The endothelial blood–brain barrier (BBB) allows the separation between blood and neural tissue, restricting diffusion of molecules and ensuring the proper function of the central nervous system (CNS). Migration of activated T cells across the BBB is essential for CNS immune surveillance, occurring preferentially through tight junctions (TJs) between adjacent BBB endothelial cells. Although it is known that neuroinflammation increases transcellular T cell diapedesis, the underlying mechanism directing T cells to paracellular or transcellular migration sites of the BBB remains unclear. Here, Britta Engelhardt and colleagues (Dias et al., 2021) combine live-cell imaging and serial block face scanning electron microscopy (SBF-SEM) to identify the structures involved in this migration process. Using a well-characterised in vitro model of the BBB coupled with microfluidics, the authors are able to track transendothelial migration and identify tricellular junctions as novel sites of T cell diapedesis. Moreover, inflammatory conditions or targeted inhibition of TJ proteins correlates with reduced paracellular diapedesis. By contrast, an abluminal chemokine gradient increases diapedesis at tricellular junctions, suggesting that chemotactic signals may guide T cells to these junctional sites. This study represents an important contribution to understanding immune cell infiltration into the CNS and identifies tricellular junctions as key players in this process.
