Brain endothelial tricellular junctions as novel sites for T-cell diapedesis across the blood-brain barrier

The migration of activated T cells across the blood-brain barrier (BBB) is a critical step in central nervous system (CNS) immune surveillance and inflammation. While T-cell diapedesis across the intact BBB seems to occur preferentially through the BBB cellular junctions, impaired BBB integrity during neuroinflammation is accompanied by increased transcellular T-cell diapedesis. The underlying mechanisms directing T cells to paracellular versus transcellular sites of diapedesis across the BBB remain to be explored. Combining in vitro live cell imaging of T-cell migration across primary mouse brain microvascular endothelial cells (pMBMECs) under physiological flow with serial block face scanning electron microscopy (SBF-SEM) we have identified BBB tricellular junctions as novel sites for T-cell diapedesis across the BBB. Downregulated expression of tricellular junctional proteins or protein-based targeting of their interactions in pMBMEC monolayers correlated with enhanced transcellular T-cell diapedesis while abluminal presence of chemokines increased T-cell diapedesis through tricellular junctions. Our observations assign an entirely novel role to BBB tricellular junctions in regulating T-cell entry into the CNS.