An interplay between hypervariable region 1 of the hepatitis C virus E2 glycoprotein, the scavenger receptor BI, and high-density lipoprotein promotes both enhancement of infection and protection against neutralizing antibodies

Lipoprotein receptors: A little grease for enveloped viruses to open the lock?

Several studies recently highlighted the role of lipoprotein receptors in viral entry. These receptors are evolutionarily ancient proteins, key for the transport of lipids as well as other signaling molecules across the plasma membrane. Here, we discuss the different families of lipoprotein receptors and how they are hijacked by enveloped viruses to promote their entry into infected cells. While the usage of lipoprotein receptors was known for members of the Flaviviridae family and vesicular stomatitis virus, the last 4 years have seen the discovery that these receptors are used by many genetically unrelated viruses. We also emphasize how viral particles interact with these receptors and the possible targeting of these host factors as antiviral strategies.

HDL-based therapeutics: A promising frontier in combating viral and bacterial infections

Low levels of high-density lipoprotein (HDL) and impaired HDL functionality have been consistently associated with increased susceptibility to infection and its serious consequences. This has been attributed to the critical role of HDL in maintaining cellular lipid homeostasis, which is essential for the proper functioning of immune and structural cells. HDL, a multifunctional particle, exerts pleiotropic effects in host defense against pathogens. It functions as a natural nanoparticle, capable of sequestering and neutralizing potentially harmful substances like bacterial lipopolysaccharides. HDL possesses antiviral activity, preventing viruses from entering or fusing with host cells, thereby halting their replication cycle. Understanding the complex relationship between HDL and the immune system may reveal innovative targets for developing new treatments to combat infectious diseases and improve patient outcomes. This review aims to emphasize the role of HDL in influencing the course of bacterial and viral infections and its and its therapeutic potential.

An innovative viewpoint on the existing and prospectiveness of SR-B1

Scavenger Receptor Class B Type 1 (SR-B1), a receptor protein expressed on the cell membrane, plays a crucial role in the metabolism and transport of cholesterol and other lipids, contributing significantly to the homeostasis of lipid levels within the body. Bibliometric analysis involves the application of mathematical and statistical methods to quantitatively analyze different types of documents. It involves the analysis of structural and temporal trends in scholarly articles, coupled with the identification of subject emphasis and variations. Through a bibliometric analysis, this study examines the historical background, current research trends, and future directions in the exploration of SR-B1. By offering insights into the research status and development of SR-B1, this paper aims to assist researchers in identifying novel pathways and areas of investigation in this field of study. Following the screening process, it can be concluded that research on SR-B1 has consistently remained a topic of significant interest over the past 17 years. Interestingly, SR-B1 has recently garnered attention in areas beyond its traditional research focus, including the field of cancer. The primary objective of this review is to provide a concise and accessible overview of the development process of SR-B1 that can help readers who are not well-versed in SR-B1 research quickly grasp its key aspects. Furthermore, this review aims to offer insights and suggestions to researchers regarding potential future research directions and areas of emphasis relating to SR-B1.

Hepatitis C virus E1 recruits high-density lipoprotein to support infectivity and evade antibody recognition

Hepatitis C virus (HCV) is a member of the Flaviviridae family; however, unlike other family members, the HCV virion has an unusually high lipid content. HCV has two envelope glycoproteins, E1 and E2. E2 contributes to receptor binding, cell membrane attachment, and immune evasion. In contrast, the functions of E1 are poorly characterized due, in part, to challenges in producing the protein. This manuscript describes the expression and purification of a soluble E1 ectodomain (eE1) that is recognized by conformational, human monoclonal antibodies. eE1 forms a complex with apolipoproteins AI and AII, cholesterol, and phospholipids by recruiting high-density lipoprotein (HDL) from the extracellular media. We show that HDL binding is a function specific to eE1 and HDL hinders recognition of E1 by a neutralizing monoclonal antibody. Either low-density lipoprotein or HDL increases the production and infectivity of cell culture-produced HCV, but E1 preferentially selects HDL, influencing both viral life cycle and antibody evasion.IMPORTANCEHepatitis C virus (HCV) infection is a significant burden on human health, but vaccine candidates have yet to provide broad protection against this infection. We have developed a method to produce high quantities of soluble E1 or E2, the viral proteins located on the surface of HCV. HCV has an unusually high lipid content due to the recruitment of apolipoproteins. We found that E1 (and not E2) preferentially recruits host high-density lipoprotein (HDL) extracellularly. This recruitment of HDL by E1 prevents binding of E1 by a neutralizing antibody and furthermore prevents antibody-mediated neutralization of the virus. By comparison, low-density lipoprotein does not protect the virus from antibody-mediated neutralization. Our findings provide mechanistic insight into apolipoprotein recruitment, which may be critical for vaccine development.

Polyvalent immunization elicits a synergistic broadly neutralizing immune response to hypervariable region 1 variants of hepatitis C virus

A hepatitis C virus (HCV) vaccine is urgently needed. Vaccine development has been hindered by HCV's genetic diversity, particularly within the immunodominant hypervariable region 1 (HVR1). Here, we developed a strategy to elicit broadly neutralizing antibodies to HVR1, which had previously been considered infeasible. We first applied a unique information theory-based measure of genetic distance to evaluate phenotypic relatedness between HVR1 variants. These distances were used to model the structure of HVR1's sequence space, which was found to have five major clusters. Variants from each cluster were used to immunize mice individually, and as a pentavalent mixture. Sera obtained following immunization neutralized every variant in a diverse HCVpp panel (n = 10), including those resistant to monovalent immunization, and at higher mean titers (1/ID50 = 435) than a glycoprotein E2 (1/ID50 = 205) vaccine. This synergistic immune response offers a unique approach to overcoming antigenic variability and may be applicable to other highly mutable viruses.

The role of human lipoproteins for hepatitis C virus persistence

Hepatitis C virus (HCV) is a hepatotropic virus that establishes a chronic infection in most individuals. Effective treatments are available; however, many patients are not aware of their infection. Consequently, they do not receive treatment and HCV transmission remains high, particularly among groups at high risk of exposure such as people who inject intravenous drugs. A prophylactic vaccine may reduce HCV transmission, but is currently not available. HCV has evolved immune evasion strategies, which facilitate persistence and complicate development of a protective vaccine. The peculiar association of HCV particles with human lipoproteins is thought to facilitate evasion from humoral immune response and viral homing to liver cells. A better understanding of these aspects provides the basis for development of protective vaccination strategies. Here, we review key information about the composition of HCV particles, the mechanisms mediating lipoprotein incorporation, and the functional consequences of this interaction.

Severe Acute Hepatitis Outbreaks Associated with a Novel Hepacivirus in Rhizomys pruinosus in Hainan, China

Members of the genus Hepacivirus have a broad range of hosts, with at least 14 species identified. To date, a highly pathogenic hepacivirus causing severe disease in animals has not been found. Here, by using high-throughput sequencing, a new hepacivirus was identified as the dominant and highly pathogenic virus in severe acute hepatitis outbreaks in bamboo rats (Rhizomys pruinosus), with ≈80% mortality; this virus emerged in February 2020 in two bamboo rat farms in China. Hepaciviral genome copies in bamboo rat liver were significantly higher than in other organs. Genomic sequences of hepacivirus strains from 12 sick bamboo rats were found to share 85.3 to 100% nucleotide (nt) identity and 94.9 to 100% amino acid (aa) identity and to share 79.7 to 87.8% nt and 90.4 to 97.8% aa identities with previously reported bamboo rat hepaciviruses of Vietnam and China. Sequence analysis further revealed the simultaneous circulation of genetically divergent hepacivirus variants within the two outbreaks. Phylogenetic analysis showed that hepacivirus strains from the present and previous studies formed an independent clade comprised of at least two genotypes, clearly different from all other known species, suggesting a novel species within the genus Hepacivirus. This is the first report of a non-human-infecting hepacivirus causing potentially fatal infection of bamboo rats, and the associated hepatitis in the animals potentially can be used to develop a surrogate model for the study of hepatitis C virus infection in humans and for the development of therapeutic strategies. IMPORTANCE Members of the genus Hepacivirus have a broad host range, with at least 14 species identified, but none is highly pathogenic to its host except for hepatitis C virus, which causes severe liver diseases in humans. In this study, a new liver-tropic hepacivirus species was identified by high-throughput sequencing as the pathogen associated with two outbreaks of severely acute hepatitis in hoary bamboo rats (Rhizomys pruinosus) on two farms in Hainan Province, China; this is the first reported highly pathogenic animal hepacivirus to our knowledge. Further phylogenetic analysis suggested that the hepaciviruses derived from hoary bamboo rats in either the current or previous studies represent a novel species within the genus Hepacivirus. This finding is a breakthrough that has significantly updated our understanding about the pathogenicity of animal hepaciviruses, and the hepacivirus-associated hepatitis in bamboo rats may have a use as an animal infection model to understand HCV infection and develop therapeutic strategies.

Scavenger Receptors in the Pathogenesis of Viral Infections

Scavenger receptors (SR) are not only pattern recognition receptors involved in the immune response against pathogens but are also important receptors exploited by different virus to enter host cells, and thus represent targets for antiviral therapy. The high mutation rates of viruses, as well as their small genomes are partly responsible for the high rates of virus resistance and effective treatments remain a challenge. Most currently approved formulations target viral-encoded factors. Nevertheless, host proteins may function as additional targets. Thus, there is a need to explore and develop new strategies aiming at cellular factors involved in virus replication and host cell entry. SR-virus interactions have implications in the pathogenesis of several viral diseases and in adenovirus-based vaccination and gene transfer technologies, and may function as markers of severe progression. Inhibition of SR could reduce adenoviral uptake and improve gene therapy and vaccination, as well as reduce pathogenesis. In this review, we will examine the crucial role of SR play in cell entry of different types of human virus, which will allow us to further understand their role in protection and pathogenesis and its potential as antiviral molecules. The recent discovery of SR-B1 as co-factor of SARS-Cov-2 (severe acute respiratory syndrome coronavirus 2) entry is also discussed. Further fundamental research is essential to understand molecular interactions in the dynamic virus-host cell interplay through SR for rational design of therapeutic strategies.

In vitro adaptation and characterization of attenuated hypervariable region 1 swap chimeras of hepatitis C virus

Hepatitis C virus (HCV) chronically infects 70 million people worldwide with an estimated annual disease-related mortality of 400,000. A vaccine could prevent spread of this pervasive human pathogen, but has proven difficult to develop, partly due to neutralizing antibody evasion mechanisms that are inherent features of the virus envelope glycoproteins, E1 and E2. A central actor is the E2 motif, hypervariable region 1 (HVR1), which protects several non-overlapping neutralization epitopes through an incompletely understood mechanism. Here, we show that introducing different HVR1-isolate sequences into cell-culture infectious JFH1-based H77 (genotype 1a) and J4 (genotype 1b) Core-NS2 recombinants can lead to severe viral attenuation. Culture adaptation of attenuated HVR1-swapped recombinants permitted us to identify E1/E2 substitutions at conserved positions both within and outside HVR1 that increased the infectivity of attenuated HVR1-swapped recombinants but were not adaptive for original recombinants. H77 recombinants with HVR1 from multiple other isolates consistently acquired substitutions at position 348 in E1 and position 385 in HVR1 of E2. Interestingly, HVR1-swapped J4 recombinants primarily acquired other substitutions: F291I (E1), F438V (E2), F447L/V/I (E2) and V710L (E2), indicating a different adaptation pathway. For H77 recombinants, the adaptive E1/E2 substitutions increased sensitivity to the neutralizing monoclonal antibodies AR3A and AR4A, whereas for J4 recombinants, they increased sensitivity to AR3A, while having no effect on sensitivity to AR4A. To evaluate effects of the substitutions on AR3A and AR4A binding, we performed ELISAs on extracted E1/E2 protein and performed immunoprecipitation of relevant viruses. However, extracted E1/E2 protein and immunoprecipitation of HCV particles only reproduced the neutralization phenotypes of the J4 recombinants. Finally, we found that the HVR1-swap E1/E2 substitutions decrease virus entry dependency on co-receptor SR-BI. Our study identifies E1/E2 positions that could be critical for intra-complex HVR1 interactions while emphasizing the need for developing novel tools for molecular studies of E1/E2 interactions.

Hepatitis C Virus Epitope Immunodominance and B Cell Repertoire Diversity

Despite the advent of effective, curative treatments for hepatitis C virus (HCV), a preventative vaccine remains essential for the global elimination of HCV. It is now clear that the induction of broadly neutralising antibodies (bNAbs) is essential for the rational design of such a vaccine. This review details the current understanding of epitopes on the HCV envelope, characterising the potency, breadth and immunodominance of antibodies induced against these epitopes, as well as describing the interactions between B-cell receptors and HCV infection, with a particular focus on bNAb heavy and light chain variable gene usage. Additionally, we consider the importance of a public repertoire for antibodies against HCV, compiling current knowledge and suggesting that further research in this area may be critical to the rational design of an effective HCV vaccine.

Cholesterol sensing by CD81 is important for hepatitis C virus entry

CD81 plays a central role in a variety of physiological and pathological processes. Recent structural analysis of CD81 indicates that it contains an intramembrane cholesterol-binding pocket and that interaction with cholesterol may regulate a conformational switch in the large extracellular domain of CD81. Therefore, CD81 possesses a potential cholesterol-sensing mechanism; however, its relevance for protein function is thus far unknown. In this study we investigate CD81 cholesterol sensing in the context of its activity as a receptor for hepatitis C virus (HCV). Structure-led mutagenesis of the cholesterol-binding pocket reduced CD81-cholesterol association but had disparate effects on HCV entry, both reducing and enhancing CD81 receptor activity. We reasoned that this could be explained by alterations in the consequences of cholesterol binding. To investigate this further we performed molecular dynamic simulations of CD81 with and without cholesterol; this identified a potential allosteric mechanism by which cholesterol binding regulates the conformation of CD81. To test this, we designed further mutations to force CD81 into either the open (cholesterol-unbound) or closed (cholesterol-bound) conformation. The open mutant of CD81 exhibited reduced HCV receptor activity, whereas the closed mutant enhanced activity. These data are consistent with cholesterol sensing switching CD81 between a receptor active and inactive state. CD81 interactome analysis also suggests that conformational switching may modulate the assembly of CD81-partner protein networks. This work furthers our understanding of the molecular mechanism of CD81 cholesterol sensing, how this relates to HCV entry, and CD81's function as a molecular scaffold; these insights are relevant to CD81's varied roles in both health and disease.

Novel markers of endothelial dysfunction in hepatitis C virus-related cirrhosis: More than a mere prediction of esophageal varices

BACKGROUND

Hepatitis C virus (HCV) infection may affect lipid metabolism by enhancing the circulating levels of inflammatory cytokines, together with its impact on endothelial function.

AIM

To evaluate the potential correlation of changes in lipid profile, carotid intima-media thickness (CIMT), and ankle-brachial index with the severity of fibrosis, grades of esophageal varices (EVs), and fibrosis indices.

METHODS

The study included 240 subjects who were divided into 3 groups; group 1 (n = 90, HCV-related cirrhotic patients with EVs), group 2 (n = 90, HCV-related cirrhotic patients without EVs), and group 3 (n = 60, served as the healthy control group). All patients underwent routine laboratory tests, including a lipid profile assay. Low-density lipoproteins (LDL)/platelet count and platelet/splenic diameter ratios were calculated. Abdominal ultrasonography, CIMT by carotid Doppler, bedside ankle-brachial index (ABI), liver stiffness measurement, and upper gastrointestinal endoscopy were performed.

RESULTS

Multivariate logistic regression revealed that very-low-density lipoprotein (VLDL) (β = 0.988, odds ratio 2.5, P = 0.001), LDL/platelet count ratio (β = 1.178, odds ratio 3.24, P = 0.001), CIMT (β = 1.37, odds ratio 3.9, P = 0.001), and ABI (β = 2.3, odds ratio 5.9, P = 0.001) were the key variables associated with significant fibrosis, EVs and endothelial dysfunction. CIMT and LDL/platelet count ratio were predictive of advanced fibrosis and EVs at cutoff values of 1.1 mm and 1 mm, respectively, with an area under the curve (AUC) of 0.966 and 0.960 (P = 0.001), while VLDL and ABI at a cutoff of 16.5 mg/dL and 0.94 were predictive of advanced fibrosis and EVs with an AUC of 0.891 and 0.823, respectively (P = 0.001).

CONCLUSION

CIMT, ABI, VLDL, LDL/platelet count ratio are good non-invasive predictors of advanced fibrosis, presence of EVs, and endothelial dysfunction in liver cirrhosis.

HCV Glycoprotein Structure and Implications for B-Cell Vaccine Development

Despite the approval of highly efficient direct-acting antivirals in the last decade Hepatitis C virus (HCV) remains a global health burden and the development of a vaccine would constitute an important step towards the control of HCV. The high genetic variability of the viral glycoproteins E1 and E2, which carry the main neutralizing determinants, together with their intrinsic structural flexibility, the high level of glycosylation that shields conserved neutralization epitopes and immune evasion using decoy epitopes renders the design of an efficient vaccine challenging. Recent structural and functional analyses have highlighted the role of the CD81 receptor binding site on E2, which overlaps with those neutralization epitopes within E2 that have been structurally characterized to date. This CD81 binding site consists of three distinct segments including “epitope I”, “epitope II” and the “CD81 binding loop”. In this review we summarize the structural features of the HCV glycoproteins that have been derived from X-ray structures of neutralizing and non-neutralizing antibody fragments complexed with either recombinant E2 or epitope-derived linear peptides. We focus on the current understanding how neutralizing antibodies interact with their cognate antigen, the structural features of the respective neutralization epitopes targeted by nAbs and discuss the implications for informed vaccine design.

HCV Interplay with Lipoproteins: Inside or Outside the Cells?

Hepatitis C virus (HCV) infection is a major public health issue leading to chronic liver diseases. HCV particles are unique owing to their particular lipid composition, namely the incorporation of neutral lipids and apolipoproteins. The mechanism of association between HCV virion components and these lipoproteins factors remains poorly understood as well as its impact in subsequent steps of the viral life cycle, such as entry into cells. It was proposed that the lipoprotein biogenesis pathway is involved in HCV morphogenesis; yet, recent evidence indicated that HCV particles can mature and evolve biochemically in the extracellular medium after egress. In addition, several viral, cellular and blood components have been shown to influence and regulate this specific association. Finally, this specific structure and composition of HCV particles was found to influence entry into cells as well as their stability and sensitivity to neutralizing antibodies. Due to its specific particle composition, studying the association of HCV particles with lipoproteins remains an important goal towards the rational design of a protective vaccine.

Hepatitis C Virus Entry: An Intriguingly Complex and Highly Regulated Process

Hepatitis C virus (HCV) is a major cause of chronic hepatitis and liver disease worldwide. Its tissue and species tropism are largely defined by the viral entry process that is required for subsequent productive viral infection and establishment of chronic infection. This review provides an overview of the viral and host factors involved in HCV entry into hepatocytes, summarizes our understanding of the molecular mechanisms governing this process and highlights the therapeutic potential of host-targeting entry inhibitors.

Hepatitis C Virus Entry: Protein Interactions and Fusion Determinants Governing Productive Hepatocyte Invasion

Hepatitis C virus (HCV) entry is among the best-studied uptake processes for human pathogenic viruses. Uptake follows a spatially and temporally tightly controlled program. Numerous host factors including proteins, lipids, and glycans promote productive uptake of HCV particles into human liver cells. The virus initially attaches to surface proteoglycans, lipid receptors such as the scavenger receptor BI (SR-BI), and to the tetraspanin CD81. After lateral translocation of virions to tight junctions, claudin-1 (CLDN1) and occludin (OCLN) are essential for entry. Clathrin-mediated endocytosis engulfs HCV particles, which fuse with endosomal membranes after pH drop. Uncoating of the viral RNA genome in the cytoplasm completes the entry process. Here we systematically review and classify HCV entry factors by their mechanistic role, relevance, and level of evidence. Finally, we report on more recent knowledge on determinants of membrane fusion and close with an outlook on future implications of HCV entry research.

Hepatitis C Virus: 30 Years after Its Discovery

Evidence for the existence of another hepatitis-causing pathogen, other than the known hepatitis A and B viruses, emerged in the mid-1970s. A frustrating search of 15 years was ended by the identification of the hepatitis C virus in 1989 using a recombinant DNA immunoscreening method. This discovery quickly led to blood tests that eliminated posttransfusion hepatitis C and could show the partial efficacy of type 1 interferon-based therapies. Subsequent knowledge of the viral replication cycle then led to the development of effective direct-acting antivirals targeting its serine protease, polymerase, and nonstructural protein 5A that resulted in the approval of orally available drug combinations that can cure patients within a few months with few side effects. Meanwhile, vaccine strategies have been shown to be feasible, and they are still required to effectively control this global epidemic.

Quasispecies Diversity Is a Major Risk Factor for Vertical Hepatitis C Virus Transmission

BACKGROUND

Vertical transmission is the major cause of pediatric hepatitis C virus (HCV) infection. The objective of this study was to better understand HCV pathogenesis in pregnant women and provide insights into risk factors and mechanisms involved in vertical transmission.

METHODS

Evolutionary dynamics of HCV variant spectra and HCV-specific neutralizing antibody responses were examined using high-throughput sequencing and pseudoparticle-based assays in pregnant women monoinfected with HCV (n = 17) or coinfected with HCV and human immunodeficiency virus (HIV)-1 (n = 15).

RESULTS

Overall, statistically significant associations were found between HCV quasispecies diversity, selective pressure exerted on the HCV E2 envelope protein, and neutralizing activity of maternal immunoglobulins. Women with low quasispecies diversity displayed significantly higher mean aspartate aminotransferase and alanine aminotransferase levels throughout pregnancy, but this difference was restricted to monoinfected participants. Low quasispecies diversity and inefficient neutralizing activity were also significantly associated with vertical transmission, but only in the monoinfected group.

CONCLUSIONS

These results indicate that maternal neutralizing antibody responses play a role in the prevention of vertical HCV transmission, but not in presence of HIV-1 coinfection, and suggest that the mechanism of vertical transmission may be different between monoinfected and coinfected women. These findings could inform management strategies for the prevention of vertical HCV transmission.

A Recombinant Hepatitis C Virus Genotype 1a E1/E2 Envelope Glycoprotein Vaccine Elicits Antibodies That Differentially Neutralize Closely Related 2a Strains through Interactions of the N-Terminal Hypervariable Region 1 of E2 with Scavenger Receptor B1

A vaccine is still urgently needed to overcome the hepatitis C virus (HCV) epidemic. It is estimated that 1.75 million new HCV infections occur each year, many of which will go undiagnosed and untreated. Untreated HCV can lead to continued spread of the disease, progressive liver fibrosis, cirrhosis, and eventually, end-stage liver disease and/or hepatocellular carcinoma (HCC). Previously, our 1a E1/E2 glycoprotein vaccine was shown to elicit broadly cross-neutralizing antibodies; however, there remains variation in the effectiveness of these antibodies against different HCV genotypes. In this study, we investigated determinants of differential neutralization sensitivity between two highly related genotype 2a isolates, J6 and JFH-1. Our data indicate that the HVR1 region determines neutralization sensitivity to vaccine antisera through modulation of sensitivity to antibodies and interactions with SR-B1. Our results provide additional insight into optimizing a broadly neutralizing HCV vaccine.

The global health burden for hepatitis C virus (HCV) remains high, despite available effective treatments. To eliminate HCV, a prophylactic vaccine is needed. One major challenge in the development of a vaccine is the genetic diversity of the virus, with 7 major genotypes and many subtypes. A global vaccine must be effective against all HCV genotypes. Our previous data showed that the 1a E1/E2 glycoprotein vaccine component elicits broad cross-neutralizing antibodies in humans and animals. However, some variation is seen in the effectiveness of these antibodies to neutralize different HCV genotypes and isolates. Of interest was the differences in neutralizing activity against two closely related isolates of HCV genotype 2a, the J6 and JFH-1 strains. Using site-directed mutagenesis to generate chimeric viruses between the J6 and JFH-1 strains, we found that variant amino acids within the core E2 glycoprotein domain of these two HCV genotype 2a viruses do not influence isolate-specific neutralization. Further analysis revealed that the N-terminal hypervariable region 1 (HVR1) of the E2 protein determines the sensitivity of isolate-specific neutralization, and the HVR1 of the resistant J6 strain binds scavenger receptor class-B type-1 (SR-B1), while the sensitive JFH-1 strain does not. Our data provide new information on mechanisms of isolate-specific neutralization to facilitate the optimization of a much-needed HCV vaccine.

IMPORTANCE A vaccine is still urgently needed to overcome the hepatitis C virus (HCV) epidemic. It is estimated that 1.75 million new HCV infections occur each year, many of which will go undiagnosed and untreated. Untreated HCV can lead to continued spread of the disease, progressive liver fibrosis, cirrhosis, and eventually, end-stage liver disease and/or hepatocellular carcinoma (HCC). Previously, our 1a E1/E2 glycoprotein vaccine was shown to elicit broadly cross-neutralizing antibodies; however, there remains variation in the effectiveness of these antibodies against different HCV genotypes. In this study, we investigated determinants of differential neutralization sensitivity between two highly related genotype 2a isolates, J6 and JFH-1. Our data indicate that the HVR1 region determines neutralization sensitivity to vaccine antisera through modulation of sensitivity to antibodies and interactions with SR-B1. Our results provide additional insight into optimizing a broadly neutralizing HCV vaccine.

Cytokines and serum amyloid A in the pathogenesis of hepatitis C virus infection

Expression of the acute phase protein serum amyloid A (SAA) is dependent on the release of the pro-inflammatory cytokines IL-1, IL-6 and TNF-α during infection and inflammation. Hepatitis C virus (HCV) upregulates SAA-inducing cytokines. In line with this, a segment of chronically infected individuals display increased circulating levels of SAA. SAA has even been proposed to be a potential biomarker to evaluate treatment efficiency and the course of disease. SAA possesses antiviral activity against HCV via direct interaction with the viral particle, but might also divert infectivity through its function as an apolipoprotein. On the other hand, SAA shares inflammatory and angiogenic activity with chemotactic cytokines by activating the G protein-coupled receptor, formyl peptide receptor 2. These latter properties might promote chronic inflammation and hepatic injury. Indeed, up to 80 % of infected individuals develop chronic disease because they cannot completely clear the infection, due to diversion of the immune response. In this review, we summarize the interconnection between SAA and cytokines in the context of HCV infection and highlight the dual role SAA could play in this disease. Nevertheless, more research is needed to establish whether the balance between those opposing activities can be tilted in favor of the host defense.

Interferon lambda 4 impacts the genetic diversity of hepatitis C virus

Hepatitis C virus (HCV) is a highly variable pathogen that frequently establishes chronic infection. This genetic variability is affected by the adaptive immune response but the contribution of other host factors is unclear. Here, we examined the role played by interferon lambda-4 (IFN-λ4) on HCV diversity; IFN-λ4 plays a crucial role in spontaneous clearance or establishment of chronicity following acute infection. We performed viral genome-wide association studies using human and viral data from 485 patients of white ancestry infected with HCV genotype 3a. We demonstrate that combinations of host genetic variants, which determine IFN-λ4 protein production and activity, influence amino acid variation across the viral polyprotein - not restricted to specific viral proteins or HLA restricted epitopes - and modulate viral load. We also observed an association with viral di-nucleotide proportions. These results support a direct role for IFN-λ4 in exerting selective pressure across the viral genome, possibly by a novel mechanism.

Enhancing Hepatitis C virus pseudoparticles infectivity through p7NS2 cellular expression

Hepatitis C pseudoparticles (HCVpp) are used to evaluate HCV cell entry while screening for neutralizing antibodies induced upon vaccination or while screening for new antiviral drugs. In this work we explore the stable production of HCVpp aiming to reduce the variability associated with transient productions. The performance of stably produced HCVpp was assessed by evaluating the influence of Human Serum and the impact of CD81 cellular expression on the infectivity of HCVpp. After evaluating the performance of stably produced HCVpp we studied the effect of co-expressing p7NS2 openreading frame (ORF) on HCVpp infectivity. Our data clearly shows an enhanced infectivity of HCVpp_p7NS2. Even though the exact mechanism was not completely elucidated, the enhanced infectivity of HCVpp_p7NS2 is neither a result of an increase production of virus particles nor a result from increased envelope density. The inhibitory effect of p7 inhibitory molecules such as rimantadine suggests a direct contribution of p7 ion channel for the enhanced infectivity of HCVpp_p7NS2 which is coherent with a pH-dependent cell entry mechanism. In conclusion, we report the establishment of a stable production system of HCVpp with enhanced infectivity through the overexpression of p7NS2 ORF contributing to improve HCV entry assessment assays widely used in antiviral drug discovery and vaccine development.

Syntenin regulates hepatitis C virus sensitivity to neutralizing antibody by promoting E2 secretion through exosomes

BACKGROUND & AIMS

Assembly of infectious hepatitis C virus (HCV) particles is known to involve host lipoproteins, giving rise to unique lipo-viro-particles (LVPs), but proteome studies now suggest that additional cellular proteins are associated with HCV virions or other particles containing the viral envelope glycoprotein E2. Many of these host cell proteins are common markers of exosomes, most notably the intracellular adaptor protein syntenin, which is required for exosome biogenesis. We aimed to elucidate the role of syntenin/E2 in HCV infection.

METHODS

Using cell culture-derived HCV, we studied the biogenesis and function of E2-coated exosomes in both hepatoma cells and primary human hepatocytes (PHHs).

RESULTS

Knockout of syntenin had a negligible impact on HCV replication and virus production, whereas ectopic expression of syntenin at physiological levels reduced intracellular E2 abundance, while concomitantly increasing the secretion of E2-coated exosomes. Importantly, cells expressing syntenin and HCV structural proteins efficiently released exosomes containing E2 but lacking the core protein. Furthermore, infectivity of HCV released from syntenin-expressing hepatoma cells and PHHs was more resistant to neutralization by E2-specific antibodies and chronic-phase patient serum. We also found that high E2/syntenin levels in sera correlate with lower serum neutralization capability.

CONCLUSIONS

E2- and syntenin-containing exosomes are a major type of particle released from cells expressing high levels of syntenin. Efficient production of E2-coated exosomes renders HCV infectivity less susceptible to antibody neutralization in hepatoma cells and PHHs.

LAY SUMMARY

This study identifies a key role for syntenin in the regulation of E2 secretion via exosomes. Efficient production of E2-coated exosomes was shown to make hepatitis C virus less sensitive to antibody neutralization. These results may have implications for the development of a hepatitis C virus vaccine.

A serum protein factor mediates maturation and apoB-association of HCV particles in the extracellular milieu

BACKGROUND & AIMS

In the sera of infected patients, hepatitis C virus (HCV) particles display heterogeneous forms with low-buoyant densities (<1.08), underscoring their lipidation via association with apoB-containing lipoproteins, which was proposed to occur during assembly or secretion from infected hepatocytes. However, the mechanisms inducing this association remain poorly-defined and most cell culture grown HCV (HCVcc) particles exhibit higher density (>1.08) and poor/no association with apoB. We aimed to elucidate the mechanisms of lipidation and to produce HCVcc particles resembling those in infected sera.

METHODS

We produced HCVcc particles of Jc1 or H77 strains from Huh-7.5 hepatoma cells cultured in standard conditions (10%-fetal calf serum) vs. in serum-free or human serum conditions before comparing their density profiles to patient-derived virus. We also characterized wild-type and Jc1/H77 hypervariable region 1 (HVR1)-swapped mutant HCVcc particles produced in serum-free media and incubated with different serum types or with purified lipoproteins.

RESULTS

Compared to serum-free or fetal calf serum conditions, production with human serum redistributed most HCVcc infectious particles to low density (<1.08) or very-low density (<1.04) ranges. In addition, short-time incubation with human serum was sufficient to shift HCVcc physical particles to low-density fractions, in time- and dose-dependent manners, which increased their specific infectivity, promoted apoB-association and induced neutralization-resistance. Moreover, compared to Jc1, we detected higher levels of H77 HCVcc infectious particles in very-low-density fractions, which could unambiguously be attributed to strain-specific features of the HVR1 sequence. Finally, all 3 lipoprotein classes, i.e., very-low-density, low-density and high-density lipoproteins, could synergistically induce low-density shift of HCV particles; yet, this required additional non-lipid serum factor(s) that include albumin.

CONCLUSIONS

The association of HCV particles with lipids may occur in the extracellular milieu. The lipidation level depends on serum composition as well as on HVR1-specific properties. These simple culture conditions allow production of infectious HCV particles resembling those of chronically-infected patients.

LAY SUMMARY

Hepatitis C virus (HCV) particles may associate with apoB and acquire neutral lipids after exiting cells, giving them low-buoyant density. The hypervariable region 1 (HVR1) is a majorviral determinant of E2 that controls this process. Besides lipoproteins, specific serum factors including albumin promote extracellular maturation of HCV virions. HCV particle production in vitro, with media of defined serum conditions, enables production of infectious particles resembling those of chronically infected patients.

Hepatitis C Virus Escape Studies of Human Antibody AR3A Reveal a High Barrier to Resistance and Novel Insights on Viral Antibody Evasion Mechanisms

Yearly, ∼2 million people become hepatitis C virus (HCV) infected, resulting in an elevated lifetime risk for severe liver-related chronic illnesses. Characterizing epitopes of broadly neutralizing antibodies (NAbs), such as AR3A, is critical to guide vaccine development. Previously identified alanine substitutions that can reduce AR3A binding to expressed H77 envelope were introduced into chimeric cell culture-infectious HCV recombinants (HCVcc) H77(core-NS2)/JFH1. Substitutions G523A, G530A, and D535A greatly reduced fitness, and S424A, P525A, and N540A, although viable, conferred only low-level AR3A resistance. Using highly NAb-sensitive hypervariable region 1 (HVR1)-deleted HCVcc, H77/JFH1_ΔHVR1 and J6(core-NS2)/JFH1_ΔHVR1, we previously reported a low barrier to developing AR5A NAb resistance substitutions. Here, we cultured Huh7.5 cells infected with H77/JFH1, H77/JFH1_ΔHVR1, or J6/JFH1_ΔHVR1 with AR3A. We identified the resistance envelope substitutions M345T in H77/JFH1, L438S and F442Y in H77/JFH1_ΔHVR1, and D431G in J6/JFH1_ΔHVR1 M345T increased infectivity and conferred low-level AR3A resistance to H77/JFH1 but not H77/JFH1_ΔHVR1 L438S and F442Y conferred high-level AR3A resistance to H77/JFH1_ΔHVR1 but abrogated the infectivity of H77/JFH1. D431G conferred AR3A resistance to J6/JFH1_ΔHVR1 but not J6/JFH1. This was possibly because D431G conferred broadly increased neutralization sensitivity to J6/JFH1_D431G but not J6/JFH1_ΔHVR1/D431G while decreasing scavenger receptor class B type I coreceptor dependency. Common substitutions at positions 431 and 442 did not confer high-level resistance in other genotype 2a recombinants [JFH1 or T9(core-NS2)/JFH1]. Although the data indicate that AR3A has a high barrier to resistance, our approach permitted identification of low-level resistance substitutions. Also, the HVR1-dependent effects on AR3A resistance substitutions suggest a complex role of HVR1 in virus escape and receptor usage, with important implications for HCV vaccine development.IMPORTANCE Hepatitis C virus (HCV) is a leading cause of liver-related mortality, and limited treatment accessibility makes vaccine development a high priority. The vaccine-relevant cross-genotype-reactive antibody AR3A has shown high potency, but the ability of the virus to rapidly escape by mutating the AR3A epitope (barrier to resistance) remains unexplored. Here, we succeeded in inducing only low-level AR3A resistance, indicating a higher barrier to resistance than what we have previously reported for AR5A. Furthermore, we identify AR3A resistance substitutions that have hypervariable region 1 (HVR1)-dependent effects on HCV viability and on broad neutralization sensitivity. One of these substitutions increased envelope breathing and decreased scavenger receptor class B type I HCV coreceptor dependency, both in an HVR1-dependent fashion. Thus, we identify novel AR3A-specific resistance substitutions and the role of HVR1 in protecting HCV from AR3-targeting antibodies. These viral escape mechanisms should be taken into consideration in future HCV vaccine development.

Hypervariable Region 1 in Envelope Protein 2 of Hepatitis C Virus: A Linchpin in Neutralizing Antibody Evasion and Viral Entry

Chronic hepatitis C virus (HCV) infection is the cause of about 400,000 annual liver disease-related deaths. The global spread of this important human pathogen can potentially be prevented through the development of a vaccine, but this challenge has proven difficult, and much remains unknown about the multitude of mechanisms by which this heterogeneous RNA virus evades inactivation by neutralizing antibodies (NAbs). The N-terminal motif of envelope protein 2 (E2), termed hypervariable region 1 (HVR1), changes rapidly in immunoglobulin-competent patients due to antibody-driven antigenic drift. HVR1 contains NAb epitopes and is directly involved in protecting diverse antibody-specific epitopes on E1, E2, and E1/E2 through incompletely understood mechanisms. The ability of HVR1 to protect HCV from NAbs appears linked with modulation of HCV entry co-receptor interactions. Thus, removal of HVR1 increases interaction with CD81, while altering interaction with scavenger receptor class B, type I (SR-BI) in a complex fashion, and decreasing interaction with low-density lipoprotein receptor. Despite intensive efforts this modulation of receptor interactions by HVR1 remains incompletely understood. SR-BI has received the most attention and it appears that HVR1 is involved in a multimodal HCV/SR-BI interaction involving high-density-lipoprotein associated ApoCI, which may prime the virus for later entry events by exposing conserved NAb epitopes, like those in the CD81 binding site. To fully elucidate the multifunctional role of HVR1 in HCV entry and NAb evasion, improved E1/E2 models and comparative studies with other NAb evasion strategies are needed. Derived knowledge may be instrumental in the development of a prophylactic HCV vaccine.

SR-B1 involvement in keratinocytes in vitro wound closure

Skin represents the most extended organ of human body, having as main function the protection of our body from outdoor stressors. Its protective ability is compromised when the skin is disrupted as a consequence of mechanical insults. For this purpose, cutaneous tissue is equipped with an efficient and fine mechanism involved in repairing the wounded area. Among the numerous players that take part in the wound healing process, SR-B1 has been recently shown to have a role in keratinocyte re-epithelialization. SR-B1 is a mediator of cholesterol uptake from HDLs, whereas it is implicated in other cellular processes such as vitamins absorption, vesicle trafficking or pathogen identification. The aim of this study was to investigate the mechanisms involved in SR-B1 role in skin wound closure. Our in vitro data demonstrated that SR-B1 influenced keratinocyte proliferation and migration through a downregulation of nuclear cyclin D1 levels and active MMP9 expression respectively possibly in an NF-kB-dependent mechanism. In addition, SR-B1 was also able to modulate keratinocyte morphology into a pro-migratory cytoskeleton rearrangement. The present in vitro study suggests a new role of SRB1 as a possible new key player in cutaneous wound healing mechanism.

Current status and future development of infectious cell-culture models for the major genotypes of hepatitis C virus: Essential tools in testing of antivirals and emerging vaccine strategies

In this review, we summarize the relevant scientific advances that led to the development of infectious cell culture systems for hepatitis C virus (HCV) with the corresponding challenges and successes. We also provide an overview of how these systems have contributed to the study of antiviral compounds and their relevance for the development of a much-needed vaccine against this major human pathogen. An efficient infectious system to study HCV in vitro, using human hepatoma derived cells, has only been available since 2005, and was limited to a single isolate, named JFH1, until 2012. Successive developments have been slow and cumbersome, as each available system has been the result of a systematic effort for discovering adaptive mutations conferring culture replication and propagation to patient consensus clones that are inherently non-viable in vitro. High genetic heterogeneity is a paramount characteristic of this virus, and as such, it should preferably be reflected in basic, translational, and clinical studies. The limited number of efficient viral culture systems, in the context of the vast genetic diversity of HCV, continues to represent a major hindrance for the study of this virus, posing a significant barrier towards studies of antivirals (particularly of resistance) and for advancing vaccine development. Intensive research efforts, driven by isolate-specific culture adaptation, have only led to efficient full-length infectious culture systems for a few strains of HCV genotypes 1, 2, 3, and 6. Hence research aimed at identifying novel strategies that will permit universal culture of HCV will be needed to further our understanding of this unique virus causing 400 thousand deaths annually.

Conformational Flexibility in the CD81-Binding Site of the Hepatitis C Virus Glycoprotein E2

Numerous antibodies have been described that potently neutralize a broad range of hepatitis C virus (HCV) isolates and the majority of these antibodies target the binding site for the cellular receptor CD81 within the major HCV glycoprotein E2. A detailed understanding of the major antigenic determinants is crucial for the design of an efficient vaccine that elicits high levels of such antibodies. In the past 6 years, structural studies have shed additional light on the way the host’s humoral immune system recognizes neutralization epitopes within the HCV glycoproteins. One of the most striking findings from these studies is that the same segments of the E2 polypeptide chain induce antibodies targeting distinct antigen conformations. This was demonstrated by several crystal structures of identical polypeptide segments bound to different antibodies, highlighting an unanticipated intrinsic structural flexibility that allows binding of antibodies with distinct paratope shapes following an “induced-fit” mechanism. This unprecedented flexibility extends to the entire binding site for the cellular receptor CD81, underlining the importance of dynamic analyses to understand (1) the interplay between HCV and the humoral immune system and (2) the relevance of this structural flexibility for virus entry. This review summarizes the current understanding how neutralizing antibodies target structurally flexible epitopes. We focus on differences and common features of the reported structures and discuss the implications of the observed structural flexibility for the viral replication cycle, the full scope of the interplay between the virus and the host immune system and—most importantly—informed vaccine design.

Modulation of cutaneous scavenger receptor B1 levels by exogenous stressors impairs "in vitro" wound closure

Scavenger receptor B1 (SR-B1) is a trans-membrane protein, involved in tissue reverse cholesterol transport. Several studies have demonstrated that SR-B1 is also implicated in other physiological processes, such as bacteria and apoptotic cells recognition and regulation of intracellular tocopherol and carotenoids levels. Among the tissues where it is localized, SR-B1 has been shown to be significantly expressed in human epidermis. Our group has demonstrated that SR-B1 levels are down-regulated in human cultured keratinocytes by environmental stressors, such as cigarette smoke, via cellular redox imbalance. Our present study aimed to investigate whether such down-regulation was confirmed in a 3D skin model and under other environmental challengers such as particulate matter and ozone. We also investigated the association between oxidation-induced SR-B1 modulation and impaired wound closure. The data obtained showed that not only cigarette, but also the other environmental stressors reduced SR-B1 expression in epidermal cutaneous tissues and that this effect might be involved in impaired wound healing.

How have retrovirus pseudotypes contributed to our understanding of viral entry?

Study of virus entry into host cells is important for understanding viral tropism and pathogenesis. Studying the entry of in vitro cultured viruses is not always practicable. Study of highly pathogenic viruses, viruses that do not grow in culture, and viruses that rapidly change phenotype in vitro can all benefit from alternative models of entry. Retrovirus particles can be engineered to display the envelope proteins of heterologous enveloped viruses. This approach, broadly termed ‘pseudotyping’, is an important technique for interrogating virus entry. In this perspective we consider how retrovirus pseudotypes have addressed these challenges and improved our understanding of the entry pathways of diverse virus species, including Ebolavirus, human immunodeficiency virus and hepatitis C virus.

Maturation of secreted HCV particles by incorporation of secreted ApoE protects from antibodies by enhancing infectivity

BACKGROUND & AIMS

Hepatitis C virus (HCV) evades humoral immunity and establishes chronic infections. Virus particles circulate in complex with lipoproteins facilitating antibody escape. Apolipoprotein E (ApoE) is essential for intracellular HCV assembly and for HCV cell entry. We aimed to explore if ApoE released from non-infected cells interacts with and modulates secreted HCV particles.

METHODS

ApoE secreted from non-infected cells was incubated with HCV from primary human hepatocytes or Huh-7.5 cells. Co-immunoprecipitation, viral infectivity and neutralization experiments were conducted.

RESULTS

Physiological levels of secreted ApoE (10-60µg/ml) enhanced the infectivity of HCV up to 8-fold across all genotypes, which indirectly decreased virus neutralization by antibodies targeting E1 or E2 up to 10-fold. Infection enhancement was observed for particles produced in primary human hepatocytes and Huh-7.5 cells. Selective depletion of ApoE ablated infection enhancement. Addition of HA-tagged ApoE to HCV particles permitted co-precipitation of HCV virions. Serum ApoE levels ranged between 10-60µg/ml, which is ca 100-fold higher than in Huh-7.5 conditioned cell culture fluids. Serum-derived HCV particles carried much higher amounts of ApoE than cell culture-derived HCV particles. Serum ApoE levels correlated with efficiency of co-precipitation of HCV upon exogenous addition of HA-ApoE. ApoE-dependent infection enhancement was independent of the hypervariable region 1 and SR-B1, but was dependent on heparan sulfate proteoglycans (HSPGs).

CONCLUSIONS

Physiological quantities of secreted ApoE stimulate HCV infection and increase antibody escape, by incorporating into virus particles and enhancing particle interactions with cellular HSPGs. Thus, secreted particles undergo ApoE-dependent maturation to enhance infectivity and to facilitate evasion from neutralizing antibodies. Lay summary: This study shows that HCV particle infectivity is remodeled by secreted ApoE after particle release from cells. Fluctuation of the availability of ApoE likely influences HCV infectivity, antibody escape and transmission.

Chronic viral hepatitis and its association with liver cancer

Chronic infection with hepatitis viruses represents the major causative factor for end-stage liver diseases, including liver cirrhosis and primary liver cancer (hepatocellular carcinoma, HCC). In this review, we highlight the current understanding of the molecular mechanisms that drive the hepatocarcinogenesis associated with chronic hepatitis virus infections. While chronic inflammation (associated with a persistent, but impaired anti-viral immune response) plays a major role in HCC initiation and progression, hepatitis viruses can also directly drive liver cancer. The mechanisms by which hepatitis viruses induce HCC include: hepatitis B virus DNA integration into the host cell genome; metabolic reprogramming by virus infection; induction of the cellular stress response pathway by viral gene products; and interference with tumour suppressors. Finally, we summarise the limitations of hepatitis virus-associated HCC model systems and the development of new techniques to circumvent these shortcomings.

Hepatitis C virus-apolipoprotein interactions: molecular mechanisms and clinical impact

INTRODUCTION

Chronic hepatitis C virus (HCV) infection is a leading cause of cirrhosis, hepatocellular carcinoma and liver failure. Moreover, chronic HCV infection is associated with liver steatosis and metabolic disorders. With 130-150 million people chronically infected in the world, HCV infection represents a major public health problem. One hallmark on the virus is its close link with hepatic lipid and lipoprotein metabolism. Areas covered: HCV is associated with lipoprotein components such as apolipoproteins. These interactions play a key role in the viral life cycle, viral persistence and pathogenesis of liver disease. This review introduces first the role of apolipoproteins in lipoprotein metabolism, then highlights the molecular mechanisms of HCV-lipoprotein interactions and finally discusses their clinical impact. Expert commentary: While the study of virus-host interactions has resulted in a improvement of the understanding of the viral life cycle and the development of highly efficient therapies, major challenges remain: access to therapy is limited and an urgently needed HCV vaccine remains still elusive. Furthermore, the pathogenesis of disease biology is still only partially understood. The investigation of HCV-lipoproteins interactions offers new perspectives for novel therapeutic approaches, contribute to HCV vaccine design and understand virus-induced liver disease and cancer.

"Hep C, where art thou": What are the remaining (fundable) questions in hepatitis C virus research?

Hepatitis C virus (HCV) has dominated the field of hepatology for the past 25 years, and its cure in the majority of treated patients is one of the greatest achievements in all of medicine. However, the latter has led to the belief by some that HCV research should be shelved for other, more pressing areas. The mission for HCV eradication is far from accomplished. As a historical reference, we should consider that disease elimination has required vaccination with all previously controlled infections including smallpox and polio and that simple, effective treatment is not sufficient in most infections to lead to substantial control. Syphilis is the best example, for which a single dose of penicillin (which literally costs pennies and that we have had since 1945) is curative in early stages. Not only have we not eradicated syphilis, rates of infection have increased in many places within the United States in recent years. Most HCV-infected subjects are unaware of their infection, remaining at risk for transmission to others and disease progression, including cirrhosis and hepatocellular carcinoma. In the era of highly effective direct-acting antivirals (DAAs), many questions pertaining to HCV remain, but they are more complex and difficult to answer. Here, I provide my perspective on some of these salient issues: the residual risk for disease progression after sustained virologic response, the optimal approach to current DAA failures, the impact of targeting people who inject drugs with DAAs, vaccine prospects, and application of neutralizing HCV glycoprotein antibodies. (Hepatology 2017;65:341-349).

Neglected but Important Role of Apolipoprotein E Exchange in Hepatitis C Virus Infection

Hepatitis C virus (HCV) is a major cause of chronic liver disease, infecting approximately 170 million people worldwide. HCV assembly is tightly associated with the lipoprotein pathway. Exchangeable apolipoprotein E (apoE) is incorporated on infectious HCV virions and is important for infectious HCV virion morphogenesis and entry. Moreover, the virion apoE level is positively correlated with its ability to escape E2 antibody neutralization. However, the role of apoE exchange in the HCV life cycle is unclear. In this study, the relationship between apoE expression and cell permissiveness to HCV infection was assessed by infecting apoE knockdown and derived apoE rescue cell lines with HCV. Exchange of apoE between lipoproteins and HCV lipoviral particles (LVPs) was evaluated by immunoprecipitation, infectivity testing, and viral genome quantification. Cell and heparin column binding assays were applied to determine the attachment efficiency of LVPs with different levels of incorporated apoE. The results showed that cell permissiveness for HCV infection was determined by exogenous apoE-associated lipoproteins. Furthermore, apoE exchange did occur between HCV LVPs and lipoproteins, which was important to maintain a high apoE level on LVPs. Lipid-free apoE was capable of enhancing HCV infectivity for apoE knockdown cells but not apoE rescue cells. A higher apoE level on LVPs conferred more efficient LVP attachment to both the cell surface and heparin beads. This study revealed that exogenous apoE-incorporating lipoproteins from uninfected hepatocytes safeguarded the apoE level of LVPs for more efficient attachment during HCV infection.

IMPORTANCE

In this study, a neglected but important role of apoE exchange in HCV LVP infectivity after virus assembly and release was identified. The data indicated that apoE expression level in uninfected cells is important for high permissiveness to HCV infection. Secreted apoE-associated lipoprotein specifically enhances infection of HCV LVPs. apoE exchange between HCV LVP and lipoproteins is important to maintain an adequate apoE level on LVPs for their efficient attachment to cell surface. These data defined for the first time an extracellular role of exchangeable apoE in HCV infection and suggested that exchangeable apolipoproteins reach a natural equilibrium between HCV LVPs and lipoprotein particles, which provides a new perspective to the understanding of the heterogeneity of HCV LVPs in composition.

Hepatitis C virus quasispecies and pseudotype analysis from acute infection to chronicity in HIV-1 co-infected individuals

HIV-1 infected patients who acquire HCV infection have higher rates of chronicity and liver disease progression than patients with HCV mono-infection. Understanding early events in this pathogenic process is important. We applied single genome sequencing of the E1 to NS3 regions and viral pseudotype neutralization assays to explore the consequences of viral quasispecies evolution from pre-seroconversion to chronicity in four co-infected individuals (mean follow up 566 days). We observed that one to three founder viruses were transmitted. Relatively low viral sequence diversity, possibly related to an impaired immune response, due to HIV infection was observed in three patients. However, the fourth patient, after an early purifying selection displayed increasing E2 sequence evolution, possibly related to being on suppressive antiretroviral therapy. Viral pseudotypes generated from HCV variants showed relative resistance to neutralization by autologous plasma but not to plasma collected from later time points, confirming ongoing virus escape from antibody neutralization.

The physiological expression of scavenger receptor SR-B1 in canine endometrial and placental epithelial cells and its potential involvement in pathogenesis of pyometra

Pyometra, the purulent inflammation of the uterus, is a common uterine disease of bitches that has potentially life-threatening consequences. The opportunistic bacterial infection of the uterus often progresses into the serious systemic inflammatory response syndrome. In a previous study, we characterized epithelial foam cells in the canine endometrial surface occurring in metestrus, and we regularly observed pronounced epithelial foam-cell formations in pyometra-affected uteri. Therefore, it was assumed that the mechanism behind lipid droplet accumulation in surface epithelial cells might even increase bacterial binding capacity and promote pyometra development. Lipid droplet accumulation in epithelial cells is accomplished via specialized lipid receptors called scavenger receptors (SR). Scavenger receptor class B type 1 (SR-B1) is an important receptor for lipid accumulation in diverse cell types, but it is also a strong binding partner for bacteria, and thereby enhances bacterial adhesion and clinical signs of systemic inflammatory response syndrome. In the present study, after the isolation of metestrous surface epithelial cells from canine uteri by laser capture microdissection, SR-B1 was identified at the messenger RNA (mRNA) level by quantitative real time polymerase chain reaction and also at the protein level by means of immunohistochemistry. In pyometra-affected uteri, SR-B1 mRNA expression was higher than that in the healthy control samples, and SR-B1 protein was expressed in the surface and crypt epithelial cells. Furthermore, to understand the physiological role of SR-B1 expression in the metestrus surface epithelial cells, we investigated its expression in the epithelial cells of the glandular chambers of canine placenta in different stages of gestation because these cells are also characterized by lipid droplet accumulation. SR-B1 was present in the placental epithelial cells of the glandular chambers from 25 to 30 and 45 to 50 days of gestation, where basal and basal and/or apical expression patterns were observed. The physiological expression of SR-B1 in metestrous endometrial surface epithelia might be related to the implantation process and embryo development, as SR-B1 was also identified in the epithelial cells of the canine placental chambers. The increased levels of SR-B1 in pyometra-affected uteri indicate a potential role for this scavenger receptor in endometrial bacterial adhesion.

Entry inhibitors: New advances in HCV treatment

Hepatitis C virus (HCV) infection affects approximately 3% of the world's population and causes chronic liver diseases, including liver fibrosis, cirrhosis, and hepatocellular carcinoma. Although current antiviral therapy comprising direct-acting antivirals (DAAs) can achieve a quite satisfying sustained virological response (SVR) rate, it is still limited by viral resistance, long treatment duration, combined adverse reactions, and high costs. Moreover, the currently marketed antivirals fail to prevent graft reinfections in HCV patients who receive liver transplantations, probably due to the cell-to-cell transmission of the virus, which is also one of the main reasons behind treatment failure. HCV entry is a highly orchestrated process involving initial attachment and binding, post-binding interactions with host cell factors, internalization, and fusion between the virion and the host cell membrane. Together, these processes provide multiple novel and promising targets for antiviral therapy. Most entry inhibitors target host cell components with high genetic barriers and eliminate viral infection from the very beginning of the viral life cycle. In future, the addition of entry inhibitors to a combination of treatment regimens might optimize and widen the prevention and treatment of HCV infection. This review summarizes the molecular mechanisms and prospects of the current preclinical and clinical development of antiviral agents targeting HCV entry.

Network Analysis of the Chronic Hepatitis C Virome Defines Hypervariable Region 1 Evolutionary Phenotypes in the Context of Humoral Immune Responses

Hypervariable region 1 (HVR1) of hepatitis C virus (HCV) comprises the first 27 N-terminal amino acid residues of E2. It is classically seen as the most heterogeneous region of the HCV genome. In this study, we assessed HVR1 evolution by using ultradeep pyrosequencing for a cohort of treatment-naive, chronically infected patients over a short, 16-week period. Organization of the sequence set into connected components that represented single nucleotide substitution events revealed a network dominated by highly connected, centrally positioned master sequences. HVR1 phenotypes were observed to be under strong purifying (stationary) and strong positive (antigenic drift) selection pressures, which were coincident with advancing patient age and cirrhosis of the liver. It followed that stationary viromes were dominated by a single HVR1 variant surrounded by minor variants comprised from conservative single amino acid substitution events. We present evidence to suggest that neutralization antibody efficacy was diminished for stationary-virome HVR1 variants. Our results identify the HVR1 network structure during chronic infection as the preferential dominance of a single variant within a narrow sequence space.

IMPORTANCE

HCV infection is often asymptomatic, and chronic infection is generally well established in advance of initial diagnosis and subsequent treatment. HVR1 can undergo rapid sequence evolution during acute infection, and the variant pool is typically seen to diverge away from ancestral sequences as infection progresses from the acute to the chronic phase. In this report, we describe HVR1 viromes in chronically infected patients that are defined by a dominant epitope located centrally within a narrow variant pool. Our findings suggest that weakened humoral immune activity, as a consequence of persistent chronic infection, allows for the acquisition and maintenance of host-specific adaptive mutations at HVR1 that reflect virus fitness.

Ledipasvir/sofosbuvir treatment of hepatitis C virus is associated with reduction in serum apolipoprotein levels

The interaction of lipoproteins with hepatitis C virus (HCV) has pathogenic and therapeutic implications. Our aim was to evaluate changes in the apolipoprotein profile of patients with chronic hepatitis C during and after successful cure with ledipasvir and sofosbuvir (LDV/SOF) with and without ribavirin (RBV). One hundred HCV genotype 1 patients who had achieved SVR-12 after treatment with 12 weeks of LDV/SOF ± RBV were selected from the ION-1 clinical trial. Frozen serum samples from baseline, end of treatment and week 4 of follow-up were used to assay apolipoproteins (apoAI, apoAII, apoB, apoCII, apoCIII, apoE) using the Multiplex platform to assess for changes in the apolipoprotein levels. At the end of treatment compared to baseline, a significant reduction in apoAII levels (-14.97 ± 63.44 μg/mL, P = 0.0067) and apoE levels (-4.38 ± 12.19 μg/mL, P < 0.001) was noted. These declines from baseline in apoAII (-16.59 ±66.15 μg/mL, P = 0.0075) and apoE (-2.66 ± 12.64 μg/mL, P = 0.015) persisted at 4 weeks of post-treatment follow-up. In multivariate analysis, treatment with LDV/SOF + RBV was independently associated with reduction in apoE (beta = 5.31 μg/mL, P = 0.002) (compared to RBV-free LDV/SOF) (P < 0.05). In contrast, apoCII levels overall increased from baseline to end of treatment (+2.74 ±11.76 μg/mL, P = 0.03) and persisted at 4 weeks of follow-up (+4.46 ± 12.81 μg/mL from baseline, P = 0.0005). Subgroup analysis revealed an increase in apoCII during treatment only in patients receiving LDV/SOF without RBV (+5.52 ± 11.92 μg/mL, P = 0.0007) but not in patients receiving LDV/SOF + RBV (P = 0.638). Treatment with LDV/SOF ± RBV is associated with a persistent reduction in the apolipoprotein AII and E after achieving cure. These data suggest that treatment with LDV/SOF ± RBV may be associated with alterations in serum apolipoproteins which could potentially impact viral eradication.

Targeting host lipid synthesis and metabolism to inhibit dengue and hepatitis C viruses

Lipids are necessary for every step in the replication cycle of hepatitis C virus (HCV) and dengue virus (DENV), members of the family Flaviviridae. Recent studies have demonstrated that discrete steps in the replication cycles of these viruses can be inhibited by pharmacological agents that target host factors mediating lipid synthesis, metabolism, trafficking, and signal transduction. Despite this, targeting host lipid metabolism and trafficking as an antiviral strategy by blockade of entire pathways may be limited due to host toxicity. Knowledge of the molecular details of lipid structure and function in replication and the mechanisms whereby specific lipids are generated and trafficked to the relevant sites may enable more targeted antiviral strategies without global effects on the host cell. In this review, we discuss lipids demonstrated to be critical to the replication cycles of HCV and DENV and highlight potential areas for anti-viral development. This review article forms part of a symposium on flavivirus drug discovery in Antiviral Research.

Functional and Biochemical Characterization of Hepatitis C Virus (HCV) Particles Produced in a Humanized Liver Mouse Model

Lipoprotein components are crucial factors for hepatitis C virus (HCV) assembly and entry. As hepatoma cells producing cell culture-derived HCV (HCVcc) particles are impaired in some aspects of lipoprotein metabolism, it is of upmost interest to biochemically and functionally characterize the in vivo produced viral particles, particularly regarding how lipoprotein components modulate HCV entry by lipid transfer receptors such as scavenger receptor BI (SR-BI). Sera from HCVcc-infected liver humanized FRG mice were separated by density gradients. Viral subpopulations, termed HCVfrg particles, were characterized for their physical properties, apolipoprotein association, and infectivity. We demonstrate that, in contrast to the widely spread distribution of apolipoproteins across the different HCVcc subpopulations, the most infectious HCVfrg particles are highly enriched in apoE, suggesting that such apolipoprotein enrichment plays a role for entry of in vivo derived infectious particles likely via usage of apolipoprotein receptors. Consistent with this salient feature, we further reveal previously undefined functionalities of SR-BI in promoting entry of in vivo produced HCV. First, unlike HCVcc, SR-BI is a particularly limiting factor for entry of HCVfrg subpopulations of very low density. Second, HCVfrg entry involves SR-BI lipid transfer activity but not its capacity to bind to the viral glycoprotein E2. In conclusion, we demonstrate that composition and biophysical properties of the different subpopulations of in vivo produced HCVfrg particles modulate their levels of infectivity and receptor usage, hereby featuring divergences with in vitro produced HCVcc particles and highlighting the powerfulness of this in vivo model for the functional study of the interplay between HCV and liver components.

Genetic Diversity Underlying the Envelope Glycoproteins of Hepatitis C Virus: Structural and Functional Consequences and the Implications for Vaccine Design

In the 26 years since the discovery of Hepatitis C virus (HCV) a major global research effort has illuminated many aspects of the viral life cycle, facilitating the development of targeted antivirals. Recently, effective direct-acting antiviral (DAA) regimens with >90% cure rates have become available for treatment of chronic HCV infection in developed nations, representing a significant advance towards global eradication. However, the high cost of these treatments results in highly restricted access in developing nations, where the disease burden is greatest. Additionally, the largely asymptomatic nature of infection facilitates continued transmission in at risk groups and resource constrained settings due to limited surveillance. Consequently a prophylactic vaccine is much needed. The HCV envelope glycoproteins E1 and E2 are located on the surface of viral lipid envelope, facilitate viral entry and are the targets for host immunity, in addition to other functions. Unfortunately, the extreme global genetic and antigenic diversity exhibited by the HCV glycoproteins represents a significant obstacle to vaccine development. Here we review current knowledge of HCV envelope protein structure, integrating knowledge of genetic, antigenic and functional diversity to inform rational immunogen design.

Comparison of neutralization profiles for anti-HCV reactive donor samples with or without detectable HCV RNA

BACKGROUND AND OBJECTIVES

At Japanese Red Cross (JRC) Blood Centers, all donated blood is screened for hepatitis C virus (HCV) by serological and nucleic acid amplification testing. Donor plasma that tested reactive for anti-HCV by serological test is disqualified even if the donor tests negative for HCV RNA. These test results reflect both true-positive results because of past HCV infection and false-positive results because the cross-reactivity of plasma IgG, which current testing methods are unable to distinguish. To characterize these antibody test results, we examined the neutralizing activity of these plasma samples.

MATERIAL AND METHODS

Donor plasma samples that tested reactive for anti-HCV by serological test but negative for HCV RNA (n = 43) were analysed for determining their neutralizing activities measured by the inhibition of the cellular entry of pseudoparticles harbouring HCV envelope glycoproteins (HCVpp).

RESULTS

Strong and broad neutralizing activities against HCVpp entry similar to the samples that tested reactive for anti-HCV serological test and positive for HCV RNA (considered to be derived from individuals with chronic HCV infection) were observed in three of 43 plasma samples from donors who tested anti-HCV reactive but HCV RNA negative.

CONCLUSION

By examining the neutralizing activities of plasma samples, we identified individuals with a past HCV infection from those in whom we were unable to confirm HCV infection according to the current testing algorithms of JRC, which do not perform anti-HCV confirmatory tests.

Low cross-neutralization of hepatitis C correlates with liver disease in immunocompromized patients

BACKGROUND AND AIMS

Chronic hepatitis C virus (HCV) infection causes severe liver disease in HIV-infected patients and liver transplant recipients. The impact of serum and immunoglobulin on viral entry was analysed in these patients.

METHOD

Sera from 60 anti-HCV positive patients, including 30 who were also anti-HIV positive, were tested with HCVpp from different genotypes (1a, 1b, 3 and 4) and with HCVcc (H77/JFH1). Seventeen HIV-seropositive and 13 HIV-seronegative patients with decompensated liver disease were studied before and after liver transplant.

RESULTS

Serum neutralization was markedly lower after liver transplant and in HIV patients than in mono-infected immune-competent individuals. This effect was due to low antibody-mediated neutralization. In HIV patients, low neutralization was correlated with low lymphocyte T CD4 cell counts and the severity of liver disease. To characterize neutralization, we tested HCVpp lacking hypervariable region (HVR1) and SR-BI receptor cholesterol transfer inhibition by BLT-4. These experiments showed that neutralization was strongly dependent on the HVR1 and the SR-BI receptor. HVR1 sequences showed that selective pressures were low in immune-compromised patients and highly correlated to HCV neutralization after liver transplant. Neutralization experiments were reproduced with HCV strain JFH1.

CONCLUSION

Serum neutralization in HIV-coinfected patients and HCV-infected liver transplant recipients is poor enhancing HCV entry through HVR1/SR-BI interplay. This may contribute to the severity of hepatitis C in these settings.

HDL in infectious diseases and sepsis

During infection significant alterations in lipid metabolism and lipoprotein composition occur. Triglyceride and VLDL cholesterol levels increase, while reduced HDL cholesterol (HDL-C) and LDL cholesterol (LDL-C) levels are observed. More importantly, endotoxemia modulates HDL composition and size: phospholipids are reduced as well as apolipoprotein (apo) A-I, while serum amyloid A (SAA) and secretory phospholipase A2 (sPLA2) dramatically increase, and, although the total HDL particle number does not change, a significant decrease in the number of small- and medium-size particles is observed. Low HDL-C levels inversely correlate with the severity of septic disease and associate with an exaggerated systemic inflammatory response. HDL, as well as other plasma lipoproteins, can bind and neutralize Gram-negative bacterial lipopolysaccharide (LPS) and Gram-positive bacterial lipoteichoic acid (LTA), thus favoring the clearance of these products. HDLs are emerging also as a relevant player during parasitic infections, and a specific component of HDL, namely, apoL-1, confers innate immunity against trypanosome by favoring lysosomal swelling which kills the parasite. During virus infections, proteins associated with the modulation of cholesterol bioavailability in the lipid rafts such as ABCA1 and SR-BI have been shown to favor virus entry into the cells. Pharmacological studies support the benefit of recombinant HDL or apoA-I mimetics during bacterial infection, while apoL-1-nanobody complexes were tested for trypanosome infection. Finally, SR-BI antagonism represents a novel and forefront approach interfering with hepatitis C virus entry which is currently tested in clinical studies. From the coming years, we have to expect new and compelling observations further linking HDL to innate immunity and infections.

Hepatitis C virus life cycle and lipid metabolism

Hepatitis C Virus (HCV) infects over 150 million people worldwide. In most cases HCV infection becomes chronic, causing liver disease ranging from fibrosis to cirrhosis and hepatocellular carcinoma. HCV affects the cholesterol homeostasis and at the molecular level, every step of the virus life cycle is intimately connected to lipid metabolism. In this review, we present an update on the lipids and apolipoproteins that are involved in the HCV infectious cycle steps: entry, replication and assembly. Moreover, the result of the assembly process is a lipoviroparticle, which represents a peculiarity of hepatitis C virion. This review illustrates an example of an intricate virus-host interaction governed by lipid metabolism.

The mechanism of HCV entry into host cells

Hepatitis C virus (HCV) is an enveloped, positive strand RNA virus classified within the Flaviviridae family and is a major cause of liver disease worldwide. HCV life cycle and propagation are tightly linked to several aspects of lipid metabolism. HCV propagation depends on and also shapes several aspects of lipid metabolism such as cholesterol uptake and efflux through different lipoprotein receptors during its entry into cells, lipid metabolism modulating HCV genome replication, lipid droplets acting as a platform for recruitment of viral components, and very low density lipoprotein assembly pathway resulting in incorporation of neutral lipids and apolipoproteins into viral particles. During the first steps of infection, HCV enters hepatocytes through a multistep and slow process. The initial capture of HCV particles by glycosaminoglycans and/or lipoprotein receptors is followed by coordinated interactions with the scavenger receptor class B type I, a major receptor of high-density lipoprotein, the CD81 tetraspanin, and the tight junction proteins Claudin-1 and Occludin. This tight concert of receptor interactions ultimately leads to uptake and cellular internalization of HCV through a process of clathrin-dependent endocytosis. Over the years, the identification of the HCV entry receptors and cofactors has led to a better understanding of HCV entry and of the narrow tropism of HCV for the liver. Yet, the role of the two HCV envelope glycoproteins, E1 and E2, remains ill-defined, particularly concerning their involvement in the membrane fusion process. Here, we review the current knowledge and advances addressing the mechanism of HCV cell entry within hepatocytes and we highlight the challenges that remain to be addressed.