In animal cells, pathogens and other extracellular material contained in phagosomes are eventually degraded in a phagolysosome. The process of phagosome maturation is not completely understood, but it is believed that phagosomes acquire proteolytic enzymes and a lower pH through a series of fleeting interactions with endocytic compartments (Desjardins et al. 1994). This process has not been studied in Drosophila, but it seems likely that the fundamental process will resemble that of phagocytosis and endocytosis in mammalian cells. Various studies of host/pathogen interactions in Drosophila S2 cells have shown that genes involved in vesicle trafficking and fusion are required for an effective cellular immune response. Transcripts for proteins involved in vesicle formation, including clathrin and members of the COPI complex, have been detected in RNAi studies of cells phagocytosing various types of bacteria (Ramet et al. 2002; Philips et al. 2005; Stroschein-Stevenson et al. 2006). These studies and others have also identified genes involved in vesicle docking and fusion, including the t-SNAREs Snap and Syntaxin as well as the small GTPases Rab 2 and Rab 5, as being required for processing phagocytosed pathogens (Agaisse et al. 2005; Philips et al. 2005; Stroschein-Stevenson et al. 2006). An RNAi screen of phagocytosis of L. monocytogenes by S2 cells showed that vesicular trafficking complexes that interact with late endosomes are necessary for the cellular immune response (Cheng et al. 2005). Furthermore, it has been reported that CTS proteases, which are most active at pH 4.5, localize to phagosomes in S2 cells
(Kocks et al. 2003) and lysosomal proteases such as cathepsins are modified and activated in mbn-2 cells upon exposure to lipopolysaccharide (Loseva and Engstrom 2004).
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