Legionella pneumophila-Derived Outer Membrane Vesicles Promote Bacterial Replication in Macrophages.
The formation and release of outer membrane vesicles (OMVs) is a phenomenon of Gram-negative bacteria. This includes Legionella pneumophila (L. pneumophila), a causative agent of severe pneumonia. Upon its transmission into the lung, L. pneumophila primarily infects and replicates within macrophages. Here, we analyzed the influence of L. pneumophila OMVs on macrophages. To this end, differentiated THP-1 cells were incubated with increasing doses of Legionella OMVs, leading to a TLR2-dependent classical activation of macrophages with the release of pro-inflammatory cytokines. Inhibition of TLR2 and NF-κB signaling reduced the induction of pro-inflammatory cytokines. Furthermore, treatment of THP-1 cells with OMVs prior to infection reduced replication of L. pneumophila in THP-1 cells. Blocking of TLR2 activation or heat denaturation of OMVs restored bacterial replication in the first 24 h of infection. With prolonged infection-time, OMV pre-treated macrophages became more permissive for bacterial replication than untreated cells and showed increased numbers of Legionella-containing vacuoles and reduced pro-inflammatory cytokine induction. Additionally, miRNA-146a was found to be transcriptionally induced by OMVs and to facilitate bacterial replication. Accordingly, IRAK-1, one of miRNA-146a's targets, showed prolonged activation-dependent degradation, which rendered THP-1 cells more permissive for Legionella replication. In conclusion, L. pneumophila OMVs are initially potent pro-inflammatory stimulators of macrophages, acting via TLR2, IRAK-1, and NF-κB, while at later time points, OMVs facilitate L. pneumophila replication by miR-146a-dependent IRAK-1 suppression. OMVs might thereby promote spreading of L. pneumophila in the host.
Institute for Lung Research, Universities of Giessen and Marburg Lung Center, Philipps-University Marburg, Member of the German Center for Lung Research (DZL), Marburg, Germany.
Anna Lena
AL
Jung
Institute for Virology, Philipps-University Marburg, Marburg, Germany.
Cornelia
C
Stoiber
Institute for Lung Research, Universities of Giessen and Marburg Lung Center, Philipps-University Marburg, Member of the German Center for Lung Research (DZL), Marburg, Germany.
Christina E
CE
Herkt
Institute for Lung Research, Universities of Giessen and Marburg Lung Center, Philipps-University Marburg, Member of the German Center for Lung Research (DZL), Marburg, Germany.
Christine
C
Schulz
Institute for Lung Research, Universities of Giessen and Marburg Lung Center, Philipps-University Marburg, Member of the German Center for Lung Research (DZL), Marburg, Germany.
Wilhelm
W
Bertrams
Institute for Lung Research, Universities of Giessen and Marburg Lung Center, Philipps-University Marburg, Member of the German Center for Lung Research (DZL), Marburg, Germany.|Department of Medicine, Pulmonary and Critical Care Medicine, University Medical Center Giessen and Marburg, Philipps-University, Member of the German Center for Lung Research (DZL), Marburg, Germany.
Bernd
B
Schmeck
D000818:Animals; D002462:Cell Membrane; D004797:Enzyme-Linked Immunosorbent Assay; D000067128:Extracellular Vesicles / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D006801:Humans; D016952:Legionella pneumophila; D007877:Legionnaires' Disease / Q000378:metabolism*; D008262:Macrophage Activation / Q000502:physiology; D008264:Macrophages / Q000378:metabolism / Q000382:microbiology*; D060888:Real-Time Polymerase Chain Reaction