Role of Mitochondrial DNA in Septic AKI via Toll-Like Receptor 9

Toll-like receptor 9 (TLR9) contributes to the development of polymicrobial septic AKI. However, the mechanisms that activate the TLR9 pathway and cause kidney injury during sepsis remain unknown. To determine the role of mitochondrial DNA (mtDNA) in TLR9-associated septic AKI, we established a cecal ligation and puncture (CLP) model of sepsis in wild-type (WT) and Tlr9 -knockout ( Tlr9 KO) mice. We evaluated systemic circulation and peritoneal cavity dynamics and immune response and tubular mitochondrial dysfunction to determine upstream and downstream effects on the TLR9 pathway, respectively. CLP increased mtDNA levels in the plasma and peritoneal cavity of WT and Tlr9 KO mice in the early phase, but the increase in the peritoneal cavity was significantly higher in Tlr9 KO mice than in WT mice. Concomitantly, leukocyte migration to the peritoneal cavity increased, and plasma cytokine production and splenic apoptosis decreased in Tlr9 KO mice compared with WT mice. Furthermore, CLP-generated renal mitochondrial oxidative stress and mitochondrial vacuolization in the proximal tubules in the early phase were reversed in Tlr9 KO mice. To elucidate the effects of mtDNA on immune response and kidney injury, we intravenously injected mice with mitochondrial debris (MTD), including substantial amounts of mtDNA. MTD caused an immune response similar to that induced by CLP, including upregulated levels of plasma IL-12, splenic apoptosis, and mitochondrial injury, but this effect was attenuated by Tlr9 KO. Moreover, MTD-induced renal mitochondrial injury was abolished by DNase pretreatment. These findings suggest that mtDNA activates TLR9 and contributes to cytokine production, splenic apoptosis, and kidney injury during polymicrobial sepsis.