Potential conflict of interest: Nothing to report.
Interferon regulatory factor-1 (IRF-1) is a transcription factor that regulates gene expression during immunity. We hypothesized that IRF-1 plays a pivotal role in liver transplant (LTx) ischemia/reperfusion (I/R) injury. Mouse orthotopic LTx was conducted after 24 hours cold storage in University of Wisconsin (UW) solution in wildtype (WT) C57BL/6 and IRF-1 knockout (KO) mice. IRF-1 deficiency in liver grafts, but not in recipients, resulted in significant reduction of hepatocyte apoptosis and liver injury, as well as improved survival. IRF-1 mRNA up-regulation was typically seen in graft hepatocytes in WT→WT LTx. Deficiency of IRF-1 signaling in graft resulted in significantly reduced messenger RNA (mRNA) levels for death ligands and death receptors in hepatocytes, as well as decreased caspase-8 activities, indicating that IRF-1 mediates death ligand-induced hepatocyte death. Further, a smaller but significant IRF-1 mRNA up-regulation was seen in WT graft nonparenchymal cells (NPC) and associated with interferon gamma (IFN-γ) mRNA up-regulation exclusively in NPC. IFN-γ mRNA was significantly reduced in IRF-1 KO graft. Thus, IRF-1 in graft hepatocytes and NPC has distinct effects in hepatic I/R injury. However, LTx with chimeric liver grafts showed that grafts lacking hepatocellular IRF-1 had better protection compared with those lacking IRF-1 in NPC. The study identifies a critical role for IRF-1 in liver transplant I/R injury. (HEPATOLOGY 2010.)
Graft injury due to hypothermic storage and warm reperfusion is a major problem complicating liver transplantation (LTx). Although all liver grafts exhibit some degree of preservation damage, patients receiving grafts with severe preservation injury suffer poor early liver function and are more susceptible to a variety of complications.1, 2 However, the initiating events that account for early graft damage are only partially understood.3, 4
Interferon regulatory factor (IRF)-1, a transcription factor originally identified as a regulator of interferon alpha/beta (IFN-α/β), is known to be involved in many aspects of innate and adaptive immune responses.5–7 IRF-1 messenger RNA (mRNA) is expressed constitutively in all cell types, and IRF-1 protein levels are regulated at the transcriptional level. IRF-1 is up-regulated in response to various stimuli such as IFNs (type I and II), double-stranded RNA, cytokines, and hormones.8 Nuclear translocation of IRF-1 results in activation of not only IFNs,7 but also various type of immune-active genes such as inducible nitric oxide synthase (iNOS) and interleukin 12 (IL-12), particularly in immune cells.9, 10
IRF-1 is also known as a regulator of apoptosis induced in various cell types with a variety of mechanisms.11, 12 IRF-1 induces a ligand-independent caspase-8-mediated apoptosis in breast cancer cells.12 The importance of IRF-1 in apoptosis is also shown in the liver, and hepatocytes from IRF-1-deficient mice are resistant to apoptosis induction by IFN-γ.11 Further, IRF-1-deficient mice show less liver injury in a concanavalin A-induced hepatitis model and the lipopolysaccharide (LPS)/D-galactosamine hepatitis model.13, 14
In the context of hepatic ischemic injury, we have reported that IRF-1 was up-regulated in a hepatic warm ischemia model and IRF-1-deficient mice have less hepatic damage.15 However, the role of endogenous IRF-1 in liver transplant ischemia reperfusion (I/R) injury has not been examined. Further, the pathophysiology of “cold” I/R injury during liver transplantation is not identical to warm I/R injury, and the signaling mechanisms of IRF-1-mediated liver injury have not been defined. Therefore, we tested the hypothesis that IRF-1 plays a pivotal role in hepatic I/R injury associated with LTx. Using IRF-1-deficient mice as donors and/or recipients, the study demonstrates that IRF-1 expression in graft cells, but not in recipient cells, is responsible for liver transplant I/R injury, in part by promoting hepatocyte apoptosis. We also show that both hepatocytes and nonparenchymal cells (NPCs) express IRF-1 during hepatic I/R injury. However, using bone marrow (BM) chimeras to generate chimeric donor liver grafts, IRF-1 expression in graft hepatocytes is more crucial to liver I/R injury.
ALT, alanine aminotransferase; Bax, Bcl-2-associated X protein; BM, bone marrow; FADD, Fas-associated protein with death domain; DR5, death receptor 5; IFN-γ, interferon gamma; I/R, ischemia/reperfusion; IRF-1, interferon regulatory factor-1; KO, knockout; LTx, liver transplantation; NPC, nonparenchymal cells; RLU, relative luminescence units; TRAIL, TNF-related apoptosis-inducing ligand; UW, University of Wisconsin solution; WT, wildtype.
Materials and Methods
Both C57BL/6 (IRF-1+/+; wildtype [WT]) and C57BL/6 background IRF-1-deficient mice (IRF-1−/−; knockout [KO]) were purchased from the Jackson Laboratory (Bar Harbor, ME). Animals were maintained in a laminar flow, specific pathogen-free atmosphere at the University of Pittsburgh.
Orthotopic Liver Transplantation.
The basic techniques of liver harvesting and syngeneic orthotopic liver transplantation without hepatic artery reconstruction were based on the method described by Qian et al.16 Liver grafts were perfused with 1.0 mL of University of Wisconsin (UW) solution by way of the portal vein and stored in UW solution for 24 hours at 4°C. Anhepatic time averaged 19.8 ± 1.7 minutes.
WT and KO mice were used for both donors and recipients in this study. Recipient animals were sacrificed at 1-24 hours after reperfusion for serum and liver graft samples. Separate groups of animals were followed for 7 days to determine the roles of IRF-1 on graft survival. All procedures in this study were performed according to the guidelines of the National Research Council's Guide for the Care and Use of Laboratory Animals and approved by the Council on Animal Care at the University of Pittsburgh.
BM chimeras were created by BM transplantation between WT and KO mice. BM cells were collected from long bones of the extremities of WT or KO mice, and 2 × 107 cells were intravenously injected into lethally (9.5 Gy) irradiated WT or KO mice by way of the penile vein. Animals were used as liver graft donors >2 months after BM transplantation.
Alanine Aminotransferase (ALT) Levels.
Serum ALT levels were measured using the Opera Clinical Chemistry System (Bayer, Tarrytown, NY).
Routine and Immunohistopathology.
Liver graft tissues were fixed in 10% formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin (H&E). The percentage of necrotic area was estimated by random evaluation of five low-power fields (×40) per each H&E section.
Apoptosis was determined in formalin-fixed paraffin-embedded graft sections using the in situ end-labeling technique (Apop Tag Peroxidase Kit; Intergen, Purchase, NY) as described.17 For cleaved caspase-3, formalin-fixed paraffin-embedded sections of liver grafts were stained17 using anti-cleaved caspase-3 antibody (Asp175: Cell Signaling, Danvers, MA).
mRNA expression was quantified by SYBR Green real-time RT-PCR using ABI PRISM 7000 Sequence Detection System (Applied Biosystems, Foster City, CA) as described17 using primers shown in Supporting Table 1. The expression of each gene was normalized to β-actin mRNA content and calculated relative to normal liver.
Serum IFN-γ Levels.
Serum samples were analyzed for IFN-γ levels using an enzyme-linked immunosorbent assay (ELISA) kit (R&D Systems, Minneapolis, MN).
Western blot was performed using cytoplasmic protein and nuclear protein (20 μg) as described.17 Membranes were incubated with primary antibody for IRF-1 (Santa Cruz Biotechnology, Santa Cruz, CA), Sam68, cleaved caspase-3 (Cell Signaling), and actin (Sigma-Aldrich, St. Louis, MO), and developed with the SuperSignal detection systems (ThermoScientific, Rockford, IL).
Caspase Activity Assay.
Caspase-8 activity was measured using Caspase-Glo 8 assay systems (Promega, Madison, WI), following the method described.18
Isolation of Hepatocytes and NPC.
Hepatocytes and hepatic NPC were isolated from the liver grafts by the collagenase digestion method of Berry and Friend.19 The initial cell suspension was filtrated through a 70-μm nylon mesh, and hepatocytes and NPC in a crude fraction were separated by low-speed centrifugation (five times at 45g for 5 minutes).
Data are presented as the mean ± standard error of the mean (SEM). Comparisons between the groups at different timepoints were performed by using Student's t test or analysis of variance (ANOVA). A log rank test was performed to evaluate survival study. Differences were considered significant at a P-value less than 0.05.
IRF-1 Deficiency in the Graft, but Not in the Host, Ameliorated Transplant-Induced Hepatic I/R Injury.
To examine the roles of IRF-1 in transplant-induced liver I/R injury, LTx between IRF-1 KO and WT mice were performed with 24 hours of cold storage in UW solution, which is known to produce a severe preservation injury.17, 20 Serum ALT levels increased to >15,000 IU/L at 12 hours after WT→WT LTx with significant hepatic necrosis mainly in zone 2 and 3 in H&E sections (Fig. 1A-D). In contrast, KO→KO LTx resulted in significantly lower ALT levels and necrotic area when compared to WT→WT LTx. Interestingly, IRF-1 KO→WT LTx showed serum ALT levels and necrotic areas similar to those in KO→KO LTx. In contrast, WT→KO LTx was not protected, and the degree of I/R injury was similar to those seen in the WT→WT LTx. Sequential analysis of serum ALT levels confirmed that KO→WT LTx showed lower ALT levels than WT→WT at all timepoints (Fig. 1B). These results clearly indicate that IRF-1 signaling in the donor liver graft (but not the recipient) is crucial in mediating liver damage posttransplant.
IRF-1 Deficiency in the Graft Improved Animal Survival After Hepatic I/R Injury.
To further examine the extent of hepatic protection in IRF-1 KO liver grafts, we tested liver graft survival in WT→WT and KO→WT LTx. WT→WT LTx without cold storage resulted in 100% recipient survival for 7 days (Fig. 1E). However, when WT animals received WT liver grafts with 24 hours cold storage, only 17% survived for 7 days. In contrast, WT animals that received IRF-1 KO grafts with 24 hours cold storage showed significantly improved animal survival of 50%. These results further support a critical role for liver graft IRF-1 in mediating transplant I/R injury, and show that the absence of endogenous IRF-1 in the liver graft during a severe preservation injury ultimately improves survival.
Hepatic IRF-1 mRNA and Protein Expression Were Induced After LTx.
As IRF-1 is transcriptionally regulated,5, 6 we examined the expression of endogenous hepatic IRF-1 mRNA in transplant-induced liver I/R injury. Hepatic IRF-1 mRNA levels promptly increased (>10-fold) after WT→WT LTx, reached peak levels at 3 hours, and were maintained at high levels for 12 hours (Fig. 2A). As expected, hepatic IRF-1 mRNA levels were barely detectable after KO→WT LTx. Small increases in IRF-1 mRNA in IRF-1 KO grafts at 3-12 hours appeared to be caused by WT host infiltrating cells. When WT grafts were transplanted into KO recipients, the degree of IRF-1 mRNA induction in the liver graft was similar to the WT→WT group, whereas KO→KO LTx did not induce IRF-1 mRNA up-regulation (Fig. 2B). Nuclear IRF-1 protein was also markedly increased at 3-12 hours after WT→WT, but not in KO→WT, LTx (Fig. 2C).
IFN-γ mRNA and Protein Were Reduced in IRF-1 KO Graft.
IFN-γ is known to be a crucial cytokine activating IRF-1 gene transcription,6, 15, 21 and also is a downstream molecule of IRF-1.21, 22 Hence, we examined the hepatic graft IFN-γ mRNA levels as well as circulating serum IFN-γ protein levels. Hepatic I/R injury strongly up-regulated IFN-γ mRNA (≈20-fold) at 3-12 hours after WT→WT LTx (Fig. 2D). In contrast, in KO→WT LTx IFN-γ mRNA levels were significantly reduced at all timepoints examined. Among four different LTx groups, IFN-γ mRNA expression was significantly suppressed in KO→WT and KO→KO groups, whereas both WT→WT and WT→KO groups showed comparable IFN-γ mRNA up-regulation at 12 hours (Fig. 2E). Serum IFN-γ levels correspondingly increased after WT→WT LTx. In KO→WT LTx, serum IFN-γ levels were significantly lower compared to WT→WT LTx (Fig. 2F). Thus, deficiency in IRF-1 significantly decreased (but did not totally eliminate) IFN-γ mRNA and serum IFN-γ release. This suggests that there are IRF-1-independent, as well as IRF-1-dependent, pathways to produce IFN-γ.
Because production of type I IFNs and IL-12 have been shown to be regulated by IRF-1,7, 10 we also examined for type I IFNs (IFN-α and IFN-β) and IL-12 in the liver grafts at 1 to 12 hours after LTx. Using RT-PCR, all three cytokines showed significant hepatic mRNA up-regulation after LTx, peaking 3 hours after reperfusion, and decreasing by 12 hours after LTx (data not shown). Interestingly, there was no significant difference in mRNA levels of these molecules comparing WT→WT versus KO→WT LTx groups (data not shown), suggesting that production of type I IFN and IL-12 are not strongly regulated by IRF-1 in this model.
IRF-1 and IFN-γ Were Differently Up-regulated in Graft Hepatocytes Versus NPC.
To determine the specific hepatic cell fractions responsible for IRF-1 and IFN-γ up-regulation in I/R injury, we isolated hepatocytes and NPC from liver grafts in WT→WT LTx for RT-PCR analysis. Hepatocyte fractions isolated from liver grafts at 3 hours after reperfusion showed a >15-fold increase in IRF-1 mRNA levels compared to those in naïve hepatocytes isolated from normal liver (Fig. 3A). On the other hand, NPC after LTx showed ≈5-fold increase over naïve NPC (Fig. 3A). Thus, IRF-1 mRNA induction after LTx was seen in both hepatocytes and NPC cells, although to a much greater degree in hepatocytes. In contrast, IFN-γ mRNA was up-regulated exclusively in the NPC fraction (Fig. 3B). Thus, graft hepatocytes and NPC have distinctive roles in hepatic I/R injury by up-regulating mRNA for IRF-1 and IFN-γ, respectively.
Lack of IRF-1 in the Liver Graft Significantly Decreased Hepatocyte Apoptosis.
As IRF-1 signaling has been known to contribute to apoptosis, we next assessed hepatocyte apoptosis in this model. The number of transferase-mediated dUTP nick-end labeling (TUNEL)+ hepatocytes significantly increased by 12 hours after LTx in the WT→WT and WT→KO groups (Fig. 4A,B). In contrast, only small numbers of TUNEL+ hepatocytes were seen in KO→WT and KO→KO LTx. These results correlated with the hepatocellular necrosis pattern and serum ALT levels, and suggested a role for IRF-1 in promoting hepatocyte apoptosis during LTx I/R injury. The significant reduction of hepatocyte apoptosis in KO liver grafts correlated with significantly lower cleaved caspase-3 protein expression in KO→WT, compared to WT→WT at 6 and 12 hours after LTx (Fig. 4C). By immunohistochemistry, cleaved caspase-3 positive hepatocytes were seen mainly in zone 3 at 6 hours after reperfusion in WT→WT LTx (Fig. 4D). In contrast, KO→WT LTx showed substantially less cleaved caspase-3 staining.
LTx-Mediated Activation of the Extrinsic Apoptotic Pathway Was Inhibited in IRF-1-Deficient Grafts.
To define the mechanisms of hepatocyte apoptosis mediated by IRF-1 during hepatic I/R injury, we first conducted RT-PCR for molecules involved in the intrinsic pathway of apoptosis. However, Bcl-2-associated X protein (Bax), Bcl-2, and Bcl-xL mRNA expressions were not significantly different between WT→WT and KO→WT LTx (Fig. 5A).
Next we assessed mRNA expression of genes in the extrinsic pathway of apoptosis. TNF-related apoptosis-inducing ligand (TRAIL), a death ligand with high homology to FasL, triggers extensive apoptosis by way of the binding to TRAIL receptors, Death receptor 4 (DR4) and DR5, recruiting Fas-associated protein with death domain (FADD), and cleaving caspase-8.23 TRAIL mRNA was significantly increased at 3 and 6 hours after WT→WT LTx, but was suppressed in KO→WT LTx. Basal TRAIL mRNA expression was noticeably lower in naïve IRF-1 KO livers than in WT (Fig. 5B). On the other hand, TRAIL receptor DR5 mRNA levels were not different between the two groups. Likewise, Fas mRNA levels were significantly reduced in KO grafts at 3 and 6 hours after LTx compared to WT grafts. FasL mRNA had a trend toward decreased expression in the IRF-1 KO grafts. Furthermore, caspase-8 activities were significantly reduced in KO→WT than in WT→WT LTx at 6 hours after reperfusion (Fig. 5C). These results indicate that IRF-1 signaling regulates hepatocyte apoptosis by way of the extrinsic pathway during hepatic I/R injury.
Liver Graft IRF-1 Deficiency Inhibited Expression of Both Death Ligands and Receptors in Hepatocytes After LTx.
To determine the site of death ligand and receptor up-regulation in liver grafts during I/R injury, we analyzed mRNA expression for death molecules in graft hepatocyte and NPC fractions following LTx. Interestingly, the elevation of TRAIL and FasL mRNA levels were mainly seen in hepatocytes in WT→WT 6 hours after LTx. In contrast, TRAIL mRNA up-regulation was completely inhibited in hepatocytes from IRF-1 KO grafts transplanted into WT recipients. FasL mRNA levels also were lower in the IRF-1 KO hepatocytes (Fig. 6A). In NPC fractions, marginal increases of TRAIL mRNA were seen after WT→WT LTx, which was inhibited in KO→WT LTx (Fig. 6A, upper). FasL mRNA levels did not increase in NPC fractions obtained from WT→WT or KO→WT LTx (Fig. 6A, lower). NPC have been considered as the source of death ligands; however, our data suggests a role for hepatocytes in producing TRAIL and FasL during I/R injury.
As expected, death receptors, DR5 and Fas, were strongly expressed on injured hepatocytes after WT→WT LTx. These receptor expressions were significantly inhibited in hepatocytes obtained from KO→WT LTx (Fig. 6B).
BM Chimeric Mice Indicate a Role for Both Hepatocyte and NPC IRF-1 in Mediating LTx Injury.
To determine the specific role of IRF-1 in hepatocytes and NPC in mediating liver damage, we created BM radiation chimeric mice using IRF-1 KO or WT BM, and produced liver grafts lacking IRF-1 exclusively in either the parenchymal cells (hepatocytes) or NPC. These chimeric donor liver grafts were then transplanted into WT recipients with 24-hour cold storage (Fig. 7, bottom). Liver injury was greatest when both hepatocytes and NPC were WT (Fig. 7, top, WT/WT). Liver grafts from WT animals reconstituted with IRF-1 KO BM had IRF-1 KO NPC and WT hepatocytes (KO/WT). Recipients of these liver grafts exhibited moderately decreased ALT levels compared to WT/WT, indicating that IRF-1 expression in the NPC does contribute to liver injury.
In contrast, liver grafts from IRF-1 KO mice reconstituted with WT BM had WT NPC and KO hepatocytes (WT/KO), and showed significantly decreased ALT levels compared to either the WT/WT or KO/WT group, which were comparable to liver grafts totally lacking IRF-1 (KO/KO). The results of IRF-1, IFN-γ, and TRAIL mRNA expression in the liver grafts from the same chimeric LTx animals are shown in Supporting Fig. 1. The results are consistent with the pattern of liver injury. Taken together, these results suggest that, although IRF-1 in hepatocytes and NPC contributes to transplant-induced liver I/R injury, IRF-1 in hepatocytes is more crucial.
Transplant-induced liver I/R injury is characterized by initial tissue damage during a cold ischemic period, followed by progressive injury during the reperfusion period. Hypothermic and hypoxic stress during cold preservation leads to adenosine triphosphate (ATP) depletion and deterioration of the intracellular homeostasis, resulting in disruption of intercellular contact and denudation in sinusoidal endothelial cells and glycolysis in hepatocytes.24–27 A subsequent warm reperfusion period has more complex features including impairment of microcirculation, reactive oxygen species (ROS) production, cytokine production, and expression of adhesion molecules and extravasation of host leukocytes. However, the eventual dysfunction of grafts is caused by parenchymal cell injury involving both apoptosis and necrosis of hepatocytes. The major and novel findings of this study are: (1) Using IRF-1-deficient mice, this study demonstrates that IRF-1 expression in hepatic grafts plays a critical role in mediating liver transplant preservation injury; (2) IRF-1-mediated apoptosis involves activation of caspase-3 and -8, as well as induction of TRAIL/DR5 and FasL/Fas pathways in hepatocytes; (3) Cell isolation experiment and chimeric liver donor LTx confirm that IRF-1 expression in both cell fractions, but to a larger extent in hepatocytes, contributes to liver damage; and (4) IRF-1 signaling in hepatic graft NPC appears to regulate proinflammatory cytokine production of IFN-γ during hepatic I/R injury. To our knowledge, this is the first study showing a role for IRF-1 in liver transplant I/R injury.
Hepatic IRF-1 expression is known to be transcriptionally regulated,5, 6 and time course studies showed a marked induction of IRF-1 mRNA and protein in WT→WT liver grafts by 3 hours after transplant (Fig. 2). Because both donor and recipient cells participate in transplant-induced liver I/R injury, we first addressed if IRF-1 expression in donor or recipient cells was critical in mediating liver I/R injury. Our results show that a significant reduction in graft damage occurs only when liver grafts are obtained from IRF-1-deficient animals, whereas the IRF-1 status in the recipient does not influence liver injury (Fig. 1). These results indicate that IRF-1 expression in liver grafts, but not host infiltrates, is responsible for liver I/R injury.
We next investigated the specific liver cell types that expressed IRF-1 in the liver transplant setting by isolating hepatocyte and NPC fractions from liver grafts. RT-PCR assay showed that IRF-1 mRNA levels were largely increased in hepatocyte fraction (>15-fold) at 3 hours after reperfusion in WT→WT LTx, but IRF-1 mRNA was also up-regulated to a lesser extent in graft NPC. To further support a specific role for IRF-1 in hepatocytes and NPC in causing liver transplant graft damage, chimeric donors were generated by BM transplant of WT or IRF-1 KO donors yielding specific KO of IRF-1 in either the NPC or hepatocytes in the liver grafts, which were then transplanted into WT recipient animals (Fig. 7). The absence of IRF-1 in the donor hepatocytes, and to a lesser extent in the donor NPC, resulted in decreased ALT levels, confirming a functional role for hepatocyte and NPC IRF-1 in mediating transplant I/R injury.
To address the mechanism of IRF-1 mediated liver I/R injury, we analyzed apoptosis-related genes in whole liver graft samples as well as isolated hepatocyte fractions. Previous work has shown that IRF-1 expression in hepatocytes is a key inducer of apoptosis.11 In our LTx model, TUNEL staining, as well as cleaved caspase-3 expression were significantly increased in WT→WT LTx, but were abolished when IRF-1 KO liver grafts were utilized. Hepatic mRNA levels for intrinsic pathway related genes (Bax, Bcl-2, and Bcl-xL) were not different between WT and KO grafts; however, mRNA up-regulation for extrinsic pathway-related genes for death receptors and ligands seen in WT grafts was significantly reduced in IRF-1 KO grafts. Further, reduction of caspase-8 activity in IRF-1 KO grafts supports that IRF-1 deficiency suppresses the extrinsic pathway of hepatocyte apoptosis. As shown in Fig. 6, hepatocytes from WT, but not IRF-1 KO, grafts actively transcribe mRNA for death ligands and death receptors after reperfusion, supporting critical roles for hepatocyte IRF-1 expression in I/R injury-induced hepatocyte apoptosis by way of the extrinsic pathway. Although membrane-binding TRAIL and FasL are well known to be expressed on T, natural killer (NK), or NKT cells, and are crucial in several models of liver injury, accumulating evidence indicates that hepatocytes also can actively produce soluble TRAIL and FasL in response to various stimuli to promote cell death in an autocrine/paracrine manner.28–30
The importance of death receptor/ligand interaction in I/R injury has been shown in previous studies. Lack of Fas/FasL pathway strongly suppressed myocardial ischemia.31 Small interfering RNA (siRNA) targeting Fas protected mice from renal I/R injury,32 and administration of anti-FasL or anti-Fas antibodies suppressed liver I/R in rats.33 Caspase-8 siRNA has also been shown to decrease liver I/R in mice.34
IRF-1, a ubiquitous nuclear factor, is induced by both type I (IFN-α/β) and type II (IFN-γ) IFNs5, 15 and regulates the transcription of IFN-responsive genes.6 Among IFNs, IFN-γ is a potent antiviral, immunoregulatory, and antitumor cytokine critically involved in innate and adaptive immune responses. Interestingly, IFN-γ or IFN-γ receptor-deficient mice have been reported to have no reduction in liver injury following hepatic warm ischemia, whereas type I IFN receptor KO mice were protected from liver damage.3, 35 However, a recent study shows that Rag KO mice reconstituted with WT cells, but not with IFN-γ KO cells, develop hepatic injury in the warm I/R injury model, suggesting the importance of IFN-γ in liver I/R injury.36 In our study, IFN-γ mRNA is up-regulated exclusively in NPC at 3 hours after reperfusion when IRF-1 nuclear protein shows peak nuclear translocation. Considering that graft IFN-γ mRNA and serum IFN-γ protein levels are significantly reduced in the KO→WT LTx group, it is tempting to assume that IFN-γ is produced by graft NPC, at least in part, by way of IRF-1 up-regulation, and further stimulates hepatocyte IRF-1 expression to augment liver I/R injury.
It is known that IRF-1 KO mice have fewer numbers of NK and NKT cells in the liver.37 NKT cells are the major population (30%) in mouse liver NPC compared to NK cells (8%).38 They are the key producer of IFN-γ and have important roles in liver I/R injury.36, 39 The altered immune cell phenotype in the liver of IRF-1 KO mice might contribute to the resistance of IRF-1KO grafts against hepatic I/R injury. Although our study did not address the role of NKT cells during hepatic I/R injury, we have unpublished preliminary data using liver grafts from CD1d KO mice with 24 hours cold storage showing no reduction in serum ALT levels in WT recipients (19,561 ± 5,285 IU/L: n = 3), indicating that the protective effects of IRF-1 deficiency in liver grafts are not likely due to the reduction of NKT cells.
Based on findings in this study, our current understanding of the role of IRF-1 in hepatic I/R injury is shown in Fig. 8. Liver I/R injury up-regulates IRF-1 signals in hepatocytes and promotes hepatocyte apoptosis through the increased expression of death receptors (DR5 and Fas) and active production of death ligands (TRAIL and FasL). In addition, I/R injury also increases IRF-1 signals in hepatic NPC and promotes production of IFN-γ and other cytokines, which can further augment IRF-1 signal in hepatocytes and can influence the magnitude of host cell infiltration into the graft. Antagonism or silencing of IRF-1 gene expression may be a potential strategy to ameliorate liver damage associated with transplant preservation injury.
We thank Mike Tabacek, Lisa Chedwick, Lifang Shao, Nicole Martik, and Carla Forsythe for assistance.