Chinese yam polysaccharide induces the differentiation and natural antibody secretion of IgM B cells to prevent Aeromonas hydrophila infection in grass carp.

Chinese yam polysaccharide (CYP) is an effective immunostimulant, however, its efficacy in grass carp, an important commercial fish species in Asia, remains untested. Here, our study evaluated the immunostimulatory effects of CYP on IgM B cells in vitro and on humoral immunity and immune defense against Aeromonas hydrophila infection in vivo. In vitro stimulation experiments showed that CYP could induce the secretion of IgM antibodies, because it could stimulate the proliferation and differentiation of head kidney IgM B cells. Moreover, CYP could also enhance the phagocytic activity of IgM B cells. When grass carp were intragastrically administrated (i.g.) with CYP, the serum IgM level increased, as did the capacity of serum IgM to bind to peptidoglycan (PGN) and lipopolysaccharide (LPS), indicating that CYP induced natural IgM production in grass carp. Correspondingly, the serum bactericidal ability to A. hydrophila enhanced. Immune protection experiments showed that CYP could protect grass carp from A. hydrophila infection in vivo, because the mortality rate and gut Aeromonas load of grass carp significantly decreased after CYP administration. Taken together, our study reveals immunostimulatory effects of CYP in grass carp, laying important foundation for the usage of CYP to control diseases in grass carp aquaculture.