Rationally Designed Immunogens Targeting HIV-1 gp120 V1V2 Induce Distinct Conformation-Specific Antibody Responses in Rabbits

Contemporary HIV-1 consensus Env with AI-assisted redesigned hypervariable loops promote antibody binding

An effective HIV-1 vaccine must elicit broadly neutralizing antibodies (bnAbs) against highly diverse Envelope glycoproteins (Env). Since Env with the longest hypervariable (HV) loops is more resistant to the cognate bnAbs than Env with shorter HV loops, we redesigned hypervariable loops for updated Env consensus sequences of subtypes B and C and CRF01_AE. Using modeling with AlphaFold2, we reduced the length of V1, V2, and V5 HV loops while maintaining the integrity of the Env structure and glycan shield, and modified the V4 HV loop. Spacers are designed to limit strain-specific targeting. All updated Env are infectious as pseudoviruses. Preliminary structural characterization suggests that the modified HV loops have a limited impact on Env's conformation. Binding assays show improved binding to modified subtype B and CRF01_AE Env but not to subtype C Env. Neutralization assays show increases in sensitivity to bnAbs, although not always consistently across clades. Strikingly, the HV loop modification renders the resistant CRF01_AE Env sensitive to 10-1074 despite the absence of a glycan at N332.

Vaccination with immune complexes modulates the elicitation of functional antibodies against HIV-1

Introduction

Neutralizing antibodies (Abs) are one of the immune components required to protect against viral infections. However, developing vaccines capable of eliciting neutralizing Abs effective against a broad array of HIV-1 isolates has been an arduous challenge.

Objective

This study sought to test vaccines aimed to induce Abs against neutralizing epitopes at the V1V2 apex of HIV-1 envelope (Env).

Methods

Four groups of rabbits received a DNA vaccine expressing the V1V2 domain of the CRF01_AE A244 strain on a trimeric 2J9C scaffold (V1V2-2J9C) along with a protein vaccine consisting of an uncleaved prefusion-optimized A244 Env trimer with V3 truncation (UFO-BG.ΔV3) or a V1V2-2J9C protein and their respective immune complexes (ICs). These IC vaccines were made using 2158, a V1V2-specific monoclonal Ab (mAb), which binds the V2i epitope in the underbelly region of V1V2 while allosterically promoting the binding of broadly neutralizing mAb PG9 to its V2 apex epitope in vitro.

Results

Rabbit groups immunized with the DNA vaccine and uncomplexed or complexed UFO-BG.ΔV3 proteins (DNA/UFO-UC or IC) displayed similar profiles of Env- and V1V2-binding Abs but differed from the rabbits receiving the DNA vaccine and uncomplexed or complexed V1V2-2J9C proteins (DNA/V1V2-UC or IC), which generated more cross-reactive V1V2 Abs without detectable binding to gp120 or gp140 Env. Notably, the DNA/UFO-UC vaccine elicited neutralizing Abs against some heterologous tier 1 and tier 2 viruses from different clades, albeit at low titers and only in a fraction of animals, whereas the DNA/V1V2-UC or IC vaccines did not. In comparison with the DNA/UFO-UC group, the DNA/UFO-IC group showed a trend of higher neutralization against TH023.6 and a greater potency of V1V2-specific Ab-dependent cellular phagocytosis (ADCP) but failed to neutralize heterologous viruses.

Conclusion

These data demonstrate the capacity of V1V2-2J9C-encoding DNA vaccine in combination with UFO-BG.ΔV3, but not V1V2-2J9C, protein vaccines, to elicit homologous and heterologous neutralizing activities in rabbits. The elicitation of neutralizing and ADCP activities was modulated by delivery of UFO-BG.ΔV3 complexed with V2i mAb 2158.

Mapping Epitopes by Phage Display

Monoclonal antibodies (mAbs) are valuable biological molecules, serving for many applications. Therefore, it is advantageous to know the interaction pattern between antibodies and their antigens. Regions on the antigen which are recognized by the antibodies are called epitopes, and the respective molecular counterpart of the epitope on the mAbs is called paratope. These epitopes can have many different compositions and/or structures. Knowing the epitope is a valuable information for the development or improvement of biological products, e.g., diagnostic assays, therapeutic mAbs, and vaccines, as well as for the elucidation of immune responses. Most of the techniques for epitope mapping rely on the presentation of the target, or parts of it, in a way that it can interact with a certain mAb. Among the techniques used for epitope mapping, phage display is a versatile technology that allows the display of a library of oligopeptides or fragments from a single gene product on the phage surface, which then can interact with several antibodies to define epitopes. In this chapter, a protocol for the construction of a single-target oligopeptide phage library, as well as for the panning procedure for epitope mapping using phage display is given.

Mucosal Delivery of HIV-1 Glycoprotein Vaccine Candidate Enabled by Short Carbon Nanotubes

The HIV-1 envelope glycoprotein spike is the target of antibodies, and therefore represents the main viral antigen for antibody-based vaccine design. One of the challenges in HIV-1 vaccine development is finding efficient ways for the immune system to recognize and respond to HIV-1 without establishing an infection. Since HIV-1 enters the body at mucosal surfaces, induction of immune response at these sites is a preferred preventive approach. Nasal administration is a very effective route for mucosal immunization since it can stimulate mucosal immune responses both locally and distantly. In this paper, Luna develops a safe, short carbon nanotube (CNT)-based, needle-free delivery platform known as "CNTVac". The size of short CNT was controlled to possess HIV-1 particle-like morphology (100-200 nm) capable of efficiently delivering a broad range of antigens intranasally. PEG-Lipid served as the antigen conformation protector and mucosal barrier penetration enhancer (Schematic Figure) was localized between V1V2 antigens, which caused highly enhanced local IgA and systemic antibody IgG responses in mice and rabbits. The short CNT incorporated with PEG-Lipid could not only serve as efficient delivery system but also reduce the amount of lipid usage in order to balance the vaccine dosage in order to eliminate the potential adverse effect. These data suggest a promising platform technology for vaccine delivery.

Differential V2-directed antibody responses in non-human primates infected with SHIVs or immunized with diverse HIV vaccines

V2p and V2i antibodies (Abs) that are specific for epitopes in the V1V2 region of the HIV gp120 envelope (Env) do not effectively neutralize HIV but mediate Fc-dependent anti-viral activities that have been correlated with protection from, or control of HIV, SIV and SHIV infections. Here, we describe a novel molecular toolbox that allows the discrimination of antigenically and functionally distinct polyclonal V2 Ab responses. We identify different patterns of V2 Ab induction by SHIV infection and three separate vaccine regimens that aid in fine-tuning an optimized immunization protocol for inducing V2p and V2i Abs. We observe no, or weak and sporadic V2p and V2i Abs in non-vaccinated SHIV-infected NHPs, but strong V2p and/or V2i Ab responses after immunization with a V2-targeting vaccine protocol. The V2-focused vaccination is superior to both natural infection and to immunization with whole Env constructs for inducing functional V2p- and V2i-specific responses. Strikingly, levels of V2-directed Abs correlate inversely with Abs specific for peptides of V3 and C5. These data demonstrate that a V1V2-targeting vaccine has advantages over the imprecise targeting of SIV/SHIV infections and of whole Env-based immunization regimens for inducing a more focused functional V2p- and V2i-specific Ab response.

Here the authors show that an HIV vaccine in non-human primates that focuses antibodies on the V1V2 region of gp120 is superior to infection or immunization with whole envelope vaccines for inducing V1V2 antibodies with anti-viral functions that correlate with protection.

Virus Control in Vaccinated Rhesus Macaques Is Associated with Neutralizing and Capturing Antibodies against the SHIV Challenge Virus but Not with V1V2 Vaccine-Induced Anti-V2 Antibodies Alone

The role of vaccine-induced anti-V2 Abs was tested in three protection experiments in rhesus macaques. In an experiment using immunogens similar to those in the RV144 vaccine trial (Anti-envelope [Env]), nine rhesus macaques were coimmunized with gp16092TH023 DNA and SIV gag and gp120A244 and gp120MN proteins. In two V2-focused experiments (Anti-V2 and Anti-V2 Mucosal), nine macaques in each group were immunized with V1V292TH023 DNA, V1V2A244 and V1V2CasaeA2 proteins, and cyclic V2CaseA2 peptide. DNA and protein immunogens, formulated in Adjuplex, were given at 0, 4, 12, and 20 weeks, followed by intrarectal SHIVBaL.P4 challenges. Peak plasma viral loads (PVL) of 106-107 copies/ml developed in all nine sham controls. Overall, PVL was undetectable in one third of immunized macaques, and two animals tightly controlled the virus with the Anti-V2 Mucosal vaccine strategy. In the Anti-Env study, Abs that captured or neutralized SHIVBaL.P4 inversely correlated with PVL. Conversely, no correlation with PVL was found in the Anti-V2 experiments with nonneutralizing plasma Abs that only captured virus weakly. Titers of Abs against eight V1V2 scaffolds and cyclic V2 peptides were comparable between controllers and noncontrollers as were Ab-dependent cellular cytotoxicity and Ab-dependent cell-mediated virus inhibition activities against SHIV-infected target cells and phagocytosis of gp120-coated beads. The Anti-Env experiment supports the role of vaccine-elicited neutralizing and nonneutralizing Abs in control of PVL. However, the two V2-focused experiments did not support a role for nonneutralizing V2 Abs alone in controlling PVL, as neither Ab-dependent cellular cytotoxicity, Ab-dependent cell-mediated virus inhibition, nor phagocytosis correlated inversely with heterologous SHIVBaL.P4 infection.

Anti-V2 antibodies virus vulnerability revealed by envelope V1 deletion in HIV vaccine candidates

The efficacy of ALVAC-based HIV and SIV vaccines in humans and macaques correlates with antibodies to envelope variable region 2 (V2). We show here that vaccine-induced antibodies to SIV variable region 1 (V1) inhibit anti-V2 antibody-mediated cytotoxicity and reverse their ability to block V2 peptide interaction with the α4β7 integrin. SIV vaccines engineered to delete V1 and favor an α helix, rather than a β sheet V2 conformation, induced V2-specific ADCC correlating with decreased risk of SIV acquisition. Removal of V1 from the HIV-1 clade A/E A244 envelope resulted in decreased binding to antibodies recognizing V2 in the β sheet conformation. Thus, deletion of V1 in HIV envelope immunogens may improve antibody responses to V2 virus vulnerability sites and increase the efficacy of HIV vaccine candidates.

Priming with DNA Expressing Trimeric HIV V1V2 Alters the Immune Hierarchy Favoring the Development of V2-Specific Antibodies in Rhesus Macaques

The RV144 vaccine trial revealed a correlation between reduced risk of HIV infection and the level of nonneutralizing-antibody (Ab) responses targeting specific epitopes in the second variable domain (V2) of the HIV gp120 envelope (Env) protein, suggesting this region as a target for vaccine development. To favor induction of V2-specific Abs, we developed a vaccine regimen that included priming with DNA expressing an HIV V1V2 trimeric scaffold immunogen followed by booster immunizations with a combination of DNA and protein in rhesus macaques. Priming vaccination with DNA expressing the HIV recombinant subtype CRF01_AE V1V2 scaffold induced higher and broader V2-specific Ab responses than vaccination with DNA expressing CRF01_AE gp145 Env. Abs recognizing the V2 peptide that was reported as a critical target in RV144 developed only after the priming immunization with V1V2 DNA. The V2-specific Abs showed several nonneutralizing Fc-mediated functions, including ADCP and C1q binding. Importantly, robust V2-specific Abs were maintained upon boosting with gp145 DNA and gp120 protein coimmunization. In conclusion, priming with DNA expressing the trimeric V1V2 scaffold alters the hierarchy of humoral immune responses to V2 region epitopes, providing a method for more efficient induction and maintenance of V2-specific Env Abs associated with reduced risk of HIV infection.IMPORTANCE The aim of this work was to design and test a vaccine regimen focusing the immune response on targets associated with infection prevention. We demonstrated that priming with a DNA vaccine expressing only the HIV Env V1V2 region induces Ab responses targeting the critical region in V2 associated with protection. This work shows that V1V2 scaffold DNA priming immunization provides a method to focus immune responses to the desired target region, in the absence of immune interference by other epitopes. This induced immune responses with improved recognition of epitopes important for protective immunity, namely, V2-specific humoral immune responses inversely correlating with HIV risk of infection in the RV144 trial.

Multimeric Epitope-Scaffold HIV Vaccines Target V1V2 and Differentially Tune Polyfunctional Antibody Responses

The V1V2 region of the HIV-1 envelope is the target of several broadly neutralizing antibodies (bNAbs). Antibodies to V1V2 elicited in the RV144 clinical trial correlated with a reduced risk of HIV infection, but these antibodies were without broad neutralizing activity. Antibodies targeting V1V2 also correlated with a reduced viral load in immunized macaques challenged with simian immunodeficiency virus (SIV) or simian/human immunodeficiency virus (SHIV). To focus immune responses on V1V2, we engrafted the native, glycosylated V1V2 domain onto five different multimeric scaffold proteins and conducted comparative immunogenicity studies in macaques. Vaccinated macaques developed high titers of plasma and mucosal antibodies that targeted structurally distinct V1V2 epitopes. Plasma antibodies displayed limited neutralizing activity but were functionally active for ADCC and phagocytosis, which was detectable 1-2 years after immunizations ended. This study demonstrates that multivalent, glycosylated V1V2-scaffold protein immunogens focus the antibody response on V1V2 and are differentially effective at inducing polyfunctional antibodies with characteristics associated with protection.

An HIV Vaccine Targeting the V2 Region of the HIV Envelope Induces a Highly Durable Polyfunctional Fc-Mediated Antibody Response in Rhesus Macaques

The HIV vaccine field now recognizes the potential importance of generating polyfunctional antibodies (Abs). The only clinical HIV vaccine trial to date to show significant efficacy (RV144) found that reduced infection rates correlated with the level of nonneutralizing Abs specific for the V2 region of the envelope glycoprotein. We have conducted a comprehensive preclinical reverse vaccinology-based vaccine program that has included the design and production and testing of numerous scaffolded V2 region immunogens. The most immunogenic vaccine regimen in nonhuman primates among those studied as part of this program consisted of a cocktail of three immunogens presenting V2 from different viruses and clades in the context of different scaffolds. Presently we demonstrate that the V2-specific Ab response from this regimen was highly durable and functionally diverse for the duration of the study (25 weeks after the final immunization). The total IgG binding response at this late time point exhibited only an ∼5× reduction in potency. Three immunizations appeared essential for the elicitation of a strong Ab-dependent cellular cytotoxicity (ADCC) response for all animals, as opposed to the Ab-dependent cellular phagocytosis (ADCP) and virus capture responses, which were comparably potent after only 2 immunizations. All functionalities measured were highly durable through the study period. Therefore, testing this vaccine candidate for its protective capacity is warranted.IMPORTANCE The only HIV vaccine trial for which protective efficacy was detected correlated this efficacy with V2-specific Abs that were effectively nonneutralizing. This result has fueled a decade of HIV vaccine research focused on designing an HIV vaccine capable of eliciting V2-focused, polyfunctional Abs that effectively bind HIV and trigger various leukocytes to kill the virus and restrict viral spread. From the numerous vaccine candidates designed and tested as part of our V2-focused preclinical vaccine program, we have identified immunogens and a vaccine regimen that induces a highly durable and polyfunctional V2-focused Ab response in rhesus macaques, described herein.

Search for antiviral functions of potentially protective antibodies against V2 region of HIV-1

In the only successful RV144 vaccine trial to date, high levels of antibodies (Abs) against the V2 region of the virus envelope protein gp120 correlated with reduced HIV-1 infection. The protective role of V2 Abs has not yet been determined, and the antiviral function of V2 Abs that mediate protection against HIV-1 in humans or SHIV infection in rhesus macaques remains unclear. V2 Abs do not neutralize resistant tier 2 viruses; their Fc-mediated activities are modest and similar to those of another anti-envelope Abs, and inhibition of the gp120–α4β7 integrin interaction is ineffective in both animals and clinical trials. Moreover, in protection experiments in monkeys, levels of V1V2 vaccine-induced V2 Abs do not correlate with plasma viral load. Together, these observations suggest that V2 Abs may not control SHIV infection in rhesus macaques and that V2 Abs may instead be a surrogate marker of other protective immune responses.

V2-Specific Antibodies in HIV-1 Vaccine Research and Natural Infection: Controllers or Surrogate Markers

Most human immunodeficiency virus (HIV) vaccine trials have lacked efficacy and empirical vaccine lead targets are scarce. Thus far, the only independent correlate of reduced risk of HIV-1 acquisition in humans is elevated levels of V2-specific antibodies identified in the modestly protective RV144 vaccine trial. Ten years after RV144, human and non-human primate vaccine studies have reassessed the potential contribution of V2-specific antibodies to vaccine efficacy. In addition, studies of natural HIV-1 infection in humans have provided insight into the development of V1V2-directed antibody responses and their impact on clinical parameters and disease progression. Functionally diverse anti-V2 monoclonal antibodies were isolated and their structurally distinct V2 epitope regions characterized. After RV144, a plethora of research studies were performed using different model systems, immunogens, protocols, and challenge viruses. These diverse studies failed to provide a clear picture regarding the contribution of V2 antibodies to vaccine efficacy. Here, we summarize the biological functions and clinical findings associated with V2-specific antibodies and discuss their impact on HIV vaccine research.

Vaccine-induced V1V2-specific antibodies control and or protect against infection with HIV, SIV and SHIV

PURPOSE OF REVIEW

In humans, only one independent immunologic correlate of reduced risk of HIV infection has been identified: a robust antibody (Ab) response to the V1V2 domain of the gp120 envelope (Env) protein. In recent years, the presence and level of V1V2-specific Abs has also been correlated with protection from SIV and SHIV infections. Here, we review the multitude of studies showing the in-vivo protective effects of V1V2 Abs and review their immunologic characteristics and antiviral functions.

RECENT FINDINGS

Structural and immunologic studies have defined four epitope families in the V1V2 domain: one epitope family, V2q, which preferentially presents as a quaternary structure of the Env trimer, and another epitope family (V2qt) which requires the quaternary trimeric Env structure; these two epitope types are recognized by two families of monoclonal Abs (mAbs)-V2q-specific and V2qt-specific mAbs-which display broad and potent neutralizing activity. A third epitope family, V2i, is present as a discontinuous conformational structure that overlays the α4β7 integrin binding motif, and a fourth epitope family (V2p) exists on V2 peptides. Antibodies specific for V2i and V2p epitopes display only poor neutralizing activity but effectively mediate other antiviral activities and have been correlated with control of and/or protection from HIV, SIV and SHIV. Notably, V2q and V2qt Abs have not been induced by any vaccines, but V2p and V2i Abs have been readily induced with various vaccines in nonhuman primates and humans.

SUMMARY

The correlation of vaccine-induced V2p and V2i Abs with protection from HIV, SIV and SHIV suggests that these Ab types are extremely important to induce with prophylactic vaccines.

Immune Correlates of Disease Progression in Linked HIV-1 Infection

Genetic and immunologic analyses of epidemiologically-linked HIV transmission enable insights into the impact of immune responses on clinical outcomes. Human vaccine trials and animal studies of HIV-1 infection have suggested immune correlates of protection; however, their role in natural infection in terms of protection from disease progression is mostly unknown. Four HIV-1⁺ Cameroonian individuals, three of them epidemiologically-linked in a polygamous heterosexual relationship and one incidence-matched case, were studied over 15 years for heterologous and cross-neutralizing antibody responses, antibody binding, IgA/IgG levels, antibody-dependent cellular cytotoxicity (ADCC) against cells expressing wild-type or CD4-bound Env, viral evolution, Env epitopes, and host factors including HLA-I alleles. Despite viral infection with related strains, the members of the transmission cluster experienced contrasting clinical outcomes including cases of rapid progression and long-term non-progression in the absence of strongly protective HLA-I or CCR5Δ32 alleles. Slower progression and higher CD4/CD8 ratios were associated with enhanced IgG antibody binding to native Env and stronger V1V2 antibody binding responses in the presence of viruses with residue K169 in V2. ADCC against cells expressing Env in the CD4-bound conformation in combination with low Env-specific IgA/IgG ratios correlated with better clinical outcome. This data set highlights for the first time that V1V2-directed antibody responses and ADCC against cells expressing open, CD4-exposed Env, in the presence of low plasma IgA/IgG ratios, can correlate with clinical outcome in natural infection. These parameters are comparable to the major correlates of protection, identified post-hoc in the RV144 vaccine trial; thus, they may also modulate the rate of clinical progression once infected. The findings illustrate the potential of immune correlate analysis in natural infection to guide vaccine development.

Phagocytosis of a Model Human Immunodeficiency Virus Target by Human Breast Milk Leukocytes Is Predominantly Granulocyte-Driven When Elicited by Specific Antibody

BACKGROUND

Studies demonstrate a protective effect of antibodies (Abs) in breast milk (BM) against mother-to-child transmission (MTCT) of human immunodeficiency virus (HIV). Contribution of the BM cellular component has been overlooked. The only clinical HIV vaccine trial to demonstrate efficacy, RV144, correlated protection with Abs mediating functions through the constant immunoglobulin region-the crystallizable fragment (Fc). These data support induction of vaccine Abs triggering antiviral activities by leukocytes through Fc receptors (FcRs).

OBJECTIVE

To measure Ab-dependent cellular phagocytosis (ADCP), an essential Fc-mediated response, by BM phagocytes.

MATERIALS AND METHODS

Cells were isolated from five human BM samples obtained at 7-183 days postpartum and analyzed for ADCP. Fluorescent beads coated with HIV envelope (Env) epitopes were used as targets. Sixty-seven to 100 mL of milk was utilized.

RESULTS

Total cell concentrations per milliliter were 16,083-222,857, with 1.6-12.3% being CD45⁺ leukocytes. ADCP activity was measurable using the HIV-specific Ab 830A. Use of the actin inhibitor cytochalasin D and FcR blocker indicated that ADCP was actin dependent and required FcR engagement. ADCP scores were variable, but largely consistent, across the samples studied, exhibiting <4-fold difference from lowest to highest activity for CD45⁺ cells. Of the CD45⁺ ADCP, significantly more activity was granulocyte derived (72-95%), while the remaining activity was monocyte driven.

CONCLUSIONS

The data indicate that BM phagocytes can manifest antiviral activities in the presence of specific Abs and therefore may contribute to reduction of MTCT of HIV.

Identification of HIV gp41-specific antibodies that mediate killing of infected cells

Antibodies that mediate killing of HIV-infected cells through antibody-dependent cellular cytotoxicity (ADCC) have been implicated in protection from HIV infection and disease progression. Despite these observations, these types of HIV antibodies are understudied compared to neutralizing antibodies. Here we describe four monoclonal antibodies (mAbs) obtained from one individual that target the HIV transmembrane protein, gp41, and mediate ADCC activity. These four mAbs arose from independent B cell lineages suggesting that in this individual, multiple B cell responses were induced by the gp41 antigen. Competition and phage peptide display mapping experiments suggested that two of the mAbs target epitopes in the cysteine loop that are highly conserved and a common target of HIV gp41-specific antibodies. The amino acid sequences that bind these mAbs are overlapping but distinct. The two other mAbs were competed by mAbs that target the C-terminal heptad repeat (CHR) and the fusion peptide proximal region (FPPR) and appear to both target a similar unique conformational epitope. These gp41-specific mAbs mediated killing of infected cells that express high levels of Env due to either pre-treatment with interferon or deletion of vpu to increase levels of BST-2/Tetherin. They also mediate killing of target cells coated with various forms of the gp41 protein, including full-length gp41, gp41 ectodomain or a mimetic of the gp41 stump. Unlike many ADCC mAbs that target HIV gp120, these gp41-mAbs are not dependent on Env structural changes associated with membrane-bound CD4 interaction. Overall, the characterization of these four new mAbs that target gp41 and mediate ADCC provides evidence for diverse gp41 B cell lineages with overlapping but distinct epitopes within an individual. Such antibodies that can target various forms of envelope protein could represent a common response to a relatively conserved HIV epitope for a vaccine.

Rabbits and men: relating their ages

Rabbit, a member of the Lagomorpha order, is the closest phylogenetic relative to humans, next to primates. It possesses greater acceptability as a laboratory mammal than primates in terms of husbandry, breeding ease, cost effectiveness, and legal ethical conveniences. Moreover, as a laboratory animal, the rabbit also owns its advantages over mice or rats, in terms of phylogenetic resemblance to human, size, blood volume, responsiveness, and other congruences enabling them to better imitate human physiological characteristics in biomedical research. A specific research aspires to effectuate its outcome on a particular human age group, for which it is pivotal to select a laboratory rabbit of exact age, which will correlate with that specific age of a human, which is currently based on mere approximation. This article is the first ever scientific venture, focused to swap this approximation of laboratory rabbit age with accuracy by relating it with that of humans analyzing different phases of life individually. Considering the diminutive lifespan of rabbits compared to humans, the correlation of their age with respect to the entire lifespan, which we found out to be 45.625 days compared to one human year, is not enough. Thereby, like our previous articles that formulated concise relation of age of laboratory rats and mice with human age, in this article also, we aim to aid biomedical research specificity in the selection of laboratory model age, separately correlating different life phases of humans with that of rabbits, the second mostly used mammal in 2016 in the United States.

Anti-V2 antibody deficiency in individuals infected with HIV-1 in Cameroon

The results of the RV144 vaccine clinical trial showed a correlation between high level of anti-V1V2 antibodies (Abs) and a decreased risk of acquiring HIV-1 infection. This turned the focus of HIV vaccine design to the induction of elevated levels of anti-V2 Abs to increase vaccine efficacy. In plasma samples from HIV-1 infected Cameroonian individuals, we observed broad variations in levels of anti-V2 Abs, and 6 of the 79 plasma samples tested longitudinally displayed substantial deficiency of V2 Abs. Sequence analysis of the V2 region from plasma viruses and multivariate analyses of V2 characteristics showed a significant difference in several features between V2-deficient and V2-reactive plasma Abs. These results suggest that HIV vaccine immunogens containing a shorter V2 region with fewer glycosylation sites and higher electrostatic charges can be beneficial for induction of a higher level of anti-V2 Abs and thus contribute to HIV vaccine efficacy.

Epitope Mapping via Phage Display from Single-Gene Libraries

Antibodies are widely used in a large variety of research applications, for diagnostics and therapy of numerous diseases, primarily cancer and autoimmune diseases. Antibodies are binding specifically to target structures (antigens). The antigen-binding properties are not only dependent on the antibody sequence, but also on the discrete antigen region recognized by the antibody (epitope). Knowing the epitope is valuable information for the improvement of diagnostic assays or therapeutic antibodies, as well as to understand the immune response of a vaccine. While huge progress has been made in the pipelines for the generation and functional characterization of antibodies, the available technologies for epitope mapping are still lacking effectiveness in terms of time and effort. Also, no technique available offers the absolute guarantee of succeeding. Thus, research to develop and improve epitope mapping techniques is still an active field. Phage display from random peptide libraries or single-gene libraries are currently among the most exploited methods for epitope mapping. The first is based on the generation of mimotopes and it is fastened to the need of high-throughput sequencing and complex bioinformatic analysis. The second provides original epitope sequences without requiring complex analysis or expensive techniques, but depends on further investigation to define the functional amino acids within the epitope. In this book chapter, we describe how to perform epitope mapping by antigen fragment phage display from single-gene antigen libraries and how to construct these types of libraries. Thus, we also provide figures and analysis to demonstrate the actual potential of this technique and to prove the necessity of certain procedural steps.

Modulation of Antibody Responses to the V1V2 and V3 Regions of HIV-1 Envelope by Immune Complex Vaccines

Prophylactic HIV vaccines must elicit antibodies (Abs) against the virus envelope glycoproteins (Env) to effectively prevent HIV infection. We investigated a vaccine platform that utilizes immune complexes made of Env proteins gp120 and monoclonal Abs (mAbs) against different gp120 epitopes. We previously observed alterations in V3 antigenicity upon formation of certain gp120/mAb complexes and demonstrated the ability of these complexes to modulate the elicitation of V3 Ab responses. However, the effects on the V1V2 domain, an important target for Abs that correlate with vaccine-induced protection against HIV, have not been studied, nor have immune complex vaccines made with non-B subtype Env. This study compared subtypes B (JRFL) and CRF_01.AE (A244) Env gp120 proteins in complex with selected gp120-specific mAbs. Allosteric and antigenic changes were detected on these immune complexes, indicating that gp120/mAb interaction induces alterations on the Env surface that may modify the Env immunogenic properties. To evaluate this idea, mice were immunized with gp120/mAb complexes or their uncomplexed gp120 counterparts. The overall serum IgG titers elicited against gp120 were comparable, but a marked skewing toward V1V2 or V3 was evident and dependent on the gp120 strain and the specificity of the mAb used to form the complexes. Compared with uncomplexed gp120_JRFL, gp120_JRFL complexed with CD4bs or V1V2 mAbs, but not with C2 or V3 mAbs, elicited V3 Abs of greater titers and breadth, and Abs more capable of neutralizing tier 1 virus. Epitope mapping revealed a shift to a more conserved site in the V3 crown. However, the complexes did not enhance V1V2 Ab response, and the elicited V1V2 Abs were not cross-reactive. This profile contrasts with Ab responses to gp120_A244/mAb complexes. Notably, gp120_A244/mAb complexes induced higher levels of V1V2 Abs with some cross-reactivity, while also stimulating weak or strain-specific V3 Abs. Sera from gp120_A244/mAb complex-immunized animals displayed no measurable virus neutralization but did mediate Ab-dependent cellular phagocytosis, albeit at levels similar to that induced by gp120_A244 alone. These data indicate the potential utility of immune complexes as vaccines to shape Ab responses toward or away from Env sites of interest.

Development of a Stable MGAT1- CHO Cell Line to Produce Clade C gp120 With Improved Binding to Broadly Neutralizing Antibodies

The high rate of new HIV infections, particularly in Sub-Saharan Africa, emphasizes the need for a safe and effective vaccine to prevent acquired immunodeficiency syndrome (AIDS). To date, the only HIV vaccine trial that has exhibited protective efficacy in humans was the RV144 study completed in Thailand. The finding that protection correlated with antibodies to gp120 suggested that increasing the quality or magnitude of the antibody response that recognize gp120 might improve the modest yet significant protection (31.2%) achieved with this immunization regimen. However, the large-scale production of rgp120 suitable for clinical trials has been challenging due, in part, to low productivity and difficulties in purification. Moreover, the antigens that are currently available were produced largely by the same technology used in the early 1990s and fail to incorporate unique carbohydrates presented on HIV virions required for the binding of several major families of broadly neutralizing antibodies (bNAbs). Here we describe the development of a high-yielding CHO cell line expressing rgp120 from a clade C isolate (TZ97008), representative of the predominant circulating HIV subtype in Southern Africa and Southeast Asia. This cell line, produced using robotic selection, expresses high levels (1.2 g/L) of the TZ97008 rgp120 antigen that incorporates oligomannose glycans required for binding to multiple glycan dependent bNAbs. The resulting rgp120 displays a lower degree of net charge and glycoform heterogeneity as compared to rgp120s produced in normal CHO cells. This homogeneity in net charge facilitates purification by filtration and ion exchange chromatography methods, eliminating the need for expensive custom-made lectin, or immunoaffinity columns. The results described herein document the availability of a novel cell line for the large-scale production of clade C gp120 for clinical trials. Finally, the strategy used to produce a TZ97008 gp120 in the MGAT⁻ CHO cell line can be applied to the production of other candidate HIV vaccines.

HIV-1 Envelope Glycoproteins from Diverse Clades Differentiate Antibody Responses and Durability among Vaccinees

Induction of broadly cross-reactive antiviral humoral responses with the capacity to target globally diverse circulating strains is a key goal for HIV-1 immunogen design. A major gap in the field is the identification of diverse HIV-1 envelope antigens to evaluate vaccine regimens for binding antibody breadth. In this study, we define unique antigen panels to map HIV-1 vaccine-elicited antibody breadth and durability. Diverse HIV-1 envelope glycoproteins were selected based on genetic and geographic diversity to cover the global epidemic, with a focus on sexually acquired transmitted/founder viruses with a tier 2 neutralization phenotype. Unique antigenicity was determined by nonredundancy (Spearman correlation), and antigens were clustered using partitioning around medoids (PAM) to identify antigen diversity. Cross-validation demonstrated that the PAM method was better than selection by reactivity and random selection. Analysis of vaccine-elicited V1V2 binding antibody in longitudinal samples from the RV144 clinical trial revealed the striking heterogeneity among individual vaccinees in maintaining durable responses. These data support the idea that a major goal for vaccine development is to improve antibody levels, breadth, and durability at the population level. Elucidating the level and durability of vaccine-elicited binding antibody breadth needed for protection is critical for the development of a globally efficacious HIV vaccine.IMPORTANCE The path toward an efficacious HIV-1 vaccine will require characterization of vaccine-induced immunity that can recognize and target the highly genetically diverse virus envelope glycoproteins. Antibodies that target the envelope glycoproteins, including diverse sequences within the first and second hypervariable regions (V1V2) of gp120, were identified as correlates of risk for the one partially efficacious HIV-1 vaccine. To build upon this discovery, we experimentally and computationally evaluated humoral responses to define envelope glycoproteins representative of the antigenic diversity of HIV globally. These diverse envelope antigens distinguished binding antibody breadth and durability among vaccine candidates, thus providing insights for advancing the most promising HIV-1 vaccine candidates.

Towards conformational fidelity of a quaternary HIV-1 epitope: computational design and directed evolution of a minimal V1V2 antigen

Structure-based approaches to antigen design utilize insights from antibody (Ab):antigen interactions and a refined understanding of protective Ab responses to engineer novel antigens presenting epitopes with conformations relevant to eliciting or discovering protective humoral responses. For human immunodeficiency virus-1 (HIV-1), one model of protection is provided by broadly neutralizing Abs (bnAbs) against epitopes present in the closed prefusion trimeric conformation of HIV-1 envelope glycoprotein, such as the variable loops 1-2 (V1V2) apex. Here, computational design and directed evolution yielded a novel V1V2 sequence variant with potential utility for inclusion in an immunogen for eliciting bnAbs, or as an epitope probe for their detection. The computational design goal was to engineer a minimal single-chain antigen with three copies of the V1V2 loops to support maintenance of closed prefusion V1V2 trimeric conformation and presentation of bnAb epitopes. Via directed evolution of this computationally designed single-chain antigen, we isolated a V1V2 sequence variant that in monomeric form exhibited preferential recognition by quaternary-preferring and conformation-dependent mAbs. Structural context and transferability of this phenotype to V1V2 sequences from all strains of HIV-1 tested suggest a conformation-stabilizing effect. This example demonstrates the potential utility of computational design and directed evolution-based protein engineering strategies to develop minimal, conformation-stabilized epitope-specific antigens.

Predominant envelope variable loop 2-specific and gp120-specific antibody-dependent cellular cytotoxicity antibody responses in acutely SIV-infected African green monkeys

Background

The initial envelope (Env)-specific antibody response in acutely HIV-1-infected individuals and simian immunodeficiency virus (SIV)-infected rhesus monkeys (RMs) is dominated by non-neutralizing antibodies targeting Env gp41. In contrast, natural primate SIV hosts, such as African green monkeys (AGMs), develop a predominant Env gp120-specific antibody response to SIV infection. However, the fine-epitope specificity and function of SIV Env-specific plasma IgG, and their potential role on autologous virus co-evolution in SIV-infected AGMs and RMs remain unclear.

Results

Unlike the dominant linear gp41-specific IgG responses in RMs, SIV-infected AGMs demonstrated a unique linear variable loop 2 (V2)-specific plasma IgG response that arose concurrently with high gp120-directed antibody-dependent cellular cytotoxicity (ADCC) activity, and SIVsab-infected cell binding responses during acute infection. Moreover, SIV variants isolated from SIV-infected AGMs exhibited high amino acid mutation frequencies within the Env V1V2 loop compared to those of RMs. Notably, the linear V2-specific IgG epitope in AGMs overlaps with an analogous region of the HIV V2 loop containing the K169 mutation epitope identified in breakthrough viruses from RV144 vaccinees.

Conclusion

Vaccine-elicited Env V2-specific IgG responses have been proposed as an immune correlate of reduced risk in HIV-1/SIV acquisition in humans and RMs. Yet the pathways to elicit these potentially-protective V2-specific IgG responses remain unclear. In this study, we demonstrate that SIV-infected AGMs, which are the natural hosts of SIV, exhibited high plasma linear V2-specific IgG binding responses that arose concurrently with SIV Env gp120-directed ADCC-mediating, and SIV-infected cell plasma IgG binding responses during acute SIV infection, which were not present in acutely SIV-infected RMs. The linear V2-specific antibody response in AGMs targets an overlapping epitope of the proposed site of vaccine-induced immune pressure defined in the moderately protective RV144 HIV-1 vaccine trial. Identifying host factors that control the early elicitation of Env V2-specific IgG and ADCC antibody responses in these natural SIV hosts could inform vaccination strategies aimed at rapidly inducing potentially-protective HIV-1 Env-specific responses in humans.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0406-5) contains supplementary material, which is available to authorized users.

Epitope Mapping by Phage Display

Among the molecules of the immune system, antibodies, particularly monoclonal antibodies (mAbs), have been shown to be interesting for many biological applications. Due to their ability to recognize only a unique part of their target, mAbs are usually very specific. These targets can have many different compositions, but the most common ones are proteins or peptides that are usually from outside the host, although self-proteins can also be targeted in autoimmune diseases, or in some types of cancer. The parts of a mAb that interact with its target compose the paratope, while the recognized parts of the target compose the epitope. Knowing the epitope is valuable for the improvement of a biological product, e.g., a diagnostic assay, a therapeutic mAb, or a vaccine, as well as for the elucidation of immune responses. The current techniques for epitope mapping rely on the presentation of the target, or parts of it, in a way that it can interact with a certain mAb. Even though there are several techniques available, each has its pros and cons. Thus, the choice for one of them is usually dependent on the preference and availability of the researcher, opening possibility for improvement, or development of alternative techniques. Phage display, for example, is a versatile technology, which allows the presentation of many different oligopeptides that can be tested against different antibodies, fitting the need for an epitope mapping approach. In this chapter, a protocol for the construction of a single-target oligopeptide phage library, as well as for the panning procedure for epitope mapping using phage display is given.

Epitope-focused immunogens against the CD4-binding site of HIV-1 envelope protein induce neutralizing antibodies against auto- and heterologous viruses

Recent discoveries of broadly neutralizing antibodies (bnAbs) in HIV-1-infected individuals have led to the identification of several major "vulnerable sites" on the HIV-1 envelope (Env) glycoprotein. These sites have provided precise targets for HIV-1 vaccine development, but identifying and utilizing many of these targets remain technically challenging. Using a yeast surface display-based approach, we sought to identify epitope-focused antigenic domains (EADs) containing one of the "vulnerable sites," the CD4-binding site (CD4bs), through screening and selection of a combinatorial antigen library of the HIV-1 envelope glycoprotein with the CD4bs bnAb VRC01. We isolated multiple EADs and found that their trimeric forms have biochemical and structural features that preferentially bind and activate B cells that express VRC01 in vitro More importantly, these EADs could induce detectable levels of neutralizing antibodies against genetically related autologous and heterologous subtype B viruses in guinea pigs. Our results demonstrate that an epitope-focused approach involving a screen of a combinatorial antigen library is feasible. The EADs identified here represent a promising collection of possible targets in the rational design of HIV-1 vaccines and lay the foundation for harnessing the specific antigenicity of CD4bs for protective immunogenicity in vivo.

Extracellular Matrix Proteins Mediate HIV-1 gp120 Interactions with α4β7

Gut-homing α₄β₇^high CD4⁺ T lymphocytes have been shown to be preferentially targeted by human immunodeficiency virus type 1 (HIV-1) and are implicated in HIV-1 pathogenesis. Previous studies demonstrated that HIV-1 envelope protein gp120 binds and signals through α₄β₇ and that this likely contributes to the infection of α₄β₇^high T cells and promotes cell-to-cell virus transmission. Structures within the second variable loop (V2) of gp120, including the tripeptide motif LDV/I, are thought to mediate gp120-α₄β₇ binding. However, lack of α₄β₇ binding has been reported in gp120 proteins containing LDV/I, and the precise determinants of gp120-α₄β₇ binding are not fully defined. In this work, we report the novel finding that fibronectins mediate indirect gp120-α₄β₇ interactions. We show that Chinese hamster ovary (CHO) cells used to express recombinant gp120 produced fibronectins and other extracellular matrix proteins that copurified with gp120. CHO cell fibronectins were able to mediate the binding of a diverse panel of gp120 proteins to α₄β₇ in an in vitro cell binding assay. The V2 loop was not required for fibronectin-mediated binding of gp120 to α₄β₇, nor did V2-specific antibodies block this interaction. Removal of fibronectin through anion-exchange chromatography abrogated V2-independent gp120-α₄β₇ binding. Additionally, we showed a recombinant human fibronectin fragment mediated gp120-α₄β₇ interactions similarly to CHO cell fibronectin. These findings provide an explanation for the apparently contradictory observations regarding the gp120-α₄β₇ interaction and offer new insights into the potential role of fibronectin and other extracellular matrix proteins in HIV-1 biology.IMPORTANCE Immune tissues within the gut are severely damaged by HIV-1, and this plays an important role in the development of AIDS. Integrin α₄β₇ plays a major role in the trafficking of lymphocytes, including CD4⁺ T cells, into gut lymphoid tissues. Previous reports indicate that some HIV-1 gp120 envelope proteins bind to and signal through α₄β₇, which may help explain the preferential infection of gut CD4⁺ T cells. In this study, we demonstrate that extracellular matrix proteins can mediate interactions between gp120 and α₄β₇ This suggests that the extracellular matrix may be an important mediator of HIV-1 interaction with α₄β₇-expressing cells. These findings provide new insight into the nature of HIV-1-α₄β₇ interactions and how these interactions may represent targets for therapeutic intervention.

Particulate Array of Well-Ordered HIV Clade C Env Trimers Elicits Neutralizing Antibodies that Display a Unique V2 Cap Approach

The development of soluble envelope glycoprotein (Env) mimetics displaying ordered trimeric symmetry has ushered in a new era in HIV-1 vaccination. The recently reported native, flexibly linked (NFL) design allows the generation of native-like trimers from clinical isolates at high yields and homogeneity. As the majority of infections world-wide are of the clade C subtype, we examined responses in non-human primates to well-ordered subtype C 16055 trimers administered in soluble or high-density liposomal formats. We detected superior germinal center formation and enhanced autologous neutralizing antibodies against the neutralization-resistant (tier 2) 16055 virus following inoculation of liposome-arrayed trimers. Epitope mapping of the neutralizing monoclonal antibodies (mAbs) indicated major contacts with the V2 apex, and 3D electron microscopy reconstructions of Fab-trimer complexes revealed a horizontal binding angle to the Env spike. These vaccine-elicited mAbs target the V2 cap, demonstrating a means to accomplish tier 2 virus neutralization by penetrating the dense N-glycan shield.

How HIV-1 entry mechanism and broadly neutralizing antibodies guide structure-based vaccine design

PURPOSE OF REVIEW

An HIV-1 vaccine that elicits broadly neutralizing antibodies (bNAbs) remains to be developed. Here, we review how knowledge of bNAbs and HIV-1 entry mechanism is guiding the structure-based design of vaccine immunogens and immunization regimens.

RECENT FINDINGS

Isolation of bNAbs from HIV-1-infected donors has led to an unprecedented understanding of the sites of vulnerability that these antibodies target on the HIV-1 envelope (Env) as well as of the immunological pathways that these antibody lineages follow to develop broad and potent neutralization. Sites of vulnerability, however, reside in the context of diverse Env conformations required for HIV-1 entry, including a prefusion-closed state, a single-CD4-bound intermediate, a three-CD4-bound intermediate, a prehairpin intermediate and postfusion states, and it is not always clear which structural state optimally presents a particular site of vulnerability in the vaccine context. Furthermore, detailed knowledge of immunological pathways has led to debate among vaccine developers as to how much of the natural antibody-developmental pathway immunogens should mimic, ranging from only the recognized epitope to multiple antigens from the antibody-virus coevolution process.

SUMMARY

A plethora of information on bNAbs is guiding HIV-1-vaccine development. We highlight consideration of the appropriate structural context from the HIV-1-entry mechanism and extraordinary progress with replicating template B-cell ontogenies.

Comprehensive Characterization of Humoral Correlates of Human Immunodeficiency Virus 1 Superinfection Acquisition in High-risk Kenyan Women

HIV-1 superinfection, in which an infected individual acquires a second HIV-1 infection from a different partner, is one of the only settings in which HIV acquisition occurs in the context of a pre-existing immune response to natural HIV infection. There is evidence that initial infection provides some protection from superinfection, particularly after 6 months of initial infection, when development of broad immunity occurs. Comparison of the immune response of superinfected individuals at the time of superinfection acquisition to that of individuals who remain singly infected despite continued exposure can shed light on immune correlates of HIV acquisition to inform prophylactic vaccine design. We evaluated a panel of humoral immune responses in the largest published group of superinfected individuals (n = 21), compared to a set of 3:1 matched singly infected controls from the same cohort. The immune functions studied included plasma neutralization, plasma and cervical antibody-dependent cellular cytotoxicity, and plasma IgG and IgA binding to a panel of 18 envelope antigens, including correlates of HIV acquisition in the RV144 vaccine trial, IgG binding to V1V2 and IgA binding to gp140. Association between each immune function and HIV superinfection was evaluated using conditional logistic regression. No significant associations were detected between any of the immune functions and superinfection acquisition. This study constitutes the most comprehensive and detailed characterization of multiple immune correlates of superinfection to date. The results suggest that immune responses not commonly measured in current HIV studies may be important in protection from HIV infection, and these or a more robust humoral response than that seen in naturally infected women may be needed for a protective vaccine.

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We conducted a case-control study of the association between humoral immune functions and HIV superinfection (SI).

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Neutralization, antibody-dependent cellular cytotoxicity, and IgG and IgA binding to Env antigens were interrogated.

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We found no significant associations between SI acquisition and neutralizing or non-neutralizing antibody activity.

HIV superinfection (repeat infection from a second partner) is a unique situation in which infection occurs in the presence of a pre-existing HIV-specific immune response. Identification of immune deficits in superinfected individuals prior to superinfection can shed light on immune functions associated with HIV acquisition, and help inform prophylactic vaccine development. We compared various antibody measures in superinfected women vs. women who remained singly infected. We found no evidence that deficits in any of the measures analyzed were associated with superinfection risk. This suggests a prophylactic vaccine may need to elicit stronger or different immune responses than those investigated here.

Vaccine nanoparticles for protection against HIV infection

The development of a successful vaccine against HIV is a major global challenge. Antiretroviral therapy is the standard treatment against HIV-1 infection. However, only 46% of the eligible people received the therapy in 2015. Furthermore, suboptimal adherence poses additional obstacles. Therefore, there is an urgent need for an HIV-1 vaccine. The most promising clinical trial to date is Phase III RV144, which for the first time demonstrated the feasibility of vaccine-mediated immune protection against HIV-1. Nevertheless, its 31% efficacy and limited durability underscore major hurdles. Here, we discuss recent progress in HIV-1 vaccine development with a special emphasis on nanovaccines, which are at the forefront of efforts to develop a successful HIV-1 vaccine.

Rationally Designed Vaccines Targeting the V2 Region of HIV-1 gp120 Induce a Focused, Cross-Clade-Reactive, Biologically Functional Antibody Response

Strong antibody (Ab) responses against V1V2 epitopes of the human immunodeficiency virus type 1 (HIV-1) gp120 envelope (Env) correlated with reduced infection rates in studies of HIV, simian-human immunodeficiency virus (SHIV), and simian immunodeficiency virus (SIV). In order to focus the Ab response on V1V2, we used six V1V2 sequences and nine scaffold proteins to construct immunogens which were tested using various immunization regimens for their ability to induce cross-reactive and biologically active V2 Abs in rabbits. A prime/boost immunization strategy was employed using gp120 DNA and various V1V2-scaffold proteins. The rabbit polyclonal Ab responses (i) were successfully focused on the V1V2 region, with weak or only transient responses to other Env epitopes, (ii) displayed broad cross-reactive binding activity with gp120s and the V1V2 regions of diverse strains from clades B, C, and E, (iii) included V2 Abs with specificities similar to those found in HIV-infected individuals, and (iv) remained detectable ≥1 year after the last boosting dose. Importantly, sera from rabbits receiving V1V2-scaffold immunogens displayed Ab-dependent cellular phagocytosis whereas sera from rabbits receiving only gp120 did not. The results represent the first fully successful example of reverse vaccinology in the HIV vaccine field with rationally designed epitope scaffold immunogens inducing Abs that recapitulate the epitope specificity and biologic activity of the human monoclonal Abs from which the immunogens were designed. Moreover, this is the first immunogenicity study using epitope-targeting, rationally designed vaccine constructs that induced an Fc-mediated activity associated with protection from infection with HIV, SIV, and SHIV.

IMPORTANCE

Novel immunogens were designed to focus the antibody response of rabbits on the V1V2 epitopes of HIV-1 gp120 since such antibodies were associated with reduced infection rates of HIV, SIV, and SHIV. The vaccine-induced antibodies were broadly cross-reactive with the V1V2 regions of HIV subtypes B, C and E and, importantly, facilitated Fc-mediated phagocytosis, an activity not induced upon immunization of rabbits with gp120. This is the first immunogenicity study of vaccine constructs that focuses the antibody response on V1V2 and induces V2-specific antibodies with the ability to mediate phagocytosis, an activity that has been associated with protection from infection with HIV, SIV, and SHIV.