Identifying the function of genes involved in excreted vesicle formation in Acanthamoeba castellanii containing Legionella pneumophila

Background: Legionella spp. can survive and replicate inside host cells such as protozoa and macrophages. After enough growth, Legionella is released from the host cells as free legionellae or Legionella-filled vesicles. The vesicles support Legionella to survive for a long time in the environment and transmit to a new host. In this study, we identified the differentially expressed genes of Acanthamoeba infected by Legionella (ACA1_114460, ACA1_091500, and ACA1_362260) and examined their roles in the formation of the excreted vesicles and escape of Legionella from the Acanthamoeba. Methods: After ingestion of Escherichia coli and Legionella pneumophila, expression levels of target genes in Acanthamoeba were measured by real-time polymerase chain reaction (PCR) analysis. The roles of target genes were investigated by transfection of small interfering RNA (siRNA). The formation of Legionella-containing excreted vesicles and the vesicular co-localization with the lysosomes were examined by Giemsa stain and LysoTracker stain. Results: ACA1_114460, ACA1_091500, and ACA1_362260 were upregulated after ingestion of Legionella in Acanthamoeba. ACA1_114460- and ACA1_091500-silenced Acanthamoeba failed to form the Legionella-containing excreted vesicles. Legionella was released as free legionellae from the Acanthamoeba. When the ACA1_362260 of Acanthamoeba was silenced, Legionella-containing excreted vesicles were fused with the lysosome. Conclusions: These results indicated that ACA1_114460, ACA1_091500, and ACA1_362260 of Acanthamoeba played important roles in the formation of Legionella-containing excreted vesicles and inhibition of the lysosomal co-localization with the phagosome. Graphical Abstract: [Figure not available: see fulltext.]