Molecular characterization and functional analysis of IL-18 in large yellow croaker (Larimichthys crocea)

Interleukin-18 (IL-18), a pro-inflammatory cytokine of the IL-1 family, is crucial for protecting the host against pathogen infection in mammals. In this study, a IL-18 homolog gene was cloned from large yellow croaker (Larimichthys crocea) (LcIL-18), which has an open reading frame (ORF) of 609 bp that encodes a polypeptide of 202 amino acids. The LcIL-18 C-terminus contains a typical IL-1 family signature and a caspase cleavage site. Phylogenetic analysis showed that LcIL-18 was most closely related to IL-18 of Miichthys miiuy. It was found that LcIL-18 was constitutively expressed in all 12 tissues tested of large yellow croakers, with the highest expression in gills. The expression of LcIL-18 in head kidney, spleen, skin, gills, and liver showed a differential pattern following infection with Pseudomonas plecoglossicida and Vibrio alginolyticus. P. plecoglossicida strongly induced LcIL-18 expression in these tissues. Conversely, in the early stage of infection, V. alginolyticus significantly inhibited LcIL-18 expression in head kidney, spleen, skin, and gills, but not in the liver. In vitro, LPS, Poly(I:C), P. plecoglossicida, and V. alginolyticus significantly upregulated the expression of the LcIL-18 in large yellow croaker head kidney (LYCK) cells. Furthermore, recombinant LcIL-18 (rLcIL-18) significantly increased cell viability and upregulated the expression of pro-inflammatory cytokines (LcIL-1β, LcIL-6, and LcTNF-α1) in LYCK cells. Our findings therefore indicated that LcIL-18 was involved in pro-inflammatory response induced by pathogenic bacteria.