Poster Presentation Lorne Infection and Immunity 2020

Differential activation of host innate immune receptors by Bacteroides fragilis bacteria and their secreted Outer Membrane Vesicles. (#172)

William Gilmore 1 2 , Rishi Pathirana 1 2 , Natalie Bitto 1 2 , Maria Kaparakis-Liaskos 1 2
  1. Department of Physiology, Anatomy and Microbiology, La Trobe University, Bundoora, Victoria, Australia
  2. Research Centre for Extracellular Vesicles, School of Molecular Sciences, La Trobe University, Bundoora, Victoria, Australia

Outer membrane vesicles (OMVs) are secreted by Gram‑negative bacteria as part of their normal growth. Both commensal and pathogenic organisms produce OMVs that contain immunostimulatory cargo including lipopolysaccharides (LPS), nucleic acids and proteins that originate from their parent bacterium. It is well established that OMVs produced by pathogens can mediate an inflammatory immune response in the host due to their immunogenic contents. However, the inflammatory potential of OMVs derived from commensal bacteria remains poorly understood. In this study, we investigated the immunostimulatory potential of OMVs produced by the commensal Bacteroides fragilis compared to their parent bacterium.

To do this, OMVs isolated from B. fragilis were purified and fractionated using density-gradient ultracentrifugation and fractions containing highly purified OMVs were characterised using ZetaView Nanoparticle Tracking Analysis. Purified OMVs ranged between 40-300nm in size and contained immunogenic cargo including proteins, DNA and RNA. Using HEK-Blue reporter cells, we determined that OMVs produced by B. fragilis were detected by Toll-like Receptor (TLR)-2, TLR4 and the cytoplasmic immune receptor NOD1, resulting in the generation of an NF-kB response. In contrast, B. fragilis bacteria were only detected by TLR2 and not TLR4 nor NOD1, suggesting that the immunostimulatory abilities of B. fragilis and their OMVs differ. We are currently investigating whether B. fragilis and their OMVs activate other innate immune receptors including TLR7, TLR8, TLR9 and NOD2. Furthermore, we are comparing differences in the cytokine response generated by co‑culturing of colonic epithelial cells with B. fragilis bacteria or their OMVs.

Collectively, our findings indicate that the immunostimulatory potential of OMVs produced by B. fragilis differs to that mediated by their parent bacterium. These findings suggest that commensal bacteria may utilise OMV secretion as a mechanism of mediating innate immune responses within the host without the need for direct cell-cell contact.