Rescue of influenza C virus from recombinant DNA.

The rescue of influenza viruses by reverse genetics has been described only for the influenza A and B viruses. Based on a similar approach, we developed a reverse-genetics system that allows the production of influenza C viruses entirely from cloned cDNA. The complete sequences of the 3' and 5' noncoding regions of type C influenza virus C/Johannesburg/1/66 necessary for the cloning of the cDNA were determined for the seven genomic segments. Human embryonic kidney cells (293T) were transfected simultaneously with seven plasmids that direct the synthesis of each of the seven viral RNA segments of the C/JHB/1/66 virus under the control of the human RNA polymerase I promoter and with four plasmids encoding the viral nucleoprotein and the PB2, PB1, and P3 proteins of the viral polymerase complex. This strategy yielded between 10(3) and 10(4) PFU of virus per ml of supernatant at 8 to 10 days posttransfection. Additional viruses with substitutions introduced in the hemagglutinin-esterase-fusion protein were successfully produced by this method, and their growth phenotype was evaluated. This efficient system, which does not require helper virus infection, should be useful in viral mutagenesis studies and for generation of expression vectors from type C influenza virus.