Characterization of an infectious pancreatic necrosis (IPN) virus carrier cell culture with resistance to superinfection with heterologous viruses

A state of persistence of a non susceptible fish cell line with infectious pancreatic necrosis virus (IPNV) was established in vitro by experimental infection. The persistently infected culture showed sustained production of infectious virus and could be continuously passaged for months. A distinct feature of this culture is that only a very small fraction of the cells harbours virus replication, in contrast to other reported IPNV-persistently infected cells from salmonid fish, where nearly all the cells express viral antigens. In spite of the small number of detectable IPNV-infected cells, the carrier culture shows resistance to superinfection with homologous as well as heterologous viruses. Temperature shift-up experiments indicate that viral interference is due to continuous replication of IPNV in the culture. Quantitation of Mx gene expression suggested that the interference phenomenon could be mediated by the activation of the interferon (IFN) system. However, conditioned medium from the IPNV-infected cell cultures only marginally protected other cells against VHSV infection, indicating that other type I IFN-independent mechanism may be underlying the resistance of the persistently infected culture to infection with heterologous viruses. Our study defines a novel in vitro model of IPNV persistence and contributes to the understanding of the widespread distribution of aquabirnaviruses in marine and fresh water environments by establishing a carrier state in non susceptible fish species.