Induction and suppression of NF-κB signalling by a DNA virus of Drosophila

Interactions between the insect immune system and RNA viruses have been best studied in Drosophila, where RNA interference, NF-O_SCPLOWKC_SCPLOWB and JAK-STAT pathways underlie antiviral immunity. In response to these immune mechanisms, insect viruses have convergently evolved suppressors of RNA interference that act by diverse mechanisms to permit viral replication. However, interactions between the insect immune system and DNA viruses have received less attention, primarily because few Drosophila-infecting DNA virus isolates are available. Here, we use a recently-isolated DNA virus of Drosophila melanogaster, Kallithea virus, to probe known antiviral immune responses and virus evasion tactics in the context of DNA virus infection. We find that fly mutants for RNA interference and Immune deficiency (Imd), but not Toll, pathways are more susceptible to Kallithea virus infection. We identify the Kallithea virus-encoded protein gp83 as a potent inhibitor of Toll signalling, strongly suggesting that Toll mediates antiviral responses during Kallithea virus infection, but that it is suppressed by the virus. Further, we find that Kallithea gp83 inhibits Toll signalling either through NF-O_SCPLOWKC_SCPLOWB transcription factor regulation, or transcriptionally. Together, these results provide a broad description of known antiviral pathways in the context of DNA virus infection and identify the first Toll pathway inhibitor in a Drosophila virus, extending the known diversity of insect virus-encoded immune inhibitors.