Icm/dot-dependent upregulation of phagocytosis by Legionella pneumophila

Mol Microbiol. 2001 Nov;42(3):603-17. doi: 10.1046/j.1365-2958.2001.02645.x.


Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Acanthamoeba / microbiology
  • Animals
  • Anti-Bacterial Agents / pharmacology
  • Bacterial Adhesion
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Cell Line
  • Gentamicins / pharmacology
  • Humans
  • Legionella pneumophila / genetics
  • Legionella pneumophila / metabolism
  • Legionella pneumophila / physiology*
  • Macrophages / microbiology*
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism*
  • Mice
  • Microscopy, Fluorescence
  • Mutation
  • Phagocytosis / physiology*
  • Up-Regulation


  • Anti-Bacterial Agents
  • Bacterial Proteins
  • Carrier Proteins
  • Gentamicins
  • Membrane Proteins