Antibody 17b binding at the coreceptor site weakens the kinetics of the interaction of envelope glycoprotein gp120 with CD4

CoPoP liposomes displaying stabilized clade C HIV-1 Env elicit tier 2 multiclade neutralization in rabbits

One of the strategies towards an effective HIV-1 vaccine is to elicit broadly neutralizing antibody responses that target the high HIV-1 Env diversity. Here, we present an HIV-1 vaccine candidate that consists of cobalt porphyrin-phospholipid (CoPoP) liposomes decorated with repaired and stabilized clade C HIV-1 Env trimers in a prefusion conformation. These particles exhibit high HIV-1 Env trimer decoration, serum stability and bind broadly neutralizing antibodies. Three sequential immunizations of female rabbits with CoPoP liposomes displaying a different clade C HIV-1 gp140 trimer at each dosing generate high HIV-1 Env-specific antibody responses. Additionally, serum neutralization is detectable against 18 of 20 multiclade tier 2 HIV-1 strains. Furthermore, the peak antibody titers induced by CoPoP liposomes can be recalled by subsequent heterologous immunization with Ad26-encoded membrane-bound stabilized Env antigens. Hence, a CoPoP liposome-based HIV-1 vaccine that can generate cross-clade neutralizing antibody immunity could potentially be a component of an efficacious HIV-1 vaccine.

CCR5 structural plasticity shapes HIV-1 phenotypic properties

CCR5 plays immune functions and is the coreceptor for R5 HIV-1 strains. It exists in diverse conformations and oligomerization states. We interrogated the significance of the CCR5 structural diversity on HIV-1 infection. We show that envelope glycoproteins (gp120s) from different HIV-1 strains exhibit divergent binding levels to CCR5 on cell lines and primary cells, but not to CD4 or the CD4i monoclonal antibody E51. This owed to differential binding of the gp120s to different CCR5 populations, which exist in varying quantities at the cell surface and are differentially expressed between different cell types. Some, but not all, of these populations are antigenically distinct conformations of the coreceptor. The different binding levels of gp120s also correspond to differences in their capacity to bind CCR5 dimers/oligomers. Mutating the CCR5 dimerization interface changed conformation of the CCR5 homodimers and modulated differentially the binding of distinct gp120s. Env-pseudotyped viruses also use particular CCR5 conformations for entry, which may differ between different viruses and represent a subset of those binding gp120s. In particular, even if gp120s can bind both CCR5 monomers and oligomers, impairment of CCR5 oligomerization improved viral entry, suggesting that HIV-1 prefers monomers for entry. From a functional standpoint, we illustrate that the nature of the CCR5 molecules to which gp120/HIV-1 binds shapes sensitivity to inhibition by CCR5 ligands and cellular tropism. Differences exist in the CCR5 populations between T-cells and macrophages, and this is associated with differential capacity to bind gp120s and to support viral entry. In macrophages, CCR5 structural plasticity is critical for entry of blood-derived R5 isolates, which, in contrast to prototypical M-tropic strains from brain tissues, cannot benefit from enhanced affinity for CD4. Collectively, our results support a role for CCR5 heterogeneity in diversifying the phenotypic properties of HIV-1 isolates and provide new clues for development of CCR5-targeting drugs.

HIV-1-Specific IgA Monoclonal Antibodies from an HIV-1 Vaccinee Mediate Galactosylceramide Blocking and Phagocytosis

Vaccine-elicited humoral immune responses comprise an array of antibody forms and specificities, with only a fraction contributing to protective host immunity. Elucidation of antibody effector functions responsible for protective immunity against human immunodeficiency virus type 1 (HIV-1) acquisition is a major goal for the HIV-1 vaccine field. Immunoglobulin A (IgA) is an important part of the host defense against pathogens; however, little is known about the role of vaccine-elicited IgA and its capacity to mediate antiviral functions. To identify the antiviral functions of HIV-1-specific IgA elicited by vaccination, we cloned HIV-1 envelope-specific IgA monoclonal antibodies (MAbs) by memory B cell cultures from peripheral blood mononuclear cells from an RV144 vaccinee and produced two IgA clonal cell lines (HG129 and HG130) producing native, nonrecombinant IgA MAbs. The HG129 and HG130 MAbs mediated phagocytosis by monocytes, and HG129 blocked HIV-1 Env glycoprotein binding to galactosylceramide, an alternative HIV-1 receptor. These findings elucidate potential antiviral functions of vaccine-elicited HIV-1 envelope-specific IgA that may act to block HIV-1 acquisition at the portal of entry by preventing HIV-1 binding to galactosylceramide and mediating antibody Fc receptor-mediated virion phagocytosis. Furthermore, these findings highlight the complex and diverse interactions of vaccine-elicited IgA with pathogens that depend on IgA fine specificity and form (e.g., multimeric or monomeric) in the systemic circulation and mucosal compartments.

IMPORTANCE Host-pathogen interactions in vivo involve numerous immune mechanisms that can lead to pathogen clearance. Understanding the nature of antiviral immune mechanisms can inform the design of efficacious HIV-1 vaccine strategies. Evidence suggests that both neutralizing and nonneutralizing antibodies can mediate some protection against HIV in animal models. Although numerous studies have characterized the functional properties of HIV-1-specific IgG, more studies are needed on the functional attributes of HIV-1-specific IgA, specifically for vaccine-elicited IgA. Characterization of the functional properties of HIV-1 Env-specific IgA monoclonal antibodies from human vaccine clinical trials are critical toward understanding the capacity of the host immune response to block HIV-1 acquisition.

HIV-1 gp120 and Modified Vaccinia Virus Ankara (MVA) gp140 Boost Immunogens Increase Immunogenicity of a DNA/MVA HIV-1 Vaccine

An important goal of human immunodeficiency virus (HIV) vaccine design is identification of strategies that elicit effective antiviral humoral immunity. One novel approach comprises priming with DNA and boosting with modified vaccinia virus Ankara (MVA) expressing HIV-1 Env on virus-like particles. In this study, we evaluated whether the addition of a gp120 protein in alum or MVA-expressed secreted gp140 (MVAgp140) could improve immunogenicity of a DNA prime-MVA boost vaccine. Five rhesus macaques per group received two DNA primes at weeks 0 and 8 followed by three MVA boosts (with or without additional protein or MVAgp140) at weeks 18, 26, and 40. Both boost immunogens enhanced the breadth of HIV-1 gp120 and V1V2 responses, antibody-dependent cellular cytotoxicity (ADCC), and low-titer tier 1B and tier 2 neutralizing antibody responses. However, there were differences in antibody kinetics, linear epitope specificity, and CD4 T cell responses between the groups. The gp120 protein boost elicited earlier and higher peak responses, whereas the MVAgp140 boost resulted in improved antibody durability and comparable peak responses after the final immunization. Linear V3 specific IgG responses were particularly enhanced by the gp120 boost, whereas the MVAgp140 boost also enhanced responses to linear C5 and C2.2 epitopes. Interestingly, gp120, but not the MVAgp140 boost, increased peak CD4⁺ T cell responses. Thus, both gp120 and MVAgp140 can augment potential protection of a DNA/MVA vaccine by enhancing gp120 and V1/V2 antibody responses, whereas potential protection by gp120, but not MVAgp140 boosts, may be further impacted by increased CD4⁺ T cell responses.

IMPORTANCE Prior immune correlate analyses with humans and nonhuman primates revealed the importance of antibody responses in preventing HIV-1 infection. A DNA prime-modified vaccinia virus Ankara (MVA) boost vaccine has proven to be potent in eliciting antibody responses. Here we explore the ability of boosts with recombinant gp120 protein or MVA-expressed gp140 to enhance antibody responses elicited by the GOVX-B11 DNA prime-MVA boost vaccine. We found that both types of immunogen boosts enhanced potentially protective antibody responses, whereas the gp120 protein boosts also increased CD4⁺ T cell responses. Our data provide important information for HIV vaccine designs that aim for effective and balanced humoral and T cell responses.

Non-neutralizing Antibodies Targeting the V1V2 Domain of HIV Exhibit Strong Antibody-Dependent Cell-mediated Cytotoxic Activity

The development of an effective vaccine against HIV-1 has proven to be challenging. Broadly neutralizing antibodies (bNAbs), whilst exhibiting neutralization breadth and potency, are elicited only in a small subset of infected individuals and have yet to be induced by vaccination. Case-control studies of RV144 identified an inverse correlation of HIV-1 infection risk with antibodies (Abs) to the V1V2 region of gp120 with high antibody-dependent cellular cytotoxicity (ADCC) activity. The neutralizing activity of Abs was not found to contribute to this protective outcome. Using primary effector and target cells and primary virus isolates, we studied the ADCC profile of different monoclonal Abs targeting the V1V2 loop of gp120 that had low or no neutralizing activity. We compared their ADCC activity to some bNAbs targeting different regions of gp120. We found that mAbs targeting the V1V2 domain induce up to 60% NK cell mediated lysis of HIV-1 infected PBMCs in a physiologically relevant ADCC model, highlighting the interest in inducing such Abs in future HIV vaccine trials. Our data also suggest that in addition to neutralization, lysis of infected cells by Abs can effectively participate in HIV protection, as suggested by the RV144 immune correlate analysis.

Influence of the Envelope gp120 Phe 43 Cavity on HIV-1 Sensitivity to Antibody-Dependent Cell-Mediated Cytotoxicity Responses

HIV-1-infected cells presenting envelope glycoproteins (Env) in the CD4-bound conformation on their surface are preferentially targeted by antibody-dependent cellular-mediated cytotoxicity (ADCC). HIV-1 has evolved sophisticated mechanisms to avoid the exposure of Env ADCC epitopes by downregulating CD4 and by limiting the overall amount of Env on the cell surface. In HIV-1, substitution of large residues such as histidine or tryptophan for serine 375 (S375H/W) in the gp120 Phe 43 cavity, where Phe 43 of CD4 contacts gp120, results in the spontaneous sampling of an Env conformation closer to the CD4-bound state. While residue S375 is well conserved in the majority of group M HIV-1 isolates, CRF01_AE strains have a naturally occurring histidine at this position (H375). Interestingly, CRF01_AE is the predominant circulating strain in Thailand, where the RV144 trial took place. In this trial, which resulted in a modest degree of protection, ADCC responses were identified as being part of the correlate of protection. Here we investigate the influence of the Phe 43 cavity on ADCC responses. Filling this cavity with a histidine or tryptophan residue in Env with a natural serine residue at this position (S375H/W) increased the susceptibility of HIV-1-infected cells to ADCC. Conversely, the replacement of His 375 by a serine residue (H375S) within HIV-1 CRF01_AE decreased the efficiency of the ADCC response. Our results raise the intriguing possibility that the presence of His 375 in the circulating strain where the RV144 trial was held contributed to the observed vaccine efficacy.IMPORTANCE HIV-1-infected cells presenting Env in the CD4-bound conformation on their surface are preferentially targeted by ADCC mediated by HIV-positive (HIV⁺) sera. Here we show that the gp120 Phe 43 cavity modulates the propensity of Env to sample this conformation and therefore affects the susceptibility of infected cells to ADCC. CRF01_AE HIV-1 strains have an unusual Phe 43 cavity-filling His 375 residue, which increases the propensity of Env to sample the CD4-bound conformation, thereby increasing susceptibility to ADCC.

Flow virometric sorting and analysis of HIV quasispecies from plasma

Flow cytometry is utilized extensively for cellular analysis, but technical limitations have prevented its routine application for characterizing virus. The recent introduction of nanoscale fluorescence-activated cytometric cell sorting now allows analysis of individual virions. Here, we demonstrate staining and sorting of infectious HIV. Fluorescent antibodies specific for cellular molecules found on budding virions were used to label CCR5-tropic Bal HIV and CXCR4-tropic NL4.3 HIV Env-expressing pseudovirions made in THP-1 cells (monocyte/macrophage) and H9 cells (T cells), respectively. Using a flow cytometer, we resolved the stained virus beyond isotype staining and demonstrated purity and infectivity of sorted virus populations on cells with the appropriate coreceptors. We subsequently sorted infectious simian/human immunodeficiency virus from archived plasma. Recovery was approximately 0.5%, but virus present in plasma was already bound to viral-specific IgG generated in vivo, likely contributing to the low yield. Importantly, using two broadly neutralizing HIV antibodies, PG9 and VRC01, we also sorted virus from archived human plasma and analyzed the sorted populations genetically and by proteomics, identifying the quasispecies present. The ability to sort infectious HIV from clinically relevant samples provides material for detailed molecular, genetic, and proteomic analyses applicable to future design of vaccine antigens and potential development of personalized treatment regimens.

A Cinnamon-Derived Procyanidin Compound Displays Anti-HIV-1 Activity by Blocking Heparan Sulfate- and Co-Receptor- Binding Sites on gp120 and Reverses T Cell Exhaustion via Impeding Tim-3 and PD-1 Upregulation

Amongst the many strategies aiming at inhibiting HIV-1 infection, blocking viral entry has been recently recognized as a very promising approach. Using diverse in vitro models and a broad range of HIV-1 primary patient isolates, we report here that IND02, a type A procyanidin polyphenol extracted from cinnamon, that features trimeric and pentameric forms displays an anti-HIV-1 activity against CXCR4 and CCR5 viruses with 1–7 μM ED₅₀ for the trimer. Competition experiments, using a surface plasmon resonance-based binding assay, revealed that IND02 inhibited envelope binding to CD4 and heparan sulphate (HS) as well as to an antibody (mAb 17b) directed against the gp120 co-receptor binding site with an IC₅₀ in the low μM range. IND02 has thus the remarkable property of simultaneously blocking gp120 binding to its major host cell surface counterparts. Additionally, the IND02-trimer impeded up-regulation of the inhibitory receptors Tim-3 and PD-1 on CD4⁺ and CD8⁺ cells, thereby demonstrating its beneficial effect by limiting T cell exhaustion. Among naturally derived products significantly inhibiting HIV-1, the IND02-trimer is the first component demonstrating an entry inhibition property through binding to the viral envelope glycoprotein. These data suggest that cinnamon, a widely consumed spice, could represent a novel and promising candidate for a cost-effective, natural entry inhibitor for HIV-1 which can also down-modulate T cell exhaustion markers Tim-3 and PD-1.

An HIV-1 Env-Antibody Complex Focuses Antibody Responses to Conserved Neutralizing Epitopes

Elicitation of broadly neutralizing Ab (bNAb) responses to the conserved elements of the HIV-1 envelope glycoproteins (Env), including the primary receptor CD4 binding site (CD4bs), is a major focus of vaccine development yet to be accomplished. However, a large number of CD4bs-directed bNAbs have been isolated from HIV-1-infected individuals. Comparison of the routes of binding used by the CD4bs-directed bNAbs from patients and the vaccine-elicited CD4bs-directed mAbs indicates that the latter fail to neutralize primary virus isolates because they approach the Env spike with a vertical angle and contact the specific surface residues occluded in the native spike, including the bridging sheet on gp120. To preferentially expose the CD4bs and direct the immune response away from the bridging sheet, resulting in an altered angle of approach, we engineered an immunogen consisting of gp120 core in complex with the prototypic CD4-induced Ab, 17b. This mAb directly contacts the bridging sheet but not the CD4bs. The complex was further stabilized by chemical crosslinking to prevent dissociation. Rabbits immunized with the crosslinked complex displayed earlier affinity maturation, achieving tier 1 virus neutralization compared with animals immunized with gp120 core alone. Immunization with the crosslinked complex induced transient Ab responses with binding specificity similar to the CD4bs-directed bNAbs. mAbs derived from complex-immunized rabbits displayed footprints on gp120 more distal from the bridging sheet as compared with previous vaccine-elicited CD4bs Abs, indicating that Env-Ab complexes effectively dampen immune responses to undesired immunodominant bridging sheet determinants.

Guidelines for cloning, expression, purification and functional characterization of primary HIV-1 envelope glycoproteins

The trimeric HIV-1 envelope (Env) glycoproteins gp120 and gp41 mediate virus entry into target cells by engaging CD4 and the coreceptors CCR5 or CXCR4 at the cell surface and driving membrane fusion. Receptor/gp120 interactions regulate the virus life cycle, HIV infection transmission and pathogenesis. Env is also the target of neutralizing antibodies. Efforts have thus been made to produce soluble HIV-1 glycoproteins to develop vaccines and study the role and mechanisms of HIV/receptor interactions. However, production and purification of Env glycoproteins and their functional assessment has to cope with multiple obstacles. These include difficulties in amplifying and cloning env sequences and setting up receptor binding assays that are suitable for studies on large collections of glycoproteins, flexible enough to adapt to Env and receptor structural heterogeneities, and allow recapitulating the receptor binding properties of virion-associated Env trimers. Here we identify these difficulties and present protocols to produce primary gp120 and determination of their binding properties to receptors. The receptor binding assays confirmed that the produced glycoproteins are competent for binding CD4 and undergo proper CD4-induced conformational changes required for interaction with CCR5. These assays may help elucidate the role of gp120/receptor interactions in the pathophysiology of HIV infection and develop HIV-1 entry inhibitors.

HLA class II genes modulate vaccine-induced antibody responses to affect HIV-1 acquisition

In the RV144 vaccine trial, two antibody responses were found to correlate with HIV-1 acquisition. Because human leukocyte antigen (HLA) class II-restricted CD4(+) T cells are involved in antibody production, we tested whether HLA class II genotypes affected HIV-1-specific antibody levels and HIV-1 acquisition in 760 individuals. Indeed, antibody responses correlated with acquisition only in the presence of single host HLA alleles. Envelope (Env)-specific immunoglobulin A (IgA) antibodies were associated with increased risk of acquisition specifically in individuals with DQB1*06. IgG antibody responses to Env amino acid positions 120 to 204 were higher and were associated with decreased risk of acquisition and increased vaccine efficacy only in the presence of DPB1*13. Screening IgG responses to overlapping peptides spanning Env 120-204 and viral sequence analysis of infected individuals defined differences in vaccine response that were associated with the presence of DPB1*13 and could be responsible for the protection observed. Overall, the underlying genetic findings indicate that HLA class II modulated the quantity, quality, and efficacy of antibody responses in the RV144 trial.

Nef Proteins from HIV-1 Elite Controllers Are Inefficient at Preventing Antibody-Dependent Cellular Cytotoxicity

Impairment of Nef function, including reduced CD4 downregulation, was described in a subset of HIV-1-infected individuals that control viral replication without antiretroviral treatment (elite controllers [EC]). Elimination of HIV-1-infected cells by antibody-dependent cellular cytotoxicity (ADCC) requires the presence of envelope glycoproteins (Env) in the CD4-bound conformation, raising the possibility that accumulating CD4 at the surface of virus-infected cells in EC could interact with Env and thereby sensitize these cells to ADCC. We observed a significant increase in the exposure of Env epitopes targeted by ADCC-mediating antibodies at the surface of cells expressing Nef isolates from EC; this correlated with enhanced susceptibility to ADCC. Altogether, our results suggest that enhanced susceptibility of HIV-1-infected cells to ADCC may contribute to the EC phenotype.

IMPORTANCE

Nef clones derived from elite controllers (EC) have been shown to be attenuated for CD4 downregulation; how this contributes to the nonprogressor phenotype of these infected individuals remains uncertain. Increasing evidence supports a role for HIV-specific antibody-dependent cellular cytotoxicity (ADCC) in controlling viral infection and replication. Here, we show that residual CD4 left at the surface of cells expressing Nef proteins isolated from ECs are sufficient to allow Env-CD4 interaction, leading to increased exposure of Env CD4-induced epitopes and increased susceptibility of infected cells to ADCC. Our results suggest that ADCC might be an active immune mechanism in EC that helps to maintain durable suppression of viral replication and low plasma viremia level in this rare subset of infected individuals. Therefore, targeting Nef's ability to downregulate CD4 could render HIV-1-infected cells susceptible to ADCC and thus have therapeutic utility.

Glucocorticoid Receptor-DNA Dissociation Kinetics Measured in Vitro Reveal Exchange on the Second Time Scale

The glucocorticoid receptor (GR) is a member of the steroid receptor family of ligand-activated transcription factors. Recent live cell imaging studies have revealed that interactions of GR with chromatin are highly dynamic, with average receptor residence times of only seconds. These findings were surprising because early kinetic studies found that GR-DNA interactions in vitro were much slower, having calculated residence times of minutes to hours. However, these latter analyses were conducted at a time when it was possible to work with only either partially purified holoreceptor or its purified but isolated DNA binding domain. Noting these limitations, we reexamined GR-DNA dissociation kinetics using a highly purified holoreceptor shown to be amenable to rigorous study. We first observe that GR-DNA interactions in vitro are not slow as previously thought but converge with in vivo behavior, having residence times of only seconds to tens of seconds. This rapid exchange is seen at six individual response elements and the multisite MMTV promoter used in live cell imaging. Second, GR dissociation rates are identical for all response elements. Thus, previously observed differences in receptor affinity toward these sequences are not due to differences in off rate but in on rate. Finally, dissociation kinetics are biphasic in character. A minimal kinetic model consistent with the data is that in which DNA-bound GR interconverts between states on a second time scale, with dissociation occurring via a multistep process. We speculate that receptor interconversion in this time frame can be recognized by the coregulatory proteins that interact with GR, leading to unique transcriptional responses.

A single-residue change in the HIV-1 V3 loop associated with maraviroc resistance impairs CCR5 binding affinity while increasing replicative capacity

Background

Maraviroc (MVC) is an allosteric CCR5 inhibitor used against HIV-1 infection. While MVC-resistant viruses have been identified in patients, it still remains incompletely known how they adjust their CD4 and CCR5 binding properties to resist MVC inhibition while preserving their replicative capacity. It is thought that they maintain high efficiency of receptor binding. To date however, information about the binding affinities to receptors for inhibitor-resistant HIV-1 remains limited.

Results

Here, we show by means of viral envelope (gp120) binding experiments and virus-cell fusion kinetics that a MVC-resistant virus (MVC-Res) that had emerged as a dominant viral quasispecies in a patient displays reduced affinities for CD4 and CCR5 either free or bound to MVC, as compared to its MVC-sensitive counterpart isolated before MVC therapy. An alanine insertion within the GPG motif (G310_P311insA) of the MVC-resistant gp120 V3 loop is responsible for the decreased CCR5 binding affinity, while impaired binding to CD4 is due to sequence changes outside V3. Molecular dynamics simulations of gp120 binding to CCR5 further emphasize that the Ala insertion alters the structure of the V3 tip and weakens interaction with CCR5 ECL2. Paradoxically, infection experiments on cells expressing high levels of CCR5 also showed that Ala allows MVC-Res to use CCR5 efficiently, thereby improving viral fusion and replication efficiencies. Actually, although we found that the V3 loop of MVC-Res is required for high levels of MVC resistance, other regions outside V3 are sufficient to confer a moderate level of resistance. These sequence changes outside V3, however, come with a replication cost, which is compensated for by the Ala insertion in V3.

Conclusion

These results indicate that changes in the V3 loop of MVC-resistant viruses can augment the efficiency of CCR5-dependent steps of viral entry other than gp120 binding, thereby compensating for their decreased affinity for entry receptors and improving their fusion and replication efficiencies. This study thus sheds light on unsuspected mechanisms whereby MVC-resistant HIV-1 could emerge and grow in treated patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0177-1) contains supplementary material, which is available to authorized users.

Bioinformatic analysis of neurotropic HIV envelope sequences identifies polymorphisms in the gp120 bridging sheet that increase macrophage-tropism through enhanced interactions with CCR5

Macrophages express low levels of the CD4 receptor compared to T-cells. Macrophage-tropic HIV strains replicating in brain of untreated patients with HIV-associated dementia (HAD) express Envs that are adapted to overcome this restriction through mechanisms that are poorly understood. Here, bioinformatic analysis of env sequence datasets together with functional studies identified polymorphisms in the β3 strand of the HIV gp120 bridging sheet that increase M-tropism. D197, which results in loss of an N-glycan located near the HIV Env trimer apex, was detected in brain in some HAD patients, while position 200 was estimated to be under positive selection. D197 and T/V200 increased fusion and infection of cells expressing low CD4 by enhancing gp120 binding to CCR5. These results identify polymorphisms in the HIV gp120 bridging sheet that overcome the restriction to macrophage infection imposed by low CD4 through enhanced gp120-CCR5 interactions, thereby promoting infection of brain and other macrophage-rich tissues.

Global shape and ligand binding efficiency of the HIV-1-neutralizing antibodies differ from those of antibodies that cannot neutralize HIV-1

Asymmetric disposition of Fab arms in the structures solved for the broadly neutralizing monoclonal antibody (nmAb) IgG1 b12 raised the question of whether the unusual shape observed for b12 is common for all IgG1 mAbs or if there is a difference in the overall shape of nmAbs versus non-nmAbs. We compared small angle x-ray scattering (SAXS) data-based models and limited proteolysis profiles of some IgG1 mAbs known to be having and lacking HIV-1 neutralizing potency. In non-nmAbs, the Fab arms were found to be symmetrically disposed in space relative to central Fc, but in most nmAbs, the Fab arms were asymmetrically disposed, as seen for IgG1 b12. The only exceptions were 2G12 and 4E10, where both Fab arms were closed above Fc, suggesting some Fab-Fc and/or Fab-Fab interaction in the nmAbs that constrained extension of the Fab-Fc linker. Interestingly, these observations were correlated with differential proteolysis profiles of the mAbs by papain. Under conditions when papain could cut both Fab arms of non-nmAbs, only one Fab arm could be removed from neutralizing ones (except for 2G12 and 4E10). Chromatography and small angle x-ray scattering results of papain-digested products revealed that 1) the Fab-Fc or Fab-Fab interactions in unliganded mAbs are retained in digested products, and 2) whereas anti-gp120 non-nmAbs could bind two gp120 molecules, nmAbs could bind only one gp120. Additional experiments showed that except for 2G12 and 4E10, unopen shapes of nmAbs remain uninfluenced by ionic strength but can be reversibly opened by low pH of buffer accompanied by loss of ligand binding ability.

Interactions of peptide triazole thiols with Env gp120 induce irreversible breakdown and inactivation of HIV-1 virions

Background

We examined the underlying mechanism of action of the peptide triazole thiol, KR13 that has been shown previously to specifically bind gp120, block cell receptor site interactions and potently inhibit HIV-1 infectivity.

Results

KR13, the sulfhydryl blocked KR13b and its parent non-sulfhydryl peptide triazole, HNG156, induced gp120 shedding but only KR13 induced p24 capsid protein release. The resulting virion post virolysis had an altered morphology, contained no gp120, but retained gp41 that bound to neutralizing gp41 antibodies. Remarkably, HIV-1 p24 release by KR13 was inhibited by enfuvirtide, which blocks formation of the gp41 6-helix bundle during membrane fusion, while no inhibition of p24 release occurred for enfuvirtide-resistant virus. KR13 thus appears to induce structural changes in gp41 normally associated with membrane fusion and cell entry. The HIV-1 p24 release induced by KR13 was observed in several clades of HIV-1 as well as in fully infectious HIV-1 virions.

Conclusions

The antiviral activity of KR13 and its ability to inactivate virions prior to target cell engagement suggest that peptide triazole thiols could be highly effective in inhibiting HIV transmission across mucosal barriers and provide a novel probe to understand biochemical signals within envelope that are involved in membrane fusion.

Antigenicity and immunogenicity of transmitted/founder, consensus, and chronic envelope glycoproteins of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) vaccine development requires selection of appropriate envelope (Env) immunogens. Twenty HIV-1 Env glycoproteins were examined for their ability to bind human anti-HIV-1 monoclonal antibodies (MAbs) and then used as immunogens in guinea pigs to identify promising immunogens. These included five Envs derived from chronically infected individuals, each representing one of five common clades and eight consensus Envs based on these five clades, as well as the consensus of the entire HIV-1 M group, and seven transmitted/founder (T/F) Envs from clades B and C. Sera from immunized guinea pigs were tested for neutralizing activity using 36 HIV-1 Env-pseudotyped viruses. All Envs bound to CD4 binding site, membrane-proximal, and V1/V2 MAbs with similar apparent affinities, although the T/F Envs bound with higher affinity to the MAb 17b, a CCR5 coreceptor binding site antibody. However, the various Envs differed in their ability to induce neutralizing antibodies. Consensus Envs elicited the most potent responses, but neutralized only a subset of viruses, including mostly easy-to-neutralize tier 1 and some more-difficult-to-neutralize tier 2 viruses. T/F Envs elicited fewer potent neutralizing antibodies but exhibited greater breadth than chronic or consensus Envs. Finally, chronic Envs elicited the lowest level and most limited breadth of neutralizing antibodies overall. Thus, each group of Env immunogens elicited a different antibody response profile. The complementary benefits of consensus and T/F Env immunogens raise the possibility that vaccines utilizing a combination of consensus and T/F Envs may be able to induce neutralizing responses with greater breadth and potency than single Env immunogens.

Neutralization sensitivity of HIV-1 subtype B' clinical isolates from former plasma donors in China

BACKGROUND

HIV-1 subtype B' isolates have been predominantly circulating in China. Their intra- and inter-subtype neutralization sensitivity to autologous and heterologous plasmas has not been well studied.

RESULTS

Twelve HIV-1 B' clinical isolates obtained from patients were tested for their intra- and inter-subtype neutralization sensitivity to the neutralization antibodies in the plasmas from patients infected by HIV-1 B' and CRF07_BC subtypes, respectively. We found that the plasmas from the HIV-1 B'-infected patients could potently neutralize heterologous viruses of subtype B' with mean ID50 titer (1/x) of about 67, but they were not effective in neutralizing autologous viruses of subtype B' with mean ID50 titer (1/x) of about 8. The plasmas from HIV-1 CRF07_BC-infected patients exhibited weak inter-subtype neutralization activity against subtype B' viruses with ID50 titer (1/x) is about 22. The neutralization sensitivity of HIV-1 B' isolates was inversely correlated with the neutralizing activity of plasmas from HIV-1 B'-infected patients (Spearman's r = -0.657, P = 0.020), and with the number of potential N-glycosylation site (PNGS) in V1-V5 region (Spearman's r = -0.493, P = 0.034), but positively correlated with the viral load (Spearman's r = 0.629, P = 0.028). It had no correlation with the length of V1-V5 regions or the CD4+ T cell count. Virus AH259V has low intra-subtype neutralization sensitivity, it can be neutralized by 17b (IC50: 10μg/ml) and 447-52D (IC50: 1.6μg/ml), and the neutralizing antibodies (nAbs) in plasma AH259P are effective in neutralizing infection by the primary HIV-1 isolates with different subtypes with ID50 titers (1/x) in the range of 32-396.

CONCLUSIONS

These findings suggest that the HIV-1 subtype B' viruses may mutate under the immune pressure, thus becoming resistant to the autologous nAbs, possibly by changing the number of PNGS in the V1-V5 region of the viral gp120. Some of primary HIV-1 isolates are able to induce both intra- and inter-subtype cross-neutralizing antibody responses.

PGV04, an HIV-1 gp120 CD4 binding site antibody, is broad and potent in neutralization but does not induce conformational changes characteristic of CD4

Recently, several broadly neutralizing monoclonal antibodies (bnMAbs) directed to the CD4-binding site (CD4bs) of gp120 have been isolated from HIV-1-positive donors. These include VRC01, 3BNC117, and NIH45-46, all of which are capable of neutralizing about 90% of circulating HIV-1 isolates and all of which induce conformational changes in the HIV-1 gp120 monomer similar to those induced by the CD4 receptor. In this study, we characterize PGV04 (also known as VRC-PG04), a MAb with potency and breadth that rivals those of the prototypic VRC01 and 3BNC117. When screened on a large panel of viruses, the neutralizing profile of PGV04 was distinct from those of CD4, b12, and VRC01. Furthermore, the ability of PGV04 to neutralize pseudovirus containing single alanine substitutions exhibited a pattern distinct from those of the other CD4bs MAbs. In particular, substitutions D279A, I420A, and I423A were found to abrogate PGV04 neutralization. In contrast to VRC01, PGV04 did not enhance the binding of 17b or X5 to their epitopes (the CD4-induced [CD4i] site) in the coreceptor region on the gp120 monomer. Furthermore, in contrast to CD4, none of the anti-CD4bs MAbs induced the expression of the 17b epitope on cell surface-expressed cleaved Env trimers. We conclude that potent CD4bs bnMAbs can display differences in the way they recognize and access the CD4bs and that mimicry of CD4, as assessed by inducing conformational changes in monomeric gp120 that lead to enhanced exposure of the CD4i site, is not uniquely correlated with effective neutralization at the site of CD4 binding on HIV-1.

DC-SIGN increases the affinity of HIV-1 envelope glycoprotein interaction with CD4

Mannose-binding C-type lectin receptors, expressed on Langerhans cells and subepithelial dendritic cells (DCs) of cervico-vaginal tissues, play an important role in HIV-1 capture and subsequent dissemination to lymph nodes. DC-SIGN has been implicated in both productive infection of DCs and the DC-mediated trans infection of CD4⁺ T cells that occurs in the absence of replication. However, the molecular events that underlie this efficient transmission have not been fully defined. In this study, we have examined the effect of the extracellular domains of DC-SIGN and Langerin on the stability of the interaction of the HIV-1 envelope glycoprotein with CD4 and also on replication in permissive cells. Surface plasmon resonance analysis showed that DC-SIGN increases the binding affinity of trimeric gp140 envelope glycoproteins to CD4. In contrast, Langerin had no effect on the stability of the gp140:CD4 complex. In vitro infection experiments to compare DC-SIGN enhancement of CD4-dependent and CD4-independent strains demonstrated significantly lower enhancement of the CD4-independent strain. In addition DC-SIGN increased the relative rate of infection of the CD4-dependent strain but had no effect on the CD4-independent strain. DC-SIGN binding to the HIV envelope protein effectively increases exposure of the CD4 binding site, which in turn contributes to enhancement of infection.

Envelope deglycosylation enhances antigenicity of HIV-1 gp41 epitopes for both broad neutralizing antibodies and their unmutated ancestor antibodies

The HIV-1 gp41 envelope (Env) membrane proximal external region (MPER) is an important vaccine target that in rare subjects can elicit neutralizing antibodies. One mechanism proposed for rarity of MPER neutralizing antibody generation is lack of reverted unmutated ancestor (putative naive B cell receptor) antibody reactivity with HIV-1 envelope. We have studied the effect of partial deglycosylation under non-denaturing (native) conditions on gp140 Env antigenicity for MPER neutralizing antibodies and their reverted unmutated ancestor antibodies. We found that native deglycosylation of clade B JRFL gp140 as well as group M consensus gp140 Env CON-S selectively increased the reactivity of Env with the broad neutralizing human mAbs, 2F5 and 4E10. Whereas fully glycosylated gp140 Env either did not bind (JRFL), or weakly bound (CON-S), 2F5 and 4E10 reverted unmutated ancestors, natively deglycosylated JRFL and CON-S gp140 Envs did bind well to these putative mimics of naive B cell receptors. These data predict that partially deglycoslated Env would bind better than fully glycosylated Env to gp41-specific naïve B cells with improved immunogenicity. In this regard, immunization of rhesus macaques demonstrated enhanced immunogenicity of the 2F5 MPER epitope on deglyosylated JRFL gp140 compared to glycosylated JRFL gp140. Thus, the lack of 2F5 and 4E10 reverted unmutated ancestor binding to gp140 Env may not always be due to lack of unmutated ancestor antibody reactivity with gp41 peptide epitopes, but rather, may be due to glycan interference of binding of unmutated ancestor antibodies of broad neutralizing mAb to Env gp41.

Critical to the design of an effective HIV-1 vaccine that will induce long-lasting broadly neutralizing antibodies is to understand why broad neutralizing antibodies are not induced. One hypothesis is that there are “holes” in the naïve B cell repertoires for unmutated B cell receptors that can bind to HIV-1 envelope (Env) neutralizing epitopes. In this paper, we test this hypothesis for the rare HIV-1 envelope gp41 broad neutralizing monoclonal antibodes (mAbs), called 2F5 and 4E10, and show that indeed, fully glycosylated Env does not bind to inferred unmutated ancestor antibodies (mimics of naïve B cell receptors) of mAbs 2F5 and 4E10, but that partially deglycosylated Envs that have had glycans removed under non-denaturing conditions, did bind to 2F5 and 4E10 unmutated ancestor antibodies. Thus, rather than there being a lack of existence of germline B cell receptors for gp41 broad neutralizing antibodies, one impediment to induction of gp41 broad neutralizing antibodies may be glycan interference with unmutated antibody binding to gp41 envelope.

Peptidomimetic inhibitors targeting the CCR5-binding site on the human immunodeficiency virus type-1 gp120 glycoprotein complexed to CD4

A helical tyrosine-sulfated epitope in CCR5 that is recognized by the human immunodeficiency virus type-1 envelope glycoprotein gp120 in its CD4-induced conformation can be mimicked structurally by a cyclic β-hairpin peptide containing two sulfated tyrosine residues at positions i and i + 2 along one β-strand.

Surface plasmon resonance for vaccine design and efficacy studies: recent applications and future trends

The lack of a clear correlation between design and protection continues to present a barrier to progress in vaccine research. In this article, we outline how surface plasmon resonance (SPR) biosensors are emerging as tools to help resolve some of the key biophysical determinants of protection and, thereby, facilitate more rational vaccine design campaigns. SPR technology has contributed significantly to our understanding of the complex biophysical determinants of HIV neutralization and offers a platform for preclinical evaluation of vaccine candidates. In particular, the concept of reverse-engineering HIV vaccine targets based on known broadly neutralizing antibody modalities is explored and extended to include other infectious diseases, such as malaria and influenza, and other diseases such as cancer. The analytical capacity afforded by SPR includes serum screening to monitor immune responses and highly efficient quality-control surveillance measures. These are discussed alongside key technological advances, such as developments in sample throughput, and a perspective predicting continued growth and diversification of the role of SPR in vaccine development is proposed.

Soluble CD4 and CD4-mimetic compounds inhibit HIV-1 infection by induction of a short-lived activated state

Binding to the CD4 receptor induces conformational changes in the human immunodeficiency virus (HIV-1) gp120 exterior envelope glycoprotein. These changes allow gp120 to bind the coreceptor, either CCR5 or CXCR4, and prime the gp41 transmembrane envelope glycoprotein to mediate virus–cell membrane fusion and virus entry. Soluble forms of CD4 (sCD4) and small-molecule CD4 mimics (here exemplified by JRC-II-191) also induce these conformational changes in the HIV-1 envelope glycoproteins, but typically inhibit HIV-1 entry into CD4-expressing cells. To investigate the mechanism of inhibition, we monitored at high temporal resolution inhibitor-induced changes in the conformation and functional competence of the HIV-1 envelope glycoproteins that immediately follow engagement of the soluble CD4 mimics. Both sCD4 and JRC-II-191 efficiently activated the envelope glycoproteins to mediate infection of cells lacking CD4, in a manner dependent on coreceptor affinity and density. This activated state, however, was transient and was followed by spontaneous and apparently irreversible changes of conformation and by loss of functional competence. The longevity of the activated intermediate depended on temperature and the particular HIV-1 strain, but was indistinguishable for sCD4 and JRC-II-191; by contrast, the activated intermediate induced by cell-surface CD4 was relatively long-lived. The inactivating effects of these activation-based inhibitors predominantly affected cell-free virus, whereas virus that was prebound to the target cell surface was mainly activated, infecting the cells even at high concentrations of the CD4 analogue. These results demonstrate the ability of soluble CD4 mimics to inactivate HIV-1 by prematurely triggering active but transient intermediate states of the envelope glycoproteins. This novel strategy for inhibition may be generally applicable to high–potential-energy viral entry machines that are normally activated by receptor binding.

Human immunodeficiency virus type 1 (HIV-1) is the cause of the global AIDS epidemic. HIV-1 gains entry into its target cells by fusing with the cell membrane, a process that begins with the interaction of the viral envelope glycoproteins with cell-surface receptors. HIV-1 uses two receptors on the target cell: CD4 and CCR5/CXCR4. Binding of the virus to the primary receptor, CD4, primes the viral envelope glycoproteins to mediate the fusion of the viral membrane and the membrane of the target cell. Soluble forms of the CD4 receptor and small molecules that mimic the effects of CD4 can inhibit virus infection; however, how this inhibition occurs is still unknown. In this report, we show that soluble mimics of CD4 inhibit HIV-1 infection by prematurely triggering the viral envelope glycoproteins. The unstable activated state of the virus lasts only a few minutes, after which the virus loses the ability to infect cells. This novel strategy for inhibition may be generally applicable to other viruses besides HIV-1, some of which are also activated by binding to their receptors.

B cell recognition of the conserved HIV-1 co-receptor binding site is altered by endogenous primate CD4

The surface HIV-1 exterior envelope glycoprotein, gp120, binds to CD4 on the target cell surface to induce the co-receptor binding site on gp120 as the initial step in the entry process. The binding site is comprised of a highly conserved region on the gp120 core, as well as elements of the third variable region (V3). Antibodies against the co-receptor binding site are abundantly elicited during natural infection of humans, but the mechanism of elicitation has remained undefined. In this study, we investigate the requirements for elicitation of co-receptor binding site antibodies by inoculating rabbits, monkeys and human-CD4 transgenic (huCD4) rabbits with envelope glycoprotein (Env) trimers possessing high affinity for primate CD4. A cross-species comparison of the antibody responses showed that similar HIV-1 neutralization breadth was elicited by Env trimers in monkeys relative to wild-type (WT) rabbits. In contrast, antibodies against the co-receptor site on gp120 were elicited only in monkeys and huCD4 rabbits, but not in the WT rabbits. This was supported by the detection of high-titer co-receptor antibodies in all sera from a set derived from human volunteers inoculated with recombinant gp120. These findings strongly suggest that complexes between Env and (high-affinity) primate CD4 formed in vivo are responsible for the elicitation of the co-receptor-site-directed antibodies. They also imply that the naïve B cell receptor repertoire does not recognize the gp120 co-receptor site in the absence of CD4 and illustrate that conformational stabilization, imparted by primary receptor interaction, can alter the immunogenicity of a type 1 viral membrane protein.

A major goal of HIV-1 vaccine research is to design novel candidates capable of neutralizing the vast array of viruses circulating in the human population. One approach is to base the vaccine upon the HIV-1 outer surface envelope glycoproteins to generate antibodies. However, during persistent infection in humans, the HIV-1 envelope glycoproteins have evolved structural features that limit the elicitation of broadly neutralizing antibodies. These immune “decoys” divert the antibody response resulting in virus subpopulations that can escape the host response. A potential means by which the virus elicits these decoy responses comes as a by-product of the entry process. Binding of the HIV-1 envelope glycoproteins to the primary receptor, human CD4, induces the formation of a second co-receptor binding site on the envelope glycoproteins, which then binds to another protein required for viral entry. Antibodies to the co-receptor binding site are generally ineffective at neutralizing HIV-1 patient isolates. Here, we demonstrate the mechanism by which antibodies to the HIV-1 co-receptor binding site are elicited in animals and humans injected with HIV-1 envelope glycoproteins and describe the implications of their formation regarding natural HIV-1 infection and vaccine design.

The HIV-1 envelope glycoprotein gp120 features four heparan sulfate binding domains, including the co-receptor binding site

It is well established that the human immunodeficiency virus-1 envelope glycoprotein surface unit, gp120, binds to cell-associated heparan sulfate (HS). Virus infectivity is increased by such interaction, and a variety of soluble polyanions efficiently neutralize immunodeficiency virus-1 in vitro. This interaction has been mainly attributed to the gp120 V3 loop. However, although evidence suggested that this particular domain does not fully recapitulate the binding activity of the protein, the ability of HS to bind to other regions of gp120 has not been completely addressed, and the exact localizations of the polysaccharide binding sites are not known. To investigate in more detail the structural basis of the HS-gp120 interaction, we used a mapping strategy and compared the heparin binding activity of wild type and mutant gp120 using surface plasmon resonance-based binding assays. Four heparin binding domains (1-4) were identified in the V2 and V3 loops, in the C-terminal domain, and within the CD4-induced bridging sheet. Interestingly, three of them were found in domains of the protein that undergo structural changes upon binding to CD4 and are involved in co-receptor recognition. In particular, Arg(419), Lys(421), and Lys(432), which directly interact with the co-receptor, are targeted by heparin. This study provides a complete account of the gp120 residues involved in heparin binding and identified several binding surfaces that constitute potential target for viral entry inhibition.

The role of antibody polyspecificity and lipid reactivity in binding of broadly neutralizing anti-HIV-1 envelope human monoclonal antibodies 2F5 and 4E10 to glycoprotein 41 membrane proximal envelope epitopes

Two neutralizing human mAbs, 2F5 and 4E10, that react with the HIV-1 envelope gp41 membrane proximal region are also polyspecific autoantibodies that bind to anionic phospholipids. To determine the autoantibody nature of these Abs, we have compared their reactivities with human anti-cardiolipin mAbs derived from a primary antiphospholipid syndrome patient. To define the role of lipid polyreactivity in binding of 2F5 and 4E10 mAbs to HIV-1 envelope membrane proximal epitopes, we determined the kinetics of binding of mAbs 2F5 and 4E10 to their nominal gp41 epitopes vs liposome-gp41 peptide conjugates. Both anti-HIV-1 mAbs 2F5 and 4E10 bound to cardiolipin with K(d) values similar to those of autoimmune anti-cardiolipin Abs, IS4 and IS6. Binding kinetics studies revealed that mAb 2F5 and 4E10 binding to their respective gp41 peptide-lipid conjugates could best be defined by a two-step (encounter-docking) conformational change model. In contrast, binding of 2F5 and 4E10 mAbs to linear peptide epitopes followed a simple Langmuir model. A mouse mAb, 13H11, that cross-blocks mAb 2F5 binding to the gp41 epitope did not cross-react with lipids nor did it neutralize HIV-1 viruses. Taken together, these data demonstrate the similarity of 2F5 and 4E10 mAbs to known anti-cardiolipin Abs and support the model that mAb 2F5 and 4E10 binding to HIV-1 involves both viral lipid membrane and gp41 membrane proximal epitopes.

Stability of a receptor-binding active human immunodeficiency virus type 1 recombinant gp140 trimer conferred by intermonomer disulfide bonding of the V3 loop: differential effects of protein disulfide isomerase on CD4 and coreceptor binding

Stable trimeric forms of human immunodeficiency virus recombinant gp140 (rgp140) are important templates for determining the structure of the glycoprotein to assist in our understanding of HIV infection and host immune response. Such information will aid the design of therapeutic drugs and vaccines. Here, we report the production of a highly stable and trimeric rgp140 derived from a HIV type 1 (HIV-1) subtype D isolate that may be suitable for structural studies. The rgp140 is functional in terms of binding to CD4 and three human monoclonal antibodies (17b, b12, and 2G12) that have broad neutralizing activities against a range of HIV-1 isolates from different subtypes. Treatment of rgp140 with protein disulfide isomerase (PDI) severely restricted 17b binding capabilities. The stable nature of the rgp140 was due to the lack of processing at the gp120/41 boundary and the presence of an intermonomer disulfide bond formed by the cysteines of the V3 loop. Further characterization showed the intermonomer disulfide bond to be a target for PDI processing. The relevance of these findings to the roles of the V3 domain and the timing of PDI action during the HIV infection process are discussed.

Restraining the conformation of HIV-1 gp120 by removing a flexible loop

The trimeric HIV/SIV envelope glycoprotein, gp160, is cleaved to noncovalently associated fragments, gp120 and gp41. Binding of gp120 to viral receptors leads to large structural rearrangements in both fragments. The unliganded gp120 core has a disordered beta3-beta5 loop, which reconfigures upon CD4 binding into an ordered, extended strand. Molecular modeling suggests that residues in this loop may contact gp41. We show here that deletions in the beta3-beta5 loop of HIV-1 gp120 weaken the binding of CD4 and prevent formation of the epitope for monoclonal antibody (mAb) 17b (which recognizes the coreceptor site). Formation of an encounter complex with CD4 binding and interactions of gp120 with mAbs b12 and 2G12 are not affected by these deletions. Thus, deleting the beta3-beta5 loop blocks the gp120 conformational change and may offer a strategy for design of restrained immunogens. Moreover, mutations in the SIV beta3-beta5 loop lead to greater spontaneous dissociation of gp120 from cell-associated trimers. We suggest that the CD4-induced rearrangement of this loop releases structural constraints on gp41 and thus potentiates its fusion activity.

Dynamic tradeoffs in the raft-mediated entry of human immunodeficiency virus type 1 into cells

To initiate an infection human immunodeficiency virus type 1 (HIV-1) particles must first bind to receptors on the surface of their host cells, a process that eventually leads to fusion of viral and cellular membranes and release of the viral genome into the cytoplasm. Understanding the molecular mechanisms of these processes may enable the development of new anti-HIV strategies. Disagreement currently prevails on the role in virus entry of microdomains within the cellular plasma membrane known as lipid rafts. Experiments have suggested that lipid rafts, in their interactions with cellular receptors and viral particles, either promote or have minimal effect on viral entry. Here we develop a dynamic model for HIV-1 entry that enables us to identify and quantitatively assess tradeoffs that can arise from the clustering of receptors in rafts. Specifically, receptor clustering can be detrimental to the initiation of viral infection by reducing the probability that a virus particle finds its primary receptor, CD4. However, receptor clustering can also enable a virus particle, once bound, to rapidly form multivalent interactions with receptors and co-receptors that are required for virus-cell membrane fusion. We show how the resolution of such tradeoffs hinges on the level and spatial distribution of receptors and co-receptors on the cell surface, and we discuss implications of these effects for the design of therapeutics that inhibit HIV-1 entry.

Kinetic rates of antibody binding correlate with neutralization sensitivity of variant simian immunodeficiency virus strains

Increasing evidence suggests that an effective AIDS vaccine will need to elicit both broadly reactive humoral and cellular immune responses. Potent and cross-reactive neutralization of simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) by polyclonal and monoclonal antibodies is well documented. However, the mechanisms of antibody-mediated neutralization have not been defined. The current study was designed to determine whether the specificity and quantitative properties of antibody binding to SIV envelope proteins correlate with neutralization. Using a panel of rhesus monoclonal antibodies previously characterized for their ability to bind and neutralize variant SIVs, we compared the kinetic rates and affinity of antibody binding to soluble envelope trimers by using surface plasmon resonance. We identified significant differences in the kinetic rates but not the affinity of monoclonal antibody binding to the neutralization-sensitive SIV/17E-CL and neutralization-resistant SIVmac239 envelope proteins that correlated with the neutralization sensitivities of the corresponding virus strains. These results suggest for the first time that neutralization resistance may be related to quantitative differences in the rates but not the affinity of the antibody-envelope interaction and may provide one mechanism for the inherent resistance of SIVmac239 to neutralization in vitro. Further, we provide evidence that factors in addition to antibody binding, such as epitope specificity, contribute to the mechanisms of neutralization of SIV/17E-CL in vitro. This study will impact the method by which HIV/SIV vaccines are evaluated and will influence the design of candidate AIDS vaccines capable of eliciting effective neutralizing antibody responses.

Interaction with CD4 and antibodies to CD4-induced epitopes of the envelope gp120 from a microglial cell-adapted human immunodeficiency virus type 1 isolate

We previously showed that the envelope glycoprotein from an in vitro microglia-adapted human immunodeficiency virus type 1 isolate (HIV-1(Bori-15)) is able to use lower levels of CD4 for infection and demonstrates greater exposure of the CD4-induced epitope recognized by the 17b monoclonal antibody than the envelope of its parental, peripheral isolate (HIV-1(Bori)). We investigated whether these phenotypic changes were related to a different interaction of their soluble monomeric gp120 proteins with CD4 or 17b. Equilibrium binding analyses showed no difference between Bori and Bori-15 gp120s. However, kinetic analysis of surface plasmon resonance-based, real-time binding experiments showed that while both proteins have similar association rates, Bori-15 gp120 has a statistically significant, 3-fold-lower dissociation rate from immobilized CD4 than Bori and a statistically significant, 14-fold-lower dissociation rate from 17b than Bori in the absence of soluble CD4. In addition, using the sensitivity to inhibition by anti-CD4 antibodies as a surrogate for CD4:trimeric envelope interaction, we found that Bori-15 envelope-pseudotyped viruses were significantly less sensitive than Bori pseudotypes, with four- to sixfold-higher 50% inhibitory concentration values for the three anti-CD4 antibodies tested. These differences, though small, suggest that adaptation to microglia correlates with the generation of a gp120 that forms a more stable interaction with CD4. Nonetheless, the observation of limited binding changes leaves open the possibility that HIV-1 adaptation to microglia and HIV-associated dementia may be related not only to diminished CD4 dependence but also to changes in other molecular factors involved in the infection process.

Characterization of gp120 and its single-chain derivatives, gp120-CD4D12 and gp120-M9: implications for targeting the CD4i epitope in human immunodeficiency virus vaccine design

Single-chain derivatives of JRFL gp120 linked to the first two domains of human CD4 (gp120-CD4D12) or to the CD4 miniprotein analog CD4M9 (gp120-M9), have been constructed. Biacore studies revealed that gp120-CD4D12 and gp120-M9 bound to antibody 17b with dissociation constants of 0.8 and 25 nM, respectively, at pH 7.0, while gp120 alone did not bind. The binding of gp120-CD4D12 to 17b is not affected by the addition of excess soluble CD4D12, while the binding of gp120-M9 is enhanced. This finding indicates that the M9 component of the single chain interacts relatively weakly with gp120 and can be displaced by soluble CD4D12. Immunogenicity studies of gp120, gp120-CD4D12, and gp120-M9 were carried out with guinea pigs. All three molecules were highly immunogenic. The resulting antisera were examined for neutralizing activities against various human immunodeficiency virus type 1 isolates. Broadly neutralizing activity was observed only with sera generated against gp120-CD4D12. These antisera were depleted of anti-CD4D12 antibodies by being passed over a column containing immobilized CD4D12. The depleted sera showed a loss of broadly neutralizing activity. Sera that were affinity purified over a column containing immobilized gp120-M9 also lacked such neutralizing activity. This finding suggests that the broadly neutralizing response observed is exclusively due to anti-CD4 antibodies. Competition experiments showed that only antisera generated against gp120-CD4D12 competed with the CD4i antibody 17b and that this activity was not affected by depletion of anti-CD4 antibodies. The data indicate that although antibodies targeting the CD4i epitope were generated by the gp120-CD4D12 immunogen, these antibodies were nonneutralizing.

Antigenicity and immunogenicity of a synthetic human immunodeficiency virus type 1 group m consensus envelope glycoprotein

Genetic variation of human immunodeficiency virus (HIV-1) represents a major obstacle for AIDS vaccine development. To decrease the genetic distances between candidate immunogens and field virus strains, we have designed and synthesized an artificial group M consensus env gene (CON6 gene) to be equidistant from contemporary HIV-1 subtypes and recombinants. This novel envelope gene expresses a glycoprotein that binds soluble CD4, utilizes CCR5 but not CXCR4 as a coreceptor, and mediates HIV-1 entry. Key linear, conformational, and glycan-dependent monoclonal antibody epitopes are preserved in CON6, and the glycoprotein is recognized equally well by sera from individuals infected with different HIV-1 subtypes. When used as a DNA vaccine followed by a recombinant vaccinia virus boost in BALB/c mice, CON6 env gp120 and gp140CF elicited gamma interferon-producing T-cell responses that recognized epitopes within overlapping peptide pools from three HIV-1 Env proteins, CON6, MN (subtype B), and Chn19 (subtype C). Sera from guinea pigs immunized with recombinant CON6 Env gp120 and gp140CF glycoproteins weakly neutralized selected HIV-1 primary isolates. Thus, the computer-generated "consensus" env genes are capable of expressing envelope glycoproteins that retain the structural, functional, and immunogenic properties of wild-type HIV-1 envelopes.

Characterization of the outer domain of the gp120 glycoprotein from human immunodeficiency virus type 1

The core of the gp120 glycoprotein from human immunodeficiency virus type 1 (HIV-1) is comprised of three major structural domains: the outer domain, the inner domain, and the bridging sheet. The outer domain is exposed on the HIV-1 envelope glycoprotein trimer and contains binding surfaces for neutralizing antibodies such as 2G12, immunoglobulin G1b12, and anti-V3 antibodies. We expressed the outer domain of HIV-1(YU2) gp120 as an independent protein, termed OD1. OD1 efficiently bound 2G12 and a large number of anti-V3 antibodies, indicating its structural integrity. Immunochemical studies with OD1 indicated that antibody responses against the outer domain of the HIV-1 gp120 envelope glycoprotein are rare in HIV-1-infected human sera that potently neutralize the virus. Surprisingly, such outer-domain-directed antibody responses are commonly elicited by immunization with recombinant monomeric gp120. Immunization with soluble, stabilized HIV-1 envelope glycoprotein trimers elicited antibody responses that more closely resembled those in the sera of HIV-1-infected individuals. These results underscore the qualitatively different humoral immune responses elicited during natural infection and after gp120 vaccination and help to explain the failure of gp120 as an effective vaccine.

An inducible HIV type 1 gp41 HR-2 peptide-binding site on HIV type 1 envelope gp120

Synthetic peptides of sequences within the HIV-1 gp41 heptad repeat-regions (HR-1 and HR-2) can effectively inhibit cell fusion and viral entry. DP178 (T-20), an HR-2 peptide, acts by inhibiting the association between HR-1 and HR-2, thereby interfering with HIV-1 fusion and viral entry. HR-2 peptide binding is predicted to be an important indicator of the presence of Env gp41 fusion intermediate conformation. A stabilized HR-2/Env conjugate might be an HIV-1 vaccine candidate and have the potential for inducing antibodies against transiently exposed epitopes on HIV-1 Env. To explore the possibility of design of HR-2 stabilized-HIV-1 immunogens, we studied the ability of HIV-1 Env to bind to HR-2 peptides. Using surface plasmon resonance (SPR)-binding assays and precipitation of soluble Env gp120 proteins with HR-2 peptide DP178, we have found that there is an HR-2 peptide-binding site on soluble HIV-1 recombinant gp120. Binding of DP178 was induced by sCD4 and by the anti-gp120 human mAb A32. The induction of DP178 binding was inhibited > 80% by the HIV-1 coreceptor-binding site mAb 17b. Binding of DP178 to gp120 was also inhibited by gp120 C4 peptides with sequences that are centrally located within the HIV-1 coreceptor-binding site. Thus, in addition to interactions with the gp41 HR-1 region, the fusion inhibitor peptide DP178 binds to triggered soluble HIV-1 recombinant gp120 following its interaction with sCD4 or CD4 mimic mAb A32. This may prove to be an important consideration when designing an HIV vaccine that utilizes constrained HIV Env proteins.

Biochemical and genetic characterizations of a novel human immunodeficiency virus type 1 inhibitor that blocks gp120-CD4 interactions

BMS-378806 is a recently discovered small-molecule human immunodeficiency virus type 1 (HIV-1) attachment inhibitor with good antiviral activity and pharmacokinetic properties. Here, we demonstrate that the compound targets viral entry by inhibiting the binding of the HIV-1 envelope gp120 protein to cellular CD4 receptors via a specific and competitive mechanism. BMS-378806 binds directly to gp120 at a stoichiometry of approximately 1:1, with a binding affinity similar to that of soluble CD4. The potential BMS-378806 target site was localized to a specific region within the CD4 binding pocket of gp120 by using HIV-1 gp120 variants carrying either compound-selected resistant substitutions or gp120-CD4 contact site mutations. Mapping of resistance substitutions to the HIV-1 envelope, and the lack of compound activity against a CD4-independent viral infection confirm the gp120-CD4 interactions as the target in infected cells. BMS-378806 therefore serves as a prototype for this new class of antiretroviral agents and validates gp120 as a viable target for small-molecule inhibitors.

A novel human antibody against human immunodeficiency virus type 1 gp120 is V1, V2, and V3 loop dependent and helps delimit the epitope of the broadly neutralizing antibody immunoglobulin G1 b12

The V1/V2 and V3 loops are proximal to the CD4 binding site (CD4bs) of human immunodeficiency virus type 1 (HIV-1) gp120 and undergo conformational change upon CD4 receptor engagement by the HIV-1 envelope spike. Nearly all of the reported monoclonal antibodies (MAbs) against the CD4bs exhibit a very limited capacity to neutralize HIV-1. However, one such human MAb, immunoglobulin G1 (IgG1) b12, is uniquely able to neutralize primary isolates across subtypes with considerable potency. The molecular basis for the anti-HIV-1 activity of b12 is not fully understood but is relevant to vaccine design. Here we describe a novel human MAb, 4KG5, whose binding to monomeric gp120 is moderately enhanced by IgG1 b12. In sharp contrast, 4KG5 binding to gp120 is inhibited by soluble CD4 (sCD4) and by all other (n = 14) anti-CD4bs MAbs tested. 4KG5 is unable to recognize gp120 in which either V1, V2, or V3 has been deleted, and MAbs against the V2 or V3 loops inhibit the binding of 4KG5 to gp120. Moreover, 4KG5 is able to inhibit the binding of the CD4-induced MAbs 17b and X5 in the absence of sCD4, whereas 17b and X5 only weakly inhibit the binding of 4KG5 to gp120. Mutagenesis of gp120 provides further evidence of a discontinuous epitope of 4KG5 that is formed by the V1/V2 loop, the V3 loop, and a portion of the bridging sheet (C4). 4KG5 was isolated as a single-chain Fv from a phage display library constructed from the bone marrow of an HIV-1-seropositive subject (FDA2) whose serum neutralizes HIV-1 across subtypes. Despite its source, we observed no significant neutralization with 4KG5 against the autologous (R2) virus and several other strains of HIV-1. The results suggest a model in which antibody access to the CD4bs on the envelope spike of HIV-1 is restricted by the orientation and/or dynamics of the V1/V2 and V3 loops, and b12 avoids these restrictions.

Spying on HIV with SPR

The application of surface plasmon resonance (SPR)-based optical biosensors has contributed extensively to our understanding of functional aspects of HIV. SPR biosensors allow the analysis of real-time interactions of any biomolecule, be it protein, nucleic acid, lipid, carbohydrate or small molecule, without the need for intrinsic or extrinsic probes. As such, the technology has been used to analyze molecular interactions associated with every aspect of the viral life cycle, from basic studies of binding events occurring during docking, replication, budding and maturation to applied research related to vaccine and inhibitory drug development. Along the way, SPR biosensors have provided a unique and detailed view into the inner workings of HIV.

Concordant modulation of neutralization resistance and high infectivity of the primary human immunodeficiency virus type 1 MN strain and definition of a potential gp41 binding site in gp120

Efforts to develop a vaccine against human immunodeficiency virus type 1 (HIV-1) are complicated by resistance of virus to neutralization. The neutralization resistance phenotype of HIV-1 has been linked to high infectivity. We studied the mechanisms determining this phenotype using clones of the T-cell-line-adapted (TCLA) MN strain (MN-TCLA) and the neutralization-resistant, primary MN strain (MN-P). Mutations in the amino- and carboxy-terminal halves of gp120 and the carboxy terminus of gp41 contributed to the neutralization resistance, high-infectivity phenotype but depended upon sequences in the leucine zipper (LZ) domain of gp41. Among 23 clones constructed to map the contributing mutations, there was a very strong correlation between infectivity and neutralization resistance (R(2) = 0.81; P < 0.0001). Mutations that distinguished the gp120s of MN-P and MN-TCLA clones were clustered in or near the CD4 and coreceptor binding sites and in regions distant from those binding sites. To test the hypothesis that some of these distant mutations may interact with gp41, we determined which of them contributed to high infectivity and whether those mutations modulated gp120-gp41 association in the context of MN-P LZ sequences. In one clone, six mutations in the amino terminus of gp120, at least four of which clustered closely on the inner domain, modulated infectivity. This clone had a gp120-gp41 association phenotype like MN-P: in comparison to MN-TCLA, spontaneous dissociation was low, and dissociation induced by soluble CD4 binding was high. These results identify a region of the gp120 inner domain that may be a binding site for gp41. Our studies clarify mechanisms of primary virus neutralization resistance.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

Simian immunodeficiency virus encephalitis: analysis of envelope sequences from individual brain multinucleated giant cells and tissue samples

Simian immunodeficiency virus (SIV)-infected macaques develop an encephalitis (SIVE) that is pathologically virtually indistinguishable from that associated with HIV infection, with multinucleated giant cells (MNGCs) being the principal histopathological manifestation. To dissect SIV variants responsible for MNGC development, we examined the relationships between env sequences transcribed in individual MNGCs and those from genomic DNA of brain and spleen tissues. The brain-specific variant found in all brain clones was dominant among the clones from MNGCs, suggesting a role in the formation of giant cells. Furthermore, two additional minor groups of sequences were present in MNGCs. One group consisted of sequences closely related to those from spleen, indicating recent and probably multiple episodes of neuroinvasion. The second group represented clones similar or identical to the initial inoculum. The survival of archival sequences and their activation presumably by the fusion of productively and quiescently infected macrophages/microglia identify the central nervous system as a possible anatomical reservoir for latent infection.

Expression, purification, and characterization of recombinant HIV gp140. The gp41 ectodomain of HIV or simian immunodeficiency virus is sufficient to maintain the retroviral envelope glycoprotein as a trimer

Efforts to understand the molecular basis of human immunodeficiency virus (HIV) envelope glycoprotein function have been hampered by the inability to generate sufficient quantities of homogeneous material. We now report on the high level expression, purification, and characterization of soluble HIV gp140 ectodomain proteins in Chinese hamster ovary-Lec3.2.8.1 cells. Gel filtration and analytical ultracentrifugation show that the uncleaved ADA strain-derived gp140 proteins are trimeric without further modification required to maintain oligomers. These spike proteins are native as judged by soluble CD4 (sCD4) (K(D) = 1-2 nm) and monoclonal antibody binding studies using surface plasmon resonance. CD4 ligation induces conformational change in the trimer, exposing the chemokine receptor binding site as assessed by 17b monoclonal antibody reactivity. Lack of anti-cooperativity in sCD4-ADA trimer interaction distinct from that observed with sCD4-SIV mac32H implies quaternary structural differences in ground states of their respective spike proteins.