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Extracellular vesicles from eukaryotic cells and outer membrane vesicles (OMVs) released from gram-negative bacteria have been described as mediators of pathogen-host interaction and intercellular communication.  Legionella pneumophila  ( L .  pneumophila ) is a causative agent of severe pneumonia. The differential effect of bacterial and host cell vesicles in  L .  pneumophila  infection is unknown so far. We infected THP-1-derived or primary human macrophages with  L .  pneumophila  and isolated supernatant vesicles by differential centrifugation. We observed an increase of exosomes in the 100 k pellet by nanoparticle tracking analysis, electron microscopy, and protein markers. This fraction additionally contained Legionella LPS, indicating also the presence of OMVs. In contrast, vesicles in the 16 k pellet, representing microparticles, decreased during infection. The 100 k vesicle fraction activated uninfected primary human alveolar epithelial cells, A549 cells, and THP-1 cells. Epithelial cell activation was reduced by exosome depletion (anti-CD63, or GW4869), or blocking of IL-1β in the supernatant. In contrast, the response of THP-1 cells to vesicles was reduced by a TLR2-neutralizing antibody, UV-inactivation of bacteria, or – partially – RNase-treatment of vesicles. Taken together, we found that during  L .  pneumophila  infection, neighbouring epithelial cells were predominantly activated by exosomes and cytokines, whereas myeloid cells were activated by bacterial OMVs.

Legionella pneumophila infection activates bystander cells differentially by bacterial and host cell vesicles