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

Allosteric targeting resolves limitations of earlier LFA-1 directed modalities

Integrins are a family of cell surface receptors well-recognized for their therapeutic potential in a wide range of diseases. However, the development of integrin targeting medications has been impacted by unexpected downstream effects, reflecting originally unforeseen interference with the bidirectional signalling and cross-communication of integrins. We here selected one of the most severely affected target integrins, the integrin lymphocyte function-associated antigen-1 (LFA-1, α_Lβ₂, CD11a/CD18), as a prototypic integrin to systematically assess and overcome these known shortcomings. We employed a two-tiered ligand-based virtual screening approach to identify a novel class of allosteric small molecule inhibitors targeting this integrin's αI domain. The newly discovered chemical scaffold was derivatized, yielding potent bis-and tris-aryl-bicyclic-succinimides which inhibit LFA-1 in vitro at low nanomolar concentrations. The characterisation of these compounds in comparison to earlier LFA-1 targeting modalities established that the allosteric LFA-1 inhibitors (i) are devoid of partial agonism, (ii) selectively bind LFA-1 versus other integrins, (iii) do not trigger internalization of LFA-1 itself or other integrins and (iv) display oral availability. This profile differentiates the new generation of allosteric LFA-1 inhibitors from previous ligand mimetic-based LFA-1 inhibitors and anti-LFA-1 antibodies, and is projected to support novel immune regulatory regimens selectively targeting the integrin LFA-1. The rigorous computational and experimental assessment schedule described here is designed to be adaptable to the preclinical discovery and development of novel allosterically acting compounds targeting integrins other than LFA-1, providing an exemplary approach for the early characterisation of next generation integrin inhibitors.

Mechanically Regulated Outside-In Activation of an I-Domain-Containing Integrin

Integrins are heterodimeric transmembrane proteins that mediate cellular adhesion and bidirectional mechanotransductions through their conformational allostery. The allosteric pathway of an I-domain-containing integrin remains unclear because of its complexity and lack of effective experiments. For a typical I-domain-containing integrin α_Xβ₂, molecular dynamics simulations were employed here to investigate the conformational dynamics in the first two steps of outside-in activation, the bindings of both the external and internal ligands. Results showed that the internal ligand binding is a prerequisite to the allosteric transmission from the α- to β-subunits and the exertion of external force to integrin-ligand complex. The opening state of αI domain with downward movement and lower half unfolding of α₇-helix ensures the stable intersubunit conformational transmission through external ligand binding first and internal ligand binding later. Reverse binding order induces a, to our knowledge, novel but unstable swingout of β-subunit Hybrid domain with the retained close states of both αI and βI domains. Prebinding of external ligand greatly facilitates the following internal ligand binding and vice versa. These simulations furthered the understanding in the outside-in activation of I-domain-containing integrins from the viewpoint of internal allosteric pathways.

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.

Ligand- and cation-induced structural alterations of the leukocyte integrin LFA-1

In αI integrins, including leukocyte function-associated antigen 1 (LFA-1), ligand-binding function is delegated to the αI domain, requiring extra steps in the relay of signals that activate ligand binding and coordinate it with cytoplasmic signals. Crystal structures reveal great variation in orientation between the αI domain and the remainder of the integrin head. Here, we investigated the mechanisms involved in signal relay to the αI domain, including whether binding of the ligand intercellular adhesion molecule-1 (ICAM-1) to the αI domain is linked to headpiece opening and engenders a preferred αI domain orientation. Using small-angle X-ray scattering and negative-stain EM, we define structures of ICAM-1, LFA-1, and their complex, and the effect of activation by Mn²⁺ Headpiece opening was substantially stabilized by substitution of Mg²⁺ with Mn²⁺ and became complete upon ICAM-1 addition. These agents stabilized αI-headpiece orientation, resulting in a well-defined orientation of ICAM-1 such that its tandem Ig-like domains pointed in the opposite direction from the β-subunit leg of LFA-1. Mutations in the integrin βI domain α1/α1' helix stabilizing either the open or the closed βI-domain conformation indicated that α1/α1' helix movements are linked to ICAM-1 binding by the αI domain and to the extended-open conformation of the ectodomain. The LFA-1-ICAM-1 orientation described here with ICAM-1 pointing anti-parallel to the LFA-1 β-subunit leg is the same orientation that would be stabilized by tensile force transmitted between the ligand and the actin cytoskeleton and is consistent with the cytoskeletal force model of integrin activation.

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.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

Leukocyte integrin αLβ2 headpiece structures: The αI domain, the pocket for the internal ligand, and concerted movements of its loops

High-resolution crystal structures of the headpiece of lymphocyte function-associated antigen-1 (integrin αLβ2) reveal how the αI domain interacts with its platform formed by the α-subunit β-propeller and β-subunit βI domains. The αLβ2 structures compared with αXβ2 structures show that the αI domain, tethered through its N-linker and a disulfide to a stable β-ribbon pillar near the center of the platform, can undergo remarkable pivoting and tilting motions that appear buffered by N-glycan decorations that differ between αL and αX subunits. Rerefined β2 integrin structures reveal details including pyroglutamic acid at the β2 N terminus and bending within the EGF1 domain. Allostery is relayed to the αI domain by an internal ligand that binds to a pocket at the interface between the β-propeller and βI domains. Marked differences between the αL and αX subunit β-propeller domains concentrate near the binding pocket and αI domain interfaces. Remarkably, movement in allostery in the βI domain of specificity determining loop 1 (SDL1) causes concerted movement of SDL2 and thereby tightens the binding pocket for the internal ligand.

A novel multi-parameter assay to dissect the pharmacological effects of different modes of integrin αLβ2 inhibition in whole blood

BACKGROUND AND PURPOSE

The integrin αLβ2 plays central roles in leukocyte adhesion and T cell activation, rendering αLβ2 an attractive therapeutic target. Compounds with different modes of αLβ2 inhibition are in development, currently. Consequently, there is a foreseeable need for bedside assays, which allow assessment of the different effects of diverse types of αLβ2 inhibitors in the peripheral blood of treated patients.

EXPERIMENTAL APPROACH

Here, we describe a flow cytometry-based technology that simultaneously quantitates αLβ2 conformational change upon inhibitor binding, αLβ2 expression and T cell activation at the single-cell level in human blood. Two classes of allosteric low MW inhibitors, designated α I and α/β I allosteric αLβ2 inhibitors, were investigated. The first application revealed intriguing inhibitor class-specific profiles.

KEY RESULTS

Half-maximal inhibition of T cell activation was associated with 80% epitope loss induced by α I allosteric inhibitors and with 40% epitope gain induced by α/β I allosteric inhibitors. This differential establishes that inhibitor-induced αLβ2 epitope changes do not directly predict the effect on T cell activation. Moreover, we show here for the first time that α/β I allosteric inhibitors, in contrast to α I allosteric inhibitors, provoked partial downmodulation of αLβ2, revealing a novel property of this inhibitor class.

CONCLUSIONS AND IMPLICATIONS

The multi-parameter whole blood αLβ2 assay described here may enable therapeutic monitoring of αLβ2 inhibitors in patients' blood. The assay dissects differential effect profiles of different classes of αLβ2 inhibitors.

Anchorage-dependent binding of integrin I-domain to adhesion ligands

The dynamic interactions between leukocyte integrin receptors and ligands in the vascular endothelium, extracellular matrix, or invading pathogens result in leukocyte adhesion, extravasation, and phagocytosis. This work examined the mechanical strength of the connection between iC3b, a complement component that stimulates phagocytosis, and the ligand-binding domain, the I-domain, of integrin αMβ2. Single-molecule force measurements of αM I-domain-iC3b complexes were conducted by atomic force microscope. Strikingly, depending on loading rates, immobilization of the I-domain via its C-terminus resulted in a 1.3-fold to 1.5-fold increase in unbinding force compared with I-domains immobilized via the N-terminus. The force spectra (unbinding force versus loading rate) of the I-domain-iC3b complexes revealed that the enhanced mechanical strength is due to a 2.4-fold increase in the lifetime of the I-domain-iC3b bond. Given the structural and functional similarity of all integrin I-domains, our result supports the existing allosteric regulatory model by which the ligand binding strength of integrin can be increased rapidly when a force is allowed to stretch the C-terminus of the I-domain. This type of mechanism may account for the rapid ligand affinity adjustment during leukocyte migration.

An internal ligand-bound, metastable state of a leukocyte integrin, αXβ2

The crystal structure of a metastable, internal ligand-bound conformation of the α_Xβ₂ integrin suggests it enables rapid equilibration between the bent-closed and extended-open conformational states.

How is massive conformational change in integrins achieved on a rapid timescale? We report crystal structures of a metastable, putative transition state of integrin α_Xβ₂. The α_Xβ₂ ectodomain is bent; however, a lattice contact stabilizes its ligand-binding αI domain in a high affinity, open conformation. Much of the αI α7 helix unwinds, loses contact with the αI domain, and reshapes to form an internal ligand that binds to the interface between the β propeller and βI domains. Lift-off of the αI domain above this platform enables a range of extensional and rotational motions without precedent in allosteric machines. Movements of secondary structure elements in the β₂ βI domain occur in an order different than in β₃ integrins, showing that integrin β subunits can be specialized to assume different intermediate states between closed and open. Mutations demonstrate that the structure trapped here is metastable and can enable rapid equilibration between bent and extended-open integrin conformations and up-regulation of leukocyte adhesiveness.

Modulation of integrin activation and signaling by α1/α1′-helix unbending at the junction

How conformational signals initiated from one end of the integrin are transmitted to the other end remains elusive. At the ligand-binding βI domain, the α1/α1′-helix changes from a bent to a straightened α-helical conformation upon integrin headpiece opening. We demonstrated that a conserved glycine at the α1/α1′ junction is critical for maintaining the bent conformation of the α1/α1′-helix in the resting state. Mutations that facilitate α1/α1′-helix unbending rendered integrin constitutively active. However, mutations that block the α1/α1′-helix unbending abolished soluble ligand binding upon either outside or inside stimuli. Such mutations also blocked ligand-induced integrin extension from outside the cell, but had no effect on talin-induced integrin extension from inside the cell. In addition, integrin mediated cell spreading, F-actin stress fiber and focal adhesion formation, and focal adhesion kinase activation were also defective in these mutant integrins, although the cells still adhered to immobilized ligands at a reduced level. Our data establish the structural role of the α1/α1′ junction that allows relaxation of the α1/α1′-helix in the resting state and transmission of bidirectional conformational signals by helix unbending upon integrin activation.

Identification, characterization, and epitope mapping of human monoclonal antibody J19 that specifically recognizes activated integrin α4β7

Integrin α(4)β(7) is a lymphocyte homing receptor that mediates both rolling and firm adhesion of lymphocytes on vascular endothelium, two of the critical steps in lymphocyte migration and tissue-specific homing. The rolling and firm adhesions of lymphocytes rely on the dynamic shift between the inactive and active states of integrin α(4)β(7), which is associated with the conformational rearrangement of integrin molecules. Activation-specific antibodies, which specifically recognize the activated integrins, have been used as powerful tools in integrin studies, whereas there is no well characterized activation-specific antibody to integrin α(4)β(7). Here, we report the identification, characterization, and epitope mapping of an activation-specific human mAb J19 against integrin α(4)β(7). J19 was discovered by screening a human single-chain variable fragment phage library using an activated α(4)β(7) mutant as target. J19 IgG specifically bound to the high affinity α(4)β(7) induced by Mn(2+), DTT, ADP, or CXCL12, but not to the low affinity integrin. Moreover, J19 IgG did not interfere with α(4)β(7)-MAdCAM-1 interaction. The epitope of J19 IgG was mapped to Ser-331, Ala-332, and Ala-333 of β(7) I domain and a seven-residue segment from 184 to 190 of α(4) β-propeller domain, which are buried in low affinity integrin with bent conformation and only exposed in the high affinity extended conformation. Taken together, J19 is a potentially powerful tool for both studies on α(4)β(7) activation mechanism and development of novel therapeutics targeting the activated lymphocyte expressing high affinity α(4)β(7).