Bath immunostimulation of rainbow trout (Oncorhynchus mykiss) fry induces enhancement of inflammatory cytokine transcripts, while repeated bath induce no changes

The application of immunostimulants may be a cost-effective practice in production of delicate and fragile fish fry since it may confer disease resistance. Bath administration to fish fry is considered an ideal delivery route for mass manipulation since there is no need for individual handling. In the current study, rainbow trout fry were bathed with three different immunostimulants: Plasmid DNA, lactoferrin and β-glucan at two different doses (0.1 μM and 1.0 μM) for 45 min, four times with an interval of 1 week. Ten fish per treatment group were sampled, and RNA of pooled tissues consisting of eye, tongue, skin, gill, thymus, spleen, head kidney, liver, small and large intestine were isolated from the fish obtained at first, second and fourth bathing (24 and 72 h post-bathing). Before cDNA transcription, two parallel samples were pooled giving a total of 5 parallels in each treatment group. Results showed that plasmid DNA and lactoferrin as well as β-glucan treated fish possessed higher gene expression with regard to the pro-inflammatory cytokines IL-1β, TNF-α, IL-6 and the anti-inflammatory cytokines IL-10 and TGF-β after the first bathing, especially 24 h post-bathing in the high-dose groups. This indicates that bath delivery of immunostimulants can induce pro-inflammatory responses. No significant changes of the pro-inflammatory cytokine IL-17A transcripts, compared to the respective controls, were observed. Gene expression levels in the immunostimulated fish after the fourth bathing did not show significant differences compared to controls.