Induction of Antiviral Mechanisms by Interferon-Related Genes in Rock Bream (<em>Oplegnathus fasciatus</em>) Infected with Rock Bream Iridovirus (RBIV)

We evaluated the transcriptional response of interferon (IFN)-related genes in rock bream iridovirus (RBIV)-infected rock bream under high-, low-, or no-mortality conditions induced by different stocking water temperatures. Under the high susceptibility condition (group A, water temperature 26 °C, 100% mortality), only the Mx gene was expressed early, with prolonged expression, and with heavy viral loads of approximately 106–107 major capsid protein gene copies/μL from 4 to 10 days post infection (dpi). However, IRF1, IRF3, IRF8, STAT1, ISG15, PKR, Viperin, GVIN1, IFI44, and ISG56 were activated at later time points (8 dpi) and then quickly decreased (10 dpi). For the low susceptibility condition, the water temperature was set at 23 °C for 7 days (group B) and then reduced to 17 °C. Group B exhibited a 28% mortality rate, in which persistent and effective antiviral responses were observed for long periods of time. In particular, at 20 and 22 dpi, when virus replication was peaked at approximately 107/μL, the expressions of most of the IFN-related genes (IRF1, IRF3, IRF8, Mx, STAT1, ISG15, PKR, Viperin, GVIN1, IFI44, and ISG56) were significantly higher in group B than in the control group. Moreover, prolonged and higher levels of IRF3 (at least 30 dpi), IRF8 (at least 30 dpi), ISG15 (at least 30 dpi), PKR (at least 28 dpi), Viperin (at least 30 dpi), and IFI44 (at least 30 dpi) were also observed in the recovery stage of infection. Under the no-susceptibility condition at 17 °C (0% mortality), significantly elevated levels of IRF3, Mx, ISG15, and PKR were observed mostly until 20 dpi. The findings indicate that RBIV infection can induce an efficient IFN-mediated antiviral immune response in low- and no-susceptibility conditions. The findings could be valuable for effective control of viral pathogens in fish.