Outer membrane vesicles (OMVs) are nanoparticles produced by bacteria as part of their normal growth. OMVs can package cargo from their parent bacterium including proteins, lipids and nucleic acids, which ultimately determine their function. OMVs produced by pathogenic bacteria have been extensively characterised as vehicles for the delivery of toxins and immunogenic molecules, thereby modulating host immune responses. By contrast, whilst commensal bacteria are recognised to modulate host immunity, the role of OMV secretion by the human microbiota remains poorly characterised. The commensal gut bacterium Bacteroides fragilis plays an integral role in maintaining gut homeostasis and modulating inflammation, in addition to their OMVs that can also mediate anti-inflammatory effects. In this study, we aimed to delineate the pathways by which B. fragilis and their OMVs mediate host innate immune responses.
To do this, B. fragilis OMVs were purified using density-gradient centrifugation and their size, concentration and contents were determined. OMVs ranged from 40‑300 nm in size and contained immunogenic cargo including protein, DNA and RNA. Using HEK-Blue reporter cell lines, we determined that B. fragilis OMVs are detected by Toll-like receptor (TLR)-2, TLR4 and the cytoplasmic immune receptor NOD1. Preliminary results indicate that B. fragilis OMVs may also activate TLR7 and NOD2. By contrast, B. fragilis cells were only detected by TLR2. Using colonic epithelial cells, we are currently elucidating the ability of B. fragilis OMVs and their parent bacteria to modulate cytokine responses and to induce potential changes in epithelial cell barrier integrity.
Collectively, our findings indicate that the commensal bacterium B. fragilis produces OMVs which may exert anti-inflammatory effects via pathways independent of those activated by their parent bacterium. Furthermore, these findings highlight a role for OMVs produced by the human microbiota in shaping immune responses in the gut without the need for direct cell-to-cell interactions.