Objective: Neutrophil extracellular traps (NETs) have been shown to play an important role in systemic lupus erythematosus (SLE) pathogenesis by activating plasmacytoid dendritic cells (PDCs) and the type I interferon (IFN) pathway. NETs composed of self-DNA are considered to be of nuclear origin and are a major source of anti-DNA autoantibody generation. This study was undertaken to evaluate whether mitochondrial DNA (mtDNA) resides in NETs, to evaluate whether mtDNA and anti-mtDNA antibodies cause dysregulation of the PDC-IFNα pathway, and to investigate the clinical implication in SLE.
Methods: Patients with SLE (n = 102), patients with rheumatoid arthritis (n = 30), and healthy donors (n = 40) were enrolled in in vitro studies. NETs were generated from phorbol 12-myristate 13-acetate (PMA)-stimulated peripheral neutrophils. Immunofluorescence staining was used to detect NET formation ex vivo and in lupus nephritis renal biopsy samples. The mtDNA levels and type I IFN-inducible gene scores were measured by quantitative polymerase chain reaction. Anti-mtDNA antibodies, anti-double-stranded DNA (anti-dsDNA) antibodies, and IFNα were detected by enzyme-linked immunosorbent assay. Purified PDCs were stimulated by isolated NETs, mtDNA, or dsDNA, combined with anti-mtDNA or dsDNA IgG, or other culture conditions. Additional patients with SLE (n = 113) were enrolled in a proof-of-concept trial. We evaluated the efficacy and safety of metformin on a background of corticosteroids and conventional immunosuppressive agents in patients with mild or moderate lupus. The primary end point was the efficacy of metformin for reducing disease flare.
Results: We detected mtDNA in NETs, and anti-mtDNA antibody levels were elevated in SLE patients compared with controls and significantly correlated with IFN scores and the disease activity index. The presence of anti-mtDNA antibodies was disproportionately associated with lupus nephritis, and correlated better than anti-dsDNA antibody levels with the lupus nephritis activity index. Mitochondrial DNA was deposited in NETs in lupus nephritis renal biopsy specimens. In addition, mtDNA/anti-mtDNA were greater inducers of PDC IFNα production via Toll-like receptor engagement than dsDNA/anti-dsDNA. We assessed the effect of metformin on down-regulating the NET mtDNA-PDC-IFNα pathway. Metformin decreased PMA-induced NET formation and CpG-stimulated PDC IFNα generation. A proof-of-concept trial of metformin add-on treatment of mild or moderate SLE resulted in decreases in clinical flares, prednisone exposure, and body weight.
Conclusion: Our findings establish a link between mtDNA in NETs, anti-mtDNA antibodies, and PDC IFNα pathogenesis in SLE, and highlight that specific strategies to down-regulate this pathway, such as treatment with metformin, may be new approaches to treat SLE.
© 2015, American College of Rheumatology.