Biological characteristics of a cold-adapted influenza A virus mutation residing on a polymerase gene

Artificial hybrids of influenza A virus RNA polymerase reveal PA subunit modulates its thermal sensitivity

BACKGROUND

Influenza A virus can infect a variety of different hosts and therefore has to adapt to different host temperatures for its efficient viral replication. Influenza virus codes for an RNA polymerase of 3 subunits: PB1, PB2 and PA. It is well known that the PB2 subunit is involved in temperature sensitivity, such as cold adaptation. On the other hand the role of the PA subunit in thermal sensitivity is still poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

To test which polymerase subunit(s) were involved in thermal stress we reconstituted artificial hybrids of influenza RNA polymerase in ribonucleoprotein (RNP) complexes and measured steady-state levels of mRNA, cRNA and vRNA at different temperatures. The PA subunit was involved in modulating RNP activity under thermal stress. Residue 114 of the PA subunit was an important determinant of this activity.

CONCLUSIONS/SIGNIFICANCE

These findings suggested that influenza A virus may acquire an RNA polymerase adapted to different body temperatures of the host by reassortment of the RNA polymerase genes.

Four viral genes independently contribute to attenuation of live influenza A/Ann Arbor/6/60 (H2N2) cold-adapted reassortant virus vaccines

Clinical studies previously demonstrated that live influenza A virus vaccines derived by genetic reassortment from the mating of influenza A/Ann Arbor/6/60 (H2N2) cold-adapted (ca) donor virus with epidemic wild-type influenza A viruses are reproducibly safe, infectious, immunogenic, and efficacious in the prevention of illness caused by challenge with virulent wild-type virus. These influenza A reassortant virus vaccines also express the ca and temperature sensitivity (ts) phenotypes in vitro, but the genes of the ca virus parent which specify the ca, ts, and attenuation (att) phenotypes have not adequately been defined. To identify the genes associated with each of these phenotypes, we isolated six single-gene substitution reassortant viruses, each of which inherited only one RNA segment from the ca parent virus and the remaining seven RNA segments from the A/Korea/1/82 (H3N2) wild-type virus parent. These were evaluated in vitro for their ca and ts phenotypes and in ferrets, hamsters, and seronegative adult volunteers for the att phenotype. We found that the polymerase PA gene of the ca parent specifies the ca phenotype and that the PB2 and PB1 genes independently specify the ts phenotype. The PA, M, PB2, and PB1 genes of the ca donor virus each contribute to the att phenotype. The finding that four genes of the ca donor virus contribute to the att phenotype provides a partial explanation for the observed phenotypic stability of ca reassortant viruses following replication in humans.

Temperature-sensitive defect of influenza A/Ann Arbor/6/60 cold-adapted variant leads to a blockage of matrix polypeptide incorporation into the plasma membrane of the infected cells

A temperature-sensitive (ts) defect in growth of the A/Ann Arbor/6/60 (A/AA/60) cold-adapted (ca) and ts variant strain has been studied. At the restrictive temperature of 38.5 degrees C, the variant synthesized all the viral polypeptides in normal amounts within the infected cells, but the virions released into the culture fluid contained greatly reduced amounts of the matrix (M1) polypeptide and showed significantly low infectivity per unit hemagglutinin activity. Cell fractionation experiments revealed that incorporation of the M1 polypeptide into plasma membranes of the variant-infected cells was selectively reduced at 38.5 degrees C, whilst it occurred normally at 34 degrees C. The ts reassortants between the A/AA/60 variant and the A/AA/1/80 wild type (wt) strain (non-ts), which had the M gene derived from the wt parent, also showed similar patterns. These results suggest that the ts defect of the variant and its ts reassortants involves the process of incorporation of the M1 polypeptide into the plasma membranes of the infected cells and that this defect is not attributable to the M gene of the variant.