Immunologic studies on the influenza A virus nonstructural protein NS1

Novel influenza virus NS1 antagonists block replication and restore innate immune function

The innate immune system guards against virus infection through a variety of mechanisms including mobilization of the host interferon system, which attacks viral products mainly at a posttranscriptional level. The influenza virus NS1 protein is a multifunctional facilitator of virus replication, one of whose actions is to antagonize the interferon response. Since NS1 is required for efficient virus replication, it was reasoned that chemical inhibitors of this protein could be used to further understand virus-host interactions and also serve as potential new antiviral agents. A yeast-based assay was developed to identify compounds that phenotypically suppress NS1 function. Several such compounds exhibited significant activity specifically against influenza A virus in cell culture but had no effect on the replication of another RNA virus, respiratory syncytial virus. Interestingly, cells lacking an interferon response were drug resistant, suggesting that the compounds block interactions between NS1 and the interferon system. Accordingly, the compounds reversed the inhibition of beta interferon mRNA induction during infection, which is known to be caused by NS1. In addition, the compounds blocked the ability of NS1 protein to inhibit double-stranded RNA-dependent activation of a transfected beta interferon promoter construct. The effects of the compounds were specific to NS1, because they had no effect on the ability of the severe acute respiratory syndrome coronavirus papainlike protease protein to block beta interferon promoter activation. These data demonstrate that the function of NS1 can be modulated by chemical inhibitors and that such inhibitors will be useful as probes of biological function and as starting points for clinical drug development.

Expression of the nonstructural protein NS1 of equine influenza A virus: detection of anti-NS1 antibody in post infection equine sera

The nucleotide sequence of the nonstructural protein NS1 of the influenza virus A/equine 2/Suffolk/89 was determined and found to be 97% identical to that of A/equine 2/Miami/63. A similar level of identity was shown for the deduced NS1 amino acid sequence. The NS1 gene was expressed, in its entirety and in part, as fusion proteins with glutathione S-transferase using the pGEX-3X expression vector. Antibodies to NS1 protein were detected in serum samples from ponies experimentally infected with influenza virus, but not in animals vaccinated with whole inactivated virus or in unprimed control animals. The antigenic determinant(s) of NS1 protein appear to be located in the C-terminal half of the protein. The implications of these findings are discussed with reference to the use of NS1 protein as a differential diagnostic marker for influenza virus infection in the presence of high levels of circulating antibody to influenza haemagglutinin generated by recent vaccination.

Influenza A subtype cross-protection after immunization of outbred mice with a purified chimeric NS1/HA2 influenza virus protein

Influenza A/PR/8/34-derived chimeric (D) protein (SK&F 106160) composed of the first 81 amino acids (aa) of NS1 fused to the conserved 157 C-terminal aa of HA2 (NS1 1-81-HA2 65-222) was previously shown to induce H-2d-restricted protective cytotoxic T-lymphocyte (CTL) immunity in inbred mice. However, D protein, like other small peptides, exhibited haplotype dependence and was not immunogenic in H-2b and H-2K mice. A potential use of this antigen in humans and the role of T cells in any protection were evaluated in outbred Swiss and inbred CBF6F1 (H-2d/b) mice. Mice immunized with D protein and challenged by small-particle aerosol with a lethal dose of influenza virus were significantly protected against mortality from influenza A/H1N1 and A/H2N2 (p < 0.05-< 0.0000001), but not from A/H3N2 and influenza B viruses when compared with control mice. D protein did not induce serum virus-neutralizing antibody but caused virus to be cleared faster in immunized mice. Protection was long-lasting. In vivo depletion of either Lyt2 (CD8+) or L3T4 (CD4+) T cells with monoclonal antibodies led to abrogation of in vitro-generated CTL activity in CF6F1 mice and significant reduction in the protective efficacy of D protein against virus challenge in both Swiss and CF6F1 mice. These results suggest that protection was mediated by CD8+ and/or CD4+ cells and not antibody. Thus D protein, via a conserved sequence on the HA2 polypeptide, has the potential to induce partially cross-reactive CTL that may protect against influenza virus disease in humans.

Antigenic variation in the influenza A virus nonstructural protein, NS1

The antigenic structure of the nonstructural (NS1) protein encoded by influenza type A virus was examined using monoclonal antibodies prepared against purified NS1 inclusions isolated from the cytoplasm of infected cells. Topographical analysis by competitive radioimmunoassay indicated that three different overlapping antigenic regions were present on the NS1 of A/WSN/33 (H1N1). Immunoprecipitation studies using infected cell lysates showed that antigenic determinants on A/WSN/33 NS1 are common to NS1 proteins encoded by a wide range of viruses of human, swine, equine, and avian origin. Several avian strains, however, were found to encode antigenically variant NS1 proteins which had either extensive changes in one or more antigenic regions or small changes in epitopes within a region suggestive of antigenic drift. There was no correlation between surface antigen subtype and the antigenic profile of the NS1 protein. The antigenic relationships of NS1 proteins shown in this study are in agreement with the available sequence data.

Nonsense mutations affecting the lengths of the NS1 nonstructural proteins of influenza A virus isolates

The proteins from cells infected with influenza A virus field isolates were labeled with [35S]methionine and analyzed by SDS-polyacrylamide gel electrophoresis. By screening more than 100 field isolates, it was found that the NS1 proteins had the greatest mobility differences, far exceeding those observed among other corresponding viral polypeptides. Partial sequence determination of RNA segment eight from 12 viruses revealed the existence of nonsense mutations at six different positions in their NS1 coding regions. The termination codons consisted of opal, ochre, and amber mutations. The sizes predicted from these sequences of 202, 217, 219, 220, 230, and 237 amino acids were in agreement with the observed mobilities of the viral polypeptides on SDS-polyacrylamide gels. The observation of large deletions in the carboxy termini of the NS1 proteins of field virus isolates would suggest that a high degree of variation can be tolerated in this polypeptide without affecting its functional capability.

The segregation of specific clones of cytotoxic lymphocytes in an in vitro primary response against influenza virus

CBA spleen cells have been stimulated in vitro with A/Jap influenza virus-infected CBA spleen cells to generate a 'primary' cytotoxic lymphocyte (CL) response. The culture conditions were devised to allow the segregation of individual clones of CL and cytotoxicity measured by the lysis of infected or non-infected L-929 cells. The specificity was assessed by splitting clones and measuring the ability of the clones to discriminate between pairs of targets. Influenza A/FMI and A/Jap strains were used. Subsets of clones were detected which could lyze either A/Jap-infected or A/FMI-infected target cells. In addition CL clones were found which lyzed uninfected L-929 cells and a fourth category were clones which could not discriminate between A/FMI-and A/Jap-infected targets.