Anti-apoptotic signalling by the Dot/Icm secretion system ofL. pneumophila

Abstract

SummaryThe 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 antiapoptotic 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-kB). Our data show that L. pneumophila induces a Dot/Icm-dependent sustained nuclear translocation of the p50 and p65 subunits of NF-kB 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 IKKa -/-b -/-double knockout mouse embryonic fibroblast cell lines, we show that nuclear translocation of NF-kB is required for the resistance of L. pneumophila-infected cells to apoptosis-inducing agents. In addition, the L. pneumophila-induced nuclear translocation of NF-kB requires the activity of IKKa and/or IKKb. 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-kB, which renders the cells refractory to external potent apoptotic stimuli.

Legionella pneumophila inhibits macrophage apoptosis by targeting pro-death members of the Bcl2 protein family

“…These observations indicate that an apoptotic process is initiated in macrophages in response to L. pneumophila infection and replication, but the bacteria are able to inhibit or reverse the process in a Dot/Icm-dependent manner, possibly by injecting effectors that can interfere with apoptotic pathways. Consistent with this hypothesis, two recent studies showed that a large number of antiapoptotic genes were induced in cells infected by virulent L. pneumophila strains via the activation of the multifunctional transcriptional regulator NF-B (17,18). Moreover, the Dot/Icm substrate SdhA was recently shown to be required for protecting macrophages from cell death by a yet-unknown mechanism (19).…”

“…Such modulation can occur at the transcriptional level because infection by L. pneumophila leads to the induction of several antiapoptotic genes via the activation of an NF-B-dependent pathway (17,18). Transcription of a number of pro-death genes, including the BH-3-only protein BNIP3 and many caspases, also was elevated in response to intracellular growth of the bacterium (17).…”

“…Such modulation can occur at the transcriptional level because infection by L. pneumophila leads to the induction of several antiapoptotic genes via the activation of an NF-B-dependent pathway (17,18). Transcription of a number of pro-death genes, including the BH-3-only protein BNIP3 and many caspases, also was elevated in response to intracellular growth of the bacterium (17). The induction of pro-death genes poses a problem for infected cells because these proteins sensitize cells to environmental insults, which, during L. pneumophila infection, can be represented by intracellular growth of the bacterium or the translocation of toxic substrates into host cells (19).…”

The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

“…A second facet of innate immunity that controls cytokine transcription and is triggered by L. pneumophila infection in murine and human cells is signaling through NF-B (32,33,(35)(36)(37)(38)(39). Thus, we assessed levels of I B␣, the inhibitor of NF-B that keeps the transcription factor sequestered in the cytoplasm of the cell.…”

A Legionella pneumophila -translocated substrate that is required for growth within macrophages and protection from host cell death

“…Although the killing of target cells may play some role in the pathogenesis of disease within the animal, host cell death is distinctly antagonistic to intracellular replication (23), and it is known that the bacterium limits host cell death by the induction of macrophage anti-apoptotic proteins (24). L. pneumophila, however, also uses other strategies to interfere with host cell death, as there seem to be Dot/Icmdependent signals that antagonize host cell death (25).…”