Neuroinflammation: a common pathway in CNS diseases as mediated at the blood-brain barrier.

The blood-brain barrier (BBB) is not simply a physical barrier but a regulatory interface between the central nervous system (CNS) and immune system. The BBB both affects and is affected by the immune system and connects at many levels with the CNS, including the following: (1) the BBB transports cytokines and secretes various substances with neuroinflammatory properties; (2) transporters are altered in disease states including traumatic injury, Alzheimer's disease and inflammatory processes; (3) cytokines and other immune secretions from the cells comprising the BBB are both constitutive and inducible; (4) immune cells are transported across the BBB by the highly regulated process termed diapedesis, which involves communication and interactions between the brain endothelial cells and the immune cells; (5) the neuroimmune system has various effects on the BBB, including modulation of important transport systems and in extreme pathological conditions even disruption of the BBB, and (6) the brain-to-blood efflux transporter P-glycoprotein is altered in inflammatory conditions, thus affecting drug delivery to the brain. In summary, the BBB is an interactive interface that regulates and defines many of the ways that the CNS and the immune system communicate with one another.