Trends in Immunology
Volume 37, Issue 3, March 2016, Pages 221-232
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Review
HIV-1 Envelope Trimer Design and Immunization Strategies To Induce Broadly Neutralizing Antibodies

https://doi.org/10.1016/j.it.2016.01.007Get rights and content

Trends

The structure of the HIV-1 Env trimer has been solved by both X-ray crystallography and cryo-EM, guiding immunogen design improvements.

Recombinant native-like HIV-1 Env trimers, based on the SOSIP.664 design, induce autologous Tier-2 neutralizing antibodies in rabbits and macaques.

Stabilizing HIV-1 Env trimers in the closed pre-fusion state reduces their propensity to undergo reversible conformational transitions (‘breathing’), and this may be important for reducing their induction of immunodominant non-NAb responses.

Knock-in mice that express specific inferred germline B cell receptors (BCRs) or the complete human germline BCR repertoire are valuable for evaluating Env immunogens.

The identification of multiple broadly neutralizing antibodies (bNAbs) against the HIV-1 envelope glycoprotein (Env) trimer has facilitated its structural characterization and guided Env immunogen design. Several recent studies constitute progress in utilizing this knowledge for the development of an HIV-1 vaccine that induces bNAbs. Native-like Env trimers can induce autologous NAb responses against resistant (Tier-2) viruses in several animal models. Here we review recent studies aimed at addressing the challenge of driving the strong but narrowly focused NAb responses to Env trimers towards ones with much greater breadth. Among strategies that merit pursuing are using multiple trimers as sequential or simultaneous immunogens, targeting the germline precursors of bNAbs, delivering sequential lineages of trimers derived from infected individuals who developed bNAbs, and presenting trimers as particulate antigens.

Introduction

More than 30 years of research have failed to yield a vaccine that provides protection against HIV-1. Most licensed antiviral vaccines work through the induction of neutralizing antibodies (NAbs) [1]. To tackle the unprecedented global sequence diversity of HIV-1, a vaccine will most likely need to induce bNAbs. Recent studies have shown that ∼20–30% of HIV-1-infected individuals develop bNAbs and, furthermore, that passive transfer of bNAbs confers protective immunity in macaques, supporting the feasibility of developing a vaccine to induce such antibodies 2, 3, 4, 5, 6, 7, 8, 9. In general, HIV-1 Env vaccine development strategies parallel, and are often guided by, next-generation approaches to designing immunogens against other ‘difficult’ pathogens such as respiratory syncytial virus (RSV) and influenza virus 10, 11, 12, 13.

bNAbs target native Env on the HIV-1 surface and impede viral infection of target cells. HIV-1 Env is processed as a gp160 precursor that is proteolytically cleaved by furin into non-covalently linked gp120 and gp41 subunits, which assemble into a trimer of heterodimers. The instability and conformationally flexibility of this six-subunit trimer hindered determination of its atomic level structure for 15 years after the core of the HIV-1 gp120 monomer was structurally characterized [10]. The structure of a nearly complete form of the trimer, known as BG505 SOSIP.664, was finally solved in 2013 11, 12. Rapid progress then provided additional insight into the intricacies of the trimer 11, 12, 13, 14, 15, 16. These cumulative observations have extensively refined our understanding of the metastability of the trimer, and of how the gp41 fusion machinery functions to drive HIV-1 entry into cells. Furthermore, the trimer structures have allowed the detailed characterization of bNAb epitopes that is now guiding new structure-based immunogen design programs 16, 17, 18, 19, 20, 21, 22, 23, 24.

The receptor-binding gp120 subunits of the trimer mediate the initial attachment of HIV-1 to target cells (most commonly CD4+ T cells), while the gp41 components that anchor the trimer in the virus membrane drive the fusion process. The conformational changes in gp120 driven by CD4 binding, followed by the dissociation of gp120 from gp41 upon coreceptor binding, lead to the step-wise transition of the gp41 subunits from the pre-fusion structure to the energetically favorable six-helical bundle (6-HB); the energy released by this conformational transition is a crucial driver of virus–cell membrane fusion [25].

HIV-1 trimers have evolved to resist both the binding of NAbs and their induction, and when NAbs are induced, the mutation rate of the viral genome rapidly drives the emergence of escape mutants. The five highly variable loops (V1–V5) on gp120 shield the more conserved domains associated with receptor binding, a defense mechanism that is dramatically reinforced by the shielding effect of the 25–30 glycan moieties per gp120–gp41 protomer that decorate the trimer surface (half the mass of gp120 is carbohydrate). During HIV-1 infection, non-functional Env proteins that expose predominantly immunodominant non-NAb epitopes, such as uncleaved or otherwise defective trimers, dissociated gp120 monomers, the post-fusion 6-HB form of gp41, and assorted degradation products, also elicit antibodies 26, 27. It is currently unknown whether non-NAbs impede the NAb response to trimers, or whether they are irrelevant. The induction and binding of NAbs might also be influenced by the conformational flexibility of the trimer, which fluctuates between closed and more-open conformations 28, 29, 30, 31.

Despite these viral defenses, the co-evolution between escape variants and NAb affinity maturation drives the development of bNAbs in ∼20% of HIV-1 infected individuals 2, 5, 6. In general, bNAbs have acquired unusual characteristics that help to overcome the defenses of the trimer against antibody binding and neutralization. For example, bNAbs have almost invariably undergone extensive somatic hypermutation (SHM), they frequently have extremely long CDR-H3 loops, they are often polyreactive, and some of them are derived from rare precursor genes [32]. These intrinsic characteristics play a major role in understanding why it has been, and no doubt will remain, so hard to induce bNAbs by immunization with Env proteins.

We provide here an overview of the currently known classes of bNAb and their epitope specificities on the Env trimer. In this context we discuss how the design and use of native-like trimers may play a role in bNAb induction, and review the strategies that are currently being pursued based around the design and use of native-like, soluble recombinant envelope trimers. Other approaches, such as those using peptides, gp120 subunits, or scaffolded epitopes, are beyond the scope of this article, but are reviewed in detail elsewhere [110].

Section snippets

Broadly Neutralizing Antibodies

Because bNAbs can neutralize a large proportion of circulating viruses from different clades they are valuable templates for Env immunogen design. For many years, only four bNAbs were known: 2G12, b12, 2F5, and 4E10. A major advance in bNAb isolation and characterization was the development of single antigen-specific B cell cloning methods that allowed the rapid isolation of monoclonal antibodies (MAbs) 19, 20, 24, 33, 34, 35, 36, 37, 38, 39, 40.

Based on their target epitopes, bNAbs can be

Design of Env Trimer Immunogens

There are multiple ways to design immunogens intended to induce bNAbs, including, but not limited to, gp120 monomer-lineages, non-native gp140 proteins, gp120 core and outer domain (OD) proteins, epitope-specific scaffolds, and epitope-based peptides. All these approaches are beyond the scope of this article, but have been reviewed elsewhere [110]. We will focus here on the design and use of immunogens based on recombinant, soluble native-like trimers that closely mimic the native Env complex

Steering bNAb Development Pathways With Lineage Vaccines

Although the use of combinations of genetically diverse trimers, delivered sequentially or simultaneously, is an approach that is being pursued in animal studies, it should be recognized that bNAbs are not produced in the earliest stages of HIV-1 infection but emerge through co-evolutionary processes. Thus, viral escape from a rapid, autologous NAb response drives renewed cycles of B cell activation and affinity maturation in a process that is unlikely to be mimicked by the use of a few

Concluding Remarks

The development of native-like trimers during the past few years has opened several areas of active investigation that are developing a bNAb-based vaccine (see Outstanding Questions). Progress in this area melds well with the ever-increasing knowledge based on the very many bNAbs that have been isolated and characterized over approximately the same period; their collective existence strongly underpins the concept of a bNAb vaccine. The determination of what are now atomic-level structures of

Acknowledgments

We thank Gabe Ozorowski and Andrew Ward for providing the image used in Figure 1. We also thank the many colleagues who have contributed greatly to our understanding of native-like trimers and their potential as immunogens over the past few years. Work in our laboratories is supported by a National Institutes of Health Grant P01 AI110657, a Vidi grant from the Netherlands Organization for Scientific Research (NWO), and a Starting Investigator Grant from the European Research Council

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