Purified, proteolytically mature HIV type 1 SOSIP gp140 envelope trimers

High-Throughput Protein Engineering Improves the Antigenicity and Stability of Soluble HIV-1 Envelope Glycoprotein SOSIP Trimers

Soluble envelope glycoprotein (Env) trimers (SOSIP.664 gp140) are attractive HIV-1 vaccine candidates, with structures that mimic the native membrane-bound Env spike (gp160). Since engineering trimers can be limited by the difficulty of rationally predicting beneficial mutations, here we used a more comprehensive mutagenesis approach with the goal of identifying trimer variants with improved antigenic and stability properties. We created 341 cysteine pairs at predicted points of stabilization throughout gp140, 149 proline residue substitutions at every residue of the gp41 ectodomain, and 362 space-filling residue substitutions at every hydrophobic and aromatic residue in gp140. The parental protein target, the clade B strain B41 SOSIP.664 gp140, does not bind the broadly neutralizing antibody PGT151 and so was used here to identify improved variants that also provide insight into the structural basis for Env antigenicity. Each of the 852 mutants was expressed in human cells and screened for antigenicity using four different monoclonal antibodies (MAbs), including PGT151. We identified 29 trimer variants with antigenic improvements derived from each of the three mutagenesis strategies. We selected four variants (Q203F, T538F, I548F, and M629P) for more comprehensive biochemical, structural, and antigenicity analyses. The T538F substitution had the most beneficial effect overall, including restoration of the PGT151 epitope. The improved B41 SOSIP.664 trimer variants identified here may be useful for vaccine and structural studies.IMPORTANCE Soluble Env trimers have become attractive HIV-1 vaccine candidates, but the prototype designs are capable of further improvement through protein engineering. Using a high-throughput screening technology (shotgun mutagenesis) to create and evaluate 852 variants, we were able to identify sequence changes that were beneficial to the antigenicity and stability of soluble trimers based on the clade B B41 env gene. The strategies described here may be useful for identifying a wider range of antigenically and structurally improved soluble trimers based on multiple genotypes for use in programs intended to create a broadly protective HIV-1 vaccine.

Native-like Env trimers as a platform for HIV-1 vaccine design

We describe the development and potential use of various designs of recombinant HIV-1 envelope glycoprotein trimers that mimic the structure of the virion-associated spike, which is the target for neutralizing antibodies. The goal of trimer development programs is to induce broadly neutralizing antibodies with the potential to intervene against multiple circulating HIV-1 strains. Among the topics we address are the designs of various constructs; how native-like trimers can be produced and purified; the properties of such trimers in vitro and their immunogenicity in various animals; and the immunization strategies that may lead to the eventual elicitation of broadly neutralizing antibodies. In summary, native-like trimers are a now a platform for structure- and immunology-based design improvements that could eventually yield immunogens of practical value for solving the long-standing HIV-1 vaccine problem.

Potential To Streamline Heterologous DNA Prime and NYVAC/Protein Boost HIV Vaccine Regimens in Rhesus Macaques by Employing Improved Antigens

In a follow-up to the modest efficacy observed in the RV144 trial, researchers in the HIV vaccine field seek to substantiate and extend the results by evaluating other poxvirus vectors and combinations with DNA and protein vaccines. Earlier clinical trials (EuroVacc trials 01 to 03) evaluated the immunogenicity of HIV-1 clade C GagPolNef and gp120 antigens delivered via the poxviral vector NYVAC. These showed that a vaccination regimen including DNA-C priming prior to a NYVAC-C boost considerably enhanced vaccine-elicited immune responses compared to those with NYVAC-C alone. Moreover, responses were improved by using three as opposed to two DNA-C primes. In the present study, we assessed in nonhuman primates whether such vaccination regimens can be streamlined further by using fewer and accelerated immunizations and employing a novel generation of improved DNA-C and NYVAC-C vaccine candidates designed for higher expression levels and more balanced immune responses. Three different DNA-C prime/NYVAC-C+ protein boost vaccination regimens were tested in rhesus macaques. All regimens elicited vigorous and well-balanced CD8(+)and CD4(+)T cell responses that were broad and polyfunctional. Very high IgG binding titers, substantial antibody-dependent cellular cytotoxicity (ADCC), and modest antibody-dependent cell-mediated virus inhibition (ADCVI), but very low neutralization activity, were measured after the final immunizations. Overall, immune responses elicited in all three groups were very similar and of greater magnitude, breadth, and quality than those of earlier EuroVacc vaccines. In conclusion, these findings indicate that vaccination schemes can be simplified by using improved antigens and regimens. This may offer a more practical and affordable means to elicit potentially protective immune responses upon vaccination, especially in resource-constrained settings.

IMPORTANCE

Within the EuroVacc clinical trials, we previously assessed the immunogenicity of HIV clade C antigens delivered in a DNA prime/NYVAC boost regimen. The trials showed that the DNA prime crucially improved the responses, and three DNA primes with a NYVAC boost appeared to be optimal. Nevertheless, T cell responses were primarily directed toward Env, and humoral responses were modest. The aim of this study was to assess improved antigens for the capacity to elicit more potent and balanced responses in rhesus macaques, even with various simpler immunization regimens. Our results showed that the novel antigens in fact elicited larger numbers of T cells with a polyfunctional profile and a good Env-GagPolNef balance, as well as high-titer and Fc-functional antibody responses. Finally, comparison of the different schedules indicates that a simpler regimen of only two DNA primes and one NYVAC boost in combination with protein may be very efficient, thus showing that the novel antigens allow for easier immunization protocols.

HIV-1 envelope glycoprotein immunogens to induce broadly neutralizing antibodies

The long pursuit for a vaccine against human immunodeficiency virus 1 (HIV-1) has recently been boosted by a number of exciting developments. An HIV-1 subunit vaccine ideally should elicit potent broadly neutralizing antibodies (bNAbs), but raising bNAbs by vaccination has proved extremely difficult because of the characteristics of the HIV-1 envelope glycoprotein complex (Env). However, the isolation of bNAbs from HIV-1-infected patients demonstrates that the human humoral immune system is capable of making such antibodies. Therefore, a focus of HIV-1 vaccinology is the elicitation of bNAbs by engineered immunogens and by using vaccination strategies aimed at mimicking the bNAb maturation pathways in HIV-infected patients. Important clues can also be taken from the successful subunit vaccines against hepatitis B virus and human papillomavirus. Here, we review the different types of HIV-1 immunogens and vaccination strategies that are being explored in the search for an HIV-1 vaccine that induces bNAbs.

Assembly and characterization of gp160-nanodiscs: A new platform for biochemical characterization of HIV envelope spikes

The human immunodeficiency virus (HIV) is the causative agent of acquired immune deficiency syndrome (AIDS) and is thus responsible for significant morbidity and mortality worldwide. Despite considerable effort, preparation of an effective vaccine for AIDS has been elusive and it has become clear that a fundamental understanding of the relevant antigenic targets on HIV is essential. The Env trimer spike is the only viral antigen present on the surface of the viral particle and it is the target of all broadly neutralizing antibodies isolated to date. Thus, a soluble, homogeneous, and well-defined preparation of Env trimers is an important first step toward biochemical and structural characterization of the antigenic spike. Phospholipid bilayer nanodiscs represent a relatively new technology that can serve as a platform for the assembly of membrane proteins into a native membrane-like environment. Here we describe the preparation and characterization of unprocessed full-length, natively glycoslyated gp160 Env proteins incorporated into nanodiscs (gp160-ND). The particles are soluble and well defined in the absence of detergent, and possess a morphology anticipated of Env incorporated into a lipid ND. Importantly, the gp160-NDs retain CD4 and Env antibody binding characteristics expected of a functional trimer spike and their incorporation into a lipid membrane allows interrogation of epitopes associated with the membrane-proximal ectodomain region of gp41. These studies provide the groundwork for the use of gp160-ND in more detailed biochemical and structural studies that may set the stage for their use in vaccine development.

Comparative Analysis of the Glycosylation Profiles of Membrane-Anchored HIV-1 Envelope Glycoprotein Trimers and Soluble gp140

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer, which consists of the gp120 and gp41 subunits, is the focus of multiple strategies for vaccine development. Extensive Env glycosylation provides HIV-1 with protection from the immune system, yet the glycans are also essential components of binding epitopes for numerous broadly neutralizing antibodies. Recent studies have shown that when Env is isolated from virions, its glycosylation profile differs significantly from that of soluble forms of Env (gp120 or gp140) predominantly used in vaccine discovery research. Here we show that exogenous membrane-anchored Envs, which can be produced in large quantities in mammalian cells, also display a virion-like glycan profile, where the glycoprotein is extensively decorated with high-mannose glycans. Additionally, because we characterized the glycosylation with a high-fidelity profiling method, glycopeptide analysis, an unprecedented level of molecular detail regarding membrane Env glycosylation and its heterogeneity is presented. Each glycosylation site was characterized individually, with about 500 glycoforms characterized per Env protein. While many of the sites contain exclusively high-mannose glycans, others retain complex glycans, resulting in a glycan profile that cannot currently be mimicked on soluble gp120 or gp140 preparations. These site-level studies are important for understanding antibody-glycan interactions on native Env trimers. Additionally, we report a newly observed O-linked glycosylation site, T606, and we show that the full O-linked glycosylation profile of membrane-associated Env is similar to that of soluble gp140. These findings provide new insight into Env glycosylation and clarify key molecular-level differences between membrane-anchored Env and soluble gp140.

IMPORTANCE

A vaccine that protects against human immunodeficiency virus type 1 (HIV-1) infection should elicit antibodies that bind to the surface envelope glycoproteins on the membrane of the virus. The envelope glycoproteins have an extensive coat of carbohydrates (glycans), some of which are recognized by virus-neutralizing antibodies and some of which protect the virus from neutralizing antibodies. We found that the HIV-1 membrane envelope glycoproteins have a unique pattern of carbohydrates, with many high-mannose glycans and also, in some places, complex glycans. This pattern was very different from the carbohydrate profile seen for a more easily produced soluble version of the envelope glycoprotein. Our results provide a detailed characterization of the glycans on the natural membrane envelope glycoproteins of HIV-1, a carbohydrate profile that would be desirable to mimic with a vaccine.

Comparable Antigenicity and Immunogenicity of Oligomeric Forms of a Novel, Acute HIV-1 Subtype C gp145 Envelope for Use in Preclinical and Clinical Vaccine Research

Eliciting broadly reactive functional antibodies remains a challenge in human immunodeficiency virus type 1 (HIV-1) vaccine development that is complicated by variations in envelope (Env) subtype and structure. The majority of new global HIV-1 infections are subtype C, and novel antigenic properties have been described for subtype C Env proteins. Thus, an HIV-1 subtype C Env protein (CO6980v0c22) from an infected person in the acute phase (Fiebig stage I/II) was developed as a research reagent and candidate immunogen. The gp145 envelope is a novel immunogen with a fully intact membrane-proximal external region (MPER), extended by a polylysine tail. Soluble gp145 was enriched for trimers that yielded the expected "fan blade" motifs when visualized by cryoelectron microscopy. CO6980v0c22 gp145 reacts with the 4E10, PG9, PG16, and VRC01 HIV-1 neutralizing monoclonal antibodies (MAbs), as well as the V1/V2-specific PGT121, 697, 2158, and 2297 MAbs. Different gp145 oligomers were tested for immunogenicity in rabbits, and purified dimers, trimers, and larger multimers elicited similar levels of cross-subtype binding and neutralizing antibodies to tier 1 and some tier 2 viruses. Immunized rabbit sera did not neutralize the highly resistant CO6980v0c22 pseudovirus but did inhibit the homologous infectious molecular clone in a peripheral blood mononuclear cell (PBMC) assay. This Env is currently in good manufacturing practice (GMP) production to be made available for use as a clinical research tool and further evaluation as a candidate vaccine.

IMPORTANCE

At present, the product pipeline for HIV vaccines is insufficient and is limited by inadequate capacity to produce large quantities of vaccine to standards required for human clinical trials. Such products are required to evaluate critical questions of vaccine formulation, route, dosing, and schedule, as well as to establish vaccine efficacy. The gp145 Env protein presented in this study forms physical trimers, binds to many of the well-characterized broad neutralizing MAbs that target conserved Env epitopes, and induce cross-subtype neutralizing antibodies as measured in both cell line and primary cell assays. This subtype C Env gp145 protein is currently undergoing good manufacturing practice production for use as a reagent for preclinical studies and for human clinical research. This product will serve as a reagent for comparative studies and may represent a next-generation candidate HIV immunogen.

Antibody potency relates to the ability to recognize the closed, pre-fusion form of HIV Env

HIV’s envelope glycoprotein (Env) is the sole target for neutralizing antibodies. The structures of many broadly neutralizing antibodies (bNAbs) in complex with truncated Env subunits or components have been reported. However, their interaction with the intact Env trimer, and the structural determinants that underlie neutralization resistance in this more native context are less well understood. Here we use hydrogen/deuterium-exchange to examine the interactions between a panel of bNAbs and native-like Env trimers (SOSIP.664 trimers). Highly potent bNAbs cause only localized effects at their binding interface, while the binding of less potent antibodies is associated with elaborate changes throughout the trimer. In conjunction with binding kinetics, our results suggest that poorly neutralizing antibodies can only bind when the trimer transiently samples an open state. We propose that the kinetics of such opening motions varies among isolates, with Env from neutralization-sensitive viruses opening more frequently than Env from resistant viruses.

CD4-induced activation in a soluble HIV-1 Env trimer

The HIV envelope glycoprotein (Env) trimer undergoes receptor-induced conformational changes that drive fusion of the viral and cellular membranes. Env conformational changes have been observed using low-resolution electron microscopy, but only large-scale rearrangements have been visible. Here, we use hydrogen-deuterium exchange and oxidative labeling to gain a more precise understanding of the unliganded and CD4-bound forms of soluble Env trimers (SOSIP.664), including their glycan composition. CD4 activation induces the reorganization of bridging sheet elements, V1/V2 and V3, much of the gp120 inner domain, and the gp41 fusion subunit. Two CD4 binding site-targeted inhibitors have substantially different effects: NBD-556 partially mimics CD4-induced destabilization of the V1/V2 and V3 crown, whereas BMS-806 only affects regions around the gp120/gp41 interface. The structural information presented here increases our knowledge of CD4- and small molecule-induced conformational changes in Env and the allosteric pathways that lead to membrane fusion.

Immunogen design for HIV-1 and influenza

Vaccines provide the most cost effective defense against pathogens. Although vaccines have been designed for a number of viral diseases, a vaccine against HIV-1 still remains elusive. In contrast, while there are excellent influenza vaccines, these need to be changed every few years because of antigenic drift and shift. The recent discovery of a large number of broadly neutralizing antibodies (bNAbs) and structural characterization of the conserved epitopes targeted by them presents an opportunity for structure based HIV-1 and influenza A vaccine design. We discuss strategies to design immunogens either targeting a particular antigenic region or focusing on native structure stabilization. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

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.

Purification of recombinant vaccinia virus-expressed monomeric HIV-1 gp120 to apparent homogeneity

Vaccinia virus (VV) has been used to express a variety of heterologous proteins, including HIV envelope (Env) glycoproteins. The Env protein is synthesized as a precursor molecule, gp160, which is cleaved into the surface antigen gp120 and the transmembrane protein gp41. Even though production of gp160 by the VV expression system has been described, its use for gp120 production is not well documented. Here we report a new procedure for the purification of gp120 from serum-containing culture supernatant of VV-infected cells. The gp120 protein was enriched to a purity better than 60% on a snowdrop (Galanthus nivalis) lectin affinity column in the presence of 0.25% zwitterionic detergent Empigen BB. After additional DEAE anion exchange and Superdex size exclusion chromatography steps, the gp120 monomer was purified free of contamination as determined by SDS-PAGE. The retention of structural integrity was confirmed by determining the affinity constant of purified gp120s to soluble CD4 and a monoclonal antibody IgG1b12, using surface plasmon resonance analysis. The purification procedure is robust and reproducible, and may find general use for glycoprotein purifications from sources where the presence of serum is desirable.

Diverse specificity and effector function among human antibodies to HIV-1 envelope glycoprotein epitopes exposed by CD4 binding

The HIV-1 envelope glycoprotein (Env) undergoes conformational transitions consequent to CD4 binding and coreceptor engagement during viral entry. The physical steps in this process are becoming defined, but less is known about their significance as targets of antibodies potentially protective against HIV-1 infection. Here we probe the functional significance of transitional epitope exposure by characterizing 41 human mAbs specific for epitopes exposed on trimeric Env after CD4 engagement. These mAbs recognize three epitope clusters: cluster A, the gp120 face occluded by gp41 in trimeric Env; cluster B, a region proximal to the coreceptor-binding site (CoRBS) and involving the V1/V2 domain; and cluster C, the coreceptor-binding site. The mAbs were evaluated functionally by antibody-dependent, cell-mediated cytotoxicity (ADCC) and for neutralization of Tiers 1 and 2 pseudoviruses. All three clusters included mAbs mediating ADCC. However, there was a strong potency bias for cluster A, which harbors at least three potent ADCC epitopes whose cognate mAbs have electropositive paratopes. Cluster A epitopes are functional ADCC targets during viral entry in an assay format using virion-sensitized target cells. In contrast, only cluster C contained epitopes that were recognized by neutralizing mAbs. There was significant diversity in breadth and potency that correlated with epitope fine specificity. In contrast, ADCC potency had no relationship with neutralization potency or breadth for any epitope cluster. Thus, Fc-mediated effector function and neutralization coselect with specificity in anti-Env antibody responses, but the nature of selection is distinct for these two antiviral activities.

Partial enzymatic deglycosylation preserves the structure of cleaved recombinant HIV-1 envelope glycoprotein trimers

The trimeric envelope glycoprotein complex (Env) is the focus of vaccine development programs aimed at generating protective humoral responses to human immunodeficiency virus type 1 (HIV-1). N-Linked glycans, which constitute almost half of the molecular mass of the external Env domains, produce considerable structural heterogeneity and are a major impediment to crystallization studies. Moreover, by shielding the peptide backbone, glycans can block attempts to generate neutralizing antibodies against a substantial subset of potential epitopes when Env proteins are used as immunogens. Here, we describe the partial deglycosylation of soluble, cleaved recombinant Env trimers by inhibition of the synthesis of complex N-glycans during Env production, followed by treatment with glycosidases under conditions that preserve Env trimer integrity. The partially deglycosylated trimers are stable, and neither abnormally sensitive to proteolytic digestion nor prone to aggregation. Moreover, the deglycosylated trimers retain or increase their ability to bind CD4 and antibodies that are directed to conformational epitopes, including the CD4-binding site and the V3 region. However, as expected, they do not react with glycan-dependent antibodies 2G12 and PGT123, or the C-type lectin receptor DC-SIGN. Electron microscopic analysis shows that partially deglycosylated trimers have a structure similar to fully glycosylated trimers, indicating that removal of glycans does not substantially perturb the structural integrity of the trimer. The glycan-depleted Env trimers should be useful for structural and immunogenicity studies.

Structural comparison of HIV-1 envelope spikes with and without the V1/V2 loop

We have used cryoelectron tomography of vitreous-ice-embedded HIV-1 virions to compare the envelope (Env) spikes of a wild-type strain with those of a mutant strain in which the V1/V2 loop has been deleted. Deletion of V1/V2 results in a spike with far more structural heterogeneity than is observed in the wild type, likely reflecting greatly enhanced gp120 protomer flexibility. A major difference between the two forms is a pronounced loss of mass from the "peak" of the native Env spike. The apparent loss of contact among three gp120 protomers likely accounts for the more open structure, heterogeneity in configuration, and previous observations that broadly neutralizing epitopes and reactive sites on other structural elements are more exposed in such constructs.

Single-particle cryoelectron microscopy analysis reveals the HIV-1 spike as a tripod structure

The HIV-1 spike is a trimer of the transmembrane gp41 and the peripheral gp120 subunit pair. It is activated for virus-cell membrane fusion by binding sequentially to CD4 and to a chemokine receptor. Here we have studied the structural transition of the trimeric spike during the activation process. We solubilized and isolated unliganded and CD4-bound spikes from virus-like particles and used cryoelectron microscopy to reconstruct their 3D structures. In order to increase the yield and stability of the spike, we used an endodomain deleted and gp120-gp41 disulfide-linked variant. The unliganded spike displayed a hollow cage-like structure where the gp120-gp41 protomeric units formed a roof and bottom, and separated lobes and legs on the sides. The tripod structure was verified by fitting the recent atomic core structure of gp120 with intact N- and C-terminal ends into the spike density map. This defined the lobe as gp120 core, showed that the legs contained the polypeptide termini, and suggested the deleted variable loops V1/V2 and V3 to occupy the roof and gp41 the bottom. CD4 binding shifted the roof density peripherally and condensed the bottom density centrally. Fitting with a V3 containing gp120 core suggested that the V1/V2 loops in the roof were displaced laterally and the V3 lifted up, while the core and leg were kept in place. The loop displacements probably prepared the spike for coreceptor interaction and roof opening so that a new fusion-active gp41 structure, assembled at the center of the cage bottom, could reach the target membrane.

Effect of trimerization motifs on quaternary structure, antigenicity, and immunogenicity of a noncleavable HIV-1 gp140 envelope glycoprotein

The external domains of the HIV-1 envelope glycoprotein (gp120 and the gp41 ectodomain, collectively known as gp140) contain all known viral neutralization epitopes. Various strategies have been used to create soluble trimers of the envelope to mimic the structure of the native viral protein, including mutation of the gp120-gp41 cleavage site, introduction of disulfide bonds, and fusion to heterologous trimerization motifs. We compared the effects on quaternary structure, antigenicity, and immunogenicity of three such motifs: T4 fibritin, a GCN4 variant, and the Escherichia coli aspartate transcarbamoylase catalytic subunit. Fusion of each motif to the C-terminus of a noncleavable JRCSF gp140(-) envelope protein led to enhanced trimerization but had limited effects on the antigenic profile and CD4-binding ability of the trimers. Immunization of rabbits provided no evidence that the trimerized gp140(-) constructs induced significantly improved neutralizing antibodies to several HIV-1 pseudoviruses, compared to gp140 lacking a trimerization motif. However, modest differences in both binding specificity and neutralizing antibody responses were observed among the various immunogens.

Structural and immunogenicity studies of a cleaved, stabilized envelope trimer derived from subtype A HIV-1

SOSIP gp140 trimers represent a soluble, stabilized, proteolytically cleaved form of the HIV-1 envelope (Env) glycoproteins. SOSIP gp140 derived from a subtype A HIV-1 isolate, KNH1144, forms exceptionally stable trimers that resemble virion-associated Env in antigenicity and topology. Here, we used electron microscopy to demonstrate that KNH1144 SOSIP gp140 trimers bound three soluble CD4 molecules in a symmetrical orientation similar to that seen for native Env. We compared the immunogenicities of KNH1144 SOSIP gp140 trimers and gp120 monomers in rabbits and found that the trimers were superior at eliciting neutralizing antibodies (NAbs) to homologous virus as well as neutralization-sensitive subtype B and C viruses. The NAb specificities for SOSIP antisera mapped in part to the CD4 binding site on gp120. We also observed adjuvant-dependent induction of antibodies to the residual levels of host cell proteins (HCPs) contained in the purified Env preparations. When present, HCP antibodies enhanced pseudovirus infection. Our findings are relevant for the further development of Env-based vaccines for HIV-1.

Mapping the immune response to the outer domain of a human immunodeficiency virus-1 clade C gp120

The outer domain (OD) of human immunodeficiency virus (HIV)-1 gp120 represents an attractive, if difficult, target for a beneficial immune response to HIV infection. Unlike the entire gp120, the OD is structurally stable and contains the surfaces that interact with both the primary and secondary cellular receptors. The primary strain-specific neutralizing target, the V3 loop, lies within the OD, as do epitopes for two cross-reactive neutralizing monoclonal antibodies (mAbs), b12 and 2G12, and the contact sites for a number of inhibitory lectins. The OD is poorly immunogenic, at least in the context of complete gp120, but purposeful OD immunization can lead to a substantial antibody response. Here, we map the antibody generated following immunization with a clade C OD. In contrast to published data for the clade B OD, the majority of the polyclonal response to the complete clade C OD is to the V3 loop; deletion of the loop substantially reduces immunogenicity. When the loop sequence was substituted for the epitope for 2F5, a well-characterized human cross-neutralizing mAb, a polyclonal response to the epitope was generated. A panel of mAbs against the clade C OD identified two mAbs that reacted with the loop and were neutralizing for clade C but not B isolates. Other mAbs recognized both linear and conformational epitopes in the OD. We conclude that, as for complete gp120, V3 immunodominance is a property of OD immunogens, that the responses can be neutralizing and that it could be exploited for the presentation of other epitopes.

Comparative evaluation of trimeric envelope glycoproteins derived from subtype C and B HIV-1 R5 isolates

We previously reported that an envelope (Env) glycoprotein immunogen (o-gp140DeltaV2SF162) containing a partial deletion in the second variable loop (V2) derived from the R5-tropic HIV-1 isolate SF162 partially protected vaccinated rhesus macaques against pathogenic SHIV(SF162P4) virus. Extending our studies to subtype C isolate TV1, we have purified o-gp140DeltaV2TV1 (subtype C DeltaV2 trimer) to homogeneity, performed glycosylation analysis, and determined its ability to bind CD4, as well as a panel of well-characterized neutralizing monoclonal antibodies (mAb). In general, critical epitopes are preserved on the subtype C DeltaV2 trimer; however, we did not observe significant binding for the b12 mAb. The molecular mass of subtype C DeltaV2 trimer was found to be 450 kDa, and the hydrodynamic radius was found to be 10.87 nm. Our data suggest that subtype C DeltaV2 trimer binds to CD4 with an affinity comparable to o-gp140DeltaV2SF162 (subtype B DeltaV2 trimer). Using isothermal titration calorimetric (ITC) analysis, we demonstrated that all three CD4 binding sites (CD4-BS) in both subtype C and B trimers are exposed and accessible. However, compared to subtype B trimer, the three CD4-BS in subtype C trimer have different affinities for CD4, suggesting a cooperativity of CD4 binding in subtype C trimer but not in subtype B trimer. Negative staining electron microscopy of the subtype C DeltaV2 trimer has demonstrated that it is in fact a trimer. These results highlight the importance of studying subtype C Env, and also of developing appropriate subtype C-specific reagents that may be used for better immunological characterization of subtype C Env for developing an AIDS vaccine.

AIDS virus envelope spike structure

The envelope (Env) spikes on HIV-1 and closely related SIV define the viral tropism, mediate the fusion process and are the prime target of the humoral response. Despite intensive efforts, Env has been slow to reveal its structural and functional secrets. Three gp120 subunits comprise the 'head' of Env and three gp41 subunits comprise the 'stalk' and other membrane-associated elements. The recent description of the core structure of unliganded (untriggered) gp120, compared to earlier CD4-liganded atomic structures, reveals dramatic conformational reorganization of the components and suggests a mechanism for the initiation of fusion. The structure of the key V3 loop, both in isolation and in association with the liganded core, helps define its role in fusion and as a prime target of neutralizing antibodies. Additional details are emerging regarding the structure of gp41 as it transitions from the preliganded configuration to the fusion intermediate (fusion-active or prehairpin intermediate) configuration, although much remains speculative. Recent advances in cryoelectron tomography are giving us the first glimpses of the overall three-dimensional structure of Env, which, when fitted with the available component atomic structures, provides new insights into the organization of the structural elements within the trimeric spike.