Using an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, studies have demonstrated that a lack of CD4+ lymphocytes attenuates dopaminergic cell death,16 as does administration of Treg cells.32 In mice that overexpress alpha-synuclein, knockout of major histocompatability class II (MHC) class II prevents both microglial activation and dopaminergic neurodegeneration.33 Furthermore, ciclosporin, a widely used immunosuppressant, is effective in improving motor and cognitive deficits in multiple mouse models of PD.34 This evidence concerns the gene CD4 and Parkinson disease.