Signaling and Dynamics of Activation of LFA-1 and Mac-1 by Immobilized IL-8

Endoplasmic Reticulum Protein 72 Regulates Integrin Mac-1 Activity to Influence Neutrophil Recruitment

BACKGROUND

Integrins mediate the adhesion, crawling, and migration of neutrophils during vascular inflammation. Thiol exchange is important in the regulation of integrin functions. ERp72 (endoplasmic reticulum-resident protein 72) is a member of the thiol isomerase family responsible for the catalysis of disulfide rearrangement. However, the role of ERp72 in the regulation of Mac-1 (integrin αMβ2) on neutrophils remains elusive.

METHODS

Intravital microscopy of the cremaster microcirculation was performed to determine in vivo neutrophil movement. Static adhesion, flow chamber, and flow cytometry were used to evaluate in vitro integrin functions. Confocal fluorescent microscopy and coimmunoprecipitation were utilized to characterize the interactions between ERp72 and Mac-1 on neutrophil surface. Cell-impermeable probes and mass spectrometry were used to label reactive thiols and identify target disulfide bonds during redox exchange. Biomembrane force probe was performed to quantitatively measure the binding affinity of Mac-1. A murine model of acute lung injury induced by lipopolysaccharide was utilized to evaluate neutrophil-associated vasculopathy.

RESULTS

ERp72-deficient neutrophils exhibited increased rolling but decreased adhesion/crawling on inflamed venules in vivo and defective static adhesion in vitro. The defect was due to defective activation of integrin Mac-1 but not LFA-1 (lymphocyte function-associated antigen-1) using blocking or epitope-specific antibodies. ERp72 interacted with Mac-1 in lipid rafts on neutrophil surface leading to the reduction of the C654-C711 disulfide bond in the αM subunit that is critical for Mac-1 activation. Recombinant ERp72, via its catalytic motifs, increased the binding affinity of Mac-1 with ICAM-1 (intercellular adhesion molecule-1) and rescued the defective adhesion of ERp72-deficient neutrophils both in vitro and in vivo. Deletion of ERp72 in the bone marrow inhibited neutrophil infiltration, ameliorated tissue damage, and increased survival during murine acute lung injury.

CONCLUSIONS

Extracellular ERp72 regulates integrin Mac-1 activity by catalyzing disulfide rearrangement on the αM subunit and may be a novel target for the treatment of neutrophil-associated vasculopathy.

Monocyte-endothelial cell interactions in vascular and tissue remodeling

Monocytes are circulating leukocytes of innate immunity derived from the bone marrow that interact with endothelial cells under physiological or pathophysiological conditions to orchestrate inflammation, angiogenesis, or tissue remodeling. Monocytes are attracted by chemokines and specific receptors to precise areas in vessels or tissues and transdifferentiate into macrophages with tissue damage or infection. Adherent monocytes and infiltrated monocyte-derived macrophages locally release a myriad of cytokines, vasoactive agents, matrix metalloproteinases, and growth factors to induce vascular and tissue remodeling or for propagation of inflammatory responses. Infiltrated macrophages cooperate with tissue-resident macrophages during all the phases of tissue injury, repair, and regeneration. Substances released by infiltrated and resident macrophages serve not only to coordinate vessel and tissue growth but cellular interactions as well by attracting more circulating monocytes (e.g. MCP-1) and stimulating nearby endothelial cells (e.g. TNF-α) to expose monocyte adhesion molecules. Prolonged tissue accumulation and activation of infiltrated monocytes may result in alterations in extracellular matrix turnover, tissue functions, and vascular leakage. In this review, we highlight the link between interactions of infiltrating monocytes and endothelial cells to regulate vascular and tissue remodeling with a special focus on how these interactions contribute to pathophysiological conditions such as cardiovascular and chronic liver diseases.

Purified complement C3b triggers phagocytosis and activation of human neutrophils via complement receptor 1

The complement system provides vital immune protection against infectious agents by labeling them with complement fragments that enhance phagocytosis by immune cells. Many details of complement-mediated phagocytosis remain elusive, partly because it is difficult to study the role of individual complement proteins on target surfaces. Here, we employ serum-free methods to couple purified complement C3b onto E. coli bacteria and beads and then expose human neutrophils to these C3b-coated targets. We examine the neutrophil response using a combination of flow cytometry, confocal microscopy, luminometry, single-live-cell/single-target manipulation, and dynamic analysis of neutrophil spreading on opsonin-coated surfaces. We show that purified C3b can potently trigger phagocytosis and killing of bacterial cells via Complement receptor 1. Comparison of neutrophil phagocytosis of C3b- versus antibody-coated beads with single-bead/single-target analysis exposes a similar cell morphology during engulfment. However, bulk phagocytosis assays of C3b-beads combined with DNA-based quenching reveal that these are poorly internalized compared to their IgG1 counterparts. Similarly, neutrophils spread slower on C3b-coated compared to IgG-coated surfaces. These observations support the requirement of multiple stimulations for efficient C3b-mediated uptake. Together, our results establish the existence of a direct pathway of phagocytic uptake of C3b-coated targets and present methodologies to study this process.

Ultrarapid lytic granule release from CTLs activates Ca2+-dependent synaptic resistance pathways in melanoma cells

Human cytotoxic T lymphocytes (CTLs) exhibit ultrarapid lytic granule secretion, but whether melanoma cells mobilize defense mechanisms with commensurate rapidity remains unknown. We used single-cell time-lapse microscopy to offer high spatiotemporal resolution analyses of subcellular events in melanoma cells upon CTL attack. Target cell perforation initiated an intracellular Ca²⁺ wave that propagated outward from the synapse within milliseconds and triggered lysosomal mobilization to the synapse, facilitating membrane repair and conferring resistance to CTL induced cytotoxicity. Inhibition of Ca²⁺ flux and silencing of synaptotagmin VII limited synaptic lysosomal exposure and enhanced cytotoxicity. Multiplexed immunohistochemistry of patient melanoma nodules combined with automated image analysis showed that melanoma cells facing CD8⁺ CTLs in the tumor periphery or peritumoral area exhibited significant lysosomal enrichment. Our results identified synaptic Ca²⁺ entry as the definitive trigger for lysosomal deployment to the synapse upon CTL attack and highlighted an unpredicted defensive topology of lysosome distribution in melanoma nodules.

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.

Protective Activities of Dendrobium huoshanense C. Z. Tang et S. J. Cheng Polysaccharide against High-Cholesterol Diet-Induced Atherosclerosis in Zebrafish

Cardiovascular disease is the highest cause of death, and atherosclerosis (AS) is the primary pathogenesis of many cardiovascular diseases. In this study, we aim to investigate the possible pharmaceutical effects of Dendrobium huoshanense C. Z. Tang et S. J. Cheng polysaccharide (DHP) in AS. We fed zebrafish with high-cholesterol diet (HCD) to establish a zebrafish AS model and treated with DHP and observed plaque formation and neutrophil counts under a fluorescence microscope. Next, a parallel flow chamber was utilized to establish low shear stress- (LSS-) induced endothelial cell (EC) dysfunction model. We observed that DHP significantly improved HCD-induced lipid deposition, oxidative stress, and inflammatory response, mainly showing that DHP significantly increased superoxide dismutase (SOD) activity, decreased plaque formation, and decreased neutrophil recruitment and the levels of total cholesterol (TC), triglyceride (TG), malondialdehyde (MDA), and reactive oxygen species (ROS). Furthermore, DHP significantly improved LSS-induced oxidative stress and EC dysfunction. Our results indicated that DHP can exert treatment effects on AS, which may attribute to its hypolipidemic, antioxidant, anti-inflammatory activities and improving LSS-induced EC dysfunction. DHP has promising potential for further development as a functional natural medicine source targeted at AS prevention.

Diaphanous-related formin mDia2 regulates beta2 integrins to control hematopoietic stem and progenitor cell engraftment

Bone marrow engraftment of the hematopoietic stem and progenitor cells (HSPCs) involves homing to the vasculatures and lodgment to their niches. How HSPCs transmigrate from the vasculature to the niches is unclear. Here, we show that loss of diaphanous-related formin mDia2 leads to impaired engraftment of long-term hematopoietic stem cells and loss of competitive HSPC repopulation. These defects are likely due to the compromised trans-endothelial migration of HSPCs since their homing to the bone marrow vasculatures remained intact. Mechanistically, loss of mDia2 disrupts HSPC polarization and induced cytoplasmic accumulation of MAL, which deregulates the activity of serum response factor (SRF). We further reveal that beta2 integrins are transcriptional targets of SRF. Knockout of beta2 integrins in HSPCs phenocopies mDia2 deficient mice. Overexpression of SRF or beta2 integrins rescues HSPC engraftment defects associated with mDia2 deficiency. Our findings show that mDia2-SRF-beta2 integrin signaling is critical for HSPC lodgment to the niches.

Bone marrow engraftment of haematopoietic stem and progenitor cells (HSPCs) requires homing and lodgement to the niche. Here, the authors show that mDia2 is required for HSPC polarization, nuclear MAL, and SRF-induced beta2 integrin expression during transendothelial migration of HSPCs required for engraftment.

ICAM-1: A master regulator of cellular responses in inflammation, injury resolution, and tumorigenesis

ICAM-1 is a cell surface glycoprotein and an adhesion receptor that is best known for regulating leukocyte recruitment from circulation to sites of inflammation. However, in addition to vascular endothelial cells, ICAM-1 expression is also robustly induced on epithelial and immune cells in response to inflammatory stimulation. Importantly, ICAM-1 serves as a biosensor to transduce outside-in-signaling via association of its cytoplasmic domain with the actin cytoskeleton following ligand engagement of the extracellular domain. Thus, ICAM-1 has emerged as a master regulator of many essential cellular functions both at the onset and at the resolution of pathologic conditions. Because the role of ICAM-1 in driving inflammatory responses is well recognized, this review will mainly focus on newly emerging roles of ICAM-1 in epithelial injury-resolution responses, as well as immune cell effector function in inflammation and tumorigenesis. ICAM-1 has been of clinical and therapeutic interest for some time now; however, several attempts at inhibiting its function to improve injury resolution have failed. Perhaps, better understanding of its beneficial roles in resolution of inflammation or its emerging function in tumorigenesis will spark new interest in revisiting the clinical value of ICAM-1 as a potential therapeutic target.

Ca2+-activated cleavage of ezrin visualised dynamically in living myeloid cells during cell surface area expansion

The intracellular events underlying phagocytosis, a crucial event for innate immunity, are still unresolved. In order to test whether the reservoir of membrane required for the formation of the phagocytic pseudopodia is maintained by cortical ezrin, and that its cleavage is a key step in releasing this membrane, the cleavage of cortical ezrin was monitored within living phagocytes (the phagocytically competent cell line RAW264.7) through expressing two ezrin constructs with fluorescent protein tags located either inside the FERM or at the actin-binding domains. When ezrin is cleaved in the linker region by the Ca2+-activated protease calpain, separation of the two fluorophores would result. Experimentally induced Ca2+ influx triggered cleavage of peripherally located ezrin, which was temporally associated with cell expansion. Ezrin cleavage was also observed in the phagocytic pseudopodia during phagocytosis. Thus, our data demonstrates that peripheral ezrin is cleaved during Ca2+-influx-induced membrane expansion and locally within the extending pseudopodia during phagocytosis. This is consistent with a role for intact ezrin in maintaining folded membrane on the cell surface, which then becomes available for cell spreading and phagocytosis.

Intravascular adhesion and recruitment of neutrophils in response to CXCL1 depends on their TRPC6 channels

Abstract

Here we report a novel role for TRPC6, a member of the transient receptor potential (TRPC) channel family, in the CXCL1-dependent recruitment of murine neutrophil granulocytes. Representing a central element of the innate immune system, neutrophils are recruited from the blood stream to a site of inflammation. The recruitment process follows a well-defined sequence of events including adhesion to the blood vessel walls, migration, and chemotaxis to reach the inflammatory focus. A common feature of the underlying signaling pathways is the utilization of Ca²⁺ ions as intracellular second messengers. However, the required Ca²⁺ influx channels are not yet fully characterized. We used WT and TRPC6^−/− neutrophils for in vitro and TRPC6^−/− chimeric mice (WT mice with WT or TRPC6^−/− bone marrow cells) for in vivo studies. After renal ischemia and reperfusion injury, TRPC6^−/− chimeric mice had an attenuated TRPC6^−/− neutrophil recruitment and a better outcome as judged from the reduced increase in the plasma creatinine concentration. In the cremaster model CXCL1-induced neutrophil adhesion, arrest and transmigration were also decreased in chimeric mice with TRPC6^−/− neutrophils. Using atomic force microscopy and microfluidics, we could attribute the recruitment defect of TRPC6^−/− neutrophils to the impact of the channel on adhesion to endothelial cells. Mechanistically, TRPC6^−/− neutrophils exhibited lower Ca²⁺ transients during the initial adhesion leading to diminished Rap1 and β₂ integrin activation and thereby reduced ICAM-1 binding. In summary, our study reveals that TRPC6 channels in neutrophils are crucial signaling modules in their recruitment from the blood stream in response to CXCL1.

Key point

Neutrophil TRPC6 channels are crucial for CXCL1-triggered activation of integrins during the initial steps of neutrophil recruitment.

Mechanical strength determines Ca2+ transients triggered by the engagement of β2 integrins to their ligands

Lymphocyte function-associated antigen-1 (LFA-1) and macrophage-1 antigen (Mac-1) are key adhesion receptors to mediate neutrophil (PMN) recruitment and intracellular calcium (Ca²⁺) signaling. Binding of LFA-1 and Mac-1 to their ligands is essential in triggering Ca²⁺ transients and activating Ca²⁺-dependent kinases involved in cytoskeletal remodeling and migratory function. While mechanical forces are critical in regulating integrin-mediated Ca²⁺ transients, it is still unclear how the bond strength of β₂-integrin-ligand pair affects Ca²⁺ responses. Here three typical ligands with known mechanical features with LFA-1 and Mac-1 in our previous work were adopted to quantify their capabilities in inducing Ca²⁺ transients in adherent PMNs under shear flow. Data indicated that LFA-1 dominates Ca²⁺ transients in PMNs on intercellular adhesive molecule 1 (ICAM-1) and junctional adhesion molecule-A (JAM-A), while Mac-1 mediates Ca²⁺ transients induced by receptor for advanced glycation end products (RAGE), consistent with their corresponding bond strengths. These results link β₂ integrin-ligand bond strength with Ca²⁺ transients in PMNs, suggesting high bond strength gives rise to strong Ca²⁺ response especially under physiological-like shear flow. The outcomes provide a new insight in understanding the mechanical regulatory mechanisms of PMN recruitment.

Neutrophil Mechanosignaling Promotes Integrin Engagement With Endothelial Cells and Motility Within Inflamed Vessels

Neutrophils are the most motile of mammalian cells, a feature that enables them to protect the host against the rapid spread of pathogens from tissue into the circulatory system. A critical process is the recruitment of neutrophils to inflamed endothelium within post-capillary venules. This occurs through cooperation between at least four families of adhesion molecules and G-protein coupled signaling receptors. These adhesion molecules convert the drag force induced by blood flow acting on the cell surface into bond tension that resists detachment. A common feature of selectin-glycoprotein tethering and integrin-ICAM bond formation is the mechanics by which force acting on these specific receptor-ligand pairs influences their longevity, strength, and topographic organization on the plasma membrane. Another distinctly mechanical aspect of neutrophil guidance is the capacity of adhesive bonds to convert external mechanical force into internal biochemical signals through the transmission of force from the outside-in at focal sites of adhesive traction on inflamed endothelium. Within this region of the plasma membrane, we denote the inflammatory synapse, Ca²⁺ release, and intracellular signaling provide directional cues that guide actin assembly and myosin driven motive force. This review provides an overview of how bond formation and outside-in signaling controls neutrophil recruitment and migration relative to the hydrodynamic shear force of blood flow.

Mechanistic Understanding of Single-Cell Behavior is Essential for Transformative Advances in Biomedicine

Most current efforts to advance medical technology proceed along one of two tracks. The first is dedicated to the improvement of clinical tasks through the incremental refinement of medical instruments. The second comprises engineering endeavors to support basic science studies that often only remotely relate to human medicine. Here we survey emerging research approaches that aim to populate the sprawling frontier between these tracks. We focus on interdisciplinary single-live-cell techniques that have overcome limitations of traditional biological methods to successfully address vital questions about medically relevant cellular behavior. Most of the presented case studies are based on the controlled manipulation of nonadherent human immune cells using one or more micropipettes. The included studies have (i) examined one-on-one encounters of immune cells with real or model pathogens, (ii) assessed the physiological role of the expandable surface area of immune cells, and (iii) started to dissect the spatiotemporal organization of signaling processes within these cells. The unique aptitude of such single-live-cell studies to fill conspicuous gaps in our quantitative understanding of medically relevant cause-effect relationships provides a sound basis for new insights that will inform and drive future biomedical innovation.

Binding of intercellular adhesion molecule 1 to β2-integrin regulates distinct cell adhesion processes on hepatic and cerebral endothelium

Flowing polymorphonuclear neutrophils (PMNs) are forced to recruit toward inflamed tissue and adhere to vascular endothelial cells, which is primarily mediated by the binding of β₂-integrins to ICAM-1. This process is distinct among different organs such as liver and brain; however, the underlying kinetic and mechanical mechanisms regulating tissue-specific recruitment of PMNs remain unclear. Here, binding kinetics measurement showed that ICAM-1 on murine hepatic sinusoidal endothelial cells (LSECs) bound to lymphocyte function-associated antigen-1 (LFA-1) with higher on- and off-rates but lower effective affinity compared with macrophage-1 antigen (Mac-1), whereas ICAM-1 on cerebral endothelial cells (BMECs or bEnd.3 cells) bound to LFA-1 with higher on-rates, similar off-rates, and higher effective affinity compared with Mac-1. Physiologically, free crawling tests of PMN onto LSEC, BMEC, or bEnd.3 monolayers were consistent with those kinetics differences between two β₂-integrins interacting with hepatic sinusoid or cerebral endothelium. Numerical calculations and Monte Carlo simulations validated tissue-specific contributions of β₂-integrin-ICAM-1 kinetics to PMN crawling on hepatic sinusoid or cerebral endothelium. Thus, this work first quantified the biophysical regulation of PMN adhesion in hepatic sinusoids compared with cerebral endothelium.

Regulator of G protein signaling 5 restricts neutrophil chemotaxis and trafficking

Neutrophils are white blood cells that are mobilized to damaged tissues and to sites of pathogen invasion, providing the first line of host defense. Chemokines displayed on the surface of blood vessels promote migration of neutrophils to these sites, and tissue- and pathogen-derived chemoattractant signals, including N-formylmethionylleucylphenylalanine (fMLP), elicit further migration to sites of infection. Although nearly all chemoattractant receptors use heterotrimeric G proteins to transmit signals, many of the mechanisms lying downstream of chemoattractant receptors that either promote or limit neutrophil motility are incompletely defined. Here, we show that regulator of G protein signaling 5 (RGS5), a protein that modulates G protein activity, is expressed in both human and murine neutrophils. We detected significantly more neutrophils in the airways of Rgs5^-/- mice than WT counterparts following acute respiratory virus infection and in the peritoneum in response to injection of thioglycollate, a biochemical proinflammatory stimulus. RGS5-deficient neutrophils responded with increased chemotaxis elicited by the chemokines CXC motif chemokine ligand 1 (CXCL1), CXCL2, and CXCL12 but not fMLP. Moreover, adhesion of these cells was increased in the presence of both CXCL2 and fMLP. In summary, our results indicate that RGS5 deficiency increases chemotaxis and adhesion, leading to more efficient neutrophil mobilization to inflamed tissues in mice. These findings suggest that RGS5 expression and activity in neutrophils determine their migrational patterns in the complex microenvironments characteristic of inflamed tissues.

Genetic ablation of IP3 receptor 2 increases cytokines and decreases survival of SOD1G93A mice

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disease characterized by the selective death of motor neurons. Disease pathophysiology is complex and not yet fully understood. Higher gene expression of the inositol 1,4,5-trisphosphate receptor 2 gene (ITPR2), encoding the IP₃ receptor 2 (IP₃R2), was detected in sporadic ALS patients. Here, we demonstrate that IP₃R2 gene expression was also increased in spinal cords of ALS mice. Moreover, an increase of IP₃R2 expression was observed in other models of chronic and acute neurodegeneration. Upregulation of IP₃R2 gene expression could be induced by lipopolysaccharide (LPS) in murine astrocytes, murine macrophages and human fibroblasts indicating that it may be a compensatory response to inflammation. Preventing this response by genetic deletion of ITPR2 from SOD1^G93A mice had a dose-dependent effect on disease duration, resulting in a significantly shorter lifespan of these mice. In addition, the absence of IP₃R2 led to increased innate immunity, which may contribute to the decreased survival of the SOD1^G93A mice. Besides systemic inflammation, IP₃R2 knockout mice also had increased IFNγ, IL-6 and IL1α expression. Altogether, our data indicate that IP₃R2 protects against the negative effects of inflammation, suggesting that the increase in IP₃R2 expression in ALS patients is a protective response.

A simple approach for bioactive surface calibration using evanescent waves

When investigating the interaction of cells with surfaces, it is becoming increasingly important to perform quantitative measurements of surface protein density to understand reaction kinetics. Previously, to calibrate a surface for an experiment one would have to use a radiometric assay or strip the surface with acid and perform a mass quantification. Although both of these methodologies have been proven to be effective measurement techniques for surface quantification, they can be time consuming and require substantial amounts of material. The latter is particularly problematic when working with specialized molecules or constructs that may be expensive to produce and/or only available in small quantities. Here we present a simple method to measure the intensity and penetration depth of an evanescent wave, and use this information to quantify the density of surface molecules in a microscopic region of a transparent surface.

Human severe sepsis cytokine mixture increases β2-integrin-dependent polymorphonuclear leukocyte adhesion to cerebral microvascular endothelial cells in vitro

Introduction

Sepsis-associated encephalopathy (SAE) is a state of acute brain dysfunction in response to a systemic infection. We propose that systemic inflammation during sepsis causes increased adhesion of leukocytes to the brain microvasculature, resulting in blood-brain barrier dysfunction. Thus, our objectives were to measure inflammatory analytes in plasma of severe sepsis patients to create an experimental cytokine mixture (CM), and to use this CM to investigate the activation and interactions of polymorphonuclear leukocytes (PMN) and human cerebrovascular endothelial cells (hCMEC/D3) in vitro.

Methods

The concentrations of 41 inflammatory analytes were quantified in plasma obtained from 20 severe sepsis patients and 20 age- and sex-matched healthy controls employing an antibody microarray. Two CMs were prepared to mimic severe sepsis (SSCM) and control (CCM), and these CMs were then used for PMN and hCMEC/D3 stimulation in vitro. PMN adhesion to hCMEC/D3 was assessed under conditions of flow (shear stress 0.7 dyn/cm²).

Results

Eight inflammatory analytes elevated in plasma obtained from severe sepsis patients were used to prepare SSCM and CCM. Stimulation of PMN with SSCM led to a marked increase in PMN adhesion to hCMEC/D3, as compared to CCM. PMN adhesion was abolished with neutralizing antibodies to either β2 (CD18), α_L/β₂ (CD11α/CD18; LFA-1) or α_M/β₂ (CD11β/CD18; Mac-1) integrins. In addition, immune-neutralization of the endothelial (hCMEC/D3) cell adhesion molecule, ICAM-1 (CD54) also suppressed PMN adhesion.

Conclusions

Human SSCM up-regulates PMN pro-adhesive phenotype and promotes PMN adhesion to cerebrovascular endothelial cells through a β2-integrin-ICAM-1-dependent mechanism. PMN adhesion to the brain microvasculature may contribute to SAE.

Functional aspects of the interaction between interleukin-8 and sulfated glycosaminoglycans

During the immune response, the cytokine interleukin 8 (IL-8, CXCL8) functions as a strong chemoattractant for polymorphonuclear leukocytes helping to direct these cells to infected/injured sites. This review focuses on the interaction of IL-8 with sulfated glycosaminoglycans expressed on cell surfaces and the extracellular matrix. This interaction contributes to the recruitment of polymorphonuclear cells from blood, penetration of these cells through the vessel wall, and their directed migration to inflammatory sites. Regulatory aspects of the interplay between IL-8 and heparan sulfate, the most abundant glycosaminoglycan, are highlighted. In this field, the large natural heterogeneity of glycosaminoglycans represents a great challenge that impedes the modeling of IL-8 functions. The interaction of IL-8 with newly developed artificial sulfated hyaluronan derivatives is also considered as these artificial substrates are an important tool for development of new materials in regenerative medicine.

T cell receptor signaling can directly enhance the avidity of CD28 ligand binding

T cell activation takes place in the context of a spatial and kinetic reorganization of cell surface proteins and signaling molecules at the contact site with an antigen presenting cell, termed the immunological synapse. Coordination of the activation, recruitment, and signaling from T cell receptor (TCR) in conjunction with adhesion and costimulatory receptors regulates both the initiation and duration of signaling that is required for T cell activation. The costimulatory receptor, CD28, is an essential signaling molecule that determines the quality and quantity of T cell immune responses. Although the functional consequences of CD28 engagement are well described, the molecular mechanisms that regulate CD28 function are largely unknown. Using a micropipet adhesion frequency assay, we show that TCR signaling enhances the direct binding between CD28 and its ligand, CD80. Although CD28 is expressed as a homodimer, soluble recombinant CD28 can only bind ligand monovalently. Our data suggest that the increase in CD28-CD28 binding is mediated through a change in CD28 valency. Molecular dynamic simulations and in vitro mutagenesis indicate that mutations at the base of the CD28 homodimer interface, distal to the ligand-binding site, can induce a change in the orientation of the dimer that allows for bivalent ligand binding. When expressed in T cells, this mutation allows for high avidity CD28–CD80 interactions without TCR signaling. Molecular dynamic simulations also suggest that wild type CD28 can stably adopt a bivalent conformation. These results support a model whereby inside-out signaling from the TCR can enhance CD28 ligand interactions by inducing a change in the CD28 dimer interface to allow for bivalent ligand binding and ultimately the transduction of CD28 costimulatory signals that are required for T cell activation.

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.

Caffeic acid disturbs monocyte adhesion onto cultured endothelial cells stimulated by adipokine resistin

Adipokines have been implicated in the pathogenesis of atherosclerosis via pro-inflammatory mechanisms contributing to insulin resistance. The adipokine resistin causes endothelium dysfunction, which plays an important role in sustaining atherogenesis. This study investigated whether resistin induced expression of cell adhesion molecules and integrins in endothelial cells and THP-1 monocytes and whether such induction was attenuated by 1-20 μM caffeic acid. Resistin enhanced endothelial expression of vascular cell adhesion molecule 1 (VCAM-1), intercellular cell adhesion molecule 1 (ICAM-1), and E-selectin and monocyte expression of β1, β2, and α4 integrins. The enhancement of these proteins was diminished by caffeic acid with reduced THP-1 cell adhesion on activated endothelium. Caffeic acid at ≤20 μM demoted resistin-stimulated interleukin 8 (IL-8) production responsible for ICAM-1 and β2 integrin induction. The endothelial up-regulation of IL-8 secretion by resistin entailed toll-like receptor 4 (TLR4) activation, but caffeic acid diminished IL-8 production and TLR4 induction. Furthermore, caffeic acid encumbered resistin-activated nuclear factor κB (NF-κB) signaling. These results demonstrate that caffeic acid blocked monocyte trafficking to resistin-activated endothelium via disturbing NF-κB signaling responsive to IL-8. Therefore, caffeic acid may have therapeutic potential in preventing obesity-associated atherosclerosis.

Blurred line between chemotactic chase and phagocytic consumption: an immunophysical single-cell perspective

An innate immune cell can sense a pathogen, either from a distance by recognizing chemoattractant stimuli or by direct physical contact. The pathogen is subsequently neutralized, which usually occurs through its phagocytic internalization. By investigating chemotaxis and phagocytosis from an immunophysical single-cell perspective, it now appears that the demarcation between these two processes is less distinct than originally thought. Several lines of evidence support this notion. First, chemotactic stimulation does not cease at the moment of initial contact between the cell and the pathogenic target. Second, even when classical chemotaxis of neutrophils is suppressed, the early cell response to contact with typical chemoattractant targets, such as zymosan, fungal spores or chemokine-coated particles, can still involve morphological attributes of chemotaxis. Recognizing that the changing morphology of motile cells is inextricably linked to physical cell behavior, this Commentary focuses on the mechanical aspects of the early response of innate immune cells to chemotactic and phagocytic stimuli. On the basis of this perspective, we propose that the combined study of chemotaxis and phagocytosis will, potentially, not only advance our grasp of the mechanisms underlying immune-cell motility but also open new lines of research that will promote a deeper understanding of the innate recognition of pathogens.

Conformational stability analyses of alpha subunit I domain of LFA-1 and Mac-1

β₂ integrin of lymphocyte function-associated antigen-1 (LFA-1) or macrophage-1 antigen (Mac-1) binds to their common ligand of intercellular adhesion molecule-1 (ICAM-1) and mediates leukocyte-endothelial cell (EC) adhesions in inflammation cascade. Although the two integrins are known to have distinct functions, the corresponding micro-structural bases remain unclear. Here (steered-)molecular dynamics simulations were employed to elucidate the conformational stability of α subunit I domains of LFA-1 and Mac-1 in different affinity states and relevant I domain-ICAM-1 interaction features. Compared with low affinity (LA) Mac-1, the LA LFA-1 I domain was unstable in the presence or absence of ICAM-1 ligand, stemming from diverse orientations of its α₇-helix with different motifs of zipper-like hydrophobic junction between α₁- and α₇-helices. Meanwhile, spontaneous transition of LFA-1 I domain from LA state to intermediate affinity (IA) state was first visualized. All the LA, IA, and high affinity (HA) states of LFA-1 I domain and HA Mac-1 I domain were able to bind to ICAM-1 ligand effectively, while LA Mac-1 I domain was unfavorable for binding ligand presumably due to the specific orientation of S144 side-chain that capped the MIDAS ion. These results furthered our understanding in correlating the structural bases with their functions of LFA-1 and Mac-1 integrins from the viewpoint of I domain conformational stability and of the characteristics of I domain-ICAM-1 interactions.