Stability of neuraminidase in inactivated influenza vaccines

Antibody-independent surface plasmon resonance assays for influenza vaccine quality control

Surface plasmon resonance (SPR)-based biosensors have emerged as a powerful platform for bioprocess monitoring due to their ability to detect biointeractions in real time, without the need for labeling. Paramount for the development of a robust detection platform is the immobilization of a ligand with high specificity and affinity for the in-solution species of interest. Following the 2009 H1N1 pandemic, much effort has been made toward the development of quality control platforms for influenza A vaccine productions, many of which have employed SPR for detection. Due to the rapid antigenic drift of influenza's principal surface protein, hemagglutinin, antibodies used for immunoassays need to be produced seasonally. The production of these antibodies represents a 6-8-week delay in immunoassay and, thus, vaccine availability. This review focuses on SPR-based assays that do not rely on anti-HA antibodies for the detection, characterization, and quantification of influenza A in bioproductions and biological samples. KEY POINTS: • The single radial immunodiffusion assay (SRID) has been the gold standard for the quantification of influenza vaccines since 1979. Due to antigenic drift of influenza's hemagglutinin protein, new antibody reagents for the SRID assay must be produced each year, requiring 6-8 weeks. The resulting delay in immunoassay availability is a major bottleneck in the influenza vaccine pipeline. This review highlights ligand options for the detection and quantification of influenza viruses using surface plasmon resonance biosensors.

Evaluation of Vaccine Immunogenicity-Correlates to Real-World Protection: Influenza

Recent events highlighted that, despite decades of studying vaccine immunogenicity and efforts toward finding correlates of protection, evaluating real-world vaccine efficacy as well as establishing meaningful licensing criteria still represents a significant challenge. In this paper, we review all aspects of influenza vaccine immunogenicity, including animal and human challenge studies, humoral and cellular immunity parameters, and their potential correlation with real-life protection from disease.

Multivalent next generation influenza virus vaccines protect against seasonal and pre-pandemic viruses

Each year, new influenza virus vaccine formulations are generated to keep up with continuously circulating and mutating viral variants. A next-generation influenza virus vaccine would provide long-lasting, broadly-reactive immune protection against current and future influenza virus strains for both seasonal and pre-pandemic viruses. Next generation immunogens were designed using computationally optimized broadly reactive antigen (COBRA) methodology to protect against a broad range of strains over numerous seasons. Novel HA and NA amino acid sequences were derived from multilayered consensus sequence alignment for multiple subtypes of influenza. This multivalent formulation was hypothesized to elicit broadly protective immune responses against both seasonal and pre-pandemic influenza viruses. Mice were vaccinated with multivalent mixtures of HA and NA (H1, H2, H3, H5, H7, N1, N2) proteins. Multivalent COBRA vaccinations elicited antibodies that recognized a broad panel of strains and vaccinated mice were protected against viruses representing multiple subtypes. This is a promising candidate for a universal influenza vaccine that elicits protective immune responses against seasonal and pre-pandemic strains over multiple seasons.

Influenza B virus neuraminidase: a potential target for next-generation vaccines?

INTRODUCTION

Influenza B viruses (IBV) cause a significant health and economic burden annually. Due to lower antigenic drift rate, less extensive antigenic diversity, and lack of animal reservoirs, the development of highly effective universal vaccines against IBV might be in reach. Current seasonal influenza vaccines are formulated to induce antibodies against the Hemagglutinin (HA) protein, but their effectiveness is reduced by mismatch between vaccine and circulating strains.

AREAS COVERED

Given antibodies against the Neuraminidase (NA) have been associated with protection during influenza infection, there is considerable interest in the development of NA-based influenza vaccines. This review summarizes insights into the role of NA-based immunity against IBV and highlights knowledge gaps that should be addressed to inform the design of next-generation influenza B vaccines. We discuss how antibodies recognize broadly cross-reactive epitopes on the NA and the lack of understanding of IBV NA antigenic evolution which would benefit vaccine development in the future.

EXPERT OPINION

Demonstrating NA antibodies as correlates of protection for IBV in humans would be paramount. Determining the extent of IBV NA antigenic evolution will be informative. Finally, it will be critical to determine optimal strategies for incorporating the appropriate NA antigens in existing clinically approved vaccine formulations.

Longitudinal Analysis of Neuraminidase and Hemagglutinin Antibodies to Influenza A Viruses after Immunization with Seasonal Inactivated Influenza Vaccines

Neuraminidase (NA)-based immunity could reduce the harmful impact of novel antigenic variants of influenza viruses. The detection of neuraminidase-inhibiting (NI) antibodies in parallel with anti-hemagglutinin (HA) antibodies may enhance research on the immunogenicity and duration of antibody responses to influenza vaccines. To assess anti-NA antibodies after vaccination with seasonal inactivated influenza vaccines, we used the enzyme-linked lectin assay, and anti-HA antibodies were detected in the hemagglutination inhibition assay. The dynamics of the anti-NA antibody response differed depending on the virus subtype: antibodies to A/H3N2 virus neuraminidase increased later than antibodies to A/H1N1pdm09 subtype neuraminidase and persisted longer. In contrast to HA antibodies, the fold increase in antibody titers to NA after vaccination poorly depended on the preexisting level. At the same time, NA antibody levels after vaccination directly correlated with titers before vaccination. A difference was found in response to NA antigen between split and subunit-adjuvanted vaccines and in NA functional activity in the vaccine formulations.

Antibody-mediated NK cell activation as a correlate of immunity against influenza infection

Antibodies play a critical role in protection against influenza; yet titers and viral neutralization represent incomplete correlates of immunity. Instead, the ability of antibodies to leverage the antiviral power of the innate immune system has been implicated in protection from and clearance of influenza infection. Here, post-hoc analysis of the humoral immune response to influenza is comprehensively profiled in a cohort of vaccinated older adults (65 + ) monitored for influenza infection during the 2012/2013 season in the United States (NCT: 01427309). While robust humoral immune responses arose against the vaccine and circulating strains, influenza-specific antibody effector profiles differed in individuals that later became infected with influenza, who are deficient in NK cell activating antibodies to both hemagglutinin and neuraminidase, compared to individuals who remained uninfected. Furthermore, NK cell activation was strongly associated with the NK cell senescence marker CD57, arguing for the need for selective induction of influenza-specific afucosylated NK activating antibodies in older adults to achieve protection. High dose vaccination, currently used for older adults, was insufficient to generate this NK cell-activating humoral response. Next generation vaccines able to selectively bolster NK cell activating antibodies may be required to achieve protection in the setting of progressively senescent NK cells.

Anti-neuraminidase and anti-hemagglutinin immune serum can confer inter-lineage cross protection against recent influenza B

Influenza B viruses (IBV) are responsible for a considerable part of the burden caused by influenza virus infections. Since their emergence in the 1980s, the Yamagata and Victoria antigenic lineages of influenza B circulate in alternate patterns across the globe. Furthermore, their evolutionary divergence and the appearance of new IBV subclades complicates the prediction of future influenza vaccines compositions. It has been proposed that the addition of the neuraminidase (NA) antigen could potentially induce a broader protection and compensate for hemagglutinin (HA) mismatches in the current vaccines. Here we show that anti-NA and -HA sera against both Victoria and Yamagata lineages have limited inter-lineage cross-reactivity. When transferred to mice prior to infection with a panel of IBVs, anti-NA sera were as potent as anti-HA sera in conferring protection against homologous challenge and, in some cases, conferred superior protection against challenge with heterologous IBV strains.

Immunological Correlates of Prevention of the Onset of Seasonal H3N2 Influenza

On influenza virus infection or vaccination, immune responses occur, including the production of antibodies with various functions that contribute to protection from seasonal influenza virus infection. In the current study, we attempted to identify the antibody functions that play a central role in preventing the onset of seasonal influenza by comparing the levels of several antibody titers for different antibody functions between 5 subclinically infected individuals and 16 patients infected with seasonal H3N2 virus. For antibody titers before influenza virus exposure, we found that the nAb titers and enzyme-linked immunosorbent assay titers against hemagglutinin and neuraminidase (NA) proteins in the subclinically infected individuals were significantly higher than those in the patients, whereas the NA inhibition titers and antibody-dependent cellular cytotoxicity activities did not significantly differ between subclinically infected individuals and infected patients. These results suggest that nAb and enzyme-linked immunosorbent assay titers against hemagglutinin and NA serve as correlates of symptomatic influenza infection.

Antigenic Characterization of Neuraminidase of Influenza A/H7N9 Viruses Isolated in Different Years

Influenza outbreaks caused by A/H7N9 viruses have occurred since 2013. After 2016, A/H7N9 influenza viruses underwent evolutionary changes. In this study, we examined the antigenic properties of influenza neuraminidase (NA) of A/H7N9 viruses as part of a live influenza vaccine (LAIV). It was shown that neuraminidase inhibiting (NI) antibodies obtained after A/Anhui/1/2013(H7N9)-based LAIV vaccination did not inhibit A/Hong Kong/125/2017(H7N9) NA and vice versa. The A/Hong Kong/125/2017(H7N9)-based LAIV elicited higher levels of NI antibodies compared to the A/Anhui/1/2013(H7N9)-based LAIV after two doses. Thelow degree of coincidence of the antibody response to hemagglutinin (HA) and NA after LAIV vaccination allows us to consider an enzyme-linked lectin assay (ELLA) as an additional measure for assessing the immunogenicity of influenza vaccines. In mice, N9-reactive monoclonal antibodies (mABs) for the A/environment/Shanghai/RL01/2013(H7N9) influenza virus partially protected against lung infection from the A/Guangdong/17SF003/2016 IDCDC-RG56N(H7N9) virus, thus showing the cross-protective properties of monoclonal antibodies against the drift variant.

Recent Progress in Recombinant Influenza Vaccine Development Toward Heterosubtypic Immune Response

Flu, a viral infection caused by the influenza virus, is still a global public health concern with potential to cause seasonal epidemics and pandemics. Vaccination is considered the most effective protective strategy against the infection. However, given the high plasticity of the virus and the suboptimal immunogenicity of existing influenza vaccines, scientists are moving toward the development of universal vaccines. An important property of universal vaccines is their ability to induce heterosubtypic immunity, i.e., a wide immune response coverage toward different influenza subtypes. With the increasing number of studies and mounting evidence on the safety and efficacy of recombinant influenza vaccines (RIVs), they have been proposed as promising platforms for the development of universal vaccines. This review highlights the current progress and advances in the development of RIVs in the context of heterosubtypic immunity induction toward universal vaccine production. In particular, this review discussed existing knowledge on influenza and vaccine development, current hemagglutinin-based RIVs in the market and in the pipeline, other potential vaccine targets for RIVs (neuraminidase, matrix 1 and 2, nucleoprotein, polymerase acidic, and basic 1 and 2 antigens), and deantigenization process. This review also provided discussion points and future perspectives in looking at RIVs as potential universal vaccine candidates for influenza.

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.

Contribution of neuraminidase to the efficacy of seasonal split influenza vaccines in the ferret model

Seasonal influenza vaccination takes into account primarily hemagglutinin (HA)-specific neutralizing antibody responses. However, the accumulation of substitutions in the antigenic regions of HA (i.e., antigenic drift) occasionally results in a mismatch between the vaccine and circulating strains. To prevent poor vaccine performance, we investigated whether an antigenically matched neuraminidase (NA) may compensate for reduced vaccine efficacy due to a mismatched HA. Ferrets were vaccinated twice with adjuvanted split inactivated influenza vaccines containing homologous HA and NA (vacH3N2), only homologous HA (vacH3N1), only homologous NA (vacH1N2), heterologous HA and NA (vacH1N1), or phosphate-buffered saline (vacPBS), followed by challenge with H3N2 virus (A/Netherlands/16190/1968). Ferrets vaccinated with homologous HA (vacH3N2 and vacH3N1) displayed minimum fever and weight loss compared to vacH1N1 and vacPBS ferrets, while ferrets vaccinated with NA-matched vacH1N2 displayed intermediate fever and weight loss. Vaccination with vacH1N2 further led to a reduction in virus shedding from the nose and undetectable virus titers in the lower respiratory tract, similarly to when the homologous vacH3N2 was used. Some protection was observed upon vacH1N1 vaccination, but this was not comparable to that observed for vacH1N2, again highlighting the important role of NA in vaccine-induced protection. These results illustrate that NA antibodies can prevent severe disease caused by influenza virus infection and that an antigenically matched NA in seasonal vaccines might prevent lower respiratory tract complications. This underlines the importance of considering NA during the yearly vaccine strain selection process, which may be particularly beneficial in seasons when the HA component of the vaccine is mismatched.

IMPORTANCE Despite the availability of vaccines, influenza virus infections continue to cause substantial morbidity and mortality in humans. Currently available influenza vaccines take primarily the hemagglutinin (HA) into account, but the highly variable nature of this protein as a result of antigenic drift has led to a recurrent decline in vaccine effectiveness. While the protective effect of neuraminidase (NA) antibodies has been highlighted by several studies, there are no requirements with regard to quantity or quality of NA in licensed vaccines, and NA immunity remains largely unexploited. Since antigenic changes in HA and NA are thought to occur asynchronously, NA immunity could compensate for reduced vaccine efficacy when drift in HA occurs. By matching and mismatching the HA and NA components of monovalent split inactivated vaccines, we demonstrated the potential of NA immunity to protect against disease, virus replication in the lower respiratory tract, and virus shedding in the ferret model.

Pre-existing antibodies directed against a tetramerizing domain enhance the immune response against artificially stabilized soluble tetrameric influenza neuraminidase

The neuraminidase (NA) is an abundant antigen at the surface of influenza virions. Recent studies have highlighted the immune-protective potential of NA against influenza and defined anti-NA antibodies as an independent correlate of protection. Even though NA head domain changes at a slightly slower pace than hemagglutinin (HA), NA is still subject to antigenic drift, and therefore an NA-based influenza vaccine antigen may have to be updated regularly and thus repeatedly administered. NA is a tetrameric type II membrane protein, which readily dissociates into dimers and monomers when expressed in a soluble form. By using a tetramerizing zipper, such as the tetrabrachion (TB) from Staphylothermus marinus, it is possible to stabilize soluble NA in its active tetrameric conformation, an imperative for the optimal induction of protective NA inhibitory antibodies. The impact of repetitive immunizations with TB-stabilized antigens on the immunogenicity of soluble TB-stabilized NA is unknown. We demonstrate that TB is immunogenic in mice. Interestingly, preexisting anti-TB antibodies enhance the anti-NA antibody response induced by immunization with TB-stabilized NA. This immune-enhancing effect was transferable by serum and operated independently of activating Fcγ receptors. We also demonstrate that priming with TB-stabilized NA antigens, enhances the NA inhibitory antibody responses against a heterosubtypic TB-stabilized NA. These findings have implications for the clinical development of oligomeric vaccine antigens that are stabilized by a heterologous oligomerizing domain.

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.

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.

Targeting Antigens for Universal Influenza Vaccine Development

Traditional influenza vaccines generate strain-specific antibodies which cannot provide protection against divergent influenza virus strains. Further, due to frequent antigenic shifts and drift of influenza viruses, annual reformulation and revaccination are required in order to match circulating strains. Thus, the development of a universal influenza vaccine (UIV) is critical for long-term protection against all seasonal influenza virus strains, as well as to provide protection against a potential pandemic virus. One of the most important strategies in the development of UIVs is the selection of optimal targeting antigens to generate broadly cross-reactive neutralizing antibodies or cross-reactive T cell responses against divergent influenza virus strains. However, each type of target antigen for UIVs has advantages and limitations for the generation of sufficient immune responses against divergent influenza viruses. Herein, we review current strategies and perspectives regarding the use of antigens, including hemagglutinin, neuraminidase, matrix proteins, and internal proteins, for universal influenza vaccine development.

In silico design of a multi-epitope peptide construct as a potential vaccine candidate for Influenza A based on neuraminidase protein

Designing an effective vaccine against different subtypes of Influenza A virus is a critical issue in the field of medical biotechnology. At the current study, a novel potential multi-epitope vaccine candidate based on the neuraminidase proteins for seven subtypes of Influenza virus was designed, using the in silico approach. Potential linear B-cell and T-cell binding epitopes from each neuraminidase protein (N1, N2, N3, N4, N6, N7, N8) were predicted by in silico tools of epitope prediction. The selected epitopes were joined by three different linkers, and physicochemical properties, toxicity, and allergenecity were investigated. The final multi-epitope construct was modeled using GalaxyWEB server, and the molecular interactions with immune receptors were investigated and the immune response simulation assay was performed. A multi-epitope construct with GPGPGPG linker with the lowest allergenicity and highest stability was selected. The molecular docking assay indicated the interactions with immune system receptors, including HLA1, HLA2, and TLR-3. Immune response simulation detected both humoral and cellular response, including the elevated count of B-cells, T-cell, and Nk-cells.

The effect of calcium and magnesium on activity, immunogenicity, and efficacy of a recombinant N1/N2 neuraminidase vaccine

Despite the importance of immunity against neuraminidase (NA), NA content and immunogenicity are neglected in current influenza vaccines. To address this, a recombinant N1/N2 NA vaccine (NAV) was developed. Stability assays were used to determine optimal temperature and buffer conditions for vaccine storage. The effect of divalent cation-related enhancement of NA stability and activity on N1 and N2 immunogenicity and efficacy against viral challenge was assessed. Differences in activity between N1 and N2 and cation-related activity enhancement did not translate into differences in immunogenicity or efficacy. NAV-vaccinated mice showed robust antibody titers against N1 and N2, and after challenge with influenza A (H1N1) virus, decreased viral titers and decreased antiviral and inflammatory responses by transcriptomic analysis. These findings provide guidance for optimal storage and assessment of NA-based vaccines and confirm the importance of NA in influenza vaccination strategies in attenuating viral replication and limiting inflammatory responses necessary to clear infection.

Influenza Neuraminidase: A Neglected Protein and Its Potential for a Better Influenza Vaccine

Neuraminidase (NA) is an influenza surface protein that helps to free viruses from mucin-associated decoy receptors and to facilitate budding from infected cells. Experiments have demonstrated that anti-NA antibodies protect animals against lethal influenza challenge by numerous strains, while decreasing pulmonary viral titers, symptoms, and lung lesions. Studies in humans during the influenza A/H3N2 pandemic and in healthy volunteers challenged with influenza A/H1N1 showed that anti-NA immunity reduced symptoms, nasopharyngeal viral shedding, and infection rates. Despite the benefits of anti-NA immunity, current vaccines focus on immunity against hemagglutinin and are not standardized to NA content leading to limited and variable NA immunogenicity. Purified NA has been shown to be safe and immunogenic in humans. Supplementing current vaccines with NA may be a simple strategy to improve suboptimal effectiveness. Immunity against NA is likely to be an important component of future universal influenza vaccines.

Neuraminidase antigenic drift of H3N2 clade 3c.2a viruses alters virus replication, enzymatic activity and inhibitory antibody binding

In the 2014-2015 influenza season a novel neuraminidase (NA) genotype was detected in global human influenza A surveillance. This novel genotype encoded an N-linked glycosylation site at position 245-247 in the NA protein from clade 3c.2a H3N2 viruses. In the years following the 2014-2015 season, this novel NA glycosylation genotype quickly dominated the human H3N2 population of viruses. To assess the effect this novel N-linked glycan has on virus fitness and antibody binding, recombinant viruses with (NA Gly+) or without (NA Gly-) the 245 NA glycan were created. Viruses with the 245 NA Gly+ genotype grew to a significantly lower infectious virus titer on primary, differentiated human nasal epithelial cells (hNEC) compared to viruses with the 245 NA Gly- genotype, but growth was similar on immortalized cells. The 245 NA Gly+ blocked human and rabbit monoclonal antibodies that target the enzymatic site from binding to their epitope. Additionally, viruses with the 245 NA Gly+ genotype had significantly lower enzymatic activity compared to viruses with the 245 NA Gly- genotype. Human monoclonal antibodies that target residues near the 245 NA glycan were less effective at inhibiting NA enzymatic activity and virus replication of viruses encoding an NA Gly+ protein compared to ones encoding NA Gly- protein. Additionally, a recombinant H6N2 virus with the 245 NA Gly+ protein was more resistant to enzymatic inhibition from convalescent serum from H3N2-infected humans compared to viruses with the 245 NA Gly- genotype. Finally, the 245 NA Gly+ protected from NA antibody mediated virus neutralization. These results suggest that while the 245 NA Gly+ decreases virus replication in hNECs and decreases enzymatic activity, the 245 NA glycan blocks the binding of monoclonal and human serum NA specific antibodies that would otherwise inhibit enzymatic activity and virus replication.

Is a Universal Influenza Virus Vaccine Possible?

Influenza viruses remain a severe burden to human health because of their contribution to overall morbidity and mortality. Current seasonal influenza virus vaccines do not provide sufficient protection to alleviate the annual impact of influenza and cannot confer protection against potentially pandemic influenza viruses. The lack of protection is due to rapid changes of the viral epitopes targeted by the vaccine and the often suboptimal immunogenicity of current immunization strategies. Major efforts to improve vaccination approaches are under way. The development of a universal influenza virus vaccine may be possible by combining the lessons learned from redirecting the immune response toward conserved viral epitopes, as well as the use of adjuvants and novel vaccination platforms.

Extending the Stalk Enhances Immunogenicity of the Influenza Virus Neuraminidase

Influenza viruses express two surface glycoproteins, the hemagglutinin (HA) and the neuraminidase (NA). Anti-NA antibodies protect from lethal influenza virus challenge in the mouse model and correlate inversely with virus shedding and symptoms in humans. Consequently, the NA is a promising target for influenza virus vaccine design. Current seasonal vaccines, however, poorly induce anti-NA antibodies, partly because of the immunodominance of the HA over the NA when the two glycoproteins are closely associated. To address this issue, here we investigated whether extending the stalk domain of the NA could render it more immunogenic on virus particles. Two recombinant influenza viruses based on the H1N1 strain A/Puerto Rico/8/1934 (PR8) were rescued with NA stalk domains extended by 15 or 30 amino acids. Formalin-inactivated viruses expressing wild-type NA or the stalk-extended NA variants were used to vaccinate mice. The virus with the 30-amino-acid stalk extension induced significantly higher anti-NA IgG responses (characterized by increased in vitro antibody-dependent cellular cytotoxicity [ADCC] activity) than the wild-type PR8 virus, while anti-HA IgG levels were unaffected. Similarly, extending the stalk domain of the NA of a recent H3N2 virus enhanced the induction of anti-NA IgGs in mice. On the basis of these results, we hypothesize that the subdominance of the NA can be modulated if the protein is modified such that its height surpasses that of the HA on the viral membrane. Extending the stalk domain of NA may help to enhance its immunogenicity in influenza virus vaccines without compromising antibody responses to HA.IMPORTANCE The efficacy of influenza virus vaccines could be improved by enhancing the immunogenicity of the NA protein. One of the reasons for its poor immunogenicity is the immunodominance of the HA over the NA in many seasonal influenza virus vaccines. Here we demonstrate that, in the mouse model, extending the stalk domain of the NA protein can enhance its immunogenicity on virus particles and overcome the immunodominance of the HA without affecting antibody responses to the HA. The antibody repertoire is broadened by the extended NA and includes additional ADCC-active antibodies. Our findings may assist in the efforts toward more effective influenza virus vaccines.

Structural restrictions for influenza neuraminidase activity promote adaptation and diversification

Influenza neuraminidase (NA) is a sialidase that contributes to viral mobility by removing the extracellular receptors for the haemagglutinin (HA) glycoprotein. However, it remains unclear why influenza NAs evolved to function as Ca²⁺-dependent tetramers that display variable stability. Here, we show that the Ca²⁺ ion located at the centre of the NA tetramer is a major stability determinant, as this Ca²⁺ ion is required for catalysis and its binding affinity varies between NAs. By examining NAs from 2009 pandemic-like H1N1 viruses, we traced the affinity variation to local substitutions that cause residues in the central Ca²⁺-binding pocket to reposition. A temporal analysis revealed that these local substitutions predictably alter the stability of the 2009 pandemic-like NAs and contribute to the tendency for the stability to vary up and down over time. In addition to the changes in stability, the structural plasticity of NA was also shown to support the formation of heterotetramers, which creates a mechanism for NA to obtain hybrid properties and propagate suboptimal mutants. Together, these results demonstrate how the structural restrictions for activity provide influenza NA with several mechanisms for adaptation and diversification.

A neuraminidase potency assay for quantitative assessment of neuraminidase in influenza vaccines

Neuraminidase (NA) immunity leads to decreased viral shedding and reduced severity of influenza disease; however, NA content in influenza vaccines is currently not regulated, resulting in inconsistent quality and quantity of NA that can vary from manufacturer to manufacturer, from year to year, and from lot to lot. To address this problem, we have developed an assay for NA quantification that could be used by the industry to move toward developing influenza vaccines that induce a predictable immune response to NA. The VaxArray Influenza Seasonal NA Potency Assay (VXI-sNA) is a multiplexed sandwich immunoassay that relies on six subtype-specific monoclonal antibodies printed in microarray format and a suite of fluor-conjugated “label” antibodies. The performance of the assay as applied to a wide range of influenza vaccines is described herein. The assay demonstrated high NA subtype specificity and high sensitivity, with quantification limits ranging from 1 to 60 ng/mL and linear dynamic ranges of 24–500-fold. When compared to an enzymatic activity assay for samples exposed to thermal degradation conditions, the assay was able to track changes in protein stability over time and exhibited good correlation with enzyme activity. The assay also demonstrated excellent analytical precision with relative error ranging from 6 to 12% over day-to-day, user-to-user, and lot-to-lot variation. The high sensitivity and reproducibility of the assay enabled robust detection and quantification of NA in crude in-process samples and low-dose, adjuvanted vaccines with an accuracy of 100 ± 10%.

Influenza vaccines that contain neuraminidase (NA) are associated with lower disease severity and better prognosis in vaccinated individuals, but the amount and quality of NA present in vaccines remains difficult to determine. Here, Rose Byrne-Nash and colleagues present the VaxArray Influenza Seasonal NA Potency Assay (VXI-sNA), a multiplexed sandwich immunoassay for the quantification of NA of all subtypes and for the determination of its potency. Featuring multiple NA subtype-specific antibodies printed in microarray format, the VXI-sNA showed high precision, dynamic range and reproducibility, and its results correlated well with NA enzymatic activity. This method is a step forward towards standardization of NA quantification for the assessment of stability, batch-to-batch variation and immunogenicity of NA in influenza vaccine formulations, and may help to develop influenza vaccines that trigger predictable immune responses to NA for increased protection against influenza infections.

Neuraminidase as an influenza vaccine antigen: a low hanging fruit, ready for picking to improve vaccine effectiveness

Neuraminidase (NA) plays an essential role in influenza virus replication, facilitating multicycle infection predominantly by releasing virions from infected cells. NA-inhibiting antibodies provide resistance to disease and NA-specific antibodies contribute to vaccine efficacy. The primary reason NA vaccine content and immunogenicity was not routinely measured in the past, was the lack of suitable assays to quantify NA and NA-specific antibodies. These are now available and with recent appreciation of its contribution to immunity, NA content of seasonal and pandemic vaccines is being considered. An added benefit of NA as a vaccine antigen is that many NA-specific antibodies bind to domains that are well conserved within a subtype, protecting against heterologous viruses. This suggests NA may be a good choice for inclusion in universal influenza vaccines.

Influenza Infection in Humans Induces Broadly Cross-Reactive and Protective Neuraminidase-Reactive Antibodies

Antibodies to the hemagglutinin (HA) and neuraminidase (NA) glycoproteins are the major mediators of protection against influenza virus infection. Here, we report that current influenza vaccines poorly display key NA epitopes and rarely induce NA-reactive B cells. Conversely, influenza virus infection induces NA-reactive B cells at a frequency that approaches (H1N1) or exceeds (H3N2) that of HA-reactive B cells. NA-reactive antibodies display broad binding activity spanning the entire history of influenza A virus circulation in humans, including the original pandemic strains of both H1N1 and H3N2 subtypes. The antibodies robustly inhibit the enzymatic activity of NA, including oseltamivir-resistant variants, and provide robust prophylactic protection, including against avian H5N1 viruses, in vivo. When used therapeutically, NA-reactive antibodies protected mice from lethal influenza virus challenge even 48 hr post infection. These findings strongly suggest that influenza vaccines should be optimized to improve targeting of NA for durable and broad protection against divergent influenza strains.

NAction! How Can Neuraminidase-Based Immunity Contribute to Better Influenza Virus Vaccines?

Neuraminidase is one of the two surface glycoproteins of influenza A and B viruses. It has enzymatic activity that cleaves terminal sialic acid from glycans, and that activity is essential at several points in the virus life cycle. While neuraminidase is a major target for influenza antivirals, it is largely ignored in vaccine development. Current inactivated influenza virus vaccines might contain neuraminidase, but the antigen quantity and quality are varied and not standardized. While there are data that show a protective role of anti-neuraminidase immunity, many questions remain unanswered. These questions, among others, concern the targeted epitopes or antigenic sites, the potential for antigenic drift, and, connected to that, the breadth of protection, differences in induction of immune responses by vaccination versus infection, mechanisms of protection, the role of mucosal antineuraminidase antibodies, stability, and the immunogenicity of neuraminidase in vaccine formulations. Reagents for analysis of neuraminidase-based immunity are scarce, and assays are not widely used for clinical studies evaluating vaccines. However, efforts to better understand neuraminidase-based immunity have been made recently. A neuraminidase focus group, NAction!, was formed at a Centers of Excellence for Influenza Research and Surveillance meeting at the National Institutes of Health in Bethesda, MD, to promote research that helps to understand neuraminidase-based immunity and how it can contribute to the design of better and broadly protective influenza virus vaccines. Here, we review open questions and knowledge gaps that have been identified by this group and discuss how the gaps can be addressed, with the ultimate goal of designing better influenza virus vaccines.

Vaccination with Recombinant Parainfluenza Virus 5 Expressing Neuraminidase Protects against Homologous and Heterologous Influenza Virus Challenge

Seasonal human influenza virus continues to cause morbidity and mortality annually, and highly pathogenic avian influenza (HPAI) viruses along with other emerging influenza viruses continue to pose pandemic threats. Vaccination is considered the most effective measure for controlling influenza; however, current strategies rely on a precise vaccine match with currently circulating virus strains for efficacy, requiring constant surveillance and regular development of matched vaccines. Current vaccines focus on eliciting specific antibody responses against the hemagglutinin (HA) surface glycoprotein; however, the diversity of HAs across species and antigenic drift of circulating strains enable the evasion of virus-inhibiting antibody responses, resulting in vaccine failure. The neuraminidase (NA) surface glycoprotein, while diverse, has a conserved enzymatic site and presents an appealing target for priming broadly effective antibody responses. Here we show that vaccination with parainfluenza virus 5 (PIV5), a promising live viral vector expressing NA from avian (H5N1) or pandemic (H1N1) influenza virus, elicited NA-specific antibody and T cell responses, which conferred protection against homologous and heterologous influenza virus challenges. Vaccination with PIV5-N1 NA provided cross-protection against challenge with a heterosubtypic (H3N2) virus. Experiments using antibody transfer indicate that antibodies to NA have an important role in protection. These findings indicate that PIV5 expressing NA may be effective as a broadly protective vaccine against seasonal influenza and emerging pandemic threats.IMPORTANCE Seasonal influenza viruses cause considerable morbidity and mortality annually, while emerging viruses pose potential pandemic threats. Currently licensed influenza virus vaccines rely on the antigenic match of hemagglutinin (HA) for vaccine strain selection, and most vaccines rely on HA inhibition titers to determine efficacy, despite the growing awareness of the contribution of neuraminidase (NA) to influenza virus vaccine efficacy. Although NA is immunologically subdominant to HA, and clinical studies have shown variable NA responses to vaccination, in this study, we show that vaccination with a parainfluenza virus 5 recombinant vaccine candidate expressing NA (PIV5-NA) from a pandemic influenza (pdmH1N1) virus or highly pathogenic avian influenza (H5N1) virus elicits robust, cross-reactive protection from influenza virus infection in two animal models. New vaccination strategies incorporating NA, including PIV5-NA, could improve seasonal influenza virus vaccine efficacy and provide protection against emerging influenza viruses.

Kinetics, Longevity, and Cross-Reactivity of Antineuraminidase Antibody after Natural Infection with Influenza A Viruses

The kinetics, longevity, and breadth of antibodies to influenza virus neuraminidase (NA) in archival, sequential serum/plasma samples from influenza A virus (IAV) H5N1 infection survivors and from patients infected with the 2009 pandemic IAV (H1N1) virus were determined using an enzyme-linked lectin-based assay. The reverse-genetics-derived H4N1 viruses harboring a hemagglutinin (HA) segment from A/duck/Shan Tou/461/2000 (H4N9) and an NA segment derived from either IAV H5N1 clade 1, IAV H5N1 clade 2.3.4, the 2009 pandemic IAV (H1N1) (H1N1pdm), or A/Puerto Rico/8/1934 (H1N1) virus were used as the test antigens. These serum/plasma samples were also investigated by microneutralization (MN) and/or hemagglutination inhibition (HI) assays. Neuraminidase-inhibiting (NI) antibodies against N1 NA of both homologous and heterologous viruses were observed in H5N1 survivors and H1N1pdm patients. H5N1 survivors who were never exposed to H1N1pdm virus developed NI antibodies to H1N1pdm NA. Seroconversion of NI antibodies was observed in 65% of the H1N1pdm patients at day 7 after disease onset, but an increase in titer was not observed in serum samples obtained late in infection. On the other hand, an increase in seroconversion rate with the HI assay was observed in the follow-up series of sera obtained on days 7, 14, 28, and 90 after infection. The study also showed that NI antibodies are broadly reactive, while MN and HI antibodies are more strain specific.

Assessment of influenza A neuraminidase (subtype N1) potency by ELISA

Antibodies that inhibit neuraminidase (NA) activity of influenza virus provide resistance against disease and have been associated with milder epidemics. Although studies have demonstrated a correlation between NA inhibition antibody titers and vaccine efficacy, neither the quantity nor form of NA is measured in seasonal and pandemic influenza vaccines. In this report, we describe development of enzyme-linked immunosorbent assays (ELISAs) that are suitable for quantitation of the native form of NA of subtype N1. The assays use mouse monoclonal antibodies (mAbs) 1H5 and CD6 to capture NAs of viruses, and a different mAb 4E9 to detect bound antigen. The 1H5-capture ELISA detects NAs of seasonal and pandemic H1N1 viruses as well as H5N1 viruses and has a limit of quantitation (LOQ) of 5.5ng/mL for seasonal H1N1A/Brisbane/59/2007 NA. The CD6-capture ELISA is specific for NA of the 2009 pandemic viruses with a LOQ of 67ng/mL for A/California/07/2009 NA. The ELISA signals in both assays are proportional to NA enzymatic activity and correlate with NA immunogenicity. The ELISAs we describe may expedite the development of NA-based influenza vaccines by providing a practical assay to measure NA potency.

Antibody Responses to Trivalent Inactivated Influenza Vaccine in Health Care Personnel Previously Vaccinated and Vaccinated for The First Time

Inactivated influenza vaccination induces a hemagglutinin-specific antibody response to the strain used for immunization. Annual vaccination is strongly recommended for health care personnel. However, it is debatable if repeated vaccination would affect the antibody response to inactivated influenza vaccine through the time. We enrolled health care personnel who had repeated and first trivalent inactivated influenza vaccination in 2005–2008. Serological antibody responses were measured by hemagglutination-inhibition (HI) test. Subjects with repeated vaccination had higher pre-vaccination and lower post-vaccination HI titer than those with first vaccination, although serological responses between groups might vary with different antigen types and while the drifted strain was introduced in the vaccine. Higher fold rise in the HI titer was observed in the group with first than repeated vaccination and the fold increase in the HI titer was inversely correlated with pre-vaccination titer in 2007 and 2008. Nevertheless, no significant difference in the day 28 seroprotection rate was observed between groups with repeated and first vaccination in most circumstances. Further studies are needed to understand the long-term effect of repeated vaccination on the antibody response both at the serological and repertoire levels among health care personnel.

Influenza virus neuraminidase (NA): a target for antivirals and vaccines

Influenza, the most common infectious disease, poses a great threat to human health because of its highly contagious nature and fast transmissibility, often leading to high morbidity and mortality. Effective vaccination strategies may aid in the prevention and control of recurring epidemics and pandemics associated with this infectious disease. However, antigenic shifts and drifts are major concerns with influenza virus, requiring effective global monitoring and updating of vaccines. Current vaccines are standardized primarily based on the amount of hemagglutinin, a major surface antigen, which chiefly constitutes these preparations along with the varying amounts of neuraminidase (NA). Anti-influenza drugs targeting the active site of NA have been in use for more than a decade now. However, NA has not been approved as an effective antigenic component of the influenza vaccine because of standardization issues. Although some studies have suggested that NA antibodies are able to reduce the severity of the disease and induce a long-term and cross-protective immunity, a few major scientific issues need to be addressed prior to launching NA-based vaccines. Interestingly, an increasing number of studies have shown NA to be a promising target for future influenza vaccines. This review is an attempt to consolidate studies that reflect the strength of NA as a suitable vaccine target. The studies discussed in this article highlight NA as a potential influenza vaccine candidate and support taking the process of developing NA vaccines to the next stage.

Emerging influenza viruses and the prospect of a universal influenza virus vaccine

Influenza viruses cause annual seasonal epidemics and pandemics at irregular intervals. Several cases of human infections with avian and swine influenza viruses have been detected recently, warranting enhanced surveillance and the development of more effective countermeasures to address the pandemic potential of these viruses. The most effective countermeasure against influenza virus infection is the use of prophylactic vaccines. However, vaccines that are currently in use for seasonal influenza viruses have to be re-formulated and re-administered in a cumbersome process every year due to the antigenic drift of the virus. Furthermore, current seasonal vaccines are ineffective against novel pandemic strains. This paper reviews zoonotic influenza viruses with pandemic potential and technological advances towards better vaccines that induce broad and long lasting protection from influenza virus infection. Recent efforts have focused on the development of broadly protective/universal influenza virus vaccines that can provide immunity against drifted seasonal influenza virus strains but also against potential pandemic viruses.

Advances in the development of influenza virus vaccines

Influenza virus infections are a major public health concern and cause significant morbidity and mortality worldwide. Current influenza virus vaccines are an effective countermeasure against infection but need to be reformulated almost every year owing to antigenic drift. Furthermore, these vaccines do not protect against novel pandemic strains, and the timely production of pandemic vaccines remains problematic because of the limitations of current technology. Several improvements have been made recently to enhance immune protection induced by seasonal and pandemic vaccines, and to speed up production in case of a pandemic. Importantly, vaccine constructs that induce broad or even universal influenza virus protection are currently in preclinical and clinical development.

Antibody to Influenza Virus Neuraminidase: An Independent Correlate of Protection

BACKGROUND

Laboratory correlates of influenza vaccine protection can best be identified by examining people who are infected despite vaccination. While the importance of antibody to viral hemagglutinin (HA) has long been recognized, the level of protection contributed independently by antibody to viral neuraminidase (NA) has not been determined.

METHODS

Sera from a controlled trial of the efficacies of inactivated influenza vaccine (IIV) and live attenuated influenza vaccine (LAIV) were tested by hemagglutination inhibition (HAI) assay, microneutralization (MN) assay, and a newly standardized lectin-based neuraminidase inhibition (NAI) assay.

RESULTS

The NAI assay detected a vaccine response in 37% of IIV recipients, compared with 77% and 67% of participants in whom responses were detected by the HAI and MN assays, respectively. For LAIV recipients, the NAI, HAI, and MN assays detected responses in 6%, 21%, and 17%, respectively. In IIV recipients, as NAI assay titers rose, the frequency of infection fell, similar to patterns seen with HAI and MN assays. HAI and MN assay titers were highly correlated, but NAI assay titers exhibited less of a correlation. Analyses suggested an independent role for NAI antibody in protection, which was similar in the IIV, LAIV, and placebo groups.

CONCLUSIONS

While NAI antibody is not produced to a large extent in response to current IIV, it appears to have an independent role in protection. As new influenza vaccines are developed, NA content should be considered.

CLINICAL TRIALS REGISTRATION

NCT00538512.

Biological and protective properties of immune sera directed to the influenza virus neuraminidase

The envelope of influenza A viruses contains two large antigens, hemagglutinin (HA) and neuraminidase (NA). Conventional influenza virus vaccines induce neutralizing antibodies that are predominantly directed to the HA globular head, a domain that is subject to extensive antigenic drift. Antibodies directed to NA are induced at much lower levels, probably as a consequence of the immunodominance of the HA antigen. Although antibodies to NA may affect virus release by inhibiting the sialidase function of the glycoprotein, the antigen has been largely neglected in past vaccine design. In this study, we characterized the protective properties of monospecific immune sera that were generated by vaccination with recombinant RNA replicon particles encoding NA. These immune sera inhibited hemagglutination in an NA subtype-specific and HA subtype-independent manner and interfered with infection of MDCK cells. In addition, they inhibited the sialidase activities of various influenza viruses of the same and even different NA subtypes. With this, the anti-NA immune sera inhibited the spread of H5N1 highly pathogenic avian influenza virus and HA/NA-pseudotyped viruses in MDCK cells in a concentration-dependent manner. When chickens were immunized with NA recombinant replicon particles and subsequently infected with low-pathogenic avian influenza virus, inflammatory serum markers were significantly reduced and virus shedding was limited or eliminated. These findings suggest that NA antibodies can inhibit virus dissemination by interfering with both virus attachment and egress. Our results underline the potential of high-quality NA antibodies for controlling influenza virus replication and place emphasis on NA as a vaccine antigen.

IMPORTANCE

The neuraminidase of influenza A viruses is a sialidase that acts as a receptor-destroying enzyme facilitating the release of progeny virus from infected cells. Here, we demonstrate that monospecific anti-NA immune sera inhibited not only sialidase activity, but also influenza virus hemagglutination and infection of MDCK cells, suggesting that NA antibodies can interfere with virus attachment. Inhibition of both processes, virus release and virus binding, may explain why NA antibodies efficiently blocked virus dissemination in vitro and in vivo. Anti-NA immune sera showed broader reactivity than anti-HA sera in hemagglutination inhibition tests and demonstrated cross-subtype activity in sialidase inhibition tests. These remarkable features of NA antibodies highlight the importance of the NA antigen for the development of next-generation influenza virus vaccines.

Overview of Serological Techniques for Influenza Vaccine Evaluation: Past, Present and Future

Serological techniques commonly used to quantify influenza-specific antibodies include the Haemagglutination Inhibition (HI), Single Radial Haemolysis (SRH) and Virus Neutralization (VN) assays. HI and SRH are established and reproducible techniques, whereas VN is more demanding. Every new influenza vaccine needs to fulfil the strict criteria issued by the European Medicines Agency (EMA) in order to be licensed. These criteria currently apply exclusively to SRH and HI assays and refer to two different target groups-healthy adults and the elderly, but other vaccine recipient age groups have not been considered (i.e., children). The purpose of this timely review is to highlight the current scenario on correlates of protection concerning influenza vaccines and underline the need to revise the criteria and assays currently in use. In addition to SRH and HI assays, the technical advantages provided by other techniques such as the VN assay, pseudotype-based neutralization assay, neuraminidase and cell-mediated immunity assays need to be considered and regulated via EMA criteria, considering the many significant advantages that they could offer for the development of effective vaccines.

Virus-vectored influenza virus vaccines

Despite the availability of an inactivated vaccine that has been licensed for >50 years, the influenza virus continues to cause morbidity and mortality worldwide. Constant evolution of circulating influenza virus strains and the emergence of new strains diminishes the effectiveness of annual vaccines that rely on a match with circulating influenza strains. Thus, there is a continued need for new, efficacious vaccines conferring cross-clade protection to avoid the need for biannual reformulation of seasonal influenza vaccines. Recombinant virus-vectored vaccines are an appealing alternative to classical inactivated vaccines because virus vectors enable native expression of influenza antigens, even from virulent influenza viruses, while expressed in the context of the vector that can improve immunogenicity. In addition, a vectored vaccine often enables delivery of the vaccine to sites of inductive immunity such as the respiratory tract enabling protection from influenza virus infection. Moreover, the ability to readily manipulate virus vectors to produce novel influenza vaccines may provide the quickest path toward a universal vaccine protecting against all influenza viruses. This review will discuss experimental virus-vectored vaccines for use in humans, comparing them to licensed vaccines and the hurdles faced for licensure of these next-generation influenza virus vaccines.

In the shadow of hemagglutinin: a growing interest in influenza viral neuraminidase and its role as a vaccine antigen

Despite the availability of vaccine prophylaxis and antiviral therapeutics, the influenza virus continues to have a significant, annual impact on the morbidity and mortality of human beings, highlighting the continued need for research in the field. Current vaccine strategies predominantly focus on raising a humoral response against hemagglutinin (HA)—the more abundant, immunodominant glycoprotein on the surface of the influenza virus. In fact, anti-HA antibodies are often neutralizing, and are used routinely to assess vaccine immunogenicity. Neuraminidase (NA), the other major glycoprotein on the surface of the influenza virus, has historically served as the target for antiviral drug therapy and is much less studied in the context of humoral immunity. Yet, the quest to discern the exact importance of NA-based protection is decades old. Also, while antibodies against the NA glycoprotein fail to prevent infection of the influenza virus, anti-NA immunity has been shown to lessen the severity of disease, decrease viral lung titers in animal models, and reduce viral shedding. Growing evidence is intimating the possible gains of including the NA antigen in vaccine design, such as expanded strain coverage and increased overall immunogenicity of the vaccine. After giving a tour of general influenza virology, this review aims to discuss the influenza A virus neuraminidase while focusing on both the historical and present literature on the use of NA as a possible vaccine antigen.