Date Published: April 22, 2016
Publisher: Public Library of Science
Author(s): Pascal Ziltener, Thomas Reinheckel, Annette Oxenius, Zhao-Qing Luo.
Legionella pneumophila is a facultative intracellular bacterium that lives in aquatic environments where it parasitizes amoeba. However, upon inhalation of contaminated aerosols it can infect and replicate in human alveolar macrophages, which can result in Legionnaires’ disease, a severe form of pneumonia. Upon experimental airway infection of mice, L. pneumophila is rapidly controlled by innate immune mechanisms. Here we identified, on a cell-type specific level, the key innate effector functions responsible for rapid control of infection. In addition to the well-characterized NLRC4-NAIP5 flagellin recognition pathway, tumor necrosis factor (TNF) and reactive oxygen species (ROS) are also essential for effective innate immune control of L. pneumophila. While ROS are essential for the bactericidal activity of neutrophils, alveolar macrophages (AM) rely on neutrophil and monocyte-derived TNF signaling via TNFR1 to restrict bacterial replication. This TNF-mediated antibacterial mechanism depends on the acidification of lysosomes and their fusion with L. pneumophila containing vacuoles (LCVs), as well as caspases with a minor contribution from cysteine-type cathepsins or calpains, and is independent of NLRC4, caspase-1, caspase-11 and NOX2. This study highlights the differential utilization of innate effector pathways to curtail intracellular bacterial replication in specific host cells upon L. pneumophila airway infection.
L. pneumophila is a Gram-negative bacterium with global distribution in freshwater environments, where it replicates intracellularly mainly in amoebae [1–3]. L. pneumophila commonly causes community acquired and nosocomial pneumonia. Although it is normally controlled by the innate immune response, L. pneumophila has the potential to cause a severe pneumonia known as Legionnaires’ disease with mortality rates of up to 30% if early bacterial replication is not controlled [4–6]. Infection occurs through inhalation of L. pneumophila contaminated aerosols, mostly generated by manmade technologies such as cooling towers, air conditioners or even car windshield wipers [7–9]. In the lung L. pneumophila initially exclusively infects alveolar macrophages (AM), using a type IV secretion system (T4SS) to inject over 300 effector proteins into the cytosol [7,10–12]. These effectors block phagosomal maturation and fusion with lysosomes, thus preventing L. pneumophila degradation, and promoting the establishment of a Legionella containing vacuole (LCV), the intracellular niche in which L. pneumophila replicates [13–16].
In this study, we identified cell-type specific key innate immune effector functions responsible for effective control of pulmonary L. pneumophila lung infection. Neutrophil-mediated mechanisms that lead to L. pneumophila clearance in vivo are twofold. On the one hand, neutrophils directly kill L. pneumophila via ROS-mediated mechanisms, and on the other hand, neutrophil and monocyte-derived TNF initiates microbicidal mechanisms in AM via TNFR1, which increase their capacity to inhibit L. pneumophila replication. The latter involves rerouting the bacteria to lysosomal compartments despite the presence of T4SS effectors, and requires at least one caspase other than caspase-1 or 11. The importance of TNF and NOX2-mediated mechanisms in the control of L. pneumophila infection are underscored by the marked susceptibility of TNF-/- and CYBB-/- mice to L. pneumophila infection.