion ligands to emulate the efficacy of selectin-mediated leukocyte homing mechanisms. Ligands for P-selectin, an endothelial cell inflammatory marker, were coupled to PEG spacers of two sizes (MW 3400 and 10,000 Da) to investigate the effects on adhesion kinetics to P-selectin substrates. Under shear flow 80 nm PEG spacers improved P-selectin-antibody adhesion frequency by up to 4.5-fold and bond lifetimes by 7-fold compared to microparticles bearing chemisorbed antibody. Presentation of the glycosulfopeptide P-selectin ligands (2-GSP-6) and its nonsulfated low affinity form (2-GP-6) by long PEG spacers led to improved lifetimes of stressed bonds formed with P-selectin in shear flow and the rolling fluxes. Thus, structural features far removed from the binding pocket of a receptor that increase molecular contour length may enhance affinity in mechanically stressed environments such as those existing within the confines of the blood vessel. Such features may be useful for improving the performance of vascular-targeted micro- and nanoparticles used for drug, gene, and image contrast delivery. Ligand presentation on molecularly extended stalks may also serve to enhance any particle-surface interaction that takes place in laminar shear flow.

Validation, In-Depth Analysis, and Modification of the Micropipette Aspiration Technique

The micropipette aspiration technique (MAT) has been successfully applied to many studies in cell adhesion such as leukocyte-endothelium interactions. However, this technique has never been validated experimentally and it has been only employed to impose constant forces. In this study, we validated the force measurement of the MAT with the optical trap and analyzed two technical issues of the MAT, force-transducer offset and cell-micropipette gap, with finite element simulation. We also modified the MAT so that increasing or decreasing forces can be applied. With the modified MAT, we studied tether extraction from endothelial cells by pulling single tethers at increasing velocities and constant force loading rates. Before the onset of tether extraction, an apparently-linear surface protrusion of a few hundred nanometers was observed, which is likely related to membrane receptors pulling on the underlying cytoskeleton. The strength of the modified MAT lies in its capability and consistency to apply a wide range of force loading rates from several piconewtons per second up to thousands of piconewtons per second. With this modification, the MAT becomes more versatile in the study of single molecule and single cell biophysics.

Adhesive dynamics simulation of G-protein-mediated chemokine-activated neutrophil adhesion

To reach sites of inflammation, a blood-borne neutrophil first rolls over the vessel wall, becoming firmly adherent on activation, and then transmigrates through the endothelium. In this study, we simulate the transition to firm adhesion via chemokine-induced integrin activation. To recreate the transition from rolling to firm adhesion, we use an integrated signaling adhesive dynamics simulation that includes selectin, integrin, and chemokine interactions between the cell and an adhesive substrate. Integrin bonds are of low affinity until activated by chemokine binding to G-protein coupled receptors on the model cell. The signal propagates within the cell through probabilistic diffusion and reaction of the signaling elements to induce the high-affinity integrins required for firm adhesion. This model showed that integrins become progressively active as cells roll and interact with chemokines, leading to a slight slowing before firm adhesion on a timescale similar to that observed in experiments. Increasing the density of chemokine resulted in decreases in the rolling time before stopping, consistent with experimental observations. However, a limit is reached where further increases in chemokine density do not increase adhesion. We found that the timescale for integrin activation correlated with the time to stop. Further, altering parameters within the intracellular signaling cascade that changed the speed of integrin activation, such as effector activation and dissociation rates, correspondingly affected the time to firm adhesion. For all conditions tested, the number of active integrin bonds at the point of firm adhesion was relatively constant. The model predicts that the time to stop would be relatively independent of selectin or integrin density, but strongly dependent on the shear rate because higher shear rates limit the intrinsic activation rate of integrins and require more integrins for adhesion.

Dynamics of Microvillus Extension and Tether Formation in Rolling Leukocytes

P-selectin glycoprotein ligand-1 (PSGL-1) binding to P-selectin mediates leukocyte rolling under conditions of flow. In human neutrophils, a type of leukocyte belonging to the innate immune system, PSGL-1 molecules are located on the neutrophil's surface ruffles, called microvilli. Each newly formed P-selectin-PSGL-1 bond can become load bearing, imposing on its microvillus a pulling force that deforms the microvillus. Depending on the magnitude of the bond force, a microvillus can be extended, or a thin membrane cylinder (a tether) can be formed at the tip of the microvillus. Here we propose a Kelvin-Voigt viscoelastic material as an improved model for microvillus extension. Using a modified version of our Event-Tracking Model of Adhesion (ETMA), we demonstrate how P-selectin-PSGL-1 load-bearing bonds shape microvillus deformation during neutrophil rolling at low shear (wall shear rate of 50 s(-1), P-selectin site density of 150 molecules μm(-2)). We also discuss the impact of microvillus deformability on neutrophil rolling. We find that the average microvillus extension constitutes 65% of the total microvillus-tether complex extension, and that the rolling neutrophil may never fully rest. A quantitative comparison with the corresponding non-deformable microvilli case supports a concept that the ability of the microvillus to deform stabilizes cell rolling.

Microcontact printing of P-selectin increases the rate of neutrophil recruitment under shear flow

The local variation of P-selectin expression on inflamed endothelial layers affects leukocyte recruitment in vivo. As an initial study of the spatially heterogeneous presentation of P-selectin in vitro, the influence of microcontact printing (microCP) of P-selectin on a planar surface in neutrophil recruitment was investigated using a parallel-plate flow chamber. Microline patterned and nonpatterned P-selectin were prepared using PDMS, Poly(dimethylsiloxane) (PMDS) stamps and isolated neutrophils perfused over the surface to quantify the level of neutrophil recruitment. We first found a significant increase in cell rolling flux and a decrease in cell rolling velocity on the microcontact printed P-selectin-surfaces compared with a randomly adsorbed P-selectin-surface. However, the increase in rolling adhesion under shear on the surfaces prepared by microCP was not proportional to the number of functional sites of P-selectin transferred using immunofluorescent labeling. Interestingly, the relative immunofluorescent intensities of the nonfunctional regions of microcontact printed P-selectin-surfaces were substantially lower than that that of randomly adsorbed P-selectin. Taken together, these data indicate that the microCP of selectin increases the transfer rate of the adhesion molecule on a surface in the functionally correct orientation and, consequently, improves the recruitment of leukocytes to the selectin surface under flow. It is concluded that microCP may be a more general technique to control protein orientation on a substrate.

Versatile horizontal force probe for mechanical tests on pipette-held cells, particles, and membrane capsules

We present a multipurpose nanomechanical force probe that combines a sideways-mounted elastic cantilever and an optical-lever detection module with automated micropipette manipulation. It allows us to apply and measure compression, stretching, adhesion, and dissociation forces in the horizontal direction while providing a "side view" of ongoing experiments. The integrated micropipette setup facilitates the easy manipulation and mechanical interrogation of individual cells, functionalized particles, and synthetic membrane capsules. Pipette-held test objects are translated perpendicularly to and from the stationary cantilever, eliminating the need to attach them to a carrier surface and substantially reducing unwanted hydrodynamic coupling effects. Moreover, the test objects can be brought into contact with the cantilever anywhere along its length, which considerably enlarges the range of forces that can be applied with a single cantilever. Advantages of this instrument are demonstrated in example measurements of single-cell compression, membrane-tether extrusion, oligonucleotide stretching, and extraction of individual lipids from surfactant-monolayer surfaces of microbubbles.

Selectin catch-slip kinetics encode shear threshold adhesive behavior of rolling leukocytes

The selectin family of leukocyte adhesion receptors is principally recognized for mediating transient rolling interactions during the inflammatory response. Recent studies using ultrasensitive force probes to characterize the force-lifetime relationship between P- and L-selectin and their endogenous ligands have underscored the ability of increasing levels of force to initially extend the lifetime of these complexes before disrupting bond integrity. This so-called "catch-slip" transition has provided an appealing explanation for shear threshold phenomena in which increasing levels of shear stress stabilize leukocyte rolling under flow. We recently incorporated catch-slip kinetics into a mechanical model for cell adhesion and corroborated this hypothesis for neutrophils adhering via L-selectin. Here, using adhesive dynamics simulations, we demonstrate that biomembrane force probe measurements of various P- and L-selectin catch bonds faithfully predict differences in cell adhesion patterns that have been described extensively in vitro. Using phenomenological parameters to characterize the dominant features of molecular force spectra, we construct a generalized phase map that reveals that robust shear-threshold behavior is possible only when an applied force very efficiently stabilizes the bound receptor complex. This criteria explains why only a subset of selectin catch bonds exhibit a shear threshold and leads to a quantitative relationship that may be used to predict the magnitude of the shear threshold for families of catch-slip bonds directly from their force spectra. Collectively, our results extend the conceptual framework of adhesive dynamics as a means to translate complex single-molecule biophysics to macroscopic cell behavior.

Event-tracking model of adhesion identifies load-bearing bonds in rolling leukocytes

OBJECTIVES

P-selectin binding to P-selectin glycoprotein ligand-1 (PSGL)-1 mediates leukocyte rolling under conditions of inflammation and injury. The aims of this study were to develop an efficient, high temporal resolution model for direct simulation of leukocyte rolling and conduct a study of load-bearing bonds using the model.

MATERIALS AND METHODS

A stochastic pi-calculus-driven event-tracking model of adhesion (ETMA) was developed and compared with experimental data. Multiple simulations for each case were conducted to obtain high-confidence numerical characteristics of leukocyte rolling.

RESULTS

Leukocyte rolling and the underlying P-selectin-PSGL-1 bonds were studied under low wall shear rate (25-50 s(-1)) conditions from measured parameters of leukocyte rolling and bond properties. For the first time, the location, number, lifetime, history, and kinetics of load-bearing bonds and their influence on cell rolling were identified and instantaneous cell displacements, translational and rotational velocities, and cell-substrate distances derived. The model explains the commonly observed "stop-start" type rolling behavior and reveals that a few load-bearing bonds are sufficient to support rolling, while a large number of bonds dissociate before becoming load bearing.

CONCLUSIONS

ETMA provides a method for more precise, direct simulation of leukocyte rolling at low wall shear rates and sets a foundation upon which further refinements can be introduced.

Catch bonds in adhesion

One of the most exciting discoveries in biological adhesion is the recent and counter-intuitive observation that the lifetimes of some biological adhesive bonds, called catch bonds, are enhanced by tensile mechanical force. At least two types of adhesive proteins have been shown to form catch bonds--blood proteins called selectins and a bacterial protein called FimH. Both mediate shear-enhanced adhesion, in which cells bind more strongly at high shear than at low shear. Single-molecule experiments and cell-free assays have now clearly demonstrated that catch bonds exist and mediate shear-enhanced adhesion. However, the mechanics of cellular organelles also contribute to shear-enhanced adhesion by modulating the force applied to catch bonds. This review examines how individual catch bond behavior contributes to shear-enhanced cellular adhesion for the two best-understood examples. The lessons from these systems offer design principles for understanding other types of shear-enhanced adhesion and for engineering nanostructured force-dependent adhesives out of catch bonds.

Ethanol enhances neutrophil membrane tether growth and slows rolling on P-selectin but reduces capture from flow and firm arrest on IL-1-treated endothelium

The effects of ethanol at physiological concentrations on neutrophil membrane tether pulling, adhesion lifetime, rolling, and firm arrest behavior were studied in parallel-plate flow chamber assays with adherent 1-microm-diameter P-selectin-coated beads, P-selectin-coated surfaces, or IL-1-stimulated human endothelium. Ethanol (0.3% by volume) had no effect on P-selectin glycoprotein ligand-1 (PSGL-1), L-selectin, or CD11b levels but caused PSGL-1 redistribution. Also, ethanol prevented fMLP-induced CD11b up-regulation. During neutrophil collisions with P-selectin-coated beads at venous wall shear rates of 25-100 s(-1), ethanol increased membrane tether length and membrane growth rate by 2- to 3-fold but reduced the adhesion efficiency (detectable bonding per total collisions) by 2- to 3-fold, compared with untreated neutrophils. Without ethanol treatment, adhesion efficiency and adhesion lifetime declined as wall shear rate was increased, whereas ethanol caused the adhesion lifetime over all events to increase from 0.1 s to 0.5 s as wall shear rate was increased, an example of pharmacologically induced hydrodynamic thresholding. Consistent with this increased membrane fluidity and reduced capture, ethanol reduced rolling velocity by 37% and rolling flux by 55% on P-selectin surfaces at 100 s(-1), compared with untreated neutrophils. On IL-1-stimulated endothelium, rolling velocity was unchanged by ethanol treatment, but the fraction of cells converting to firm arrest was reduced from 35% to 24% with ethanol. Overall, ethanol caused competing biophysical and biochemical effects that: 1) reduced capture due to PSGL-1 redistribution, 2) reduced rolling velocity due to increased membrane tether growth, and 3) reduced conversion to firm arrest.

Human neutrophil surface protrusion under a point load: location independence and viscoelasticity

Mechanical properties of neutrophils have been recognized as key contributors to stabilizing neutrophil rolling on the endothelium during the inflammatory response. In particular, accumulating evidence suggests that surface protrusion and tether extraction from neutrophils facilitate stable rolling by relieving the disruptive forces on adhesive bonds. Using a customized optical trap setup, we applied piconewton-level pulling forces on targeted receptors that were located either on the microvillus tip (CD162) or intermicrovillus surface of neutrophils (CD18 and CD44). Under a constant force-loading rate, there always occurred an initial tent-like surface protrusion that was terminated either by rupture of the adhesion or by a "yield" or "crossover" to tether extraction. The corresponding protrusional stiffness of neutrophils was found to be between 0.06 and 0.11 pN/nm, depending on the force-loading rate and the cytoskeletal integrity, but not on the force location, the medium osmolality, nor the temperature increase from 22 degrees C to 37 degrees C. More importantly, we found that neutrophil surface protrusion was accompanied by force relaxation and hysteresis. In addition, the crossover force did not change much in the range of force-loading rates studied, and the protrusional stiffness of lymphocytes was similar to that of neutrophils. These results show that neutrophil surface protrusion is essentially viscoelastic, with a protrusional stiffness that stems primarily from the actin cortex, and the crossover force is independent of the receptor-cytoskeleton interaction.

Low spring constant regulates P-selectin-PSGL-1 bond rupture

Forced dissociation of selectin-ligand bonds is crucial to such biological processes as leukocyte recruitment, thrombosis formation, and tumor metastasis. Although the bond rupture has been well known at high loading rate r(f) (>or=10(2) pN/s), defined as the product of spring constant k and retract velocity v, how the low r(f) (<10(2) pN/s) or the low k regulates the bond dissociation remains unclear. Here an optical trap assay was used to quantify the bond rupture at r(f) <or= 20 pN/s with low k ( approximately 10(-3)-10(-2) pN/nm) when P-selectin and P-selectin glycoprotein ligand 1 (PSGL-1) were respectively coupled onto two glass microbeads. Our data indicated that the bond rupture force f retained the similar values when r(f) increased up to 20 pN/s. It was also found that f varied with different combinations of k and v even at the same r(f). The most probable force, f*, was enhanced with the spring constant when k < 47.0 x 10(-3) pN/nm, indicating that the bond dissociation at low r(f) was spring constant dependent and that bond rupture force depended on both the loading rate and the mechanical compliance of force transducer. These results provide new insights into understanding the P-selectin glycoprotein ligand 1 bond dissociation at low r(f) or k.

Roles of cell and microvillus deformation and receptor-ligand binding kinetics in cell rolling

Polymorphonuclear leukocyte (PMN) recruitment to sites of inflammation is initiated by selectin-mediated PMN tethering and rolling on activated endothelium under flow. Cell rolling is modulated by bulk cell deformation (mesoscale), microvillus deformability (microscale), and receptor-ligand binding kinetics (nanoscale). Selectin-ligand bonds exhibit a catch-slip bond behavior, and their dissociation is governed not only by the force but also by the force history. Whereas previous theoretical models have studied the significance of these three "length scales" in isolation, how their interplay affects cell rolling has yet to be resolved. We therefore developed a three-dimensional computational model that integrates the aforementioned length scales to delineate their relative contributions to PMN rolling. Our simulations predict that the catch-slip bond behavior and to a lesser extent bulk cell deformation are responsible for the shear threshold phenomenon. Cells bearing deformable rather than rigid microvilli roll slower only at high P-selectin site densities and elevated levels of shear (>or=400 s(-1)). The more compliant cells (membrane stiffness=1.2 dyn/cm) rolled slower than cells with a membrane stiffness of 3.0 dyn/cm at shear rates >50 s(-1). In summary, our model demonstrates that cell rolling over a ligand-coated surface is a highly coordinated process characterized by a complex interplay between forces acting on three distinct length scales.

3D computational modeling and simulation of leukocyte rolling adhesion and deformation

A 3D computational fluid dynamic (CFD) model is presented to simulate transient rolling adhesion and deformation of leukocytes over a P-selectin coated surface in shear flow. The computational model is based on immersed boundary method for cell deformation, and stochastic Monte Carlo simulation for receptor/ligand interaction. The model is shown to predict the characteristic 'stop-and-go' motion of rolling leukocytes. Here we examine the effect of cell deformation, shear rate, and microvilli distribution on the rolling characteristics. Comparison with experimental measurements is presented throughout the article. We observe that compliant cells roll more stably, and have longer pause times due to reduced bond force and increased bond lifetime. Microvilli presentation is shown to affect rolling characteristics by altering the step size, but not pause times. Our simulations predict a significant sideway motion of the cell arising purely due to receptor/ligand interaction, and discrete nature of microvilli distribution. Adhesion is seen to occur via multiple tethers, each of which forms multiple selectin bonds, but often one tether is sufficient to support rolling. The adhesion force is concentrated in only 1-3 tethered microvilli in the rear-most part of a cell. We also observe that the number of selectin bonds that hold the cell effectively against hydrodynamic shear is significantly less than the total adhesion bonds formed between a cell and the substrate. The force loading on individual microvillus and selectin bond is not continuous, rather occurs in steps. Further, we find that the peak force on a tethered microvillus is much higher than that measured to cause tether extrusion.

Adhesive dynamics simulation of neutrophil arrest with stochastic activation

The transition from rolling to firm adhesion is a key step in the adhesion cascade that permits a neutrophil to exit the bloodstream and make its way to a site of inflammation. In this work, we construct an integrated model of neutrophil activation and arrest that combines a biomechanical model of neutrophil adhesion and adhesive dynamics, with fully stochastic signal transduction modeling, in the form of kinetic Monte Carlo simulation within the microvilli. We employ molecular binding parameters gleaned from the literature and from simulation of cell-free rolling mediated by selectin molecules. We create a simplified model of lymphocyte function-associated antigen-1 activation that links P-selectin glycoprotein ligand-1 ligation to integrin activation. The model utilizes an energy profile of various integrin activation states drawn from literature data and permits manipulation of signal diffusivity within the microvillus. Our integrated model recreates neutrophil arrest within physiological timescales, and we demonstrate that increasing signal diffusivity within a microvillus accelerates arrest. If the energy barrier between free unactivated and free activated lymphocyte function-associated antigen-1 increases, the period of rolling before arrest increases. We further demonstrate that, within our model, modification of endothelial ligand surface densities can control arrest. In addition, the relative concentrations of signaling molecules control the fractional activation of the overall signaling pathway and the rolling time to arrest. This work presents the first, to our knowledge, fully stochastic model of neutrophil activation, which, though simplified, can recapitulate significant physiological details of neutrophil arrest yet retains the capacity to incorporate additional information regarding mechanisms of neutrophil signal transduction as they are elucidated.

What is the biological relevance of the specific bond properties revealed by single-molecule studies?

During the last decade, many authors took advantage of new methodologies based on atomic force microscopy (AFM), biomembrane force probes (BFPs), laminar flow chambers or optical traps to study at the single-molecule level the formation and dissociation of bonds between receptors and ligands attached to surfaces. Experiments provided a wealth of data revealing the complexity of bond response to mechanical forces and the dependence of bond rupture on bond history. These results supported the existence of multiple binding states and/or reaction pathways. Also, single bond studies allowed us to monitor attachments mediated by a few bonds. The aim of this review is to discuss the impact of this new information on our understanding of biological molecules and phenomena. The following points are discussed: (i) which parameters do we need to know in order to predict the behaviour of an encounter between receptors and ligands, (ii) which information is actually yielded by single-molecule studies and (iii) is it possible to relate this information to molecular structure?

The viscoelasticity of membrane tethers and its importance for cell adhesion

Cell adhesion mechanically couples cells to surfaces. The durability of individual bonds between the adhesive receptors and their ligands in the presence of forces determines the cellular adhesion strength. For adhesive receptors such as integrins, it is a common paradigm that the cell regulates its adhesion strength by altering the affinity state of the receptors. However, the probability distribution of rupture forces is dependent not only on the affinity of individual receptor-ligand bonds but also on the mechanical compliance of the cellular anchorage of the receptor. Hence, by altering the anchorage, the cell can regulate its adhesion strength without changing the affinity of the receptor. Here, we analyze the anchorage of the integrin VLA-4 with its ligand VCAM-1. For this purpose, we develop a model based on the Kelvin body, which allows one to quantify the mechanical properties of the adhesive receptor's anchorage using atomic force microscopy on living cells. As we demonstrate, the measured force curves give valuable insight into the mechanics of the cellular anchorage of the receptor, which is described by the tether stiffness, the membrane rigidity, and the membrane viscosity. The measurements relate to a tether stiffness of k(t) = 1.6 microN/m, an initial membrane rigidity of k(i) = 260 microN/m, and a viscosity of mu = 5.9 microN x s/m. Integrins exist in different activation states. When activating the integrin with Mg(2+), we observe altered viscoelastic parameters of k(t) = 0.9 microN/m, k(i) = 190 microN/m, and mu = 6.0 microN x s/m. Based on our model, we postulate that anchorage-related effects are common regulating mechanisms for cellular adhesion beyond affinity regulation.

Dynamic states of cells adhering in shear flow: from slipping to rolling

Motivated by rolling adhesion of white blood cells in the vasculature, we study how cells move in linear shear flow above a wall to which they can adhere via specific receptor-ligand bonds. Our computer simulations are based on a Langevin equation accounting for hydrodynamic interactions, thermal fluctuations, and adhesive interactions. In contrast to earlier approaches, our model not only includes stochastic rules for the formation and rupture of bonds, but also fully resolves both receptor and ligand positions. We identify five different dynamic states of motion in regard to the translational and angular velocities of the cell. The transitions between the different states are mapped out in a dynamic state diagram as a function of the rates for bond formation and rupture. For example, as the cell starts to adhere under the action of bonds, its translational and angular velocities become synchronized and the dynamic state changes from slipping to rolling. We also investigate the effect of nonmolecular parameters. In particular, we find that an increase in viscosity of the medium leads to a characteristic expansion of the region of stable rolling to the expense of the region of firm adhesion, but not to the expense of the regions of free or transient motion. Our results can be used in an inverse approach to determine single bond parameters from flow chamber data on rolling adhesion.

Vesicles and red blood cells in shear flow

We describe the similarities and the specificities of the behaviour of individual soft particles, namely, drops, lipid vesicles and red blood cells subjected to a shear flow. We highlight that their motion depends in a non-trivial way on the particle mechanical properties. We detail the effect of the presence of a wall with or without wall-particle attractive interaction from a biological perspective.

A Novel Technique of Quantifying Flexural Stiffness of Rod-Like Structures

In cellular and molecular biomechanics, extensional stiffness of rod-like structures such as leukocyte microvilli can be easily measured with many techniques, but not many techniques are available for measuring their flexural stiffness. In this paper, we report a novel technique of measuring the flexural stiffness of rod-like structures. This technique is based on image deconvolution and, as an example, it was used for determining the flexural stiffness of neutrophil microvilli. The probes we used were 40-nm-diameter fluorescent beads, which were bound to the tips of neutrophil microvilli by anti-L-selectin antibody. The fluorescent images of the bead, which was positioned at the center of the cell bottom, were acquired with high magnification and long exposure time (3 s). Using a Gaussian function as the point spread function of our imaging system, we established a convolution equation based on Boltzmann's law, which yields an analytical expression that relates the bead image profile to the flexural stiffness of the microvillus. The flexural stiffness was then obtained by the least squares regression. On average, the flexural stiffness was determined to be 7 pN/mum for single neutrophil microvilli. With the resolution of our imaging system, this technique can be used for measuring any flexural stiffness smaller than 34 pN/mum and it has great potential in single molecule biomechanics.

How Cells feel their environment: a focus on early dynamic events

It is now well demonstrated that cell adhesion to a foreign surface strongly influences prominent functions such as survival, proliferation, differentiation, migration or mediator release. Thus, a current challenge of major practical and theoretical interest is to understand how cells process and integrate environmental cues to determine future behaviour. The purpose of this review is to summarize some pieces of information that might serve this task. Three sequential points are discussed. First, selected examples are presented to illustrate the influence of substratum chemistry, topography and mechanical properties on nearly all aspects of cell behaviour observed during the days following adhesion. Second, we review reported evidence that long term cell behaviour is highly dependent on the alterations of cell shape and cytoskeletal organization that are often initiated during the minutes to hours following adhesion. Third, we review recently obtained information on cell membrane roughness and dynamics, as well as kinetics and mechanics of molecular interactions. This knowledge is required to understand the influence of substratum structure on cell signaling during the first minute following contact, before the appearance of detectable structural changes. It is suggested that unraveling the earliest phenomena following cell-to-substratum encounter might provide a tractable way of better understanding subsequent events.

How cells tiptoe on adhesive surfaces before sticking

Cell membranes are studded with protrusions that were thoroughly analyzed with electron microscopy. However, the nanometer-scale three-dimensional motions generated by cell membranes to fit the topography of foreign surfaces and initiate adhesion remain poorly understood. Here, we describe the dynamics of surface deformations displayed by monocytic cells bumping against fibronectin-coated surfaces. We observed membrane undulations with typically 5 nm amplitude and 5-10 s lifetime. Cell membranes behaved as independent units of micrometer size. Cells detected the presence of foreign surfaces at 50 nm separation, resulting in time-dependent amplification of membrane undulations. Molecular contact then ensued with apparent cell-membrane separation of 30-40 nm, and this distance steadily decreased during the following tens of seconds. Contact maturation was associated with in-plane egress of bulky molecules and robust membrane fluctuations. Thus, membrane undulations may be the major determinant of cell sensitivity to substrate topography, outcome of interaction, and initial kinetics of contact extension.

Dual role of melanoma cell adhesion molecule (MCAM)/CD146 in lymphocyte endothelium interaction: MCAM/CD146 promotes rolling via microvilli induction in lymphocyte and is an endothelial adhesion receptor

The melanoma cell adhesion molecule (MCAM)/CD146 is expressed as two isoforms differing by their cytoplasmic domain (MCAM long (MCAM-l) and MCAM short (MCAM-s)). MCAM being expressed by endothelial cells and activated T cells, we analyzed its involvement in lymphocyte trafficking. The NK cell line NKL1 was transfected by MCAM isoforms and submitted to adhesion on both the endothelial cell monolayer and recombinant molecules under shear stress. MCAM-l transfection reduced rolling velocity and increased NKL1 adhesion on the endothelial cell monolayer and VCAM-1. Scanning electron microscopy revealed that MCAM-l induced microvilli formation and extension. In contrast, MCAM short or mock transfection had no effect on adhesion of NKL1 cells and microvilli formation. As shown by mutagenesis, serine 32 of the MCAM-l cytoplasmic tail, belonging to a putative protein kinase C phosphorylation site, was necessary for MCAM-l-actin cytoskeleton interaction and microvilli induction. Accordingly, chelerythrine chloride, a protein kinase C inhibitor, abolished MCAM-l-induced microvilli and rolling of MCAM-l-transfected NKL1 cells. Inhibition of adhesion under shear stress by anti-MCAM Abs suggested that both lymphoid MCAM-l and endothelial MCAM were also directly involved in lymphocyte endothelium interaction. MCAM-l-transfected NKL1 and activated CD4 T cells adhered to rMCAM under shear stress whereas anti-MCAM Ab treatment inhibited this process. Taken together, these data establish that MCAM is involved in the initial steps of lymphocyte endothelium interaction. By promoting the rolling on the inflammation marker VCAM-1 via microvilli induction and displaying adhesion receptor activity involving possible homophilic MCAM-l-MCAM-l interactions, MCAM might be involved in the recruitment of activated T cells to inflammation sites.