Virus-specific cofactor requirement and chimeric hepatitis C virus/GB virus B nonstructural protein 3

Running With Scissors: Evolutionary Conflicts Between Viral Proteases and the Host Immune System

Many pathogens encode proteases that serve to antagonize the host immune system. In particular, viruses with a positive-sense single-stranded RNA genome [(+)ssRNA], including picornaviruses, flaviviruses, and coronaviruses, encode proteases that are not only required for processing viral polyproteins into functional units but also manipulate crucial host cellular processes through their proteolytic activity. Because these proteases must cleave numerous polyprotein sites as well as diverse host targets, evolution of these viral proteases is expected to be highly constrained. However, despite this strong evolutionary constraint, mounting evidence suggests that viral proteases such as picornavirus 3C, flavivirus NS3, and coronavirus 3CL, are engaged in molecular ‘arms races’ with their targeted host factors, resulting in host- and virus-specific determinants of protease cleavage. In cases where protease-mediated cleavage results in host immune inactivation, recurrent host gene evolution can result in avoidance of cleavage by viral proteases. In other cases, such as recently described examples in NLRP1 and CARD8, hosts have evolved ‘tripwire’ sequences that mimic protease cleavage sites and activate an immune response upon cleavage. In both cases, host evolution may be responsible for driving viral protease evolution, helping explain why viral proteases and polyprotein sites are divergent among related viruses despite such strong evolutionary constraint. Importantly, these evolutionary conflicts result in diverse protease-host interactions even within closely related host and viral species, thereby contributing to host range, zoonotic potential, and pathogenicity of viral infection. Such examples highlight the importance of examining viral protease-host interactions through an evolutionary lens.

Experimental Infections of the Common Marmoset (Callithrix jacchus)

Interest in the use of marmosets for experimental infectious disease has dramatically increased in the last decade. These animals are native to the Atlantic coastal forests in northeastern Brazil. The majority of experimental animals come from the National Primate Research Centers and other breeding facilities. They are advantageous because of their relative small size, weighting 350–400 g as adults, their life span is compact compared with other nonhuman primate (NHP), and they produce offspring by 3 years of age. They are free of Herpes B virus and, it is believed, to date, other dangerous human pathogens (Abbot et al., 2003) [1]. We describe here the experimental infections of marmosets to human pathogens. While it is always interesting to compare various NHPs with each other, the importance of an animal model is always in comparing its similarities to human infections.

Infection of Common Marmosets with GB Virus B Chimeric Virus Encoding the Major Nonstructural Proteins NS2 to NS4A of Hepatitis C Virus

A lack of immunocompetent-small-primate models has been an obstacle for developing hepatitis C virus (HCV) vaccines and affordable antiviral drugs. In this study, HCV/GB virus B (GBV-B) chimeric virus carrying the major nonstructural proteins NS2 to NS4A (HCV NS2 to -4A chimera) was produced and used to infect common marmosets, since HCV NS2 to NS4A proteins are critical proteases and major antigens. Seven marmosets were inoculated intrahepatically with HCV NS2 to -4A chimera RNA for primary infection or intravenously injected with chimera-containing serum for passage infection. Three animals used as controls were injected with phosphate-buffered saline (PBS) or GBV-B, respectively. Six of seven HCV NS2 to -4A chimera-infected marmosets exhibited consistent viremia and one showed transient viremia during the course of follow-up detection. All six infected animals with persistent circulating viremia presented characteristics typical of viral hepatitis, including viral RNA and proteins in hepatocytes and histopathological changes in liver tissue. Viremia was consistently detected for 5 to 54 weeks of follow-up. FK506 immunosuppression facilitated the establishment of persistent chimera infection in marmosets. An animal with chimera infection spontaneously cleared the virus in blood 7 weeks following the first inoculation, but viral-RNA persistence, low-level viral protein, and mild necroinflammation remained in liver tissue. The specific antibody and T-cell response to HCV NS3 in this viremia-resolved marmoset was boosted by rechallenging, but no viremia was detected during 57 weeks of follow-up. The chimera-infected marmosets described can be used as a suitable small-primate animal model for studying novel antiviral drugs and T-cell-based vaccines against HCV infection.

IMPORTANCE

HCV infection causes approximately 70% of chronic hepatitis and is frequently associated with primary liver cancer globally. Chimpanzees have been used as a reliable primate model for HCV infection, but ethical considerations have restricted their utility in biomedical research. GB virus B (GBV-B) is a flavivirus related to HCV. It can infect common marmosets, a New World small primate, and induces viral hepatitis similar to HCV infection in humans. To minimize differences between GBV-B and HCV, we generated HCV NS2 to -4A/GBV-B chimeric viruses and established a chimera-infected marmoset model. HCV NS2 to -4A chimera-infected marmosets provide a small-animal model for evaluating novel antiviral drugs targeting HCV NS3-NS4A protease and T-cell-based HCV vaccines.

Modeling HCV disease in animals: virology, immunology and pathogenesis of HCV and GBV-B infections

Hepatitis C virus (HCV) infection has become a global public health burden costing billions of dollars in health care annually. Even with rapidly advancing scientific technologies this disease still poses a significant threat due to a lack of vaccines and affordable treatment options. The immune correlates of protection and predisposing factors toward chronicity remain major obstacles to development of HCV vaccines and immunotherapeutics due, at least in part, to lack of a tangible infection animal model. This review discusses the currently available animal models for HCV disease with a primary focus on GB virus B (GBV-B) infection of New World primates that recapitulates the dual Hepacivirus phenotypes of acute viral clearance and chronic pathologic disease. HCV and GBV-B are also closely phylogenetically related and advances in characterization of the immune systems of New World primates have already led to the use of this model for drug testing and vaccine trials. Herein, we discuss the benefits and caveats of the GBV-B infection model and discuss potential avenues for future development of novel vaccines and immunotherapies.

Changes in immune cell populations in the periphery and liver of GBV-B-infected and convalescent tamarins (Saguinus labiatus)

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GBV-B infection of tamarins is a valuable model for acute HCV infection.

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We observed distinct expression patterns of PD-1, a marker of T-cell activation, on peripheral and hepatic lymphocytes.

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Differential PD-1 expression is coincident with reduction in peripheral GBV-B.

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Liver-associated viral RNA in the absence of peripheral viraemia indicates maintenance of occult infection.

Flaviviruses related to hepatitis C virus (HCV) in suitable animal models may provide further insight into the role that cellular immunity contributes to spontaneous clearance of HCV. We characterised changes in lymphocyte populations in tamarins with an acute GBV-B infection, a hepatitis virus of the flaviviridae. Major immune cell populations were monitored in peripheral and intra-hepatic lymphocytes at high viraemia or following a period when peripheral virus was no longer detected. Limited changes in major lymphocyte populations were apparent during high viraemia; however, the proportions of CD3⁺ lymphocytes decreased and CD20⁺ lymphocytes increased once peripheral viraemia became undetectable. Intrahepatic lymphocyte populations increased at both time points post-infection. Distinct expression patterns of PD-1, a marker of T-cell activation, were observed on peripheral and hepatic lymphocytes; notably there was elevated PD-1 expression on hepatic CD4⁺ T-cells during high viraemia, suggesting an activated phenotype, which decreased following clearance of peripheral viraemia. At times when peripheral vRNA was not detected, suggesting viral clearance, we were able to readily detect GBV-B RNA in the liver, indicative of long-term virus replication. This study is the first description of changes in lymphocyte populations during GBV-B infection of tamarins and provides a foundation for more detailed investigations of the responses that contribute to the control of GBV-B infection.

Development and characterization of a replicon-based phenotypic assay for assessing HCV NS4B from clinical isolates

The hepatitis C virus (HCV) NS4B inhibitors have shown potent inhibition of HCV replication in vitro. To assess the effect of viral diversity on the susceptibility to NS4B inhibitors, genotype (GT)-specific GT1a and GT1b replicon shuttle vectors were designed and created for cloning HCV NS4B genes from clinical isolates. For the GT1b NS4B shuttle vector, the S2204I adaptive mutation was introduced in NS5A to improve replication due to the replacement of the K1846T adaptive mutation in NS4B with NS4B from the clinical isolates. In addition to the adaptive mutations, a newly identified Huh-7 cell line, Huh-7-1C, which is highly permissive for both GT1a and GT1b replication, was used to further enhance the replication levels. HCV NS4B gene from clinical isolates was amplified and inserted into the corresponding GT1a and GT1b modified lab strain chimeric replicons. GT1a and GT1b chimeric replicons expressing diverse NS4B genes from corresponding subtypes of clinical isolates replicated at highly efficient levels for phenotypic analysis. Due to natural variation in their amino acid residues in NS4B, these isolates displayed varying drug susceptibilities to an NS4B inhibitor. In mixed populations with wild-type, the sensitivity of resistance detection of NS4B resistant mutants H94R and V105M was between 20% and 80%. The chimeric shuttle vectors can be used to characterize the activity of antiviral drugs targeting NS4B from diverse natural clinical isolates and aid in the development of novel compounds against HCV NS4B.

Combined X-ray, NMR, and kinetic analyses reveal uncommon binding characteristics of the hepatitis C virus NS3-NS4A protease inhibitor BI 201335

Hepatitis C virus infection, a major cause of liver disease worldwide, is curable, but currently approved therapies have suboptimal efficacy. Supplementing these therapies with direct-acting antiviral agents has the potential to considerably improve treatment prospects for hepatitis C virus-infected patients. The critical role played by the viral NS3 protease makes it an attractive target, and despite its shallow, solvent-exposed active site, several potent NS3 protease inhibitors are currently in the clinic. BI 201335, which is progressing through Phase IIb trials, contains a unique C-terminal carboxylic acid that binds noncovalently to the active site and a bromo-quinoline substitution on its proline residue that provides significant potency. In this work we have used stopped flow kinetics, x-ray crystallography, and NMR to characterize these distinctive features. Key findings include: slow association and dissociation rates within a single-step binding mechanism; the critical involvement of water molecules in acid binding; and protein side chain rearrangements, a bromine-oxygen halogen bond, and profound pK(a) changes within the catalytic triad associated with binding of the bromo-quinoline moiety.

A comparative analysis of the substrate permissiveness of HCV and GBV-B NS3/4A proteases reveals genetic evidence for an interaction with NS4B protein during genome replication

The hepatitis C virus (HCV) serine protease (NS3/4A) processes the NS3-NS5B segment of the viral polyprotein and also cleaves host proteins involved in interferon signaling, making it an important target for antiviral drug discovery and suggesting a wide breadth of substrate specificity. We compared substrate specificities of the HCV protease with that of the GB virus B (GBV-B), a distantly related nonhuman primate hepacivirus, by exchanging amino acid sequences at the NS4B/5A and/or NS5A/5B cleavage junctions between these viruses within the backbone of subgenomic replicons. This mutagenesis study demonstrated that the GBV-B protease had a broader substrate tolerance, a feature corroborated by structural homology modeling. However, despite efficient polyprotein processing, GBV-B RNAs containing HCV sequences at the C-terminus of NS4B had a pseudo-lethal replication phenotype. Replication-competent revertants contained second-site substitutions within the NS3 protease or NS4B N-terminus, providing genetic evidence for an essential interaction between NS3 and NS4B during genome replication.

Homology modeling and molecular dynamics simulations of Dengue virus NS2B/NS3 protease: insight into molecular interaction

The pathogenic West Nile virus (WNV) and Dengue virus (DV) are growing global threats for which there are no specific treatments. Both viruses possess a two component NS2B/NS3 protease which cleaves viral precursor proteins. Whereas for the WNV protease two crystal structures in complex with an inhibitor have been solved recently, no such information is available for the DV protease. Here, we report the generation of a homology model of DV NS2B/NS3 protease. Since it is known from the related WNV protease that it adopts a distinct conformation in free and in inhibitor-complexed form, a special emphasis was given to the analysis of the protease flexibility. Therefore, several models of DV NS2B/NS3 protease complexed with the peptidic inhibitor (Bz-Nle(P4)-Lys(P3)-Arg(P2)-Arg(P1)-H) were generated. The first DV protease model (DV-1) was constructed using the available crystal structure of the apo DV NS2B/NS3 protease. The second model (DV-2) was built taking the WNV NS3/NS2B protease in the inhibitor-complexed form as the template structure. Molecular dynamics simulations which were carried out for the WNV crystal structures as well as for the DV models provided an understanding of the role of NS2B for maintaining the protease in the active conformation. It was also demonstrated that NS2B is not only important for maintaining NS3 in the active form, but is also essential for establishing the interaction between residues from the S2 pocket and the peptidic inhibitor. The DV NS2B/NS3 model in the productive conformation can now be used for structure-based design purposes.

Antiviral activity and host gene induction by tamarin and marmoset interferon-alpha and interferon-gamma in the GBV-B primary hepatocyte culture model

GBV-B induces hepatitis in tamarins and marmosets and is a surrogate model for HCV infections. Here, we cloned and characterized the antiviral activity of tamarin and marmoset interferon (IFN)alpha and IFN gamma. Potent antiviral activity was observed for tamarin and marmoset IFN alpha in primary hepatocyte cultures infected with GBV-B. The antiviral activity was greater in cultures exposed to IFN alpha prior to GBV-B infection, suggesting that either GBV-B was capable of inhibition of the antiviral activity of exogenous IFN alpha or that the preexisting endogenous IFN response to the virus reduced efficacy to exogenous IFN alpha. IFN gamma also exhibited antiviral activity in GBV-B infected hepatocytes. The transcriptional response to IFN alpha in marmoset hepatocytes was characterized using human genome microarrays. Since the GBV-B hepatocyte culture model possesses a functional innate immune response, it will provide opportunities to explore the nature of the antiviral response to a virus closely related to HCV.

A cooperative interaction between nontranslated RNA sequences and NS5A protein promotes in vivo fitness of a chimeric hepatitis C/GB virus B

GB virus B (GBV-B) is closely related to hepatitis C virus (HCV), infects small non-human primates, and is thus a valuable surrogate for studying HCV. Despite significant differences, the 5′ nontranslated RNAs (NTRs) of these viruses fold into four similar structured domains (I-IV), with domains II-III-IV comprising the viral internal ribosomal entry site (IRES). We previously reported the in vivo rescue of a chimeric GBV-B (vGB/III^HC) containing HCV sequence in domain III, an essential segment of the IRES. We show here that three mutations identified within the vGB/III^HC genome (within the 3′NTR, upstream of the poly(U) tract, and NS5A coding sequence) are necessary and sufficient for production of this chimeric virus following intrahepatic inoculation of synthetic RNA in tamarins, and thus apparently compensate for the presence of HCV sequence in domain III. To assess the mechanism(s) underlying these compensatory mutations, and to determine whether 5′NTR subdomains participating in genome replication do so in a virus-specific fashion, we constructed and evaluated a series of chimeric subgenomic GBV-B replicons in which various 5′NTR subdomains were substituted with their HCV homologs. Domains I and II of the GBV-B 5′NTR could not be replaced with HCV sequence, indicating that they contain essential, virus-specific RNA replication elements. In contrast, domain III could be swapped with minimal loss of genome replication capacity in cell culture. The 3′NTR and NS5A mutations required for rescue of the related chimeric virus in vivo had no effect on replication of the subgenomic GBneoD/III^HC RNA in vitro. The data suggest that in vivo fitness of the domain III chimeric virus is dependent on a cooperative interaction between the 5′NTR, 3′NTR and NS5A at a step in the viral life cycle subsequent to genome replication, most likely during particle assembly. Such a mechanism may be common to all hepaciviruses.

Virus-specific T-cell immunity correlates with control of GB virus B infection in marmosets

GB virus B (GBV-B) is a hepatotropic virus that is closely related to hepatitis C virus (HCV). GBV-B causes acute hepatitis in infected marmosets and tamarins and is therefore a useful small-animal model for the study of HCV. We investigated virus-specific T-cell responses in marmosets infected with GBV-B. Gamma interferon (IFN-gamma) enzyme-linked immunospot (ELISPOT) assay responses in the peripheral blood of two marmosets were assessed throughout the course of GBV-B infection. These T-cell responses were directed against the GBV-B nonstructural proteins 3 (NS3), 4A (NS4A), and 5B (NS5B), and their appearance was temporally associated with clearance of viremia. These marmosets were then rechallenged with GBV-B at least 3 months after clearance of the primary infection to determine if the animals were protected from reinfection. There was no detectable viremia following reinfection, although a sharp increase in T-cell responses against GBV-B proteins was observed. Epitope mapping of T-cell responses to GBV-B was performed with liver and blood samples from both marmosets after rechallenge with GBV-B. Three shared, immunodominant T-cell epitopes within NS3 were identified in animals with multiple common major histocompatibility complex class I alleles. IFN-gamma ELISPOT responses were also detected in the livers of two marmosets that had resolved a primary GBV-B infection. These responses were high in frequency and were directed against epitopes within GBV-B NS3, NS4A, and NS5B proteins. These results indicate that virus-specific T-cell responses are detectable in the liver and blood of GBV-B-infected marmosets and that the clearance of GBV-B is associated with the appearance of these responses.

Mutagenesis analysis of the NS2B determinants of the Alkhurma virus NS2B-NS3 protease activation

Alkhurma virus (ALKV) is a tick-borne class 4 flavivirus responsible for several human cases of haemorrhagic fever in Saudi Arabia, with no specific treatment currently available. The viral RNA encodes a serine protease (NS2B-NS3), essential for virus replication in infected cells, that constitutes an attractive target for antiviral compounds. In an attempt to identify residues and motifs on NS2B that are necessary for protease activity of the ALKV NS2B-NS3 complex, a series of modified NS2B-NS3 proteins was constructed, with point mutations on particular residues or with the NS2B domain derived from two different viruses. Four mutants and the two chimeric proteins exhibited reduction of protease activity against BAPNA (a p-nitroanilide substrate). The results demonstrate that tight complementarity of the protein sequences is necessary for NS2B-dependent activation of NS3. The results also determine residues in the ALKV NS2B cofactor essential for protease activation, giving new insights into protease function in flaviviruses.

GB virus B disrupts RIG-I signaling by NS3/4A-mediated cleavage of the adaptor protein MAVS

Understanding the mechanisms of hepatitis C virus (HCV) pathogenesis and persistence has been hampered by the lack of small, convenient animal models. GB virus B (GBV-B) is phylogenetically the closest related virus to HCV. It causes generally acute and occasionally chronic hepatitis in small primates and is used as a surrogate model for HCV. It is not known, however, whether GBV-B has evolved strategies to circumvent host innate defenses similar to those of HCV, a property that may contribute to HCV persistence in vivo. We show here in cultured tamarin hepatocytes that GBV-B NS3/4A protease, but not a related catalytically inactive mutant, effectively blocks innate intracellular antiviral responses signaled through the RNA helicase, retinoic acid-inducible gene I (RIG-I), an essential sensor molecule that initiates host defenses against many RNA viruses, including HCV. GBV-B NS3/4A protease specifically cleaves mitochondrial antiviral signaling protein (MAVS; also known as IPS-1/Cardif/VISA) and dislodges it from mitochondria, thereby disrupting its function as a RIG-I adaptor and blocking downstream activation of both interferon regulatory factor 3 and nuclear factor kappa B. MAVS cleavage and abrogation of virus-induced interferon responses were also observed in Huh7 cells supporting autonomous replication of subgenomic GBV-B RNAs. Our data indicate that, as in the case of HCV, GBV-B has evolved to utilize its major protease to disrupt RIG-I signaling and impede innate antiviral defenses. These data provide further support for the use of GBV-B infection in small primates as an accurate surrogate model for deciphering virus-host interactions in hepacivirus pathogenesis.

The active essential CFNS3d protein complex

The NS2B-NS3 protease complex is essential for the replication of dengue virus, which is the etiologic agent of dengue and hemorrhagic fevers, diseases that are a burden for the tropical and subtropical areas of the world. The active form of the NS3 protease linked to the 40 residues of the NS2B cofactor shows highly flexible and disordered region(s) that are responsible for its high propensity to aggregate at the concentrations necessary for NMR spectroscopy studies or for crystallization. Limited proteolysis of this active form of the protease enabled us to obtain a folded and new essential form of the NS2B-NS3 protease complex. We found that the region from residues D50 to E80 of NS2B interacts directly and strongly with the NS3 protease domain. The proteolytic activity of the noncovalently binding complex was determined by a rapid and continuous fluorescence resonance energy transfer activity assay using a depsipeptide substrate. The new protein-cofactor complex obtained, encompassing the NS2B fragment (D50-E80) and the NS3 protease, shows proteolytic activity. The (1)H-(15)N-heteronuclear single quantum coherence spectrum of the isotopically enriched protein complex shows good cross-peak dispersion; this is indicative of a stable folded state. Our results significantly complement the X-ray structure of the NS2B-NS3pro complex published recently. Moreover, these results open the way to performing direct structural and interaction studies in solution on a new active NS2B-NS3pro complex with libraries of substrates and inhibitors in order to identify new drugs that prevent viral polyprotein processing.

Robust translocation along a molecular monorail: the NS3 helicase from hepatitis C virus traverses unusually large disruptions in its track

The NS3 helicase is essential for replication of the hepatitis C virus. This multifunctional Superfamily 2 helicase protein unwinds nucleic acid duplexes in a stepwise, ATP-dependent manner. Although kinetic features of its mechanism are beginning to emerge, little is known about the physical determinants for NS3 translocation along a strand of nucleic acid. For example, it is not known whether NS3 can traverse covalent or physical discontinuities on the tracking strand. Here we provide evidence that NS3 translocates with a mechanism that is different from its well-studied relative, the Vaccinia helicase NPH-II. Like NPH-II, NS3 translocates along the loading strand (the strand bearing the 3'-overhang) and it fails to unwind substrates that contain nicks, or covalent discontinuities in the loading strand. However, unlike NPH-II, NS3 readily unwinds RNA duplexes that contain long stretches of polyglycol, which are moieties that bear no resemblance to nucleic acid. Whether located on the tracking strand, the top strand, or both, long polyglycol regions fail to disrupt the function of NS3. This suggests that NS3 does not require the continuous formation of specific contacts with the ribose-phosphate backbone as it translocates along an RNA duplex, which is an observation consistent with the large NS3 kinetic step size (18 base-pairs). Rather, once NS3 loads onto a substrate, the helicase can translocate along the loading strand of an RNA duplex like a monorail train following a track. Bumps in the track do not significantly disturb NS3 unwinding, but a break in the track de-rails the helicase.

Identification of residues in the dengue virus type 2 NS2B cofactor that are critical for NS3 protease activation

Proteolytic processing of the dengue virus polyprotein is mediated by host cell proteases and the virus-encoded NS2B-NS3 two-component protease. The NS3 protease represents an attractive target for the development of antiviral inhibitors. The three-dimensional structure of the NS3 protease domain has been determined, but the structural determinants necessary for activation of the enzyme by the NS2B cofactor have been characterized only to a limited extent. To test a possible functional role of the recently proposed Phix(3)Phi motif in NS3 protease activation, we targeted six residues within the NS2B cofactor by site-specific mutagenesis. Residues Trp62, Ser71, Leu75, Ile77, Thr78, and Ile79 in NS2B were replaced with alanine, and in addition, an L75A/I79A double mutant was generated. The effects of these mutations on the activity of the NS2B(H)-NS3pro protease were analyzed in vitro by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of autoproteolytic cleavage at the NS2B/NS3 site and by assay of the enzyme with the fluorogenic peptide substrate GRR-AMC. Compared to the wild type, the L75A, I77A, and I79A mutants demonstrated inefficient autoproteolysis, whereas in the W62A and the L75A/I79A mutants self-cleavage appeared to be almost completely abolished. With exception of the S71A mutant, which had a k(cat)/K(m) value for the GRR-AMC peptide similar to that of the wild type, all other mutants exhibited drastically reduced k(cat) values. These results indicate a pivotal function of conserved residues Trp62, Leu75, and Ile79 in the NS2B cofactor in the structural activation of the dengue virus NS3 serine protease.

In vivo analysis of the 3' untranslated region of GB virus B after in vitro mutagenesis of an infectious cDNA clone: persistent infection in a transfected tamarin

GB virus B (GBV-B), the virus most closely related to hepatitis C virus (HCV), infects tamarins and causes acute hepatitis. The 3' untranslated region (UTR) of an infectious GBV-B clone (pGBB) has a proximal short sequence followed by a poly(U) tract and a 3' terminal sequence. Our investigators previously demonstrated that the 3' terminal sequence was critical for in vivo infectivity. Here, we tested the effect of deleting the short sequence and/or the poly(U) tract from pGBB; infectivity of each mutant was tested by intrahepatic transfection of two tamarins with transcribed RNA. A mutant lacking both regions was not viable. However, mutants lacking either the short sequence or the poly(U) tract were viable. All four tamarins had a wild-type-like acute infection and developed acute hepatitis. Whereas we found that five tamarins transfected with the wild-type clone pGBB had acute resolving infection, one tamarin transfected with the poly(U) deletion mutant became persistently infected. This animal had viremia and hepatitis until its death at week 90. The genomes recovered at weeks 2, 7, 15, 20, 60, and 90 lacked the poly(U) stretch. Eight amino acid changes were identified at week 90. One change, in the putative p7 protein, was dominant at week 15. Thus, persistence of GBV-B, like persistence of HCV, was associated with the emergence of virus variants. Four tamarins inoculated with serum collected at weeks 2 and 90 from the tamarin with persistent infection had an acute resolving infection. Nonetheless, the demonstration that GBV-B can persist in tamarins strengthens its relevance as a surrogate model for the study of HCV.

Characterization of GB virus B polyprotein processing reveals the existence of a novel 13-kDa protein with partial homology to hepatitis C virus p7 protein

Although responsible for a major health problem worldwide, hepatitis C virus is difficult to study because of the absence of fully permissive cell cultures or experimental animal models other than the chimpanzee. GB virus B (GBV-B), a closely related hepatotropic virus that infects small New World primates and replicates efficiently in primary hepatocyte cultures, is an attractive surrogate model system. However, little is known about processing of the GBV-B polyprotein. Because an understanding of these events is critical to further development of model GBV-B systems, we characterized signal peptidase processing of the polyprotein segment containing the putative structural proteins. We identified the exact N termini of the mature GBV-B envelope proteins, E1 and E2, and the first nonstructural protein, NS2, by direct amino acid sequencing. Interestingly, these studies document the existence of a previously unrecognized 13-kDa protein (p13) located between E2 and NS2 within the polyprotein. We compared the sequence of the p13 protein to that of hepatitis C virus p7, a small membrane-spanning protein with a similar location in the polyprotein and recently identified ion channel activity. The C-terminal half of p13 shows clear homology with p7, suggesting a common function, but the substantially larger size of p13, with 4 rather than 2 predicted transmembrane segments, indicates a different structural organization and/or additional functions. The identification of p13 in the GBV-B polyprotein provides strong support for the hypothesis that ion channel-forming proteins are essential for the life cycle of flaviviruses, possibly playing a role in virion morphogenesis and/or virus entry into cells.

Development of a GB virus B marmoset model and its validation with a novel series of hepatitis C virus NS3 protease inhibitors

GB virus B (GBV-B), a flavivirus closely related to HCV, has previously been shown to infect and replicate to high titers in tamarins (Saguinus sp.). This study describes the use of GBV-B infection and replication in the common marmoset (Callithrix jacchus) for the successful development and validation of a surrogate animal model for hepatitis C virus (HCV). Infection of marmosets with GBV-B produced a viremia that peaked at 10(8) to 10(9) genome copies/ml for a period of 40 to 60 days followed by viral clearance at 60 to 80 days postinfection. Passage of the initial tamarin-derived GBV-B in marmosets produced an infectious stock that gave a more reproducible and consistent infection in the marmoset. Titration of the virus stocks in vivo indicated that they contained 1 infectious unit for every 1,000 genome copies. Cultures of primary marmoset hepatocytes were also successfully infected with GBV-B, with high levels of virus detected in supernatants and cells for up to 14 days postinfection. Treatment of GBV-B-infected hepatocyte cultures with a novel class of HCV protease inhibitor (pyrrolidine 5,5 trans-lactams) reduced viral levels by more than 2 logs. Treatment of GBV-B-infected marmosets with one such inhibitor resulted in a 3-log drop in serum viral titer over 4 days of therapy. These studies provide the first demonstration of the in vivo efficacy of a small-molecule inhibitor for HCV in an animal model and illustrate the utility of GBV-B as a surrogate animal model system for HCV.

Hep3B human hepatoma cells support replication of the wild-type and a 5'-end deletion mutant GB virus B replicon

Hepatitis C virus (HCV) and GB virus B (GBV-B) replicons have been reported to replicate only in Huh7 cells. Here we demonstrate that subpopulations of another human hepatoma cell line, Hep3B, are permissive for the GBV-B replicon, showing different levels of enhancement of replication from those of the unselected parental cell population. Adaptive mutations are not required for replication of the GBV-B replicon in these cells, as already demonstrated for Huh7 cells. Nonetheless, we identified a mutant replicon in one of the selected cell lines, which, although lacking the 5' end proximal stem-loop, is able to replicate in Hep3B cells as well as in Huh7 cells. This mutant indeed shows a higher replication efficiency than does wild-type replicon, especially in the Hep3B cell clone from which it was originally recovered. This indicates that the stem-loop Ia is not necessary for replication of the GBV-B replicon in human cells, unlike what occurs with HCV, and that its absence can even provide a selective advantage.

Chronic hepatitis associated with GB virus B persistence in a tamarin after intrahepatic inoculation of synthetic viral RNA

Progress in understanding the pathogenesis of hepatitis C virus (HCV) has been slowed by the absence of tractable small animal models. Whereas GB virus B (GBV-B, an unclassified flavivirus) shares a phylogenetic relationship and several biologic attributes with HCV, including hepatotropism, it is not known to cause persistent infection, a hallmark of HCV. Here, we document persistent GBV-B infection in one of two healthy tamarins (Saguinus oedipus) inoculated intrahepatically with infectious synthetic RNA. High-titer viremia (108 to 109 genome equivalents per ml) and transiently elevated serum alanine transaminase activities were present from weeks 4 to 12 postinoculation in both animals. However, whereas GBV-B was eliminated from one animal by 20 weeks, the second animal remained viremic (103 to 107 genome equivalents per ml) for >2 years, with alanine transaminase levels becoming elevated again before spontaneous resolution of the infection. A liver biopsy taken late in the course of infection demonstrated hepatitis with periportal mononuclear infiltrates, hepatocellular microvesicular changes, cytoplasmic lipid droplets, and disordered mitochondrial ultrastructure, findings remarkably similar to chronic hepatitis C. GBV-B-infected hepatocytes contained numerous small vesicular membranous structures resembling those associated with expression of HCV nonstructural proteins, and sequencing of GBV-B RNA demonstrated a rate of molecular evolution comparable to that of HCV. We conclude that GBV-B is capable of establishing persistent infections in healthy tamarins, a feature that substantially enhances its value as a model for HCV. Mitochondrial structural changes and altered lipid metabolism leading to steatosis are conserved features of the pathogenesis of chronic hepatitis caused by these genetically distinct flaviviruses.

Enzymatic activities of the GB virus-B RNA-dependent RNA polymerase

The GB virus-B (GBV-B) nonstructural protein 5B (NS5B) encodes an RNA-dependent RNA polymerase (RdRp) with greater than 50% sequence similarity to the hepatitis C virus (HCV) NS5B. Recombinant GBV-B NS5B was reported to possess RdRp activity (W. Zhong et al., 2000, J. Viral Hepat. 7, 335-342). In this study, the GBV-B RdRp was examined more thoroughly for different RNA synthesis activities, including primer-extension, de novo initiation, template switch, terminal nucleotide addition, and template specificity. The results can be compared with previous characterizations of the HCV RdRp. The two RdRps share similarities in terms of metal ion and template preference, the abilities to add nontemplated nucleotides, perform both de novo initiation and extension from a primer, and switch templates. However, several differences in RNA synthesis between the GBV-B and HCV RdRps were observed, including (i) optimal temperatures for activity, (ii) ranges of Mn(2+) concentration tolerated for activity, and (iii) cation requirements for de novo RNA synthesis and terminal transferase activity. To assess whether the recombinant GBV-B RdRp may represent a relevant surrogate system for testing HCV antiviral agents, two compounds demonstrated to be active at nanomolar concentrations against HCV NS5B were tested on the GBV RdRp. A chain terminating nucleotide analog could prevent RNA synthesis, while a nonnucleoside HCV inhibitor was unable to affect RNA synthesis by the GBV RdRp.

Comparison of tamarins and marmosets as hosts for GBV-B infections and the effect of immunosuppression on duration of viremia

GBV-B virus is a close relative to hepatitis C virus (HCV) that causes hepatitis in tamarins, and thus, is an attractive surrogate model for HCV. In this study, we demonstrate that the host range of GBV-B extends to the common marmoset with an infection profile similar to that observed for tamarins. Marmoset hepatocytes were susceptible to in vitro infection with GBV-B. Virus was efficiently secreted into the medium, and approximately 25% of hepatocytes were positive for NS3 staining. In an attempt to induce persistent infections, tamarins were immunosuppressed with FK506 and inoculated with GBV-B. Although no chronic infections were induced, the duration of viremia was increased in most animals. In one animal, the duration of viremia was extended to 46 weeks, but viral clearance occurred 18 weeks after stopping FK506 therapy. The greater availability of marmosets in comparison to tamarins will greatly facilitate future research efforts with this model.

Tissue culture and animal models for hepatitis C virus

In recent years, significant advances have been achieved both in the development of animal- and tissue-culture models for HCV. Among all the new systems, the small animal model based on transgenic mice with chimeric mouse-human livers and the replicon system will presumably have the most profound impact on future HCV research. Yet, in spite of this progress, much more work will be required to optimizse both systems. In case of the mouse model, breeding homozygous Alb-uPa animals is difficult because of the toxicity of the transgene, and the transplantation of primary human hepatocytes into mice a few days after birth is technically challenging. These are immunodeficient, and, therefore, it will be desirable to furnish them with components of the human immune system in order to expand the applicability of this in vivo model to questions related to pathogenesis. Advances in cryopreservation techniques are urgently needed, moreover, as this would improve the availability of primary hepatocytes and in turn also the accessibility of this small animal model. As regards the replicon system, a number of open questions remain that will hopefully be answered by future research. Why, for instance, has replication in cell culture so far been achieved only with genotype 1b isolates, whereas an isolate with proven infectivity derived from genotype 1a failed to replicate in Huh-7 cells? And why can replicons so far be propagated only in this particular cell line? Is this attributable to the lack of certain inhibitory factors, or the presence of specific activators? What are the mechanisms underlying cell-culture adaptation. and what determines whether a certain Huh-7 cell replicates HCV RNA more efficiently? Finally, the replicon system may also lead the way to the development of systems for efficient virus production in cell culture, and ultimately also a permissive cell line. These developments would at last allow us to model the complete viral life cycle, something researchers have been struggling with ever since the first identification of HCV.

Hepatitis C virus therapies: current treatments, targets and future perspectives

Chronic hepatitis C virus (HCV) infection is the cause of an emerging global epidemic of chronic liver disease. Current combination therapies are at best 80% efficacious and are often poorly tolerated. Strategies to improve the therapeutic response include the development of novel interferons, nucleoside analogues with reduced haemolysis compared with ribavirin and inosine 5'-monophosphate dehydrogenase inhibitors. Compounds in preclinical or early clinical trials include small molecules that inhibit virus-specific enzymes (such as the serine proteases, RNA polymerase and helicase) or interfere with translation (including anti-sense molecules, iRNA and ribozymes). Advances in understanding HCV replication, obtaining a sub-genomic replicon and contriving potential small animal models, in addition to solving crystallographic structures for the replication enzymes, have improved prospects for developing novel therapies. This review summarizes current and evolving treatments for chronic hepatitis C infection. In addition, progress in HCV targets and drug discovery tools valuable in the search for novel anti-HCV agents is detailed.

Cell clones selected from the Huh7 human hepatoma cell line support efficient replication of a subgenomic GB virus B replicon

Tamarins (Saguinus species) infected by GB virus B (GBV-B) have recently been proposed as an acceptable surrogate model for hepatitis C virus (HCV) infection. The availability of infectious genomic molecular clones of both viruses will permit chimeric constructs to be tested for viability in animals. Studies in cells with parental and chimeric constructs would also be very useful for both basic research and drug discovery. For this purpose, a convenient host cell type supporting replication of in vitro-transcribed GBV-B RNA should be identified. We constructed a GBV-B subgenomic selectable replicon based on the sequence of a genomic molecular clone proved to sustain infection in tamarins. The corresponding in vitro-transcribed RNA was used to transfect the Huh7 human hepatoma cell line, and intracellular replication of transfected RNA was shown to occur, even though in a small percentage of transfected cells, giving rise to antibiotic-resistant clones. Sequence analysis of GBV-B RNA from some of those clones showed no adaptive mutations with respect to the input sequence, whereas the host cells sustained higher GBV-B RNA replication than the original Huh7 cells. The enhancement of replication depending on host cell was shown to be a feature common to the majority of clones selected. The replication of GBV-B subgenomic RNA was susceptible to inhibition by known inhibitors of HCV to a level similar to that of HCV subgenomic RNA.

Hepatitis C virus cell culture replication systems: their potential use for the development of antiviral therapies

Hepatitis C virus is a significant public health problem. Current drug regimens have low efficacy against some hepatitis C virus genotypes, while no vaccine is available. The absence of an efficient cell culture system and an accessible small animal model to study hepatitis C virus replication and pathogenesis are major obstacles to the development of effective antiviral therapies. Studies of surrogate model systems, either related viruses or chimeric viruses containing part of the hepatitis C virus genome, have given insight into hepatitis C virus replication, in addition to being a powerful tool for drug discovery. The recent development of an efficient system for the initiation of replication in cell culture provides a viable screen for inhibitors of hepatitis C virus replication. It also brings us much closer to the ultimate goal of an infectious cell culture system for hepatitis C virus.

Generation of infectious and transmissible virions from a GB virus B full-length consensus clone in tamarins

The strong similarity between GB virus B (GBV-B) and hepatitis C virus (HCV) makes tamarins infected by GBV-B an acceptable surrogate animal model for HCV infection. Even more attractive, for drug discovery purposes, is the idea of constructing chimeric viruses by inserting HCV genes of interest into a GBV-B genome frame. To accomplish this, infectious cDNA clones of both viruses must be available. The characterization of several HCV molecular clones capable of infecting chimpanzees has been published, whereas only one infectious GBV-B clone inducing hepatitis in tamarins has been reported so far. Here we describe the infection of tamarins by intrahepatic injection of RNA transcribed from a genomic GBV-B clone (FL-3) and transmission of the disease from infected to naive tamarins via serum inoculation. The disease resulting from both direct and secondary infection was characterized for viral RNA titre and hepatitis parameters as well as for viral RNA distribution in the hepatic tissue. Host humoral immune response to GBV-B antigens was also monitored. The progression of the disease was compared to that induced by intravenous injection of different amounts of the non-recombinant virus.

Ribavirin induces error-prone replication of GB virus B in primary tamarin hepatocytes

GB virus B (GBV-B) is the closest relative of hepatitis C virus (HCV) and is an attractive surrogate model for HCV antiviral studies. GBV-B induces an acute, resolving hepatitis in tamarins. Utilizing primary cultures of tamarin hepatocytes, we have previously developed a tissue culture system that exhibits high levels of GBV-B replication. In this report, we have extended the utility of this system for testing antiviral compounds. Treatment with human interferon provided only a marginal antiviral effect, while poly(I-C) yielded >3 and 4 log units of reduction of cell-associated and secreted viral RNA, respectively. Interestingly, treatment of GBV-B-infected hepatocytes with ribavirin resulted in an approximately 4-log decrease in viral RNA levels. Guanosine blocked the antiviral effect of ribavirin, suggesting that inhibition of IMP dehydrogenase (IMPDH) and reduction of intracellular GTP levels were essential for the antiviral effect. However, mycophenolic acid, another IMPDH inhibitor, had no antiviral effect. Virions harvested from ribavirin-treated cultures exhibited a dramatically reduced specific infectivity. These data suggest that incorporation of ribavirin triphosphate induces error-prone replication with concomitant reduction in infectivity and that reduction of GTP pools may be required for incorporation of ribavirin triphosphate. In contrast to the in vitro studies, no significant reduction in viremia was observed in vivo following treatment of tamarins with ribavirin during acute infection with GBV-B. These findings are consistent with the observation that ribavirin monotherapy for HCV infection decreases liver disease without a significant reduction in viremia. Our data suggest that nucleoside analogues that induce error-prone replication could be an attractive approach for the treatment of HCV infection if administered at sufficient levels to result in efficient incorporation by the viral polymerase.

GB virus B as a model for hepatitis C virus

GB viruses A and B (GBV-A and GBV-B) are members of the Flaviviridae family and are isolated from tamarins injected with serum from a human hepatitis patient. Along with a related human virus, GB virus C, or alternatively, hepatitis G virus (GBV-C/HGV), the three viruses represent the GB agents. Of the three viruses, GBV-B has been proposed as a potential surrogate model for the study of hepatitis C virus (HCV) infections of humans. GBV-B is phylogenetically most closely related to HCV and causes an acute, self-resolving hepatitis in tamarins as indicated by an increase in alanine aminotransferase and changes in liver histology. Similarities between GBV-B and HCV are found at the nucleotide sequence level with the two viruses sharing 28% amino acid homology over the lengths of their open reading frames. Short regions have even higher levels of homology that are functionally significant as shown by the ability of the GBV-B NS3 protease to cleave recombinant HCV polyprotein substrates. The shared protease substrate specificities suggest that GBV-B may be useful in testing antiviral compounds for activity against HCV. Although there are numerous similarities between GBV-B and HCV, there are important differences in that HCV frequently causes chronic infections in people, whereas GBV-B appears to cause only acute infections. The acute versus chronic course of infection may point to important differences between the two viruses that, along with the numerous similarities, will make GBV-B in tamarins a good surrogate model for HCV.

Tamarin alpha-interferon is active in mouse liver upon intramuscular gene delivery

BACKGROUND

The hepatitis C virus (HCV) is responsible for a severe and widespread form of hepatitis for which a durable and effective therapy has not yet been established. The only approved therapy against hepatitis C, alpha-interferon protein intramuscular administration, presents numerous drawbacks that might be overcome by adopting a gene therapy approach. HCV exclusively infects humans and chimpanzees, hence an acceptable animal model for hepatitis C pharmacological studies is not available. Recently, tamarins infected by GB virus B (GBV-B) have been proposed as a surrogate animal model for HCV infection. The aim of the present study was the production of tamarin interferon (tIFN) through delivery of tIFN-coding DNA to evaluate the feasibility of a gene therapy approach based on IFN electro-gene transfer (EGT) in future studies with primates.

METHODS

Production and biological activity of cloned tamarin interferon was monitored in cultured cells upon transfection and in mice upon muscle EGT of the corresponding plasmid DNA, respectively.

RESULTS

A tamarin gene encoding a protein homologous to human interferon-alpha2 (hIFN-alpha2) has been cloned. The tamarin IFN-alpha (tIFN-alpha) protein shows antiviral activity in a cell-based assay. Upon EGT of the corresponding gene in mouse muscles, tIFN-alpha is detectable at high levels in serum for at least 4 months. Most important, activity of tIFN, measured as enhancement of mRNA levels of genes induced by type I IFNs, is also detectable in the liver of EGT-treated mice.

CONCLUSION

The present study demonstrates that the delivery of tIFN-alpha DNA via intramuscular injection yields a functional protein able to produce biological effects inside a remote target organ, the liver. This finding, besides the specific purpose of the present study, is of general relevance with a view to establishing therapeutic protocols based on EGT.

Development of a primary tamarin hepatocyte culture system for GB virus-B: a surrogate model for hepatitis C virus

GB virus-B (GBV-B) causes an acute hepatitis in tamarins characterized by increased alanine transaminase levels that quickly return to normal as the virus is cleared. Phylogenetically, GBV-B is the closest relative to hepatitis C virus (HCV), and thus GBV-B infection of tamarins represents a powerful surrogate model system for the study of HCV. In this study, the course of infection of GBV-B in tamarins was followed using a real-time 5' exonuclease (TaqMan) reverse transcription-PCR assay to determine the level of GBV-B in the serum. Peak viremia levels exceeded 10(9) genome equivalents/ml, followed by viral clearance within 14 to 16 weeks. Rechallenge of animals that had cleared infection resulted in viremia that was limited to 1 week, suggestive of a strong protective immune response. A robust tissue culture system for GBV-B was developed using primary cultures of tamarin hepatocytes. Hepatocytes obtained from a GBV-B-infected animal maintained high levels of cell-associated viral RNA and virion secretion for 42 days of culture. In vitro infection of normal hepatocytes resulted in rapid amplification of cell-associated viral RNA and secretion of up to 10(7) genome equivalents/ml of culture supernatant. In addition, infection could be monitored by immunofluorescence staining for GBV-B nonstructural NS3 protein. This model system overcomes many of the current obstacles to HCV research, including low levels of viral replication, lack of a small primate animal model, and lack of a reproducible tissue culture system.

Processing of GB virus B non-structural proteins in cultured cells requires both NS3 protease and NS4A cofactor

The identification of antivirals and vaccines against hepatitis C virus (HCV) infection is hampered by the lack of convenient animal models. The need to develop surrogate models has recently drawn attention to GB virus B (GBV-B), which produces hepatitis in small primates. In a previous study in vitro, it was shown that GBV-B NS3 protease shares substrate specificity with the HCV enzyme, known to be crucial for virus replication. In this report, GBV-B NS3 activity on GBV-B precursor proteins has been analysed in a cell-based system. It is shown that mature protein products are obtained that are compatible with the cleavage sites proposed on the basis of sequence homology with HCV and that GBV-B NS4A protein is required as a cofactor for optimal enzymatic activity. Experiments in vitro supported by a structural model mapped the region of NS4A that interacts with NS3 and showed that the GBV-B cofactor cannot be substituted for by its HCV analogue.