Mechanisms of microglia accumulation in Alzheimer's disease: therapeutic implications

In Alzheimer's disease (AD), and other conditions affecting integrity of the blood-brain barrier, microglia can originate in the bone marrow, migrate into the blood and enter the brain in a chemokine-dependent manner. CCR2, a chemokine receptor that controls mononuclear phagocyte infiltration into the brain in multiple sclerosis, bacterial meningitis and neuropathic pain, also regulates microglia accumulation in mouse models of AD. CCR2 deficiency leads to lower microglia accumulation and higher brain beta-amyloid (A beta) levels, indicating that early microglial accumulation promotes A beta clearance. In support of this protective role, enhancing microglia accumulation delays progression of AD. AD mice that constitutively express interieukin-1 in the brain, or that are deficient in peripheral mononuclear phagocyte transforming growth factor-beta signaling, have increased microglia accumulation around beta-amyloid plaques and reduced AD-like pathology. Regulating microglia recruitment into the brain is a novel therapeutic strategy to delay or stop progression of AD. Here, we review the role of microglia in AD and the mechanisms of their accumulation and discuss implications for AD therapy.