This calls for improved methods for protection of farmed salmon against virus diseases. The discovery of type I IFNs in fish opens a possibility for using them in prophylaxis against virus infections in fish. Type I IFNs are induced upon host cell recognition of viral nucleic acids [2], and protect other cells against infection by inducing numerous antiviral proteins such as Mx, ISG15, IFIT5 (ISG58) and Viperin [3], [4] and [5].

In fish, four Vorinostat cost type I IFN subtypes, named IFNa, IFNb, IFNc and IFNd, have so far been characterized [6] and [7]. IFNa and IFNd contain 2 cysteines (2C-IFNs) while IFNb and IFNc contain 4 cysteines (4C-IFNs). The largest cluster of IFN genes has been found in Atlantic salmon, encoding two IFNa, four IFNb and five IFNc genes [6]. Atlantic salmon IFNa, IFNb and IFNc and IFNd have only 22–37% amino acid sequence identity and show major differences in cellular expression properties and antiviral activities [6] and [8]. IFNa1 and IFNc induced similar strong antiviral activity against IPNV and induced similar transcript levels of antiviral genes in cell lines,

IFNb was less active and IFNd showed no antiviral activity [8]. IFNa1, IFNb and IFNc provided only transient inhibition of ISAV replication in TO cells [9]. In humans, pegylated recombinant IFN-α, mostly in combination with ribavirin, is used for treatment of chronic hepatitis C virus infections [10]. IFN-α treatment has also shown protective effects against influenza virus infection in mammals and chicken [11], [12] and [13]. However, IFN prophylaxis to next combat virus diseases Lumacaftor in vivo in domestic animals and human has apparently had limited success due to the costs of recombinant IFNs, their rapid degradation in the body and side effects. Reports on effects of IFNs against virus infection in live fish are scarce. Treatment of rainbow trout with recombinant Atlantic salmon IFNa2 injected intraperitoneally (i.p.) provided protection against IHNV infection for up to 7 days, which is not enough for prophylaxis of farmed

fish [14]. In the present work we have used a more novel approach by studying antiviral effects of intramuscular (i.m.) injection of IFN expressing plasmids in Atlantic salmon. The results showed surprising differences among IFNa, IFNb and IFNc plasmids in their ability to induce systemic expression of antiviral genes and to protect salmon from infection with a high virulent strain of ISAV. Notably, i.m. injection of IFNc plasmid provided systemic up-regulation of antiviral genes in salmon for at least 8 weeks accompanied by a high level of protection against ISAV infection. Atlantic salmon (Salmo salar L.) presmolts (35–45 g) of the strain Aquagen standard (Aquagen, Kyrksæterøra, Norway) were kept at Tromsø Aquaculture Research Station, Norway in 300 l tanks supplied with fresh water at 10 °C and were fed commercial dry food. Prior to treatments, the fish were anesthetized with 0.