Effects of dietary pyridoxine on the skin immunity, tight junctions, antioxidants and apoptosis of grass carp (Ctenopharyngodon idella) infected with Aeromonas hydrophila

In this study, fish were fed six diets containing different levels of pyridoxine (PN) for 10 weeks, and then infected with Aeromonas hydrophila for 2 weeks. The results showed that dietary supplementation with PN had a powerful effect on the inhibition of skin haemorrhage and lesion morbidity, and the enhancement of immunity after fish were infected with A. hydrophila. For example, PN increased the levels of immunoglobulin M (IgM), acid phosphatase (ACP), complement 3 (C3), lysozyme (LZ), C4, antimicrobial peptides, target of rapamycin (TOR), anti-inflammatory cytokines, ribosomal protein S6 kinases 1 (S6K1) and inhibitor of kappa B alpha (I kappa B alpha), while decreasing the levels of proinflammatory cytokines [except interleukin-12 p40 (IL-12p40)], eIF4E-binding proteins (4E-BP), I kappa B kinase beta (IKK beta) and nuclear factor kappa B p65 (NF-kappa Bp65) in grass carp skin. Additionally, dietary supplementation with PN increased the levels of tight junction proteins (not claudin-15 and claudin-12), antioxidant enzymes [except manganese superoxide dismutase (MnSOD) activity and gene expression, glutathione peroxidase 4a (GPx4a) and GPx4b gene expression], NF-E2-related Factor 2 (Nrf2), myeloid cell leukaemia-1 (Mcl-1) and inhibitor of apoptosis proteins (IAP), while decreasing the levels of malondialdehyde (MDA), reactive oxygen species (ROS), protein carbonyl (PC), Kelch-like ECH-associating protein 1a (Keap1a) myosin light chain kinase (MLCK), cysteinyl aspartic acid protease-9 (caspase-9), caspase-8, caspase-7, caspase-3, Bcl-2-associated X protein (Bax), p38 mitogen-activated protein kinase (p38MAPK) and apoptotic protease-activating factor 1 (Apaf-1) in grass carp skin. In summary, dietary PN can enhance the immunity of fish skin via elevated immune substances, an eased inflammatory response and an improved barrier integrity.