Structure of the integrin VLA-4 and its cell-cell and cell-matrix adhesion functions

Discovery and evaluation of potent, cysteine-based alpha4beta1 integrin antagonists

Acyclic, disulphide derivatives of cysteine have been identified as moderately potent antagonists of alpha4beta1-mediated leukocyte cell adhesion to VCAM. This communication describes how they were discovered from a simple L-cystine derivative and using the structure-activity data of C*DThioPC* related cyclic peptides.

Production of recombinant soluble human integrin alpha4beta1

Integrin alpha4beta1 is a major leukocyte adhesion receptor that is a key target for the development of anti-inflammatory therapeutics. With the dual long-term goals of developing a reagent for use in high-throughput inhibitor screening assays and for crystallisation trials and subsequent structure determination, we have generated a recombinant soluble alpha4beta1 receptor. Both subunits were truncated prior to the transmembrane domains by site-directed mutagenesis and expressed using baculovirus infection of insect cells. The molecular weights of the recombinant subunits were as expected for post-translationally unmodified protein. In addition, as observed for the native subunit, a proportion of the alpha4 subunit was proteolytically processed into two fragments. ELISA and solid phase ligand-binding assays were performed to investigate the folding and functionality of the soluble integrin. The data suggest that the receptor was correctly folded and that it bound recombinant ligands with similar kinetics to the native molecule.

Integrin expression on neutrophils in a rabbit model of Group B Streptococcal meningitis

Products released by polymorphonuclear cells (PMNs) during an acute inflammatory response can result in diffuse tissue injury. Integrins are cell surface adhesion proteins that play a pivotal role in inflammation by allowing PMNs to adhere to the endothelium and migrate through the extracellular matrix. We examined the expression of beta1 and beta2 integrins on neutrophils from blood and cerebrospinal fluid (CSF) in an animal model of Group B Streptococcal meningitis. We further evaluated whether integrin expression correlates with pathophysiologic markers of central nervous system inflammation. Our data demonstrate that beta3 and beta2 integrin expression on circulating neutrophils does not significantly increase as a consequence of meningitis. In extravesated CSF neutrophils, a significant increase in expression of both beta1 and beta2 integrins is noted. Furthermore, a majority of the beta1 integrins on extravesated neutrophils have undergone affinity modulation. Using regression analysis, we demonstrated that increasing beta1 integrin expression correlates with decreasing CSF glucose concentration and serum/CSF glucose ratio. Regression analysis approached significance when CSF protein was compared to PMN beta1 integrin expression. Polymorphonuclear leukocytes beta1 integrin expression also showed a direct correlation to myeloperoxidase activity in brain tissue. Beta2 expression on CSF PMNs did not correlate with these markers of inflammation/sequestration. These data demonstrate integrin expression on extravesated neutrophils markedly increases during meningitis and support a role for beta1 integrins on neutrophils in the pathophysiologic consequences of meningitis.

Increased infectivity of HIV type 1 particles bound to cell surface and solid-phase ICAM-1 and VCAM-1 through acquired adhesion molecules LFA-1 and VLA-4

HIV-1 incorporates a variety of host membrane proteins during budding. We have previously shown that adhesion molecules are acquired by the virus in their activated or functional states. Our studies and those of others indicate that adhesion molecules can have profound effects on virus infectivity and its resistance to neutralization by antiviral antibodies. In this study we have examined the effect on infectivity of immobilization or margination of HIV-1 through acquired integrins LFA-1 and VLA-4 onto nonsusceptible cells and solid-phase adhesion ligands (ICAM-1 and VCAM-1, respectively). LFA-1- and VLA-4-mediated HIV-1 binding was supported by ICAM-1 and VCAM-1 immunoglobulin Fc chimeras, respectively. Integrin-mediated HIV-1 binding was also supported by 293 cells transfected with ICAM-1. In both cases the specificity of binding was confirmed with the appropriate blocking monoclonal antibodies or soluble adhesion ligands. We used a sensitive single-cycle infection assay based on a cell line expressing an LTR-luciferase cDNA construct to compare the infectivity of bound virus with that of free virus. Our results show that the binding of HIV-1 to nonsusceptible cells or immobilized adhesion ligands through acquired integrins can increase its infectivity by as much as two orders of magnitude. These results have implications for in vivo dissemination and transmission of HIV-1 and may also explain the high level of virus replication seen in solid lymphoid organs.

Cloning, sequence analysis, and chromosomal localization of the novel human integrin alpha11 subunit (ITGA11)

The integrins are a large family of cell adhesion molecules consisting of noncovalently associated alphabeta heterodimers. We have cloned and sequenced the cDNA of a novel human integrin alpha-subunit, designated alpha11. The alpha11 cDNA encodes a mature protein with a large 1120-residue extracellular domain that contains an I-domain of 207 residues and is linked by a transmembrane domain to a short cytoplasmic domain of 24 amino acids. The deduced alpha11 protein shows the typical structural features of integrin alpha-subunits and is similar to a distinct group of alpha-subunits from collagen-binding integrins. However, it differs from most integrin alpha-chains by an incompletely preserved cytoplasmic GFFKR motif. The human ITGA11 gene was localized to bands q22.3-q23 on chromosome 15, and its transcripts were found in a variety of tissues, but predominantly in bone, cartilage, cardiac muscle, and skeletal muscle. Expression of a 5.5-kb alpha11 mRNA was detectable in small intestine.

Characterization of Eosinophil Adhesion to TNF-α-Activated Endothelium Under Flow Conditions: α4 Integrins Mediate Initial Attachment, and E-Selectin Mediates Rolling

The multistep model of leukocyte adhesion reveals that selectins mediate rolling interactions and that integrins mediate firm adhesion processes. In this study, the interaction between eosinophils and TNF-α-activated HUVEC (second or third passage) was studied under flow conditions (0.8 and 3.2 dynes/cm2). Especially the role of α4 integrins on eosinophils and E-selectin on HUVEC was studied. Inhibition of the integrin α4 chain on eosinophils reduced the number of firmly adhered resting eosinophils to TNF-α-stimulated endothelium by 43% whereas the percentage rolling cells increased 2.2-fold compared with untreated control eosinophils. Blocking of E-selectin on the endothelium reduced the number of adherent eosinophils by only 23% and 16%. In this situation, however, hardly any rolling adhesion was observed, and the few rolling cells showed a low rolling velocity. Blocking both α4 integrin on eosinophils and E-selectin on HUVEC reduced the number of adhered eosinophils by 95%. P-selectin did not significantly participate in eosinophil adhesion to TNF-α-activated HUVEC. Inhibition of both α4 integrins and β2 integrins on eosinophils resulted in a reduction of adhered cells by 65% and a 3-fold increase in percentage rolling cells. Taken together, these results clearly show that resting eosinophils preferentially use constitutively active α4 integrins (α4β1, α4β7) for the first attachment to TNF-α-activated HUVEC. In addition, α4 integrins and E-selectin work synergistically in eosinophil adherence to TNF-α-activated HUVEC. Although E-selectin is important for eosinophil rolling under these conditions, P-selectin plays only a minor role.

Multiple activation states of integrin alpha4beta1 detected through their different affinities for a small molecule ligand

We have used the highly specific alpha4beta1 inhibitor 4-((N'-2-methylphenyl)ureido)-phenylacetyl-leucine-aspartic acid-valine-proline (BIO1211) as a model LDV-containing ligand to study alpha4beta1 integrin-ligand interactions on Jurkat cells under diverse conditions that affect the activation state of alpha4beta1. Observed KD values for BIO1211 binding ranged from a value of 20-40 nM in the non-activated state of the integrin that exists in 1 mM Mg2+, 1 mM Ca2+ to 100 pM in the activated state seen in 2 mM Mn2+ to 18 pM when binding was measured after co-activation by 2 mM Mn2+ plus 10 microgram/ml of the integrin-activating monoclonal antibody TS2/16. The large range in KD values was governed almost exclusively by differences in the dissociation rates of the integrin-BIO1211 complex, which ranged from 0.17 x 10(-4) s-1 to >140 x 10(-4) s-1. Association rate constants varied only slightly under the same conditions, all falling in the narrow range from 0.9 to 2.7 x 10(6) M-1 s-1. The further increase in affinity observed upon co-activation by divalent cations and TS2/16 compared with that observed at saturating concentrations of metal ions or TS2/16 alone indicates that the mechanism by which these factors bring about activation are distinct and identified a previously unrecognized high affinity state on alpha4beta1 that had not been detected by conventional assay methods. Similar changes in affinity were observed when the binding properties of vascular cell adhesion molecule-1 and CS1 to alpha4beta1 were studied, indicating that the different affinity states detected with BIO1211 are an inherent property of the integrin.

Rodent models of lymphocyte migration

The ability of leukocytes to migrate out of blood into tissues enables them to perform their surveillance functions. Understanding the molecular mechanisms by which this migration is accomplished has the potential of unveiling new methods of regulating immune responses. The existing knowledge of rodent physiology and the recent development of knockout mice makes rodents attractive models for studying the mechanisms of leukocyte migration in vivo. This review considers the existing rodent models in light of the knowledge gained from them in lymphocyte migration, and in addition, shows the advantages and limitations of using rodent models in studying lymphocyte migration.

Selective, tight-binding inhibitors of integrin alpha4beta1 that inhibit allergic airway responses

Integrin alpha4beta1 mediates leukocyte recruitment, activation, mediator release, and apoptosis inhibition, and it plays a central role in inflammatory pathophysiology. High-affinity, selective inhibitors of alpha4beta1, based on the Leu-Asp-Val (LDV) sequence from the alternatively spliced connecting segment-1 (CS-1) peptide of cellular fibronectin, are described that employ a novel N-terminal peptide "cap" strategy. One inhibitor, BIO-1211, was approximately 10(6)-fold more potent than the starting peptide and exhibited tight-binding properties (koff = 1.4 x 10(-4) s-1, KD = 70 pM), a remarkable finding for a noncovalent, small-molecule inhibitor of a protein receptor. BIO-1211 was also 200-fold selective for the activated form of alpha4beta1, and it stimulated expression of ligand-induced epitopes on the integrin beta1 subunit, a property consistent with occupancy of the receptor's ligand-binding site. Pretreatment of allergic sheep with a 3-mg nebulized dose of BIO-1211 inhibited early and late airway responses following antigen challenge and prevented development of nonspecific airway hyperresponsiveness to carbachol. These results show that highly selective and potent small-molecule antagonists can be identified to integrins with primary specificity for peptide domains other than Arg-Gly-Asp (RGD); they confirm the generality of integrins as small molecule targets; and they validate alpha4beta1 as a therapeutic target for asthma.

Fibronectin augments monocyte adhesion to low-density lipoprotein-stimulated mesangial cells

BACKGROUND

Glomerular monocyte infiltration is an early feature of lipid-mediated renal injury in animal models. Interactions between mesangial and infiltrating mononuclear cells may contribute to the development of glomerular scarring.

METHODS

Adherence of U-937 monocytes to low-density lipoprotein (LDL)- or tumor necrosis factor alpha (TNFalpha)-prestimulated human mesangial cells was assessed by colorimetry of nuclear staining with crystal violet. Blocking antibodies were added to examine the mechanisms of binding. Adhesion molecule expression and fibronectin synthesis were measured by ELISA.

RESULTS

Preincubation of mesangial cells for 24 hours with LDL (100 micrograms/ml) or mildly oxidized (minimally modified) LDL (MM-LDL) increased monocyte adhesion by 207% and 240%, respectively, compared with control nonstimulated cells (100%). TNFalpha (100 U/ml) enhanced binding by 335% and up-regulated intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression by 505% and 179%, respectively, as compared with MM-LDL (120% and 116%) and LDL, which had no effect. Blocking antibodies to these adhesion molecules inhibited monocyte binding to TNFalpha- and, to a lesser extent, MM-LDL-primed mesangial cells, but had no effect after LDL pretreatment. In contrast to TNFalpha, MM-LDL and LDL increased mesangial cell-associated fibronectin, whereas antibodies to fibronectin inhibited monocyte binding to lipoprotein-stimulated but not TNFalpha-stimulated cells.

CONCLUSIONS

Although enhanced monocyte adhesion to TNFalpha- and, to a lesser extent, MM-LDL-stimulated mesangial cells is mediated by changes in ICAM-1 and VCAM-1 expression, both LDL and MM-LDL promote similar cellular interactions as a result of increased fibronectin production.

Sequence Motifs in the Integrin α4 Cytoplasmic Tail Required for Regulation of In Vivo Expansion of Murine Lymphoma Cells

The binding of integrins to cognate ligands is tightly controlled by intracellular signals. Conversely, integrin occupancy generates biochemical signals inside the cell. The present study examined whether concepts of integrin function established by in vitro analysis apply to regulation of receptor function in complex biologic settings in vivo using a mouse model of tumor metastasis. Integrin α4 subunits were truncated at amino acid Gln1014 (A4-1014), preserving the conserved GFFKR motif, and at position Glu1021 (A4-1021). In vitro adhesion assays revealed that cytoplasmic tail truncations did not affect constitutive ligand binding of α4 integrins, while agonist-induced adhesion was abolished by the A4-1014, but not by the A4-1021, mutation. Inducible ligand binding of α4 integrins was dependent on cytoskeletal function, whereas constitutive adhesion was not. In vivo metastasis formation assays demonstrated that expansion of murine T lymphoma cells in spleen is strongly inhibited by the wild-type α4 subunit and the A4-1021 mutant. In contrast, the in vivo phenotype of α4 integrin expression in lymphoma cells was completely abrogated by the A4-1014 mutation. Cross-linking of α4 integrins in vitro inhibited proliferation and induced apoptosis of LB cells expressing wild-type α4 subunits or the A4-1021 mutant, but not of LB-A4-1014 cells. In summary, these results demonstrate that sequence motifs regulating cytoskeleton-dependent α4 integrin activation in vitro are essential for the control of LB lymphoma cell expansion both in vitro and in vivo.

Serum levels of soluble intercellular adhesion molecule-1 and soluble vascular cell adhesion molecule-1 in asymptomatic carriers of hepatitis C virus

High levels of serum-soluble intercellular adhesion molecule-1 (sICAM-1) and soluble vascular cell adhesion molecule-1 (sVCAM-1) have been noted in patients with chronic hepatitis C. This study aimed to measure serum levels of sICAM-1 and sVCAM-1 in asymptomatic hepatitis C virus carriers and clarify the clinical significance of measuring soluble forms. Serum levels of sICAM-1 were significantly higher than in healthy controls but serum sVCAM-1 levels did not differ statistically from those in healthy controls. Liver biopsy obtained from 12 asymptomatic hepatitis C virus carriers showed evidence of hepatitis. Estimating sICAM-1 and sVCAM-1 in asymptomatic carriers may be helpful, especially in cases in which liver biopsy is not possible.

Sickle Cell Adhesion to Laminin: Potential Role for the 5 Chain

Sickle red blood cell (RBC) adhesion to the endothelium and to exposed, underlying subendothelial proteins is believed to contribute to vascular occlusion in sickle cell disease. Laminin, a major component of the subendothelium, supports significant adhesion of sickle, but not normal RBCs. The purpose of this study was to define the adhesive region for sickle RBCs within a human laminin preparation using a flow adhesion assay designed to mimic physiologic flow through postcapillary venules. Because sickle RBCs did not adhere to the common laminin contaminants entactin or collagen type IV, neither of these proteins are likely to contribute to the observed adhesion to laminin. Known adhesive regions of laminin neither supported nor inhibited sickle RBC adhesion to laminin, suggesting a mechanism of adhesion previously uncharacterized in other laminin adhesion studies. Moreover, sickle RBCs did not adhere to mouse EHS laminin or to human laminin-2 (merosin), eliminating the 1, 2, β1, and γ1 chains as mediators of sickle cell adhesion. The monoclonal antibody 4C7, which binds at or near the G-domain of the laminin 5 chain, significantly inhibited sickle RBC adhesion. These results suggest that an adhesive region for sickle RBCs is contained within the laminin 5 chain.

© 1998 by The American Society of Hematology.

Sickle Cell Adhesion to Laminin: Potential Role for the 5 Chain

Sickle red blood cell (RBC) adhesion to the endothelium and to exposed, underlying subendothelial proteins is believed to contribute to vascular occlusion in sickle cell disease. Laminin, a major component of the subendothelium, supports significant adhesion of sickle, but not normal RBCs. The purpose of this study was to define the adhesive region for sickle RBCs within a human laminin preparation using a flow adhesion assay designed to mimic physiologic flow through postcapillary venules. Because sickle RBCs did not adhere to the common laminin contaminants entactin or collagen type IV, neither of these proteins are likely to contribute to the observed adhesion to laminin. Known adhesive regions of laminin neither supported nor inhibited sickle RBC adhesion to laminin, suggesting a mechanism of adhesion previously uncharacterized in other laminin adhesion studies. Moreover, sickle RBCs did not adhere to mouse EHS laminin or to human laminin-2 (merosin), eliminating the 1, 2, β1, and γ1 chains as mediators of sickle cell adhesion. The monoclonal antibody 4C7, which binds at or near the G-domain of the laminin 5 chain, significantly inhibited sickle RBC adhesion. These results suggest that an adhesive region for sickle RBCs is contained within the laminin 5 chain.

© 1998 by The American Society of Hematology.

Pathogenic mechanisms in the rheumatoid nodule: comparison of proinflammatory cytokine production and cell adhesion molecule expression in rheumatoid nodules and synovial membranes from the same patient

OBJECTIVE

To investigate the production of proinflammatory cytokines and expression of cell adhesion molecules in the rheumatoid nodule.

METHODS

Cytokine content (tumor necrosis factor alpha [TNFalpha], interleukin-1beta [IL-1beta], and IL-1 receptor antagonist [IL-1Ra]), at the messenger RNA (mRNA) and protein levels, and cell adhesion molecule expression were studied in 16 rheumatoid nodules and 6 synovial membranes.

RESULTS

Macrophages in the rheumatoid nodules contained TNFalpha, IL-1beta, and IL-1Ra mRNA and protein, particularly in perivascular cells of the stroma and in the palisading layer. All cell adhesion molecules studied were expressed in both the rheumatoid nodules and synovial membranes, with increased expression of E-selectin in the rheumatoid nodule compared with the synovial membrane, and with the absence of vascular cell adhesion molecule 1 expression on cells of the palisading layer in the rheumatoid nodule.

CONCLUSION

The presence of similar proinflammatory cytokines and cell adhesion molecules in the rheumatoid nodule and synovial membrane suggests that similar pathogenic processes result in the chronic inflammation and tissue destruction in these lesions.

Structural specializations of immunoglobulin superfamily members for adhesion to integrins and viruses

The circulation and migration of leukocytes are critical for immune surveillance and immune response to infection or injury. The key step of leukocyte recruitment involves the adhesion between immunoglobulin superfamily (IgSF) proteins on endothelium and integrin molecules on leukocyte surfaces. Some of the IgSF members are subverted as virus receptors. Four crystal structures of N-terminal two-domain fragments of these IgSF proteins have been determined: intercellular adhesion molecule-1 (ICAM-1), ICAM-2, vascular adhesion molecule-1 (VCAM-1), and mucosal addressin cell adhesion molecule-1 (MAdCAM-1). An acidic residue near the bottom of domain 1 plays a key role in integrin binding. For ICAM-1 and ICAM-2, this glutamic acid residue is located on a flat surface, complementary to the flat surface of the I domain of the integrin to which they bind, lymphocyte function-associated antigen-1 (LFA-1). For VCAM-1 and MAdCAM-1, the acidic residue is aspartic acid, and it resides on a protruded CD loop which may be complementary to a more pocket-like structure in the alpha 4 integrins to which they bind, which lack I domains. A number of unique structural features of this subclass of IgSF have been identified which are proposed to consolidate the domain structure to resist force during adhesion to integrins. Different mechanisms are proposed for the different CAMs to present the integrin-binding surface toward the opposing cell for adhesion, and prevent cis interaction with integrins on the same cell. Finally, CD4 and ICAM-1 are compared in the context of ligand binding and virus binding, which shows how human immunodeficiency virus and rhinovirus fit well with the distinct structural feature of their cognate receptors.

Blockade of very late antigen-4 integrin binding to fibronectin in allograft recipients: I. Treatment with connecting segment-1 peptides prevents acute rejection by suppressing intragraft mononuclear cell accumulation, endothelial activation, and cytokine expression

BACKGROUND

Allograft rejection is associated with infiltration of inflammatory cells and local deposition of fibronectin (FN). This study was carried out to examine the hypothesis that peptides known to specifically block adhesive interactions between the connecting segment-1 (CS1)-binding domain of FN and alpha4beta1 integrin on circulating cells may interfere with the immune cascade, which would lead to acute rejection in transplant recipients.

METHODS AND RESULTS

Cardiac allografts from Lewis x Brown Norway F1 hybrids were rejected in 7+/-1 days in Lewis rats. Treatment with bioactive CS1 peptides (4 mg/kg/day i.v. for 7 days) abrogated acute rejection and prolonged cardiac allograft survival to 13+/-1 days (P<0.001). This effect correlated with decreased expression of total fibronectin and cell adhesion molecules, such as alpha4beta1, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, as well as reduced infiltration by CD4+ and CD8+ T cells at the graft site. Treatment with CS1 peptides decreased alloantigen activation, as evidenced by decreased intragraft infiltration by CD25+ cells, and diminished expression of mRNA coding for Th1 (interleukin [IL]-2, interferon-gamma)- and Th2 (IL-4, IL-5, IL-6)-type cytokines. CS1-mediated immunosuppressive effects could be reversed and acute rejection recreated after adjunctive treatment of rats with recombinant IL-2.

CONCLUSION

Our data are consistent with the model in which in vivo interaction between the alpha4beta1 integrin receptor and the cell-associated CS1 motif of FN is critical for rejection cascade. The novel therapeutic approach of selectively blocking the alpha4beta1-FN activation pathway with CS1 peptides prevents acute allograft rejection by inhibiting expansion of antigen-specific T cells and inducing a transient state of cytokine-responsive anergy in the residual T-cell population.

alpha 4 integrins and tumor metastasis

Taken together, alpha 4 integrins may influence metastatic process at various stages (Fig. 1). The detachment of tumor cells from the primary tumor and the invasion of the surrounding tissue represent the onset of tumor metastasis. There is good experimental evidence that at the primary tumor site expression of alpha 4 integrins inhibits the ability of melanoma cells to break loose. This could be achieved either by strengthening of homotypic adhesion to adjacent tumor cells or by down regulation of matrix metalloproteases that are required for tumor cell migration through the extracellular matrix. After entering the blood circulation, alpha 4 integrins on tumor cells derived from melanomas, sarcomas or lymphomas rather promote than inhibit accumulation of disseminated cells in distant organs. The positive effects of alpha 4 integrins at this stage of metastasis formation appear to depend on alpha 4 integrin interactions with ligands expressed on the surface of endothelial cells. While VCAM-1 is expressed on endothelial cells exposed to inflammatory cytokines, MAdCAM-1 is constitutively expressed on mucosal endothelium. In addition, it is conceivable that tumor cell aggregates trapped in the microcirculation may trigger local inflammatory reactions that result in VCAM-1 up-regulation. Tumor cell-bound alpha 4 integrins may strengthen adhesion to endothelium and promote trans-endothelial migration (HAUZENBERGER et al. 1997; MEERSCHAERT and FURIE 1994). Successful formation of new tumor colonies in distant organs is the final step in the metastatic cascade. Interestingly, alpha 4 integrin dependent mechanisms may either promote or inhibit this process. Thus, it was observed that alpha 4 integrins may direct cancer cells like CHO and lymphoma cells to organ compartments, where ligands for alpha 4 integrins are expressed (e.g., bone marrow). Depending on the tumor type this event may result in enhanced metastasis formation. However, as was documented for murine lymphoma cells alpha 4 integrins may also inhibit tumor cell growth either by inducing apoptosis or by reducing the proliferation rate. Based on numerous studies on human cancers and experimental tumor models, alpha 4 integrins may represent attractive target molecules for therapeutic manipulation of tumor cell behavior. To this end, however, it will be of great importance to precisely define the molecular basis for the adverse effects of alpha 4 integrins on metastasis formation.

Leukocyte adhesion molecules

Recruitment of leukocytes is critical to many of the processes studied in oral biology. With the development of new tools such as monoclonal antibody production and transgenic mice, the specific adhesion molecules thought to be important in leukocyte recruitment have been identified and their function examined. These molecules can be divided into three major classes: selectins, members of the immunoglobulin superfamily, and integrins. They mediate interactions between leukocytes and endothelial cells, facilitating the initial process of leukocyte rolling, firm attachment to endothelium, transendothelial migration, diapedesis, and migration along connective tissue. The goal of this paper is to provide an understanding of which molecules are involved in the above processes by discussing their cellular distribution, counter-receptors, and physiologic function.

Roles of adhesion molecules ICAM-1 and alpha4 integrin in antigen-induced changes in microvascular permeability associated with lung inflammation in sensitized brown Norway rats

Increased microvascular permeability and mucosal edema are pathological features of airway inflammation in asthma. In this study, we investigated the characteristics of the edema response occurring in a model of antigen-induced lung inflammation in sensitized brown Norway rats and examined the effects of monoclonal antibodies (mAbs) to adhesion molecules on this response. Ovalbumin (OA) challenge-induced increases in lung permeability were determined by the leakage of 125I-labeled bovine serum albumin (BSA) into the extravascular tissues of the lungs 24 h after challenge in animals intravenously injected (prechallenge) with this tracer. Inflammatory cell infiltration into the alveolar space was determined by bronchoalveolar lavage (BAL). Mean extravascular plasma volume in the lung increased 233% as compared with control (P < 0.005) at 24 h and increased to 517% by 72 h. The 24-h edema response was completely inhibited by two oral doses (0.1 mg/kg) of dexamethasone 1 h before, and 7 h after, challenge. Intraperitoneal administration of the anti-rat ICAM-1 mAb 1A29, or anti-rat alpha4 integrin mAb TA-2 (2 mg/kg at 12 and 1 h before, and 7 h after, antigen challenge), significantly suppressed eosinophil infiltration into the alveolar space without inhibiting the enhanced microvascular leakage and lung edema. Determination of plasma antibody concentrations by ELISA of mouse IgG1 indicated that sufficient concentrations of the appropriate mAb were present to block alpha4- or ICAM-1-dependent adhesion. The results suggest that increases in microvascular permeability and plasma leakage occurred independently of eosinophil accumulation.

Modifications of experimental bronchopulmonary hyperresponsiveness

Bronchopulmonary hyperresponsiveness (BHR) is a hallmark of asthma and other inflammatory diseases of the airways. Animal models of BHR are available in which systemic or local immunizations, followed by acute allergenic provocations into the airways, augment responses to intravenous or intratracheal nonspecific bronchoconstrictor agents. Guinea-pig models are easy to manipulate but have serious handicaps: lack of proper genetics, lack of biomolecular tools, and frequent excess of eosinophils in the bronchoalveolar lavage fluid (BALF). Murine models have proper genetics and molecular tools, and they have the further advantage of being widely used for the study of other pathologies. In many of these studies, interleukin (IL)-5 appears as a major cytokine, produced by Th2 lymphocytes. Interleukin-5 promotes eosinophil differentiation and maturation, recruitment to the airways, and possibly activation. The presence of eosinophils in the airways and in the BALF may be necessary but is not sufficient to support BHR, since intense eosinophilia may be present in its absence. Bronchopulmonary hyperresponsiveness is also induced by the administration of lipopolysaccharide (LPS); in that case, eosinophils are not involved, and the role of neutrophils and of tumor necrosis factor-alpha, even though likely, has not been proven. Comparison of BHR induced by allergen (Th2- and largely eosinophil-dependent) and by LPS (probably macrophage-dependent) should allow for a better understanding of the mechanisms of BHR and for the development of important remedies.

Obliterative bronchiolitis

Obliterative bronchiolitis following lung transplantation is common and potentially devastating. Its exact cause is undefined, but multiple immune and nonimmune processes contribute to its pathogenesis. Severe acute rejection and recurrent acute rejection have been shown to confer the greatest risk for obliterative bronchiolitis, signifying the central importance of alloimmunity in the disease process. Treatment of established disease with intensification of immune suppression has been of limited benefit, so current clinical strategies include early detection and minimization of risk. As our understanding of the disease evolves, it is hoped that effective interventions targeted at specific pathogenetic steps will emerge. In the meantime, obliterative bronchiolitis remains the most important and sinister long-term complication of lung transplantation.

Regulation of alpha 4 integrin-mediated adhesion of human eosinophils to fibronectin and vascular cell adhesion molecule-1

BACKGROUND

Eosinophils selectively accumulate at sites of allergic inflammation. Their recruitment is dependent on both the expression and functional activity of cell adhesion molecules. How the functional activity of cell adhesion molecules on eosinophils is regulated is poorly understood.

OBJECTIVE

Our objective was to examine the functional activity of alpha 4 integrins on human eosinophils and its regulation by various agents.

METHODS

Function of alpha 4 integrins on human eosinophils was examined by testing adhesion to immobilized fibronection and vascular cell adhesion molecule-1 (VCAM-1) in the presence or absence of a monoclonal antibody (mAb) (8A2) that activates beta 1 integrin function.

RESULTS

Spontaneous eosinophil adhesion to VCAM-1 was enhanced by 8A2, but adhesion to fibronectin could only be detected in the presence of 8A2. Concentrations of 8A2 that were approximately 100-fold less than saturating induced maximal eosinophil adhesion. Adhesion to VCAM-1 in the presence of 8A2 was effectively inhibited by alpha 4 and beta 1 integrin mAbs: beta 7 mAb had partial inhibitory activity. Connecting segment-1 peptide and alpha 4 mAb blocked 8A2-dependent fibronectin binding: beta 1, beta 2, and beta 7 integrin mAbs had partial inhibitory activity. Eosinophils obtained from bronchoalveolar lavage fluids and blood eosinophils stimulated with IL-5, platelet-activating factor, or RANTES displayed increased beta 2 integrin-dependent, not alpha 4 integrin-dependent, attachment. Spontaneous adhesion of eosinophils to VCAM-1 was significantly reduced by the tyrosine kinase inhibitor tyrphostin B46 (inhibitory concentration of 50% approximately equal to 20 mumol/L); this effect was reversed by 8A2.

CONCLUSIONS

The functional activity of integrins on eosinophils can be positively and negatively regulated. Altered integrin avidity may influence eosinophil recruitment in vivo.

Cytoplasmic tails of beta 1, beta 2, and beta 7 integrins differentially regulate LFA-1 function in K562 cells

The beta 2 integrin lymphocyte function-associated antigen 1 (LFA-1) mediates activation-dependent adhesion of lymphocytes. To investigate whether lymphocyte-specific elements are essential for LFA-1 function, we expressed LFA-1 in the erythroleukemic cell line K562, which expresses only the integrin very late antigen 5. We observed that LFA-1-expressing K562 cannot bind to intercellular adhesion molecule 1-coated surfaces when stimulated by phorbol 12-myristate 13-acetate (PMA), whereas the LFA-1-activating antibody KIM185 markedly enhanced adhesion. Because the endogenously expressed beta 1 integrin very late antigen 5 is readily activated by PMA, we investigated the role of the cytoplasmic domain of distinct beta subunits in regulating LFA-1 function. Transfection of chimeric LFA-1 receptors in K562 cells reveals that replacement of the beta 2 cytoplasmic tail with the beta 1 but not the beta 7 cytoplasmic tail completely restores PMA responsiveness of LFA-1, whereas a beta 2 cytoplasmic deletion mutant of LFA-1 is constitutively active. Both deletion of the beta 2 cytoplasmic tail or replacement by the beta 1 cytoplasmic tail alters the localization of LFA-1 into clusters, thereby regulating LFA-1 activation and LFA-1-mediated adhesion to intercellular adhesion molecule 1. These data demonstrate that distinct signaling routes activate beta 1 and beta 2 integrins through the beta-chain and hint at the involvement of lymphocyte-specific signal transduction elements in beta 2 and beta 7 integrin activation that are absent in the nonlymphocytic cell line K562.

VLA-4-mediated interactions between normal human hematopoietic progenitors and stromal cells

The alpha 4 beta 1 integrin very late activation antigen-4 (VLA-4) has been implicated to play a role in the adhesive interactions between hematopoietic progenitor cells (HPC) and bone marrow stromal cells which express the vascular cell adhesion molecule-1 (VCAM-1) or produce fibronectin (FN). Here, we summarize some of the recent advances made in the elucidation of the role of these particular adhesive interactions for the regulation of normal hematopoiesis. HPC bind to stroma mainly through VLA-4/VCAM-1 interactions. There is evidence which suggests that more primitive HPC constitutively express VLA-4 in a high-affinity state. In vitro studies in the mouse have shown that monoclonal antibodies (mAb) against VLA-4 partly block the development of lymphocytes, myelopoietic cells, and erythropoiesis, whereas in the human system outgrowth of TdT+ B cells is severely retarded by such mAb. In vivo studies revealed that VLA-4 is involved in erythropoietic development, and is particularly important for homing and lodgement of HPC in the bone marrow. Hematopoiesis in mice with deficient expression of alpha 4 integrin or VLA-4's ligand VCAM-1 appears to develop normally. However, chimeras developed from wild-type blastocysts and beta 1 -/- embryonic stem cells do not contain beta 1 -/- hematopoietic cells, although these are present as blood islands in the yolk sac. These beta 1 -/- hematopoietic cells are capable of forming colonies, indicating that beta 1-integrin is not involved in hematopoietic differentiation, but is primarily important for migration of hematopoietic cells into the fetal hematopoietic organs. In addition to the role of VLA-4 in migration, it may also have other regulatory functions. It has been demonstrated that ligation of VLA-4 induces phosphorylation of the protein tyrosine kinase (PTK) pp125FAK as well as other proteins which may be involved in the regulation of ligand affinity. Indeed, it has been shown that tyrosine kinase-dependent stimulation of CD34+ hematopoietic cell lines with c-kit ligand (KL), IL-3 or GM-CSF transiently activates the ability of VLA-4 to bind to VCAM-1 or FN. These events are most probably involved in the induction of quiescence in HPC which adhere to stromal cells. This claim was recently substantiated: when HPC were treated with Fab fragments of an anti-VLA-4 mAb, entry into S-phase of the cell cycle was prevented. Taken together, the present data point to a role for VLA-4 in HPC migration, cell cycle regulation, erythropoiesis and B-lymphopoiesis. Moreover, these insights may explain how defects in adhesive behavior of leukemic HPC through VLA-4 contribute to their dysregulated growth and provide a rationale for therapeutically correcting those defects.

A quantitative assay for intercellular aggregation

In an earlier communication (Munn et al., J Immunol. Methods 166: 11-25, 1993), we presented the initial development of a quantitative assay for monitoring the rates of cellular aggregation based on digital image processing and video microscopy. This study describes some important enhancements and modifications to the procedure. A new index is introduced to characterize the three-dimensional morphology of the aggregates. This index is based on temporal changes in the projected area of the cells and cell aggregates during the course of the experiment. By drawing an analogy with the kinetic theory of gases, we have also introduced a procedure to normalize for variations in cell seeding density among different experiments. In addition, the image analysis technique has been improved by introducing a background subtraction algorithm to remove illumination defects and an adaptive segmentation procedure. These improvements allowed us to completely automate the image analysis procedure, thus minimizing user intervention and improving the reproducibility of the measurements. The enhanced visual assay is evaluated using some recent results from our studies on homotypic lymphocyte aggregation.

Interaction of monocytoid cells with the mucosal addressin MAdCAM-1 via the integrins VLA-4 and LPAM-1

The differentiation of myeloid cells into macrophages and granulocytes is accompanied by marked changes in adhesive phenotype. Here we seek to understand the regulation of expression and functionality of the VLA-4 (alpha 4 beta 1), LPAM-1 (alpha 4 beta 7) and HML-1 (alpha E beta 7) integrins on monocytes/macrophages and granulocytes, given that these integrins including LFA-1 (alpha L beta 2) mediate the entry, retention and signalling events of pathogenic leucocytes within chronically inflamed tissues. Phorbol ester-induced monocytic differentiation of the promyelocyte cell line HL60 led to increases in the steady-state levels of beta 2 and beta 7 mRNA transcripts, requiring a period of 10 and 24 h, respectively, of de novo protein synthesis. There was a parallel de novo expression of LPAM-1 on the cell surface, despite the fact that alpha 4 mRNA transcripts were rapidly down-regulated. At 72 h, HML-1 was not coexpressed with LPAM-1 on HL60 cells, although it was weakly expressed on peripheral blood monocytes/macrophages after a prolonged period of in vitro culture. Retinoic acid-induced granulocytic differentiation of HL60 cells led to the appearance of low levels of LPAM-1 at the cell surface. LPAM-1 was not found expressed on peripheral blood neutrophils, raising the possibility that it is transiently expressed during granulocyte differentiation. In accord with the above findings, differentiated monocytes and HL60 cells bound to recombinant MAdCAM-1 in an alpha 4- and beta 7-integrin-dependent fashion, whereas a population of undifferentiated HL60 cells and Mn(+2)-activated monocytes bound in an alpha 4-integrin-dependent beta 7-integrin-independent manner via VLA-4 expressed abundantly at all stages of differentiation. Four h after attachment, some of these VLA-4+ LPAM-1- HL60 cells could be seen to start spreading. These finding suggest that MAdCAM-1 can bind to VLA-4 when LPAM-1 is absent, and thus has the potential to recruit both VLA-4-bearing monocytes and VLA-4+ LPAM-1+ macrophages into chronically inflamed tissues.

Multiple nuclear regulatory proteins bind a single cis-acting promoter element to control basal transcription of the human alpha 4 integrin gene in corneal epithelial cells

Expression of the fibronectin-binding integrin alpha 4 beta 1 has been postulated to be an important event in the process of corneal epithelial wound healing. In a previous study, we identified upstream positive and negative cis-acting regulatory elements that are needed to modulate the transcriptional activity of the human alpha 4 integrin subunit gene promoter in primary cultures of rabbit corneal epithelial cells. We have shown that most of the basal activity directed by this promoter was dependent on the presence of a cis-acting DNA sequence designated the alpha 4.1 element, centered at position -45 relative to the human alpha 4 mRNA start site. Here, we demonstrate that five distinct nuclear regulatory proteins (designated Bp1 to Bp5) from rabbit corneal epithelial cells possess the ability to bind the alpha 4.1 element in a specific manner in vitro. However, when they are combined together, only two of them (Bp2 and Bp5) retained their ability to interact with their specific target sequence in in vitro assays. The apparent molecular masses of the Bp1 to Bp5 proteins were determined and found to be of 91, 74, 59, 45, and 39 kD, respectively. Electrophoretic mobility-shift assays (EMSAs) indicated that only Bp2 also possesses the ability to bind the alpha 4.2 element, a site homologous to alpha 4.1 which plays a minor role in alpha 4 gene expression. Despite the presence of three Ets binding sites in the immediate vicinity of alpha 4.1, competition experiments in EMSA clearly indicate that Bp1, Bp2, Bp4, and Bp5 do not belong to the Ets family of transcription factors. Insertion of both alpha 4.1 and alpha 4.2 upstream from the basal promoter of the mouse p12 gene provided evidence that both elements have the ability to modulate basal expression driven from a heterologous promoter. alpha 4.1 was shown to function as an activator, whereas alpha 4.2 acted as a repressor in a manner that is dependent on its orientation, further stressing the critical regulatory function played by these two elements on alpha 4 gene basal expression.

Proteolysis regulates exposure of the IIICS-1 adhesive sequence in plasma fibronectin

The alternatively spliced type III connecting segment (IIICS) of fibronectin (Fn) contains an amino acid sequence, CS-1, which is recognized by the integrin receptor, alpha 4 beta 1. Plasma Fn inhibits alpha 4 beta 1-dependent binding of lymphocytes and monocytes to CS-1 containing Fn derivatives poorly, suggesting limited exposure of the CS-1 sequence in Fn. To test the availability of CS-1 in plasma Fn, an antibody was raised to the synthetic peptide CS-1. The CS-1 sequence was found to be minimally exposed in plasma Fn; and immobilization of Fn, a model of matrix deposition, caused only a modest increase in its exposure. Digestion of Fn with selected proteases, however, induced substantial expression of the CS-1 sequence. The acid protease cathepsin D generated fragments of 31-33.5 kDa from the COOH-terminal heparin-binding domain of Fn which possessed high immunoreactivity with anti-CS-1. Digestion of Fn with cathepsin B also resulted in the exposure of CS-1 sequence in a 140 kDa fragment. Although the digestion of Fn with neutral proteases (neutrophil elastase, cathepsin G, chymotrypsin, trypsin) generated fragments from the COOH-terminal heparin-binding domain of similar molecular weight as with cathepsin D, the exposure of CS-1 did not occur. Exposure of the CS-1 region by the cathepsins was supported by cell adhesion experiments; digestion of Fn with cathepsins D and B transformed inert plasma Fn to an effective inhibitor of adhesion of lymphoblastoid B and T cells (Ramos, Jurkat, Molt-4) to an immobilized CS-1 conjugate. These results suggest that exposure of the CS-1 sequence in plasma Fn by proteolysis with cathepsins D and B, enzymes implicated in several pathological processes, may serve a regulatory function in cell adhesion. The adhesive function of the CS-1 region in intact Fn appears to be suppressed by the native conformation of the molecule.

New insights into tumor-host interactions in lymphoma metastasis

The metastatic process is characterized by a complex series of sequential steps involving constant interactions (mutual "cross-talks") of metastasized tumor cells with their microenvironment (lymphocyte, macrophages, endothelial cells, etc.) in target organs. These interactions determine the outcome of metastasis (either the eradication of metastatic cells or their increased proliferation and invasion). Recently developed methods of tumor and host cell analysis at the molecular level allow better elucidation of molecular mechanisms of metastasis and of immune mechanisms involved in antitumor responses. Direct modulation of these processes will probably increase the success of clinical cancer treatment. Here we review data (a) on the expression of some costimulatory (MHC class II, CD80, sialoadhesin) and adhesion (LFA-1, ICAM-1, VLA-4) molecules on both metastasized tumor cells and host cells and (b) on the production of a cytotoxic molecule, nitric oxide, by in situ activated Kupffer and endothelial cells in the process of liver metastasis. This study was performed with well-characterized murine ESbL T lymphoma cells transduced with the bacterial lacZ gene, which allows detection and quantification of metastases at the single cell level throughout lymphoma growth and metastasis. Experimental results are discussed in the context of recent literature.

The genomic structure of the murine alpha 4 integrin gene

The VLA-4 (alpha 4 beta 1) integrin is a leukocyte glycoprotein involved in both cell-extracellular matrix and cell-cell interactions. We report here the cloning of the murine alpha 4 gene whose protein product is antigenically related to the human VLA-4 alpha chain. The alpha 4 m gene is about 75 kb long and consists of 28 exons, ranging in size from 46 bp (exon 13) to 437 bp (exon 1). The introns varied from 79 bp (intron 8) to more than 17 kb (intron 2). Three mRNA transcripts from this alpha 4 m gene can be visualized on Northern blot. After cloning the 3' untranslated region (3' UTR), four polyadenylation sites could be identified, presumably responsible for the presence of three to four transcripts of the alpha 4 gene, differing substantially in length.

The alpha 4-integrin supports leukocyte rolling and adhesion in chronically inflamed postcapillary venules in vivo

A role for the alpha 4-integrin (alpha 4 beta 1 or alpha 4 beta 7), has been implicated in the recruitment of peripheral blood mononuclear cells (PBMCs) to sites of inflammation. However, the adhesive interactions (i.e., tethering, rolling, and adhesion) mediated by the alpha 4-integrin have not been characterized in vivo. The objective of this study was to establish a model wherein postcapillary venules were chronically inflamed, and then use intravital microscopy to identify the adhesive interactions mediated by the alpha 4-integrin in vivo. Between 4 and 20 d after immunization with Mycobacterium butyricum, animals developed a systemic vasculitis characterized by large increases in the numbers of rolling and adhering leukocytes within mesenteric venules. The selectins could only account for approximately 50% of the leukocyte rolling whereas the remaining cells rolled exclusively via the alpha 4-integrin. Anti-alpha 4 therapy also eliminated the increase in leukocyte adhesion observed in this model, whereas selectin therapies and an anti-CD18 (beta 2-integrin) monoclonal antibody (mAb) did not reduce adhesion. A serum against polymorphonuclear leukocytes (PMNs) was used to confirm that a significant proportion of rolling cells, and most of the adhering cells were PBMCs. Sequential treatment with anti-PMN serum and the anti-alpha 4 mAb demonstrated that alpha 4-dependent rolling was distinct from PMN rolling populations. Initial leukocyte tethering via the alpha 4-integrin could not be demonstrated in this model, whereas L-selectin did support leukocyte tethering. These data suggest that the alpha 4-integrin can mediate both rolling and adhesion in the multistep recruitment of PMBCs in vivo, and these interactions occur independently of the selectins and beta 2-integrins.

Defense mechanisms in Peyer's patches and mesenteric lymph nodes against Yersinia enterocolitica involve integrins and cytokines

Adhesion molecules and cytokines are involved in regulation of cellular host responses in infection processes. In this study the roles of the integrins Mac-1 and VLA-4, as well as those of the cytokines tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma), in defense mechanisms against Yersinia enterocolitica in Peyer's patches (PP) and mesenteric lymph nodes (MLN) were investigated by blocking these molecules with antibodies in vivo prior to orogastric Yersinia infection. Intestinal Yersinia infection caused abscesses composed of polymorphonuclear (Mac-1+ VLA-4+ Pgp-1+ ICAM-1-) and mononuclear (Mac-1+ VLA-4+ Pgp-1+ ICAM-inhibited phagocytosis of yersiniae by macrophages, (ii) reduced Yersinia-specific proliferation and IFN-gamma production of T cells from PP and MLN, and (iii) caused increased bacterial growth in PP and MLN followed by profound tissue destruction. Neutralization of TNF-alpha or IFN-gamma had comparable effects, suggesting that cell-mediated host responses including activated macrophages are required for control of yersiniae in intestinal tissues. The number of Mac-1+ cells in PP and MLN increased after yersinia infection, and recruitment of these cells was not blocked by administration of anticytokine or anti-integrin antibodies. While anti-VLA-4, -TNF-alpha, or -IFN-gamma antibody treatment caused an increased dissemination of yersiniae from PP to the spleen systemic dissemination was reduced by anti-Mac-1 antibodies. The results of this study suggest that the cytokines IFN-gamma and TNF-alpha as well as the integrins Mac-1 and VLA-4 are involved in protective cellular host defense mechanisms in PP and MLN against Y. enterocolitica, the latter probably being involved in both cell-cell and cell-pathogen interactions.

The platelet cytoskeleton stabilizes the interaction between alphaIIbbeta3 and its ligand and induces selective movements of ligand-occupied integrin

Previously, we showed that a subpopulation of the major platelet integrin, alphaIIbbeta3, co-sediments from detergent lysates with talin and other membrane skeleton proteins. Once alphaIIbbeta3 has bound adhesive ligand in a platelet aggregate, the detergent-insoluble alphaIIbbeta3 redistributes (along with the detergent-insoluble membrane skeleton proteins and a variety of signaling molecules) to a fraction that contains cytoplasmic actin filaments. Concomitantly, certain signaling molecules are activated. The present study shows that, in intact platelets, alphaIIbbeta3 forms clusters when occupied by ligand and is selectively moved into the open canalicular system; alphaIIbbeta3 that has not bound ligand remains diffusely distributed at the periphery of the cell. When cytoplasmic actin filaments are depolymerized by cytochalasins, the ability of alphaIIbbeta3 to bind ligand is decreased, and the movement of ligand-occupied alphaIIbbeta3 is prevented. Together with the previous findings, these results suggest that (i) membrane skeleton-associated alphaIIbbeta3 is selectively induced to bind ligand in activated platelets, (ii) ligand-induced transmembrane signaling causes an altered association of membrane skeleton-associated alphaIIbbeta3 with the cytoplasmic component of the cytoskeleton, (iii) ligand-induced cytoskeletal reorganizations stabilize the interaction between ligand and integrin, and (iv) ligand-occupancy triggers cytoskeletal reorganizations that result in selective movements of occupied ligand.

A region of the integrin VLA alpha 4 subunit involved in homotypic cell aggregation and in fibronectin but not vascular cell adhesion molecule-1 binding

The VLA-4 (alpha 4 beta 1) integrin is involved in the adhesion of cells to fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In order to study alpha 4 structure-function relationships, we have expressed mutated alpha 4 subunit by transfection into VLA-4-negative K562 cells. Substitutions at alpha 4 residues Arg89-Asp90, which show the highest surface probability indexes inside the N-terminal alpha 4/80 fragment, resulted in a reduction in the reactivity of all anti-alpha 4 epitope A monoclonal antibodies (mAbs) tested, compared with the reactivity with anti-alpha 4 epitopes B1, B2, and C mAb, both by transfectant flow cytometry, and by immunoprecipitation and SDS-polyacrylamide gel electrophoresis analysis of transfectant surface-iodinated proteins. In contrast, substitutions at nearby residues, Gln101, Pro102, and Ile105 did not affect the reactivity of any anti-alpha 4 mAb representing the known alpha 4 epitopes. Homotypic cell aggregation triggered by anti-alpha 4 epitope A mAb was prevented in the transfectants expressing mutated alpha 4 Arg89-Asp90Asp residues, while cell aggregation was fully achieved with either anti-alpha 4 epitope B2 or anti-beta 1 mAb. Mutations at alpha 4 residues Gln101, Pro102, and Ile108 did not affect the homotypic cell aggregation of the transfectants expressing these mutations. In addition, the adhesion of mutant Arg89-Asp90 alpha 4 transfectants to the connecting segment-1-containing fibronectin-40 (FN-40) fragment of fibronectin was diminished compared to wild type alpha 4 transfectants, as well as to other mutant alpha 4 transfectants. This adhesion to FN-40 was restored when the activating anti-beta 1 TS2/16 mAb was present in the adhesion assays. In contrast, adhesion to VCAM-1 was not affected by mutations at Arg89-Asp90, nor at Gln101, Pro102, and Ile108 alpha 4 residues. Altogether, these results indicate that alpha 4 residues Arg89 and Asp90 are included in a region involved in homotypic cell aggregation, as well as in adhesion to FN-40, but not to VCAM-1.

Integrin alpha 4 cysteines 278 and 717 modulate VLA-4 ligand binding and also contribute to alpha 4/180 formation

Here we describe experiments in which we mutated four of the six integrin alpha 4 subunit cysteine residues that are not present in most other integrin alpha subunits that lack an I domain. In four different types of ligand binding assay we found that optimal integrin alpha 4 beta 1 and/or to CS1 peptide required the presence of both alpha 4 Cys 278 and Cys 717. In addition, optimal ligand binding required divalent cations and reduced cysteines, as evidenced by EDTA and N-ethylmaleimide inhibition results. In a control experiment, an alpha 4 mutation that completely eliminated the alpha 4 80/70 proteolytic cleavage site had no effect on ligand binding. Notably, although Cys 278 an Cys 717 mutations markedly altered ligand binding, they had no adverse effect on cell adhesion. Thus, compared with cell adhesion, ligand binding is a distinct and apparently more stringent test of VLA-4 integrin-ligand interactions. In addition, we have established that the formation of the previously described alpha 4/180 [Parker, Pujades, Brenner and Hemler (1993) J. Biol. Chem. 268, 7028-2035] also requires Cys 278 and Cys 717, divalent cations and reduced cysteines. thus alpha 4/180 appears to be more functionally relevant than alpha 4/150.

Endothelin-1 modulates the expression of adhesion molecules on fibroblast-like synovial cells (FLS)

Endothelin-1 is known to possess various biological properties. In the present study we have investigated the effects of Endothelin-1 (Et-1) on the expression of adhesion molecules ICAM-1, VCAM-1 and CD-44 by fibroblast-like synoviocytes. Cultured fibroblast-like synoviocytes were treated with Et-1 in the absence or presence of C1306, a specific endothelin-A-receptor antagonist. Cell surface expression of ICAM-1, VCAM-1 and CD-44 was determined by immunofluorescence studies, Cyto-ELISA and FACS-analysis. ICAM-1, VCAM-1 and CD-44 were constitutively expressed on cultured FLS. After incubation with Et-1 the expression of ICAM-1, VCAM-1 and CD-44 increased. The level of expression of adhesion molecules after Et-1 stimulation was similar to cytokine mediated effects (IL-1 beta, TNF-alpha). IL-1 beta showed the strongest stimulatory effect on the expression of ICAM-1, TNF-alpha preferentially induced the expression of VCAM-1 and CD-44, and Et-1 strongly stimulated the upregulation of CD-44 expression. In addition, Et-1 enhanced significantly the IL-1 beta mediated upregulation of VCAM-1 expression, whereas TNF-alpha mediated expression of VCAM-1 was downregulated by Et-1. Furthermore, the Et-1 induced expression of adhesion molecules on FLS was mediated via the endothelin-A-receptor (EtA-receptor), since C-1306, a selective endothelin-A-receptor antagonist, could block this effect. These results indicate that Et-1 has stimulating effects on FLS in vitro. The expression of adhesion molecules can be upregulated by Et-1 similar to proinflammatory cytokines. The modulating effect of Et-1 can be inhibited by the pretreatment with a selective EtA-receptor antagonist. Thus, Et-1 may have immunoregulatory functions in the recruitment of cells infiltrating the inflamed tissue.

Carboxyfluorescein diacetate labeling does not affect adhesion molecule expression or function in human neutrophils or eosinophils

Fluorescently labeled leukocytes are commonly used in in vitro and in vivo experimental systems. However, the effects of fluorescent labeling on the expression and function of leukocyte adhesion molecules has not been examined in part because the extreme intensity of fluorescence tends to obscure signals from other fluorochromes used for dual color analysis. We have utilized a novel technique involving a 7-amino-4-methylcoumarin-3-acetic acid (AMCA) fluorophore-conjugated F(ab')2 fragment excitable in the ultraviolet wavelength range (350-450 nm) and dual-laser flow cytometry to determine if labeling of human neutrophils and eosinophils with the fluorescent dye 5-(6)-carboxyfluorescein diacetate (CFDA) alters surface expression of the primary leukocyte adhesion molecules involved in leukocyte-endothelial interactions. Simultaneously, adhesion molecule function was assessed by comparing the ability of CFDA-labeled vs. control cells to adhere to cultured human umbilical vein endothelial cells (HUVEC) and purified immobilized adhesion molecules. Isolated human eosinophils and neutrophils were fluorescently labeled by incubation with CFDA. Flow cytometric comparisons of labeled and unlabeled cells demonstrated that fluorescence labeling of neutrophils and eosinophils with CFDA did not alter basal surface expression of the beta 2 integrins (i.e., CD11a CD11b or, CD18). Stimulation of neutrophils with fMLP and eosinophils with PMA resulted in increased surface expression of CD11b and CD18 which was not altered by CFDA labeling. Likewise, CFDA labeling of neutrophils and eosinophils did not significantly alter their integrin-dependent adhesion to activated HUVEC under static or rotational conditions. Similarly, adhesion to immobilized recombinant E- and P-selectin was unaltered. These data demonstrate that fluorescent labeling of human neutrophils and eosinophils with CFDA does not alter surface expression or function of several adhesion molecules necessary for leukocyte-endothelial interactions. The use of CFDA-labeled cells in experiments employing intravital microscopy should therefore provide valid information on adhesion molecule function in vivo.

Mononuclear leukocytes invade rabbit arterial intima during thickening formation via CD18-and VLA-4-dependent mechanisms and stimulate smooth muscle migration

The role of mononuclear leukocytes for the migration of smooth muscle cells (SMCs) during intimal thickening was investigated in the rabbit model of electrically stimulated carotid artery. The approach was to inhibit leukocyte entry into the arterial intima with antibodies against the adhesion molecules very late activation antigen-4 (VLA-4) and CD11/CD18. In electrically stimulated control rabbits treated either with saline or a nonspecific antibody, all types of granulocytes, monocytes, and lymphocytes migrated across an intact endothelium into the acellular subendothelial space, followed by the movement of SMCs from the media into the intima within 36 hours of applying electrical current. Treatment of the rabbits with monoclonal antibody (mAb) HP1/2 directed toward the alpha 4 subunit (CD49d) of VLA-4 inhibited mononuclear leukocyte invasion (consisting of monocytes and lymphocytes) by approximately 70% compared with the IgG-treated control rabbits and completely abolished the minimal influx of basophils and eosinophils after 36 hours. Neutrophil infiltration, however, remained unaffected by anti-VLA-alpha 4 treatment. Under these conditions, SMC migration across the internal elastic lamina was reduced by 50%. The use of mAb HP1/2 together with mAb 60.3 (directed to the beta 2 chain of CD11/CD18) completely abolished the influx of monocytes, lymphocytes, and all types of granulocytes into the arterial intima. This complete blockade of leukocyte infiltration resulted in a 70% reduction of intimal SMC accumulation. Together with our previous findings excluding neutrophils as stimulators of SMC migration, the present results indicate that mononuclear leukocyte promote lesion development by stimulating SMC migration.

Interaction of the cell-binding domain of fibronectin with VLA-5 integrin induces monokine production in cultured human monocytes

The effect of fibronectin on IL-1 alpha, IL-1 beta, tumour necrosis factor-alpha (TNF-alpha), and IL-6 production was investigated with cultured monocytes isolated from human peripheral blood. Monokine concentrations were determined by both ELISA and bioassay. Fibronectin markedly stimulated the secretion of IL-1 alpha, IL-1 beta, TNF-alpha and IL-6 from cultured monocytes in a dose-dependent manner, with the maximal effect apparent within 24 h. Northern blot analysis revealed a marked increase in the abundance of mRNA specific for each monokine on exposure of monocytes to fibronectin. Monoclonal antibodies to the alpha chain of very late antigen (VLA)-5, the beta 1 integrin, the alpha chain of Mac-1, and the beta 2 integrin, as well as the synthetic peptide of GRGDSP (which corresponds to the cell-binding domain of fibronectin), inhibited (> 50%) fibronectin-induced monokine production. Monoclonal antibodies to the alpha chain of VLA-4, and the alpha chain of LFA-1, as well as the synthetic peptide CS-1 (which corresponds to the alternatively spliced connecting segment of fibronectin) and the control peptide GRADSP, had no inhibitory effect on monokine production. A MoAb, R60, that recognizes an epitope of the fibronectin molecule that includes the RGD sequence, inhibited monokine production, whereas the MoAb Y16, which recognizes another epitope of fibronectin not including RGD, did not. These results indicate that fibronectin-induced production of IL-1 alpha, IL-1 beta, TNF-alpha and IL-6 from cultured monocytes is mediated predominantly by interaction of the cell-binding domain of fibronectin with VLA-5, although Mac-1 also may contribute to this effect of fibronectin. Our results indicate that the interaction of fibronectin with integrins may contribute to the cytokine network in inflammatory response.

MCP-1-stimulated monocytes preferentially utilize beta 2-integrins to migrate on laminin and fibronectin

Recruitment of monocytes to inflammatory sites involves a series of sequential attachments and detachments to extracellular matrix proteins in response to a chemoattractant gradient. In this study we compared the migration of human peripheral blood monocytes on different extracellular matrix proteins in response to monocyte chemoattractant protein-1 (MCP-1) and N-formylmethionyl-leucyl-phenylalanine. Monocytes migrated more effectively on laminin compared with other extracellular matrix proteins. In contrast, this preference was not observed with neutrophils, suggesting that the monocytes and neutrophils may have differences in their migration on extracellular matrix proteins. To study this further, function-blocking monoclonal antibodies were used to examine mechanistically whether beta 1- and beta 2-integrins were involved in monocyte migration on fibronectin or laminin in response to MCP-1. Monocyte migration on both laminin and fibronectin was blocked 100% (P < 0.05) by intact monoclonal antibody, F(ab') fragments, and F(ab')2 fragments to beta 2-integrins. We also determined that antibodies to beta 2-integrins block monocyte migration that has already been initiated. In contrast, antibody to the beta 1-integrins inhibited monocyte migration by approximately 40% (P < 0.05). Thus monocytes that express both beta 1- and beta 2-integrins require utilization of beta 2-integrins in migration on extracellular matrix proteins. The results also suggest that beta 1-integrins facilitate monocyte migration but that monocyte migration is not absolutely dependent on the interaction of beta 1-integrins with extracellular matrix proteins. In contrast, neutrophil migration is beta 2-integrin dependent and is not facilitated by beta 1-integrins.

Differential regulation of alpha 4 integrin-dependent binding to domains 1 and 4 of vascular cell adhesion molecule-1

The vascular cell adhesion molecule-1 (VCAM-1) plays an important role in diverse physiological and pathological processes. The homologous first and fourth immunoglobulin-like domains of the seven domain form of VCAM-1 present binding motifs for alpha 4 beta 1 integrin. Using a panel of VCAM-1 domain deletion mutants we show that alpha 4 beta 7 integrin interacts with both domains 1 and 4. In contrast to their identical domain usage, alpha 4 beta 1 and alpha 4 beta 7 integrins differ in the activation states required for binding to domains 1 and 4 of VCAM-1. We show that integrin alpha 4 beta 1 required significantly higher concentrations of Mn2+ than integrin alpha 4 beta 7 to support half-maximal adhesion to domain 4. Moreover, a clear difference in the capacity of integrins alpha 4 beta 1 and alpha 4 beta 7 to interact with domain 4 was detected in the presence of Ca2+ and Mg2+ cations. Adhesion to domain 1 of VCAM-1, however, was not affected by integrin heterodimer composition. Instead, the activity level of integrin alpha 4 beta 1 for domain 1 binding was regulated by CD24 expression. Binding to seven domain VCAM-1 was not altered significantly by beta 1 and beta 7 subunits or CD24. These data indicate that integrin heterodimer composition and CD24 expression differentially modulate integrin binding to domains 1 and 4 of VCAM-1. Mechanisms that alter integrin binding specificity or monovalent versus divalent interactions may affect the strength of adhesion as well as signal transmission in adherent cells and may therefore be critical to controlling the cellular response to integrin occupancy.

Possible significance of VLA-4 (alpha 4 beta 1) for hematogenous metastasis of renal-cell cancer

Very late antigen-4 (VLA-4) composed of alpha 4 and beta 1, a member of the beta 1-integrin subfamily, facilitates cell-to-cell interaction with vascular cell-adhesion molecule-1 (VCAM-1) on endothelial cells (EC). Attachment of blood-borne tumor cells to EC is a crucial step for hematogenous metastasis, and VLA-4-positive tumor cells can attach to EC by binding to VCAM-1. Renal-cell cancer (RCC) reveals proportionally greater percentages of metastases than other carcinomas at initial diagnosis. We investigated whether VLA-4 is expressed on RCC, and how such expression on RCC correlates with the metastatic potential of RCC. Immunohistochemical staining on 66 primary and 4 metastatic RCC showed that 4 out of 4 metastatic and 5 out of 8 primary RCC from patients with lung and/or brain metastases expressed alpha 4 and beta 1 chains. On the other hand, 13 of 58 RCC without metastases expressed alpha 4 chain, alpha 4 and beta 1 expressions were also detected on 5 out of 5 human RCC cell lines, ACHN, KRC/Y, A498, Caki1 and Caki2, by flow-cytometric analysis. Reverse-transcriptase-polymerase-chain-reaction (RT-PCR), followed by Southern-blot hybridization with cDNA probe for a alpha 4 chain, also confirmed mRNA production in 4 out of 5 RCC cell lines. Furthermore, adhesion of alpha 4-positive RCC cell lines to human umbilical-vein endothelial cells (HUVEC) was augmented by treatment of HUVEC with tumor necrosis factor-alpha (TNF-alpha). This adhesion was inhibited by anti-alpha 4 or anti-VCAM-1 antibodies, suggesting that VLA-4-VCAM-1 interaction was involved in the adhesion between RCC cells and HUVEC. Taken together, VLA-4 on RCC cells might play a crucial role in their hematogenous metastasis.

Synovial cells are potent antigen-presenting cells for superantigen, staphylococcal enterotoxin B (SEB)

There is ample evidence suggesting that superantigens may act as a triggering factor in the pathogenesis of rheumatoid arthritis (RA). We investigated whether superantigen could activate T cells in the presence of synovial cells. T cells were cultured with SEB in the presence of interferon-gamma (IFN-gamma)-treated synovial cells. T cell proliferation and activation were assessed by 3H-thymidine incorporation and IL-2 production. The expression of HLA class II antigens and adhesion molecules on synovial cells was detected by flow cytometer. In the presence of IFN-gamma-treated synovial cells, T cells proliferated vigorously and produced IL-2 in response to SEB. A low SEB-induced T cell response was noticed in the presence of untreated synovial cells. Allogeneic as well as autologous IFN-gamma-treated synovial cells markedly enhanced SEB-induced T cell proliferation. IFN-gamma-treated synovial cells had increased expression of HLA class II antigens and intercellular adhesion molecule-1 (ICAM-1) adhesion molecules. MoAbs towards these antigens markedly inhibited the SEB-induced T cell response. These results indicate that activated synovial cells are potent antigen-presenting cells for SEB to T cells, and that superantigens may play a critical role in the pathogenesis of RA through activated synovial cells.