Icm/Dot‐dependent upregulation of phagocytosis by Legionella pneumophila

Abstract

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 (HMMΦ). 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 HMMΦ. 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.