The stimulation of the innate immune system by SJNNV protects juvenile european sea bass (Dicentrarchus labrax) against superinfection with RGNNV

Abstract

In this study, viral replication and transcription of innate immunogenes have been determined by RT-real time-PCR (RT-qPCR) in the course of an experimental infection. Three different experimental conditions were considered: i) RGNNVinoculated animals; ii) SJNNV-inoculated animals and iii) animals inoculated with SJNNV and superinfected with RGNNV. Superinfection was performed 24 h after the SJNNV inoculation. Control animals were mocked-injected with L-15 medium. The RGNNV-infected group showed typical symptoms of the disease and displayed 76% cumulative mortality at the end of the experiment, whereas the mortality in the superinfected group was 4%, and no mortality was recorded in the SJNNV-inoculated group. The analysis of the Mx transcription by RT-qPCR showed a clearly differential induction of the sea bass innate immune system by RGNNV and SJNNV, since no transcription was recorded at any time tested (from 0 h to 48 h p.i.) after the RGNNV inoculation, whereas the injection of SJNNV resulted in an important increase of the Mx transcription from 24 h p.i. onwards. In the superinfected group the induction of the Mx gene transcription follows the same patterns that the ones described for the groups inoculated with SJNNV and RGNNV separately. These results suggest that the induction of the IFN mediated system by the previous infection with SJNNV could be responsible for the decrease in the mortality recorded in the superinfected group, protecting sea bass of the posterior infection with RGNNV.