Evidence that Dot‐dependent and ‐independent factors isolate the Legionella pneumophila phagosome from the endocytic network in mouse macrophages

Legionella pneumophila survives within macrophages by evading phagosome–lysosome fusion. To determine whether L. pneumophila resides in an intermediate endosomal compartment or is isolated from the endosomal pathway and to investigate what bacterial factors contribute to establishment of its vacuole, we applied a series of fluorescence microscopy assays. The majority of vacuoles, aged 2.5 min to 4 h containing post‐exponential phase (PE) L. pneumophila , appeared to be separate from the endosomal pathway, as judged by the absence of transferrin receptor, LAMP‐1, cathepsin D and each of four fluorescent probes used to label the endocytic pathway either before or after infection. In contrast, more than 70% of phagosomes that contained Escherichia coli , polystyrene beads, or exponential phase (E) L. pneumophila matured to phagolysosomes, as judged by co‐localization with LAMP‐1, cathepsin D and fluorescent endosomal probes. Surprisingly, neither bacterial viability nor the putative Dot/Icm transport complex was absolutely required for vacuole isolation; although phagosomes containing either formalin‐killed PE wild‐type or live PE dotA or dotB mutant L. pneumophila rapidly accumulated LAMP‐1, less than 20% acquired lysosomal cathepsin D or fluorescent endosomal probes. Therefore, a Dot‐dependent factor(s) isolates the L. pneumophila phagosome from a LAMP‐1‐containing compartment, and a formalin‐resistant Dot‐independent activity inhibits vacuolar accumulation of endocytosed material and delivery to the degradative lysosomes.