Cell-free nuclear, but not mitochondrial, DNA concentrations correlate with the early host inflammatory response after severe trauma

Abstract

Severe blunt trauma is associated with an early 'genomic storm' which causes simultaneous up- and down-regulation of host protective immunity. Excessive inflammation can lead to organ injury. In the absence of infection, the inflammatory response is presumably driven by release of endogenous alarmins called danger-associated molecular patterns (DAMPs), which initiate immune responses through pattern-recognition receptors (PRR). Here we examined the relationship between concentrations of cell-free (cf) nuclear DNA (ncDNA) and mitochondrial DNA (mtDNA) within 24 hours post trauma with circulating leukocyte transcriptomics and plasma IL-6 concentrations, as well as the patients' clinical trajectories. In 104 patients enrolled from two level-1 trauma centers, ncDNA and mtDNA concentrations were increased within 24 hours of severe trauma, but only ncDNA concentrations correlated with leukocyte gene expression and outcomes. Surprisingly, ncDNA, not mtDNA concentrations, were significantly elevated in trauma patients who developed chronic critical illness versus rapid clinical recovery. Plasma IL-6 and leukocyte transcriptomics were better predictors of outcomes than cfDNA levels. Although mtDNA and ncDNA are significantly increased in the immediate post-trauma period, the dramatic inflammatory and gene expression changes seen after severe trauma are only weakly correlated with ncDNA concentrations, and more importantly, mtDNA concentrations are not associated with adverse clinical trajectories.

Figure 1

Plasma cfDNA, IL-6 concentrations and blood leukocyte transcriptomics in healthy subjects and blunt trauma patients at ≤12 and 24 hours after injury. Panel A. Cell-free mtDNA copy number at ≤12 and 24 hours. Values represent medians, quartiles (box plots) and 95%iles (whiskers). On the left side are healthy controls and the total cohort of trauma patients in whom values were obtained. The right side contains the trauma cohort broken into subjects who died during hospitalization, rapidly recovered (RAP) or developed chronic critical illness (CCI). *p < 0.05, **p < 0.01. Panel B. Cell-free ncDNA concentrations at 12 and 24 hours. Panel C. Plasma IL-6 concentrations. Median, quartiles for patients who suffered an early death have not been included on the figure because it would expand the y-axis and compress the appearance of the other groups. In the early death patients, the median IL-6 concentration was 1501 pgs/ml (812, 4349) at ≤12 hours and 372 pgs/ml (238, 3822) at 24 hours. Panel D. s63 leukocyte transcriptomics. The expression of 63 genes was reduced to a single metric as described in the Materials and Methods, and previously published.

Figure 2

Areas under the receiver operating curves for individual metrics at 24 hours. The predictive ability of cfDNA copy number, plasma IL-6 concentrations and leukocyte transcriptomics (s63 DFR) to predict CCI and a rapid recovery are presented. Values are obtained at 24 hours and compared to APACHE II scores. The accompanying table provides AUCs and relative risks.