Effects of chronic alkalinity stress on immune response in Nile tilapia (Oreochromis niloticus) infected with Aeromonas hydrophila

Under suitable environmental conditions, fish have a functional immune system and are therefore expected to have high disease resistance, growth rate and reproductive activity. To explore the effects of chronic alkalinity stress on the immune response in Nile tilapia, tilapia (291.1 ± 47.2 g) were exposed to 10 mmol/L and 30 mmol/L of sodium bicarbonate alkaline water (AW). At 31 d of alkalinity stress, the fish were infected by Aeromonas hydrophila and the cumulative mortality was calculated within 7 d. The immune-related indexes of peripheral blood cells, enzyme activities (Lys, Alp, C3 and IgM) and gene expression (tlr5, il-1β and γ-ifn) in the liver, spleen and gill of Nile tilapia were assessed at 1 and 31 d of alkalinity stress and 1 and 7 d post-infection. The results showed that: 1) the cumulative mortality of tilapia in 10 mmol/L AW was 26.7%, which was significantly lower than 66.7% in 30 mmol/L AW and 51.11% in freshwater (FW); 2) At 1 d post-infection, the respiratory burst and lymphocyte proliferation rates at 10 mmol/L AW were higher than at 30 mmol/L AW and FW; 3) Alp activity at 10 mmol/L AW was higher than at 30 mmol/L AW in the liver and spleen after infection; 4) At 31 d of alkalinity stress and 7 d post-infection, the tlr5 mRNA expression level in the liver and spleen at 10 mmol/L AW was higher than at 30 mmol/L AW and FW. At 7 d post-infection, the γ-ifn mRNA expression level in the gill of tilapia in 10 mmol/L AW was higher than in 30 mmol/L AW and FW. The il-1β mRNA expression level at 30 mmol/L AW was significantly higher than at 10 mmol/L AW at most time points in the spleen and gill; 5) At 7 d post-infection, the activities or expression level of Lys, Alp, C3, IgM, tlr5 and γ-ifn in the spleen of tilapia in 30 mmol/L AW were lower than in 10 mmol/L AW and FW. The above results showed that long-term feeding of Nile tilapia in 10 mmol/L carbonate AW can promote immune function and reduce mortality from bacterial infection. •Long-term low carbonate alkalinity (10 mmol/L) can reduce mortality from Aeromonas hydrophila infection in tilapia.•Long-term low carbonate alkalinity (10 mmol/L) can improve immune performance in tilapia by improving leukocyte activity.•Immune function in the spleen of tilapia was inhibited upon high alkalinity stress (30 mmol/L).•Long-term high carbonate alkalinity (30 mmol/L) did not increase resistance to pathogens.