Advantages to the use of rodent hepadnavirus core proteins as vaccine platforms

Design of hepadnavirus core protein-based chimeric virus-like particles carrying epitopes from respiratory syncytial virus

Respiratory syncytial virus (RSV) is one of the most important pathogens causing respiratory tract infection in humans, especially in infants and the elderly. The identification and structural resolution of the potent neutralizing epitopes on RSV fusion (F) protein enable an "epitope-focused" vaccine design. However, the display of RSV F epitope II on the surface of the widely-used human hepatitis B virus core antigen (HBcAg) has failed to induce neutralizing antibody response in mice. Here, we used the hepadnavirus core protein (HcAg) from different mammalian hosts as scaffolds to construct chimeric virus-like particles (VLPs) presenting the RSV F epitope II. Mouse immunization showed that different HcAg-based chimeric VLPs elicited significantly different neutralizing antibody responses, among which the HcAg derived from roundleaf bat (RBHcAg) is the most immunogenic. Furthermore, RBHcAg was used as the scaffold platform to present multiple RSV F epitopes, and the immunogenicity was further improved in comparison to that displaying a single epitope II. The designed RBHcAg-based multiple-epitope-presenting VLP formulated with MF59-like adjuvant elicited a potent and balanced Th1/Th2 immune response, and offered substantial protection in mice against the challenge of live RSV A2 virus. The designed chimeric VLPs may serve as the potential starting point for developing epitope-focused vaccines against RSV. Our study also demonstrated that RBHcAg is an effective VLP carrier for presenting foreign epitopes, providing a promising platform for epitope-focused vaccine design.

Sequential administration of virus-like particle-based nanomedicine to elicit enhanced tumor chemotherapy

Protein cages have played a long-standing role in biomedicine applications, especially in tumor chemotherapy. Among protein cages, virus like particles (VLPs) have received attention for their potential applications in vaccine development and targeted drug delivery. However, most of the existing protein-based platform technologies are plagued with immunological problems that may limit their systemic delivery efficiency as drug carriers. Here, we show that using immune-orthogonal protein cages sequentially and modifying the dominant loop epitope can circumvent adaptive immune responses and enable effective drug delivery using repeated dosing. We genetically modified three different hepadnavirus core protein derived VLPs as delivery vectors for doxorubicin (DOX). These engineered VLPs have similar assembly characteristics, particle sizes, and immunological properties. Our results indicated that there was negligible antibody cross-reactivity in either direction between these three RGD-VLPs in mice that were previously immunized against HBc VLPs. Moreover, the sequential administration of multiple RGD-VLP-based nanomedicine (DOX@RGD-VLPs) could effectively reduce immune clearance and inhibited tumor growth. Hence, this study could provide an attractive protein cage-based platform for therapeutic drug delivery.

Designing a therapeutic hepatitis B vaccine to circumvent immune tolerance

An effective prophylactic hepatitis B virus (HBV) vaccine has long been available but is ineffective for chronic infection. The primary cause of chronic hepatitis B (CHB) and greatest impediment for a therapeutic vaccine is the direct and indirect effects of immune tolerance to HBV antigens. The resulting defective CD4⁺/CD8⁺ T cell response, poor cytokine production, insufficient neutralizing antibody (nAb) and poor response to HBsAg vaccination characterize CHB infection. The objective of this study was to develop virus-like-particles (VLPs) that elicit nAb to prevent viral spread and prime CD4⁺/CD8⁺ T cells to eradicate intracellular HBV. Eight neutralizing B cell epitopes from the envelope PreS1 region were consolidated onto a species-variant of the HBV core protein, the woodchuck hepatitis core antigen (WHcAg). PreS1-specific B cell epitopes were chosen because of preferential expression on HBV virions. Because WHcAg and HBcAg are not crossreactive at the B cell level and only partially cross-reactive at the CD4⁺/CD8⁺ T cell level, CD4⁺ T cells specific for WHcAg-unique T cell sites can provide cognate T-B cell help for anti-PreS1 Ab production that is not curtailed by immune tolerance. Immunization of immune tolerant HBV transgenic (Tg) mice with PreS1-WHc VLPs elicited levels of high titer anti-PreS1 nAbs equivalent to wildtype mice. Passive transfer of PreS1 nAbs into human-liver chimeric mice prevented acute infection and cleared serum HBV from mice previously infected with HBV in a model of CHB. At the T cell level, PreS1-WHc VLPs and hybrid WHcAg/HBcAg DNA immunogens elicited HBcAg-specific CD4⁺ Th and CD8⁺ CTL responses.

A two-step heat treatment of cell disruption supernatant enables efficient removal of host cell proteins before chromatographic purification of HBc particles

The thermal stability of HBc particles was systematically investigated for efficient removal of host cell proteins (HCP) by heat treatment before chromatographic step. The HBc particles were found stable up to 80°C for 30 min without any noticeable change in circular dichroism spectra, fluorescence spectra and transmission electron microscope observation. When heating was applied to precipitate the HCP in the cell disruption supernatant of HBc fermentation, the HCP removal effect was more obvious as the temperature went higher. However, a phenomenon was found beyond 70°C where the recovered HBc particles had larger than normal size and molecular weight as observed by dynamic light scattering and multi-angle laser light scattering. Analysis found that the HBc particles possess nanopores which expand with temperature. When the temperature was above 70℃, the pores were large enough for some HCP to penetrate in, but not being able to get out after cooling down. To fully utilize the thermal stability and avoid the interference of HCP entering, a two-step heat treatment strategy was designed. The supernatant was firstly heated up to 60°C for 30 min to precipitate most HCP, then another 30 min at 70°C was used to remove the rest impurities. The two-step heat treatment effectively avoided the HCP entering problem, achieving 85.8% particle recovery and 74.7% purity. With further one-step hydrophobic interaction chromatography, the purity was increased to 99.0% with overall process recovery of 77.7%, considerably higher than those reported in the literature. The same process design was applied to purify three HBc-related products, including OVA-HBc, M2e-HBc and NP-HBc. All recoveries were higher than 50% with purity greater than 97%.

Virus-like particles that display Zika virus envelope protein domain III induce potent neutralizing immune responses in mice

Several Zika virus (ZIKV) vaccine candidates have recently been described which use inactivated whole virus, DNA or RNA that express the virus' Envelope (E) glycoprotein as the antigen. These were successful in stimulating production of virus-targeted antibodies that protected animals against ZIKV challenges, but their use potentially will predispose vaccinated individuals to infection by the related Dengue virus (DENV). We have devised a virus like particle (VLP) carrier based on the hepatitis B core antigen (HBcAg) that displays the ZIKV E protein domain III (zDIII), and shown that it can be produced quickly and easily purified in large quantities from Nicotiana benthamiana plants. HBcAg-zDIII VLPs are shown to be highly immunogenic, as two doses elicited potent humoral and cellular responses in mice that exceed the threshold correlated with protective immunity against multiple strains of Zika virus. Notably, HBcAg-zDIII VLPs-elicited antibodies did not enhance the infection of DENV in Fc gamma receptor-expressing cells, offsetting the concern of ZIKV vaccines inducing cross-reactive antibodies and sensitizing people to subsequent DENV infection. Thus, our zDIII-based vaccine offers improved safety and lower cost production than other current alternatives, with equivalent effectiveness.

A non-human hepadnaviral adjuvant for hepatitis C virus-based genetic vaccines

Human hepatitis B virus (HBV) core antigen (HBcAg) can act as an adjuvant in hepatitis C virus (HCV)-based DNA vaccines. Since two billion people are, or have been, in contact with HBV, one may question the use of human HBV sequences as adjuvant. We herein evaluated non-human stork hepatitis B virus core gene-sequences from stork as DNA vaccine adjuvants. Full-length and fragmented stork HBcAg gene-sequences were added to an HCV non-structural (NS) 3/4A gene (NS3/4A-stork-HBcAg). This resulted in an enhanced priming of HCV-specific IFN-γ and IL-2 responses in both wild-type (wt)- and NS3/4A-transgenic (Tg) mice, the latter with dysfunctional NS3/4A-specific T cells. The NS3/4A-stork-HBcAg vaccine primed NS3/4A-specific T cells in hepatitis B e antigen (HBeAg)-Tg mice with dysfunctional T cells to HBcAg and HBeAg. Repeated immunizations boosted expansion of IFN-γ and IL-2-producing NS3/4A-specific T cells in wt- and NS3/4A-Tg mice. Importantly, NS3/4A-stork-HBcAg-DNA induced in vivo long-term functional memory T cell responses, whose maintenance required CD4(+) T cells. Thus, avian HBcAg gene-sequences from stork can effectively act as a DNA vaccine adjuvant. This technology can most likely be universally expanded to other genetic vaccine antigens, as this completely avoids the use of sequences from a human virus where a pre-existing immunity may interfere with its adjuvant effect.

P. falciparum and P. vivax Epitope-Focused VLPs Elicit Sterile Immunity to Blood Stage Infections

In order to design P. falciparum preerythrocytic vaccine candidates, a library of circumsporozoite (CS) T and B cell epitopes displayed on the woodchuck hepatitis virus core antigen (WHcAg) VLP platform was produced. To test the protective efficacy of the WHcAg-CS VLPs, hybrid CS P. berghei/P. falciparum (Pb/Pf) sporozoites were used to challenge immunized mice. VLPs carrying 1 or 2 different CS repeat B cell epitopes and 3 VLPs carrying different CS non-repeat B cell epitopes elicited high levels of anti-insert antibodies (Abs). Whereas, VLPs carrying CS repeat B cell epitopes conferred 98% protection of the liver against a 10,000 Pb/Pf sporozoite challenge, VLPs carrying the CS non-repeat B cell eptiopes were minimally-to-non-protective. One-to-three CS-specific CD4/CD8 T cell sites were also fused to VLPs, which primed CS-specific as well as WHcAg-specific T cells. However, a VLP carrying only the 3 T cell domains failed to protect against a sporozoite challenge, indicating a requirement for anti-CS repeat Abs. A VLP carrying 2 CS repeat B cell epitopes and 3 CS T cell sites in alum adjuvant elicited high titer anti-CS Abs (endpoint dilution titer >1x10⁶) and provided 80–100% protection against blood stage malaria. Using a similar strategy, VLPs were constructed carrying P. vivax CS repeat B cell epitopes (WHc-Pv-78), which elicited high levels of anti-CS Abs and conferred 99% protection of the liver against a 10,000 Pb/Pv sporozoite challenge and elicited sterile immunity to blood stage infection. These results indicate that immunization with epitope-focused VLPs carrying selected B and T cell epitopes from the P. falciparum and P. vivax CS proteins can elicit sterile immunity against blood stage malaria. Hybrid WHcAg-CS VLPs could provide the basis for a bivalent P. falciparum/P. vivax malaria vaccine.

Palivizumab epitope-displaying virus-like particles protect rodents from RSV challenge

Respiratory syncytial virus (RSV) is the most common cause of serious viral bronchiolitis in infants, young children, and the elderly. Currently, there is not an FDA-approved vaccine available for RSV, though the mAb palivizumab is licensed to reduce the incidence of RSV disease in premature or at-risk infants. The palivizumab epitope is a well-characterized, approximately 24-aa helix-loop-helix structure on the RSV fusion (F) protein (F254-277). Here, we genetically inserted this epitope and multiple site variants of this epitope within a versatile woodchuck hepadnavirus core-based virus-like particle (WHcAg-VLP) to generate hybrid VLPs that each bears 240 copies of the RSV epitope in a highly immunogenic arrayed format. A challenge of such an epitope-focused approach is that to be effective, the conformational F254-277 epitope must elicit antibodies that recognize the intact virus. A number of hybrid VLPs containing RSV F254-277 were recognized by palivizumab in vitro and elicited high-titer and protective neutralizing antibody in rodents. Together, the results from this proof-of-principle study suggest that the WHcAg-VLP technology may be an applicable approach to eliciting a response to other structural epitopes.

Hepatitis C virus hypervariable region 1 variants presented on hepatitis B virus capsid-like particles induce cross-neutralizing antibodies

Hepatitis C virus (HCV) infection is still a serious global health burden. Despite improved therapeutic options, a preventative vaccine would be desirable especially in undeveloped countries. Traditionally, highly conserved epitopes are targets for antibody-based prophylactic vaccines. In HCV-infected patients, however, neutralizing antibodies are primarily directed against hypervariable region I (HVRI) in the envelope protein E2. HVRI is the most variable region of HCV, and this heterogeneity contributes to viral persistence and has thus far prevented the development of an effective HVRI-based vaccine. The primary goal of an antibody-based HCV vaccine should therefore be the induction of cross-reactive HVRI antibodies. In this study we approached this problem by presenting selected cross-reactive HVRI variants in a highly symmetric repeated array on capsid-like particles (CLPs). SplitCore CLPs, a novel particulate antigen presentation system derived from the HBV core protein, were used to deliberately manipulate the orientation of HVRI and therefore enable the presentation of conserved parts of HVRI. These HVRI-CLPs induced high titers of cross-reactive antibodies, including neutralizing antibodies. The combination of only four HVRI CLPs was sufficient to induce antibodies cross-reactive with 81 of 326 (24.8%) naturally occurring HVRI peptides. Most importantly, HVRI CLPs with AS03 as an adjuvant induced antibodies with a 10-fold increase in neutralizing capability. These antibodies were able to neutralize infectious HCVcc isolates and 4 of 19 (21%) patient-derived HCVpp isolates. Taken together, these results demonstrate that the induction of at least partially cross-neutralizing antibodies is possible. This approach might be useful for the development of a prophylactic HCV vaccine and should also be adaptable to other highly variable viruses.

Deletion modification enhances anthrax specific immunity and protective efficacy of a hepatitis B core particle-based anthrax epitope vaccine

Protective antigen (PA) is one of the major virulence factors of anthrax and is also the major constituent of the current anthrax vaccine. Previously, we found that the 2β2-2β3 loop of PA contains a dominant neutralizing epitope, the SFFD. We successfully inserted the 2β2-2β3 loop of PA into the major immunodominant region (MIR) of hepatitis B virus core (HBc) protein. The resulting fusion protein, termed HBc-N144-PA-loop2 (HBcL2), can effectively produce anthrax specific protective antibodies in an animal model. However, the protective immunity caused by HBcL2 could still be improved. In this research, we removed amino acids 79-81 from the HBc MIR of the HBcL2. This region was previously reported to be the major B cell epitope of HBc, and in keeping with this finding, we observed that the short deletion in the MIR not only diminished the intrinsic immunogenicity of HBc but also stimulated a higher titer of anthrax specific immunity. Most importantly, this deletion led to the full protection of the immunized mice against a lethal dose anthrax toxin challenge. We supposed that the conformational changes which occurred after the short deletion and foreign insertion in the MIR of HBc were the most likely reasons for the improvement in the immunogenicity of the HBc-based anthrax epitope vaccine.

Hepatitis B core-based virus-like particles to present heterologous epitopes

Since the first effort to recombinantly express the hepatitis B core protein (HBc) in bacteria, the remarkable virion-like structure has fuelled interest in unraveling the structural and antigenic properties of this protein. Initial studies proved HBc virus-like particles to possess strong immunogenic properties, which can be conveyed to linked antigens. More than 35 years later, numerous studies have been performed using HBc as a carrier protein for antigens derived from over a dozen different pathogens and diseases. In this review, the authors highlight the intriguing features of HBc as carrier and antigen, illustrated by some examples and experimental results that underscore the value of HBc as an antigen-presenting platform. Two of these HBc fusions, targeting influenza A and malaria, have even progressed into clinical testing. In the future, the HBc-based virus-like particles platform will probably continue to be used for the display of poorly immunogenic antigens, mainly because virus-like particle formation by HBc capsomers is compatible with nearly any available recombinant gene expression system.

Significance of expression of caspase-3 in chronic hepatitis B and C and nonalcoholic steatohepatitis

AIM: To determine the expression of caspase-3 in liver tissues of patients with chronic hepatitis B (CHB), chronic hepatitis C (CHC), or nonalcoholic steatohepatitis (NASH) and to analyze their clinical and pathological significance.

METHODS: Immunohistochemistry was used to determine the expression of caspase-3 in liver tissues from 70 patients with CHB, 50 patients with CHC, 50 patients with NASH, and 15 normal people.

RESULTS: The positive rates of caspase-3 expression in CHB, CHC and NASH were significantly higher than that in normal hepatic tissue (57.1%, 72.0%, 82.0% vs 18.8%; all P < 0.05). Caspase-3 expression was closely related with hepatic cell apoptosis (r = 0.356, P < 0.001). The positive rates of caspase-3 expression increased with the increase in the degree of pathological injury (all P < 0.05).

CONCLUSION: The expression of caspase-3 in CHB, CHC and NASH is closely related with liver cell apoptosis. Overexpression of caspase-3 may play an important role in the development of hepatitis.

SplitCore: an exceptionally versatile viral nanoparticle for native whole protein display regardless of 3D structure

Nanoparticles displaying native proteins are attractive for many applications, including vaccinology. Virus-based nanoparticles are easily tailored by genetic means, commonly by inserting heterologous sequences into surface-exposed loops. The strategy works well with short peptides but is incompatible with the structures of most native proteins, except those with closely juxtaposed termini. Here we overcome this constraint by splitting the capsid protein of hepatitis B virus, one of the most advanced and most immunogenic display platforms, inside the insertion loop (SplitCore). The split parts, coreN and coreC, efficiently form capsid-like particles (CLPs) in E. coli and so do numerous fusions to coreN and/or coreC of differently structured proteins, including human disease related antigens of >300 amino acids in length. These CLPs induced high-titer antibodies, including neutralizing ones, in mice. The concept was easily expanded to triple-layer CLPs carrying reporter plus targeting domains, and should be applicable to protein-based nanoparticle design in general.

Delivery of woodchuck hepatitis virus-like particle presented influenza M2e by recombinant attenuated Salmonella displaying a delayed lysis phenotype

The use of live recombinant attenuated Salmonella vaccines (RASV) is a promising approach for controlling infections by multiple pathogens. The highly conserved extracellular domain of the influenza M2 protein (M2e) has been shown to provide broad spectrum protection against multiple influenza subtypes sharing similar M2e sequences. An M2e epitope common to a number of avian influenza subtypes was inserted into the core antigen of woodchuck hepatitis virus and expressed in two different recombinant attenuated Salmonella Typhimurium strains. One strain was attenuated via deletion of the cya and crp genes. The second strain was engineered to exhibit a programmed delayed lysis phenotype. Both strains were able to produce both monomeric fusion proteins and fully assembled core particles. Mice orally immunized with the strain exhibiting delayed lysis induced significantly greater antibody titers than the Δcya Δcrp strain and provided moderate protection against weight loss to a low level challenge with the influenza strain A/WSN/33 modified to express the M2e sequence common to avian viruses. Further studies indicated that the Salmonella expressed core antigen induced comparable antibody levels to the purified core antigen injected with an alum adjuvant and that both are able to reduce viral replication in the lungs. To our knowledge this is the first report demonstrating Salmonella-mediated delivery of influenza virus M2e protein in a mammalian host to induce a protective immune response against viral challenge.

Use of hepadnavirus core proteins as vaccine platforms

The first virus-like particle to be tested for use as a vaccine carrier was based on the hepatitis B virus nucleocapsid protein. This viral subunit, while not infectious on its own, is a 36-nm particle that is highly immunogenic during a natural infection. The self-assembly and high degree of immunogenicity is maintained when expressed as a recombinant protein and, moreover, can confer a high degree of immunogenicity on foreign antigens linked to the particle, either chemically or genetically. This review describes the current state of the hepadnaviral core protein as a vaccine carrier.

Interaction of the hepatitis B core antigen and the innate immune system

Previous studies demonstrated that the primary APCs for the hepatitis B core Ag (HBcAg) were B cells and not dendritic cells (DC). We now report that splenic B1a and B1b cells more efficiently present soluble HBcAg to naive CD4(+) T cells than splenic B2 cells. This was demonstrated by direct HBcAg-biotin-binding studies and by HBcAg-specific T cell activation in vitro in cultures of naive HBcAg-specific T cells and resting B cell subpopulations. The inability of DC to function as APCs for exogenous HBcAg relates to lack of uptake of HBcAg, not to processing or presentation, because HBcAg/anti-HBc immune complexes can be efficiently presented by DC. Furthermore, HBcAg-specific CD4(+) and CD8(+) T cell priming with DNA encoding HBcAg does not require B cell APCs. TLR activation, another innate immune response, was also examined. Full-length (HBcAg(183)), truncated (HBcAg(149)), and the nonparticulate HBeAg were screened for TLR stimulation via NF-kappaB activation in HEK293 cells expressing human TLRs. None of the HBc/HBeAgs activated human TLRs. Therefore, the HBc/HBeAg proteins are not ligands for human TLRs. However, the ssRNA contained within HBcAg(183) does function as a TLR-7 ligand, as demonstrated at the T and B cell levels in TLR-7 knockout mice. Bacterial, yeast, and mammalian ssRNA encapsidated within HBcAg(183) all function as TLR-7 ligands. These studies indicate that innate immune mechanisms bridge to and enhance the adaptive immune response to HBcAg and have important implications for the use of hepadnavirus core proteins as vaccine carrier platforms.

Universal influenza A M2e-HBc vaccine protects against disease even in the presence of pre-existing anti-HBc antibodies

The extracellular domain of influenza A virus matrix protein 2 (M2e) is strongly conserved. Therefore, vaccines based on M2e can induce broad-spectrum immunity against influenza. We have mainly used recombinant virus-like particles derived from Hepatitis B virus core (HBc) as carrier for efficacious presentation of the M2e antigen. Here, we address whether pre-existing HBc-specific immunity interferes with the protective immune response obtained by M2e-HBc vaccination. Anti-HBc antibodies were induced by immunizing mice with unsubstituted HBc virus-like particles in the presence of two different adjuvants. We demonstrate that pre-existing HBc-specific antibodies affect neither the induction of M2e-specific antibody responses to vaccination with M2e-HBc particles, nor the protective efficacy of the resulting response. These results suggest that vaccination with M2e-HBc can induce protective anti-M2e antibodies even in anti-HBc positive individuals. The implications of these findings are discussed in the context of the clinical development of an M2e-based universal influenza vaccine, which recently successfully completed a Phase I trial.

Hepatitis B virus core protein inhibits TRAIL-induced apoptosis of hepatocytes by blocking DR5 expression

Hepatitis B virus (HBV) causes chronic hepatitis in hundreds of millions of people worldwide, which can eventually lead to hepatocellular carcinoma (HCC). The molecular mechanisms underlying HBV persistence are not well understood. TRAIL, the TNF-related apoptosis-inducing ligand, has recently been implicated in hepatocyte death during HBV infection. We report here that the HBV core protein (HBc) is a potent inhibitor of TRAIL-induced apoptosis. Overexpressing HBc significantly decreased TRAIL-induced apoptosis of human hepatoma cells, whereas knocking-down HBc expression in hepatoma cells transfected with HBV genome enhanced it. When present in the same cell, HBc blocked the pro-apoptotic effect of the HBV X protein (HBx). The resistance of HBc-expressing cells to TRAIL-induced apoptosis was associated with a significant reduction in death receptor 5 (DR5) expression. Upon transfection, HBc significantly repressed the promoter activity of the human DR5 gene. Importantly, HBc gene transfer inhibited hepatocyte death in a mouse model of HBV-induced hepatitis; and in patients with chronic hepatitis, DR5 expression in the liver was significantly reduced. These results indicate that HBc may prevent hepatocytes from TRAIL-induced apoptosis by blocking DR5 expression, which in turn contributes to the development of chronic hepatitis and HCC. They also call into question the potential side effects of HBc-based vaccines.