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

Deletion of Talin1 in Myeloid Cells Facilitates Atherosclerosis in Mice

BACKGROUND

Integrin-regulated monocyte recruitment and cellular responses of monocyte-derived macrophages are critical for the pathogenesis of atherosclerosis. In the canonical model, talin1 controls ligand binding to integrins, a prerequisite for integrins to mediate leukocyte recruitment and induce immune responses. However, the role of talin1 in the development of atherosclerosis has not been studied. Our study investigated how talin1 in myeloid cells regulates the progression of atherosclerosis.

METHODS

On an Apoe-/- background, myeloid talin1-deficient mice and the control mice were fed with a high-fat diet for 8 or 12 weeks to induce atherosclerosis. The atherosclerosis development in the aorta and monocyte recruitment into atherosclerotic lesions were analyzed.

RESULTS

Myeloid talin1 deletion facilitated the formation of atherosclerotic lesions and macrophage deposition in lesions. Talin1 deletion abolished integrin β2-mediated adhesion of monocytes but did not impair integrin α4β1-dependent cell adhesion in a flow adhesion assay. Strikingly, talin1 deletion did not prevent Mn2+- or chemokine-induced activation of integrin α4β1 to the high-affinity state for ligands. In an in vivo competitive homing assay, monocyte infiltration into inflamed tissues was prohibited by antibodies to integrin α4β1 but was not affected by talin1 deletion or antibodies to integrin β2. Furthermore, quantitative polymerase chain reaction and ELISA (enzyme-linked immunosorbent assay) analysis showed that macrophages produced cytokines to promote inflammation and the proliferation of smooth muscle cells. Ligand binding to integrin β3 inhibited cytokine generation in macrophages, although talin1 deletion abolished the negative effects of integrin β3.

CONCLUSIONS

Integrin α4β1 controls monocyte recruitment during atherosclerosis. Talin1 is dispensable for integrin α4β1 activation to the high-affinity state and integrin α4β1-mediated monocyte recruitment. Yet, talin1 is required for integrin β3 to inhibit the production of inflammatory cytokines in macrophages. Thus, intact monocyte recruitment and elevated inflammatory responses cause enhanced atherosclerosis in talin1-deficient mice. Our study provides novel insights into the roles of myeloid talin1 and integrins in the progression of atherosclerosis.

LFA-1 Activation in T-Cell Migration and Immunological Synapse Formation

Integrin LFA-1 plays a critical role in T-cell migration and in the formation of immunological synapses. LFA-1 functions through interacting with its ligands with differing affinities: low, intermediate, and high. Most prior research has studied how LFA-1 in the high-affinity state regulates the trafficking and functions of T cells. LFA-1 is also presented in the intermediate-affinity state on T cells, however, the signaling to activate LFA-1 to the intermediate-affinity state and the role of LFA-1 in this affinity state both remain largely elusive. This review briefly summarizes the activation and roles of LFA-1 with varied ligand-binding affinities in the regulation of T-cell migration and immunological synapse formation.

Integrin Regulators in Neutrophils

Neutrophils are the most abundant leukocytes in humans and are critical for innate immunity and inflammation. Integrins are critical for neutrophil functions, especially for their recruitment to sites of inflammation or infections. Integrin conformational changes during activation have been heavily investigated but are still not fully understood. Many regulators, such as talin, Rap1-interacting adaptor molecule (RIAM), Rap1, and kindlin, are critical for integrin activation and might be potential targets for integrin-regulating drugs in treating inflammatory diseases. In this review, we outline integrin activation regulators in neutrophils with a focus on the above critical regulators, as well as newly discovered modulators that are involved in integrin activation.

Mitofusin-2 regulates leukocyte adhesion and β2 integrin activation

Neutrophils are critical for inflammation and innate immunity, and their adhesion to vascular endothelium is a crucial step in neutrophil recruitment. Mitofusin-2 (MFN2) is required for neutrophil adhesion, but molecular details are unclear. Here, we demonstrated that β₂ -integrin-mediated slow-rolling and arrest, but not PSGL-1-mediated cell rolling, are defective in MFN2-deficient neutrophil-like HL60 cells. This adhesion defect is associated with reduced expression of fMLP (N-formylmethionyl-leucyl-phenylalanine) receptor FPR1 as well as the inhibited β₂ integrin activation, as assessed by conformation-specific monoclonal antibodies. MFN2 deficiency also leads to decreased actin polymerization, which is important for β2 integrin activation. Mn²⁺ -induced cell spreading is also inhibited after MFN2 knockdown. MFN2 deficiency limited the maturation of β2 integrin activation during the neutrophil-directed differentiation of HL60 cells, which is indicated by CD35 and CD87 markers. MFN2 knockdown in β2-integrin activation-matured cells (CD87^high population) also inhibits integrin activation, indicating that MFN2 directly affects β2 integrin activation. Our study illustrates the function of MFN2 in leukocyte adhesion and may provide new insights into the development and treatment of MFN2 deficiency-related diseases.

Neutrophils lacking ERM proteins polarize and crawl directionally but have decreased adhesion strength

Ezrin/radixin/moesin (ERM) proteins are adaptors that link the actin cytoskeleton to the cytoplasmic domains of membrane proteins. Leukocytes express mostly moesin with lower levels of ezrin but no radixin. When leukocytes are activated, ERMs are postulated to redistribute membrane proteins from microvilli into uropods during polarization and to transduce signals that influence adhesion and other responses. However, these functions have not been tested in leukocytes lacking all ERMs. We used knockout (KO) mice with neutrophils lacking ezrin, moesin, or both proteins (double knockout [DKO]) to probe how ERMs modulate cell shape, adhesion, and signaling in vitro and in vivo. Surprisingly, chemokine-stimulated DKO neutrophils still polarized and redistributed ERM-binding proteins such as PSGL-1 and CD44 to the uropods. Selectin binding to PSGL-1 on moesin KO or DKO neutrophils activated kinases that enable integrin-dependent slow rolling but not those that generate neutrophil extracellular traps. Flowing neutrophils of all genotypes rolled normally on selectins and, upon chemokine stimulation, arrested on integrin ligands. However, moesin KO and DKO neutrophils exhibited defective integrin outside-in signaling and reduced adhesion strength. In vivo, DKO neutrophils displayed normal directional crawling toward a chemotactic gradient, but premature detachment markedly reduced migration from venules into inflamed tissues. Our results demonstrate that stimulated neutrophils do not require ERMs to polarize or to move membrane proteins into uropods. They also reveal an unexpected contribution of moesin to integrin outside-in signaling and adhesion strengthening.

Biphasic Force-Regulated Phosphorylation Site Exposure and Unligation of ERM Bound with PSGL-1: A Novel Insight into PSGL-1 Signaling via Steered Molecular Dynamics Simulations

The PSGL-1-actin cytoskeleton linker proteins ezrin/radixin/moesin (ERM), an adaptor between P-selectin glycoprotein ligand-1 (PSGL-1) and spleen tyrosine kinase (Syk), is a key player in PSGL-1 signal, which mediates the adhesion and recruitment of leukocytes to the activated endothelial cells in flow. Binding of PSGL-1 to ERM initials intracellular signaling through inducing phosphorylation of Syk, but effects of tensile force on unligation and phosphorylation site exposure of ERM bound with PSGL-1 remains unclear. To answer this question, we performed a series of so-called “ramp-clamp” steered molecular dynamics (SMD) simulations on the radixin protein FERM domain of ERM bound with intracellular juxtamembrane PSGL-1 peptide. The results showed that, the rupture force of complex pulled with constant velocity was over 250 pN, which prevented the complex from breaking in front of pull-induced exposure of phosphorylation site on immunoreceptor tyrosine activation motif (ITAM)-like motif of ERM; the stretched complex structure under constant tensile forces <100 pN maintained on a stable quasi-equilibrium state, showing a high mechano-stabilization of the clamped complex; and, in consistent with the force-induced allostery at clamped stage, increasing tensile force (<50 pN) would decrease the complex dissociation probability but facilitate the phosphorylation site exposure, suggesting a force-enhanced biophysical connectivity of PSGL-1 signaling. These force-enhanced characters in both phosphorylation and unligation of ERM bound with PSGL-1 should be mediated by a catch-slip bond transition mechanism, in which four residue interactions on binding site were involved. This study might provide a novel insight into the transmembrane PSGL-1 signal, its biophysical connectivity and molecular structural basis for cellular immune responses in mechano-microenvironment, and showed a rational SMD-based computer strategy for predicting structure-function relation of protein under loads.

Th1 Cells Rolling on Selectins Trigger DAP12-Dependent Signals That Activate Integrin αLβ2

During inflammation, both neutrophils and effector T cells use selectins to roll and integrins to arrest in postcapillary venules. In both cell types, chemokines can transduce signals that convert integrin αLβ2 to a high-affinity conformation, which interacts with ICAM-1 to mediate arrest. In neutrophils, selectins also trigger an immunoreceptor-like signaling cascade that converts integrin αLβ2 to an intermediate-affinity conformation, which interacts with ICAM-1 to slow rolling. It is not known whether selectins induce similar signaling events in T cells. Ag engagement causes phosphorylation of ITAMs on the TCR; these motifs recruit kinases and adaptors that lead to the activation of αLβ2. We found that mouse Th1 cells rolling on P- or E-selectin triggered signals that promoted αLβ2-dependent slow rolling on ICAM-1 in vitro and in vivo. The selectin signaling cascade resembled that used by the TCR, except that unexpectedly, Th1 cells employed the ITAM-bearing protein DAP12, which was not known to be expressed in these cells. Importantly, outside-in signaling through ligand-occupied αLβ2 also required DAP12. Cooperative selectin and chemokine signaling in Th1 cells promoted αLβ2-dependent slow rolling and arrest in vitro and in vivo and migration into Ag-challenged tissues in vivo. Our findings reveal an important function for DAP12 in Th1 cells and a new mechanism to recruit effector T cells to sites of inflammation.

The function of P-selectin glycoprotein ligand-1 is conserved from ancestral fishes to mammals

PSGL-1 is a mucin-like glycoprotein that supports, in mammals, leukocyte rolling on selectins. However, we have limited knowledge whether its function is conserved in non-mammals and how its structure adapted during evolution. To identify conserved amino acid sequences required for selectin binding, we performed multiple alignments of PSGL-1 sequences from 18 mammals, 4 birds, 3 reptiles, 1 amphibian, and 15 fishes. The amino-terminal T[D/E]PP[D/E] motif, which identifies in mammals a core-2 O-glycosylated threonine required for selectin-binding, is partially conserved in some fishes (e.g., T. rubripes) and birds (e.g., G. gallus), however, most non-mammals do not display it. The sulfated tyrosine residues of human PSGL-1, which bind L- and P-selectin, are not observed in non-mammals, suggesting that they are dispensable for selectin-binding or that other amino acids play their role. A mucin-like domain is present in all species. Interestingly, the alignment of cytoplasmic sequences of non-mammals reveals the conservation of ezrin/radixin/moesin binding site and two new motifs (M1 and M2). To examine the conservation of PSGL-1 function, we cloned PSGL-1 cDNA sequences of zebrafish and fugu, and established their cross-reactivity with human selectins under flow conditions. Importantly, deleting the well-conserved M1 motif strongly decreased PSGL-1 expression at leukocyte surface and induced retention of the precursor molecule in the endoplasmic reticulum, indicating that M1 motif provides a signal required to export PSGL-1 precursors to the Golgi complex. These data show for the first time the conservation of PSGL-1 function from fishes to mammals and reveal the function of a new motif.

Adhesive dynamics simulations quantitatively predict effects of kindlin-3 deficiency on T-cell homing

Leukocyte adhesion is important for the proper functioning of the immune system. While leukocyte homing is mediated by adhesion receptors, the activation of these receptors is modulated by intracellular signaling molecules. In Leukocyte Adhesion Deficiency Type 3, the loss of the kindlin-3 prevents the activation of Leukocyte Function-associated Antigen-1 (LFA-1), which leads to a defect in adhesion, causing recurrent infections and bleeding disorders. Here, we use Integrated Signaling Adhesive Dynamics, a computer model of leukocyte rolling and adhesion combined with a simulated intracellular signaling cascade, to predict the response of T cells to depletion of kindlin-3. Our model predicts that cell adhesion is hypersensitive to the amount of kindlin-3 in the cell, while the rolling velocity is independent of kindlin-3 concentration. In addition, our simulation predicted that the time to stop, an important metric of adhesion, would increase with decreasing kindlin-3 expression. These predictions were confirmed experimentally in experiments using Jurkat cells with reduced expression of kindlin-3. These results suggest that Adhesive Dynamics is a versatile tool for quantifying adhesion in the immune response and predicting the effects of engineering cellular components.

Cooperative PSGL-1 and CXCR2 signaling in neutrophils promotes deep vein thrombosis in mice

Inflammation is a major contributor to deep vein thrombosis (DVT). Flow restriction of the inferior vena cava (IVC) in mice induces DVT like that in humans. In this model, P-selectin-dependent adhesion of neutrophils and monocytes leads to release of neutrophil extracellular traps (NETs) and expression of tissue factor. However, it is not known what signals cause myeloid cells to generate these procoagulant effectors. Using ultrasonography and spinning-disk intravital microscopy in genetically engineered mice, we found that engagement of P-selectin glycoprotein ligand-1 (PSGL-1) and the chemokine receptor CXCR2 on rolling neutrophils propagated signals that cooperated to induce β2 integrin-dependent arrest in flow-restricted IVCs. Unlike previous reports, PSGL-1 signaling in neutrophils did not require L-selectin, and it used tyrosine 145 rather than tyrosines 112 and 128 on the adaptor Src homology domain-containing leukocyte phosphoprotein of 76 kDa. PSGL-1 and CXCR2 signaling cooperated to increase the frequency and size of thrombi, in part by stimulating release of NETs. Unlike in neutrophils, blocking PSGL-1 or CXCR2 signaling in monocytes did not affect their recruitment into thrombi or their expression of tissue factor. Our results demonstrate that neutrophils cooperatively signal through PSGL-1 and CXCR2 to promote DVT.

Thyroid cancer cells in space during the TEXUS-53 sounding rocket mission - The THYROID Project

Human follicular thyroid cancer cells (FTC-133) were sent to space via a sounding rocket during the TEXUS-53 mission to determine the impact of short-term microgravity on these cells. To enable cell culture and fixation in real microgravity, an automated experiment container (EC) was constructed. In order to ensure safe cell culture, cell-chambers consisting of polycarbonate (PC) material were used. They were highly biocompatible as proved by measuring cell survival using Annexin V flow cytometry. In the follow-up experiment, FTC-133 cells were sent to space via a sounding rocket and were fixed before and after the microgravity (µg) phase with RNAlater. In addition, cells were tested for reactions on hypergravity (hyper-g) as much as 18 g to determine whether worst case acceleration during launch can have an influence on the cells. We investigated genes belonging to biological processes such as cytoskeleton, cell adhesion, tumor growth, angiogenesis and apoptosis. Pathway analyses revealed central functions of VEGFA and EGF. EGF upregulates aspartate beta-hydroxylase (ASPH) which is influencing CASP3. Hyper-g induced a significant up-regulation of TUBB1, VIM, RDX, CAV1, VEGFA and BCL2. FTC-133 cells grown in an automated EC exposed to µg revealed moderate gene expression changes indicating their survival in orbit.

Microfluidics-based super-resolution microscopy enables nanoscopic characterization of blood stem cell rolling

Hematopoietic stem/progenitor cell (HSPC) homing occurs via cell adhesion mediated by spatiotemporally organized ligand-receptor interactions. Although molecules and biological processes involved in this multistep cellular interaction with endothelium have been studied extensively, molecular mechanisms of this process, in particular the nanoscale spatiotemporal behavior of ligand-receptor interactions and their role in the cellular interaction, remain elusive. We introduce a microfluidics-based super-resolution fluorescence imaging platform and apply the method to investigate the initial essential step in the homing, tethering, and rolling of HSPCs under external shear stress that is mediated by selectins, expressed on endothelium, with selectin ligands (that is, CD44) expressed on HSPCs. Our new method reveals transient nanoscale reorganization of CD44 clusters during cell rolling on E-selectin. We demonstrate that this mechanical force-induced reorganization is accompanied by a large structural reorganization of actin cytoskeleton. The CD44 clusters were partly disrupted by disrupting lipid rafts. The spatial reorganization of CD44 and actin cytoskeleton was not observed for the lipid raft-disrupted cells, demonstrating the essential role of the spatial clustering of CD44 on its reorganization during cell rolling. The lipid raft disruption causes faster and unstable cell rolling on E-selectin compared with the intact cells. Together, our results demonstrate that the spatial reorganization of CD44 and actin cytoskeleton is the result of concerted effect of E-selectin-ligand interactions, external shear stress, and spatial clustering of the selectin ligands, and has significant effect on the tethering/rolling step in HSPC homing. Our new experimental platform provides a foundation for characterizing complicated HSPC homing.

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.

An Experimentally Determined State Diagram for Human CD4+ T Lymphocyte CXCR4-Stimulated Adhesion Under Shear Flow

Introduction

The leukocyte adhesion cascade is important for the maintenance of homeostasis and the ability of immune cells to access sites of infection and inflammation. Despite much work identifying the molecular components of the cascade, and numerous simulations to predict the relationship between molecule density, identity, and adhesion, these relationships have not been measured experimentally.

Methods

Using surfaces functionalized with recombinant ICAM-1 and/or E-selectin along with immobilized SDF-1α, we used a flow chamber to measure rates of tethering, rolling and arrest of primary naïve human CD4⁺ T lymphocytes on different surface densities of ligand.

Results

Cells required a minimum level of ligand density to progress beyond tethering. E-selectin and ICAM-1 were found to have a synergistic relationship in promoting cell arrest. Surfaces with both ligands had the highest levels of arrest, while surfaces containing only E-selectin hindered the cell's ability to progress beyond rolling. In contrast, surfaces of ICAM-1 allowed only tethering or arrest. Cells maintained constant rolling velocity and time to stop over large variations in surface density and composition. In addition, surface densities of only O(10¹) sites/μm² allowed for rolling while surface densities of O(10²) sites/μm² promoted arrest, approximately equal to previously determined simulated values.

Conclusions

We have systematically and experimentally mapped out the state diagram of T-cell adhesion under flow, directly demonstrating the quantitative requirements for each dynamic state of adhesion, and showing how multiple adhesion molecules can act in synergy to secure arrest.

Coronin 1A, a novel player in integrin biology, controls neutrophil trafficking in innate immunity

Trafficking of polymorphonuclear neutrophils (PMNs) during inflammation critically depends on the β₂ integrins lymphocyte function-associated antigen 1 (LFA-1) (CD11a/CD18) and macrophage-1 antigen (CD11b/CD18). Here, we identify coronin 1A (Coro1A) as a novel regulator of β₂ integrins that interacts with the cytoplasmic tail of CD18 and is crucial for induction of PMN adhesion and postadhesion events, including adhesion strengthening, spreading, and migration under flow conditions. Transition of PMN rolling to firm adhesion critically depends on Coro1A by regulating the accumulation of high-affinity LFA-1 in focal zones of adherent cells. Defective integrin affinity regulation in the genetic absence of Coro1A impairs leukocyte adhesion and extravasation in inflamed cremaster muscle venules in comparison with control animals. In a Helicobacter pylori mouse infection model, PMN infiltration into the gastric mucosa is dramatically reduced in Coro1A^-/- mice, resulting in an attenuated gastric inflammation. Thus, Coro1A represents an important novel player in integrin biology, with key functions in PMN trafficking during innate immunity.

Leukocytes Crossing the Endothelium: A Matter of Communication

Leukocytes cross the endothelial vessel wall in a process called transendothelial migration (TEM). The purpose of leukocyte TEM is to clear the causing agents of inflammation in underlying tissues, for example, bacteria and viruses. During TEM, endothelial cells initiate signals that attract and guide leukocytes to sites of tissue damage. Leukocytes react by attaching to these sites and signal their readiness to move back to endothelial cells. Endothelial cells in turn respond by facilitating the passage of leukocytes while retaining overall integrity. In this review, we present recent findings in the field and we have endeavored to synthesize a coherent picture of the intricate interplay between endothelial cells and leukocytes during TEM.

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.

Selectins: initiators of leucocyte adhesion and signalling at the vascular wall

The selectins are transmembrane, Ca(2+)-dependent lectins that mediate leucocyte rolling on vascular surfaces, the first adhesive step during inflammation and immune surveillance. Leucocytes express L-selectin, activated platelets express P-selectin, and activated endothelial cells express E- and P-selectin. Rolling involves force-regulated, rapidly reversible interactions of selectins with a limited number of glycosylated cell surface ligands. Rolling permits leucocytes to interact with immobilized chemokines that convert β2 integrins to high-affinity conformations, which mediate arrest, post-arrest adhesion strengthening, and transendothelial migration. However, rolling leucocytes also transduce signals through selectin ligands, the focus of this review. These signals include serial activation of kinases and recruitment of adaptors that convert integrins to intermediate-affinity conformations, which decrease rolling velocities. In vitro, selectin signalling enables myeloid cells to respond to suboptimal levels of chemokines and other agonists. This cooperative signalling triggers effector responses such as degranulation, superoxide production, chemokine synthesis, and release of procoagulant/proinflammatory microparticles. In vivo, selectin-mediated adhesion and signalling likely contributes to atherosclerosis, arterial and deep vein thrombosis, ischaemia-reperfusion injury, and other cardiovascular diseases.

Blocking neutrophil integrin activation prevents ischemia-reperfusion injury

Neutrophil recruitment, mediated by β2 integrins, combats pyogenic infections but also plays a key role in ischemia-reperfusion injury and other inflammatory disorders. Talin induces allosteric rearrangements in integrins that increase affinity for ligands (activation). Talin also links integrins to actin and other proteins that enable formation of adhesions. Structural studies have identified a talin1 mutant (L325R) that perturbs activation without impairing talin's capacity to link integrins to actin and other proteins. Here, we found that mice engineered to express only talin1(L325R) in myeloid cells were protected from renal ischemia-reperfusion injury. Dissection of neutrophil function in vitro and in vivo revealed that talin1(L325R) neutrophils had markedly impaired chemokine-induced, β2 integrin-mediated arrest, spreading, and migration. Surprisingly, talin1(L325R) neutrophils exhibited normal selectin-induced, β2 integrin-mediated slow rolling, in sharp contrast to the defective slow rolling of neutrophils lacking talin1 or expressing a talin1 mutant (W359A) that blocks talin interaction with integrins. These studies reveal the importance of talin-mediated activation of integrins for renal ischemia-reperfusion injury. They further show that neutrophil arrest requires talin recruitment to and activation of integrins. However, although neutrophil slow rolling requires talin recruitment to integrins, talin-mediated integrin activation is dispensable.

O-glycans direct selectin ligands to lipid rafts on leukocytes

Palmitoylated cysteines typically target transmembrane proteins to domains enriched in cholesterol and sphingolipids (lipid rafts). P-selectin glycoprotein ligand-1 (PSGL-1), CD43, and CD44 are O-glycosylated proteins on leukocytes that associate with lipid rafts. During inflammation, they transduce signals by engaging selectins as leukocytes roll in venules, and they move to the raft-enriched uropods of polarized cells upon chemokine stimulation. It is not known how these glycoproteins associate with lipid rafts or whether this association is required for signaling or for translocation to uropods. Here, we found that loss of core 1-derived O-glycans in murine C1galt1(-/-) neutrophils blocked raft targeting of PSGL-1, CD43, and CD44, but not of other glycosylated proteins, as measured by resistance to solubilization in nonionic detergent and by copatching with a raft-resident sphingolipid on intact cells. Neuraminidase removal of sialic acids from wild-type neutrophils also blocked raft targeting. C1galt1(-/-) neutrophils or neuraminidase-treated neutrophils failed to activate tyrosine kinases when plated on immobilized anti-PSGL-1 or anti-CD44 F(ab')2. Furthermore, C1galt1(-/-) neutrophils incubated with anti-PSGL-1 F(ab')2 did not generate microparticles. In marked contrast, PSGL-1, CD43, and CD44 moved normally to the uropods of chemokine-stimulated C1galt1(-/-) neutrophils. These data define a role for core 1-derived O-glycans and terminal sialic acids in targeting glycoprotein ligands for selectins to lipid rafts of leukocytes. Preassociation of these glycoproteins with rafts is required for signaling but not for movement to uropods.

Cytoskeletal regulation of CD44 membrane organization and interactions with E-selectin

Interactions of CD44 on neutrophils with E-selectin on activated endothelial cells mediate rolling under flow, a prerequisite for neutrophil arrest and migration into perivascular tissues. How CD44 functions as a rolling ligand despite its weak affinity for E-selectin is unknown. We examined the nanometer scale organization of CD44 on intact cells. CD44 on leukocytes and transfected K562 cells was cross-linked within a 1.14-nm spacer. Depolymerizing actin with latrunculin B reduced cross-linking. Fluorescence resonance energy transfer (FRET) revealed tight co-clustering between CD44 fused to yellow fluorescent protein (YFP) and CD44 fused to cyan fluorescent protein on K562 cells. Latrunculin B reduced FRET-reported co-clustering. Number and brightness analysis confirmed actin-dependent CD44-YFP clusters on living cells. CD44 lacking binding sites for ankyrin and for ezrin/radixin/moesin (ERM) proteins on its cytoplasmic domain (ΔANKΔERM) did not cluster. Unexpectedly, CD44 lacking only the ankyrin-binding site (ΔANK) formed larger but looser clusters. Fluorescence recovery after photobleaching demonstrated increased CD44 mobility by latrunculin B treatment or by deleting the cytoplasmic domain. ΔANKΔERM mobility increased only modestly, suggesting that the cytoplasmic domain engages the cytoskeleton by an additional mechanism. Ex vivo differentiated CD44-deficient neutrophils expressing exogenous CD44 rolled on E-selectin and activated Src kinases after binding anti-CD44 antibody. In contrast, differentiated neutrophils expressing ΔANK had impaired rolling and kinase activation. These data demonstrate that spectrin and actin networks regulate CD44 clustering and suggest that ankyrin enhances CD44-mediated neutrophil rolling and signaling.

Agonist leukadherin-1 increases CD11b/CD18-dependent adhesion via membrane tethers

Integrin CD11b/CD18 is a key adhesion receptor that mediates leukocyte migration and immune functions. Leukadherin-1 (LA1) is a small molecule agonist that enhances CD11b/CD18-dependent cell adhesion to its ligand ICAM-1. Here, we used single-molecule force spectroscopy to investigate the biophysical mechanism by which LA1-activated CD11b/CD18 mediates leukocyte adhesion. Between the two distinct populations of CD11b/CD18:ICAM-1 complex that participate in cell adhesion, the cytoskeleton(CSK)-anchored elastic elements and the membrane tethers, we found that LA1 enhanced binding of CD11b/CD18 on K562 cells to ICAM-1 via the formation of long membrane tethers, whereas Mn(2+) additionally increased ICAM-1 binding via CSK-anchored bonds. LA1 activated wild-type and LFA1(-/-) neutrophils also showed longer detachment distances and time from ICAM-1-coated atomic force microscopy tips, but significantly lower detachment force, as compared to the Mn(2+)-activated cells, confirming that LA1 primarily increased membrane-tether bonds to enhance CD11b/CD18:ICAM-1 binding, whereas Mn(2+) induced additional CSK-anchored bond formation. The results suggest that the two types of agonists differentially activate integrins and couple them to the cellular machinery, providing what we feel are new insights into signal mechanotransduction by such agents.

Expression of CD11b (MAC-1) and CD162 (PSGL-1) on monocytes is decreased under conditions of deep hypothermic circulatory arrest

Deep hypothermic circulatory arrest (DHCA) is a common technique used to protect vital organs during surgical interventions on the thoracic aorta or during surgery for complex congenital heart disease. Activated leukocytes are key mediators of inflammatory responses during ischemia. Intercellular crosstalk between leukocytes, platelets and endothelial cells is mediated by cell adhesion molecules. These molecules trigger complex cell-cell interaction mechanisms and initiate the release of proinflammatory molecules. One parameter that is known to have a significant impact on inflammatory cell activation and the production of proinflammatory markers is temperature. However, to the best of our knowledge, no data have yet been published on the effect of hypothermia on leukocyte surface markers during DHCA. Thus, the aim of the present study was to investigate the effect of hypothermia on the expression of cell adhesion molecules on monocytes under DHCA conditions in vitro. Blood samples collected from 11 healthy volunteers were incubated in a well-established model simulating circulatory arrest at 36°C and 18°C for 30 min. The expression of cluster of differentiation (CD) molecule 11B (CD11b), CD54 and CD162 on monocytes was measured as the mean fluorescence intensity (MFI) using flow cytometry. The expression level of CD11b on monocytes was significantly decreased following the incubation of the blood samples at 18°C compared with the level in blood samples incubated at 36°C (P<0.001). After 30 min of blood stasis in the circulatory arrest model, the expression level of CD162 on monocytes was significantly lower in the blood samples incubated at 18°C than in those incubated at 36°C (P<0.001). No association was identified between temperature and the surface expression of CD54 on monocytes following 30 min of stasis. These findings demonstrate that deep hypothermia decreases the expression of CD11b and CD162 on monocytes in an experimental setup simulating the conditions of DHCA. This may be the result of the inhibition of leukocyte-endothelial and leukocyte-platelet interactions, which may be a beneficial aspect of deep hypothermia that affects the inflammatory response and tissue damage during DHCA.

Mechanical regulation of T-cell functions

T cells are key players of the mammalian adaptive immune system. They experience different mechanical microenvironments during their life cycle, from the thymus, secondary lymph organs, and peripheral tissues that are free of externally applied force, but display variable substrate rigidities to the blood and lymphatic circulation systems, where complicated hydrodynamic forces are present. Regardless of whether T cells are subject to external forces or generate their own internal forces, they respond and adapt to different biomechanical cues to modulate their adhesion, migration, trafficking, and triggering of immune functions through mechanical regulation of various molecules that bear force. These include adhesive receptors, immunoreceptors, motor proteins, cytoskeletal proteins, and their associated molecules. Here, we discuss the forces acting on various surface and cytoplasmic proteins of a T cell in different mechanical milieus. We review existing data on how force regulates protein conformational changes and interactions with counter molecules, including integrins, actin, and the T-cell receptor, and how each relates to T-cell functions.

MD-2-dependent human Toll-like receptor 4 monoclonal antibodies detect extracellular association of Toll-like receptor 4 with extrinsic soluble MD-2 on the cell surface

MD-2 is essential for lipopolysaccharide (LPS) recognition of Toll-like receptor 4 (TLR4) but not for cell surface expression. The TLR4/MD-2 complex is formed intracellularly through co-expression. Extracellular complex formation remains a matter for debate because of the aggregative nature of secreted MD-2 in the absence of TLR4 co-expression. We demonstrated extracellular complex formation using three independent monoclonal antibodies (mAbs), all of which are specific for complexed TLR4 but unreactive with free TLR4 and MD-2. These mAbs bound to TLR4-expressing Ba/F3 cells only when co-cultured with MD-2-secreting Chinese hamster ovary cells or incubated with conditioned medium from these cells. All three mAbs bound the extracellularly formed complex indistinguishably from the intracellularly formed complex in titration studies. In addition, we demonstrated that two mAbs lost their affinity for TLR4/MD-2 on LPS stimulation, suggesting that these mAbs bound to conformation-sensitive epitopes. This was also found when the extracellularly formed complex was stimulated with LPS. Additionally, we showed that cell surface TLR4 and extrinsically secreted MD-2 are capable of forming the functional complex extracellularly, indicating an additional or alternative pathway for the complex formation.

Effects of microfluidic channel geometry on leukocyte rolling assays

Microfluidic cell adhesion assays have emerged as a means to increase throughput as well as reduce the amount of costly reagents. However as dimensions of the flow chamber are reduced and approach the diameter of a cell (D(c)), theoretical models have predicted that mechanical stress, force, and torque on a cell will be amplified. We fabricated a series of microfluidic devices that have a constant width:height ratio (10:1) but with varying heights. The smallest microfluidic device (200 μm ×20 μm) requires perfusion rates as low as 40 nL/min to generate wall shear stresses of 0.5 dynes/cm(2). When neutrophils were perfused through P-selectin coated chambers at equivalent wall shear stress, rolling velocities decreased by approximately 70 % as the ratio of cell diameter to chamber height (D(c)/H) increased from 0.08 (H = 100 μm) to 0.40 (H = 20 μm). Three-dimensional numerical simulations of neutrophil rolling in channels of different heights showed a similar trend. Complementary studies with PSGL-1 coated microspheres and paraformaldehyde-fixed neutrophils suggested that changes in rolling velocity were related to cell deformability. Using interference reflection microscopy, we observed increases in neutrophil contact area with increasing chamber height (9-33 %) and increasing wall shear stress (28-56 %). Our results suggest that rolling velocity is dependent not only on wall shear stress but also on the shear stress gradient experienced by the rolling cell. These results point to the D(c)/H ratio as an important design parameter of leukocyte microfluidic assays, and should be applicable to rolling assays that involve other cell types such as platelets or cancer cells.

Lessons from rare maladies: leukocyte adhesion deficiency syndromes

PURPOSE OF REVIEW

The leukocyte adhesion deficiency (LAD) syndromes are rare genetically determined conditions with challenging clinical features. These immunodeficiencies also provide insights that are broadly relevant to the biology of leukocytes, platelets, intercellular interactions, and intracellular signaling. Recent discoveries merit their review in the context of existing knowledge.

RECENT FINDINGS

New activities of β(2) integrins, which are deficient or absent in LAD-I, and new β(2) integrin-dependent functions of neutrophils and other leukocytes have recently been identified. Genetic defects and mechanisms accounting for impaired fucosylation of selectin ligands and defective selectin binding and signaling in LAD-II are now apparent. LAD-III, which presents with bleeding similar to that in Glanzmann thrombasthenia and platelet dysfunction in addition to impaired leukocyte adhesion, is now known to be due to absence of KINDLIN-3, a cytoplasmic protein that acts cooperatively with TALIN-1 in activating β(1), β(2), and β(3) integrins. Understanding of the leukocyte adhesion cascade and interactions of leukocytes with inflamed endothelium, which are impaired in each of the LAD syndromes, continues to be refined.

SUMMARY

Although LAD syndromes are rare maladies, their investigation is generating new knowledge directly applicable to the diagnosis and care of patients and to fundamental paradigms in immunobiology and hemostasis.

P-selectin glycoprotein ligand-1 forms dimeric interactions with E-selectin but monomeric interactions with L-selectin on cell surfaces

Interactions of selectins with cell surface glycoconjugates mediate the first step of the adhesion and signaling cascade that recruits circulating leukocytes to sites of infection or injury. P-selectin dimerizes on the surface of endothelial cells and forms dimeric bonds with P-selectin glycoprotein ligand-1 (PSGL-1), a homodimeric sialomucin on leukocytes. It is not known whether leukocyte L-selectin or endothelial cell E-selectin are monomeric or oligomeric. Here we used the micropipette technique to analyze two-dimensional binding of monomeric or dimeric L- and E-selectin with monomeric or dimeric PSGL-1. Adhesion frequency analysis demonstrated that E-selectin on human aortic endothelial cells supported dimeric interactions with dimeric PSGL-1 and monomeric interactions with monomeric PSGL-1. In contrast, L-selectin on human neutrophils supported monomeric interactions with dimeric or monomeric PSGL-1. Our work provides a new method to analyze oligomeric cross-junctional molecular binding at the interface of two interacting cells.

Neutrophil rolling at high shear: flattening, catch bond behavior, tethers and slings

Neutrophil recruitment to sites of inflammation involves neutrophil rolling along the inflamed endothelium in the presence of shear stress imposed by blood flow. Neutrophil rolling in post-capillary venules in vivo is primarily mediated by P-selectin on the endothelium binding to P-selectin glycoprotein ligand-1 (PSGL-1) constitutively expressed on neutrophils. Blood flow exerts a hydrodynamic drag on the rolling neutrophil which is partially or fully balanced by the adhesive forces generated in the P-selectin-PSGL-1 bonds. Rolling is the result of rapid formation and dissociation of P-selectin-PSGL-1 bonds at the center and rear of the rolling cell, respectively. Neutrophils roll stably on P-selectin in post-capillary venules in vivo and flow chambers in vitro at wall shear stresses greater than 6 dyn cm(-2). However, the mechanisms that enable neutrophils to roll at such high shear stress are not completely understood. In vitro and in vivo studies have led to the discovery of four potential mechanisms, viz. cell flattening, catch bond behavior, membrane tethers, and slings. Rolling neutrophils undergo flattening at high shear stress, which not only increases the size of the cell footprint but also reduces the hydrodynamic drag experienced by the rolling cell. P-selectin-PSGL-1 bonds behave as catch bonds at small detachment forces and thus become stronger with increasing force. Neutrophils rolling at high shear stress form membrane tethers which can be longer than the cell diameter and promote the survival of P-selectin-PSGL-1 bonds. Finally, neutrophils rolling at high shear stress form 'slings', which act as cell autonomous adhesive substrates and support step-wise peeling. Tethers and slings act together and contribute to the forces balancing the hydrodynamic drag. How the synergy between the four mechanisms leads to stable rolling at high shear stress is an area that needs further investigation.

Neutrophil arrest by LFA-1 activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimeric integrin consisting of α_L (gene name, Itgal) and β₂ (gene name, Itgb2) subunits expressed in all leukocytes. LFA-1 is essential for neutrophil recruitment to inflamed tissue. Activation of LFA-1 by chemokines allows neutrophils and other leukocytes to undergo arrest, resulting in firm adhesion on endothelia expressing intercellular adhesion molecules (ICAMs). In mice, CXCR2 is the primary chemokine receptor involved in triggering neutrophil arrest, and it does so through “inside-out” activation of LFA-1. CXCR2 signaling induces changes in LFA-1 conformation that are coupled to affinity upregulation of the ligand-binding headpiece (extended with open I domain). Unlike naïve lymphocytes, engagement of P-selectin glycoprotein ligand-1 (PSGL-1) on neutrophils stimulates a slow rolling behavior that is mediated by LFA-1 in a distinct activation state (extended with closed I domain). How inside-out signaling cascades regulate the structure and function of LFA-1 is being studied using flow chambers, intravital microscopy, and flow cytometry for ligand and reporter antibody binding. Here, we review how LFA-1 activation is regulated by cellular signaling and ligand binding. Two FERM domain-containing proteins, talin-1 and Kindlin-3, are critical integrin co-activators and have distinct roles in the induction of LFA-1 conformational rearrangements. This review integrates these new results into existing models of LFA-1 activation.