Long-term crowding stress induces chronic inflammatory response and declines the immunity of grass carp (Ctenopharyngodon idella)

This study is aimed to explore the mechanism of overcrowding stress on the immunity of grass carp Ctenopharyngodon idella. Fish were reared for 120 days at 10 kg/m3 (low stocking density, LSD), 15 kg/m3 (medium stocking density, MSD) and 20 kg/m3 (high stocking density, HSD). The results showed that HSD significantly increased the serum cortisol level, implying that fish reared at higher density were subjected to physiological stress. HSD led to a remarkable decline in serum lysozyme activity. The conspicuous decrease of serum complement 3 (C3) and immunoglobulin M (IgM) contents also appeared in the HSD group. The serum antioxidant enzymes activity included total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), glutathione peroxidase (GPx), and catalase (CAT) of fish reared at HSD group were clearly decreased. The serum glutathione (GSH) concentration of fish reared at HSD group was notably reduced. The malondialdehyde (MDA) level was obviously increased in HSD group. Long-term crowding stress gave rise to the constitution of melano-macrophage centers in spleen. The transcriptional levels of TLR4/NF-κB signaling pathway were notably up-regulated except for iκbα and mTOR pathway related genes were prominently down-regulated in fish reared at HSD group. The notably up-regulated pro-inflammatory cytokines (tnf-α, ifnγ2, il1-β, and il-8) except for il-6 and prominently down-regulated anti-inflammatory cytokines (il-10, il4/13a, il4/13b, tgf-β1 and tgf-β2) appeared in HSD group. All of the above results revealed that long-term overcrowding stress induced unbalanced inflammatory response via TLR4/NF-κB and mTOR signaling pathway and decreased the immunity in grass carp.