A Wolbachia pipientis protein confers resistance to virus infection in Drosophila melanogaster
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The intracellular bacterium Wolbachia pipientis alters the biology of its arthropod hosts in many ways, and can increase resistance to RNA viruses in both Drosophila and mosquitoes. Wolbachia -induced pathogen blocking has generated much interest because of its potential to restrict insect vector transmission of human diseases caused by RNA viruses. However, the molecular mechanisms by which Wolbachia affects host viral resistance are still mostly elusive. We used dilp2-3,5 mutant Drosophila , which are long-lived, but only in the presence of Wolbachia , to show that the presence of Wolbachia also increased the resistance of the mutant flies to infection with Drosophila C virus (DCV), relative both to mutants lacking Wolbachia and to wild type flies with and without Wolbachia . The insulin mutant flies had higher Wolbachia titres than wild type flies. By RNA-seq analysis of the Wolbachia transcriptome, we identified Wolbachia genes that were more strongly expressed in dilp2-3,5 mutant flies. Ankyrin-domain-containing proteins were among the most strongly up-regulated and they are predicted to be secreted effector proteins. To address their effect on host physiology, we generated 4 transgenic fly lines each with inducible expression of a different genes encoding ankyrin-domain-containing proteins. Expression of 3 of these did not cause obvious effects, but expression of WD0754 at high levels severely shortened fly survival. Interestingly, however, chronic low-level induction of WD0754 increased the resistance of the flies to DCV infection. Proteomics analysis showed a robust, tissue-specific, anti-viral response upon WD0754 induction, and identified the NFkB-like IMD pathway as a potential mediator of the antiviral activity of WD0754. Consistently, WD0754 expression activated Relish, a key downstream transcriptional mediator of IMD signalling, while loss-of Relish blocked the antiviral effect of WD0754. In summary, we identified a Wolbachia -derived ankyrin domain containing protein that modulates host immunity through the IMD pathway.
Authors’ summary
Wolbachia is an intracellular bacterium that is able to proliferate in many insect species. Part of its evolutionary success is due to its ability to protect its host from infection with RNA viruses, but how Wolbachia does this is unknown. Drosophila that are mutant for insulin signaling are often long-lived, but only in the presence of Wolbachia . We found that flies lacking several of the Drosophila insulin ligands were protected against infection by Drosophila C virus (DCV), but again only in the presence of Wolbachia . A group of Wolbachia genes were upregulated in the Drosophila insulin mutants, suggesting that they could be responsible for the anti-viral protection. When we expressed one of these possible effectors, the ankyrin-domain-containing protein WD0754, in flies it could provide protection from DCV infection and induced upregulation of anti-viral immunity proteins. We further characterised this WD0754-induced anti-viral immunity and found that it required Relish, a well-known Drosophila immune transcription factor. This work marks an important step in understanding Wolbachia -induced pathogen blocking, especially in the context of Wolbachia’s use as a biological control agent for vector-borne, pathogenic RNA viruses of humans.
