Epitope mapping of monoclonal antibody to integrin alphaL beta2 hybrid domain suggests different requirements of affinity states for intercellular adhesion molecules (ICAM)-1 and ICAM-3 binding

Extracellular vesicles from the lung pro-thrombotic niche drive cancer-associated thrombosis and metastasis via integrin beta 2

Cancer is a systemic disease with complications beyond the primary tumor site. Among them, thrombosis is the second leading cause of death in patients with certain cancers (e.g., pancreatic ductal adenocarcinoma [PDAC]) and advanced-stage disease. Here, we demonstrate that pro-thrombotic small extracellular vesicles (sEVs) are secreted by C-X-C motif chemokine 13 (CXCL13)-reprogrammed interstitial macrophages in the non-metastatic lung microenvironment of multiple cancers, a niche that we define as the pro-thrombotic niche (PTN). These sEVs package clustered integrin β2 that dimerizes with integrin αX and interacts with platelet-bound glycoprotein (GP)Ib to induce platelet aggregation. Blocking integrin β2 decreases both sEV-induced thrombosis and lung metastasis. Importantly, sEV-β2 levels are elevated in the plasma of PDAC patients prior to thrombotic events compared with patients with no history of thrombosis. We show that lung PTN establishment is a systemic consequence of cancer progression and identify sEV-β2 as a prognostic biomarker of thrombosis risk as well as a target to prevent thrombosis and metastasis.

THE CIS ASSOCIATION OF CD47 WITH INTEGRIN Mac-1 REGULATES MACROPHAGE RESPONSES BY STABILIZING THE EXTENDED INTEGRIN CONFORMATION

CD47 is a ubiquitously expressed cell surface integrin-associated protein. Recently, we have demonstrated that integrin Mac-1 (α_Mβ₂, CD11b/CD18, CR3), the major adhesion receptor on the surface of myeloid cells, can be coprecipitated with CD47. However, the molecular basis for the CD47-Mac-1 interaction and its functional consequences remain unclear. Here, we demonstrated that CD47 regulates macrophage functions directly interacting with Mac-1. In particular, adhesion, spreading, migration, phagocytosis, and fusion of CD47-deficient macrophages were significantly decreased. We validated the functional link between CD47 and Mac-1 by co-immunoprecipitation analysis using various Mac-1-expressing cells. In HEK293 cells expressing individual α_M and β₂ integrin subunits, CD47 was found to bind both subunits. Interestingly, a higher amount of CD47 was recovered with the free β₂ subunit than in the complex with the whole integrin. Furthermore, activating Mac-1-expressing HEK293 cells with PMA, Mn²⁺_, and activating antibody MEM48 increased the amount of CD47 in complex with Mac-1, suggesting CD47 has a greater affinity for the extended integrin conformation. Notably, on the surface of cells lacking CD47, fewer Mac-1 molecules could convert into an extended conformation in response to activation. Additionally, we identified the binding site in CD47 for Mac-1 in its constituent IgV domain. The complementary binding sites for CD47 in Mac-1 were localized in integrin epidermal growth factor-like domains 3 and 4 of the β₂ and calf-1 and calf-2 domains of the α subunits. These results indicate that Mac-1 forms a lateral complex with CD47, which regulates essential macrophage functions by stabilizing the extended integrin conformation.

Intersecting Mechanisms of Hypoxia and Prostaglandin E2-Mediated Inflammation in the Comparative Biology of Oral Squamous Cell Carcinoma

The importance of inflammation in the pathogenesis of cancer was first proposed by Rudolph Virchow over 150 years ago, and our understanding of its significance has grown over decades of biomedical research. The arachidonic acid pathway of inflammation, including cyclooxygenase (COX) enzymes, PGE2 synthase enzymes, prostaglandin E2 (PGE2) and PGE2 receptors has been extensively studied and has been associated with different diseases and different types of cancers, including oral squamous cell carcinoma (OSCC). In addition to inflammation in the tumour microenvironment, low oxygen levels (hypoxia) within tumours have also been shown to contribute to tumour progression. Understandably, most of our OSCC knowledge comes from study of this aggressive cancer in human patients and in experimental rodent models. However, domestic animals develop OSCC spontaneously and this is an important, and difficult to treat, form of cancer in veterinary medicine. The primary goal of this review article is to explore the available evidence regarding interaction between hypoxia and the arachidonic acid pathway of inflammation during malignant behaviour of OSCC. Overlapping mechanisms in hypoxia and inflammation can contribute to tumour growth, angiogenesis, and, importantly, resistance to therapy. The benefits and controversies of anti-inflammatory and anti-angiogenic therapies for human and animal OSCC patients will be discussed, including conventional pharmaceutical agents as well as natural products.

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Understanding the Synergy of NKp46 and Co-Activating Signals in Various NK Cell Subpopulations: Paving the Way for More Successful NK-Cell-Based Immunotherapy

The NK cell population is characterized by distinct NK cell subsets that respond differently to the various activating stimuli. For this reason, the determination of the optimal cytotoxic activation of the different NK cell subsets can be a crucial aspect to be exploited to counter cancer cells in oncologic patients. To evaluate how the triggering of different combination of activating receptors can affect the cytotoxic responses of different NK cell subsets, we developed a microbead-based degranulation assay. By using this new assay, we were able to detect CD107a⁺ degranulating NK cells even within the less cytotoxic subsets (i.e., resting CD56^bright and unlicensed CD56^dim NK cells), thus demonstrating its high sensitivity. Interestingly, signals delivered by the co-engagement of NKp46 with 2B4, but not with CD2 or DNAM-1, strongly cooperate to enhance degranulation on both licensed and unlicensed CD56^dim NK cells. Of note, 2B4 is known to bind CD48 hematopoietic antigen, therefore this observation may provide the rationale why CD56^dim subset expansion correlates with successful hematopoietic stem cell transplantation mediated by alloreactive NK cells against host T, DC and leukemic cells, while sparing host non-hematopoietic tissues and graft versus host disease. The assay further confirms that activation of LFA-1 on NK cells leads to their granule polarization, even if, in some cases, this also takes to an inhibition of NK cell degranulation, suggesting that LFA-1 engagement by ICAMs on target cells may differently affect NK cell response. Finally, we observed that NK cells undergo a time-dependent spontaneous (cytokine-independent) activation after blood withdrawal, an aspect that may strongly bias the evaluation of the resting NK cell response. Altogether our data may pave the way to develop new NK cell activation and expansion strategies that target the highly cytotoxic CD56^dim NK cells and can be feasible and useful for cancer and viral infection treatment.

Understanding the Synergy of NKp46 and Co-Activating Signals in Various NK Cell Subpopulations: Paving the Way for More Successful NK-Cell-Based Immunotherapy

The NK cell population is characterized by distinct NK cell subsets that respond differently to the various activating stimuli. For this reason, the determination of the optimal cytotoxic activation of the different NK cell subsets can be a crucial aspect to be exploited to counter cancer cells in oncologic patients. To evaluate how the triggering of different combination of activating receptors can affect the cytotoxic responses of different NK cell subsets, we developed a microbead-based degranulation assay. By using this new assay, we were able to detect CD107a⁺ degranulating NK cells even within the less cytotoxic subsets (i.e., resting CD56^bright and unlicensed CD56^dim NK cells), thus demonstrating its high sensitivity. Interestingly, signals delivered by the co-engagement of NKp46 with 2B4, but not with CD2 or DNAM-1, strongly cooperate to enhance degranulation on both licensed and unlicensed CD56^dim NK cells. Of note, 2B4 is known to bind CD48 hematopoietic antigen, therefore this observation may provide the rationale why CD56^dim subset expansion correlates with successful hematopoietic stem cell transplantation mediated by alloreactive NK cells against host T, DC and leukemic cells, while sparing host non-hematopoietic tissues and graft versus host disease. The assay further confirms that activation of LFA-1 on NK cells leads to their granule polarization, even if, in some cases, this also takes to an inhibition of NK cell degranulation, suggesting that LFA-1 engagement by ICAMs on target cells may differently affect NK cell response. Finally, we observed that NK cells undergo a time-dependent spontaneous (cytokine-independent) activation after blood withdrawal, an aspect that may strongly bias the evaluation of the resting NK cell response. Altogether our data may pave the way to develop new NK cell activation and expansion strategies that target the highly cytotoxic CD56^dim NK cells and can be feasible and useful for cancer and viral infection treatment.

Activated integrins identify functional antigen-specific CD8+ T cells within minutes after antigen stimulation

Immediate β2-integrin activation upon T cell receptor stimulation is critical for effective interaction between T cells and their targets and may therefore be used for the rapid identification and isolation of functional T cells. We present a simple and sensitive flow cytometry-based assay to assess antigen-specific T cells using fluorescent intercellular adhesion molecule (ICAM)-1 multimers that specifically bind to activated β2-integrins. The method is compatible with surface and intracellular staining; it is applicable for monitoring of a broad range of virus-, tumor-, and vaccine-specific CD8+ T cells, and for isolating viable antigen-reacting cells. ICAM-1 binding correlates with peptide-MHC multimer binding but, notably, it identifies the fraction of antigen-specific CD8+ T cells with immediate and high functional capability (i.e., expressing high levels of cytotoxic markers and cytokines). Compared with the currently available methods, staining of activated β2-integrins presents the unique advantage of requiring activation times of only several minutes, therefore delivering functional information nearly reflecting the in vivo situation. Hence, the ICAM-1 assay is most suitable for rapid and precise monitoring of functional antigen-specific T cell responses, including for patient samples in a variety of clinical settings, as well as for the isolation of functional T cells for adoptive cell-transfer immunotherapies.

Selectins and chemokines use shared and distinct signals to activate β2 integrins in neutrophils

Rolling neutrophils receive signals while engaging P- and E-selectin and chemokines on inflamed endothelium. Selectin signaling activates β2 integrins to slow rolling velocities. Chemokine signaling activates β2 integrins to cause arrest. Despite extensive study, key aspects of these signaling cascades remain unresolved. Using complementary in vitro and in vivo assays, we found that selectin and chemokine signals in neutrophils triggered Rap1a-dependent and phosphatidylinositol-4-phosphate 5-kinase γ (PIP5Kγ90)-dependent pathways that induce integrin-dependent slow rolling and arrest. Interruption of both pathways, but not either pathway alone, blocked talin-1 recruitment to and activation of integrins. An isoform of PIP5Kγ90 lacking the talin-binding domain (PIP5Kγ87) could not activate integrins. Chemokines, but not selectins, used phosphatidylinositol-4,5-bisphosphate 3-kinase γ (PI3Kγ) in cooperation with Rap1a to mediate integrin-dependent slow rolling (at low chemokine concentrations), as well as arrest (at high chemokine concentrations). High levels of chemokines activated β2 integrins without selectin signals. When chemokines were limiting, they synergized with selectins to activate β2 integrins.

Hypoxia Enhances Fusion of Oral Squamous Carcinoma Cells and Epithelial Cells Partly via the Epithelial-Mesenchymal Transition of Epithelial Cells

Increasing evidence and indications showed that cell fusion is crucial in tumor development and metastasis, and hypoxia, a closely linked factor to tumor microenvironment, which can lead to EMT, induces angiogenesis and metastasis in tumor growth. However, the relationship between hypoxia and fusion has not been reported yet. EMT will change some proteins in the epithelial cell surface and the changes of proteins in cell surface may increase cell fusion. This study found that hypoxia promotes the spontaneous cell fusion between Oral Squamous Carcinoma Cells (OSCCs) and Human Immortalized Oral Epithelial Cells (HIOECs). At the same time, Hypoxia can lead to EMT, and hypoxia-pretreated HIOECs increased fusion rate with OSCC, while the fusion rate was significantly reduced by DAPT, a kind of EMT blocker. Therefore, epithelial cells can increase spontaneously cell fusion with OSCC by EMT. Our study may provide a new insight to link among tumor microenvironment, cell fusion, and cancer.

Downstream effect profiles discern different mechanisms of integrin αLβ2 inhibition

The integrin leucocyte function-associated antigen-1 (αLβ2, LFA-1) plays crucial roles in T cell adhesion, migration and immunological synapse (IS) formation. Consequently, αLβ2 is an important therapeutic target in autoimmunity. Three major classes of αLβ2 inhibitors with distinct modes of action have been described to date: Monoclonal antibodies (mAbs), small molecule α/β I allosteric and small molecule α I allosteric inhibitors. The objective of this study was to systematically compare these three modes of αLβ2 inhibition for their αLβ2 inhibitory as well as their potential agonist-like effects. All inhibitors assessed were found to potently block αLβ2-mediated leucocyte adhesion. None of the inhibitors induced ZAP70 phosphorylation, indicating absence of agonistic outside-in signalling. Paradoxically, however, the α/β I allosteric inhibitor XVA143 induced conformational changes within αLβ2 characteristic for an intermediate affinity state. This effect was not observed with the α I allosteric inhibitor LFA878 or the anti-αLβ2 mAb efalizumab. On the other hand, efalizumab triggered the unscheduled internalization of αLβ2 in CD4+ and CD8+ T cells while LFA878 and XVA143 did not affect or only mildly reduced αLβ2 surface expression, respectively. Moreover, efalizumab, in contrast to the small molecule inhibitors, disturbed the fine-tuned internalization/recycling of engaged TCR/CD3, concomitantly decreasing ZAP70 expression levels. In conclusion, different modes of αLβ2 inhibition are associated with fundamentally different biologic effect profiles. The differential established here is expected to provide important translational guidance as novel αLβ2 inhibitors will be advanced from bench to bedside.

LFA-1 integrin antibodies inhibit leukocyte α4β1-mediated adhesion by intracellular signaling

Binding of intercellular adhesion molecule-1 to the β2-integrin leukocyte function associated antigen-1 (LFA-1) is known to induce cross-talk to the α4β1 integrin. Using different LFA-1 monoclonal antibodies, we have been able to study the requirement and mechanism of action for the cross-talk in considerable detail. LFA-1-activating antibodies and those inhibitory antibodies that signal to α4β1 induce phosphorylation of Thr-758 on the β2-chain, which is followed by binding of 14-3-3 proteins and signaling through the G protein exchange factor Tiam1. This results in dephosphorylation of Thr-788/789 on the β1-chain of α4β1 and loss of binding to its ligand vascular cell adhesion molecule-1. The results show that with LFA-1 antibodies, we can activate LFA-1 and inhibit α4β1, inhibit both LFA-1 and α4β1, inhibit LFA-1 but not α4β1, or not affect LFA-1 or α4β1 These findings are important for the understanding of integrin regulation and for the interpretation of the effect of integrin antibodies and their use in clinical applications.

Multi-Inhibitory Effects of A2A Adenosine Receptor Signaling on Neutrophil Adhesion Under Flow

A2A adenosine receptor (A2AAR) signaling negatively regulates inflammatory responses in many disease models, but the detailed mechanisms remain unclear. We used the selective A2AAR agonist, ATL313, to examine how A2AAR signaling affects human and murine neutrophil adhesion under flow. Treating neutrophils with ATL313 inhibited selectin-induced, β2 integrin-dependent slow rolling and chemokine-induced, β2 integrin-dependent arrest on ICAM-1. ATL313 inhibited selectin-induced β2 integrin extension, which supports slow rolling, and chemokine-induced hybrid domain "swing-out," which supports arrest. Furthermore, ATL313 inhibited integrin outside-in signaling as revealed by reduced neutrophil superoxide production and spreading on immobilized anti-β2 integrin Ab. ATL313 suppressed selectin-triggered activation of Src family kinases (SFKs) and p38 MAPK, chemokine-triggered activation of Ras-related protein 1, and β2 integrin-triggered activation of SFKs and Vav cytoskeletal regulatory proteins. ATL313 activated protein kinase A and its substrate C-terminal Src kinase, an inhibitor of SFKs. Treating neutrophils with a protein kinase A inhibitor blocked the actions of ATL313. In vivo, ATL313-treated neutrophils rolled faster and arrested much less frequently in postcapillary venules of the murine cremaster muscle after TNF-α challenge. Furthermore, ATL313 markedly suppressed neutrophil migration into the peritoneum challenged with thioglycollate. ATL313 did not affect A2AAR-deficient neutrophils, confirming its specificity. Our findings provide new insights into the anti-inflammatory mechanisms of A2AAR signaling and the potential utility of A2AAR agonists in inflammatory diseases.

A novel multi-parameter assay to dissect the pharmacological effects of different modes of integrin αLβ2 inhibition in whole blood

BACKGROUND AND PURPOSE

The integrin αLβ2 plays central roles in leukocyte adhesion and T cell activation, rendering αLβ2 an attractive therapeutic target. Compounds with different modes of αLβ2 inhibition are in development, currently. Consequently, there is a foreseeable need for bedside assays, which allow assessment of the different effects of diverse types of αLβ2 inhibitors in the peripheral blood of treated patients.

EXPERIMENTAL APPROACH

Here, we describe a flow cytometry-based technology that simultaneously quantitates αLβ2 conformational change upon inhibitor binding, αLβ2 expression and T cell activation at the single-cell level in human blood. Two classes of allosteric low MW inhibitors, designated α I and α/β I allosteric αLβ2 inhibitors, were investigated. The first application revealed intriguing inhibitor class-specific profiles.

KEY RESULTS

Half-maximal inhibition of T cell activation was associated with 80% epitope loss induced by α I allosteric inhibitors and with 40% epitope gain induced by α/β I allosteric inhibitors. This differential establishes that inhibitor-induced αLβ2 epitope changes do not directly predict the effect on T cell activation. Moreover, we show here for the first time that α/β I allosteric inhibitors, in contrast to α I allosteric inhibitors, provoked partial downmodulation of αLβ2, revealing a novel property of this inhibitor class.

CONCLUSIONS AND IMPLICATIONS

The multi-parameter whole blood αLβ2 assay described here may enable therapeutic monitoring of αLβ2 inhibitors in patients' blood. The assay dissects differential effect profiles of different classes of αLβ2 inhibitors.

Distinct binding affinities of Mac-1 and LFA-1 in neutrophil activation

Macrophage-1 Ag (Mac-1) and lymphocyte function-associated Ag-1 (LFA-1), two β2 integrins expressed on neutrophils (PMNs), mediate PMN recruitment cascade by binding to intercellular adhesive molecule 1. Distinct functions of LFA-1-initiating PMN slow rolling and firm adhesion but Mac-1-mediating cell crawling are assumed to be governed by the differences in their binding affinities and kinetic rates. In this study, we applied an adhesion frequency approach to compare their kinetics in the quiescent and activated states using three molecular systems, constitutively expressed receptors on PMNs, wild-type and high-affinity (HA) full-length constructs transfected on 293T cells, and wild-type and HA recombinant extracellular constructs. Data indicate that the difference in binding affinity between Mac-1 and LFA-1 is on-rate dominated with slightly or moderately varied off-rate. This finding was further confirmed when both β2 integrins were activated by chemokines (fMLF or IL-8), divalent cations (Mg(2+) or Mn(2+)), or disulfide bond lockage on an HA state. Structural analyses reveal that such the kinetics difference is likely attributed to the distinct conformations at the interface of Mac-1 or LFA-1 and intercellular adhesive molecule 1. This work furthers the understandings in the kinetic differences between Mac-1 and LFA-1 and in their biological correlations with molecular activation and structural bases.

Isoflurane binds and stabilizes a closed conformation of the leukocyte function-associated antigen-1

We previously demonstrated that isoflurane targets lymphocyte function-associated antigen-1 (LFA-1), a critical adhesion molecule for leukocyte arrest. However, it remains to be determined how isoflurane interacts with the full ectodomain LFA-1 and modulates its conformation and function. Isoflurane binding sites on the full ectodomain LFA-1 were probed by photolabeling using photoactivatable isoflurane (azi-isoflurane). The adducted residues were determined by liquid chromatography/mass spectrometry analysis. Separately, docking simulations were performed to predict binding sites. Point mutations were introduced around isoflurane binding sites. The significance of isoflurane's effect was assessed in both intracellular adhesion molecule-1 (ICAM-1) binding assays and epitope mapping of activation-sensitive antibodies using flow cytometry. Two isoflurane binding sites were identified using photolabeling and were further validated by the docking simulation: one at the hydrophobic pocket in the ICAM-1 binding domain (the αI domain); the other at the βI domain. Mutagenesis of the α'1 helix showed that isoflurane binding sites at the βI domain were significantly important in modulating LFA-1 function and conformation. Epitope mapping using activation-sensitive antibodies suggested that isoflurane stabilized LFA-1 in the closed conformation. This study suggested that isoflurane binds to both the αI and βI domains allosteric to the ICAM-1 binding site, and that isoflurane binding stabilizes LFA-1 in the closed conformation.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

L-plastin regulates polarization and migration in chemokine-stimulated human T lymphocytes

Chemokines such as SDF-1α play a crucial role in orchestrating T lymphocyte polarity and migration via polymerization and reorganization of the F-actin cytoskeleton, but the role of actin-associated proteins in this process is not well characterized. In this study, we have investigated a role for L-plastin, a leukocyte-specific F-actin-bundling protein, in SDF-1α-stimulated human T lymphocyte polarization and migration. We found that L-plastin colocalized with F-actin at the leading edge of SDF-1α-stimulated T lymphocytes and was also phosphorylated at Ser(5), a site that when phosphorylated regulates the ability of L-plastin to bundle F-actin. L-plastin phosphorylation was sensitive to pharmacological inhibitors of protein kinase C (PKC), and several PKC isoforms colocalized with L-plastin at the leading edge of SDF-1α-stimulated lymphocytes. However, PKC ζ, an established regulator of cell polarity, was the only isoform that regulated L-plastin phosphorylation. Knockdown of L-plastin expression with small interfering RNAs demonstrated that this protein regulated the localization of F-actin at the leading edge of chemokine-stimulated cells and was also required for polarization, lamellipodia formation, and chemotaxis. Knockdown of L-plastin expression also impaired the Rac1 activation cycle and Akt phosphorylation in response to SDF-1α stimulation. Furthermore, L-plastin also regulated SDF-1α-mediated lymphocyte migration on the integrin ligand ICAM-1 by influencing velocity and persistence, but in a manner that was independent of LFA-1 integrin activation or adhesion. This study, therefore, demonstrates an important role for L-plastin and the signaling pathways that regulate its phosphorylation in response to chemokines and adds L-plastin to a growing list of proteins implicated in T lymphocyte polarity and migration.

Signal-dependent slow leukocyte rolling does not require cytoskeletal anchorage of P-selectin glycoprotein ligand-1 (PSGL-1) or integrin αLβ2

In inflamed venules, neutrophils roll on P- or E-selectin, engage P-selectin glycoprotein ligand-1 (PSGL-1), and signal extension of integrin α(L)β(2) in a low affinity state to slow rolling on intercellular adhesion molecule-1 (ICAM-1). Cytoskeleton-dependent receptor clustering often triggers signaling, and it has been hypothesized that the cytoplasmic domain links PSGL-1 to the cytoskeleton. Chemokines cause rolling neutrophils to fully activate α(L)β(2), leading to arrest on ICAM-1. Cytoskeletal anchorage of α(L)β(2) has been linked to chemokine-triggered extension and force-regulated conversion to the high affinity state. We asked whether PSGL-1 must interact with the cytoskeleton to initiate signaling and whether α(L)β(2) must interact with the cytoskeleton to extend. Fluorescence recovery after photobleaching of transfected cells documented cytoskeletal restraint of PSGL-1. The lateral mobility of PSGL-1 similarly increased by depolymerizing actin filaments with latrunculin B or by mutating the cytoplasmic tail to impair binding to the cytoskeleton. Converting dimeric PSGL-1 to a monomer by replacing its transmembrane domain did not alter its mobility. By transducing retroviruses expressing WT or mutant PSGL-1 into bone marrow-derived macrophages from PSGL-1-deficient mice, we show that PSGL-1 required neither dimerization nor cytoskeletal anchorage to signal β(2) integrin-dependent slow rolling on P-selectin and ICAM-1. Depolymerizing actin filaments or decreasing actomyosin tension in neutrophils did not impair PSGL-1- or chemokine-mediated integrin extension. Unlike chemokines, PSGL-1 did not signal cytoskeleton-dependent swing out of the β(2)-hybrid domain associated with the high affinity state. The cytoskeletal independence of PSGL-1-initiated, α(L)β(2)-mediated slow rolling differs markedly from the cytoskeletal dependence of chemokine-initiated, α(L)β(2)-mediated arrest.

Co-culture of hematopoietic stem cells with mesenchymal stem cells increases VCAM-1-dependent migration of primitive hematopoietic stem cells

Hematopoietic stem cells (HSC) lose their capacity for engraftment during ex vivo cytokine expansion. It has been shown that mesenchymal stem cells (MSC) improve HSC transplantability; however, the molecular mechanisms responsible for this effect have not yet been completely elucidated. This paper reports that expanding HSC in co-culture with MSC enhances a vascular cell adhesion molecule (VCAM-1)-dependent pro-migratory phenotype. MSC did not regulate the HSC expression of CD49d (VCAM-1 counter-receptor molecule), but did decrease the cytokine-induced HSC VCAM-1-mediated pro-adhesive phenotype. Co-culture with MSC reduced the expression of the inactive conformation of lymphocyte function-associated antigen (LFA-1) at the HSC uropod, and induced higher expression of an LFA-1 activation epitope. Interestingly, VCAM-1-dependent HSC migration was modulated by targeting this LFA-1 high affinity form, suggesting integrin cross-regulation. VCAM-1-mediated HSC transmigration appeared to favor the more primitive HSC immunophenotype. Our results suggested that co-culture with MSC improved VCAM-1-dependent migration of primitive HSC, which was affected in ex vivo cytokine-expanded HSCs by a mechanism involving LFA-1 modulation.

Regulation of integrin affinity on cell surfaces

Lymphocyte activation triggers adhesiveness of lymphocyte function-associated antigen-1 (LFA-1; integrin α(L)β(2)) for intercellular adhesion molecules (ICAMs) on endothelia or antigen-presenting cells. Whether the activation signal, after transmission through multiple domains to the ligand-binding αI domain, results in affinity changes for ligand has been hotly debated. Here, we present the first comprehensive measurements of LFA-1 affinities on T lymphocytes for ICAM-1 under a broad array of activating conditions. Only a modest increase in affinity for soluble ligand was detected after activation by chemokine or T-cell receptor ligation, conditions that primed LFA-1 and robustly induced lymphocyte adhesion to ICAM-1 substrates. By stabilizing well-defined LFA-1 conformations by Fab, we demonstrate the absolute requirement of the open LFA-1 headpiece for adhesiveness and high affinity. Interaction of primed LFA-1 with immobilized but not soluble ICAM-1 triggers energy-dependent affinity maturation of LFA-1 to an adhesive, high affinity state. Our results lend support to the traction or translational motion dependence of integrin activation.

RGS10 restricts upregulation by chemokines of T cell adhesion mediated by α4β1 and αLβ2 integrins

Chemokines rapidly and transiently upregulate α4β1 and αLβ2 integrin-mediated adhesion during T lymphocyte extravasation by activating Gα-dependent inside-out signaling. To limit and terminate Gα-mediated signaling, cells can use several mechanisms, including the action of regulator of G protein signaling (RGS) proteins, which accelerate the GTPase activity of Gα subunits. Using human T cells silenced for or overexpressing RGS10, we show in this article that RGS10 functions as an inhibitor of Gα(i)-dependent, chemokine-upregulated T cell adhesion mediated by α4β1 and αLβ2. Shear stress-dependent detachment and cell spreading analyses revealed that RGS10 action mainly targets the adhesion strengthening and spreading phases of α4β1-mediated cell attachment. Associated with these observations, chemokine-stimulated Vav1-Rac1 activation was longer sustained and of higher intensity in RGS10-silenced T cells, or inhibited in cells overexpressing RGS10. Of importance, expression of constitutively activated Rac1 forms in cells overexpressing RGS10 led to the rescue of CXCL12-stimulated adhesion to VCAM-1 to levels similar to those in control transfectants. Instead, adhesion under flow conditions, soluble binding experiment, flow cytometry, and biochemical analyses revealed that the earlier chemokine-triggered integrin activation step was mostly independent of RGS10 actions. The data strongly suggest that RGS10 opposes activation by chemokines of the Vav1-Rac1 pathway in T cells, leading to repression of adhesion strengthening mediated by α4β1. In addition to control chemokine-upregulated T cell attachment, RGS10 also limited adhesion-independent cell chemotaxis and activation of cdc42. These results identify RGS10 as a key molecule that contributes to the termination of Gα-dependent signaling during chemokine-activated α4β1- and αLβ2-dependent T cell adhesion.

Overview: structural biology of integrins

Integrins are cell adhesion molecules that play important roles in many biological processes including hemostasis, immune responses, development, and cancer. Their adhesiveness is dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces outside-in signals from the extracellular domain to the cytoplasm. Advances in the past several years have shed light on structural basis for integrin regulation and signaling, especially how the large-scale reorientations of the ectodomain are related to the inter-domain and intra-domain shape shifting that changes ligand-binding affinity. Experiments have also shown how the conformational changes of the ectodomain are linked to changes in the α- and β-subunit transmembrane and cytoplasmic domains.

The effect of rhG-CSF on the conformation of LFA-1 on CD4+ T cells in hemopoietic stem cell transplantation

Recombinant human granulocyte colony-stimulating factor (rhG-CSF) modulates donor T cell function in hemopoietic stem cell transplantation. The effects of rhG-CSF on the activation of CD4(+) T cells have been poorly investigated. We investigated whether rhG-CSF mobilization influenced the activation and proliferation capacity of CD4(+) T cells. Cell treatment with phorbol 12-myristate 13-acetate (PMA) plus ionomycin or the CD3 mAb OKT3 plus intercellular cell adhesion molecule-1 (ICAM-1) at 37 degrees C for 6 h induced a dramatic increase in CD25, CD69 and MEM148 epitope exposure. rhG-CSF mobilization decreased CD25, CD69 and MEM148 epitope expression on activated CD4(+) T cells compared with cells before mobilization. The transcription factor Jun activation domain-binding protein 1 (JAB1) plays a role in the activation of CD4(+) T cells, and the rhG-CSF mobilization changed the level of nuclear JAB1 protein. rhG-CSF mobilization also decreased the adhesion of CD4(+) T cells to ICAM-1, but had no effect on the levels of donor CD4+CD25+ regulatory T cells. Overall, these data suggest the rhG-CSF mobilization can influence CD4(+) T cell activation through LFA-1/ICAM-1 costimulatory signaling in HSC transplantation.

Requirement of open headpiece conformation for activation of leukocyte integrin alphaXbeta2

Negative stain electron microscopy (EM) and adhesion assays show that alpha(X)beta(2) integrin activation requires headpiece opening as well as extension. An extension-inducing Fab to the beta(2) leg, in combination with representative activating and inhibitory Fabs, were examined for effect on the equilibrium between the open and closed headpiece conformations. The two activating Fabs stabilized the open headpiece conformation. Conversely, two different inhibitory Fabs stabilized the closed headpiece conformation. Adhesion assays revealed that alpha(X)beta(2) in the extended-open headpiece conformation had high affinity for ligand, whereas both the bent conformation and the extended-closed headpiece conformation represented the low affinity state. Intermediate integrin affinity appears to result not from a single conformational state, but from a mixture of equilibrating conformational states.

Modulation of integrin activation by an entropic spring in the {beta}-knee

We show that the length of a loop in the β-knee, between the first and second cysteines (C1-C2) in integrin EGF-like (I-EGF) domain 2, modulates integrin activation. Three independent sets of mutants, including swaps among different integrin β-subunits, show that C1-C2 loop lengths of 12 and longer favor the low affinity state and masking of ligand-induced binding site (LIBS) epitopes. Shortening length from 12 to 4 residues progressively increases ligand binding and LIBS epitope exposure. Compared with length, the loop sequence had a smaller effect, which was ascribable to stabilizing loop conformation, and not interactions with the α-subunit. The data together with structural calculations support the concept that the C1-C2 loop is an entropic spring and an emerging theme that disordered regions can regulate allostery. Diversity in the length of this loop may have evolved among integrin β-subunits to adjust the equilibrium between the bent and extended conformations at different set points.

Structure of an integrin with an alphaI domain, complement receptor type 4

We report the structure of an integrin with an alphaI domain, alpha(X)beta(2), the complement receptor type 4. It was earlier expected that a fixed orientation between the alphaI domain and the beta-propeller domain in which it is inserted would be required for allosteric signal transmission. However, the alphaI domain is highly flexible, enabling two betaI domain conformational states to couple to three alphaI domain states, and greater accessibility for ligand recognition. Although alpha(X)beta(2) is bent similarly to integrins that lack alphaI domains, the terminal domains of the alpha- and beta-legs, calf-2 and beta-tail, are oriented differently than in alphaI-less integrins. Linkers extending to the transmembrane domains are unstructured. Previous mutations in the beta(2)-tail domain support the importance of extension, rather than a deadbolt, in integrin activation. The locations of further activating mutations and antibody epitopes show the critical role of extension, and conversion from the closed to the open headpiece conformation, in integrin activation. Differences among 10 molecules in crystal lattices provide unprecedented information on interdomain flexibility important for modelling integrin extension and activation.

Rap1 controls lymphocyte adhesion cascade and interstitial migration within lymph nodes in RAPL-dependent and -independent manners

The small GTPase Rap1 and its effector RAPL regulate lymphocyte adhesion and motility. However, their precise regulatory roles in the adhesion cascade preceding entry into lymph nodes and during interstitial migration are unclear. Here, we show that Rap1 is indispensably required for the chemokine-triggered initial arrest step of rolling lymphocytes through LFA-1, whereas RAPL is not involved in rapid arrest. RAPL and talin play a critical role in stabilizing lymphocyte arrest to the endothelium of blood vessels under flow or to the high endothelial venules of peripheral lymph nodes in vivo. Further, mutagenesis and peptide studies suggest that release of a trans-acting restraint from the beta2 cytoplasmic region of LFA-1 is critical for Rap1-dependent initial arrest. Rap1 or RAPL deficiency severely impaired lymphocyte motility over lymph node stromal cells in vitro, and RAPL deficiency impaired high-velocity directional movement within lymph nodes. These findings reveal the several critical steps of Rap1, which are RAPL-dependent and -independent, in lymphocyte trafficking.

Functional and structural characterization of the talin F0F1 domain

The globular head domain of talin, a large multi-domain cytoplasmic protein, is required for inside-out activation of the integrins, a family of heterodimeric transmembrane cell adhesion molecules. Talin head contains a FERM domain that is composed of F1, F2, and F3 subdomains. A F0 subdomain is located N-terminus to F1. The F3 contains a canonical phosphotyrosine binding (PTB) fold that directly interacts with the membrane proximal NPxY/F motif in the integrin beta cytoplasmic tail. This interaction is stabilized by the F2 that interacts with the lipid head-groups of the plasma membrane. In comparison to F2 and F3, the properties of the F0F1 remains poorly characterized. Here, we showed that F0F1 is essential for talin-induced activation of integrin alphaLbeta2 (LFA-1). F0F1 has a high content of beta-sheet secondary structure, and it tends to homodimerize that may provide stability against proteolysis and chaotrope induced unfolding.

Urokinase-type plasminogen activator receptor induces conformational changes in the integrin alphaMbeta2 headpiece and reorientation of its transmembrane domains

The glycosylphosphatidylinositol-linked urokinase-type plasminogen activator receptor (uPAR) interacts with the heterodimer cell adhesion molecules integrins to modulate cell adhesion and migration. Devoid of a cytoplasmic domain, uPAR triggers intracellular signaling via its associated molecules that contain cytoplasmic domains. Interestingly, uPAR changes the ectodomain conformation of one of its partner molecules, integrin alpha(5)beta(1), and elicits cytoplasmic signaling. The separation or reorientation of integrin transmembrane domains and cytoplasmic tails are required for integrin outside-in signaling. However, there is a lack of direct evidence showing these conformational changes of an integrin that interacts with uPAR. In this investigation we used reporter monoclonal antibodies and fluorescence resonance energy transfer analyses to show conformational changes in the alpha(M)beta(2) headpiece and reorientation of its transmembrane domains when alpha(M)beta(2) interacts with uPAR.

Intercellular adhesion molecule-3 binding of integrin alphaL beta2 requires both extension and opening of the integrin headpiece

The leukocyte-restricted integrin alpha(L)beta(2) is required in immune processes such as leukocyte adhesion, migration, and immune synapse formation. Activation of alpha(L)beta(2) by conformational changes promotes alpha(L)beta(2) binding to its ligands, ICAMs. It was reported that different affinity states of alpha(L)beta(2) are required for binding ICAM-1 and ICAM-3. Recently, the bent, extended with a closed headpiece, and extended with open headpiece conformations of alpha(L)beta(2), was reported. To address the overall conformational requirements of alpha(L)beta(2) that allow selective binding of these ICAMs, we examined the adhesion properties of these alpha(L)beta(2) conformers. alpha(L)beta(2) with different conformations were generated by mutations, and verified by using a panel of reporter mAbs that detect alpha(L)beta(2) extension, hybrid domain movement, or I-like domain activation. We report a marked difference between extended alpha(L)beta(2) with closed and open headpieces in their adhesive properties to ICAM-1 and ICAM-3. Our data show that the extension of alpha(L)beta(2) alone is sufficient to mediate ICAM-1 adhesion. By contrast, an extended alpha(L)beta(2) with an open headpiece is required for ICAM-3 adhesion.

Permissive transmembrane helix heterodimerization is required for the expression of a functional integrin

The current paradigm is that integrin is activated via inside-out signalling when its cytoplasmic tails and TMs (transmembrane helices) are separated by specific cytosolic protein(s). Perturbations of the helical interface between the α- and β-TMs of an integrin, as a result of mutations, affect its function. Previous studies have shown the requirement for specific pairing between integrin subunits by ectodomain-exchange analyses. It remains unknown whether permissive α/β-TM pairing of an integrin is also required for pairing specificity and the expression of a functionally regulated receptor. We performed scanning replacement of integrin β2-TM with a TM of other integrin β-subunits. With the exception of β4 substitution, others presented β2-integrins with modified phenotypes, either in their expression or ligand-binding properties. Subsequently, we adopted αLβ2 for follow-on experiments because its conformation and affinity-state transitions have been well defined as compared with other members of the β2-integrins. Replacement of β2- with β3-TM generated a chimaeric αLβ2 of an intermediate affinity that adhered to ICAM-1 (intercellular adhesion molecule 1) but not to ICAM-3 constitutively. Replacing αL-TM with αIIb-TM, forming a natural αIIb/β3-TM pair, reversed the phenotype of the chimaera to that of wild-type αLβ2. Interestingly, the replacement of αLβ2- with β3-TM showed neither an extended conformation nor the separation of its cytoplasmic tails, which are well-reported hallmarks of an activated αLβ2, as determined by reporter mAb (monoclonal antibody) KIM127 reactivity and FRET (fluorescence resonance energy transfer) measurements respectively. Collectively, our results suggest that TM pairing specificity is required for the expression of a functionally regulated integrin.

A structural hypothesis for the transition between bent and extended conformations of the leukocyte beta2 integrins

Integrins mediate cell adhesion in response to activation signals that trigger conformational changes within their ectodomain. It is thought that a compact bent conformation of the molecule represents its physiological low affinity state and extended conformations its active state. We have determined the structure of two integrin fragments of the beta2 subunit. The first structure, consisting of the plexin-semaphorin-integrin domain, hybrid, integrin-epidermal growth factor 1 (I-EGF1), and I-EGF2 domains (PHE2), showed an L-shaped conformation with the bend located between the I-EGF1 and I-EGF2 domains. The second structure, which includes, in addition, the I-EGF3 domain, showed an extended conformation. The major reorientation of I-EGF2 with respect to the other domains in the two structures is accompanied by a change of torsion angle of the disulfide bond between Cys(461)-Cys(492) by 180 degrees and the conversion of a short alpha-helix (residues Ser(468)-Cys(475)) into a flexible coil. Based on the PHE2 structure, we introduced a disulfide bond between the plexin-semaphorin-integrin domain and I-EGF2 domains in the beta2 subunit. The resultant alphaLbeta2 integrin (leukocyte function-associated antigen-1) variant was locked in a bent state and could not be detected with the monoclonal antibody KIM127 in Mg(2+)/EGTA. However, it retained the binding activity to ICAM-1. These results provide a structural hypothesis for our understanding of the transition between the resting and active states of leukocyte function-associated antigen-1.

The cytosolic protein talin induces an intermediate affinity integrin alphaLbeta2

The integrin alphaLbeta2 mediates leukocyte adhesion and migration that are required for a functional immune system. It is known that inside-out signaling triggers alphaLbeta2 conformational changes, which affect its ligand-binding affinity. At least three alphaLbeta2 affinity states (low, intermediate, and high) were described. The cytosolic protein talin connects alphaLbeta2 to the actin filament. The talin head domain is also known to activate alphaLbeta2 ligand binding. However, it remains to be determined whether talin promotes an intermediate or high affinity alphaLbeta2. In this study using transfectants and T cells, we showed that talin induced an intermediate affinity alphaLbeta2 that adhered constitutively to its ligand intercellular adhesion molecule (ICAM)-1 but not ICAM-3. Adhesion to ICAM-3 was induced when an additional exogenous activating agent was included. Similar profiles were observed with soluble ICAMs. In addition, the intermediate affinity alphaLbeta2 induced by talin allowed adhesion and migration of T cells on immobilized ICAMs.

Structural basis of integrin regulation and signaling

Integrins are cell adhesion molecules that mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. They play critical roles for the immune system in leukocyte trafficking and migration, immunological synapse formation, costimulation, and phagocytosis. Integrin adhesiveness can be dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces signals from the extracellular domain to the cytoplasm in the classical outside-in direction. Recent structural, biochemical, and biophysical studies have greatly advanced our understanding of the mechanisms of integrin bidirectional signaling across the plasma membrane. Large-scale reorientations of the ectodomain of up to 200 A couple to conformational change in ligand-binding sites and are linked to changes in alpha and beta subunit transmembrane domain association. In this review, we focus on integrin structure as it relates to affinity modulation, ligand binding, outside-in signaling, and cell surface distribution dynamics.

Mutation of a conserved asparagine in the I-like domain promotes constitutively active integrins alphaLbeta2 and alphaIIbbeta3

The leukocyte beta2 integrins are heterodimeric adhesion receptors required for a functional immune system. Many leukocyte adhesion deficiency-1 (LAD-1) mutations disrupt the expression and function of beta2 integrins. Herein, we further characterized the LAD-1 mutation N329S in the beta2 inserted (I)-like domain. This mutation converted alphaLbeta2 from a resting into a high affinity conformer because alphaLbeta2N329S transfectants adhered avidly to ligand intercellular adhesion molecule (ICAM)-3 in the absence of additional activating agent. An extended open conformation is adopted by alphaLbeta2N329S because of its reactivity with the beta2 activation reporter monoclonal antibodies MEM148 and KIM127. A corresponding mutation in beta3 generated constitutively active alphaIIbbeta3 that adhered to fibrinogen. This Asn is conserved in all human beta subunits, and it resides before the last helix of the I-like domain, which is known to be important in activation signal propagation. By mutagenesis studies and review of existing integrin structures, we conjectured that this conserved Asn may have a primary role in shaping the I-like domain by stabilizing the conformation of the alpha7 helix and the beta6-alpha7 loop in the I-like domain.

Tests of the extension and deadbolt models of integrin activation

Despite extensive evidence that integrin conformational changes between bent and extended conformations regulate affinity for ligands, an alternative hypothesis has been proposed in which a "deadbolt" can regulate affinity for ligand in the absence of extension. Here, we tested both the deadbolt and the extension models. According to the deadbolt model, a hairpin loop in the beta3 tail domain could act as a deadbolt to restrain the displacement of the beta3 I domain beta6-alpha7 loop and maintain integrin in the low affinity state. We found that mutating or deleting the beta3 tail domain loop has no effect on ligand binding by either alphaIIbbeta 3 or alphaVbeta3 integrins. In contrast, we found that mutations that lock integrins in the bent conformation with disulfide bonds resist inside-out activation induced by cytoplasmic domain mutation. Furthermore, we demonstrated that extension is required for accessibility to fibronectin but not smaller fragments. The data demonstrate that integrin extension is required for ligand binding during integrin inside-out signaling and that the deadbolt does not regulate integrin activation.

Integrin structures and conformational signaling

Integrins are cell adhesion molecules that play critical roles in development, wound healing, hemostasis, immunity and cancer. Advances in the past two years have shed light on the structural basis for integrin regulation and signaling, especially on how global conformational changes between bent and extended conformations relate to the inter-domain and intra-domain shape shifting that regulates affinity for ligand. The downward movements of the C-terminal helices of the alpha I and beta I domains and the swing-out of the hybrid domain play pivotal roles in integrin conformational signaling. Experiments have also shown that integrins transmit bidirectional signals across the plasma membrane by coupling extracellular conformational change with an unclasping and separation of the alpha and beta transmembrane and cytoplasmic domains.

Targeting integrin structure and function in disease

Initially linked to the pathogenesis of inflammatory and hematologic diseases, integrins have become validated drug targets with the approval of five drugs. Moreover, there are several promising drug candidates in preclinical and clinical stages of development for multiple clinical indications. Integrins are attractive drug targets as their antagonism can block several steps in disease progression or maintenance. Integrin inhibitors can block the proliferation, migration, or tissue localization of inflammatory, angiogenic, and tumor cells, as well as signaling and gene expression contributing to disease. There has been a rapid increase in the elucidation of integrin structure, their allosteric mechanisms of bidirectional signaling, and the structure of complexes with drugs. This information brings greater focus to how integrins support various cellular functions and how they have been and may be targeted to develop novel drugs. Here we review conformational switches, including an internal ligand, which allosterically regulate the transition from low- to high-affinity ligand binding. We address some of the successes, disappointments, and challenges in targeting competitive or allosteric sites to develop therapeutics. We also discuss new opportunities, including a structure-based approach to discover novel drugs to treat inflammatory and other diseases. This approach targets structural relatives of the von Willebrand factor A-domain present in integrins and many functionally diverse proteins.

Two Functional Active Conformations of the Integrin α2β1, Depending on Activation Condition and Cell Type

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin alpha2beta1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of alpha2beta1 because it only binds to the activated state of alpha2beta1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of alpha2beta1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of alpha2beta1 are dependent on the cell type where alpha2beta1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and alpha2beta1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of alpha2beta1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells.