Gyrodactylus derjavini infection elicits IL-1β expression in rainbow trout skin

Specific gene expression of the pro-inflammatory cytokine IL-1β and the type II IL-1β receptor (IL-1RII, ‘decoy receptor’) was studied in skin of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) fry during infection with the ectoparasitic monogenean Gyrodactylus derjavini Mikailov, 1975. Cytokine expression was studied in both primary and secondary infections using a semi-quantitative RT–PCR technique. Generally, low levels of specific IL-1β1, IL-1β2 and IL-1RII gene transcription were found in uninfected hosts. In contrast, a clear induction of both IL-1β isoforms could be observed during the initial phases of primary G. derjavini infections. A less obvious induction of IL-1β expression was seen in secondary infections imposed just after recovery from the priming infection. However, challenge infections given one-month post full recovery of the primary infections did not elicit transcription levels above those seen in skin samples from uninfected hosts. Despite this, hosts carrying a secondary infection elicited a stronger response toward G. derjavini thus expelling parasites quicker than their naı̈ve counterparts. When induced, indications of a more tight regulation of the IL-1β1 isoform compared to IL-1β2 could be observed. Increased expression of IL-1RII was seen later during the course of a primary infection and could be involved in the observed down regulation of IL-1, especially the β1 isoform. The current study represents the first example of cytokine expression in fish induced by an ectoparasitic infection and indicates the importance of localised mucosal immune reactions in responses of fish towards gyrodactylids.