Our study investigated the effect of dietary
methionine hydroxy analogue (MHA) on growth and immunity (head kidney, spleen and skin) of young grass carp (Ctenopharyngodon idella). A total of 630 grass carp (259.70 ± 0.47 g) were fed graded levels of MHA (0, 2.4, 4.4, 6.4, 8.5 and 10.5 g/kg diet) and one dl-
methionine (DLM) group (6.4 g/kg diet) for 8 weeks. At the end of the feeding trial, fish were challenged with Aeromonas hydrophila for 14 days. The results indicated that optimal MHA increased
lysozyme (LZ) and
acid phosphatase (ACP) activities,
complement 3 (C3), C4 and
immunoglobulin M (
IgM) contents and up-regulated
mRNA levels of
liver expressed antimicrobial peptide 2,
hepcidin (head kidney), β-defensin-1 in the immune organs (P < 0.05), suggesting that MHA could enhance antimicrobial ability of fish. Meanwhile, optimal MHA enhanced the immune function of immune organs via down-regulating pro-inflammatory
cytokines mRNA levels and up-regulated anti-inflammatory
cytokines mRNA levels, which might be attributed to the down-regulation of nuclear factor κB p65, c-Rel, IκB
kinase β,
p38 mitogen activated protein kinase, eIF4E-binding protein1 (4E-BP1) and 4E-BP2
mRNA levels and up-regulation of inhibitor of κBα,
ribosomal protein S6 kinase 1 and target of
rapamycin mRNA levels (P < 0.05). In addition, optimal MHA improved cellular structure integrity of immune organs via repressing
death receptor and mitochondria pathways induced apoptosis, which might be related to the down-regulation of
c-Jun-N-terminal kinase mRNA levels (P < 0.05). Simultaneously, optimal MHA improved cellular structure integrity of immune organs via elevating
glutathione contents,
antioxidant enzymes activities and corresponding
isoforms mRNA levels to attenuate oxidative damage, which might be to the up-regulation of
NF-E2-related factor 2 mRNA levels and down-regulation of Kelch-like ECH-associating
protein 1a
mRNA levels (P < 0.05). Besides, optimal MHA improved intercellular structure integrity of immune organs via up-regulating the
mRNA levels of intercellular tight junctions-related genes, which might be owing to the down-regulation of
myosin light chain kinase mRNA levels (P < 0.05). In conclusion, MHA exerted a positive effect on the immune function and structural integrity of immune organs in fish. Furthermore, according to the positive effect, MHA was superior to DLM in grass carp. However, based on the growth performance, the efficacy of MHA relative to DLM was 97%. Finally, on the premise of the basal diet containing 4.01 g/kg
methionine, the optimal MHA supplementation levels based on feed intake, PWG, defense against skin
hemorrhage and lesion, LZ and ACP activities,
IgM content, against
malondialdehyde,
protein carbonyl and ROS in the head kidney of young grass carp were 5.07, 5.21, 5.76, 5.90, 5.88, 5.80, 6.22, 5.68 and 6.85 g/kg diet, respectively.