A schiff base with mildly oxidized carbohydrate ligands stabilizes L-selectin and not P-selectin or E-selectin rolling adhesions in shear flow

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

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

Absence of the endothelial oxidase AOC3 leads to abnormal leukocyte traffic in vivo

Leukocyte migration from the blood to tissues is a prerequisite for normal immune responses. We produced mice deficient in an endothelial cell-surface oxidase (amine oxidase, copper containing-3 [AOC3], also known as vascular adhesion protein-1 [VAP-1]) and found that this enzyme is needed for leukocyte extravasation in vivo. Real-time imaging shows that AOC3 mediates slow rolling, firm adhesion, and transmigration of leukocytes in vessels at inflammatory sites and lymphoid tissues. Absence of AOC3 results in reduced lymphocyte homing into lymphoid organs and in attenuated inflammatory response in peritonitis. These data alter the paradigm of leukocyte extravasation cascade by providing the first physiological proof for the concept that endothelial cell surface enzymes regulate the development of inflammatory reactions in vivo and suggest that this enzyme should be useful as an anti-inflammatory target.

Oxidative modification of leukocyte adhesion

In this issue of Immunity, Marko Salmi and colleagues describe mice lacking AOC3, an endothelial cell monoaminooxidase that is involved in modulating leukocyte rolling, adhesion, and migration (Stolen et al., 2005). Their data demonstrate the importance of oxidative modification of (unknown) adhesion molecules in regulating inflammation and lymphocyte homing.

Environmental contaminant and disinfection by-product trichloroacetaldehyde stimulates T cells in vitro

It had been shown previously that MRL+/+ mice exposed to occupationally relevant doses of the environmental contaminant trichloroethylene in their drinking water developed lupus-like symptoms and autoimmune hepatitis in association with activation of Interferon-gamma (IFN-gamma)-producing CD4+ T cells. Since trichloroethylene must be metabolized in order to promote the T-cell activation associated with autoimmunity, the present study was initiated to determine whether the immunoregulatory effects of trichloroethylene could be mimicked by one of its major metabolites, trichloroacetaldehyde (TCAA). At concentrations ranging from 0.04 to 1 mM TCAA co-stimulated proliferation of murine T-helper type 1 (Th1) cells treated with anti-CD3 antibody or antigen in vitro. TCAA at similar concentrations also induced phenotypic alterations commensurate with activation (upregulation of CD28 and downregulation of CD62L) in both cloned memory Th1 cells, as well as naïve CD4+ T cells from MRL+/+ mice. TCAA-induced Th1 cell activation was accompanied by phoshorylation of activating transcription factor 2 (ATF-2) and c-Jun, two components of the activator protein-1 (AP-1) transcription factor. TCAA at higher concentrations was also shown to form a Schiff base on T cells, and inhibition of Schiff base formation suppressed the ability of TCAA to phosphorylate ATF-2. Taken together, these results suggest that TCAA promotes T-cell activation via stimulation of the mitogen-activated protein (MAP) kinase pathway in association with Schiff base formation on T-cell surface proteins. By demonstrating that TCAA can stimulate T-cell function directly, these results may explain how the environmental toxicant trichloroethylene promotes T-cell activation and related autoimmunity in vivo.

Sickle erythrocyte adherence to endothelium at low shear: role of shear stress in propagation of vaso-occlusion

Under venular flow conditions, sickle cell adherence to endothelium is mediated by cell adhesion molecules and adhesive proteins associated with inflammation, coagulation, and endothelial perturbation. Periodic and reduced blood flow are observed in sickle microcirculation during hematologic steady state, suggesting that blood flow is compromised in sickle microcirculation. We tested the hypothesis that low blood flow enhances adherence by quantifying sickle cell adhesion to endothelium under venular flow (1.0 dyne/cm(2) shear stress) and low flow (0.1 dyne/cm(2) shear stress), with and without addition of adhesion promoting agonists. Under low flow, sickle cell adherence to endothelium increases with contact time in the absence of endothelial activation or adhesive protein addition. In contrast, at venular shear stress, sickle cell adherence only occurs following endothelial activation with TNF-alpha or addition of thrombospondin. Analysis of these data with a mathematical model reveals that at low flow adherence is "transport-controlled," meaning that contact time between sickle cells and endothelium is a more important determinant of adherence than high-affinity receptor-ligand interactions. Low-affinity interactions are sufficient for adhesion at low flow. In contrast, at venular flow (1 dyne/cm(2) shear stress) adherence is "affinity-controlled," meaning that adherence requires induction of specific high-affinity receptor-ligand interactions. These findings demonstrate that in addition to activating factors and adherence proteins, microvascular shear stress is an important determinant of sickle cell adhesion to endothelium. This suggests that in vivo, erythrostasis is an important determinant of adhesion that can act either independently or concurrently with ongoing acute events to induce adhesive interactions and vaso-occlusion.

Chemical and biological strategies for engineering cell surface glycosylation

Oligosaccharides play a crucial role in many of the recognition, signaling, and adhesion events that take place at the surface of cells. Abnormalities in the synthesis or presentation of these carbohydrates can lead to misfolded and inactive proteins, as well as to several debilitating disease states. However, their diverse structures, which are the key to their function, have hampered studies by biologists and chemists alike. This review presents an overview of techniques for examining and manipulating cell surface oligosaccharides through genetic, enzymatic, and chemical strategies.

Selectin receptor-ligand bonds: Formation limited by shear rate and dissociation governed by the Bell model

We have studied the principles that govern the formation and dissociation of an adhesive bond between a cell moving in shear flow and a substrate and tested different theories of how force affects bond dissociation. Viscosity relates the kinematics of fluid movement (shear rate, units of time(-1)) to shear stress (units of force/area, the product of shear rate and viscosity). At different medium viscosities, the formation of receptor-ligand bonds between a cell in the flowstream and P-selectin on the vessel wall showed a similar efficiency as a function of shear rate but not of shear stress. Therefore, bond formation was a function of shear rate and hence of the kinematics of receptor and ligand movement. By contrast, the kinetics of bond dissociation was a function of shear stress and hence of force on the bond. The different requirements for bond formation and dissociation allowed dissociation kinetics to be measured at higher forces on the bond by increasing medium viscosity. Data over an extended range of forces on the bond therefore could be collected that enabled five different proposed equations, relating force to bond dissociation, to be compared for fit to experimental data. The relationship proposed by Bell [Bell, G. I. (1978) Science 200, 618-627] fit the data significantly the best and also predicted an off-rate in the absence of force that best matched an independent measurement [Mehta, P., Cummings, R. D. & McEver, R. P. (1998) J. Biol. Chem. 273, 32506-32513].

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.

Modifying the mechanical property and shear threshold of L-selectin adhesion independently of equilibrium properties

Interactions between adhesion molecules on two different cells differ from interactions between receptors and soluble ligands in that the adhesion molecule interaction (bond) is often subjected to force. It is widely assumed by cell biologists that the 'strength' of a bond is a simple function of the affinity of one adhesion molecule for the other, whereas biophysicists suggest that bonds have 'mechanical properties' that affect their strength. Mechanical properties are a function of the shape of the energy landscape related to bond formation and dissociation, whereas affinity is related only to the net energy change. Mechanical properties determine the amount by which the kinetics and affinity of bonds are altered by applied force. To date there has been no experimental manipulation of an adhesion molecule that has been shown to affect mechanical properties. L-selectin is an adhesion molecule that mediates lymphocyte binding to, and rolling on, high endothelial venules; these are prerequisites for the emigration of lymphocytes from the bloodstream into lymph nodes. Here we report a selective and reversible chemical modification of a mucin-like ligand that alters the mechanical properties of its bond with L-selectin. The effect of force on the rate of bond dissociation, that is, on a mechanical property, is altered, whereas there is little or no effect of the modification on the rate of bond dissociation in the absence of force. Moreover, the puzzling requirement for hydrodynamic shear flow above a threshold level for L-selectin interactions is dramatically altered.

Mechanism of ligand binding to E- and P-selectin analyzed using selectin/mannose-binding protein chimeras

The mechanism of oligosaccharide binding to the selectin cell adhesion molecules has been analyzed by transferring regions of the carbohydrate-recognition domains of E- and P-selectin into corresponding sites in the homologous rat serum mannose-binding protein. Insertion of two basic regions and an adjacent glutamic acid residue leads to efficient binding of HL-60 cells and sialyl-Lewisx-conjugated serum albumin. Substitution of glycine for a histidine residue known to stabilize mannose in the binding site of wild type mannose-binding protein results in dramatic loss of affinity for mannose without decreasing binding to sialyl-Lewisx. The accumulated effect of these changes is to alter the ligand binding selectivity of the domain so that it resembles E- or P-selectin more closely than it resembles the parental mannose-binding domain. Affinity labeling using sialyl-Lewisx in which the sialic acid has been mildly oxidized has been used to verify this switch in specificity and to show that the sialic acid-containing portion of the ligand interacts near the sequence Lys-Lys-Lys corresponding to residues 111-113 of E-selectin. The binding of sialyl-Lewisx-serum albumin is inhibited dramatically at physiological and higher salt concentrations, consistent with a significant electrostatic component to the binding interaction. The binding characteristics of these gain-of-function chimeras suggest that they contain many of the selectin residues responsible for selective ligand binding.

Enhancement of cytolytic activity of human peripheral blood lymphocytes by sodium periodate (IO4) possible involvement of protein kinase C

Sodium periodate (IO4) exerts a number of biological effects including the enhancement of lymphocyte activation. In this study, we investigated its effects on cytotoxicity of human peripheral blood lymphocytes (PBL) and explored the mechanism whereby it exerted these effects. In vitro treatment of human PBL with IO4 augmented their cytotoxicity against K562 myelogenous leukemia cells. IO4 oxidative treatment increased the frequency of effector-to-target cell binding. It also increased cellular ATP levels in effector cells, suggesting that the post-binding cytolytic functions of these cells were also enhanced after treatment with IO4. Moreover, IO4 treatment significantly increased the protein kinase C (PKC) activity of effector cells and induced the translocation of activity in the membrane fraction from the cytosol. H-7, a potent PKC inhibitor, significantly reduced this enhancement of membrane-associated PKC activity at 10 microM and significantly reduced the enhanced cytotoxicity of PBL at the same concentration. These results indicated that IO4 enhanced the binding capacity and post-binding cytolytic functions of PBL and that PKC activation was one mechanism to explain the IO4-induced cellular activation.