Development of a Mouse-Adapted Live Attenuated Influenza Virus That Permits In Vivo Analysis of Enhancements to the Safety of Live Attenuated Influenza Virus Vaccine

The live attenuated influenza virus vaccine (LAIV) is preferentially recommended for use in persons 2 through 49 years of age but has not been approved for children under 2 or asthmatics due to safety concerns. Therefore, increasing safety is desirable. Here we describe a murine LAIV with reduced pathogenicity that retains lethality at high doses and further demonstrate that we can enhance safety in vivo through mutations within NS1. This model may permit preliminary safety analysis of improved LAIVs.     

In Vitro Analysis of Virus Particle Subpopulations in Candidate Live-Attenuated Influenza Vaccines Distinguishes Effective from Ineffective Vaccines

Two effective (vac⁺) and two ineffective (vac⁻) candidate live-attenuated influenza vaccines (LAIVs) derived from naturally selected genetically stable variants of A/TK/OR/71-delNS1[1-124] (H7N3) that differed only in the length and kind of amino acid residues at the C terminus of the nonstructural NS1 protein were analyzed for their content of particle subpopulations. These subpopulations included total physical particles (measured as hemagglutinating particles [HAPs]) with their subsumed biologically active particles of infectious virus (plaque-forming particles [PFPs]) and different classes of noninfectious virus, namely, interferon-inducing particles (IFPs), noninfectious cell-killing particles (niCKPs), and defective interfering particles (DIPs). The vac⁺ variants were distinguished from the vac⁻ variants on the basis of their content of viral subpopulations by (i) the capacity to induce higher quantum yields of interferon (IFN), (ii) the generation of an unusual type of IFN-induction dose-response curve, (iii) the presence of IFPs that induce IFN more efficiently, (iv) reduced sensitivity to IFN action, and (v) elevated rates of PFP replication that resulted in larger plaques and higher PFP and HAP titers. These in vitro analyses provide a benchmark for the screening of candidate LAIVs and their potential as effective vaccines. Vaccine design may be improved by enhancement of attributes that are dominant in the effective (vac⁺) vaccines.Live-attenuated vaccines are considered more effective than their inactive or single-component counterparts because they activate both the innate and adaptive immune systems and elicit responses to a broader range of antigens for longer periods of time (2, 10, 25, 28). Influenza virus variants with alterations in the reading frame of the nonstructural NS1 protein gene (delNS1), which express truncated NS1 proteins, characteristically induce enhanced yields of type I interferon (IFN) relative to the yields of their isogenic parental virus encoding full-length NS1 proteins (11, 13, 21, 33, 39). Many of these delNS1 variants have proved to be effective as live-attenuated influenza vaccines (LAIVs), providing protection against challenge virus in a broad range of species (33, 46), including chickens (39, 44). The IFN-inducing capacity of the virus is considered an important element in the effectiveness of LAIVs (33). In that context, influenza viruses are intrinsically sensitive to the antiviral action of IFN (31, 32, 36), although they may display a nongenetic-based transient resistance (36). In addition, IFN sensitizes cells to the initiation of apoptosis by viruses (42) and by double-stranded RNA (40), which may be spontaneously released in the course of influenza virus replication (14). Furthermore, IFN functions as an adjuvant to boost the adaptive immune response in mammals (3, 4, 11, 26, 41, 43, 46) and in chickens when administered perorally in the drinking water of influenza virus-infected birds (19). This raises the question: does the enhanced induction of IFN by delNS1 variants suffice to render an infectious influenza virus preparation sufficiently attenuated to function as an effective live vaccine? To address that question, we turned to a recent report that described the selection of several variants of influenza virus with a common backbone of A/TK/OR/71-SEPRL (Southeast Poultry Research Laboratory) that contained NS1 protein genes which were unusual in the length and nature of the amino acid residues at the C termini of the truncated NS1 proteins that they expressed because of the natural introduction of a frameshift and stop codon by the deletion in the NS1 protein gene (44). delNS1 variants were isolated from serial low-inoculum passages of TK/OR/71-delNS1[1-124] (H7N3) in eggs (44). Four of these genetically stable plaque-purified variants, each encoding a truncated NS1 protein of a particular length, were tested as a candidate LAIV in 2-week-old chickens. Two of the delNS1 variants were effective as live vaccines (double deletions [D-del] pc3 and pc4) (phenotypically vac⁺), and two were not (D-del pc1 and pc2) (phenotypically vac⁻) (44), despite only subtle differences in their encoded delNS1 proteins. Why were they phenotypically different?The present study addresses this question by analyzing and comparing the different virus particles that constitute the subpopulations of these two effective (vac⁺) and two ineffective (vac⁻) live vaccine candidates. These analyses are based on recent reports in which noninfectious but biologically active particles (niBAPs) in subpopulations of influenza virus particles were defined and quantified (20, 21, 29). The study described in this report reveals several quantitative and qualitative differences between the particle subpopulations of the four candidate LAIVs, including the different types of IFN-induction dose-response curves, the quantum (maximum) yields (QY) of IFN induced, the efficacy of the interferon-inducing particles (IFPs), the replication efficiency of the virus, and the size of the plaques that they produced. Evidence is presented that the in vitro analysis of virus particle subpopulations may be useful to distinguish vac⁺ from vac⁻ LAIV candidates and provide a basis for identifying and enhancing the performance of particles with desirable phenotypes.     

Attenuation of equine influenza viruses through truncations of the NS1 protein

Equine influenza is a common disease of the horse, causing significant morbidity worldwide. Here we describe the establishment of a plasmid-based reverse genetics system for equine influenza virus. Utilizing this system, we generated three mutant viruses encoding carboxy-terminally truncated NS1 proteins. We have previously shown that a recombinant human influenza virus lacking the NS1 gene (delNS1) could only replicate in interferon (IFN)-incompetent systems, suggesting that the NS1 protein is responsible for IFN antagonist activity. Contrary to previous findings with human influenza virus, we found that in the case of equine influenza virus, the length of the NS1 protein did not correlate with the level of attenuation of that virus. With equine influenza virus, the mutant virus with the shortest NS1 protein turned out to be the least attenuated. We speculate that the basis for attenuation of the equine NS1 mutant viruses generated is related to their level of NS1 protein expression. Our findings show that the recombinant mutant viruses are impaired in their ability to inhibit IFN production in vitro and they do not replicate as efficiently as the parental recombinant strain in embryonated hen eggs, in MDCK cells, or in vivo in a mouse model. Therefore, these attenuated mutant NS1 viruses may have potential as candidates for a live equine influenza vaccine.     

A GFP Expressing Influenza A Virus to Report In Vivo Tropism and Protection by a Matrix Protein 2 Ectodomain-Specific Monoclonal Antibody

The severity of influenza-related illness is mediated by many factors, including in vivo cell tropism, timing and magnitude of the immune response, and presence of pre-existing immunity. A direct way to study cell tropism and virus spread in vivo is with an influenza virus expressing a reporter gene. However, reporter gene-expressing influenza viruses are often attenuated in vivo and may be genetically unstable. Here, we describe the generation of an influenza A virus expressing GFP from a tri-cistronic NS segment. To reduce the size of this engineered gene segment, we used a truncated NS1 protein of 73 amino acids combined with a heterologous dimerization domain to increase protein stability. GFP and nuclear export protein coding information were fused in frame with the truncated NS1 open reading frame and separated from each other by 2A self-processing sites. The resulting PR8-NS1(1–73)GFP virus was successfully rescued and replicated as efficiently as the parental PR8 virus in vitro and was slightly attenuated in vivo. Flow cytometry-based monitoring of cells isolated from PR8-NS1(1–73)GFP virus infected BALB/c mice revealed that GFP expression peaked on day two in all cell types tested. In particular respiratory epithelial cells and myeloid cells known to be involved in antigen presentation, including dendritic cells (CD11c⁺) and inflammatory monocytes (CD11b⁺ GR1⁺), became GFP positive following infection. Prophylactic treatment with anti-M2e monoclonal antibody or oseltamivir reduced GFP expression in all cell types studied, demonstrating the usefulness of this reporter virus to analyze the efficacy of antiviral treatments in vivo. Finally, deep sequencing analysis, serial in vitro passages and ex vivo analysis of PR8-NS1(1–73)GFP virus, indicate that this virus is genetically and phenotypically stable.     

The contribution of systemic and pulmonary immune effectors to vaccine-induced protection from H5N1 influenza virus infection

Live attenuated influenza vaccines (LAIVs) are effective in providing protection against influenza challenge in animal models and in preventing disease in humans. We previously showed that LAIVs elicit a range of immune effectors and that successful induction of pulmonary cellular and humoral immunity in mice requires pulmonary replication of the vaccine virus. An upper respiratory tract immunization (URTI) model was developed in mice to mimic the human situation, in which the vaccine virus does not replicate in the lower respiratory tract, allowing us to assess the protective efficacy of an H5N1 LAIV against highly pathogenic H5N1 virus challenge in the absence of significant pulmonary immunity. Our results show that, after one dose of an H5N1 LAIV, pulmonary influenza-specific lymphocytes are the main contributors to clearance of challenge virus from the lungs and that contributions of influenza-specific enzyme-linked immunosorbent assay (ELISA) antibodies in serum and splenic CD8(+) T cells were negligible. Complete protection from H5N1 challenge was achieved after two doses of H5N1 LAIV and was associated with maturation of the antibody response. Although passive transfer of sera from mice that received two doses of vaccine prevented lethality in naive recipients following challenge, the mice showed significant weight loss, with high pulmonary titers of the H5N1 virus. These data highlight the importance of mucosal immunity in mediating optimal protection against H5N1 infection. Understanding the requirements for effective induction and establishment of these protective immune effectors in the respiratory tract paves the way for a more rational and effective vaccine approach in the future.     

A reassortment-incompetent live attenuated influenza virus vaccine for protection against pandemic virus strains

Although live-attenuated influenza vaccines (LAIV) are safe for use in protection against seasonal influenza strains, concerns regarding their potential to reassort with wild-type virus strains have been voiced. LAIVs have been demonstrated to induce enhanced mucosal and cell-mediated immunity better than inactivated vaccines while also requiring a smaller dose to achieve a protective immune response. To address the need for a reassortment-incompetent live influenza A virus vaccine, we have designed a chimeric virus that takes advantage of the fact that influenza A and B viruses do not reassort. Our novel vaccine prototype uses an attenuated influenza B virus that has been manipulated to express the ectodomain of the influenza A hemagglutinin protein, the major target for eliciting neutralizing antibodies. The hemagglutinin RNA segment is modified such that it contains influenza B packaging signals, and therefore it cannot be incorporated into a wild-type influenza A virus. We have applied our strategy to different influenza A virus subtypes and generated chimeric B/PR8 HA (H1), HK68 (H3), and VN (H5) viruses. All recombinant viruses were attenuated both in vitro and in vivo, and immunization with these recombinant viruses protected mice against lethal influenza A virus infection. Overall, our data indicate that the chimeric live-attenuated influenza B viruses expressing the modified influenza A hemagglutinin are effective LAIVs.     

Mutations in the NS1 protein of swine influenza virus impair anti-interferon activity and confer attenuation in pigs

It has been shown previously that the nonstructural protein NS1 of influenza virus is an alpha/beta interferon (IFN-alpha/beta) antagonist, both in vitro and in experimental animal model systems. However, evidence of this function in a natural host has not yet been obtained. Here we investigated the role of the NS1 protein in the virulence of a swine influenza virus (SIV) isolate in pigs by using reverse genetics. The virulent wild-type A/Swine/Texas/4199-2/98 (TX/98) virus and various mutants encoding carboxy-truncated NS1 proteins were rescued. Growth properties of TX/98 viruses with mutated NS1, induction of IFN in tissue culture, and virulence-attenuation in pigs were analyzed and compared to those of the recombinant wild-type TX/98 virus. Our results indicate that deletions in the NS1 protein decrease the ability of the TX/98 virus to prevent IFN-alpha/beta synthesis in pig cells. Moreover, all NS1 mutant viruses were attenuated in pigs, and this correlated with the amount of IFN-alpha/beta induced in vitro. These data suggest that the NS1 protein of SIV is a virulence factor. Due to their attenuation, NS1-mutated swine influenza viruses might have a great potential as live attenuated vaccine candidates against SIV infections of pigs.     

Contribution of NS1 Effector Domain Dimerization to Influenza A Virus Replication and Virulence

Conserved tryptophan-187 facilitates homodimerization of the influenza A virus NS1 protein effector domain. We generated a mutant influenza virus strain expressing NS1-W187R to destabilize this self-interaction. NS1-W187R protein exhibited lower double-stranded RNA (dsRNA)-binding activity, showed a temporal redistribution during infection, and was minimally compromised for interferon antagonism. The mutant virus replicated similarly to the wild type in vitro, but it was slightly attenuated for replication in mice, causing notably reduced morbidity and mortality. These data suggest biological relevance for the W187-mediated homotypic interaction of NS1.     

Transfectant Influenza A Viruses with Long Deletions in the NS1 Protein Grow Efficiently in Vero Cells

We established a reverse genetics system for the nonstructural (NS) gene segment of influenza A virus. This system is based on the use of the temperature-sensitive (ts) reassortant virus 25A-1. The 25A-1 virus contains the NS gene from influenza A/Leningrad/134/57 virus and the remaining gene segments from A/Puerto Rico (PR)/8/34 virus. This particular gene constellation was found to be responsible for the ts phenotype. For reverse genetics of the NS gene, a plasmid-derived NS gene from influenza A/PR/8/34 virus was ribonucleoprotein transfected into cells that were previously infected with the 25A-1 virus. Two subsequent passages of the transfection supernatant at 40°C selected viruses containing the transfected NS gene derived from A/PR/8/34 virus. The high efficiency of the selection process permitted the rescue of transfectant viruses with large deletions of the C-terminal part of the NS1 protein. Viable transfectant viruses containing the N-terminal 124, 80, or 38 amino acids of the NS1 protein were obtained. Whereas all deletion mutants grew to high titers in Vero cells, growth on Madin-Darby canine kidney (MDCK) cells and replication in mice decreased with increasing length of the deletions. In Vero cells expression levels of viral proteins of the deletion mutants were similar to those of the wild type. In contrast, in MDCK cells the level of the M1 protein was significantly reduced for the deletion mutants.     

Efficacy of live attenuated and inactivated influenza vaccines among children in rural India: A 2-year,randomized, triple-blind,placebo-controlled trial

BackgroundInfluenza is a cause of febrile acute respiratory infection (FARI) in India; however, few influenza vaccine trials have been conducted in India. We assessed absolute and relative efficacy of live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV) among children aged 2 to 10 years in rural India through a randomized, triple-blind, placebo-controlled trial conducted over 2 years.Methods and findingsIn June 2015, children were randomly allocated to LAIV, IIV, intranasal placebo, or inactivated polio vaccine (IPV) in a 2:2:1:1 ratio. In June 2016, vaccination was repeated per original allocation. Overall, 3,041 children received LAIV (n = 1,015), IIV (n = 1,010), nasal placebo (n = 507), or IPV (n = 509). Mean age of children was 6.5 years with 20% aged 9 to 10 years.Through weekly home visits, nasal and throat swabs were collected from children with FARI and tested for influenza virus by polymerase chain reaction. The primary outcome was laboratory-confirmed influenza-associated FARI; vaccine efficacy (VE) was calculated using modified intention-to-treat (mITT) analysis by Cox proportional hazards model (PH) for each year.In Year 1, VE was 40.0% (95% confidence interval (CI) 25.2 to 51.9) for LAIV and 59.0% (95% CI 47.8 to 67.9) for IIV compared with controls; relative efficacy of LAIV compared with IIV was −46.2% (95% CI −88.9 to −13.1). In Year 2, VE was 51.9% (95% CI 42.0 to 60.1) for LAIV and 49.9% (95% CI 39.2 to 58.7) for IIV; relative efficacy of LAIV compared with IIV was 4.2% (95% CI −19.9 to 23.5). No serious adverse vaccine-attributable events were reported. Study limitations include differing dosage requirements for children between nasal and injectable vaccines (single dose of LAIV versus 2 doses of IIV) in Year 1 and the fact that immunogenicity studies were not conducted.ConclusionsIn this study, we found that LAIV and IIV vaccines were safe and moderately efficacious against influenza virus infection among Indian children.Trial registrationClinical Trials Registry of India CTRI/2015/06/005902.

Anand Krishnan and co-workers study the efficacy and safety of influenza vaccines for children in India.     

Functional replacement of the carboxy-terminal two-thirds of the influenza A virus NS1 protein with short heterologous dimerization domains

The NS1 protein of influenza A/WSN/33 virus is a 230-amino-acid-long protein which functions as an interferon alpha/beta (IFN-alpha/beta) antagonist by preventing the synthesis of IFN during viral infection. In tissue culture, the IFN inhibitory function of the NS1 protein has been mapped to the RNA binding domain, the first 73 amino acids. Nevertheless, influenza viruses expressing carboxy-terminally truncated NS1 proteins are attenuated in mice. Dimerization of the NS1 protein has previously been shown to be essential for its RNA binding activity. We have explored the ability of heterologous dimerization domains to functionally substitute in vivo for the carboxy-terminal domains of the NS1 protein. Recombinant influenza viruses were generated that expressed truncated NS1 proteins of 126 amino acids, fused to 28 or 24 amino acids derived from the dimerization domains of either the Saccharomyces cerevisiae PUT3 or the Drosophila melanogaster Ncd (DmNcd) proteins. These viruses regained virulence and lethality in mice. Moreover, a recombinant influenza virus expressing only the first 73 amino acids of the NS1 protein was able to replicate in mice lacking three IFN-regulated antiviral enzymes, PKR, RNaseL, and Mx, but not in wild-type (Mx-deficient) mice, suggesting that the attenuation was mainly due to an inability to inhibit the IFN system. Remarkably, a virus with an NS1 truncated at amino acid 73 but fused to the dimerization domain of DmNcd replicated and was also highly pathogenic in wild-type mice. These results suggest that the main biological function of the carboxy-terminal region of the NS1 protein of influenza A virus is the enhancement of its IFN antagonist properties by stabilizing the NS1 dimeric structure.     

A Single Amino Acid Substitution in the Novel H7N9 Influenza A Virus NS1 Protein Increases CPSF30 Binding and Virulence

Although an effective interferon antagonist in human and avian cells, the novel H7N9 influenza virus NS1 protein is defective at inhibiting CPSF30. An I106M substitution in H7N9 NS1 can restore CPSF30 binding together with the ability to block host gene expression. Furthermore, a recombinant virus expressing H7N9 NS1-I106M replicates to higher titers in vivo, and is subtly more virulent, than the parental virus. Natural polymorphisms in H7N9 NS1 that enhance CPSF30 binding may be cause for concern.     

PDlim2 selectively interacts with the PDZ binding motif of highly pathogenic avian H5N1 influenza A virus NS1

The multi-functional NS1 protein of influenza A virus is a viral virulence determining factor. The last four residues at the C-terminus of NS1 constitute a type I PDZ domain binding motif (PBM). Avian influenza viruses currently in circulation carry an NS1 PBM with consensus sequence ESEV, whereas human influenza viruses bear an NS1 PBM with consensus sequence RSKV or RSEV. The PBM sequence of the influenza A virus NS1 is reported to contribute to high viral pathogenicity in animal studies. Here, we report the identification of PDlim2 as a novel binding target of the highly pathogenic avian influenza virus H5N1 strain with an NS1 PBM of ESEV (A/Chicken/Henan/12/2004/H5N1, HN12-NS1) by yeast two-hybrid screening. The interaction was confirmed by in vitro GST pull-down assays, as well as by in vivo mammalian two-hybrid assays and bimolecular fluorescence complementation assays. The binding was also confirmed to be mediated by the interaction of the PDlim2 PDZ domain with the NS1 PBM motif. Interestingly, our assays showed that PDlim2 bound specifically with HN12-NS1, but exhibited no binding to NS1 from a human influenza H1N1 virus bearing an RSEV PBM (A/Puerto Rico/8/34/H1N1, PR8-NS1). A crystal structure of the PDlim2 PDZ domain fused with the C-terminal hexapeptide from HN12-NS1, together with GST pull-down assays on PDlim2 mutants, reveals that residues Arg16 and Lys31 of PDlim2 are critical for the binding between PDlim2 and HN12-NS1. The identification of a selective binding target of HN12-NS1 (ESEV), but not PR8-NS1 (RSEV), enables us to propose a structural mechanism for the interaction between NS1 PBM and PDlim2 or other PDZ-containing proteins.     

Generation of Live Attenuated Novel Influenza Virus A/California/7/09 (H1N1) Vaccines with High Yield in Embryonated Chicken Eggs

Several live attenuated influenza virus A/California/7/09 (H1N1) (CA09) candidate vaccine variants that possess the hemagglutinin (HA) and neuraminidase (NA) gene segments from the CA09 virus and six internal protein gene segments from the cold-adapted influenza virus A/Ann Arbor/6/60 (H2N2) virus were generated by reverse genetics. The reassortant viruses replicated relatively poorly in embryonated chicken eggs. To improve virus growth in eggs, reassortants expressing the HA and NA of CA09 were passaged in MDCK cells and variants exhibiting large-plaque morphology were isolated. These variants replicated at levels approximately 10-fold higher than the rate of replication of the parental strains in embryonated chicken eggs. Sequence analysis indicated that single amino acid changes at positions 119, 153, 154, and 186 were responsible for the improved growth properties in MDCK cells and eggs. In addition, the introduction of a mutation at residue 155 that was previously shown to enhance the replication of a 1976 swine influenza virus also significantly improved the replication of the CA09 virus in eggs. Each variant was further evaluated for receptor binding preference, antigenicity, attenuation phenotype, and immunogenicity. Mutations at residues 153, 154, and 155 drastically reduced viral antigenicity, which made these mutants unsuitable as vaccine candidates. However, changes at residues 119 and 186 did not affect virus antigenicity or immunogenicity, justifying their inclusion in live attenuated vaccine candidates to protect against the currently circulating 2009 swine origin H1N1 viruses.Human infections with the swine origin influenza virus A (H1N1) were first detected in April 2009 and spread across the globe, resulting in WHO declaring a pandemic on 12 June 2009 for the first time in the past 41 years. More than 296,471 people have had confirmed infections with this novel H1N1 virus, and there have been at least 3,486 deaths as of September 18, 2009. In the last century, an influenza H1N1 virus caused the devastating 1918-1919 pandemic; this pandemic was characterized by a mild outbreak in the spring of 1918, followed by a lethal wave globally in the fall of that year which killed as many as 50 million people worldwide (20, 29). The 2009 H1N1 viruses circulating globally since April 2009 have not caused a significant rise in mortality related to influenza. Nucleotide sequence analysis suggested that E627 in PB2, a deletion of the PDZ ligand domain in NS1, and the lack of the PB1-F2 open reading frame in the 2009 H1N1 viruses may contribute to the relatively mild virulence (20, 26, 27). Recent animal studies have shown that the 2009 H1N1 influenza viruses did not replicate in tissues beyond the respiratory tract and did not cause significant mortality in the ferret model; however, the 2009 H1N1 viruses are capable of infecting deep in the lung tissues and caused more significant lesions in the lung tissues of animals, including nonhuman primates, than typical seasonal strains (13, 17, 19). Children and young adults are particularly susceptible to the 2009 H1N1 virus infection because they have no or low immunity to the novel 2009 H1N1 strains (11, 13). The widespread and rapid distribution of the 2009 H1N1 viruses in humans raises a concern about the evolution of more virulent strains during passage in the population. One fear is that mutant forms of the 2009 H1N1 viruses may exhibit significantly increased virulence (2, 19). Therefore, there is an urgent need to develop an effective vaccine to control the influenza pandemic caused by the swine origin H1N1 viruses.Live attenuated influenza vaccine (LAIV) has been licensed in the United States annually since 2003. The seasonal vaccine protects against influenza illness and elicits both systemic and mucosal immune responses, including serum hemagglutination inhibition (HAI) antibodies that react to antigenically drifted strains (3, 4). A critical attribute of an effective pandemic vaccine is its capability to elicit an immune response in immunonaive individuals; LAIV has been shown to offer protection following a single dose in young children. However, two doses of vaccines are recommended for children younger than 9 years of age who have never been immunized with influenza vaccines. In order to produce LAIV to protect against the newly emerged swine origin H1N1 influenza virus, we have produced several 6:2 reassortant candidate vaccine strains that express the hemagglutinin (HA) and neuraminidase (NA) gene segments from influenza virus A/California/4/09 (A/CA/4/09) (H1N1) or A/CA/7/09 (H1N1), as well as the six internal protein gene segments (PB1, PB2, PA, NP, M, and NS) from cold-adapted A/Ann Arbor/6/60 (H2N2) (AA60) virus, which is the master donor virus for all influenza virus A strains in trivalent seasonal LAIV. Initial evaluation of these candidate vaccine strains indicated that they did not replicate as efficiently as seasonal H1N1 influenza vaccine strains in embryonated chicken eggs. In this report, we describe directed modifications of the HA gene segment that improved vaccine yields in eggs, resulting in a number of vaccine candidates that are available for human use.     

The N- and C-terminal domains of the NS1 protein of influenza B virus can independently inhibit IRF-3 and beta interferon promoter activation

The NS1 proteins of influenza A and B viruses (A/NS1 and B/NS1 proteins) have only approximately 20% amino acid sequence identity. Nevertheless, these proteins show several functional similarities, such as their ability to bind to the same RNA targets and to inhibit the activation of protein kinase R in vitro. A critical function of the A/NS1 protein is the inhibition of synthesis of alpha/beta interferon (IFN-alpha/beta) during viral infection. Recently, it was also found that the B/NS1 protein inhibits IFN-alpha/beta synthesis in virus-infected cells. We have now found that the expression of the B/NS1 protein complements the growth of an influenza A virus with A/NS1 deleted. Expression of the full-length B/NS1 protein (281 amino acids), as well as either its N-terminal RNA-binding domain (amino acids 1 to 93) or C-terminal domain (amino acids 94 to 281), in the absence of any other influenza B virus proteins resulted in the inhibition of IRF-3 nuclear translocation and IFN-beta promoter activation. A mutational analysis of the truncated B/NS1(1-93) protein showed that RNA-binding activity correlated with IFN-beta promoter inhibition. In addition, a recombinant influenza B virus with NS1 deleted induces higher levels of IRF-3 activation, as determined by its nuclear translocation, and of IFN-alpha/beta synthesis than wild-type influenza B virus. Our results support the hypothesis that the NS1 protein of influenza B virus plays an important role in antagonizing the IRF-3- and IFN-induced antiviral host responses to virus infection.     

Identification of a Highly Conserved Epitope on Avian Influenza Virus Non-Structural Protein 1 Using a Peptide Microarray

Avian influenza virus (AIV) non-structural protein 1 (NS1) is a multifunctional protein. It is present at high levels in infected cells and can be used for AIV detection and diagnosis. In this study, we generated monoclonal antibody (MAb) D7 against AIV NS1 protein by immunization of BALB/c mice with purified recombinant NS1 protein expressed in Escherichia coli. Isotype determination revealed that the MAb was IgG₁/κ-type subclass. To identify the epitope of the MAb D7, the NS1 protein was truncated into a total of 225 15-mer peptides with 14 amino acid overlaps, which were spotted for a peptide microarray. The results revealed that the MAb D7 recognized the consensus DAPF motif. Furthermore, the AIV NS1 protein with the DAPF motif deletion was transiently expressed in 293T cells and failed to react with MAb D7. Subsequently, the DAPF motif was synthesized with an elongated GSGS linker at both the C- and N-termini. The MAb D7 reacted with the synthesized peptide both in enzyme-linked immunosorbent assay (ELISA) and dot-blot assays. From these results, we concluded that DAPF motif is the epitope of MAb D7. To our knowledge, this is the first report of a 4-mer epitope on the NS1 protein of AIV that can be recognized by MAb using a peptide microarray, which is able to simplify epitope identification, and that could serve as the basis for immune responses against avian influenza.     

Cellular immune responses in children and adults receiving inactivated or live attenuated influenza vaccines

The patterns of cellular immune responses induced by live attenuated influenza vaccine (LAIV) versus those of the trivalent inactivated influenza vaccine (TIV) have not been studied extensively, especially in children. The goals of this study were to evaluate the effects of TIV and LAIV immunization on cellular immunity to live influenza A virus in children and adults and to explore factors associated with variations in responses to influenza vaccines among individuals. A gamma interferon (IFN-gamma) flow cytometry assay was used to measure IFN-gamma-producing (IFN-gamma+) NK and T cells in peripheral blood mononuclear cell cultures stimulated with a live influenza A virus strain before and after LAIV or TIV immunization of children and adults. The mean percentages of influenza A virus-specific IFN-gamma+ CD4 and CD8 T cells increased significantly after LAIV, but not TIV, immunization in children aged 5 to 9 years. No increases in the mean levels of influenza A virus-reactive IFN-gamma+ T cells and NK cells were observed in adults given LAIV or TIV. TIV induced a significant increase in influenza A virus-reactive T cells in 6-month- to 4-year-old children; LAIV was not evaluated in this age group. The postvaccination changes (n-fold) in the percentages of influenza A virus-reactive IFN-gamma+ T and NK cells in adults were highly variable and correlated inversely with the prevaccination percentages, in particular with that of the CD56(dim) NK cell subset. In conclusion, our findings identify age, type of vaccine, and prevaccination levels of immune reactivity to influenza A virus as factors significantly associated with the magnitude of cellular immune responses to influenza vaccines.