Inhibition of virus attachment to CD4+ target cells is a major mechanism of T cell line-adapted HIV-1 neutralization

Antibody-mediated control mechanisms of viral infections

Antibodies generated after vaccination or natural pathogen exposure are essential mediators of protection against many infections. Most studies with viruses have focused on antibody neutralization, in which protection is conferred by the fragment antigen binding region (Fab) through targeting of different steps in the viral lifecycle including attachment, internalization, fusion, and egress. Beyond neutralization, the fragment crystallizable (Fc) region of antibodies can integrate innate and adaptive immune responses by engaging complement components and distinct Fc gamma receptors (FcγR) on different host immune cells. In this review, we discuss recent advances in our understanding of antibody neutralization and Fc effector functions, and the assays used to measure them. Additionally, we describe the contexts in which these mechanisms are associated with protection against viruses and highlight how Fc-FcγR interactions can improve the potency of antibody-based therapies.

Antiviral neutralizing antibodies: from in vitro to in vivo activity

Neutralizing antibodies (nAbs) are being increasingly used as passive antiviral reagents in prophylactic and therapeutic modalities and to guide viral vaccine design. In vivo, nAbs can mediate antiviral functions through several mechanisms, including neutralization, which is defined by in vitro assays in which nAbs block viral entry to target cells, and antibody effector functions, which are defined by in vitro assays that evaluate nAbs against viruses and infected cells in the presence of effector systems. Interpreting in vivo results in terms of these in vitro assays is challenging but important in choosing optimal passive antibody and vaccine strategies. Here, I review findings from many different viruses and conclude that, although some generalizations are possible, deciphering the relative contributions of different antiviral mechanisms to the in vivo efficacy of antibodies currently requires consideration of individual antibody-virus interactions.

To bnAb or Not to bnAb: Defining Broadly Neutralising Antibodies Against HIV-1

Since their discovery, antibodies capable of broad neutralisation have been at the forefront of HIV-1 research and are of particular interest due to in vivo passive transfer studies demonstrating their potential to provide protection. Currently an exact definition of what is required for a monoclonal antibody to be classed as a broadly neutralising antibody (bnAb) has not yet been established. This has led to hundreds of antibodies with varying neutralisation breadth being studied and has given insight into antibody maturation pathways and epitopes targeted. However, even with this knowledge, immunisation studies and vaccination trials to date have had limited success in eliciting antibodies with neutralisation breadth. For this reason there is a growing need to identify factors specifically associated with bnAb development, yet to do this a set of criteria is necessary to distinguish bnAbs from non-bnAbs. This review aims to define what it means to be a HIV-1 bnAb by comparing neutralisation breadth, genetic features and epitopes of bnAbs, and in the process highlights the challenges of comparing the array of antibodies that have been isolated over the years.

Diverse antiviral IgG effector activities are predicted by unique biophysical antibody features

BACKGROUND

The critical role of antibody Fc-mediated effector functions in immune defense has been widely reported in various viral infections. These effector functions confer cellular responses through engagement with innate immune cells. The precise mechanism(s) by which immunoglobulin G (IgG) Fc domain and cognate receptors may afford protection are poorly understood, however, in the context of HIV/SHIV infections. Many different in vitro assays have been developed and utilized to measure effector functions, but the extent to which these assays capture distinct antibody activities has not been fully elucidated.

RESULTS

In this study, six Fc-mediated effector function assays and two biophysical antibody profiling assays were performed on a common set of samples from HIV-1 infected and vaccinated subjects. Biophysical antibody profiles supported robust prediction of diverse IgG effector functions across distinct Fc-mediated effector function assays. While a number of assays showed correlated activities, supervised machine learning models indicated unique antibody features as primary contributing factors to the associated effector functions. Additional experiments established the mechanistic relevance of relationships discovered using this unbiased approach.

CONCLUSIONS

In sum, this study provides better resolution on the diversity and complexity of effector function assays, offering a clearer perspective into this family of antibody mechanisms of action to inform future HIV-1 treatment and vaccination strategies.

Enfuvirtide-Protoporphyrin IX Dual-Loaded Liposomes: In Vitro Evidence of Synergy against HIV-1 Entry into Cells

We have developed a nanocarrier consisting of large unilamellar vesicles (LUVs) for combined delivery of two human immunodeficiency virus type 1 (HIV-1) entry inhibitors, enfuvirtide (ENF) and protoporphyrin IX (PPIX). The intrinsic lipophilicity of ENF and PPIX, a fusion inhibitor and an attachment inhibitor, respectively, leads to their spontaneous incorporation into the lipid bilayer of the LUVs nanocarrier. Both entry inhibitors partition significantly toward LUVs composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and a 9:1 mixture of POPC:1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DPPE-PEG₂₀₀₀), representative of conventional and immune-evasive drug delivery formulations, respectively. These colocalize in the core of lipid membranes. Dual-loaded nanocarriers are monodispersed and retain the size distribution, thermotropic behavior, and surface charge of the unloaded form. Combination of the two entry inhibitors in the nanocarrier resulted in improved synergy against HIV-1 entry compared to combination in free form, strongly when immune-evasive formulations are used. We propose that the improved action of the entry inhibitors when loaded into the nanocarriers results from their slow release at the site of viral entry. Overall, liposomes remain largely unexplored platforms for combination of viral entry inhibitors, with potential for improvement of current antiretroviral therapy drug safety and application. Our work calls for a reappraisal of the potential of entry inhibitor combinations and delivery for clinical use in antiretroviral therapy.

Closing and Opening Holes in the Glycan Shield of HIV-1 Envelope Glycoprotein SOSIP Trimers Can Redirect the Neutralizing Antibody Response to the Newly Unmasked Epitopes

In HIV-1 vaccine research, native-like, soluble envelope glycoprotein SOSIP trimers are widely used for immunizing animals. The epitopes of autologous neutralizing antibodies (NAbs) induced by the BG505 and B41 SOSIP trimers in rabbits and macaques have been mapped to a few holes in the glycan shields that cover most of the protein surfaces. For BG505 trimers, the dominant autologous NAb epitope in rabbits involves residues that line a cavity caused by the absence of a glycan at residue 241. Here, we blocked this epitope in BG505 SOSIPv4.1 trimer immunogens by knocking in an N-linked glycan at residue 241. We then opened holes elsewhere on the trimer by knocking out single N-linked glycans at residues 197, 234, 276, 332, and 355 and found that NAb responses induced by the 241-glycan-bearing BG505 trimers were frequently redirected to the newly opened sites. The strongest evidence for redirection of the NAb response to neoepitopes, through the opening and closing of glycan holes, was obtained from trimer immunogen groups with the highest occupancy of the N241 site. We also attempted to knock in the N289-glycan to block the sole autologous NAb epitope on the B41 SOSIP.v4.1 trimer. Although a retrospective analysis showed that the new N289-glycan site was substantially underoccupied, we found some evidence for redirection of the NAb response to a neoepitope when this site was knocked in and the N356-glycan site knocked out. In neither study, however, was redirection associated with increased neutralization of heterologous tier 2 viruses.IMPORTANCE Engineered SOSIP trimers mimic envelope-glycoprotein spikes, which stud the surface of HIV-1 particles and mediate viral entry into cells. When used for immunizing test animals, they elicit antibodies that neutralize resistant sequence-matched HIV-1 isolates. These neutralizing antibodies recognize epitopes in holes in the glycan shield that covers the trimer. Here, we added glycans to block the most immunogenic neutralization epitopes on BG505 and B41 SOSIP trimers. In addition, we removed selected other glycans to open new holes that might expose new immunogenic epitopes. We immunized rabbits with the various glycan-modified trimers and then dissected the specificities of the antibody responses. Thus, in principle, the antibody response might be diverted from one site to a more cross-reactive one, which would help in the induction of broadly neutralizing antibodies by HIV-1 vaccines based on envelope glycoproteins.

Aptamers in HIV research diagnosis and therapy

Aptamers are high affinity single-stranded nucleic acid or protein ligands which exhibit specificity and avidity comparable to, or exceeding that of antibodies and can be generated against most targets. The functionality of aptamers is based on their unique tertiary structure, complexity and their ability to attain unique binding pockets by folding. Aptamers are selected in vitro by a process called Systematic Evolution of Ligands by Exponential enrichment (SELEX). The Kd values for the selected aptamer are often in the picomolar to low nanomolar range. Stable and nontoxic aptamers could be selected for a wide range of ligands including small molecules to large proteins. Aptamers have shown tremendous potential and have found multipurpose application in the field of therapeutic, diagnostic, biosensor and bio-imaging. While their mechanism of action can be similar to that of monoclonal antibodies, aptamers provide additional advantages in terms of production cost, simpler regulatory approval and lower immunogenicity as they are synthesized chemically. Human immunodeficiency virus (HIV) is the primary cause of acquired immune deficiency syndrome (AIDS), which causes significant morbidity and mortality with a significant consequent decrease in the quality of patient's lives. While cART has led to good viral control, people living with HIV now suffer from non-HIV comorbidities due to viral protein expression that cannot be controlled by cART. Hence pathophysiological mechanisms that govern these comorbidities with a focus on therapies that neutralize these HIV effects gained increased attention. Recent advances in HIV/AIDS research have identified several molecular targets and for the development of therapeutic and diagnostic using aptamers against HIV/AIDS. This review presents recent advances in aptamers technology for potential application in HIV diagnostics and therapeutics towards improving the quality of life of people living with HIV.

Broadly Neutralizing Antibodies to HIV and Their Role in Vaccine Design

HIV employs multiple means to evade the humoral immune response, particularly the elicitation of and recognition by broadly neutralizing antibodies (bnAbs). Such antibodies can act antivirally against a wide spectrum of viruses by targeting relatively conserved regions on the surface HIV envelope trimer spike. Elicitation of and recognition by bnAbs are hindered by the arrangement of spikes on virions and the relatively difficult access to bnAb epitopes on spikes, including the proximity of variable regions and a high density of glycans. Yet, in a small proportion of HIV-infected individuals, potent bnAb responses do develop, and isolation of the corresponding monoclonal antibodies has been facilitated by identification of favorable donors with potent bnAb sera and by development of improved methods for human antibody generation. Molecular studies of recombinant Env trimers, alone and in interaction with bnAbs, are providing new insights that are fueling the development and testing of promising immunogens aimed at the elicitation of bnAbs.

Stabilized HIV-1 envelope glycoprotein trimers for vaccine use

Purpose of review

To provide an update on the latest developments in the field of HIV-1 antibody-based soluble envelope glycoprotein (Env) trimer design for vaccine use.

Recent findings

The development of soluble native-like HIV-1 Env trimer immunogens has moved the field of antibody-based vaccine design forward dramatically over the past few years with refinement of various stabilizing approaches. However, despite this progress, significant challenges remain. Firstly, although trimers are relatively stable in solution, they nevertheless sample different conformational states, some of which may be less relevant to binding and induction of broadly neutralizing antibodies (bNAbs). Secondly, these trimers expose unwanted immunodominant surfaces that may distract the adaptive immune response from recognizing more immunorecessive but conserved neutralization-relevant surfaces on the trimer. The availability of atomic-resolution structural information has allowed guided design of mutations that have further stabilized trimers and allowed reduced exposure of unwanted epitopes. Moreover, chemical cross-linking approaches that do not require structural information have also contributed to trimer stabilization and selection of particular conformational forms. However, current knowledge suggests that strategies additional to trimer stabilization will be required to elicit bNAb, including targeting naïve B cell receptors with specific immunogens, and guiding B cell lineages toward recognizing conserved surfaces on Env with high affinity.

Summary

This review will give a perspective on these challenges, and summarize current approaches to overcoming them with the aim of developing immunogens to elicit bNAb responses in humans by active vaccination.

Chlamydia trachomatis Infection of Endocervical Epithelial Cells Enhances Early HIV Transmission Events

Chlamydia trachomatis causes a predominantly asymptomatic, but generally inflammatory, genital infection that is associated with an increased risk for HIV acquisition. Endocervical epithelial cells provide the major niche for this obligate intracellular bacterium in women, and the endocervix is also a tissue in which HIV transmission can occur. The mechanism by which CT infection enhances HIV susceptibility at this site, however, is not well understood. Utilizing the A2EN immortalized endocervical epithelial cell line grown on cell culture inserts, we evaluated the direct role that CT-infected epithelial cells play in facilitating HIV transmission events. We determined that CT infection significantly enhanced the apical-to-basolateral migration of cell-associated, but not cell-free, HIVBaL, a CCR5-tropic strain of virus, across the endocervical epithelial barrier. We also established that basolateral supernatants from CT-infected A2EN cells significantly enhanced HIV replication in peripheral mononuclear cells and a CCR5+ T cell line. These results suggest that CT infection of endocervical epithelial cells could facilitate both HIV crossing the mucosal barrier and subsequent infection or replication in underlying target cells. Our studies provide a mechanism by which this common STI could potentially promote the establishment of founder virus populations and the maintenance of local HIV reservoirs in the endocervix. Development of an HIV/STI co-infection model also provides a tool to further explore the role of other sexually transmitted infections in enhancing HIV acquisition.

Fc receptors and their influence on efficacy of therapeutic antibodies for treatment of viral diseases

The lack of vaccines against several important viral diseases necessitates the development of therapeutics to save lives and control epidemics. In recent years, therapeutic antibodies have received considerable attention due to their good safety profiles and clinical success when used against viruses such as respiratory syncytial virus, Ebola virus and Hendra virus. The binding affinity of these antibodies can directly impact their therapeutic efficacy. However, we and others have also demonstrated that the subtype of Fc-gamma receptors (FcγRs) engaged influences the stoichiometric requirement for virus neutralization. Hence, the development of therapeutic antibodies against infectious diseases should consider the FcγRs engaged and Fc-effector functions involved. This review highlights the current state of knowledge about FcγRs and FcγR effector functions involved in virus neutralization, with emphasis on factors that can affect FcγR engagement. A better understanding of Fc-FcγR interactions during virus neutralization will allow development of therapeutic antibodies that are efficacious and can be administered with minimal side effects.

Capacity of Broadly Neutralizing Antibodies to Inhibit HIV-1 Cell-Cell Transmission Is Strain- and Epitope-Dependent

An increasing number of broadly neutralizing antibodies (bnAbs) are considered leads for HIV-1 vaccine development and novel therapeutics. Here, we systematically explored the capacity of bnAbs to neutralize HIV-1 prior to and post-CD4 engagement and to block HIV-1 cell-cell transmission. Cell-cell spread is known to promote a highly efficient infection with HIV-1 which can inflict dramatic losses in neutralization potency compared to free virus infection. Selection of bnAbs that are capable of suppressing HIV irrespective of the transmission mode therefore needs to be considered to ascertain their in vivo activity in therapeutic use and vaccines. Employing assay systems that allow for unambiguous discrimination between free virus and cell-cell transmission to T cells, we probed a panel of 16 bnAbs for their activity against 11 viruses from subtypes A, B and C during both transmission modes. Over a wide range of bnAb-virus combinations tested, inhibitory activity against HIV-1 cell-cell transmission was strongly decreased compared to free virus transmission. Activity loss varied considerably between virus strains and was inversely associated with neutralization of free virus spread for V1V2- and V3-directed bnAbs. In rare bnAb-virus combinations, inhibition for both transmission modes was comparable but no bnAb potently blocked cell-cell transmission across all probed virus strains. Mathematical analysis indicated an increased probability of bnAb resistance mutations to arise in cell-cell rather than free virus spread, further highlighting the need to block this pathway. Importantly, the capacity to efficiently neutralize prior to CD4 engagement correlated with the inhibition efficacy against free virus but not cell-cell transmitted virus. Pre-CD4 attachment activity proved strongest amongst CD4bs bnAbs and varied substantially for V3 and V1V2 loop bnAbs in a strain-dependent manner. In summary, bnAb activity against divergent viruses varied depending on the transmission mode and differed depending on the window of action during the entry process, underscoring that powerful combinations of bnAbs are needed for in vivo application.

When selecting broadly neutralizing antibodies (bnAbs) for clinical application, potency and breadth against free viruses are vital, but additional features may be needed to ensure in vivo efficacy. Considering that HIV-1 can utilize free virus and cell-cell transmission to infect, the efficacy of neutralizing antibodies in vivo may depend on their ability to block both pathways. While breadth and potency of bnAbs against free viruses have been intensely studied, their precise activity during cell-cell spread remains uncertain. Our analysis of the cell-cell neutralization capacity of a large selection of bnAbs against a spectrum of HIV-1 strains revealed that while bnAbs showed an overall decreased activity during cell-cell transmission, losses varied substantially depending on bnAb and virus strain probed. Although bnAbs occasionally retained activity during cell-cell transmission for individual viruses, this ability was rare and generally not associated with a high potency against free virus spread. Notably, neutralization of free virus but not cell-cell transmission was linked with the activity of bnAbs to inhibit prior to CD4 engagement, highlighting the functional differences of the processes. Since no single bnAb combines the entire range of mechanistic features anticipated to support in vivo efficacy, our study adds further evidence that combinations of bnAbs need to be considered for human application.

Gp120/CD4 blocking antibodies are frequently elicited in ART-naïve chronically HIV-1 infected individuals

Antibodies with the ability to block the interaction of HIV-1 envelope glycoprotein (Env) gp120 with CD4, including those overlapping the CD4 binding site (CD4bs antibodies), can protect from infection by HIV-1, and their elicitation may be an interesting goal for any vaccination strategy. To identify gp120/CD4 blocking antibodies in plasma samples from HIV-1 infected individuals we have developed a competitive flow cytometry-based functional assay. In a cohort of treatment-naïve chronically infected patients, we showed that gp120/CD4 blocking antibodies were frequently elicited (detected in 97% plasma samples) and correlated with binding to trimeric HIV-1 envelope glycoproteins. However, no correlation was observed between functional CD4 binding blockade data and titer of CD4bs antibodies determined by ELISA using resurfaced gp120 proteins. Consistently, plasma samples lacking CD4bs antibodies were able to block the interaction between gp120 and its receptor, indicating that antibodies recognizing other epitopes, such as PGT126 and PG16, can also play the same role. Antibodies blocking CD4 binding increased over time and correlated positively with the capacity of plasma samples to neutralize the laboratory-adapted NL4.3 and BaL virus isolates, suggesting their potential contribution to the neutralizing workforce of plasma in vivo. Determining whether this response can be boosted to achieve broadly neutralizing antibodies may provide valuable information for the design of new strategies aimed to improve the anti-HIV-1 humoral response and to develop a successful HIV-1 vaccine.

Neutralisation of HIV-1 cell-cell spread by human and llama antibodies

BACKGROUND

Direct cell-cell spread of HIV-1 is a very efficient mode of viral dissemination, with increasing evidence suggesting that it may pose a considerable challenge to controlling viral replication in vivo. Much current vaccine research involves the study of broadly neutralising antibodies (bNabs) that arise during natural infection with the aims of eliciting such antibodies by vaccination or incorporating them into novel therapeutics. However, whether cell-cell spread of HIV-1 can be effectively targeted by bNabs remains unclear, and there is much interest in identifying antibodies capable of efficiently neutralising virus transmitted by cell-cell contact.

RESULTS

In this study we have tested a panel of bNAbs for inhibition of cell-cell spread, including some not previously evaluated for inhibition of this mode of HIV-1 transmission. We found that three CD4 binding site antibodies, one from an immunised llama (J3) and two isolated from HIV-1-positive patients (VRC01 and HJ16) neutralised cell-cell spread between T cells, while antibodies specific for glycan moieties (2G12, PG9, PG16) and the MPER (2F5) displayed variable efficacy. Notably, while J3 displayed a high level of potency during cell-cell spread we found that the small size of the llama heavy chain-only variable region (VHH) J3 is not required for efficient neutralisation since recombinant J3 containing a full-length human heavy chain Fc domain was significantly more potent. J3 and J3-Fc also neutralised cell-cell spread of HIV-1 from primary macrophages to CD4+ T cells.

CONCLUSIONS

In conclusion, while bNabs display variable efficacy at preventing cell-cell spread of HIV-1, we find that some CD4 binding site antibodies can inhibit this mode of HIV-1 dissemination and identify the recently described llama antibody J3 as a particularly potent inhibitor. Effective neutralisation of cell-cell spread between physiologically relevant cell types by J3 and J3-Fc supports the development of VHH J3 nanobodies for therapeutic or prophylactic applications.

Neutralization of Virus Infectivity by Antibodies: Old Problems in New Perspectives

Neutralizing antibodies (NAbs) can be both sufficient and necessary for protection against viral infections, although they sometimes act in concert with cellular immunity. Successful vaccines against viruses induce NAbs but vaccine candidates against some major viral pathogens, including HIV-1, have failed to induce potent and effective such responses. Theories of how antibodies neutralize virus infectivity have been formulated and experimentally tested since the 1930s; and controversies about the mechanistic and quantitative bases for neutralization have continually arisen. Soluble versions of native oligomeric viral proteins that mimic the functional targets of neutralizing antibodies now allow the measurement of the relevant affinities of NAbs. Thereby the neutralizing occupancies on virions can be estimated and related to the potency of the NAbs. Furthermore, the kinetics and stoichiometry of NAb binding can be compared with neutralizing efficacy. Recently, the fundamental discovery that the intracellular factor TRIM21 determines the degree of neutralization of adenovirus has provided new mechanistic and quantitative insights. Since TRIM21 resides in the cytoplasm, it would not affect the neutralization of enveloped viruses, but its range of activity against naked viruses will be important to uncover. These developments bring together the old problems of virus neutralization-mechanism, stoichiometry, kinetics, and efficacy-from surprising new angles.

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

Background

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

Results

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

Conclusions

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

The molecular basis of HIV entry

Infection by HIV starts when the virus attaches to a susceptible cell. For viral replication to continue, the viral envelope must fuse with a cellular membrane, thereby delivering the viral core to the cytoplasm, where the RNA genome is reverse-transcribed. The key players in this entry by fusion are the envelope glycoprotein, on the viral side, and CD4 and a co-receptor, CCR5 or CXCR4, on the cellular side. Here, the interplay of these molecules is reviewed from cell-biological, structural, mechanistic, and modelling-based perspectives. Hypotheses are evaluated regarding the cellular compartment for entry, the transfer of virus through direct cell-to-cell contact, the sequence of molecular events, and the number of molecules involved on each side of the virus-cell divide. An emerging theme is the heterogeneity among the entry mediators on both sides, a diversity that affects the efficacy of entry inhibitors, be they small-molecule ligands, peptides or neutralizing antibodies. These insights inform rational strategies for therapy as well as vaccination.

Potent and broad neutralization of HIV-1 by a llama antibody elicited by immunization

A heavy chain–only antibody isolated from a llama repeatedly immunized with trimeric HIV-1 Env neutralizes 96% of tested HIV-1 strains.

Llamas (Lama glama) naturally produce heavy chain–only antibodies (Abs) in addition to conventional Abs. The variable regions (VHH) in these heavy chain–only Abs demonstrate comparable affinity and specificity for antigens to conventional immunoglobulins despite their much smaller size. To date, immunizations in humans and animal models have yielded only Abs with limited ability to neutralize HIV-1. In this study, a VHH phagemid library generated from a llama that was multiply immunized with recombinant trimeric HIV-1 envelope proteins (Envs) was screened directly for HIV-1 neutralization. One VHH, L8CJ3 (J3), neutralized 96 of 100 tested HIV-1 strains, encompassing subtypes A, B, C, D, BC, AE, AG, AC, ACD, CD, and G. J3 also potently neutralized chimeric simian-HIV strains with HIV subtypes B and C Env. The sequence of J3 is highly divergent from previous anti–HIV-1 VHH and its own germline sequence. J3 achieves broad and potent neutralization of HIV-1 via interaction with the CD4-binding site of HIV-1 Env. This study may represent a new benchmark for immunogens to be included in B cell–based vaccines and supports the development of VHH as anti–HIV-1 microbicides.

Kinetic mechanism for HIV-1 neutralization by antibody 2G12 entails reversible glycan binding that slows cell entry

Despite structural knowledge of broadly neutralizing monoclonal antibodies (NMAbs) complexed to HIV-1 gp120 and gp41 envelope glycoproteins, virus inactivation mechanisms have been difficult to prove, in part because neutralization assays are complex and were previously not understood. Concordant with recent evidence that HIV-1 titers are determined by a race between entry of cell-attached virions and competing inactivation processes, we show that NMAb 2G12, which binds to gp120 N-glycans with α (1, 2)-linked mannose termini and inhibits replication after passive transfer into patients, neutralizes by slowing entry of adsorbed virions. Accordingly, apparent neutralization is attenuated when a kinetically competing virus inactivation pathway is blocked. Moreover, removing 2G12 from media causes its dissociation from virions coupled to accelerated entry and restored infectivity, demonstrating the reversibility of neutralization. A difference between 2G12 dissociation and infectivity recovery rates implies that the inhibited complexes at virus-cell junctions contain several 2G12's that must dissociate before entry commences. Quantitative microscopy of 2G12 binding and dissociation from single virions and studies using a split CCR5 coreceptor suggest that 2G12 competitively inhibits interactions between gp120's V3 loop and the tyrosine sulfate-containing CCR5 amino terminus, thereby reducing assembly of complexes that catalyze entry. These results reveal a unique reversible kinetic mechanism for neutralization by an antibody that binds near a critical V3 region in the glycan shield of gp120.

Env-glycoprotein heterogeneity as a source of apparent synergy and enhanced cooperativity in inhibition of HIV-1 infection by neutralizing antibodies and entry inhibitors

We measured the inhibition of infectivity of HIV-1 isolates and derivative clones by combinations of neutralizing antibodies (NAbs) and other entry inhibitors in a single-cycle-replication assay. Synergy was analyzed both by the current linear and a new non-linear method. The new method reduced spurious indications of synergy and antagonism. Synergy between NAbs was overall weaker than between other entry inhibitors, and no stronger where one ligand is known to enhance the binding of another. However, synergy was stronger for a genetically heterogeneous HIV-1 R5 isolate than for its derivative clones. Enhanced cooperativity in inhibition by combinations, compared with individual inhibitors, correlated with increased synergy at higher levels of inhibition, while being less variable. Again, cooperativity enhancement was stronger for isolates than clones. We hypothesize that genetic, post-translational or conformational heterogeneity of the Env protein and of other targets for inhibitors can yield apparent synergy and increased cooperativity between inhibitors.

A cell-to-cell HIV transfer assay identifies humoral responses with broad neutralization activity

BACKGROUND

Cell-to-cell HIV spread through virological synapses proceeds in two steps, first HIV particles are rapidly transferred to target cells in a CD4-dependent manner and then coreceptor-dependent events allow for infection or death of single target cells and cell-to-cell fusion.

METHODS

293T or MOLT cells producing HIV particles were cocultured with primary CD4 T-cells or reporter cell lines. The extent of HIV transfer, cell fusion and target cell death was assessed. Inhibition by sera from 19 HIV-infected patients was evaluated and compared with cell-free HIV neutralization using different envelopes from clades A, B, C and E.

RESULTS

Sera showed different abilities to protect CD4 T-cells from cell-to-cell transfer, fusion or death when cocultured with HIV producing 293T cells. Some sera were able to block all parameters (a property of IgGb12), while other showed lower activity against HIV transfer despite being able to block fusion and death (a property of antibodies blocking post-CD4 binding steps). Neutralization of cell-to-cell HIV transfer strongly correlated with IgG binding to native Env. Interestingly, sera that efficiently blocked HIV transfer showed broader neutralizing response, as they neutralized a higher percentage of the viruses tested compared with sera showing low CD4 binding site responses (P=0.01). Similar results were observed in a model of T cell-T cell HIV transmission, although this experimental model showed lower capacity to discriminate broadly neutralizing responses.

CONCLUSION

Cell-to-cell HIV transfer assays identify sera with broadly neutralizing capacity and may help to characterize anti-HIV humoral responses.

A post-entry role for CD63 in early HIV-1 replication

Macrophages and CD4(+) lymphocytes are the major reservoirs for HIV-1 infection. CD63 is a tetraspanin transmembrane protein, which has been shown to play an essential role during HIV-1 replication in macrophages. In this study, we further confirm the requirement of CD63 in early HIV-1 replication events in both macrophages and a CD4(+) cell line. Further analysis revealed that viral attachment and cell-cell fusion were unaffected by CD63 silencing. However, CD63-depleted macrophages showed a significant decrease in the initiation and completion of HIV-1 reverse transcription, affecting subsequent events of the HIV-1 life cycle. Integration of HIV-1 cDNA as well as the formation of 2-LTR circles was notably reduced. Reporter assays showed that CD63 down regulation reduced production of the early HIV protein Tat. In agreement, CD63 silencing also inhibited production of the late protein p24. These findings suggest that CD63 plays an early post-entry role prior to or at the reverse transcription step.

Variations in autologous neutralization and CD4 dependence of b12 resistant HIV-1 clade C env clones obtained at different time points from antiretroviral naïve Indian patients with recent infection

Background

Limited information is available on HIV-1 Indian clade C sensitivities to autologous antibodies during the course of natural infection. In the present study, a total of 37 complete envelope clones (Env) were amplified at different time points predominantly from the plasma of five Indian patients with recent HIV-1 infection and envelope-pseudotyped viruses were examined for their magnitude of sensitivity to autologous plasma antibodies during natural course of infection.

Results

Variable low levels of neutralization were consistently detected with contemporaneous autologous plasma. In contrast to clade B and African clade C HIV-1 envelopes, Env clones obtained from four patients were found to be resistant to IgG1b12. The majority of the Env clones were resistant to 2G12 and 2F5 due to the absence of the minimal motifs required for antibody recognition, but were sensitive to 4E10. Nonetheless, Env clones from one patient were found to be sensitive to 2G12, atypical for clade C, and one Env clone exhibited unusual sensitivity to 17b, suggesting spontaneous exposure of CD4i epitopes. Phylogenetic analysis revealed that Env clones were closely clustered within patients. Variation in the potential N-linked glycosylation pattern also appeared to be different in patients over the course of infection. Interestingly, we found that the sensitivity of Envs to contemporaneous autologous NAbs correlated positively with increased sensitivity to soluble CD4 and inversely with anti-CD4 antibody and Envs with increased NAb sensitivity were able to efficiently infect HeLa cells expressing low CD4.

Conclusion

Our data showed considerable variations in autologous neutralization of these early HIV-1 clade C Envs in each of these patients and indicate greater exposure to CD4 of Envs that showed increased autologous neutralization. Interestingly, Env clones obtained from a single patient at different time points were found to retain sensitivity to b12 antibody that binds to CD4 binding site in Env in contrast to Envs obtained from other patients. However, we did not find any association between increased b12 sensitivity of Envs obtained from this particular patient with their degree of exposure to CD4.

Role of gp120 trimerization on HIV binding elucidated with Brownian adhesive dynamics

We simulated the docking of human immunodeficiency virus (HIV) with a cell membrane using Brownian adhesive dynamics. The main advance in the current version of Brownian adhesive dynamics is that we use a simple bead-spring model to coarsely approximate the role of gp120 trimerization on HIV docking. We used our simulations to elucidate the effect of env spike density on the rate and probability of HIV binding, as well as the probability that each individual gp120 trimer is fully engaged. We found that for typical CD4 surface densities, viruses expressing as few as 8 env spikes will dock with binding rate constants comparable to viruses expressing 72 spikes. We investigated the role of cellular receptor diffusion on the degree of binding achieved by the virus on both short timescales (where binding has reached steady state but before substantial receptor accumulation in the viral-cell contact zone has occurred) and long timescales (where the system has reached steady state). On short timescales, viruses with 10-23 env trimers most efficiently form fully engaged trimers. On long timescales, all gp120 in the contact area will become bound to CD4. We found that it takes seconds for engaged trimers to cluster CD4 molecules in the contact zone, which partially explains the deleay in viral entry.

Cost-effective production of a vaginal protein microbicide to prevent HIV transmission

A series of small-molecule microbicides has been developed for vaginal delivery to prevent heterosexual HIV transmission, but results from human clinical trials have been disappointing. Protein-based microbicides, such as HIV-specific monoclonal antibodies, have been considered as an alternative approach. Despite their promising safety profile and efficacy, the major drawback of such molecules is the economy of large-scale production in mammalian cells, the current system of choice. Here, we show that an alternative biomanufacturing platform is now available for one of the most promising anti-HIV antibodies (2G12). Our data show that the HIV-neutralization capability of the antibody is equal to or superior to that of the same antibody produced in CHO cells. We conclude that this protein production system may provide a means to achieve microbicide ingredient manufacture at costs that would allow product introduction and manufacture in the developing world.

Antibody binding in proximity to the receptor/glycoprotein complex leads to a basal level of virus neutralization

Hypothetically, antibodies may neutralize enveloped viruses by diverse mechanisms, such as disruption of receptor binding, interference with conformational changes required for virus entry, steric hindrance, or virus aggregation. Here, we demonstrate that retroviral infection mediated by the avian sarcoma-leukosis virus (ASLV-A) envelope glycoproteins can be neutralized by an antibody directed against a functionally unimportant component of a chimeric receptor protein. Thus, the binding of an antibody in proximity to the retroviral envelope glycoprotein-receptor complex, without binding to the entry machinery itself, results in neutralization. This finding provides additional support for the hypothesis that steric hindrance is sufficient for antibody-mediated neutralization of retroviruses.

T cell line passage can select for pre-existing neutralization-sensitive variants from the quasispecies of primary human immunodeficiency virus type-1 isolates

Primary human immunodeficiency type 1 viruses (HIV-1) resist antibody neutralization but become sensitive after passage through T cell lines. We and others previously reported an increased neutralization sensitivity of HIV-1 after prolonged culture on primary peripheral blood mononuclear cells (PBMC). Hence we hypothesized that adaptation to growth in T cell lines is in fact selection of a pre-existing neutralization-sensitive HIV-1 variant from the quasispecies in the PBMC culture. Indeed, increased neutralization sensitivity was associated with largely identical synonymous and non-synonymous mutations between progeny of parallel H9 passages from the same split inoculum from 2 of 3 viruses. H9 T cell line adaptation of molecular cloned HIV-1 was less successful and associated with only a few de novo mutations that varied between parallel H9-adapted progeny from the same split inoculum. We conclude that T cell line adaptation of HIV-1 can indeed select for a pre-existing variant but that this most likely depends on the viral diversity in the inoculum.

Time frames for neutralization during the human immunodeficiency virus type 1 entry phase, as monitored in synchronously infected cell cultures

Characterization of the neutralizing interaction between antibody and virus is hindered by the nonsynchronized progression of infection in cell cultures. Discrete steps of the viral entry sequence cannot be discerned, and thus, the mode of antibody-mediated interference with virus infectivity remains undefined. Here, we magnetically synchronize the motion and cell attachment of human immunodeficiency virus type 1 (HIV-1) to monitor the progression of neutralization, both in solution and following virus attachment to the cell. By simultaneous transfer of all viral particles from reaction solution with antibody to the cell-bound state, the precise rate of neutralization of cell-free virus could be determined for each antibody. HIV-1 neutralization by both monoclonal and polyclonal antibody preparations followed distinct pseudo-first-order kinetics. For all antibodies, cell types, and HIV-1 strains examined, postattachment interference served a major role in the neutralizing effect. To monitor the progression of postattachment interference, we synchronized the entry process at initiation and measured the escape of cell-bound virus from antibody. We found that different antibodies neutralized the virus over different time frames during the entry phase. Virus was observed to progress through a sequence of shifting sensitivities to different antibodies during entry, suggested here to correlate with the exposure time of the target epitope on receptor-activated viral envelope proteins. Thus, by monitoring the progression of HIV-1 entry under synchronized conditions, we identify a new and significant determinant of antibody neutralization capacity, namely, the time frames for neutralization during the course of the viral entry phase.

Genetic and neutralization properties of subtype C human immunodeficiency virus type 1 molecular env clones from acute and early heterosexually acquired infections in Southern Africa

A standard panel of subtype C human immunodeficiency virus type 1 (HIV-1) Env-pseudotyped viruses was created by cloning, sequencing, and characterizing functional gp160 genes from 18 acute and early heterosexually acquired infections in South Africa and Zambia. In general, the gp120 region of these clones was shorter (most evident in V1 and V4) and less glycosylated compared to newly transmitted subtype B viruses, and it was underglycosylated but no different in length compared to chronic subtype C viruses. The gp120s also exhibited low amino acid sequence variability (12%) in V3 and high variability (39%) immediately downstream of V3, a feature shared with newly transmitted subtype B viruses and chronic viruses of both subtypes. When tested as Env-pseudotyped viruses in a luciferase reporter gene assay, all clones possessed an R5 phenotype and resembled primary isolates in their sensitivity to neutralization by HIV-1-positive plasmas. Results obtained with a multisubtype plasma panel suggested partial subtype preference in the neutralizing antibody response to infection. The clones were typical of subtype C in that all were resistant to 2G12 (associated with loss of N-glycosylation at position 295) and most were resistant to 2F5, but all were sensitive to 4E10 and many were sensitive to immunoglobulin G1b12. Finally, conserved neutralization epitopes in the CD4-induced coreceptor binding domain of gp120 were poorly accessible and were difficult to induce and stabilize with soluble CD4 on Env-pseudotyped viruses. These results illustrate key genetic and antigenic properties of subtype C HIV-1 that might impact the design and testing of candidate vaccines. A subset of these gp160 clones are suitable for use as reference reagents to facilitate standardized assessments of vaccine-elicited neutralizing antibody responses.

Characterization of HIV-1 envelope gp41 genetic diversity and functional domains following perinatal transmission

Background

HIV-1 envelope gp41 is a transmembrane protein that promotes fusion of the virus with the plasma membrane of the host cells required for virus entry. In addition, gp41 is an important target for the immune response and development of antiviral and vaccine strategies, especially when targeting the highly variable envelope gp120 has not met with resounding success. Mutations in gp41 may affect HIV-1 entry, replication, pathogenesis, and transmission. We, therefore, characterized the molecular properties of gp41, including genetic diversity, functional motifs, and evolutionary dynamics from five mother-infant pairs following perinatal transmission.

Results

The gp41 open reading frame (ORF) was maintained with a frequency of 84.17% in five mother-infant pairs' sequences following perinatal transmission. There was a low degree of viral heterogeneity and estimates of genetic diversity in gp41 sequences. Both mother and infant gp41 sequences were under positive selection pressure, as determined by ratios of non-synonymous to synonymous substitutions. Phylogenetic analysis of 157 mother-infant gp41 sequences revealed distinct clusters for each mother-infant pair, suggesting that the epidemiologically linked mother-infant pairs were evolutionarily closer to each other as compared with epidemiologically unlinked sequences. The functional domains of gp41, including fusion peptide, heptad repeats, glycosylation sites and lentiviral lytic peptides were mostly conserved in gp41 sequences analyzed in this study. The CTL recognition epitopes and motifs recognized by fusion inhibitors were also conserved in the five mother-infant pairs.

Conclusion

The maintenance of an intact envelope gp41 ORF with conserved functional domains and a low degree of genetic variability as well as positive selection pressure for adaptive evolution following perinatal transmission is consistent with an indispensable role of envelope gp41 in HIV-1 replication and pathogenesis.

Inhibition of HIV Env binding to cellular receptors by monoclonal antibody 2G12 as probed by Fc-tagged gp120

During natural HIV infection, an array of host receptors are thought to influence virus attachment and the kinetics of infection. In this study, to probe the interactions of HIV envelope (Env) with various receptors, we assessed the inhibitory properties of various anti-Env monoclonal antibodies (mAbs) in binding assays. To assist in detecting Env in attachment assays, we generated Fc fusions of full-length wild-type gp120 and several variable loop-deleted gp120s. Through investigation of the inhibition of Env binding to cell lines expressing CD4, CCR5, DC-SIGN, syndecans or combinations thereof, we found that the broadly neutralizing mAb, 2G12, directed to a unique carbohydrate epitope of gp120, inhibited Env-CCR5 binding, partially inhibited Env-DC-SIGN binding, but had no effect on Env-syndecan association. Furthermore, 2G12 inhibited Env attachment to primary monocyte-derived dendritic cells, that expressed CD4 and CCR5 primary HIV receptors, as well as DC-SIGN, and suggested that the dual activities of 2G12 could be valuable in vivo for inhibiting initial virus dissemination and propagation.

Induction of neutralizing antibodies against human immunodeficiency virus type 1 primary isolates by Gag-Env pseudovirion immunization

A major challenge for the development of an effective HIV vaccine is to elicit neutralizing antibodies against a broad array of primary isolates. Monomeric gp120-based vaccine approaches have not been successful in inducing this type of response, prompting a number of approaches designed to recreate the native glycoprotein complex that exists on the viral membrane. Gag-Env pseudovirions are noninfectious viruslike particles that recreate the native envelope glycoprotein structure and have the potential to generate neutralizing antibody responses against primary isolates. In this study, an inducible cell line was created in order to generate Gag-Env pseudovirions for examination of neutralizing antibody responses in guinea pigs. Unadjuvanted pseudovirions generated relatively weak anti-gp120 responses, while the use of a block copolymer water-in-oil emulsion or aluminum hydroxide combined with CpG oligodeoxynucleotides resulted in high levels of antibodies that bind to gp120. Sera from immunized animals neutralized a panel of human immunodeficiency virus (HIV) type 1 primary isolate viruses at titers that were significantly higher than that of the corresponding monomeric gp120 protein. Interpretation of these results was complicated by the occurrence of neutralizing antibodies directed against cellular (non-envelope protein) components of the pseudovirion. However, a major component of the pseudovirion-elicited antibody response was directed specifically against the HIV envelope. These results provide support for the role of pseudovirion-based vaccines in generating neutralizing antibodies against primary isolates of HIV and highlight the potential confounding role of antibodies directed at non-envelope cell surface components.

Interactions between natural killer cells and antibody Fc result in enhanced antibody neutralization of human immunodeficiency virus type 1

Antibodies can prevent lentivirus infections in animals and may play a role in controlling viral burden in established infection. In preventing and particularly in controlling infection, antibodies likely function in the presence of large quantities of virus. In this study, we explored the mechanisms by which antibodies neutralize large inocula of human immunodeficiency virus type 1 (HIV-1) on different target cells. Immunoglobulin G (IgG) from HIV-infected patients was tested for neutralizing activity against primary R5 strains of HIV-1 at inocula ranging from 100 to 20,000 50% tissue culture infective doses. At all virus inocula, inhibition by antibody was enhanced when target cells for virus growth were monocyte-depleted, peripheral blood mononuclear cells (PBMCs) rather than CD4(+) lymphocytes. However, enhanced inhibition on PBMCs was greatest with larger amounts of virus. Depleting PBMCs of natural killer (NK) cells, which express Fc receptors for IgG (FcgammaRs), abrogated the enhanced antibody inhibition, whereas adding NK cells to CD4(+) lymphocytes restored inhibition. There was no enhanced inhibition on PBMCs when F(ab')(2) was used. Further experiments demonstrated that the release of beta-chemokines, most likely through FcgammaR triggering of NK cells, contributed modestly to the antiviral activity of antibody on PBMCs and that antibody-coated virus adsorbed to uninfected cells provided a target for NK cell-mediated inhibition of HIV-1. These results indicate that Fc-FcgammaR interactions enhance the ability of antibody to neutralize HIV-1. Since FcgammaR-bearing cells are always present in vivo, FcgammaR-mediated antibody function may play a role in the ability of antibody to control lentivirus infection.

The C108g epitope in the V2 domain of gp120 functions as a potent neutralization target when introduced into envelope proteins derived from human immunodeficiency virus type 1 primary isolates

Monoclonal antibodies (MAbs) directed against epitopes in the V2 domain of human immunodeficiency virus type 1 gp120 often possess neutralizing activity, but these generally are highly type specific, neutralize only laboratory isolates, or have low potency. The most potent of these is C108g, directed against a type-specific epitope in HXB2 and BaL gp120s, which is glycan dependent and, in contrast to previous reports, dependent on intact disulfide bonds. This epitope was introduced into two primary Envs, derived from a neutralization-sensitive (SF162) and a neutralization-resistant (JR-FL) isolate, by substitution of two residues and, for SF162, addition of an N-linked glycosylation site. C108g effectively neutralized both variant Envs with considerably higher potency than standard MAbs against the V3 and CD4-binding domains and the broadly neutralizing MAbs 2G12 and 2F5. These amino acid substitutions also introduced the epitope recognized by a second V2-specific MAb, 10/76b, but this MAb possessed potent neutralizing activity only in the absence of the glycan required for C108g reactivity. In contrast to other gp120-specific neutralizing MAbs, C108g did not block binding of soluble Env proteins to either the CD4 or the CCR5 receptor, but studies with a fusion-arrested Env indicated that C108g neutralized at a step preceding the one blocked by the gp41-specific MAb, 2F5. These results indicate that the V1/V2 domain possesses targets that mediate potent neutralization of primary viral isolates via a novel mechanism and suggest that inclusion of carbohydrate determinants into these epitopes may help overcome the indirect masking effects that limit the neutralizing potency of antibodies commonly produced after infection.

Enfuvirtide resistance mutations: impact on human immunodeficiency virus envelope function, entry inhibitor sensitivity, and virus neutralization

Enfuvirtide (ENF/T-20/Fuzeon), the first human immunodeficiency virus (HIV) entry inhibitor to be licensed, targets a structural intermediate of the entry process. ENF binds the HR1 domain in gp41 after Env has bound CD4, preventing conformational changes needed for membrane fusion. Mutations in HR1 that confer ENF resistance can arise following ENF therapy. ENF resistance mutations were introduced into an R5- and X4-tropic Env to examine their impact on fusion, infection, and sensitivity to different classes of entry inhibitors and neutralizing antibodies. HR1 mutations could reduce infection and fusion efficiency and also delay fusion kinetics, likely accounting for their negative impact on viral fitness. HR1 mutations had minimal effect on virus sensitivity to other classes of entry inhibitors, including those targeting CD4 binding (BMS-806 and a CD4-specific monoclonal antibody [MAb]), coreceptor binding (CXCR4 inhibitor AMD3100 and CCR5 inhibitor TAK-779), or fusion (T-1249), indicating that ENF-resistant viruses can remain sensitive to other entry inhibitors in vivo. Some HR1 mutations conferred increased sensitivity to a subset of neutralizing MAbs that likely target fusion intermediates or with epitopes preferentially exposed following receptor interactions (17b, 48D, 2F5, 4E10, and IgGb12), as well as sera from some HIV-positive individuals. Mechanistically, enhanced neutralization correlated with reduced fusion kinetics, indicating that, in addition to steric constraints, kinetics may also limit virus neutralization by some antibodies. Therefore, escape from ENF comes at a cost to viral fitness and may confer enhanced sensitivity to humoral immunity due to prolonged exposure of epitopes that are not readily accessible in the native Env trimer. Resistance to other entry inhibitors was not observed.

The prolonged culture of human immunodeficiency virus type 1 in primary lymphocytes increases its sensitivity to neutralization by soluble CD4

Primary strains of human immunodeficiency virus type 1 (HIV-1) are known to adapt to replication in cell lines in vitro by becoming sensitive to soluble CD4 (sCD4) and neutralizing antibodies (NAb). T-cell lines favor isolation of variants that use CXCR4 as a co-receptor, while primary isolates predominantly use CCR5. We have now studied how a primary R5 isolate, CC1/85, adapts to prolonged replication in primary human peripheral blood mononuclear cells (PBMC). After 19 passages, a variant virus, CCcon.19, had increased sensitivity to both sCD4 and NAb b12 that binds to a CD4-binding site (CD4BS)-associated epitope, but decreased sensitivity to anti-CD4 antibodies. CCcon.19 retains the R5 phenotype, its resistance to other NAbs was unaltered, its sensitivity to various entry inhibitors was unchanged, and its ability to replicate in macrophages was modestly increased. We define CCcon.19 as a primary T-cell adapted (PTCA) variant. Genetic sequence analysis combined with mutagenesis studies on clonal, chimeric viruses derived from CC1/85 and the PTCA variant showed that the most important changes were in the V1/V2 loop structure, one of them involving the loss of an N-linked glycosylation site. Monomeric gp120 proteins expressed from CC1/85 and the PTCA variant did not differ in their affinities for sCD4, suggesting that the structural consequences of the sequence changes were manifested at the level of the native, trimeric Env complex. Overall, the adaptation process probably involves selection for variants with higher CD4 affinity and hence greater fusion efficiency, but this also involves the loss of some resistance to neutralization by agents directed at or near to the CD4BS. The loss of neutralization resistance is of no relevance under in vitro conditions, but NAbs would presumably be a counter-selection pressure against such adaptive changes in vivo, at least when the humoral immune response is intact.

Inhibiting sexual transmission of HIV-1 infection

The worldwide infection rate for HIV-1 is estimated to be 14,000 per day, but only now, more than 20 years into the epidemic, are the immediate events between exposure to infectious virus and the establishment of infection becoming clear. Defining the mechanisms of HIV-1 transmission, the target cells involved and how the virus attaches to and fuses with these cells, could reveal ways to block the sexual spread of the virus. In this review, we will discuss how our increasing knowledge of the ways in which HIV-1 is transmitted is shaping the development of new, more sophisticated intervention strategies based on the application of vaginal or rectal microbicides.

Identifying epitopes of HIV-1 that induce protective antibodies

As with most pathogens, HIV-1 induces a polyclonal antibody response to a wide array of epitopes on different viral proteins. Studies of polyclonal sera have helped to identify several epitopes on HIV-1 envelope glycoproteins that induce protective antibodies.

Antibodies to several constant regions of the virus envelope induce neutralizing antibodies, but because of the poor immunogenicity of some of these epitopes, the rare structure of neutralizing antibodies to these epitopes, or the preponderance of antibodies to particular epitopes that are non-neutralizing rather than neutralizing, targeting each of these epitopes with vaccine constructs presents difficult challenges.

Antibodies to variable regions of gp120, such as V1, V2 and V3, have long been considered irrelevant to vaccine design. However, there are conserved features in the stem of the V1/V2 loop and in the V3 loop that have crucial functions in virus infectivity and explain how antibodies to these regions can be crossreactive. These conserved elements within the variable regions might therefore be relevant targets for vaccines.

HIV-1 strains exist that are not neutralized by monoclonal antibodies but are neutralized by pooled sera from HIV-1⁺ individuals. This indicates that there might be neutralizing epitopes that have not yet been identified.

Present vaccine protocols induce antibodies to many epitopes rather than focusing the immune response on epitopes that will induce protective antibodies. Given that several neutralizing epitopes in gp120 and gp41 have been identified, it might be advantageous to direct the antibody response to these protective epitopes.

It is highly unlikely that a single construct will protect against all subtypes of HIV-1. Given the continuing evolution of the virus and the spread of subtypes throughout the world, the question is how to choose which strains, and how many, need to be represented in a vaccine to give maximum protection.

During the past 20 years, the pendulum of opinion in the HIV-1 vaccine field has swung between two extremes, initially favouring the induction of antibodies only, and subsequently favouring the induction of cell-mediated immune responses only. At present, the consensus seems to be that induction of both humoral and cellular immunity by an HIV-1 vaccine will be required to achieve maximum protection. One obstacle to the development of an effective HIV-1 vaccine has been the difficulty in inducing broadly reactive, potent antibodies with protective functions. Defining epitopes and designing immunogens that will induce these antibodies is one of the main challenges that now confronts the HIV-1 vaccine field.

A single injection of recombinant measles virus vaccines expressing human immunodeficiency virus (HIV) type 1 clade B envelope glycoproteins induces neutralizing antibodies and cellular immune responses to HIV

The anchored and secreted forms of the human immunodeficiency virus type 1 (HIV-1) 89.6 envelope glycoprotein, either complete or after deletion of the V3 loop, were expressed in a cloned attenuated measles virus (MV) vector. The recombinant viruses grew as efficiently as the parental virus and expressed high levels of the HIV protein. Expression was stable during serial passages. The immunogenicity of these recombinant vectors was tested in mice susceptible to MV and in macaques. High titers of antibodies to both MV and HIV-Env were obtained after a single injection in susceptible mice. These antibodies neutralized homologous SHIV89.6p virus, as well as several heterologous HIV-1 primary isolates. A gp160 mutant in which the V3 loop was deleted induced antibodies that neutralized heterologous viruses more efficiently than antibodies induced by the native envelope protein. A high level of CD8+ and CD4+ cells specific for HIV gp120 was also detected in MV-susceptible mice. Furthermore, recombinant MV was able to raise immune responses against HIV in mice and macaques with a preexisting anti-MV immunity. Therefore, recombinant MV vaccines inducing anti-HIV neutralizing antibodies and specific T lymphocytes responses deserve to be tested as a candidate AIDS vaccine.

Polyclonal immunoglobulin G from patients neutralizes human immunodeficiency virus type 1 primary isolates by binding free virions, but without interfering with an initial CD4-independent attachment of the virus to primary blood mononuclear cells

We investigated the relationship between human immunodeficiency virus type 1 (HIV-1) primary isolate (PI) antibody-mediated neutralization and attachment to primary blood mononuclear cells (PBMC). Incubation of PIs with immunoglobulin G (IgG) purified from infected patients did not inhibit attachment of the viruses with PBMC, but partial to complete neutralization was achieved. Neutralization of PIs already fixed on the cells was achieved by some IgG samples only and was of limited intensity compared to the former neutralization protocol. On the contrary, the binding of IgG to free virions was shown to be sufficient to reach potent neutralization, as the infectivity of IgG-PI complexes purified from the bulk of antibodies before addition to PBMC was strongly diminished compared to mock-treated controls. Monoclonal antibodies to the CDR2 domain of CD4 completely inhibited the infection of PBMC without interfering with the attachment of PIs to the cells, suggesting that, under these experimental conditions, the initial attachment of viruses to PBMC involves alternative cellular receptors. This initial interaction may also involve other components of the viral envelope than gp120, as partial depletion of the surface glycoproteins of primary viral particles that resulted in an almost complete loss of infectivity did not impair attachment to PBMC. A limited inhibition of attachment was observed when interfering with putative interactions with cellular heparan sulfate, whereas no effect was observed for cellular CD147 or nucleolin or for virion-incorporated cyclophilin A. Altogether, our results favor a mechanism of neutralization of HIV-1 PIs by polyclonal IgG where antibodies predominantly bind free virions and neutralize without interfering with the attachment to PBMC, which, in this model, is mainly CD4 independent.

Access of antibody molecules to the conserved coreceptor binding site on glycoprotein gp120 is sterically restricted on primary human immunodeficiency virus type 1

Anti-human immunodeficiency virus type 1 (HIV-1) antibodies whose binding to gp120 is enhanced by CD4 binding (CD4i antibodies) are generally considered nonneutralizing for primary HIV-1 isolates. However, a novel CD4i-specific Fab fragment, X5, has recently been found to neutralize a wide range of primary isolates. To investigate the precise nature of the extraordinary neutralizing ability of Fab X5, we evaluated the abilities of different forms (immunoglobulin G [IgG], Fab, and single-chain Fv) of X5 and other CD4i monoclonal antibodies to neutralize a range of primary HIV-1 isolates. Our results show that, for a number of isolates, the size of the neutralizing agent is inversely correlated with its ability to neutralize. Thus, the poor ability of CD4i-specific antibodies to neutralize primary isolates is due, at least in part, to steric factors that limit antibody access to the gp120 epitopes. Studies of temperature-regulated neutralization or fusion-arrested intermediates suggest that the steric effects are important in limiting the binding of IgG to the viral envelope glycoproteins after HIV-1 has engaged CD4 on the target cell membrane. The results identify hurdles in using CD4i epitopes as targets for antibody-mediated neutralization in vaccine design but also indicate that the CD4i regions could be efficiently targeted by small molecule entry inhibitors.

Virion-bound ICAM-1 and activated LFA-1: a combination of factors conferring resistance to neutralization by sera from human immunodeficiency virus type 1-infected individuals independently of the disease status and phase

The role of the supplementary interaction between virion-bound host ICAM-1 and LFA-1 on target cells in sensitivity to neutralization of human immunodeficiency virus type 1 (HIV-1) is poorly studied. Serum samples from four long-term nonprogressors (LTNPs) and sequential sera from one progressor were used to assess neutralization sensitivity of isogenic ICAM-1-negative and ICAM-1-bearing HIV-1(NL4-3), a prototype of T-cell-line-adapted viruses. We found that virus neutralization sensitivity to the studied sera is not modified by the additional interaction between virally embedded ICAM-1 and LFA-1 under an inactive state. However, expression on the target cell surface of an activated LFA-1 form renders ICAM-1-bearing virus particles, but not viruses devoid of ICAM-1, more refractory to neutralization by sera from three out of four LTNPs and all sequential sera from the person who has experienced a progression of the HIV-1-associated disease. Although no conclusive correlation could be drawn between virus susceptibility to neutralization and the disease status or stages of HIV-1 infection, these findings demonstrate that other nonspecific virus-cell interactions mediated by virion-anchored host proteins and their normal cognate ligands on target cells represent factors that can affect the mechanism of HIV-1 neutralization.

Structural analysis of the epitope of the anti-HIV antibody 2F5 sheds light into its mechanism of neutralization and HIV fusion

Inhibition of human immunodeficiency virus (HIV) fusion with the host cell has emerged as a viable therapeutic strategy, and rational design of inhibitors and vaccines, interfering with this process, is a prime target for antiviral research. To advance our knowledge of the structural biology of HIV fusion, we have studied the membrane-proximal region of the fusogenic envelope subunit gp41, which includes the epitope ELDKWA of the broadly neutralizing human antibody 2F5. The structural evidence available for this region is contradictory, with some studies suggesting an overall helical conformation, while the X-ray structure of the ELDKWAS peptide bound to the antibody shows it folded in a type I beta turn. We used a two-step strategy: Firstly, by a competition binding assay, we identified the proper boundaries of the domain recognized by 2F5, which we found considerably larger than the ELDKWAS hexapeptide. Secondly, we studied the structure of the resulting 13 amino acid residue peptide by collecting NMR data and analyzing them by our previously developed statistical method (NAMFIS). Our study revealed that the increase in binding affinity goes in parallel with stabilization of specific local and global conformational propensities, absent from the shorter epitope. When compounded with the available biological evidence, our structural analysis allows us to propose a specific role for the membrane-proximal region during HIV fusion, in terms of a conformational transition between the turn and the helical structure. At the same time, our hypothesis offers a structural explanation for the mechanism of neutralization of mAb 2F5.

Unmutated immunoglobulin M can protect mice from death by influenza virus infection

To elucidate the role of class switch recombination (CSR) and somatic hypermutation (SHM) in virus infection, we have investigated the influence of the primary and secondary infections of influenza virus on mice deficient of activation-induced cytidine deaminase (AID), which is absolutely required for CSR and SHM. In the primary infection, AID deficiency caused no significant difference in mortality but did cause difference in morbidity. In the secondary infection with a lethal dose of influenza virus, both AID-/- and AID+/- mice survived completely. However, AID-/- mice could not completely block replication of the virus and their body weights decreased severely whereas AID+/- mice showed almost complete prevention from the reinfection. Depletion of CD8+ T cells by administration of an anti-CD8 monoclonal antibody caused slightly severer body weight loss but did not alter the survival rate of AID-/- mice in secondary infection. These results indicate that unmutated immunoglobulin (Ig)M alone is capable of protecting mice from death upon primary and secondary infections. Because the titers of virus-neutralizing antibodies were comparable between AID-/- and AID+/- mice at the time of the secondary infection, a defect of AID-/- mice in protection of morbidity might be due to the absence of either other Ig classes such as IgG, high affinity antibodies with SHM, or both.

Inter-subtype cross-neutralizing antibodies recognize epitopes on cell-associated HIV-1 virions

HIV-1 infected individuals with cross-neutralizing antibodies against primary HIV-1 isolates belonging to Group M (envA-H) and O, are identified. To investigate the neutralization-kinetics of primary isolates with these antibodies, different neutralization assay conditions are compared. Each set is summarized as a/b/c where a is the time in hours for which antibody is incubated with virus, b is the time in hours allowed for virus to absorb to cells, c is the total culture period in days, from the cells' first exposure to virus, before antigen production (peripheral blood mononuclear cells) or number of fluorescent cells (GHOST) are measured. In HIV-infected individuals, neutralizing antibodies can be detected against a wide range of primary isolates (Group M; A-H and Group O) in PBMC-assays with short incubation phases (1/2/7 or 1/24/7). If cultures are extended (1/2/14 or 1/24/14), however, neutralization can be lost. In kinetic experiments, neutralization can even be seen without pre-incubation (a=0 hr). This study shows that neutralization of primary HIV isolates by cross-reactive antibodies can continue after the virus has bound to its target cell. This neutralization, however, is not an all or nothing loss in virus infectivity. Most often it leads only to a reduction in viral replication rates.

Nonneutralizing antibodies to the CD4-binding site on the gp120 subunit of human immunodeficiency virus type 1 do not interfere with the activity of a neutralizing antibody against the same site

We have investigated whether nonneutralizing monoclonal antibodies (MAbs) to the gp120 subunit of the envelope glycoprotein (Env) complex of human immunodeficiency virus type 1 (HIV-1) can interfere with HIV-1 neutralization by another anti-gp120 MAb. We used neutralizing (b12) and nonneutralizing (205-42-15, 204-43-1, 205-46-9) MAbs to the epitope cluster overlapping the CD4-binding site (CD4BS) on gp120. All the MAbs, neutralizing or otherwise, cross-competed for binding to monomeric gp120, indicating the close topological proximity of their epitopes. However, the nonneutralizing CD4BS MAbs did not interfere with the neutralization activity of MAb b12. In contrast, in a binding assay using oligomeric Env expressed on the surface of Env-transfected cells, the nonneutralizing MAbs did partially compete with b12 for Env binding. The surface of Env-transfected cells contains two categories of binding site for CD4BS MAbs. One type of site is recognized by both b12 and nonneutralizing CD4BS MAbs; the other is recognized by only b12. Binding assays for Env-gp120 interactions based on the use of monomeric gp120 or Env-transfected cells do not predict the outcome of HIV-1 neutralization assays, and they should therefore be used only with caution when gauging the properties of anti-Env MAbs.

Heterogeneity of envelope molecules expressed on primary human immunodeficiency virus type 1 particles as probed by the binding of neutralizing and nonneutralizing antibodies

Virion capture assays, in which immobilized antibodies (Abs) capture virus particles, have been used to suggest that nonneutralizing Abs bind effectively to human immunodeficiency virus type 1 (HIV-1) primary viruses. Here, we show that virion capture assays, under conditions commonly reported in the literature, give a poor indication of epitope expression on the surface of infectious primary HIV-1. First, estimation of primary HIV-1 capture by p24 measurements shows a very poor correlation with an estimation based on infectivity measurements. Second, virion capture appears to require relatively low Ab affinity for the virion, as shown by the ability of a monoclonal Ab to capture a wild-type and a neutralization escape variant virus equally well. Nevertheless, in a more interpretable competition format, it is shown that nonneutralizing anti-CD4 binding site (CD4bs) Abs compete with a neutralizing anti-CD4bs Ab (b12) for virus capture, suggesting that the nonneutralizing anti-CD4bs Abs are able to bind to the envelope species that is involved in virion capture in these experiments. However, the nonneutralizing anti-CD4bs Abs do not inhibit neutralization by b12 even at considerable excess. This suggests that the nonneutralizing Abs are unable to bind effectively to the envelope species required for virus infectivity. The results were obtained for three different primary virus envelopes. The explanation that we favor is that infectious HIV-1 primary virions can express two forms of gp120, an accessible nonfunctional form and a functional form with limited access. Binding to the nonfunctional form, which needs only to be present at relatively low density on the virion, permits capture but does not lead to neutralization. The expression of a nonfunctional but accessible form of gp120 on virions may contribute to the general failure of HIV-1 infection to elicit cross-neutralizing Abs and may represent a significant problem for vaccines based on viruses or virus-like particles.

CD4 binding site antibodies inhibit human immunodeficiency virus gp120 envelope glycoprotein interaction with CCR5

The human immunodeficiency virus type 1 (HIV-1) gp120 exterior glycoprotein is conformationally flexible. Upon binding the host cell receptor, CD4, gp120 assumes a conformation that is able to bind the chemokine receptors CCR5 or CXCR4, which act as coreceptors for the virus. CD4-binding-site (CD4BS) antibodies are neutralizing antibodies elicited during natural infection that are directed against gp120 epitopes that overlap the binding site for CD4. Recent studies (S. H. Xiang et al., J. Virol. 76:9888-9899, 2002) suggest that CD4BS antibodies recognize conformations of gp120 distinct from the CD4-bound conformation. This predicts that the binding of CD4BS antibodies will inhibit chemokine receptor binding. Here, we show that Fab fragments and complete immunoglobulin molecules of CD4BS antibodies inhibit CD4-independent gp120 binding to CCR5 and cell-cell fusion mediated by CD4-independent HIV-1 envelope glycoproteins. These results are consistent with a model in which the binding of CD4BS antibodies limits the ability of gp120 to assume a conformation required for coreceptor binding.