Affinity and kinetic analysis of L-selectin (CD62L) binding to glycosylation-dependent cell-adhesion molecule-1

Construction of multivalent sialyl Lex from the type Ia group B Streptococcus capsular polysaccharide

The type Ia group B Streptococcus (GBSIa) capsular polysaccharide was specifically degraded by partial Smith oxidation of 2,3-diol of the Glc in the backbone to fragments representing asialo core repeating units. Sialylation of these oligomers furnished GBSIa multiple repeating units. One, two and three repeating units of GBSIa were obtained pure, and the higher oligomers were obtained as mixtures. After enzymatic fucosylation oligosaccharides carrying bivalent, trivalent and other multivalent sialyl Le(x) epitopes presented as appendages on an oligolactoside scaffold were obtained.

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].

Carbohydrate sulfotransferases: novel therapeutic targets for inflammation, viral infection and cancer

Effective direct inhibition of adhesion receptors by small molecules has been hampered by extended receptor-ligand interfaces as well as the entropic penalties often associated with inhibition of cell adhesion. Therefore, alternative strategies have targeted enzymes that are centrally involved in the biosynthesis of recognition epitopes, which are crucial for productive adhesion. Two classes of enzymes shown to play a pivotal role in cell-cell and cell-matrix adhesions are the protein-tyrosine and carbohydrate sulfotransferases, which impart crucial sulfate moieties onto glycoproteins. The carbohydrate sulfotransferases will be discussed in terms of target validation and small-molecule inhibitor discovery.

Differences in zero-force and force-driven kinetics of ligand dissociation from beta-galactoside-specific proteins (plant and animal lectins, immunoglobulin G) monitored by plasmon resonance and dynamic single molecule force microscopy

Protein-carbohydrate interactions are involved in diverse regulatory processes. To help understand the mechanics and kinetics of dissociation of receptor-ligand complexes, we have analyzed the separation of lactose and the N-glycan chains of asialofetuin (ASF) from three lectins and an immunoglobulin G fraction by surface plasmon resonance at zero force and by atomic force microscopy with variations of the external force. While the (AB)2 agglutinins from Ricinus communis (RCA) and Viscum album (VAA) show structural homology, the homodimeric galectin-1 from bovine heart (BHL) has no similarity to the two plant lectins except for sharing this monosaccharide specificity. The beta-galactoside-binding immunoglobulin G (IgG) fraction from human serum provides a further model system with distinct binding-site architecture. The k(off) constants for the two plant agglutinins were independent of the nature of the ligand at 1.1-1.3 x 10(-3) s(-1), whereas the geometry of ligand and binding site presentation affected this parameter for BHL (0.5 x 10(-3) s(-1) for lactose and 1 x 10(-3) s(-1) for ASF) and IgG (1.3 x 10(-3) s(-1) for lactose and 0.55 x 10(-3) s(-1) for ASF). When assessing comparatively the rupture forces at a loading rate of 3 nN/s with lactose as ligand, 34 +/- 6 pN (BHL), 36 +/- 4 pN (IgG), 47 +/- 7 pN (VAA), and 58 +/- 9 pN (RCA) were measured. For the same loading rate the rupture forces for the receptor-ASF interactions were found to be 37 +/- 3 pN (BHL), 43 +/- 5 pN (VAA), 45 +/- 6 pN (IgG), and 65 +/- 9 pN (RCA). The variation of the pulling velocity revealed in all cases a linear dependence between the rupture force and the natural logarithm of the loading rate. Performing probability density and Monte Carlo calculations, the potential barrier widths, which determine the inverse dynamic dependence with the rate of force elevation, increased from 4 A (RCA) and 7 A (VAA and IgG) to 10 A (BHL) for the receptor-lactose interactions. Presenting ASF as ligand potential widths of 4 A for RCA and IgG and 6 A for VAA and BHL were obtained. Since the dissociation kinetics at zero force apparently cannot predict the behavior in force-driven experiments, these results reveal new insights into biological functions. The dissociation kinetics under force helps to explain the difference in the toxic potency of VAA and RCA and points to a function of the galectin in cis-crosslinking and in transient trans-bridging.

L-selectin gene T668C mutation in type 1 diabetes patients and their first degree relatives

There have been some studies published recently which have suggested that L-selectin and/or other adhesion molecules could be the new markers for diabetes type 1 risk development in humans and animal models of the disease. The alterations of soluble L-selectin have been found not only in overt but also in the preclinical stage of disease development and were independent from the presence of ICA - a marker of ongoing autoimmunity, but associated with HLA related genetic predisposition to insulin-dependent diabetes mellitus (IDDM). The aim of our study was to evaluate the frequency of the L-selectin gene T668C mutation (from thymine to cytosine at position 668) resulted in F206L an amino acid substitution in patients with overt diabetes and their unaffected first degree relatives in comparison to the unselected control population. In the unaffected siblings of IDDM subjects we have observed a significantly higher frequency of the L-selectin gene T668C mutation in comparison to their relatives with type 1 diabetes and healthy controls. It was also shown that there is an association between T668C mutation and low HLA related risk of IDDM development, the highest frequency of F206L mutation in the EGF domain of L-selectin was observed in relatives with 'protective' HLA DQB1*0602 allele and nonDRB1*03-nonDRB1*04 haplotype, while in subjects with highest risk of IDDM haplotype the frequency of T668C mutation was similar to the controls. We would like to hypothesise that the T668C L-selectin gene mutation could have a (protective?) role in the development of IDDM, but further studies concerning their role in type 1 diabetes are needed.

Animal lectins as cell adhesion molecules

Protein-carbohydrate interaction is exploited in cell adhesion mechanisms besides the recognition of peptide motifs. The sugar code thus significantly contributes to the intriguing specificity of cellular selection of binding partners. Focusing on two classes of lectins (selectins and galectins), it is evident that their functionality for mediation of adhesive contacts is becoming increasingly appreciated, as is the integration of this type of interaction with other recognition modes to yield the noted specificity. The initial contact formation between leukocytes and activated endothelium makes use of selectins to guide lymphocyte trafficking. In addition to the three selectins which bind a distinct array of ligands, galectin-1 and galectin-3 and possibly other members of this family are involved in cell-cell or cell-matrix interactions. This review summarizes structural and functional aspects of these two classes of endogenous lectins relevant for cell adhesion.

Thirty-six views of T-cell recognition

While much is known about the signalling pathways within lymphocytes that are triggered during activation, much less is known about how the various cell surface molecules on T cells initiate these events. To address this, we have focused on the primary interaction that drives T-cell activation, namely the binding of a particular T-cell receptor (TCR) to peptide-MHC ligands, and find a close correlation between biological activity and off-rate; that is, the most stimulatory TCR ligands have the slowest dissociation rates. In general, TCRs from multiple histocompatibility complex (MHC) class-II-restricted T cells have half-lives of 1-11s at 25 degrees C, a much narrower range than found with antibodies and suggesting a strong selection for an optimum dissociation rate. TCR ligands with even faster dissociation rates tend to be antagonists. To observe the effects of these different ligands in their physiological setting, we made gene fusions of various molecules with green fluorescent protein (GFP), transfected them into the relevant lymphocytes, and observed their movements during T-cell recognition using multicolour video microscopy. We find that clustering of CD3zeta-GFP and CD4-GFP on the Tcell occurs concomitantly or slightly before the first rise in calcium by the T cell, and that various GFP-labelled molecules on the B-cell side cluster shortly thereafter (ICAM-1, class II MHC, CD48), apparently driven byT-cell molecules. Most of this movement towards the interface is mediated by signals through the co-stimulatory receptors, CD28 and LFA-1, and involves myosin motors and the cortical actin cytoskeleton. Thus, we have proposed that the principal mechanism by which co-stimulation enhances T-cell responsiveness is by increasing the local density of T-cell activation molecules, their ligands and their attendant signalling apparatus. In collaboration with Michael Dustin and colleagues, we have also found that the formation and stability of the TCR-peptide-MHC cluster at the centre of the interaction cap between T and B cells is highly dependent on the dissociation rate of the TCR and its ligand. Thus, we are able to link this kinetic parameter to the formation of a cell surface structure that is linked to and probably causal with respect to T-cell activation.

The LFA-1 integrin supports rolling adhesions on ICAM-1 under physiological shear flow in a permissive cellular environment

The LFA-1 integrin is crucial for the firm adhesion of circulating leukocytes to ICAM-1-expressing endothelial cells. In the present study, we demonstrate that LFA-1 can arrest unstimulated PBL subsets and lymphoblastoid Jurkat cells on immobilized ICAM-1 under subphysiological shear flow and mediate firm adhesion to ICAM-1 after short static contact. However, LFA-1 expressed in K562 cells failed to support firm adhesion to ICAM-1 but instead mediated K562 cell rolling on the endothelial ligand under physiological shear stress. LFA-1-mediated rolling required an intact LFA-1 I-domain, was enhanced by Mg2+, and was sharply dependent on ICAM-1 density. This is the first indication that LFA-1 can engage in rolling adhesions with ICAM-1 under physiological shear flow. The ability of LFA-1 to support rolling correlates with decreased avidity and impaired time-dependent adhesion strengthening. A beta2 cytoplasmic domain-deletion mutant of LFA-1, with high avidity to immobilized ICAM-1, mediated firm arrests of K562 cells interacting with ICAM-1 under shear flow. Our results suggest that restrictions in LFA-1 clustering mediated by cytoskeletal attachments may lock the integrin into low-avidity states in particular cellular environments. Although low-avidity LFA-1 states fail to undergo adhesion strengthening upon contact with ICAM-1 at stasis, these states are permissive for leukocyte rolling on ICAM-1 under physiological shear flow. Rolling mediated by low-avidity LFA-1 interactions with ICAM-1 may stabilize rolling initiated by specialized vascular rolling receptors and allow the leukocyte to arrest on vascular endothelium upon exposure to stimulatory endothelial signals.

Bivalent sequential binding model of a Bacillus thuringiensis toxin to gypsy moth aminopeptidase N receptor

Specificity for target insects of Bacillus thuringiensis insecticidal Cry toxins is largely determined by toxin affinity for insect midgut receptors. The mode of binding for one such toxin-receptor complex was investigated by extensive toxin mutagenesis, followed by real-time receptor binding analysis using an optical biosensor (BIAcore). Wild-type Cry1Ac, a three-domain, lepidopteran-specific toxin, bound purified gypsy moth (Lymantria dispar) aminopeptidase N (APN) biphasically. Site 1 displayed fast association and dissociation kinetics, while site 2 possessed slower kinetics, yet tighter affinity. We empirically determined that two Cry1Ac surface regions are involved in in vivo toxicity and APN binding. Mutations within domain III affected binding rates to APN site 1, whereas mutations in domain II affected binding rates to APN site 2. Furthermore, domain III contact is completely inhibited in the presence of N-acetylgalactosamine, indicating loss of domain III binding eliminates all APN binding. Based upon these observations, the following model is proposed. A cavity in lectin-like domain III initiates docking through recognition of an N-acetylgalactosamine moiety on L. dispar APN. Following primary docking, a higher affinity domain II binding mechanism occurs, which is critical for insecticidal activity.

Soluble L-selectin levels in type I diabetes mellitus: a surrogate marker for disease activity?

L-selectin (CD62L) is a cell adhesion molecule which plays a key role in the initiation of leucocyte migration from blood vessels to sites of local inflammation. The aim of this study was to investigate T-lymphocyte expression of CD62L antigen and serum levels of soluble L-selectin (sL-selectin) in subjects with clinical and preclinical type I diabetes to determine whether they could provide surrogate markers for disease activity. CD62L selectin expression on memory T lymphocytes was studied by cytometric analysis in 22 patients with newly diagnosed type I diabetes, 20 first-degree relatives of patients with type I diabetes, 14 patients with Graves' disease, and 22 healthy controls. sL-selectin levels were measured by enzyme-linked immunosorbent assay (ELISA) in enlarged groups of subjects in these categories, as well as in patients with long-standing type I diabetes, treated Graves' disease and type II (non-insulin dependent) diabetes. L-selectin levels were also related to islet autoantibodies, human leucocyte antigen (HLA) genotype and L-selectin T668C gene polymorphisms. L-selectin expression on memory T lymphocytes was reduced in newly diagnosed diabetes and islet autoantibody positive siblings compared with controls. sL-selectin levels were significantly raised in newly diagnosed type I diabetes compared with controls, with intermediate levels in family members, both with and without islet autoantibodies, and in long-standing type I diabetes. Levels were also raised in patients with untreated Graves' disease. Patients with type II diabetes had sL-selectin levels which did not differ from controls. sL-selectin levels correlated with the presence of diabetes-associated HLA alleles in both family members and controls; levels also fell with increasing age in family members. Multiple regression analysis showed that HLA genotype and age were independent determinants of sL-selectin levels. sL-selectin levels are raised at the time of diagnosis of type I diabetes and Graves' disease and appear to be modulated by disease activity, but levels are determined predominantly by HLA-associated genetic susceptibility and age. sL-selectin may provide a late marker of autoimmune destruction of islets and sequential measurement may be useful in monitoring disease activity and the effect of interventions preceding type I diabetes.

Continuous weak-affinity immunosensing

A multitude of weak biological interactions, either working alone or in concert, occur frequently throughout biological systems. We have used this natural feature of readily reversible interactions as the basis for continuous immunosensing. In a model system, a set of weak monoclonal antibodies directed towards a carbohydrate epitope was studied with the aid of surface plasmon resonance. Because the system requires no regeneration, it can be used as a truly on-line immunosensing device. This principle should have wide application in all areas where there is a need for the continuous evaluation of a molecule.

Selectin-carbohydrate interactions in shear flow

Hydrodynamic shear creates mechanical stresses on selectin bonds, modulating affinity and kinetic parameters. Chemical modification of sialyl Lewis(x) increases the strength of L-selectin bonds without altering recognition, suggesting that mechanical and biorecognition characteristics are separable. L-selectin bond formation rates may be strongly influenced by sulfate orientation in sulfo sialyl Lewis(x).

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

A novel glycosulfopeptide binds to P-selectin and inhibits leukocyte adhesion to P-selectin

P-selectin glycoprotein ligand-1 (PSGL-1) is a dimeric membrane mucin on leukocytes that binds selectins. The molecular features of PSGL-1 that determine this high affinity binding are unclear. Here we demonstrate the in vitro synthesis of a novel glycosulfopeptide (GSP-6) modeled after the extreme N terminus of PSGL-1, which has been predicted to be important for P-selectin binding. GSP-6 contains three tyrosine sulfate (TyrSO(3)) residues and a monosialylated, core 2-based O-glycan with a sialyl Lewis x (C2-O-sLe(x)) motif at a specific Thr residue. GSP-6 binds tightly to immobilized P-selectin, whereas glycopeptides lacking either TyrSO(3) or C2-O-sLe(x) do not detectably bind. Remarkably, an isomeric glycosulfopeptide to GSP-6, termed GSP-6', which contains sLe(x) on an extended core 1-based O-glycan, does not bind immobilized P-selectin. Equilibrium gel filtration analysis revealed that GSP-6 binds to soluble P-selectin with a K(d) of approximately 350 nM. GSP-6 (<5 microM) substantially inhibits neutrophil adhesion to P-selectin in vitro, whereas free sLe(x) (5 mM) only slightly inhibits adhesion. In contrast to the inherent heterogeneity of post-translational modifications of recombinant proteins, glycosulfopeptides permit the placement of sulfate groups and glycans of precise structure at defined positions on a polypeptide. This approach should expedite the probing of structure-function relationships in sulfated and glycosylated proteins, and may facilitate development of novel drugs to treat inflammatory diseases involving P-selectin-mediated leukocyte adhesion.

Distinct selectin ligands on colon carcinoma mucins can mediate pathological interactions among platelets, leukocytes, and endothelium

Selectins are adhesion molecules that mediate calcium-dependent cell-cell interactions among leukocytes, platelets, and endothelial cells. The naturally occurring vascular ligands for the selectins are mostly mucin-type glycoproteins. Increased expression and altered glycosylation of mucins are known to be prominent features of carcinoma progression. We have previously shown that all three selectins bind to colon carcinoma cell lines in a calcium-dependent fashion and that carcinoma growth and metastasis formation are attenuated in P-selectin-deficient mice. Here we show that the three recombinant soluble selectins recognize ligands within primary colon carcinoma tissue samples. Affinity chromatography showed that the ligands for all three selectins are O-sialoglycoprotease-sensitive mucins that are recognized in a calcium- and sialic acid-dependent manner. Furthermore, there are separate binding sites on the mucins for each selectin, allowing cross-binding of a single mucin molecule by more than one selectin. We also show that the selectin ligands on purified carcinoma mucins can mediate at least four different pathological interactions among platelets, leukocytes, and endothelial cells. These findings could explain some of the adhesive events of blood-borne tumor cells reported to occur with leukocytes, platelets, and endothelial cells, which are believed to play a part in modulating some early events in tumor metastases.

Kinetic measurements of binding of galectin 3 to a laminin substratum

Galectin 3, a beta-galactoside binding protein, contains a C-terminal carbohydrate recognition domain (CRD) and an N-terminal segment including multiple repeats of a proline/tyrosine/glycine-rich motif. Previous work has shown that galectin 3 but not the isolated CRD binds to laminin, a multivalent ligand, with positive cooperativety indicating the formation of multiple interactions although the lectin in solution is monomeric. Using surface plasmon resonance, we find that hamster galectin 3 at sub-micromolar concentrations or its isolated CRD at all concentrations binds to a laminin substratum with similar association (k(ass); 10-30,000 M(-1) S(-1)) and dissociation (k(diss); 0.2-0.3 S1(-1)) rates and weak affinity (Ka; 1-3 x 10(5) M(-1)). At higher concentrations of galectin 3 the off rate decreases ten fold leading to increased affinity. Ligation of an N-terminal epitope of galectin 3 with a monoclonal Fab fragment increases association and dissociation rates ten fold. A recombinant protein obtained by deletion of the first 93 N-terminal residues binds to laminin with positive cooperativity and a slowly dissociating fraction (K(diss); 0.002 S(-1)) accumulates on the substratum. The data suggest that homophilic interactions between CRD as well as N terminal domains are implicated in galectin 3 aggregation on the substratum leading to positive binding cooperativity.

Structure of a heterophilic adhesion complex between the human CD2 and CD58 (LFA-3) counterreceptors

Interaction between CD2 and its counterreceptor, CD58 (LFA-3), on opposing cells optimizes immune recognition, facilitating contacts between helper T lymphocytes and antigen-presenting cells as well as between cytolytic effectors and target cells. Here, we report the crystal structure of the heterophilic adhesion complex between the amino-terminal domains of human CD2 and CD58. A strikingly asymmetric, orthogonal, face-to-face interaction involving the major beta sheets of the respective immunoglobulin-like domains with poor shape complementarity is revealed. In the virtual absence of hydrophobic forces, interdigitating charged amino acid side chains form hydrogen bonds and salt links at the interface (approximately 1200 A2), imparting a high degree of specificity albeit with low affinity (K(D) of approximately microM). These features explain CD2-CD58 dynamic binding, offering insights into interactions of related immunoglobulin superfamily receptors.

Leukocyte adhesion: High-speed cells with ABS

In order to decide where to exit blood vessels and enter tissues, leukocytes roll along endothelial surfaces. Recent studies suggest that an 'automatic braking system' (ABS), involving selectin cell-adhesion molecules, enables leukocytes to roll at a fairly constant velocity despite large variations in blood flow rate.

Sulfotransferases of two specificities function in the reconstitution of high endothelial cell ligands for L-selectin

L-selectin, a lectin-like receptor, mediates rolling of lymphocytes on high endothelial venules (HEVs) in secondary lymphoid organs by interacting with HEV ligands. These ligands consist of a complex of sialomucins, candidates for which are glycosylation- dependent cell adhesion molecule 1 (GlyCAM-1), CD34, and podocalyxin. The ligands must be sialylated, fucosylated, and sulfated for optimal recognition by L-selectin. Our previous structural characterization of GlyCAM-1 has demonstrated two sulfation modifications, Gal-6-sulfate and GlcNAc-6-sulfate in the context of sialyl Lewis x. We now report the cloning of a Gal-6-sulfotransferase and a GlcNAc-6-sulfotransferase, which can modify GlyCAM-1 and CD34. The Gal-6-sulfotransferase shows a wide tissue distribution. In contrast, the GlcNAc-6-sulfotransferase is highly restricted to HEVs, as revealed by Northern analysis and in situ hybridization. Expression of either enzyme in Chinese hamster ovary cells, along with CD34 and fucosyltransferase VII, results in ligand activity, as detected by binding of an L-selectin/IgM chimera. When coexpressed, the two sulfotransferases synergize to produce strongly enhanced chimera binding.

Structure and function of the selectin ligand PSGL-1

P-selectin glycoprotein ligand-1 (PSGL-1) is a dimeric mucin-like 120-kDa glycoprotein on leukocyte surfaces that binds to P- and L-selectin and promotes cell adhesion in the inflammatory response. The extreme amino terminal extracellular domain of PSGL-1 is critical for these interactions, based on site-directed mutagenesis, blocking monoclonal antibodies, and biochemical analyses. The current hypothesis is that for high affinity interactions with P-selectin, PSGL-1 must contain O-glycans with a core-2 branched motif containing the sialyl Lewis x antigen (NeuAc alpha 2-->3Gal beta 1-->4[Fuc alpha 1-->3]GlcNAc beta 1-->R). In addition, high affinity interactions require the co-expression of tyrosine sulfate on tyrosine residues near the critical O-glycan structure. This review addresses the biochemical evidence for this hypothesis and the evidence that PSGL-1 is an important in vivo ligand for cell adhesion.

Adhesion energy of receptor-mediated interaction measured by elastic deformation

We investigated the role of receptor binding affinity in surface adhesion. A sensitive technique was developed to measure the surface energy of receptor-mediated adhesion. The experimental system involved a functionalized elastic agarose bead resting on a functionalized glass coverslip. Attractive intersurface forces pulled the two surfaces together, deforming the bead to produce an enlarged contact area. The Johnson-Kendall-Roberts (JKR) model was used to relate the surface energy of the interaction to the elasticity of the bead and the area of contact. The surface energies for different combinations of modified surfaces in solution were obtained from reflection interference contrast microscopy (RICM) measurements of the contact area formed by the bead and the coverslip. Studies with surfaces functionalized with ligand-receptor pairs showed that the relationship between surface energy and the association constant of the ligand binding has two regimes. At low binding affinity, surface energy increased linearly with the association constant, while surface energy increased logarithmically with the association constant in the high affinity regime.

Inhibition of L-selectin-mediated leukocyte rolling by synthetic glycoprotein mimics

Synthetic carbohydrate and glycoprotein mimics displaying sulfated saccharide residues have been assayed for their L-selectin inhibitory properties under static and flow conditions. Polymers displaying the L-selectin recognition epitopes 3',6-disulfo Lewis x(Glc) (3-O-SO3-Galbeta1alpha4(Fucalpha1alpha3)-6-O-SO3-Glcbeta+ ++-OR) and 3',6'-disulfo Lewis x(Glc) (3, 6-di-O-SO3-Galbeta1alpha4(Fucalpha1alpha3)Glcbeta-OR) both inhibit L-selectin binding to heparin under static, cell-free binding conditions with similar efficacies. Under conditions of shear flow, however, only the polymer displaying 3',6-disulfo Lewis x(Glc) inhibits the rolling of L-selectin-transfected cells on the glycoprotein ligand GlyCAM-1. Although it has been shown to more effective than sialyl Lewis x at blocking the L-selectin-GlyCAM-1 interaction in static binding studies, the corresponding monomer had no effect in the dynamic assay. These data indicate that multivalent ligands are far more effective inhibitors of L-selectin-mediated rolling than their monovalent counterparts and that the inhibitory activities are dependent on the specific sulfation pattern of the recognition epitope. Importantly, our results indicate the L-selectin specificity for one ligand over another found in static, cell-free binding assays is not necessarily retained under the conditions of shear flow. The results suggest that monovalent or polyvalent carbohydrate or glycoprotein mimetics that inhibit selectin binding in static assays may not block the more physiologically relevant process of selectin-mediated rolling.

T cell receptor and coreceptor CD8 alphaalpha bind peptide-MHC independently and with distinct kinetics

The T cell surface glycoprotein CD8 enhances T cell antigen recognition by binding to MHC class I molecules. We show that human CD8 alphaalpha binds to the MHC class I molecule HLA-A2 with an extremely low affinity (Kd approximately 0.2 mM at 37 degrees C) and with kinetics that are between 2 and 3 orders of magnitude faster than reported for T cell receptor/peptide-MHC interactions. Furthermore, CD8 alphaalpha had no detectable effect on a T cell receptor (TCR) binding to the same peptide-MHC class I complex. These binding properties provide an explanation as to why the CD8/MHC class I interaction is unable to initiate cell-cell adhesion and how it can enhance TCR recognition without interfering with its specificity.

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.

Mechanisms that regulate the function of the selectins and their ligands

Selectins are a family of three cell adhesion molecules (L-, E-, and P-selectin) specialized in capturing leukocytes from the bloodstream to the blood vessel wall. This initial cell contact is followed by the selectin-mediated rolling of leukocytes on the endothelial cell surface. This represents the first step in a cascade of molecular interactions that lead to leukocyte extravasation, enabling the processes of lymphocyte recirculation and leukocyte migration into inflamed tissue. The central importance of the selectins in these processes has been well documented in vivo by the use of adhesion-blocking antibodies as well as by studies on selectin gene-deficient mice. This review focuses on the molecular mechanisms that regulate expression and function(s) of the selectins and their ligands. Cell-surface expression of the selectins is regulated by a variety of different mechanisms. The selectins bind to carbohydrate structures on glycoproteins, glycolipids, and proteoglycans. Glycoproteins are the most likely candidates for physiologically relevant ligands. Only a few glycoproteins are appropriately glycosylated to allow strong binding to the selectins. Recently, more knowledge about the structure and the regulated expression of some of the carbohydrates on these ligands necessary for selectin binding has been accumulated. For at least one of these ligands, the physiological function is now well established. A novel and exciting aspect is the signaling function of the selectins and their ligands. Especially in the last two years, convincing data have been published supporting the idea that selectins and glycoprotein ligands of the selectins participate in the activation of leukocyte integrins.

GlyCAM-1 supports leukocyte rolling in flow: evidence for a greater dynamic stability of L-selectin rolling of lymphocytes than of neutrophils

L-selectin plays a major role in leukocyte traffic through lymph node high endothelial venules (HEV). We have investigated the role of GlyCAM-1, a major L-selectin ligand produced by HEV, in mediating leukocyte rolling under in vitro flow conditions. Purified GlyCAM-1 was found to support tethering and rolling in physiological shear flow of both human and murine L-selectin expressing leukocytes at an efficiency comparable to the HEV-derived L-selectin ligands termed peripheral node addressin (PNAd). Major dynamic differences between L-selectin rolling of peripheral blood T lymphocytes and neutrophils expressing similar L-selectin level were observed on GlyCAM-1. Lymphocytes established slower and more shear resistant rolling than neutrophils and could roll on GlyCAM-1 at shear stresses lower than the threshold values required for L-selectin-mediated neutrophil rolling. Notably, high stability of L-selectin rolling of lymphocytes requires intact cellular energy, although initial lymphocyte tethering to L-selectin ligands is energy-independent. By contrast, L-selectin mediated rolling of neutrophils is insensitive to energy depletion. The distinct dynamic behavior and energy-dependence of L-selectin rolling in different leukocytes suggest that L-selectin adhesiveness in shear flow is regulated in a cell-type specific manner. The greater stability of L-selectin rolling of lymphocytes on surface-adsorbed GlyCAM-1 may contribute to their selective recruitment at peripheral lymph nodes.

Affinity and kinetic analysis of P-selectin binding to P-selectin glycoprotein ligand-1

Leukocytes use the cell-surface mucin P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin on activated endothelial cells and platelets. By using surface plasmon resonance, we measured the affinity and kinetics of binding of soluble monomeric human P-selectin to immobilized PSGL-1 from human neutrophils. Binding was specific, as documented by its Ca2+-dependence, its inhibition by specific monoclonal antibodies to P-selectin and PSGL-1, and its abrogation by treating PSGL-1 with sialidase. Similar binding was observed for soluble P-selectin that contained the lectin and epidermal growth factor domains plus all nine consensus repeats, and for a soluble construct that contained only the lectin and epidermal growth factor domains. Soluble P-selectin bound saturably to a single class of sites on PSGL-1 with a dissociation constant (Kd) of 320 +/- 20 nM. The measured koff was 1.4 +/- 0.1 s-1, and the calculated kon was 4.4 x 10(6) M-1 s-1. We conclude that monomeric P-selectin binds to PSGL-1 with fast association and dissociation rates and relatively high affinity. These features may be important for efficient tethering and rolling of leukocytes at physiologic densities of PSGL-1 and P-selectin.

Affinity and Kinetic Analysis of the Molecular Interaction of ICAM-1 and Leukocyte Function-Associated Antigen-1

LFA-1 is a member of the β2 integrin family, and interacts with ICAM-1, a member of the Ig superfamily containing five Ig-like domains. Interaction of LFA-1 with ICAM-1 is important in a number of cellular events, including Ag-specific T cell activation and leukocyte transendothelial migration, which are known to be typically transient and highly regulated. In this study, we have used surface plasmon resonance technology to study the ICAM-1/LFA-1 interaction at the molecular level. A soluble form of LFA-1 (sLFA-1), normally expressed as two noncovalently associated membrane-bound subunits, has been produced, and its interaction with ICAM-1 has been examined. The kinetic analysis of a monomeric sLFA-1 binding to the first two domains of ICAM-1 expressed as a chimeric IgG fusion protein (D1D2-IgG) revealed that sLFA-1 was bound to the D1D2-IgG chimera with a Kd of 500 nM and dissociated with a kdiss of 0.1 s−1. Monomeric membrane-bound LFA-1 purified from plasma membranes showed a similar kinetic to sLFA-1. These results suggest that the monovalent interaction between ICAM-1 and LFA-1 has a primarily high affinity and a slow dissociation rate constant as compared with other adhesion molecules, suggesting a potential mechanism for firm adhesion.

Regulation of L-selectin-mediated rolling through receptor dimerization

L-selectin binding activity for its ligand expressed by vascular endothelium is rapidly and transiently increased after leukocyte activation. To identify mechanisms for upregulation and assess how this influences leukocyte/endothelial cell interactions, cell-surface dimers of L-selectin were induced using the coumermycin-GyrB dimerization strategy for cross-linking L-selectin cytoplasmic domains in L-selectin cDNA-transfected lymphoblastoid cells. Coumermycin- induced L-selectin dimerization resulted in an approximately fourfold increase in binding of phosphomanan monoester core complex (PPME), a natural mimic of an L-selectin ligand, comparable to that observed after leukocyte activation. Moreover, L-selectin dimerization significantly increased (by approximately 700%) the number of lymphocytes rolling on vascular endothelium under a broad range of physiological shear stresses, and significantly slowed their rolling velocities. Therefore, L-selectin dimerization may explain the rapid increase in ligand binding activity that occurs after leukocyte activation and may directly influence leukocyte migration to peripheral lymphoid tissues or to sites of inflammation. Inducible oligomerization may also be a common mechanism for rapidly upregulating the adhesive or ligand-binding function of other cell-surface receptors.

The kinetics and shear threshold of transient and rolling interactions of L-selectin with its ligand on leukocytes

The kinetics of rolling and transient adhesions through selectins may depend on the kinetics and mechanical properties of the selectin:ligand bond, as well as on cellular properties including receptor-anchoring to the cell membrane and cytoskeleton. Kinetics are known to depend on the selectin and may also be ligand dependent. Here, we study the kinetics of transient and rolling interactions of leukocytes with L-selectin immobilized on a substrate. Remarkably, all properties examined are similar to those seen when the sidedness is opposite, i.e., when the L-selectin ligand is on the substrate and when the ligand is isolated from HEV rather than present on leukocytes. The similar properties include rolling velocity, a threshold shear stress above 0.4 dyn/cm2 required to support rolling, a k degreesoff of 7.0 to 6.8 s-1 for the L-selectin tether bond, and a mechanical bond length of 0.24 to 0.20 A. Our results argue against a model in which L-selectin shedding mediates rolling. Furthermore, the fast and force-resistant kinetic properties suggest that L-selectin is specialized dynamically for tethering leukocytes to vessel walls and adherent leukocytes.

Kinetics of interaction of HLA-C ligands with natural killer cell inhibitory receptors

The recognition of HLA-C molecules by specific inhibitory receptors is a crucial step in the regulation of natural killer (NK) cell function. Using soluble, recombinant HLA-C molecules and NK inhibitory receptors (NKIR, members of the immunoglobulin superfamily), we show that HLA-C binds to NKIR molecules with extremely fast association and dissociation rates, among the fastest of the immune system interactions so far studied. These kinetics may be essential for the biological function of NK cells, i.e., to facilitate the rapid immunosurveillance of cells for absent or diminished expression of class I MHC proteins.

Dimerization of P-selectin glycoprotein ligand-1 (PSGL-1) required for optimal recognition of P-selectin

Interactions between P-selectin, expressed on endothelial cells and activated platelets, and its leukocyte ligand, a homodimer termed P-selectin glycoprotein ligand-1 (PSGL-1), mediate the earliest adhesive events during an inflammatory response. To investigate whether dimerization of PSGL-1 is essential for functional interactions with P-selectin, a mutant form of PSGL-1 was generated in which the conserved membrane proximal cysteine was mutated to alanine (designated C320A). Western blotting under both denaturing and native conditions of the C320A PSGL-1 mutant isolated from stably transfected cells revealed expression of only a monomeric form of PSGL-1. In contrast to cells cotransfected with alpha1-3 fucosyltransferase-VII (FucT-VII) plus PSGL-1, K562 cells expressing FucT-VII plus C320A failed to bind COS cells transfected with P-selectin in a low shear adhesion assay, or to roll on CHO cells transfected with P-selectin under conditions of physiologic flow. In addition, C320A transfectants failed to bind chimeric P-selectin fusion proteins. Both PSGL-1 and C320A were uniformly distributed on the surface of transfected K562 cells. Thus, dimerization of PSGL-1 through the single, conserved, extracellular cysteine is essential for functional recognition of P-selectin.

CD2 and the nature of protein interactions mediating cell-cell recognition

Rapid progress has recently been made in characterising the structures of leukocyte cell-surface molecules. Detailed analyses of the structure and interactions of CD2 were the first involving a molecule that has not been directly linked to antigen recognition in the manner of antigen receptors or co-receptors. It seems highly likely that the properties of ligand binding by CD2 are relevant to the general mechanisms of cell-cell recognition. As an example of biological recognition, the defining characteristic of cell-cell contact is that it involves the simultaneous interaction of hundreds, if not thousands, of molecules. Affinity and kinetic analyses of ligand binding by CD2 indicated that the protein interactions mediating cell-cell contact, whilst highly specific, are much weaker than initially anticipated, probably due to the requirement that such contacts be easily reversible. Simultaneously, in addressing the mechanism of this mode of recognition, structural and mutational studies focussed on the role of charged residues clustered in the ligand-binding face of CD2, yielding the concept that electrostatic complementarity, rather than surface-shape complementarity, is the dominant feature of specific, low-affinity protein recognition at the cell surface by CD2. The crystallographic analysis of the CD2-binding domain of CD58 strongly supports this concept.

The role of charged residues mediating low affinity protein-protein recognition at the cell surface by CD2

Insights into the structural basis of protein-protein recognition have come principally from the analysis of proteins such as antibodies, hormone receptors, and proteases that bind their ligands with relatively high affinity (Ka approximately 10(9) M-1). In contrast, few studies have been done on the very low affinity interactions mediating cell adhesion and cell-cell recognition. As a site of protein-protein recognition, the ligand binding face of the T lymphocyte cell-cell recognition molecule, CD2, which binds its ligands 10(4)- to 10(5)-fold more weakly than do antibodies and proteases, is unusual in being both very flat and highly charged. An analysis of the effect of mutations and ionic strength on CD2 binding to its ligand, CD48, indicates that these charged residues contribute little, if any, binding energy to this interaction. However, the loss of these charged residues is shown to markedly reduce ligand-binding specificity. Thus, the charged residues increase the specificity of CD2 binding without increasing the affinity. This phenomenon is likely to result from a requirement for electrostatic complementarity between charged binding surfaces to compensate for the removal, upon binding, of water interacting with the charged residues. It is proposed that this mode of recognition is highly suited to biological interactions requiring a low affinity because it uncouples increases in specificity from increases in affinity.

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.