Targeting glial PD-1/PD-L1 restores microglial homeostasis and reduces neuronal hyperactivity in an Alzheimer's disease model.

Alzheimer's disease (AD) involves complex neuroimmune dysregulation, and the role of immune checkpoint pathways in regulating neuro-glial interactions and intrinsic glial homeostasis remains unclear. In AD, glial expression of programmed cell death protein 1 (PD-1) and its ligand (PD-L1) is elevated in mouse models and human patients, suggesting involvement of immune checkpoint signaling in glial function. To define the role of this pathway in the AD brain, we locally modulated PD-1/PD-L1 signaling by intracortical anti-PD-L1 injection in 8-10-month-old 5xFAD mice, combined with in vivo two-photon imaging and quantitative functional analyses. Brain-intrinsic PD-L1 blockade reshaped the local glial microenvironment, restoring impaired microglial process convergence and increasing P2RY12 expression, a key marker of homeostatic function, with concomitant attenuation of aberrant neuronal hyperactivity. Astrocyte-specific PD-L1 knockdown produced similar effects, indicating a key role of astrocytic PD-L1 in regulating microglia-neuron interactions. These findings suggest that the glial PD-1/PD-L1 axis functions as a brain-intrinsic regulator of glial homeostasis linked to neuronal dysfunction in AD.