Determination of the human antibody response to the epitope defined by the hepatitis C virus-neutralizing monoclonal antibody AP33

Current Hepatitis C Vaccine Candidates Based on the Induction of Neutralizing Antibodies

The introduction of direct-acting antivirals (DAAs) has revolutionized hepatitis C treatment. Short courses of treatment with these drugs are highly beneficial to patients, eliminating hepatitis C virus (HCV) without adverse effects. However, this outstanding success is tempered by the continuing difficulty of eradicating the virus worldwide. Thus, access to an effective vaccine against HCV is strongly needed to reduce the burden of the disease and contribute to the elimination of viral hepatitis. The recent failure of a T-cell vaccine based on the use of viral vectors expressing the HCV non-structural protein sequences to prevent chronic hepatitis C in drug users has pointed out that the induction of neutralizing antibodies (NAbs) will be essential in future vaccine candidates. To induce NAbs, vaccines must contain the main target of this type of antibody, the HCV envelope glycoproteins (E1 and E2). In this review, we summarize the structural regions in E1 and E2 proteins that are targeted by NAbs and how these proteins are presented in the vaccine candidates currently under development.

Effect of Glycan Shift on Antibodies against Hepatitis C Virus E2 412-425 Epitope Elicited by Chimeric sHBsAg-Based Virus-Like Particles

Two of the most important mechanisms of hepatitis C virus (HCV) immune evasion are the high variability of the amino acid sequence and epitope shielding via heavy glycosylation of the envelope (E) proteins. Previously, we showed that chimeric sHBsAg (hepatitis B virus [HBV] small surface antigen)-based virus-like particles (VLPs) carrying highly conserved epitope I from the HCV E2 glycoprotein (sHBsAg_412-425) elicit broadly neutralizing antibodies (bnAbs). However, many reports have identified escape mutations for such bnAbs that shift the N-glycosylation site from N417 to N415. This shift effectively masks the recognition of epitope I by antibodies raised against the wild-type glycoprotein. To investigate if glycan-shift-mediated immune evasion could be overcome by targeted vaccination strategies, we designed sHBsAg-based VLPs carrying epitope I with an N417S change (sHBsAg_N417S). Studies in BALB/c mice revealed that both sHBsAg_412-425 and sHBsAg_N417S VLPs were immunogenic, eliciting antibodies that recognized peptides encompassing epitope I regardless of the N417S change. However, we observed substantial differences in E1E2 glycoprotein binding and cell culture-derived HCV (HCVcc) neutralization between the sera elicited by sHBsAg_412-425 and those elicited by sHBsAg_N417S VLPs. Our results suggest a complex interplay among antibodies targeting epitope I, the E1E2 glycosylation status, and the epitope or global E1E2 conformation. Additionally, we observed striking similarities in the E1E2 glycoprotein binding patterns and HCVcc neutralization between sHBsAg_412-425 sera and AP33, suggesting that the immunization of mice with sHBsAg_412-425 VLPs can elicit AP33-like antibodies. This study emphasizes the role of antibodies against epitope I and represents an initial effort toward designing an antigen that elicits an immune response against epitope I with a glycan shift change. IMPORTANCE Epitope I, located within amino acids 412 to 423 of the HCV E2 glycoprotein, is an important target for an epitope-based HCV vaccine. One interesting feature of epitope I is the N417 glycosylation site, where a single change to S417 or T417 can shift the glycosylation site to position N415. This shift can effectively prevent the binding of broadly neutralizing antibodies targeting epitope I. Aiming to overcome glycan-shift-mediated immune evasion, we constructed sHBsAg_N417S VLPs carrying E2 epitope I, with N417S, and compared them with VLPs carrying wild-type epitope I. We show that antibodies elicited by the sHBsAg-based VLPs presenting two variants of the 412-425 epitope targeted two distinct glycan variants of the HCV E1E2 heterodimer. Our study suggests that due to the conformational flexibility of the E2 glycoprotein and epitope I, future vaccine antigens should elicit antibodies targeting more than one conformation and glycosylation variant of the 412-423 epitope.

To Include or Occlude: Rational Engineering of HCV Vaccines for Humoral Immunity

Direct-acting antiviral agents have proven highly effective at treating existing hepatitis C infections but despite their availability most countries will not reach the World Health Organization targets for elimination of HCV by 2030. A prophylactic vaccine remains a high priority. Whilst early vaccines focused largely on generating T cell immunity, attention is now aimed at vaccines that generate humoral immunity, either alone or in combination with T cell-based vaccines. High-resolution structures of hepatitis C viral glycoproteins and their interaction with monoclonal antibodies isolated from both cleared and chronically infected people, together with advances in vaccine technologies, provide new avenues for vaccine development.

HCV Glycoprotein Structure and Implications for B-Cell Vaccine Development

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

Specific Antibodies Induced by Immunization with Hepatitis B Virus-Like Particles Carrying Hepatitis C Virus Envelope Glycoprotein 2 Epitopes Show Differential Neutralization Efficiency

Hepatitis C virus (HCV) infection with associated chronic liver diseases is a major health problem worldwide. Here, we designed hepatitis B virus (HBV) small surface antigen (sHBsAg) virus-like particles (VLPs) presenting different epitopes derived from the HCV E2 glycoprotein (residues 412-425, 434-446, 502-520, and 523-535 of isolate H77C). Epitopes were selected based on their amino acid sequence conservation and were previously reported as targets of HCV neutralizing antibodies. Chimeric VLPs obtained in the Leishmania tarentolae expression system, in combination with the adjuvant Addavax, were used to immunize mice. Although all VLPs induced strong humoral responses, only antibodies directed against HCV 412-425 and 523-535 epitopes were able to react with the native E1E2 glycoprotein complexes of different HCV genotypes in ELISA. Neutralization assays against genotype 1-6 cell culture infectious HCV (HCVcc), revealed that only VLPs carrying the 412-425 epitope induced efficient HCV cross-neutralizing antibodies, but with isolate specific variations in efficacy that could not necessarily be explained by differences in epitope sequences. In contrast, antibodies targeting 434-446, 502-520, and 523-535 epitopes were not neutralizing HCVcc, highlighting the importance of conformational antibodies for efficient virus neutralization. Thus, 412-425 remains the most promising linear E2 epitope for further bivalent, rationally designed vaccine research.

B cell immunodominance in primary hepatitis C virus infection

BACKGROUND & AIMS

Neutralising antibodies (NAbs) play a key role in clearance of HCV. NAbs have been isolated and mapped to several domains on the HCV envelope proteins. However, the immunodominance of these epitopes in HCV infection remains unknown, hindering efforts to elicit optimal epitope-specific responses. Furthermore, it remains unclear which epitope-specific responses are associated with broad NAb (bNAb) activity in primary HCV infection. The aim of this study was to define B cell immunodominance in primary HCV, and its implications on neutralisation breadth and clearance.

METHODS

Using samples from 168 patients with primary HCV infection, the antibody responses targeted 2 immunodominant domains, termed domains B and C. Genotype 1 and 3 infections were associated with responses targeted towards different bNAb domains.

RESULTS

No epitopes were uniquely targeted by clearers compared to those who developed chronic infection. Samples with bNAb activity were enriched for multi-specific responses directed towards the epitopes antigenic region 3, antigenic region 4, and domain D, and did not target non-neutralising domains.

CONCLUSIONS

This study outlines for the first time a clear NAb immunodominance profile in primary HCV infection, and indicates that it is influenced by the infecting virus. It also highlights the need for a vaccination strategy to induce multi-specific responses that do not target non-neutralising domains.

LAY SUMMARY

Neutralising antibodies will likely form a key component of a protective hepatitis C virus vaccine. In this work we characterise the predominant neutralising and non-neutralising antibody (epitope) targets in acute hepatitis C virus infection. We have defined the natural hierarchy of epitope immunodominance, and demonstrated that viral genotype can impact on this hierarchy. Our findings highlight key epitopes that are associated with broadly neutralising antibodies, and the deleterious impact of mounting a response towards some of these domains on neutralising breadth. These findings should guide future efforts to design immunogens aimed at generating neutralising antibodies with a vaccine candidate.

Improving the aqueous solubility of HCV-E2 glycoprotein epitope mimics by cyclization using POLAR hinges

In this research we describe the improvement of the water-solubility of cyclic epitope mimics based on the HCV E2 glycoprotein by incorporation of suitable polar hinges. The poor solubility of epitope mimics based on peptide sequences in the envelope (E2) protein hampered their synthesis and purification and made it very difficult to prepare the molecular constructs for evaluation of their bioactivity. Since changes in the amino acid composition are hardly possible in these epitope mimics in order to increase water-solubility, a polar cyclization hinge may offer a remedy leading to a significant increase of polarity and therefore water solubility. These polar hinges were applied in the synthesis of better water-soluble HCV-E2 epitopes. An azide functionality in the polar hinges allowed attachment of a tetraethylene glycol linker by Cu-catalyzed azide-alkyne cyclo-addition (CuAAC) for a convenient conjugation to ELISA plates in order to evaluate the bio-activity of the epitope mimics. The immunoassays showed that the use of more polar cyclization hinges still supported anti-HCV antibody recognition and did not negatively influence their binding. This significantly increased solubility induced by polar hinges should therefore allow for the molecular construction and ultimate evaluation of synthetic vaccine molecules.

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.

Hepatitis C Virus Envelope Glycoproteins: A Balancing Act of Order and Disorder

Chronic hepatitis C virus infection often leads to liver cirrhosis and primary liver cancer. In 2015, an estimated 71 million people were living with chronic HCV. Although infection rates have decreased in many parts of the world over the last several decades, incidence of HCV infection doubled between 2010 and 2014 in the United States mainly due to increases in intravenous drug use. The approval of direct acting antiviral treatments is a necessary component in the elimination of HCV, but inherent barriers to treatment (e.g., cost, lack of access to healthcare, adherence to treatment, resistance, etc.) prevent dramatic improvements in infection rates. An effective HCV vaccine would significantly slow the spread of the disease. Difficulties in the development of an HCV culture model system and expression of properly folded- and natively modified-HCV envelope glycoproteins E1 and E2 have hindered vaccine development efforts. The recent structural and biophysical studies of these proteins have demonstrated that the binding sites for the cellular receptor CD-81 and neutralizing antibodies are highly flexible in nature, which complicate vaccine design. Furthermore, the interactions between E1 and E2 throughout HCV infection is poorly understood, and structural flexibility may play a role in shielding antigenic epitopes during infection. Here we discuss the structural complexities of HCV E1 and E2.

Immunogenetic and structural analysis of a class of HCV broadly neutralizing antibodies and their precursors

Elicitation of broadly neutralizing antibodies (bnAbs) is a leading strategy in rational vaccine design against antigenically diverse pathogens. Here, we studied a panel of monoclonal antibodies (mAbs) from mice immunized with the hepatitis C virus (HCV) envelope glycoproteins E1E2. Six of the mAbs recognize the conserved E2 antigenic site 412-423 (AS412) and cross-neutralize diverse HCV genotypes. Immunogenetic and structural analysis revealed that the antibodies originated from two different germline (GL) precursors and bind AS412 in a β-hairpin conformation. Intriguingly, the anti-HCV activity of one antibody lineage is associated with maturation of the light chain (LC), whereas the other lineage is dependent on heavy-chain (HC) maturation. Crystal structures of GL precursors of the LC-dependent lineage in complex with AS412 offer critical insights into the maturation process of bnAbs to HCV, providing a scientific foundation for utilizing the mouse model to study AS412-targeting vaccine candidates.

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

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

The Neutralizing Face of Hepatitis C Virus E2 Envelope Glycoprotein

The high genetic variability of hepatitis C virus, together with the high level of glycosylation on the viral envelope proteins shielding potential neutralizing epitopes, pose a difficult challenge for vaccine development. An effective hepatitis C virus (HCV) vaccine must target conserved epitopes and the HCV E2 glycoprotein is the main target for such neutralizing antibodies (NAbs). Recent structural investigations highlight the presence of a highly conserved and accessible surface on E2 that is devoid of N-linked glycans and known as the E2 neutralizing face. This face is defined as a hydrophobic surface comprising the front layer (FL) and the CD81 binding loop (CD81bl) that overlap with the CD81 receptor binding site on E2. The neutralizing face consists of highly conserved residues for recognition by cross-NAbs, yet it appears to be high conformationally flexible, thereby presenting a moving target for NAbs. Three main overlapping neutralizing sites have been identified in the neutralizing face: antigenic site 412 (AS412), antigenic site 434 (AS434), and antigenic region 3 (AR3). Here, we review the structural analyses of these neutralizing sites, either as recombinant E2 or epitope-derived linear peptides in complex with bNAbs, to understand the functional and preferred conformations for neutralization, and for viral escape. Collectively, these studies provide a foundation and molecular templates to facilitate structure-based approaches for HCV vaccine development.

Intrinsic structural versatility of the highly conserved 412-423 epitope of the Hepatitis C Virus E2 protein

HCV infection is a major threaten for human health as it affects hundreds of million people worldwide. Here we investigated the conformational properties of the 412-423 fragment of the envelope E2 protein, one of the most immunogenic regions of the virus proteome whose characterization may provide interesting insights for anti-HCV vaccine development. The spectroscopic characterization of the polypeptide unravels its unexpected tendency to form amyloid-like aggregates. When kept in monomeric state, it shows a limited tendency to adopt regular secondary structure. Enhanced molecular dynamics simulations, starting from four distinct conformational states, highlight its structural versatility. Interestingly, all multiform conformational states of the polypeptide detected in crystallographic complexes with antibodies are present in the structural ensemble of all simulations. This observation corroborates the idea that known antibodies recognize this region through a conformational selection mechanism. Accordingly, the design of effective anti-HCV vaccines should consider the intrinsic flexibility of this region. The structural versatility of the 412-423 region is particularly puzzling if its remarkable sequence conservation is considered. It is likely that flexibility and sequence conservation are important features that endow this epitope with the ability to accomplish distinct functions such as immunity escape and interaction with host receptors.

Escape of Hepatitis C Virus from Epitope I Neutralization Increases Sensitivity of Other Neutralization Epitopes

The hepatitis C virus (HCV) E2 glycoprotein is a major target of the neutralizing antibody (nAb) response, with multiple type-specific and broadly neutralizing antibody (bnAb) epitopes identified. The 412-to-423 region can generate bnAbs that block interaction with the cell surface receptor CD81, with activity toward multiple HCV genotypes. In this study, we reveal the structure of rodent monoclonal antibody 24 (MAb24) with an extensive contact area toward a peptide spanning the 412-to-423 region. The crystal structure of the MAb24–peptide 412-to-423 complex reveals the paratope bound to a peptide hairpin highly similar to that observed with human MAb HCV1 and rodent MAb AP33, but with a different angle of approach. In viral outgrowth experiments, we demonstrated three distinct genotype 2a viral populations that acquired resistance to MAb24 via N415D, N417S, and N415D/H386R mutations. Importantly, the MAb24-resistant viruses exhibited significant increases in sensitivity to the majority of bnAbs directed to epitopes within the 412-to-423 region and in additional antigenic determinants located within E2 and the E1E2 complex. This study suggests that modification of N415 causes a global change in glycoprotein structure that increases its vulnerability to neutralization by other antibodies. This finding suggests that in the context of an antibody response to viral infection, acquisition of escape mutations in the 412-to-423 region renders the virus more susceptible to neutralization by other specificities of nAbs, effectively reducing the immunological fitness of the virus. A vaccine for HCV that generates polyspecific humoral immunity with specificity for the 412-to-423 region and at least one other region of E2 is desirable.

IMPORTANCE Understanding how antibodies neutralize hepatitis C virus (HCV) is essential for vaccine development. This study reveals for the first time that when HCV develops resistance to a major class of bnAbs targeting the 412-to-423 region of E2, this results in a concomitant increase in sensitivity to neutralization by a majority of other bnAb specificities. Vaccines for the prevention of HCV infection should therefore generate bnAbs directed toward the 412-to-423 region of E2 and additional bnAb epitopes within the viral glycoproteins.

Structure-Based Design of Hepatitis C Virus Vaccines That Elicit Neutralizing Antibody Responses to a Conserved Epitope

Despite recent advances in therapeutic options, hepatitis C virus (HCV) remains a severe global disease burden, and a vaccine can substantially reduce its incidence. Due to its extremely high sequence variability, HCV can readily escape the immune response; thus, an effective vaccine must target conserved, functionally important epitopes. Using the structure of a broadly neutralizing antibody in complex with a conserved linear epitope from the HCV E2 envelope glycoprotein (residues 412 to 423; epitope I), we performed structure-based design of immunogens to induce antibody responses to this epitope. This resulted in epitope-based immunogens based on a cyclic defensin protein, as well as a bivalent immunogen with two copies of the epitope on the E2 surface. We solved the X-ray structure of a cyclic immunogen in complex with the HCV1 antibody and confirmed preservation of the epitope conformation and the HCV1 interface. Mice vaccinated with our designed immunogens produced robust antibody responses to epitope I, and their serum could neutralize HCV. Notably, the cyclic designs induced greater epitope-specific responses and neutralization than the native peptide epitope. Beyond successfully designing several novel HCV immunogens, this study demonstrates the principle that neutralizing anti-HCV antibodies can be induced by epitope-based, engineered vaccines and provides the basis for further efforts in structure-based design of HCV vaccines.IMPORTANCE Hepatitis C virus is a leading cause of liver disease and liver cancer, with approximately 3% of the world's population infected. To combat this virus, an effective vaccine would have distinct advantages over current therapeutic options, yet experimental vaccines have not been successful to date, due in part to the virus's high sequence variability leading to immune escape. In this study, we rationally designed several vaccine immunogens based on the structure of a conserved epitope that is the target of broadly neutralizing antibodies. In vivo results in mice indicated that these antigens elicited epitope-specific neutralizing antibodies, with various degrees of potency and breadth. These promising results suggest that a rational design approach can be used to generate an effective vaccine for this virus.

The core domain of hepatitis C virus glycoprotein E2 generates potent cross-neutralizing antibodies in guinea pigs

A vaccine that prevents hepatitis C virus (HCV) infection is urgently needed to support an emerging global elimination program. However, vaccine development has been confounded because of HCV's high degree of antigenic variability and the preferential induction of type-specific immune responses with limited potency against heterologous viral strains and genotypes. We showed previously that deletion of the three variable regions from the E2 receptor-binding domain (Δ123) increases the ability of human broadly neutralizing antibodies (bNAbs) to inhibit E2-CD81 receptor interactions, suggesting improved bNAb epitope exposure. In this study, the immunogenicity of Δ123 was examined. We show that high-molecular-weight forms of Δ123 elicit distinct antibody specificities with potent and broad neutralizing activity against all seven HCV genotypes. Antibody competition studies revealed that immune sera raised to high-molecular-weight Δ123 was poly specific, given that it inhibited the binding of human bNAbs directed to three major neutralization epitopes on E2. By contrast, the immune sera raised to monomeric Δ123 predominantly blocked the binding of a non-neutralizing antibody to Δ123, while having reduced ability to block bNAb binding to E2, and neutralization was largely toward the homologous genotype. This increased ability of oligomeric Δ123 to generate bNAbs correlates with occlusion of the non-neutralizing face of E2 in this glycoprotein form.

CONCLUSION

The results from this study reveal new information on the antigenic and immunogenic potential of E2-based immunogens and provide a pathway for the development of a simple, recombinant protein-based prophylactic vaccine for HCV with potential for universal protection. (Hepatology 2017;65:1117-1131).

Limited naturally occurring escape in broadly neutralizing antibody epitopes in hepatitis C glycoprotein E2 and constrained sequence usage in acute infection

Broadly neutralizing antibodies have been associated with spontaneous clearance of the hepatitis C infection as well as viral persistence by immune escape. Further study of neutralizing antibody epitopes is needed to unravel pathways of resistance to virus neutralization, and to identify conserved regions for vaccine design. All reported broadly neutralizing antibody (BNAb) epitopes in the HCV Envelope (E2) glycoprotein were identified. The critical contact residues of these epitopes were mapped onto the linear E2 sequence. All publicly available E2 sequences were then downloaded and the contact residues within the BNAb epitopes were assessed for the level of conservation, as well as the frequency of occurrence of experimentally-proven resistance mutations. Epitopes were also compared between two sequence datasets obtained from samples collected at well-defined time points from acute (<180days) and chronic (>180days) infections, to identify any significant differences in residue usage. The contact residues for all BNAbs were contained within 3 linear regions of the E2 protein sequence. An analysis of 1749 full length E2 sequences from public databases showed that only 10 out of 29 experimentally-proven resistance mutations were present at a frequency >5%. Comparison of subtype 1a viral sequences obtained from samples collected during acute or chronic infection revealed significant differences at positions 610 and 655 with changes in residue (p<0.05), and at position 422 (p<0.001) with a significant difference in variability (entropy). The majority of experimentally-described escape variants do not occur frequently in nature. The observed differences between acute and chronically isolated sequences suggest constraints on residue usage early in infection.

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).

Viral evasion and challenges of hepatitis C virus vaccine development

Hepatitis C virus (HCV) is a major global disease burden, often leading to chronic liver diseases, cirrhosis, cancer, and death in those infected. Despite the recent approval of antiviral therapeutics, a preventative vaccine is recognized as the most effective means to control HCV globally, particularly in at-risk and developing country populations. Here we describe the efforts and challenges related to the development of an HCV vaccine, which after decades of research have not been successful. Viral sequence variability poses a major challenge, yet recent research has provided unprecedented views of the atomic structure of HCV epitopes and immune recognition by antibodies and T cell receptors. This, coupled with insights from deep sequencing, robust neutralization assays, and other technological advances, is spurring research toward rationally HCV designed vaccines that preferentially elicit responses toward conserved epitopes of interest that are associated with viral neutralization and clearance.

Bivalent vaccine platform based on Japanese encephalitis virus (JEV) elicits neutralizing antibodies against JEV and hepatitis C virus

Directly acting antivirals recently have become available for the treatment of hepatitis C virus (HCV) infection, but there is no prophylactic vaccine for HCV. In the present study, we took advantage of the properties of Japanese encephalitis virus (JEV) to develop antigens for use in a HCV vaccine. Notably, the surface-exposed JEV envelope protein is tolerant of inserted foreign epitopes, permitting display of novel antigens. We identified 3 positions that permitted insertion of the HCV E2 neutralization epitope recognized by HCV1 antibody. JEV subviral particles (SVP) containing HCV-neutralization epitope (SVP-E2) were purified from culture supernatant by gel chromatography. Sera from mice immunized with SVP-E2 inhibited infection by JEV and by trans-complemented HCV particles (HCVtcp) derived from multi-genotypic viruses, whereas sera from mice immunized with synthetic E2 peptides did not show any neutralizing activity. Furthermore, sera from mice immunized with SVP-E2 neutralized HCVtcp with N415K escape mutation in E2. As with the SVP-E2 epitope-displaying particles, JEV SVPs with HCV E1 epitope also elicited neutralizing antibodies against HCV. Thus, this novel platform harboring foreign epitopes on the surface of the particle may facilitate the development of a bivalent vaccine against JEV and other pathogens.

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.

Monoclonal anti-envelope antibody AP33 protects humanized mice against a patient-derived hepatitis C virus challenge

End-stage liver disease (ESLD) caused by hepatitis C virus (HCV) infection is a major indication for liver transplantation. However, immediately after transplantation, the liver graft of viremic patients universally becomes infected by circulating virus, resulting in accelerated liver disease progression. Currently available direct-acting antiviral therapies have reduced efficacy in patients with ESLD and prophylactic strategies to prevent HCV recurrence are still highly needed. In this study, we compared the ability of two broadly reactive monoclonal antibodies (mAbs), designated 3/11 and AP33, recognizing a distinct, but overlapping, epitope in the viral E2 glycoprotein to protect humanized mice from a patient-derived HCV challenge. Their neutralizing activity was assessed using the HCV pseudoparticles and cell-culture-derived HCV systems expressing multiple patient-derived envelopes and a human-liver chimeric mouse model. HCV RNA was readily detected in all control mice challenged with a patient-derived HCV genotype 1b isolate, whereas 3 of 4 AP33-treated mice were completely protected. In contrast, only one of four 3/11-treated mice remained HCV-RNA negative throughout the observation period, whereas the other 3 had a viral load that was indistinguishable from that in the control group. The increased in vivo efficacy of AP33 was in line with its higher affinity and neutralizing capacity observed in vitro.

CONCLUSIONS

Although mAbs AP33 and 3/11 target the same region in E2, only mAb AP33 can efficiently protect from challenge with a heterologous HCV population in vivo. Given that mAb AP33 efficiently neutralizes viral variants that escaped the humoral immune response and reinfected the liver graft of transplant patients, it may be a valuable candidate to prevent HCV recurrence. In addition, our data are valuable for the design of a prophylactic vaccine.

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

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

Antibodies to an interfering epitope in hepatitis C virus E2 can mask vaccine-induced neutralizing activity

Hepatitis C virus (HCV) neutralization occurring at the E2 region 412-426 (EP-I) could be enhanced when antibodies directed specifically to the E2 region 434-446 (EP-II) were removed from serum samples of persistently infected patients and vaccinated chimpanzees, a phenomenon of so-called antibody interference. Here, we show that this type of interference can be observed in individuals after immunization with recombinant E1E2 proteins. One hundred twelve blinded serum samples from a phase I, placebo-controlled, dose escalation trial using recombinant HCV E1E2 with MF59C.1 adjuvant in healthy HCV-negative adults were tested in enzyme-linked immunosorbent assay for binding reactivity to peptides representing the E2 regions 412-426 (EP-I) and 434-446 (EP-II). All samples were subsequently tested for neutralizing activity using cell-culture HCV 1a(H77)/2a chimera, HCV pseudotype particles (HCVpp) H77, and HCVpp HCV-1 after treatment to remove EP-II-specific antibodies or mock treatment with a control peptide. Among the 112 serum samples, we found 22 double positive (EP-I and EP-II), 6 EP-II positive only, 14 EP-I positive only, and 70 double negative. Depleting EP-II antibodies from double-positive serum samples increased 50% inhibitory dose (ID50) neutralizing antibody titers (up to 4.9-fold) in up to 72% of samples (P ≤ 0.0005), contrasting with ID50 neutralization titer increases in 2 of 70 double-negative samples (2.9%; P > 0.5). In addition, EP-I-specific antibody levels in serum samples showed a significant correlation with ID50 neutralization titers when EP-II antibodies were removed (P < 0.0003).

CONCLUSION

These data show that antibodies to the region 434-446 are induced during immunization of individuals with recombinant E1E2 proteins, and that these antibodies can mask effective neutralizing activity from EP-I-specific antibodies. Elicitation of EP-II-specific antibodies with interfering capacity should be avoided in producing an effective cross-neutralizing vaccine aimed at the HCV envelope proteins.

Antibodies Targeting Novel Neutralizing Epitopes of Hepatitis C Virus Glycoprotein Preclude Genotype 2 Virus Infection

Currently, there is no effective vaccine to prevent hepatitis C virus (HCV) infection, partly due to our insufficient understanding of the virus glycoprotein immunology. Most neutralizing antibodies (nAbs) were identified using glycoprotein immunogens, such as recombinant E1E2, HCV pseudoparticles or cell culture derived HCV. However, the fact that in the HCV acute infection phase, only a small proportion of patients are self-resolved accompanied with the emergence of nAbs, indicates the limited immunogenicity of glycoprotein itself to induce effective antibodies against a highly evolved virus. Secondly, in previous reports, the immunogen sequence was mostly the genotype of the 1a H77 strain. Rarely, other genotypes/subtypes have been studied, although theoretically one genotype/subtype immunogen is able to induce cross-genotype neutralizing antibodies. To overcome these drawbacks and find potential novel neutralizing epitopes, 57 overlapping peptides encompassing the full-length glycoprotein E1E2 of subtype 1b were synthesized to immunize BALB/c mice, and the neutralizing reactive of the induced antisera against HCVpp genotypes 1–6 was determined. We defined a domain comprising amino acids (aa) 192–221, 232–251, 262–281 and 292–331 of E1, and 421–543, 564–583, 594–618 and 634–673 of E2, as the neutralizing regions of HCV glycoprotein. Peptides PUHI26 (aa 444–463) and PUHI45 (aa 604–618)-induced antisera displayed the most potent broad neutralizing reactive. Two monoclonal antibodies recognizing the PUHI26 and PUHI45 epitopes efficiently precluded genotype 2 viral (HCVcc JFH and J6 strains) infection, but they did not neutralize other genotypes. Our study mapped a neutralizing epitope region of HCV glycoprotein using a novel immunization strategy, and identified two monoclonal antibodies effective in preventing genotype 2 virus infection.

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

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

Capitalizing on knowledge of hepatitis C virus neutralizing epitopes for rational vaccine design

Hepatitis C virus infects nearly 3% of the world's population and is often referred as a silent epidemic. It is a leading cause of liver cirrhosis and hepatocellular carcinoma in endemic countries. Although antiviral drugs are now available, they are not readily accessible to marginalized social groups and developing nations that are disproportionally impacted by HCV. To stop the HCV pandemic, a vaccine is needed. Recent advances in HCV research have provided new opportunities for studying HCV neutralizing antibodies and their subsequent use for rational vaccine design. It is now recognized that neutralizing antibodies to conserved antigenic sites of the virus can cross-neutralize diverse HCV genotypes and protect against infection in vivo. Structural characterization of the neutralizing epitopes has provided valuable information for design of candidate immunogens.

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.

HCV envelope glycoproteins in evirion assembly and entry

ABSTRACT 

HCV encodes two envelope glycoproteins, E1 and E2, which assemble as a non-covalent heterodimer in infected cells. During HCV morphogenesis, these proteins are incorporated into viral particles and they are the major viral determinants of HCV entry. Functional studies have revealed unique features in these viral envelope glycoproteins. Indeed, E1–E2 interaction, mediated by their transmembrane domain, is essential for HCV assembly and entry. Furthermore, recent data also show that these glycoproteins interact with apolipoproteins. Recent crystallography data provide some structural support to better understand how these proteins interact with the host. In this review, we summarize the biogenesis of HCV envelope glycoproteins and their role in HCV morphogenesis in the context of the hijacking of the very low-density lipoprotein assembly pathway by this virus. We also describe the functions of HCV glycoproteins during virus entry with a special focus on the unexpected structural features of E2 glycoprotein. Finally, we discuss the major neutralizing epitopes in the light of E2 structure.

Structural flexibility of a conserved antigenic region in hepatitis C virus glycoprotein E2 recognized by broadly neutralizing antibodies

Neutralizing antibodies (NAbs) targeting glycoprotein E2 are important for the control of hepatitis C virus (HCV) infection. One conserved antigenic site (amino acids 412 to 423) is disordered in the reported E2 structure, but a synthetic peptide mimicking this site forms a β-hairpin in complex with three independent NAbs. Our structure of the same peptide in complex with NAb 3/11 demonstrates a strikingly different extended conformation. We also show that residues 412 to 423 are essential for virus entry but not for E2 folding. Together with the neutralizing capacity of the 3/11 Fab fragment, this indicates an unexpected structural flexibility within this epitope. NAbs 3/11 and AP33 (recognizing the extended and β-hairpin conformations, respectively) display similar neutralizing activities despite converse binding kinetics. Our results suggest that HCV utilizes conformational flexibility as an immune evasion strategy, contributing to the limited immunogenicity of this epitope in patients, similar to the conformational flexibility described for other enveloped and nonenveloped viruses.

IMPORTANCE

Approximately 180 million people worldwide are infected with hepatitis C virus (HCV), and neutralizing antibodies play an important role in controlling the replication of this major human pathogen. We show here that one of the most conserved antigenic sites within the major glycoprotein E2 (amino acids 412 to 423), which is disordered in the recently reported crystal structure of an E2 core fragment, can adopt different conformations in the context of the infectious virus particle. Recombinant Fab fragments recognizing different conformations of this antigenic site have similar neutralization activities in spite of converse kinetic binding parameters. Of note, an antibody response targeting this antigenic region is less frequent than those targeting other more immunogenic regions in E2. Our results suggest that the observed conformational flexibility in this conserved antigenic region contributes to the evasion of the humoral host immune response, facilitating chronicity and the viral spread of HCV within an infected individual.

The Humoral Immune Response to HCV: Understanding is Key to Vaccine Development

Hepatitis C virus (HCV) remains a global problem, despite advances in treatment. The low cost and high benefit of vaccines have made them the backbone of modern public health strategies, and the fight against HCV will not be won without an effective vaccine. Achievement of this goal will benefit from a robust understanding of virus-host interactions and protective immunity in HCV infection. In this review, we summarize recent findings on HCV-specific antibody responses associated with chronic and spontaneously resolving human infection. In addition, we discuss specific epitopes within HCV's envelope glycoproteins that are targeted by neutralizing antibodies. Understanding what prompts or prevents a successful immune response leading to viral clearance or persistence is essential to designing a successful vaccine.

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.

Challenges to the development of vaccines to hepatitis C virus that elicit neutralizing antibodies

Despite 20 years of research, a vaccine to prevent hepatitis C virus (HCV) infection has not been developed. A vaccine to prevent HCV will need to induce broadly reactive immunity able to prevent infection by the 7 genetically and antigenically distinct genotypes circulating world-wide. HCV encodes two surface exposed glycoproteins, E1 and E2 that function as a heterodimer to mediate viral entry. Neutralizing antibodies (NAbs) to both E1 and E2 have been described with the major NAb target being E2. The function of E2 is to attach virions to host cells via cell surface receptors that include, but is not limited to, the tetraspanin CD81 and scavenger receptor class B type 1. However, E2 has developed a number of immune evasion strategies to limit the effectiveness of the NAb response and possibly limit the ability of the immune system to generate potent NAbs in natural infection. Hypervariable regions that shield the underlying core domain, subdominant neutralization epitopes and glycan shielding combine to make E2 a difficult target for the immune system. This review summarizes recent information on the role of NAbs to prevent HCV infection, the targets of the NAb response and structural information on glycoprotein E2 in complex with neutralizing antibodies. This new information should provide a framework for the rational design of new vaccine candidates that elicit highly potent broadly reactive NAbs to prevent HCV infection.

The past, present and future of neutralizing antibodies for hepatitis C virus

Hepatitis C virus (HCV) is a major cause of liver disease and hepatocellular carcinoma worldwide. HCV establishes a chronic infection in the majority of cases. However, some individuals clear the virus, demonstrating a protective role for the host immune response. Although new all-oral drug combinations may soon replace traditional ribavirin-interferon therapy, the emerging drug cocktails will be expensive and associated with side-effects and resistance, making a global vaccine an urgent priority. T cells are widely accepted to play an essential role in clearing acute HCV infection, whereas the role antibodies play in resolution and disease pathogenesis is less well understood. Recent studies have provided an insight into viral neutralizing determinants and the protective role of antibodies during infection. This review provides a historical perspective of the role neutralizing antibodies play in HCV infection and discusses the therapeutic benefits of antibody-based therapies. This article forms part of a symposium in Antiviral Research on "Hepatitis C: next steps toward global eradication."

Structural and functional characterization of the single-chain Fv fragment from a unique HCV E1E2-specific monoclonal antibody

The nucleotide sequence of the unique neutralizing monoclonal antibody D32.10 raised against a conserved conformational epitope shared between E1 and E2 on the serum-derived hepatitis C virus (HCV) envelope was determined. Subsequently, the recombinant single-chain Fv fragment (scFv) was cloned and expressed in Escherichia coli, and its molecular characterization was assessed using multi-angle laser light scattering. The scFv mimicked the antibody in binding to the native serum-derived HCV particles from patients, as well as to envelope E1E2 complexes and E1, E2 glycoproteins carrying the viral epitope. The scFv D32.10 competed with the parental IgG for binding to antigen, and therefore could be a promising candidate for therapeutics and diagnostics.

Structural and antigenic definition of hepatitis C virus E2 glycoprotein epitopes targeted by monoclonal antibodies

Hepatitis C virus (HCV) is the major cause of chronic liver disease as well as the major indication for liver transplantation worldwide. Current standard of care is not completely effective, not administrable in grafted patients, and burdened by several side effects. This incomplete effectiveness is mainly due to the high propensity of the virus to continually mutate under the selective pressure exerted by the host immune response as well as currently administered antiviral drugs. The E2 envelope surface glycoprotein of HCV (HCV/E2) is the main target of the host humoral immune response and for this reason one of the major variable viral proteins. However, broadly cross-neutralizing monoclonal antibodies (mAbs) directed against HCV/E2 represent a promising tool for the study of virus-host interplay as well as for the development of effective prophylactic and therapeutic approaches. In the last few years many anti-HCV/E2 mAbs have been evaluated in preclinical and clinical trials as possible candidate antivirals, particularly for administration in pre- and post-transplant settings. In this review we summarize the antigenic and structural characteristics of HCV/E2 determined through the use of anti-HCV/E2 mAbs, which, given the absence of a crystal structure of this glycoprotein, represent currently the best tool available.

Determination of the human antibody response to the neutralization epitopes encompassing amino acids 313-327 and 432-443 of hepatitis C virus E1E2 glycoproteins

It has been reported that monoclonal antibodies (MAbs) to the E1E2 glycoproteins may have the potential to prevent hepatitis C virus (HCV) infection. The protective epitopes targeted by these MAbs have been mapped to the regionsencompassing amino acids 313–327 and 432–443. In this study, we synthesized these two peptides and tested the reactivity of serum samples from 336 patients, 210 of whichwere from Chronic Hepatitis C (CHC) patients infected with diverse HCV genotypes.The remaining 126 samples were isolated from patients who had spontaneously clearedHCV infection.In the chronic HCV-infected group (CHC group), the prevalence of human serum antibodies reactive to epitopes 313–327 and 432–443was 24.29%(51 of 210) and4.76%(10 of 210),respectively. In thespontaneousclearance group (SC group),the prevalence was 0.79%(1 of 126) and 12.70%(16 of 126), respectively.The positive serum samples that contained antibodies reactive to epitope 313–327 neutralizedHCV pseudoparticles (HCVpp) bearing the envelope glycoproteins of genotypes 1a or 1b and/or 4, but genotypes 2a, 3a, 5 and 6 were not neutralized. The neutralizing activity of these serum samples could not be inhibited by peptide 313–327. Six samples (SC17, SC38, SC86, SC92, CHC75 and CHC198) containing antibodies reactive to epitope 432–443 had cross-genotype neutralizing activities. Theneutralizing activityof SC38, SC86, SC92 and CHC75waspartiallyinhibited by peptide 432–443. However,the neutralizing activity of sample SC17 for genotype 4HCVpp and sample CHC198 for genotype 1b HCVppwere notinhibited by the peptide.This study identifies the neutralizing ability of endogenous anti-HCV antibodies and warrants the exploration of antibodies reactive to epitope432–443as sources for future antibody therapies.

Cooperativity in virus neutralization by human monoclonal antibodies to two adjacent regions located at the amino terminus of hepatitis C virus E2 glycoprotein

A challenge for hepatitis C virus (HCV) vaccine development is defining conserved epitopes that induce protective antibodies against this highly diverse virus. An envelope glycoprotein (E2) segment located at amino acids (aa) 412 to 423 contains highly conserved neutralizing epitopes. While polyclonal antibodies to aa 412 to 423 from HCV-infected individuals confirmed broad neutralization, conflicting findings have been reported on polyclonal antibodies to an adjacent region, aa 434 to 446, that may or may not interfere with neutralization by antibodies to aa 412 to 423. To define the interplay between these antibodies, we isolated human monoclonal antibodies (HMAbs) to aa 412 to 423, designated HC33-related HMAbs (HC33 HMAbs), and characterized their interactions with other HMAbs to aa 434 to 446. A subset of the HC33 HMAbs neutralized genotype 1 to 6 infectious cell culture-derived HCV virions (HCVcc) with various activities. Although nonneutralizing HC33 HMAbs were isolated, they had lower binding affinities than neutralizing HC33 HMAbs. These antibodies could be converted to neutralizing antibodies by affinity maturation. Unidirectional competition for binding to E2 was observed between HC33 HMAbs and HMAbs to aa 434 to 446. When HMAbs to aa 434 to 446, which mediated neutralization, were combined with neutralizing HC33 HMAbs, biphasic patterns in neutralization were observed. A modest degree of antagonism was observed at lower concentrations, and a modest degree of synergism was observed at higher concentrations. However, the overall effect was additive neutralization. A similar pattern was observed when these antibodies were combined to block E2 binding to the HCV coreceptor, CD81. These findings demonstrate that both of these E2 regions participate in epitopes mediating virus neutralization and that the antibodies to aa 412 to 423 and aa 434 to 446 do not hinder their respective virus-neutralizing activities.

Neutralization interfering antibodies: a "novel" example of humoral immune dysfunction facilitating viral escape?

The immune response against some viral pathogens, in particular those causing chronic infections, is often ineffective notwithstanding a robust humoral neutralizing response. Several evasion mechanisms capable of subverting the activity of neutralizing antibodies (nAbs) have been described. Among them, the elicitation of non-neutralizing and interfering Abs has been hypothesized. Recently, this evasion mechanism has acquired an increasing interest given its possible impact on novel nAb-based antiviral therapeutic and prophylactic approaches. In this review, we illustrate the mechanisms of Ab-mediated interference and the viral pathogens described in literature as able to adopt this "novel" evasion strategy.

Toward a hepatitis C virus vaccine: the structural basis of hepatitis C virus neutralization by AP33, a broadly neutralizing antibody

The E2 envelope glycoprotein of hepatitis C virus (HCV) binds to the host entry factor CD81 and is the principal target for neutralizing antibodies (NAbs). Most NAbs recognize hypervariable region 1 on E2, which undergoes frequent mutation, thereby allowing the virus to evade neutralization. Consequently, there is great interest in NAbs that target conserved epitopes. One such NAb is AP33, a mouse monoclonal antibody that recognizes a conserved, linear epitope on E2 and potently neutralizes a broad range of HCV genotypes. In this study, the X-ray structure of AP33 Fab in complex with an epitope peptide spanning residues 412 to 423 of HCV E2 was determined to 1.8 Å. In the complex, the peptide adopts a β-hairpin conformation and docks into a deep binding pocket on the antibody. The major determinants of antibody recognition are E2 residues L413, N415, G418, and W420. The structure is compared to the recently described HCV1 Fab in complex with the same epitope. Interestingly, the antigen-binding sites of HCV1 and AP33 are completely different, whereas the peptide conformation is very similar in the two structures. Mutagenesis of the peptide-binding residues on AP33 confirmed that these residues are also critical for AP33 recognition of whole E2, confirming that the peptide-bound structure truly represents AP33 interaction with the intact glycoprotein. The slightly conformation-sensitive character of the AP33-E2 interaction was explored by cross-competition analysis and alanine-scanning mutagenesis. The structural details of this neutralizing epitope provide a starting point for the design of an immunogen capable of eliciting AP33-like antibodies.

Human monoclonal antibody HCV1 effectively prevents and treats HCV infection in chimpanzees

Hepatitis C virus (HCV) infection is a leading cause of liver transplantation and there is an urgent need to develop therapies to reduce rates of HCV infection of transplanted livers. Approved therapeutics for HCV are poorly tolerated and are of limited efficacy in this patient population. Human monoclonal antibody HCV1 recognizes a highly-conserved linear epitope of the HCV E2 envelope glycoprotein (amino acids 412–423) and neutralizes a broad range of HCV genotypes. In a chimpanzee model, a single dose of 250 mg/kg HCV1 delivered 30 minutes prior to infusion with genotype 1a H77 HCV provided complete protection from HCV infection, whereas a dose of 50 mg/kg HCV1 did not protect. In addition, an acutely-infected chimpanzee given 250 mg/kg HCV1 42 days following exposure to virus had a rapid reduction in viral load to below the limit of detection before rebounding 14 days later. The emergent virus displayed an E2 mutation (N415K/D) conferring resistance to HCV1 neutralization. Finally, three chronically HCV-infected chimpanzees were treated with a single dose of 40 mg/kg HCV1 and viral load was reduced to below the limit of detection for 21 days in one chimpanzee with rebounding virus displaying a resistance mutation (N417S). The other two chimpanzees had 0.5–1.0 log₁₀ reductions in viral load without evidence of viral resistance to HCV1. In vitro testing using HCV pseudovirus (HCVpp) demonstrated that the sera from the poorly-responding chimpanzees inhibited the ability of HCV1 to neutralize HCVpp. Measurement of antibody responses in the chronically-infected chimpanzees implicated endogenous antibody to E2 and interference with HCV1 neutralization although other factors may also be responsible. These data suggest that human monoclonal antibody HCV1 may be an effective therapeutic for the prevention of graft infection in HCV-infected patients undergoing liver transplantation.

The majority of individuals infected with hepatitis C virus (HCV) become chronically infected and many go on to develop liver failure requiring liver transplantation. Unfortunately, the transplanted liver becomes infected with HCV in nearly 100% of transplant patients. Current treatments for HCV are poorly tolerated after liver transplantation and graft health is compromised by infection. We have developed a monoclonal antibody called HCV1 that blocks HCV from infecting liver cells in culture. Using chimpanzees as a model for HCV infection, we demonstrate that HCV1 has the ability to prevent HCV infection. We also show that HCV1 can treat chimpanzees chronically infected with HCV and reduce plasma viral load to below the level of detection for a period of 7 to 21 days. The virus that reemerges in the treated chimpanzees was resistant to HCV1 neutralization demonstrating target engagement. Given the ability of HCV1 to protect chimpanzees from HCV infection, we speculate that HCV1 may be beneficial in HCV- infected patients undergoing liver transplant.

Structural basis of hepatitis C virus neutralization by broadly neutralizing antibody HCV1

Hepatitis C virus (HCV) infects more than 2% of the global population and is a leading cause of liver cirrhosis, hepatocellular carcinoma, and end-stage liver diseases. Circulating HCV is genetically diverse, and therefore a broadly effective vaccine must target conserved T- and B-cell epitopes of the virus. Human mAb HCV1 has broad neutralizing activity against HCV isolates from at least four major genotypes and protects in the chimpanzee model from primary HCV challenge. The antibody targets a conserved antigenic site (residues 412-423) on the virus E2 envelope glycoprotein. Two crystal structures of HCV1 Fab in complex with an epitope peptide at 1.8-Å resolution reveal that the epitope is a β-hairpin displaying a hydrophilic face and a hydrophobic face on opposing sides of the hairpin. The antibody predominantly interacts with E2 residues Leu(413) and Trp(420) on the hydrophobic face of the epitope, thus providing an explanation for how HCV isolates bearing mutations at Asn(415) on the same binding face escape neutralization by this antibody. The results provide structural information for a neutralizing epitope on the HCV E2 glycoprotein and should help guide rational design of HCV immunogens to elicit similar broadly neutralizing antibodies through vaccination.

Naturally occurring antibodies that recognize linear epitopes in the amino terminus of the hepatitis C virus E2 protein confer noninterfering, additive neutralization

Chronic hepatitis C virus (HCV) infection can persist even in the presence of a broadly neutralizing antibody response. Various mechanisms that underpin viral persistence have been proposed, and one of the most recently proposed mechanisms is the presence of interfering antibodies that negate neutralizing responses. Specifically, it has been proposed that antibodies targeting broadly neutralizing epitopes located within a region of E2 encompassing residues 412 to 423 can be inhibited by nonneutralizing antibodies binding to a less conserved region encompassing residues 434 to 446. To investigate this phenomenon, we characterized the neutralizing and inhibitory effects of human-derived affinity-purified immunoglobulin fractions and murine monoclonal antibodies and show that antibodies to both regions neutralize HCV pseudoparticle (HCVpp) and cell culture-infectious virus (HCVcc) infection albeit with different breadths and potencies. Epitope mapping revealed the presence of overlapping but distinct epitopes in both regions, which may explain the observed differences in neutralizing phenotypes. Crucially, we failed to demonstrate any inhibition between these two groups of antibodies, suggesting that interference by nonneutralizing antibodies, at least for the region encompassing residues 434 to 446, does not provide a mechanism for HCV persistence in chronically infected individuals.

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.

The role of neutralizing antibodies in hepatitis C virus infection

Hepatitis C virus (HCV) is a blood-borne virus estimated to infect around 170 million people worldwide and is, therefore, a major disease burden. In some individuals the virus is spontaneously cleared during the acute phase of infection, whilst in others a persistent infection ensues. Of those persistently infected, severe liver diseases such as cirrhosis and primary liver cancer may develop, although many individuals remain asymptomatic. A range of factors shape the course of HCV infection, not least host genetic polymorphisms and host immunity. A number of studies have shown that neutralizing antibodies (nAb) arise during HCV infection, but that these antibodies differ in their breadth and mechanism of neutralization. Recent studies, using both mAbs and polyclonal sera, have provided an insight into neutralizing determinants and the likely protective role of antibodies during infection. This understanding has helped to shape our knowledge of the overall structure of the HCV envelope glycoproteins--the natural target for nAb. Most nAb identified to date target receptor-binding sites within the envelope glycoprotein E2. However, there is some evidence that other viral epitopes may be targets for antibody neutralization, suggesting the need to broaden the search for neutralization epitopes beyond E2. This review provides a comprehensive overview of our current understanding of the role played by nAb in HCV infection and disease outcome and explores the limitations in the study systems currently used. In addition, we briefly discuss the potential therapeutic benefits of nAb and efforts to develop nAb-based therapies.

Hepatitis C patient-derived glycoproteins exhibit marked differences in susceptibility to serum neutralizing antibodies: genetic subtype defines antigenic but not neutralization serotype

Neutralizing antibodies have a role in controlling hepatitis C virus (HCV) infection. A successful vaccine will need to elicit potently neutralizing antibodies that are capable of preventing the infection of genetically diverse viral isolates. However, the specificity of the neutralizing antibody response in natural HCV infection still is poorly understood. To address this, we examined the reactivity of polyclonal antibodies isolated from chronic HCV infection to the diverse patient-isolated HCV envelope glycoproteins E1 and E2 (E1E2), and we also examined the potential to neutralize the entry of pseudoparticles bearing these diverse E1E2 proteins. The genetic type of the infection was found to determine the pattern of the antibody recognition of these E1E2 proteins, with the greatest reactivity to homologous E1E2 proteins. This relationship was strongest when the component of the antibody response directed only to linear epitopes was analyzed. In contrast, the neutralization serotype did not correlate with genotype. Instead, serum-derived antibodies displayed a range of neutralization breadth and potency, while different E1E2 glycoproteins displayed different sensitivities to neutralization, such that these could be divided broadly into neutralization-sensitive and -resistant phenotypes. An important additional observation was that entry mediated by some E1E2 proteins was enhanced in the presence of some of the polyclonal antibody fractions isolated during chronic infection. These data highlight the need to use diverse E1E2 isolates, which represent extremes of neutralization sensitivity, when screening antibodies for therapeutic potential and for testing antibodies generated following immunization as part of vaccine development.

Oral immunization with attenuated Salmonella carrying a co-expression plasmid encoding the core and E2 proteins of hepatitis C virus capable of inducing cellular immune responses and neutralizing antibodies in mice

Hepatitis C virus (HCV) core protein has long been considered an attractive candidate for inclusion in a protective vaccine. However, this protein may hamper the development of systemic immune responses because of its immune suppressive properties. We previously reported that immune responses to HCV core protein could be efficiently induced by attenuated Salmonella carrying the HCV core protein, but not the HCV core DNA vaccine. To optimize the combination of the core protein and envelope protein 2 (E2) into a vaccine formulation to induce cellular immune responses and neutralizing antibodies, we constructed a plasmid containing two expression cassettes. One expression cassette was included to regulate the expression of HCV core protein by an inducible in vivo-activated Salmonella promoter, the other was included to regulate the expression of HCV E2 protein by the cytomegalovirus enhancer/promoter. Oral immunization of BALB/c mice with the attenuated Salmonella strain SL7207 carrying this plasmid efficiently induced HCV core and E2-specific cellular immune responses and antibodies. IgG purified from immunized mice could neutralize the infectivity of HCV pseudoparticles (HCVpp) of both the autologous Con 1 isolate and the heterologous H77 isolate, and cell culture produced HCV (HCVcc) of Con1-JFH1 chimera. These results indicated that this vaccine strategy can effectively deliver core and E2 protein to the immune system and provide a promising approach for the development of prophylactic and therapeutic vaccines against HCV infection.

Immunotherapeutic potential of neutralizing antibodies targeting conserved regions of the HCV envelope glycoprotein E2

HCV is a major cause of chronic liver disease worldwide. There is no vaccine available and the current antiviral therapies fail to cure approximately half of treated patients. Liver disease caused by HCV infection is the most common indication for orthotopic liver transplantation. Unfortunately, reinfection of the new liver is universal and often results in an aggressive form of the disease leading to graft loss and the need for retransplantation. Immunotherapies using antibodies that potently inhibit HCV infection have the potential to control or even prevent graft reinfection. The virion envelope glycoproteins E1 and E2, which are involved in HCV entry into host cells, are the targets of neutralizing antibodies. To date, a number of monoclonal antibodies targeting conserved regions of E2 have been described that display outstanding neutralizing capabilities against HCV infection in both in vitro and in vivo systems. This article will summarize the current literature on these neutralizing anti-E2 antibodies and discuss their potential immunotherapeutic efficacy.