HLA-DRB5 and Parkinson disease: Alpha-synuclein specific T cell reactivity is associated with HLA-DRB1*15:01 and -DRB5*01:01 (22, 23), and with preclinical and early PD (24, 25), and the infiltration of CD4+ lymphocytes into the brain contributes to neurodegeneration in a mouse model of PD (15, 26).