Characterization of a conformationally sensitive murine monoclonal antibody directed to the metal ion-dependent adhesion site face of integrin CD11b

Hemolytic Uremic Syndrome-Induced Acute Kidney Injury Treated via Immunomodulation with the Selective Cytopheretic Device

INTRODUCTION

Shiga-toxin associated-hemolytic uremic syndrome (STEC-HUS) is a severe cause of acute kidney injury (AKI) in children. Although most children recover, about 5% die and 30% develop chronic renal morbidity. HUS pathophysiology includes activated neutrophils damaging vascular endothelial cells. Therapeutic immunomodulation of activated neutrophils may alter the progression of disease. We present 3 pediatric patients treated with the selective cytopheretic device (SCD).

METHODS

We describe a 12 y.o. (patient 1) and two 2 y.o. twins (patients 2 and 3) with STEC-HUS requiring continuous renal replacement therapy (CRRT) who were enrolled in two separate studies of the SCD.

RESULTS

Patient 1 presented with STEC-HUS causing AKI and multisystem organ failure and received 7 days of SCD and CRRT treatment. After SCD initiation, the patient had gradual recovery of multi-organ dysfunction, with normal kidney and hematologic parameters at 60-day follow-up. Patients 2 and 3 presented with STEC-HUS with AKI requiring dialysis. Each received 24 h of SCD therapy. Thereafter, both gradually improved, with normalization (patient 2) and near-normalization (patient 3) of kidney function at 60-day follow-up.

CONCLUSION

Immunomodulatory treatment with the SCD was associated with improvements in multisystem stigmata of STEC-HUS-induced AKI and was well-tolerated without any device-related adverse events.

Use of extracorporeal immunomodulation in a toddler with hemophagocytic lymphohistiocytosis and multisystem organ failure

INTRODUCTION

Hemophagocytic lymphohistiocytosis (HLH) is a dysregulated immune disorder in children, associated with Epstein-Barr virus (EBV) infection or malignancies. In severe forms, HLH presents with signs and symptoms of hyperinflammation that progress to life-threatening multiorgan failure. Intervention with an extracorporeal immunomodulatory treatment utilizing a selective cytopheretic device (SCD) could be beneficial. The SCD with regional citrate anticoagulation selectively binds the most highly activated circulating neutrophils and monocytes and deactivates them before release to the systemic circulation. Multiple clinical studies, including a multicenter study in children, demonstrate SCD therapy attenuates hyperinflammation, resolves ongoing tissue injury and allows progression to functional organ recovery. We report the first case of SCD therapy in a patient with HLH and multi-organ failure.

CASE DIAGNOSIS/TREATMENT

A previously healthy 22-month-old toddler presented with fever, abdominal distension, organomegaly, pancytopenia, and signs of hyperinflammation. EBV PCR returned at > 25 million copies. The clinical and laboratory pictures were consistent with systemic EBV-positive T-cell lymphoma with symptoms secondary to HLH. The patient met inclusion criteria for an ongoing study of integration of the SCD with a continuous kidney replacement therapy (CKRT) as part of standard of care. The patient received CKRT-SCD for 4 days with normalization of serum markers of sepsis and inflammation. The patient underwent hematopoietic stem cell transplantation 52 days after presentation and has engrafted with normal kidney function 8 months later.

CONCLUSIONS

SCD treatment resulted in improvement of poor tissue perfusion reflected by rapid decline in serum lactate levels, lessened systemic capillary leak with discontinuation of vasoactive agents, and repair and recovery of lung and kidney function with extubation and removal of hemodialysis support.

Integrin CD11b Contributes to Hypertension and Vascular Dysfunction Through Mediating Macrophage Adhesion and Migration

BACKGROUND

Leukocyte adhesion to endothelium is an early inflammatory response and is mainly controlled by the β2-integrins. However, the role of integrin CD11b/CD18 in the pathogenesis of hypertension and vascular dysfunction is unclear.

METHODS

Hypertension was established by angiotensin II (490 ng/kg·per min) or deoxycorticosterone acetate salt. Hypertensive responses were studied in CD11b-deficient (CD11b^-/-) mice, bone marrow transplanted and wild-type (WT) mice that were administered anti-CD11b neutralizing antibody or agonist leukadherin-1. Blood pressure was monitored with tail-cuff method and radiotelemetry. Blood and vascular inflammatory cells were assessed by flow cytometry. Aortic remodeling and function were examined using histology and aortic ring analysis. Cell adhesion and migration were evaluated in vitro. The relationship between circulating CD11b⁺ immune cells and hypertension was analyzed in patients with hypertension.

RESULTS

We found that CD11b and CD18 expression as well as the CD45⁺CD11b⁺CD18⁺ myeloid cells were highly increased in the aorta of angiotensin II-infused mice. Ablation or pharmacological inhibition of CD11b in mice significantly alleviated hypertension, aortic remodeling, superoxide generation, vascular dysfunction, and the infiltration of CD11b⁺ macrophages through reducing macrophage adhesion and migration. These effects were confirmed in WT mice reconstituted with CD11b-deficient bone marrow cells. Conversely, angiotensin II-induced hypertensive response was exacerbated by CD11b agonist leukadherin-1. Notably, circulating CD45⁺CD11b⁺CD18⁺ myeloid cells and the ligand levels in hypertensive patients were significantly higher than in normotensive controls.

CONCLUSIONS

We demonstrated a critical significance of CD11b⁺ myeloid cells in hypertension and vascular dysfunction. Targeting CD11b may represent a novel therapeutic option for hypertension.

Structural Immunology of Complement Receptors 3 and 4

Complement receptors (CR) 3 and 4 belong to the family of beta-2 (CD18) integrins. CR3 and CR4 are often co-expressed in the myeloid subsets of leukocytes, but they are also found in NK cells and activated T and B lymphocytes. The heterodimeric ectodomain undergoes considerable conformational change in order to switch the receptor from a structurally bent, ligand-binding in-active state into an extended, ligand-binding active state. CR3 binds the C3d fragment of C3 in a way permitting CR2 also to bind concomitantly. This enables a hand-over of complement-opsonized antigens from the cell surface of CR3-expressing macrophages to the CR2-expressing B lymphocytes, in consequence acting as an antigen presentation mechanism. As a more enigmatic part of their functions, both CR3 and CR4 bind several structurally unrelated proteins, engineered peptides, and glycosaminoglycans. No consensus motif in the proteinaceous ligands has been established. Yet, the experimental evidence clearly suggest that the ligands are primarily, if not entirely, recognized by a single site within the receptors, namely the metal-ion dependent adhesion site (MIDAS). Comparison of some recent identified ligands points to CR3 as inclined to bind positively charged species, while CR4, by contrast, binds strongly negative-charged species, in both cases with the critical involvement of deprotonated, acidic groups as ligands for the Mg²⁺ ion in the MIDAS. These properties place CR3 and CR4 firmly within the realm of modern molecular medicine in several ways. The expression of CR3 and CR4 in NK cells was recently demonstrated to enable complement-dependent cell cytotoxicity toward antibody-coated cancer cells as part of biological therapy, constituting a significant part of the efficacy of such treatment. With the flexible principles of ligand recognition, it is also possible to propose a response of CR3 and CR4 to existing medicines thereby opening a possibility of drug repurposing to influence the function of these receptors. Here, from advances in the structural and cellular immunology of CR3 and CR4, we review insights on their biochemistry and functions in the immune system.

Prophylactic orthosteric inhibition of leukocyte integrin CD11b/CD18 prevents long-term fibrotic kidney failure in cynomolgus monkeys

Ischaemic acute kidney injury (AKI), an inflammatory disease process, often progresses to chronic kidney disease (CKD), with no available effective prophylaxis. This is in part due to lack of clinically relevant CKD models in non-human primates. Here we demonstrate that inhibition of the archetypal innate immune receptor CD11b/CD18 prevents progression of AKI to CKD in cynomolgus monkeys. Severe ischaemia-reperfusion injury of the right kidney, with subsequent periods of the left ureter ligation, causes irreversible right kidney failure 3, 6 or 9 months after AKI. Moreover, prophylactic inactivation of CD11b/CD18, using the orthosteric CD11b/CD18 inhibitor mAb107, improves microvascular perfusion and histopathology, reduces intrarenal pro-inflammatory mediators and salvages kidney function long term. These studies reveal an important early role of CD11b⁺ leukocytes in post-ischaemic kidney fibrosis and failure, and suggest a potential early therapeutic intervention to mitigate progression of ischaemic AKI to CKD in humans.

Effects of tranexamic acid on coagulation indexes of patients undergoing heart valve replacement surgery under cardiopulmonary bypass

This study aims to explore the effects of tranexamic acid on the coagulation indexes of patients undergoing heart valve replacement surgery under the condition of cardiopulmonary bypass (CPB). One hundred patients who conformed to the inclusive criteria were selected and divided into a tranexamic acid group and a non-tranexamic acid group. They all underwent heart valve replacement surgery under CPB. Patients in the tranexamic acid group were intravenously injected with 1 g of tranexamic acid (100 mL) at the time point after anesthesia induction and before skin incision and at the time point after the neutralization of heparin. Patients in the non-tranexamic acid group were given 100 mL of normal saline at corresponding time points, respectively. Then the coagulation indexes of the two groups were analyzed. The activated blood clotting time (ACT) of the two groups was within normal scope before CPB, while four coagulation indexes including prothrombin time (PT), activated partial thromboplastin time (APTT), international normalized ratio (INR), and fibrinogen (FIB) had significant increases after surgery; the PT and INR of the tranexamic acid group had a remarkable decline after surgery. All the findings suggest that the application of tranexamic acid in heart valve replacement surgery under CPB can effectively reduce intraoperative and postoperative blood loss.

EM structure of the ectodomain of integrin CD11b/CD18 and localization of its ligand-binding site relative to the plasma membrane

One-half of the integrin α-subunit Propeller domains contain and extra vWFA domain (αA domain), which mediates integrin binding to extracellular physiologic ligands via its metal-ion-dependent adhesion site (MIDAS). We used electron microscopy to determine the 3D structure of the αA-containing ectodomain of the leukocyte integrin CD11b/CD18 (αMβ2) in its inactive state. A well defined density for αA was observed within a bent ectodomain conformation, while the structure of the ectodomain in complex with the Fab fragment of mAb107, which binds at the MIDAS face of CD11b and stabilizes the inactive state, further revealed that αA is restricted to a relatively small range of orientations relative to the Propeller domain. Using Fab 107 as probe in fluorescent lifetime imaging microscopy (FLIM) revealed that αA is positioned relatively far from the membrane surface in the inactive state, and a systematic orientation search revealed that the MIDAS face would be accessible to extracellular ligand in the inactive state of the full-length cellular integrin. These studies are the first to define the 3D EM structure of an αA-containing integrin ectodomain and to position the ligand-binding face of αA domain in relation to the plasma membrane, providing new insights into current models of integrin activation.

Neutrophil Inhibitory Factor Selectively Inhibits the Endothelium-Driven Transmigration of Eosinophils In Vitro and Airway Eosinophilia in OVA-Induced Allergic Lung Inflammation

Leukocyte adhesion molecules are involved in cell recruitment in an allergic airway response and therefore provide a target for pharmaceutical intervention. Neutrophil inhibitory factor (NIF), derived from canine hookworm (Ancylostoma caninum), binds selectively and competes with the A-domain of CD11b for binding to ICAM-1. The effect of recombinant NIF was investigated. Intranasal administration of rNIF reduced pulmonary eosinophilic infiltration, goblet cell hyperplasia, and Th₂ cytokine production in OVA-sensitized mice. In vitro, transendothelial migration of human blood eosinophils across IL-4-activated umbilical vein endothelial cell (HUVEC) monolayers was inhibited by rNIF (IC₅₀: 4.6 ± 2.6 nM; mean ± SEM), but not across TNF or IL-1-activated HUVEC monolayers. Treatment of eosinophils with rNIF together with mAb 60.1 directed against CD11b or mAb 107 directed against the metal ion-dependent adhesion site (MIDAS) of the CD11b A-domain resulted in no further inhibition of transendothelial migration suggesting shared functional epitopes. In contrast, rNIF increased the inhibitory effect of blocking mAbs against CD18, CD11a, and VLA-4. Together, we show that rNIF, a selective antagonist of the A-domain of CD11b, has a prominent inhibitory effect on eosinophil transendothelial migration in vitro, which is congruent to the in vivo inhibition of OVA-induced allergic lung inflammation.

Stable coordination of the inhibitory Ca2+ ion at the metal ion-dependent adhesion site in integrin CD11b/CD18 by an antibody-derived ligand aspartate: implications for integrin regulation and structure-based drug design

A central feature of integrin interaction with physiologic ligands is the monodentate binding of a ligand carboxylate to a Mg(2+) ion hexacoordinated at the metal ion-dependent adhesion site (MIDAS) in the integrin A domain. This interaction stabilizes the A domain in the high-affinity state, which is distinguished from the default low-affinity state by tertiary changes in the domain that culminate in cell adhesion. Small molecule ligand-mimetic integrin antagonists act as partial agonists, eliciting similar activating conformational changes in the A domain, which has contributed to paradoxical adhesion and increased patient mortality in large clinical trials. As with other ligand-mimetic integrin antagonists, the function-blocking mAb 107 binds MIDAS of integrin CD11b/CD18 A domain (CD11bA), but in contrast, it favors the inhibitory Ca(2+) ion over the Mg(2+) ion at MIDAS. We determined the crystal structures of the Fab fragment of mAb 107 complexed to the low- and high-affinity states of CD11bA. Favored binding of the Ca(2+) ion at MIDAS is caused by the unusual symmetric bidentate ligation of a Fab-derived ligand Asp to a heptacoordinated MIDAS Ca(2+) ion. Binding of the Fab fragment of mAb 107 to CD11bA did not trigger the activating tertiary changes in the domain or in the full-length integrin. These data show that the denticity of the ligand Asp/Glu can modify the divalent cation selectivity at MIDAS and hence integrin function. Stabilizing the Ca(2+) ion at MIDAS by bidentate ligation to a ligand Asp/Glu may provide one approach for designing pure integrin antagonists.

Affinity, lateral mobility, and clustering contribute independently to beta 2-integrin-mediated adhesion

Affinity changes and avidity modulation both contribute to activation of beta(2)-integrin-mediated adhesion, an essential, early step in inflammation. Avidity modulation, defined as an increase in adhesiveness independent of integrin conformational changes, might be due to integrin clustering, motion, or both. Increased integrin diffusion upon leukocyte activation has been demonstrated, but whether it is proadhesive in itself, or just constitutes a mechanism for integrin clustering, remains unclear. To understand the proadhesive effects of integrin affinity changes, clustering, and motion, an experimental system was devised to separate them. Clustering and integrin motion together were induced by cytochalasin D (CD) without inducing high-affinity; integrin motion could then be frozen by fixation; and high affinity was induced independently by Mn(2+). Adhesion was equivalent for fixed and unfixed cells except following pretreatment with CD or Mn(2+), which increased adhesion for both. However, fixed cells were less adhesive than unfixed cells after CD, even though integrin clustering was similar. A simple explanation is that CD induces both clustering and integrin motion, fixation then stops motion on fixed cells, but integrins continue to diffuse on unfixed cells, increasing the kinetics of integrin/ICAM-1 interactions to enhance adhesion. Affinity changes are then independent of, and additive to, avidity effects.

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.

Role of the beta-subunit arginine/lysine finger in integrin heterodimer formation and function

Formation of the integrin alphabeta heterodimer is essential for cell surface expression and function. At the core of the alphabeta interface is a conserved Arg/Lys "finger" from the beta-subunit that inserts into a cup-like "cage" formed of two layers of aromatic residues in the alpha-subunit. We evaluated the role of this residue in heterodimer formation in an alphaA-lacking and an alphaA-containing integrin alphaVbeta3 and alphaMbeta2 (CD11b/CD18), respectively. Arg261 of beta3 was mutated to Ala or Glu; the corresponding Lys252 of beta2 was mutated to Ala, Arg, Glu, Asp, or Phe; and the effects on heterodimer formation in each integrin examined by ELISA and immunoprecipitation in HEK 293 cells cotransfected with plasmids encoding the alpha- and beta-subunits. The Arg261Glu (but not Arg261Ala) substitution significantly impaired cell surface expression and heterodimer formation of alphaVbeta3. Although Lys252Arg, and to a lesser extent Lys252Ala, were well tolerated, each of the remaining substitutions markedly reduced cell surface expression and heterodimer formation of CD11b/CD18. Lys252Arg and Lys252Ala integrin heterodimers displayed a significant increase in binding to the physiologic ligand iC3b. These data demonstrate an important role of the Arg/Lys finger in formation of a stable integrin heterodimer, and suggest that subtle changes at this residue affect the activation state of the integrin.

Characterization of a novel monoclonal antibody with restricted specificity to the free beta 2 integrin alpha M CD11b subunit

Leukocyte cell surface expression and function of beta2 integrins require the intracellular association of alpha subunits, CD11a, b, c, d, respectively, with the common CD18 beta2 subunit. We have raised and characterized a murine MAb -- ME-MDF -- directed against the low affinity form of the human integrin alphaM subunit CD11b A-domain. MAb ME-MDF is an IgG2a that has a kDa of 2,45461 +/- 0.12 x 10(-9) M. MAb ME-MDF recognizes both the low and high affinity forms of the CD11b A-domain. Flow cytometry showed that ME-MDF does not recognize the heterodimeric CD11b/CD18 molecule at the surface of polymorphonuclear cells and the human monoblast cell line U937. Western blot analysis of U937 cell line cell surface proteins demonstrated that ME-MDF reacts specifically with the CD11b subunit but does not react with the heterodimeric CD11b/CD18 complex, a feature that differentiates it from other CD11b A-dom-specific MAbs. These observations suggest that ME-MDF recognizes an epitope that is involved in the association of the two subunits and hence is not accessible within the heterodimeric form of the CD11b/CD18 molecule. These data show that the CD11b A-dom engages not only the MIDAS but also the ME-MDF-specific epitope to associate with the CD18 subunit. We have also constructed, and expressed in the yeast Pichia pastoris, the corresponding recombinant scFv form of MAb ME-MDF and characterized the CDRs. MAb ME-MDF is characterized by short VH and VL CDR3. MAb ME-MDF and/or its recombinant scFv form would be very useful to study the structural basis of the association between the alpha and beta2 integrin subunits and to investigate the possibility of modulating CR3 cell surface expression by preventing subunit association.

A simple, no-wash cell adhesion-based high-throughput assay for the discovery of small-molecule regulators of the integrin CD11b/CD18

The leukocyte-specific integrin CD11b/CD18 plays a key role in the biological function of these cells and represents a validated therapeutic target for inflammatory diseases. Currently, the low affinity interaction between CD11b/CD18 integrin and its respective ligand poses a challenge in the development of cell-based adhesion assays for the high-throughput screening (HTS) environment. Here the authors describe a simple cell-based adhesion assay that can be readily used for HTS for the discovery of functional regulators of CD11b/CD18. The assay consistently produces acceptable Z' values (> 0.5) for HTS. After testing the assay using 2 established blocking antibodies as reference biologicals, the authors performed a proof-of-concept primary screen using a library of 6612 compounds and identified both agonist and antagonist hits.

The beta-tail domain (betaTD) regulates physiologic ligand binding to integrin CD11b/CD18

Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (betaTD) and the ligand-binding betaA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alphaMbeta2) with its physiologic ligand iC3b. We replaced the betaTD CD loop residues D658GMD of the CD18 (beta2) subunit with the equivalent D672SSG of the beta3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca(2+) plus Mg(2+) (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the betaTD-modified integrins bound in Ca(2+)Mg(2+) to the betaA-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal betaTD as an allosteric modulator of integrin activation.

AL-57, a ligand-mimetic antibody to integrin LFA-1, reveals chemokine-induced affinity up-regulation in lymphocytes

Affinity of integrin lymphocyte function-associated antigen 1 (LFA-1) is enhanced by conformational changes from the low-affinity closed form to the high-affinity (HA) open form of the ligand-binding inserted (I) domain as shown by work with purified I domains. However, affinity up-regulation of LFA-1 on the cell surface by physiological agonists such as chemokines has yet to be demonstrated by monovalent reagents. We characterize a mAb, AL-57 (activated LFA-1 clone 57), that has been developed by phage display that selectively targets the HA open conformation of the LFA-1 I domain. AL-57 discriminates among low-affinity, intermediate-affinity, and HA states of LFA-1. Furthermore, AL-57 functions as a ligand mimetic that binds only upon activation and requires Mg2+ for binding. Compared with the natural ligand intercellular adhesion molecule-1, AL-57 shows a tighter binding to the open I domain and a 250-fold slower off rate. Monovalent Fab AL-57 demonstrates affinity increases on a subset (approximately 10%) of lymphocyte cell surface LFA-1 molecules upon stimulation with CXCL-12 (CXC chemokine ligand 12). Affinity up-regulation correlates with global conformational changes of LFA-1 to the extended form. Affinity increase stimulated by CXCL-12 is transient and peaks 2 to 5 min after stimulation.

Integrin structure, allostery, and bidirectional signaling

Alphabeta heterodimeric integrins mediate dynamic adhesive cell-cell and cell-extracellular matrix (ECM) interactions in metazoa that are critical in growth and development, hemostasis, and host defense. A central feature of these receptors is their capacity to change rapidly and reversibly their adhesive functions by modulating their ligand-binding affinity. This is normally achieved through interactions of the short cytoplasmic integrin tails with intracellular proteins, which trigger restructuring of the ligand-binding site through long-range conformational changes in the ectodomain. Ligand binding in turn elicits conformational changes that are transmitted back to the cell to regulate diverse responses. The publication of the integrin alphaVbeta3 crystal structure has provided the context for interpreting decades-old biochemical studies. Newer NMR, crystallographic, and EM data, reviewed here, are providing a better picture of the dynamic integrin structure and the allosteric changes that guide its diverse functions.

A novel simple and rapid PCR-based site-directed mutagenesis method

Site-directed mutagenesis (SDM) is a powerful tool for exploring protein structure and function, and several procedures adjusted to specific purposes are still being developed. Herein we describe a straightforward and efficient method with versatile applications for introducing site-specific alterations in any deoxyribonucleic acid (DNA) sequence cloned in a plasmidic expression vector. In this polymerase chain reaction (PCR)-based SDM method, forward and reverse primers are used to amplify the plasmid containing the sequence of interest. The primers are designed so that the desired modifications are introduced at the 5' end of one of the primers, whereas the other primer starts with the nucleotide at position (-1) of the one to be modified. The PCR is carried out using Pfu DNA polymerase. The blunt-ended PCR-generated DNA fragment is self-ligated and used to transform Escherichia coli. Mutant clones are screened by colony hybridization using the mutagenic primer as probe and the presence of the mutation is confirmed by direct DNA sequencing. This procedure was used efficiently to introduce substitutions, deletions, and insertions in the DNA sequences coding for a recombinant form (scFv) of antibody 107 specific of the human CR3 molecule, the rat alpha integrin CD11b A-domain and the human CD8beta cloned in pPICZalphaB, pGEX-2T, and CDM8 expression vectors, respectively.

A survey of the year 2002 commercial optical biosensor literature

We have compiled 819 articles published in the year 2002 that involved commercial optical biosensor technology. The literature demonstrates that the technology's application continues to increase as biosensors are contributing to diverse scientific fields and are used to examine interactions ranging in size from small molecules to whole cells. Also, the variety of available commercial biosensor platforms is increasing and the expertise of users is improving. In this review, we use the literature to focus on the basic types of biosensor experiments, including kinetics, equilibrium analysis, solution competition, active concentration determination and screening. In addition, using examples of particularly well-performed analyses, we illustrate the high information content available in the primary response data and emphasize the impact of including figures in publications to support the results of biosensor analyses.

CD157 is an important mediator of neutrophil adhesion and migration

CD157, a glycosylphosphatidylinositol (GPI)-anchored protein encoded by a member of the CD38 NADase/ADP-ribosyl cyclase gene family, is expressed on the surface of most human circulating neutrophils. This work demonstrates that CD157 is a receptor that induces reorganization of the cytoskeleton and significant changes in cell shape, and that signals mediated by CD157 act through modulation of cytosolic Ca(2+) concentration. These signals are independent of the products of CD157's enzymatic activities (ie, cyclic adenosine diphosphate [ADP]-ribose and ADP-ribose). Indeed, the enzymatic activities of CD157 in circulating neutrophils as well as in dimethyl sulfoxide (DMSO)-differentiated (CD157(+)/CD38(-)) HL-60 cells, are hardly detectable. This work also shows that the receptorial activity relies on cross-talk between CD157 and beta(2) integrin. CD157 localizes in GM1-enriched lipid rafts and, upon activation, it migrates to the uropod, a structure specialized in motility and adhesive functions. Indeed, CD157 is involved in adhesion to extracellular matrix proteins and in chemotaxis induced in vitro by formyl-methionyl-leucyl-phenylalanine (fMLP). These findings were consistent with the results obtained in neutrophils from patients with paroxysmal nocturnal hemoglobinuria (PNH), in which CD157 is deficient. These neutrophils showed constant defects in adhesion and migration. Our data attribute specific and crucial roles to CD157 in the regulation of innate immunity during inflammation.

Binding Affinity of Metal Ions to the CD11b A-domain Is Regulated by Integrin Activation and Ligands

The divalent cations Mg(2+) and Ca(2+) regulate the interaction of integrins with their cognate ligands, with Mg(2+) uniformly facilitating and Ca(2+) generally inhibiting such interactions in vitro. Because both cations are present in mm concentrations in vivo, the physiologic relevance of the in vitro observations is unclear. We measured the affinity of both cations to the inactive and active states of the ligand- and cation-binding A-domain (CD11bA) from integrin CD11b/CD18 in the absence and presence of the single-chain 107 antibody (scFv107), an activation-insensitive ligand-mimetic antibody. Using titration calorimetry, we found that Mg(2+) and Ca(2+) display equivalent (mm) affinities to inactive CD11bA. Activation induced a approximately 10-fold increase in the binding affinity of Mg(2+) to CD11bA with no change in that of Ca(2+) (106 microm +/- 16 and 2.1 mm +/- 0.19, respectively, n = 4). This increase is largely driven by favorable enthalpy. scFv107 induced a 50-80-fold increase in the binding affinity of Ca(2+) (but not Mg(2+) or Mn(2+)) to either form of CD11bA. Thus the affinity of metal ions to integrins is itself regulated by the activation state of these receptors and by certain ligands. These findings, which we expect will be applicable in vivo, elucidate a new level of regulation of the integrin-metal-ligand ternary complex and help explain some of the discrepant effects of Ca(2+) on integrin-ligand interactions.

Crystal structure of the alpha1beta1 integrin I domain in complex with an antibody Fab fragment

The alpha1beta1 (VLA-1) integrin is a cell-surface receptor for collagen and laminin and has been implicated in biological pathways involved in several pathological processes. These processes may be inhibited by the monoclonal antibody AQC2, which binds with high affinity to human alpha1beta1 integrin. To understand the structural basis of the inhibition we determined the crystal structure of the complex of a chimeric rat/human I domain of the alpha1beta1 integrin and the Fab fragment of humanized AQC2 antibody. The structure of the complex shows that the antibody blocks the collagen binding site of the I domain. An aspartate residue, from the CDR3 loop of the antibody heavy chain, coordinates the MIDAS metal ion in a manner similar to that of a glutamate residue from collagen. Substitution of the aspartate residue by alanine or arginine results in significant reduction of antibody binding affinity. Interestingly, although the mode of metal ion coordination resembles that of the open conformation, the I domain maintains an overall closed conformation previously observed only for unliganded I domains.

Structures of the alpha L I domain and its complex with ICAM-1 reveal a shape-shifting pathway for integrin regulation

The structure of the I domain of integrin alpha L beta 2 bound to the Ig superfamily ligand ICAM-1 reveals the open ligand binding conformation and the first example of an integrin-IgSF interface. The I domain Mg2+ directly coordinates Glu-34 of ICAM-1, and a dramatic swing of I domain residue Glu-241 enables a critical salt bridge. Liganded and unliganded structures for both high- and intermediate-affinity mutant I domains reveal that ligand binding can induce conformational change in the alpha L I domain and that allosteric signals can convert the closed conformation to intermediate or open conformations without ligand binding. Pulling down on the C-terminal alpha 7 helix with introduced disulfide bonds ratchets the beta 6-alpha 7 loop into three different positions in the closed, intermediate, and open conformations, with a progressive increase in affinity.