Mitochondria are evolutionary endosymbionts derived from bacteria, and they contain DNA similar to bacterial DNA. In the present study, we analyzed the potential role of mitochondrial DAMPs, particularly mitochondrial DNA, as ET inducers in human monocytes using an in vitro model of ET and samples from patients with ACS. Our data indicate that exposure to mitochondrial DAMPs suffices to induce endotoxin tolerance in human monocytes. The production of several pro-inflammatory cytokines was impaired when mtLys-pretreated cultures were challenged with LPS. In contrast, TGFb levels increased as did the cell Sulopenem surface expression of CD163; both are well known markers of the ET or M2 phenotype. We also observed the expression of a very RS 56812 hydrochloride important negative regulator of the TLR-NFkB pathway: IRAKM. This pseudokinase is one of the genes that is consistently induced into ET. Here we verified that IRAK-M is upregulated in LPS-treated monocytes preconditioned with mtLys. In line with these findings, the analysis of p65 translocation into the nucleus revealed an inhibition of this process in mtLyspretreated cultures. Subsequently, the transcription of proinflammatory cytokines was also impaired. Strict controls were established in our assays to ensure the absence of endotoxin traces. All these features prompted us to postulate that mitochondrial DAMPs provoke a refractory state in human monocytes that match the M2 phenotype. Similar results were obtained when mtLys mtDNA, not nuDNA, were used in our in vitro model. In most cases, ET has been associated with previous endotoxin contact, which induces a refractory state in a second endotoxin challenge. These data now indicate that endogenous ligands activate TLR signaling and induce endotoxin tolerancelike features or an M2 phenotype. Therefore, DAMPs play a crucial role in the development of inflammation and the refractory state observed in ����sterile���� pathologies. Published data suggest that mitochondria can also be a source of DAMPs, as they carry bacterial motifs due to their endosymbiotic origin. To explore the pathophysiological implications of our data, we studied monocytes from ACS patients. We previously reported an early increase in IRAK-M expression in circulating cells from ACS patients without any infection history. Therefore, we hypothesized that IRAK-M induction could be due to the presence of mitochondrial DAMPs and, subsequently, explain a manifest ET status of these patients hours after they were diagnosed.