Abstract

Interleukin-21 (IL-21) is a pleiotropic cytokine that regulates immune responses, but its role in fish intestinal immunity remains poorly understood. In this study, we produced recombinant grass carp IL-21 (r Ci IL-21) and investigated its protective effects against Aeromonas hydrophila- induced enteritis. Administration of r Ci IL-21 significantly reduced intestinal bacterial load and mitigated histopathological damage caused by A. hydrophila infection. Furthermore, r Ci IL-21 treatment enhanced intestinal barrier integrity by increasing mucus layer thickness, promoting goblet cell proliferation, and upregulating the expression of mucin-related genes ( agr2 , muc2 , and klf4 ) and tight junction proteins ( zo-1 , claudin-3 , and occludin ) after infection. Additionally, r Ci IL-21 attenuated apoptosis and oxidative stress while promoting the expression of antimicrobial factors ( c-lysozyme , β-defensin 1 and β-defensin 2 ) and anti-inflammatory factor ( il-10 ), collectively leading to reduced bacterial load and improved survival rates. Collectively, these results demonstrate that r Ci IL-21 plays a multiple role in alleviating bacterial enteritis by reinforcing intestinal barrier integrity, modulating inflammatory responses, and enhancing innate antimicrobial defense, highlighting its potential as a therapeutic agent for bacterial enteritis in aquaculture.