PSEN1 and Alzheimer disease: PD-L1 levels are increased in the cerebrospinal fluid of AD patients, and expression of PD-L1 and PD-1 in astrocytes and microglia, respectively, was upregulated in proximity to amyloid plaques in AD patients and APP/PS1 mice [81], consistent with PD-L1 and PD-1 being expressed under conditions of chronic inflammation and a role for this pathway in the microglial phagocytosis of amyloid.