Potential conflict of interest: Nothing to report.
This work was supported by the intramural program of NIAAA, NIH (to B.G.).
Address reprint requests to: Bin Gao, M.D., Ph.D., Laboratory of Liver Diseases, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, MD 20892. E-mail: email@example.com
Alpha-Galactosylceramide (α-Galcer), a specific agonist for invariant natural killer T (iNKT) cells, is being evaluated in clinical trials for the treatment of viral hepatitis and liver cancer. However, the results from α-Galcer treatment are mixed, partially because of the variety of cytokines produced by activated iNKT cells that have an unknown synergistic effect on the progression of liver disease. It is well documented that injection of α-Galcer induces mild hepatitis with a rapid elevation in the levels of interleukin (IL)−4 and a delayed elevation in the levels of interferon-gamma (IFN-γ), and both of these cytokines are thought to mediate many functions of iNKT cells. Surprisingly, genetic deletion of both IL-4 and IFN-γ aggravated, rather than abolished, α-Galcer-induced iNKT hepatitis. Moreover, genetic ablation of IL-4, the IL-4 receptor, or its downstream signaling molecule signal transducer and activator of transcription (STAT)6 ameliorated α-Galcer-induced neutrophil infiltration, liver injury, and hepatitis. In contrast, genetic deletion of IFN-γ, the IFN-γ receptor, or its downstream signaling molecule STAT1 enhanced liver neutrophil accumulation, thereby exacerbating liver injury and hepatitis. Moreover, depletion of neutrophils eradicated α-Galcer-induced liver injury in wild-type, STAT1 knockout, and IFN-γ knockout mice. Conclusion: Our results propose a model in which activated iNKT cells rapidly release IL-4, which promotes neutrophil survival and hepatitis but also sequentially produce IFN-γ, which acts in a negative feedback loop to ameliorate iNKT hepatitis by inducing neutrophil apoptosis. Thus, modification of iNKT production of IL-4 and IFN-γ may have the potential to improve the efficacy of α-Galcer in the treatment of liver disease. (Hepatology 2013;58:1474–1485)
Natural killer T (NKT) cells are a heterogeneous group of nonconventional T lymphocytes that recognize, through their T-cell receptors (TCRs), glycolipid antigens presented by the nonclassical major histocompatibility complex (MHC) class I-like molecule CD1. CD1d-restricted NKT cells can be divided into two subsets: type I and type II NKT cells. Type I NKT (invariant NKT or iNKT) cells represent the predominant subset and exclusively express an invariant TCR-α chain, whereas type II NKT cells express more diverse TCRs. The naturally occurring glycolipid α-Galactosylceramide (α-Galcer), originally isolated from a marine sponge, was discovered in 1993 during a screen for novel cancer therapeutic agents and was later found to be a specific agonist for mouse and human iNKT cells. It is now well established that α-Galcer is a strong ligand capable of inducing iNKT activation and the rapid production of T helper (Th)1 (interferon-gamma [IFN-γ]) and Th2 (interleukin [IL]-4) cytokines as well as many other cytokines, such as IL-17 and TNF-α, thereby affecting a wide variety of functions in innate and adaptive immunity. Owing to its potent immunomodulatory properties, α-Galcer has been actively investigated in preclinical and clinical studies for the treatment of cancer, infections, and autoimmune and inflammatory diseases.
The therapeutic potential of α-Galcer for the treatment of liver disease has received particular attention because of the enrichment of iNKT cells in the liver. Mouse and human liver lymphocytes contain 20%-35% and 10%-15% iNKT cells, respectively, whereas peripheral blood lymphocytes contain less than 5% iNKT cells. Accumulating evidence suggests that iNKT cells play complex and even opposing roles in controlling liver injury, regeneration, fibrosis, and liver tumor transformation in different animal models and in patients with different stages or types of liver diseases.[6-8] This involvement is likely a result of the wide array of cytokines produced by iNKT cells. For example, iNKT cells not only can produce antifibrotic cytokines such as IFN-γ, to inhibit liver fibrosis, but also can produce IL-4, IL-13, hedgehog, and osteopontin to exacerbate liver fibrosis. The production of both type I (IFN-γ) and type II (IL-4) cytokines is a hallmark of iNKT activation, which mediates many important functions in the liver.[6-8] The action of IFN-γ is mediated by way of the binding of IFN-γ receptor 1 (IFNGR1) and IFNGR2, whereas IL-4 exerts its effects by way of the binding of IL-4Rα and the gp140/γc chain or IL-4Rα and the IL-13Rα1 chain. These cytokines then activate predominantly signal transducer and activator of transcription (STAT)1 and STAT6, respectively, in hepatocytes, liver nonparenchymal cells, and immune cells and thereby play important roles in the pathogenesis of liver disease.
Despite its complex and obscure immunomodulatory properties in the liver, α-Galcer is being evaluated in clinical trials for the treatment of viral hepatitis and liver cancer.[5, 12-14] In these studies, patients tolerated α-Galcer treatment well, although the results of treatment in patients with liver diseases are inconsistent.[5, 12-14] These inconclusive results are likely the result of α-Galcer inducing iNKT production of a wide variety of cytokines, the synergistic effects of which remain largely unknown for the control of hepatitis. These questions urgently need to be addressed prior to the application of α-Galcer in additional clinical trials for treating liver disease. Injection of α-Galcer into mice induces iNKT activation, with rapid production of IL-4 but delayed production of IFN-γ, which results in mild hepatitis and liver injury.[15, 16] In the current study, we found that after α-Galcer injection, iNKT cells rapidly produce IL-4, which promotes liver neutrophil accumulation and hepatitis by way of a STAT6-dependent mechanism, whereas the subsequent production of IFN-γ acts in a negative feedback loop to control α-Galcer-induced hepatitis by inducing neutrophil apoptosis by way of a STAT1-dependent mechanism.
Materials and Methods
Eight- to 10-week-old male mice were used in this study. C57BL/6J, IFN-γ−/−, IFNGR−/−, IL-4−/−, and STAT6−/− mice on a C57B/6J background were purchased from the Jackson Laboratory (Bar Harbor, ME). Balb/c and IL-4R−/− mice on a Balb/c background were also purchased from the Jackson Laboratory. STAT1−/− mice were originally purchased from Taconic (Hudson, NY) and backcrossed into a C57BL/6J background for at least 11 generations. IFN-γ−/− IL-4−/− double knockout (dKO) mice were generated as a result of several steps of cross-breeding between IFN-γ−/− and IL-4−/− mice. All animals were maintained in a specific pathogen-free facility and were cared for in accordance with National Institutes of Health guidelines; this study was approved by the National Institute on Alcohol Abuse and Alcoholism Animal Care and Use Committee.
Murine iNKT Cell-Driven Hepatitis
To induce murine iNKT cell-driven acute experimental hepatitis, a single intravenous injection of α-Galcer (3 μg in 300 μL vehicle) was administered to each mouse. Control mice received 300 μL of vehicle. α-Galcer (KRN7000) was purchased from Alexis Biochemicals (San Diego, CA) and dissolved in 0.5% polysorbate-20 (Tween-20) and diluted in sterile phosphate-buffered saline (PBS).
Data are expressed as the mean ± SEM for each group and were analyzed using GraphPad Prism software (v. 5.0a; GraphPad Software, La Jolla, CA). To compare values obtained from two groups, Student t test was performed. To compare values obtained from three or more groups, single-factor analysis of variance (ANOVA) was used, followed by Tukey's post-hoc test. Statistical significance was designated at the P < 0.05 level.
Additional Materials and Methods are included in the Supporting Materials.
Deletion of Both IFN-γ and IL-4 Exacerbates α-Galcer-Induced Liver Injury and Hepatitis
Injection of α-Galcer rapidly increased the serum levels of IL-4, with a peak effect at 3 hours postinjection, whereas the elevation of serum IFN-γ was delayed, with a peak effect at 16 hours postinjection (Fig. 1A). Administration of α-Galcer resulted in a mild elevation of serum aspartate aminotransferase (ALT) and alanine aminotransferase (AST) levels (Fig. 1A) and spotted necrosis (Supporting Fig. 1) in wild-type (WT) C57BL/6 mice by 16 hours postinjection. However, to our surprise, α-Galcer induced 5- to 6-fold higher serum ALT and AST levels and a larger area of necrosis in IL-4−/−IFN-γ−/− dKO mice than those in WT mice at 16 hours after α-Galcer injection (Fig. 1; Supporting Fig. 1). In addition, administration of α-Galcer induced an accumulation of inflammatory foci in the livers of WT mice, with the peak effect occurring at 72 hours postinjection (Supporting Fig. 1), and the number of inflammatory foci was also much higher in dKO mice than that in WT mice (Supporting Fig. 1).
α-Galcer-Induced Hepatocellular Damage Is Reduced in IL-4−/− or IL-4R−/− Mice
To determine the role of early production of IL-4 in α-Galcer-induced liver injury, we examined the effects in IL-4−/− and IL-4R−/− mice. As illustrated in Fig. 2A,B, α-Galcer-induced elevation of serum ALT and AST was lower in IL-4−/− and IL-4R−/− mice than in WT controls. Liver histology analyses further revealed that IL-4−/− and IL-4R−/− mice had reduced liver necrosis and fewer inflammatory foci than WT control mice after α-Galcer administration (Figs. 2C-D). The number of myeloperoxidase (MPO)-positive neutrophils was also lower in IL-4−/− and IL-4R−/− mice than in WT mice 72 hours after α-Galcer administration (Fig. 2C,D).
IL-4 Is Required for Neutrophil Accumulation Post-α-Galcer Administration, Which Contributes to Liver Injury
The above findings indicated that the number of inflammatory foci (iNKT expansion) in the liver was lower in IL-4−/− or IL-4R−/− mice than in WT mice 72 hours post-α-Galcer injection, which may have been due to IL-4-mediated promotion of iNKT proliferation, as demonstrated previously. Fluorescence-activated cell sorting (FACS) analyses of liver MNCs revealed that WT and IL-4−/− mice had a similar number of iNKT cells at the early timepoints post-α-Galcer injection (data not shown), which does not explain the reduced liver injury in IL-4−/− mice. To further explore the mechanisms underlying α-Galcer-induced liver injury, we examined NK cells and neutrophils in the liver. In this case, FACS analyses revealed that the number of NK cells was not increased post-α-Galcer injection and that depletion of NK cells using an anti-ASGM1 antibody did not affect α-Galcer-induced liver injury in mice (data not shown), suggesting that NK cells are not involved in this process. In contrast, there was a striking increase in the percentage and total number of neutrophils in the liver after α-Galcer injection. As illustrated in Fig. 3A,B, the percentage of neutrophils was elevated 4-fold, whereas the total number of neutrophils was elevated 30-fold at 3 hours post-α-Galcer administration. Moreover, depletion of neutrophils markedly reduced serum ALT and AST levels (Fig. 3C), suggesting that the accumulation of neutrophils contributes to α-Galcer-induced hepatocellular damage.
Figure 3D shows that the percentage and total number of hepatic neutrophils were lower in IL-4−/− mice than in WT mice at 3 hours post-α-Galcer administration. Furthermore, Fig. 3E shows that neutrophils from α-Galcer-treated IL-4−/− mice demonstrated higher levels of apoptosis (Annexin V staining) than those of WT mice, which suggests that the reduced neutrophil accumulation in α-Galcer-treated IL-4−/− mice was due to increased neutrophil apoptosis.
STAT6, the Major Downstream Signaling Molecule of IL-4, Is Required for α-Galcer-Induced Neutrophil Accumulation and Liver Injury
To further understand the mechanisms through which IL-4 contributes to α-Galcer-induced liver injury, we examined the role of STAT6 in this model. As illustrated in Fig. 4A, STAT6 was activated in neutrophils from the livers of α-Galcer-treated WT mice, whereas this activation was diminished in neutrophils from α-Galcer-treated IL-4−/− mice. This result suggests that IL-4 is responsible for the observed STAT6 activation in neutrophils.
In agreement with the data from IL-4−/− mice, STAT6−/− mice had lower serum levels of ALT and AST (Fig. 4B), fewer inflammatory foci (Supporting Fig. 2), and a reduced number of neutrophils in the liver (Fig. 4C) compared to WT mice post-α-Galcer administration. In addition, neutrophils from α-Galcer-treated STAT6−/− mice demonstrated higher levels of apoptosis than those from WT mice (Fig. 4D). Collectively, our findings suggest that IL-4/STAT6 inhibit neutrophil apoptosis.
To understand the mechanisms underlying the IL-4/STAT6-mediated inhibition of neutrophil apoptosis, we investigated the expression of antiapoptotic genes in these cells and identified that the expression of survivin and Bcl-2 was significantly up-regulated in hepatic neutrophils from α-Galcer-treated WT mice, whereas this up-regulation was reduced in hepatic neutrophils from α-Galcer-treated IL-4−/− or STAT6−/− mice (Fig. 4E).
α-Galcer-Induced Liver Injury Is Exacerbated in IFN-γ−/− or IFNGR−/− Mice
The finding that deletion of IL-4 abolished α-Galcer-induced hepatitis cannot explain the exacerbated α-Galcer-induced liver injury observed in IL-4−/−IFN-γ−/− dKO mice. To further understand the mechanisms by which IL-4−/−IFN-γ−/− dKO mice are more susceptible to α-Galcer-induced hepatitis, we examined this model in IFN-γ−/− or IFNGR−/− mice. As illustrated in Fig. 5A, IFN-γ−/− or IFNGR−/− mice were more sensitive to α-Galcer-induced liver injury, as reflected by the higher levels of serum ALT and AST than WT mice. In agreement with the biochemical data, histological examination, as shown in Fig. 5B, confirmed more severe liver injury and inflammation (larger area of necrosis and a larger number of inflammatory foci) in IFN-γ−/− and IFNGR−/− mice at both 16 hours and 72 hours after α-Galcer administration than in WT mice. In addition, the number of MPO+ neutrophils was higher in the livers of IFN-γ−/− or IFNGR−/− mice post-α-Galcer injection (Fig. 5B).
Enhanced α-Galcer-Induced Liver Injury in IFN-γ−/− Mice Is Dependent on Neutrophils But Not NK Cells
Because it has been shown that NKT and NK cells can kill hepatocytes and contribute to liver injury,[18, 19] we hypothesized that the differences in α-Galcer-induced liver injury in WT and IFN-γ−/− mice were due to varying degrees of NKT and NK activation. The data in Supporting Fig. 3 show that mononuclear cells (MNCs) from α-Galcer-treated WT and IFN-γ−/− mice had similar levels of cytotoxicity towards hepatocytes, which could not account for the difference in liver injury between WT and KO mice and suggests that enhanced liver injury in IFN-γ−/− mice is not due to increased MNC (NKT) cytotoxicity. In addition, depletion of NK cells did not affect serum ALT and AST levels in IFN-γ−/− mice (Supporting Fig. 4), indicating that NK cells are not the cause of the severe liver injury in these animals.
FACS analyses showed that the percentage of iNKT cells was markedly decreased, whereas the percentage of macrophages was slightly increased 3 hours after α-Galcer injection. Such changes were similar in WT and IFN-γ−/− mice (data not shown). Interestingly, the percentage and total number of neutrophils were much higher in IFN-γ−/− mice than in WT mice 3 hours after α-Galcer injection (Fig. 6A), which was likely due to reduced apoptosis as demonstrated by Annexin V staining (Fig. 6B). Moreover, depletion of neutrophils with an anti-Ly6G antibody reduced α-Galcer-induced elevation of serum ALT and AST levels by 80% in IFN-γ−/− mice (Fig. 6C), and liver histology revealed that depletion of neutrophils completely prevented α-Galcer-induced necrosis in IFN-γ−/− mice (Supporting Fig. 5).
Next we investigated the mechanisms through which neutrophils contribute to liver injury by examining liver leukocyte cytotoxicity against hepatocytes. As shown in Fig. 6D, liver polymorphonuclear cells (PMNs) isolated from α-Galcer-treated IFN-γ−/− mice demonstrated higher levels of cytotoxic activity against mouse hepatocytes than those from α-Galcer-treated WT mice.
STAT1−/− Mice Are More Susceptible to α-Galcer-Induced Liver Injury and Neutrophil Infiltration
Figure 7A shows that STAT1 was activated in neutrophils from α-Galcer-treated WT mice but not in neutrophils from IFN-γ−/− mice, suggesting that STAT1 is the key downstream signaling molecule of IFN-γ in this process. To examine the role of STAT1 in α-Galcer-induced hepatitis, we compared the α-Galcer-induced liver injury in STAT1−/− and WT mice. As illustrated in Fig. 7B, α-Galcer administration induced higher levels of serum ALT and AST in STAT1−/− mice than in WT mice at 16 hours postinjection. Moreover, the liver histology revealed that STAT1−/− mice had larger areas of necrosis in the liver than WT mice at 24 hours post-α-Galcer injection (data not shown). In agreement with the data from IFN-γ−/− mice, the STAT1−/− mice also had a larger number of liver neutrophils than WT mice 3 hours after α-Galcer injection (Fig. 7C). Additionally, liver neutrophils from α-Galcer-treated STAT1−/− mice demonstrated reduced levels of apoptosis compared with those of WT mice (Fig. 7D). Finally, the depletion of neutrophils with an anti-Ly6G antibody markedly abolished α-Galcer-induced liver injury in STAT1−/− mice (Fig. 7E).
To understand the mechanisms underlying decreased neutrophil apoptosis in IFN-γ−/− mice post-α-Galcer injection, we investigated the expression of proapoptotic genes and identified that the expression of caspase 3, Bax, and Fas was up-regulated in neutrophils from α-Galcer-treated WT mice, and this result was observed to a lesser extent in neutrophils from IFN-γ−/− or STAT1−/− mice (Fig. 7F).
Antagonism Between IL-4 and IFN-γ in Controlling α-Galcer-Induced Liver Injury and Neutrophil Accumulation
The above data suggest that IL-4 and IFN-γ play opposing roles in controlling α-Galcer-induced liver injury. Next we examined whether IL-4 and IFN-γ antagonize each other to control iNKT-mediated liver injury in vivo by comparing α-Galcer-induced hepatic neutrophil accumulation and injury among IL-4−/−IFN-γ−/−, IL-4−/−, IFN-γ−/−, and WT mice. As shown in Fig. 8A,B, IFN-γ−/− mice had the highest levels of serum ALT and AST and the greatest number of liver neutrophils, whereas IL-4−/− mice had the lowest levels of serum ALT and AST and the lowest number of liver neutrophils. The values from IL-4−/−IFN-γ−/− mice were between those from IFN-γ−/− and IL-4−/− mice. These findings suggest that IL-4 and IFN-γ antagonize each other to control α-Galcer-induced liver neutrophil infiltration and injury in vivo.
It has long been known that injection of α-Galcer activates iNKT cells, inducing a rapid elevation in the levels of IL-4 and a delayed elevation in the levels of IFN-γ. In the present study, we demonstrate (1) that the rapid production of IL-4 by iNKT cells induces liver neutrophil accumulation, which contributes to liver injury, and (2) that the delayed production of IFN-γ attenuates hepatic neutrophil accumulation by inducing neutrophil apoptosis, thereby preventing iNKT-mediated liver injury. We have integrated these findings into a model depicting the opposing roles of IFN-γ and IL-4 in controlling iNKT-mediated neutrophil accumulation and liver injury (Fig. 8C).
iNKT Activation Induces Hepatic Neutrophil Accumulation, Which Contributes to Liver Injury
Although it is well documented that injection of the iNKT ligand α-Galcer induces mild hepatitis, the underlying mechanisms have not been fully understood. Previous studies have suggested that Kupffer cells do not contribute to α-Galcer-induced hepatitis. In the current study we observed a striking increase (30-fold) in neutrophils in the liver 3 hours after α-Galcer injection and found that depletion of neutrophils prevented α-Galcer-induced liver injury, which suggests that the accumulation of neutrophils contributes to liver injury. However, the mechanism through which neutrophils induce liver injury in this model was not investigated. It has also previously been shown that neutrophils induce hepatocellular damage in several models of liver injury by way of the oxidative killing of hepatocytes or the induction of liver lymphocyte recruitment.[21-23] These mechanisms also likely mediate the neutrophil-mediated liver injury induced by α-Galcer because liver neutrophil-enriched PMNs from α-Galcer-treated mice were able to kill primary hepatocytes in vitro (Fig. 6D).
IL-4 Exacerbates α-Galcer-Induced Liver Injury by Promoting Hepatic Neutrophil Survival and Infiltration
Activation of iNKT cells has been shown to induce neutrophil accumulation in the lung, ischemic kidneys by way of an IL-17-dependent mechanism, and in Listeria-infected livers by way of an IL-17-independent mechanism but inhibit neutrophil infiltration in cholestatic liver damage. Although α-Galcer injection rapidly stimulates iNKT cells to produce IL-17, the blockade of IL-17 increased, rather than reduced, α-Galcer-induced hepatic neutrophil recruitment, suggesting that IL-17 is not involved in iNKT-induced hepatic neutrophil accumulation and injury. Our current findings demonstrate that IL-4/STAT6 signaling plays a critical role in inducing liver neutrophil accumulation by inhibiting neutrophil apoptosis because genetic deletion of IL-4, the IL-4R, or its downstream signaling molecule STAT6 increased neutrophil apoptosis and suppressed neutrophil accumulation in α-Galcer-treated mice (Fig. 3). Although IL-4 has been shown to suppress neutrophil apoptosis in human neutrophils, the underlying mechanisms are not fully understood. Here, we demonstrated that the expression of survivin and Bcl-2 in neutrophils was up-regulated in α-Galcer-treated WT mice but not in IL-4−/− or STAT6−/− mice (Fig. 4). Because survivin and Bcl-2 play an important role in promoting neutrophil survival and proliferation,[28, 29] the induction of survivin and Bcl-2 by IL-4 and STAT6 likely promotes neutrophil survival and accumulation in the liver during α-Galcer-induced iNKT hepatitis.
Additionally, IL-4 has been shown to promote hepatic leukocyte recruitment by augmenting the expression of chemokines in Con A-induced hepatitis by way of a STAT6-dependent mechanism. This mechanism may also apply to IL-4/STAT6 promotion of neutrophil accumulation in α-Galcer-induced iNKT hepatitis because hepatic expression of several chemokines was lower in IL-4−/− or STAT6−/− mice than in WT mice after α-Galcer administration (Supporting Fig. 6). Additionally, hepatic expression of IFN-γ was also lower in IL-4−/− mice than that in WT mice after α-Galcer (Supporting Fig. 7), suggesting IL-4 enhances IFN-γ production. However, this unlikely contributes to IL-4 promotion of hepatic neutrophil accumulation because IFN-γ attenuates hepatic neutrophil infiltration (see below).
IFN-γ Prevents Neutrophil Infiltration and Protects Against α-Galcer-Induced Liver Injury by Accelerating Hepatic Neutrophil Apoptosis
The detrimental effects of IFN-γ/STAT1 signaling have been documented in several models of liver injury, including Con A-induced hepatitis[31-33] and LPS/D-galactosamine-induced liver injury. However, a previous study found that inhibition of IFN-γ exacerbated α-Galcer-induced liver injury, but the underlying mechanisms of this protective effect remain enigmatic. In the present study, we found that genetic ablation of the IFN-γ, IFNGR, or STAT1 genes also exacerbated α-Galcer-induced hepatocellular damage. Our additional findings suggest that the beneficial effect of IFN-γ in α-Galcer-induced liver injury is mediated by the prevention of hepatic neutrophil accumulation. First, as shown in Fig. 6A, the total number of neutrophils in the liver was much higher in α-Galcer-treated IFN-γ−/− and STAT1−/− mice than in WT mice. Second, liver PMNs from α-Galcer-treated IFN-γ−/− mice had higher levels of cytotoxicity against primary mouse hepatocytes than those from WT mice (Fig. 6D). Finally, depletion of neutrophils ameliorated α-Galcer-induced severe liver injury by 90% in both IFN-γ−/− and STAT1−/− mice (Figs. 6, 7), suggesting that the accumulation of neutrophils contributes to the exacerbated liver injury observed in α-Galcer-treated IFN-γ−/− and STAT1−/− mice.
IFN-γ−/− mice had lower levels of hepatic expression of IL-4 compared to WT mice after α-Galcer injection (Supporting Fig. 7), suggesting that IFN-γ is required for the production of IL-4. However, this unlikely attributes to IFN-γ prevention of hepatic neutrophil infiltration because IL-4 promotes hepatic neutrophil accumulation (see above). Our further findings indicate that IFN-γ attenuates hepatic neutrophil accumulation by inducing neutrophil apoptosis after α-Galcer injection, as neutrophil apoptosis was suppressed in IFN-γ−/− mice (Fig. 6). Mechanistic studies suggest that the proapoptotic effect of IFN-γ is mediated by the induction of several proapoptotic genes by way of a STAT1-dependent mechanism (Fig. 7F). Collectively, these findings suggest that IFN-γ stimulates the expression of proapoptotic genes in neutrophils by way of a STAT1-dependent mechanism, thereby playing an important role in preventing hepatic neutrophil accumulation in α-Galcer-induced liver injury.
In addition to their opposing roles in the control of hepatic neutrophil accumulation, IL-4 and IFN-γ have been shown to inversely control NKT cell proliferation in vitro. During the course of our studies, we observed the percentage and total number of liver iNKT cells in WT, IL-4−/−, IFN-γ−/−, and IL-4−/−IFN-γ−/− dKO mice were comparable before α-Galcer injection. After α-Galcer injection, liver iNKT cells rapidly disappeared within 24 hours. This disappearance was similar among these four strains of mice (data not shown). These findings suggest that the differences in hepatic neutrophil accumulation 3 hours post-α-Galcer injection among WT, IL-4−/−, IFN-γ−/−, and IL-4−/−IFN-γ−/− dKO mice were not caused by the changes in iNKT cells at the early timepoints after α-Galcer injection. Additionally, expression of activation markers (CD11b and CD62L) and production of reactive oxygen species (ROS) were comparable in neutrophils from α-Galcer-treated WT, IL-4−/−, and IFN-γ−/− mice (Supporting Fig. 8), suggesting IL-4 and IFN-γ regulate hepatic neutrophil accumulation but not activation.
Although IL-4 and IFN-γ mediate many crucial functions of iNKT cells in the liver,[6-8] IL-4−/−IFN-γ−/− dKO mice still had significant liver injury after α-Galcer injection, suggesting that mechanisms other than IL-4 and IFN-γ are involved. It was previously reported that α-Galcer treatment induces TNF-α production by iNKT cells and that inhibition of TNF-α ameliorated α-Galcer-induced liver injury and diminished the aggravating effects of IFN-γ neutralization in this liver injury. These findings suggest that TNF-α likely contributes to the α-Galcer-induced liver injury in IL-4−/−IFN-γ−/− dKO mice.
Because activation of iNKT cells by α-Galcer has been shown to inhibit hepatitis viral replication and liver cancer growth in animals,[36-38] α-Galcer has been tested in clinical trials for the treatment of viral hepatitis and liver cancer.,[12-14] In general, treatment with α-Galcer in patients was well tolerated but showed few beneficial effects.[12-14] Our findings that α-Galcer-induced production of IL-4 and IFN-γ antagonize each other to control liver injury suggest that manipulation of these cytokines may improve the therapeutic potential of α-Galcer in the treatment of liver disease. For example, α-Galcer injection stimulates iNKT cell production of IFN-γ, which is not only absolutely required for the antitumor and antiviral activities of α-Galcer in vivo,[35, 38] but also protects against α-Galcer-induced liver injury, as demonstrated in this and another study. In contrast, IL-4 produced by iNKT cells not only impairs iNKT antitumor activities but also exacerbates iNKT-mediated liver injury. Thus, the development of ligands that activate iNKT cells to preferentially produce IFN-γ may have higher antiviral and antitumor activities but lower hepatotoxicity than α-Galcer. Indeed, there is an ongoing intensive effort to identify α-Galcer analogs that stimulate iNKT cells to preferentially secrete IFN-γ or IL-4, which may lead to the identification of better iNKT activators for the treatment of liver disease.