Legionella pneumophila is an intracellular pathogen that is the dominant aetiological agent of Legionnaires’ Disease (LD), a particularly severe form of pneumonia. Upon inhalation of aerosols containing the bacteria, alveolar macrophages (AM) engulf the pathogen and initiate an inflammatory response. Despite this, L. pneumophila survives within AM by establishing a replicative vacuole through the injection of more than 300 bacterial virulence proteins termed Dot/Icm effector proteins into the macrophage cytosol.
Cigarette smoke (CS) is strongly linked to increased susceptibility to LD as well as many other infections. Here, we aimed to identify why cigarette smoking renders individuals more susceptible to LD. We characterised a model of concurrent acute CS exposure and L. pneumophila infection. CS-exposed mice displayed increased pulmonary bacterial loads and developed more severe disease when infected with L. pneumophila. Using flow cytometry, we found that one of the main effects of CS on lung leukocytes was a marked reduction of AM numbers, in contrast to the current belief that CS exposure causes expansion of AM. Therefore, we confirmed this observation using light-sheet microscopy to image whole lungs in order to avoid any potential artifacts caused by extraction of leukocytes. Although it was expected that AM depletion would ameliorate disease, as they are the cellular niche for Legionella replication, depletion of AM in mice with clodronate liposomes mimicked the effects of CS exposure.
In summary, our results show that despite being the cellular niche for Legionella replication, AM contribute towards clearance of infection. We hypothesize that one of the primary ways by which CS causes more severe LD is via depleting AM. Hence, targeting AM during LD may be detrimental to disease severity, and restoration of AM numbers or function in cigarette smokers may be a potential avenue of investigation for therapeutic intervention.