Fig. 2.

Schematic representation of the CNS at steady-state and during a relapse. a. Healthy CNS. The endothelial cells of the BBB are ensheeted by astrocytic end-feet. The BBB is impermeable notably to leukocytes. Oligodendrocytes form the myelin layer that surrounds the axon. b CNS during MS. The blood-brain barrier is disrupted and the endothelial permeability is increased. The astrocytic end-feet are detached, allowing leukocytes to transmigrate and trigger an inflammatory cascade. Inflammatory signals produced by leukocytes activate astrocytes. The myelin sheet is disrupted and phagocytes start to remove myelin debris. Neurons are further activated during the inflammatory reaction