Cytoplasmic anchorage of L-selectin controls leukocyte capture and rolling by increasing the mechanical stability of the selectin tether

Wound Healing from an Actin Cytoskeletal Perspective

Wound healing requires a complex cascade of highly controlled and conserved cellular and molecular processes. These involve numerous cell types and extracellular matrix molecules regulated by the actin cytoskeleton. This microscopic network of filaments is present within the cytoplasm of all cells and provides the shape and mechanical support required for cell movement and proliferation. Here, an overview of the processes of wound healing are described from the perspective of the cell in relation to the actin cytoskeleton. Key points of discussion include the role of actin, its binding proteins, signaling pathways, and events that play significant roles in the phases of wound healing. The identification of cytoskeletal targets that can be used to manipulate and improve wound healing is included as an emerging area of focus that may inform future therapeutic approaches to improve healing of complex wounds.

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

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

PTENα promotes neutrophil chemotaxis through regulation of cell deformability

Neutrophils are a major component of immune defense and are recruited through neutrophil chemotaxis in response to invading pathogens. However, the molecular mechanism that controls neutrophil chemotaxis remains unclear. Here, we report that PTENα, the first isoform identified in the PTEN family, regulates neutrophil deformability and promotes chemotaxis of neutrophils. A high level of PTENα is detected in neutrophils and lymphoreticular tissues. Homozygous deletion of PTENα impairs chemoattractant-induced migration of neutrophils. We show that PTENα physically interacts with cell membrane cross-linker moesin through its FERM domain and dephosphorylates moesin at Thr558, which disrupts the association of filamentous actin with the plasma membrane and subsequently induces morphologic changes in neutrophil pseudopodia. These results demonstrate that PTENα acts as a phosphatase of moesin and modulates neutrophil-mediated host immune defense. We propose that PTENα signaling is a potential target for the treatment of infections and immune diseases.

Neutrophil Cell Shape Change: Mechanism and Signalling during Cell Spreading and Phagocytosis

Perhaps the most important feature of neutrophils is their ability to rapidly change shape. In the bloodstream, the neutrophils circulate as almost spherical cells, with the ability to deform in order to pass along narrower capillaries. Upon receiving the signal to extravasate, they are able to transform their morphology and flatten onto the endothelium surface. This transition, from a spherical to a flattened morphology, is the first key step which neutrophils undergo before moving out of the blood and into the extravascular tissue space. Once they have migrated through tissues towards sites of infection, neutrophils carry out their primary role-killing infecting microbes by performing phagocytosis and producing toxic reactive oxygen species within the microbe-containing phagosome. Phagocytosis involves the second key morphology change that neutrophils undergo, with the formation of pseudopodia which capture the microbe within an internal vesicle. Both the spherical to flattened stage and the phagocytic capture stage are rapid, each being completed within 100 s. Knowing how these rapid cell shape changes occur in neutrophils is thus fundamental to understanding neutrophil behaviour. This article will discuss advances in our current knowledge of this process, and also identify an important regulated molecular event which may represent an important target for anti-inflammatory therapy.

A head-to-tail view of L-selectin and its impact on neutrophil behaviour

L-selectin is a type I transmembrane cell adhesion molecule expressed on most circulating leukocytes, including neutrophils. Engagement of L-selectin with endothelial-derived ligands initiates neutrophil tethering and rolling behaviour along luminal walls of post-capillary venules, constituting the first step of the multi-step adhesion cascade. There is a large body of evidence to suggest that signalling downstream of L-selectin can influence neutrophil behaviour: adhesion, migration and priming. This review will cover aspects of L-selectin form and function and introduce the “triad of L-selectin regulation”, highlighting the inextricable links between adhesion, signalling and ectodomain shedding and also highlighting the cytosolic proteins that interconnect them. Recent advances in how L-selectin impacts priming, transendothelial migration (TEM) and cell polarity will also be discussed.

Detection of bidirectional signaling during integrin activation and neutrophil adhesion

Neutrophil arrest and migration on inflamed endothelium is dependent upon a conformational shift in CD11a/CD18 (LFA-1) from a low to high affinity and clustered state which determines the strength and lifetime of bond formation with intracellular adhesion molecule 1 (ICAM-1). Cytoskeletal adaptor proteins kindlin-3 and talin-1 anchor clustered LFA-1 to the cytoskeleton and support the transition from neutrophil rolling to arrest. We employ microfluidic flow channels and total internal reflection fluorescence microscopy to evaluate the spatiotemporal regulation of LFA-1 affinity and bond formation that facilitate the transition from neutrophil rolling to arrest. Methodology is presented to correlate the relationship between integrin conformation, bond formation with ICAM-1, and cytoskeletal engagement and adhesion strengthening necessary to achieve a migratory phenotype.

The PSGL-1-L-selectin signaling complex regulates neutrophil adhesion under flow

Neutrophils are recruited from the blood to sites of inflammation, where they contribute to immune defense but may also cause tissue damage. During inflammation, neutrophils roll along the microvascular endothelium before arresting and transmigrating. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen 1 (LFA-1), which can be induced by selectin engagement. Here, we demonstrate that a subset of P-selectin glycoprotein ligand-1 (PSGL-1) molecules is constitutively associated with L-selectin. Although this association does not require the known lectin-like interaction between L-selectin and PSGL-1, the signaling output is dependent on this interaction and the cytoplasmic tail of L-selectin. The PSGL-1-L-selectin complex signals through Src family kinases, ITAM domain-containing adaptor proteins, and other kinases to ultimately result in LFA-1 activation. The PSGL-1-L-selectin complex-induced signaling effects on neutrophil slow rolling and recruitment in vivo demonstrate the functional importance of this pathway. We conclude that this is a signaling complex specialized for sensing adhesion under flow.

Signals regulating L-selectin-dependent leucocyte adhesion and transmigration

L-selectin is a type I transmembrane cell adhesion molecule that is expressed on the surface of most circulating leukocytes. Studies in L-selectin knockout mice reveal a prominent role for this glycoprotein in health and disease, regulating leucocyte recruitment to peripheral lymph nodes (e.g. naïve T-cells) and sites of acute and chronic inflammation (e.g. monocytes and neutrophils). Clinical trials have revealed L-selectin as a promising target in some acute and chronic inflammatory diseases. Unearthing the intracellular signals that act directly downstream of L-selectin may also expose novel therapeutic targets in a cell type/disease-specific manner. This review will focus on L-selectin-dependent signalling - exploring the different signals that potentially arise from distinct phases of the multi-step adhesion cascade and the contribution of known binding partners of L-selectin in this response.

Chemokines, selectins and intracellular calcium flux: temporal and spatial cues for leukocyte arrest

Leukocyte trafficking to acute sites of injury or infection requires spatial and temporal cues that fine tune precise sites of firm adhesion and guide migration to endothelial junctions where they undergo diapedesis to sites of insult. Many detailed studies on the location and gradient of chemokines such as IL-8 and other CXCR ligands reveal that their recognition shortly after selectin-mediated capture and rolling exerts acute effects on integrin activation and subsequent binding to their ligands on the endothelium, which directs firm adhesion, adhesion strengthening, and downstream migration. In this process, G-protein coupled receptor (GPCR) signaling has been found to play an integral role in activating and mobilizing intracellular stores of calcium, GTPases such as Rap-1 and Rho and cytokeletal proteins such as Talin and F-actin to facilitate cell polarity and directional pseudopod formation. A critical question remaining is how intracellular Ca(2+) flux from CRAC channels such as Orai1 synergizes with cytosolic stores to mediate a rapid flux which is critical to the onset of PMN arrest and polarization. Our review will highlight a specific role for calcium as a signaling messenger in activating focal clusters of integrins bound to the cytoskeleton which allows the cell to attain a migratory phenotype. The precise interplay between chemokines, selectins, and integrins binding under the ubiquitous presence of shear stress from blood flow provides an essential cooperative signaling mechanism for effective leukocyte recruitment.

The shedding of CD62L (L-selectin) regulates the acquisition of lytic activity in human tumor reactive T lymphocytes

CD62L/L-selectin is a marker found on naïve T cells and further distinguishes central memory (Tcm, CD62L+) from effector memory (Tem, CD62L-) T cells. The regulation of CD62L plays a pivotal role in controlling the traffic of T lymphocytes to and from peripheral lymph nodes. CD62L is shed from the cell membrane following T cell activation, however, the physiological significance of this event remains to be elucidated. In this study, we utilized in vitro generated anti-tumor antigen T cells and melanoma lines as a model to evaluate the dynamics of CD62L shedding and expression of CD107a as a marker of lytic activity. Upon encounter, with matched tumor lines, antigen reactive T cells rapidly lose CD62L expression and this was associated with the acquisition of CD107a. By CD62L ELISA, we confirmed that this transition was mediated by the shedding of CD62L when T cells encountered specific tumor antigen. The introduction of a shedding resistant mutant of CD62L into the tumor antigen-reactive T cell line JKF6 impaired CD107a acquisition following antigen recognition and this was correlated with decreased lytic activity as measured by (51)Cr release assays. The linkage of the shedding of CD62L from the surface of anti-tumor T cells and acquisition of lytic activity, suggests a new function for CD62L in T cell effector functions and anti-tumor activity.

Kindlin-3 is required for the stabilization of TCR-stimulated LFA-1:ICAM-1 bonds critical for lymphocyte arrest and spreading on dendritic cells

Kindlin-3 is a key lymphocyte function–associated antigen-1 (LFA-1) coactivator deleted in leukocyte adhesion deficiency-III (LAD-III). In the present study, we investigated the involvement of this adaptor in lymphocyte motility and TCR-triggered arrest on ICAM-1 or on dendritic cells (DCs). Kindlin-3–null primary T cells from a LAD-III patient migrated normally on the major lymph node chemokine CCL21 and engaged in normal TCR signaling. However, TCR activation of Kindlin-3–null T lymphocytes failed to trigger the robust LFA-1–mediated T-cell spreading on ICAM-1 and ICAM-1–expressing DCs that is observed in normal lymphocytes. Kindlin-3 was also essential for cytoskeletal anchorage of the LFA-1 heterodimer and for microclustering of LFA-1 within ventral focal dots of TCR-stimulated lymphocytes spread on ICAM-1. Surprisingly, LFA-1 on Kindlin-3–null lymphocytes migrating over CCL21 acquired normal expression of an epitope associated with the conformational activation of the key headpiece domain, β I. This activated LFA-1 was highly responsive to TCR-triggered ICAM-1–driven stop signals in normal T cells locomoting on CCL21, but not in their Kindlin-3–null T-cell counterparts. We suggest that Kindlin-3 selectively contributes to a final TCR-triggered outside-in stabilization of bonds generated between chemokine-primed LFA-1 molecules and cell-surface ICAM-1.

Adam17-dependent shedding limits early neutrophil influx but does not alter early monocyte recruitment to inflammatory sites

TNF-α-converting enzyme (TACE, herein denoted as Adam17) proteolytically sheds several cell-surface inflammatory proteins, but the physiologic importance of the cleavage of these substrates from leukocyte subsets during inflammation is incompletely understood. In this study, we show that Adam17-null neutrophils have a 2-fold advantage in their initial recruitment during thioglycollate-induced peritonitis, and they roll slower and adhere more readily in the cremaster model than wild-type neutrophils. Although CD44 and ICAM-1 are both in vitro substrates of Adam17, their surface levels are not altered on Adam17-null neutrophils. In contrast, L-selectin levels are elevated up to 10-fold in Adam17-null circulating neutrophils, and their accelerated peritoneal influx, slower rolling, and increased adhesion in the cremaster muscle are dependent on L-selectin. Analysis of mixed chimeras shows that enhanced L-selectin levels and accelerated influx were both cell-intrinsic properties of neutrophils lacking Adam17. In contrast, Adam17-null monocytes display no acceleration of infiltration into the peritoneum in spite of elevated L-selectin surface levels, and their peritoneal influx was independent of L-selectin. Therefore, our data demonstrate substrate and myeloid cell-type specificity of Adam17-mediated cleavage of its substrates, and show that neutrophils and monocytes use distinct mechanisms for infiltration of tissues.

Role of c-Abl in L-selectin shedding from the neutrophil surface

L-selectin is a key molecule that participates in neutrophil tethering and subsequent rolling. It is cleaved from the surface of neutrophils activated in the presence of lipopolysaccharides, N-formyl-methionine-leucine-phenylalanine (fMLP), or Interleukin-8 (IL-8). We previously showed that L-selectin is also shed from the neutrophil surface during rolling on sialyl Lewis-x coated surfaces in a force-, ADAM-17 sheddase-, and p38 MAP kinase-dependent manner under flow. c-Abl tyrosine kinase is phosphorylated when L-selectin on the surface of neutrophils is cross-linked with anti-L-selectin antibodies. Here, we study the effect of c-Abl inhibition on L-selectin shedding from primary human neutrophils in static conditions following exposure to fMLP, IL-8, and hypotonic buffer and under flow through sialyl Lewis-x coated microtubes. Results indicate that c-Abl inhibition by STI571 significantly affects neutrophil adhesion via L-selectin, by decreasing the average rolling velocity and increasing the flux of rolling cells. The change in surface receptor expression was verified by flow cytometry. Interestingly, other forms of L-selectin shedding induced by fMLP, IL-8 or osmotic swelling were unaffected by STI571 treatment. These findings implicate the c-Abl signaling molecule in regulating L-selectin mechanical shedding in response to shear stress, setting this type of signaling apart from those triggered by the presence of a hypotonic environment, fMLP, or IL-8. This study sheds light on the role of c-Abl in neutrophil adhesion not previously reported in the literature.

Alpha-actinin: a multidisciplinary protein with important role in B-cell driven autoimmunity

Alpha-actinin (α-actinin) is a ubiquitous cytoskeletal protein, which belongs to the superfamily of filamentous actin (F-actin) crosslinking proteins. It is present in multiple subcellular regions of both muscle and non-muscle cells, including cell-cell and cell-matrix contact sites, cellular protrusions and stress fiber dense regions and thus, it seems to bear multiple important roles in the cell by linking the cytoskeleton to many different transmembrane proteins in a variety of junctions. Four isoforms of human α-actinin have already been identified namely, the "muscles" α-actinin-2 and α-actinin-3 and the "non-muscles" α-actinin-1 and α-actinin-4. The precise functions of α-actinin isoforms as well as the precise role and significance of their binding to F-actin particularly in-vivo, have been elusive. They are generally believed to represent key structural components of large-scale F-actin cohesion in cells required for cell shape and motility. α-Actinin-2 has been implicated in myopathies such as nemalin body myopathy, hypertrophic and dilated cardiomyopathy and it may have at least an indirect pathogenetic role in diseases of the central nervous system (CNS) like schizophrenia, epilepsy, ischemic brain damage, CNS lupus and neurodegenerative disorders. The role of "non-muscle" α-actinins in the kidney seems to be crucial as an essential component of the glomerular filtration barrier. Therefore, they have been implicated in the pathogenesis of familial focal segmental glomerulosclerosis, nephrotic syndrome, IgA nephropathy, focal segmental glomerulosclerosis and minimal change disease. α-Actinin is also expressed on the membrane and cytosol of parenchymal and ductal cells of the liver and it seems that it interacts with hepatitis C virus in an essential way for the replication of the virus. Finally α-actinin, especially α-actinin-4, has been implicated in cancer cell progression and metastasis, as well as the migration of several cell types participating in the immune response. Based on these functions, the accumulating reported evidence of the importance of α-actinin as a target autoantigen in the pathogenesis of autoimmune diseases, particularly systemic lupus erythematosus and autoimmune hepatitis, is also discussed along with the possible perspectives that are potentially emerging from the study of this peculiar molecule in health and disease.

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.

A congenital activating mutant of WASp causes altered plasma membrane topography and adhesion under flow in lymphocytes

Leukocytes rely on dynamic actin-dependent changes in cell shape to pass through blood vessels, which is fundamental to immune surveillance. Wiskott-Aldrich Syndrome protein (WASp) is a hematopoietic cell-restricted cytoskeletal regulator important for modulating cell shape through Arp2/3-mediated actin polymerization. A recently identified WASp(I294T) mutation was shown to render WASp constitutively active in vivo, causing increased filamentous (F)-actin polymerization, high podosome turnover in macrophages, and myelodysplasia. The aim of this study was to determine the effect of WASp(I294T) expression in lymphocytes. Here, we report that lymphocytes isolated from a patient with WASp(I294T), and in a cellular model of WASp(I294T), displayed abnormal microvillar architecture, associated with an increase in total cellular F-actin. Microvillus function was additionally altered as lymphocytes bearing the WASp(I294T) mutation failed to roll normally on L-selectin ligand under flow. This was not because of defects in L-selectin expression, shedding, cytoskeletal anchorage, or membranal positioning; however, under static conditions of adhesion, WASp(I294T)-expressing lymphocytes exhibited altered dynamic interaction with L-selectin ligand, with a significantly reduced rate of adhesion turnover. Together, our results demonstrate that WASp(I294T) significantly affects lymphocyte membrane topography and L-selectin-dependent adhesion, which may be linked to defective hematopoiesis and leukocyte function in affected patients.

Myosin 1G is an abundant class I myosin in lymphocytes whose localization at the plasma membrane depends on its ancient divergent pleckstrin homology (PH) domain (Myo1PH)

Class I myosins, which link F-actin to membrane, are largely undefined in lymphocytes. Mass spectrometric analysis of lymphocytes identified two short tail forms: (Myo1G and Myo1C) and one long tail (Myo1F). We investigated Myo1G, the most abundant in T-lymphocytes, and compared key findings with Myo1C and Myo1F. Myo1G localizes to the plasma membrane and associates in an ATP-releasable manner to the actin-containing insoluble pellet. The IQ+tail region of Myo1G (Myo1C and Myo1F) is sufficient for membrane localization, but membrane localization is augmented by the motor domain. The minimal region lacks IQ motifs but includes: 1) a PH-like domain; 2) a "Pre-PH" region; and 3) a "Post-PH" region. The Pre-PH predicted alpha helices may contribute electrostatically, because two conserved basic residues on one face are required for optimal membrane localization. Our sequence analysis characterizes the divergent PH domain family, Myo1PH, present also in long tail myosins, in eukaryotic proteins unrelated to myosins, and in a probable ancestral protein in prokaryotes. The Myo1G Myo1PH domain utilizes the classic lipid binding site for membrane association, because mutating either of two basic residues in the "signature motif" destroys membrane localization. Mutation of each basic residue of the Myo1G Myo1PH domain reveals another critical basic residue in the beta3 strand, which is shared only by Myo1D. Myo1G differs from Myo1C in its phosphatidylinositol 4,5-bisphosphate dependence for membrane association, because membrane localization of phosphoinositide 5-phosphatase releases Myo1C from the membrane but not Myo1G. Thus Myo1PH domains likely play universal roles in myosin I membrane association, but different isoforms have diverged in their binding specificity.

Molecular basis of leukocyte-endothelium interactions during the inflammatory response

The process of leukocyte extravasation, a critical step in the inflammatory response, involves the migration of leukocytes from the bloodstream towards target tissues, where they exert their effector function. Leukocyte extravasation is orchestrated by the combined action of cellular adhesion receptors and chemotactic factors, and involves radical morphological changes in both leukocytes and endothelial cells. Thus, it constitutes an active process for both cell types and promotes the rapid and efficient influx of leukocytes to inflammatory foci without compromising the integrity of the endothelial barrier. This article provides a review of leukocyte extravasation from both molecular and mechanical points of view, with a particular emphasis on the most recent findings on the topic. It includes a description of newly revealed steps in the adhesion cascade, such as slow rolling motion, intraluminal crawling and alternative pathways for transcellular migration, and discusses the functional role of novel adhesion receptors, the spatiotemporal organization of receptors at the plasma membrane and the signaling pathways that control different phases of the extravasation process.

In Vitro and in Vivo Characterization of Molecular Interactions between Calmodulin, Ezrin/Radixin/Moesin, and L-selectin

L-selectin is a cell adhesion molecule that tethers leukocytes to the luminal walls of venules during inflammation and enables them to roll under the force of blood flow. Clustering of L-selectin during rolling is thought to promote outside-in signals that lead to integrin activation and chemokine receptor expression, ultimately contributing to leukocyte arrest. Several studies have underscored the importance of the L-selectin cytoplasmic tail in functionally regulating adhesion and signaling. Interestingly, the L-selectin tail comprises only 17 amino acids, and yet it is thought to bind simultaneously to several proteins. For example, constitutive association of calmodulin (CaM) and ezrin/radixin/moesin (ERM) to L-selectin confers resistance to proteolysis and microvillar positioning, respectively. In this report we found that recombinant purified CaM and ERM bound non-competitively to the same tail of L-selectin. Furthermore, molecular modeling supported the possibility that CaM, L-selectin, and moesin could form a heterotrimeric complex. Finally, using fluorescence lifetime imaging microscopy to measure fluorescence resonance energy transfer, it was shown that CaM, L-selectin, and ERM could interact simultaneously in vivo. Moreover, L-selectin clustering promoted CaM/ERM interaction in cis (i.e. derived from neighboring L-selectin tails). These results highlight a novel intracellular event that occurs as a consequence of L-selectin clustering, which could participate in transducing signals that promote the transition from rolling to arrest.

T cell adhesion mechanisms revealed by receptor lateral mobility

Cell surface receptors mediate the exchange of information between cells and their environment. In the case of adhesion receptors, the spatial distribution and molecular associations of the receptors are critical to their function. Therefore, understanding the mechanisms regulating the distribution and binding associations of these molecules is necessary to understand their functional regulation. Experiments characterizing the lateral mobility of adhesion receptors have revealed a set of common mechanisms that control receptor function and thus cellular behavior. The T cell provides one of the most dynamic examples of cellular adhesion. An individual T cell makes innumerable intercellular contacts with antigen presenting cells, the vascular endothelium, and many other cell types. We review here the mechanisms that regulate T cell adhesion receptor lateral mobility as a window into the molecular regulation of these systems, and we present a general framework for understanding the principles and mechanisms that are likely to be common among these and other cellular adhesion systems. We suggest that receptor lateral mobility is regulated via four major mechanisms-reorganization, recruitment, dispersion, and anchoring-and we review specific examples of T cell adhesion receptor systems that utilize one or more of these mechanisms.

Mechanical regulation of cell adhesion

Cellular adhesion against external forces is governed by both the equilibrium affinity of the involved receptor-ligand bonds and the mechanics of the cell. Certain receptors like integrins change their affinity as well as the mechanics of their anchorage to tune the adhesiveness. Whereas in the last few years the focus of integrin research has lain on the affinity regulation of the adhesion receptors, more recently the importance of cellular mechanics became apparent. Here, we focus on different aspects of the mechanical regulation of the cellular adhesiveness.

Separable requirements for cytoplasmic domain of PSGL-1 in leukocyte rolling and signaling under flow

In inflamed venules, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to roll on P-selectin and E-selectin and to activate integrin alphaLbeta2 (lymphocyte function-associated antigen-1, LFA-1) to slow rolling on intercellular adhesion molecule-1 (ICAM-1). Studies in cell lines have suggested that PSGL-1 requires its cytoplasmic domain to localize in membrane domains, to support rolling on P-selectin, and to signal through spleen tyrosine kinase (Syk). We generated "DeltaCD" mice that express PSGL-1 without the cytoplasmic domain. Unexpectedly, neutrophils from these mice localized PSGL-1 normally in microvilli, uropods, and lipid rafts. DeltaCD neutrophils expressed less PSGL-1 on their surfaces because of inefficient export from the endoplasmic reticulum. Limited digestion of wild-type neutrophils with O-sialoglycoprotein endopeptidase was used to reduce the PSGL-1 density to that on DeltaCD neutrophils. At matched PSGL-1 densities, both DeltaCD and wild-type neutrophils rolled similarly on P-selectin. However, DeltaCD neutrophils rolling on P-selectin did not trigger Syk-dependent activation of LFA-1 to slow rolling on ICAM-1. These data demonstrate that the PSGL-1 cytoplasmic domain is dispensable for leukocyte rolling on P-selectin but is essential to activate beta2 integrins to slow rolling on ICAM-1.

Enrichment of distinct microfilament-associated and GTP-binding-proteins in membrane/microvilli fractions from lymphoid cells

Lymphocyte microvilli mediate initial adhesion to endothelium during lymphocyte transition from blood into tissue but their molecular organization is incompletely understood. We modified a shear-based procedure to prepare biochemical fractions enriched for membrane/microvilli (MMV) from both human peripheral blood T-lymphocytes (PBT) and a mouse pre-B lymphocyte line (300.19). Enrichment of proteins in MMV relative to post nuclear lysate was determined by LC/MS/MS analysis and label-free quantitation. Subsequent analysis emphasized the 291 proteins shared by PBT and 300.19 and estimated by MS peak area to be highest abundance. Validity of the label-free quantitation was confirmed by many internal consistencies and by comparison with Western blot analyses. The MMV fraction was enriched primarily for subsets of cytoskeletal proteins, transmembrane proteins and G-proteins, with similar patterns in both lymphoid cell types. The most enriched cytoskeletal proteins were microfilament-related proteins NHERF1, Ezrin/Radixin/Moesin (ERMs), ADF/cofilin and Myosin1G. Other microfilament proteins such as talin, gelsolin, myosin II and profilin were markedly reduced in MMV, as were intermediate filament- and microtubule-related proteins. Heterotrimeric G-proteins and some small G-proteins (especially Ras and Rap1) were enriched in the MMV preparation. Two notable general observations also emerged. There was less overlap between the two cells in their transmembrane proteins than in other classes of proteins, consistent with a special role of plasma membrane proteins in differentiation. Second, unstimulated primary T-lymphocytes have an unusually high concentration of actin and other microfilament related proteins, consistent with the singular role of actin-mediated motility in the immunological surveillance performed by these primary cells.

Cells on the run: shear-regulated integrin activation in leukocyte rolling and arrest on endothelial cells

The arrest of rolling leukocytes on various target vascular beds is mediated by specialized leukocyte integrins and their endothelial immunoglobulin superfamily (IgSF) ligands. These integrins are kept in largely inactive states and undergo in situ activation upon leukocyte-endothelial contact by both biochemical and mechanical signals from flow-derived shear forces. In vivo and in vitro studies suggest that leukocyte integrin activation involves conformational alterations through inside-out signaling followed by ligand-induced rearrangements accelerated by external forces. This activation process takes place within fractions of seconds by in situ signals transduced to the rolling leukocyte as it encounters specialized endothelial-displayed chemoattractants, collectively termed arrest chemokines. In neutrophils, selectin rolling engagements trigger intermediate affinity integrins to support reversible adhesions before chemokine-triggered arrest. Different leukocyte subsets appear to use different modalities of integrin activation during rolling and arrest at distinct endothelial sites.

Identification of novel isoforms of mouse L-selectin with different carboxyl-terminal tails

The leukocyte adhesion molecule L-selectin mediates the recruitment of lymphocytes to secondary lymphoid organs and is involved in the accumulation of neutrophils at sites of inflammation. In this study, we report the identification of novel isoforms of the mouse L-selectin gene, termed L-selectin-v1 and L-selectin-v2. Sequence analysis revealed that these isoforms are generated by alternative splicing: the L-selectin-v2 transcript includes a previously unknown exon of 100 bp located between the 7th and 8th exons of the mouse L-selectin gene, while the L-selectin-v1 transcript contains the first 49-bp sequence of this new exon. The insertion of each new sequence adds a downstream reading frame, giving rise to predicted proteins that differ in their carboxyl-terminal tails. These splice variants were found in cells that express conventional L-selectin, termed L-selectin-c, including B and T lymphocytes and granulocytes. Functionally, like L-selectin-c, both L-selectin-v1 and L-selectin-v2 expressed in cultured cells underwent phorbol ester-induced shedding, although L-selectin-v1 and L-selectin-v2 were shed to a greater and lesser degree, respectively, than L-selectin-c. Under flow conditions, both L-selectin-v1 and L-selectin-v2 mediated faster cell rolling than did L-selectin-c. In addition, ligation of L-selectin-c and L-selectin-v1, but not L-selectin-v2, induced p38 mitogen-activated protein kinase phosphorylation. These results suggest that alternative splicing is one mechanism for generating functional diversity in L-selectin.

c-Abl is required for the signaling transduction induced by L-selectin ligation

Lymphocyte recruitment onto inflamed tissues requires cells tethering to and rolling on vascular surfaces under flow. L-selectin is constitutively expressed on leukocytes to mediate the leukocytes' initial capture and subsequent rolling along the vessel. Apart from its adhesive function, engagement of L-selectin also results in cell activation, which is related to the completed signaling transduction. Here we show that ligation of L-selectin with its mAb increases c-Abl kinase activity, and that the activated c-Abl kinase can be recruited to and phosphorylate the cytoplasmic domain of L-selectin. In addition, the activated c-Abl kinase can regulate Zap70 kinase by increasing the phosphorylation of the Y319 site of Zap70 kinase and connect with Zap70 kinase through its SH2 domain. These results indicate that c-Abl kinase plays an important role in accepting and transferring the upstream activation events induced by L-selectin ligation.

Mechanisms and consequences of neutrophil interaction with the endothelium

Leukocyte recruitment into inflamed tissue proceeds in a cascade-like fashion. The first contact of neutrophils with the endothelium is mediated by selectins and their counterreceptors, followed by rolling of neutrophils along the endothelial wall of postcapillary venules and integrin-mediated arrest. While rolling, neutrophils collect different inflammatory signals that can activate several pathways. In addition to activation of neutrophils by ligation of G-protein-coupled receptors with chemokines and other chemoattractants, integrins and selectin ligands are also able to signal into the cell, where they initiate neutrophil extravasation, promote cytoskeletal rearrangement, and ultimately induce superoxide production and degranulation. These signaling pathways may be targeted by therapeutic interventions to inhibit specific functions of neutrophils without affecting others. This Review is focused on the signaling events during the interaction of neutrophils with the endothelium.

Clustering endothelial E-selectin in clathrin-coated pits and lipid rafts enhances leukocyte adhesion under flow

During inflammation, E-selectin expressed on cytokine-activated endothelial cells mediates leukocyte rolling under flow. E-selectin undergoes endocytosis and may associate with lipid rafts. We asked whether distribution of E-selectin in membrane domains affects its functions. E-selectin was internalized in transfected CHO cells or cytokine-activated human umbilical vein endothelial cells (HUVECs). Confocal microscopy demonstrated colocalization of E-selectin with alpha-adaptin, a clathrin-associated protein. Deleting the cytoplasmic domain of E-selectin or disrupting clathrin-coated pits with hypertonic medium blocked internalization of E-selectin, reduced colocalization of E-selectin with alpha-adaptin, and inhibited E-selectin-mediated neutrophil rolling under flow. Unlike CHO cells, HUVECs expressed a small percentage of E-selectin in lipid rafts. Even fewer neutrophils rolled on E-selectin in HUVECs treated with hypertonic medium and with methyl-beta-cyclodextrin, which disrupts lipid rafts. These data demonstrate that E-selectin clusters in both clathrin-coated pits and lipid rafts of endothelial cells but is internalized in clathrin-coated pits. Distribution in both domains markedly enhances E-selectin's ability to mediate leukocyte rolling under 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.

Membranal cholesterol is not required for L-selectin adhesiveness in primary lymphocytes but controls a chemokine-induced destabilization of L-selectin rolling adhesions

Cholesterol-enriched lipid microdomains regulate L-selectin signaling, but the role of membrane cholesterol in L-selectin adhesion is unclear. Arrest chemokines are a subset of endothelial chemokines that rapidly activate leukocyte integrin adhesiveness under shear flow. In the absence of integrin ligands, these chemokines destabilize L-selectin-mediated leukocyte rolling. In the present study, we investigated how cholesterol extraction from the plasma membrane of peripheral blood T or B cells affects L-selectin adhesions and their destabilization by arrest chemokines. Unlike the Jurkat T cell line, whose L-selectin-mediated adhesion is cholesterol dependent, in primary human PBLs and in murine B cells and B cell lines, cholesterol depletion did not impair any intrinsic adhesiveness of L-selectin, consistent with low selectin partitioning into lipid rafts in these cells. However, cholesterol raft disruption impaired the ability of two arrest chemokines, CXCL12 and CXCL13, but not of a third arrest chemokine, CCL21, to destabilize L-selectin-mediated rolling of T lymphocytes. Actin capping by brief incubation with cytochalasin D impaired the ability of all three chemokines to destabilize L-selectin rolling. Blocking of the actin regulatory phosphatidylinositol lipid, phosphatidylinositol 4,5-bisphosphate, did not affect chemokine-mediated destabilization of L-selectin adhesions. Collectively, our results suggest that L-selectin adhesions are inhibited by actin-associated, cholesterol-stabilized assemblies of CXCL12- and CXCL13-binding receptors on both T and B lymphocytes. Thus, the regulation of L-selectin by cholesterol-enriched microdomains varies with the cell type as well as with the identity of the destabilizing chemokine.

Integrin modulation and signaling in leukocyte adhesion and migration

The movement of leukocytes from the blood into peripheral tissues plays a key role in immunity as well as chronic inflammatory and autoimmune diseases. The shear force of blood flow presents special challenges to leukocytes as they establish adhesion on the vascular endothelium and migrate into the underlying tissues. Integrins are a family of cell adhesion and signaling molecules, whose function can be regulated to meet these challenges. The affinity of integrins for their vascular ligands can be stimulated in subseconds by chemoattractant signaling. This aids in inducing leukocyte adhesion under flow conditions. Further, linkage of these integrins to the actin cytoskeleton also helps to establish adhesion to the endothelium under flow conditions. In the case of alpha4beta1 integrins, this linkage of the integrin to the cytoskeleton is mediated in part by the binding of paxillin to the alpha4 integrin subunit and the subsequent binding of paxillin to the cytoskeleton molecule talin. The movement of leukocytes along the vascular endothelium and in between endothelial cells requires the temporal and spatial regulation of small guanosine triphosphatases, such as Rac1. We describe mechanisms through which alpha4beta1 integrin signaling regulates appropriate Rac activation to drive leukocyte migration.

Talin 1 and Paxillin Facilitate Distinct Steps in Rapid VLA-4-mediated Adhesion Strengthening to Vascular Cell Adhesion Molecule 1

VLA-4 (alpha4beta1) is a key integrin in lymphocytes, interacting with endothelial vascular cell adhesion molecule 1 (VCAM-1) on blood vessels and stroma. To dissect the contribution of the two cytoskeletal VLA-4 adaptor partners paxillin and talin to VLA-4 adhesiveness, we transiently knocked them down in Jurkat T cells and primary resting human T cells by small interfering RNA silencing. Paxillin was required for VLA-4 adhesiveness to low density VCAM-1 under shear stress conditions and was found to control mechanical stability of bonds mediated by the alpha4 subunit but did not affect the integrin affinity or avidity to VCAM-1 in shear-free conditions. Talin 1 maintained VLA-4 in a high affinity conformation, thereby promoting rapid VLA-4 adhesion strengthening to VCAM-1 under both shear stress and shear-free conditions. Talin 1, but not paxillin, was required for VLA-4 to undergo optimal stimulation by the prototypic chemokine, CXCL12, under shear stress conditions. Interestingly, talin 1 and paxillin played the same distinct roles in VLA-4 adhesions of primary T lymphocytes, although VLA-4 affinity to VCAM-1 was at least 200-fold lower in these cells than in Jurkat cells. Collectively, our results suggest that whereas paxillin is a mechanical regulator of VLA-4 bonds generated in the absence of chemokine signals and low VCAM-1 occupancy, talin 1 is a versatile VLA-4 affinity regulator implicated in both spontaneous and chemokine-triggered rapid adhesions to VCAM-1.

Leucocyte-endothelial interactions in health and disease

The emigration of leucocytes into the tissue as a crucial step in the response to inflammatory signals has been acknowledged for more than 100 years. The endothelium does not only represent a mechanical barrier between blood and tissue, the circulatory system also connects different organ systems with each other, thus allowing the communication between remote systems. Leukocytes can function as messengers and messages at the same time. Failure or dysregulation of leucocyte-endothelial communication can severely affect the integrity of the organism. The interaction between leucocytes and the vascular endothelium has been recognised as an attractive target for the therapy of numerous disorders and diseases, including excessive inflammatory responses and autoimmune diseases, both associated with enormous consequences for patients and the health care system. There is promising evidence that the success rate of such treatments will increase as the understanding of the molecular mechanisms keeps improving. This chapter reviews the current knowledge about leucocyte-endothelial interaction. It will also display examples of both physiological and dysregulated leucocyte-endothelial interactions and identify potential therapeutical approaches.

Impact of fever-range thermal stress on lymphocyte-endothelial adhesion and lymphocyte trafficking

The evolutionarily conserved febrile response has been associated with improved survival during infection in endothermic and ectothermic species although its protective mechanism of action is not fully understood. Temperatures within the range of physiologic fever influence multiple parameters of the immune response including lymphocyte proliferation and cytotoxic activity, neutrophil and dendritic cell migration, and production or bioactivity of proinflammatory cytokines. This review focuses on the emerging role of fever-range thermal stress in promoting lymphocyte trafficking to secondary lymphoid organs that are major sites for launching effective immune responses during infection or inflammation. Specific emphasis will be on the molecular basis of thermal control of lymphocyte-endothelial adhesion, a critical checkpoint controlling lymphocyte extravasation, as well as the contribution of interleukin-6 (IL-6) trans-signaling to thermal activities. New results are presented indicating that thermal stimulation of lymphocyte homing potential is evident in evolutionarily distant endothermic vertebrate species. These observations support the view that the evolutionarily conserved febrile response contributes to immune protection and host survival by amplifying lymphocyte access to peripheral lymphoid organs.

Intercellular protein transfer at the NK cell immune synapse: mechanisms and physiological significance

Immune synapses (IS) are supramolecular clusters providing intercellular communication among cells of the immune system. While the physiological role and consequences of IS formation are beginning to be understood, these studies have given rise to a new research topic in the biology of lymphocyte interactions: synaptic transfer of proteins between lymphocytes. During natural killer (NK) cell immunosurveillance, clustering and transfer of receptor and ligand molecules have been observed at both the inhibitory and cytotoxic NK cell immune synapse (NK-IS). The transfer of activating receptors seems to be associated with receptor distribution to thin membrane connective structures (MCS)/nanotubes that communicate effector and susceptible target cells. Strikingly, bidirectional transfer of the activating receptor NKG2D and its cellular ligand MICB correlates with a reduction in NK cell cytotoxic function. In this regard, synaptic uptake of MICB may represent a novel strategy of tumor immune evasion. Finally, synaptic acquisition of receptors by NK cells may also favor the spread of pathogens. In this review we discuss possible mechanisms of synaptic protein transfer and propose different testable hypotheses about the physiological and pathological significance of this process for NK cell function.

Force as a Facilitator of Integrin Conformational Changes during Leukocyte Arrest on Blood Vessels and Antigen-Presenting Cells

Integrins comprise a large family of cell-cell and cell-matrix adhesion receptors that rapidly modulate their adhesiveness. The arrest of leukocyte integrins on target vascular beds involves instantaneous conformational switches generating shear-resistant adhesions. Structural data suggest that these integrins are maintained in low-affinity conformations and must rapidly undergo conformational switches transduced via cytoplasmic changes ("inside-out" signaling) and simultaneous ligand-induced rearrangements ("outside-in"). This bidirectional activation is accelerated by signals from endothelial chemoattractants (chemokines). Recent studies predict that shear forces in the piconewton (pN) range per integrin can facilitate these biochemical switches. After extravasation, antigen recognition involves smaller internal forces from cytoskeletal motors and actin polymers forming the immune synapse. In this review, we address how forces facilitate allosteric integrin activation by biochemical signals. Evidence suggests that preformed cytoskeletal anchorage rather than free integrin mobility is key for force-enhanced integrin activation by chemokines and TCR signals.

Synapses: sites of cell recognition, adhesion, and functional specification

Synapses are specialized adhesive contacts characteristic of many types of cell-cell interactions involving neurons, immune cells, epithelial cells, and even pathogens and host cells. Cell-cell adhesion is mediated by structurally diverse classes of cell-surface glycoproteins, which form homophilic or heterophilic interactions across the intercellular space. Adhesion proteins bind to a cytoplasmic network of scaffolding proteins, regulators of the actin cytoskeleton, and signal transduction pathways that control the structural and functional organization of synapses. The themes of this review are to compare the organization of synapses in different cell types and to understand how different classes of cell adhesion proteins and cytoplasmic protein networks specify the assembly of functionally distinct synapses in different cell contexts.

Hurdles to lymphocyte trafficking in the tumor microenvironment: implications for effective immunotherapy

An important consideration in the development of T cell-based cancer immunotherapy is that effector T cells must efficiently traffic to the tumor microenvironment in order to control malignant progression. T cell trafficking to target tissues is orchestrated by dynamic interactions between circulating lymphocytes and endothelial cells lining blood vessels. It is informative, in this regard, to compare and contrast the molecular mechanisms governing lymphocyte extravasation at distinct vascular sites: (1) high endothelial venules (HEV) of secondary lymphoid organs, which are portals for efficient trafficking of naive and central memory T lymphocytes; (2) non-activated endothelium of normal tissues that mediate relatively low basal levels of trafficking but are rapidly transformed into HEV-like vessels in response to local inflammatory stimuli; and (3) vessels within the intratumoral region and the surrounding peritumoral areas. These vessels can be distinguished by differential expression of hallmark trafficking molecules that function as molecular beacons directing lymphocyte migration across vascular barriers. This article reviews evidence that recruitment of effector T cells to the intratumoral microenvironment is impeded by sub-threshold expression of trafficking molecules on tumor microvessels. Emerging data support the thesis that when considered from the perspective of extravasation, vessels embedded within the intratumoral microenvironment of established tumors do not exhibit stereotypical characteristics of a chronic inflammatory state. A major challenge will be to develop therapeutic approaches to improve trafficking of effector T lymphocytes to tumor sites without skewing the balance in favor of a chronic inflammatory milieu that facilitates tumor maintenance and progression.

Glycosylation dependent cell adhesion molecule 1-like protein and L-selectin expression in sheep interplacentomal and placentomal endometrium

Glycosylation dependent cell adhesion molecule 1 (GlyCAM-1), a mucin component of sheep histotroph produced by glandular epithelium (GE) during early pregnancy, is hypothesized to function in implantation. However, GlyCAM-1 is present in uterine tissues subsequent to implantation suggesting additional functions of this l-selectin-binding ligand. This study focused on uterine GlyCAM-1 expression during placentome development in sheep. Western blot analysis of day 50 pregnant sheep identified 45, 40, and 25 kDa bands in interplacentomal endometrium, 40 and 25 kDa bands in placentomes, and 80 and 40 kDa bands in chorioallantois. The GlyCAM-1 proteins in interplacentomal regions were comparable to those detected in day 15-19 pregnant sheep, however, the 80 kDa form was unique to chorioallantois, and the absence of the 45 kDa GlyCAM-1 in placentomes indicated differences between interplacentomal and placentomal endometrium. Immunofluorescence identified GlyCAM-1 in lumenal epithelium (LE), stromal fibroblasts, and vascular smooth muscle cells. To better define its cellular distribution, GlyCAM-1 was co-localized with either epithelium-specific cytokeratin, smooth muscle-specific alpha-smooth muscle actin (alpha SMA), or stromal-specific vimentin. In interplacentomal endometrium, GlyCAM-1 co-localized with cytokeratin in LE but not in GE. GlyCAM-1 did not co-localize with alpha SMA, and was localized in the extracellular matrix of vimentin-positive stroma. In placentomes, GlyCAM-1 did not co-localize with cytokeratin, but did co-localize with alpha SMA and vimentin. Thus, in contrast to interplacentomal regions, GlyCAM-1 in placentomes was predominantly localized in vasculature rather than epithelial cells. Further, leukocytes expressing L-selectin were localized to the endothelial surface of GlyCAM-1-expressing vessels within placentomes. These data suggest that GlyCAM-1 assumes distinct functions in compartment-specific regions of the sheep uterus.

c-Abl is involved in the F-actin assembly triggered by L-selectin crosslinking

L-selectin is a cell adhesion molecule mediating the initial capture and subsequent rolling of leukocytes along the endothelial cells expressing L-selectin ligands. In addition to its action in adhesion, an intracellular signaling role for L-selectin has been recognized. Its cytoplasmic domain is involved in signal transduction following antibody crosslinking and in the regulation of receptor binding activity in response to intracellular signals. In this work, we demonstrated that L-selectin crosslinking led to F-actin polymerization and redistribution in human neutrophils. Using immuno-fluorescence microscopy, we observed that F-actin redistribution spatiotemporally related to the polarization of L-selectin. STI571, a specific inhibitor for cytoplasmic tyrosine kinase c-Abl, can inhibit F-actin polymerization and c-Abl redistribution in the activated neutrophils. Furthermore, we determined that c-Abl redistributed to the region where L-selectin polarized and associated with L-selectin in the activated neutrophils. The association between L-selectin and c-Abl was reduced by cytochalasin B. These results suggested that c-Abl was involved in the F-actin alteration triggered by L-selectin crosslinking in human neutrophils.

Alpha4beta1-dependent adhesion strengthening under mechanical strain is regulated by paxillin association with the alpha4-cytoplasmic domain

The capacity of integrins to mediate adhesiveness is modulated by their cytoplasmic associations. In this study, we describe a novel mechanism by which α₄-integrin adhesiveness is regulated by the cytoskeletal adaptor paxillin. A mutation of the α₄ tail that disrupts paxillin binding, α₄(Y991A), reduced talin association to the α₄β₁ heterodimer, impaired integrin anchorage to the cytoskeleton, and suppressed α₄β₁-dependent capture and adhesion strengthening of Jurkat T cells to VCAM-1 under shear stress. The mutant retained intrinsic avidity to soluble or bead-immobilized VCAM-1, supported normal cell spreading at short-lived contacts, had normal α₄-microvillar distribution, and responded to inside-out signals. This is the first demonstration that cytoskeletal anchorage of an integrin enhances the mechanical stability of its adhesive bonds under strain and, thereby, promotes its ability to mediate leukocyte adhesion under physiological shear stress conditions.

Dynamic control of lymphocyte trafficking by fever-range thermal stress

Migration of blood-borne lymphocytes into tissues involves a tightly orchestrated sequence of adhesion events. Adhesion molecules and chemokine receptors on the surface of circulating lymphocytes initiate contact with specialized endothelial cells under hemodynamic shear prior to extravasation across the vascular barrier into tissues. Lymphocyte-endothelial adhesion occurs preferentially in high endothelial venules (HEV) of peripheral lymphoid organs. The continuous recirculation of naïve and central memory lymphocytes across lymph node and Peyer's patch HEV underlies immune surveillance and immune homeostasis. Lymphocyte-endothelial interactions are markedly enhanced in HEV-like vessels of extralymphoid organs during physiological responses associated with acute and chronic inflammation. Similar adhesive mechanisms must be invoked for efficient trafficking of immune effector cells to tumor sites in order for the immune system to have an impact on tumor progression. Here we discuss recent evidence for the role of fever-range thermal stress in promoting lymphocyte-endothelial adhesion and trafficking across HEV in peripheral lymphoid organs. Findings are also presented that support the hypothesis that lymphocyte-endothelial interactions are limited within tumor microenvironments. Further understanding of the molecular mechanisms that dynamically promote lymphocyte trafficking in HEV may provide the basis for novel approaches to improve recruitment of immune effector cells to tumor sites.

L-selectin: adhesion, signalling and its importance in pathologic posttraumatic endotoxemia and non-septic inflammation

The leucocyte expressed surface-bound L-selectin belongs to the selectin family of adhesion molecules. It exhibits adhesive as well as signalling functions. Mainly, it is of importance in lymphocyte homing and in the extravasation of leucocytes into the surrounding tissue during inflammation. Acting in the initial step of the cell adhesion cascade, L-selectin is responsible for the rolling of leucocytes on endothelial layers. Therefore, L-selectin is thought to be an adequate target for pharmacological interventions. Beneath the discussion of the molecules' general features like molecule structure and its regulation, the review focuses firstly on L-selectin in the context of posttraumatic inflammatory disorders, and secondly on the importance of L-selectin specific signalling events.

Cytoskeletal interactions regulate inducible L-selectin clustering

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

L-Selectin ligands in lymphoid tissues and models of inflammation

Both lymphocyte recirculation through the lymphoid tissues and leukocyte recruitment to sites of inflammation are essential components of immune surveillance, and are necessary for sustained protection against pathogens. This process is mediated by the leukocyte-endothelial adhesion cascade of which the interaction of leukocyte L-Selectin with its endothelial ligand initiates the first critical tethering and rolling step. As well as discussing the constitutive L-Selectin ligands in lymphoid tissues, this review examines the literature regarding their induction in inflammation, and draws attention to recent findings regarding soluble L-Selectin ligands that suggest an emerging multifunctional role in leukocyte recirculation and inflammation.

Beta-1 integrin-mediated adhesion may be initiated by multiple incomplete bonds, thus accounting for the functional importance of receptor clustering

The regulation of cell integrin receptors involves modulation of membrane expression, shift between different affinity states, and topographical redistribution on the cell membrane. Here we attempted to assess quantitatively the functional importance of receptor clustering. We studied beta-1 integrin-mediated attachment of THP-1 cells to fibronectin-coated surfaces under low shear flow. Cells displayed multiple binding events with a half-life of the order of 1 s. The duration of binding events after the first second after arrest was quantitatively accounted for by a model assuming the existence of a short-time intermediate binding state with 3.6 s(-1) dissociation rate and 1.3 s(-1) transition frequency toward a more stable state. Cell binding to surfaces coated with lower fibronectin densities was concluded to be mediated by single molecular interactions, whereas multiple bonds were formed <1 s after contact with higher fibronectin surface densities. Cell treatment with microfilament inhibitors or a neutral antiintegrin antibody decreased bond number without changing aforementioned kinetic parameters whereas a function enhancing antibody increased the rate of bond formation and/or the lifetime of intermediate state. Receptor aggregation was induced by treating cells with neutral antiintegrin antibody and antiimmunoglobulin antibodies. A semiquantitative confocal microscopy study suggested that this treatment increased between 40% and 100% the average number of integrin receptors located in a volume of approximately 0.045 microm(3) surrounding each integrin. This aggregation induced up to 2.7-fold increase of the average number of bonds. Flow cytometric analysis of fluorescent ligand binding showed that THP-1 cells displayed low-affinity beta-1 integrins with a dissociation constant in the micromolar range. It is concluded that the initial step of cell adhesion was mediated by multiple incomplete bonds rather than a single equilibrium-state ligand receptor association. This interpretation accounts for the functional importance of integrin clustering.