–          Background/Purpose: Eukaryotic cells contain organelles called mitochondria that govern energy supply and control of cell death. Whereas damaged organs or activated cells can extrude their mitochondria, which is suggested to trigger innate immunity, it is less well known whether extracellular mitochondria are also recognized by the adaptive immune system.

–          Methods: We improved methodologies for isolation of pure mitochondria from human and mouse cells and tissues, and designed ELISA to measure the presence of antibodies directed against mitochondrial outer membrane and mtDNA in mouse models, and patients with systemic lupus erythematosus (SLE)  (n=170, Table 1), compared to controls (n=44) and patients with antiphospholipid syndrome (APS) (n=12) or primary biliary cirrhosis (PBC) (n=12). Carotid ultrasounds were available in 113 patients. SpearmanÕs correlation analysis was used to explore whether antibody levels were associated with the following outcomes: SLEDAI-2K, SDI and carotid ultrasound plaques and intima-media thickness (CIMT) as surrogates for vascular damage.

–          Results: Antibodies against mitochondrial outer membrane and mtDNA were increased in a mouse model of SLE. In humans, the levels of antibodies recognizing the outer mitochondrial membrane and mtDNA were significantly higher in SLE patients than in controls, APS patients and PBC patients (Table 2). Antibodies against mtDNA were associated with a lower likelihood of previous thrombotic events and higher disease activity measured by SLEDAI=2K, the latter being entirely explained by the high correlation between commercial assays for antibodies to double stranded DNA and our assay for mtDNA antibodies. While antibodies against the mitochondrial outer membrane were not associated with the measured disease characteristics, their presence was not completely explained by the presence of anti-cardiolipin, suggesting that other mitochondrial targets exist. Oxidation of the mitochondrial surface had no impact on autoantibody recognition of mitochondrial antigens.

–          Conclusion: This study provides useful insights into optimized methodologies for assessing mitochondrial antibodies in mice and humans, providing an appreciation of the anti-mitochondrial antibody repertoire and its role in autoimmunity. These findings further support the concept that extracellular mitochondria provide an important source of circulating autoantigens that may be clinically relevant in SLE.

Table 1 : Demographic and clinical variables for SLE patients