Safety and economic aspects of continuous mammalian cell culture

Mammalian perfusion cultivation at high L-Arginine concentration for efficient production of recombinant protein by increasing perfusion filter transmission

Cultivations of Chinese Hamster Ovary (CHO) cells in a perfusion setup were conducted in the presence of super physiological concentrations of L-Arginine to investigate the impact on transmission through the perfusion filter for production of a recombinant domain antibody. Our study revealed that the presence of L-Arginine within the range of 30 to 50mM had a positive impact on transmission. However, the higher concentrations were found to have a negative correlation with cell viability, and an optimal concentration of approximately 40mM was identified. The supplementation of L-Arginine improved overall cultivation performance and enhanced product quality attributes. As a result, our findings demonstrate that the supplementation of L-Arginine to mammalian perfusion cultivations stands as an effective method to address transmission issues, exerting a broad impact on process and production of recombinant proteins.

In-silico media optimization for continuous cultures using genome scale metabolic networks: The case of CHO-K1

The cell culture is the central piece of a biotechnological industrial process. It includes upstream (e.g. media preparation, fixed costs, etc.) and downstream steps (e.g. product purification, waste disposal, etc.). In the continuous mode of cell culture, a constant flow of fresh media replaces culture fluid until the system reaches a steady state. This steady state is the standard operation mode which, under very general conditions, is a function of the ratio between the cell density and the dilution rate and depends on the media supplied to the culture. To optimize the production process it is widely accepted that the concentration of the metabolites in this media should be carefully tuned. A poor media may not provide enough nutrients to the culture, while a media too rich in nutrients may be a waste of resources because, either the cells do not use all of the available nutrients, or worse, they over-consume them producing toxic byproducts. In this study, we show how an in-silico study of a genome scale metabolic network coupled to the dynamics of a chemostat could guide the strategy to optimize the media to be used in a continuous process. Given a known media we model the concentrations of the cells in a chemostat as a function of the dilution rate. Then, we cast the problem of optimizing the production process within a linear programming framework in which the goal is to minimize the cost of the media keeping fixed the cell concentration for a given dilution rate in the chemostat. We evaluate our results in two metabolic models: first a simplified model of mammalian cell metabolism, and then in a realistic genome-scale metabolic network of mammalian cells, the Chinese hamster ovary cell line. We explore the latter in more detail given specific meaning to the predictions of the concentrations of several metabolites.

Modeling of copy number variability in Pichia pastoris

Development of continuous biopharmaceutical manufacturing processes is an area of active research. This study considers the long-term transgene copy number stability of Pichia pastoris in continuous bioreactors. We propose a model of copy number loss that quantifies population heterogeneity. An analytical solution is derived and compared with existing experimental data. The model is then used to provide guidance for stable operating timescales. The model is extended to consider copy number dependent growth such as in the case of Zeocin supplementation. The model is also extended to analyze a continuous seeding strategy. This study is a critical step towards understanding the impact of continuous processing on the stability of Pichia pastoris and the resultant products.

Maximum entropy and population heterogeneity in continuous cell cultures

Continuous cultures of mammalian cells are complex systems displaying hallmark phenomena of nonlinear dynamics, such as multi-stability, hysteresis, as well as sharp transitions between different metabolic states. In this context mathematical models may suggest control strategies to steer the system towards desired states. Although even clonal populations are known to exhibit cell-to-cell variability, most of the currently studied models assume that the population is homogeneous. To overcome this limitation, we use the maximum entropy principle to model the phenotypic distribution of cells in a chemostat as a function of the dilution rate. We consider the coupling between cell metabolism and extracellular variables describing the state of the bioreactor and take into account the impact of toxic byproduct accumulation on cell viability. We present a formal solution for the stationary state of the chemostat and show how to apply it in two examples. First, a simplified model of cell metabolism where the exact solution is tractable, and then a genome-scale metabolic network of the Chinese hamster ovary (CHO) cell line. Along the way we discuss several consequences of heterogeneity, such as: qualitative changes in the dynamical landscape of the system, increasing concentrations of byproducts that vanish in the homogeneous case, and larger population sizes.

Single-cell cloning enables the selection of more productive Drosophila melanogaster S2 cells for recombinant protein expression

The generation of monoclonal cell lines is an important early process development step for recombinant protein production. Although single-cell cloning is an established method in mammalian cell lines, straightforward protocols are not yet available for insect cells. We describe a new method for the generation of monoclonal insect cells without using fetal bovine serum and/or feeder cells pretreated by irradiation or exposure to mitomycin. Highly productive clones of Drosophila melanogaster S2 cells were prepared in a two-step procedure, comprising the establishment of a polyclonal population and subsequent single cell isolation by limiting dilution. Necessary growth factors were provided by co-cultivation of single transformants with untransfected feeder cells, which were later removed by antibiotic selection. Enhanced expression of EGFP and two target peptides was confirmed by flow cytometry and dot/western blotting. Highly productive clones were stable, showed a uniform expression profile and typically a sixfold to tenfold increase in cell-specific productivity.

Continuous and Integrated Expression and Purification of Recombinant Antibodies

This chapter introduces the necessary concepts to design continuous expression and purification processes for monoclonal antibodies. The operation of a perfusion bioreactor is discussed containing the preparation procedures, the seeding train and the preparation and control of a long-term production run. The downstream processes exploit the benefits of countercurrent chromatography. Their design from batch experiments is presented. The CaptureSMB process is introduced for continuous capturing while for polishing applications the design of the two-column MCSGP process is described. The chapter also puts these processes together in the context of their integration to an end-to-end production process.

Characterizing steady states of genome-scale metabolic networks in continuous cell cultures

In the continuous mode of cell culture, a constant flow carrying fresh media replaces culture fluid, cells, nutrients and secreted metabolites. Here we present a model for continuous cell culture coupling intra-cellular metabolism to extracellular variables describing the state of the bioreactor, taking into account the growth capacity of the cell and the impact of toxic byproduct accumulation. We provide a method to determine the steady states of this system that is tractable for metabolic networks of arbitrary complexity. We demonstrate our approach in a toy model first, and then in a genome-scale metabolic network of the Chinese hamster ovary cell line, obtaining results that are in qualitative agreement with experimental observations. We derive a number of consequences from the model that are independent of parameter values. The ratio between cell density and dilution rate is an ideal control parameter to fix a steady state with desired metabolic properties. This conclusion is robust even in the presence of multi-stability, which is explained in our model by a negative feedback loop due to toxic byproduct accumulation. A complex landscape of steady states emerges from our simulations, including multiple metabolic switches, which also explain why cell-line and media benchmarks carried out in batch culture cannot be extrapolated to perfusion. On the other hand, we predict invariance laws between continuous cell cultures with different parameters. A practical consequence is that the chemostat is an ideal experimental model for large-scale high-density perfusion cultures, where the complex landscape of metabolic transitions is faithfully reproduced.

Comparison of use of Vero cell line and suspension culture of murine macrophage to attenuation of virulence of Neospora caninum

In this study the tachyzoite yields of Neospora caninum were compared in two cell lines: Vero (African Green Monkey Kidney) and suspension culture of murine macrophage (J774) cell lines. Then, N. caninum were continuously passaged in these cell lines for 3 months and the effect of host cells on virulence of tachyzoites was assessed by broiler chicken embryonated eggs. Inoculation was performed in the chorioallantoic (CA) liquid of the embryonated eggs with different dilutions (0.5 × 10(4), 1.0 × 10(4), 1.5 × 10(4)) of tachtzoites isolated from these cell cultures. The mortality pattern and pathological changes of the dead embryos and hatched chickens were noted. Tissue samples of brain, liver and heart were examined by histopathological and detection of DNA of parasite by polymerase chain reaction (PCR). Also, consecutive sections of the tissues examined histologically were used for immunohistochemical (IHC) examination. Embryos inoculated with tachyzoites derived from Vero cell line (group V) showed a higher mortality rate (100%) than the embryos that received tachyzoites derived from J774 cell line (group J) (10% mortality rate). The results of this study indicated that the culture of N. caninum in J774 cell led to a marked increase in the number of tachyzoite yields and rapid attenuation in comparison to Vero, so the results were confirmed by IHC and PCR. This study is the first report of the significant effect of host cell on the attenuation of virulence of N. caninum tachyzoites. These findings could potentially provide a practical approach in the mass production of N. caninum tachyzoites, and also in producing live attenuated vaccine.

On-line immunoanalysis of monoclonal antibodies during a continuous culture of hybridoma cells

The monoclonal-antibody production of an immobilized hybridoma cell line cultivated in a fluidized-bed reactor was monitored on-line for nearly 900 h. The monoclonal antibody concentration was determined by an immuno affinity-chromatography method (ABICAP). Antibodies directed against the product, e.g. IgG, were immobilized on a micro-porous gel and packed in small columns. After all IgG present in the sample was bound to the immobilized antibodies, unbound proteins were removed by rinsing the column. Elution of the bound antibodies followed and the antibodies were determined by fluorescence. The analytical procedure was automated with a robotic device to enable on-line measurements. The correlation between the on-line determined data and antibody concentrations measured by HPLC was linear.A sampling system was constructed, which was based on a pneumatically actuated in-line membrane valve integrated into the circulation loop of the reactor. Separation of the cells from the sample stream was achieved by a depth filter made of glass-fibre, situated outside the reactor. Rapid obstruction of the filter by cells or cell debris and contamination of the sample system was avoided by intermittent rinsing of the sample system with a chemical solution. The intermittent rinsing of the filter, which had a surface of 4.8 cm(2), resulted in an operational capacity of up to 40 samples (1.0 l total sample volume). Both the sampling system and the analytical device functioned without failure during this long-term culture.The culture temperature was varied between 34 and 40 °C. Raising the temperature from 34 up to 37 °C resulted in a simultaneous increase of growth and specific antibody production rate. Specific metabolic rates of glucose, lactate, glutamine and ammonium stayed constant in this temperature range. A further enhancement of temperature up to 40 °C had a negative effect on the growth rate, whereas the specific monoclonal antibody production rate showed a small increase. The other specific metabolic rates also increased in the temperature range between 38 to 40 °C.

Rapid authentication of animal cell lines using pyrolysis mass spectrometry and auto-associative artificial neural networks

Pyrolysis mass spectrometry (PyMS) was used to produce biochemical fingerprints from replicate frozen cell cultures of mouse macrophage hybridoma 2C11-12, human leukaemia K562, baby hamster kidney BHK 21/C13, and mouse tumour BW-O, and a fresh culture of Chinese hamster ovary CHO cells. The dimensionality of these data was reduced by the unsupervised feature extraction pattern recognition technique of auto-associative neural networks. The clusters observed were compared with the groups obtained from the more conventional statistical approaches of hierarchical cluster analysis. It was observed that frozen and fresh cell line cultures gave very different pyrolysis mass spectra. When only the frozen animal cells were analysed by PyMS, auto-associative artificial neural networks (ANNs) were employed to discriminate between them successfully. Furthermore, very similar classifications were observed when the same spectral data were analysed using hierarchical cluster analysis. We demonstrate that this approach can detect the contamination of cell lines with low numbers of bacteria and fungi; this approach could plausibly be extended for the rapid detection of mycoplasma infection in animal cell lines. The major advantages that PyMS offers over more conventional methods used to type cell lines and to screen for microbial infection, such as DNA fingerprinting, are its speed, sensitivity and the ability to analyse hundreds of samples per day. We conclude that the combination of PyMS and ANNs can provide a rapid and accurate discriminatory technique for the authentication of animal cell line cultures.

Special Engineering Aspects

The specific characteristics of mammalian cells discussed in Chap. 2 require adapted solutions for bioreactor design and operation. Especially, cell damage by shear stress and aeration has to be considered. Therefore this chapter starts with a detailed discussion of shear stress effects on mammalian cells (anchorage-dependent and suspendable cells) in model systems and bioreactors, respectively, and consequences for reactor design. Appropriate oxygen supply is another critical issue, as adapted oxygen supply systems are required. Techniques for immobilization of cells, either grown on microcarriers in suspension culture or within macroporous carriers in fixed bed or fluidized bed reactors, are discussed as well. With respect to the operation of bioreactors, the characteristics of different culture modes (batch, fed-batch, chemostat, perfusion) are introduced and practical examples are given. Finally, concepts for monitoring of bioreactors, including offline and online methods as well as control loops (e.g. O₂, pH), are considered.

The “Push-to-Low” Approach for Optimization of High-Density Perfusion Cultures of Animal Cells

High product titer is considered a strategic advantage of fed-batch over perfusion cultivation mode. The titer difference has been experimentally demonstrated and reported in the literature. However, the related theoretical aspects and strategies for optimization of perfusion processes with respect to their fed-batch counterparts have not been thoroughly explored. The present paper introduces a unified framework for comparison of fed-batch and perfusion cultures, and proposes directions for improvement of the latter. The comparison is based on the concept of "equivalent specific perfusion rate", a variable that conveniently bridges various cultivation modes. The analysis shows that development of economically competitive perfusion processes for production of stable proteins depends on our ability to dramatically reduce the dilution rate while keeping high cell density, i.e., operating at low specific perfusion rates. Under these conditions, titer increases significantly, approaching the range of fed-batch titers. However, as dilution rate is decreased, a limit is reached below which performance declines due to poor growth and viability, specific productivity, or product instability. To overcome these limitations, a strategy referred to as "push-to-low" optimization has been developed. This approach involves an iterative stepwise decrease of the specific perfusion rate, and is most suitable for production of stable proteins where increased residence time does not compromise apparent specific productivity or product quality. The push-to-low approach was successfully applied to the production of monoclonal antibody against tumor necrosis factor (TNF). The experimental results followed closely the theoretical prediction, providing a multifold increase in titer. Despite the medium improvement, reduction of the specific growth rate along with increased apoptosis was observed at low specific perfusion rates. This phenomenon could not be explained with limitation or inhibition by the known nutrients and metabolites. Even further improvement would be possible if the cause of apoptosis were understood. In general, a strategic target in the optimization of perfusion processes should be the decrease of the cell-specific perfusion rate to below 0.05 nL/cell/day, resulting in high, batch-like titers. The potential for high titer, combined with high volumetric productivity, stable performance over many months, and superior product/harvest quality, make perfusion processes an attractive alternative to fed-batch production, even in the case of stable proteins.

Optimization of virus yield as a strategy to improve rabies vaccine production by Vero cells in a bioreactor

To improve rabies vaccine production by Vero cells, we have developed a strategy based on high cell density culture and optimization of virus yield. We have first optimized cell growth in spinner flask using a Taguchi's L8 experimental design. We analyzed the effects of the following factors: initial glucose and glutamine concentrations, Cytodex 1 concentration and the regulation of glucose level at 1 g l(-1). We have also investigated the effect of the following factor interactions: Cytodex 1 concentration/glutamine concentration, Cytodex 1 concentration/glucose concentration and glucose concentration/glutamine concentration. Statistical analysis of the collected data pointed to the initial glucose concentration, the regulation of glucose level at 1 g l(-1) and the interactions between Cytodex 1 concentration/initial glucose concentration and Cytodex 1 concentration/initial glutamine concentration as the parameters that affected cell growth. Using the optimal conditions determined earlier, we have studied Vero cell growth in a 7-l bioreactor and in batch culture, and obtained a cell density level equal to 3.6 +/- 0.2 x 10(6) cells ml-1. Cell infection with rabies virus (LP 2061/Vero strain) at a multiplicity of infection (MOI) of 0.3 using M199 medium supplemented with 0.2% bovine serum albumin (BSA), yielded a maximal virus titer equal to 8 +/- 1.6 x 10(7) Fluorescent Focus Units (FFU) ml-1. We have also studied Vero cell growth in a 7-l bioreactor using recirculation as a perfusion culture mode during cell proliferation step and perfusion for virus multiplication phase. In comparison to batch culture, we reached a higher cell density level that was equal to 10.1 +/- 0.5 x 10(6) cells ml-1. Cell infection under the conditions previously indicated, yielded 14l of virus harvest that had a virus titer equal to 2.6 +/- 0.5 x 10(7) FFU ml-1. The activity of the inactivated virus harvest showed a protective activity that meets WHO requirements.

Performance of small-scale CHO perfusion cultures using an acoustic cell filtration device for cell retention: characterization of separation efficiency and impact of perfusion on product quality

Several small-scale Chinese hamster ovary (CHO) suspension cultures were grown in perfusion mode using a new acoustic filtration system. The separation performance was evaluated at different cell concentrations and perfusion rates for two different CHO cell lines. It was found that the separation performance depends inversely on the cell concentration and perfusion rate. High media flow rates as well as high cell concentrations resulted in a significant drop in the separation performance, which limited the maximal cell concentration achievable. However, packed cell volumes of 10% to 16% (corresponding to 3 to 6. 10(7) cells/mL) could be reached and were maintained without additional bleeding after shifting the temperature to 33 degrees C. Perfusion, up to 50 days, did not harm the cells and did not result in a loss of performance of the acoustic filter as often seen with other perfusion systems. Volumetric productivities in perfusion mode were 2- to 12-fold higher for two cell lines producing two different glycoproteins when compared to fed-batch or batch processes using the same cell lines. Product concentrations were in the range of 20% to 80% of batch or fed-batch culture, respectively. In addition, using the protease-sensitive product rhesus thrombopoietin, we could show that cultivation in perfusion mode drastically reduced proteolysis when compared to a batch culture without addition of protease inhibitors such as leupeptin.

Reactor design for large scale suspension animal cell culture

The scale of operation of freely suspended animal cell culture has been increasing and in order to meet the demand for recombinant therapeutic products, this increase is likely to continue. The most common reactor types used are stirred tanks. Air lift fermenters are also used, albeit less commonly. No specific guidelines have been published for large scale (>/=10 000 L) animal cell culture and reactor designs are often based on those used for microbial systems. However, due to the large difference in energy inputs used for microbial and animal cell systems such designs may be far from optimal. In this review the importance of achieving a balance between mixing, mass transfer and shear effects is emphasised. The implications that meeting this balance has on design of vessels and operation, particularly in terms of strategies to ensure adequate mixing to achieve homogeneity in pH and dissolved gas concentrations are discussed.

Some myths and messages concerning the batch and continuous culture of animal cells

lt is often assumed that continuous processes are more difficult and less productive than a suite of batch processes for the production of a particular biomolecule. This paper cites two papers which have appeared in the literature which propound this view and examines in detaü the justification for the support of this contention. After reviewing those features where it is alleged that continuous processes are at a disadvantage, the authors of this paper conclude that the opposite is the case and that for suitable processes the most effective way of generating product is by the use of fully continuous processes. The choice of a particular process dependends on a variety of fixed and variable factors which are unique to the process. These factors are discussed and two decision trees are presented which are designed to facilitate the choice of the appropriate process technology.

Clarification of animal cell cultures on a large scale by continuous centrifugation

Animal cells from 80-L and 2000-L fed batch fermentations were removed by a prototype disc stack centrifuge in order to achieve a fast and reliable separation of solids from large quantities of cell culture fluids. The clarification capacity was excellent for animal cells but particles remained in the liquid phase and affected further downstream processing of the cell-free harvest fluid. No significant loss of product was observed. A number of parameters were monitored to optimize process conditions for use with animal cells.

Interaction of cell culture with downstream purification: a case study

Separation of product from secreting mammalian cells in the culture both means the transition from product generation to product isolation. This interface within a biotech production process has to perform a proper solid/liquid phase separation of the cell suspension to make the product containing fluid amenable for further purification. These subsequent steps require fluid with low occurrence of contaminants in order to function properly. The goal of this study was to evaluate some economic and fast cell separation methods for the preparation of a product fluid ready for use in further ultrafiltration and chromatographic processes. We have performed experiments to test the usefulness of disc stack centrifuges and tangential flow microfiltration units at large scale. Both systems revealed outstanding prospects with regard to throughput and scale up properties. However, the centrificgation did not lead to a fluid sufficiently free of particles for direct ultrafiltration or chromatography. Thus, an additional filtration step was necessary. On the other hand microfiltration led to an acceptable quality of process fluid directly. By optimisation of process parameters an effective, reproducible and robust cell separation can be obtained. However, our experience has been that such optimal conditions are somewhat specific for a narrow range. Thus, even the equipment functioning well with one type of cell would possibly not perform as well with another cell or even with the same cell under conditions slightly different to the usual situation.

Software sensors for the monitoring of perfusion cultures: evaluation of the hybridoma density and the medium composition from glucose concentration measurements

New software sensors based on the Extended Kalman Filter technique have been developed for the monitoring of animal cell perfusion cultures. They use a kinetic model describing the growth, death and metabolism of hybridoma cells as a function of the medium composition. The model was initially validated on a batch culture and found to correctly predict the continuous perfusion culture kinetics, except for the production of ammonia and lactate. Using the measurement of a single component in the culture medium, in this case glucose, the Extended Kalman Filter provides an excellent evaluation of the time variation of the concentrations of living and dead cells, of glutamine and antibodies, during the whole perfusion culture for a retained cell density rising from 1 to 11 x 10(6) cells.ml-1 inside the reactor.

Biological safety considerations in the production of health care products from recombinant organisms

Safety considerations in the field of recombinant technology and rDNA production of health care products have been under discussion since the beginning of this technology in 1973 and will certainly go on. However no adverse effects, which could have been attributed to rDNA technology have been observed. On the other hand many life-saving and life-improving drugs have been on the market for many years to the benefit of many patients. New technologies and products thereof often provoke uncertainties about their impact on the environment or society. This article discusses some potential risks in the application of rDNA technology to drugs as well as some benefits for patients, society and environment.

Problems in scale-up of biotechnology production processes

The unique nature of biotechnology processes adds to the complexity and difficulty of scale-up. Successful scale-up means a shortened cycle to full-scale production, competitive advantage, and cost savings. The many pitfalls as well as actual and potential scale-up problems are reviewed. Emphasis is placed on covering all areas of concern in planning, executing, and documenting key studies. Needs in technology transfer are discussed and regulatory requirements are incorporated into scale-up needs. A review of the recent literature is coupled with actual case studies; problem avoidance is stressed. Problems in asepsis, in construction, and in validation are discussed and potential solutions given. Organizational problems are noted. Finally, checklists are given for project planning, for a safety audit, and for timely attainment of successful scale-up. Eighty-two references are included.