Increased neutrophils in inflammatory bowel disease accelerate the accumulation of amyloid plaques in the mouse model of Alzheimer's disease.

Background: Alzheimer's disease (AD) is one of the neurodegenerative diseases and characterized by the appearance and accumulation of amyloid-β (Aβ) aggregates and phosphorylated tau with aging. The aggregation of Aβ, which is the main component of senile plaques, is closely associated with disease progression. App^NL-G-F mice, a mouse model of AD, have three familial AD mutations in the amyloid-β precursor gene and exhibit age-dependent AD-like symptoms and pathology. Gut-brain interactions have attracted considerable attention and inflammatory bowel disease (IBD) has been associated with a higher risk of dementia, especially AD, in humans. However, the underlying mechanisms and the effects of intestinal inflammation on the brain in AD remain largely unknown. Therefore, we aimed to investigate the effects of intestinal inflammation on AD pathogenesis.

Methods: Wild-type and App^NL-G-F mice at three months of age were fed with water containing 2% dextran sulfate sodium (DSS) to induce colitis. Immune cells in the brain were analyzed using single-cell RNA sequencing (scRNA-seq) analysis, and the aggregation of Aβ protein in the brain was analyzed via immunohistochemistry.

Results: An increase in aggregated Aβ was observed in the brains of App^NL-G-F mice with acute intestinal inflammation. Detailed scRNA-seq analysis of immune cells in the brain showed that neutrophils in the brain increased after acute enteritis. Eliminating neutrophils by antibodies suppressed the accumulation of Aβ, which increased because of intestinal inflammation.

Conclusion: These results suggest that neutrophils infiltrate the AD brain parenchyma when acute colitis occurs, and this infiltration is significantly related to disease progression. Therefore, we propose that neutrophil-targeted therapies could reduce Aβ accumulation observed in early AD and prevent the increased risk of AD due to colitis.