# Extracellular mitochondrial DNA activates complement and is associated with complement activation in patients with out-of-hospital cardiac arrest

## TL;DR

Extracellular mitochondrial DNA activates the complement system and is linked to complement activation in patients who experience cardiac arrest outside of a hospital.

## Contribution

This study demonstrates that extracellular mitochondrial DNA, but not nuclear DNA, activates the complement system in vitro and is clinically relevant in out-of-hospital cardiac arrest patients.

## Key findings

- Mitochondrial DNA, but not nuclear DNA, activates complement components C3bc, C3bBbP, and sC5b-9 in a dose- and time-dependent manner.
- Complement activation markers C3bc and sC5b-9 correlate with mitochondrial and nuclear DNA levels in out-of-hospital cardiac arrest patients.
- Extracellular mitochondrial DNA may drive sterile inflammation and could inform new therapeutic strategies.

## Abstract

Synthetic analogues of mitochondrial DNA (mtDNA) have been reported as potent complement system activators. This study investigated the impact of endogenous mtDNA on complement activation. mtDNA and nuclear DNA (nDNA) were extracted from human placental tissue and complement activation was determined using ELISA. When incubated in lepirudin-anticoagulated human blood and plasma, increasing concentrations of mtDNA, but not nDNA, resulted in dose- and time-dependent increases in C3bc, C3bBbP and soluble C5b-9 (sC5b-9). In a clinical context, mtDNA, nDNA, and complement levels of 55 resuscitated out-of-hospital cardiac arrest (OHCA) patients were determined using qPCR and ELISA. C3bc and sC5b-9 correlated significantly with mtDNA and nDNA in OHCA patients. We conclude that mtDNA, but not nDNA, triggers in vitro complement activation, which holds significance in a clinical context in OHCA patients. This novel mode of human complement activation might explain pathophysiological mechanisms of sterile inflammation and could be relevant for innovative therapeutic strategies.

The online version contains supplementary material available at 10.1038/s41598-025-24705-1.