Protection from the 2009 H1N1 pandemic influenza by an antibody from combinatorial survivor-based libraries

How Broadly Neutralising Antibodies Are Redefining Immunity to Influenza

Recent avian influenza outbreaks have heightened global concern over viral threats with the potential to significantly impact human health. Influenza is particularly alarming due to its history of causing pandemics and zoonotic reservoirs. In response, significant progress has been made toward the development of universal influenza vaccines, largely driven by the discovery of broadly neutralising antibodies (bnAbs), which have the potential to neutralise a broad range of influenza viruses, extending beyond the traditional strain-specific response. This could lead to longer-lasting immunity, reducing the need for seasonal vaccinations, and improve preparedness for future pandemics. This review offers a comprehensive analysis of these antibodies, their application in clinical studies, and both their potential and possible shortcomings in managing future influenza outbreaks.

Influenza vaccination-induced H3 stalk-reactive memory B-cell clone expansion

Current vaccine formulations elicit a recall immune response against viruses by targeting epitopes on the globular head of hemagglutinin (HA), and stalk-reactive antibodies are rarely found. However, stalk-specific memory B-cell expansion after influenza vaccination is poorly understood. In this study, B cells were isolated from individuals immunized with seasonal tetravalent influenza vaccines at days 0 and 28 for H7N9 stimulation in vitro. Plasma and supernatants were collected for the analysis of anti-HA IgG using ELISA and a Luminex assay. Memory B cells were positively enriched, and total RNA was extracted for B cell receptor (BCR) H-CDR3 sequencing. All subjects displayed increased anti-H3 antibody secretion after vaccination, whereas no increase in cH5/3-reactive IgG levels was detected. The number of shared memory B-cell clones among individuals dropped dramatically from 593 to 37. Four out of 5 subjects displayed enhanced frequencies of the VH3-23 and VH3-30 genes, and one exhibited an increase in the frequency of VH1-18, which are associated with the stalk of HA. An increase in H3 stalk-specific antibodies produced by B cells stimulated with H7N9 viruses was detected after vaccination. These results demonstrated that H3 stalk-specific memory B cells can expand and secrete antibodies that bind to the stalk in vitro, although no increase in serum H3 stalk-reactive antibodies was found after vaccination, indicating potential for developing a universal vaccine strategy.

Analysis of the conserved protective epitopes of hemagglutinin on influenza A viruses

The conserved protective epitopes of hemagglutinin (HA) are essential to the design of a universal influenza vaccine and new targeted therapeutic agents. Over the last 15 years, numerous broadly neutralizing antibodies (bnAbs) targeting the HA of influenza A viruses have been isolated from B lymphocytes of human donors and mouse models, and their binding epitopes identified. This work has brought new perspectives for identifying conserved protective epitopes of HA. In this review, we succinctly analyzed and summarized the antigenic epitopes and functions of more than 70 kinds of bnAb. The highly conserved protective epitopes are concentrated on five regions of HA: the hydrophobic groove, the receptor-binding site, the occluded epitope region of the HA monomers interface, the fusion peptide region, and the vestigial esterase subdomain. Our analysis clarifies the distribution of the conserved protective epitope regions on HA and provides distinct targets for the design of novel vaccines and therapeutics to combat influenza A virus infection.

Development of Neutralizing Nanobodies to the Hemagglutinin Stem Domain of Influenza A Viruses

The influenza virus infection claims ~650,000 lives annually. Taking into account the evolving resistance of the pathogen to antiviral drugs and the waning effectiveness of vaccination among certain populations, new approaches to the treatment of influenza are needed. The current study is aimed at obtaining single-domain antibodies (Nanobodies®) to the highly conserved stem domain of influenza A virus hemagglutinin by phage display. Two high-affinity neutralizing clones of Nanobodies® with a particular specificity were selected; they ensured 100% neutralization of the H1N1 and H5N2 influenza viruses in vivo. The obtained data demonstrate that it is possible to develop highly effective VHH-based drugs for the treatment of influenza.

Canonical features of human antibodies recognizing the influenza hemagglutinin trimer interface

Broadly reactive antibodies targeting the influenza A virus hemagglutinin (HA) head domain are thought to be rare and to require extensive somatic mutations or unusual structural features to achieve breadth against divergent HA subtypes. Here we describe common genetic and structural features of protective human antibodies from several individuals recognizing the trimer interface (TI) of the influenza A HA head, a recently identified site of vulnerability. We examined the sequence of TI-reactive antibodies, determined crystal structures for TI antibody-antigen complexes, and analyzed the contact residues of the antibodies on HA to discover common genetic and structural features of TI antibodies. Our data reveal that many TI antibodies are encoded by a light chain variable gene segment incorporating a shared somatic mutation. In addition, these antibodies have a shared acidic residue in the heavy chain despite originating from diverse heavy chain variable gene segments. These studies show that the TI region of influenza A HA is a major antigenic site with conserved structural features that are recognized by a common human B cell public clonotype. The canonical nature of this antibody-antigen interaction suggests that the TI epitope might serve as an important target for structure-based vaccine design.

Influenza Virus and SARS-CoV-2 Vaccines

Seasonal influenza and the current COVID-19 pandemic represent looming global health challenges. Efficacious and safe vaccines remain the frontline tools for mitigating both influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-induced diseases. This review will discuss the existing strategies for influenza vaccines and how these strategies have informed SARS-CoV-2 vaccines. It will also discuss new vaccine platforms and potential challenges for both viruses.

Identification and Structure of a Multidonor Class of Head-Directed Influenza-Neutralizing Antibodies Reveal the Mechanism for Its Recurrent Elicitation

Multidonor antibodies are of interest for vaccine design because they can in principle be elicited in the general population by a common set of immunogens. For influenza, multidonor antibodies have been observed against the hemagglutinin (HA) stem, but not the immunodominant HA head. Here, we identify and characterize a multidonor antibody class (LPAF-a class) targeting the HA head. This class exhibits potent viral entry inhibition against H1N1 A/California/04/2009 (CA09) virus. LPAF-a class antibodies derive from the HV2-70 gene and contain a "Tyr-Gly-Asp"-motif, which occludes the HA-sialic acid binding site as revealed by a co-crystal structure with HA. Both germline-reverted and mature LPAF antibodies potently neutralize CA09 virus and have nanomolar affinities for CA09 HA. Moreover, increased frequencies for LPFA-a class antibodies are observed in humans after a single vaccination. Overall, this work highlights the identification of a multidonor class of head-directed influenza-neutralizing antibodies and delineates the mechanism of their recurrent elicitation in humans.

Convergent antibody evolution and clonotype expansion following influenza virus vaccination

Recent advances in high-throughput single cell sequencing have opened up new avenues into the investigation of B cell receptor (BCR) repertoires. In this study, PBMCs were collected from 17 human participants vaccinated with the split-inactivated influenza virus vaccine during the 2016-2017 influenza season. A combination of Immune Repertoire Capture (IRCTM) technology and IgG sequencing was performed on ~7,800 plasmablast (PB) cells and preferential IgG heavy-light chain pairings were investigated. In some participants, a single expanded clonotype accounted for ~22% of their PB BCR repertoire. Approximately 60% (10/17) of participants experienced convergent evolution, possessing public PBs that were elicited independently in multiple participants. Binding profiles of one private and three public PBs confirmed they were all subtype-specific, cross-reactive hemagglutinin (HA) head-directed antibodies. Collectively, this high-resolution antibody repertoire analysis demonstrated the impact evolution can have on BCRs in response to influenza virus vaccination, which can guide future universal influenza prophylactic approaches.

Hemagglutinin stalk-based monoclonal antibody elicits broadly reactivity against group 1 influenza A virus

Background

Influenza virus remains a continuous and severe threat to public health worldwide, and its prevention and treatment have always been a major international issue. Because of its ability to evade immune surveillance through rapid antigenic drift and antigenic shift, broad-spectrum vaccines seem increasingly important.

Methods

A mAb named 3C12 from an immortalized hybrid cell was generated via immunizing mice with HA2 protein from A/chicken/Anhui/BRI99/2016 (AH/BRI99/16, H9N2) generated by prokaryotic expression. Then, its broad-spectrum activity was analyzed by WB and IFA. Next, the minimal linear epitope was identified via analyzing the reaction of a series of HA truncations with 3C12. Finally, the protective effects of 3C12 were evaluated in vitro and in vivo infection experiments.

Results

The mAb could react with the viruses of subtypes H1, H2, H5, H8, H9, H12, H13, H16, and HA protein of H18 in group 1, but failed to react with viruses in group 2. The minimal linear epitope targeted by the mAb was ⁴³³NAELLVL⁴³⁹ in full length of HA and localized in the C-helix region of HA2 (residue 95-101, HA2 numbering). What’s more, the mAb 3C12 inhibited H1, H2, H5, H8, H9, H12, H13 and H16 virus-replication in vitro and also has shown effectiveness in preventing and treating disease in mice challenged with lethal dose of AH/BRI99/16 (H9N2) virus in vivo. These results suggested that the broadly reactive anti-HA stem mAb 3C12 exhibited prophylactic and therapeutic efficacy.

Conclusions

Here, we have demonstrated that the linear epitope identified in this study could be a novel target for developing broad-spectrum influenza diagnostics or vaccine design, and the HA2-based monoclonal antibody is indeed a promising strategy for broad-spectrum protection against seasonal and pandemic influenza viruses.

Influenza Hemagglutinin Structures and Antibody Recognition

Hemagglutinin (HA) is most abundant glycoprotein on the influenza virus surface. Influenza HA promotes viral entry by engaging the receptor and mediating virus-host membrane fusion. At the same time, HA is the major antigen of the influenza virus. HA antigenic shift can result in pandemics, whereas antigenic drift allows human circulating strains to escape herd immunity. Most antibody responses against HA are strain-specific. However, antibodies that have neutralizing activities against multiple strains or even subtypes have now been discovered and characterized. These broadly neutralizing antibodies (bnAbs) target conserved regions on HA, such as the receptor-binding site and the stem domain. Structural studies of such bnAbs have provided important insight into universal influenza vaccine and therapeutic design. This review discusses the HA functions as well as HA-antibody interactions from a structural perspective.

Neutralizing Antibodies Targeting the Conserved Stem Region of Influenza Hemagglutinin

Influenza continues to be a public health threat despite the availability of annual vaccines. While vaccines are generally effective at inducing strain-specific immunity, they are sub-optimal or ineffective when drifted or novel pandemic strains arise due to sequence changes in the major surface glycoprotein hemagglutinin (HA). The discovery of a large number of antibodies targeting the highly conserved stem region of HAs that are capable of potently neutralizing a broad range of virus strains and subtypes suggests new ways to protect against influenza. The structural characterization of HA stem epitopes and broadly neutralizing antibody paratopes has enabled the design of novel proteins, mini-proteins, and peptides targeting the HA stem, thus providing a foundation for the design of new vaccines. In this narrative, we comprehensively review the current knowledge about stem-directed broadly neutralizing antibodies and the structural features contributing to virus neutralization.

Development of broadly reactive influenza vaccines by targeting the conserved regions of the hemagglutinin stem and head domains

INTRODUCTION

Influenza virus infections cause serious illness in millions of people each year. Although influenza virus vaccines are available, they are not optimally effective due to mismatches between the influenza virus strains used for the vaccine and the circulating strains. To improve protection by vaccines, a broadly protective or universal vaccine may be required. Strategies to develop universal vaccines aim to elicit broadly reactive antibodies, which target regions on the viral hemagglutinin (HA) protein which are conserved between strains. Broadly reactive antibodies have helped to identify such targets and can guide the design of such a vaccine.

AREAS COVERED

The first part of this review provides an in-depth overview of broadly reactive anti-HA antibodies, discussing their origin, breadth and their mechanisms of protection. The second part discusses the technical design and mode of action of potential universal vaccine candidates that aim to elicit these broadly reactive antibodies and provide protection against a majority of influenza strains.

EXPERT OPINION

While great strides have been made in the development of universal influenza vaccine candidates, real-life use still requires improvement of stability, enhancement of their breadth of protection and ease of production, while efficacies need to be determined in human trials.

A Site of Vulnerability on the Influenza Virus Hemagglutinin Head Domain Trimer Interface

Here, we describe the discovery of a naturally occurring human antibody (Ab), FluA-20, that recognizes a new site of vulnerability on the hemagglutinin (HA) head domain and reacts with most influenza A viruses. Structural characterization of FluA-20 with H1 and H3 head domains revealed a novel epitope in the HA trimer interface, suggesting previously unrecognized dynamic features of the trimeric HA protein. The critical HA residues recognized by FluA-20 remain conserved across most subtypes of influenza A viruses, which explains the Ab's extraordinary breadth. The Ab rapidly disrupted the integrity of HA protein trimers, inhibited cell-to-cell spread of virus in culture, and protected mice against challenge with viruses of H1N1, H3N2, H5N1, or H7N9 subtypes when used as prophylaxis or therapy. The FluA-20 Ab has uncovered an exceedingly conserved protective determinant in the influenza HA head domain trimer interface that is an unexpected new target for anti-influenza therapeutics and vaccines.

A Complex Dance: Measuring the Multidimensional Worlds of Influenza Virus Evolution and Anti-Influenza Immune Responses

The human antibody response to influenza virus infection or vaccination is as complicated as it is essential for protection against flu. The constant antigenic changes of the virus to escape human herd immunity hinder the yearly selection of vaccine strains since it is hard to predict which virus strains will circulate for the coming flu season. A "universal" influenza vaccine that could induce broad cross-influenza subtype protection would help to address this issue. However, the human antibody response is intricate and often obscure, with factors such as antigenic seniority or original antigenic sin (OAS), and back-boosting ensuring that each person mounts a unique immune response to infection or vaccination with any new influenza virus strain. Notably, the effects of existing antibodies on cross-protective immunity after repeated vaccinations are unclear. More research is needed to characterize the mechanisms at play, but traditional assays such as hemagglutinin inhibition (HAI) and microneutralization (MN) are excessively limited in scope and too resource-intensive to effectively meet this challenge. In the past ten years, new multiple dimensional assays (MDAs) have been developed to help overcome these problems by simultaneously measuring antibodies against a large panel of influenza hemagglutinin (HA) proteins with a minimal amount of sample in a high throughput way. MDAs will likely be a powerful tool for accelerating the study of the humoral immune response to influenza vaccination and the development of a universal influenza vaccine.

Structural Insights for Anti-Influenza Vaccine Design

Influenza A virus are a persistent and significant threat to human health, and current vaccines do not provide sufficient protection due to antigenic drift, which allows influenza viruses to easily escape immune surveillance and antiviral drug activity. Influenza hemagglutinin (HA) is a glycoprotein needed for the entry of enveloped influenza viruses into host cells and is a potential target for anti-influenza humoral immune responses. In recent years, a number of broadly neutralizing antibodies (bnAbs) have been isolated, and their relative structural information obtained from the crystallization of influenza antigens in complex with bnAbs has provided some new insights into future influenza vaccine research. Here, we review the current knowledge of the HA-targeted bnAbs and the structure-based mechanisms contributing to neutralization. We also discuss the potential for this structure-based approach to overcome the challenge of obtaining a highly desired "universal" influenza vaccine, especially on small proteins and peptides.

What Are the Most Powerful Immunogen Design Vaccine Strategies? A Structural Biologist's Perspective

The ability of structure-based design to control the shape and reactivity-the atomic-level chemistry-of an immunogen argues for it being one of the "most powerful" immunogen-design strategies. But antigenic reactivity is only one of the properties required to induce a protective immune response. Here, a multidimensional approach is used to exemplify the enabling role atomic-level information can play in the development of immunogens against three viral pathogens, respiratory syncytial virus, influenza A virus, and human immunodeficiency virus (HIV), which have resisted standard approaches to vaccine development. Overall, structure-based strategies incorporating B-cell ontogenies and viral evasion mechanisms appear exceptionally powerful.

Potent peptidic fusion inhibitors of influenza virus

Influenza therapeutics with new targets and mechanisms of action are urgently needed to combat potential pandemics, emerging viruses, and constantly mutating strains in circulation. We report here on the design and structural characterization of potent peptidic inhibitors of influenza hemagglutinin. The peptide design was based on complementarity-determining region loops of human broadly neutralizing antibodies against the hemagglutinin (FI6v3 and CR9114). The optimized peptides exhibit nanomolar affinity and neutralization against influenza A group 1 viruses, including the 2009 H1N1 pandemic and avian H5N1 strains. The peptide inhibitors bind to the highly conserved stem epitope and block the low pH-induced conformational rearrangements associated with membrane fusion. These peptidic compounds and their advantageous biological properties should accelerate the development of new small molecule- and peptide-based therapeutics against influenza virus.

The road to a more effective influenza vaccine: Up to date studies and future prospects

Influenza virus causes an acute respiratory infection in humans. Frequent point mutations in the influenza genome and occasional exchange of genetic segments between virus strains help the virus evade the pre-existing immunity, resulting in epidemics and pandemics. Although vaccination is the most effective intervention, mismatches between circulating viruses and vaccine strains reduce vaccine efficacy. Furthermore, current injectable vaccines induce IgG antibodies in serum (which limit progression of influenza symptoms) but not secretory IgA antibodies in the respiratory mucosa (which prevent virus infection efficiently). Therefore, numerous studies have attempted to improve influenza vaccines. The discovery of broadly neutralizing antibodies has progressed research into antigen design. Studies designed to improve vaccine efficacy by changing the vaccine administration route have also been conducted. A thorough understanding of the mechanisms underlying the action of various vaccines is essential if we are to develop a universal influenza vaccine. Therefore, evaluating the quality and quantity of antibodies induced by vaccines, which determine vaccine efficacy, is critical. However, at present vaccine evaluation relies on hemagglutination inhibition tests, which only measure the quantity of antibody produced. Antibody repertoires comprise a set of antibodies with specific genetic or molecular features that correspond to their functions. Genetically and functionally similar antibodies may be produced by multiple individuals exposed to an identical stimulus. Therefore, it may be possible to evaluate and compare multiple vaccine strategies in terms of the quality and quantity of an antibody response induced by a vaccine by examining antibody repertoires. Recent studies have used single cell expression and high-throughput immunoglobulin sequencing to provide a detailed picture of antibody responses. These novel methods may be critical for detailed characterization of antibody repertoires induced by various vaccination strategies.

A Broadly Reactive Human Anti-hemagglutinin Stem Monoclonal Antibody That Inhibits Influenza A Virus Particle Release

Many broadly reactive human monoclonal antibodies against the hemagglutinin (HA) stem of influenza A virus have been developed for therapeutic applications. These antibodies typically inhibit viral entry steps, especially the HA conformational change that is required for membrane fusion. To better understand the mechanisms by which such antibodies inhibit viral replication, we established broadly reactive human anti-HA stem antibodies and determined the properties of these antibodies by examining their reactivity with 18 subtypes of HA, evaluating their in vivo protective efficacy, identifying their epitopes, and characterizing their inhibitory mechanisms. Among the eight human monoclonal antibodies we generated, which recognized at least 3 subtypes of the soluble HA antigens tested, clone S9-1-10/5-1 reacted with 18 subtypes of HA and protected mice from lethal infection with H1N1pdm09, H3N2, H5N1, and H7N9 viruses. This antibody recognized the HA2 helix A in the HA stem, and inhibited virus particle release from infected cells but did not block viral entry completely. These results show that broadly reactive human anti-HA stem antibodies can exhibit protective efficacy by inhibiting virus particle release. These findings expand our knowledge of the mechanisms by which broadly reactive stem-targeting antibodies inhibit viral replication and provide valuable information for universal vaccine development.

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A broadly mouse-protective anti-HA stem antibody, S9-1-10/5-1, was isolated.

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S9-1-10/5-1 mainly inhibited virus release rather than virus entry.

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S9-1-10/5-1 tethers virions via crosslinking HA molecules between neighboring virions.

Broadly reactive human monoclonal antibodies against the influenza HA stem have received attention because of their potential utility against multiple HA subtypes. Some of these antibodies inhibit virus entry and/or protect mice via antibody-dependent cellular cytotoxicity. Here, we identified a human monoclonal antibody that suppresses virus propagation in vitro and in vivo by primarily inhibiting virus particle release. This finding provides another inhibitory mechanism of action for the anti-HA stem antibodies, indicating that the anti-HA stem antibodies could be potent anti-virals due to their pluripotency.

Vaccine-Induced Antibodies that Neutralize Group 1 and Group 2 Influenza A Viruses

Antibodies capable of neutralizing divergent influenza A viruses could form the basis of a universal vaccine. Here, from subjects enrolled in an H5N1 DNA/MIV-prime-boost influenza vaccine trial, we sorted hemagglutinin cross-reactive memory B cells and identified three antibody classes, each capable of neutralizing diverse subtypes of group 1 and group 2 influenza A viruses. Co-crystal structures with hemagglutinin revealed that each class utilized characteristic germline genes and convergent sequence motifs to recognize overlapping epitopes in the hemagglutinin stem. All six analyzed subjects had sequences from at least one multidonor class, and-in half the subjects-multidonor-class sequences were recovered from >40% of cross-reactive B cells. By contrast, these multidonor-class sequences were rare in published antibody datasets. Vaccination with a divergent hemagglutinin can thus increase the frequency of B cells encoding broad influenza A-neutralizing antibodies. We propose the sequence signature-quantified prevalence of these B cells as a metric to guide universal influenza A immunization strategies.

Association between Hemagglutinin Stem-Reactive Antibodies and Influenza A/H1N1 Virus Infection during the 2009 Pandemic

The discovery of influenza virus broadly neutralizing (BrN) antibodies prompted efforts to develop universal vaccines. Influenza virus stem-reactive (SR) broadly neutralizing antibodies have been detected by screening antibody phage display libraries. However, studies of SR BrN antibodies in human serum, and their association with natural infection, are limited. To address this, pre- and postpandemic sera from a prospective community cohort study in Vietnam were assessed for antibodies that inhibit SR BrN monoclonal antibody (MAb) (C179) binding to H1N1 pandemic 2009 virus (H1N1pdm09). Of 270 households, 33 with at least one confirmed H1N1pdm09 illness or at least two seroconverters were included. The included households comprised 71 infected and 41 noninfected participants. Sera were tested as 2-fold dilutions between 1:5 and 1:40. Fifty percent C179 inhibition (IC₅₀) titers did not exceed 10, although both IC₅₀ titers and percent C179 inhibition by sera diluted 1:5 or 1:10 correlated with hemagglutination inhibition (HI) and microneutralization (MN) titers (all P < 0.001). Thirteen (12%) participants had detectable prepandemic IC₅₀ titers, but only one reached a titer of 10. This proportion increased to 44% after the pandemic, when 39 participants had a titer of 10, and 67% of infected compared to 44% of noninfected had detectable IC₅₀ titers (P < 0.001). The low levels of SR antibodies in prepandemic sera were not associated with subsequent H1N1pdm09 infection (P = 0.241), and the higher levels induced by H1N1pdm09 infection returned to prepandemic levels within 2 years. The findings indicate that natural infection induces only low titers of SR antibodies that are not sustained.

IMPORTANCE Universal influenza vaccines could have substantial health and economic benefits. The focus of universal vaccine research has been to induce antibodies that prevent infection by diverse influenza virus strains. These so-called broadly neutralizing antibodies are readily detected in mice and ferrets after infection with a series of distinct influenza virus strains. The 2009 H1N1 pandemic provided an opportunity to investigate whether infection with a novel strain induced broadly neutralizing antibodies in humans. We found that broadly neutralizing antibodies were induced, but levels were low and poorly maintained. This could represent an obstacle for universal vaccine development and warrants further investigation.

Cross-protective potential of anti-nucleoprotein human monoclonal antibodies against lethal influenza A virus infection

The nucleoprotein (NP) possesses regions that are highly conserved among influenza A viruses, and has therefore been one of the target viral proteins for development of a universal influenza vaccine. It has been expected that human or humanized antibodies will be made available for the prophylaxis, pre-emptive and acute treatment of viral infection. However, it is still unclear whether anti-NP human antibody can confer protection against influenza virus infection. In this study, we generated transgenic mice expressing anti-NP human mAbs derived from lymphocytes of a patient infected with H5N1 highly pathogenic avian influenza (HPAI) virus, and experimental infections were conducted to examine antiviral effects of the anti-NP antibodies against H5N1 HPAI viral infections with a high fatality rate in mammals. Transgenic mouse lines expressing the anti-NP human mAbs at more than 1 mg ml-1 showed marked resistance to H5N1 virus infections. In addition, resistance to infection with an H1N1 subtype that shows strong pathogenicity to mice was also confirmed. Although the anti-NP mAbs expressed in the transgenic mice did not neutralize the virus, the mAbs could bind to NP located on the surface of infected cells. These results suggested a possibility that the non-neutralizing anti-NP human mAbs could induce indirect antiviral effects, such as antibody-dependent cellular cytotoxicity or complement-dependent cytotoxicity. Taken together, these results demonstrated that anti-NP human mAbs play an important role in heterosubtypic protection against lethal influenza virus infections in vivo.

Identification of a CD4 T-cell epitope in the hemagglutinin stalk domain of pandemic H1N1 influenza virus and its antigen-driven TCR usage signature in BALB/c mice

The stalk region of the influenza virus hemagglutinin is relatively well conserved compared with the globular head domain, which makes it a potential target for use as a universal vaccine against influenza. However, the role of CD4 T cells in the hemagglutinin stalk-specific immune response is not clear. Here we identified a mouse CD4 T-cell epitope that encompasses residues HA2_113-131 from the hemagglutinin stalk domain after a sub-lethal infection of influenza. In response to stimulation with the identified epitope, splenocytes derived from the infected mice showed significant polyfunctionality as shown by IL-2, TNF-α and IFN-γ production as well as degranulation. Moreover, mice immunized with the peptide corresponding to this CD4 T-cell epitope exhibited interindividual sharing of the CD4 T-cell receptor β sequences, and they had a higher survival rate following a challenge with a lethal dose of pandemic H1N1 influenza virus. Thus, our data demonstrated a crucial role of hemagglutinin stalk-specific CD4 T cells in the host immune response against influenza virus infection.

Safety and Upper Respiratory Pharmacokinetics of the Hemagglutinin Stalk-Binding Antibody VIS410 Support Treatment and Prophylaxis Based on Population Modeling of Seasonal Influenza A Outbreaks

Background

Seasonal influenza is a major public health concern in vulnerable populations. Here we investigated the safety, tolerability, and pharmacokinetics of a broadly neutralizing monoclonal antibody (VIS410) against Influenza A in a Phase 1 clinical trial. Based on these results and preclinical data, we implemented a mathematical modeling approach to investigate whether VIS410 could be used prophylactically to lessen the burden of a seasonal influenza epidemic and to protect at-risk groups from associated complications.

Methods

Using a single-ascending dose study (n = 41) at dose levels from 2 mg/kg–50 mg/kg we evaluated the safety as well as the serum and upper respiratory pharmacokinetics of a broadly-neutralizing antibody (VIS410) against influenza A (ClinicalTrials.gov identifier NCT02045472). Our primary endpoints were safety and tolerability of VIS410 compared to placebo. We developed an epidemic microsimulation model testing the ability of VIS410 to mitigate attack rates and severe disease in at risk-populations.

Findings

VIS410 was found to be generally safe and well-tolerated at all dose levels, from 2–50 mg/kg. Overall, 27 of 41 subjects (65.9%) reported a total of 67 treatment emergent adverse events (TEAEs). TEAEs were reported by 20 of 30 subjects (66.7%) who received VIS410 and by 7 of 11 subjects (63.6%) who received placebo. 14 of 16 TEAEs related to study drug were considered mild (Grade 1) and 2 were moderate (Grade 2). Two subjects (1 subject who received 30 mg/kg VIS410 and 1 subject who received placebo) experienced serious AEs (Grade 3 or 4 TEAEs) that were not related to study drug. VIS410 exposure was approximately dose-proportional with a mean half-life of 12.9 days. Mean VIS410 C_max levels in the upper respiratory tract were 20.0 and 25.3 μg/ml at the 30 mg/kg and 50 mg/kg doses, respectively, with corresponding serum C_max levels of 980.5 and 1316 μg/mL. Using these pharmacokinetic data, a microsimulation model showed that median attack rate reductions ranged from 8.6% (interquartile range (IQR): 4.7%–11.0%) for 2% coverage to 22.6% (IQR: 12.7–30.0%) for 6% coverage. The overall benefits to the elderly, a vulnerable subgroup, are largest when VIS410 is distributed exclusively to elderly individuals, resulting in reductions in hospitalization rates between 11.4% (IQR: 8.2%–13.3%) for 2% coverage and 30.9% (IQR: 24.8%–35.1%) for 6% coverage among those more than 65 years of age.

Interpretation

VIS410 was generally safe and well tolerated and had good relative exposure in both serum and upper respiratory tract, supporting its use as either a single-dose therapeutic or prophylactic for influenza A. Including VIS410 prophylaxis among the public health interventions for seasonal influenza has the potential to lower attack rates and substantially reduce hospitalizations in individuals over the age of 65.

Funding

Visterra, Inc.

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VIS410, a broadly neutralizing monoclonal antibody, neutralizes seasonal strains of influenza A.

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VIS410 was found to be safe and well tolerated in a phase 1 clinical study.

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VIS410 drug levels in the upper respiratory tract support treatment and prophylaxis of influenza A.

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Epidemic modeling of VIS410 as a prophylactic therapy demonstrated substantial reduction of hospitalizations.

Influenza infection results in significant morbidity and mortality especially in high risk groups such as the elderly. VIS410 is a broadly neutralizing antibody engineered to bind the influenza A virus. VIS410 was shown to be safe and well tolerated in a phase 1 clinical trial in healthy adult volunteers. Measurements of drug levels of VIS410 in the upper respiratory tract demonstrated that protective levels were achieved at the site of infection. Epidemic modeling indicate that for an antibody such as VIS410 prophylactic administration to 4–6% of the population would be sufficient to substantially suppress hospitalizations related to severe influenza.

Structural characterization of viral epitopes recognized by broadly cross-reactive antibodies

Influenza hemagglutinin (HA) is the major surface glycoprotein on influenza viruses and mediates viral attachment and subsequent fusion with host cells. The HA is the major target of the immune response, but due to its high level of variability, as evidenced by substantial antigenic diversity, it had been historically considered to elicit only a narrow, strain-specific antibody response. However, a recent explosion in the discovery of broadly neutralizing antibodies (bnAbs) to influenza virus has identified two major supersites of vulnerability on the HA through structural characterization of HA-antibody complexes. These commonly targeted epitopes are involved with receptor binding as well as the fusion machinery and, hence, are functionally conserved and less prone to mutation. These bnAbs can neutralize viruses by blocking infection or the spread of infection by preventing progeny release. Structural analyses of these bnAbs show they exhibit striking similarities and trends in recognition of the HA and use recurring recognition motifs, despite substantial differences in their germline genes. This information can be utilized in design of novel therapeutics as well as in immunogens for improved vaccines with greater breadth and efficacy.

Prospects of HA-based universal influenza vaccine

Current influenza vaccines afford substantial protection in humans by inducing strain-specific neutralizing antibodies (Abs). Most of these Abs target highly variable immunodominant epitopes in the globular domain of the viral hemagglutinin (HA). Therefore, current vaccines may not be able to induce heterosubtypic immunity against the divergent influenza subtypes. The identification of broadly neutralizing Abs (BnAbs) against influenza HA using recent technological advancements in antibody libraries, hybridoma, and isolation of single Ab-secreting plasma cells has increased the interest in developing a universal influenza vaccine as it could provide life-long protection. While these BnAbs can serve as a source for passive immunotherapy, their identification represents an important step towards the design of such a universal vaccine. This review describes the recent advances and approaches used in the development of universal influenza vaccine based on highly conserved HA regions identified by BnAbs.

An anti-H5N1 influenza virus FcDART antibody is a highly efficacious therapeutic agent and prophylactic against H5N1 influenza virus infection

Highly pathogenic H5N1 avian influenza viruses are associated with severe disease in humans and continue to be a pandemic threat. While vaccines are available, other approaches are required for patients that typically respond poorly to vaccination, such as the elderly and the immunocompromised. To produce a therapeutic agent that is highly efficacious at low doses and is broadly specific against antigenically drifted H5N1 influenza viruses, we developed two neutralizing monoclonal antibodies and combined them into a single bispecific Fc fusion protein (the Fc dual-affinity retargeting [FcDART] molecule). In mice, a single therapeutic or prophylactic dose of either monoclonal antibody at 2.5 mg/kg of body weight provided 100% protection against challenge with A/Vietnam/1203/04 (H5N1) or the antigenically drifted strain A/Whooper swan/Mongolia/244/05 (H5N1). In ferrets, a single 1-mg/kg prophylactic dose provided 100% protection against A/Vietnam/1203/04 challenge. FcDART was also effective, as a single 2.5-mg/kg therapeutic or prophylactic dose in mice provided 100% protection against A/Vietnam/1203/04 challenge. Antibodies bound to conformational epitopes in antigenic sites on the globular head of the hemagglutinin protein, on the basis of analysis of mutants with antibody escape mutations. While it was possible to generate escape mutants in vitro, they were neutralized by the antibodies in vivo, as mice infected with escape mutants were 100% protected after only a single therapeutic dose of the antibody used to generate the escape mutant in vitro. In summary, we have combined the antigen specificities of two highly efficacious anti-H5N1 influenza virus antibodies into a bispecific FcDART molecule, which represents a strategy to produce broadly neutralizing antibodies that are effective against antigenically diverse influenza viruses.

IMPORTANCE

Highly pathogenic H5N1 avian influenza viruses are associated with severe disease in humans and are a pandemic threat. A vaccine is available, but other approaches are required for patients that typically respond poorly to vaccination, such as the elderly and the immunocompromised. The variability of the virus means that such an approach must be broad spectrum. To achieve this, we developed two antibodies that neutralize H5N1 influenza viruses. In mice, these antibodies provided complete protection against a spectrum of H5N1 influenza viruses at a single low dose. We then combined the two antibodies into a single molecule, FcDART, which combined the broad-spectrum activity and protective efficacy of both antibodies. This treatment provides a novel and effective therapeutic agent or prophylactic with activity against highly pathogenic H5N1 avian influenza viruses.

Antiviral strategies against influenza virus: towards new therapeutic approaches

Influenza viruses are major human pathogens responsible for respiratory diseases affecting millions of people worldwide and characterized by high morbidity and significant mortality. Influenza infections can be controlled by vaccination and antiviral drugs. However, vaccines need annual updating and give limited protection. Only two classes of drugs are currently approved for the treatment of influenza: M2 ion channel blockers and neuraminidase inhibitors. However, they are often associated with limited efficacy and adverse side effects. In addition, the currently available drugs suffer from rapid and extensive emergence of drug resistance. All this highlights the urgent need for developing new antiviral strategies with novel mechanisms of action and with reduced drug resistance potential. Several new classes of antiviral agents targeting viral replication mechanisms or cellular proteins/processes are under development. This review gives an overview of novel strategies targeting the virus and/or the host cell for counteracting influenza virus infection.

Influenza A HA's conserved epitopes and broadly neutralizing antibodies: a prediction method

A conserved epitope is an epitope retained by multiple strains of influenza as the key target of a broadly neutralizing antibody. Identification of conserved epitopes is of strong interest to help design broad-spectrum vaccines against influenza. Conservation score measures the evolutionary conservation of an amino acid position in a protein based on the phylogenetic relationships observed amongst homologous sequences. Here, Average Amino Acid Conservation Score (AAACS) is proposed as a method to identify HA's conserved epitopes. Our analysis shows that there is a clear distinction between conserved epitopes and nonconserved epitopes in terms of AAACS. This method also provides an excellent classification performance on an independent dataset. In contrast, alignment-based comparison methods do not work well for this problem, because conserved epitopes to the same broadly neutralizing antibody are usually not identical or similar. Location-based methods are not successful either, because conserved epitopes are located at both the less-conserved globular head (HA1) and the more-conserved stem (HA2). As a case study, two conserved epitopes on HA are predicted for the influenza A virus H7N9: One should match the broadly neutralizing antibodies CR9114 or FI6v3, while the other is new and requires validation by wet-lab experiments.

Options and obstacles for designing a universal influenza vaccine

Since the discovery of antibodies specific to a highly conserved stalk region of the influenza virus hemagglutinin (HA), eliciting such antibodies has been considered the key to developing a universal influenza vaccine that confers broad-spectrum protection against various influenza subtypes. To achieve this goal, a prime/boost immunization strategy has been heralded to redirect host immune responses from the variable globular head domain to the conserved stalk domain of HA. While this approach has been successful in eliciting cross-reactive antibodies against the HA stalk domain, protective efficacy remains relatively poor due to the low immunogenicity of the domain, and the cross-reactivity was only within the same group, rather than among different groups. Additionally, concerns are raised on the possibility of vaccine-associated enhancement of viral infection and whether multiple boost immunization protocols would be considered practical from a clinical standpoint. Live attenuated vaccine hitherto remains unexplored, but is expected to serve as an alternative approach, considering its superior cross-reactivity. This review summarizes recent advancements in the HA stalk-based universal influenza vaccines, discusses the pros and cons of these approaches with respect to the potentially beneficial and harmful effects of neutralizing and non-neutralizing antibodies, and suggests future guidelines towards the design of a truly protective universal influenza vaccine.

Modified vaccinia virus ankara (MVA) as production platform for vaccines against influenza and other viral respiratory diseases

Respiratory viruses infections caused by influenza viruses, human parainfluenza virus (hPIV), respiratory syncytial virus (RSV) and coronaviruses are an eminent threat for public health. Currently, there are no licensed vaccines available for hPIV, RSV and coronaviruses, and the available seasonal influenza vaccines have considerable limitations. With regard to pandemic preparedness, it is important that procedures are in place to respond rapidly and produce tailor made vaccines against these respiratory viruses on short notice. Moreover, especially for influenza there is great need for the development of a universal vaccine that induces broad protective immunity against influenza viruses of various subtypes. Modified Vaccinia Virus Ankara (MVA) is a replication-deficient viral vector that holds great promise as a vaccine platform. MVA can encode one or more foreign antigens and thus functions as a multivalent vaccine. The vector can be used at biosafety level 1, has intrinsic adjuvant capacities and induces humoral and cellular immune responses. However, there are some practical and regulatory issues that need to be addressed in order to develop MVA-based vaccines on short notice at the verge of a pandemic. In this review, we discuss promising novel influenza virus vaccine targets and the use of MVA for vaccine development against various respiratory viruses.

Human monoclonal antibodies derived from a patient infected with 2009 pandemic influenza A virus broadly cross-neutralize group 1 influenza viruses

Influenza viruses are a continuous threat to human public health because of their ability to evolve rapidly through genetic drift and reassortment. Three human monoclonal antibodies (HuMAbs) were generated in this study, 1H11, 2H5 and 5G2, and they cross-neutralize a diverse range of group 1 influenza A viruses, including seasonal H1N1, 2009 pandemic H1N1 (H1N1pdm) and avian H5N1 and H9N2. The three HuMAbs were prepared by fusing peripheral blood lymphocytes from an H1N1pdm-infected patient with a newly developed fusion partner cell line, SPYMEG. All the HuMAbs had little hemagglutination inhibition activity but had strong membrane-fusion inhibition activity against influenza viruses. A protease digestion assay showed the HuMAbs targeted commonly a short α-helix region in the stalk of the hemagglutinin. Furthermore, Ile45Phe and Glu47Gly double substitutions in the α-helix region made the HA unrecognizable by the HuMAbs. These two amino acid residues are highly conserved in the HAs of H1N1, H5N1 and H9N2 viruses. The HuMAbs reported here may be potential candidates for the development of therapeutic antibodies against group 1 influenza viruses.

Advances in universal influenza virus vaccine design and antibody mediated therapies based on conserved regions of the hemagglutinin

The threat of novel influenza viruses emerging into the human population from animal reservoirs, as well as the short duration of protection conferred by licensed vaccines against human seasonal strains has spurred research efforts to improve upon current vaccines and develop novel therapeutics against influenza viruses. In recent years these efforts have resulted in the identification of novel, highly conserved epitopes for neutralizing antibodies on the influenza virus hemagglutinin protein, which are present in both the stalk and globular head domains of the molecule. The existence of such epitopes may allow for generation of novel therapeutic antibodies, in addition to serving as attractive targets of novel vaccine design. The aims of developing improved vaccines include eliciting broader protection from drifted strains, inducing long-lived immunity against seasonal strains, and allowing for the rational design of vaccines that can be stockpiled for use as pre-pandemic vaccines. In addition, an increased focus on influenza virus vaccine research has prompted an improved understanding of how the immune system responds to influenza virus infection.

DBA/2 mouse as an animal model for anti-influenza drug efficacy evaluation

Influenza viruses are seasonally recurring human pathogens. Vaccines and antiviral drugs are available for influenza. However, the viruses, which often change themselves via antigenic drift and shift, demand constant efforts to update vaccine antigens every year and develop new agents with broad-spectrum antiviral efficacy. An animal model is critical for such efforts. While most human influenza viruses are unable to kill BALB/c mice, some strains have been shown to kill DBA/2 mice without prior adaptation. Therefore, in this study, we explored the feasibility of employing DBA/2 mice as a model in the development of anti-influenza drugs. Unlike the BALB/c strain, DBA/2 mice were highly susceptible and could be killed with a relatively low titer (50% DBA/2 lethal dose = 10(2.83) plaque-forming units) of the A/Korea/01/2009 virus (2009 pandemic H1N1 virus). When treated with a neuraminidase inhibitor, oseltamivir phosphate, infected DBA/2 mice survived until 14 days post-infection. The reduced morbidity of the infected DBA/2 mice was also consistent with the oseltamivir treatment. Taking these data into consideration, we propose that the DBA/2 mouse is an excellent animal model to evaluate antiviral efficacy against influenza infection and can be further utilized for combination therapies or bioactivity models of existing and newly developed anti-influenza drugs.

Therapeutics against influenza

Despite 75 years of research into prevention and treatment of influenza, the viruses that cause this disease continue to rank as some of the most important pathogens afflicting humans today. Progress in development of therapeutics for influenza has been slow for much of that time, but has accelerated in pace over the last two decades. Two classes of antiviral medications are used in humans at present, but each has limitations in scope and effectiveness of use. New strategies involving these licensed agents, including alternate forms of delivery and combination therapy with other drugs, are currently being explored. In addition, several novel antiviral compounds are in various clinical phases of development. Together with strategies designed to target the virus itself, new approaches to interrupt host–pathogen interactions or modulate detrimental aspects of the immune response have been proposed. Therapy for influenza will likely undergo substantial changes in the decades to come, evolving with our knowledge of pathogenesis as new approaches become viable and are validated clinically.

A Combinatorial approach: To design inhibitory molecules on Hemagglutinin protein of H1N1 virus (Swine Flu)

The Hemagglutinin (HA) is a protein of influenza A virus. It is present on the surface of influenza A virus and it is a glycoprotein. The HA is identified as potential drug target. H1N1 thiazolides, proved to be a potent drug in the inhibition of H1N1 replication. It is also known as inhibitor of other strains of influenza A virus. Thiazolide drug represses viral HA's maturation at a level which exists just before the resistance from digestion of endoglycosidase-H and thereby it hampers, HA insertion in host membrane. Blocking the appropriate active site of hemagglutinin protein helps in the disease control. In the present work, we have generated diverse combinatorial library based ligands on known inhibitor thiazolides and they were used for virtual screening by Molegro virtual docker program. K-means clustering approach was used for finding new inhibitory molecules with more appropriate features. These resulted molecules are may be helpful in the treatment of swine flu and many other related diseases.

Immune responses to infection with H5N1 influenza virus

Influenza A H5N1 viruses remain a substantial threat to global public health. In particular, the expanding genetic diversity of H5N1 viruses and the associated risk for human adaptation underscore the importance of better understanding host immune responses that may protect against disease or infection. Although much emphasis has been placed on investigating early virus-host interactions and the induction of innate immune responses, little is known of the consequent adaptive immune response to H5N1 virus infection. In this review, we describe the H5N1 virus-specific and cross-reactive antibody and T cell responses in humans and animal models. Data from limited studies suggest that although initially robust, there is substantial waning of the serum antibody responses in survivors of H5N1 virus infection. Characterization of monoclonal antibodies generated from memory B cells of survivors of H5N1 virus infection has provided an understanding of the fine specificity of the human antibody response to H5N1 virus infection and identified strategies for immunotherapy. Human T cell responses induced by infection with seasonal influenza viruses are directed to relatively conserved internal proteins and cross-react with the H5N1 subtype. A role for T cell-based heterosubtypic immunity against H5N1 viruses is suggested in animal studies. Further studies on adaptive immune responses to H5N1 virus infection in both humans and animals are needed to inform the design of optimal immunological treatment and prevention modalities.

Broadly neutralizing antibodies against influenza viruses

Despite available antivirals and vaccines, influenza continues to be a major cause of mortality worldwide. Vaccination generally induces an effective, but strain-specific antibody response. As the virus continually evolves, new vaccines have to be administered almost annually when a novel strain becomes dominant. Furthermore, the sporadic emerging resistance to neuraminidase inhibitors among circulating strains suggests an urgent need for new therapeutic agents. Recently, several cross-reactive antibodies have been described, which neutralize an unprecedented spectrum of influenza viruses. These broadly neutralizing antibodies generally target conserved functional regions on the major influenza surface glycoprotein hemagglutinin (HA). The characterization of their neutralization breadth and epitopes on HA could stimulate the development of new antibody-based antivirals and broader influenza vaccines. This article forms part of a symposium in Antiviral Research on "Treatment of influenza: targeting the virus or the host."

Structure of a classical broadly neutralizing stem antibody in complex with a pandemic H2 influenza virus hemagglutinin

We report the structural characterization of the first antibody identified to cross-neutralize multiple subtypes of influenza A viruses. The crystal structure of mouse antibody C179 bound to the pandemic 1957 H2N2 hemagglutinin (HA) reveals that it targets an epitope on the HA stem similar to those targeted by the recently identified human broadly neutralizing antibodies. C179 also inhibits the low-pH conformational change of the HA but uses a different angle of approach and both heavy and light chains.

Unraveling of a neutralization mechanism by two human antibodies against conserved epitopes in the globular head of H5 hemagglutinin

The rapid spread of highly pathogenic avian influenza (HPAI) H5N1 virus underscores the importance of effective antiviral treatment. Previously, we developed human monoclonal antibodies 65C6 and 100F4 that neutralize almost all (sub)clades of HPAI H5N1. The conserved 65C6 epitope was mapped to the globular head of HA. However, neither the 100F4 epitope nor the neutralization mechanism by these antibodies was known. In this study, we determined the 100F4 epitope and unraveled a neutralization mechanism by antibodies 65C6 and 100F4.

Influenza B-cells protective epitope characterization: a passkey for the rational design of new broad-range anti-influenza vaccines

The emergence of new influenza strains causing pandemics represents a serious threat to human health. From 1918, four influenza pandemics occurred, caused by H1N1, H2N2 and H3N2 subtypes. Moreover, in 1997 a novel influenza avian strain belonging to the H5N1 subtype infected humans. Nowadays, even if its transmission is still circumscribed to avian species, the capability of the virus to infect humans directly from avian reservoirs can result in fatalities. Moreover, the risk that this or novel avian strains could adapt to inter-human transmission, the development of resistance to anti-viral drugs and the lack of an effective prevention are all incumbent problems for the world population. In this scenario, the identification of broadly neutralizing monoclonal antibodies (mAbs) directed against conserved regions shared among influenza isolates has raised hopes for the development of monoclonal antibody-based immunotherapy and "universal" anti-influenza vaccines.

Heterosubtypic antibody recognition of the influenza virus hemagglutinin receptor binding site enhanced by avidity

Continual and rapid mutation of seasonal influenza viruses by antigenic drift necessitates the almost annual reformulation of flu vaccines, which may offer little protection if the match to the dominant circulating strain is poor. S139/1 is a cross-reactive antibody that neutralizes multiple HA strains and subtypes, including those from H1N1 and H3N2 viruses that currently infect humans. The crystal structure of the S139/1 Fab in complex with the HA from the A/Victoria/3/1975 (H3N2) virus reveals that the antibody targets highly conserved residues in the receptor binding site and contacts antigenic sites A, B, and D. Binding and plaque reduction assays show that the monovalent Fab alone can protect against H3 strains, but the enhanced avidity from binding of bivalent IgG increases the breadth of neutralization to additional strains from the H1, H2, H13, and H16 subtypes. Thus, antibodies making relatively low affinity Fab interactions with the receptor binding site can have significant antiviral activity when enhanced by avidity through bivalent interactions of the IgG, thereby extending the breadth of binding and neutralization to highly divergent influenza virus strains and subtypes.

Cross-neutralization of influenza A viruses mediated by a single antibody loop

Immune recognition of protein antigens relies on the combined interaction of multiple antibody loops, which provide a fairly large footprint and constrain the size and shape of protein surfaces that can be targeted. Single protein loops can mediate extremely high-affinity binding, but it is unclear whether such a mechanism is available to antibodies. Here we report the isolation and characterization of an antibody called C05, which neutralizes strains from multiple subtypes of influenza A virus, including H1, H2 and H3. X-ray and electron microscopy structures show that C05 recognizes conserved elements of the receptor-binding site on the haemagglutinin surface glycoprotein. Recognition of the haemagglutinin receptor-binding site is dominated by a single heavy-chain complementarity-determining region 3 loop, with minor contacts from heavy-chain complementarity-determining region 1, and is sufficient to achieve nanomolar binding with a minimal footprint. Thus, binding predominantly with a single loop can allow antibodies to target small, conserved functional sites on otherwise hypervariable antigens.

Viva la revolución: rethinking influenza a virus antigenic drift

Rapid antigenic evolution of the influenza A virus hemagglutinin has precluded developing vaccines that provide durable protection. The yearly costs of influenza (circa $10(11) in the USA alone) easily justify investments in better understanding the interaction of influenza with antibodies and other inducible elements of the immune system that potentially limit or circumvent antigenic variation. Here, I summarize exciting new findings that offer the possibility of a quantum improvement in vaccine efficacy, focusing on studies clearly documenting robust neutralizing antibody responses to the conserved stem region of the hemagglutinin.

Phage display-based strategies for cloning and optimization of monoclonal antibodies directed against human pathogens

In the last two decades, several phage display-selected monoclonal antibodies (mAbs) have been described in the literature and a few of them have managed to reach the clinics. Among these, the anti-respiratory syncytial virus (RSV) Palivizumab, a phage-display optimized mAb, is the only marketed mAb directed against microbial pathogens. Palivizumab is a clear example of the importance of choosing the most appropriate strategy when selecting or optimizing an anti-infectious mAb. From this perspective, the extreme versatility of phage-display technology makes it a useful tool when setting up different strategies for the selection of mAbs directed against human pathogens, especially when their possible clinical use is considered. In this paper, we review the principal phage display strategies used to select anti-infectious mAbs, with particular attention focused on those used against hypervariable pathogens, such as HCV and influenza viruses.

Passive immune neutralization strategies for prevention and control of influenza A infections

Although vaccination significantly reduces influenza severity, seasonal human influenza epidemics still cause more than 250,000 deaths annually. Vaccine efficacy is limited in high-risk populations such as infants, the elderly and immunosuppressed individuals. In the event of an influenza pandemic (such as the 2009 H1N1 pandemic), a significant delay in vaccine availability represents a significant public health concern, particularly in high-risk groups. The increasing emergence of strains resistant to the two major anti-influenza drugs, adamantanes and neuraminidase inhibitors, and the continuous circulation of avian influenza viruses with pandemic potential in poultry, strongly calls for alternative prophylactic and treatment options. In this review, we focus on passive virus neutralization strategies for the prevention and control of influenza type A viruses.

A non-VH1-69 heterosubtypic neutralizing human monoclonal antibody protects mice against H1N1 and H5N1 viruses

Influenza viruses are among the most important human pathogens and are responsible for annual epidemics and sporadic, potentially devastating pandemics. The humoral immune response plays an important role in the defense against these viruses, providing protection mainly by producing antibodies directed against the hemagglutinin (HA) glycoprotein. However, their high genetic variability allows the virus to evade the host immune response and the potential protection offered by seasonal vaccines. The emergence of resistance to antiviral drugs in recent years further limits the options available for the control of influenza. The development of alternative strategies for influenza prophylaxis and therapy is therefore urgently needed. In this study, we describe a human monoclonal antibody (PN-SIA49) that recognizes a highly conserved epitope located on the stem region of the HA and able to neutralize a broad spectrum of influenza viruses belonging to different subtypes (H1, H2 and H5). Furthermore, we describe its protective activity in mice after lethal challenge with H1N1 and H5N1 viruses suggesting a potential application in the treatment of influenza virus infections.

Pandemic Influenza A H1N1 (2009) Virus: Lessons from the Past and Implications for the Future

The recent pandemic by novel influenza A (H1N1) 2009 (pH1N1) virus is an emerging viral infection, being of significant international concern and requires intensive research. The virus spread in pandemic proportions, and continues to be in the post-pandemic phase. Since, the pH1N1 is still circulating in the community, monitoring is required during the post-pandemic period. The pH1N1 defied influenza seasonality and rapidly became dominant over the seasonal influenza viruses. This new strain was antigenically different from the seasonal H1N1 influenza strains due to the genetic re-assortment. Surprisingly, this new reassortant virus emerged at the end of influenza season, caused a sudden toll of mild illness and is now co-circulating with the seasonal strains. The recent outbreak of pH1N1 consolidates the fact that a new reassortant virus may have originated in animal reservoirs and got transferred to human who were in close contact with these animals. There is a continued need for multisite surveillance to detect potentially dangerous influenza strains, which may emerge and establish themselves in human population. This review is an attempt to address the lessons learnt from the recent influenza pandemic and the future implications for prevention and control of influenza.

A human antibody recognizing a conserved epitope of H5 hemagglutinin broadly neutralizes highly pathogenic avian influenza H5N1 viruses

Influenza A virus infection is a persistent threat to public health worldwide due to its ability to evade immune surveillance through rapid genetic drift and shift. Current vaccines against influenza A virus provide immunity to viral isolates that are similar to vaccine strains. High-affinity neutralizing antibodies against conserved epitopes could provide immunity to diverse influenza virus strains and protection against future pandemic viruses. In this study, by using a highly sensitive H5N1 pseudotype-based neutralization assay to screen human monoclonal antibodies produced by memory B cells from an H5N1-infected individual and molecular cloning techniques, we developed three fully human monoclonal antibodies. Among them, antibody 65C6 exhibited potent neutralization activity against all H5 clades and subclades except for subclade 7.2 and prophylactic and therapeutic efficacy against highly pathogenic avian influenza H5N1 viruses in mice. Studies on hemagglutinin (HA)-antibody complexes by electron microscopy and epitope mapping indicate that antibody 65C6 binds to a conformational epitope comprising amino acid residues at positions 118, 121, 161, 164, and 167 (according to mature H5 numbering) on the tip of the membrane-distal globular domain of HA. Thus, we conclude that antibody 65C6 recognizes a neutralization epitope in the globular head of HA that is conserved among almost all divergent H5N1 influenza stains.