DNA is a potent damage-associated molecular pattern (DAMP) signaling that, once in the extracellular space, triggers the activation of the innate immune system. Indeed, several studies demonstrated that there is cytosolic accumulation of nuclear genome-derived DNA in senescent cells. Mitochondrial dysfunction is a crucial feature of senescence, a critical biological determinant of aging and tumorigenesis. Mitochondrial DNA (mtDNA) can be released from mitochondria and promote inflammation as a cell stress response. A recent report proposes that senescent cells induced by sub-lethal apoptotic stress release mtDNA through BAX/BAK macropores that promote SASP. However, all these studies focused on the effect of cytosolic DNA when the senescence has been established. It remains elusive whether mtDNA is released during the initial stage of senescence induction and how it contributes to senescence. Moreover, mtDNA has been shown to be biologically active as a DAMP, and its extracellular release can directly trigger innate immune response.
Here, by using three different senescent models, we find that senescent cells release mtDNA to both the cytosol and the extracellular space. This process occurs even in the initial stage of senescence induction and precedes that of nuclear DNA, resulting in the activation of the cGAS/STING pathway and the establishment of cellular senescence. Intriguingly, by exploiting co-culture and in vivo cross-species experiments, we show that extracellular mtDNA released by senescent tumor cells is packaged in extracellular vesicles (EVs) and is specifically captured by polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) in the tumor microenvironment. Mechanistically, PMN-MDSCs uptake mtDNA leads to enhanced immunosuppressive ability through cGAS-STING-NF-κB activation. Pharmacological inhibition of mtDNA released from senescent tumor cells blocks the PMN-MDSCs immunosuppressive activity, improving the efficacy of therapy-induced senescence (TIS) in cancer. These results reveal the crucial role of mtDNA in initiating cellular senescence and immunosuppression independently of the SASP. Thus, targeting the mtDNA release-mediated pathway may hold promise to reprogram the immune suppressive microenvironment in patients treated with chemotherapy.