Increased natural cytotoxicity receptor expression and relevant IL-10 production in NK cells from chronically infected viremic HCV patients

Peginterferon Alfa-2a/Ribavirin treatment efficacy in chronic hepatitis C patients is related to natural killer group 2D gene rs1049174 GC polymorphism

Natural killer group 2D (NKG2D), as an activating receptor, plays pivotal role in viral infectious diseases. Several single nucleotide polymorphisms (SNPs) in the NKG2D gene have characterized that the rs1049174G/C SNP of NKG2D is in the spotlight of notice because of its role in activating of human T cells. This study aimed to investigate rs1049174G/C genetic polymorphism in Chronic Hepatitis C (CHC) patients. The study compromised 107 CHC patients with genotype 1a and 1b. All recruited patients were under treatment with Peginterferon Alfa-2a/Ribavirin according to standard protocol. After completing treatment, 67 patients showed sustained virologic response (SVR) and the rest of patients did not respond to the treatment and considered as non-responder (NR). Genotyping of NKG2D rs1049174G/C SNP was performed using PCR-RFLP method in SVR and NR patients. The NKG2D rs1049174 genotypes frequency for GG, GC and CC were 45, 41 and 14 % respectively. Genotypes distribution were significantly different between SVR and NR groups (p = 0.005). So that the patients with the homozygous GG genotype demonstrated a higher response to Peginterferon Alfa-2a/Ribavirin therapy against HCV infection (OR = 6.0, 95 %CI 1.71-21.08, p = 0.005). In conclusion, the rs1049174 GG genotype of NKG2D receptor is an effective factor in successfully treatment of CHC patients by Peginterferon Alfa-2a/Ribavirin.

Spontaneous and natural cytotoxicity receptor-mediated cytotoxicity are effector functions of distinct natural killer subsets in hepatitis C virus-infected chimpanzees

In humans, CD16 and CD56 are used to identify functionally distinct natural killer (NK) subsets. Due to ubiquitous CD56 expression, this marker cannot be used to distinguish between NK cell subsets in chimpanzees. Therefore, functional analysis of distinct NK subsets during hepatitis C virus (HCV) infection has never been performed in these animals. In the present study an alternative strategy was used to identify four distinct NK subsets on the basis of the expression of CD16 and CD94. The expression of activating and inhibiting surface receptors showed that these subsets resemble human NK subsets. CD107 expression was used to determine degranulation of the different subsets in naive and HCV-infected chimpanzees. In HCV-infected chimpanzees increased spontaneous cytotoxicity was observed in CD94(high/dim) CD16(pos) and CD94(low) CD16(pos) subsets. By contrast, increased natural cytotoxicity receptor (NCR)- mediated degranulation after NKp30 and NKp44 triggering was demonstrated in the CD94(dim) CD16(neg) subset. Our findings suggest that spontaneous and NCR-mediated cytotoxicity are effector functions of distinct NK subsets in HCV-infected chimpanzees.

Increased CD56(bright) NK cells in HIV-HCV co-infection and HCV mono-infection are associated with distinctive alterations of their phenotype

Background

HIV-HCV co-infection is associated with accelerated progression to hepatic fibrosis, cirrhosis and hepatocellular carcinoma than HCV mono-infection. The contribution of innate immunity during HIV-HCV co-infection has been a relatively under-investigated area. Natural killer (NK) cells are pivotal sentinels of innate immunity against viruses and tumour cells. In this study we evaluated the effect of HIV-HCV co-infection on peripheral blood NK cell subsets with emphasis on the phenotype of CD56^bright NK cells.

Methods

Sixty patients were included in the study; HIV mono-infected (n = 12), HCV mono-infected (n = 15), HCV-HIV co-infected (n = 21) and healthy controls (n = 16). PBMCs were isolated and immunophenotyping of NK cells was performed by flowcytometry.

Results

We observed an expansion of CD56^bright NK cell subset in HIV-HCV co-infection as compared to healthy controls and HIV mono-infected group. All the infected groups had an upregulated expression of the activating receptor NKG2D on CD56^bright NK cells in comparison to healthy controls while not differing amongst themselves.

The expression of NKp46 in HIV-HCV co-infected group was significantly upregulated as compared to both HIV as well as HCV mono-infections while NKp30 expression in the HIV-HCV co-infected group significantly differed as compared to HIV mono-infection. The CD56^bright NK cell subset was activated in HIV-HCV co-infection as assessed by the expression of CD69 as compared to healthy controls but was significantly downregulated in comparison to HIV mono-infection. CD95 expression on CD56^bright NK cells followed the same pattern where there was an increased expression of CD95 in HIV mono-infection and HIV-HCV co-infection as compared to healthy controls. In contrast to CD69 expression, CD95 expression in HCV mono-infection was decreased when compared to HIV mono-infection and HIV-HCV co-infection. Finally, expression of CXCR3 on CD56^bright NK cells was increased in HIV-HCV co-infection in comparison to HIV mono-infection while remaining similar to HCV mono-infection.

Conclusion

Thus, HIV-HCV co-infection is able to modulate the phenotype of CD56^bright NK cell subset in a unique way such that NKp46 and CXCR3 expressions are distinct for co-infection while both mono-infections have an additive effect on CD56^bright, CD69 with CD95 expressions. HCV mono-infection has a dominant effect on NKp30 expression while NKG2D and CD127 expressions remained same in all the groups.

Impact of natural killer cells on chronic hepatitis C and hepatocellular carcinoma

Natural killer (NK) cells are involved in the pathogenesis of hepatitis C viral (HCV) infection and hepatocellular carcinoma (HCC). Recent immunological progresses have revealed the molecular mechanisms of activation or inhibition of NK cells. In patients infected with HCV, the percentages of NK cells are decreased and the NK receptor expression and function of NK cells including cytotoxicity and cytokine production are altered. These alterations in NK cells are associated with persistent infection with HCV, liver injury, liver fibrosis and liver carcinogenesis. In HCV treatment, NK cells play a role in the eradication of HCV in both interferon (IFN)-based therapy and IFN-free therapy using direct-acting antivirals (DAA). In HCC patients, the exhaustion of NK cells that represents lower cytotoxicity and impaired cytokine production may contribute to the progression of HCC. Several immunotherapies targeting NK cells have been reported. NK cell transfer and NK-activating gene therapy have been demonstrated to be effective in mouse liver cancer models and several clinical trials are ongoing. Recently, the role of major histocompatibility complex class I-related chain A (MICA), a human ligand of NKG2D, has attracted attention in the development of HCC. The expression of MICA could be controlled by anti-HCC drugs including sorafenib. A new chemo-immunotherapy may be expected in the treatment of HCC. In this review, we summarize the impact of NK cells on chronic hepatitis C and HCC.

Natural killer cells in hepatitis C: Current progress

Patients infected with the hepatitis C virus (HCV) are characterized by a high incidence of chronic infection, which results in chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. The functional impairment of HCV-specific T cells is associated with the evolution of an acute infection to chronic hepatitis. While T cells are the important effector cells in adaptive immunity, natural killer (NK) cells are the critical effector cells in innate immunity to virus infections. The findings of recent studies on NK cells in hepatitis C suggest that NK cell responses are indeed important in each phase of HCV infection. In the early phase, NK cells are involved in protective immunity to HCV. The immune evasion strategies used by HCV may target NK cells and might contribute to the progression to chronic hepatitis C. NK cells may control HCV replication and modulate hepatic fibrosis in the chronic phase. Further investigations are, however, needed, because a considerable number of studies observed functional impairment of NK cells in chronic HCV infection. Interestingly, the enhanced NK cell responses during interferon-α-based therapy of chronic hepatitis C indicate successful treatment. In spite of the advances in research on NK cells in hepatitis C, establishment of more physiological HCV infection model systems is needed to settle unsolved controversies over the role and functional status of NK cells in HCV infection.

Clinical impact of the NKp30/B7-H6 axis in high-risk neuroblastoma patients

The immunosurveillance mechanisms governing high-risk neuroblastoma (HR-NB), a major pediatric malignancy, have been elusive. We identify a potential role for natural killer (NK) cells, in particular the interaction between the NK receptor NKp30 and its ligand, B7-H6, in the metastatic progression and survival of HR-NB after myeloablative multimodal chemotherapy and stem cell transplantation. NB cells expressing the NKp30 ligand B7-H6 stimulated NK cells in an NKp30-dependent manner. Serum concentration of soluble B7-H6 correlated with the down-regulation of NKp30, bone marrow metastases, and chemoresistance, and soluble B7-H6 contained in the serum of HR-NB patients inhibited NK cell functions in vitro. The expression of distinct NKp30 isoforms affecting the polarization of NK cell functions correlated with 10-year event-free survival in three independent cohorts of HR-NB in remission from metastases after induction chemotherapy (n = 196, P < 0.001), adding prognostic value to known risk factors such as N-Myc amplification and age >18 months. We conclude that the interaction between NKp30 and B7-H6 may contribute to the fate of NB patients and that both the expression of NKp30 isoforms on circulating NK cells and the concentration of soluble B7-H6 in the serum may be clinically useful as biomarkers for risk stratification.

Immunopathogenesis of Hepatitis C Virus Infection

Despite advances in therapy, hepatitis C virus infection remains a major global health issue with 3 to 4 million incident cases and 170 million prevalent chronic infections. Complex, partially understood, host-virus interactions determine whether an acute infection with hepatitis C resolves, as occurs in approximately 30% of cases, or generates a persistent hepatic infection, as occurs in the remainder. Once chronic infection is established, the velocity of hepatocyte injury and resultant fibrosis is significantly modulated by immunologic as well as environmental factors. Immunomodulation has been the backbone of antiviral therapy despite poor understanding of its mechanism of action.

Roles of natural killer cells in antiviral immunity

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NK cells can kill virus-infected cells and protect against severe infections.

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Long-lived memory NK cells may develop after vaccination or infection.

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NK cells are potent regulatory of antiviral T and B cell responses.

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The role of NK cells in human infection is complex and context-dependent.

Natural killer (NK) cells are important in immune defense against virus infections. This is predominantly considered a function of rapid, innate NK-cell killing of virus-infected cells. However, NK cells also prime other immune cells through the release of interferon gamma (IFN-γ) and other cytokines. Additionally, NK cells share features with long-lived adaptive immune cells and can impact disease pathogenesis through the inhibition of adaptive immune responses by virus-specific T and B cells. The relative contributions of these diverse and conflicting functions of NK cells in humans are poorly defined and likely context-dependent, thereby complicating the development of therapeutic interventions. Here we focus on the contributions of NK cells to disease in diverse virus infections germane to human health.

Contradicting roles of miR-182 in both NK cells and their host target hepatocytes in HCV

BACKGROUND AND AIM

Natural killer cells are part of the innate immunity involved in viral eradication and were shown to be greatly affected by HCV infection. Epigenetic regulation of NK cell function by microRNAs was not efficiently studied before and was never studied in HCV infection; therefore the aim of this study was to assess for the first time the role of microRNAs in regulating the function of NK cells of HCV-infected patients and hence viral replication in the target HCV-infected Huh7 cells.

METHODOLOGY

NK cells were isolated from PBMCs of HCV-infected patients as well as controls, and HCV-infected liver biopsies as well as Huh7 cells infected with the virus were used. For the infection of Huh7 cells, first viral vector was in-vitro transcribed into viral RNA that was then used to infect naїve Huh7 cells. Supernatant from the infected cells was then collected and used for further infection. For manipulation of NK cells or Huh7 cells, miR-182 mimics and inhibitors were transfected via lipofection method. RNA was extracted from each cell population, reverse transcribed. Gene expression as well as viral load was quantified using qPCR.

RESULTS

Screening of NKG2A and NKG2D between patients and controls showed no difference in expression of NKG2A, while NKG2D was found to be downregulated. In view of that, bioinformatics analysis was performed and showed that miR-182 has potential binding sites on both the inhibitory receptor NKG2A and the activating receptor NKG2D, and on its ligand ULBP2, as well as on the viral genome itself. In NK cells of HCV-infected patients, miR-182 was found to be over-expressed compared to controls; its ectopic expression was found to decrease NKG2D mRNA level, while miR-182 inhibitors were able to decrease NKG2A mRNA compared to untransfected cells. In addition, co-culturing genotype 4 or 2 HCV-infected Huh7 cells with miR-182 mimicked NK cells of HCV-infected patients showed decreased viral replication, suggesting an enhanced NK cell function. On the other hand, miR-182 and ULBP2 were both found to be downregulated in HCV liver tissues and HCV-infected Huh7 cells compared to their controls. miR-182 mimics were found to decrease ULBP2 mRNA and increase viral replication in genotypes 4 and 2 HCV-infected target (Huh7) cells compared to controls, while miR-182 inhibitor decreased viral replication in the cell models.

CONCLUSION

miR-182 was never investigated before, neither in HCV infection nor in NK cells, and we found it to have dysregulated expression in both liver tissues and NK cells of HCV-infected patients compared to control. In addition to that, miR-182 was found to have a contradicting effect in both effector cell and its HCV-infected target cell regarding HCV replication.

'Emergency exit' of bone-marrow-resident CD34(+)DNAM-1(bright)CXCR4(+)-committed lymphoid precursors during chronic infection and inflammation

During chronic inflammatory disorders, a persistent natural killer (NK) cell derangement is observed. While increased cell turnover is expected, little is known about whether and how NK-cell homeostatic balance is maintained. Here, flow cytometric analysis of peripheral blood mononuclear cells in chronic inflammatory disorders, both infectious and non-infectious, reveals the presence of a CD34(+)CD226(DNAM-1)(bright)CXCR4(+) cell population displaying transcriptional signatures typical of common lymphocyte precursors and giving rise to NK-cell progenies with high expression of activating receptors and mature function and even to α/β T lymphocytes. CD34(+)CD226(bright)CXCR4(+) cells reside in bone marrow, hardly circulate in healthy donors and are absent in cord blood. Their proportion correlates with the degree of inflammation, reflecting lymphoid cell turnover/reconstitution during chronic inflammation. These findings provide insight on intermediate stages of NK-cell development, a view of emergency recruitment of cell precursors, and upgrade our understanding and monitoring of chronic inflammatory conditions.

Natural killer cell dysfunction in hepatocellular carcinoma and NK cell-based immunotherapy

The mechanisms linking hepatitis B virus (HBV) and hepatitis C virus (HCV) infection to hepatocellular carcinoma (HCC) remain largely unknown. Natural killer (NK) cells account for 25%–50% of the total number of liver lymphocytes, suggesting that NK cells play an important role in liver immunity. The number of NK cells in the blood and tumor tissues of HCC patients is positively correlated with their survival and prognosis. Furthermore, a group of NK cell-associated genes in HCC tissues is positively associated with the prolonged survival. These facts suggest that NK cells and HCC progression are strongly associated. In this review, we describe the abnormal NK cells and their functional impairment in patients with chronic HBV and HCV infection, which contribute to the progression of HCC. Then, we summarize the association of NK cells with HCC based on the abnormalities in the numbers and phenotypes of blood and liver NK cells in HCC patients. In particular, the exhaustion of NK cells that represents lower cytotoxicity and impaired cytokine production may serve as a predictor for the occurrence of HCC. Finally, we present the current achievements in NK cell immunotherapy conducted in mouse models of liver cancer and in clinical trials, highlighting how chemoimmunotherapy, NK cell transfer, gene therapy, cytokine therapy and mAb therapy improve NK cell function in HCC treatment. It is conceivable that NK cell-based anti-HCC therapeutic strategies alone or in combination with other therapies will be great promise for HCC treatment.

NK cell function and receptor diversity in the context of HCV infection

Hepatitis C virus (HCV) infects over 170 million people in the world. While a minority of individuals are able to naturally clear this hepatotropic virus using their immune system, most people go on to develop a lifetime chronic infection that can result in severe liver pathology, potentially leading to liver cirrhosis and hepatic cellular carcinoma. Investigations into acute immune responses and spontaneous clearance of the virus are severely hampered by difficulties in identification of relevant patient cohorts. While the role for the adaptive immune response in viral clearance is well established, it is becoming clear that the innate immune system also impacts on HCV outcome. The innate immune response to infection is likely to influence the type of adaptive immune response that develops and will ultimately influence if the virus is cleared or develops into a chronic infection. Natural Killer (NK) cells are lymphocytes that have important anti-viral functions including direct cytotoxicity of infected cells and the production of inflammatory cytokines, e.g., IFN-γ. They are generally considered to be cells of the innate immune system, although there is increasing evidence that NK cells adapt and persist in response to particular viral infections. NK cells are altered in patients with acute and chronic HCV infection. There is increasing evidence from both cellular and genetic studies that NK cells modulate HCV outcome. This review will describe and discuss the current experimental and clinical evidence of a role for NK cells in HCV infection and describe recent discoveries that are likely to play a role in future research.

Natural Killer Cells in Viral Hepatitis

Natural killer (NK) cells are traditionally regarded as first-line effectors of the innate immune response, but they also have a distinct role in chronic infection. Here, we review the role of NK cells against hepatitis C virus (HCV) and hepatitis B virus (HBV), two agents that cause acute and chronic hepatitis in humans. Interest in NK cells was initially sparked by genetic studies that demonstrated an association between NK cell-related genes and the outcome of HCV infection. Viral hepatitis also provides a model to study the NK cell response to both endogenous and exogenous type I interferon (IFN). Levels of IFN-stimulated genes increase in both acute and chronic HCV infection and pegylated IFNα has been the mainstay of HCV and HBV treatment for decades. In chronic viral hepatitis, NK cells display decreased production of antiviral cytokines. This phenotype is found in both HCV and HBV infection but is induced by different mechanisms. Potent antivirals now provide the opportunity to study the reversibility of the suppressed cytokine production of NK cells in comparison with the antigen-induced defect in IFNγ and tumor necrosis factor-α production of virus-specific T cells. This has implications for immune reconstitution in other conditions of chronic inflammation and immune exhaustion, such as human immunodeficiency virus infection and cancer.

Role of NK cells in immunotherapy and virotherapy of solid tumors

Although natural killer (NK) cells are endowed with powerful cytolytic activity against cancer cells, their role in different therapies against solid tumors has not yet been fully elucidated. Their interactions with various elements of the tumor microenvironment as well as their possible effects in contributing to and/or limiting oncolytic virotherapy render this potential immunotherapeutic tool still difficult to exploit at the bedside. Here, we will review the current literature with the aim of providing new hints to manage this powerful cell type in future innovative therapies, such as the use of NK cells in combination with new cytokines, specific mAbs (inducing ADCC), Tyr-Kinase inhibitors, immunomodulatory drugs and/or the design of oncolytic viruses aimed at optimizing the effect of NK cells in virotherapy.

Boosting vaccine efficacy the natural (killer) way

Coordination of the innate and adaptive immune systems is paramount to the development of protective humoral and cellular immunity following vaccination. Natural killer (NK) cells are front-line soldiers of the innate immune system, and recent studies have revealed functions for NK cells in long-lived immune memory and the regulation of adaptive immune responses. These findings suggest that NK cells may play important roles in the development of efficacious vaccines, as well as, in some contexts, failed immunizations. Here, we review the current understanding of the immunomodulatory and memory differentiation capabilities of NK cells. We examine the context dependency of the mechanisms and the nature of NK cell-mediated modulation of the immune response, and discuss how these insights may impact immunization strategies and the development of next-generation vaccines.

NKp30 isoforms in patients with chronic hepatitis C virus infection

Natural killer (NK) cells play an important role in virus infection, their action being regulated by several activating and inhibitory receptors. The NKp30 activating receptor and its isoforms have recently emerged as important determinants of efficient NK cell responses. We determined the relative proportions of NKp30 isoforms in patients with chronic hepatitis C virus (HCV) infection and healthy donors (HD). NK cell function (degranulation and cytokine production) and correlations with clinical parameters were assessed following unsupervised hierarchical clustering of patients according to isoform expression. NKp30 receptor expression on NK cells and all isoforms were reduced in HCV-infected patients. Patients were clustered into two groups: the HCV-1 group had similar isoform expression to the HD group, whereas the HCV-2 group had lower expression. The latter showed a better functional activity, and a higher proportion of the activating a isoform and of the NKp30 isoform a/c ratio compared with the HCV-1 cluster. There was a positive correlation between the activating a isoform and liver stiffness and an inverse relationship between the immunosuppressive c isoform and the fibrosis 4 score, suggesting a potentially important role of NKp30 isoforms in influencing liver damage and ensuing fibrosis.

Successful Interferon-Free Therapy of Chronic Hepatitis C Virus Infection Normalizes Natural Killer Cell Function

BACKGROUND & AIMS

Chronic hepatitis C virus infection activates an intrahepatic immune response, leading to increased expression of interferon (IFN)-stimulated genes and activation of natural killer (NK) cells-the most prevalent innate immune cell in the liver. We investigated whether the elimination of hepatitis C virus with direct-acting antiviral normalizes expression of IFN-stimulated genes and NK cell function.

METHODS

We used multicolor flow cytometry to analyze NK cells from the liver and blood of 13 HCV-infected patients who did not respond to treatment with pegylated interferon and ribavirin. Samples were collected before and during IFN-free treatment with daclatasvir and asunaprevir and compared with samples from the blood of 13 healthy individuals (controls). Serum levels of chemokine C-X-C motif ligand (CXCL) 10 or CXCL11 were measured by enzyme-linked immunosorbent assay.

RESULTS

Before treatment, all patients had increased levels of CXCL10 or CXCL11 and a different NK cell phenotype from controls, characterized by increased expression of HLA-DR, NKp46, NKG2A, CD85j, signal transducer and activator of transcription 1 (STAT1), phosphorylated STAT1, and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). NK cells from patients also had increased degranulation and decreased production of IFNγ and tumor necrosis factor α compared with NK cells from controls. Nine patients had an end-of-treatment response (undetectable virus) and 4 had virologic breakthrough between weeks 4 and 12 of therapy. A rapid decrease in viremia and level of inflammatory cytokines in all patients was associated with decreased activation of intrahepatic and blood NK cells; it was followed by restoration of a normal NK cell phenotype and function by week 8 in patients with undetectable viremia. This normalized NK cell phenotype was maintained until week 24 (end of treatment).

CONCLUSIONS

Direct-acting antiviral-mediated clearance of HCV is associated with loss of intrahepatic immune activation by IFNα, which is indicated by decreased levels of CXCL10 and CXCL11 and normalization of NK cell phenotype and function.

NK cell receptor imbalance and NK cell dysfunction in HBV infection and hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is currently the third leading cause of cancer mortality and a common poor-prognosis malignancy due to postoperative recurrence and metastasis. There is a significant correlation between chronic hepatitis B virus (HBV) infection and hepatocarcinogenesis. As the first line of host defense against viral infections and tumors, natural killer (NK) cells express a large number of immune recognition receptors (NK receptors (NKRs)) to recognize ligands on hepatocytes, liver sinusoidal endothelial cells, stellate cells and Kupffer cells, which maintain the balance between immune response and immune tolerance of NK cells. Unfortunately, the percentage and absolute number of liver NK cells decrease significantly during the development and progression of HCC. The abnormal expression of NK cell receptors and dysfunction of liver NK cells contribute to the progression of chronic HBV infection and HCC and are significantly associated with poor prognosis for liver cancer. In this review, we focus on the role of NK cell receptors in anti-tumor immune responses in HCC, particularly HBV-related HCC. We discuss specifically how tumor cells evade attack from NK cells and how emerging understanding of NKRs may aid the development of novel treatments for HCC. Novel mono- and combination therapeutic strategies that target the NK cell receptor-ligand system may potentially lead to successful and effective immunotherapy in HCC.Cellular & Molecular Immunology advance online publication, 6 October 2014; doi:10.1038/cmi.2014.91.

Immunoregulatory cytokine networks: 60 years of learning from murine cytomegalovirus

Innate immunity defends against infection but also mediates immunoregulatory effects shaping innate and adaptive responses. Studies of murine cytomegalovirus (MCMV) infections have helped elucidate the mechanisms inducing, as well as the elicited soluble and cellular networks contributing to, innate immunity. Specialized receptors are engaged by infection-induced structures to stimulate production of key innate cytokines. These then stimulate cytokine and cellular responses such as activation of natural killer (NK) cells to mediate elevated killing by type 1 interferon (IFN) and/or to produce the pro-inflammatory and antiviral cytokine IFN-γ by interleukin 12 (IL-12). An inter-systemic loop, with IL-6 inducing glucocorticoid release, negatively regulates these early cytokine responses. As infections advance into periods of overlapping innate and adaptive responses, however, the cells are intrinsically conditioned to modify the biological effects of exposure to individual cytokines. Some pathways are turned off to inhibit an existing, whereas others are broadened for acquisition of a new, response function. Remarkably, extended NK cell proliferation during MCMV infection is associated with epigenetic modifications shifting the state of the inhibitory cytokine IL-10 gene from closed to open and results in their becoming equipped to produce this cytokine. When induced, NK cell IL-10 negatively regulates the magnitude of adaptive responses to protect against immune pathology. Thus, innate immunoregulatory cytokine networks are integral to pro-inflammatory and defense functions, but responding cells have the flexibility to undergo cell intrinsic conditioning with changing network characteristics to result in a new negative immunoregulatory function, and consequently, both promote beneficial and limit detrimental immune responses.

Hepatitis C viral infection is associated with activated cytolytic natural killer cells expressing high levels of T cell immunoglobulin- and mucin-domain-containing molecule-3

T cell immunoglobulin- and mucin-domain-containing molecule-3 (Tim-3) is an inhibitory receptor implicated in T cell exhaustion characteristic of chronic viral infection. Limited data exist on NK cell Tim-3 expression and functional consequences. In chronic hepatitis C virus (HCV)-infected subjects, we found increased Tim-3 on NKs, which was associated with an activated phenotype. The high level of Tim-3 was not reversed by successful IFN-alpha-based antiviral therapy. Tim-3(high) NK cells up-regulated TRAIL in response to IFN-alpha to a greater extent and demonstrated greater lymphokine-activated killing activity, viral control, and degranulation but similar cytokine production than their Tim-3(low) counterparts. Our results suggest that Tim-3 on NKs is associated with activation of this innate lymphocyte population that is polarized towards cytotoxicity in chronic HCV. These findings reveal roles for Tim-3 in the regulation of NKs that might represent targets for treatment of chronic viral infections.

Inherent transcriptional signatures of NK cells are associated with response to IFNα + rivabirin therapy in patients with Hepatitis C Virus

BACKGROUND

Differences in the expression of Natural Killer cell receptors have been reported to reflect divergent clinical courses in patients with chronic infections or tumors. However, extensive molecular characterization at the transcriptional level to support this view is lacking. The aim of this work was to characterize baseline differences in purified NK cell transcriptional activity stratified by response to treatment with PEG-IFNα/RBV in patients chronically infected with HCV.

METHODS

To this end we here studied by flow cytometer and gene expression profile, phenotypic and transcriptional characteristics of purified NK cells in patients chronically infected with HCV genotype-1 virus who were subsequently treated with PEG-IFNα/RBV. Results were further correlated with divergent clinical response obtained after treatment.

RESULTS

The pre-treatment transcriptional patterns of purified NK cells from patients subsequently undergoing a sustained virologic response (SVR) clearly segregated from those of non-responder (NR) patients. A set of 476 transcripts, including molecules involved in RNA processing, ubiquitination pathways as well as HLA class II signalling were differently expressed among divergent patients. In addition, treatment outcome was associated with differences in surface expression of NKp30 and NKG2D. A complex relationship was observed that suggested for extensive post-transcriptional editing. Only a small number of the NK cell transcripts identified were correlated with chronic HCV infection/replication indicating that inherent transcriptional activity prevails over environment effects such as viral infection.

CONCLUSIONS

Collectively, inherent/genetic modulation of NK cell transcription is involved in setting the path to divergent treatment outcomes and could become useful to therapeutic advantage.

Tropism of human pegivirus (formerly known as GB virus C/hepatitis G virus) and host immunomodulation: insights into a highly successful viral infection

Human pegivirus (HPgV; originally called GB virus C/hepatitis G virus) is an RNA virus within the genus Pegivirus of the family Flaviviridae that commonly causes persistent infection. Worldwide, ~750 million people are actively infected (viraemic) and an estimated 0.75-1.5 billion people have evidence of prior HPgV infection. No causal association between HPgV and disease has been identified; however, several studies described a beneficial relationship between persistent HPgV infection and survival in individuals infected with human immunodeficiency virus. The beneficial effect appeared to be related to a reduction in host immune activation. HPgV replicates well in vivo (mean plasma viral loads typically >1×107 genome copies ml-1); however, the virus grows poorly in vitro and systems to study this virus are limited. Consequently, mechanisms of viral persistence and host immune modulation remain poorly characterized, and the primary permissive cell type(s) has not yet been identified. HPgV RNA is found in liver, spleen, bone marrow and PBMCs, including T- and B-lymphocytes, NK-cells, and monocytes, although the mechanism of cell-to-cell transmission is unclear. HPgV RNA is also present in serum microvesicles with properties of exosomes. These microvesicles are able to transmit viral RNA to PBMCs in vitro, resulting in productive infection. This review summarizes existing data on HPgV cellular tropism and the effect of HPgV on immune activation in various PBMCs, and discusses how this may influence viral persistence. We conclude that an increased understanding of HPgV replication and immune modulation may provide insights into persistent RNA viral infection of humans.

Mechanisms regulating NK cell activation during viral infection

ABSTRACT 

NK cells constitute a population of lymphocytes involved in innate immune functions. They play a critical role in antiviral immune surveillance. Viruses have evolved with their host species for millions of years, each exerting a selective pressure upon the other. As a corollary, the pathways used by the immune system that are critical to control viral infection can be revealed by defining the role of viral gene products that are nonessential for virus replication. We relate here the battery of resources available to NK cells to recognize and eliminate viruses and reciprocally the immune evasion mechanisms developed by viruses to prevent NK cell activation.

Immunological aspects of antiviral therapy of chronic hepatitis B virus and hepatitis C virus infections

Hepatitis B virus (HBV) and hepatitis C virus (HCV) cause a large proportion of acute and chronic liver disease worldwide. Over the past decades many immunological studies defined host immune responses that mediate spontaneous clearance of acute HBV and HCV infection. However, host immune responses are also relevant in the context of treatment-induced clearance of chronic HBV and HCV infection. First, the pretreatment level of interferon-stimulated genes as well as genetic determinants of innate immune responses, such as single nucleotide polymorphisms near the IFNL3 gene, are strong predictors of the response to interferon-alpha (IFN-α)-based therapy. Second, IFN-α, which has been a mainstay of HBV and HCV therapy over decades, and ribavirin, which has also been included in interferon-free direct antiviral therapy for HCV, modulate host immune responses. Third, both IFN-α-based and IFN-α-free treatment regimens of HBV and HCV infection alter the short-term and long-term adaptive immune response against these viruses. Finally, treatment studies have not just improved the clinical outcomes, but also provided opportunities to study virus-host interaction. This review summarizes our current knowledge on how a patient's immune response affects the treatment outcome of HBV and HCV infection and how innate and adaptive immune responses themselves are altered by the different treatment regimens.

NK cells and interferons

The role of Natural Killer cells in host defense against infections as well as in tumour surveillance has been widely appreciated for a number of years. Upon recognition of "altered" cells, NK cells release the content of cytolytic granules, leading to the death of target cells. Moreover, NK cells are powerful producers of chemokines and cytokines, particularly Interferon-γ (IFN-γ), of which they are the earliest source upon a variety of infections. Despite being armed to fight against pathogens, NK cells become fully functional upon an initial phase of activation that requires the action of several cytokines, including type I IFNs. Type I IFNs are now recognized as key players in antiviral defense and immune regulation, and evidences from both mouse models of disease and in vitro studies support the existence of an alliance between type I IFNs and NK cells to ensure effective protection against viral infections. This review will focus on the role of type I IFNs in regulating NK cell functions to elicit antiviral response and on NK cell-produced IFN-γ beneficial and pathological effects.

Analysis of NKp30/NCR3 isoforms in untreated HIV-1-infected patients from the ANRS SEROCO cohort

Natural killer (NK) cells play a prominent role at the intersection between innate and cognate immunity, thus influencing the development of multiple pathological conditions including HIV-1-induced AIDS. Not only NK cells directly kill HIV-1-infected cells, but also control the maturation and/or elimination of dendritic cells (DCs). These functions are regulated by the delicate balance between activating and inhibiting receptors expressed at the NK-cell surface. Among the former, NKp30 has raised significant interest since the alternative splicing of its intracellular domain leads to differential effector functions, dictating the prognosis of patients bearing gastrointestinal sarcoma, and B7-H6 has recently been identified as its main ligand. Since NKp30 is downregulated in CD56^-/CD16⁺ NK cells expanded in viremic, chronically infected HIV-1⁺ patients, we decided to investigate the predictive value of NKp30 splice variants for spontaneous disease progression in 89 therapy-naïve HIV-1-infected individuals enrolled in an historical cohort of patients followed since diagnosis (ANRS SEROCO cohort). We found no difference in the representation of NK-cell subsets (CD56^bright, CD56^dim, CD56^neg) in HIV-1-infected patients as compared with healthy subjects. NKp30 downregulation was detected in CD56^dim and CD56^neg NK-cell subsets, yet this did not convey any prognostic value. None of the NKp30 isoforms did affect disease progression, as measured in terms of time-to-loss of circulating CD4⁺ T cells, time-to-AIDS-defining events and overall survival. NKp30 isoforms do not seem to play a major role in the outcome of HIV-1 infection, but the heterogeneity of the immuno-virological status of patients at enrollment could have to be taken into account.

Extracellular HCV-core protein induces an immature regulatory phenotype in NK cells: implications for outcome of acute infection

Background

Hepatitis C viral (HCV) proteins, including core, demonstrate immuno-modulatory properties; however, the effect of extracellular core on natural killer (NK) cells has not previously been investigated.

Aims

To characterise NKs in acute HCV infection over time, and, to examine the effect of exogenous HCV-core protein on NK cell phenotype and function.

Methods

Acute HCV patients (n = 22), including 10 subjects who spontaneously recovered, were prospectively studied. Flow-cytometry was used to measure natural cytotoxicity and to phenotype NKs directly ex vivo and after culture with HCV-core protein. Microarray analysis was used to identify pathways involved in the NK cell response to exogenous HCV-core.

Results

Direct ex vivo analysis demonstrated an increased frequency of immature/regulatory CD56^bright NKs early in acute HCV infection per se which normalized with viral clearance. Natural cytotoxicity was reduced and did not recover after viral clearance. There was a statistically significant correlation between the frequency of CD56^bright NKs and circulating serum levels of HCV core protein. In vitro culture of purified CD56^bright NK cells with HCV-core protein in the presence of IL-15 maintained a significant proportion of NKs in the CD56^bright state. The in vitro effect of core closely correlates with NK characteristics measured directly ex vivo in acute HCV infection. Pathway analysis suggests that HCV-core protein attenuates NK interferon type I responses.

Conclusions

Our data suggest that HCV-core protein alters NK cell maturation and may influence the outcome of acute infection.

Chronic viral infection promotes sustained Th1-derived immunoregulatory IL-10 via BLIMP-1

During the course of many chronic viral infections, the antiviral T cell response becomes attenuated through a process that is regulated in part by the host. While elevated expression of the immunosuppressive cytokine IL-10 is involved in the suppression of viral-specific T cell responses, the relevant cellular sources of IL-10, as well as the pathways responsible for IL-10 induction, remain unclear. In this study, we traced IL-10 production over the course of chronic lymphocytic choriomeningitis virus (LCMV) infection in an IL-10 reporter mouse line. Using this model, we demonstrated that virus-specific T cells with reduced inflammatory function, particularly Th1 cells, display elevated and sustained IL-10 expression during chronic LCMV infection. Furthermore, ablation of IL-10 from the T cell compartment partially restored T cell function and reduced viral loads in LCMV-infected animals. We found that viral persistence is needed for sustained IL-10 production by Th1 cells and that the transcription factor BLIMP-1 is required for IL-10 expression by Th1 cells. Restimulation of Th1 cells from LCMV-infected mice promoted BLIMP-1 and subsequent IL-10 expression, suggesting that constant antigen exposure likely induces the BLIMP-1/IL-10 pathway during chronic viral infection. Together, these data indicate that effector T cells self-limit their responsiveness during persistent viral infection via an IL-10-dependent negative feedback loop.

Innate immune cell networking in hepatitis C virus infection

Persistent viral infection, such as HCV infection, is the result of the inability of the host immune system to mount a successful antiviral response, as well as the escape strategies devised by the virus. Although each individual component of the host immune system plays important roles in antiviral immunity, the interactive network of immune cells as a whole acts against the virus. The innate immune system forms the first line of host defense against viral infection, and thus, virus elimination or chronic HCV infection is linked to the direct outcome of the interactions between the various innate immune cells and HCV. By understanding how the distinct components of the innate immune system function both individually and collectively during HCV infection, potential therapeutic targets can be identified to overcome immune dysfunction and control chronic viral infection.

Proliferation conditions promote intrinsic changes in NK cells for an IL-10 response

Constitutively found at high frequencies, the role for NK cell proliferation remains unclear. In this study, a shift in NK cell function from predominantly producing IFN-γ, a cytokine with proinflammatory and antimicrobial functions, to producing the immunoregulatory cytokine IL-10 was defined during extended murine CMV infection. The response occurred at times subsequent to IL-12 production, but the NK cells elicited acquired responsiveness to IL-12 and IL-21 for IL-10 production. Because neither IL-12 nor IL-21 was required in vivo, however, additional pathways appeared to be available to promote NK cell IL-10 expression. In vitro studies with IL-2 to support proliferation and in vivo adoptive transfers into murine CMV-infected mice demonstrated that NK cell proliferation and further division enhanced the change. In contrast to the sustained open profile of the IFN-γ gene, NK cells responding to infection acquired histone modifications in the IL-10 gene indicative of changing from a closed to an open state. The IL-10 response to IL-12 was proliferation dependent ex vivo if the NK cells had not yet expanded in vivo but independent if they had. Thus, a novel role for proliferation in supporting changing innate cell function is reported.

Natural cytotoxicity receptor-dependent natural killer cytolytic activity directed at hepatitis C Virus (HCV) is associated with liver inflammation, African American race, IL28B genotype, and response to pegylated interferon/ribavirin therapy in chronic HCV infection

BACKGROUND

Natural killer (NK) cells are implicated in the pathogenesis of hepatitis C virus (HCV) infection and outcome of interferon (IFN)-α based therapy, although mechanisms remain unclear.

METHODS

To evaluate NK ability to control HCV infection, we analyzed healthy donor and HCV-infected donor NK-cell cytolytic activity directed at HCV-infected target cells.

RESULTS

HCV-infected subjects' natural cytotoxicity receptor (NCR)-dependent NK-cell cytolytic activity directed at HCV-infected and uninfected Huh7.5 target cells was greater than that of cells from healthy donors, and this localized to the African American subset. However, IFN-α-enhanced NK cytolytic function was lower in HCV-infected subjects, again localized mainly to the African American subset. Additionally, whereas HCV-infected Huh7.5 cells were more readily targeted than uninfected cells, the selectivity of cytolytic activity for infected targets was lower during HCV infection and after IFN-α stimulation, and lower selectivity was in part attributable to greater NKp46 expression. Furthermore, cytolytic activity was associated with higher serum aspartate aminotransferase, rs12979860 IL28B genotype, and in vivo response to pegylated IFN/ribavirin therapy.

CONCLUSIONS

These data indicate that during chronic HCV infection, race-associated increase in NCR expression and IL28B-associated cytolytic activity may participate in host response to IFN-α-containing HCV therapy.

Immune responses to HCV and other hepatitis viruses

Five human hepatitis viruses cause most of the acute and chronic liver disease worldwide. Over the past 25 years, hepatitis C virus (HCV) in particular has received much interest because of its ability to persist in most immunocompetent adults and because of the lack of a protective vaccine. Here we examine innate and adaptive immune responses to HCV infection. Although HCV activates an innate immune response, it employs an elaborate set of mechanisms to evade interferon (IFN)-based antiviral immunity. By comparing innate and adaptive immune responses to HCV with those to hepatitis A and B viruses, we suggest that prolonged innate immune activation by HCV impairs the development of successful adaptive immune responses. Comparative immunology provides insights into the maintenance of immune protection. We conclude by discussing prospects for an HCV vaccine and future research needs for the hepatitis viruses.

Structural basis for recognition of cellular and viral ligands by NK cell receptors

Natural killer (NK) cells are key components of innate immune responses to tumors and viral infections. NK cell function is regulated by NK cell receptors that recognize both cellular and viral ligands, including major histocompatibility complex (MHC), MHC-like, and non-MHC molecules. These receptors include Ly49s, killer immunoglobulin-like receptors, leukocyte immunoglobulin-like receptors, and NKG2A/CD94, which bind MHC class I (MHC-I) molecules, and NKG2D, which binds MHC-I paralogs such as the stress-induced proteins MICA and ULBP. In addition, certain viruses have evolved MHC-like immunoevasins, such as UL18 and m157 from cytomegalovirus, that act as decoy ligands for NK receptors. A growing number of NK receptor–ligand interaction pairs involving non-MHC molecules have also been identified, including NKp30–B7-H6, killer cell lectin-like receptor G1–cadherin, and NKp80–AICL. Here, we describe crystal structures determined to date of NK cell receptors bound to MHC, MHC-related, and non-MHC ligands. Collectively, these structures reveal the diverse solutions that NK receptors have developed to recognize these molecules, thereby enabling the regulation of NK cytolytic activity by both host and viral ligands.

An effective interferon-gamma-mediated inhibition of hepatitis C virus replication by natural killer cells is associated with spontaneous clearance of acute hepatitis C in human immunodeficiency virus-positive patients

Hepatitis C virus (HCV) coinfection is an increasing health problem in human immunodeficiency virus-positive (HIV(+) ) individuals. However, a considerable proportion of HIV(+) patients manage to overcome acute hepatitis C (AHC) spontaneously. In the present study, we analyzed the role of natural killer (NK) cells in modulating the course of AHC in HIV(+) patients. Twenty-seven HIV(+) patients with AHC (self-limited course: n = 10; chronic course: n = 17), 12 HIV(+) patients with chronic hepatitis C (CHC), 8 HIV monoinfected individuals, and 12 healthy controls were studied. NK cells were phenotypically analyzed by flow cytometry. Interferon-gamma (IFN-γ) secretion, degranulation (CD107a), and anti-HCV (= inhibition of HCV replication) activity of NK subpopulations were analyzed using the HuH7A2 HCVreplicon cell system. NK cell frequency did not differ significantly between HIV(+) patients with chronic and self-limited course of AHC. However, we found NK cells from patients with self-limiting infection to be significantly more effective in inhibiting HCV replication in vitro than NK cells from patients developing CHC. Of note, antiviral NK cell activity showed no significant correlation with NK cell degranulation, but was positively correlated with IFN-γ secretion, and blocking experiments confirmed an important role for IFN-γ in NK cell-mediated inhibition of HCV replication. Accordingly, NK cells from patients that spontaneously cleared the virus displayed a stronger IFN-γ secretion than those developing chronic infection. Finally, we observed high expression of NKG2D and NKp46, respectively, to be associated with self-limiting course of aHCV. Accordingly, we found that blocking of these NK cell receptors significantly impaired antiviral NK cell activity.

CONCLUSION

Our data suggest a strong IFN-γ-mediated antiviral NK cell response to be associated with a self-limited course of AHC in HIV(+) patients.

Limited hepatitis B virus replication space in the chronically hepatitis C virus-infected liver

We compared the kinetics and magnitude of hepatitis B virus (HBV) infection in hepatitis C virus (HCV)-naive and chronically HCV-infected chimpanzees in whose livers type I interferon-stimulated gene (ISG) expression is strongly induced. HBV infection was delayed and attenuated in the HCV-infected animals, and the number of HBV-infected hepatocytes was drastically reduced. These results suggest that establishment of HBV infection and its replication space is limited by the antiviral effects of type I interferon in the chronically HCV-infected liver.

Baseline and Dynamic Expression of Activating NK Cell Receptors in the Control of Chronic Viral Infections: The Paradigm of HIV-1 and HCV

Natural killer (NK) cell function is regulated by a balance between the triggering of activating and inhibitory receptors expressed on their surface. A relevant effort has been focused so far on the study of KIR carriage/expression setting the basis for NK cell education and self-tolerance. Focus on the evolution and regulation of activating NK receptors has lagged behind so far. Our understanding of activating receptor expression and regulation has recently improved by evidences derived from in vitro and in vivo studies. Virus infection - either acute or chronic - determines preferential expansion of NK cells with specific phenotype, activating receptors, and with recall-like functional activity. Studies on patients with viral infection (HIV and HCV) and specific diverging clinical courses confirm that inter-individual differences may exist in baseline expression of natural cytotoxicity receptors (NKp46 and NKp30). The findings that patients with divergent clinical courses have different kinetics of activating receptor density expression upon NK cell activation in vitro provide an additional, time-dependent, functional parameter. Kinetic changes in receptor expression thus represent an additional parameter to basal receptor density expression. Different expression and inducibilities of activating receptors on NK cells contribute to the high diversity of NK cell populations and may help our understanding of the inter-individual differences in innate responses that underlie divergent disease courses.

Between Scylla and Charybdis: The role of the human immune system in the pathogenesis of hepatitis C

Hepatitis C virus (HCV) frequently elicits only mild immune responses so that it can often establish chronic infection. In this case HCV antigens persist and continue to stimulate the immune system. Antigen persistence then leads to profound changes in the infected host’s immune responsiveness, and eventually contributes to the pathology of chronic hepatitis. This topic highlight summarizes changes associated with chronic hepatitis C concerning innate immunity (interferons, natural killer cells), adaptive immune responses (immunoglobulins, T cells, and mechanisms of immune regulation (regulatory T cells). Our overview clarifies that a strong anti-HCV immune response is frequently associated with acute severe tissue damage. In chronic hepatitis C, however, the effector arms of the immune system either become refractory to activation or take over regulatory functions. Taken together these changes in immunity may lead to persistent liver damage and cirrhosis. Consequently, effector arms of the immune system will not only be considered with respect to antiviral defence but also as pivotal mechanisms of inflammation, necrosis and progression to cirrhosis. Thus, avoiding Scylla - a strong, sustained antiviral immune response with inital tissue damage - takes the infected host to virus-triggered immunopathology, which ultimately leads to cirrhosis and liver cancer - the realm of Charybdis.

The natural killer cell response to HCV infection

In the last few years major progress has been made in better understanding the role of natural killer (NK) cells in hepatitis C virus (HCV) infection. This includes multiple pathways by which HCV impairs or limits NK cells activation. Based on current genetic and functional data, a picture is emerging where only a rapid and strong NK cell response early on during infection which results in strong T cell responses and possible subsequent clearance, whereas chronic HCV infection is associated with dysfunctional or biased NK cells phenotypes. The hallmark of this NK cell dysfunction is persistent activation promoting ongoing hepatitis and hepatocyte damage, while being unable to clear HCV due to impaired IFN-γ responses. Furthermore, some data suggests certain chronically activated subsets that are NKp46^high may be particularly active against hepatic stellate cells, a key player in hepatic fibrogenesis. Finally, the role of NK cells during HCV therapy, HCV recurrence after liver transplant and hepatocellular carcinoma are discussed.

Pathogenesis of chronic viral hepatitis: differential roles of T cells and NK cells

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infections account for 57% of cases of liver cirrhosis and 78% of cases of primary liver cancer worldwide and cause a million deaths per year. Although HBV and HCV differ in their genome structures, replication strategies and life cycles, they have common features, including their noncytopathic nature and their capacity to induce chronic liver disease, which is thought to be immune mediated. However, the rate of disease progression from chronic hepatitis to cirrhosis varies greatly among infected individuals, and the factors that regulate it are largely unknown. This review summarizes our current understanding of the roles of antigen-specific and nonspecific immune cells in the pathogenesis of chronic hepatitis B and C and discusses recent findings that identify natural killer cells as regulators of T cell function and liver inflammation.

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 cells in human autoimmune disorders

Natural killer (NK) cells are innate lymphocytes that play a critical role in early host defense against viruses. Through their cytolytic capacity and generation of cytokines and chemokines, NK cells modulate the activity of other components of the innate and adaptive immune systems and have been implicated in the initiation or maintenance of autoimmune responses. This review focuses on recent research elucidating a potential immunoregulatory role for NK cells in T-cell and B-cell-mediated autoimmune disorders in humans, with a particular focus on multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematous. A better understanding of the contributions of NK cells to the development of autoimmunity may lead to novel therapeutic targets in these diseases.

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 HIV controller patients express an activated effector phenotype and do not up-regulate NKp44 on IL-2 stimulation

Control of HIV replication in elite controller (EC) and long-term nonprogressor (LTNP) patients has been associated with efficient CD8(+)cytotoxic T-lymphocyte function. However, innate immunity may play a role in HIV control. We studied the expression of natural cytotoxicity receptors (NKp46, NKp30, and NKp44) and their induction over a short time frame (2-4 d) on activation of natural killer (NK) cells in 31 HIV controller patients (15 ECs, 16 LTNPs). In EC/LTNP, induction of NKp46 expression was normal but short (2 d), and NKp30 was induced to lower levels vs. healthy donors. Notably, in antiretroviral-treated aviremic progressor patients (TAPPs), no induction of NKp46 or NKp30 expression occurred. More importantly, EC/LTNP failed to induce expression of NKp44, a receptor efficiently induced in activated NK cells in TAPPs. The specific lack of NKp44 expression resulted in sharply decreased capability of killing target cells by NKp44, whereas TAPPs had conserved NKp44-mediated lysis. Importantly, conserved NK cell responses, accompanied by a selective defect in the NKp44-activating pathway, may result in lack of killing of uninfected CD4(+)NKp44Ligand(+) cells when induced by HIVgp41 peptide-S3, representing a relevant mechanism of CD4(+) depletion. In addition, peripheral NK cells from EC/LTNP had increased NKG2D expression, significant HLA-DR up-regulation, and a mature (NKG2A-CD57(+)killer cell Ig-like receptor(+)CD85j(+)) phenotype, with cytolytic function also against immature dendritic cells. Thus, NK cells in EC/LTNP can maintain substantially unchanged functional capabilities, whereas the lack of NKp44 induction may be related to CD4 maintenance, representing a hallmark of these patients.

Human NK cell receptors/markers: a tool to analyze NK cell development, subsets and function

Natural killer (NK) cells are important components of the innate immunity and play a key role in host defense by virtue of their ability to release cytokines and to mediate cytolytic activity against tumor cells and virus-infected cells. NK cells were first described more than 30 years ago on the basis of their peculiar functional capabilities. Subsequently, thanks to the production of a variety of monoclonal antibodies, it became possible to identify surface receptors and markers expressed by NK cells as well as to characterize their functional properties. Here, we provide a brief historical overview about the discovery of human NK cell receptors and we delineate the main phenotypic features of differentiating and mature NK cells in healthy donors as well as their alterations in certain pathologic conditions.

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.

Structural insights into activation of antiviral NK cell responses

Natural killer (NK) cells are key components of innate immune responses, providing surveillance against cells undergoing tumorigenesis or infection, by viruses or internal pathogens. NK cells can directly eliminate compromised cells and regulate downstream responses of the innate and acquired immune systems through the release of immune modulators (cytokines, interferons). The importance of the role NK cells play in immune defense was demonstrated originally in herpes viral infections, usually mild or localized, which become severe and life threatening in NK-deficient patients . NK cell effector functions are governed by balancing opposing signals from a diverse array of activating and inhibitory receptors. Many NK receptors occur in paired activating and inhibitory isoforms and recognize major histocompatibility complex (MHC) class I proteins with varying degrees of peptide specificity. Structural studies have made considerable inroads into understanding the molecular mechanisms employed to broadly recognize multiple MHC ligands or specific pathogen-associated antigens and the strategies employed by viruses to thwart these defenses. Although many details of NK development, signaling, and integration remain mysterious, it is clear that NK receptors are key components of a system exquisitely tuned to sense any dysregulation in MHC class I expression, or the expression of certain viral antigens, resulting in the elimination of affected 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.