Reply
Authors: Jerzy W. Kupiec-Weglinski, Naoko Kamo
Journal: Hepatology (2013)

We thank Inoue et al. for their insightful comments on our Hepatology article,1 in which we used a well-established murine model of liver ischemia-reperfusion injury (IRI) to show that ASC-mediated caspase-1/interleukin (IL)−1β signaling promoted HMGB1 induction and facilitated Toll-like receptor (TLR)4-dependent inflammatory phenotype, leading to hepatocellular damage. We agree that an apparent discrepancy between our published and Inoue et al.'s ongoing studies on the role of the ASC pathway may relate to putative differences between the models. The 90-minute warm ischemia in our hands triggered an approximate 6-fold higher level of hepatocellular damage (assessed by serum alkaline aminotransferase levels), as compared with 60-minute warm ischemia in their studies. The idea that the putative pathogenic function of inflammasome signaling may depend on the extent or severity of IR insult is interesting and intriguing. However, such a conclusion requires rigorous testing of wild-type and ASC-deficient animals under “one roof” by the same microsurgeon and under identical experimental conditions. We find that even minor differences in room temperature may significantly affect hepatocellular damage in this seemingly straightforward liver IRI model. Another explanation for disparate findings may relate to phase- and cell-type–specific immune activation during the reperfusion phase. We favor the idea that different TLRs and inflammasome components may operate at distinct stages and in different cell types during hepatic IRI.2 In support of our in vivo findings, we documented that ASC signaling was required for TLR4-mediated inflammatory response in lipopolysaccharide-stimulated bone marrow cell cultures. Although this well-controlled in vitro system partially mimics the real-life IRI scenario, we find it useful in addressing mechanistic questions raised by in vivo observations. We are aware of somewhat conflicting data on inflammasome signaling in other IR-stressed organ systems. Although ASC absence failed to protect mice from renal IRI in one study,3 others reported that ASC-deficient kidneys were largely resistant against IRI.4 A more severe epithelial injury after 30-minute renal ischemia in the latter study may have progressed through an ASC-dependent mechanism (versus an ASC-independent pathway after 25-minute ischemia in the former study). There may also be organ-specific differences in inflammasome sensitivity in IR-triggered immune cascades. The fact that the severity of liver IRI peaks at 6 hours of reperfusion in our model, whereas that of kidney IRI lasts for days, may explain these discrepancies as well. Moreover, the liver might be unique in TLR/inflammasome activation in such a way that endogenous ligands may readily tolerize the hepatic, but not the renal, innate immune environment. As to the role of IL-1β, consistent with our findings,1 others have provided evidence that IL-1 receptor blockade ameliorated hepatocellular damage in rat liver IRI models.5, 6 Jerzy W. Kupiec-Weglinski, M.D., Ph.D.1 Naoko Kamo, M.D.1,2 1Dumont-UCLA Transplant Center Department of Surgery Division of Liver and Pancreas Transplantation David Geffen School of Medicine at UCLA Los Angeles, CA 2Department of Surgery Graduate School of Medicine Kyoto University Kyoto, Japan