Threonine deficiency decreased intestinal immunity and aggravated inflammation associated with NF-κBand target of rapamycinsignalling pathways in juvenile grass carp ( Ctenopharyngodon idella) after infection with Aeromonas hydrophila

Abstract

This study aimed to investigate the impacts of dietary threonine on intestinal immunity and inflammation in juvenile grass carp. Six iso-nitrogenous semi-purified diets containing graded levels of threonine (3·99–21·66 g threonine/kg) were formulated and fed to fishes for 8 weeks, and then challenged with Aeromonas hydrophilafor 14 d. Results showed that, compared with optimum threonine supplementation, threonine deficiency (1) decreased the ability of fish against enteritis, intestinal lysozyme activities (except in the distal intestine), acid phosphatase activities, complement 3 (C3) and C4 contents and IgM contents (except in the proximal intestine (PI)), and it down-regulated the transcript abundances of liver-expressed antimicrobial peptide( LEAP) -2A, LEAP-2B, hepcidin, IgZ, IgMand β-defensin1(except in the PI) ( P<0·05); (2) could up-regulate intestinal pro-inflammatory cytokines TNF-α, IL-1β, IL-6, IL-8and IL-17DmRNA levels partly related to NF-κBsignalling; (3) could down-regulate intestinal anti-inflammatory cytokine transforming growth factor( TGF) -β1, TGF-β2, IL-4/13A(not IL-4/13B) and IL-10mRNA levels partly by target of rapamycin signalling. Finally, on the basis of the specific growth rate, against the enteritis morbidity and IgM contents, the optimum threonine requirements were estimated to be 14·53 g threonine/kg diet (4·48 g threonine/100 g protein), 15.05 g threonine/kg diet (4·64 g threonine/100 g protein) and 15·17 g threonine/kg diet (4·68 g threonine/100 g protein), respectively.