Synergistic inhibitory activity of α- and β-LFA-1 peptides on LFA-1/ICAM-1 interaction

Interactions of cell-adhesion molecule LFA-1 and its ligand ICAM-1 play important roles during immune and inflammatory responses. Critical residues of LFA-1 for ICAM-1 binding are known to be in the I-domain of the α-subunit and the I-like domain of the β-subunit. On the basis of our previous work demonstrating the inhibitory activity of I-domain cyclic peptide cLAB.L on LFA-1/ICAM-1 interaction, here we have explored the activity of I-like-domain peptide LBE on the binding mechanism of cLAB.L. LBE enhances cLAB.L binding to T-cells and epithelial cells. The adherence of T-cells to epithelial monolayers was suppressed by the two peptides. The addition of LBE to the monolayers prior to the addition cLAB.L produced a better inhibitory effect than the reverse procedure. LBE, but not cLAB.L, changes the ICAM-1 conformation, suggesting that LBE binds to ICAM-1 at sites that are distinct from these of cLAB.L and induces improved conformation in ICAM-1 for binding to cLAB.L.     

Targeting ICAM-1/LFA-1 interaction for controlling autoimmune diseases: designing peptide and small molecule inhibitors

This review describes the role of modulation of intracellular adhesion molecule-1 (ICAM-1)/leukocyte function-associated antigen-1 (LFA-1) interaction in controlling autoimmune diseases or inducing immunotolerance. ICAM-1/LFA-1 interaction is essential for T-cell activation as well as for migration of T-cells to target tissues. This interaction also functions, along with Signal-1, as a co-stimulatory signal (Signal-2) for T-cell activation, which is delivered by the T-cell receptors (TCR)-major histocompatibility complex (MHC)-peptide complex. Therefore, blocking ICAM-1/LFA-1 interaction can suppress T-cell activation in autoimmune diseases and organ transplantation. Many types of inhibitors (i.e. antibodies, peptides, small molecules) have been developed to block ICAM-1/LFA-1 interactions, and some of these molecules have reached clinical trials. Peptides derived from ICAM-1 and LFA-1 sequences have been shown to inhibit T-cell adhesion and activation. In addition, these inhibitors have been useful in elucidating the mechanism of ICAM-1/LFA-1 interaction. Besides binding to LFA-1, the ICAM-1 peptide can be internalized by LFA-1 receptors into the cytoplasmic domain of T-cells. Therefore, this ICAM-1 peptide can be utilized to selectively target toxic drugs to T-cells, thus avoiding harmful side effects. Finally, bi-functional inhibitory peptide (BPI), which is made by conjugating the antigenic peptide and an LFA-1 peptide, can alter the T-cell commitment from T-helper-1 (Th1) to T-helper-2 (Th2)-like cells, suggesting that this peptide may have a role in blocking the formation of the "immunological synapse."     

Structural and ICAM-1-docking properties of a cyclic peptide from the I-domain of LFA-1: an inhibitor of ICAM-1/LFA- 1-mediated T-cell adhesion

The purpose of this work was to study the conformation of cyclic peptide 1, cyclo(1,12)-Pen1-Ile2-Thr3-Asp4-Gly5-Glu6-Ala7- Thr8-Asp9-Ser10-Gly11-Cys12-OH, derived from the I-domain of the LFA-1 alpha-subunit. We found that cyclic peptide 1 can bind to the D1-domain of ICAM-1 and inhibit ICAM-1/LFA-1-mediated homotypic and heterotypic T-cell adhesion. To understand the bioactive conformation and binding requirements for cyclic peptide 1, its solution structure was studied using NMR, CD, and molecular dynamics simulations. Furthermore, possible binding properties between the cyclic peptide and the D1-domain of ICAM-1 were evaluated using docking experiments. This cyclic peptide has a stable betaII -turn at Asp4- Gly5-Glu6-Ala7 and a betaI-turn at Pen1-Ile2-Thr3-Asp4; a less stable betaV-turn is found at the C-terminal region. The beta-turn at Asp4- Gly5-Glu6-Ala7 was also found in the X-ray structure of the I-domain of LFA-1. Our CD studies showed that the peptide binds to calcium/magnesium and forms a 1:1 (peptide:calcium/magnesium) complex with low cation concentrations and multiple types of complexes with higher cation concentrations. Binding to divalent cations causes a conformational change in peptide 1; this is consistent with our previous study that binding of peptide 1 to ICAM-1 was influenced by divalent cations. Docking studies show the interaction between cyclic peptide 1 and the D1-domain of ICAM-1; it indicates that the Ile2-Thr3-Asp4-Gly4-Glu6-Ala7-Thr8 sequence interacts with the F and C strands of the D1-domain. Finally, these studies will help us design a new generation of selective peptides that may bind better to the D1-domain of ICAM-1.     

Structural and ICAM-1-Docking Properties of a Cyclic Peptide from the I-domain of LFA-1: An inhibitor of ICAM-1/LFA-1-mediated T-cell adhesion

Abstract

The purpose of this work was to study the conformation of cyclic peptide 1, cyclo(1,12)- Pen1-Ile2-Thr3-Asp4-Gly5-Glu6-Ala7-Thr8-Asp9-Ser10-Gly11-Cys12-OH, derived from the I-domain of the LFA-1 α-subunit. We found that cyclic peptide 1 can bind to the D1- domain of ICAM-1 and inhibit ICAM-1/LFA-1-mediated homotypic and heterotypic T-cell adhesion. To understand the bioactive conformation and binding requirements for cyclic peptide 1, its solution structure was studied using NMR, CD, and molecular dynamics simulations. Furthermore, possible binding properties between the cyclic peptide and the D1- domain of ICAM-1 were evaluated using docking experiments. This cyclic peptide has a stable βII'-turn at Asp4-Gly5-Glu6-Ala7 and a βI-turn at Pen1-Ile2-Thr3-Asp4; a less stable βV-turn is found at the C-terminal region. The β-turn at Asp4-Gly5-Glu6-Ala7 was also found in the X-ray structure of the I-domain of LFA-1. Our CD studies showed that the peptide binds to calcium/magnesium and forms a 1:1 (peptide:calcium/magnesium) complex with low cation concentrations and multiple types of complexes with higher cation concentrations. Binding to divalent cations causes a conformational change in peptide 1; this is consistent with our previous study that binding of peptide 1 to ICAM-1 was influenced by divalent cations. Docking studies show the interaction between cyclic peptide 1 and the D1- domain of ICAM-1; it indicates that the Ile2-Thr3-Asp4-Gly4-Glu6-Ala7-Thr8 sequence interacts with the F and C strands of the D1-domain. Finally, these studies will help us design a new generation of selective peptides that may bind better to the D1-domain of ICAM-1.     

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.     

A mechanism for antibody-mediated outside-in activation of LFA-1

MEM83 is an inserted domain (I-domain)-specific antibody that up-regulates the interaction of LFA-1 with ICAM-1 through an outside-in activation mechanism. We demonstrate here that there is no change in the affinity of the MEM83 antibody for the I-domain in either its low (wild-type) or high affinity form and that MEM83 does not enhance the binding of the wild-type I-domain to ICAM-1. Furthermore, we show that the antibody acts as an activating agent to induce LFA-1/ICAM-1-dependent homotypic cell aggregation only as an IgG, but not as a Fab fragment. On the basis of these data, we propose an avidity-based mechanism that requires no direct activation of the LFA-1 I-domain by the binding of the antibody; rather, activation is enhanced when there is an interaction with both arms of the IgG. A molecular model of the antibody interaction with LFA-1 illustrates the symmetry and accessibility of the two MEM83 epitopes across the LFA-1/ICAM-1 heterotetramer. We hypothesize that MEM83 stabilizes adjacent LFA-1 molecules in their active form by the free energy that is gained from the binding of the I-domains to each arm of the IgG. This leads to stabilization of the open state of the integrin and outside-in signaling. Our model supports a mechanism in which both affinity and avidity regulation are required in the activation of LFA-1.     

2E8 Binds to the High Affinity I-domain in a Metal Ion-dependent Manner: A SECOND GENERATION MONOCLONAL ANTIBODY SELECTIVELY TARGETING ACTIVATED LFA-1*

The activation of leukocyte function-associated antigen-1 (LFA-1) plays a critical role in regulating immune responses. The metal ion-dependent adhesion site on the I-domain of LFA-1 α_L subunit is the key recognition site for ligand binding. Upon activation, conformation changes in the I-domain can lead LFA-1 from the low affinity state to the high affinity (HA) state. Using the purified HA I-domain locked by disulfide bonds for immunization, we developed an mAb, 2E8, that specifically binds to cells expressing the HA LFA-1. The surface plasmon resonance analysis has shown that 2E8 only binds to the HA I-domain and that the dissociation constant (K_D) for HA I-domain is 197 nm. The binding of 2E8 to the HA I-domain is metal ion-dependent, and the affinity decreased as Mn²⁺ was replaced sequentially by Mg²⁺ and Ca²⁺. Surface plasmon resonance analysis demonstrates that 2E8 inhibits the interaction of HA I-domain and ICAM-1. Furthermore, we found that 2E8 can detect activated LFA-1 on both JY and Jurkat cells using flow cytometry and parallel plate adhesion assay. In addition, 2E8 inhibits JY cell adhesion to human umbilical vein endothelial cells and homotypic aggregation. 2E8 treatment reduces the proliferation of both human CD4⁺ and CD8⁺ T cells upon OKT3 stimulation without the impairment of their cytolytic function. Taken together, these data demonstrate that 2E8 is specific for the high affinity form of LFA-1 and that 2E8 inhibits LFA-1/ICAM-1 interactions. As a novel activation-specific monoclonal antibody, 2E8 is a potentially useful reagent for blocking high affinity LFA-1 and modulating T cell activation in research and therapeutics.     

Inhibition of ICAM-1/LFA-1-mediated heterotypic T-cell adhesion to epithelial cells: design of ICAM-1 cyclic peptides

In this work, we have designed cyclic peptides (cIBL, cIBR, cIBC, CH4 and CH7) derived from the parent IB peptide (ICAM-1(1-21)) that are inhibitors of ICAM-1/LFA-1-mediated T-cell adhesion to Caco-2 cell monolayers. Cyclic peptide cIBR has the best activity of any of the peptides evaluated. The active ICAM-1 peptides have a common Pro-Arg-Gly sequence that may be important for binding to LFA-1.     

I-domain of lymphocyte function-associated antigen-1 mediates rolling of polystyrene particles on ICAM-1 under flow

In their active state, beta(2)-integrins, such as LFA-1, mediate the firm arrest of leukocytes by binding intercellular adhesion molecules (ICAMs) expressed on endothelium. Although the primary function of LFA-1 is assumed to be the ability to mediate firm adhesion, recent work has shown that LFA-1 can contribute to cell tethering and rolling under hydrodynamic flow, a role previously largely attributed to the selectins. The inserted (I) domain of LFA-1 has recently been crystallized in the wild-type (wt) and locked-open conformations and has been shown to, respectively, support rolling and firm adhesion under flow when expressed in alpha(L)beta(2) heterodimers or as isolated domains on cells. Here, we report results from cell-free adhesion assays where wt I-domain-coated polystyrene particles were allowed to interact with ICAM-1-coated surfaces in shear flow. We show that wt I-domain can independently mediate the capture of particles from flow and support their rolling on ICAM-1 surfaces in a manner similar to how carbohydrate-selectin interactions mediate rolling. Adhesion is specific and blocked by appropriate antibodies. We also show that the rolling velocity of I-domain-coated particles depends on the wall shear stress in flow chamber, I-domain site density on microsphere surfaces, and ICAM-1 site density on substrate surfaces. Furthermore, we show that rolling is less sensitive to wall shear stress and ICAM-1 substrate density at high density of I-domain on the microsphere surface. Computer simulations using adhesive dynamics can recreate bead rolling dynamics and show that the mechanochemical properties of ICAM-1-I-domain interactions are similar to those of carbohydrate-selectin interactions. Understanding the biophysics of adhesion mediated by the I-domain of LFA-1 can elucidate the complex roles this integrin plays in leukocyte adhesion in inflammation.     

Design of LFA-1 antagonists based on a 2,3-dihydro-1H-pyrrolizin-5(7aH)-one scaffold

A new class of lymphocyte function-associated antigen-1 (LFA-1) antagonists is described. Elaboration of the 2,3-dihydro-1H-pyrrolizin-5(7aH)-one scaffold resulted in the synthesis of potent inhibitors of the LFA-1/ICAM-1 interaction. Along with the in vitro activity, we present the X-ray crystal structure of the complex of compound 9b, in a novel binding mode to the I-domain of LFA-1.     

Dynamic control of allosteric antagonism of leukocyte function antigen-1 and intercellular adhesion molecule-1 interaction

Leukocyte function associated antigen-1 (LFA-1) plays a critical role in T cell migration and has been recognized as a therapeutic target for immune disorders. Several classes of small molecule antagonists have been developed to block LFA-1 interaction with intercellular adhesion molecule-1 (ICAM-1). Recent structural studies show that the antagonists bind to an allosteric site in the I-domain of LFA-1. However, it is not yet clear how these small molecules work as antagonists since no significant conformational change is observed in the I-domain-antagonist complex structures. Here we present a computational study suggesting how these allosteric antagonists affect the dynamics of the I-domain. The lowest frequency vibrational mode calculated from an LFA-1 I-domain structure shows large scale "coil-down" motion of the C-terminal alpha7 helix, which may lead to the open form of the I-domain. The presence of an allosteric antagonist greatly reduces this motion of the alpha7 helix as well as other parts of the I-domain. Thus, our study suggests that allosteric antagonists work by eliminating breathing motion that leads to the open conformation of the I-domain.     

A Ca2+ binding cyclic peptide derived from the alpha-subunit of LFA-1: inhibitor of ICAM-1/LFA-1-mediated T-cell adhesion.

The objective of this work is to study the conformation of cyclic peptide (1), cyclo (1, 12) Pen1-Gly2-Val3-Asp4-Val5-Asp6-Gln7-+ ++Asp8-Gly9-Glu10-Thr11-Cys12, in the presence and absence of calcium. Cyclic peptide 1 is derived from the divalent cation binding sequence of the alpha-subunit of LFA-1. This peptide has been shown to inhibit ICAM-1-LFA-1 mediated T-cell adhesion. In order to understand the structural requirements for this biologically active peptide, its solution structure was studied by nuclear magnetic resonance (NMR), circular dichroism (CD) and molecular dynamics simulations. This cyclic peptide exhibits two types of possible conformations in solution. Structure I is a loop-turn-loop type of structure, which is suitable to bind cations such as EF hand proteins. Structure II is a more extended structure with beta-hairpin bend at Asp4-Val5-Asp6-Gln7. There is evidence that alterations in the conformation of LFA-1 upon binding to divalent cations cause LFA-1 to bind to ICAM-1. To understand this mechanism, the cation-binding properties of the peptide were studied by CD and NMR. CD studies indicated that the peptide binds to calcium and forms a 1 : 1 (peptide: calcium) complex at low calcium concentrations and multiple types of complexes at higher cation concentrations. NMR studies indicated that the conformation of the peptide is not significantly altered upon binding to calcium. The peptide can inhibit T-cell adhesion by directly binding to ICAM-1 or by disrupting the interaction of the alpha and beta-subunits of LFA-1 protein. This study will help us to understand the mechanism(s) of action of this peptide and will improve our ability to design a better inhibitor of T-cell adhesion.     

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.     

NMR solution structure of a potent cyclic nonapeptide inhibitor of ICAM-1-mediated leukocyte adhesion produced by homologous amino acid substitution.

We have previously described a disulfide-linked cyclic nonapeptide (inhibitory peptide-01, IP01), with the sequence CLLRMRSIC, which binds to intercellular adhesion molecule-1 (ICAM-1), and blocks binding to its counter-structure, the integrin alphaLbeta2 (leukocyte functional antigen-1, LFA-1) (Sillerud et al., J. Peptide Res. 62, 2003: 97). We now report the optimization of this peptide by means of single homologous amino acid substitutions to yield a new peptide (IP02-K6; CLLRMKSAC) which shows an approximately sixfold improvement in inhibitory activity of multivalent leukocyte binding (inhibition constant for 50% inhibition, IC50 = 90 microm) compared with IP01 (IC50 = 580 microm). This improvement in activity gives IP02-K6 potent in vivo activity in a murine model of ischemia reperfusion injury (Merchant et al., Am. J. Physiol. Heart Circ. 284, 2003: H1260). In order to determine the structural features relevant to ICAM-1-binding, we have determined the structure of IP02-K6 using proton nuclear magnetic resonance (NMR) spectroscopy and restrained molecular modeling. In our previously reported study of solution models of IP01, we observed three interconverting conformations during low-temperature molecular dynamics simulation. In the present study, we find a single conformation of IP02-K6 similar to one of the previously found conformations of IP01 (family C). In particular, an R4-S7 beta-turn is present in similar proportions in both conformation C of IP01 and in IP02-K6; this motif is important in binding to ICAM-1 because this turn enables the IP02-K6 backbone to drape over proline-36 on ICAM-1. The NMR-derived solution model of IP02-K6 was found to dock at the alphaLbeta2-binding site on ICAM-1 with no changes in peptide backbone conformation. This docking model displaced five of the 15 alphaLbeta2 residues at the ICAM-1-binding site and provided a rationale for understanding the quantitative relationship between IP02-K6 structure and biologic activity.     

Divalent cation regulation of the function of the leukocyte integrin LFA-1

The integrin lymphocyte function-associated antigen-1 (LFA-1) expressed on T cells serves as a useful model for analysis of leukocyte integrin functional activity. We have assessed the role of divalent cations Mg2+, Ca2+, and Mn2+ in LFA-1 binding to ligand intercellular adhesion molecule-1 (ICAM-1) and induction of the divalent cation-dependent epitope recognized by mAb 24. Manganese strongly promoted both expression of the 24 epitope and T cell binding to ICAM-1 via LFA-1, suggesting that Mn2+ is able to directly alter the conformation of LFA-1 in a manner that favors ligand binding. Since Mn2+ also promotes functional activity of other integrins, parallels in mechanism of ligand binding may span the integrin family. In contrast, induction of 24 epitope expression by Mg2+ required removal of Ca2+ from T cell LFA-1 with EGTA. Furthermore, binding of mAb 24 to T cell LFA-1 in the presence of either Mn2+ or Mg2+ was found to be specifically inhibited by Ca2+, suggestive of a negative regulatory role for Ca2+ in the control of leukocyte integrin function. Analysis of T cell binding to ICAM-1 via LFA-1 in the presence of Mg2+ or Mn2+, confirmed that Ca2+ exerted inhibitory effects upon LFA-1 function. The implication of our findings is that Ca2+ bound with relatively high affinity to LFA-1 may serve to maintain an inactive state. Thus induction of function and 24 epitope expression may occur as a result of displacement of Ca2+ from leukocyte integrins or alternatively, such activators may be able to impose the required conformational change in the presence of bound Ca2+.     

The LFA-1 integrin supports rolling adhesions on ICAM-1 under physiological shear flow in a permissive cellular environment

The LFA-1 integrin is crucial for the firm adhesion of circulating leukocytes to ICAM-1-expressing endothelial cells. In the present study, we demonstrate that LFA-1 can arrest unstimulated PBL subsets and lymphoblastoid Jurkat cells on immobilized ICAM-1 under subphysiological shear flow and mediate firm adhesion to ICAM-1 after short static contact. However, LFA-1 expressed in K562 cells failed to support firm adhesion to ICAM-1 but instead mediated K562 cell rolling on the endothelial ligand under physiological shear stress. LFA-1-mediated rolling required an intact LFA-1 I-domain, was enhanced by Mg2+, and was sharply dependent on ICAM-1 density. This is the first indication that LFA-1 can engage in rolling adhesions with ICAM-1 under physiological shear flow. The ability of LFA-1 to support rolling correlates with decreased avidity and impaired time-dependent adhesion strengthening. A beta2 cytoplasmic domain-deletion mutant of LFA-1, with high avidity to immobilized ICAM-1, mediated firm arrests of K562 cells interacting with ICAM-1 under shear flow. Our results suggest that restrictions in LFA-1 clustering mediated by cytoskeletal attachments may lock the integrin into low-avidity states in particular cellular environments. Although low-avidity LFA-1 states fail to undergo adhesion strengthening upon contact with ICAM-1 at stasis, these states are permissive for leukocyte rolling on ICAM-1 under physiological shear flow. Rolling mediated by low-avidity LFA-1 interactions with ICAM-1 may stabilize rolling initiated by specialized vascular rolling receptors and allow the leukocyte to arrest on vascular endothelium upon exposure to stimulatory endothelial signals.     

Identification of the binding site in intercellular adhesion molecule 1 for its receptor, leukocyte function-associated antigen 1.

Intercellular adhesion molecule 1 (ICAM-1, CD54) is a member of the Ig superfamily and is a counterreceptor for the beta 2 integrins: lymphocyte function-associated antigen 1 (LFA-1, CD11a/CD18), complement receptor 1 (MAC-1, CD11b/CD18), and p150,95 (CD11c/CD18). Binding of ICAM-1 to these receptors mediates leukocyte-adhesive functions in immune and inflammatory responses. In this report, we describe a cell-free assay using purified recombinant extracellular domains of LFA-1 and a dimeric immunoadhesin of ICAM-1. The binding of recombinant secreted LFA-1 to ICAM-1 is divalent cation dependent (Mg2+ and Mn2+ promote binding) and sensitive to inhibition by antibodies that block LFA-1-mediated cell adhesion, indicating that its conformation mimics that of LFA-1 on activated lymphocytes. We describe six novel anti-ICAM-1 monoclonal antibodies, two of which are function blocking. Thirty-five point mutants of the ICAM-1 immunoadhesin were generated and residues important for binding of monoclonal antibodies and purified LFA-1 were identified. Nineteen of these mutants bind recombinant LFA-1 equivalently to wild type. Sixteen mutants show a 66-2500-fold decrease in LFA-1 binding yet, with few exceptions, retain binding to the monoclonal antibodies. These mutants, along with modeling studies, define the LFA-1 binding site on ICAM-1 as residues E34, K39, M64, Y66, N68, and Q73, that are predicted to lie on the CDFG beta-sheet of the Ig fold. The mutant G32A also abrogates binding to LFA-1 while retaining binding to all of the antibodies, possibly indicating a direct interaction of this residue with LFA-1. These data have allowed the generation of a highly refined model of the LFA-1 binding site of ICAM-1.     

Cellular activation of leukocyte function-associated antigen-1 and its affinity are regulated at the I domain allosteric site

The I domain of the integrin LFA-1 possesses a ligand binding interface that includes the metal ion-dependent adhesion site. Binding of the LFA-1 ligand, ICAM-1 to the metal ion-dependent adhesion site is regulated by the I domain allosteric site (IDAS). We demonstrate here that intracellular signaling leading to activation of LFA-1 binding to ICAM-1 is regulated at the IDAS. Inhibitory mutations in or proximal to the IDAS are dominant to cytoplasmic signals that activate binding to ICAM-1. In addition, mutational activation at the IDAS greatly increases the binding of lymphocyte-expressed LFA-1 to ICAM-1 in response to PMA, but does not result in constitutive binding. Binding of a novel CD18 activation epitope mAb to LFA-1 in response to soluble ICAM-1 binding was also blocked by inhibitory and was enhanced by activating IDAS mutations. Surface plasmon resonance using soluble wild-type LFA-1 and an IDAS mutant of LFA-1 indicate that the IDAS can regulate a 6-fold change in the K(d) of ICAM-1 binding. The K(d) of wild-type LFA-1 (1.2 x 10(-1) s(-1)) differed with that of the activating IDAS mutant (1.9 x 10(-2) s(-1)), but their K(a) values were identical (2.2 x 10(5) M(-1)s(-1)). We propose that IDAS regulates the binding of LFA-1 to ICAM-1 activated by intracellular signals. IDAS can control the affinity state of LFA-1 with concomitant I domain and CD18 conformational changes.     

Comparative normal mode analysis of LFA-1 integrin I-domains

The conformational dynamics of the Inserted domain (I-domain) from the lymphocyte function-associated antigen-1 (LFA-1) was investigated by normal mode analysis of multiple structures of the low, intermediate, and high affinity states. LFA-1 is an integrin expressed on leukocytes and is of critical importance in adhesion reactions, like antigen-specific responses, homing, and diapedesis. The main ligand binding site of LFA-1 is the I-domain, which recognizes intercellular adhesion molecules (ICAMs), members of the immunoglobulin superfamily. From experimental crystal structures, a large-scale conformational change of, among others, the α7 helix of the I-domain has been observed leading to the proposal that these structural changes are linked to the conformational regulation of LFA-1. The results from the present calculations show that structural changes of the α7 helix consistent with those observed in the crystal structures are significantly sampled by the low frequency modes. This was found to be particularly true for the low affinity state of the I-domain, indicating that low frequency motions favor the conformational transition implicated in activation. However, beyond the simple downward shift of the helix implied by the crystal structures, the calculations further show that there is a noticeable swinging-out motion of the helix. The consequences of this motion are discussed in the context of integrin activation and inhibition. Moreover, significant changes in the atomic-level dynamics and in long-range correlated motions of the I-domain were found to occur upon binding of the natural ligand ICAM. These changes were more local upon binding of an allosteric inhibitor. The present study opens the question of how changes in dynamics may contribute to the long-range transmission of signal upon ICAM binding by the LFA-1 I-domain.     

Thymocyte LFA-1 and thymic epithelial cell ICAM-1 molecules mediate binding of activated human thymocytes to thymic epithelial cells.

We have investigated the binding in vitro of activated thymocytes to thymic epithelial (TE) cells, and studied the effect of up-regulation of TE cell surface intracellular adhesion molecule 1 (ICAM-1) and HLA-DR by IFN-gamma on the ability of TE cells to bind to both resting and activated human thymocytes. TE cell binding to activated and resting thymocytes was studied by using our previously described suspension assay of TE-thymocyte conjugate formation. We found that activated mature and immature thymocytes bound maximally at 37 degrees C to IFN-gamma-treated ICAM-1+ and HLA-DR+ TE cells and this TE-activated thymocyte binding was inhibited by antibodies to LFA-1 alpha-chain (CD11a) (68.1 +/- 5.6% inhibition, p less than 0.01) and ICAM-1 (73.9 +/- 7.7% inhibition, p less than 0.05). Neither anti-HLA-DR antibody L243 nor anti-MHC class I antibody 3F10 inhibited IFN-gamma-treated TE binding to activated thymocytes. As with antibodies to LFA-3 and CD2, antibodies to LFA-1 and ICAM-1 also inhibited PHA-induced mature thymocyte activation when accessory signals were provided by TE cells in vitro. Finally, LFA-1 and ICAM-1 were expressed early on in human thymic fetal ontogeny in patterns similar to those seen in postnatal thymus. Taken together, these data suggest that resting mature and immature thymocytes bind to TE cells via the CD2/LFA-3 ligand pair, whereas activated thymocytes bind via both CD2/LFA-3 and LFA-1/ICAM-1 ligand systems. We postulate that IFN-gamma produced intrathymically may regulate TE expression of ICAM-1 and therefore potentially may regulate TE cell binding to activated thymocytes beginning in the earliest stages of human thymic development.