An emerging role of guanidine acetic acid in rescuing immune function injured by Aeromonas hydrophila in grass carp (Ctenopharyngodon idella)

Guanidine acetic acid (GAA) is a safe feed additive and the only precursor for the creatine synthesis of vertebrates. Aeromonas hydrophila is a major fish pathogen causing severe morbidity and mortality in the aquaculture sector. Under the current intensive farming system, A feed additive that can improve fish disease resistance is urgently needed. In teleost fish, the spleen (SP) and head kidney (HK) play a crucial role in peripheral immunity organs and fish skin (SK) serves as one of the most important mucosal barriers. Thus, the study focused on immune defense substances, inflammatory process, and mechanisms of molecular action in the HK, SP, and SK of on-growing grass carp (169.46–600.89 g). The fish were fed graded levels of GAA (0, 150, 300, 450 and 600 mg/kg) for 60 days, and then conducted a 14-day challenge experiment by intraperitoneal injection of Aeromonas hydrophila. In our results, we found that 300 or 450 mg/kg GAA could down-regulate the pro-inflammatory cytokines IL-(1β, 6, 12p40 and 17D), TNF-α and IFN-γ2 mRNA expression, and up-regulate the pro-inflammatory cytokines IL-(10, 11 and 4/13 A) and TGF-β1 mRNA expression in the HK, SP and SK. Furthermore, 300 or 450 mg/kg GAA could up-regulate the mRNA expression and promote the protein phosphorylation of STAT3 in the HK, SP and SK. Indicating that in the case of Aeromonas hydrophila. challenge, GAA could enhance the immune defense and mediated the inflammatory process, possibly via the JAKs-STATs signaling pathway in fish.