CD4 and Parkinson disease: CD4+ and CD8+ lymphocytes have been shown to be present in increased numbers in the SN at postmortem in patients with PD,16 25 as well as in ex vivo cerebrospinal fluid (CSF) samples.31 The precise mechanism by which peripheral immune cells drive neuronal damage in PD is still unclear, but it has recently been demonstrated that Th17 cells from patients with PD drive cell death in autologous induced pluripotent stem cell (iPSC) derived dopaminergic midbrain neurons.25