Paxillin binding to the alpha 4 integrin subunit stimulates LFA-1 (integrin alpha L beta 2)-dependent T cell migration by augmenting the activation of focal adhesion kinase/proline-rich tyrosine kinase-2

Ectopic expression of vascular cell adhesion molecule-1 as a new mechanism for tumor immune evasion

Immune escape is an important reason why the immune system cannot control tumor growth, but how escape variants emerge during immunotherapy remains poorly understood. Here, we identify a new mechanism of tumor immune escape using an in vivo selection strategy. We generated a highly immune-resistant cancer cell line (P3) by subjecting a susceptible cancer cell line (P0/TC-1) to multiple rounds of in vivo immune selection. Microarray analysis of P0 and P3 revealed that vascular cell adhesion molecule-1 (VCAM-1) is up-regulated in the P3-resistant variant. Retroviral transfer of VCAM-1 into P0 significantly increased its resistance against a vaccine-induced immune response. Analysis of tumors showed a dramatic decrease in the number of tumor-infiltrating cluster of differentiation 8(+) (CD8(+)) T cells in the tumors expressing VCAM-1. In vitro transwell migration assays showed that VCAM-1 can promote the migration of CD8(+) T cells through its interaction with the alpha(4)beta(1) integrin. Site-directed mutagenesis of VCAM-1 at amino acid residues required for interaction with alpha(4)beta(1) integrin completely abolished the immune resistance conferred by VCAM-1 in vivo. Surface staining showed that most renal cell carcinomas (RCC) express VCAM-1, whereas an RCC that responded to vaccination was VCAM-1 negative. These data provide evidence that tumor expression of VCAM-1 represents a new mechanism of immune evasion and has important implications for the development of immunotherapy for human RCC.

In vivo CD44-CD49d complex formation in autoimmune disease has consequences on T cell activation and apoptosis resistance

CD44 is involved in leukocyte migration and activation and has recently been reported to contribute to leukocyte extravasation by associating with CD49d. We explored whether similar changes in CD44 activity are seen in vivo using murine alopecia areata (AA) as a chronic, organ-related autoimmune disease model system. Expression of the activated, hyaluronan-binding form of CD44, and of CD49d, was elevated in draining lymph node cells (LNC) of AA-affected mice as compared to control mice. LNC of AA mice displayed increased motility, proliferative activity and apoptosis resistance, which were equally well inhibited by anti-CD44 and anti-CD49d. The latter is the sequelae of the association between CD44 and CD49d that is seen in activated lymphocytes. Significantly, due to CD44-CD49d complex formation, CD44 gains access to focal adhesion kinase and CD49d gains access to CD44-associated lck and ezrin, such that downstream kinases become activated via CD44 or CD49d engagement. Thus, by their association, CD44 and CD49d mutually avail themselves of the partner's signaling pathways and the ligand binding of each one triggers signaling pathways of both. This strongly influences the lymphocytes' activation state and function.

Activation, coactivation, and costimulation of resting human natural killer cells

Natural killer (NK) cells possess potent perforin- and interferon-gamma-dependent effector functions that are tightly regulated. Inhibitory receptors for major histocompatibility complex class I display variegated expression among NK cells, which confers specificity to individual NK cells. Specificity is also provided by engagement of an array of NK cell activation receptors. Target cells may express ligands for a multitude of activation receptors, many of which signal through different pathways. How inhibitory receptors intersect different signaling cascades is not fully understood. This review focuses on advances in understanding how activation receptors cooperate to induce cytotoxicity in resting NK cells. The role of activating receptors in determining specificity and providing redundancy of target cell recognition is discussed. Using Drosophila insect cells as targets, we have examined the contribution of individual receptors. Interestingly, the strength of activation is not determined simply by additive effects of parallel activation pathways. Combinations of signals from different receptors can have different outcomes: synergy, no enhancement over individual signals, or additive effects. Cytotoxicity requires combined signals for granule polarization and degranulation. The integrin leukocyte function-associated antigen-1 contributes a signal for polarization but not for degranulation. Conversely, CD16 alone or in synergistic combinations, such as NKG2D and 2B4, signals for phospholipase-C-gamma- and phosphatidylinositol-3-kinase-dependent degranulation.

The p110delta isoform of PI3K differentially regulates beta1 and beta2 integrin-mediated monocyte adhesion and spreading and modulates diapedesis

OBJECTIVE

Leukocyte diapedesis is misregulated in inflammatory disease and depends on the binding of monocytic LFA-1 and VLA-4 to endothelial ICAM-1 and VCAM-1, respectively. The authors hypothesized that these different molecular interactions elicit specific signaling cascades within monocytes regulating specific steps in adhesion, motility, and diapedesis.

METHODS

The authors employed the PI3K p110delta catalytic subunit specific inhibitor IC87114 (2 microM) and the broad-spectrum PI3K inhibitory agents LY294002 (50 microM) and wortmannin (100 nM), to examine the role of PI3Kdelta in monocyte diapedesis through endothelial monolayers and its role in monocyte adhesion and spreading upon carpets of ICAM-1 or VCAM-1. They further explored the effects of PI3Kdelta inhibition on the activation state of beta1 and beta2 integrins with immunocytochemistry and flow cytometry.

RESULTS

In human peripheral blood monocytes IC87114 was as effective as wortmannin and LY294002 at inhibiting diapedesis, however, in THP-1 cells LY294002 and wortmannin caused a 5-fold reduction in diapedesis, while IC87114 only decreased diapedesis 2-fold. PI3Kdelta activity was specifically required for THP-1 cell adhesion and spreading on VCAM-1, but not on ICAM-1 protein substrates. Flow cytometric analysis demonstrated that PI3Kdelta inhibition decreased the amount of conformationally active beta 1-integrins, while having no effect on the prevalence of conformationally active beta 2-integrins expressed on the cell surface. In addition, PI3Kdelta inhibition resulted in a 4-fold decrease in the activation state of Rac-1 and Cdc42.

CONCLUSIONS

These results demonstrate the specific necessity of PI3Kdelta in regulating monocytic integrin activation and the general role of PI3K signaling during diapedesis, implicating PI3K as a target for therapeutic intervention.

Differential engagement of modules 1 and 4 of vascular cell adhesion molecule-1 (CD106) by integrins alpha4beta1 (CD49d/29) and alphaMbeta2 (CD11b/18) of eosinophils

We have studied adhesion of eosinophils to various forms of vascular cell adhesion molecule 1 (VCAM-1, CD106), an integrin counter-receptor implicated in eosinophil recruitment to the airway in asthma. Full-length 7d-VCAM-1, with seven immunoglobulin-like modules, contains integrin-binding sites in modules 1 and 4. The alternatively spliced six-module protein, 6d-VCAM-1, lacks module 4. In static assays, unactivated purified human blood eosinophils adhered similarly to recombinant soluble human 6d-VCAM-1 and 7d-VCAM-1 coated onto polystyrene microtiter wells. Further experiments, however, revealed differences in recognition of modules 1 and 4. Antibody blocking indicated that eosinophil adhesion to 6d-VCAM-1 or a VCAM-1 construct containing only modules 1-3, 1-3VCAM-1, is mediated by alpha4beta1 (CD49d/29), whereas adhesion to a construct containing modules 4-7, 4-7VCAM-1, is mediated by bothalpha4beta1 andalphaMbeta2 (CD11b/18). Inhibitors of phosphoinositide 3-kinase, which block adhesion of eosinophils mediated by alphaMbeta2, blocked adhesion to 4-7VCAM-1 but had no effect on adhesion to 6d-VCAM-1. Consistent with the antibody and pharmacological blocking experiments, eosinophilic leukemic cell lines lacking alphaMbeta2 did not adhere to 4-7VCAM-1 but did adhere to 6d-VCAM-1 or 1-3VCAM-1. Activation of eosinophils by interleukin (IL)-5 enhanced static adhesion to 6d-VCAM-1, 7d-VCAM-1, or 4-7VCAM-1; IL-5-enhanced adhesion to all 3 constructs was blocked by anti-alphaMbeta2. Adhesion of unstimulated eosinophils to 7d-VCAM-1 under flow conditions was inhibited by anti-alpha4 or anti-alphaM. IL-5 treatment decreased eosinophil adhesion to 7d-VCAM-1 under flow, and anti-alphaM had the paradoxical effect of increasing adhesion. These results demonstrate that alphaMbeta2 modulatesalpha4beta1-mediated eosinophil adhesion to VCAM-1 under both static and flow conditions.

New approaches to blockade of alpha4-integrins, proven therapeutic targets in chronic inflammation

The recruitment of leukocytes into tissue is a pivotal step in inflammation. alpha4-Integrins are adhesion receptors on circulating leukocytes that mediate attachment to the endothelium and facilitate their migration into the inflamed tissue. This multistep process is mediated by the interaction of alpha4-integrins with their counter receptors VCAM-1 and MadCAM-1 that are expressed on endothelial cells. alpha4-Integrins act as both adhesive and signaling receptors. Paxillin, a signaling adaptor molecule, binds directly to the alpha4 cytoplasmic tail and its binding is important for cell migration. Blocking the adhesive functions of alpha4-integrins has been shown to be an effective therapeutic approach in the treatment of autoimmune diseases, but also carries the risk of defects in development, hematopoiesis and immune surveillance. Interfering with alpha4 signaling by inhibiting the alpha4-paxillin interaction decreases alpha4-mediated cell migration and adhesion to VCAM-1 and MadCAM under shear flow. These in vitro effects are accompanied by a selective impairment of leukocyte migration into inflammatory sites when the alpha4-paxillin interaction is blocked in vivo. Thus, blockade of alpha4-integrin signaling may offer a novel strategy for interfering with the functions of these receptors in pathological events while sparing important physiological functions.

Contrasting Roles for Domain 4 of VCAM-1 in the Regulation of Cell Adhesion and Soluble VCAM-1 Binding to Integrin α4β1

Cell adhesion mediated by the interaction between integrin alpha4beta1 and VCAM-1 is important in normal physiologic processes and in inflammatory and autoimmune disease. Numerous studies have mapped the alpha4beta1 binding sites in VCAM-1 that mediate cell adhesion; however, little is known about the regions in VCAM-1 important for regulating soluble binding. In the present study, we demonstrate that 6D VCAM-1 (an alternatively spliced isoform of VCAM-1 lacking Ig-like domain 4) binds alpha4beta1 with a higher relative affinity than does the full-length form of VCAM-1 containing 7 Ig-like extracellular domains (7D VCAM-1). In indirect binding assays, the EC50 of soluble 6D VCAM-1 binding to alpha4beta1 on Jurkat cells (in 1 mM MnCl2) was 2 x 10(-9) M, compared with 7D VCAM-1 at 11 x 10(-9) M. When used in solution to inhibit alpha4beta1 mediated cell adhesion, the IC50 of 6D VCAM-1 was 13 x 10(-9) M, compared with 7D VCAM-1 measured at 150 x 10(-9) M. Removal of Ig-like domains 4, 5, or 6, or simply substituting Asp328 in domain 4 of 7D VCAM-1 with alanine, caused increased binding of soluble 7D VCAM-1 to alpha4beta1. In contrast, cells adhered more avidly to 7D VCAM-1 under shear force, as it induced cell spreading at lower concentrations than did 6D VCAM-1. Finally, soluble 6D VCAM-1 acts as an agonist through alpha4beta1 by augmenting cell migration and inducing cell aggregation. These results indicate that the domain 4 of VCAM-1 plays a contrasting role when VCAM-1 is presented in solution or as a cell surface-expressed adhesive substrate.

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.

Blocking the alpha 4 integrin-paxillin interaction selectively impairs mononuclear leukocyte recruitment to an inflammatory site

Antagonists to alpha4 integrin show promise for several autoimmune and inflammatory diseases but may exhibit mechanism-based toxicities. We tested the capacity of blockade of alpha4 integrin signaling to perturb functions involved in inflammation, while limiting potential adverse effects. We generated and characterized mice bearing a Y991A mutation in alpha4 integrin [alpha4(Y991A) mice], which blocks paxillin binding and inhibits alpha4 integrin signals that support leukocyte migration. In contrast to the embryonic-lethal phenotype of alpha4 integrin-null mice, mice bearing the alpha4(Y991A) mutation were viable and fertile; however, they exhibited defective recruitment of mononuclear leukocytes into thioglycollate-induced peritonitis. Alpha4 integrins are essential for definitive hematopoiesis; however, the alpha4(Y991A) mice had intact lymphohematopoiesis and, with the exception of reduced Peyer's patches, normal architecture and cellularity of secondary lymphoid tissues. We conclude that interference with alpha4 integrin signaling can selectively impair mononuclear leukocyte recruitment to sites of inflammation while sparing vital functions of alpha4 integrins in development and hematopoiesis.

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.

GPI-anchored CEA family glycoproteins CEA and CEACAM6 mediate their biological effects through enhanced integrin α5β1-fibronectin interaction

Carcinoembryonic antigen (CEA) and CEA family member CEACAM6 are glycophosphatidyl inositol (GPI)-anchored, intercellular adhesion molecules that are up-regulated in a wide variety of human cancers, including colon, breast, and lung. When over-expressed in a number of cellular systems, these molecules are capable of inhibiting cellular differentiation and anoikis, as well as disrupting cell polarization and tissue architecture, thus increasing tumorigenicity. The present study shows that perturbation of the major fibronectin receptor, integrin alpha5beta1, underlies some of these biological effects. Using confocal microscopy and specific antibodies, CEA and CEACAM6 were demonstrated to co-cluster with integrin alpha5beta1 on the cell surface. The presence of CEA and CEACAM6 was shown to lead to an increase in the binding of the integrin alpha5beta1 receptor to its ligand fibronectin, without changing its cell surface levels, resulting in increased adhesion of CEA/CEACAM6-expressing cells to fibronectin. More tenacious binding of free fibronectin to cells led to enhanced fibronectin matrix assembly and the formation of a polymerized fibronectin "cocoon" around the cells. Disruption of this process with specific monoclonal antibodies against either fibronectin or integrin alpha5beta1 led to the restoration of cellular differentiation and anoikis in CEA/CEACAM6 producing cells.

Integrin alpha4beta1 promotes focal adhesion kinase-independent cell motility via alpha4 cytoplasmic domain-specific activation of c-Src

The fibronectin binding integrins alpha5beta1 and alpha4beta1 generate signals pivotal for cell migration through distinct yet undefined mechanisms. For alpha5beta1, beta1-mediated activation of focal adhesion kinase (FAK) promotes c-Src recruitment to FAK and the formation of a FAK-Src signaling complex. Herein, we show that FAK expression is essential for alpha5beta1-stimulated cell motility and that exogenous expression of human alpha4 in FAK-null fibroblasts forms a functional alpha4beta1 receptor that promotes robust cell motility equal to the alpha5beta1 stimulation of wild-type and FAK-reconstituted fibroblasts. alpha4beta1-stimulated FAK-null cell spreading and motility were dependent on the integrity of the alpha4 cytoplasmic domain, independent of direct paxillin binding to alpha4, and were not affected by PRNK expression, a dominant-negative inhibitor of Pyk2. alpha4 cytoplasmic domain-initiated signaling led to a approximately 4-fold activation of c-Src which did not require paxillin binding to alpha4. Notably, alpha4-stimulated cell motility was inhibited by catalytically inactive receptor protein-tyrosine phosphatase alpha overexpression and blocked by the p50Csk phosphorylation of c-Src at Tyr-529. alpha4beta1-stimulated cell motility of triple-null Src(-/-), c-Yes(-/-), and Fyn(-/-) fibroblasts was dependent on c-Src reexpression that resulted in p130Cas tyrosine phosphorylation and Rac GTPase loading. As p130Cas phosphorylation and Rac activation are common downstream targets for alpha5beta1-stimulated FAK activation, our results support the existence of a novel alpha4 cytoplasmic domain connection leading to c-Src activation which functions as a FAK-independent linkage to a common motility-promoting signaling pathway.

Prolactin stimulates integrin-mediated adhesion of circulating mononuclear cells to endothelial cells

Attachment of leukocytes to endothelial cells is an essential step for the extravasation and recruitment of cells at sites of inflammation. The pituitary hormone prolactin (PRL) is involved in the inflammatory process. Here, we show that treatment with PRL of human peripheral blood mononuclear cells (PBMC) stimulates their adhesion to human umbilical vein endothelial cells (HUVEC) activated by interleukin-1beta. Stimulation of adhesion by PRL is mediated via integrins leukocyte functional antigen-1 (LFA-1) and very late antigen-4 (VLA-4), because immunoneutralization of both integrins prevents PRL action. Also, PRL promotes the adhesion of PBMC to immobilized intercellular adhesion molecule-1 and fibronectin, ligands for LFA-1 and VLA-4, respectively. Stimulation of integrin-mediated cell adhesion by PRL may involve the activation of chemokine receptors, because PRL upregulates the expression of the G-protein-coupled chemokine receptor CXCR3 in PBMC, and pertussis toxin, a specific G-protein inhibitor, blocks PRL stimulation of PBMC adhesion to HUVEC. In addition, PRL stimulates tyrosine phosphorylation pathways leading to leukocyte adhesion. PRL triggered the tyrosine phosphorylation of Janus kinase-2, of signal transducer and activator of transcription-3 and 5, and of the focal adhesion protein paxillin. Furthermore, genistein, a tyrosine kinase inhibitor, blocked PRL-stimulated adhesion of PBMC and Jurkat T-cells to HUVEC. These results suggest that PRL promotes integrin-mediated leukocyte adhesion to endothelial cells via chemokine receptors and tyrosine phosphorylation signaling pathways.

Ligation of lymphocyte function-associated antigen-1 on monocytes decreases very late antigen-4-mediated adhesion through a reactive oxygen species-dependent pathway

Monocyte-endothelial adhesion plays an important role in monocyte trafficking and hence is important for immune responses and pathogenesis of inflammatory diseases including atherosclerosis. The cross-talk between different integrins on monocytes may be crucial for a coordinated regulation of the cellular adhesion during the complex process of transendothelial migration. By using monoclonal antibodies and recombinant intercellular adhesion molecule 1 (ICAM-1) to engage lymphocyte function-associated antigen 1 (LFA-1) on monocytic cells, we found that the cellular adhesion to vascular cell adhesion molecule 1 (VCAM-1) mediated by very late antigen 4 (VLA-4) was suppressed after this treatment and the suppression depended on the presence of reactive oxygen species (ROSs). Inhibition of production of ROSs through the use of inhibitor of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, but not inhibitors of mitochondrial electron transport chain or xanthine oxidase, revealed that this suppression on VLA-4-mediated cellular binding was mediated by ROSs produced by phagocyte NADPH oxidase. Activation of phosphoinositol-3 kinase and Akt appears to mediate this NADPH oxidase activation through p47phox phosphorylation and Rac-1 activation. Our results provide a novel pathway in which ROSs play a critical role in integrin cross-talk in monocytes. This signaling pathway may be important for cellular transition from firm arrest to diapedesis during monocyte trafficking. (Blood. 2004;104:4046-4053)

Paxillin selectively associates with constitutive and chemoattractant-induced high-affinity α4β1 integrins: implications for integrin signaling

Leukocyte α4β1 integrins regulate hematopoietic and lymphoid development, as well as the emigration of circulating cells to sites of inflammation. Because vascular cell adhesion molecule-1 (VCAM-1) binding to high-affinity α4β1 is stable, these integrins can be detected and selectively precipitated from cell lysates using VCAM-1/Fc. With this approach, high-affinity α4β1 integrin expression was demonstrated on lymphocytes in the bone marrow, thymus, spleen, and the peritoneal cavity of normal mice, but not in peripheral lymph nodes. Immature lymphocytes preferentially expressed high-affinity α4β1 in the bone marrow and thymus. Paxillin is a cytoplasmic adaptor molecule that can bind to the α4 tail and initiate signaling. Paxillin was associated selectively with high-affinity integrins that were isolated from human Jurkat T cells or from murine tissues, and blotting with a phospho-specific antibody demonstrated that Ser988 in the α4 cytoplasmic tail was dephosphorylated in high-affinity but not low-affinity integrins. A rapid and transient α4β1 affinity up-regulation in formyl peptide receptor-transfected U937 cells stimulated with N-formyl-methyonyl-leucyl-phenylalanine (fMLP) correlated temporally with induced paxillin binding to α4 integrins. These data suggest that ligand binding to high-affinity α4β1 integrins may initiate outside-in signaling cascades through paxillin that regulate leukocyte maturation and emigration. (Blood. 2004;104:2818-2824)

Phosphatidylinositol-3-kinase activation mediates proline-rich tyrosine kinase 2 phosphorylation and recruitment to β1-integrins in human CD34+ cells

OBJECTIVE

beta1-integrins mediate hematopoietic stem and progenitor cell homing and retention in the bone marrow (BM) and inhibit hematopoietic proliferation and differentiation. Having no intrinsic kinase activity, integrins recruit intracellular kinases, such as the focal adhesion kinase (FAK) or the related proline-rich tyrosine kinase 2 (PYK2), to initiate signal transduction. Phosphatidylinositol-3-kinase (PI3K), which is involved in beta1-integrin signaling in many cell types, is physically and functionally associated with FAK in anchorage-dependent cells. Because PYK2 is the principal focal adhesion kinase expressed in primary human CD34+ cells, we assessed its functional relationship with PI3K in CD34+ cells in response to integrin engagement.

METHODS

beta1-integrins on primary mobilized peripheral blood CD34+ cells and CD34+ KG1A cells were engaged by adhesion to fibronectin (FN) or by cross-linking with an anti-beta1 integrin antibody, respectively. PI3K activity and PYK2 phosphorylation were then assessed in the presence or absence of the PI3K inhibitor, wortmannin. Association between PI3K, PYK2, and the beta1-integrin subunit were also evaluated in co-immunoprecipitation experiments.

RESULTS

beta1-integrin engagement induced PI3K activation, which was required for, and temporally preceded, PYK2 phosphorylation, indicating that PI3K lies upstream of PYK2 in CD34+ cells. Furthermore, although PYK2 and PI3K were constitutively associated, interaction of the PYK2/PI3K complex with beta1-integrins required prior integrin engagement and PI3K activation.

CONCLUSION

Activation of PI3K following beta1-integrin engagement on human CD34+ cells results in subsequent phosphorylation of PYK2, and is required for the recruitment of the PI3K/PYK2 complex to beta1-integrins at the cell surface.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

Integrins and T cell-mediated immunity

Integrin receptors mediate adhesive events that are critical for a specific and effective immune response to foreign pathogens. Integrin-dependent interactions of lymphocytes and antigen-presenting cells (APCs) to endothelium regulate the efficiency and specificity of trafficking into secondary lymphoid organs and peripheral tissue. Within these sites, integrins facilitate cell movement via interactions with the extracellular matrix, and promote and stabilize antigen-specific interactions between T lymphocytes and APCs that are critical for initiating T cell-activation events. In this review, we discuss the role of integrins in T cell-mediated immunity, with a focus on how these receptors participate in lymphocyte recirculation and T cell activation, how antigen stimulation regulates integrin activity, and how integrins define functionally unique subsets of T cells and APCs.

Conformational regulation of alpha 4 beta 1-integrin affinity by reducing agents. "Inside-out" signaling is independent of and additive to reduction-regulated integrin activation

The alpha(4)beta(1)-integrin (very late antigen-4 (VLA-4), CD49d/CD29) is an adhesion receptor involved in the interaction of lymphocytes, dendritic cells, and stem cells with the extracellular matrix and endothelial cells. This and other integrins have the ability to regulate their affinity for ligands through a process termed "inside-out" signaling that affects cell adhesion avidity. Several mechanisms are known to regulate integrin affinity and conformation: conformational changes induced by separation of the C-terminal tails, divalent ions, and reducing agents. Recently, we described a fluorescent LDV-containing small molecule that was used to monitor VLA-4 affinity changes in live cells (Chigaev, A., Blenc, A. M., Braaten, J. V., Kumaraswamy, N., Kepley, C. L., Andrews, R. P., Oliver, J. M., Edwards, B. S., Prossnitz, E. R., Larson, R. S., and Sklar, L. A. (2001) J. Biol. Chem. 276, 48670-48678). Using the same molecule, we also developed a fluorescence resonance energy transfer-based assay to probe the "switchblade-like" opening of VLA-4 upon activation. Here, we investigated the effect of reducing agents on the affinity and conformational state of the VLA-4 integrin simultaneously with cell activation initiated by inside-out signaling through G protein-coupled receptors or Mn(2+) in live cells in real time. We found that reducing agents (dithiothreitol and 2,3-dimercapto-1-propanesulfonic acid) induced multiple states of high affinity of VLA-4, where the affinity change was accompanied by an extension of the integrin molecule. Bacitracin, an inhibitor of the reductive function of the plasma membrane, diminished the effect of dithiothreitol, but had no effect on inside-out signaling. Based on this result and differences in the kinetics of integrin activation, we conclude that conformational activation of VLA-4 by inside-out signaling is independent of and additive to reduction-regulated integrin activation.

Competition for talin results in trans-dominant inhibition of integrin activation

The ability of integrin adhesion receptors to undergo rapid changes in affinity for their extracellular ligands (integrin activation) is essential for the development and function of multicellular animals and is dependent on interactions between the integrin beta subunit-cytoplasmic tail and the cytoskeletal protein talin. Cross-talk among different integrins and between integrins and other receptors impacts many cellular processes including adhesion, spreading, migration, clot retraction, proliferation, and differentiation. One form of integrin cross-talk, transdominant inhibition of integrin activation, occurs when ligand binding to one integrin inhibits the activation of a second integrin. This may be relevant clinically in a number of settings such as during platelet adhesion, leukocyte trans-migration, and angiogenesis. Here we report that competition for talin underlies the trans-dominant inhibition of integrin activation. This conclusion is based on our observations that (i). beta tails selectively defective in talin binding are unable to mediate trans-dominant inhibition, (ii). trans-dominant inhibition can be reversed by overexpression of integrin binding and activating fragments of talin, and (iii). expression of another non-integrin talin-binding protein, phosphatidylinositol phosphate kinase type Igamma-90, also inhibits integrin activation. Thus, the sequestration of talin by the suppressive species is both necessary and sufficient for trans-dominant inhibition of integrin activation.

NMR Solution Structure of the Focal Adhesion Targeting Domain of Focal Adhesion Kinase in Complex with a Paxillin LD Peptide

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is regulated by integrins. Upon activation, FAK generates signals that modulate crucial cell functions, including cell proliferation, migration, and survival. The C-terminal focal adhesion targeting (FAT) sequence mediates localization of FAK to discrete regions in the cell called focal adhesions. Several binding partners for the FAT domain of FAK have been identified, including paxillin. We have determined the solution structure of the avian FAT domain in complex with a peptide mimicking the LD2 motif of paxillin by NMR spectroscopy. The FAT domain retains a similar fold to that found in the unliganded form when complexed to the paxillin-derived LD2 peptide, an antiparallel four-helix bundle. However, noticeable conformational changes were observed upon the LD2 peptide binding, especially the position of helix 4. Multiple lines of evidence, including the results obtained from isothermal titration calorimetry, intermolecular nuclear Overhauser effects, mutagenesis, and protection from paramagnetic line broadening, support the existence of two distinct paxillin-binding sites on the opposite faces of the FAT domain. The structure of the FAT domain-LD2 complex was modeled using the program HADDOCK based on our solution structure of the LD2-bound FAT domain and mutagenesis data. Our model of the FAT domain-LD2 complex provides insight into the molecular basis of FAK-paxillin binding interactions, which will aid in understanding the role of paxillin in FAK targeting and signaling.

T-cell integrins: more than just sticking points

T cells use integrins in essentially all of their functions. They use integrins to migrate in and out of lymph nodes and, following infection, to migrate into other tissues. At the beginning of an immune response, integrins also participate in the immunological synapse formed between T cells and antigen-presenting cells. Because the ligands for integrins are widely expressed, integrin activity on T cells must be tightly controlled. Integrins become active following signalling through other membrane receptors, which cause both affinity alteration and an increase in integrin clustering. Lipid raft localization may increase integrin activity. Signalling pathways involving ADAP, Vav-1 and SKAP-55, as well as Rap1 and RAPL, cause clustering of leukocyte function-associated antigen-1 (LFA-1; integrin αLβ2). T-cell integrins can also signal, and the pathways dedicated to the migratory activity of T cells have been the most investigated so far. Active LFA-1 causes T-cell attachment and lamellipodial movement induced by myosin light chain kinase at the leading edge, whereas RhoA and ROCK cause T-cell detachment at the trailing edge. Another important signalling pathway acts through CasL/Crk, which might regulate the activity of the GTPases Rac and Rap1 that have important roles in T-cell migration.

Integrin alpha 4 beta 1-dependent T cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin

alpha 4 integrins mediate increased cell migration and decreased cell spreading because the alpha 4 cytoplasmic domain (tail) binds tightly to paxillin, a signaling adaptor protein. Paxillin binding to the alpha 4 tail is blocked by alpha 4 phosphorylation at Ser988. To establish the biological role of alpha 4 phosphorylation, we reconstituted alpha 4-deficient Jurkat T cells with phosphorylation-mimicking (alpha 4(S988D)) or non-phosphorylatable (alpha 4(S988A)) mutants. alpha 4(S988D) disrupted paxillin binding and also inhibited cell migration and promoted cell spreading. In contrast, the non-phosphorylatable alpha 4(S988A) resulted in a further reduction in cell spreading; however, this mutation led to an unexpected suppression of cell migration. The suppression of cell migration by alpha 4(S988A) was ascribable to enhanced alpha 4-paxillin association, because enforced association by an alpha 4-paxillin fusion led to a phenotype similar to that of the non-phosphorylatable alpha 4(S988A) mutant. These data establish that optimal alpha 4-mediated cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin.