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

Broadly neutralizing antibodies for HIV prevention: a comprehensive review and future perspectives

SUMMARYThe human immunodeficiency virus (HIV) epidemic remains a formidable global health concern, with 39 million people living with the virus and 1.3 million new infections reported in 2022. Despite anti-retroviral therapy's effectiveness in pre-exposure prophylaxis, its global adoption is limited. Broadly neutralizing antibodies (bNAbs) offer an alternative strategy for HIV prevention through passive immunization. Historically, passive immunization has been efficacious in the treatment of various diseases ranging from oncology to infectious diseases. Early clinical trials suggest bNAbs are safe, tolerable, and capable of reducing HIV RNA levels. Although challenges such as bNAb resistance have been noted in phase I trials, ongoing research aims to assess the additive or synergistic benefits of combining multiple bNAbs. Researchers are exploring bispecific and trispecific antibodies, and fragment crystallizable region modifications to augment antibody efficacy and half-life. Moreover, the potential of other antibody isotypes like IgG3 and IgA is under investigation. While promising, the application of bNAbs faces economic and logistical barriers. High manufacturing costs, particularly in resource-limited settings, and logistical challenges like cold-chain requirements pose obstacles. Preliminary studies suggest cost-effectiveness, although this is contingent on various factors like efficacy and distribution. Technological advancements and strategic partnerships may mitigate some challenges, but issues like molecular aggregation remain. The World Health Organization has provided preferred product characteristics for bNAbs, focusing on optimizing their efficacy, safety, and accessibility. The integration of bNAbs in HIV prophylaxis necessitates a multi-faceted approach, considering economic, logistical, and scientific variables. This review comprehensively covers the historical context, current advancements, and future avenues of bNAbs in HIV prevention.

Ultrasensitive quantification of HIV-1 cell-to-cell transmission in primary human CD4+ T cells measures viral sensitivity to broadly neutralizing antibodies

IMPORTANCE

HIV-1 can efficiently transmit from one cell to another but accurate quantification of this mode of transmission is still challenging. Here, we developed an ultrasensitive assay to measure HIV-1 transmission between cells and to evaluate HIV-1 escape from broadly neutralizing antibodies in primary human T cells. This assay will contribute to understanding the fundamental mechanisms of HIV-1 cell-to-cell transmission, allow evaluation of pre-existing or acquired HIV-1 resistance in clinical trials, and can be adapted to study the biology of other retroviruses.

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.

Mucosal application of the broadly neutralizing antibody 10-1074 protects macaques from cell-associated SHIV vaginal exposure

Passive immunization using broadly neutralizing antibodies (bNAbs) is investigated in clinical settings to inhibit HIV-1 acquisition due to the lack of a preventive vaccine. However, bNAbs efficacy against highly infectious cell-associated virus transmission has been overlooked. HIV-1 transmission mediated by infected cells present in body fluids likely dominates infection and aids the virus in evading antibody-based immunity. Here, we show that the anti-N-glycans/V3 loop HIV-1 bNAb 10-1074 formulated for topical vaginal application in a microbicide gel provides significant protection against repeated cell-associated SHIV162P3 vaginal challenge in non-human primates. The treated group has a significantly lower infection rate than the control group, with 5 out of 6 animals fully protected from the acquisition of infection. The findings suggest that mucosal delivery of potent bnAbs may be a promising approach for preventing transmission mediated by infected cells and support the use of anti-HIV-antibody-based strategies as potential microbicides in human clinical trials.

Engineered CD4 T cells expressing a membrane anchored viral inhibitor restrict HIV-1 through cis and trans mechanisms

HIV-1 infection of target cells can occur through either cell-free virions or cell-cell transmission in a virological synapse, with the latter mechanism of infection reported to be 100- to 1,000-fold more efficient. Neutralizing antibodies and entry inhibitors effectively block cell-free HIV-1, but with few exceptions, they display much less inhibitory activity against cell-mediated HIV-1 transmission. Previously, we showed that engineering HIV-1 target cells by genetically linking single-chain variable fragments (scFvs) of antibodies to glycosyl phosphatidylinositol (GPI) potently blocks infection by cell-free virions and cell-mediated infection by immature dendritic cell (iDC)-captured HIV-1. Expression of scFvs on CD4+ cell lines by transduction with X5 derived anti-HIV-1 Env antibody linked to a GPI attachment signal directs GPI-anchored scFvs into lipid rafts of the plasma membrane. In this study, we further characterize the effect of GPI-scFv X5 on cell-cell HIV-1 transmission from DCs to target cells. We report that expression of GPI-scFv X5 in transduced CD4+ cell lines and human primary CD4+ T cells potently restricts viral replication in iDC- or mDC-captured HIV-1 in trans. Using live-cell imaging, we observed that when GPI-GFP or GPI-scFv X5 transduced T cells are co-cultured with iDCs, GPI-anchored proteins enrich in contact zones and subsequently migrate from T cells into DCs, suggesting that transferred GPI-scFv X5 interferes with HIV-1 infection of iDCs. We conclude that GPI-scFv X5 on the surface of transduced CD4+ T cells not only potently blocks cell-mediated infection by DCs, but it transfers from transduced cells to the surface of iDCs and neutralizes HIV-1 replication in iDCs. Our findings have important implications for HIV-1 antibody-based immunotherapies as they demonstrate a viral inhibitory effect that extends beyond the transduced CD4+ T cells to iDCs which can enhance HIV-1 replication.

Trapping the HIV-1 V3 loop in a helical conformation enables broad neutralization

The third variable (V3) loop on the human immunodeficiency virus 1 (HIV-1) envelope glycoprotein trimer is indispensable for virus cell entry. Conformational masking of V3 within the trimer allows efficient neutralization via V3 only by rare, broadly neutralizing glycan-dependent antibodies targeting the closed prefusion trimer but not by abundant antibodies that access the V3 crown on open trimers after CD4 attachment. Here, we report on a distinct category of V3-specific inhibitors based on designed ankyrin repeat protein (DARPin) technology that reinstitute the CD4-bound state as a key neutralization target with up to >90% breadth. Broadly neutralizing DARPins (bnDs) bound V3 solely on open envelope and recognized a four-turn amphipathic α-helix in the carboxy-terminal half of V3 (amino acids 314-324), which we termed 'αV3C'. The bnD contact surface on αV3C was as conserved as the CD4 binding site. Molecular dynamics and escape mutation analyses underscored the functional relevance of αV3C, highlighting the potential of αV3C-based inhibitors and, more generally, of postattachment inhibition of HIV-1.

The Host Cytoskeleton Functions as a Pleiotropic Scaffold: Orchestrating Regulation of the Viral Life Cycle and Mediating Host Antiviral Innate Immune Responses

Viruses are obligate intracellular parasites that critically depend on their hosts to initiate infection, complete replication cycles, and generate new progeny virions. To achieve these goals, viruses have evolved numerous elegant strategies to subvert and utilize different cellular machinery. The cytoskeleton is often one of the first components to be hijacked as it provides a convenient transport system for viruses to enter the cell and reach the site of replication. The cytoskeleton is an intricate network involved in controlling the cell shape, cargo transport, signal transduction, and cell division. The host cytoskeleton has complex interactions with viruses during the viral life cycle, as well as cell-to-cell transmission once the life cycle is completed. Additionally, the host also develops unique, cytoskeleton-mediated antiviral innate immune responses. These processes are also involved in pathological damages, although the comprehensive mechanisms remain elusive. In this review, we briefly summarize the functions of some prominent viruses in inducing or hijacking cytoskeletal structures and the related antiviral responses in order to provide new insights into the crosstalk between the cytoskeleton and viruses, which may contribute to the design of novel antivirals targeting the cytoskeleton.

Mapping the interplay between NK cells and HIV: therapeutic implications

Although highly effective at durably suppressing plasma HIV-1 viremia, combination antiretroviral therapy (ART) treatment regimens do not eradicate the virus, which persists in long-lived CD4+ T cells. This latent viral reservoir serves as a source of plasma viral rebound following treatment interruption, thus requiring lifelong adherence to ART. Additionally, challenges remain related not only to access to therapy but also to a higher prevalence of comorbidities with an inflammatory etiology in treated HIV-1+ individuals, underscoring the need to explore therapeutic alternatives that achieve sustained virologic remission in the absence of ART. Natural killer (NK) cells are uniquely positioned to positively impact antiviral immunity, in part due to the pleiotropic nature of their effector functions, including the acquisition of memory-like features, and, therefore, hold great promise for transforming HIV-1 therapeutic modalities. In addition to defining the ability of NK cells to contribute to HIV-1 control, this review provides a basic immunologic understanding of the impact of HIV-1 infection and ART on the phenotypic and functional character of NK cells. We further delineate the qualities of "memory" NK cell populations, as well as the impact of HCMV on their induction and subsequent expansion in HIV-1 infection. We conclude by highlighting promising avenues for optimizing NK cell responses to improve HIV-1 control and effect a functional cure, including blockade of inhibitory NK receptors, TLR agonists to promote latency reversal and NK cell activation, CAR NK cells, BiKEs/TriKEs, and the role of HIV-1-specific bNAbs in NK cell-mediated ADCC activity against HIV-1-infected cells.

Design of a Bispecific HIV Entry Inhibitor Targeting the Cell Receptor CD4 and Viral Fusion Protein Gp41

Given the high variability and drug-resistance problem by human immunodeficiency virus type 1 (HIV-1), the development of bispecific or multi-specific inhibitors targeting different steps of HIV entry is highly appreciated. We previously generated a very potent short-peptide-based HIV fusion inhibitor 2P23. In this study, we designed and characterized a bifunctional inhibitor termed 2P23-iMab by genetically conjugating 2P23 to the single-chain variable fragment (scFv) of ibalizumab (iMab), a newly approved antibody drug targeting the cell receptor CD4. As anticipated, 2P23-iMab could bind to the cell membrane through CD4 anchoring and inhibit HIV-1 infection as well as viral Env-mediated cell-cell fusion efficiently. When tested against a large panel of HIV-1 pseudoviruses with different subtypes and phenotypes, 2P23-iMab exhibited dramatically improved inhibitory activity than the parental inhibitors; especially, it potently inhibited the viruses not being susceptible to iMab. Moreover, 2P23-iMab had a dramatically increased potency in inhibiting two panels of HIV-1 mutants that are resistant to T-20 or 2P23 and the infections of HIV-2 and simian immunodeficiency virus (SIV). In conclusion, our studies have provided new insights into the design of novel bispecific HIV entry inhibitors with highly potent and broad-spectrum antiviral activity.

A cellular trafficking signal in the SIV envelope protein cytoplasmic domain is strongly selected for in pathogenic infection

The HIV/SIV envelope glycoprotein (Env) cytoplasmic domain contains a highly conserved Tyr-based trafficking signal that mediates both clathrin-dependent endocytosis and polarized sorting. Despite extensive analysis, the role of these functions in viral infection and pathogenesis is unclear. An SIV molecular clone (SIVmac239) in which this signal is inactivated by deletion of Gly-720 and Tyr-721 (SIVmac239ΔGY), replicates acutely to high levels in pigtail macaques (PTM) but is rapidly controlled. However, we previously reported that rhesus macaques and PTM can progress to AIDS following SIVmac239ΔGY infection in association with novel amino acid changes in the Env cytoplasmic domain. These included an R722G flanking the ΔGY deletion and a nine nucleotide deletion encoding amino acids 734-736 (ΔQTH) that overlaps the rev and tat open reading frames. We show that molecular clones containing these mutations reconstitute signals for both endocytosis and polarized sorting. In one PTM, a novel genotype was selected that generated a new signal for polarized sorting but not endocytosis. This genotype, together with the ΔGY mutation, was conserved in association with high viral loads for several months when introduced into naïve PTMs. For the first time, our findings reveal strong selection pressure for Env endocytosis and particularly for polarized sorting during pathogenic SIV infection in vivo.

The HIV Env Glycoprotein Conformational States on Cells and Viruses

The HIV Env glycoprotein is the surface glycoprotein responsible for viral entry into CD4+ immune cells. During infection, Env also serves as a primary target for antibody responses, which are robust but unable to control virus replication. Immune evasion by HIV-1 Env appears to employ complex mechanisms to regulate what antigenic states are presented to the immune system. Immunodominant features appear to be distinct from epitopes that interfere with Env functions in mediating infection. Further, cell-cell transmission studies indicate that vulnerable conformational states are additionally hidden from recognition on infected cells, even though the presence of Env at the cell surface is required for viral infection through the virological synapse. Cell-cell infection studies support that Env on infected cells is presented in distinct conformations from that on virus particles. Here we review data regarding the regulation of conformational states of Env and assess how regulated sorting of Env within the infected cell may underlie mechanisms to distinguish Env on the surface of virus particles versus Env on the surface of infected cells. These mechanisms may allow infected cells to avoid opsonization, providing cell-to-cell infection by HIV with a selective advantage during evolution within an infected individual. Understanding how distinct Env conformations are presented on cells versus viruses may be essential to designing effective vaccine approaches and therapeutic strategies to clear infected cell reservoirs.

So Pathogenic or So What?-A Brief Overview of SIV Pathogenesis with an Emphasis on Cure Research

HIV infection requires lifelong antiretroviral therapy (ART) to control disease progression. Although ART has greatly extended the life expectancy of persons living with HIV (PWH), PWH nonetheless suffer from an increase in AIDS-related and non-AIDS related comorbidities resulting from HIV pathogenesis. Thus, an HIV cure is imperative to improve the quality of life of PWH. In this review, we discuss the origins of various SIV strains utilized in cure and comorbidity research as well as their respective animal species used. We briefly detail the life cycle of HIV and describe the pathogenesis of HIV/SIV and the integral role of chronic immune activation and inflammation on disease progression and comorbidities, with comparisons between pathogenic infections and nonpathogenic infections that occur in natural hosts of SIVs. We further discuss the various HIV cure strategies being explored with an emphasis on immunological therapies and "shock and kill".

A Replication-Competent HIV Clone Carrying GFP-Env Reveals Rapid Env Recycling at the HIV-1 T Cell Virological Synapse

HIV-1 infection is enhanced by cell-cell adhesions between infected and uninfected T cells called virological synapses (VS). VS are initiated by the interactions of cell-surface HIV-1 envelope glycoprotein (Env) and CD4 on target cells and act as sites of viral assembly and viral transfer between cells. To study the process that recruits and retains HIV-1 Env at the VS, a replication-competent HIV-1 clone carrying an Env-sfGFP fusion protein was designed to enable live tracking of Env within infected cells. Combined use of surface pulse-labeling of Env and fluorescence recovery after photobleaching (FRAP) studies, enabled the visualization of the targeted accumulation and sustained recycling of Env between endocytic compartments (EC) and the VS. We observed dynamic exchange of Env at the VS, while the viral structural protein, Gag, was largely immobile at the VS. The disparate exchange rates of Gag and Env at the synapse support that the trafficking and/or retention of a majority of Env towards the VS is not maintained by entrapment by a Gag lattice or immobilization by binding to CD4 on the target cell. A FRAP study of an Env endocytosis mutant showed that recycling is not required for accumulation at the VS, but is required for the rapid exchange of Env at the VS. We conclude that the mechanism of Env accumulation at the VS and incorporation into nascent particles involves continuous internalization and targeted secretion rather than irreversible interactions with the budding virus, but that this recycling is largely dispensable for VS formation and viral transfer across the VS.

Single-chain variable fragments of broadly neutralizing antibodies prevent HIV cell-cell transmission

Broadly neutralizing antibodies (bNAbs) are able to prevent HIV infection following passive administration. Single-chain variable fragments (scFv) may have advantages over IgG as their smaller size permits improved diffusion into mucosal tissues. We have previously shown that scFv of bNAbs retain significant breadth and potency against cell-free viral transmission in a TZM-bl assay. However, scFv have not been tested for their ability to block cell-cell transmission, a model in which full-sized bNAbs lose potency. We tested 4 scFv (CAP256.25, PGT121, 3BNC117 and 10E8v4) compared to IgG, in free-virus and cell-cell neutralization assays in A3.01 cells, against a panel of seven heterologous viruses. We show that free-virus neutralization titers in the TZM-bl and A3.01 assays were not significantly different, and confirm that scFv show a 1 to 32-fold reduction in activity in the cell-free model, compared to IgG. However, whereas IgG show 3.4 to 19-fold geometric mean potency loss in cell-cell neutralization compared to free-virus transmission, scFv had more comparable activity in the two assays, with only a 1.3 to 2.3-fold reduction. Geometric mean IC₅₀ of scFv for cell-cell transmission ranged from 0.65 μg/ml (10E8v4) to 2.3 μg/ml (3BNC117) with IgG and scFv neutralization showing similar potency against cell-associated transmission. Therefore, despite the reduced activity of scFv in cell-free assays, their retention of activity in the cell-cell format may make scFv useful for the prevention of both modes of transmission in HIV prevention studies. Importance Broadly neutralizing antibodies (bNAbs) are a major focus for passive immunization against HIV, with the recently concluded HVTN AMP (Antibody Mediated Protection) trial providing proof of concept. Most studies focus on cell-free HIV, however cell-associated virus may play a significant role in HIV infection, pathogenesis and latency. Single-chain variable fragments (scFv) of antibodies may have increased tissue penetration, and reduced immunogenicity. We previously demonstrated that scFv of four HIV-directed bNAbs (CAP256-VRC26.25, PGT121, 3BNC117 and 10E8v4) retain significant potency and breadth against cell-free HIV. As some bNAbs have been shown to lose potency against cell-associated virus, we investigated the ability of bNAb scFv to neutralize this mode of transmission. We demonstrate that unlike IgG, scFv of bNAbs are able to neutralize cell-free and cell-associated virus with similar potency. These scFv, which show functional activity in the therapeutic range, may therefore be suitable for further development as passive immunity for HIV prevention.

Cell membrane-anchored anti-HIV single-chain antibodies and bifunctional inhibitors targeting the gp41 fusion protein: new strategies for HIV gene therapy

Emerging studies indicate that infusion of HIV-resistant cells could be an effective strategy to achieve a sterilizing or functional cure. We recently reported that glycosylphosphatidylinositol (GPI)-anchored nanobody or a fusion inhibitory peptide can render modified cells resistant to HIV-1 infection. In this study, we comprehensively characterized a panel of newly isolated HIV-1-neutralizing antibodies as GPI-anchored inhibitors. Fusion genes encoding the single-chain variable fragment (scFv) of 3BNC117, N6, PGT126, PGT128, 10E8, or 35O22 were constructed with a self-inactivating lentiviral vector, and they were efficiently expressed in the lipid raft sites of target cell membrane without affecting the expression of HIV-1 receptors (CD4, CCR5 and CXCR4). Significantly, transduced cells exhibited various degrees of resistance to cell-free HIV-1 infection and cell-associated HIV-1 transmission, as well as viral Env-mediated cell–cell fusion, with the cells modified by GPI-10E8 showing the most potent and broad anti-HIV activity. In mechanism, GPI-10E8 also interfered with the processing of viral Env in transduced cells and attenuated the infectivity of progeny viruses. By genetically linking 10E8 with a fusion inhibitor peptide, we subsequently designed a group of eight bifunctional constructs as cell membrane-based inhibitors, designated CMI01∼CMI08, which rendered cells completely resistant to HIV-1, HIV-2, and simian immunodeficiency virus (SIV). In human CD4⁺ T cells, GPI-10E8 and its bifunctional derivatives blocked both CCR5- and CXCR4-tropic HIV-1 isolates efficiently, and the modified cells displayed robust survival selection under HIV-1 infection. Therefore, our studies provide new strategies for generating HIV-resistant cells, which can be used alone or with other gene therapy approaches.

Distinct conformations of the HIV-1 V3 loop crown are targetable for broad neutralization

The V3 loop of the HIV-1 envelope (Env) protein elicits a vigorous, but largely non-neutralizing antibody response directed to the V3-crown, whereas rare broadly neutralizing antibodies (bnAbs) target the V3-base. Challenging this view, we present V3-crown directed broadly neutralizing Designed Ankyrin Repeat Proteins (bnDs) matching the breadth of V3-base bnAbs. While most bnAbs target prefusion Env, V3-crown bnDs bind open Env conformations triggered by CD4 engagement. BnDs achieve breadth by focusing on highly conserved residues that are accessible in two distinct V3 conformations, one of which resembles CCR5-bound V3. We further show that these V3-crown conformations can, in principle, be attacked by antibodies. Supporting this conclusion, analysis of antibody binding activity in the Swiss 4.5 K HIV-1 cohort (n = 4,281) revealed a co-evolution of V3-crown reactivities and neutralization breadth. Our results indicate a role of V3-crown responses and its conformational preferences in bnAb development to be considered in preventive and therapeutic approaches.

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.

Preventive HIV Vaccines-Leveraging on Lessons from the Past to Pave the Way Forward

Almost four decades on, since the 1980’s, with hundreds of HIV vaccine candidates tested in both non-human primates and humans, and several HIV vaccines trials later, an efficacious HIV vaccine continues to evade us. The enormous worldwide genetic diversity of HIV, combined with HIV’s inherent recombination and high mutation rates, has hampered the development of an effective vaccine. Despite the advent of antiretrovirals as pre-exposure prophylaxis and preventative treatment, which have shown to be effective, HIV infections continue to proliferate, highlighting the great need for a vaccine. Here, we provide a brief history for the HIV vaccine field, with the most recent disappointments and advancements. We also provide an update on current passive immunity trials, testing proof of the concept of the most clinically advanced broadly neutralizing monoclonal antibodies for HIV prevention. Finally, we include mucosal immunity, the importance of vaccine-elicited immune responses and the challenges thereof in the most vulnerable environment–the female genital tract and the rectal surfaces of the gastrointestinal tract for heterosexual and men who have sex with men transmissions, respectively.

Bystander CD4 T-cell death is inhibited by broadly neutralizing anti-HIV antibodies only at levels blocking cell-to-cell viral transmission

The progressive loss of CD4+ T cells during HIV infection of lymphoid tissues involves both the apoptotic death of activated and productively infected CD4 T cells and the pyroptotic death of large numbers of resting and abortively infected bystander CD4 T cells. HIV spreads both through cellular release of virions and cell-to-cell transmission involving the formation of virological synapses. Cell-to-cell transmission results in high-level transfer of large quantities of virions to the target cell exceeding that achieved with cell-free virions. Broadly neutralizing anti-HIV antibodies (bNAbs) binding to HIV envelope protein capably block cell-free virus spread, and when added at higher concentrations can also interdict cell-to-cell transmission. Exploiting these distinct dose–response differences, we now show that four different bNAbs block the pyroptotic death of bystander cells, but only when added at concentrations sufficient to block cell-to-cell transmission. These findings further support the conclusion that HIV killing of abortively infected bystander CD4 T cells requires cell-to-cell transfer of virions. As bNAbs attract more interest as potential therapeutics, it will be important to consider the higher concentrations of these antibodies required to block the inflammatory death of bystander CD4 T cells.

Broadly Neutralizing Antibodies for HIV-1 Prevention

Given the absence of an effective vaccine for protection against HIV-1 infection, passive immunization strategies that utilize potent broadly neutralizing antibodies (bnAbs) to block acquisition of HIV-1 are being rigorously pursued in the clinical setting. bnAbs have demonstrated robust protection in preclinical animal models, and several leading bnAb candidates have shown favorable safety and pharmacokinetic profiles when tested individually or in combinations in early phase human clinical trials. Furthermore, passive administration of bnAbs in HIV-1 infected individuals has resulted in prolonged suppression of viral rebound following interruption of combination antiretroviral therapy, and robust antiviral activity when administered to viremic individuals. Recent results from the first efficacy trials testing repeated intravenous administrations of the anti-CD4 binding site bnAb VRC01 have demonstrated positive proof of concept that bnAb passive immunization can confer protection against HIV-1 infection in humans, but have also highlighted the considerable barriers that remain for such strategies to effectively contribute to control of the epidemic. In this review, we discuss the current status of clinical studies evaluating bnAbs for HIV-1 prevention, highlight lessons learned from the recent Antibody Mediated Prevention (AMP) efficacy trials, and provide an overview of strategies being employed to improve the breadth, potency, and durability of antiviral protection.

Employing Broadly Neutralizing Antibodies as a Human Immunodeficiency Virus Prophylactic & Therapeutic Application

Despite the discovery that the human immunodeficiency virus 1 (HIV-1) is the pathogen of acquired immunodeficiency syndrome (AIDS) in 1983, there is still no effective anti-HIV-1 vaccine. The major obstacle to the development of HIV-1 vaccine is the extreme diversity of viral genome sequences. Nonetheless, a number of broadly neutralizing antibodies (bNAbs) against HIV-1 have been made and identified in this area. Novel strategies based on using these bNAbs as an efficacious preventive and/or therapeutic intervention have been applied in clinical. In this review, we summarize the recent development of bNAbs and its application in HIV-1 acquisition prevention as well as discuss the innovative approaches being used to try to convey protection within individuals at risk and being treated for HIV-1 infection.

Antibody-based CCR5 blockade protects Macaques from mucosal SHIV transmission

In the absence of a prophylactic vaccine, the use of antiretroviral therapy (ART) as pre-exposure prophylaxis (PrEP) to prevent HIV acquisition by uninfected individuals is a promising approach to slowing the epidemic, but its efficacy is hampered by incomplete patient adherence and ART-resistant variants. Here, we report that competitive inhibition of HIV Env-CCR5 binding via the CCR5-specific antibody Leronlimab protects rhesus macaques against infection following repeated intrarectal challenges of CCR5-tropic SHIVSF162P3. Injection of Leronlimab weekly at 10 mg/kg provides significant but partial protection, while biweekly 50 mg/kg provides complete protection from SHIV acquisition. Tissue biopsies from protected macaques post challenge show complete CCR5 receptor occupancy and an absence of viral nucleic acids. After Leronlimab washout, protected macaques remain aviremic, and adoptive transfer of hematologic cells into naïve macaques does not transmit viral infection. These data identify CCR5 blockade with Leronlimab as a promising approach to HIV prophylaxis and support initiation of clinical trials.

Advancing HIV Broadly Neutralizing Antibodies: From Discovery to the Clinic

Despite substantial progress in confronting the global HIV-1 epidemic since its inception in the 1980s, better approaches for both treatment and prevention will be necessary to end the epidemic and remain a top public health priority. Antiretroviral therapy (ART) has been effective in extending lives, but at a cost of lifelong adherence to treatment. Broadly neutralizing antibodies (bNAbs) are directed to conserved regions of the HIV-1 envelope glycoprotein trimer (Env) and can block infection if present at the time of viral exposure. The therapeutic application of bNAbs holds great promise, and progress is being made toward their development for widespread clinical use. Compared to the current standard of care of small molecule-based ART, bNAbs offer: (1) reduced toxicity; (2) the advantages of extended half-lives that would bypass daily dosing requirements; and (3) the potential to incorporate a wider immune response through Fc signaling. Recent advances in discovery technology can enable system-wide mining of the immunoglobulin repertoire and will continue to accelerate isolation of next generation potent bNAbs. Passive transfer studies in pre-clinical models and clinical trials have demonstrated the utility of bNAbs in blocking or limiting transmission and achieving viral suppression. These studies have helped to define the window of opportunity for optimal intervention to achieve viral clearance, either using bNAbs alone or in combination with ART. None of these advances with bNAbs would be possible without technological advancements and expanding the cohorts of donor participation. Together these elements fueled the remarkable growth in bNAb development. Here, we review the development of bNAbs as therapies for HIV-1, exploring advances in discovery, insights from animal models and early clinical trials, and innovations to optimize their clinical potential through efforts to extend half-life, maximize the contribution of Fc effector functions, preclude escape through multiepitope targeting, and the potential for sustained delivery.

Generation of HIV-resistant cells with a single-domain antibody: implications for HIV-1 gene therapy

The cure or functional cure of the "Berlin patient" and "London patient" indicates that infusion of HIV-resistant cells could be a viable treatment strategy. Very recently, we genetically linked a short-peptide fusion inhibitor with a glycosylphosphatidylinositol (GPI) attachment signal, rendering modified cells fully resistant to HIV infection. In this study, GPI-anchored m36.4, a single-domain antibody (nanobody) targeting the coreceptor-binding site of gp120, was constructed with a lentiviral vector. We verified that m36.4 was efficiently expressed on the plasma membrane of transduced TZM-bl cells and targeted lipid raft sites without affecting the expression of HIV receptors (CD4, CCR5, and CXCR4). Significantly, TZM-bl cells expressing GPI-m36.4 were highly resistant to infection with divergent HIV-1 subtypes and potently blocked HIV-1 envelope-mediated cell-cell fusion and cell-cell viral transmission. Furthermore, we showed that GPI-m36.4-modified human CEMss-CCR5 cells were nonpermissive to both CCR5- and CXCR4-tropic HIV-1 isolates and displayed a strong survival advantage over unmodified cells. It was found that GPI-m36.4 could also impair HIV-1 Env processing and viral infectivity in transduced cells, underlying a multifaceted mechanism of antiviral action. In conclusion, our studies characterize m36.4 as a powerful nanobody that can generate HIV-resistant cells, offering a novel gene therapy approach that can be used alone or in combination.

HIV-1 Entry and Prospects for Protecting against Infection

Human Immunodeficiency Virus type-1 (HIV-1) establishes a latent viral reservoir soon after infection, which poses a major challenge for drug treatment and curative strategies. Many efforts are therefore focused on blocking infection. To this end, both viral and host factors relevant to the onset of infection need to be considered. Given that HIV-1 is most often transmitted mucosally, strategies designed to protect against infection need to be effective at mucosal portals of entry. These strategies need to contend also with cell-free and cell-associated transmitted/founder (T/F) virus forms; both can initiate and establish infection. This review will discuss how insight from the current model of HIV-1 mucosal transmission and cell entry has highlighted challenges in developing effective strategies to prevent infection. First, we examine key viral and host factors that play a role in transmission and infection. We then discuss preventive strategies based on antibody-mediated protection, with emphasis on targeting T/F viruses and mucosal immunity. Lastly, we review treatment strategies targeting viral entry, with focus on the most clinically advanced entry inhibitors.

Mechanistic Analysis of the Broad Antiretroviral Resistance Conferred by HIV-1 Envelope Glycoprotein Mutations

Despite the effectiveness of antiretroviral (ARV) therapy, virological failure can occur in some HIV-1-infected patients in the absence of mutations in drug target genes. We previously reported that, in vitro, the lab-adapted HIV-1 NL4-3 strain can acquire resistance to the integrase inhibitor dolutegravir (DTG) by acquiring mutations in the envelope glycoprotein (Env) that enhance viral cell-cell transmission. In this study, we investigated whether Env-mediated drug resistance extends to ARVs other than DTG and whether it occurs in other HIV-1 isolates. We demonstrate that Env mutations can reduce susceptibility to multiple classes of ARVs and also increase resistance to ARVs when coupled with target-gene mutations. We observe that the NL4-3 Env mutants display a more stable and closed Env conformation and lower rates of gp120 shedding than the WT virus. We also selected for Env mutations in clinically relevant HIV-1 isolates in the presence of ARVs. These Env mutants exhibit reduced susceptibility to DTG, with effects on replication and Env structure that are HIV-1 strain dependent. Finally, to examine a possible in vivo relevance of Env-mediated drug resistance, we performed single-genome sequencing of plasma-derived virus from five patients failing an integrase inhibitor-containing regimen. This analysis revealed the presence of several mutations in the highly conserved gp120-gp41 interface despite low frequency of resistance mutations in integrase. These results suggest that mutations in Env that enhance the ability of HIV-1 to spread via a cell-cell route may increase the opportunity for the virus to acquire high-level drug resistance mutations in ARV target genes.IMPORTANCE Although combination antiretroviral (ARV) therapy is highly effective in controlling the progression of HIV disease, drug resistance can be a major obstacle. Recent findings suggest that resistance can develop without ARV target gene mutations. We previously reported that mutations in the HIV-1 envelope glycoprotein (Env) confer resistance to an integrase inhibitor. Here, we investigated the mechanism of Env-mediated drug resistance and the possible contribution of Env to virological failure in vivo We demonstrate that Env mutations can reduce sensitivity to major classes of ARVs in multiple viral isolates and define the effect of the Env mutations on Env subunit interactions. We observed that many Env mutations accumulated in individuals failing integrase inhibitor therapy despite a low frequency of resistance mutations in integrase. Our findings suggest that broad-based Env-mediated drug resistance may impact therapeutic strategies and provide clues toward understanding how ARV-treated individuals fail therapy without acquiring mutations in drug target genes.

The importance of semen leukocytes in HIV-1 transmission and the development of prevention strategies

HIV-1 sexual transmission occurs mostly through contaminated semen, which is a complex mixture of soluble factors with immunoregulatory functions and cells. It is well established that semen cells from HIV-1-infected men are able to produce the virus and that are harnessed to efficiently interact with mucosal barriers exposed during sexual intercourse. Several cofactors contribute to semen infectivity and may enhance the risk of HIV-1 transmission to a partner by increasing local HIV-1 replication in the male genital tract, thereby increasing the number of HIV-1-infected cells and the local HIV-1 shedding in semen. The introduction of combination antiretroviral therapy has improved the life expectancy of HIV-1 infected individuals; however, there is evidence that systemic viral suppression does not always reflect full viral suppression in the seminal compartment. This review focus on the role semen leukocytes play in HIV-1 transmission and discusses implications of the increased resistance of cell-mediated transmission to immune-based prevention strategies.

Broadly neutralizing antibodies potently inhibit cell-to-cell transmission of semen leukocyte-derived SHIV162P3

BACKGROUND

HIV-1 sexual transmission occurs mostly through infected semen, which contains both free virions and infected leukocytes. Transmission initiated by infected cells has been shown by several in vitro and in vivo studies and a reduced capacity of broadly neutralizing antibodies (bNAbs) to inhibit cell-to-cell transmission has also been reported. However, due to limitations of available experimental models, there is yet no clarity to which extend bNAbs can prevent transmission mediated by semen leukocytes.

METHODS

We developed a novel in vitro assay to measure cell-cell transmission that makes use of splenocytes or CD45+ semen leukocytes collected from acutely SHIV162P3-infected cynomolgus macaques. A panel of 11 bNAbs was used either alone or in combination to assess their inhibitory potential against both cell-free and cell-cell infection.

FINDINGS

Splenocytes and semen leucocytes displayed a similar proportion of CD4+T-cell subsets. Either cell type transferred infection in vitro to target TZM-bl cells and PBMCs. Moreover, infection of macaques was achieved following intravaginal challenge with splenocytes. The anti-N-glycans/V3 loop bNAb 10-1074 was highly efficient against cell-associated transmission mediated by infected spleen cells and its potency was maintained when transmission was mediated by CD45+ semen leukocytes.

INTERPRETATION

These results support the use of bNAbs in preventative or therapeutic studies aiming to block transmission events mediated not only by free viral particles but also by infected cells. Our experimental system could be used to predict in vivo efficacy of bNAbs.

FUNDING

This work was funded by the ANRS and the European Commission.

Broadly neutralizing antibodies for HIV-1: efficacies, challenges and opportunities

Combination antiretroviral therapy (cART) is effective but not curative, and no successful vaccine is currently available for human immunodeficiency virus-1 (HIV-1). Broadly neutralizing antibodies (bNAbs) provide a new approach to HIV-1 prevention and treatment, and these promising candidates advancing into clinical trials have shown certain efficacies in infected individuals. In addition, bNAbs have the potential to kill HIV-1-infected cells and to affect the course of HIV-1 infection by directly engaging host immunity. Nonetheless, challenges accompany the use of bNAbs, including transient suppression of viraemia, frequent emergence of resistant viruses in rebound viraemia, suboptimal efficacy in virus cell-to-cell transmission, and unclear effects on the cell-associated HIV-1 reservoir. In this review, we discuss opportunities and potential strategies to address current challenges to promote the future use of immunotherapy regimens.

EWI-2 Inhibits Cell-Cell Fusion at the HIV-1 Virological Presynapse

Cell-to-cell transfer of virus particles at the Env-dependent virological synapse (VS) is a highly efficient mode of HIV-1 transmission. While cell–cell fusion could be triggered at the VS, leading to the formation of syncytia and preventing exponential growth of the infected cell population, this is strongly inhibited by both viral (Gag) and host (ezrin and tetraspanins) proteins. Here, we identify EWI-2, a protein that was previously shown to associate with ezrin and tetraspanins, as a host factor that contributes to the inhibition of Env-mediated cell–cell fusion. Using quantitative fluorescence microscopy, shRNA knockdowns, and cell–cell fusion assays, we show that EWI-2 accumulates at the presynaptic terminal (i.e., the producer cell side of the VS), where it contributes to the fusion-preventing activities of the other viral and cellular components. We also find that EWI-2, like tetraspanins, is downregulated upon HIV-1 infection, most likely by Vpu. Despite the strong inhibition of fusion at the VS, T cell-based syncytia do form in vivo and in physiologically relevant culture systems, but they remain small. In regard to that, we demonstrate that EWI-2 and CD81 levels are restored on the surface of syncytia, where they (presumably) continue to act as fusion inhibitors. This study documents a new role for EWI-2 as an inhibitor of HIV-1-induced cell–cell fusion and provides novel insight into how syncytia are prevented from fusing indefinitely.

A Membrane-Anchored Short-Peptide Fusion Inhibitor Fully Protects Target Cells from Infections of Human Immunodeficiency Virus Type 1 (HIV-1), HIV-2, and Simian Immunodeficiency Virus

Emerging studies demonstrate that the antiviral activity of viral fusion inhibitor peptides can be dramatically improved when being chemically or genetically anchored to the cell membrane, where viral entry occurs. We previously reported that the short-peptide fusion inhibitor 2P23 and its lipid derivative possess highly potent antiviral activities against human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus (SIV). To develop a sterilizing or functional-cure strategy, here we genetically linked 2P23 and two control peptides (HIV-1 fusion inhibitor C34 and hepatitis B virus [HBV] entry inhibitor 4B10) with a glycosylphosphatidylinositol (GPI) attachment signal. As expected, GPI-anchored inhibitors were efficiently expressed on the plasma membrane of transduced TZM-bl cells and primarily directed to the lipid raft site without interfering with the expression of CD4, CCR5, and CXCR4. GPI-anchored 2P23 (GPI-2P23) completely protected TZM-bl cells from infections of divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide (T20)-resistant mutants. GPI-2P23 also rendered the cells resistant to viral envelope-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission. Moreover, GPI-2P23-modified human CD4⁺ T cells (CEMss-CCR5) fully blocked both R5- and X4-tropic HIV-1 isolates and displayed a robust survival advantage over unmodified cells during HIV-1 infection. In contrast, it was found that GPI-anchored C34 was much less effective in inhibiting HIV-2, SIV, and T20-resistant HIV-1 mutants. Therefore, our studies have demonstrated that genetically anchoring a short-peptide fusion inhibitor to the target cell membrane is a viable strategy for gene therapy of both HIV-1 and HIV-2 infections.IMPORTANCE Antiretroviral therapy with multiple drugs in combination can efficiently suppress HIV replication and dramatically reduce the morbidity and mortality associated with AIDS-related illness; however, antiretroviral therapy cannot eradiate the HIV reservoirs, and lifelong treatment is required, which often results in cumulative toxicities, drug resistance, and a multitude of complications, thus necessitating the development of sterilizing-cure or functional-cure strategies. Here, we report that genetically anchoring the short-peptide fusion inhibitor 2P23 to the cell membrane can fully prevent infections from divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide-resistant mutants. Membrane-bound 2P23 also effectively blocks HIV-1 Env-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission, renders CD4⁺ T cells nonpermissive to infection, and confers a robust survival advantage over unmodified cells. Thus, our studies verify a powerful strategy to generate resistant cells for gene therapy of both the HIV-1 and HIV-2 infections.

Negative and Positive Selection Pressure During Sexual Transmission of Transmitted Founder HIV-1

Sexual transmission of HIV-1 consists of processes that exert either positive or negative selection pressure on the virus. The sum of these selection pressures lead to the transmission of only one specific HIV-1 strain, termed the transmitted founder virus. Different dendritic cell subsets are abundantly present at mucosal sites and, interestingly, these DC subsets exert opposite pressure on viral selection during sexual transmission. In this review we describe receptors and cellular compartments in DCs that are involved in HIV-1 communication leading to either viral restriction by the host or further dissemination to establish a long-lived reservoir. We discuss the current understanding of host antiretroviral restriction factors against HIV-1 and specifically against the HIV-1 transmitted founder virus. We will also discuss potential clinical implications for exploiting these intrinsic restriction factors in developing novel therapeutic targets. A better understanding of these processes might help in developing strategies against HIV-1 infections by targeting dendritic cells.

Why viruses sometimes disperse in groups?†

Many organisms disperse in groups, yet this process is understudied in viruses. Recent work, however, has uncovered different types of collective infectious units, all of which lead to the joint delivery of multiple viral genome copies to target cells, favoring co-infections. Collective spread of viruses can occur through widely different mechanisms, including virion aggregation driven by specific extracellular components, cloaking inside lipid vesicles, encasement in protein matrices, or binding to cell surfaces. Cell-to-cell viral spread, which allows the transmission of individual virions in a confined environment, is yet another mode of clustered virus dissemination. Nevertheless, the selective advantages of dispersing in groups remain poorly understood in most cases. Collective dispersal might have emerged as a means of sharing efficacious viral transmission vehicles. Alternatively, increasing the cellular multiplicity of infection may confer certain short-term benefits to viruses, such as overwhelming antiviral responses, avoiding early stochastic loss of viral components required for initiating infection, or complementing genetic defects present in different viral genomes. However, increasing infection multiplicity may also entail long-term costs, such as mutation accumulation and the evolution of defective particles or other types of cheater viruses. These costs and benefits, in turn, should depend on the genetic relatedness among collective infectious unit members. Establishing the genetic basis of collective viral dispersal and performing controlled experiments to pinpoint fitness effects at different spatial and temporal scales should help us clarify the implications of these spread modes for viral fitness, pathogenicity, and evolution.

Mechanisms of HIV-1 cell-to-cell transmission and the establishment of the latent reservoir

HIV-1 spreads through both the release of cell-free particles and by cell-to-cell transmission. Mounting evidence indicates that cell-to-cell transmission is more efficient than cell-free transmission of particles and likely influences the pathogenesis of HIV-1 infection. This mode of viral transmission also influences the generation and maintenance of the latent reservoir, which represents the main obstacle for curing the infection. In this review we will discuss general cell contact-dependent mechanisms that HIV-1 utilizes for its spread and the evidence pointing to cell-to-cell transmission as a mechanism for the establishment and maintenance of latent infection.

Broadly Neutralizing Antibodies against HIV: Back to Blood

After years of continuous exposure to HIV envelope antigens, a minority of HIV-infected individuals develop a cognate polyclonal humoral response comprising very potent and extremely cross-reactive neutralizing antibodies [broadly neutralizing antibodies (bNAbs)]. Isolated bNAbs derived from memory B cell pools have been the focus of intense studies over the past decade. However, it is not yet known how to translate the features of bNAbs into practical HIV prevention methods. In this review, we attempt to seek insights from emerging information about the human broadly neutralizing plasma response as well as its frequency, clonal composition, specificity, potency, and commonality among infected subjects. We also consider how this information points to selecting and prioritizing certain epitope targets and strategies for HIV vaccine design.

CD4 occupancy triggers sequential pre-fusion conformational states of the HIV-1 envelope trimer with relevance for broadly neutralizing antibody activity

During the entry process, the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer undergoes a sequence of conformational changes triggered by both CD4 and coreceptor engagement. Resolving the conformation of these transient entry intermediates has proven challenging. Here, we fine-mapped the antigenicity of entry intermediates induced by increasing CD4 engagement of cell surface–expressed Env. Escalating CD4 triggering led to the sequential adoption of different pre-fusion conformational states of the Env trimer, up to the pre-hairpin conformation, that we assessed for antibody epitope presentation. Maximal accessibility of the coreceptor binding site was detected below Env saturation by CD4. Exposure of the fusion peptide and heptad repeat 1 (HR1) required higher CD4 occupancy. Analyzing the diverse antigenic states of the Env trimer, we obtained key insights into the transitions in epitope accessibility of broadly neutralizing antibodies (bnAbs). Several bnAbs preferentially bound CD4-triggered Env, indicating a potential capacity to neutralize both pre- and post-CD4 engagement, which needs to be explored. Assessing binding and neutralization activity of bnAbs, we confirm antibody dissociation rates as a driver of incomplete neutralization. Collectively, our findings highlight a need to resolve Env conformations that are neutralization-relevant to provide guidance for immunogen development.

Comprehensive mapping of conformational stages adopted by the HIV‐1 envelope glycoprotein trimer during entry into the cell reveals the preference of broadly neutralizing antibodies for distinct pre-fusion states of the trimer.

The trimeric human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) mediates HIV-1 entry into its target cells. Entry is initiated by sequential triggering of Env upon interaction with its primary receptor CD4 and a coreceptor on target cells. The ensuing structural rearrangements of the Env trimer bring the viral membrane in close vicinity of the cellular membrane, enabling fusion. Resolving the structural differences between the consecutive conformations Env adopts during the entry process is of high interest, as different antigenic domains are exposed, which may affect the capacity of neutralizing antibodies to bind to Env and inhibit entry. Here, we compared the conformation of unliganded closed Env with the transitional CD4-bound Env forms by studying the antigenicity of cell surface–expressed Env with and without CD4 triggering. We show that incremental triggering by soluble CD4 allows the capture of the full continuum of conformational changes, including events that follow coreceptor interaction. Thus, the setup we introduce here turns a simple binding assay into a powerful tool to study transitional conformation changes in HIV-1 Env. Analyzing the capacity of Env-reactive antibodies to recognize the diverse Env stages, our study reveals novel aspects of the binding preferences of neutralizing antibodies that affect their inhibitory activity.

Fc-dependent functions are redundant to efficacy of anti-HIV antibody PGT121 in macaques

A considerable body of evidence suggests that Fc-dependent functions improve the capacity of broadly neutralizing antibodies (BnAbs) to protect against and control HIV-1 infection. This phenomenon, however, has not been formally tested in robust cell-associated macaque simian-human immunodeficiency virus (SHIV) models with newer-generation BnAbs. We studied both the WT BnAb PGT121 and a LALA mutant of PGT121 (which has impaired Fc-dependent functions) for their ability to protect pigtail macaques from an i.v. high-dose cell-associated SHIVSF162P3 challenge. We found that both WT and LALA PGT121 completely protected all 12 macaques studied. Further, partial depletion of NK cells, key mediators of Fc-dependent functions, did not abrogate the protective efficacy of PGT121 in 6 macaques. Additionally, in animals with established SHIVSF162P3 infection, SHIV viremia levels were equally rapidly reduced by LALA and WT PGT121. Our studies suggest that the potent neutralizing capacity of PGT121 renders the Fc-dependent functions of the Ab at least partially redundant. These findings have implications for Ab-mediated protection from and control of HIV-1 infection.

Variable infectivity and conserved engagement in cell-to-cell viral transfer by HIV-1 Env from Clade B transmitted founder clones

HIV-1 transmission is usually initiated by a single viral strain called transmitted/ founder (T/F) virus. In in vitro models, HIV-1 can efficiently spread via cell-free and virological synapse (VS)-mediated cell-to-cell infection. Both modes of infection require the viral glycoprotein Envelope (Env). The efficiency with which T/F Envs initiate VS and mediate cell-to-cell infection has not been well characterized. Here we tested a panel of isogenic HIV-1 molecular clones that carry different Clade B T/F Envs. We found that despite variable infectivity among different Env clones in the two modes of infection, T/F Envs generally mediated efficient VS formation and subsequent cell-to-cell transfer. In contrast, in vitro infectivity of the T/F Env clones was more variable and strongly correlated with intrinsic fusogenicity of various Envs. We speculate that the conservation of cell-to-cell transfer by T/F Env is indicative of a biologically important function of Env.

Recent progress in broadly neutralizing antibodies to HIV

In this Review, we highlight some recent developments in the discovery and application of broadly neutralizing antibodies (bnAbs) to human immunodeficiency virus (HIV); i.e., antibodies able to neutralize diverse isolates of HIV. We consider the characterization of novel bnAbs, recent data on the effects of bnAbs in vivo in humans and animal models, and the importance of both kinds of data for the application of Abs to prophylaxis and therapy and to guide vaccine design. We seek to place newly discovered bnAbs in the context of existing bnAbs, and we explore the various characteristics of the antibodies that are most desirable for different applications.

HIV Entry and Its Inhibition by Bifunctional Antiviral Proteins

HIV entry is a highly specific and time-sensitive process that can be divided into receptor binding, coreceptor binding, and membrane fusion. Bifunctional antiviral proteins (bAVPs) exploit the multi-step nature of the HIV entry process by binding to two different extracellular targets. They are generated by expressing a fusion protein containing two entry inhibitors with a flexible linker. The resulting fusion proteins exhibit exceptional neutralization potency and broad cross-clade inhibition. In this review, we summarize the HIV entry process and provide an overview of the design, antiviral potency, and methods of delivery of bAVPs. Additionally, we discuss the advantages and limitations of bAVPs for HIV prevention and treatment.

A defucosylated bispecific multivalent molecule exhibits broad HIV-1-neutralizing activity and enhanced antibody-dependent cellular cytotoxicity against reactivated HIV-1 latently infected cells

OBJECTIVE

Current treatments cannot completely eradicate HIV-1 owing to the presence of latently infected cells, which harbor transcriptionally silent HIV-1. However, defucosylated antibodies can readily kill latently infected cells after their activation to express envelope glycoprotein (Env) through antibody-dependent cellular cytotoxicity (ADCC). We herein aimed to test a defucosylated bispecific multivalent molecule consisting of domain-antibody and single-domain CD4, LSEVh-LS-F, for its HIV-1 neutralizing activity and ADCC against the reactivated latently infected cells, compared with the nondefucosylated molecule LSEVh-LS.

METHODS

LSEVh-LS-F's neutralizing activity against a panel of newly characterized Chinese HIV-1 clinical isolates was assessed by using TZM-bl-based and PBMC-based assays. LSEVh-LS-F-mediated ADCC in the presence of natural killer cells against cell lines that stably express Env proteins, HIV-1-infected cells and LRA-reactivated HIV-1 latent cells, was measured using a lactate dehydrogenase (LDH) cytotoxicity assay or flow cytometry.

RESULTS

LSEVh-LS-F and LSEVh-LS were equally effective in neutralized infection of all HIV-1 isolates tested with IC50 and IC90 values 3∼4-fold lower than those of VRC01. LSEVh-LS-F was more effective in natural killer-mediated killing of HIV-1 Env-expressing cell lines, HIV-1-infected cells, latency reactivation agents-reactivated ACH2 cells and reactivated latently infected resting CD4+ T cell line as well as resting CD4+ T lymphocytes isolated from patients receiving HAART.

CONCLUSION

LSEVh-LS-F exhibits broad HIV-1 neutralizing activity and enhanced ADCC against HIV-1-infected cells, reactivated latently infected cell lines and primary CD4+ T cells, thus being a promising candidate therapeutic for eradicating the HIV-1 reservoir.

Importance of Fc-mediated functions of anti-HIV-1 broadly neutralizing antibodies

Anti-HIV-1 broadly neutralizing antibodies (BnAbs) exhibit an impressive capacity to protect against chimeric SIV-HIV (SHIV) challenges in macaques and potently reduce viremia in both SHIV-infected macaques and HIV-1-infected humans. There is a body of evidence suggesting Fc-mediated functions of anti-HIV-1 binding antibodies are important in protecting from infection and controlling viremia. The degree to which the efficacy of BnAbs is assisted by Fc-mediated functions is of great interest. Challenge experiments with the older generation BnAb b12 showed that mutating the Fc region to abrogate Fcγ receptor binding reduced protective efficacy in macaques. Similar data have been generated with newer BnAbs using murine models of HIV-1. In addition, the degree to which therapeutically administered BnAbs reduce viremia suggests that elimination of infected cells through Fc-mediated functions may contribute to their efficacy. Fc-mediated functions that eliminate infected cells may be particularly important for challenge systems involving cell-associated virus. Herein we review data regarding the importance of Fc-mediated functions of BnAbs in mediating protective immunity and control of viremia.

HIV-1 cell-to-cell transmission and broadly neutralizing antibodies

HIV-1 spreads through contacts between infected and target cells. Polarized viral budding at the contact site forms the virological synapse. Additional cellular processes, such as nanotubes, filopodia, virus accumulation in endocytic or phagocytic compartments promote efficient viral propagation. Cell-to-cell transmission allows immune evasion and likely contributes to HIV-1 spread in vivo. Anti-HIV-1 broadly neutralizing antibodies (bNAbs) defeat the majority of circulating viral strains by binding to the viral envelope glycoprotein (Env). Several bNAbs have entered clinical evaluation during the last years. It is thus important to understand their mechanism of action and to determine how they interact with infected cells. In experimental models, HIV-1 cell-to-cell transmission is sensitive to neutralization, but the effect of antibodies is often less marked than during cell-free infection. This may be due to differences in the conformation or accessibility of Env at the surface of virions and cells. In this review, we summarize the current knowledge on HIV-1 cell-to-cell transmission and discuss the role of bNAbs during this process.

Neutralizing Antibody-Based Prevention of Cell-Associated HIV-1 Infection

Improved vaccine-mediated protection against HIV-1 requires a thorough understanding of the mode of HIV-1 transmission and how various immune responses control transmission. Cell-associated HIV-1 is infectious and contributes to HIV-1 transmission in humans. Non-human primate models of cell-associated SIV infection demonstrate that cell-associated SIV is more infectious than cell-free SIV. In a recently described chimeric simian–human immunodeficiency virus (SHIV) macaque model, it was demonstrated that an occult infection with cell-associated SHIV can be established that evades passive protection with a broadly neutralizing antibody (bnAb). Indeed, considerable in vitro data shows that bnAbs have less efficacy against cell-associated HIV-1 than cell-free HIV-1. Optimizing the protective capacity of immune responses such as bnAbs against cell-associated infections may be needed to maximize their protective efficacy.

Accounting for Space—Quantification of Cell-To-Cell Transmission Kinetics Using Virus Dynamics Models

Mathematical models based on ordinary differential equations (ODE) that describe the population dynamics of viruses and infected cells have been an essential tool to characterize and quantify viral infection dynamics. Although an important aspect of viral infection is the dynamics of viral spread, which includes transmission by cell-free virions and direct cell-to-cell transmission, models used so far ignored cell-to-cell transmission completely, or accounted for this process by simple mass-action kinetics between infected and uninfected cells. In this study, we show that the simple mass-action approach falls short when describing viral spread in a spatially-defined environment. Using simulated data, we present a model extension that allows correct quantification of cell-to-cell transmission dynamics within a monolayer of cells. By considering the decreasing proportion of cells that can contribute to cell-to-cell spread with progressing infection, our extension accounts for the transmission dynamics on a single cell level while still remaining applicable to standard population-based experimental measurements. While the ability to infer the proportion of cells infected by either of the transmission modes depends on the viral diffusion rate, the improved estimates obtained using our novel approach emphasize the need to correctly account for spatial aspects when analyzing viral spread.

Incomplete inhibition of HIV infection results in more HIV infected lymph node cells by reducing cell death

HIV has been reported to be cytotoxic in vitro and in lymph node infection models. Using a computational approach, we found that partial inhibition of transmissions of multiple virions per cell could lead to increased numbers of live infected cells. If the number of viral DNA copies remains above one after inhibition, then eliminating the surplus viral copies reduces cell death. Using a cell line, we observed increased numbers of live infected cells when infection was partially inhibited with the antiretroviral efavirenz or neutralizing antibody. We then used efavirenz at concentrations reported in lymph nodes to inhibit lymph node infection by partially resistant HIV mutants. We observed more live infected lymph node cells, but with fewer HIV DNA copies per cell, relative to no drug. Hence, counterintuitively, limited attenuation of HIV transmission per cell may increase live infected cell numbers in environments where the force of infection is high.

The HIVvirus infects cells of the immune system. Once inside, it hijacks the cellular molecular machineries to make more copies of itself, which are then transmitted to new host cells. HIV eventually kills most cells it infects, either in the steps leading to the infection of the cell, or after the cell is already producing virus. HIV can spread between cells in two ways, known as cell-free or cell-to-cell. In the first, individual viruses are released from infected cells and move randomly through the body in the hope of finding new cells to infect. In the second, infected cells interact directly with uninfected cells. The second method is often much more successful at infecting new cells since they are exposed to multiple virus particles.

HIV infections can be controlled by using combinations of antiretroviral drugs, such as efavirenz, to prevent the virus from making more of itself. With a high enough dose, the drugs can in theory completely stop HIV infections, unless the virus becomes resistant to treatment. However, some patients continue to use these drugs even after the virus they are infected with develops resistance. It is not clear what effect taking ineffective, or partially effective, drugs has on how HIV progresses.

Using efavirenz, Jackson, Hunter et al. partially limited the spread of HIV between human cells grown in the laboratory. The experiments mirrored the situation where a partially resistant HIV strain spreads through the body. The results show that the success of cell-free infection is reduced as drug dose increases. Yet paradoxically, in cell-to-cell infection, the presence of drug caused more cells to become infected. This can be explained by the fact that, in cell-to-cell spread, each cell is exposed to multiple copies of the virus. The drug dose reduced the number of viral copies per cell without stopping the virus from infecting completely. The reduced number of viral copies per cell made it more likely that infected cells would survive the infection long enough to produce virus particles themselves.

Viruses that can kill cells, such as HIV, must balance the need to make more of themselves against the speed that they kill their host cell to maximize the number of infected cells. If transmission between cells is too effective and too many virus particles are delivered to the new cell, the virus may not manage to infect new hosts before killing the old ones. These findings highlight this delicate balance. They also indicate a potential issue in using drugs to treat partially resistant virus strains. Without care, these treatments could increase the number of infected cells in the body, potentially worsening the effects of living with HIV.

Glycosyl-Phosphatidylinositol-Anchored Anti-HIV Env Single-Chain Variable Fragments Interfere with HIV-1 Env Processing and Viral Infectivity

In previous studies, we demonstrated that single-chain variable fragments (scFvs) from anti-human immunodeficiency virus (HIV) Env monoclonal antibodies act as entry inhibitors when tethered to the surface of target cells by a glycosyl-phosphatidylinositol (GPI) anchor. Interestingly, even if a virus escapes inhibition at entry, its replication is ultimately controlled. We hypothesized that in addition to functioning as entry inhibitors, anti-HIV GPI-scFvs may also interact with Env in an infected cell, thereby interfering with the infectivity of newly produced virions. Here, we show that expression of the anti-HIV Env GPI-scFvs in virus-producing cells reduced the release of HIV from cells 5- to 22-fold, and infectivity of the virions that were released was inhibited by 74% to 99%. Additionally, anti-HIV Env GPI-scFv X5 inhibited virion production and infectivity after latency reactivation and blocked transmitter/founder virus production and infectivity in primary CD4⁺ T cells. In contrast, simian immunodeficiency virus (SIV) production and infectivity were not affected by the anti-HIV Env GPI-scFvs. Loss of infectivity of HIV was associated with a reduction in the amount of virion-associated Env gp120. Interestingly, an analysis of Env expression in cell lysates demonstrated that the anti-Env GPI-scFvs interfered with processing of Env gp160 precursors in cells. These data indicate that GPI-scFvs can inhibit Env processing and function, thereby restricting production and infectivity of newly synthesized HIV. Anti-Env GPI-scFvs therefore appear to be unique anti-HIV molecules as they derive their potent inhibitory activity by interfering with both early (receptor binding/entry) and late (Env processing and incorporation into virions) stages of the HIV life cycle.IMPORTANCE The restoration of immune function and persistence of CD4⁺ T cells in HIV-1-infected individuals without antiretroviral therapy requires a way to increase resistance of CD4⁺ T cells to infection by both R5- and X4-tropic HIV-1. Previously, we reported that anchoring anti-HIV-1 single-chain variable fragments (scFvs) via glycosyl-phosphatidylinositol (GPI) to the surface of permissive cells conferred a high level of resistance to HIV-1 variants at the level of entry. Here, we report that anti-HIV GPI-scFvs also derive their potent antiviral activity in part by blocking HIV production and Env processing, which consequently inhibits viral infectivity even in primary infection models. Thus, we conclude that GPI-anchored anti-HIV scFvs derive their potent blocking activity of HIV replication by interfering with successive stages of the viral life cycle. They may be effectively used in genetic intervention of HIV-1 infection.

Distinct functions for the membrane-proximal ectodomain region (MPER) of HIV-1 gp41 in cell-free and cell-cell viral transmission and cell-cell fusion

HIV-1 is spread by cell-free virions and by cell-cell viral transfer. We asked whether the structure and function of a broad neutralizing antibody (bNAb) epitope, the membrane-proximal ectodomain region (MPER) of the viral gp41 transmembrane glycoprotein, differ in cell-free and cell-cell-transmitted viruses and whether this difference could be related to Ab neutralization sensitivity. Whereas cell-free viruses bearing W666A and I675A substitutions in the MPER lacked infectivity, cell-associated mutant viruses were able to initiate robust spreading infection. Infectivity was restored to cell-free viruses by additional substitutions in the cytoplasmic tail (CT) of gp41 known to disrupt interactions with the viral matrix protein. We observed contrasting effects on cell-free virus infectivity when W666A was introduced to two transmitted/founder isolates, but both mutants could still mediate cell-cell spread. Domain swapping indicated that the disparate W666A phenotypes of the cell-free transmitted/founder viruses are controlled by sequences in variable regions 1, 2, and 4 of gp120. The sequential passaging of an MPER mutant (W672A) in peripheral blood mononuclear cells enabled selection of viral revertants with loss-of-glycan suppressor mutations in variable region 1, suggesting a functional interaction between variable region 1 and the MPER. An MPER-directed bNAb neutralized cell-free virus but not cell-cell viral spread. Our results suggest that the MPER of cell-cell-transmitted virions has a malleable structure that tolerates mutagenic disruption but is not accessible to bNAbs. In cell-free virions, interactions mediated by the CT impose an alternative MPER structure that is less tolerant of mutagenic alteration and is efficiently targeted by bNAbs.