Astrocyte immunosenescence and deficits in interleukin 10 signaling in the aged brain disrupt the regulation of microglia following innate immune activation

Microglia, the innate immune cells of the brain, develops a pro-inflammatory, "primed" profile with age. Using single-cell RNA-sequencing, we confirmed hippocampal microglia of aged mice (18 m.o.) had an amplified (4 h) and prolonged (24 h) neuroinflammatory response to peripheral lipopolysaccharide (LPS) challenge compared to adults (2 m.o.). Overall, there were several unique cell-, age-, and time-dependent differences in the clusters of microglia identified. Analysis of upstream regulators and canonical pathways revealed impaired regulation of an activated, neuroinflammatory state within microglia. Moreover, microglia in the aged hippocampus failed to turn over during the resolving phase of neuroinflammation. Concomitantly, astrocytes in the aged hippocampus were "immunosenescent" both 4 and 24 h after LPS challenge. For example, aged astrocytes had reduced anti-inflammatory signaling and cholesterol biosynthesis, two pathways by which astrocytes regulate the inflammatory profile of microglia. One of the pathways reduced in the aged hippocampus was interleukin (IL)-10 signaling. This pathway increases astrocytic expression of transforming growth factor (TGF)-β, an anti-inflammatory cytokine with abundant receptor expression on microglia. Therefore, transgenic astrocytic Il10ra