Inducers of salmon innate immunity: An in vitro and in vivo approach.

Maintaining fish health is one of the most important aims in aquaculture. Prevention of fish diseases therefore is crucial and can be achieved by various different strategies, including most often a combination of different methods such as optimal feed and fish density, as well as strengthening the immune system. Understanding the fish innate immune system and developing methods to activate it, in an effort to prevent infections in the first place, has been a goal in recent years. In this study we choose different inducers of the innate immune system and examined their effects in vitro on the salmon cell line CHSE-214. We found that the butyrate derivatives 4-phenyl butyrate (PBA) and β-hydroxy-β-methyl butyrate (HMB) induce the expression of various innate immune genes differentially over 24-72 h. Similarly, lipids generated from fish oils were found to have an effect on the expression of the antimicrobial peptides cathelicidin and hepcidin, as well as iNOS and the viral receptor RIG-1. Interestingly we found that vitamin D3, similar as in mammals, was able to increase cathelicidin expression in fish cells. The observed induction of these different innate immune factors correlated with antibacterial activity against Aeromonas salmonicida and antiviral activity against IPNV and ISAV in vitro. To relate this data to the in vivo situation we examined cathelicidin expression in juvenile salmon and found that salmon families vary greatly in their basal cathelicidin levels. Examining cathelicidin levels in families known to be resistant to IPNV showed that these QTL-families had lower basal levels of cathelicidin in gills, than non QTL-families. Feeding fish with HMB caused a robust increase in cathelicidin expression in gills, but not skin and this was independent of the fish being resistant to IPNV. These findings support the use of fish cell lines as a tool to develop new inducers of the fish innate immune system, but also highlight the importance of the tissue studied in vivo. Understanding the response of the innate immune system in different tissues and what effect this might have on infections and downstream cellular pathways is an interesting research topic for the future.