Structural Basis of Protection against H7N9 Influenza Virus by Human Anti-N9 Neuraminidase Antibodies

Isolation of human antibodies against influenza B neuraminidase and mechanisms of protection at the airway interface

Influenza B viruses (IBVs) comprise a substantial portion of the circulating seasonal human influenza viruses. Here, we describe the isolation of human monoclonal antibodies (mAbs) that recognized the IBV neuraminidase (NA) glycoprotein from an individual following seasonal vaccination. Competition-binding experiments suggested the antibodies recognized two major antigenic sites. One group, which included mAb FluB-393, broadly inhibited IBV NA sialidase activity, protected prophylactically in vivo, and bound to the lateral corner of NA. The second group contained an active site mAb, FluB-400, that broadly inhibited IBV NA sialidase activity and virus replication in vitro in primary human respiratory epithelial cell cultures and protected against IBV in vivo when administered systemically or intranasally. Overall, the findings described here shape our mechanistic understanding of the human immune response to the IBV NA glycoprotein through the demonstration of two mAb delivery routes for protection against IBV and the identification of potential IBV therapeutic candidates.

Protective human monoclonal antibodies target conserved sites of vulnerability on the underside of influenza virus neuraminidase

Continuously evolving influenza viruses cause seasonal epidemics and pose global pandemic threats. Although viral neuraminidase (NA) is an effective drug and vaccine target, our understanding of the NA antigenic landscape still remains incomplete. Here, we describe NA-specific human antibodies that target the underside of the NA globular head domain, inhibit viral propagation of a wide range of human H3N2, swine-origin variant H3N2, and H2N2 viruses, and confer both pre- and post-exposure protection against lethal H3N2 infection in mice. Cryo-EM structures of two such antibodies in complex with NA reveal non-overlapping epitopes covering the underside of the NA head. These sites are highly conserved among N2 NAs yet inaccessible unless the NA head tilts or dissociates. Our findings help guide the development of effective countermeasures against ever-changing influenza viruses by identifying hidden conserved sites of vulnerability on the NA underside.

Targeting neuraminidase: the next frontier for broadly protective influenza vaccines

Current seasonal influenza vaccines, which mainly target hemagglutinin (HA), require annual updates due to the continuous antigenic drift of the influenza virus. Developing an influenza vaccine with increased breadth of protection will have significant public health benefits. The recent discovery of broadly protective antibodies to neuraminidase (NA) has provided important insights into developing a universal influenza vaccine, either by improving seasonal influenza vaccines or designing novel immunogens. However, further in-depth molecular characterizations of NA antibody responses are warranted to fully leverage broadly protective NA antibodies for influenza vaccine designs. Overall, we posit that focusing on NA for influenza vaccine development is synergistic with existing efforts targeting HA, and may represent a cost-effective approach to generating a broadly protective influenza vaccine.

Broadly neutralizing antibodies to combat influenza virus infection

The diversified classification and continuous alteration of influenza viruses underscore for antivirals and vaccines that can counter a broad range of influenza subtypes. Hemagglutinin (HA) and neuraminidase (NA) are two principle viral surface targets for broadly neutralizing antibodies. A series of monoclonal antibodies, targeting HA and NA, have been discovered and characterized with a wide range of neutralizing activity against influenza viruses. Clinical studies have demonstrated the safety and efficacy of some HA stem-targeting antibodies against influenza viruses. Broadly neutralizing antibodies (bnAbs) can serve as both prophylactic and therapeutic agents, as well as play a critical role in identifying antigens and epitopes for the development of universal vaccines. In this review, we described and summarized the latest discoveries and advancements of bnAbs against influenza viruses in both pre- and clinical development. Additionally, we assess whether bnAbs can serve as a viable alternative to vaccination against influenza. Finally, we discussed the rationale behind reverse vaccinology, a structure-guided universal vaccine design strategy that efficiently identifies candidate antigens and conserved epitopes that can be targeted by antibodies.

Leveraging vaccination-induced protective antibodies to define conserved epitopes on influenza N2 neuraminidase

There is growing appreciation for neuraminidase (NA) as an influenza vaccine target; however, its antigenicity remains poorly characterized. In this study, we isolated three broadly reactive N2 antibodies from the plasmablasts of a single vaccinee, including one that cross-reacts with NAs from seasonal H3N2 strains spanning five decades. Although these three antibodies have diverse germline usages, they recognize similar epitopes that are distant from the NA active site and instead involve the highly conserved underside of NA head domain. We also showed that all three antibodies confer prophylactic and therapeutic protection in vivo, due to both Fc effector functions and NA inhibition through steric hindrance. Additionally, the contribution of Fc effector functions to protection in vivo inversely correlates with viral growth inhibition activity in vitro. Overall, our findings advance the understanding of NA antibody response and provide important insights into the development of a broadly protective influenza vaccine.

Immune response in influenza virus infection and modulation of immune injury by viral neuraminidase

Influenza A viruses cause severe respiratory illnesses in humans and animals. Overreaction of the innate immune response to influenza virus infection results in hypercytokinemia, which is responsible for mortality and morbidity. The influenza A virus surface glycoprotein neuraminidase (NA) plays a vital role in viral attachment, entry, and virion release from infected cells. NA acts as a sialidase, which cleaves sialic acids from cell surface proteins and carbohydrate side chains on nascent virions. Here, we review progress in understanding the role of NA in modulating host immune response to influenza virus infection. We also discuss recent exciting findings targeting NA protein to interrupt influenza-induced immune injury.

Alloreactivity and autoreactivity converge to support B cell epitope targeting in transplant rejection

Antibody (Ab) responses against human leukocyte antigen (HLA) proteins mismatched between donor and recipient are leading cause of allograft loss in kidney transplantation. However, therapies targeting alloreactive B cell and Ab-secreting cell (ASC) are lacking, motivating the need to understand how to prevent and abrogate these alloresponses. Using molecular, structural, and proteomic techniques, we profiled the B cell response in a kidney transplant recipient with antibody-mediated rejection and graft loss. We found that this response spanned the rejected organ and peripheral blood, stimulated the differentiation of multiple B cell subsets, and produced a high-affinity, donor-specific, anti-HLA response. We found epitopic immunodominance that relied on highly exposed, solvent-accessible mismatched HLA residues as well as structural and biomolecular evidence of autoreactivity against the recipient's self-HLA allele. These alloreactive and autoreactive signatures converged in the recipient's circulating donor-specific Ab repertoire, suggesting that rejection requires both the recognition of non-self and breaches of tolerance to lead to alloinjury and graft loss.

Antiviral Approaches against Influenza Virus

Preventing and controlling influenza virus infection remains a global public health challenge, as it causes seasonal epidemics to unexpected pandemics. These infections are responsible for high morbidity, mortality, and substantial economic impact. Vaccines are the prophylaxis mainstay in the fight against influenza. However, vaccination fails to confer complete protection due to inadequate vaccination coverages, vaccine shortages, and mismatches with circulating strains. Antivirals represent an important prophylactic and therapeutic measure to reduce influenza-associated morbidity and mortality, particularly in high-risk populations. Here, we review current FDA-approved influenza antivirals with their mechanisms of action, and different viral- and host-directed influenza antiviral approaches, including immunomodulatory interventions in clinical development. Furthermore, we also illustrate the potential utility of machine learning in developing next-generation antivirals against influenza.

Mutational fitness landscape of human influenza H3N2 neuraminidase

Influenza neuraminidase (NA) has received increasing attention as an effective vaccine target. However, its mutational tolerance is not well characterized. Here, the fitness effects of >6,000 mutations in human H3N2 NA are probed using deep mutational scanning. Our result shows that while its antigenic regions have high mutational tolerance, there are solvent-exposed regions with low mutational tolerance. We also find that protein stability is a major determinant of NA mutational fitness. The deep mutational scanning result correlates well with mutational fitness inferred from natural sequences using a protein language model, substantiating the relevance of our findings to the natural evolution of circulating strains. Additional analysis further suggests that human H3N2 NA is far from running out of mutations despite already evolving for >50 years. Overall, this study advances our understanding of the evolutionary potential of NA and the underlying biophysical constraints, which in turn provide insights into NA-based vaccine design.

Broadly Protective Neuraminidase-Based Influenza Vaccines and Monoclonal Antibodies: Target Epitopes and Mechanisms of Action

Neuraminidase (NA) is an important surface protein on influenza virions, playing an essential role in the viral life cycle and being a key target of the immune system. Despite the importance of NA-based immunity, current vaccines are focused on the hemagglutinin (HA) protein as the target for protective antibodies, and the amount of NA is not standardized in virion-based vaccines. Antibodies targeting NA are predominantly protective, reducing infection severity and viral shedding. Recently, NA-specific monoclonal antibodies have been characterized, and their target epitopes have been identified. This review summarizes the characteristics of NA, NA-specific antibodies, the mechanism of NA inhibition, and the recent efforts towards developing NA-based and NA-incorporating influenza vaccines.

In vitro and in vivo effects of 3-indoleacetonitrile-A potential new broad-spectrum therapeutic agent for SARS-CoV-2 infection

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak has resulted in significant global morbidity, mortality, and societal disruption. Currently, effective antiviral drugs for the treatment of SARS-CoV-2 infection are limited. Therefore, safe and effective antiviral drugs to combat COVID-19 are urgently required. In previous studies, we showed that 3-indoleacetonitrile, a plant growth hormone produced by cruciferous (Brassica) vegetables, is effective in treating influenza A virus infection. However, the molecular mechanisms underlying these effects remain unclear. Herein, we demonstrated that 3-indoleacetonitrile exhibits broad-spectrum antiviral activity and is effective against HSV-1 and VSV infections in vitro. This phenomenon prompted us to study its role in the anti-SARS-CoV-2 process. Interestingly, 3-indoleacetonitrile exhibited antiviral activity against SARS-CoV-2 in vitro. Importantly, tail vein injection of 3-indoleacetonitrile resulted in good antiviral activity in mouse models infected with WBP-1 (a mouse adaptation of the SARS-CoV-2 strain). Mechanistically, 3-indoleacetonitrile promoted the host interferon signalling pathway response and inhibited autophagic flux. Furthermore, we demonstrated that 3-indoleacetonitrile induced an increase in mitochondrial antiviral-signalling (MAVS) protein levels, which might be attributed to its inhibition of the interaction between MAVS and the selective autophagy receptor SQSTM1. Overall, our results demonstrate that 3-indoleacetonitrile is potently active against SARS-CoV-2 in vitro and in vivo, which may provide a foundation for further clinical testing for the treatment of COVID-19. In addition, considering its broad-spectrum antiviral effect, it should be explored whether it also has an effect on other viruses that threaten human health.

Prevalence and mechanisms of evolutionary contingency in human influenza H3N2 neuraminidase

Neuraminidase (NA) of human influenza H3N2 virus has evolved rapidly and been accumulating mutations for more than half-century. However, biophysical constraints that govern the evolutionary trajectories of NA remain largely elusive. Here, we show that among 70 natural mutations that are present in the NA of a recent human H3N2 strain, >10% are deleterious for an ancestral strain. By mapping the permissive mutations using combinatorial mutagenesis and next-generation sequencing, an extensive epistatic network is revealed. Biophysical and structural analyses further demonstrate that certain epistatic interactions can be explained by non-additive stability effect, which in turn modulates membrane trafficking and enzymatic activity of NA. Additionally, our results suggest that other biophysical mechanisms also contribute to epistasis in NA evolution. Overall, these findings not only provide mechanistic insights into the evolution of human influenza NA and elucidate its sequence-structure-function relationship, but also have important implications for the development of next-generation influenza vaccines.

Monoclonal antibodies targeting the influenza virus N6 neuraminidase

Influenza A viruses are a diverse species that include 16 true hemagglutinin (HA) subtypes and 9 true neuraminidase (NA) subtypes. While the antigenicity of many HA subtypes is reasonably well studied, less is known about NA antigenicity, especially when it comes to non-human subtypes that only circulate in animal reservoirs. The N6 subtype NAs are mostly found in viruses infecting birds. However, they have also been identified in viruses that infect mammals, such as swine and seals. More recently, highly pathogenic H5N6 subtype viruses have caused rare infections and mortality in humans. Here, we generated murine mAbs to the N6 NA, characterized their breadth and antiviral properties in vitro and in vivo and mapped their epitopes by generating escape mutant viruses. We found that the antibodies had broad reactivity across the American and Eurasian N6 lineages, but relatively little binding to the H5N6 NA. Several of the antibodies exhibited strong NA inhibition activity and some also showed activity in the antibody dependent cellular cytotoxicity reporter assay and neutralization assay. In addition, we generated escape mutant viruses for six monoclonal antibodies and found mutations on the lateral ridge of the NA. Lastly, we observed variable protection in H4N6 mouse challenge models when the antibodies were given prophylactically.

PSMD12-Mediated M1 Ubiquitination of Influenza A Virus at K102 Regulates Viral Replication

The M1 of influenza A virus (IAV) is important for the virus life cycle, especially for the assembly and budding of viruses, which is a multistep process that requires host factors. Identifying novel host proteins that interact with M1 and understanding their functions in IAV replication are of great interest in antiviral drug development. In this study, we identified 19 host proteins in DF1 cells suspected to interact with the M1 protein of an H5N6 virus through immunoprecipitation (IP)/mass spectrometry. Among them, PSMD12, a 26S proteasome regulatory subunit, was shown to interact with influenza M1, acting as a positive host factor in IAV replication in avian and human cells. The data showed that PSMD12 promoted K63-linked ubiquitination of M1 at the K102 site. H5N6 and PR8 with an M1-K102 site mutant displayed a significantly weaker replication ability than the wild-type viruses. Mechanistically, PSMD12 promoted M1-M2 virus-like particle (VLP) release, and an M1-K102 mutation disrupted the formation of supernatant M1-M2 VLPs. An H5N6 M1-K102 site mutation or knockdown PSMD12 disrupted the budding release of the virus in chicken embryo fibroblast (CEF) cells, which was confirmed by transmission electron microscopy. Further study confirmed that M1-K102 site mutation significantly affected the virulence of H5N6 and PR8 viruses in mice. In conclusion, we report the novel host factor PSMD12 which affects the replication of influenza virus by mediating K63-linked ubiquitination of M1 at K102. These findings provide novel insight into the interactions between IAV and host cells, while suggesting an important target for anti-influenza virus drug research.

IMPORTANCE M1 is proposed to play multiple biologically important roles in the life cycle of IAV, which relies largely on host factors. This study is the first one to identify that PSMD12 interacts with M1, mediates K63-linked ubiquitination of M1 at the K102 site, and thus positively regulates influenza virus proliferation. PSMD12 promoted M1-M2 VLP egress, and an M1-K102 mutation affected the M1-M2 VLP formation. Furthermore, we demonstrate the importance of this site to the morphology and budding of influenza viruses by obtaining mutant viruses, and the M1 ubiquitination regulator PSMD12 has a similar function to the M1 K102 mutation in regulating virus release and virus morphology. Additionally, we confirm the reduced virulence of H5N6 and PR8 (H1N1) viruses carrying the M1-K102 site mutation in mice. These findings provide novel insights into IAV interactions with host cells and suggest a valid and highly conserved candidate target for antiviral drug development.

Antigenic comparison of the neuraminidases from recent influenza A vaccine viruses and 2019-2020 circulating strains

Although viral-based influenza vaccines contain neuraminidase (NA or N) antigens from the recommended seasonal strains, NA is not extensively evaluated like hemagglutinin (H) during the strain selection process. Here, we compared the antigenicity of NAs from recently recommended H1N1 (2010–2021 seasons) and H3N2 (2015–2021 seasons) vaccine strains and viruses that circulated between September 2019 and December 2020. The antigenicity was evaluated by measuring NA ferret antisera titers that provide 50% inhibition of NA activity in an enzyme-linked lectin assay. Our results show that NAs from circulating H1N1 viruses and vaccine strains for the 2017–2021 seasons are all antigenically similar and distinct from the NA in the H1N1 strain recommended for the 2010–2017 seasons. Changes in N1 antigenicity were attributed to the accumulation of substitutions over time, especially the loss of an N-linked glycosylation site (Asn386) in current N1s. The NAs from circulating H3N2 viruses and the 2020–2021 vaccine strains showed similar antigenicity that varied across the N2s in the 2016–2020 vaccine strains and was distinct from the N2 in the 2015–2016 vaccine strain. These data suggest that the recent N1 antigenicity has remained similar since the loss of the head domain N-linked glycosylation site, whereas N2 antigenicity has changed more incrementally each season.

Influenza chimeric hemagglutinin structures in complex with broadly protective antibodies to the stem and trimer interface

Influenza virus hemagglutinin (HA) has been the primary target for influenza vaccine development. Broadly protective antibodies targeting conserved regions of the HA unlock the possibility of generating universal influenza immunity. Two group 2 influenza A chimeric HAs, cH4/3 and cH15/3, were previously designed to elicit antibodies to the conserved HA stem. Here, we show by X-ray crystallography and negative-stain electron microscopy that a broadly protective antistem antibody can stably bind to cH4/3 and cH15/3 HAs, thereby validating their potential as universal vaccine immunogens. Furthermore, flexibility was observed in the head domain of the chimeric HA structures, suggesting that antibodies could also potentially interact with the head interface epitope. Our structural and binding studies demonstrated that a broadly protective antihead trimeric interface antibody could indeed target the more open head domain of the cH15/3 HA trimer. Thus, in addition to inducing broadly protective antibodies against the conserved HA stem, chimeric HAs may also be able to elicit antibodies against the conserved trimer interface in the HA head domain, thereby increasing the vaccine efficacy.

Novel Epitopes of the Influenza Virus N1 Neuraminidase Targeted by Human Monoclonal Antibodies

Influenza virus neuraminidase (NA)-targeting antibodies are an independent correlate of protection against influenza. Antibodies against the NA act by blocking enzymatic activity, preventing virus release and transmission. As we advance the development of improved influenza virus vaccines that incorporate standard amounts of NA antigen, it is important to identify the antigenic targets of human monoclonal antibodies (mAbs). Here, we describe escape mutants generated by serial passage of A/Netherlands/602/2009 (H1N1)pdm09 in the presence of human anti-N1 mAbs. We observed escape mutations on the head domain of the N1 protein around the enzymatic site (S364N, N369T, and R430Q) and also detected escape mutations located on the sides and bottom of the NA (N88D, N270D, and Q313K/R). This work increases our understanding of how human antibody responses target the N1 protein. IMPORTANCE As improved influenza virus vaccines are being developed, the influenza virus neuraminidase (NA) is becoming an important new target for immune responses. By identifying novel epitopes of anti-NA antibodies, we can improve vaccine design. Additionally, characterizing escape mutations in these epitopes aids in identifying NA antigenic drift in circulating viruses.

Structure-based design of stabilized recombinant influenza neuraminidase tetramers

Influenza virus neuraminidase (NA) is a major antiviral drug target and has recently reemerged as a key target of antibody-mediated protective immunity. Here we show that recombinant NAs across non-bat subtypes adopt various tetrameric conformations, including an "open" state that may help explain poorly understood variations in NA stability across viral strains and subtypes. We use homology-directed protein design to uncover the structural principles underlying these distinct tetrameric conformations and stabilize multiple recombinant NAs in the "closed" state, yielding two near-atomic resolution structures of NA by cryo-EM. In addition to enhancing thermal stability, conformational stabilization improves affinity to protective antibodies elicited by viral infection, including antibodies targeting a quaternary epitope and the broadly conserved catalytic site. Stabilized NAs can also be integrated into viruses without affecting fitness. Our findings provide a deeper understanding of NA structure, stability, and antigenicity, and establish design strategies for reinforcing the conformational integrity of recombinant NA proteins.

Librator: a platform for the optimized analysis, design, and expression of mutable influenza viral antigens

Artificial mutagenesis and protein engineering have laid the foundation for antigenic characterization and universal vaccine design for influenza viruses. However, many methods used in this process require manual sequence editing and protein expression, limiting their efficiency and utility in high-throughput applications. More streamlined in silico tools allowing researchers to properly analyze and visualize influenza viral protein sequences with accurate nomenclature are necessary to improve antigen design and productivity. To address this need, we developed Librator, a system for analyzing and designing custom protein sequences of influenza virus hemagglutinin (HA) and neuraminidase (NA) glycoproteins. Within Librator's graphical interface, users can easily interrogate viral sequences and phylogenies, visualize antigen structures and conservation, mutate target residues and design custom antigens. Librator also provides optimized fragment design for Gibson Assembly of HA and NA expression constructs based on peptide conservation of all historical HA and NA sequences, ensuring fragments are reusable and compatible across related subtypes, thereby promoting reagent savings. Finally, the program facilitates single-cell immune profiling, epitope mapping of monoclonal antibodies and mosaic protein design. Using Librator-based antigen construction, we demonstrate that antigenicity can be readily transferred between HA molecules of H3, but not H1, lineage viruses. Altogether, Librator is a valuable tool for analyzing influenza virus HA and NA proteins and provides an efficient resource for optimizing recombinant influenza antigen synthesis.

Human Antibodies for Viral Infections

Antibodies have been used to prevent or treat viral infections since the nineteenth century, but the full potential to use passive immunization for infectious diseases has yet to be realized. The advent of efficient methods for isolating broad and potently neutralizing human monoclonal antibodies is enabling us to develop antibodies with unprecedented activities. The discovery of IgG Fc region modifications that extend antibody half-life in humans to three months or more suggests that antibodies could become the principal tool with which we manage future viral epidemics. Antibodies for members of most virus families that cause severe disease in humans have been isolated, and many of them are in clinical development, an area that has accelerated during the effort to prevent or treat COVID-19 (coronavirus disease 2019). Broad and potently neutralizing antibodies are also important research reagents for identification of protective epitopes that can be engineered into active vaccines through structure-based reverse vaccinology. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Novel Recombinant Influenza Virus Neuraminidase Vaccine Candidate Stabilized by a Measles Virus Phosphoprotein Tetramerization Domain Provides Robust Protection from Virus Challenge in the Mouse Model

Current seasonal influenza virus vaccines do not induce robust immune responses to neuraminidase. Several factors, including immunodominance of hemagglutinin over neuraminidase, instability of neuraminidase in vaccine formulations, and variable, nonstandardized amounts of neuraminidase in the vaccines, may contribute to this effect. However, vaccines that induce strong antineuraminidase immune responses would be beneficial, as they are highly protective. Furthermore, antigenic drift is slower for neuraminidase than for hemagglutinin, potentially providing broader coverage. Here, we designed stabilized recombinant versions of neuraminidase by replacing the N-terminal cytoplasmic domain, transmembrane, and extracellular stalk with tetramerization domains from the measles or Sendai virus phosphoprotein or from an Arabidopsis thaliana transcription factor. The measles virus tetramerization domain-based construct, termed N1-MPP, was chosen for further evaluation, as it retained antigenicity, neuraminidase activity, and structural integrity and provided robust protection in vivo against lethal virus challenge in the mouse model. We tested N1-MPP as a standalone vaccine, admixed with seasonal influenza virus vaccines, or given with seasonal influenza virus vaccines but in the other leg of the mouse. Admixture with different formulations of seasonal vaccines led to a weak neuraminidase response, suggesting a dominant effect of hemagglutinin over neuraminidase when administered in the same formulation. However, administration of neuraminidase alone or with seasonal vaccine administered in the alternate leg of the mouse induced robust antibody responses. Thus, this recombinant neuraminidase construct is a promising vaccine antigen that may enhance and broaden protection against seasonal influenza viruses. IMPORTANCE Influenza virus infections remain a high risk to human health, causing up to 650,000 deaths worldwide every year, with an enormous burden on the health care system. Since currently available seasonal vaccines are only partially effective and often mismatched to the circulating strains, a broader protective influenza virus vaccine is needed. Here, we generated a recombinant influenza virus vaccine candidate based on the more conserved neuraminidase surface glycoprotein in order to induce a robust and broader protective immune response against a variety of circulating influenza virus strains.

Antigenic evolution of human influenza H3N2 neuraminidase is constrained by charge balancing

As one of the main influenza antigens, neuraminidase (NA) in H3N2 virus has evolved extensively for more than 50 years due to continuous immune pressure. While NA has recently emerged as an effective vaccine target, biophysical constraints on the antigenic evolution of NA remain largely elusive. Here, we apply combinatorial mutagenesis and next-generation sequencing to characterize the local fitness landscape in an antigenic region of NA in six different human H3N2 strains that were isolated around 10 years apart. The local fitness landscape correlates well among strains and the pairwise epistasis is highly conserved. Our analysis further demonstrates that local net charge governs the pairwise epistasis in this antigenic region. In addition, we show that residue coevolution in this antigenic region is correlated with the pairwise epistasis between charge states. Overall, this study demonstrates the importance of quantifying epistasis and the underlying biophysical constraint for building a model of influenza evolution.

Influenza Neuraminidase Characteristics and Potential as a Vaccine Target

Neuraminidase of influenza A and B viruses plays a critical role in the virus life cycle and is an important target of the host immune system. Here, we highlight the current understanding of influenza neuraminidase structure, function, antigenicity, immunogenicity, and immune protective potential. Neuraminidase inhibiting antibodies have been recognized as correlates of protection against disease caused by natural or experimental influenza A virus infection in humans. In the past years, we have witnessed an increasing interest in the use of influenza neuraminidase to improve the protective potential of currently used influenza vaccines. A number of well-characterized influenza neuraminidase-specific monoclonal antibodies have been described recently, most of which can protect in experimental challenge models by inhibiting the neuraminidase activity or by Fc receptor-dependent mechanisms. The relative instability of the neuraminidase poses a challenge for protein-based antigen design. We critically review the different solutions that have been proposed to solve this problem, ranging from the inclusion of stabilizing heterologous tetramerizing zippers to the introduction of inter-protomer stabilizing mutations. Computationally engineered neuraminidase antigens have been generated that offer broad, within subtype protection in animal challenge models. We also provide an overview of modern vaccine technology platforms that are compatible with the induction of robust neuraminidase-specific immune responses. In the near future, we will likely see the implementation of influenza vaccines that confront the influenza virus with a double punch: targeting both the hemagglutinin and the neuraminidase.

Evolution and Antigenic Advancement of N2 Neuraminidase of Swine Influenza A Viruses Circulating in the United States following Two Separate Introductions from Human Seasonal Viruses

Two separate introductions of human seasonal N2 neuraminidase genes were sustained in U.S. swine since 1998 (N2-98) and 2002 (N2-02). Herein, we characterized the antigenic evolution of the N2 of swine influenza A virus (IAV) across 2 decades following each introduction. The N2-98 and N2-02 expanded in genetic diversity, with two statistically supported monophyletic clades within each lineage. To assess antigenic drift in swine N2 following the human-to-swine spillover events, we generated a panel of swine N2 antisera against representative N2 and quantified the antigenic distance between wild-type viruses using enzyme-linked lectin assay and antigenic cartography. The antigenic distance between swine and human N2 was smallest between human N2 circulating at the time of each introduction and the archetypal swine N2. However, sustained circulation and evolution in swine of the two N2 lineages resulted in significant antigenic drift, and the N2-98 and N2-02 swine N2 lineages were antigenically distinct. Although intralineage antigenic diversity was observed, the magnitude of antigenic drift did not consistently correlate with the observed genetic differences. These data represent the first quantification of the antigenic diversity of neuraminidase of IAV in swine and demonstrated significant antigenic drift from contemporary human seasonal strains as well as antigenic variation among N2 detected in swine. These data suggest that antigenic mismatch may occur between circulating swine IAV and vaccine strains. Consequently, consideration of the diversity of N2 in swine IAV for vaccine selection may likely result in more effective control and aid public health initiatives for pandemic preparedness. IMPORTANCE Antibodies inhibiting the neuraminidase (NA) of IAV reduce clinical disease, virus shedding, and transmission, particularly in the absence of neutralizing immunity against hemagglutinin. To understand antibody recognition of the genetically diverse NA in U.S. swine IAV, we characterized the antigenic diversity of N2 from swine and humans. N2 detected in swine IAV were derived from two distinct human-to-swine spillovers that persisted, are antigenically distinct, and underwent antigenic drift. These findings highlight the need for continued surveillance and vaccine development in swine with increased focus on the NA. Additionally, human seasonal N2 isolated after 2005 were poorly inhibited by representative swine N2 antisera, suggesting a lack of cross-reactive NA antibody-mediated immunity between contemporary swine and human N2. Bidirectional transmission between humans and swine represents a One Health challenge, and determining the correlates of immunity to emerging IAV strains is critical to mitigating zoonotic and reverse-zoonotic transmission.

Optimisation of Neuraminidase Expression for Use in Drug Discovery by Using HEK293-6E Cells

Influenza virus is a highly contagious virus that causes significant human mortality and morbidity annually. The most effective drugs for treating influenza are the neuraminidase inhibitors, but resistance to these inhibitors has emerged, and additional drug discovery research on neuraminidase and other targets is needed. Traditional methods of neuraminidase production from embryonated eggs are cumbersome, while insect cell derived protein is less reflective of neuraminidase produced during human infection. Herein we describe a method for producing neuraminidase from a human cell line, HEK293-6E, and demonstrate the method by producing the neuraminidase from the 1918 H1N1 pandemic influenza strain. This method produced high levels of soluble neuraminidase expression (>3000 EU/mL), was enhanced by including a secretion signal from a viral chemokine binding protein, and does not require co-expression of additional proteins. The neuraminidase produced was of sufficient quantity and purity to support high resolution crystal structure determination. The structure solved using this protein conformed to the previously reported structure. Notably the glycosylation at three asparagine residues was superior in quality to that from insect cell derived neuraminidase. This method of production of neuraminidase should prove useful in further studies, such as the characterisation of inhibitor binding.

Improved epitope resolution of the prefusion trimer-specific antibody AM14 bound to the RSV F glycoprotein

Respiratory syncytial virus (RSV) is the most common cause of acute lower respiratory tract infections resulting in medical intervention and hospitalizations during infancy and early childhood, and vaccination against RSV remains a public health priority. The RSV F glycoprotein is a major target of neutralizing antibodies, and the prefusion stabilized form of F (DS-Cav1) is under investigation as a vaccine antigen. AM14 is a human monoclonal antibody with the exclusive capacity of binding an epitope on prefusion F (PreF), which spans two F protomers. The quality of recognizing a trimer-specific epitope makes AM14 valuable for probing PreF-based immunogen conformation and functionality during vaccine production. Currently, only a low-resolution (5.5 Å) X-ray structure is available of the PreF-AM14 complex, revealing few reliable details of the interface. Here, we perform complementary structural studies using X-ray crystallography and cryo-electron microscopy (cryo-EM) to provide improved resolution structures at 3.6 Å and 3.4 Å resolutions, respectively. Both X-ray and cryo-EM structures provide clear side-chain densities, which allow for accurate mapping of the AM14 epitope on DS-Cav1. The structures help rationalize the molecular basis for AM14 loss of binding to RSV F monoclonal antibody-resistant mutants and reveal flexibility for the side chain of a key antigenic residue on PreF. This work provides the basis for a comprehensive understanding of RSV F trimer specificity with implications in vaccine design and quality assessment of PreF-based immunogens.

Influenza vaccine: progress in a vaccine that elicits a broad immune response

INTRODUCTION

The licensed seasonal influenza vaccines predominantly induce neutralizing antibodies against immunodominant hypervariable epitopes of viral surface proteins, with limited protection against antigenically distant influenza viruses. Strategies have been developed to improve vaccines' performance in terms of broadly reactive and long-lasting immune response induction.

AREAS COVERED

We have summarized the advancements in the development of cross-protective influenza vaccines and discussed the challenges in evaluating them in preclinical and clinical trials. Here, the literature regarding the current stage of development of universal influenza vaccine candidates was reviewed.

EXPERT OPINION

Although various strategies aim to redirect adaptive immune responses from variable immunodominant to immunosubdominant antigens, more conserved epitopes are being investigated. Approaches that improve antibody responses to conserved B cell epitopes have increased the protective efficacy of vaccines within a subtype or phylogenetic group of influenza viruses. Vaccines that elicit significant levels of T cells recognizing highly conserved viral epitopes possess a high cross-protective potential and may cover most circulating influenza viruses. However, the development of T cell-based universal influenza vaccines is challenging owing to the diversity of MHCs in the population, unpredictable degree of immunodominance, lack of adequate animal models, and difficulty in establishing T cell immunity in humans.

ABBREVIATIONS

cHA: chimeric HA; HBc: hepatitis B virus core protein; HA: hemagglutinin; HLA: human leucocyte antigen; IIV: inactivated influenza vaccine; KLH: keyhole limpet hemocyanin; LAH: long alpha helix; LAIV: live attenuated influenza vaccine; M2e: extracellular domain of matrix 2 protein; MHC: major histocompatibility complex; mRNA: messenger ribonucleic acid; NA: neuraminidase; NS1: non-structural protein 1; qNIV: quadrivalent nanoparticle influenza vaccine; T_RM: tissue-resident memory T cells; VE: vaccine effectiveness; VLP: virus-like particles; VSV: vesicular stomatitis virus.

Identification of key residues involved in the neuraminidase antigenic variation of H9N2 influenza virus

Influenza A H9N2 virus causes economic loss to the poultry industry and has likely contributed to the genesis of H5N1 and H7N9 viruses. The neuraminidase (NA) of H9N2 virus, like haemagglutinin, is under antibody selective pressure and may undergo antigenic change; however, its antigenic structure remains to be elucidated. In this study, we used monoclonal antibodies (mAbs) to probe the H9N2 viral NA residues that are key for antibody binding/inhibition. These mAbs fell into three groups based on their binding/inhibition of the NA of H9N2 viruses isolated during 1999–2019: group I only bounded the NA of the early 2000 H9N2 viruses but possessed no neutralizing ability, group II bounded and inhibited the NA of H9N2 viruses isolated before 2012, and group III reacted with most or all tested H9N2 viruses. We showed that NA residue 356 is key for the recognition by group I mAbs, residues 344, 368, 369, and 400 are key for the binding/inhibition of NA by group II antibodies, whereas residues 248, 253, and the 125/296 combination are key for neutralizing antibodies in group III. Our findings highlighted NA antigenic change of the circulating H9N2 viruses, and provided data for a more complete picture of the antigenic structure of H9N2 viral NA.

Universal Influenza Virus Neuraminidase Vaccine Elicits Protective Immune Responses against Human Seasonal and Pre-pandemic Strains

The hemagglutinin (HA) surface protein is the primary immune target for most influenza vaccines. The neuraminidase (NA) surface protein is often a secondary target for vaccine designs. In this study, computationally optimized broadly reactive antigen (COBRA) methodology was used to generate the N1-I NA vaccine antigen that was designed to cross-react with avian, swine, and human influenza viruses of the N1 NA subtype. The elicited antibodies bound to NA proteins derived from A/California/07/2009 (H1N1)pdm09, A/Brisbane/59/2007 (H1N1), A/Swine/North Carolina/154074/2015 (H1N1), and A/Viet Nam/1203/2004 (H5N1) influenza viruses, with NA-neutralizing activity against a broad panel of HXN1 influenza strains. Mice vaccinated with the N1-I COBRA NA vaccine were protected from mortality and viral lung titers were lower when challenged with four different viral challenges (A/California/07/2009, A/Brisbane/59/2007, A/Swine/North Carolina/154074/2015, and A/Viet Nam/1203/2004). Vaccinated mice had little to no weight loss against both homologous, but also cross-NA, genetic clade challenges. Lung viral titers were lower than the mock-vaccinated mice and, at times, equivalent to the homologous control. Thus, the N1-I COBRA NA antigen has the potential to be a complementary component in a multiantigen universal influenza virus vaccine formulation that also contains HA antigens. IMPORTANCE The development and distribution of a universal influenza vaccine would alleviate global economic and public health stress from annual influenza virus outbreaks. The influenza virus NA vaccine antigen allows for protection from multiple HA subtypes and virus host origins, but it has not been the focus of vaccine development. The N1-I NA antigen described here protected mice from direct challenge of four distinct influenza viruses and inhibited the enzymatic activity of an N1 influenza virus panel. The use of the NA antigen in combination with the HA antigen widens the breadth of protection against various virus strains. Therefore, this research opens the door to the development of a longer-lasting vaccine with increased protective breadth.

The Human Antibody Response to the Influenza Virus Neuraminidase Following Infection or Vaccination

The influenza virus neuraminidase (NA) is primarily involved in the release of progeny viruses from infected cells—a critical role for virus replication. Compared to the immuno-dominant hemagglutinin, there are fewer NA subtypes, and NA experiences a slower rate of antigenic drift and reduced immune selection pressure. Furthermore, NA inhibiting antibodies prevent viral egress, thus preventing viral spread. Anti-NA immunity can lessen disease severity, reduce viral shedding, and decrease viral lung titers in humans and various animal models. As a result, there has been a concerted effort to investigate the possibilities of incorporating immunogenic forms of NA as a vaccine antigen in future vaccine formulations. In this review, we discuss NA-based immunity and describe several human NA-specific monoclonal antibodies (mAbs) that have a broad range of protection. We also review vaccine platforms that are investigating NA antigens in pre-clinical models and their potential use for next-generation influenza virus vaccines. The evidence presented here supports the inclusion of immunogenic NA in future influenza virus vaccines.

Analysis of the Evolution of Pandemic Influenza A(H1N1) Virus Neuraminidase Reveals Entanglement of Different Phenotypic Characteristics

The influenza A virus (IAV) neuraminidase (NA) is essential for virion release from cells and decoy receptors and an important target of antiviral drugs and antibodies. Adaptation to a new host sialome and escape from the host immune system are forces driving the selection of mutations in the NA gene. Phylogenetic analysis shows that until 2015, 16 amino acid substitutions in NA became fixed in the virus population after introduction in the human population of the pandemic IAV H1N1 (H1N1pdm09) in 2009. The accumulative effect of these substitutions, in the order in which they appeared, was analyzed using recombinant proteins and viruses in combination with different functional assays. The results indicate that NA activity did not evolve to a single optimum but rather fluctuated within a certain bandwidth. Furthermore, antigenic and enzymatic properties of NA were intertwined, with several residues affecting multiple properties. For example, the substitution K432E in the second sialic acid binding site, next to the catalytic site, was shown to affect catalytic activity, substrate specificity, and the pH optimum for maximum activity. This substitution also altered antigenicity of NA, which may explain its selection. We propose that the entanglement of NA phenotypes may be an important determining factor in the evolution of NA.IMPORTANCE Since its emergence in 2009, the pandemic H1N1 influenza A virus (IAV) has caused significant disease and mortality in humans. IAVs contain two envelope glycoproteins, the receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA). NA is essential for virion release from cells and decoy receptors, is an important target of antiviral drugs, and is increasingly being recognized as an important vaccine antigen. Not much is known, however, about the evolution of this protein upon the emergence of the novel pandemic H1N1 virus, with respect to its enzymatic activity and antigenicity. By reconstructing the evolutionary path of NA, we show that antigenic and enzymatic properties of NA are intertwined, with several residues affecting multiple properties. Understanding the entanglement of NA phenotypes will lead to better comprehension of IAV evolution and may help the development of NA-based vaccines.

Recent Advances in the Chemical Biology of N-Glycans

Asparagine-linked N-glycans on proteins have diverse structures, and their functions vary according to their structures. In recent years, it has become possible to obtain high quantities of N-glycans via isolation and chemical/enzymatic/chemoenzymatic synthesis. This has allowed for progress in the elucidation of N-glycan functions at the molecular level. Interaction analyses with lectins by glycan arrays or nuclear magnetic resonance (NMR) using various N-glycans have revealed the molecular basis for the recognition of complex structures of N-glycans. Preparation of proteins modified with homogeneous N-glycans revealed the influence of N-glycan modifications on protein functions. Furthermore, N-glycans have potential applications in drug development. This review discusses recent advances in the chemical biology of N-glycans.

Identification and Characterization of Novel Antibody Epitopes on the N2 Neuraminidase

The influenza virus neuraminidase (NA) is becoming a focus for novel vaccine designs. However, the epitopes of human anti-NA antibodies have been poorly defined. Using a panel of 10 anti-N2 monoclonal antibodies (MAbs) that bind the H3N2 virus A/Switzerland/9715293/2013, we generated five escape mutant viruses. These viruses contained mutations K199E/T, E258K, A272D, and S331N. We found that mutations at K199 and E258 had the largest impact on MAb binding, NA inhibition and neutralization activity. In addition, a natural isolate from the 2017-2018 season was found to contain the E258K mutation and was resistant to numerous antibodies tested. The mutation S331N, was identified in virus passaged in the presence of antibody; however, it had little impact on MAb activity and greatly decreased viral fitness. This information aids in identifying novel human MAb epitopes on the N2 and helps with the detection of antigenically drifted NAs.

IMPORTANCE The influenza virus neuraminidase is an emerging target for universal influenza virus vaccines. However, in contrast to influenza virus hemagglutinin, we know little about antibody epitopes and antigenic sites on the neuraminidase. Characterizing and defining these sites is aiding vaccine development and helping to understand antigenic drift of NA.

Structure-Based Modification of an Anti-neuraminidase Human Antibody Restores Protection Efficacy against the Drifted Influenza Virus

The immune system produces antibodies to protect the human body from harmful invaders. The monoclonal antibody (MAb) is one kind of effective antivirals. In this study, we isolated an antibody (Z2B3) from an H7N9 influenza virus-infected child. It shows cross-reactivity to both group 1 (N1) and group 2 (N9) neuraminidases (NAs) but is sensitive to N1 NA with a K432E substitution. Structural analysis of the NA-antibody fragment antigen-binding (Fab) complex provides a clue for antibody modification, and the modified antibody restored binding and inhibition to recently drifted N1 NA and regained protection against the variant influenza strain. This finding suggests that antibodies to NA may be a useful therapy and can be in principle edited to defeat drifted influenza virus.

Here, we investigate a monoclonal antibody, Z2B3, isolated from an H7N9-infected patient, that exhibited cross-reactivity to both N9 (group 2) and a broad range of seasonal and avian N1 (group 1) proteins but lost activity to the N1 with the substitution K432E. This substitution exists in 99.25% of seasonal influenza strains after 2013. The NA-Z2B3 complex structures indicated that Z2B3 binds within the conserved active site of the neuraminidase (NA) protein. A salt bridge between D102 in Z2B3 and K432 in NA plays an important role in binding. Structure-based modification of Z2B3 with D102R in heavy chain reversed the salt bridge and restored the binding and inhibition of N1 with E432. Furthermore, Z2B3-D102R can protect mice from A/Serbia/NS-601/2014 H1N1 virus (NA contains E432) infection while the wild-type Z2B3 antibody shows no protection. This study demonstrates that a broadly reactive and protective antibody to NA can be in principle edited to restore binding and inhibition to recently drifted N1 NA and regain protection against the variant influenza strain.

Human Antibodies Targeting Influenza B Virus Neuraminidase Active Site Are Broadly Protective

Influenza B virus (IBV) infections can cause severe disease in children and the elderly. Commonly used antivirals have lower clinical effectiveness against IBV compared to influenza A viruses (IAV). Neuraminidase (NA), the second major surface protein on the influenza virus, is emerging as a target of broadly protective antibodies that recognize the NA active site of IAVs. However, similarly broadly protective antibodies against IBV NA have not been identified. Here, we isolated and characterized human monoclonal antibodies (mAbs) that target IBV NA from an IBV-infected patient. Two mAbs displayed broad and potent capacity to inhibit IBV NA enzymatic activity, neutralize the virus in vitro, and protect against lethal IBV infection in mice in prophylactic and therapeutic settings. These mAbs inserted long CDR-H3 loops into the NA active site, engaging residues highly conserved among IBV NAs. These mAbs provide a blueprint for the development of improved vaccines and therapeutics against IBVs.

Drivers of recombinant soluble influenza A virus hemagglutinin and neuraminidase expression in mammalian cells

Recombinant soluble trimeric influenza A virus hemagglutinins (HA) and tetrameric neuraminidases (NAs) have proven to be excellent tools to decipher biological properties. Receptor binding and sialic acid cleavage by recombinant proteins correlate satisfactorily compared to whole viruses. Expression of HA and NA can be achieved in a plethora of different laboratory hosts. For immunological and receptor interaction studies however, insect and mammalian cell expressed proteins are preferred due to the presence of N-linked glycosylation and disulfide bond formation. Because mammalian-cell expression is widely applied, an increased expression yield is an important goal. Here we report that using codon-optimized genes and sfGFP fusions, the expression yield of HA can be significantly improved. sfGFP also significantly increased expression yields when fused to the N-terminus of NA. In this study, a suite of different hemagglutinin and neuraminidase constructs are described, which can be valuable tools to study a wide array of different HAs, NAs and their mutants.

Imprinting, immunodominance, and other impediments to generating broad influenza immunity

Natural influenza virus infections and seasonal vaccinations often do not confer broadly neutralizing immunity across diverse influenza strains. In addition, the virus is capable of rapid antigenic drift in order to evade pre-existing immunity. The surface glycoproteins, hemagglutinin, and neuraminidase can easily mutate their immunodominant epitopes without impacting fitness. Skewing human antibody repertoires to target more conserved epitopes is thus an expanding area of research: Many groups are attempting to produce universal influenza vaccines that can protect across a wide variety of strains. Achieving this goal will require a detailed understanding of how infection history impacts humoral responses. It will also require the ability to manipulate or enhance B cell selection in order to expand clones that can recognize subdominant but protective epitopes. In this review, we will discuss what immune imprinting means to immunologists and describe efforts to overcome or silence imprinting in order to improve vaccination efficiency.

The data universe of structural biology

The Protein Data Bank (PDB) has grown from a small data resource for crystallographers to a worldwide resource serving structural biology. The history of the growth of the PDB and the role that the community has played in developing standards and policies are described. This article also illustrates how other biophysics communities are collaborating with the worldwide PDB to create a network of interoperating data resources. This network will expand the capabilities of structural biology and enable the determination and archiving of increasingly complex structures.

Innovations in structure-based antigen design and immune monitoring for next generation vaccines

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Immunofocusing as a strategy for structure-based vaccine design.

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Structure-based vaccine design for RSV F, influenza HA, HIV-1 Env.

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Structure-based vaccine design translated into the clinic.

The recent explosion of atomic-level structures of glycoproteins that comprise the surface antigens of human enveloped viruses, such as RSV, influenza, and HIV, provide tremendous opportunities for rational, structure-based vaccine design. Several concepts in structure-based vaccine design have been put into practice and are are well along preclinical and clinical implementation. Testing of these designed immunogens will provide key insights into the ability to induce the desired immune responses, namely neutralizing antibodies. Many of these immunogens in human clinical trials represent only the first wave of designs and will likely require continued tweaking and elaboration to achieve the ultimate goal of enhanced breadth and potency. Considerable effort is now being invested in germline targeting, epitope focusing, and improved immune presentation such as multivalent nanoparticle incorporation. This review highlights some of the recent advances in these areas as we prepare for the next generation of immunogens for subsequent rounds of iterative vaccine development.

Host immune response-inspired development of the influenza vaccine

Objective

To assess the current and future development of influenza vaccines.

Data Sources

PubMed searches were performed cross-referencing the keywords influenza, influenza vaccine, host immune response, correlates of protection, vaccine development, vaccine efficacy. Articles were reviewed for additional citations.

Study Selections

Articles were reviewed and selected on the basis of relevance to subject matter.

Results

In this review, we first introduce the influenza virus, its nomenclature, and the concepts of antigenic drift and shift. Second, we discuss the status of currently licensed influenza virus vaccines. We briefly focus on influenza vaccine responses beyond hemagglutination inhibition that may correlate with protection against influenza viruses of different subtypes. Third, we explain how studying host responses to influenza infection and vaccination with advanced serologic methods, B-cell receptor sequencing, and transcriptomic profiling can guide the development of improved influenza virus vaccines. Fourth, we provide 2 suggestions on how current influenza vaccines can be optimized by redirecting immune responses toward conserved viral antigens and the use of adjuvants.

Conclusion

Influenza vaccine design can benefit from novel insights obtained from the study of host responses to influenza virus infection and vaccination. Integration of the large amount of available clinical and preclinical data requires systems approaches that can elucidate novel correlates of protection and will guide further development of influenza vaccine.

Host-Virus Interaction: How Host Cells Defend against Influenza A Virus Infection

Influenza A viruses (IAVs) are highly contagious pathogens infecting human and numerous animals. The viruses cause millions of infection cases and thousands of deaths every year, thus making IAVs a continual threat to global health. Upon IAV infection, host innate immune system is triggered and activated to restrict virus replication and clear pathogens. Subsequently, host adaptive immunity is involved in specific virus clearance. On the other hand, to achieve a successful infection, IAVs also apply multiple strategies to avoid be detected and eliminated by the host immunity. In the current review, we present a general description on recent work regarding different host cells and molecules facilitating antiviral defenses against IAV infection and how IAVs antagonize host immune responses.

Emerging from the Shadow of Hemagglutinin: Neuraminidase Is an Important Target for Influenza Vaccination

Gilchuk et al. (2019) and Zhu et al. (2019) provide detailed structural information about epitopes on influenza virus N9 neuraminidase and characterize breadth, potency, mechanism of action, and in vivo protective effects of human antibodies targeting these epitopes. The findings highlight the importance of neuraminidase as a target for improved influenza virus vaccines.

Influenza H7N9 Virus Neuraminidase-Specific Human Monoclonal Antibodies Inhibit Viral Egress and Protect from Lethal Influenza Infection in Mice

H7N9 avian influenza virus causes severe infections and might have the potential to trigger a major pandemic. Molecular determinants of human humoral immune response to N9 neuraminidase (NA) proteins, which exhibit unusual features compared with seasonal influenza virus NA proteins, are ill-defined. We isolated 35 human monoclonal antibodies (mAbs) from two H7N9 survivors and two vaccinees. These mAbs react to NA in a subtype-specific manner and recognize diverse antigenic sites on the surface of N9 NA, including epitopes overlapping with, or distinct from, the enzyme active site. Despite recognizing multiple antigenic sites, the mAbs use a common mechanism of action by blocking egress of nascent virions from infected cells, thereby providing an antiviral prophylactic and therapeutic protection in vivo in mice. Studies of breadth, potency, and diversity of antigenic recognition from four subjects suggest that vaccination with inactivated adjuvanted vaccine induce NA-reactive responses comparable to that of H7N9 natural infection.