This situation permits the recruitment of immune cells, to the specific region of the brain where infiltrated (e.g., MOs, DCs, CD4+ T cells, and CD8+ T cells) and residential immune cells (e.g., MGCs) meet and amplify their activation, and the resultant massive generation of pro-inflammatory cytokines (e.g., IFNγ, TNFα, IL-1β, IL-6, IL-8, IL-12, and IL-17), which are all lethal to DA neurons, and this condition develops neurodegeneration in PD. Here, CD8A is linked to Parkinson disease.