Heterogeneous neutralizing antibody and antibody-dependent cell cytotoxicity responses in HIV-1 elite controllers

Broadly cross-neutralizing antibodies in HIV-1 patients with undetectable viremia

Several recent studies have identified HIV-infected patients able to produce a broad neutralizing response, and the detailed analyses of their sera have provided valuable information to improve future vaccine design. All these studies have excluded patients on antiretroviral treatment and with undetectable viral loads, who have an improved B cell profile compared to untreated patients. To better understand the induction of neutralizing antibodies in patients on antiretroviral treatment with undetectable viremia, we have screened 508 serum samples from 364 patients (173 treated and 191 untreated) for a broadly neutralizing antibody (bNAb) response using a new strategy based on the use of recombinant viruses. Sera able to neutralize a minipanel of 6 recombinant viruses, including envelopes from 5 different subtypes, were found in both groups. After IgG purification, we were able to confirm the presence of IgG-associated broadly neutralizing activity in 3.7% (7 of 191) of untreated patients with detectable viremia and 1.7% (3 of 174) of aviremic patients receiving antiretroviral treatment. We thus confirm the possibility of induction of a broad IgG-associated neutralizing response in patients on antiretroviral treatment, despite having undetectable viremia. This observation is in stark contrast to the data obtained from long-term nonprogressors, whose little neutralizing activity has been attributed to the low levels of viral replication.

Antibody-dependent cell-mediated viral inhibition emerges after simian immunodeficiency virus SIVmac251 infection of rhesus monkeys coincident with gp140-binding antibodies and is effective against neutralization-resistant viruses

Antibody-dependent cell-mediated viral inhibition (ADCVI) is an attractive target for vaccination because it takes advantage of both the anamnestic properties of an adaptive immune response and the rapid early response characteristics of an innate immune response. Effective utilization of ADCVI in vaccine strategies will depend on an understanding of the natural history of ADCVI during acute and chronic human immunodeficiency virus type 1 (HIV-1) infection. We used the simian immunodeficiency virus (SIV)-infected rhesus monkey as a model to study the kinetics of ADCVI in early infection, the durability of ADCVI through the course of infection, and the effectiveness of ADCVI against viruses with envelope mutations that are known to confer escape from antibody neutralization. We demonstrate the development of ADCVI, capable of inhibiting viral replication 100-fold, within 3 weeks of infection, preceding the development of a comparable-titer neutralizing antibody response by weeks to months. The emergence of ADCVI was temporally associated with the emergence of gp140-binding antibodies, and in most animals, ADCVI persisted through the course of infection. Highly evolved viral envelopes from viruses isolated at late time points following infection that were resistant to plasma neutralization remained susceptible to ADCVI, suggesting that the epitope determinants of neutralization escape are not shared by antibodies that mediate ADCVI. These findings suggest that despite the ability of SIV to mutate and adapt to multiple immunologic pressures during the course of infection, SIV envelope may not escape the binding of autologous antibodies that mediate ADCVI.

Alterations of the B-cell response by HIV-1 replication

While the hallmark of HIV-1 infection is the progressive depletion of CD4(+) T cells, extensive B-cell dysfunction ensues that impairs the quality of the humoral response. HIV-1 infection causes hypergammaglobulinemia, polyclonal activation, loss of memory B-cell subsets, B-cell exhaustion, aberrant B-cell surface markers, and impaired humoral responses against infections and vaccinations. The totality of the mechanisms that contribute to B-cell dysfunction in vivo is unknown, although roles for HIV proteins (Env, Tat, and Nef) and virions binding to CD21 on B cells have been identified. Recent studies suggest that early antiretroviral therapy, that minimizes virus replication, can profoundly preserve the early B-cell response to HIV-1. Thus, it is clear that there is an intricate interplay between HIV replication and stimulation of the host B-cell response to infection. A better understanding of how HIV-1 subverts a productive B-cell response is needed to inform vaccine strategies that aim to elicit long-lived plasma cells and memory B-cell responses that can act quickly upon antigen stimulation.

Rapid SIV Env-specific mucosal and serum antibody induction augments cellular immunity in protecting immunized, elite-controller macaques against high dose heterologous SIV challenge

Three Indian rhesus macaques, Ad-SIV primed/protein boosted and exposed twice to high-dose mucosal SIV(mac251) challenges, exhibited elite control of viremia over 6.5 years. They were negative for host factors associated with control of SIV infection. After a third intrarectal challenge with SIV(smE660), all controlled viremia, with one (macaque #5) maintaining undetectable viremia in blood. Acquisition was not blocked, but virus was contained in the jejunum and draining lymph nodes. Polyfunctional memory T cell responses and high-titered neutralizing and non-neutralizing serum and mucosal antibodies were present before and maintained post-challenge. The level of protection seen for animal #5 was predicted from analyses of gene transcription in jejunum 2 weeks post-challenge. Macaques #7 and #9, exhibiting lower pre-challenge cellular and humoral immunity, partially controlled the SIV(smE660) challenge. Initial vaccine-induced control by macaque #5 extended to the SIV(smE660) challenge due to multiple immune mechanisms that were boosted and augmented by cryptic SIV exposure.

B cell responses to HIV-1 infection and vaccination: pathways to preventing infection

The B cell arm of the immune response becomes activated soon after HIV-1 transmission, yet the initial antibody response does not control HIV-1 replication, and it takes months for neutralizing antibodies to develop against the autologous virus. Antibodies that can be broadly protective are made only in a minority of subjects and take years to develop--too late to affect the course of disease. New studies of the earliest stages of HIV-1 infection, new techniques to probe the human B cell repertoire, the modest degree of efficacy in a vaccine trial and new studies of human monoclonal antibodies that represent the types of immune responses an HIV-1 vaccine should induce are collectively illuminating paths that a successful HIV-1 vaccine might take.

Immune mechanisms of HIV control

HIV-1 can be contained by the immune system, as demonstrated by the existence of rare individuals who spontaneously control HIV-1 replication in the absence of antiretroviral therapy. Emerging evidence points to the importance of a very active cellular immune response in mediating HIV-1 control. The rapid induction of interferon-dependent HIV restriction factors, the presence of protective MHC class I alleles, and the development of a high avidity T-cell response may all cooperate in limiting HIV replication at an early stage. This review will focus on recent advances in understanding the immune mechanisms of HIV control, and on the lessons that may be drawn for the development of candidate HIV vaccines.

Induction of immunity to human immunodeficiency virus type-1 by vaccination

Recent findings have brought optimism that development of a successful human immunodeficiency virus type-1 (HIV-1) vaccine lies within reach. Studies of early events in HIV-1 infection have revealed when and where HIV-1 is potentially vulnerable to vaccine-targeted immune responses. With technical advances in human antibody production, clues about how antibodies recognize HIV-1 envelope proteins have uncovered new targets for immunogen design. A recent vaccine regimen has shown modest efficacy against HIV-1 acquisition. However, inducing long-term T and B cell memory and coping with HIV-1 diversity remain high priorities. Mediators of innate immunity may play pivotal roles in blocking infection and shaping immunity; vaccine strategies to capture these activities are under investigation. Challenges remain in integrating basic, preclinical and clinical research to improve predictions of types of immunity associated with vaccine efficacy, to apply these insights to immunogen design, and to accelerate evaluation of vaccine efficacy in persons at-risk for infection.

HIV-1 continues to replicate and evolve in patients with natural control of HIV infection

Elucidating mechanisms leading to the natural control of HIV-1 infection is of great importance for vaccine design and for understanding viral pathogenesis. Rare HIV-1-infected individuals, termed HIV-1 controllers, have plasma HIV-1 RNA levels below the limit of detection by standard clinical assays (<50 to 75 copies/ml) without antiretroviral therapy. Although several recent studies have documented persistent low-grade viremia in HIV-1 controllers at a level not significantly different from that in HIV-1-infected individuals undergoing treatment with combination antiretroviral therapy (cART), it is unclear if plasma viruses are undergoing full cycles of replication in vivo or if the infection of new cells is completely blocked by host immune mechanisms. We studied a cohort of 21 HIV-1 controllers with a median level of viremia below 1 copy/ml, followed for a median of 11 years. Less than half of the cohort carried known protective HLA types (B*57/27). By isolating HIV-1 RNA from large volumes of plasma, we amplified single genome sequences of both pro-rt and env longitudinally. This study is the first to document that HIV-1 pro-rt and env evolve in this patient group, albeit at rates somewhat lower than in HIV-1 noncontrollers, in HLA B*57/27-positive, as well as HLA B*57/27-negative, individuals. Viral diversity and adaptive events associated with immune escape were found to be restricted in HIV-1 controllers, suggesting that replication occurs in the face of less overall immune selection.

Strategies for eliciting HIV-1 inhibitory antibodies

PURPOSE OF REVIEW

Major roadblocks persist in the development of vaccines that elicit potent neutralizing antibodies targeting diverse HIV-1 strains, similar to known broadly neutralizing HIV-1 human monoclonal antibodies. Alternatively, other types of anti-HIV-1 envelope antibodies that may not neutralize HIV-1 in traditional neutralization assays but have other anti-HIV-1 activities (hereafter termed HIV-1 inhibitory antibodies) can be elicited by current vaccine strategies, and numerous studies are exploring their roles in preventing HIV-1 acquisition. We review examples of strategies for eliciting potentially protective HIV-1 inhibitory antibodies.

RECENT FINDINGS

Heterologous prime-boost strategies can yield anti-HIV immune responses, although only one (canarypox prime, Env protein boost) has been tested and shown positive results in an efficacy trial (RV144). Although the immune correlates of protection are as yet undefined, the reduced rate of acquisition without a significant effect on initial viral loads or CD4 T-cell counts, have raised the hypothesis of an RV144 vaccine-elicited transient protective B-cell response.

SUMMARY

In light of the RV144 trial, there is a critical need to define the entire functional spectrum of anti-HIV-1 antibodies, how easily each can be elicited, and how effective different types of antibody effector mechanisms can be in prevention of HIV-1 transmission.

Role of humoral immunity in host defense against HIV

Individuals infected with HIV-1 and nearly everyone vaccinated with HIV-1 vaccines will, in time, generate antibodies against viral proteins. These antibodies do not resolve natural infection, and vaccine candidates that successfully stimulate the production of high titers of neutralizing antibodies have failed to protect against infection. In spite of this, antibodies continue to be a focus of vaccine research. One reason for the continued interest in antibodies is the failure of a vaccine engineered to generate cell-mediated immunity against HIV. Successful protective immunity against most intracellular pathogens involves several arms of the immune response. A successful vaccine should also stimulate both protective cell-mediated immunity and specific antibody. Efforts should be directed toward making a vaccine that will stimulate the production of 1) more antibody, 2) more broadly cross-reactive neutralizing antibody (broadly neutralizing antibodies), and 3) antibody with a particular functional activity (antibody-dependent cell-mediated cytotoxicity; catalytic antibodies).

IgG subclass profiles in infected HIV type 1 controllers and chronic progressors and in uninfected recipients of Env vaccines

We have studied IgG subclass responses to the HIV-1 proteins gp120, gp41, p24, and Tat in individuals who control their infection without using antiretroviral drugs (HIV-1 controllers; HC) or who progress to disease (chronic progressors; CP). We also measured IgG subclass titers to gp120 in vaccinated individuals. In all cases, the IgG1 subclass dominated the overall response to each antigen. The only IgG titer that differed significantly between the HC and CP groups was to the p24 Gag protein, which was higher in the HC group. IgG1 titers to both p24 and gp120 were significantly higher in the HC group, and IgG3 anti-gp120 antibodies, although rare, were detected more frequently in that group than in CP. Overall, significantly more patients had IgG2 antibodies to gp120 than to gp41. Antibodies to other IgG subclasses were infrequent and their frequency or titers did not differ between the two patient groups. Anti-gp41 and anti-Tat responses also did not correlate with immune control, and anti-Tat antibodies were infrequently detected. Although we found isotypic differences in IgG responses to HIV-1 antigens among vaccinees and the HC and CP individuals, there were no indications of differential T(H)1:T(H)2 polarization between the different groups.

Elucidating the elite: mechanisms of control in HIV-1 infection

In patients with progressive disease, untreated HIV-1 infection is characterized by high viral loads and decreasing CD4(+)T cell counts which lead to opportunistic infection and other AIDS-defining illness. A rare subset of patients termed 'elite controllers' (ECs) maintain control over viremia and often retain normal CD4(+)T cell levels without treatment with antiretroviral drugs. For the most part these patients are infected with replication-competent, fit virus. Factors such as strong, polyfunctional cytotoxic T lymphocyte (CTL) responses and retention of T cell proliferative ability appear to be important in control of HIV-1. Defining what enables ECs to control viral replication will aid in the development of effective vaccine and treatment regimens. This review will discuss differences between ECs and progressors while emphasizing recent findings on the immunological response of ECs to HIV-1.

The implications of patterns in HIV diversity for neutralizing antibody induction and susceptibility

PURPOSE OF REVIEW

Designing an HIV vaccine capable of eliciting broadly cross-reactive neutralizing antibodies is an extraordinarily difficult challenge. Here we focus on the implications of HIV diversity for vaccine design, detailing the impact of levels of variation in epitopes of known potent neutralizing antibodies, and summarizing patterns of overall variation in regional domains within gp120. Strategies for rational vaccine design, to enhance coverage of HIV's natural diversity, are considered.

RECENT FINDINGS

Each amino acid in an envelope gp120 three-dimensional structure was grouped with its 10 nearest neighbors and classified by their natural sequence variability. Within-subtype variation is superimposed on patterns of subtype-specific variation. Regions under selection with moderate diversity are realistic vaccine targets; their variation reflects the value of escape in these regions, whereas the level of diversity is potentially approachable with a vaccine.

SUMMARY

HIV diversity is so extensive that vaccine design strategies may benefit by factoring in diversity from the earliest stages, even for vaccines that target relatively conserved regions.

Fc receptor-mediated antiviral antibodies

PURPOSE OF REVIEW

We summarize current information on Fc receptor-mediated antiviral activities of antibodies. These activities include Fcgamma receptor-mediated inhibition and neutralization of HIV on antigen-presenting cells, antibody-dependent cellular cytotoxicity, and antibody-dependent cell-mediated virus inhibition (ADCVI).

RECENT FINDINGS

An Fcgamma receptor-mediated mechanism that results in augmented neutralization and may render nonneutralizing antibodies inhibitory has been demonstrated in antigen-presenting cell. Antibody-dependent cellular cytotoxicity antibody activity correlates inversely with HIV disease progression in humans, and higher vaccine-induced antibody-dependent cellular cytotoxicity antibody responses are associated with lower acute simian immunodeficiency virus viremia levels in macaques. Following vaccination with rgp120, ADCVI antibody levels are higher among those with a lower rate of sexually acquired HIV infection. Nonneutralizing simian immunodeficiency virus immune serum that prevents infection of newborn macaques after oral challenge has potent ADCVI antibody activity. Abrogating the ability of the Fc segment of the broadly neutralizing mAb b12 to bind to Fcgamma receptors and to mediate ADCVI substantially reduces b12's protective effect in a simian/human immunodeficiency virus vaginal challenge model.

SUMMARY

Fc-FcgammaR interactions play a critical role in the biological function of antibody and are likely to be instrumental in preventing or modulating lentiviral infection. Exploiting antibody responses that depend on Fc-FcgammaR interactions may help widen the breadth and increase the potency of vaccine-induced antibody. Although the importance of generating optimal Fab-antigen interactions cannot be overestimated, improving Fc-FcgammaR interactions through adjuvants or other strategies provides another option for improving HIV vaccines and immunotherapies.

HIV-1-specific antibody responses during acute and chronic HIV-1 infection

PURPOSE OF REVIEW

The humoral immune response to HIV-1 throughout infection is comprised of complex mixtures of antibody isotypes with numerous HIV-1 specificities. However, unlike antibody responses to most infections, protective antibody responses are delayed and do not arise until long after HIV-1 latency is established. We review recent data on HIV-1-specific antibody isotypes induced following HIV-1 transmission: to understand the effects of HIV-1 on B cell and T cell effector responses, to understand the timing of the rise and fall of different anti-HIV-1 antibodies and to understand how antibodies could contribute to protective immunity if they were either pre-existing or elicited immediately after HIV-1 transmission.

RECENT FINDINGS

Studies of the earliest events following infection by the transmitted/founder virus have recently revealed that early destruction of B cell generative microenvironments may be responsible for delay of potentially protective anti-HIV-1 antibody responses. Unlike the initial CD8 T cell response to HIV-1, the initial induced antibody response is usually ineffective in controlling virus replication during acute HIV-1 infection.

SUMMARY

The antibody isotypes and specificities elicited during HIV-1 infection can provide a window into deciphering the detrimental effects of HIV-1 on B cell and T cell responses. Additionally, further characterization of the virus inhibitory capabilities of anti-HIV-1 antibody isotypes can define the spectrum of potential protective HIV-1 antibodies that could be readily elicited by experimental vaccines and adjuvants.

Antibody-Mediated Fcγ Receptor-Based Mechanisms of HIV Inhibition: Recent Findings and New Vaccination Strategies

The HIV/AIDS pandemic is one of the most devastating pandemics worldwide. Today, the major route of infection by HIV is sexual transmission. One of the most promising strategies for vaccination against HIV sexual infection is the development of a mucosal vaccine, which should be able to induce strong local and systemic protective immunity. It is believed that both humoral and cellular immune responses are needed for inducing a sterilizing protection against HIV. Recently, passive administration of monoclonal neutralizing antibodies in macaques infected by vaginal challenge demonstrated a crucial role of FcγRs in the protection afforded by these antibodies. This questioned about the role of innate and adaptive immune functions, including ADCC, ADCVI, phagocytosis of opsonized HIV particles and the production of inflammatory cytokines and chemokines, in the mechanism of HIV inhibition in vivo. Other monoclonal antibodies - non-neutralizing inhibitory antibodies - which recognize immunogenic epitopes, have been shown to display potent FcγRs-dependent inhibition of HIV replication in vitro. The potential role of these antibodies in protection against sexual transmission of HIV and their biological relevance for the development of an HIV vaccine therefore need to be determined. This review highlights the potential role of FcγRs-mediated innate and adaptive immune functions in the mechanism of HIV protection.

Breadth of human immunodeficiency virus-specific neutralizing activity in sera: clustering analysis and association with clinical variables

Induction of antibodies that neutralize a broad range of human immunodeficiency virus type 1 (HIV-1) isolates is a major goal of vaccine development. To study natural examples of broad neutralization, we analyzed sera from 103 HIV-1-infected subjects. Among progressor patients, 20% of sera neutralized more than 75% of a panel of 20 diverse viral isolates. Little activity was observed in sera from long-term nonprogressors (elite controllers). Breadth of neutralization was correlated with viral load, but not with CD4 count, history of past antiretroviral use, age, gender, race/ethnicity, or route of exposure. Clustering analysis of sera by a novel method identified a statistically robust subgrouping of sera that demonstrated broad and potent neutralization activity.

Breadth of neutralizing antibody response to human immunodeficiency virus type 1 is affected by factors early in infection but does not influence disease progression

The determinants of a broad neutralizing antibody (NAb) response and its effect on human immunodeficiency virus type 1 (HIV-1) disease progression are not well defined, partly because most prior studies of a broad NAb response were cross-sectional. We examined correlates of NAb response breadth among 70 HIV-infected, antiretroviral-naïve Kenyan women from a longitudinal seroincident cohort. NAb response breadth was measured 5 years after infection against five subtype A viruses and one subtype B virus. Greater NAb response breadth was associated with a higher viral load set point and greater HIV-1 env diversity early in infection. However, greater NAb response breadth was not associated with a delayed time to a CD4(+) T-cell count of <200, antiretroviral therapy, or death. Thus, a broad NAb response results from a high level of antigenic stimulation early in infection, which likely accounts for prior observations that greater NAb response breadth is associated with a higher viral load later in infection.

Effector mechanisms in HIV-1 infected elite controllers: highly active immune responses?

Elite controllers (EC) are HIV-1 infected patients control viral replication to a level of <50 copies/ml without antiretroviral therapy. These patients are also known as elite suppressors, or HIV controllers, and they differ from traditional long-term non-progressors (LTNPs) who maintain stable CD4 counts and are asymptomatic without antiretroviral therapy. Recent studies suggest that many EC are infected with replication-competent virus. Thus it appears that host factors such as innate immunity, the humoral immune response, and the cellular immune response are involved in the suppression of viral replication in EC. This article will review the effector mechanisms that are thought to play a role in the remarkable control of viral replication seen in these patients. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.