Two interleukin-17C-like genes exist in rainbow trout Oncorhynchus mykiss that are differentially expressed and modulated.

Interleukin (IL)-17 family members (IL-17A-F) are key players in adaptive immune responses and have a central role in coordinating innate and adaptive immunity. Here, we report on two novel IL-17 homologues in rainbow trout Oncorhynchus mykiss, trout (t) IL-17C1 and tIL-17C2, that share 73.7% amino acid identity. The two tIL-17C-like molecules have relatively higher sequence identities to IL-17Cs from fish and mammals and the fish IL-17C-like molecules phylogenetically form a specific clade that groups with the mammalian IL-17C and IL-17E clades. However, the gene organisation of the fish IL-17C-like molecules is closer to mammalian IL-17Es than to IL-17Cs, and this taken together with other factors suggest the fish IL-17C-like genes may have arisen from an ancestral gene that gave rise to mammalian IL-17C and IL-17E. The expression of tIL-17Cs was detectable in all the eight tissues examined, with the expression of tIL-17 mainly contributed by tIL-17C1 in gills and skin, and by tIL-17C2 in spleen, head kidney and brain. The expression of tIL-17Cs was modulated by inflammatory stimulants, including IL-1beta, interferon-gamma, LPS and PolyIC, in a trout macrophage cell line (RTS-11). IL-1beta was the most potent inducer of tIL-17C2 but only had a minor effect on the expression of tIL-17C1. LPS and PolyIC were also potent inducers of tIL-17C2. The expression of tIL-17Cs was also up-regulated by bacterial infection, with the extent and increase more dramatic for tIL-17C2. The broad distribution of expression and differential modulation of tIL-17Cs by inflammatory stimulants and infection suggest important roles of the two tIL-17Cs in the salmonid immune system.