Fig. 2

T lymphocytes in the healthy CSF. CSF is produced by ependymal cells in the choroid plexus within brain ventricles and circulates in the subarachnoid space around the brain and spinal cord. CSF provides mechanical protection and supports the flow of nutrients and neurotransmitters. In healthy condition, CSF contains only 1000–3000 cells/mL, including T cells that contribute to immunosurveillance. These are mainly CD4 + , with a ratio of 4:1 versus CD8 + and exhibit a memory phenotype: 90% are CD45RO + CCR7 + central memory T cells (Tcm), and 10% are CDR45RO + CCR7− effector memory T cells (Tem) [47]. Among CD4 + T cells, Th17 are usually present expressing receptors that interact with the choroid plexus, allowing Th17 cell trafficking in the CSF: a main example is CCR6, that binds CCL20 on the choroid plexus [53]. CSF Th17 also may produce low levels of CKs, such as IFN-γ and GM-CSF [52]. Parts of the figure were drawn by using pictures from Servier Medical Art. Servier Medical Art by Servier is licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by/3.0/)