Intracellular pathogens and antigen presentation-new challenges with Legionella pneumophila.

In this issue of Immunity, examine the intracellular life of Legionella pneumophila in dendritic cells (DC) and macrophages, as well as the presentation of its antigens to CD4 T cells. Legionella is a particularly interesting bacterium because of the peculiarities inherent in its intracellular sojourn in phagocytes: it resides in an unusual vesicle characterized by ribosomes studded along its walls. In this compartment, Legionella proteins encoded by the dot gene inhibit phagosome-lysosome fusion and endosomal acidification, yielding a vesicular structure conducive to the multiplication of Legionella, poor in lysosomal contents, and in MHC molecules.