Betaine protects ayu (Plecoglossus altivelis) against Vibrio anguillarum infection in salinity by regulating the immunomodulatory activity of monocytes/macrophages

Abstract Salinity stress have detrimental effect on teleosts and may increase susceptibility to disease. Betaine is a compound utilized by organisms to regulate osmotic pressure and plays an important role in the immune response and anti-inflammatory functions. Here, the protective effects of betaine against Vibrio anguillarum infection of ayu under conditions of salinity was determined. A total of 60 ayu were cultured at 10 ppt salinity for 14 days, all fish were intraperitoneally injected with 100 μL of 100 or 200 mg/kg of betaine or saline on days 0, 2, 4, and 6. The fish were subsequently infected with V. anguillarum. The results showed that betaine treatment improved the survival rate of V. anguillarum-infected ayu. Betaine treatment decreased the bacterial load and histological damage in the head-kidney, liver, and spleen. Besides, the inflammatory response was decreased by betaine in V. anguillarum-infected ayu tissues and monocytes/macrophages (MO/MФ). Furthermore, in hypertonic-exposed MO/MФ (19 mOsm), betaine treatment enhanced phagocytosis activity and promoted the bacterial killing capability. Finally, we found that betaine mediated the inflammatory response through BGT-1 in V. anguillarum-infected ayu MO/MФ. In conclusion, when exposed to osmotic stress, betaine was able to mediate the host defense in ayu and modulate the antibacterial response through MO/MФ.