Fc receptor-mediated phagocytosis in tissues as a potent mechanism for preventive and therapeutic HIV vaccine strategies

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

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

Platform-Specific Fc N-Glycan Profiles of an Antisperm Antibody

IgG Fc N-glycosylation is necessary for effector functions and is an important component of quality control. The choice of antibody manufacturing platform has the potential to significantly influence the Fc glycans of an antibody and consequently alter their activity and clinical profile. The Human Contraception Antibody (HCA) is an IgG1 antisperm monoclonal antibody (mAb) currently in clinical development as a novel, non-hormonal contraceptive. Part of its development is selecting a suitable expression platform to manufacture HCA for use in the female reproductive tract. Here, we compared the Fc glycosylation of HCA produced in two novel mAb manufacturing platforms, namely transgenic tobacco plants (Nicotiana benthamiana; HCA-N) and mRNA-mediated expression in human vaginal cells (HCAmRNA). The Fc N-glycan profiles of the two HCA products were determined using mass spectrometry. Major differences in site occupancy, glycan types, and glycoform distributions were revealed. To address how these differences affect Fc function, antibody-dependent cellular phagocytosis (ADCP) assays were performed. The level of sperm phagocytosis was significantly lower in the presence of HCA-N than HCAmRNA. This study provides evidence that the two HCA manufacturing platforms produce functionally distinct HCAs; this information could be useful for the selection of an optimal platform for HCA clinical development and for mAbs in general.

Antibody surface mobility amplifies FcγR signaling via Arp2/3 during phagocytosis

We report herein that the anti-CD20 therapeutic antibody, rituximab, is rearranged into microclusters within the phagocytic synapse by macrophage Fcγ receptors (FcγR) during antibody-dependent cellular phagocytosis. These microclusters were observed to potently recruit Syk and to undergo rearrangements that were limited by the cytoskeleton of the target cell, with depolymerization of target-cell actin filaments leading to modest increases in phagocytic efficiency. Total internal reflection fluorescence analysis revealed that FcγR total phosphorylation, Syk phosphorylation, and Syk recruitment were enhanced when IgG-FcγR microclustering was enabled on fluid bilayers relative to immobile bilayers in a process that required Arp2/3. We conclude that on fluid surfaces, IgG-FcγR microclustering promotes signaling through Syk that is amplified by Arp2/3-driven actin rearrangements. Thus, the surface mobility of antigens bound by IgG shapes the signaling of FcγR with an unrecognized complexity beyond the zipper and trigger models of phagocytosis.

Antibody Fc-chimerism and effector functions: When IgG takes advantage of IgA

Recent advances in the development of therapeutic antibodies (Abs) have greatly improved the treatment of otherwise drug-resistant cancers and autoimmune diseases. Antibody activities are mediated by both their Fab and the Fc. However, therapeutic Abs base their protective mechanisms on Fc-mediated effector functions resulting in the activation of innate immune cells by FcRs. Therefore, Fc-bioengineering has been widely used to maximise the efficacy and convenience of therapeutic antibodies. Today, IgG remains the only commercially available therapeutic Abs, at the expense of other isotypes. Indeed, production, sampling, analysis and related in vivo studies are easier to perform with IgG than with IgA due to well-developed tools. However, interest in IgA is growing, despite a shorter serum half-life and a more difficult sampling and purification methods than IgG. Indeed, the paradigm that the effector functions of IgG surpass those of IgA has been experimentally challenged. Firstly, IgA has been shown to bind to its Fc receptor (FcR) on effector cells of innate immunity with greater efficiency than IgG, resulting in more robust IgA-mediated effector functions in vitro and better survival of treated animals. In addition, the two isotypes have been shown to act synergistically. From these results, new therapeutic formats of Abs are currently emerging, in particular chimeric Abs containing two tandemly expressed Fc, one from IgG (Fcγ) and one from IgA (Fcα). By binding both FcγR and FcαR on effector cells, these new chimeras showed improved effector functions in vitro that were translated in vivo. Furthermore, these chimeras retain an IgG-like half-life in the blood, which could improve Ab-based therapies, including in AIDS. This review provides the rationale, based on the biology of IgA and IgG, for the development of Fcγ and Fcα chimeras as therapeutic Abs, offering promising opportunities for HIV-1 infected patients. We will first describe the main features of the IgA- and IgG-specific Fc-mediated signalling pathways and their respective functional differences. We will then summarise the very promising results on Fcγ and Fcα containing chimeras in cancer treatment. Finally, we will discuss the impact of Fcα-Fcγ chimerism in prevention/treatment strategies against infectious diseases such as HIV-1.

Monoclonal Antibodies for Bacterial Pathogens: Mechanisms of Action and Engineering Approaches for Enhanced Effector Functions

Monoclonal antibody (mAb) therapy has opened a new era in the pharmaceutical field, finding application in various areas of research, from cancer to infectious diseases. The IgG isoform is the most used therapeutic, given its long half-life, high serum abundance, and most importantly, the presence of the Fc domain, which can be easily engineered. In the infectious diseases field, there has been a rising interest in mAbs research to counteract the emerging crisis of antibiotic resistance in bacteria. Various pathogens are acquiring resistance mechanisms, inhibiting any chance of success of antibiotics, and thus may become critically untreatable in the near future. Therefore, mAbs represent a new treatment option which may complement or even replace antibiotics. However, very few antibacterial mAbs have succeeded clinical trials, and until now, only three mAbs have been approved by the FDA. These failures highlight the need of improving the efficacy of mAb therapeutic activity, which can also be achieved with Fc engineering. In the first part of this review, we will describe the mechanisms of action of mAbs against bacteria, while in the second part, we will discuss the recent advances in antibody engineering to increase efficacy of pre-existing anti-bacterial mAbs.

Antibody-mediated prevention of vaginal HIV transmission is dictated by IgG subclass in humanized mice

HIV broadly neutralizing antibodies (bNAbs) are capable of both blocking viral entry and driving innate immune responses against HIV-infected cells through their Fc region. Vaccination or productive infection results in a polyclonal mixture of class-switched immunoglobulin G (IgG) antibodies composed of four subclasses, each encoding distinct Fc regions that differentially engage innate immune functions. Despite evidence that innate immunity contributes to protection, the relative contribution of individual IgG subclasses is unknown. Here, we used vectored immunoprophylaxis in humanized mice to interrogate the efficacy of individual IgG subclasses during prevention of vaginal HIV transmission by VRC07, a potent CD4-binding site-directed bNAb. We find that VRC07 IgG2, which lacks Fc-mediated functionality, exhibited substantially reduced protection in vivo relative to other subclasses. Low concentrations of highly functional VRC07 IgG1 yielded substantial protection against vaginal challenge, suggesting that interventions capable of eliciting modest titers of functional IgG subclasses may provide meaningful benefit against infection.

Impact of IgG Isotype on the Induction of Antibody-Dependent Cellular Phagocytosis of HIV by Human Milk Leukocytes

Approximately 100,000 mother-to-child transmission (MTCT) events of HIV via human milk feeding occur each year. However, only about 15% of infants milk-fed by untreated HIV+ mothers become infected, suggesting a protective effect of the milk itself. Infants ingest 105-108 maternal leukocytes daily via milk, which remain functional beyond ingestion. Such function may be elicited by maternal milk antibody (Ab). Though IgA is dominant in milk, most HIV-specific milk Abs are of the IgG subclass, highlighting the importance of investigating the function of each IgG isotype in the milk context. Though Ab effector function mediated by the constant (Fc) domain via interaction with Fc Receptors (FcRs), such as Ab-dependent cellular phagocytosis (ADCP), are critical in protecting against HIV infection, ADCP is largely unexplored as it relates to mitigation of MTCT. Presently we report the ADCP activity of milk leukocytes against HIV particles and immune complexes (ICs), using 57 unique samples from 34 women, elicited by IgG1/2/3/4 of monoclonal (m)Ab 246-D. Granulocyte ADCP of HIV was most potent compared to other phagocytes when elicited by IgG1/3/4. IgG1/3 activated granulocytes similarly, exhibiting 1.6x-4.4x greater activity compared to IgG2/4, and a preference for virus compared to ICs. Notably, CD16- monocyte ADCP of a given target were unaffected by isotype, and CD16+ monocytes were poorly stimulated by IgG1. IgG2/4 elicited potent IC ADCP, and in terms of total leukocyte IC ADCP, IgG4 and IgG3 exhibited similar function, with IgG4 eliciting 1.6x-2.1x greater activity compared to IgG1/IgG2, and CD16+ monocytes most stimulated by IgG2. These data contribute to a more comprehensive understanding of Fc-mediated functionality of milk leukocytes, which is critical in order to develop therapeutic approaches to eliminating this route of MTCT, including mucosal administration of mAbs and/or a maternal vaccination aimed to elicit a potent milk Ab response.

Improving Antibody Therapeutics by Manipulating the Fc Domain: Immunological and Structural Considerations

Interactions between the crystallizable fragment (Fc) domain of antibodies and a plethora of cellular Fc receptors (FcRs) or soluble proteins form a critical link between humoral and innate immunity. In particular, the immunoglobulin G Fc domain is critical for the clearance of target cells by processes that include (a) cytotoxicity, phagocytosis, or complement lysis; (b) modulation of inflammation; (c) antigen presentation; (d) antibody-mediated receptor clustering; and (e) cytokine release. More than 30 Fc-engineered antibodies aimed primarily at tailoring these effects for optimal therapeutic outcomes are in clinical evaluation or have already been approved. Nonetheless, our understanding of how FcR engagement impacts various immune cell phenotypes is still largely incomplete. Recent insights into FcR biology coupled with advances in Fc:FcR structural analysis, Fc engineering, and mouse models that recapitulate human biology are helping to fill in existing knowledge gaps. These advances will provide a blueprint on how to fine-tune the Fc domain to achieve optimal therapeutic efficacy. Expected final online publication date for the Annual Review of Biomedical Engineering, Volume 24 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Leveraging Antibody, B Cell and Fc Receptor Interactions to Understand Heterogeneous Immune Responses in Tuberculosis

Despite over a century of research, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), continues to kill 1.5 million people annually. Though less than 10% of infected individuals develop active disease, the specific host immune responses that lead to Mtb transmission and death, as well as those that are protective, are not yet fully defined. Recent immune correlative studies demonstrate that the spectrum of infection and disease is more heterogenous than has been classically defined. Moreover, emerging translational and animal model data attribute a diverse immune repertoire to TB outcomes. Thus, protective and detrimental immune responses to Mtb likely encompass a framework that is broader than T helper type 1 (Th1) immunity. Antibodies, Fc receptor interactions and B cells are underexplored host responses to Mtb. Poised at the interface of initial bacterial host interactions and in granulomatous lesions, antibodies and Fc receptors expressed on macrophages, neutrophils, dendritic cells, natural killer cells, T and B cells have the potential to influence local and systemic adaptive immune responses. Broadening the paradigm of protective immunity will offer new paths to improve diagnostics and vaccines to reduce the morbidity and mortality of TB.

Phagocytosis by an HIV antibody is associated with reduced viremia irrespective of enhanced complement lysis

Increasingly, antibodies are being used to treat and prevent viral infections. In the context of HIV, efficacy is primarily attributed to dose-dependent neutralization potency and to a lesser extent Fc-mediated effector functions. It remains unclear whether augmenting effector functions of broadly neutralizing antibodies (bNAbs) may improve their clinical potential. Here, we use bNAb 10E8v4 targeting the membrane external proximal region (MPER) to examine the role of antibody-mediated effector and complement (C’) activity when administered prophylactically against SHIV challenge in rhesus macaques. With sub-protective dosing, we find a 78–88% reduction in post-acute viremia that is associated with 10E8v4-mediated phagocytosis acting at the time of challenge. Neither plasma nor tissue viremic outcomes in vivo is improved with an Fc-modified variant of 10E8v4 enhanced for C’ functions as determined in vitro. These results suggest that effector functions inherent to unmodified 10E8v4 contribute to efficacy against SHIV_SF162P3 in the absence of plasma neutralizing titers, while C’ functions are dispensable in this setting, informing design of bNAb modifications for improving protective efficacy.

While antibodies neutralize HIV via Fab recognition of viral surface antigens, antibody Fc domains mediate effector functions, including antibody-dependent cellular phagocytosis (ADCP) and cytotoxicity (ADCC), and complement (C') activity. Here, Spencer et al. modify bNAb 10E8v4 to enhance C'-mediated potency in SHIV challenged rhesus macaques to probe its function in protection, showing that in the absence of neutralization, enhancing C' activities in vitro adds no value toward reducing viremia in either blood or tissue.

Building a better antibody through the Fc: advances and challenges in harnessing antibody Fc effector functions for antiviral protection

Antibodies can provide antiviral protection through neutralization and recruitment of innate effector functions through the Fc domain. While neutralization has long been appreciated for its role in antibody-mediated protection, a growing body of work indicates that the antibody Fc domain also significantly contributes to antiviral protection. Recruitment of innate immune cells such as natural killer cells, neutrophils, monocytes, macrophages, dendritic cells and the complement system by antibodies can lead to direct restriction of viral infection as well as promoting long-term antiviral immunity. Monoclonal antibody therapeutics against viruses are increasingly incorporating Fc-enhancing features to take advantage of the Fc domain, uncovering a surprising breadth of mechanisms through which antibodies can control viral infection. Here, we review the recent advances in our understanding of antibody-mediated innate immune effector functions in protection from viral infection and review the current approaches and challenges to effectively leverage innate immune cells via antibodies.

HIV Broadly Neutralizing Antibodies Expressed as IgG3 Preserve Neutralization Potency and Show Improved Fc Effector Function

The ability of several broadly neutralizing antibodies (bNAbs) to protect against HIV infection is enhanced through Fc receptor binding. Antibody isotype modulates this effect, with IgG3 associated with improved HIV control and vaccine efficacy. We recently showed that an IgG3 variant of bNAb CAP256-VRC26.25 exhibited more potent neutralization and phagocytosis than its IgG1 counterpart. Here, we expanded this analysis to include additional bNAbs targeting all major epitopes. A total of 15 bNAbs were expressed as IgG1 or IgG3, and pairs were assessed for neutralization potency against the multi-subtype global panel of 11 HIV strains. Binding to the neonatal Fc receptor (FcRn) and Fcγ receptors were measured using ELISA and antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis were measured using infectious viruses and global panel Env SOSIP trimers, respectively. IgG3 bNAbs generally showed similar or increased (up to 60 fold) neutralization potency than IgG1 versions, though the effect was virus-specific. This improvement was statistically significant for CAP256-VRC26.25, 35022, PGT135 and CAP255.G3. IgG3 bNAbs also showed significantly improved binding to FcγRIIa which correlated with enhanced phagocytosis of all trimeric Env antigens. Differences in ADCC were epitope-specific, with IgG3 bNAbs to the MPER, CD4 binding site and gp120-gp41 interface showing increased ADCC. We also explored the pH dependence of IgG1 and IgG3 variants for FcRn binding, as this determines the half-life of antibodies. We observed reduced pH dependence, associated with shorter half-lives for IgG3 bNAbs, with κ-light chains. However, IgG3 bNAbs that use λ-light chains showed similar pH dependence to their IgG1 counterparts. This study supports the manipulation of the constant region to improve both the neutralizing and Fc effector activity of bNAbs, and suggests that IgG3 versions of bNAbs may be preferable for passive immunity given their polyfunctionality.

Modeling HIV-1 Within-Host Dynamics After Passive Infusion of the Broadly Neutralizing Antibody VRC01

VRC01 is a broadly neutralizing antibody that targets the CD4 binding site of HIV-1 gp120. Passive administration of VRC01 in humans has assessed the safety and the effect on plasma viremia of this monoclonal antibody (mAb) in a phase 1 clinical trial. After VRC01 infusion, the plasma viral load in most of the participants was reduced but had particular dynamics not observed during antiretroviral therapy. In this paper, we introduce different mathematical models to explain the observed dynamics and fit them to the plasma viral load data. Based on the fitting results we argue that a model containing reversible Ab binding to virions and clearance of virus-VRC01 complexes by a two-step process that includes (1) saturable capture followed by (2) internalization/degradation by phagocytes, best explains the data. This model predicts that VRC01 may enhance the clearance of Ab-virus complexes, explaining the initial viral decay observed immediately after antibody infusion in some participants. Because Ab-virus complexes are assumed to be unable to infect cells, i.e., contain neutralized virus, the model predicts a longer-term viral decay consistent with that observed in the VRC01 treated participants. By assuming a homogeneous viral population sensitive to VRC01, the model provides good fits to all of the participant data. However, the fits are improved by assuming that there were two populations of virus, one more susceptible to antibody-mediated neutralization than the other.

Antibodies for Human Immunodeficiency Virus-1 Cure Strategies

Human immunodeficiency virus (HIV) infection leads to the establishment of a long-lived latent cellular reservoir. One strategy to eliminate quiescent reservoir cells is to reactivate virus replication to induce HIV envelope glycoprotein (Env) expression on the cell surface exposing them to subsequent antibody targeting. Via the interactions between the antibody Fc domain and Fc-γ receptors (FcγRs) that are expressed on innate effector cells, such as natural killer cells, monocytes, and neutrophils, antibodies can mediate the elimination of infected cells. Over the last decade, a multitude of human monoclonal antibodies that are broadly neutralizing across many HIV-1 subtypes have been identified and are currently being explored for HIV eradication strategies. Antibody development also includes novel Fc engineering approaches to increase engagement of effector cells and optimize antireservoir efficacy. In this review, we discuss the usefulness of antibodies for HIV eradication approaches specifically focusing on antibody-mediated strategies to target latently infected cells and options to increase antibody efficacy.

Passive immunization of the human vagina

The vagina is an excellent site for topical passive immunization, as access is relatively easy, and it is an enclosed space that has been shown to retain bioactive antibodies for several hours. A number of sexually transmitted infections could potentially be prevented by delivery of specific monoclonal antibodies to the vagina. Furthermore, our group is developing antisperm antibodies for vaginally delivered on-demand topical contraception. In this article, we describe physical features of the vagina that could play a role in antibody deployment, and antibody modifications that could affect mAb retention and function in the female reproductive tract. We also review results of recent Phase 1 clinical trials of vaginal passive immunization with antibodies against sexually transmitted pathogens, and describe our current studies on the use of anti-sperm mAbs for contraception.

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.

Protective efficacy of the anti-HIV broadly neutralizing antibody PGT121 in the context of semen exposure

BACKGROUND

HIV-1 infections occur following viral exposure at anogenital mucosal surfaces in the presence of semen. Semen contains immunosuppressive and pro-inflammatory factors. Semen from HIV-1-infected donors contains anti-HIV-1 antibodies. We assessed if passively infused anti-HIV-1 neutralizing antibody conferred protection from rectal SHIV_SF162P3 challenge at semen exposed mucosae.

METHODS

We pooled seminal plasma from HIV-1-infected donors. The pool was screened by ELISA for antibodies against HIV-1_SF162 gp140. The ability of seminal plasma to inhibit macaque NK cells from responding to direct and antibody-dependent stimulation was assessed. The ability of seminal plasma to inhibit macaque granulocytes from mediating oxidative burst was also assessed. To demonstrate viral infectivity in the presence of seminal plasma, macaques (n = 4) were rectally challenged with SHIV_SF162P3 following exposure to 2.5 mL of seminal plasma. To evaluate if anti-HIV-1 neutralizing antibody confers protection against rectal SHIV challenge at semen exposed mucosae, eight macaques were intravenously infused with PGT121, either wild type (n = 4) or the Fc receptor binding deficient LALA variant (n = 4), and rectally challenged with SHIV_SF162P3 following exposure to 2.5 mL of seminal plasma.

FINDINGS

Anti-HIV-1_SF162 gp140 antibodies were detected in seminal plasma. Seminal plasma inhibited direct and antibody-dependent NK cell activation and granulocyte oxidative burst in vitro. Rectal SHIV_SF162P3 challenge of control macaques following seminal plasma exposure resulted in infection of all animals. All macaques infused with wild type or LALA PGT121 and challenged with SHIV_SF162P3 following seminal plasma exposure were protected.

INTERPRETATION

PGT121 conferred protection against rectal SHIV_SF162P3 challenge at semen exposed mucosae. Future research should investigate if semen alters protection conferred by antibodies more dependent on non-neutralizing functions.

FUNDING

This work was supported by a grant from the Australian National Health and Medical Research Council (APP1124680).

The current and future role of nanovaccines in HIV-1 vaccine development

Introduction: An efficacious vaccine for HIV-1 has been sought for over 30 years to eliminate the virus from the human population. Many challenges have occurred in the attempt to produce a successful immunogen, mainly caused by the basic biology of the virus. Immunogens have been developed focusing on inducing one or more of the following types of immune responses; neutralizing antibodies, non-neutralizing antibodies, and T-cell mediated responses. One way to better present and develop an immunogen for HIV-1 is through the use of nanotechnology and nanoparticles.Areas covered: This article gives a basic overview of the HIV-1 vaccine field, as well as nanotechnology, specifically nanovaccines. It then covers the application of nanovaccines made from biological macromolecules to HIV-1 vaccine development for neutralizing antibodies, non-neutralizing antibodies, and T-cell-mediated responses.Expert opinion: Nanovaccines are an area that is ripe for further exploration in HIV-1 vaccine field. Not only are nanovaccines capable of carrying and presenting antigens in native-like conformations, but they have also repeatedly been shown to increase immunogenicity over recombinant antigens alone. Only through further research can the true role of nanovaccines in the development of an efficacious HIV-1 vaccine be established.

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.

Enhancement of Antibody-Dependent Cellular Cytotoxicity and Phagocytosis in Anti-HIV-1 Human-Bovine Chimeric Broadly Neutralizing Antibodies

No prophylactic vaccine has provided robust protection against human immunodeficiency virus type 1 (HIV-1). Vaccine-induced broadly neutralizing antibodies (bNAbs) have not been achieved in humans and most animals; however, cows vaccinated with HIV-1 envelope trimers produce bNAbs with unusually long third heavy complementarity-determining regions (CDRH3s). Alongside neutralization, Fc-mediated effector functions, including antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADP), may be critical for in vivo bNAb antiviral activity. Here, we aimed to augment the Fc-dependent effector functions of a chimeric human-bovine bNAb, NC-Cow1, which binds the CD4 binding site (CD4bs) and exhibits broader and more potent neutralization than most human CD4bs bNAbs by using an exceptionally long 60-amino acid (aa) CDRH3. The bovine variable region of NC-Cow1 was paired with a human IgG1 Fc region mutated to create the following three variants: G236R/L328R (GRLR) that abrogates Fc-gamma receptor (FcγR) binding, and two variants that enhance binding, namely, G236A/S239D/I332E (GASDIE) and G236A/S239D/A330L/I332E (GASDALIE). Both GASDIE and GASDALIE improved binding to human FcγRIIA and FcγRIIIA, enhanced human natural killer (NK) cell activation, and mediated higher levels of ADCC and ADP activity than the wild-type human IgG1 Fc. GASDALIE mediated higher phagocytic activity than GASDIE. As expected, GRLR eliminated binding to FcγRs and did not mediate ADCC or ADP. We demonstrated that mutations in the human Fc region of bovine chimeric antibodies with ultralong CDRH3s could enhance antibody effector functions while maintaining envelope binding and neutralization. This study will have significant implications in the development of multifunctional anti-HIV antibodies, which may be important to prevent HIV-1 transmission in an antibody-based topical microbicide. IMPORTANCE Despite successful antiviral chemotherapy, human immunodeficiency virus (HIV) is still a lifelong persistent virus, and no vaccine yet prevents HIV transmission. Topical microbicides offer an important alternative method to prevent sexual transmission of HIV-1. With the production of highly potent anti-HIV-1 broadly neutralizing antibodies (bNAbs) and multifunctional antibodies, monoclonal antibodies are now important prophylactic agents. Recently discovered anti-HIV-1 bovine bNAbs (with higher potency and breadth than most human bNAbs) could be novel candidates as potent topical microbicides. Our study is significant as it demonstrates the compatibility of combining bovine-derived neutralization with human-derived antibody-effector functions. This study is a new approach to antibody engineering that strengthens the feasibility of using high-potency bovine variable region bNAbs with augmented Fc function and promotes them as a strong candidate for antibody-mediated therapies.

Mining HIV controllers for broad and functional antibodies to recognize and eliminate HIV-infected cells

HIV monoclonal antibodies for viral reservoir eradication strategies will likely need to recognize reactivated infected cells and potently drive Fc-mediated innate effector cell activity. We systematically characterize a library of 185 HIV-envelope-specific antibodies derived from 15 spontaneous HIV controllers (HCs) that selectively exhibit robust serum Fc functionality and compared them to broadly neutralizing antibodies (bNAbs) in clinical development. Within the 10 antibodies with the broadest cell-recognition capability, seven originated from HCs and three were bNAbs. V3-loop-targeting antibodies are enriched among the top cell binders, suggesting the V3-loop may be selectively exposed and accessible on the cell surface. Fc functionality is more variable across antibodies, which is likely influenced by distinct binding topology and corresponding Fc accessibility, highlighting not only the importance of target-cell recognition but also the need to optimize for Fc-mediated elimination. Ultimately, our results demonstrate that this comprehensive selection process can identify monoclonal antibodies poised to eliminate infected cells.

Rossignol et al. characterize 185 HIV-envelope-specific antibodies derived from spontaneous HIV controllers, downselecting antibodies based on their ability to broadly recognize infected cells and potently drive Fc-mediated innate effector cell activity. This comprehensive selection process can identify monoclonal antibodies poised to eliminate infected cells for viral reservoir eradication strategies.

Adjuvant-mediated enhancement of the immune response to HIV vaccines

Protection from human immunodeficiency virus (HIV) acquisition will likely require an effective vaccine that elicits antibodies against the HIV-1 envelope glycoproteins (Envs), which are the sole target of neutralizing antibodies and a main focus of vaccine development. Adjuvants have been widely used to augment the magnitude and longevity of the adaptive immune responses to immunizations with HIV-1 Envs and to guide the development of specific immune responses. Here, we review the adjuvants that have been used in combination with HIV-1 Envs in several preclinical and human clinical trials in recent years. We summarize the interactions between the HIV-1 Envs and adjuvants, and highlight the routes of vaccine administration for various formulations. We then discuss the use of combinations of different adjuvants, the potential effect of adjuvants on the elicitation of antibodies enriched in somatic hypermutation and containing long complementarity-determining region 3 of the antibody heavy chain, and the elicitation of non-neutralizing antibodies.

Accelerated Clearance and Degradation of Cell-Free HIV by Neutralizing Antibodies Occurs via FcγRIIb on Liver Sinusoidal Endothelial Cells by Endocytosis

Neutralizing Abs suppress HIV infection by accelerating viral clearance from blood circulation in addition to neutralization. The elimination mechanism is largely unknown. We determined that human liver sinusoidal endothelial cells (LSEC) express FcγRIIb as the lone Fcγ receptor, and using humanized FcγRIIb mouse, we found that Ab-opsonized HIV pseudoviruses were cleared considerably faster from circulation than HIV by LSEC FcγRIIb. Compared with humanized FcγRIIb-expressing mice, HIV clearance was significantly slower in FcγRIIb knockout mice. Interestingly, a pentamix of neutralizing Abs cleared HIV faster compared with hyperimmune anti-HIV Ig (HIVIG), although the HIV Ab/Ag ratio was higher in immune complexes made of HIVIG and HIV than pentamix and HIV. The effector mechanism of LSEC FcγRIIb was identified to be endocytosis. Once endocytosed, both Ab-opsonized HIV pseudoviruses and HIV localized to lysosomes. This suggests that clearance of HIV, endocytosis, and lysosomal trafficking within LSEC occur sequentially and that the clearance rate may influence downstream events. Most importantly, we have identified LSEC FcγRIIb-mediated endocytosis to be the Fc effector mechanism to eliminate cell-free HIV by Abs, which could inform development of HIV vaccine and Ab therapy.

Brain tissue transcriptomic analysis of SIV-infected macaques identifies several altered metabolic pathways linked to neuropathogenesis and poly (ADP-ribose) polymerases (PARPs) as potential therapeutic targets

Despite improvements in antiretroviral therapy, human immunodeficiency virus type 1 (HIV-1)-associated neurocognitive disorders (HAND) remain prevalent in subjects undergoing therapy. HAND significantly affects individuals' quality of life, as well as adherence to therapy, and, despite the increasing understanding of neuropathogenesis, no definitive diagnostic or prognostic marker has been identified. We investigated transcriptomic profiles in frontal cortex tissues of Simian immunodeficiency virus (SIV)-infected Rhesus macaques sacrificed at different stages of infection. Gene expression was compared among SIV-infected animals (n = 11), with or without CD8+ lymphocyte depletion, based on detectable (n = 6) or non-detectable (n = 5) presence of the virus in frontal cortex tissues. Significant enrichment in activation of monocyte and macrophage cellular pathways was found in animals with detectable brain infection, independently from CD8+ lymphocyte depletion. In addition, transcripts of four poly (ADP-ribose) polymerases (PARPs) were up-regulated in the frontal cortex, which was confirmed by real-time polymerase chain reaction. Our results shed light on involvement of PARPs in SIV infection of the brain and their role in SIV-associated neurodegenerative processes. Inhibition of PARPs may provide an effective novel therapeutic target for HIV-related neuropathology.

HIV Antibody Fc N-Linked Glycosylation Is Associated with Viral Rebound

Changes in antibody glycosylation are linked to inflammation across several diseases. Alterations in bulk antibody galactosylation can predict rheumatic flares, act as a sensor for immune activation, predict gastric cancer relapse, track with biological age, shift with vaccination, change with HIV reservoir size on therapy, and decrease in HIV and HCV infections. However, whether changes in antibody Fc biology also track with reservoir rebound time remains unclear. The identification of a biomarker that could forecast viral rebound time could significantly accelerate the downselection and iterative improvement of promising HIV viral eradication strategies. Using a comprehensive antibody Fc-profiling approach, the level of HIV-specific antibody Fc N-galactosylation is significantly associated with time to rebound after treatment discontinuation across three independent cohorts. Thus virus-specific antibody glycosylation may represent a promising, simply measured marker to track reservoir reactivation.

Recent advances in high-throughput flow cytometry for drug discovery

INTRODUCTION

High-throughput flow cytometry (HTFC) has proven to be an important technology in drug discovery. The use of HTFC enables multi-parametric screening of suspension cells containing heterogenous cell populations and coated particles for screening proteins of interest. Novel targets, novel cell markers and compound clusters for drug development have been identified from HTFC screens.

AREAS COVERED

In this article, the authors focus on reviewing the recent HTFC applications reported during the last 5-6 years, including drug discovery screens and studies for immune, immune-oncology, infectious and inflammatory diseases. The main HTFC approaches, development of HTFC systems, and automated sample preparation systems for HTFC are also discussed.

EXPERT OPINION

The advance of HTFC technology coupled with automated sample acquisition and sample preparation has demonstrated its utility in screening large numbers of compounds using suspension cells, facilitated screening of disease-relevant human primary cells, and enabled deep understanding of mechanism of action by analyzing multiple parameters. The authors see HTFC as a very valuable tool in immune, immune-oncology, infectious and inflammatory diseases where immune cells play essential roles.

Adjuvanted HIV-1 vaccine promotes antibody-dependent phagocytic responses and protects against heterologous SHIV challenge

To augment HIV-1 pox-protein vaccine immunogenicity using a next generation adjuvant, a prime-boost strategy of recombinant modified vaccinia virus Ankara and multimeric Env gp145 was evaluated in macaques with either aluminum (alum) or a novel liposomal monophosphoryl lipid A (MPLA) formulation adsorbed to alum, ALFA. Binding antibody responses were robust and comparable between arms, while antibody-dependent neutrophil and monocyte phagocytotic responses were greatly enhanced by ALFA. Per-exposure vaccine efficacy against heterologous tier 2 SHIV mucosal challenge was 90% in ALFA-adjuvanted males (P = 0.002), while alum conferred no protection. Half of the ALFA-adjuvanted males remained uninfected after the full challenge series, which spanned seven months after the last vaccination. Antibody-dependent monocyte and neutrophil phagocytic responses both strongly correlated with protection. Significant sex differences in infection risk were observed, with much lower infection rates in females than males. In humans, MPLA-liposome-alum adjuvanted gp120 also increased HIV-1-specific phagocytic responses relative to alum. Thus, next-generation liposome-based adjuvants can drive vaccine elicited antibody effector activity towards potent phagocytic responses in both macaques and humans and these responses correlate with protection. Future protein vaccination strategies aiming to improve functional humoral responses may benefit from such adjuvants.

Survivors of Ebola Virus Disease Develop Polyfunctional Antibody Responses

Monoclonal antibodies can mediate protection against Ebola virus (EBOV) infection through direct neutralization as well as through the recruitment of innate immune effector functions. However, the antibody functional response following survival of acute EBOV disease has not been well characterized. In this study, serum antibodies from Ebola virus disease (EVD) survivors from Sierra Leone were profiled to capture variation in overall subclass/isotype abundance, neutralizing activity, and innate immune effector functions. Antibodies from EVD survivors exhibited robust innate immune effector functions, mediated primarily by IgG1 and IgA1. In conclusion, development of functional antibodies follows survival of acute EVD.

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.

Neutralizing antibodies for HIV-1 prevention

PURPOSE OF REVIEW

In the absence of a protective vaccine against HIV-1, passive immunization using novel broadly neutralizing antibodies (bNAbs) is an attractive concept for HIV-1 prevention. Here, we summarize the results of preclinical and clinical studies of bNAbs, discuss strategies for optimizing bNAb efficacy and lay out current pathways for the development of bNAbs as prophylaxis.

RECENT FINDINGS

Passive transfer of second-generation bNAbs results inpotent protection against infection in preclinical animal models. Furthermore, multiple bNAbs targeting different epitopes on the HIV-1 envelope trimer are in clinical evaluation and have demonstrated favorable safety profiles and robust antiviral activity in chronically infected individuals. The confirmation that passive immunization with bNAb(s) will prevent HIV-1 acquisition in humans is pending and the focus of ongoing investigations. Given the global diversity of HIV-1, bNAb combinations or multispecific antibodies will most likely be required to produce the necessary breadth for effective protection.

SUMMARY

Encouraging results from preclinical and clinical studies support the development of bNAbs for prevention and a number of antibodies with exceptional breadth and potency are available for clinical evaluation. Further optimization of viral coverage and antibody half-life will accelerate the clinical implementation of bNAbs as a critical tool for HIV-1 prevention strategies.

Vaccinal effect of HIV-1 antibody therapy

PURPOSE OF REVIEW

The review recalls recent findings regarding the induction of vaccinal effects by HIV-1 broadly neutralizing antibodies (bNAbs) and highlights potential therapeutic strategies to exploit such immunomodulatory properties.

RECENT FINDINGS

Studies in different animal models have shown that mAbs can generate long-lasting protective immunity. Induction of this vaccinal effect by HIV-1 bNAbs has also been more recently reported in animal models of HIV-1 infection. Notably, bNAbs treatment of macaques infected with the chimeric simian-human immunodeficiency virus (SHIV) improved both humoral and cellular adaptive immune responses that contributed to disease control. Importantly, this concept has been extended to HIV-1-infected patients as enhancement of humoral responses was recently reported in HIV-1 patients treated with bNAbs. Studies aiming at elucidating the mechanisms underlying these immunomodulatory properties of bNAbs have identified a role for immune complexes in shaping immune responses against HIV-1. They also highlight different Fc (fragment crystallizable) region effector functions that might be required for the enhancement of HIV-1 immune responses upon bNAbs treatment.

SUMMARY

HIV-1 bNAbs can elicit protective adaptive immune responses through mechanisms involving multiple cellular and molecular actors of the immune system. Harnessing these mechanisms will be crucial to achieve protective immunity against HIV-1 infection by bNAbs.

Update on Fc-Mediated Antibody Functions Against HIV-1 Beyond Neutralization

Antibodies (Abs) are the major component of the humoral immune response and a key player in vaccination. The precise Ab-mediated inhibitory mechanisms leading to in vivo protection against HIV have not been elucidated. In addition to the desired viral capture and neutralizing Ab functions, complex Ab-dependent mechanisms that involve engaging immune effector cells to clear infected host cells, immune complexes, and opsonized virus have been proposed as being relevant. These inhibitory mechanisms involve Fc-mediated effector functions leading to Ab-dependent cellular cytotoxicity, phagocytosis, cell-mediated virus inhibition, aggregation, and complement inhibition. Indeed, the decreased risk of infection observed in the RV144 HIV-1 vaccine trial was correlated with the production of non-neutralizing inhibitory Abs, highlighting the role of Ab inhibitory functions besides neutralization. Moreover, Ab isotypes and subclasses recognizing specific HIV envelope epitopes as well as pecular Fc-receptor polymorphisms have been associated with disease progression. These findings further support the need to define which Fc-mediated Ab inhibitory functions leading to protection are critical for HIV vaccine design. Herein, based on our previous review Su & Moog Front Immunol 2014, we update the different inhibitory properties of HIV-specific Abs that may potentially contribute to HIV protection.

An Inflammatory Story: Antibodies in Tuberculosis Comorbidities

Mycobacterium tuberculosis (Mtb) resides in a quarter of the world's population and is the causative agent for tuberculosis (TB), the most common infectious reason of death in humans today. Although cellular immunity has been firmly established in the control of Mtb, there is growing evidence that antibodies may also modulate the infection. More specifically, certain antibody features are associated with inflammation and are divergent in different states of human infection and disease. Importantly, TB impacts not just the healthy but also those with chronic conditions. While HIV represents the quintessential comorbid condition for TB, recent epidemiological evidence shows that additional chronic conditions such as diabetes and kidney disease are rising. In fact, the prevalence of diabetes as a comorbid TB condition is now higher than that of HIV. These chronic diseases are themselves independently associated with pro-inflammatory immune states that encompass antibody profiles. This review discusses isotypes, subclasses, post-translational modifications and Fc-mediated functions of antibodies in TB infection and in the comorbid chronic conditions of HIV, diabetes, and kidney diseases. We propose that inflammatory antibody profiles, which are a marker of active TB, may be an important biomarker for detection of TB disease progression within comorbid individuals. We highlight the need for future studies to determine which inflammatory antibody profiles are the consequences of comorbidities and which may potentially contribute to TB reactivation.

IgG3 enhances neutralization potency and Fc effector function of an HIV V2-specific broadly neutralizing antibody

Broadly neutralizing antibodies (bNAbs) protect against HIV infection in non-human primates and their efficacy may be enhanced through interaction with Fc receptors on immune cells. Antibody isotype is a modulator of this binding with the IgG3 subclass mediating potent Fc effector function and is associated with HIV vaccine efficacy and HIV control. BNAb functions are typically assessed independently of the constant region with which they are naturally expressed. To examine the role of natural isotype in the context of a bNAb lineage we studied CAP256, an HIV-infected individual that mounted a potent V2-specific bNAb response. CAP256 expressed persistently high levels of plasma IgG3 which we found mediated both broad neutralizing activity and potent Fc function. Sequencing of germline DNA and the constant regions of V2-directed bNAbs from this donor revealed the expression of a novel IGHG3 allele as well as IGHG3*17, an allele that produces IgG3 antibodies with increased plasma half-life. Both allelic variants were used to generate CAP256-VRC26.25 and CAP256-VRC26.29 IgG3 bNAbs and these were compared to IgG1 versions. IgG3 variants were shown to have significantly higher phagocytosis and trogocytosis compared to IgG1 versions, which corresponded to increased affinity for FcγRIIa. Neutralization potency was also significantly higher for IgG3 bNAbs, particularly against viruses lacking the N160 glycan. By exchanging hinge regions between subclass variants, we showed that hinge length modulated both neutralization potency and Fc function. This study showed that co-operation between the variable and natural IgG3 constant regions enhanced the polyfunctionality of antibodies, indicating the value of leveraging genetic variation which could be exploited for passive immunity.

Terminal Effector CD8 T Cells Defined by an IKZF2+IL-7R- Transcriptional Signature Express FcγRIIIA, Expand in HIV Infection, and Mediate Potent HIV-Specific Antibody-Dependent Cellular Cytotoxicity

Chronic HIV-1 is associated with increased levels of FcγRIIIA⁺ CD8 T cells.

FcγRIIIA⁺ CD8 T cells display an innate transcriptomic profile akin to NK cells.

ADCC is mediated by FcγRIIIA⁺ CD8 T cells at levels comparable with NK cells.

HIV-1 infection expands large populations of late-stage differentiated CD8 T cells that may persist long after viral escape from TCR recognition. In this study, we investigated whether such CD8 T cell populations can perform unconventional innate-like antiviral effector functions. Chronic untreated HIV-1 infection was associated with elevated numbers of CD45RA⁺CD57⁺ terminal effector CD8 T cells expressing FcγRIIIA (CD16). The FcγRIIIA⁺ CD8 T cells displayed a distinctive transcriptional profile between conventional CD8 T cells and NK cells, characterized by high levels of IKZF2 and low expression of IL7R. This transcriptional profile translated into a distinct NKp80⁺ IL-7Rα⁻ surface phenotype with high expression of the Helios transcription factor. Interestingly, the FcγRIIIA⁺ CD8 T cells mediated HIV-specific Ab-dependent cellular cytotoxicity (ADCC) activity at levels comparable with NK cells on a per cell basis. The FcγRIIIA⁺ CD8 T cells were highly activated in a manner that correlated positively with expansion of the CD8 T cell compartment and with plasma levels of soluble mediators of antiviral immunity and inflammation such as IP-10, TNF, IL-6, and TNFRII. The frequency of FcγRIIIA⁺ CD8 T cells persisted as patients initiated suppressive antiretroviral therapy, although their activation levels declined. These data indicate that terminally differentiated effector CD8 T cells acquire enhanced innate cell-like characteristics during chronic viral infection and suggest that HIV-specific ADCC is a function CD8 T cells use to target HIV-infected cells. Furthermore, as the FcγRIIIA⁺ CD8 T cells persist in treatment, they contribute significantly to the ADCC-capable effector cell pool in patients on antiretroviral therapy.

Boosting of Markers of Fcγ Receptor Function in Anti-HIV Antibodies During Structured Treatment Interruption

Anti-HIV envelope (Env) antibodies elicit important Fc receptor functions, including FcγRIIIa-mediated natural killer cell killing of opsonized infected targets. How these antibodies evolve during HIV infection and treatment remains poorly understood. We describe changes in anti-HIV Env IgG using longitudinal samples from seroconverter subjects treated soon after infection and later during periods of structured treatment interruption (STI). Our well-validated dimeric rsFcγR binding assays combine effects of opsonizing antibody subclasses, epitopes, and geometries to provide a measure of FcγR (Fcγ receptor)-mediated functionality. IgG1 anti-Env titers diminished rapidly during antiretroviral therapy (ART; t_1/2 3.0 ± 0.8 months), while the dimeric rsFcγRIIIa activity persisted longer (t_1/2 33 ± 11 months), suggesting that there is maintenance of functional antibody specificities within the diminished pool of anti-HIV Env Abs. The initial antibody response to infection in two subjects was characterized by approximately fivefold higher FcγRIIIa compared with FcγRIIa binding activity. Uncoupling of FcγRIIa and FcγRIIIa activities may be a distinct feature of the early antibody response that preferentially engages FcγRIIIa-mediated effector functions. Two to three STI cycles, even with low viremia, were sufficient to boost dimeric FcγR activity in these seroconverter subjects. We hypothesize that increased humoral immunity induced by STI is a desirable functional outcome potentially achievable by therapeutic immunization during ART. We conclude that controlled viral antigen exposure under the protection of suppressive ART may be effective in eliciting FcγR-dependent function in support of viral reactivation and kill strategies.

Vaccine-induced V1V2-specific antibodies control and or protect against infection with HIV, SIV and SHIV

PURPOSE OF REVIEW

In humans, only one independent immunologic correlate of reduced risk of HIV infection has been identified: a robust antibody (Ab) response to the V1V2 domain of the gp120 envelope (Env) protein. In recent years, the presence and level of V1V2-specific Abs has also been correlated with protection from SIV and SHIV infections. Here, we review the multitude of studies showing the in-vivo protective effects of V1V2 Abs and review their immunologic characteristics and antiviral functions.

RECENT FINDINGS

Structural and immunologic studies have defined four epitope families in the V1V2 domain: one epitope family, V2q, which preferentially presents as a quaternary structure of the Env trimer, and another epitope family (V2qt) which requires the quaternary trimeric Env structure; these two epitope types are recognized by two families of monoclonal Abs (mAbs)-V2q-specific and V2qt-specific mAbs-which display broad and potent neutralizing activity. A third epitope family, V2i, is present as a discontinuous conformational structure that overlays the α4β7 integrin binding motif, and a fourth epitope family (V2p) exists on V2 peptides. Antibodies specific for V2i and V2p epitopes display only poor neutralizing activity but effectively mediate other antiviral activities and have been correlated with control of and/or protection from HIV, SIV and SHIV. Notably, V2q and V2qt Abs have not been induced by any vaccines, but V2p and V2i Abs have been readily induced with various vaccines in nonhuman primates and humans.

SUMMARY

The correlation of vaccine-induced V2p and V2i Abs with protection from HIV, SIV and SHIV suggests that these Ab types are extremely important to induce with prophylactic vaccines.

The Dual Role of Neutrophils in HIV Infection

PURPOSE OF REVIEW

We summarize what is known about neutrophils in HIV infection, focusing on their potential roles in HIV protection, acquisition, and pathogenesis.

RECENT FINDINGS

Recent studies have demonstrated that neutrophil-associated proteins and cytokines in genital tissue pre-infection associate with HIV acquisition. However, recent in vivo assessment of highly exposed seronegative individuals and in vitro studies of anti-HIV functions of neutrophils add to older literature evidence that neutrophils may be important in a protective response to HIV infection. Neutrophils are important for containment of pathogens but can also contribute to tissue damage due to their release of reactive oxygen species, proteases, and other potentially harmful effector molecules. Overall, there is a clear evidence for both helpful and harmful roles of neutrophils in HIV acquisition and pathogenesis. Further study, particularly of tissue neutrophils, is needed to elucidate the kinetics, phenotype, and functionality of neutrophils in HIV infection to better understand this dichotomy.

A versatile high-throughput assay to characterize antibody-mediated neutrophil phagocytosis

Neutrophils, the most abundant white blood cell, play a critical role in anti-pathogen immunity via phagocytic clearance, secretion of enzymes and immunomodulators, and the release of extracellular traps. Neutrophils non-specifically sense infection through an array of innate immune receptors and inflammatory sensors, but are also able to respond in a pathogen/antigen-specific manner when leveraged by antibodies via Fc-receptors. Among neutrophil functions, antibody-dependent neutrophil phagocytosis (ADNP) results in antibody-mediated opsonization, enabling neutrophils to sense and respond to infection in a pathogen-appropriate manner. Here, we describe a high-throughput flow cytometric approach to effectively visualize and quantify ADNP and its downstream consequences. The assay is easily adaptable, supporting both the use of purified neutrophils or white blood cells, the use of purified Ig or serum, and the broad utility of any target antigen. Thus, this ADNP assay represents a high-throughput platform for the in-depth characterization of neutrophil function.

•

A high-throughput antibody-dependent neutrophil phagocytosis (ADNP) assay was developed.

•

This flow cytometry assay is flexible and can be easily adapted to any pathogen.

•

Analysis of sample sets by ADNP assay is fast, robust and cost-effective.

•

Additional neutrophil functions can be profiled in secondary analyses.

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.

The Human FcγRII (CD32) Family of Leukocyte FcR in Health and Disease

FcγRs have been the focus of extensive research due to their key role linking innate and humoral immunity and their implication in both inflammatory and infectious disease. Within the human FcγR family FcγRII (activatory FcγRIIa and FcγRIIc, and inhibitory FcγRIIb) are unique in their ability to signal independent of the common γ chain. Through improved understanding of the structure of these receptors and how this affects their function we may be able to better understand how to target FcγR specific immune activation or inhibition, which will facilitate in the development of therapeutic monoclonal antibodies in patients where FcγRII activity may be desirable for efficacy. This review is focused on roles of the human FcγRII family members and their link to immunoregulation in healthy individuals and infection, autoimmunity and cancer.

Antibody-Dependent Cellular Phagocytosis in Antiviral Immune Responses

Antiviral activities of antibodies may either be dependent only on interactions between the antibody and cognate antigen, as in binding and neutralization of an infectious virion, or instead may require interactions between antibody-antigen immune complexes and immunoproteins or Fc receptor expressing immune effector cells. These Fc receptor-dependent antibody functions provide a direct link between the innate and adaptive immune systems by combining the potent antiviral activity of innate effector cells with the diversity and specificity of the adaptive humoral response. The Fc receptor-dependent function of antibody-dependent cellular phagocytosis (ADCP) provides mechanisms for clearance of virus and virus-infected cells, as well as for stimulation of downstream adaptive immune responses by facilitating antigen presentation, or by stimulating the secretion of inflammatory mediators. In this review, we discuss the properties of Fc receptors, antibodies, and effector cells that influence ADCP. We also provide and interpret evidence from studies that support a potential role for ADCP in either inhibiting or enhancing viral infection. Finally, we describe current approaches used to measure antiviral ADCP and discuss considerations for the translation of studies performed in animal models. We propose that additional investigation into the role of ADCP in protective viral responses, the specific virus epitopes targeted by ADCP antibodies, and the types of phagocytes and Fc receptors involved in ADCP at sites of virus infection will provide insight into strategies to successfully leverage this important immune response for improved antiviral immunity through rational vaccine design.

Monocytes Latently Infected with Human Cytomegalovirus Evade Neutrophil Killing

One site of latency of human cytomegalovirus (HCMV) in vivo is in undifferentiated cells of the myeloid lineage. Although latently infected cells are known to evade host T cell responses by suppression of T cell effector functions, it is not known if they must also evade surveillance by other host immune cells. Here we show that cells latently infected with HCMV can, indeed, be killed by host neutrophils but only in a serum-dependent manner. Specifically, antibodies to the viral latency-associated US28 protein mediate neutrophil killing of latently infected cells. To address this mechanistically, a full proteomic screen was carried out on latently infected monocytes. This showed that latent infection downregulates the neutrophil chemoattractants S100A8/A9, thus suppressing neutrophil recruitment to latently infected cells. The ability of latently infected cells to inhibit neutrophil recruitment represents an immune evasion strategy of this persistent human pathogen, helping to prevent clearance of the latent viral reservoir.

Immune Correlate-Guided HIV Vaccine Design

Although vaccines have been successfully developed against several pathogens, designing an effective vaccine to protect against human immunodeficiency virus (HIV) has remained an intractable challenge. To address this, the research community has looked to human and non-human primate studies to understand the correlates of protective immunity, based on which a targeted vaccine strategy may be designed. Two distinct approaches, focused on different immune correlates of protection, have emerged. The first focuses on structure-based design of HIV envelope immunogens that are able to induce antibodies that neutralize the virus. The second focuses on strategies aimed at driving non-neutralizing polyclonal and polyfunctional antibodies that engage other arms of immunity to clear the virus. Here we review these two different vaccine design strategies and posit that ultimately the convergence of these two efforts will likely be necessary for the development of a globally protective HIV vaccine.

HEV ORF3 downregulatesCD14 and CD64 to impair macrophages phagocytosis through inhibiting JAK/STAT pathway

Hepatitis E virus (HEV) could induce chronic hepatitis and liver failure with high mortality through unknown mechanisms. The previous study showed that the HEV open reading frames 3 (ORF3) could inhibit macrophages inflammatory response. Impaired macrophages phagocytosis was also found in patients infected with HEV and its nucleic acids could be detected in macrophages. To elucidate the role of HEV ORF3 on phagocytosis, the phagocytosis activation was measured by phagocytosis test, flow cytometry, and phalloidin staining. Meanwhile, the expression of key phagocytic receptors and the activation of transduction pathway were investigated by using reverse transcription real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot analysis. Results of phagocytosis test showed that the HEV ORF3 could significantly impair the absorption capacity of latex beads. Furthermore, results of RT-qPCR and Western blot analysis showed that the expression of CD14 and CD64 decreased. Afterward, the present study showed that the activation of Janus kinase-signal transducer and activator of transcription (JAK/STAT) signaling pathway was inhibited by HEV ORF3 and downregulation of CD14 and CD64 could be reversed by interferon γ, one activator of the JAK1/STAT1 signaling pathway. In conclusion, HEV ORF3 could significantly impair the phagocytosis of macrophage by downregulating expression of CD14 and CD64, which may function by inhibiting the activation of the JAK1/STAT1 signaling pathway.

The alternatively spliced porcine FcγRI regulated PRRSV-ADE infection and proinflammatory cytokine production

Receptors for the Fc region of IgG (FcγRs) play a key role in protecting the immune system and host from infection. In this study, we described the cloning, sequencing and characterization of porcine FcγRI, and reported six different FcγRI isoforms, four of which have never been reported before. Further analysis revealed that FcγR isoforms are generated by alternative splicing mechanisms, including two membrane isoforms and four soluble isoforms. Importantly, we found FcγRI splice variants differentially influence PRRSV antibody-dependent enhancement (ADE) effects. Membrane pCD64-T1 promotes endocytosis of the PRRSV-antibody complex to enhance PRRSV replication, and soluble pCD64-T3 has no ADE effect on PRRSV proliferation, but shows an inflammation enhancement effect. The differential expression of selective splicing in primary PAM cells and 3D4/21 cell lines are altered and regulated by PRRSV infection and inflammatory environment. Our results indicated that porcine FcγRI plays dual regulatory roles in PRRSV multiplication and PRRSV inflammation process by the alternatively spliced mechanism, which will be a new target in PRRSV prevention and control.

Mucosal Vaccine Approaches for Prevention of HIV and SIV Transmission

Optimal protective immunity to HIV will likely require that plasma cells, memory B cells and memory T cells be stationed in mucosal tissues at portals of viral entry. Mucosal vaccine administration is more effective than parenteral vaccine delivery for this purpose. The challenge has been to achieve efficient vaccine uptake at mucosal surfaces, and to identify safe and effective adjuvants, especially for mucosally administered HIV envelope protein immunogens. Here, we discuss strategies used to deliver potential HIV vaccine candidates in the intestine, respiratory tract, and male and female genital tract of humans and nonhuman primates. We also review mucosal adjuvants, including Toll-like receptor agonists, which may adjuvant both mucosal humoral and cellular immune responses to HIV protein immunogens.

Primary Human Neutrophils Exhibit a Unique HIV-Directed Antibody-Dependent Phagocytosis Profile

The only clinical HIV vaccine trial to demonstrate efficacy, RV144, correlated protection with the antibodies (Abs) mediating function via the "constant" immunoglobulin region, the crystallizable fragment (Fc). These data have supported a focus on the induction of Abs by vaccines that trigger antiviral activities by relevant leukocytes via Fc receptors (FcRs). Neutrophils are phagocytes that comprise > 50% of leukocytes and display unique FcRs. We sought to compare the Ab-dependent cellular phagocytosis (ADCP) activity of human neutrophils to the commonly assayed THP-1 cell line. HIV-specific Abs were employed to elicit ADCP of beads coated with HIV envelope protein. Overall, trends were noted among neutrophil donors and the ADCP profile was different from that of THP-1 cells. mAb ELISA titers correlated with ADCP by THP-1 cells but not neutrophils. Monoclonal (m)Abs were also tested with primary monocytes. Donor-to-donor variation was high, and hindered the analysis of this dataset, but it was, in itself, an important finding. This study illustrates the concept that the assessment of FcR-mediated Ab activity with a frequently used cell line such as THP-1 is not necessarily indicative of relevant Ab functionality in vivo, and this calls for in-depth study of the properties of the HIV antibodies best-suited to eliciting antiviral activities by primary cells.

Preventive and therapeutic features of broadly neutralising monoclonal antibodies against HIV-1

The viral plasticity and the vast diversity of HIV-1 circulating strains necessitates the identification of new approaches to control this global pandemic. New generation broadly neutralising monoclonal antibodies (bnMAbs) against the HIV-1 viral envelope protein (Env) can prevent virus acquisition, reduce viraemia, enhance immunity, and induce the killing of infected cells in animal models of HIV-1 infection. Most importantly, passively administered bnMAbs are effective at decreasing viraemia and delaying viral rebound in people chronically infected with HIV-1. Single antibody treatment is associated with the emergence of viral escape mutants, and virus suppression is not maintained in the long term. However, a combination of bnMAbs and bioengineered multivalent antibodies that target different sites on Env might increase the efficacy of immunotherapy, adding a new relevant tool for clinical use. The aim of this Review is to highlight the potential benefits of this novel prophylactic and therapeutic approach to fight HIV-1.

Neutrophil Vaccination Dynamics and Their Capacity To Mediate B Cell Help in Rhesus Macaques

Neutrophils are the most abundant leukocyte and play a critical role in the initial response to an Ag. Recently, their ability to contribute to adaptive immunity has been highlighted. We evaluated the ability of neutrophils from blood to contribute to the adaptive immune response in a preclinical rhesus macaque SIV vaccine trial. Replication-competent adenovirus-SIV recombinants induced neutrophil activation, B cell help markers, and enhanced ability to generate reactive oxygen species. Boosting with SIV vaccines (adjuvant together with ALVAC or DNA plus envelope protein) elicited significant neutrophil responses. Serum cytokine and chemokine levels induced correlated with the frequency of neutrophil subsets expressing IL-21, myeloperoxidase, and CD64. Post-SIV infection, neutrophils exhibited dysfunction, both phenotypically and functionally. B cells from protected and infected macaques cocultured with autologous polymorphonuclear cells, consisting primarily of neutrophils, were activated, underwent class switching, and produced Abs. This B cell help was not aided by addition of IL-10 and was largely contact dependent. Numerous genes associated with inflammation, Ab production, and chemotaxis were upregulated in the cocultured B cells. We conclude that immune stimulation by vaccination or antigenic exposure imparts a greater ability of neutrophils to contribute to the adaptive immune response. Harnessing this granulocytic response has the potential to improve vaccine efficacy.