Dietary Astragalus polysaccharides enhance potency of inactivated Pseudomonas plecoglossicida vaccine in large yellow croaker (Larimichthys crocea)

Dietary Astragalus polysaccharides (APS) get wide application in aquaculture due to their excellent immunoregulatory effects. However, little is known about the effects of dietary APS on vaccine potency in fish. In the present study, large yellow croakers (Larimichthys crocea) were injected with formalin-inactivated Pseudomonas plecoglossicida after APS feeding for 14 d and then challenged by live P. plecoglossicida on 28 d post-vaccination. The results showed that dietary APS combined with inactivated vaccine could improve the survival rate, and alleviate splenic lesions and bacteria load post-challenge, thus exhibiting a better protection in large yellow croaker against P. plecoglossicida infection than inactivated vaccine treatment alone. Fish in APS + P. plecoglossicida vaccine group expressed a better antioxidant status by possessing a relatively higher serum total antioxidant capacity (T-AOC) and superoxide dismutase (SOD) activity and a significantly lower malondialdehyde (MDA) content than those in vaccine alone group. Serum lysozyme (LZM) and alkaline phosphatase (AKP) activities, and immunoglobulin M (IgM) titers were all improved in fish of APS + P. plecoglossicida vaccine group compared to fish in vaccine group. Furthermore, fish in APS + P. plecoglossicida vaccine group showed a lower down-regulation of pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-6, and a higher up-regulation of anti-inflammatory cytokine IL-10, immunoglobulin (IgM) and T cell immunity-related cytokines, interferon-γ (IFN-γ), IL-4/13A, and IL-4/13B, when compared with those in fish of vaccine group. These results suggested that dietary APS could assist inactivated vaccine to trigger stronger innate and adaptive immune responses against P. plecoglossicida infection. These findings further uncover the immunoregulatory mechanism of dietary APS, and provide valuable information for prevention and control of bacteriosis in fish.