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Live attenuated influenza A virus vaccines are currently produced by the transfer of attenuating genes from a donor virus to new epidemic variants of influenza A virus, with the selection of reassortant viruses that possess the protective antigens (i.e., the two surface glycoproteins) of the epidemic virus and the attenuating genes from the donor virus. The previously studied attenuated donor viruses were produced by conventional methods such as passage of virus at low temperature or chemical mutagenesis. The present paper describes a new strategy for the generation of a donor virus bearing an attenuating, non-surface-glycoprotein gene. This strategy involves the introduction of attenuating mutations into the cDNA copy of the PB2 polymerase gene by site-directed mutagenesis, transfection of in vitro RNA transcripts of PB2 cDNA, and recovery of the transfected PB2 gene into an infectious virus. An avian-human influenza A virus PB2 single-gene reassortant virus (with an avian influenza A virus PB2 gene) that replicates efficiently in avian tissue but poorly in mammalian cells was used as a helper virus to rescue a transfected synthetic RNA derived from a human influenza A virus PB2 gene. The desired human influenza A virus mutant PB2 transfectant was favored in this situation because the avian influenza A virus PB2 gene restricts viral replication in mammalian cells in culture, the system used for rescue, thereby providing strong selection for the virus bearing the human influenza A virus PB2 gene. We validated the feasibility of this approach by rescuing the PB2 gene of the wild-type influenza A/Ann Arbor/6/60 virus and a mutant derivative that had a single amino acid substitution introduced at position 265 by site-directed mutagenesis. Previously, this amino acid substitution had been shown to specify both a temperature-sensitive (ts) and an attenuation (att) phenotype. The rescued mutant 265 PB2 transfectant virus exhibited the ts and att phenotypes, which confirms that these phenotypes were specified by this single amino acid substitution. The transfectant virus was immunogenic and protected hamsters from subsequent challenge with wild-type virus. The cDNA copy of this influenza A/Ann Arbor/6/60 virus mutant 265 PB2 gene will be used as a substrate for the introduction of additional attenuating mutations by site-directed mutagenesis.

Rescue of an influenza A virus wild-type PB2 gene and a mutant derivative bearing a site-specific temperature-sensitive and attenuating mutation