ExoU Activates NF-κB and Increases IL-8/KC Secretion during Pseudomonas aeruginosa Infection

ExoU, a Pseudomonas aeruginosa cytotoxin injected into host cytosol by type III secretion system, exhibits a potent proinflammatory activity that leads to a marked recruitment of neutrophils to infected tissues. To evaluate the mechanisms that account for neutrophil infiltration, we investigated the effect of ExoU on IL-8 secretion and NF-kappa B activation. We demonstrate that ExoU increases IL-8 mRNA and protein levels in P. aeruginosa-infected epithelial and endothelial cell lines. Also, ExoU induces the nuclear translocation of p65/p50 NF-kappa B transactivator heterodimer as well as NF-kappa B-dependent transcriptional activity. ChIP assays clearly revealed that ExoU promotes p65 binding to NF-kappa B site in IL-8 promoter and the treatment of cultures with the NF-kappa B inhibitor Bay 11-7082 led to a significant reduction in IL-8 mRNA levels and protein secretion induced by ExoU. These results were corroborated in a murine model of pneumonia that revealed a significant reduction in KC secretion and neutrophil infiltration in bronchoalveolar lavage when mice were treated with Bay 11-7082 before infection with an ExoU-producing strain. In conclusion, our data demonstrate that ExoU activates NF-kappa B, stimulating IL-8 expression and secretion during P. aeruginosa infection, and unveils a new mechanism triggered by this important virulence factor to interfere in host signaling pathways.