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Signal transduction during Legionella pneumophila entry into human monocytes
P Y Coxon1, J T Summersgill, J A Ramirez
1Department of Microbiology and Immunology, University of Louisville, Louisville, Kentucky 40292, USA. pycoxo01@ulkyvm.louisville.edu
Infection and Immunity
|May 29, 1998
Summary
Legionella pneumophila entry into monocytes triggers actin polymerization via tyrosine kinase (TK) and protein kinase C (PKC) signaling. This tyrosine phosphorylation is a general phagocytosis mechanism, not specific to L. pneumophila virulence.
Area of Science:
- Immunology
- Cell Biology
- Microbiology
Background:
- Legionella pneumophila causes Legionnaires' disease by replicating within human monocytes and macrophages.
- Bacterial uptake by these cells is primarily mediated by complement receptor 3 (CR3)-dependent phagocytosis.
- The molecular mechanisms, particularly the roles of actin polymerization and intracellular signaling, governing this process require further elucidation.
Purpose of the Study:
- To investigate the involvement of actin polymerization and specific phosphorylation events in L. pneumophila uptake by monocytes.
- To identify the key signaling kinases activated during bacterial internalization.
- To determine if the observed phosphorylation patterns are specific to L. pneumophila or part of a general phagocytic pathway.
Main Methods:
- Uptake inhibition assays were employed to study bacterial internalization.
- Confocal microscopy was utilized to visualize cellular responses at the site of bacterial entry.
- Western blotting and kinase inhibitors (genistein, tyrphostin, staurosporine) were used to analyze protein phosphorylation and identify involved kinases.
Main Results:
- L. pneumophila entry induced actin polymerization and activated tyrosine kinase (TK) and protein kinase C (PKC) signaling.
- Six major cellular proteins were identified as TK phosphorylated upon bacterial entry, with three associating with the cytoskeleton.
- Similar tyrosine phosphorylation patterns were observed during CR3-mediated uptake of non-pathogenic bacteria (E. coli) and yeast (zymosan), suggesting a conserved mechanism.
Conclusions:
- Phagocytosis of L. pneumophila by monocytes involves TK and PKC signaling pathways that activate actin polymerization.
- The tyrosine phosphorylation observed during L. pneumophila uptake is a general host cell response to CR3-mediated phagocytosis, not a specific virulence factor.
- These findings contribute to understanding the early host-pathogen interactions in Legionnaires' disease and innate immune cell responses.