Primers used in this study.

While robust interferon (IFN) responses in fish are critical for viral clearance, dysregulated signalling can trigger detrimental hyperinflammation, necessitating precise immunoregulatory mechanisms. This study identified Toll-interacting protein (TOLLIP) as a pivotal negative regulator of IFN production in grass carp (Ctenopharyngodon idella). Upon grass carp reovirus (GCRV) infection, TOLLIP expression increases significantly in tissues and cells. Furthermore, TOLLIP overexpression reduced GCRV- and polyinosinic-polycytidylic acid (poly I:C)-induced IFN expression, whereas tollip knockdown increased the cellular IFN production capacity. TOLLIP subsequently binds and degrades STING. Further mechanistic studies revealed that TOLLIP degrades STING in a dose-dependent manner via an autophagy-lysosome-dependent pathway. Interestingly, autophagy-related protein 5 (ATG5) was found to interact with TOLLIP and reduce TOLLIP-mediated STING degradation after atg5 knockdown. In addition, TOLLIP attenuated STING-driven IFN activation and compromised antiviral efficacy. These findings demonstrate that fish TOLLIP plays a specialized regulatory role in antiviral innate immunity, balancing immune defence with homeostasis maintenance.