Icm/Dot‐dependent upregulation of phagocytosis by Legionella pneumophila

Abstract

SummaryLegionella 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 monocytederived macrophages (HMMF). 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 HMMF. 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.

Use of <em>Galleria mellonella</em> as a Model Organism to Study <em>Legionella pneumophila</em> Infection

“…Previously it has been shown that L. pneumophila virulence in macrophages, amoebae and mammalian models is dependent on the presence of the Dot/Icm secretion system [41][42][43] . G. mellonella larvae were infected as described above and the virulence of the wild type (WT) and a Dot/Icm-deficient strain compared.…”

Rab1 Guanine Nucleotide Exchange Factor SidM Is a Major Phosphatidylinositol 4-Phosphate-binding Effector Protein of Legionella pneumophila

“…3 LCVs avoid fusion with lysosomes (6), intercept vesicular traffic at endoplasmic reticulum (ER) exit sites (7), and fuse with the ER (8 -10). The uptake of L. pneumophila and formation of LCVs in macrophages and amoebae depends on the Icm/Dot type IV secretion system (T4SS) (11)(12)(13)(14). Although more than 100 Icm/Dot substrates ("effector" proteins) have been identified to date, only few are functionally characterized, including effectors that interfere with host cell signal transduction, vesicle trafficking, or apoptotic pathways (15)(16)(17)(18).…”

Expression of Legionella pneumophila paralogous lipid A biosynthesis genes under different growth conditions

“…Phagocytosis and intracellular replication of L. pneumophila is promoted by the intracellular multiplication/ defective organelle trafficking (Icm/Dot) type IV secretion system (T4SS) (Hilbi et al, 2001;Segal et al, 1998;Vogel et al, 1998), a conjugation apparatus which translocates more than 40 putative 'effector' proteins into host cells (Brüggemann et al, 2006;Nagai & Roy, 2003). The Icm/Dot T4SS is also required for L. pneumophila to survive and grow on agar plates impregnated with Acanthamoeba castellanii (Albers et al, 2005).…”