Mitochondrial DNA Released by Trauma Induces Neutrophil Extracellular Traps

Abstract

Neutrophil extracellular traps (NETs) are critical for anti-bacterial activity of the innate immune system. We have previously shown that mitochondrial damage-associated molecular patterns (mtDAMPs), including mitochondrial DNA (mtDNA), are released into the circulation after injury. We therefore questioned whether mtDNA is involved in trauma-induced NET formation. Treatment of human polymorphoneutrophils (PMN) with mtDNA induced robust NET formation, though in contrast to phorbol myristate acetate (PMA) stimulation, no NADPH-oxidase involvement was required. Moreover, formation of mtDNA-induced NETs was completely blocked by TLR9 antagonist, ODN-TTAGGG. Knowing that infective outcomes of trauma in elderly people are more severe than in young people, we measured plasma mtDNA and NET formation in elderly and young trauma patients and control subjects. MtDNA levels were significantly higher in the plasma of elderly trauma patients than young patients, despite lower injury severity scores in the elderly group. NETs were not visible in circulating PMN isolated from either young or old control subjects. NETs were however, detected in PMN isolated from young trauma patients and to a lesser extent from elderly patients. Stimulation by PMA induced widespread NET formation in PMN from both young volunteers and young trauma patients. NET response to PMA was much less pronounced in both elderly volunteers’ PMN and in trauma patients’ PMN. We conclude that mtDNA is a potent inducer of NETs that activates PMN via TLR9 without NADPH-oxidase involvement. We suggest that decreased NET formation in the elderly regardless of higher mtDNA levels in their plasma may result from decreased levels of TLR9 and/or other molecules, such as neutrophil elastase and myeloperoxidase that are involved in NET generation. Further study of the links between circulating mtDNA and NET formation may elucidate the mechanisms of trauma-related organ failure as well as the greater susceptibility to secondary infection in elderly trauma patients.

Lymphocytes eject interferogenic mitochondrial DNA webs in response to CpG and non-CpG oligodeoxynucleotides of class C

“…Moreover, oxidized mtDNA binds to, and activates, the NLRP3 inflammasome leading to production of IL-1β (13). mtDNA also promotes neutrophil adhesion and, noteworthy, acts as a potent inducer of NETs that further raise the inflammatory pressure (3,20,43). West and co-workers highlight the role of mtDNA as a participant in cell-intrinsic triggering of innate immune responses and antiviral signaling (4).…”

“…Later, it was observed that extracellular traps (ETs) could be generated by live cells that released mtDNA instead of nuclear DNA (16)(17)(18)(19). Notably, Itagaki et al found that cell-free mtDNA, released in trauma patients, itself induced classical, suicidal, NETosis via a ROS-dependent TLR9-pathway (20). Although beneficial for pathogen clearance, circulating DNA constitute a potential risk for autoantibody induction if not rapidly cleared.…”

The Contribution of Innate Immunity to the Pathogenesis of ANCA-associated Vasculitis

“…The conclusion was drawn from histological observations of co-localizations of DNA, histones and the granule proteins PR3, LL37, NE and MPO in various combinations. The presence of NETs in glomeruli in AAV have then been confirmed by others (Yoshida et al 2013, O'Sullivan et al 2015, Tang et al 2015, and NETs have also been shown to be present in skin lesions (Abreu-Velez et al 2009, Sangaletti et al 2012) and thrombi , Imamoto et al 2014) from patients. In addition to the histological findings of NETs in biopsies, elevated levels of NETs in the circulations have also been identified in patients (Table I) (Kessenbrock et al 2009, Surmiak et al 2015, Surmiak et al 2016).…”

“…It has been shown that mtDNA also can induce NETs (Itagaki et al 2015). This induced NET formation was shown to be ROS independent but whether the NETs were of nuclear or mitochondrial origin was not investigated (Itagaki et al 2015).…”

“…Several studies have also shown that neutrophils in patients possess increased spontaneous NETosis compared with HC (Grayson et al 2015, Tang et al 2015, which could contribute to the elevated levels in the circulation. One of these studies showed that both normal density neutrophils and LDGs in patients released more NETs than normaldensity neutrophils in HC, but also showed that LDGs appear to be the main source of NETs in the patients (Grayson et al 2015).…”