Genomic characterization and expression analysis of five novel IL-17 genes in the Pacific oyster, Crassostrea gigas.

Interleukin-17 (IL-17) is a proinflammatory cytokine that plays an important role in clearing extracellular bacteria and contributes to the pathology of many autoimmune and allergic conditions. In the present study, five novel IL-17 homologs were identified by searching and analyzing the Pacific oyster genome. All six CgIL-17 members (including a previously reported homolog) contained four conserved cysteines that were used in the formation of disulfide bonds. Phylogenetic analysis showed that all invertebrate IL-17s were clustered into one group, implying that invertebrate IL-17s evolved from one common ancestral gene and subsequently diversified. All CgIL-17s shared the same genomic structure, containing two exons and one intron, except for the CgIL-17-3 and CgIL-17-5 genes, which each had only one exon. The expression pattern of the CgIL-17 genes was analyzed by qRT-PCR in a variety of tissues and at different developmental stages, and these genes were highly expressed in the gill and digestive gland tissues. Moreover, the expression of the CgIL-17 family genes was significantly up-regulated in hemocytes challenged with Pathogen-Associated Molecular Patterns (PAMPs). CgIL-17-3 had a strong response to lipopolysaccharide (LPS) and heat-killed Vibrio alginolyticus (HKVA) challenge, while CgIL-17-5 and CgIL-17-6 can be activated by peptidoglycan (PGN), but not by heat-killed Listeria monocytogenes (HKLM). The distinct, up-regulated transcript levels of the CgIL-17s in response to PAMPs challenge further indicate that CgIL-17s are likely to be significant components of immune responses by playing diversified roles in host defense in the Pacific oyster. These findings suggest that CgIL-17s are involved in innate immune responses and further supports their conserved function in mollusks immunity.