Recombinant Epinephelus lanceolatus serum amyloid A as a feed additive: Effects on immune gene expression and resistance to Vibrio alginolyticus infection in Epinephelus lanceolatus

Recombinant Epinephelus lanceolatus serum amyloid A (rElSAA) exhibits strong immunostimulant activity and enhances phagocytic clearance of bacteria by macrophages. However, the effects of dietary rElSAA supplementation on growth performance, immunomodulation and disease resistance in giant grouper have not been previously evaluated. To test whether oral administration of rElSAA affects growth, fish were fed with 0, 0.88, 4.4 or 22 mg/kg rElSAA-containing diet for 28 days. No statistically significant differences in body weight were observed between groups. Next, we tested whether oral administration of rElSAA may enhance disease resistance. Fish were fed with 0, 0.88, 4.4 or 22 mg/kg rElSAA-containing diet for 3, 7, 14, 21 or 28 days, followed by challenge with Vibrio alginolyticus. Survival was then monitored for 4 days. Fish that were fed with rElSAA-containing diet for 28 days showed significantly improved survival after infection. In addition, the expression levels of immune defense-associated genes in hepatic tissue were assessed by quantitative real-time polymerase chain reaction before and after V. alginolyticus infection. Oral administration of rElSAA increased the expression level of toll-like receptor 5, whereas the expression levels of CC chemokine 1, SAA and C reactive protein were decreased. Thus, the data suggest that rElSAA may enhance host immunity by attenuating regulatory T cell-mediated suppression of inflammation. Together, our results demonstrate that rElSAA is a promising candidate as a feed additive for giant grouper, which may effectively enhance disease resistance after being administered for several weeks.