Mitochondrial DNA that escapes from autophagy causes inflammation and heart failure. Researchers found that when mitochondrial DNA is not properly degraded by autophagy in heart cells, it can trigger inflammatory responses through Toll-like receptor 9 (TLR9), leading to myocarditis and dilated cardiomyopathy. Cardiac-specific deletion of lysosomal deoxyribonuclease (DNase) II, which normally degrades mitochondrial DNA, did not cause heart issues under normal conditions but increased mortality and severe heart failure after pressure overload. DNase II-deficient hearts showed infiltration of inflammatory cells and increased expression of inflammatory cytokines, with accumulation of mitochondrial DNA in autolysosomes. Administration of TLR9 inhibitors or Tlr9 ablation reduced cardiomyopathy. Tlr9 ablation also improved pressure overload-induced heart dysfunction and inflammation. Mitochondrial DNA, similar to bacterial DNA, contains inflammatogenic CpG motifs. Damaged mitochondria are degraded by autophagy, which involves sequestration in autophagosomes and fusion with lysosomes. Pressure overload impairs mitochondrial function. DNase II, found in lysosomes, degrades DNA in macrophages. In this study, DNase II in cardiomyocytes digests mitochondrial DNA in the autophagy system to prevent inflammation. DNase II activity was upregulated in hypertrophied hearts but not in failing hearts. DNase II-deficient mice showed increased mortality and severe heart failure after pressure overload. TAC-operated DNase2a^-/- mice had cell infiltration, fibrosis, and increased cytokine expression. TLR9 signaling was involved in inflammatory responses in failing hearts. Mitochondrial DNA released into the extracellular space can trigger inflammation. The study shows that mitochondrial DNA escaping autophagy leads to TLR9-mediated inflammation and heart failure. The findings suggest that autophagy is crucial for preventing mitochondrial DNA from causing inflammation in the heart. The study also highlights the role of TLR9 in the pathogenesis of heart failure. The results provide new insights into the mechanisms of chronic inflammation in failing hearts.Mitochondrial DNA that escapes from autophagy causes inflammation and heart failure. Researchers found that when mitochondrial DNA is not properly degraded by autophagy in heart cells, it can trigger inflammatory responses through Toll-like receptor 9 (TLR9), leading to myocarditis and dilated cardiomyopathy. Cardiac-specific deletion of lysosomal deoxyribonuclease (DNase) II, which normally degrades mitochondrial DNA, did not cause heart issues under normal conditions but increased mortality and severe heart failure after pressure overload. DNase II-deficient hearts showed infiltration of inflammatory cells and increased expression of inflammatory cytokines, with accumulation of mitochondrial DNA in autolysosomes. Administration of TLR9 inhibitors or Tlr9 ablation reduced cardiomyopathy. Tlr9 ablation also improved pressure overload-induced heart dysfunction and inflammation. Mitochondrial DNA, similar to bacterial DNA, contains inflammatogenic CpG motifs. Damaged mitochondria are degraded by autophagy, which involves sequestration in autophagosomes and fusion with lysosomes. Pressure overload impairs mitochondrial function. DNase II, found in lysosomes, degrades DNA in macrophages. In this study, DNase II in cardiomyocytes digests mitochondrial DNA in the autophagy system to prevent inflammation. DNase II activity was upregulated in hypertrophied hearts but not in failing hearts. DNase II-deficient mice showed increased mortality and severe heart failure after pressure overload. TAC-operated DNase2a^-/- mice had cell infiltration, fibrosis, and increased cytokine expression. TLR9 signaling was involved in inflammatory responses in failing hearts. Mitochondrial DNA released into the extracellular space can trigger inflammation. The study shows that mitochondrial DNA escaping autophagy leads to TLR9-mediated inflammation and heart failure. The findings suggest that autophagy is crucial for preventing mitochondrial DNA from causing inflammation in the heart. The study also highlights the role of TLR9 in the pathogenesis of heart failure. The results provide new insights into the mechanisms of chronic inflammation in failing hearts.