Infectious salmon anemia (ISA) is a serious disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), belonging to the genus Isavirus, family Orthomyxoviridae. There is an urgent need to understand the virulence factors and pathogenic mechanisms of ISAV and to develop new vaccine approaches. Using a recombinant molecular biology approach, we report the development of a plasmid-based reverse genetic system for ISAV, which includes the use of a novel fish promoter, the Atlantic salmon internal transcribed spacer region 1 (ITS-1). Salmon cells cotransfected with pSS-URG-based vectors expressing the eight viral RNA segments and four cytomegalovirus (CMV)-based vectors that express the four proteins of the ISAV ribonucleoprotein complex allowed the generation of infectious recombinant ISAV (rISAV). We generated three recombinant viruses, wild-type rISAV 901_09 and rISAVr S6-NotI-HPR containing a NotI restriction site and rISAV S6/EGFP-HPR harboring the open reading frame of enhanced green fluorescent protein (EGFP), both within the highly polymorphic region (HPR) of segment 6. All rescued viruses showed replication activity and cytopathic effect in Atlantic salmon kidney-infected cells. The fluorescent recombinant viruses also showed a characteristic cytopathic effect in salmon cells, and the viruses replicated to a titer of 6.5 ؋ 10 5 PFU/ml, similar to that of the wild-type virus. This novel reverse genetics system offers a powerful tool to study the molecular biology of ISAV and to develop a new generation of ISAV vaccines to prevent and mitigate ISAV infection, which has had a profound effect on the salmon industry.
Infectious salmon anemia (ISA) is a disease that principally affects Atlantic salmon (Salmo salar), which has caused enormous losses to salmon farming worldwide (1, 2). ISA was first reported in 1984 in Norway (3) and subsequently was identified in Canada (4), Scotland (5), the United States (6), the Faroe Islands (Denmark) (6), and Chile (7). The etiological agent of this disease is the infectious salmon anemia virus (ISAV), a pleomorphic enveloped virus with a diameter of 45 to 140 nm, which belongs to the family Orthomyxoviridae (8). Its genome has 8 negative-polarity singlestranded RNA (ssRNA) segments that encode 10 proteins. Sequence similarity analyses of the genomes of different ISAV isolates show significant variation in the coding regions. In contrast, the untranslated regions (UTR) at both ends of each segment are highly conserved (9-11), as has been described for other orthomyxoviruses as well (e.g., influenza A and B viruses) (12).There is little knowledge at present about the functions of the ISAV proteins. Bioinformatic analyses of the sequences predict that segments 1, 2, and 4 encode the subunits of the RNA-dependent RNA polymerase (RdRp), analogous to PB2, PB1, and PA, respectively, of the influenza A virus. Segment 3 encodes the nucleoprotein (NP), which has been reported to have the capacity to bind ssRNA (13). For influenza virus, it has been shown that t...