Pseudomonas aeruginosa induces type-III-secretion-mediated apoptosis of macrophages and epithelial cells

Pseudomonas aeruginosa is a gram-negative opportunistic pathogen that is cy totoxic towards a variety of eukaryotic cells. To investigate the effect of this bacterium on macrophages, we infected J774A.1 cells and primary bone- marrow-derived murine macrophages with the P. aeruginosa strain PA103 in vi tro, PA103 caused type-III-secretion-dependent killing of macrophages withi n 2 h of infection. Only a portion of the killing required the putative cyt otoxin ExoU. By three criteria, terminal deoxynucleotidyltransferase-mediat ed dUTP-biotin nick end labeling assays, cytoplasmic nucleosome assays, and Hoechst staining, the ExoU-independent but type-III-secretion-dependent ki lling exhibited features of apoptosis. Extracellular bacteria were capable of inducing apoptosis, and some laboratory and clinical isolates of P. aeru ginosa induced significantly higher levels of this form of cell death than others. Interestingly, HeLa cells but not Madin-Darby canine kidney cells w ere susceptible to type-III-secretion-mediated apoptosis under the conditio ns of these assays. These findings are consistent with a model in which the P. aeruginosa type III secretion system transports at least two factors th at kill macrophages: ExoU, which causes necrosis, and a second, as yet unid entified, effector protein, which induces apoptosis. Such killing may contr ibute to the ability of this organism to persist and disseminate within inf ected patients.