Microglial PD‐1 stimulation by astrocytic PD‐L1 suppresses neuroinflammation and Alzheimer’s disease pathology

• Published in: The EMBO journal 2021-12, Vol.40 (24), p.e108662-n/a
• Main Authors: Kummer, Markus P, Ising, Christina, Kummer, Christiane, Sarlus, Heela, Griep, Angelika, Vieira‐Saecker, Ana, Schwartz, Stephanie, Halle, Annett, Brückner, Matthias, Händler, Kristian, Schultze, Joachim L, Beyer, Marc, Latz, Eicke, Heneka, Michael T
• Format: Article
• Language: English
• Subjects: Aged; Aged, 80 and over; Alzheimer Disease - genetics; Alzheimer Disease - immunology; Alzheimer's disease; Amyloid beta-Protein Precursor - genetics; Amyloid beta-Protein Precursor - toxicity; Animals; APP; Astrocytes; Astrocytes - metabolism; B7-H1 Antigen - metabolism; Brain; Case-Control Studies; CD36 antigen; CD36 Antigens - metabolism; Cell culture; Deposition; Disease Models, Animal; EMBO19; EMBO27; Female; Gene Deletion; HEK293 Cells; HeLa Cells; Homeostasis; Humans; Immune clearance; Immune response; Immune system; Immunoregulation; Inflammation; Inflammatory response; innate immune system; Male; Mice; Mice, Transgenic; Microglia; Microglia - metabolism; Middle Aged; PD-L1 protein; PD‐1 knockout mice; Peptides; Phagocytosis; Presenilin 1; Programmed Cell Death 1 Receptor - genetics; Programmed Cell Death 1 Receptor - metabolism; PS1 mice; Receptors; Senile plaques; Up-Regulation; β-Amyloid
• ISSN: 0261-4189; 1460-2075
• DOI: 10.15252/embj.2021108662

Chronic neuroinflammation is a pathogenic component of Alzheimer’s disease (AD) that may limit the ability of the brain to clear amyloid deposits and cellular debris. Tight control of the immune system is therefore key to sustain the ability of the brain to repair itself during homeostasis and disease. The immune‐cell checkpoint receptor/ligand pair PD‐1/PD‐L1, known for their inhibitory immune function, is expressed also in the brain. Here, we report upregulated expression of PD‐L1 and PD‐1 in astrocytes and microglia, respectively, surrounding amyloid plaques in AD patients and in the APP/PS1 AD mouse model. We observed juxtamembrane shedding of PD‐L1 from astrocytes, which may mediate ectodomain signaling to PD‐1‐expressing microglia. Deletion of microglial PD‐1 evoked an inflammatory response and compromised amyloid‐β peptide (Aβ) uptake. APP/PS1 mice deficient for PD‐1 exhibited increased deposition of Aβ, reduced microglial Aβ uptake, and decreased expression of the Aβ receptor CD36 on microglia. Therefore, ineffective immune regulation by the PD‐1/PD‐L1 axis contributes to Aβ plaque deposition during chronic neuroinflammation in AD. SYNOPSIS Neuroinflammation, a hallmark of Alzheimer’s disease (AD), is involved in beta‐amyloid peptide (Aβ) plaque deposition and clearance. Here, the PD‐1/PD‐L1 axis is found as an important pathway for regulating the immune system in the brain, sustaining microglial Aβ uptake and reducing chronic neuroinflammation. Astrocytic PD‐L1 and microglial PD‐1 are upregulated around Aβ plaques in Alzheimer’s disease (AD) and APP/PS1 mice. PD‐L1 is secreted in a soluble form by astrocytes in culture. PD‐1 deficiency compromises Aβ phagocytosis and induces an inflammatory response in vitro . Loss of PD‐1 increases Aβ plaque deposition and reduces microglial Aβ uptake in APP/PS1 mice. Graphical Abstract The immune‐checkpoint receptor/ligand pair PD‐1/PD‐L1 sustains phagocytic function of microglia to prevent Aβ plaque deposition in APP/PS mouse models and patient tissues.