Membrane vesicles derived from Streptococcus suis serotype 2 induce cell pyroptosis in endothelial cells via the NLRP3/Caspase-1/GSDMD pathway

Streptococcus suis serotype 2 (S. suis 2) is a zoonotic pathogen that clinically causes severe swine and human infections (such as meningitis, endocarditis, and septicemia). In order to cause widespread diseases in different organs, S. suis 2 must colonize the host, break the blood barrier, and cause exaggerated inflammation. In the last few years, most studies have focused on a single virulence factor and its influences on the host. Membrane vesicles (MVs) can be actively secreted into the extracellular environment contributing to bacteria-host interactions. Gram-negative bacteria-derived outer membrane vesicles (OMVs) were recently shown to activate host Caspase-11-mediated non-canonical inflammasome pathway via deliverance of OMV-bound lipopolysaccharide (LPS), causing host cell pyroptosis. However, little is known about the effect of the MVs from S. suis 2 (Gram-positive bacteria without LPS) on cell pyroptosis. Thus, we investigated the molecular mechanism by which S. suis 2 MVs participate in endothelial cell pyroptosis. In this study, we used proteomics, electron scanning microscopy, fluorescence microscope, Western blotting, and bioassays, to investigate the MVs secreted by S. suis 2. First, we demonstrated that S. suis 2 secreted MVs with an average diameter of 72.04 nm, and 200 proteins in MVs were identified. Then, we showed that MVs were transported to cells via mainly dynamin-dependent endocytosis. The S. suis 2 MVs activated NLRP3/Caspase-1/GSDMD canonical inflammasome signaling pathway, resulting in cell pyroptosis, but it did not activate the Caspase-4/-5 pathway. More importantly, endothelial cells produce large amounts of reactive oxygen species (ROS) and lost their mitochondrial membrane potential under induction by S. suis 2 MVs. The results in this study suggest for the first time that MVs from S. suis 2 were internalized by endothelial cells via mainly dynamin-dependent endocytosis and might promote NLRP3/Caspase-1/GSDMD pathway by mitochondrial damage, which produced mtDNA and ROS under induction, leading to the pyroptosis of endothelial cells.