Passive immunization of macaques with polyclonal anti-SHIV IgG against a heterologous tier 2 SHIV: outcome depends on IgG dose

Mucosal AIDS virus transmission is enhanced by antiviral IgG isolated early in infection

OBJECTIVE

Antibody-dependent enhancement (ADE) affects host-virus dynamics in fundamentally different ways: i) enhancement of initial virus acquisition, and/or ii) increased disease progression/severity. Here we address the question whether anti-HIV-1 antibodies can enhance initial infection. While cell-culture experiments hinted at this possibility, in-vivo proof remained elusive.

DESIGN

We used passive immunization in nonhuman primates challenged with simian-human immunodeficiency virus (SHIV), a chimera expressing HIV-1 envelope. We purified IgG from rhesus monkeys with early-stage SHIV infection - before cross-neutralizing anti-HIV-1 antibodies had developed - and screened for maximal complement-mediated antibody-dependent enhancement (C'-ADE) of viral replication with a SHIV strain phylogenetically distinct from that harbored by IgG donor macaques. IgG fractions with maximal C'-ADE but lacking neutralization were combined to yield enhancing anti-SHIV IgG (enSHIVIG).

RESULTS

We serially enrolled naive macaques (Group 1) to determine the minimal and 50% animal infectious doses required to establish persistent infection after intrarectal SHIV challenge. The first animal was inoculated with a 1 : 10 virus-stock dilution; after this animal's viral RNA load was >104copies/ml, the next macaque was challenged with 10x less virus, a process repeated until viremia no longer ensued. Group 2 was pretreated intravenously with enSHIVIG 24 h before SHIV challenge. Overall, Group 2 macaques required 3.4-fold less virus compared to controls (P = 0.002). This finding is consistent with enhanced susceptibility of the passively immunized animals to mucosal SHIV challenge.

CONCLUSION

These passive immunization data give proof of IgG-mediated enhanced virus acquisition after mucosal exposure - a potential concern for antibody-based AIDS vaccine development.

What Is the Predictive Value of Animal Models for Vaccine Efficacy in Humans? The Importance of Bridging Studies and Species-Independent Correlates of Protection

Animal models have played a pivotal role in all stages of vaccine development. Their predictive value for vaccine effectiveness depends on the pathogen, the robustness of the animal challenge model, and the correlates of protection (if known). This article will cover key questions regarding bridging animal studies to efficacy trials in humans. Examples include human papillomavirus (HPV) vaccine in which animal protection after vaccination with heterologous prototype virus-like particles (VLPs) predicted successful efficacy trials in humans, and a recent approval of anthrax vaccine in accordance with the "Animal Rule." The establishment of animal models predictive of vaccine effectiveness in humans has been fraught with difficulties with low success rate to date. Challenges facing the use of animal models for vaccine development against Ebola and HIV will be discussed.

Passive and active antibody studies in primates to inform HIV vaccines

INTRODUCTION

Prevention of infection remains the ultimate goal for HIV vaccination, and there is compelling evidence that antibodies directed to Envelope are necessary to block infection. Generating antibodies that are sufficiently broad, potent, and sustained to block infection by the diverse HIV-1 strains circulating worldwide remains an area of intense study.

AREAS COVERED

In this review, we have summarized progress from publications listed as PubMed citations in 2016-17 in the areas of passive antibody studies using human neutralizing monoclonal antibodies in nonhuman primates, HIV Envelope vaccine development and active vaccination studies to generate potent neutralizing antibodies.

EXPERT COMMENTARY

Passive transfer studies in nonhuman primates using human neutralizing monoclonal antibodies have informed the potency, specificity, and cooperativity of antibodies needed to prevent infection, leading to clinical studies now testing potent antibodies for prevention of HIV. Progress in understanding the structure of Envelope has led to novel vaccine constructs, including mimetics, scaffolds and native-like proteins. As yet, no single approach ensures protection against the circulating global HIV-1 strains, but there is progress in understanding why, and intense research continues in these and other areas for a solution. We offer perspectives on how this knowledge may shape the design of future HIV vaccines.

Nonhuman primate models for the evaluation of HIV-1 preventive vaccine strategies: model parameter considerations and consequences

PURPOSE OF REVIEW

Nonhuman primate (NHP) models of AIDS are powerful systems for evaluating HIV vaccine approaches in vivo. Authentic features of HIV-1 transmission, dissemination, target cell tropism, and pathogenesis, and aspects of anti-HIV-1 immune responses, can be recapitulated in NHPs provided the appropriate, specific model parameters are considered. Here, we discuss key model parameter options and their implications for HIV-1 vaccine evaluation.

RECENT FINDINGS

With the availability of several different NHP host species/subspecies, different challenge viruses and challenge stock production methods, and various challenge routes and schemata, multiple NHP models of AIDS exist for HIV vaccine evaluation. The recent development of multiple new challenge viruses, including chimeric simian-human immunodeficiency viruses and simian immunodeficiency virus clones, improved characterization of challenge stocks and production methods, and increased insight into specific challenge parameters have resulted in an increase in the number of available models and a better understanding of the implications of specific study design choices.

SUMMARY

Recent progress and technical developments promise new insights into basic disease mechanisms and improved models for better preclinical evaluation of interventions to prevent HIV transmission.

The role of Fc receptors in HIV prevention and therapy

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

Prospects for engineering HIV-specific antibodies for enhanced effector function and half-life

PURPOSE OF REVIEW

A wealth of recent animal model data suggests that as exciting possibilities for the use of antibodies in passive immunotherapy strategies continue to develop, it will be important to broadly consider how antibodies achieve anti-HIV-1 effect in vivo.

RECENT FINDINGS

Beyond neutralization breadth and potency, substantial evidence from natural infection, vaccination, and studies in animal models points to a critical role for antibody Fc receptor (FcR) engagement in reducing risk of infection, decreasing postinfection viremia, and delaying viral rebound. Supporting these findings in the setting of HIV, the clinical maturation of recombinant antibody therapeutics has reinforced the importance of Fc-driven activity in vivo across many disease settings, as well as opportunely resulted in the development and exploration of a number of engineered Fc sequence and glycosylation variants that possess differential binding to FcRs. Exploiting these variants as tools, the individual and concerted effects of antibody effector functions such as antibody-dependent cellular cytotoxicity, antibody-dependent cell-mediated virus inhibition, phagocytosis, complement-dependent cytotoxicity, antibody half-life, and compartmentalization are now being explored. As exciting molecular therapies are advanced, these studies promise to provide insight into optimal in-vivo antibody activity profiles.

SUMMARY

Careful consideration of recent progress in understanding protective antibody activities in vivo can point toward how tailoring antibody activity via Fc domain modification may enable optimization of HIV prevention and eradication strategies.

Protective Efficacy and Mechanism of Passive Immunization with Polyclonal Antibodies in a Sepsis Model of Staphylococcus aureus Infection

Staphylococcus aureus (S. aureus) is an opportunistic bacterial pathogen responsible for a diverse spectrum of human diseases, resulting in considerable yearly mortality rates. Due to its rapid acquisition of antibiotic resistance, it becomes increasingly difficult to cure S. aureus infections with conventional antibiotics. Immunotherapy represents a promising alternative strategy to prevent and/or treat the infection. In the present study, passive immunization with polyclonal antibodies targeting three possible S. aureus antigens, Hla, SEB and MntC (termed "SAvac-pcAb") after challenge with lethal dose of S. aureus resulted in reduced bacterial loads, inflammatory cell infiltration and decreased pathology, and was able to provide nearly complete protection in a murine sepsis model. In vitro studies confirmed the direct interaction of SAvac-pcAb with S. aureus bacteria. Additional studies validated that SAvac-pcAb contained both opsonic and neutralizing antibodies that contributed to its protective efficacy. The former mediated opsonophagocytosis in a neutrophil-dependent manner, while the later inhibited the biological functions of Hla and SEB, two major virulence factors secreted by S. aureus. Critically, we demonstrated that SAvac-pcAb was cross-reactive with different clinical strains of S. aureus. These results confirmed the efficacy for treatment of S. aureus infection by passive immunization as an important therapeutic option.

Defense-in-depth by mucosally administered anti-HIV dimeric IgA2 and systemic IgG1 mAbs: complete protection of rhesus monkeys from mucosal SHIV challenge

Although IgA is the most abundantly produced immunoglobulin in humans, its role in preventing HIV-1 acquisition, which occurs mostly via mucosal routes, remains unclear. In our passive mucosal immunizations of rhesus macaques (RMs), the anti-HIV-1 neutralizing monoclonal antibody (nmAb) HGN194, given either as dimeric IgA1 (dIgA1) or dIgA2 intrarectally (i.r.), protected 83% or 17% of the RMs against i.r. simian-human immunodeficiency virus (SHIV) challenge, respectively. Data from the RV144 trial implied that vaccine-induced plasma IgA counteracted the protective effector mechanisms of IgG1 with the same epitope specificity. We thus hypothesized that mucosal dIgA2 might diminish the protection provided by IgG1 mAbs targeting the same epitope. To test our hypothesis, we administered HGN194 IgG1 intravenously (i.v.) either alone or combined with i.r. HGN194 dIgA2. We enrolled SHIV-exposed, persistently aviremic RMs protected by previously administered nmAbs; RM anti-human IgG responses were undetectable. However, low-level SIV Gag-specific proliferative T-cell responses were found. These animals resemble HIV-exposed, uninfected humans, in which local and systemic cellular immune responses have been observed. HGN194 IgG1 and dIgA2 used alone and the combination of the two neutralized the challenge virus equally well in vitro. All RMs given only i.v. HGN194 IgG1 became infected. In contrast, all RMs given HGN194 IgG1+dIgA2 were completely protected against high-dose i.r. SHIV-1157ipEL-p challenge. These data imply that combining suboptimal defenses at the mucosal and systemic levels can completely prevent virus acquisition. Consequently, active vaccination should focus on defense-in-depth, a strategy that seeks to build up defensive fall-back positions well behind the fortified frontline.

Multimodality vaccination against clade C SHIV: partial protection against mucosal challenges with a heterologous tier 2 virus

We sought to test whether vaccine-induced immune responses could protect rhesus macaques (RMs) against upfront heterologous challenges with an R5 simian-human immunodeficiency virus, SHIV-2873Nip. This SHIV strain exhibits many properties of transmitted HIV-1, such as tier 2 phenotype (relatively difficult to neutralize), exclusive CCR5 tropism, and gradual disease progression in infected RMs. Since no human AIDS vaccine recipient is likely to encounter an HIV-1 strain that exactly matches the immunogens, we immunized the RMs with recombinant Env proteins heterologous to the challenge virus. For induction of immune responses against Gag, Tat, and Nef, we explored a strategy of immunization with overlapping synthetic peptides (OSP). The immune responses against Gag and Tat were finally boosted with recombinant proteins. The vaccinees and a group of ten control animals were given five low-dose intrarectal (i.r.) challenges with SHIV-2873Nip. All controls and seven out of eight vaccinees became systemically infected; there was no significant difference in viremia levels of vaccinees vs. controls. Prevention of viremia was observed in one vaccinee which showed strong boosting of virus-specific cellular immunity during virus exposures. The protected animal showed no challenge virus-specific neutralizing antibodies in the TZM-bl or A3R5 cell-based assays and had low-level ADCC activity after the virus exposures. Microarray data strongly supported a role for cellular immunity in the protected animal. Our study represents a case of protection against heterologous tier 2 SHIV-C by vaccine-induced, virus-specific cellular immune responses.

Low Multiplicity of HIV-1 Infection and No Vaccine Enhancement in VAX003 Injection Drug Users

BACKGROUND

We performed human immunodeficiency virus type 1 (HIV-1) transmitted/founder (T/F) virus analysis of the VAX003 vaccine efficacy trial participants to characterize the transmission bottleneck and test for vaccine-associated reduction or enhancement of infection in this injection drug user (IDU) cohort.

METHODS

We performed single genome sequencing of plasma vRNA from 50 subjects sampled in early HIV infection. Sequences were analyzed phylogenetically, T/F viruses enumerated, and a sieve analysis performed.

RESULTS

Eight of 19 (42%) placebo recipients were productively infected by more than 1 virus (range 1-5, median 1, mean 1.7). This frequency of multiple virus transmission was greater than reported for heterosexual cohorts (19%, P = .03) but not statistically different from vaccine recipients (22.6%, P > .05), where the range was 1-3, median 1, and mean 1.3 (P > .05 for all comparisons). An atypical sieve effect was detected in Env V2 but was not associated with reduction or enhancement of virus acquisition.

CONCLUSIONS

The number of T/F viruses in IDUs was surprising low, with 95% of individuals infected by only 1-3 viruses. This finding suggests that a successful vaccine or other prevention modality generally needs to protect against only one or a few viruses regardless of risk behavior. T/F analysis identified an atypical genetic sieve in the V2 region of Envelope and found no evidence for vaccine-mediated enhancement in VAX003.

Lack of protection following passive transfer of polyclonal highly functional low-dose non-neutralizing antibodies

Recent immune correlates analysis from the RV144 vaccine trial has renewed interest in the role of non-neutralizing antibodies in mediating protection from infection. While neutralizing antibodies have proven difficult to induce through vaccination, extra-neutralizing antibodies, such as those that mediate antibody-dependent cellular cytotoxicity (ADCC), are associated with long-term control of infection. However, while several non-neutralizing monoclonal antibodies have been tested for their protective efficacy in vivo, no studies to date have tested the protective activity of naturally produced polyclonal antibodies from individuals harboring potent ADCC activity. Because ADCC-inducing antibodies are highly enriched in elite controllers (EC), we passively transferred highly functional non-neutralizing polyclonal antibodies, purified from an EC, to assess the potential impact of polyclonal non-neutralizing antibodies on a stringent SHIV-SF162P3 challenge in rhesus monkeys. Passive transfer of a low-dose of ADCC inducing antibodies did not protect from infection following SHIV-SF162P3 challenge. Passively administered antibody titers and gp120-specific, but not gp41-specific, ADCC and antibody induced phagocytosis (ADCP) were detected in the majority of the monkeys, but did not correlate with post infection viral control. Thus these data raise the possibility that gp120-specific ADCC activity alone may not be sufficient to control viremia post infection but that other specificities or Fc-effector profiles, alone or in combination, may have an impact on viral control and should be tested in future passive transfer experiments.