Weak influence of light and time of day on transcriptional response to SAV3 infection in Atlantic salmon (Salmo salar) smolts

We began with two hypotheses. First, that compared with a regular light-dark cycle (long photoperiod; LP), constant light (LL) would have a weaker resistance to viral challenge in Atlantic salmon smolts. Second, that Atlantic salmon smolts kept in a regular light-dark cycle would have altered immune sensitivity between the light and dark phases.

To test our first hypothesis, we photostimulated smoltification in parr through exposure to either LP or LL conditions for 6 weeks then challenged them by SAV3 infection. LP and LL groups showed indistinguishable measurements in classical smolt characteristics and comparable levels of SAV3 mRNA in the heart after 8 and 14 days post infection. They also showed comparable transcriptional responses of select type I interferons (IFNa, IFNb, IFNc) and both class I and class II major histocompatibility complexes (MHC I and MHC II) in both heart and head kidney tissue.

IFNγ induction in the heart, however, was marked higher in the LL compared to the LP group. While this data suggests a heightened sensitivity of the LL group to SAV3 viral challenge, the consistent induction of MHC II, a key target gene of IFNγ signaling, to SAV3 infection in both LL and LP groups indicates that photoperiod may only have a minor impact on interferon regulation, rather than a more general influence on the viral immune response.

To test our second hypothesis, we compared the transcriptomic response in the heart to SAV3 infection delivered either at the mid-light or mid-dark phase in the LP group. Both mid-light and mid-dark groups showed an upregulation of immune related genes and down-regulation of structural genes of mitochondria, but an analysis of the interaction between infection and time of day identified no differentially regulated genes.

This second experiment suggests that, unlike in mammals, daily timing of viral infection may not play a major role in the immune response of Atlantic salmon.