The defective clearance of amyloid-β (Aβ) 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 Aβ but only inefficient clearance of Aβ deposits. Neurons and neuronal nuclei, respectively, express interleukin-1β and the chemokine RANTES, which could induce the inflammatory cell infiltration. To clarify the pathophysiology of Aβ clearance, we examined Aβ 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 Aβ followed by Aβ degradation or export. AD monocytes show poor differentiation and only surface uptake of Aβ 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 Aβ 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.
In both HIVE and AD, blood-borne activated monocyte/macrophages and lymphocytes appear to migrate through a disrupted blood-brain barrier. The lacunae around macrophages in amyloid-beta plaques but not in vessel walls are consistent with the ability of macrophages to phagocytize and clear amyloid-beta deposits in vitro.
Background: Aside from numerous parenchymal and vascular deposits of amyloid ((AO) peptide, neurofibrillary tangles, and neuronal and synaptic loss, the neuropathology of Alzheimer's disease is accompanied by a subtle and chronic inflammatory reaction that manifests itself as microglial activation. However, in Alzheimer's disease, alterations in the permeability of the blood-brain barrier and chemotaxis, in part mediated by chemokines and cytokines, may permit the recruitment and transendothelial passage of peripheral cells into the brain parenchyma. Materials and Methods: Human monocytes from different donors were tested for their capacity to differentiate into macrophages and their ability to secrete cytokines and chemokines in the presence of AO3 1-42. A paradigm of the blood-brain barrier was constructed utilizing human brain endothelial and astroglial cells with the anatomical and physiological characteristics observed in vivo. This model was used to test the ability of monocytes/macrophages to transmigrate when challenged by A,B 1-42 on the brain side of the blood-brain barrier model. Results: In cultures of peripheral monocytes, AP3 1-42 induced the secretion of proinflammatory cytokines TNF-a, IL-6, IL-1(3, and IL-12, as well as CC chemokines MCP-1, MIP-la, and MIP-1,B3, and CXC chemokine IL-8 in a doserelated fashion. In the blood-brain barrier model, AP3 1-42 and monocytes on the brain side potentiated monocyte transmigration from the blood side to the brain side. AP3 1-42 stimulated differentiation of monocytes into adherent macrophages in a dose-related fashion. The magnitude of these proinflammatory effects of AP3 1-42 varied dramatically with monocytes from different donors. Conclusion: In some individuals, circulating monocytes/macrophages, when recruited by chemokines produced by activated microglia and macrophages, could add to the inflammatory destruction of the brain in Alzheimer's disease.
Early after the infection, the blood-brain barrier protects the brain from HIV-1. Immune mediators, such as TNF-alpha, open a paracellular route for the virus into the brain. The virus and viral proteins stimulate brain microglia and macrophages to attract monocytes into the brain. Infiltrating macrophages cause progression of HIV-1 encephalitis.
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