For cell culture-based influenza vaccine production virus yield optimisation is of crucial importance. In particular, with the recent threat of the new H1N1 pandemic, not only seasonal vaccines but also pre-/pandemic vaccines have to be supplied in large quantities. In vivo influenza replication is limited by the immune system, but for production cell lines the impact of cellular defence mechanisms on virus yield is unknown. In influenza-infected adherent Madin-Darby canine kidney (MDCK) cells the interferon (IFN) response and subsequent induction of the antiviral state was monitored. Virus yield and host cell signalling intensity were strain-dependent. By over-expression of viral antagonists IFN-signalling could be reduced up to 90%. However, maximum virus titre determined by real-time PCR and HA-assay was not altered significantly. Stimulation of the antiviral state by conditioned medium led to enhanced IFN-signalling, which initially slowed down virus replication but had only minor effects on final virus titres. Interestingly, minireplicon assays revealed that canine Mx proteins are lacking the antiviral activity against influenza of their human or mouse counterparts. In summary, for MDCK cell culture-based influenza virus production host cell defence mechanisms seem to play only a minor role for final virus yields. Antiviral mechanisms of these epithelial cells may slow down influenza replication, which in vivo gains time for the immune system to be activated, but do not reduce maximum virus titres obtained in the bioprocess
