| Pseudomonas aeruginosa is a bacterium found ubiquitously in the environment, but which is also medically important for its ability to cause disease in humans, including severe pneumonia. P. aeruginosa utilizes a number of potent virulence determinants to cause disease; of these, the type III secretion system (TTSS) has been shown to be important in development of severe pneumonia in animal models of infection and human patients. Some P. aeruginosa strains secrete a potent cytotoxin, ExoU, into host cells via the TTSS. Inside of host cells, ExoU is activated and cleaves phospholipids of the plasma membrane resulting in cell death. Interestingly, P. aeruginosa isolates that secrete ExoU are associated with severe disease and worse clinical outcomes in patients with ventilator-associated pneumonia. However, the mechanism by which ExoU contributes to the pathophysiology of severe pneumonia remains unclear. This dissertation addresses the mechanism by which ExoU-secreting P. aeruginosa cause severe acute lung infection, specifically by examining the interaction of ExoU with innate immune cells. We describe the consequences of ExoU secretion on disease progression using a mouse model of acute pneumonia. We provide evidence that recruited phagocytic cells are impaired by ExoU, thereby preventing bacterial eradication from the lung. Furthermore, we directly examine the cell types intoxicated with ExoU in vivo. We demonstrate that a significant proportion of phagocytic cells are injected with ExoU in the lung. Early during infection, resident alveolar macrophages are intoxicated with ExoU, which may incapacitate these sentinel cells. As neutrophils and monocytes are recruited to the lung to eliminate the bacteria, they are injected with ExoU. The types of cells injected are largely dictated by the abundance of each cell type in the lung. Taken together, the results described in this dissertation propose a model for the mechanism of ExoU during early pneumonia. Incapacitation of phagocytic cells by intoxication with ExoU creates an environment in which P. aeruginosa can survive and cause the pathology observed during severe pneumonia. |
