SLE is a prototypical nucleic acid-triggered autoimmune disease during which pathogenic, IgG anti-dsDNA autoantibodies form glomerular and vascular deposits and cause tissue damage. Recent studies suggest that deregulated lysosomal degradation of cytoplasmic DNA may result in its accumulation and sensing by intracellular receptors culminating in antiDNA autoimmunity. In SLE, circulating IFNαinduces peripheral blood monocytes to differentiate into inflammatory DCs, a process implemented through autophagy and mitophagy. Mitochondrial DNA – a DNA of bacterial origin – is a known cytoplasmic danger signal but it has never been explored as a potential source for the triggering of DNA autoimmunity. We sought to explore this possibility for SLE autoimmunity in humans.
Serum and peripheral blood monocytes were isolated from active, newly diagnosed or off-treatment SLE patients and matched healthy donors. IFNα signalling was induced by 10% SLE serum or rIFNα. Autophagy was assessed by QPCR, immunoblotting and confocal microscopy for Atg5, LC3 and P62. Autoreactivity was assayed in MLRs and assessed by ELISA for IL6 and TNFα and flow cytometry for HLADR, CD86 and Tcell CFSE dillution. Autophagolysosomal pH, mtROS production, mitochondrial polarisation and mtDNA accumulation were assessed by confocal microscopy and flow cytometry using specific fluorescent dyes (Lysotracker RedDND99, mitoSOX, JC1, MitotrackerRed CMXRos and picogreen respectively).
Freshly isolated SLE monocytes and healthy monocytes exposed to SLE serum or rIFNαdisplay autophagy induction (Atg5 and LC3II increase) but defective autophagolysosomal degradation (increase in autophagosomal P62). Lysosomalalkalinization (decreased Lysotracker intensity P < 0.005) coincided with increased mtROS production (3fold higher mitoSOX intensity P < 0.05) leading to accumulation of damaged, oxidising mitochondria (increase in JC1 aggregates, 4fold higher Mitotracker intensity P < 0.005) and increased survival of mtDNA inside cytoplasmic compartments fused with lysosomes (picogreen-lysotrackercolocalisation). mtROS scavenging (mitoTEMPO) and inversion of autophagy completion (rapamycin) rescued the autoreactive phenotype of IFNαeducated monocytes in all cases.
This study unveils a molecular mechanism that leads to the generation of a pool of immunogenic DNA autoantigens in SLE. IFNα mediated mitophagy deregulation leads to accumulation of oxmtDNA, a defect that can be targeted in order to avoid the development of the anti-DNA inflammatory response that drives SLE.