Francisella tularensis Outer Membrane Vesicles Participate in the Early Phase of Interaction With Macrophages

Abstract

Francisella tularensis is known to release unusually shaped tubular outer membrane vesicles (OMV) containing a number of previously identified virulence factors and immunomodulatory proteins. In this study, we present that OMV isolated from the F. tularensis subsp. holarctica strain FSC200 enter readily into primary bone marrow-derived macrophages (BMDM) and seem to reside in structures resembling late endosomes in the later intervals. The isolated OMV enter BMDM generally via macropinocytosis and clathrin-dependent endocytosis, with a minor role played by lipid raft-dependent endocytosis. OMVs proved to be non-toxic and had no negative impact on the viability of BMDM. Unlike the parent bacterium itself, isolated OMV induced massive and dose-dependent proinflammatory responses in BMDM. Using transmission electron microscopy, we also evaluated OMV release from the bacterial surface during several stages of the interaction of Francisella with BMDM. During adherence and the early phase of the uptake of bacteria, we observed numerous tubular OMV-like protrusions bulging from the bacteria in close proximity to the macrophage plasma membrane. This suggests a possible role of OMV in the entry of bacteria into host cells. On the contrary, the OMV release from the bacterial surface during its cytosolic phase was negligible. We propose that OMV play some role in the extracellular phase of the interaction of Francisella with the host and that they are involved in the entry mechanism of the bacteria into macrophages.

Outer Membrane Vesicles Released from Garlic Exosome‐like Nanoparticles (GaELNs) Train Gut Bacteria that Reverses Type 2 Diabetes via the Gut‐Brain Axis

“…[ 53–55 ] OMVs, being extracellular nanovesicles released by bacteria, exhibit heterogeneity in size (40–300 nm diameter), [ 11,56–60 ] where the size dictates both the entry into host cells and the protein composition. [ 61–63 ] In our investigation, we noted that a fraction of orally administered A. muciniphila OMVs evades uptake during the journey from the gut to the brain, escaping uptake by other cell types and ultimately being taken up by microglial cells. We propose that a specific subset of OMVs possesses both an “eat me” signal tailored for brain microglial cells and a “do not eat me” signal that is universal for resident cells such as macrophages in other tissues.…”

Bacterial membrane vesicles in the pathogenesis and treatment of inflammatory bowel disease

“…The MVs of Francisella tularensis were reported to be involved in the entry of the bacteria into macrophages. 135 Furthermore, dysbiosis enables the overgrowth of harmful bacteria in the gut, resulting in the increased secretion of their MVs in the gut lumen. Studies have also shown that IBD patients exhibited elevated levels of MVs in their feces compared to healthy individuals, 142 which has been attributed to the dysbiotic gut microbiota associated with IBD.…”

Tiny but mighty: Possible roles of bacterial extracellular vesicles in gut‐liver crosstalk for non‐alcoholic fatty liver disease

“…Francisella tularensis bEVs were found to be involved in the entry mechanism of the bacteria into macrophages. 78 Secondly, bEVs induce immune responses against pathogens in macrophages. bEVs secreted by Paracoccidioides brasiliensis induced the production of proinflammatory mediators and M1 polarization in murine macrophages.…”

“…Firstly, bEVs play a role in entrance and spreading of pathogens and act directly on macrophages. Francisella tularensis bEVs were found to be involved in the entry mechanism of the bacteria into macrophages 78 . Secondly, bEVs induce immune responses against pathogens in macrophages.…”