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

Generation of hepatitis B virus PreS2-S antigen in Hansenula polymorpha

Despite the long availability of a traditional prophylactic vaccine containing the HBV surface antigen (HBsAg) and aluminum adjuvant, nearly 10% of the population remains unable to generate an effective immune response. Previous studies have indicated that hepatitis B virus (HBV) PreS2-S is abundant in T/B cell epitopes, which induces a stronger immune response than HBsAg, particularly in terms of cytotoxic T lymphocyte (CTL) reaction. In the current study, the HBV PreS2-S gene encoding an extra 26 amino acids (PreS2 C-terminus) located at the N-terminus of HBsAg was cloned into the pVCH1300 expression vector. PreS2-S expressed in the methylotrophic yeast, Hansenula polymorpha, was produced at a yield of up to 250 mg/L. Subsequent purification steps involved hydrophobic adsorption to colloidal silica, ion-exchange chromatography and density ultracentrifugation. The final product was obtained with a total yield of ∼ 15% and purity of ∼ 99%. In keeping with previous studies, ∼ 22 nm viruslike particles were detected using electron microscopy. The generated PreS2-S antigen will be further studied for efficacy and safty in animals.

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.

A mimotope of pre-S2 region of surface antigen of viral hepatitis B screened by phage display

To acquire the phage-displayed mimotopes which mimic the specificity of hepatitis B virus surface antigen (HBsAg), a random peptide library expressing linear peptide with 12 amino acids in length were used to screen with the serum from a hepatitis B virus infected patient in the recovery phase. After 3 rounds of biopanning, the positive phages were confirmed by competitive ELISA using HBsAg/P33. Two phagotopes were identified and one of them was confirmed as mimotope by competition experiment. Based on the mimotpe, a multiple antigenic peptide with four branches was synthesized by solid phase peptide synthesis. The antiginicity and specificity of the synthesized antigen was tested in BALB/c mice compared with the native epitope-based antigen. The results showed that the mimotope-based antigen could evoke higher titer of antibodies with the same specificity of the epitope-based antigen. Those findings indicate mimotopes can be used in antigen and vaccine design.

Structure and glycosylation patterns of surface proteins from woodchuck hepatitis virus

Woodchucks chronically infected with woodchuck hepatitis virus (WHV) are a valuable model for human hepatitis B virus (HBV) in studies of pathogenesis, immunity, and antiviral therapy. For this reason, substantial efforts to characterize both the similarities and the differences between HBV and WHV are being made. The structure of the WHV surface proteins (WHs proteins) has not yet been adequately elucidated. The bands that would be expected for glycosylated and nonglycosylated small (S) WHs protein are found by sodium dodecyl sulfate gel electrophoresis of purified WHs protein, but the bands corresponding to the middle (M) and large (L) WHs proteins of HBV are not seen at the expected sizes, even though the sequences of the WHV and HBV surface protein genes are 60% homologous. By amino-terminal sequencing we have identified two bands at 41 and 45 kDa as the MWHs proteins, 8 kDa larger than expected. We have also confirmed that two bands at 24 and 27 kDa are SWHs proteins. A protein of 49 kDa was blocked at the N terminus, which using immunoblotting with an antiserum against WHV pre-S1 (positions 126 to 146) was identified, together with a part of the 45-kDa protein, as glycosylated and nonglycosylated LWHs protein of the expected size. Sialidase and O-glycosidase digestion showed that the larger size of MWHs protein results from the presence of O glycoside groups which are probably in the pre-S2 domain of MWHs protein. Since the pre-S2 domains of HBV and WHV have similar numbers of potential O glycosylation sites, it appears to be likely that the glycosyltransferases act differently on the viral proteins in woodchucks and humans.

The human beta 2-adrenergic receptor expressed in Schizosaccharomyces pombe retains its pharmacological properties

We have developed a rapid and efficient expression system to study the human beta 2 adrenergic receptor (hu beta 2AR) in the fission yeast Schizosaccharomyces pombe. This was achieved by cloning the hu beta 2AR gene, modified by replacement of the 5' untranslated and a small part of the N-terminal coding sequence (first 14 amino acids) with the corresponding region of the yeast Saccharomyces cerevisiae STE2 (alpha-factor receptor) gene. The gene was then placed under the control of a S. pombe constitutive promoter for alcohol dehydrogenase (adh). Hu beta 2AR expression was assessed by immunoblot analysis of the chimeric protein with an anti-STE2 serum raised against a dodecapeptide homologous to the N-terminal amino acids of STE2 and ligand binding was assayed using [125I]cyanopindolol. We demonstrate here that the chimeric receptor expressed in S. pombe exhibits the same characteristic ligand specificity and affinity as that of the authentic hu beta 2AR. This system constitutes a convenient alternative to existing methods for studying seven transmembrane domain receptors due to its simplicity and high reproducibility.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

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.

Concomitant cellular expression of heat shock regulated genes of hepatitis B virus surface antigen and of human growth hormone by a NIH-3T3 cell line

A plasmid carrying a DNA fragment of hepatitis B virus, coding for the pre-S2 and the entire S region of the surface antigen (HBsAg), placed under the control of the promoter of the human 70 kDa heat shock protein gene (hsp70), was introduced into Line 6, a recombinant cell line that was selected from NIH-3T3 cells previously transfected with a similar construct coding for the human growth hormone cDNA gene (chGH) and with the plasmid pEJ carrying the Ha-rasEJ activated cellular oncogene. The resulting cell line, EMS8, expressed: (1) hsp70/HBsAg and hsp70/hGH hybrid genes, (2) the human Ha-rasEJ oncogene, and (3) the neomycin resistance gene, the two last plasmid markers being used for cell selection. EMS8 cells were able to carry out post-translational modifications of the middle M and the major S envelope proteins of HBV, such as assembly and glycosylation. Accordingly, the cells synthesized and secreted both free and glycosylated M and S viral proteins, and the human growth hormone protein. In addition concomitant expression of HBsAg and hGH proteins as well as their mRNA were detected in EMS8 cells at least up to 72 hr after heat induction instead of 24 hr in the case of hGH in Line 6 cells.

Serology of acute exacerbation in chronic hepatitis B virus infection

BACKGROUND

Liver injury in many patients with chronic hepatitis B is sporadic and is often characterized by acute exacerbations alternating with relatively normal liver function. The aim of this study was to perform detailed serological and biochemical analysis during periods of active liver disease to better understand the mechanisms responsible for the cyclic nature of liver injury.

METHODS

A series of serum samples from 19 hepatitis B e antigen (HBeAg)-positive patients were analyzed for alterations in serum hepatitis B virus (HBV) DNA, HBeAg, anti-HBe production, and HBeAg-specific immune complex formation before, during, and after spontaneous acute exacerbations of liver injury.

RESULTS

This analysis revealed significant correlations between increasing levels of serum HBV DNA, HBeAg, HBeAg-specific immune complexes, and liver injury. These results suggest that increases in viral replication and accumulation of viral proteins in the serum and intracellularly and the subsequent immune response play an important role in initiating acute exacerbations of liver injury in chronic hepatitis B infection.

CONCLUSIONS

We propose that production of the secreted and cellular forms of the nucleoprotein reaches a threshold level and elicits specific immune responses, which mediate liver injury.

The serology of chronic hepatitis B infection revisited

The serology of chronic hepatitis B infection has been established through the use of commercial immunoassays to measure the structural antigens of the hepatitis B virus and their respective antibodies in serum. However, the commercial assays have not been designed to detect serum antibodies in the presence of an excess of circulating antigens. A series of serum samples from 200 HBeAg-positive, chronically infected hepatitis B patients with varying degrees of liver disease were analyzed using novel immunoassays designed to detect antibodies in the presence of circulating viral antigens. All patients, regardless of their liver disease, were seronegative for antibodies specific for the envelope antigens or the secreted nucleoprotein antigen (HBeAg) when the commercial assays were used. In contrast, virtually all chronically infected patients with liver disease and approximately 50% of patients without liver disease demonstrated anti-HBe and anti-envelope antibodies when sera were tested in the more sensitive immunoassays. Furthermore, asymptomatic patients could be serologically distinguished from symptomatic patients based on antibody fine specificity, titer, and IgG subclass. This study revealed that the majority of chronically infected hepatitis B patients produce a variety of antibodies for many years, and are not immunologically unresponsive, as suggested by the current assays.

Fermentation study for the production of hepatitis B virus pre-S2 antigen by the methylotrophic yeastHansenula polymorpha

Various physico-chemical parameters have been studied in order to improve the production of hepatitis B virus pre-S2 antigen (middle surface antigen) by the methylotrophic yeast Hansenula polymorpha. Antigen production was done in two steps: first, production of cells on glycerol (Phase 1), followed by induction of antigen expression with methanol (Phase 2). Dense cultures of H. polymorpha, equivalent to 35-40 g/l (dry weight), were readily obtained in small fermenters using minimal medium containing glycerol as carbon source. Antigen expression in this minimal medium, after induction with methanol, was however, low and never exceeded 1.6 mg/l of culture. Antigen production was greatly enhanced by adding complex organic nitrogen sources along with methanol at induction time; yeast extract was the best of all the sources tested. In shake flasks, antigen production was proportional to yeast extract concentration up to 7% (w/v) yeast extract, it became clear the the nutritional conditions for good antigen expression were different from those for good biomass production. The effects of yeast extract were reproduced in small fermenters: antigen levels reached 8-9 mg/l in medium containing 6% (w/v) yeast extract during induction with methanol. The mechanisms of yeast extract's effects are still unknown but are probably nutritional. The recombinant H. polymorpha strain produced both periplasmic and intracellular antigen. The periplasmic antigen was shown to be present as 20-22-nm particles and was therefore immunogenic. Immunoblotting indicated that part of the pre-S2 antigen was present as a 24-kDa degradation product. These studies have led to a 140-fold increase in volumetric productivity of antigen and to a 4.6-fold increase in specific production.

Expression of Torpedo nicotinic acetylcholine receptor subunits in yeast is enhanced by use of yeast signal sequences

We have produced the four subunits of the nicotinic acetylcholine receptor of Torpedo californica, an integral membrane protein, in the yeast Saccharomyces cerevisiae. Two of the subunits (alpha and delta) were readily produced from their cDNAs after simply subcloning them into a yeast shuttle vector adjacent to a yeast promoter. The other two protein subunits (beta and gamma) were not produced by this strategy, although the amounts of mRNA produced from these expression constructs are similar to those for alpha and delta. Replacing the DNA coding for the normal N-terminal signal sequences for the beta and gamma subunits with DNA coding for the signal sequence of yeast invertase results in successful protein synthesis. The yeast signal sequence allows these subunits to be translocated across the membrane of the endoplasmic reticulum and to be glycosylated. The appropriate final size of the subunit proteins suggests that the yeast signal sequence has been properly cleaved after translocation.

Synthesis and secretion of hepatitis B middle surface antigen by the methylotrophic yeast Hansenula polymorpha

The methylotrophic yeast, Hansenula polymorpha, has been developed as a host system for the synthesis of heterologous proteins. The middle surface antigen of hepatitis B virus (preS2-HBsAg) has been synthesized under the control of a methanol-regulated promoter derived from the methanol oxidase-encoding gene. The synthesized preS2-HBsAg protein was found to be secreted outside the cell membrane into the periplasm and further excreted into the culture medium following permeabilization of the cell wall with beta-1,3-glucanase (beta Glu). Cell cultures treated with beta Glu were able to continuously synthesize and secrete 22-nm particles of preS2-HBsAg into the medium for several days. The overall yield of antigen from treated cultures was found to be over threefold greater than that of untreated controls. The observation that complex supramolecular structures, such as the 22-nm particles of preS2-HBsAg, can be secreted by H. polymorpha and released into the medium, suggests the potential for these yeasts to be an alternative secretory host.

Simultaneous synthesis and assembly of various hepatitis B surface proteins in Saccharomyces cerevisiae

Yeast transposon of class-1-based vectors, allowing integration at a series of chromosomal loci by homologous recombination with resident transposons, were constructed. Using such vectors, we have introduced several copies of an expression cassette encoding the major hepatitis B surface protein as well as expression cassettes encoding the middle (M) or/and the large (L) surface protein into Saccharomyces cerevisiae. In extracts of such strains, coassembly of the different proteins into a single lipoprotein structure is observed. This was demonstrated by immunoprecipitation of the major protein using monoclonal antibodies directed specifically against epitopes that are present only on the M or the L protein. These results show that hepatitis B surface antigen envelope proteins synthesized in yeast are able to assemble into structures composed of different polypeptides. This opens the possibility of producing in yeast a variety of particles carrying well-defined amounts of preS epitopes on their surface. Also, one can envisage the production of mixed particles containing different foreign epitopes on their surface, in defined relative abundance, which could be useful for vaccine applications.

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.