Inducible expression of amplified human beta interferon genes in CHO cells

Pharmacokinetic considerations in the treatment of multiple sclerosis with interferon-β

INTRODUCTION

Interferon-β (IFNβ) is well established as a disease-modifying treatment for patients with multiple sclerosis. Several preparations of the biopharmaceutical are available differing in protein structure, formulation, dose as well as frequency and route of administration. Recently, a pegylated form of IFNβ has been marketed.

AREAS COVERED

Following a PubMed database search, we provide an overview of what is presently known about the pharmacokinetics (PK) of IFNβ including its absorption, distribution, metabolism and elimination. Also, we discuss the association with clinically relevant issues such as treatment efficacy, adverse events and anti-drug antibodies.

EXPERT OPINION

IFNβ has a bioavailability of ∼ 30% after subcutaneous or intramuscular administration, shows peak serum concentrations within several hours, has a half-life of < 1 day and is eliminated by a renal and hepatic pathway. PK parameters do not substantially differ between the types of IFNβ and routes of administration; only pegylation of IFNβ results in substantially increased and prolonged PK. Although no clinical dose-effect relationship could be established, there is an association of IFNβ dose with magnetic resonance imaging outcome parameters. Furthermore, there is an association of IFNβ serum levels with the occurrence of adverse events and anti-drug antibodies.

Establishment of a stable CHO cell line with high level expression of recombinant porcine IFN-β

A CHO cell clone (CHO-PoIFN-β) with stable porcine IFN-β expression under control of CMV promoter was selected under G418 pressure. In a 25 cm² cell culture flask (5 ml culture medium), the cumulative protein yield of recombinant PoIFN-β reached 2.3 × 10⁶ IU/ml. This cells clone maintained stable expression for at least 20 generations even in the absence of G418 selection pressure. The expressed recombinant PoIFN-β could induce the expression of porcine Mx protein in PK15 cells, and activate the chicken Mx promoter-controlled luciferase reporter gene expression, confirming that the recombinant PoIFN-β has the biological activity of natural porcine type-I interferon. In addition, the recombinant PoIFN-β fully protected PK15 cells against 1000 TCID₅₀ of porcine transmissible gastroenteritis virus and pseudo-rabies virus infection, demonstrating its high potential in therapeutic applications. This is the first report of establishing a mammalian cell line with stable expression of porcine IFN-β.

25 years of recombinant proteins from reactor-grown cells - where do we go from here?

The purpose of this review is to describe the current status and to highlight several emerging trends in the manufacture of recombinant therapeutic proteins in cultivated mammalian cells, focusing on Chinese hamster ovary cells as the major production host. Over the past 25 years, specific and volumetric productivities for recombinant cell lines have increased about 20-fold as the result of improvements in media and bioprocess design. Future yield increases are expected to come from further developments in gene delivery and genetic selection for more efficient recovery of high-producing cell lines and in high-throughput cultivation systems to simplify medium design and bioprocess development. Other emerging trends in protein manufacturing that are discussed include the use of disposal bioreactors and transient gene expression. We specifically highlight current research in our own laboratories.

Cloning, high expression and purification of recombinant human intereferon-beta-1b in Escherichia coli

Sequential evaluation and process control strategy were employed for impurity profile and high recovery with quality of rhIFN-beta-1b expressed in Escherichia coli. The high-level expression was achieved by using codon substitution (AT content of 52.6% at N-terminal region) and optimization of culture conditions. The addition of rifampicin at a concentration of 200 microg/ml has increased the specific product yield of 66 mg optical density(-1) l(-1) (43.5% of total cellular protein). Eighty-three percent of lipopolysaccharides, 32% of host deoxyribonucleic acid (DNA), and 78% of host cell proteins were removed by 0.75% Triton X-100 and 2 M urea wash. Eleven percent of lipopolysaccharides, 39% of host DNA, and 12% of host cell proteins were removed at the solubilization step. Ninety-two percent of protein refolding was achieved by high-pressure diafiltration method. Refolding by high-pressure diafiltration, bed height, and height equivalent to the theoretical plate value in chromatography column were identified as key parameters for high recovery with purity. Finally, the established process yielded 34% of purified protein with greater than 99% purity and is acceptable for preclinical toxicological studies. The purified rhIFN-beta-1b obtained in this study is the highest that has been reported so far.

Bioprocess applications of a Sindbis virus-based temperature-inducible expression system

The production and study of toxic proteins requires inducible expression systems with low basal level expression and high inducibility. Here, we describe bioprocess applications of the pCytTS temperature-regulatable Sindbis virus replicon-based expression system. We used green fluorescent protein as a marker protein to optimize the selection of stable transfected clones with increased expression levels. Using the optimized protocol, clones were constructed that produced the growth-inhibiting, anti-viral protein interferon beta (beta-IFN). Selected clones were analyzed for temperature-dependent beta-IFN production in adherent and suspension cultures in serum free medium. Specific expression levels were around 1.0 x 10(5) IU/10(6) cells/day (0.5 microg/10(6) cells/day) in suspension cultures and over 1.5 x 10(6) IU/mL/day (7.5 microg/mL/day) in hollow fiber reactors using adherent cells. Hexahistidine-tagged beta-IFN purified from T-flask cultures was highly glycosylated and showed high specific activity. beta-IFN mRNA amplified by the viral replicase for 10 days did not show an accumulation of mutations. These data suggest the applicability of the pCytTS-inducible expression system for the production of high-quality glycoproteins in different reactors.

Expression of recombinant human granulocyte colony-stimulating factor in CHO dhfr- cells: new insights into the in vitro amplification expression system

The in vitro amplification method for heterologous gene expression in mammalian cells is based on the stable transfection of cells with long, linear DNA molecules having several copies of complete expression units, coding for the gene of interest, linked to one terminal unit, coding for the selectable marker. DNA concatenamers containing additional expression units can also be prepared: we exploited this feature by co-polymerizing expression units coding for granulocyte colony-stimulating factor (G-CSF) with cassettes for dihydrofolate reductase (DHFR) and for neomycin (Nm) resistance, as selectable markers. We were thus able to obtain high level production of G-CSF in chinese hamster ovary (CHO) dhfr- cells by combining in vitro amplification to just one step of in vivo amplification. This approach required a considerably shorter time than the classical, stepwise amplification by methotrexate.

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.

High-level expression of recombinant human soluble thrombomodulin in serum-free medium by CHO-K1 cells

Cultivation of gene-engineered Chinese hamster ovary (CHO-K1) cells that produce recombinant human soluble thrombomodulin (rsTM) was investigated to optimize conditions for high-level expression of the protein in a serum-free medium. For economic protein production, oxygenation of cultures with pure O2 permitted sufficient cell growth for high rsTM production with only 1 g/l of microcarriers and a low foetal bovine serum concentration. A longer growth phase (over 5 days) with serum was important to establish sufficient growth of this cell line on the microcarriers for subsequent serum-free culture, and to support a long-term production phase (about 2 months). In the production phase, a high glucose concentration (6.15 g/l) in the serum-free medium was very effective for prolonging the harvest cycle interval. Under these conditions, up to 100 mg/l rsTM was expressed in the conditioned medium. The rates of glucose consumption (G) and lactate production (L) were measured periodically and their ratio (L/G ratio) correlated with rsTM productivity. When the average L/G ratio was lower, reflecting a lower lactate production rate due to appropriate oxygenation of the culture, the specific rsTM production rate increased. Thus it may be possible to estimate protein productivity from L/G ratios calculated from the glucose and lactate measurements.

Quantitative mapping of the N-linked sialyloligosaccharides of recombinant erythropoietin: combination of direct high-performance anion-exchange chromatography and 2-aminopyridine derivatization

A rapid quantitative analysis of the sialylated N-linked oligosaccharides of recombinant erythropoietin (EPO) expressed in Chinese hamster ovary (CHO) cells has been developed. The procedure utilizes a glycoamidase (glycopeptidase F) to release all of the N-linked oligosaccharides from the native glycoprotein, followed by direct chromatographic analysis using high-performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection. The eight sialyloligosaccharides isolated from HPAEC were characterized by derivatizing with 2-aminopyridine followed by two-dimensional HPLC mapping of the pyridylaminated asialooligosaccharides (Tomiya et al., 1988, Anal. Biochem. 171, 73-90). Seven kinds of complex-type asialooligosaccharides were identified ranging from a biantennary structure to N-acetyllactosamine-extended tetraantennary structure. Approximately 3% of the terminal galactose residues of the oligosaccharides released from EPO were not sialylated whereas 97% contained an alpha(2-->3)-linked sialic acid. Quantitative oligosaccharide mapping of four different lots of EPO from CHO cells was performed to quantify the molar balance and distribution of the N-linked oligosaccharides. The sialyloligosaccharides were distributed with approximately 5% disialylated (single type), 20% trisialylated (six types), and 75% tetrasialylated (four types) oligosaccharides with an average molar recovery of 85% starting from 750 pmol of EPO.

Site-directed mutagenesis of an apolipoprotein E mutant, apo E5(Glu3----Lys) and its binding to low density lipoprotein receptors

Apo E5(Glu3----Lys) is a naturally occurring apolipoprotein E (apo E) mutant found in patients with hyperlipoproteinemia and atherosclerosis. It has been shown to have a high affinity for low density lipoprotein (LDL) receptors. In this study, mutant apo E5 was produced by Chinese hamster ovary cells by means of an in vitro site-directed mutagenesis technique, and its LDL receptor binding activity was assessed. The apo E5 obtained from gene expression bound more readily to the LDL receptor than did plasma apo E3. The concentrations required for 50% competitive binding of 125I-labeled LDL to the LDL receptors were 58.9 ng/ml for plasma apo E3 and 25.7 ng/ml for the expressed apo E5. The expressed apo E5 displayed 229% normal binding. This result is highly consistent with that obtained with plasma apo E5, which showed 217% normal binding. Although the experimental apo E isoproteins contained more sialic acid than plasma apo E, the extent of sialylation had no effect on the receptor binding of apo E.

Purification of recombinant human interferon-beta by immobilized antisense peptides

Synthetic antisense peptides encoded in the antisense strands of DNA corresponding to the 1-14, 42-54 and 103-115 fragments of the human interferon-beta sequence were applied in the purification of recombinant human interferon-beta from a mammalian cell culture. The protein fragments were selected on the basis of their computer-predicted exposure on the surface of the protein. The antisense peptides were synthetized by the solid-phase method directly on the resin used as the stationary phase in affinity chromatography. All the tested antisense peptides showed a selective affinity for human interferon-beta, permitting a ten-fold purification of the protein.

Generation of recombinant CHO(dhfr-) cell lines by single selection for dhfr+ transformants

In order to establish a mammalian cell expression system with a minimum of selection steps and a stable expression of microgram amounts of recombinant protein (human tissue-type plasminogen activator mutants and chimeric proteins) per 10(6) cells per day, we investigated Chinese hamster ovary cells and the dihydrofolate reductase-deficient Chinese hamster ovary cell line CHO(dhfr-). The 1tPA expression vector pCMVtPA was cotransfected either with the SV40 enhancer sequence containing dhfr expression vector pMT2 or with the enhancerless dhfr expression vector pAdD26SV(A) into CHO(dhfr-) cells. With both dhfr expression plasmids, selection for dhfr+ transformants followed by single dilution cloning was sufficient to generate cell lines with a production level of up to 4.6 micrograms tPA/10(6) cells.day. This approach is useful if gene amplification procedures are time-consuming and impracticable because of a large number of recombinant proteins. In order to establish CHO cell lines with a tPA expression level as high as that in the case of CHO(dhfr-) cells, repeated dilution cloning is necessary.

Tunicamycin treatment of CHO cells abrogates multiple blocks to retrovirus infection, one of which is due to a secreted inhibitor

Chinese hamster ovary (CHO) cells are resistant to infection by all of the major classes of murine retroviruses and are partially resistant to infection by gibbon ape leukemia virus. Treatment of CHO cells with the glycosylation inhibitor tunicamycin rendered these cells susceptible to infection by retroviral vectors with ecotropic, xenotropic, and amphotropic host ranges and increased the titer of gibbon ape leukemia virus pseudotyped vectors 10-fold. Vectors having a polytropic host range did not infect CHO cells in the presence or absence of tunicamycin, showing that the effect of tunicamycin was specific and related to the pseudotype of the vector. We present evidence for three mechanisms of resistance to infection: lack of viral receptors on CHO cells, the presence of nonfunctional receptors which can be made functional by treatment with tunicamycin, and the secretion of a protein factor that blocks retroviral infection of CHO cells. Several criteria indicate that the secreted inhibitor is not an interferon, and secretion of this factor was not detected in several other cell lines that were examined.

Expression of a human monoclonal anti-HIV-1 antibody in CHO cells

The cDNA coding for the light and heavy chains, respectively, of the human monoclonal antibody 3D6 (IgG1, kappa), which binds specifically to human immunodeficiency virus-1 (HIV-1) gp41, was inserted into three different mammalian expression vectors and transfected into Chinese hamster ovary (CHO) cells. Transcription was under the control of Rous sarcoma virus long terminal repeat (RSV LTR), human cytomegalovirus major immediate early (CMV IE) promoter, and mouse mammary tumor virus long terminal repeat (MMTV LTR), respectively. Antibody productivity was monitored in the supernatants of selected clones. The binding characteristics of the CHO-derived antibody to HIV-1 gp41 were found to be identical to that of the original antibody produced by hybridoma cells.

Efficient selection for high-expression transfectants with a novel eukaryotic vector

We have developed a new expression vector which allows efficient selection for transfectants that express foreign genes at high levels. The vector is composed of a ubiquitously strong promoter based on the beta-actin promoter, a 69% subregion of the bovine papilloma virus genome, and a mutant neomycin phosphotransferase II-encoding gene driven by a weak promoter, which confers only marginal resistance to G418. Thus, high concentrations of G418 (approx. 800 micrograms/ml) effectively select for transfectants containing a high vector copy number (greater than 300). We tested this system by producing human interleukin-2 (IL-2) in L cells and Chinese hamster ovary (CHO) cells, and the results showed that high concentrations of G418 efficiently yielded L cell and CHO cell transfectants stably producing IL-2 at levels comparable with those previously attained using gene amplification. The vector sequences were found to have integrated into the host chromosome, and were stably maintained in the transfectants for several months.

Production of recombinant protein C in serum-containing and serum-free perfusion culture

For the development of a perfusion culture producing recombinant human protein C, the effects of fetal calf serum and growth factors on cell growth and recombinant protein production were investigated. Although the growth of recombinant cells was stimulated by serum in a dose-dependent manner, a lower concentration of serum (2%) could support both synthesis and post-translational modification of protein C as efficiently as 10% serum. Among the growth factors tested, transferrin enhanced protein C production to the level comparable with 10% serum, while insulin was effective in maintaining cellular metabolism. Based on these results, a perfusion culture for a scale-up production of recombinant protein C was done using an Opticell culture system. A good productivity of the recombinant protein was obtained in low serum or serum-free medium for more than one month.

Posttranscriptional regulation of beta interferon expression in erythroid Friend cells treated with gamma interferon

Treatment of Friend erythroleukemia cells (FLC) with gamma interferon (IFN-gamma) in the presence of anti-IFN-beta antibodies reduces the effectiveness of the antiviral state and the induction of 2'-5'-oligoadenylate synthetase activity, indicating that the antiviral activity of IFN-gamma in FLC is in part mediated by the production of IFN-beta. Accordingly, IFN-gamma induces a less pronounced antiviral state in FLC resistant to IFN-alpha/beta than in wild-type cells. Moreover, while results of run-on assays indicate that both IFN-alpha and -beta genes are constitutively transcribed in these cells, FLC treatment with IFN-gamma induces only IFN-beta mRNA accumulation. These results indicate that posttranscriptional mechanisms are involved in the regulation of IFN-beta and -alpha expression by IFN-gamma. The low amounts of the induced IFN-beta synergize with IFN-gamma in mounting the potent antiviral effect.

Chinese hamster ovary cell growth and interferon production kinetics in stirred batch culture

Recombinant human interferon-gamma production by Chinese hamster ovary cells was restricted to the growth phase of batch cultures in serum-free medium. The specific interferon production rate was highest during the initial period of exponential growth but declined subsequently in parallel with specific growth rate. This decline in specific growth rate and interferon productivity was associated with a decline in specific metabolic activity as determined by the rate of glucose uptake and the rates of lactate and ammonia production. The ammonia and lactate concentrations that had accumulated by the end of the batch culture were not inhibitory to growth. Glucose was exhausted by the end of the growth phase but increased glucose concentrations did not improve the cell yield or interferon production kinetics. Analysis of amino acid metabolism showed that glutamine and asparagine were exhausted by the end of the growth phase, but supplementation of these amino acids did not improve either cell or product yields. When glutamine was omitted from the growth medium there was no cell proliferation but interferon production occurred, suggesting that recombinant protein production can be uncoupled from cell proliferation.

Production of recombinant proteins in serum-free media

The advantages of serum-free culture for the manufacture of recombinant biopharmaceuticals from mammalian cells are reviewed. The process favoured is fed-batch serum-free cell culture. This process is applicable to the majority of cell lines, is practical on the large scale, gives the lowest manufacturing cost, and can be carried out without the use of any serum.

Spin filter perfusion system for high density cell culture: production of recombinant urinary type plasminogen activator in CHO cells

We have used a 20 liter stirred tank fermentor, equipped with a 127 mesh ethylene-tetrafluoroethylene rotating screen for cell recycle, for the continuous production of recombinant single chain urokinase-type plasminogen activator (rscu-PA) from Chinese hamster ovary (CHO) cells. Viable cell densities between 60 and 74 million per ml were maintained at medium perfusion rates of 3.0 to 4.0 fermentor volumes per day. Cells were retained by the 120 micron nominal opening filter through the formation of "clumped" cell aggregates of 200 to 600 microns in size, which did not foul the filter. In 31 days of culture, a total of 51 grams of rscu-PA were produced in 1,000 liters of medium. The rscu-PA produced over the course of this continuous culture was purified and characterized both in vitro and in vivo and shown to be comparable to natural scu-PA produced from the transformed human kidney cell line, TCL-598.

Efficient expression of human beta-interferon in Namalwa KJM-1 cells adapted to serum-free medium by a dhfr gene coamplification method

We previously reported the expression of human beta-interferon (beta-IFN) (Miyaji et al., 1989) and human lymphotoxin (Miyaji et al., 1990) in Namalwa KJM-1 cells adapted to serum-free medium. To establish an efficient gene expression system, a dihydrofolate reductase (dhfr) gene coamplification method was applied to this cell line. A beta-IFN expression plasmid was introduced with a dhfr expression plasmid into KJM-1 and methotrexate (MTX)-resistant derivatives were selected by a stepwise increase of MTX concentration. Among them, derivatives which showed higher expression levels of beta-IFN than that achieved by the parental transformants were obtained, suggesting that a dhfr gene coamplification method can be used for efficient expression of foreign genes in KJM-1 which contains endogenous dhfr genes. Then, an improved beta-IFN expression vector was constructed, which contains a dhfr transcription unit. This plasmid was introduced into KJM-1 and then, MTX-resistant derivatives were selected. Among them, the highest producer, clone 40-10-24, secreted beta-IFN at a level as high as 5 micrograms/ml, which is about 100-fold higher than that obtained by the G418-resistant parental transformants. In addition, beta-IFN produced by recombinant KJM-1 cells had the same molecular weight of that produced by fibroblasts.

Introduction of stable high-copy-number DNA into Chinese hamster ovary cells by electroporation

A new gene transfer protocol has been developed that introduces up to 800 copies of an expression vector into Chinese hamster ovary cells in a single step by electroporation. The DNA typically integrates in tandem repeats so that the restriction endonuclease site used to linearize the input DNA remains intact. This is likely due to ligation of vector DNA via cohesive ends prior to integration. This high-copy-number procedure is far more rapid than the conventional stepwise gene amplification method used to generate stable eukaryotic protein production cell lines. By employing the expression vector pJODtPA, in which the selectable marker dihydrofolate reductase (DHFR) and the human tissue plasminogen activator (tPA) casettes are separated by a spacer and an RNA polymerase II terminator, cell lines secreting as much as 24 pg/cell.day tPA were isolated following electroporation and a single methotrexate selection. Gene copies and expression levels are stable over long periods of growth. A single round of gene amplification was performed following the high-copy-number procedure to yield a clone having a tPA production level of 45 pg/cell.day.

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.

Characterization of four different mammalian-cell-derived recombinant human interferon-beta 1s. Identical polypeptides and non-identical carbohydrate moieties compared to natural ones

In order to evaluate the structural identification of various recombinant human interferon-beta 1s, the recombinant proteins were produced in four different mammalian cells (human PC12 and PC8 lung adenocarcinoma cells, Chinese hamster ovary cells and mouse C127 cells) and characterized. Each mammalian-cell-derived recombinant human interferon-beta 1 represented a single band of 23 kDa on sodium dodecyl sulphate/polyacrylamide gel electrophoresis, the same molecular mass as fibroblast-derived natural human interferon-beta 1. Specific activities, amino acid compositions, amino-terminal sequences, peptide maps on C18 reversed-phase high-performance liquid chromatography and circular dichroic spectra of recombinant proteins were in good agreement with natural ones. On the other hand, the patterns of isoelectric focusing were different between mammalian-cell-derived recombinant human interferon-beta 1s and natural human interferon-beta 1. Sugar composition analysis revealed that the recombinant protein from Chinese hamster ovary cells has a similar sugar composition to that of natural protein and the other recombinant proteins have increased amounts of galactose and glucosamine in comparison to the natural protein. Furthermore, there is no galactosamine in the natural protein, while small amounts of galactosamine were detected in the oligosaccharides released from PC8- and C127-derived recombinant proteins by N-glycanase. These results indicate that mammalian-cell-derived recombinant human interferon-beta 1s have identical polypeptides to those of natural human interferon-beta 1 but their carbohydrate moieties, including unusual N-linked oligosaccharides, are individually different from natural ones and depend on the host cell.

Production of interferon-beta upon induction with polyinosinic acid:polycytidylic acid during the cell cycle of human fibroblasts

The production of interferon-beta was examined at various stages of the cell cycle in synchronized and unsynchronized cell populations induced by poly(I):poly(C). Human fibroblasts were synchronized with mitotic detachment and, at different stages of the cell cycle, poly(I):poly(C) was added for induction of interferon-beta. One hour after induction, cell-free medium was collected and assayed for secreted interferon-beta. The cells were then fixed and stained with a DNA-specific fluorochrome, 4',6-diamidino-2-phenylindole (DAPI), for cell cycle analysis by microfluorometry. The data indicated that interferon-beta was produced in every stage examined of the cell cycle. In addition, the level of intracellular interferon-beta was quantitatively measured in single cells of an unsynchronized cell population using a specific antibody. In the same individual cell, DAPI fluorescence intensity was measured for determination of the cell cycle position. The results show that interferon-beta protein can be detected throughout the cell cycle.

Effect of recombinant hybrid human interferon on replication and morphogenesis of HSV-1 in monkey cells

Human recombinant alpha interferon, A/D, significantly reduced the replication and cell fusion induced by herpes simplex virus type 1 in monkey cells. Thin-section electron microscopy of interferon-treated monkey cells showed distinct assembly of nucleocapsids within the nucleus. Analysis of virus-specific proteins by the immunoblot technique confirmed that A/D interferon had no significant effect on the expression of major nucleocapsid proteins, although the expression of glycoproteins B and D was reduced in interferon-treated cells. The possibility of an interferon-induced block at a late stage in virus morphogenesis is discussed.

Constitutive and inducible expression of cloned human interferon-b1 gene in HeLa cells through an episomal eukaryotic vector

An EcoRI fragment (1.83 Kb) of human DNA containing B1-interferon coding sequences was cloned in the episomal eukaryotic vector pBK-TK. The recombinant plasmid was transfected into thymidine kinase-deficient HeLa cells. Thymidine kinase-positive transformants were obtained and the cells were found to express B-interferon both constitutively and after induction with Newcastle Disease Virus or poly(rI)-poly(rC). The recombinant plasmid was present in stably transformed HeLa cells in an episomal state in hundreds of copies per cell.

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.

Interferon-alpha-(IFN) producing CHO cell lines are resistant to the antiproliferative activity of IFN: a correlation with gene expression

CHO cell lines that constitutively produce the murine interferon-alpha (IFN-alpha) subspecies alpha 4 and alpha 6 were constructed. The producer cell lines were protected against viral (vesicular stomatitis virus) infection by the IFN species secreted, but were resistant to the growth inhibitory activity of the IFN species. As compared with alpha 4, the alpha 6 protein displayed a high antiproliferative activity when added to normal CHO cells, which correlates completely with the high antiviral activity of alpha 6 on these cells. Three messenger ribonucleic acid (mRNA) species, which are normally induced in CHO cells by IFN treatment (1-8, 2-5A synthetase, and ISG 15) were constitutively present in CHO producer cell lines. The level of another mRNA (ISG 54), however, was very low in the producer cells as compared with its expression in short-term IFN-treated cells. These data indicate that 1-8, 2-5A synthetase and ISG 15 are not involved in the antigrowth activity of IFN in this system, but rather suggest a function of ISG 54 in this respect.

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.

Reconstitution of functionally active antibody directed against creatine kinase from separately expressed heavy and light chains in non-lymphoid cells

We report here for the first time reconstitution and secretion of functionally active antibody in non-lymphoid cells. Expression vectors for the light and the heavy chain of a monoclonal antibody directed against creatine kinase (EC 2.7.3.2) were introduced into COS and CHO Chinese hamster ovary dhfr- cells. Introduction of the expression vectors separately gave rise to immuno-reactive material in the culture supernatants, but only cotransfection of the expression plasmids resulted in secretion of protein with immuno-reactivity against antibodies directed against mouse heavy and light chains as well as specific antigen-binding affinity, as determined by enzyme-linked immunosorbent assay. Secreted kappa and gamma chains from reconstituted antibody were characterized by immunoadsorption and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In COS cells, reconstituted antibody was transiently secreted; cotransfection of kappa and gamma chain expression plasmids with a dihydrofolate reductase (DHFR)-expression plasmid into CHO dhfr- cells gave rise to stable transformants secreting functionally active antibody.

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.

Vectors for expression and amplification of cDNA in mammalian cells: expression of rat phenylalanine hydroxylase

We have constructed two recombinant plasmid vectors for direct expression and amplification of cDNA in mammalian cells. Each vector carries two dominant selectable markers (the bacterial neo gene and the mouse DHFR gene), a promoter sequence (viral LTR in pAV009/A+, and sheep metallothionein promoter in pMT010/A+), a polyadenylation signal sequence, and a Bam HI site to allow insertion of cDNA. We have used these vectors to prepare recombinant clones for the expression of rat phenylalanine hydroxylase (PH) in LTK- cells. Selection of transformants with neomycin followed by selection of the transformants in methotrexate led to a 30- to 60-fold amplification of the DHFR marker and co-amplification of the PH cDNA, with a corresponding increase in the level of PH mRNA and enzyme polypeptide. The expressed enzyme has a subunit molecular weight of 50,000 which corresponds to the W- allele of rat liver PH. PH activity was detected in the transfected cells by enzymatic measurement of the conversion of [14C]phenylalanine to [14C]tyrosine, and by growth of these cells in a tyrosine-free culture medium. Expression of rat PH in cell culture should facilitate the analysis of the biochemical properties of this enzyme.