Cecal ligation and puncture followed by MRSA pneumonia increases mortality in mice and blunts production of local and systemic cytokines

Mortality in the ICU frequently results from the synergistic effect of two temporally-distinct infections. This study examined the pathophysiology of a new model of intraabdominal sepsis followed by methicillin-resistant Staphylococcus aureus (MRSA) pneumonia. Mice underwent cecal ligation and puncture (CLP) or sham laparotomy followed three days later by an intratracheal injection of MRSA or saline. Both CLP/saline and sham/MRSA mice had 100% survival while animals with CLP followed by MRSA pneumonia had 67% seven-day survival. Animals subjected to CLP/MRSA had increased bronchoalveolar lavage (BAL) concentrations of MRSA compared to sham/MRSA animals. Animals subjected to sham/MRSA pneumonia had increased BAL levels of IL-6, TNF-α, and G-CSF compared to those given intratracheal saline while CLP/MRSA mice had a blunted local inflammatory response with markedly decreased cytokine levels. Similarly, animals subjected to CLP/saline had increased peritoneal lavage levels of IL-6 and IL-1β compared to those subjected to sham laparotomy while this response was blunted in CLP/MRSA mice. Systemic cytokines were upregulated in both CLP/saline and sham/MRSA mice, and this was blunted by the combination of CLP/MRSA. In contrast, no synergistic effect on pneumonia severity, white blood cell count or lymphocyte apoptosis was identified in CLP/MRSA mice compared to animals with either insult in isolation. These results indicate that a clinically relevant model of CLP followed by MRSA pneumonia causes higher mortality than could have been predicted from studying either infection in isolation, and this was associated with a blunted local (pulmonary and peritoneal) and systemic inflammatory response and decreased ability to clear infection.