Monitoring of mammalian cell growth and virus production process using on-line oxygen uptake rate measurement

Real-time characterization of mammalian cell culture bioprocesses by magnetic sector MS

Mammalian cell culture processes were characterized upon the analysis of the exhaust-gas composition achieved through the on-line integration of a magnetic sector MS analyser with benchtop bioreactors. The non-invasive configuration of the magnetic sector MS provided continuous evaluation of the bioreactor's exhaust gas filter integrity and facilitated the accurate quantification of O2 and CO2 levels in the off-gas stream which ensured preserved bioreactor sterility prior to cell inoculation and provided evidence of the ongoing cellular respiratory activity throughout the cultures. Real-time determination of process parameters such as the Respiratory Quotient (RQ) allowed for precise pin-pointing of the occurrence of shifts in cellular metabolism which were correlated to depletion of key nutrients in the growth medium, demonstrating the suitability of this technology for tracking cell culture process performance.

New developments in online OUR monitoring and its application to animal cell cultures

The increasing demand for biopharmaceuticals produced in mammalian cells has driven the industry to enhance the productivity of bioprocesses through intensification of culture process. Fed-batch and perfusion culturing strategies are considered the most attractive choices, but the application of these processes requires the availability of reliable online measuring systems for the estimation of cell density and metabolic activity. This manuscript reviews the methods (and the devices used) for monitoring of the oxygen consumption, also known as oxygen uptake rate (OUR), since it is a straightforward parameter to estimate viable cell density and the physiological state of cells. Furthermore, as oxygen plays an important role in the cell metabolism, OUR has also been very useful to estimate nutrient consumption, especially the carbon (glucose and glutamine) and nitrogen (glutamine) sources. Three different methods for the measurement of OUR have been developed up to date, being the dynamic method the golden standard, even though DO and pH perturbations generated in the culture during each measurement. For this, many efforts have been focused in developing non-invasive methods, such as global mass balance or stationary liquid mass balance. The low oxygen consumption rates by the cells and the high accuracy required for oxygen concentration measurement in the gas streams (inlet and outlet) have limited the applicability of the global mass balance methodology in mammalian cell cultures. In contrast, stationary liquid mass balance has successfully been implemented showing very similar OUR profiles compared with those obtained with the dynamic method. The huge amount of studies published in the last years evidence that OUR have become a reliable alternative for the monitoring and control of high cell density culturing strategies with very high productivities.

Optimization of HEK-293S cell cultures for the production of adenoviral vectors in bioreactors using on-line OUR measurements

The culture of HEK-293S cells in a stirred tank bioreactor for adenoviral vectors production for gene therapy is studied. Process monitoring using oxygen uptake rate (OUR) was performed. The OUR was determined on-line by the dynamic method, providing good information of the process evolution. OUR enabled cell activity monitoring, facilitating as well the determination of the feeding rate in perfusion cultures and when to infect the culture. Batch cultures were used to validate the monitoring methodology. A cell density of 10×10(5)cell/mL was infected, producing 1.3×10(9) infectious viral particles/mL (IVP/mL). To increase cell density values maintaining cell specific productivity, perfusion cultures, based on tangential flow filtration, were studied. In this case, OUR measurements were used to optimize the dynamic culture medium feeding strategy, addressed to avoid any potential nutrient limitation. Furthermore, the infection protocol was defined in order to optimize the use of the viral inoculum, minimizing the uncontrolled release of particles through the filter unit mesh. All these developments enabled an infection at 78×10(5)cell/mL with the consequent production of 44×10(9)IVP/mL, representing a cell specific productivity 4.3 times higher than for the batch culture.

Scalable production of influenza virus in HEK-293 cells for efficient vaccine manufacturing

Cell culture processes offer an attractive alternative to conventional chicken egg-based influenza vaccine production methods. However, most protocols still rely on the use of adherent cells, which makes process scale-up a challenging issue. In this study, it is demonstrated that the HEK-293 human cell line is able to efficiently replicate influenza virus. Production in serum-free suspension of HEK-293 cultures resulted in high titers of infectious influenza viruses for different subtypes and variants including A/H1, A/H3 and B strains. After virus adaptation and optimization of infection conditions, production in 3-L bioreactor resulted in titers of up to 10(9)IVP/mL demonstrating the scale-up potential of the process.

On-line monitoring of responses to nutrient feed additions by multi-frequency permittivity measurements in fed-batch cultivations of CHO cells

Changes in the nutrient availability of mammalian cell cultures are reflected in the beta-dispersion parameter characteristic frequency (f ( C )) and the on-line dual frequency permittivity signal. Multi-frequency permittivity measurements were therefore evaluated in fed-batch cultivations of two different CHO cell lines. Similar responses to nutrient depletions and discontinuous feed additions were monitored in different cultivation phases and experimental setups. Sudden increases in permittivity and f ( C ) occurred when feed additions were conducted. A constant or declining permittivity value in combination with a decrease in f ( C ) indicated nutrient limitations. f ( C ) correlated well with changes in oxygen uptake rate when cell diameter remained constant, indicating that metabolic activity is reflected in the value of f ( C ). When significant cell size changes occurred during the cultivations, the analysis of the beta-dispersion parameters was rendered complex. For the application of our findings in other systems it will be hence required to conduct additional off-line measurements. Based on these results, it is hypothesized that multi-frequency permittivity measurements can give information on the intracellular or physiological state in fed-batch mode. Similar observations were made when using different cell lines and feeding strategies, indicating that the findings are transferable to other cell lines and systems. The results should lead to an improved understanding of routine fed-batch processes. Additional studies are, however, required to explore how these observations can be used for fed-batch process development and optimization.

Bioprocess monitoring and computer control: key roots of the current PAT initiative

This review article has been written for the journal, Biotechnology and Bioengineering, to commemorate the 70th birthday of Daniel I.C. Wang, who served as doctoral thesis advisor to each of the co-authors, but a decade apart. Key roots of the current PAT initiative in bioprocess monitoring and control are described, focusing on the impact of Danny Wang's research as a professor at MIT. The history of computer control and monitoring in biochemical processing has been used to identify the areas that have already benefited and those that are most likely to benefit in the future from PAT applications. Past applications have included the use of indirect estimation methods for cell density, expansion of on-line/at-line and on-line/in situ measurement techniques, and development of models and expert systems for control and optimization. Future applications are likely to encompass additional novel measurement technologies, measurements for multi-scale and disposable bioreactors, real time batch release, and more efficient data utilization to achieve process validation and continuous improvement goals. Dan Wang's substantial contributions in this arena have been one key factor in steering the PAT initiative towards realistic and attainable industrial applications.

Metabolic flux analysis of HEK-293 cells in perfusion cultures for the production of adenoviral vectors

To meet increasing needs of adenovirus vectors for gene therapy programs, development of efficient and reproducible production processes is required. Perfusion cultures were employed to allow infection at greater cell concentrations. In an effort to define culture conditions resulting in enhanced productivities, experiments performed at different feed rates and infected at various cell densities were compared using metabolic flux analysis. The highest specific product yields were achieved in experiments performed at high perfusion rates and/or low cell concentrations. The intracellular flux analysis revealed that these experiments exhibited greater glycolytic fluxes, slightly higher TCA fluxes, and greater ATP production rates at the time of infection. In contrast, cultures infected at high cell density and/or low medium renewal rates were characterized by a more efficient utilization of glucose at the time of infection, but the specific product yields achieved were lower. The intracellular flux analysis provided a rational basis for the implementation of a feeding strategy that allowed successful infection at a density of 5x10(6)cells/ml.

Insights into adenoviral vector production kinetics in acoustic filter-based perfusion cultures

One of the major limitations in the production of adenoviral vectors is the reduction in cell-specific productivity observed for increasing cell density at infection in batch cultures. This observation strongly suggests some nutrient depletion and/or metabolite inhibition in the media. These limitations have been partially overcome through other feeding strategies, such as fed-batch and sequential batch operations. To improve these results, we evaluated perfusion as a strategy to increase the volumetric productivity of HEK-293 cell cultures, by allowing productive infection at higher cell densities. An acoustic cell separator was employed in consideration of the increased shear sensitivity of the cells during the infection phase. The effects of perfusion rate and cell density at infection on the production of a recombinant adenovirus expressing the GFP were investigated. The perfusion mode allowed successful infection at cell densities in the range of 2.4-3 x 10(6) cell/mL, while maintaining a similar cell specific productivity (17,900 +/- 2400 VP/cell) to that of a batch infected at a low cell density (5 x 10(5) cell/mL). The highest virus concentrations (4.1 +/- 0.6 x 10(10) VP/mL) were attained for a feed rate of 2 vol/d and constituted a fivefold increase compared to a batch with medium replacement. Rapid assessment of the infection status was achieved through the use of on-line monitoring of respiration, fluorescence, and biovolume. Analysis of the kinetics of nutrient consumption and metabolite production revealed that a reduction in specific productivity is correlated with reduced metabolic activity.

Serum-free suspension cultivation of PER.C6(R) cells and recombinant adenovirus production under different pH conditions

PER.C6(R) cell growth, metabolism, and adenovirus production were studied in head-to-head comparisons in stirred bioreactors under different pH conditions. Cell growth rate was found to be similar in the pH range of 7.1-7.6, while a long lag phase and a slower growth rate were observed at pH 6.8. The specific consumption rates of glucose and glutamine decreased rapidly over time during batch cell growth, as did the specific lactate and ammonium production rates. Cell metabolism in both infected and uninfected cultures was very sensitive to culture pH, resulting in dramatic differences in glucose/glutamine consumption and lactate/ammonium production under different pH conditions. It appeared that glucose metabolism was suppressed at low pH but the efficiency of energy production from glucose was enhanced. Adenovirus infection resulted in profound changes in cell growth and metabolism. Cell growth was largely arrested under all pH conditions, while glucose consumption and lactate production were elevated post virus infection. Virus infection induced a reduction in glutamine consumption at low pH but an increase at high pH. The optimal pH for adenovirus production was found to be 7.3 under the experimental conditions used in the study. Deviations from this optimum resulted in significant reductions of virus productivity. The results indicate that culture pH is a very critical process parameter in PER.C6(R) cell culture and adenovirus production.

Large-scale mammalian cell culture

Mammalian cell culture continues to draw major research efforts. A great deal of progress has recently been made in cellular physiology, especially in factors adversely affecting cell growth or viability. Through molecular genetic manipulation, cells are more readily cultivated in a medium free of animal proteins. Achieving a high cell concentration and high viability continue to be common themes in engineering research. The need to implement a control policy for fed-batch and perfusion cultures has prompted increased efforts in process monitoring and control. Integrating these advances will be beneficial for ensuring product quality and process consistency.