The study shows that both normal and slightly damaged mitochondrial RNA can spark inflammation in human immune cells, and the natural peptide LL‑37 makes this effect stronger. Even RNA floating in our blood can act as an inflammation trigger. This helps explain how cellular damage might fuel chronic inflammation.

Inflammation is implicated in a wide range of disorders, and thought to be involved in most leading causes of death today in the United States with high associated costs. New insights into better understanding its etiology, detection and prevention are thus of major importance in health care. One emerging field providing such insights has been the identification of DAMPs, or damage-associated molecular patterns. We have studied DAMPs within the context of degraded and oxidized mitochondrial DNA and RNA ("DeMP"), most recently demonstrating potent mitochondrial RNA (mtRNA) immunogenic response in mouse macrophages. Here, we extend these studies to assess the proinflammatory role of mitochondrial control (native) and oxidized RNA using human RNA and cells. THP-1 macrophage mtRNA triggered a proinflammatory response (induction of IL-6 and TNFα) when transfected into the same cells. Modestly oxidized mtRNA (DeMP RNA) but not cytoplasmic RNA induced a similar response, in contrast to attenuated immunogenicity previously observed with more oxidized DeMP RNA. This DeMP RNA may also cause a mild prooxidant stress. The proinflammatory effects of mtRNA was significantly reduced following pretreatment with RNases specific for single and double stranded RNA, implicating these forms of mtRNA in proinflammatory response. The natural nucleic acid-encapsulating peptide LL-37 also triggered a proinflammatory effect in the presence of control mtRNA and DeMP RNA. Finally, human blood plasma RNA exhibits proinflammatory activity. These results provide new insights into the immunostimulation of mitochondrial RNA including its activity in human cells; identify human plasma RNA as proinflammatory; and provide further evidence that oxidized DeMP mtRNA acts as a sensitive and broad-spectrum sensor and regulator of mitochondrial oxidative stress.