Lipoproteins Are Responsible for the Pro-Inflammatory Property of Staphylococcus aureus Extracellular Vesicles

Abstract

<i>Staphylococcal aureus</i> (<i>S. aureus</i>), a Gram-positive bacteria, is known to cause various infections. Extracellular vesicles (EVs) are a heterogeneous array of membranous structures secreted by cells from all three domains of life, i.e., eukaryotes, bacteria, and archaea. Bacterial EVs are implied to be involved in both bacteria-bacteria and bacteria-host interactions during infections. It is still unclear how <i>S. aureus</i> EVs interact with host cells and induce inflammatory responses. In this study, EVs were isolated from <i>S. aureus</i> and mutant strains deficient in either prelipoprotein lipidation (Δ<i>lgt</i>) or major surface proteins (Δ<i>srtAB</i>). Their immunostimulatory capacities were assessed both in vitro and in vivo. We found that <i>S. aureus</i> EVs induced pro-inflammatory responses both in vitro and in vivo. However, this activity was dependent on lipidated lipoproteins (Lpp), since EVs isolated from the Δ<i>lgt</i> showed no stimulation. On the other hand, EVs isolated from the Δ<i>srtAB</i> mutant showed full immune stimulation, indicating the cell wall anchoring of surface proteins did not play a role in immune stimulation. The immune stimulation of <i>S. aureus</i> EVs was mediated mainly by monocytes/macrophages and was TLR2 dependent. In this study, we demonstrated that not only free Lpp but also EV-imbedded Lpp had high pro-inflammatory activity.