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.
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.
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.
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.