Entry and intracellular localization of Legionella dumoffii in Vero cells

Organisms of some Legionella species are known to internalize and multiply within epithelial cell lines. During the study on interaction between Legionella spp. and Vero cells, we found that L. dumoffii Tex-KL (ATCC 33343) can enter into Vero cells approximately four to 20 times more often than five other strains of four species of legionella. The mode of entry between L. dumoffii Tex-KL and L. pneumophila Philadelphia-1 was compared and studied by treating Vero cells with reagents which inhibit phagocytosis and endocytosis. Monodansylcadaverine, cytochalasin D and nocodazol were used as inhibitors of receptor-mediated endocytosis, microfilament-dependent phagocytosis and polymerization of microtubules, respectively. The uptake of L. dumoffii Tex-KL required receptor-mediated endocytosis by Vero cells, while the uptake of L. pneumophila Philadelphia-1 used mainly microfilament-dependent phagocytosis. Polymerization of microtubules was necessary for Vero cells for the uptake of both strains of legionella. An electron microscopic examination revealed that some organisms of the L. dumoffii strain Tex-KL escaped from endosomal vacuoles into cytoplasm in the early stage of infection, and proliferated in the cytoplasm. At that period, most of the bacteria were surrounded by rough endoplasmic reticula. In contrast, L. pneumophila Philadelphia-1 proliferated only within ribosome-lined endosome. We suggest that L. dumoffii Tex-KL internalize and proliferate in Vero cells in a different way to L. pneumophila Philadelphia-1, and that there is a variety of the mode of interaction between Legionella spp. and epithelial cells.