Inflammatogenic properties of endogenous DNA

Abstract

Bacterial DNA and CpG-containing oligodeoxynucleotides (ODNs) induce host inflammatory responses as a result of their content of unmethylated ¡§CpG motifs¡¨. Since the CpG motifs in the mitochondrial (mt) genome are in this respect similar to bacterial DNA, in that they are non-methylated, we investigated whether mtDNA also displays inflammatogenic potential. In addition, we addressed the question as to whether oxidatively damaged DNA displays pro-inflammatory properties in vivo.We report that intra-articular injections of mtDNA as well as oxidized DNA, but not nuclear DNA (nDNA) induce arthritis in mice. Mitochondrial DNA-induced arthritis (MDIA) occurred within 3 days of instillation of mtDNA in mouse joints and lasted for at least 14 days. MDIA was characterized by a thickening of the synovial membrane and the presence of large numbers of synovial Mac1+ and Mac3+ mononuclear phagocytic cells. The presence of macrophages was required for the induction of MDIA, since MDIA was significantly suppressed in macrophage-depleted mice. In contrast, neither B nor T cells participate in the induction of MDIA, since arthritis was apparent in SCID mice. Importantly, mtDNA, but not nDNA, stimulated the proliferation of mouse splenocytes and the production of the pro-inflammatory cytokine TNF- Ñ. NF- ÛB signalling seems to play a regulatory role in cytokine induction by mtDNA, since the administration in vivo of an antisense ODN to the p65 subunit of NF- ÛB blocked MDIA. Two regions of the human mitochondrial genome with different CpG frequencies were amplified by PCR, and shown to also induce arthritis following intra-articular injection.In terms of human arthritis, extracellular mtDNA and oxidized DNA were observed in significantly high levels in the synovial fluid (SF) samples of rheumatoid arthritis (RA) patients compared to SF control samples. In addition to RA, we found that Alzheimer¡¦s disease (AD) and vascular dementia (VaD) patients displayed significantly increased intrathecal levels of oxidized DNA than healthy controls.Our results suggest that endogenous mtDNA and oxidized DNA have potent inflammatogenic properties, since these molecules are capable of inducing arthritis in a healthy host, and that these reactive species are present in the joints of RA patients and in brains of demented patients.