Pathological protection and molecular regulation of cystatin B in Nile tilapia (Oreochromis niloticus) bacterial disease.

Cystatin B (CSTB) is an endogenous cysteine protease inhibitor that plays a critical role in the modulation of numerous biological processes, involving immune responses, apoptosis, and inflammation in mammals. However, its immunological functions in teleost, particularly Nile tilapia (Oreochromis niloticus) remain unclear. In the present study, the CSTB was cloned and characterized from Nile tilapia (On-CSTB) and its role in bacterial infection was revealed. The On-CSTB open reading frame is 300 bp encoding 99 amino acids that contains a conserved cystatin domain. Multiple sequence alignment analysis revealed that On-CSTB share over 75% identity with fish lineages, and 45% identity with mammals. qPCR analysis showed that On-CSTB is widely expressed across various tissues and highly expressed in blood cells and intestines and can be significantly inducted by Streptococcus agalactiae (S. agalactiae) and Aeromonas hydrophila (A. hydrophila). In vivo experiments demonstrated that On-CSTB protein could suppress inflammation while participating in the MyD88/NF-κB signaling pathway and inflammasome activation, affecting apoptosis and pyroptosis processes. Moreover, On-CSTB contributed to enhanced tissue integrity and alleviated pathological damage. These findings collectively highlight On-CSTB as a crucial immunomodulator that contributes to host defense and tissue protection in fish. The present study offers novel perspectives on the immunomodulatory role of CSTB in tilapia, providing a basis for disease resistance strategies in aquaculture.