NS-based live attenuated H1N1 pandemic vaccines protect mice and ferrets

Development of Live Vaccine Candidates for Canine Influenza H3N2 Using Naturally Truncated NS1 Gene

The NS1 influenza protein of influenza A virus is a viral nonstructural protein encoded by the NS gene segment that has multiple accessory functions during viral infection. In recent years, the major role ascribed to NS1 has been its inhibition of host immune responses, especially the limitation of both interferon (IFN) production and the antiviral effects of IFN-induced protein. We isolated an equine influenza virus with a naturally truncated NS1 gene in our previous study. In this current research, we inserted this partially truncated NS gene into the H3N2 canine influenza virus using reverse genetics to develop a live attenuated vaccine strain. To evaluate whether the developed strain is suitable as a live vaccine candidate, we compared its replication kinetics with wild-type virus in MDCK cells and specific pathogen-free eggs. Additionally, we investigated host antiviral gene expression, viral replication in the respiratory system, and associated lung tissue damage in mice experiments. To confirm the efficacy of the vaccine candidate, we evaluated the immunogenicity and protectivity of the developed vaccine strain against canine influenza H3N2, compared with a commercial inactivated vaccine. Through these experiments, it was confirmed that the naturally truncated NS1 inserted virus has sufficient potential as a live vaccine candidate, and we hopefully expect that this study would make a great contribution to the development of a live vaccine for canine influenza H3N2.

Optimizing the Live Attenuated Influenza A Vaccine Backbone for High-Risk Patient Groups

Together with inactivated influenza vaccines (IIV), live attenuated influenza vaccines (LAIV) are an important tool to prevent influenza A virus (IAV) illnesses in patients. LAIVs present the advantages to have a needle-free administration and to trigger a mucosal immune response. LAIV is approved for healthy 2- to 49-year old individuals. However, due to its replicative nature and higher rate of adverse events at-risk populations are excluded from the benefits of this vaccine. Using targeted mutagenesis, we modified the nonstructural protein 1 of the currently licensed LAIV in order to impair its ability to bind the host cellular protein CPSF30 and thus its ability to inhibit host mRNA poly-adenylation. We characterized our optimized LAIV (optiLAIV) in three different mouse models mimicking healthy and high-risk patients. Using a neonatal mouse model, we show faster clearance of our optimized vaccine compared to the licensed LAIV. Despite lower replication, optiLAIV equally protected mice against homosubtypic and hetesubtypic influenza strain challenges. We confirmed the safer profile of optiLAIV in Stat1^-/- mice (highly susceptible to viral infections) by showing no signs of morbidity compared to a 50% mortality rate observed following LAIV inoculation. Using a human nasal 3D tissue model, we showed an increased induction of ER stress-related genes following immunization with optiLAIV. Induction of ER stress was previously shown to improve antigen-specific immune responses and is proposed as the mechanism of action of the licensed adjuvant AS03. This study characterizes a safer LAIV candidate in two mouse models mimicking infants and severely immunocompromised patients and proposes a simple attenuation strategy that could broaden LAIV application and reduce influenza burden in high-risk populations. IMPORTANCE Live attenuated influenza vaccine (LAIV) is a needle-free, mucosal vaccine approved for healthy 2- to 49-year old individuals. Its replicative nature and higher rate of adverse events excludes at-risk populations. We propose a strategy to improve LAIV safety and explore the possibility to expand its applications in children under 2-year old and immunocompromised patients. Using a neonatal mouse model, we show faster clearance of our optimized vaccine (optiLAIV) compared to the licensed LAIV. Despite lower replication, optiLAIV equally protected mice against influenza virus challenges. We confirmed the safer profile of optiLAIV in Stat1^-/- mice (highly susceptible to viral infections) by showing no signs of morbidity compared to a 50% mortality rate from LAIV. OptiLAIV could expand the applications of the current LAIV and help mitigate the burden of IAV in susceptible populations.

Influenza virus causes lung immunopathology through down-regulating PPARγ activity in macrophages

Fatal influenza (flu) virus infection often activates excessive inflammatory signals, leading to multi-organ failure and death, also referred to as cytokine storm. PPARγ (Peroxisome proliferator-activated receptor gamma) agonists are well-known candidates for cytokine storm modulation. The present study identified that influenza infection reduced PPARγ expression and decreased PPARγ transcription activity in human alveolar macrophages (AMs) from different donors. Treatment with PPARγ agonist Troglitazone ameliorated virus-induced proinflammatory cytokine secretion but did not interfere with the IFN-induced antiviral pathway in human AMs. In contrast, PPARγ antagonist and knockdown of PPARγ in human AMs further enhanced virus-stimulated proinflammatory response. In a mouse model of influenza infection, flu virus dose-dependently reduced PPARγ transcriptional activity and decreased expression of PPARγ. Moreover, PPARγ agonist troglitazone significantly reduced high doses of influenza infection-induced lung pathology. In addition, flu infection reduced PPARγ expression in all mouse macrophages, including AMs, interstitial macrophages, and bone-marrow-derived macrophages but not in alveolar epithelial cells. Our results indicate that the influenza virus specifically targets the PPARγ pathway in macrophages to cause acute injury to the lung.

Development of a T Cell-Based COVID-19 Vaccine Using a Live Attenuated Influenza Vaccine Viral Vector

The COVID-19 pandemic emerged in 2020 and has caused an unprecedented burden to all countries in the world. SARS-CoV-2 continues to circulate and antigenically evolve, enabling multiple reinfections. To address the issue of the virus antigenic variability, T cell-based vaccines are being developed, which are directed to more conserved viral epitopes. We used live attenuated influenza vaccine (LAIV) virus vector to generate recombinant influenza viruses expressing various T-cell epitopes of SARS-CoV-2 from either neuraminidase (NA) or non-structural (NS1) genes, via the P2A self-cleavage site. Intranasal immunization of human leukocyte antigen-A*0201 (HLA-A2.1) transgenic mice with these recombinant viruses did not result in significant SARS-CoV-2-specific T-cell responses, due to the immunodominance of NP₃₆₆ influenza T-cell epitope. However, side-by-side stimulation of peripheral blood mononuclear cells (PBMCs) of COVID-19 convalescents with recombinant viruses and LAIV vector demonstrated activation of memory T cells in samples stimulated with LAIV/SARS-CoV-2, but not LAIV alone. Hamsters immunized with a selected LAIV/SARS-CoV-2 prototype were protected against challenge with influenza virus and a high dose of SARS-CoV-2 of Wuhan and Delta lineages, which was confirmed by reduced weight loss, milder clinical symptoms and less pronounced histopathological signs of SARS-CoV-2 infection in the lungs, compared to LAIV- and mock-immunized animals. Overall, LAIV is a promising platform for the development of a bivalent vaccine against influenza and SARS-CoV-2.

Pathobiology of an NS1-Truncated H3N2 Swine Influenza Virus Strain in Pigs

Virus strains in the live attenuated influenza vaccine (LAIV) for swine in the United States that was on the market until 2020 encode a truncated nonstructural protein 1 of 126 amino acids (NS1del126). Their attenuation is believed to be due to an impaired ability to counteract the type I interferon (IFN)-mediated antiviral host response. However, this mechanism has been documented only in vitro for H3N2 strain A/swine/Texas/4199-2/98 NS1del126 (lvTX98), and several cases of clinical respiratory disease in the field were associated with the LAIV strains. We therefore further examined the pathobiology, including type I IFN induction, of lvTX98 in pigs and compared it with IFN induction in pig kidney-15 (PK-15) cells. lvTX98 induced up to 3-fold-higher type I IFN titers than wild-type TX98 (wtTX98) after inoculation of PK-15 cells at a high multiplicity of infection, while virus replication kinetics were similar. Mean nasal lvTX98 excretion by intranasally inoculated pigs was on average 50 times lower than that for wtTX98 but still reached titers of up to 4.3 log10 50% tissue culture infective doses/mL. After intratracheal inoculation, mean lvTX98 titers in the lower respiratory tract were significantly reduced at 18 to 48 h postinoculation (hpi) but similar to wtTX98 titers at 72 hpi. lvTX98 caused milder clinical signs than wtTX98 but induced comparable levels of microscopic and macroscopic lung lesions, peak neutrophil infiltration, and peak type I IFN. Thus, lvTX98 was partly attenuated in pigs, but this could not be associated with higher type I IFN levels. IMPORTANCE Swine influenza A viruses (swIAVs) with a truncated NS1del126 protein were strongly attenuated in previous laboratory-based safety studies and therefore approved for use as LAIVs for swine in the United States. In the field, however, the LAIV strains were detected in diagnostic samples and could regain a wild-type NS1 via reassortment with endemic swIAVs. This suggests a significant degree of LAIV replication and urges further investigation of the level and mechanism of attenuation of these LAIV strains in vivo. Here, we show that H3N2 LAIV strain lvTX98 is only partly attenuated in pigs and is excreted at significant titers after intranasal vaccination. Attenuation and restricted replication of lvTX98 in vivo seemed to be associated with the loss of NS1 functions other than type I IFN antagonism. Our findings can help to explain the occurrence of clinical respiratory disease and reassortment events associated with NS1del126-based LAIV strains in the field.

Intranasal Immunization with the Influenza A Virus Encoding Truncated NS1 Protein Protects Mice from Heterologous Challenge by Restraining the Inflammatory Response in the Lungs

Influenza viruses with an impaired NS1 protein are unable to antagonize the innate immune system and, therefore, are highly immunogenic because of the self-adjuvating effect. Hence, NS1-mutated viruses are considered promising candidates for the development of live-attenuated influenza vaccines and viral vectors for intranasal administration. We investigated whether the immunogenic advantage of the virus expressing only the N-terminal half of the NS1 protein (124 a.a.) can be translated into the induction of protective immunity against a heterologous influenza virus in mice. We found that immunization with either the wild-type A/PR/8/34 (H1N1) influenza strain (A/PR8/NSfull) or its NS1-shortened counterpart (A/PR8/NS124) did not prevent the viral replication in the lungs after the challenge with the A/Aichi/2/68 (H3N2) virus. However, mice immunized with the NS1-shortened virus were better protected from lethality after the challenge with the heterologous virus. Besides showing the enhanced influenza-specific CD8⁺ T-cellular response in the lungs, immunization with the A/PR8/NS124 virus resulted in reduced concentrations of proinflammatory cytokines and the lower extent of leukocyte infiltration in the lungs after the challenge compared to A/PR8/NSfull or the control group. The data show that intranasal immunization with the NS1-truncated virus may better induce not only effector T-cells but also certain immunoregulatory mechanisms, reducing the severity of the innate immune response after the heterologous challenge.

Conserved T-cell epitopes of respiratory syncytial virus (RSV) delivered by recombinant live attenuated influenza vaccine viruses efficiently induce RSV-specific lung-localized memory T cells and augment influenza-specific resident memory T-cell responses

Respiratory syncytial virus (RSV) can cause recurrent infection in people because it does not stimulate a long-lived immunological memory. There is an urgent need to develop a safe and efficacious vaccine against RSV that would induce immunological memory without causing immunopathology following natural RSV infection. We have previously generated two recombinant live attenuated influenza vaccine (LAIV) viruses that encode immunodominant T-cell epitopes of RSV M2 protein in the neuraminidase or NS1 genes. These chimeric vaccines afforded protection against influenza and RSV infection in mice, without causing pulmonary eosinophilia or inflammatory RSV disease. The current study assessed the formation of influenza-specific and RSV-specific CD4 and CD8 T-cell responses in the lungs of mice, with special attention to the lung tissue-resident memory T cell subsets (T_RM). The RSV epitopes did not affect influenza-specific CD4 effector memory T cell (Tem) levels in the lungs. The majority of these cells formed by LAIV or LAIV-RSV viruses had CD69⁺CD103^- phenotype. Both LAIV+NA/RSV and LAIV+NS/RSV recombinant viruses induced significant levels of RSV M2₈₂ epitope-specific lung-localized CD8 Tem cells expressing both CD69 and CD103 T_RM markers. Surprisingly, the CD69⁺CD103⁺ influenza-specific CD8 Tem responses were augmented by the addition of RSV epitopes, possibly as a result of the local microenvironment formed by the RSV-specific memory T cells differentiating to T_RM in the lungs of mice immunized with LAIV-RSV chimeric viruses. This study provides evidence that LAIV vector-based vaccination can induce robust lung-localized T-cell immunity to the inserted T-cell epitope of a foreign pathogen, without altering the immunogenicity of the viral vector itself.

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Two LAIV-RSV vaccine viruses induced RSV M2₈₂-specific effector memory CD8 T cells producing both IFNγ and TNFα cytokines.

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The inserted RSV epitopes did not affect influenza-specific CD4 Tem levels in the lungs of immunized mice.

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LAIV-RSV viruses induced RSV M2₈₂-specific lung-localized CD8 Tem cells expressing both CD69 and CD103 T_RM markers.

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The magnitude of RSV M2₈₂-specific CD8 Tem responses correlates with protection against RSV-induced lung pathology.

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The addition of RSV epitopes into the LAIV strain augmented CD69⁺CD103⁺ influenza-specific CD8 Tem responses in the lungs.

Recombinant Live Attenuated Influenza Vaccine Viruses Carrying Conserved T-cell Epitopes of Human Adenoviruses Induce Functional Cytotoxic T-Cell Responses and Protect Mice against Both Infections

Human adenoviruses (AdVs) are one of the most common causes of acute respiratory viral infections worldwide. Multiple AdV serotypes with low cross-reactivity circulate in the human population, making the development of an effective vaccine very challenging. In the current study, we designed a cross-reactive AdV vaccine based on the T-cell epitopes conserved among various AdV serotypes, which were inserted into the genome of a licensed cold-adapted live attenuated influenza vaccine (LAIV) backbone. We rescued two recombinant LAIV-AdV vaccines by inserting the selected AdV T-cell epitopes into the open reading frame of full-length NA and truncated the NS1 proteins of the H7N9 LAIV virus. We then tested the bivalent vaccines for their efficacy against influenza and human AdV5 in a mouse model. The vaccine viruses were attenuated in C57BL/6J mice and induced a strong influenza-specific antibody and cell-mediated immunity, fully protecting the mice against virulent influenza virus infection. The CD8 T-cell responses induced by both LAIV-AdV candidates were functional and efficiently killed the target cells loaded either with influenza NP366 or AdV DBP418 peptides. In addition, high levels of recall memory T cells targeted to an immunodominant H2b-restricted CD8 T-cell epitope were detected in the immunized mice after the AdV5 challenge, and the magnitude of these responses correlated with the level of protection against pulmonary pathology caused by the AdV5 infection. Our findings suggest that the developed recombinant vaccines can be used for combined protection against influenza and human adenoviruses and warrant further evaluation on humanized animal models and subsequent human trials.

Cell-to-Cell Variation in Defective Virus Expression and Effects on Host Responses during Influenza Virus Infection

Defective influenza virus particles generated during viral replication carry incomplete viral genomes and can interfere with the replication of competent viruses. These defective genomes are thought to modulate the disease severity and pathogenicity of an influenza virus infection. Different defective viral genomes also introduce another source of variation across a heterogeneous cell population. Evaluating the impact of defective virus genomes on host cell responses cannot be fully resolved at the population level, requiring single-cell transcriptional profiling. Here, we characterized virus and host transcriptomes in individual influenza virus-infected cells, including those of defective viruses that arise during influenza A virus infection. We established an association between defective virus transcription and host responses and validated interfering and immunostimulatory functions of identified dominant defective viral genome species in vitro. This study demonstrates the intricate effects of defective viral genomes on host transcriptional responses and highlights the importance of capturing host-virus interactions at the single-cell level.

Virus and host factors contribute to cell-to-cell variation in viral infections and determine the outcome of the overall infection. However, the extent of the variability at the single-cell level and how it impacts virus-host interactions at a system level are not well understood. To characterize the dynamics of viral transcription and host responses, we used single-cell RNA sequencing to quantify at multiple time points the host and viral transcriptomes of human A549 cells and primary bronchial epithelial cells infected with influenza A virus. We observed substantial variability in viral transcription between cells, including the accumulation of defective viral genomes (DVGs) that impact viral replication. We show (i) a correlation between DVGs and virus-induced variation of the host transcriptional program and (ii) an association between differential inductions of innate immune response genes and attenuated viral transcription in subpopulations of cells. These observations at the single-cell level improve our understanding of the complex virus-host interplay during influenza virus infection.

Tyr82 Amino Acid Mutation in PB1 Polymerase Induces an Influenza Virus Mutator Phenotype

In various positive-sense single-stranded RNA viruses, a low-fidelity viral RNA-dependent RNA polymerase (RdRp) confers attenuated phenotypes by increasing the mutation frequency. We report a negative-sense single-stranded RNA virus RdRp mutant strain with a mutator phenotype. Based on structural data of RdRp, rational targeting of key residues, and screening of fidelity variants, we isolated a novel low-fidelity mutator strain of influenza virus that harbors a Tyr82-to-Cys (Y82C) single-amino-acid substitution in the PB1 polymerase subunit. The purified PB1-Y82C polymerase indeed showed an increased frequency of misincorporation compared with the wild-type PB1 in an in vitro biochemical assay. To further investigate the effects of position 82 on PB1 polymerase fidelity, we substituted various amino acids at this position. As a result, we isolated various novel mutators other than PB1-Y82C with higher mutation frequencies. The structural model of influenza virus polymerase complex suggested that the Tyr82 residue, which is located at the nucleoside triphosphate entrance tunnel, may influence a fidelity checkpoint. Interestingly, although the PB1-Y82C variant replicated with wild-type PB1-like kinetics in tissue culture, the 50% lethal dose of the PB1-Y82C mutant was 10 times lower than that of wild-type PB1 in embryonated chicken eggs. In conclusion, our data indicate that the Tyr82 residue of PB1 has a crucial role in regulating polymerase fidelity of influenza virus and is closely related to attenuated pathogenic phenotypes in vivo IMPORTANCE Influenza A virus rapidly acquires antigenic changes and antiviral drug resistance, which limit the effectiveness of vaccines and drug treatments, primarily owing to its high rate of evolution. Virus populations formed by quasispecies can contain resistance mutations even before a selective pressure is applied. To study the effects of the viral mutation spectrum and quasispecies, high- and low-fidelity variants have been isolated for several RNA viruses. Here, we report the discovery of a low-fidelity RdRp variant of influenza A virus that contains a substitution at Tyr82 in PB1. Viruses containing the PB1-Y82C substitution showed growth kinetics and viral RNA synthesis levels similar to those of the wild-type virus in cell culture; however, they had significantly attenuated phenotypes in a chicken egg infection experiment. These data demonstrated that decreased RdRp fidelity attenuates influenza A virus in vivo, which is a desirable feature for the development of safer live attenuated vaccine candidates.

Recombinant live attenuated influenza vaccine viruses carrying CD8 T-cell epitopes of respiratory syncytial virus protect mice against both pathogens without inflammatory disease

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory disease in young children, elderly and immunocompromised adults. There is no licensed vaccine against RSV although development of an effective and safe RSV vaccine has been a high priority for several decades. Among the various vaccine platforms, the viral-vectored RSV vaccines based on licensed cold-adapted live attenuated influenza vaccine (LAIV) might offer an advantage of inducing adequate mucosal CD8 T cell immunity at the infection site of respiratory pathogens. We constructed two recombinant LAIV viruses expressing immunodominant T-cell epitopes of RSV M2-1 protein. The results in this study provide evidence that RSV CD8 T cell epitopes delivered by LAIV viral vector could confer protection against RSV infection without causing pulmonary eosinophilia and inflammatory RSV disease in mice. In addition, these chimeric LAIV-RSV vaccines retained their attenuated phenotype and ability to protect against virulent influenza virus, thus providing a unique approach to fight against two dangerous respiratory viral pathogens using a single vaccine preparation.

Modulation of Innate Immune Responses by the Influenza A NS1 and PA-X Proteins

Influenza A viruses (IAV) can infect a broad range of animal hosts, including humans. In humans, IAV causes seasonal annual epidemics and occasional pandemics, representing a serious public health and economic problem, which is most effectively prevented through vaccination. The defense mechanisms that the host innate immune system provides restrict IAV replication and infection. Consequently, to successfully replicate in interferon (IFN)-competent systems, IAV has to counteract host antiviral activities, mainly the production of IFN and the activities of IFN-induced host proteins that inhibit virus replication. The IAV multifunctional proteins PA-X and NS1 are virulence factors that modulate the innate immune response and virus pathogenicity. Notably, these two viral proteins have synergistic effects in the inhibition of host protein synthesis in infected cells, although using different mechanisms of action. Moreover, the control of innate immune responses by the IAV NS1 and PA-X proteins is subject to a balance that can determine virus pathogenesis and fitness, and recent evidence shows co-evolution of these proteins in seasonal viruses, indicating that they should be monitored for enhanced virulence. Importantly, inhibition of host gene expression by the influenza NS1 and/or PA-X proteins could be explored to develop improved live-attenuated influenza vaccines (LAIV) by modulating the ability of the virus to counteract antiviral host responses. Likewise, both viral proteins represent a reasonable target for the development of new antivirals for the control of IAV infections. In this review, we summarize the role of IAV NS1 and PA-X in controlling the antiviral response during viral infection.

Live attenuated influenza vaccine viral vector induces functional cytotoxic T-cell immune response against foreign CD8+ T-cell epitopes inserted into NA and NS1 genes using the 2A self-cleavage site

The development of viral vector vaccines against various pathogens for which conventional vaccination approaches are not applicable has been a priority for a number of years. One promising approach is the insertion of immunodominant conservative cytotoxic T-cell (CTL) epitopes into the genome of a viral vector, which then delivers these epitopes to target cells, inducing immunity. Many different viruses have been assessed as viral vectors for CTL-based vaccines, but only a few of them are clinically relevant, mainly because of safety issues and limited knowledge about their performance in humans. In this regard, the use of licensed cold-adapted live attenuated influenza vaccine (LAIV) viruses as a vector delivery system has clear advantages for CTL-based vector vaccines against other respiratory pathogens: LAIV is known to induce all arms of the adaptive immune system and is administered via nasal spray, and its production process is relatively easy and inexpensive. Here we present the first results of the use of an LAIV backbone for designing a CTL epitope-based vaccine against respiratory syncytial virus (RSV). The chimeric LAIV-RSV vaccine candidates were attenuated in mice and induced strong, fully functional CTL immunity in this animal model.

Generation of a broadly reactive influenza H1 antigen using a consensus HA sequence

H1N1, one of the most prevalent influenza A virus subtypes affecting the human population, can cause infections varying from mild respiratory syndrome to severe pneumonia. The current H1N1 vaccine needs to be updated annually and does not protect against future outbreaks. Here, we downloaded 2,656 HA protein sequences of human H1N1 viruses from the NCBI influenza database (up to the date of Aug. 2012) and constructed a phylogenetic tree of these H1 proteins via the neighbor-joining method using MEGA 5.0 software. A consensus H1 protein (CH1) was generated and was further modified with published conserved T-cell and B-cell epitopes. Interestingly, this CH1 protein is genetically similar to an H1 isolate obtained during the 1980s (A/Memphis/7/1980), indicating that a universal HA antigen may exist in nature. Vaccination with a DNA vaccine expressing CH1 elicited broadly reactive T-cell and B-cell responses to heterologous H1N1 viruses, though this vaccine did not successfully neutralize pdm09 H1N1 viruses. A combination of CH1 and pdm09 HA in a DNA vaccination neutralized pdm09 H1N1 viruses and protected mice from lethal infections by all representative H1N1 viruses. Moreover, a recombinant chimeric PR8-CH1 virus carrying HA sequence of the consensus H1 and all other seven genes from the PR8 strain was highly attenuated in mice, with a lethal dose (LD₅₀) of more than 10⁶ pfu. Vaccination with PR8-CH1 virus provided complete protection against infections by heterologous H1N1 strains. Taken together, a universal H1 antigen, CH1, was developed by constructing a consensus HA sequence, and the PR8-CH1 virus containing this consensus sequence elicited broadly protective immunity against heterologous H1N1 viruses.

A Naturally Occurring Deletion in the Effector Domain of H5N1 Swine Influenza Virus Nonstructural Protein 1 Regulates Viral Fitness and Host Innate Immunity

Nonstructural protein 1 (NS1) of influenza A virus regulates innate immune responses via various mechanisms. We previously showed that a naturally occurring deletion (the EALQR motif) in the NS1 effector domain of an H5N1 swine-origin avian influenza virus impairs the inhibition of type I interferon (IFN) in chicken fibroblasts and attenuates virulence in chickens. Here we found that the virus bearing this deletion in its NS1 effector domain showed diminished inhibition of IFN-related cytokine expression and attenuated virulence in mice. We further showed that deletion of the EALQR motif disrupted NS1 dimerization, impairing double-stranded RNA (dsRNA) sequestration and competitive binding with RIG-I. In addition, the EALQR-deleted NS1 protein could not bind to TRIM25, unlike full-length NS1, and was less able to block TRIM25 oligomerization and self-ubiquitination, further impairing the inhibition of TRIM25-mediated RIG-I ubiquitination compared to that with full-length NS1. Our data demonstrate that the EALQR deletion prevents NS1 from blocking RIG-I-mediated IFN induction via a novel mechanism to attenuate viral replication and virulence in mammalian cells and animals.IMPORTANCE H5 highly pathogenic avian influenza viruses have infected more than 800 individuals across 16 countries, with an overall case fatality rate of 53%. Among viral proteins, nonstructural protein 1 (NS1) of influenza virus is considered a key determinant for type I interferon (IFN) antagonism, pathogenicity, and host range. However, precisely how NS1 modulates virus-host interaction, facilitating virus survival, is not fully understood. Here we report that a naturally occurring deletion (of the EALQR motif) in the NS1 effector domain of an H5N1 swine-origin avian influenza virus disrupted NS1 dimerization, which diminished the blockade of IFN induction via the RIG-I signaling pathway, thereby impairing virus replication and virulence in the host. Our study demonstrates that the EALQR motif of NS1 regulates virus fitness to attain a virus-host compromise state in animals and identifies this critical motif as a potential target for the future development of small molecular drugs and attenuated vaccines.

Pathogenicity testing of influenza candidate vaccine viruses in the ferret model

The development of influenza candidate vaccine viruses (CVVs) for pre-pandemic vaccine production represents a critical step in pandemic preparedness. The multiple subtypes and clades of avian or swine origin influenza viruses circulating world-wide at any one time necessitates the continuous generation of CVVs to provide an advanced starting point should a novel zoonotic virus cross the species barrier and cause a pandemic. Furthermore, the evolution and diversity of novel influenza viruses that cause zoonotic infections requires ongoing monitoring and surveillance, and, when a lack of antigenic match between circulating viruses and available CVVs is identified, the production of new CVVs. Pandemic guidelines developed by the WHO Global Influenza Program govern the design and preparation of reverse genetics-derived CVVs, which must undergo numerous safety and quality tests prior to human use. Confirmation of reassortant CVV attenuation of virulence in ferrets relative to wild-type virus represents one of these critical steps, yet there is a paucity of information available regarding the relative degree of attenuation achieved by WHO-recommended CVVs developed against novel viruses with pandemic potential. To better understand the degree of CVV attenuation in the ferret model, we examined the relative virulence of six A/Puerto Rico/8/1934-based CVVs encompassing five different influenza A subtypes (H2N3, H5N1, H5N2, H5N8, and H7N9) compared with the respective wild-type virus in ferrets. Despite varied virulence of wild-type viruses in the ferret, all CVVs examined showed reductions in morbidity and viral shedding in upper respiratory tract tissues. Furthermore, unlike the wild-type counterparts, none of the CVVs spread to extrapulmonary tissues during the acute phase of infection. While the magnitude of virus attenuation varied between virus subtypes, collectively we show the reliable and reproducible attenuation of CVVs that have the A/Puerto Rico/9/1934 backbone in a mammalian model.

Efficacy of Live-Attenuated H9N2 Influenza Vaccine Candidates Containing NS1 Truncations against H9N2 Avian Influenza Viruses

H9N2 avian influenza virus is a zoonotic agent with a broad host range that can contribute genetic information to H5 or H7N9 subtype viruses, which are significant threats to both humans and birds. Thus, there is a great need for a vaccine to control H9N2 avian influenza. Three mutant viruses of an H9N2 virus A/chicken/Taixing/10/2010 (rTX-NS1-73, rTX-NS1-100, and rTX-NS1-128) were constructed with different NS1 gene truncations and confirmed by western blot analysis. The genetic stability, pathogenicity, transmissibility, and host immune responses toward these mutants were evaluated. The mutant virus rTX-NS1-128 exhibited the most attenuated phenotype and lost transmissibility. The expression levels of interleukin 12 in the nasal and tracheal tissues from chickens immunized with rTX-NS1-128 were significantly upregulated on day 3 post-immunization and the IgA and IgG antibody levels were significantly increased on days 7, 14, and 21 post-immunization when compared to chickens that received an inactivated vaccine. rTX-NS1-128 also protected chickens from challenge by homologous and heterologous H9N2 avian influenza viruses. The results indicate that rTX-NS1-128 can be used as a potential live-attenuated vaccine against H9N2 avian influenza.

Preclinical Toxicology of Vaccines1

Immunity to targeted infectious diseases may be conferred or enhanced by vaccines, which are manufactured from recombinant forms as well as inactivated or attenuated organisms. These vaccines have to meet requirements for safety, quality, and efficacy. In addition to antigenic components, various adjuvants may be included in vaccines to evoke an effective immune response. To ensure the safety of new vaccines, preclinical toxicology studies are conducted prior to the initiation of, and concurrently with, clinical studies. There are five different types of preclinical toxicology study in the evaluation of vaccine safety: single and/or repeat dose, reproductive and developmental, mutagenicity, carcinogenicity, and safety pharmacology. If any adverse effects are observed in the course of these studies, they should be fully evaluated and a final safety decision made accordingly. Successful preclinical toxicology studies depend on multiple factors including using the appropriate study designs, using the right animal model, and evoking an effective immune response. Additional in vivo and in vitro assays that establish the identity, purity, safety, and potency of the vaccine play a significant role in assessing critical characteristics of vaccine safety.

Reversion of Cold-Adapted Live Attenuated Influenza Vaccine into a Pathogenic Virus

The only licensed live attenuated influenza A virus vaccines (LAIVs) in the United States (FluMist) are created using internal protein-coding gene segments from the cold-adapted temperature-sensitive master donor virus A/Ann Arbor/6/1960 and HA/NA gene segments from circulating viruses. During serial passage of A/Ann Arbor/6/1960 at low temperatures to select the desired attenuating phenotypes, multiple cold-adaptive mutations and temperature-sensitive mutations arose. A substantial amount of scientific and clinical evidence has proven that FluMist is safe and effective. Nevertheless, no study has been conducted specifically to determine if the attenuating temperature-sensitive phenotype can revert and, if so, the types of substitutions that will emerge (i.e., compensatory substitutions versus reversion of existing attenuating mutations). Serial passage of the monovalent FluMist 2009 H1N1 pandemic vaccine at increasing temperatures in vitro generated a variant that replicated efficiently at higher temperatures. Sequencing of the variant identified seven nonsynonymous mutations, PB1-E51K, PB1-I171V, PA-N350K, PA-L366I, NP-N125Y, NP-V186I, and NS2-G63E. None occurred at positions previously reported to affect the temperature sensitivity of influenza A viruses. Synthetic genomics technology was used to synthesize the whole genome of the virus, and the roles of individual mutations were characterized by assessing their effects on RNA polymerase activity and virus replication kinetics at various temperatures. The revertant also regained virulence and caused significant disease in mice, with severity comparable to that caused by a wild-type 2009 H1N1 pandemic virus.

IMPORTANCE

The live attenuated influenza vaccine FluMist has been proven safe and effective and is widely used in the United States. The phenotype and genotype of the vaccine virus are believed to be very stable, and mutants that cause disease in animals or humans have never been reported. By propagating the virus under well-controlled laboratory conditions, we found that the FluMist vaccine backbone could regain virulence to cause severe disease in mice. The identification of the responsible substitutions and elucidation of the underlying mechanisms provide unique insights into the attenuation of influenza virus, which is important to basic research on vaccines, attenuation reversion, and replication. In addition, this study suggests that the safety of LAIVs should be closely monitored after mass vaccination and that novel strategies to continue to improve LAIV vaccine safety should be investigated.

Influenza B viruses: not to be discounted

In contrast to influenza A viruses, which have been investigated extensively, influenza B viruses have attracted relatively little attention. However, influenza B viruses are an important cause of morbidity and mortality in the human population and full understanding of their biological and epidemiological properties is imperative to better control this important pathogen. However, some of its characteristics are still elusive and warrant investigation. Here, we review evolution, epidemiology, pathogenesis and immunity and identify gaps in our knowledge of influenza B viruses. The divergence of two antigenically distinct influenza B viruses is highlighted. The co-circulation of viruses of these two lineages necessitated the development of quadrivalent influenza vaccines, which is discussed in addition to possibilities to develop universal vaccination strategies.

Swine Influenza Virus PA and Neuraminidase Gene Reassortment into Human H1N1 Influenza Virus Is Associated with an Altered Pathogenic Phenotype Linked to Increased MIP-2 Expression

Swine are susceptible to infection by both avian and human influenza viruses, and this feature is thought to contribute to novel reassortant influenza viruses. In this study, the influenza virus reassortment rate in swine and human cells was determined. Coinfection of swine cells with 2009 pandemic H1N1 virus (huH1N1) and an endemic swine H1N2 (A/swine/Illinois/02860/09) virus (swH1N2) resulted in a 23% reassortment rate that was independent of α2,3- or α2,6-sialic acid distribution on the cells. The reassortants had altered pathogenic phenotypes linked to introduction of the swine virus PA and neuraminidase (NA) into huH1N1. In mice, the huH1N1 PA and NA mediated increased MIP-2 expression early postinfection, resulting in substantial pulmonary neutrophilia with enhanced lung pathology and disease. The findings support the notion that swine are a mixing vessel for influenza virus reassortants independent of sialic acid distribution. These results show the potential for continued reassortment of the 2009 pandemic H1N1 virus with endemic swine viruses and for reassortants to have increased pathogenicity linked to the swine virus NA and PA genes which are associated with increased pulmonary neutrophil trafficking that is related to MIP-2 expression.

IMPORTANCE

Influenza A viruses can change rapidly via reassortment to create a novel virus, and reassortment can result in possible pandemics. Reassortments among subtypes from avian and human viruses led to the 1957 (H2N2 subtype) and 1968 (H3N2 subtype) human influenza pandemics. Recent analyses of circulating isolates have shown that multiple genes can be recombined from human, avian, and swine influenza viruses, leading to triple reassortants. Understanding the factors that can affect influenza A virus reassortment is needed for the establishment of disease intervention strategies that may reduce or preclude pandemics. The findings from this study show that swine cells provide a mixing vessel for influenza virus reassortment independent of differential sialic acid distribution. The findings also establish that circulating neuraminidase (NA) and PA genes could alter the pathogenic phenotype of the pandemic H1N1 virus, resulting in enhanced disease. The identification of such factors provides a framework for pandemic modeling and surveillance.

Current and emerging cell culture manufacturing technologies for influenza vaccines

Annually, influenza virus infects millions of people worldwide. Vaccination programs against seasonal influenza infections require the production of hundreds of million doses within a very short period of time. The influenza vaccine is currently produced using a technology developed in the 1940s that relies on replicating the virus in embryonated hens' eggs. The monovalent viral preparation is inactivated and purified before being formulated in trivalent or tetravalent influenza vaccines. The production process has depended on a continuous supply of eggs. In the case of pandemic outbreaks, this mode of production might be problematic because of a possible drastic reduction in the egg supply and the low flexibility of the manufacturing process resulting in a lack of supply of the required vaccine doses in a timely fashion. Novel production systems using mammalian or insect cell cultures have emerged to overcome the limitations of the egg-based production system. These industrially well-established production systems have been primarily selected for a faster and more flexible response to pandemic threats. Here, we review the most important cell culture manufacturing processes that have been developed in recent years for mass production of influenza vaccines.

The NS1 protein: a multitasking virulence factor

The non-structural protein 1 of influenza virus (NS1) is a relatively small polypeptide with an outstanding number of ascribed functions. NS1 is the main viral antagonist of the innate immune response during influenza virus infection, chiefly by inhibiting the type I interferon system at multiple steps. As such, its role is critical to overcome the first barrier the host presents to halt the viral infection. However, the pro-viral activities of this well-studied protein go far beyond and include regulation of viral RNA and protein synthesis, and disruption of the host cell homeostasis by dramatically affecting general gene expression while tweaking the PI3K signaling network. Because of all of this, NS1 is a key virulence factor that impacts influenza pathogenesis, and adaptation to new hosts, making it an attractive target for control strategies. Here, we will overview the many roles that have been ascribed to the NS1 protein, and give insights into the sequence features and structural properties that make them possible, highlighting the need to understand how NS1 can actually perform all of these functions during viral infection.

Characterization of uncultivable bat influenza virus using a replicative synthetic virus

Bats harbor many viruses, which are periodically transmitted to humans resulting in outbreaks of disease (e.g., Ebola, SARS-CoV). Recently, influenza virus-like sequences were identified in bats; however, the viruses could not be cultured. This discovery aroused great interest in understanding the evolutionary history and pandemic potential of bat-influenza. Using synthetic genomics, we were unable to rescue the wild type bat virus, but could rescue a modified bat-influenza virus that had the HA and NA coding regions replaced with those of A/PR/8/1934 (H1N1). This modified bat-influenza virus replicated efficiently in vitro and in mice, resulting in severe disease. Additional studies using a bat-influenza virus that had the HA and NA of A/swine/Texas/4199-2/1998 (H3N2) showed that the PR8 HA and NA contributed to the pathogenicity in mice. Unlike other influenza viruses, engineering truncations hypothesized to reduce interferon antagonism into the NS1 protein didn't attenuate bat-influenza. In contrast, substitution of a putative virulence mutation from the bat-influenza PB2 significantly attenuated the virus in mice and introduction of a putative virulence mutation increased its pathogenicity. Mini-genome replication studies and virus reassortment experiments demonstrated that bat-influenza has very limited genetic and protein compatibility with Type A or Type B influenza viruses, yet it readily reassorts with another divergent bat-influenza virus, suggesting that the bat-influenza lineage may represent a new Genus/Species within the Orthomyxoviridae family. Collectively, our data indicate that the bat-influenza viruses recently identified are authentic viruses that pose little, if any, pandemic threat to humans; however, they provide new insights into the evolution and basic biology of influenza viruses.

The identification of influenza virus-like sequences in two different bat species has generated great interest in understanding their biology, ability to mix with other influenza viruses, and their public health threat. Unfortunately, bat-influenza viruses couldn't be cultured from the samples containing the influenza-like nucleic acids. We used synthetic genomics strategies to create wild type bat-influenza, or bat-influenza modified by substituting the surface glycoproteins with those of model influenza A viruses. Although influenza virus-like particles were produced from both synthetic genomes, only the modified bat-influenza viruses could be cultured. The modified bat-influenza viruses replicated efficiently in vitro and an H1N1 modified version caused severe disease in mice. Collectively our data show: (1) the two bat-flu genomes identified in other studies are replication competent, suggesting that host cell specificity is the major limitation for propagation of bat-influenza, (2) bat-influenza NS1 antagonizes host interferon response more efficiently than that of a model influenza A virus, (3) bat-influenza has both genetic and protein incompatibility with influenza A or B viruses, and (4) that these bat-influenza lineages pose little pandemic threat.

Influenza virus non-structural protein NS1: interferon antagonism and beyond

Most viruses express one or several proteins that counter the antiviral defences of the host cell. This is the task of non-structural protein NS1 in influenza viruses. Absent in the viral particle, but highly expressed in the infected cell, NS1 dramatically inhibits cellular gene expression and prevents the activation of key players in the IFN system. In addition, NS1 selectively enhances the translation of viral mRNAs and may regulate the synthesis of viral RNAs. Our knowledge of the virus and of NS1 has increased dramatically during the last 15 years. The atomic structure of NS1 has been determined, many cellular partners have been identified and its multiple activities have been studied in depth. This review presents our current knowledge, and attempts to establish relationships between the RNA sequence, the structure of the protein, its ligands, its activities and the pathogenicity of the virus. A better understanding of NS1 could help in elaborating novel antiviral strategies, based on either live vaccines with altered NS1 or on small-compound inhibitors of NS1.

STAT2 signaling and dengue virus infection

Dengue virus (DENV) is an important human pathogen whose byzantine relationship with the immune response is poorly understood. DENV causes dengue fever and dengue hemorrhagic fever/dengue shock syndrome, diseases for which palliative care is the only treatment. DENV immunopathogenesis studies are complicated by the lack of an immunocompetent small-animal model, and this has hindered anti-DENV drug and vaccine development. This review describes strategies that DENV uses to evade the type I interferon response and focuses on how data gained from the study of DENV NS5-mediated STAT2 degradation may be used to create immunocompetent DENV mouse models and design anti-DENV therapeutics.

An evaluation of the emerging vaccines against influenza in children

BACKGROUND

Influenza is an under-appreciated cause of acute lower respiratory infections (ALRI) in children. It is estimated to cause approximately 20 million new episodes of ALRI in children annually, 97% of these occurring in developing countries. It is also estimated to result in 28000 to 112000 deaths annually in young children. Apart from hospitalisations and deaths, influenza has significant economic consequences. The current egg-based inactivated influenza vaccines have several limitations: annual vaccination, high production costs, and cannot respond adequately to meet the demand during pandemics.

METHODS

We used a modified CHNRI methodology for setting priorities in health research investments. This was done in two stages. In Stage I, we systematically reviewed the literature related to emerging cross-protective vaccines against influenza relevant to several criteria of interest: answerability; cost of development, production and implementation; efficacy and effectiveness; deliverability, affordability and sustainability; maximum potential impact on disease burden reduction; acceptability to the end users and health workers; and effect on equity. In Stage II, we conducted an expert opinion exercise by inviting 20 experts (leading basic scientists, international public health researchers, international policy makers and representatives of pharmaceutical companies). They answered questions from the CHNRI framework and their "collective optimism" towards each criterion was documented on a scale from 0 to 100%.

RESULTS

The experts expressed very high level of optimism for deliverability, impact on equity, and acceptability to health workers and end users. However, they expressed concerns over the criteria of answerability, low development cost, low product cost, low implementation cost, affordability and, to a lesser extent sustainability. In addition they felt that the vaccine would have higher efficacy and impact on disease burden reduction on overall influenza-associated disease rather than specifically influenza-associated pneumonia.

CONCLUSION

Although the landscape of emerging influenza vaccines shows several promising candidates, it is unlikely that the advancements in the newer vaccine technologies will be able to progress through to large scale production in the near future. The combined effects of continued investments in researching new vaccines and improvements of available vaccines will hopefully shorten the time needed to the development of an effective seasonal and pandemic influenza vaccine suitable for large scale production.

The influenza virus NS1 protein as a therapeutic target

Nonstructural protein 1 (NS1) of influenza A virus plays a central role in virus replication and blockade of the host innate immune response, and is therefore being considered as a potential therapeutic target. The primary function of NS1 is to dampen the host interferon (IFN) response through several distinct molecular mechanisms that are triggered by interactions with dsRNA or specific cellular proteins. Sequestration of dsRNA by NS1 results in inhibition of the 2'-5' oligoadenylate synthetase/RNase L antiviral pathway, and also inhibition of dsRNA-dependent signaling required for new IFN production. Binding of NS1 to the E3 ubiquitin ligase TRIM25 prevents activation of RIG-I signaling and subsequent IFN induction. Cellular RNA processing is also targeted by NS1, through recognition of cleavage and polyadenylation specificity factor 30 (CPSF30), leading to inhibition of IFN-β mRNA processing as well as that of other cellular mRNAs. In addition NS1 binds to and inhibits cellular protein kinase R (PKR), thus blocking an important arm of the IFN system. Many additional proteins have been reported to interact with NS1, either directly or indirectly, which may serve its anti-IFN and additional functions, including the regulation of viral and host gene expression, signaling pathways and viral pathogenesis. Many of these interactions are potential targets for small-molecule intervention. Structural, biochemical and functional studies have resulted in hypotheses for drug discovery approaches that are beginning to bear experimental fruit, such as targeting the dsRNA-NS1 interaction, which could lead to restoration of innate immune function and inhibition of virus replication. This review describes biochemical, cell-based and nucleic acid-based approaches to identifying NS1 antagonists.

Asparagine substitution at PB2 residue 701 enhances the replication, pathogenicity, and transmission of the 2009 pandemic H1N1 influenza A virus

The 2009/2010 pandemic influenza virus (H1N1pdm) contains an avian-lineage PB2 gene that lacks E627K and D701N substitutions important in the pathogenesis and transmission of avian-origin viruses in humans or other mammals. Previous studies have shown that PB2-627K is not necessary because of a compensatory Q591R substitution. The role that PB2-701N plays in the H1N1pdm phenotype is not well understood. Therefore, PB2-D701N was introduced into an H1N1pdm virus (A/New York/1682/2009 (NY1682)) and analyzed in vitro and in vivo. Mini-genome replication assay, in vitro replication characteristics in cell lines, and analysis in the mouse and ferret models demonstrated that PB2-D701N increased virus replication rates and resulted in more severe pathogenicity in mice and more efficient transmission in ferrets. In addition, compared to the NY1682-WT virus, the NY1682-D701N mutant virus induced less IFN-λ and replicated to a higher titer in primary human alveolar epithelial cells. These findings suggest that the acquisition of the PB2-701N substitution by H1N1pdm viruses may result in more severe disease or increase transmission in humans.

Perspectives on replication-incompetent nasal influenza virus vaccines

Influenza is an emerging as well as resurging contagious disease with a worldwide impact on public health. Although broad administration of the licensed influenza virus (IFV) vaccines has mitigated the disease in many countries over the years, there are intrinsic problems associated with them. The study under evaluation reports that a novel PB2-knockout nonreplicating nasal IFV vaccine has been generated with the capacity to confer protection of mice against live IFV challenges. Moreover, an exogenous gene expressed from the bioengineered PB2-knockout IFV could elicit an immune response against the exogenous protein, showing its potential to deliver transgenes as a vector. The risk-benefit ratio of this new influenza vaccine vector is discussed.

NS1-truncated live attenuated virus vaccine provides robust protection to aged mice from viral challenge

Immunological changes associated with age contribute to the high rates of influenza virus morbidity and mortality in the elderly. Compounding this problem, aged individuals do not respond to vaccination as well as younger, healthy adults. Efforts to increase protection to this demographic group are of utmost importance, as the proportion of the population above the age of 65 is projected to increase in the coming decade. Using a live influenza virus with a truncated nonstructural protein 1 (NS1), we are able to stimulate cellular and humoral immune responses of aged mice comparable to levels seen in young mice. Impressively, a single vaccination provided protection following stringent lethal challenge in aged mice.

Live attenuated H5N1 vaccine with H9N2 internal genes protects chickens from infections by both highly pathogenic H5N1 and H9N2 influenza viruses

Please cite this paper as: Nang et al. (2013) Live attenuated H5N1 vaccine with H9N2 internal genes protects chickens from infections by both Highly Pathogenic H5N1 and H9N2 Influenza Viruses. Influenza and Other Respiratory Viruses 7(2) 120–131.

Background  The highly pathogenic H5N1 and H9N2 influenza viruses are endemic in many countries around the world and have caused considerable economic loss to the poultry industry.

Objectives  We aimed to study whether a live attenuated H5N1 vaccine comprising internal genes from a cold‐adapted H9N2 influenza virus could protect chickens from infection by both H5N1 and H9N2 viruses.

Methods  We developed a cold‐adapted H9N2 vaccine virus expressing hemagglutinin and neuraminidase derived from the highly pathogenic H5N1 influenza virus using reverse genetics.

Results and Conclusions  Chickens immunized with the vaccine were protected from lethal infections with homologous and heterologous H5N1 or H9N2 influenza viruses. Specific antibody against H5N1 virus was detected up to 11 weeks after vaccination (the endpoint of this study). In vaccinated chickens, IgA and IgG antibody subtypes were induced in lung and intestinal tissue, and CD4+ and CD8+ T lymphocytes expressing interferon‐gamma were induced in the splenocytes. These data suggest that a live attenuated H5N1 vaccine with cold‐adapted H9N2 internal genes can protect chickens from infection with H5N1 and H9N2 influenza viruses by eliciting humoral and cellular immunity.

Two years after pandemic influenza A/2009/H1N1: what have we learned?

The world had been anticipating another influenza pandemic since the last one in 1968. The pandemic influenza A H1N1 2009 virus (A/2009/H1N1) finally arrived, causing the first pandemic influenza of the new millennium, which has affected over 214 countries and caused over 18,449 deaths. Because of the persistent threat from the A/H5N1 virus since 1997 and the outbreak of the severe acute respiratory syndrome (SARS) coronavirus in 2003, medical and scientific communities have been more prepared in mindset and infrastructure. This preparedness has allowed for rapid and effective research on the epidemiological, clinical, pathological, immunological, virological, and other basic scientific aspects of the disease, with impacts on its control. A PubMed search using the keywords "pandemic influenza virus H1N1 2009" yielded over 2,500 publications, which markedly exceeded the number published on previous pandemics. Only representative works with relevance to clinical microbiology and infectious diseases are reviewed in this article. A significant increase in the understanding of this virus and the disease within such a short amount of time has allowed for the timely development of diagnostic tests, treatments, and preventive measures. These findings could prove useful for future randomized controlled clinical trials and the epidemiological control of future pandemics.

New strategies for the development of H5N1 subtype influenza vaccines: progress and challenges

The emergence and spread of highly pathogenic avian influenza (H5N1) viruses among poultry in Asia, the Middle East, and Africa have fueled concerns of a possible human pandemic, and spurred efforts towards developing vaccines against H5N1 influenza viruses, as well as improving vaccine production methods. In recent years, promising experimental reverse genetics-derived H5N1 live attenuated vaccines have been generated and characterized, including vaccines that are attenuated through temperature-sensitive mutation, modulation of the interferon antagonist protein, or disruption of the M2 protein. Live attenuated influenza virus vaccines based on each of these modalities have conferred protection against homologous and heterologous challenge in animal models of influenza virus infection. Alternative vaccine strategies that do not require the use of live virus, such as virus-like particle (VLP) and DNA-based vaccines, have also been vigorously pursued in recent years. Studies have demonstrated that influenza VLP vaccination can confer homologous and heterologous protection from lethal challenge in a mouse model of infection. There have also been improvements in the formulation and production of vaccines following concerns over the threat of H5N1 influenza viruses. The use of novel substrates for the growth of vaccine virus stocks has been intensively researched in recent years, and several candidate cell culture-based systems for vaccine amplification have emerged, including production systems based on Madin-Darby canine kidney, Vero, and PerC6 cell lines. Such systems promise increased scalability of product, and reduced reliance on embryonated chicken eggs as a growth substrate. Studies into the use of adjuvants have shown that oil-in-water-based adjuvants can improve the immunogenicity of inactivated influenza vaccines and conserve antigen in such formulations. Finally, efforts to develop more broadly cross-protective immunization strategies through the inclusion of conserved influenza virus antigens in vaccines have led to experimental vaccines based on the influenza hemagglutinin (HA) stem domain. Such vaccines have been shown to confer protection from lethal challenge in mouse models of influenza virus infection. Through further development, vaccines based on the HA stem have the potential to protect vaccinated individuals against unanticipated pandemic and epidemic influenza virus strains. Overall, recent advances in experimental vaccines and in vaccine production processes provide the potential to lower mortality and morbidity resulting from influenza infection.

Engineering temperature sensitive live attenuated influenza vaccines from emerging viruses

The licensed live attenuated influenza A vaccine (LAIV) in the United States is created by making a reassortant containing six internal genes from a cold-adapted master donor strain (ca A/AA/6/60) and two surface glycoprotein genes from a circulating/emerging strain (e.g., A/CA/7/09 for the 2009/2010 H1N1 pandemic). Technologies to rapidly create recombinant viruses directly from patient specimens were used to engineer alternative LAIV candidates that have genomes composed entirely of vRNAs from pandemic or seasonal strains. Multiple mutations involved in the temperature-sensitive (ts) phenotype of the ca A/AA/6/60 master donor strain were introduced into a 2009 H1N1 pandemic strain rA/New York/1682/2009 (rNY1682-WT) to create rNY1682-TS1, and additional mutations identified in other ts viruses were added to rNY1682-TS1 to create rNY1682-TS2. Both rNY1682-TS1 and rNY1682-TS2 replicated efficiently at 30°C and 33°C. However, rNY1682-TS1 was partially restricted, and rNY1682-TS2 was completely restricted at 39°C. Additionally, engineering the TS1 or TS2 mutations into a distantly related human seasonal H1N1 influenza A virus also resulted pronounced restriction of replication in vitro. Clinical symptoms and virus replication in the lungs of mice showed that although rNY1682-TS2 and the licensed FluMist(®)-H1N1pdm LAIV that was used to combat the 2009/2010 pandemic were similarly attenuated, the rNY1682-TS2 was more protective upon challenge with a virulent mutant of pandemic H1N1 virus or a heterologous H1N1 (A/PR/8/1934) virus. This study demonstrates that engineering key temperature sensitive mutations (PB1-K391E, D581G, A661T; PB2-P112S, N265S, N556D, Y658H) into the genomes of influenza A viruses attenuates divergent human virus lineages and provides an alternative strategy for the generation of LAIVs.

Influenza virus interferon-inducing particle efficiency is reversed in avian and mammalian cells, and enhanced in cells co-infected with defective-interfering particles

Naturally selected variants of influenza virus encoding truncated NS1 proteins were tested in chickens as candidate live-attenuated influenza vaccines. Their effectiveness correlated with the amount of interferon (IFN) induced in chicken cells. Effective variants induced large amounts of IFN and contained subpopulations with high ratios of defective-interfering particles:IFN-inducing particles (DIP:IFP). Ineffective variants induced less IFN and contained lower ratios of DIP:IFP. Unexpectedly, there was a reversal of phenotypes in mammalian cells. Variants that induced low amounts of IFN and had low DIP:IFP ratios in chicken cells were excellent IFN inducers with high DIP:IFP ratios in mammalian cells, and vice versa. The high DIP:IFP ratios and computer-simulated dynamics of infection suggested that DIP, as an individual particle, did not function as an IFP. The higher efficiency of IFPs in the presence of DIPs was attributed to reduced amounts of newly synthesized viral polymerase known to result from out-competition by defective-interfering RNAs, and the subsequent failure of that polymerase to turn-off cellular mRNA transcription-including IFN-mRNA.

Immunomodulatory therapy for severe influenza

Influenza A virus is a significant cause of morbidity and mortality worldwide. Severe influenza is recognized as a clinical syndrome, characterized by hyperinduction of proinflammatory cytokine production, otherwise known as hypercytokinemia or a 'cytokine storm'. Research focused on therapeutics to modulate influenza virus-induced inflammation is currently underway. In this review, we discuss the limitations of current antiviral drug treatment strategies, describe the influenza viral and host pathogenicity determinants, and present the evidence supporting the use of immunomodulatory therapy to target the host inflammatory response as a means to improve clinical outcome in severe influenza. We then review the experimental data on investigational immunomodulatory agents targeting the host inflammatory response in severe influenza, including anti-TNF therapy, statins, glucocorticoids, cyclooxygenase-2 inhibitors, macrolides, peroxisome proliferator-activated receptor agonists, AMP-activated protein kinase agonists and high mobility group box 1 antagonists. We then conclude with a rationale for the use of mesenchymal stromal (stem) cells and angiopoietin-1 therapy against deleterious influenza-induced host responses that mediate end-organ injury and dysfunction.

Comparative analyses of pandemic H1N1 and seasonal H1N1, H3N2, and influenza B infections depict distinct clinical pictures in ferrets

Influenza A and B infections are a worldwide health concern to both humans and animals. High genetic evolution rates of the influenza virus allow the constant emergence of new strains and cause illness variation. Since human influenza infections are often complicated by secondary factors such as age and underlying medical conditions, strain or subtype specific clinical features are difficult to assess. Here we infected ferrets with 13 currently circulating influenza strains (including strains of pandemic 2009 H1N1 [H1N1pdm] and seasonal A/H1N1, A/H3N2, and B viruses). The clinical parameters were measured daily for 14 days in stable environmental conditions to compare clinical characteristics. We found that H1N1pdm strains had a more severe physiological impact than all season strains where pandemic A/California/07/2009 was the most clinically pathogenic pandemic strain. The most serious illness among seasonal A/H1N1 and A/H3N2 groups was caused by A/Solomon Islands/03/2006 and A/Perth/16/2009, respectively. Among the 13 studied strains, B/Hubei-Wujiagang/158/2009 presented the mildest clinical symptoms. We have also discovered that disease severity (by clinical illness and histopathology) correlated with influenza specific antibody response but not viral replication in the upper respiratory tract. H1N1pdm induced the highest and most rapid antibody response followed by seasonal A/H3N2, seasonal A/H1N1 and seasonal influenza B (with B/Hubei-Wujiagang/158/2009 inducing the weakest response). Our study is the first to compare the clinical features of multiple circulating influenza strains in ferrets. These findings will help to characterize the clinical pictures of specific influenza strains as well as give insights into the development and administration of appropriate influenza therapeutics.

Efficacy of live attenuated vaccines against 2009 pandemic H1N1 influenza in ferrets

The advent of the H1N1 influenza pandemic (pH1N1) in 2009 triggered the rapid production of pandemic influenza vaccines, since seasonal influenza vaccines were expected and demonstrated not to provide significant cross-protection against the newly emerged pandemic virus. To increase vaccine production capacity and further evaluate the effectiveness of different candidate pandemic influenza vaccines, the World Health Organization stimulated the evaluation of different vaccination concepts including the use of live attenuated influenza vaccines (LAIVs). Therefore, we have immunized ferrets intranasally with a single dose of pH1N1-LAIV from different manufacturers. They all induced adequate serum HI antibody titers in the ferrets and protected them against intratracheal wild-type pH1N1 virus challenge: pH1N1 virus replication in the upper respiratory tract and lungs was reduced and no disease signs or severe broncho-interstitial pneumonia were observed in any of the vaccinated ferrets. These data together with the relatively efficient production process emphasize the potential of the LAIV concept for pandemic preparedness.

Animal models to assess the toxicity, immunogenicity and effectiveness of candidate influenza vaccines

INTRODUCTION

Every year, > 100 million doses of licensed influenza vaccine are administered worldwide, with relatively few serious adverse events reported. Initiatives to manufacture influenza vaccines on different platforms have come about to ensure timely production of strain-specific as well as universal vaccines. To prevent adverse events that may be associated with these new vaccines, it is important to evaluate the toxicity of new formulations in animal models.

AREAS COVERED

This review outlines preclinical studies that evaluate safety, immunogenicity and effectiveness of novel products to support further development and clinical trials. This has been done through a review of the latest literature describing vaccines under development.

EXPERT OPINION

The objective of preclinical safety tests is to demonstrate the absence of toxic contaminants and adventitious agents. Additional tests that characterize vaccine content more completely, or demonstrate the absence of exacerbated disease following virus challenge in vaccinated animals, may provide additional data to ensure the safety of new vaccine strategies.

Pulmonary pathology of pandemic influenza A/H1N1 virus (2009)-infected ferrets upon longitudinal evaluation by computed tomography

We investigated the development of pulmonary lesions in ferrets by means of computed tomography (CT) following infection with the 2009 pandemic A/H1N1 influenza virus and compared the scans with gross pathology, histopathology and immunohistochemistry. Ground-glass opacities observed by CT scanning in all infected lungs corresponded to areas of alveolar oedema at necropsy. These areas were most pronounced on day 3 and gradually decreased from days 4 to 7 post-infection. This pilot study shows that the non-invasive imaging procedure allows quantification and characterization of influenza-induced pulmonary lesions in living animals under biosafety level 3 conditions and can thus be used in pre-clinical pharmaceutical efficacy studies.

Reverse genetics plasmid for cloning unstable influenza A virus gene segments

Reverse genetics approaches that enable the generation of recombinant influenza A viruses entirely from plasmids are invaluable for studies on virus replication, morphogenesis, pathogenesis, or transmission. Furthermore, influenza virus reverse genetics is now critical for the development of new vaccines for this human and animal pathogen. Periodically, influenza gene segments are unstable within plasmids in bacteria. The PB2 gene segment of a highly pathogenic avian H5 influenza virus A/Turkey/Ontario/7732/1966 (Ty/Ont) was unstable in commonly available cloning plasmids (e.g., pcDNA3.1/V5-His-TOPO) and in standard influenza virus reverse genetics plasmids (e.g., pHH21), which contain high copy origins of replication. Thus, a low-copy influenza reverse genetics plasmid (pGJ3C3) was developed to enable rapid cloning of unstable influenza A virus genes using ligation-independent recombination-based cloning. The unstable Ty/Ont PB2 gene segment was efficiently cloned using the pGJ3C3 plasmid and this clone was used to rescue a recombinant Ty/Ont virus. This low copy reverse genetics plasmid will be useful for cloning other unstable segments of influenza A viruses in order to rescue recombinant viruses, which will facilitate basic studies and vaccine seed stock production.