A Legionella pneumophila effector impedes host gene silencing to promote virulence

RNA silencing is a gene silencing mechanism directed by small RNAs. Human miRNAs act as central regulators of host-bacteria interactions. However, it is unknown whether human pathogenic bacteria could impede RNA silencing to promote virulence. Here, we show that the Legionella pneumophila type IV-secreted effector LegK1 efficiently suppresses siRNA and miRNA activities in human cells. This effect depends on its known kinase activity, but also on its novel capacity, found here, to bind Argonaute (Ago) proteins. We further demonstrate that the ability of LegK1 to activate NF-{kappa}B signaling is required for RNA silencing suppression, establishing a link between effector-mediated NF-{kappa}B signaling and RNA silencing suppression. LegK1 also promotes L. pneumophila growth in both amoeba and human macrophages, supporting a role for this effector in virulence. Finally, we show that, in infected-macrophages, the latter activity relies, in part, on the genetic targeting of human Ago4. These findings indicate that a L. pneumophila effector has evolved to suppress RNA silencing to promote virulence. Significance StatementIt is now well established that mammalian viruses suppress RNAi to promote their replication in host cells. However, whether mammalian pathogenic bacteria use a similar virulence strategy remains unknown. Here, we show that the LegK1 effector from Legionella pneumophia, the causal agent of Legionnaires disease, efficiently suppresses RNAi in human cells. This effect depends on its ability to interact with Argonaute (Ago) proteins and to activate NF-{kappa}B signaling. In addition, LegK1 promotes virulence in infected-macrophages through the genetic targeting of human Ago4. Based on the lack of NF-{kappa}B-related factors in amoebae, and on the presence of canonical Ago proteins in these natural L. pneumophila hosts, we propose that the RNAi suppression activity of LegK1 represents its primary virulence function.