In vivo gp41 antibodies targeting the 2F5 monoclonal antibody epitope mediate human immunodeficiency virus type 1 neutralization breadth

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

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

Cholesterol Constrains the Antigenic Configuration of the Membrane-Proximal Neutralizing HIV-1 Epitope

The envelope glycoprotein (Env) enables HIV-1 cell entry through fusion of host-cell and viral membranes induced by the transmembrane subunit gp41. Antibodies targeting the C-terminal sequence of the membrane-proximal external region (C-MPER) block the fusogenic activity of gp41 and achieve neutralization of divergent HIV-1 strains and isolates. Thus, recreating the structure that generates broadly neutralizing C-MPER antibodies during infection is a major goal in HIV vaccine development. Here, we have reconstituted a peptide termed CpreTM-TMD in a membrane environment. This peptide contains the C-MPER epitope and the minimum TMD residues required for the anchorage of the Env glycoprotein to the viral membrane. In addition, we have used antibody 10E8 variants to gauge the antigenic configuration attained by CpreTM-TMD as a function of the membrane cholesterol content, a functional determinant of the HIV envelope and liposome-based vaccines. Differential binding of the 10E8 variants and the trend of the IgG responses recovered from rabbits immunized with liposome-peptide formulations, suggested that cholesterol may restrict 10E8 accessibility to the C-MPER epitope. Our data ruled out the destabilization of the lipid bilayer architecture in CpreTM-TMD-containing membranes, and pointed to the perturbation of the helical conformation by lipid packing as the cause of the antigenic configuration loss induced by cholesterol. Overall, our results provide additional insights into the structural basis of the Env complex anchoring to membranes, and suggest new approaches to the design of effective immunogens directed against the near pan-neutralizing HIV-1 epitope C-MPER.

Identification of a conserved linear epitope using monoclonal antibody against non-structural protein 3A of foot-and-mouth disease virus with potential for differentiation between infected and vaccinated animals

Foot-and-mouth disease (FMD) is a highly contagious and economically devastating viral disease of cloven-hoofed animals. Vaccination is a key element in the control of FMD among countries where the disease is enzootic. Differentiating infected from vaccinated animals in herds after immunization is an important component of effective eradication strategies. Non-structural protein (NSP) 3A of FMDV is as part of a larger detected antigen that is used for this differential diagnosis. Here, we generated a specific monoclonal antibody (MAb) against FMDV non-structural protein called 3A10, and further defined the linear epitopes recognized by the MAb 3A10 using a series of peptides that expressed GST-fused protein. Using Western blot, it was showed that the 5-aa peptide ¹²⁶ERTLP¹³⁰ of 3A was the minimal epitope reactive to MAb 3A10. Alanine-scanning mutagenesis analysis revealed that Arg127 and Leu129 were crucial for MAb 3A10 binding to ¹²⁶ERTLP¹³⁰. Furthermore, sequence alignment analysis, indicated that the epitope ¹²⁶ERTLP¹³⁰ recognized by 3A10 was shown to be conserved among seven serotypes of FMDV strains. The synthetic peptide Elisa demonstrated that this epitope peptide could be recognized by sera from FMDV-infected pigs and cattle, but negative reactivity to unvaccinated and vaccinated healthy animal sera. Thus, the MAb reagents and the linear epitopes defined herein provide theoretical and technical support for the development of diagnostic tools for infection differentiating FMDV infected from vaccinated animals.

Mapping of Neutralizing Antibody Epitopes on the Envelope of Viruses Obtained from Plasma Samples Exhibiting Broad Cross-Clade Neutralization Potential Against HIV-1

Several broadly neutralizing antibodies (bNAbs) that can target HIV strains with large degrees of variability have recently been identified. However, efforts to induce synthesis of such bNAbs that can protect against HIV infection have not met with much success. Identification of specific epitopes encoded in the HIV-1 envelope (Env) that can direct the host to synthesize bNAbs remains a challenge. In a previous study, we identified 12 antiretroviral therapy-naive HIV-1-infected individuals whose plasma exhibited broad cross-clade neutralization property against different clades of HIV-1. In this study, we sequenced the full-length HIV-1 gp160 from 11 of these individuals and analyzed the sequences to identify bNAb epitopes. We identified critical residues in the viral envelopes that contribute to the formation of conformational epitopes and possibly induce the production of bNAbs, using in silico methods. We found that many of the sequences had conserved glycans at positions N160 (10/11) and N332 (9/11), which are known to be critical for the binding of PG9/PG16-like and PGT128-like bNAbs, respectively. We also observed conservation of critical glycans at positions N234 and N276 critical for the interaction with CD4 binding site bNAbs in 8/11 and 11/11 sequences, respectively. We modeled the three-dimensional structure of the 11 HIV-1 envelopes and found that though each had structural differences, the critical residues were mostly present on the surface of the Env structures. The identified critical residues are proposed as candidates for further evaluation as bNAb epitopes.

Using High-Throughput Sequencing to Characterize the Development of the Antibody Repertoire During Infections: A Case Study of HIV-1

High throughput sequencing (HTS) approaches have only recently been applied to describing the antibody/B-cell repertoire in fine detail, but these data sets have already become critical to the design of vaccines and therapeutics, and monitoring of cancer immunotherapy. As a case study, we describe the potential and present limitations of HTS studies of the Ab repertoire during infection with HIV-1. Most of the present studies restrict their analyses to lineages of specific bnAbs. We discuss future initiatives to expand this type of analysis to more complete repertoires and to improve comparing and sharing of these Ab repertoire data across studies and institutions.

Molecular Basis of Unusually High Neutralization Resistance in Tier 3 HIV-1 Strain 253-11

Understanding the mechanisms used by HIV-1 to evade antibody neutralization may contribute to the design of a high-coverage vaccine. The tier 3 virus 253-11 is poorly neutralized by subtype-matched and subtype C sera, even compared to other tier 3 viruses, and is also recognized poorly by V3/glycan-targeting monoclonal antibodies (MAbs). We found that sequence polymorphisms in the V3 loop and N-linked glycosylation sites contribute only minimally to the high neutralization resistance of 253-11. Interestingly, the 253-11 membrane-proximal external region (MPER) is rarely recognized by sera in the context of the wild-type virus but is commonly recognized in the context of an HIV-2 chimera, suggesting steric or kinetic hindrance of binding to MPER in the native envelope (Env). Mutations in the 253-11 MPER, which were previously reported to increase the lifetime of the prefusion Env conformation, affected the resistance of 253-11 to antibodies targeting various epitopes on HIV-1 Env, presumably destabilizing its otherwise stable, closed trimer structure. To gain insight into the structure of 253-11, we constructed and crystallized a recombinant 253-11 SOSIP trimer. The resulting structure revealed that the heptad repeat helices in gp41 are drawn in close proximity to the trimer axis and that gp120 protomers also showed a relatively compact disposition around the trimer axis. These observations give substantial insight into the molecular features of an envelope spike from a tier 3 virus and into possible mechanisms that may contribute to its unusually high neutralization resistance.IMPORTANCE HIV-1 isolates that are highly resistant to broadly neutralizing antibodies could limit the efficacy of an antibody-based vaccine. We studied 253-11, which is highly resistant to commonly elicited neutralizing antibodies. To further understand its resistance, we made mutations that are known to delay fusion and thus increase the time that the virus spends in the open conformation following CD4 binding. Interestingly, we found that these mutations affect the 253-11 envelope (Env) spike before CD4 binding, presumably by destabilizing the trimer structure. To gain further information about the structure of the 253-11 Env trimer, we generated a recombinant 253-11 SOSIP trimer. The crystal structure of the SOSIP trimer revealed that the gp41 helices and the gp120 protomers were drawn in toward the center of the molecule compared to most solved HIV-1 Env structures. These observations provide insight into the distinct molecular features of a tier 3 envelope spike.

Antibody-Mediated Therapy against HIV/AIDS: Where Are We Standing Now?

Acquired immunodeficiency syndrome (AIDS) cases are on the rise globally. To date, there is still no effective measure to eradicate the causative agent, human immunodeficiency virus (HIV). Highly active antiretroviral therapy (HAART) is being used in HIV/AIDS management, but it results in long-term medication and has major drawbacks such as multiple side effects, high cost, and increasing the generation rate of escape mutants. In addition, HAART does not control HIV-related complications, and hence more medications and further management are required. With this, other alternatives are urgently needed. In the past, small-molecule inhibitors have shown potent antiviral effects, and some of them are now being evaluated in clinical trials. The challenges in developing these small molecules for clinical use include the off-target effect, poor stability, and low bioavailability. On the other hand, antibody-mediated therapy has emerged as an important therapeutic modality for anti-HIV therapeutics development. Many antiviral antibodies, namely, broad neutralizing antibodies (bnAbs) against multiple strains of HIV, have shown promising effects in vitro and in animal studies; further studies are ongoing in clinical trials to evaluate their uses in clinical applications. This short review aims to discuss the current development of therapeutic antibodies against HIV and the challenges in adopting them for clinical use.

Optimization of the EC26-2A4 Epitope in the gp41 Membrane Proximal External Region Targeted by Neutralizing Antibodies from an Elite Controller

The analysis of patient derived HIV neutralizing antibodies (nAbs) and their target epitopes in the viral envelope (Env) protein provides important basic information for vaccine design. In this study we optimized an epitope, EC26-2A4, that is targeted by neutralizing antibodies from an elite controller (EC26) and localizes in the membrane-proximal external region from the gp41 transmembrane protein. Due to its overlap with the epitope of the first generation broadly neutralizing monoclonal Ab (mAb) 2F5 associated with autoreactivity, we first defined the minimal core epitope reacting with antibodies from EC26 plasma, but not with mAb 2F5. The optimized minimal epitope, EC26-2A4ΔM, was able to induce neutralizing antibodies in vaccinated mice. We further analyzed the frequency of antibodies against the EC26-2A4ΔM peptide in HIV-positive patient sera from a treated cohort and an untreated long-term nonprogressor (LTNP) cohort. Interestingly, 27% of the LTNP sera reacted with the peptide, whereas only 9% showed reactivity in the treated cohort. Although there was no association between the presence of antibodies against the EC26-2A4ΔM epitope and viral load or CD4 count in these patients, the CD4 nadir in the treated cohort was higher in patients positive for EC26-2A4ΔM antibodies, in particular in patients having such antibodies at an early and a late timepoint after infection.

Identification of a serotype-independent linear epitope of foot-and-mouth disease virus

Foot-and-mouth disease (FMD), caused by foot-and-mouth disease virus (FMDV), is a highly contagious infectious disease that affects domestic and wild cloven-hoofed animals worldwide. VP2 is a structural protein of FMDV. In this study, an FMDV serotype-independent monoclonal antibody (MAb), 10B10, against the viral capsid protein VP2 was generated, and a series of GST fusion proteins expressing a truncated peptide of VP2 was subjected to Western blot analysis using MAb 10B10. Their results indicated that the peptide ⁸TLLEDRILT¹⁶ of VP2 is the minimal requirement of the epitope recognized by MAb 10B10. Importantly, this linear epitope was highly conserved among all seven serotypes of FMDV in a sequence alignment analysis. Subsequent alanine-scanning mutagenesis analysis revealed that the residues Thr⁸ and Asp¹² of the epitope were crucial for MAb-10B10 binding. Furthermore, Western blot analysis also revealed that the MAb 10B10-directed epitope could be recognized by positive sera from FMDV-infected cattle. The discovery that MAb 10B10 recognizes a serotype-independent linear epitope of FMDV suggests potential applications for this MAb in the development of serotype-independent tests for FMDV.

Antigenic characterization of the human immunodeficiency virus (HIV-1) envelope glycoprotein precursor incorporated into nanodiscs

The entry of human immunodeficiency virus (HIV-1) into host cells is mediated by the viral envelope glycoproteins (Envs), which are derived by the proteolytic cleavage of a trimeric gp160 Env precursor. The mature Env trimer is a major target for entry inhibitors and vaccine-induced neutralizing antibodies. Env interstrain variability, conformational flexibility and heavy glycosylation contribute to evasion of the host immune response, and create challenges for structural characterization and vaccine development. Here we investigate variables associated with reconstitution of the HIV-1 Env precursor into nanodiscs, nanoscale lipid bilayer discs enclosed by membrane scaffolding proteins. We identified detergents, as well as lipids similar in composition to the viral lipidome, that allowed efficient formation of Env-nanodiscs (Env-NDs). Env-NDs were created with the full-length Env precursor and with an Env precursor with the majority of the cytoplasmic tail intact. The self-association of Env-NDs was decreased by glutaraldehyde crosslinking. The Env-NDs exhibited an antigenic profile expected for the HIV-1 Env precursor. Env-NDs were recognized by broadly neutralizing antibodies. Of note, neutralizing antibody epitopes in the gp41 membrane-proximal external region and in the gp120:gp41 interface were well exposed on Env-NDs compared with Env expressed on cell surfaces. Most Env epitopes recognized by non-neutralizing antibodies were masked on the Env-NDs. This antigenic profile was stable for several days, exhibiting a considerably longer half-life than that of Env solubilized in detergents. Negative selection with weak neutralizing antibodies could be used to improve the antigenic profile of the Env-NDs. Finally, we show that lipid adjuvants can be incorporated into Env-NDs. These results indicate that Env-NDs represent a potentially useful platform for investigating the structural, functional and antigenic properties of the HIV-1 Env trimer in a membrane context.

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

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

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

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

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

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

Engineering Recombinant Reoviruses To Display gp41 Membrane-Proximal External-Region Epitopes from HIV-1

Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

The gp41 membrane-proximal external region (MPER) is a target for broadly neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). However, replication-defective virus vaccines currently under evaluation in clinical trials do not efficiently elicit MPER-specific antibodies. Structural modeling suggests that the MPER forms an α-helical coiled coil that is required for function and immunogenicity. To maintain the native MPER conformation, we used reverse genetics to engineer replication-competent reovirus vectors that displayed MPER sequences in the α-helical coiled-coil tail domain of viral attachment protein σ1. Sequences in reovirus strain type 1 Lang (T1L) σ1 were exchanged with sequences encoding HIV-1 strain Ba-L MPER epitope 2F5 or the entire MPER. Individual 2F5 or MPER substitutions were introduced at virion-proximal or virion-distal sites in the σ1 tail. Recombinant reoviruses containing heterologous HIV-1 sequences were viable and produced progeny yields comparable to those with wild-type virus. HIV-1 sequences were retained following 10 serial passages in cell culture, indicating that the substitutions were genetically stable. Recombinant viruses engineered to display the 2F5 epitope or full-length MPER in σ1 were recognized by purified 2F5 antibody. Inoculation of mice with 2F5-containing vectors or rabbits with 2F5- or MPER-containing vectors elicited anti-reovirus antibodies, but HIV-1-specific antibodies were not detected. Together, these findings indicate that heterologous sequences that form α-helices can functionally replace native sequences in the α-helical tail domain of reovirus attachment protein σ1. However, although these vectors retain native antigenicity, they were not immunogenic, illustrating the difficulty of experimentally inducing immune responses to this essential region of HIV-1.

IMPORTANCE Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

Antibodies Targeting the Envelope of HIV-1

Antibodies (Abs) are a critical component of the human immune response against viral infections. In HIV-infected patients, a robust Ab response against the virus develops within months of infection; however, due to numerous strategies, the virus usually escapes the biological effects of the various Abs. Here we provide an overview of the different viral evasion mechanisms, including glycosylation, high mutation rate, and conformational masking by the envelope glycoproteins of the virus. In response to virus infection and to its evolution within a host, "conventional Abs" are generated, and these can also be induced by immunization; generally, these Abs are limited in their neutralization breadth and potency. In contrast, "exceptional Abs" require extended exposure to virus to generate the required hypermutation in the immunoglobulin variable regions, and they occur only in rare HIV-infected individuals, but they display impressive breadth and potency. In this review, we describe the major regions of the HIV envelope spike that are targeted by conventional and exceptional Abs. These include the first, second, and third variable loops (V1, V2, and V3) located at the apex of the envelope trimer, the CD4 binding site, and the membrane-proximal external region of the gp41 ectodomain. Lastly, we discuss the challenging task of HIV immunogen design and approaches for choosing which immunogens might be used to elicit protective Abs.

N-terminal residues of an HIV-1 gp41 membrane-proximal external region antigen influence broadly neutralizing 2F5-like antibodies

The Human immunodeficiency virus type 1 (HIV-1) gp41 membrane proximal external region (MPER) is targeted by broadly neutralizing antibodies (e.g. 2F5, 4E10, Z13e and m66.6), which makes this region a promising target for vaccine design. One strategy to elicit neutralizing antibodies against the MPER epitope is to design peptide immunogens mimicking neutralization structures. To probe 2F5-like neutralizing antibodies, two yeast-displayed antibody libraries from peripheral blood mononuclear cells from a HIV-1 patient were screened against the 2F5 epitope peptide SP62. Two 2F5-like antibodies were identified that specifically recognized SP62. However, these antibodies only weakly neutralized HIV-1 primary isolates. The epitopes recognized by these two 2F5-like antibodies include not only the 2F5 epitope (amino acids (aa) 662-667 in the MPER) but also several other residues (aa 652-655) locating at the N-terminus in SP62. Experimental results suggest that residues of SP62 adjacent to the 2F5 epitope influence the response of broadly neutralizing 2F5-like antibodies in vaccination. Our findings may aid the design of vaccine immunogens and development of therapeutics against HIV-1 infection.

Identification of a conserved linear epitope using a monoclonal antibody against non-structural protein 3B of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) is a member of the family Picornaviridae that has caused severe economic losses in many countries of the world. Regular vaccinations have been effectively used to control foot-and-mouth disease (FMD) in countries where the disease is enzootic. Distinguishing between infected and vaccinated animals in herds after immunization is an important component of effective eradication strategies. Nonstructural protein (NSP) 3B of FMDV is part of a larger antigen that is used for this differential diagnosis. In this study, an FMDV serotype-independent monoclonal antibody (MAb) against NSP 3B, 5D12, was generated. Using western blot, it was revealed that MAb 5D12 binds to three fragments of 3B displaying the motifs G(1)PYAGPLERQKPLK(14), K(18)LPQQEGPYAGPMER(32) and V(45)KEGPYEGPVKKPVA(59). The motif G(1)PYAGPLERQKPLK(14) was chosen for further mapping. Different truncated motifs derived from the motif G(1)PYAGPLERQKPLK(14) were expressed as GST-fusion constructs for western blot analysis. The results showed that the 5-aa peptide P(2)YAGP(6) was the minimal epitope reactive to MAb 5D12. Subsequent alanine-scanning mutagenesis analysis revealed that Pro(2), Gly(5) and Pro(6) were crucial for MAb 5D12 binding to P(2)YAGP(6). Furthermore, through sequence alignment analysis, the epitope PxxGP recognized by 5D12 was found to be present not only in 3B-1 but also in 3B2 and 3B3 and was highly conserved in seven serotypes of FMDV strains. Western blot analysis also revealed that the peptide epitope could be recognized by sera from FMDV-infected pigs and cattle. Thus, the 5D12-recognized 3B epitope identified here provides theoretical support for the development of MAb 5D12 as a differential diagnosis reagent for FMDV infection.

Rapid Development of gp120-Focused Neutralizing B Cell Responses during Acute Simian Immunodeficiency Virus Infection of African Green Monkeys

The initial phases of acute human immunodeficiency virus type 1 (HIV-1) infection may be critical for development of effective envelope (Env)-specific antibodies capable of impeding the establishment of the latent pool of HIV-1-infected CD4(+) T cells, preventing virus-induced immune hyperactivation to limit disease progression and blocking vertical virus transmission. However, the initial systemic HIV-1 Env-specific antibody response targets gp41 epitopes and fails to control acute-phase viremia. African-origin, natural simian immunodeficiency virus (SIV) hosts do not typically progress to AIDS and rarely postnatally transmit virus to their infants, despite high milk viral loads. Conversely, SIV-infected rhesus macaques (RMs), Asian-origin nonnatural SIV hosts, sustain pathogenic SIV infections and exhibit higher rates of postnatal virus transmission. In this study, of acute SIV infection, we compared the initial systemic Env-specific B cell responses of AGMs and RMs in order to probe potential factors influencing the lack of disease progression observed in AGMs. AGMs developed higher-magnitude plasma gp120-specific IgA and IgG responses than RMs, whereas RMs developed more robust gp140-directed IgG responses. These gp120-focused antibody responses were accompanied by rapid autologous neutralizing responses during acute SIV infection in AGMs compared to RMs. Moreover, acute SIV infection elicited a higher number of circulating Env-specific memory B cells in peripheral blood of AGMs than in the blood of RMs. These findings indicate that AGMs have initial systemic Env-specific B cell responses to SIV infection distinct from those of a nonnatural SIV host, resulting in more functional SIV-specific humoral responses, which may be involved in impairing pathogenic disease progression and minimizing postnatal transmission.

IMPORTANCE

Due to the worldwide prevalence of HIV-1 infections, development of a vaccine to prevent infection or limit the viral reservoir remains an important goal. HIV-1-infected humans, as well as SIV-infected nonnatural SIV hosts, develop pathogenic infections and readily transmit the virus to their infants. Conversely, natural SIV hosts do not develop pathogenic infections and rarely transmit the virus to their infants. The immunologic factors contributing to these favorable outcomes in natural SIV hosts could prove invaluable for directing HIV-1 vaccine and therapy design. This study identified distinctions in the specificity and function of the initial systemic SIV envelope-specific B cell response that developed during acute SIV infection in natural and nonnatural SIV host species. Identification of distinct acute B cell responses in natural SIV hosts may inform vaccine strategies seeking to elicit similar responses prior to or during the initial phases of acute HIV-1 infection.

Immunoglobulin gene insertions and deletions in the affinity maturation of HIV-1 broadly reactive neutralizing antibodies

Induction of HIV-1 broad neutralizing antibodies (bnAbs) is a goal of HIV-1 vaccine development but has remained challenging partially due to unusual traits of bnAbs, including high somatic hypermutation (SHM) frequencies and in-frame insertions and deletions (indels). Here we examined the propensity and functional requirement for indels within HIV-1 bnAbs. High-throughput sequencing of the immunoglobulin (Ig) VHDJH genes in HIV-1 infected and uninfected individuals revealed that the indel frequency was elevated among HIV-1-infected subjects, with no unique properties attributable to bnAb-producing individuals. This increased indel occurrence depended only on the frequency of SHM point mutations. Indel-encoded regions were generally proximal to antigen binding sites. Additionally, reconstruction of a HIV-1 CD4-binding site bnAb clonal lineage revealed that a large compound VHDJH indel was required for bnAb activity. Thus, vaccine development should focus on designing regimens targeted at sustained activation of bnAb lineages to achieve the required SHM and indel events.

MABGEL 1: first phase 1 trial of the anti-HIV-1 monoclonal antibodies 2F5, 4E10 and 2G12 as a vaginal microbicide

BACKGROUND

Monoclonal antibodies (mAbs) which potently neutralize a broad range of HIV isolates are potential microbicide candidates. To date, topical application of mAbs in humans and their stability in vaginal secretions has not been studied.

OBJECTIVES

To assess the pharmacokinetics and safety of the mAbs 2F5, 4E10 and 2G12 when applied vaginally in women.

DESIGN

A randomized, double-blind, placebo-controlled phase 1 trial.

METHODS

Twenty-eight healthy, sexually abstinent women administered 2.5 g of gel daily for 12 days containing either 10 or 20 mg/g of each mAb (MABGEL) or placebo. Main clinical evaluations and sampling occurred at baseline, 1, 8, and 24 hours post-1st dose and 12 and 36 hours post-12th dose.

RESULTS

After adjustment for dilution factors, median levels of 2F5, 4E10 and 2G12 in vaginal secretions at 1 hour post high-dose MABGEL were 7.74, 5.28 and 7.48 mg/ml respectively. Levels of 2F5 and 4E10 declined exponentially thereafter with similar estimated half-lives (4.6 and 4.3 hours). In contrast, 2G12 levels declined more rapidly in the first 8 hours, with an estimated half-life of 1.4 hours during this period. There was no evidence of systemic absorption. There were no significant differences in local or systemic adverse event rates or vaginal flora changes (by qPCR) between active and placebo gel arms. Whilst at least 1 adverse event was recorded in 96% of participants, 95% were mild and none were serious.

CONCLUSIONS

Vaginal application of 50 mg of each mAb daily was safe over a 12 day period. Median mAb concentrations detected at 8 hours post dose were potentially sufficient to block HIV transmission.2G12 exhibited more rapid elimination from the human vagina than 4E10 and 2F5, likely due to poor stability of 2G12 in acidic human vaginal secretions. Further research is needed to develop mAb-based vaginal microbicides and delivery systems.

TRIAL REGISTRATION

ISRCTN 64808733 UK CRN Portfolio 6470.

Autoreactivity in HIV-1 broadly neutralizing antibodies: implications for their function and induction by vaccination

PURPOSE OF REVIEW

This review discusses progress in understanding the impact of immune tolerance on inducing broadly neutralizing antibodies (bnAbs), and how such knowledge can be incorporated into novel immunization approaches.

RECENT FINDINGS

Over 120 bnAbs have now been isolated, all of which bear unusual features associated with host tolerance controls, but paradoxically may also be required for their function. Evidence that poly/autoreactivity of membrane proximal external region bnAbs can invoke such controls has been demonstrated by knock-in technology, highlighting its potential for studying the impact of tolerance in the generation of bnAb lineages to distinct HIV-1 envelope targets. The requirement for extensive affinity maturation in developing neutralization breadth/potency during infection is being examined, and similar studies in the setting of immunization will be aided by testing novel vaccine approaches in knock-in models that either selectively express reverted V(D)J rearrangements, or unrearranged germline segments, from which bnAb lineages originate.

SUMMARY

It is increasingly apparent that immune tolerance, sometimes invoked by self-reactivity that overlaps with bnAb epitope specificity, adds to a formidable set of roadblocks impeding bnAb induction. The path to an effective HIV-1 vaccine may thus benefit from a deeper understanding of host controls, including categorizing those that are unique or common at distinct bnAb targets, and ranking those most feasible to overcome by immunization. Ultimately, such emerging information will be critical to incorporate into new vaccine approaches that can be tested in human trials.

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.

Drift of the HIV-1 envelope glycoprotein gp120 toward increased neutralization resistance over the course of the epidemic: a comprehensive study using the most potent and broadly neutralizing monoclonal antibodies

Extending our previous analyses to the most recently described monoclonal broadly neutralizing antibodies (bNAbs), we confirmed a drift of HIV-1 clade B variants over 2 decades toward higher resistance to bNAbs targeting almost all the identified gp120-neutralizing epitopes. In contrast, the sensitivity to bNAbs targeting the gp41 membrane-proximal external region remained stable, suggesting a selective pressure on gp120 preferentially. Despite this evolution, selected combinations of bNAbs remain capable of neutralizing efficiently most of the circulating variants.

Progress in HIV-1 vaccine development

The past 2 years have seen a number of basic and translational science advances in the quest for development of an effective HIV-1 vaccine. These advances include discovery of new envelope targets of potentially protective antibodies, demonstration that CD8(+) T cells can control HIV-1 infection, development of immunogens to overcome HIV-1 T-cell epitope diversity, identification of correlates of transmission risk in an HIV-1 efficacy trial, and mapping of the coevolution of HIV-1 founder envelope mutants in infected subjects with broad neutralizing antibodies, thereby defining broad neutralizing antibody developmental pathways. Despite these advances, a promising HIV-1 vaccine efficacy trial published in 2013 did not prevent infection, and the HIV-1 vaccine field is still years away from deployment of an effective vaccine. This review summarizes what some of the scientific advances have been, what roadblocks still remain, and what the most promising approaches are for progress in design of successful HIV-1 vaccine candidates.

Epitope specificity of human immunodeficiency virus-1 antibody dependent cellular cytotoxicity [ADCC] responses

Antibody dependent cellular cytotoxicity [ADCC] has been suggested to play an important role in control of Human Immunodeficiency Virus-1 [HIV-1] viral load and protection from infection. ADCC antibody responses have been mapped to multiple linear and conformational epitopes within the HIV-1 envelope glycoproteins gp120 and gp41. Many epitopes targeted by antibodies that mediate ADCC overlap with those recognized by antibodies capable of virus neutralization. In addition, recent studies conducted with human monoclonal antibodies derived from HIV-1 infected individuals and HIV-1 vaccine-candidate vaccinees have identified a number of antibodies that lack the ability to capture primary HIV-1 isolates or mediate neutralizing activity, but are able to bind to the surface of infected CD4+ T cells and mediate ADCC. Of note, the conformational changes in the gp120 that may not exclusively relate to binding of the CD4 molecule are important in exposing epitopes recognized by ADCC responses. Here we discuss the HIV-1 envelope epitopes targeted by ADCC antibodies in the context of the potential protective capacities of ADCC.

Immunization with hybrid proteins containing the membrane proximal external region of HIV-1

The transmembrane envelope (TM) protein gp41 of HIV-1 is an attractive target when designing a vaccine to induce neutralizing antibodies. A few broadly neutralizing antibodies (2F5, 4E10, and 10E8) that target conserved epitopes in the membrane proximal external region (MPER) of gp41 have been isolated from infected individuals. However, attempts to induce such antibodies by immunizations with gp41 and Env derivatives containing the MPER were successful only to some extent. In contrast, immunizations with the ectodomain of the TM protein p15E of different gamma retroviruses resulted in the induction of neutralizing antibodies. These sera recognized epitopes located in the MPER and in the fusion peptide proximal region (FPPR) of p15E. Based on these results, both regions of p15E were substituted with the corresponding sequences derived from gp41 of HIV-1. Thus, four different hybrid antigens were produced. One of the inserted sequences contained the epitopes of 2F5 and 4E10 in the MPER; the other corresponded to the FPPR. Vaccination of rats, guinea pigs, and a goat induced binding antibodies directed against the FPPR of gp41 and the 2F5 epitope (ELDKWA) located in the MPER. Despite the exact recognition of the 2F5 epitope, no or very weak neutralization of HIV-1NL4-3 by the immune sera was demonstrated. Nonetheless, using the strategy of hybrid proteins, antibodies targeting the desired epitope were successfully induced.

Enhanced antibody responses to an HIV-1 membrane-proximal external region antigen in mice reconstituted with cultured lymphocytes

We have shown that the protective HIV-1 Ab, 2F5, avidly reacts with a conserved mammalian self-Ag, kynureninase, and that the development of B cells specific for the 2F5 epitope is constrained by immunological tolerance. These observations suggest that the capacity to mount Ab responses to the 2F5 epitope is mitigated by tolerance, but such capacity may be latent in the pretolerance and/or anergic B cell pools. In this study, we use B cell tetramer reagents to track the frequencies of B cells that recognize the HIV-1 2F5 epitope (SP62): in C57BL/6 mice, SP62-binding transitional B cells are readily identified in bone marrow but are lost during subsequent development. Unsurprisingly then, immunization with SP62 immunogen does not elicit significant humoral responses in normal C57BL/6 mice. Reconstitution of Rag1(null) mice with normal congenic B cells that have matured in vitro restores the capacity to mount significant serum Ab and germinal center responses to this HIV-1 epitope. These B cell cultures are permissive for the development of autoreactive B cells and support the development of SP62-specific B cell compartments normally lost in 2F5 Ab knockin mice. The recovery of humoral responses to the 2F5/SP62 epitope of HIV-1 by reconstitution with B cells containing forbidden, autoreactive clones provides direct evidence that normal C57BL/6 mice latently possess the capacity to generate humoral responses to a conserved, neutralizing HIV-1 epitope.

An autoreactive antibody from an SLE/HIV-1 individual broadly neutralizes HIV-1

Broadly HIV-1-neutralizing antibodies (BnAbs) display one or more unusual traits, including a long heavy chain complementarity-determining region 3 (HCDR3), polyreactivity, and high levels of somatic mutations. These shared characteristics suggest that BnAb development might be limited by immune tolerance controls. It has been postulated that HIV-1-infected individuals with autoimmune disease and defective immune tolerance mechanisms may produce BnAbs more readily than those without autoimmune diseases. In this study, we identified an HIV-1-infected individual with SLE who exhibited controlled viral load (<5,000 copies/ml) in the absence of controlling HLA phenotypes and developed plasma HIV-1 neutralization breadth. We collected memory B cells from this individual and isolated a BnAb, CH98, that targets the CD4 binding site (CD4bs) of HIV-1 envelope glycoprotein 120 (gp120). CH98 bound to human antigens including dsDNA, which is specifically associated with SLE. Anti-dsDNA reactivity was also present in the patient's plasma. CH98 had a mutation frequency of 25% and 15% nt somatic mutations in the heavy and light chain variable domains, respectively, a long HCDR3, and a deletion in the light chain CDR1. The occurrence of anti-dsDNA reactivity by a HIV-1 CD4bs BnAb in an individual with SLE raises the possibility that some BnAbs and SLE-associated autoantibodies arise from similar pools of B cells.

Immune response to HIV

PURPOSE OF REVIEW

Major advances have been made in the delineation of HIV-specific immune response and in the mechanisms of virus escape. The kinetics of the immunological and virological events occurring during primary HIV infection indicate that the establishment of the latent HIV reservoir, the major obstacle to HIV eradication likely occurs during the very early stages of primary infection, that is, the 'eclipse phase', prior to the development of the HIV-specific immune response which has limited efficacy in the control of the early events of infection. Therefore, the window of opportunity to develop effective interventions either to clear HIV during primary infection or to prevent rebound of HIV in patients successfully treated who stop antiretroviral therapy is very narrow.

RECENT FINDINGS

Genetic factors most strongly associated with nonprogressive infection are human leukocyte antigen (HLA) class I alleles and particularly HLA-B5701. CD4 and CD8 T-cell responses with polyfunctional profile are associated with nonprogressive infection. Broader neutralizing antibodies are detected 3-4 years after infection, generated only in 20% of individuals but show no efficacy in the control of HIV replication.

SUMMARY

In the present review, we shall discuss the different components of the HIV-specific immune response elicited by the infection, the kinetics of these responses during primary infection and the changes following transition to the chronic phase of infection, and the functional profile of 'effective' versus 'noneffective' HIV-specific immune responses.

Interactions of peptide triazole thiols with Env gp120 induce irreversible breakdown and inactivation of HIV-1 virions

Background

We examined the underlying mechanism of action of the peptide triazole thiol, KR13 that has been shown previously to specifically bind gp120, block cell receptor site interactions and potently inhibit HIV-1 infectivity.

Results

KR13, the sulfhydryl blocked KR13b and its parent non-sulfhydryl peptide triazole, HNG156, induced gp120 shedding but only KR13 induced p24 capsid protein release. The resulting virion post virolysis had an altered morphology, contained no gp120, but retained gp41 that bound to neutralizing gp41 antibodies. Remarkably, HIV-1 p24 release by KR13 was inhibited by enfuvirtide, which blocks formation of the gp41 6-helix bundle during membrane fusion, while no inhibition of p24 release occurred for enfuvirtide-resistant virus. KR13 thus appears to induce structural changes in gp41 normally associated with membrane fusion and cell entry. The HIV-1 p24 release induced by KR13 was observed in several clades of HIV-1 as well as in fully infectious HIV-1 virions.

Conclusions

The antiviral activity of KR13 and its ability to inactivate virions prior to target cell engagement suggest that peptide triazole thiols could be highly effective in inhibiting HIV transmission across mucosal barriers and provide a novel probe to understand biochemical signals within envelope that are involved in membrane fusion.

Structural basis for HIV-1 neutralization by 2F5-like antibodies m66 and m66.6

Antibodies m66.6 and 2F5 are the only effective human HIV-1-neutralizing antibodies reported thus far to recognize the N-terminal region of the membrane-proximal external region (MPER) of the gp41 subunit of the HIV-1 envelope glycoprotein. Although 2F5 has been extensively characterized, much less is known about antibody m66.6 or antibody m66, a closely related light-chain variant. Here, we report the crystal structure of m66 in complex with its gp41 epitope, along with unbound structures of m66 and m66.6. We used mutational and binding analyses to decipher antibody elements critical for their recognition of gp41 and determined the molecular basis that underlies their neutralization of HIV-1. When bound by m66, the N-terminal region of the gp41 MPER adopts a conformation comprising a helix, followed by an extended loop. Comparison of gp41-bound m66 to unbound m66.6 identified three light-chain residues of m66.6 that were confirmed through mutagenesis to underlie the greater breadth of m66.6-mediated virus neutralization. Recognition of gp41 by m66 also revealed similarities to antibody 2F5 both in the conformation of crucial epitope residues as well as in the angle of antibody approach. Aromatic residues at the tip of the m66.6 heavy-chain third complementarity-determining region, as in the case of 2F5, were determined to be critical for virus neutralization in a manner that correlated with antibody recognition of the MPER in a lipid context. Antibodies m66, m66.6, and 2F5 thus utilize similar mechanistic elements to recognize a common gp41-MPER epitope and to neutralize HIV-1.

Antigenic analysis of monoclonal antibodies against different epitopes of σB protein of avian reovirus

BACKGROUND

Avian reovirus (ARV) causes arthritis, tenosynovitis, runting-stunting syndrome (RSS), malabsorption syndrome (MAS) and immunosuppression in chickens. σB is one of the major structural proteins of ARV, which is able to induce group-specific antibodies against the virus.

METHODS AND RESULTS

The present study described the identification of two linear B-cell epitopes in ARV σB through expressing a set of partially overlapping and consecutive truncated peptides spanning σB screened with two monoclonal antibodies (mAbs) 1F4 and 1H3-1.The data indicated that (21)KTPACW(26) (epitope A) and (32)WDTVTFH(38) (epitope B) were minimal determinants of the linear B cell epitopes. Antibodies present in the serum of ARV-positive chickens recognized the minimal linear epitopes in Western blot analyses. By sequence alignment analysis, we determined that the epitopes A and B were not conserved among ARV, duck reovirus (DRV) and turkey reovirus (TRV) strains. Western blot assays, confirmed that epitopes A and B were ARV-specific epitopes, and they could not react with the corresponding peptides of DRV and TRV.

CONCLUSIONS AND SIGNIFICANCE

We identified (21)KTPACW(26) and (32)WDTVTFH(38) as σB -specific epitopes recognized by mAbs 1F4 and 1H3-1, respectively. The results in this study may have potential applications in development of diagnostic techniques and epitope-based marker vaccines against ARV groups.

Expression and immunological characterization of cardamom mosaic virus coat protein displaying HIV gp41 epitopes

The coat protein of cardamom mosaic virus (CdMV), a member of the genus Macluravirus, assembles into virus-like particles when expressed in an Escherichia coli expression system. The N and C-termini of the coat protein were engineered with the Kennedy peptide and the 2F5 and 4E10 epitopes of gp41 of HIV. The chimeric proteins reacted with sera from HIV positive persons and also stimulated secretion of cytokines by peripheral blood mononuclear cells from these persons. Thus, a system based on the coat protein of CdMV can be used to display HIV-1 antigens.

Epitope specificity of cross-clade neutralizing sera from Chinese HIV-1-positive individuals

Induction of broadly neutralizing antibody is considered important for an effective HIV-1 vaccine. Identification and characterization of broadly neutralizing antibodies in HIV-1-infected patients will facilitate our understanding of the immune correlates to protection and the design of an effective prophylactic vaccine. A number of studies to probe the specificity of antibodies in broadly neutralizing sera in the United States and Europe have been reported. However, little is known about and would be interesting to investigate the immunological characteristics of HIV-1-positive sera in China where non-clade B viruses are prevalent and the circulating viral subtypes are distinct and more complex than both the United States and Europe. Here, we screened 80 Chinese HIV-1-positive sera against a minipanel of pseudoviruses representing various circulating HIV-1 subtypes in China and identified 8 cross-clade neutralizing sera (CNsera). Immunological characterization of the sera showed that gp120-targeting antibodies with multiple epitope specificities contributed to the cross-clade neutralizing activity of these CNsera. V3-directed antibodies were prevalent in these CNsera, but did not mainly contribute to their neutralization breath and potency while CD4bs-specific, 2F5- and 4E10-like antibodies were rarely detected. 2G12-like neutralizing antibodies were more frequently detected in HIV-1 patients from China where recombinant subtype viruses are prevalent than in United States and Europe. One broadly neutralizing serum (Serum 45) was identified to contain antibodies with unknown epitope specificities that were sensitive to terminal glycan modifications on virus Env and insensitive to N160K mutagenesis, and correlated with the cross-clade neutralization activity of Serum 45.

HIV-1 gp41 envelope IgA is frequently elicited after transmission but has an initial short response half-life

Prevention of HIV-1 transmission at mucosal surfaces will likely require durable pre-existing mucosal anti-HIV-1 antibodies (Abs). Defining the ontogeny, specificities and potentially protective nature of the initial mucosal virus-specific B-cell response will be critical for understanding how to induce protective Ab responses by vaccination. Genital fluids from patients within the earliest stages of acute HIV-1 infection (Fiebig I-VI) were examined for multiple anti-HIV specificities. Gp41 (but not gp120) Env immunoglobulin (Ig)A Abs were frequently elicited in both plasma and mucosal fluids within the first weeks of transmission. However, shortly after induction, these initial mucosal gp41 Env IgA Abs rapidly declined with a t(½) of ∼2.7 days. B-cell-activating factor belonging to the TNF family (BAFF) was elevated immediately preceding the appearance of gp41 Abs, likely contributing to an initial T-independent Ab response. HIV-1 transmission frequently elicits mucosal HIV-1 envelope-specific IgA responses targeted to gp41 that have a short half-life.

Infectious virion capture by HIV-1 gp120-specific IgG from RV144 vaccinees

The detailed examination of the antibody repertoire from RV144 provides a unique template for understanding potentially protective antibody functions. Some potential immune correlates of protection were untested in the correlates analyses due to inherent assay limitations, as well as the need to keep the correlates analysis focused on a limited number of endpoints to achieve statistical power. In an RV144 pilot study, we determined that RV144 vaccination elicited antibodies that could bind infectious virions (including the vaccine strains HIV-1 CM244 and HIV-1 MN and an HIV-1 strain expressing transmitted/founder Env, B.WITO.c). Among vaccinees with the highest IgG binding antibody profile, the majority (78%) captured the infectious vaccine strain virus (CM244), while a smaller proportion of vaccinees (26%) captured HIV-1 transmitted/founder Env virus. We demonstrated that vaccine-elicited HIV-1 gp120 antibodies of multiple specificities (V3, V2, conformational C1, and gp120 conformational) mediated capture of infectious virions. Although capture of infectious HIV-1 correlated with other humoral immune responses, the extent of variation between these humoral responses and virion capture indicates that virion capture antibodies occupy unique immunological space.

A gp41 MPER-specific llama VHH requires a hydrophobic CDR3 for neutralization but not for antigen recognition

The membrane proximal external region (MPER) of the HIV-1 glycoprotein gp41 is targeted by the broadly neutralizing antibodies 2F5 and 4E10. To date, no immunization regimen in animals or humans has produced HIV-1 neutralizing MPER-specific antibodies. We immunized llamas with gp41-MPER proteoliposomes and selected a MPER-specific single chain antibody (VHH), 2H10, whose epitope overlaps with that of mAb 2F5. Bi-2H10, a bivalent form of 2H10, which displayed an approximately 20-fold increased affinity compared to the monovalent 2H10, neutralized various sensitive and resistant HIV-1 strains, as well as SHIV strains in TZM-bl cells. X-ray and NMR analyses combined with mutagenesis and modeling revealed that 2H10 recognizes its gp41 epitope in a helical conformation. Notably, tryptophan 100 at the tip of the long CDR3 is not required for gp41 interaction but essential for neutralization. Thus bi-2H10 is an anti-MPER antibody generated by immunization that requires hydrophobic CDR3 determinants in addition to epitope recognition for neutralization similar to the mode of neutralization employed by mAbs 2F5 and 4E10.

Identification of autoantigens recognized by the 2F5 and 4E10 broadly neutralizing HIV-1 antibodies

Many human monoclonal antibodies that neutralize multiple clades of HIV-1 are polyreactive and bind avidly to mammalian autoantigens. Indeed, the generation of neutralizing antibodies to the 2F5 and 4E10 epitopes of HIV-1 gp41 in man may be proscribed by immune tolerance because mice expressing the V(H) and V(L) regions of 2F5 have a block in B cell development that is characteristic of central tolerance. This developmental blockade implies the presence of tolerizing autoantigens that are mimicked by the membrane-proximal external region of HIV-1 gp41. We identify human kynureninase (KYNU) and splicing factor 3b subunit 3 (SF3B3) as the primary conserved, vertebrate self-antigens recognized by the 2F5 and 4E10 antibodies, respectively. 2F5 binds the H4 domain of KYNU which contains the complete 2F5 linear epitope (ELDKWA). 4E10 recognizes an epitope of SF3B3 that is strongly dependent on hydrophobic interactions. Opossums carry a rare KYNU H4 domain that abolishes 2F5 binding, but they retain the SF3B3 4E10 epitope. Immunization of opossums with HIV-1 gp140 induced extraordinary titers of serum antibody to the 2F5 ELDKWA epitope but little or nothing to the 4E10 determinant. Identification of structural motifs shared by vertebrates and HIV-1 provides direct evidence that immunological tolerance can impair humoral responses to HIV-1.

Antibody-based candidate therapeutics against HIV-1: implications for virus eradication and vaccine design

INTRODUCTION

The currently available anti-HIV-1 drugs can control the infection but do not eradicate the virus. Their long-term use can lead to side effects and resistance to therapy. Therefore, eradication of the virus has been a major goal of research. Biological therapeutics including broadly neutralizing monoclonal antibodies (bnAbs) are promising tools to reach this goal. They could also help design novel vaccine immunogens potentially capable of eliciting bnAbs targeting the HIV-1 envelope glycoproteins (Envs).

AREAS COVERED

We review HIV-1 bnAbs and their potential as candidate prophylactics and therapeutics used individually, in combination, or as bispecific fusion proteins. We also discuss their potential use in the 'activation-elimination' approach for HIV-1 eradication in infected patients receiving antiretroviral treatment as well as current vaccine design efforts based on understanding of interactions of candidate vaccine immunogens with matured bnAbs and their putative germline predecessors, and related antibody maturation pathways.

EXPERT OPINION

Exploration of HIV-1 bnAbs has provided and will continue to provide useful knowledge that helps develop novel types of biotherapeutics and vaccines. It is possible that bnAb-based candidate therapeutics could help eradicate HIV-1. Development of vaccine immunogens capable of eliciting potent bnAbs in humans remains a fundamental challenge.

Identification of a new epitope for HIV-neutralizing antibodies in the gp41 membrane proximal external region by an Env-tailored phage display library

HIV controllers are a valuable source for the identification of HIV-neutralizing antibodies, as chronic infection over decades allows extensive affinity maturation of antibodies for improved Ag recognition. We analyzed a small cohort of elite controllers (ECs) for HIV-neutralizing antibodies using a panel of standardized HIV-1 pseudovirions on TZM-bl cells. An HIV-1 Env-tailored phage display library was generated to select epitopes targeted by neutralizing antibodies in the EC26 plasma sample showing the broadest neutralizing activity. Selected Env fragments were mostly allocated to the membrane proximal external region of gp41. After preabsorbing the EC26 plasma with the selected phage EC26-2A4, we achieved 50% depletion of its neutralizing activity. Furthermore, antibodies affinity-purified with the EC26-2A4 epitope from EC26 plasma showed neutralizing activity, proving that the selected phage indeed contains an epitope targeted by neutralizing plasma antibodies. Epitope fine mapping of the purified plasma antibodies on peptide arrays identified a new epitope overlapping, but clearly distinct, from the prominent 2F5 epitope. Of note, the purified antibodies did not show autoreactivity with cardiolipin, whereas low reactivity with phosphatidylserine comparable to mAb 2F5 was observed. Thus, this new epitope represents a promising candidate for further analysis in view of HIV vaccine development.

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.

Proteolytic processing of the human immunodeficiency virus envelope glycoprotein precursor decreases conformational flexibility

The mature envelope glycoprotein (Env) spike on the surface of human immunodeficiency virus type 1 (HIV-1) virions is derived by proteolytic cleavage of a trimeric gp160 glycoprotein precursor. Remarkably, proteolytic processing of the HIV-1 Env precursor results in changes in Env antigenicity that resemble those associated with glutaraldehyde fixation. Apparently, proteolytic processing of the HIV-1 Env precursor decreases conformational flexibility of the Env trimeric complex, differentially affecting the integrity/accessibility of epitopes for neutralizing and nonneutralizing antibodies.

Non-neutralizing antibodies in prevention of HIV infection

INTRODUCTION

One of the challenges facing the development of an AIDS vaccine is eliciting antibody (Ab) capable of preventing the acquisition of HIV. Broadly neutralizing Ab (bnAb) that can prevent HIV infection has proven to be difficult to elicit. Here, we consider the potential for protective non-neutralizing Ab (pnnAb) to provide the much needed Ab component for an HIV vaccine. Such Ab acts by "tagging" virus or infected cells for destruction by the innate immune system.

AREAS COVERED

We review interactions between the Fc region of immunoglobulin G (IgG) and Fcϒ receptors or complement that can lead to the destruction of HIV or HIV-infected cells, correlations between the presence of pnnAb and the prevention of HIV and simian immunodeficiency virus (SIV) infections, differences between classical HIV-specific bnAb and HIV-specific pnnAb, HIV envelope antigens and adjuvants which have been hypothesized to generate pnnAb, and the use of avidity as a serological correlate for pnnAb.

EXPERT OPINION

We hypothesize that selection of HIV for the poor ability to elicit bnAb has also selected it for slow entry into cells and a window of opportunity for pnnAb to tag virus for destruction by innate immune responses.

HIV-1 antibodies from infection and vaccination: insights for guiding vaccine design

Attempts to formulate a protective HIV-1 vaccine through classic vaccine design strategies have not been successful. Elicitation of HIV-1-specific broadly neutralizing antibodies (bnAbs) at high titers that are present before exposure might be required to achieve protection. Recently, the application of new technologies has facilitated the study of clonal lineages of HIV-1 envelope (Env) antibodies, which have provided insights into HIV-1 antibody development during infection and upon vaccination. Strategies are being developed for the analysis of infection and vaccine candidate-induced antibodies, their gene usage, and their maturation pathways such that this information can be used to attempt to guide rational vaccine design.

Antibody-dependent cellular cytotoxicity-mediating antibodies from an HIV-1 vaccine efficacy trial target multiple epitopes and preferentially use the VH1 gene family

The ALVAC-HIV/AIDSVAX-B/E RV144 vaccine trial showed an estimated efficacy of 31%. RV144 secondary immune correlate analysis demonstrated that the combination of low plasma anti-HIV-1 Env IgA antibodies and high levels of antibody-dependent cellular cytotoxicity (ADCC) inversely correlate with infection risk. One hypothesis is that the observed protection in RV144 is partially due to ADCC-mediating antibodies. We found that the majority (73 to 90%) of a representative group of vaccinees displayed plasma ADCC activity, usually (96.2%) blocked by competition with the C1 region-specific A32 Fab fragment. Using memory B-cell cultures and antigen-specific B-cell sorting, we isolated 23 ADCC-mediating nonclonally related antibodies from 6 vaccine recipients. These antibodies targeted A32-blockable conformational epitopes (n = 19), a non-A32-blockable conformational epitope (n = 1), and the gp120 Env variable loops (n = 3). Fourteen antibodies mediated cross-clade target cell killing. ADCC-mediating antibodies displayed modest levels of V-heavy (VH) chain somatic mutation (0.5 to 1.5%) and also displayed a disproportionate usage of VH1 family genes (74%), a phenomenon recently described for CD4-binding site broadly neutralizing antibodies (bNAbs). Maximal ADCC activity of VH1 antibodies correlated with mutation frequency. The polyclonality and low mutation frequency of these VH1 antibodies reveal fundamental differences in the regulation and maturation of these ADCC-mediating responses compared to VH1 bNAbs.

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

Although human immunodeficiency type 1 (HIV-1) infection induces strong antibody responses to the viral envelope glycoprotein (Env) only a few of these antibodies possess the capacity to neutralize a broad range of strains. The induction of such antibodies represents an important goal in the development of a preventive vaccine against the infection. Among the broadly neutralizing monoclonal antibodies discovered so far, three (2F5, Z13 and 4E10) target the short and hidden membrane proximal external region (MPER) of the gp41 transmembrane protein. Antibody responses to MPER are rarely observed in HIV-infected individuals or after immunization with Env immunogens. To initiate antibody responses to MPER in its membrane-embedded native conformation, we generated expression plasmids encoding the membrane-anchored ectodomain of gp41 with N-terminal deletions of various sizes. Following transfection of these plasmids, the MPER domains are displayed on the cell surface and incorporated into HIV virus like particles (VLP). Transfected cells displaying MPER mutants bound as efficiently to both 2F5 and 4E10 as cells transfected with a plasmid encoding full-length Env. Mice immunized with VLPs containing the MPER mutants produced MPER-specific antibodies, the levels of which could be increased by the trimerization of the displayed proteins as well as by a DNA prime-VLP boost immunization strategy. Although 2F5 competed for binding to MPER with antibodies in sera of some of the immunized mice, neutralizing activity could not be detected. Whether this is due to inefficient binding of the induced antibodies to MPER in the context of wild type Env or whether the overall MPER-specific antibody response induced by the MPER display mutants is too low to reveal neutralizing activity, remains to be determined.

HIV-1 superinfection in women broadens and strengthens the neutralizing antibody response

Identifying naturally-occurring neutralizing antibodies (NAb) that are cross-reactive against all global subtypes of HIV-1 is an important step toward the development of a vaccine. Establishing the host and viral determinants for eliciting such broadly NAbs is also critical for immunogen design. NAb breadth has previously been shown to be positively associated with viral diversity. Therefore, we hypothesized that superinfected individuals develop a broad NAb response as a result of increased antigenic stimulation by two distinct viruses. To test this hypothesis, plasma samples from 12 superinfected women each assigned to three singly infected women were tested against a panel of eight viruses representing four different HIV-1 subtypes at matched time points post-superinfection (~5 years post-initial infection). Here we show superinfected individuals develop significantly broader NAb responses post-superinfection when compared to singly infected individuals (RR = 1.68, CI: 1.23-2.30, p = 0.001). This was true even after controlling for NAb breadth developed prior to superinfection, contemporaneous CD4+ T cell count and viral load. Similarly, both unadjusted and adjusted analyses showed significantly greater potency in superinfected cases compared to controls. Notably, two superinfected individuals were able to neutralize variants from four different subtypes at plasma dilutions >1∶300, suggesting that their NAbs exhibit elite activity. Cross-subtype breadth was detected within a year of superinfection in both of these individuals, which was within 1.5 years of their initial infection. These data suggest that sequential infections lead to augmentation of the NAb response, a process that may provide insight into potential mechanisms that contribute to the development of antibody breadth. Therefore, a successful vaccination strategy that mimics superinfection may lead to the development of broad NAbs in immunized individuals.

Induction of antibodies in rhesus macaques that recognize a fusion-intermediate conformation of HIV-1 gp41

A component to the problem of inducing broad neutralizing HIV-1 gp41 membrane proximal external region (MPER) antibodies is the need to focus the antibody response to the transiently exposed MPER pre-hairpin intermediate neutralization epitope. Here we describe a HIV-1 envelope (Env) gp140 oligomer prime followed by MPER peptide-liposomes boost strategy for eliciting serum antibody responses in rhesus macaques that bind to a gp41 fusion intermediate protein. This Env-liposome immunization strategy induced antibodies to the 2F5 neutralizing epitope ⁶⁶⁴DKW residues, and these antibodies preferentially bound to a gp41 fusion intermediate construct as well as to MPER scaffolds stabilized in the 2F5-bound conformation. However, no serum lipid binding activity was observed nor was serum neutralizing activity for HIV-1 pseudoviruses present. Nonetheless, the Env-liposome prime-boost immunization strategy induced antibodies that recognized a gp41 fusion intermediate protein and was successful in focusing the antibody response to the desired epitope.

Binding of anti-membrane-proximal gp41 monoclonal antibodies to CD4-liganded and -unliganded human immunodeficiency virus type 1 and simian immunodeficiency virus virions

The broadly neutralizing monoclonal antibodies (MAbs) 4E10, 2F5, and Z13e1 target membrane-proximal external region (MPER) epitopes of HIV-1 gp41 in a manner that remains controversial. The requirements for initial lipid bilayer binding and/or CD4 ligation have been proposed. To further investigate these issues, we probed for binding of these MAbs to human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) virions with protein A-conjugated gold (PAG) nanoparticles using negative-stain electron microscopy. We found moderate levels of PAG associated with unliganded HIV-1 and SIV virions incubated with the three MAbs. Significantly higher levels of PAG were associated with CD4-liganded HIV-1 (epitope-positive) but not SIV (epitope-negative) virions. A chimeric SIV virion displaying the HIV-1 4E10 epitope also showed significantly higher PAG association after CD4 ligation and incubation with 4E10. MAbs accumulated rapidly on CD4-liganded virions and slowly on unliganded virions, although both reached similar levels in time. Anti-MPER epitope-specific binding was stable to washout. Virions incubated with an irrelevant MAb or CD4-only (no MAb) showed negligible PAG association, as did a vesicle-rich fraction devoid of virions. Preincubation with Fab 4E10 inhibited both specific and nonspecific 4E10 IgG binding. Our data provide evidence for moderate association of anti-MPER MAbs to viral surfaces but not lipid vesicles, even in the absence of cognate epitopes. Significantly greater MAb interaction occurs in epitope-positive virions following long incubation or CD4 ligation. These findings are consistent with a two-stage binding model where these anti-MPER MAbs bind first to the viral lipid bilayer and then to the MPER epitopes following spontaneous or induced exposure.