Targeting antibody responses to the membrane proximal external region of the envelope glycoprotein of human immunodeficiency virus

Liposome-based peptide vaccines to elicit immune responses against the membrane active domains of the HIV-1 Env glycoprotein

The fusion peptide (FP) and the Trp-rich membrane proximal external region (MPER) display membrane activity during HIV-1 fusion. These domains are highly conserved in the envelope glycoprotein (Env) and, consequently, antibodies targeting these regions block entry of divergent HIV strains and isolates into target cells. With the aim of recovering concurrent responses against the membrane-active Env domains, we have produced hybrid peptides that connect FP and MPER sequences via flexible aminohexanoic acid tethers, and tested their potential as immunogens. We demonstrate that liposome-based formulations containing FP-MPER hybrid peptides could elicit in rabbits, antibodies with the binding sequence specificity of neutralizing antibodies that engage with the N-terminal MPER sub-region. Determination of the thermodynamic parameters of binding using the Fab 2F5 as an N-terminal MPER antibody model, revealed that the hydrophobic interaction surface for epitope engagement appears to be optimal in the FP-MPER hybrid. In general, our data support: i) the use of liposomes as carriers for membrane active peptides; ii) the capacity of these liposome-based vaccines to focus humoral responses to N-terminal MPER epitopes; and iii) the need to include lipid membranes in immunogens to elicit such specific responses.

Intrastructural Help: Harnessing T Helper Cells Induced by Licensed Vaccines for Improvement of HIV Env Antibody Responses to Virus-Like Particle Vaccines

Induction of persistent antibody responses by vaccination is generally thought to depend on efficient help by T follicular helper cells. Since the T helper cell response to HIV Env may not be optimal, we explored the possibility of improving the HIV Env antibody response to virus-like particle (VLP) vaccines by recruiting T helper cells induced by commonly used licensed vaccines to provide help for Env-specific B cells. B cells specific for the surface protein of a VLP can internalize the entire VLP and thus present peptides derived from the surface and core proteins on their major histocompatibility complex class II (MHC-II) molecules. This allows T helper cells specific for the core protein to provide intrastructural help for B cells recognizing the surface protein. Consistently, priming mice with an adjuvanted Gag protein vaccine enhanced the HIV Env antibody response to subsequent booster immunizations with HIV VLPs. To harness T helper cells induced by the licensed Tetanolpur vaccines, HIV VLPs that contained T helper cell epitopes of tetanus toxoid were generated. Tetanol-immunized mice raised stronger antibody responses to immunizations with VLPs containing tetanus toxoid T helper cell epitopes but not to VLPs lacking these epitopes. Depending on the priming immunization, the IgG subtype response to HIV Env after the VLP immunization could also be modified. Thus, harnessing T helper cells induced by other vaccines appears to be a promising approach to improve the HIV Env antibody response to VLP vaccines.

IMPORTANCE Induction of HIV Env antibodies at sufficient levels with optimal Fc effector functions for durable protection remains a challenge. Efficient T cell help may be essential to induce such a desirable antibody response. Here, we provide proof of concept that T helper cells induced by a licensed vaccine can be harnessed to provide help for HIV Env-specific B cells and to modulate the Env-specific IgG subtype response.

Immunologic Insights on the Membrane Proximal External Region: A Major Human Immunodeficiency Virus Type-1 Vaccine Target

Broadly neutralizing antibodies (bNAbs) targeting conserved regions within the human immunodeficiency virus type-1 (HIV-1) envelope glycoprotein (Env) can be generated by the human immune system and their elicitation by vaccination will be a key point to protect against the wide range of viral diversity. The membrane proximal external region (MPER) is a highly conserved region within the Env gp41 subunit, plays a major role in membrane fusion and is targeted by naturally induced bNAbs. Therefore, the MPER is considered as an attractive vaccine target. However, despite many attempts to design MPER-based immunogens, further study is still needed to understand its structural complexity, its amphiphilic feature, and its limited accessibility by steric hindrance. These particular features compromise the development of MPER-specific neutralizing responses during natural infection and limit the number of bNAbs isolated against this region, as compared with other HIV-1 vulnerability sites, and represent additional hurdles for immunogen development. Nevertheless, the analysis of MPER humoral responses elicited during natural infection as well as the MPER bNAbs isolated to date highlight that the human immune system is capable of generating MPER protective antibodies. Here, we discuss the recent advances describing the immunologic and biochemical features that make the MPER a unique HIV-1 vulnerability site, the different strategies to generate MPER-neutralizing antibodies in immunization protocols and point the importance of extending our knowledge toward new MPER epitopes by the isolation of novel monoclonal antibodies. This will be crucial for the redesign of immunogens able to skip non-neutralizing MPER determinants.

Modulating immunogenic properties of HIV-1 gp41 membrane-proximal external region by destabilizing six-helix bundle structure

The C-terminal alpha-helix of gp41 membrane-proximal external region (MPER; (671)NWFDITNWLWYIK(683)) encompassing 4E10/10E8 epitopes is an attractive target for HIV-1 vaccine development. We previously reported that gp41-HR1-54Q, a trimeric protein comprised of the MPER in the context of a stable six-helix bundle (6HB), induced strong immune responses against the helix, but antibodies were directed primarily against the non-neutralizing face of the helix. To better target 4E10/10E8 epitopes, we generated four putative fusion intermediates by introducing double point mutations or deletions in the heptad repeat region 1 (HR1) that destabilize 6HB in varying degrees. One variant, HR1-∆10-54K, elicited antibodies in rabbits that targeted W672, I675 and L679, which are critical for 4E10/10E8 recognition. Overall, the results demonstrated that altering structural parameters of 6HB can influence immunogenic properties of the MPER and antibody targeting. Further exploration of this strategy could allow development of immunogens that could lead to induction of 4E10/10E8-like antibodies.

Enhancing the Quality of Antibodies to HIV-1 Envelope by GagPol-Specific Th Cells

The importance of Fc-dependent effector functions of Abs induced by vaccination is increasingly recognized. However, vaccination of mice against HIV envelope (Env) induced a skewed Th cell response leading to Env-specific Abs with reduced effector function. To overcome this bias, GagPol-specific Th cells were harnessed to provide intrastructural help for Env-specific B cells after immunization with virus-like particles containing GagPol and Env. This led to a balanced Env-specific humoral immune response with a more inflammatory Fc glycan profile. The increased quality in the Ab response against Env was confirmed by FcγR activation assays. Because the Env-specific Th cell response was also biased in human vaccinees, intrastructural help is an attractive novel approach to increase the efficacy of prophylactic HIV Env-based vaccines and may also be applicable to other particulate vaccines.

Immunogenic properties of a trimeric gp41-based immunogen containing an exposed membrane-proximal external region

The membrane-proximal external region (MPER) of HIV-1 gp41 is an attractive target for vaccine development. Thus, better understanding of its immunogenic properties in various structural contexts is important. We previously described the crystal structure of a trimeric protein complex named gp41-HR1-54Q, which consists of the heptad repeat regions 1 and 2 and the MPER. The protein was efficiently recognized by broadly neutralizing antibodies. Here, we describe its immunogenic properties in rabbits. The protein was highly immunogenic, especially the C-terminal end of the MPER containing 4E10 and 10E8 epitopes ((671)NWFDITNWLWYIK(683)). Although antibodies exhibited strong competition activity against 4E10 and 10E8, neutralizing activity was not detected. Detailed mapping analyses indicated that amino acid residues critical for recognition resided on faces of the alpha helix that are either opposite of or perpendicular to the epitopes recognized by 4E10 and 10E8. These results provide critical information for designing the next generation of MPER-based immunogens.

Eliciting neutralizing antibodies against the membrane proximal external region of HIV-1 Env by chimeric live attenuated influenza A virus vaccines

Despite significant efforts directed toward research on HIV-1 vaccines, a truly effective immunogen has not been achieved. However, the broadly neutralizing antibodies (BnAbs) 2F5 and 4E10, targeting the highly conserved membrane proximal external region (MPER) of HIV-1, are two promising tools for vaccine development. Here we engrafted the MPER into the linker domain between the trimeric core structure and the transmembrane domain of influenza A virus HA2 to investigate the potential of such chimeric viruses to elicit HIV-1 neutralizing antibodies. In the context of proliferating attenuated influenza A viruses, these HIV-1 neutralizing antibody epitopes could be continuously expressed and mimicked their native conformation to induce humoral immune responses. While MPER-specific antibodies could be detected in serum of guinea pigs vaccinated with the chimeric viruses, they exhibited only weakly neutralizing activities. These antisera from vaccinated animals neutralized viruses of clades B and BC (tier 1), but not of clades AE (tier 1) and C (tier 2). These results suggest that influenza A virus can be used as a vehicle for displaying MPER and inducing BnAbs, but it provides limited protection against HIV-1 infection. In the future development of HIV-1 vaccines by rational design, a more effective live virus vector or multiple antigens should be chosen to facilitate the process of neutralizing antibody maturation.

Liposomal vaccines incorporating molecular adjuvants and intrastructural T-cell help promote the immunogenicity of HIV membrane-proximal external region peptides

An HIV vaccine capable of inducing high and durable levels of broadly neutralizing antibodies has thus far proven elusive. A promising antigen is the membrane-proximal external region (MPER) from gp41, a segment of the viral envelope recognized by a number of broadly neutralizing antibodies. Though an attractive vaccine target due to the linear nature of the epitope and its highly conserved sequence, MPER peptides are poorly immunogenic and may require display on membranes to achieve a physiological conformation matching the native virus. Here we systematically explored how the structure and composition of liposomes displaying MPER peptides impacts the strength and durability of humoral responses to this antigen as well as helper T-cell responses in mice. Administration of MPER peptides anchored to the surface of liposomes induced MPER-specific antibodies whereas MPER administered in oil-based emulsion adjuvants or alum did not, even when combined with Toll-like receptor agonists. High-titer IgG responses to liposomal MPER required the inclusion of molecular adjuvants such as monophosphoryl lipid A. Anti-MPER humoral responses were further enhanced by incorporating high-Tm lipids in the vesicle bilayer and optimizing the MPER density to a mean distance of ∼10-15 nm between peptides on the liposomes' surfaces. Encapsulation of helper epitopes within the vesicles allowed efficient "intrastructural" T-cell help, which promoted IgG responses to MPER while minimizing competing B-cell responses against the helper sequence. These results define several key properties of liposome formulations that promote durable, high-titer antibody responses against MPER peptides, which will be a prerequisite for a successful MPER-targeting vaccine.

Multimeric scaffolds displaying the HIV-1 envelope MPER induce MPER-specific antibodies and cross-neutralizing antibodies when co-immunized with gp160 DNA

Developing a vaccine that overcomes the diversity of HIV-1 is likely to require a strategy that directs antibody (Ab) responses toward conserved regions of the viral Envelope (Env). However, the generation of neutralizing Abs (NAbs) targeting these regions through vaccination has proven to be difficult. One conserved region of particular interest is the membrane proximal external region (MPER) of Env located within the gp41 ectodomain. In order to direct the immune response to this region, the MPER and gp41 ectodomain were expressed separately as N-terminal fusions to the E2 protein of Geobacillus stearothermophilus. The E2 protein acts as a scaffold by self-assembling into 60-mer particles, displaying up to 60 copies of the fused target on the surface. Rabbits were immunized with E2 particles displaying MPER and/or the gp41 ectodomain in conjunction with DNA encoding full-length gp160. Only vaccines including E2 particles displaying MPER elicited MPER-specific Ab responses. NAbs were elicited after two immunizations that largely targeted the V3 loop. To overcome V3 immunodominance in the DNA component, E2 particles displaying MPER were used in conjunction with gp160 DNA lacking hypervariable regions V2, V3, or combined V1V2V3. All rabbits had HIV binding Ab responses and NAbs following the second vaccination. Using HIV-2/HIV-1 MPER chimeric viruses as targets, NAbs were detected in 12/16 rabbits after three immunizations. Low levels of NAbs specific for Tier 1 and 2 viruses were observed in all groups. This study provides evidence that co-immunizing E2 particles displaying MPER and gp160 DNA can focus Ab responses toward conserved regions of Env.

A fusion intermediate gp41 immunogen elicits neutralizing antibodies to HIV-1

The membrane-proximal external region (MPER) of the human immunodeficiency virus, type 1 (HIV-1) envelope glycoprotein subunit gp41 is targeted by potent broadly neutralizing antibodies 2F5, 4E10, and 10E8. These antibodies recognize linear epitopes and have been suggested to target the fusion intermediate conformation of gp41 that bridges viral and cellular membranes. Anti-MPER antibodies exert different degrees of membrane interaction, which is considered to be the limiting factor for the generation of such antibodies by immunization. Here we characterize a fusion intermediate conformation of gp41 (gp41(int)-Cys) and show that it folds into an elongated ∼ 12-nm-long extended structure based on small angle x-ray scattering data. Gp41(int)-Cys was covalently linked to liposomes via its C-terminal cysteine and used as immunogen. The gp41(int)-Cys proteoliposomes were administered alone or in prime-boost regimen with trimeric envelope gp140(CA018) in guinea pigs and elicited high anti-gp41 IgG titers. The sera interacted with a peptide spanning the MPER region, demonstrated competition with broadly neutralizing antibodies 2F5 and 4E10, and exerted modest lipid binding, indicating the presence of MPER-specific antibodies. Although the neutralization potency generated solely by gp140(CA018) was higher than that induced by gp41(int)-Cys, the majority of animals immunized with gp41(int)-Cys proteoliposomes induced modest breadth and potency in neutralizing tier 1 pseudoviruses and replication-competent simian/human immunodeficiency viruses in the TZM-bl assay as well as responses against tier 2 HIV-1 in the A3R5 neutralization assay. Our data thus demonstrate that liposomal gp41 MPER formulation can induce neutralization activity, and the strategy serves to improve breadth and potency of such antibodies by improved vaccination protocols.

Development and immunological assessment of VLP-based immunogens exposing the membrane-proximal region of the HIV-1 gp41 protein

Background

The membrane-proximal external region (MPER) of HIV-1 gp41 is particularly conserved and target for the potent broadly neutralizing monoclonal antibodies (bnMAbs) 2F5, 4E10 and 10E8. Epitope focusing and stabilization present promising strategies to enhance the quality of immune responses to specific epitopes.

Results

The aim of this work was to design and evaluate novel immunogens based on the gp41 MPER with the potential to elicit cross-clade neutralizing antibodies. For that purpose, gp41 was truncated N-terminally in order to dispose immunodominant, non-neutralizing sites and enhance the exposure of conserved regions. To stabilize a trimeric conformation, heterologous GCN4 and HA2 zipper domains were fused based on an in silico “best-fit” model to the protein’s amino terminus. Cell surface exposure of resulting proteins and their selective binding to bnMAbs 2F5 and 4E10 could be shown by cytometric analyses. Incorporation into VLPs and preservation of antigenic structures were verified by electron microscopy, and the oligomeric state was successfully stabilized by zipper domains. These gp41 immunogens were evaluated for antigenicity in an immunization study in rabbits primed with homologous DNA expression plasmids and boosted with virus-like particle (VLP) proteins. Low titers of anti-MPER antibodies were measured by IgG ELISA, and low neutralizing activity could be detected against a clade C and B viral isolate in sera.

Conclusions

Thus, although neutralizing titers were very moderate, induction of cross-clade neutralizing antibodies seems possible following immunization with MPER-focusing immunogens. However, further refinement of MPER presentation and immunogenicity is clearly needed to induce substantial neutralization responses to these epitopes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-014-0079-x) contains supplementary material, which is available to authorized users.

Multi-Parameter Exploration of HIV-1 Virus-Like Particles as Neutralizing Antibody Immunogens in Guinea Pigs, Rabbits and Macaques

Virus-like particles (VLPs) offer a platform to test the hypothesis that, since antibody binding to native envelope glycoprotein (Env) trimers results in HIV-1 neutralization, that native Env trimers presented in membranes may be useful for inducing neutralizing antibodies (nAbs) in a vaccine setting. So far, VLPs have not fulfilled this potential. Here, using a "shotgun" approach, we evaluated a wide cross-section of variables in a series of VLP immunizations. We identified 3 tentative leads. First, that VLP doses may not have been sufficient for optimal nAb induction. Second, that dampening the antigenicity of non-functional Env (for example uncleaved gp160) using either protease digests or IgG masking may be useful. Third, that guinea pig sera preferentially target non-conserved epitopes and exhibit relatively high background activity, suggesting that rabbits may be preferable as small animal vaccine models. Recent immunogenicity studies in rabbits appear to bear out all 3 of these leads.

HIV-derived lentiviral particles promote T-cell independent activation and differentiation of naïve cognate conventional B2-cells in vitro

In animal models, lentiviral particles (LP) were shown to be promising HIV vaccine candidates. Since little is known about the direct impact of LP on antigen-specific B cells, we incorporated Hen Egg Lysozyme (HEL) into LP (HEL-LP) derived from HIV to study their effect on HEL-specific, B cell receptor-transgenic B-cells (HEL(+)B-cells) in vitro. We observed preferential binding of HEL-LP to HEL(+)B-cells and their efficient internalization. HEL-LP were able to effectively cross-link B-cell receptors as indicated by the loss of surface CD62L. In the absence of CD4(+) T-cells, other activation events induced by LP in cognate naïve B-cells included increased expression of activation and co-stimulatory molecules as well as an enhanced proliferative response. Additionally, the B-cell phenotype shifted toward a germinal center pattern with further differentiation into memory and IgG3- and IgA-producing cells. The observed CD4(+) T-cell independent activation and differentiation may be due to LP-induced expression of CD40L by a subset of cognate B-cells. Thus, even in the absence of CD4(+) T-cells LP provide strong direct activation signals to cognate naïve B-cells, which may contribute to the strong humoral immune responses observed after LP immunization.

Forced virus evolution reveals functional crosstalk between the disulfide bonded region and membrane proximal ectodomain region of HIV-1 gp41

Background

The disulfide-bonded region (DSR) of HIV-1 gp41 mediates association with gp120 and plays a role in transmission of receptor-induced conformational changes in gp120 to gp41 that activate membrane fusion function. In this study, forced viral evolution of a DSR mutant that sheds gp120 was employed to identify domains within gp120-gp41 that are functionally linked to the glycoprotein association site.

Results

The HIV-1_AD8 mutant, W596L/K601D, was serially passaged in U87.CD4.CCR5 cells until replication was restored. Whereas the W596L mutation persisted throughout the cultures, a D601H pseudoreversion in the DSR partially restored cell-free virus infectivity and virion gp120-gp41 association, with further improvements to cell-free virus infectivity following a 2nd-site D674E mutation in the membrane-proximal external region (MPER) of gp41. In an independent culture, D601H appeared with a deletion in V4 (Thr-394-Trp-395) and a D674N substitution in the MPER, however this MPER mutation was inhibitory to W596L/K601H cell-free virus infectivity. While cell-free virus infectivity was not fully restored for the revertant genotypes, their cell-to-cell transmission approached the levels observed for WT. Interestingly, the functional boost associated with the addition of D674E to W596L/K601H was not observed for cell-cell fusion where the cell-surface expressed glycoproteins function independently of virion assembly. The W596L/K601H and W596L/K601H/D674E viruses exhibited greater sensitivity to neutralization by the broadly reactive MPER directed monoclonal antibodies, 2F5 and 4E10, indicating that the reverting mutations increase the availability of conserved neutralization epitopes in the MPER.

Conclusions

The data indicate for the first time that functional crosstalk between the DSR and MPER operates in the context of assembled virions, with the Leu-596-His-601-Glu-674 combination optimizing viral spread via the cell-to-cell route. Our data also indicate that changes in the gp120-gp41 association site may increase the exposure of conserved MPER neutralization epitopes in virus.

Recognition of membrane-bound fusion-peptide/MPER complexes by the HIV-1 neutralizing 2F5 antibody: implications for anti-2F5 immunogenicity

The membrane proximal external region (MPER) of the fusogenic HIV-1 glycoprotein-41 harbors the epitope sequence recognized by 2F5, a broadly neutralizing antibody isolated from an infected individual. Structural mimicry of the conserved MPER 2F5 epitope constitutes a pursued goal in the field of anti-HIV vaccine development. It has been proposed that 2F5 epitope folding into its native state is attained in the vicinity of the membrane interface and might involve interactions with other viral structures. Here we present results indicating that oligomeric complexes established between MPER and the conserved amino-terminal fusion peptide (FP) can partition into lipid vesicles and be specifically bound by the 2F5 antibody at their surfaces. Cryo-transmission electron microscopy of liposomes doped with MPER:FP peptide mixtures provided the structural grounds for complex recognition by antibody at lipid bilayer surfaces. Supporting the immunogenicity of the membrane-bound complex, these MPER:FP peptide-vesicle formulations could trigger cross-reactive anti-MPER antibodies in rabbits. Thus, our observations suggest that contacts with N-terminal regions of gp41 may stabilize the 2F5 epitope as a membrane-surface antigen.