Immune cell entry into the central nervous system: Involvement of adhesion molecules and chemokines

In multiple sclerosis and in its animal model experimental autoimmune encephalomyelitis (EAE), inflammatory cells migrate across the highly specialized endothelial blood–brain barrier (BBB) and gain access to the central nervous system (CNS). It is well established that leukocyte recruitment across this vascular bed is unique due to the predominant involvement of α4-integrins in mediating the initial contact to as well as firm adhesion with the endothelium. In contrast, the involvement of the selectins, L-selectin, E- and P-selectin and their respective carbohydrate ligands such as P-selectin glycoprotein (PSGL)-1 in this process has been controversially discussed. Intravital microscopic analysis of immune cell interaction with superficial brain vessels demonstrates a role for E- and P-selectin and their common ligand PSGL-1 in lymphocyte rolling. However, E- and P-selectin-deficient SJL- or C57Bl/6 mice or PSGL-1-deficient C57Bl/6 mice develop EAE indistinguishable from wild-type mice. Considering these apparently discrepant observations, it needs to be discussed whether the molecular mechanisms involved in leukocyte trafficking across superficial brain vessels are irrelevant for EAE pathogenesis or whether the therapeutic efficacy of targeting α4-integrins in EAE is truly dependent on the inhibition of leukocyte trafficking across the BBB.