Legionella pneumophila, an intracellular pathogen and a common cause of community-acquired pneumonia, employs sophisticated strategies to infect and replicate in alveolar macrophages (AMs). These strategies include translocating over 300 bacterial effector proteins into the host cytosol, where they manipulate various cellular pathways. However, infected cells have developed numerous strategies to detect and counteract the infection. The outcome of the host-pathogen interaction, whether it results in bacterial clearance or an extensive infection, depends on the balance between the pathogen’s virulence strategies and the host’s defense mechanisms. Several studies have investigated the host-pathogen interaction in vitro, mainly using murine and human hematopoietic cell culture systems. However, the exact mechanisms of bacterial detection by the innate immune system are incompletely explored, and little is known about how tissue-resident AMs respond to the infection. Within the first part of this study, the role of the C-type-lectin receptor CLEC12A, which binds to L. pneumophila, in the immune response was examined. The findings from infection experiments in a murine in vivo model and in murine and human macrophages in vitro indicate that the receptor has no significant impact on the outcome of the infection with L. pneumophila. The second part of the study investigated the response of in vivo L. pneumophila-infected and uninfected bystander AMs. Transcriptome analysis revealed a robust upregulation of various proinflammatory and immunoregulatory genes in infected AMs, while uninfected bystander cells seem to be only activated towards the end of the first replication cycle of L. pneumophila (20 h post infection). Proteome analyses further indicate that several proinflammatory proteins are impaired in their translation in virulent infected AMs (e.g., IL-1β, CCL6, CCL9) and that only a limited number of proteins including IL-1⍺, ATF3, GDF15, and A20 were found to be expressed on protein level in infected AMs. Furthermore, L. pneumophila seems to affect the cholesterol homeostasis of AMs in vivo. In conclusion, this study enables a deeper understanding of the immune response against L. pneumophila and provides a unique view of the overall cellular response in tissue-resident AMs towards the infection in vivo.
