Making a little affinity go a long way: a topological view of LFA-1 regulation

Size-dependent protein segregation at membrane interfaces

Membrane interfaces formed at cell-cell junctions are associated with characteristic patterns of membrane protein organization, such as E-cadherin enrichment in epithelial junctional complexes and CD45 exclusion from the signaling foci of immunological synapses. To isolate the role of protein size in these processes, we reconstituted membrane interfaces in vitro using giant unilamellar vesicles decorated with synthetic binding and non-binding proteins. We show that size differences between binding and non-binding proteins can dramatically alter their organization at membrane interfaces in the absence of active contributions from the cytoskeleton, with as little as a ~5 nm increase in non-binding protein size driving its exclusion from the interface. Combining in vitro measurements with Monte Carlo simulations, we find that non-binding protein exclusion is also influenced by lateral crowding, binding protein affinity, and thermally-driven membrane height fluctuations that transiently limit access to the interface. This simple, sensitive, and highly effective means of passively segregating proteins has implications for signaling at cell-cell junctions and protein sorting at intracellular contact points between membrane-bound organelles.

The dendritic cell cytoskeleton promotes T cell adhesion and activation by constraining ICAM-1 mobility

Integrity of the dendritic cell (DC) actin cytoskeleton is essential for T cell priming, but the underlying mechanisms are poorly understood. We show that the DC F-actin network regulates the lateral mobility of intracellular cell adhesion molecule 1 (ICAM-1), but not MHCII. ICAM-1 mobility and clustering are regulated by maturation-induced changes in the expression and activation of moesin and α-actinin-1, which associate with actin filaments and the ICAM-1 cytoplasmic domain. Constrained ICAM-1 mobility is important for DC function, as DCs expressing a high-mobility ICAM-1 mutant lacking the cytoplasmic domain exhibit diminished antigen-dependent conjugate formation and T cell priming. These defects are associated with inefficient induction of leukocyte functional antigen 1 (LFA-1) affinity maturation, which is consistent with a model in which constrained ICAM-1 mobility opposes forces on LFA-1 exerted by the T cell cytoskeleton, whereas ICAM-1 clustering enhances valency and further promotes ligand-dependent LFA-1 activation. Our results reveal an important new mechanism through which the DC cytoskeleton regulates receptor activation at the immunological synapse.

F-actin flow drives affinity maturation and spatial organization of LFA-1 at the immunological synapse

The T cell actin network generates mechanical forces that regulate LFA-1 activity at the immunological synapse.

Integrin-dependent interactions between T cells and antigen-presenting cells are vital for proper T cell activation, effector function, and memory. Regulation of integrin function occurs via conformational change, which modulates ligand affinity, and receptor clustering, which modulates valency. Here, we show that conformational intermediates of leukocyte functional antigen 1 (LFA-1) form a concentric array at the immunological synapse. Using an inhibitor cocktail to arrest F-actin dynamics, we show that organization of this array depends on F-actin flow and ligand mobility. Furthermore, F-actin flow is critical for maintaining the high affinity conformation of LFA-1, for increasing valency by recruiting LFA-1 to the immunological synapse, and ultimately for promoting intracellular cell adhesion molecule 1 (ICAM-1) binding. Finally, we show that F-actin forces are opposed by immobilized ICAM-1, which triggers LFA-1 activation through a combination of induced fit and tension-based mechanisms. Our data provide direct support for a model in which the T cell actin network generates mechanical forces that regulate LFA-1 activity at the immunological synapse.

Membrane protein dynamics and functional implications in mammalian cells

The organization of the plasma membrane is both highly complex and highly dynamic. One manifestation of this dynamic complexity is the lateral mobility of proteins within the plane of the membrane, which is often an important determinant of intermolecular protein-binding interactions, downstream signal transduction, and local membrane mechanics. The mode of membrane protein mobility can range from random Brownian motion to immobility and from confined or restricted motion to actively directed motion. Several methods can be used to distinguish among the various modes of protein mobility, including fluorescence recovery after photobleaching, single-particle tracking, fluorescence correlation spectroscopy, and variations of these techniques. Here, we present both a brief overview of these methods and examples of their use to elucidate the dynamics of membrane proteins in mammalian cells-first in erythrocytes, then in erythroblasts and other cells in the hematopoietic lineage, and finally in non-hematopoietic cells. This multisystem analysis shows that the cytoskeleton frequently governs modes of membrane protein motion by stably anchoring the proteins through direct-binding interactions, by restricting protein diffusion through steric interactions, or by facilitating directed protein motion. Together, these studies have begun to delineate mechanisms by which membrane protein dynamics influence signaling sequelae and membrane mechanical properties, which, in turn, govern cell function.

Functional anatomy of T cell activation and synapse formation

T cell activation and function require a structured engagement of antigen-presenting cells. These cell contacts are characterized by two distinct dynamics in vivo: transient contacts resulting from promigratory junctions called immunological kinapses or prolonged contacts from stable junctions called immunological synapses. Kinapses operate in the steady state to allow referencing to self-peptide-MHC (pMHC) and searching for pathogen-derived pMHC. Synapses are induced by T cell receptor (TCR) interactions with agonist pMHC under specific conditions and correlate with robust immune responses that generate effector and memory T cells. High-resolution imaging has revealed that the synapse is highly coordinated, integrating cell adhesion, TCR recognition of pMHC complexes, and an array of activating and inhibitory ligands to promote or prevent T cell signaling. In this review, we examine the molecular components, geometry, and timing underlying kinapses and synapses. We integrate recent molecular and physiological data to provide a synthesis and suggest ways forward.

An extended vision for dynamic high-resolution intravital immune imaging

The past few years have seen the application of confocal and especially two-photon microscopy to the dynamic high-resolution imaging of lymphocytes and antigen presenting cells within organs such as lymph nodes and thymus. After summarizing some of the published results obtained to date using these methods, we describe our view of how this technology will develop and be applied in the near future. This includes its extension to a wide variety of non-lymphoid tissues, to the tracking of functional responses in addition to migratory behavior, to the analysis of molecular events previously studied only in vitro, to dissection of the interplay between hematopoietic and stromal elements, to visualization of a wider array of cell types including neutrophils, macrophages, NK cells, NKT cells and others, and to the interaction of the host with infectious agents. Reaching these goals will depend on a combination of new tools for genetic manipulations, novel fluorescent reporters, enhanced instrumentation, and better surgical techniques for the extended imaging of live animals. The end result will be a new level of understanding of how orchestrated cell movement and interaction contribute to the physiological and pathological activities of the immune system.

Molecular mechanics and dynamics of leukocyte recruitment during inflammation

Discovery of new genes and proteins directly supporting leukocyte adhesion is waning, whereas there is heightened interest in the cell mechanics and receptor dynamics that lead from transient tethering via selectins to affinity shifts and adhesion strengthening through integrins. New optical tools enable real-time imaging of leukocyte rolling and arrest in parallel plate flow channels (PPFCs), and detection of single-molecule force spectroscopy provides an inner view of the intercellular adhesive contact region. Leukocyte recruitment during acute inflammation is triggered by ligation of G protein-coupled chemotactic receptors (GPCRs) and clustering of selectins. This, in turn, activates beta(2)-integrin (CD18), which facilitates cell capture and arrest in shear flow. This review provides a conceptual model for the molecular events supporting leukocyte recruitment.

A Model for CD2/CD58-Mediated Adhesion Strengthening

Stable cell adhesion is vital for structural integrity and functional efficacy. Yet how low affinity adhesion molecules such as CD2 and CD58 can produce stable cell adhesion is still not completely understood. In this paper, we present a theoretical model that simulates the accumulation of CD2 and CD58 in the contact area of a Jurkat T lymphoblast and a CD58-containing substrate. The cell is assumed to have a spherical shape initially and it is allowed to spread gradually on a circular substrate. Mobile CD2 and CD58 can diffuse freely on both the cell and substrate. Their binding in the contact area is controlled by first-order kinetics. The contact area grows linearly with the total number of CD2/CD58 bonds. Cellular deformation and cytoskeleton involvement were not considered. This time-dependent moving-boundary problem was solved with the Crank-Nicolson finite difference scheme and the variable space grid method. Our simulated results are in reasonable agreement with the experimental observations. The role of diffusion becomes more and more prominent during the contact area increase, which is not sensitive to the kinetic rate constants tested in this study. However, it is very sensitive to the dissociation equilibrium constant and the concentrations of CD2 and CD58.

Immunological synapse formation licenses CD40-CD40L accumulations at T-APC contact sites

The maintenance of tolerance is likely to rely on the ability of a T cell to polarize surface molecules providing "help" to only specific APCs. The formation of a mature immunological synapse leads to concentration of the TCR at the APC interface. In this study, we show that the CD40-CD154 receptor-ligand pair is also highly concentrated into a central region of the synapse on mouse lymphocytes only after the formation of the TCR/CD3 c-SMAC. Concentration of this ligand was strictly dependent on TCR recognition, the binding of ICAM-1 to T cell integrins and the presence of an intact cytoskeleton in the T cells. This may provide a novel explanation for the specificity of T cell help directing the help signal to the site of Ag receptor signal. It may also serve as a site for these molecular aggregates to coassociate and/or internalize alongside other signaling receptors.

Cooperative adhesion of ligand-receptor bonds

Cooperative (simultaneous) breakage of multiple adhesive bonds has been proposed as a mechanism for enhanced binding strength between adhesion molecules on apposing cell surfaces. In this report, we used the atomic force microscopy (AFM) to study how changes in binding affinity and separation rate of force-induced ligand-receptor dissociation affect binding cooperativity. The AFM force measurements were carried out using (strept)avidin-functionalized cantilever tips and biotinylated agarose beads under conditions where multiple (strept)avidin-biotin linkages were formed following surface contact. At slow surface separation of the AFM cantilever from the bead's surface, the (strept)avidin-biotin linkages appeared to rupture sequentially. Increasing the separation rate from 210 to 1950 nm/s led to a linear increase in the average rupture force. Moreover, force histograms revealed a quantized force distribution that shifted toward higher values with increasing separation rate. In measurements of streptavidin-iminobiotin adhesion, the force distribution also shifted toward higher values when the buffer was adjusted to a higher pH to raise the binding affinity. Together, these results demonstrate that the cooperativity of ligand-receptor bonds is significantly enhanced by increases in surface separation rate and/or binding affinity.

The immunological synapse: integrins take the stage

Adhesive interactions play important roles in coordinating T-cell migration and activation, specifically in the formation of the immunological synapse (IS), a specialized cell-cell junction. Recent demonstrations show several molecules implicated in T-cell signaling, including Vav, ADAP, and Rap-1, have major roles in integrin regulation and place adhesion molecules at center stage in addressing the question: what are the signals involved in the formation of the IS and full T-cell activation? This review focuses on the role of integrins as an essential system for both physical adhesion and signaling in T-cell activation. The role of integrins appears to be quite distinct from classical costimulation and has been largely overlooked due to the ubiquitous use of serum in lymphocyte functional assays. Each major signal transduction pathway has branches leading to the nucleus and others that feed back on cytoskeletal and membrane regulation at the IS.

Vav1 controls integrin clustering and MHC/peptide-specific cell adhesion to antigen-presenting cells

Integrin-mediated adhesion is essential for the formation of stable contacts between T cells and antigen-presenting cells (APCs). We show that Vav1 controls integrin-mediated adhesion of thymocytes and T cells to ECM proteins and ICAM1 following TCR stimulation. In a peptide-specific system, Vav1 is required for T cell adhesion to peptide-loaded APCs. Intriguingly, TCR-induced cell adhesion and aggregation of integrins occurs independent of WASP. Whereas LFA-1 and actin caps colocalize in wasp(-/-) T cells in response to TCR stimulation, loss of WASP uncouples TCR caps from actin patches. Our data reveal a novel role for Vav1 and WASP in the regulation of TCR-induced integrin clustering and cell adhesion and show that integrin and TCR clustering are controlled by distinct pathways.

CD18 activation epitopes induced by leukocyte activation

The cell surface adhesion molecule LFA-1 coordinates leukocyte trafficking and is a costimulatory molecule for T cell activation. We developed a panel of mAbs that recognize activation epitopes on the CD18 subunit, and show that stimulation of T lymphocytes appears to be accompanied by a conformational change in a subpopulation of LFA-1 that does not require ligand binding. Activation epitope up-regulation requires divalent cations, is sensitive to cellular signal transduction events, and correlates with cell adhesion. In addition, the stimulated appearance of these activation epitopes is absent in cell lines from patients with leukocyte adhesion deficiency-1/variant that has previously been shown to be defective in LFA-1 activation. Thus, these activation epitope Abs can be used to dissect signal transmission to CD18. Evidence suggests that these CD18 activation epitopes are induced early in cellular activation and are independent of actin rearrangement necessary for avid adhesion. We have also determined that function-blocking CD18 Abs inhibit the induction of activation epitopes. One activation epitope Ab binds to a site on CD18 distinct from that of the blocking Abs, indicating that the blocking Abs suppress a conformational change in LFA-1. We also find that these neoepitopes are present on rLFA-1 with high affinity for ICAM-1 and their binding is modulated in parallel with the affinity of LFA-1 for ICAM-1. Collectively, these neoepitope Abs identify a subpopulation of LFA-1 most likely with high affinity for ICAM-1 and necessary for LFA-1 function.

Rho and Rho-associated kinase modulate the tyrosine kinase PYK2 in T-cells through regulation of the activity of the integrin LFA-1

We have examined the role of the small GTPase Rho and its downstream effector, the Rho-associated kinase (ROCK), in the control of the adhesive and signaling function of the lymphocyte function-associated antigen-1 (LFA-1) integrin in human T-lymphocytes. Inhibition of Rho (either by treatment with C3-exoenzyme or transfection with a dominant-negative form of Rho (N19Rho)) or ROCK (by treatment with Y-27632) results in the following: (a) partial disorganization and aggregation of cortical filamentous actin (F-actin); (b) induction of LFA-1-mediated cellular adhesion to the LFA-1 ligand intercellular adhesion molecule-1 (ICAM-1) through a mechanism involving clustering of LFA-1 molecules, rather than alterations in the level of expression or in the affinity state of this integrin; and (c) induction of cellular polarization and activation of the tyrosine kinase PYK2. Transfection of T-cells with a constitutively active form of Rho (V14Rho) blocks the clustering of LFA-1 on the membrane and the LFA-1-mediated activation of PYK2. Importantly, the activation of PYK2 caused by inhibition of Rho or ROCK takes place only when the T-cells are plated onto ICAM-1 but not when they are either prevented from interacting with ICAM-1 with anti-LFA-1 blocking antibodies or when they are plated on the nonspecific poly-l-lysine substrate. These results indicate that the small GTPase Rho regulates the tyrosine kinase PYK2 in T-cells through the F-actin-mediated control of the activity of the integrin LFA-1. These findings represent a novel paradigm for the regulation of the activity of a cytoplasmic tyrosine kinase by the small GTPase Rho.

Effects of IL-8, Gro-alpha, and LTB(4) on the adhesive kinetics of LFA-1 and Mac-1 on human neutrophils

Firm adhesion of rolling neutrophils on inflamed endothelium is dependent on beta(2) (CD18)-integrins and activating stimuli. LFA-1 (CD11a/CD18) appears to be more important than Mac-1 (CD11b/CD18) in neutrophil emigration at inflammatory sites, but little is known of the relative binding characteristics of these two integrins under conditions thought to regulate firm adhesion. The present study examined the effect of chemoattractants on the kinetics of LFA-1 and Mac-1 adhesion in human neutrophils. We found that subnanomolar concentrations of interleukin-8, Gro-alpha, and leukotriene B(4) (LTB(4)) induced rapid and optimal rates of LFA-1-dependent adhesion of neutrophils to intercellular adhesion molecule (ICAM)-1-coated beads. These optimal rates of LFA-1 adhesion were transient and decayed within 1 min after chemoattractant stimulation. Mac-1 adhesion was equally rapid initially but continued to rise for >/=6 min after stimulation. A fourfold higher density of ICAM-1 on beads markedly increased the rate of binding to LFA-1 but did not change the early and narrow time window for the optimal rate of adhesion. Using well-characterized monoclonal antibodies, we showed that activation of LFA-1 and Mac-1 by Gro-alpha was completely blocked by anti-CXC chemokine receptor R2, but activation of these integrins by interleukin-8 was most effectively blocked by anti-CXC chemokine receptor R1. The topographical distribution of beads also reflected significant differences between LFA-1 and Mac-1. Beads bound to Mac-1 translocated to the cell uropod within 4 min, but beads bound to LFA-1 remained bound to the lamellipodial regions at the same time. These kinetic and topographical differences may indicate distinct functional contributions of LFA-1 and Mac-1 on neutrophils.

Cellular activation of leukocyte function-associated antigen-1 and its affinity are regulated at the I domain allosteric site

The I domain of the integrin LFA-1 possesses a ligand binding interface that includes the metal ion-dependent adhesion site. Binding of the LFA-1 ligand, ICAM-1 to the metal ion-dependent adhesion site is regulated by the I domain allosteric site (IDAS). We demonstrate here that intracellular signaling leading to activation of LFA-1 binding to ICAM-1 is regulated at the IDAS. Inhibitory mutations in or proximal to the IDAS are dominant to cytoplasmic signals that activate binding to ICAM-1. In addition, mutational activation at the IDAS greatly increases the binding of lymphocyte-expressed LFA-1 to ICAM-1 in response to PMA, but does not result in constitutive binding. Binding of a novel CD18 activation epitope mAb to LFA-1 in response to soluble ICAM-1 binding was also blocked by inhibitory and was enhanced by activating IDAS mutations. Surface plasmon resonance using soluble wild-type LFA-1 and an IDAS mutant of LFA-1 indicate that the IDAS can regulate a 6-fold change in the K(d) of ICAM-1 binding. The K(d) of wild-type LFA-1 (1.2 x 10(-1) s(-1)) differed with that of the activating IDAS mutant (1.9 x 10(-2) s(-1)), but their K(a) values were identical (2.2 x 10(5) M(-1)s(-1)). We propose that IDAS regulates the binding of LFA-1 to ICAM-1 activated by intracellular signals. IDAS can control the affinity state of LFA-1 with concomitant I domain and CD18 conformational changes.

Analysis of fertilin alpha (ADAM1)-mediated sperm-egg cell adhesion during fertilization and identification of an adhesion-mediating sequence in the disintegrin-like domain

Fertilin alpha (also known as ADAM1) is a member of the ADAM (A disintegrin and A metalloprotease domain) family of proteins. In this study, we examine the mechanism of mouse fertilin alpha's in adhesion of sperm to the egg plasma membrane during fertilization. We find that recombinant forms of fertilin alpha corresponding to either the disintegrin-like domain or the cysteine-rich domain and the EGF-like repeat can perturb sperm-egg binding, suggesting that both of these domains can participate in fertilin alpha-mediated adhesion events. In further examination of the fertilin alpha disintegrin-like domain, we find that a subdomain of disintegrin-like domain with the sequence DLEECDCG outside the putative disintegrin loop but with homology to the fertilin beta disintegrin loop can inhibit the binding of both sperm and recombinant fertilin alpha to eggs, suggesting that this is an adhesion-mediating motif of the fertilin alpha disintegrin-like domain. This sequence also inhibits the binding of recombinant fertilin beta to eggs and thus is the first peptide sequence found to block two different sperm ligands. Finally, a monoclonal antibody to the tetraspanin protein CD9, KMC.8, inhibited the binding of recombinant fertilin alpha to eggs in one type of binding assay, suggesting that, under certain conditions, fertilin alpha may interact with a KMC.8-sensitive binding site on the egg plasma membrane.

Fertilin beta and other ADAMs as integrin ligands: insights into cell adhesion and fertilization

One of the most important cell-cell interactions is that of the sperm with the egg. This interaction, which begins with cell adhesion and culminates with membrane fusion, is mediated by multiple molecules on the gametes. One of the best-characterized of these molecules is fertilin beta, a ligand on mammalian sperm and one of the first ADAMs (A Disintegrin and A Metalloprotease domain) to be identified. Fertilin beta (also known as ADAM2) participates in sperm-egg membrane binding, and it has long been hypothesized that this function is achieved through the interaction of the disintegrin domain of fertilin beta with an integrin on the egg surface. There are now approximately 30 members of the ADAM family and, to date, five different ADAMs (fertilin beta, ADAM9, ADAM12, ADAM15, ADAM23) have been described to interact with integrins (specifically alpha(6)beta(1), alpha(v)beta(3), alpha(9)beta(1), alpha(v)beta(5), and/or alpha(5)beta(1)). This field will be discussed with respect to what is known about specific ADAMs and the integrins with which they interact, and what the implications are for sperm-egg interactions and for integrin function. These data will also be discussed in the context of recent knockout studies, which show that eggs lacking the alpha(6) integrin subunit can be fertilized, and eggs lacking the integrin-associated tetraspanin protein CD9 fail to fertilize. Key issues in cell adhesion that pertain to gametes and fertilization will also be highlighted.

Interaction of T cells with APCs: the serial encounter model

Primary immune responses are initiated by specific physical interaction of antigen-specific T cells and professional antigen-presenting cells (APCs). Productive interactions can be a dynamic process that combines physical T-cell binding to APCs with vigorous crawling across and scanning of the APC surface, resulting in signal induction. After T-cell detachment, subsequent migratory contacts to the same or neighboring dendritic cells (DCs) allow the accumulation of sequential signals and interaction time. Here, we develop a serial encounter model of T-cell activation and discuss how the summation of multiple signals provides an efficient strategy to control an ongoing immune response.

Cytohesin-1 regulates beta-2 integrin-mediated adhesion through both ARF-GEF function and interaction with LFA-1

Intracellular signaling pathways, which regulate the interactions of integrins with their ligands, affect a wide variety of biological functions. Here we provide evidence of how cytohesin-1, an integrin-binding protein and guanine-nucleotide exchange factor (GEF) for ARF GTPases, regulates cell adhesion. Mutational analyses of the beta-2 cytoplasmic domain revealed that the adhesive function of LFA-1 depends on its interaction with cytohesin-1, unless the integrin is activated by exogenous divalent cations. Secondly, cytohesin-1 induces expression of an extracellular activation epitope of LFA-1, and the exchange factor function is not essential for this activity. In contrast, LFA-1-mediated cell adhesion and spreading on intercellular cell adhesion molecule 1 is strongly inhibited by a cytohesin-1 mutant, which fails to catalyze ARF GDP-GTP exchange in vitro. Thus, cytohesin-1 is involved in the activation of LFA-1, most probably through direct interaction with the integrin, and induces cell spreading by its ARF-GEF activity. We therefore propose that both direct regulation of the integrin and concomitant changes in the membrane topology of adherent T cells are modulated by dissectable functions of cytohesin-1.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

Mechanisms and consequences of affinity modulation of integrin alpha(V)beta(3) detected with a novel patch-engineered monovalent ligand

Integrin alpha(V)beta(3) mediates diverse responses in vascular cells, ranging from cell adhesion, migration, and proliferation to uptake of adenoviruses. However, the extent to which alpha(V)beta(3) is regulated by changes in receptor conformation (affinity), receptor diffusion/clustering (avidity), or post-receptor events is unknown. Affinity regulation of the related integrin, alpha(IIb)beta(3), has been established using a monovalent ligand-mimetic antibody, PAC1 Fab. To determine the role of affinity modulation of alpha(V)beta(3), a novel monovalent ligand-mimetic antibody (WOW-1) was created by replacing the heavy chain hypervariable region 3 of PAC1 Fab with a single alpha(V) integrin-binding domain from multivalent adenovirus penton base. Both WOW-1 Fab and penton base bound selectively to activated alpha(V)beta(3), but not to alpha(IIb)beta(3), in receptor and cell binding assays. alpha(V)beta(3) affinity varied with the cell type. Unstimulated B-lymphoblastoid cells bound WOW-1 Fab poorly (apparent K(d) = 2.4 microM), but acute stimulation with phorbol 12-myristate 13-acetate increased receptor affinity >30-fold (K(d) = 80 nM), with no change in receptor number. In contrast, alpha(V)beta(3) in melanoma cells was constitutively active, but ligand binding could be suppressed by overexpression of beta(3) cytoplasmic tails. Up-regulation of alpha(V)beta(3) affinity had functional consequences in that it increased cell adhesion and spreading and promoted adenovirus-mediated gene transfer. These studies establish that alpha(V)beta(3) is subject to rapid regulated changes in affinity that influence the biological functions of this integrin.

Type IV pili of pathogenic Neisseriae elicit cortical plaque formation in epithelial cells

The pathogenic Neisseriae Neisseria meningitidis and Neisseria gonorrhoeae, initiate colonization by attaching to host cells using type IV pili. Subsequent adhesive interactions are mediated through the binding of other bacterial adhesins, in particular the Opa family of outer membrane proteins. Here, we have shown that pilus-mediated adhesion to host cells by either meningococci or gonococci triggers the rapid, localized formation of dramatic cortical plaques in host epithelial cells. Cortical plaques are enriched in both components of the cortical cytoskeleton and a subset of integral membrane proteins. These include: CD44v3, a heparan sulphate proteoglycan that may serve as an Opa receptor; EGFR, a receptor tyrosine kinase; CD44 and ICAM-1, adhesion molecules known to mediate inflammatory responses; f-actin; and ezrin, a component that tethers membrane components to the actin cytoskeleton. Genetic analyses reveal that cortical plaque formation is highly adhesin specific. Both pilE and pilC null mutants fail to induce cortical plaques, indicating that neisserial type IV pili are required for cortical plaque induction. Mutations in pilT, a gene required for pilus-mediated twitching motility, confer a partial defect in cortical plaque formation. In contrast to type IV pili, many other neisserial surface structures are not involved in cortical plaque induction, including Opa, Opc, glycolipid GgO4-binding adhesins, polysialic acid capsule or a particular lipooligosaccharide variant. Furthermore, it is shown that type IV pili allow gonococci to overcome the inhibitory effect of heparin, a soluble receptor analogue, on gonococcal invasion of Chang and A431 epithelial cells. These and other observations strongly suggest that type IV pili play an active role in initiating neisserial infection of the mucosal surface in vivo. The functions of type IV pili and other neisserial adhesins are discussed in the specific context of the mucosal microenvironment, and a multistep model for neisserial colonization of mucosal epithelia is proposed.