Symbiont-derived sphingolipids regulate inflammatory responses in rainbow trout (Oncorhynchus mykiss)

Farmed fish live in association with diverse bacterial communities that produce wide arrays of metabolites. In rainbow trout, the skin and the gills are colonized by Flectobacillus major, a bacterium known to produce sphingolipids (SLs). The goal of this study is to evaluate the ability of F. major SLs to regulate rainbow trout inflammatory responses. F. major SLs were delivered by themselves or in combination with Freund's Complete Adjuvant (FCA), an oil-based adjuvant known to cause severe abdominal inflammation when injected to fish. Trout injected with SL + FCA showed decreased severity of FCA toxic effects including necrosis, granuloma formation and presence of oil droplets. However, inclusion of SLs in the FCA preparation did not decrease infiltration of immune cells intramuscularly at the site of injection. Intraperitoneal or intravenous delivery of F. major SLs resulted in increased expression of IgT, IgM and TGFβ transcripts in the gills but not the head-kidney and had no effects on IL-10 expression. These results indicate the F. major SLs regulate rainbow trout inflammatory responses and indicate that this compound can have important applications in farmed fish health management.