Functional characterization of an interleukin-20 homolog in channel catfish

Interleukin-20 (IL-20), a member of the IL-10 cytokine family, exhibits dual immunoregulatory roles in mammals, balancing both anti-inflammatory and pro-inflammatory responses. However, its function in antimicrobial immunity in teleost fish remains poorly characterized. In this study, we identified and functionally characterized an IL-20-like homolog in channel catfish (Ictalurus punctatus), designated IcIL-20. The IcIL-20 protein consists of 184 amino acids, contains six conserved cysteine residues, and displays structural features typical of the IL-10 family. Transcript analysis revealed constitutive expression of IcIL-20 across multiple tissues. Following infection with Yersinia ruckeri or Streptococcus iniae, IcIL-20 expression was significantly upregulated in immune-relevant tissues including the gill, skin, hindgut, spleen, trunk kidney, and head kidney. In vitro stimulation of channel catfish kidney (CCK) cells with phytohemagglutinin (PHA), lipopolysaccharide (LPS), or phorbol 12-myristate 13-acetate (PMA) also induced IcIL-20 expression. Functional assays demonstrated that recombinant IcIL-20 protein promotes the expression of pro-inflammatory cytokines TNF-α and IL-1β in both CCK and channel catfish ovary (CCO) cells. These findings indicate that IcIL-20 is involved in the immune defense against bacterial infections in channel catfish, providing a basis for further exploration of IL-20-mediated immunity in teleosts.