The long and short of antiviral defense: small RNA-based immunity in insects

The host RNA interference (RNAi) pathway of insects senses virus infection and induces an antiviral response to restrict virus replication. Dicer-2 detects viral double-stranded RNA, produced by RNA and DNA viruses, and generates viral small interfering RNAs (vsiRNAs). Recent small RNA profiling studies provided new insights into the viral RNA substrates that trigger vsiRNA biogenesis. The importance of the antiviral RNAi pathway is underscored by the observation that viruses have evolved sophisticated mechanisms to counteract this small RNA-based immune response. More recently, it was proposed that another small RNA silencing mechanism, the piRNA pathway, also processes viral RNAs in Drosophila and mosquitoes. Here, we review recent insights into the mechanism of antiviral RNAi, viral small RNA profiles, and viral counter-defense mechanisms in insects.