Antigenicity-defined conformations of an extremely neutralization-resistant HIV-1 envelope spike

Broadly neutralizing monoclonal antibodies for HIV prevention

Introduction

The last 12 years have seen remarkable progress in the isolation and characterization of at least five different epitope classes of HIV‐specific broadly neutralizing antibodies (bnAbs). Detailed analyses of these bnAb lineages, maturation pathways and epitopes have created new opportunities for vaccine development. In addition, interest exists in passive administration of monoclonal antibodies as a viable option for HIV prevention.

Discussion

Recently, two antibody‐mediated prevention (AMP) trials of a passively administered monoclonal antibody targeting the HIV envelope CD4 binding site, called VRC01, provided proof‐of‐concept that monoclonal antibody infusion could offer protection against HIV acquisition. While the trials failed to show overall protection against HIV acquisition, sub‐analyses revealed that VRC01 infusion provided a 75% prevention efficacy against HIV strains that were susceptible to the antibody. The study also demonstrated that in vitro neutralizing activity, measured by the TZM‐bl/pseudovirus assay, was able to predict HIV prevention efficacy in humans. In addition, the AMP trials defined a threshold protective concentration, or neutralization titer, for the VRC01 class of bnAbs, explaining the observed low overall efficacy and serving as a benchmark for the clinical testing of new bnAbs, bnAb cocktails and neutralizing antibody‐inducing vaccines. Newer bnAbs that exhibit greater potency and breadth of neutralization in vitro than VRC01 are available for clinical testing. Combinations of best‐in‐class bnAbs with complementary magnitude, breadth and extent of complete neutralization are predicted to far exceed the prevention efficacy of VRC01. Some engineered bi‐ and trispecific mAbs exhibit similar desirable neutralizing activity and afford advantages for manufacturing and delivery. Modifications that prolong the serum half‐life and improve genital tissue persistence offer additional advantages.

Conclusions

Iterative phase 1 trials are acquiring safety and pharmacokinetic data on dual and triple bnAbs and bi‐ and trispecific antibodies in preparation for future AMP studies that seek to translate findings from the VRC01 efficacy trials and achieve acceptable levels of overall prevention efficacy.

HIV-1 Subtype C Tier 3 Viruses Have Increased Infectivity Compared to Tier 2 Viruses

A primary concern of an antibody-based HIV-1 therapy is the virus' ability to rapidly escape antibody responses. Therefore, we investigated the relationships between antibody neutralization sensitivity, viral phenotype, and infectivity in 13 subtype C viruses using a HeLa transfectant-based assay. We observed that the seven tier 3 viruses exhibited higher infectivity than the tier 2 viruses, suggesting that higher neutralization resistance did not have a substantial entry cost. There was no relationship between neutralization resistance and susceptibility to entry inhibitors Maraviroc, PSC RANTES, or the fusion inhibitor T20, indicating that neutralization resistance may not alter these inhibitor target sites. By analyzing glycosylation patterns in 82 subtype C viruses, we found that the presence of an N-linked glycan motif at position N413 and its absence at N332 were the most important predictors of neutralization resistance. In a set of 200 subtype C viruses, tier 3 strains were more resistant than tier 2 or 1B viruses to several broadly neutralizing monoclonal antibodies targeting three different epitopes. This suggests that it is unlikely that resistance to antibodies targeting a single epitope drives overall resistance. In the context of an antibody-based intervention, highly resistant viruses with increased infectivity, circulating in the population, could hinder HIV-1 control since entry of tier 3 viruses is not always selected against. Therefore, for any long-term antibody-based intervention to be globally relevant, it must elicit responses that limit the occurrence of resistance.

A Rare Mutation in an Infant-Derived HIV-1 Envelope Glycoprotein Alters Interprotomer Stability and Susceptibility to Broadly Neutralizing Antibodies Targeting the Trimer Apex

The envelope glycoprotein (Env) of human immunodeficiency virus type 1 (HIV-1) is the sole target of broadly neutralizing antibodies (bnAbs). Several mechanisms, such as the acquisition of mutations, variability of the loop length, and alterations in the glycan pattern, are employed by the virus to shield neutralizing epitopes on Env to sustain survival and infectivity within the host. The identification of mutations that lead to viral evasion of the host immune response is essential for the optimization and engineering of Env-based trimeric immunogens. Here, we report a rare leucine-to-phenylalanine escape mutation (L184F) at the base of hypervariable loop 2 (population frequency of 0.0045%) in a 9-month-old perinatally HIV-1-infected infant broad neutralizer. The L184F mutation altered the trimer conformation by modulating intramolecular interactions stabilizing the trimer apex and led to viral escape from autologous plasma bnAbs and known N160 glycan-targeted bnAbs. The L184F amino acid change led to the acquisition of a relatively open trimeric conformation, often associated with tier 1 HIV-1 isolates and increased susceptibility to neutralization by polyclonal plasma antibodies of weak neutralizers. While there was no impact of the L184F mutation on free virus transmission, a reduction in cell-to-cell transmission was observed. In conclusion, we report a naturally selected viral mutation, L184F, that influenced a change in the conformation of the Env trimer apex as a mechanism of escape from contemporaneous plasma V2 apex-targeted nAbs. Further studies should be undertaken to define viral mutations acquired during natural infection, to escape selection pressure exerted by bnAbs, to inform vaccine design and bnAb-based therapeutic strategies.IMPORTANCE The design of HIV-1 envelope-based immunogens capable of eliciting broadly neutralizing antibodies (bnAbs) is currently under active research. Some of the most potent bnAbs target the quaternary epitope at the V2 apex of the HIV-1 Env trimer. By studying naturally circulating viruses from a perinatally HIV-1-infected infant with plasma neutralizing antibodies targeted to the V2 apex, we identified a rare leucine-to-phenylalanine substitution, in two out of six functional viral clones, that destabilized the trimer apex. This single-amino-acid alteration impaired the interprotomeric interactions that stabilize the trimer apex, resulting in an open trimer conformation and escape from broadly neutralizing autologous plasma antibodies and known V2 apex-directed bnAbs, thereby favoring viral evasion of the early bnAb response of the infected host. Defining the mechanisms by which naturally occurring viral mutations influence the sensitivity of HIV-1 to bnAbs will provide information for the development of vaccines and bnAbs as anti-HIV-1 reagents.

Sensitivity to monoclonal antibody 447-52D and an open env trimer conformation correlate poorly with inhibition of HIV-1 infectivity by SERINC5

The host protein SERINC5 inhibits the infectivity of HIV-1 virions in an Env-dependent manner and is counteracted by Nef. The conformation of the Env trimer reportedly correlates with sensitivity to SERINC5. Here, we tested the hypothesis that the "open" conformation of the Env trimer revealed by sensitivity to the V3-loop specific antibody 447-52D directly correlates with sensitivity to SERINC5. Of five Envs tested, SF162 was the most sensitive to neutralization by 447-52D, but it was not the most sensitive to SERINC5; instead the Env of LAI was substantially more sensitive to SERINC5 than all the other Envs. Mutational opening of the trimer by substitution of two tyrosines that mediate interaction between the V2 and V3 loops sensitized the Envs of JRFL and LAI to 447-52D as previously reported, but only BaL was sensitized to SERINC5. These data suggest that trimer "openness" is not sufficient for sensitivity to SERINC5.

Cryo-EM Structure of Full-length HIV-1 Env Bound With the Fab of Antibody PG16

The HIV-1 envelope protein (Env) is the target of neutralizing antibodies and the template for vaccine immunogen design. The dynamic conformational equilibrium of trimeric Env influences its antigenicity and potential immunogenicity. Antibodies that bind at the trimer apex stabilize a "closed" conformation characteristic of the most difficult to neutralize isolates. A goal of vaccine development is therefore to mimic the closed conformation in a designed immunogen. A disulfide-stabilized, trimeric Env ectodomain-the "SOSIP" construct-has many of the relevant properties; it is also particularly suitable for structure determination. Some single-molecule studies have, however, suggested that the SOSIP trimer is not a good representation of Env on the surface of a virion or an infected cell. We isolated Env (fully cleaved to gp120 and gp41) from the surface of expressing cells using tagged, apex-binding Fab PG16 and determined the structure of the PG16-Env complex by cryo-EM to an overall resolution of 4.6 Å. Placing the only purification tag on the Fab ensured that the isolated Env was continuously stabilized in its closed, native conformation. The Env structure in this complex corresponds closely to the SOSIP structures determined by both x-ray crystallography and cryo-EM. Although the membrane-interacting elements are not resolved in our reconstruction, we can make inferences about the connection between ectodomain and membrane-proximal external region (MPER) by reference to the published cryo-tomography structure of an Env "spike" and the NMR structure of the MPER-transmembrane segment. We discuss these results in view of the conflicting interpretations in the literature.

HIV-1 Env gp41 Transmembrane Domain Dynamics Are Modulated by Lipid, Water, and Ion Interactions

The gp41 transmembrane domain (TMD) of the envelope glycoprotein of the human immunodeficiency virus modulates the conformation of the viral envelope spike, the only druggable target on the surface of the virion. Targeting the envelope glycoprotein with small-molecule and antibody therapies requires an understanding of gp41 TMD dynamics, which is often challenging given the difficulties in describing native membrane properties. Here, atomistic molecular dynamics simulations of a trimeric, prefusion gp41 TMD in a model, asymmetric viral membrane that mimics the native viral envelope were performed. Water and chloride ions were observed to permeate the membrane and interact with the highly conserved arginine bundle, (R696)₃, at the center of the membrane and influenced TMD stability by creating a network of hydrogen bonds and electrostatic interactions. We propose that this (R696)₃ - water - anion network plays an important role in viral fusion with the host cell by modulating protein conformational changes within the membrane. Additionally, R683 and R707 at the exofacial and cytofacial membrane-water interfaces, respectively, are anchored in the lipid headgroup region and serve as a junction point for stabilization of the termini. The membrane thins as a result of the tilting of the gp41 trimer with nearby lipids increasing in volume, leading to an entropic driving force for TMD conformational change. These results provide additional detail and perspective on the influence of certain lipid types on TMD dynamics and a rationale for targeting key residues of the TMD for therapeutic design. These insights into the molecular details of TMD membrane anchoring will build toward a greater understanding of the dynamics that lead to viral fusion with the host cell.

Molecular Mechanism of HIV-1 Entry

HIV-1 envelope glycoprotein [Env; trimeric (gp160)₃ cleaved to (gp120/gp41)₃] attaches the virion to a susceptible cell and induces fusion of viral and cell membranes to initiate infection. It interacts with the primary receptor CD4 and coreceptor (e.g., chemokine receptor CCR5 or CXCR4) to allow viral entry by triggering large structural rearrangements and unleashing the fusogenic potential of gp41 to induce membrane fusion. Recent advances in structural biology of HIV-1 Env and its complexes with the cellular receptors have revealed molecular details of HIV-1 entry and yielded new mechanistic insights. In this review, I summarize our latest understanding of the HIV-1 membrane fusion process and discuss possible pathways for productive viral entry.

Neutralization tiers of HIV-1

Purpose of review

HIV-1 isolates are often classified on the basis of neutralization ‘tier’ phenotype. Tier classification has important implications for the monitoring and interpretation of vaccine-elicited neutralizing antibody responses. The molecular basis that distinguishes the multiple neutralization phenotypes of HIV-1 has been unclear. We present a model based on the dynamic nature of the HIV-1 envelope glycoproteins and its impact on epitope exposure. We also describe a new approach for ranking HIV-1 vaccine-elicited neutralizing antibody responses.

Recent findings

The unliganded trimeric HIV-1 envelope glycoprotein spike spontaneously transitions through at least three conformations. Neutralization tier phenotypes correspond to the frequency by which the trimer exists in a closed (tiers 2 and 3), open (tier 1A), or intermediate (tier 1B) conformation. An increasing number of epitopes become exposed as the trimer opens, making the virus more sensitive to neutralization by certain antibodies. The closed conformation is stabilized by many broadly neutralizing antibodies.

Summary

The tier 2 neutralization phenotype is typical of most circulating strains and is associated with a predominantly closed Env trimer configuration that is a high priority to target with vaccines. Assays with tier 1A viruses should be interpreted with caution and with the understanding that they detect many antibody specificities that do not neutralize tier 2 viruses and do not protect against HIV-1 infection.

Structural basis of coreceptor recognition by HIV-1 envelope spike

HIV-1 envelope glycoprotein (Env), which consists of trimeric (gp160)₃ cleaved to (gp120 and gp41)₃, interacts with the primary receptor CD4 and a coreceptor (such as chemokine receptor CCR5) to fuse viral and target-cell membranes. The gp120-coreceptor interaction has previously been proposed as the most crucial trigger for unleashing the fusogenic potential of gp41. Here we report a cryo-electron microscopy structure of a full-length gp120 in complex with soluble CD4 and unmodified human CCR5, at 3.9 Å resolution. The V3 loop of gp120 inserts into the chemokine-binding pocket formed by seven transmembrane helices of CCR5, and the N terminus of CCR5 contacts the CD4-induced bridging sheet of gp120. CCR5 induces no obvious allosteric changes in gp120 that can propagate to gp41; it does bring the Env trimer close to the target membrane. The N terminus of gp120, which is gripped by gp41 in the pre-fusion or CD4-bound Env, flips back in the CCR5-bound conformation and may irreversibly destabilize gp41 to initiate fusion. The coreceptor probably functions by stabilizing and anchoring the CD4-induced conformation of Env near the cell membrane. These results advance our understanding of HIV-1 entry into host cells and may guide the development of vaccines and therapeutic agents.

Molecular Basis of Unusually High Neutralization Resistance in Tier 3 HIV-1 Strain 253-11

Understanding the mechanisms used by HIV-1 to evade antibody neutralization may contribute to the design of a high-coverage vaccine. The tier 3 virus 253-11 is poorly neutralized by subtype-matched and subtype C sera, even compared to other tier 3 viruses, and is also recognized poorly by V3/glycan-targeting monoclonal antibodies (MAbs). We found that sequence polymorphisms in the V3 loop and N-linked glycosylation sites contribute only minimally to the high neutralization resistance of 253-11. Interestingly, the 253-11 membrane-proximal external region (MPER) is rarely recognized by sera in the context of the wild-type virus but is commonly recognized in the context of an HIV-2 chimera, suggesting steric or kinetic hindrance of binding to MPER in the native envelope (Env). Mutations in the 253-11 MPER, which were previously reported to increase the lifetime of the prefusion Env conformation, affected the resistance of 253-11 to antibodies targeting various epitopes on HIV-1 Env, presumably destabilizing its otherwise stable, closed trimer structure. To gain insight into the structure of 253-11, we constructed and crystallized a recombinant 253-11 SOSIP trimer. The resulting structure revealed that the heptad repeat helices in gp41 are drawn in close proximity to the trimer axis and that gp120 protomers also showed a relatively compact disposition around the trimer axis. These observations give substantial insight into the molecular features of an envelope spike from a tier 3 virus and into possible mechanisms that may contribute to its unusually high neutralization resistance.IMPORTANCE HIV-1 isolates that are highly resistant to broadly neutralizing antibodies could limit the efficacy of an antibody-based vaccine. We studied 253-11, which is highly resistant to commonly elicited neutralizing antibodies. To further understand its resistance, we made mutations that are known to delay fusion and thus increase the time that the virus spends in the open conformation following CD4 binding. Interestingly, we found that these mutations affect the 253-11 envelope (Env) spike before CD4 binding, presumably by destabilizing the trimer structure. To gain further information about the structure of the 253-11 Env trimer, we generated a recombinant 253-11 SOSIP trimer. The crystal structure of the SOSIP trimer revealed that the gp41 helices and the gp120 protomers were drawn in toward the center of the molecule compared to most solved HIV-1 Env structures. These observations provide insight into the distinct molecular features of a tier 3 envelope spike.

Envelope proteins of two HIV-1 clades induced different epitope-specific antibody response

Using HIV-1 envelope protein (Env)-based immunogens that closely mimic the conformation of functional HIV-1 Envs and represent the isolates prevalent in relevant geographical region is considered a rational approach towards developing HIV vaccine. We recently reported that like clade B Env, JRFL, membrane bound Indian clade C Env, 4-2.J41 is also efficiently cleaved and displays desirable antigenic properties for plasmid DNA immunization. Here, we evaluated the immune response in rabbit by injecting the animals with plasmid expressing membrane bound efficiently cleaved 4-2.J41 Env followed by its gp140-foldon (gp140-fd) protein boost. The purified 4-2.J41-gp140-fd protein is recognized by a wide panel of broadly neutralizing antibodies (bNAbs) including the quaternary conformation-dependent antibody, PGT145 with high affinity. We have also evaluated and compared the quality of antibody response elicited in rabbits after immunizing with plasmid DNA expressing the membrane bound efficiently cleaved Env followed by gp140-fd proteins boost with either of clade C Env, 4-2.J41 or clade B Env, JRFL or in combination. In comparison to JRFL group, 4-2.J41 group elicited autologous as well as limited low level cross clade neutralizing antibody response. Preliminary epitope-mapping of sera from animals show that in contrast to JRFL group, no reactivity to either linear peptides or V3-loop is detected in 4-2.J41 group. Furthermore, the presence of conformation-specific antibody in sera from animals immunized with 4-2.J41 Env is observed. However, unlike JRFL group, in 4-2.J41 group of animals, CD4-binding site-directed antibodies cannot be detected. Additionally, we have demonstrated that the quality of antibody response in combination group is guided by JRFL Env-based immunogen suggesting that the selection and the quality of Envs in multicade candidate vaccine are important factors to elicit desirable response.

Prediction of HIV-1 sensitivity to broadly neutralizing antibodies shows a trend towards resistance over time

Treatment with broadly neutralizing antibodies (bNAbs) has proven effective against HIV-1 infections in humanized mice, non-human primates, and humans. Due to the high mutation rate of HIV-1, resistance testing of the patient’s viral strains to the bNAbs is still inevitable. So far, bNAb resistance can only be tested in expensive and time-consuming neutralization experiments. Here, we introduce well-performing computational models that predict the neutralization response of HIV-1 to bNAbs given only the envelope sequence of the virus. Using non-linear support vector machines based on a string kernel, the models learnt even the important binding sites of bNAbs with more complex epitopes, i.e., the CD4 binding site targeting bNAbs, proving thereby the biological relevance of the models. To increase the interpretability of the models, we additionally provide a new kind of motif logo for each query sequence, visualizing those residues of the test sequence that influenced the prediction outcome the most. Moreover, we predicted the neutralization sensitivity of around 34,000 HIV-1 samples from different time points to a broad range of bNAbs, enabling the first analysis of HIV resistance to bNAbs on a global scale. The analysis showed for many of the bNAbs a trend towards antibody resistance over time, which had previously only been discovered for a small non-representative subset of the global HIV-1 population.

Several sequence-based approaches exist to predict the epitope of broadly neutralizing antibodies (bNAbs) against HIV based on the correlation between variation in the viral sequence and neutralization response to the antibody. Though the potential epitope sites can be used to predict the neutralization response, the methods are not optimized for the task, using additional structural information, additional preselection steps to identify the epitope sites, and assuming independence and/or only linear relationship between the potential sites and the neutralization response. To model also the neutralization response to bNAbs with more complex binding sites, including for example several non-consecutive residues or accompanying conformational changes, we used non-linear, multivariate machine learning techniques. Though we used only the viral sequence information, the models learnt the corresponding binding sites of the bNAbs. In general only few residues were learnt to be responsible for a change in neutralization response, which can additionally reduce the sequencing cost for application in clinical routine. We propose our tailored models to aid the patient selection process for current clinical trials for bNAb immunotherapy, but also as a basis to predict the best combinations of bNAbs, which will be required for routine clinical practice in the future.