Extending the cytoplasmic tail of the influenza a virus M2 protein leads to reduced virus replication in vivo but not in vitro

A comparison of RSV and influenza in vitro kinetic parameters reveals differences in infecting time

Influenza and respiratory syncytial virus (RSV) cause acute infections of the respiratory tract. Since the viruses both cause illnesses with similar symptoms, researchers often try to apply knowledge gleaned from study of one virus to the other virus. This can be an effective and efficient strategy for understanding viral dynamics or developing treatment strategies, but only if we have a full understanding of the similarities and differences between the two viruses. This study used mathematical modeling to quantitatively compare the viral kinetics of in vitro RSV and influenza virus infections. Specifically, we determined the viral kinetics parameters for RSV A2 and three strains of influenza virus, A/WSN/33 (H1N1), A/Puerto Rico/8/1934 (H1N1), and pandemic H1N1 influenza virus. We found that RSV viral titer increases at a slower rate and reaches its peak value later than influenza virus. Our analysis indicated that the slower increase of RSV viral titer is caused by slower spreading of the virus from one cell to another. These results provide estimates of dynamical differences between influenza virus and RSV and help provide insight into the virus-host interactions that cause observed differences in the time courses of the two illnesses in patients.

Defining Viral Defective Ribosomal Products: Standard and Alternative Translation Initiation Events Generate a Common Peptide from Influenza A Virus M2 and M1 mRNAs

Influenza A virus gene segment 7 encodes two proteins: the M1 protein translated from unspliced mRNA and the M2 protein produced by mRNA splicing and largely encoded by the M1 +1 reading frame. To better understand the generation of defective ribosomal products relevant to MHC class I Ag presentation, we engineered influenza A virus gene segment 7 to encode the model H-2 K(b) class I peptide ligand SIINFEKL at the M2 protein C terminus. Remarkably, after treating virus-infected cells with the RNA splicing inhibitor spliceostatin A to prevent M2 mRNA generation, K(b)-SIINFEKL complexes were still presented on the cell surface at levels ≤60% of untreated cells. Three key findings indicate that SIINFEKL is produced by cytoplasmic translation of unspliced M1 mRNA initiating at CUG codons within the +1 reading frame: 1) synonymous mutation of CUG codons in the M2-reading frame reduced K(b)-SIINFEKL generation; 2) K(b)-SIINFEKL generation was not affected by drug-mediated inhibition of AUG-initiated M1 synthesis; and 3) K(b)-SIINFEKL was generated in vitro and in vivo from mRNA synthesized in the cytoplasm by vaccinia virus, and hence cannot be spliced. These findings define a viral defective ribosomal product generated by cytoplasmic noncanonical translation and demonstrate the participation of CUG-codon-based translation initiation in pathogen immunosurveillance.

Recombinant influenza A H3N2 viruses with mutations of HA transmembrane cysteines exhibited altered virological characteristics

Influenza A H3N2 virus as the cause of 1968 pandemic has since been circulating in human and swine. Our earlier study has shown that mutations of one or two cysteines in the transmembrane domain of H3 hemagglutinin (HA) affected the thermal stability and fusion activity of recombinant HA proteins. Here, we report the successful generation of three recombinant H3N2 mutant viruses (C540S, C544L, and 2C/SL) with mutations of one or two transmembrane cysteines of HA in the background of A/swine/Guangdong/01/98 [H3N2] using reverse genetics, indicating that the mutated cysteines were not essential for virus assembly and growth. Further characterization revealed that recombinant H3N2 mutant viruses exhibited larger plaque sizes, increased growth rate in cells, enhanced fusion activity, reduced thermal and acidic resistances, and increased virulence in embryonated eggs. These results demonstrated that the transmembrane cysteines (C540 and C544) in H3 HA have profound effects on the virological features of H3N2 viruses.

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.

Mechanisms of sex disparities in influenza pathogenesis

Epidemiological evidence from influenza outbreaks and pandemics reveals that morbidity and mortality are often higher for women than men. Sex differences in the outcome of influenza are age-dependent, often being most pronounced among adults of reproductive ages (18-49 years of age) and sometimes reflecting the unique state of pregnancy in females, which is a risk factor for severe disease. Small animal models of influenza virus infection illustrate that inflammatory immune responses also differ between the sexes and impact the outcome of infection, with females generating higher proinflammatory cytokine and chemokine responses and experiencing greater morbidity and mortality than males. Males and females also respond differently to influenza vaccines, with women initiating higher humoral immune responses but experiencing more adverse reactions to seasonal influenza vaccines than men. Small animal models further show that elevated immunity following vaccination in females leads to greater cross-protection against novel influenza viruses in females compared with males. Sex steroid hormones, including estradiol and testosterone, as well as genetic differences between the sexes may play roles in modulating sex differences in immune responses to influenza virus infection and vaccination. Future studies must elucidate the pathways and cellular responses that differ between the sexes and determine how best to use this knowledge to inform public health policy-makers about prophylaxis and therapeutic treatments of influenza virus infections to ensure adequate protection in both males and females.

Antibody responses and cross protection against lethal influenza A viruses differ between the sexes in C57BL/6 mice

A mouse model was used to determine if protective immunity to influenza A virus infection differs between the sexes. The median lethal dose of H1N1 or H3N2 was lower for naïve females than males. After a sublethal, primary infection with H1N1 or H3N2, females and males showed a similar transient morbidity, but females generated more neutralizing and total anti-influenza A virus antibodies. Immunized males and females showed similar protection against secondary challenge with a homologous virus, but males experienced greater morbidity and had higher lung viral titers after infection with a lethal dose of heterologous virus. Females develop stronger humoral immune responses and greater cross protection against heterosubtypic virus challenge.

The cholesterol recognition/interaction amino acid consensus motif of the influenza A virus M2 protein is not required for virus replication but contributes to virulence

Influenza A virus particles assemble and bud from plasma membrane domains enriched with the viral glycoproteins but only a small fraction of the total M2 protein is incorporated into virus particles when compared to the other viral glycoproteins. A membrane proximal cholesterol recognition/interaction amino acid consensus (CRAC) motif was previously identified in M2 and suggested to play a role in protein function. We investigated the importance of the CRAC motif on virus replication by generating recombinant proteins and viruses containing amino acid substitutions in this motif. Alteration or completion of the M2 CRAC motif in two different virus strains caused no changes in virus replication in vitro. Viruses lacking an M2 CRAC motif had decreased morbidity and mortality in the mouse model of infection, suggesting that this motif is a virulence determinant which may facilitate virus replication in vivo but is not required for basic virus replication in tissue culture.

Cellular networks involved in the influenza virus life cycle

Influenza viruses cause epidemics and pandemics. Like all viruses, influenza viruses rely on the host cellular machinery to support their life cycle. Accordingly, identification of the host functions co-opted for viral replication is of interest to understand the mechanisms of the virus life cycle and to find new targets for the development of antiviral compounds. Among the various approaches used to explore host factor involvement in the influenza virus replication cycle, perhaps the most powerful is RNAi-based genome-wide screening, which has shed new light on the search for host factors involved in virus replication. In this review, we examine the cellular genes identified to date as important for influenza virus replication in genome-wide screens, assess pathways that were repeatedly identified in these studies, and discuss how these pathways might be involved in the individual steps of influenza virus replication, ultimately leading to a comprehensive understanding of the virus life cycle.

Influenza virus assays based on virus-inducible reporter cell lines

BACKGROUND

Virus-inducible reporter genes have been used as the basis of virus detection and quantitation assays for a number of viruses. A strategy for influenza A virus-induction of a reporter gene was recently described. In this report, we describe the extension of this strategy to influenza B virus, the generation of stable cell lines with influenza A and B virus-inducible reporter genes, and the use of these cells in various clinically relevant viral assays. Each of the cell lines described herein constitutively express an RNA transcript that contains a reporter gene coding region flanked by viral 5￠- and 3￠-untranslated regions (UTR) and therefore mimics an influenza virus genomic segment. Upon infection of the cells with influenza virus the virus-inducible reporter gene segment (VIRGS) is replicated and transcribed by the viral polymerase complex resulting in reporter gene expression.

FINDINGS

Reporter gene induction occurs after infection with a number of laboratory strains and clinical isolates of influenza virus including several H5N1 strains. The induction is dose-dependent and highly specific for influenza A or influenza B viruses.

CONCLUSIONS

These cell lines provide the basis of simple, rapid, and objective assays that involve virus quantitation such as determination of viral titer, assessment of antiviral susceptibility, and determination of antibody neutralization titer. These cell lines could be very useful for influenza virus researchers and vaccine manufacturers.

The cytoplasmic domain of influenza M2 protein interacts with caveolin-1

The cytoplasmic domain of influenza M2 protein (M2c) consists of 54 amino acid (aa) residues from aa44 to aa97. In this paper, M2c and its deletion mutant M2c(delta47-55) were expressed using prokaryotic expression system. First, glutaraldehyde crosslinking assay showed that M2c had multimerization potential mediated by aa47-55. Then, M2c, instead of M2c(delta47-55), directed eGFP from the whole cell localization to a predominately perinuclear region in CHO cells, which indicated that aa47-55 of M2c mediated the localization. Moreover, M2c colocalized with caveolin-1 (Cav) when CHO cells were cotransfected with Cav. A caveolin-1 binding motif phixxxxphixxphi (phi represents aromatic amino acid residues) in aa47-55 of M2c was found by sequence alignment and analysis. Further overlay ELISA result showed that M2c, but not M2c(delta47-55), bound to prokaryotically expressed cholesterol-free Cav(2-101), which illustrated the interaction could be cholesterol-independent. That was the first report of cellular protein bound to M2c.

Palmitoylation of the influenza A virus M2 protein is not required for virus replication in vitro but contributes to virus virulence

The influenza A virus M2 protein has important roles during virus entry and in the assembly of infectious virus particles. The cytoplasmic tail of the protein can be palmitoylated at a cysteine residue, but this residue is not conserved in a number of human influenza A virus isolates. Recombinant viruses encoding M2 proteins with a serine substituted for the cysteine at position 50 were generated in the A/WSN/33 (H1N1) and A/Udorn/72 (H3N2) genetic backgrounds. The recombinant viruses were not attenuated for replication in MDCK cells, Calu-3 cells, or in primary differentiated murine trachea epithelial cell cultures, indicating there was no significant contribution of M2 palmitoylation to virus replication in vitro. The A/WSN/33 M2C50S virus displayed a slightly reduced virulence after infection of mice, suggesting that there may be novel functions for M2 palmitoylation during in vivo infection.