EXTRACELLULAR MITOCHONDRIA RELEASED FROM LIVER ISCHEMIA REPERFUSION INJURY ACT AS ALARMINS TO EVOKE INNATE IMMUNE SYSTEM AND TRIGGER INFLAMMATION
Ischemia reperfusion injury (IRI) during liver transplantation enhances the immunogenicity of allografts and thus impacts overall graft outcome. This sterile inflammatory insult is known to activate innate immunity and propagate organ damage through the recognition of endogenous alarmins. The purpose of this study was to investigate the role of mitochondrial alarmins in the pathogenesis of hepatic IRI and the mechanism of immune activation by mitochondria. Mitochondria are vital cellular organelles for energy generation. However, owing to their similarity to bacteria, extracellular mitochondria function as alarmins. Recent studies revealed that mitochondria are critically involved in the initiation and progression of multiple diseases and associated with immune dysregulation. Increased levels of mtDNA and free mitochondria (FM) release were observed in both transplantation-associated and warm IR. Co-culture of mitochondrial components with hepatocytes significantly increased cell death. Mast cells (MC) and Kupffer cells (KC) are versatile immune sentinel cells known to respond to both external and internal stimuli as a first line of defense in liver. However, the response of MC and KC to mitochondria was unknown. Injection of FM into mice led to neutrophil recruitment and macrophage activation within the peritoneal cavity at 16h. MC-deficient mice had significantly reduced neutrophil responses to FM although macrophage activation was still observed. Culture of MC or macrophages with mitochondria resulted in increased production of multiple mediators. FM induced histamine release from human CBMC and mouse BMMC. This degranulation process was rapid and largely independent of extracellular calcium. Pharmacological approaches revealed that FPR1 was important for mitochondria-mediated in vitro degranulation and mediator production, in addition to in vivo sterile inflammation. FM also enhanced macrophage activation, although this was in an FPR1-independent manner. FM were potent activators of both human and mouse innate immune cells, such as MC and macrophages, in vitro and induced substantial MC-dependent, neutrophilic inflammation in vivo. Regulation of mitochondria-mediated immune responses may be important during the initiation or progression of human disease such as acute graft failure or chronic graft versus host disease in liver transplantation, rheumatoid arthritis, or in patients with blood transfusion, which have all been reported to demonstrate enhanced FM release.