Different fates of Legionella pneumophila pmi and mil mutants within macrophages and alveolar epithelial cells

Alveolar epithelial cells, which constitute the majority of the alveolar su rface, may represent a potential niche for intracellular replication of Leg ionella pneumophila that has been largely overlooked. We examined the pheno types of a bank of 121 macrophage-defective mutants of L. pneumophila (desi gnated as pmi and mil) for their cytopathogenicity to and intracellular sur vival and replication within human alveolar epithelial cells. Our data show ed that 91 of 121 mutants that were defective (modest-severe) in macrophage s exhibited wild type-like phenotypes in human type I alveolar epithelial c ells. In contrast, the other 30 mutants were defective in both macrophages and alveolar epithelial cells. Transmission electron microscopy of the intr acellular infection by three mutants showed that the defect in intracellula r replication in macrophages and epithelial cells was associated with a def ect in recruitment of the RER around the phagosome. Differences in attachme nt to macrophages and epithelial cells were also exhibited by some of the m utants. Pulmonary infection studies of A/J mice showed that a mutant defect ive in macrophages but not in alveolar epithelial cells replicated like the wild type strain in the lungs of A/J mice. In contrast, a mutant defective in both macrophages and alveolar epithelial cells failed to replicate and was killed. We conclude that certain distinct genetic loci of L. pneumophil a are uniquely required for intracellular survival and replication within p hagocytic but not epithelial cells, which may be important in vivo. (C) 199 8 Academic Press.