Characterization and expression analysis of an interleukin-7 homologue in the Japanese pufferfish, Takifugu rubripes

An interleukin-7 (IL-7) gene has been cloned and sequenced within the Japanese pufferfish, Takifugu rubripes (Fugu), providing the first conclusive evidence for the existence of IL-7 in teleosts. The IL-7 cDNA was composed of 1438 bp, with a 117 bp 5'-UTR, an 826 bp 3'-UTR containing two mRNA instability motifs, and a 498 bp ORF which translates into a peptide of 166 amino acid residues. The Fugu IL-7 peptide contained a predicted signal peptide of 19 amino acid residues and the IL-7/9 family signature (Arg140-Val149). Homology analysis of Fugu IL-7 with other known IL-2 family members showed some similarity to the known mammalian IL-7 and Fugu IL-21 molecules, and phylogenetic tree analysis showed that the Fugu IL-7 clustered with the chicken and mammalian IL-7 and mammalian IL-9, away from other IL-2 family members (IL-2, IL-4, IL-15 and IL-21). The organization of the Fugu IL-7 gene was the same as in mouse, and consisted of five exons and four introns, but differed from the human gene, which is composed of six exons and five introns. Comparison of the Fugu and human genomes showed that some synteny existed around the IL-7 gene, with the presence of both the protein kinase inhibitor-alpha and chromosome 8 ORF 70 (C8orf70) genes, with IL-7 and C8orf70 having the same transcriptional orientation. The Fugu IL-7 gene was expressed within unstimulated tissues, such as the head kidney, spleen, liver, intestine, gill and muscle. Stimulation of head kidney cells with lipopolysaccharide, polyinosinic polycytidylic acid or phytohaemagglutinin increased this expression. This discovery of IL-7 in Fugu will allow a more complete analysis of fish immune responses in the future.