Betanodavirus Quantification and IFN I-Antagonism Detection Using Luciferase Reporter Systems Based on Fish mx Promoters.

Mx genes display strong and quick induction in response to viral infections, which varies according to the viral virulence; furthermore, mx transcription is blocked by several viruses as part of their immune evasion strategies. Therefore, the level and time course of mx induction reflect virus-host interplay. This idea prompted the development of in vitro experimental systems consisting of cells stably expressing luciferase under the control of fish mx promoters. In this study, two RTG-2 cell lines, expressing luciferase under the control of Senegalese sole (Solea senegalensis) mx (ssmx) promoter, and under the control of gilthead sea bream (Sparus aurata) mx2 (saumx2) promoter, have been applied to study betanodavirus-host interaction. Both systems were inoculated with nervous necrosis virus, NNV, isolates belonging to different genotypes. The combination of both systems has been proved to be useful to detect all of them, although each isolate triggered a characteristic profile in each system. In addition, a protocol to estimate the titre of the RGNNV isolate in sea bass (Dicentrarchus labrax) brain has been established, and a clear dose-dependent antagonistic effect of this NNV isolate was recorded. Thus, both cell lines are useful tools to study betanodavirus-host interaction and, therefore, contribute to developing measures to fight viral infections in aquaculture.