An ApoE-Associated Low-Inflammatory Microglial State Emerges After Inflammatory Challenge in Alzheimer's Disease Mice.

Patients with Alzheimer's disease (AD) frequently experience inflammatory insults; however, the mechanisms by which microglia respond to these challenges remain unclear. Although AD microglia have been proposed to be primed for exaggerated inflammatory responses, single-cell evidence remains limited. To investigate microglial responses to inflammation in AD, we challenged AD mouse models with intraperitoneal lipopolysaccharide (LPS) and used single-cell RNA sequencing to characterize microglial states, along with in vivo immunostaining and in vitro models to define their features and underlying mechanisms. We found that, in response to an inflammatory challenge, microglia adopted a low-inflammatory state accompanied by elevated expression of mitochondrial respiratory chain genes. This state was associated with the phagocytosis of dystrophic neurites and was recapitulated in vitro using an efferocytosis-based model, with apolipoprotein E implicated in its underlying mechanism. In summary, we identified a distinct microglial state that provides new insights into the dynamic role of microglia in AD.