Mechanisms of Pseudomonas aeruginosa-induced injury to the alveolar epithelium

Bacterial pneumonia is associated with a high morbidity and mortality, especially when it occurs in critically-ill patients. P. aeruginosa is one of the major pathogens causing pneumonia in critically-ill patients and is associated with the highest mortalities because of its antibiotic resistance and its bacterial products. We have investigated the major virulence factors of P. aeruginosa that cause acute lung injury; recently, we documented the involvement of the type III secretion system in the pathogenesis of P. aeruginosa pneumonia and sepsis. We determined that the major virulence factor of P. aeruginosa causing cell cytotoxicity was exoenzyme U (ExoU), a new cytotoxin regulated by the exoenzyme S regulon. The instillation of P. aeruginosa strains that produced ExoU caused necrosis of lung epithelial cells and also blocked phagocytosis of the bacteria. In the recent publication of the P. aeruginosa genome, it was shown that exoenzyme S-associated toxins, including ExoU, are type III secreted toxins. The type III secretion system is utilized by all major Gram negative pathogens. This system allows P. aeruginosa to intoxicate eukaryotic cells by directly injected toxins, including ExoU, into the cytosol of cells. Based on these facts, we investigated the role of PcrV in the translocation of type III secreted toxins by A aeruginosa. We were the first to document that PcrV was responsible for the translocation of the type III secreted-toxins of P. aeruginosa. Mice were vaccinated with endotoxin-free, recombinant PcrV, and immune mice were resistant to P. aeruginosa lung infections. The administration of immune anti-PcrV antibody prior to lung infection prevented airspace-infected mice from lung injury and death. Therefore, we concluded that PcrV was an effective antigen, that antibodies blocking PcrV protected mice from subsequent lung injury and septic death. We hope to establish that antibody therapy blocking a major bacterial toxin system is a good therapeutic strategy; the success of this project could lead to other antibody therapies for other highly resistant bacteria.