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

Ivona Pavkova; Jana Klimentova; Jan Bavlovic; Lenka Horcickova; Klara Kubelkova; Erik Vlcak; Helena Raabova; Vlada Filimonenko; Ondrej Ballek; Jiri Stulik

doi:10.3389/fmicb.2021.748706

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

Front Microbiol. 2021 Oct 15:12:748706. doi: 10.3389/fmicb.2021.748706. eCollection 2021.

Authors

Affiliations

¹ Department of Molecular Pathology and Biology, Faculty of Military Health Sciences, University of Defence, Hradec Kralove, Czechia.
² Electron Microscopy Core Facility, Institute of Molecular Genetics of the Czech Academy of Sciences, Prague, Czechia.
³ Department of Biology of the Cell Nucleus, Institute of Molecular Genetics of the Czech Academy of Sciences, Prague, Czechia.
⁴ Laboratory of Immunobiology, Institute of Molecular Genetics of the Czech Academy of Sciences, Prague, Czechia.

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.

Keywords: FSC200; Francisella tularensis; cell entry; host–pathogen interaction; macrophage; outer membrane vesicles.