Conformational characterization of aberrant disulfide-linked HIV-1 gp120 dimers secreted from overexpressing cells

Stoichiometry for entry and binding properties of the Env protein of R5 T cell-tropic HIV-1 and its evolutionary variant of macrophage-tropic HIV-1

Human immunodeficiency virus type 1 typically requires a high density of CD4 for efficient entry as a mechanism to target CD4+ T cells (T-tropic), with CCR5 being used most often as the coreceptor. When target T cells are limiting, the virus can evolve to infect cells with a low density of CD4 such as macrophages (M-tropic). The entry phenotype is known to be encoded in the viral Env protein on the surface of the virus particle. Using data showing a dose response for infectivity based on CD4 surface density, we built a model consistent with T-tropic viruses requiring multiple CD4 molecules to mediate infection, whereas M-tropic viruses can infect cells using a single CD4 receptor molecule interaction. We also found that T-tropic viruses bound to the surface of cells with a low density of CD4 are released more slowly than M-tropic viruses which we modeled to be due to multiple interactions of the T-tropic virus with multiple CD4 molecules to allow the initial stable binding. Finally, we found that some M-tropic Env proteins, as the gp120 subunit, possess an enhanced affinity for CD4 compared with their T-tropic pair, indicating that the evolution of macrophage tropism can be reflected both in the closed Env trimer conformation on the virion surface and, in some cases, also in the open confirmation of gp120 Env. Collectively, these studies reveal differences in the stoichiometry of interaction of T-tropic and M-tropic viruses with CD4 and start to identify the basis of binding differences at the biochemical level.

IMPORTANCE

Human immunodeficiency virus type 1 normally targets CD4+ T cells for viral replication. When T cells are limiting, the virus can evolve to infect myeloid cells. The evolutionary step involves a change from requiring a high surface density of CD4 for entry to being able to infect cells with a low density of CD4, as is found on myeloid lineage cells such as macrophage and microglia. Viruses able to infect macrophages efficiently are most often found in the CNS late in the disease course, and such viruses may contribute to neurocognitive impairment. Here, we examine the CD4 binding properties of the viral Env protein to explore these two different entry phenotypes.

Stabilizing the HIV-1 envelope glycoprotein State 2A conformation

The HIV-1 envelope glycoprotein (Env) trimer [(gp120/gp41)₃] is a metastable complex expressed at the surface of viral particles and infected cells that samples different conformations. Before engaging CD4, Env adopts an antibody-resistant "closed" conformation (State 1). CD4 binding triggers an intermediate conformation (State 2) and then a more "open" conformation (State 3) that can be recognized by non-neutralizing antibodies (nnAbs) such as those that recognize the coreceptor binding site (CoRBS). Binding of antibodies to the CoRBS permits another family of nnAbs, the anti-cluster A family of Abs which target the gp120 inner domain, to bind and stabilize an asymmetric conformation (State 2A). Cells expressing Env in this conformation are susceptible to antibody-dependent cellular cytotoxicity (ADCC). This conformation can be stabilized by small-molecule CD4 mimetics (CD4mc) or soluble CD4 (sCD4) in combination with anti-CoRBS Ab and anti-cluster A antibodies. The precise stoichiometry of each component that permits this sequential opening of Env remains unknown. Here, we used a cell-based ELISA (CBE) assay to evaluate each component individually. In this assay we used a "trimer mixing" approach by combining wild-type (wt) subunits with subunits impaired for CD4 or CoRBS Ab binding. This enabled us to show that State 2A requires all three gp120 subunits to be bound by sCD4/CD4mc and anti-CoRBS Abs. Two of these subunits can then bind anti-cluster A Abs. Altogether, our data suggests how this antibody vulnerable Env conformation is stabilized.Importance Stabilization of HIV-1 Env State 2A has been shown to sensitize infected cells to ADCC. State 2A can be stabilized by a "cocktail" composed of CD4mc, anti-CoRBS and anti-cluster A Abs. We present evidence that optimal State 2A stabilization requires all three gp120 subunits to be bound by both CD4mc and anti-CoRBS Abs. Our study provides valuable information on how to stabilize this ADCC-vulnerable conformation. Strategies aimed at stabilizing State 2A might have therapeutic utility.

A Systematic Approach to HIV-1 Vaccine Immunogen Selection

A tremendous loss of financial and human resources from seven large-scale HIV vaccine efficacy trials suggest a need for a systematic approach to vaccine selection. We conducted a systematic analysis of three important envelope glycoprotein (Env) vaccine candidates: BG505 SOSIP.664, 1086.C gp140, and 1086.C gp120 to determine the most promising by comparing their structure and antigenicity. We found that the BG505 SOSIP trimer and 1086.C gp140 clearly outperformed the 1086.C gp120 monomer. BG505 SOSIP.664 bound the strongest to the most potent and broadest broadly neutralizing antibodies (bnAbs) PG9, PGT145, VRC01, and PGT121. Of interest, although BG505 SOSIP.664 did not bind to the CH58 mAb, 1086.C gp140 bound strongly to this mAb, which belongs to a class of non-neutralizing antibodies that may be protective based on correlates of protection studies of the RV144 HIV vaccine trial. The 1086.C gp120 monomer was the least antigenic of the three vaccine immunogens, binding the weakest to bnAbs and CH58 mAb. Taken together, the evidence provided here combined with previous preclinical immunogenicity and efficacy data strongly argue that the BG505 SOSIP.664 trimer and 1086.C gp140 are likely to be better vaccine immunogens than the monomeric 1086.C gp120, which was just recently tested and shown to be nonefficacious in a phase IIb/III trial. Thus, to best utilize our financial and valuable human resources, we propose a systematic approach by not only comparing structure and antigenicity, but also immunogenicity and efficacy of Env vaccine candidates in the preclinical phase to the selection of only the most promising vaccine candidates for clinical testing.

Optimized Hepatitis C Virus (HCV) E2 Glycoproteins and their Immunogenicity in Combination with MVA-HCV

Hepatitis C virus (HCV) represents a major global health challenge and an efficient vaccine is urgently needed. Many HCV vaccination strategies employ recombinant versions of the viral E2 glycoprotein. However, recombinant E2 readily forms disulfide-bonded aggregates that might not be optimally suited for vaccines. Therefore, we have designed an E2 protein in which we strategically changed eight cysteines to alanines (E2.C8A). E2.C8A formed predominantly monomers and virtually no aggregates. Furthermore, E2.C8A also interacted more efficiently with broadly neutralizing antibodies than conventional E2. We used mice to evaluate different prime/boost immunization strategies involving a modified vaccinia virus Ankara (MVA) expressing the nearly full-length genome of HCV (MVA-HCV) in combination with either the E2 aggregates or the E2.C8A monomers. The combined MVA-HCV/E2 aggregates prime/boost strategy markedly enhanced HCV-specific effector memory CD4⁺ T cell responses and antibody levels compared to MVA-HCV/MVA-HCV. Moreover, the aggregated form of E2 induced higher levels of anti-E2 antibodies in vaccinated mice than E2.C8A monomers. These antibodies were cross-reactive and mainly of the IgG1 isotype. Our findings revealed how two E2 viral proteins that differ in their capacity to form aggregates are able to enhance to different extent the HCV-specific cellular and humoral immune responses, either alone or in combination with MVA-HCV. These combined protocols of MVA-HCV/E2 could serve as a basis for the development of a more effective HCV vaccine.

A Prime-Boost Immunization Strategy with Vaccinia Virus Expressing Novel gp120 Envelope Glycoprotein from a CRF02_AG Isolate Elicits Cross-Clade Tier 2 HIV-1 Neutralizing Antibodies

Development of new immunogens eliciting broadly neutralizing antibodies (bNAbs) is a main priority for the HIV-1 vaccine field. Envelope glycoproteins from non-B-non-C HIV-1clades have not been fully explored as components of a vaccine. We produced Vaccinia viruses expressing a truncated version of gp120 (gp120t) from HIV-1 clades CRF02_AG, H, J, B, and C and examined their immunogenicity in mice and rabbits. Mice primed with the recombinant Vaccinia viruses and boosted with the homologous gp120t or C2V3C3 polypeptides developed antibodies that bind potently to homologous and heterologous envelope glycoproteins. Notably, a subset of mice immunized with the CRF02_AG-based envelope immunogens developed a cross-reactive neutralizing response against tier 2 HIV-1 Env-pseudoviruses and primary isolates. Rabbits vaccinated with the CRF02_AG-based envelope immunogens also generated potent binding antibodies, and one animal elicited antibodies that neutralized almost all (13 of 16, 81.3%) tier 2 HIV-1 isolates tested. Overall, the results suggest that the novel CRF02_AG-based envelope immunogens and prime-boost immunization strategy elicit the type of immune responses required for a preventive HIV-1 vaccine.

Calcium Phosphate Nanoparticle-Based Vaccines as a Platform for Improvement of HIV-1 Env Antibody Responses by Intrastructural Help

Incorporation of immunodominant T-helper epitopes of licensed vaccines into virus-like particles (VLP) allows to harness T-helper cells induced by the licensed vaccines to provide intrastructural help (ISH) for B-cell responses against the surface proteins of the VLPs. To explore whether ISH could also improve antibody responses to calcium phosphate (CaP) nanoparticle vaccines we loaded the nanoparticle core with a universal T-helper epitope of Tetanus toxoid (p30) and functionalized the surface of CaP nanoparticles with stabilized trimers of the HIV-1 envelope (Env) resulting in Env-CaP-p30 nanoparticles. In contrast to soluble Env trimers, Env containing CaP nanoparticles induced activation of naïve Env-specific B-cells in vitro. Mice previously vaccinated against Tetanus raised stronger humoral immune responses against Env after immunization with Env-CaP-p30 than mice not vaccinated against Tetanus. The enhancing effect of ISH on anti-Env antibody levels was not attended with increased Env-specific IFN-γ CD4 T-cell responses that otherwise may potentially influence the susceptibility to HIV-1 infection. Thus, CaP nanoparticles functionalized with stabilized HIV-1 Env trimers and heterologous T-helper epitopes are able to recruit heterologous T-helper cells induced by a licensed vaccine and improve anti-Env antibody responses by intrastructural help.

Antibody-dependent cellular cytotoxicity in HIV infection

: Interactions between the Fc segment of IgG and its receptors (FcγRs) found on cells such as natural killer cells, monocytes, macrophages and neutrophils can potentially mediate antiviral effects in the setting of HIV and related infections. We review the potential role of FcγR interactions in HIV, SIV and SHIV infections, with an emphasis on antibody-dependent cellular cytotoxicity (ADCC). Notably, these viruses employ various strategies, including CD4 down-regulation and BST-2/tetherin antagonism to limit the effect of ADCC. Although correlative data suggest that ADCC participates in both protection and control of established infection, there is little direct evidence in support of either role. Direct evidence does, however, implicate an FcγR-dependent function in augmenting the beneficial in vivo activity of neutralizing antibodies.

cGMP production and analysis of BG505 SOSIP.664, an extensively glycosylated, trimeric HIV-1 envelope glycoprotein vaccine candidate

We describe the properties of BG505 SOSIP.664 HIV‐1 envelope glycoprotein trimers produced under current Good Manufacturing Practice (cGMP) conditions. These proteins are the first of a new generation of native‐like trimers that are the basis for many structure‐guided immunogen development programs aimed at devising how to induce broadly neutralizing antibodies (bNAbs) to HIV‐1 by vaccination. The successful translation of this prototype demonstrates the feasibility of producing similar immunogens on an appropriate scale and of an acceptable quality for Phase I experimental medicine clinical trials. BG505 SOSIP.664 trimers are extensively glycosylated, contain numerous disulfide bonds and require proteolytic cleavage, all properties that pose a substantial challenge to cGMP production. Our strategy involved creating a stable CHO cell line that was adapted to serum‐free culture conditions to produce envelope glycoproteins. The trimers were then purified by chromatographic methods using a 2G12 bNAb affinity column and size‐exclusion chromatography. The chosen procedures allowed any adventitious viruses to be cleared from the final product to the required extent of >12 log₁₀. The final cGMP production run yielded 3.52 g (peptidic mass) of fully purified trimers (Drug Substance) from a 200 L bioreactor, a notable yield for such a complex glycoprotein. The purified trimers were fully native‐like as judged by negative‐stain electron microscopy, and were stable over a multi‐month period at room temperature or below and for at least 1 week at 50°C. Their antigenicity, disulfide bond patterns, and glycan composition were consistent with trimers produced on a research laboratory scale. The methods reported here should pave the way for the cGMP production of other native‐like Env glycoprotein trimers of various designs and genotypes.

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.

A Highly Conserved gp120 Inner Domain Residue Modulates Env Conformation and Trimer Stability

Previous studies have shown that highly conserved residues in the inner domain of gp120 are required for HIV-1 envelope glycoprotein (Env) transitions to the CD4-bound conformation (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656-667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012; A. Desormeaux, M. Coutu, H. Medjahed, B. Pacheco, A. Herschhorn, et al., J Virol 87:2549-2562, 2013, http://dx.doi.org/10.1128/JVI.03104-12). Moreover, W69, a highly conserved residue located at the interface between layer 1 and layer 2 of the inner domain, was recently shown to be important for efficient Env recognition by CD4-induced (CD4i) antibodies capable of potent antibody-dependent cellular cytotoxicity (W. D. Tolbert, N. Gohain, M. Veillette, J. P. Chapleau, C. Orlandi, et al., 2016, Structure 24:697-709, http://dx.doi.org/10.1016/j.str.2016.03.005; S. Ding, M. Veillette, M. Coutu, J. Prevost, L. Scharf, et al., 2016, J Virol 90:2127-2134, http://dx.doi.org/10.1128/JVI.02779-15). We evaluated the contribution of the hydrophobicity of W69 to conformational changes of Env by replacing it with a series of residues with aliphatic or aromatic side chains of decreasing chain length. We have found that the hydrophobicity of residue 69 is important for Env processing, CD4 binding, and its transition to the CD4-bound conformation. The most deleterious effect was observed when W69 was replaced with alanine or glycine residues. However, the functions lost due to W69 mutations could be progressively restored with amino acids of increasing aliphatic chain length and fully recovered with residues bearing an aromatic ring. Interestingly, poor CD4 binding of W69A could be fully restored by introducing a compensatory mutation within layer 2 (S115W). Structural studies of HIV-1 gp120 coree W69A/S115W mutant bound to the CD4 peptide mimetic M48U1 and Fab of anti-cluster A antibody N60-i3 revealed no perturbations to the overall structure of the double mutant compared to the wild-type protein but identified higher mobility within the interface between layer 1 and layer 2, the bridging sheet region, and the CD4 binding site.IMPORTANCE HIV-1 Env transitions to the CD4-bound conformation are required for viral entry. Previous studies identified a highly conserved residue of the inner domain, W69, as being involved in these conformational transitions (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656-667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012). Here, we show that W69, located at the interface between gp120 and gp41 in the PGT151-bound trimer, plays a critical role in the interprotomer signaling induced by CD4 binding. This new information might be useful in immunogen design.

Generation and Characterization of a Bivalent HIV-1 Subtype C gp120 Protein Boost for Proof-of-Concept HIV Vaccine Efficacy Trials in Southern Africa

The viral envelope glycoprotein (Env) is the major target for antibody (Ab)-mediated vaccine development against the Human Immunodeficiency Virus type 1 (HIV-1). Although several recombinant Env antigens have been evaluated in clinical trials, only the surface glycoprotein, gp120, (from HIV-1 subtype B, MN, and subtype CRF_01AE, A244) used in the ALVAC prime-AIDSVAX gp120 boost RV144 Phase III HIV vaccine trial was shown to contribute to protective efficacy, although modest and short-lived. Hence, for clinical trials in southern Africa, a bivalent protein boost of HIV-1 subtype C gp120 antigens composed of two complementary gp120s, from the TV1.C (chronic) and 1086.C (transmitted founder) HIV-1 strains, was selected. Stable Chinese Hamster Cell (CHO) cell lines expressing these gp120s were generated, scalable purification methods were developed, and a detailed analytical analysis of the purified proteins was conducted that showed differences and complementarity in the antigenicity, glycan occupancy, and glycan content of the two gp120 molecules. Moreover, mass spectrometry revealed some disulfide heterogeneity in the expressed proteins, particularly in V1V2-C1 region and most prominently in the TV1 gp120 dimers. These dimers not only lacked binding to certain key CD4 binding site (CD4bs) and V1V2 epitope-directed ligands but also elicited reduced Ab responses directed to those epitopes, in contrast to monomeric gp120, following immunization of rabbits. Both monomeric and dimeric gp120s elicited similarly high titer Tier 1 neutralizing Abs as measured in standard virus neutralization assays. These results provide support for clinical evaluations of bivalent preparations of purified monomeric TV1.C and 1086.C gp120 proteins.

Comprehensive Characterization of Reference Standard Lots of HIV-1 Subtype C Gp120 Proteins for Clinical Trials in Southern African Regions

Two HIV-1 subtype C gp120 protein candidates were the selected antigens for several experimental vaccine regimens now under evaluation in HVTN 100 Phase I/II clinical trial aiming to support the start of the HVTN 702 Phase IIb/III trial in southern Africa, which is designed to confirm and extend the partial protection seen against HIV-1 infection in the RV144 Thai trial. Here, we report the comprehensive physicochemical characterization of the gp120 reference materials that are representative of the clinical trial materials. Gp120 proteins were stably expressed in Chinese Hamster Ovary (CHO) cells and subsequently purified and formulated. A panel of analytical techniques was used to characterize the physicochemical properties of the two protein molecules. When formulated in the AS01 Adjuvant System, the bivalent subtype C gp120 antigens elicited 1086.C- and TV1.C-specific binding antibody and CD4+ T cell responses in mice. All the characteristics were highly representative of the Clinical Trial Materials (CTM). Data from this report demonstrate the immunogenicity of the gp120 antigens, provide comprehensive characterization of the molecules, set the benchmark for assessment of current and future CTM lots, and lay the physicochemical groundwork for interpretation of future clinical trial data.

Range of CD4-Bound Conformations of HIV-1 gp120, as Defined Using Conditional CD4-Induced Antibodies

The HIV envelope binds cellular CD4 and undergoes a range of conformational changes that lead to membrane fusion and delivery of the viral nucleocapsid into the cellular cytoplasm. This binding to CD4 reveals cryptic and highly conserved epitopes, the molecular nature of which is still not fully understood. The atomic structures of CD4 complexed with gp120 core molecules (a form of gp120 in which the V1, V2, and V3 loops and N and C termini have been truncated) have indicated that a hallmark feature of the CD4-bound conformation is the bridging sheet minidomain. Variations in the orientation of the bridging sheet hairpins have been revealed when CD4-liganded gp120 was compared to CD4-unliganded trimeric envelope structures. Hence, there appears to be a number of conformational transitions possible in HIV-1 monomeric gp120 that are affected by CD4 binding. The spectrum of CD4-bound conformations has been interrogated in this study by using a well-characterized panel of conditional, CD4-induced (CD4i) monoclonal antibodies (MAbs) that bind HIV-1 gp120 and its mutations under various conditions. Two distinct CD4i epitopes of the outer domain were studied: the first comprises the bridging sheet, while the second contains elements of the V2 loop. Furthermore, we show that the unliganded extended monomeric core of gp120 (coree) assumes an intermediate CD4i conformation in solution that further undergoes detectable rearrangements upon association with CD4. These discoveries impact both accepted paradigms concerning gp120 structure and the field of HIV immunogen design.

IMPORTANCE

Elucidation of the conformational transitions that the HIV-1 envelope protein undergoes during the course of entry into CD4(+)cells is fundamental to our understanding of HIV biology. The binding of CD4 triggers a range of gp120 structural rearrangements that could present targets for future drug design and development of preventive vaccines. Here we have systematically interrogated and scrutinized these conformational transitions using a panel of antibody probes that share a specific preference for the CD4i conformations. These have been employed to study a collection of gp120 mutations and truncations. Through these analyses, we propose 4 distinct sequential steps in CD4i transitions of gp120 conformations, each defined by antibody specificities and structural requirements of the HIV envelope monomer. As a result, we not only provide new insights into this dynamic process but also define probes to further investigate HIV infection.

Native Conformation and Canonical Disulfide Bond Formation Are Interlinked Properties of HIV-1 Env Glycoproteins

We investigated whether there is any association between a native-like conformation and the presence of only the canonical (i.e., native) disulfide bonds in the gp120 subunits of a soluble recombinant human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein. We used a mass spectrometry (MS)-based method to map the disulfide bonds present in nonnative uncleaved gp140 proteins and native-like SOSIP.664 trimers based on the BG505 env gene. Our results show that uncleaved gp140 proteins were not homogeneous, in that substantial subpopulations (20 to 80%) contained aberrant disulfide bonds. In contrast, the gp120 subunits of the native-like SOSIP.664 trimer almost exclusively retained the canonical disulfide bond pattern. We also observed that the purification method could influence the proportion of an Env protein population that contained aberrant disulfide bonds. We infer that gp140 proteins may always contain a variable but substantial proportion of aberrant disulfide bonds but that the impact of this problem can be minimized via design and/or purification strategies that yield native-like trimers. The same factors may also be relevant to the production and purification of monomeric gp120 proteins that are free of aberrant disulfide bonds.

IMPORTANCE

It is widely thought that a successful HIV-1 vaccine will include a recombinant form of the Env protein, a trimer located on the virion surface. To increase yield and simplify purification, Env proteins are often made in truncated, soluble forms. A consequence, however, can be the loss of the native conformation concomitant with the virion-associated trimer. Moreover, some soluble recombinant Env proteins contain aberrant disulfide bonds that are not expected to be present in the native trimer. To assess whether these observations are linked, to determine the extent of disulfide bond scrambling, and to understand why scrambling occurs, we determined the disulfide bond profiles of two soluble Env proteins with different designs that are being assessed as vaccine candidates. We found that uncleaved gp140 forms heterogeneous mixtures in which aberrant disulfide bonds abound. In contrast, BG505 SOSIP.664 trimers are more homogeneous, native-like entities that contain predominantly the native disulfide bond profile.

Targeting HIV Reservoir in Infected CD4 T Cells by Dual-Affinity Re-targeting Molecules (DARTs) that Bind HIV Envelope and Recruit Cytotoxic T Cells

HIV reservoirs and production of viral antigens are not eliminated in chronically infected participants treated with combination antiretroviral therapy (cART). Novel therapeutic strategies aiming at viral reservoir elimination are needed to address chronic immune dysfunction and non-AIDS morbidities that exist despite effective cART. The HIV envelope protein (Env) is emerging as a highly specific viral target for therapeutic elimination of the persistent HIV-infected reservoirs via antibody-mediated cell killing. Dual-Affinity Re-Targeting (DART) molecules exhibit a distinct mechanism of action via binding the cell surface target antigen and simultaneously engaging CD3 on cytotoxic T lymphocytes (CTLs). We designed and evaluated Env-specific DARTs (HIVxCD3 DARTs) derived from known antibodies recognizing diverse Env epitopes with or without broadly neutralizing activity. HIVxCD3 DARTs derived from PGT121, PGT145, A32, and 7B2, but not VRC01 or 10E8 antibodies, mediated potent CTL-dependent killing of quiescent primary CD4 T cells infected with diverse HIV isolates. Similar killing activity was also observed with DARTs structurally modified for in vivo half-life extension. In an ex vivo model using cells isolated from HIV-infected participants on cART, combinations of the most potent HIVxCD3 DARTs reduced HIV expression both in quiescent and activated peripheral blood mononuclear cell cultures isolated from HIV-infected participants on suppressive cART. Importantly, HIVxCD3 DARTs did not induce cell-to-cell virus spread in resting or activated CD4 T cell cultures. Collectively, these results provide support for further development of HIVxCD3 DARTs as a promising therapeutic strategy for targeting HIV reservoirs.

Current HIV therapies prevent AIDS by dramatically reducing, but not eliminating, HIV infection. A reservoir of HIV-infected cells persists during long-term antiviral therapy, and individuals are at increased risk to develop non-AIDS illnesses, e.g., accelerated heart, bone, or kidney disease. Novel strategies are thus needed to safely kill HIV-infected cells and reduce or eliminate the HIV reservoir. An emerging strategy to kill HIV-infected cells involves antibodies (Abs) that bind the HIV envelope protein (Env). Env can distinguish HIV-infected cells from uninfected cells, and some Env-specific Abs can kill HIV-infected cells by recruiting immune cells, e.g., NK cells and macrophages. Here, we developed a strategy to kill HIV-infected cells that is complementary to Env-specific Abs. We designed and evaluated Dual-Affinity Re-Targeting (DART) molecules that incorporate Env-binding specificities with a CD3-binding specificity to recruit and activate cytotoxic T cells. We report that HIVxCD3 DARTs potently and selectively kill HIV-infected cells. Furthermore, HIV DARTs perturb resting and activated viral reservoirs in cells isolated from individuals on antiviral therapy. This novel strategy may be an important element of future antiviral therapies that target the HIV reservoir.

Influences on the Design and Purification of Soluble, Recombinant Native-Like HIV-1 Envelope Glycoprotein Trimers

We have investigated factors that influence the production of native-like soluble, recombinant trimers based on the env genes of two isolates of human immunodeficiency virus type 1 (HIV-1), specifically 92UG037.8 (clade A) and CZA97.012 (clade C). When the recombinant trimers based on the env genes of isolates 92UG037.8 and CZA97.012 were made according to the SOSIP.664 design and purified by affinity chromatography using broadly neutralizing antibodies (bNAbs) against quaternary epitopes (PGT145 and PGT151, respectively), the resulting trimers are highly stable and they are fully native-like when visualized by negative-stain electron microscopy. They also have a native-like (i.e., abundant) oligomannose glycan composition and display multiple bNAb epitopes while occluding those for nonneutralizing antibodies. In contrast, uncleaved, histidine-tagged Foldon (Fd) domain-containing gp140 proteins (gp140UNC-Fd-His), based on the same env genes, very rarely form native-like trimers, a finding that is consistent with their antigenic and biophysical properties and glycan composition. The addition of a 20-residue flexible linker (FL20) between the gp120 and gp41 ectodomain (gp41ECTO) subunits to make the uncleaved 92UG037.8 gp140-FL20 construct is not sufficient to create a native-like trimer, but a small percentage of native-like trimers were produced when an I559P substitution in gp41ECTO was also present. The further addition of a disulfide bond (SOS) to link the gp120 and gp41 subunits in the uncleaved gp140-FL20-SOSIP protein increases native-like trimer formation to ∼20 to 30%. Analysis of the disulfide bond content shows that misfolded gp120 subunits are abundant in uncleaved CZA97.012 gp140UNC-Fd-His proteins but very rare in native-like trimer populations. The design and stabilization method and the purification strategy are, therefore, all important influences on the quality of trimeric Env proteins and hence their suitability as vaccine components.

IMPORTANCE

Soluble, recombinant multimeric proteins based on the HIV-1 env gene are current candidate immunogens for vaccine trials in humans. These proteins are generally designed to mimic the native trimeric envelope glycoprotein (Env) that is the target of virus-neutralizing antibodies on the surfaces of virions. The underlying hypothesis is that an Env-mimetic protein may be able to induce antibodies that can neutralize the virus broadly and potently enough for a vaccine to be protective. Multiple different designs for Env-mimetic trimers have been put forth. Here, we used the CZA97.012 and 92UG037.8 env genes to compare some of these designs and determine which ones best mimic virus-associated Env trimers. We conclude that the most widely used versions of CZA97.012 and 92UG037.8 oligomeric Env proteins do not resemble the trimeric Env glycoprotein on HIV-1 viruses, which has implications for the design and interpretation of ongoing or proposed clinical trials of these proteins.

HIV-1 gp120 dimers decrease the overall affinity of gp120 preparations for CD4-induced ligands

For several years, tools to study the conformational changes of HIV-1 envelope glycoproteins have been developed in order to comprehend those changes and their role in the fusion process and immunogenicity of HIV-1. To facilitate these studies, expression of the HIV-1 gp120 envelope glycoprotein has been done in several over-expression settings. However, over-expression of HIV-1 gp120 in mammalian cells leads to the formation of aberrant disulfide-linked dimers that can bias the results of experiments aimed at measuring gp120 affinity with different ligands. The presence of these gp120 dimers, generated in a widely used gp120 expression system, affects the affinity of gp120 for CD4-induced ligands, as evaluated by surface plasmon resonance. Upon monomeric gp120 purification, neither the removal of potential glycosylation sites on V4 nor the removal of the V5 variable region affect the overall affinity of gp120 for 17b and A32 CD4-induced ligands. Removal of these aberrant disulfide-linked gp120 dimers by standard size exclusion chromatography is sufficient to restore the overall affinity of gp120 preparations for these ligands.

Engineering and exploitation of a fluorescent HIV-1 gp120 for live cell CD4 binding assays

The HIV-1 envelope glycoprotein, gp120, binds the host cell receptor, CD4, in the initial step of HIV viral entry and infection. This process is an appealing target for the development of inhibitory drugs and neutralizing antibodies. To study gp120 binding and intracellular trafficking, we engineered a fluorescent fusion of the humanized gp120 JRFL HIV-1 variant and GFP. Gp120-sfGFP is glycosylated with human sugars, robustly expressed, and secreted from cultured human cells. Protein dynamics, quality control, and trafficking can be visualized in live cells. The fusion protein can be readily modified with different gp120 variants or fluorescent proteins. Finally, secreted gp120-sfGFP enables a sensitive and easy binding assay that can quantitatively screen potential inhibitors of gp120-CD4 binding on live cells via fluorescence imaging or laser scanning cytometry. This adaptable research tool should aid in studies of gp120 cell biology and the development of novel anti-HIV drugs.

Glycosylation and disulfide bond analysis of transiently and stably expressed clade C HIV-1 gp140 trimers in 293T cells identifies disulfide heterogeneity present in both proteins and differences in O-linked glycosylation

The HIV-1 envelope protein (Env) mediates viral entry into host cells to initiate infection and is the sole target of antibody-based vaccine development. Significant efforts have been made toward the design, engineering, and expression of various soluble forms of HIV Env immunogen, yet a highly effective immunogen remains elusive. One of the key challenges in the development of an effective HIV vaccine is the presence of the complex set of post-translational modifications (PTMs) on Env, namely, glycosylation and disulfide bonds, that affect protein folding, epitope accessibility, and immunogenecity. Although these PTMs vary with expression systems, variations in Env's PTMs due to changes in the expression method are not yet well established. In this study, we compared the disulfide bond network and glycosylation profiles of clade C recombinant HIV-1 Env trimers, C97ZA012 gp140, expressed by stable and transient transfections using an integrated mass mapping workflow that combines collision induced dissociation (CID) and electron transfer dissociation (ETD). Site-specific analysis of the N- and O-glycosylation profiles revealed that C97ZA012 gp140 produced by both transfection methods displayed a high degree of similarity in N-glycosylation profiles and site occupancy except for one site. By contrast, different O-glycosylation profiles were detected. Analysis of the disulfide bond networks of the Env revealed that both transfection methods yielded C97ZA012 gp140 adopting the expected disulfide bond pattern identified for the monomeric gp120 and gp41 as well as alternative disulfide bond patterns in the C1, V1/V2, and C2 regions. The finding that disulfide bonding is consistently heterogeneous in these proteins is perhaps the most significant outcome of these studies; this disulfide heterogeneity has been reported for multiple other recombinant gp140s, and it is likely present in most recombinantly expressed Env immunogens.

Differential binding of neutralizing and non-neutralizing antibodies to native-like soluble HIV-1 Env trimers, uncleaved Env proteins, and monomeric subunits

Background

The trimeric envelope glycoproteins (Env) on the surface of HIV-1 virions are the targets for neutralizing antibodies (NAbs). No candidate HIV-1 immunogen has yet induced potent, broadly active NAbs (bNAbs). Part of the explanation may be that previously tested Env proteins inadequately mimic the functional, native Env complex. Trimerization and the proteolytic processing of Env precursors into gp120 and gp41 profoundly alter antigenicity, but soluble cleaved trimers are too unstable to serve as immunogens. By introducing stabilizing mutations (SOSIP), we constructed soluble, cleaved Env trimers derived from the HIV-1 subtype A isolate BG505 that resemble native Env spikes on virions both structurally and antigenically.

Results

We used surface plasmon resonance (SPR) to quantify antibody binding to different forms of BG505 Env: the proteolytically cleaved SOSIP.664 trimers, cleaved gp120-gp41_ECTO protomers, and gp120 monomers. Non-NAbs to the CD4-binding site bound only marginally to the trimers but equally well to gp120-gp41_ECTO protomers and gp120 monomers, whereas the bNAb VRC01, directed to the CD4bs, bound to all three forms. In contrast, bNAbs to V1V2 glycan-dependent epitopes bound preferentially (PG9 and PG16) or exclusively (PGT145) to trimers. We also explored the antigenic consequences of three different features of SOSIP.664 gp140 trimers: the engineered inter-subunit disulfide bond, the trimer-stabilizing I559P change in gp41_ECTO, and proteolytic cleavage at the gp120-gp41_ECTO junction. Each of these three features incrementally promoted native-like trimer antigenicity. We compared Fab and IgG versions of bNAbs and validated a bivalent model of IgG binding. The NAbs showed widely divergent binding kinetics and degrees of binding to native-like BG505 SOSIP.664. High off-rate constants and low stoichiometric estimates of NAb binding were associated with large amounts of residual infectivity after NAb neutralization of the corresponding BG505.T332N pseudovirus.

Conclusions

The antigenicity and structural integrity of cleaved BG505 SOSIP.664 trimers render these proteins good mimics of functional Env spikes on virions. In contrast, uncleaved gp140s antigenically resemble individual gp120-gp41_ECTO protomers and gp120 monomers, but not native trimers. Although NAb binding to functional trimers may thus be both necessary and sufficient for neutralization, the kinetics and stoichiometry of the interaction influence the neutralizing efficacy of individual NAbs.

A functional interaction between gp41 and gp120 is observed for monomeric but not oligomeric, uncleaved HIV-1 Env gp140

The envelope glycoprotein (Env) is the sole antigenic feature on the surface of HIV and the target for the humoral immune system. Soluble, uncleaved gp140 Env constructs truncated at the transmembrane domain are being investigated intensively as potential vaccine immunogens by many groups, and understanding their structural properties is essential. We used hydrogen/deuterium-exchange mass spectrometry and small-angle X-ray scattering to probe structural order in a panel of commonly used gp140 constructs and matched gp120 monomers. We observed that oligomeric forms of uncleaved gp140, generally presumed to be trimeric, contain a protease-resistant form of gp41 akin to the postfusion, helical bundle conformation and appear to lack specific interactions between gp120 and gp41. In contrast, the monomeric form of gp140 shows significant stabilization of the gp120 inner domain imparted by the gp41 region, demonstrating excellent agreement with past mutagenesis studies. Moreover, the gp140 monomers respond to CD4 binding in manner that is consistent with the initial stages of Env activation: CD4 binding induces structural ordering throughout gp120 while loosening its association with gp41. The results indicate that uncleaved gp140 oligomers do not represent an authentic prefusion form of Env, whereas gp140 monomers isolated from the same glycoprotein preparations in many ways exhibit function and internal structural order that are consistent with expectations for certain aspects of native Env. gp140 monomers may thus be a useful reagent for advancing structural and functional studies.

Recognition of synthetic glycopeptides by HIV-1 broadly neutralizing antibodies and their unmutated ancestors

Current HIV-1 vaccines elicit strain-specific neutralizing antibodies. Broadly neutralizing antibodies (BnAbs) are not induced by current vaccines, but are found in plasma in ∼20% of HIV-1-infected individuals after several years of infection. One strategy for induction of unfavored antibody responses is to produce homogeneous immunogens that selectively express BnAb epitopes but minimally express dominant strain-specific epitopes. Here we report that synthetic, homogeneously glycosylated peptides that bind avidly to variable loop 1/2 (V1V2) BnAbs PG9 and CH01 bind minimally to strain-specific neutralizing V2 antibodies that are targeted to the same envelope polypeptide site. Both oligomannose derivatization and conformational stabilization by disulfide-linked dimer formation of synthetic V1V2 peptides were required for strong binding of V1V2 BnAbs. An HIV-1 vaccine should target BnAb unmutated common ancestor (UCA) B-cell receptors of naïve B cells, but to date no HIV-1 envelope constructs have been found that bind to the UCA of V1V2 BnAb PG9. We demonstrate herein that V1V2 glycopeptide dimers bearing Man5GlcNAc2 glycan units bind with apparent nanomolar affinities to UCAs of V1V2 BnAbs PG9 and CH01 and with micromolar affinity to the UCA of a V2 strain-specific antibody. The higher-affinity binding of these V1V2 glycopeptides to BnAbs and their UCAs renders these glycopeptide constructs particularly attractive immunogens for targeting subdominant HIV-1 envelope V1V2-neutralizing antibody-producing B cells.

A next-generation cleaved, soluble HIV-1 Env trimer, BG505 SOSIP.664 gp140, expresses multiple epitopes for broadly neutralizing but not non-neutralizing antibodies

A desirable but as yet unachieved property of a human immunodeficiency virus type 1 (HIV-1) vaccine candidate is the ability to induce broadly neutralizing antibodies (bNAbs). One approach to the problem is to create trimeric mimics of the native envelope glycoprotein (Env) spike that expose as many bNAb epitopes as possible, while occluding those for non-neutralizing antibodies (non-NAbs). Here, we describe the design and properties of soluble, cleaved SOSIP.664 gp140 trimers based on the subtype A transmitted/founder strain, BG505. These trimers are highly stable, more so even than the corresponding gp120 monomer, as judged by differential scanning calorimetry. They are also homogenous and closely resemble native virus spikes when visualized by negative stain electron microscopy (EM). We used several techniques, including ELISA and surface plasmon resonance (SPR), to determine the relationship between the ability of monoclonal antibodies (MAbs) to bind the soluble trimers and neutralize the corresponding virus. In general, the concordance was excellent, in that virtually all bNAbs against multiple neutralizing epitopes on HIV-1 Env were highly reactive with the BG505 SOSIP.664 gp140 trimers, including quaternary epitopes (CH01, PG9, PG16 and PGT145). Conversely, non-NAbs to the CD4-binding site, CD4-induced epitopes or gp41ECTO did not react with the trimers, even when their epitopes were present on simpler forms of Env (e.g. gp120 monomers or dissociated gp41 subunits). Three non-neutralizing MAbs to V3 epitopes did, however, react strongly with the trimers but only by ELISA, and not at all by SPR and to only a limited extent by EM. These new soluble trimers are useful for structural studies and are being assessed for their performance as immunogens.

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.

Topological analysis of HIV-1 glycoproteins expressed in situ on virus surfaces reveals tighter packing but greater conformational flexibility than for soluble gp120

In natural infection, antibodies interact with HIV-1 primarily through nonfunctional forms of envelope glycoproteins (Env), including uncleaved (UNC) gp160 and gp41 stumps. These antigens are important to fully characterize, as they may be decoys that promote nonneutralizing responses and may also be targets for nonneutralizing effector responses. In this study, we compared the antigenic properties of Env expressed in situ on pseudovirion virus-like particle (VLP) surfaces and soluble gp120 using harmonized enzyme-linked immunosorbent assays (ELISAs) and a panel of 51 monoclonal antibodies (MAbs). Only 32 of 46 soluble gp120-reactive MAbs recognized the primary UNC gp160 antigen of VLPs. Indeed, many epitopes were poorly exposed (C1, V2, C1-C4, C4, C4-V3, CD4 induced [CD4i], and PGT group 3) or obscured (C2, C5, and C1-C5) on VLPs. In further studies, VLP Env exhibited an increased degree of inter-MAb competition, the epicenter of which was the base of the V3 loop, where PGT, 2G12, V3, and CD4 binding site specificities competed. UNC gp160 also underwent more drastic soluble CD4 (sCD4)-induced conformational changes than soluble gp120, exposing CD4i, C1-C4, and V2 epitopes. A greater propensity of UNC gp160 to undergo conformational changes was also suggested by the induction of CD4i MAb binding to VLPs by a V3 MAb as well as by soluble CD4. The same effect was not observed for soluble gp120. Taken together, our data suggest that membrane-expressed UNC gp160 exists in a less "triggered" conformational state than soluble gp120 and that MAb binding to UNC gp160 tends to have greater conformational consequences.

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.

Identification of an HIV-1 clade A envelope that exhibits broad antigenicity and neutralization sensitivity and elicits antibodies targeting three distinct epitopes

Broadly neutralizing antibodies (bNAbs) PG9 and PG16 were isolated from an International AIDS Vaccine Initiative (IAVI) Protocol G subject infected with human immunodeficiency virus type 1 (HIV-1) clade A. Both antibodies are highly potent and neutralize greater than 70% of viruses tested. We sought to begin immunogen design based on viral sequences from this patient; however, pseudoviruses prepared with 19 envelope sequences from this subject were resistant to neutralization by PG9 and PG16. Therefore, we used a bioinformatics approach to identify closely related viruses that were potentially sensitive to PG9 and PG16. A most-recent common ancestor (MRCA) sequence for the viral envelope (Env) was determined and aligned with 99 subtype A gp160 sequences from the Los Alamos HIV database. Virus BG505.W6M.ENV.C2 (BG505) was found to have the highest degree of homology (73%) to the MRCA sequence. Pseudoviruses prepared with this Env were sensitive to neutralization with a broad panel of bNAbs, including PG9 and PG16. When expressed by 293T cells as soluble gp120, the BG505 monomer bound well to both PG9 and PG16. We further showed that a point mutation (L111A) enabled more efficient production of a stable gp120 monomer that preserves the major neutralization epitopes. Finally, we showed that an adjuvanted formulation of this gp120 protein elicited neutralizing antibodies in rabbits (following a gp120 DNA vaccine prime) and that the antisera competed with bNAbs from 3 classes of nonoverlapping epitopes. Thus, the BG505 Env protein warrants further investigation as an HIV vaccine candidate, as a stand-alone protein, or as a component of a vaccine vector.

The highly conserved layer-3 component of the HIV-1 gp120 inner domain is critical for CD4-required conformational transitions

The trimeric envelope glycoprotein (Env) of human immunodeficiency virus type 1 (HIV-1) mediates virus entry into host cells. CD4 engagement with the gp120 exterior envelope glycoprotein subunit represents the first step during HIV-1 entry. CD4-induced conformational changes in the gp120 inner domain involve three potentially flexible topological layers (layers 1, 2, and 3). Structural rearrangements between layer 1 and layer 2 have been shown to facilitate the transition of the envelope glycoprotein trimer from the unliganded to the CD4-bound state and to stabilize gp120-CD4 interaction. However, our understanding of CD4-induced conformational changes in the gp120 inner domain remains incomplete. Here, we report that a highly conserved element of the gp120 inner domain, layer 3, plays a pivot-like role in these allosteric changes. In the unliganded state, layer 3 modulates the association of gp120 with the Env trimer, probably by influencing the relationship of the gp120 inner and outer domains. Importantly, layer 3 governs the efficiency of the initial gp120 interaction with CD4, a function that can also be fulfilled by filling the Phe43 cavity. This work defines the functional importance of layer 3 and completes a picture detailing the role of the gp120 inner domain in CD4-induced conformational transitions in the HIV-1 Env trimer.

Escherichia coli-expressed near full length HIV-1 envelope glycoprotein is a highly sensitive and specific diagnostic antigen

Background

The Human Immunodeficiency Virus type 1 (HIV-1) envelope glycoprotein gp160, useful in detecting anti-HIV-1 antibodies, is difficult to express in heterologous hosts. The major hurdles are its signal sequence, strong hydrophobic regions and heavy glycosylation. While it has not been possible to express full length recombinant (r)-gp160 in E. coli, it can be expressed in insect and mammalian cells, but at relatively higher cost. In this work, we report E. coli-based over-expression of r-gp160 variant and evaluate its performance in diagnostic immunoassays for the detection of anti-HIV-1 antibodies.

Methods

A deletion variant of r-gp160 lacking hydrophobic regions of the parent full length molecule was expressed in E. coli and purified to near homogeneity using single-step Ni(II)-affinity chromatography. Biotinylated and europium(III) chelate-labeled versions of this antigen were used to set up one- and two-step time-resolved fluorometric double antigen sandwich assays. The performance of these assays was evaluated against a collection of well-characterized human sera (n=131), that included an in-house panel and four commercially procured panels.

Results

In-frame deletion of three hydrophobic regions, spanning amino acid residues 1–43, 519–538 and 676–706, of full length HIV-1 gp160 resulted in its expression in E. coli. Both the one- and two-step assays manifested high sensitivity unambiguously identifying 75/77 and 77/77 HIV-1 positive sera, respectively. Both assays also identified all 52 HIV-seronegative sera correctly. Between the two assays, the mean signal-to-cutoff value of the two-step assay was an order of magnitude greater than that of the one-step assay. Both assays were highly specific manifesting no cross-reactivity towards antibodies specific to other viruses like hepatitis B, C, and human T cell leukemia viruses.

Conclusions

This study has demonstrated the expression of r-gp160 variant in E. coli, by deletion of hydrophobic regions, and its purification in reasonable yields. This underscores the potential for cost saving in antigen production. Evaluation of this antigen in a double antigen sandwich two-step assay showed it to be a highly sensitive and specific HIV-1 diagnostic reagent. The amenability of this assay to the one-step format suggests its potential utility in developing a rapid point-of-care HIV-1 diagnostic test.

Antigenicity and immunogenicity of RV144 vaccine AIDSVAX clade E envelope immunogen is enhanced by a gp120 N-terminal deletion

An immune correlates analysis of the RV144 HIV-1 vaccine trial revealed that antibody responses to the gp120 V1/V2 region correlated inversely with infection risk. The RV144 protein immunogens (A244-rp120 and MN-rgp120) were modified by an N-terminal 11-amino-acid deletion (Δ11) and addition of a herpes simplex virus (HSV) gD protein-derived tag (gD). We investigated the effects of these modifications on gp120 expression, antigenicity, and immunogenicity by comparing unmodified A244 gp120 with both Δ11 deletion and gD tag and with Δ11 only. Analysis of A244 gp120, with or without Δ11 or gD, demonstrated that the Δ11 deletion, without the addition of gD, was sufficient for enhanced antigenicity to gp120 C1 region, conformational V2, and V1/V2 gp120 conformational epitopes. RV144 vaccinee serum IgGs bound more avidly to A244 gp120 Δ11 than to the unmodified gp120, and their binding was blocked by C1, V2, and V1/V2 antibodies. Rhesus macaques immunized with the three different forms of A244 gp120 proteins gave similar levels of gp120 antibody titers, although higher antibody titers developed earlier in A244 Δ11 gp120-immunized animals. Conformational V1/V2 monoclonal antibodies (MAbs) gave significantly higher levels of blocking of plasma IgG from A244 Δ11 gp120-immunized animals than IgG from animals immunized with unmodified A244 gp120, thus indicating a qualitative difference in the V1/V2 antibodies induced by A244 Δ11 gp120. These results demonstrate that deletion of N-terminal residues in the RV144 A244 gp120 immunogen improves both envelope antigenicity and immunogenicity.

Solution structure, conformational dynamics, and CD4-induced activation in full-length, glycosylated, monomeric HIV gp120

The gp120 subunit of the HIV Env glycoprotein is responsible for receptor interactions leading to viral entry and is a primary target for neutralizing antibodies. Most structural studies have focused on the heavily truncated, deglycosylated gp120 core, leaving fundamental aspects of the glycoprotein that are responsible for immune evasion and receptor-induced activation unresolved. Here we investigate full-length, glycosylated HIV gp120 in unliganded and CD4-bound forms by using small-angle X-ray scattering to visualize global structural reorganization and hydrogen/deuterium exchange to track changes in local conformational dynamics. The studies revealed unliganded full-length gp120 to be considerably more dynamic, particularly at the CD4 binding site, than suggested by previous studies of the subunit core alone. The large V1/V2 loops, previously unmapped, are positioned to mask the coreceptor binding site in an orientation that recapitulates that observed in the Env trimer. CD4 binding shifts V1/V2 to unmask the coreceptor binding site and triggers profound dynamic changes in gp120 spanning from the binding site to the gp41-interactive face of gp120. These findings provide further insights on the structural basis of Env antigenicity and immunogenicity and of allosteric effects upon receptor binding.

Lineage-specific differences between human and simian immunodeficiency virus regulation of gp120 trimer association and CD4 binding

Metastable conformations of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) must be maintained in the unliganded state of the envelope glycoprotein trimer. Binding of gp120 to the primary receptor, CD4, triggers the transition to an open conformation of the trimer, promoting interaction with the CCR5 chemokine receptor and ultimately leading to gp41-mediated virus-cell membrane fusion and entry. Topological layers in the gp120 inner domain contribute to gp120-trimer association in the unliganded state and to CD4 binding. Here we describe similarities and differences between HIV-1 and SIVmac gp120. In both viruses, the gp120 N/C termini and the inner domain β-sandwich and layer 2 support the noncovalent association of gp120 with the envelope glycoprotein trimer. Layer 1 of the SIVmac gp120 inner domain contributes more to trimer association than the corresponding region of HIV-1 gp120. On the other hand, layer 1 plays an important role in stabilizing the CD4-bound conformation of HIV-1 but not SIVmac gp120 and thus contributes to HIV-1 binding to CD4. In SIVmac, CD4 binding is instead enhanced by tryptophan 375, which fills the Phe 43 cavity of gp120. Activation of SIVmac by soluble CD4 is dependent on tryptophan 375 and on layer 1 residues that determine a tight association of gp120 with the trimer. Distinct biological requirements for CD4 usage have resulted in lineage-specific differences in the HIV-1 and SIV gp120 structures that modulate trimer association and CD4 binding.

Stabilization of HIV-1 envelope in the CD4-bound conformation through specific cross-linking of a CD4 mimetic

CD4 binding on gp120 leads to the exposure of highly conserved regions recognized by the HIV co-receptor CCR5 and by CD4-induced (CD4i) antibodies. A covalent gp120-CD4 complex was shown to elicit CD4i antibody responses in monkeys, which was correlated with control of the HIV virus infection (DeVico, A., Fouts, T., Lewis, G. K., Gallo, R. C., Godfrey, K., Charurat, M., Harris, I., Galmin, L., and Pal, R. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 17477-17482). Because the inclusion of CD4 in a vaccine formulation should be avoided, due to potential autoimmune reactions, we engineered small sized CD4 mimetics (miniCD4s) that are poorly immunogenic and do not induce anti-CD4 antibodies. We made covalent complexes between such an engineered miniCD4 and gp120 or gp140, through a site-directed coupling reaction. These complexes were recognized by CD4i antibodies as well as by the HIV co-receptor CCR5. In addition, they elicit CD4i antibody responses in rabbits and therefore represent potential vaccine candidates that mimic an important HIV fusion intermediate, without autoimmune hazard.

Analysis of the disulfide bond arrangement of the HIV-1 envelope protein CON-S gp140 ΔCFI shows variability in the V1 and V2 regions

Disulfide bonding of cysteines is one of the most important protein modifications, and it plays a key role in establishing/maintaining protein structures in biologically active forms. Therefore, the determination of disulfide bond arrangement is one important aspect to understanding the chemical structure of a protein and defining its functional domains. Herein, aiming to understand how the HIV-1 envelope protein's structure influences its immunogenicity, we used an MS-based approach, liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance (LC/ESI-FTICR) mass spectrometry, to determine the disulfide linkages on an oligomeric form of the group M consensus HIV-1 envelope protein (Env), CON-S gp140 ΔCFI. This protein has marked improvement in its immunogenicity compared to monomeric gp120 and wild-type forms of gp140 Envs. Our results demonstrate that the disulfide connectivity in the N-terminal region of CON-S gp140 ΔCFI is different from the disulfide bonding previously reported in the monomeric form of gp120 HIV-1 Env. Additionally, heterogeneity of the disulfide bonding was detected in this region. These data suggest that the V1/V2 region does not have a single, conserved disulfide bonding pattern and that variability could impact immunogenicity of expressed Envs.