Trichomonas vaginalis kills and eats--evidence for phagocytic activity as a cytopathic effect

This study reports that the cytopathic effect of Trichomonas vaginalis, an important human parasite of the urogenital tract, occurs due to mechanical stress and subsequent phagocytosis of the necrotic cells. The investigation was done using a primary culture of bovine oviduct epithelial cells (BOECs), grown either in monolayers or as floating cells. Trophozoites displaying different virulence levels were co-incubated with BOECs for times varying between 1 min and 48 h. Analyses were performed using videomicroscopy, scanning and transmission electron microscopy, colourimetric assays and cytochemistry. Injury was observed as early as 1 h after incubation, while after 12 h the host cells were severely damaged when a fresh trichomonad isolate was used. Trichomonads attack the host cells by clustering around them. Mechanical stress on the microvilli of the host cells was observed and appeared to induce plasma membrane damage and cell death. After membrane injury and lysis, fragments of the necrotic cells were ingested by trichomonads. Phagocytosis occurred by trichomonads avidly eating large portions of epithelial cells containing the nucleus and other organelles, but living or intact cells were not ingested. Necrotic fragments were rapidly digested in lysosomes, as shown by acid phosphatase and ruthenium red assays where only the BOECs were labelled. The lytic capacity of the trichomonads was more pronounced in host cell suspensions.