Granzyme K provides protection for Nile tilapia (Oreochromis niloticus) from acute bacterial infection.

Granzyme K (GzmK) is a trypsin-like serine protease that functions as a pro-apoptotic protease, has non-cytotoxic functions, and is involved in various physiopathological processes. However, the impact of GzmK on the immunological response of Nile tilapia against bacterial challenge is still poorly explored. In this study, we identified the GzmK gene from Nile tilapia ( On-GzmK ) and investigated its immunologic roles through in vivo experiments. On-GzmK has an open reading frame of 786 bp and encodes 255 amino acids. On-GzmK has over 60 % genetic similarity with other fish and over 30 % similarity with mammals. Its transcript levels were highest in the liver and vastly increased after being challenged with Streptococcus agalactiae and Aeromonas hydrophila. In vivo experiments implied that On-GzmK might promote inflammatory responses and regulate apoptosis and pyroptosis by participating in the activation of multiple signaling pathways, such as JAK-STAT, MAPK, and NF-κB. The present observations offer additional theoretical support for investigating the processes by which GzmK shields fish against bacterial infections. • GzmK is widely distributed in various tissues in healthy Nile tilapia. • GzmK transcript levels increased in Nile tilapia after bacterial infection, indicating its immunological involvement. • rOn-GzmK regulates the expression of immune genes and participates in the activation of signalling pathways. • On-GzmK is mainly expressed in non-specific cytotoxic cells (NCC) and induces apoptosis and pyroptosis.