Temporal resolution of two-tracked NF-κB activation by Legionella pneumophila

Abstract

The intracellular pathogen Legionella pneumophila activates the transcription factor NF-κB in macrophages and human epithelial cells, contributing to cytokine production and anti-apoptosis. The former is important for the innate immune response to infection, the latter for intracellular replication by securing host cell survival. Here, we demonstrate biphasic activation of NF-κB by L. pneumophila in human epithelial cells, using a p65-GFP expressing variant of A549 cells. Early in infection, a strong but transient nuclear translocation of p65 was observed. Only flagellindeficient (ΔfliA and ΔflaA) mutants could not induce this first, TLR5 and MyD88-dependent activation. The second p65 translocation event, however, is a long-term activation, independent of flagellin, TLR5 and MyD88, and marked by permanent nuclear localization of p65-GFP without oscillation for 30 h. Persistent p65 translocation also involved degradation of IκBa and upregulation of anti-apoptotic genes. L. pneumophila mutants lacking a functional Dot/Icm secretion system (ΔdotA; ΔicmB/dotO), Dot/Icm effectors (ΔsdbA; ΔlubX) and two bacterial effector mutants (ΔenhC; ΔptsP) could not induce persistent p65 translocation. Strikingly, all these mutants were deficient in intracellular replication in A549 cells. Our data underline the strong connection between NF-κB activation and intracellular replication and hints at an active interference of NF-κB signalling by L. pneumophila. © 2009 Blackwell Publishing Ltd.