Antiviral activity produced by an IPNV-carrier EPC cell culture confers resistance to VHSV infection

Infectious pancreatic necrosis virus (IPNV), a fish birnavirus, can establish a persistent infection on epithelioma papulosum cyprinid (EPC) cells producing a carrier state where a small fraction of IPNV-infected cells is maintained in the culture after continuous subculture. The EPCIPNV cells are resistant to challenge with IPNV as well as to challenge with viral hemorrhagic septicemia virus (VHSV), a rhabdovirus. In this work, the antiviral effect of the IPNV carrier culture conditioned medium (EPCIPNV-CM) was tested and analyzed in detail. EPC cells treated with the carrier culture supernatant become protected against VHSV challenge. Size-fractionation by filtration and acid and heat treatment showed that the IPNV persistently infected cells release an acid-resistant soluble factor in the molecular weight fraction bellow 50 kDa. The capacity of the EPCIPNV-CM to induce cytokine genes in EPC cells was also determined by real-time RT-PCR. We found that there is a positive correlation between up-regulation of mx gene expression in EPC cells treated with EPCIPNV-CM and protection against VHSV challenge. Our findings indicate that the control of IPNV multiplication in the carrier culture as well as the interference with rhabdovirus replication are connected to the production and release of an antiviral (interferon-like) factor to the medium.