Tau, amyloid-beta and alpha-synuclein co-pathologies synergistically enhance neuroinflammation and neuropathology.

Alzheimer's and Parkinson disease pathology often co-occur, with amyloid-β and phosphorylated tau, found in 30-50% of idiopathic Parkinson disease cases. α -synuclein inclusions, a hallmark of Parkinson disease, are present in 50% of Alzheimer's cases and the co-expression of these pathologies is linked to faster cognitive decline and earlier death. Immune activation is a hallmark of both diseases, but current model systems primarily examine each pathology in isolation. As such, how these co-pathologies interact to drive inflammation and neuronal loss remain poorly understood. To address this, we developed a co-pathology mouse model combining tau, amyloid-β, and α-synuclein. Here, we show that co-pathologies synergistically trigger a distinct and amplified neuroimmune response, marked by robust expansion of CD4 and CD8 tissue-resident memory T cells and increased CD68 microglia, a population of activated, phagocytosing microglia, compared to single pathology brains. These changes were abundant in the hippocampus and cortex, regions showing elevated amyloid-β protein pathology load and enhanced neuronal loss with co-pathology expression. Our findings demonstrate that co-pathologies act synergistically to enhance immune activation prior to neurodegeneration. This model provides a platform for assessing mixed-pathology mechanisms and identifies key immune cell populations that may drive disease acceleration across Alzheimer's, Parkinson disease and their related dementias.