Viperin suppresses goose astrovirus type 2 replication through inhibition of viral RdRP activity

Goose astrovirus 2 (GAstV-2) infection induces visceral gout in goslings, resulting in high mortality rates in birds aged 3-15 days. Despite extensive research since the virus was first identified in 2018, the innate immune defense mechanisms of the host against GAstV-2 remain incompletely characterized, impeding the development of effective vaccines and therapeutics. Here, we demonstrated that GAstV-2 infection upregulated Viperin expression both in vitro and in vivo, suggesting a potential role for Viperin as a host restriction factor. Further studies revealed that the P2 domain of viral ORF2 contributed to stimulate Viperin. Overexpression and knockdown experiments confirmed that Viperin exerts antiviral activity against GAstV-2. However, Viperin did not affect attachment or internalization. Mechanistic studies revealed that Viperin inhibits GAstV-2 by attenuating viral RNA-dependent RNA polymerase (RdRp) activity in a manner dependent on its S-adenosylmethionine (SAM) domain. The SAM domain was found to promote the production of 3'-deoxy-3',4'-didehydro-cytidine-5'-triphosphate (ddhCTP), which correlated with reduced RdRp activity and suppression of viral replication. Furthermore, intramuscular administration of ddhCTP reduced mortality and viral loads, and ameliorated GAstV-2-induced pathological injury in the kidney, spleen, and liver of goslings. Collectively, these findings identify Viperin as a key host restriction factor against GAstV-2 and highlight its potential as a target for antiviral strategies.