The quest for an antibody‐based <scp>HIV</scp> vaccine (2017)

This article introduces a series of reviews covering B cells and Immunity to HIV appearing in Volume 275 of Immunological Reviews. Despite major advances in our understanding of the biology of HIV-1 infection, and advances in anti-retroviral therapy to treat the disease, there were 2.1 million new cases of HIV-1 infection in 2015, and 36.7 million people living with AIDS (http://www.unaids.org/en/resources/fact-sheet). Thus, a vaccine that can prevent HIV infection remains a global priority. Thirty-three years after the discovery of HIV-1,1 and the demonstration that it was the cause of AIDS,2 and after six HIV-1 vaccine efficacy trials,3-8 no HIV-1 candidate vaccine has shown enough efficacy to be approved for clinical use. Of several vaccine concepts tested in efficacy trials, only one, the RV144 pox virus prime, protein boost (ALVAC/AIDSVAX B/E) vaccine, showed a low level of vaccine protection with an estimated 31% vaccine efficacy.8 Candidate vaccines have sought to elicit both antibody and T-cell responses, but to fully prevent the acquisition of infection, a major focus has been on the induction of protective antibody responses.9, 10 Hence, the focus of this issue of Immunologic Reviews is “Antibodies and Immunity to HIV”. Animal models have demonstrated that passive administration of HIV-1-neutralizing antibodies can fully protect against infection, but the induction of such antibodies via immunization remains a major scientific challenge. With recent advances in the isolation and characterization of broadly neutralizing antibodies (bnAbs) from HIV-1-infected subjects, in elucidating structures of the HIV-1 envelope glycoprotein (Env), in defining novel approaches to immunogen design, and in improved understanding of the immunological pathways leading to bNAb elicitation, the challenge developing an HIV-1 vaccine appears to be more tractable. The articles in this issue highlight both major areas of HIV-1 vaccine development progress and remaining obstacles, and provide context for the renewed optimism that a highly effective vaccine, while not imminent, is possible. The HIV-1 vaccine field has now realized that development of a safe and globally effective HIV-1 vaccine will require a greater level of understanding of host-virus interactions than has ever been required or achieved for any previous vaccine.10-12 This is necessitated by a unique combination of virological and immunological characteristics that make protection against HIV-1 infection particularly difficult. Upon infection, HIV-1 genes are incorporated into the host genome, leading to a latent pool of infected CD4+ T cells that are resistant to removal by either anti-retroviral treatment (ART) or by the host immune system. Chronic ongoing replication outpaces the ability of the immune system to control HIV-1 infection and outfits the virus with unfavorable characteristics, including extraordinary antigenic diversity, heavy glycosylation, and the shielding of potentially vulnerable Env epitopes. The induction of effective antibodies to HIV-1 Env is further complicated by the high level of somatic hypermutation (SHM) associated with potent neutralization, and the limitation of bnAb B-cell lineage development by host immune tolerance mechanisms. The only HIV-1 vaccine efficacy trial that showed some protection (estimated 31% vaccine efficacy at 42 months) was carried out by the US Army and the government of Thailand using a canarypox vector (ALVAC) and a bivalent gp120 boost (AIDSVAX B/E).8 The trial, called RV144, was conducted in Thailand where the predominant HIV-1 strain was CRF01_AE. Studies of the immune correlates of decreased transmission risk in RV144 demonstrated that antibodies to the second variable (V2) loop were associated with lower risk of infection. These antibodies were not able to neutralize (tier 2) HIV-1 primary isolate strains, but were able to mediate antibody-dependent cellular cytotoxicity (ADCC).13 To follow up on the RV144 trial, a new trial using a similar design but with clade C envelope inserts (ALVAC-C prime, ALVAC + C/C gp120 boost) is now planned for South Africa where the epidemic is most pressing.13 In parallel with these plans, vaccine researchers continue to focus on the difficult task of developing a vaccine that is able to induce broadly neutralizing antibodies (bnAbs), that is, antibodies that can potently neutralize the majority of diverse circulating strains of HIV-1.9, 10 Such bnAbs can be made, but thus far, only in the setting of HIV infection, and only years after HIV transmission. Thus, the HIV-1 vaccine field is pursuing a two-pronged approach for antibody-based vaccine development—first to design immunogens and adjuvants that will result in increased vaccine efficacy from that seen in RV144, and second to design immunogens and immunization strategies that will result in the induction of the difficult-to-induce antibodies that can neutralize tier 2 HIV-1 primary isolates. Critical questions addressed in this volume of Immunological Reviews are: (i) Why do bnAbs develop in HIV-infected individuals but not in the setting of vaccination? (ii) How are bnAbs regulated compared to easily induced non-neutralizing antibodies, or tier 1-virus-neutralizing antibodies? (iii) Are there characteristics of HIV-infected individuals who make bnAbs that can help define strategies to induce bnAbs? (iv) What are preferred structures or forms of immunogens that are needed to induce bnAbs? (v) Are there additional strategies for use of antibodies to prevent HIV transmission other than via vaccine induction of bnAbs? and (vi) What are strategies for improvement on the vaccine efficacy seen in the RV144 trial? The reviews in this volume provide an in depth series of discussions on these questions. While there was initial enthusiasm in 1988-1990 for identification of the third variable loop (V3) of gp120 as the principle neutralization determinant of HIV-1,14-16 the exposure of the V3 loops was soon found to be unusually characteristic of viruses adapted to growth in T-cell lines. Data emerged from 1990 to 1994 that HIV-1 primary isolates grown in CD4 T cells were much more resistant to soluble CD4 and to gp120-induced neutralizing antibodies, including easy-to-induce gp120V3 loop and CD4-binding site antibodies.17-20 Over the last 10 years, progress has been made both in defining the vulnerable and relatively conserved neutralizing antibody sites on the HIV-1 envelope (Env), and in isolating large numbers of bnAbs from HIV-1 infected individuals. Initial progress involved recruiting large numbers of HIV-1-infected individuals, identifying those with plasma bnAb activity, and defining the specificities of bnAbs in plasma.9, 10 McCoy and Burton21 open the volume with a review of the five major specificities of bnAbs, and discuss the techniques used to isolate them from HIV-1-infected individuals. Critical to HIV-1 vaccine development has been the observation that bnAbs can indeed be made by the human immune system in the setting of infection, and the elucidation of their structural mode of recognition. Ward and Wilson22 review the history of structural analysis of the HIV-1 Env and discuss the remarkable journey over the past 10 years resulting in an understanding of the structural mode of recognition of bnAbs, culminating with co-crystal and cryo-electron microscopy structures of soluble native and a membrane-bound Env trimers. It has been known for over 30 years that HIV-1 infection perturbs the immune system, with profound effects on antibody responses.23 Moir and Fauci24 review their work on the effects of HIV-1 infection on the human immune system, and in particular on B-cell function. They provide a brief review of normal B-cell development and an in depth review of the effects of HIV-1 on B-cell functions. In particular, they note the dramatic changes during HIV-1 infection in memory B and plasmablast populations, and the profound dysfunction that can occur in antigen-specific responses. Nonetheless, in approximately 50% of HIV-1-infected individuals, cross-reactive neutralizing antibodies do develop in the setting of the profound perturbations caused by HIV-1.25 T follicular helper cells (Tfh) are key CD4+ T cells, found in germinal centers, that promote induction of high affinity antibodies. Havenar-Daughton et al.26 review their work on changes in Tfh levels in those that make bnAbs and the role of Tfh in the germinal center in generation of bnAbs. Moreover, they discuss their work on development of fine needle aspiration as a tool for study of Tfh over time in the setting of vaccination.26 Borrow and Moody discuss work characterizing Tfh and T regulatory cells (Treg) in the large CHAVI cohort of HIV-1-infected individuals who make bnAbs vs those that do not. The immunologic phenotype they discuss of elevated Tfh, low Treg, exhausted Treg, and high frequency of plasma autoantibodies is similar to findings in autoimmune disease patients such as systemic lupus erythematosus.27-29 They go on to discuss the relevance of these findings to bnAb regulation.30 Critical to our understanding of how to elicit bnAbs via vaccination has been investigation into the host controls limiting bnAb induction. In 2005 it was found that three of the known bnAbs were polyreactive for host antigens.29 This generated the hypothesis that, due to bnAb unusual traits such as long HCDR3 regions and polyreactivity, they were controlled by immune tolerance mechanisms. Tolerance control of bnAb lineages implies that bnAb precursors have been culled from bone marrow and the periphery or made anergic at tolerance checkpoints, and therefore are subdominant and disfavored. Here, Garnett Kelsoe and one of us (B. F. H.) review evidence for host immune tolerance control of bnAb development, describe host molecules that cross-react with bnAbs, and describe the rationale and methods for enhancing bnAb development with immune regulatory strategies.31 Understanding bnAb development and targeting naive B-cell receptors by vaccine antigens has required the development of new computational biology methods and programs for inferring germline antibody sequences and constructing evolutionary trees of bnAbs. Kwong et al.32 outline their multiple ‘antibodyome’ approaches to computational analysis of the antibody response to HIV-1. Next, Kepler and Wiehe33 discuss programs for immunoglobulin gene annotation, and strategies for inference of bnAb lineage intermediate and germline antibodies. Moreover, they go on to discuss progress in the genetic and structural analysis of HIV-1-neutralizing antibody affinity maturation. One technology that has propelled the study of HIV-1 vaccine development forward has been the use of human gene knock-in (KI) mice to study both tolerance control of bnAb development by host mechanisms and the ability of immunogens to stimulate the appropriate B cells and to drive bnAb lineage development. Verkoczy et al. review work done by them and the field with bnAb KI mice, including bnAb KI models expressing deduced precursor V(D)J rearrangements of mature bnAbs or unrearranged germline V, D, J segments (that can be assembled into variable region exons that encode bnAb precursors). These new mouse models allow the evaluation of vaccine immunogens and regimens for inducing bnAbs. They also review progress made in using immunogens to activate bnAb germline B cells in KI mice.34 For over 20 years the HIV-1 vaccine development field has been trying to induce protective antibodies with a variety of individual Env immunogens, or with mixtures of unrelated Envs, but without consistent success.9, 10 As noted above, the realization that immune tolerance can limit the induction or development of bnAb lineages, suggests that such lineages are disfavored and may require unique approaches to stimulate and induce affinity maturation.12, 35-37 Over the past 7 years the concept has arisen that specific Env immunogens will be needed to target the appropriate naive B cells that expresses the unmutated ancestor (UA) of a neutralizing antibody lineage. Furthermore, sequential immunogens may be needed to mature such lineages and to drive subdominant and disfavored B-cell lineages toward effective virus neutralization (Fig. 1). Bonsignori et al.38 describe the strategy of B-cell lineage design, wherein the co-evolution of HIV-1 and bnAb B-cell lineages are mapped and specific Envs are chosen from an individual that can be used in sequence for inducing similar bnAb B-cell lineages. They describe the results of mapping the co-evolution of CD4-binding site, V1V2-glycan- and V3-glycan-targeted bnAbs in HIV-1-infected individuals, and describe vaccine strategies that derive from this work. Penny Moore et al.39 describe elegant studies in an African HIV-1-infected individual that made V1V2-glycan bnAbs and describe strategies for recapitulating bnAb induction in the setting of vaccination from their work. Rogier Sanders and John Moore describe the design of the soluble recombinant Env trimeric proteins that include a disulfide bond between gp120 and gp41, and additional stabilizing mutations (SOSIP Env). Such SOSIP trimers have antigenic characteristics similar to native Env trimers. Sanders and Moore40 review both technical aspects of trimer purification, and data that these trimers can induce tier 2 autologous neutralizing antibodies better than Env monomers or non-SOSIP oligomers. The review by Hedestam et al. describes an additional approach to constructing stable recombinant Env trimers by replacing the native gp120-41 cleavage site with a native flexible linker (NFL). They describe new and evolving tools to evaluate B-cell vaccine response, including approaches to use next generation sequencing for antibody lineage tracing.41 Stamatatos et al. discuss the issue of necessity of targeting B cells expressing the unmutated common ancestor (UCA) and sometimes called germline ancestor of a bnAb lineage. It was originally observed that germline reverted variants of a bnAb did not bind HIV-1 Env and thus speculated that this was a prime reason for the inability of HIV Env to induce bnAbs.42, 43 With studies of bnAb co-evolution beginning with the transmitted/founder (TF) virus Envs from the time of transmission, we now know that for some bnAb specificities, TF Envs can bind the UCA and initiate bnAb lineages.44, 45 Nonetheless, structurally designed immunogens, particularly those for the VRC01 class of CD4-binding site bnAbs, have proven highly immunogenic for initiation of UA BCR-bearing B-cell activation and beginning of affinity maturation in bnAb knock-in mice.46-52 Thus, Stamatatos et al.53 review the need to target bnAb germlines, and discuss strategies of immunogen design that can meet this purpose. For many years, the Los Alamos National Laboratory HIV-1 Sequence Database has served the HIV vaccine field by collecting now over 500,000 HIV sequences from all over the world (http://www.hiv.lanl.gov/content/sequence/HIV/mainpage.html). Korber et al.54 review use of the LANL database, structural biology data sets, and new sets of HIV-1 Env sequences from HIV-1/bnAb co-evolution studies can inform polyvalent Env vaccine design. As noted above, the only HIV-1 vaccine trial to show any vaccine efficacy (RV144) utilized a canarypox vector (ALVAC-AE) and a bivalent B/E gp120 + ALVAC boost.8 Phase I trials with a new ALVAC-C prime, ALVAC-C/gp120 C/C boost vaccine have been completed and a new vaccine efficacy trial in Africa is planned. An immune correlates analysis of the RV144 trial showed that the decreased risk of transmission was associated with high IgG antibodies to the V2 region of Env.55 Furthermore, such antibodies displayed FcR-mediated ADCC as an effector function.55 Bournazos and Ravetch56 provide an in-depth review of anti-retroviral antibody FcγR-mediated effector functions and explain how antibodies can control and/or prevent HIV-1 infection. Tomaras and Plotkin57 review immune correlate principles in general and discuss that the immune correlates work on the RV144 trial over the past 7 years. This work reveals the complexity of protectively immunity—with the suggestion that multiple types of immune responses together, comprise an immune correlate—thus implicating polyfunctional immune control of HIV-1 transmission. Ackerman et al.58 describe new techniques for system serology analysis of vaccine trials for more comprehensive analysis of vaccine trial immune correlates that incorporates multiple antibody characteristic and effector functions. Lewis et al.59 describe an interesting new hypothesis of how to find targets of FcR-mediated effector function on HIV-1 Env that can directly impact vaccine design. They review examples of protection in animal models of ADCC and other non-neutralizing effector functions. One of us (J. R. M.) and coauthors Pegu Hessell and Haigwood60, discuss our current understanding of the role of bnAbs in mediating protection against infection. The newest generation of natural and engineered bnAbs demonstrate remarkable potency and breadth—providing the opportunity to use such products clinically, as pre-exposure prophylaxis, to prevent HIV infection in high-risk individuals. Brady et al.61, discuss an alternative approach to antibody immunoprophylaxis involving vector-mediated gene transfer by adeno-associated virus. They provide insights into the potential advantages, and obstacles to overcome, for the gene-based delivery of antibodies. Finally, Margolis et al. review the role and potential advantages of antibodies to treat HIV infection. They highlight antibody functions such as the ability to opsonize virus-infected cells and mediate ADCC, and ultimately, the potential to clear HIV-infected cells. In particular, they discuss the construction of bispecific antibodies as well as other antibody designs that may be superior to naturally occurring bnAbs for their ability to target the latently infected pool of CD4 T cells.62 This volume of Immunological Reviews brings together an extraordinary group of investigators who are conducting research on antibody-based immunity. It is fascinating to see the fields of virology, structural biology, humoral immunology, and vaccinology converge to provide new insights into vaccine design and immunization strategies that have the potential to induce protective antibody reposes against HIV-1. There is a major focus on understanding how the immune system generates bnAbs during natural infection and learning from both the isolation and structural characterization of such bnAbs, and the dissection of the immune pathways leading to the evolution of broad and potent antibodies, including virus-host co-evolution and host tolerance mechanisms that limit the induction and maturation of bnAbs. Substantial progress has been made in eliciting neutralizing antibodies in humanized mouse models, and this provides insights into how to design vaccines for human trials. In addition to antibody-based prevention, there is recent interest in the use of bnAbs to treat, or eradicate, HIV-1 infection. To verify and potentially improve on the RV144 vaccine trial, the strategy has centered on understanding immune correlates of protection and increasing the breadth of ADCC and other FcR-mediated responses to strains of HIV-1 circulating in South Africa. New approaches to understanding the quality of the antibody response, including IgG isotype, subclass, glycosylation patterns, and Fc-mediated effector functions are adding a new level of analysis, and potentially understanding, to in vivo protection. For induction of broadly neutralizing antibodies, the field is taking advantage of rapid progress in the structural understanding of the HIV-1 Env and its neutralization epitopes, and recent data on immune pathways of bnAbs, and is focused on design of sequential immunogens as well as recreating the immunologic milieu of HIV-1 infection in the setting of vaccination such that bnAbs will be allowed by the immune system to be made (Fig. 1). This work represents the interface between basic and translational research to evaluate vaccine and therapeutic candidates in the context of the human immune system. No other vaccine has required such a detailed understanding about fundamental aspects of the human immune system and its interaction with novel antigens. Several years ago, there was little optimism that a broadly neutralizing antibody-based HIV-1 vaccine could be designed, or would be feasible. The reviews in this issue provide a rational for have observed how the immune system generates in response to and we the of the initial interaction between and the naive B cells that will bnAb lineages. These are the toward the of broadly protective antibodies, and to development of an HIV-1 vaccine that will be globally This work was an of AIDS to F. for the for R. was by to the for and are on for HIV-1 broadly neutralizing antibodies.