Molecular cloning, expression analysis and functional characterization of interleukin-22 in So-iny mullet, Liza haematocheila

• First a.a. in the family signature of mullet IL-22 was different from the ordinary.
• The expression of mullet IL-22 mRNA was up-regulated by S. dysgalactiae infection.
• Recombinant mullet IL-22 improved the expression of mullet β-defensin transcripts.
• mrIL-22 increased the survival rate of mullets infected with S. dysgalactiae.
• The in vivo function of fish IL-22 in bacterial diseases was firstly demonstrated.

In the present study, interleukin-22 (IL-22) from So-iny mullet (Liza haematocheila) was identified, and its tissue expression in both healthy and Streptococcus dysgalactiae-infected fish was examined. The full length cDNA sequence of mullet IL-22 was 1070 bp, containing an open reading frame of 555 bp. The deduced amino acid sequence shared high similarity (45.1–67.9%) with IL-22 from other fish species. Mullet IL-22 also contained an IL-10 family signature and four cysteine residues that were well conserved in other vertebrate IL-22 molecules. Mullet IL-22 mRNA was highly expressed in kidney, moderately expressed in liver and gut, and relatively weakly expressed in spleen, and its expression was significantly up-regulated in all the examined tissues following S. dysgalactiae infection. Furthermore, recombinant mullet IL-22 protein was shown to promote the expression of β-defensin in the four tissues and to increase the survival rate of the fish infected with S. dysgalactiae. Our results suggest mullet IL-22 plays an important role in the immune defense against bacterial infection and has the potential to be used to treat bacterial diseases in fish.