Cold-adapted reassortants of influenza A virus in MDCK cells. II. Role of the temperature-sensitive property of cold-adapted reassortants in mice

Characterization of live influenza vaccine donor strain derived from cold-adaptation of X-31 virus

A human influenza A virus X-31 (high-yielding strain) was cold-adapted for possible future use as live attenuated vaccine. Mutant influenza viruses were selected during successive serial passage in embryonated hens' eggs at progressively lower sub-optimal temperature (30, 27 degrees C followed by 24 degrees C). The cold-passaged mutant exhibited both temperature-sensitivity (ts) and cold-adapted (ca) phenotypes. The pathogenicity and immunogenicity of X-31 ca virus were studied in mice following intranasal inoculation. The mice did not show clinical signs even at high titer infection. Immunization of mice with X-31 ca virus elicited high titers of neutralizing antibody and provided complete protection against homologous and heterologous virus challenges. To assess the genetic stability, the X-31 ca virus was passaged at 37 degrees C in MDCK cells or inoculated into mice. Revertant virus was not found in the lungs of any of the mice and the supernatants of the MDCK culture. We conclude that the X-31 ca candidate vaccine virus exhibits the desired level of attenuation, immunogenicity, and protective efficacy required for live attenuated vaccine and merits further evaluation at clinical level.

Genetics of cold-adapted B/Ann Arbor/1/66 influenza virus reassortants: the acidic polymerase (PA) protein gene confers temperature sensitivity and attenuated virulence

The cold-adapted B/Ann Arbor/1/66 influenza virus (ca B/AA/1/66) expresses temperature-sensitive (ts), cold-adapted (ca) and attenuation phenotypes. Reassortants which inherit one or more genes from ca B/AA/1/66 and all other genes from a virulent, wild-type influenza virus, B/Houston/1732/76, were produced and evaluated in order to identify the gene(s) responsible for the ts, ca and attenuation phenotypes. Only reassortants which inherited the PA gene from ca B/AA/1/66 expressed the ts phenotype in MDCK cells at 39 degrees C. None of the reassortants tested expressed the ca phenotype in embryonated eggs at 25 degrees C. The virulence of several reassortants was evaluated in ferrets. Inheritance of the PA gene from ca B/AA/1/66 was correlated with significant febrile attenuation and the apparent restriction of viral replication in the lower respiratory tract. Isolation of a virulent, non-ts revertant virus inheriting only the PA gene from ca B/AA/1/66 established a direct relationship between expression of the ts phenotype and attenuated virulence. Evidence for the contribution of at least one other gene from ca B/AA/1/66 to attenuation was observed. Thus, based on the methods used to determine reassortant gene compositions, these results indicate that the PA gene is primarily responsible for attenuation of ca B/AA/1/66 and its reassortants.

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.

Cold-adapted reassortants of influenza A virus: pathogenicity of A/Ann Arbor/6/60 X A/Alaska/6/77 reassortant viruses in vivo and in vitro

Cold-adapted reassortants of A/Ann Arbor/6/60 X A/Alaska/6/77 viruses made in MDCK cells have recently been assessed genotypically and for temperature-sensitive and cold-adapted phenotypes. These reassortants were used to infect ferrets and hamsters and to inoculate organ cultures of hamster tracheal rings, in order to assess their degree of virulence. Virulence in the three model systems corresponded quite well, and a correlation between loss of virulence and particular A/AA/6/60 genes present in the reassortants was noted. Two different reassortants containing either RNA 2 or RNA 5 (NA gene) alone from A/AA/6/60 showed little attenuation from the wild-type parent. A reassortant containing both RNA 2 and the NA gene from A/AA/6/60 and all remaining wild-type genes showed some small decrease in virulence compared to the wild-type virus. However a reassortant containing these two A/AA/6/60 genes and RNA 3 as an additional gene from this parent, had a level of attenuation comparable to that of the cold-adapted virus.

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

The biological function of a cold-adapted (ca) mutation residing on the PB2 gene of an influenza A/Ann Arbor/6/60 (A/AA/6/60) ca variant virus in the viral replication cycle at 25 degrees C was studied. The viral polypeptide synthesis of A/AA/6/60 ca variant at 25 degrees C was evident approximately 6 hours earlier than the wild type (wt) virus and yielded twice as many products. The quantitative analysis of viral complementary RNA (cRNA), synthesized in the presence of cycloheximide, revealed that A/AA/6/60 ca variant and a single gene reassortant that contains only the PB2 gene of the ca variant with remaining genes of the wt virus produced equal amount of cRNA at 25 degrees and 33 degrees C, which was an amount approximately four fold greater than the wt virus' cRNA synthesized at 25 degrees C. These results strongly suggest that the ca mutation residing on the PB2 gene of A/AA/6/60 ca variant affects the messenger RNA synthesis at 25 degrees C in the primary transcription.