Depletion of gut bacteria resulted in a significant reduction of Il-17a-expressing CD4-positive cells in the gut and spleen as well as bacterial DNA in brain tissue, suggesting that gut bacteria regulate brain pathology in AD mice via at least two pathways: 1) regulating peripheral Il-17a-expressing T lymphocytes and 2) directly modulating cells in the brain with bacterial components (not necessary living cells). This evidence concerns the gene CD4 and Alzheimer disease.