Effect of the complete replacement of dietary fish meal by soybean meal on histopathology and immune response of the hindgut in grass carp (Ctenopharyngodon idellus)

A 14-day experiment was conducted to explore the pathological process and immune response of soybean meal (SBM) induced enteritis (SBMIE) in grass carp (Ctenopharyngodon idellus). The complete replacement of dietary fish meal (FM) with SBM resulted in a remarkable reduction in final body weight, weight gain ratio, and feed conversion efficiency (p < 0.05). The typical histopathological changes of SBMIE appeared starting at day 4, and progressively increased in severity until day 8, then gradually subsided after day 11. The course of SBMIE could be divided into incubation period (days 1-2), prodromal period (days 3-6), symptomatic period (days 7-10), and convalescent period (days 11-14). Transcription levels of pro-inflammatory cytokines, including IL-1 beta, TNF-alpha, IL-6, IL-8, IL-17A/F1 and IFN-gamma 2, were up-regulated during the prodromal period, and then down-regulated during the convalescent period. Transcript levels of anti-inflammatory cytokines (IL-10 and TGF beta 1) and their receptors (IL-10RI and T beta RII), were up-regulated during the prodromal and convalescent periods. Transcript levels of MHCII beta, lg mu, Ig tau, TCR delta, TCR beta, CD4, and CD8 alpha were altered in SBMIE. Furthermore, expression levels of T-bet, IFN-gamma 2, ROR gamma 2 and IL-17A/F1 were significantly increased in the initiation of enteritis, whereas the transcript levels of Foxp3 and IL-2/15Ra were significantly up-regulated in the repair of enteritis. In conclusion, grass carp SBMIE is regulated by the adjustment of SBM-based diet intake, and the changes of the above-mentioned genes expression suggest that these genes may be involved in SBMIE.