Antiviral immunity in Drosophila requires systemic RNA interference spread

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Abstract
Drosophila and other insects are known to be able to mount a local antiviral defence involving RNA interference (RNAi). It was previously thought that Drosophila is unable to systemically spread an RNAi response, based on observations that endogenously expressed RNA hairpins did not spread from cell to cell. But experiments involving challenge with Sindbis and Drosophila C viruses now show that D. melanogaster can also generate a systemic RNAi response. This suggests that the RNA silencing component of immunity in vertebrates and invertebrates may be more highly conserved than was thought. RNA silencing is an important player in antiviral defence mechanisms. This paper provides evidence that RNA silencing possesses a systemic arm not only in plants but also in insects; systemic spread of dsRNA from cell to cell is an important component of the antiviral immune response in Drosophila. Multicellular organisms evolved sophisticated defence systems to confer protection against pathogens. An important characteristic of these immune systems is their ability to act both locally at the site of infection and at distal uninfected locations1,2,3,4. In insects, such as Drosophila melanogaster, RNA interference (RNAi) mediates antiviral immunity5,6,7. However, the antiviral RNAi defence in flies seems to be a local, cell-autonomous process, as flies are thought to be unable to generate a systemic RNAi response8. Here we show that a recently defined double-stranded RNA (dsRNA) uptake pathway9 is essential for effective antiviral RNAi immunity in adult flies. Mutant flies defective in this dsRNA uptake pathway were hypersensitive to infection with Drosophila C virus and Sindbis virus. Mortality in dsRNA-uptake-defective flies was accompanied by 100-to 105-fold increases in viral titres and higher levels of viral RNA. Furthermore, inoculating naked dsRNA into flies elicited a sequence-specific antiviral immune response that required an intact dsRNA uptake pathway. These findings suggest that spread of dsRNA to uninfected sites is essential for effective antiviral immunity. Notably, infection with green fluorescent protein (GFP)-tagged Sindbis virus suppressed expression of host-encoded GFP at a distal site. Thus, similar to protein-based immunity in vertebrates, the antiviral RNAi response in flies also relies on the systemic spread of a virus-specific immunity signal.