Mammalian target of rapamycin regulates a hyperresponsive state in pulmonary neutrophils late after burn injury.

Bacterial pneumonia is a leading cause of death late after burn injury due to the severe immune dysfunction that follows this traumatic injury. The Mechanistic/Mammalian Target of Rapamycin (mTOR) pathway drives many effector functions of innate immune cells required for bacterial clearance. Studies have demonstrated alterations in multiple cellular processes in patients and animal models following burn injury in which mTOR is a central component. Goals of this study were to (1) investigate the importance of mTOR signaling in antimicrobial activity by neutrophils and (2) therapeutically target mTOR to promote normalization of the immune response. We utilized a murine model of 20% total body surface area burn and the mTOR-specific inhibitor rapamycin. Burn injury led to innate immune hyperresponsiveness in the lung including recruitment of neutrophils with greater ex vivo oxidative activity compared with neutrophils from sham-injured mice. Elevated oxidative function correlated with improved clearance of Pseudomonas aeruginosa, despite down-regulated expression of the bacterial-sensing TLR molecules. Rapamycin administration reversed the burn injury-induced lung innate immune hyperresponsiveness and inhibited enhanced bacterial clearance in burn mice compared with untreated burn mice, resulting in significantly higher mortality. Neutrophil ex vivo oxidative burst was decreased by rapamycin treatment. These data indicate that (1) neutrophil function within the lung is more important than recruitment for bacterial clearance following burn injury and (2) mTOR inhibition significantly impacts innate immune hyperresponsiveness, including neutrophil effector function, allowing normalization of the immune response late after burn injury.

Direct detection of blood nitric oxide reveals a burn-dependent decrease of nitric oxide in response to Pseudomonas aeruginosa infection.

PURPOSE: Burn is associated with severe immune dysfunction, including an anti-inflammatory state that occurs late after burn. While increased nitric oxide (NO) production is associated with severe infection and sepsis, the effect of burn trauma on these levels during a non-lethal infection remains unknown. We hypothesized that in a mouse model, (1) NO levels would be increased after infection without trauma and (2) burn would lead to decreased NO production even during infection. METHODS: Mice were infected via intra-tracheal inoculation with Pseudomonas aeruginosa 14 d following a 20% total body surface area contact burn. At 48h following infection, blood was drawn to quantify NO concentrations using a microfluidic electrochemical sensor. SIGNIFICANT FINDINGS: In uninjured mice, infection caused a significant increase in blood NO levels. Increases in NO occurred in a dose-dependent response to the bacterial inoculum. Following burn, an identical infection did not elicit increases in NO. CONCLUSIONS: While increases in NO are expected over the course of an infection without prior trauma, burn and subsequent immune suppression decreases NO levels even in the presence of infection.