CD4 and Parkinson disease: • Increased frequencies of IL-17 producing CD4+ T cells in PD patients, no significant difference in IFN-γ or IL-4 producing cells• Co-culture of iPSC-midbrain neurons with Th17 cells/IL-17 increased neuronal cell death and levels of IL-17, IL-1β, TNF-α, and IL-6 in PD cells• Neuronal cell death in PD co-cultures occurred via IL-17/IL-17 receptor signalling, and potential activation of the NF-κB signalling pathway and preventing IL-17/IL-17 receptor interaction attenuated this