Grass carp reovirus-II NS38 mediates immune evasion by rescuing VP41 from ISG15-mediated degradation

Grass carp (Ctenopharyngodon idella) hemorrhagic disease, mainly caused by type II grass carp reovirus (GCRV-II), poses a major threat to grass carp aquaculture. Although interferon-stimulated genes (ISGs) are central to host antiviral defense, the ISGs that restrict GCRV-II infection and the mechanisms by which the virus counteracts them remain poorly understood. Here, we identify nine ISGs induced by GCRV-II infection, seven of which suppress viral replication. Among these, the ISG15 homologs ISG15-1 and ISG15-2 (ISG15s) exhibit potent antiviral activity and function primarily in their free forms rather than through ISGylation. Mechanistically, free ISG15s interact with viral NS79 and VP41 and promote their degradation through the autophagy- lysosomal pathway. As a countermeasure, GCRV-II uses NS38 to competitively bind to ISG15s for rescuing ISG15s-mediated degradation of VP41. Our findings provide mechanistic insight into the antiviral activity of grass carp ISG15s against GCRV-II and reveal how GCRV-II selectively evades this host defense, which will be helpful for the development of antiviral drugs against GCRV-II infection. This study investigates the antiviral functions of grass carp ISG15 homologs and the immune evasion strategy of grass carp reovirus II. The findings reveal that free ISG15 proteins restrict viral replication by promoting degradation of viral proteins through the autophagy-lysosomal pathway, while GCRV-II uses NS38 to counteract ISG15-mediated antiviral defense by rescuing VP41 stability.