Legionella pneumophila Strain 130b Evades Macrophage Cell Death Independent of the Effector SidF in the Absence of Flagellin

Abstract

The human pathogen <i>Legionella pneumophila</i> must evade host cell death signaling to enable replication in lung macrophages and to cause disease. After bacterial growth, however, <i>L. pneumophila</i> is thought to induce apoptosis during egress from macrophages. The bacterial effector protein, SidF, has been shown to control host cell survival and death by inhibiting pro-apoptotic BNIP3 and BCL-RAMBO signaling. Using live-cell imaging to follow the <i>L. pneumophila</i>-macrophage interaction, we now demonstrate that <i>L. pneumophila</i> evades host cell apoptosis independent of SidF. In the absence of SidF, <i>L. pneumophila</i> was able to replicate, cause loss of mitochondria membrane potential, kill macrophages, and establish infections in lungs of mice. Consistent with this, deletion of BNIP3 and BCL-RAMBO did not affect intracellular <i>L. pneumophila</i> replication, macrophage death rates, and <i>in vivo</i> bacterial virulence. Abrogating mitochondrial cell death by genetic deletion of the effectors of intrinsic apoptosis, BAX, and BAK, or the regulator of mitochondrial permeability transition pore formation, cyclophilin-D, did not affect bacterial growth or the initial killing of macrophages. Loss of BAX and BAK only marginally limited the ability of <i>L. pneumophila</i> to efficiently kill all macrophages over extended periods. <i>L. pneumophila</i> induced killing of macrophages was delayed in the absence of capsase-11 mediated pyroptosis. Together, our data demonstrate that <i>L. pneumophila</i> evades host cell death responses independently of SidF during replication and can induce pyroptosis to kill macrophages in a timely manner.