CD4 and Parkinson disease: In a PD-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model, dopaminergic cell death was almost exclusively caused by the substantia nigra-infiltrating CD4+T cells by a FasL-mediated cytolytic pathway, but not by IFNγ-dependent mechanisms, in an environment replete with activated microglia and pro-inflammatory cytokines (144, 149, 150).