This study investigated the effects of
dietary protein levels on
disease resistance, immune function and structural integrity in the head kidney, spleen and skin of grass carp (Ctenopharyngodon idella). A total of 540 grass carp (264.11 ± 0.76 g) were fed six diets containing graded levels of
protein (143.1, 176.7, 217.2, 257.5, 292.2 and 322.8 g digestible
protein kg-1 diet) for 8 weeks. After the growth trial, fish were challenged with Aeromonas hydrophila for 14 days. The results indicated that optimal levels of
dietary protein: (1) (1) increased the
lysozyme (LA) and
acid phosphatase (ACP) activities and the
complement 3 (C3) and C4 contents, up-regulated
antimicrobial peptides, anti-inflammatory
cytokines, inhibitor of κBα, target of
rapamycin and
ribosomal protein S6 kinases 1
mRNA levels, whereas down-regulated pro-inflammatory
cytokines,
nuclear factor kappa B (NF-κB) P65, IKKβ, IKKγ, eIF4E-binding
proteins (4E-BP) 1 and 4E-BP2
mRNA levels in the head kidney, spleen and skin of grass carp (P < 0.05), suggesting that optimal level of
dietary protein could enhance immune function in the head kidney, spleen and skin of fish; (2) increased the activities and
mRNA levels of
antioxidant enzymes, enhanced the
glutathione content, decreased
reactive oxygen species,
malondialdehyde (MDA) and
protein carbonyl contents, and up-regulated the
mRNA levels of
NF-E2-related factor 2,
B-cell lymphoma protein-2,
inhibitor of apoptosis proteins, myeloid cell leukemia-1 and tight junction complexes, whereas down-regulated Kelch-like-ECH-associated
protein (Keap) 1b, cysteinyl
aspartic acid-protease 3, 8, 9,
Fas ligand, apoptotic
protease activating factor-1, Bcl-2 associated X
protein and
myosin light chain kinase mRNA levels in the head kidney, spleen and skin of grass carp (P < 0.05), indicating that optimal level of
dietary protein could improve structural integrity in the head kidney, spleen and skin of fish. Finally, based on the skin
hemorrhage and lesion morbidity, LA activity and MDA content, the optimal levels of
dietary protein for grass carp (264 g-787 g) were estimated to be 241.45 g kg-1 diet (217.68 g digestible
protein kg-1 diet), 301.68 g kg-1 diet (265.48 g digestible
protein kg-1 diet) and 307.84 g kg-1 diet (272.71 g digestible
protein kg-1 diet), respectively.