Mucosal immune response of Nile tilapia Oreochromis niloticus during Gyrodactylus cichlidarum infection.

Abstract Monogenean Gyrodactylus cichlidarum can cause severe mortality of Nile tilapia (Oreochromis niloticus) fry. To date, reports about mucosal immunity of O. niloticus against this parasite have been rare. In order to explore the mucosal immunity of Nile tilapia against G. cichlidarum infection, the expressions of six adaptive immune-related genes and the contents of specific immunoglobulin IgM and IgT in the skin-associated lymphoid tissues (SALT) were dynamically analyzed after primary and secondary infections. The abundances of G. cichlidarum on the hosts after secondary infection were lower than those after primary parasite infection, which implied that hosts could initiate immune protection against G. cichlidarum reinfection to some degree. The transcription levels of TCR-β and CD4 genes in the skin tissue were significantly up-regulated after primary G. cichlidarum infection, while genes pIgR and IgT were only detected with significant up-regulations during secondary infection. With the exception of pIgR, which had remarkably higher expression in the fish with low parasite loads, all other genes studied tended to have higher mRNA level in the fish with higher parasite loads. The specific IgM content in the skin mucus increased significantly on the 2nd day after the primary exposure, higher than the corresponding value during the secondary exposure, and had significantly positive correlation with the parasite loads during the first parasite infection. These results manifested that acquired immune responses in the SALT of Nile tilapia participated in the resistance against G. cichlidarum infection, underscoring the involvement of mucosal immunity in fish against monogenean infection, and suggesting potential prophylactic treatment of gyrodactylid disease of tilapia.