Antigen recognition detains CD8⁺ T cells at the blood-brain barrier and contributes to its breakdown

Blood-brain barrier (BBB) breakdown and immune cell infiltration into the central nervous system (CNS) are early hallmarks of multiple sclerosis (MS). High numbers of CD8⁺ T cells are found in MS lesions, and antigen (Ag) presentation at the BBB has been proposed to promote CD8⁺ T cell entry into the CNS. Here, we show that brain endothelial cells process and cross-present Ag, leading to effector CD8⁺ T cell differentiation. Under physiological flow in vitro, endothelial Ag presentation prevented CD8⁺ T cell crawling and diapedesis resulting in brain endothelial cell apoptosis and BBB breakdown. Brain endothelial Ag presentation in vivo was limited due to Ag uptake by CNS-resident macrophages but still reduced motility of Ag-specific CD8⁺ T cells within CNS microvessels. MHC class I-restricted Ag presentation at the BBB during neuroinflammation thus prohibits CD8⁺ T cell entry into the CNS and triggers CD8⁺ T cell-mediated focal BBB breakdown.