Vero/CHOK1, a novel mixture of cell lines that is optimal for the rescue of influenza A vaccine seeds

•Universal cloning strategy for influenza vRNA expression.•Animal-free reverse genetics system for influenza A viruses.•Reverse genetics using regulatory-compliant Vero/CHOK1 cells.•Infection of CHO cells with A and B influenza viruses. Seasonal and pandemic influenza vaccine manufacturing is challenged with a tight production schedule. Reverse genetics constitutes a rapid method for creating viruses. Vero and CHOK1 cells were found to be an appropriate cell mixture for the generation of influenza reassortants by reverse genetics under the constraints of vaccine production, such as the use of regulatory-compliant cells and culture media devoid of components of animal origin. In addition, no further amplification in cell or egg substrates was required, thus reducing the time needed to obtain reassortant seed virus. In parallel, the cloning step was shown to be dramatically improved, permitting the rapid vRNA expression of influenza viruses. In addition, nucleoporation of the cells was conducted to more efficiently target the nucleus and avoid the use of chemical reagents containing proteins of animal origin. In conclusion, the reverse genetics system for influenza A viruses reported in this study was shown to be rapid, simple to perform and totally animal component-free to best comply with the requirements of health authorities for the production of a vaccine seed.