We hypothesize that in a subset of cases, PD is initiated by an autoimmune event involving recognition of alpha-synuclein in the gut, and that interactions between the immune system and the peripheral and central nervous systems establish the disease in the brain. We propose integrating cutting-edge technologies in neuroimmunology, single cell genomics, gut microbiome, and computational biology to determine at unprecedented depth whether PD has the signature of autoimmune processes and explore how immune reactions initiate a process, from the gut, that spreads through the peripheral nervous system, and finally to the brain where neurodegeneration results in PD. First, we will characterize T cell-mediated autoimmunity in PD; second, we will evaluate the role of the microbiome in the initiation of PD and progression along the gut-to-brain axis; and third, we will define perturbations in the neuro-immune interactome in the PD brain.