Antigenic sites on the arginine-rich carboxyl-terminal domain of the capsid protein of hepatitis B virus distinct from hepatitis B core or e antigen

The Hepatitis B Virus Core Variants that Expose Foreign C-Terminal Insertions on the Outer Surface of Virus-Like Particles

The major immunodominant region (MIR) and N-terminus of the hepatitis B virus (HBV) core (HBc) protein were used to expose foreign insertions on the outer surface of HBc virus-like particles (VLPs). The additions to the HBc positively charged arginine-rich C-terminal (CT) domain are usually not exposed on the VLP surface. Here, we constructed a set of recombinant HBcG vectors in which CT arginine stretches were substituted by glycine residues. In contrast to natural HBc VLPs and recombinant HBc VLP variants carrying native CT domain, the HBcG VLPs demonstrated a lowered capability to pack bacterial RNA during expression in Escherichia coli cells. The C-terminal addition of a model foreign epitope from the HBV preS1 sequence to the HBcG vectors resulted in the exposure of the inserted epitope on the VLP surface, whereas the same preS1 sequences added to the native CT of the natural HBc protein remained buried within the HBc VLPs. Based on the immunisation of mice, the preS1 epitope added to the HBcG vectors as a part of preS1(20-47) and preS1phil sequences demonstrated remarkable immunogenicity. The same epitope added to the original C-terminus of the HBc protein did not induce a notable level of anti-preS1 antibodies. HBcG vectors may contribute to the further development of versatile HBc VLP-based vaccine and gene therapy applications.

Preparation by alkaline treatment and detailed characterisation of empty hepatitis B virus core particles for vaccine and gene therapy applications

Hepatitis B virus (HBV) core (HBc) virus-like particles (VLPs) are one of the most powerful protein engineering tools utilised to expose immunological epitopes and/or cell-targeting signals and for the packaging of genetic material and immune stimulatory sequences. Although HBc VLPs and their numerous derivatives are produced in highly efficient bacterial and yeast expression systems, the existing purification and packaging protocols are not sufficiently optimised and standardised. Here, a simple alkaline treatment method was employed for the complete removal of internal RNA from bacteria- and yeast-produced HBc VLPs and for the conversion of these VLPs into empty particles, without any damage to the VLP structure. The empty HBc VLPs were able to effectively package the added DNA and RNA sequences. Furthermore, the alkaline hydrolysis technology appeared efficient for the purification and packaging of four different HBc variants carrying lysine residues on the HBc VLP spikes. Utilising the introduced lysine residues and the intrinsic aspartic and glutamic acid residues exposed on the tips of the HBc spikes for chemical coupling of the chosen peptide and/or nucleic acid sequences ensured a standard and easy protocol for the further development of versatile HBc VLP-based vaccine and gene therapy applications.

A VLP library of C-terminally truncated Hepatitis B core proteins: correlation of RNA encapsidation with a Th1/Th2 switch in the immune responses of mice

An efficient pBR327- and Ptrp-based E. coli expression system was used to generate a large-scale library of virus like particles (VLP) formed by recombinant hepatitis B virus (HBV) core (HBc) protein derivatives. To construct the library, the gene of HBc protein of the genotype D/subtype ayw2 virus was gradually truncated from the 3`-end and twenty-two HBc variants (with truncation up to 139 aa) were expressed at high levels. The proteins were purified by salt precipitation and gel filtration. Background RNA binding was observed for VLPs formed by HBc1-149, which lacked all C-terminal Arg blocks, and the addition of three Arg residues (HBc1-152) only slightly increased RNA binding. The presence of two Arg blocks (proteins HBc1-162 and HBc1-163) resulted in approximately half of the typical level of RNA binding, and the presence of three blocks (protein HBc1-171) led to approximately 85% of the typical level of binding. Only a small increase in the level of RNA binding was found for the HBc1-175 VLPs, which contained all four Arg blocks but lacked the last 8 aa of the full-length HBc protein. VLPs containing high levels of RNA had higher antigenicity according to an ELISA with anti-HBc mAbs than the VLPs formed by HBc variants without C-terminal Arg blocks and lacking RNA. The results indicate that the VLPs were stabilised by nucleic acids. The immunogenicity in BALB/c mice was comparable for VLPs formed by different HBc proteins, but a clear switch from a Th1 response to a Th2 response occurred after the loss of encapsidated RNA. We did not observe significant differences in lymphocyte proliferation in vitro for the tested VLP variants; however, the loss of RNA encapsidation correlated with a decreased level of IFN-γ induction, which is a measure of the potential CTL activity of immunogens.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

The affinities of monoclonal antibodies against core antigen of hepatitis B virus

Four monoclonal antibodies generated against the recombinant core antigen of hepatitis B virus are investigated for antigen binding. All exhibit a similar affinity to polystyrene-sorbed antigen but only one of them interacts with native form of HBcAg (an assembled particle) in solution. The presence of 0.1% sodium dodecylsulphate is required for the binding of other three antibodies. The phenomenon can be interpreted as inaccessibility of the corresponding epitopes unless the multimeric antigen structure is disrupted. The core antigen coated on polystyrene is considered as a similar exposed structure.

A protease-sensitive hinge linking the two domains of the hepatitis B virus core protein is exposed on the viral capsid surface

Core particles of hepatitis B virus are assembled from dimers of a single 185-residue (subtype adw) viral capsid or core protein (p21.5) which possesses two distinct domains: residues 1 to 144 form a minimal capsid assembly domain, and the arginine-rich, carboxyl-terminal residues 150 to 185 form a protamine-like domain that mediates nucleic acid binding. Little is known about the topography of the p21.5 polypeptide within either the p21.5 capsids or dimers. Here, using site-specific proteases and monoclonal antibodies, we have defined the accessibility of p21.5 residues in dimers and capsids assembled from wild-type and mutant hepatitis B virus core proteins in Xenopus oocytes and in vitro. The data reveal the protamine region to be accessible to external reagents in p21.5 dimers but largely cryptic in wild-type capsids. Strikingly, in capsids the only protease target region was a 9-residue peptide covering p21.5 residues Glu-145 to Asp-153, which falls largely between the two core protein domains. By analogy with protease-sensitive interdomain regions in other proteins, we propose that this peptide constitutes a hinge between the assembly and nucleic acid binding domains of p21.5. We further found that deletion or replacement of the terminal Cys-185 residue greatly increased surface exposure of the protamine tails in capsids, suggesting that a known disulfide linkage involving this residue tethers the protamine region inside the core particles. We propose that disruption of this disulfide linkage allows the protamine region to appear transiently on the surface of the core particle.

The structure of the variable regions of mouse monoclonal antibodies to hepatitis B virus core antigen

From a panel of monoclonal antibodies (MoAbs) directed against E. coli-derived native and denatured hepatitis B virus (HBV) core antigen we have selected a set of specific MoAbs which recognize different linear antigenic determinants: MoAb C1-5--cl epitope; MoAb 14K8--less immunogenic N-terminal region; and MoAbs 13C9, 10F10 and 14E11, 14G3--the immunodominant region between amino acids 134 and 140. We have applied the polymerase chain reaction technique to clone Ig VH and VL region genes, and appropriate full-length cDNA clones were obtained and characterized by nucleotide sequence analysis. Among the six heavy chain variable region sequences examined, three VH families were represented. Two of them belong to the 7183 (MoAb C1-5) and 3609 (14B8) families respectively and four, having only two amino acid changes in the CDR2 region, to the J558 family. These four probably are derived from a single expanded B-cell clone. The light chain sequences indicate that their VL are encoded by V kappa 21, V kappa 19 and V kappa 3 germline genes. Unlike VH genes, light chain genes are closely related to known representatives of mouse kappa light chain families and are employed also by MoAbs raised against other antigens.

A micromolar pool of antigenically distinct precursors is required to initiate cooperative assembly of hepatitis B virus capsids in Xenopus oocytes

Assembly of hepatitis B virus capsid-like (core) particles occurs efficiently in a variety of heterologous systems via aggregation of approximately 180 molecules of a single 21.5-kDa core protein (p21.5), resulting in an icosahedral capsid structure with T = 3 symmetry. Recent studies on the assembly of hepatitis B virus core particles in Xenopus oocytes suggested that dimers of p21.5 represent the major building block from which capsids are generated. Here we determined the concentration dependence of this assembly process. By injecting serially diluted synthetic p21.5 mRNA into Xenopus oocytes, we expressed different levels of intracellular p21.5 and monitored the production of p21.5 dimers and capsids by radiolabeling and immunoprecipitation, by radioimmunoassay, or by quantitative enzyme-linked immunosorbent assay analysis. The data revealed that (i) p21.5 dimers and capsids are antigenically distinct, (ii) capsid assembly is a highly cooperative and concentration-dependent process, and (iii) p21.5 must accumulate to a signature concentration of approximately 0.7 to 0.8 microM before capsid assembly initiates. This assembly process is strikingly similar to the assembly of RNA bacteriophage R17 as defined by in vitro studies.

Characterization of hepatitis B virus capsid particle assembly in Xenopus oocytes

Little is known about the assembly of the 28-nm nucleocapsid or core particle of hepatitis B virus. Here we show that this assembly process can be reconstituted in Xenopus oocytes injected with a synthetic mRNA encoding the hepatitis B virus capsid protein (p21.5). Injected oocytes produce both a nonparticulate p21.5 species (free p21.5) and capsid particles. We describe rapid and simple methods for fractionating these species on a small scale either with step gradients of 10 to 60% (wt/vol) sucrose or by centrifugation to pellet the particles, and we characterize the oocyte core particles. Free p21.5 exhibits chemical and physical properties distinctly different from those of particles. Free p21.5 is partially cleaved by proteinase K, whereas core particles are almost completely resistant to cleavage. This suggests that the carboxyl-terminal protamine region, the main target for proteases within p21.5, is exposed in free p21.5 but faces the interior of the p21.5 core particle. Finally, pulse-chase experiments demonstrated that free p21.5 can be chased almost quantitatively into core particles, establishing that free p21.5 is fully competent to form particles and represents an assembly intermediate on the pathway for core particle formation. However, core particle assembly appears very dependent on p21.5 concentration and is rapidly compromised if the p21.5 concentration is lowered. The advantages of oocytes for studying assembly are discussed.

Phosphorylation in the carboxyl-terminal domain of the capsid protein of hepatitis B virus: evaluation with a monoclonal antibody

The capsid protein of hepatitis B virus (p21c) is made of 183 amino acids coded for by the C gene. By using p21c isolated from Dane particles (hepatitis B virus) as an immunogen, a monoclonal antibody (no. 2212) which recognized an epitope dependent on the phosphorylation of p21c was raised. The binding of no. 2212 antibody to authentic p21c was completely inhibited by a synthetic undecapeptide with a sequence of RRRSQSPRRRR, representing amino acids 165 to 175 of p21c, only when the peptide was phosphorylated. Either or both of Ser-168 and Ser-170 were phosphorylated in p21c in vivo, therefore, and contributed to the manifestation of the epitope. No. 2212 antibody bound to p21c from core particles derived from Dane particles or hepatocellular carcinoma tissues (PLC/342) propagated in nude mice but did not bind to p21c from core particles expressed in Escherichia coli or yeast cells, indicating different states of phosphorylation in them. Nonphosphorylated p21c showed a higher affinity for the viral DNA than did phosphorylated p21c. Since the serum from an asymptomatic carrier, with a high titer for antibody to hepatitis B core antigen, specifically bound to phosphorylated undecapeptide (amino acids 165 to 175), the epitope would stimulate humoral antibody responses in the human host.

Human and murine B-cells recognize the HBeAg/beta (or HBe2) epitope as a linear determinant

The complete amino acid (aa) sequence of the hepatitis B virus (HBV) core protein (HBcAg), deduced from the genome of the HBV ayw subtype, was synthesized as decapeptides with five overlapping aas. The peptides were tested for reactivity with monoclonal antibodies (mAbs) to the beta (or HBe2) epitope of hepatitis B e antigen (HBe/b mAbs; 57/8, 78/3, 141/158 and 141/207). Cross-competition between the mAbs with a mAb to the HBe/alpha epitope (or HBe1) and an anti-HBc mAb showed that all the HBe/b mAbs specifically inhibited human anti-HBe/b binding. Screening the HBc/e peptides showed that all anti-HBe/b mAbs recognized a peptide covering the residues 126-135. Three of the mAbs, 78/3, 141/152 and 141/207, had a less restricted reactivity than the other two, suggesting the recognition of the HBe/b as a discontinuous determinant. Fine mapping of the region aa 126-135 was performed by synthesizing decapeptides with nine overlapping aas, covering residues aa 121-140. All mAbs, except 78/3, reacted with the linear sequence TPPAYR, at residues 128-133. An additional set of peptides was synthesized, where the six aas within the epitope 128-133 were substituted in turn by the other 19 possible aas. By this approach, the essential aas for mAb 57/8 were found to be the sequence of PPA at residues 129-131, and for mAb 141/158 the sequence PP-Y, at residues 129, 130 and 132, respectively. Human recognition of the linear HBe/b epitope was investigated by using a peptide covering residues 121-140 (p 33). Thirty-one sera from chronic carriers of HBsAg, of which seven were positive for HBeAg and the remaining 24 for anti-HBe, were investigated. Of the sera with HBeAg, two had low levels of anti/-HBe/b in the p 33 assay. Out of the sera with anti-HBe, eight were positive in the p 33 EIA. Thus, murine monoclonals and human sera may recognize the HBe/b epitope as a linear determinant residing around aa 130.