Hepatic ischemia and reperfusion causes neutrophildependent liver injury. Although the mechanisms of ischemia/reperfusion-induced liver neutrophil recruitment are somewhat understood, less is known regarding the early events that initiate the inflammatory injury. Using a murine model of partial hepatic ischemia and reperfusion, we evaluated the role of endogenous interleukin (IL)-12 in this inflammatory response. Hepatic ischemia for 90 minutes and reperfusion for up to 4 hours resulted in hepatocyte expression of IL-12. By 8 hours of reperfusion there were large increases in serum levels of interferon-␥ (IFN␥) and tumor necrosis factor-␣ (TNF␣). In addition, hepatic ischemia/reperfusion caused significant increases in liver neutrophil recruitment, hepatocellular injury, and liver edema, as defined by liver myeloperoxidase content, serum alanine aminotransferase, and liver wet to dry weight ratios, respectively. In mice treated with neutralizing antibody to IL-12 and in mice deficient in the IL-12 p40 gene, ischemia/ reperfusion-induced increases in IFN␥ and TNF␣ were greatly diminished. These conditions also caused significant reductions in liver myeloperoxidase content and attenuated the parameters of liver injury. The data suggest that IL-12 is required for the full induction of injury after hepatic ischemia and reperfusion. (HEPATOLOGY 1999;30: 1448-1453.)
Hepatic ischemia occurring during trauma or hemorrhage, or under controlled situations such as liver resectional surgery or transplantation, may lead to inflammatory liver injury during hepatic reperfusion. [1][2][3] Local organ injury and dysfunction caused by ischemia/reperfusion is likely a direct result of the released products (oxidants, proteases, lipids, etc.) of activated neutrophils that have accumulated in the liver parenchyma. 4,5 In the initial stages of this injury the proinflammatory cytokines, tumor necrosis factor-␣ (TNF␣) and interleukin (IL)-1, are induced in liver. 6,7 TNF␣ is known to upregulate liver expression of vascular endothelial cell adhesion molecules (i.e., intracellular adhesion molecule-1), and CXC chemokines, such as epithelial neutrophil-activating protein and macrophage inflammatory protein-2. [8][9][10] The combined effects of adhesion molecules and CXC chemokines results in neutrophil adhesion, transmigration and accumulation in liver. Although the downstream mechanisms responsible for neutrophil recruitment and liver injury are somewhat understood, less is known regarding the early events that propagate this inflammatory response.
The cornerstone of most inflammatory responses, including hepatic ischemia/reperfusion injury appears to be the production of the proinflammatory cytokines TNF␣ and IL-1. Blockade of either TNF␣ or IL-1 ameliorates inflammatory injury in a number of animal models. However, clinical trials involving humans with sepsis and involving blocking interventions for TNF␣ or IL-1 have been largely unsuccessful, 11,12 suggesting the participation of other mediators in the development of inflammatory tissue injury. One potential candidate is IL-12. Expression of IL-12 is increased in liver during endotoxemia. IL-12 is required for optimal clearance of bacterial endotoxin and Escherichia coli. 13 In addition, exogenous administration of IL-12 in normal mice induces Kupffer cell activation, upregulation of hepatic vascular cell adhesion molecules, and leukocyte accumulation in liver over the course of several days. 14 Although much is known regarding the function of IL-12 in the host defense against gram-positive and gram-negative bacteria, [15][16][17] relatively little is known regarding the participation of IL-12 in the onset of acute hepatic inflammatory events.
In the present studies, we have investigated the role of endogenous IL-12 in the development and regulation of inflammatory injury of liver induced by ischemia and reperfusion, using antibody blockade of IL-12 and mice lacking the IL-12 p40 gene. The data suggest that IL-12 plays a central role in the induction of hepatic ischemia/reperfusion injury by regulating the production of TNF␣ and IFN␥. We show that, in wild-type mice, hepatic expression of IL-12 is upregulated during the period of ischemia. Mice treated with neutralizing antibody to IL-12 as well as IL-12 p40 Ϫ/Ϫ mice displayed greatly reduced production of TNF␣ and IFN␥. Furthermore, hepatic neutrophil accumulation and hepatocellular injury were greatly reduced in mice treated with anti-IL-12 or in p40 Ϫ/Ϫ mice. Thus, the data suggest that IL-12 plays an important role in the liver inflammatory response to ischemia and reperfusion.