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Legionella pneumophila is internalized by a macropinocytotic uptake pathway controlled by the Dot/Icm system and the
1Howard Hughes Medical Institute, Tufts University School of Medicine, Boston, MA 02111, USA.
Abstract:
The products of the Legionella pneumophila dot/icm genes enable the bacterium to replicate within a macrophage vacuole. This study demonstrates that the Dot/Icm machinery promotes macropinocytotic uptake of L. pneumophila into mouse macrophages. In mouse strains harboring a permissive Lgn1 allele, L. pneumophila promoted formation of vacuoles that were morphologically similar to macropinosomes and dependent on the presence of an intact Dot/Icm system. Macropinosome formation appeared to occur during, rather than after, the closure of the plasma membrane about the bacterium, since a fluid-phase marker preloaded into the macrophage endocytic path failed to label the bacterium-laden macropinosome. The resulting macropinosomes were rich in GM1 gangliosides and glycosylphosphatidylinositol-linked proteins. The Lgn1 allele restrictive for L. pneumophila intracellular replication prevented dot/icm-dependent macropinocytosis, with the result that phagosomes bearing the microorganism were targeted into the endocytic network. Analysis of macrophages from recombinant inbred mouse strains support the model that macropinocytotic uptake is controlled by the Lgn1 locus. These results indicate that the products of the dot/icm genes and Lgn1 are involved in controlling an internalization route initiated at the time of bacterial contact with the plasma membrane.
Insights
Legionella pneumophila uses its Dot/Icm machinery to trigger macropinocytosis, a specific uptake process in macrophages. This bacterial entry mechanism is controlled by the Lgn1 gene, influencing intracellular replication.
Area of Science:
- Microbiology
- Cell Biology
- Immunology
Background:
- Legionella pneumophila requires a functional Dot/Icm secretion system for intracellular replication within host macrophages.
- Bacterial entry into host cells can occur through various mechanisms, including phagocytosis and macropinocytosis, influencing intracellular fate.
Purpose of the Study:
- To investigate the role of the Legionella pneumophila Dot/Icm machinery in the uptake process by macrophages.
- To determine whether Dot/Icm influences the specific internalization route, particularly macropinocytosis, and its dependence on host factors like Lgn1.
Main Methods:
- Utilized mouse macrophages from different strains, including those with permissive and restrictive Lgn1 alleles.
- Employed microscopy and fluid-phase markers to analyze vacuole formation and bacterial internalization.
- Investigated the composition of bacterium-containing vacuoles, including GM1 gangliosides and GPI-linked proteins.
Main Results:
- Demonstrated that the Dot/Icm system actively promotes macropinocytotic uptake of L. pneumophila into permissive macrophages.
- Observed that macropinosome formation occurs concurrently with bacterial engulfment, indicating a Dot/Icm-mediated internalization process.
- Showed that restrictive Lgn1 alleles block Dot/Icm-dependent macropinocytosis, redirecting bacteria to a different endocytic pathway.
Conclusions:
- The Dot/Icm machinery is crucial for inducing macropinocytosis, a specific internalization route for L. pneumophila.
- The host Lgn1 locus plays a significant role in controlling this Dot/Icm-dependent macropinocytotic uptake.
- Bacterial entry via Dot/Icm-mediated macropinocytosis is initiated at the plasma membrane upon bacterial contact.
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