Potentiation of Cytokine-Mediated Restriction of Intracellular Replication by a Dot/Icm-Translocated Effector.

is ubiquitous in freshwater environments, where it replicates within unicellular protozoa. However, is also an accidental human pathogen that can cause Legionnaires' disease in immunocompromised individuals by uncontrolled replication within alveolar macrophages. To replicate within eukaryotic phagocytes, utilizes a Dot/Icm type IV secretion system to translocate a large arsenal of over 300 effector proteins directly into host cells. In mammals, translocated effectors contribute to innate immune restriction of We found previously that the effector LegC4 is important for replication within a natural host protist but is deleterious to replication in a mouse model of Legionnaires' disease. In the present study, we used cultured mouse primary macrophages to investigate how LegC4 attenuates replication. We found that LegC4 enhanced restriction of replication within macrophages activated with tumor necrosis factor (TNF) or interferon gamma (IFN-γ). In addition, expression of was sufficient to restrict replication within TNF- or IFN-γ-activated macrophages. Thus, this study demonstrates that LegC4 contributes to clearance from healthy hosts by potentiating cytokine-mediated host defense mechanisms. spp. are natural pathogens of protozoa and accidental pathogens of humans. Innate immunity in healthy individuals effectively controls infection due in part to rapid and robust production of proinflammatory cytokines resulting from detection of Dot/Icm-translocated substrates, including effectors. Here, we demonstrate that the effector LegC4 enhances proinflammatory host restriction of by macrophages. These data suggest that LegC4 may augment proinflammatory signaling or antimicrobial activity of macrophages, a function that has not previously been observed for another bacterial effector. Further insight into LegC4 function will likely reveal novel mechanisms to enhance immunity against pathogens.