Characterisation of the polymerised and monomeric human serum albumin binding sites on hepatitis B surface antigen

Elucidation of the early infection machinery of hepatitis B virus by using bio-nanocapsule

Currently, hepatitis B virus (HBV), upon attaching to human hepatocytes, is considered to interact first with heparan sulfate proteoglycan (HSPG) via an antigenic loop of HBV envelope S protein. Then, it is promptly transferred to the sodium taurocholate cotransporting polypeptide (NTCP) via the myristoylated N-terminal sequence of pre-S1 region (from Gly-2 to Gly-48, HBV genotype D), and it finally enters the cell by endocytosis. However, it is not clear how HSPG passes HBV to NTCP and how NTCP contributes to the cellular entry of HBV. Owing to the poor availability and the difficulty of manipulations, including fluorophore encapsulation, it has been nearly impossible to perform biochemical and cytochemical analyses using a substantial amount of HBV. A bio-nanocapsule (BNC), which is a hollow nanoparticle consisting of HBV envelope L protein, was efficiently synthesized in Saccharomyces cerevisiae. Since BNC could encapsulate payloads (drugs, genes, proteins) and specifically enter human hepatic cells utilizing HBV-derived infection machinery, it could be used as a model of HBV infection to elucidate the early infection machinery. Recently, it was demonstrated that the N-terminal sequence of pre-S1 region (from Asn-9 to Gly-24) possesses low pH-dependent fusogenic activity, which might play a crucial role in the endosomal escape of BNC payloads and in the uncoating process of HBV. In this minireview, we describe a model in which each domain of the HBV L protein contributes to attachment onto human hepatic cells through HSPG, initiation of endocytosis, interaction with NTCP in endosomes, and consequent provocation of membrane fusion followed by endosomal escape.

Development of a virus-mimicking nanocarrier for drug delivery systems: The bio-nanocapsule

As drug delivery systems, nanocarriers should be capable of executing the following functions: evasion of the host immune system, targeting to the diseased site, entering cells, escaping from endosomes, and releasing payloads into the cytoplasm. Since viruses perform some or all of these functions, they are considered naturally occurring nanocarriers. To achieve biomimicry of the hepatitis B virus (HBV), we generated the "bio-nanocapsule" (BNC)-which deploys the human hepatocyte-targeting domain, fusogenic domain, and polymerized-albumin receptor domain of HBV envelope L protein on its surface-by overexpressing the L protein in yeast cells. BNCs are capable of delivering various payloads to the cytoplasm of human hepatic cells specifically in vivo, which is achieved via formation of complexes with various materials (e.g., drugs, nucleic acids, and proteins) by electroporation, fusion with liposomes, or chemical modification. In this review, we describe BNC-related technology, discuss retargeting strategies for BNCs, and outline other virus-inspired nanocarriers.

Targeted destruction of the polymerized human serum albumin binding site within the preS2 region of the HBV surface antigen while retaining full immunogenicity for this epitope

The 55-amino acid (a.a.) preS2 region of the hepatitis B virus (HBV) envelope protein is highly immunogenic, and antibodies against this epitope confer seroprotection against HBV infections. Accordingly, various experimental and clinical studies for developing and evaluating HBV vaccines that include this particular epitope have been reported. However, a pitfall in using preS2 epitopes as part of a vaccinating antigen is that polymerized human serum albumin (pHSA), which is a normal constituent of the human serum, binds to and makes complexes with this particular region. Consequently, it is most likely that the antigen epitope is masked by serum pHSA and subsequently not detected by the immune system. To overcome these limitations, a novel single a.a substitute of the preS2 region was designed that corresponds to a tyrosine to serine exchange at position 140 of preS2. Competitive enzyme-linked immunosorbent assay showed that this substitution completely abolishes pHSA-binding activities in the mutated preS2 peptide, and CD spectra analysis revealed that this property might have been induced by slight conformational changes in its secondary structure. Nevertheless, the original B-cell epitope was still preserved in the mutated preS2 as determined by experimental immunization in mice. In this regard, the preS2(120-145/Y140S) sequence may be an HBV vaccine where epitopes, with intrinsic properties have been deleted without affecting the immunogenicity of the epitope itself. It is expected that the inclusion of this point mutated preS2 epitope will improve the efficacy of conventional preS2-containing HBV vaccines.

Transfection of a rat hepatoma cell line with a construct expressing human liver annexin V confers susceptibility to hepatitis B virus infection

Previously, we have found that human liver annexin V (hA-V; in earlier reports referred as Endonexin II) is a specific hepatitis B surface antigen (HBsAg) binding protein. In this study, we demonstrate that transfection of rat hepatoma FTO 2B cells, a cell line that is not infectable by hepatitis B virus (HBV) and does not express hA-V, with a construct containing the hA-V gene, resulted in hA-V expressing cells susceptible to HBV infection. After in vitro infection, transfected FTO cells (assigned as FTO 9.1 cells) expressing hA-V in cultures were shown to contain HBV-precore/core, X mRNAs, and covalently closed circular (ccc) DNA as detected by polymerase chain reaction (PCR). The presence of HBV ccc and replicative intermediate DNA was also demonstrated by Southern blot hybridization assay. HBV DNA secreted in the culture medium was also evident as determined by quantitative branched DNA (bDNA) assay. HBsAg and hepatitis B core antigen (HBcAg) could also be detected by an immunocytochemical method in 10% to 15% of the cells at day 3 and day 5 after infection. Infectivity of in vitro-propagated HBV was demonstrated by infection of the naive FTO 9.1 cells with the culture supernatant from HBV-carrier cultures. In contrast to primary cultures of human hepatocytes and FTO 9.1 cells, primary rat and mouse hepatocytes, as well as rat hepatoma cell lines that do not express hA-V, are not susceptible to HBV infection. These findings suggest that hA-V plays a key role in the initial step of HBV infection and that the species-specific susceptibility to HBV infection and replication in hepatocytes is associated with the expression of hA-V.

Analysis of the hepatitis B virus genome and immune response in HBsAg, anti-HBs positive chronic hepatitis

Although the development of antibodies against the hepatitis B virus surface antigen generally leads to the clearance of the infecting virus, anti-HBs reactivity has been reported in patients with chronic hepatitis. In the present study we analyzed the viral genome and the antibody specificity in a series of serum samples collected from a patient who seroconverted to anti-HBs during interferon therapy without clearing HBsAg. The appearance of an anti-HBs response was accompanied by the emergence of a pre-S1 defective viral genome. However, the wild-type adw2 molecular species remained largely dominant during follow up. The patient's antibody response to the surface viral antigens was directed towards the heterologous y subdeterminant and the pre-S1 fragment deleted in the variant hepatitis B virus. These results suggest that the selection of the escape viral mutant does not play a major role in viral persistence.

Immunogenicity of a recombinant Pre-S2-containing hepatitis B vaccine versus plasma-derived vaccine administered as a booster

GenHevac B Pasteur is a recombinant hepatitis B vaccine derived from a mammalian cell line and containing HBs as well as pre-S2 antigens. Its immunogenicity was compared to that of the plasma-derived vaccine Hevac B Pasteur in a population primovaccinated 5.5 years earlier with four injections of the same plasma vaccine. The booster injection with either GenHevac or Hevac was administered to 295 subjects with residual anti-HBs titres below 500 IU/l (group 1: 0-9; group 2: 10-99; group 3: 100-499 IU/l). After four weeks, GenHevac had induced higher anti-HBs responses than Hevac in all groups, particularly among the low responders of group 1. Response to the vaccine occurred earlier with GenHevac. Mean anti-pre-S2 production was moderate in all groups for both vaccines (GenHevac: 60 IU/l; Hevac: 31 IU/l) and was not found in the 32 subjects who produced less than 100 IU/l anti-HBs. The results of the present study indicate that GenHevac is at least as immunogenic as Hevac.

Anti-pre-S2 antibody response in subjects vaccinated against hepatitis B and in naturally immunized subjects

Serum samples from individuals immunized with a pepsinized or non-pepsinized vaccine and from patients who had recovered from acute hepatitis B or who developed a chronic form of the disease, were analysed for the presence of antibody against the pre-S2 epitope of the hepatitis B virus. Anti-pre-S2 antibody was absent in all but one individual immunized with the pepsinized vaccine. Thirty-eight percent of the subjects who responded by anti-HBs production to the non-pepsinized preparation showed anti-pre-S2 antibody one year after complete vaccination. Among subjects who did not produce anti-HBs after immunization with this vaccine, 1 single individual produced anti-pre-S2 antibody. Anti-preS2 antibody was detectable after one year in 38% of the patients who recovered from acute hepatitis B, but in none of those with chronic hepatitis B. The kinetics of anti-pre-S2 antibody response to a booster injection was also analysed 1 month and 1 year after the 3rd injection and 1 month after the 4th injection of the non-pepsinized vaccine.

Interaction between hepatitis B surface proteins and monomeric human serum albumin

HBsAg is known to bind to human serum albumin polymerized by glutaraldehyde, human serum albumin has been found in preparations of HBsAg by several investigators. However, it is not yet known whether natural human serum albumin binds to hepatitis B virus under physiological conditions. We studied the binding between natural or recombinant HBsAg and monomeric human serum albumin by immunological, biochemical and biophysical methods. The binding capacity of 20-nm HBs spheres was variable but ranged up to six molecules HSA/sphere. A reversible binding site for human serum albumin was exclusively localized in the preS2 domain, whereas the S domain was inactive in vitro. Human serum albumin copurified with HBsAg of human origin during gel chromatography or sucrose-gradient centrifugation. This human serum albumin was monomeric in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The preS2-bound part of the human serum albumin could be removed from HBsAg by high-salt, such as CsCl centrifugation, but another part could only be removed by treatment with a disulfide cleaving reagent. Most of this covalently bound human serum albumin was retained at the HBsAg particle after complete cleavage of medium-sized HBs protein with trypsin. This indicates a second way in which albumin binds irreversible to cysteine(s) of the small HBs protein (SHBs, P24 and GP27).

The hepatitis B virus and the host response

The clearance of hepatitis B virus infected cells from the liver is probably dependent on an interplay between the interferon system and the cellular limb of the host immune response. Although the importance of the nucleocapsid proteins as targets for sensitized cytotoxic T cells is established in chronic hepatitis B infection, further studies are needed during the early phase of acute infection. The relative importance of pre-S sequences as inducers and targets of the virus neutralizing humoral immune response is becoming established but their precise place will await the development of in vitro models of hepadna virus infection and precise definition of the mechanism of viral uptake. In adult life, deficient production of alpha-interferon and suppression of the ability of the host to respond to interferon are probably important factors underlying the development of chronic infection. In the neonate, however, specific suppression of the cell-mediated immune response may be involved. The presence of a mutation in the pre-core region of some virus isolates has recently been described. Hepatocytes infected with this virus cannot produce hepatitis B e antigen and the course of the liver disease is fairly rapid. Whether this mutant causes liver damage in the same way as the wild virus or is directly cytopathic needs further study. In adult-acquired chronic hepatitis B virus infection, alpha-interferon produces hepatitis B e antigen clearance in 26-88% of cases and is followed by resolution of the hepatic inflammation. Results in neonatally acquired infection are less impressive and prednisolone priming followed by interferon may be needed.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the binding sites for glutaraldehyde-polymerized albumin on purified woodchuck hepatocyte plasma membranes

Highly purified woodchuck hepatocyte plasma membranes demonstrated tight specific binding to glutaraldehyde-polymerized serum albumin immobilized on Sepharose macrobeads. This phenomenon was characterized in detail and used for recognition of the plasma membrane constituents involved in binding of the albumin polymer. The hepatocyte membrane-polyalbumin interaction was found to be ligand-specific, saturable, and time-dependent. Other characteristics of a specific receptor-ligand interaction were also noted, including a dependence on the temperature, pH, and ionic strength of the binding medium. Kinetic studies revealed the presence of two classes of binding sites for glutaraldehyde-polymerized albumin on purified membranes. The sites mediating the saturable high-affinity binding of polymer to hepatocyte membranes could not be solubilized by Triton X-100. Binding activity of Triton-insoluble membrane residues was inhibited by heat treatment and proteolysis, and was significantly suppressed by neuroaminidase digestion. These findings suggest a glycoprotein nature for the high-affinity binding sites and indicate that the corresponding receptors apparently are tightly associated with the plasma membrane matrix. In contrast, low-affinity binding of polymeric albumin was inhibited by both Triton X-100 and pronase, was resistant to neuraminidase, and was activated by lipase, suggesting that membrane lipids are important for the binding conduct. In conclusion, these results provide clear evidence that hepatocyte plasma membranes are endowed with at least two classes of chemically distinct binding components, which are able to specifically recognize serum albumin artificially modified by glutaraldehyde treatment. Therefore, they suggest that in vivo hepatocytes may perform a specific receptor-dependent uptake of ligands expressing glutaraldehyde-polymerized albumin specificity. This phenomenon may play an important role in the proposed participation of naturally modified human serum albumin as a bridge in the attachment and penetration into host hepatocyte of hepatitis B virus, which is known to possess a receptor that is specific for glutaraldehyde-cross-linked human serum albumin.

The interaction between native serum albumin and hepatitis B virus

Purified hepatitis B virus particles were obtained from HBeAg positive serum by sucrose gradient ultracentrifugation and sephadex G-200 gel filtration. These virions formed a precipitation line in counterimmune electrophoresis with anti-albumin antibody, but the reaction could be inhibited by anti-HBs. After two months at 4 degrees C, another precipitating line was formed under the same condition which could not be inhibited by anti-HBs and was, thus, due to free albumin. When that sample was incubated at 37 degrees C overnight, the line of free albumin disappeared. The virion bound albumin was monomeric in non-denaturing gel electrophoresis. These results suggest that a reversible binding between virion and albumin may occur in vivo and does not require chemical modification or cross-linking.