Synthesis in yeast of hepatitis B virus surface antigen modified P31 particles by gene modification

Hepatitis B Virus (HBV) Subviral Particles as Protective Vaccines and Vaccine Platforms

Hepatitis B remains one of the major global health problems more than 40 years after the identification of human hepatitis B virus (HBV) as the causative agent. A critical turning point in combating this virus was the development of a preventative vaccine composed of the HBV surface (envelope) protein (HBsAg) to reduce the risk of new infections. The isolation of HBsAg sub-viral particles (SVPs) from the blood of asymptomatic HBV carriers as antigens for the first-generation vaccines, followed by the development of recombinant HBsAg SVPs produced in yeast as the antigenic components of the second-generation vaccines, represent landmark advancements in biotechnology and medicine. The ability of the HBsAg SVPs to accept and present foreign antigenic sequences provides the basis of a chimeric particulate delivery platform, and resulted in the development of a vaccine against malaria (RTS,S/AS01, Mosquirix^TM), and various preclinical vaccine candidates to overcome infectious diseases for which there are no effective vaccines. Biomedical modifications of the HBsAg subunits allowed the identification of strategies to enhance the HBsAg SVP immunogenicity to build potent vaccines for preventative and possibly therapeutic applications. The review provides an overview of the formation and assembly of the HBsAg SVPs and highlights the utilization of the particles in key effective vaccines.

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.

Efficient and rapid purification of drug- and gene-carrying bio-nanocapsules, hepatitis B virus surface antigen L particles, from Saccharomyces cerevisiae

Bio-nanocapsules (BNCs) are hollow particles (approx. 50 nm diameter) consisting of hepatitis B virus surface antigen (HBsAg) large (L, pre-S1+pre-S2+S) proteins embedded in a unilamellar liposome, sharing the same transmembrane S region with an immunogen of hepatitis B vaccine (i.e., HBsAg small (S) protein particle). BNCs can incorporate drugs and genes into the hollow space and systemic administration of the BNCs can deliver the products to human liver via the human hepatocyte-specific receptor within the pre-S (pre-S1+pre-S2) region displayed on BNC's surface. Thus, BNCs are expected to offer efficient and safe non-viral nanocarriers to deliver human liver-specific genes and drugs. To date, BNCs have been purified from the crude extract of BNC-overexpressing yeast cells by fractionation with polyethylene glycol followed by one CsCl equilibrium and two sucrose density gradient ultracentrifugation steps. However, the process was inefficient in terms of yield and time, and was not suitable for mass production because of the ultracentrifugation step. Furthermore, trace contamination with yeast-derived proteinases degraded the pre-S region, which is indispensable for liver-targeting, during long-term storage. In this study, we developed a new purification method involving heat treatment and sulfated cellulofine column chromatography to facilitate rapid purification, completely remove proteinases, and enable mass production. In addition, the BNCs were functional for at least 14 months after lyophilization with 5% (w/v) sucrose as an excipient. This new process will significantly contribute to the development of forthcoming BNC-based nanomedicines as well as hepatitis B vaccines.

Expression of hepatitis B surface antigen major subtypes in Pichia pastoris and purification for in vitro diagnosis

This study describes the expression in Pichia pastoris of hepatitis B surface antigens (HBsAg) corresponding to the S region of the four major subtypes: adr, adw2, ayr and ayw3 and to the preS2-S region of the two subtypes adr and adw2. The recombinant yeast strains have been selected amongst methanol utilization positive (Mut+) or sensitive strains (Mut s) and cultivated to high cell density in bioreactor using a short protocol. Our results prove the efficiency of P. pastoris to produce all the major HBsAg subtypes and confirm the ability of the methanol regulated promoter of alcohol oxidase I gene (AOX) to express heterologous protein through phenotype Mut+ or Mut s strains. All these recombinant HBsAg proteins, including subtype ayr, whose production has never been presented, have been highly purified using a short original sequence of steps which includes high-pressure cell disruption associated with detergent treatment, ultrafiltration and immunopurification chromatography using a mAb anti-HBs. The whole process avoids possible alterations of antigenic properties and allows to obtain with high yield, high quality reagents for in vitro diagnosis.

Glucose dependent [correction of dependant] negative translational control of the heterologous expression of the preS2 HBV antigen in yeast

In the present study, we have compared the expression level of the genes encoding the viral Hepatitis B S and preS2 using two different yeast vectors, a constitutive one (YepIPT) and a galactose-inducible one (YepDP1-8). We showed that the S and preS2 mRNAs were present in equivalent amounts in both systems, while the corresponding proteins showed a different pattern. The S and preS2 proteins were efficiently produced after galactose induction. However, under the constitutive promoter, the S protein was synthesized at the same level in presence of 2% glucose or galactose whereas the preS2 protein was efficiently synthesized on galactose but absent on 2% glucose. The substitution of glucose by non-repressive carbon sources such as 2% galactose, ethanol or glycerol led to a significant expression of preS2. A high level of preS2 expression was also achieved by lowering the glucose concentration. Our data suggest that glucose exerts a concentration dependent negative translational control on the preS2 mRNA.

Safety and efficacy of a recombinant yeast-derived pre-S2 + S-containing hepatitis B vaccine (TGP-943): phase 1, 2 and 3 clinical testing

A recombinant yeast-derived pre-S2 + S-containing hepatitis B vaccine (TGP-943) was clinically evaluated through three phases of testing in a total of 2137 volunteers. We observed the immunogenic purity of TGP-943 (phase 1), the inter-lot reproducibility of both safety and immunogenicity (phase 2), no significant side-effects, a high capability of inducing both anti-HBs and anti-pre-S2 antibodies (phases 1, 2 and 3), and an ability to induce seroconversion in the majority of vaccines who had been non-responsive to conventional hepatitis B vaccines (phases 2 and 3). In conclusion, TGP-943 is a safe and tolerable vaccine, with special merits: the ability to induce an early anti-pre-S2 response that circumvents the problem of delayed appearance of anti-HBs, and efficacy in non-responders to previous vaccination.

Saccharomyces cerevisiae can release hepatitis B virus surface antigen (HBsAg) particles into the medium by its secretory apparatus

We constructed a plasmid that directs the synthesis and secretion of hepatitis B virus (HBV) surface antigen (HBsAg) particles by Saccharomyces cerevisiae. This plasmid contains a proteinase-resistant HBsAg M (M-P31c) gene fused at its 5'-terminus with a chicken-lysozyme signal peptide (C-SIG) gene, which is placed under the yeast GLD (glyceraldehyde-3-phosphate dehydrogenase gene) promoter. The products encoded by the "C-SIG+M-P31c" (LM-P31c) gene were synthesized and assembled themselves into HBsAg particles in yeast cells, and the particles were released into the medium along with poly-HSA (polymerized human serum albumin) binding activity. The HBsAg particles purified from the medium were very similar in density (1.19 g cm-3), size (19.2 +/- 0.8 nm in diameter) and shape (sphere) to human-plasma-derived HBsAg particles. When several sec (temperature-sensitive secretion-defective) mutants were used as host cells, the release of HBsAg particles into the medium was blocked at 37 degrees C but not at 25 degrees C, indicating that the HBsAg particles are exported through the normal yeast secretion pathway. To our knowledge, this is the first report that yeast cells are capable of secreting particles into the medium.

Characterization of two differently glycosylated molecular species of yeast-derived hepatitis B vaccine carrying the pre-S2 region

Modified hepatitis B virus surface antigen M protein particles (HBsAg M-P31c) produced in yeast is mainly composed of two differently glycosylated proteins, GP37 and GP34. GP37 has an N-linked sugar chain and O-linked sugar chains; and GP34 has an N-linked sugar chain bound to the peptide backbone P31. Although M-P31c vaccine elicits both anti-S and anti-pre-S2 antibodies, whether there are any differences between GP37 and GP34 in the ability to elicit these antibodies is still unknown. To clarify this issue, we prepared particles which were composed solely of GP37 or GP34 by affinity chromatography, using polymerized human serum albumin as a ligand and digestion with alpha-mannosidase. We also prepared particles composed solely of P31 by successive digestion with alpha-manosidase and endo-beta-N-acetylglycosaminidase H. The vaccines derived from these three kinds of particles elicited both anti-S and anti-pre-S2 antibodies in mice to the same extent as the original M-P31c vaccine. These results suggest that the N- and O-linked sugar chains of M-P31c component proteins produced in the host yeast cells have no effect on the ability to elicit anti-S and anti-pre-S2 antibodies and that there are no differences with respect to antibody response in mice between the two major components of M-P31c, GP37 and GP34.

Characterization of recombinant human and rat pancreatic phospholipases A2 secreted from Saccharomyces cerevisiae: difference in proteolytic processing

An expression plasmid for human pancreatic phospholipase A2 in Saccharomyces cerevisiae was constructed by insertion of cDNA encoding its preprophospholipase A2 into a yeast expression vector pAM82. The resulting product secreted in the yeast culture medium was mainly prophospholipase A2, which was the same as the natural proenzyme in all aspects examined, including the higher order structure. However, when the rat preprophospholipase A2 cDNA was manipulated in the same manner, the active phospholipase A2 of the intact mature form was secreted with the proenzyme being hardly detected in the medium. This unexpected favorable result would occur due to cleavage of rat phospholipase A2 pro-peptide by a trypsin-like proteinase in S. cerevisiae. Based on this finding, we constructed a plasmid carrying the sequence coding for the prepro-peptide of rat pancreatic phospholipase A2 behind the PHO5 promoter in the pAM82 vector, which leads to the secretion of heterologous proteins as their mature form. The use of this plasmid led to secretion of biologically active human pancreatic secretory trypsin inhibitor and a glutamic acid-specific endopeptidase from Staphylococcus aureus ATCC 12600, which are eukaryote and prokaryote proteins, respectively, in the culture medium of S. cerevisiae.

The oligosaccharides in a recombinant hepatitis B virus surface antigen (HBsAg) carrying the pre-S2 region derived from yeast

The N- and O-linked oligosaccharides in a yeast-derived HBsAg M protein (pre-S2 + S) were analyzed. Two major structures of the N-linked oligosaccharides bound to residue Asn4 were determined to be high-mannose type oligosaccharides, Man7GlcNAc2 and Man8GlcNAc2, by two-dimensional sugar mapping of the corresponding pyridylamino oligosaccharides. Peptide mapping of the M protein, sequence analysis of the glycopeptides after beta-elimination under reducing conditions and sugar-composition analysis revealed that the O-linked oligosaccharides were composed solely of mannose and bound to residue Ser5, Thr6, Thr7, Ser27, Ser28, Ser29 and Thr31 in the pre-S2 region.

High-level expression and characterization of hepatitis B virus surface antigen in silkworm using a baculovirus vector

The Bombyx mori nuclear polyhedrosis virus (BmNPV) in the silkworm was used successfully for mass production of biologically active foreign genes under the control of the polyhedrin promoter. This system was adapted for the production of large amounts of hepatitis B virus surface antigens (HBsAg). The DNA fragments coded for the middle protein, which is composed of the S protein with the pre-S2 region, were cloned, the signal protein gene of beta-IFN was added, and both were inserted into a cloning vector. After co-transfection with wild-type BmNPV, stable recombinant viruses were isolated by the limiting dilution method. Infected silkworm larvae with the recombinants expressed HBsAg at high levels (400-600 micrograms/ml). These products, consisting of two polypeptides with molecular weights of approximately 25,000 (p25) and glycosylated P25 (GP28), were purified as assembled 22-nm particles. We demonstrated that HBsAg from silkworms consists of S protein with 7 amino acids of Pre-S2.

Protective efficacy of a novel hepatitis B vaccine consisting of M (pre-S2 + S) protein particles (a third generation vaccine)

The protective efficacy of a new type of yeast-derived hepatitis B (HB) vaccine (TGP-943, subtype adr), which was formulated from modified M (pre-S2 + S; P31) protein (M-P31c) particles, was investigated in chimpanzees. Animals were injected intramuscularly three times at 4-week intervals with doses of 10 or 40 micrograms (as a protein) of TGP-943. There were no significant differences in the immunogenicity of 10 micrograms compared to that of 40 micrograms of TGP-943 in terms of anti-S antibody response, while the induction and persistence of anti-pre-S2 antibodies seemed dose-related. Chimpanzees, vaccinated with 40 micrograms of TGP-943, produced anti-pre-S2 antibodies 2 weeks after the first injection, which appeared earlier than anti-HBs (S) antibodies. A maximum level of the anti-pre-S2 antibodies was reached 2 weeks after the second injection. Apart from immunization with TGP-943, chimpanzees injected with denatured TGP-943, consisting of 10 micrograms (as a protein) of non-particulate M-P31c antigen, produced anti-pre-S2 antibodies with a non-protecting level of anti-S antibodies (less than 10 mIU ml-1). Five weeks after the third injection, all animals were challenged intravenously with 1000 chimpanzee infectious units of HBV subtype (ayw) and were protected as confirmed by normal serological markers, no signs of infection in the sera and liver biopsies, and no detection of HBV-DNA by PCR method. No side effects from inoculation with TGP-943 or denatured TGP-943 were also encountered in any animals.

Efficient expression of genetically engineered hepatitis B virus surface antigen P31 proteins in yeast

We have constructed plasmids that express modified hepatitis B virus surface antigen (HBsAg) P31-coding genes (M-P31c, d, e, f, and i) having various genetically engineered pre-S2 regions. The plasmids contain the GAPDH (gene coding for glyceraldehyde-3-phosphate dehydrogenase) promoter and the PGK (gene coding for 3-phosphoglycerate kinase) terminator, both isolated from sake brewing yeast, Saccharomyces cerevisiae Kyokai III. Expression levels of the modified HBsAg P31 proteins in yeast are greatly increased from 0.4% to 11.7% of total cell protein. However, the specific mRNAs are expressed at equal levels and the degradation rates of the modified P31 proteins do not vary significantly. Therefore, we considered that different expression levels of the modified P31 proteins are attributed to the changes of the post-translational efficiency. And it was suggested that the conformational stability of the N-terminal peptide (Met-1-Phe-46) in the endoplasmic reticulum membrane determines the expression level of modified P31 proteins.

Characterization of purified hepatitis B surface antigen containing pre-S(2) epitopes expressed in Saccharomyces cerevisiae

The cloning and expression of the hepatitis B middle-protein surface antigen gene in the yeast Saccharomyces cerevisiae is described. A generalized expression vector carrying the yeast glyceraldehyde-3-phosphate dehydrogenase gene promoter was used. Expressed material, in the form of supramolecular particles, was purified and characterized. Severe proteolysis within the pre-S(2) region was observed for material expressed in a wild-type yeast host. This proteolysis was substantially reduced by utilization of a protease-deficient host. Immunoblotting of sodium dodecyl sulfate-polyacrylamide gels with several antibodies of differing specificity was performed to characterize the various protein species present. All species were analyzed by N-terminal sequencing after electroelution from gels. Carbohydrate staining of gels and glycosidase treatments of the purified antigen material indicated that full-length antigen was present in both glycosylated and unglycosylated forms. Glycosylation appeared to be of both asparagine-linked and threonine/serine-linked types. Site-directed mutagenesis was used to convert two arginine residues in the pre-S(2) region of the antigen to glutamine residues. The changes abolished reactivity with one polyclonal and two monoclonal antibodies specific for epitopes within the pre-S(2) region.

A supersensitive dot-hybridization method: rapid and quantitative detection of host-derived DNA in recombinant products at the one picogram level

We have developed a highly sensitive method of DNA dot-blotting hybridization to detect host-derived DNA (nuclear DNA and plasmid DNA) in a recombinant product. This method has two distinctive features compared to the conventional hybridization method: firstly, a highly specific radioactive probe is prepared by using ultrasonicated DNA, instead of untreated DNA, as a template for the oligo-labeling reaction; secondly, the signal to noise ratio is increased by the use of lambda phage DNA as non-homologous DNA. This method enabled us to detect host-derived DNA at the one picogram level without using a radioisotope of high specific activity and long exposure times.

Antibodies recognizing human serum albumin are not elicited by immunization with preS2 sequences of the hepatitis B virus envelope protein

Antibodies to the preS2 region of the hepatitis B virus (HBV) envelope protein and to human serum albumin (HSA) were allegedly detected at about the same level in sera of humans with acute or chronic hepatitis B [Hellström et al., 1986]. It was claimed that anti-HSA arises as a result of an immune response to the preS2 sequence and that it was involved in hepatocellular damage. Over 100 sera from animals and humans immunized with HBsAg containing preS2 sequences, or with synthetic peptides from the preS1, preS2, and S regions of the HBV env protein were assayed for anti-HSA. The results revealed the following: 1) Immunization with the native preS2 sequence or with unconjugated synthetic peptides derived from that sequence does not result in elicitation of anti-HSA. Therefore the alleged appearance of anti-HSA during hepatitis B cannot be directly related to an anti-preS2-specific immune response. 2) Some synthetic peptides, whether or not they were derived from the preS2 sequence, when linked to certain carriers, but not to others, elicited in rabbits an anti-HSA response, which was markedly lower than the response to the homologous peptide. These anti-HSA antibodies could be separated from anti-preS2-specific antibodies by affinity chromatography and did not recognize the synthetic peptide used for immunization. The use in active immunoprophylaxis of hepatitis B of unconjugated peptides from the preS2 sequence with proven high immunogenicity will avoid carrier/linker-mediated induction of antibodies not relevant to protection against HBV.