A Novel Role for A20-Regulated CNS Endothelial ICOSL in Immune Cell Adhesion and Autoimmune Neuroinflammation

A20 is a ubiquitin-modifying protein that negatively regulates canonical NF-?B signaling and mutations in A20/TNFAIP3 have been associated with a variety of autoimmune diseases, including multiple sclerosis (MS). We found that deletion of A20 in central nervous system (CNS) endothelial cells (BEC) enhances experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. These mice showed increased numbers of CNS-infiltrating immune cells during neuroinflammation and in the steady state. While the integrity of the barrier was not impaired, we observed a strong activation of the endothelium in A20?BEC mice, with dramatically increased levels of the adhesion molecules ICAM-1 and VCAM-1. We furthermore discovered ICOSL as novel adhesion molecule expressed by A20-deficient BECs. Silencing of ICOSL in BECs ameliorated the severity of an active EAE disease in wildtype mice and significantly delayed the onset of symptom development. Furthermore, blocking of ICOSL on primary CNS-derived endothelial cells impaired the adhesion of different T cell populations to the monolayer. Taken together, we here report a novel function of A20-regulated ICOSL expressed on CNS endothelial cells during inflammation. We propose that BEC-ICOSL contributes to the firm adhesion of T cells to the barrier, promoting T cell transmigration into the CNS and eventually driving autoimmune neuroinflammation. Overall design: CNS-endothelial knockout of A20 versus wildtype-like control and wildype-like EAE control