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

Anti-HIV-1 ADCC Antibodies following Latency Reversal and Treatment Interruption

There is growing interest in utilizing antibody-dependent cellular cytotoxicity (ADCC) to eliminate infected cells following reactivation from HIV-1 latency. A potential barrier is that HIV-1-specific ADCC antibodies decline in patients on long-term antiretroviral therapy (ART) and may not be sufficient to eliminate reactivated latently infected cells. It is not known whether reactivation from latency with latency-reversing agents (LRAs) could provide sufficient antigenic stimulus to boost HIV-1-specific ADCC. We found that treatment with the LRA panobinostat or a short analytical treatment interruption (ATI), 21 to 59 days, was not sufficient to stimulate an increase in ADCC-competent antibodies, despite viral rebound in all subjects who underwent the short ATI. In contrast, a longer ATI, 2 to 12 months, among subjects enrolled in the Strategies for Management of Antiretroviral Therapy (SMART) trial robustly boosted HIV-1 gp120-specific Fc receptor-binding antibodies and ADCC against HIV-1-infected cells in vitro These results show that there is a lag between viral recrudescence and the boosting of ADCC antibodies, which has implications for strategies toward eliminating latently infected cells.IMPORTANCE The "shock and kill" HIV-1 cure strategy aims to reactivate HIV-1 expression in latently infected cells and subsequently eliminate the reactivated cells through immune-mediated killing. Several latency reversing agents (LRAs) have been examined in vivo, but LRAs alone have not been able to achieve HIV-1 remission and prevent viral rebound following analytical treatment interruption (ATI). In this study, we examined whether LRA treatment or ATI can provide sufficient antigenic stimulus to boost HIV-1-specific functional antibodies that can eliminate HIV-1-infected cells. Our study has implications for the antigenic stimulus required for antilatency strategies and/or therapeutic vaccines to boost functional antibodies and assist in eliminating the latent reservoir.

BST-2 Expression Modulates Small CD4-Mimetic Sensitization of HIV-1-Infected Cells to Antibody-Dependent Cellular Cytotoxicity

Antibodies recognizing conserved CD4-induced (CD4i) epitopes on human immunodeficiency virus type 1 (HIV-1) Env and able to mediate antibody-dependent cellular cytotoxicity (ADCC) have been shown to be present in sera from most HIV-1-infected individuals. These antibodies preferentially recognize Env in its CD4-bound conformation. CD4 downregulation by Nef and Vpu dramatically reduces exposure of CD4i HIV-1 Env epitopes and therefore reduce the susceptibility of HIV-1-infected cells to ADCC mediated by HIV-positive (HIV+) sera. Importantly, this mechanism of immune evasion can be circumvented with small-molecule CD4 mimetics (CD4mc) that are able to transition Env into the CD4-bound conformation and sensitize HIV-1-infected cells to ADCC mediated by HIV+ sera. However, HIV-1 developed additional mechanisms to avoid ADCC, including Vpu-mediated BST-2 antagonism, which decreases the overall amount of Env present at the cell surface. Accordingly, BST-2 upregulation in response to alpha interferon (IFN-α) was shown to increase the susceptibility of HIV-1-infected cells to ADCC despite the activity of Vpu. Here we show that BST-2 upregulation by IFN-β and interleukin-27 (IL-27) also increases the surface expression of Env and thus boosts the ability of CD4mc to sensitize HIV-1-infected cells to ADCC by sera from HIV-1-infected individuals.IMPORTANCE HIV-1 evolved sophisticated strategies to conceal Env epitopes from ADCC-mediating antibodies present in HIV+ sera. Vpu-mediated BST-2 downregulation was shown to decrease ADCC responses by limiting the amount of Env present at the cell surface. This effect of Vpu was shown to be attenuated by IFN-α treatment. Here we show that in addition to IFN-α, IFN-β and IL-27 also affect Vpu-mediated BST-2 downregulation and greatly enhance ADCC responses against HIV-1-infected cells in the presence of CD4mc. These findings may inform strategies aimed at HIV prevention and eradication.

Short Communication: Small-Molecule CD4 Mimetics Sensitize HIV-1-Infected Cells to Antibody-Dependent Cellular Cytotoxicity by Antibodies Elicited by Multiple Envelope Glycoprotein Immunogens in Nonhuman Primates

Recent studies have linked antibody Fc-mediated effector functions with control of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus infections. Interestingly, the presence of antibodies with potent antibody-dependent cellular cytotoxicity (ADCC) activity in RV144 vaccine trial participants correlated inversely with HIV-1 acquisition risk. These antibodies were recently found to recognize epitopes on the HIV-1 envelope (Env) glycoprotein exposed upon Env-CD4 binding. Accordingly, small-molecule CD4 mimetics (CD4mc) that induce Env to sample the CD4-bound conformation were shown to sensitize HIV-1-infected cells to ADCC mediated by sera from HIV-1-infected individuals. However, it remains unknown whether antibodies elicited through immunization can also mediate CD4mc-induced ADCC. In this study, we tested the capacity of CD4mc to sensitize HIV-1-infected cells to ADCC by sera from Env-vaccinated nonhuman primates using a FACS-based ADCC assay. In parallel, we evaluated the ability of CD4mc to sensitize HIV-1 viral particles to neutralization by sera from these immunized animals. We found that the vaccine-induced antibodies were able to mediate ADCC and viral neutralization in the presence, but not the absence, of CD4mc. Thus, CD4mc are capable of sensitizing HIV-1-infected cells to ADCC and infectious viral particles to neutralization by easy-to-elicit antibodies that are otherwise unable to mediate these activities.

Lack of ADCC Breadth of Human Nonneutralizing Anti-HIV-1 Antibodies

Anti-human immunodeficiency virus type 1 (HIV-1) nonneutralizing antibodies (nnAbs) capable of antibody-dependent cellular cytotoxicity (ADCC) have been identified as a protective immune correlate in the RV144 vaccine efficacy trial. Broadly neutralizing antibodies (bNAbs) also mediate ADCC in cell culture and rely on their Fc region for optimal efficacy in animal models. Here, we selected 9 monoclonal nnAbs and 5 potent bNAbs targeting various epitopes and conformations of the gp120/41 complex and analyzed the potency of the two types of antibodies to bind and eliminate HIV-1-infected cells in culture. Regardless of their neutralizing activity, most of the selected antibodies recognized and killed cells infected with two laboratory-adapted HIV-1 strains. Some nnAbs also bound bystander cells that may have captured viral proteins. However, in contrast to the bNAbs, the nnAbs bound poorly to reactivated infected cells from 8 HIV-positive individuals and did not mediate effective ADCC against these cells. The nnAbs also inefficiently recognize cells infected with 8 different transmitted-founder (T/F) isolates. The addition of a synthetic CD4 mimetic enhanced the binding and killing efficacy of some of the nnAbs in an epitope-dependent manner without reaching the levels achieved by the most potent bNAbs. Overall, our data reveal important qualitative and quantitative differences between nnAbs and bNAbs in their ADCC capacity and strongly suggest that the breadth of recognition of HIV-1 by nnAbs is narrow.IMPORTANCE Most of the anti-HIV antibodies generated by infected individuals do not display potent neutralizing activities. These nonneutralizing antibodies (nnAbs) with antibody-dependent cellular cytotoxicity (ADCC) have been identified as a protective immune correlate in the RV144 vaccine efficacy trial. However, in primate models, the nnAbs do not protect against simian-human immunodeficiency virus (SHIV) acquisition. Thus, the role of nnAbs with ADCC activity in protection from infection remains debatable. In contrast, broadly neutralizing antibodies (bNAbs) neutralize a large array of viral strains and mediate ADCC in cell culture. We analyzed the capacities of 9 nnAbs and 5 bNAbs to eliminate infected cells. We selected 18 HIV-1 strains, including virus reactivated from the reservoir of HIV-positive individuals and transmitted-founder isolates. We report that the nnAbs bind poorly to cells infected with primary HIV-1 strains and do not mediate potent ADCC. Overall, our data show that the breadth of recognition of HIV-1 by nnAbs is narrow.

Influence of the Envelope gp120 Phe 43 Cavity on HIV-1 Sensitivity to Antibody-Dependent Cell-Mediated Cytotoxicity Responses

HIV-1-infected cells presenting envelope glycoproteins (Env) in the CD4-bound conformation on their surface are preferentially targeted by antibody-dependent cellular-mediated cytotoxicity (ADCC). HIV-1 has evolved sophisticated mechanisms to avoid the exposure of Env ADCC epitopes by downregulating CD4 and by limiting the overall amount of Env on the cell surface. In HIV-1, substitution of large residues such as histidine or tryptophan for serine 375 (S375H/W) in the gp120 Phe 43 cavity, where Phe 43 of CD4 contacts gp120, results in the spontaneous sampling of an Env conformation closer to the CD4-bound state. While residue S375 is well conserved in the majority of group M HIV-1 isolates, CRF01_AE strains have a naturally occurring histidine at this position (H375). Interestingly, CRF01_AE is the predominant circulating strain in Thailand, where the RV144 trial took place. In this trial, which resulted in a modest degree of protection, ADCC responses were identified as being part of the correlate of protection. Here we investigate the influence of the Phe 43 cavity on ADCC responses. Filling this cavity with a histidine or tryptophan residue in Env with a natural serine residue at this position (S375H/W) increased the susceptibility of HIV-1-infected cells to ADCC. Conversely, the replacement of His 375 by a serine residue (H375S) within HIV-1 CRF01_AE decreased the efficiency of the ADCC response. Our results raise the intriguing possibility that the presence of His 375 in the circulating strain where the RV144 trial was held contributed to the observed vaccine efficacy.IMPORTANCE HIV-1-infected cells presenting Env in the CD4-bound conformation on their surface are preferentially targeted by ADCC mediated by HIV-positive (HIV⁺) sera. Here we show that the gp120 Phe 43 cavity modulates the propensity of Env to sample this conformation and therefore affects the susceptibility of infected cells to ADCC. CRF01_AE HIV-1 strains have an unusual Phe 43 cavity-filling His 375 residue, which increases the propensity of Env to sample the CD4-bound conformation, thereby increasing susceptibility to ADCC.

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.

Broadly Neutralizing Antibodies Display Potential for Prevention of HIV-1 Infection of Mucosal Tissue Superior to That of Nonneutralizing Antibodies

Definition of the key parameters mediating effective antibody blocking of HIV-1 acquisition within mucosal tissue may prove critical to effective vaccine development and the prophylactic use of monoclonal antibodies. Although direct antibody-mediated neutralization is highly effective against cell-free virus, antibodies targeting different sites of envelope vulnerability may display differential activity against mucosal infection. Nonneutralizing antibodies (nnAbs) may also impact mucosal transmission events through Fc-gamma receptor (FcγR)-mediated inhibition. In this study, a panel of broadly neutralizing antibodies (bnAbs) and nnAbs, including those associated with protection in the RV144 vaccine trial, were screened for the ability to block HIV-1 acquisition and replication across a range of cellular and mucosal tissue models. Neutralization potency, as determined by the TZM-bl infection assay, did not fully predict activity in mucosal tissue. CD4-binding site (CD4bs)-specific bnAbs, in particular VRC01, were consistent in blocking HIV-1 infection across all cellular and tissue models. Membrane-proximal external region (MPER) (2F5) and outer domain glycan (2G12) bnAbs were also efficient in preventing infection of mucosal tissues, while the protective efficacy of bnAbs targeting V1-V2 glycans (PG9 and PG16) was more variable. In contrast, nnAbs alone and in combinations, while active in a range of cellular assays, were poorly protective against HIV-1 infection of mucosal tissues. These data suggest that tissue resident effector cell numbers and low FcγR expression may limit the potential of nnAbs to prevent establishment of the initial foci of infection. The solid protection provided by specific bnAbs clearly demonstrates their superior potential over that of nonneutralizing antibodies for preventing HIV-1 infection at the mucosal portals of infection.

IMPORTANCE Key parameters mediating effective antibody blocking of HIV-1 acquisition within mucosal tissue have not been defined. While bnAbs are highly effective against cell-free virus, they are not induced by current vaccine candidates. However, nnAbs, readily induced by vaccines, can trigger antibody-dependent cellular effector functions, through engagement of their Fc-gamma receptors. Fc-mediated antiviral activity has been implicated as a secondary correlate of decreased HIV-1 risk in the RV144 vaccine efficacy trial, suggesting that protection might be mediated in the absence of classical neutralization. To aid vaccine design and selection of antibodies for use in passive protection strategies, we assessed a range of bnAbs and nnAbs for their potential to block ex vivo challenge of mucosal tissues. Our data clearly indicate the superior efficacy of neutralizing antibodies in preventing mucosal acquisition of infection. These results underscore the importance of maintaining the central focus of HIV-1 vaccine research on the induction of potently neutralizing antibodies.

First Phase I human clinical trial of a killed whole-HIV-1 vaccine: demonstration of its safety and enhancement of anti-HIV antibody responses

Background

Vaccination with inactivated (killed) whole-virus particles has been used to prevent a wide range of viral diseases. However, for an HIV vaccine this approach has been largely negated due to inherent safety concerns, despite the ability of killed whole-virus vaccines to generate a strong, predominantly antibody-mediated immune response in vivo. HIV-1 Clade B NL4-3 was genetically modified by deleting the nef and vpu genes and substituting the coding sequence for the Env signal peptide with that of honeybee melittin signal peptide to produce a less virulent and more replication efficient virus. This genetically modified virus (gmHIV-1_NL4-3) was inactivated and formulated as a killed whole-HIV vaccine, and then used for a Phase I human clinical trial (Trial Registration: Clinical Trials NCT01546818). The gmHIV-1_NL4-3 was propagated in the A3.01 human T cell line followed by virus purification and inactivation with aldrithiol-2 and γ-irradiation. Thirty-three HIV-1 positive volunteers receiving cART were recruited for this observer-blinded, placebo-controlled Phase I human clinical trial to assess the safety and immunogenicity.

Results

Genetically modified and killed whole-HIV-1 vaccine, SAV001, was well tolerated with no serious adverse events. HIV-1_NL4-3-specific PCR showed neither evidence of vaccine virus replication in the vaccine virus-infected human T lymphocytes in vitro nor in the participating volunteers receiving SAV001 vaccine. Furthermore, SAV001 with adjuvant significantly increased the pre-existing antibody response to HIV-1 proteins. Antibodies in the plasma of vaccinees were also found to recognize HIV-1 envelope protein on the surface of infected cells as well as showing an enhancement of broadly neutralizing antibodies inhibiting tier I and II of HIV-1 B, D, and A subtypes.

Conclusion

The killed whole-HIV vaccine, SAV001, is safe and triggers anti-HIV immune responses. It remains to be determined through an appropriate trial whether this immune response prevents HIV infection.

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.

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.

Comparison of Antibody-Dependent Cell-Mediated Cytotoxicity and Virus Neutralization by HIV-1 Env-Specific Monoclonal Antibodies

Although antibodies to the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein have been studied extensively for their ability to block viral infectivity, little data are currently available on nonneutralizing functions of these antibodies, such as their ability to eliminate virus-infected cells by antibody-dependent cell-mediated cytotoxicity (ADCC). HIV-1 Env-specific antibodies of diverse specificities, including potent broadly neutralizing and nonneutralizing antibodies, were therefore tested for ADCC against cells infected with a lab-adapted HIV-1 isolate (HIV-1NL4-3), a primary HIV-1 isolate (HIV-1JR-FL), and a simian-human immunodeficiency virus (SHIV) adapted for pathogenic infection of rhesus macaques (SHIVAD8-EO). In accordance with the sensitivity of these viruses to neutralization, HIV-1NL4-3-infected cells were considerably more sensitive to ADCC, both in terms of the number of antibodies and magnitude of responses, than cells infected with HIV-1JR-FL or SHIVAD8-EO ADCC activity generally correlated with antibody binding to Env on the surfaces of virus-infected cells and with viral neutralization; however, neutralization was not always predictive of ADCC, as instances of ADCC in the absence of detectable neutralization, and vice versa, were observed. These results reveal incomplete overlap in the specificities of antibodies that mediate these antiviral activities and provide insights into the relationship between ADCC and neutralization important for the development of antibody-based vaccines and therapies for combating HIV-1 infection.

IMPORTANCE

This study provides fundamental insights into the relationship between antibody-dependent cell-mediated cytotoxicity (ADCC) and virus neutralization that may help to guide the development of antibody-based vaccines and immunotherapies for the prevention and treatment of HIV-1 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.

Nef Proteins from HIV-1 Elite Controllers Are Inefficient at Preventing Antibody-Dependent Cellular Cytotoxicity

Impairment of Nef function, including reduced CD4 downregulation, was described in a subset of HIV-1-infected individuals that control viral replication without antiretroviral treatment (elite controllers [EC]). Elimination of HIV-1-infected cells by antibody-dependent cellular cytotoxicity (ADCC) requires the presence of envelope glycoproteins (Env) in the CD4-bound conformation, raising the possibility that accumulating CD4 at the surface of virus-infected cells in EC could interact with Env and thereby sensitize these cells to ADCC. We observed a significant increase in the exposure of Env epitopes targeted by ADCC-mediating antibodies at the surface of cells expressing Nef isolates from EC; this correlated with enhanced susceptibility to ADCC. Altogether, our results suggest that enhanced susceptibility of HIV-1-infected cells to ADCC may contribute to the EC phenotype.

IMPORTANCE

Nef clones derived from elite controllers (EC) have been shown to be attenuated for CD4 downregulation; how this contributes to the nonprogressor phenotype of these infected individuals remains uncertain. Increasing evidence supports a role for HIV-specific antibody-dependent cellular cytotoxicity (ADCC) in controlling viral infection and replication. Here, we show that residual CD4 left at the surface of cells expressing Nef proteins isolated from ECs are sufficient to allow Env-CD4 interaction, leading to increased exposure of Env CD4-induced epitopes and increased susceptibility of infected cells to ADCC. Our results suggest that ADCC might be an active immune mechanism in EC that helps to maintain durable suppression of viral replication and low plasma viremia level in this rare subset of infected individuals. Therefore, targeting Nef's ability to downregulate CD4 could render HIV-1-infected cells susceptible to ADCC and thus have therapeutic utility.

Antibody-Dependent Cellular Cytotoxicity against Reactivated HIV-1-Infected Cells

Lifelong antiretroviral therapy (ART) for HIV-1 does not diminish the established latent reservoir. A possible cure approach is to reactivate the quiescent genome from latency and utilize immune responses to eliminate cells harboring reactivated HIV-1. It is not known whether antibodies within HIV-1-infected individuals can recognize and eliminate cells reactivated from latency through antibody-dependent cellular cytotoxicity (ADCC). We found that reactivation of HIV-1 expression in the latently infected ACH-2 cell line elicited antibody-mediated NK cell activation but did not result in antibody-mediated killing. The lack of CD4 expression on these HIV-1 envelope (Env)-expressing cells likely resulted in poor recognition of CD4-induced antibody epitopes on Env. To examine this further, cultured primary CD4(+) T cells from HIV-1(+) subjects were used as targets for ADCC. These ex vivo-expanded primary cells were modestly susceptible to ADCC mediated by autologous or heterologous HIV-1(+) serum antibodies. Importantly, ADCC mediated against these primary cells could be enhanced following incubation with a CD4-mimetic compound (JP-III-48) that exposes CD4-induced antibody epitopes on Env. Our studies suggest that with sufficient reactivation and expression of appropriate Env epitopes, primary HIV-1-infected cells can be targets for ADCC mediated by autologous serum antibodies and innate effector cells. The results of this study suggest that further investigation into the potential of ADCC to eliminate reactivated latently infected cells is warranted.

IMPORTANCE

An HIV-1 cure remains elusive due to the persistence of long-lived latently infected cells. An HIV-1 cure strategy, termed "shock and kill," aims to reactivate HIV-1 expression in latently infected cells and subsequently eliminate the reactivated cells through immune-mediated killing. While recent research efforts have focused on reversing HIV-1 latency, it remains unclear whether preexisting immune responses within HIV-1(+) individuals can efficiently eliminate the reactivated cells. HIV-1-specific antibodies can potentially eliminate cells reactivated from latency via Fc effector functions by recruiting innate immune cells. Our study highlights the potential role that antibody-dependent cellular cytotoxicity might play in antilatency cure approaches.