Hepatitis E virus seropositivity in an ethnically diverse community blood donor population

BACKGROUND AND OBJECTIVES

Hepatitis E virus (HEV) is an underrecognized and emerging infectious disease that may threaten the safety of donor blood supply in many parts of the world. We sought to elucidate whether our local community blood supply is at increased susceptibility for transmission of transfusion-associated HEV infections.

MATERIALS AND METHODS

We screened 10,002 randomly selected donations over an 8-month period between 2017 and 2018 at the Stanford Blood Center for markers of HEV infection using commercial IgM/IgG serological tests and reverse transcriptase quantitative polymerase chain reaction assays (RT-qPCR). Donor demographic information, including gender, age, self-identified ethnicity, location of residence and recent travel, were obtained from the donor database and used to generate multivariate binary logistic regressions for risk factors of IgG seropositivity.

RESULTS

A total of 10,002 blood donations from 7507 unique donors were screened, and there was no detectable HEV RNA by RT-qPCR. The overall seropositivity rate was 12.1% for IgG and 0.56% for IgM. Multivariate analysis of unique donors revealed a significantly higher risk of IgG seropositivity with increasing age, White/Asian ethnicities and residence in certain local counties.

CONCLUSION

Although HEV IgG seroprevalence in the San Francisco Bay Area is consistent with ongoing infection, the screening of a large donor population did not identify any viraemic blood donors. While HEV is an underrecognized and emerging infection in other regions, there is no evidence to support routine blood screening for HEV in our local blood supply currently; however, periodic monitoring may still be required to assess the ongoing risk.

Induction of broadly neutralizing antibodies using a secreted form of the hepatitis C virus E1E2 heterodimer as a vaccine candidate

SignificanceHepatitis C virus chronically infects approximately 1% of the world's population, making an effective vaccine for hepatitis C virus a major unmet public health need. The membrane-associated E1E2 envelope glycoprotein has been used in clinical studies as a vaccine candidate. However, limited neutralization breadth and difficulty in producing large amounts of homogeneous membrane-associated E1E2 have hampered efforts to develop an E1E2-based vaccine. Our previous work described the design and biochemical validation of a native-like soluble secreted form of E1E2 (sE1E2). Here, we describe the immunogenic characterization of the sE1E2 complex. sE1E2 elicited broadly neutralizing antibodies in immunized mice, with increased neutralization breadth relative to the membrane-associated E1E2, thereby validating this platform as a promising model system for vaccine development.

An Antigenically Diverse, Representative Panel of Envelope Glycoproteins for Hepatitis C Virus Vaccine Development

BACKGROUND & AIMS

Development of a prophylactic hepatitis C virus (HCV) vaccine will require accurate and reproducible measurement of neutralizing breadth of vaccine-induced antibodies. Currently available HCV panels may not adequately represent the genetic and antigenic diversity of circulating HCV strains, and the lack of standardization of these panels makes it difficult to compare neutralization results obtained in different studies. Here, we describe the selection and validation of a genetically and antigenically diverse reference panel of 15 HCV pseudoparticles (HCVpps) for neutralization assays.

METHODS

We chose 75 envelope (E1E2) clones to maximize representation of natural polymorphisms observed in circulating HCV isolates, and 65 of these clones generated functional HCVpps. Neutralization sensitivity of these HCVpps varied widely. HCVpps clustered into 15 distinct groups based on patterns of relative sensitivity to 7 broadly neutralizing monoclonal antibodies. We used these data to select a final panel of 15 antigenically representative HCVpps.

RESULTS

Both the 65 and 15 HCVpp panels span 4 tiers of neutralization sensitivity, and neutralizing breadth measurements for 7 broadly neutralizing monoclonal antibodies were nearly equivalent using either panel. Differences in neutralization sensitivity between HCVpps were independent of genetic distances between E1E2 clones.

CONCLUSIONS

Neutralizing breadth of HCV antibodies should be defined using viruses spanning multiple tiers of neutralization sensitivity rather than panels selected solely for genetic diversity. We propose that this multitier reference panel could be adopted as a standard for the measurement of neutralizing antibody potency and breadth, facilitating meaningful comparisons of neutralization results from vaccine studies in different laboratories.

Crystal Structure of a Bivalent Antibody Fab Fragment

We determined the crystal structure to 1.8 Å resolution of the Fab fragment of an affinity-matured human monoclonal antibody (HC84.26.5D) that recognizes the E2 envelope glycoprotein of hepatitis C virus (HCV). Unlike conventional Fabs, which are monovalent monomers, Fab HC84.26.5D assembles into a bivalent domain-swapped dimer in which the two V_L/V_H modules are separated by ~25 Å. In solution, Fab HC84.26.5D exists predominantly as a dimer (~80%) in equilibrium with the monomeric form of the Fab (~20%). Dimerization is mediated entirely by deletion of a single residue, V_HSer113 (Kabat numbering), in the elbow region linking the V_H and C_H1 domains. In agreement with the crystal structure, dimeric Fab HC84.26.5D is able to bind two HCV E2 molecules in solution. This is only the second example of a domain-swapped Fab dimer from among >3000 Fab crystal structures determined to date. Moreover, the architecture of the doughnut-shaped Fab HC84.26.5D dimer is completely different from that of the previously reported Fab 2G12 dimer. We demonstrate that the highly identifiable shape of dimeric Fab HC84.26.5D makes it useful as a fiducial marker for single-particle cryoEM analysis of HCV E2. Bivalent domain-swapped Fab dimers engineered on the basis of HC84.26.5D may also serve as a means of doubling the effective size of conventional Fab-protein complexes for cryoEM.

Design of a native-like secreted form of the hepatitis C virus E1E2 heterodimer

Hepatitis C virus (HCV) is a major worldwide health burden, and a preventive vaccine is needed for global control or eradication of this virus. A substantial hurdle to an effective HCV vaccine is the high variability of the virus, leading to immune escape. The E1E2 glycoprotein complex contains conserved epitopes and elicits neutralizing antibody responses, making it a primary target for HCV vaccine development. However, the E1E2 transmembrane domains that are critical for native assembly make it challenging to produce this complex in a homogenous soluble form that is reflective of its state on the viral envelope. To enable rational design of an E1E2 vaccine, as well as structural characterization efforts, we have designed a soluble, secreted form of E1E2 (sE1E2). As with soluble glycoprotein designs for other viruses, it incorporates a scaffold to enforce assembly in the absence of the transmembrane domains, along with a furin cleavage site to permit native-like heterodimerization. This sE1E2 was found to assemble into a form closer to its expected size than full-length E1E2. Preservation of native structural elements was confirmed by high-affinity binding to a panel of conformationally specific monoclonal antibodies, including two neutralizing antibodies specific to native E1E2 and to its primary receptor, CD81. Finally, sE1E2 was found to elicit robust neutralizing antibodies in vivo. This designed sE1E2 can both provide insights into the determinants of native E1E2 assembly and serve as a platform for production of E1E2 for future structural and vaccine studies, enabling rational optimization of an E1E2-based antigen.

Structure-Based Design of Hepatitis C Virus E2 Glycoprotein Improves Serum Binding and Cross-Neutralization

An effective vaccine for hepatitis C virus (HCV) is a major unmet need, and it requires an antigen that elicits immune responses to key conserved epitopes. Based on structures of antibodies targeting HCV envelope glycoprotein E2, we designed immunogens to modulate the structure and dynamics of E2 and favor induction of broadly neutralizing antibodies (bNAbs) in the context of a vaccine. These designs include a point mutation in a key conserved antigenic site to stabilize its conformation, as well as redesigns of an immunogenic region to add a new N-glycosylation site and mask it from antibody binding. Designs were experimentally characterized for binding to a panel of human monoclonal antibodies (HMAbs) and the coreceptor CD81 to confirm preservation of epitope structure and preferred antigenicity profile. Selected E2 designs were tested for immunogenicity in mice, with and without hypervariable region 1, which is an immunogenic region associated with viral escape. One of these designs showed improvement in polyclonal immune serum binding to HCV pseudoparticles and neutralization of isolates associated with antibody resistance. These results indicate that antigen optimization through structure-based design of the envelope glycoproteins is a promising route to an effective vaccine for HCV.IMPORTANCE Hepatitis C virus infects approximately 1% of the world's population, and no vaccine is currently available. Due to the high variability of HCV and its ability to actively escape the immune response, a goal of HCV vaccine design is to induce neutralizing antibodies that target conserved epitopes. Here, we performed structure-based design of several epitopes of the HCV E2 envelope glycoprotein to engineer its antigenic properties. Designs were tested in vitro and in vivo, demonstrating alteration of the E2 antigenic profile in several cases, and one design led to improvement of cross-neutralization of heterologous viruses. This represents a proof of concept that rational engineering of HCV envelope glycoproteins can be used to modulate E2 antigenicity and optimize a vaccine for this challenging viral target.

An alternate conformation of HCV E2 neutralizing face as an additional vaccine target

To achieve global elimination of hepatitis C virus (HCV), an effective cross-genotype vaccine is needed. The HCV envelope glycoprotein E2 is the main target for neutralizing antibodies (nAbs), which aid in HCV clearance and protection. E2 is structurally flexible and functions in engaging host receptors. Many nAbs bind to the "neutralizing face" on E2, including several broadly nAbs encoded by the V_H1-69 germline gene family that bind to a similar conformation (A) of this face. Here, a previously unknown conformation (B) of the neutralizing face is revealed in crystal structures of two of four additional E2-V_H1-69 nAb complexes. In this conformation, the E2 front-layer region is displaced upon antibody binding, exposing residues in the back layer for direct antibody interaction. This E2 B structure may represent another conformational state in the viral entry process that is susceptible to antibody neutralization and thus provide a new target for rational vaccine development.

Broadly neutralizing antibodies from an individual that naturally cleared multiple hepatitis C virus infections uncover molecular determinants for E2 targeting and vaccine design

Cumulative evidence supports a role for neutralizing antibodies contributing to spontaneous viral clearance during acute hepatitis C virus (HCV) infection. Information on the timing and specificity of the B cell response associated with clearance is crucial to inform vaccine design. From an individual who cleared three sequential HCV infections with genotypes 1b, 1a and 3a strains, respectively, we employed peripheral B cells to isolate and characterize neutralizing human monoclonal antibodies (HMAbs) to HCV after the genotype 1 infections. The majority of isolated antibodies, designated as HMAbs 212, target conformational epitopes on the envelope glycoprotein E2 and bound broadly to genotype 1–6 E1E2 proteins. Further, some of these antibodies showed neutralization potential against cultured genotype 1–6 viruses. Competition studies with defined broadly neutralizing HCV HMAbs to epitopes in distinct clusters, designated antigenic domains B, C, D and E, revealed that the selected HMAbs compete with B, C and D HMAbs, previously isolated from subjects with chronic HCV infections. Epitope mapping studies revealed domain B and C specificity of these HMAbs 212. Sequential serum samples from the studied subject inhibited the binding of HMAbs 212 to autologous E2 and blocked a representative domain D HMAb. The specificity of this antibody response appears similar to that observed during chronic infection, suggesting that the timing and affinity maturation of the antibody response are the critical determinants in successful and repeated viral clearance. While additional studies should be performed for individuals with clearance or persistence of HCV, our results define epitope determinants for antibody E2 targeting with important implications for the development of a B cell vaccine.

Studies of hepatitis C virus (HCV) infected individuals spontaneously clearing acute infections provide an opportunity to characterize the specificities of associated protective antibody responses. In an individual who resolved three separate HCV infections with different HCV genotypes, the antibodies induced during these acute infection episodes were similar to those induced during chronic infection. Surprisingly, the earliest detected antibodies were directed against conformational HCV epitopes on the envelope glycoprotein E2 (including polyprotein residues 434–446) known to be targeted by broadly neutralizing antibodies. Taken together, the key B-cell determinants in spontaneous clearance are the timing and affinity maturation of broadly neutralizing antibody responses.

Antigenicity and Immunogenicity of Differentially Glycosylated Hepatitis C Virus E2 Envelope Proteins Expressed in Mammalian and Insect Cells

The development of a prophylactic vaccine for hepatitis C virus (HCV) remains a global health challenge. Cumulative evidence supports the importance of antibodies targeting the HCV E2 envelope glycoprotein to facilitate viral clearance. However, a significant challenge for a B cell-based vaccine is focusing the immune response on conserved E2 epitopes capable of eliciting neutralizing antibodies not associated with viral escape. We hypothesized that glycosylation might influence the antigenicity and immunogenicity of E2. Accordingly, we performed head-to-head molecular, antigenic, and immunogenic comparisons of soluble E2 (sE2) produced in (i) mammalian (HEK293) cells, which confer mostly complex- and high-mannose-type glycans; and (ii) insect (Sf9) cells, which impart mainly paucimannose-type glycans. Mass spectrometry demonstrated that all 11 predicted N-glycosylation sites were utilized in both HEK293- and Sf9-derived sE2, but that N-glycans in insect sE2 were on average smaller and less complex. Both proteins bound CD81 and were recognized by conformation-dependent antibodies. Mouse immunogenicity studies revealed that similar polyclonal antibody responses were generated against antigenic domains A to E of E2. Although neutralizing antibody titers showed that Sf9-derived sE2 induced moderately stronger responses than did HEK293-derived sE2 against the homologous HCV H77c isolate, the two proteins elicited comparable neutralization titers against heterologous isolates. Given that global alteration of HCV E2 glycosylation by expression in different hosts did not appreciably affect antigenicity or overall immunogenicity, a more productive approach to increasing the antibody response to neutralizing epitopes may be complete deletion, rather than just modification, of specific N-glycans proximal to these epitopes.IMPORTANCE The development of a vaccine for hepatitis C virus (HCV) remains a global health challenge. A major challenge for vaccine development is focusing the immune response on conserved regions of the HCV envelope protein, E2, capable of eliciting neutralizing antibodies. Modification of E2 by glycosylation might influence the immunogenicity of E2. Accordingly, we performed molecular and immunogenic comparisons of E2 produced in mammalian and insect cells. Mass spectrometry demonstrated that the predicted glycosylation sites were utilized in both mammalian and insect cell E2, although the glycan types in insect cell E2 were smaller and less complex. Mouse immunogenicity studies revealed similar polyclonal antibody responses. However, insect cell E2 induced stronger neutralizing antibody responses against the homologous isolate used in the vaccine, albeit the two proteins elicited comparable neutralization titers against heterologous isolates. A more productive approach for vaccine development may be complete deletion of specific glycans in the E2 protein.

Standardized Method for the Study of Antibody Neutralization of HCV Pseudoparticles (HCVpp)

Hepatitis C virus (HCV) pseudoparticles (HCVpp) are generated by cotransfection of HCV envelope (E1 and E2) genes along with a retroviral packaging/reporter construct into HEK293T cells. Enveloped particles bearing HCV E1E2 proteins on their surface are released through a retroviral budding process into the supernatant. Viral E1E2 glycoproteins facilitate a single round of receptor-mediated entry of HCVpp into hepatoma cells, which can be quantified by reporter gene expression. These HCVpp have been employed to study mechanisms of HCV entry into hepatoma cells, as well as HCV neutralization by immune sera or HCV-specific monoclonal antibodies.

In vivo combination of human anti-envelope glycoprotein E2 and -Claudin-1 monoclonal antibodies for prevention of hepatitis C virus infection

Despite the development of direct-acting antivirals (DAAs), hepatitis C virus (HCV) infection remains a major cause for liver disease and cancer worldwide. Entry inhibitors block virus host cell entry and, therefore, prevent establishment of chronic infection and liver disease. Due to their unique mechanism of action, entry inhibitors provide an attractive antiviral strategy in organ transplantation. In this study, we developed an innovative approach in preventing HCV infection using a synergistic combination of a broadly neutralizing human monoclonal antibody (HMAb) targeting the HCV E2 protein and a host-targeting anti-claudin 1 (CLDN1) humanized monoclonal antibody. An in vivo proof-of-concept study in human liver-chimeric FRG-NOD mice proved the efficacy of the combination therapy at preventing infection by an HCV genotype 1b infectious serum. While administration of individual antibodies at lower doses only showed a delay in HCV infection, the combination therapy was highly protective. Furthermore, the combination proved to be effective in preventing infection of primary human hepatocytes by neutralization-resistant HCV escape variants selected during liver transplantation, suggesting that a combination therapy is suited for the neutralization of difficult-to-treat variants. In conclusion, our findings suggest that the combination of two HMAbs targeting different steps of virus entry improves treatment efficacy while simultaneously reducing treatment duration and costs. Our approach not only provides a clinical perspective to employ HMAb combination therapies to prevent graft re-infection and its associated liver disease but may also help to alleviate the urgent demand for organ transplants by allowing the transplantation of organs from HCV-positive donors.

Hepatitis C Virus (HCV)-Apolipoprotein Interactions and Immune Evasion and Their Impact on HCV Vaccine Design

With more than 71 million people chronically infected, hepatitis C virus (HCV) is one of the leading causes of liver disease and hepatocellular carcinoma. While efficient antiviral therapies have entered clinical standard of care, the development of a protective vaccine is still elusive. Recent studies have shown that the HCV life cycle is closely linked to lipid metabolism. HCV virions associate with hepatocyte-derived lipoproteins to form infectious hybrid particles that have been termed lipo-viro-particles. The close association with lipoproteins is not only critical for virus entry and assembly but also plays an important role during viral pathogenesis and for viral evasion from neutralizing antibodies. In this review, we summarize recent findings on the functional role of apolipoproteins for HCV entry and assembly. Furthermore, we highlight the impact of HCV-apolipoprotein interactions for evasion from neutralizing antibodies and discuss the consequences for antiviral therapy and vaccine design. Understanding these interactions offers novel strategies for the development of an urgently needed protective vaccine.

Mapping Determinants of Virus Neutralization and Viral Escape for Rational Design of a Hepatitis C Virus Vaccine

Hepatitis C virus (HCV) continues to spread worldwide with an annual increase of 1.75 million new infections. The number of HCV cases in the U.S. is now greater than the number of HIV cases and is increasing in young adults because of the opioid epidemic sweeping the country. HCV-related liver disease is the leading indication of liver transplantation. An effective vaccine is of paramount importance to control and prevent HCV infection. While this vaccine will need to induce both cellular and humoral immunity, this review is focused on the required antibody responses. For highly variable viruses, such as HCV, isolation and characterization of monoclonal antibodies mediating broad virus neutralization are an important guide for vaccine design. The viral envelope glycoproteins, E1 and E2, are the main targets of these antibodies. Epitopes on the E2 protein have been studied more extensively than epitopes on E1, due to higher antibody targeting that reflects these epitopes having higher degrees of immunogenicity. E2 epitopes are overall organized in discrete clusters of overlapping epitopes that ranged from high conservation to high variability. Other epitopes on E1 and E1E2 also are targets of neutralizing antibodies. Taken together, these regions are important for vaccine design. Another element in vaccine design is based on information on how the virus escapes from broadly neutralizing antibodies. Escape mutations can occur within the epitopes that are involved in antibody binding and in regions that are not involved in their epitopes, but nonetheless reduce the efficiency of neutralizing antibodies. An understanding on the specificities of a protective B cell response, the molecular locations of these epitopes on E1, E2, and E1E2, and the mechanisms, which enable the virus to negatively modulate neutralizing antibody responses to these regions will provide the necessary guidance for vaccine design.

Designing a B Cell-Based Vaccine against a Highly Variable Hepatitis C Virus

The ability to use structure-based design and engineering to control the molecular shape and reactivity of an immunogen to induce protective responses shows great promise, along with corresponding advancements in vaccine testing and evaluation systems. We describe in this review new paradigms for the development of a B cell-based HCV vaccine. Advances in test systems to measure in vitro and in vivo antibody-mediated virus neutralization include retroviral pseudotype particles expressing HCV E1E2 glycoproteins (HCVpp), infectious cell culture-derived HCV virions (HCVcc), and surrogate animal models mimicking acute HCV infection. Their applications have established the role of broadly neutralizing antibodies to control HCV infection. However, the virus has immunogenic regions in the viral envelope glycoproteins that are associated with viral escape or non-neutralizing antibodies. These regions serve as immunologic decoys that divert the antibody response from less prominent conserved regions mediating virus neutralization. This review outlines the immunogenic regions on E2, which are roughly segregated into the hypervariable region 1 (HVR1), and five clusters of overlapping epitopes designated as antigenic domains A-E. Understanding the molecular architecture of conserved neutralizing epitopes within these antigenic domains, and how other antigenic regions or decoys deflect the immune response from these conserved regions will provide a roadmap for the rational design of an HCV vaccine.

A novel neutralizing human monoclonal antibody broadly abrogates hepatitis C virus infection in vitro and in vivo

Infections with hepatitis C virus (HCV) represent a worldwide health burden and a prophylactic vaccine is still not available. Liver transplantation (LT) is often the only option for patients with HCV-induced end-stage liver disease. However, immediately after transplantation, the liver graft becomes infected by circulating virus, resulting in accelerated progression of liver disease. Although the efficacy of HCV treatment using direct-acting antivirals has improved significantly, immune compromised LT-patients and patients with advanced liver disease remain difficult to treat. As an alternative approach, interfering with viral entry could prevent infection of the donor liver. We generated a human monoclonal antibody (mAb), designated 2A5, which targets the HCV envelope. The neutralizing activity of mAb 2A5 was assessed using multiple prototype and patient-derived HCV pseudoparticles (HCVpp), cell culture produced HCV (HCVcc), and a human-liver chimeric mouse model. Neutralization levels observed for mAb 2A5 were generally high and mostly superior to those obtained with AP33, a well-characterized HCV-neutralizing monoclonal antibody. Using humanized mice, complete protection was observed after genotype 1a and 4a HCV challenge, while only partial protection was achieved using gt1b and 6a isolates. Epitope mapping revealed that mAb 2A5 binding is conformation-dependent and identified the E2-region spanning amino acids 434 to 446 (epitope II) as the predominant contact domain.

CONCLUSION

mAb 2A5 shows potent anti-HCV neutralizing activity both in vitro and in vivo and could hence represent a valuable candidate to prevent HCV recurrence in LT-patients. In addition, the detailed identification of the neutralizing epitope can be applied for the design of prophylactic HCV vaccines.

Conformational Flexibility in the Immunoglobulin-Like Domain of the Hepatitis C Virus Glycoprotein E2

The hepatitis C virus (HCV) glycoprotein E2 is the major target of neutralizing antibodies and is therefore highly relevant for vaccine design. Its structure features a central immunoglobulin (Ig)-like β-sandwich that contributes to the binding site for the cellular receptor CD81. We show that a synthetic peptide corresponding to a β-strand of this Ig-like domain forms an α-helix in complex with the anti-E2 antibody DAO5, demonstrating an inside-out flip of hydrophobic residues and a secondary structure change in the composite CD81 binding site. A detailed interaction analysis of DAO5 and cross-competing neutralizing antibodies with soluble E2 revealed that the Ig-like domain is trapped by different antibodies in at least two distinct conformations. DAO5 specifically captures retrovirus particles bearing HCV glycoproteins (HCVpp) and infectious cell culture-derived HCV particles (HCVcc). Infection of cells by DAO5-captured HCVpp can be blocked by a cross-competing neutralizing antibody, indicating that a single virus particle simultaneously displays E2 molecules in more than one conformation on its surface. Such conformational plasticity of the HCV E2 receptor binding site has important implications for immunogen design.IMPORTANCE Recent advances in the treatment of hepatitis C virus (HCV) infection with direct-acting antiviral drugs have enabled the control of this major human pathogen. However, due to their high costs and limited accessibility in combination with the lack of awareness of the mostly asymptomatic infection, there is an unchanged urgent need for an effective vaccine. The viral glycoprotein E2 contains regions that are crucial for virus entry into the host cell, and antibodies that bind to these regions can neutralize infection. One of the major targets of neutralizing antibodies is the central immunoglobulin (Ig)-like domain within E2. We show here that this Ig-like domain is conformationally flexible at the surface of infectious HCV particles and pseudoparticles. Our study provides novel insights into the interactions of HCV E2 with the humoral immune system that should aid future vaccine development.

Affinity maturation of a broadly neutralizing human monoclonal antibody that prevents acute hepatitis C virus infection in mice

Direct-acting antivirals (DAAs) have led to a high cure rate in treated patients with chronic hepatitis C virus (HCV) infection, but this still leaves a large number of treatment failures secondary to the emergence of resistance-associated variants (RAVs). To increase the barrier to resistance, a complementary strategy is to use neutralizing human monoclonal antibodies (HMAbs) to prevent acute infection. However, earlier efforts with the selected antibodies led to RAVs in animal and clinical studies. Therefore, we identified an HMAb that is less likely to elicit RAVs for affinity maturation to increase potency and, more important, breadth of protection. Selected matured antibodies show improved affinity and neutralization against a panel of diverse HCV isolates. Structural and modeling studies reveal that the affinity-matured HMAb mediates virus neutralization, in part, by inducing conformational change to the targeted epitope, and that the maturated light chain is responsible for the improved affinity and breadth of protection. A matured HMAb protected humanized mice when challenged with an infectious HCV human serum inoculum for a prolonged period. However, a single mouse experienced breakthrough infection after 63 days when the serum HMAb concentration dropped by several logs; sequence analysis revealed no viral escape mutation.

CONCLUSION

The findings suggest that a single broadly neutralizing antibody can prevent acute HCV infection without inducing RAVs and may complement DAAs to reduce the emergence of RAVs. (Hepatology 2016;64:1922-1933).

Hypervariable region 1 shielding of hepatitis C virus is a main contributor to genotypic differences in neutralization sensitivity

There are 3-4 million new hepatitis C virus (HCV) infections yearly. The extensive intergenotypic sequence diversity of envelope proteins E1 and E2 of HCV and shielding of important epitopes by hypervariable region 1 (HVR1) of E2 are believed to be major hindrances to developing universally protective HCV vaccines. Using cultured viruses expressing the E1/E2 complex of isolates H77 (genotype 1a), J6 (2a), or S52 (3a), with and without HVR1, we tested HVR1-mediated neutralization occlusion in vitro against a panel of 12 well-characterized human monoclonal antibodies (HMAbs) targeting diverse E1, E2, and E1/E2 epitopes. Surprisingly, HVR1-mediated protection was greatest for S52, followed by J6 and then H77. HCV pulldown experiments showed that this phenomenon was caused by epitope shielding. Moreover, by regression analysis of HMAb binding and neutralization titer of HCV we found a strong correlation for HVR1-deleted viruses but not for parental viruses retaining HVR1. The intergenotype neutralization sensitivity of the parental viruses to HMAb antigenic region (AR) 2A, AR3A, AR4A, AR5A, HC84.26, and HC33.4 varied greatly (>24-fold to >130-fold differences in 50% inhibitory concentration values). However, except for AR5A, these differences decreased to less than 6.0-fold when comparing the corresponding HVR1-deleted viruses. Importantly, this simplified pattern of neutralization sensitivity in the absence of HVR1 was also demonstrated in a panel of HVR1-deleted viruses of genotypes 1a, 2a, 2b, 3a, 5a, and 6a, although for all HMAbs, except AR4A, an outlier was observed. Finally, unique amino acid residues in HCV E2 could explain these outliers in the tested cases of AR5A and HC84.26.

CONCLUSION

HVR1 adds complexity to HCV neutralization by shielding a diverse array of unexpectedly cross-genotype-conserved E1/E2 epitopes. Thus, an HVR1-deleted antigen could be a better HCV vaccine immunogen. (Hepatology 2016;64:1881-1892).

Hepatitis C virus vaccine candidates inducing protective neutralizing antibodies

INTRODUCTION

With more than 150 million chronically infected people, hepatitis C virus (HCV) remains a substantial global health burden. Direct-acting antivirals have dramatically improved viral cure. However, limited access to therapy, late stage detection of infection and re-infection following cure illustrate the need for a vaccine for global control of infection. Vaccines with induction of neutralizing antibodies (nAbs) have been shown to protect successfully against infections by multiple viruses and are currently developed for HCV. Areas covered: Here we review the progress towards the development of vaccines aiming to confer protection against chronic HCV infection by inducing broadly nAbs. The understanding or viral immune evasion in infected patients, the development of novel model systems and the recent structural characterization of viral envelope glycoprotein E2 has markedly advanced our understanding of the molecular mechanisms of virus neutralization with the concomitant development of several vaccine candidates. Expert commentary: While HCV vaccine development remains challenged by the high viral diversity and immune evasion, marked progress in HCV research has advanced vaccine design. Several vaccine candidates have shown robust induction of nAbs in animal models and humans. Randomized clinical trials are the next step to assess their clinical efficacy for protection against chronic infection.

Antibody Treatment of Ebola and Sudan Virus Infection via a Uniquely Exposed Epitope within the Glycoprotein Receptor-Binding Site

Previous efforts to identify cross-neutralizing antibodies to the receptor-binding site (RBS) of ebolavirus glycoproteins have been unsuccessful, largely because the RBS is occluded on the viral surface. We report a monoclonal antibody (FVM04) that targets a uniquely exposed epitope within the RBS; cross-neutralizes Ebola (EBOV), Sudan (SUDV), and, to a lesser extent, Bundibugyo viruses; and shows protection against EBOV and SUDV in mice and guinea pigs. The antibody cocktail ZMapp™ is remarkably effective against EBOV (Zaire) but does not cross-neutralize other ebolaviruses. By replacing one of the ZMapp™ components with FVM04, we retained the anti-EBOV efficacy while extending the breadth of protection to SUDV, thereby generating a cross-protective antibody cocktail. In addition, we report several mutations at the base of the ebolavirus glycoprotein that enhance the binding of FVM04 and other cross-reactive antibodies. These findings have important implications for pan-ebolavirus vaccine development and defining broadly protective antibody cocktails.

Apolipoprotein E Mediates Evasion From Hepatitis C Virus Neutralizing Antibodies

BACKGROUND & AIMS

Efforts to develop an effective vaccine against hepatitis C virus (HCV) have been hindered by the propensity of the virus to evade host immune responses. HCV particles in serum and in cell culture associate with lipoproteins, which contribute to viral entry. Lipoprotein association has also been proposed to mediate viral evasion of the humoral immune response, though the mechanisms are poorly defined.

METHODS

We used small interfering RNAs to reduce levels of apolipoprotein E (apoE) in cell culture-derived HCV-producing Huh7.5-derived hepatoma cells and confirmed its depletion by immunoblot analyses of purified viral particles. Before infection of naïve hepatoma cells, we exposed cell culture-derived HCV strains of different genotypes, subtypes, and variants to serum and polyclonal and monoclonal antibodies isolated from patients with chronic HCV infection. We analyzed the interaction of apoE with viral envelope glycoprotein E2 and HCV virions by immunoprecipitation.

RESULTS

Through loss-of-function studies on patient-derived HCV variants of several genotypes and subtypes, we found that the HCV particle apoE allows the virus to avoid neutralization by patient-derived antibodies. Functional studies with human monoclonal antiviral antibodies showed that conformational epitopes of envelope glycoprotein E2 domains B and C were exposed after depletion of apoE. The level and conformation of virion-associated apoE affected the ability of the virus to escape neutralization by antibodies.

CONCLUSIONS

In cell-infection studies, we found that HCV-associated apoE helps the virus avoid neutralization by antibodies against HCV isolated from chronically infected patients. This method of immune evasion poses a challenge for the development of HCV vaccines.

Structural basis for penetration of the glycan shield of hepatitis C virus E2 glycoprotein by a broadly neutralizing human antibody

Hepatitis C virus (HCV) is a major cause of liver cirrhosis and hepatocellular carcinoma. A challenge for HCV vaccine development is to identify conserved epitopes able to elicit protective antibodies against this highly diverse virus. Glycan shielding is a mechanism by which HCV masks such epitopes on its E2 envelope glycoprotein. Antibodies to the E2 region comprising residues 412-423 (E2(412-423)) have broadly neutralizing activities. However, an adaptive mutation in this linear epitope, N417S, is associated with a glycosylation shift from Asn-417 to Asn-415 that enables HCV to escape neutralization by mAbs such as HCV1 and AP33. By contrast, the human mAb HC33.1 can neutralize virus bearing the N417S mutation. To understand how HC33.1 penetrates the glycan shield created by the glycosylation shift to Asn-415, we determined the structure of this broadly neutralizing mAb in complex with its E2(412-423) epitope to 2.0 Å resolution. The conformation of E2(412-423) bound to HC33.1 is distinct from the β-hairpin conformation of this peptide bound to HCV1 or AP33, because of disruption of the β-hairpin through interactions with the unusually long complementarity-determining region 3 of the HC33.1 heavy chain. Whereas Asn-415 is buried by HCV1 and AP33, it is solvent-exposed in the HC33.1-E2(412-423) complex, such that glycosylation of Asn-415 would not prevent antibody binding. Furthermore, our results highlight the structural flexibility of the E2(412-423) epitope, which may serve as an immune evasion strategy to impede induction of antibodies targeting this site by reducing its antigenicity.

Naturally selected hepatitis C virus polymorphisms confer broad neutralizing antibody resistance

For hepatitis C virus (HCV) and other highly variable viruses, broadly neutralizing mAbs are an important guide for vaccine development. The development of resistance to anti-HCV mAbs is poorly understood, in part due to a lack of neutralization testing against diverse, representative panels of HCV variants. Here, we developed a neutralization panel expressing diverse, naturally occurring HCV envelopes (E1E2s) and used this panel to characterize neutralizing breadth and resistance mechanisms of 18 previously described broadly neutralizing anti-HCV human mAbs. The observed mAb resistance could not be attributed to polymorphisms in E1E2 at known mAb-binding residues. Additionally, hierarchical clustering analysis of neutralization resistance patterns revealed relationships between mAbs that were not predicted by prior epitope mapping, identifying 3 distinct neutralization clusters. Using this clustering analysis and envelope sequence data, we identified polymorphisms in E2 that confer resistance to multiple broadly neutralizing mAbs. These polymorphisms, which are not at mAb contact residues, also conferred resistance to neutralization by plasma from HCV-infected subjects. Together, our method of neutralization clustering with sequence analysis reveals that polymorphisms at noncontact residues may be a major immune evasion mechanism for HCV, facilitating viral persistence and presenting a challenge for HCV vaccine development.

Breadth of neutralization and synergy of clinically relevant human monoclonal antibodies against HCV genotypes 1a, 1b, 2a, 2b, 2c, and 3a

Human monoclonal antibodies (HMAbs) with neutralizing capabilities constitute potential immune-based treatments or prophylaxis against hepatitis C virus (HCV). However, lack of cell culture-derived HCV (HCVcc) harboring authentic envelope proteins (E1/E2) has hindered neutralization investigations across genotypes, subtypes, and isolates. We investigated the breadth of neutralization of 10 HMAbs with therapeutic potential against a panel of 16 JFH1-based HCVcc-expressing patient-derived Core-NS2 from genotypes 1a (strains H77, TN, and DH6), 1b (J4, DH1, and DH5), 2a (J6, JFH1, and T9), 2b (J8, DH8, and DH10), 2c (S83), and 3a (S52, DBN, and DH11). Virus stocks used for in vitro neutralization analysis contained authentic E1/E2, with the exception of full-length JFH1 that acquired the N417S substitution in E2. The 50% inhibition concentration (IC50) for each HMAb against the HCVcc panel was determined by dose-response neutralization assays in Huh7.5 cells with antibody concentrations ranging from 0.0012 to 100 μg/mL. Interestingly, IC50 values against the different HCVcc's exhibited large variations among the HMAbs, and only three HMAbs (HC-1AM, HC84.24, and AR4A) neutralized all 16 HCVcc recombinants. Furthermore, the IC50 values for a given HMAb varied greatly with the HCVcc strain, which supports the use of a diverse virus panel. In cooperation analyses, HMAbs HC84.24, AR3A, and, especially HC84.26, demonstrated synergistic effects towards the majority of the HCVcc's when combined individually with AR4A.

CONCLUSION

Through a neutralization analysis of 10 clinically relevant HMAbs against 16 JFH1-based Core-NS2 recombinants from genotypes 1a, 1b, 2a, 2b, 2c, and 3a, we identified at least three HMAbs with potent and broad neutralization potential. The neutralization synergism obtained when pooling the most potent HMAbs could have significant implications for developing novel strategies to treat and control HCV.

Non-random escape pathways from a broadly neutralizing human monoclonal antibody map to a highly conserved region on the hepatitis C virus E2 glycoprotein encompassing amino acids 412-423

A challenge for hepatitis C virus (HCV) vaccine development is to define epitopes that are able to elicit protective antibodies against this highly diverse virus. The E2 glycoprotein region located at residues 412-423 is conserved and antibodies to 412-423 have broadly neutralizing activities. However, an adaptive mutation, N417S, is associated with a glycan shift in a variant that cannot be neutralized by a murine but by human monoclonal antibodies (HMAbs) against 412-423. To determine whether HCV escapes from these antibodies, we analyzed variants that emerged when cell culture infectious HCV virions (HCVcc) were passaged under increasing concentrations of a specific HMAb, HC33.1. Multiple nonrandom escape pathways were identified. Two pathways occurred in the context of an N-glycan shift mutation at N417T. At low antibody concentrations, substitutions of two residues outside of the epitope, N434D and K610R, led to variants having improved in vitro viral fitness and reduced sensitivity to HC33.1 binding and neutralization. At moderate concentrations, a S419N mutation occurred within 412-423 in escape variants that have greatly reduced sensitivity to HC33.1 but compromised viral fitness. Importantly, the variants generated from these pathways differed in their stability. N434D and K610R-associated variants were stable and became dominant as the virions were passaged. The S419N mutation reverted back to N419S when immune pressure was reduced by removing HC33.1. At high antibody concentrations, a mutation at L413I was observed in variants that were resistant to HC33.1 neutralization. Collectively, the combination of multiple escape pathways enabled the virus to persist under a wide range of antibody concentrations. Moreover, these findings pose a different challenge to vaccine development beyond the identification of highly conserved epitopes. It will be necessary for a vaccine to induce high potency antibodies that prevent the formation of escape variants, which can co-exist with lower potency or levels of neutralizing activities.

Neutralization resistance of hepatitis C virus can be overcome by recombinant human monoclonal antibodies

Immunotherapy and vaccine development for hepatitis C virus (HCV) will depend on broadly reactive neutralizing antibodies (NAbs). However, studies in infectious strain JFH1-based culture systems expressing patient-derived Core-NS2 proteins have suggested neutralization resistance for specific HCV strains, in particular, of genotype 2. To further examine this phenomenon, we developed a panel of HCV genotype 2 recombinants for testing of sensitivity to neutralization by chronic-phase patient sera and lead human monoclonal antibodies (HMAbs). The novel Core-NS2 recombinants, with patient-derived genotype 2a (strain T9), 2b (strains DH8 and DH10), and 2c (strain S83) consensus sequences, were viable in Huh7.5 hepatoma cells without requirement for adaptive mutations, reaching HCV infectivity titers of 3.9-4.5 log10 focus-forming units per milliliter. In in vitro neutralization assays, we demonstrated that the novel genotype 2 viruses as well as prototype strains J6/JFH1(2a) and J8/JFH1(2b), all with authentic envelope proteins, were resistant to neutralization by genotype 2a, 2b, 2c, 2j, 2i, and 2q patient sera. However, these patient sera had high titers of HCV-specific NAbs, because they efficiently reduced the infectivity of J6(2a) and J8(2b) with deleted hypervariable region 1. The genotype 2a, 2b, and 2c viruses, found resistant to polyclonal patient sera neutralization, were efficiently neutralized by two lead HMAbs (AR4A and HC84.26).

CONCLUSION

Using novel 2a, 2b, and 2c cell-culture systems, expressing authentic envelope proteins, we demonstrated resistance of HCV to patient-derived polyclonal high-titer NAbs. However, the same genotype 2 culture viruses were all sensitive to HMAbs recognizing conformational epitopes, indicating that neutralization resistance of HCV can be overcome by applying recombinant antibodies. These findings have important implications for HCV immunotherapy and vaccine development.

Structural basis of HCV neutralization by human monoclonal antibodies resistant to viral neutralization escape

The high mutation rate of hepatitis C virus allows it to rapidly evade the humoral immune response. However, certain epitopes in the envelope glycoproteins cannot vary without compromising virus viability. Antibodies targeting these epitopes are resistant to viral escape from neutralization and understanding their binding-mode is important for vaccine design. Human monoclonal antibodies HC84-1 and HC84-27 target conformational epitopes overlapping the CD81 receptor-binding site, formed by segments aa434-446 and aa610-619 within the major HCV glycoprotein E2. No neutralization escape was yet observed for these antibodies. We report here the crystal structures of their Fab fragments in complex with a synthetic peptide comprising aa434-446. The structures show that the peptide adopts an α-helical conformation with the main contact residues F⁴⁴² and Y⁴⁴³ forming a hydrophobic protrusion. The peptide retained its conformation in both complexes, independently of crystal packing, indicating that it reflects a surface feature of the folded glycoprotein that is exposed similarly on the virion. The same residues of E2 are also involved in interaction with CD81, suggesting that the cellular receptor binds the same surface feature and potential escape mutants critically compromise receptor binding. In summary, our results identify a critical structural motif at the E2 surface, which is essential for virus propagation and therefore represents an ideal candidate for structure-based immunogen design for vaccine development.

Characterization of hepatitis C virus recombinants with chimeric E1/E2 envelope proteins and identification of single amino acids in the E2 stem region important for entry

The hepatitis C virus (HCV) envelope proteins E1 and E2 play a key role in host cell entry and represent important targets for vaccine and drug development. Here, we characterized HCV recombinants with chimeric E1/E2 complexes in vitro. Using genotype 1a/2a JFH1-based recombinants expressing 1a core-NS2, we exchanged E2 with functional isolate sequences of genotypes 1a (alternative isolate), 1b, and 2a. While the 1a-E2 exchange did not impact virus viability, the 2a-E2 recombinant was nonviable. After E2 exchange from three 1b isolates, long delays were observed before spread of infection. For recovered 1b-E2 recombinants, single E2 stem region amino acid changes were identified at residues 706, 707, and 710. In reverse genetic studies, these mutations increased infectivity titers by ~100-fold, apparently without influencing particle stability or cell binding although introducing slight decrease in particle density. In addition, the 1b-E2 exchange led to a decrease in secreted core protein of 25 to 50%, which was further reduced by the E2 stem region mutations. These findings indicated that compensatory mutations permitted robust infectious virus production, without increasing assembly/release. Studies of E1/E2 heterodimerization showed no differences in intracellular E1/E2 interaction for chimeric constructs with or without E2 stem region mutations. Interestingly, the E2 stem region mutations allowed efficient entry, which was verified in 1a-E1/1b-E2 HCV pseudoparticle assays. A CD81 inhibition assay indicated that the mutations influenced a late step of the HCV entry pathway. Overall, this study identified specific amino acids in the E2 stem region of importance for HCV entry and for production of infectious virus particles.

Transient activation of the PI3K-AKT pathway by hepatitis C virus to enhance viral entry

The PI3K-AKT signaling pathway plays an important role in cell growth and metabolism. Here we report that hepatitis C virus (HCV) transiently activates the PI3K-AKT pathway. This activation was observed as early as 15 min postinfection, peaked by 30 min, and became undetectable at 24 h postinfection. The activation of AKT could also be mediated by UV-inactivated HCV, HCV pseudoparticle, and the ectodomain of the HCV E2 envelope protein. Because antibodies directed against CD81 and claudin-1, but not antibodies directed against scavenger receptor class B type I or occludin, could also activate AKT, the interaction between HCV E2 and its two co-receptors CD81 and claudin-1 probably triggered the activation of AKT. This activation of AKT by HCV was important for HCV infectivity, because the silencing of AKT by siRNA or the treatment of cells with its inhibitors or with the inhibitor of its upstream regulator PI3K significantly inhibited HCV infection, whereas the expression of constitutively active AKT enhanced HCV infection. The PI3K-AKT pathway is probably involved in HCV entry, because the inhibition of this pathway could inhibit the entry of HCV pseudoparticle but not the VSV pseudoparticle into cells. Furthermore, the treatment of cells with the AKT inhibitor AKT-V prior to HCV infection inhibited HCV infection, whereas the treatment after HCV infection had no obvious effect. Taken together, our studies indicated that HCV transiently activates the PI3K-AKT pathway to facilitate its entry. These results provide important information for understanding HCV replication and pathogenesis and raised the possibility of targeting this cellular pathway to treat HCV patients.

Human monoclonal antibodies to a novel cluster of conformational epitopes on HCV E2 with resistance to neutralization escape in a genotype 2a isolate

The majority of broadly neutralizing antibodies to hepatitis C virus (HCV) are against conformational epitopes on the E2 glycoprotein. Many of them recognize overlapping epitopes in a cluster, designated as antigenic domain B, that contains residues G530 and D535. To gain information on other regions that will be relevant for vaccine design, we employed yeast surface display of antibodies that bound to genotype 1a H77C E2 mutant proteins containing a substitution either at Y632A (to avoid selecting non-neutralizing antibodies) or D535A. A panel of nine human monoclonal antibodies (HMAbs) was isolated and designated as HC-84-related antibodies. Each HMAb neutralized cell culture infectious HCV (HCVcc) with genotypes 1-6 envelope proteins with varying profiles, and each inhibited E2 binding to the viral receptor CD81. Five of these antibodies neutralized representative genotypes 1-6 HCVcc. Epitope mapping identified a cluster of overlapping epitopes that included nine contact residues in two E2 regions encompassing aa418-446 and aa611-616. Effect on virus entry was measured using H77C HCV retroviral pseudoparticles, HCVpp, bearing an alanine substitution at each of the contact residues. Seven of ten mutant HCVpp showed over 90% reduction compared to wild-type HCVpp and two others showed approximately 80% reduction. Interestingly, four of these antibodies bound to a linear E2 synthetic peptide encompassing aa434-446. This region on E2 has been proposed to elicit non-neutralizing antibodies in humans that interfere with neutralizing antibodies directed at an adjacent E2 region from aa410-425. The isolation of four HC-84 HMAbs binding to the peptide, aa434-446, proves that some antibodies to this region are to highly conserved epitopes mediating broad virus neutralization. Indeed, when HCVcc were passaged in the presence of each of these antibodies, virus escape was not observed. Thus, the cluster of HC-84 epitopes, designated as antigenic domain D, is relevant for vaccine design for this highly diverse virus.

Affinity maturation to improve human monoclonal antibody neutralization potency and breadth against hepatitis C virus

A potent neutralizing antibody to a conserved hepatitis C virus (HCV) epitope might overcome its extreme variability, allowing immunotherapy. The human monoclonal antibody HC-1 recognizes a conformational epitope on the HCV E2 glycoprotein. Previous studies showed that HC-1 neutralizes most HCV genotypes but has modest potency. To improve neutralization, we affinity-matured HC-1 by constructing a library of yeast-displayed HC-1 single chain Fv (scFv) mutants, using for selection an E2 antigen from one of the poorly neutralized HCVpp. We developed an approach by parallel mutagenesis of the heavy chain variable (VH) and κ-chain variable (Vk) genes separately, then combining the optimized VH and Vk mutants. This resulted in the generation of HC-1-related scFv variants exhibiting improved affinities. The best scFv variant had a 92-fold improved affinity. After conversion to IgG1, some of the antibodies exhibited a 30-fold improvement in neutralization activity. Both surface plasmon resonance and solution kinetic exclusion analysis showed that the increase in affinity was largely due to a lowering of the dissociation rate constant, Koff. Neutralization against a panel of HCV pseudoparticles and infectious 2a HCV virus improved with the affinity-matured IgG1 antibodies. Interestingly, some of these antibodies neutralized a viral isolate that was not neutralized by wild-type HC-1. Moreover, propagating 2a HCVcc under the selective pressure of WT HC-1 or affinity-matured HC-1 antibodies yielded no viral escape mutants and, with the affinity-matured IgG1, needed 100-fold less antibody to achieve complete virus elimination. Taken together, these findings suggest that affinity-matured HC-1 antibodies are excellent candidates for therapeutic development.

Neutralizing antibody response to hepatitis C virus

A critical first step in a "rational vaccine design" approach for hepatitis C virus (HCV) is to identify the most relevant mechanisms of immune protection. Emerging evidence provides support for a protective role of virus neutralizing antibodies, and the ability of the B cell response to modify the course of acute HCV infection. This has been made possible by the development of in vitro cell culture models, based on HCV retroviral pseudotype particles expressing E1E2 and infectious cell culture-derived HCV virions, and small animal models that are robust tools in studies of antibody-mediated virus neutralization. This review is focused on the immunogenic determinants on the E2 glycoprotein mediating virus neutralization and the pathways in which the virus is able to escape from immune containment. Encouraging findings from recent studies provide support for the existence of broadly neutralization antibodies that are not associated with virus escape. The identification of conserved epitopes mediating virus neutralization that are not associated with virus escape will facilitate the design of a vaccine immunogen capable of eliciting broadly neutralizing antibodies against this highly diverse virus.

Mapping a region of hepatitis C virus E2 that is responsible for escape from neutralizing antibodies and a core CD81-binding region that does not tolerate neutralization escape mutations

Understanding the interaction between broadly neutralizing antibodies and their epitopes provides a basis for the rational design of a preventive hepatitis C virus (HCV) vaccine. CBH-2, HC-11, and HC-1 are representatives of antibodies to overlapping epitopes on E2 that mediate neutralization by blocking virus binding to CD81. To obtain insights into escape mechanisms, infectious cell culture virus, 2a HCVcc, was propagated under increasing concentrations of a neutralizing antibody to isolate escape mutants. Three escape patterns were observed with these antibodies. First, CBH-2 escape mutants that contained mutations at D431G or A439E, which did not compromise viral fitness, were isolated. Second, under the selective pressure of HC-11, escape mutations progressed from a single L438F substitution at a low antibody concentration to double substitutions, L438F and N434D or L438F and T435A, at higher antibody concentrations. Escape from HC-11 was associated with a loss of viral fitness. An HCV pseudoparticle (HCVpp) containing the L438F mutation bound to CD81 half as efficiently as did wild-type (wt) HCVpp. Third, for HC-1, the antibody at a critical concentration completely suppressed viral replication and generated no escape mutants. Epitope mapping revealed contact residues for CBH-2 and HC-11 in two regions of the E2 glycoprotein, amino acids (aa) 425 to 443 and aa 529 to 535. Interestingly, contact residues for HC-1 were identified only in the region encompassing aa 529 to 535 and not in aa 425 to 443. Taken together, these findings point to a region of variability, aa 425 to 443, that is responsible primarily for viral escape from neutralization, with or without compromising viral fitness. Moreover, the region aa 529 to 535 is a core CD81 binding region that does not tolerate neutralization escape mutations.

Comprehensive linker-scanning mutagenesis of the hepatitis C virus E1 and E2 envelope glycoproteins reveals new structure-function relationships

Despite extensive research, many details about the structure and functions of hepatitis C virus (HCV) glycoproteins E1 and E2 are not fully understood, and their crystal structure remains to be determined. We applied linker-scanning mutagenesis to generate a panel of 34 mutants, each containing an insertion of 5 aa at a random position within the E1E2 sequence. The mutated glycoproteins were analysed by using a range of assays to identify regions critical for maintaining protein conformation, E1E2 complex assembly, CD81 receptor binding, membrane fusion and infectivity. The results, while supporting previously published data, provide several interesting new findings. Firstly, insertion at amino acid 587 or 596 reduced E1E2 heterodimerization without affecting reactivity with some conformation-sensitive mAbs or with CD81, thus implicating these residues in glycoprotein assembly. Secondly, insertions within a conserved region of E2, between amino acid residues 611 and 631, severely disrupted protein conformation and abrogated binding of all conformation-sensitive antibodies, suggesting that the structural integrity of this region is critical for the correct folding of E2. Thirdly, an insertion at Leu-682 specifically affected membrane fusion, providing direct evidence that the membrane-proximal ‘stem’ of E2 is involved in the fusion mechanism. Overall, our results show that the HCV glycoproteins generally do not tolerate insertions and that there are a very limited number of sites that can be changed without dramatic loss of function. Nevertheless, we identified two E2 insertion mutants, at amino acid residues 408 and 577, that were infectious in the murine leukemia virus-based HCV pseudoparticle system.

Hepatitis C virus e2 protein ectodomain is essential for assembly of infectious virions

The Hepatitis C virus E1 and E2 envelope proteins are the major players in all events required for virus entry into target cells. In addition, the recently developed HCV cell culture system has indicated that E1E2 heterodimer formation is a prerequisite for viral particle production. In this paper, we explored a new genetic approach to construct intergenotypic 2a/1b chimeras, maintaining the structural region of the infectious strain JFH1 and substituting the soluble portion of E1 and/or E2 proteins. This strategy provides useful information on the role of the surface-exposed domain of the envelope proteins in virus morphogenesis and allows comparative analysis of different HCV genotypes. We found that substituting the E2 protein ectodomain region abolishes the production of chimeric infectious particles. Our data indicate that the soluble part of the E2 protein is involved in a genotype-specific interplay with remaining viral proteins that affect the HCV assembly process.

CD81 is dispensable for hepatitis C virus cell-to-cell transmission in hepatoma cells

Hepatitis C virus (HCV) infects cells by the direct uptake of cell-free virus following virus engagement with specific cell receptors such as CD81. Recent data have shown that HCV is also capable of direct cell-to-cell transmission, although the role of CD81 in this process is disputed. Here, we generated cell culture infectious strain JFH1 HCV (HCVcc) genomes carrying an alanine substitution of E2 residues W529 or D535 that are critical for binding to CD81 and infectivity. Co-cultivation of these cells with naïve cells expressing enhanced green fluorescent protein (EGFP) resulted in a small number of cells co-expressing both EGFP and HCV NS5A, showing that the HCVcc mutants are capable of cell-to-cell spread. In contrast, no cell-to-cell transmission from JFH1_ΔE1E2-transfected cells occurred, indicating that the HCV glycoproteins are essential for this process. The frequency of cell-to-cell transmission of JFH1_W529A was unaffected by the presence of neutralizing antibodies that inhibit E2–CD81 interactions. By using cell lines that expressed little or no CD81 and that were refractive to infection with cell-free virus, we showed that the occurrence of viral cell-to-cell transmission is not influenced by the levels of CD81 on either donor or recipient cells. Thus, our results show that CD81 plays no role in the cell-to-cell spread of HCVcc and that this mode of transmission is shielded from neutralizing antibodies. These data suggest that therapeutic interventions targeting the entry of cell-free HCV may not be sufficient in controlling an ongoing chronic infection, but need to be complemented by additional strategies aimed at disrupting direct cell-to-cell viral transmission.

Broadly neutralizing human monoclonal antibodies to the hepatitis C virus E2 glycoprotein

The humoral response to hepatitis C virus (HCV) may contribute to controlling infection. We previously isolated human monoclonal antibodies to conformational epitopes on the HCV E2 glycoprotein. Here, we report on their ability to inhibit infection by retroviral pseudoparticles incorporating a panel of full-length E1E2 clones representing the full spectrum of genotypes 1-6. We identified one antibody, CBH-5, that was capable of neutralizing every genotype tested. It also potently inhibited chimeric cell culture-infectious HCV, which had genotype 2b envelope proteins in a genotype 2a (JFH-1) background. Analysis using a panel of alanine-substitution mutants of HCV E2 revealed that the epitope of CBH-5 includes amino acid residues that are required for binding of E2 to CD81, a cellular receptor essential for virus entry. This suggests that CBH-5 inhibits HCV infection by competing directly with CD81 for a binding site on E2.

Structural elucidation of critical residues involved in binding of human monoclonal antibodies to hepatitis C virus E2 envelope glycoprotein

Human monoclonal antibodies derived from B cells of HCV-infected individuals provide information on the immune response to native HCV envelope proteins as they are recognized during infection. Monoclonal antibodies have been useful in the determination of the function and structure of specific immunogenic domains of proteins and should also be useful for the structure/function characterization of HCV E1 and E2 envelope glycoproteins. The HCV E2 envelope glycoprotein has at least three immunodistinctive conformation domains, designated A, B, and C. Conformational epitopes within domain B and C are neutralizing antibody targets on HCV pseudoparticles as well as from infectious cell culture virus. In this study, a combination of differential surface modification and mass spectrometric limited proteolysis followed by alanine mutagenesis was used to provide insight into potential conformational changes within the E2 protein upon antibody binding. The arginine guanidine groups in the E2 protein were modified with CHD in both the affinity bound and free states followed by mass spectrometric analysis, and the regions showing protection upon antibody binding were identified. This protection can arise by direct contact between the residues and the monoclonal antibody, or by antibody-induced conformational changes. Based on the mass spectrometric data, site-directed mutagenesis experiments were performed which clearly identified additional amino acid residues on E2 distant from the site of antibody interaction, whose change to alanine inhibited antibody recognition by inducing conformational changes within the E2 protein.

Therapeutic control of hepatitis C virus: the role of neutralizing monoclonal antibodies

Liver failure associated with hepatitis C virus (HCV) accounts for a substantial portion of liver transplantation. Although current therapy helps some patients with chronic HCV infection, adverse side effects and a high relapse rate are major problems. These problems are compounded in liver transplant recipients as reinfection occurs shortly after transplantation. One approach to control reinfection is the combined use of specific antivirals together with HCV-specific antibodies. Indeed, a number of human and mouse monoclonal antibodies to conformational and linear epitopes on HCV envelope proteins are potential candidates, since they have high virus neutralization potency and are directed to epitopes conserved across diverse HCV genotypes. However, a greater understanding of the factors contributing to virus escape and the role of lipoproteins in masking virion surface domains involved in virus entry will be required to help define those protective determinants most likely to give broad protection. An approach to immune escape is potentially caused by viral infection of immune cells leading to the induction hypermutation of the immunoglobulin gene in B cells. These effects may contribute to HCV persistence and B cell lymphoproliferative diseases.

Immunogenic and functional organization of hepatitis C virus (HCV) glycoprotein E2 on infectious HCV virions

Development of full-length hepatitis C virus (HCV) RNAs replicating efficiently and producing infectious cell-cultured virions, HCVcc, in hepatoma cells provides an opportunity to characterize immunogenic domains on viral envelope proteins involved in entry into target cells. A panel of immunoglobulin G1 human monoclonal antibodies (HMAbs) to three immunogenic conformational domains (designated A, B, and C) on HCV E2 glycoprotein showed that epitopes within two domains, B and C, mediated HCVcc neutralization, whereas HMAbs to domain A were all nonneutralizing. For the neutralizing antibodies to domain B (with some to conserved epitopes among different HCV genotypes), the inhibitory antibody concentration reducing HCVcc infection by 90%, IC90, ranged from 0.1 to 4 microg/ml. For some neutralizing HMAbs, HCVcc neutralization displayed a linear correlation with an antibody concentration between the IC50 and the IC90 while others showed a nonlinear correlation. The differences between IC50/IC90 ratios and earlier findings that neutralizing HMAbs block E2 interaction with CD81 suggest that these antibodies block different facets of virus-receptor interaction. Collectively, these findings support an immunogenic model of HCV E2 having three immunogenic domains with distinct structures and functions and provide added support for the idea that CD81 is required for virus entry.

Preclinical evaluation of two neutralizing human monoclonal antibodies against hepatitis C virus (HCV): a potential treatment to prevent HCV reinfection in liver transplant patients

Passive immunotherapy is potentially effective in preventing reinfection of liver grafts in hepatitis C virus (HCV)-associated liver transplant patients. A combination of monoclonal antibodies directed against different epitopes may be advantageous against a highly mutating virus such as HCV. Two human monoclonal antibodies (HumAbs) against the E2 envelope protein of HCV were developed and tested for the ability to neutralize the virus and prevent human liver infection. These antibodies, designated HCV-AB 68 and HCV-AB 65, recognize different conformational epitopes on E2. They were characterized in vitro biochemically and functionally. Both HumAbs are immunoglobulin G1 and have affinity constants to recombinant E2 constructs in the range of 10(-10) M. They are able to immunoprecipitate HCV particles from infected patients' sera from diverse genotypes and to stain HCV-infected human liver tissue. Both antibodies can fix complement and form immune complexes, but they do not activate complement-dependent or antibody-dependent cytotoxicity. Upon complement fixation, the monoclonal antibodies induce phagocytosis of the immune complexes by neutrophils, suggesting that the mechanism of viral clearance includes endocytosis. In vivo, in the HCV-Trimera model, both HumAbs were capable of inhibiting HCV infection of human liver fragments and of reducing the mean viral load in HCV-positive animals. The demonstrated neutralizing activities of HCV-AB 68 and HCV-AB 65 suggest that they have the potential to prevent reinfection in liver transplant patients and to serve as prophylactic treatment in postexposure events.

Analysis of a highly flexible conformational immunogenic domain a in hepatitis C virus E2

Hepatitis C (HCV) E2 glycoprotein is involved in virus attachment and entry, and its structural organization is largely unknown. Characterization of a panel of human monoclonal antibodies (HMAbs) to HCV by competition studies has led to an immunogenic organization model of E2 with three domains designated A, B, and C and epitopes in each domain having similar structural and functional properties. Domain A contains nonneutralizing epitopes, and domains B and C contain neutralizing epitopes. The isolation and characterization of three new HMAbs within domain A for a total of six provide support for this model. All six domain A HMAbs do not neutralize HCV retroviral pseudotype particle (HCVpp) infection on Huh-7 cells, and all six HMAbs have similar binding affinity and maximum binding, B(max), a relative indicator of epitope density, as other neutralizing HMAbs, suggesting that neutralization is epitope specific and not by binding to any surface epitope. The dose-dependent neutralizing activity of CBH-7, an HMAb to a domain C epitope in spatial proximity to domain A, and of CBH-5, a domain B HMAb to a more distant epitope, were tested in the presence and absence of each domain A HMAb. No enhancement or reduction in CBH-7 or CBH-5 neutralizing activity was observed, indicating that the potential induction of nonneutralizing antibodies should not be a central issue for HCV vaccine design. To assess whether domain A is involved in the structural changes as part of a pH-dependent virus envelope fusion process, changes in antibody binding patterns to normal pH and acid pH-treated HCVpp were measured. Antibody binding affinity of HMAbs to HCVpp was not affected by low pH. However, the B(max) values for low-pH-treated HCVpp with antibodies to domain A increased 46%, for domain C (CBH-7) they increased 23%, and for domain B (CBH-5) there was a decrease of 12%. Collectively, the organization and function of HCV E2 antigenic domains are roughly analogous to the large envelope glycoprotein E organizational structure for other flaviviruses with three distinct structural and functional domains.

Folding and dimerization of hepatitis C virus E1 and E2 glycoproteins in stably transfected CHO cells

The recombinant E1E2 heterodimer of the hepatitis C virus is a candidate for a subunit vaccine. Folding analysis of E1 and E2 glycoproteins, stably expressed in CHO cells, showed that E1 folding was faster and more efficient than E2. The oxidized DTT-resistant conformation of E1 was completed within 2 h post-synthesis, while E2 not only required up to 6 h but also generated non-native species. Calnexin was found to assist E1 folding, whereas no chaperone association was found with E2. The assembly of E1 and E2 was assessed by co-immunoprecipitation and sedimentation velocity analysis. We found that the formation of native E1E2 heterodimers paralleled E2 oxidation kinetics, suggesting that E2 completed its folding process after association with E1. Once formed, sedimentation of the native E1E2 heterodimers was consistent with the absence of additional associated factors. Taken together, our data strongly suggest that productive folding of the major HCV spike protein E2 is assisted by E1.

Hepatitis C virus E2 has three immunogenic domains containing conformational epitopes with distinct properties and biological functions

Mechanisms of virion attachment, interaction with its receptor, and cell entry are poorly understood for hepatitis C virus (HCV) because of a lack of an efficient and reliable in vitro system for virus propagation. Infectious HCV retroviral pseudotype particles (HCVpp) were recently shown to express native E1E2 glycoproteins, as defined in part by HCV human monoclonal antibodies (HMAbs) to conformational epitopes on E2, and some of these antibodies block HCVpp infection (A. Op De Beeck, C. Voisset, B. Bartosch, Y. Ciczora, L. Cocquerel, Z. Y. Keck, S. Foung, F. L. Cosset, and J. Dubuisson, J. Virol. 78:2994-3002, 2004). Why some HMAbs are neutralizing and others are nonneutralizing is looked at in this report by a series of studies to determine the expression of their epitopes on E2 associated with HCVpp and the role of antibody binding affinity. Antibody cross-competition defined three E2 immunogenic domains with neutralizing HMAbs restricted to two domains that were also able to block E2 interaction with CD81, a putative receptor for HCV. HCVpp immunoprecipitation showed that neutralizing and nonneutralizing domains are expressed on E2 associated with HCVpp, and affinity studies found moderate-to-high-affinity antibodies in all domains. These findings support the perspective that HCV-specific epitopes are responsible for functional steps in virus infection, with specific antibodies blocking distinct steps of virus attachment and entry, rather than the perspective that virus neutralization correlates with increased antibody binding to any virion surface site, independent of the epitope recognized by the antibody. Segregation of virus neutralization and sensitivity to low pH to specific regions supports a model of HCV E2 immunogenic domains similar to the antigenic structural and functional domains of other flavivirus envelope E glycoproteins.

Antibody light chain-catalyzed hydrolysis of a hepatitis C virus peptide

A panel of human monoclonal and recombinant antibody light chains was screened for cleavage of the synthetic peptide corresponding to a neutralizing epitope of hepatitis C virus (residues 192-205 of envelope glycoprotein E1). One of the 39 light chains studied hydrolyzed the Val197-Ser198 bond of the peptide with Km and kcat values of 223 +/- 7 microM and 0.087 +/- 0.001 min(-1).

Human monoclonal antibody to hepatitis C virus E1 glycoprotein that blocks virus attachment and viral infectivity

Human antibodies elicited in response to hepatitis C virus (HCV) infection are anticipated to react with the native conformation of the viral envelope structure. Isolation of these antibodies as human monoclonal antibodies that block virus binding and entry will be useful in providing potential therapeutic reagents and for vaccine development. H-111, an antibody to HCV envelope 1 protein (E1) that maps to the YEVRNVSGVYH sequence and is located near the N terminus of E1 and is able to immunoprecipitate E1E2 heterodimers, is described. Binding of H-111 to HCV E1 genotypes 1a, 1b, 2b, and 3a indicates that the H-111 epitope is highly conserved. Sequence analysis of antibody V regions showed evidence of somatic and affinity maturation of H-111. Finally, H-111 blocks HCV-like particle binding to and HCV virion infection of target cells, suggesting the involvement of this epitope in virus binding and entry.

Incomplete Humoral Immunity against Hepatitis C Virus Is Linked with Distinct Recognition of Putative Multiple Receptors by E2 Envelope Glycoprotein

Little is known about the role of the humoral immune response to hepatitis C virus (HCV). This study provides molecular evidence for the mechanism by which neutralizing Abs from the sera of chronic HCV patients have lower inhibitory activities against the binding of HCV E2 envelope protein to human hepatoma cell lines than to a lymphoma cell line. E2 binds to several putative receptors, specifically human CD81; human scavenger receptor, class B, type 1; and heparan sulfate. We have shown that E2 binds to target cells via these receptors in a noncompetitive manner. Thus, incomplete inhibition of one of the receptors leads to only a partial E2 blockade and, possibly, evasion of the host immune response. We demonstrated that the difference in and reduction of inhibition was closely related to impaired blockade of E2 binding to scavenger receptor, class B, type 1, and heparan sulfate. We have also shown that soluble E2 protein binds to multiple soluble receptors via separate binding domains on E2, providing further evidence for the distinct recognition of multiple cellular receptors by E2. This report suggests a novel finding that biased humoral immune responses to HCV E2 might provide an alternative mechanism for viral escape without the involvement of mutation. Additionally, our data give crucial consideration to the development of HCV vaccines that stimulate protective humoral immune responses.

Recognition of native hepatitis C virus E1E2 heterodimers by a human monoclonal antibody

The majority of hepatitis C virus (HCV)-infected individuals progress from acute to chronic disease, despite the presence of a strong humoral immune response to the envelope glycoproteins E1 and E2. When expressed in mammalian cells, E1 and E2 form both noncovalently linked E1E2 heterodimers, believed to be properly folded, and disulfide-linked, high-molecular-weight aggregates that are misfolded. Previously, we identified 10 human monoclonal antibodies (HMAbs) that bind E2 glycoproteins from different genotypes. Here we demonstrate that one of these HMAbs, CBH-2, is unique in its ability to distinguish between properly folded and misfolded envelope proteins. This HMAb recognizes HCV-E2 only when complexed with E1. The E1E2 complexes recognized by CBH-2 are noncovalently linked heterodimers and not misfolded disulfide-linked, high-molecular-weight aggregates. The E1E2 heterodimers seen by CBH-2 no longer associate with the endoplasmic reticulum chaperone calnexin and are likely to represent the prebudding form of the HCV virion.