Altered T-lymphocyte responsiveness to polyclonal cell activators is responsible for liver cell necrosis in alcohol-fed rats

Thymic NF-κB-inducing kinase regulates CD4+ T cell-elicited liver injury and fibrosis in mice

BACKGROUND & AIMS

The liver is an immunologically-privileged organ. Breakdown of liver immune privilege has been reported in chronic liver disease; however, the role of adaptive immunity in liver injury is poorly defined. Nuclear factor-κB-inducing kinase (NIK) is known to regulate immune tissue development, but its role in maintaining liver homeostasis remains unknown. This study aimed to assess the role of NIK, particularly thymic NIK, in regulating liver adaptive immunity.

METHODS

NIK was deleted systemically or conditionally using the Cre/loxp system. Cluster of differentiation [CD]4+ or CD8+ T cells were depleted using anti-CD4 or anti-CD8 antibody. Donor bone marrows or thymi were transferred into recipient mice. Immune cells were assessed by immunohistochemistry and flow cytometry.

RESULTS

Global, but not liver-specific or hematopoietic lineage cell-specific, deletion of NIK induced fatal liver injury, inflammation, and fibrosis. Likewise, adoptive transfer of NIK-null, but not wild-type, thymi into immune-deficient mice induced liver inflammation, injury, and fibrosis in recipients. Liver inflammation was characterized by a massive expansion of T cells, particularly the CD4+ T cell subpopulation. Depletion of CD4+, but not CD8+, T cells fully protected against liver injury, inflammation, and fibrosis in NIK-null mice. NIK deficiency also resulted in inflammation in the lung, kidney, and pancreas, but to a lesser degree relative to the liver.

CONCLUSIONS

Thymic NIK suppresses development of autoreactive T cells against liver antigens, and NIK deficiency in the thymus results in CD4+ T cell-orchestrated autoimmune hepatitis and liver fibrosis. Thus, thymic NIK is essential for the maintenance of liver immune privilege and liver homeostasis.

LAY SUMMARY

We found that global or thymus-specific ablation of the NIK gene results in fatal autoimmune liver disease in mice. NIK-deficient mice develop liver inflammation, injury, and fibrosis. Our findings indicate that thymic NIK is essential for the maintenance of liver integrity and homeostasis.

Hepatocyte-mediated cytotoxicity and host defense mechanisms in the alcohol-injured liver

The consumption of alcohol is associated with many health issues including alcoholic liver disease (ALD). The natural history of ALD involves the development of steatosis, inflammation (steatohepatitis), fibrosis and cirrhosis. During the stage of steatohepatitis, the combination of inflammation and cellular damage can progress to a severe condition termed alcoholic hepatitis (AH). Unfortunately, the pathogenesis of AH remains uncharacterized. Some modulations have been identified in host defense and liver immunity mechanisms during AH that highlight the role of intrahepatic lymphocyte accumulation and associated inflammatory cytokine responses. Also, it is hypothesized that alcohol-induced injury to liver cells may significantly contribute to the aberrant lymphocytic distribution that is seen in AH. In particular, the regulation of lymphocytes by hepatocytes may be disrupted in the alcoholic liver resulting in altered immunologic homeostasis and perpetuation of disease. In recent studies, it was demonstrated that the direct killing of activated T lymphocytes by hepatocytes is facilitated by the asialoglycoprotein receptor (ASGPR). The ASGPR is a well-characterized glycoprotein receptor that is exclusively expressed by hepatocytes. This hepatic receptor is known for its role in the clearance of desialylated glycoproteins or cells, yet neither its physiological function nor its role in disease states has been determined. Interestingly, alcohol markedly impairs ASGPR function; however, the effect alcohol has on ASGPR-mediated cytotoxicity of lymphocytes remains to be elucidated. This review discusses the contribution of hepatocytes in immunological regulation and, importantly, how pathological effects of ethanol disrupt hepatocellular-mediated defense mechanisms.

Immunology in alcoholic liver disease

Hepatic fibrosis is a known consequence of long-term use of alcohol and is regarded as a turning point in alcohol-induced liver disease because it can lead to cirrhosis. The mechanisms of injury are not well understood, but recent studies have helped advance the understanding of the earliest events in the process that eventually leads to hepatic injury and, in some cases, fibrosis. It is hoped that increasing understanding of the role played by the immune system in the process will lead to the development of new therapies for these patients.

Immunological response in alcoholic liver disease

The development of alcoholic liver disease (ALD) can be attributed to many factors that cause damage to the liver and alter its functions. Data collected over the last 30 years strongly suggests that an immune component may be involved in the onset of this disease. This is best evidenced by the detection of circulating autoantibodies, infiltration of immune cells in the liver, and the detection of hepatic aldehyde modified proteins in patients with ALD. Experimentally, there are numerous immune responses that occur when proteins are modified with the metabolites of ethanol. These products are formed in response to the high oxidative state of the liver during ethanol metabolism, causing the release of many inflammatory processes and potential of necrosis or apoptosis of liver cells. Should cellular proteins become modified with these reactive alcohol metabolites and be recognized by the immune system, then immune responses may be initiated. Therefore, it was the purpose of this article to shed some insight into how the immune system is involved in the development and/or progression of ALD.

Chronic alcohol consumption accelerates liver injury in T cell-mediated hepatitis: alcohol disregulation of NF-kappaB and STAT3 signaling pathways

Alcohol consumption is a major risk factor accelerating the progression of liver disease in patients with chronic hepatitis virus infection. However, the mechanism underlying the enhanced susceptibility of alcoholics to liver injury is not fully understood. Here, we demonstrate that chronic ethanol consumption increases the susceptibility of C57BL/6 mice to concanavalin A (Con A)-induced T cell-mediated hepatitis. Injection of a low dose of Con A (5 microg/g) causes severe liver damage in ethanol-fed mice as evidenced by a significant elevation of serum alanine aminotransaminase levels, massive necrosis, and infiltration of leukocytes but only slightly induces liver injury in control pair-fed mice. In ethanol-fed mice, the activation and cytotoxicity of natural killer T cells, cells that play key roles in Con A-induced T cell hepatitis, are not significantly enhanced relative to pair-fed mice. Moreover, Con A-induced activation of hepatic NF-kappaB is increased, whereas activation of STAT1 and STAT3 is attenuated in ethanol-fed mice. Consistent with this result, the expression of chemokines and adhesion molecules [such as ICAM-1, macrophage inflammatory protein (MIP)-1, MIP-2, and MCP-1] controlled by NF-kappaB is upregulated, whereas STAT1-controlled expression of chemokines (such as MIG and IP-10) is downregulated in ethanol-fed mice compared with pair-fed mice. In conclusion, chronic alcohol consumption accelerates T cell-mediated hepatitis via upregulation of the NF-kappaB signaling pathway and subsequently enhances expression of chemokines/adhesive molecules and recruitment of leukocytes into the liver. Downregulation of the antiapoptotic STAT3 signal may also contribute to alcohol potentiation of T cell hepatitis.

Recent advances in alcoholic liver disease III. Role of the innate immune response in alcoholic hepatitis

Ethanol consumption is known to cause significant acute liver damage resulting in hepatic fibrosis and eventual cirrhosis when consumed chronically. The mechanism(s) by which ethanol exerts its damaging effects on the liver are not well understood; however, recent scientific investigation has begun to delineate the earliest events in alcoholic liver disease. From these studies, it is apparent that components of the innate immune system and, in particular, Kupffer cells, play a significant role in this process. It is also becoming clear that other parts of the immune system including T cells may also be responsible for mediating the devastating effects of chronic alcohol consumption on the liver. This review will highlight recent experiments demonstrating a role for the innate immune response in the initiation and progression of alcohol-induced liver hepatitis and subsequent organ damage.

Activated natural killer T cells induce liver injury by Fas and tumor necrosis factor-alpha during alcohol consumption

BACKGROUND & AIMS

Chronic alcohol abuse induces liver injury and increases the severity of viral hepatitis, but the precise mechanisms responsible are not well understood. In particular, little is known about the role of natural killer T cells in alcohol-induced liver injury. Natural killer T cells are mediators of important regulator and effector functions making use of Fas and tumor necrosis factor (TNF)-alpha in apoptosis induction. This report analyzes the role of natural killer T cells, Fas, and TNF-alpha in a model of chronic alcohol consumption.

METHODS

Mice fed alcohol by intragastric tube were assayed for serum alanine aminotransferase values, liver histology, and liver mononuclear cells before and after activation of natural killer T cells by ligand alpha-galactosylceramide.

RESULTS

In alcohol-consuming animals, liver natural killer T cells increase, and further activation by alpha-galactosylceramide causes lethal liver injury. This is explained by alcohol-induced hepatocyte sensitization to cell-mediated lysis, which develops concomitant to increased cytolytic activity of natural killer T cells. Natural killer T cell-mediated apoptosis proceeds by the Fas pathway, and Fas is essential for alcohol-associated liver injury. TNF-alpha plays an additional role as a defect in TNF receptor-1 inhibits alcohol-associated liver injury. Alcohol-fed natural killer T cell-deficient Jalpha281(-/-) mice express a delay in alcohol-induced liver injury.

CONCLUSIONS

Alcohol consumption induces an increase of natural killer T cells in the liver and a high sensitivity of hepatocytes to cell-mediated lysis. Stimulation of natural killer T cells during alcohol consumption induces serious liver injury by a mechanism that involves concomitant signals by Fas and tumor necrosis factor receptor-1 on alcohol-stressed hepatocytes.

Portal lymphocytic infiltrate in alcoholic liver disease

The aim of this study was to estimate the frequency of lymphocytic portal infiltrate in human alcoholic liver disease and to determine whether it was correlated with liver injury. This retrospective study included 200 consecutive patients referred between February 1996 and March 2001 to the Service Central d'Hépato-Gastroentérologie at Groupe Hospitalier Pitié-Salpêtrière Hospital, Paris, France. The inclusions criteria were as follows: (1) daily alcohol consumption greater than 40 g; (2) appropriate serological exclusion of B and C viral hepatitis; and (3) available liver biopsy specimen. Autoimmune hepatitis was excluded in all patients with the aid of a consensus scoring system. Forty percent of the patients had a predominantly lymphocytic portal infiltrate. On stepwise logistic regression of the predictive value of the 5 variables (gender, age, daily alcohol consumption, steatosis, and portal and septal fibrosis index), only 2 statistically independent predictors of predominant lymphocytic portal infiltrate were identified: portal and septal fibrosis index (P <.05) and age (P <.05). In conclusion, predominantly portal lymphocytic infiltrate occurs frequently in alcoholic liver disease, even in patients without serological markers of B and C viral hepatitis. Moreover, after adjustment for age, portal and septal fibrosis index remains significantly associated with the presence of a predominantly lymphocytic portal infiltrate. These results suggest that lymphocytes may play a role in the pathogenesis of alcohol-induced liver fibrosis.

Lymphocyte-mediated liver injury in alcohol-related hepatitis

The pathogenesis of alcohol-related liver disease (ALD) remains inadequately explained. Increasing alcohol intake is associated with an increased risk of ALD, but many heavy drinkers develop no liver damage. An explanation for ALD susceptibility requires theories that extend beyond a biochemical understanding of alcohol metabolism. Several hepatic cell populations are involved in the pathogenesis of liver injury. The liver-associated lymphocyte (LAL) response to alcohol intake plus immune stimulation may determine susceptibility to liver damage. We have isolated rat LALs and demonstrated the following: (1) Liver-associated lymphocytes differ from the peripheral blood lymphocyte pool; the CD8:CD4 ratio is higher in the LAL population than in peripheral blood. (2) Tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 production by these cells is suppressed by regular alcohol intake. (3) Tumor necrosis factor-alpha and interleukin-6 production by LALs is increased after parenteral administration of concanavalin A (Con A) and by Con A in in vitro LAL cultures obtained from healthy (control) and ethanol-consuming rats. (4) In vivo stimuli that lead to increased cytokine production by LALs lead, within 12-24 h, to increased hepatocyte necrosis [elevated alanine aminotransferase (ALT) levels] and apoptosis. (5) Liver-associated lymphocytes isolated from ethanol-consuming rats, transferred to non-ethanol-consuming rats, confer on the latter animals an ethanol-consuming response to Con A. (6) Cytokine release by LALs is quantitatively as significant as that from Kupffer cells after exposure to lipopolysaccharide. (7) In co-culture studies inhibition of TNF-alpha activity reduces hepatocyte apoptosis induced in the presence of activated LALs. (8) Finally, nuclear factor-kappa B inhibition decreases production of nitric oxide and TNF-alpha, with an associated reduction in hepatocyte apoptosis. In summary, our study findings support the suggestion that a role for LALs exists in the pathogenesis of alcohol and Con A-mediated liver disease.

Current concepts in the pathogenesis of alcoholic liver injury

Alcoholic liver disease (ALD) develops as a consequence of priming and sensitizing mechanisms rendered by cross-interactions of primary mechanistic factors and secondary risk factors. This concept, albeit not novel, is becoming widely accepted by the field, and more research is directed toward identifying and characterizing the interfaces of the cross-interactions to help understand individual predisposition to the disease. Another pivotal development is the beginning of cell type-specific research to elucidate specific contributions not only of hepatocytes, but also of hepatic macrophages, liver-associated lymphocytes, sinusoidal endothelial cells, and hepatic stellate cells to sensitizing and priming mechanisms. In particular, the critical role of hepatic macrophages has been highlighted and the priming mechanisms concerning this paracrine effect have been proposed. Glutathione depletion in hepatocyte mitochondria is considered the most important sensitizing mechanism. One of the contributing factors is decreased methionine metabolism. Remaining key questions include how altered methionine metabolism contribute to the pathogenesis of ALD; how cross-talk among nonparenchymal liver cells or between nonparenchymal cells and hepatocytes leads to ALD; how dysfunctional mitochondria determine the type of cell death in ALD; and what secondary factors are critical for the development of advanced ALD such as alcoholic hepatitis and cirrhosis.

Comparative effects of acute ethanol dosage on liver and muscle protein metabolism

Experiments were performed to address some outstanding issues and investigate possible mechanisms relating to the acute comparative effects of ethanol on liver and skeletal muscle protein metabolism. Ethanol (EtOH)-treated rats were injected (i.p.) with a bolus of EtOH (75 mmol/kg body weight) and sacrificed at 20 min, 1-, 2.5-, 6-, and 24-hr time points. Control rats were injected with saline (Con-Sal; 0.15 mmol/L NaCl). All 24-hr ethanol-treated animals were compared with saline-injected rats subjected to controlled feeding (i.e. pair-fed controls for 24 hr EtOH). At 24 hr, there was no measurable alcohol in the plasma, whereas high levels were seen from 20 min to 6 hr (up to 448 mg/dL). Plasma levels of albumin were reduced at initial time points, and activities of aspartate aminotransferase increased, but there was no histological evidence of overt tissue damage either in muscle or liver. Hepatic protein and RNA contents and indices of tissue (C(s) and k(s)) and whole-body (V(s)) protein synthesis were significantly increased in ethanol-dosed rats relative to saline-injected pair-fed controls at 24 hr. In the liver, four of the seven cytoplasmic proteases investigated (alanyl-, arginyl-, and pyroglutamyl-aminopeptidases and proline-endopeptidase) showed significant increases in activity at 24 hr relative to pair-fed controls; four of the six lysosomal proteases showed significant decreases in activity (dipeptidyl-aminopeptidase II and cathepsins B, L, and H). In skeletal muscle, k(s) fell progressively between 1 and 24 hr (-25 to -69%; P < 0.001), but no significant changes in skeletal muscle protease activities were seen at 24 hr. At 24 hr after ethanol dosage in vivo, there were no significant increases in protein carbonyl content in liver or skeletal muscle compared to pair-fed controls (muscle levels actually decreased slightly). However, using either rat or human tissue, both liver and muscle carbonyl increased in vitro in response to superoxide and hydroxyl radicals: muscle was more susceptible to carbonyl formation than liver and both tissues were more sensitive to hydroxyl compared to superoxide radicals. These results show divergent effects of acute ethanol treatment on liver and skeletal muscle protein metabolism, which may not be linked to in vivo free radical-mediated protein damage (as indicated by carbonyl formation), at least in the short term.

Impact of alcohol on the ability of Kupffer cells to produce chemokines and its role in alcoholic liver disease

Chemokines are implicated in the pathogenesis of alcoholic liver disease in humans and in experimental models of alcohol intoxication. The major sources of these chemokines are Kupffer cells which represent more than 80% of tissue macrophages in the body. Kupffer cells are highly responsive to the effects of ethanol, endotoxin and human immunodeficiency virus (HIV)-1 glycoprotein120. These agents, either independently or in combination, may exacerbate the production of chemokines. Chemokines are agents that are highly chemotactic to mononuclear cells and granulocytes. The levels of these chemokines in sera and tissue are elevated in patients with alcoholic hepatitis, alcoholic cirrhosis, diseased livers, viral hepatitis, and in experimental models of chronic alcohol intoxication. Alcohol-induced influx of endotoxin from the gut into the portal circulation is suggested to play an important role in the activation of Kupffer cells which leads to enhanced chemokine release. The up-regulation of chemokines during alcohol consumption is selective. During the early phase of alcoholic liver disease, C-X-C or alpha-chemokines predominate. This is also associated with neutrophilic infiltration of the liver. In the later stage, up-regulation of C-C or beta-chemokine production and migration of mononuclear cells into the liver are observed, and this may lead to liver cirrhosis. Selective up-regulation of chemokine synthesis and release may involve differential modulation of the transcription factors required for chemokine gene expression. Increased cytokine release following alcohol consumption may also regulate chemokine secretion in Kupffer cells via paracrine and autocrine mechanisms and vice versa. In addition, infection with HIV-1 may further compromise the liver to more damage. During HIV-1 infection, a pre-existing liver disease superimposed on chronic alcohol consumption may also exacerbate HIV-1 replication and lymphocytic infiltration in the liver, because of the ability of HIV-1 gp120 to stimulate chemokine production by Kupffer cells and stimulate migration of inflammatory leucocytes in the liver.

Ethanol-altered liver-associated T cells mediate liver injury in rats administered Concanavalin A (Con A) or lipopolysaccharide (LPS)

BACKGROUND

Recent work from our laboratory implicates T cells in the pathogenesis of alcoholic liver disease. We have studied the role of liver-associated T cells in acute hepatitis produced in control rats administered Concanavalin A (Con A) after adoptive transfer of T cells from alcohol-consuming animals.

METHODS

Liver-associated T cells from ethanol-consuming rats were transferred via tail vein to nonethanol-consuming rats. They then received Con A (20 mg/kg body weight) intravenously. This produced a severe hepatitis. Serum was collected for the assay of alanine aminotransferase (ALT) and cytokines.

RESULTS

Hepatic necrosis was accompanied by an increase in plasma levels of ALT, interleukin-6, and tumor necrosis factor-alpha. These increases correlated with increased production of interleukin-6 and tumor necrosis factor-alpha in culture of liver-associated T cells stimulated or unstimulated with Con A. Immunohistology staining showed increased infiltration of inflammatory cells comprised of neutrophils and mononuclear cells, which included greater numbers of CD4+ T cells in the portal tract areas and around the central vein. Focal and lobular necrosis was seen with inflammatory cells in the necrotic area. Hepatocytes isolated from the liver showed increased apoptosis compared with rats that received liver-associated T cells from nonethanol-consuming rats. Injection of endotoxin LPS, in the same model, was associated with less hepatocyte injury indicating a distinct role for T cells as opposed to Kupffer cells in this model of liver disease.

CONCLUSIONS

Chronic ethanol consumption induces a lesion in a pool of liver-associated T cells which can mediate liver injury after polyclonal mitogen activation.

Alcohol abuse, alcoholism, and damage to the immune system--a review

Chronic alcohol abuse exacts a major social and medical toll in the United States and other Western countries. One of the least appreciated medical complications of alcohol abuse is altered immune regulation leading to immunodeficiency and autoimmunity. The consequences of the immunodeficiency include increased susceptibility to bacterial pneumonia, tuberculosis, and other infectious diseases. In addition, the chronic alcoholic often has circulating autoantibodies, and recent investigations indicate that the most destructive complications of alcoholism, such as liver disease and liver failure, may have a component of autoimmunity. Current research on altered cytokine balance produced by alcohol is leading to new insights on the regulation of the immune system in the chronic alcoholic. There is also recent development of exciting new techniques designed to improve or restore immune function by manipulation of cytokine balance. Although much remains to be learned, both in the abnormalities produced by alcohol and in the techniques to reverse those abnormalities, current progress reflects a rapidly improving understanding of the basic immune disorders of the alcoholic.