Escape of a highly defective influenza a virus mutant from its temperature sensitive phenotype by extragenic suppression and other types of mutation

TLDR: The A/Victoria/75-ts-l A2 recombinant proved to be more attenuated and more stable genetically in hamsters, adults, and children than a series of ts-1 [El recombinants studied previously], and single-lesion ts mutants, which exhibited an unusual degree of genetic stability.

Abstract: Genetic instability represents one of the major obstacles impeding current efforts to develop safe, effective mutants for use in live virus vaccines. Although certain temperature-sensitive (1s) and cold-adapted ( c a ) mutants, with defined mutations responsible for attenuation, have been shown to be safe and immunogenic in man, on occasion infected individuals have yielded virus with altered ts or ca This type of genetic instability has been observed most frequently during unrestricted viral replication in a completely susceptible For example, the influenza A/Hong Kong/68-ts-l [El (H3N2) recombinant, which exhibited ii 38\" C shutoff temperature for plaque formation, retained its ts phenotype after replication in adult volunteers who had detectable immunity to the viral neuraminidase surface-glycoprotein but not to the viral hemagglutinin surface-gly~oprotein.~. In contrast, when the ts-1 [El recombinant was given to young vaccinees who had not been previously infected with an influenza A virus, 25% of these individuals shed virus that had lost its temperature sensitivity ( t ~ ) .1 In an attempt to produce a ts virus more stable than the influenza A/Hong Kong/68 ts-1 m] donor, two single-lesion ts mutants, which exhibited an unusual degree of genetic stability. were mated and their rs genes were transferred into a recombinant.? The resulting influenza A/ Udorn/ 72-ts-1A2 recombinant possessed ts mutations on the P3 and P1 polymerase genes and exhibited a 37\" C shutoff temperature for plaque f~ rma t ion .~ , * The two mutant genes were then transferred by genetic reassortment to the influenza A/Victoria/ 75 (H3N2) wild-type virus.9 The A/Victoria/75-ts-l A2 recombinant proved to be more attenuated and more stable genetically in hamsters, adults, and children than a series of ts-1 [El recombinants studied previously. (Wright, et al. Un-