Immune response of Exopalaemon carinicauda infected with an AHPND-causing strain of Vibrio parahaemolyticus

To investigate the immune response of Exopalaemon carinicauda infected with an AHPND-causing strain of Vibrio parahaemolyticus (VPAHPND), three-generation breeding of shrimp selected for their survival to VPAHPND infection was applied to explore the relationship between immune parameters and AHPND-resistant capacity of E. carinicauda. In this study, the LD50 dose of 48 h and survival rates at 144 h of shrimp to VPAHPND increased from 106.0 to 106.6 cfu ml−1 and from 26.67% to 36.67% by three successive generations selection, respectively, while there was no significant difference between the first and second generation (p > .05). Then the immune parameters including vibrio density, total hemocyte counts (THCs), hemocyanin (HEM) concentration, antibacterial activity, activities of four immune enzymes, and expressions of eight immune-related genes were determined in the shrimp of the first (G1) and the third selective generation (G3). The results showed that the shrimp in G1 and G3 generation cleared most of VPAHPND infecting hepatopancreas during 24 h and 6 h post injection, respectively. The levels of THCs, HEM concentration, antibacterial activity, immune enzymes including lysozyme (LZM) activity, alkaline phosphatase (AKP) activity in cell-free hemolymph, and the expression levels of Tollip, ALF, cathepsin B in hemocytes and hepatopancreas, crustin, LZM, SR in hepatopancreas and LGBP in hemocytes were higher in G3 generation than in G1 generation after infection with VPAHPND, suggesting that these parameters may serve as potential disease-resistant indicators for evaluating the physiological status and disease-resistant capability of shrimp when infected with VPAHPND. To further test the role of above genes in the shrimp immune response, RNAi was used to suppress their expressions and a significant decrease in survival was observed in knockdown shrimp infected with VPAHPND as compared to controls.