Cell- and Protein-Directed Glycosylation of Native Cleaved HIV-1 Envelope.
ABSTRACT: The gp120/gp41 HIV-1 envelope glycoprotein (Env) is highly glycosylated, with up to 50% of its mass consisting of N-linked glycans. This dense carbohydrate coat has emerged as a promising vaccine target, with its glycans acting as epitopes for a number of potent and broadly neutralizing antibodies (bnAbs). Characterizing the glycan structures present on native HIV-1 Env is thus a critical goal for the design of Env immunogens. In this study, we used a complementary, multistep approach involving ion mobility mass spectrometry and high-performance liquid chromatography to comprehensively characterize the glycan structures present on HIV-1 gp120 produced in peripheral blood mononuclear cells (PBMCs). The capacity of different expression systems, including pseudoviral particles and recombinant cell surface trimers, to reproduce native-like glycosylation was then assessed. A population of oligomannose glycans on gp120 was reproduced across all expression systems, supporting this as an intrinsic property of Env that can be targeted for vaccine design. In contrast, Env produced in HEK 293T cells failed to accurately reproduce the highly processed complex-type glycan structures observed on PBMC-derived gp120, and in particular the precise linkage of sialic acid residues that cap these glycans. Finally, we show that unlike for gp120, the glycans decorating gp41 are mostly complex-type sugars, consistent with the glycan specificity of bnAbs that target this region. These findings provide insights into the glycosylation of native and recombinant HIV-1 Env and can be used to inform strategies for immunogen design and preparation.Development of an HIV vaccine is desperately needed to control new infections, and elicitation of HIV bnAbs will likely be an important component of an effective vaccine. Increasingly, HIV bnAbs are being identified that bind to the N-linked glycans coating the HIV envelope glycoproteins gp120 and gp41, highlighting them as important targets for vaccine design. It is therefore important to characterize the glycan structures present on native, virion-associated gp120 and gp41 for development of vaccines that accurately mimic native-Env glycosylation. In this study, we used a number of analytical techniques to precisely study the structures of both the oligomannose and complex-type glycans present on native Env to provide a reference for determining the ability of potential HIV immunogens to accurately replicate the glycosylation pattern on these native structures.
Project description:Many broadly neutralizing antibodies (bnAbs) against HIV-1 recognize and/or penetrate the glycan shield on native, virion-associated envelope glycoprotein (Env) spikes. The same bnAbs also bind to recombinant, soluble trimeric immunogens based on the SOSIP design. While SOSIP trimers are close structural and antigenic mimics of virion Env, the extent to which their glycan structures resemble ones on infectious viruses is undefined. Here, we compare the overall glycosylation of gp120 and gp41 subunits from BG505 (clade A) virions produced in a lymphoid cell line with those from recombinant BG505 SOSIP trimers, including CHO-derived clinical grade material. We also performed detailed site-specific analyses of gp120. Glycans relevant to key bnAb epitopes are generally similar on the recombinant SOSIP and virion-derived Env proteins, although the latter do contain hotspots of elevated glycan processing. Knowledge of native versus recombinant Env glycosylation will guide vaccine design and manufacturing programs.
Project description:A highly glycosylated, trimeric envelope glycoprotein (Env) mediates HIV-1 cell entry. The high density and heterogeneity of the glycans shield Env from recognition by the immune system, but paradoxically, many potent broadly neutralizing antibodies (bNAbs) recognize epitopes involving this glycan shield. To better understand Env glycosylation and its role in bNAb recognition, we characterized a soluble, cleaved recombinant trimer (BG505 SOSIP.664) that is a close structural and antigenic mimic of native Env. Large, unprocessed oligomannose-type structures (Man8-9GlcNAc2) are notably prevalent on the gp120 components of the trimer, irrespective of the mammalian cell expression system or the bNAb used for affinity purification. In contrast, gp41 subunits carry more highly processed glycans. The glycans on uncleaved, non-native oligomeric gp140 proteins are also highly processed. A homogeneous, oligomannose-dominated glycan profile is therefore a hallmark of a native Env conformation and a potential Achilles' heel that can be exploited for bNAb recognition and vaccine design.
Project description:The extensive glycosylation of HIV-1 envelope (Env) glycoprotein leaves few glycan-free holes large enough to admit broadly neutralizing antibodies (bnAb). Consequently, most bnAbs must inevitably make some glycan contacts and avoid clashes with others. To investigate how Env glycan maturation regulates HIV sensitivity to bnAbs, we modified HIV-1 pseudovirus (PV) using various glycoengineering (GE) tools. Promoting the maturation of ?-2,6 sialic acid (SA) glycan termini increased PV sensitivity to two bnAbs that target the V2 apex and one to the interface between Env surface gp120 and transmembrane gp41 subunits, typically by up to 30-fold. These effects were reversible by incubating PV with neuraminidase. The same bnAbs were unusually potent against PBMC-produced HIV-1, suggesting similar ?-2,6 hypersialylated glycan termini may occur naturally. Overexpressing ?-galactosyltransferase during PV production replaced complex glycans with hybrid glycans, effectively 'thinning' trimer glycan coverage. This increased PV sensitivity to some bnAbs but ablated sensitivity to one bnAb that depends on complex glycans. Other bnAbs preferred small glycans or galactose termini. For some bnAbs, the effects of GE were strain-specific, suggesting that GE had context-dependent effects on glycan clashes. GE was also able to increase the percent maximum neutralization (i.e. saturation) by some bnAbs. Indeed, some bnAb-resistant strains became highly sensitive with GE-thus uncovering previously unknown bnAb breadth. As might be expected, the activities of bnAbs that recognize glycan-deficient or invariant oligomannose epitopes were largely unaffected by GE. Non-neutralizing antibodies were also unaffected by GE, suggesting that trimers remain compact. Unlike mature bnAbs, germline-reverted bnAbs avoided or were indifferent to glycans, suggesting that glycan contacts are acquired as bnAbs mature. Together, our results suggest that glycovariation can greatly impact neutralization and that knowledge of the optimal Env glycoforms recognized by bnAbs may assist rational vaccine design.
Project description: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.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.
Project description:The HIV-1 envelope gp160 glycoprotein (Env) is a trimer of gp120 and gp41 heterodimers that mediates cell entry and is the primary target of the humoral immune response. Broadly neutralizing antibodies (bNAbs) to HIV-1 have revealed multiple epitopes or sites of vulnerability, but mapping of most of these sites is incomplete owing to a paucity of structural information on the full epitope in the context of the Env trimer. Here, a crystal structure of the soluble BG505 SOSIP gp140 trimer at 4.6 Å resolution with the bNAbs 8ANC195 and PGT128 reveals additional interactions in comparison to previous antibody-gp120 structures. For 8ANC195, in addition to previously documented interactions with gp120, a substantial interface with gp41 is now elucidated that includes extensive interactions with the N637 glycan. Surprisingly, removal of the N637 glycan did not impact 8ANC195 affinity, suggesting that the antibody has evolved to accommodate this glycan without loss of binding energy. PGT128 indirectly affects the N262 glycan by a domino effect, in which PGT128 binds to the N301 glycan, which in turn interacts with and repositions the N262 glycan, thereby illustrating the important role of neighboring glycans on epitope conformation and stability. Comparisons with other Env trimer and gp120 structures support an induced conformation for glycan N262, suggesting that the glycan shield is allosterically modified upon PGT128 binding. These complete epitopes of two broadly neutralizing antibodies on the Env trimer can now be exploited for HIV-1 vaccine design.
Project description:Broadly neutralizing antibodies (bNAbs) to HIV-1 envelope glycoprotein (Env) can prevent infection in animal models. Characterized bNAb targets, although key to vaccine and therapeutic strategies, are currently limited. We defined a new site of vulnerability by solving structures of bNAb 8ANC195 complexed with monomeric gp120 by X-ray crystallography and trimeric Env by electron microscopy. The site includes portions of gp41 and N-linked glycans adjacent to the CD4-binding site on gp120, making 8ANC195 the first donor-derived anti-HIV-1 bNAb with an epitope spanning both Env subunits. Rather than penetrating the glycan shield by using a single variable-region CDR loop, 8ANC195 inserted its entire heavy-chain variable domain into a gap to form a large interface with gp120 glycans and regions of the gp120 inner domain not contacted by other bNAbs. By isolating additional 8ANC195 clonal variants, we identified a more potent variant, which may be valuable for therapeutic approaches using bNAb combinations with nonoverlapping epitopes.
Project description:The HIV envelope glycoprotein (Env) is extensively modified with host-derived N-linked glycans. The high density of glycosylation on the viral spike limits enzymatic processing, resulting in numerous underprocessed oligomannose-type glycans. This extensive glycosylation not only shields conserved regions of the protein from the immune system but also acts as a target for anti-HIV broadly neutralizing antibodies (bnAbs). In response to the host immune system, the HIV glycan shield is constantly evolving through mutations affecting both the positions and numbers of potential N-linked glycosylation sites (PNGSs). Here, using longitudinal Env sequences from a clade C-infected individual (CAP256), we measured the impact of the shifting glycan shield during HIV infection on the abundance of oligomannose-type glycans. By analyzing the intrinsic mannose patch from a panel of recombinant CAP256 gp120s displaying high protein sequence variability and changes in PNGS number and positioning, we show that the intrinsic mannose patch persists throughout the course of HIV infection and correlates with the number of PNGSs. This effect of the glycan density on the processing state was also supported by the analysis of a cross-clade panel of recombinant gp120 glycoproteins. Together, these observations underscore the importance of glycan clustering for the generation of carbohydrate epitopes for anti-HIV bnAbs. The persistence of the intrinsic mannose patch over the course of HIV infection further highlights this epitope as an important target for HIV vaccine strategies.<h4>Importance</h4>Development of an HIV vaccine is critical for control of the HIV pandemic, and elicitation of broadly neutralizing antibodies (bnAbs) is likely to be a key component of a successful vaccine response. The HIV envelope glycoprotein (Env) is covered in an array of host-derived N-linked glycans often referred to as the glycan shield. This glycan shield is a target for many of the recently isolated anti-HIV bnAbs and is therefore under constant pressure from the host immune system, leading to changes in both glycan site frequency and location. This study aimed to determine whether these genetic changes impacted the eventual processing of glycans on the HIV Env and the susceptibility of the virus to neutralization. We show that despite this variation in glycan site positioning and frequency over the course of HIV infection, the mannose patch is a conserved feature throughout, making it a stable target for HIV vaccine design.
Project description:Broadly neutralizing antibodies (bNAbs) isolated from HIV-1-infected individuals inform HIV-1 vaccine design efforts. Developing bNAbs with increased efficacy requires understanding how antibodies interact with the native oligomannose and complex-type N-glycan shield that hides most protein epitopes on HIV-1 envelope (Env). Here we present crystal structures, including a 3.8-Å X-ray free electron laser dataset, of natively glycosylated Env trimers complexed with BG18, the most potent V3/N332gp120 glycan-targeting bNAb reported to date. Our structures show conserved contacts mediated by common D gene-encoded residues with the N332gp120 glycan and the gp120 GDIR peptide motif, but a distinct Env-binding orientation relative to PGT121/10-1074 bNAbs. BG18's binding orientation provides additional contacts with N392gp120 and N386gp120 glycans near the V3-loop base and engages protein components of the V1-loop. The BG18-natively-glycosylated Env structures facilitate understanding of bNAb-glycan interactions critical for using V3/N332gp120 bNAbs therapeutically and targeting their epitope for immunogen design.
Project description:HIV-1 vaccine design is informed by structural studies elucidating mechanisms by which broadly neutralizing antibodies (bNAbs) recognize and/or accommodate N-glycans on the trimeric envelope glycoprotein (Env). Variability in high-mannose and complex-type Env glycoforms leads to heterogeneity that usually precludes visualization of the native glycan shield. We present 3.5-Å- and 3.9-Å-resolution crystal structures of the HIV-1 Env trimer with fully processed and native glycosylation, revealing a glycan shield of high-mannose and complex-type N-glycans, which we used to define complete epitopes of two bNAbs. Env trimer was complexed with 10-1074 (against the V3-loop) and IOMA, a new CD4-binding site (CD4bs) antibody. Although IOMA derives from VH1-2*02, the germline gene of CD4bs-targeting VRC01-class bNAbs, its light chain lacks the short CDRL3 that defines VRC01-class bNAbs. Thus IOMA resembles 8ANC131-class/VH1-46-derived CD4bs bNAbs, which have normal-length CDRL3s. The existence of bNAbs that combine features of VRC01-class and 8ANC131-class antibodies has implications for immunization strategies targeting VRC01-like bNAbs.
Project description:The elicitation of broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoprotein (Env) trimer remains a major vaccine challenge. Most cross-conserved protein determinants are occluded by self-N-glycan shielding, limiting B cell recognition of the underlying polypeptide surface. The exceptions to the contiguous glycan shield include the conserved receptor CD4 binding site (CD4bs) and glycoprotein (gp)41 elements proximal to the furin cleavage site. Accordingly, we performed heterologous trimer-liposome prime:boosting in rabbits to drive B cells specific for cross-conserved sites. To preferentially expose the CD4bs to B cells, we eliminated proximal N-glycans while maintaining the native-like state of the cleavage-independent NFL trimers, followed by gradual N-glycan restoration coupled with heterologous boosting. This approach successfully elicited CD4bs-directed, cross-neutralizing Abs, including one targeting a unique glycan-protein epitope and a bNAb (87% breadth) directed to the gp120:gp41 interface, both resolved by high-resolution cryoelectron microscopy. This study provides proof-of-principle immunogenicity toward eliciting bNAbs by vaccination.