Outer membrane vesicles (OMVs) are secreted by Gram-negative bacteria and can package bacterial cargo including peptidoglycan, lipids and nucleic acids for their delivery to host cells. Therefore, the cargo composition of OMVs can determine their detection by host innate immune receptors and how they modulate host immune responses. Recently, the release of OMVs by the intestinal microbiota, including the commensal Bacteroides fragilis, have emerged as novel mechanisms to dampen inflammation in the host. In this study, we aimed to delineate the pathways by which B. fragilis OMVs mediate host innate immune responses compared to their parent bacteria.
To do this, the size and composition of purified B. fragilis OMVs was characterised, revealing that immunostimulatory products including peptidoglycan and nucleic acids were associated with B. fragilis OMVs. Additionally, we observed the enrichment of specific protein and LPS cargo into OMVs compared to their parent bacteria. Furthermore, the ability of OMVs to enter and deliver their cargo intracellularly to intestinal epithelial cells was determined using confocal microscopy, and their potential to activate innate immune receptors compared to their parent bacteria was determined using HEK-Blue reporter cell lines. Whilst B. fragilis bacteria could only activate Toll-like receptor (TLR)-2, B. fragilis OMVs induced the activation of TLR2 as well as TLR4, TLR7 and NOD1 that detect bacterial LPS, RNA and peptidoglycan, respectively. Currently, we are elucidating the mechanisms underpinning the differences in innate immune receptor activation between B. fragilis OMVs and their bacteria, and the subsequent immunological outcomes.
Collectively, our results demonstrate that B. fragilis OMVs activate different immune signalling pathways compared to their parent bacteria, revealing novel roles for OMVs secreted by the intestinal microbiota in activating host immune responses.