The binding sites for competitive antagonistic, allosteric antagonistic, and agonistic antibodies to the I domain of integrin LFA-1

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Peptides of neuron specific enolase as potential ASD biomarkers: From discovery to epitope mapping

Autism spectrum disorder (ASD) is an important health issue and affects 1 in 59 children in the US. Prior studies determined that maternal autoantibody related (MAR) autism is thought to be associated with ~23% of ASD cases. We previously identified seven MAR-specific autoantigens including CRMP1, CRMP2, GDA, LDHA, LDHB, STIP1, and YBX1. We subsequently described the epitope peptide sequences recognized by maternal autoantibodies for each of the seven ASD-specific autoantigens. The aim of the current study was to expand upon our previous work and identify additional antigens recognized by the ASD-specific maternal autoantibodies, as well as to map the unique ASD-specific epitopes using microarray technology. Fetal Rhesus macaque brain tissues were separated by molecular weight and a fraction containing bands between 37 and 45 kDa was analyzed using 2-D gel electrophoresis, followed by peptide mass mapping using MALDI-TOF MS and TOF/TOF tandem MS/MS. Using this methodology, Neuron specific enolase (NSE) was identified as a target autoantigen and selected for epitope mapping. The full NSE sequence was translated into 15-mer peptides with an overlap of 14 amino acids onto microarray slides and probed with maternal plasma from mothers with an ASD child and from mothers with a Typically Developing child (TD) (ASD = 27 and TD = 21). The resulting data were analyzed by T-test. We found 16 ASD-specific NSE-peptide sequences for which four sequences were statistically significant (p < 0.05) using both the t-test and SAM t-test: DVAASEFYRDGKYDL (p = 0.047; SAM score 1.49), IEDPFDQDDWAAWSK (p = 0.049; SAM score 1.49), ERLAKYNQLMRIEEE (p = 0.045; SAM score 1.57), and RLAKYNQLMRIEEEL (p = 0.017; SAM score 1.82). We further identified 5 sequences that were recognized by both ASD and TD antibodies suggesting a large immunodominant epitope (DYPVVSIEDPFDQDDWAAW). While maternal autoantibodies against the NSE protein are present both in mothers with ASD and mothers of TD children, there are several ASD-specific epitopes that can potentially be used as MAR ASD biomarkers. Further, studies including analysis of NSE as a target protein in combination with the previously identified MAR ASD autoantigens are currently underway.

Anti-αLβ2 antibodies reveal novel endocytotic cross-modulatory functionality

BACKGROUND AND PURPOSE

Antibodies targeting cell surface receptors are considered to enable highly selective therapeutic interventions for immune disorders and cancer. Their biological profiles are found, generally, to represent the net effects of antibody-target interactions. The former therapeutic anti-integrin αLβ2 antibody efalizumab seems to defeat this paradigm by eliciting, via mechanisms currently unknown, much broader effects than would be predicted based on its target specificity.

EXPERIMENTAL APPROACH

To elucidate the mechanisms behind these broad effects, we investigated in primary human lymphocytes in vitro the effects of anti-αLβ2 antibodies on the expression of αLβ2 as well as unrelated α4 integrins, in comparison to Fab fragments and small-molecule inhibitors.

KEY RESULTS

We demonstrate that anti-αLβ2 mAbs directly induce the internalization of α4 integrins. The endocytotic phenomenon is a direct consequence of their antibody nature. It is inhibited when monovalent Fab fragments or small-molecule inhibitors are used. It is independent of crosslinking via anti-Fc mAbs and of αLβ2 activation. The cross-modulatory effect is unidirectional and not observed in a similar fashion with the α4 integrin antibody natalizumab.

CONCLUSION AND IMPLICATIONS

The present study identifies endocytotic cross-modulation as a hitherto unknown non-canonical functionality of anti-αLβ2 antibodies. This cross-modulation has the potential to fundamentally alter an antibody's benefit risk profile, as evident with efalizumab. The newly described phenomenon may be of relevance to other therapeutic antibodies targeting cluster-forming receptors. Thus, pharmacologists should be cognizant of this action when investigating such antibodies.

Lipid Nanoparticles for Delivery of Therapeutic RNA Oligonucleotides

Gene therapy is an exciting field that has the potential to address emerging scientific and therapeutic tasks. RNA-based gene therapy has made remarkable progress in recent decades. Nevertheless, efficient targeted delivery of RNA therapeutics is still a prerequisite for entering the clinics. In this review, we introduce current delivery methods for RNA gene therapeutics based on lipid nanoparticles (LNPs). We focus on the clinical appeal of recent RNA NPs and discuss existing challenges of fabrication and screening LNP candidates for effective translation into drugs of human metabolic diseases and cancer.

The promise and challenges of immune agonist antibody development in cancer

Immune cell functions are regulated by co-inhibitory and co-stimulatory receptors. The first two generations of cancer immunotherapy agents consist primarily of antagonist antibodies that block negative immune checkpoints, such as programmed cell death protein 1 (PD1) and cytotoxic T lymphocyte protein 4 (CTLA4). Looking ahead, there is substantial promise in targeting co-stimulatory receptors with agonist antibodies, and a growing number of these agents are making their way through various stages of development. This Review discusses the key considerations and potential pitfalls of immune agonist antibody design and development, their differentiating features from antagonist antibodies and the landscape of agonist antibodies in clinical development for cancer treatment.

Integrins as Therapeutic Targets: Successes and Cancers

Integrins are transmembrane receptors that are central to the biology of many human pathologies. Classically mediating cell-extracellular matrix and cell-cell interaction, and with an emerging role as local activators of TGFβ, they influence cancer, fibrosis, thrombosis and inflammation. Their ligand binding and some regulatory sites are extracellular and sensitive to pharmacological intervention, as proven by the clinical success of seven drugs targeting them. The six drugs on the market in 2016 generated revenues of some US$3.5 billion, mainly from inhibitors of α4-series integrins. In this review we examine the current developments in integrin therapeutics, especially in cancer, and comment on the health economic implications of these developments.

Single- and double-walled carbon nanotubes enhance atherosclerogenesis by promoting monocyte adhesion to endothelial cells and endothelial progenitor cell dysfunction

BACKGROUND

The use of carbon nanotubes has increased lately. However, the cardiovascular effect of exposure to carbon nanotubes remains elusive. The present study investigated the effects of pulmonary exposure to single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) on atherosclerogenesis using normal human aortic endothelial cells (HAECs) and apolipoprotein E-deficient (ApoE^-/-) mice, a model of human atherosclerosis.

METHODS

HAECs were cultured and exposed to SWCNTs or DWCNTs for 16 h. ApoE^-/- mice were exposed to SWCNTs or DWCNTs (10 or 40 μg/mouse) once every other week for 10 weeks by pharyngeal aspiration.

RESULTS

Exposure to CNTs increased the expression level of adhesion molecule (ICAM-1) and enhanced THP-1 monocyte adhesion to HAECs. ApoE^-/- mice exposed to CNTs showed increased plaque area in the aorta by oil red O staining and up-regulation of ICAM-1 expression in the aorta, compared with vehicle-treated ApoE^-/- mice. Endothelial progenitor cells (EPCs) are mobilized from the bone marrow into the circulation and subsequently migrate to the site of endothelial damage and repair. Exposure of ApoE^-/- mice to high-dose SWCNTs or DWCNTs reduced the colony-forming units of EPCs in the bone marrow and diminished their migration function.

CONCLUSION

The results suggested that SWCNTs and DWCNTs enhanced atherosclerogenesis by promoting monocyte adhesion to endothelial cells and inducing EPC dysfunction.

Downstream effect profiles discern different mechanisms of integrin αLβ2 inhibition

The integrin leucocyte function-associated antigen-1 (αLβ2, LFA-1) plays crucial roles in T cell adhesion, migration and immunological synapse (IS) formation. Consequently, αLβ2 is an important therapeutic target in autoimmunity. Three major classes of αLβ2 inhibitors with distinct modes of action have been described to date: Monoclonal antibodies (mAbs), small molecule α/β I allosteric and small molecule α I allosteric inhibitors. The objective of this study was to systematically compare these three modes of αLβ2 inhibition for their αLβ2 inhibitory as well as their potential agonist-like effects. All inhibitors assessed were found to potently block αLβ2-mediated leucocyte adhesion. None of the inhibitors induced ZAP70 phosphorylation, indicating absence of agonistic outside-in signalling. Paradoxically, however, the α/β I allosteric inhibitor XVA143 induced conformational changes within αLβ2 characteristic for an intermediate affinity state. This effect was not observed with the α I allosteric inhibitor LFA878 or the anti-αLβ2 mAb efalizumab. On the other hand, efalizumab triggered the unscheduled internalization of αLβ2 in CD4+ and CD8+ T cells while LFA878 and XVA143 did not affect or only mildly reduced αLβ2 surface expression, respectively. Moreover, efalizumab, in contrast to the small molecule inhibitors, disturbed the fine-tuned internalization/recycling of engaged TCR/CD3, concomitantly decreasing ZAP70 expression levels. In conclusion, different modes of αLβ2 inhibition are associated with fundamentally different biologic effect profiles. The differential established here is expected to provide important translational guidance as novel αLβ2 inhibitors will be advanced from bench to bedside.

LFA-1 integrin antibodies inhibit leukocyte α4β1-mediated adhesion by intracellular signaling

Binding of intercellular adhesion molecule-1 to the β2-integrin leukocyte function associated antigen-1 (LFA-1) is known to induce cross-talk to the α4β1 integrin. Using different LFA-1 monoclonal antibodies, we have been able to study the requirement and mechanism of action for the cross-talk in considerable detail. LFA-1-activating antibodies and those inhibitory antibodies that signal to α4β1 induce phosphorylation of Thr-758 on the β2-chain, which is followed by binding of 14-3-3 proteins and signaling through the G protein exchange factor Tiam1. This results in dephosphorylation of Thr-788/789 on the β1-chain of α4β1 and loss of binding to its ligand vascular cell adhesion molecule-1. The results show that with LFA-1 antibodies, we can activate LFA-1 and inhibit α4β1, inhibit both LFA-1 and α4β1, inhibit LFA-1 but not α4β1, or not affect LFA-1 or α4β1 These findings are important for the understanding of integrin regulation and for the interpretation of the effect of integrin antibodies and their use in clinical applications.

Heterotropic modulation of selectin affinity by allosteric antibodies affects leukocyte rolling

Selectins are a family of adhesion receptors designed for efficient leukocyte tethering to the endothelium under shear. As a key property to resist premature bond disruption, selectin adhesiveness is enhanced by tensile forces that promote the conversion of a bent into an extended conformation of the N-terminal lectin and epidermal growth factor-like domains. Conformation-specific Abs have been invaluable in deciphering the activation mechanism of integrins, but similar reagents are not available for selectins. In this study, we show that the anti-human L-selectin mAbs DREG-55 and LAM1-5 but not DREG-56, DREG-200, or LAM1-1 heterotropically modulate adhesion presumably by stabilizing the extended receptor conformation. Force-free affinity assays, flow chamber, and microkinetic studies reveal a ligand-specific modulation of L-selectin affinity by DREG-55 mAb, resulting in a dramatic decrease of rolling velocity under flow. Furthermore, secondary tethering of polymorphonuclear cells was blocked by DREG-200 but significantly boosted by DREG-55 mAb. The results emphasize the need for a new classification for selectin Abs and introduce the new concept of heterotropic modulation of receptor function.

Propofol shares the binding site with isoflurane and sevoflurane on leukocyte function-associated antigen-1

BACKGROUND

We previously demonstrated that propofol interacted with the leukocyte adhesion molecule leukocyte function-associated antigen-1 (LFA-1) and inhibited the production of interleukin-2 via LFA-1 in a dependent manner. However, the binding site(s) of propofol on LFA-1 remains unknown.

METHODS

First, the inhibition of LFA-1's ligand binding by propofol was confirmed in an enzyme-linked immunosorbent assay (ELISA) ELISA-type assay. The binding site of propofol on LFA-1 was probed with a photolabeling experiment using a photoactivatable propofol analog called azi-propofol-m. The adducted residues of LFA-1 by this compound were determined using liquid chromatography-mass spectrometry. In addition, the binding of propofol to the ligand-binding domain of LFA-1 was examined using 1-aminoanthracene (1-AMA) displacement assay. Furthermore, the binding site(s) of 1-AMA and propofol on LFA-1 was studied using the docking program GLIDE.

RESULTS

We demonstrated that propofol impaired the binding of LFA-1 to its ligand intercellular adhesion molecule-1. The photolabeling experiment demonstrated that the adducted residues were localized in the allosteric cavity of the ligand-binding domain of LFA-1 called "lovastatin site." The shift of fluorescence spectra was observed when 1-AMA was coincubated with the low-affinity conformer of LFA-1 ligand-binding domain (wild-type [WT] αL I domain), not with the high-affinity conformer, suggesting that 1-AMA bound only to WT αL I domain. In the 1-AMA displacement assay, propofol decreased 1-AMA fluorescence signal (at 520 nm), suggesting that propofol competed with 1-AMA and bound to the WT αL I domain. The docking simulation demonstrated that both 1-AMA and propofol bound to the lovastatin site, which agreed with the photolabeling experiment.

CONCLUSIONS

We demonstrated that propofol bound to the lovastatin site in LFA-1. Previously we showed that the volatile anesthetics isoflurane and sevoflurane bound to this site. Taken together, the lovastatin site is an example of the common binding sites for anesthetics currently used clinically.

Small molecule agonist of very late antigen-4 (VLA-4) integrin induces progenitor cell adhesion

Activation of the integrin family of cell adhesion receptors on progenitor cells may be a viable approach to enhance the effects of stem cell-based therapies by improving cell retention and engraftment. Here, we describe the synthesis and characterization of the first small molecule agonist identified for the integrin α4β1 (also known as very late antigen-4 or VLA-4). The agonist, THI0019, was generated via two structural modifications to a previously identified α4β1 antagonist. THI0019 greatly enhanced the adhesion of cultured cell lines and primary progenitor cells to α4β1 ligands VCAM-1 and CS1 under both static and flow conditions. Furthermore, THI0019 facilitated the rolling and spreading of cells on VCAM-1 and the migration of cells toward SDF-1α. Molecular modeling predicted that the compound binds at the α/β subunit interface overlapping the ligand-binding site thus indicating that the compound must be displaced upon ligand binding. In support of this model, an analog of THI0019 modified to contain a photoreactive group was used to demonstrate that when cross-linked to the integrin, the compound behaves as an antagonist instead of an agonist. In addition, THI0019 showed cross-reactivity with the related integrin α4β7 as well as α5β1 and αLβ2. When cross-linked to αLβ2, the photoreactive analog of THI0019 remained an agonist, consistent with it binding at the α/β subunit interface and not at the ligand-binding site in the inserted ("I") domain of the αL subunit. Co-administering progenitor cells with a compound such as THI0019 may provide a mechanism for enhancing stem cell therapy.

Leukocyte function antigen-1, kindlin-3, and calcium flux orchestrate neutrophil recruitment during inflammation

Neutrophil arrest and migration on inflamed endothelium involves a conformational shift in CD11a/CD18 (leukocyte function antigen-1; LFA-1) to a high-affinity and clustered state that determines the strength and lifetime of bond formation with ICAM-1. Cytoskeletal adapter proteins Kindlin-3 and Talin-1 anchor clustered LFA-1 to the cytoskeleton and facilitate the transition from neutrophil rolling to arrest. We recently reported that tensile force acts on LFA-1 bonds inducing their colocalization with Orai1, the predominant membrane store operated Ca(2+) channel that cooperates with the endoplasmic reticulum to elicit cytosolic flux. Because Kindlin-3 was recently reported to initiate LFA-1 clustering in lymphocytes, we hypothesized that it cooperates with Orai1 and LFA-1 in signaling local Ca(2+) flux necessary for shear-resistant neutrophil arrest. Using microfluidic flow channels combined with total internal reflection fluorescence microscopy, we applied defined shear stress to low- or high-affinity LFA-1 and imaged the spatiotemporal regulation of bond formation with Kindlin-3 recruitment and Ca(2+) influx. Orai1 and Kindlin-3 genes were silenced in neutrophil-like HL-60 cells to assess their respective roles in this process. Kindlin-3 was enriched within focal clusters of high-affinity LFA-1, which promoted physical linkage with Orai1. This macromolecular complex functioned to amplify inside-out Ca(2+) signaling in response to IL-8 stimulation by catalyzing an increased density of Talin-1 and consolidating LFA-1 clusters within sites of contact with ICAM-1. In this manner, neutrophils use focal adhesions as mechanosensors that convert shear stress-mediated tensile force into local bursts of Ca(2+) influx that catalyze cytoskeletal engagement and an adhesion-strengthened migratory phenotype.

The C-terminal αI domain linker as a critical structural element in the conformational activation of αI integrins

The activation of α/β heterodimeric integrins is the result of highly coordinated rearrangements within both subunits. The molecular interactions between the two subunits, however, remain to be characterized. In this study, we use the integrin α(L)β(2) to investigate the functional role of the C-linker polypeptide that connects the C-terminal end of the inserted (I) domain with the β-propeller domain on the α subunit and is located at the interface with the βI domain of the β chain. We demonstrate that shortening of the C-linker by eight or more amino acids results in constitutively active α(L)β(2) in which the αI domain is no longer responsive to the regulation by the βI domain. Despite this intersubunit uncoupling, both I domains remain individually sensitive to intrasubunit conformational changes induced by allosteric modulators. Interestingly, the length and not the sequence of the C-linker appears to be critical for its functionality in α/β intersubunit communication. Using two monoclonal antibodies (R7.1 and CBR LFA-1/1) we further demonstrate that shortening of the C-linker results in the gradual loss of combinational epitopes that require both the αI and β-propeller domains for full reactivity. Taken together, our findings highlight the role of the C-linker as a spring-like element that allows relaxation of the αI domain in the resting state and controlled tension of the αI domain during activation, exerted by the β chain.

Regulation of integrin affinity on cell surfaces

Lymphocyte activation triggers adhesiveness of lymphocyte function-associated antigen-1 (LFA-1; integrin α(L)β(2)) for intercellular adhesion molecules (ICAMs) on endothelia or antigen-presenting cells. Whether the activation signal, after transmission through multiple domains to the ligand-binding αI domain, results in affinity changes for ligand has been hotly debated. Here, we present the first comprehensive measurements of LFA-1 affinities on T lymphocytes for ICAM-1 under a broad array of activating conditions. Only a modest increase in affinity for soluble ligand was detected after activation by chemokine or T-cell receptor ligation, conditions that primed LFA-1 and robustly induced lymphocyte adhesion to ICAM-1 substrates. By stabilizing well-defined LFA-1 conformations by Fab, we demonstrate the absolute requirement of the open LFA-1 headpiece for adhesiveness and high affinity. Interaction of primed LFA-1 with immobilized but not soluble ICAM-1 triggers energy-dependent affinity maturation of LFA-1 to an adhesive, high affinity state. Our results lend support to the traction or translational motion dependence of integrin activation.

The interaction of LFA-1 on mononuclear cells and ICAM-1 on tubular epithelial cells accelerates TGF-β1-induced renal epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) of renal epithelial cells (RTECs) has pivotal roles in the development of renal fibrosis. Although the interaction of lymphocyte function-associated antigen 1 (LFA-1) on leukocytes and its ligand, intracellular adhesion molecule 1 (ICAM-1), plays essential roles in most inflammatory reactions, its pathogenetic role in the EMT of RTECs remains to be clarified. In the present study, we investigated the effect of the interaction of LFA-1 on peripheral blood mononuclear cells (PBMCs) and ICAM-1 on HK-2 cells after stimulation with TGF-β(1) on the EMT of RTECs. ICAM-1 was highly expressed in HK-2 cells. After TGF-β(1) stimulation, the chemokines CCL3 and CXCL12 increased on HK-2 cells. After co-culture of PBMCs and HK-2 cells pre-stimulated with TGF-β(1) (0.1 ng/ml) (HK-2-TGF-β(1) (0.1)), the expression of the active form of LFA-1 increased on PBMCs; however, total LFA-1 expression did not change. The expression of the active form of LFA-1 on PBMCs did not increase after co-culture with not CCL3 but CXCL12 knockdown HK-2-TGF-β(1) (0.1). The expression of epithelial cell junction markers (E-cadherin and occludin) further decreased and that of mesenchymal markers (vimentin and fibronectin) further increased in HK-2-TGF-β(1) (0.1) after co-culture with PBMCs for 24 hrs (HK-2-TGF-β(1) (0.1)-PBMCs). The phosphorylation of ERK 1/2 but not smad2 and smad3 increased in HK-2-TGF-β(1) (0.1)-PBMCs. The snail and slug signaling did not increase HK-2-TGF-β(1) (0.1)-PBMCs. Although the migration and invasion of HK-2 cells induced full EMT by a high dose (10.0 ng/ml) and long-term (72-96 hrs) TGF-β(1) stimulation increased, that of HK-2-TGF-β(1) (0.1)-PBMCs did not increase. These results suggested that HK-2 cells stimulated with TGF-β(1) induced conformational activation of LFA-1 on PBMCs by increased CXCL12. Then, the direct interaction of LFA-1 on PBMCs and ICAM-1 on HK-2 cells activated ERK1/2 signaling to accelerate the part of EMT of HK-2 cells induced by TGF-β(1).

Sedative drug modulates T-cell and lymphocyte function-associated antigen-1 function

BACKGROUND

Sedative drugs modify immune cell functions via several mechanisms. However, the effects of sedatives on immune function have been primarily investigated in neutrophils and macrophages, and to the lesser extent lymphocytes. Lymphocyte function-associated antigen-1 (LFA-1) is an adhesion molecule that has a central role in regulating immune function of lymphocytes including interleukin-2 (IL-2) production and lymphocyte proliferation. Previous clinical studies reported that propofol and isoflurane reduced IL-2 level in patients, but midazolam did not. We previously demonstrated that isoflurane inhibited LFA-1 binding to its counter ligand, intercellular adhesion molecule-1 (ICAM-1), which might contribute to the reduction of IL-2 levels. In the current study, we examined the effect of propofol, midazolam, and dexmedetomidine on LFA-1/ICAM-1 binding, and the subsequent biological effects.

METHODS

The effect of sedative drugs on T-cell proliferation and IL-2 production was measured by calorimetric assays on human peripheral blood mononuclear cells. Because LFA-1/ICAM-1 binding has an important role in T-cell proliferation and IL-2 production, we measured the effect of sedative drugs on ICAM-1 binding to LFA-1 protein (cell-free assay). This analysis was followed by flow cytometric analysis of LFA-1 expressing T-cell binding to ICAM-1 (cell-based assay). To determine whether the drug/LFA-1 interaction is caused by competitive or allosteric inhibition, we analyzed the sedative drug effect on wild-type and high-affinity LFA-1 and a panel of monoclonal antibodies that bind to different regions of LFA-1.

RESULTS

Propofol at 10 to 100 μM inhibited ICAM-1 binding to LFA-1 in cell-free assays and cell-based assays (P < 0.05). However, dexmedetomidine and midazolam did not affect LFA-1/ICAM-1 binding. Propofol directly inhibits LFA-1 binding to ICAM-1 by binding near the ICAM-1 contact area in a competitive manner. At clinically relevant concentrations, propofol, but not dexmedetomidine or midazolam, inhibited IL-2 production (P < 0.05). Additionally, propofol inhibited lymphocyte proliferation (P < 0.05).

CONCLUSIONS

Our study suggests that propofol competitively inhibits LFA-1 binding to ICAM-1 on T-cells and suppresses T-cell proliferation and IL-2 production, whereas dexmedetomidine and midazolam do not significantly influence these immunological assays.

RNAi-mediated CCR5 silencing by LFA-1-targeted nanoparticles prevents HIV infection in BLT mice

RNA interference (RNAi)-mediated knockdown of gene expression offers a novel treatment strategy for human immunodeficiency virus (HIV) infection. However, the major hurdle for clinical use is a practical strategy for small interfering RNA (siRNA) delivery to the multiple immune cell types important in viral pathogenesis. We have developed a novel immunoliposome method targeting the lymphocyte function-associated antigen-1 (LFA-1) integrin expressed on all leukocytes and evaluated it for systemic delivery of siRNA in a humanized mouse model. We show that in vivo administration of the LFA-1 integrin-targeted and stabilized nanoparticles (LFA-1 I-tsNPs) results in selective uptake of siRNA by T cells and macrophages, the prime targets of HIV. Further, in vivo administration of anti-CCR5 siRNA/LFA-1 I-tsNPs resulted in leukocyte-specific gene silencing that was sustained for 10 days. Finally, humanized mice challenged with HIV after anti-CCR5 siRNA treatment showed enhanced resistance to infection as assessed by the reduction in plasma viral load and disease-associated CD4 T-cell loss. This study demonstrates the potential in vivo applicability of LFA-1-directed siRNA delivery as anti-HIV prophylaxis.

Structural basis of activation-dependent binding of ligand-mimetic antibody AL-57 to integrin LFA-1

The activity of integrin LFA-1 (alpha(L)beta(2)) to its ligand ICAM-1 is regulated through the conformational changes of its ligand-binding domain, the I domain of alpha(L) chain, from an inactive, low-affinity closed form (LA), to an intermediate-affinity form (IA), and then finally, to a high-affinity open form (HA). A ligand-mimetic human monoclonal antibody AL-57 (activated LFA-1 clone 57) was identified by phage display to specifically recognize the affinity-upregulated I domain. Here, we describe the crystal structures of the Fab fragment of AL-57 in complex with IA, as well as in its unligated form. We discuss the structural features conferring AL-57's strong selectivity for the high affinity, open conformation of the I domain. The AL-57-binding site overlaps the ICAM-1 binding site on the I domain. Furthermore, an antibody Asp mimics an ICAM Glu by forming a coordination to the metal-ion dependent adhesion site (MIDAS). The structure also reveals better shape complementarity and a more hydrophobic interacting interface in AL-57 binding than in ICAM-1 binding. The results explain AL-57's antagonistic mimicry of LFA-1's natural ligands, the ICAM molecules.

LFA-1 affinity regulation is necessary for the activation and proliferation of naive T cells

The activation of LFA-1 (lymphocyte function-associated antigen) is a critical event for T cell co-stimulation. The mechanism of LFA-1 activation involves both affinity and avidity regulation, but the role of each in T cell activation remains unclear. We have identified antibodies that recognize and block different affinity states of the mouse LFA-1 I-domain. Monoclonal antibody 2D7 preferentially binds to the low affinity conformation, and this specific binding is abolished when LFA-1 is locked in the high affinity conformation. In contrast, M17/4 can bind both the locked high and low affinity forms of LFA-1. Although both 2D7 and M17/4 are blocking antibodies, 2D7 is significantly less potent than M17/4 in blocking LFA-1-mediated adhesion; thus, blocking high affinity LFA-1 is critical for preventing LFA-1-mediated adhesion. Using these reagents, we investigated whether LFA-1 affinity regulation affects T cell activation. We found that blocking high affinity LFA-1 prevents interleukin-2 production and T cell proliferation, demonstrated by TCR cross-linking and antigen-specific stimulation. Furthermore, there is a differential requirement of high affinity LFA-1 in the activation of CD4(+) and CD8(+) T cells. Although CD4(+) T cell activation depends on both high and low affinity LFA-1, only high affinity LFA-1 provides co-stimulation for CD8(+) T cell activation. Together, our data demonstrated that the I-domain of LFA-1 changes to the high affinity state in primary T cells, and high affinity LFA-1 is critical for facilitating T cell activation. This implicates LFA-1 activation as a novel regulatory mechanism for the modulation of T cell activation and proliferation.

Efalizumab binding to the LFA-1 alphaL I domain blocks ICAM-1 binding via steric hindrance

Lymphocyte function-associated antigen 1 (LFA-1) plays important roles in immune cell adhesion, trafficking, and activation and is a therapeutic target for the treatment of multiple autoimmune diseases. Efalizumab is one of the most efficacious antibody drugs for treating psoriasis, a very common skin disease, through inhibition of the binding of LFA-1 to the ligand intercellular adhesion molecule 1 (ICAM-1). We report here the crystal structures of the Efalizumab Fab alone and in complex with the LFA-1 alpha(L) I domain, which reveal the molecular mechanism of inhibition of LFA-1 by Efalizumab. The Fab binds with an epitope on the inserted (I) domain that is distinct from the ligand-binding site. Efalizumab binding blocks the binding of LFA-1 to ICAM-1 via steric hindrance between its light chain and ICAM-1 domain 2 and thus inhibits the activities of LFA-1. These results have important implications for the development of improved antibodies and new therapeutic strategies for the treatment of autoimmune diseases.

Novel tools for functional analysis of CD11c: activation-specific, activation-independent, and activating antibodies

Functions and binding properties of four CD11c-specific mAbs are described here. The mAb 496B stimulated, while 496K inhibited ligand binding of CD11c. The stimulatory mAb, 496B, as well as the inhibitory mAbs BU15 and 496 K appear to act allosterically, as they do not bind the CD11c I domain. The mAb 3.9 bound preferentially to activated forms of CD11c and the binding was divalent cation dependent. CD11c binding to 3.9 recapitulates many of the integrin-ligand interactions. Our data suggest that 3.9 is a competitive antagonist, BU15 and 496K are allosteric antagonists, and 496B is an allosteric agonist of CD11c. These mAbs provide a set of tools to study the functions of the dendritic cell marker, CD11c.

Regulation of LFA-1-dependent inflammatory cell recruitment by Cbl-b and 14-3-3 proteins

Inside-out signaling regulation of the beta2-integrin leukocyte function-associated antigen-1 (LFA-1) by different cytoplasmic proteins, including 14-3-3 proteins, is essential for adhesion and migration of immune cells. Here, we identify a new pathway for the regulation of LFA-1 activity by Cbl-b, an adapter molecule and ubiquitin ligase that modulates several signaling pathways. Cbl-b-/- mice displayed increased macrophage recruitment in thioglycollate-induced peritonitis, which was attributed to Cbl-b deficiency in macrophages, as assessed by bone marrow chimera experiments. In vitro, Cbl-b-/- bone marrow-derived mononuclear phagocytes (BMDMs) displayed increased adhesion to endothelial cells. Activation of LFA-1 in Cbl-b-deficient cells was responsible for their increased endothelial adhesion in vitro and peritoneal recruitment in vivo, as the phenotype of Cbl-b deficiency was reversed in Cbl-b-/-LFA-1-/- mice. Consistently, LFA-1-mediated adhesion of BMDM to ICAM-1 but not VLA-4-mediated adhesion to VCAM-1 was enhanced by Cbl-b deficiency. Cbl-b deficiency resulted in increased phosphorylation of T758 in the beta2-chain of LFA-1 and thereby in enhanced association of 14-3-3beta protein with the beta2-chain, leading to activation of LFA-1. Consistently, disruption of the 14-3-3/beta2-integrin interaction abrogated the enhanced ICAM-1 adhesion of Cbl-b-/- BMDMs. In conclusion, Cbl-b deficiency activates LFA-1 and LFA-1-mediated inflammatory cell recruitment by stimulating the interaction between the LFA-1 beta-chain and 14-3-3 proteins.

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.

Selective gene silencing in activated leukocytes by targeting siRNAs to the integrin lymphocyte function-associated antigen-1

Silencing gene expression by RNAi is a powerful method for exploring gene function and validating drug targets and potentially for therapy. Lymphocytes and other primary blood cells are resistant to lipid-based transfection in vitro and are difficult to target in vivo. We show here that antibody-protamine fusion proteins targeting the human integrin lymphocyte function-associated antigen-1 (LFA-1) efficiently deliver siRNAs and specifically induce silencing in primary lymphocytes, monocytes, and dendritic cells. Moreover, a fusion protein constructed from an antibody that preferentially recognizes activation-dependent conformational changes in LFA-1 selectively targets activated leukocytes and can be used to suppress gene expression and cell proliferation only in activated lymphocytes. The siRNA-fusion protein complexes do not cause lymphocyte activation or induce IFN responses. K562 cells expressing latent WT or constitutively activated LFA-1 engrafted in the lungs of SCID mice are selectively targeted by intravenously injected fusion protein-siRNA complexes, demonstrating the potential in vivo applicability of LFA-1-directed siRNA delivery.

Integrin Regulation of Lymphocyte Trafficking: Lessons from Structural and Signaling Studies

High trafficking capability of lymphocytes is crucial in immune surveillance and antigen responses. Central to this regulatory process is a dynamic control of lymphocyte adhesion behavior regulated by chemokines and adhesion receptors such as integrins. Modulation of lymphocyte adhesive responses occurs in a wide range of time window from less than a second to hours, enabling rolling lymphocyte to attach to and migrate through endothelium and interact with antigen-presenting cells. While there has been a rapid progress in the understanding of integrin structure, elucidation of signaling events to relay extracellular signaling to integrins in physiological contexts has recently emerged from studies using gene-targeting and gene-silencing technique. Regulatory molecules critical for integrin activity control distribution of integrins, polarized cell morphology and motility, suggesting a signaling network that coordinates integrin function with lymphocyte migration. Here, I review recent studies of integrin structural changes and intracellular signal molecules that trigger integrin activation (inside-out signals), and discuss molecular mechanisms that control lymphocyte integrins and how inside-out signals coordinately modulate adhesive reactions and cell shape and migration.

A small molecule agonist of an integrin, alphaLbeta2

The binding of integrin alpha(L)beta(2) to its ligand intercellular adhesion molecule-1 is required for immune responses and leukocyte trafficking. Small molecule antagonists of alpha(L)beta(2) are under intense investigation as potential anti-inflammatory drugs. We describe for the first time a small molecule integrin agonist. A previously described alpha/beta I allosteric inhibitor, compound 4, functions as an agonist of alpha(L)beta(2) in Ca(2+) and Mg(2+)and as an antagonist in Mn(2+). We have characterized the mechanism of activation and its competitive and noncompetitive inhibition by different compounds. Although it stimulates ligand binding, compound 4 nonetheless inhibits lymphocyte transendothelial migration. Agonism by compound 4 results in accumulation of alpha(L)beta(2) in the uropod, extreme uropod elongation, and defective de-adhesion. Small molecule integrin agonists open up novel therapeutic possibilities.

Evidence that a significant number of naive T cells enter non-lymphoid organs as part of a normal migratory pathway

Only activated and effector memory T cells are thought to access non-lymphoid tissues. In contrast, naive T cells are thought to circulate only between the blood, lymph and secondary lymphoid organs. We examined the phenotype of endogenous T cells in various non-lymphoid organs and showed that a subset of cells exhibited an apparently naive phenotype and were functionally inactive. FTY720 treatment selectively depleted this population from the non-lymphoid tissues. In addition, RAG-deficient TCR transgenic CD4 and CD8 T cells were present in non-lymphoid tissues in bone marrow chimeric mice and in situ imaging analysis revealed their location in the parenchymal tissues. Moreover, migration of TCR transgenic T cells to non-lymphoid tissues after adoptive transfer was pertussis-toxin resistant. Overall, the results suggest that naive T cells may circulate through non-lymphoid tissues as part of their normal migratory pathway.

Characterization of Binding Properties of ICAM-1 Peptides to LFA-1: Inhibitors of T-cell Adhesion

In the present study, we characterized the binding site of two intercellular adhesion molecule-1-derived cyclic peptides, cIBC and cIBR, to the LFA-1 on the surface of T cells. These peptides had been able to inhibit LFA-1/intercellular adhesion molecule-1 signal by blocking the signal-2 of immune synapse. Both peptides prefer to bind to the closed form of LFA-1 I-domain, indicating that two peptides act as allosteric inhibitors against intercellular adhesion molecule-1. Binding site mapping using monoclonal antibodies proposes that cIBC binds to around residues 266-272 of LFA-1 I-domain where this site is adjacent to the metal ion-dependent adhesion site. On the other hand, cIBR binds to the pocket called L-site where is distant from metal ion-dependent adhesion site. Cross-inhibition mapping between two peptides show that cIBR could inhibit the binding of cIBC but not vice versa, suggesting that cIBR has some properties that allow this peptide bind to more than one site. Structural comparison between cIBC and cIBR reveals that cIBR is more flexible than cIBC, allowing this peptide bind to exposed region, such as cIBC-binding site as well as cramped pocket like L-site. Our findings are important for understanding the selectivity of cIBC and cIBR peptides; thus, they can be conjugated with drugs and transported specifically to the target.

Rolling adhesion of alphaL I domain mutants decorrelated from binding affinity

Activated lymphocyte function-associated antigen-1 (LFA-1, alphaLbeta2 integrin) found on leukocytes facilitates firm adhesion to endothelial cell layers by binding to intercellular adhesion molecule-1 (ICAM-1), which is up-regulated on endothelial cells at sites of inflammation. Recent work has shown that LFA-1 in a pre-activation, low-affinity state may also be involved in the initial tethering and rolling phase of the adhesion cascade. The inserted (I) domain of LFA-1 contains the ligand-binding epitope of the molecule, and a conformational change in this region during activation increases ligand affinity. We have displayed wild-type I domain on the surface of yeast and validated expression using I domain specific antibodies and flow cytometry. Surface display of I domain supports yeast rolling on ICAM-1-coated surfaces under shear flow. Expression of a locked open, high-affinity I domain mutant supports firm adhesion of yeast, while yeast displaying intermediate-affinity I domain mutants exhibit a range of rolling phenotypes. We find that rolling behavior for these mutants fails to correlate with ligand binding affinity. These results indicate that unstressed binding affinity is not the only molecular property that determines adhesive behavior under shear flow.

Directed evolution to probe protein allostery and integrin I domains of 200,000-fold higher affinity

Understanding allostery may serve to both elucidate mechanisms of protein regulation and provide a basis for engineering active mutants. Herein we describe directed evolution applied to the integrin alpha(L) inserted domain for studying allostery by using a yeast surface display system. Many hot spots for activation are identified, and some single mutants exhibit remarkable increases of 10,000-fold in affinity for a physiological ligand, intercellular adhesion molecule-1. The location of activating mutations traces out an allosteric interface in the interior of the inserted domain that connects the ligand binding site to the alpha7-helix, which communicates allostery to neighboring domains in intact integrins. The combination of two activating mutations (F265S/F292G) leads to an increase of 200,000-fold in affinity to intercellular adhesion molecule-1. The F265S/F292G mutant is potent in antagonizing lymphocyte function-associated antigen 1-dependent lymphocyte adhesion, aggregation, and transmigration.

Specific integrin alpha and beta chain phosphorylations regulate LFA-1 activation through affinity-dependent and -independent mechanisms

Integrins are adhesion receptors that are crucial to the functions of multicellular organisms. Integrin-mediated adhesion is a complex process that involves both affinity regulation and cytoskeletal coupling, but the molecular mechanisms behind this process have remained incompletely understood. In this study, we report that the phosphorylation of each cytoplasmic domain of the leukocyte function-associated antigen-1 integrin mediates different modes of integrin activation. α Chain phosphorylation on Ser1140 is needed for conformational changes in the integrin after chemokine- or integrin ligand–induced activation or after activation induced by active Rap1 (Rap1V12). In contrast, the β chain Thr758 phosphorylation mediates selective binding to 14-3-3 proteins in response to inside-out activation through the T cell receptor, resulting in cytoskeletal rearrangements. Thus, site-specific phosphorylation of the integrin cytoplasmic domains is important for the dynamic regulation of these complex receptors in cells.

Targeting integrin structure and function in disease

Initially linked to the pathogenesis of inflammatory and hematologic diseases, integrins have become validated drug targets with the approval of five drugs. Moreover, there are several promising drug candidates in preclinical and clinical stages of development for multiple clinical indications. Integrins are attractive drug targets as their antagonism can block several steps in disease progression or maintenance. Integrin inhibitors can block the proliferation, migration, or tissue localization of inflammatory, angiogenic, and tumor cells, as well as signaling and gene expression contributing to disease. There has been a rapid increase in the elucidation of integrin structure, their allosteric mechanisms of bidirectional signaling, and the structure of complexes with drugs. This information brings greater focus to how integrins support various cellular functions and how they have been and may be targeted to develop novel drugs. Here we review conformational switches, including an internal ligand, which allosterically regulate the transition from low- to high-affinity ligand binding. We address some of the successes, disappointments, and challenges in targeting competitive or allosteric sites to develop therapeutics. We also discuss new opportunities, including a structure-based approach to discover novel drugs to treat inflammatory and other diseases. This approach targets structural relatives of the von Willebrand factor A-domain present in integrins and many functionally diverse proteins.

MEKK1 controls neurite regrowth after experimental injury by balancing ERK1/2 and JNK2 signaling

After injury, peripheral neuronal cells initiate complex signaling cascades to promote survival and regeneration. In the present study, we have identified the mitogen-activated protein kinase (MAPK) isoforms which are necessary for nerve growth factor (NGF)-induced neurite regrowth after injury of differentiated PC12 cells. Extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the usually pro-apoptotic c-Jun N-terminal kinase 2 (JNK2) are crucial for neurite regrowth, while p38 plays no role in this context. Surprisingly, the MEK1 inhibitors PD 98059 and U 0126 blocked both ERK1/2 and JNK phosphorylation, indicating a novel form of balancing MAPK cascade cross-talk. Results from RNAi experiments excluded direct ERK/JNK interactions. We identified the upstream kinase MEKK1 as an activator of both the ERK1/2 and JNK2 pathways, whereby the ERK1/2 kinase MEK1 and the JNK kinase MKK7 bind to MEKK1 in a competing fashion. Our findings suggest an important role of JNK2 and MAPK pathway cross-talk in neurite regeneration.

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.

An atomic resolution view of ICAM recognition in a complex between the binding domains of ICAM-3 and integrin alphaLbeta2

Within the Ig superfamily (IgSF), intercellular adhesion molecules (ICAMs) form a subfamily that binds the leukocyte integrin alphaLbeta2. We report a 1.65-A-resolution crystal structure of the ICAM-3 N-terminal domain (D1) in complex with the inserted domain, the ligand-binding domain of alphaLbeta2. This high-resolution structure and comparisons among ICAM subfamily members establish that the binding of ICAM-3 D1 onto the inserted domain represents a common docking mode for ICAM subfamily members. The markedly different off-rates of ICAM-1, -2, and -3 appear to be determined by the hydrophobicity of residues that surround a metal coordination bond in the alphaLbeta2-binding interfaces. Variation in composition of glycans on the periphery of the interfaces influences on-rate.