A 56 days feeding trial was conducted to investigate the effects of
enzyme-treated
soy protein (ETSP) supplementation in
low protein diets on immune function of immune organs (head kidney, spleen and skin) in on-growing grass carp. A total of 540 on-growing grass carp (initial average weight: 325.72 ± 0.60 g) were fed six diets, which included a normal
protein diet (28% crude
protein) and five
low protein diets (26% crude
protein) supplemented with graded levels of ETSP (0.0, 0.8, 1.2, 1.6 and 2.0%). At the end of feeding period, a challenge test was performed by
infection with Aeromonas hydrophila for two weeks. The results indicated that (1) reducing
dietary protein content from 28 to 26% decreased antibacterial substances and aggravated inflammatory responses of above three immune organs; (2) under the condition of reducing
protein level in diet, 0.8-1.2% ETSP supplementation reversed these above adverse effects on immune function of above three immune organs; (3) suitable ETSP supplementation-decreased inflammatory responses were partly associated with [IκB
kinase β (IKKβ)/inhibitor of κBα (IκBα)/
nuclear factor kappa B (NF-κB) p65 and p52 or NF-κB p65] signaling and [target of
rapamycin (TOR)/(S6K1, 4E-BP)] signaling in above three immune organs. (4) On the basis of C3 content (head kidney), C4 content (spleen) and skin
hemorrhage and lesion, the optimal ETSP supplementation levels in
low protein diets were estimated to be 1.48%, 1.61% and 1.03%, respectively. In summary, ETSP supplementation in
low protein diets improved immune function of head kidney, spleen and skin in on-growing grass carp.