CD4 and Parkinson disease: Both CD8+ and CD4+ T cells (but not B cells) were identified in the brain in both postmortem human PD specimens and in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, and evidence obtained in the animal model pointed to CD4+ T cells as main determinants of T cell-mediated dopaminergic cell death11.