Molecular characterization of fish cytokine IL-17C from Amphiprion clarkii and its immunomodulatory effects on the responses to pathogen-associated molecular patterns and bacterial challenges

Interleukin 17C (IL17C) is a cytokine that regulates innate immunity by recruiting antimicrobial peptides and pro-inflammatory cytokines. In this study, we characterized properties of IL-17C from Amphiprion clarkii also known as yellowtail clownfish (AcIL-17C). The AcIL-17C gene is 489 base pairs long and encodes a 163 amino acid long protein. AcIL-17C includes a signal peptide for localization in the extracellular space and comprises the IL-17 domain. The transcription analysis revealed that AcIL-17C mRNA was ubiquitously expressed in 12 tested tissues. Blood cells treated with polyinosinic:polycytidylic acid (poly (I:C)), lipopolysaccharides (LPS), and Vibrio harveyi, AcIL-17C mRNA expression was upregulated at 6 h (following poly (I:C) and LPS treatments) and at 24 h post-injection (following all treatments). The downstream gene analysis of the epithelial fathead minnow (FHM) cells showed upregulated expression of genes, such as FHM_NK-Lysin, FHM_Hepcidin-1, FHM_Defensin-β, encoding antimicrobial peptides, as well as of FHM_IL-1β, FHM_TNF-A, FHM_IL-11, and FHM_STAT3 genes encoding inflammation-related proteins and IL-17C receptor genes FHM_IL-17RA, and FHM_IL-17RE at 12 and 24 h after treatment with AcIL-17C. The bacterial colony counting assay showed lower colony counts of Escherichia coli grown on FHM cells transfected with AcIL-17C carrying vector compared to those grown on control FHM cells. Further, AcIL-17C had a concentration-dependent positive effect on the survival of FHM cells infected with E. coli compared to the percentage of survived control cells. There has been a lack of studies characterizing the functions of teleost IL-17C. Therefore, these findings provide important information about the teleost host defense mechanisms and insights on the IL-17C-mediated antibacterial immunity.