Background: Historically, fever, pneumonia, and sepsis after trauma are ascribed to pain and poor pulmonary toilet. No evidence supports that assertion however, and no known biologic mechanisms link injury to infection. Our studies show that injured tissues release mitochondria (MT). Mitochondrial damage-associated molecular patterns (mtDAMPs) however can mimic bacterial pathogen-associated danger molecules and attract neutrophils (PMN). We hypothesized that mtDAMPs from traumatized tissue divert neutrophils from the lung, causing susceptibility to infection.
Methods: Anesthetized rats (6-10 per group) underwent pulmonary contusion (PC) by chest percussion. When modeling traumatic MT release, some rats had MT isolated from the liver (equal to 5% liver necrosis) injected intraperitoneally (IPMT). Negative controls had PC plus buffer intraperitoneally. Positive controls underwent PC plus cecal ligation and puncture. At 16 hours, bronchoalveolar and peritoneal lavages were performed. Bronchoalveolar lavage fluid (BALF) and peritoneal lavage fluid were assayed for PMN count, albumin, interleukin β, (IL-β), and CINC-1. Assays were normalized to blood urea nitrogen to calculate absolute concentrations.
Results: PC caused alveolar IL-1β and CINC production and a 34-fold increase in BALF neutrophils. As expected, IPMT increased peritoneal IL-1β and CINC and attracted PMN to the abdomen. However, remarkably, IPMT after PC attenuated BALF cytokine accumulation and decreased BALF PMN. Cecal ligation and puncture had no direct effect on BALF PMNs but, like IPMT, blunted BALF leukocytosis after PC.
Conclusion: Rather than acting as a "second hit" to enhance PMN-mediated lung injury, mtDAMPs from trauma and pathogen-associated danger molecules from peritoneal infection diminish PMN accumulation in a contused lung. This may make the lung susceptible to pneumonia. This paradigm provides a novel mechanistic model of the relationship among blunt tissue trauma, systemic inflammation, and pneumonia that can be studied to improve trauma outcomes.