Alphavirus Infection Triggers Antiviral RNAi Immunity In Mammals

RNAi（RNA interference）is a well-established antiviral immunity in eukaryotes.However,for mammalian somatic cells,the common targets of viral infections,antiviral RNAi only becomes evident when viral suppressors of RNAi（VSR）are disabled by mutations or VSR-targeting drugs,thereby limiting its scope as a mammalian immunity.We uncovered that a wild-type alphavirus,Semliki Forest virus（SFV）,triggered the Dicer-dependent production of viral（v）si RNAs in both mammalian somatic cells and adult mice.These SFV-vsi RNAs are located at a particular region within the 5’-terminus of SFV genome and Argonaute-loaded.One pair of vsi RNAs account for up to 99%among them,which named vsi SFV.These vsi RNAs can specifically silence cognate viral RNAs and confer effective anti-SFV activity.Sindbis virus（SINV）,another alphavirus,also induced vsi RNA production in mammalian somatic cells.We find that these vsi RNAs are produced from conserved sequences at the 5’terminus of viarl genomic RNAs,termed AC Motif.Further,the AC Motif mutant SFV_{mut AA} is rescued and we find that SFV_{mut AA} infection can not induce RNAi antiviral immune responses,suggesting that AC Motif is required for the production of vsi RNAs.Moreover,We find enoxacin,an RNAi enhancer,can inhibit the replication of viral RNA dependently of RNAi response,and protect mice from SFV-induced neuropathogenesis and lethality These findings demonstrate that alphaviruses trigger the production of active vsi RNAs in mammalian somatic cells and adult mice,and the RNAi enhancer,enoxacin,has potent antiviral activity both in vitro and in vivo,highlighting the functional importance and therapeutic potential of antiviral RNAi in mammals.