ABSTRACT
The endothelial cells (ECs) that constitute the blood-brain barrier (BBB) have been described as the gatekeeper to the brain. This analogy is correct in as much as brain ECs actively regulate the passage of molecules and cells to and from the central nervous system (CNS). In the context of immune cell traffic, the level of control exerted by the cerebral vascular ECs appears complex. It has been recognized for several decades that EC surface adhesion molecules are responsible for the capture of circulating leukocytes. In recent years, however, it has been realized that the same adhesion molecules are capable of triggering outside-in signaling cascades, which are critically required for successful leukocyte diapedesis. These observations lend strong support to the proposition that ECs respond to adherent activated leukocytes and that leukocyte emigration is dependent on a close partnership with the ECs. This concept has led to the focus of attention being directed more towards ECs and their role during leukocyte extravasation.
