Somatic mutations of the immunoglobulin framework are generally required for broad and potent HIV-1 neutralization

Manipulating the selection forces during affinity maturation to generate cross-reactive HIV antibodies

Generation of potent antibodies by a mutation-selection process called affinity maturation is a key component of effective immune responses. Antibodies that protect against highly mutable pathogens must neutralize diverse strains. Developing effective immunization strategies to drive their evolution requires understanding how affinity maturation happens in an environment where variants of the same antigen are present. We present an in silico model of affinity maturation driven by antigen variants which reveals that induction of cross-reactive antibodies often occurs with low probability because conflicting selection forces, imposed by different antigen variants, can frustrate affinity maturation. We describe how variables such as temporal pattern of antigen administration influence the outcome of this frustrated evolutionary process. Our calculations predict, and experiments in mice with variant gp120 constructs of the HIV envelope protein confirm, that sequential immunization with antigen variants is preferred over a cocktail for induction of cross-reactive antibodies focused on the shared CD4 binding site epitope.

Insights into the trimeric HIV-1 envelope glycoprotein structure

The HIV-1 envelope glycoprotein (Env) trimer is responsible for receptor recognition and viral fusion with CD4(+) T cells, and is the sole target for neutralizing antibodies. Thus, understanding its molecular architecture is of significant interest. However, the Env trimer has proved to be a challenging target for 3D structure determination. Recent electron microscopy (EM) and X-ray structures have at last enabled us to decipher the structural complexity and unique features of the Env trimer, and how it is recognized by an ever-expanding arsenal of potent broadly neutralizing antibodies. We describe our current knowledge of the Env trimer structure in the context of exciting recent developments in the identification and characterization of HIV broadly neutralizing antibodies.

Affinity enhancement of antibodies: how low-affinity antibodies produced early in immune responses are followed by high-affinity antibodies later and in memory B-cell responses

The antibodies produced initially in response to most antigens are high molecular weight (MW) immunoglobulins (IgM) with low affinity for the antigen, while the antibodies produced later are lower MW classes (e.g., IgG and IgA) with, on average, orders of magnitude higher affinity for that antigen. These changes, often termed affinity maturation, take place largely in small B-cell clusters (germinal center; GC) in lymphoid tissues in which proliferating antigen-stimulated B cells express the highly mutagenic cytidine deaminase that mediates immunoglobulin class-switching and sequence diversification of the immunoglobulin variable domains of antigen-binding receptors on B cells (BCR). Of the large library of BCR-mutated B cells thus rapidly generated, a small minority with affinity-enhancing mutations are selected to survive and differentiate into long-lived antibody-secreting plasma cells and memory B cells. BCRs are also endocytic receptors; they internalize and cleave BCR-bound antigen, yielding peptide-MHC complexes that are recognized by follicular helper T cells. Imperfect correlation between BCR affinity for antigen and cognate T-cell engagement may account for the increasing affinity heterogeneity that accompanies the increasing average affinity of antibodies. Conservation of mechanisms underlying mutation and selection of high-affinity antibodies over the ≈200 million years of evolution separating bird and mammal lineages points to the crucial role of antibody affinity enhancement in adaptive immunity.

HIV antibodies. Antigen modification regulates competition of broad and narrow neutralizing HIV antibodies

Some HIV-infected individuals develop broadly neutralizing antibodies (bNAbs), whereas most develop antibodies that neutralize only a narrow range of viruses (nNAbs). bNAbs, but not nNAbs, protect animals from experimental infection and are likely a key component of an effective vaccine. nNAbs and bNAbs target the same regions of the viral envelope glycoprotein (Env), but for reasons that remain unclear only nNAbs are elicited by Env immunization. We show that in contrast to germline-reverted (gl) bNAbs, glnNAbs recognized diverse recombinant Envs. Moreover, owing to binding affinity differences, nNAb B cell progenitors had an advantage in becoming activated and internalizing Env compared with bNAb B cell progenitors. We then identified an Env modification strategy that minimized the activation of nNAb B cells targeting epitopes that overlap those of bNAbs.

Epitope target structures of Fc-mediated effector function during HIV-1 acquisition

PURPOSE OF REVIEW

This review analyzes recent studies suggesting that highly conserved epitopes in the HIV-1 Env trimer are targets of potentially protective nonneutralizing antibodies that mediate antibody-dependent cellular cytotoxicity.

RECENT FINDINGS

Recent studies in both non-human primates and humans suggest that nonneutralizing antibodies play a role in blocking infection with hybrid simian HIV (SHIV)/simian immunodeficiency virus (SIV) or HIV-1 by Fc-mediated effector function, in particular antibody-dependent cellular cytotoxicity. Further, several studies implicate highly conserved epitopes in the C1 region of gp120 as targets of these antibodies. However, these suggestions are controversial, as passive immunization studies do not indicate that such antibodies can block acquisition in non-human primates. Potential reasons for this discrepancy are discussed in the structural context of potent antibody-dependent cellular cytotoxicity epitopes on target cells during the narrow window of opportunity when antibodies can block HIV-1 acquisition.

SUMMARY

Cumulative evidence suggests that, in addition to virus neutralization, Fc-mediated effector responses to highly conserved epitopes in the HIV-1 trimer play distinct as well as overlapping roles in blocking HIV-1 acquisition. Evidence will be discussed as to whether nonneutralizing antibodies specific for epitopes on the HIV-1 Env trimer that become exposed during viral entry contribute significantly to blocking HIV-1 acquisition.

Immunogen design to focus the B-cell repertoire

PURPOSE OF REVIEW

This review aims to bring together recent developments relevant to the design of HIV-1 envelope glycoprotein-based immunogens to elicit broadly neutralizing antibodies (bNAbs).

RECENT FINDINGS

The combined use of structural biology and deep sequencing of antigen-specific B-cell lineages has allowed cross-sectional and longitudinal views of antibody evolution towards broad and potent neutralization of HIV-1. Recent advances in molecular modelling allied with protein and glycoprotein engineering have fuelled the design of new-generation viral envelope glycoproteins (Env)-based antigens.

SUMMARY

Although proof-of-principle for vaccine elicitation of bNAbs to HIV-1 is still lacking, many of the conceptual hurdles are being addressed.

Autoreactivity in HIV-1 broadly neutralizing antibodies: implications for their function and induction by vaccination

PURPOSE OF REVIEW

This review discusses progress in understanding the impact of immune tolerance on inducing broadly neutralizing antibodies (bnAbs), and how such knowledge can be incorporated into novel immunization approaches.

RECENT FINDINGS

Over 120 bnAbs have now been isolated, all of which bear unusual features associated with host tolerance controls, but paradoxically may also be required for their function. Evidence that poly/autoreactivity of membrane proximal external region bnAbs can invoke such controls has been demonstrated by knock-in technology, highlighting its potential for studying the impact of tolerance in the generation of bnAb lineages to distinct HIV-1 envelope targets. The requirement for extensive affinity maturation in developing neutralization breadth/potency during infection is being examined, and similar studies in the setting of immunization will be aided by testing novel vaccine approaches in knock-in models that either selectively express reverted V(D)J rearrangements, or unrearranged germline segments, from which bnAb lineages originate.

SUMMARY

It is increasingly apparent that immune tolerance, sometimes invoked by self-reactivity that overlaps with bnAb epitope specificity, adds to a formidable set of roadblocks impeding bnAb induction. The path to an effective HIV-1 vaccine may thus benefit from a deeper understanding of host controls, including categorizing those that are unique or common at distinct bnAb targets, and ranking those most feasible to overcome by immunization. Ultimately, such emerging information will be critical to incorporate into new vaccine approaches that can be tested in human trials.

Complementary and synergistic activities of anti-V3, CD4bs and CD4i antibodies derived from a single individual can cover a wide range of HIV-1 strains

Antibodies with modest neutralizing activity and narrow breadth are commonly elicited in HIV-1. Here, we evaluated the complementary and synergistic activities of a set of monoclonal antibodies (MAb) isolated from a single patient, directed to V3, CD4 binding site (CD4bs), and CD4 induced (CD4i) epitopes. Despite low somatic hypermutation percentages in the variable regions, these MAbs covered viral strains from subtypes B, C, A and CRF01_AE and transmitted/founder viruses in terms of binding, neutralizing and antibody-dependent cell-mediated cytotoxicity (ADCC) activities. In addition, a combination of the anti-V3 and CD4bs MAbs showed a synergistic effect over the neutralization of HIV-1JR-FL. A humoral response from a single patient covered a wide range of viruses by complementary and synergistic activities of antibodies with different specificities. Inducing a set of narrow neutralizing antibodies, easier to induce than the broadly neutralizing antibodies, could be a strategy for developing an effective vaccine against HIV-1.

An Outdated Notion of Antibody Specificity is One of the Major Detrimental Assumptions of the Structure-Based Reverse Vaccinology Paradigm, Which Prevented It from Helping to Develop an Effective HIV-1 Vaccine

The importance of paradigms for guiding scientific research is explained with reference to the seminal work of Karl Popper and Thomas Kuhn. A prevalent paradigm, followed for more than a decade in HIV-1 vaccine research, which gave rise to the strategy known as structure-based reverse vaccinology is described in detail. Several reasons why this paradigm did not allow the development of an effective HIV-1 vaccine are analyzed. A major reason is the belief shared by many vaccinologists that antibodies possess a narrow specificity for a single epitope and are not polyspecific for a diverse group of potential epitopes. When this belief is abandoned, it becomes obvious that the one particular epitope structure observed during the crystallographic analysis of a neutralizing antibody–antigen complex does not necessarily reveal, which immunogenic structure should be used to elicit the same type of neutralizing antibody. In the physical sciences, scientific explanations are usually presented as logical deductions derived from a relevant law of nature together with certain initial conditions. In immunology, causal explanations in terms of a single cause acting according to a law of nature are not possible because numerous factors always play a role in bringing about an effect. The implications of this state of affairs for the rational design of HIV vaccines are outlined. An alternative approach to obtain useful scientific understanding consists in intervening empirically in the immune system and it is suggested that manipulating the system experimentally is needed to learn to control it and achieve protective immunity by vaccination.

Targeting B-cell germlines and focusing affinity maturation: the next hurdles in HIV-1-vaccine development?

Vaccines that protect against viral infection usually elicit neutralizing antibodies, but HIV-1 vaccine candidates have failed to induce broad and potent such responses. Broadly active neutralizing antibodies (bNAbs) do, however, slowly emerge in a minority of HIV-1-infected subjects; and passive immunization with bNAbs protects against viral acquisition in animal models of HIV-1 infection. New techniques have made it possible to interrogate human B cells and thereby to isolate highly potent bNAbs to uncharted epitope clusters. Furthermore, recent high-resolution structure determinations of near-native soluble envelope glycoprotein trimers in complex with different bNAbs reveal the molecular basis for neutralization. Such trimer structures may serve as blueprints for vaccine design. Here we discuss how a vaccine might bridge a reactivity gap from germline antibody to bNAb and simulate the intricate stimuli of affinity maturation that sometimes prevail in chronic infection.

Antibody persistence and T-cell balance: two key factors confronting HIV vaccine development

The quest for a prophylactic AIDS vaccine is ongoing, but it is now clear that the successful vaccine must elicit protective antibody responses. Accordingly, intense efforts are underway to identify immunogens that elicit these responses. Regardless of the mechanism of antibody-mediated protection, be it neutralization, Fc-mediated effector function, or both, antibody persistence and appropriate T-cell help are significant problems confronting the development of a successful AIDS vaccine. Here, we discuss the evidence illustrating the poor persistence of antibody responses to Env, the envelope glycoprotein of HIV-1, and the related problem of CD4(+) T-cell responses that compromise vaccine efficacy by creating excess cellular targets of HIV-1 infection. Finally, we propose solutions to both problems that are applicable to all Env-based AIDS vaccines regardless of the mechanism of antibody-mediated protection.

A fusion intermediate gp41 immunogen elicits neutralizing antibodies to HIV-1

The membrane-proximal external region (MPER) of the human immunodeficiency virus, type 1 (HIV-1) envelope glycoprotein subunit gp41 is targeted by potent broadly neutralizing antibodies 2F5, 4E10, and 10E8. These antibodies recognize linear epitopes and have been suggested to target the fusion intermediate conformation of gp41 that bridges viral and cellular membranes. Anti-MPER antibodies exert different degrees of membrane interaction, which is considered to be the limiting factor for the generation of such antibodies by immunization. Here we characterize a fusion intermediate conformation of gp41 (gp41(int)-Cys) and show that it folds into an elongated ∼ 12-nm-long extended structure based on small angle x-ray scattering data. Gp41(int)-Cys was covalently linked to liposomes via its C-terminal cysteine and used as immunogen. The gp41(int)-Cys proteoliposomes were administered alone or in prime-boost regimen with trimeric envelope gp140(CA018) in guinea pigs and elicited high anti-gp41 IgG titers. The sera interacted with a peptide spanning the MPER region, demonstrated competition with broadly neutralizing antibodies 2F5 and 4E10, and exerted modest lipid binding, indicating the presence of MPER-specific antibodies. Although the neutralization potency generated solely by gp140(CA018) was higher than that induced by gp41(int)-Cys, the majority of animals immunized with gp41(int)-Cys proteoliposomes induced modest breadth and potency in neutralizing tier 1 pseudoviruses and replication-competent simian/human immunodeficiency viruses in the TZM-bl assay as well as responses against tier 2 HIV-1 in the A3R5 neutralization assay. Our data thus demonstrate that liposomal gp41 MPER formulation can induce neutralization activity, and the strategy serves to improve breadth and potency of such antibodies by improved vaccination protocols.

Characteristics of memory B cells elicited by a highly efficacious HPV vaccine in subjects with no pre-existing immunity

Licensed human papillomavirus (HPV) vaccines provide near complete protection against the types of HPV that most commonly cause anogenital and oropharyngeal cancers (HPV 16 and 18) when administered to individuals naive to these types. These vaccines, like most other prophylactic vaccines, appear to protect by generating antibodies. However, almost nothing is known about the immunological memory that forms following HPV vaccination, which is required for long-term immunity. Here, we have identified and isolated HPV 16-specific memory B cells from female adolescents and young women who received the quadrivalent HPV vaccine in the absence of pre-existing immunity, using fluorescently conjugated HPV 16 pseudoviruses to label antigen receptors on the surface of memory B cells. Antibodies cloned and expressed from these singly sorted HPV 16-pseudovirus labeled memory B cells were predominantly IgG (>IgA>IgM), utilized diverse variable genes, and potently neutralized HPV 16 pseudoviruses in vitro despite possessing only average levels of somatic mutation. These findings suggest that the quadrivalent HPV vaccine provides an excellent model for studying the development of B cell memory; and, in the context of what is known about memory B cells elicited by influenza vaccination/infection, HIV-1 infection, or tetanus toxoid vaccination, indicates that extensive somatic hypermutation is not required to achieve potent vaccine-specific neutralizing antibody responses.

There is an urgent need to better understand how to reliably generate effective vaccines, particularly subunit vaccines, as certain pathogens are considered to pose too great of a safety risk to be developed as live, attenuated or killed vaccines (e.g., HIV-1). The human papillomavirus (HPV) vaccines are two of the most effective subunit vaccines ever developed and have continued to show protection against HPV associated disease up to and beyond five years post-vaccination. Moreover, the target population for these vaccines have essentially no pre-existing immunity to the HPV types covered by the vaccine; therefore, these vaccines provide an excellent model for studying the immunity elicited by a highly effective subunit vaccine. As the HPV vaccines, like most vaccines, protect by generating antibodies, we are interested in characterizing the memory B cells elicited by the HPV vaccine. Memory B cells help to sustain antibody levels over time by rapidly differentiating into antibody secreting cells upon pathogen re-exposure. Although previous studies have provided evidence that the HPV vaccines elicit memory B cells, they did not characterize these cells. Here, we have isolated HPV-specific memory B cells from adolescent females and women who received the quadrivalent HPV vaccine and have cloned antibodies from these cells. Importantly, we find that these antibodies potently inhibit HPV and that the memory B cells from which they derive exhibit hallmarks of long-lived memory B cells.

Human immunodeficiency virus vaccines

Although some success was achieved in recent years in HIV prevention, an effective vaccine remains the means with the most potential of curtailing HIV-1 infections worldwide. Despite multiple failed attempts, a recent HIV vaccine regimen demonstrated modest protection from infection. Although the protective efficacy in this trial was not sufficient to warrant licensure, it spurred renewed optimism in the field and has provided valuable insights for improving future vaccine designs. This review summarizes the pertinent details of vaccine development and discusses ways the field is moving forward to develop a vaccine to prevent HIV infection and disease progression.

Rapid development of broadly influenza neutralizing antibodies through redundant mutations

The neutralizing antibody response to influenza virus is dominated by antibodies that bind to the globular head of haemagglutinin, which undergoes a continuous antigenic drift, necessitating the re-formulation of influenza vaccines on an annual basis. Recently, several laboratories have described a new class of rare influenza-neutralizing antibodies that target a conserved site in the haemagglutinin stem. Most of these antibodies use the heavy-chain variable region VH1-69 gene, and structural data demonstrate that they bind to the haemagglutinin stem through conserved heavy-chain complementarity determining region (HCDR) residues. However, the VH1-69 antibodies are highly mutated and are produced by some but not all individuals, suggesting that several somatic mutations may be required for their development. To address this, here we characterize 197 anti-stem antibodies from a single donor, reconstruct the developmental pathways of several VH1-69 clones and identify two key elements that are required for the initial development of most VH1-69 antibodies: a polymorphic germline-encoded phenylalanine at position 54 and a conserved tyrosine at position 98 in HCDR3. Strikingly, in most cases a single proline to alanine mutation at position 52a in HCDR2 is sufficient to confer high affinity binding to the selecting H1 antigen, consistent with rapid affinity maturation. Surprisingly, additional favourable mutations continue to accumulate, increasing the breadth of reactivity and making both the initial mutations and phenylalanine at position 54 functionally redundant. These results define VH1-69 allele polymorphism, rearrangement of the VDJ gene segments and single somatic mutations as the three requirements for generating broadly neutralizing VH1-69 antibodies and reveal an unexpected redundancy in the affinity maturation process.

Ontogeny of recognition specificity and functionality for the broadly neutralizing anti-HIV antibody 4E10

The process of antibody ontogeny typically improves affinity, on-rate, and thermostability, narrows polyspecificity, and rigidifies the combining site to the conformer optimal for binding from the broader ensemble accessible to the precursor. However, many broadly-neutralizing anti-HIV antibodies incorporate unusual structural elements and recognition specificities or properties that often lead to autoreactivity. The ontogeny of 4E10, an autoreactive antibody with unexpected combining site flexibility, was delineated through structural and biophysical comparisons of the mature antibody with multiple potential precursors. 4E10 gained affinity primarily by off-rate enhancement through a small number of mutations to a highly conserved recognition surface. Controverting the conventional paradigm, the combining site gained flexibility and autoreactivity during ontogeny, while losing thermostability, though polyspecificity was unaffected. Details of the recognition mechanism, including inferred global effects due to 4E10 binding, suggest that neutralization by 4E10 may involve mechanisms beyond simply binding, also requiring the ability of the antibody to induce conformational changes distant from its binding site. 4E10 is, therefore, unlikely to be re-elicited by conventional vaccination strategies.

Early preservation of CXCR5+ PD-1+ helper T cells and B cell activation predict the breadth of neutralizing antibody responses in chronic HIV-1 infection

Much is known about the characteristics of broadly neutralizing antibodies (bNAbs) generated during HIV-1 infection, but little is known about immunological mechanisms responsible for their development in only a minority of those infected by HIV-1. By monitoring longitudinally a cohort of HIV-1-infected subjects, we observed that the preservation of CXCR5(+) CD4(+) T helper cell frequencies and activation status of B cells during the first year of infection correlates with the maximum breadth of plasma neutralizing antibody responses during chronic infection independently of viral load. Although, during the first year of infection, no differences were observed in the abilities of peripheral CXCR5(+) CD4(+) T helper cells to induce antibody secretion by autologous naive B cells, higher frequencies of class-switched antibodies were detected in cocultures of CXCR5(+) CD4(+) T and B cells from the subjects who later developed broadly neutralizing antibody responses than those who did not. Furthermore, B cells from the former subjects had higher expression of AICDA than B cells from the latter subjects, and transcript levels correlated with the frequency of CXCR5(+) CD4(+) T cells. Thus, the early preservation of CXCR5(+) CD4(+) T cells and B cell function are central to the development of bNAbs. Our study provides a possible explanation for their infrequent generation during HIV-1 infection.

IMPORTANCE

Broadly neutralizing antibodies are developed by HIV-1-infected subjects, but so far (and despite intensive efforts over the past 3 decades) they have not been elicited by immunization. Understanding how bNAbs are generated during natural HIV-1 infection and why only some HIV-1-infected subjects generate such antibodies will assist our efforts to elicit bNAbs by immunization. CXCR5(+) PD-1(+) CD4(+) T cells are critical for the development of high-affinity antigen-specific antibody responses. In our study, we found that the HIV-1-infected subjects who develop bNAbs have a higher frequency of peripheral CXCR5(+) PD-1(+) CD4(+) T cells in early infection and also that this frequency mirrored what was observed in uninfected subjects and correlated with the level of B cell activation across subjects. Our study highlights the critical role helper T cell function has in the elicitation of broadly neutralizing antibody responses in the context of HIV infection.

HIV-1 Env-specific memory and germinal center B cells in C57BL/6 mice

Continued efforts to define the immunogenic properties of the HIV-1 envelope glycoproteins (Env) are needed to elicit effective antibody (Ab) responses by vaccination. HIV-1 is a highly neutralization-resistant virus due to conformational and glycan shielding of conserved Ab determinants on the virus spike. Elicitation of broadly neutralizing Abs that bind poorly accessible epitope regions on Env is therefore extremely challenging and will likely require selective targeting of specific sub-determinants. To evaluate such approaches there is a pressing need for in vivo studies in both large and small animals, including mice. Currently, most mouse immunization studies are performed in the BALB/c strain; however, the C57BL/6 strain offers improved possibilities for mechanistic studies due to the availability of numerous knock-out strains on this genetic background. Here, we compared Env immunogenicity in BALB/c and C57BL/6 mice and found that the magnitude of the antigen-specific response was somewhat lower in C57BL/6 than in BALB/c mice by ELISA but not significantly different by B cell ELISpot measurements. We then established protocols for the isolation of single Env-specific memory B cells and germinal center (GC) B cells from immunized C57BL/6 mice to facilitate future studies of the elicited response at the monoclonal Ab level. We propose that these protocols can be used to gain an improved understanding of the early recruitment of Env-specific B cells to the GC as well as the archiving of such responses in the memory B cell pool following immunization.

Broad and potent HIV-1 neutralization by a human antibody that binds the gp41-gp120 interface

The isolation of human monoclonal antibodies (mAbs) is providing important insights regarding the specificities that underlie broad neutralization of HIV-1 (reviewed in¹). Here we report a broad and extremely potent HIV-specific mAb, termed 35O22, which binds novel HIV-1 envelope glycoprotein (Env) epitope. 35O22 neutralized 62% of 181 pseudoviruses with an IC₅₀<50 μg/ml. The median IC₅₀ of neutralized viruses was 0.033 μg/ml, among the most potent thus far described. 35O22 did not bind monomeric forms of Env tested, but did bind the trimeric BG505 SOSIP.664. Mutagenesis and a reconstruction by negative-stain electron microscopy of the Fab in complex with trimer revealed it to bind a conserved epitope, which stretched across gp120 and gp41. The specificity of 35O22 represents a novel site of vulnerability on HIV Env, which serum analysis indicates to be commonly elicited by natural infection. Binding to this new site of vulnerability may thus be an important complement to current mAb-based approaches to immunotherapies, prophylaxis, and vaccine design.

The Antibody Germline/Maturation Hypothesis, Elicitation of Broadly Neutralizing Antibodies Against HIV-1 and Cord Blood IgM Repertoires

We have previously observed that all known potent broadly neutralizing antibodies (bnAbs) against HIV-1 are highly divergent from their putative germline predecessors in contrast to bnAbs against viruses causing acute infections such as henipaviruses and SARS CoV, which are much less divergent from their germline counterparts. Consequently, we have hypothesized that germline antibodies may not bind to the HIV-1 envelope glycoprotein (Env) because they are so different compared to the highly somatically mutated HIV-1-specific bnAbs. We have further hypothesized that the immunogenicity of highly conserved epitopes on the HIV-1 envelope glycoproteins (Envs) may be reduced or eliminated by their very weak or absent interactions with germline antibodies and immune responses leading to the elicitation of bnAbs may not be initiated and/or sustained. Even if such responses are initiated, the maturation pathways are so extraordinarily complex that prolonged periods of time may be required for elicitation of bnAbs with defined unique sequences. We provided the initial evidence supporting this antibody germline/maturation hypothesis, which prompted a number of studies to design vaccine immunogens that could bind putative germline predecessors of known bnAbs and to explore complex B cell lineages. However, guiding the immune system through the exceptionally complex antibody maturation pathways to elicit known bnAbs remains a major challenge. Here, we discuss studies exploring the antibody germline/maturation hypothesis as related to elicitation of bnAbs against HIV-1 and present our recent data demonstrating the existence of germline-like precursors of VRC01 antibodies in a human cord blood IgM library.

HIV-1 receptor binding site-directed antibodies using a VH1-2 gene segment orthologue are activated by Env trimer immunization

Broadly neutralizing antibodies (bNAbs) isolated from chronically HIV-1 infected individuals reveal important information regarding how antibodies target conserved determinants of the envelope glycoprotein (Env) spike such as the primary receptor CD4 binding site (CD4bs). Many CD4bs-directed bNAbs use the same heavy (H) chain variable (V) gene segment, VH1-2*02, suggesting that activation of B cells expressing this allele is linked to the generation of this type of Ab. Here, we identify the rhesus macaque VH1.23 gene segment to be the closest macaque orthologue to the human VH1-2 gene segment, with 92% homology to VH1-2*02. Of the three amino acids in the VH1-2*02 gene segment that define a motif for VRC01-like antibodies (W50, N58, flanking the HCDR2 region, and R71), the two identified macaque VH1.23 alleles described here encode two. We demonstrate that immunization with soluble Env trimers induced CD4bs-specific VH1.23-using Abs with restricted neutralization breadth. Through alanine scanning and structural studies of one such monoclonal Ab (MAb), GE356, we demonstrate that all three HCDRs are involved in neutralization. This contrasts to the highly potent CD4bs-directed VRC01 class of bNAb, which bind Env predominantly through the HCDR2. Also unlike VRC01, GE356 was minimally modified by somatic hypermutation, its light (L) chain CDRs were of average lengths and it displayed a binding footprint proximal to the trimer axis. These results illustrate that the Env trimer immunogen used here activates B cells encoding a VH1-2 gene segment orthologue, but that the resulting Abs interact distinctly differently with the HIV-1 Env spike compared to VRC01.

Passive transfer of modest titers of potent and broadly neutralizing anti-HIV monoclonal antibodies block SHIV infection in macaques

Five potent and broadly anti-HIV neutralizing monoclonal antibodies are able to block infection by two different SHIVs in monkeys. The authors show that antibodies targeting the outer glycan coat were the most effective and determined that titers of roughly 1:100 protected half the animals.

It is widely appreciated that effective human vaccines directed against viral pathogens elicit neutralizing antibodies (NAbs). The passive transfer of anti–HIV-1 NAbs conferring sterilizing immunity to macaques has been used to determine the plasma neutralization titers, which must be present at the time of exposure, to prevent acquisition of SIV/HIV chimeric virus (SHIV) infections. We administered five recently isolated potent and broadly acting anti-HIV neutralizing monoclonal antibodies (mAbs) to rhesus macaques and challenged them intrarectally 24 h later with either of two different R5-tropic SHIVs. By combining the results obtained from 60 challenged animals, we determined that the protective neutralization titer in plasma preventing virus infection in 50% of the exposed monkeys was relatively modest (∼1:100) and potentially achievable by vaccination.

HIV vaccines: a brief overview

The scope of the article is to review the different approaches that have been used for HIV vaccines. The review is based on articles retrieved by PubMed and clinical trials from 1990 up to date. The article discusses virus complexity, protective and non-protective immune responses against the virus, and the most important approaches for HIV vaccine development.

HIV infection: epidemiology, pathogenesis, treatment, and prevention

HIV prevalence is increasing worldwide because people on antiretroviral therapy are living longer, although new infections decreased from 3.3 million in 2002, to 2.3 million in 2012. Global AIDS-related deaths peaked at 2.3 million in 2005, and decreased to 1.6 million by 2012. An estimated 9.7 million people in low-income and middle-income countries had started antiretroviral therapy by 2012. New insights into the mechanisms of latent infection and the importance of reservoirs of infection might eventually lead to a cure. The role of immune activation in the pathogenesis of non-AIDS clinical events (major causes of morbidity and mortality in people on antiretroviral therapy) is receiving increased recognition. Breakthroughs in the prevention of HIV important to public health include male medical circumcision, antiretrovirals to prevent mother-to-child transmission, antiretroviral therapy in people with HIV to prevent transmission, and antiretrovirals for pre-exposure prophylaxis. Research into other prevention interventions, notably vaccines and vaginal microbicides, is in progress.

A critical question for HIV vaccine development: which antibodies to induce?

A vaccine against HIV-1 must prevent infection against genetically diverse virus strains. Two approaches are currently being pursued to elicit antibody-mediated protection: vaccines that induce potent and broadly reactive neutralizing antibodies (bnAbs) or vaccines that induce "conventional antibodies," which are less potent and broadly neutralizing in comparison. Although bnAbs may provide the greatest level of protection, their structural and genetic characteristics make their elicitation through vaccination a major challenge. In contrast, conventional HIV-1 antibodies have been induced by vaccination and correlated with reduced HIV-1 infection in a phase III vaccine trial. Here, I present evidence that both approaches should be pursued with equal vigor.

Affinity maturation in an HIV broadly neutralizing B-cell lineage through reorientation of variable domains

Rapidly evolving pathogens, such as human immunodeficiency and influenza viruses, escape immune defenses provided by most vaccine-induced antibodies. Proposed strategies to elicit broadly neutralizing antibodies require a deeper understanding of antibody affinity maturation and evolution of the immune response to vaccination or infection. In HIV-infected individuals, viruses and B cells evolve together, creating a virus-antibody "arms race." Analysis of samples from an individual designated CH505 has illustrated the interplay between an antibody lineage, CH103, and autologous viruses at various time points. The CH103 antibodies, relatively broad in their neutralization spectrum, interact with the CD4 binding site of gp120, with a contact dominated by CDRH3. We show by analyzing structures of progenitor and intermediate antibodies and by correlating them with measurements of binding to various gp120s that there was a shift in the relative orientation of the light- and heavy-chain variable domains during evolution of the CH103 lineage. We further show that mutations leading to this conformational shift probably occurred in response to insertions in variable loop 5 (V5) of the HIV envelope. The shift displaced the tips of the light chain away from contact with V5, making room for the inserted residues, which had allowed escape from neutralization by the progenitor antibody. These results, which document the selective mechanism underlying this example of a virus-antibody arms race, illustrate the functional significance of affinity maturation by mutation outside the complementarity determining region surface of the antibody molecule.

Emerging concepts on T follicular helper cell dynamics in HIV infection

Inducing cross-reactive broadly neutralizing antibody (bNAb) responses to HIV through vaccination remains an insurmountable challenge. T follicular helper (TFH) cells are fundamental for the development of antigen-specific antibody responses and therefore crucial for anti-HIV vaccine design. Here, we review recent studies supporting an intricate involvement of TFH cells in HIV pathogenesis and bNAb development during HIV infection. We also examine emerging data suggesting that TFH cell responses may be traceable in peripheral blood, and discuss the implications of these findings in the context of vaccine design and future research in TFH cell immunobiology.

Differential binding of neutralizing and non-neutralizing antibodies to native-like soluble HIV-1 Env trimers, uncleaved Env proteins, and monomeric subunits

Background

The trimeric envelope glycoproteins (Env) on the surface of HIV-1 virions are the targets for neutralizing antibodies (NAbs). No candidate HIV-1 immunogen has yet induced potent, broadly active NAbs (bNAbs). Part of the explanation may be that previously tested Env proteins inadequately mimic the functional, native Env complex. Trimerization and the proteolytic processing of Env precursors into gp120 and gp41 profoundly alter antigenicity, but soluble cleaved trimers are too unstable to serve as immunogens. By introducing stabilizing mutations (SOSIP), we constructed soluble, cleaved Env trimers derived from the HIV-1 subtype A isolate BG505 that resemble native Env spikes on virions both structurally and antigenically.

Results

We used surface plasmon resonance (SPR) to quantify antibody binding to different forms of BG505 Env: the proteolytically cleaved SOSIP.664 trimers, cleaved gp120-gp41_ECTO protomers, and gp120 monomers. Non-NAbs to the CD4-binding site bound only marginally to the trimers but equally well to gp120-gp41_ECTO protomers and gp120 monomers, whereas the bNAb VRC01, directed to the CD4bs, bound to all three forms. In contrast, bNAbs to V1V2 glycan-dependent epitopes bound preferentially (PG9 and PG16) or exclusively (PGT145) to trimers. We also explored the antigenic consequences of three different features of SOSIP.664 gp140 trimers: the engineered inter-subunit disulfide bond, the trimer-stabilizing I559P change in gp41_ECTO, and proteolytic cleavage at the gp120-gp41_ECTO junction. Each of these three features incrementally promoted native-like trimer antigenicity. We compared Fab and IgG versions of bNAbs and validated a bivalent model of IgG binding. The NAbs showed widely divergent binding kinetics and degrees of binding to native-like BG505 SOSIP.664. High off-rate constants and low stoichiometric estimates of NAb binding were associated with large amounts of residual infectivity after NAb neutralization of the corresponding BG505.T332N pseudovirus.

Conclusions

The antigenicity and structural integrity of cleaved BG505 SOSIP.664 trimers render these proteins good mimics of functional Env spikes on virions. In contrast, uncleaved gp140s antigenically resemble individual gp120-gp41_ECTO protomers and gp120 monomers, but not native trimers. Although NAb binding to functional trimers may thus be both necessary and sufficient for neutralization, the kinetics and stoichiometry of the interaction influence the neutralizing efficacy of individual NAbs.

Immunoglobulin somatic hypermutation by APOBEC3/Rfv3 during retroviral infection

Somatic hypermutation (SHM) is an integral process in the development of high-affinity antibodies that are important for recovery from viral infections and vaccine-induced protection. Ig SHM occurs predominantly in germinal centers (GC) via the enzymatic activity of activation-induced deaminase (AID). In contrast, the evolutionarily related apolipoprotein B mRNA-editing enzyme, catalytic polypeptide 3 (APOBEC3) proteins are known to restrict retroviruses, including HIV-1. We previously reported that mouse APOBEC3 encodes Recovery from Friend virus 3 (Rfv3), a classical resistance gene in mice that promotes the neutralizing antibody response against retrovirus infection. We now show that APOBEC3/Rfv3 complements AID in driving Ig SHM during retrovirus infection. Analysis of antibody sequences from retrovirus-specific hybridomas and GC B cells from infected mice revealed Ig heavy-chain V genes with significantly increased C-to-T and G-to-A transitions in wild-type as compared with APOBEC3-defective mice. The context of the mutations was consistent with APOBEC3 but not AID mutational activity. These findings help explain the role of APOBEC3/Rfv3 in promoting the neutralizing antibody responses essential for recovery from retroviral infection and highlight APOBEC3-mediated deamination as a previously unidentified mechanism for antibody diversification in vivo.

AIDS/HIV. Host controls of HIV neutralizing antibodies

The unusual traits of broadly neutralizing antibodies for HIV-1 are stimulating new strategies to induce their production through vaccination.

Discovery of protective B-cell epitopes for development of antimicrobial vaccines and antibody therapeutics

Protective antibodies play an essential role in immunity to infection by neutralizing microbes or their toxins and recruiting microbicidal effector functions. Identification of the protective B-cell epitopes, those parts of microbial antigens that contact the variable regions of the protective antibodies, can lead to development of antibody therapeutics, guide vaccine design, enable assessment of protective antibody responses in infected or vaccinated individuals, and uncover or localize pathogenic microbial functions that could be targeted by novel antimicrobials. Monoclonal antibodies are required to link in vivo or in vitro protective effects to specific epitopes and may be obtained from experimental animals or from humans, and their binding can be localized to specific regions of antigens by immunochemical assays. The epitopes are then identified with mapping methods such as X-ray crystallography of antigen-antibody complexes, antibody inhibition of hydrogen-deuterium exchange in the antigen, antibody-induced alteration of the nuclear magnetic resonance spectrum of the antigen, and experimentally validated computational docking of antigen-antibody complexes. The diversity in shape, size and structure of protective B-cell epitopes, and the increasing importance of protective B-cell epitope discovery to development of vaccines and antibody therapeutics are illustrated through examples from different microbe categories, with emphasis on epitopes targeted by broadly neutralizing antibodies to pathogens of high antigenic variation. Examples include the V-shaped Ab52 glycan epitope in the O-antigen of Francisella tularensis, the concave CR6261 peptidic epitope in the haemagglutinin stem of influenza virus H1N1, and the convex/concave PG16 glycopeptidic epitope in the gp120 V1/V2 loop of HIV type 1.

Development of broadly neutralizing antibodies from autologous neutralizing antibody responses in HIV infection

PURPOSE OF REVIEW

Detailed genetic and structural characterization has revealed that broadly neutralizing antibodies (bnAbs) against HIV-1 have unusually high levels of somatic hypermutation, long CDRH3 domains, and the ability to target one of four sites of vulnerability on the HIV-1 envelope (Env) glycoproteins. A current priority is to understand how bnAbs are generated during natural infection, and translate this information into immunogens that can elicit bnAb following vaccination.

RECENT FINDINGS

Strain-specific neutralizing antibodies can acquire broad neutralizing capacity when the transmitted/founder Env or a specific Env variant is recognized by an unmutated rearranged germline that has the capacity to develop bnAb-like features. This event could be relatively infrequent, as only certain germlines appear to possess inherent features needed for bnAb activity. Furthermore, the glycosylation pattern and diversity of circulating HIV-1 Envs, as well as the state of the B-cell compartment, may influence the activation and maturation of certain antibody lineages.

SUMMARY

Collectively, studies over the last year have suggested that the development of HIV-1 Env immunogens that bind and activate bnAb-like germlines is feasible. However, more information about the features of Env variants and the host factors that lead to breadth during natural infection are needed to elicit bnAbs through immunization.

Antibody 8ANC195 reveals a site of broad vulnerability on the HIV-1 envelope spike

Broadly neutralizing antibodies (bNAbs) to HIV-1 envelope glycoprotein (Env) can prevent infection in animal models. Characterized bNAb targets, although key to vaccine and therapeutic strategies, are currently limited. We defined a new site of vulnerability by solving structures of bNAb 8ANC195 complexed with monomeric gp120 by X-ray crystallography and trimeric Env by electron microscopy. The site includes portions of gp41 and N-linked glycans adjacent to the CD4-binding site on gp120, making 8ANC195 the first donor-derived anti-HIV-1 bNAb with an epitope spanning both Env subunits. Rather than penetrating the glycan shield by using a single variable-region CDR loop, 8ANC195 inserted its entire heavy-chain variable domain into a gap to form a large interface with gp120 glycans and regions of the gp120 inner domain not contacted by other bNAbs. By isolating additional 8ANC195 clonal variants, we identified a more potent variant, which may be valuable for therapeutic approaches using bNAb combinations with nonoverlapping epitopes.

Preventive and therapeutic applications of neutralizing antibodies to Human Immunodeficiency Virus Type 1 (HIV-1)

The development of a preventive vaccine to neutralize the highly variable and antigenically diverse human immunodeficiency virus type 1 (HIV-1) has been an indomitable goal. The recent discovery of a number of cross-neutralizing and potent monoclonal antibodies from elite neutralizers has provided important insights in this field. Neutralizing antibodies (NAbs) are useful in identifying neutralizing epitopes of vaccine utility and for understanding the mechanism of potent and broad cross-neutralization thus providing a modality of preventive and therapeutic value. In this article we review the current understanding on the potential use of broadly neutralizing antibodies (bNAbs) in their full-length IgG structure, engineered domain antibody or bispecific versions towards preventive and therapeutic applications. The potential implications of NAbs are discussed in the light of the recent developments as key components in vaccination against HIV-1. The development of a vaccine immunogen which elicits bNAbs and confers protective immunity remains a real challenge.

Structural insights on the role of antibodies in HIV-1 vaccine and therapy

Despite 30 years of effort, there is no effective vaccine for HIV-1. However, antibodies can prevent HIV-1 infection in humanized mice and macaques when passively transferred. New single-cell-based methods have uncovered many broad and potent donor-derived antibodies, and structural studies have revealed the molecular bases for their activities. The new data suggest why such antibodies are difficult to elicit and inform HIV-1 vaccine development efforts. In addition to protecting against infection, the newly identified antibodies can suppress active infections in mice and macaques, suggesting they could be valuable additions to anti-HIV-1 therapies and to strategies to eradicate HIV-1 infection.

CD4-mimetic small molecules sensitize human immunodeficiency virus to vaccine-elicited antibodies

Approaches to prevent human immunodeficiency virus (HIV-1) transmission are urgently needed. Difficulties in eliciting antibodies that bind conserved epitopes exposed on the unliganded conformation of the HIV-1 envelope glycoprotein (Env) trimer represent barriers to vaccine development. During HIV-1 entry, binding of the gp120 Env to the initial receptor, CD4, triggers conformational changes in Env that result in the formation and exposure of the highly conserved gp120 site for interaction with the coreceptors, CCR5 and CXCR4. The DMJ compounds (+)-DMJ-I-228 and (+)-DMJ-II-121 bind gp120 within the conserved Phe 43 cavity near the CD4-binding site, block CD4 binding, and inhibit HIV-1 infection. Here we show that the DMJ compounds sensitize primary HIV-1, including transmitted/founder viruses, to neutralization by monoclonal antibodies directed against CD4-induced (CD4i) epitopes and the V3 region, two gp120 elements involved in coreceptor binding. Importantly, the DMJ compounds rendered primary HIV-1 sensitive to neutralization by antisera elicited by immunization of rabbits with HIV-1 gp120 cores engineered to assume the CD4-bound state. Thus, small molecules like the DMJ compounds may be useful as microbicides to inhibit HIV-1 infection directly and to sensitize primary HIV-1 to neutralization by readily elicited antibodies.

IMPORTANCE

Preventing HIV-1 transmission is a priority for global health. Eliciting antibodies that can neutralize many different strains of HIV-1 is difficult, creating problems for the development of a vaccine. We found that certain small-molecule compounds can sensitize HIV-1 to particular antibodies. These antibodies can be elicited in rabbits. These results suggest an approach to prevent HIV-1 sexual transmission in which a virus-sensitizing microbicide is combined with a vaccine.

Thermodynamic stability contributes to immunoglobulin specificity

Antigen-binding specificity of immunoglobulins is important for their function in immune defense. However, immune repertoires contain a considerable fraction of immunoglobulins with promiscuous binding behavior, the physicochemical basis of which is not well understood. Evolution of immunoglobulin specificity occurs through iterative processes of mutation and selection, referred to as affinity maturation. Recent studies reveal that some somatic mutations could compromise the thermodynamic stability of the variable regions of immunoglobulins. By integrating this observation with the wealth of data on the evolution of novel enzyme activities, we propose that antibody specificity is linked to the thermodynamic stability of the antigen-binding regions, which provides a quantitative distinction between highly specific and promiscuous antibodies.

Clonal and cellular dynamics in germinal centers

Germinal centers (GCs) are the site of antibody affinity maturation, a process that involves complex clonal and cellular dynamics. Selection of B cells bearing higher-affinity immunoglobulins proceeds via a stereotyped pattern whereby B cells migrate cyclically between the GC's two anatomical compartments. This process occurs in a timeframe that is well suited to analysis by intravital microscopy, and much has been learned in recent years by use of these techniques. On a longer time scale, the diversity of B cell clones and variants within individual GCs is also thought to change as affinity maturation progresses; however, our understanding of clonal dynamics in individual GCs is limited. We discuss recent progress in the elucidation of clonal and cellular dynamics patterns.

Single-cell and deep sequencing of IgG-switched macaque B cells reveal a diverse Ig repertoire following immunization

The nonhuman primate model is important for preclinical evaluation of prophylactic and therapeutic intervention strategies. The recent description of the rhesus macaque germline Ig loci and establishment of a database of germline gene segments offer improved opportunities to delineate Ig gene usage in the overall B cell repertoire as well as in response to vaccination. We applied 454-pyrosequencing and single-cell RT-PCR of bulk and sorted memory B cells, respectively, to investigate IGHV gene segment expression in rhesus macaques. The two methods gave remarkably concordant results and identified groups of gene segments that are frequently or rarely used. We further examined the VH repertoire of Ag-specific memory B cells induced by immunization with recombinant HIV-1 envelope glycoproteins, an important vaccine component. We demonstrate that HIV-1 envelope glycoprotein immunization activates a highly polyclonal response composed of most of the expressed VH gene segments, illustrating the considerable genetic diversity of responding B cells following vaccination.

Antibody engineering and therapeutics, The Annual Meeting of the Antibody Society: December 8-12, 2013, Huntington Beach, CA

The 24^th Antibody Engineering & Therapeutics meeting brought together a broad range of participants who were updated on the latest advances in antibody research and development. Organized by IBC Life Sciences, the gathering is the annual meeting of The Antibody Society, which serves as the scientific sponsor. Preconference workshops on 3D modeling and delineation of clonal lineages were featured, and the conference included sessions on a wide variety of topics relevant to researchers, including systems biology; antibody deep sequencing and repertoires; the effects of antibody gene variation and usage on antibody response; directed evolution; knowledge-based design; antibodies in a complex environment; polyreactive antibodies and polyspecificity; the interface between antibody therapy and cellular immunity in cancer; antibodies in cardiometabolic medicine; antibody pharmacokinetics, distribution and off-target toxicity; optimizing antibody formats for immunotherapy; polyclonals, oligoclonals and bispecifics; antibody discovery platforms; and antibody-drug conjugates.

Elicitation of HIV-1-neutralizing antibodies against the CD4-binding site

PURPOSE OF REVIEW

The HIV-1 site of binding for the CD4 receptor has long attracted attention as a potential supersite of vulnerability to antibody-mediated neutralization. We review recent findings related to effective CD4-binding site antibodies isolated from HIV-1-infected individuals and discuss implications for immunogen design.

RECENT FINDINGS

Highly effective CD4-binding site antibodies such as antibody VRC01 have the ability to neutralize over 90% of circulating HIV-1 strains. Sequence and structural analysis of these antibodies from over half a dozen HIV-1-infected donors reveals remarkable similarity in their ontogenies and their modes of recognition, all of which involve mimicry of CD4 receptor by antibody-heavy chain. Meanwhile, other effective CD4-binding site neutralizers such as antibody CH103 have been shown to utilize a different mode of recognition, with next-generation sequencing of both virus and antibody suggesting co-evolution to drive the development of antibody-neutralization breadth.

SUMMARY

The nexus of information concerning the CD4-binding site and its recognition by human antibodies capable of effective neutralization has expanded remarkably in the last few years. Although barriers are substantial, new insights from donor-serum responses, atomic-level structures of antibody-Env complexes, and next-generation sequencing of B-cell transcripts are invigorating vaccine-design efforts to elicit effective CD4-binding site antibodies.

Vaccine-elicited primate antibodies use a distinct approach to the HIV-1 primary receptor binding site informing vaccine redesign

HIV-1 neutralization requires Ab accessibility to the functional envelope glycoprotein (Env) spike. We recently reported the isolation of previously unidentified vaccine-elicited, CD4 binding site (CD4bs)-directed mAbs from rhesus macaques immunized with soluble Env trimers, indicating that this region is immunogenic in the context of subunit vaccination. To elucidate the interaction of the trimer-elicited mAbs with gp120 and their insufficient interaction with the HIV-1 primary isolate spike, we crystallized the Fab fragments of two mAbs, GE136 and GE148. Alanine scanning of their complementarity-determining regions, coupled with epitope scanning of their epitopes on gp120, revealed putative contact residues at the Ab/gp120 interface. Docking of the GE136 and GE148 Fabs to gp120, coupled with EM reconstructions of these nonbroadly neutralizing mAbs (non-bNAbs) binding to gp120 monomers and EM modeling to well-ordered trimers, suggested Ab approach to the CD4bs by a vertical angle of access relative to the more lateral mode of interaction used by the CD4bs-directed bNAbs VRC01 and PGV04. Fitting the structures into the available cryo-EM native spike density indicated clashes between these two vaccine-elicited mAbs and the topside variable region spike cap, whereas the bNAbs duck under this quaternary shield to access the CD4bs effectively on primary HIV isolates. These results provide a structural basis for the limited neutralizing breadth observed by current vaccine-induced, CD4bs-directed Abs and highlight the need for better ordered trimer immunogens. The analysis presented here therefore provides valuable information to guide HIV-1 vaccine immunogen redesign.

Modulation of nonneutralizing HIV-1 gp41 responses by an MHC-restricted TH epitope overlapping those of membrane proximal external region broadly neutralizing antibodies

A goal of HIV-1 vaccine development is to elicit broadly neutralizing Abs (BnAbs), but current immunization strategies fail to induce BnAbs, and for unknown reasons, often induce nonneutralizing Abs instead. To explore potential host genetic contributions controlling Ab responses to the HIV-1 Envelope, we have used congenic strains to identify a critical role for MHC class II restriction in modulating Ab responses to the membrane proximal external region (MPER) of gp41, a key vaccine target. Immunized H-2(d)-congenic strains had more rapid, sustained, and elevated MPER(+) Ab titers than those bearing other haplotypes, regardless of immunogen, adjuvant, or prime or boost regimen used, including formulations designed to provide T cell help. H-2(d)-restricted MPER(+) serum Ab responses depended on CD4 TH interactions with class II (as revealed in immunized intra-H-2(d/b) congenic or CD154(-/-) H-2(d) strains, and by selective abrogation of MPER restimulated, H-2(d)-restricted primed splenocytes by class II-blocking Abs), and failed to neutralize HIV-1 in the TZM-b/l neutralization assay, coinciding with lack of specificity for an aspartate residue in the neutralization core of BnAb 2F5. Unexpectedly, H-2(d)-restricted MPER(+) responses functionally mapped to a core TH epitope partially overlapping the 2F5/z13/4E10 BnAb epitopes as well as nonneutralizing B cell-Ab binding residues. We propose that class II restriction contributes to the general heterogeneity of nonneutralizing gp41 responses induced by Envelope. Moreover, the proximity of TH and B cell epitopes in this restriction may have to be considered in redesigning minimal MPER immunogens aimed at exclusively binding BnAb epitopes and triggering MPER(+) BnAbs.

Prevalence of broadly neutralizing antibody responses during chronic HIV-1 infection

OBJECTIVE

Studies of neutralizing antibodies in HIV-1 infected individuals provide insights into the quality of the response that should be possible to elicit with vaccines and ways to design effective immunogens. Some individuals make high titres of exceptional broadly reactive neutralizing antibodies that are of particular interest; however, more modest responses may be a reasonable goal for vaccines. We performed a large cross-sectional study to determine the spectrum of neutralization potency and breadth that is seen during chronic HIV-1 infection.

DESIGN

Neutralization potency and breadth were assessed with genetically and geographically diverse panels of 205 chronic HIV-1 sera and 219 Env-pseudotyped viruses representing all major genetic subtypes of HIV-1.

METHODS

Neutralization was measured by using Tat-regulated luciferase reporter gene expression in TZM-bl cells. Serum-neutralizing activity was compared with a diverse set of human mAbs that are widely considered to be broadly neutralizing.

RESULTS

We observed a uniform continuum of responses, with most sera displaying some level of cross-neutralization, and approximately 50% of sera neutralizing more than 50% of viruses. Titres of neutralization (potency) were highly correlated with breadth. Many sera had breadth comparable to several of the less potent broadly neutralizing human mAbs.

CONCLUSION

These results help clarify the spectrum of serum-neutralizing activity induced by HIV-1 infection and that should be possible to elicit with vaccines. Importantly, most people appear capable of making low to moderate titres of broadly neutralizing antibodies. Additional studies of these relatively common responses might provide insights for practical and feasible vaccine designs.

Antibodies VRC01 and 10E8 neutralize HIV-1 with high breadth and potency even with Ig-framework regions substantially reverted to germline

Abs capable of effectively neutralizing HIV-1 generally exhibit very high levels of somatic hypermutation, both in their CDR and framework-variable regions. In many cases, full reversion of the Ab-framework mutations back to germline results in substantial to complete loss of HIV-1-neutralizing activity. However, it has been unclear whether all or most of the observed framework mutations would be necessary or whether a small subset of these mutations might be sufficient for broad and potent neutralization. To address this issue and to explore the dependence of neutralization activity on the level of somatic hypermutation in the Ab framework, we applied a computationally guided framework-reversion procedure to two broadly neutralizing anti-HIV-1 Abs, VRC01 and 10E8, which target two different HIV-1 sites of vulnerability. Ab variants in which up to 78% (38 of 49 for VRC01) and 89% (31 of 35 for 10E8) of framework mutations were reverted to germline retained breadth and potency within 3-fold of the mature Abs when evaluated on a panel of 21 diverse viral strains. Further, a VRC01 variant with an ∼50% framework-reverted L chain showed a 2-fold improvement in potency over the mature Ab. Our results indicate that only a small number of Ab-framework mutations may be sufficient for high breadth and potency of HIV-1 neutralization by Abs VRC01 and 10E8. Partial framework revertants of HIV-1 broadly neutralizing Abs may present advantages over their highly mutated counterparts as Ab therapeutics and as targets for immunogen design.