Phagocytosis of Escherichia coli by amoeboid microglial cells in the developing brain.

Amoeboid microglial cells (AMC) in the corpus callosum of 1-day-old rats receiving a single intracerebral injection of Escherichia coli ( E. Coli) were examined at various time intervals following the injection. A large number of E. coli were internalized by the AMC at 1-3 h after the injection. However, no E. coli were identifiable at 1 day after the injection, but large phagosomes were observed in the cytoplasm of AMC. With time, the phagosomes in the AMC were reduced so that by 7 days the cells appeared comparable to the controls. Apoptotic or necrotic AMC were not encountered during the study period. This is consistent with the results of cell counts, which showed no significant change in the AMC population following E. coli injection compared with controls. The present results suggest that AMC are capable of removing live bacteria from their vicinity. Up-regulation of complement type 3 receptors and induction of major histocompatibility complex class II antigens were observed in the AMC at days 1-3 and 7 following E. coli administration. This may be related to their involvement in mediating endocytosis and their possible role in antigen presentation.