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To determine whether pathogenicAcanthamoeba culbertsonitrophozoites and lysate can induce cytopathic changes in primary-culture microglial cells, morphological changes were observed by transmission electron microscopy (TEM). In addition, the secretion of two kinds of cytokines, tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β), from microglial cells was observed. Trophozoites of pathogenicA. culbertsonimade contact with microglial cells and produced digipodia. TEM revealed that microglial cells cocultured with amoebic trophozoites underwent a necrotic process, accompanied by lysis of the cell membrane. TEM of microglial cells cocultured with amoebic lysate showed that the membranes of the small cytoplasmic vacuoles as well as the cell membrane were lysed. The amounts of TNF-α secreted from microglial cells cocultured withA. culbertsonitrophozoites or lysate increased at 6 h of incubation. The amounts of IL-1β secreted from microglial cells cocultured withA. culbertsonitrophozoites at 6 h of incubation was similar to those secreted from the control group, but the amounts decreased during cultivation withA. culbertsonilysate. These results suggest that pathogenicA. culbertsoniinduces the cytopathic effects in primary-culture rat microglial cells, with the effects characterized by necrosis of microglial cells and changes in levels of secretion of TNF-α and IL-1β from microglial cells.