Expression of cytokines IL-1β and TNF-α in tissues and cysts surrounding Didymocystis wedli (Digenea, Didymozoidae) in the Pacific bluefin tuna (Thunnus orientalis)

Tuna long distance migrations and exposure to wide range of ambient water temperatures facilitate infections with several parasitic groups. This is reflected in the remarkable diversity of tuna parasite communities, especially members of Didymozoidae superfamily (Poche, 1907) (Trematoda, Digenea). Didymocystis wedli is the most frequent species encountered in bluefin tuna parasitizing gill filaments, therefore suggested as a biological marker to differentiate between discrete tuna Atlantic stocks. Because of its high occurrence in gill tissue and inflammatory reaction as the consequence, the aim of our study was to asses if inflammatory madiation through expression of IL-1β and TNF-α is present locally at the site of D. wedli encystment, as well as if the systematic expression of cytokines can be detected in different tissues of infected versus uninfected fish. Quantification of localized cytokine expression was done on paraffine embedded gill sections by in situ hybridization, while quantitative PCR was used to mesured cytokine transcripts in skin mucus, kidney, spleen, gills and liver. Our results suggest that tuna constitutive expression of IL-1β and TNF-α in gills and skin implies a well-adapted innate immunity present at the barrier between the organism and environment. Upregulation of both cytokines in Didymocystis-infected gills not followed by a systematic response evidences the ongoing of an inflammatory process specific for the parasitation site. However, the lack of intensive cytokines response to D. wedli observed by molecular and histological data that fails to eliminate the parasite, could be related to the “old” age of the parasitic process.