A signature of SLE patients is activation of the type I Interferon (IFN} pathway in peripheral blood lymphocytes (PBL). A major pathway to type I IFN activation is the RNA-sensing family of RIG-I-like Receptor (RLR} helicases, which includes RIG-I and MDA5. These sensors are triggered by uncapped viral RNAs, but not by capped mammalian RNA. We have determined that the RLR pathway is also regulated by caspase-8 and its paralogue, c-FLIP. c-FLIP levels are elevated in PBL of SLE patients. Of particular interest is that mitochondria contain uncapped RNA, which we have determined triggers the RLR pathway. Co-investigator Dr. Andras Perl has demonstrated that PBL of SLE patients manifest mitochondrial hyperpolarization, leading to cell death. We hypothesize that this mitochondrial stress will lead to release of mitochondrial RNA and activation of the RLR pathway. This could explain the striking type I IFN signature found in PBL of SLE patients. The Specific Aims would be to: 1. Determine which mitochondrial RNA sequences from stressed hyperpolarized mitochondria activate the RLR pathway for type I IFN production; and 2. Determine whether mitochondrial hyperpolarization seen in SLE lymphocytes results in release of mitochondrial RNA and reflects their metabolic state.