Structural analysis of the unmutated ancestor of the HIV-1 envelope V2 region antibody CH58 isolated from an RV144 vaccine efficacy trial vaccinee

B cell somatic hypermutation following COVID-19 vaccination with Ad26.COV2.S

The viral vector-based COVID-19 vaccine Ad26.COV2.S has been recommended by the WHO since 2021 and has been administered to over 200 million people. Prior studies have shown that Ad26.COV2.S induces durable neutralizing antibodies (NAbs) that increase in coverage of variants over time, even in the absence of boosting or infection. Here, we studied humoral responses following Ad26.COV2.S vaccination in individuals enrolled in the initial Phase 1/2a trial of Ad26.COV2.S in 2020. Through 8 months post vaccination, serum NAb responses increased to variants, including B.1.351 (Beta) and B.1.617.2 (Delta), without additional boosting or infection. The level of somatic hypermutation, measured by nucleotide changes in the VDJ region of the heavy and light antibody chains, increased in Spike-specific B cells. Highly mutated mAbs from these sequences neutralized more SARS-CoV-2 variants than less mutated comparators. These findings suggest that the increase in NAb breadth over time following Ad26.COV2.S vaccination is mediated by affinity maturation.

Determination of Binding Affinity of Antibodies to HIV-1 Recombinant Envelope Glycoproteins, Pseudoviruses, Infectious Molecular Clones, and Cell-Expressed Trimeric gp160 Using Microscale Thermophoresis

Developing a preventative vaccine for HIV-1 has been a global priority. The elicitation of broadly neutralizing antibodies (bNAbs) against a broad range of HIV-1 envelopes (Envs) from various strains appears to be a critical requirement for an efficacious HIV-1 vaccine. To understand their ability to neutralize HIV-1, it is important to characterize the binding characteristics of bNAbs. Our work is the first to utilize microscale thermophoresis (MST), a rapid, economical, and flexible in-solution temperature gradient method to quantitatively determine the binding affinities of bNAbs and non-neutralizing monoclonal antibodies (mAbs) to HIV-1 recombinant envelope monomer and trimer proteins of different subtypes, pseudoviruses (PVs), infectious molecular clones (IMCs), and cells expressing the trimer. Our results demonstrate that the binding affinities were subtype-dependent. The bNAbs exhibited a higher affinity to IMCs compared to PVs and recombinant proteins. The bNAbs and mAbs bound with high affinity to native-like gp160 trimers expressed on the surface of CEM cells compared to soluble recombinant proteins. Interesting differences were seen with V2-specific mAbs. Although they recognize linear epitopes, one of the antibodies also bound to the Envs on PVs, IMCs, and a recombinant trimer protein, suggesting that the epitope was not occluded. The identification of epitopes on the envelope surface that can bind to high affinity mAbs could be useful for designing HIV-1 vaccines and for down-selecting vaccine candidates that can induce high affinity antibodies to the HIV-1 envelope in their native conformation.

Neutralization titer biomarker for antibody-mediated prevention of HIV-1 acquisition

The Antibody Mediated Prevention trials showed that the broadly neutralizing antibody (bnAb) VRC01 prevented acquisition of human immunodeficiency virus-1 (HIV-1) sensitive to VRC01. Using AMP trial data, here we show that the predicted serum neutralization 80% inhibitory dilution titer (PT80) biomarker-which quantifies the neutralization potency of antibodies in an individual's serum against an HIV-1 isolate-can be used to predict HIV-1 prevention efficacy. Similar to the results of nonhuman primate studies, an average PT80 of 200 (meaning a bnAb concentration 200-fold higher than that required to reduce infection by 80% in vitro) against a population of probable exposing viruses was estimated to be required for 90% prevention efficacy against acquisition of these viruses. Based on this result, we suggest that the goal of sustained PT80 <200 against 90% of circulating viruses can be achieved by promising bnAb regimens engineered for long half-lives. We propose the PT80 biomarker as a surrogate endpoint for evaluatinon of bnAb regimens, and as a tool for benchmarking candidate bnAb-inducing vaccines.

HIV vaccine delayed boosting increases Env variable region 2-specific antibody effector functions

In the RV144 HIV-1 phase III trial, vaccine efficacy directly correlated with the magnitude of the variable region 2-specific (V2-specific) IgG antibody response, and in the presence of low plasma IgA levels, with the magnitude of plasma antibody-dependent cellular cytotoxicity. Reenrollment of RV144 vaccinees in the RV305 trial offered the opportunity to define the function, maturation, and persistence of vaccine-induced V2-specific and other mAb responses after boosting. We show that the RV144 vaccine regimen induced persistent V2 and other HIV-1 envelope-specific memory B cell clonal lineages that could be identified throughout the approximately 11-year vaccination period. Subsequent boosts increased somatic hypermutation, a critical requirement for antibody affinity maturation. Characterization of 22 vaccine-induced V2-specific mAbs with epitope specificities distinct from previously characterized RV144 V2-specific mAbs CH58 and CH59 found increased in vitro antibody-mediated effector functions. Thus, when inducing non-neutralizing antibodies, one method by which to improve HIV-1 vaccine efficacy may be through late boosting to diversify the V2-specific response to increase the breadth of antibody-mediated anti-HIV-1 effector functions.

V2-Directed Vaccine-like Antibodies from HIV-1 Infection Identify an Additional K169-Binding Light Chain Motif with Broad ADCC Activity

Antibodies that bind residue K169 in the V2 region of the HIV-1 envelope correlated with reduced risk of infection in the RV144 vaccine trial but were restricted to two ED-motif-encoding light chain genes. Here, we identify an HIV-infected donor with high-titer V2 peptide-binding antibodies and isolate two antibody lineages (CAP228-16H/19F and CAP228-3D) that mediate potent antibody-dependent cell-mediated cytotoxicity (ADCC). Both lineages use the IGHV5-51 heavy chain germline gene, similar to the RV144 antibody CH58, but one lineage (CAP228-16H/19F) uses a light chain without the ED motif. A cocrystal structure of CAP228-16H bound to a V2 peptide identified a IGLV3-21 gene-encoded DDxD motif that is used to bind K169, with a mechanism that allows CAP228-16H to recognize more globally relevant V2 immunotypes. Overall, these data further our understanding of the development of cross-reactive, V2-binding, antiviral antibodies and effectively expand the human light chain repertoire able to respond to RV144-like immunogens.

Common helical V1V2 conformations of HIV-1 Envelope expose the α4β7 binding site on intact virions

The α4β7 integrin is a non-essential HIV-1 adhesion receptor, bound by the gp120 V1V2 domain, facilitating rapid viral dissemination into gut-associated lymphoid tissues. Antibodies blocking this interaction early in infection can improve disease outcome, and V1V2-targeted antibodies were correlated with moderate efficacy reported from the RV144 HIV-1 vaccine trial. Monoclonal α4β7-blocking antibodies recognise two slightly different helical V2 conformations, and current structural data suggests their binding sites are occluded in prefusion envelope trimers. Here, we report cocrystal structures of two α4β7-blocking antibodies from an infected donor complexed with scaffolded V1V2 or V2 peptides. Both antibodies recognised the same helix-coil V2 conformation as RV144 antibody CH58, identifying a frequently sampled alternative conformation of full-length V1V2. In the context of Envelope, this α-helical form of V1V2 displays highly exposed α4β7-binding sites, potentially providing a functional role for non-native Envelope on virion or infected cell surfaces in HIV-1 dissemination, pathogenesis, and vaccine design.

Antibodies blocking the V1V2 domain of HIV Envelope from binding integrin are associated with positive disease outcomes. Here, Wibmer et al. determine the structure of full length V1V2 bound to these antibodies, revealing an alternative fold of V1V2 with exposed integrin-binding sites that functions on non-native Envelope.

Multi-Donor Longitudinal Antibody Repertoire Sequencing Reveals the Existence of Public Antibody Clonotypes in HIV-1 Infection

Characterization of single antibody lineages within infected individuals has provided insights into the development of Env-specific antibodies. However, a systems-level understanding of the humoral response against HIV-1 is limited. Here, we interrogated the antibody repertoires of multiple HIV-infected donors from an infection-naive state through acute and chronic infection using next-generation sequencing. This analysis revealed the existence of "public" antibody clonotypes that were shared among multiple HIV-infected individuals. The HIV-1 reactivity for representative antibodies from an identified public clonotype shared by three donors was confirmed. Furthermore, a meta-analysis of publicly available antibody repertoire sequencing datasets revealed antibodies with high sequence identity to known HIV-reactive antibodies, even in repertoires that were reported to be HIV naive. The discovery of public antibody clonotypes in HIV-infected individuals represents an avenue of significant potential for better understanding antibody responses to HIV-1 infection, as well as for clonotype-specific vaccine development.

Insights into the Structural Basis of Antibody Affinity Maturation from Next-Generation Sequencing

Affinity maturation is the process whereby the immune system generates antibodies of higher affinities during a response to antigen. It is unique in being the only evolutionary mechanism known to operate on a molecule in an organism's own body. Deciphering the structural mechanisms through which somatic mutations in antibody genes increase affinity is critical to understanding the evolution of immune repertoires. Next-generation sequencing (NGS) has allowed the reconstruction of antibody clonal lineages in response to viral pathogens, such as HIV-1, which was not possible in earlier studies of affinity maturation. Crystal structures of antibodies from these lineages bound to their target antigens have revealed, at the atomic level, how antibodies evolve to penetrate the glycan shield of envelope glycoproteins, and how viruses in turn evolve to escape neutralization. Collectively, structural studies of affinity maturation have shown that increased antibody affinity can arise from any one or any combination of multiple diverse mechanisms, including improved shape complementarity at the interface with antigen, increased buried surface area upon complex formation, additional interfacial polar or hydrophobic interactions, and preorganization or rigidification of the antigen-binding site.

Potential contribution of gut microbiota and systemic inflammation on HIV vaccine effectiveness and vaccine design

The quest for an effective HIV-1 vaccine began as soon as the virus causing AIDS was identified. After several disappointing attempts, results of the Phase-III RV144 trial in Thailand were a beacon of hope for the field demonstrating correlation between protection and immunological markers. In order to optimize vaccine response, we underline results from yellow fever and hepatitis B vaccines, where protective responses were predicted by the pre-vaccination level of immune activation in healthy individuals. Such findings support the assessment and reduction of pre-vaccine immune activation in order to optimize vaccine response. Immune activation in healthy individuals can be influenced by age, presence of CMV infection, gut dysbiosis and microbial translocation. We speculate that the level of immune activation should therefore be assessed to better select participants in vaccine trials, and interventions to reduce inflammation should be used to increase protective HIV vaccine response.

Genetic and structural analyses of affinity maturation in the humoral response to HIV-1

Most broadly neutralizing antibodies (BNAbs) elicited in response to HIV-1 infection are extraordinarily mutated. One goal of HIV-1 vaccine development is to induce antibodies that are similar to the most potent and broad BNAbs isolated from infected subjects. The most effective BNAbs have very high mutation frequencies, indicative of the long periods of continual activation necessary to acquire the BNAb phenotype through affinity maturation. Understanding the mutational patterns that define the maturation pathways in BNAb development is critical to vaccine design efforts to recapitulate through vaccination the successful routes to neutralization breadth and potency that have occurred in natural infection. Studying the mutational changes that occur during affinity maturation, however, requires accurate partitioning of sequence data into B-cell clones and identification of the starting point of a B-cell clonal lineage, the initial V(D)J rearrangement. Here, we describe the statistical framework we have used to perform these tasks. Through the recent advancement of these and similar computational methods, many HIV-1 ancestral antibodies have been inferred, synthesized and their structures determined. This has allowed, for the first time, the investigation of the structural mechanisms underlying the affinity maturation process in HIV-1 antibody development. Here, we review what has been learned from this atomic-level structural characterization of affinity maturation in HIV-1 antibodies and the implications for vaccine design.

Glycosylation and oligomeric state of envelope protein might influence HIV-1 virion capture by α4β7 integrin

The α4ß7 integrin present on host cells recognizes the V1V2 domain of the HIV-1 envelope protein. This interaction might be involved in virus transmission. Administration of α4ß7-specific antibodies inhibit acquisition of SIV in a macaque challenge model. But the molecular details of V1V2: α4ß7 interaction are unknown and its importance in HIV-1 infection remains controversial. Our biochemical and mutational analyses show that glycosylation is a key modulator of V1V2 conformation and binding to α4ß7. Partially glycosylated, but not fully glycosylated, envelope proteins are preferred substrates for α4ß7 binding. Surprisingly, monomers of the envelope protein bound strongly to α4ß7 whereas trimers bound poorly. Our results suggest that a conformationally flexible V1V2 domain allows binding of the HIV-1 virion to the α4ß7 integrin, which might impart selectivity for the poorly glycosylated HIV-1 envelope containing monomers to be more efficiently captured by α4ß7 integrin present on mucosal cells at the time of HIV-1 transmission.

Immunodominance of Antibody Recognition of the HIV Envelope V2 Region in Ig-Humanized Mice

In the RV144 gp120 HIV vaccine trial, decreased transmission risk was correlated with Abs that reacted with a linear epitope at a lysine residue at position 169 (K169) in the HIV-1 envelope (Env) V2 region. The K169 V2 response was restricted to Abs bearing Vλ rearrangements that expressed aspartic acid/glutamic acid in CDR L2. The AE.A244 gp120 in AIDSVAX B/E also bound to the unmutated ancestor of a V2-glycan broadly neutralizing Ab, but this Ab type was not induced in the RV144 trial. In this study, we sought to determine whether immunodominance of the V2 linear epitope could be overcome in the absence of human Vλ rearrangements. We immunized IgH- and Igκ-humanized mice with the AE.A244 gp120 Env. In these mice, the V2 Ab response was focused on a linear epitope that did not include K169. V2 Abs were isolated that used the same human VH gene segment as an RV144 V2 Ab but paired with a mouse λ L chain. Structural characterization of one of these V2 Abs revealed how the linear V2 epitope could be engaged, despite the lack of aspartic acid/glutamic acid encoded in the mouse repertoire. Thus, despite the absence of the human Vλ locus in these humanized mice, the dominance of Vλ pairing with human VH for HIV-1 Env V2 recognition resulted in human VH pairing with mouse λ L chains instead of allowing otherwise subdominant V2-glycan broadly neutralizing Abs to develop.

Structure-based Design of Cyclically Permuted HIV-1 gp120 Trimers That Elicit Neutralizing Antibodies

A major goal for HIV-1 vaccine development is an ability to elicit strong and durable broadly neutralizing antibody (bNAb) responses. The trimeric envelope glycoprotein (Env) spikes on HIV-1 are known to contain multiple epitopes that are susceptible to bNAbs isolated from infected individuals. Nonetheless, all trimeric and monomeric Env immunogens designed to date have failed to elicit such antibodies. We report the structure-guided design of HIV-1 cyclically permuted gp120 that forms homogeneous, stable trimers, and displays enhanced binding to multiple bNAbs, including VRC01, VRC03, VRC-PG04, PGT128, and the quaternary epitope-specific bNAbs PGT145 and PGDM1400. Constructs that were cyclically permuted in the V1 loop region and contained an N-terminal trimerization domain to stabilize V1V2-mediated quaternary interactions, showed the highest homogeneity and the best antigenic characteristics. In guinea pigs, a DNA prime-protein boost regimen with these new gp120 trimer immunogens elicited potent neutralizing antibody responses against highly sensitive Tier 1A isolates and weaker neutralizing antibody responses with an average titer of about 115 against a panel of heterologous Tier 2 isolates. A modest fraction of the Tier 2 virus neutralizing activity appeared to target the CD4 binding site on gp120. These results suggest that cyclically permuted HIV-1 gp120 trimers represent a viable platform in which further modifications may be made to eventually achieve protective bNAb responses.

Structural basis for germline antibody recognition of HIV-1 immunogens

Efforts to elicit broadly neutralizing antibodies (bNAbs) against HIV-1 require understanding germline bNAb recognition of HIV-1 envelope glycoprotein (Env). The VRC01-class bNAb family derived from the VH1-2*02 germline allele arose in multiple HIV-1–infected donors, yet targets the CD4-binding site on Env with common interactions. Modified forms of the 426c Env that activate germline-reverted B cell receptors are candidate immunogens for eliciting VRC01-class bNAbs. We present structures of germline-reverted VRC01-class bNAbs alone and complexed with 426c-based gp120 immunogens. Germline bNAb–426c gp120 complexes showed preservation of VRC01-class signature residues and gp120 contacts, but detectably different binding modes compared to mature bNAb-gp120 complexes. Unlike typical antibody-antigen interactions, VRC01–class germline antibodies exhibited preformed antigen-binding conformations for recognizing immunogens. Affinity maturation introduced substitutions increasing induced-fit recognition and electropositivity, potentially to accommodate negatively-charged complex-type N-glycans on gp120. These results provide general principles relevant to the unusual evolution of VRC01–class bNAbs and guidelines for structure-based immunogen design.

DOI:http://dx.doi.org/10.7554/eLife.13783.001

When human immunodeficiency virus-1 (HIV-1) infects humans it can cause a serious disease that damages the immune system. Currently there is no cure for this disease and there are no vaccines available to halt the spread of the virus. Researchers are hoping to be able to develop a single vaccine that can protect individuals against every form (or strain) of HIV-1, but this has proved difficult because many different versions of the virus exist.

An effective vaccine triggers long-lasting immunity to a particular virus or microbe by activating the production of proteins called antibodies that identify and help to destroy the threat. Research has shown that most individuals infected with HIV-1 produce antibodies that can only recognize a few HIV strains. However, there are rare individuals who produce “broadly neutralizing antibodies”; that is, antibodies that can recognize and help to kill 90% or more of HIV-1 strains. Understanding how broadly neutralizing antibodies are produced in infected individuals may aid the development of a vaccine that can protect others from the many circulating strains of HIV.

When an individual encounters a virus, immature antibodies are modified to generate mature antibodies that bind more effectively to specific virus proteins. Here, Scharf et al. investigated how a class of broadly neutralizing antibodies called VRC01-class antibodies, which bind to an HIV protein called gp120, are produced. The experiments used a technique called X-ray crystallography to reveal the three-dimensional structures of immature versions of these antibodies when they are bound to gp120.

Scharf et al. discovered that, unlike most antibodies, the overall final structure of VRC01 antibodies is formed before the antibody matures. Instead of making large changes to the structure of these antibodies, the maturation process makes VRC01-class antibodies become more positively charged, which allows them to bind to gp120 proteins on a wider variety of HIV viruses. These findings suggest that it may be possible to use modified gp120 proteins in vaccines to trigger the production of broadly neutralizing antibodies against HIV.

DOI:http://dx.doi.org/10.7554/eLife.13783.002