Legionella pneumophila is internalized by a macropinocytotic uptake pathway controlled by the Dot/Icm system and the mouse Lgn1 locus

The products of the Legionella pneumophila dot/icm genes enable the bacteri um to replicate within a macrophage vacuole. This study demonstrates that t he Dot/Icm machinery promotes macropinocytotic uptake of L. pneumophila int o mouse macrophages. In mouse strains harboring a permissive Lgn1 allele, L . pneumophila promoted formation of vacuoles that were morphologically simi lar to macropinosomes and dependent on the presence of an intact Dot/ Icm s ystem. Macropinosome formation appeared to occur during, rather than after, the closure of the plasma membrane about the bacterium, since a fluid-phas e marker preloaded into the macrophage endocytic path failed to label the b acterium-laden macropinosome. The resulting macropinosomes were rich in GM1 gangliosides and glycosylphosphatidylinositol-linked proteins. The Lgn1 al lele restrictive for L. pneumophila intracellular replication prevented dot /icm-dependent macropinocytosis, with the result that phagosomes bearing th e microorganism were targeted into the endocytic network. Analysis of macro phages from recombinant inbred mouse strains support the model that macropi nocytotic uptake is controlled by the Lgn1 locus. These results indicate th at the products of the dot/icm genes and Lgn1 are involved in controlling a n internalization route initiated at the time of bacterial contact with the plasma membrane.