Anti‐apoptotic signalling by the Dot/Icm secretion system of L. pneumophila

Summary The Dot/Icm type IV secretion system of Legionella pneumophila triggers robust activation of caspase‐3 during early and exponential stages of proliferation within human macrophages, but apoptosis is delayed till late stages of infection, which is novel. As caspase‐3 is the executioner of the cell, we tested the hypothesis that L. pneumophila triggers anti‐apoptotic signalling within the infected human macrophages to halt caspase‐3 from dismantling the cells. Here we show that during early and exponential replication, L. pneumophila ‐infected human monocyte‐derived macrophages (hMDMs) exhibit a remarkable resistance to induction of apoptosis, in a Dot/Icm‐dependent manner. Microarray analyses and real‐time PCR reveal that during exponential intracellular replication, L. pneumophila triggers upregulation of 12 anti‐apoptotic genes that are linked to activation of the nuclear transcription factor kappa‐B (NF‐κB). Our data show that L. pneumophila induces a Dot/Icm‐dependent sustained nuclear translocation of the p50 and p65 subunits of NF‐κB during exponential intracellular replication. Bacterial entry is essential both for the anti‐apoptotic phenotype of infected hMDMs and for nuclear translocation of the p65. Using p65 –/– and IKKα –/– β –/– double knockout mouse embryonic fibroblast cell lines, we show that nuclear translocation of NF‐κB is required for the resistance of L. pneumophila ‐infected cells to apoptosis‐inducing agents. In addition, the L. pneumophila ‐induced nuclear translocation of NF‐κB requires the activity of IKKα and/or IKKβ. We conclude that although the Dot/Icm secretion system of L. pneumophila elicits an early robust activation of caspase‐3 in human macrophages, it triggers a strong anti‐apoptotic signalling cascade mediated, at least in part by NF‐κB, which renders the cells refractory to external potent apoptotic stimuli.