The leukocyte function-associated antigen-1 (LFA-1)-binding site on ICAM-3 comprises residues on both faces of the first immunoglobulin domain

ICAM-3 (CD50), a member of the Ig superfamily, is a major ligand for the leukocyte integrin LFA-1 (CD11a/CD18). This interaction represents one of several Ig superfamily/integrin ligand-receptor pairs that have been described to date. ICAM-3 is highly expressed on resting leukocytes and on APCs. In addition to an adhesive function, ICAM-3 can act as a signal-transducing molecule on T cells, providing a costimulatory signal for cell proliferation. Eighteen point mutations in ICAM-3 were generated, and residues important for binding of functional blocking Abs were identified. Mutation of seven of the residues reduced or abrogated adhesion to LFA-1, including three residues that are located on strand A of the ABED face of domain 1. In contrast, extensive mutagenesis analysis of ICAM-1 has shown that only residues on the GFC face interact with LFA-1. Our results provide evidence for a more extensive binding interface between ICAM-3 and LFA-1 than has previously been described. ICAM-3 appears to be unique among the ICAMs in utilizing residues on both faces of domain 1 for interaction with its ligand LFA-1.     

The role of LFA-1 (CD11a/CD18) cytoplasmic domains in binding to intercellular adhesion molecule-1 (CD54) and in postreceptor cell spreading

We have investigated the role of the cytoplasmic domains of LFA-1 in binding to ICAM-1 and in postadhesion events. Various truncated and chimeric forms of LFA-1 alpha (CD11a) and beta (CD18) chains were generated and transfected into murine fibroblast TNR-2 cells. Transfected fibroblasts expressing wild-type LFA-1 adhered only weakly to ICAM-1 immobilized on plastic, and phorbol ester pretreatment enhanced this adhesion significantly. In contrast, transfected cells expressing LFA-1 lacking both the alpha and the beta cytoplasmic domains, the beta cytoplasmic domain alone, or GPI-anchored LFA-1 adhered to immobilized ICAM-1 without prior activation. Truncation of the alpha cytoplasmic domain alone resulted in much reduced cell adhesion which could be only weakly upregulated by PMA. The presence of manganese dramatically enhanced the binding to ICAM-1 of LFA-1 lacking the alpha cytoplasmic domain or both cytoplasmic domains, whereas it had relatively little effect on wild-type LFA-1 or the mutant lacking the beta cytoplasmic domain. Soluble LFA-1, generated by phosphatidylinositol-specific phospholipase-C treatment of GPI-anchored LFA-1, was capable of binding ICAM-1+ cells. Although doubly truncated or GPI-anchored LFA-1 mediated cell adhesion to immobilized ICAM-1, cells expressing these mutants, as well as those expressing individual alpha and beta chain truncations, failed to spread out following this adhesion, whereas the wild-type transfectants did so readily. Manganese had no effect on cell spreading. Fluorescent staining of these cells indicated no significant variation in the distribution of LFA-1 on the cell surface. From these results we conclude that (1) cells expressing LFA-1 lacking both the alpha and the beta cytoplasmic domains adhere to ICAM-1 without prior stimulation, indicating the importance of LFA-1 cytoplasmic domains in inside-out signaling, (2) truncation of the alpha cytoplasmic domain alone inhibits cell adhesion by making LFA-1 nonresponsive to inside-out signaling, and (3) both cytoplasmic domains are required for cell spreading following adhesion to immobilized ICAM-1.     

Dominant-negative effect of the lymphocyte function-associated antigen-1 beta (CD18) cytoplasmic domain on leukocyte adhesion to ICAM-1 and fibronectin

The cytoplasmic domains of LFA-1 (CD11a/CD18) are thought to play an important role in the regulation of LFA-1 function. To further elucidate the role of the LFA-1 cytoplasmic domains, we transfected chimeric proteins consisting of the extracellular domain of CD4 fused with the transmembrane and cytoplasmic domains of LFA-1 into T and B cell lines, EL-4 and A20, respectively, and examined their effects on LFA-1-mediated cell adhesion. The CD4/18, but not CD4/11a, chimera profoundly inhibited LFA-1-mediated cell adhesion to ICAM-1, as well as cell spreading following cell adhesion. Unexpectedly, cell adhesion to fibronectin was also inhibited by the CD4/18 chimera. The CD4/18 chimera did not affect the expression of endogenous LFA-1 or the association of CD11a and CD18. Truncation of the carboxyl-terminal 13 amino acid residues of the CD18 cytoplasmic domain of the chimera completely abrogated the inhibitory effect on LFA-1. Among these amino acid residues, the carboxyl-terminal six residues were dispensable for the inhibitory effect in EL-4 cells, whereas it significantly reduced the inhibitory activity of CD4/18 in A20 cells. A larger truncation of the CD18 cytoplasmic domain was needed to fully abrogate the inhibitory effects of CD4/18 on the adhesion to fibronectin. These results show that 1) the CD4/18 chimera has dominant-negative effects on cell adhesion mediated by LFA-1 as well as fibronectin receptors, and 2) amino acid residues of the CD18 cytoplasmic domain involved in the inhibition of LFA-1 seem to be different from those for fibronectin receptors.     

Cloning and sequencing of a cDNA encoding bovine intercellular adhesion molecule 3 (ICAM-3)

A cDNA encoding a putative bovine intercellular adhesion molecule (ICAM)-3, a ligand of the leukocyte integrin LFA-1 (CD11a/CD18), was sequenced and compared with human ICAM sequences. The 1635-bp bovine sequence codes for a protein of 544 amino acids (aa). This putative bovine ICAM-3 has five immunoglobulin (Ig)-like domains similar to human ICAM-1 and ICAM-3, and belongs to the Ig gene superfamily. The overall identities of the deduced aa sequence with those of human ICAM-3 and ICAM-1 are 61% and 58%, respectively. The predicted number and positions of Cys residues are all conserved between the bovine and human ICAM 3 aa sequences.     

Contribution of CD11a/CD18, CD11b/CD18, ICAM-1 (CD54) and -2 (CD102) to human monocyte migration through endothelium and connective tissue fibroblast barriers

Recently we reported that monocyte migration through a barrier of human synovial fibroblasts (HSF) is mediated by the CD11/CD18 (beta2) integrins, and the beta1 integrins VLA-4 and VLA-5 on monocytes. Here we investigated in parallel the role of beta2 integrin family members, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) on monocytes, and the immunoglobulin supergene family members, ICAM-1 and ICAM-2 on HSF and on human umbilical vein endothelial cells (HUVEC), in monocyte migration through HSF and HUVEC monolayers. Using function blocking monoclonal antibodies (mAb), when both VLA-4 and VLA-5 on monocytes were blocked, treatment of monocytes with mAb to both LFA-1 and to Mac-1 completely inhibited monocyte migration across HSF barriers, although blocking either of these beta2 integrins alone had no effect on migration, even when VLA-4 and VLA-5 were blocked. This indicates that optimal beta2 integrin-dependent monocyte migration in synovial connective tissue may be mediated by either LFA-1 or Mac-1. Both ICAM-1 and ICAM-2 were constitutively expressed on HSF and on HUVEC, although ICAM-2 was only minimally expressed on HSF. Based on results of mAb blockade, ICAM-1 appeared to be the major ligand for LFA-1-dependent migration through the HSF. In contrast, both ICAM-1 and ICAM-2 mediated LFA-1-dependent monocyte migration through HUVEC. However, neither ICAM-1 nor ICAM-2 was required for Mac-1 -dependent monocyte migration through either cell barrier, indicating that Mac-1 can utilize ligands distinct from ICAM-1 and ICAM-2 on HSF and on HUVEC during monocyte transmigration.     

Increased LFA-1-mediated homotypic cell adhesion is associated with the G1 growth arrest induced by rapamycin in a T cell lymphoma

The immunosuppressive macrolide, rapamycin, impedes the G1 to S cell cycle progression in cytokine-stimulated normal lymphocytes and in certain autonomously proliferating cell lines. Here, we found that the rapamycin-induced growth arrest augments homotypic aggregation in the YAC-1 T cell lymphoma. The growth arrest and increased aggregation were both blocked by the rapamycin antagonist, L-685,818, which interacts with the intracellular binding proteins mediating rapamycin's biochemical action. Moreover, rapamycin-induced aggregation was not seen in YAC-1 cells mutants selected for resistance to the drug's antiproliferative effect. Although the inhibition of G1/S progression induced by serum deprivation also resulted in increased cellular aggregation, cell cycle blockade in late G1 by mimosine, early S phase by hydroxyurea, or G2/M by nocodazole all failed to do so. Furthermore, the aggregation induced by rapamycin was blocked by antibodies to the alpha (CD11a) or beta (CD18) subunits of the integrin, LFA-1, or to its ligands, ICAM-1 and ICAM-2, and did not occur in LFA-1-deficient YAC mutants. However, the surface expression of LFA-1, ICAM-1, or ICAM-2 was not augmented in cells aggregated by rapamycin. Finally, the serine/threonine protein phosphatase inhibitor, okadaic acid, was found to abrogate rapamycin-induced aggregation. Therefore, rapamycin's impairment of YAC-1 cell growth in G1 is accompanied by enhanced LFA-1-mediated homotypic cell adhesion that may reflect an increase of the integrin's avidity for its ligands and may involve protein phosphorylation/dephosphorylation events. This suggests the existence of a link between cell cycle progression and "inside-out" LFA-1 signaling, possibly regulated by rapamycin's biochemical targets.     

Inhibition and stimulation of LFA-1 and Mac-1 functions by antibodies against murine CD18. Evidence that the LFA-1 binding sites for ICAM-1, -2, and -3 are distinct

The murine CD18 monoclonal antibody (mAb) M18/2 was reported to inhibit lymphoma metastasis [Zahalka, M. A. et al. (1993) J. Immunol. 150, 4466]. To identify the pathways potentially blocked, we studied the effects of M18/2 compared with two new mAb against murine CD18, GAME-46, and -245. Whereas the GAME mAb blocked most Mac-1-mediated interactions, M18/2 had no effect, or even stimulated. The same was true for adhesion of LFA-1 to ICAM-1. To test effects on interactions with different ICAMs, we used L cells transfected with human ICAM-1, -2, and -3. As previously described, mouse LFA-1 does not bind to human ICAM-1 but we show here that mouse LFA-1 does bind to human ICAM-2 and -3. Again, the GAME mAb blocked completely, but M18/2 did not. These results indicate that the LFA-1 binding sites for ICAM-1 and ICAM-2 and -3, although in close vicinity, are distinct. Furthermore, effects of M18/2 on metastasis cannot be ascribed to blocking of any known beta2-integrin activity.     

The I domain of integrin LFA-1 interacts with ICAM-1 domain 1 at residue Glu-34 but not Gln-73

Using a solid phase assay, we show that isolated LFA-1 I domain binds ICAM-1 in a Mg2+-dependent manner and is blocked by anti-I domain monoclonal antibodies. This activity mirrors that of the intact receptor (Dransfield, I., Caba?as, C., Craig, A., and Hogg, N. (1992) J. Cell Biol. 116, 219-226) and suggests that the I domain controls divalent cation-dependent receptor function. In ICAM-1, domain 1 residues Glu-34 and Gln-73 have been identified as critical for binding of LFA-1 as an intact receptor (Staunton, D. E., Dustin, M. L., Erickson, H. P., and Springer, T. A. (1990) Cell 61, 243-254). For the first time, we show that isolated I domain binds to domain 1 of ICAM-1 and that this interaction is inhibited partially by mutation of Glu-34 but not by Gln-73. The anti-ICAM-1 monoclonal antibody RR1/1, which maps to Gln-73 (Staunton, D. E., Dustin, M. L., Erickson, H. P., and Springer, T. A. (1990) Cell 61, 243-254), enhances I domain binding, suggesting potential allosteric control or coordinate binding by this region. Finally, I domain binding inhibited by Glu-34 ICAM-1 mutation correlates with divalent cation dependence, indicating that this residue might be in direct contact with the metal ion-dependent adhesion site. Thus, we describe the interaction between the LFA-1 I domain and ICAM-1, an event that controls the function of the intact receptor but includes only part of the complete ligand binding site.     

Molecular comparison of soluble intercellular adhesion molecule (sICAM)-1 and sICAM-3 binding to lymphocyte function-associated antigen-1

The interactions of intercellular adhesion molecules-1 and -3 (ICAM-1 and ICAM-3) with lymphocyte function-associated antigen-1 (LFA-1) have been characterized and compared on the molecular and cellular level. Enzyme-linked immunosorbent-based molecular assays have been utilized to calculate the binding affinities of soluble ICAM-1 (sICAM-1) and soluble ICAM-3 (sICAM-3) for LFA-1. Consistent with previously published data, we found that sICAM-1 binds to LFA-1 with an affinity of approximately 60 nM. In contrast, sICAM-3 binds to LFA-1 with an affinity approximately 9 times weaker ( approximately 550 nM). Both sICAM-1 and sICAM-3 require divalent cations for binding. Specifically, both Mg2+ and Mn2+ support high affinity adhesion, although interestingly, high concentrations of Ca2+ decrease the affinity of each molecule for LFA-1 substantially. Furthermore, a panel of anti-LFA-1 monoclonal antibodies were characterized for their ability to block sICAM-1 and sICAM-3/LFA-1 interactions in molecular and cellular assays to help distinguish binding sites on LFA-1 for both molecules. Finally, molecular and cellular competition experiments demonstrate that sICAM-1 and sICAM-3 compete with each other for binding to LFA-1. The above data demonstrate that sICAM-1 and sICAM-3 share a common binding site or an overlapping binding site on LFA-1 and that the apparent differences in binding sites can be attributed to different affinities of sICAM-1 and sICAM-3 for LFA-1.     

A novel LFA-1 activation epitope maps to the I domain

A panel of 21 alpha-subunit (CD11a) and 10 beta-subunit (CD18) anti-LFA-1 mAbs was screened for ability to activate LFA-1. A single anti-CD11a mAb, MEM-83, was identified which was able to directly induce the binding of T cells to purified ICAM-1 immobilized on plastic. This ICAM-1 binding could be achieved by monovalent Fab fragments of mAb MEM-83 at concentrations equivalent to whole antibody, was associated with appearance of the "activation reporter" epitope detected by mAb 24, and was completely inhibited by anti-ICAM-1 and LFA-1 blocking mAbs. The epitope recognized by mAb MEM-83 was distinct from that recognized by mAb NKI-L16, an anti-CD11a mAb previously reported to induce LFA-1 activation, in that it was constitutively present on freshly isolated peripheral blood mononuclear cells and was not divalent cation dependent for expression. The ICAM-1 binding activity induced by mAb MEM-83 was, however, dependent on the presence of Mg2+ divalent cations. Using an in vitro-translated CD11a cDNA deletion series, we have mapped the MEM-83 activation epitope to the "I" domain of the LFA-1 alpha subunit. These studies have therefore identified a novel LFA-1 activation epitope mapping to the I domain of LFA-1, thereby implicating this domain in the regulation of LFA-1 binding to ICAM-1.     

CD50 monoclonal antibodies inhibit neutrophil activation

The constitutive high expression of CD50 (ICAM-3) on resting leukocytes, coupled with the observation that CD50 is the primary LFA-1 ligand on resting T cells, suggests that CD50 may be an important LFA-1 ligand in the initiation of the immune/inflammatory response. CD50 mAbs have been reported to increase homotypic adhesion of lymphocytes, and lymphocyte adhesion to HUVEC and extracellular matrix proteins. In this study, the effects of CD50 mAbs on neutrophil activation were examined. CD50 mAbs were found to inhibit neutrophil adhesion induced by FMLP and 12-O-tetradecanoyl-phorbol-13-acetate to resting and TNF-activated HUVEC. CD50 mAbs also inhibited neutrophil adhesion stimulated by CD66a, CD66b, CD66c, and CD66d mAbs to HUVEC. CD50 mAbs inhibited the up-regulation of CD11b/CD18 to the neutrophil surface, and the down-regulation of surface CD62L expression. The potential contribution of src family kinases to the previously described tyrosine kinase activity associated with CD50 in neutrophils was also examined. hck and lyn were found to account for much of the tyrosine kinase activity associated with CD50 in neutrophils. The data indicate that CD50 in neutrophils functions not only as a potential ligand for LFA-1, but also regulates the surface expression and activity of CD11b/CD18 and CD62L. In contrast to the effects in lymphocytes, CD50 appears to function as a negative regulator of neutrophil activation.     

Characteristics of cation binding to the I domains of LFA-1 and MAC-1. The LFA-1 I domain contains a Ca2+-binding site

The crystal structures of the I domains of integrins MAC-1 (alphaM beta2; CD11b/CD18) and LFA-1 (alphaL beta2; CD11a/CD18) show that a single conserved cation-binding site is present in each protein. Purified recombinant I domains have intrinsic ligand binding activity, and in several systems this interaction has been demonstrated to be cation-dependent. It has been proposed that the I domain cation-binding site represents a general metal ion-dependent adhesion motif utilized for binding protein ligands. Here we show that the purified recombinant I domain of LFA-1 (alphaLI) binds cations, but with significantly different characteristics compared with the I domain of MAC-1 (alphaMI). Both alphaLI and alphaMI bind 54Mn2+ in a conformation-dependent manner, and in general, cations with charge and size characteristics similar to Mn2+ most effectively inhibit 54Mn2+ binding. Surprisingly, however, physiological levels of Ca2+ (1-2 mM) inhibited 54Mn2+ binding to purified alphaLI, but not to alphaMI. Using 45Ca2+ and 54Mn2+ in direct binding studies, the dissociation constants (KD) for the interactions between these cations and alphaLI were estimated to be 5-6 x 10(-5) and 1-2 x 10(-5) M, respectively. Together with the available structural information, the data suggest differential affinities for Mn2+ and Ca2+ binding to the single conserved site within alphaLI. Antagonism of LFA-1, but not MAC-1, -mediated cell adhesion by Ca2+ may be related to the Ca2+ binding activity of the LFA-1 I domain.     

Involvement of lymphocyte function-associated antigen-1 and intercellular adhesion molecule-1 in osteoclastogenesis: a possible role in direct interaction between osteoclast precursors

In our search for molecules involved in the process of osteoclast differentiation, we examined the surface phenotypes of the preosteoclast-like cells and osteoclast-like multinucleated cells (MNCs) formed in bone marrow cultures, using monoclonal antibodies recognizing different antigen molecules expressed on hematopoietic cells. Among these cell surface antigens, lymphocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) were highly expressed on mononuclear cells in the cultures for forming preosteoclast-like mononuclear cells. The double detection of these two antigen molecules with osteoclast-specific antigen and with calcitonin receptor, using a fluorescence-activated cell sorter or autoradiography technique, revealed that LFA-1 and ICAM-1 were expressed on the preosteoclasts. The expression of ICAM-1 was detected on both preosteoclasts and osteoclast-like MNCs, whereas the expression of LFA-1 was restricted to preosteoclasts. We designed a peptide with the sequence of the binding site of ICAM-1 against the ligand LFA-1. In the whole bone marrow culture system for forming osteoclast-like MNCs, a significant inhibition of MNC formation was observed by the addition of this peptide. These results strongly suggest the involvement of an LFA-1/ICAM-1-interaction in osteoclastogenesis.     

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The expression of adhesion molecules was studied on CD34+ hematopoietic precursors in cord blood, bone marrow and mobilized blood. The samples were labeled in a double immunofluorescence procedure with a CD34 monoclonal antibody and with antibodies against maturation and differentiation antigens and adhesion molecules. Myeloid precursors formed the majority of the CD34+ cells in all samples. In bone marrow a separate cluster of B-cell precursors with low forward scatter was present. Nearly all CD34+ cells in normal bone marrow expressed VLA-4 and VLA-5, PECAM-1, LFA-3 and HCAM. The majority of the CD34+ cells also had LFA -1 and L-selectin on the surface membrane. A small subset was VLA-2, VLA-3, ICAM-1 or Mac-1 positive. CD34+ cells expressing the vitronectin receptor or the CD11c antigen were rare. Cord blood and mobilized blood CD34+ cells had a lower expression of VLA-2, VLA-3 and VLA-5 and a higher expression of LFA-1, ICAM-1 and L-selectin than bone marrow CD34+ cells. Except for LFA-1, this was not due to the presence of more myeloid precursors in these samples. Low beta1 integrin expression may lead to less adhesion to the extracellular matrix. High expression of L-selectin may facilitate interaction with endothelial cells. Therefore, this phenotype may favour mobilization.     

Soluble E-selectin enhances intercellular adhesion molecule-1 (ICAM-1) expression in human tumor cell lines

E-selectin mediates neovascularization via its soluble form, while its membrane-bound form initiates binding of tumor cells to vascular endothelium. Therefore, it was studied whether soluble E-selectin regulates further adhesion molecules on tumor cells. In tumor cells but not in related nonmalignant cells, intercellular adhesion molecule (ICAM)-1 expression was strikingly increased from 5 to 68% positive cells by in vitro inoculation of a recombinant E-selectin-IgG1 within 24 h, as analyzed by flow cytometry. The absence of changes in the expression of vascular cell adhesion molecule, integrin ligands (CD11a, CD18, integrin alpha 4), and sialyl-Lewis X indicates a specific effect of soluble E-selectin on ICAM-1. A cell adhesion assay revealed that the enhanced adhesion on T-cells to tumor cells mediated by soluble E-selectin-induced ICAM-1 expression was at a maximum after a 12-h incubation period. Therefore, ICAM-1 regulation on tumor cells might be a mechanism of immune escape.     

Monoclonal antibodies against ICAM-1 and CD18 attenuate cerebral vasospasm after experimental subarachnoid hemorrhage in rabbits

BACKGROUND AND PURPOSE: Inflammatory responses have been implicated in the elaboration of several forms of central nervous system injury, including cerebral vasospasm after subarachnoid hemorrhage (SAH). A critical event participating in such responses is the recruitment of circulating leukocytes into the inflammatory site. Two of the key adhesion molecules responsible for the attachment of leukocytes to endothelial cells are intercellular adhesion molecule-1 (ICAM-1) and the common beta chain of the integrin superfamily (CD18). This study examined the effects of monoclonal antibodies on ICAM-1 and the effects of CD18 on cerebral vasospasm after SAH. METHODS: A rabbit model of SAH was utilized to test the influence of intracisternally administered antibodies to ICAM-1 and CD18 on cerebral vasospasm. Antibodies were administered alone or in combination, and the cross-sectional area of basilar arteries was assessed histologically on day 2 post-SAH. RESULTS: Treatment with antibodies to ICAM-1 or CD18 inhibited vasospasm by 22% and 27%, respectively. When administered together, the attenuation of vasospasm increased to 56%. All of these effects achieved statistical significance. CONCLUSIONS: These findings provide the first evidence that the severity of cerebral vasospasm can be attenuated using monoclonal antibodies against ICAM-1 and CD18. The results reinforce the concept that cell-mediated inflammation plays an important role in cerebral vasospasm after SAH and suggest that therapeutic targeting of cellular adhesion molecules can be of benefit in treating cerebral vasospasm.     

A dimeric crystal structure for the N-terminal two domains of intercellular adhesion molecule-1

The 3.0-A structure of a 190-residue fragment of intercellular adhesion molecule-1 (ICAM-1, CD54) reveals two tandem Ig-superfamily (IgSF) domains. Each of two independent molecules dimerizes identically with a symmetry-related molecule over a hydrophobic interface on the BED sheet of domain 1, in agreement with dimerization of ICAM-1 on the cell surface. The residues that bind to the integrin LFA-1 are well oriented for bivalent binding in the dimer, with the critical Glu-34 residues pointing away from each other on the periphery. Residues that bind to rhinovirus are in the flexible BC and FG loops at the tip of domain 1, and these and the upper half of domain 1 are well exposed in the dimer for docking to virus. By contrast, a residue important for binding to Plasmodium falciparum-infected erythrocytes is in the dimer interface. The presence of A' strands in both domains 1 and 2, conserved hydrogen bonds at domain junctions, and elaborate hydrogen bond networks around the key integrin binding residues in domain 1 make these domains suited to resist tensile forces during adhesive interactions. A subdivision of the intermediate (I) set of IgSF domains is proposed in which domain 1 of ICAM-1 and previously described I set domains belong to the I1 set and domain 2 of ICAM-1, ICAM-2, and vascular cell adhesion molecule-1 belong to the I2 set.     

Monocyte adherence induced by lipopolysaccharide involves CD14, LFA-1, and cytohesin-1. Regulation by Rho and phosphatidylinositol 3-kinase

Mechanisms regulating lipopolysaccharide (LPS)-induced adherence to intercellular adhesion molecule (ICAM)-1 were examined using THP-1 cells transfected with CD14-cDNA (THP-1wt). THP-1wt adherence to ICAM-1 was LPS dose-related, time-dependent, and inhibited by antibodies to either CD14 or leukocyte function associated antigen (LFA)-1, but was independent of any change in the number of surface expressed LFA-1 molecules. A potential role for phosphatidylinositol (PI) 3-kinase (PI 3-kinase) in LPS-induced adherence was examined using the PI 3-kinase inhibitors LY294002 and Wortmannin. Both inhibitors selectively attenuated LPS-induced, but not phorbol 12-myristate 13-acetate-induced adherence. Inhibition by these agents was unrelated to any changes in either LPS binding to or LFA-1 expression by THP-1wt cells. LPS-induced adherence was also abrogated in U937 cells transfected with a dominant negative mutant of of PI 3-kinase. Toxin B from Clostridium difficile, an inhibitor of the Rho family of GTP-binding proteins, abrogated both PI-3 kinase activation and adherence induced by LPS. Cytohesin-1, a phosphatidylinositol 3,4,5-triphosphate-regulated adaptor molecule for LFA-1 activation, was found to be expressed in THP-1wt cells. In addition, treatment of THP-1wt with cytohesin-1 antisense attenuated LPS-induced adherence. These findings suggest a model in which LPS induces adherence through a process of "inside-out" signaling involving CD14, Rho, and PI 3-kinase. This converts low avidity LFA-1 into an active form capable of increased binding to ICAM-1. This change in LFA-1 appears to be cytohesin-1-dependent.     

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Membrane molecules such as CD36 (OKM5), intercellular adhesion molecule-1 (ICAM-1, CD54), gamma interferon-induced protein 10 (gamma-IP10) and IL-1 are induced and/or upregulated in psoriatic epidermis. These molecules have important accessory, trafficking or signalling functions in the immune system and also play a role in the pathophysiology of psoriasis. The relevance of adhesion molecules, CD36 and epidermal IL-1 in psoriasis was studied in vitro in the autologous mixed epidermal cell - T lymphocyte reaction (MECLR). Their level of expression was quantitated in epidermal cell suspensions (ECS) from patients with psoriasis and their function was assessed by blocking with specific mAbs and antisera or by depleting CD36+ cells from the ECS prior to the MECLR. ECS from psoriatic lesions contained increased numbers of CD36+ (23 +/- 12%), ICAM-1(+) (31 +/- 14%) and IL-1(+) (57 +/- 21%) cells. The autologous MECLR was inhibited in samples from all patients by mAb to CD2 (LFA-2), CD11a (LFA-1alpha), CD18 (LFA-1beta), ICAM-1, CD58 (LFA-3) and an antiserum to IL-1beta. Thus, adhesion molecules facilitate inflammation in psoriasis not only via adhesion and recruitment of T lymphocyte in psoriatic lesions, but also via activation of T cells. Furthermore CD36 molecules on psoriatic epidermal cells do not costimulate autologous T lymphocytes in psoriasis. The observed costimulatory function of IL-1beta in the MECLR emphasizes its relevance in psoriasis.     

Beta2-integrins mediate stable adhesion in collisional interactions between neutrophils and ICAM-1-expressing cells

The aggregation of human neutrophils in suspension has features that are analogous to their attachment to activated endothelium in that both involve selectin and beta2-integrin adhesion receptors. For the collisional interaction that forms neutrophil aggregates in suspension, there is a tethering step in which L-selectin on neutrophils binds PSGL-1. At relatively low shear rates (100-200 s(-1)) firm adhesion is mediated in equal measure by LFA-1 binding to ICAM-3, and Mac-1 binding to an as yet undefined ligand. In this report we used a mouse melanoma cell line expressing an estimated 700,000 ICAM-1 (CD54) to examine the relative roles of LFA-1 and Mac-1 over the kinetics of heterotypic cell adhesion in shear mixed suspensions. Neither heterotypic nor homotypic neutrophil aggregates formed with application of shear alone. However, the rate of aggregation peaked within seconds of chemotactic stimulation. In contrast to homotypic aggregation, neither L-selectin nor its O-glycoprotein ligands on neutrophils contributed to heterotypic adhesion. Adhesion was inhibited in a dose-dependent manner as ICAM-1 was titrated with blocking mAb. A direct interaction between LFA-1 and ICAM-1 was preferred over the first minute of stimulation, whereas at later times adhesion was supported equally by Mac-1. Activation with MnCl2 also favored participation of the constitutively expressed LFA-1. Application of defined shear in a cone and plate viscometer showed that adhesion to the ICAM-1 cells decreased from a maximum level to baseline as shear rate increased up to 400 s(-1) in a manner typical of integrin adhesion alone. In contrast, homotypic aggregation supported by the transition from selectin to integrin binding exhibited an increase in efficiency up to 800 s(-1). The pathophysiological significance of receptor site density and duration of contact in collisional interactions relevant to leukocyte recruitment compared to leukocyte-endothelial cell interactions on surfaces is discussed.