Effects of an exopolysaccharide from Lactococcus lactis Q-9 on innate immune response, antioxidant activity, and disease resistance against Aeromonas hydrophila in Cyprinus carpio

To investigate the effects of the oral administration of exopolysaccharides from Lactococcus lactis Q-9 (EPS-9) on the immunoregulatory properties, antioxidant activities and resistance against Aeromonas hydrophila in Cyprinus carpio, the purified EPS-9 (250, 500 and 1 000 μg/mL) were co-cultured with the head kidney cells of C. carpio. The proliferation and phagocytosis activities of the head kidney cells, and the concentration of nitric oxide (NO) and cytokines in the culture medium were determined. Next, 300 C. carpio (47.66±0.43) g) were randomly divided into five groups; the two control groups (negative and positive) were administered sterile PBS and the three treatment groups were administered different concentrations of EPS-9 (250, 500 and 1 000 μg/mL) for seven days. Subsequently, the positive and treatment groups were infected with A. hydrophila, and the negative group was treated with sterile PBS for 24 h. The concentration of NO, cytokines, lysozyme (LZM) and alkaline phosphatase (AKP) in serum, the antioxidative indexes (T-AOC, T-SOD, CAT, GSH, GSH-Px, MDA) in the hepatopancreas of the C. carpio were tested. We observed that EPS-9 could significantly enhance the proliferation and phagocytosis activities, besides inducing the production of NO, pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and anti-inflammatory cytokines (IL-10, TGF-β) in vitro. The in vivo experimental results revealed that EPS-9 significantly enhanced NO production, protein levels of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), LZM and AKP activities in the serum, and the antioxidant level in the hepatopancreas compared to that observed in the negative group prior to A. hydrophila infection. NO, pro-inflammatory cytokines, LZM and AKP activities were significantly lower than that in the positive group after infection. However, whether infected or not, the expression of anti-inflammatory cytokines (IL-10, TGF-β) increased significantly in the EPS-9-treated groups. Therefore, the results suggested that EPS-9 could enhance the non-specific immunity, antioxidant activities and anti-A. hydrophila activity of C. carpio in vitro and in vivo, and could be used as a safe and effective feed additive in aquaculture.