Background In systemic lupus erythematosus (SLE), lupus nephritis (LN) is associated with chronic inflammation and perpetuated fibroblast activation, both determined by epigenetic mechanisms involving aberrant CpG DNA promoter methylation. During SLE progression, global methylation patterns are commonly lost. These CpG DNA promoter methylation patterns are not limited to the kidney, circulating CpG-rich DNA is also detectable in the blood allowing for biomonitoring (“liquid biopsy”). However, little is known about its specific contribution to determining disease progression. In the kidney, CpG-rich DNA activates TLR9 signaling mechanisms involved in inflammation and fibrogenesis. Based on these observations, we hypothesized that CpG-rich DNA promoter fragments potentially accelerate renal inflammation and fibrogenesis in SLE-associated LN.
Objectives To analyze the role of circulating CpG-rich DNA on endothelial TLR9 signalling and the effect of experimental modification of oligodinucleotides on kidney inflammation in the Pristane-induced murine model of SLE.
Methods We isolated circulating CpG-rich DNA from blood samples in a cohort of SLE patients. Then, we tested how these DNA promoter fragments influenced the LN phenotype in a TMPD (“pristane”)-induced mouse model. Further, we investigated how this renal response could be influenced by the administration of either human or synthetic methylated/unmethylated CpG-rich DNA oligodinucleotides. Additionally, the effects of the administration of cicruclating CpG-rich DNA fragments on TLR9-signalling was analyzed in endothelial cell cultures.
Results We show that circulating CpG-rich DNA promoter fragments are detectable in SLE patients' blood. Furthermore, SLE-associated LN is associated with accumulation of unmethylated CpG-rich DNA promoter fragments, implicating a mechanistic connection. These observations were further corroborated in a rodent model of TMPD-induced SLE where administration of CpG-rich DNA (isolated from LN patients or synthetic unmethylated CpG-rich DNA oligonucleotides) worsened the renal phenotype in terms of inflammation and fibrogenesis.
Causal contribution of TLR9 was further confirmed in Tlr9+/--knockout mice with protection from renal inflammation and kidney fibrosis after administration of unmethylated CpG-rich DNA promoter oligonucleotides. TLR9-mediated intrarenal inflammation can be therapeutically targeted by administration of synthetic methylated CpG-rich DNA oligonucleotides, ultimately associated with suppression of TLR9-mediated signaling responses and renal injury in experimental SLE/LN.
Conclusions Collectively, our results implicate accumulation of unmethylated CpG-rich promoter DNA fragments in SLE-associated LN. Furthermore. these unmethylated CpG-rich promoter DNA fragments causally contribute to TLR9-mediated inflammation and renal fibrogenesis. Administration of methylated CpG-rich oligonucleotides antagonized intrarenal TLR9-mediated inflammatory signaling responses and fibrogenesis. Therefore, biomonitoring of CpG-rich promoter DNA fragments and modulation of intrarenal TLR9 signaling is a promising therapeutical target in SLE-associated LN.
Disclosure of Interest None declared