Isolation and characterization of human fetal brain-derived microglia in in vitro culture

Human brain microglia may play a central role in immunopathogenesis of CNS diseases including HIV infection, multiple sclerosis and Alzheimer's disease. In order to investigate the possible relationship between microglia and the mononuclear phagocyte system, human brain microglia were isolated from 14-18-week-old fetal brains, and maintained in in vitro culture. Enriched fetal brain microglia were stained for different monocyte/macrophage and glial cell markers. Fresh dissociated brain cells lacked macrophage surface markers. Isolated microglial cells stained positive for complement receptor C3bi, Class II [human leukocyte antigen-DR (HLA-DR)] antigen and with the lectin Ricinus communis. Microglia also share several functional properties with monocyte/macrophages, which include generation of superoxide anion and histochemically demonstrable intracellular acid phosphatase and non-specific esterase. Primary human dissociated brain cultures were maintained in culture for at least 28 weeks. Although microglia were not observed above the astrocyte cell layer after 5 weeks in culture, microglia-like cells appear below the astrocyte layer after 12 weeks in culture. These cells stained positive for non-specific esterase and displayed oxidative burst activity upon activation with phorbol myristate acetate. Thus, we have successfully isolated an enriched population of microglia from human fetal brain and have demonstrated that these cells possess markers and properties which are characteristics of mononuclear phagocytes.