Pneumonia after cecal ligation and puncture: a clinically relevant "two-hit" model of sepsis

Sepsis continues to be the primary cause of death among patients in surgical intensive care units. In many cases, death does not result from the initial septic event but rather from subsequent nosocomial infection with pneumonia being the most common etiology. In addition, most deaths in patients with sepsis occur after the first 72 h. By contrast, in most animal models of sepsis, most deaths occur within the first 72 h. The purpose of this study was to develop a clinically relevant "two-hit" model of sepsis that would reflect delayed mortality because of secondary nosocomial infection. The well-accepted and widely used cecal ligation and puncture (CLP) model was used as the "first hit". Pseudomonas aeruginosa or Streptococcus pneumoniae was used to induce pneumonia in mice 72 h after CLP as a "second hit." In this study, mortality in mice undergoing CLP followed by pneumonia was significantly higher than in mice receiving pneumonia or CLP alone. S. pneumoniae pneumonia after CLP resulted in a 95% mortality compared with a 20% mortality for pneumonia alone, P < 0.0001. Similarly, mortality of P. aeruginosa pneumonia after CLP (85%) was significantly higher than P. aeruginosa alone (20%), P < 0.0001. Mice undergoing CLP followed by P. aeruginosa pneumonia also had significantly higher levels of B- and T-cell apoptotic death. Finally, mice undergoing CLP followed by P. aeruginosa or S. pneumoniae pneumonia had significantly decreased concentrations of proinflammatory mediators monocyte chemoattractant protein-1 and interleukin (IL)-6 compared with mice undergoing CLP or pneumonia alone. In conclusion, a primary sublethal infection impairs the immune system thus rendering the host more susceptible to secondary infection and death. Double injury, that is, CLP followed by pneumonia, provides a useful tool in the study of sepsis, creating a prolonged period of infection as opposed to CLP alone. The extended duration of infection may lead to a better understanding of the mechanism of the immune dysregulation seen in clinical sepsis and therefore provides for evaluation of potential therapies that target specific stages of the immune response.