Natural killer cells: primary target for hepatitis C virus immune evasion strategies?

Liver natural killer and natural killer T cells: immunobiology and emerging roles in liver diseases

Hepatic lymphocytes are enriched in NK and NKT cells that play important roles in antiviral and antitumor defenses and in the pathogenesis of chronic liver disease. In this review, we discuss the differential distribution of NK and NKT cells in mouse, rat, and human livers, the ultrastructural similarities and differences between liver NK and NKT cells, and the regulation of liver NK and NKT cells in a variety of murine liver injury models. We also summarize recent findings about the role of NK and NKT cells in liver injury, fibrosis, and repair. In general, NK and NKT cells accelerate liver injury by producing proinflammatory cytokines and killing hepatocytes. NK cells inhibit liver fibrosis via killing early-activated and senescent-activated stellate cells and producing IFN-gamma. In regulating liver fibrosis, NKT cells appear to be less important than NK cells as a result of hepatic NKT cell tolerance. NK cells inhibit liver regeneration by producing IFN-gamma and killing hepatocytes; however, the role of NK cells on the proliferation of liver progenitor cells and the role of NKT cells in liver regeneration have been controversial. The emerging roles of NK/NKT cells in chronic human liver disease will also be discussed.Understanding the role of NK and NKT cells in the pathogenesis of chronic liver disease may help us design better therapies to treat patients with this disease.

Adoptive immunotherapy with liver allograft-derived lymphocytes induces anti-HCV activity after liver transplantation in humans and humanized mice

After liver transplantation in HCV-infected patients, the virus load inevitably exceeds pre-transplantation levels. This phenomenon reflects suppression of the host-effector immune responses that control HCV replication by the immunosuppressive drugs used to prevent rejection of the transplanted liver. Here, we describe an adoptive immunotherapy approach, using lymphocytes extracted from liver allograft perfusate (termed herein liver allograft-derived lymphocytes), which includes an abundance of NK/NKT cells that mounted an anti-HCV response in HCV-infected liver transplantation recipients, despite the immunosuppressive environment. This therapy involved intravenously injecting patients 3 days after liver transplantation with liver allograft-derived lymphocytes treated with IL-2 and the CD3-specific mAb OKT3. During the first month after liver transplantation, the HCV RNA titers in the sera of recipients who received immunotherapy were markedly lower than those in the sera of recipients who did not receive immunotherapy. We further explored these observations in human hepatocyte-chimeric mice, in which mouse hepatocytes were replaced by human hepatocytes. These mice unfailingly developed HCV infections after inoculation with HCV-infected human serum. However, injection of human liver-derived lymphocytes treated with IL-2/OKT3 completely prevented HCV infection. Furthermore, an in vitro study using genomic HCV replicon-containing hepatic cells revealed that IFN-gamma-secreting cells played a pivotal role in such anti-HCV responses. Thus, our study presents what we believe to be a novel paradigm for the inhibition of HCV replication in HCV-infected liver transplantation recipients.

The effect of donor race on the survival of Black Americans undergoing liver transplantation for chronic hepatitis C

The purpose of this study was to determine the effect of donor race on the outcome of black patients with chronic hepatitis C infection who undergo liver transplantation. The records for deceased donor liver transplants that occurred in the United States between January 1998 and December 2007 were obtained from the United Network for Organ Sharing. 26,212 records contained sufficient data to be included in the analysis. Of these, 11,989 (45.7%) records were for patients positive for hepatitis C virus (HCV) and 1292 (4.9%) were for patients both HCV-positive and black. Black recipients with white donors were found to have significantly worse outcomes than all other recipient-donor race combinations (P < 0.001). The crude 5-year survival rate for black recipients who had a black donor was 14% higher than the 5-year survival rate for black recipients who had a white donor. Multivariate regression analysis determined that a graft from a race-unmatched donor was an independent risk factor for graft failure (hazard ratio = 1.41, 95% confidence interval = 1.11-1.79) among HCV-positive black recipients but not among HCV-negative black recipients after adjustments for donor age, recipient age, cold ischemia time, serum creatinine, serum bilirubin, diabetes mellitus, body mass index, and donor cytomegalovirus status. The observation that race-unmatched grafts are a risk factor in HCV-positive black recipients, but not in HCV-negative black recipients, suggests an alteration of the graft-host relationship by HCV. In conclusion, our results suggest that HCV-positive black recipients who undergo liver transplantation can have increased graft survival if their donors are black, with survival rates approaching those of white liver transplant recipients.

Coinfection with HIV-1 and HCV--a one-two punch

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease, cirrhosis, and death; it is estimated that 180 million persons are infected with HCV worldwide. The consequences of HCV are worse in those who are coinfected with human immunodeficiency virus 1 (HIV-1), which is unfortunately a common scenario because of shared risk factors of the viruses. More studies into effects of HCV/HIV-1 coinfection are needed, but efforts have been hampered by limitations in our understanding of the combined pathogenesis of the 2 viruses. Gaining insight into the mechanisms that underlie the immunopathogenesis of these persistent viral infections could lead to new therapeutic strategies for patients with HCV/HIV-1 coinfection.

Microanatomy of the liver immune system

The critical metabolic functions of the liver often eclipse any perception of its role as an immune organ. However, the liver as a mediator of systemic and local innate immunity and an important site of immune regulation is now an accepted concept. Complex repertoires of lymphoid and non-lymphoid cells are key to hepatic defense and immunoregulation. Hepatic cells of myeloid lineage include Kupffer cells and dendritic cells. Intrahepatic lymphocytes are distinct both in phenotype and function from their counterparts in any other organ and include both conventional (CD4+ and CD8+ alphabeta T cell receptor (TCR)+ T cells, B cells, natural killer (NK) cells) and nonconventional lymphoid cells (natural killer T (NKT) cells, gamma delta TCR+ T cells, CD4- CD8- T cells). Many hepatic T cells express the TCR at an intermediate level and the great majority of them either coexpress NK cell markers (NKT cells) or they are apoptosing peripheral T cells. The percentage of activated (CD69+) and memory (CD45RB low+) lymphocytes is much higher while naive (CD62L high) and resting T cells as well as B lymphocytes are underrepresented in the liver. The discovery of major populations of lymphoid cells in the liver that differ phenotypically, functionally and even perhaps developmentally from populations in other regions has been key to the evolving perception of the liver as a regulatory lymphoid organ. This chapter will focus on these populations and how they contribute to immune surveillance against malignant, infectious and autoimmune disease of the liver.

Natural killer cell receptors and their ligands in liver diseases

The liver is a distinctive immune organ with predominant innate immunity, being rich in innate immune cells such as natural killer (NK) cells. In humans, NK cells comprise about 30%-50% of intrahepatic lymphocytes, whereas peripheral blood lymphocytes contain about 5%-20% NK cells. Accumulating evidence suggests that NK cells play an important role not only in host defense against invading microorganisms and tumor transformation in the liver but also in liver injury and repair. In recent years, significant progress has been made in terms of understanding how NK cells recognize their target cells and carry out their effector functions. It is now clear that NK cells are strictly regulated by numerous activating and inhibitory NK cell receptors that recognize various classes of cell surface ligands, some of which are expressed by normal healthy cells. Therefore, to further elucidate the involvement of NK cells in the pathogenesis of liver diseases, an understanding of recent advances in NK cell biology is crucial. This review provides an overview of recent advances in our knowledge of human NK cell receptors and their ligands in the context of liver diseases.

Natural killer cell function is intact after direct exposure to infectious hepatitis C virions

Although hepatitis C virus (HCV) has been shown to readily escape from virus-specific T and B cell responses, its effects on natural killer (NK) cells are less clear. Based on two previous reports that recombinant, truncated HCV E2 protein inhibits NK cell functions via crosslinking of CD81, it is now widely believed that HCV impairs NK cells as a means to establish persistence. However, the relevance of these findings has not been verified with HCV E2 expressed as part of intact virions. Here we employed a new cell culture system generating infectious HCV particles with genotype 1a and 2a structural proteins, and analyzed direct and indirect effects of HCV on human NK cells. Antibody-mediated crosslinking of CD16 stimulated and antibody-mediated crosslinking of CD81 inhibited NK cell activation and interferon gamma (IFN-gamma) production. However, infectious HCV itself had no effect even at titers that far exceeded HCV RNA and protein concentrations in the blood of infected patients. Consistent with these results, anti-CD81 but not HCV inhibited NK cell cytotoxicity. These results were independent of the presence or absence of HCV-binding antibodies and independent of the presence or absence of other peripheral blood mononuclear cell populations.

CONCLUSION

HCV 1a or 2a envelope proteins do not modulate NK cell function when expressed as a part of infectious HCV particles. Without direct inhibition by HCV, NK cells may become activated by cytokines in acute HCV infection and contribute to infection outcome and disease pathogenesis.

A synergistic interferon-gamma production is induced by mouse hepatitis virus in interleukin-12 (IL-12)/IL-18-activated natural killer cells and modulated by carcinoembryonic antigen-related cell adhesion molecules (CEACAM) 1a receptor

The production of interferon-gamma (IFN-gamma) by infiltrating natural killer (NK) cells in liver is involved in the control of mouse hepatitis virus (MHV) infection. The objectives of this study were to identify the mechanisms used by MHV type 3 to modulate the production of IFN-gamma by NK cells during the acute hepatitis in susceptible C57BL/6 mice. Ex vivo and in vitro experiments revealed that NK cells, expressing carcinoembryonic antigen-related cell adhesion molecules (CEACAM) 1a (the MHV receptor), can produce a higher level of IFN-gamma in the presence of both L2-MHV3 and interleukin-12 (IL-12)/IL-18. The synergistic production of IFN-gamma by NK cells depends on viral replication rather than viral fixation only, because it is inhibited or not induced in cells infected with ultraviolet-inactivated viruses and in cells from Ceacam1a(-/-) mice infected with virulent viruses. The synergistic IFN-gamma production involves the p38 mitogen-activated protein kinase (MAPK) rather than the extracellular signal-regulated kinase-1/2 MAPK signalling pathway. However, the signal triggered through the engagement of CEACAM1a decreases the production of IFN-gamma, when these molecules are cross-linked using specific monoclonal antibodies. These results suggest that control of acute hepatitis by IFN-gamma-producing NK cells may depend on both production of IL-12 and IL-18 in the liver environment and viral infection of NK cells.

Increased frequency of CD56Bright NK-cells, CD3-CD16+CD56- NK-cells and activated CD4+T-cells or B-cells in parallel with CD4+CDC25High T-cells control potentially viremia in blood donors with HCV

A detailed phenotypic analysis of major and minor circulating lymphocyte subsets is described in potential blood donors with markers of hepatitis C virus (HCV), including non-viremic and viremic groups. Although there were no changes in the hematological profile of either group, increased the levels of pre-NK cells (CD3-CD16+CD56-) and a lower frequency of mature NK cells (CD3-CD16+CD56+) characterized innate immunity in the non-viremic group. Both non-viremic and viremic groups displayed significantly increased levels of CD56(Bright) NK cells. Furthermore, this subset was significantly elevated in the viremic subgroup with a low viral load. In addition, an increase in the NKT2 subset was observed only in this subgroup. An enhanced frequency of activated CD4+ T-cells (CD4+HLA-DR+) was a characteristic feature of the non-viremic group, whereas elevated CD19+ B-cells and CD19+CD86+ cell populations were the major phenotypic features of the viremic group, particularly in individuals with a low viral load. Although CD4+CD25High T-cells were significantly elevated in both the viremic and non-viremic groups, it was particularly evident in the viremic low viral load subgroup. A parallel increase in CD4+CD25High T-cells, pre-NK, and activated CD4+ T-cells was observed in the non-viremic group, whereas a parallel increase in CD4+CD25High T-cells and CD19+ B-cells was characteristic of the low viral load subgroup. These findings suggest that CD56Bright NK cells, together with pre-NK cells and activated CD4+ T-cells in combination with CD4+CD25High T-cells, might play an important role in controlling viremia. Elevated CD56(Bright) NK cells, B-cell responses and a T-regulated immunological profile appeared to be associated with a low viral load.

Hepatitis C and innate immunity: recent advances

Eradication of hepatitis C virus (HCV) infection requires a complex and coordinated interplay between innate and adaptive immune responses that, when it fails, leads to chronic infection. In this review, the innate immune mechanisms by which HCV is sensed and by which HCV undermines host defense are discussed. The critical role of dendritic cells in antigen presentation and T-cell activation in addition to type I interferon production and interference of HCV with innate immune cell functions are reviewed. Finally, current and emerging therapeutic approaches targeting innate immune pathways are evaluated.

A novel role for adiponectin in regulating the immune responses in chronic hepatitis C virus infection

Adipose tissue releases pro-inflammatory and anti-inflammatory mediators, including adiponectin, which elicit a broad range of metabolic and immunological effects. The study aim was to determine in subjects infected with chronic hepatitis C virus (HCV) the effects of total adiponectin and its high-molecular-weight (HMW) and low-molecular-weight isoforms on HCV-specific immune responses. Serum levels of total adiponectin and its isoforms were determined by immunoassay. The ex vivo effect of adiponectin on the HCV-specific T-cell response was examined by interferon gamma (IFN-gamma) enzyme-linked immunosorbent spot and enzyme-linked immunosorbent assay cytokine assays. The role of the mitogen-activated protein kinase (MAPK) signaling pathway in mediating the adiponectin effect on T cells was also evaluated. We found that serum levels of total and HMW adiponectin were significantly decreased in subjects with chronic HCV and increased body mass index (BMI) compared with HCV-infected lean subjects. The presence of an anti-HCV specific immune response was strongly associated with lower BMI (P = 0.004) and higher serum total (P = 0.01) and HMW (P = 0.02) adiponectin. In ex vivo assays, total adiponectin and the HMW adiponectin isoform enhanced HCV-specific IFN-gamma production (P = 0.02 and 0.03, respectively). Adiponectin-R1 receptors were expressed on T cells and monocytes. In depletion experiments, the IFN-gamma response to adiponectin was entirely dependent on the simultaneous presence of both CD4 and CD8 T cells, and to a lesser extent, natural killer cells. Selective inhibition of p38MAPK activity by SB203580 abrogated the IFN-gamma response to adiponectin, whereas extracellular signal-regulated kinase 1/2 inhibition by PD98059 did not affect the response.

CONCLUSION

In chronic HCV, a reciprocal association exists between BMI, adiponectin, and the anti-HCV immune responses, emphasizing the important role played by adiposity in regulating the immune response in HCV infection.

Sustained response to interferon-alpha plus ribavirin therapy for chronic hepatitis C is closely associated with increased dynamism of intrahepatic natural killer and natural killer T cells

AIM

Previous studies have revealed that functional impairment of innate immune cells, including natural killer (NK) and natural killer T (NKT) cells, might be associated with the persistence of hepatitis C virus (HCV) infection. However, the involvement of innate immune cells, which predominate in the liver, in therapeutic HCV clearance is still unclear.

METHODS

To clarify the role of intrahepatic innate immune cells in the clinical outcome of patients with chronic hepatitis C (CHC) treated with interferon-alpha plus ribavirin (IFN/RBV), we prospectively investigated the status of NK and NKT cells in paired liver biopsy and peripheral blood (PB) samples obtained from 21 CHC patients before and immediately after IFN/RBV treatment by flow cytometry. Normal liver and PB samples were obtained from 10 healthy donors for living donor liver transplantation.

RESULTS

Before treatment, intrahepatic NK and NKT cells constituted a significantly lower proportion in CHC patients than in healthy individuals (P < 0.05). After IFN/RBV treatment, the proportions and absolute numbers of CD3(-)CD161(+) NK and CD3(+)CD56(+) NKT cells in the liver, but not in PB, were significantly increased in sustained responders (SR) as compared with poor responders (P < 0.05). The proportion of CD3(+)CD161(+) NKT cells was also increased in the liver of SR after the treatment. Moreover, there was a striking increase of activated CD152(+) cells among CD3(+)CD56(+) NKT cells in the liver of SR (P = 0.041).

CONCLUSION

These findings demonstrate that sustained response to IFN/RBV treatment for patients with CHC is closely associated with increased dynamism of NK and NKT cells in the liver.

Transplantation immunology: what the clinician needs to know for immunotherapy

The liver is unique among transplanted organs with respect to its interaction with the host immune system. There is evidence, both anecdotal and documented, that some liver recipients who cease taking immunosuppressive drugs maintain allograft function, suggesting robust tolerance is in place. Moreover, recipients of human liver allografts require less immunosuppression than do other organ recipients, and liver transplants confer protection on other organ grafts from the same donor. Hence, the liver shows features of immune privilege. Still, the liver can display destructive immunologic processes such as rejection in approximately one quarter of patients. The understanding of the cellular and molecular mechanisms operant in tolerance vs allograft rejection is important for developing new agents to improve long-term outcome, minimize infectious complications (including recurrence of hepatotropic viruses), and deliver immunosuppression without long-term toxicity. This review describes the unique aspects of the hepatic immune response, the pathways involved in T-cell activation and alloantigen recognition, effector cells and pathways mediating liver allograft rejection, the role of regulatory T cells, and targets of current and future immunosuppressive agents.

Activation of natural killer cells inhibits liver fibrosis: a novel strategy to treat liver fibrosis

Liver lymphocytes are enriched in natural killer (NK) cells, which are involved in innate immune defenses against viral infection and tumor transformation in the liver. Recent evidence indicates that NK cell activation by IFN-alpha, IFN-gamma or dsRNA attenuates liver fibrosis through the direct killing of activated hepatic stellate cells (HSCs). Interestingly, NK cells do not kill quiescent or fully activated HSCs, but only early-activated HSCs, as only these cells express elevated levels of the NK cell-activating ligand retinoic acid-induced early transcript (RAE)-1 and TNF-related apoptosis-inducing ligand receptors, in addition to downregulated levels of the NK-cell inhibitory ligand, MHC-I. Inhibition of liver fibrosis by NK cells can also be achieved through production of IFN-gamma, which induces HSC cell cycle arrest and apoptosis in a STAT1-dependent manner. Clinically, it has also been observed that NK cell activity is negatively correlated with liver fibrosis in patients with chronic hepatitis C infection. Therefore, since NK cells inhibit liver fibrosis, stimulating NK activity could potentially be a novel strategy to treat liver fibrosis. Clinical studies will be required to confirm whether stimulating NK cell activity is effective and safe in treating human liver fibrosis.

Immunopathogenesis of hepatitis C virus infection and hepatic fibrosis: New insights into antifibrotic therapy in chronic hepatitis C

Fibrosis and cirrhosis represent the consequences of a sustained wound-healing response to chronic liver injury of any cause. Chronic hepatitis C virus (HCV) has emerged as a leading cause of cirrhosis in the USA and throughout the world. HCV may induce fibrogenesis directly by hepatic stellate cell activation or indirectly by promoting oxidative stress and apoptosis of infected cells. The ultimate result of chronic HCV injury is the accumulation of extracellular matrix with high density type I collagen within the subendothelial space of Disse, culminating in cirrhosis with hepatocellular dysfunction. The treatment of hepatitis C with the combination of pegylated interferon and ribavirin is still both problematic and costly, has suboptimal efficacy, serious side effects and a high level of intolerance, and is contraindicated in many patients. Hence, new approaches have assumed greater importance, for which there is an urgent need. The sustained progress in understanding the pathophysiology of hepatic fibrosis in the past two decades has increased the possibility of developing drugs specifically targeting the fibrogenic process. Future efforts should identify genetic markers associated with fibrosis risk in order to tailor the treatment of HCV infection based on genetically regulated pathways of injury and/or fibrosis. Such advances will expand the arsenal to overcome liver fibrosis, particularly in patients with hepatic diseases who have limited treatment options, such as those patients with chronic hepatitis C who have a high risk of fibrosis progression and recurrent HCV disease after liver transplantation.

Phenotypic and functional changes of cytotoxic CD56pos natural T cells determine outcome of acute hepatitis C virus infection

Innate CD56(pos) natural killer (NK) and natural T (NT) cells comprise important hepatic antiviral effector lymphocytes whose activity is fine-tuned through surface NK receptors (NKRs). Dysregulation of NKRs in patients with long-standing hepatitis C virus (HCV) infection has been shown, but little is known regarding NKRs in acute infection. Treatment-naïve patients with acute HCV (n = 22), including 10 with spontaneous recovery, were prospectively studied. CD56(pos) NT levels were reduced early in acute HCV infection and did not fluctuate over time. In resolving HCV infection, NT cells with a more activated phenotype (lower CD158A and higher natural cytotoxicity receptor expression) at baseline predated spontaneous recovery. Moreover, NKG2A expression on CD56(+) NT cells correlated directly with circulating HCV RNA levels. Deficient interleukin-13 (IL-13) production by NT cells and reduced IL-2-activated killing (LAK) at baseline were associated with the ultimate development of persistence. These results indicate a previously unappreciated role for NT cells in acute HCV infection and identify a potential target for pharmacologic manipulation.

Hepatitis viruses: live and let die

Viral hepatitis is a diffuse inflammatory reaction of the liver caused by hepatotropic viruses. Among the hepatitis viruses, only hepatitis B virus and hepatitis C virus are able to persist in the host and cause chronic hepatitis. In the course of persistent infection, inflammation forms the pathogenetic basis of chronic hepatitis that can lead to nodular fibrosis, which can progress to cirrhosis and, eventually, hepatocellular carcinoma (HCC). Of the different antiviral defense systems employed by the host, apoptosis significantly contributes to the prevention of viral replication, dissemination, and persistence. Pathomorphologic studies have shown acidophilic bodies and hepatocyte dropout, features that are compatible with apoptosis. The number of hepatocytes showing features of apoptosis in patients with chronic hepatitis B and C was found to be higher than in healthy subjects, indicating that apoptosis is involved in the pathogenesis of these diseases. There are various data suggesting that hepatitis B and C viral proteins may modulate apoptosis. Vice versa, mechanisms of apoptosis inhibition might represent central survival strategies employed by the virus which, in the end, may contribute to HCC development. While the expression and retention of viral proteins in hepatocytes may influence the severity and progression of liver disease, the mechanisms of liver injury in viral hepatitis are defined to be due not only to the direct cytopathic effects of viruses, but also to the host immune response to viral proteins expressed by infected hepatocytes. However, the exact role of these observations in relation to pathogenesis remains to be established. The mechanism and systems are complex. This report aims to provide an overview and intends to cite only a small number of pertinent references.

Natural killer cell and hepatic cell interaction via NKG2A leads to dendritic cell-mediated induction of CD4 CD25 T cells with PD-1-dependent regulatory activities

Natural killer (NK) cells have the ability to control dendritic cell (DC)-mediated T cell responses. However, the precise mechanisms by which NK receptor-mediated regulation of NK cells determines the magnitude and direction of DC-mediated T cell responses remain unclear. In the present study, we applied an in vitro co-culture system to examine the impact of NK cells cultured with hepatic cells on DC induction of regulatory T cells. We found that interaction of NK cells and non-transformed hepatocytes (which express HLA-E) via the NKG2A inhibitory receptor resulted in priming of DCs to induce CD4(+) CD25(+) T cells with regulatory properties. NKG2A triggering led to characteristic changes of the cytokine milieu of co-cultured cells; an increase in the transforming growth factor (TGF)-beta involved in the generation of this specific type of DC, and a decrease in the tumour necrosis factor-alpha capable of antagonizing the effect of TGF-beta. The regulatory cells induced by NK cell-primed DCs exert their suppressive actions through a negative costimulator programmed death-1 (PD-1) mediated pathway, which differs from freshly isolated CD4(+) CD25(+) T cells. These findings provide new insight into the role of NK receptor signals in the DC-mediated induction of regulatory T cells.

Prediction of venous metastases, recurrence, and prognosis in hepatocellular carcinoma based on a unique immune response signature of the liver microenvironment

Hepatocellular carcinoma (HCC) is an aggressive malignancy mainly due to metastases or postsurgical recurrence. We postulate that metastases are influenced by the liver microenvironment. Here, we show that a unique inflammation/immune response-related signature is associated with noncancerous hepatic tissues from metastatic HCC patients. This signature is principally different from that of the tumor. A global Th1/Th2-like cytokine shift in the venous metastasis-associated liver microenvironment coincides with elevated expression of macrophage colony-stimulating factor (CSF1). Moreover, a refined 17 gene signature was validated as a superior predictor of HCC venous metastases in an independent cohort, when compared to other clinical prognostic parameters. We suggest that a predominant humoral cytokine profile occurs in the metastatic liver milieu and that a shift toward anti-inflammatory/immune-suppressive responses may promote HCC metastases.