CD4 and Parkinson disease: Moreover, patients with overt PD exhibit: (1) a reduction in the levels of circulating naïve T cells and TREG cells116, (2) an increase in the ratio of IFNG- over IL4-producing CD4+ T cells in the periphery117, and (3) circulating T cells responding to SNCA-derived peptides118, an autoreactivity that appears to develop even prior to clinical manifestations of the disease119.