Nora Virus Persistent Infections Are Not Affected by the RNAi Machinery

Drosophila melanogaster is widely used to decipher the innate immune system in response to various pathogens. The innate immune response towards persistent virus infections is among the least studied in this model system. We recently discovered a picorna-like virus, the Nora virus which gives rise to persistent and essentially symptom-free infections in Drosophila melanogaster. Here, we have used this virus to study the interaction with its host and with some of the known Drosophila antiviral immune pathways. First, we find a striking variability in the course of the infection, even between flies of the same inbred stock. Some flies are able to clear the Nora virus but not others. This phenomenon seems to be threshold-dependent; flies with a high-titer infection establish stable persistent infections, whereas flies with a lower level of infection are able to clear the virus. Surprisingly, we find that both the clearance of low-level Nora virus infections and the stability of persistent infections are unaffected by mutations in the RNAi pathways. Nora virus infections are also unaffected by mutations in the Toll and Jak-Stat pathways. In these respects, the Nora virus differs from other studied Drosophila RNA viruses

Clearance is threshold-dependent.

<p>Viral RNA secreted during 24 hrs in feces of single flies, serially transferred to clean food. Every line represents a single fly followed during 19 days. n.d: not detected.</p

Nora virus control is siRNA-independent.

<p>Viral RNA was quantified in Nora virus-infected mutant flies, serially transferred for four days to clean food. <i>AGO2^51B</i>, <i>AGO2⁴¹⁴</i>, <i>Dcr-2^L811fsx</i>, <i>r2d2^S165fsX</i>, <i>piwi¹ and piwi²</i>, are null homozygous mutants. <i>Dcr</i>-<i>1^Q1147X</i> is a heterozygous null mutant. <i>hop^Tum</i> is a constitutive active mutant and <i>pll²</i> is a strong loss-of-function allele. Every dot represents the viral load in a single fly. n.d: not detected.</p

Nora virus clearance.

<p>Viral RNA in single flies kept (A) four days on contaminated food (unflip), or serially transferred for (B) four and (C) fourteen days to clean food (flip). Every dot represents the quantification from a single fly. OR: Oregon R, CS. Canton S, n.d: not detected.</p