Figure 2.

Phagosome maturation. The new phagosome quickly develops the characteristics of early endosomes, through a series of fusion and fission events with sorting and recycling endosomes (5, 6). The early phagosome is marked by the presence of the small GTPase Rab5 (13, 14), early endosome antigen 1 (EEA1) (15), and the class III PI-3K human vacuolar protein-sorting 34 (hvPS34) (16). The early phagosome also becomes a little acidic (pH 6.1–6.5) by the action of V-ATPase accumulating on its membrane (17, 18). The late phagosome is marked by the presence of Rab7 (19–21) and lysosomal-associated membrane proteins (LAMPs) (22, 23). Proteins that will be recycled are separated in sorting (recycling) vesicles, while proteins intended for degradation are eliminated in intraluminal vesicles (ILVs), directed into the lumen of the phagosome (24). The lumen of the late phagosome gets more acidic (pH 5.5–6.0), due to the action of more V-ATPase molecules on the membrane. Phagolysosomes are formed when late phagosomes fuse with lysosomes. Phagolysosomes are acidic (pH 5–5.5) and contain many degradative enzymes, including various cathepsins, proteases, lysozymes, and lipases. Other microbicidal component of the phagosome is the NADPH oxidase that generates reactive oxygen species (25).