Furthermore, mechanistic studies have revealed that virally encoded suppressors can act at different steps in the silencing pathway, including Dicer-2 processing and Ago2 slicing [4],

suggesting that indeed, the entire pathway is required for defense. In contrast to RNA viruses, very little is known about the interactions of DNA viruses with the antiviral RNA-silencing machinery, particularly in arthropods. If these Z-VAD-FMK nmr viruses were restricted by the RNAi machinery, the DNA genome could not be targeted directly; rather, RNA transcripts from the viral genome would form structures with double-stranded character that would be recognized and processed by Dicer-2 (Fig. 1A). In Drosophila, a recent study by Bronkhorst et al. [15] found that overlapping bidirectional transcription of the dsDNA virus invertebrate iridescent virus 6 (IIV-6) likely leads to the formation of dsRNA in trans, which

is processed by Dicer-2 into small RNAs. Conversely, small RNAs produced in wild-caught mosquitoes infected with a ssDNA densovirus, which has no overlapping convergent transcripts, map predominantly to the viral RNA transcripts, suggesting that local interactions within a single-stranded RNA strand form dsRNA in cis that are targeted by antiviral RNAi [16]. ICG-001 However, the mechanism by which the insect RNAi pathway restricts infection of DNA viruses remains poorly understood, and is an important subject of future study. Shrimp are arthropods of agricultural and ecological importance, and white spot syndrome virus (WSSV) is a highly pathogenic dsDNA virus that impacts aquaculture and is thought to have caused over $15 billion in losses [17]. It has been demonstrated that sequence-specific long dsRNAs could confer antiviral immunity against WSSV, as well as against the shrimp RNA virus Taura syndrome virus [18]. Moreover, injection of a synthetic siRNA against WSSV VP28, a viral envelope protein, conferred sequence-specific antiviral resistance [19]. Therefore, both long dsRNAs and synthetic siRNAs induce sequence-specific antiviral immunity in shrimp. Whether the shrimp RNAi pathway

naturally targets RNA or DNA viral pathogens remained unclear. However, in this issue of the European Journal of Immunology, Huang and Zhang examine whether the RNAi pathway directs an antiviral immune response against the dsDNA virus WSSV in shrimp [20]. Since a synthetic siRNA designed to target VP28 (vp28-siRNA) Casein kinase 1 is capable of controlling infection, Huang and Zhang first asked whether vp28-siRNA is produced naturally during infection of the shrimp Marsupenaeus japonicus with WSSV. Indeed, vp28-siRNA can be detected by northern blotting and small RNA sequencing of infected tissues. Expression of vp28-siRNA in various shrimp tissues is dependent upon WSSV infection, as the siRNA cannot be detected in tissues where WSSV does not replicate to detectable levels. Thus, vp28-siRNA is a virus-derived small RNA that is generated from WSSV transcripts during infection.