Intrabacterial regulation of a cytotoxic effector by its cognate metaeffector promotes Legionella pneumophila virulence

Abstract

Legionella pneumophila is a natural pathogen of unicellular protozoa that can opportunistically infect macrophages and cause Legionnaires Disease. Intracellular replication is driven by hundreds of bacterial effector proteins that are translocated into infected host cells by a Dot/Icm type IV secretion system. L. pneumophila effectors are temporally regulated in part by a unique family of translocated regulatory effectors, termed metaeffectors, which bind and modulate the function of a cognate effector in host cells. We discovered that regulation of the cytotoxic effector SidI by its metaeffector, MesI, is critical for L. pneumophila virulence in natural and opportunistic hosts. MesI binds and negatively regulates SidI activity in vitro but how dysregulation of SidI impairs L. pneumophila intracellular replication is unclear. Using a chromosomally-encoded inducible expression system, we discovered that dysregulation of SidI, via loss of MesI, was toxic to L. pneumophila. SidI enzymatic activity was required for toxicity since L. pneumophila growth was unaffected by induced expression of a catalytically inactive sidI allele. We found MesI translocation into host cells was dispensable for intracellular replication and that MesI-deficient bacteria were rapidly degraded within host cells. Together, our data suggest a unique role for intrabacterial effector regulation by a translocated metaeffector in L. pneumophila virulence. ImportanceLegionella pneumophila replicates within phagocytic host cells using hundreds of effector protein virulence factors, which canonically subvert the function of host proteins and pathways. L. pneumophila encodes a unique family of translocated effectors called metaeffectors, which bind and regulate the function of a cognate effector in host cells. The metaeffector MesI promotes L. pneumophila virulence by regulating the cytotoxic effector SidI; however, the MesI regulatory mechanism is poorly understood. We discovered a unique intrabacterial role for MesI in L. pneumophila virulence. When uncoupled from MesI, SidI was toxic to L. pneumophila in vitro and triggered robust bacterial degradation in host cells. Importantly, translocation of MesI was dispensable for intracellular replication, demonstrating that intrabacterial regulation of SidI contributes to L. pneumophila virulence. These data show a unique and important role for MesI regulation of SidI within the bacterium, which challenges the dogma that effectors function exclusively within host cells.