Microglial PD-1 modulates neuroinflammation and Alzheimer's disease pathology by astrocytic PD-L1

Neuroinflammation, a pathogenic component of Alzheimer disease (AD), may limit the ability of the brain to clear deposits and debris. Tight control of the immune system may therefore be key to sustain the ability of the brain for repair and clearance. Here we report that PD-L1 on astrocytes and its receptor PD-1 on microglia, known for its inhibitory immune function, are upregulated around amyloid plaques in AD and APP/PS1 mice. Juxtamembrane shedding of PD-L1 was observed from astrocytes suggesting ectodomain signaling to microglial PD-1. Deletion of PD-1 in microglia evoked an inflammatory response and compromised Aβ uptake. Likewise, in APP/PS1 PD-1-/- mice increased deposition of amyloid β (Aβ), reduced microglial Aβ uptake, and decreased expression of the Aβ receptor CD36 on microglia were detected. Therefore, ineffective immune regulation by the PD-1/PD-L1 axis contributes to Aβ plaque deposition during chronic, unresolved neuroinflammation in AD.