Alveolar Macrophages and Neutrophils Are the Primary Reservoirs for Legionella pneumophila and Mediate Cytosolic Surveillance of Type IV Secretion

Abstract

ABSTRACTLegionella pneumophila, an intracellular pathogen responsible for the severe pneumonia Legionnaires' disease, uses itsdot/icm-encoded type IV secretion system (T4SS) to translocate effector proteins that promote its survival and replication into the host cell cytosol. However, by introducing bacterial products into the host cytosol,L. pneumophilaalso activates cytosolic immunosurveillance pathways, thereby triggering robust proinflammatory responses that mediate the control of infection. Thus, the pulmonary cell types thatL. pneumophilainfects not only may act as an intracellular niche that facilitates its pathogenesis but also may contribute to the immune response againstL. pneumophila. The identity of these host cells remains poorly understood. Here, we developed a strain ofL. pneumophilaproducing a fusion protein consisting of β-lactamase fused to the T4SS-translocated effector RalF, which allowed us to track cells injected by the T4SS. Our data reveal that alveolar macrophages and neutrophils both are the primary recipients of T4SS-translocated effectors and harbor viableL. pneumophiladuring pulmonary infection of mice. Moreover, both alveolar macrophages and neutrophils from infected mice produced tumor necrosis factor and interleukin-1α in response to T4SS-sufficient, but not T4SS-deficient,L. pneumophila. Collectively, our data suggest that alveolar macrophages and neutrophils are both an intracellular reservoir forL. pneumophilaand a source of proinflammatory cytokines that contribute to the host immune response againstL. pneumophiladuring pulmonary infection.