Influenza A virus lacking the NS1 gene replicates in interferon-deficient systems

V and C proteins of measles virus function as virulence factors in vivo

The measles virus (MV) P gene encodes three proteins: the P protein and two nonstructural proteins, C and V. Because the functions of both the C and V protein are unknown, we used MV C (C-) and V (V-) deletion recombinants generated by the MV reverse genetics system (F. Radecke, P. Spielhofer, H. Schnieder, K. Kaelin, M. Huber, C. Dotsch, G. Christiansen, and M. A. Billeter 1995. EMBO J. 14, 5773-5784). Compared to parental vaccine strain, Edmonston (Ed) MV, both had normal growth and cytopathic effects in Vero cells and showed similar growth kinetics in human neuroblastoma SK-N-MC cells and in primary mouse neurons expressing the MV receptor, CD46. However, in vivo, using YAC-CD46 transgenic mice as a model for MV induced CNS disease (M. B. A. Oldstone, H. Lewicki, D. Thomas, A. Tishon, S. Dales, J. Patterson, M. Manchester, D. Homann, D. Naniche, and A. Holz 1999. Cell 98, 629-640), C- and V- viruses differed markedly from wt Ed(V(+)C(+)) virus. Newborn mice inoculated with as little as 10(3) PFU of Ed strain became ill and died after 10-15 days. In contrast, those inoculated with 10(3) or 10(4) PFU of MV C- or MV V- showed significantly fewer and milder clinical symptoms and had a lower mortality. A total of 10(5) PFU V- virus were required to kill most YAC-CD46 mice, and less than half (44%) were killed with a corresponding dose of MV C-. Immunohistochemical staining for MV antigens showed similar extents of spread for MV C- and MV Ed but restricted spread for MV V- throughout the brain. Viral load and transcription were markedly reduced for V- but not for C-. Multiple cytokines and chemokines were equivalently upregulated for all three viruses. Therefore, MV C and V proteins encode virulence functions in vivo and likely operate via separate mechanisms.

Polyuridylated mRNA synthesized by a recombinant influenza virus is defective in nuclear export

The poly(A) tail of influenza virus mRNA is synthesized by reiterative copying of a U track near the 5' end of the virion RNA (vRNA) template by the viral RNA polymerase. We have engineered a novel influenza A/WSN/33 virus which contains a neuraminidase (NA) vRNA with its U track mutated into an A track. Instead of synthesizing poly(A)-tailed NA mRNA, this novel virus synthesizes poly(U)-tailed NA mRNA. In infected cells, most poly(U)-tailed NA mRNA was retained in the nucleus, while most control polyadenylated NA mRNA was transported to the cytoplasm. These results suggest that the poly(A) tail is important for efficient nuclear export of NA mRNA. The mutant virus produced a reduced amount of NA and showed an attenuated phenotype, suggesting that poly(A) signal mutants of this type might be useful as potential live attenuated virus vaccines. In addition, this virus mutant might provide a useful model to further elucidate the basic mechanisms of mRNA nuclear export.

Rescue of influenza A virus from recombinant DNA

We have rescued influenza A virus by transfection of 12 plasmids into Vero cells. The eight individual negative-sense genomic viral RNAs were transcribed from plasmids containing human RNA polymerase I promoter and hepatitis delta virus ribozyme sequences. The three influenza virus polymerase proteins and the nucleoprotein were expressed from protein expression plasmids. This plasmid-based reverse genetics technique facilitates the generation of recombinant influenza viruses containing specific mutations in their genes.

Sendai virus C proteins counteract the interferon-mediated induction of an antiviral state

We have studied the relationship between the Sendai virus (SeV) C proteins (a nested set of four proteins initiated at different start codons) and the interferon (IFN)-mediated antiviral response in IFN-competent cells in culture. SeV strains containing wild-type or various mutant C proteins were examined for their ability (i) to induce an antiviral state (i.e., to prevent the growth of vesicular stomatitis virus [VSV] following a period of SeV infection), (ii) to induce the elevation of Stat1 protein levels, and (iii) to prevent IFN added concomitant with the SeV infection from inducing an antiviral state. We find that expression of the wild-type C gene and, specifically, the AUG114-initiated C protein prevents the establishment of an antiviral state: i.e., cells infected with wild-type SeV exhibited little or no increase in Stat1 levels and were permissive for VSV replication, even in the presence of exogenous IFN. In contrast, in cells infected with SeV lacking the AUG114-initiated C protein or containing a single amino acid substitution in the C protein, the level of Stat1 increased and VSV replication was inhibited. The prevention of the cellular IFN-mediated antiviral response appears to be a key determinant of SeV pathogenicity.

Approaches and strategies for the treatment of influenza virus infections

Influenza A and B viruses belong to the Orthomyxoviridae family of viruses. These viruses are responsible for severe morbidity and significant excess mortality each year. Infection with influenza viruses usually leads to respiratory involvement and can result in pneumonia and secondary bacterial infections. Vaccine approaches to the prophylaxis of influenza virus infections have been problematic owing to the ability of these viruses to undergo antigenic shift by exchanging genomic segments or by undergoing antigenic drift, consisting of point mutations in the haemagglutinin (HA) and neuraminidase (NA) genes as a result of an error-prone viral polymerase. Historically, antiviral approaches for the therapy of both influenza A and B viruses have been largely unsuccessful until the elucidation of the X-ray crystallographic structure of the viral NA, which has permitted structure-based drug design of inhibitors of this enzyme. In addition, recent advances in the elucidation of the structure and complex function of influenza HA have resulted in the discovery of a number of diverse compounds that target this viral protein. This review article will focus largely on newer antiviral agents including those that inhibit the influenza virus NA and HA. Other novel approaches that have entered clinical trials or been considered for their clinical utility will be mentioned.

Interaction of influenza virus NS1 protein and the human homologue of Staufen in vivo and in vitro

A screening for human proteins capable of interacting with influenza virus NS1 has been carried out using the two-hybrid genetic trap in yeast. A cDNA corresponding to the human homologue of Drosophila melanogaster Staufen protein (hStaufen) was isolated that fulfilled all genetic controls of the two-hybrid protocol. Using a hStaufen cDNA isolated from a lambda human library, the interaction of hStaufen and NS1 proteins was characterised in vivo and in vitro. Co-transfection of NS1 cDNA and a partial cDNA of hStaufen led to the relocalisation of recombinant hStaufen protein from its normal accumulation site in the cytoplasm to the nuclear location of NS1 protein. NS1 and hStaufen proteins could be co-immunoprecipitated from extracts of co-transfected cells and from mixtures of extracts containing either protein, as well as from extracts of influenza virus-infected cells. Furthermore, both proteins co-localised in the ribosomal and polysomal fractions of influenza virus-infected cells. The interaction was also detected in pull-down experiments using a resin containing purified hStaufen and NS1 protein translated in vitro. Deletion mapping of the NS1 gene indicated that a mutant protein containing the N-terminal 81 amino acids is unable to interact with hStaufen, in spite of retaining full RNA-binding capacity. These results are discussed in relation to the possible mechanisms of action of hStaufen and its relevance for influenza virus infection.

Learning from our foes: a novel vaccine concept for influenza virus

Concerted efforts to study the molecular biology of influenza viruses and the ability to genetically engineer them have dramatically advanced our understanding of the functions of influenza viral genes and gene products. The only nonstructural protein (NS1) coded for by the influenza virus was shown to possess interferon antagonist activity and thus to play an important role in countering the interferon (antiviral) response of the host following infection. Influenza A and B virus mutants with "weak" anti-interferon activity are highly attenuated because the host is able to mount an effective interferon response. It is suggested that these NS1-modified attenuated influenza viruses can induce a protective immune response and that they are ideal live virus vaccine candidates against influenza.