CD8A and Parkinson disease: Several studies indicate that T-cell subpopulations contribute to PD pathophysiology.175 For example, CD4 and CD8 T cells significantly infiltrated into the SN of patients with PD,160 with CD8 T-cell concentrations particularly high.176,177 Because this infiltration occurs in early-stage PD and subsides with disease progression, CD8 T cells seem to be important at the beginning of the disease.