Fig. 1

Structure and function of meninges in homeostatic brain. a Meningeal structure and immune composition from perspective of the coronal section of human’s skull. The dura mater residing underneath the skull bone consists two layers: the periosteal and meningeal layer. Dural sinuses reside in specific sites, where these two layers separate from each other, and drain cerebral venous blood towards the systemic circulation. Meningeal lymphatic vessels run along the dural sinuses. The arachnoid mater is the middle layer of meninges, which effectively separates the dura from the subarachnoid space filled with the cerebrospinal fluid (CSF). The arachnoid mater penetrates deep into the dural sinuses and forms numerous arachnoid villi though which part of CSF flows into the dural sinuses and further into the systemic circulation. The pia mater intimately associated with the brain and spinal cord is the deepest layer of meninges. In addition, diverse immune cells are present in the distinct meningeal layers in homeostasis. The schemes are based on evidence in experimental animal models. b Partial enlarged perspective of the coronal section centered on dural sinuses. Dural sinuses are immune hubs in which immune surveillance of homeostatic CNS works. Circulating T cells are recruited to the dura by chemokines released from dural stromal cells, such as mural cells, and accumulate around the dural sinuses in homeostasis. Both dural lymphatic vessels and arachnoid villi are involved in CSF circulation, which may allow the drainage of CNS-derived antigens in the CSF to perisinusal dura. Sinus-associated antigen-presenting cells including macrophages and dendritic cells capture CNS-derived antigens and present these antigens to perisinusal T cells, thus allowing meningeal immune surveillance of homeostatic CNS