Extracellular MicroRNAs Induce Potent Innate Immune Responses via TLR7/MyD88-Dependent Mechanisms

Tissue ischemia, such as transient myocardial ischemia, leads to release of cellular RNA including microRNA(miRNA) into the circulation and extracellular (ex-) space, but the biological function of the ex-RNA is poorly understood. We recently reported that cardiac RNA of both human and rodent origins induced cytokine production and immune cell activation. However, the identity of the ex-RNA responsible for the proinflammatory effect remains unclear. In the current study, using an miRNA array, we profiled the plasma miRNAs 4 h after transient myocardial ischemia (45 min) or sham procedure. Among 38 plasma miRNAs that were elevated following ischemia, eight were tested for their ability to induce cytokine response in macrophages and cardiomyocytes. We found that six miRNA mimics (miR-34a, -122, -133a, -142, -146a, and -208a) induced cytokine production in a dose-dependent manner. The effects of miRNAs (miR-133a, -146a, and -208a) were diminished by uridine→adenosine mutation and by RNase pretreatment. The miRNA-induced cytokine (MIP-2, TNF-α, and IL-6) production was abolished in cells deficient of TLR7 or MyD88, or by a TLR7 antagonist, but remained the same in TLR3- or Trif-deficient cells. In vivo, mice i.p. injected with miR-133a or miR-146a had marked peritoneal neutrophil and monocyte migration, which was significantly attenuated in TLR7^-/- mice. Moreover, locked nucleic acid anti-miRNA inhibitors of these six miRNAs markedly reduced cardiac RNA-induced cytokine production. Taken together, these data demonstrate that ex-miRNA mimics (miR-34a, -122, -133a, -142, -146a, and -208a) are potent innate immune activators and that the miRNAs most likely induce cytokine production and leukocyte migration through TLR7 signaling.