Membrane vesicle delivery of a streptococcal M protein disrupts the blood–brain barrier by inducing autophagic endothelial cell death

Significance

Streptococcus equi subsp. zooepidemicus (SEZ) are Group C streptococci that cause meningitis in animals and humans. Here, we show SEZ releases membrane vesicles (MVs) that contain the SEZ M protein, SzM. MV provides an efficient means of delivery of SzM to host cells. Endocytosis of these vesicles results in autophagic cell death in hBMECs (human brain endothelial microvascular cells) and disruption of the BBB (blood–brain barrier) in mice. Blockade of MV endocytosis or inactivation of autophagic death attenuated SEZ pathogenicity in mice. Together, these findings provide therapeutic targets for treatment of SEZ infection and extend our knowledge of streptococcal virulence mechanisms.

Abstract

M family proteins are critical virulence determinants of Streptococci. Streptococcus equi subsp. zooepidemicus (SEZ) are Group C streptococci that cause meningitis in animals and humans. SzM, the M protein of SEZ, has been linked to SEZ brain invasion. Here, we demonstrate that SzM is important in SEZ disruption of the blood–brain barrier (BBB). SEZ release SzM-bound membrane vesicles (MVs), and endocytosis of these vesicles by human brain endothelial microvascular cells (hBMECs) results in SzM-dependent cytotoxicity. Furthermore, administration of SzM-bound MVs disrupted the murine BBB. A CRISPR screen revealed that SzM cytotoxicity in hBMECs depends on PTEN-related activation of autophagic cell death. Pharmacologic inhibition of PTEN activity prevented SEZ disruption of the murine BBB and delayed mortality. Our data show that MV delivery of SzM to host cells plays a key role in SEZ pathogenicity and suggests that MV delivery of streptococcal M family proteins is likely a common streptococcal virulence mechanism.