Association of enhanced HIV-1 neutralization by a single Y681H substitution in gp41 with increased gp120-CD4 interaction and macrophage infectivity

The membrane-proximal external region of human immunodeficiency virus (HIV-1) envelope glycoprotein trimers in A18-lipid nanodiscs

During human immunodeficiency virus (HIV-1) entry, the metastable pretriggered envelope glycoprotein (Env) trimer ((gp120/gp41)3) opens asymmetrically. We present cryo-EM structures of cleaved asymmetric Env trimers in amphipol-lipid nanodiscs. The gp41 membrane-proximal external region (MPER) could be traced in Env protomers that remained close to the nanodisc despite Env tilting. The MPER interacts with the gp120 C-termini and gp41 α9 helices at the base of the Env trimer. MPER conformation is coupled with the tilt angles of the α9 helices, the helicity of the gp41 heptad repeat (HR1N) regions, and the opening angles between the protomers of the asymmetric trimers. Our structural models explain the stabilizing effects of MPER integrity and Env proteolytic maturation on the pretriggered Env conformation. Superimposed on the asymmetry of the Env protomers, variation in the glycans at the trimer apex creates substantial structural heterogeneity in the V2 quaternary epitopes of difficult-to-elicit broadly neutralizing antibodies.

Assessing bnAb potency in the context of HIV-1 envelope conformational plasticity

For use in prevention and treatment, HIV-1 broadly neutralizing antibodies (bnAbs) have to overcome Env conformational heterogeneity of viral quasispecies and neutralize with constant high potency. Comparative analysis of neutralization data from the CATNAP database revealed a nuanced relationship between bnAb activity and Env conformational flexibility, with substantial epitope-specific variation of bnAb potency ranging from increased to decreased activity against open, neutralization-sensitive Env. To systematically investigate the impact of variability in Env conformation on bnAb potency we screened 126 JR-CSF point mutants for generalized neutralization sensitivity to weakly neutralizing antibodies (weak-nAbs) depending on trimer opening and plasma from people with chronic HIV-1 infection. 23 mutations resulted in a highly neutralization sensitive phenotype, which was associated with de-stabilization of the closed, prefusion conformation. Including 19 of these mutants into a Sensitivity Env mutant panel (SENSE-19), we classified bnAbs according to potency variations in response to trimer opening. To verify that these sensitivity patterns are independent of the in vitro assay system, replication-competent SENSE-19 mutant viruses were tested on primary CD4 T cells. While loss of potency on SENSE-19 was registered for bnAbs from several classes recognizing quaternary epitopes on pre-triggered Env, structural destabilization benefitted MPER bnAbs and other inhibitors known to have post-CD4 attachment neutralization activity. Importantly, for a subset of CD4bs bnAbs, and the interface bnAb PGT151, particularly low potency variation was noted, suggesting that Env conformational tolerance can be achieved but is not the rule. In summary, SENSE-19 screens revealed distinct tolerance levels to Env conformational intermediates between bnAbs that provide mechanistic insights in their function and broaden current neutralization breadth assessments.

Inducible cell lines producing replication-defective human immunodeficiency virus particles containing envelope glycoproteins stabilized in a pretriggered conformation

During the process by which human immunodeficiency virus (HIV-1) enters cells, the envelope glycoprotein (Env) trimer on the virion surface engages host cell receptors. Binding to the receptor CD4 induces Env to undergo transitions from a pretriggered, "closed" (State-1) conformation to more "open" (State 2/3) conformations. Most broadly neutralizing antibodies (bNAbs), which are difficult to elicit, recognize the pretriggered (State-1) conformation. More open Env conformations are recognized by poorly neutralizing antibodies (pNAbs), which are readily elicited during natural infection and vaccination with current Env immunogens. Env heterogeneity likely contributes to HIV-1 persistence by skewing antibody responses away from the pretriggered conformation. The conformationally flexible gp160 Env precursor on the infected cell or virion surface potentially presents multiple pNAb epitopes to the host immune system. Although proteolytic cleavage to produce the functional, mature Env trimer [(gp120/gp41)3] stabilizes State-1, many primary HIV-1 Envs spontaneously sample more open conformations. Here, we establish inducible cell lines that produce replication-defective HIV-1 particles with Env trimers stabilized in a pretriggered conformation. The mature Env is enriched on virus-like particles (VLPs). Using complementary approaches, we estimate an average of 25-50 Env trimers on each VLP. The stabilizing changes in Env limit the natural conformational heterogeneity of the VLP Env trimers, allowing recognition by bNAbs but not pNAbs. These defective VLPs provide a more homogeneous source of pretriggered Env trimers in a native membrane environment. Thus, these VLPs may facilitate the characterization of this functionally important Env conformation and its interaction with the immune system.IMPORTANCEA major impediment to the development of an effective HIV/AIDS vaccine is the inefficiency with which human immunodeficiency virus (HIV-1) envelope glycoproteins elicit antibodies that neutralize multiple virus strains. Neutralizing antibodies recognize a particular shape of the envelope glycoproteins that resides on the viral membrane before the virus engages the host cell. Here, we report the creation of stable cell lines that inducibly produce non-infectious HIV-like particles. The normally flexible envelope glycoprotein spikes on these virus-like particles have been stabilized in a conformation that is recognized by broadly neutralizing antibodies. These virus-like particles allow the study of the envelope glycoprotein conformation, its modification by sugars, and its ability to elicit desired neutralizing antibodies.

Conformations of membrane human immunodeficiency virus (HIV-1) envelope glycoproteins solubilized in Amphipol A18 lipid-nanodiscs

Upon binding to the host cell receptor, CD4, the pretriggered (State-1) conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer undergoes transitions to downstream conformations important for virus entry. State 1 is targeted by most broadly neutralizing antibodies (bNAbs), whereas downstream conformations elicit immunodominant, poorly neutralizing antibody (pNAb) responses. Extraction of Env from the membranes of viruses or Env-expressing cells disrupts the metastable State-1 Env conformation, even when detergent-free approaches like styrene-maleic acid lipid nanoparticles (SMALPs) are used. Here, we combine three strategies to solubilize and purify mature membrane Envs that are antigenically native (i.e., recognized by bNAbs and not pNAbs): (1) solubilization of Env with a novel amphipathic copolymer, Amphipol A18; (2) use of stabilized pretriggered Env mutants; and (3) addition of the State-1-stabilizing entry inhibitor, BMS-806. Amphipol A18 was superior to the other amphipathic copolymers tested (SMA and AASTY 11-50) for preserving a native Env conformation. A native antigenic profile of A18 Env-lipid-nanodiscs was maintained for at least 7 days at 4°C and 2 days at 37°C in the presence of BMS-806 and was also maintained for at least 1 h at 37°C in a variety of adjuvants. The damaging effects of a single cycle of freeze-thawing on the antigenic profile of the A18 Env-lipid-nanodiscs could be prevented by the addition of 10% sucrose or 10% glycerol. These results underscore the importance of the membrane environment to the maintenance of a pretriggered (State-1) Env conformation and provide strategies for the preparation of lipid-nanodiscs containing native membrane Envs.IMPORTANCEThe human immunodeficiency virus (HIV-1) envelope glycoproteins (Envs) mediate virus entry into the host cell and are targeted by neutralizing antibodies elicited by natural infection or vaccines. Detailed studies of membrane proteins like Env rely on purification procedures that maintain their natural conformation. In this study, we show that an amphipathic copolymer A18 can directly extract HIV-1 Env from a membrane without the use of detergents. A18 promotes the formation of nanodiscs that contain Env and membrane lipids. Env in A18-lipid nanodiscs largely preserves features recognized by broadly neutralizing antibodies (bNAbs) and conceals features potentially recognized by poorly neutralizing antibodies (pNAbs). Our results underscore the importance of the membrane environment to the native conformation of HIV-1 Env. Purification methods that bypass the need for detergents could be useful for future studies of HIV-1 Env structure, interaction with receptors and antibodies, and immunogenicity.

Membrane HIV-1 envelope glycoproteins stabilized more strongly in a pretriggered conformation than natural virus Envs

The pretriggered conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer ((gp120/gp41)3) is targeted by virus entry inhibitors and broadly neutralizing antibodies (bNAbs). The lability of pretriggered Env has hindered its characterization. Here, we produce membrane Env variants progressively stabilized in pretriggered conformations, in some cases to a degree beyond that found in natural HIV-1 strains. Pretriggered Env stability correlated with stronger trimer subunit association, increased virus sensitivity to bNAb neutralization, and decreased capacity to mediate cell-cell fusion and virus entry. For some highly stabilized Env mutants, after virus-host cell engagement, the normally inaccessible gp120 V3 region on an Env intermediate became targetable by otherwise poorly neutralizing antibodies. Thus, evolutionary pressure on HIV-1 Env to maintain trimer integrity, responsiveness to the CD4 receptor, and resistance to antibodies modulates pretriggered Env stability. The strongly stabilized pretriggered membrane Envs reported here will facilitate further characterization of this functionally important conformation.

Conformations of Human Immunodeficiency Virus Envelope Glycoproteins in Detergents and Styrene-Maleic Acid Lipid Particles

The mature human immunodeficiency virus (HIV) envelope glycoprotein (Env) trimer, which consists of noncovalently associated gp120 exterior and gp41 transmembrane subunits, mediates virus entry into cells. The pretriggered (State-1) Env conformation is the major target for broadly neutralizing antibodies (bNAbs), whereas receptor-induced downstream Env conformations elicit immunodominant, poorly neutralizing antibody (pNAb) responses. To examine the contribution of membrane anchorage to the maintenance of the metastable pretriggered Env conformation, we compared wild-type and State-1-stabilized Envs solubilized in detergents or in styrene-maleic acid (SMA) copolymers. SMA directly incorporates membrane lipids and resident membrane proteins into lipid nanoparticles (styrene-maleic acid lipid particles [SMALPs]). The integrity of the Env trimer in SMALPs was maintained at both 4°C and room temperature. In contrast, Envs solubilized in Cymal-5, a nonionic detergent, were unstable at room temperature, although their stability was improved at 4°C and/or after incubation with the entry inhibitor BMS-806. Envs solubilized in ionic detergents were relatively unstable at either temperature. Comparison of Envs solubilized in Cymal-5 and SMA at 4°C revealed subtle differences in bNAb binding to the gp41 membrane-proximal external region, consistent with these distinct modes of Env solubilization. Otherwise, the antigenicity of the Cymal-5- and SMA-solubilized Envs was remarkably similar, both in the absence and in the presence of BMS-806. However, both solubilized Envs were recognized differently from the mature membrane Env by specific bNAbs and pNAbs. Thus, detergent-based and detergent-free solubilization at 4°C alters the pretriggered membrane Env conformation in consistent ways, suggesting that Env assumes default conformations when its association with the membrane is disrupted. IMPORTANCE The human immunodeficiency virus (HIV) envelope glycoproteins (Envs) in the viral membrane mediate virus entry into the host cell and are targeted by neutralizing antibodies elicited by natural infection or vaccines. Detailed studies of membrane proteins rely on purification procedures that allow the proteins to maintain their natural conformation. In this study, we show that a styrene-maleic acid (SMA) copolymer can extract HIV-1 Env from a membrane without the use of detergents. The Env in SMA is more stable at room temperature than Env in detergents. The purified Env in SMA maintains many but not all of the characteristics expected of the natural membrane Env. Our results underscore the importance of the membrane environment to the native conformation of HIV-1 Env. Purification methods that bypass the need for detergents could be useful tools for future studies of HIV-1 Env structure and its interaction with receptors and antibodies.

Characterization of the Human Immunodeficiency Virus (HIV-1) Envelope Glycoprotein Conformational States on Infectious Virus Particles

Human immunodeficiency virus (HIV-1) entry into cells involves triggering of the viral envelope glycoprotein (Env) trimer ([gp120/gp41]3) by the primary receptor, CD4, and coreceptors, CCR5 or CXCR4. The pretriggered (State-1) conformation of the mature (cleaved) Env is targeted by broadly neutralizing antibodies (bNAbs), which are inefficiently elicited compared with poorly neutralizing antibodies (pNAbs). Here, we characterize variants of the moderately triggerable HIV-1AD8 Env on virions produced by an infectious molecular proviral clone; such virions contain more cleaved Env than pseudotyped viruses. We identified three types of cleaved wild-type AD8 Env trimers on virions: (i) State-1-like trimers preferentially recognized by bNAbs and exhibiting strong subunit association; (ii) trimers recognized by pNAbs directed against the gp120 coreceptor-binding region and exhibiting weak, detergent-sensitive subunit association; and (iii) a minor gp41-only population. The first Env population was enriched and the other Env populations reduced by introducing State-1-stabilizing changes in the AD8 Env or by treatment of the virions with crosslinker or the State-1-preferring entry inhibitor, BMS-806. These stabilized AD8 Envs were also more resistant to gp120 shedding induced by a CD4-mimetic compound or by incubation on ice. Conversely, a State-1-destabilized, CD4-independent AD8 Env variant exhibited weaker bNAb recognition and stronger pNAb recognition. Similar relationships between Env triggerability and antigenicity/shedding propensity on virions were observed for other HIV-1 strains. State-1 Envs on virions can be significantly enriched by minimizing the adventitious incorporation of uncleaved Env; stabilizing the pretriggered conformation by Env modification, crosslinking or BMS-806 treatment; strengthening Env subunit interactions; and using CD4-negative producer cells. IMPORTANCE Efforts to develop an effective HIV-1 vaccine have been frustrated by the inability to elicit broad neutralizing antibodies that recognize multiple virus strains. Such antibodies can bind a particular shape of the HIV-1 envelope glycoprotein trimer, as it exists on a viral membrane but before engaging receptors on the host cell. Here, we establish simple yet powerful assays to characterize the envelope glycoproteins in a natural context on virus particles. We find that, depending on the HIV-1 strain, some envelope glycoproteins change shape and fall apart, creating decoys that can potentially divert the host immune response. We identify requirements to keep the relevant envelope glycoprotein target for broad neutralizing antibodies intact on virus-like particles. These studies suggest strategies that should facilitate efforts to produce and use virus-like particles as vaccine immunogens.

Global Increases in Human Immunodeficiency Virus Neutralization Sensitivity Due to Alterations in the Membrane-Proximal External Region of the Envelope Glycoprotein Can Be Minimized by Distant State 1-Stabilizing Changes

Binding to the receptor, CD4, drives the pretriggered, "closed" (State-1) conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer ([gp120/gp41]₃) into more "open" conformations. HIV-1 Env on the viral membrane is maintained in a State-1 conformation that resists binding and neutralization by commonly elicited antibodies. Premature triggering of Env before the virus engages a target cell typically leads to increased susceptibility to spontaneous inactivation or ligand-induced neutralization. Here, we showed that single amino acid substitutions in the gp41 membrane-proximal external region (MPER) of a primary HIV-1 strain resulted in viral phenotypes indicative of premature triggering of Env to downstream conformations. Specifically, the MPER changes reduced viral infectivity and globally increased virus sensitivity to poorly neutralizing antibodies, soluble CD4, a CD4-mimetic compound, and exposure to cold. In contrast, the MPER mutants exhibited decreased sensitivity to the State 1-preferring inhibitor, BMS-806, and to the PGT151 broadly neutralizing antibody. Depletion of cholesterol from virus particles did not produce the same State 1-destabilizing phenotypes as MPER alterations. Notably, State 1-stabilizing changes in Env distant from the MPER could minimize the phenotypic effects of MPER alteration but did not affect virus sensitivity to cholesterol depletion. Thus, membrane-proximal gp41 elements contribute to the maintenance of the pretriggered Env conformation. The conformationally disruptive effects of MPER changes can be minimized by distant State 1-stabilizing Env modifications, a strategy that may be useful in preserving the native pretriggered state of Env. IMPORTANCE The pretriggered shape of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) is a major target for antibodies that can neutralize many strains of the virus. An effective HIV-1 vaccine may need to raise these types of antibodies, but this goal has proven difficult. One reason is that the pretriggered shape of Env is unstable and dependent on interactions near the viral membrane. Here, we showed that the membrane-proximal external region (MPER) of Env plays an important role in maintaining Env in a pretriggered shape. Alterations in the MPER resulted in global changes in Env conformation that disrupted its pretriggered shape. We also found that these disruptive effects of MPER changes could be minimized by distant Env modifications that stabilized the pretriggered shape. These modifications may be useful for preserving the native shape of Env for structural and vaccine studies.

Geospatial HIV-1 subtype C gp120 sequence diversity and its predicted impact on broadly neutralizing antibody sensitivity

Evolving diversity in globally circulating HIV-1 subtypes presents a formidable challenge in defining and developing neutralizing antibodies for prevention and treatment. HIV-1 subtype C is responsible for majority of global HIV-1 infections. In the present study, we examined the diversity in genetic signatures and attributes that differentiate region-specific HIV-1 subtype C gp120 sequences associated with virus neutralization outcomes to key bnAbs having distinct epitope specificities. A total of 1814 full length HIV-1 subtype C gp120 sequence from 37 countries were retrieved from Los Alamos National Laboratory HIV database (www.hiv.lanl.gov). The amino acid sequences were assessed for their phylogenetic association, variable loop lengths and prevalence of potential N-linked glycosylation sites (pNLGS). Responses of these sequences to bnAbs were predicted with a machine learning algorithm ‘bNAb-ReP’ and compared with those reported in the CATNAP database. Subtype C sequences from Asian countries including India differed phylogenetically when compared with that from African countries. Variable loop lengths and charges within Indian and African clusters were also found to be distinct from each other, specifically for V1, V2 and V4 loops. Pairwise analyses at each of the 25 pNLG sites indicated distinct country specific profiles. Highly significant differences (p<0.001***) were observed in prevalence of four pNLGS (N130, N295, N392 and N448) between South Africa and India, having most disease burden associated with subtype C. Our findings highlight that distinctly evolving clusters within global intra-subtype C gp120 sequences are likely to influence the disparate region-specific sensitivity of circulating HIV-1 subtype C to bnAbs.

Effects of the SOS (A501C/T605C) and DS (I201C/A433C) Disulfide Bonds on HIV-1 Membrane Envelope Glycoprotein Conformation and Function

Most broadly neutralizing antibodies and many entry inhibitors target the pretriggered (state 1) conformation of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env). Here we examine two previously reported Env mutants designed to be stabilized in this conformation by the introduction of artificial disulfide bonds: A501C/T605C (called SOS) and I201C/A433C (called DS). SOS Env supported virus entry and cell-cell fusion only after exposure to a reducing agent, dithiothreitol (DTT). Deletion of the Env cytoplasmic tail improved the efficiency with which the SOS Env supported virus infection in a reducing environment. The antigenicity of the SOS Env was similar to that of the unmodified Env, except for greater sensitivity to some state 1-preferring ligands. In contrast, viruses with the DS Env were not infectious, even after DTT treatment. The proteolytic maturation of the DS Env on both cell surfaces and virions was severely compromised compared with that of the unmodified Env. The DS Env exhibited detectable cell-fusing activity when DTT was present. However, the profiles of cell-surface Env recognition and cell-cell fusion inhibition by antibodies differed for the DS Env and the unmodified Env. Thus, the DS Env appears to be stabilized in an off-pathway conformation that is nonfunctional on the virus. The SOS change exerted more subtle, context-dependent effects on Env conformation and function.IMPORTANCE The human immunodeficiency virus type 1 (HIV-1) envelope proteins (Envs) bind receptors on the host cell and change shape to allow the virus to enter the cell. Most virus-inhibiting antibodies and drugs recognize a particular shape of Env called state 1. Disulfide bonds formed by cysteine residues have been introduced into soluble forms of the flexible envelope proteins in an attempt to lock them into state 1 for use in vaccines and as research tools. We evaluated the effect of these cysteine substitutions on the ability of the membrane Env to support virus entry and on susceptibility to inhibition by antibodies and small molecules. We found that the conformation of the envelope proteins with the cysteine substitutions differed from that of the unmodified membrane envelope proteins. Awareness of these effects can assist efforts to create stable HIV-1 Env complexes that more closely resemble the state 1 conformation.

Prediction of VRC01 neutralization sensitivity by HIV-1 gp160 sequence features

The broadly neutralizing antibody (bnAb) VRC01 is being evaluated for its efficacy to prevent HIV-1 infection in the Antibody Mediated Prevention (AMP) trials. A secondary objective of AMP utilizes sieve analysis to investigate how VRC01 prevention efficacy (PE) varies with HIV-1 envelope (Env) amino acid (AA) sequence features. An exhaustive analysis that tests how PE depends on every AA feature with sufficient variation would have low statistical power. To design an adequately powered primary sieve analysis for AMP, we modeled VRC01 neutralization as a function of Env AA sequence features of 611 HIV-1 gp160 pseudoviruses from the CATNAP database, with objectives: (1) to develop models that best predict the neutralization readouts; and (2) to rank AA features by their predictive importance with classification and regression methods. The dataset was split in half, and machine learning algorithms were applied to each half, each analyzed separately using cross-validation and hold-out validation. We selected Super Learner, a nonparametric ensemble-based cross-validated learning method, for advancement to the primary sieve analysis. This method predicted the dichotomous resistance outcome of whether the IC50 neutralization titer of VRC01 for a given Env pseudovirus is right-censored (indicating resistance) with an average validated AUC of 0.868 across the two hold-out datasets. Quantitative log IC50 was predicted with an average validated R2 of 0.355. Features predicting neutralization sensitivity or resistance included 26 surface-accessible residues in the VRC01 and CD4 binding footprints, the length of gp120, the length of Env, the number of cysteines in gp120, the number of cysteines in Env, and 4 potential N-linked glycosylation sites; the top features will be advanced to the primary sieve analysis. This modeling framework may also inform the study of VRC01 in the treatment of HIV-infected persons.

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.

Evaluation of the contribution of the transmembrane region to the ectodomain conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein

Background

The human immunodeficiency virus (HIV-1) envelope glycoprotein (Env), a Type 1 transmembrane protein, assembles into a trimeric spike complex that mediates virus entry into host cells. The high potential energy of the metastable, unliganded Env trimer is maintained by multiple non-covalent contacts among the gp120 exterior and gp41 transmembrane Env subunits. Structural studies suggest that the gp41 transmembrane region forms a left-handed coiled coil that contributes to the Env trimer interprotomer contacts. Here we evaluate the contribution of the gp41 transmembrane region to the folding and stability of Env trimers.

Methods

Multiple polar/charged amino acid residues, which hypothetically disrupt the stop-transfer signal, were introduced in the proposed lipid-interactive face of the transmembrane coiled coil, allowing release of soluble cleavage-negative Envs containing the modified transmembrane region (TM_mod). We also examined effects of cleavage, the cytoplasmic tail and a C-terminal fibritin trimerization (FT) motif on oligomerization, antigenicity and functionality of soluble and membrane-bound Envs.

Results

The introduction of polar/charged amino acids into the transmembrane region resulted in the secretion of soluble Envs from the cell. However, these TM_mod Envs primarily formed dimers. By contrast, control cleavage-negative sgp140 Envs lacking the transmembrane region formed soluble trimers, dimers and monomers. TM_mod and sgp140 trimers were stabilized by the addition of a C-terminal FT sequence, but still exhibited carbohydrate and antigenic signatures of a flexible ectodomain structure. On the other hand, detergent-solubilized cleaved and uncleaved Envs isolated from the membranes of expressing cells exhibited "tighter” ectodomain structures, based on carbohydrate modifications. These trimers were found to be unstable in detergent solutions, but could be stabilized by the addition of a C-terminal FT moiety. The C-terminal FT domain decreased Env cleavage and syncytium-forming ability by approximately three-fold; alteration of the FT trimerization interface restored Env cleavage and syncytium formation to near-wild-type levels.

Conclusion

The modified transmembrane region was not conducive to trimerization of soluble Envs. However, for HIV-1 Env ectodomains that are minimally modified, membrane-anchored Envs exhibit the most native structures and can be stabilized by appropriately positioned FT domains.

Residues in the gp41 Ectodomain Regulate HIV-1 Envelope Glycoprotein Conformational Transitions Induced by gp120-Directed Inhibitors

Interactions between the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer maintain the metastable unliganded form of the viral spike. Binding of gp120 to the receptor, CD4, changes the Env conformation to promote gp120 interaction with the second receptor, CCR5 or CXCR4. CD4 binding also induces the transformation of Env into the prehairpin intermediate, in which the gp41 heptad repeat 1 (HR1) coiled coil is assembled at the trimer axis. In nature, HIV-1 Envs must balance the requirements to maintain the noncovalent association of gp120 with gp41 and to evade the host antibody response with the need to respond to CD4 binding. Here we show that the gp41 HR1 region contributes to gp120 association with the unliganded Env trimer. Changes in particular amino acid residues in the gp41 HR1 region decreased the efficiency with which Env moved from the unliganded state. Thus, these gp41 changes decreased the sensitivity of HIV-1 to cold inactivation and ligands that require Env conformational changes to bind efficiently. Conversely, these gp41 changes increased HIV-1 sensitivity to small-molecule entry inhibitors that block Env conformational changes induced by CD4. Changes in particular gp41 HR1 amino acid residues can apparently affect the relative stability of the unliganded state and CD4-induced conformations. Thus, the gp41 HR1 region contributes to the association with gp120 and regulates Env transitions from the unliganded state to downstream conformations.IMPORTANCE The development of an efficient vaccine able to prevent HIV infection is a worldwide priority. Knowledge of the envelope glycoprotein structure and the conformational changes that occur after receptor engagement will help researchers to develop an immunogen able to elicit antibodies that block HIV-1 transmission. Here we identify residues in the HIV-1 transmembrane envelope glycoprotein that stabilize the unliganded state by modulating the transitions from the unliganded state to the CD4-bound state.

Amino Acid Changes in the HIV-1 gp41 Membrane Proximal Region Control Virus Neutralization Sensitivity

Most HIV-1 vaccines elicit neutralizing antibodies that are active against highly sensitive (tier-1) viruses or rare cases of vaccine-matched neutralization-resistant (tier-2) viruses, but no vaccine has induced antibodies that can broadly neutralize heterologous tier-2 viruses. In this study, we isolated antibodies from an HIV-1-infected individual that targeted the gp41 membrane-proximal external region (MPER) that may have selected single-residue changes in viral variants in the MPER that resulted in neutralization sensitivity to antibodies targeting distal epitopes on the HIV-1 Env. Similarly, a single change in the MPER in a second virus from another infected-individual also conferred enhanced neutralization sensitivity. These gp41 single-residue changes thus transformed tier-2 viruses into tier-1 viruses that were sensitive to vaccine-elicited tier-1 neutralizing antibodies. These data demonstrate that Env amino acid changes within the MPER bnAb epitope of naturally-selected escape viruses can increase neutralization sensitivity to multiple types of neutralizing antibodies, and underscore the critical importance of the MPER for maintaining the integrity of the tier-2 HIV-1 trimer.

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Amino acid changes in the HIV gp41 MPER can regulate neutralization sensitivity of distal epitopes.

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MPER antibodies isolated early are resistant to MPER changes that enhance neutralization sensitivity.

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HIV gp41 MPER is critical for determining overall HIV envelope conformations.

The HIV-1 envelope protein (Env) is the primary target for neutralizing antibodies. Most HIV-1 vaccines elicit neutralizing antibodies that are active against highly neutralization-sensitive (tier-1) or rare vaccine-matched more neutralization-resistant (tier-2) viruses, but no vaccine has induced antibodies that can broadly neutralize heterologous tier-2 viruses. In this study, we identified changes that occurred in two HIV-1-infected individuals in the membrane proximal region of the HIV-1 Env that resulted in neutralization sensitivity to antibodies targeting distal epitopes on the HIV Env. These single-residue changes thus transformed tier-2 viruses into tier-1 viruses, highlighting the importance of MPER residues in maintaining neutralization-resistant virus.

Determinants in V2C2 region of HIV-1 clade C primary envelopes conferred altered neutralization susceptibilities to IgG1b12 and PG9 monoclonal antibodies in a context-dependent manner

In the present study by examining pseudoviruses expressing patient chimeric envelopes (Envs) made between an IgG1b12 (b12)-sensitive (2-5.J3) and a b12-resistant (4.J22) HIV-1 clade C envelope, we identified determinants in the V2C2 region that governed susceptibility to b12 monoclonal antibody, but not to other CD4 binding site antibodies. Interestingly, when the V2C2 sequence of the 2-5.J3 Env was transferred to other b12-resistant primary clade C Envs, their susceptibility to b12 varied, indicating that this effect was context dependent. In addition, we identified determinants within the V2 region in the b12-resistant envelope that significantly modulated the neutralization of Env-pseudotyped viruses to PG9/PG16 MAbs. The enhanced neutralization susceptibilities of Envs to b12 and PG9 MAbs were correlated with increased exposure of their corresponding epitopes highlighting vulnerabilities in the V2C2 region that altered Env conformation necessary for the efficient accessibility of b12 and PG9 antibodies.

Preventive and therapeutic applications of neutralizing antibodies to Human Immunodeficiency Virus Type 1 (HIV-1)

The development of a preventive vaccine to neutralize the highly variable and antigenically diverse human immunodeficiency virus type 1 (HIV-1) has been an indomitable goal. The recent discovery of a number of cross-neutralizing and potent monoclonal antibodies from elite neutralizers has provided important insights in this field. Neutralizing antibodies (NAbs) are useful in identifying neutralizing epitopes of vaccine utility and for understanding the mechanism of potent and broad cross-neutralization thus providing a modality of preventive and therapeutic value. In this article we review the current understanding on the potential use of broadly neutralizing antibodies (bNAbs) in their full-length IgG structure, engineered domain antibody or bispecific versions towards preventive and therapeutic applications. The potential implications of NAbs are discussed in the light of the recent developments as key components in vaccination against HIV-1. The development of a vaccine immunogen which elicits bNAbs and confers protective immunity remains a real challenge.

Influences on trimerization and aggregation of soluble, cleaved HIV-1 SOSIP envelope glycoprotein

We describe methods to improve the properties of soluble, cleaved gp140 trimers of the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins (Env) for use in structural studies and as immunogens. In the absence of nonionic detergents, gp140 of the KNH1144 genotype, terminating at residue 681 in gp41 (SOSIP.681), has a tendency to form higher-order complexes or aggregates, which is particularly undesirable for structure-based research. We found that this aggregation in the absence of detergent does not involve the V1, V2, or V3 variable regions of gp120. Moreover, we observed that detergent forms micelles around the membrane-proximal external region (MPER) of the SOSIP.681 gp140 trimers, whereas deletion of most of the MPER residues by terminating the gp140 at residue 664 (SOSIP.664) prevented the aggregation that otherwise occurs in SOSIP.681 in the absence of detergent. Although the MPER can contribute to trimer formation, truncation of most of it only modestly reduced trimerization and lacked global adverse effects on antigenicity. Thus, the MPER deletion minimally influenced the kinetics of the binding of soluble CD4 and a CD4-binding site antibody to immobilized trimers, as detected by surface plasmon resonance. Furthermore, the MPER deletion did not alter the overall three-dimensional structure of the trimers, as viewed by negative-stain electron microscopy. Homogeneous and aggregate-free MPER-truncated SOSIP Env trimers are therefore useful for immunogenicity and structural studies.

Structural characterization of cleaved, soluble HIV-1 envelope glycoprotein trimers

Human immunodeficiency virus type 1 (HIV-1) infection is a significant global public health problem for which development of an effective prophylactic vaccine remains a high scientific priority. Many concepts for a vaccine are focused on induction of appropriate titers of broadly neutralizing antibodies (bNAbs) against the viral envelope (Env) glycoproteins gp120 and gp41, but no immunogen has yet accomplished this goal in animals or humans. One approach to induction of bNAbs is to design soluble, trimeric mimics of the native viral Env trimer. Here, we describe structural studies by negative-stain electron microscopy of several variants of soluble Env trimers based on the KNH1144 subtype A sequence. These Env trimers are fully cleaved between the gp120 and gp41 components and stabilized by specific amino acid substitutions. We also illustrate the structural consequences of deletion of the V1/V2 and V3 variable loops from gp120 and the membrane-proximal external region (MPER) from gp41. All of these variants adopt a trimeric configuration that appropriately mimics native Env spikes, including the CD4 receptor-binding site and the epitope for the VRC PG04 bNAb. These cleaved, soluble trimer designs can be adapted for use with multiple different env genes for both vaccine and structural studies.

The griffithsin dimer is required for high-potency inhibition of HIV-1: evidence for manipulation of the structure of gp120 as part of the griffithsin dimer mechanism

Griffithsin (Grft) is a protein lectin derived from red algae that tightly binds the HIV envelope protein gp120 and effectively inhibits virus infection. This inhibition is due to the binding by Grft of high-mannose saccharides on the surface of gp120. Grft has been shown to be a tight dimer, but the role of the dimer in Grft's anti-HIV function has not been fully explored. To investigate the role of the Grft dimer in anti-HIV function, an obligate dimer of Grft was designed by expressing the protein with a peptide linker between the two subunits. This "Grft-linker-Grft" is a folded protein dimer, apparently nearly identical in structural properties to the wild-type protein. A "one-armed" obligate dimer was also designed (Grft-linker-Grft OneArm), with each of the three carbohydrate binding sites of one subunit mutated while the other subunit remained intact. While both constructed dimers retained the ability to bind gp120 and the viral surface, Grft-linker-Grft OneArm was 84- to 1,010-fold less able to inhibit HIV than wild-type Grft, while Grft-linker-Grft had near-wild-type antiviral potency. Furthermore, while the wild-type protein demonstrated the ability to alter the structure of gp120 by exposing the CD4 binding site, Grft-linker-Grft OneArm largely lost this ability. In experiments to investigate gp120 shedding, it was found that Grft has different effects on gp120 shedding for strains from subtype B and subtype C, and this might correlate with Grft function. Evidence is provided that the dimer form of Grft is critical to the function of this protein in HIV inhibition.

Selected amino acid changes in HIV-1 subtype-C gp41 are associated with specific gp120(V3) signatures in the regulation of co-receptor usage

The majority of studies have characterized the tropism of HIV-1 subtype-B isolates, but little is known about the determinants of tropism in other subtypes. So, the goal of the present study was to genetically characterize the envelope of viral proteins in terms of co-receptor usage by analyzing 356 full-length env sequences derived from HIV-1 subtype-C infected individuals. The co-receptor usage of V3 sequences was inferred by using the Geno2Pheno and PSSM algorithms, and also analyzed to the "11/25 rule". All reported env sequences were also analyzed with regard to N-linked glycosylation sites, net charge and hydrophilicity, as well as the binomial correlation phi coefficient to assess covariation among gp120(V3) and gp41 signatures and the average linkage hierarchical agglomerative clustering were also performed. Among env sequences present in Los Alamos Database, 255 and 101 sequences predicted as CCR5 and CXCR4 were selected, respectively. The classical V3 signatures at positions 11 and 25, and other specific V3 and gp41 amino acid changes were found statistically associated with different co-receptor usage. Furthermore, several statistically significant associations between V3 and gp41 signatures were also observed. The dendrogram topology showed a cluster associated with CCR5-usage composed by five gp41 mutated positions, A22V, R133M, E136G, N140L, and N166Q that clustered with T2V(V3) and G24T(V3) (bootstrap=1). Conversely, a heterogeneous cluster with CXCR4-usage, involving S11GR(V3), 13-14insIG/LG(V3), P16RQ(V3), Q18KR(V3), F20ILV(V3), D25KRQ(V3), Q32KR(V3) along with A30T(gp41), S107N(gp41), D148E(gp41), A189S(gp41) was identified (bootstrap=0.86). Our results show that as observed for HIV-1 subtype-B, also in subtype-C specific and different gp41 and gp120V3 amino acid changes are associated individually or together with CXCR4 and/or CCR5 usage. These findings strengthen previous observations that determinants of tropism may also reside in the gp41 protein.