Rat mAbs were raised against murine intercellular adhesion molecule-2 (ICAM-2). Immune precipitation and purification reveal that the murine ICAM-2 glycoprotein is 55 kDa and is similar in size to human ICAM-2. ICAM-2 is expressed on a variety of leukocyte cell lines, including T and B lymphoma, mastocytoma, and macrophage lines. ICAM-2 is well expressed on endothelioma cell lines, and in contrast to ICAM-1, expression is not increased by inflammatory cytokines. One of the mAb to ICAM-2 partially or completely inhibits binding of cells expressing LFA-1 to purified ICAM-2, and binding of cells expressing ICAM-2 to purified LFA-1. The findings in the mouse are congruent with those in the human, suggesting functional conservation of ICAM-2 across species.

We isolated a partial genomic clone encoding ITGAD, a novel beta 2-integrin alpha subunit. The ITGAD gene is highly homologous to the three previously known alpha subunit-encoding genes, that compose the beta 2 integrin family, in deduced amino acid sequence, intron/exon structure and mapping location (chromosome 16p11).

The CC chemokines regulated on activation normal T expressed and secreted (RANTES) and monocyte chemotactic protein 3 (MCP-3), and the anaphylatoxin C5a, induce activation, degranulation, chemotaxis, and transendothelial migration of eosinophils. Adhesion assays on purified ligands showed differential regulation of beta 1 and beta 2 integrin avidity in eosinophils. Adhesiveness of VLA-4 (alpha 4 beta 1, CD29/CD49d) for vascular cell adhesion molecule 1 or fibronectin was rapidly increased but subsequently reduced by RANTES, MCP-3, or C5a. The deactivation of VLA-4 lead to cell detachment, whereas phorbol 12-myristate 13-acetate induced sustained activation of VLA-4. In contrast, chemoattractants stimulated a prolonged increase in the adhesiveness of Mac-1 (alpha M beta 2, CD11b/CD18) for intercellular adhesion molecule 1. Inhibition by pertussis toxin confirmed signaling via G protein-coupled receptors. Chemoattractants induced transient, while phorbol 12-myristate 13-acetate induced sustained actin polymerization. Disruption of actin filaments by cytochalasins inhibited increases in avidity of VLA-4 but not of Mac-1. Chemoattractants did not upregulate a Mn2+-inducible beta 1 neoepitope defined by the mAb 9EG7, but induced prolonged expression of a Mac-1 activation epitope recognized by the mAb CBRM1/5. This mAb inhibited chemoattractant-stimulated adhesion of eosinophils to intercellular adhesion molecule 1. Thus, regulation of VLA-4 was dependent on the actin cytoskeleton, whereas conformational changes appeared to be crucial for activation of Mac-1. To our knowledge, this is the first demonstration that physiological agonists, such as chemoattractants, can differentially regulate the avidity of a beta 1 and a beta 2 integrin expressed on the same leukocyte.

We investigated the role of neutrophil microvilli in interactions with E-selectin and P-selectin in hydrodynamic shear flow by disruption with cytochalasin B, hypotonic swelling, and chilling. Cytochalasin B only marginally reduced microvilli numbers (from 30 +/- 6 to 16 +/- 6 per cell perimeter, p < 0.005) as shown by electron microscopy, completely disrupted tethering in shear flow to E-selectin and P-selectin, increased the strength of rolling adhesions on E-selectin and P-selectin, and increased cell deformability in shear flow with a likely increase in the area of cell:substrate contact. Hypoosmotic swelling markedly reduced microvilli number (to 6 +/- 5 per perimeter, p < 0.005), almost completely inhibited tethering on E- and P-selectin, and increased the strength of rolling adhesions on P-selectin but not on E-selectin. Chilling almost completely abolished microvilli (to 3 +/- 3 per perimeter, p < 0.005), but pseudopod-like structures were present, and had little effect on tethering in flow. Immunogold labeling of L-selectin, which is normally clustered on tips of microvilli, showed that in the absence of microvilli it remained in small clusters. Our studies show that alterations in cell morphology and viscoelasticity can have opposing effects on tethering and rolling, showing that they are independently regulatable. Furthermore, our results suggest that the association of molecules that mediate rolling with microvilli tips may be important not just to enhance presentation, but for other functions such as to promote resistance to extraction from the membrane or cooperative interactions among clustered receptors.

BACKGROUND:Epithelial dysfunction and patient symptoms in inflammatory intestinal diseases such as ulcerative colitis and Crohn's disease correlate with migration of neutrophils (PMN) across the intestinal epithelium. In vitro modeling of PMN transepithelial migration has revealed distinct differences from transendothelial migration. By using polarized monolayers of human intestinal epithelia (T84), PMN transepithelial migration has been shown to be dependent on the leukocyte integrin CD11b/CD18 (Mac-1), but not on CD11a/CD18 (LFA-1). Since intercellular adhesion molecule-I (ICAM-1) is an important endothelial counterreceptor for these integrins, its expression in intestinal epithelia and role in PMN-intestinal epithelial interactions was investigated.
MATERIALS AND METHODS: A panel of antibodies against different domains of ICAM-1, polarized monolayers of human intestinal epithelia (T84), and natural human colonic epithelia were used to examine the polarity of epithelial ICAM-1 surface expression and the functional role of ICAM-1 in neutrophil-intestinal epithelial adhesive interactions.
RESULTS: While no surface expression of ICAM-1 was detected on unstimulated T84 cells, interferon-gamma (IFN gamma) elicited a marked expression of ICAM-1 that selectively polarized to the apical epithelial membrane. Similarly, apically restricted surface expression of ICAM-1 was detected in natural human colonic epithelium only in association with active inflammation. With or without IFN gamma pre-exposure, physiologically directed (basolateral-to-apical) transepithelial migration of PMN was unaffected by blocking monoclonal antibodies (mAbs) to ICAM-1. In contrast, PMN migration across IFN gamma-stimulated monolayers in the reverse (apical-to-basolateral) direction was inhibited by anti-ICAM-1 antibodies. Adhesion studies revealed that T84 cells adhered selectively to purified CD11b/CD18 and such adherence, with or without IFN gamma pre-exposure, was unaffected by ICAM-1 mAb. Similarly, freshly isolated epithelial cells from inflamed human intestine bound to CD11b/CD18 in an ICAM-1-independent fashion.
CONCLUSIONS: These data indicate that ICAM-1 is strictly polarized in intestinal epithelia and does not represent a counterreceptor for neutrophil CD11b/CD18 during physiologically directed transmigration, but may facilitate apical membrane-PMN interactions after the arrival of PMN in the intestinal lumen.

Chemotactic factors are postulated to direct emigration of lymphocytes from the blood stream into sites of inflammation. Members of a family of chemotactic cytokines, termed chemokines, have been shown to attract lymphocytes but efficacy, i.e., the maximal percentage of attracted cells, has been low. We have identified a highly efficacious lymphocyte chemotactic activity in the supernatants of the murine bone marrow stroma cell line MS-5 which attracts 10-fold more lymphocytes in vitro than currently described lymphocyte chemoattractants. Purification of this chemotactic activity revealed identity to stromal cell-derived factor 1 (SDF-1). SDF-1 acts on lymphocytes and monocytes but not neutrophils in vitro and is both a highly efficacious and highly potent mononuclear cell attractant in vivo. In addition, SDF-1 induces intracellular actin polymerization in lymphocytes, a process that is thought to be a prerequisite for cell motility. Since SDF-1 is expressed constitutively in a broad range of tissues it may have a role in immune surveillance and in basal extravasation of lymphocytes and monocytes rather than in inflammation.

We have compared the ability of human alpha/beta and gamma/delta T lymphocytes to adhere to selectin-bearing substrates, an interaction thought to be essential for homing and localization at sites of inflammation. Both T cell populations form rolling adhesions on E- and P-selectin substrates under physiologic flow conditions. Although equivalent to alpha/beta T cells in binding to E-selectin, gamma/delta T cells demonstrated greater ability to adhere to P-selectin that was purified or expressed on the surface of activated, adherent platelets. Under static conditions, 80% of gamma/delta T cells and 53% of alpha/beta T cells formed shear-resistant adhesions to P-selectin, whereas only 30% of gamma/delta and alpha/beta T cells adhered to E-selectin. The enhance ability of gamma/delta T cells to adhere to P-selectin cannot be attributed to differences in expression of the P-selectin glycoprotein ligand (PSGL-1), as all alpha/beta T cells versus approximately 75% of gamma/delta T cells expressed PSGL-1. Both cell populations expressed a similar percentage of the carbohydrate antigens sialyl LewisX and cutaneous lymphocyte-associated antigen. Depletion of lymphocyte populations or T cell clones bearing these oligosaccharides with the monoclonal antibody CSLEX-1 and HECA-452, respectively, resulted in a substantial reduction in adhesion to E-selectin and slight reduction in adhesion to P-selectin under flow conditions. Treatment of cells with an endopeptidase that selectively degrades O-sialomucins such as PSGL-1, abolished P-selectin but not E-selectin adhesion. Removal of terminal sialic acids with neuraminidase or protease treatment of cells abrogated cell adhesion to both selectin substrates. These results provide direct evidence for the presence of distinct E- and P-selectin ligands on T lymphocytes and suggest that gamma/delta T cells may be preferentially recruited to inflammatory sites during the early stages of an immune response when P-selectin is upregulated.

We demonstrate an additional step and a positive feedback loop in leukocyte accumulation on inflamed endothelium. Leukocytes in shear flow bind to adherent leukocytes through L-selectin/ligand interactions and subsequently bind downstream and roll on inflamed endothelium, purified E-selectin, P-selectin, L-selectin, VCAM-1, or peripheral node addressin. Thus adherent leukocytes nucleate formation of strings of rolling cells and synergistically enhance leukocyte accumulation. Neutrophils, monocytes, and activated T cell lines, but not peripheral blood T lymphocytes, tether to each other through L-selectin. L-selectin is not involved in direct binding to either E- or P-selectin and is not a major counterreceptor of endothelial selectins. Leukocyte-leukocyte tethers are more tolerant to high shear than direct tethers to endothelial selectins and, like other L-selectin-mediated interactions, require a shear threshold. Synergism between leukocyte-leukocyte and leukocyte-endothelial interactions introduces novel regulatory mechanisms in recruitment of leukocytes in inflammation.

Acute neutrophil (PMN) recruitment to postischemic cardiac or pulmonary tissue has deleterious effects in the early reperfusion period, but the mechanisms and effects of neutrophil influx in the pathogenesis of evolving stroke remain controversial. To investigate whether PMNs contribute to adverse neurologic sequelae and mortality after stroke, and to study the potential role of the leukocyte adhesion molecule intercellular adhesion molecule-1 (ICAM-1) in the pathogenesis of stroke, we used a murine model of transient focal cerebral ischemia consisting of intraluminal middle cerebral artery occlusion for 45 min followed by 22 h of reperfusion. PMN accumulation, monitored by deposition of 111In-labeled PMNs in postischemic cerebral tissue, was increased 2.5-fold in the ipsilateral (infarcted) hemisphere compared with the contralateral (noninfarcted) hemisphere (P < 0.01). Mice immunodepleted of neutrophils before surgery demonstrated a 3.0-fold reduction in infarct volumes (P < 0.001), based on triphenyltetrazolium chloride staining of serial cerebral sections, improved ipsilateral cortical cerebral blood flow (measured by laser Doppler), and reduced neurological deficit compared with controls. In wild-type mice subjected to 45 min of ischemia followed by 22 h of reperfusion, ICAM-1 mRNA was increased in the ipsilateral hemisphere, with immunohistochemistry localizing increased ICAM-1 expression on cerebral microvascular endothelium. The role of ICAM-1 expression in stroke was investigated in homozygous null ICAM-1 mice (ICAM-1 -/-) in comparison with wild-type controls (ICAM-1 +/+). ICAM-1 -/- mice demonstrated a 3.7-fold reduction in infarct volume (P < 0.005), a 35% increase in survival (P < 0.05), and reduced neurologic deficit compared with ICAM-1 +/+ controls. Cerebral blood flow to the infarcted hemisphere was 3.1-fold greater in ICAM-1 -/- mice compared with ICAM-1 +/+ controls (P < 0.01), suggesting an important role for ICAM-1 in the genesis of postischemic cerebral no-reflow. Because PMN-depleted and ICAM-1-deficient mice are relatively resistant to cerebral ischemia-reperfusion injury, these studies suggest an important role for ICAM-1-mediated PMN adhesion in the pathophysiology of evolving stroke.

Studies in the rat have pointed to a role for intercellular adhesion molecule-1 (ICAM-1) in the pathogenesis of acute tubular necrosis. These studies used antibodies, which may have nonspecific effects. We report that renal ICAM-1 mRNA levels and systemic levels of the cytokines IL-1 and TNF-alpha increase 1 h after ischemia/ reperfusion in the mouse. We sought direct proof for a critical role for ICAM-1 in the pathophysiology of ischemic renal failure using mutant mice genetically deficient in ICAM-1. ICAM-1 is undetectable in mutant mice in contrast with normal mice, in which ICAM-1 is prominent in the endothelium of the vasa recta. Mutant mice are protected from acute renal ischemic injury as judged by serum creatinine, renal histology, and animal survival . Renal leukocyte infiltration, quantitated morphologically and by measuring tissue myeloperoxidase, was markedly less in ICAM-1-deficient than control mice. To evaluate whether prevention of neutrophil infiltration could be responsible for the protection observed in the mutant mice, we treated normal mice with antineutrophil serum to reduce absolute neutrophil counts to < 100 cells/mm3. These neutrophil-depleted animals were protected against ischemic renal failure. Anti-1CAm-1 antibody protected normal mice against renal ischemic injury but did not provide additional protection to neutrophil-depleted animals. Thus, ICAM-1 is a key mediator of ischemic acute renal failure likely acting via potentiation of neutrophilendothelial interactions.

Intercellular adhesion molecule 3 (ICAM-3; CD50) is the predominant counter-receptor on resting T cells and monocytes for the leukocyte integrin, lymphocyte function-associated antigen 1 (LFA-1; CD11a/CD18), and may play an important role in the initial stages of the T cell-dependent immune response. Deletion of individual immunoglobulin superfamily (IgSF) domains of ICAM-3 and ICAM-3 IgSF domain chimeras with CD21 showed there is a single LFA-1 binding site in ICAM-3 and that IgSF domain 1 is necessary and sufficient for LFA-1 binding. Epitope mapping and functional studies performed with 17 anti-ICAM-3 monoclonal antibodies demonstrated that only some monoclonal antibodies, with epitopes wholly within domain 1 of ICAM-3, were able to block binding of ICAM-3 bearing cells to purified LFA-1, in agreement with the data obtained from the domain deletion mutants and CD21 chimeras. Analysis of a panel of 45 point mutants of domain 1 of ICAM-3 identified five residues that may contact LFA-1 as part of the binding site, Asn23, Ser25, Glu37, Phe54, and Gln75. These five residues are predicted by molecular modeling, based on the structure of vascular cell adhesion molecule 1 (VCAM-1), to cluster in two distinct locations on domain 1 of ICAM-3 on the BED face (Asn23 and Ser25) and on the C strand or CD loop (E37), the E strand (F54), and the FG loop (Q75). The residues, Asn23 and Ser25, comprise a consensus N-linked glycosylation site.

Chemokines are chemotactic cytokines that activate and direct the migration of leukocytes. There are two subfamilies, the CXC and the CC chemokines. We recently found that the CXC-chemokine stromal cell-derived factor-1 (SDF-1) is a highly efficacious lymphocyte chemoattractant. Chemokines act on responsive leukocyte subsets through G-protein-coupled seven-transmembrane receptors, which are also used by distinct strains of HIV-1 as cofactors for viral entry. Laboratory-adapted and some T-cell-line-tropic (T-tropic) primary viruses use the orphan chemokine receptor LESTR/fusin (also known as fusin), whereas macrophage-tropic primary HIV-1 isolates use CCR-5 and CCR-3 (refs 7-11), which are receptors for known CC chemokines. Testing of potential receptors demonstrated that SDF-1 signalled through, and hence 'adopted', the orphan receptor LESTR, which we therefore designate CXC-chemokine receptor-4 (CXCR-4). SDF-1 induced an increase in intracellular free Ca2+ and chemotaxis in CXCR-4-transfected cells. Because SDF-1 is a biological ligand for the HIV-1 entry cofactor LESTR, we tested whether it inhibited HIV-1. SDF-1 inhibited infection by T-tropic HIV-1 of HeLa-CD4 cells, CXCR-4 transfectants, and peripheral blood mononuclear cells (PBMCs), but did not affect CCR-5-mediated infection by macrophage-tropic (M-tropic) and dual-tropic primary HIV-1.

High endothelial venules (HEV) are specialized plump postcapillary venules in lymphoid tissues that support high levels of lymphocyte extravasation from the blood. We have recently identified a novel human transcript, expressed to high levels in HEV, that encodes a secreted, acidic protein closely related to the anti-adhesive extracellular matrix protein known as BM-40, osteonectin, and SPARC (secreted protein acidic and rich in cysteine). Here, we show that this protein, designated hevin, is associated with basal, lateral, and apical surfaces of HEV cells, and unlike MECA-79 antigen, is not expressed on the underlying basement membrane. In contrast to fibronectin or other adhesive extracellular matrix proteins, purified hevin does not support endothelial cell adhesion in vitro. Moreover, addition of soluble exogenous hevin inhibits attachment and spreading of endothelial cells on fibronectin substrates. Hevin-treated cells do not form focal adhesions and exhibit a rounded morphology. Together, these results suggest that hevin is an abundant extracellular protein that modulates high endothelial cell adhesion to the basement membrane.

Pre-B-cell growth-stimulating factor/ stromal cell-derived factor 1 (PBSF/SDF-1) is a member of the CXC group of chemokines that is initially identified as a bone marrow stromal cell-derived factor and as a pre-B-cell stimulatory factor. Although most chemokines are thought to be inducible inflammatory mediators, PBSF/SDF-1 is essential for perinatal viability,. B lymphopoiesis, bone marrow myelopoiesis, and cardiac ventricular septal formation, and it has chemotactic activities on resting lymphocytes and monocytes. In this paper, we have isolated a cDNA that encodes a seven transmembrane-spanning-domain receptor, designated pre-B-cell-derived chemokine receptor (PB-CKR) from a murine pre-B-cell clone, DW34. The deduced amino acid sequence has 90% identity with that of a HUMSTSR/fusin, a human immunodeficiency virus 1 (HIV-1) entry coreceptor. However, the second extracellular region has lower identity (67%) compared with HUMSTSR/fusin. PB-CKR is expressed during embryo genesis and in many organs and T cells of adult mice. Murine PBSF/SDF-1 induced an increase in intracellular free Ca2+ in DW34 cells and PB-CKR-transfected Chinese hamster ovary (CHO) cells, suggesting that PB-CKR is a functional receptor for murine PBSF/SDF-1. Murine PBSF/ SDF-1 also induced Ca2+ influx in fusin-transfected CHO cells. On the other hand, considering previous results that HIV-1 does not enter murine T cells that expressed human CD4, PB-CKR may not support HIV-1 infection. Thus, PB-CKR will be an important tool for functional mapping of HIV-1 entry coreceptor fusin and for understanding the function of PBSF/SDF-1 further.

Platelets bound to thrombogenic surfaces have been shown to support activation-dependent firm adhesion of neutrophils in flow following selectin-mediated tethering and rolling. The specific receptor(s) responsible for mediating adhesion-strengthening interactions between neutrophils and platelets has not previously been identified. Furthermore, the ability of adherent platelets to support the migration of bound neutrophils has not been tested. We studied neutrophil interactions with activated, surface-adherent platelets as a model for leukocyte binding in vascular shear flow and emigration at thrombogenic sites. Our results demonstrate that the beta 2-integrin Mac-1 (CD11b/CD18) is required for both firm attachment to and transmigration of neutrophils across surface-adherent platelets. In flow assays, neutrophils from patients with leukocyte adhesion deficiency-1 (LAD-I), which lack beta 2-integrin receptors, formed P-selectin-mediated rolling interactions, but were unable to develop firm adhesion to activated platelets, in contrast to healthy neutrophils, which developed firm adhesion within 5 to 30 seconds after initiation of rolling. Furthermore, the adhesion-strengthening interaction observed for healthy neutrophils could be specifically inhibited by monoclonal antibodies (mAbs) to Mac-1, but not to lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) or intercellular adhesion molecule-2 (ICAM-2; CD102). Further evidence for a beta 2-integrin-dependent neutrophil/platelet interaction is demonstrated by the complete inhibition of interleukin (IL)-8-induced neutrophil transmigration across platelets bound to fibronectin-coated polycarbonate filters by mAbs to Mac-1. Thus, Mac-1 is required for firm adhesion of neutrophils to activated, adherent platelets and may play an important role in promoting neutrophil accumulation on and migration across platelets deposited at sites of vascular injury.

Circulating lymphocytes gain access to lymph nodes owing to their ability to initiate rolling along specialized high endothelial venules (HEVs). One mechanism of rolling involves L-selectin binding to peripheral node addressin (PNAd) on HEVs. Activated platelets are shown to bind to circulating lymphocytes and to mediate rolling in HEVs, in vivo, through another molecule, P-selectin, which also interacts with PNAd. In vitro, activated platelets enhanced tethering of lymphocytes to PNAd and sustained lymphocyte rolling, even in the absence of functional L-selectin. Thus, a platelet pathway operating through P-selectin provides a second mechanism for lymphocyte delivery to HEVs.

L-Selectin is a leukocyte cell adhesion receptor that contributes to neutrophil (PMN) rolling on activated endothelium at sites of inflammation and mediates lymphocyte attachment to high endothelial venules in peripheral lymph nodes. Localization of this receptor to the tips of PMN and lymphocyte microvilli has been demonstrated. However, its distribution on these cells has not been quantified, and its localization on other leukocytes and the morphometry of microvilli on different leukocyte subpopulations have not been previously examined. In this study, PMN and mononuclear leukocytes were isolated from anticoagulated blood by dextran sedimentation and density centrifugation, fixed in 2% paraformaldehyde and 0.05% glutaraldehyde, immunogold-labeled for L-selectin, and embedded in Epon resin. The distribution of L-selectin was determined by counting gold particles on the plasma membrane of sectioned cells, and the surface microstructure of these cells was surveyed on two-dimensional transmission electron micrographs. On average, 78% of PMN, 72% of monocyte, and 71% of lymphocyte L-selectin was observed on the microvilli, with more variance on lymphocytes than the other cell types. Typical PMN and monocyte sections had 26 microvilli, whereas typical lymphocyte sections had 23. Quantitation of the distribution of L-selectin and leukocyte surface topology offers a foundation from which to study the requirement of microvilli or microvillus-localized L-selectin for leukocyte tethering and rolling in model systems that mimic microvascular environments.

Intercellular adhesion molecule 1 (ICAM-1) is a member of the immunoglobulin superfamily that interacts with two integrins, LFA-1 and Mac-1. These interactions are critical for leukocyte extravasation into inflamed tissue. To assess the role of ICAM-1 expression in the pathogenesis of bacterial infection, homozygously mutant mice lacking the ICAM-1 gene were exposed to Staphylococcus aureus. Within 6 days after inoculation 50% of the animals in the ICAM-1(-/-) group, but none of the controls, had died. Despite the high level of mortality, ICAM-1(-/-) mice developed less frequent and less severe arthritis than their wild-type littermates. In agreement, normal mice inoculated with staphylococci and administered anti-ICAM-1 antibodies exhibited a higher frequency of mortality but less severe arthritis than the controls. Our results indicate that ICAM-1 on the one hand provides protection against systemic disease but on the other hand aggravates the local disease manifestation.

Selectins are a family of lectins, that mediate tethering and rolling of leukocytes on endothelium in vascular shear flow. Mild periodate oxidation of the L-selectin ligand CD34, or L-selectin ligands on leukocytes, enhanced resistance to detachment in shear and decreased rolling velocity equivalent to an 8-fold increase in ligand density, yet had little effect on the rate of tethering. Enhanced interactions were also seen with mildly oxidized sialyl Lewisa and sialyl Lewisx glycolipids. Enhancement was completely reversed by borohydride reduction, yielding a strength of interaction equivalent to that with the native ligands. No effect on the strength of P-selectin and E-selectin interactions was seen after mild oxidation of their ligands. Completeness of modification of sialic acid by mild periodate was verified with monoclonal antibody to sialyl Lewisx-related structures and resistance to neuraminidase. The addition of cyanoborohydride to leukocytes rolling through L-selectin on mildly oxidized but not native CD34 caused arrest of rolling cells and formation of EDTA-resistant bonds to the substrate, suggesting that a Schiff base was reduced. Cyanoborohydride reduction of mildly oxidized cells rolling on P-selectin and E-selectin also caused arrest and formation of EDTA-resistant bonds but with slower kinetics. These data suggest that interactions with a sialic acid aldehyde group on mildly oxidized ligands that include interconversion to a Schiff base can occur with three selectins yet only stabilize binding through the selectin with the fastest koff, L-selectin.

Leukocyte emigration possibly requires dynamic regulation of integrin adhesiveness for endothelial and extracellular matrix ligands. Adhesion assays on purified vascular cell adhension molecule (VCAM)-1, fibronectin, and fibronectin fragments revealed distinct kinetic patterns for the regulation of very late antigen (VLA)-4 (alpha 4 beta 1) and VLA-5 (alpha 5 beta 1) avidity by the CC chemokines monocyte inflammatory protein (MIP)-1 alpha, RANTES (regulated on activation, normal T expressed and secreted), or monocyte chemoattractant protein (MCP)-1 in monocytes. CC chemokines induced early activation and subsequent deactivation of VLA-4, whereas upregulation of VLA-5 avidity occurred later and persisted. Controlled detachment assays in shear flow suggested that adhesive strength of VLA-4 for VCAM-1 or the 40-kD fragment of fibronectin (FN40) is more rapidly increased and subsequently reduced by MCP-1 than by MIP-1 alpha, and confirmed late and sustained activation of the adhesive strength of VLA-5 for the 120-kD fragment of fibronectin (FN120). Mn2+ or the stimulating beta 1 mAb TS2/16 strongly and stably enhanced monocyte binding to VCAM-1 or fibronectin, and locked beta 1 integrins in a high avidity state, which was not further modulated by CC chemokines. Mn2+ and mAb TS2/16 inhibited CC chemokine-induced transendothelial migration, particularly chemotaxis across stimulated endothelium that involved VLA-4 and VCAM-1. VLA-4 on Jurkat cells is of constitutively high avidity and interfered with migration across barriers expressing VCAM-1. Low but not high site densities of VCAM-1 or FN40 promoted, while FN120 impaired, beta 1 integrin-dependent monocyte chemotaxis to MCP-1 across filters coated with these substrates. Thus, we show that CC chemokines can differentially and selectively regulate avidity of integrins sharing common beta subunits. Transient activation and deactivation of VLA-4 may serve to facilitate transendothelial diapedesis, whereas late and prolonged activation of VLA-5 may mediate subsequent interactions with the basement membrane and extracellular matrix.