NOT ALL CELL-FREE MITOCHONDRIAL DNA IS EQUAL IN TRAUMA PATIENTS

Mitochondrial DNA (mtDNA) acts as a proinflammatory damage-associated molecular pattern that stimulates innate immune activation via Toll-like receptor 9, similarly to bacterial DNA. A number of clinical studies have measured elevated cell-free mtDNA in the plasma of trauma patients, thought to originate from tissue injury and inflammatory processes; however, the magnitude of this increase, the absolute concentration, and the association with poor outcomes varies considerably across studies. Measurements of cell-free mtDNA in healthy individuals have shown that the majority of "cell-free" mtDNA (>95%) can be centrifuged/filtered from plasma in the size range of 0.45 to 5 μm, suggesting that there are larger forms of mtDNA-containing complexes in the plasma that could be considered cell-free. Whether this is true for trauma patients (and other relevant disease states) and the clinical relevance of the larger forms of mtDNA is unknown. These findings from healthy individuals also suggest that the centrifugation speeds used to generate cell-free plasma (which are rarely consistent among studies) could result in mixed populations of cell-free mtDNA that could confound associations with outcomes. We demonstrate in this study of 25 major trauma patients that the majority of the cell-free mtDNA in trauma patient plasma (>95%) is removed after centrifugation at 16,000g. Despite the larger forms of mtDNA being predominant, they do not correlate with outcomes or expected parameters such as injury/shock severity, multiple organ failure, and markers of inflammation, whereas low-molecular-weight cell-free mtDNA correlates strongly with these variables.