Ineffective phagocytosis of amyloid-β by macrophages of Alzheimer's disease patients

The defective clearance of amyloid-beta (A beta) in the brain of Alzheimer's disease (AD) patients is unexplained. The immunohistochemical studies of the frontal lobe and hippocampus show perivascular and intraplaque infiltration by blood-borne macrophages containing intracellular A beta but only inefficient clearance of A beta deposits. Neurons and neuronal nuclei, respectively, express interleukin-1 beta and the chemokine RANTES, which could induce the inflammatory cell infiltration. To clarify the pathophysiology of A beta clearance, we examined A beta phagocytosis by monocytes and macrophages isolated from the blood of age-matched patients and controls. Control monocytes display excellent differentiation into macrophages and intracellular phagocytosis of A)3 followed by A,3 degradation or export. AD monocytes show poor differentiation and only surface uptake of A beta and suffer apoptosis. HLA DR and cyclooxygenase-2 are abnormally expressed on neutrophils and monocytes of AD patients. AD patients have higher levels of intracellular cytokines compared to controls. Thus A beta clearance is not restricted to brain microglia and involves systemic innate immune responses. In AD, however, macrophage phagocytosis is defective, which may elicit compensatory response by the adaptive immune system.