Decreased NK cell frequency in chronic hepatitis C does not affect ex vivo cytolytic killing

Natural killer cells: multifaceted players with key roles in hepatitis C immunity

Natural killer cells (NKs) are involved in every stage of hepatitis C viral (HCV) infection, from protection against HCV acquisition and resolution in the acute phase to treatment-induced clearance. In addition to their direct antiviral actions, NKs are involved in the induction and priming of appropriate downstream T-cell responses. In the setting of chronic HCV, overall NK cell levels are decreased, subset distribution is altered, and changes in NK receptor (NKR) expression have been demonstrated, although the contribution of individual NKRs to viral clearance or persistence remains to be clarified. Enhanced NK cell cytotoxicity accompanied by insufficient interferon-γ production may promote liver damage in the setting of chronic infection. Treatment-induced clearance is associated with activation of NK cells, and it will be of interest to monitor NK cell responses to triple therapy. Activated NK cells also have anti-fibrotic properties, and the same hepatic NK cell populations that are actively involved in control of HCV may also be involved in control of HCV-associated liver damage. We still have much to learn, in particular: how do liver-derived NKs influence the outcome of HCV infection? Do NK receptors recognize HCV-specific components? And, are HCV-specific memory NK populations generated?

Natural killer cell dynamic profile is associated with treatment outcome in patients with chronic HCV infection

BACKGROUND & AIMS

A substantial proportion of patients with chronic hepatitis C virus infection treated with pegylated interferon α/ribavirin fail to achieve sustained virological response (SVR). Since growing evidence suggests that innate immunity may influence treatment responses, we examined natural killer (NK) cell phenotypic and functional changes during standard antiviral therapy.

METHODS

Expression of several NK-cell regulatory molecules was evaluated by flow cytometry in 37 consecutive patients with chronic HCV infection at baseline and at different time points during and after discontinuation of treatment. Cytokine production was evaluated by intracellular staining. Cytolytic potential was assessed as degranulation and as antibody-dependent cytotoxicity.

RESULTS

Baseline frequencies of CD56(dim) NK cells and perforin content were significantly higher, whereas CD16 expression was lower in SVR vs. non-responder subjects. Analysis by linear regression for repeated measures during the first 12 weeks showed significantly increased frequencies of activated (CD69(+)) NK cells in rapid virological responders (RVR) and identified a typical NK cell profile associated with SVR, featuring higher NK perforin content, lower CD16 expression, and higher proportion of CD56(dim)/CD16(-) cells. Moreover, SVR patients displayed higher natural and antibody-dependent NK cell cytotoxicity. IL28B rs12979860 CC homozygosis was significantly associated with SVR, independently of NK-cell phenotype and function.

CONCLUSIONS

Different NK-cell phenotypic and functional features, in patients with chronic hepatitis C treated with standard therapy, were observed between non-responder vs. SVR patients, suggesting a potential role of NK cells in the response to treatment.

Natural killer cells in hepatitis C virus infection

Hepatitis C virus (HCV) infection induces the long-term risk of liver cirrhosis or hepatocellular carcinoma and in adults represents the most common cause of liver transplantation. Natural killer (NK) cells participate in innate immune responses with efficient direct antitumor and antiviral defense. Over the years, their complex interaction with downstream adaptive responses and with the regulation of immune responses has been increasingly recognized. Considerable advances have been made particularly in understanding the role of NK cells in the pathophysiology of HCV infection and their possible use as biological markers for clinical purposes. This review summarizes the available data on the role of NK cells in the natural history of HCV infection and their role in the outcome of treatment. The main objective of this review is to summarize recent advancements in the basic understanding of NK cell function highlighting their possible translational use in clinical practice. An integrated practical view on the possible use of currently available predictive immunogenetic and NK cell functional tests is provided, to support clinical management choices for optimal treatment of patients with both standard and new drug regimens.

The role of natural killer cells in hepatitis C infection

HCV infection is an exponentially growing health burden worldwide, with an estimated 170 million people infected. Although therapies for HCV are continually improving, there remain a considerable proportion of patients who do not achieve viral eradication and develop liver disease. Natural killer (NK) cells are crucial for T-cell activation and are one of the first-line sentinel cell responders to viral infection. A recent explosion in studies exploring the role of NK cells in HCV infection has yielded important mechanistic information and intriguing potential therapeutic options for HCV infection. This review provides a general overview of normal NK cell function and outlines some of the important mechanisms characterizing the immune interplay between NK cells and HCV infection.

Chronic bystander infections and immunity to unrelated antigens

Chronic infections with persistent pathogens such as helminths, mycobacteria, Plasmodium, and hepatitis viruses affect more than a third of the human population and are associated with increased susceptibility to other pathogens as well as reduced vaccine efficacy. Although these observations suggest an impact of chronic infections in modulating immunity to unrelated antigens, little is known regarding the underlying mechanisms. Here, we summarize evidence of the most prevalent infections affecting immunity to unrelated pathogens and vaccines, and discuss potential mechanisms of how different bystander chronic infections might impact immune responses. We suggest that bystander chronic infections affect different stages of host responses and may impact transmission and recognition of other pathogens, innate immune responses, priming and differentiation of adaptive effector responses, as well as the development and maintenance of immunological memory. Further understanding of the immunological effects of coinfection should provide opportunities to enhance vaccine efficacy and control of infectious diseases.

Dendritic Cells in HIV-1 and HCV Infection: Can They Help Win the Battle?

Persistent infections with human immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV) are a major cause of morbidity and mortality worldwide. As sentinels of our immune system, dendritic cells (DCs) play a central role in initiating and regulating a potent antiviral immune response. Recent advances in our understanding of the role of DCs during HIV-1 and HCV infection have provided crucial insights into the mechanisms employed by these viruses to impair DC functions in order to evade an effective immune response against them. Modulation of the immunological synapse between DC and T-cell, as well as dysregulation of the crosstalk between DCs and natural killer (NK) cells, are emerging as two crucial mechanisms. This review focuses on understanding the interaction of HIV-1 and HCV with DCs not only to understand the immunopathogenesis of chronic HIV-1 and HCV infection, but also to explore the possibilities of DC-based immunotherapeutic approaches against them. Host genetic makeup is known to play major roles in infection outcome and rate of disease progression, as well as response to anti-viral therapy in both HIV-1 and HCV-infected individuals. Therefore, we highlight the genetic variations that can potentially affect DC functions, especially in the setting of chronic viral infection. Altogether, we address if DCs’ potential as critical effectors of antiviral immune response could indeed be utilized to combat chronic infection with HIV-1 and HCV.

Suppression of NK cells and regulatory T lymphocytes in cats naturally infected with feline infectious peritonitis virus

A strong cell-mediated immunity (CMI) is thought to be indispensable for protection against infection with feline infectious peritonitis virus (FIPV) in cats. In this study, the role of natural killer (NK) cells and regulatory T cells (Tregs), central players in the innate and adaptive CMI respectively, was examined during natural FIPV infection. When quantified, both NK cells and Tregs were drastically depleted from the peripheral blood, mesenteric lymph node (LN) and spleen in FIP cats. In contrast, mesentery and kidney from FIP cats did not show any difference when compared to healthy non-infected control animals. In addition, other regulatory lymphocytes (CD4+CD25−Foxp3+ and CD3+CD8+Foxp3+) were found to be depleted from blood and LN as well. Phenotypic analysis of blood-derived NK cells in FIP cats revealed an upregulation of activation markers (CD16 and CD25) and migration markers (CD11b and CD62L) while LN-derived NK cells showed upregulation of only CD16 and CD62L. LN-derived NK cells from FIPV-infected cats were also significantly less cytotoxic when compared with healthy cats. This study reveals for the first time that FIPV infection is associated with severe suppression of NK cells and Tregs, which is reflected by cell depletion and lowered cell functionality (only NK cells). This will un-doubtfully lead to a reduced capacity of the innate immune system (NK cells) to battle FIPV infection and a decreased capacity (Tregs) to suppress the immunopathology typical for FIP. However, these results will also open possibilities for new therapies targeting specifically NK cells and Tregs to enhance their numbers and/or functionality during FIPV infection.

Fuzheng Huayu inhibits carbon tetrachloride-induced liver fibrosis in mice through activating hepatic NK cells

AIM OF THE STUDY

Fuzheng Huayu (FZHY) is a Chinese compound herbal preparation which consists of six Chinese herbs. This study examines the preventative effects of FZHY on liver fibrosis induced by carbon tetrachloride (CCl(4)) and explores its possible mechanisms of action.

MATERIALS AND METHODS

Liver fibrosis was induced in male C57BL/6N mice by injecting a 10% CCl(4) solution intraperitoneal twice a week for six weeks. After 6 weeks of treatment, serum ALT and AST assay, liver tissue histological examination and immunostaining were carried out to examine the liver function and fibrosis degree. The expression levels of alpha-smooth muscle actin (SMA) were measured by quantitative real-time PCR and western blot. Hepatic natural killer (NK) cells were isolated from liver and evaluated by FACS.

RESULTS

Upon pathological examination, the FZHY-treated mice showed significantly reduced liver damage. The expression of α-SMA increased markedly upon treatment with CCl(4) and the increase was reversed by FZHY treatment. FZHY treatment also enhanced the activation of hepatic NK cells and the production of interferon-gamma (IFN-γ). The protective effects of FZHY were reversed in the mice that were depleted of NK cells by anti-ASGM-1 Ab treatment.

CONCLUSIONS

FZHY can efficiently inhibit CCl(4)-induced liver fibrosis. Furthermore, the depletion of NK cells attenuates the protective effects of FZHY. We conclude that FZHY could be an effective drug for liver fibrosis, and its mechanism of action involves the activation of hepatic NK cells.

Control of human viral infections by natural killer cells

Natural killer (NK) cells are effector cells of the innate immune system and are important in the control of viral infections. Their relevance is reflected by the multiple mechanisms evolved by viruses to evade NK cell-mediated immune responses. Over recent years, our understanding of the interplay between NK cell immunity and viral pathogenesis has improved significantly. Here, we review the role of NK cells in the control of four important viral infections in humans: cytomegalovirus, influenza virus, HIV-1, and hepatitis C virus.

Natural killer cell functional dichotomy: a feature of chronic viral hepatitis?

Natural killer (NK) cells are involved in innate immune responses to viral infections either via direct cytotoxicity which destroys virus-infected cells or production of immunoregulatory cytokines which modulate adaptive immunity and directly inhibit virus replication. These functions are mediated by different NK subpopulations, with cytotoxicity being generally performed by CD56(dim) NK cells, whereas CD56(bright) NK cells are mainly involved in cytokine secretion. NK functional defects are usually combined so that impaired degranulation is often associated with deficient cytokine production. Innate immunity is thought to be relevant in the control of hepatitis virus infections such as hepatitis B virus (HBV) and hepatitis C virus (HCV), and recent findings reproducibly indicate that NK cells in chronic viral hepatitis are characterized by a functional dichotomy, featuring a conserved or enhanced cytotoxicity and a reduced production of interferon (IFN)-γ and tumor necrosis factor-α. In chronic HCV infection this appears to be caused by altered IFN-α signaling resulting from increased signal transducer and activator of transcription 1 (STAT1) phosphorylation, which polarizes NK cells toward cytotoxicity, and a concomitantly reduced IFN-α induced STAT4 phosphorylation yielding reduced IFN-γ mRNA levels. These previously unappreciated findings are compatible on the one hand with the inability to clear HCV and HBV from the liver and on the other they may contribute to understand why these patients are often resistant to IFN-α-based therapies.

Natural killer cell responses during viral infections: flexibility and conditioning of innate immunity by experience

Natural killer (NK) cells mediate innate defense against viral infections, but the mechanisms in place to access their functions as needed during diverse challenges while limiting collateral damage are poorly understood. Recent molecular characterization of effects mediated through infection-induced inhibitory/activating NK receptor-ligand pairs and cytokines are providing new insights into pathways regulating their responses and revealing unexpected consequences for NK cell subset effects, maintenance, proliferation and function through times overlapping with adaptive and long-lived immunity. The observations define flexible pathways for experience-induced 'conditioning' and challenge narrowly defined roles for NK cells and innate immunity as first responders with prescribed functions. They suggest that individual experiences as well as genes influence the innate immune resources available to fight off an infection.

Differentially regulated gene expression associated with hepatitis C virus clearance

Human chronic hepatitis C virus (HCV) infections pose a significant public health threat, necessitating the development of novel treatments and vaccines. HCV infections range from spontaneous resolution to end-stage liver disease. Approximately 10-30% of HCV infections undergo spontaneous resolution independent of treatment by yet-to-be-defined mechanisms. These individuals test positive for anti-HCV antibodies in the absence of detectable viral serum RNA. To identify genes associated with HCV clearance, this study compared gene expression profiles between current drug users chronically infected with HCV and drug users who cleared their HCV infection. This analysis identified 91 differentially regulated (up- or downregulated by twofold or more) genes potentially associated with HCV clearance. The majority of genes identified were associated with immune function, with the remaining genes categorized either as cancer related or 'other'. Identification of factors and pathways that may influence virus clearance will be essential to the development of novel treatment strategies.

Hepatitis C virus-mediated modulation of cellular immunity

The hepatitis C virus (HCV) is a major cause of chronic liver disease globally. A chronic infection can result in liver fibrosis, liver cirrhosis, hepatocellular carcinoma and liver failure in a significant ratio of the patients. About 170 million people are currently infected with HCV. Since 80 % of the infected patients develop a chronic infection, HCV has evolved sophisticated escape strategies to evade both the innate and the adaptive immune system. Thus, chronic hepatitis C is characterized by perturbations in the number, subset composition and/or functionality of natural killer cells, natural killer T cells, dendritic cells, macrophages and T cells. The balance between HCV-induced immune evasion and the antiviral immune response results in chronic liver inflammation and consequent immune-mediated liver injury. This review summarizes our current understanding of the HCV-mediated interference with cellular immunity and of the factors resulting in HCV persistence. A profound knowledge about the intrinsic properties of HCV and its effects on intrahepatic immunity is essential to be able to design effective immunotherapies against HCV such as therapeutic HCV vaccines.

The CXCR3(+)CD56Bright phenotype characterizes a distinct NK cell subset with anti-fibrotic potential that shows dys-regulated activity in hepatitis C

BACKGROUND

In mouse models, natural killer (NK) cells have been shown to exert anti-fibrotic activity via killing of activated hepatic stellate cells (HSC). Chemokines and chemokine receptors critically modulate hepatic recruitment of NK cells. In hepatitis C, the chemokine receptor CXCR3 and its ligands have been shown to be associated with stage of fibrosis suggesting a role of these chemokines in HCV associated liver damage by yet incompletely understood mechanisms. Here, we analyzed phenotype and function of CXCR3 expressing NK cells in chronic hepatitis C.

METHODS

Circulating NK cells from HCV-infected patients (n = 57) and healthy controls (n = 27) were analyzed with respect to CXCR3 and co-expression of different maturation markers. Degranulation and interferon-γ secretion of CXCR3(+) and CXCR3(-) NK cell subsets were studied after co-incubation with primary human hepatic stellate cells (HSC). In addition, intra-hepatic frequency of CXCR3(+) NK cells was correlated with stage of liver fibrosis (n = 15).

RESULTS

We show that distinct NK cell subsets can be distinguished based on CXCR3 surface expression. In healthy controls CXCR3(+)CD56Bright NK cells displayed strongest activity against HSC. Chronic hepatitis C was associated with a significantly increased frequency of CXCR3(+)CD56Bright NK cells which showed impaired degranulation and impaired IFN-γ secretion in response to HSC. Of note, we observed intra-hepatic accumulation of this NK cell subset in advanced stages of liver fibrosis.

CONCLUSION

We show that distinct NK cell subsets can be distinguished based on CXCR3 surface expression. Intra-hepatic accumulation of the functionally impaired CXCR3(+)CD56Bright NK cell subset might be involved in HCV-induced liver fibrosis.

NK cells from HCV-infected patients effectively induce apoptosis of activated primary human hepatic stellate cells in a TRAIL-, FasL- and NKG2D-dependent manner

In mouse models it has been shown that natural killer (NK) cells can attenuate liver fibrosis via killing of activated hepatic stellate cells (HSCs) in a NKG2D- and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-dependent manner. However, only little data exist regarding interactions of human NK cells with HSCs and their potential role in hepatitis C virus (HCV)-associated fibrogenesis. Therefore, purified NK cells from untreated HCV RNA(+) patients (n=33), interferon-α (IFN-α)-treated patients (n=17) and healthy controls (n=18) were coincubated with activated primary HSCs, and were tested for degranulation (CD107a expression) and secretion of IFN-γ and TNF-α, respectively. Induction of HSC apoptosis was analyzed using an active caspase-3 assay. We found that following coincubation with HSCs a significant increase in CD107a expression could be observed in both NK cells from HCV(+) patients and healthy controls, whereas only negligible secretion of IFN-γ and TNF-α could be detected. More importantly, NK cells from untreated HCV RNA(+) patients were significantly more effective in induction of HSC apoptosis (17.8 ± 9.2%) than NK cells from healthy controls (6.2 ± 2.1%; P<0.0001). Additionally, we observed an inverse correlation of liver fibrosis stage and the ability of NK cells to induce HSC apoptosis. Induction of HSC apoptosis was contact dependent and could partly be blocked by antibodies specific for TRAIL, NKG2D and FasL, respectively. It is noteworthy that NK cells from IFN-α-treated HCV(+) patients displayed the highest capability to kill HSCs (27.6 ± 10.5%). Accordingly, pre-stimulation of NK cells with recombinant IFN-α significantly increased the ability of NK cells to induce cell death in primary HSCs and was dependent on upregulated expression of TRAIL. Here we demonstrate that NK cells from HCV-infected patients are highly efficient in inducing apoptosis of activated HSCs. Thus, NK cells may have an important anti-fibrotic role in chronic hepatitis C.

Genetically associated CD16(+)56(-) natural killer cell interferon (IFN)-αR expression regulates signaling and is implicated in IFN-α-induced hepatitis C virus decline

BACKGROUND

Natural killer (NK) cells likely contribute to outcome of acute hepatitis C virus (HCV) infection and interferon (IFN)-induced control of chronic HCV infection. We previously observed IFN-αR and NKp30 expression associated with IFN-α-dependent NK cell activity.

METHODS

Here, we examined CD16(+)56(-), CD16(+)56(+), and CD16(-)56(+) NK cell subset IFN-αR and NKp30 expression in relation to magnitude of HCV genotype 1 decrease during pegylated IFN-α plus ribavirin therapy.

RESULTS

We observed greater baseline IFN-αR and NKp30 expression on CD16(+)56(+) and CD16(-)56(+) NK subsets in HCV-infected patients than in healthy control subjects. Baseline CD16(+)56(-) NK IFN-αR expression was associated with IFN-α-induced pSTAT1, and both were associated with magnitude of HCV decrease during pegylated IFN-α plus ribavirin therapy. Baseline CD16(+)56(-) NK IFN-αR expression was associated with race and interleukin 28B genotype, negatively associated with aspartate aminotransferase-to platelet ratio index, and positively associated with increase in NKp30 expression after in vivo IFN-α exposure. Finally, in vitro IFN-α2a-activated NK cytolysis of HCV-infected target cells was in part dependent on NKp30, and CD16(+)56(-) NK cell IFN-αR expression correlated with cytolytic activity.

CONCLUSIONS

IFN-αR expression on CD16(+)56(-) NK cells during chronic HCV infection may in part be genetically determined, and level of expression regulates IFN-α signaling, which in turn may contribute to control of HCV infection.

The effects of hepatitis C virus core protein on functional responses in the NK cell line YTS

Hepatitis C virus infection affects more than 170 million people worldwide. More than 80% of the patients are not able to eliminate the virus and progress to a chronic infection that usually culminates in complications such as cirrhosis and/or hepatocellular carcinoma. Although the adaptive immune response has been widely shown to be essential for viral clearance, the role of natural killer (NK) cells is not clearly understood. In this study, the effect of HCV core protein is examined on NK cell function, i.e., cytotoxicity and cytokine secretion. The expression of core protein in the YTS NK cell line led to an increase in the percentage of apoptotic cells soon after transduction. The surviving cells exhibited decreased cytotoxicity associated with decreases in perforin and granzyme B expression. Furthermore, the HCV core protein-transduced YTS NK cells had reduced IFNγ production as well as an altered surface receptor expression pattern. These features may correspond to a state of functional anergy similar to that seen in T cells transduced with HCV core protein. Together, these data suggest that HCV core protein may alter NK cell function.

Liver fibrosis: mechanisms of immune-mediated liver injury

Liver fibrosis and its end-stage consequence, cirrhosis, represent the final common pathway of virtually all chronic liver diseases. Research into hepatic stellate cell activation, imbalance of the extracellular matrix synthesis and degradation and the contribution of cytokines and chemokines has further elucidated the mechanisms underlying fibrosis. Furthermore, clarification of changes in host adaptive and innate immune systems has accelerated our understanding of the association between liver inflammation and fibrosis. Continued elucidation of the mechanisms of hepatic fibrosis has provided a comprehensive model of fibrosis progression and regression. This review summarizes the current concepts of improvements that have been made in the field of fibrosis.

NK cells prevalence, subsets and function in viral hepatitis C

Innate immunity appears to play an important role in the pathogenesis of viral hepatitis C. Among various cell subsets of this immunity natural killer (NK) cells raised particular interest. These cells are abundant in liver, possess significant cytotoxic potential and show links with adaptive immunity. They play important role, particularly in the acute phase of viral infections, including hepatitis C. They exhibit various types of receptors, either inhibitory or activating, that are able to react with distinct ligands on infected cells. Homozygosity of some receptors, namely KIR2DL3 reacting with recipient HLA-C1 antigens is a herald of good prognosis in hepatitis C virus (HCV) infection. In the early stage of the latter, both the prevalence and the cytotoxicity of NK cells are increased. Their inhibitory receptors are down regulated whereas activating ones are up regulated. Interferon-γ secreted by NK56(+bright) NK cells has a direct cytotoxic effect on infected hepatocytes. In contrast, in the chronic phase of HCV liver disease both, the prevalence and function of NK cells are impaired. Nevertheless, their cytotoxicity contributes to liver injury. Cells show change in the polarization profile from NK1 to NK2, manifested by secretion of immunosuppressive cytokines. Some HCV peptides are inhibitory for NK cells leading to the reduction of their antiviral activity. The unwanted effects of HCV peptides can be at least partly reversed by the antiviral therapy.

IL-10 restricts activation-induced death of NK cells during acute murine cytomegalovirus infection

IL-10 is an immunomodulatory cytokine that acts to antagonize T cell responses elicited during acute and chronic infections. Thus, the IL-10R signaling pathway provides a potential therapeutic target in strategies aimed at combating infectious diseases. In this study, we set out to investigate whether IL-10 expression had an effect on NK cells. Murine CMV infection provides the best characterized in vivo system to evaluate the NK cell response, with NK cells being critical in the early control of acute infection. Blockade of IL-10R during acute murine CMV infection markedly reduced the accumulation of cytotoxic NK cells in the spleen and lung, a phenotype associated with a transient elevation of virus DNA load. Impaired NK cell responsiveness after IL-10R blockade was attributed to elevated levels of apoptosis observed in NK cells exhibiting an activated phenotype. Therefore, we conclude that IL-10 contributes to antiviral innate immunity during acute infection by restricting activation-induced death in NK cells.

Involvement of activating NK cell receptors and their modulation in pathogen immunity

Natural Killer (NK) cells are endowed with cell-structure-sensing receptors providing inhibitory protection from self-destruction (inhibitory NK receptors, iNKRs, including killer inhibitory receptors and other molecules) and rapid triggering potential leading to functional cell activation by Toll-like receptors (TLRs), cytokine receptors, and activating NK cell receptors including natural cytotoxicity receptors (NCRs, i.e., NKp46, NKp46, and NKp44). NCR and NKG2D recognize ligands on infected cells which may be endogenous or may directly bind to some structures derived from invading pathogens. In this paper, we address the known direct or indirect interactions between activating receptors and pathogens and their expression during chronic HIV and HCV infections.

Reduced frequencies of NKp30+NKp46+, CD161+, and NKG2D+ NK cells in acute HCV infection may predict viral clearance

BACKGROUND & AIMS

While the majority of HCV-infected patients progress to chronic hepatitis, a small fraction of individuals are able to clear the virus. Resolution of infection occurs within the first few weeks to months of infection, suggesting that innate immune functions may be critical for early control. Epidemiologic data support a role for particular NK cell receptor bearing populations in this control, yet the mechanism by which NK cells respond to HCV early in infection is unknown.

METHODS

Changes in the phenotype and function of NK cells were investigated in a cohort of 43 individuals identified during various stages of HCV infection with different clinical outcomes.

RESULTS

Acute, chronic, and resolved HCV infections were characterized by an expansion of CD56(neg) NK cells. Furthermore, increased levels of HLA-C-binding KIR(+) NK cells were observed in HCV resolvers, while all stages of HCV infection were associated with reduced percentages of NKG2D(+), NKp30(+), and NKp46(+) NK cells, and a slight increase in the ability of NK cells to respond to target cells bearing the ligands for these receptors. In contrast, NKG2A(+) and CD94(+) NK cells were elevated in acute and chronic HCV infection, but not in resolved infection. Most importantly, in acute infection, lower frequencies of NKp30(+), NKp46(+), CD161(+), and NKG2D(+) NK cells were observed in patients who were subsequently able to clear HCV infection than in those becoming chronically infected.

CONCLUSIONS

These data implicate particular populations of NK cells in the early control and clearance of HCV infection.

Differential alteration of CD56(bright) and CD56 (dim) natural killer cells in frequency, phenotype, and cytokine response in chronic hepatitis C virus infection

BACKGROUND

Natural killer (NK) cells play an important role in immune responses to virus infection. The cell population consists of CD56(bright) (bright-subset) and CD56(dim) (dim-subset) subsets that possess armed functions of cytokine production and cytolysis, respectively. How these subsets are involved in chronic hepatitis C virus infection (CHC) remains obscure.

METHODS

We investigated the frequency, phenotype, and cytokine response of these subsets in blood from CHC patients and healthy subjects (HS).

RESULTS

Dim-subset, but not bright-subset, showed lower frequency in the patients than in HS. Bright-subset from the patients more frequently expressed the NKG2A/CD94 inhibitory receptor than that from HS, while both subsets from the patients expressed lower levels of the NKG2D activating receptor. Both subsets from the patients displayed a significantly higher level of the signal transducer and activator of transcription (STAT) 1, compared with the HS. Upon stimulation with interferon-α, bright-subset activated less STAT4, required for interferon-γ production, and dim-subset activated more STAT1, required for cytolysis, in the patients than in HS.

CONCLUSIONS

These results indicate alterations of NK cell subsets in frequency, phenotype, and cytokine response in CHC, which might be associated with the immune pathogenesis of CHC.

Activation of natural killer cells by hepatitis C virus particles in vitro

Little is known about the ability of hepatitis C virus (HCV) to alter early innate immune responses in infected patients. Previous studies have shown that natural killer (NK) cells are functionally impaired after interaction of recombinant HCV glycoprotein E2 with the co-stimulatory CD81 molecule in vitro; however, the functional consequences of a prolonged contact of NK cells with HCV particles have remained unclear. We have examined the phenotypes of purified, interleukin-2-activated NK cells from healthy donors and HCV genotype 1b patients after culture for 5 days with HCV pseudoparticles (HCVpp) and serum samples containing HCV genotype 1b. NK cells from healthy donors and chronic HCV patients were found to up-regulate receptors associated with activation (NKp46, NKp44, NKp30, NKG2D), while NK receptors from the killer cell immunoglobulin-like receptor family (KIR/CD158), predominantly having an inhibitory function, were significantly down-modulated after culture in the presence of HCV particles compared with control cultures of NK cells. HCV-infected sera and HCVpp elicited significantly higher secretion of the NK effector lymphokines interferon-γ and tumour necrosis factor-α. Furthermore, HCV stimulated the cytotoxic potential of NK cells from normal donors and patients. The enhanced activation of NK cells after prolonged culture with HCVpp or HCV-containing sera for 5 days suggests that these innate effector cells may play an important role in viral control during early phases of HCV infection.

Lymphocyte subset characterization associated with persistent hepatitis C virus infection and subsequent progression of liver fibrosis

This study aims to deepen the understanding of lymphocyte phenotypes related to the course of hepatitis C virus (HCV) infection and progression of liver fibrosis in a cohort of atomic bomb survivors. The study subjects comprise 3 groups: 162 HCV persistently infected, 145 spontaneously cleared, and 3,511 uninfected individuals. We observed increased percentages of peripheral blood T(H)1 and total CD8 T cells and decreased percentages of natural killer (NK) cells in the HCV persistence group compared with the other 2 groups after adjustment for age, gender, and radiation exposure dose. Subsequently, we determined that increased T(H)1 cell percentages in the HCV persistence group were significantly associated with an accelerated time-course reduction in platelet counts-accelerated progression of liver fibrosis-whereas T(C)1 and NK cell percentages were inversely associated with progression. This study suggests that T(H)1 immunity is enhanced by persistent HCV infection and that percentages of peripheral T(H)1, T(C)1, and NK cells may help predict progression of liver fibrosis.

Dysfunction of Immune Systems and Host Genetic Factors in Hepatitis C Virus Infection with Persistent Normal ALT

Patients with chronic hepatitis C (CHC) virus infection who have persistently normal alanine aminotransferase levels (PNALT) have mild inflammation and fibrosis in comparison to those with elevated ALT levels. The cellular immune responses to HCV are mainly responsible for viral clearance and the disease pathogenesis during infection. However, since the innate and adaptive immune systems are suppressed by various kinds of mechanisms in CHC patients, the immunopathogenesis of CHC patients with PNALT is still unclear. In this review, we summarize the representative reports about the immune suppression in CHC to better understand the immunopathogenesis of PNALT. Then, we summarize and speculate on the immunological aspects of PNALT including innate and adaptive immune systems and genetic polymorphisms of HLA and cytokines.

KIR/HLA interactions and pathogen immunity

The innate immune system is the first line of defence in response to pathogen infection. Natural killer (NK) cells perform a vital role in this response with the ability to directly kill infected cells, produce cytokines, and cross-talk with the adaptive immune system. These effector functions are dependent on activation of NK cells which is determined by surface receptor interactions with ligands on target cells. Of these receptors, the polymorphic killer immunoglobulin-like receptors (KIRs), which interact with MHC class 1 (also highly polymorphic), are largely inhibitory, and exhibit substantial genetic diversity. The result is a significant variation of NK cell repertoire between individuals and also between populations, with a multitude of possible KIR:HLA combinations. As each KIR:ligand interaction may have differential effects on NK cell activation and inhibition, this diversity has important potential influences on the host response to infections. Genetic studies have demonstrated associations between specific KIR:ligand combinations and the outcome of viral (and other) infections, in particular hepatitis C and HIV infection. Detailed functional studies are not required to define the mechanisms underpinning these disease associations.

Molecular mechanisms of alcoholic liver disease: innate immunity and cytokines

Alcohol consumption is a predominant etiological factor in the pathogenesis of chronic liver diseases worldwide, causing fatty liver, alcoholic hepatitis, fibrosis/cirrhosis, and hepatocellular carcinoma. In the past few decades, significant progress has been made in our understanding of the molecular mechanisms underlying alcoholic liver injury. Activation of innate immunity components such as Kupffer cells, LPS/TLR4, and complements in response to alcohol exposure plays a key role in the development and progression of alcoholic liver disease (ALD). LPS activation of Kupffer cells also produces IL-6 and IL-10 that may play a protective role in ameliorating ALD. IL-6 activates signal transducer and activator of transcription 3 (STAT3) in hepatocytes and sinusoidal endothelial cells, while IL-10 activates STAT3 in Kupffer cells/macrophages, subsequently protecting against ALD. In addition, alcohol consumption also inhibits some components of innate immunity such as natural killer (NK) cells, a type of cells that play key roles in anti-viral, anti-tumor, and anti-fibrotic defenses in the liver. Ethanol inhibition of NK cells likely contributes significantly to the pathogenesis of ALD. Understanding the roles of innate immunity and cytokines in alcoholic liver injury may provide insight into novel therapeutic targets in the treatment of alcoholic liver disease.

Hepatitis C virus (HCV) evades NKG2D-dependent NK cell responses through NS5A-mediated imbalance of inflammatory cytokines

Understanding how hepatitis C virus (HCV) induces and circumvents the host's natural killer (NK) cell-mediated immunity is of critical importance in efforts to design effective therapeutics. We report here the decreased expression of the NKG2D activating receptor as a novel strategy adopted by HCV to evade NK-cell mediated responses. We show that chronic HCV infection is associated with expression of ligands for NKG2D, the MHC class I-related Chain (MIC) molecules, on hepatocytes. However, NKG2D expression is downmodulated on circulating NK cells, and consequently NK cell-mediated cytotoxic capacity and interferon-γ production are impaired. Using an endotoxin-free recombinant NS5A protein, we show that NS5A stimulation of monocytes through Toll-like Receptor 4 (TLR4) promotes p38- and PI3 kinase-dependent IL-10 production, while inhibiting IL-12 production. In turn, IL-10 triggers secretion of TGFβ which downmodulates NKG2D expression on NK cells, leading to their impaired effector functions. Moreover, culture supernatants of HCV JFH1 replicating Huh-7.5.1 cells reproduce the effect of recombinant NS5A on NKG2D downmodulation. Exogenous IL-15 can antagonize the TGFβ effect and restore normal NKG2D expression on NK cells. We conclude that NKG2D-dependent NK cell functions are modulated during chronic HCV infection, and demonstrate that this alteration can be prevented by exogenous IL-15, which could represent a meaningful adjuvant for therapeutic intervention.

Increased natural killer cell cytotoxicity and NKp30 expression protects against hepatitis C virus infection in high-risk individuals and inhibits replication in vitro

CD56(pos) natural killer (NK)/natural T (NT) cells are important innate effectors providing the first line of defense against viral infection. Enhanced NK activity has been shown to protect from human immunodeficiency virus-1 infection. However, the role played by these innate effectors in protection against or development of hepatitis C virus (HCV) infection is unknown. We characterized CD56(pos) populations in 11 injection drug users (IDUs) who remained uninfected despite being repeatedly exposed to HCV. NK profiles in exposed but uninfected (EU) individuals were compared with preinfection samples (median 90 days prior to HCV seroconversion) collected from 14 IDUs who were exposed and subsequently became infected (EI) and unexposed normal control subjects (n = 8). Flow cytometric analysis of CD56(pos) populations demonstrated that EUs had a higher proportion of CD56(low) mature (P = 0.0011) NK cells compared with EI subjects. Bead-isolated NKs ( > 90% purity) from EUs had significantly higher interleukin-2 (IL-2)-induced cytolytic activity against the NK-sensitive cell line K562 at an effector-to-target ratio of 10:1 (P < 0.0001). NKp30, a natural cytotoxicity receptor involved in NK activation, is highest on NK/NT cells in EUs relative to infected subjects. Using the JFH-1 infection system, we demonstrated that NKp30(high) cells in the absence of exogenous stimulation significantly reduce infection of hepatocytes.

CONCLUSION

CD56(pos) populations in EUs are enriched for effector NKs displaying enhanced IL-2-induced cytolytic activity and higher levels of the natural cytotoxicity receptor NKp30-activating receptor. In addition, NKp30(high) cells are more effective in preventing infection of Huh-7.5 cells than their NKp30(low/neg) counterparts. These data support the hypothesis that NK cells contribute to anti-HCV defense in vivo in the earliest stages of infection, providing innate protection from HCV acquisition.

Increased degranulation of natural killer cells during acute HCV correlates with the magnitude of virus-specific T cell responses

BACKGROUND & AIMS

Natural killer (NK) cells provide early defense against viral infections by killing infected cells and producing cytokines that inhibit viral replication. NK cells also interact with dendritic cells (DCs) and this reciprocal interaction regulates both innate and adaptive immunity. Genetic studies have suggested that NK cell activity is a determinant of HCV infectious outcome but a functional correlation has not been established. We hypothesized that increased NK cell activity during acute HCV infection correlates with spontaneous viral clearance.

METHODS

We used multiparametric flow cytometry to monitor longitudinally the phenotype and the activity of NK cells in a cohort of intravenous drug users following HCV exposure. Three groups were studied: acute HCV with chronic evolution (n = 13), acute resolving HCV (n = 11), and exposed un-infected individuals (n = 10). We examined the expression of several NK cell-activating and -inhibiting receptors, IFN-γ production and CD107a degranulation upon stimulation, and the kinetics of NK cell responses relative to T cell responses.

RESULTS

We observed decreased expression of the inhibitory NKG2A receptor in NK cells following spontaneous HCV clearance. In addition, we observed increased NK cell degranulation during acute HCV irrespective of infectious outcome. NK cell peak responses preceded or coincided with peak T cell responses. Furthermore, NK cell degranulation correlated with the magnitude of HCV-specific T cells.

CONCLUSIONS

Our results demonstrate that NK cells are activated during acute HCV regardless of infection outcome and may play an indirect role through induction and priming of T cell responses.

Natural killer cells in viral hepatitis: facts and controversies

BACKGROUND

Hepatitis B virus (HBV) and hepatitis C virus (HCV) are major human hepatotropic pathogens responsible for a large number of chronic infections worldwide. Their persistence is thought to result from inefficiencies of innate and adaptive immune responses; however, very little information is available on the former. Natural killer (NK) cells are a major component of innate immunity and their activity is tightly regulated by several inhibitory and activating receptors.

DESIGN

In this review, we examine controversial findings regarding the role of NK cells in the pathogenesis of acute and chronic liver disease caused by HCV and HBV.

RESULTS

Recent studies built up on technical advances to identify NK receptors and their functional correlates in this setting. While NK cells seem to behave correctly during acute hepatitis, it would appear that the NK cytotoxic potential is generally conserved in chronic hepatitis, if not increased in the case of HCV. In contrast, their ability to secrete antiviral cytokines such as interferon ex vivo or after cytokine stimulation is severely impaired.

CONCLUSIONS

Current evidence suggests the existence of an NK cell functional dichotomy, which may contribute to virus persistence, while maintaining low-level chronic liver inflammation. The study of liver-infiltrating NK cells is still at the very beginning, but it is likely that it will shed more light on the role of this simple and at the same time complex innate immune cell in liver disease.

Association of NKG2A with treatment for chronic hepatitis C virus infection

Natural killer (NK) cells are critical to the immune response to viral infections. Their functions are controlled by receptors for major histocompatibility complex (MHC) class I, including NKG2A and killer-cell immunoglobulin-like receptors (KIR). In order to evaluate the role of MHC class I receptors in the immune response to hepatitis C virus infection we have studied patients with chronic HCV infection by multi-parameter flow cytometry directly ex vivo. This has permitted evaluation of combinatorial expression of activating and inhibitory receptors on single NK cells. Individuals with chronic HCV infection had fewer CD56(dim) NK cells than healthy controls (4.9 +/- 3.4% versus 9.0 +/- 5.9%, P < 0.05). Expression levels of the inhibitory receptor NKG2A was up-regulated on NK cells from individuals with chronic hepatitis C virus (HCV) (NKG2A mean fluorescence intensity 5692 +/- 2032 versus 4525 +/- 1646, P < 0.05). Twelve individuals were treated with pegylated interferon and ribavirin. This resulted in a down-regulation of NKG2A expression on CD56(dim) NK cells. Individuals with a sustained virological response (SVR) had greater numbers of NKG2A-positive, KIR-negative NK cells than those without SVR (27.6 +/- 9.6% NK cells versus 17.6 +/- 5.7, P < 0.02). Our data show that NKG2A expression is dysregulated in chronic HCV infection and that NKG2A-positive NK cells are associated with a beneficial response to pegylated interferon and ribavirin therapy.

Here today--not gone tomorrow: roles for activating receptors in sustaining NK cells during viral infections

The conclusive evidence supporting a role for NK cells in defense against viruses has been obtained under conditions of NK cell deficiencies prior to infections. NK cell proliferation can be induced during infections, but the advantages of resulting expansion have been unclear because NK cell basal frequency is already high. However, NK cell decreases are also observed during certain conditions of viral infection. Given the range of potent antiviral and immunoregulatory functions of NK cells, such "disappearance" dramatically changes the resources available to the host. New studies demonstrate that proliferation dependent on activating receptors for virus-induced ligands is key for NK cell maintenance, and allows their continued availability for control of adaptive immune responses and immunopathology. This pathway for sustaining NK cells may represent a system used generally to select subsets for rescue during homeostatic purging. In the case of NK cells, though, nonselection limits continued access to the many beneficial functions of NK cells. The observations resolve the long-standing conundrum of reported NK cell increases and decreases during viral infections. Moreover, they demonstrate a previously unappreciated role for activating receptors, i.e. to keep NK cells here today and also tomorrow.

Increased proportion of the CD56(bright) NK cell subset in patients chronically infected with hepatitis C virus (HCV) receiving interferon-alpha and ribavirin therapy

Natural killer (NK) cells are implicated in the regulation of a protective immune response in patients chronically infected with hepatitis C virus (HCV), but effects of interferon-alpha/ribavirin therapy on NK cell subsets and the consequences of viral clearance during therapy remain unclear. Samples were collected from chronically infected patients (n = 34) at baseline and from a subset after 3-10 months on pegylated interferon-alpha and ribavirin therapy (n = 19). NK cells present in cryopreserved PBMC were characterized by flow cytometry. Before therapy, the frequency of CD3-CD56+ NK cells was lower in patients than uninfected controls. Therapy increased proportions of CD56(bright) NK cells. Frequencies of CD56(dim) NK cells declined slightly while perforin and CD16 expression on CD56(dim) NK cells decreased compared to baseline samples. Evaluation of NK cell subsets at baseline did not identify patients able to achieve sustained virological response following therapy. However, therapy may promote the expansion of NK cells able to produce interferon-gamma, while minimizing cytotoxicity to limit liver damage.

Consistent beneficial effects of killer cell immunoglobulin-like receptor 2DL3 and group 1 human leukocyte antigen-C following exposure to hepatitis C virus

Natural killer cells are a key component in the immune control of viral infections. Their functions are controlled by inhibitory receptors for major histocompatability complex (MHC) class I, including the killer cell immunoglobulin-like receptors (KIR). KIR2DL3 in combination with its cognate human leukocyte antigen (HLA)-C ligand has been shown to be associated with spontaneous resolution of viremia following hepatitis C virus (HCV) infection. In order to determine if this gene combination is advantageous across all potential outcomes following HCV exposure, we studied individuals with apparent resistance to HCV infection who remain seronegative and aviremic despite long-term injection drug use and also individuals chronically infected with HCV who successfully clear HCV with treatment. Homozygosity for KIR2DL3 in combination with group 1 HLA-C allotypes was more frequent in exposed seronegative aviremic individuals as compared to those with chronic HCV (25.0% versus 9.7%, P = 0.003, odds ratio [OR] = 3.1, 95% confidence interval [CI] = 1.3-7.1) in a model similar to that found for those spontaneously resolving HCV. In individuals undergoing treatment for HCV, those with KIR2DL3 and group 1 HLA-C were more likely to make a sustained virological response (SVR) (P = 0.013, OR = 2.3, 95% CI = 1.1-4.5). KIR and HLA-C protection in both treatment response and spontaneously resolving HCV was validated at the allelic level, in which KIR2DL3-HLA-Cw*03 was associated with SVR (P = 0.004, OR = 3.4, 95% CI = 1.5-8.7) and KIR2DL3/KIR2DL3-HLA-Cw*03 was associated with spontaneous resolution of HCV infection (P = 0.01, OR = 2.3, 95% CI = 1.2-4.4).

CONCLUSION

KIR and HLA-C genes are consistently beneficial determinants in the outcome of HCV infection. This advantage extends to the allelic level for both gene families.

Activation of natural killer cells during acute infection with hepatitis C virus

BACKGROUND & AIMS

Natural killer (NK) cells are essential early after infection, not only for viral containment but also for timely and efficient induction of adaptive responses. An inhibitory effect of hepatitis C virus (HCV)-E2 proteins on NK cells has been reported, but the features of NK cell responses in the acute phase of hepatitis C are still largely undefined. Therefore, the aim of this study was to characterize the function and phenotype of NK cells in the acute phase of infection and compare individuals with chronic and self-limited outcomes.

METHODS

Twenty-two individuals with acute HCV infection, 14 with chronic evolution, and 8 with self-limited infection, were studied using NK phenotypic and functional assays.

RESULTS

An increased expression of NKG2D on both CD56(bright) and CD56(dim) NK cells was detected in patients with acute HCV, irrespective of the outcome, as compared with healthy controls. Also, interferon gamma production and cytotoxicity by NK cells were higher in individuals with acute HCV infection than in healthy controls. Subset analysis showed increased interferon gamma production in both NK cell subsets carrying group 1 and group 2 HLA-C-specific killer cell immunoglobulin-like receptors. However, increased CD107a was noted only on NK cells expressing the group 1 HLA-C-specific killer cell immunoglobulin-like receptor and was maximal in self-limited infection.

CONCLUSIONS

Our data show that in the acute phase of HCV infection, NK cells are activated regardless of outcome, with no evidence of a suppressive effect of HCV on NK cell function.

Chronic hepatitis C viral infection reduces NK cell frequency and suppresses cytokine secretion: Reversion by anti-viral treatment

Impaired activity of NK (natural killer) cells has been proposed as a mechanism contributing to viral persistence and chronic infection in hepatitis C (HCV) infection. We aimed to assess the impact of HCV infection on NK cells regarding frequency, subset distribution, and cytotoxic and cytokine secretion functions, as well as IFN-alpha and ribavirin therapeutic effects on NK cells. Significant reduction of total NK frequency and the CD56(dim)16(+) subset was observed in chronic HCV patients. IFN-gamma expression upon stimulation with K562 was severely suppressed but cytotoxicity measured by CD107a expression was maintained. These adverse effects were reversed after treatment with pegylated IFN-alpha and ribavirin; however, these skewed functions were not recovered in treatment-resistant patients. Thus, HCV chronic infection severely affects NK functions, except for cytotoxicity. Altered NK cell frequency and cytokine secretion by HCV infection may contribute to impaired cellular immune response and virus persistence.

Natural killer cells are polarized toward cytotoxicity in chronic hepatitis C in an interferon-alfa-dependent manner

BACKGROUND & AIMS

Patients with chronic hepatitis C virus (HCV) infection display great variability in disease activity and progression. Although virus-specific adaptive immune responses have been characterized extensively and found to be impaired in chronic hepatitis C, the role of innate immune responses in disease activity and progression of chronic hepatitis C is not well understood.

METHODS

We studied 42 HCV-infected patients and 12 healthy uninfected controls.

RESULTS

We found an increased frequency of natural killer (NK) cells expressing tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), NKp44, NKG2C, and CD122 in chronic hepatitis C as compared with healthy controls (P < .05 for all markers) and stronger activation of NK cells in the liver than in the blood (P < .05). This NK cell phenotype was associated with polarization of NK cell function toward CD107a expression as a marker of degranulation, but with not increased interferon (IFN)-gamma production of CD56(dim) NK cells. The polarized NK cell phenotype correlated with alanine aminotransferase levels (r(2) = 0.312, P = .03). To investigate whether in vivo exposure of NK cells to HCV-induced type I IFN was causing this NK cell phenotype, peripheral blood mononuclear cells from 10 healthy controls and 8 HCV-infected patients were stimulated in the presence of IFN-alfa, which resulted in increased NK cell expression of TRAIL and CD107a (P < .001), but not IFN-gamma.

CONCLUSIONS

Collectively, these results describe a polarized NK cell phenotype induced by chronic exposure to HCV-induced IFN-alfa. This phenotype may contribute to liver injury through TRAIL expression and cytotoxicity, whereas the lacking increase in IFN-gamma production may facilitate the inability to clear HCV.

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.

Natural killer cell functional dichotomy in chronic hepatitis B and chronic hepatitis C virus infections

BACKGROUND & AIMS

The phenotypic and functional characteristics of natural killer (NK) cells in chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are incompletely defined and largely controversial.

METHODS

We studied NK cell receptor expression, cytotoxic activity, and cytokine production in peripheral blood mononuclear cells from 35 patients with chronic hepatitis C, 22 with chronic hepatitis B, and 30 healthy controls.

RESULTS

Patients with chronic HBV infection had an increased proportion of NKG2C(+) NK cells with normal inhibitory receptor expression and a lower proportion of activated NK cells compared with HCV(+) patients, which was associated with normal or reduced cytolytic activity and markedly dysfunctional tumor necrosis factor-alpha and interferon-gamma production. Patients with chronic HCV infection showed a predominantly activating phenotype, featuring a decreased percentage of cells expressing the inhibitory receptor KIR3DL1 and a concomitant increase in the proportion of NKG2D(+) NK cells. Expression of the CD69 early activation antigen on NK cells positively correlated with serum alanine aminotransferase and HCV RNA values, suggesting participation of virus-induced effector NK cells in liver necroinflammation. Phenotypic changes in HCV(+) patients were associated with enhanced cytokine-induced cytolytic activity and increased usage of natural cytotoxicity and NKG2D receptor pathways, accompanied by defective cytokine production, although to a lesser extent than patients with chronic HBV infection.

CONCLUSIONS

These findings provide evidence for a functional dichotomy in patients with chronic HBV and HCV infections, featuring conserved or enhanced cytolytic activity and dysfunctional cytokine production, which may contribute to virus persistence.

Fine characterization of intrahepatic NK cells expressing natural killer receptors in chronic hepatitis B and C

BACKGROUND/AIMS

The fate of intrahepatic NK cell subsets in the course of HCV and HBV infections is not clearly understood.

METHODS

Blood and intrahepatic CD56(+) NK cell subsets (expressing NKG2A, CD158a,h or CD158b,j receptors) from HCV or HBV patients were quantified by flow cytometry and localized by immunohistochemistry in liver biopsies.

RESULTS

A significant reduction in NK cell frequency and a quantitative imbalance between CD56(bright) and CD56(dim) subsets were observed in chronic HCV patients as compared to HBV patients, underlining that the inflammatory environment is not the only cause of these phenomena. The proportions of intrahepatic NK cells expressing either NKG2A, and/or CD158a,h, CD158b,j differed significantly between HCV and HBV patients. A higher frequency of perforin among intrahepatic CD56(+)CD3(-) cells was observed in HCV compared to HBV patients. Double immunohistochemical staining showed that CD56(+)CD3(-) cells were localized within necrotic areas. Immune monitoring of circulating CD56 subsets revealed that CD3(-)CD56(bright)NKG2A(+) and CD3(-)CD56(dim)NKG2A(+) cells were positively correlated with the necroinflammatory score and inversely correlated with viral load, respectively, in HCV patients.

CONCLUSIONS

HCV and HBV affect NK cell subsets according to the status of the diseases, especially CD3(-)CD56(dim)NKG2A(+) and CD3(-)CD56(bright)NKG2A(+) cells, may be of interest for disease monitoring.

Activating receptors promote NK cell expansion for maintenance, IL-10 production, and CD8 T cell regulation during viral infection

Natural killer (NK) cells have the potential to deliver both direct antimicrobial effects and regulate adaptive immune responses, but NK cell yields have been reported to vary greatly during different viral infections. Activating receptors, including the Ly49H molecule recognizing mouse cytomegalovirus (MCMV), can stimulate NK cell expansion. To define Ly49H's role in supporting NK cell proliferation and maintenance under conditions of uncontrolled viral infection, experiments were performed in Ly49h^−/−, perforin 1 (Prf1)^−/−, and wild-type (wt) B6 mice. NK cell numbers were similar in uninfected mice, but relative to responses in MCMV-infected wt mice, NK cell yields declined in the absence of Ly49h and increased in the absence of Prf1, with high rates of proliferation and Ly49H expression on nearly all cells. The expansion was abolished in mice deficient for both Ly49h and Prf1 (Ly49h^−/−Prf1^−/−), and negative consequences for survival were revealed. The Ly49H-dependent protection mechanism delivered in the absence of Prf1 was a result of interleukin 10 production, by the sustained NK cells, to regulate the magnitude of CD8 T cell responses. Thus, the studies demonstrate a previously unappreciated critical role for activating receptors in keeping NK cells present during viral infection to regulate adaptive immune responses.

Impaired plasmacytoid dendritic cell (PDC)-NK cell activity in viremic human immunodeficiency virus infection attributable to impairments in both PDC and NK cell function

Human immunodeficiency virus (HIV) and hepatitis C virus (HCV) infections impair plasmacytoid dendritic cell (PDC) and natural killer (NK) cell subset numbers and functions, though little is known about PDC-NK cell interactions during these infections. We evaluated PDC-dependent NK cell killing and gamma interferon (IFN-gamma) and granzyme B production, using peripheral blood mononuclear cell (PBMC)-based and purified cell assays of samples from HCV- and HIV-infected subjects. CpG-enhanced PBMC killing and IFN-gamma and granzyme B activity (dependent on PDC and NK cells) were impaired in viremic HIV infection. In purified PDC-NK cell culture experiments, CpG-enhanced, PDC-dependent NK cell activity was cell contact and IFN-alpha dependent, and this activity was impaired in viremic HIV infection but not in HCV infection. In heterologous PDC-NK cell assays, impaired PDC-NK cell killing activity was largely attributable to an NK cell defect, while impaired PDC-NK cell IFN-gamma-producing activity was attributable to both PDC and NK cell defects. Additionally, the response of NK cells to direct IFN-alpha stimulation was defective in viremic HIV infection, and this defect was not attributable to diminished IFN-alpha receptor expression, though IFN-alpha receptor and NKP30 expression was closely associated with killer activity in viremic HIV infection but not in healthy controls. These data indicate that during uncontrolled HIV infection, PDC-dependent NK cell function is impaired, which is in large part attributable to defective IFN-alpha-induced NK cell activity and not to altered IFN-alpha receptor, NKP30, NKP44, NKP46, or NKG2D expression.