Involvement of the choroid plexus in central nervous system inflammation

During inflammatory conditions in the central nervous system (CNS), immune cells immigrate into the CNS and can be detected in the CNS parenchyma and in the cerebrospinal fluid (CSF). The most comprehensively investigated model for CNS inflammation is experimental autoimmune encephalomyelitis (EAE), which is considered the prototype model for the human disease multiple sclerosis (MS). In EAE autoagressive CD4(+), T cells gain access to the CNS and initiate the molecular and cellular events leading to edema, inflammation, and demyelination in the CNS. The endothelial blood-brain barrier (BBB) has been considered the obvious place of entry for the circulating immune cells into the CNS. A role of the choroid plexus in the pathogenesis of EAE or MS, i.e., as an alternative entry site for circulating lymphocytes directly into the CSF, has not been seriously considered before. However, during EAE, we observed massive ultrastructural changes within the choroid plexus, which are different from changes observed during hypoxia. Using immunohistochemistry and in situ hybridization, we observed expression of VCAM-1 and ICAM-1 in the choroid plexus and demonstrated their upregulation and also de novo expression of MAdCAM-1 during EAE. Ultrastructural studies revealed polar localization of ICAM-1, VCAM-1, and MAdCAM-1 on the apical surface of choroid plexus epithelial cells and their complete absence on the fenestrated endothelial cells within the choroid plexus parenchyme. Furthermore, ICAM-1, VCAM-1, and MAdCAM-1 expressed in choroid plexus epithelium mediated binding of lymphocytes via their known ligands. In vitro, choroid plexus epithelial cells can be induced to express ICAM-1, VCAM-1, MAdCAM-1, and, additionally, MHC class I and II molecules on their surface. Taken together, our observations imply a previously unappreciated function of the choroid plexus in the immunosurveillance of the CNS.