Expression of a foreign gene by stable recombinant influenza viruses harboring a dicistronic genomic segment with an internal promoter.

Based on the observation that an internally located 3' promoter sequence can be functional (R. Flick and G. Hobom, Virology, 1999, 262(1), 93-103), we generated transfectant influenza A viruses harboring a dicistronic segment containing the CAT gene (660 nt) or a fragment of the Mengo virus VP0 capsid gene (306 nt) under the control of a duplicated 3' promoter sequence. Despite slightly reduced NA expression, the transfectant viruses replicated efficiently and proved to be stable upon both serial passage in vitro in MDCK cells and in vivo replication in the pulmonary tissue of infected mice. Internal initiation of replication and transcription from the second, internal, 3' promoter directed the synthesis of subgenomic vRNA and mRNA and therefore permitted expression of the foreign gene product, e.g., the CAT enzyme. The design of this vector may prove particularly appropriate for the utilization of influenza virus for the expression of heterologous proteins in their native form.