Serum-free growth medium for the cultivation of a wide spectrum of mammalian cells in stirred bioreactors

Development of a modified serum-free medium for Vero cell cultures: effects of protein hydrolysates, l-glutamine and SITE liquid media supplement on cell growth

Vero cells have been widely used in the viral vaccine production due to the recommendation of the World Health Organization regarding its safety and non-tumorigenicity. The aim of this study was to describe the development a modified serum-free medium for Vero cell cultures. Two protein hydrolysates (Bacto™ soytone and Bacto™ yeast extract), vitamin C, vitamin B12, SITE liquid media supplement, and recombinant human epidermal growth factor (rEGF) were investigated as serum substitutes. A sequential experiment of fractional factorial and central composite design was applied. A modified serum-free medium obtained (named as SFM01-M) was verified. Contrary to P0, the cell yields obtained at P1, P2, and P3 decreased continuously during the verification experiments indicating that Vero cells could not adapt to SFM01-M as expected according to the empirical mathematical model. To improve cell growth after P0, protein hydrolysates, l-glutamine, and SITE liquid media supplement were further investigated. The results showed that cell yields gradually decreased from P1 to P3 when a fixed concentration of Bacto™ yeast extract (7.0 g/L) combined with various concentrations of Bacto™ soytone (0.1-7.0 g/L) in SFM01-M were used. Similarly, cell yields also gradually decreased from P1 to P3 when a fixed concentration of Bacto™ soytone (7.0 g/L) combined with various concentrations of Bacto™ yeast extract (0.1-7.0 g/L) in SFM01-M were used. However, the combination of Bacto™ soytone at 0.1 g/L and Bacto™ yeast extract at 7.0 g/L or Bacto™ soytone at 7.0 g/L and Bacto™ yeast extract at 0.1 g/L in SFM01-M could give the maximum cell yield at P3 when compared with other combinations. In addition, the addition of SITE liquid media supplement (0.1-2.0% v/v) in SFM01-M in which the concentrations of Bacto™ soytone, Bacto™ yeast extract, and l-glutamine were fixed at 0.1 g/L, 0.1 g/L, and 4.0 mM, respectively, the results showed that the cell yields obtained at P3 were not significantly different. From this study, the optimum concentrations of SFM01-M components were as follows: Bacto™ soytone (0.1 g/L), Bacto™ yeast extract (0.1 g/L), vitamin C (9.719 mg/L), vitamin B12 (0.1725 mg/L), SITE liquid media supplement (0.1-2.0% v/v), rEGF (0.05756 mg/L), l-glutamine (4.0 mM), MEM non-essential amino acids (1.0% v/v), sodium pyruvate (1.0 mM), MEM (9.4 g/L), and sodium hydrogen carbonate (2.2 g/L). However, to evaluate SFM01-M in the long-term subculture of Vero cells, the efficiency of SFM01-M will be further investigated.

Objectives, benefits and challenges of bioreactor systems for the clinical-scale expansion of T lymphocyte cells

Cell therapies based on T cell have gathered interest over the last decades for treatment of cancers, becoming recently the most investigated lineage for clinical trials. Although results of adoptive cell therapies are very promising, obtaining large batches of T cell at clinical scale is still challenging nowadays. We propose here a review study focusing on how bioreactor systems could increase expansion rates of T cell culture specifically towards efficient, reliable and reproducible cell therapies. After describing the specificities of T cell culture, in particular activation, phenotypical characterization and cell density considerations, we detail the main objectives of bioreactors in this context, namely scale-up, GMP-compliance and reduced time and costs. Then, we report recent advances on the different classes of bioreactor systems commonly investigated for non-adherent cell expansion, in comparison with the current "gold standard" of T cell culture (flasks and culture bag). Results obtained with hollow fibres, G-Rex® flasks, Wave bioreactor, multiple-step bioreactors, spinner flasks as well as original homemade designs are discussed to highlight advantages and drawbacks in regards to T cells' specificities. Although there is currently no consensus on an optimal bioreactor, overall, most systems reviewed here can improve T cell culture towards faster, easier and/or cheaper protocols. They also offer strong outlooks towards automation, process control and complete closed systems, which could be mandatory developments for a massive clinical breakthrough. However, proper controls are sometimes lacking to conclude clearly on the features leading to the progresses regarding cell expansion, and the field could benefit from process engineering methods, such as quality by design, to perform multi parameters studies and face these challenges.

Albumin promotes proliferation of G1 arrested serum starved hepatocellular carcinoma cells

Albumin is the most abundant plasma protein and functions as a transport molecule that continuously interacts with various cell types. Because of these properties, albumin has been exploited by the pharmaceutical industry to improve drug delivery into target cells. The immediate effects of albumin on cells, however, require further understanding. The cell interacting properties and pharmaceutical applications of albumin incentivises continual research into the immediate effects of albumin on cells. The HepG2/C3A hepatocellular carcinoma cell line is used as a model for studying cancer pathology as well as liver biosynthesis and cellular responses to drugs. Here we investigated the direct effect of purified albumin on HepG2/C3A cell proliferation in the absence of serum, growth factors and other serum originating albumin bound molecules. We observed that the reduced cell counts in serum starved HepG2/C3A cultures were increased by the inclusion of albumin. Cell cycle analysis demonstrated that the percentage of cells in G1 phase during serum starvation was reduced from 86.4 ± 2.3% to 78.3 ± 3.2% by the inclusion of albumin whereas the percentage of cells in S phase was increased from 6.5 ± 1.5% to 14.3 ± 3.6%. A significant reduction in the cell cycle inhibitor protein, P21, accompanied the changes in the proportions of cell cycle phases upon treatment with albumin. We have also observed that the levels of dead cells determined by DNA fragmentation and membrane permeabilization caused by serum starvation (TUNEL: 16.6 ± 7.2%, ethidium bromide: 13.8 ± 4.8%) were not significantly altered by the inclusion of albumin (11.6 ± 10.2%, ethidium bromide: 16.9 ± 8.9%). Therefore, the increase in cell number was mainly caused by albumin promoting proliferation rather than protection against cell death. These primary findings demonstrate that albumin has immediate effects on HepG2/C3A hepatocellular carcinoma cells. These effects should be taken into consideration when studying the effects of albumin bound drugs or pathological ligands bound to albumin on HepG2/C3A cells.

Scaling and systems biology for integrating multiple organs-on-a-chip

Coupled systems of in vitro microfabricated organs-on-a-chip containing small populations of human cells are being developed to address the formidable pharmacological and physiological gaps between monolayer cell cultures, animal models, and humans that severely limit the speed and efficiency of drug development. These gaps present challenges not only in tissue and microfluidic engineering, but also in systems biology: how does one model, test, and learn about the communication and control of biological systems with individual organs-on-chips that are one-thousandth or one-millionth of the size of adult organs, or even smaller, i.e., organs for a milliHuman (mHu) or microHuman (μHu)? Allometric scaling that describes inter-species variation of organ size and properties provides some guidance, but given the desire to utilize these systems to extend and validate human pharmacokinetic and pharmacodynamic (PK/PD) models in support of drug discovery and development, it is more appropriate to scale each organ functionally to ensure that it makes the suitable physiological contribution to the coupled system. The desire to recapitulate the complex organ-organ interactions that result from factors in the blood and lymph places a severe constraint on the total circulating fluid (~5 mL for a mHu and ~5 μL for a μHu) and hence on the pumps, valves, and analytical instruments required to maintain and study these systems. Scaling arguments also provide guidance on the design of a universal cell-culture medium, typically without red blood cells. This review presents several examples of scaling arguments and discusses steps that should ensure the success of this endeavour.

Adaptation of BHK cells producing a recombinant protein to serum-free media and protein-free medium

In this work a recombinant BHK21 clone producing a fusion protein with potential application in tumour target therapy was adapted to five different serum-free media (SFM) and to a protein-free medium (PFM). Only the PFM did not require a gradual adaptation to cell growth in the absence of serum. All tested SFM required a gradual adaptation (up to 35 days). For the majority of the SFM tested, cell specific productivity was not affected by the decrease in serum concentration during adaptation; however, cell growth was significantly affected by the serum decrease. Both cell growth and productivity were increased when PFM SMIF6 was used instead of the control medium. Long term measurements (approximately 100 days) of cell specific productivity for PFM and the two best SFM showed that productivity was maintained. This indicates the media capability to be used in long term production processes.

Effects of dissolved oxygen levels and the role of extra- and intracellular amino acid concentrations upon the metabolism of mammalian cell lines during batch and continuous cultures

The effects of dissolved oxygen and the concentration of essential amino acids upon the metabolism of two mammalian cell lines (rCHO producing human active (t-PA) and a mouse-mouse hybridoma) were investigated in batch, chemostat, and perfusion cultures. Intracellular amino acid concentrations were measured for both cell lines during repeated batch cultures and the K(S)-values for the essential amino acids were calculated using Monod equations via computer simulation. The K(S)-values were in the range of 10 mmol L(-1) and the pool of most intracellular amino acids remained constant at about 10-100 fold higher in concentration than in the medium. No significant differences were observed between the hybridoma and CHO cell. The specific nutrient uptake rates corresponded with the cell specific growth rate and the effects of reduced dissolved oxygen concentrations only became evident when the DO dropped below 5% of air saturation (critical concentration below 1%). Nevertheless, a correlation between nutrient concentration and specific oxygen uptake was detected.

A serum-free medium formulation supporting growth of human umbilical cord vein endothelial cells in long-term cultivation

A serum-free medium formulation - TUD-1 - was developed supporting growth of HUVEC in tissue culture. Special features of the basal medium formulation are highly elevated levels of glutamine and serine as well as the inclusion of N-acetylcysteine and phosphoascorbic acid. The cellular mitogenic needs are satisfied by bFGF, VEGF, EGF and liver growth factor. Further hormone supplementation consists of insulin and hydrocortisone. A protocoll for serum-free passage of HUVEC was established for serum-free long-term cultivation of freshly isolated HUVEC for up to 20 cumulative population doublings without significant differences in final cell density compared to controls cultivated with serum.

New technologies for automated cell counting based on optical image analysis ;The Cellscreen'

A prototype of a newly developed apparatus for measuring cell growth characteristics of suspension cells in micro titre plates over a period of time was examined. Fully automated non-invasive cell counts in small volume cultivation vessels, e.g. 96 well plates, were performed with the Cellscreen system by Innovatis AG, Germany. The system automatically generates microscopic images of suspension cells which had sedimented on the base of the well plate. The total cell number and cell geometry was analysed without staining or sampling using the Cedex image recognition technology. Thus, time course studies of cell growth with the identical culture became possible. Basic parameters like the measurement range, the minimum number of images which were required for statistically reliable results, as well as the influence of the measurement itself and the effect of evaporation in 96 well plates on cell proliferation were determined. A comparison with standard methods including the influence of the cultured volume per well (25 mul to 200 mul) on cell growth was performed. Furthermore, the toxic substances ammonia, lactate and butyrate were used to show that the Cellscreen system is able to detect even the slightest changes in the specific growth rate.

Linoleic acid improves the robustness of cells in agitated cultures

The murine hybridoma (CC9C10) was subjected to high shear rates in a spinner flask to determine the effect of various culture additives on cell survival. At 500 rpm, the half-life of the viable cell concentration in a low protein serum-free medium was 50 min. Both bovine serum albumin and Pluronic F-68 had a significant effect in protecting cells under these conditions. The effects of the two supplements were additive, so that in the presence of both supplements there was minimal cell damage at 500 rpm. The survival rate of cells grown in media supplemented with linoleic acid improved significantly under high stirring rates. Cells grown for one passage in 50 muM linoleic acid and stirred at 500 rpm had a significantly higher survival rate than control cells. For cells grown over 5 passages in 25 muM linoleic acid, the survival rate at 470 rpm was x3 greater than that determined for control cells. This difference gradually decreased at higher stirring rates up to 610 rpm when the half-life of the viable cell population was reduced to approximately 10 min. Supplementation of cultures with linoleic acid has previously been shown to result in incorporation into all three cellular lipid fractions - polar, non-polar and free fatty acid (Butler et al., 1997). Our explanation for the increased survivability of the cells at high agitation rates in the presence of linoleic acid is that the structural lipid components of the cell including the outer membrane attained a higher unsaturated/saturated ratio which was more robust than that of control cells.

Production of recombinant human antithrombin III on 20-L bioreactor scale: correlation of supernatant neuraminidase activity, desialylation, and decrease of biological activity of recombinant glycoprotein

Chinese hamster ovary (CHO) cells producing the recombinant glycoprotein human antithrombin III (rhAT III) were batch cultivated in a 20-L bioreactor for 13 days. Neuraminidase activity in cell-free supernatant was monitored during cultivation and free sialic acid was determined by HPLC. Neu5Acalpha(2-->3)Gal-specific Maackia amurensis and Galbeta(1-->4)GlcNAc-specific Datura stramonium agglutinin were used for determination of sialylated and desialylated rhAT III, respectively. A commercial test kit was used for evaluation of functional rhAT III activity. Supernatant neuraminidase as well as lactate dehydrogenase activity increased significantly during batch growth. The enhanced number of dead cells correlated with increased neuraminidase activity, which seemed to be principally due to cell lysis, resulting in release of cytosolic neuraminidase. Loss of terminally alpha(2-->3) linked sialic acids of the oligosaccharide portions of rhAT III, analyzed in lectin-based Western blot and lectin-adsorbent assays, correlated with a decrease of activity of rhAT III produced throughout long-term batch cultivation. Thus, structural oligosaccharide integrity as well as the functional activity of recombinant glycoprotein depend on the viability and mortality of the bioreactor culture, and batches with a high number of viable cells are required to guarantee production of glycoproteins with maximum biological activity. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 441-448, 1997.

Promotion of stem cell proliferation by vegetable peptone

OBJECTIVES

Technical limitations and evolution of therapeutic applications for cell culture-derived products have accelerated elimination of animal-derived constituents from such products to minimize inadvertent introduction of microbial contaminants, such as fungi, bacteria or viruses. The study described here was conducted to investigate the proliferative effect of vegetable peptone on adult stem cells in the absence of serum, and its possible mechanisms of action.

MATERIALS AND METHODS

Cell viability and proliferation were determined using the MTT assay and Click-iT EdU flow cytometry, respectively. In addition, changes in expression of cytokine genes were analysed using MILLIPLEX human cytokine enzyme-linked immunosorbent assay kit.

RESULTS

Viability of cord blood-derived mesenchymal stem cells (CB-MSC) and adipose tissue-derived stem cells (ADSC) increased significantly when treated with the peptone. In addition, median value of the group treated with peptone shifted to the right when compared to the untreated control group. Furthermore, quantitative analysis of the cytokines revealed that production of vascular endothelial growth factor (VEGF), transforming growth factor-beta1 (TGF-beta1), and interleukin-6 (IL-6) increased significantly in response to treatment with our vegetable peptone in both CB-MSCs and ADSCs.

CONCLUSIONS

Our findings revealed that the vegetable peptone promotes proliferation of CB-MSCs and ADSCs. In addition, results of this study suggest that induction of stem cell proliferation by vegetable peptone is likely to be related to its induction of VEGF, TGF-beta1, and IL-6 expression.

Effect of seeding technique and scaffold material on bone formation in tissue-engineered constructs

The aim of the present study was to test the hypothesis that both scaffold material and the type of cell culturing contribute to the results of in vivo osteogenesis in tissue-engineered constructs in an interactive manner. CaCO3 scaffolds and mineralized collagen scaffolds were seeded with human trabecular bone cells at a density of 5 x 10(6) cells/cm(3) and were left to attach under standard conditions for 24 h. Subsequently, they were submitted to static and dynamic culturing for 14 days (groups III and IV, respectively). Dynamic culturing was carried out in a continuous flow perfusion bioreactor. Empty scaffolds and scaffolds that were seeded with cells and kept under standard conditions for 24 h served as controls (groups I and II, respectively). Five scaffolds of each biomaterial and from each group were implanted into the gluteal muscles of rnu rats for 6 weeks. Osteogenesis was assessed quantitatively by histomorphometry and expression of osteocalcin (OC) and vascular endothelial growth factor (VEGF) was determined by immunohistochemistry. CaCO3 scaffolds exhibited 15.8% (SD 3.1) of newly formed bone after static culture and 22.4% (SD 8.2) after dynamic culture. Empty control scaffolds did not show bone formation, and scaffolds after 24 h of standard conditions produced 8.2% of newly formed bone (SD 4.0). Differences between the controls and the scaffolds cultured for 14 days were significant, but there was no significant difference between static and dynamic culturing. Mineralized collagen scaffolds did not show bone formation in any group. There was a significant difference in the expression of OC within the scaffolds submitted to static versus dynamic culturing in the CaCO3 scaffolds. VEGF expression did not show significant differences between static and dynamic culturing in the two biomaterials tested. It is concluded that within the limitations of the study the type of biomaterial had the dominant effect on in vivo bone formation in small tissue-engineered scaffolds. The culture period additionally affected the amount of bone formed, whereas the type of culturing may have had a positive effect on the expression of osteogenic markers but not on the quantity of bone formation.

Bone formation in trabecular bone cell seeded scaffolds used for reconstruction of the rat mandible

This study tested whether different in vitro cultivation techniques for tissue-engineered scaffolds seeded with human trabecular bone cells affect in vivo bone formation when implanted into critical-size defects in rat mandibles. Human trabecular cells were isolated and seeded into three types of scaffolds (porous CaCO(3), mineralized collagen, porous tricalcium phosphate). Four in vitro groups were produced: empty control scaffolds incubated with cell culture medium for 24 h; scaffolds seeded with trabecular bone cells, cultivated under static conditions for 24 h; scaffolds seeded with trabecular bone cells, cultivated for 14 days under static conditions; scaffolds seeded with trabecular bone cells, cultivated for 14 days in a continuous flow perfusion bioreactor. The scaffolds were implanted press fit into non-healing defects, 5 mm diameter, in rat mandibles. After 6 weeks the presence of human cells was assessed; none were detected. Histomorphometric evaluation showed that neither seeding human trabecular bone cells nor the culturing technique increased the amount of early bone formation compared with the level provided by osteoconductive bone ingrowth from the defect edges. It is concluded that human bone marrow stroma cells in tissue-engineered scaffolds and associated in vitro technology are difficult to test in the mandible in animal models.

Effects on growth behavior in continuous hybridoma cell cultures: The role of viral contamination

This article describes the retrovirus expression with optimal nutrient supply and its potential growth inhibition effects in continuous hybridoma cell cultivation. A special reactor setup with total cell retention was developed to examine growth inhibition effects. Using this fermentation strategy we observed a decrease of viability cell rate which occurred at a defined state of the process despite sufficient nutrient supply. Therefore we assume that inhibitory substances are responsible for these effects. The molecular weight range of the inhibitory substances and the possible retrovirus cooperation of these growth inhibition effects were examined. To determine the molecular weight range we used the following methods: ultrafiltration, gelfiltration, ultracentrifugation and gel electrophoresis. Furthermore, RT-PCR and western-/immunoblot are used to detect retrovirus particles in the supernatant and to show a retrovirus participation on growth inhibition effects. The possible growth modulation was tested in a biological assay (MTT-assay).

Endogenous retrovirus particles and their repercussion effects on the growth behaviour of continuous hybridoma cultivation processes

Hybridoma cells used for the production of monoclonal antibodies are also known to form growth inhibitory substances. Growth inhibitors already described in the literature belong to the class of peptides and proteins likeTGF-ss (Transforming Growth Factor-ss). The endogenous retrovirus particles - a further potential substance producing this kind of effect - are described here. To examine whether the retrovirus particles participated in growth inhibitory effects hybridoma cells were cultivated in continuous perfusion mode by using a special reactor set-up. A rapid increase of the signal in the supernatant which coincided with a decrease of viability could be observed by monitoring the reverse transcriptase-activity during this type of fermentation process. The examination of concentrated and fractionated supernatant from this period showed growth inhibitory effects in the biological assay (MTT-assay). Investigations of respective fractions demonstrated retrovirus particles with reverse transcriptase-activity. Based on RT-PCR data it was shown that only inhibitory fractions contain retrovirus particles which were of E-MuLV and MCF origin.

Development and optimisation of a procedure for the production of Parapoxvirus ovis by large-scale microcarrier cell culture in a non-animal, non-human and non-plant-derived medium

For the production of a chemically inactivated Parapoxvirus ovis (PPVO), an adherent bovine kidney cell line was cultivated on Cytodex-3 microcarriers in suspension culture. The inactivated and purified virus particles have shown immune modulatory activity in several animal models. PPVO was produced by a biphasic batch process at the 3.5 and 10 L scale. Aeration was realised by bubble-free membrane oxygenation via a tube stator with a central two-blade anchor impeller. In order to increase efficiency, process robustness and safety, the established process was optimised. The cell line was adapted to a protein-free medium (except recombinant insulin) in order to increase biosafety. A scale up to a 50 L pilot plant with direct cell expansion was performed successfully. In parallel, the biphasic batch process was optimised with special emphasis on different operating conditions (cell number, Multiplicity of Infection (MOI), etc.) and process management (fed-batch, dialysis, etc.). The quality and concentration of the purified virus particles was assessed by quantitative electron microscopy, residual host cell protein and DNA-content and, finally, biologic activity in a transgenic mouse model. This integrated approach led to a new, safe, robust and highly productive large-scale production process, called "Volume-Expanded-Fed" Batch with cell densities up to 6-7e06 cells/mL. By subsequent dilution of infected cells into the next process scale, an increase in total productivity by a factor of 40 (related to an established biphasic batch process) was achieved.

The role of recombinant proteins in the development of serum-free media

Early developments in serum-free media led to a variety of formulations in which components normally provided in serum and required for growth (insulin, transferrin, lipid supplements, trace elements) and poorly defined components (extracts, hydrolysates) were added to defined basal media. These additives were mostly animal-derived. Given recent concerns about TSEs (transmissible spongiform encephalopathies) and other adventitious agents, the drive in media formulations must be towards elimination of animal-origin materials while maintaining cell line productivity. The progress made towards removing animal-derived components and the use of recombinant proteins in serum-free media for mammalian cells is reviewed.

Integrative recombination between adenovirus type 12 DNA and mammalian DNA in a cell-free system: joining by short sequence homologies

A cell-free system was developed to investigate the mechanism of how junctions are formed between viral and cellular DNAs during adenoviral DNA integration into the hamster cell genome. Recombination between the segment of adenovirus type 12 (Ad12) DNA, that comprises sequence coordinates 20885-24053, subsequently termed PstI-D fragment and the hamster preinsertion DNA sequence p7 was studied in a cell-free system. The p7 DNA segment had served as viral DNA integration site in the Ad12-induced tumor CLAC1. Nuclear extracts initially from uninfected BHK21 hamster cells were fractionated by a series of chromatographic steps. DNAs of the in vitro generated recombinants were analyzed in detail. In the course of the recombination reaction, the two linear molecules were joined. The reaction took place between two short homologous sequences one of which was always at or very close to a DNA terminus, the second one could be several kilobase pairs remote from a DNA terminus. Apparently, the nucleotide sequence at the terminus of one recombining molecule determined the point of junction by searching for short homologies in the partner molecule. The recombination reaction was not conservative, the sequences in-between the short sequence homologies and one of the short sequence homologies were deleted in the in vitro recombinants. Two main criteria influenced the choice of interacting short sequence homologies: perfect homologies of 8-9 bp were most frequently found, they were preferred over more extended, but less perfect homologies. Comparing different short sequence homologies with similar stabilities, those combinations seemed to be chosen in the reaction which led to a minimal loss of nucleotides in the recombinants. The in vitro activity was found in nuclear extracts from both hamster and human cells. The activity was, hence, available for Ad12 DNA in productively infected human and abortively infected hamster cells. The specific recombination activity was increased in nuclear extracts of hamster cells abortively infected with Ad12. The junction sites in the recombinants, which were generated by the cell-free system, were very similar to junctions between adenoviral and cellular DNAs cloned from Ad12-induced tumor cells and Ad12-transformed cell lines.

Evaluation of various serum and animal protein free media for the production of a veterinary rabies vaccine in BHK-21 cells

We have carried out the adaptation of BHK-21 cells to two serum free (Ex Cell 520 and HyQ PF CHO) and three animal protein free media: Ex Cell 302, HyQ PF CHO MPS and Rencyte BHK. After a direct switch or a gradual adaptation, we have achieved BHK-21 cells growth in the following media: HyQ PF CHO, HyQ PF CHO MPS, Rencyte BHK and Ex Cell 302. The most suitable media for BHK-21 cells growth, with respect to cell density and specific growth rate, were HyQ PF CHO and HyQ PF CHO MPS. Hence we have selected these media to study cell growth and the production of rabies virus. Kinetic studies of cell growth in spinner flasks using the selected media have shown that a maximal cell density of 2x10(6) cells x ml(-1) was reached in both media. For rabies virus production, the viral titer obtained was 1.7x10(6) FFU x ml(-1) in HyQ PF CHO as well as in HyQ PF CHO MPS medium. The optimization of rabies virus production by BHK-21 cells grown in a 2 l bioreactor using the selected media, pointed to the following parameters: culture mode, perfusion rate and multiplicity of infection (MOI), as being the critical factors for achieving a good virus yield. When tested in mice, the activity of the experimental vaccines prepared on HyQ PF CHO MPS medium has shown a protective activity that meets WHO requirements.

Isolation and characterization of a mouse monoclonal antibody against human corneal endothelial cells

In vitro cultivation of human corneal endothelial cells (HCEC) is associated with loss of typical cobblestone-like appearance during successive passages. Thus far morphology was the sole criterion for the cell's endothelial nature. Mouse monoclonal antibodies (mabs) to human corneal endothelial cells were raised using standard immunization and hybridoma isolation procedures. The specificity of mabs for human corneal endothelial cells was tested in comparison to other endothelial cell types, to fibroblasts, corneal keratocytes and to human retinal pigmented epithelial cells. In addition immunofluorescence or immunoperoxidase staining was performed with frozen tissue sections of human corneas and with various other human tissues. The mab 9.3.E reacts with cultured human corneal endothelial cells, but not with cultured human fibroblasts and human keratocytes. In frozen sections selective positivity of corneal endothelium in contrast to negativity of the other corneal cell types was confirmed. In investigated extraocular tissues positivity was observed in smooth muscle cells including related cells (i.e. Ito and mesangial cells) and in Schwann's cells and adipocytes, but apparently not in vascular endothelial cells. The mab is human-specific and binds to a protein with a molecular weight of 130 kDa mainly accumulating along cell membranes. A mouse monoclonal antibody against human corneal endothelial cells was established in vitro and was shown to be capable of differentiating corneal endothelial cells from other corneal cell types, especially from corneal keratocytes. It is, however, not cornea-specific, but also reacts with certain extraocular cell types.

Developments and improvements in the manufacturing of human therapeutics with mammalian cell cultures

During recent years, biopharmaceutical products manufactured by processes that use mammalian cell cultures have gained increasing importance. At the same time, a strong awareness of the importance of the safety and quality of such products has also emerged. This has led to improvements in cultivation and production technology, validation procedures and process organization.

Estimation of cell damage in bench- and pilot-scale affinity expanded-bed chromatography for the purification of monoclonal antibodies

The first step in downstream processing of mammalian cell culture includes the separation of the cells without cell damage to avoid the release of intracellular enzymes, which could potentially cause proteolytic degradation of the target protein and may increase the impurities for further chromatographic steps. This especially includes the reduction of host DNA for therapeutic proteins. The aim of this investigation was to examine the extent of cell damage at the bench and pilot scale using a stabilized fluidized bed (expanded bed) for direct recovery of IgG from cell culture broth. For this purpose, Streamline-25 and -200 columns containing 75 mL and 5 L of rProtein A matrix, respectively, were used. The repeated batch cultivations resulted in high cell viabilities of about 90% prior purification. The pH was gently adjusted to pH 8 before the broth was applied to the gel. In bench scale, 1 to 6 L of unclarified feed was applied to the Streamline-25 column. In pilot scale, up to 95 L was processed using the Streamline-200 column. The antibodies from 95 L of unclarified feed were recovered after approximately 1.5 h. The possible cell damage, caused either by the equipment or by the cells' passage through the expanded bed, was detected by the following assays: microscopic count of the cells using trypan blue dye exclusion to determine viability; monitoring of intracellular components (i.e., DNA concentration); activity of lactate dehydrogenase (LDH); and, finally, the particle load in the flow through and the eluate. Despite the sensitivity of hybridoma cells to shear forces, neither the high flow rate (300 to 450 cm/h) nor the passage of the cells through the expanded bed caused any relevant cell damage or clogging of the gel. Excellent DNA depletion was observed.

The new medium MDSS2N, free of any animal protein supports cell growth and production of various viruses

The development of media free of serum and animal or human proteins is of utmost importance for increasing the safety of biologicals produced for therapy and vaccination. In order to reduce the risk of contamination, we have modified the serum free medium MDSS2, a very efficient serum free medium for the production of various biologicals including experimental vaccines using different cell lines (Merten et al., 1994), by replacing the animal derived products by plant extracts. The new serum and animal protein free medium (MDSS2N) can be efficiently used for biomass production of various cell lines. These cells grow equally well or better in this new serum-free medium than in the old formulation (MDSS2):* BHK-21/BRS cells, adapted to MDSS2N, showed an overall specific growth rate of 0.0197 h-1 (mu_max = 0.0510+/-0.0058 h-1), whereas those cultivated in MDSS2 grew with an average specific growth rate of 0.0179 h-1 (mu_max = 0.0305+/-0.0177 h-1).* Vero cells grew with an average specific growth rate of 0.0159 h-1 and 0.0153 h-1 in MDSS2 and MDSS2N, respectively. Very similar growth rates were obtained in microcarrier cultures in stirred tank reactors: the specific growth rates were 0.0161 h-1 and 0.0166 h-1 for MDSS2 and MDSS2N cultures, respectively.* For MDCK cells, when cultured on microcarriers in bioreactors, a higher average specific growth rate was observed in MDSS2N than in MDSS2; values of 0.0248 h-1 and 0.0168 h-1, respectively, were obtained.The capacity of MDSS2N to support the production of different viruses was equally evaluated and it could be established that for certain viruses there are no or insignificant differences between MDSS2N and MDSS2 (influenza and polio virus), whereas, the production of rabies virus is somewhat reduced in MDSS2N when compared to MDSS2. The use of MDSS2N for cell culture and the production of various viruses is discussed.

Recombinant Human Albumin in Cell Culture: Evaluation of Growth-Promoting Potential for NRK and SCC-9 Cells In Vitro

Serum-derived albumin has for a long time been used in cell culture media, but the exact role of albumin and/or impurities bound to albumin has not been precisely defined. In this study, recombinant human albumin was evaluated for its growth-promoting activity on two cell lines, NRK and SCC-9. For NRK cells, the recombinant human albumin was found to exert an inhibitory effect. The fact that fatty acid free HSA was also inhibitory while HSA fraction V was stimulatory suggested a role for fatty acids or some other bound moieties in growth stimulation by HSA fraction V. Addition of oleic acid, cholesterol, phosphatidylcholine, phosphatidylserine or a combination of these lipids, however, did not significantly improve the growth stimulating activity of either fatty acid free HSA or the recombinant human albumin. For SCC-9 cells, both recombinant human albumin and fatty acid free HSA showed slight stimulation (although they were not as active as HSA fraction V), suggesting that in some cell systems, the albumin molecule per se may promote cell growth and survival.

Expression and purification of recombinant human apolipoprotein A-I in Chinese hamster ovary cells

The expression of apolipoprotein A-I (apoA-I) has been shown to be very difficult due to its amphiphilic character, autoaggregation, and degradation. We have expressed apoA-I using CHO cells, Baculovirus, and Escherichia coli [Schmidt et al., J. Biol. Chem. (1995) 270, 469-475]. Here we report about optimized conditions for the expression of proapoA-I in CHO cells, testing various serum-free media. We were able to yield apoA-I expression up to 80 micrograms/ml, by far the highest ever reported. However, immunoblot analysis revealed degraded apoA-I. The best apoA-I expression testing various conditions was about 20-30 micrograms/ml without any evidence of degradation. Interestingly, the apoA-I expression resulted in reproducible apoA-I fragments of 26 and 14 kDa. These fragments are consistent with already reported in vivo findings, in which carboxy-terminal proteolysis was suggested. The use of the protease inhibitors pepstatin and chymostatin, both carboxy-peptidase inhibitors, did result in contrast to other studied protease inhibitors in increased apoA-I yield. Therefore, limited carboxy-terminal proteolysis contributes to the degradation of CHO cell-secreted apoA-I. In addition, we evaluated various purification methods for the preparative isolation of recombinant apoA-I. In our hands we obtained the best recovery and no degradation with reversed-phase chromatography using a FPLC system.

Characterization of changes in the glycosylation pattern of recombinant proteins from BHK-21 cells due to different culture conditions

The N-glycosylation patterns of a genetically engineered human interleukin-2 variant glycoprotein (IL-Mu6), produced by BHK-21 cells from long-term suspension and microcarrier cultures in the presence and absence of fetal calf serum were compared. IL-Mu6 was used as a model protein in studying the effect of different controlled cell culture conditions on the expression of N-glycans in recombinant glycoproteins. IL-Mu6 contains a single amino acid substitution (Glu100<==>Asn) generating a potential N-glycosylation recognition site (Asn100-Xxx-Thr/Ser) in addition to the natural O-glycosylation at position Thr3. Parallel cell cultivations were carried out in two continuously perfused 2.5-liter stirred bioreactors under defined culture conditions. Major differences were found in the glycoprotein products obtained during these different cultivation conditions. Serum-free cultures resulted in a higher level of terminal sialylation and proximal alpha 1-6 fucosylation. The ratio of O- to N-glycans as well as the amount of nonglycosylated product and the antennarity of N-linked carbohydrates in the model protein exhibited major differences depending on the presence or absence of serum, the condition of growth and the cultivation procedure.

An experimental rabies vaccine produced with a new BHK-21 suspension cell culture process: use of serum-free medium and perfusion-reactor system

An experimental rabies vaccine was prepared from the BHK-21 cell line adapted to culture in suspension using bioreactors. A new serum-free medium (MDSS2) (Merten et al., Cytotechnology, 1994, 14, 47) developed for the culture of various cell lines and for the production of several biologicals, was used for cell culture and virus production. The PV-Paris/BHK-21 rabies virus strain (adapted to the BHK-21 grown in monolayer) was adapted to BHK-21 cells cultivated in suspension and in the serum-free medium. High titres of rabies virus were obtained with bioreactors equipped with a perfusion system using BHK-21 cells grown in suspension in MDSS2. Experimental vaccines were prepared and had satisfactory protective activity when tested in mice. This new and low cost technology for rabies vaccine production could be suitable for developing countries where rabies is an important health problem.

The effect of fatty acids on hybridoma cell growth and antibody productivity in serum-free cultures

A murine B-lymphocyte hybridoma (CC9C10) was adapted for growth in a serum-free medium. Supplementation of the medium with cis-unsaturated fatty acids (10-50 microM) improved the cell yield in the order oleic/linoleic > linoleic > oleic. Initial supplementation with the fatty acids also caused a significant increase (58%) in the volumetric Mab titre. Continued growth of the cells in the fatty acid supplemented media over five culture passages resulted in a gradual deterioration of the Mab yield concomitant with the appearance of lipid inclusions in the cytosol. The higher Mab yield could be restored by a limited period of growth of the lipid-loaded cells in fatty acid-free medium. These effects were independent of growth rate. This suggests that the optimal intracellular lipid content is finely balanced between a reduced and an overloaded state. Specific glucose and glutamine utilisation rates were unaffected by the presence of fatty acids. Also, the optimal glucose and glutamine concentrations for growth were independent of the fatty acids.

Significant inhibition of hybridoma cells by exogenous application of ganglioside GM3, a possible modulator of cell growth in vitro

Gangliosides of the mouse-rat hybridoma cell line 187.1, which secretes an antibody against kappa-light chain of mouse IgG, were isolated and structurally characterized by biochemical and immunological methods (overlay technique), and fast atom bombardment-mass spectrometry. Exclusively GM3, substituted with C24:1 and C16:0 fatty acid and C18:1 sphingosine, was found in this B cell derived cell line. A GM3(NeuGc) to GM3(NeuAc) ratio (80 to 20), was characteristic for 187.1 cells, and absolute GM3 amounts of about 0.3 mg 10(-9) viable cells were determined. Exogenous application of GM3, which has been isolated from large cell preparations, to 187.1 cells showed growth inhibition in a concentration dependent manner. Using the MTT-assay and the [3H]thymidine incorporation assay, the cells exhibited a strong reduction in metabolic and proliferative activity, respectively, after exposure of cells to GM3. GM3 was applied in concentrations between 3 microM and 30 microM, giving evidence for strong inhibitory effects at 30 microM GM3 and less but significant suppression after application of GM3 concentrations lower than 20 microM. No cellular response was observed at the lowest concentration (3 microM) used in this study. Hybridoma cells as well as other cell types like fibroblasts, muscle cells and endothelial cells, are in general characterized by high expression of the GM3 ganglioside, which is known to act as a modulator of cellular growth in monolayer cultures of adherent cells. Since gangliosides are released to the culture medium by cell lysis, i.e. cell death, and/or by active membrane shedding, the results obtained in this study suggest a growth regulatory role of GM3 in high density hybridoma cell cultures.

HBs-MAb production in perfusion culture with selective ammonia removal system

Conventional serum-free perfusion cultures of hybridoma TO-405 cells using medium supplemented with additional amounts of glucose, glutamine, beta-mercaptoethanol and growth factors failed to yield cell densities and monoclonal antibody (MAb) concentrations which were significantly different from the results of unsupplemented perfusion cultures. Ammonia building-up to inhibitory concentrations in all the cultures was regarded as one of the primary reasons. When perfusion cultures and medium supplementation were done coupled to the ammonia removing system, the viable cell density grew to a maximum of 2.5 x 10(7) cells per ml at high percentage viability. This value is more than a 300% increase from that of conventional perfusion cultures and better compared to serum-supplemented cultures. The monoclonal antibody accumulated to a concentration as high as 26.3 x 10(5) mIU per ml. This is about 10-times when compared to that which can be achieved in ordinary perfusion cultures.

On-line monitoring of monoclonal antibody production with regenerable flow-injection immuno systems

In this paper two systems for the observation of the production of mouse-IgG during the cultivation of hybridoma cells in a perfusion reactor are presented. The direct immunosystem is based on the detection of changes in capacitance of a dielectric layer (tantalum oxide) on a metal surface (tantalum) when antibodies bind to immobilized anti-antibodies. The sensor consisted of a 25 nm tantalum oxide layer, electrochemically grown onto a laser patternized 1 micron thick tantalum layer. The indirect system is based on an automated fluorimetric sandwich ELISA system with beta-galactosidase conjugated secondary antibodies. Two cultivations of mouse hybridoma cells in a 2-1 perfusion reactor were performed. The first cultivation was monitored with the capacitance system, the second cultivation was monitored with the fluorimetric system.

Evaluation of the proteolytic potential of in vitro-cultivated hybridoma and recombinant mammalian cells

The proteolytic potential of culture supernatants derived from recombinant baby hamster kidney (BHK) 21 and mouse-mouse hybridoma cells have been characterized. Several assays using enzyme specific chromogenic artificial peptides, as well as a radioactive test for the detection of the total activity, have been established and were adapted to the special conditions existing in culture media of mammalian cells. Proteolytic activity was detected in human serum albumin containing media which was specific for peptides ending with a terminal arginine. The addition of N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer to the culture media resulted in a significant peptide cleavage potential, supporting the fact that this compound is not recommended as a supplement in animal cell culture media. Medium shock protease activity has been detected in culture supernatants of BHK cells when medium was changed completely, caused by a switch from a serum containing state of growth to a serum-free state of growth which is often used in processes with microcarriers. However, this proteolytic activity showed a transient behaviour whereby its secretion stopped when the cells had adapted to the serum-free medium conditions. Characterization of the proteolytic activities using different specific inhibitors and activators supported the assumption that the proteolytic activity reflects a cell specific composition of proteases which can also change dependent on the culture conditions used.

The Super-Spinner: a low cost animal cell culture bioreactor for the CO2 incubator

The production of small quantities of monoclonal antibodies and recombinant proteins was carried out using a new low cost production system, the Super Spinner. Into a 1 1 standard Duran flask a membrane stirrer equipped with a polypropylene hollow fiber membrane was installed to improve the oxygen supply by bubble-free aeration. The aeration was facilitated by using the CO2 conditioned incubator gas, which was pumped through the membrane stirrer via a small membrane pump. The maximal oxygen transfer rate (OTRmax) of the Super Spinner was detected. For this purpose one spinner flask was equipped with an oxygen electrode. The OTRmax was measured by the dynamic method. The ratio of membrane length to culture volume was adapted corresponding to the oxygen uptake rate of the cells according to the desired cell density. A balanced nutrient supply resulted in an optimal formation and yield of products.

Evaluation of the new serum-free medium (MDSS2) for the production of different biologicals: use of various cell lines

The presence of serum in cell culture raises safety problems for the production of biologicals, thus a new serum-free medium (MDSS2) was developed. The evaluation of this medium for the growth of different cell lines (BHK-21 C13, BSR and Vero) has shown that cells grew in this medium similarly to standard serum-containing medium, independently of the culture system used: in static (as monolayer) as well as in agitated systems (in suspension in spinner and perfusion reactors). BHK-21 and BSR cells grew as aggregate cultures and could proliferate in both static and agitated culture systems. Vero cells stayed attached to a substrate and proliferated equally in static and in agitated microcarrier-culture systems. The cell densities obtained with BHK-21 cells depended only on the culture system used. They ranged from 2-3 x 10(6) to 6-12 x 10(6) cells per ml for static batch and perfusion reactor cultures respectively. The cell concentration was 3 to 6 times higher than in classical cultures performed in serum-containing medium. The cell densities obtained with Vero cells were indistinguishable from those obtained in serum-containing medium, whatever the cell culture system used. These cell lines have been used for the production of rabies virus. With respect to BHK-21 and BSR, similar production rates of rabies glycoprotein have been found as in the standard roller bottle process. The production of rabies virus and of viral glycoprotein by Vero cells cultivated in serum-free medium was augmented 1.5-fold and 2.5-fold, respectively, when compared to serum-containing medium. A recombinant BHK-21 cell line, producing human IL-2, can also proliferate in MDSS2, after addition of insulin. The specific IL-2 production rate was augmented 3-4-fold in comparison to serum-containing medium. For the cells tested, the MDSS2 serum-free medium is a good growth and production medium. Its use for cultivating other cell lines and/or for the production of other biologicals is discussed.

Design of a bubble-swarm bioreactor for animal cell culture

A stationary bubble-swarm has been used to aerate a mammalian cell culture bioreactor with an extremely low gas flow rate. Prolonging the residence time of the gas bubbles within the medium improved the efficiency of the gas transfer into the liquid phase and suppressed foam formation. An appropriate field of speed gradients prevented the bubbles from rising to the surface. This aeration method achieves an almost 90% transfer of oxygen supplied by the bubbles. Consequently, it is able to supply cells with oxygen even at high cell densities, while sparging with a gas flow of only 0.22 x 10(-3) -1.45 x 10(-3) vvm (30-200 ml/h). The reactor design, the oxygen transfer rates and the high efficiency of the system are presented. Two repeated batch cultures of a rat-mouse hybridoma cell line are compared with a surface-aerated spinner culture. The used cell culture medium was serum-free, either with or without BSA and did not contain surfactants or other cell protecting agents. One batch is discussed in detail for oxygen supply, amino acid consumption and specific antibody production.

Ganglioside alterations in YAC-1 cells cultivated in serum-supplemented and serum-free growth medium

Gangliosides of the 'GM1b-pathway' (GM1b and GalNAc-GM1b) have been found to be highly expressed by the mouse T lymphoma YAC-1 grown in serum-supplemented medium, whereas GM2 and GM1 ('GM1a-pathway') occurred only in low amounts [Müthing, J., Peter-Katalinić, J., Hanisch, F.-G., Neumann, U. (1991) Glycoconjugate J 8:414-23]. Considerable differences in the ganglioside composition of YAC-1 cells grown in serum-supplemented and in well defined serum-free medium were observed. After transfer of the cells from serum-supplemented medium (RPMI 1640 with 10% fetal calf serum) to serum-free medium (RPMI 1640 with well defined supplements), GM1b and GalNAc-GM1b decreased and only low amounts of these gangliosides could be detected in serum-free growing cells. The expression of GM1a was also diminished but not as strongly as that of GM1b and GalNAc-GM1b. These growth medium mediated ganglioside alterations were reversible, and the original ganglioside expression was achieved by readaptation of serum-free growing cells to the initial serum-supplemented medium. On the other hand, a 'new' ganglioside, supposed to represent GalNAc-GD1a and not expressed by serum-supplemented growing cells, was induced during serum-free cultivation, and increased strongly after readaptation. These observations reveal that the ganglioside composition of in vitro cultivated cells can be modified by the extracellular environment due to different supplementation of the basal growth medium.

Surface-modified membranes as a matrix for protein purification

Recently introduced membrane-based chromatographic supports for protein separation are available either with a coupled ligand, e.g., protein A, protein G or ion-exchange groups, or as activated matrices for coupling a desired ligand. The coupling conditions for protein A and immunoglobulin G to an epoxy-activated membrane were determined. The performance of the prepared affinity membranes was investigated using pure rabbit immunoglobulin G and protein A as a model system. For practical application monoclonal antibodies from cell culture supernatant were purified with a prepared protein A membrane and for comparison with a sulphonic acid ion exchange membrane.

Enzyme sensor-FIA-system for on-line monitoring of glucose, lactate and glutamine in animal cell cultures

Enzyme sensors for glucose, lactate and glutamine were connected via flow-injection analysis (FIA) devices to two different bioprocesses. They were used for on-line process control of perfused bioreactor systems containing mammalian cell lines producing a monoclonal antibody and recombinant interleukin-2. The biosensor system gives direct access to important process data which can be used as control parameters for long term cell cultivation systems.