ALCAM regulates BBB integrity and neuroinflammation

Abstract

The loss of blood–brain barrier (BBB) integrity is a hallmark of multiple sclerosis and is associated with increased leukocyte trafficking into the central nervous system (CNS). During neuroinflammation, BBB endothelial dysfunction correlates with the upregulation of cell adhesion molecules essential for immune cell transmigration. In this study, the specific role of the adhesion molecule ALCAM on BBB endothelial cells (BBB-ECs) and its effects on leukocyte transmigration during the course of experimental autoimmune encephalomyelitis (EAE) were assessed. Using a modified adhesion assay under shear stress, we demonstrated a significant reduction in human monocytes and T helper cell adhesion to BBB-ECs following ALCAM blockade. Anti-ALCAM antibody also reduced significantly the migration of monocytes, Th1 and Th17 lymphocytes across both BBB-ECs and meningeal endothelial cells. Furthermore, in vivo ALCAM neutralization reduced leukocyte infiltration and EAE severity. However, ALCAM KO mice developed a more severe active EAE associated with a significant increase in perivascular infiltration of pro-inflammatory lymphocytes (Th1/Th17) and M1 monocytes/macrophages, as compared to WT controls. ALCAM KO mice also displayed more extensive CNS demyelination. In addition, EAE transfer experiments in which ALCAM KO mice received WT MOG reactive splenocytes suggest that the pathology observed in active EAE is linked to the absence of ALCAM on BBB-ECs. Phenotypic characterization of un-immunized ALCAM KO mice revealed a reduced expression of BBB junctional proteins contributing to the severity of EAE clinical course. Furthermore, our results also demonstrate that ALCAM indirectly associate with tight junction molecules at the BBB, suggesting ALCAM's important role in maintaining BBB stability. Collectively, our data provide evidence of the implication of ALCAM in leukocyte transmigration across the brain endothelium in human and point to a biologically relevant function of ALCAM in BBB integrity in mouse.