Astragalus polysaccharides mediate the immune response and intestinal microbiota in grass carp (Ctenopharyngodon idellus)

The immunomodulatory effect of Astragalus polysaccharides (APS) has been revealed in aquatic animals. However, the fundamental mechanisms in grass carp (Ctenopharyngodon idellus) are not well characterized. This research has explored the impacts of APS on immune response, gut microbes and transcriptome profiles of grass carp. The eight week feeding trial was executed with the basal diet (Control) complemented with APS at diverse concentrations (0 and 1 g/kg). Firstly, the enzyme activity assay results indicated that dietary APS could considerably increase the activities of antioxidant and digestive enzymes. Furthermore, gut microbial analysis data showed that the intestinal microbial diversity and composition of grass carp were altered at the phylum level between the Control and APS groups. Additionally, beta-diversity revealed that APS had influenced the bacterial assembly in the foregut and hindgut of grass carp. The transcriptomic profiling was used to further explicate the mechanism fundamental to the effects of APS on molecular evidences in the intestines; the results revealed that the APS could activate immune related genes. Further study results revealed that the immune response stimulated by APS, the mRNA expression levels of TNF-α, IL-1β, CSF, ILF2, DCLK1 and TAGAP genes were all significantly up-regulated after dietary APS supplementation. Finally, histopathological changes of different tissues and bacterial load levels in the liver between the A. hydrophila and A. hydrophila + APS groups illustrated that APS protected the tissues under A. hydrophila challenge. Overall, the present study revealed that APS could modulate the intestinal microbiota and activate the immune response to protect the grass carp from bacterial infection.