Prion diseases or Transmissible Spongiform Encephalopathies are transmissible neurodegenerative diseases in humans and animals that have no treatment and are 100% lethal. The transformation of glia into reactive states is recognized as one of the major hallmarks of neurodegenerative diseases including prion, Alzheimer’s, and Parkinson’s diseases. Conflicting data exist on whether in prion diseases, reactive microglia are protective or deleterious. Microglia are regarded as the main cells in CNS responsible for phagocytosis, yet the role of microglia-driven phagocytosis in prion disease has never been systematically examined. The current project is designed to fill this knowledge gap. Our recent studies supported by a previous award revealed that microglia not only effectively phagocytose prions or PrPSc but also engulf viable neurons that do not display apoptotic markers. Our work suggests that PrPSc primes microglia for phagocytosis via activating phagocytic pathways that sense exposed galactose as an ‘eat-me’ signal. We hypothesize that the phenotype of microglia transforms with disease progression, changing from primarily protective at the early stages when microglia uptake PrPSc, to predominantly detrimental at the later stages when microglia also target viable neurons. Aim 1 will establish the timeline of PrPSc uptake and neuronal engulfment in prion-infected animals, and rigorously test whether reactive microglia associated with prion disease phagocytose viable neurons. Aim 2 will assess the role of the P2y12 receptor in microglial phagocytosis of neurons. This aim will use P2ry12 inhibitors approved by the FDA and a mouse model for the conditional knockout of P2ry12 in myeloid cells to test whether the depletion of P2ry12 slows down or reverses disease progression in prion-infected animals. Aim 3 will employ primary microglia isolated from prion-infected animals to test whether the engulfment of viable neurons is mediated by phagocytic pathways that sense exposed galactose. Furthermore, Aim 3 will examine the effect of a conditional knockout of CD11b, which constitutes the MAC-1 phagocytic receptor in myeloid cells, on disease progression in prion- infected animals. As a whole, this study will inform us as to whether phagocytic activity should be targeted as a potential therapeutic strategy for treating prion diseases. Among the goals of this application is a translational component that will test the effectiveness of FDA-approved inhibitors of P2y12 receptors applied after clinical onset.