Cell surface expression of major histocompatibility complex class I molecules is reduced in hepatitis C virus subgenomic replicon-expressing cells

Endoplasmic Reticulum Stress in Hepatitis B Virus and Hepatitis C Virus Infection

Endoplasmic reticulum (ER) stress, a type of cellular stress, always occurs when unfolded or misfolded proteins accumulating in the ER exceed the protein folding capacity. Because of the demand for rapid viral protein synthesis after viral infection, viral infections become a risk factor for ER stress. The hepatocyte is a cell with large and well-developed ER, and hepatitis virus infection is widespread in the population, indicating the interaction between hepatitis viruses and ER stress may have significance for managing liver diseases. In this paper, we review the process that is initiated by the hepatocyte through ER stress against HBV and HCV infection and explain how this information can be helpful in the treatment of HBV/HCV-related diseases.

CXCL14 suppresses human papillomavirus-associated head and neck cancer through antigen-specific CD8+ T-cell responses by upregulating MHC-I expression

Evasion of the host immune responses is critical for both persistent human papillomavirus (HPV) infection and associated cancer progression. We have previously shown that expression of the homeostatic chemokine CXCL14 is significantly downregulated by the HPV oncoprotein E7 during cancer progression. Restoration of CXCL14 expression in HPV-positive head and neck cancer (HNC) cells dramatically suppresses tumor growth and increases survival through an immune-dependent mechanism in mice. While CXCL14 recruits natural killer (NK) and T cells to the tumor microenvironment, the mechanism by which CXCL14 mediates tumor suppression through NK and/or T cells remained undefined. Here, we report that CD8⁺ T cells are required for CXCL14-mediated tumor suppression. Using a CD8⁺ T cell receptor transgenic model, we show that the CXCL14-mediated antitumor CD8⁺ T cell responses require antigen specificity. Interestingly, CXCL14 expression restores major histocompatibility complex class I (MHC-I) expression on HPV-positive HNC cells downregulated by HPV, and knockdown of MHC-I expression in HNC cells results in loss of tumor suppression even with CXCL14 expression. These results suggest that CXCL14 enacts antitumor immunity through restoration of MHC-I expression on tumor cells and promoting antigen-specific CD8⁺ T cell responses to suppress HPV-positive HNC.

Conserved and variable natural killer cell receptors: diverse approaches to viral infections

Natural killer (NK) cells are lymphocytes of the innate immune system with essential roles during viral infections. NK cell functions are mediated through a repertoire of non-rearranging inhibitory and activating receptors that interact with major histocompatibility complex (MHC)-peptide complexes on the surface of infected cells. Recent work studying the conserved CD94-NKG2A and variable killer cell immunoglobulin-like receptor-MHC systems suggest that these two receptor families may have subtly different properties in terms of interactions with MHC class I bound peptides, and in recognition of down-regulation of MHC class I. In this review, we discuss how these properties generate diversity in the NK cell response to viruses.

Malaria Parasite Liver Infection and Exoerythrocytic Biology

In their infection cycle, malaria parasites undergo replication and population expansions within the vertebrate host and the mosquito vector. Host infection initiates with sporozoite invasion of hepatocytes, followed by a dramatic parasite amplification event during liver stage parasite growth and replication within hepatocytes. Each liver stage forms up to 90,000 exoerythrocytic merozoites, which are in turn capable of initiating a blood stage infection. Liver stages not only exploit host hepatocyte resources for nutritional needs but also endeavor to prevent hepatocyte cell death and detection by the host's immune system. Research over the past decade has identified numerous parasite factors that play a critical role during liver infection and has started to delineate a complex web of parasite-host interactions that sustain successful parasite colonization of the mammalian host. Targeting the parasites' obligatory infection of the liver as a gateway to the blood, with drugs and vaccines, constitutes the most effective strategy for malaria eradication, as it would prevent clinical disease and onward transmission of the parasite.

Modulation of Human Leukocyte Antigen-C by Human Cytomegalovirus Stimulates KIR2DS1 Recognition by Natural Killer Cells

The interaction of inhibitory killer cell Ig-like receptors (KIRs) with human leukocyte antigen (HLA) class I molecules has been characterized in detail. By contrast, activating members of the KIR family, although closely related to inhibitory KIRs, appear to interact weakly, if at all, with HLA class I. KIR2DS1 is the best studied activating KIR and it interacts with C2 group HLA-C (C2-HLA-C) in some assays, but not as strongly as KIR2DL1. We used a mouse 2B4 cell reporter system, which carries NFAT-green fluorescent protein with KIR2DS1 and a modified DAP12 adaptor protein. KIR2DS1 reporter cells were not activated upon coculture with 721.221 cells transfected with different HLA-C molecules, or with interferon-γ stimulated primary dermal fibroblasts. However, KIR2DS1 reporter cells and KIR2DS1⁺ primary natural killer (NK) cells were activated by C2-HLA-C homozygous human fetal foreskin fibroblasts (HFFFs) but only after infection with specific clones of a clinical strain of human cytomegalovirus (HCMV). Active viral gene expression was required for activation of both cell types. Primary NKG2A^-KIR2DS1⁺ NK cell subsets degranulated after coculture with HCMV-infected HFFFs. The W6/32 antibody to HLA class I blocked the KIR2DS1 reporter cell interaction with its ligand on HCMV-infected HFFFs but did not block interaction with KIR2DL1. This implies a differential recognition of HLA-C by KIR2DL1 and KIR2DS1. The data suggest that modulation of HLA-C by HCMV is required for a potent KIR2DS1-mediated NK cell activation.

Organ system view of the hepatic innate immunity in HCV infection

An orchestration of innate and adaptive immunity determines the infection outcome and whether the host achieves clearance or allows the pathogen to establish persistent infection. The robust activation of the innate immune response plays the most critical role in both limiting viral replication and halting the spread of the pathogen immediately after infection. The magnitude of innate immune activation is coupled with the efficient mounting of the adaptive immunity. Although immunity against HCV infection is known to be inadequate as most cases transitions to chronicity, approximately 25% of acute infection cases result in spontaneous clearance. The exact immune mechanisms that govern the infection outcome remain largely unknown; recent discoveries suggest that the innate immune system facilitates this event. Both infected hepatocytes and local innate immune cells trigger the front line defense program of the liver as well as the recruitment of diverse adaptive immune cells to the site of infection. Although hepatocyte is the target of HCV infection, nearly all cell types that exist in the liver are involved in the innate defense and contribute to the pathophysiology of hepatic inflammation. The main focus of this comprehensive review is to discuss the current knowledge on how each hepatic cell type contributes to the organ system level innate immunity against HCV infection as well as interplays with the viral evasion program. Furthermore, this review article also aims to synchronize the observations from both molecular biological studies and clinical studies with the ultimate goal of improving our understanding of HCV mediated hepatitis. J. Med. Virol. 88:2025-2037, 2016. © 2016 Wiley Periodicals, Inc.

Influence of Glycosylation Inhibition on the Binding of KIR3DL1 to HLA-B*57:01

Viral infections can affect the glycosylation pattern of glycoproteins involved in antiviral immunity. Given the importance of protein glycosylation for immune function, we investigated the effect that modulation of the highly conserved HLA class I N-glycan has on KIR:HLA interactions and NK cell function. We focused on HLA-B*57:01 and its interaction with KIR3DL1, which has been shown to play a critical role in determining the progression of a number of human diseases, including human immunodeficiency virus-1 infection. 721.221 cells stably expressing HLA-B*57:01 were treated with a panel of glycosylation enzyme inhibitors, and HLA class I expression and KIR3DL1 binding was quantified. In addition, the functional outcomes of HLA-B*57:01 N-glycan disruption/modulation on KIR3DL1ζ+ Jurkat reporter cells and primary human KIR3DL1+ NK cells was assessed. Different glycosylation enzyme inhibitors had varying effects on HLA-B*57:01 expression and KIR3DL1-Fc binding. The most remarkable effect was that of tunicamycin, an inhibitor of the first step of N-glycosylation, which resulted in significantly reduced KIR3DL1-Fc binding despite sustained expression of HLA-B*57:01 on 721.221 cells. This effect was paralleled by decreased activation of KIR3DL1ζ+ Jurkat reporter cells, as well as increased degranulation of primary human KIR3DL1+ NK cell clones when encountering HLA-B*57:01-expressing 721.221 cells that were pre-treated with tunicamycin. Overall, these results demonstrate that N-glycosylation of HLA class I is important for KIR:HLA binding and has an impact on NK cell function.

Influence of HIV and HCV on T cell antigen presentation and challenges in the development of vaccines

Some of the central challenges for developing effective vaccines against HIV and hepatitis C virus (HCV) are similar. Both infections are caused by small, highly mutable, rapidly replicating RNA viruses with the ability to establish long-term chronic pathogenic infection in human hosts. HIV has caused 60 million infections globally and HCV 180 million and both viruses may co-exist among certain populations by virtue of common blood-borne, sexual, or vertical transmission. Persistence of both pathogens is achieved by evasion of intrinsic, innate, and adaptive immune defenses but with some distinct mechanisms reflecting their differences in evolutionary history, replication characteristics, cell tropism, and visibility to mucosal versus systemic and hepatic immune responses. A potent and durable antibody and T cell response is a likely requirement of future HIV and HCV vaccines. Perhaps the single biggest difference between the two vaccine design challenges is that in HCV, a natural model of protective immunity can be found in those who resolve acute infection spontaneously. Such spontaneous resolvers exhibit durable and functional CD4⁺ and CD8⁺ T cell responses (Diepolder et al., 1995; Cooper et al., 1999; Thimme et al., 2001; Grakoui et al., 2003; Lauer et al., 2004; Schulze Zur Wiesch et al., 2012). However, frequent re-infection suggests partial or lack of protective immunity against heterologous HCV strains, possibly indicative of the degree of genetic diversity of circulating HCV genotypes and subtypes. There is no natural model of protective immunity in HIV, however, studies of “elite controllers,” or individuals who have durably suppressed levels of plasma HIV RNA without antiretroviral therapy, has provided the strongest evidence for CD8⁺ T cell responses in controlling viremia and limiting reservoir burden in established infection. Here we compare and contrast the specific mechanisms of immune evasion used by HIV and HCV, which subvert adaptive human leukocyte antigen (HLA)-restricted T cell immunity in natural infection, and the challenges these pose for designing effective preventative or therapeutic vaccines.

A cognitive computational model inspired by the immune system response

The immune system has a cognitive ability to differentiate between healthy and unhealthy cells. The immune system response (ISR) is stimulated by a disorder in the temporary fuzzy state that is oscillating between the healthy and unhealthy states. However, modeling the immune system is an enormous challenge; the paper introduces an extensive summary of how the immune system response functions, as an overview of a complex topic, to present the immune system as a cognitive intelligent agent. The homogeneity and perfection of the natural immune system have been always standing out as the sought-after model we attempted to imitate while building our proposed model of cognitive architecture. The paper divides the ISR into four logical phases: setting a computational architectural diagram for each phase, proceeding from functional perspectives (input, process, and output), and their consequences. The proposed architecture components are defined by matching biological operations with computational functions and hence with the framework of the paper. On the other hand, the architecture focuses on the interoperability of main theoretical immunological perspectives (classic, cognitive, and danger theory), as related to computer science terminologies. The paper presents a descriptive model of immune system, to figure out the nature of response, deemed to be intrinsic for building a hybrid computational model based on a cognitive intelligent agent perspective and inspired by the natural biology. To that end, this paper highlights the ISR phases as applied to a case study on hepatitis C virus, meanwhile illustrating our proposed architecture perspective.

Hepatitis C virus attenuates interferon-induced major histocompatibility complex class I expression and decreases CD8+ T cell effector functions

BACKGROUND & AIMS

Major histocompatibility complex (MHC) class I-restricted CD8(+) T cells are required for clearance of hepatitis C virus (HCV) infection. MHC class I expression is up-regulated by type I and II interferons (IFNs). However, little is known about the effects of HCV infection on IFN-induced expression of MHC class I.

METHODS

We used the HCV cell culture system (HCVcc) with the genotype 2a Japanese fulminant hepatitis-1 strain to investigate IFN-induced expression of MHC class I and its regulatory mechanisms. HCVcc-infected Huh-7.5 cells were analyzed by flow cytometry, metabolic labeling, immunoprecipitation, and immunoblotting analyses. Protein kinase R (PKR) was knocked down with lentiviruses that express small hairpin RNAs. The functional effects of MHC class I regulation by HCV were demonstrated in co-culture studies, using HCV-specific CD8(+) T cells.

RESULTS

Although the baseline level of MHC class I was not affected by HCV infection, IFN-induced expression of MHC class I was notably attenuated in HCV-infected cells. This was associated with replicating HCV RNA, not with viral protein. HCV infection reduced IFN-induced synthesis of MHC class I protein and induced phosphorylation of PKR and eIF2α. IFN-induced MHC class I expression was restored by small hairpin RNA-mediated knockdown of PKR in HCV-infected cells. Co-culture of HCV-specific CD8(+) T cells and HCV-infected cells that expressed HLA-A2 demonstrated that HCV infection reduced the effector functions of HCV-specific CD8(+) T cells; these functions were restored by small hairpin RNA-mediated knockdown of PKR.

CONCLUSIONS

IFN-induced expression of MHC class I is attenuated in HCV-infected cells by activation of PKR, which reduces the effector functions of HCV-specific CD8(+) T cells. This appears to be an important mechanism by which HCV circumvents antiviral adaptive immune responses.

The effect of bovine viral diarrhea virus (BVDV) strains on bovine monocyte-derived dendritic cells (Mo-DC) phenotype and capacity to produce BVDV

Background

Dendritic cells (DC) are important antigen presentation cells that monitor, process, and present antigen to T cells. Viruses that infect DC can have a devastating impact on the immune system. In this study, the ability of bovine viral diarrhea virus (BVDV) to replicate and produce infectious virus in monocyte-derived dendritic cells (Mo-DC) and monocytes was studied. The study also examined the effect of BVDV infection on Mo-DC expression of cell surface markers, including MHCI, MHCII, and CD86, which are critical for DC function in immune response.

Methods

Peripheral blood mononuclear cells (PBMCs) were isolated from bovine blood through gradient centrifugation. The adherent monocytes were isolated from PBMCs and differentiated into Mo-DC using bovine recombinant interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GMCSF). To determine the effect of BVDV on Mo-DC, four strains of BVDV were used including the severe acute non-cytopathic (ncp) BVDV2a-1373; moderate acute ncp BVDV2a 28508-5; and a homologous virus pair [i.e., cytopathic (cp) BVDV1b TGAC and ncp BVDV1b TGAN]. The Cooper strain of bovine herpesvirus 1 (BHV1) was used as the control virus. Mo-DC were infected with one of the BVDV strains or BHV-1 and were subsequently examined for virus replication, virus production, and the effect on MHCI, MHCII, and CD86 expression.

Results

The ability of monocytes to produce infectious virus reduced as monocytes differentiated to Mo-DC, and was completely lost at 120 hours of maturation. Interestingly, viral RNA increased throughout the course of infection in Mo-DC, and the viral non-structural (NS5A) and envelope (E2) proteins were expressed. The ncp strains of BVDV down-regulated while cp strain up-regulated the expression of the MHCI, MHCII, and CD86 on Mo-DC.

Conclusions

The study revealed that the ability of Mo-DC to produce infectious virus was reduced with its differentiation from monocytes to Mo-DC. The inability to produce infectious virus may be due to a hindrance of virus packaging or release mechanisms. Additionally, the study demonstrated that ncp BVDV down-regulated and cp BVDV up-regulated the expression of Mo-DC cell surface markers MHCI, MHCII, and CD86, which are important in the mounting of immune responses.

Hepatitis B virus X protein inhibits autophagic degradation by impairing lysosomal maturation

Deficiency in autophagy, a lysosome-dependent cell degradation pathway, has been associated with a variety of diseases especially cancer. Recently, the activation of autophagy by hepatitis B virus X (HBx) protein, which is implicated in hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC), has been identified in hepatic cells. However, the underlying mechanism and the relevance of HBx-activated autophagy to the carcinogenesis caused by HBV remain elusive. Here, by transfection of HBV genomic DNA and HBx in hepatic and hepatoma cells, we showed that HBV- or HBx-induced autophagosome formation was accompanied by unchanged MTOR (mechanistic target of rapamycin) activity and decreased degradation of LC3 and SQSTM1/p62, the typical autophagic cargo proteins. Further functional and morphological analysis indicated that HBx dramatically impaired lysosomal acidification leading to a drop in lysosomal degradative capacity and the accumulation of immature lysosomes possibly through interaction with V-ATPase affecting its lysosome targeting. Moreover, clinical specimen test showed increased SQSTM1 and immature lysosomal hydrolase CTSD (cathepsin D) in human liver tissues with chronic HBV infection and HBV-associated liver cancer. These data suggest that a repressive effect of HBx on lysosomal function is responsible for the inhibition of autophagic degradation, and this may be critical to the development of HBV-associated HCC.

Host glycans and antigen presentation

The cell-mediated adaptive immune response depends upon the activation of T cells via recognition of antigen in the context of a major histocompatibility complex (MHC) molecule. Although studies have shown that alterations in T cell receptor glycosylation reduces the activation threshold, the data on MHC is far less definitive. Here, we discuss the data on MHC glycosylation and the role the glycans might play during the adaptive host response.

Roles for major histocompatibility complex glycosylation in immune function

The major histocompatibility complex (MHC) glycoprotein family, also referred to as human leukocyte antigens, present endogenous and exogenous antigens to T lymphocytes for recognition and response. These molecules play a central role in enabling the immune system to distinguish self from non-self, which is the basis for protective immunity against pathogenic infections and disease while at the same time representing a serious obstacle for tissue transplantation. All known MHC family members, like the majority of secreted, cell surface, and other immune-related molecules, carry asparagine (N)-linked glycans. The immune system has evolved increasing complexity in higher-order organisms along with a more complex pattern of protein glycosylation, a relationship that may contribute to immune function beyond the early protein quality control events in the endoplasmic reticulum that are commonly known. The broad MHC family maintains peptide sequence motifs for glycosylation at sites that are highly conserved across evolution, suggesting importance, yet functional roles for these glycans remain largely elusive. In this review, we will summarize what is known about MHC glycosylation and provide new insight for additional functional roles for this glycoprotein modification in mediating immune responses.

Virus-induced ER stress and the unfolded protein response

The accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum (ER) results in ER stress that triggers cytoprotective signaling pathways, termed the unfolded protein response (UPR), to restore and maintain homeostasis in the ER or to induce apoptosis if ER stress remains unmitigated. The UPR signaling network encompasses three core elements, i.e., PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring protein-1 (IRE1). Activation of these three branch pathways of the UPR leads to the translation arrest and degradation of misfolded proteins, the expression of ER molecular chaperones, and the expansion of the ER membrane to decrease the load of proteins and increase the protein-folding capacity in the ER. Recently, the essential roles of the UPR have been implicated in a number of mammalian diseases, particularly viral diseases. In virus-infected cells, the cellular translation machinery is hijacked by the infecting virus to produce large amounts of viral proteins, which inevitably perturbs ER homeostasis and causes ER stress. This review summarizes current knowledge about the UPR signaling pathways, highlights two identified UPR pathways in plants, and discuss progress in elucidating the UPR in virus-infected cells and its functional roles in viral infection.

The unfolded protein response (UPR)-activated transcription factor X-box-binding protein 1 (XBP1) induces microRNA-346 expression that targets the human antigen peptide transporter 1 (TAP1) mRNA and governs immune regulatory genes

To identify endoplasmic reticulum (ER) stress-induced microRNAs (miRNA) that govern ER protein influx during the adaptive phase of unfolded protein response, we performed miRNA microarray profiling and analysis in human airway epithelial cells following ER stress induction using proteasome inhibition or tunicamycin treatment. We identified miR-346 as the most significantly induced miRNA by both classic stressors. miR-346 is encoded within an intron of the glutamate receptor ionotropic delta-1 gene (GRID1), but its ER stress-associated expression is independent of GRID1. We demonstrated that the spliced X-box-binding protein-1 (sXBP1) is necessary and sufficient for ER stress-associated miR-346 induction, revealing a novel role for this unfolded protein response-activated transcription factor. In mRNA profiling arrays, we identified 21 mRNAs that were reduced by both ER stress and miR-346. The target genes of miR-346 regulate immune responses and include the major histocompatibility complex (MHC) class I gene products, interferon-induced genes, and the ER antigen peptide transporter 1 (TAP1). Although most of the repressed mRNAs appear to be indirect targets because they lack specific seeding sites for miR-346, we demonstrate that the human TAP1 mRNA is a direct target of miR-346. The human TAP1 mRNA 3'-UTR contains a 6-mer canonical seeding site for miR-346. Importantly, the ER stress-associated reduction in human TAP1 mRNA and protein levels could be reversed with an miR-346 antagomir. Because TAP function is necessary for proper MHC class I-associated antigen presentation, our results provide a novel mechanistic explanation for reduced MHC class I-associated antigen presentation that was observed during ER stress.

Dysfunction of autophagy participates in vacuole formation and cell death in cells replicating hepatitis C virus

Hepatitis C virus (HCV) is a major cause of chronic liver diseases. A high risk of chronicity is the major concern of HCV infection, since chronic HCV infection often leads to liver cirrhosis and hepatocellular carcinoma. Infection with the HCV genotype 1 in particular is considered a clinical risk factor for the development of hepatocellular carcinoma, although the molecular mechanisms of the pathogenesis are largely unknown. Autophagy is involved in the degradation of cellular organelles and the elimination of invasive microorganisms. In addition, disruption of autophagy often leads to several protein deposition diseases. Although recent reports suggest that HCV exploits the autophagy pathway for viral propagation, the biological significance of the autophagy to the life cycle of HCV is still uncertain. Here, we show that replication of HCV RNA induces autophagy to inhibit cell death. Cells harboring an HCV replicon RNA of genotype 1b strain Con1 but not of genotype 2a strain JFH1 exhibited an incomplete acidification of the autolysosome due to a lysosomal defect, leading to the enhanced secretion of immature cathepsin B. The suppression of autophagy in the Con1 HCV replicon cells induced severe cytoplasmic vacuolation and cell death. These results suggest that HCV harnesses autophagy to circumvent the harmful vacuole formation and to maintain a persistent infection. These findings reveal a unique survival strategy of HCV and provide new insights into the genotype-specific pathogenicity of HCV.

Cell-to-cell contact with hepatitis C virus-infected cells reduces functional capacity of natural killer cells

The distinct feature of hepatitis C virus (HCV) infection is a high incidence of chronicity. The reason for chronic HCV infection has been actively investigated, and impairment of innate and adaptive immune responses against HCV is proposed as a plausible cause. Whereas functional impairment of HCV-specific T cells is well characterized, the role and functional status of natural killer (NK) cells in each phase of HCV infection are still elusive. We therefore investigated whether direct interaction between NK cells and HCV-infected cells modulates NK cell function. HCV-permissive human hepatoma cell lines were infected with cell culture-generated HCV virions and cocultured with primary human NK cells. Cell-to-cell contact between NK cells and HCV-infected cells reduced NK cells' capacity to degranulate and lyse target cells, especially in the CD56(dim) NK cell subset, which is characterized by low-density surface expression of CD56. The decrease in degranulation capacity was correlated with downregulated expression of NK cell-activating receptors, such as NKG2D and NKp30, on NK cells. The ability of NK cells to produce and secrete gamma interferon (IFN-γ) also diminished after exposure to HCV-infected cells. The decline of IFN-γ production was consistent with the reduction of NK cell degranulation. In conclusion, cell-to-cell contact with HCV-infected cells negatively modulated functional capacity of NK cells, and the inhibition of NK cell function was associated with downregulation of NK-activating receptors on NK cell surfaces. These observations suggest that direct cell-to-cell interaction between NK cells and HCV-infected hepatocytes may impair NK cell function in vivo and thereby contribute to the establishment of chronic infection.

Hepatitis C virus-induced oxidative stress and mitochondrial dysfunction: a focus on recent advances in proteomics

The natural history of chronic hepatitis C virus (HCV) infection presents two major aspects. On one side, the illness is by itself benign, whereas, on the other side, epidemiological evidence clearly identifies chronic HCV infection as the principal cause of cirrhosis, hepatocellular carcinoma, and extrahepatic diseases, such as autoimmune type II mixed cryoglobulinemia and some B cell non-Hodgkin's lymphomas. The mechanisms responsible for the progression of liver disease to severe liver injury are still poorly understood. Nonetheless, considerable biological data and studies from animal models suggest that oxidative stress contributes to steatohepatitis and that the increased generation of reactive oxygen and nitrogen species, together with the decreased antioxidant defense, promotes the development of hepatic and extrahepatic complications of HCV infection. The principal mechanisms causing oxidative stress in HCV-positive subjects have only been partially elucidated and have identified chronic inflammation, iron overload, ER stress, and a direct activity of HCV proteins in increasing mitochondrial ROS production, as key events. This review summarizes current knowledge regarding mechanisms of HCV-induced oxidative stress with its long-term effects in the context of HCV-related diseases, and includes a discussion of recent contributions from proteomics studies.

Dendritic cells in hepatitis C infection: can they (help) win the battle?

Infection with hepatitis C virus (HCV) is a public health problem; it establishes a chronic course in ~85% of infected patients and increases their risk for developing liver cirrhosis, hepatocellular carcinoma, and significant extrahepatic manifestations. The mechanisms of HCV persistence remain elusive and are largely related to inefficient clearance of the virus by the host immune system. Dendritic cells (DCs) are the most efficient inducers of immune responses; they are capable of triggering productive immunity and maintaining the state of tolerance to self- and non-self antigens. During the past decade, multiple research groups have focused on DCs, in hopes of unraveling an HCV-specific DC signature or DC-dependent mechanisms of antiviral immunity which would lead to a successful HCV elimination strategy. This review incorporates the latest update in the current status of knowledge on the role of DCs in anti-HCV immunity as it relates to several challenging questions: (a) the phenotype and function of diverse DC subsets in HCV-infected patients; (b) the characteristics of non-human HCV infection models from the DCs' point of view; (c) how can in vitro systems, ranging from HCV protein- or peptide-exposed DC to HCV protein-expressing DCs, and in vivo systems, ranging from HCV protein-expressing transgenic mice to HCV-infected non-human primates, be employed to dissect the role of DCs in triggering/maintaining a robust antiviral response; and (d) the prospect of DC-based strategy for managing and finding a cure for HCV infection.

Inhibition of major histocompatibility complex Class I antigen presentation by hepatitis C virus core protein in myeloid dendritic cells

Hepatitis C virus core (HCVcore) protein was expressed in myeloid dendritic cells (DC) from C57/B6 mice (H-2K(b)) by electroporation of HCVcore mRNA to investigate its effect on the ability of DC to prime CD8+ T cells displaying a T cell receptor specific for OVA(257-264) peptide (SIINFEKL)/H-2K(b) complex. Expression of full length HCVcore(191), which is directed to the endoplasmic reticulum (ER) membrane by a C-terminal signal sequence, but not a truncated variant HCVcore(152), which has a wider subcellular localization including the nucleus, significantly reduced surface levels of the H-2K(b)/SIINFEKL complex and impaired the ability of DC to prime naïve CD8+ T cells when they had to process endogenous antigen but not when MHC class I molecules were loaded directly with SIINFEKL peptide. Exploitation of the MHC class I antigen-processing pathway by HCVcore(191) impairs the ability of DC to stimulate CD8+ T cells and may contribute to the persistence of HCV infection.

Parasites lead to evolution of robustness against gene loss in host signaling networks

Many biological networks can maintain their function against single gene loss. However, the evolutionary mechanisms responsible for such robustness remain unclear. Here, we demonstrate that antagonistic host–parasite interactions can act as a selective pressure driving the emergence of robustness against gene loss. Using a model of host signaling networks and simulating their coevolution with parasites that interfere with network function, we find that networks evolve both redundancy and specific architectures that allow them to maintain their response despite removal of proteins. We show that when the parasite pressure is removed, subsequent evolution can lead to loss of redundancy while architecture-based robustness is retained. Contrary to intuition, increased parasite virulence hampers evolution of robustness by limiting the generation of population level diversity in the host. However, when robustness emerges under high virulence, it tends to be stronger. These findings predict an increased presence of robustness mechanisms in biological networks operating under parasite interference. Conversely, the presence of such mechanisms could indicate current or past parasite interference.

Therapy with interferon-alpha and ribavirin for chronic hepatitis C virus infection upregulates membrane HLA-ABC, CD86, and CD28 on peripheral blood mononuclear cells

Multiple interferon-stimulated genes (ISGs) involving T-cell activation are upregulated during initial interferon-alpha-based therapy for chronic hepatitis C virus (HCV) infection. However, the long-term impact on therapeutic outcome in patients remains unknown. In this study, the effects of anti-HCV therapy on the surface expression of HLA-ABC, CD86, and CD28 were longitudinally assessed. These proteins are integral membrane receptors of antigen presentation and triggering of costimulatory signals for activating CD8+ T cells. Peripheral blood mononuclear cells were collected at baseline and post-treatment for 1 day, and 2, 4, 12, and 24 weeks, respectively. This treatment led to a time-related elevation of membrane levels of HLA-ABC and CD86 on B-cells and monocytes in patients with a sustained response (n = 23), but not in those without (n = 8). Meanwhile, upregulation of CD28 on CD4+ and CD8+ T cells was comparable in both groups of sustained responders and non-responders. Steady increases in the B cells' surface and intracellular HLA-ABC were observed, thus, the surface-to-intracellular ratios did not alter over the period of treatment. Furthermore, multivariate analysis shows that increased HLA-ABC on monocytes by week 12 correlates significantly with sustained response (P = 0.033). In conclusion, differential modulation of T-cell activation ISGs, such as HLA-ABC and CD86 might correlate with the outcome of interferon-alpha-based therapy in chronic hepatitis C patients.

The unfolded protein response in hereditary haemochromatosis

To cope with the accumulation of unfolded or misfolded proteins the endoplasmic reticulum (ER) has evolved specific signalling pathways collectively called the unfolded protein response (UPR). Elucidation of the mechanisms governing ER stress signallinghas linked this response to the regulation of diverse physiologic processes as well as to the progression of a number of diseases. Interest in hereditary haemochromatosis (HH) has focused on the study of proteins implicated in iron homeostasis and on the identification of new alleles related with the disease. HFE has been amongst the preferred targets of interest, since the discovery that its C282Y mutation was associated with HH. However, the discrepancies between the disease penetrance and the frequency of this mutation have raised the possibility that its contribution to disease progression might go beyond the mere involvement in regulation of cellular iron uptake. Recent findings revealed that activation of the UPR is a feature of HH and that this stress response may be involved in the genesis of immunological anomalies associated with the disease. This review addresses the connection of the UPR with HH, including its role in MHC-I antigen presentation pathway and possible implications for new clinical approaches to HH.

ER stress: can the liver cope?

Hepatocytes contain abundant endoplasmic reticulum (ER) which is essential for protein metabolism and stress signaling. Hepatic viral infections, metabolic disorders, mutations of genes encoding ER-resident proteins, and abuse of alcohol or drugs can induce ER stress. Liver cells cope with ER stress by an adaptive protective response termed unfolded protein response (UPR), which includes enhancing protein folding and degradation in the ER and down-regulating overall protein synthesis. When the UPR adaptation to ER stress is insufficient, the ER stress response unleashes pathological consequences including hepatic fat accumulation, inflammation and cell death which can lead to liver disease or worsen underlying causes of liver injury, such as viral or diabetes-obesity-related liver disease.

Viruses, endoplasmic reticulum stress, and interferon responses

Viral infection induces endoplasmic reticulum (ER) stress and interferon responses. While viral double-stranded RNA intermediates trigger interferon responses, viral polypeptides synthesized during infection stimulate ER stress. Among the interferon-regulated gene products, the double-stranded RNA-dependent protein kinase (PKR) plays a key role in limiting viral replication. Thus, to establish productive infection, viruses have evolved mechanisms to overcome the deleterious effects of PKR. It has become clear that ER stress causes translational attenuation and transcriptional upregulation of genes encoding proteins that facilitate folding or degradation of proteins. Notably, prolonged ER stress triggers apoptosis. Therefore, viruses are confronted with the consequences of ER stress. Emerging evidence suggests that viruses not only interfere with the interferon system involving PKR but also manipulate the programs emanating from the ER in a complex way, which may facilitate viral replication or pathogenesis. This review highlights recent progress in these areas.

Hepatitis C virus, ER stress, and oxidative stress

Hepatitis C virus (HCV) replication is associated with the endoplasmic reticulum (ER), where the virus causes stress. Cells cope with ER stress by activating an adaptive program called the unfolded protein response (UPR), which alleviates this stress by stimulating protein folding and degradation in the ER and down-regulating overall protein synthesis. Recent work suggests that HCV also alters ER calcium homeostasis, inducing oxidative stress. Future progress in understanding the control that HCV exerts over the ER will provide insight into viral strategies for pathogenesis and persistence in chronically infected patients.

Novel insights into hepatitis C virus replication and persistence

Hepatitis C virus (HCV) is a small enveloped RNA virus that belongs to the family Flaviviridae. A hallmark of HCV is its high propensity to establish a persistent infection that in many cases leads to chronic liver disease. Molecular studies of the virus became possible with the first successful cloning of its genome in 1989. Since then, the genomic organization has been delineated, and viral proteins have been studied in some detail. In 1999, an efficient cell culture system became available that recapitulates the intracellular part of the HCV life cycle, thereby allowing detailed molecular studies of various aspects of viral RNA replication and persistence. This chapter attempts to summarize the current state of knowledge in these most actively worked on fields of HCV research.

Virus subversion of protective immunity

The major histocompatibility (MHC) class I antigen presentation pathway plays a pivotal role in immunity to viruses. MHC class I molecules are expressed on the cell surface of all nucleated cells and present peptides derived from intracellular proteins to cytotoxic T lymphocytes (CTLs), which then eliminate virally infected cells. However, many viruses have evolved proteins to inhibit the MHC class I pathway, thus enabling virally infected cells to escape CTL lysis. In this review, we summarize recent findings about viral inhibition of the MHC class I pathway.

Iron, the HFE gene, and hepatitis C

Intrahepatic iron overload is commonly seen in chronic hepatitis C infection. High levels of intrahepatic iron may lead to accelerated liver injury and development of fibrosis and cirrhosis. This is frequently seen in hereditary hemochromatosis, which in most of the cases is caused by homozygous mutations in the HFE gene. In patients suffering from chronic hepatitis C, the presence of heterozygous HFE mutations associates with higher hepatic iron scores and advanced stages of fibrosis. HFE mutations must therefore be considered as important comorbidity factors in chronic hepatitis C infection.

Role of viral and host factors in HCV persistence: which lesson for therapeutic and preventive strategies?

Several lines of evidence support the view that hepatitis C virus is not directly cytopathic for infected host cells and that the immune response plays a central role in the pathogenesis of liver damage. Innate and adaptive immune responses are induced in most individuals infected with hepatitis C virus but are insufficient to eliminate the virus. The mechanisms responsible for this failure are largely unknown but the kinetics of hepatitis C virus replication relative to the priming of the adaptive responses may exert a profound influence on the balance between virus and host. Immediately after hepatitis C virus infection, the virus replicates efficiently, inducing the production of type I interferons. However, the rapid increase in viral replication seems to be ignored by the adaptive immune response, and after a short interval from exposure, viral load can reach levels comparable to those of patients with established persistent infection. The CD8-mediated response shows functional defects, with impaired production of interferon-gamma, low perforin content, decreased capacity of expansion and lysis of target cells. Late appearance and functional defects of T cells in hepatitis C virus infection might be the result of the rapid increase of the viral load that could create the conditions for exhaustion of the adaptive response or reflect an insufficient function of the innate immune response. This possibility is suggested by in vitro studies showing that hepatitis C virus gene products can interfere with the anti-viral activity of type I interferons and natural killer cells as well as with the maturation of dendritic cells. While T-cell defects are reversed in a minority of infected individuals who succeed in controlling the infection, the T-cell impairment becomes progressively more profound as infection progresses to chronicity. In this situation, therapeutic restoration of adaptive responses may represent a rational strategy to obtain resolution of infection and to complement available therapies. The peculiar kinetics of hepatitis C virus replication and T-cell induction soon after infection may have important implications also for the design of protective vaccines since memory responses may not be able to precede the early peak of viral replication. Therefore, vaccines against hepatitis C virus may be unable to prevent infection but may rather be effective in facilitating a self-limited evolution of infection.

Cell-Mediated Immunity and the Outcome of Hepatitis C Virus Infection

The hepatitis C virus (HCV) infects approximately three percent of the world's population. Some individuals resolve the infection spontaneously, but the majority develop persistent viremia that often causes progressive liver disease. There is an emerging consensus that cellular immune responses are essential for spontaneous resolution of acute hepatitis C and long-term protection from persistent infection. This review focuses on the recent advances in understanding mechanisms of protective immunity and why they fail in most infected individuals. The distinct yet complementary role of CD4+ and CD8+ T lymphocytes in this process is highlighted.

Hyperhomocysteinemia, endoplasmic reticulum stress, and alcoholic liver injury

Deficiencies in vitamins or other factors (B6, B12, folic acid, betaine) and genetic disorders for the metabolism of the non-protein amino acid-homocysteine (Hcy) lead to hyperhomocysteinemia (HHcy). HHcy is an integral component of several disorders including cardiovascular disease, neurodegeneration, diabetes and alcoholic liver disease. HHcy unleashes mediators of inflammation such as NFkappaB, IL-1beta, IL-6, and IL-8, increases production of intracellular superoxide anion causing oxidative stress and reducing intracellular level of nitric oxide (NO), and induces endoplasmic reticulum (ER) stress which can explain many processes of Hcy-promoted cell injury such as apoptosis, fat accumulation, and inflammation. Animal models have played an important role in determining the biological effects of HHcy. ER stress may also be involved in other liver diseases such as alpha (1)-antitrypsin (alpha(1)-AT) deficiency and hepatitis C and/or B virus infection. Future research should evaluate the possible potentiative effects of alcohol and hepatic virus infection on ER stress-induced liver injury, study potentially beneficial effects of lowering Hcy and preventing ER stress in alcoholic humans, and examine polymorphism of Hcy metabolizing enzymes as potential risk-factors for the development of HHcy and liver disease.

Hepatitis C Virus Suppresses the IRE1-XBP1 Pathway of the Unfolded Protein Response

Hepatitis C virus (HCV) gene expression disrupts normal endoplasmic reticulum (ER) functions and induces ER stress. ER stress results from the accumulation of unfolded or misfolded proteins in the ER; cells can alleviate this stress by degrading or refolding these proteins. The IRE1-XBP1 pathway directs both protein refolding and degradation in response to ER stress. Like IRE1-XBP1, other branches of the ER stress response mediate protein refolding. However, IRE1-XBP1 can also specifically activate protein degradation. We show here that XBP1 expression is elevated in cells carrying HCV subgenomic replicons, but XBP1 trans-activating activity is repressed. This prevents the IRE1-XBP1 transcriptional induction of EDEM (ER degradation-enhancing alpha-mannosidase-like protein). The mRNA expression of EDEM is required for the degradation of misfolded proteins. Consequently, misfolded proteins are stable in cells expressing HCV replicons. HCV may suppress the IRE1-XBP1 pathway to stimulate the synthesis of its viral proteins. IRE1alpha-null MEFs, a cell line with a defective IRE1-XBP1 pathway, show elevated levels of HCV IRES-mediated translation. Therefore, HCV may suppress the IRE1-XBP1 pathway to not only promote HCV expression but also to contribute to the persistence of the virus in infected hepatocytes.