Activation of adult human derived microglia by myelin phagocytosis in vitro

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Abstract
The present study was designed to determine the extent to which cultured glial cells phagocytose normal central nervous system (CNS) myelin and CNS myelin opsonized with serum or purified antibody against myelin basic protein (MBP). Glial cells studied were mixed cultures (consisting of astrocytes, microglia, and oligodendrocytes) and enriched microglia established from adult human brain specimens and enriched astrocytes from fetal human brain. A human monocytic cell line, THP‐1, was included as a control. Uptake of 125I‐labelled myelin was followed over a 24 hr time period. An assay of oxidative burst (30 min) and cytokine bioassays measuring IL‐1, IL‐6, and tumor necrosis factor (TNF) production (6‐‐48 hr) were used to investigate short‐ and longterm activation of phagocytosing cells. Maximum myelin uptake by phagocytosing glial cells occurred within 12‐‐24 hr following myelin incubation. Opsonization of myelin prior to the phagocytosis assay resulted in greater myelin uptake by mixed glial cell cultures, microglia, and THP‐1 cells over that of nontreated myelin. The magnitude of myelin phagocytosis by astrocytes was considerably lower than microglia and THP‐, and was not affected by myelin opsonization. Within 30 min of myelin phagocytosis, microglia and THP‐1 cells underwent oxidative burst; opsonization of myelin by purified anti‐MBP IgG and heat‐inactivated serum enhanced the microglial oxidative burst activity. Production of IL‐1, TNF, and most markedly IL‐6 by microglia was increased following 12‐‐24 hr of myelin ingestion. Our data demonstrate that myelin phagocytosis by adult human‐derived microglia occurs in vitro, is augmented when myelin is opsonized, and results in the activation of microglia as assessed by oxidative burst and cytokine production.