Expression of Amplified Hepatitis B Virus Surface Antigen Genes in Chinese Hamster Ovary Cells

Enhanced cell disruption strategy in the release of recombinant hepatitis B surface antigen from Pichia pastoris using response surface methodology

BACKGROUND

Cell disruption strategies by high pressure homogenizer for the release of recombinant Hepatitis B surface antigen (HBsAg) from Pichia pastoris expression cells were optimized using response surface methodology (RSM) based on the central composite design (CCD). The factors studied include number of passes, biomass concentration and pulse pressure. Polynomial models were used to correlate the above mentioned factors to project the cell disruption capability and specific protein release of HBsAg from P. pastoris cells.

RESULTS

The proposed cell disruption strategy consisted of a number of passes set at 20 times, biomass concentration of 7.70 g/L of dry cell weight (DCW) and pulse pressure at 1,029 bar. The optimized cell disruption strategy was shown to increase cell disruption efficiency by 2-fold and 4-fold for specific protein release of HBsAg when compared to glass bead method yielding 75.68% cell disruption rate (CDR) and HBsAg concentration of 29.20 mg/L respectively.

CONCLUSIONS

The model equation generated from RSM on cell disruption of P. pastoris was found adequate to determine the significant factors and its interactions among the process variables and the optimum conditions in releasing HBsAg when validated against a glass bead cell disruption method. The findings from the study can open up a promising strategy for better recovery of HBsAg recombinant protein during downstream processing.

A mechanistic understanding of production instability in CHO cell lines expressing recombinant monoclonal antibodies

One of the most significant problems in industrial bioprocessing of recombinant proteins using engineered mammalian cells is the phenomenon of cell line instability, where a production cell line suffers a loss of specific productivity (qP). This phenomenon occurs with unpredictable kinetics and has been widely observed in Chinese hamster ovary (CHO) cell lines and with all commonly used gene expression systems. The underlying causes (both genetic and physiological) and the precise molecular mechanisms underpinning cell line instability have yet to be fully elucidated, although recombinant gene silencing and loss of recombinant gene copies have been shown to cause qP loss. In this work we have investigated the molecular mechanisms underpinning qP instability over long-term sub-culture in CHO cell lines producing recombinant IgG1 and IgG2 monoclonal antibodies (Mab's). We demonstrate that production instability derives from two primary mechanisms: (i) epigenetic--methylation-induced transcriptional silencing of the CMV promoter driving Mab gene transcription and (ii) genetic--progressive loss of recombinant Mab gene copies in a proliferating CHO cell population. We suggest that qP decline resulting from loss of recombinant genes is a consequence of the inherent genetic instability of recombinant CHO cell lines.

Production of recombinant proteins in mammalian cells

The best strategy for consistent production of larger quantities of pure protein is stable expression. Popular hosts for stable expression are Chinese hamster ovary (CHO) cells, baby hamster kidney (BHK-21) cells, myeloma cells, and the transformed kidney cell line 293. Protocols for stable production in CHO cells are described in this unit. Typical methods for transfection using commercially available plasmid expression vectors are described, along with methods to select for stable expression and methods for amplifying the expression level in the transfected cell. Following this, procedures are presented for efficient cell growth to obtain significant amounts of protein product. Support protocols describe freezing of cells, determination of growth rates, determination of specific productivity of cells, preparing samples for assay, and setting up a 10-day shaker-flask growth curve.

Characterisation of recombinant CHO cell lines by investigation of protein productivities and genetic parameters

We have generated a recombinant CHO cell line expressing the fusion protein EpoFc. After selection and screening, protein expression, gene and mRNA copy numbers were analysed in order to gain more information on the influence of genetic parameters on the productivity and stability of production cells. Results from semi-quantitative blot methods were compared to quantitative PCR (qPCR) analyses, whose advantage mainly lies in their higher sensitivity, and the cheaper and faster methodology. We developed stable and high producing clones with low gene copy numbers, in contrast to other cell lines where multiple steps of methotrexate amplification have lead to hundreds of copies of inserts with the risk of karyotypic instabilities and decreased growth rates that overcome the benefits of increased productivities. When comparing genetic parameters to productivity, a good correlation of mRNA levels with specific productivity was observed, whereas high gene copy numbers were not always accompanied by high protein expressions. Based on our data derived from a typical example of a cell line development process, genetic parameters are useful tools for the selection of scalable production clones. Nevertheless, a wider range of cell lines has to be investigated in order to implement genetic analyses into a screening process.

Cloning and expression of the human tumor-specific antibody GM4

The human monoclonal antibody GM4 was generated by fusing pooled lymphocytes from cancer patients with the lymphoblastoid cell line SHFP-1. Immunohistochemical staining of tumor and normal tissue indicated that this human IgG4 antibody preferentially reacted with melanomas and neuroblastomas. In this study, we demonstrate that GM4 recognizes a "vimentin-like" peptide sequence that we have termed AgGM4. To generate a recombinant derivative of this human antibody, we isolated and expressed the complete heavy and light chain genes. The entire coding sequence for both the heavy and light chains was isolated by RT-PCR using a set of degenerate 5' signal sequence specific primers and a 3' constant region primer. High level antibody synthesis and secretion was achieved in Chinese hamster ovary (CHO) cells using a vector designed to maximize expression. Western blot and FACS analysis indicated recombinant GM4 reacted with human tumor cell lines and AgGM4 in a manner similar to the antibody produced by the hybridoma cell line, demonstrating that the specificity of the antibody was not altered during molecular cloning.

High-copy expression vector based on amplification-promoting sequences

We describe a new vector system that allows efficient expression of heterologous proteins in transformed mouse L fibroblasts. This is due to its persistence at high copy numbers, achieved by a 370-bp amplification promoting element (muNTS1) derived from the nontranscribed spacer of murine rDNA. Copy number determination showed that this sequence mediates a 40- to 800-fold amplification of the vector DNA in transfected L cells. High copy number was accompanied by increased expression levels of the reporter gene secreted alkaline phosphatase (SEAP). Analyzing the structural organization of multicopy plasmid DNA in mouse L cells revealed that plasmid DNA is integrated as reiterated head-to-tail concatamers into the chromosomal DNA. The vector described here can be used as a versatile high-copy expression system for heterologous proteins overcoming any limitation to enzyme-deficient cell lines.

Safety and economic aspects of continuous mammalian cell culture

Mammalian cell cultures are the most appropriate host cells for recombinant DNA derived products if complex protein structures have to be synthesized in their native form. Due to their physiological behaviour they grow either adherent or in suspension. For the attachment of adherent cells, microcarriers or wire springs can be applied to increase the internal surface of the bioreactor. Both systems provide a simplified media exchange but, however, show some limitations in scale up. In contrast, suspension culture systems as homogeneous systems independent of any carrier have not shown any limitation in scale up. Because most cell lines which are of commercial interest grow in suspension, this technology is best advanced and used in batch and continuous mode. Although mammalian cell cultures are sensitive to hydrodynamic shear forces, technologies for deep tank production are developed which allow stirrer tip speed of up to 1.5 m s-1 sufficient for oxygen uptake, suspension of cells and homogeneous supply with nutrients. For long term bioprocesses without selection pressure it has to be considered that transformed cell lines might show genetic instability due to their variations of chromosomes. In addition, sterile technology becomes an important factor in long term bioprocesses. The decision as to which cell culture system should be chosen, whether batch or continuous processes should be applied essentially is based on the capital investment, the amount of material to be produced, genetic stability of the production cell line, reliability of sterile technology and the flexibility required in the production plant. Under the assumption that 20 kg of a protein have to be produced per year and the same product concentrations in the harvest fluid are reached in the batch process and for instance in the chemostat, it can be considered that the capital investment for one 10,000 l batch process and a 2 x 2,000 l continuous process, necessary to produce the amount of material, is comparable. Risk of microbial contamination or technical failure can be considered to be fairly low in the batch process. The economic risk for long term bioprocess in the chemostat can be expected to be medium and high in the perfusion system which is in the large scale not technically fully satisfactory. In addition, due to the longer down time period after contaminations and the start up of the continuous process, the annual yield of the batch process can be considered to be higher.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth behavior of Chinese hamster ovary cells in a compact loop bioreactor: 1. Effects of physical and chemical environments

Chinese hamster ovary (CHO) cells were cultivated in a compact loop bioreactor using MEM-alpha medium supplemented with 10% fetal calf serum. Effects of physical and chemical environments, i.e., pH in the medium, stirring speed of impellers, temperature and partial pressure of oxygen (pO2) upon growth of suspended cells in the bioreactor were determined in batch cultures. Growth behavior was characterized by specific rates of growth (mu), glucose consumption (qG) and lactate production (qL), and the yield coefficients (cell yield from glucose, YX/G, and lactate yield from glucose, YL/G). An effect of medium osmolality was also evaluated with T-flask monolayer cultivation. The best growth was observed at pH 7.6, 37 degrees C, 400 rpm, 50-100% saturation with oxygen and 320 mOsmol kg-1. Corresponding to the previous work with a human melanoma cell line, the sophisticated cultivation and process control systems have been improved for CHO cells.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

Amplification of multicistronic plasmids in the human 293 cell line and secretion of correctly processed recombinant human protein C

We have constructed multicistronic vectors containing the cDNAs for murine dihydrofolate reductase (DHFR), hygromycin phosphotransferase (HyPR), and human protein C (HPC), an antithrombotic factor. Using a sequential selection protocol with hygromycin (Hy) and methotrexate (MTX), we demonstrate the selective amplification of the murine dhfr cDNA in the adenovirus-transformed human kidney cell line 293, and the coamplification of the cDNA for HPC. Such recombinant 293 cell lines secreted HPC at levels as high as 25 micrograms/10(6) cells/day. In addition, we found that the complex vitamin K-dependent posttranslational modification of gamma-carboxylation of glutamate was not limiting at these high secretion levels, although the proteolytic processing of the protein was slightly reduced. Further, the HPC secreted from the gene-amplified cell lines had full anticoagulant activity when compared to plasma-derived HPC.

Expression of hepatitis B virus surface antigen containing the pre-S region in mammalian cell culture system

The surface antigen of hepatitis B virus (HBV) has been expressed in a mouse cell line using metallothionein-bovine papilloma virus vectors. Four different recombinant plasmids were used and it was found that the expression level varies significantly from one plasmid to another. Removing the HBV polyadenylation signal from the plasmid drastically reduced the expression level. Providing even a heterologous polyadenylation signal improved the expression level from the reduced one by at least tenfold. The steady-state level of cytoplasmic HBV-specific RNA was much higher when the polyadenylation signal was present. One of the constructs gave a very high expression level and has been characterized further. It was possible to detect any plasmid in the episomal form in the producing clones and all the HBV DNA was found to be integrated in the mouse chromosome at more than one site. The copy number of the plasmid varied greatly between different clones and in the best one it is approximately 150 copies per haploid genome. Cells can be grown continuously in the presence of cadmium chloride without splitting for more than two months and the excreted surface antigen can be harvested during this period. Western blot analysis showed that the antigen contains the pre-S region.

A new expression system for mammalian cells based on putative replicator sequences of the mouse and a truncated thymidine kinase gene

We have constructed a new expression vector for mammalian cells. The vector contains a truncated tk gene for amplification under selective conditions, a sequence putatively supporting the replication of plasmid DNA in eukaryotic cells (murine autonomously replicating sequence) and an expression cassette for the cDNA to be studied. As a model cDNA we have used that of human tissue-type plasminogen activator (t-PA). Analysis of Hirt supernatants and chromosomal DNA from L cells, prepared six weeks after isolation of the clones indicated a 50- to 500-fold amplification of the expression construct in the cells. Concomitantly, the expression of t-PA was dramatically increased. Our data are consistent with episomal persistence of the expression construct, with a head-to-tail mode of integration into the mouse genome and with coexistence of both episomal plasmids and head-to-tail integrates. In tk-deficient cell lines other then L-cells, such as mouse mastocytoma or rat hepatoma cells, a strong selection against the persistence of the expression construct was noted. After long-term propagation of the L-cells under selective conditions the expression of the indicator gene continually decreases, but finally a constant plateau level of expression is established. Expression could be restored to the original level by blocking more efficiently the de novo synthesis of nucleosides.

Recombinant versus plasma-derived hepatitis B vaccines: issues of safety, immunogenicity and cost-effectiveness

Any attempt to reduce the incidence of hepatitis B on a worldwide scale requires the availability of large quantities of potent, safe and affordable hepatitis B vaccine. However, ongoing doubts or concerns--justified or not--persist about the comparative safety, immunogenicity and cost-effectiveness of commercially available hepatitis B vaccines, whether derived from plasma or produced via recombinant expression systems. This review compares plasma versus recombinant hepatitis B vaccines in terms of these alleged differences and in the light of increasing clinical data acquired following administration of recombinant yeast-derived hepatitis B vaccines.

Amplified expression constructs for human tissue-type plasminogen activator in Chinese hamster ovary cells: instability in the absence of selective pressure

By linking an expression cassette for human tissue-type plasminogen activator (t-PA) to an amplifiable marker gene, its introduction into Chinese hamster ovary dhfr- cells and subsequent amplification with methotrexate, we have generated cell lines that overproduce the heterologous protein and contain 300-1100 copies of the expression constructs integrated into the hamster genome. We present a detailed investigation of the fate of amplified sequences in the presence and absence of selective pressure by parallel examination of three producer cell lines with respect to relevant parameters. These include the determination of t-PA production upon continuous propagation in culture, the genomic organization of the integrated expression constructs by Southern blotting, and the localization of homogeneously staining regions by in-situ hybridization with biotinylated probes and visualization by interference reflection microscopy. We conclude that in the three cell lines examined, the decrease in production of t-PA in the absence of methotrexate selection is accompanied by decreases in the number of integrated expression constructs and the size of the amplified regions, whereas all these parameters are stable when selective pressure is maintained. The instability is probably due to the head-to-tail mode of integration of the expression constructs in the hamster genome, which increases the frequency of homologous recombination between the integrated plasmids in recombination-proficient cells in the absence of selective pressure.

Amplifiable expression vectors for mammalian cell lines

We have constructed a prototype of a general expression vector for mammalian cells, which carriers a strong promoter active in a variety of tissues and animal species for expression of the gene of interest and a truncated gene for amplification in selective medium. Expression of the S gene of HBV encoding the surface antigen was used to evaluate various versions of this vector. The human metallothionein IIA promoter was found to be particularly efficient for expression of this gene, both in mouse L and monkey Vero cells. Including a promoterless tk gene in the vector led to gene amplification in Ltk- cells by selection of TK+ variants in selective medium containing hypoxanthine, aminopterine and thymidine (HAT medium). Concomitant increases of the S gene expression levels were initially 50-100 fold. Although many clones were unstable, even in selective medium, some maintained the high expression levels for over one year.

A recombinant Chinese hamster ovary cell line containing a 300-fold amplified tetramer of the hepatitis B genome together with a double selection marker expresses high levels of viral protein

A new series of double-selection plasmids containing recombinant genes expressing the neomycin phosphotransferase (NEO) of transposon Tn5 and mouse dihydrofolate reductase (DHFR) in mammalian cells is described. Activity of the recombinant DHFR gene varied more than 50-fold, depending on the location of the simian virus 40 72 base-pair repeat or enhancer, which is part of the promoter of the NEO unit. A NEO-DHFR module with the enhancer located at the 3' end of the DHFR gene was inserted into a plasmid containing four tandem head-to-tail copies of the hepatitis B virus (HBV) genome and the new plasmid was used to transform DHFR- Chinese hamster ovary cells. In one of the cell lines obtained, an unrearranged copy of the HBV tetramer could be amplified 300-fold by increasing selective pressure with methotrexate, resulting in a proportional increase of the synthesis of HBV surface antigen. Four different mRNAs detected in the amplified cell line probably encode HBV core protein, pre-S and surface antigens, and the X protein. As a result of the DNA amplification, synthesis of HBV proteins is no longer restricted to resting cells. Integrated plasmid sequences appear to be stable during the amplification process.