Immunoelectron microscopic detection of the hepatitis B virus major surface protein in dilated perinuclear membranes of yeast cells

Guidelines for Small-Scale Production and Purification of Hepatitis B Surface Antigen Virus-Like Particles from Recombinant Pichia pastoris

Virus-like particle (VLP)-based vaccines have been in the market since decades for preventing viral infection and have proven their usefulness also in other areas of biotechnology. Here, we describe in detail simple small-scale production and purification procedures for the generation of hepatitis B surface antigen (HBsAg) VLPs using Pichia pastoris as expression host. This protocol may also be applicable with variations to other HBsAg-based VLPs additionally carrying antigens of other pathogens.

Dengue-specific subviral nanoparticles: design, creation and characterization

Background

Dengue is today the most significant of arboviral diseases. Novel tools are necessary to effectively address the problem of dengue. Virus-like particles (VLP) offer a versatile nanoscale platform for developing tools with potential biomedical applications. From the perspective of a potentially useful dengue-specific tool, the dengue virus envelope protein domain III (EDIII), endowed with serotype-specificity, host receptor recognition and the capacity to elicit virus-neutralizing antibodies, is an attractive candidate.

Methods

We have developed a strategy to co-express and co-purify Hepatitis B virus surface (S) antigen in two forms: independently and as a fusion with EDIII. We characterized these physically and functionally.

Results

The two forms of the S antigen associate into VLPs. The ability of these to display EDIII in a functionally accessible manner is dependent upon the relative levels of the two forms of the S antigen. Mosaic VLPs containing the fused and un-fused components in 1:4 ratio displayed maximal functional competence.

Conclusions

VLPs armed with EDIII may be potentially useful in diagnostic, therapeutic and prophylactic applications.

Algae as protein factories: expression of a human antibody and the respective antigen in the diatom Phaeodactylum tricornutum

Microalgae are thought to offer great potential as expression system for various industrial, therapeutic and diagnostic recombinant proteins as they combine high growth rates with all benefits of eukaryotic expression systems. Moreover, microalgae exhibit a phototrophic lifestyle like land plants, hence protein expression is fuelled by photosynthesis, which is CO₂-neutral and involves only low production costs. So far, however, research on algal bioreactors for recombinant protein expression is very rare calling for further investigations in this highly promising field. In this study, we present data on the expression of a monoclonal human IgG antibody against the Hepatitis B surface protein and the respective antigen in the diatom Phaeodactylum tricornutum. Antibodies are fully-assembled and functional and accumulate to 8.7% of total soluble protein, which complies with 21 mg antibody per gram algal dry weight. The Hepatitis B surface protein is functional as well and is recognized by algae-produced and commercial antibodies.

Virus-like particle production with yeast: ultrastructural and immunocytochemical insights into Pichia pastoris producing high levels of the hepatitis B surface antigen

BACKGROUND

A protective immune response against Hepatitis B infection can be obtained through the administration of a single viral polypeptide, the Hepatitis B surface antigen (HBsAg). Thus, the Hepatitis B vaccine is generated through the utilization of recombinant DNA technology, preferentially by using yeast-based expression systems. However, the polypeptide needs to assemble into spherical particles, so-called virus-like particles (VLPs), to elicit the required protective immune response. So far, no clear evidence has been presented showing whether HBsAg assembles in vivo inside the yeast cell into VLPs or later in vitro during down-stream processing and purification.

RESULTS

High level production of HBsAg was carried out with recombinant Pichia pastoris using the methanol inducible AOX1 expression system. The recombinant vaccine was isolated in form of VLPs after several down-stream steps from detergent-treated cell lysates. Search for the intracellular localization of the antigen using electron microscopic studies in combination with immunogold labeling revealed the presence of HBsAg in an extended endoplasmic reticulum where it was found to assemble into defined multi-layered, lamellar structures. The distance between two layers was determined as ~6 nm indicating that these lamellas represent monolayers of well-ordered HBsAg subunits. We did not find any evidence for the presence of VLPs within the endoplasmic reticulum or other parts of the yeast cell.

CONCLUSIONS

It is concluded that high level production and intrinsic slow HBsAg VLP assembly kinetics are leading to retention and accumulation of the antigen in the endoplasmic reticulum where it assembles at least partly into defined lamellar structures. Further transport of HBsAg to the Golgi apparatus is impaired thus leading to secretory pathway disfunction and the formation of an extended endoplasmic reticulum which bulges into irregular cloud-shaped formations. As VLPs were not found within the cells it is concluded that the VLP assembly process must take place during down-stream processing after detergent-mediated disassembly of HBsAg lamellas and subsequent reassembly of HBsAg into spherical VLPs.

Exploiting the intracellular compartmentalization characteristics of the S. cerevisiae host cell for enhancing primary purification of lipid-envelope virus-like particles

This article demonstrates how the intracellular compartmentalization of the S. cerevisiae host cell can be exploited to impart selectivity during the primary purification of lipid-envelope virus-like particles (VLPs). The hepatitis B surface antigen (HBsAg) was used as the VLP model in this study. Expressed HBsAg remain localized on the endoplasmic reticulum and the recovery process involves treating cell homogenate with a detergent for HBsAg liberation. In our proposed strategy, a centrifugation step is introduced immediately following cell disruption but prior to the addition of detergent to allow the elimination of bulk cytosolic contaminants in the supernatant, achieving approximately 70% reduction of contaminating yeast proteins, lipids, and nucleic acids. Recovery and subsequent treatment of the solids fraction with detergent then releases the HBsAg into a significantly enriched product stream with a yield of approximately 80%. The selectivity of this approach is further enhanced by operating under moderate homogenization pressure conditions ( approximately 400 bar). Observed improvements in the recovery of active HBsAg and reduction of contaminating host lipids were attributed to the low-shear conditions experienced by the HBsAg product and reduced cell fragmentation, which led to lower coextraction of lipids during the detergent step. As a result of the cleaner process stream, the level of product capture during the loading stage of a downstream hydrophobic interaction chromatography stage increased by two-fold leading to a concomitant increase in the chromatography step yield. The lower level of exposure to contaminants is also expected to improve column integrity and lifespan.

Conformational analysis of hepatitis B surface antigen fusions in an Agrobacterium-mediated transient expression system

Vaccine antigens have been successfully produced in transgenic plants for oral immunization. Recently, a fusion strategy has been adopted to produce multicomponent vaccines and to target antigens to mucosal sites for enhanced oral immunogenicity. However, antigen fusions may not be folded correctly due to steric hindrance and may thus lose their potency. Here, we describe an Agrobacterium-mediated transient assay that provides enough antigen-expressing material at 2 days post-transfection to evaluate antigen conformation. Using the hepatitis B surface antigen (HBsAg) as a model antigen and the green fluorescent protein (GFP) as a model fusion partner, we showed that transiently expressed HBsAg and an HBsAg fusion with GFP at the N-terminus (GFP:HBsAg), but not the HBsAg fusion with GFP at the C-terminus (HBsAg:GFP), formed the 'a' determinant and virus-like particles (VLPs), similar to yeast-derived vaccine HBsAg. Thus, it is feasible to modify the HBsAg with an N-terminal fusion of up to 239 amino acids without altering its major antigenic properties. Our results also demonstrate that the Agrobacterium-mediated transient expression system can be used to evaluate the conformation of plant-based vaccines or other pharmaceutical proteins in a high-throughput manner.

Epitope Tagging of the Yeast K+ Carrier Trk2p Demonstrates Folding That Is Consistent with a Channel-like Structure

TRK family proteins, which mediate the concentrative uptake of potassium by plant cells, fungi, and bacteria, resemble primitive potassium channels in sequence and have recently been proposed actually to fold like potassium channels in a 4-MPM motif (Durell, S. R., and Guy, H. R. (1999) Biophys. J. 77, 789 - 807), instead of like conventional substrate porters in the 12-TM motif (Gaber, R. F., Styles, C. A., and Fink, G. R. (1988) Mol. Cell. Biol. 8, 2848-2859). The known fungal members of this family possess a very long hydrophilic loop, positioned intracellularly in the K(+)-channel model and extracellularly in the substrate porter model. This and two shorter hydrophilic segments have been tested as topological markers for the true folding pattern of TRK proteins using Saccharomyces cerevisiae Trk2p. Hemagglutinin epitope tags were inserted into all three segments, and the enhanced green fluorescent protein (EGFP) was fused to the C terminus of Trk2p. The gene constructs were expressed from a high copy plasmid, and sidedness of the tags was determined by native fluorescence (EGFP), indirect immunofluorescence, and immunoelectron microscopy. Both the long-loop tag and the C-terminal EGFP fusion allowed abundant protein to reach the plasma membrane and support normal yeast growth. In all determinations, the long-loop tag was localized to the inner surface of the yeast cell plasma membrane, thus strongly supporting the channel-like folding model. Additional observations showed (i). membrane-associated Trk2p to lie in proteolipid rafts; (ii). significant tagged protein, expressed from the plasmid, to be sequestered in cytoplasmic vesicular-tubular clusters; and (iii). suppression of such clusters by yeast growth in 5-10% glycerol. This chaperone-like effect may assist other membrane proteins (overexpressed or heterologously expressed) to function within the yeast plasma membrane.

A plant signal peptide-hepatitis B surface antigen fusion protein with enhanced stability and immunogenicity expressed in plant cells

The use of transgenic plants to express orally immunogenic protein antigens is an emerging strategy for vaccine biomanufacturing and delivery. This concept has particular suitability for developing countries. One factor that has limited the development of this technology is the relatively modest levels of accumulation of some antigenic proteins in plant tissues. We used fusion protein design to improve expression of the hepatitis B surface antigen (HBsAg) by attempting to mimic the process of HBsAg targeting to the endoplasmic reticulum of human liver cells during hepatitis B virus infection. We created a gene encoding a recombinant HBsAg modified to contain a plant signal peptide fused to its amino terminus. The signal peptide from soybean vegetative storage protein vspA (VSP alpha S) directed endoplasmic reticulum targeting of HBsAg in plant cells, but was not cleaved and resulted in enhanced VSP alpha S-HBsAg fusion accumulation. This product was more stable and presented the protective "a" antigenic determinant to significantly higher levels than unmodified native HBsAg expressed in plant cells. It also showed a greater extent of intermolecular disulfide bond formation and formation of virus-like particles. Moreover, VSP alpha S-HBsAg stimulated higher levels of serum IgG than native HBsAg when injected into mice. We conclude that HBsAg tolerates a polypeptide fusion at the amino terminus and that VSP alpha S-HBsAg is an improved antigen for plant-based expression of a subunit vaccine for hepatitis B virus.

Hepatitis B surface antigen (HBsAg) expression in plant cell culture: Kinetics of antigen accumulation in batch culture and its intracellular form

The production of edible vaccines in transgenic plants and plant cell culture may be improved through a better understanding of antigen processing and assembly. The hepatitis B surface antigen (HBsAg) was chosen for study because it undergoes substantial and complex post-translational modifications, which are necessary for its immunogenicity. This antigen was expressed in soybean (Glycine max L. Merr. cv Williams 82) and tobacco NT1 (Nicotiana tabacum L.) cell suspension cultures, and HBsAg production in batch culture was characterized. The plant-derived antigen consisted predominantly of disulfide cross-linked HBsAg protein (p24(s)) dimers, which were all membrane associated. Similar to yeast, the plant-expressed HBsAg was retained intracellularly. The maximal HBsAg titers were obtained with soybean suspension cultures (20-22 mg/L) with titers in tobacco cultures being approximately 10-fold lower. For soybean cells, electron microscopy and immunolocalization demonstrated that all the HBsAg was localized to the endoplasmic reticulum (ER) and provoked dilation and proliferation of the ER network. Sucrose gradient analysis of crude extracts showed that HBsAg had a complex size distribution uncharacteristic of the antigen's normal structure of uniform 22-nm virus-like particles. The extent of authentic epitope formation was assessed by comparing total p24(s) synthesized to that reactive by polyclonal and monoclonal immunoassays. Depending on culture age, between 40% and 100% of total p24(s) was polyclonal antibody reactive whereas between 6% and 37% was recognized by a commercial monoclonal antibody assay. Possible strategies to increase HBsAg production and improve post-translational processing are discussed.

Oral immunization with hepatitis B surface antigen expressed in transgenic plants

Oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) derived from yeast (purified product) or in transgenic potatoes (uncooked unprocessed sample) was compared. An oral adjuvant, cholera toxin, was used to increase immune responses. Transgenic plant material containing HBsAg was the superior means of both inducing a primary immune response and priming the mice to respond to a subsequent parenteral injection of HBsAg. Electron microscopy of transgenic plant samples revealed evidence that the HBsAg accumulated intracellularly; we conclude that natural bioencapsulation of the antigen may provide protection from degradation in the digestive tract until plant cell degradation occurs near an immune effector site in the gut. The correlate of protection from hepatitis B virus infection is serum antibody titers induced by vaccination; the protective level in humans is 10 milliunits/ml or greater. Mice fed HBsAg-transgenic potatoes produced HBsAg-specific serum antibodies that exceeded the protective level and, on parenteral boosting, generated a strong long-lasting secondary antibody response. We have also shown the effectiveness of oral delivery by using a parenteral prime-oral boost immunization schedule. The demonstrated success of oral immunization for hepatitis B virus with an "edible vaccine" provides a strategy for contributing a means to achieve global immunization for hepatitis B prevention and eradication.

aly/aly mice: a unique model of biliary disease

An autosomal recessive murine mutation, coined "aly/aly" or "alymphoplasia," was recently reported. Homozygotes for aly are defective in both humoral and cell-mediated immune function and have diffuse lymphoid cell infiltration of various tissues, particularly around the conduit ducts of the pancreas and salivary glands. In pilot studies in our laboratories, aly/aly mice were found to have peculiar biliary tract lesions, which were analyzed histologically and immunohistochemically in the present study. The livers of aly/aly mice older than 8 weeks consistently showed a variable lymphoid cell infiltration with lymph follicle formation in portal tracts; intrahepatic biliary epithelial cells showed various types of damage including pseudopyloric gland metaplasia and proliferative changes. In addition, the extrahepatic bile duct and intrahepatic large bile duct were found to contain an acidophilic substance in their epithelial cytoplasm. In the lumen and occasionally in the cytoplasm of these bile ducts, acidophilic crystals were also seen. Ultrastructurally, the intracytoplasmic acidophilic substances consisted of membrane-bound intracytoplasmic inclusions with homogeneous electron density, likely derived from rough endoplasmic reticulum (ER). Immunohistochemically, the cytoplasmic acidophilic substances were simultaneously positive for cystatin C, gastrin, serotonin, and somatostatin. In contrast, the acidophilic crystals did not react with any of these antibodies. These findings suggest that the intracytoplasmic acidophilic substances may contain a precursor of the peptide hormones, possibly because of defective secretion or intracellular transport. We believe that the aly/aly mouse is a useful model for the analysis of biliary metabolic events, and for studies of the interaction of the immune system and biliary destruction.

Formation of poliomyelitis subviral particles in the yeast Saccharomyces cerevisiae

The sequence of the poliovirus genome encoding 3CD (a protease) was transferred to the yeast Saccharomyces cerevisiae on expression vectors with either a constitutive or an inducible promoter. Transformants could only be obtained with vectors carrying the inducible transcription unit. Extracts of induced cells were able to cleave cell-free synthesized P1, the precursor of the poliovirus capsid proteins, into VP0, VP3 and VP1. In yeast cells constitutively expressing P1, induction of 3CD expression resulted in only trace amounts of processed products. Processing could be improved considerably by simultaneous induction of both P1 and 3CD expression. Analysis of extracts of such induced cells revealed the presence of particles that resembled authentic subviral particles.

Cloning and growth-regulated expression of the gene encoding the hepatitis B virus middle surface antigen in Yarrowia lipolytica

Expression of the gene encoding the hepatitis B virus middle surface antigen (pre-HBsAg) in the yeast Yarrowia lipolytica has been studied. The preS2-HBsAg gene was expressed from the alkaline extracellular protease-encoding gene (XPR2) promoter. In the fusion construct, the membrane-spanning 'a' domain of preS2-HBsAg has been replaced by the leader peptide and the proI region of the alkaline protease, thus eliminating the epitope responsible for the immune escape mechanism. Expression has been found to be growth-stage dependent with the highest protein accumulation during the stationary phase, accounting for around 2.35% of the total soluble intracellular proteins. The produced protein was assembled into Dane particles and was immunogenic in mice. The expression vector was found to be stable for at least 100 generations under non-selective conditions.