A time-dependent interactive effect of nitrite and ammonia on inflammatory and immune response in the head kidney of silver carp (Hypophthalmichthys molitrix)

Nitrite and ammonia stress frequently have harmful effects on aquatic animals. However, the effect of ammonia combined with nitrite on immunity is unclear. Aimed to determine their interactive effect on head kidney, silver carp were exposed to ammonia (15 mg/L), nitrite (10 mg/L), or ammonia+nitrite (15 mg/L + 10 mg/L), and control conditions for 48 h. The results showed that exposure to nitrite and ammonia caused loss of cytoplasm and vacuolar degeneration of cells in head kidney. Following exposure to nitrite and ammonia, tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β) levels decreased significantly, while immunoglobulin M (IgM) and complement 3 (C3) levels increased significantly (P < 0.05). Additionally, TNF-α and C3 were significantly affected synergistically by 24 h of exposure to ammonia+nitrite, while significantly antagonistic effect on IL-1β and C3 was observed after 48 h of exposure. After 48 h of exposure, differently expressed genes (DEGs) induced by ammonia, nitrite, and their combination were mainly assigned to molecular function and biological process, and complement and coagulation cascade pathway was enriched with the highest number of immunity-associated DEGs. Integrated biomarker response (IBR) in nitrite group was higher than that in ammonia group, and the combination of ammonia and nitrite increased and decreased the IBR induced by ammonia and nitrite after 24 and 48 h of exposure, respectively. In conclusion, the toxicities of ammonia and nitrite toward head kidney displayed inflammatory suppression and immune activation, and their had synergistic and antagonistic effects on head kidney at 24 h and 48 h, respectively.