Ligand binding specificity of the leukocyte response integrin expressed by human neutrophils

Biomaterial functionalization with triple-helical peptides for tissue engineering

In the growing field of tissue engineering, providing cells in biomaterials with the adequate biological cues represents an increasingly important challenge. Yet, biomaterials with excellent mechanical properties often are often biologically inert to many cell types. To address this issue, researchers resort to functionalization, i.e. the surface modification of a biomaterial with active molecules or substances. Functionalization notably aims to replicate the native cellular microenvironment provided by the extracellular matrix, and in particular by collagen, its major component. As our understanding of biological processes regulating cell behaviour increases, functionalization with biomolecules binding cell surface receptors constitutes a promising strategy. Amongst these, triple-helical peptides (THPs) that reproduce the architectural and biological properties of collagen are especially attractive. Indeed, THPs containing binding sites from the native collagen sequence have successfully been used to guide cell response by establishing cell-biomaterial interactions. Notably, the GFOGER motif recognising the collagen-binding integrins is extensively employed as a cell adhesive peptide. In biomaterials, THPs efficiently improved cell adhesion, differentiation and function on biomaterials designed for tissue repair (especially for bone, cartilage, tendon and heart), vascular graft fabrication, wound dressing, drug delivery or immunomodulation. This review describes the key characteristics of THPs, their effect on cells when combined to biomaterials and their strong potential as biomimetic tools for regenerative medicine. STATEMENT OF SIGNIFICANCE: This review article describes how triple-helical peptides constitute efficient tools to improve cell-biomaterial interactions in tissue engineering. Triple helical peptides are bioactive molecules that mimic the architectural and biological properties of collagen. They have been successfully used to specifically recognize cell-surface receptors and provide cells seeded on biomaterials with controlled biological cues. Functionalization with triple-helical peptides has enabled researchers to improve cell function for regenerative medicine applications, such as tissue repair. However, despite encouraging results, this approach remains limited and under-exploited, and most functionalization strategies reported in the literature rely on biomolecules that are unable to address collagen-binding receptors. This review will assist researchers in selecting the correct tools to functionalize biomaterials in efforts to guide cellular response.

Tumor-Targeting Immune System Engagers (ISErs) Activate Human Neutrophils after Binding to Cancer Cells

Immune system engagers (ISErs) make up a new class of immunotherapeutics against cancer. They comprise two or more tumor-targeting peptides and an immune-stimulating effector peptide connected by inert polymer linkers. They are produced by solid phase peptide synthesis and share the specific targeting activities of antibodies (IgGs) but are much smaller in size and exploit a different immune-stimulating mechanism. Two ISErs (Y-9 and Y-59) that bind to the cancer cell markers integrin α3 and EphA2, respectively, are analyzed here with respect to their immune cell stimulation. We have previously shown that they activate formyl peptide receptors on myeloid immune cells and induce respiratory burst in neutrophils and myeloid chemotaxis in solution. It remained, however, unclear whether these molecules can stimulate immune cells while bound to tumor cells, an essential step in the hypothesized mode of action. Here, we demonstrate that ISEr Y-9 induced respiratory burst and caused a change in the shape of neutrophils when bound to the surface of protein A beads as a model of tumor cells. More importantly, tumor cell lines carrying receptor-bound Y-9 or Y-59 also activated neutrophils, evidenced by a significant change in shape. Interestingly, similar activation was induced by the supernatants of the cells incubated with ISEr, indicating that ISErs released from tumor cells, intact or degraded into fragments, significantly contributed to immune stimulation. These findings provide new evidence for the mode of action of ISErs, namely by targeting cancer cells and subsequently provoking an innate immune response against them.

EGFL7 is expressed in bone microenvironment and promotes angiogenesis via ERK, STAT3, and integrin signaling cascades

Angiogenesis plays a pivotal role in bone formation, remodeling, and fracture healing. The regulation of angiogenesis in the bone microenvironment is highly complex and orchestrated by intercellular communication between bone cells and endothelial cells. Here, we report that EGF-like domain 7 (EGFL7), a member of the epidermal growth factor (EGF) repeat protein superfamily is expressed in both the osteoclast and osteoblast lineages, and promotes endothelial cell activities. Addition of exogenous recombinant EGFL7 potentiates SVEC (simian virus 40-transformed mouse microvascular endothelial cell line) cell migration and tube-like structure formation in vitro. Moreover, recombinant EGFL7 promotes angiogenesis featuring web-like structures in ex vivo fetal mouse metatarsal angiogenesis assay. We show that recombinant EGFL7 induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3), and focal adhesion kinase (FAK) in SVEC cells. Inhibition of ERK1/2 and STAT3 signaling impairs EGFL7-induced endothelial cell migration, and angiogenesis in fetal mouse metatarsal explants. Bioinformatic analyses indicate that EGFL7 contains a conserved RGD/QGD motif and EGFL7-induced endothelial cell migration is significantly reduced in the presence of RGD peptides. Moreover, EGFL7 gene expression is significantly upregulated during growth plate injury repair. Together, these results demonstrate that EGFL7 expressed by bone cells regulates endothelial cell activities through integrin-mediated signaling. This study highlights the important role that EGFL7, like EGFL6, expressed in bone microenvironment plays in the regulation of angiogenesis in bone.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.

RGD-labeled USPIO inhibits adhesion and endocytotic activity of alpha v beta3-integrin-expressing glioma cells and only accumulates in the vascular tumor compartment

PURPOSE

To investigate the biologic effect of arginine-glycine-aspartic acid (RGD)-labeled ultrasmall superparamagnetic iron oxide (USPIO) (referred to as RGD-USPIO) on human umbilical vein endothelial cells (HUVECs), ovarian carcinoma (MLS) cells, and glioblastoma (U87MG) cells and on U87MG xenografts in vivo.

MATERIALS AND METHODS

All experiments were approved by the governmental review committee on animal care.USPIOs were coated with integrin-specific (RGD) or unspecific (arginine-alanine-aspartic acid [RAD]) peptides. USPIO uptake in HUVECs, MLS cells, and U87MG cells and in U87MG tumor xenografts was determined with T2 magnetic resonance (MR) relaxometry in 16 nude mice. Cells and tumors were characterized by using immunofluorescence microscopy. Trypan blue staining and lactate dehydrogenase assay were used to assess cytotoxicity. Statistical evaluation was performed by using a Mann-Whitney test or a linear mixed model with random intercept for the comparison of data from different experiments. Post hoc pairwise comparisons were adjusted according to a Tukey test.

RESULTS

HUVECs and MLS cells internalized RGD-USPIOs significantly more than unspecific probes. Controversially, U87MG cells accumulated RGD-USPIOs to a lesser extent than USPIO. Furthermore, only in U87MG cells, free RGD and alpha(v)beta(3) integrin-blocking antibodies strongly reduced endocytosis of nonspecific USPIOs. This was accompanied by a loss of cadherin-dependent intercellular contacts, which could not be attributed to cell damage. In U87MG tumors, RGD-USPIO accumulated exclusively at the neovasculature but not within tumor cells. The vascular accumulation of RGD-USPIO caused significantly higher changes of the R2 relaxation rate of tumors than observed for USPIO.

CONCLUSION

In glioma cells with unstable intercellular contacts, inhibition of alpha(v)beta(3) integrins by antibodies and RGD and RGD-USPIO disintegrated intercellular contacts and reduced endocytotic activity, illustrating the risk of inducing biologic effects by using molecular MR probes.

Recombinant human activated protein C inhibits integrin-mediated neutrophil migration

Integrin-mediated cell migration is central to many biologic and pathologic processes. During inflammation, tissue injury results from excessive infiltration and sequestration of activated leukocytes. Recombinant human activated protein C (rhAPC) has been shown to protect patients with severe sepsis, although the mechanism underlying this protective effect remains unclear. Here, we show that rhAPC directly binds to beta(1) and beta(3) integrins and inhibits neutrophil migration, both in vitro and in vivo. We found that human APC possesses an Arg-Gly-Asp (RGD) sequence, which is critical for the inhibition. Mutation of this sequence abolished both integrin binding and inhibition of neutrophil migration. In addition, treatment of septic mice with a RGD peptide recapitulated the beneficial effects of rhAPC on survival. Thus, we conclude that leukocyte integrins are novel cellular receptors for rhAPC and the interaction decreases neutrophil recruitment into tissues, providing a potential mechanism by which rhAPC may protect against sepsis.

Quantitative measurements of the strength of adhesion of human neutrophils to a substratum in a microfluidic device

We describe a quantitative assay of the strength of adhesion of activated and nonactivated human neutrophils to a substratum, which is carried out in a custom-made microfluidic device. The strength of adhesion is quantified by the fraction of cells remaining adherent (ACF) after a given time of exposure to shear stress in a test microchannel. The microfluidic device is made of two layers of poly(dimethylsiloxane) with integrated membrane valves. This construction allows concurrent testing of two different populations of cells, as well as setting well-defined times of exposure of cells to stress and of their incubation prior to the exposure. The test microchannels have a tapered profile, exposing cells to nearly an order of magnitude range of shear stress. ACF is measured periodically by computer-controlled videomicroscopy scans of the device, with up to 60,000 individual cells identified within a 90 seconds scan. The high throughput of the scans allows reliable quantitative assessment of the ACF. Adhesion of untreated neutrophils and neutrophils activated with formyl-Met-Leu-Phe was tested concurrently in a series of experiments with a fibrinogen-coated glass substratum. At optimized testing conditions, the ACF of activated cells was consistently found to be three times higher than that of nonactivated cells. An adhesion assay could be completed within 11 min from the loading of cells into the device without any intervention by the operator. The proposed device and assay could be used to assess the state of activation of neutrophils in human blood with a potential application to diagnostics of inflammation.

Novel CD47-dependent intercellular adhesion modulates cell migration

CD47 is a ubiquitously expressed plasma membrane protein, also known as Integrin Associated Protein, that modulates cell adhesion both through alteration of the avidity of integrin binding and through interaction with its own ligands, the extracellular matrix protein thrombospondin (TSP) and the plasma membrane response regulator SIRPalpha1. We now show that CD47 expression on fibroblasts can induce intercellular adhesion resulting in cell aggregation in the absence of active integrins, SIRPalpha1 binding, and detectable TSP. CD47-expressing cells preferentially bind to other CD47-expressing cells, and intercellular adhesion requires stimulation by serum or a CD47-binding peptide from TSP. Cell-cell adhesion is inhibited by pertussis toxin and C. difficile toxin B, and both adherent and aggregating CD47-expressing fibroblasts have more rac in the GTP bound state than CD47-deficient cells. Spontaneous migration of Jurkat lymphocytes through a fibroblast monolayer is decreased by fibroblast expression of CD47, consistent with an increased barrier function of the CD47 expressing cells. The lymphocyte chemoattractant SDF-1alpha stimulates migration of Jurkat cells through this monolayer only if both the lymphocytes and fibroblasts express CD47, and the inhibition of migration by a CD47-interacting peptide from TSP similarly requires CD47 expression on both cell types. Thus, signaling dependent on both heterotrimeric and rho family GTPases can induce CD47 to participate in cell-cell interactions independent of known ligands that enhance intercellular adhesion and modulate cell migration.

Role of phosphatidylinositol 3-kinase in the binding of Bordetella pertussis to human monocytes

Bordetella pertussis, the causative agent of whooping cough, adheres to human monocytes by means of filamentous haemagglutinin (FHA), a bacterial surface protein that is recognized by complement receptor type 3 (CR3, alphaMbeta2 integrin). Previous work has shown that an FHA Arg-Gly-Asp (RGD, residues 1097-1099) site interacts with a complex composed of leucocyte response integrin (LRI, alphavbeta3 integrin) and integrin-associated protein (IAP, CD47) on human monocytes, resulting in enhancement of CR3-mediated bacterial binding. However, the pathway that mediates alphavbeta3-alphaMbeta2 integrin signalling remains to be characterized. Here we describe the involvement of phosphatidylinositol 3-kinase (PI3-K) in this pathway. Wortmannin and LY294002, inhibitors of PI3-K, reduced alphavbeta3/IAP-upregulated, CR3-associated bacterial binding to human monocytes. B. pertussis infection of human monocytes resulted in a marked recruitment of cellular PI3-K to the sites of B. pertussis contact. In contrast, cells infected with an isogenic strain carrying a G1098A mutation at the FHA RGD site did not show any recruitment of PI3-K. We found that ligation of FHA by alphavbeta3/IAP induced RGD-dependent tyrosine phosphorylation of a 60 kDa protein, which associated with IAP and PI3-K in human monocytes. These results suggest that PI3-K and a tyrosine phosphorylated 60 kDa protein may be involved in this biologically important integrin signalling pathway.

Gene for integrin-associated protein (IAP, CD47): physical mapping, genomic structure, and expression studies in skeletal muscle

Integrin-associated protein (IAP) is a widely expressed membrane protein with multiple functions in immunological and neuronal processes. Having physically mapped the IAP gene into a BAC/PAC contig covering approximately 1 Mb on human chromosome 3ql3.1-q13.2, we determined the genomic organization of the gene, established its expression in skeletal muscle, and identified a novel splice variant. Our expression studies demonstrate expression of integrin-associated protein in the t-tubular system and the euchromatin of skeletal muscle cells where its function thus far is not known.

An RGD sequence in the P2Y(2) receptor interacts with alpha(V)beta(3) integrins and is required for G(o)-mediated signal transduction

The P2Y(2) nucleotide receptor (P2Y(2)R) contains the integrin-binding domain arginine-glycine-aspartic acid (RGD) in its first extracellular loop, raising the possibility that this G protein-coupled receptor interacts directly with an integrin. Binding of a peptide corresponding to the first extracellular loop of the P2Y(2)R to K562 erythroleukemia cells was inhibited by antibodies against alpha(V)beta(3)/beta(5) integrins and the integrin-associated thrombospondin receptor, CD47. Immunofluorescence of cells transfected with epitope-tagged P2Y(2)Rs indicated that alpha(V) integrins colocalized 10-fold better with the wild-type P2Y(2)R than with a mutant P2Y(2)R in which the RGD sequence was replaced with RGE. Compared with the wild-type P2Y(2)R, the RGE mutant required 1,000-fold higher agonist concentrations to phosphorylate focal adhesion kinase, activate extracellular signal-regulated kinases, and initiate the PLC-dependent mobilization of intracellular Ca(2+). Furthermore, an anti-alpha(V) integrin antibody partially inhibited these signaling events mediated by the wild-type P2Y(2)R. Pertussis toxin, an inhibitor of G(i/o) proteins, partially inhibited Ca(2+) mobilization mediated by the wild-type P2Y(2)R, but not by the RGE mutant, suggesting that the RGD sequence is required for P2Y(2)R-mediated activation of G(o), but not G(q). Since CD47 has been shown to associate directly with G(i/o) family proteins, these results suggest that interactions between P2Y(2)Rs, integrins, and CD47 may be important for coupling the P2Y(2)R to G(o).

Tethered-TGF-beta increases extracellular matrix production of vascular smooth muscle cells

Biomaterials developed for tissue engineering and wound healing applications need to support robust cell adhesion, yet also need to be replaced by new tissue synthesized by those cells. In order to maintain mechanical integrity of the tissue, the cells must generate sufficient extracellular matrix before the scaffold is degraded. We have previously shown that materials containing cell adhesive ligands to promote or improve cell adhesion can decrease extracellular matrix production (Mann et al., Modification of surfaces with cell adhesion peptides alters extracellular matrix deposition. Biomaterials 1999;20:2281-6). Such decreased matrix production by cells in tissue engineering scaffolds may result in tissue failure. However, we have found that TGF-beta1 can be used in scaffolds to dramatically increase matrix production. Matrix production by vascular smooth muscle cells grown on adhesive ligand-modified glass surfaces and in PEG hydrogels containing covalently bound adhesive ligands was increased in the presence of 0.04 pmol/ml (1 ng/ml) TGF-beta1. TGF-beta1 can counteract the effect of these adhesive ligands on matrix production; matrix production could be increased even above that observed in the absence of adhesive peptides. Further, TGF-beta1 covalently immobilized to PEG retained its ability to increase matrix production. Tethering TGF-beta1 to the polymer scaffold resulted in a significant increase in matrix production over the same amount of soluble TGF-beta1.

Integrin-associated protein (CD47/IAP) contributes to T cell arrest on inflammatory vascular endothelium under flow

Integrin-associated protein (CD47/IAP) is a pentaspan molecule that regulates integrin functions. We prepared a CD47-deficient Jurkat T cell line to assess its role in the arrest of T cells on inflammatory endothelium. Under flow conditions, constitutive arrest of CD47-deficient cells is strongly decreased as compared to the original cell line, whereas reexpression of CD47 reestablishes their ability to stop. Moreover, cells transfected with a chimera made with the extracellular portion of CD47 and the transmembrane domain of CD7 or several truncated forms of CD47 show that the first transmembrane domain and a short cytoplasmic loop are sufficient for this process. CD47 effect is indirect and depends mainly on the alpha4beta1/VCAM-1 pathway, as shown by blocking antibodies. We detected on endothelium the two CD47 counter receptors known to date: thrombospondin and SIRP1alpha. Blocking experiments show that both are involved. Overall, CD47 participates in the constitutive arrest of T lymphocytes on inflamed vascular endothelium by up-regulating alpha 4beta1 integrins.

alpha(v)beta(3) integrin and bacterial lipopolysaccharide are involved in Coxiella burnetii-stimulated production of tumor necrosis factor by human monocytes

Coxiella burnetii, the agent of Q fever, enters human monocytes through alpha(v)beta(3) integrin and survives inside host cells. In addition, C. burnetii stimulates the synthesis of inflammatory cytokines including tumor necrosis factor (TNF) by monocytes. We studied the role of the interaction of C. burnetii with THP-1 monocytes in TNF production. TNF transcripts and TNF release reached maximum values within 4 h. Almost all monocytes bound C. burnetii after 4 h, while the percentage of phagocytosing monocytes did not exceed 20%. Cytochalasin D, which prevented the uptake of C. burnetii without interfering with its binding, did not affect the expression of TNF mRNA. Thus, bacterial adherence, but not phagocytosis, is necessary for TNF production by monocytes. The monocyte alpha(v)beta(3) integrin was involved in TNF synthesis since peptides containing RGD sequences and blocking antibodies against alpha(v)beta(3) integrin inhibited TNF transcripts induced by C. burnetii. Nevertheless, the cross-linking of alpha(v)beta(3) integrin by specific antibodies was not sufficient to induce TNF synthesis. The signal delivered by C. burnetii was triggered by bacterial lipopolysaccharide (LPS). Polymyxin B inhibited the TNF production stimulated by C. burnetii, and soluble LPS isolated from C. burnetii largely mimicked viable bacteria. On the other hand, avirulent variants of C. burnetii induced TNF production through an increased binding to monocytes rather than through the potency of their LPS. We suggest that the adherence of C. burnetii to monocytes via alpha(v)beta(3) integrin enables surface LPS to stimulate TNF production in THP-1 monocytes.

Subversion of Monocyte Functions by Coxiella burnetii: Impairment of the Cross-Talk Between αvβ3 Integrin and CR3

Several intracellular pathogens exploit macrophages as a niche for survival and replication. The success of this strategy requires the subversion or the avoidance of microbicidal functions of macrophages. Coxiella burnetii, the agent of Q fever, is a strictly intracellular bacterium that multiplies in myeloid cells. The survival of C. burnetii may depend on the selective use of macrophage receptors. Virulent C. burnetii organisms were poorly internalized but survived successfully in human monocytes, whereas avirulent variants were efficiently phagocytosed but were also rapidly eliminated. The uptake of avirulent organisms was mediated by leukocyte response integrin (αvβ3 integrin) and CR3 (αMβ2 integrin), as demonstrated by using specific Abs and RGD sequence-containing peptides. The phagocytic efficiency of CR3 depends on its activation via αvβ3 integrin and integrin-associated protein. Indeed, CR3-mediated phagocytosis of avirulent C. burnetii was abrogated in macrophages from integrin-associated protein−/− mice. In contrast, the internalization of virulent C. burnetii organisms involved the engagement of αvβ3 integrin but not that of CR3. The pretreatment of monocytes with virulent C. burnetii organisms prevented the CR3-mediated phagocytosis of zymosan particles and CR3 activation assessed by the expression of the 24 neo-epitope. We conclude that the virulence of C. burnetii is associated with the engagement of αvβ3 integrin and the impairment of CR3 activity, which probably results from uncoupling αvβ3 integrin from integrin-associated protein. This study describes a strategy not previously reported of phagocytosis modulation by intracellular pathogens.

Enterococcus faecalis aggregation substance promotes opsonin-independent binding to human neutrophils via a complement receptor type 3-mediated mechanism

Enterococcus faecalis aggregation substance (AS) mediates efficient adhesion between bacteria, thereby facilitating plasmid exchange as an integral part of a bacterial sex pheromone system. We examined the interaction of AS-bearing E. faecalis with human neutrophils (PMNs), an important component of the host defense system. AS promoted a markedly increased opsonin-independent bacterial binding to PMNs. Adhesion was dependent on the expression of the enterococcal Asc10 protein, which contains two Arg-Gly-Asp (RGD) sequences, and addition of exogenous RGD-containing peptides inhibited AS-mediated binding by 66%. AS-mediated adhesion was inhibited by 85% by anti-human complement receptor type 3 (CR3) monoclonal antibodies or by use of PMNs from a patient with leukocyte adhesion deficiency. However, AS-bearing E. faecalis cells were unable to bind to CHO-Mac-1 cells, expressing functionally active CR3, suggesting the potential need for additional PMN surface receptors for bacterial adhesion. Monoclonal antibodies against integrin-associated protein (CD47) and L-selectin, both of which may interact with CR3 and bind to ligands on E. faecalis, also inhibited AS-dependent binding. The non-opsonic binding of E. faecalis to PMNs may play an important role in this organism's pathogenesis.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Role of cholesterol in formation and function of a signaling complex involving alphavbeta3, integrin-associated protein (CD47), and heterotrimeric G proteins

Integrin-associated protein (CD47) is a multiply membrane spanning member of the immunoglobulin superfamily that regulates some adhesion-dependent cell functions through formation of a complex with alphavbeta3 integrin and trimeric G proteins. Cholesterol is critical for the association of the three protein components of the supramolecular complex and for its signaling. The multiply membrane spanning domain of IAP is required for complex formation because it binds cholesterol. The supramolecular complex forms preferentially in glycosphingolipid-enriched membrane domains. Binding of mAb 10G2 to the IAP Ig domain, previously shown to be required for association with alphavbeta3, is affected by both the multiply membrane spanning domain and cholesterol. These data demonstrate that cholesterol is an essential component of the alphavbeta3/IAP/G protein signaling complex, presumably acting through an effect on IAP conformation.

Diverse effects of neutrophil integrin occupation on respiratory burst activation

Integrin occupation can alter the function of neutrophils (PMN), but the mechanism(s) involved is still unclear. This study demonstrated that the occupation of PMN integrins (especially those of the beta(3) subfamily) strongly enhances TNF stimulation of the respiratory burst but down-regulates that induced by PMA, fMLP, Con A, and serum treated zymosan. Treatment of PMN with genistein, staurosporine, and wortmannin, inhibitors of tyrosine kinases, protein kinase C, and phosphotidylinostol 3-kinase (PI 3-kinase) respectively, completely blocked the TNF-stimulated respiratory burst in PMN. Genistein and wortmannin enhanced the PMA-stimulated respiratory burst but only in cells adherent to RGD peptide. These findings suggest that PMN integrins (beta(3) subfamily) can generate signals that regulate the PMN agonist responses, probably through the activities of tyrosine kinases and PI 3-kinase.

Cell adhesion molecules in the pathogenesis of and host defence against microbial infection

Eukaryotic cell adhesion molecules (CAMs) are used by various cells and extracellular molecules in host defence against infection. They are involved in many processes including recognition by circulating phagocytes of a site of inflammation, transmigration through the endothelial barrier, diapedesis through basement membrane and extracellular matrix, and release of effector mechanisms at the infected site. CAMs involved in leucocyte-endothelial cell interaction include the selectins, integrins, and members of the immunoglobulin superfamily. However, CAMs are also used by various microorganisms (protozoa, fungi, bacteria, and viruses) during their pathogenesis. For example, bacteria that utilise CAMs include Mycobacterium tuberculosis, Listeria monocytogenes, Yersinia spp, enteropathogenic Escherichia coli, Shigella spp, Neisseria spp, Bordetella spp, and Borrelia burgdorferi. In addition, CAMs are involved in the pathogenetic effects of the RTX toxins of Pasteurella haemolytica, Actinobacillus actinomycetemcomitans, and the superantigen exotoxins of Staphylococcus aureus and Streptococcus pyogenes. A recurrent and topical theme of potential importance within the bacterial group is the intimate relation between CAMs, bacterial protein receptors, and type III secretion systems. For example, the IpaBCD protein complex is secreted by the type III system of Shigella flexneri and interacts with alpha 5 beta 1 integrin on the eukaryotic cell surface, followed by Rho mediated internalisation; this illustrates the relevance of cellular microbiology. CAMs might prove to be novel therapeutic targets. Comparative genomics has provided the knowledge of shared virulence determinants among diverse bacterial genera, and will continue to deepen our understanding of microbial pathogenesis, particularly in the context of the interaction of prokaryotic and eukaryotic molecules.

Platelet and Fibrin Deposition at the Damaged Vessel Wall: Cooperative Substrates for Neutrophil Adhesion Under Flow Conditions

At sites of vessel wall damage, the primary hemostatic reaction involves platelet and fibrin deposition. At these sites, circulating leukocytes marginate and become activated. Adhered platelets can support leukocyte localization; however, the role of fibrin in this respect is not known. We studied the adhesion of human neutrophils (polymorphonuclear leukocytes [PMNs]) to endothelial extracellular matrix (ECM)-bound fibrin and platelets under flow conditions. ECM alone did not show PMN adhesion. ECM-coated cover slips were perfused with plasma to form a surface-bound fibrin network, and/or with whole blood to allow platelet adhesion. Unstimulated PMNs adhered to fibrin at moderate shear stress (20 to 200 mPa). ECM-bound platelets induced rolling adhesion and allowed more PMNs to adhere at higher shear (320 mPa). ECM coated with both platelets and fibrin induced more static and shear-resistant PMN adhesion. PMN adhesion to fibrin alone but not to platelet/fibrin surfaces was inhibited by soluble fibrinogen. Adhesion to fibrin alone was inhibited by CD11b and CD18 blocking antibodies. Furthermore, fibrin formed under flow conditions showed up to threefold higher PMN adhesion compared with fibrin formed under static conditions, due to structural differences. These results indicate that circulating PMNs adhere to fibrin in an integrin-dependent manner at moderate shear stresses. However, at higher shear rates (<200 mPa), additional mechanisms (ie, activated platelets) are necessary for an interaction of PMNs with a fibrin network.

Platelet and Fibrin Deposition at the Damaged Vessel Wall: Cooperative Substrates for Neutrophil Adhesion Under Flow Conditions

At sites of vessel wall damage, the primary hemostatic reaction involves platelet and fibrin deposition. At these sites, circulating leukocytes marginate and become activated. Adhered platelets can support leukocyte localization; however, the role of fibrin in this respect is not known. We studied the adhesion of human neutrophils (polymorphonuclear leukocytes [PMNs]) to endothelial extracellular matrix (ECM)-bound fibrin and platelets under flow conditions. ECM alone did not show PMN adhesion. ECM-coated cover slips were perfused with plasma to form a surface-bound fibrin network, and/or with whole blood to allow platelet adhesion. Unstimulated PMNs adhered to fibrin at moderate shear stress (20 to 200 mPa). ECM-bound platelets induced rolling adhesion and allowed more PMNs to adhere at higher shear (320 mPa). ECM coated with both platelets and fibrin induced more static and shear-resistant PMN adhesion. PMN adhesion to fibrin alone but not to platelet/fibrin surfaces was inhibited by soluble fibrinogen. Adhesion to fibrin alone was inhibited by CD11b and CD18 blocking antibodies. Furthermore, fibrin formed under flow conditions showed up to threefold higher PMN adhesion compared with fibrin formed under static conditions, due to structural differences. These results indicate that circulating PMNs adhere to fibrin in an integrin-dependent manner at moderate shear stresses. However, at higher shear rates (<200 mPa), additional mechanisms (ie, activated platelets) are necessary for an interaction of PMNs with a fibrin network.

Domain-specific interactions between entactin and neutrophil integrins. G2 domain ligation of integrin alpha3beta1 and E domain ligation of the leukocyte response integrin signal for different responses

Extracellular matrix proteins activate neutrophils to up-regulate many physiologic functions that are necessary at sites of tissue injury. To elucidate the ligand-receptor interactions that mediate these functions, we examined neutrophil activation by the basement membrane protein, entactin. Entactin is structurally and functionally organized into distinct domains; therefore, we utilized glutathione S-transferase -fusion proteins encompassing its four major domains, G1, G2, E, and G3, to assess interactions between entactin and neutrophil integrin receptors. We show that the E domain, which contains the single RGD sequence of entactin, is sufficient for ligation of the beta3-like integrin, leukocyte response integrin, and signaling for chemotaxis. Moreover, the G2 domain signals for stimulation of Fc receptor-mediated phagocytosis via ligation of alpha3beta1. This receptor-ligand interaction was revealed only after stimulation of neutrophil by immune complexes or phorbol esters. Interestingly, the E domain does not enhance phagocytosis, and the G2 domain is not chemotactic. Furthermore, cleavage of entactin with the matrix metalloproteinase, matrilysin, liberates peptides that retain E domain-mediated chemotaxis and G2 domain-mediated enhancement of phagocytosis. These studies indicate that multiple domains of entactin have the ability to ligate individual integrins expressed by neutrophils and to activate distinct functions.

Decreased resistance to bacterial infection and granulocyte defects in IAP-deficient mice

Granulocyte [polymorphonuclear leucocyte (PMN)] migration to sites of infection and subsequent activation is essential for host defense. Gene-targeted mice deficient for integrin-associated protein (IAP, also termed CD47) succumbed to Escherichia coli peritonitis at inoccula survived by heterozygous littermates. In vivo, they had an early defect in PMN accumulation at the site of infection. In vitro, IAP-/- PMNs were deficient in beta3 integrin-dependent ligand binding, activation of an oxidative burst, and Fc receptor-mediated phagocytosis. Thus, IAP plays a key role in host defense by participating both in PMN migration in response to bacterial infection and in PMN activation at extravascular sites.

Neutrophil activation by adhesion: mechanisms and pathophysiological implications

Neutrophil adhesion plays an essential role in the formation of an inflammatory exudate. Moreover, adhesion activates selective neutrophil functions and regulates the cell response to additional stimuli. In this review we summarize the information available on adhesion molecules involved in neutrophil adhesion to endothelial cells and extracellular matrix proteins and the experimental approaches which have been developed to block neutrophil adhesion and neutrophil mediated tissue damage. We also address the mechanisms of activation of selective neutrophil functions by adhesion molecules and, in particular the mechanisms of signal transduction by neutrophil integrins. On the basis of recent results obtained in our and other laboratories we propose a model hypothesizing mechanisms of signaling by neutrophil integrins involved in regulation of selective functions.

Leucocyte adhesion molecules in host defence against infection

The destructive potential of leucocytes needs to be fully held in check in the circulation, while being released at the site of tissue destruction and infection. Leucocyte adhesion molecules are the key to this regulation, the recognition by the leucocytes of areas of tissue damage, and to transendothelial migration of leucocytes into these areas. The three classes of leucocyte adhesion molecules-the integrins, selectins and immunoglobulin superfamily members-are reviewed with their ligands and presented in the context of leucocyte migration, activation and host defence.

Phagocytosis

Phagocytosis is the process of recognition and engulfment of microorganisms or tissue debris that accumulate during infection, inflammation or wound repair. This ingestion, which is performed most efficiently by migrating, bone marrow-derived cells called 'professional phagocytes', is essential for successful host defense. Ingestion results when an invading microorganism is recognized by specific receptors on the phagocyte surface and requires multiple, successive interactions between the phagocyte and the target. Each of these interactions results in a signal transduction event, which is confined to the membrane and cytoskeleton around the ligated receptor and which is required for successful phagocytosis. Many molecules found at sites of inflammation or infection stimulate phagocytosis, so that efficient ingestion is confined to the site of infection or inflammation, which in turn limits the proinflammatory and tissue-destructive processes that accompany phagocytosis. This review summarizes current understanding of this critical component of host defense and of its regulation.

Adhesion of activated platelets to polymorphonuclear leukocytes

Polymorphonuclear leukocytes (PMNL) are components of the blood which as such interact extensively with other blood cells, with endothelial cells or with plasma. Here, we consider the interaction between PMNL and platelets which is efficient during adhesion of platelets to PMNL and which can be studied in vitro using the rosette formation assay. The adhesion of activated platelets to PMNL seems to be mediated mainly by a protein of platelets (CD62) and its counterreceptor on PMNL, but also other platelet receptors are involved. Here we demonstrate the participation of the glycoprotein IIb-IIIa complex (CD41a) in the adhesion of activated platelets to PMNL due to the following findings: a) inhibition of adhesion by monoclonal antibodies raised against CD41a, b) inhibition of adhesion by peptides such as RGDS and echistatin, c) inhibition of adhesion by dissociation of CD41a with EGTA and d) inhibition of adhesion using platelets from a thrombasthenic patient which have almost no CD41a in the surface membrane but a normal expression of CD62 upon activation. The adhesion of activated platelets to PMNL via CD41a seems to be mediated by fibrinogen due to the following findings: a) addition of fibrinogen to ADP-stimulated and fixed platelets increases significantly the rosette formation and b) the incubation of unstimulated platelets with fibrinogen and an antibody raised against glycoprotein IIIa which stimulates fibrinogen binding to the platelet surface results in an enlarged rosette formation.

Mechanism of fibronectin enhancement of group B streptococcal phagocytosis by human neutrophils and culture-derived macrophages

In previous studies, we reported that fibronectin (FN) markedly enhances phagocytic uptake of antibody-coated group B streptococci (GBS) by human polymorphonuclear leukocytes. Furthermore, administration of FN along with a GBS type-specific monoclonal or polyclonal antibody to infected neonatal rats significantly enhances survival. In this study, we have examined the molecular mechanism of this enhancement through phagocyte receptors which recognize the Arg-Gly-Asp (RGD) peptide sequences contained within the FN molecule. Incubation of human polymorphonuclear leukocytes or culture-derived macrophages on coverslips coated with GRGDSP but not GRGESP markedly enhanced uptake of immunoglobulin G-coated GBS. The enhancing effect of the RGD-containing peptides was blocked by monoclonal antibodies B6H12 (directed against the integrin-associated protein) and 7G2 (directed against the beta 3-integrin receptor for RGD). These data suggest that FN enhancement of antibody-coated GBS uptake is mediated by the critical RGD sequence. Furthermore, this active peptide sequence may have an important role in immunotherapy of bacterial infections, especially in patients with decreased plasma FN concentrations.