CD4 and Parkinson disease: While evidence exists that CD4 + and CD8 + T cells contribute to dopaminergic neuron loss in PD (Galiano-Landeira et al., 2020; Brochard et al., 2009; Subbarayan et al., 2020; Kurkowska-Jastrzebska et al., 1999b), other studies suggest that specific T cell subsets, such as Tregs, may exert neuroprotective effects (Reynolds et al., 2007).