Acute osmotic stress affects Tilapia (Oreochromis mossambicus) innate immune responses.

Most of the tilapia studies are focused on its osmoregulatory mechanism. Meanwhile, less information is available about its innate immune response on fish faced with hyperosmolality. In the present study, in vivo analyses were carried out to investigate the innate immune response of Oreochromis mossambicus, transferred from freshwater to 25 ppt seawater (SW). In vivo, lysozyme activities of plasma and head kidney (HK) were increased at 1h and at 24h after transfer to SW but decreased at 8h after SW transfer. Surprisingly, the alternative complement pathway in plasma increased 8h after SW transfer. The phagocytic capacity of spleen and HK immune cells increased modestly at 1h and at 4h, after SW transfer, but the respiratory burst activity of immune cells in both HK and spleen shows an increase in superoxide release at 8h after SW transfer. Our results reveal that the transfer of fish from conditions of hypoosmolarity to hyperosmolality significantly enhances plasma lysozyme, ACP activity, and both phagocytic and respiratory burst activity. Taken together, the results indicate that exposure of tilapia to hyperosmotic conditions has immunostimulatory effects on its cellular immune reactions (phagocytosis and respiratory burst activity) and humoral reactions (lysozyme activity and complement activity).