Constitutive and stimulus-induced phosphorylation of CD11/CD18 leukocyte adhesion molecules

β2 Integrin CD11d/CD18: From Expression to an Emerging Role in Staged Leukocyte Migration

CD11d/CD18 is the most recently discovered and least understood β2 integrin. Known CD11d adhesive mechanisms contribute to both extravasation and mesenchymal migration – two key aspects for localizing peripheral leukocytes to sites of inflammation. Differential expression of CD11d induces differences in monocyte/macrophage mesenchymal migration including impacts on macrophage sub-set migration. The participation of CD11d/CD18 in leukocyte localization during atherosclerosis and following neurotrauma has sparked interest in the development of CD11d-targeted therapeutic agents. Whereas the adhesive properties of CD11d have undergone investigation, the signalling pathways induced by ligand binding remain largely undefined. Underlining each adhesive and signalling function, CD11d is under unique transcriptional control and expressed on a sub-set of predominately tissue-differentiated innate leukocytes. The following review is the first to capture the nearly three decades of CD11d research and discusses the emerging role of CD11d in leukocyte migration and retention during the progression of a staged immune response.

Complement Receptors and Their Role in Leukocyte Recruitment and Phagocytosis

The complement system is deeply embedded in our physiology and immunity. Complement activation generates a multitude of molecules that converge simultaneously on the opsonization of a target for phagocytosis and activation of the immune system via soluble anaphylatoxins. This response is used to control microorganisms and to remove dead cells, but also plays a major role in stimulating the adaptive immune response and the regeneration of injured tissues. Many of these effects inherently depend on complement receptors expressed on leukocytes and parenchymal cells, which, by recognizing complement-derived molecules, promote leukocyte recruitment, phagocytosis of microorganisms and clearance of immune complexes. Here, the plethora of information on the role of complement receptors will be reviewed, including an analysis of how this functionally and structurally diverse group of molecules acts jointly to exert the full extent of complement regulation of homeostasis.

PPM1F controls integrin activity via a conserved phospho-switch

Control of integrin activity is vital during development and tissue homeostasis, while derailment of integrin function contributes to pathophysiological processes. Phosphorylation of a conserved threonine motif (T788/T789) in the integrin β cytoplasmic domain increases integrin activity. Here, we report that T788/T789 functions as a phospho-switch, which determines the association with either talin and kindlin-2, the major integrin activators, or filaminA, an integrin activity suppressor. A genetic screen identifies the phosphatase PPM1F as the critical enzyme, which selectively and directly dephosphorylates the T788/T789 motif. PPM1F-deficient cell lines show constitutive integrin phosphorylation, exaggerated talin binding, increased integrin activity, and enhanced cell adhesion. These gain-of-function phenotypes are reverted by reexpression of active PPM1F, but not a phosphatase-dead mutant. Disruption of the ppm1f gene in mice results in early embryonic death at day E10.5. Together, PPM1F controls the T788/T789 phospho-switch in the integrin β1 cytoplasmic tail and constitutes a novel target to modulate integrin activity.

Regulation of cell adhesion: a collaborative effort of integrins, their ligands, cytoplasmic actors, and phosphorylation

Integrins are large heterodimeric type 1 membrane proteins expressed in all nucleated mammalian cells. Eighteen α-chains and eight β-chains can combine to form 24 different integrins. They are cell adhesion proteins, which bind to a large variety of cellular and extracellular ligands. Integrins are required for cell migration, hemostasis, translocation of cells out from the blood stream and further movement into tissues, but also for the immune response and tissue morphogenesis. Importantly, integrins are not usually active as such, but need activation to become adhesive. Integrins are activated by outside-in activation through integrin ligand binding, or by inside-out activation through intracellular signaling. An important question is how integrin activity is regulated, and this topic has recently drawn much attention. Changes in integrin affinity for ligand binding are due to allosteric structural alterations, but equally important are avidity changes due to integrin clustering in the plane of the plasma membrane. Recent studies have partially solved how integrin cell surface structures change during activation. The integrin cytoplasmic domains are relatively short, but by interacting with a variety of cytoplasmic proteins in a regulated manner, the integrins acquire a number of properties important not only for cell adhesion and movement, but also for cellular signaling. Recent work has shown that specific integrin phosphorylations play pivotal roles in the regulation of integrin activity. Our purpose in this review is to integrate the present knowledge to enable an understanding of how cell adhesion is dynamically regulated.

Phosphorylation of the α-chain in the integrin LFA-1 enables β2-chain phosphorylation and α-actinin binding required for cell adhesion

The integrin leukocyte function-associated antigen-1 (LFA-1) plays a pivotal role in leukocyte adhesion and migration, but the mechanism(s) by which this integrin is regulated has remained incompletely understood. LFA-1 integrin activity requires phosphorylation of its β2-chain and interactions of its cytoplasmic tail with various cellular proteins. The α-chain is constitutively phosphorylated and necessary for cellular adhesion, but how the α-chain regulates adhesion has remained enigmatic. We now show that substitution of the α-chain phosphorylation site (S1140A) in T cells inhibits the phosphorylation of the functionally important Thr-758 in the β2-chain, binding of α-actinin and 14-3-3 protein, and expression of an integrin-activating epitope after treatment with the stromal cell-derived factor-1α. The presence of this substitution resulted in a loss of cell adhesion and directional cell migration. Moreover, LFA-1 activation through the T-cell receptor in cells expressing the S1140A LFA-1 variant resulted in less Thr-758 phosphorylation, α-actinin and talin binding, and cell adhesion. The finding that the LFA-1 α-chain regulates adhesion through the β-chain via specific phosphorylation at Ser-1140 in the α-chain has not been previously reported and emphasizes that both chains are involved in the regulation of LFA-1 integrin activity.

Double-sided effect of tumor microenvironment on platelets targeting nanoparticles

The cancer cells and stromal cells in tumor microenvironment secrete cytokines and recruit "homing" cells (macrophage, lymphocytes, platelets, etc.). Platelets can interact with tumor microenvironment and specifically aggregate at tumor sites. Surprising, we observed different "homing" effects of activated platelets in breast cancer model and pancreatic cancer model which is highly related with the blood supply of tumors. Besides, platelets targeting magnetic nanoparticles (MNPs) can home to breast cancer but be repelled from pancreatic cancer. We observed the targeting effect of MNPs is highly related with the expressions of collagen Ⅰ (marker of extracellular matrix) and CD34 (marker of tumor neovascularization). The homing nanoparticles such as platelets targeting MNPs could realize the tumor targeting ability, photo-thermal effect and tumor immunotherapeutic ability in the accessible tumor (e.g. breast cancer) but not the hypovascular tumor (e.g. pancreatic cancer).

Melatonin inhibits granulocyte adhesion to ICAM via MT3/QR2 and MT2 receptors

Neutrophils are cells of the innate immune system that first respond and arrive to the site of infection. Melatonin modulates acute inflammatory responses by interfering with leukocyte recruitment. It is known that melatonin modulates granulocyte migration though the endothelial layer thereby acting on the endothelial cell. Here we investigated whether melatonin could modulate granulocyte infiltration by acting directly on granulocytes. Granulocyte infiltration into the peritoneal cavity was investigated in mice kept at normal light/dark conditions and mice kept under constant lighting. To induce migration of neutrophils from the blood into the injury site via the endothelial layer, a bacterial product N-formyl-l-methionyl- l-leucyl- l-phenylalanine (fMLP) was injected into the peritoneal cavity. We found that the number of infiltrated granulocytes during the dark time was lower than that during the light time. It did not depend on circadian time. Moreover, the expression of an adhesion molecule, CD18, on granulocytes, was also lower during the dark time as compared with the light time. We have found that melatonin inhibited fMLP-induced CD18 up-regulation. Importantly, melatonin also inhibited the integrin-mediated granulocyte adhesion to intercellular adhesion molecule-coated plates. This study additionally showed that melatonin receptors MT2 and MT3/quinone reductase 2 (QR2) are expressed on granulocytes. Interestingly, melatonin increases the expression of its MT3/QR2 receptor. The fMLP-mediated CD18 up-regulation was inhibited by melatonin via MT2 receptor and the integrin-mediated granulocyte adhesion was inhibited by melatonin via MT3/QR2 and MT2 receptors. In conclusion, we show that melatonin suppresses granulocyte migration via endothelium by acting directly on granulocytes.

Regulation of integrin activity by phosphorylation

Integrins are heterodimeric complex type I membrane proteins involved in cellular adhesion and signaling. They exist as inactive molecules in resting cells, and need activation to become adhesive. Although much is known about their structure, and a large number of interacting molecules have been described, we still only partially understand how their activities are regulated. In this review we focus on the leukocyte-specific β2-integrins and, specifically, on the role of integrin phosphorylation in the regulation of activity. Phosphorylation reactions can be fast and reversible, thus enabling strictly directed regulatory activities both time-wise and locally in specific regions of the plasma membrane in different leukocytes.

Integrin CD11c/CD18 α-chain phosphorylation is functionally important

CD11c/CD18 (αXβ2, p150/95, or complement receptor 4, CR4) is a monocyte/macrophage-enriched integrin that has been reported to bind to a variety of ligands. These include cell surface proteins, extracellular matrix proteins, and soluble ligands. The regulation of ligand binding to CD11c/CD18 has remained poorly understood. Previous work has shown that both α-chain and β-chain phosphorylations of CD11a/CD18 and CD11b/CD18 are needed for activity, but no corresponding studies on CD11c/CD18 have been performed. In this study, we have identified the phosphorylation site of CD11c as Ser-1158 and show that it is pivotal for adherence and phagocytosis.

Adiponectin inhibits neutrophil phagocytosis of Escherichia coli by inhibition of PKB and ERK 1/2 MAPK signalling and Mac-1 activation

Full length adiponectin is a potent immune modulatory adipokine, impacting upon the actions of several immune cells. Neutrophil oxidative burst has been shown to decrease in response to adiponectin, and we speculated that it could have other effects on neutrophil function. Here we report that adiponectin reduces the phagocytic ability of human neutrophils, decreasing significantly the ingestion of opsonised E. coli by these cells in whole blood (p<0.05) and as isolated neutrophils (p<0.05). We then determined the mechanisms involved. We observed that the activation of Mac-1, the receptor engaged in complement-mediated phagocytosis, was decreased by adiponectin in response to E. coli stimulation. Moreover, treatment of neutrophils with adiponectin prior to incubation with E. coli significantly inhibited signalling through the PI3K/PKB and ERK 1/2 pathways, with a parallel reduction of F-actin content. Studies with pharmacological inhibitors showed that inhibition of PI3K/PKB, but not ERK 1/2 signalling was able to prevent the activation of Mac-1. In conclusion, we propose that adiponectin negatively affects neutrophil phagocytosis, reducing the uptake of E. coli and inhibiting Mac-1 activation, the latter by blockade of the PI3K/PKB signal pathway.

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

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

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

RAGE and ICAM-1 differentially control leukocyte recruitment during acute inflammation in a stimulus-dependent manner

Background

The receptor for advanced glycation endproducts, RAGE, is involved in the pathogenesis of many inflammatory conditions, which is mostly related to its strong activation of NF-κB but also due to its function as ligand for the β₂-integrin Mac-1. To further dissect the stimulus-dependent role of RAGE on leukocyte recruitment during inflammation, we investigated β₂-integrin-dependent leukocyte adhesion in RAGE^-/- and Icam1^-/- mice in different cremaster muscle models of inflammation using intravital microscopy.

Results

We demonstrate that RAGE, but not ICAM-1 substantially contributes to N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced leukocyte adhesion in TNF-α-pretreated cremaster muscle venules in a Mac-1-dependent manner. In contrast, fMLP-stimulated leukocyte adhesion in unstimulated cremaster muscle venules is independent of RAGE, but dependent on ICAM-1 and its interaction with LFA-1. Furthermore, chemokine CXCL1-stimulated leukocyte adhesion in surgically prepared cremaster muscle venules was independent of RAGE but strongly dependent on ICAM-1 and LFA-1 suggesting a differential and stimulus-dependent regulation of leukocyte adhesion during inflammation in vivo.

Conclusion

Our results demonstrate that RAGE and ICAM-1 differentially regulate leukocyte adhesion in vivo in a stimulus-dependent manner.

Characterization of the caprine (Capra hircus) beta-2 integrin CD18-encoding cDNA and identification of mutations potentially responsible for the ruminant-specific virulence ofMannheimia haemolytica

The leukocyte integrins play a critical role in a great number of cellular adhesive interactions during the immune response. We describe here the isolation and characterization of the caprine beta(2) (CD18) sub-unit, common to the leukocyte beta(2)-integrin family. The deduced 770-amino-acid sequence reveals a transmembrane protein with 80, 81, 83, 96 and 99% identity with its canine, murine, human, bovine and ovine homologues respectively. Analysis of CD18 sequences emphasizes the functional importance of the beta(2) sub-unit I-like domain, and included metal ion-dependent adhesion site-like motif and confirms that of the cytoplasmic tail. Moreover, comparisons of ruminant versus non-ruminant CD18 sequences allowed the identification of 16 potential mutation sites that could be held responsible for the unique virulence of Mannheimia haemolytica for ruminants. Mannheimiosis is known to be the major respiratory disease among ruminants, whereas it is not pathogenic for other mammals, an observation that has been attributed to a specific interaction between M. haemolytica leukotoxin and ruminants' CD18. Therefore, the data provided here offer the possibility to explore new avenues in studies based on the caprine model and provide key information for future studies aimed at elucidating the molecular mechanisms underlying the ruminant-specific virulence of M. haemolytica.

Phagocytosis: a repertoire of receptors and Ca(2+) as a key second messenger

Receptor-mediated phagocytosis is a complex process that mediates the internalization, by a cell, of other cells and large particles; this is an important physiological event not only in mammals, but in a wide diversity of organisms. Of simple unicellular organisms that use phagocytosis to extract nutrients, to complex metazoans in which phagocytosis is essential for the innate defence system, as a first line of defence against invading pathogens, as well as for the clearance of damaged, dying or dead cells. Evolution has armed multicellular organisms with a range of receptors expressed on many cells that serve as the molecular basis to bring about phagocytosis, regardless of the organism or the specific physiological event concerned. Key to all phagocytic processes is the finely controlled rearrangement of the actin cytoskeleton, in which Ca(2+) signals play a major role. Ca(2+) is involved in cytoskeletal changes by affecting the actions of a number of contractile proteins, as well as being a cofactor for the activation of a number of intracellular signalling molecules, which are known to play important roles during the initiation, progression and resolution of the phagocytic process. In mammals, the requirement of Ca(2+) for the initial steps in phagocytosis, and the subsequent phagosome maturation, can be quite different depending on the type of cell and on the type of receptor that is driving phagocytosis. In this review we discuss the different receptors that mediate professional and non-professional phagocytosis, and discuss the role of Ca(2+) in the different steps of this complex process.

Regulation of integrin activity and signalling

The ability of cells to attach to each other and to the extracellular matrix is of pivotal significance for the formation of functional organs and for the distribution of cells in the body. Several molecular families of proteins are involved in adhesion, and recent work has substantially improved our understanding of their structures and functions. Also, these molecules are now being targeted in the fight against disease. However, less is known about how their activity is regulated. It is apparent that among the different classes of adhesion molecules, the integrin family of adhesion receptors is unique in the sense that they constitute a large group of widely distributed receptors, they are unusually complex and most importantly their activities are strictly regulated from the inside of the cell. The activity regulation is achieved by a complex interplay of cytoskeletal proteins, protein kinases, phosphatases, small G proteins and adaptor proteins. Obviously, we are only in the beginning of our understanding of how the integrins function, but already now fascinating details have become apparent. Here, we describe recent progress in the field, concentrating mainly on mechanistical and structural studies of integrin regulation. Due to the large number of articles dealing with integrins, we focus on what we think are the most exciting and rewarding directions of contemporary research on cell adhesion and integrins.

Increased extracellular pressure enhances cancer cell integrin-binding affinity through phosphorylation of beta1-integrin at threonine 788/789

Increased extracellular pressure stimulates beta1-integrin-dependent cancer cell adhesion. We asked whether pressure-induced adhesion is mediated by changes in beta1-integrin binding affinity or avidity and whether these changes are phosphorylation dependent. We evaluated integrin affinity and clustering in human SW620 colon cancer cells by measuring differences in binding between soluble Arg-Gly-Asp (RGD)-Fc ligands and RGD-Fc-F(ab')2 multimeric complexes under ambient and 15-mmHg increased pressures. Phosphorylation of beta1-integrin S785 and T788/9 residues in SW620 and primary malignant colonocytes was assessed in parallel. We further used GD25-beta1-integrin-null murine fibroblasts stably transfected with either wild-type beta1A-integrin, S785A, TT788/9AA, or T788D mutants to investigate the role of beta1-integrin site-specific phosphorylation. SW620 binding of RGD-Fc-F(ab')2 multimeric complexes, but not soluble RGD-Fc ligands, was sensitive to integrin clustering. RGD-Fc ligand binding was significantly increased under elevated pressure, suggesting that pressure modulates beta1-integrin affinity. Pressure stimulated both beta1-integrin S785 and T788/9 phosphorylation. GD25-beta1A-integrin wild-type and S785A cells displayed an increase in adhesion to fibronectin under elevated pressure, an effect absent in beta1-integrin-null and TT788/9AA cells. T788D substitution significantly elevated basal cell adhesion but displayed no further increase under pressure. These results suggest pressure-induced cell adhesion is mediated by beta1-integrin T788/9 phosphorylation-dependent changes in integrin binding affinity.

The wild boar (Sus scrofa) lymphocyte function-associated antigen-1 (CD11a/CD18) receptor: cDNA sequencing, structure analysis and comparison with homologues

Background

The most predominant beta2-integrin lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18, alphaLbeta2), expressed on all leukocytes, is essential for many adhesive functions of the immune system. Interestingly, RTX toxin-producing bacteria specifically target this leukocyte beta2-integrin which exacerbates lesions and disease development.

Results

This study reports the sequencing of the wild boar beta2-integrin CD11a and CD18 cDNAs. Predicted CD11a and CD18 subunits share all the main structural characteristics of their mammalian homologues, with a larger interspecies conservation for the CD18 than the CD11a. Besides these strong overall similarities, wild boar and domestic pig LFA-1 differ by 2 (CD18) and 1 or 3 (CD11a) substitutions, of which one is located in the crucial I-domain (CD11a, E168D).

Conclusion

As most wild boars are seropositive to the RTX toxin-producing bacterium Actinobacillus pleuropneumoniae and because they have sustained continuous natural selection, future studies addressing the functional impact of these polymorphisms could bring interesting new information on the physiopathology of Actinobacillus pleuropneumoniae-associated pneumonia in domestic pigs.

Characterization of hyaluronan-binding proteins on guinea pig polymorphonuclear leukocytes: possible involvement of complement receptor type 3 (CR3, CD11b/CD18) in the hyaluronan-leukocyte interaction

Hyaluronan (HA), a high-molecular-weight glycosaminoglycan ubiquitously present in the extracellular matrices (ECMs) of animals, plays important roles in ECM organization and cell behavior through binding to hyaluronan-binding proteins (HABPs). We previously reported that HA has anti-inflammatory effects on guinea pig phagocytes, although the nature of guinea pig HABPs was unknown. In this study, we characterized guinea pig HABPs on peritoneal polymorphonuclear leukocytes (PMNs) and blood neutrophils by flow cytometry and affinity chromatography. It was found that PMNs express diverse HABPs with different molecular weights. These HABPs maximally bound with HA over a wide pH range (6-8), and recognized HAs as small as the pentadisaccharide units of d-glucuronic acid and N-acetyl-d-glucosamine. Furthermore, they could be divided into Mg(2+)-dependent and Ca(2+)/Mg(2+)-independent groups. Interestingly, two proteins in the Mg(2+)-dependent group were found to be the two subunits of complement receptor type 3 (CR3, CD11b/CD18). Unlike PMNs, blood neutrophils expressed several functionally inactive HABPs. Among these inactive HABPs, Mg(2+)-dependent proteins including CR3 but not Ca(2+)/Mg(2+)-independent proteins were activated on phorbol ester-stimulation. These results show the existence of diverse HABPs on guinea pig neutrophils and the cell activation-dependent activation of HABPs. It is also suggested that the CR3-HA interaction is possibly involved in the regulation of neutrophil function.

alpha-Chain phosphorylation of the human leukocyte CD11b/CD18 (Mac-1) integrin is pivotal for integrin activation to bind ICAMs and leukocyte extravasation

The promiscuous CD11b/CD18 (Mac-1) integrin has important roles in regulating many immunologic functions such as leukocyte adhesion and emigration from the bloodstream via interactions with the endothelial ligands ICAM-1 and ICAM-2, iC3b-mediated phagocytosis, and apoptosis. However, the mechanisms for Mac-1 inside-out activation have remained poorly understood. Phosphorylation of integrin cytoplasmic domains is emerging as an important mechanism of regulating integrin functions. Here, we have studied phosphorylation of human CD11b, which takes place on the cytoplasmic Ser1126 in neutrophils. We show that mutation of the serine phosphorylation site leads to inability of Mac-1 to become activated to bind the cellular ligands ICAM-1 and ICAM-2. However, CD11b-mutant cells are fully capable of binding other studied CD11b ligands (ie, iC3b and denatured BSA). Activation epitopes expressed in the extracellular domain of the integrin and affinity for soluble ICAM ligands were decreased for the mutated integrin. Additionally, the mutation resulted in inhibition of chemokine-induced migration in a transendothelial assay in vitro and significantly reduced the accumulation of intravenously administered cells in the spleen and lungs of Balb/c mice. These results characterize a novel selective mechanism of Mac-1-integrin activation, which mediates leukocyte emigration from the bloodstream to the tissues.

Impaired complement-mediated phagocytosis by HIV type-1-infected human monocyte-derived macrophages involves a cAMP-dependent mechanism

HIV-1 infection of cells of macrophage lineage impairs a number of effector functions performed by these cells, including phagocytosis of opsonized pathogens. In this study we investigate the effects of HIV-1 on the mechanism of complement (C')-mediated phagocytosis by human monocyte-derived macrophages (MDM). Using C'-opsonized sheep red blood cells (sRBC) as targets, we demonstrate that phagocytosis is inhibited by HIV-1 infection in vitro. Inhibition is not due to downregulation of surface C' receptors (R) or altered binding of C'-opsonized targets to HIV-1-infected MDM, suggesting a postreceptor-mediated mechanism of suppression. Having shown that increased levels of intracellular cAMP in uninfected MDM inhibit phagocytosis, we demonstrate that HIV-1 infection of MDM is associated with increased intracellular cAMP. Using the adenylate cyclase inhibitors 2',5'-dideoxyadenosine and MDL-12,330A, we show that phagocytosis by HIV-1- infected MDM can be restored by inhibition of cAMP production. Defective phagocytosis by HIV-1-infected MDM did not correlate with prostaglandin secretion, and was less in uninfected MDM within the HIV-1-infected cell culture suggesting a minimal bystander effect. Inhibition required viral entry but not active viral replication, as shown by use of the antiretroviral drug lamivudine. Hence, our study suggests that HIV-1 impairs C'R-mediated phagocytosis in MDM by elevating intracellular cAMP levels, independent of prostaglandin secretion, and contributes to our understanding of how HIV-1 impairs cell-mediated immunity.

Specific integrin alpha and beta chain phosphorylations regulate LFA-1 activation through affinity-dependent and -independent mechanisms

Integrins are adhesion receptors that are crucial to the functions of multicellular organisms. Integrin-mediated adhesion is a complex process that involves both affinity regulation and cytoskeletal coupling, but the molecular mechanisms behind this process have remained incompletely understood. In this study, we report that the phosphorylation of each cytoplasmic domain of the leukocyte function-associated antigen-1 integrin mediates different modes of integrin activation. α Chain phosphorylation on Ser1140 is needed for conformational changes in the integrin after chemokine- or integrin ligand–induced activation or after activation induced by active Rap1 (Rap1V12). In contrast, the β chain Thr758 phosphorylation mediates selective binding to 14-3-3 proteins in response to inside-out activation through the T cell receptor, resulting in cytoskeletal rearrangements. Thus, site-specific phosphorylation of the integrin cytoplasmic domains is important for the dynamic regulation of these complex receptors in cells.

LFA-1 (CD11a) as a therapeutic target

Leukocyte function associated antigen-1 (LFA-1) was one of the earliest of cell-surface molecules identified by monoclonal antibodies generated against leukocyte immunogens. This integrin heterodimer is perhaps best known as a classic adhesion molecule facilitating the interaction between T cells and antigen-presenting cells. However, varied studies indicate that LFA-1 has multi-faceted roles in the immune response including adhesion, activation and trafficking of leukocyte populations. While there has been long-standing interest in LFA-1 as a therapeutic target for regulating immunity, anti-LFA-1 therapy is still not a first-line indication for any clinical condition. Antagonism of LFA-1 with monoclonal antibodies, either alone or in combination with other agents, can result in regulatory tolerance in vivo. Furthermore, new generation humanized anti-LFA-1 monoclonal antibodies (Efalizumab) show at least modest promise for continued application in clinical trials. Thus, anti-LFA-1 forms a potential, but still largely unexploited, immunotherapy which may find its greatest application as an agent which augments other therapies.

An in vitro model of Fabry disease

Fabry disease is an X-linked inherited loss of alpha-galactosidase A (alpha-Gal A). Affected patients experience complications that include neuropathy, renal failure, and cardiovascular disease. Although the genetic and biochemical basis of this sphingolipidosis is well studied, the basis for the vascular disease remains poorly understood. In an attempt to create a suitable in vitro model of this disease, conditions for the growth of primary cultures of aortic endothelial cells from wild-type and alpha-Gal A -/0 mice were established. The cultured cells demonstrated CD-31 expression by flow cytometry and LDL binding by immunofluorescence. The glycolipid expression patterns were compared between wild-type and alpha-Gal A null cells. Importantly, cells from alpha-Gal A -/0 mice but not alpha-Gal A +/0 mice expressed high levels of the globo-series glycosphingolipid globotriaosylceramide (Gb3). The age-dependent elevation in Gb3 was measured. By 4 mo of age, alpha-Gal A -/0 mouse aortic endothelial cells achieved their peak Gb3 levels. The ability to lower Gb3 levels pharmacologically was assessed next. The glucosylceramide synthase inhibitor ethylenedioxyphenyl-P4 significantly lowered but did not eliminate Gb3 levels by 96 h of treatment. Gb3 synthesis was completely blocked as measured by [14C]galactose labeling. Recombinant alpha-Gal A more significantly lowered Gb3 levels by 48 h but had a more limited effect on de novo synthesis. Together, both agents eliminated detectable Gb3. In summary, primary cultures of aortic endothelial cells from Fabry mice retain the phenotype of elevated globo-series glycosphingolipids. These cells provide a useful model for comparing pharmacologic agents used for glycolipid reduction.

RhoH is required to maintain the integrin LFA-1 in a nonadhesive state on lymphocytes

Lymphocyte function-associated antigen 1 (LFA-1) is relatively nonadhesive on resting lymphocytes; however, the mechanisms underlying changes in its adhesiveness are poorly understood. In this study, we generated a Jurkat T cell clone, J+hi1.14, that contained low amounts of mRNA for RhoH, a leukocyte-specific inhibitory Rho family member. J+hi1.14 cells expressed constitutively adhesive LFA-1 and the cells bound spontaneously to intracellular adhesion molecules 1, 2 and 3. Reconstitution of RhoH mRNA expression in J+hi1.14 cells reverted the adhesion phenotype to that of wild-type. We obtained similar results using RNA interference in peripheral blood lymphocytes. These data demonstrate that RhoH is required for maintenance of lymphocyte LFA-1 in a nonadhesive state.

Molecular cloning and characterisation of the CD18 partner in ovine (Ovis aries) β2-integrins

The leukocyte integrins play a critical role in a number of cellular adhesive interactions during the immune response. We describe here the isolation and characterization of the ovine beta(2) (CD18) subunit, common to the leukocyte beta(2)-integrin family. The deduced 770-amino-acid sequence reveals a transmembrane protein with 81%, 83% and 95% identity with its murine, human and bovine homologues, respectively. Comparisons of CD18 sequences emphasize the functional importance of the beta(2) subunit I-like domain and included metal ion-dependent adhesion site (MIDAS)-like motif and confirm that of the cytoplasmic tail. The data provided here will offer the possibility to explore new avenues in studies based on the ovine model.

Threonine phosphorylation sites in the beta 2 and beta 7 leukocyte integrin polypeptides

The cytoplasmic domains of integrins play a key role in a variety of integrin-mediated events including adhesion, migration, and signaling. The molecular mechanisms that enhance integrin function are still incompletely understood. Because protein kinases are known to be involved in the signaling and the activation of integrins, the role of phosphorylation has been studied by several groups. The beta(2) leukocyte integrin subunit has previously been shown to become phosphorylated in leukocytes on cytoplasmic serine and functionally important threonine residues. We have now mapped the phosphorylated threonine residues in activated T cells. After phorbol ester stimulation, all three threonine residues (758-760) of the threonine triplet became phosphorylated but only two at a time. CD3 stimulation leads to a strong threonine phosphorylation of the beta(2) integrin, but differed from phorbol ester activation in that phosphorylation occurred only on threonine 758. The other leukocyte-specific integrin, beta(7), has also been shown to need the cytoplasmic domain and leukocyte-specific signal transduction elements for integrin activation. Cell activation with phorbol ester, and interestingly, through the TCR-CD3 complex, caused beta(7) integrin binding to VCAM-1. Additionally, cell activation led to increased phosphorylation of the beta(7) subunit, and phosphoamino acid analysis revealed that threonine residues became phosphorylated after cell activation. Sequence analysis by manual radiosequencing by Edman degradation established that threonine phosphorylation occurred in the same threonine triplet as in beta(2) phosphorylation.

Defective phagocytosis by human monocyte/macrophages following HIV-1 infection: underlying mechanisms and modulation by adjunctive cytokine therapy

Defective immunological function of cells of the macrophage lineage contributes considerably to the pathogenesis of HIV-1 infection. Impairment of phagocytosis of opportunistic pathogens such as Mycobacterium avium complex (MAC), Pneumocystis carinii, Toxoplasma gondii or Candida albicans by peripheral blood monocytes, tissue macrophages and monocyte-derived macrophages following in vivo and in vitro HIV-1 infection is well documented. The development of opportunistic infections due to these pathogens in HIV-infected individuals at late stages of disease is attributed to defective monocyte/macrophage function. The mechanisms whereby HIV-1 impairs phagocytosis are not well known. A number of phagocytic receptors normally mediate engulfment of specific opportunistic pathogens by cells of macrophage lineage; distinct mechanisms are triggered by pathogen-receptor binding to promote cytoskeletal rearrangements and engulfment. This review focuses on the signalling events occurring during Fcgamma receptor- and complement receptor-mediated phagocytosis, and considers the mechanisms by which HIV-1 inhibits those signalling events. Since macrophage function is enhanced by cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma), the use of these immunomodulators is of potential interest as adjunctive immunotherapy in immunosuppressed individuals. In this review we present examples of clinical applications of GM-CSF and IFN-gamma therapy for the treatment of opportunistic infections in HIV-infected individuals receiving antiretroviral drugs.

The immunological synapse: integrins take the stage

Adhesive interactions play important roles in coordinating T-cell migration and activation, specifically in the formation of the immunological synapse (IS), a specialized cell-cell junction. Recent demonstrations show several molecules implicated in T-cell signaling, including Vav, ADAP, and Rap-1, have major roles in integrin regulation and place adhesion molecules at center stage in addressing the question: what are the signals involved in the formation of the IS and full T-cell activation? This review focuses on the role of integrins as an essential system for both physical adhesion and signaling in T-cell activation. The role of integrins appears to be quite distinct from classical costimulation and has been largely overlooked due to the ubiquitous use of serum in lymphocyte functional assays. Each major signal transduction pathway has branches leading to the nucleus and others that feed back on cytoskeletal and membrane regulation at the IS.

Phagocytosis--the mighty weapon of the silent warriors

Professional phagocytes, comprising polymorphonuclear neutrophils and monocyte/macrophage cells, play an important role in the host defense. Any defect in their function exposes the organism to microbial intruders terminating in fatal diseases. The functional responses of the phagocytes to bacterial and fungal infections include chemotaxis, actin assembly, migration, adhesion, aggregation, phagocytosis, degranulation, and reactive oxygen species production. Superoxide generation by phagocytic NADPH oxidase is an imperative step toward bacterial killing. Phagocytes participate in inflammatory reactions and exert tumoricidal activity. They are supported by serum factors such as immunoglobulins, cytokines, complement, the acute phase reactant C-reactive protein, production of antibacterial proteins, and others. In addition to their principal task to eliminate bacteria, they are engaged in removing damaged, senescent, and apoptotic cells. Engulfed cell debris, large particles such as latex beads, fat, and oil droplets, are examples of phagocytic activity illustrated in the present review with transmission and scanning electron microscope micrographs. Numerous factors, such as diseases and stressful conditions, affect the engulfing activity of the professional phagocytes. Our experience regarding the impaired phagocytic capacity of cells in patients with diabetes and chronic renal failure is discussed. The results obtained in our laboratory from experiments detecting the effect of strenuous physical exercise, hypothermia, fasting, and abdominal photon irradiation on the phagocytic capacity of human polymorphonuclear neutrophils and rat peritoneal macrophages are hereby summarized and the reports on those subjects in the recent literature are reviewed. A variety of assays are applied for quantifying phagocytosis. Flow cytometry based on incubation of phagocytic cells with fluorescent conjugated particles and measuring the amount of fluorescence as an indicator of the engulfing capacity of the cells is a useful method. A direct visualization of the ingested particles using light or electron microscopy is a valuable tool for estimation of phagocytic function. In our hands, the use of semithin sections of embedded phagocytes following their incubation with latex particles provided satisfactory results for measuring the total number of phagocytic cells, as well as the internalizing capacity of each individual cell. Microbiological assays, the nitroblue tetrazolium test, quantitation of antibody- and antigen-mediated phagocytosis, as well as methods reviewed in detail in other reports are additional applications for determination of this intricate process.

Phosphorylation of the cytoplasmic domain of the integrin CD18 chain by protein kinase C isoforms in leukocytes

The CD11/CD18 (beta(2)) integrins are leukocyte-specific adhesion receptors, and their ability to bind ligands on other cells can be activated by extracellular stimuli. During cell activation, the CD18 chain is known to become phosphorylated on serine and functionally important threonine residues located in the intracellular C-terminal tail. Here, we identify catalytic domain fragments of protein kinase C (PKC) delta and PKCbetaI/II as the major protein kinases in leukocyte extracts that phosphorylate a peptide corresponding to the cytoplasmic tail of the integrin CD18 chain. The sites phosphorylated in vitro were identified as Ser-745 and Thr-758. PKCalpha and PKCeta also phosphorylated these residues, and PKCalpha additionally phosphorylated Thr-760. Ser-745, a novel site, was shown to become phosphorylated in T cells in response to phorbol ester stimulation. Ser-756, a residue not phosphorylated by PKC isoforms, also became phosphorylated in T cells after phorbol ester stimulation. When leukocyte extracts were subjected to affinity chromatography on agarose to which residues 751-761 of the CD18 chain phosphorylated at Thr-758 were bound covalently, the only proteins that bound specifically were identified as isoforms of 14-3-3 proteins. Thus, PKC-mediated phosphorylation of CD18 after cell stimulation could lead to the recruitment of 14-3-3 proteins to the activated integrin, which may play a role in regulating its adhesive state or ability to signal.

Phosphorylation of the integrin alpha 4 cytoplasmic domain regulates paxillin binding

alpha4 integrins are essential for embryogenesis, hematopoiesis, inflammation, and immune response possibly because alpha4 integrins have distinct signaling properties from other integrins. Specifically, the alpha4 cytoplasmic domain binds tightly to paxillin, a signaling adaptor protein, leading to increased cell migration and an altered cytoskeletal organization that results in reduced cell spreading. The alpha4 tail contains potential phosphorylation sites clustered in its core paxillin binding region. We now report that the alpha4 tail is phosphorylated in vitro and in vivo. Furthermore, Ser(988) is a major phosphorylation site. Using antibodies specific for Ser(988)-phosphorylated alpha4, we found the stoichiometry of alpha4 phosphorylation varied in different cells. However, >60% of alpha4 was phosphorylated in Jurkat T cells. Phosphorylation at Ser(988) blocked paxillin binding to the alpha4 tail. A phosphorylation-mimicking mutant of alpha4 (alpha4S988D) blocked paxillin binding and reversed the inhibitory effect of alpha4 on cell spreading. Consequently, alpha4 phosphorylation is a biochemical mechanism to modulate paxillin binding to alpha4 integrins with consequent regulation of alpha4 integrin-dependent cellular functions.

Mac-1-dependent tyrosine phosphorylation during neutrophil adhesion

Activated neutrophils display an array of physiological responses, including initiation of the oxidative burst, phagocytosis, and cell migration, that are associated with cellular adhesion. Under conditions that lead to cellular adhesion, we observed rapid tyrosine phosphorylation of an intracellular protein with an approximate relative molecular mass of 92 kDa (p92). Phosphorylation of p92 was inducible when Mac-1 was activated by phorbol 12-myristate 13-acetate, the beta(2)-specific activating antibody CBR LFA-1/2, or interleukin-8 (77 amino acids). In addition, tyrosine phosphorylation of p92 was dependent on engagement of Mac-1 with ligand. Several observations suggest that this event may be an important step in the signaling pathway initiated by Mac-1 binding. p92 phosphorylation was specifically blocked with antibodies to CD11b, the alpha-subunit of Mac-1, and was rapidly reversible on disengagement of the integrin ligand interaction. Integrin-stimulated phosphorylation of p92 created binding sites that were recognized in vitro by the SH2 domains of c-CrkII and Src. Our observations suggest that neutrophil adhesion mediated through the binding of the beta(2)-integrin Mac-1 initiates a signaling cascade that involves the activation of protein tyrosine kinases and leads to the regulation of protein-protein interactions via SH2 domains, a key process shared with growth factor signaling pathways.

Phagocytosis and the actin cytoskeleton

The process of engulfing a foreign particle - phagocytosis - is of fundamental importance for a wide diversity of organisms. From simple unicellular organisms that use phagocytosis to obtain their next meal, to complex metazoans in which phagocytic cells represent an essential branch of the immune system, evolution has armed cells with a fantastic repertoire of molecules that serve to bring about this complex event. Regardless of the organism or specific molecules concerned, however, all phagocytic processes are driven by a finely controlled rearrangement of the actin cytoskeleton. A variety of signals can converge to locally reorganise the actin cytoskeleton at a phagosome, and there are significant similarities and differences between different organisms and between different engulfment processes within the same organism. Recent advances have demonstrated the complexity of phagocytic signalling, such as the involvement of phosphoinostide lipids and multicomponent signalling complexes in transducing signals from phagocytic receptors to the cytoskeleton. Similarly, a wide diversity of ‘effector molecules’ are now implicated in actin-remodelling downstream of these receptors.

Phosphorylation of a conserved integrin alpha 3 QPSXXE motif regulates signaling, motility, and cytoskeletal engagement

Integrin alpha 3A cytoplasmic tail phosphorylation was mapped to amino acid S1042, as determined by mass spectrometry, and confirmed by mutagenesis. This residue occurs within a "QPSXXE" motif conserved in multiple alpha chains (alpha 3A, alpha 6A, alpha 7A), from multiple species. Phosphorylation of alpha 3A and alpha 6A did not appear to be directly mediated by protein kinase C (PKC) alpha, beta, gamma, delta, epsilon, zeta, or mu, or by any of several other known serine kinases, although PKC has an indirect role in promoting phosphorylation. A S1042A mutation did not affect alpha 3-Chinese hamster ovary (CHO) cell adhesion to laminin-5, but did alter 1) alpha 3-dependent tyrosine phosphorylation of focal adhesion kinase and paxillin (in the presence or absence of phorbol 12-myristate 13 acetate stimulation), and p130(CAS) (in the absence of phorbol 12-myristate 13 acetate stimulation), 2) the shape of cells spread on laminin-5, and 3) alpha 3-dependent random CHO cell migration on laminin-5. In addition, S1042A mutation altered the PKC-dependent, ligand-dependent subcellular distribution of alpha 3 and F-actin in CHO cells. Together, the results demonstrate clearly that alpha 3A phosphorylation is functionally relevant. In addition, the results strongly suggest that alpha 3 phosphorylation may regulate alpha 3 integrin interaction with the cytoskeleton.

Disruption of beta(2)-integrin-cytoskeleton coupling abolishes the signaling capacity of these integrins on granulocytes

Integrin-dependent adhesion and dynamic modulations of the actin network are prerequisites for normal cell locomotion. To investigate whether the actin microfilamentous system does play a role in regulation of beta(2)-integrin-induced signalling, we pretreated granulocytes with staurosporine, a well-known protein kinase inhibitor that has also been shown to disrupt the cytoskeleton of intact cells. Pretreatment with staurosporine completely inhibited the beta(2)-integrin-induced Ca(2+) signal and also its ability to trigger actin polymerisation. This inhibition was not related to phosphorylation of the CD18-chain of the beta(2)-integrin, nor to inhibition of protein kinases. Instead, association of beta(2)-integrins with the cortical cytoskeleton, which was observed in untreated cells, was abolished after exposure to staurosporine, indicating that beta(2)-integrin signalling depends on integrin-cytoskeleton interaction. These results suggest not only that the actin network provides an adhesive link to the extracellular matrix and a driving force for the locomotory response, but also that it participates in regulation of beta(2)-integrin signalling during granulocyte locomotion.

The cytoskeletal association of CD11/CD18 leukocyte integrins in phorbol ester-activated cells correlates with CD18 phosphorylation

Leukocyte adhesion is a regulated process, which involves CD11/CD18 leukocyte integrins. CD11/CD18 acidity may be regulated intracellularly, and the CD18 polypeptide has previously been shown to become phosphorylated on serine and threonine after phorbol ester activation of T cells. Increased adhesiveness is believed to be mediated by regulating the overall avidity of cellular contact. CD11/CD18 integrins have earlier been reported to interact with several cytoskeletal proteins. We have now studied the involvement of the CD18 phosphorylation in cytoskeletal associations. We have investigated the distribution of phosphorylated CD18 between soluble, cytoskeletal and nuclear fractions of T cell detergent lysates. A significant amount of phosphorylated CD18 polypeptides was observed to fraction along with the cytoskeleton, while the majority of the cell surface CD18 molecules remained in the soluble fraction. Putative candidates for this altered cytoskeletal binding of CD11/CD18 were shown to be talin and filamin, which were observed to bind to CD18 cytoplasmic peptides and co-precipitate with CD18. The importance of the CD18 cytoplasmic domain in the regulation of the leukocyte adhesion was further strengthened by inhibition of phorbol ester-induced T cell adhesion with a phosphorylated lipopeptide corresponding to the cytoplasmic portion of the CD18. These results indicate that the induced CD18 phosphorylation and the altered cytoskeletal binding of the phosphorylated integrin complex may contribute to the increased avidity of CD11/CD18-mediated leukocyte adhesion.

Phospholipase D-derived phosphatidic acid is involved in the activation of the CD11b/CD18 integrin in human eosinophils

Priming of human eosinophils is an essential event for the respiratory burst induced by serum-opsonized particles [serum-treated zymosan (STZ)]. In this study we have found that treatment of eosinophils with platelet-activating factor (PAF) leads to activation of phospholipase D. Inhibition of the formation of phospholipase D-derived products by ethanol resulted in about 90% inhibition of PAF-induced binding of fluorescent STZ particles to the cells, but only when ethanol was added to the cells before treatment with PAF. When ethanol was added after treatment with PAF, only a minor inhibition of the STZ binding and STZ-induced response was observed. These results indicate that phospholipase D-derived phosphatidic acid is involved in PAF priming, without having an effect on STZ stimulation. In the presence of propranolol, which inhibits phosphatidic acid-phosphatase activity, binding of STZ particles to human eosinophils induced by suboptimal concentrations of PAF was enhanced, indicating that phosphatidic acid and not diradylglyceride is the relevant molecule derived from phospholipase D activity. Addition of cell-permeant diC8-phosphatidic acid (DiC8-PA) to human eosinophils resulted in CD11b/CD18-dependent adhesion, both to STZ particles and fibronectin-coated wells, without significant upregulation of CD11b/CD18. The DiC8-PA-induced adhesion was not mediated via the fatty acid moiety, because other C8-lipids such as 1,2-diC8-phosphatidylcholine, 1-C8-monoacylglycerol or C8-ceramide were without effect. Activation of protein kinase C with PMA or 1,2-diC8-diacylglycerol did result in enhanced STZ binding. However, under these latter conditions upregulation of CD11b/CD18 was observed. Taken together, these results suggest that phospholipase D-derived PA is involved in changing the affinity of the CD11b/CD18 integrin for its ligands.

Signaling pathways in phagocytosis

Phagocytosis is an uptake of large particles governed by the actin-based cytoskeleton. Binding of particles to specific cell surface receptors is the first step of phagocytosis. In higher Eucaryota, the receptors able to mediate phagocytosis are expressed almost exclusively in macrophages, neutrophils, and monocytes, conferring immunodefence properties to these cells. Receptor clustering is thought to occur upon particle binding, that in turn generates a phagocytic signal. Several pathways of phagocytic signal transduction have been identified, including the activation of tyrosine kinases and (or) serine/threonine kinase C in pivotal roles. Kinase activation leads to phosphorylation of the receptors and other proteins, recruited at the sites of phagocytosis. Monomeric GTPases of the Rho and ARF families are likely to be engaged downstream of activated receptors. The GTPases, in cooperation with phosphatidylinositol 4-phosphate 5-kinase and phosphatidylinositol 3-kinase lipid modifying enzymes, can modulate locally the assembly of the submembranous actin filament system leading to particle internalization.

Leukocyte adhesion--a fundamental process in leukocyte physiology

Leukocyte adhesion is of pivotal functional importance. The adhesion involves several different adhesion molecules, the most important of which are the leukocyte beta 2-integrins (CD11/CD18), the intercellular adhesion molecules, and the selectins. We and others have extensively studied the specificity and binding sites in the integrins and the intercellular adhesion molecules for their receptors and ligands. The integrins have to become activated to exert their functions but the possible mechanisms of activation remain poorly understood. Importantly, a few novel intercellular adhesion molecules have been recently described, which seem to function only in specific tissues. Furthermore, it is becoming increasingly apparent that changes in integrins and intercellular adhesion molecules are associated with a number of acute and chronic diseases.

Studies on a haemolymph lectin isolated from Rhodnius prolixus and its interaction with Trypanosoma rangeli

We demonstrated that in Rhodnius prolixus haemocyte monolayers, both Trypanosoma cruzi and Trypanosoma rangeli are capable of inducing haemocyte/parasite clump formation. We also purified, by one-step affinity chromatography, a haemolymph galactoside-binding lectin from R. prolixus which we believe could play an important role in the development of T. rangeli in the haemocoel of the insect vector. This lectin markedly enhanced the activation of clump formation by T. rangeli in R. prolixus haemocyte monolayers, with an increase in clump size and haemocyte aggregation. The haemolymph lectin also significantly affected the motilitity and survival of T. rangeli culture short forms, but not the long forms, when they were incubated in vitro. This molecule is also one of the few described in insects with agglutination activity independent of calcium ions. The partial N-terminal amino acid sequence of this lectin demonstrated similarity to a bacterial xylulose kinase and in preliminary experiments the purified haemolymph lectin phosphorylated a tyrosine kinase substrate in a dose-dependent manner. The possible role of this haemolymph lectin in the life cycle of T. rangeli is discussed.

Characterization of beta2 (CD18) integrin phosphorylation in phorbol ester-activated T lymphocytes

Integrins are transmembrane proteins involved in cell-cell and cell-extracellular-matrix interactions. The affinity and avidity of integrins for their ligands change in response to cytoplasmic signals. This 'inside-out' activation has been reported to occur also with beta2 integrins (CD18). The beta2 integrin subunit has previously been shown to become phosphorylated in T lymphocytes on cytoplasmic serine and the functionally important threonine residues after treatment with phorbol esters or on triggering of T-cell receptors. We have now characterized the phosphorylation of beta2 integrins in T-cells in more detail. When T-cells were activated by phorbol esters the phosphorylation was mainly on Ser756. After inhibition of serine/threonine phosphatases, phosphorylation was also found in two of the threonine residues in the threonine triplet 758-760 of the beta2 cytoplasmic domain. Activation of T-cells by phorbol esters resulted in phosphorylation in only approx. 10% of the integrin molecules. Okadaic acid increased this phosphorylation to approx. 30% of the beta2 molecules, assuming three phosphorylation sites. This indicates that a strong dynamic phosphorylation exists in serine and threonine residues of the beta2 integrins.

Adhesion molecules in clinical medicine

Cellular adhesion molecules (CAMs) are critical components in the processes of embryogenesis, tissue repair and organization, lymphocyte function, lymphocyte homing and tumor metastasis, as well as being central to the interactions between hemopoietic progenitors and bone marrow microenvironment, and between leukocytes and platelets with vascular endothelium. Expression of CAMs regulates normal hemopoiesis and migration and function of mature hemopoietic cells. CAMs are an important part of the inflammatory response and may regulate cytokine synthesis. In addition, CAM expression may be critical for tumorigenesis. Monoclonal antibodies to CAMs have been developed for clinical use; initial results suggest that these agents have great potential in the prevention and treatment of inflammation, thrombosis, reperfusion injury, and graft rejection.

Granulocyte macrophage colony-stimulating factor augments ICAM-1 and VCAM-1 activation of eosinophil function

Intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) are members of the immunoglobulin superfamily adhesion molecules on vascular endothelium and important in the development of eosinophil (EOS) accumulation in allergic inflammation. To define the role of these adhesion proteins in EOS inflammation, peripheral blood EOS from allergic donors were incubated in either buffer (control)-, recombinant human (rh)-VCAM-1-, or rh-ICAM-1-coated plates, and the effects of these adhesion proteins on EOS effector functions were determined. VCAM-1 induced spontaneous EOS adhesion whereas EOS adhesion to ICAM-1 required a second signal, such as granulocyte macrophage colony-stimulating factor (GM-CSF). Although only VCAM-1 stimulated EOS superoxide anion (O2-) generation, the addition of GM-CSF (100 pM) to the reactions resulted in a greater and equivalent production of O2- with VCAM-1 and ICAM-1. In the presence of GM-CSF, ICAM-1 and VCAM-1 caused significant release of EOS-derived neurotoxin (EDN). Moreover, only ICAM-1 (no GM-CSF) promoted calcium ionophore A23187 (0.2 microM)-induced EOS leukotriene C4 (LTC4). Enhanced O2- generation, EDN release, and LTC4 generation observed with ICAM-1 and VCAM-1 were significantly inhibited by anti-beta2-integrin antibody. These results suggest that ICAM-1 and VCAM-1 are important in determining the eventual function of airway EOS.

Differential Requirements for LFA-1 Binding to ICAM-1 and LFA-1-Mediated Cell Aggregation

Cellular adhesion through the β2 integrin lymphocyte function-associated Ag (LFA)-1 is a complex event involving activation, ligand binding, and cell shape changes that ultimately result in enhanced adhesion. In this report we define requirements for ligand binding and post receptor signaling by comparing two mechanisms of activation of LFA-1: 1) inside-out signaling and 2) direct activation by the β2 Ab, CBR LFA-1/2. Our results demonstrate that activation of LFA-1 binding to ICAM-1 by CBR LFA-1/2, in contrast to inside-out signaling mechanisms, does not require protein kinase C activation or protein phosphatase 2A activity nor is it affected by agents that interfere with reorganization of the cytoskeleton. Inhibition of protein tyrosine kinase activity does not affect ICAM-1 binding by either mechanism of activation. However, activation by either mode does require the presence of the β cytoplasmic domain; deletion of the C-terminal phenylalanine or the five amino acid stretch between 756–762 abolished activation of LFA-1. This, combined with the observation that intracellular energy pools must be preserved, implicates the β cytoplasmic domain in a key energy-dependent conformational change in LFA-1 that is required to achieve enhanced ligand binding. Post ligand binding events induced by both PMA and Ab stimulation, as measured by homotypic aggregation, require protein tyrosine kinase, phosphatase, and RhoA activities. By examining both ligand binding and aggregation, we have been able to dissect the signaling components critical in the multistep process of LFA-1-mediated cellular adhesion.