The broadly neutralizing HIV-1 IgG 2F5 elicits gp41-specific antibody-dependent cell cytotoxicity in a FcγRI-dependent manner

Roles of fragment crystallizable-mediated effector functions in broadly neutralizing antibody activity against HIV

PURPOSE OF REVIEW

'Broadly neutralizing antibodies' (bNAbs), are rare HIV-specific antibodies which exhibit the atypical ability to potently neutralize diverse viral isolates. While efforts to elicit bNAbs through vaccination have yet to succeed, recent years have seen remarkable preclinical and clinical advancements of passive immunization approaches targeting both HIV prevention and cure. We focus here on the potential to build upon this success by moving beyond neutralization to additionally harness the diverse effector functionalities available to antibodies via fragment crystallizable-effector (Fc) functions.

RECENT FINDINGS

Recent studies have leveraged the ability to engineer bNAb Fc domains to either enhance or abrogate particular effector functions to demonstrate that activities such as antibody-dependent cell-mediated cytotoxicity contribute substantially to in-vivo antiviral activity. Intriguingly, recent studies in both nonhuman primates and in humans have suggested that passive bNAb infusion can lead to durable immunity by enhancing virus-specific T-cell responses through a 'vaccinal effect'.

SUMMARY

The combination of antibody engineering strategies designed to enhance effector functions, with the broad and potent antigen recognition profile of bNAbs, has the potential to give rise to powerful new therapeutics for HIV. We aim to provide a timely review of recent advances to catalyze this development.

Identification of a CD4-binding site-directed antibody with ADCC activity from a chronic HIV-1B'-infected Chinese donor

Antibody-dependent cell-mediated cytotoxicity (ADCC) plays an important role in controlling HIV-1 invasion and replication in vivo. Isolation and identification of monoclonal antibodies (mAbs) with ADCC activity help design effective vaccines and develop novel treatment strategies. In this study, we first identified a broad neutralizer who had been infected with an HIV-1B' strain for over 10 years. Next, through probe-specific single-B-cell sorting and PCR amplification, we obtained genes for variable regions of the heavy chain (VHs) and light chain (VLs) of six antibodies and ligated them into expression vectors. After antibody expression and ELISA screening, we obtained a CD4-binding site-directed antibody (451-B4), whose VH and VL originated from the IGHV1-24 and IGLV1-40 germlines, respectively. Although 451-B4 neutralized only the SF162 tier 1 pseudovirus and 398F1 tier 2 pseudovirus, it could mediate comparable ADCC activity to a broadly neutralizing antibody, VRC01. The 451-B4 antibody will be a useful candidate for developing an ADCC-based treatment strategy against HIV-1 replication or latent infection in vivo.

Antibody-Dependent Cellular Phagocytosis of HIV-1-Infected Cells Is Efficiently Triggered by IgA Targeting HIV-1 Envelope Subunit gp41

Antibodies mediate a broad array of non-neutralizing Fc-mediated functions against HIV-1 including antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). Accordingly, ADCC and ADCP induced by anti-HIV envelope gp120 IgG have been correlated to the limited success of the HIV-1 phase III vaccine trial RV144. It remains elusive whether ADCP can also be triggered by IgA, the isotype predominant at mucosal surfaces through which HIV-1 is mainly transmitted. Yet, we have previously shown that the HIV envelope subunit gp41-specific broadly neutralizing antibody 2F5 under the IgA isotype (2F5-IgA) triggers ADCC and cooperates with 2F5-IgG to increase HIV-1-infected cell lysis. Here, we now demonstrate that 2F5-IgA, more efficiently than 2F5-IgG, induces ADCP not only of gp41-coated beads but also of primary HIV-1-infected cells in a FcαRI-dependent manner. Both primary monocytes and neutrophils can act as effector cells of 2F5-IgA-mediated ADCP, although with different kinetics with faster neutrophil phagocytosis. However, unlike for ADCC, 2F5-IgA and 2F5-IgG do not cooperate to increase ADCP. Altogether, our results reveal that gp41-specific IgA mediate the efficient phagocytosis of HIV-1-infected cells. Inducing such ADCC and ADCP-prone IgA response by vaccination in addition to anti-HIV envelope IgG, might increase the protection against HIV acquisition at mucosal level.

Activity of human serum antibodies in an influenza virus hemagglutinin stalk-based ADCC reporter assay correlates with activity in a CD107a degranulation assay

The stalk of the influenza virus hemagglutinin (HA) is an attractive target for antibody-based universal influenza virus vaccine development. While antibodies that target this part of the virus can be neutralizing, it has been shown in recent years that Fc receptor-mediated effector functions are of significant importance for the protective effect of anti-stalk antibodies. Several assays to measure Fc-Fc receptor interaction-based effector functions like antibody-dependent cellular cytotoxicity and antibody-dependent cellular phagocytosis exist, but they suffer from limitations such as low throughput and high run-to-run variability. Reporter assays for antibody-dependent cellular cytotoxicity based on reporter cells that express luciferase upon engagement of human FcγRIIIa with the Fc of antigen-bound antibodies have been developed as well. These reporter assays can be used in a higher throughput setting with limited run-to-run assay variability but since they express only one Fc receptor, their biological relevance is unclear. Here we optimized an antibody-dependent cellular cytotoxicity reporter assay to measure the activity of antibodies to the conserved stalk domain of H1 hemagglutinin. The assay was then correlated to a CD107a-based degranulation assay, and a strong and significant correlation could be observed. This data suggests that the FcγRIIIa-based reporter assay is a good substitute for functional assays, especially in settings where larger sample numbers need to be analyzed.

Knowns and Unknowns of Assaying Antibody-Dependent Cell-Mediated Cytotoxicity Against HIV-1

It is now well-accepted that Fc-mediated effector functions, including antibody-dependent cellular cytotoxicity (ADCC), can contribute to vaccine-elicited protection as well as post-infection control of HIV viremia. This picture was derived using a wide array of ADCC assays, no two of which are strictly comparable, and none of which is qualified at the clinical laboratory level. An earlier comparative study of assay protocols showed that while data from different ADCC assay formats were often correlated, they remained distinct in terms of target cells and the epitopes and antigen(s) available for recognition by antibodies, the effector cells, and the readout of cytotoxicity. This initial study warrants expanded analyses of the relationships among all current assay formats to determine where they detect overlapping activities and where they do not. Here we summarize knowns and unknowns of assaying ADCC against HIV-1.

Identification of a HIV Gp41-Specific Human Monoclonal Antibody With Potent Antibody-Dependent Cellular Cytotoxicity

Antibody-Dependent Cellular Cytotoxicity (ADCC) is a major mechanism of protection against viral infections in vivo. Identification of HIV-1-specific monoclonal antibodies (mAbs) with potent ADCC activity may help develop an effective HIV-1 vaccine. In present study, we isolated such human mAb, designated E10, from an HIV-1-infected patient sample by single B cell sorting and single cell PCR. E10 bound to gp140 trimer and linear peptides derived from gp41 membrane proximal external region (MPER). E10 epitope (QEKNEQELLEL) overlapped with mAb 2F5 epitope. However, E10 differentiated from 2F5 in neutralization breadth and potency, as well as ADCC activity. E10 showed low neutralization activity and narrow spectrum of neutralization compared to 2F5, but it mediated higher ADCC activity than 2F5 at low antibody concentration. Fine mapping of E10 epitope may potentiate MPER-based subunit vaccine development.

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

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

IgA Targeting Human Immunodeficiency Virus-1 Envelope gp41 Triggers Antibody-Dependent Cellular Cytotoxicity Cross-Clade and Cooperates with gp41-Specific IgG to Increase Cell Lysis

The protective efficacy of human immunodeficiency virus-1 (HIV-1) antibodies (Abs) remains mostly correlated with their in vitro neutralizing activity engaging their Fab region. However, anti-HIV-1 Abs also mediate a broad array of Fc-mediated effector functions including Ab-dependent cellular cytotoxicity (ADCC), which depend primarily on the Ab isotype. While ADCC is commonly associated with HIV-1 gp120 envelope-specific IgGs, whether IgAs, especially those targeting the HIV-1 gp41 envelope, also mediate ADCC remains elusive. Therefore, to assess the capacity of IgA specific for HIV-1 to induce Fcα-mediated ADCC, we used the gp41 envelope-specific IgA transformed from the broadly neutralizing 2F5-IgG we have previously reported to induce ADCC. We demonstrate that 2F5-IgA engages FcαRI (CD89), expressed on human monocytes used as effector cells, to induce the lysis of HIV-1 Clade A- and B-infected target cells by ADCC. Furthermore, the 2F5-IgA and 2F5-IgG cooperate to enhance target cells lysis by ADCC. Cooperation in ADCC is also observed between 2F5-IgA and the broadly neutralizing 10E8-IgG. These results provide a new perspective for IgA in protection against HIV-1 acquisition or reservoir eradication and suggest that inducing IgA by vaccination, in particular when targeting gp41, in combination with IgG could strengthen protection by complementary and cooperative activities with IgG.

Evolution of Neutralization Response in HIV-1 Subtype C-Infected Individuals Exhibiting Broad Cross-Clade Neutralization of HIV-1 Strains

Strain-specific neutralizing antibodies develop in all human immunodeficiency virus type 1 (HIV-1)-infected individuals. However, only 10–30% of infected individuals produce broadly neutralizing antibodies (bNAbs). Identification and characterization of these bNAbs and understanding their evolution dynamics are critical for obtaining useful clues for the development of an effective HIV vaccine. Very recently, we published a study in which we identified 12 HIV-1 subtype C-infected individuals from India whose plasma showed potent and broad cross-clade neutralization (BCN) ability (1). In the present study, we report our findings on the evolution of host bNAb response over a period of 4 years in a subset of these individuals. Three of the five individuals (NAB033, NAB059, and NAB065) demonstrated a significant increase (p < 0.05) in potency. Interestingly, two of the three samples also showed a significant increase in CD4 binding site-specific antibody response, maintained stable CD4+ T cell counts (>350 cells/mm³) and continued to remain ART-naïve for more than 10 years after initial diagnosis, implying a strong clinical correlation with the development and evolution of broadly neutralizing antibody response against HIV-1.

Conserved HIV Epitopes for an Effective HIV Vaccine

Despite major advances in antiretroviral therapy against HIV-1, an effective HIV vaccine is urgently required to reduce the number of new cases of HIV infections in the world. Vaccines are the ultimate tool in the medical arsenal to control and prevent the spread of infectious diseases such as HIV/AIDS. Several failed phase-IIb to –III clinical vaccine trials against HIV-1 in the past generated a plethora of information that could be used for better designing of an effective HIV vaccine in the future. Most of the tested vaccine candidates produced strong humoral responses against the HIV proteins; however, failed to protect due to: 1) the low levels and the narrow breadth of the HIV-1 neutralizing antibodies and the HIV-specific antibody-dependent Fc-mediated effector activities, 2) the low levels and the poor quality of the anti-HIV T-cell responses, and 3) the excessive responses to immunodominant non-protective HIV epitopes, which in some cases blocked the protective immunity and/or enhanced HIV infection. The B-cell epitopes on HIV for producing broadly neutralizing antibodies (bNAbs) against HIV have been extensively characterized, and the next step is to develop bNAb epitope immunogen for HIV vaccine. The bNAb epitopes are often conformational epitopes and therefore more difficult to construct as vaccine immunogen and likely to include immunodominant non-protective HIV epitopes. In comparison, T-cell epitopes are short linear peptides which are easier to construct into vaccine immunogen free of immunodominant non-protective epitopes. However, its difficulty lies in identifying the T-cell epitopes conserved among HIV subtypes and induce long-lasting, potent polyfunctional T-cell and cytotoxic T lymphocyte (CTL) activities against HIV. In addition, these protective T-cell epitopes must be recognized by the HLA prevalent in the country(s) targeted for the vaccine trial. In conclusion, extending from the findings from previous vaccine trials, future vaccines should combine both T- and B-cell epitopes as vaccine immunogen to induce multitude of broad and potent immune effector activities required for sterilizing protection against global HIV subtypes.

Antiviral Functions of Human Immunodeficiency Virus Type 1 (HIV-1)-Specific IgG Antibodies: Effects of Antiretroviral Therapy and Implications for Therapeutic HIV-1 Vaccine Design

Contemporary antiretroviral therapy (ART) is effective and tolerable for long periods of time but cannot eradicate human immunodeficiency virus type 1 (HIV-1) infection by either elimination of viral reservoirs or enhancement of HIV-1-specific immune responses. Boosting "protective" HIV-1-specific immune responses by active or passive immunization will therefore be necessary to control or eradicate HIV-1 infection and is currently the topic of intense investigation. Recently reported studies conducted in HIV patients and non-human primate (NHP) models of HIV-1 infection suggest that HIV-1-specific IgG antibody responses may contribute to the control of HIV-1 infection. However, production of IgG antibodies with virus neutralizing activity by vaccination remains problematic and while vaccine-induced natural killer cell-activating IgG antibodies have been shown to prevent the acquisition of HIV-1 infection, they may not be sufficient to control or eradicate established HIV-1 infection. It is, therefore, important to consider other functional characteristics of IgG antibody responses. IgG antibodies to viruses also mediate opsonophagocytic antibody responses against virions and capsids that enhance the function of phagocytic cells playing critical roles in antiviral immune responses, particularly conventional dendritic cells and plasmacytoid dendritic cells. Emerging evidence suggests that these antibody functions might contribute to the control of HIV-1 infection. In addition, IgG antibodies contribute to the intracellular degradation of viruses via binding to the cytosolic fragment crystallizable (Fc) receptor tripartite motif containing-21 (TRIM21). The functional activity of an IgG antibody response is influenced by the IgG subclass content, which affects binding to antigens and to Fcγ receptors on phagocytic cells and to TRIM21. The IgG subclass content and avidity of IgG antibodies is determined by germinal center (GC) reactions in follicles of lymphoid tissue. As HIV-1 infects cells in GCs and induces GC dysfunction, which may persist during ART, strategies for boosting HIV-1-specific IgG antibody responses should include early commencement of ART and possibly the use of particular antiretroviral drugs to optimize drug levels in lymphoid follicles. Finally, enhancing particular functions of HIV-1-specific IgG antibody responses by using adjuvants or cytokines to modulate the IgG subclass content of the antibody response might be investigated in NHP models of HIV-1 infection and during trials of therapeutic vaccines in HIV patients.

A heterologous prime-boosting strategy with replicating Vaccinia virus vectors and plant-produced HIV-1 Gag/dgp41 virus-like particles

Showing modest efficacy, the RV144 HIV-1 vaccine clinical trial utilized a non-replicating canarypox viral vector and a soluble gp120 protein boost. Here we built upon the RV144 strategy by developing a novel combination of a replicating, but highly-attenuated Vaccinia virus vector, NYVAC-KC, and plant-produced HIV-1 virus-like particles (VLPs). Both components contained the full-length Gag and a membrane anchored truncated gp41 presenting the membrane proximal external region with its conserved broadly neutralizing epitopes in the pre-fusion conformation. We tested different prime/boost combinations of these components in mice and showed that the group primed with NYVAC-KC and boosted with both the viral vectors and plant-produced VLPs have the most robust Gag-specific CD8 T cell responses, at 12.7% of CD8 T cells expressing IFN-γ in response to stimulation with five Gag epitopes. The same immunization group elicited the best systemic and mucosal antibody responses to Gag and dgp41 with a bias towards IgG1.

Immune recruitment or suppression by glycan engineering of endogenous and therapeutic antibodies

Human serum IgG contains multiple glycoforms which exhibit a range of binding properties to effector molecules such as cellular Fc receptors. Emerging knowledge of how the Fc glycans contribute to the antibody structure and effector functions has opened new avenues for the exploitation of defined antibody glycoforms in the treatment of diseases. Here, we review the structure and activity of antibody glycoforms and highlight developments in antibody glycoengineering by both the manipulation of the cellular glycosylation machinery and by chemoenzymatic synthesis. We discuss wide ranging applications of antibody glycoengineering in the treatment of cancer, autoimmunity and inflammation. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Immunological Characterization of Plant-Based HIV-1 Gag/Dgp41 Virus-Like Particles

It is widely anticipated that a prophylactic vaccine may be needed to control the HIV/AIDS epidemic worldwide. Despite over two decades of research, a vaccine against HIV-1 remains elusive, although a recent clinical trial has shown promising results. Recent studies have focused on highly conserved domains within HIV-1 such as the membrane proximal external region (MPER) of the envelope glycoprotein, gp41. MPER has been shown to play critical roles in mucosal transmission of HIV-1, though this peptide is poorly immunogenic on its own. Here we provide evidence that plant-produced HIV-1 enveloped virus-like particles (VLPs) consisting of Gag and a deconstructed form of gp41 comprising the MPER, transmembrane, and cytoplasmic domains (Dgp41) provides an effective platform to display MPER for use as an HIV vaccine candidate. Prime-boost strategies combining systemic and mucosal priming with systemic boosting using two different vaccine candidates (VLPs and CTB-MPR--a fusion of MPER and the B-subunit of cholera toxin) were investigated in BALB/c mice. Serum antibody responses against both the Gag and gp41 antigens were elicited when systemically primed with VLPs. These responses could be recalled following systemic boosting with VLPs. In addition, mucosal priming with VLPs allowed for a boosting response against Gag and gp41 when boosted with either candidate. Importantly, the VLPs also induced Gag-specific CD4 and CD8 T-cell responses. This report on the immunogenicity of plant-based Gag/Dgp41 VLPs may represent an important milestone on the road towards a broadly efficacious and inexpensive subunit vaccine against HIV-1.

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.

A High Throughput Protein Microarray Approach to Classify HIV Monoclonal Antibodies and Variant Antigens

In recent years, high throughput discovery of human recombinant monoclonal antibodies (mAbs) has been applied to greatly advance our understanding of the specificity, and functional activity of antibodies against HIV. Thousands of antibodies have been generated and screened in functional neutralization assays, and antibodies associated with cross-strain neutralization and passive protection in primates, have been identified. To facilitate this type of discovery, a high throughput-screening tool is needed to accurately classify mAbs, and their antigen targets. In this study, we analyzed and evaluated a prototype microarray chip comprised of the HIV-1 recombinant proteins gp140, gp120, gp41, and several membrane proximal external region peptides. The protein microarray analysis of 11 HIV-1 envelope-specific mAbs revealed diverse binding affinities and specificities across clades. Half maximal effective concentrations, generated by our chip analysis, correlated significantly (P<0.0001) with concentrations from ELISA binding measurements. Polyclonal immune responses in plasma samples from HIV-1 infected subjects exhibited different binding patterns, and reactivity against printed proteins. Examining the totality of the specificity of the humoral response in this way reveals the exquisite diversity, and specificity of the humoral response to HIV.

Calcitonin gene-related peptide inhibits human immunodeficiency type 1 transmission by Langerhans cells via an autocrine/paracrine feedback mechanism

AIM

Peripheral neurones innervating mucosal epithelia are in direct contact with resident immune cells, including Langerhans cells (LCs). Such neurones secrete the neuropeptide calcitonin gene-related peptide (CGRP) that modulates LCs function. We recently found that CGRP strongly inhibits human immunodeficiency virus type 1 (HIV-1) transmission, by interfering with multiple steps of mucosal LC-mediated HIV-1 transfer, including increased expression of the LC-specific lectin langerin. Herein, we investigated the anti-HIV-1 mechanism of CGRP.

METHODS

In the presence of CGRP, HIV-1 transfer from LCs to CD4+ T cells was tested with viral clones using either the HIV-1 co-receptor CCR5 (R5) or CXCR4 (X4). Surface expression of CCR5, CXCR4 and langerin was evaluated by flow cytometry. CGRP secretion by LCs was measured with an enzyme immunoassay. Expression of the multimeric CGRP receptor was examined by quantitative real-time RT-PCR and immuno-fluorescent microscopy.

RESULTS

Calcitonin gene-related peptide decreased transfer of HIV-1 R5, but increased that of X4. These opposing effects correlated with decreased CCR5 vs. increased CXCR4 surface expression in LCs. Inhibition of HIV-1 R5 transfer by CGRP involved signal transducer and activator of transcription 4 (STAT4) activation. Both αCGRP and βCGRP were similarly efficient in decreasing HIV-1 R5 transfer and increasing langerin expression. LCs secreted low basal levels of endogenous CGRP, which increased markedly following CGRP treatment. CGRP also increased expression of its cognate receptor in LCs.

CONCLUSION

CGRP engages a positive feedback mechanism that would further enhance its anti-HIV-1 activity. This information might be relevant for the therapeutic use of CGRP as a prophylactic agent against HIV-1.

Epitope target structures of Fc-mediated effector function during HIV-1 acquisition

PURPOSE OF REVIEW

This review analyzes recent studies suggesting that highly conserved epitopes in the HIV-1 Env trimer are targets of potentially protective nonneutralizing antibodies that mediate antibody-dependent cellular cytotoxicity.

RECENT FINDINGS

Recent studies in both non-human primates and humans suggest that nonneutralizing antibodies play a role in blocking infection with hybrid simian HIV (SHIV)/simian immunodeficiency virus (SIV) or HIV-1 by Fc-mediated effector function, in particular antibody-dependent cellular cytotoxicity. Further, several studies implicate highly conserved epitopes in the C1 region of gp120 as targets of these antibodies. However, these suggestions are controversial, as passive immunization studies do not indicate that such antibodies can block acquisition in non-human primates. Potential reasons for this discrepancy are discussed in the structural context of potent antibody-dependent cellular cytotoxicity epitopes on target cells during the narrow window of opportunity when antibodies can block HIV-1 acquisition.

SUMMARY

Cumulative evidence suggests that, in addition to virus neutralization, Fc-mediated effector responses to highly conserved epitopes in the HIV-1 trimer play distinct as well as overlapping roles in blocking HIV-1 acquisition. Evidence will be discussed as to whether nonneutralizing antibodies specific for epitopes on the HIV-1 Env trimer that become exposed during viral entry contribute significantly to blocking HIV-1 acquisition.

Identification of antibody glycosylation structures that predict monoclonal antibody Fc-effector function

OBJECTIVE

To determine monoclonal antibody (mAb) features that predict fragment crystalizable (Fc)-mediated effector functions against HIV.

DESIGN

Monoclonal antibodies, derived from Chinese hamster ovary cells or Epstein-Barr virus-immortalized mouse heteromyelomas, with specificity to key regions of the HIV envelope including gp120-V2, gp120-V3 loop, gp120-CD4(+) binding site, and gp41-specific antibodies, were functionally profiled to determine the relative contribution of the variable and constant domain features of the antibodies in driving robust Fc-effector functions.

METHODS

Each mAb was assayed for antibody-binding affinity to gp140(SR162), antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) and for the ability to bind to FcγRIIa, FcγRIIb and FcγRIIIa receptors. Antibody glycan profiles were determined by HPLC.

RESULTS

Neither the specificity nor the affinity of the mAbs determined the potency of Fc-effector function. FcγRIIIa binding strongly predicted ADCC and decreased galactose content inversely correlated with ADCP, whereas N-glycolylneuraminic acid-containing structures exhibited enhanced ADCP. Additionally, the bi-antenary glycan arm onto which galactose was added predicted enhanced binding to FcγRIIIa and ADCC activity, independent of the specificity of the mAb.

CONCLUSIONS

Our studies point to the specific Fc-glycan structures that can selectively promote Fc-effector functions independently of the antibody specificity. Furthermore, we demonstrated antibody glycan structures associated with enhanced ADCP activity, an emerging Fc-effector function that may aid in the control and clearance of HIV infection.

Recombinant expression, purification, and biophysical characterization of the transmembrane and membrane proximal domains of HIV-1 gp41

The transmembrane subunit (gp41) of the envelope glycoprotein of HIV-1 associates noncovalently with the surface subunit (gp120) and together they play essential roles in viral mucosal transmission and infection of target cells. The membrane proximal region (MPR) of gp41 is highly conserved and contains epitopes of broadly neutralizing antibodies. The transmembrane (TM) domain of gp41 not only anchors the envelope glycoprotein complex in the viral membrane but also dynamically affects the interactions of the MPR with the membrane. While high-resolution X-ray structures of some segments of the MPR were solved in the past, they represent the post-fusion forms. Structural information on the TM domain of gp41 is scant and at low resolution. Here we describe the design, expression and purification of a protein construct that includes MPR and the transmembrane domain of gp41 (MPR-TMTEV-6His), which reacts with the broadly neutralizing antibodies 2F5 and 4E10 and thereby may represent an immunologically relevant conformation mimicking a prehairpin intermediate of gp41. The expression level of MPR-TMTEV-6His was improved by fusion to the C-terminus of Mistic protein, yielding ∼ 1 mg of pure protein per liter. The isolated MPR-TMTEV-6His protein was biophysically characterized and is a monodisperse candidate for crystallization. This work will enable further investigation into the structure of MPR-TMTEV-6His, which will be important for the structure-based design of a mucosal vaccine against HIV-1.

Low antibody-dependent cellular cytotoxicity responses in Zambians prior to HIV-1 intrasubtype C superinfection

We have previously shown that HIV-1 superinfected Zambian seroconverters mount low binding and neutralizing antibody responses to their primary HIV-1 infecting virus, which could increase susceptibility to re-infection. Here, we investigated if antibody-dependent cellular cytotoxicity (ADCC), a process by which virus-infected cells are killed, was also reduced. Superinfected individuals exhibited low ADCC activity compared to non-superinfected individuals, but similar levels of CMV-reactive binding antibodies, suggesting superinfected individuals are capable of generating and maintaining virus-specific antibodies.

Nonneutralizing functional antibodies: a new "old" paradigm for HIV vaccines

Animal and human data from various viral infections and vaccine studies suggest that nonneutralizing antibodies (nNAb) without neutralizing activity in vitro may play an important role in protection against viral infection in vivo. This was illustrated by the recent human immunodeficiency virus (HIV) RV144 vaccine efficacy trial, which demonstrated that HIV-specific IgG-mediated nNAb directed against the V2 loop of HIV type 1 envelope (Env) were inversely correlated with risk for HIV acquisition, while Env-specific plasma IgA-mediated antibodies were directly correlated with risk. However, tier 1 NAb in the subset of responders with a low level of plasma Env-specific IgA correlated with decreased risk. Nonhuman primate simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus (SHIV) challenge studies suggest that Env-mediated antibodies are essential and sufficient for protection. A comparison of immune responses generated in human efficacy trials reveals subtle differences in the fine specificities of the antibody responses, in particular in HIV-specific IgG subclasses. The underlying mechanisms that may have contributed to protection against HIV acquisition in humans, although not fully understood, are possibly mediated by antibody-dependent cell-mediated cytotoxicity (ADCC) and/or other nonneutralizing humoral effector functions, such as antibody-mediated phagocytosis. The presence of such functional nNAb in mucosal tissues and cervico-vaginal and rectal secretions challenges the paradigm that NAb are the predominant immune response conferring protection, although this does not negate the desirability of evoking neutralizing antibodies through vaccination. Instead, NAb and nNAb should be looked upon as complementary or synergistic humoral effector functions. Several HIV vaccine clinical trials to study these antibody responses in various prime-boost modalities in the systemic and mucosal compartments are ongoing. The induction of high-frequency HIV-specific functional nNAb at high titers may represent an attractive hypothesis-testing strategy in future HIV vaccine efficacy trials.

Anti-MPER antibodies with heterogeneous neutralization capacity are detectable in most untreated HIV-1 infected individuals

Background

The MPER region of the HIV-1 envelope glycoprotein gp41 is targeted by broadly neutralizing antibodies. However, the localization of this epitope in a hydrophobic environment seems to hamper the elicitation of these antibodies in HIV infected individuals.

We have quantified and characterized anti-MPER antibodies by ELISA and by flow cytometry using a collection of mini gp41-derived proteins expressed on the surface of 293T cells. Longitudinal plasma samples from 35 HIV-1 infected individuals were assayed for MPER recognition and MPER-dependent neutralizing capacity using HIV-2 viruses engrafted with HIV-1 MPER sequences.

Results

Miniproteins devoid of the cysteine loop of gp41 exposed the MPER on 293T cell membrane. Anti-MPER antibodies were identified in most individuals and were stable when analyzed in longitudinal samples. The magnitude of the responses was strongly correlated with the global response to the HIV-1 envelope glycoprotein, suggesting no specific limitation for anti-MPER antibodies. Peptide mapping showed poor recognition of the C-terminal MPER moiety and a wide presence of antibodies against the 2F5 epitope. However, antibody titers failed to correlate with 2F5-blocking activity and, more importantly, with the specific neutralization of HIV-2 chimeric viruses bearing the HIV-1 MPER sequence; suggesting a strong functional heterogeneity in anti-MPER humoral responses.

Conclusions

Anti-MPER antibodies can be detected in the vast majority of HIV-1 infected individuals and are generated in the context of the global anti-Env response. However, the neutralizing capacity is heterogeneous suggesting that eliciting neutralizing anti-MPER antibodies by immunization might require refinement of immunogens to skip nonneutralizing responses.

Epitope specificity of human immunodeficiency virus-1 antibody dependent cellular cytotoxicity [ADCC] responses

Antibody dependent cellular cytotoxicity [ADCC] has been suggested to play an important role in control of Human Immunodeficiency Virus-1 [HIV-1] viral load and protection from infection. ADCC antibody responses have been mapped to multiple linear and conformational epitopes within the HIV-1 envelope glycoproteins gp120 and gp41. Many epitopes targeted by antibodies that mediate ADCC overlap with those recognized by antibodies capable of virus neutralization. In addition, recent studies conducted with human monoclonal antibodies derived from HIV-1 infected individuals and HIV-1 vaccine-candidate vaccinees have identified a number of antibodies that lack the ability to capture primary HIV-1 isolates or mediate neutralizing activity, but are able to bind to the surface of infected CD4+ T cells and mediate ADCC. Of note, the conformational changes in the gp120 that may not exclusively relate to binding of the CD4 molecule are important in exposing epitopes recognized by ADCC responses. Here we discuss the HIV-1 envelope epitopes targeted by ADCC antibodies in the context of the potential protective capacities of ADCC.

Neutralizing IgG at the portal of infection mediates protection against vaginal simian/human immunodeficiency virus challenge

Neutralizing antibodies may have critical importance in immunity against human immunodeficiency virus type 1 (HIV-1) infection. However, the amount of protective antibody needed at mucosal surfaces has not been fully established. Here, we evaluated systemic and mucosal pharmacokinetics (PK) and pharmacodynamics (PD) of 2F5 IgG and 2F5 Fab fragments with respect to protection against vaginal challenge with simian-human immunodeficiency virus-BaL in macaques. Antibody assessment demonstrated that 2F5 IgG was more potent than polymeric forms (IgM and IgA) across a range of cellular and tissue models. Vaginal challenge studies demonstrated a dose-dependent protection for 2F5 IgG and no protection with 2F5 Fab despite higher vaginal Fab levels at the time of challenge. Animals receiving 50 or 25 mg/kg of body weight 2F5 IgG were completely protected, while 3/5 animals receiving 5 mg/kg were protected. In the control animals, infection was established by a minimum of 1 to 4 transmitted/founder (T/F) variants, similar to natural human infection by this mucosal route; in the two infected animals that had received 5 mg 2F5 IgG, infection was established by a single T/F variant. Serum levels of 2F5 IgG were more predictive of sterilizing protection than measured vaginal levels. Fc-mediated antiviral activity did not appear to influence infection of primary target cells in cervical explants. However, PK studies highlighted the importance of the Fc portion in tissue biodistribution. Data presented in this study may be important in modeling serum levels of neutralizing antibodies that need to be achieved by either vaccination or passive infusion to prevent mucosal acquisition of HIV-1 infection in humans.

Progesterone and a phospholipase inhibitor increase the endosomal bis(monoacylglycero)phosphate content and block HIV viral particle intercellular transmission

Progesterone, the cationic amphiphile U18666A and a phospholipase inhibitor (Methyl Arachidonyl Fluoro Phosphonate, MAFP) inhibited by 70%-90% HIV production in viral reservoir cells, i.e. human THP-1 monocytes and monocyte-derived macrophages (MDM). These compounds triggered an inhibition of fluid phase endocytosis (macropinocytosis) and modified cellular lipid homeostasis since endosomes accumulated filipin-stained sterols and Bis(Monoacylglycero)Phosphate (BMP). BMP was quantified using a new cytometry procedure and was increased by 1.25 times with MAFP, 1.7 times with U18666A and 2.5 times with progesterone. MAFP but not progesterone or U18666A inhibited the hydrolysis of BMP by the Pancreatic Lipase Related Protein 2 (PLRP2) as shown by in-vitro experiments. The possible role of sterol transporters in steroid-mediated BMP increase is discussed. Electron microscopy showed the accumulation of viral particles either into large intracellular viral-containing compartments or outside the cells, indicating that endosomal accumulation of BMP could block intracellular biogenesis of viral particles while inhibition of macropinocytosis would prevent viral particle uptake. This is the first report linking BMP metabolism with a natural steroid such as progesterone or with involvement of a phospholipase A1 activity. BMP cellular content could be used as a biomarker for efficient anti-viral drugs.

A nonparametric procedure for defining a new humoral immunologic profile in a pilot study on HIV infected patients

This work aims at identifying a set of humoral immunologic parameters that improve prediction of the activation process in HIV patients. Starting from the well-known impact of humoral immunity in HIV infection, there is still a lack of knowledge in defining the role of the modulation of functional activity and titers of serum antibodies from early stage of infection to the development of AIDS. We propose an integrated approach that combines humoral and clinical parameters in defining the host immunity, implementing algorithms associated with virus control. A number of humoral parameters were simultaneously evaluated in a whole range of serum samples from HIV-positive patients. This issue has been afforded accounting for estimation problems typically related to "feasibility" studies where small sample size in each group and large number of parameters are jointly estimated. We used nonparametric statistical procedures to identify biomarkers in our study which included 42 subjects stratified on five different stages of HIV infection, i.e., Elite Controllers (EC), Long Term Non Progressors (LTNP), HAART, AIDS and Acute Infection (AI). The main goal of the paper is to illustrate a novel profiling method for helping to design a further confirmatory study. A set of seventeen different HIV-specific blood humoral factors were analyzed in all subjects, i.e. IgG and IgA to gp120IIIB, to gp120Bal, to whole gp41, to P1 and T20 gp41 epitopes of the MPER-HR2 region, to QARILAV gp41 epitope of the HR1 region and to CCR5; neutralization activity against five different virus strains and ADCC were also evaluated. Patients were selected on the basis of CD4 cell counts, HIV/RNA and clinical status. The Classification and Regression Trees (CART) approach has been used to uncover specific patterns of humoral parameters in different stages of HIV disease. Virus neutralization of primary virus strains and antibodies to gp41 were required to classify patients, suggesting that clinical profiles strongly rely on functional activity against HIV.

Closing the door to human immunodeficiency virus

The pandemic of human immunodeficiency virus type one (HIV-1), the major etiologic agent of acquired immunodeficiency disease (AIDS), has led to over 33 million people living with the virus, among which 18 million are women and children. Until now, there is neither an effective vaccine nor a therapeutic cure despite over 30 years of efforts. Although the Thai RV144 vaccine trial has demonstrated an efficacy of 31.2%, an effective vaccine will likely rely on a breakthrough discovery of immunogens to elicit broadly reactive neutralizing antibodies, which may take years to achieve. Therefore, there is an urgency of exploring other prophylactic strategies. Recently, antiretroviral treatment as prevention is an exciting area of progress in HIV-1 research. Although effective, the implementation of such strategy faces great financial, political and social challenges in heavily affected regions such as developing countries where drug resistant viruses have already been found with growing incidence. Activating latently infected cells for therapeutic cure is another area of challenge. Since it is greatly difficult to eradicate HIV-1 after the establishment of viral latency, it is necessary to investigate strategies that may close the door to HIV-1. Here, we review studies on non-vaccine strategies in targeting viral entry, which may have critical implications for HIV-1 prevention.

Enhanced phagocytic activity of HIV-specific antibodies correlates with natural production of immunoglobulins with skewed affinity for FcγR2a and FcγR2b

While development of an HIV vaccine that can induce neutralizing antibodies remains a priority, decades of research have proven that this is a daunting task. However, accumulating evidence suggests that antibodies with the capacity to harness innate immunity may provide some protection. While significant research has focused on the cytolytic properties of antibodies in acquisition and control, less is known about the role of additional effector functions. In this study, we investigated antibody-dependent phagocytosis of HIV immune complexes, and we observed significant differences in the ability of antibodies from infected subjects to mediate this critical effector function. We observed both quantitative differences in the capacity of antibodies to drive phagocytosis and qualitative differences in their FcγR usage profile. We demonstrate that antibodies from controllers and untreated progressors exhibit increased phagocytic activity, altered Fc domain glycosylation, and skewed interactions with FcγR2a and FcγR2b in both bulk plasma and HIV-specific IgG. While increased phagocytic activity may directly influence immune activation via clearance of inflammatory immune complexes, it is also plausible that Fc receptor usage patterns may regulate the immune response by modulating downstream signals following phagocytosis--driving passive degradation of internalized virus, release of immune modulating cytokines and chemokines, or priming of a more effective adaptive immune response.

Anti-HIV-1 antibody-dependent cellular cytotoxicity mediated by hyperimmune bovine colostrum IgG

Antibodies with antibody-dependent cellular cytotoxicity (ADCC) activity play an important role in protection against HIV-1 infection, but generating sufficient amounts of antibodies to study their protective efficacy is difficult. HIV-specific IgG can be easily and inexpensively produced in large quantities using bovine colostrum. We previously vaccinated cows with HIV-1 envelope gp140 and elicited high titers of anti-gp140-binding IgG in colostrum. In the present study, we determined whether bovine antibodies would also demonstrate specific cytotoxic activity. We found that bovine IgG bind to Fcγ-receptors (FcγRs) on human neutrophils, monocytes, and NK cells in a dose-dependent manner. Antibody-dependent killing was observed in the presence of anti-HIV-1 colostrum IgG but not nonimmune colostrum IgG. Killing was dependent on Fc and FcγR interaction since ADDC activity was not seen with F(ab')(2) fragments. ADCC activity was primarily mediated by CD14(+) monocytes with FcγRIIa (CD32a) as the major receptor responsible for monocyte-mediated ADCC in response to bovine IgG. In conclusion, we demonstrate that bovine anti-HIV colostrum IgG have robust HIV-1-specific ADCC activity and therefore offer a useful source of antibodies able to provide a rapid and potent response against HIV-1 infection. This could assist the development of novel Ab-mediated approaches for prevention of HIV-1 transmission.

Isotype modulates epitope specificity, affinity, and antiviral activities of anti-HIV-1 human broadly neutralizing 2F5 antibody

The constant heavy chain (CH1) domain affects antibody affinity and fine specificity, challenging the paradigm that only variable regions contribute to antigen binding. To investigate the role of the CH1 domain, we constructed IgA2 from the broadly neutralizing anti-HIV-1 2F5 IgG1, and compared 2F5 IgA2 and IgG binding affinity and functional activities. We found that 2F5 IgA2 bound to the gp41 membrane proximal external region with higher affinity than IgG1. Functionally, compared with IgG1, 2F5 IgA2 more efficiently blocked HIV-1 transcytosis across epithelial cells and CD4(+) cell infection by R5 HIV-1. The 2F5 IgG1 and IgA2 acted synergistically to fully block HIV-1 transfer from Langerhans to autologous CD4(+) T cells and to inhibit CD4(+) T-cell infection. Epitope mapping performed by screening a random peptide library and in silico docking modeling suggested that along with the 2F5 IgG canonical ELDKWA epitope on gp41, the IgG1 recognized an additional 3D-conformational epitope on the gp41 C-helix. In contrast, the IgA2 epitope included a unique conformational motif on the gp41 N-helix. Overall, the CH1 region of 2F5 contributes to shape its epitope specificity, antibody affinity, and functional activities. In the context of sexually transmitted infections such as HIV-1/AIDS, raising a mucosal IgA-based vaccine response should complement an IgG-based vaccine response in blocking HIV-1 transmission.

Antibody-dependent cellular cytotoxicity against primary HIV-infected CD4+ T cells is directly associated with the magnitude of surface IgG binding

Antibody (Ab)-dependent cellular cytotoxicity (ADCC) is thought to potentially play a role in vaccine-induced protection from HIV-1. The characteristics of such antibodies remain incompletely understood. Furthermore, correlates between ADCC and HIV-1 immune status are not clearly defined. We screened the sera of 20 HIV-1-positive (HIV-1(+)) patients for ADCC. Normal human peripheral blood mononuclear cells were used to derive HIV-infected CD4(+) T cell targets and autologous, freshly isolated, natural killer (NK) cells in a novel assay that measures granzyme B (GrB) and HIV-1-infected CD4(+) T cell elimination (ICE) by flow cytometry. We observed that complex sera mediated greater levels of ADCC than anti-HIV-1 envelope glycoprotein (Env)-specific monoclonal antibodies and serum-mediated ADCC correlated with the amount of IgG and IgG1 bound to HIV-1-infected CD4(+) T cells. No correlation between ADCC and viral load, CD4(+) T cell count, or neutralization of HIV-1(SF162) or other primary viral isolates was detected. Sera pooled from clade B HIV-1(+) individuals exhibited breadth in killing targets infected with HIV-1 from clades A/E, B, and C. Taken together, these data suggest that the total amount of IgG bound to an HIV-1-infected cell is an important determinant of ADCC and that polyvalent antigen-specific Abs are required for a robust ADCC response. In addition, Abs elicited by a vaccine formulated with immunogens from a single clade may generate a protective ADCC response in vivo against a variety of HIV-1 species. Increased understanding of the parameters that dictate ADCC against HIV-1-infected cells will inform efforts to stimulate ADCC activity and improve its potency in vaccinees.

Virus like particle based strategy to elicit HIV-protective antibodies to the alpha-helic regions of gp41

Natural antibodies to gp41 inhibit HIV-1 replication through the recognition of two different regions, corresponding to the leucine zipper motif in the HR1 alpha-helix and to another motif within HR2 region, hosting 2F5 and 4E10 epitope. This study aimed at reproducing such protective responses through VLP vaccination. Six regions covering the alpha-helical regions of gp41 were conjugated to the surface of AP205 phage-based VLPs. Once administered in mice via systemic or mucosal route, these immunogens elicited high titers of gp41-specific IgG. Immunogenicity and HIV infectivity reduction were obtained either with HR2 regions or with peptides where aminoacid strings were added to either the C-terminus or N-terminus of core epitope in HR1 region. Antibody-dependent cell cytotoxicity (ADCC) activity was induced by one of the HR2 epitopes only. These results may have relevant implications for the development of new vaccinal approaches against HIV infection.

A nonfucosylated variant of the anti-HIV-1 monoclonal antibody b12 has enhanced FcγRIIIa-mediated antiviral activity in vitro but does not improve protection against mucosal SHIV challenge in macaques

Eliciting neutralizing antibodies is thought to be a key activity of a vaccine against human immunodeficiency virus (HIV). However, a number of studies have suggested that in addition to neutralization, interaction of IgG with Fc gamma receptors (FcγR) may play an important role in antibody-mediated protection. We have previously obtained evidence that the protective activity of the broadly neutralizing human IgG1 anti-HIV monoclonal antibody (MAb) b12 in macaques is diminished in the absence of FcγR binding capacity. To investigate antibody-dependent cellular cytotoxicity (ADCC) as a contributor to FcγR-associated protection, we developed a nonfucosylated variant of b12 (NFb12). We showed that, compared to fully fucosylated (referred to as wild-type in the text) b12, NFb12 had higher affinity for human and rhesus macaque FcγRIIIa and was more efficient in inhibiting viral replication and more effective in killing HIV-infected cells in an ADCC assay. Despite these more potent in vitro antiviral activities, NFb12 did not enhance protection in vivo against repeated low-dose vaginal challenge in the simian-human immunodeficiency virus (SHIV)/macaque model compared to wild-type b12. No difference in protection, viral load, or infection susceptibility was observed between animals given NFb12 and those given fully fucosylated b12, indicating that FcγR-mediated activities distinct from FcγRIIIa-mediated ADCC may be important in the observed protection against SHIV challenge.

Neutralizing antibodies and control of HIV: moves and countermoves

It is now evident that powerful antibodies directed to conserved regions of HIV-1 envelope protein develop during chronic infection in some individuals and that these antibodies can neutralize a broad array of diverse isolates in vitro, so termed broadly neutralizing antibodies (bNAbs). A great deal of effort is directed internationally at understanding the ontogeny of NAbs during infection as well as in designing and testing immunogens that can elicit bNAbs in animal models and in humans. Given the parrying tactics of Env, multiple approaches, along with high-resolution structural studies, will be needed to reach a degree of understanding sufficient to design an effective vaccine. We discuss and note here some of the most important recent advances in our knowledge of how neutralizing antibodies develop in vivo, the recent discovery of extremely powerful neutralizing monoclonal antibodies isolated from natural infection, enhanced methodologies that have accelerated discoveries on both fronts, and the progress made in eliciting potent NAbs with limited breadth by vaccination.

A monoclonal antibody directed against a conformational epitope of the HIV-1 trans-activator (Tat) protein neutralizes cross-clade

The identification of a neutralizing mAb against extracellular HIV-1 transactivator of transcription (Tat) is important for the development of an efficient HIV-1 treatment. Tat plays an essential role in HIV-1 pathogenesis, not only for HIV-1 replication but also as an extracellular toxin able to disrupt the immune system. We showed previously that immunization of rabbits with Tat Oyi, a variant cloned from an African woman who did not develop AIDS following HIV-1 infection, raised antibodies able to recognize different Tat variants. We carried out mice immunization with Tat Oyi and selected a mAb named 7G12, which had the capacity to cross-recognize heterologous Tat variants by a common three-dimensional epitope. These results highlighted that Tat variants were able to acquire a structure, in contrast to a number of studies showing Tat as an unfolded protein. mAb 7G12 also had the capacity to neutralize the biological activities of these Tat variants by blocking the cellular uptake of extracellular Tat. This is the first study using Tat Oyi to produce a mAb able to neutralize effectively activities of extracellular Tats from different HIV-1 subtypes. This mAb has an important potential in therapeutic passive immunization and could help HIV-1 infected patients to restore their immunity.