Rab5 regulates the kiss and run fusion between phagosomes and endosomes and the acquisition of phagosome leishmanicidal properties in RAW 264.7 macrophages.

Phagolysosome biogenesis is essential for the killing and degradation of intracellular pathogens. It involves the fusion of phagosomes with various endocytic organelles, a process known to be regulated in part by Rab proteins. We generated RAW 264.7 macrophages expressing an active mutant of Rab5 (Rab5(Q79L)) to determine the role of Rab5 in phagocytosis and phagolysosome biogenesis. Our results indicate that Rab5 stimulates phagocytosis of latex beads but not Fc or C3 receptor-mediated phagocytosis. Rab5 also acts to restrict the complete fusion of phagosomes with endosomes, a phenomenon allowing exchange of solutes from the two compartments without complete intermixing of their membrane (kiss and run). In Rab5(Q79L)-expressing macrophages, uncontrolled fusion events occurred, leading to the appearance of giant phagosomes. These phagosomes could initiate their maturation and acquire LAMP1, but failed to generate the microbicidal conditions needed to kill intracellular parasites. These results identify Rab5 as a key molecule regulating phagosome-endosome fusion and as an essential component in the innate ability of macrophages to restrict the growth of intracellular parasites.