Cell-free mitochondrial DNA (cf-mtDNA) is a potent driver of postinjury systemic inflammatory response syndrome (SIRS) and correlates with major trauma patients’ outcomes. Plasma cf-mtDNA concentration increases days after injury, unrelated to cell death and severity of the original tissue injury. The source and the release mechanism of cf-mtDNA remains unknown, which impedes potential therapeutic strategies to harness SIRS. Mitochondria as intact organelles have recently been detected in the circulation of healthy humans, however, cf-mitochondria have not yet been studied in the context of post-injury inflammation. We hypothesised that cf-mitochondrial count and activity in plasma correlate with markers of inflammation, tissue injury, resuscitation and outcomes.
Plasma samples from 25 trauma patients, collected preoperative to major orthopaedic surgery, postoperatively and 1, 3 and 5 days postoperatively were stained with markers of the mitochondrial outer membrane (TOM70) and mitochondrial activity (MitoTracker Deep Red) and were analysed with flow cytometry. Patient values were compared to healthy controls using a Mann-Whitney test. Correlations with clinical data were performed using a Spearman’s Test.
Cell free mitochondria (TOM70+/CD42-) were detected at 94050 (95%CI 51700-160100)/mL plasma in trauma patients at a 5-fold greater concentration to that of controls. Mitohondrial count did not significantly increase after major orthopaedic surgery. In trauma patients 14% of the cf-mitochondria were active (TOM70+/CD42-/Mitotracker+). Cf-Mitochondrial count was only associated with clotting factor transfusions but not with tissue injury or inflammation. Cf-mitochondrial activity however, was associated with tissue injury markers, injury severity, SIRS and all components of allogenic transfusion.
Major trauma patients have more cell-free mitochondria in their plasma than healthy controls. The larger number of functioning extracellular mitochondria was observed in more severe tissue injury and systemic inflammation. Our findings also prompt further study into the role of blood and blood component transfusions in producing higher numbers of cell free mitochondria.