Measures of Systemic Innate Immune Function Predict the Risk of Nosocomial Infection in Pediatric Burn Patients

ALTERED PROFILES OF EXTRACELLULAR MITOCHONDRIAL DNA IN IMMUNOPARALYZED PEDIATRIC PATIENTS AFTER THERMAL INJURY

ABSTRACT

Background: Thermal injury is a major cause of morbidity and mortality in the pediatric population worldwide with secondary infection being the most common acute complication. Suppression of innate and adaptive immune function is predictive of infection in pediatric burn patients, but little is known about the mechanisms causing these effects. Circulating mitochondrial DNA (mtDNA), which induces a proinflammatory signal, has been described in multiple disease states but has not been studied in pediatric burn injuries. This study examined the quantity of circulating mtDNA and mtDNA mutations in immunocompetent (IC) and immunoparalyzed (IP) pediatric burn patients. Methods: Circulating DNA was isolated from plasma of pediatric burn patients treated at Nationwide Children's Hospital Burn Center at early (1-3 days) and late (4-7 days) time points postinjury. These patients were categorized as IP or IC based on previously established immune function testing and secondary infection. Three mitochondrial genes, D loop, ND1, and ND4, were quantified by multiplexed qPCR to assess both mtDNA quantity and mutation load. Results: At the early time point, there were no differences in plasma mtDNA quantity; however, IC patients had a progressive increase in mtDNA over time when compared with IP patients (change in ND1 copy number over time 3,880 vs. 87 copies/day, P = 0.0004). Conversely, the IP group had an increase in mtDNA mutation burden over time. Conclusion: IC patients experienced a significant increase in circulating mtDNA quantity over time, demonstrating an association between increased mtDNA release and proinflammatory phenotype in the burn patients. IP patients had significant increases in mtDNA mutation load likely representative of degree of oxidative damage. Together, these data provide further insight into the inflammatory and immunological mechanisms after pediatric thermal injury.

A combination of burn wound injury and Pseudomonas infection elicits unique gene expression that enhances bacterial pathogenicity

IMPORTANCE

The interaction between an underlying disease process and a specific pathogen may lead to the unique expression of genes that affect bacterial pathogenesis. These genes may not be observed during infection in the absence of, or with a different underlying process or infection during the underlying process with a different pathogen. To test this hypothesis, we used Nanostring technology to compare gene transcription in a murine-burned wound infected with P. aeruginosa. The Nanostring probeset allowed the simultaneous direct comparison of immune response gene expression in both multiple host tissues and P. aeruginosa in conditions of burn alone, infection alone, and burn with infection. While RNA-Seq is used to discover novel transcripts, NanoString could be a technique to monitor specific changes in transcriptomes between samples and bypass the additional adjustments for multispecies sample processing or the need for the additional steps of alignment and assembly required for RNASeq. Using Nanostring, we identified arginine and IL-10 as important contributors to the lethal outcome of burned mice infected with P. aeruginosa. While other examples of altered gene transcription are in the literature, our study suggests that a more systematic comparison of gene expression in various underlying diseases during infection with specific bacterial pathogens may lead to the identification of unique host-pathogen interactions and result in more precise therapeutic interventions.

Early detection of soluble CD27, BTLA, and TIM-3 predicts the development of nosocomial infection in pediatric burn patients

Thermal injury induces concurrent inflammatory and immune dysfunction, which is associated with adverse clinical outcomes. However, these effects in the pediatric population are less studied and there is no standard method to identify those at risk for developing infections. Our goal was to better understand immune dysfunction and identify soluble protein markers following pediatric thermal injury. Further we wanted to determine which early inflammatory, soluble, or immune function markers are most predictive of the development of nosocomial infections (NI) after burn injury. We performed a prospective observational study at a single American Burn Association-verified Pediatric Burn Center. A total of 94 pediatric burn subjects were enrolled and twenty-three of those subjects developed a NI with a median time to diagnosis of 8 days. Whole blood samples, collected within the first 72 hours after injury, were used to compare various markers of inflammation, immune function, and soluble proteins between those who recovered without developing an infection and those who developed a NI after burn injury. Within the first three days of burn injury, innate and adaptive immune function markers (ex vivo lipopolysaccharide-induced tumor necrosis factor alpha production capacity, and ex vivo phytohemagglutinin-induced interleukin-10 production capacity, respectively) were decreased for those subjects who developed a subsequent NI. Further analysis of soluble protein targets associated with these pathways displayed significant increases in soluble CD27, BTLA, and TIM-3 for those who developed a NI. Our findings indicate that suppression of both the innate and adaptive immune function occurs concurrently within the first 72 hours following pediatric thermal injury. At the same time, subjects who developed NI have increased soluble protein biomarkers. Soluble CD27, BTLA, and TIM-3 were highly predictive of the development of subsequent infectious complications. This study identifies early soluble protein makers that are predictive of infection in pediatric burn subjects. These findings should inform future immunomodulatory therapeutic studies.

Measures of Adaptive Immune Function Predict the Risk of Nosocomial Infection in Pediatric Burn Patients

Thermal injury results in changes in the inflammatory and innate immune response of pediatric patients. Plasma cytokines, cellular profiles, and reduction in innate immune function following burn injury has also been correlated to adverse outcomes (e.g., mortality and infectious complications). Changes in adaptive immune function following thermal injury are not as well characterized. Our goal was to better understand if adaptive immune dysfunction occurs early after pediatric thermal injury and is a risk factor for nosocomial infections (NI). A prospective, longitudinal immune function observational study was performed at a single ABA-verified pediatric burn center. Eighty burn patients were enrolled with 20 developing a NI, defined using CDC criteria. We collected whole blood samples from pediatric burn patients within the first 72 hours from injury and between days 4-7, where applicable to analyze adaptive immune function. We compared immune function between burn patients who went on to develop NI and those that did not. Within the first 72 hours of injury, burn patients who developed NI had significantly lower absolute CD4+ lymphocyte counts and whole blood ex vivo phytohemagglutinin (PHA)-induced IFNγ and IL-10 production capacity compared to those that did not develop infection. Further analysis using receiver operating characteristic curve revealed that PHA-induced IL-10 production capacity had the highest area under the curve. Our data demonstrates early adaptive immune suppression occurs following pediatric thermal injury and PHA-induced IL-10 production capacity appears to be a predictor for the development of NI.

The role of macrophages in thermal injury

Macrophages, first discovered for their phagocytic ability, are a complicated and heterogeneous cell type. The unique properties of macrophages allow them to perform a vast array of functions, including phagocytosis, cytokine production, antigen presentation, and wound healing. Some macrophage populations are derived from monocytes and are induced into specific phenotypes by the local tissue microenvironment, while other macrophages form during early embryonic development. The exposure of the host to local pathogens and/or traumatic injury alters the tissue microenvironment and, in turn, influences changes in macrophage phenotype and function. Perhaps the most significant change in the local tissue microenvironment and subsequent macrophage phenotype occurs after thermal injury, which causes localized tissue damage and a massive systemic inflammatory response. However, few studies have explored the influence of burn injury on the host macrophages and macrophage function in burn wounds. Furthermore, the literature is scant regarding the impact macrophage function has on outcomes in thermal injury. This review will focus on the current knowledge of macrophage function in burn wounds and the phenotypic changes in macrophages during thermal injury while identifying knowledge gaps.