Receptor-mediated phagocytosis of myelin by macrophages and microglia: Effect of opsonization and receptor blocking agents
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Myelin is phagocytosed by microglia (MG) and to a somewhat lesser extent by peritoneal macrophages (Mϕ) in a dose- and time-dependent manner. In serum-free medium opsonization of rat myelin significantly enhances binding and ingestion, more by rat macrophages than by microglia. Furthermore the requirement for opsonization is not restricted to anti-myelin antibodies as the difference in the rate of myelin uptake by macrophages is largely eliminated when they are cultured in 10% fetal calf serum. Binding and ingestion of both myelin and opsonized myelin are inhibited to the same dose-dependent extent by zymosan, oxidized LDL, peroxidase-antiperoxidase (PAP), opsonized erythrocytes and the anti-CR3 antibody OX42 implicating lectin, scavenger, Fc and complement receptors in the phagocytosis of myelin. Thus while the differential uptake of myelin and opsonized myelin by macrophages would indicate a central role for the Fc receptor, binding inhibition studies implicate a range of membrane receptors which would obviate the need for antigen-antibody complexing to stimulate phagocytosis. Uptake of both myelin preparations by macrophages or microglia is stimulated by interferon-γ and inhibited by TGF-β, and the process of ingestion results in increased nitric oxide release and decreased superoxide production, the effect being more pronounced when myelin is opsonized.
Key WordsMyelin microglia macrophages free radicals zymosan CR3 cytokines
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