Release of gp120 Restraints Leads to an Entry-Competent Intermediate State of the HIV-1 Envelope Glycoproteins

Novel ApeC-containing protein mediates the recognition and internalization of Vibrio splendidus in Apostichopus japonicus

Pattern recognition receptors (PRRs) mediate the innate immune responses and play a crucial role in host defense against pathogen infections. Apextrin C-terminal (ApeC)-containing proteins (ACPs), a newly discovered class of PRRs specific to invertebrates, recognize pathogens through their ApeC domain as intracellular or extracellular effectors. However, the other immunological functions of ACPs remain unclear. In this study, a membrane-localized ACP receptor was identified in the sea cucumber Apostichopus japonicus (denoted as AjACP1). The ApeC domain of AjACP1, which was located outside of its cell membrane, exhibited the capability to recognize and aggregate Vibrio splendidus. AjACP1 was upregulated upon V. splendidus infection, internalizing into the cytoplasm of coelomocytes. AjACP1 overexpression enhanced the phagocytic activity of coelomocytes against V. splendidus, while knockdown of AjACP1 by RNA interfere inhibited coelomocyte endocytosis. Inhibitor experiments indicated that AjACP1 regulated coelomocyte phagocytosis through the actin-dependent endocytic signaling pathway. Further investigation revealed that AjACP1 interacted with the subunit of the actin-related protein 2/3 complex ARPC2, promoting F-actin polymerization and cytoskeletal rearrangement and thereby affecting the coelomocyte phagocytosis of V. splendidus via the actin-dependent endocytic signaling pathway. As a novel membrane PRR, AjACP1 mediates the recognition and phagocytic activity of coelomocytes against V. splendidus through the AjACP1-ARPC2-F-actin polymerization and cytoskeletal rearrangement pathway.

Membrane HIV-1 envelope glycoproteins stabilized more strongly in a pretriggered conformation than natural virus Envs

The pretriggered conformation of the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer ((gp120/gp41)3) is targeted by virus entry inhibitors and broadly neutralizing antibodies (bNAbs). The lability of pretriggered Env has hindered its characterization. Here, we produce membrane Env variants progressively stabilized in pretriggered conformations, in some cases to a degree beyond that found in natural HIV-1 strains. Pretriggered Env stability correlated with stronger trimer subunit association, increased virus sensitivity to bNAb neutralization, and decreased capacity to mediate cell-cell fusion and virus entry. For some highly stabilized Env mutants, after virus-host cell engagement, the normally inaccessible gp120 V3 region on an Env intermediate became targetable by otherwise poorly neutralizing antibodies. Thus, evolutionary pressure on HIV-1 Env to maintain trimer integrity, responsiveness to the CD4 receptor, and resistance to antibodies modulates pretriggered Env stability. The strongly stabilized pretriggered membrane Envs reported here will facilitate further characterization of this functionally important conformation.

mRNA-based HIV-1 vaccines

SUMMARYThe success of the Severe Acute Respiratory Syndrome Coronavirus 2 mRNA vaccines to lessen/prevent severe COVID-19 opened new opportunities to develop RNA vaccines to fight other infectious agents. HIV-1 is a lentivirus that integrates into the host cell genome and persists for the lifetime of infected cells. Multiple mechanisms of immune evasion have posed significant obstacles to the development of an effective HIV-1 vaccine over the last four decades since the identification of HIV-1. Recently, attempts to address some of these challenges have led to multiple studies that manufactured, optimized, and tested, in different animal models, mRNA-based HIV-1 vaccines. Several clinical trials have also been initiated or are planned to start soon. Here, we review the current strategies applied to HIV-1 mRNA vaccines, discuss different targeting approaches, summarize the latest findings, and offer insights into the challenges and future of HIV-1 mRNA vaccines.

Insights from HIV-1 vaccine and passive immunization efficacy trials

An effective HIV-1 vaccine is still not available, and most vaccine efficacy trials conducted over the years resulted in no significant overall protection. Here we highlight several insights gained from these trials as well as emerging questions that may be important for further guidance to advance current research directions.

The combination of three CD4-induced antibodies targeting highly conserved Env regions with a small CD4-mimetic achieves potent ADCC activity

The majority of naturally-elicited antibodies against the HIV-1 envelope glycoproteins (Env) are non-neutralizing (nnAbs), because they are unable to recognize the Env timer in its native "closed" conformation. Nevertheless, it has been shown that nnAbs have the potential to eliminate HIV-1-infected cells by Antibody-Dependent Cellular Cytotoxicity (ADCC) provided that Env is present on the cell surface in its "open" conformation. This is because most nnAbs recognize epitopes that become accessible only after Env interaction with CD4 and the exposure of epitopes that are normally occluded in the closed trimer. HIV-1 limits this vulnerability by downregulating CD4 from the surface of infected cells, thus preventing a premature encounter of Env with CD4. Small CD4-mimetics (CD4mc) sensitize HIV-1-infected cells to ADCC by opening the Env glycoprotein and exposing CD4-induced (CD4i) epitopes. There are two families of CD4i nnAbs, termed anti-cluster A and anti-CoRBS Abs, which are known to mediate ADCC in the presence of CD4mc. Here, we performed Fab competition experiments and found that anti-gp41 cluster I antibodies comprise a major fraction of the plasma ADCC activity in people living with HIV (PLWH). Moreover, addition of gp41 cluster I antibodies to cluster A and CoRBS antibodies greatly enhanced ADCC mediated cell killing in the presence of a potent indoline CD4mc, CJF-III-288. This cocktail outperformed broadly-neutralizing antibodies and even showed activity against HIV-1 infected monocyte-derived macrophages. Thus, combining CD4i antibodies with different specificities achieves maximal ADCC activity, which may be of utility in HIV cure strategies.

Alternative substitutions of N332 in HIV-1AD8 gp120 differentially affect envelope glycoprotein function and viral sensitivity to broadly neutralizing antibodies targeting the V3-glycan

The envelope glycoprotein (Env) trimer on the surface of human immunodeficiency virus type I (HIV-1) mediates viral entry into host CD4+ T cells and is the sole target of neutralizing antibodies. Broadly neutralizing antibodies (bnAbs) that target gp120 V3-glycan of HIV-1 Env trimer are potent and block the entry of diverse HIV-1 strains. Most V3-glycan bnAbs interact, to a different extent, with a glycan attached to N332, but Asn at this position is not absolutely conserved or required for HIV-1 entry based on the prevalence of N332 in different circulating HIV-1 strains from diverse clades. Here, we studied the effects of amino acid changes at position 332 of HIV-1AD8 Envs on HIV-1 sensitivity to antibodies, cold exposure, and soluble CD4. We further investigated how these changes affect Env function and HIV-1 infectivity in vitro. Our results suggest robust tolerability of HIV-1AD8 Env N332 to changes, with specific changes that resulted in extended exposure of gp120 V3 loop, which is typically concealed in most primary HIV-1 isolates. Viral evolution leading to Asn at position 332 of HIVAD8 Envs is supported by the selection advantage of high levels of cell-cell fusion, transmission, and infectivity with high levels of cell surface expression and slightly higher gp120 shedding than most N332 variants. Thus, tolerance of HIV-1AD8 Envs to different amino acids at position 332 provides increased flexibility to respond to changing conditions/environments and evade the immune system. Modeling studies of the distance between N332 glycan and specific bnAbs were in agreement with N332 glycan dependency on bnAb neutralization. Overall, our studies provide insights into the contribution of specific amino acids at position 332 to Env antigenicity, stability on ice, and conformational states.

IMPORTANCE

Glycan attached to amino acid asparagine at position 332 of HIV-1 envelope glycoproteins is a main target of a subset of broadly neutralizing antibodies that block HIV-1 infection. Here, we defined the contribution of different amino acids at this position to Env antigenicity, stability on ice, and conformational states.

Bioorthogonal click labeling of an amber-free HIV-1 provirus for in-virus single molecule imaging

Structural dynamics of human immunodeficiency virus 1 (HIV-1) envelope (Env) glycoprotein mediate cell entry and facilitate immune evasion. Single-molecule FRET using peptides for Env labeling revealed structural dynamics of Env, but peptide use risks potential effects on structural integrity/dynamics. While incorporating noncanonical amino acids (ncAAs) into Env by amber stop-codon suppression, followed by click chemistry, offers a minimally invasive approach, this has proved to be technically challenging for HIV-1. Here, we develope an intact amber-free HIV-1 system that overcomes hurdles of preexisting viral amber codons. We achieved dual-ncAA incorporation into Env on amber-free virions, enabling single-molecule Förster resonance energy transfer (smFRET) studies of click-labeled Env that validated the previous peptide-based labeling approaches by confirming the intrinsic propensity of Env to dynamically sample multiple conformational states. Amber-free click-labeled Env also enabled real-time tracking of single virion internalization and trafficking in cells. Our system thus permits in-virus bioorthogonal labeling of proteins, compatible with studies of virus entry, trafficking, and egress from cells.

HIResist: a database of HIV-1 resistance to broadly neutralizing antibodies

MOTIVATION

Changing the course of the human immunodeficiency virus type I (HIV-1) pandemic is a high public health priority with approximately 39 million people currently living with HIV-1 (PLWH) and about 1.5 million new infections annually worldwide. Broadly neutralizing antibodies (bnAbs) typically target highly conserved sites on the HIV-1 envelope glycoproteins (Envs), which mediate viral entry, and block the infection of diverse HIV-1 strains. But different mechanisms of HIV-1 resistance to bnAbs prevent robust application of bnAbs for therapeutic and preventive interventions.

RESULTS

Here we report the development of a new database that provides data and computational tools to aid the discovery of resistant features and may assist in analysis of HIV-1 resistance to bnAbs. Bioinformatic tools allow identification of specific patterns in Env sequences of resistant strains and development of strategies to elucidate the mechanisms of HIV-1 escape; comparison of resistant and sensitive HIV-1 strains for each bnAb; identification of resistance and sensitivity signatures associated with specific bnAbs or groups of bnAbs; and visualization of antibody pairs on cross-sensitivity plots. The database has been designed with a particular focus on user-friendly and interactive interface. Our database is a valuable resource for the scientific community and provides opportunities to investigate patterns of HIV-1 resistance and to develop new approaches aimed to overcome HIV-1 resistance to bnAbs.

AVAILABILITY AND IMPLEMENTATION

HIResist is freely available at https://hiresist.ahc.umn.edu/.

Epistatic pathways can drive HIV-1 escape from integrase strand transfer inhibitors

People living with human immunodeficiency virus (HIV) receiving integrase strand transfer inhibitors (INSTIs) have been reported to experience virological failure in the absence of resistance mutations in integrase. To elucidate INSTI resistance mechanisms, we propagated HIV-1 in the presence of escalating concentrations of the INSTI dolutegravir. HIV-1 became resistant to dolutegravir by sequentially acquiring mutations in the envelope glycoprotein (Env) and the nucleocapsid protein. The selected Env mutations enhance the ability of the virus to spread via cell-cell transfer, thereby increasing the multiplicity of infection (MOI). While the selected Env mutations confer broad resistance to multiple classes of antiretrovirals, the fold resistance is ~2 logs higher for INSTIs than for other classes of drugs. We demonstrate that INSTIs are more readily overwhelmed by high MOI than other classes of antiretrovirals. Our findings advance the understanding of how HIV-1 can evolve resistance to antiretrovirals, including the potent INSTIs, in the absence of drug-target gene mutations.

Inhibition of human immunodeficiency virus (HIV-1) infectivity by expression of poorly or broadly neutralizing antibodies against Env in virus-producing cells

The human immunodeficiency virus (HIV-1) envelope (Env) glycoprotein precursor (gp160) trimerizes, is modified by high-mannose glycans in the endoplasmic reticulum, and is transported via Golgi and non-Golgi secretory pathways to the infected cell surface. In the Golgi, gp160 is partially modified by complex carbohydrates and proteolytically cleaved to produce the mature functional Env trimer, which is preferentially incorporated into virions. Broadly neutralizing antibodies (bNAbs) generally recognize the cleaved Env trimer, whereas poorly neutralizing antibodies (pNAbs) bind the conformationally flexible gp160. We found that expression of bNAbs, pNAbs, or soluble/membrane forms of the receptor, CD4, in cells producing HIV-1 all decreased viral infectivity. Four patterns of co-expressed ligand:Env were observed: (i) ligands (CD4, soluble CD4-Ig, and some pNAbs) that specifically recognize the CD4-bound Env conformation resulted in uncleaved Envs lacking complex glycans that were not incorporated into virions; (ii) other pNAbs produced Envs with some complex carbohydrates and severe defects in cleavage, which were relieved by brefeldin A treatment; (iii) bNAbs that recognize gp160 as well as mature Envs resulted in Envs with some complex carbohydrates and moderate decreases in virion Env cleavage; and (iv) bNAbs that preferentially recognize mature Envs produced cleaved Envs with complex glycans in cells and on virions. The low infectivity observed upon co-expression of pNAbs or CD4 could be explained by disruption of Env trafficking, reducing the level of Env and/or increasing the fraction of uncleaved Env on virions. In addition to bNAb effects on virion Env cleavage, the secreted bNAbs neutralized the co-expressed viruses.IMPORTANCEThe Env trimers on the HIV-1 mediate virus entry into host cells. Env is synthesized in infected cells, modified by complex sugars, and cleaved to form a mature, functional Env, which is incorporated into virus particles. Env elicits antibodies in infected individuals, some of which can neutralize the virus. We found that antibodies co-expressed in the virus-producing cell can disrupt Env transit to the proper compartment for cleavage and sugar modification and, in some cases, block incorporation into viruses. These studies provide insights into the processes by which Env becomes functional in the virus-producing cell and may assist attempts to interfere with these events to inhibit HIV-1 infection.

Next-generation bNAbs for HIV-1 cure strategies

Despite the ability to suppress viral replication using anti-retroviral therapy (ART), HIV-1 remains a global public health problem. Curative strategies for HIV-1 have to target and eradicate latently infected cells across the body, i.e. the viral reservoir. Broadly neutralizing antibodies (bNAbs) targeting the HIV-1 envelope glycoprotein (Env) have the capacity to neutralize virions and bind to infected cells to initiate elimination of these cells. To improve the efficacy of bNAbs in terms of viral suppression and viral reservoir eradication, next generation antibodies (Abs) are being developed that address the current limitations of Ab treatment efficacy; (1) low antigen (Env) density on (reactivated) HIV-1 infected cells, (2) high viral genetic diversity, (3) exhaustion of immune cells and (4) short half-life of Abs. In this review we summarize and discuss preclinical and clinical studies in which anti-HIV-1 Abs demonstrated potent viral control, and describe the development of engineered Abs that could address the limitations described above. Next generation Abs with optimized effector function, avidity, effector cell recruitment and immune cell activation have the potential to contribute to an HIV-1 cure or durable control.

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.

Emerging role of cyclophilin A in HIV-1 infection: from producer cell to the target cell nucleus

The HIV-1 genome encodes a small number of proteins with structural, enzymatic, regulatory, and accessory functions. These viral proteins interact with a number of host factors to promote the early and late stages of HIV-1 infection. During the early stages of infection, interactions between the viral proteins and host factors enable HIV-1 to enter the target cell, traverse the cytosol, dock at the nuclear pore, gain access to the nucleus, and integrate into the host genome. Similarly, the viral proteins recruit another set of host factors during the late stages of infection to orchestrate HIV-1 transcription, translation, assembly, and release of progeny virions. Among the host factors implicated in HIV-1 infection, Cyclophilin A (CypA) was identified as the first host factor to be packaged within HIV-1 particles. It is now well established that CypA promotes HIV-1 infection by directly binding to the viral capsid. Mechanistic models to pinpoint CypA's role have spanned from an effect in the producer cell to the early steps of infection in the target cell. In this review, we will describe our understanding of the role(s) of CypA in HIV-1 infection, highlight the current knowledge gaps, and discuss the potential role of this host factor in the post-nuclear entry steps of HIV-1 infection.

V3 tip determinants of susceptibility to inhibition by CD4-mimetic compounds in natural clade A human immunodeficiency virus (HIV-1) envelope glycoproteins

IMPORTANCE

CD4-mimetic compounds (CD4mcs) are small-molecule inhibitors of human immunodeficiency virus (HIV-1) entry into host cells. CD4mcs target a pocket on the viral envelope glycoprotein (Env) spike that is used for binding to the receptor, CD4, and is highly conserved among HIV-1 strains. Nonetheless, naturally occurring HIV-1 strains exhibit a wide range of sensitivities to CD4mcs. Our study identifies changes distant from the binding pocket that can influence the susceptibility of natural HIV-1 strains to the antiviral effects of multiple CD4mcs. We relate the antiviral potency of the CD4mc against this panel of HIV-1 variants to the ability of the CD4mc to activate entry-related changes in Env conformation prematurely. These findings will guide efforts to improve the potency and breadth of CD4mcs against natural HIV-1 variants.

Enhancing anti-viral neutralization response to immunization with HIV-1 envelope glycoprotein immunogens

An effective human immunodeficiency virus type I (HIV-1) vaccine that robustly elicits broadly neutralizing antibodies (bnAbs) against HIV-1 envelope glycoproteins (Envs) to block viral entry is still not available. Thus, identifying triggers for elicitation of different types of anti-HIV-1 Env antibodies by vaccination could provide further guidance for immunogen design and vaccine development. Here, we studied the immune response to HIV-1 Env immunogens in rabbits. We show that sequential immunizations with conformation-specific Env immunogens can elicit low titer but broad neutralization responses against heterologous, neutralization-resistant (tier 2/3) transmitted/founder (T/F) HIV-1 strains. More importantly, an mRNA vaccine candidate that could mediate the presentation of a cytoplasmic tail-deleted (ΔCT) HIV-1AD8 Env immunogen on virus-like particles significantly increased the neutralization response. This strategy shifted the type of elicited antibodies, decreasing the level of binding to soluble Envs while significantly increasing their overall viral neutralization activity. The breadth and potency of neutralizing response against heterologous, T/F HIV-1 strains significantly increased in a subset of rabbits. Efficient neutralization activity was associated with high cellular immune responses specific to HIV-1 Envs. These results help to understand the immune response to different immunization schemes and will allow developing new approaches to selectively manipulate the type of humoral immune response by specific vaccination.

Alternative substitutions of N332 in HIV-1AD8 gp120 differentially affect envelope glycoprotein function and viral sensitivity to broadly neutralizing antibodies targeting the V3-glycan

The envelope glycoprotein (Env) trimer on the surface of human immunodeficiency virus type I (HIV-1) mediates viral entry into host CD4+ T cells and is the sole target of neutralizing antibodies. Broadly neutralizing antibodies (bnAbs) that target gp120 V3-glycan of HIV-1 Env trimer are potent and block the entry of diverse HIV-1 strains. Most V3-glycan bnAbs interact, to a different extent, with a glycan attached to N332 but Asn at this position is not absolutely conserved or required for HIV-1 entry based on prevalence of N332 in different circulating HIV-1 strains from diverse clades. Here, we studied the effects of amino acid changes at position 332 of HIV-1AD8 Envs on HIV-1 sensitivity to antibodies, cold exposure, and soluble CD4. We further investigated how these changes affect Env function and HIV-1 infectivity in vitro. Our results suggest robust tolerability of HIV-1AD8 Env N332 to changes with specific changes that resulted in extended exposure of gp120 V3 loop, which is typically concealed in most primary HIV-1 isolates. Viral evolution leading to Asn at position 332 of HIVAD8 Envs is supported by the selection advantage of high levels of cell-cell fusion, transmission, and infectivity even though cell surface expression levels are lower than most N332 variants. Thus, tolerance of HIV-1AD8 Envs to different amino acids at position 332 provides increased flexibility to respond to changing conditions/environments and to evade the immune system. Modeling studies of the distance between N332 glycan and specific bnAbs was in agreement with N332 glycan dependency on bnAb neutralization. Overall, our studies provide insights into the contribution of specific amino acids at position 332 to Env antigenicity, stability on ice, and conformational states.

Alterations in gp120 glycans or the gp41 fusion peptide-proximal region modulate the stability of the human immunodeficiency virus (HIV-1) envelope glycoprotein pretriggered conformation

The human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer mediates entry into host cells by binding receptors, CD4 and CCR5/CXCR4, and fusing the viral and cell membranes. In infected cells, cleavage of the gp160 Env precursor yields the mature Env trimer, with gp120 exterior and gp41 transmembrane Env subunits. Env cleavage stabilizes the State-1 conformation, which is the major target for broadly neutralizing antibodies, and decreases the spontaneous sampling of more open Env conformations that expose epitopes for poorly neutralizing antibodies. During HIV-1 entry into cells, CD4 binding drives the metastable Env from a pretriggered (State-1) conformation into more "open," lower-energy states. Here, we report that changes in two dissimilar elements of the HIV-1 Env trimer, namely particular gp120 glycans and the gp41 fusion peptide-proximal region (FPPR), can independently modulate the stability of State 1. Individual deletion of several gp120 glycans destabilized State 1, whereas removal of a V1 glycan resulted in phenotypes indicative of a more stable pretriggered Env conformation. Likewise, some alterations of the gp41 FPPR decreased the level of spontaneous shedding of gp120 from the Env trimer and stabilized the pretriggered State-1 Env conformation. State-1-stabilizing changes were additive and could suppress the phenotypes associated with State-1-destabilizing alterations in Env. Our results support a model in which multiple protein and carbohydrate elements of the HIV-1 Env trimer additively contribute to the stability of the pretriggered (State-1) conformation. The Env modifications identified in this study will assist efforts to characterize the structure and immunogenicity of the metastable State-1 conformation. IMPORTANCE The elicitation of antibodies that neutralize multiple strains of HIV-1 is an elusive goal that has frustrated the development of an effective vaccine. The pretriggered shape of the HIV-1 envelope glycoprotein (Env) spike on the virus surface is the major target for such broadly neutralizing antibodies. The "closed" pretriggered Env shape resists the binding of most antibodies but is unstable and often assumes "open" shapes that elicit ineffective antibodies. We identified particular changes in both the protein and the sugar components of the Env trimer that stabilize the pretriggered shape. Combinations of these changes were even more effective at stabilizing the pretriggered Env than the individual changes. Stabilizing changes in Env could counteract the effect of Env changes that destabilize the pretriggered shape. Locking Env in its pretriggered shape will assist efforts to understand the Env spike on the virus and to incorporate this shape into vaccines.

Conformations of Human Immunodeficiency Virus Envelope Glycoproteins in Detergents and Styrene-Maleic Acid Lipid Particles

The mature human immunodeficiency virus (HIV) envelope glycoprotein (Env) trimer, which consists of noncovalently associated gp120 exterior and gp41 transmembrane subunits, mediates virus entry into cells. The pretriggered (State-1) Env conformation is the major target for broadly neutralizing antibodies (bNAbs), whereas receptor-induced downstream Env conformations elicit immunodominant, poorly neutralizing antibody (pNAb) responses. To examine the contribution of membrane anchorage to the maintenance of the metastable pretriggered Env conformation, we compared wild-type and State-1-stabilized Envs solubilized in detergents or in styrene-maleic acid (SMA) copolymers. SMA directly incorporates membrane lipids and resident membrane proteins into lipid nanoparticles (styrene-maleic acid lipid particles [SMALPs]). The integrity of the Env trimer in SMALPs was maintained at both 4°C and room temperature. In contrast, Envs solubilized in Cymal-5, a nonionic detergent, were unstable at room temperature, although their stability was improved at 4°C and/or after incubation with the entry inhibitor BMS-806. Envs solubilized in ionic detergents were relatively unstable at either temperature. Comparison of Envs solubilized in Cymal-5 and SMA at 4°C revealed subtle differences in bNAb binding to the gp41 membrane-proximal external region, consistent with these distinct modes of Env solubilization. Otherwise, the antigenicity of the Cymal-5- and SMA-solubilized Envs was remarkably similar, both in the absence and in the presence of BMS-806. However, both solubilized Envs were recognized differently from the mature membrane Env by specific bNAbs and pNAbs. Thus, detergent-based and detergent-free solubilization at 4°C alters the pretriggered membrane Env conformation in consistent ways, suggesting that Env assumes default conformations when its association with the membrane is disrupted. IMPORTANCE The human immunodeficiency virus (HIV) envelope glycoproteins (Envs) in the viral membrane mediate virus entry into the host cell and are targeted by neutralizing antibodies elicited by natural infection or vaccines. Detailed studies of membrane proteins rely on purification procedures that allow the proteins to maintain their natural conformation. In this study, we show that a styrene-maleic acid (SMA) copolymer can extract HIV-1 Env from a membrane without the use of detergents. The Env in SMA is more stable at room temperature than Env in detergents. The purified Env in SMA maintains many but not all of the characteristics expected of the natural membrane Env. Our results underscore the importance of the membrane environment to the native conformation of HIV-1 Env. Purification methods that bypass the need for detergents could be useful tools for future studies of HIV-1 Env structure and its interaction with receptors and antibodies.

Asymmetric conformations of cleaved HIV-1 envelope glycoprotein trimers in styrene-maleic acid lipid nanoparticles

During virus entry, the pretriggered human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer initially transits into a default intermediate state (DIS) that remains structurally uncharacterized. Here, we present cryo-EM structures at near-atomic resolution of two cleaved full-length HIV-1 Env trimers purified from cell membranes in styrene-maleic acid lipid nanoparticles without antibodies or receptors. The cleaved Env trimers exhibited tighter subunit packing than uncleaved trimers. Cleaved and uncleaved Env trimers assumed remarkably consistent yet distinct asymmetric conformations, with one smaller and two larger opening angles. Breaking conformational symmetry is allosterically coupled with dynamic helical transformations of the gp41 N-terminal heptad repeat (HR1N) regions in two protomers and with trimer tilting in the membrane. The broken symmetry of the DIS potentially assists Env binding to two CD4 receptors-while resisting antibody binding-and promotes extension of the gp41 HR1 helical coiled-coil, which relocates the fusion peptide closer to the target cell membrane.

Temsavir blocks the immunomodulatory activities of HIV-1 soluble gp120

While HIV-1-mediated CD4 downregulation protects infected cells from antibody-dependent cellular cytotoxicity (ADCC), shed gp120 binds to CD4 on uninfected bystander CD4+ T cells, sensitizing them to ADCC mediated by HIV+ plasma. Soluble gp120-CD4 interaction on multiple immune cells also triggers a cytokine burst. The small molecule temsavir acts as an HIV-1 attachment inhibitor by preventing envelope glycoprotein (Env)-CD4 interaction and alters the overall antigenicity of Env by affecting its processing and glycosylation. Here we show that temsavir also blocks the immunomodulatory activities of shed gp120. Temsavir prevents shed gp120 from interacting with uninfected bystander CD4+ cells, protecting them from ADCC responses and preventing a cytokine burst. Mechanistically, this depends on temsavir's capacity to prevent soluble gp120-CD4 interaction, to reduce gp120 shedding, and to alter gp120 antigenicity. This suggests that the clinical benefits provided by temsavir could extend beyond blocking viral entry.

Characterization of the Human Immunodeficiency Virus (HIV-1) Envelope Glycoprotein Conformational States on Infectious Virus Particles

Human immunodeficiency virus (HIV-1) entry into cells involves triggering of the viral envelope glycoprotein (Env) trimer ([gp120/gp41]3) by the primary receptor, CD4, and coreceptors, CCR5 or CXCR4. The pretriggered (State-1) conformation of the mature (cleaved) Env is targeted by broadly neutralizing antibodies (bNAbs), which are inefficiently elicited compared with poorly neutralizing antibodies (pNAbs). Here, we characterize variants of the moderately triggerable HIV-1AD8 Env on virions produced by an infectious molecular proviral clone; such virions contain more cleaved Env than pseudotyped viruses. We identified three types of cleaved wild-type AD8 Env trimers on virions: (i) State-1-like trimers preferentially recognized by bNAbs and exhibiting strong subunit association; (ii) trimers recognized by pNAbs directed against the gp120 coreceptor-binding region and exhibiting weak, detergent-sensitive subunit association; and (iii) a minor gp41-only population. The first Env population was enriched and the other Env populations reduced by introducing State-1-stabilizing changes in the AD8 Env or by treatment of the virions with crosslinker or the State-1-preferring entry inhibitor, BMS-806. These stabilized AD8 Envs were also more resistant to gp120 shedding induced by a CD4-mimetic compound or by incubation on ice. Conversely, a State-1-destabilized, CD4-independent AD8 Env variant exhibited weaker bNAb recognition and stronger pNAb recognition. Similar relationships between Env triggerability and antigenicity/shedding propensity on virions were observed for other HIV-1 strains. State-1 Envs on virions can be significantly enriched by minimizing the adventitious incorporation of uncleaved Env; stabilizing the pretriggered conformation by Env modification, crosslinking or BMS-806 treatment; strengthening Env subunit interactions; and using CD4-negative producer cells. IMPORTANCE Efforts to develop an effective HIV-1 vaccine have been frustrated by the inability to elicit broad neutralizing antibodies that recognize multiple virus strains. Such antibodies can bind a particular shape of the HIV-1 envelope glycoprotein trimer, as it exists on a viral membrane but before engaging receptors on the host cell. Here, we establish simple yet powerful assays to characterize the envelope glycoproteins in a natural context on virus particles. We find that, depending on the HIV-1 strain, some envelope glycoproteins change shape and fall apart, creating decoys that can potentially divert the host immune response. We identify requirements to keep the relevant envelope glycoprotein target for broad neutralizing antibodies intact on virus-like particles. These studies suggest strategies that should facilitate efforts to produce and use virus-like particles as vaccine immunogens.

Indoline CD4-mimetic compounds mediate potent and broad HIV-1 inhibition and sensitization to antibody-dependent cellular cytotoxicity

Binding to the host cell receptors, CD4 and CCR5/CXCR4, triggers large-scale conformational changes in the HIV-1 envelope glycoprotein (Env) trimer [(gp120/gp41)3] that promote virus entry into the cell. CD4-mimetic compounds (CD4mcs) comprise small organic molecules that bind in the highly conserved CD4-binding site of gp120 and prematurely induce inactivating Env conformational changes, including shedding of gp120 from the Env trimer. By inducing more "open," antibody-susceptible Env conformations, CD4mcs also sensitize HIV-1 virions to neutralization by antibodies and infected cells to antibody-dependent cellular cytotoxicity (ADCC). Here, we report the design, synthesis, and evaluation of novel CD4mcs based on an indoline scaffold. Compared with our current lead indane scaffold CD4mc, BNM-III-170, several indoline CD4mcs exhibit increased potency and breadth against HIV-1 variants from different geographic clades. Viruses that were selected for resistance to the lead indane CD4mc, BNM-III-170, are susceptible to inhibition by the indoline CD4mcs. The indoline CD4mcs also potently sensitize HIV-1-infected cells to ADCC mediated by plasma from HIV-1-infected individuals. Crystal structures indicate that the indoline CD4mcs gain potency compared to the indane CD4mcs through more favorable π-π overlap from the indoline pose and by making favorable contacts with the vestibule of the CD4-binding pocket on gp120. The rational design of indoline CD4mcs thus holds promise for further improvements in antiviral activity, potentially contributing to efforts to treat and prevent HIV-1 infection.

An intact amber-free HIV-1 system for in-virus protein bioorthogonal click labeling that delineates envelope conformational dynamics

The HIV-1 envelope (Env) glycoprotein is conformationally dynamic and mediates membrane fusion required for cell entry. Single-molecule fluorescence resonance energy transfer (smFRET) of Env using peptide tags has provided mechanistic insights into the dynamics of Env conformations. Nevertheless, using peptide tags risks potential effects on structural integrity. Here, we aim to establish minimally invasive smFRET systems of Env on the virus by combining genetic code expansion and bioorthogonal click chemistry. Amber stop-codon suppression allows site-specifically incorporating noncanonical/unnatural amino acids (ncAAs) at introduced amber sites into proteins. However, ncAA incorporation into Env (or other HIV-1 proteins) in the virus context has been challenging due to low copies of Env on virions and incomplete amber suppression in mammalian cells. Here, we developed an intact amber-free virus system that overcomes impediments from preexisting ambers in HIV-1. Using this system, we successfully incorporated dual ncAAs at amber-introduced sites into Env on intact virions. Dual-ncAA incorporated Env retained similar neutralization sensitivities to neutralizing antibodies as wildtype. smFRET of click-labeled Env on intact amber-free virions recapitulated conformational profiles of Env. The amber-free HIV-1 infectious system also permits in-virus protein bioorthogonal labeling, compatible with various advanced microscopic studies of virus entry, trafficking, and egress in living cells. Amber-free HIV-1 infectious systems actualized minimal invasive Env tagging for smFRET, versatile for in-virus bioorthogonal click labeling in advanced microscopic studies of virus-host interactions.

Mechanisms of HIV-1 cell-to-cell transfer to myeloid cells

In addition to CD4+ T lymphocytes, cells of the myeloid lineage such as macrophages, dendritic cells (DCs), and osteoclasts (OCs) are emerging as important target cells for HIV-1, as they likely participate in all steps of pathogenesis, including sexual transmission and early virus dissemination in both lymphoid and nonlymphoid tissues where they can constitute persistent virus reservoirs. At least in vitro, these myeloid cells are poorly infected by cell-free viral particles. In contrast, intercellular virus transmission through direct cell-to-cell contacts may be a predominant mode of virus propagation in vivo leading to productive infection of these myeloid target cells. HIV-1 cell-to-cell transfer between CD4+ T cells mainly through the formation of the virologic synapse, or from infected macrophages or dendritic cells to CD4+ T cell targets, have been extensively described in vitro. Recent reports demonstrate that myeloid cells can be also productively infected through virus homotypic or heterotypic cell-to-cell transfer between macrophages or from virus-donor-infected CD4+ T cells, respectively. These modes of infection of myeloid target cells lead to very efficient spreading in these poorly susceptible cell types. Thus, the goal of this review is to give an overview of the different mechanisms reported in the literature for cell-to-cell transfer and spreading of HIV-1 in myeloid cells.

Broad Tricyclic Ring Inhibitors Block SARS-CoV-2 Spike Function Required for Viral Entry

The entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into host cells requires binding of the viral spike glycoprotein to the angiotensin-converting enzyme 2 (ACE2) receptor, which triggers subsequent conformational changes to facilitate viral and cellular fusion at the plasma membrane or following endocytosis. Here, we experimentally identified selective and broad inhibitors of SARS-CoV-2 entry that share a tricyclic ring (or similar) structure. The inhibitory effect was restricted to early steps during infection and the entry inhibitors interacted with the receptor binding domain of the SARS-CoV-2 spike but did not significantly interfere with receptor (ACE2) binding. Instead, some of these compounds induced conformational changes or affected spike assembly and blocked SARS-CoV-2 spike cell-cell fusion activity. The broad inhibitors define a highly conserved binding pocket that is present on the spikes of SARS-CoV-1, SARS-CoV-2, and all circulating SARS-CoV-2 variants tested and block SARS-CoV spike activity required for mediating viral entry. These compounds provide new insights into the SARS-CoV-2 spike topography, as well as into critical steps on the entry pathway, and can serve as lead candidates for the development of broad-range entry inhibitors against SARS-CoVs.

Characterization of Human Immunodeficiency Virus (HIV-1) Envelope Glycoprotein Variants Selected for Resistance to a CD4-Mimetic Compound

Binding to the host cell receptors CD4 and CCR5/CXCR4 triggers conformational changes in the human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer that promote virus entry. CD4 binding allows the gp120 exterior Env to bind CCR5/CXCR4 and induces a short-lived prehairpin intermediate conformation in the gp41 transmembrane Env. Small-molecule CD4-mimetic compounds (CD4mcs) bind within the conserved Phe-43 cavity of gp120, near the binding site for CD4. CD4mcs like BNM-III-170 inhibit HIV-1 infection by competing with CD4 and by prematurely activating Env, leading to irreversible inactivation. In cell culture, we selected and analyzed variants of the primary HIV-1_AD8 strain resistant to BNM-III-170. Two changes (S375N and I424T) in gp120 residues that flank the Phe-43 cavity each conferred an ~5-fold resistance to BNM-III-170 with minimal fitness cost. A third change (E64G) in layer 1 of the gp120 inner domain resulted in ~100-fold resistance to BNM-III-170, ~2- to 3-fold resistance to soluble CD4-Ig, and a moderate decrease in viral fitness. The gp120 changes additively or synergistically contributed to BNM-III-170 resistance. The sensitivity of the Env variants to BNM-III-170 inhibition of virus entry correlated with their sensitivity to BNM-III-170-induced Env activation and shedding of gp120. Together, the S375N and I424T changes, but not the E64G change, conferred >100-fold and 33-fold resistance to BMS-806 and BMS-529 (temsavir), respectively, potent HIV-1 entry inhibitors that block Env conformational transitions. These studies identify pathways whereby HIV-1 can develop resistance to CD4mcs and conformational blockers, two classes of entry inhibitors that target the conserved gp120 Phe-43 cavity. IMPORTANCE CD4-mimetic compounds (CD4mcs) and conformational blockers like BMS-806 and BMS-529 (temsavir) are small-molecule inhibitors of human immunodeficiency virus (HIV-1) entry into host cells. Although CD4mcs and conformational blockers inhibit HIV-1 entry by different mechanisms, they both target a pocket on the viral envelope glycoprotein (Env) spike that is used for binding to the receptor CD4 and is highly conserved among HIV-1 strains. Our study identifies changes near this pocket that can confer various levels of resistance to the antiviral effects of a CD4mc and conformational blockers. We relate the antiviral potency of a CD4mc against this panel of HIV-1 variants to the ability of the CD4mc to activate changes in Env conformation and to induce the shedding of the gp120 exterior Env from the spike. These findings will guide efforts to improve the potency and breadth of small-molecule HIV-1 entry inhibitors.

Design, synthesis, and bio-evaluation of novel triterpenoid derivatives as anti-HIV-1 compounds

Two betulinic acid derivatives, RPR103611 (2) and IC9564 (3) were previously reported to be potent HIV-1 entry inhibitors. In this current study, a SAR study of the triterpenoid moiety of 2 and 3 has been performed and an oleanolic acid derivative (4) was identified as a novel HIV-1 entry inhibitor. In addition, the combination of 4 with several-type of HIV-1 neutralizing antibodies provided significant synergistic effects. The synthetic utility of the CC double bond in the C-ring of 4 was also demonstrated to develop the 12-keto-type oleanolic acid derivative (5) as a potent anti-HIV compound. This simple transformation led to a significantly increased anti-HIV activity and a reduced cytotoxicity of the compound.

Enhancement of CD4 Binding, Host Cell Entry, and Sensitivity to CD4bs Antibody Inhibition Conferred by a Natural but Rare Polymorphism in the HIV-1 Envelope

A rare but natural polymorphism in the HIV-1 envelope (Env) glycoprotein, lysine at position 425 was selected as a mutation conferring resistance to maraviroc (MVC) in vitro. N425K has not been identified in HIV-infected individuals failing an MVC-based treatment. This study reports that the rare K425 polymorphism in an HIV-1 subtype A Env has increased affinity for CD4, resulting in faster host cell entry kinetics and the ability to scavenge for low cell surface expression of CD4 to mediate entry. Whereas the subtype A wild-type isolate-74 Env (N425) is inhibited by soluble (s) CD4, HIV-1 with K425 A74 Env shows enhanced infection and the ability to infect CCR5+ cells when pretreated with sCD4. Upon adding K425 or N425 HIV-1 to CD4⁺/CCR5+ cells along with RANTES/CCL3, only K425 HIV-1 was able to infect cells when CCR5 recycled/returned to the cell surface at 12 h post-treatment. These findings suggest that upon binding to CD4, K425 Env may maintain a stable State 2 "open" conformation capable of engaging CCR5 for entry. Only K425 was significantly more sensitivity than wild-type N425 A74 to inhibition by the CD4 binding site (bs) compound, BMS-806, the CD4bs antibody, VRC01 and N6, and the single-chain CD4i antibody, SCm9. K425 A74 was also capable of activating B cells expressing the VRC01 surface immunoglobulin. In summary, despite increased replicative fitness, we propose that K425 HIV-1 may be counterselected within infected individuals if K425 HIV-1 is rapidly eliminated by CD4bs-neutralizing antibodies. IMPORTANCE Typically, a natural amino acid polymorphism is found as the wild-type sequence in the HIV-1 population if it provides a selective advantage to the virus. The natural K425 polymorphism in HIV-1 Env results in higher host cell entry efficiency and greater replicative fitness by virtue of its high binding affinity to CD4. The studies presented herein suggest that the rare K425 HIV-1, compared to the common N425 HIV-1, may be more sensitive to inhibition by CD4bs-neutralizing antibodies (i.e., antibodies that bind to the CD4 binding pocket on the HIV-1 envelope glycoprotein). If CD4bs antibodies did emerge in an infected individual, the K425 HIV-1 may be hypersensitive to inhibition, and thus this K425 virus variant may be removed from the HIV-1 swarm despite its higher replication fitness. Studies are now underway to determine whether addition of the K425 polymorphism into the Envelope-based HIV-1 vaccines could enhance protective immunity.

HIV-1 cell-to-cell spread overcomes the virus entry block of non-macrophage-tropic strains in macrophages

Macrophages (MΦ) are increasingly recognized as HIV-1 target cells involved in the pathogenesis and persistence of infection. Paradoxically, in vitro infection assays suggest that virus isolates are mostly T-cell-tropic and rarely MΦ-tropic. The latter are assumed to emerge under CD4+ T-cell paucity in tissues such as the brain or at late stage when the CD4 T-cell count declines. However, assays to qualify HIV-1 tropism use cell-free viral particles and may not fully reflect the conditions of in vivo MΦ infection through cell-to-cell viral transfer. Here, we investigated the capacity of viruses expressing primary envelope glycoproteins (Envs) with CCR5 and/or CXCR4 usage from different stages of infection, including transmitted/founder Envs, to infect MΦ by a cell-free mode and through cell-to-cell transfer from infected CD4+ T cells. The results show that most viruses were unable to enter MΦ as cell-free particles, in agreement with the current view that non-M-tropic viruses inefficiently use CD4 and/or CCR5 or CXCR4 entry receptors on MΦ. In contrast, all viruses could be effectively cell-to-cell transferred to MΦ from infected CD4+ T cells. We further showed that viral transfer proceeded through Env-dependent cell-cell fusion of infected T cells with MΦ targets, leading to the formation of productively infected multinucleated giant cells. Compared to cell-free infection, infected T-cell/MΦ contacts showed enhanced interactions of R5 M- and non-M-tropic Envs with CD4 and CCR5, resulting in a reduced dependence on receptor expression levels on MΦ for viral entry. Altogether, our results show that virus cell-to-cell transfer overcomes the entry block of isolates initially defined as non-macrophage-tropic, indicating that HIV-1 has a more prevalent tropism for MΦ than initially suggested. This sheds light into the role of this route of virus cell-to-cell transfer to MΦ in CD4+ T cell rich tissues for HIV-1 transmission, dissemination and formation of tissue viral reservoirs.

Regulation of epitope exposure in the gp41 membrane-proximal external region through interactions at the apex of HIV-1 Env

Glycoprotein Env of human immunodeficiency virus type 1 (HIV-1) mediates viral entry through membrane fusion. Composed of gp120 and gp41 subunits arranged as a trimer-of-heterodimers, Env adopts a metastable, highly dynamic conformation on the virion surface. This structural plasticity limits the temporospatial exposure of many highly conserved, neutralizing epitopes, contributing to the difficulty in developing effective HIV-1 vaccines. Here, we employed antibody neutralization of HIV-1 infectivity to investigate how inter- and intra-gp120 interactions mediated by variable loops V1/V2 and V3 at the Env apex regulate accessibility of the gp41 membrane-proximal external region (MPER) at the Env base. Swapping the V3 loop from Env_SF162 into the Env_HXB2 background shifted MPER exposure from the prefusogenic state to a functional intermediate conformation that was distinct from the prehairpin-intermediate state sensitive to gp41-targeted fusion inhibitors. The V3-loop swap had a profound impact on global protein dynamics, biasing the equilibrium to a closed conformation resistant to most anti-gp120 antibodies, stabilizing the protein to both cold- and soluble CD4-induced Env inactivation, and increasing the CD4 requirements for viral entry. Further dissection of the Env_HXB2 V3 loop revealed that residue 306 uniquely modulated epitope exposure and trimer stability. The R306S substitution substantially decreased sensitivity to antibodies targeting the gp41 MPER and, surprisingly, the gp120 V3-loop crown (residues 312–315), but had only modest effects on exposure of intervening gp120 epitopes. Furthermore, the point mutation reduced soluble CD4-induced inactivation, but had no impact on cold inactivation. The residue appeared to exert its effects by electrostatically modifying the strength of intra-subunit interactions between the V1/V2 and V3 loops. The distinct patterns of neutralization and stability pointed to a novel prefusogenic Env conformation along the receptor activation pathway and suggested that apical Env-regulation of gp41 MPER exposure can be decoupled from much of the dynamics of gp120 subunits.

Surface glycoprotein Env is the main target for neutralizing antibodies elicited by HIV-1 vaccines. Env spontaneously fluctuates among different structures, limiting exposure of many attractive antibody-binding epitopes and, thereby, confounding vaccine development. To characterize these fluctuations, we examined how exposure of the MPER epitope found at the base of Env is regulated by interactions of the V3 loop located in the apex. Starting with an extremely flexible Env with a readily-exposed MPER, we identified two alterations that substantially restricted antibody access to the epitope. The first, a wholesale swap of V3 loops between HIV-1 strains, energetically stabilized Env in a closed structure that restricted access to antibodies throughout the protein. The second, a point mutation that altered V3-loop charge, specifically destabilized the MPER-exposed conformation but had minimal impact on antibody access to Env regions in between the apex and base. The results indicate that MPER exposure is not explicitly tied to the dynamics of Env regions between the apex and base and suggest a new structural fluctuation during Env activation.

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.

Antigenic analysis of the HIV-1 envelope trimer implies small differences between structural states 1 and 2

The conformationally dynamic HIV-1 envelope trimer (Env) is the target of broadly neutralizing antibodies (bnAbs) that block viral entry. Single-molecule Förster resonance energy transfer (smFRET) has revealed that HIV-1 Env exists in at least three conformational states on the virion. Prior to complete host-receptor engagement (State 3), Env resides most prevalently in the smFRET-defined State 1, which is preferentially recognized by most bnAbs that are elicited by natural infection. smFRET has also revealed that soluble trimers containing prefusion-stabilizing disulfide and isoleucine-to-proline substitutions reside primarily in State 2, which is a required intermediate between States 1 and 3. While high-resolution Env structures have been determined for States 2 and 3, the structure of these trimers in State 1 is unknown. To provide insight into the State 1 structure, here we characterized antigenic differences between smFRET-defined states and then correlated these differences with known structural differences between States 2 and 3. We found that cell surface-expressed Env was enriched in each state using state-enriching antibody fragments or small-molecule virus entry inhibitors and then assessed binding to HIV-1 bnAbs preferentially binding different states. We observed small but consistent differences in binding between Env enriched in States 1 and 2, and a more than 10-fold difference in binding to Env enriched in these states versus Env enriched in State 3. We conclude that structural differences between HIV-1 Env States 1 and 3 are likely more than 10-fold greater than those between States 1 and 2, providing important insight into State 1.

Functional and Highly Cross-Linkable HIV-1 Envelope Glycoproteins Enriched in a Pretriggered Conformation

Binding to the receptor, CD4, drives the pretriggered, "closed" (state-1) conformation of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer into more "open" conformations (states 2 and 3). Broadly neutralizing antibodies, which are elicited inefficiently, mostly recognize the state-1 Env conformation, whereas the more commonly elicited poorly neutralizing antibodies recognize states 2/3. HIV-1 Env metastability has created challenges for defining the state-1 structure and developing immunogens mimicking this labile conformation. The availability of functional state-1 Envs that can be efficiently cross-linked at lysine and/or acidic amino acid residues might assist these endeavors. To that end, we modified HIV-1_AD8 Env, which exhibits an intermediate level of triggerability by CD4. We introduced lysine/acidic residues at positions that exhibit such polymorphisms in natural HIV-1 strains. Env changes that were tolerated with respect to gp120-gp41 processing, subunit association, and virus entry were further combined. Two common polymorphisms, Q114E and Q567K, as well as a known variant, A582T, additively rendered pseudoviruses resistant to cold, soluble CD4, and a CD4-mimetic compound, phenotypes indicative of stabilization of the pretriggered state-1 Env conformation. Combining these changes resulted in two lysine-rich HIV-1_AD8 Env variants (E.2 and AE.2) with neutralization- and cold-resistant phenotypes comparable to those of natural, less triggerable tier 2/3 HIV-1 isolates. Compared with these and the parental Envs, the E.2 and AE.2 Envs were cleaved more efficiently and exhibited stronger gp120-trimer association in detergent lysates. These highly cross-linkable Envs enriched in a pretriggered conformation should assist characterization of the structure and immunogenicity of this labile state. IMPORTANCE The development of an efficient vaccine is critical for combating HIV-1 infection worldwide. However, the instability of the pretriggered shape (state 1) of the viral envelope glycoprotein (Env) makes it difficult to raise neutralizing antibodies against HIV-1. Here, by introducing multiple changes in Env, we derived two HIV-1 Env variants that are enriched in state 1 and can be efficiently cross-linked to maintain this shape. These Env complexes are more stable in detergent, assisting their purification. Thus, our study provides a path to a better characterization of the native pretriggered Env, which should assist vaccine development.

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

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

Conformation of HIV-1 Envelope Governs Rhesus CD4 Usage and Simian-Human Immunodeficiency Virus Replication

Infection of rhesus macaques with simian-human immunodeficiency viruses (SHIVs) is the preferred model system for vaccine development because SHIVs encode human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins (Envs)-a key target of HIV-1 neutralizing antibodies. Since the goal of vaccines is to prevent new infections, SHIVs encoding circulating HIV-1 Env are desired as challenge viruses. Development of such biologically relevant SHIVs has been challenging, as they fail to infect rhesus macaques, mainly because most circulating HIV-1 Envs do not use rhesus CD4 (rhCD4) receptor for viral entry. Most primary HIV-1 Envs exist in a closed conformation and occasionally transit to a downstream, open conformation through an obligate intermediate conformation. Here, we provide genetic evidence that open Env conformations can overcome the rhCD4 entry barrier and increase replication of SHIVs in rhesus lymphocytes. Consistent with prior studies, we found that circulating HIV-1 Envs do not use rhCD4 efficiently for viral entry. However, by using HIV-1 Envs with single amino acid substitutions that alter their conformational state, we found that transitions to intermediate and open Env conformations allow usage of physiological levels of rhCD4 for viral entry. We engineered these single amino acid substitutions in the transmitted/founder HIV-1_BG505 Envs encoded by SHIV-BG505 and found that open Env conformation enhances SHIV replication in rhesus lymphocytes. Lastly, CD4-mediated SHIV pulldown, sensitivity to soluble CD4, and fusogenicity assays indicated that open Env conformation promotes efficient rhCD4 binding and viral-host membrane fusion. These findings identify the conformational state of HIV-1 Env as a major determinant for rhCD4 usage, viral fusion, and SHIV replication. IMPORTANCE Rhesus macaques are a critical animal model for preclinical testing of HIV-1 vaccine and prevention approaches. However, HIV-1 does not replicate in rhesus macaques, and thus, chimeric simian-human immunodeficiency viruses (SHIVs), which encode HIV-1 envelope glycoproteins (Envs), are used as surrogate challenge viruses to infect rhesus macaques for modeling HIV-1 infection. Development of SHIVs encoding Envs from clinically relevant, circulating HIV-1 variants has been extremely challenging, as such SHIVs replicate poorly, if at all, in rhesus lymphocytes. This is most probably because many circulating HIV-1 Envs do not use rhesus CD4 efficiently for viral entry. In this study, we identified conformational state of HIV-1 envelope as a key determinant for rhesus CD4 usage, viral-host membrane fusion, and SHIV replication in rhesus lymphocytes.

Clinical evidence for a lack of cross-resistance between temsavir and ibalizumab or maraviroc

BACKGROUND

Temsavir (TMR), the active agent of the gp120-directed attachment inhibitor fostemsavir (FTR), the CD4-directed attachment inhibitor ibalizumab (IBA), and the CCR5 antagonist maraviroc (MVC) are antiretroviral agents that target steps in HIV-1 viral entry. Although mechanisms of inhibition of the three agents are different, it is important to understand whether there is potential for cross-resistance between these agents, as all involve interactions with gp120.

METHODS

Envelopes derived from plasma samples from participants in the BRIGHTE study who experienced protocol-derived virologic failure (PDVF) and were co-dosed with FTR and either IBA or MVC were analyzed for susceptibility to the agents. Also, CCR5-tropic MVC-resistant envelopes from the MOTIVATE trials were regenerated and studies were performed to understand whether susceptibility to multiple agents were linked.

RESULTS

The cloned envelopes exhibited reduced susceptibility to TMR and resistance to the co-dosed agent. At PDVF, emergent or preexisting amino acid substitutions were present at TMR positions of interest. When amino acid substitutions at these positions were reverted to the consensus sequence, full susceptibility to TMR was restored without effecting resistance to the co-dosed agent. In addition, five envelopes from MOTIVATE were regenerated and exhibited R5-tropic-MVC-resistance. Only one exhibited reduced susceptibility to TMR and it contained an M426L polymorphism. When reverted to 426M, full sensitivity for TMR was restored, but it remained MVC resistant.

CONCLUSION

The data confirm that decreased susceptibility to TMR and resistance to IBA or MVC are not linked and that there is no cross-resistance between either of these two agents and FTR.

Asymmetric Structures and Conformational Plasticity of the Uncleaved Full-Length Human Immunodeficiency Virus Envelope Glycoprotein Trimer

The functional human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer [(gp120/gp41)3] is produced by cleavage of a conformationally flexible gp160 precursor. gp160 cleavage or the binding of BMS-806, an entry inhibitor, stabilizes the pretriggered, "closed" (state 1) conformation recognized by rarely elicited broadly neutralizing antibodies. Poorly neutralizing antibodies (pNAbs) elicited at high titers during natural infection recognize more "open" Env conformations (states 2 and 3) induced by binding the receptor, CD4. We found that BMS-806 treatment and cross-linking decreased the exposure of pNAb epitopes on cell surface gp160; however, after detergent solubilization, cross-linked and BMS-806-treated gp160 sampled non-state-1 conformations that could be recognized by pNAbs. Cryo-electron microscopy of the purified BMS-806-bound gp160 revealed two hitherto unknown asymmetric trimer conformations, providing insights into the allosteric coupling between trimer opening and structural variation in the gp41 HR1N region. The individual protomer structures in the asymmetric gp160 trimers resemble those of other genetically modified or antibody-bound cleaved HIV-1 Env trimers, which have been suggested to assume state-2-like conformations. Asymmetry of the uncleaved Env potentially exposes surfaces of the trimer to pNAbs. To evaluate the effect of stabilizing a state-1-like conformation of the membrane Env precursor, we treated cells expressing wild-type HIV-1 Env with BMS-806. BMS-806 treatment decreased both gp160 cleavage and the addition of complex glycans, implying that gp160 conformational flexibility contributes to the efficiency of these processes. Selective pressure to maintain flexibility in the precursor of functional Env allows the uncleaved Env to sample asymmetric conformations that potentially skew host antibody responses toward pNAbs. IMPORTANCE The envelope glycoprotein (Env) trimers on the surface of human immunodeficiency virus (HIV-1) mediate the entry of the virus into host cells and serve as targets for neutralizing antibodies. The functional Env trimer is produced by cleavage of the gp160 precursor in the infected cell. We found that the HIV-1 Env precursor is highly plastic, allowing it to assume different asymmetric shapes. This conformational plasticity is potentially important for Env cleavage and proper modification by sugars. Having a flexible, asymmetric Env precursor that can misdirect host antibody responses without compromising virus infectivity would be an advantage for a persistent virus like HIV-1.

Functional differences among the spike glycoproteins of multiple emerging severe acute respiratory syndrome coronavirus 2 variants of concern

We compared the functional properties of spike (S) glycoproteins from the original SARS-CoV-2 strain (D614) (Wuhan, China), the globally dominant D614G strain, and emerging geographic variants: B.1.1.7 (United Kingdom), B.1.351 (South Africa), P.1 (Brazil), and B.1.1.248 (Brazil/Japan). Compared with D614G, the emerging variants exhibited an increased affinity for the receptor, ACE2, and increased ability to infect cells with low ACE2 levels. All variants lost infectivity similarly at room temperature and 37°C; however, in the cold, B.1.1.7 was more stable, and P.1 and B.1.1.248 were less stable. Shedding of the S1 glycoprotein from the S contributed to virus inactivation in the cold. B.1.351, P.1, and B.1.1.248 were neutralized by convalescent and vaccinee sera less efficiently than the other variants. S glycoprotein properties such as requirements for ACE2 levels on the target cell, functional stability in the cold, and resistance to host neutralizing antibodies potentially contribute to the outgrowth of emerging SARS-CoV-2 variants.

Truncation of the Cytoplasmic Tail of Equine Infectious Anemia Virus Increases Virion Production by Improving Env Cleavage and Plasma Membrane Localization

Envelope glycoproteins (Envs) of lentiviruses harbor unusually long cytoplasmic tails (CTs). Natural CT truncations always occur in vitro and are accompanied by attenuated virulence, but their effects on viral replication have not been fully elucidated. The Env in equine infectious anemia virus (EIAV) harbors the longest CT in the lentiviral family, and a truncated CT was observed in a live attenuated vaccine. This study demonstrates that CT truncation significantly increased EIAV production, as determined by comparing the virion yields from EIAV infectious clones in the presence and absence of the CT. A significant increase in a cleaved product from the CT-truncated Env precursor, but not the full-length Env, was observed. We further confirmed that the presence of the CT inhibited the cleavage of the Env precursor and found that a functional domain located at the C terminus was responsible for this function. Moreover, CT-truncated Env was mainly localized at the plasma membrane (PM), while full-length Env was mainly localized in the cytoplasm. The CT truncation caused a dramatic reduction in the endocytosis of Env. These results suggest that the CT can modulate the processing and trafficking of EIAV Env and thus regulate EIAV replication.

IMPORTANCE The mature lentivirus envelope glycoprotein (Env) is composed of a surface unit (SU) and a transmembrane unit (TM), which are cleaved products of the Env precursor. After mature Env is heterodimerically formed from the cleavage of the Env precursor, it is trafficked to the plasma membrane (PM) for incorporation and virion assembly. Env harbors a long cytoplasmic tail (CT), which has been increasingly found to play multiple roles in the Env biological cycle. Here, we revealed for the first time that the CT of equine infectious anemia virus (EIAV) Env inhibits cleavage of the Env precursor. Simultaneously, the CT promoted Env endocytosis, resulting in weakened Env localization at the PM. We also validated that the CT could significantly decrease EIAV production. These findings suggest that the CT regulates the processing and trafficking of EIAV Env to balance virion production.

HIV-1 Envelope Glycoproteins Proteolytic Cleavage Protects Infected Cells from ADCC Mediated by Plasma from Infected Individuals

The HIV-1 envelope glycoprotein (Env) is synthesized in the endoplasmic reticulum as a trimeric gp160 precursor, which requires proteolytic cleavage by a cellular furin protease to mediate virus-cell fusion. Env is conformationally flexible but controls its transition from the unbound "closed" conformation (State 1) to downstream CD4-bound conformations (States 2/3), which are required for fusion. In particular, HIV-1 has evolved several mechanisms that reduce the premature "opening" of Env which exposes highly conserved epitopes recognized by non-neutralizing antibodies (nnAbs) capable of mediating antibody-dependent cellular cytotoxicity (ADCC). Env cleavage decreases its conformational transitions favoring the adoption of the "closed" conformation. Here we altered the gp160 furin cleavage site to impair Env cleavage and to examine its impact on ADCC responses mediated by plasma from HIV-1-infected individuals. We found that infected primary CD4+ T cells expressing uncleaved, but not wildtype, Env are efficiently recognized by nnAbs and become highly susceptible to ADCC responses mediated by plasma from HIV-1-infected individuals. Thus, HIV-1 limits the exposure of uncleaved Env at the surface of HIV-1-infected cells at least in part to escape ADCC responses.

Structure dynamics of HIV-1 Env trimers on native virions engaged with living T cells

The HIV-1 envelope glycoprotein (Env) mediates viral entry into the host cell. Although the highly dynamic nature of Env intramolecular conformations has been shown with single molecule spectroscopy in vitro, the bona fide Env intra- and intermolecular mechanics when engaged with live T cells remains unknown. We used two photon fast fluorescence lifetime imaging detection of single-molecule Förster Resonance Energy Transfer occurring between fluorescent labels on HIV-1 Env on native virions. Our observations reveal Env dynamics at two levels: transitions between different intramolecular conformations and intermolecular interactions between Env within the viral membrane. Furthermore, we show that three broad neutralizing anti-Env antibodies directed to different epitopes restrict Env intramolecular dynamics and interactions between adjacent Env molecules when engaged with living T cells. Importantly, our results show that Env-Env interactions depend on efficient virus maturation, and that is disrupted upon binding of Env to CD4 or by neutralizing antibodies. Thus, this study illuminates how different intramolecular conformations and distribution of Env molecules mediate HIV-1 Env-T cell interactions in real time and therefore might control immune evasion.

Parainfluenza virus entry at the onset of infection

Parainfluenza viruses, members of the enveloped, negative-sense, single stranded RNA Paramyxoviridae family, impact global child health as the cause of significant lower respiratory tract infections. Parainfluenza viruses enter cells by fusing directly at the cell surface membrane. How this fusion occurs via the coordinated efforts of the two molecules that comprise the viral surface fusion complex, and how these efforts may be blocked, are the subjects of this chapter. The receptor binding protein of parainfluenza forms a complex with the fusion protein of the virus, remaining stably associated until a receptor is reached. At that point, the receptor binding protein actively triggers the fusion protein to undergo a series of transitions that ultimately lead to membrane fusion and viral entry. In recent years it has become possible to examine this remarkable process on the surface of viral particles and to begin to understand the steps in the transition of this molecular machine, using a structural biology approach. Understanding the steps in entry leads to several possible strategies to prevent fusion and inhibit infection.

HIV-1 Envelope Glycoprotein Cell Surface Localization Is Associated with Antibody-Induced Internalization

To minimize immune responses against infected cells, HIV-1 has evolved different mechanisms to limit the surface expression of its envelope glycoproteins (Env). Recent observations suggest that the binding of certain broadly neutralizing antibodies (bNAbs) targeting the 'closed' conformation of Env induces its internalization. On the other hand, non-neutralizing antibodies (nNAbs) that preferentially target Env in its 'open' conformation, remain bound to Env on the cell surface for longer periods of time. In this study, we attempt to better understand the underlying mechanisms behind the differential rates of antibody-mediated Env internalization. We demonstrate that 'forcing' open Env using CD4 mimetics allows for nNAb binding and results in similar rates of Env internalization as those observed upon the bNAb binding. Moreover, we can identify distinct populations of Env that are differentially targeted by Abs that mediate faster rates of internalization, suggesting that the mechanism of antibody-induced Env internalization partially depends on the localization of Env on the cell surface.

Intra- and extra-cellular environments contribute to the fate of HIV-1 infection

HIV-1 entry into host cells leads to one of the following three alternative fates: (1) HIV-1 elimination by restriction factors, (2) establishment of HIV-1 latency, or (3) active viral replication in target cells. Here, we report the development of an improved system for monitoring HIV-1 fate at single-cell and population levels and show the diverse applications of this system to study specific aspects of HIV-1 fate in different cell types and under different environments. An analysis of the transcriptome of infected, primary CD4+ T cells that support alternative fates of HIV-1 identifies differential gene expression signatures in these cells. Small molecules are able to selectively target cells that support viral replication with no significant effect on viral latency. In addition, HIV-1 fate varies in different tissues following infection of humanized mice in vivo. Altogether, these studies indicate that intra- and extra-cellular environments contribute to the fate of HIV-1 infection.

HIV-1 entry: Duels between Env and host antiviral transmembrane proteins on the surface of virus particles

Human Immunodeficiency Virus type-1 (HIV-1) is the causative agent of AIDS. Its entry step is mediated by the envelope glycoprotein (Env). During the entry process, Env vastly changes its conformation. While non-liganded Env tends to have a closed structure, receptor-binding of Env opens its conformation, which leads to virus-cell membrane fusion. Single-molecule fluorescence resonance energy transfer (smFRET) imaging allows observation of these conformational changes on the virion surface. Nascent HIV-1 particles incorporate multiple host transmembrane proteins, some of which inhibit the entry process. The Env structure or its dynamics may determine the effectiveness of these antiviral mechanisms. Here, we review recent findings about the Env conformation changes on virus particles and inhibition of Env activities by virion-incorporated host transmembrane proteins.

Susceptibility of global HIV-1 clinical isolates to fostemsavir using the PhenoSense® Entry assay

Background

Fostemsavir is a prodrug of a first-in-class HIV-1 attachment inhibitor, temsavir, that binds to gp120 and blocks attachment to the host-cell CD4 receptor, preventing entry and infection of the target cell. Previous studies using a limited number of clinical isolates showed that there was intrinsic variability in their susceptibility to temsavir.

Objectives

Here, an analysis was performed using all clinical isolates analysed in the Monogram Biosciences PhenoSense^® Entry assay as part of the development programme.

Methods

In total, 1337 individual envelopes encompassing 20 different HIV-1 subtypes were examined for their susceptibility to temsavir. However, only seven subtypes (B, C, F1, A, [B, F1], BF and A1) were present more than five times, with subtype B (881 isolates) and subtype C (156 isolates) having the largest numbers.

Results

As expected, variability in susceptibility was observed within all subtypes. However, for the great majority of these viruses, temsavir was highly potent, with most viruses exhibiting IC₅₀s <10 nM. One exception was CRF01_AE viruses, where all five isolates exhibited IC₅₀s >100 nM. For the 607 isolates where tropism data were available, geometric mean temsavir IC₅₀ values were remarkably similar for CCR5-, CXCR4- and dual mixed-tropic envelopes from infected individuals.

Conclusions

These data show that HIV-1 viruses from most subtypes are highly susceptible to temsavir and that temsavir susceptibility is independent of tropism.

Peptide Triazole Thiol Irreversibly Inactivates Metastable HIV-1 Env by Accessing Conformational Triggers Intrinsic to Virus-Cell Entry

KR13, a peptide triazole thiol previously established to inhibit HIV-1 infection and cause virus lysis, was evaluated by flow cytometry against JRFL Env-presenting cells to characterize induced Env and membrane transformations leading to irreversible inactivation. Transiently transfected HEK293T cells were preloaded with calcein dye, treated with KR13 or its thiol-blocked analogue KR13b, fixed, and stained for gp120 (35O22), MPER (10E8), 6-helix-bundle (NC-1), immunodominant loop (50-69), and fusion peptide (VRC34.01). KR13 induced dose-dependent transformations of Env and membrane characterized by transient poration, MPER exposure, and 6-helix-bundle formation (analogous to native fusion events), but also reduced immunodominant loop and fusion peptide exposure. Using a fusion peptide mutant (V504E), we found that KR13 transformation does not require functional fusion peptide for poration. In contrast, simultaneous treatment with fusion inhibitor T20 alongside KR13 prevented membrane poration and MPER exposure, showing that these events require 6-helix-bundle formation. Based on these results, we formulated a model for PTT-induced Env transformation portraying how, in the absence of CD4/co-receptor signaling, PTT may provide alternate means of perturbing the metastable Env-membrane complex, and inducing fusion-like transformation. In turn, the results show that such transformations are intrinsic to Env and can be diverted for irreversible inactivation of the protein complex.

Changes in the V1 Loop of HIV-1 Envelope Glycoproteins Can Allosterically Modulate the Trimer Association Domain and Reduce PGT145 Sensitivity

Human immunodeficiency virus (HIV-1) envelope glycoproteins (Envs) are a main focus of immunogen design and vaccine development. Broadly neutralizing antibodies (bnAbs) against HIV-1 Envs target conserved epitopes and neutralize multiple HIV-1 viral strains. Nevertheless, application of bnAbs to therapy and prevention is limited by resistant strains that are developed or preexist within the viral population. Here we studied the HIV-1_NAB9 Envs that were isolated from a person who injects drugs and exhibits high and broad resistance to multiple bnAbs. We identified an insertion of 11 amino acids in the V1 loop that allosterically modulates HIV-1_NAB9 sensitivity to the PGT145 bnAb, which targets the Env trimer association domain and supports high level viral infectivity. Our data provide new insights into the mechanisms of HIV-1 resistance to bnAbs and into allosteric connectivity between different HIV-1 Env domains.

Modulating HIV-1 envelope glycoprotein conformation to decrease the HIV-1 reservoir

Small CD4-mimetic compounds (CD4mc) sensitize HIV-1-infected cells to antibody-dependent cellular cytotoxicity (ADCC) by facilitating antibody recognition of epitopes that are otherwise occluded on the unliganded viral envelope (Env). Combining CD4mc with two families of CD4-induced (CD4i) antibodies, which are frequently found in plasma of HIV-1-infected individuals, stabilizes Env in a conformation that is vulnerable to ADCC. We employed new-generation SRG-15 humanized mice, supporting natural killer (NK) cell and Fc-effector functions to demonstrate that brief treatment with CD4mc and CD4i-Abs significantly decreases HIV-1 replication, the virus reservoir and viral rebound after ART interruption. These effects required Fc-effector functions and NK cells, highlighting the importance of ADCC. Viral rebound was also suppressed in HIV-1+-donor cell-derived humanized mice supplemented with autologous HIV-1+-donor-derived plasma and CD4mc. These results indicate that CD4mc could have therapeutic utility in infected individuals for decreasing the size of the HIV-1 reservoir and/or achieving a functional cure.