Extraintestinal pathogenic Escherichia coli (ExPEC) survives within neutrophils
Nazareth, Helen Marie
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Extraintestinal pathogenic Escherichia coli (ExPEC) are a significant cause of a variety of infections, including urinary tract infections, intra-abdominal infections, pneumonia, soft tissue infections, and neonatal meningitis. The innate immune system is the host's first line of defense against these pathogens. The binding of ExPEC and/or its components (e.g. lipid A) to macrophages and host epithelial cells triggers the release of cytokines that results in the migration of neutrophils into the infection site. Neutrophils engulf bacteria through phagocytosis and kill them by releasing toxic substances into the phagosomal compartment. In many cases, the neutrophil response will clear the infection. However, urinary tract infection (UTI) due to ExPEC is characterized by neutrophil influx without eradication of the infection. The failure of the innate immune system to clear ExPEC infections may be in part the result of ExPEC subverting neutrophil bactericidal activity. We hypothesized that ExPEC was able to subvert the neutrophil killing response and survive within neutrophils. We also hypothesized that neutrophil and bacterial factors were involved in the intracellular survival of ExPEC. The results presented here reveal for the first time that ExPEC is able to survive within neutrophils. Our model strain CP9 was found to survive within human blood-derived neutrophils, and within the neutrophils of the rat subcutaneous infection model. Other ExPEC strains were found to survive within human neutrophils in vitro , as well, and ExPEC strains that were studied ex vivo from the urine samples of human UTI patients were also found to survive. Mechanistically, we were able to rule out escape from the neutrophil phagosome as a possible survival mechanism, and uptake of CP9 via macropinocytosis does not appear to contribute to intracellular survival. We also discounted the possibility that ExPEC could be resistant to the neutrophil oxidative burst. Our CP9 mutant studies showed that the toxins hemolysin and cytotoxic necrotizing factor were not responsible for ExPEC survival. The adhesins encoded by pap and prs were also not involved, nor were the siderophore IroN, or the capsule found to contribute to survival within neutrophils. While O antigen may have a negative impact on survival, our results show that this may be in further need of study. We conclude that survival of ExPEC within neutrophils may contribute to the pathogenesis of ExPEC infections.