Structural definition of an antibody-dependent cellular cytotoxicity response implicated in reduced risk for HIV-1 infection

Initiation of HIV neutralizing B cell lineages with sequential envelope immunizations

A strategy for HIV-1 vaccine development is to define envelope (Env) evolution of broadly neutralizing antibodies (bnAbs) in infection and to recreate those events by vaccination. Here, we report host tolerance mechanisms that limit the development of CD4-binding site (CD4bs), HCDR3-binder bnAbs via sequential HIV-1 Env vaccination. Vaccine-induced macaque CD4bs antibodies neutralize 7% of HIV-1 strains, recognize open Env trimers, and accumulate relatively modest somatic mutations. In naive CD4bs, unmutated common ancestor knock-in mice Env⁺B cell clones develop anergy and partial deletion at the transitional to mature B cell stage, but become Env^- upon receptor editing. In comparison with repetitive Env immunizations, sequential Env administration rescue anergic Env⁺ (non-edited) precursor B cells. Thus, stepwise immunization initiates CD4bs-bnAb responses, but immune tolerance mechanisms restrict their development, suggesting that sequential immunogen-based vaccine regimens will likely need to incorporate strategies to expand bnAb precursor pools.

Impact of HIV-1 Envelope Conformation on ADCC Responses

HIV-1 envelope glycoproteins (Env) represent the only virus-specific antigen exposed at the surface of infected cells. In its unliganded form, Env from primary viruses samples a 'closed' conformation (State 1), which is preferentially recognized by broadly neutralizing antibodies (bNAbs). CD4 engagement drives Env into an intermediate 'partially open' (State 2) and then into the 'open' CD4-bound conformation (State 3). Emerging evidence suggests a link between Env conformation and Ab-dependent cellular cytotoxicity (ADCC). HIV-1-infected cells exposing Env in the CD4-bound conformation are susceptible to ADCC mediated by CD4-induced Abs and HIV+sera. Cells exposing State 1 Env are susceptible to ADCC mediated by bNAbs. Here, we discuss how Env conformation affects ADCC responses and in vitro measurements.

Patterns of conserved gp120 epitope presentation on attached HIV-1 virions

A complete picture of HIV antigenicity during early replication is needed to elucidate the full range of options for controlling infection. Such information is frequently gained through analyses of isolated viral envelope antigens, host CD4 receptors, and cognate antibodies. However, direct examination of viral particles and virus-cell interactions is now possible via advanced microscopy techniques and reagents. Using such methods, we recently determined that CD4-induced (CD4i) transition state epitopes in the HIV surface antigen, gp120, while not exposed on free particles, rapidly become immunoreactive upon virus-cell binding. Here, we use 3D direct stochastic optical reconstruction microscopy (dSTORM) to show that certain CD4i epitopes specific to transition state structures are exposed across the surface of cell-bound virions, thus explaining their immunoreactivity. Moreover, such structures and their marker epitopes are dispersed to regions of virions distal to CD4 contact. We further show that the appearance and positioning of distal CD4i exposures is partially dependent on Gag maturation and intact matrix-gp41 interactions within the virion. Collectively, these observations provide a unique perspective of HIV during early replication. These features may define unique insights for understanding how humoral responses target virions and for developing related antiviral countermeasures.

Targeting the Late Stage of HIV-1 Entry for Antibody-Dependent Cellular Cytotoxicity: Structural Basis for Env Epitopes in the C11 Region

Antibodies can have an impact on HIV-1 infection in multiple ways, including antibody-dependent cellular cytotoxicity (ADCC), a correlate of protection observed in the RV144 vaccine trial. One of the most potent ADCC-inducing epitopes on HIV-1 Env is recognized by the C11 antibody. Here, we present the crystal structure, at 2.9 Å resolution, of the C11-like antibody N12-i3, in a quaternary complex with the HIV-1 gp120, a CD4-mimicking peptide M48U1, and an A32-like antibody, N5-i5. Antibody N12-i3 recognizes an epitope centered on the N-terminal "eighth strand" of a critical β sandwich, which our analysis indicates to be emblematic of a late-entry state, after the gp120 detachment. In prior entry states, this sandwich comprises only seven strands, with the eighth strand instead pairing with a portion of the gp120 C terminus. The conformational gymnastics of HIV-1 gp120 thus includes altered β-strand pairing, possibly to reduce immunogenicity, although nevertheless still recognized by the human immune system.

HIV antibodies for treatment of HIV infection

The bar is high to improve on current combination antiretroviral therapy (ART), now highly effective, safe, and simple. However, antibodies that bind the HIV envelope are able to uniquely target the virus as it seeks to enter new target cells, or as it is expressed from previously infected cells. Furthermore, the use of antibodies against HIV as a therapeutic may offer advantages. Antibodies can have long half-lives, and are being considered as partners for long-acting antiretrovirals for use in therapy or prevention of HIV infection. Early studies in animal models and in clinical trials suggest that such antibodies can have antiviral activity but, as with small-molecule antiretrovirals, the issues of viral escape and resistance will have to be addressed. Most promising, however, are the unique properties of anti-HIV antibodies: the potential ability to opsonize viral particles, to direct antibody-dependent cellular cytotoxicity (ADCC) against actively infected cells, and ultimately the ability to direct the clearance of HIV-infected cells by effector cells of the immune system. These distinctive activities suggest that HIV antibodies and their derivatives may play an important role in the next frontier of HIV therapeutics, the effort to develop treatments that could lead to an HIV cure.

Beyond Viral Neutralization

It has been known for more than 30 years that HIV-1 infection drives a very potent B cell response resulting in the production of anti-HIV-1 antibodies targeting several viral proteins, particularly its envelope glycoproteins (Env). Env epitopes are exposed on the surfaces of viral particles and infected cells where they are targets of potentially protective antibodies. These antibodies can interdict infection by neutralization and there is strong evidence suggesting that Fc-mediated effector function can also contribute to protection. Current evidence suggests that Fc-mediated effector function plays a role in protection against infection by broadly neutralizing antibodies and it might be important for protection by non-neutralizing antibodies. Fc-mediated effector function includes diverse mechanisms such as antibody-dependent cellular cytotoxicity (ADCC), antibody-mediated complement activation, antibody-dependent cellular phagocytosis, antibody-dependent cell-mediated virus inhibition, antibody-mediated trancytosis inhibition, and antibody-mediated virus opsonization. All these functions could be beneficial in fighting viral infections, including HIV-1. In this perspective, we discuss the latest developments in ADCC research discussed at the HIVR4P satellite session on non-neutralizing antibodies, with emphasis on the mechanisms of ADCC resistance used by HIV-1, the structural basis of epitopes recognized by antibodies that mediate ADCC, natural killer-cell education and ADCC, and murine models to study ADCC against HIV-1.

HIV-1 Consensus Envelope-Induced Broadly Binding Antibodies

Antibodies that cross-react with multiple HIV-1 envelopes (Envs) are useful reagents for characterizing Env proteins and for soluble Env capture and purification assays. We previously reported 10 murine monoclonal antibodies induced by group M consensus Env, CON-6 immunization. Each demonstrated broad cross-reactivity to recombinant Envs. Here we characterized the Env epitopes to which they bind. Seven mapped to linear epitopes in gp120, five at the Env N-terminus, and two at the Env C-terminus. One antibody, 13D7, bound at the gp120 N-terminus (aa 30-42), reacted with HIV-1-infected CD4+ T cells, and when expressed in a human IgG1 backbone, mediated antibody-dependent cellular cytotoxicity. Antibody 18F11 bound at the gp120 C-terminus (aa 445-459) and reactivity was glycan dependent. Antibodies 13D7, 3B3, and 16H3 bound to 100 percent of HIV-1 Envs tested in ELISA and sodium dodecyl sulfate/polyacrylamide gel electrophoresis/western blot analysis. These data define the epitopes of monoclonal antibody reagents for characterization of recombinant Envs, one epitope of which is also expressed on the surface of HIV-1-infected CD4+ T cells.

Vorinostat Renders the Replication-Competent Latent Reservoir of Human Immunodeficiency Virus (HIV) Vulnerable to Clearance by CD8 T Cells

Latently human immunodeficiency virus (HIV)-infected cells are transcriptionally quiescent and invisible to clearance by the immune system. To demonstrate that the latency reversing agent vorinostat (VOR) induces a window of vulnerability in the latent HIV reservoir, defined as the triggering of viral antigen production sufficient in quantity and duration to allow for recognition and clearance of persisting infection, we developed a latency clearance assay (LCA). The LCA is a quantitative viral outgrowth assay (QVOA) that includes the addition of immune effectors capable of clearing cells expressing viral antigen. Here we show a reduction in the recovery of replication-competent virus from VOR exposed resting CD4 T cells following addition of immune effectors for a discrete period.

TAKE HOME MESSAGE

VOR exposure leads to sufficient production of viral protein on the cell surface, creating a window of vulnerability within this latent reservoir in antiretroviral therapy (ART)-suppressed HIV-infected individuals that allows the clearance of latently infected cells by an array of effector mechanisms.

Humoral and Innate Antiviral Immunity as Tools to Clear Persistent HIV Infection

Human immunodeficiency virus (HIV) type 1 uses the CD4 molecule as its principal receptor to infect T cells. HIV-1 integrates its viral genome into the host cell, leading to persistent infection wherein HIV-1 can remain transcriptionally silent in latently infected CD4⁺ T cells. On reactivation of replication-competent provirus, HIV-1 envelope glycoproteins (Env) are expressed and accumulate on the cell surface, allowing infected cells to be detected and targeted by endogenous immune responses or immune interventions. HIV-1 Env-specific antibodies have the potential to bind HIV-1 cell surface Env and promote elimination of infected CD4⁺ T cells by recruiting cytotoxic effector cells, such as natural killer cells, monocytes, and polymorphonuclear cells. Harnessing humoral and innate cellular responses has become one focus of research to develop innovative strategies to recruit and redirect cytotoxic effector cells to eliminate the HIV-1 latently infected CD4⁺ T-cell reservoir.

Crystal structures of human Fabs targeting the Bexsero meningococcal vaccine antigen NHBA

Neisserial heparin-binding antigen (NHBA) is a surface-exposed lipoprotein from Neisseria meningitidis and is a component of the meningococcus B vaccine Bexsero. As part of a study to characterize the three-dimensional structure of NHBA and the molecular basis of the human immune response to Bexsero, the crystal structures of two fragment antigen-binding domains (Fabs) isolated from human monoclonal antibodies targeting NHBA were determined. Through a high-resolution analysis of the organization and the amino-acid composition of the CDRs, these structures provide broad insights into the NHBA epitopes recognized by the human immune system. As expected, these Fabs also show remarkable structural conservation, as shown by a structural comparison of 15 structures of apo Fab 10C3 which were obtained from crystals grown in different crystallization conditions and were solved while searching for a complex with a bound NHBA fragment or epitope peptide. This study also provides indirect evidence for the intrinsically disordered nature of two N-terminal regions of NHBA.

Quaternary contact in the initial interaction of CD4 with the HIV-1 envelope trimer

Binding of the gp120 envelope (Env) glycoprotein to the CD4 receptor is the first step in the HIV-1 infectious cycle. Although the CD4-binding site has been extensively characterized, the initial receptor interaction has been difficult to study because of major CD4-induced structural rearrangements. Here we used cryogenic electron microscopy (cryo-EM) to visualize the initial contact of CD4 with the HIV-1 Env trimer at 6.8-Å resolution. A single CD4 molecule is embraced by a quaternary HIV-1-Env surface formed by coalescence of the previously defined CD4-contact region with a second CD4-binding site (CD4-BS2) in the inner domain of a neighboring gp120 protomer. Disruption of CD4-BS2 destabilized CD4-trimer interaction and abrogated HIV-1 infectivity by preventing the acquisition of coreceptor-binding competence. A corresponding reduction in HIV-1 infectivity occurred after the mutation of CD4 residues that interact with CD4-BS2. Our results document the critical role of quaternary interactions in the initial HIV-Env-receptor contact, with implications for treatment and vaccine design.

Survivors Remorse: antibody-mediated protection against HIV-1

It is clear that antibodies can play a pivotal role in preventing the transmission of HIV-1 and large efforts to identify an effective antibody-based vaccine to quell the epidemic. Shortly after HIV-1 was discovered as the cause of AIDS, the search for epitopes recognized by neutralizing antibodies became the driving strategy for an antibody-based vaccine. Neutralization escape variants were discovered shortly thereafter, and, after almost three decades of investigation, it is now known that autologous neutralizing antibody responses and their selection of neutralization resistant HIV-1 variants can lead to broadly neutralizing antibodies in some infected individuals. This observation drives an intensive effort to identify a vaccine to elicit broadly neutralizing antibodies. In contrast, there has been less systematic study of antibody specificities that must rely mainly or exclusively on other protective mechanisms, although non-human primate (NHP) studies as well as the RV144 vaccine trial indicate that non-neutralizing antibodies can contribute to protection. Here we propose a novel strategy to identify new epitope targets recognized by these antibodies for which viral escape is unlikely or impossible.

Envelope-specific antibodies and antibody-derived molecules for treating and curing HIV infection

HIV-1 is a retrovirus that integrates into host chromatin and can remain transcriptionally quiescent in a pool of immune cells. This characteristic enables HIV-1 to evade both host immune responses and antiretroviral drugs, leading to persistent infection. Upon reactivation of proviral gene expression, HIV-1 envelope (HIV-1 Env) glycoproteins are expressed on the cell surface, transforming latently infected cells into targets for HIV-1 Env-specific monoclonal antibodies (mAbs), which can engage immune effector cells to kill productively infected CD4+ T cells and thus limit the spread of progeny virus. Recent innovations in antibody engineering have resulted in novel immunotherapeutics such as bispecific dual-affinity re-targeting (DART) molecules and other bi- and trispecific antibody designs that can recognize HIV-1 Env and recruit cytotoxic effector cells to kill CD4+ T cells latently infected with HIV-1. Here, we review these immunotherapies, which are designed with the goal of curing HIV-1 infection.

Fc Receptor-Mediated Activities of Env-Specific Human Monoclonal Antibodies Generated from Volunteers Receiving the DNA Prime-Protein Boost HIV Vaccine DP6-001

HIV-1 is able to elicit broadly potent neutralizing antibodies in a very small subset of individuals only after several years of infection, and therefore, vaccines that elicit these types of antibodies have been difficult to design. The RV144 trial showed that moderate protection is possible and that this protection may correlate with antibody-dependent cellular cytotoxicity (ADCC) activity. Our previous studies demonstrated that in an HIV vaccine phase I trial, the DP6-001 trial, a polyvalent Env DNA prime-protein boost formulation could elicit potent and broadly reactive, gp120-specific antibodies with positive neutralization activities. Here we report on the production and analysis of HIV-1 Env-specific human monoclonal antibodies (hMAbs) isolated from vaccinees in the DP6-001 trial. For this initial report, 13 hMAbs from four vaccinees in the DP6-001 trial showed broad binding to gp120 proteins of diverse subtypes both autologous and heterologous to vaccine immunogens. Equally cross-reactive Fc receptor-mediated functional activities, including ADCC and antibody-dependent cellular phagocytosis (ADCP) activities, were present with both immune sera and isolated MAbs, confirming the induction of nonneutralizing functional hMAbs by the DNA prime-protein boost vaccination. Elicitation of broadly reactive hMAbs by vaccination in healthy human volunteers confirms the value of the polyvalent formulation in this HIV vaccine design.

IMPORTANCE

The roles of Fc receptor-mediated protective antibody responses are gaining more attention due to their potential contribution to the low-level protection against HIV-1 infection that they provided in the RV144 trial. At the same time, information about hMabs from other human HIV vaccine studies is very limited. In the current study, both immune sera and monoclonal antibodies from vaccinated humans showed not only high-level ADCC and ADCP activities but also cross-subtype ADCC and ADCP activities when a polyvalent DNA prime-protein boost vaccine formulation was used.

Molecular basis for epitope recognition by non-neutralizing anti-gp41 antibody F240

Antibody-dependent cell-mediated cytotoxicity (ADCC) by non-neutralizing antibodies (nnAbs) specific to the HIV envelope (Env) glycoproteins present at the surface of virus sensitized or infected cells plays a role in the effective adaptive immune response to HIV. Here, we explore the molecular basis for the epitope at the disulfide loop region (DLR) of the principal immunodominant domain of gp41, recognized by the well-known nnAb F240. Our structural studies reveal details of the F240-gp41 interface and describe a structure of DLR that is distinct from known conformations of this region studied in the context of either CD4-unliganded Env trimer or the gp41 peptide in the unbound state. These data coupled with binding and functional analyses indicate that F240 recognizes non-trimeric Env forms which are significantly overexpressed on intact virions but poorly represented at surfaces of cells infected with infectious molecular clones and endogenously-infected CD4 T cells from HIV-1-infected individuals. Furthermore, although we detect ADCC activities of F240 against cells spinoculated with intact virions, our data suggest that these activities result from F240 recognition of gp41 stumps or misfolded Env variants present on virions rather than its ability to recognize functional gp41 transition structures emerging on trimeric Env post CD4 receptor engagement.

The first 24 h: targeting the window of opportunity for antibody-mediated protection against HIV-1 transmission

PURPOSE OF REVIEW

I will review evidence that antibodies protect against HIV-1 transmission in a short window of opportunity, involving neutralization, Fc-mediated effector function, or both.

RECENT FINDINGS

The last decade witnessed a dramatic progress in the understanding of antibody-mediated protection against HIV-1, including active and passive immunization studies in nonhuman primates; association between reduced infection risk and the specificities and function of antibodies in the RV144 clinical trial; identification of potent, broadly neutralizing antibodies; high-resolution structural studies of the HIV-1 envelope trimer; and an increasing appreciation that Fc-mediated effector function is critical to protection against transmission for neutralizing and nonneutralizing antibodies. Less information is known about how antibodies protect in situ, except that they must do in the first 24 h after exposure. New evidence suggests that antibodies protect in an acute innate immune environment involving the NXLRX1 inflammasome and transforming growth factor beta (TGF-β) that favors infection and rapid dissemination of CCR6RORγ Th17 cells.

SUMMARY

These recent findings set the stage for understanding how antibodies can prevent the transmission of HIV-1. In this context, antibodies must prevent infection in an innate immune environment that strongly favors transmission. This information is key for the development of a vaccine against HIV-1.

HIV-specific CD4-induced Antibodies Mediate Broad and Potent Antibody-dependent Cellular Cytotoxicity Activity and Are Commonly Detected in Plasma From HIV-infected humans

HIV-specific antibodies (Abs) can reduce viral burden by blocking new rounds of infection or by destroying infected cells via activation of effector cells through Fc–FcR interaction. This latter process, referred to as antibody-dependent cellular cytotoxicity (ADCC), has been associated with viral control and improved clinical outcome following both HIV and SIV infections. Here we describe an HIV viral-like particle (VLP)-based sorting strategy that led to identification of HIV-specificmemory B cells encoding Abs that mediate ADCC froma subtype A-infected Kenyan woman at 914 days post-infection. Using this strategy, 12 HIV-envelope-specific monoclonal antibodies (mAbs) were isolated and three mediated potent ADCC activitywhen compared to well-characterized ADCC mAbs. The ADCC-mediating Abs also mediated antibody-dependent cell-mediated virus inhibition (ADCVI), which provides a net measure of Fc receptor-triggered effects against replicating virus. Two of the three ADCC-mediating Abs targeted a CD4-induced (CD4i) epitope also bound by the mAb C11; the third antibody targeted the N-terminus of V3. Both CD4i Abs identified here demonstrated strong cross-clade breadth with activity against 10 of 11 envelopes tested, including those from clades A, B, C, A/D and C/D, whereas the V3-specific antibody showed more limited breadth. Variants of these CD4i, C11-like mAbs engineered to interrupt binding to FcγRs inhibited a measurable percentage of the donor's ADCC activity starting as early as 189 days post-infection. C11-like antibodies also accounted for between 18–78% of ADCC activity in 9 chronically infected individuals from the same cohort study. Further, the two CD4i Abs originated from unique B cells, suggesting that antibodies targeting this epitope can be commonly produced. Taken together, these data provide strong evidence that CD4i, C11-like antibodies develop within the first 6 months of infection and they can arise fromunique B-cell lineages in the same individual. Further, thesemAbsmediate potent plasma IgG-specificADCC breadth and potency and contribute to ADCC activity in other HIV-infected individuals.

A Highly Conserved gp120 Inner Domain Residue Modulates Env Conformation and Trimer Stability

Previous studies have shown that highly conserved residues in the inner domain of gp120 are required for HIV-1 envelope glycoprotein (Env) transitions to the CD4-bound conformation (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656-667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012; A. Desormeaux, M. Coutu, H. Medjahed, B. Pacheco, A. Herschhorn, et al., J Virol 87:2549-2562, 2013, http://dx.doi.org/10.1128/JVI.03104-12). Moreover, W69, a highly conserved residue located at the interface between layer 1 and layer 2 of the inner domain, was recently shown to be important for efficient Env recognition by CD4-induced (CD4i) antibodies capable of potent antibody-dependent cellular cytotoxicity (W. D. Tolbert, N. Gohain, M. Veillette, J. P. Chapleau, C. Orlandi, et al., 2016, Structure 24:697-709, http://dx.doi.org/10.1016/j.str.2016.03.005; S. Ding, M. Veillette, M. Coutu, J. Prevost, L. Scharf, et al., 2016, J Virol 90:2127-2134, http://dx.doi.org/10.1128/JVI.02779-15). We evaluated the contribution of the hydrophobicity of W69 to conformational changes of Env by replacing it with a series of residues with aliphatic or aromatic side chains of decreasing chain length. We have found that the hydrophobicity of residue 69 is important for Env processing, CD4 binding, and its transition to the CD4-bound conformation. The most deleterious effect was observed when W69 was replaced with alanine or glycine residues. However, the functions lost due to W69 mutations could be progressively restored with amino acids of increasing aliphatic chain length and fully recovered with residues bearing an aromatic ring. Interestingly, poor CD4 binding of W69A could be fully restored by introducing a compensatory mutation within layer 2 (S115W). Structural studies of HIV-1 gp120 coree W69A/S115W mutant bound to the CD4 peptide mimetic M48U1 and Fab of anti-cluster A antibody N60-i3 revealed no perturbations to the overall structure of the double mutant compared to the wild-type protein but identified higher mobility within the interface between layer 1 and layer 2, the bridging sheet region, and the CD4 binding site.IMPORTANCE HIV-1 Env transitions to the CD4-bound conformation are required for viral entry. Previous studies identified a highly conserved residue of the inner domain, W69, as being involved in these conformational transitions (A. Finzi, S. H. Xiang, B. Pacheco, L. Wang, J. Haight, et al., Mol Cell 37:656-667, 2010, http://dx.doi.org/10.1016/j.molcel.2010.02.012). Here, we show that W69, located at the interface between gp120 and gp41 in the PGT151-bound trimer, plays a critical role in the interprotomer signaling induced by CD4 binding. This new information might be useful in immunogen design.

Co-receptor Binding Site Antibodies Enable CD4-Mimetics to Expose Conserved Anti-cluster A ADCC Epitopes on HIV-1 Envelope Glycoproteins

Human immunodeficiency virus type 1 (HIV-1) has evolved a sophisticated strategy to conceal conserved epitopes of its envelope glycoproteins (Env) recognized by antibody-dependent cellular cytotoxicity (ADCC)-mediating antibodies. These antibodies, which are present in the sera of most HIV-1-infected individuals, preferentially recognize Env in its CD4-bound conformation. Accordingly, recent studies showed that small CD4-mimetics (CD4mc) able to “push” Env into this conformation sensitize HIV-1-infected cells to ADCC mediated by HIV + sera. Here we test whether CD4mc also expose epitopes recognized by anti-cluster A monoclonal antibodies such as A32, thought to be responsible for the majority of ADCC activity present in HIV + sera and linked to decreased HIV-1 transmission in the RV144 trial. We made the surprising observation that CD4mc are unable to enhance recognition of HIV-1-infected cells by this family of antibodies in the absence of antibodies such as 17b, which binds a highly conserved CD4-induced epitope overlapping the co-receptor binding site (CoRBS). Our results indicate that CD4mc initially open the trimeric Env enough to allow the binding of CoRBS antibodies but not anti-cluster A antibodies. CoRBS antibody binding further opens the trimeric Env, allowing anti-cluster A antibody interaction and sensitization of infected cells to ADCC. Therefore, ADCC responses mediated by cluster A antibodies in HIV-positive sera involve a sequential opening of the Env trimer on the surface of HIV-1-infected cells. The understanding of the conformational changes required to expose these vulnerable Env epitopes might be important in the design of new strategies aimed at fighting HIV-1.

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CD4-mimetics fail to enhance recognition of infected cells by anti-cluster A antibodies (Abs).

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Co-receptor binding site Abs in conjunction with CD4-mimetics allow binding of Env by anti-cluster A Abs.

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Co-receptor binding site Abs help CD4-mimetics sensitize HIV-1-infected cells to ADCC.

HIV-1 developed sophisticated strategies to conceal vulnerable epitopes of its envelope glycoproteins (Env) recognized by antibody-dependent cellular cytotoxicity (ADCC)-mediating antibodies. CD4-mimetics (CD4mc) were shown to sensitize HIV-1-infected cells to ADCC induced by HIV + sera. Here we show that this response requires a sequential opening of Env at the surface of HIV-1-infected cells. Co-receptor binding site antibodies, also present in HIV + sera, are required to expose ADCC-mediating epitopes recognized by anti-cluster A antibodies upon CD4mc addition. The understanding of the conformational changes required to expose anti-cluster A epitopes might be important in the design of new strategies aimed at fighting HIV-1.

NKG2D Acts as a Co-Receptor for Natural Killer Cell-Mediated Anti-HIV-1 Antibody-Dependent Cellular Cytotoxicity

The utility of antibody-dependent cellular cytotoxicity (ADCC) for eliminating HIV-1-infected cells is of much interest for the design of both prophylactic vaccines for HIV-1 prevention and therapeutics to eliminate latently infected cells following reactivation. Significant research has been conducted to understand the antibody specificities involved in anti-HIV-1 ADCC responses. Perhaps equally important as the identity of the antibodies mediating these responses are factors regulating the ability of ADCC effector cells, in particular, natural killer (NK) cells, to respond to antibody-coated target cells. Indeed, a plethora of activating and inhibitory receptors expressed on the surface of NK cells might act in conjunction with CD16 to influence ADCC. As the expression of NKG2D and its ligands has been linked to HIV-1 disease progression, we evaluated if signals through NKG2D were involved in anti-HIV-1 ADCC. Utilizing assays measuring cytolysis, we provide the first data implicating NKG2D in antibody-dependent NK cell responses against a target cell line either pulsed with gp120 or infected with HIV-1. These observations are highly significant for understanding antibody-dependent NK cell responses against HIV-1 and might be useful for optimizing prophylactics and therapeutics aiming to utilize antibodies and optimally functional NK cells to control HIV-1.

The role of Fc receptors in HIV prevention and therapy

Over the past decade, a wealth of experimental evidence has accumulated supporting the importance of Fc receptor (FcR) ligation in antibody-mediated pathology and protection in many disease states. Here we present the diverse evidence base that has accumulated as to the importance of antibody effector functions in the setting of HIV prevention and therapy, including clinical correlates, genetic associations, viral evasion strategies, and a rapidly growing number of compelling animal model experiments. Collectively, this work identifies antibody interactions with FcR as important to both therapeutic and prophylactic strategies involving both passive and active immunity. These findings mirror those in other fields as investigators continue to work toward identifying the right antibodies and the right effectors to be present at the right sites at the right time.

Dimeric FcγR Ectodomains as Probes of the Fc Receptor Function of Anti-Influenza Virus IgG

Ab-dependent cellular cytotoxicity, phagocytosis, and Ag presentation are key mechanisms of action of Abs arising in vaccine or naturally acquired immunity, as well of therapeutic mAbs. Cells expressing the low-affinity FcγRs (FcγRII or CD32 and FcγRIII or CD16) are activated for these functions when receptors are aggregated following the binding of IgG-opsonized targets. Despite the diversity of the Fc receptor proteins, IgG ligands, and potential responding cell types, the induction of all FcγR-mediated responses by opsonized targets requires the presentation of multiple Fc regions in close proximity to each other. We demonstrated that such "near-neighbor" Fc regions can be detected using defined recombinant soluble (rs) dimeric low-affinity ectodomains (rsFcγR) that have an absolute binding requirement for the simultaneous engagement of two IgG Fc regions. Like cell surface-expressed FcγRs, the binding of dimeric rsFcγR ectodomains to Ab immune complexes was affected by Ab subclass, presentation, opsonization density, Fc fucosylation, or mutation. The activation of an NK cell line and primary NK cells by human IgG-opsonized influenza A hemagglutinin correlated with dimeric rsFcγRIIIa binding activity but not with Ab titer. Furthermore, the dimeric rsFcγR binding assay sensitively detected greater Fc receptor activity to pandemic H1N1 hemagglutinin after the swine influenza pandemic of 2009 in pooled human polyclonal IgG. Thus these dimeric rsFcγR ectodomains are validated, defined probes that should prove valuable in measuring the immune-activating capacity of IgG Abs elicited by infection or vaccination or experimentally derived IgG and its variants.

An HIV gp120-CD4 Immunogen Does Not Elicit Autoimmune Antibody Responses in Cynomolgus Macaques

A promising concept for human immunodeficiency virus (HIV) vaccines focuses immunity on the highly conserved transition state structures and epitopes that appear when the HIV glycoprotein gp120 binds to its receptor, CD4. We are developing chimeric antigens (full-length single chain, or FLSC) in which gp120 and CD4 sequences are flexibly linked to allow stable intrachain complex formation between the two moieties (A. DeVico et al., Proc Natl Acad Sci U S A 104:17477-17482, 2007, doi:10.1073/pnas.0707399104; T. R. Fouts et al., J Virol 74:11427-11436, 2000, doi:10.1128/JVI.74.24.11427-11436.2000). Proof of concept studies with nonhuman primates show that FLSC elicited heterologous protection against simian-human immunodeficiency virus (SHIV)/simian immunodeficiency virus (SIV) (T. R. Fouts et al., Proc Natl Acad Sci U S A 112:E992-E999, 2016, doi:10.1073/pnas.1423669112), which correlated with antibodies against transition state gp120 epitopes. Nevertheless, advancement of any vaccine that comprises gp120-CD4 complexes must consider whether the CD4 component breaks tolerance and becomes immunogenic in the autologous host. To address this, we performed an immunotoxicology study with cynomolgus macaques vaccinated with either FLSC or a rhesus variant of FLSC containing macaque CD4 sequences (rhFLSC). Enzyme-linked immunosorbent assay (ELISA) binding titers, primary CD3(+) T cell staining, and temporal trends in T cell subset frequencies served to assess whether anti-CD4 autoantibody responses were elicited by vaccination. We find that immunization with multiple high doses of rhFLSC did not elicit detectable antibody titers despite robust responses to rhFLSC. In accordance with these findings, immunized animals had no changes in circulating CD4(+) T cell counts or evidence of autoantibody reactivity with cell surface CD4 on primary naive macaque T cells. Collectively, these studies show that antigens using CD4 sequences to stabilize transition state gp120 structures are unlikely to elicit autoimmune antibody responses, supporting the advancement of gp120-CD4 complex-based antigens, such as FLSC, into clinical testing.

Probing the Impact of Local Structural Dynamics of Conformational Epitopes on Antibody Recognition

Antibody-antigen interactions are governed by recognition of specific residues and structural complementarity between the antigen epitope and antibody paratope. While X-ray crystallography has provided detailed insights into static conformations of antibody-antigen complexes, factors such as conformational flexibility and dynamics, which are not readily apparent in the structures, can also have an impact on the binding event. Here we investigate the contribution of dynamics in the HIV-1 gp120 glycoprotein to antibody recognition of conserved conformational epitopes, including the CD4- and coreceptor-binding sites, and an inner domain site that is targeted by ADCC-active antibodies. Hydrogen/deuterium-exchange mass spectrometry (HDX-MS) was used to measure local structural dynamics across a panel of variable loop truncation mutants of HIV-1 gp120, including full-length gp120, ΔV3, ΔV1/V2, and extended core, which includes ΔV1/V2 and V3 loop truncations. CD4-bound full-length gp120 was also examined as a reference state. HDX-MS revealed a clear trend toward an increased level of order of the conserved subunit core resulting from loop truncation. Combined with biolayer interferometry and enzyme-linked immunosorbent assay measurements of antibody-antigen binding, we demonstrate that an increased level of ordering of the subunit core was associated with better recognition by an array of antibodies targeting complex conformational epitopes. These results provide detailed insight into the influence of structural dynamics on antibody-antigen interactions and suggest the importance of characterizing the structural stability of vaccine candidates to improve antibody recognition of complex epitopes.

Range of CD4-Bound Conformations of HIV-1 gp120, as Defined Using Conditional CD4-Induced Antibodies

The HIV envelope binds cellular CD4 and undergoes a range of conformational changes that lead to membrane fusion and delivery of the viral nucleocapsid into the cellular cytoplasm. This binding to CD4 reveals cryptic and highly conserved epitopes, the molecular nature of which is still not fully understood. The atomic structures of CD4 complexed with gp120 core molecules (a form of gp120 in which the V1, V2, and V3 loops and N and C termini have been truncated) have indicated that a hallmark feature of the CD4-bound conformation is the bridging sheet minidomain. Variations in the orientation of the bridging sheet hairpins have been revealed when CD4-liganded gp120 was compared to CD4-unliganded trimeric envelope structures. Hence, there appears to be a number of conformational transitions possible in HIV-1 monomeric gp120 that are affected by CD4 binding. The spectrum of CD4-bound conformations has been interrogated in this study by using a well-characterized panel of conditional, CD4-induced (CD4i) monoclonal antibodies (MAbs) that bind HIV-1 gp120 and its mutations under various conditions. Two distinct CD4i epitopes of the outer domain were studied: the first comprises the bridging sheet, while the second contains elements of the V2 loop. Furthermore, we show that the unliganded extended monomeric core of gp120 (coree) assumes an intermediate CD4i conformation in solution that further undergoes detectable rearrangements upon association with CD4. These discoveries impact both accepted paradigms concerning gp120 structure and the field of HIV immunogen design.

IMPORTANCE

Elucidation of the conformational transitions that the HIV-1 envelope protein undergoes during the course of entry into CD4(+)cells is fundamental to our understanding of HIV biology. The binding of CD4 triggers a range of gp120 structural rearrangements that could present targets for future drug design and development of preventive vaccines. Here we have systematically interrogated and scrutinized these conformational transitions using a panel of antibody probes that share a specific preference for the CD4i conformations. These have been employed to study a collection of gp120 mutations and truncations. Through these analyses, we propose 4 distinct sequential steps in CD4i transitions of gp120 conformations, each defined by antibody specificities and structural requirements of the HIV envelope monomer. As a result, we not only provide new insights into this dynamic process but also define probes to further investigate HIV infection.

Paring Down HIV Env: Design and Crystal Structure of a Stabilized Inner Domain of HIV-1 gp120 Displaying a Major ADCC Target of the A32 Region

Evidence supports a role of antibody-dependent cellular cytotoxicity (ADCC) toward transitional epitopes in the first and second constant (C1-C2) regions of gp120 (A32-like epitopes) in preventing HIV-1 infection and in vaccine-induced protection. Here, we describe the first successful attempt at isolating the inner domain (ID) of gp120 as an independent molecule that encapsulates the A32-like region within a minimal structural unit of the HIV-1 Env. Through structure-based design, we developed ID2, which consists of the ID expressed independently of the outer domain and stabilized in the CD4-bound conformation by an inter-layer disulfide bond. ID2 expresses C1-C2 epitopes in the context of CD4-triggered full-length gp120 but without any known neutralizing epitope present. Thus, ID2 represents a novel probe for the analysis and/or selective induction of antibody responses to the A32 epitope region. We also present the crystal structure of ID2 complexed with mAb A32, which defines its epitope.

A new cell line for high throughput HIV-specific antibody-dependent cellular cytotoxicity (ADCC) and cell-to-cell virus transmission studies

Several lines of evidence indicate that antibody-dependent cellular cytotoxicity (Wren et al., 2013) is important in the pathogenesis of HIV-1 infection. Namely, ADCC is induced during natural HIV-1 infection or in HIV-1 vaccine studies, the latter demonstrated by the RV144 vaccine trial. To expedite the assessment of ADCC in studies of HIV, we have developed a high throughput assay. We have optimized the rapid fluorometric antibody-mediated cytotoxicity assay (RFADCC) by transfecting the EGFP-CEM-NKr cell line to constitutively express SNAP-tagged CCR5. This cell line can then serve as a source of HIV-specific targets when coated with monomeric gp120, spinoculated with inactivated intact virions, infected by cell-free viral diffusion or infected by cell-to-cell transmission of virus. The optimized strategy has two significant advantages over the original RFADCC method: First, the preparation of detectable target cells is less labor intensive and faster as it does not rely on multiple staining and washing steps for target cells. Second, because the target cell markers GFP and SNAP are constitutively expressed, the assay provides highly reproducible data. These strengths make the optimized RFADCC assay suitable not only for studies of HIV-1 specific cytotoxicity but also for studies of cell–cell transmission of virus. In conclusion, this assay provides a new generation T cell line that can expedite large clinical studies as well as research studies in humans or non-human primates.

The function and affinity maturation of HIV-1 gp120-specific monoclonal antibodies derived from colostral B cells

Despite the risk of transmitting HIV-1, mothers in resource-poor areas are encouraged to breastfeed their infants because of beneficial immunologic and nutritional factors in milk. Interestingly, in the absence of antiretroviral prophylaxis, the overwhelming majority of HIV-1-exposed, breastfeeding infants are naturally protected from infection. To understand the role of HIV-1 envelope (Env)-specific antibodies in breast milk in natural protection against infant virus transmission, we produced 19 HIV-1 Env-specific monoclonal antibodies (mAbs) isolated from colostrum B cells of HIV-1-infected mothers and investigated their specificity, evolution, and anti-HIV-1 functions. Despite the previously reported genetic compartmentalization and gp120-specific bias of colostrum HIV Env-specific B cells, the colostrum Env-specific mAbs described here demonstrated a broad range of gp120 epitope specificities and functions, including inhibition of epithelial cell binding and dendritic cell-mediated virus transfer, neutralization, and antibody-dependent cellular cytotoxicity. We also identified divergent patterns of colostrum Env-specific B-cell lineage evolution with respect to crossreactivity to gastrointestinal commensal bacteria, indicating that commensal bacterial antigens play a role in shaping the local breast milk immunoglobulin G (IgG) repertoire. Maternal vaccine strategies to specifically target this breast milk B-cell population may be necessary to achieve safe breastfeeding for all HIV-1-exposed infants.

Harnessing the protective potential of HIV-1 neutralizing antibodies

Recent biological, structural, and technical advances are converging within the HIV-1 vaccine field to harness the power of antibodies for prevention and therapy. Numerous monoclonal antibodies with broad neutralizing activity against diverse HIV-1 isolates have now been identified, revealing at least five sites of vulnerability on the envelope (Env) glycoproteins. While there are practical and technological barriers blocking a clear path from broadly neutralizing antibodies (bNAb) to a protective vaccine, this is not a dead end. Scientists are revisiting old approaches with new technology, cutting new trails through unexplored territory, and paving new roads in the hopes of preventing HIV-1 infection. Other promising avenues to capitalize on the power of bNAbs are also being pursued, such as passive antibody immunotherapy and gene therapy approaches. Moreover, non-neutralizing antibodies have inhibitory activities that could have protective potential, alone or in combination with bNAbs. With a new generation of bNAbs, and a clinical trial that associated antibodies with reduced acquisition, the field is closer than ever to developing strategies to use antibodies against HIV-1.

A Highly Conserved Residue of the HIV-1 gp120 Inner Domain Is Important for Antibody-Dependent Cellular Cytotoxicity Responses Mediated by Anti-cluster A Antibodies

Previous studies have shown that sera from HIV-1-infected individuals contain antibodies able to mediate antibody-dependent cellular cytotoxicity (ADCC). These antibodies preferentially recognize envelope glycoprotein (Env) epitopes induced upon CD4 binding. Here, we show that a highly conserved tryptophan at position 69 of the gp120 inner domain is important for ADCC mediated by anti-cluster A antibodies and sera from HIV-1-infected individuals.

Targeting HIV Reservoir in Infected CD4 T Cells by Dual-Affinity Re-targeting Molecules (DARTs) that Bind HIV Envelope and Recruit Cytotoxic T Cells

HIV reservoirs and production of viral antigens are not eliminated in chronically infected participants treated with combination antiretroviral therapy (cART). Novel therapeutic strategies aiming at viral reservoir elimination are needed to address chronic immune dysfunction and non-AIDS morbidities that exist despite effective cART. The HIV envelope protein (Env) is emerging as a highly specific viral target for therapeutic elimination of the persistent HIV-infected reservoirs via antibody-mediated cell killing. Dual-Affinity Re-Targeting (DART) molecules exhibit a distinct mechanism of action via binding the cell surface target antigen and simultaneously engaging CD3 on cytotoxic T lymphocytes (CTLs). We designed and evaluated Env-specific DARTs (HIVxCD3 DARTs) derived from known antibodies recognizing diverse Env epitopes with or without broadly neutralizing activity. HIVxCD3 DARTs derived from PGT121, PGT145, A32, and 7B2, but not VRC01 or 10E8 antibodies, mediated potent CTL-dependent killing of quiescent primary CD4 T cells infected with diverse HIV isolates. Similar killing activity was also observed with DARTs structurally modified for in vivo half-life extension. In an ex vivo model using cells isolated from HIV-infected participants on cART, combinations of the most potent HIVxCD3 DARTs reduced HIV expression both in quiescent and activated peripheral blood mononuclear cell cultures isolated from HIV-infected participants on suppressive cART. Importantly, HIVxCD3 DARTs did not induce cell-to-cell virus spread in resting or activated CD4 T cell cultures. Collectively, these results provide support for further development of HIVxCD3 DARTs as a promising therapeutic strategy for targeting HIV reservoirs.

Current HIV therapies prevent AIDS by dramatically reducing, but not eliminating, HIV infection. A reservoir of HIV-infected cells persists during long-term antiviral therapy, and individuals are at increased risk to develop non-AIDS illnesses, e.g., accelerated heart, bone, or kidney disease. Novel strategies are thus needed to safely kill HIV-infected cells and reduce or eliminate the HIV reservoir. An emerging strategy to kill HIV-infected cells involves antibodies (Abs) that bind the HIV envelope protein (Env). Env can distinguish HIV-infected cells from uninfected cells, and some Env-specific Abs can kill HIV-infected cells by recruiting immune cells, e.g., NK cells and macrophages. Here, we developed a strategy to kill HIV-infected cells that is complementary to Env-specific Abs. We designed and evaluated Dual-Affinity Re-Targeting (DART) molecules that incorporate Env-binding specificities with a CD3-binding specificity to recruit and activate cytotoxic T cells. We report that HIVxCD3 DARTs potently and selectively kill HIV-infected cells. Furthermore, HIV DARTs perturb resting and activated viral reservoirs in cells isolated from individuals on antiviral therapy. This novel strategy may be an important element of future antiviral therapies that target the HIV reservoir.

Chemical Cross-Linking Stabilizes Native-Like HIV-1 Envelope Glycoprotein Trimer Antigens

Major neutralizing antibody immune evasion strategies of the HIV-1 envelope glycoprotein (Env) trimer include conformational and structural instability. Stabilized soluble trimers such as BG505 SOSIP.664 mimic the structure of virion-associated Env but nevertheless sample different conformational states. Here we demonstrate that treating BG505 SOSIP.664 trimers with glutaraldehyde or a heterobifunctional cross-linker introduces additional stability with relatively modest effects on antigenicity. Thus, most broadly neutralizing antibody (bNAb) epitopes were preserved after cross-linking, whereas the binding of most weakly or nonneutralizing antibodies (non-NAb) was reduced. Cross-linking stabilized all Env conformers present within a mixed population, and individual conformers could be isolated by bNAb affinity chromatography. Both positive selection of cross-linked conformers using the quaternary epitope-specific bNAbs PGT145, PGT151, and 3BC315 and negative selection with non-NAbs against the V3 region enriched for trimer populations with improved antigenicity for bNAbs. Similar results were obtained using the clade B B41 SOSIP.664 trimer. The cross-linking method may, therefore, be useful for countering the natural conformational heterogeneity of some HIV-1 Env proteins and, by extrapolation, also vaccine immunogens from other pathogens.

IMPORTANCE The development of a vaccine to induce protective antibodies against HIV-1 is of primary public health importance. Recent advances in immunogen design have provided soluble recombinant envelope glycoprotein trimers with near-native morphology and antigenicity. However, these trimers are conformationally flexible, potentially reducing B-cell recognition of neutralizing antibody epitopes. Here we show that chemical cross-linking increases trimer stability, reducing binding of nonneutralizing antibodies while largely maintaining neutralizing antibody binding. Cross-linking followed by positive or negative antibody affinity selection of individual stable conformational variants further improved the antigenic and morphological characteristics of the trimers. This approach may be generally applicable to HIV-1 Env and also to other conformationally flexible pathogen antigens.

Dual-Affinity Re-Targeting proteins direct T cell-mediated cytolysis of latently HIV-infected cells

Enhancement of HIV-specific immunity is likely required to eliminate latent HIV infection. Here, we have developed an immunotherapeutic modality aimed to improve T cell-mediated clearance of HIV-1-infected cells. Specifically, we employed Dual-Affinity Re-Targeting (DART) proteins, which are bispecific, antibody-based molecules that can bind 2 distinct cell-surface molecules simultaneously. We designed DARTs with a monovalent HIV-1 envelope-binding (Env-binding) arm that was derived from broadly binding, antibody-dependent cellular cytotoxicity-mediating antibodies known to bind to HIV-infected target cells coupled to a monovalent CD3 binding arm designed to engage cytolytic effector T cells (referred to as HIVxCD3 DARTs). Thus, these DARTs redirected polyclonal T cells to specifically engage with and kill Env-expressing cells, including CD4+ T cells infected with different HIV-1 subtypes, thereby obviating the requirement for HIV-specific immunity. Using lymphocytes from patients on suppressive antiretroviral therapy (ART), we demonstrated that DARTs mediate CD8+ T cell clearance of CD4+ T cells that are superinfected with the HIV-1 strain JR-CSF or infected with autologous reservoir viruses isolated from HIV-infected-patient resting CD4+ T cells. Moreover, DARTs mediated CD8+ T cell clearance of HIV from resting CD4+ T cell cultures following induction of latent virus expression. Combined with HIV latency reversing agents, HIVxCD3 DARTs have the potential to be effective immunotherapeutic agents to clear latent HIV-1 reservoirs in HIV-infected individuals.

Conformational Masking and Receptor-Dependent Unmasking of Highly Conserved Env Epitopes Recognized by Non-Neutralizing Antibodies That Mediate Potent ADCC against HIV-1

The mechanism of antibody-mediated protection is a major focus of HIV-1 vaccine development and a significant issue in the control of viremia. Virus neutralization, Fc-mediated effector function, or both, are major mechanisms of antibody-mediated protection against HIV-1, although other mechanisms, such as virus aggregation, are known. The interplay between virus neutralization and Fc-mediated effector function in protection against HIV-1 is complex and only partially understood. Passive immunization studies using potent broadly neutralizing antibodies (bnAbs) show that both neutralization and Fc-mediated effector function provides the widest dynamic range of protection; however, a vaccine to elicit these responses remains elusive. By contrast, active immunization studies in both humans and non-human primates using HIV-1 vaccine candidates suggest that weakly neutralizing or non-neutralizing antibodies can protect by Fc-mediated effector function, albeit with a much lower dynamic range seen for passive immunization with bnAbs. HIV-1 has evolved mechanisms to evade each type of antibody-mediated protection that must be countered by a successful AIDS vaccine. Overcoming the hurdles required to elicit bnAbs has become a major focus of HIV-1 vaccine development. Here, we discuss a less studied problem, the structural basis of protection (and its evasion) by antibodies that protect only by potent Fc-mediated effector function.

Cocrystal Structures of Antibody N60-i3 and Antibody JR4 in Complex with gp120 Define More Cluster A Epitopes Involved in Effective Antibody-Dependent Effector Function against HIV-1

Accumulating evidence indicates a role for Fc receptor (FcR)-mediated effector functions of antibodies, including antibody-dependent cell-mediated cytotoxicity (ADCC), in prevention of human immunodeficiency virus type 1 (HIV-1) acquisition and in postinfection control of viremia. Consequently, an understanding of the molecular basis for Env epitopes that constitute effective ADCC targets is of fundamental interest for humoral anti-HIV-1 immunity and for HIV-1 vaccine design. A substantial portion of FcR effector function of potentially protective anti-HIV-1 antibodies is directed toward nonneutralizing, transitional, CD4-inducible (CD4i) epitopes associated with the gp41-reactive region of gp120 (cluster A epitopes). Our previous studies defined the A32-like epitope within the cluster A region and mapped it to the highly conserved and mobile layers 1 and 2 of the gp120 inner domain within the C1-C2 regions of gp120. Here, we elucidate additional cluster A epitope structures, including an A32-like epitope, recognized by human monoclonal antibody (MAb) N60-i3, and a hybrid A32-C11-like epitope, recognized by rhesus macaque MAb JR4. These studies define for the first time a hybrid A32-C11-like epitope and map it to elements of both the A32-like subregion and the seven-layered β-sheet of the gp41-interactive region of gp120. These studies provide additional evidence that effective antibody-dependent effector function in the cluster A region depends on precise epitope targeting--a combination of epitope footprint and mode of antibody attachment. All together these findings help further an understanding of how cluster A epitopes are targeted by humoral responses.

IMPORTANCE

HIV/AIDS has claimed the lives of over 30 million people. Although antiretroviral drugs can control viral replication, no vaccine has yet been developed to prevent the spread of the disease. Studies of natural HIV-1 infection, simian immunodeficiency virus (SIV)- or simian-human immunodeficiency virus (SHIV)-infected nonhuman primates (NHPs), and HIV-1-infected humanized mouse models, passive transfer studies in infants born to HIV-infected mothers, and the RV144 clinical trial have linked FcR-mediated effector functions of anti-HIV-1 antibodies with postinfection control of viremia and/or blocking viral acquisition. With this report we provide additional definition of the molecular determinants for Env antigen engagement which lead to effective antibody-dependent effector function directed to the nonneutralizing CD4-dependent epitopes in the gp41-reactive region of gp120. These findings have important implications for the development of an effective HIV-1 vaccine.

Human Non-neutralizing HIV-1 Envelope Monoclonal Antibodies Limit the Number of Founder Viruses during SHIV Mucosal Infection in Rhesus Macaques

HIV-1 mucosal transmission begins with virus or virus-infected cells moving through mucus across mucosal epithelium to infect CD4+ T cells. Although broadly neutralizing antibodies (bnAbs) are the type of HIV-1 antibodies that are most likely protective, they are not induced with current vaccine candidates. In contrast, antibodies that do not neutralize primary HIV-1 strains in the TZM-bl infection assay are readily induced by current vaccine candidates and have also been implicated as secondary correlates of decreased HIV-1 risk in the RV144 vaccine efficacy trial. Here, we have studied the capacity of anti-Env monoclonal antibodies (mAbs) against either the immunodominant region of gp41 (7B2 IgG1), the first constant region of gp120 (A32 IgG1), or the third variable loop (V3) of gp120 (CH22 IgG1) to modulate in vivo rectal mucosal transmission of a high-dose simian-human immunodeficiency virus (SHIV-BaL) in rhesus macaques. 7B2 IgG1 or A32 IgG1, each containing mutations to enhance Fc function, was administered passively to rhesus macaques but afforded no protection against productive clinical infection while the positive control antibody CH22 IgG1 prevented infection in 4 of 6 animals. Enumeration of transmitted/founder (T/F) viruses revealed that passive infusion of each of the three antibodies significantly reduced the number of T/F genomes. Thus, some antibodies that bind HIV-1 Env but fail to neutralize virus in traditional neutralization assays may limit the number of T/F viruses involved in transmission without leading to enhancement of viral infection. For one of these mAbs, gp41 mAb 7B2, we provide the first co-crystal structure in complex with a common cyclical loop motif demonstrated to be critical for infection by other retroviruses.

CD4 mimetics sensitize HIV-1-infected cells to ADCC

HIV-1-infected cells presenting envelope glycoproteins (Env) in the CD4-bound conformation on their surface are preferentially targeted by antibody-dependent cell-mediated cytotoxicity (ADCC). HIV-1 has evolved a sophisticated mechanism to avoid exposure of ADCC-mediating Env epitopes by down-regulating CD4 and by limiting the overall amount of Env at the cell surface. Here we report that small-molecule CD4-mimetic compounds induce the CD4-bound conformation of Env, and thereby sensitize cells infected with primary HIV-1 isolates to ADCC mediated by antibodies present in sera, cervicovaginal lavages, and breast milk from HIV-1-infected individuals. Importantly, we identified one CD4 mimetic with the capacity to sensitize endogenously infected ex vivo-amplified primary CD4 T cells to ADCC killing mediated by autologous sera and effector cells. Thus, CD4 mimetics hold the promise of therapeutic utility in preventing and controlling HIV-1 infection.

An NK Cell Population Lacking FcRγ Is Expanded in Chronically Infected HIV Patients

We previously demonstrated that NK cells from HIV-infected individuals have elevated expression of activation markers, spontaneously degranulate ex vivo, and decrease expression of a signal-transducing protein for NK-activating receptors, FcRγ. Importantly, these changes were maintained in virologically suppressed (VS) individuals receiving combination antiretroviral therapy (cART). In this study, we show that loss of FcRγ is caused by the expansion of a novel subset of FcRγ(-)CD56(dim) NK cells with an altered activation receptor repertoire and biological properties. In a cross-sectional study, FcRγ(-) NK cells as a proportion of total CD56(dim) NK cells increased in cART-naive viremic HIV-infected individuals (median [interquartile range] = 25.9 [12.6-56.1] compared with 3.80 [1.15-11.5] for HIV(-) controls, p < 0.0001) and in VS HIV-infected individuals (22.7 [13.1-56.2] compared with 3.80 [1.15-11.5], p = 0.0004), with no difference between cART-naive and VS patients (p = 0.93). FcRγ(-) NK cells expressed no NKp30 or NKp46. They showed greater Ab-dependent cellular cytotoxicity activity against rituximab-opsonized Raji cells and in a whole-blood assay measuring NK responses to overlapping HIV peptides, despite having reduced CD16 expression compared with conventional NK cells. Their prevalence correlated with CMV Ab titers in HIV(-) subjects but not in HIV(+) individuals, and with the inflammatory marker CXCL10 in both groups. The expansion of a subset of NK cells that lacks NKp30 and NKp46 to ∼90% of CD56(dim) NK cells in some VS HIV(+) individuals may influence NK-mediated immunosurveillance in patients receiving cART.

Effects of the I559P gp41 change on the conformation and function of the human immunodeficiency virus (HIV-1) membrane envelope glycoprotein trimer

The mature human immunodeficiency virus (HIV-1) envelope glycoprotein (Env) trimer is produced by proteolytic cleavage of a precursor and consists of three gp120 exterior and three gp41 transmembrane subunits. The metastable Env complex is induced to undergo conformational changes required for virus entry by the binding of gp120 to the receptors, CD4 and CCR5/CXCR4. An isoleucine-to-proline change (I559P) in the gp41 ectodomain has been used to stabilize soluble forms of HIV-1 Env trimers for structural characterization and for use as immunogens. In the native membrane-anchored HIV-1_BG505 Env, the I559P change modestly decreased proteolytic maturation, increased the non-covalent association of gp120 with the Env trimer, and resulted in an Env conformation distinctly different from that of the wild-type HIV-1_BG505 Env. Compared with the wild-type Env, the I559P Env was recognized inefficiently by polyclonal sera from HIV-1-infected individuals, by several gp41-directed antibodies, by some antibodies against the CD4-binding site of gp120, and by antibodies that preferentially recognize the CD4-bound Env. Some of the gp120-associated antigenic differences between the wild-type HIV-1_BG505 Env and the I559P mutant were compensated by the SOS disulfide bond between gp120 and gp41, which has been used to stabilize cleaved soluble Env trimers. Nonetheless, regardless of the presence of the SOS changes, Envs with proline 559 were recognized less efficiently than Envs with isoleucine 559 by the VRC01 neutralizing antibody, which binds the CD4-binding site of gp120, and the PGT151 neutralizing antibody, which binds a hybrid gp120-gp41 epitope. The I559P change completely eliminated the ability of the HIV-1_BG505 Env to mediate cell-cell fusion and virus entry, and abolished the capacity of the SOS Env to support virus infection in the presence of a reducing agent. These results suggest that differences exist between the quaternary structures of functional Env spikes and I559P Envs.

What Do Chaotrope-Based Avidity Assays for Antibodies to HIV-1 Envelope Glycoproteins Measure?

When HIV-1 vaccine candidates that include soluble envelope glycoproteins (Env) are tested in humans and other species, the resulting antibody responses to Env are sifted for correlates of protection or risk. One frequently used assay measures the reduction in antibody binding to Env antigens by an added chaotrope (such as thiocyanate). Based on that assay, an avidity index was devised for assessing the affinity maturation of antibodies of unknown concentration in polyclonal sera. Since a high avidity index was linked to protection in animal models of HIV-1 infection, it has become a criterion for evaluating antibody responses to vaccine candidates. But what does the assay measure and what does an avidity index mean? Here, we have used a panel of monoclonal antibodies to well-defined epitopes on Env (gp120, gp41, and SOSIP.664 trimers) to explore how the chaotrope acts. We conclude that the chaotrope sensitivity of antibody binding to Env depends on several properties of the epitopes (continuity versus tertiary- and quaternary-structural dependence) and that the avidity index has no simple relationship to antibody affinity for functional Env spikes on virions. We show that the binding of broadly neutralizing antibodies against quaternary-structural epitopes is particularly sensitive to chaotrope treatment, whereas antibody binding to epitopes in variable loops and to nonneutralization epitopes in gp41 is generally resistant. As a result of such biases, the avidity index may at best be a mere surrogate for undefined antibody or other immune responses that correlate weakly with protection.

IMPORTANCE

An effective HIV-1 vaccine is an important goal. Such a vaccine will probably need to induce antibodies that neutralize typically transmitted variants of HIV-1, preventing them from infecting target cells. Vaccine candidates have so far failed to induce such antibody responses, although some do protect weakly against infection in animals and, possibly, humans. In the search for responses associated with protection, an avidity assay based on chemical disruption is often used to measure the strength of antibody binding. We have analyzed this assay mechanistically and found that the epitope specificity of an antibody has a greater influence on the outcome than does its affinity. As a result, the avidity assay is biased toward the detection of some antibody specificities while disfavoring others. We conclude that the assay may yield merely indirect correlations with weak protection, specifically when Env vaccination has failed to induce broad neutralizing responses.

HIV-1 gp120 as a therapeutic target: navigating a moving labyrinth

INTRODUCTION

The HIV-1 gp120 envelope (Env) glycoprotein mediates attachment of virus to human target cells that display requisite receptors, CD4 and co-receptor, generally CCR5. Despite high-affinity interactions with host receptors and proof-of-principle by the drug maraviroc that interference with CCR5 provides therapeutic benefit, no licensed drug currently targets gp120.

AREAS COVERED

An overview of the role of gp120 in HIV-1 entry and of sites of potential gp120 vulnerability to therapeutic inhibition is presented. Viral defenses that protect these sites and turn gp120 into a moving labyrinth are discussed together with strategies for circumventing these defenses to allow therapeutic targeting of gp120 sites of vulnerability.

EXPERT OPINION

The gp120 envelope glycoprotein interacts with host proteins through multiple interfaces and has conserved structural features at these interaction sites. In spite of this, targeting gp120 for therapeutic purposes is challenging. Env mechanisms that have evolved to evade the humoral immune response also shield it from potential therapeutics. Nevertheless, substantial progress has been made in understanding HIV-1 gp120 structure and its interactions with host receptors, and in developing therapeutic leads that potently neutralize diverse HIV-1 strains. Synergies between advances in understanding, needs for therapeutics against novel viral targets and characteristics of breadth and potency for a number of gp120-targetting lead molecules bodes well for gp120 as a HIV-1 therapeutic target.

Conserved molecular signatures in gp120 are associated with the genetic bottleneck during simian immunodeficiency virus (SIV), SIV-human immunodeficiency virus (SHIV), and HIV type 1 (HIV-1) transmission

Human immunodeficiency virus (HIV) transmission typically results from infection by a single transmitted/founder (T/F) variant. Are T/F variants chosen uniformly at random from the donor pool, or are they selected based on advantageous traits facilitating transmission? Finding evidence for selection during transmission is of particular interest, because it would indicate that phenotypic and/or genetic properties of the viruses might be harnessed as potential vaccine targets or immunotherapies. Here, we systematically evaluated the differences between the Env proteins of simian immunodeficiency virus/simian HIV (SIV/SHIV) stock and T/F variants in search of "signature" sites of transmission. We also surveyed residue preferences in HIV at the SIV/SHIV signature sites. Four sites of gp120 showed significant selection, and an additional two sites showed a similar trend. Therefore, the six sites clearly differentiate T/F viruses from the majority of circulating variants in the stocks. The selection of SIV/SHIV could be inferred reasonably across both vaccinated and unvaccinated subjects, with infections resulting from vaginal, rectal, and intravenous routes of transmission and regardless of viral dosage. The evidence for selection in SIV and SHIV T/F variants is strong and plentiful, and in HIV the evidence is suggestive though commensurate with the availability of suitable data for analysis. Two of the signature residues are completely conserved across the SIV, SHIV, and HIV variants we examined. Five of the signature residues map to the C1 region of gp120 and one to the signal peptide. Our data raise the possibility that C1, while governing the association between gp120 and gp41, modulates transmission efficiency, replicative fitness, and/or host cell tropism at the level of virus-cell attachment and entry.

IMPORTANCE

The present study finds significant evidence of selection on gp120 molecules of SIV/SHIV T/F viruses. The data provide ancillary evidence suggesting the same sites are under selection in HIV. Our findings suggest that the signature residues are involved in increasing the transmissibility of infecting viruses; therefore, they are potential targets for developing a vaccine or other protective measures. A recent study identified the same T/F signature motif but interpreted it as an effect of neutralization resistance. Here, we show that the T/F motif has broader functional significance beyond neutralization sensitivity, because it is present in nonimmune subjects. Also, a vaccine regimen popular in animal trials might have increased the transmission of variants with otherwise low transmission fitness. Our observations might explain why many animal vaccine trials have not faithfully predicted outcomes in human vaccine trials and suggest that current practices in vaccine design need to be reexamined accordingly.

The role of HIV-specific antibody-dependent cellular cytotoxicity in HIV prevention and the influence of the HIV-1 Vpu protein

There is growing interest in the role of anti-HIV antibody-dependent cellular cytotoxicity (ADCC) antibodies in the prevention and control of HIV infection. Passive transfer studies in macaques support a role for the Fc region of antibodies in assisting in the prevention of simian-human immunodeficiency virus (SHIV) infection. The Thai RV144 HIV-1 vaccine trial induced anti-HIV ADCC antibodies that may have played a role in the partial protection observed. Several observational studies support a role for ADCC antibodies in slowing HIV disease progression. However, HIV evolves to escape ADCC antibodies and chronic HIV infections causes dysfunction of effector cells such as natural killer (NK) cells that mediate the ADCC functions. Further, four recent studies show that the HIV-1 Vpu protein, by promoting release of virions, reduces the capacity of ADCC antibodies to recognize HIV-infected cells. The review dissects some of the recent research on HIV-specific ADCC antibodies and discusses mechanisms to further harness ADCC antibodies in the prevention and control of HIV infection.

The HIV-1 gp120 CD4-bound conformation is preferentially targeted by antibody-dependent cellular cytotoxicity-mediating antibodies in sera from HIV-1-infected individuals

Recent studies have linked antibody Fc-mediated effector functions with protection or control of human immunodeficiency type 1 (HIV-1) and simian immunodeficiency (SIV) infections. Interestingly, the presence of antibodies with potent antibody-dependent cellular cytotoxicity (ADCC) activity in the Thai RV144 vaccine trial was suggested to correlate with decreased HIV-1 acquisition risk. These antibodies recently were found to recognize HIV envelope (Env) epitopes exposed upon Env-CD4 interaction. CD4 downregulation by Nef and Vpu, as well as Vpu-mediated BST-2 antagonism, were reported to modulate exposure of those CD4-induced HIV-1 Env epitopes and were proposed to play a role in reducing the susceptibility of infected cells to ADCC mediated by this class of antibodies. Here, we report the high prevalence of antibodies recognizing CD4-induced HIV-1 Env epitopes in sera from HIV-1-infected individuals, which correlated with their ability to mediate ADCC responses against HIV-1-infected cells, exposing these Env epitopes at the cell surface. Furthermore, our results indicate that Env variable regions V1, V2, V3, and V5 do not represent a major determinant for ADCC responses mediated by sera from HIV-1-infected individuals. Altogether, these findings suggest that HIV-1 tightly controls the exposure of certain Env epitopes at the surface of infected cells in order to prevent elimination by Fc-effector functions.

IMPORTANCE

Here, we identified a particular conformation of HIV-1 Env that is specifically targeted by ADCC-mediating antibodies present in sera from HIV-1-infected individuals. This observation suggests that HIV-1 developed sophisticated mechanisms to minimize the exposure of these epitopes at the surface of infected cells.