Conformational evaluation of HIV-1 trimeric envelope glycoproteins using a cell-based ELISA assay.

HIV-1 envelope glycoproteins (Env) mediate viral entry into target cells and are essential to the infectious cycle. Understanding how those glycoproteins are able to fuel the fusion process through their conformational changes could lead to the design of better, more effective immunogens for vaccine strategies. Here we describe a cell-based ELISA assay that allows studying the recognition of trimeric HIV-1 Env by monoclonal antibodies. Following expression of HIV-1 trimeric Env at the surface of transfected cells, conformation specific anti-Env antibodies are incubated with the cells. A horseradish peroxidase-conjugated secondary antibody and a simple chemiluminescence reaction are then used to detect bound antibodies. This system is highly flexible and can detect Env conformational changes induced by soluble CD4 or cellular proteins. It requires minimal amount of material and no highly-specialized equipment or know-how. Thus, this technique can be established for medium to high throughput screening of antigens and antibodies, such as newly-isolated antibodies.

Interaction with cellular CD4 exposes HIV-1 envelope epitopes targeted by antibody-dependent cell-mediated cytotoxicity.

UNLABELLED: Anti-HIV-1 envelope glycoprotein (Env) antibodies without broadly neutralizing activity correlated with protection in the RV144 clinical trial, stimulating interest in other protective mechanisms involving antibodies, such as antibody-dependent cell-mediated cytotoxicity (ADCC). Env epitopes targeted by many antibodies effective at mediating ADCC are poorly exposed on the unliganded Env trimer. Here we investigated the mechanism of exposure of ADCC epitopes on Env and showed that binding of Env and CD4 within the same HIV-1-infected cell effectively exposes these epitopes. Env capacity to transit to the CD4-bound conformation is required for ADCC epitope exposure. Importantly, cell surface CD4 downregulation by Nef and Vpu accessory proteins and Vpu-mediated BST-2 antagonism modulate exposure of ADCC-mediating epitopes and reduce the susceptibility of infected cells to this effector function in vitro. Significantly, Env conformational changes induced by cell surface CD4 are conserved among Env from HIV-1 and HIV-2/SIVmac lineages. Altogether, our observations describe a highly conserved mechanism required to expose ADCC epitopes that might help explain the evolutionary advantage of downregulation of cell surface CD4 by the HIV-1 Vpu and Nef proteins. IMPORTANCE: HIV-1 envelope epitopes targeted by many antibodies effective at mediating antibody-dependent cell-mediated cytotoxicity (ADCC) are poorly exposed on the unliganded envelope trimer. Here we investigated the mechanism of exposure of these epitopes and found that envelope interaction with the HIV-1 CD4 receptor is required to expose some of these epitopes. Moreover, our results suggest that HIV-1 CD4 downregulation might help avoid the killing of HIV-1-infected cells by this immune mechanism.

Molecular architecture of the uncleaved HIV-1 envelope glycoprotein trimer.

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer, a membrane-fusing machine, mediates virus entry into host cells and is the sole virus-specific target for neutralizing antibodies. Binding the receptors, CD4 and CCR5/CXCR4, triggers Env conformational changes from the metastable unliganded state to the fusion-active state. We used cryo-electron microscopy to obtain a 6-Å structure of the membrane-bound, heavily glycosylated HIV-1 Env trimer in its uncleaved and unliganded state. The spatial organization of secondary structure elements reveals that the unliganded conformations of both glycoprotein (gp)120 and gp41 subunits differ from those induced by receptor binding. The gp120 trimer association domains, which contribute to interprotomer contacts in the unliganded Env trimer, undergo rearrangement upon CD4 binding. In the unliganded Env, intersubunit interactions maintain the gp41 ectodomain helical bundles in a "spring-loaded" conformation distinct from the extended helical coils of the fusion-active state. Quaternary structure regulates the virus-neutralizing potency of antibodies targeting the conserved CD4-binding site on gp120. The Env trimer architecture provides mechanistic insights into the metastability of the unliganded state, receptor-induced conformational changes, and quaternary structure-based strategies for immune evasion.

The HIV-1 gp120 major variable regions modulate cold inactivation.

HIV-1 entry involves the viral envelope glycoproteins (Env gps) and receptors on the target cell. Receptor binding channels the intrinsic high potential energy of Env into the force required to fuse the membranes of virus and target cell. For some HIV-1 strains, prolonged incubation on ice decreases Env potential energy and results in functional inactivation. By characterizing chimeras between two primary clade C HIV-1 strains that differ in sensitivities to cold, soluble CD4, and neutralizing antibodies, we found that these properties were largely determined by discrete elements within the gp120 variable regions V1V2 and V3.

The highly conserved layer-3 component of the HIV-1 gp120 inner domain is critical for CD4-required conformational transitions.

The trimeric envelope glycoprotein (Env) of human immunodeficiency virus type 1 (HIV-1) mediates virus entry into host cells. CD4 engagement with the gp120 exterior envelope glycoprotein subunit represents the first step during HIV-1 entry. CD4-induced conformational changes in the gp120 inner domain involve three potentially flexible topological layers (layers 1, 2, and 3). Structural rearrangements between layer 1 and layer 2 have been shown to facilitate the transition of the envelope glycoprotein trimer from the unliganded to the CD4-bound state and to stabilize gp120-CD4 interaction. However, our understanding of CD4-induced conformational changes in the gp120 inner domain remains incomplete. Here, we report that a highly conserved element of the gp120 inner domain, layer 3, plays a pivot-like role in these allosteric changes. In the unliganded state, layer 3 modulates the association of gp120 with the Env trimer, probably by influencing the relationship of the gp120 inner and outer domains. Importantly, layer 3 governs the efficiency of the initial gp120 interaction with CD4, a function that can also be fulfilled by filling the Phe43 cavity. This work defines the functional importance of layer 3 and completes a picture detailing the role of the gp120 inner domain in CD4-induced conformational transitions in the HIV-1 Env trimer.

Lineage-specific differences between human and simian immunodeficiency virus regulation of gp120 trimer association and CD4 binding.

Metastable conformations of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) must be maintained in the unliganded state of the envelope glycoprotein trimer. Binding of gp120 to the primary receptor, CD4, triggers the transition to an open conformation of the trimer, promoting interaction with the CCR5 chemokine receptor and ultimately leading to gp41-mediated virus-cell membrane fusion and entry. Topological layers in the gp120 inner domain contribute to gp120-trimer association in the unliganded state and to CD4 binding. Here we describe similarities and differences between HIV-1 and SIVmac gp120. In both viruses, the gp120 N/C termini and the inner domain ß-sandwich and layer 2 support the noncovalent association of gp120 with the envelope glycoprotein trimer. Layer 1 of the SIVmac gp120 inner domain contributes more to trimer association than the corresponding region of HIV-1 gp120. On the other hand, layer 1 plays an important role in stabilizing the CD4-bound conformation of HIV-1 but not SIVmac gp120 and thus contributes to HIV-1 binding to CD4. In SIVmac, CD4 binding is instead enhanced by tryptophan 375, which fills the Phe 43 cavity of gp120. Activation of SIVmac by soluble CD4 is dependent on tryptophan 375 and on layer 1 residues that determine a tight association of gp120 with the trimer. Distinct biological requirements for CD4 usage have resulted in lineage-specific differences in the HIV-1 and SIV gp120 structures that modulate trimer association and CD4 binding.

Bradykinin forming capacity of oversulfated chondroitin sulfate contaminated heparin in vitro.

UNLABELLED: Oversulfated chondroitin sulfate (OSCS) contaminated heparin has been associated with severe anaphylactoid reaction (AR), mainly in dialysed patients. Although attributed to bradykinin (BK) released during contact system activation by OSCS, no definitive evidence exists until now for a BK release during incubation of contaminated heparin with human plasma. In this study, we investigated the kinin forming capacity of OSCS and OSCS contaminated heparin when incubated in vitro with a pool of human plasma. At 100 microg/mL, OSCS liberates BK in a profile similar but not identical to dextran sulfate, a well known activator of the plasma contact system. The results have highlighted that the quantity of BK accumulated during contact system activation depends not only on the concentration of OSCS but also on the plasma dilution and the presence of an angiotensin converting enzyme inhibitor. We demonstrate a highly significant correlation between the concentration of OSCS present in the contaminated heparin and BK released concentration. In conclusion, for the first time, we show that OSCS contaminated heparin incubated with human plasma has the capacity to liberate BK at a concentration that could explain the role of this inflammatory peptide in the pathophysiology of AR associated with OSCS contaminated heparin. DISCLAIMER: The findings and conclusions in this article have not been formally disseminated by the Food and Drug Administration and should not be construed to represent any Agency determination or policy.

Acute hypotensive transfusion reaction during liver transplantation in a patient on angiotensin converting enzyme inhibitors from low aminopeptidase P activity.

Acute hypotensive transfusion reactions are newly characterized transfusion reactions in which hypotension is the prominent feature. The pathophysiology of acute hypotensive transfusion reactions is related to the bradykinin function and its metabolism. A liver transplant recipient on treatment with an angiotensin converting enzyme inhibitor developed sudden hypotension, that is, systolic pressure of 60 mm Hg, after receiving 200 mL of a blood product mixture without significant surgical blood loss. He responded to the resuscitation measure, although hypotension developed again after a challenge transfusion of 200 mL of the blood mixture. A severe hypotensive reaction to the blood transfusion and diffuse bleeding from the dissection surfaces forced the transplantation to be aborted after the common bile duct had been divided. We hypothesized that the patient had an acute hypotensive transfusion reaction due to disordered bradykinin metabolism. Analysis of his blood showed low levels of both angiotensin converting enzyme and aminopeptidase P enzyme activity, confirming that the patient experienced an acute hypotensive transfusion reaction that was due to the use of the angiotensin converting enzyme inhibitor and was precipitated by an abnormality in the metabolic enzyme pathway. It is recommended to discontinue angiotensin converting enzyme inhibitors and switch to a different class of antihypertensive medications for patients with a high Model for End-Stage Liver Disease score on the waiting list for liver transplantation.

Lack of direct interaction between enalaprilat and the kinin B1 receptors.

It has been recently proposed that the second extracellular loop of the human bradykinin (BK) B1 receptor (B1R) contains a conserved HExxH motif also present in peptidases possessing a Zn2+ prosthetic group, such as angiotensin converting enzyme (ACE), and that ACE inhibitors directly activate B1R signaling in endothelial cells. However, the binding of ACE inhibitors to the B1Rs has never been directly evaluated. Information about binding of a radiolabeled inhibitor to natural or recombinant ACE in intact cells (physiologic ionic composition) was also collected. We used the tritiated form of an ACE inhibitor previously proposed to activate the B1R, enalaprilat, to address these questions using recombinant human B1Rs and naturally expressed or recombinant ACE. [3H]Lys-des-Arg9-BK bound to the human recombinant B1Rs with high affinity (KD 0.35 nM) in HEK 293a cells. [3H]Enalaprilat (0.25-10 nM) did not bind to cells expressing recombinant human B1R, but bound with a subnanomolar affinity to recombinant ACE or to naturally expressed ACE in human umbilical vein endothelial cells. The radioligand was further validated using a binding competition assay that involved unlabeled ACE inhibitors or their prodrug forms in endothelial cells. Membranes of HEK 293a cells that expressed B1Rs did not hydrolyze hippuryl-glycylglycine (an ACE substrate). Enalaprilat did not stimulate calcium signaling in HEK 293a cells that expressed B1Rs. A typical ACE inhibitor did not bind to nor stimulate the human B1Rs; nevertheless, several other indirect mechanisms could connect ACE inhibition to B1R stimulation in vivo.

Metallopeptidase activities in hereditary angioedema: effect of androgen prophylaxis on plasma aminopeptidase P.

BACKGROUND: Aminopeptidase P (APP) plays an important role in the catabolism of kinins in human plasma, mostly for des-Arg(9)-bradykinin. Impaired degradation of this active bradykinin metabolite was found to be associated with a decreased APP activity in hypertensive patients who experienced angioedema while being treated with angiotensin I-converting enzyme inhibitors. The pathophysiology of hereditary angioedema is presently attributed only to a quantitative/qualitative C1 inhibitor (CI-INH) defect with increased bradykinin release. OBJECTIVES: In the context of androgen prophylaxis, increased CI-INH function cannot fully explain protection from angioedema attacks alone because of the limited reversion of the CI-INH defects. Therefore we hypothesized that androgen prophylaxis could enhance plasma APP activity. METHODS: Patients with hereditary angioedema were investigated for plasma metallopeptidase activities responsible for kinin catabolism (APP, angiotensin I-converting enzyme, and carboxypeptidase N) and for CI-INH function in treated and untreated patients. RESULTS: APP activity was asymmetrically distributed in untreated patients (n = 147): the mean value was significantly lower than the value in a reference healthy and unmedicated population (n = 116; P < or = .001). Prophylaxis with androgen induced a significant increase in APP activity (P < or = .001), whereas it did not affect the other metallopeptidase activities. In both patient groups, APP activity showed a significant inverse relationship to disease severity (P < or = .001). CONCLUSION: In addition to the effect on circulating CI-INH levels, the increase in APP levels brought on by androgens could contribute to a more effective control of the kinin accumulation considered to be responsible for the symptoms of angioedema.

The effect of electronegativity and angiotensin-converting enzyme inhibition on the kinin-forming capacity of polyacrylonitrile dialysis membranes.

The combination of negatively-charged membranes and angiotensin I-converting enzyme inhibitors (ACEi) evokes hypersensitivity reactions (HSR) during hemodialysis and bradykinin (BK)-related peptides have been hypothesized as being responsible for these complications. In this study, we tested the effects of neutralizing the membrane electronegativity (zeta potential) of polyacrylonitrile AN69 membranes by coating a polyethyleneimine layer (AN69-ST membranes) over the generation of kinins induced by blood contact with synthetic membranes. We used minidialyzers with AN69 or AN69-ST membranes in an ex vivo model of plasma and we showed that plasma dialysis with AN69 membranes led to significant BK and des-Arg(9)-BK release, which was potentiated by ACEi. This kinin formation was dramatically decreased by AN69-ST membranes, even in the presence of an ACEi, and kinin recovery in the dialysates was also significantly lower with these membranes. High molecular weight kininogen and factor XII detection by immunoblotting of the protein layer coating both membranes corroborated the results: binding of these proteins and contact system activation on AN69-ST membranes were reduced. This ex vivo experimental model applied to the plasma, dialysate and dialysis membrane could be used for the characterization of the kinin-forming capacity of any biomaterial potentially used in vivo in combination with drugs which modulate the pharmacological activity of kinins.

Cardiovascular expression of inflammatory signaling molecules, the kinin B1 receptor and COX2, in the rabbit: effects of LPS, anti-inflammatory and anti-hypertensive drugs.

We first aimed to test the effect of anti-inflammatory drugs, etanercept and dexamethasone sodium phosphate (DSP), on the expression of inducible inflammatory signaling molecules (the bradykinin [BK] B(1) receptor [B(1)R], cyclooxygenase [COX]-2) in lipopolysaccharide (LPS)-treated rabbits. Preliminary experiments mostly based on a novel cellular model, rabbit dermis fibroblasts, showed that etanercept inhibited TNF-alpha-induced B(1)R expression ([(3)H]Lys-des-Arg(9)-BK binding), but that DSP also inhibited cytokine-induced B(1)R upregulation with less selectivity. LPS (100 microg/kg i.v.) induced the expression of the B(1)R (aortic contractility ex vivo, mRNA in hearts) and COX2 (immunoblots, heart extracts). However, the function of the BK B(2) receptor was unchanged (jugular vein contractility ex vivo). DSP pre-treatment profoundly reduced the induction of the B(1)R and COX2 whereas etanercept significantly inhibited only COX2 expression. The second aim was to verify whether chronic angiotensin converting enzyme (ACE) blockade in rabbits would induce B(1)R expression, as reported in other species. 14-Day enalapril oral dosing, but not treatment with the angiotensin receptor antagonist losartan, significantly increased aortic contractions mediated by B(1)Rs, however much less than LPS. Enalapril treatment did not increase COX2 expression but increased the ex vivo relaxation of the mesenteric artery mediated by endogenous prostaglandins. Chronic ACE inhibition recruits inflammatory signaling systems.

Generation of kinins during preparation and storage of whole blood-derived platelet concentrates.

BACKGROUND: Leukoreduction of platelet (PLT) concentrates (PCs) may be associated with hypotension in recipients, and a role for bradykinin (BK)-related peptides has been proposed for this side effect. STUDY DESIGN AND METHODS: The concentration of BK and one of its vasoactive metabolites, des-arginine(9)-BK (des-Arg(9)-BK), was measured in a large number of PCs as a function of leukoreduction and storage duration with specific enzyme immunoassays and complementary techniques. RESULTS: On Day 0 of storage, kinins were detected in leukoreduced and unfiltered PCs at a concentration lower than 100 pg per mL. During storage, both kinin levels peaked on Day 5 of storage, with a concentration higher than 1 ng per mL in 22 percent of PCs whether filtered on Day 0 or not. Physicochemical and pharmacologic characterizations of immunoreactive kinins confirm their nature. In vitro activation of the contact system of the corresponding PLT-poor plasma showed that a high kinin concentration on Day 5 of the storage corresponded with a low kinin-forming capacity of plasma. On Day 7, BK was no longer elevated presumably due to its degradation and the depletion of kinin-forming capacity of the plasma in stored PCs. The activities of metallopeptidases that metabolize BK-related peptides in plasma from PCs were at levels similar to those recorded in the plasma of a normal reference population and were unaffected by storage. CONCLUSION: Storage of PCs contributes to the hydrolysis of high-molecular-weight kininogen and generation of pharmacologically relevant BK levels that might pose a hazard in susceptible patients.

[Physiopathology of the acute adverse effects of angiotensin-converting-enzyme inhibitors]. / Physiopathologie des effets secondaires aigus des inhibiteurs de l'enzyme de conversion de l'angiotensine.

Angiotensin-I-converting-enzyme inhibitors are currently used to treat more than 40 million cardiovascular patients worldwide. These drugs have a variety of acute adverse effects, the nature of which depends on the clinical context, and which include angioedema, anaphylactoid reactions in hemodialysis patients, and severe hypotensive reactions during blood product transfusions. These adverse effects result from a combination of factors affecting the synthesis, metabolism and pharmacological activity of bradykinin and des-arginine9-bradykinin, two powerful vasodilatory and pro-inflammatory peptides. Experimental evidence obtained in our laboratory suggests that acquired, genetic and pharmacological factors can influence the risk of these rare but potentially life-threatening effects.

Endothelial stress induces the release of vitamin D-binding protein, a novel growth factor.

Endothelial cells (EC) under stress release paracrine mediators that facilitate accumulation of vascular smooth muscle cells (VSCM) at sites of vascular injury. We found that medium conditioned by serum-starved EC increase proliferation and migration of VSCM in vitro. Fractionation of the conditioned medium followed by mass spectral analysis identified one bioactive component as vitamin D-binding protein (DBP). DBP induced both proliferation and migration of VSMC in vitro in association with increased phosphorylation of ERK 1/2. PD 98059, a biochemical inhibitor of ERK 1/2, abrogated these proliferative and migratory responses in VSMC. DBP is an important carrier for the vitamin-D sterols, 25-hydroxyvitamin-D, and 1alpha,25-dihydroxyvitamin-D. Both sterols inhibited the activity of DBP on VSMC, suggesting that vitamin D binding sites are important for initiating the activities of DBP on VSMC. Release of DBP at sites of endothelial injury represents a novel pathway favoring accumulation of VSMC at sites of vascular injury.

Apoptosis of endothelial cells triggers a caspase-dependent anti-apoptotic paracrine loop active on VSMC.

Increased endothelial apoptosis and decreased apoptosis of vascular smooth muscle cells (VSMC) are central to initiation of myo-intimal thickening. We hypothesized that apoptosis of endothelial cells (EC) induces the release of anti-apoptotic mediator(s) active on VSMC. We found that serum-free medium conditioned by apoptotic EC decreases apoptosis of VSMC compared with fresh serum-free medium. Inhibition of endothelial apoptosis during conditioning with a pan-caspase inhibitor ZVAD-FMK blocked the release of the anti-apoptotic factor(s) active on VSMC. VSMC exposed to serum-free medium conditioned by apoptotic EC showed increased ERK 1/2 phosphorylation, enhanced Bcl-xl expression, and inhibition of p53 expression. Fractionation of the conditioned medium followed by mass spectral analysis identified one bioactive component as a C-terminal fragment of the domain V of perlecan. Serum-free medium supplemented with either a synthetic peptide containing the EGF motif of the domain V of perlecan or chondroitin 4-sulfate, a glycosaminoglycan anchored on the domain V of perlecan, increased ERK 1/2 phosphorylation and Bcl-xl protein levels while inhibiting apoptosis of VSMC. These results suggest that a proteolytic activity developing downstream of activated caspases in apoptotic EC initiates degradation of pericellular proteoglycans and liberation of bioactive fragments with a robust impact on inhibition of VSMC apoptosis.

Blockade of the apoptotic machinery by cyclosporin A redirects cell death toward necrosis in arterial endothelial cells: regulation by reactive oxygen species and cathepsin D.

Blockade of the mitochondrial permeability transition pore (mPTP) by cyclosporin A (CsA) inhibits apoptosis in various cell types. However, use of CsA in humans is associated with damage to the arterial endothelium. We evaluated whether inhibition of the apoptotic machinery by CsA promotes other forms of cell death in arterial endothelial cells (EC). Exposure of human umbilical artery EC (HUAEC) to clinically relevant concentrations of CsA for up to 24 h was associated with a significant increase in necrotic features. We detected inhibition of apoptosis and a significant increase in necrosis in HUAEC exposed concomitantly to CsA and mitomycin C, a proapoptotic DNA damaging agent. We found that CsA-induced cell death is independent of caspase activation, p53 induction, and calcineurin inhibition. However, bongkrekic acid, another mPTP blocker, also increased necrosis in HUAEC. Dihydroethidium and acridine orange staining revealed increased intracellular production of reactive oxygen species (ROS) followed by lysosomal damage in HUAEC exposed to CsA. Hydroxyl radical and superoxide scavengers and inhibition of cathepsin D activity significantly attenuated CsA-induced EC death. These results suggest that inhibition of the apoptotic machinery by CsA in arterial EC favors development of a necrotic form of cell death regulated by ROS and secondary lysosomal damage.