Regulation of LFA-1 avidity in human B cells. Requirements for dephosphorylation events for high avidity ICAM-1 binding.

Regulation of the avidity of LFA-1 (CD11a/CD18, alpha L beta 2) for its ligand ICAM-1 (CD54) was studied in human B cells by evaluating the effects of a phorbol ester, anti-IgM antibodies, staurosporine, and okadaic acid. We monitored changes in LFA-1 avidity by quantifying binding of cells to an immobilized rICAM-1 fusion protein. In this assay, the protein kinase C-activating phorbol ester PDB and anti-IgM antibodies, as well as the protein kinase inhibitor, staurosporine, were able to induce LFA-1-dependent binding to ICAM-1. This demonstrates that the high avidity state of LFA-1 can be induced by a protein kinase C-dependent and by a protein kinase C-independent pathway. Furthermore, treatment of the cells with the protein phosphatase inhibitor, okadaic acid, inhibited binding to ICAM-1. Treatment with staurosporine before addition of okadaic acid not only induced enhanced binding of cells to ICAM-1, but also dramatically reduced the ability of okadaic acid to inhibit binding. These results suggest a critical role for a protein phosphatase in inducing the high avidity state of LFA-1 as well as a role for a protein kinase in inducing the low avidity state of LFA-1.     

Structure of an allosteric inhibitor of LFA-1 bound to the I-domain studied by crystallography, NMR, and calorimetry

LFA-1 (lymphocyte function-associated antigen-1) plays a role in intercellular adhesion and lymphocyte trafficking and activation and is an attractive anti-inflammatory drug target. The alpha-subunit of LFA-1, in common with several other integrins, has an N-terminally inserted domain (I-domain) of approximately 200 amino acids that plays a central role in regulating ligand binding to LFA-1. An additional region, termed the I-domain allosteric site (IDAS), has been identified exclusively within the LFA-1 I-domain and shown to regulate the function of this protein. The IDAS is occupied by small molecule LFA-1 inhibitors when cocrystallized or analyzed by (15)N-(1)H HSQC (heteronuclear single-quantum coherence) NMR (nuclear magnetic resonance) titration experiments. We report here a novel arylthio inhibitor that binds the I-domain with a K(d) of 18.3 nM as determined by isothermal titration calorimetry (ITC). This value is in close agreement with the IC(50) (10.9 nM) derived from a biochemical competition assay (DELFIA) that measures the level of inhibition of binding of whole LFA-1 to its ligand, ICAM-1. Having established the strong affinity of the arylthio inhibitor for the isolated I-domain, we have used a range of techniques to further characterize the binding, including ITC, NMR, and X-ray crystallography. We have first developed an effective ITC binding assay for use with low-solubility inhibitors that avoids the need for ELISA-based assays. In addition, we utilized a fast NMR-based assay for the generation of I-domain-inhibitor models. This is based around the collection of HCCH-TOCSY spectra of LFA-1 in the bound form and the identification of a subset of side chain methyl groups that give chemical shift changes upon binding of LFA-1 inhibitors. This subset was used in two-dimensional (13)C-(15)N and (15)N-filtered and -edited two-dimensional NMR experiments to identify a minimal set of intraligand and ligand-protein NOEs, respectively (nuclear Overhauser enhancements). Models from the NMR data were assessed by comparison to an X-ray crystallographic structure of the complex, confirming that the method correctly predicted the essential features of the bound ligand.     

LFA-1 to LFA-1 signals involve zeta-associated protein-70 (ZAP-70) tyrosine kinase: relevance for invasion and migration of a T cell hybridoma.

We previously showed that LFA-1-dependent in vitro invasion and in vivo migration of a T cell hybridoma was blocked in cells overexpressing a truncated dominant-negative zeta-associated protein (ZAP)-70. The truncated ZAP-70 also blocked LFA-1-dependent chemotaxis through ICAM-1-coated filters induced by 1 ng/ml stromal cell-derived factor-1, but not LFA-1-independent chemotaxis induced by 100 ng/ml stromal cell-derived factor-1. This suggested that LFA-1 engagement triggers a signal that amplifies a weak chemokine signal and that dominant-negative ZAP-70 blocks this LFA-1 signal. Here we show that cross-linking of part of the LFA-1 molecules with Abs causes activation of free LFA-1 molecules (not occupied by the Ab) on the same cell, which then bind to ICAM-2 on other cells. This causes cell aggregation that was also blocked by dominant-negative ZAP-70. Thus, an LFA-1 signal involving ZAP-70 activates other LFA-1 molecules, suggesting that the chemokine signal can be amplified by multiple cycles of LFA-1 activation. The chemokine and the LFA-1 signal were both blocked by a phospholipase C inhibitor and a calpain inhibitor, suggesting that one of the amplified signals is the phospholipase C-dependent activation of calpain. Finally, we show that both Src-homology 2 domains are required for inhibition of invasion, chemotaxis, and aggregation by the truncated ZAP-70, suggesting that ZAP-70 interacts with a phosphorylated immunoreceptor tyrosine-based activation motif (ITAM) sequence. Remarkably, this is not an ITAM in the TCR/CD3 complex because this is not expressed by this T cell hybridoma.     

N-Benzoyl amino acids as LFA-1/ICAM inhibitors 1: amino acid structure-activity relationship

The association of ICAM-1 with LFA-1 plays a critical role in several autoimmune diseases. N-2-Bromobenzoyl L-tryptophan, compound 1, was identified as an inhibitor to the formation of the LFA-1/ICAM complex. The SAR of the amino acid indicates that the carboxylic acid is required for inhibition and that L-histidine is the most favored amino acid.     

Improved lymphocyte function-associated antigen-1 (LFA-1) inhibition by statin derivatives: molecular basis determined by x-ray analysis and monitoring of LFA-1 conformational changes in vitro and ex vivo

The integrin lymphocyte function-associated antigen-1 (LFA-1) (alphaLbeta2; CD11a/CD18) plays an important role in leukocyte migration and T cell activation. LFA-1 is inhibited by the cholesterol-lowering drug lovastatin, which binds to an allosteric site of the alphaL I domain termed the lovastatin site (L-site). Here we report for the first time the x-ray structures of the LFA-1 I domain complexed with derivatives of lovastatin optimized for LFA-1 inhibition. This analysis identified two new subpockets within the L-site occupied by chemical groups of the statin derivatives but not by lovastatin itself. Occupancy of these L-site subpockets led to distinct conformational changes in LFA-1, which were detectable by an epitope-monitoring assay. We utilized this assay to demonstrate improved LFA-1 inhibition in human blood in vitro and in blood samples from treated animals ex vivo. Moreover, we demonstrate that the novel lovastatin-derived LFA-1 inhibitor LFA878 exhibits potent anti-inflammatory effects in carrageenan-induced rat paw edema. In summary, the findings reported here extend the understanding of LFA-1 inhibition at the molecular level, allow for the identification and design of LFA-1 inhibitors of further enhanced potency, and support the expectation that LFA-1 inhibitors binding to the L-site will be of therapeutic value in treating inflammatory diseases.     

Regulation of the expression of leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) on a human eosinophilic leukemia cell line EoL-3.

The effects of several cytokines and phorbol myristate acetate (PMA) on LFA-1 and ICAM-1 expression on a human eosinophilic leukemia cell line, EoL-3, were investigated and compared with those of a human monocytic leukemia cell line, U937. EoL-3 cells expressed large amounts of LFA-1 and small amounts of ICAM-1, and their expression was regulated similarly in EoL-3 cells and U937 cells. Interferon-gamma (IFN-gamma) enhanced ICAM-1 expression but not LFA-1 expression, and PMA augmented both LFA-1 and ICAM-1 expression. IFN-gamma and PMA showed an additive effect on ICAM-1 expression. These results collectively suggest that expression of LFA-1 and ICAM-1 is regulated differently and that IFN-gamma and PMA regulate the expression through different mechanisms. PMA but not IFN-gamma induced homotypic adhesion of EoL-3 and U937 cells, suggesting that PMA but not IFN-gamma activated the adhesive function of these cells. Staurosporin, an inhibitor of protein kinases (PKs), partly suppressed IFN-gamma- and PMA-augmented expression of ICAM-1 on EoL-3 and U937 cells, but did not affect PMA-augmented LFA-1 expression, suggesting that staurosporin-sensitive PKs are involved in IFN-gamma- and PMA-augmented ICAM-1 expression but not in PMA-augmented LFA-1 expression. The role of protein kinase C (PK-C) in these mechanisms was not revealed because a PK-C inhibitor, H-7, did not show any definitive effect on IFN-gamma- and PMA-induced expression of LFA-1 and ICAM-1. Moreover, cyclic AMP (cAMP)- and cGMP-dependent pathways were not shown to be involved in the augmentation of the expression of these molecules.     

Dissociation of I domain and global conformational changes in LFA-1: refinement of small molecule-I domain structure-activity relationships

LFA-1 (alphalbeta2) is constitutively expressed on leukocytes, but its activity is rapidly regulated. This rapid activation has been proposed to be associated with conformation changes in the inserted ("I") domain within the headpiece of LFA-1 as well as conversion of the molecules from bent to extended forms. To study these molecular changes as they relate to affinity regulation of LFA-1, we developed and synthesized a fluorescent derivative of BIRT-377 [Kelly et al. (2001) J. Immunol.] to examine changes in LFA-1 affinity in a flow cytometer with live cells. BIRT-377 binds to the ligand-binding or "I" domain of LFA-1. Structure-activity relationships studies indicated that an aminoalkyl group could be added to the central hydantoin group without significantly affecting binding. Using this modified derivative [1-(N-fluoresceinylthioureidobutyl)-[5R]-(4-bromobenzyl)-3-(3,5-dichlorophenyl)-5-methyl-imidazolidine-2,4-dione (FBABIRT)], we analyzed the affinity of FBABIRT binding to LFA-1 on live cells. The binding affinity increases, and the dissociation rate decreases with divalent cation (Mn(2+)) stimulation. We then used FBABIRT with fluorescent resonance energy transfer (FRET) to show that LFA-1 changes its height relative to the cell surface when cells were treated with dithiothreitol (DTT) but not Mn(2+). Competition assays among FBABIRT and BIRT derivatives defined structure-affinity relationships that refine the current model of BIRT-377 binding to the I domain. Our data supports the model in which BIRT-377 binds to the I domain and stabilizes the bent structure of LFA-1, while divalent cation activation results in a small conformational change in the I domain without significant extension of LFA-1. DTT, in contrast, induces a conversion to the extended form of LFA-1 in the presence of BIRT-377 on live cells. The structure-activity studies suggest that BIRT-377 is a fully optimized inhibitor.     

Small molecule inhibitors induce conformational changes in the I domain and the I-like domain of lymphocyte function-associated antigen-1. Molecular insights into integrin inhibition.

The beta(2) integrin lymphocyte function-associated antigen-1 (LFA-1) is a conformationally flexible alpha/beta heterodimeric receptor, which is expressed on the surface of all leukocytes. LFA-1 mediates cell adhesion crucial for normal immune and inflammatory responses. Intracellular signals or cations are required to convert LFA-1 from a nonligand binding to a ligand binding state. Here we investigated the effect of small molecule inhibitors on LFA-1 by monitoring the binding of monoclonal antibodies mapped to different receptor domains. The inhibitors were found to not only induce epitope changes in the I domain of the alpha(L) chain but also in the I-like domain of the beta(2) chain depending on the individual chemical structure of the inhibitor and its binding site. For the first time, we provide strong evidence that the I-like domain represents a target for allosteric LFA-1 inhibition similar to the well established regulatory L-site on the I domain of LFA-1. Moreover, the antibody binding patterns observed in the presence of the various inhibitors establish a conformational interaction between the LFA-1 I domain and the I-like domain in the native receptor that is formed upon activation. Differentially targeting the binding sites of the inhibitors, the L-site and the I-like domain, may open new avenues for highly specific therapeutic intervention in diseases where integrins play a pathophysiological role.     

Activation of LFA-1 by ionomycin is independent of calpain-mediated talin cleavage

Activation of calpains by calcium flux leading to talin cleavage is thought to be an important process of LFA-1 activation by inside-out signalling. Here, we tested the effects of the calcium ionophore ionomycin and calpain inhibitor calpeptin on LFA-1-mediated adhesion of a T cell hybridoma line, cytotoxic T cells and primary resting T cells. Ionomycin activated LFA-1-mediated adhesion of all three types of T cells, and calpeptin inhibited the effects of ionomycin. However, calpeptin also inhibited activation of LFA-1 by PMA, which did not induce calcium flux. Cleavage of talin was undetectable in ionomycin-treated T cells. Furthermore, treatment with ionomycin and calpeptin induced apoptosis of T cells. Inhibitors of phosphatidyl Inositol-3 kinase inhibited activation of LFA-1 by ionomycin, but not by PMA, whereas the protein kinase C inhibitor inhibited the effects of PMA, but not ionomycin. Thus, activation of LFA-1 by ionomycin is independent of calpain-mediated talin cleavage.     

The leukocyte integrin LFA-1 reconstituted by cDNA transfection in a nonhematopoietic cell line is functionally active and not transiently regulated.

The functional activity of lymphocyte function-associated antigen 1 (LFA-1) on leukocytes can be regulated by T-cell receptor (TCR) stimulation and pharmacologic agents. It was of interest to determine if functionally active LFA-1 could be reconstituted on a nonhematopoietic, LFA-1-negative cell line. We report the expression of LFA-1 and diethylaminoethyl (DEAE) Mac-1 alpha beta heterodimers on the cell surface of a fibroblastoid cell line, COS, by DEAE dextran cotransfection of the alpha and beta subunit cDNAs. Immunoprecipitation studies demonstrated that the alpha and beta subunit was expressed in heterodimers. The alpha or beta subunit was expressed at lower levels after transfection with the alpha or beta subunit cDNA alone. Cotransfection of the alpha and beta subunit cDNAs, but not transfection of alpha or beta alone, was sufficient to reconstitute intercellular adhesion molecule-1 (ICAM-1) binding activity. Consistent with this observation, LFA-1 on the fibroblastoid cells possesses the activation epitope defined by the L16 monoclonal antibody (mAb). This epitope marks the conversion of LFA-1 from the low to high avidity state on peripheral blood T lymphocytes (PBLs) and is constitutively present on activated cell lines. In contrast to LFA-1 on leukocytes, the functional activity of LFA-1 on fibroblastoid cells was not influenced by phorbol ester treatment. Furthermore, the use of agents that interfere with intracellular signaling, a protein kinase C inhibitor, cAMP analogue, or the combination of a phosphodiesterase inhibitor and adenyl cyclase activator, did not affect the binding of COS cells expressing LFA-1 to purified ICAM-1.     

F-actin flow drives affinity maturation and spatial organization of LFA-1 at the immunological synapse

Integrin-dependent interactions between T cells and antigen-presenting cells are vital for proper T cell activation, effector function, and memory. Regulation of integrin function occurs via conformational change, which modulates ligand affinity, and receptor clustering, which modulates valency. Here, we show that conformational intermediates of leukocyte functional antigen 1 (LFA-1) form a concentric array at the immunological synapse. Using an inhibitor cocktail to arrest F-actin dynamics, we show that organization of this array depends on F-actin flow and ligand mobility. Furthermore, F-actin flow is critical for maintaining the high affinity conformation of LFA-1, for increasing valency by recruiting LFA-1 to the immunological synapse, and ultimately for promoting intracellular cell adhesion molecule 1 (ICAM-1) binding. Finally, we show that F-actin forces are opposed by immobilized ICAM-1, which triggers LFA-1 activation through a combination of induced fit and tension-based mechanisms. Our data provide direct support for a model in which the T cell actin network generates mechanical forces that regulate LFA-1 activity at the immunological synapse.     

Leukocyte function-associated antigen 1-mediated adhesion stability is dynamically regulated through affinity and valency during bond formation with intercellular adhesion molecule-1

Neutrophil rolling and transition to arrest on inflamed endothelium are dynamically regulated by the affinity of the beta(2) integrin CD11a/CD18 (leukocyte function associated antigen 1 (LFA-1)) for binding intercellular adhesion molecule (ICAM)-1. Conformational shifts are thought to regulate molecular affinity and adhesion stability. Also critical to adhesion efficiency is membrane redistribution of active LFA-1 into dense submicron clusters where multimeric interactions occur. We examined the influences of affinity and dimerization of LFA-1 on LFA-1/ICAM-1 binding by engineering a cell-free model in which two recombinant LFA-1 heterodimers are bound to respective Fab domains of an antibody attached to latex microspheres. Binding of monomeric and dimeric ICAM-1 to dimeric LFA-1 was measured in real time by fluorescence flow cytometry. ICAM-1 dissociation kinetics were measured while LFA-1 affinity was dynamically shifted by the addition of allosteric small molecules. High affinity LFA-1 dissociated 10-fold faster when bound to monomeric compared with dimeric ICAM-1, corresponding to bond lifetimes of 25 and 330 s, respectively. Downshifting LFA-1 into an intermediate affinity state with the small molecule I domain allosteric inhibitor IC487475 decreased the difference in dissociation rates between monomeric and dimeric ICAM-1 to 4-fold. When LFA-1 was shifted into the low affinity state by lovastatin, both monomeric and dimeric ICAM-1 dissociated in less than 1 s, and the dissociation rates were within 50% of each other. These data reveal the respective importance of LFA-1 affinity and proximity in tuning bond lifetime with ICAM-1 and demonstrate a nonlinear increase in the bond lifetime of the dimer versus the monomer at higher affinity.     

Signaling through integrin LFA-1 leads to filamentous actin polymerization and remodeling,resulting in enhanced T cell adhesion

The integrins can activate signaling pathways, but the final downstream outcome of these pathways is often unclear. This study analyzes the consequences of signaling events initiated by the interaction of the leukocyte integrin LFA-1 with its ligand, dimeric ICAM-1. We show that the active form of LFA-1 regulates its own function on primary human T cells by directing the remodeling of the F-actin cytoskeleton to strengthen T cell adhesion to ICAM-1. Confocal microscopy revealed that both F-actin bundling and overall levels of F-actin are increased in the ICAM-1-adhering T cells. This increase in F-actin levels and change in F-actin distribution was quantitated for large numbers of T cells using the technique of laser scanning cytometry and was found to be significant. The study went on to show that clustering of conformationally altered LFA-1 is essential for the changes in F-actin, and a model is proposed in which clustered, high-avidity T cell LFA-1, interacting with multivalent ICAM-1, causes LFA-1 signaling, which results in F-actin polymerization and higher-order F-actin bundling. The findings demonstrate that LFA-1 acts not only as an adhesion receptor but also as a signaling receptor by actively initiating the F-actin reorganization that is essential for many T cell-dependent processes.     

Comparison of the solution conformations of a cell-adhesive peptide LBE and its reverse sequence EBL

T-cell adhesion is mediated by an ICAM-1/LFA-1 interaction; this interaction plays a crucial role in T-cell activation during immune response. LBE peptide, which is derived from the beta-subunit of LFA-1, has been shown to inhibit ICAM-1/LFA-1-mediated T-cell adhesion. In this work, we studied the solution conformations of LBE peptide and its reverse sequence (EBL) by NMR, CD and molecular dynamics simulations. Reverse peptides have been used as controls in biological studies. The effect of reversing the sequence of LBE to EBL peptides on their respective conformations is important in understanding their biological properties in vitro or in vivo. The NMR studies for these peptides were carried out in water and in TFE/water solvent systems. In 40% TFE/water, both peptides exhibited helical conformation. CD studies suggested that the LBE exhibits 30% helical conformation, while the EBL exhibits 20% helical conformation. From the NMR and MD simulation studies, it was evident that the peptides exhibited a stable helical conformation; a stable helical structure was found at Leu6 to Leu15 for LBE and at Gly9 to Leu17 for EBL. The helical conformations of LBE and EBL may be in equilibrium with other possible conformers; the other conformers contain loop and turn structures. Both peptides bind to divalent cations because the LBE is derived from the cation-binding region of the LFA-1. This study shows that reversing the peptide sequence did not alter the secondary structure of the corresponding sequence. Hence, caution must be exercised when using reverse peptides as controls in biological studies. This report will improve our ability to design a better inhibitor of ICAM-1/LFA-1 interaction.     

Suppression of LFA-1 Expression by Spermine Is Associated with Enhanced Methylation of ITGAL,the LFA-1 Promoter Area

Spermine and spermidine, natural polyamines, suppress lymphocyte function-associated antigen 1 (LFA-1) expression and its associated cellular functions through mechanisms that remain unknown. Inhibition of ornithine decarboxylase, which is required for polyamine synthesis, in Jurkat cells by 3 mM D,L-alpha-difluoromethylornithine hydrochloride (DFMO) significantly decreased spermine and spermidine concentrations and was associated with decreased DNA methyltransferase (Dnmt) activity, enhanced demethylation of the LFA-1 gene (ITGAL) promoter area, and increased CD11a expression. Supplementation with extracellular spermine (500 µM) of cells pretreated with DFMO significantly increased polyamine concentrations, increased Dnmt activity, enhanced methylation of the ITGAL promoter, and decreased CD11a expression. It has been shown that changes in intracellular polyamine concentrations affect activities of -adenosyl-L-methionine-decaroboxylase, and, as a result, affect concentrations of the methyl group donor, S-adenosylmethionine (SAM), and of the competitive Dnmt inhibitor, decarboxylated SAM. Additional treatments designed to increase the amount of SAM and decrease the amount of decarboxylated SAM–such as treatment with methylglyoxal bis-guanylhydrazone (an inhibitor of S-adenosyl-L-methionine-decaroboxylase) and SAM supplementation–successfully decreased CD11a expression. Western blot analyses revealed that neither DFMO nor spermine supplementation affected the amount of active Ras-proximate-1, a member of the Ras superfamily of small GTPases and a key protein for regulation of CD11a expression. The results of this study suggest that polyamine-induced suppression of LFA-1 expression occurs via enhanced methylation of ITGAL.     

Extracellular Ca2+ modulates leukocyte function-associated antigen-1 cell surface distribution on T lymphocytes and consequently affects cell adhesion

Transition of leukocyte function-associated antigen-1 (LFA-1), from an inactive into an activate state depends on the presence of extracellular Mg2+ and/or Ca2+ ions. Although Mg2+ is directly involved in ligand binding, the role of Ca2+ in LFA-1 mediated adhesion remained obscure. We now demonstrate that binding of Ca2+, but not Mg2+, directly correlates with clustering of LFA-1 molecules at the cell surface of T cells, thereby facilitating LFA-1-ligand interaction. Using a reporter antibody (NKI-L16) that recognizes a Ca(2+)-dependent epitope on LFA-1, we found that Ca2+ can be bound by LFA-1 with different strength. We noticed that weak binding of Ca2+ is associated with a dispersed LFA-1 surface distribution on T cells and with non- responsiveness of these cells to stimuli known to activate LFA-1. In contrast, stable binding of Ca2+ by LFA-1 correlates with a patch-like surface distribution and vivid ligand binding after activation of LFA- 1. Mg(2+)-dependent ligand binding does not affect binding of Ca2+ by LFA-1 as measured by NKI-L16 expression, suggesting that Mg2+ binds to a distinct site, and that both cations are important to mediate adhesion. Only Sr2+ ions can replace Ca2+ to express the L16 epitope, and to induce clustering of LFA-1 at the cell surface. We conclude that Ca2+ is involved in avidity regulation of LFA-1 by clustering of LFA-1 molecules at the cell surface, whereas Mg2+ is important in regulation of the affinity of LFA-1 for its ligands.     

Proteasome activity is required for T lymphocyte aggregation after mitogen activation

The proteasome is a multicatalytic complex of proteases involved in T lymphocyte proliferation and activation through multiple mechanisms. In this study, we investigated its role in lymphocyte aggregation. We found that blocking proteasome activity by a proteasome-specific inhibitor lactacystin (LAC) prevented clustering of T lymphocytes after stimulation with various mitogens. Expression of adhesion molecules ICAM-1 and LFA-1 at cell surfaces of activated T cells was decreased after treatment with LAC. Mechanisms by which the proteasome intervenes in the expression of these adhesion molecules were different. LAC inhibited ICAM-1 expression at the mRNA level, whereas LFA-1 inhibition was probably at a post-translational level. Downregulation of these molecules after proteasome inhibition likely contributes to the observed repression of T cell aggregation. Our results show that the proteasome plays an important role in cell-cell interaction during T cell activation.     

Force spectroscopy of the leukocyte function-associated antigen-1/intercellular adhesion molecule-1 interaction

Interactions between leukocyte function-associated antigen-1 (LFA-1) with its cognate ligand, intercellular adhesion molecule-1 (ICAM-1) play a crucial role in leukocyte adhesion. Because the cell and its adhesive components are subject to external perturbation from the surrounding flow of blood, it is important to understand the binding properties of the LFA-1/ICAM-1 interaction in both steady state and in the presence of an external pulling force. Here we report on atomic force microscopy (AFM) measurements of the unbinding of LFA-1 from ICAM-1. The single molecule measurements revealed the energy landscape corresponding to the dissociation of the LFA-1/ICAM-1 complex and provided the basis for defining the energetic determinants of the complex at equilibrium and under the influence of an external force. The AFM force measurements were performed in an experimental system consisting of an LFA-1-expressing T cell hybridoma, 3A9, attached to the end of the AFM cantilever and an apposing surface expressing ICAM-1. In measurements covering three orders of magnitude change in force loading rate, the LFA-1/ICAM-1 force spectrum (i.e., unbinding force versus loading rate) revealed a fast and a slow loading regime that characterized a steep inner activation barrier and a wide outer activation barrier, respectively. The addition of Mg(2+), a cofactor that stabilizes the LFA-1/ICAM-1 interaction, elevated the unbinding force of the complex in the slow loading regime. In contrast, the presence of EDTA suppressed the inner barrier of the LFA-1/ICAM-1 complex. These results suggest that the equilibrium dissociation constant of the LFA-1/ICAM-1 interaction is regulated by the energetics of the outer activation barrier of the complex, while the ability of the complex to resist a pulling force is determined by the divalent cation-dependent inner activation barrier.     

HIV envelope gp120 activates LFA-1 on CD4 T-lymphocytes and increases cell susceptibility to LFA-1-targeting leukotoxin (LtxA)

The cellular adhesion molecule LFA-1 and its ICAM-1 ligand play an important role in promoting HIV-1 infectivity and transmission. These molecules are present on the envelope of HIV-1 virions and are integral components of the HIV virological synapse. However, cellular activation is required to convert LFA-1 to the active conformation that has high affinity binding for ICAM-1. This study evaluates whether such activation can be induced by HIV itself. The data show that HIV-1 gp120 was sufficient to trigger LFA-1 activation in fully quiescent naïve CD4 T cells in a CD4-dependent manner, and these CD4 T cells became more susceptible to killing by LtxA, a bacterial leukotoxin that preferentially targets leukocytes expressing high levels of the active LFA-1. Moreover, virus p24-expressing CD4 T cells in the peripheral blood of HIV-infected subjects were found to have higher levels of surface LFA-1, and LtxA treatment led to significant reduction of the viral DNA burden. These results demonstrate for the first time the ability of HIV to directly induce LFA-1 activation on CD4 T cells. Although LFA-1 activation may enhance HIV infectivity and transmission, it also renders the cells more susceptible to an LFA-1-targeting bacterial toxin, which may be harnessed as a novel therapeutic strategy to deplete virus reservoir in HIV-infected individuals.     

IL-27 induces Th1 differentiation via p38 MAPK/T-bet- and intercellular adhesion molecule-1/LFA-1/ERK1/2-dependent pathways

IL-27, a novel member of the IL-6/IL-12 family, activates both STAT1 and STAT3 through its receptor, which consists of WSX-1 and gp130 subunits, resulting in positive and negative regulations of immune responses. We recently demonstrated that IL-27 induces Th1 differentiation through ICAM-1/LFA-1 interaction in a STAT1-dependent, but T-bet-independent mechanism. In this study, we further investigated the molecular mechanisms by focusing on p38 MAPK and ERK1/2. IL-27-induced Th1 differentiation was partially inhibited by lack of T-bet expression or by blocking ICAM-1/LFA-1 interaction with anti-ICAM-1 and/or anti-LFA-1, and further inhibited by both. Similarly, the p38 MAPK inhibitor, SB203580, or the inhibitor of ERK1/2 phosphorylation, PD98059, partially suppressed IL-27-induced Th1 differentiation and the combined treatment completely suppressed it. p38 MAPK was then revealed to be located upstream of T-bet, and SB203580, but not PD98059, inhibited T-bet-dependent Th1 differentiation. In contrast, ERK1/2 was shown to be located downstream of ICAM-1/LFA-1, and PD98059, but not SB203580, inhibited ICAM-1/LFA-1-dependent Th1 differentiation. Furthermore, it was demonstrated that STAT1 is important for IL-27-induced activation of ERK1/2, but not p38 MAPK, and that IL-27 directly induces mRNA expression of growth arrest and DNA damage-inducible 45gamma, which is known to mediate activation of p38 MAPK. Finally, IL-12Rbeta2 expression was shown to be up-regulated by IL-27 in both T-bet- and ICAM-1/LFA-1-dependent mechanisms. Taken together, these results suggest that IL-27 induces Th1 differentiation via two distinct pathways, p38 MAPK/T-bet- and ICAM-1/LFA-1/ERK1/2-dependent pathways. This is in contrast to IL-12, which induces it via only p38 MAPK/T-bet-dependent pathway.