Organization of the integrin LFA-1 in nanoclusters regulates its activity

The multiple faces of prostaglandin E2 G-protein coupled receptor signaling during the dendritic cell life cycle

Many processes regulating immune responses are initiated by G-protein coupled receptors (GPCRs) and report biochemical changes in the microenvironment. Dendritic cells (DCs) are the most potent antigen-presenting cells and crucial for the regulation of innate and adaptive immune responses. The lipid mediator Prostaglandin E2 (PGE2) via four GPCR subtypes (EP1-4) critically regulates DC generation, maturation and migration. The role of PGE₂ signaling in DC biology was unraveled by the characterization of EP receptor subtype expression in DC progenitor cells and DCs, the identification of the signaling pathways initiated by these GPCR subtypes and the classification of DC responses to PGE₂ at different stages of differentiation. Here, we review the advances in PGE₂ signaling in DCs and describe the efforts still to be made to understand the spatio-temporal fine-tuning of PGE₂ responses by DCs.

Lymphatic endothelium forms integrin-engaging 3D structures during DC transit across inflamed lymphatic vessels

Dendritic cell (DC) transmigration across the lymphatic endothelium is critical for the initiation and sustenance of immune responses. Under noninflammatory conditions, DC transit across the lymphatic endothelial cell (LEC) has been shown to be integrin independent. In contrast, there is increasing evidence for the participation of integrins and their ligands in DC transit across lymphatic endothelium under inflammation. In this sense, we describe the formation of ICAM-1 (CD54)-enriched three-dimensional structures on LEC/DC contacts, as these DCs adhere to inflamed skin lymphatic vessels and transmigrate into them. In vitro imaging revealed that under inflammation ICAM-1 accumulated on microvilli projections surrounding 60% of adhered DCs. In contrast, these structures were scarcely formed in noninflammatory conditions. Furthermore, ICAM-1-enriched microvilli were important in promoting DC transendothelial migration and DC crawling over the LEC surface. Microvilli formation was dependent on the presence of β-integrins on the DC side and on integrin conformational affinity to ligand. Finally, we observed that LEC microvilli structures appeared in close vicinity of CCL21 depots and that their assembly was partially inhibited by CCL21-neutralizing antibodies. Therefore, under inflammatory conditions, integrin ligands form three-dimensional membrane projections around DCs. These structures offer docking sites for DC transit from the tissue toward the lymphatic vessel lumen.

Toxoplasma gondii modulates the dynamics of human monocyte adhesion to vascular endothelium under fluidic shear stress

Toxoplasma gondii actively infects circulating immune cells, including monocytes and DCs, and is thought to use these cells as Trojan horses for parasite dissemination. To investigate the interactions of T. gondii-infected human monocytes with vascular endothelium under conditions of shear stress, we developed a fluidic and time-lapse fluorescence microscopy system. Both uninfected and infected monocytes rolled, decelerated, and firmly adhered on TNF-α-activated endothelium. Interestingly, T. gondii-infected primary human monocytes and THP-1 cells exhibited altered adhesion dynamics compared with uninfected monocytes: infected cells rolled at significantly higher velocities (2.5- to 4.6-fold) and over greater distances (2.6- to 4.8-fold) than uninfected monocytes, before firmly adhering. During monocyte searching, 29-36% of infected monocytes compared with 0-11% of uninfected monocytes migrated >10 μm from the point where they initiated searching, and these "wandering" searches were predominantly in the direction of flow. As infected monocytes appeared delayed in their transition to firm adhesion, we examined the effects of infection on integrin expression and function. T. gondii did not affect the expression of LFA-1, VLA-4, or MAC-1 or the ability of Mn(2+) to activate these integrins. However, T. gondii infection impaired LFA-1 and VLA-4 clustering and pseudopod extension in response to integrin ligands. Surprisingly, a single intracellular parasite was sufficient to mediate these effects. This research has established a system for studying pathogen modulation of human leukocyte adhesion under conditions of physiological shear stress and has revealed a previously unappreciated effect of T. gondii infection on ligand-dependent integrin clustering.

On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines

Immunology often conveys the image of large molecules, either in the soluble state or in the membrane of leukocytes, forming multiple contacts with a target for actions of the immune system. Avidity names the ability of a polyvalent molecule to form multiple connections of the same kind with ligands tethered to the same surface. Polyvalent interactions are vastly stronger than their monovalent equivalent. In the present review, the functional consequences of polyvalent interactions are explored in a perspective of recent theoretical advances in understanding the thermodynamics of such binding. From insights on the structural biology of soluble pattern recognition molecules as well as adhesion molecules in the cell membranes or in their proteolytically shed form, this review documents the prominent role of polyvalent interactions in making the immune system a formidable barrier to microbial infection as well as constituting a significant challenge to the application of nanomedicines.

The tetraspanin CD37 orchestrates the α(4)β(1) integrin-Akt signaling axis and supports long-lived plasma cell survival

Signaling by the serine and threonine kinase Akt (also known as protein kinase B), a pathway that is common to all eukaryotic cells, is central to cell survival, proliferation, and gene induction. We sought to elucidate the mechanisms underlying regulation of the kinase activity of Akt in the immune system. We found that the four-transmembrane protein CD37 was essential for B cell survival and long-lived protective immunity. CD37-deficient (Cd37(-/-)) mice had reduced numbers of immunoglobulin G (IgG)-secreting plasma cells in lymphoid organs compared to those in wild-type mice, which we attributed to increased apoptosis of plasma cells in the germinal centers of the spleen, areas in which B cells proliferate and are selected. CD37 was required for the survival of IgG-secreting plasma cells in response to binding of vascular cell adhesion molecule 1 to the α(4)β(1) integrin. Impaired α(4)β(1) integrin-dependent Akt signaling in Cd37(-/-) IgG-secreting plasma cells was the underlying cause responsible for impaired cell survival. CD37 was required for the mobility and clustering of α(4)β(1) integrins in the plasma membrane, thus regulating the membrane distribution of α(4)β(1) integrin necessary for activation of the Akt survival pathway in the immune system.

Leukocyte function antigen-1, kindlin-3, and calcium flux orchestrate neutrophil recruitment during inflammation

Neutrophil arrest and migration on inflamed endothelium involves a conformational shift in CD11a/CD18 (leukocyte function antigen-1; LFA-1) to a high-affinity and clustered state that determines the strength and lifetime of bond formation with ICAM-1. Cytoskeletal adapter proteins Kindlin-3 and Talin-1 anchor clustered LFA-1 to the cytoskeleton and facilitate the transition from neutrophil rolling to arrest. We recently reported that tensile force acts on LFA-1 bonds inducing their colocalization with Orai1, the predominant membrane store operated Ca(2+) channel that cooperates with the endoplasmic reticulum to elicit cytosolic flux. Because Kindlin-3 was recently reported to initiate LFA-1 clustering in lymphocytes, we hypothesized that it cooperates with Orai1 and LFA-1 in signaling local Ca(2+) flux necessary for shear-resistant neutrophil arrest. Using microfluidic flow channels combined with total internal reflection fluorescence microscopy, we applied defined shear stress to low- or high-affinity LFA-1 and imaged the spatiotemporal regulation of bond formation with Kindlin-3 recruitment and Ca(2+) influx. Orai1 and Kindlin-3 genes were silenced in neutrophil-like HL-60 cells to assess their respective roles in this process. Kindlin-3 was enriched within focal clusters of high-affinity LFA-1, which promoted physical linkage with Orai1. This macromolecular complex functioned to amplify inside-out Ca(2+) signaling in response to IL-8 stimulation by catalyzing an increased density of Talin-1 and consolidating LFA-1 clusters within sites of contact with ICAM-1. In this manner, neutrophils use focal adhesions as mechanosensors that convert shear stress-mediated tensile force into local bursts of Ca(2+) influx that catalyze cytoskeletal engagement and an adhesion-strengthened migratory phenotype.

How membrane structures control T cell signaling

Genetic and biochemical studies have identified a large number of molecules involved in T cell signaling. They have provided us with a comprehensive understanding of protein-protein interactions and protein modifications that take place upon antigen recognition. Diffraction limited fluorescence microscopy has been used to study the distribution of signaling molecules on a cellular level. Specifically, the discovery of microclusters and the immunological synapse demonstrates that T cell signaling cascades utilizes spatial association and segregation. Recent advancements in live cell imaging have allowed us to visualize the spatio-temporal mechanisms of T cell signaling at nanometer scale resolution. This led to the discovery that proteins are organized in distinct membrane domains prior and during T cell activation. Evidently, plasma membrane structures and signaling molecule distributions at all length scales (molecular to cellular) are intrinsic to the mechanisms that govern signaling initiation, transduction, and inhibition. Here we provide an overview of possible plasma membrane models, molecular assemblies that have been described to date, how they can be visualized and how they might contribute to T cell signaling.

Quantitative modeling assesses the contribution of bond strengthening, rebinding and force sharing to the avidity of biomolecule interactions

Cell adhesion is mediated by numerous membrane receptors. It is desirable to derive the outcome of a cell-surface encounter from the molecular properties of interacting receptors and ligands. However, conventional parameters such as affinity or kinetic constants are often insufficient to account for receptor efficiency. Avidity is a qualitative concept frequently used to describe biomolecule interactions: this includes incompletely defined properties such as the capacity to form multivalent attachments. The aim of this study is to produce a working description of monovalent attachments formed by a model system, then to measure and interpret the behavior of divalent attachments under force. We investigated attachments between antibody-coated microspheres and surfaces coated with sparse monomeric or dimeric ligands. When bonds were subjected to a pulling force, they exhibited both a force-dependent dissociation consistent with Bell’s empirical formula and a force- and time-dependent strengthening well described by a single parameter. Divalent attachments were stronger and less dependent on forces than monovalent ones. The proportion of divalent attachments resisting a force of 30 piconewtons for at least 5 s was 3.7 fold higher than that of monovalent attachments. Quantitative modeling showed that this required rebinding, i.e. additional bond formation between surfaces linked by divalent receptors forming only one bond. Further, experimental data were compatible with but did not require stress sharing between bonds within divalent attachments. Thus many ligand-receptor interactions do not behave as single-step reactions in the millisecond to second timescale. Rather, they exhibit progressive stabilization. This explains the high efficiency of multimerized or clustered receptors even when bonds are only subjected to moderate forces. Our approach provides a quantitative way of relating binding avidity to measurable parameters including bond maturation, rebinding and force sharing, provided these parameters have been determined. Also, this provides a quantitative description of the phenomenon of bond strengthening.

Lateral mobility of individual integrin nanoclusters orchestrates the onset for leukocyte adhesion

Integrins are cell membrane adhesion receptors involved in morphogenesis, immunity, tissue healing, and metastasis. A central, yet unresolved question regarding the function of integrins is how these receptors regulate both their conformation and dynamic nanoscale organization on the membrane to generate adhesion-competent microclusters upon ligand binding. Here we exploit the high spatial (nanometer) accuracy and temporal resolution of single-dye tracking to dissect the relationship between conformational state, lateral mobility, and microclustering of the integrin receptor lymphocyte function-associated antigen 1 (LFA-1) expressed on immune cells. We recently showed that in quiescent monocytes, LFA-1 preorganizes in nanoclusters proximal to nanoscale raft components. We now show that these nanoclusters are primarily mobile on the cell surface with a small (ca. 5%) subset of conformational-active LFA-1 nanoclusters preanchored to the cytoskeleton. Lateral mobility resulted crucial for the formation of microclusters upon ligand binding and for stable adhesion under shear flow. Activation of high-affinity LFA-1 by extracellular Ca(2+) resulted in an eightfold increase on the percentage of immobile nanoclusters and cytoskeleton anchorage. Although having the ability to bind to their ligands, these active nanoclusters failed to support firm adhesion in static and low shear-flow conditions because mobility and clustering capacity were highly compromised. Altogether, our work demonstrates an intricate coupling between conformation and lateral diffusion of LFA-1 and further underscores the crucial role of mobility for the onset of LFA-1 mediated leukocyte adhesion.

Cytoskeletal control of CD36 diffusion promotes its receptor and signaling function

The mechanisms that govern receptor coalescence into functional clusters--often a critical step in their stimulation by ligand--are poorly understood. We used single-molecule tracking to investigate the dynamics of CD36, a clustering-responsive receptor that mediates oxidized LDL uptake by macrophages. We found that CD36 motion in the membrane was spatially structured by the cortical cytoskeleton. A subpopulation of receptors diffused within linear confinement regions whose unique geometry simultaneously facilitated freedom of movement along one axis while increasing the effective receptor density. Co-confinement within troughs enhanced the probability of collisions between unligated receptors and promoted their clustering. Cytoskeleton perturbations that inhibited diffusion in linear confinement regions reduced receptor clustering in the absence of ligand and, following ligand addition, suppressed CD36-mediated signaling and internalization. These observations demonstrate a role for the cytoskeleton in controlling signal transduction by structuring receptor diffusion within membrane regions that increase their collision frequency.

HIV envelope gp120 activates LFA-1 on CD4 T-lymphocytes and increases cell susceptibility to LFA-1-targeting leukotoxin (LtxA)

The cellular adhesion molecule LFA-1 and its ICAM-1 ligand play an important role in promoting HIV-1 infectivity and transmission. These molecules are present on the envelope of HIV-1 virions and are integral components of the HIV virological synapse. However, cellular activation is required to convert LFA-1 to the active conformation that has high affinity binding for ICAM-1. This study evaluates whether such activation can be induced by HIV itself. The data show that HIV-1 gp120 was sufficient to trigger LFA-1 activation in fully quiescent naïve CD4 T cells in a CD4-dependent manner, and these CD4 T cells became more susceptible to killing by LtxA, a bacterial leukotoxin that preferentially targets leukocytes expressing high levels of the active LFA-1. Moreover, virus p24-expressing CD4 T cells in the peripheral blood of HIV-infected subjects were found to have higher levels of surface LFA-1, and LtxA treatment led to significant reduction of the viral DNA burden. These results demonstrate for the first time the ability of HIV to directly induce LFA-1 activation on CD4 T cells. Although LFA-1 activation may enhance HIV infectivity and transmission, it also renders the cells more susceptible to an LFA-1-targeting bacterial toxin, which may be harnessed as a novel therapeutic strategy to deplete virus reservoir in HIV-infected individuals.

Analysis of molecular diffusion by first-passage time variance identifies the size of confinement zones

The diffusion of receptors within the two-dimensional environment of the plasma membrane is a complex process. Although certain components diffuse according to a random walk model (Brownian diffusion), an overwhelming body of work has found that membrane diffusion is nonideal (anomalous diffusion). One of the most powerful methods for studying membrane diffusion is single particle tracking (SPT), which records the trajectory of a label attached to a membrane component of interest. One of the outstanding problems in SPT is the analysis of data to identify the presence of heterogeneity. We have adapted a first-passage time (FPT) algorithm, originally developed for the interpretation of animal movement, for the analysis of SPT data. We discuss the general application of the FPT analysis to molecular diffusion, and use simulations to test the method against data containing known regions of confinement. We conclude that FPT can be used to identify the presence and size of confinement within trajectories of the receptor LFA-1, and these results are consistent with previous reports on the size of LFA-1 clusters. The analysis of trajectory data for cell surface receptors by FPT provides a robust method to determine the presence and size of confined regions of diffusion.

The CD157-integrin partnership controls transendothelial migration and adhesion of human monocytes

CD157, a member of the CD38 gene family, is an NAD-metabolizing ectoenzyme and a signaling molecule whose role in polarization, migration, and diapedesis of human granulocytes has been documented; however, the molecular events underpinning this role remain to be elucidated. This study focused on the role exerted by CD157 in monocyte migration across the endothelial lining and adhesion to extracellular matrix proteins. The results demonstrated that anti-CD157 antibodies block monocyte transmigration and adhesion to fibronectin and fibrinogen but that CD157 cross-linking is sufficient to overcome the block, suggesting an active signaling role for the molecule. Consistent with this is the observation that CD157 is prevalently located within the detergent-resistant membrane microdomains to which, upon clustering, it promotes the recruitment of β(1) and β(2) integrin, which, in turn, leads to the formation of a multimolecular complex favoring signal transduction. This functional cross-talk with integrins allows CD157 to act as a receptor despite its intrinsic structural inability to do so on its own. Intracellular signals mediated by CD157 rely on the integrin/Src/FAK (focal adhesion kinase) pathway, resulting in increased activity of the MAPK/ERK1/2 and the PI3K/Akt downstream signaling pathways, which are crucial in the control of monocyte transendothelial migration. Collectively, these findings indicate that CD157 acts as a molecular organizer of signaling-competent membrane microdomains and that it forms part of a larger molecular machine ruled by integrins. The CD157-integrin partnership provides optimal adhesion and transmigration of human monocytes.

Dendritic cells adhere to and transmigrate across lymphatic endothelium in response to IFN-α

Migration of DC into lymphatic vessels ferries antigenic cargo and pro-inflammatory stimuli into the draining LN. Given that tissues under the influence of viral infections produce type I IFN, it is conceivable that these cytokines enhance DC migration in order to facilitate an antiviral immune response. Cultured lymphatic endothelium monolayers pretreated with TNF-α were used to model this phenomenon under inflammatory conditions. DC differentiated in the presence of either IFN-α2b or IFN-α5 showed enhanced adhesion to cultured lymphatic endothelial cells. These pro-adhesive effects were mediated by DC, not the lymphatic endothelium, and correlated with increased DC transmigration across lymphatic endothelial cell monolayers. Transmigration was guided by chemokines acting on DC, and blocking experiments with mAb indicated a role for LFA-1. Furthermore, incubation of DC with IFN-α led to the appearance of active conformation epitopes on the CD11a integrin chains expressed by DC. Differentiation of mouse DC in the presence of IFN-α also increased DC migration from inflammed footpads toward popliteal LN. Collectively, these results indicate a role for type I IFN in directing DC toward LN under inflammatory conditions.

The ubiquitin C-terminal hydrolase UCH-L1 regulates B-cell proliferation and integrin activation

The ubiquitin C-terminal hydrolase-L1 (UCH-L1) is a deubiquitinating enzyme that catalyses the hydrolysis of polyubiquitin precursors and small ubiquitin adducts. UCH-L1 has been detected in a variety of malignant and metastatic tumours but its biological function in these cells is unknown. We have previously shown that UCH-L1 is highly expressed in Burkitt's lymphoma (BL) and is up-regulated upon infection of B lymphocytes with Epstein-Barr virus (EBV). Here we show that knockdown of UCH-L1 by RNAi inhibits the proliferation of BL cells in suspension and semisolid agar and activates strong LFA-1-dependent homotypic adhesion. Induction of cell adhesion correlated with cation-induced binding to ICAM-1, clustering of LFA-1 into lipid rafts and constitutive activation of the Rap1 and Rac1 GTPases. Expression of a catalytically active UCH-L1 promoted the proliferation of a UCH-L1-negative EBV transformed lymphoblastoid cell line (LCL) and inhibited cell adhesion, whereas a catalytic mutant had no effect, confirming the requirement of UCH-L1 enzymatic activity for the regulation of these phenotypes. Our results identify UCH-L1 as a new player in the signalling pathways that promote the proliferation and invasive capacity of malignant B cells.

LFA-1 activity state on dendritic cells regulates contact duration with T cells and promotes T-cell priming

A key event in the successful induction of adaptive immune responses is the antigen-specific activation of T cells by dendritic cells (DCs). Although LFA-1 (lymphocyte function-associated antigen 1) on T cells is considered to be important for antigen-specific T-cell activation, the role for LFA-1 on DCs remains elusive. Using 2 different approaches to activate LFA-1 on DCs, either by deletion of the αL-integrin cytoplasmic GFFKR sequence or by silencing cytohesin-1-interacting protein, we now provide evidence that DCs are able to make use of active LFA-1 and can thereby control the contact duration with naive T cells. Enhanced duration of DC/T-cell interaction correlates inversely with antigen-specific T-cell proliferation, generation of T-helper 1 cells, and immune responses leading to delayed-type hypersensitivity. We could revert normal interaction time and T-cell proliferation to wild-type levels by inhibition of active LFA-1 on DCs. Our data further suggest that cytohesin-1-interacting protein might be responsible for controlling LFA-1 deactivation on mature DCs. In summary, our findings indicate that LFA-1 on DCs needs to be in an inactive state to ensure optimal T-cell activation and suggest that regulation of LFA-1 activity allows DCs to actively control antigen-driven T-cell proliferation and effective immune responses.

Phosphorylation of paxillin at threonine 538 by PKCdelta regulates LFA1-mediated adhesion of lymphoid cells

We investigated the PKCdelta-mediated phosphorylation of paxillin within its LIM4 domain and the involvement of this phosphorylation in activation of LFA-1 integrins of the Baf3 pro-B lymphocytic cell line. Using phosphorylated-threonine-specific antibodies, phosphorylated amino acid analysis and paxillin phosphorylation mutants, we demonstrated that TPA, the pharmacological analog of the endogenous second messenger diacyl glycerol, stimulates paxillin phosphorylation at threonine 538 (T538). The TPA-responsive PKC isoform PKCdelta directly binds paxillin in a yeast two-hybrid assay and phosphorylates paxillin at T538 in vitro and also co-immunoprecipitates with paxillin and mediates phosphorylation of this residue in vivo. Recombinant wild-type paxillin, its phospho-inhibitory T538A or phospho-mimetic T538E mutants were expressed in the cells simultaneously with siRNA silencing of the endogenous paxillin. These experiments suggest that phosphorylation of paxillin T538 contributes to dissolution of the actin cytoskeleton, redistribution of LFA-1 integrins and an increase in their affinity. We also show that phosphorylation of T538 is involved in the activation of LFA-1 integrins by TPA.

Neutrophil microdomains: linking heterocellular interactions with vascular injury

PURPOSE OF REVIEW

Neutrophil infiltration is an important feature in inflammatory scenarios. Before these cells infiltrate tissues, however, they contribute to crucial intravascular events in which neutrophil microdomains mediate heterotypic interactions with endothelial cells, red blood cells and/or platelets. In vascular diseases, this can result in exacerbated neutrophil activation, subsequent vascular injury and obstruction of microcirculatory blood flow. This review discusses recent advances in elucidating these neutrophil domains and their associated functions in cell adhesion.

RECENT FINDINGS

Neutrophil recruitment is mediated by sequential interactions with the endothelium, termed rolling, adhesion and extravasation. Evidence points to novel signaling pathways induced during the rolling phase resulting in the transition to leukocyte adhesion, which appear to contribute to chemokine mediated activation. In addition, specific neutrophil microdomains are important for interactions with other hematopoietic cells inducing reductions in microvascular flow and injury.

SUMMARY

Neutrophils integrate signals received from the endothelium to act as linkers between the vessel wall and a variety of vascular components (i.e. endothelial cells, platelets, red blood cells) in acute and chronic inflammatory conditions to mediate interactions that can result in vascular injury and vasoocclusion.

Imaging individual proteins and nanodomains on intact cell membranes with a probe-based optical antenna

Optical antennas that confine and enhance electromagnetic fields in a nanometric region hold great potential for nanobioimaging and biosensing. Probe-based monopole optical antennas are fabricated to enhance fields localized to <30 nm near the antenna apex in aqueous conditions. These probes are used under appropriate excitation antenna conditions to image individual antibodies with an unprecedented resolution of 26 +/- 4 nm and virtually no surrounding background. On intact cell membranes in physiological conditions, the obtained resolution is 30 +/- 6 nm. Importantly, the method allows individual proteins to be distinguished from nanodomains and the degree of clustering to be quantified by directly measuring physical size and intensity of individual fluorescent spots. Improved antenna geometries should lead to true live cell imaging below 10-nm resolution with position accuracy in the subnanometric range.

TLR4-mediated podosome loss discriminates gram-negative from gram-positive bacteria in their capacity to induce dendritic cell migration and maturation

Chronic infections are caused by microorganisms that display effective immune evasion mechanisms. Dendritic cell (DC)-dependent T cell-mediated adaptive immunity is one of the mechanisms that have evolved to prevent the occurrence of chronic bacterial infections. In turn, bacterial pathogens have developed strategies to evade immune recognition. In this study, we show that gram-negative and gram-positive bacteria differ in their ability to activate DCs and that gram-negative bacteria are far more effective inducers of DC maturation. Moreover, we observed that only gram-negative bacteria can induce loss of adhesive podosome structures in DCs, a response necessary for the induction of effective DC migration. We demonstrate that the ability of gram-negative bacteria to trigger podosome turnover and induce DC migration reflects their capacity to selectively activate TLR4. Examining mice defective in TLR4 signaling, we show that this DC maturation and migration are mainly Toll/IL-1 receptor domain-containing adaptor-inducing IFNbeta-dependent. Furthermore, we show that these processes depend on the production of PGs by these DCs, suggesting a direct link between TLR4-mediated signaling and arachidonic metabolism. These findings demonstrate that gram-positive and gram-negative bacteria profoundly differ in their capacity to activate DCs. We propose that this inability of gram-positive bacteria to induce DC maturation and migration is part of the armamentarium necessary for avoiding the induction of an effective cellular immune response and may explain the frequent involvement of these pathogens in chronic infections.

A hidden Markov model for single particle tracks quantifies dynamic interactions between LFA-1 and the actin cytoskeleton

The extraction of hidden information from complex trajectories is a continuing problem in single-particle and single-molecule experiments. Particle trajectories are the result of multiple phenomena, and new methods for revealing changes in molecular processes are needed. We have developed a practical technique that is capable of identifying multiple states of diffusion within experimental trajectories. We model single particle tracks for a membrane-associated protein interacting with a homogeneously distributed binding partner and show that, with certain simplifying assumptions, particle trajectories can be regarded as the outcome of a two-state hidden Markov model. Using simulated trajectories, we demonstrate that this model can be used to identify the key biophysical parameters for such a system, namely the diffusion coefficients of the underlying states, and the rates of transition between them. We use a stochastic optimization scheme to compute maximum likelihood estimates of these parameters. We have applied this analysis to single-particle trajectories of the integrin receptor lymphocyte function-associated antigen-1 (LFA-1) on live T cells. Our analysis reveals that the diffusion of LFA-1 is indeed approximately two-state, and is characterized by large changes in cytoskeletal interactions upon cellular activation.

Hotspots of GPI-anchored proteins and integrin nanoclusters function as nucleation sites for cell adhesion

Recruitment of receptor proteins to lipid rafts has been proposed as an important mechanism to regulate their cellular function. In particular, rafts have been implicated in regulation of integrin-mediated cell adhesion, although the underlying mechanism remains elusive. We used single-molecule near-field optical microscopy (NSOM) with localization accuracy of approximately 3 nm, to capture the spatio-functional relationship between the integrin LFA-1 and raft components (GPI-APs) on immune cells. Dual color nanoscale imaging revealed the existence of a nanodomain GPI-AP subpopulation that further concentrated in regions smaller than 250 nm, suggesting a hierarchical prearrangement of GPI-APs on resting monocytes. We previously demonstrated that in quiescent monocytes, LFA-1 preorganizes in nanoclusters. We now show that integrin nanoclusters are spatially different but reside proximal to GPI-AP nanodomains, forming hotspots on the cell surface. Ligand-mediated integrin activation resulted in an interconversion from monomers to nanodomains of GPI-APs and the generation of nascent adhesion sites where integrin and GPI-APs colocalized at the nanoscale. Cholesterol depletion significantly affected the reciprocal distribution pattern of LFA-1 and GPI-APs in the resting state, and LFA-1 adhesion to its ligand. As such, our data demonstrate the existence of nanoplatforms as essential intermediates in nascent cell adhesion. Since raft association with a variety of membrane proteins other than LFA-1 has been documented, we propose that hotspots regions enriched with raft components and functional receptors may constitute a prototype of nanoscale inter-receptor assembly and correspond to a generic mechanism to offer cells with privileged areas for rapid cellular function and responses to the outside world.

Interplay between cell adhesion and growth factor receptors: from the plasma membrane to the endosomes

The emergence of multicellular animals could only take place once evolution had produced molecular mechanisms for cell adhesion and communication. Today, all metazoans express integrin-type adhesion receptors and receptors for growth factors. Integrins recognize extracellular matrix proteins and respective receptors on other cells and, following ligand binding, can activate the same cellular signaling pathways that are regulated by growth factor receptors. Recent reports have indicated that the two receptor systems also collaborate in many other ways. Here, we review the present information concerning the role of integrins as assisting growth factor receptors and the interplay between the receptors in cell signaling and in the orchestration of receptor recycling.

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.

Association of FcgammaRIIa (CD32a) with lipid rafts regulates ligand binding activity

Binding of Igs to myeloid cells via FcR is a key event in the control of innate and acquired immunity. FcgammaRIIa (CD32a) is a receptor for multivalent IgG expressed predominantly by myeloid cells, and its association with microdomains rich in cholesterol and sphingolipids, termed as lipid rafts, has been reported to be essential for efficient signaling. However, for many myeloid cell types, ligand binding to CD32a is suppressed by as yet undefined mechanisms. In this study, we have examined the role of CD32a-lipid raft interactions in the regulation of IgG binding to CD32a. Disruption of lipid raft structure following depletion or sequestration of membrane cholesterol greatly inhibited CD32a-mediated IgG binding. Furthermore, specific CD32a mutants, which show reduced association with lipid rafts (A224S and C241A), displayed decreased levels of IgG binding compared with wild-type CD32a. In contrast, constitutively lipid raft-associated CD32a (GPI-anchored CD32a) exhibited increased capacity for IgG binding compared with the full-length transmembrane CD32a. Our findings clearly suggest a major role for lipid rafts in the regulation of IgG binding and, more specifically, that suppression of CD32a-mediated IgG binding in myeloid cells is achieved by receptor exclusion from lipid raft membrane microdomains.

The ins and outs of leukocyte integrin signaling

Integrins are the principal cell adhesion receptors that mediate leukocyte migration and activation in the immune system. These receptors signal bidirectionally through the plasma membrane in pathways referred to as inside-out and outside-in signaling. Each of these pathways is mediated by conformational changes in the integrin structure. Such changes allow high-affinity binding of the receptor with counter-adhesion molecules on the vascular endothelium or extracellular matrix and lead to association of the cytoplasmic tails of the integrins with intracellular signaling molecules. Leukocyte functional responses resulting from outside-in signaling include migration, proliferation, cytokine secretion, and degranulation. Here, we review the key signaling events that occur in the inside-out versus outside-in pathways, highlighting recent advances in our understanding of how integrins are activated by a variety of stimuli and how they mediate a diverse array of cellular responses.

Regulation of T cell integrin function by adapter proteins

Integrins are cell surface heterodimers that bind adhesion molecules expressed on other cells or in the extracellular matrix. Integrin-mediated interactions are critical for T cell development in the thymus, migration of T cells in the periphery, and induction of T cell effector functions. In resting T cells, integrins are maintained in a low affinity state. Engagement of the T cell receptor or chemokine receptors increases integrin affinity, enabling integrins to bind their ligands and initiate a signaling cascade resulting in altered cell morphology and motility. Our laboratory is interested how adapter proteins, mediators of intracellular signal transduction, regulate both signals from the T cell receptor to integrins (inside-out signaling) and (outside-in) signals from integrins into the cell.

Integrins in immunity

A successful immune response depends on the capacity of immune cells to travel from one location in the body to another–these cells are rapid migrators, travelling at speeds of μm/minute. Their ability to penetrate into tissues and to make contacts with other cells depends chiefly on the β2 integrin known as LFA-1. For this reason, we describe the control of its activity in some detail. For the non-immunologist, the fine details of an immune response often seem difficult to fathom. However, the behaviour of immune cells, known as leukocytes (Box 1), is subject to the same biological rules as many other cell types, and this holds true particularly for the functioning of the integrins on these cells. In this Commentary, we highlight, from a cell-biology point of view, the integrin-mediated immune-cell migration and cell-cell interactions that occur during the course of an immune response.

ICAM-1-mediated endothelial nitric oxide synthase activation via calcium and AMP-activated protein kinase is required for transendothelial lymphocyte migration

As a gatekeeper of leukocyte trafficking the vasculature fulfills an essential immune function. We have recently shown that paracellular transendothelial lymphocyte migration is controlled by intercellular adhesion molecule 1 (ICAM-1)-mediated vascular endothelial cadherin (VEC) phosphorylation [Turowski et al., J. Cell Sci. 121, 29-37 (2008)]. Here we show that endothelial nitric oxide synthase (eNOS) is a critical regulator of this pathway. ICAM-1 stimulated eNOS by a mechanism that was clearly distinct from that utilized by insulin. In particular, phosphorylation of eNOS on S1177 in response to ICAM-1 activation was regulated by src family protein kinase, rho GTPase, Ca(2+), CaMKK, and AMPK, but not Akt/PI3K. Functional neutralization of any component of this pathway or its downstream effector guanylyl cyclase significantly reduced lymphocyte diapedesis across the endothelial monolayer. In turn, activation of NO signaling promoted lymphocyte transmigration. The eNOS signaling pathway was required for T-cell transmigration across primary rat and human microvascular endothelial cells and also when shear flow was applied, suggesting that this pathway is ubiquitously used. These data reveal a novel and essential role of eNOS in basic immune function and provide a key link in the molecular network governing endothelial cell compliance to diapedesis.

Endothelial adhesion receptors are recruited to adherent leukocytes by inclusion in preformed tetraspanin nanoplatforms

VCAM-1 and ICAM-1, receptors for leukocyte integrins, are recruited to cell-cell contact sites on the apical membrane of activated endothelial cells. In this study, we show that this recruitment is independent of ligand engagement, actin cytoskeleton anchorage, and heterodimer formation. Instead, VCAM-1 and ICAM-1 are recruited by inclusion within specialized preformed tetraspanin-enriched microdomains, which act as endothelial adhesive platforms (EAPs). Using advanced analytical fluorescence techniques, we have characterized the diffusion properties at the single-molecule level, nanoscale organization, and specific intradomain molecular interactions of EAPs in living primary endothelial cells. This study provides compelling evidence for the existence of EAPs as physical entities at the plasma membrane, distinct from lipid rafts. Scanning electron microscopy of immunogold-labeled samples treated with a specific tetraspanin-blocking peptide identify nanoclustering of VCAM-1 and ICAM-1 within EAPs as a novel mechanism for supramolecular organization that regulates the leukocyte integrin-binding capacity of both endothelial receptors during extravasation.

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.

Actin S-nitrosylation inhibits neutrophil beta2 integrin function

The focus of this work was to elucidate the mechanism for inhibition of neutrophil beta(2) integrin adhesion molecules by hyperoxia. Results demonstrate that exposure to high oxygen partial pressures increases synthesis of reactive species derived from type 2 nitric-oxide synthase and myeloperoxidase, leading to excessive S-nitrosylation of beta-actin and possibly profilin. Hyperoxia causes S-nitrosylation of the four cysteine moieties closest to the carboxyl-terminal end of actin, which results in formation of short actin filaments. This alters actin polymerization, network formation, and intracellular distribution, as well as inhibits beta(2) integrin clustering. If neutrophils are exposed to ultraviolet light to reverse S-nitrosylation, or are incubated with N-formyl-methionyl-leucine-phenylalanine to trigger "inside-out" activation, the effects of hyperoxia are reversed. We conclude that cytoskeletal changes triggered by hyperoxia inhibit beta(2) integrin-dependent neutrophil adhesion.

Distinct kinetic and mechanical properties govern ALCAM-mediated interactions as shown by single-molecule force spectroscopy

The activated leukocyte cell adhesion molecule (ALCAM) mediates dynamic homotypic and heterotypic cellular interactions. Whereas homotypic ALCAM-ALCAM interactions have been implicated in the development and maintenance of tissue architecture and tumor progression, heterotypic ALCAM-CD6 interactions act to initiate and stabilize T-cell–dendritic-cell interactions affecting T-cell activation. The ability to resist the forces acting on the individual bonds during these highly dynamic cellular contacts is thought to be crucial for the (patho)physiology of ALCAM-mediated cell adhesion. Here, we used atomic force microscopy to characterize the relationship between affinity, avidity and the stability of ALCAM-mediated interactions under external loading, at the single-molecule level. Disruption of the actin cytoskeleton resulted in enhanced ALCAM binding avidity, without affecting the tensile strength of the individual bonds. Force spectroscopy revealed that the ALCAM-CD6 bond displayed a significantly higher tensile strength, a smaller reactive compliance and an up to 100-fold lower dissociation rate in the physiological force window in comparison to the homotypic interaction. These results indicate that homotypic and heterotypic ALCAM-mediated adhesion are governed by significantly distinct kinetic and mechanical properties, providing novel insight into the role of ALCAM during highly dynamic cellular interactions.

Cell-matrix adhesion complexes: master control machinery of cell migration

Cell-matrix adhesion complexes (CMACs) are foci of cellular attachment to the extracellular matrix (ECM). This attachment, mediated by integrins and adaptor proteins, provides both physical and regulatory links between the ECM and the cellular microfilament system. Through continual regulation and rearrangement of both ECM adhesion and actin structures, CMACs constitute core machineries of cell migration. To fulfill this role, CMACs are exceptionally flexible and dynamic complexes, and their components undergo rapid and regulated turn-over to maintain delicately balanced streams of mechanical and chemical information. Besides the critical role of CMACs in cell migration, signaling through these complexes provides influence over virtually every major cellular function, including for example cell survival, cell differentiation and cell proliferation. This review depicts the roles of CMACs in cell migration and discusses how CMACs integrate with other sub-cellular systems involved in cell motility. Importantly, we also present a rationalized view of CMACs as information handling machines, and suggest strategies that may facilitate better understanding of the complex cell migration phenomenon as a whole, through quantitative and integrative (systems biology) approaches.

Interplay between PEO tether length and ligand spacing governs cell spreading on RGD-modified PMMA-g-PEO comb copolymers

The effects of tether length on cell adhesion to poly(methyl methacrylate)-graft-poly(ethylene oxide), PMMA-g-PEO, comb copolymer films functionalized with the adhesion peptide RGD were investigated. Copolymers having PEO tether lengths of 10 and 22 EO segments were synthesized and coupled with a synthetic peptide that contained both RGD and the synergy sequence PHSRN. Cell spreading assays revealed that the longer polymer tethers increased the rate of spreading and reduced the time required for fibroblasts to form focal adhesions. Fluorescence resonance energy transfer (FRET) measurements indicated a mean separation between integrin-bound peptides of 15.6 +/- 1.4 nm for combs with long (22-mer) tethers, compared with 17.5 +/- 1.3 nm for short (10-mer) tethers, on films of comparable peptide density (approximately 2500 peptides/microm2). The results suggest that the added mobility afforded by the more extensible tethers encouraged the formation of focal adhesions by allowing cells to reorganize tethered peptides on the nanometer length scale. In addition, adhesion peptides were selectively coupled to 10-mer or 22-mer PEO tethers within a bimodal brush to investigate stratification effects on cell adhesion. Peptides bound by short tethers in a bed of long unsubstituted chains resulted in surfaces that resisted, rather than promoted, cell adhesion. By contrast, when long peptide tethers were employed with short unsubstituted chains, cell attachment and spreading were comparable to that found on a monomodal brush of long chains at equivalent peptide density.

CD8+ dendritic cells use LFA-1 to capture MHC-peptide complexes from exosomes in vivo

Exosomes are secreted vesicles formed in late endocytic compartments. Mature dendritic cells (DCs) secrete exosomes bearing functional MHC-peptide complexes and high levels of ICAM-1. Such exosomes can activate Ag-specific naive T cells but only after recapture by recipient APCs. In this study, we addressed the molecular mechanisms of interaction between exosomes and recipient DCs. We show that exosomes can be presented by mouse DCs without the need for internalization and processing. Exosomes interact with DCs through a specific saturable receptor. Although the two major ligands of ICAM-1, LFA-1 and Mac-1, are expressed by lymphoid organ DCs, only LFA-1 is required for exosome capture by these cells. Accordingly, we show that CD8(+) DCs express higher levels of LFA-1 than CD8(-) DCs, and that they are the main recipients of exosomes in vivo. We propose a new role for LFA-1 on DCs, as a receptor for exosomes to favor Ag transfer between DCs in vivo.

Small GTPases and LFA-1 reciprocally modulate adhesion and signaling

Leukocyte-function-associated antigen-1 (LFA-1) is an integrin that is critical for T-cell adhesion and immunologic responses. As a transmembrane receptor and adhesion molecule, LFA-1 signals bidirectionally, whereby information about extracellular ligands is passed outside-in while cellular activation is transmitted inside-out to the adhesive ectodomain. Here, we review the role of small guanosine triphosphatases (GTPases) in LFA-1 signaling. Rap1, a Ras-related GTPase, appears to be central to LFA-1 function. Rap1 is regulated by receptor signaling [e.g. T-cell receptor (TCR), CD28, and cytotoxic T-lymphocyte antigen-4 (CTLA-4)] and by adapter proteins [e.g. adhesion and degranulation-promoting adapter protein (ADAP) and Src kinase-associated phosphoprotein of 55 kDa (SKAP-55)]. Inside-out signaling flows through Rap1 to regulator of adhesion and cell polarization enriched in lymphoid tissues (RAPL) and Rap1-GTP interacting adapter molecule (RIAM) that act in conjunction with the cytoskeleton on the cytosolic domain of LFA-1 to increase adhesion of the ectodomain. Outside-in signaling also relies on small GTPases such as Rho proteins. Vav-1, a guanine nucleotide exchange factor for Rho proteins, is activated as a consequence of LFA-1 engagement. Jun-activating binding protein-1 (JAB-1) and cytohesin-1 have been implicated as possible outside-in signaling intermediates. We have recently shown that Ras is also downstream of LFA-1 engagement: LFA-1 signaling through phospholipase D (PLD) to RasGRP1 was required for Ras activation on the plasma membrane following stimulation of TCR.

The role of the integrin LFA-1 in T-lymphocyte migration

A successful immune response depends on the migration of lymphocytes into lymph nodes or inflamed tissues where they make contact with antigen-presenting cells. We are interested in how one member of the integrin family, leukocyte function-associated antigen-1 (LFA-1), controls the function and, in particular, the migration of immune cells. We find that this integrin operates not only as an adhesion receptor for T lymphoblasts (T cells) but also induces their migration in vitro at approximately 15 microm/min. Migration requires active myosin light chain kinase at the leading edge and Rho kinase at the trailing edge of the cell. Two active conformations of LFA-1 are differently distributed on the T-cell membrane and regulate independent aspects of migration. High-affinity LFA-1 is located in a midcell 'focal zone' and influences the speed of migration, whereas intermediate affinity LFA-1 controls leading edge adhesions. Manipulating LFA-1 conformation in vivo can be performed, for example, by creating the active conformation in a transgenic mouse, and this model gives further insight into the role of LFA-1 in migration. In humans, the beneficial effect of functioning CD18 integrins in combating infections in vivo is illustrated by rare patients displaying two forms of leukocyte adhesion deficiency. In summary, we speculate that T cells have evolved a mode of rapid migration that is of paramount importance in achieving the high-speed immune surveillance upon which depends the body's protection against diverse invaders from pathogens to cancer cells.

Two synergistic activation mechanisms of alpha2beta1 integrin-mediated collagen binding

Activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA) induces ligand-independent aggregation of a cell surface collagen receptor, alpha2beta1 integrin. Concomitantly, TPA increases the avidity of alpha2beta1 for collagen and the number of conformationally activated alpha2beta1 integrins. The structural change was shown using a monoclonal antibody 12F1 that recognizes the "open" (active) conformation of the inserted domain in the alpha2 subunit (alpha2I). Amino acid residue Glu-336 in alpha2 subunit is proposed to mediate the interaction between alpha2I domain and beta1 subunit. Glu-336 seems to regulate a switch between open and "closed" conformations, since the mutation alpha2E336A inhibited the TPA-related increase in the number of 12F1 positive integrins. E336A also reduced cell adhesion to collagen. However, E336A did not prevent the TPA-related increase in adhesion to collagen or alpha2beta1 aggregation. Thus, alpha2beta1 integrin avidity is regulated by two synergistic mechanisms, first an alpha2E336-dependent switch to the open alpha2I conformation, and second an alpha2E336-independent mechanism temporally associated with receptor aggregation.

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.

Posterolateral approach for open reduction and internal fixation of trimalleolar ankle fractures

Cell-cell and cell-matrix interactions are of critical importance in immunobiology. Leukocytes make extensive use of a specialized repertoire of receptors to mediate such processes. Among these receptors, integrins are known to be of crucial importance. This review deals with the central role of integrins and their counterreceptors during the establishment of leukocyte-endothelium contacts, interstitial migration, and final encounter with antigen-presenting cells to develop an appropriate immune response. Particularly, we have addressed the molecular events occurring during these sequential processes, leading to the dynamic subcellular redistribution of adhesion receptors and the reorganization of the actin cytoskeleton, which is reflected in changes in cytoarchitecture, including leukocyte polarization, endothelial docking structure formation, or immune synapse organization. The roles of signaling and structural actin cytoskeleton-associated proteins and organized membrane microdomains in the regulation of receptor adhesiveness are also discussed.