The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.

In multiple sclerosis, encephalitogenic CD4(+) lymphocytes require adhesion molecules to accumulate into central nervous system inflammatory lesions. Using proteomic techniques, we identified expression of melanoma cell adhesion molecule (MCAM) on a subset of human effector memory CD4(+) lymphocytes and on human blood-brain barrier endothelium. Herein, we demonstrate that MCAM is a stable surface marker that refines the identification of interleukin 17(+), interleukin 22(+), RAR-related orphan receptor γ and interleukin 23 receptor(+) cells within the CD161(+)CCR6(+) subset of memory CD4(+) lymphocytes. We also show that MCAM(+) lymphocytes express significantly more granulocyte/macrophage colony stimulating factor and granzyme B than MCAM(-) lymphocytes. Furthermore, the proportion of MCAM(+) CD4(+) lymphocytes is significantly increased in the blood and in the central nervous system of patients with multiple sclerosis and experimental autoimmune encephalomyelitis animals compared with healthy controls or other neurological diseases, and MCAM expression is upregulated at the blood-brain barrier within inflammatory lesions. Moreover, blockade of MCAM or depletion of MCAM(+) CD4(+) T lymphocytes both restrict the migration of T(H)17 lymphocytes across blood-brain barrier endothelial cells and decrease the severity of experimental autoimmune encephalomyelitis. Our findings indicate that MCAM could serve as a potential biomarker for multiple sclerosis and represents a valuable target for the treatment of neuroinflammatory conditions.

Blood-brain barrier function is driven by the influence of astrocyte-secreted factors. During neuroinflammatory responses the blood-brain barrier is compromised resulting in central nervous system damage and exacerbated pathology. Here, we identified endothelial netrin 1 induction as a vascular response to astrocyte-derived sonic hedgehog that promotes autocrine barrier properties during homeostasis and increases with inflammation. Netrin 1 supports blood-brain barrier integrity by upregulating endothelial junctional protein expression, while netrin 1 knockout mice display disorganized tight junction protein expression and barrier breakdown. Upon inflammatory conditions, blood-brain barrier endothelial cells significantly upregulated netrin 1 levels in vitro and in situ, which prevented junctional breach and endothelial cell activation. Finally, netrin 1 treatment during experimental autoimmune encephalomyelitis significantly reduced blood-brain barrier disruption and decreased clinical and pathological indices of disease severity. Our results demonstrate that netrin 1 is an important regulator of blood-brain barrier maintenance that protects the central nervous system against inflammatory conditions such as multiple sclerosis and experimental autoimmune encephalomyelitis.

Our study uncovers an important cell-specific role for Ninjurin-1 in the transmigration of inflammatory APCs across the BBB and further emphasizes the importance of myeloid cell recruitment during the development of neuroinflammatory lesions.

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