Transient responses of hybridoma metabolism to changes in the oxygen supply rate in continuous culture

Mechanism for multiplicity of steady states with distinct cell concentration in continuous culture of mammalian cells

Continuous culture for the production of biopharmaceutical proteins offers the possibility of steady state operations and thus more consistent product quality and increased productivity. Under some conditions, multiplicity of steady states has been observed in continuous cultures of mammalian cells, wherein with the same dilution rate and feed nutrient composition, steady states with very different cell and product concentrations may be reached. At those different steady states, cells may exhibit a high glycolysis flux with high lactate production and low cell concentration, or a low glycolysis flux with low lactate and high cell concentration. These different steady states, with different cell concentration, also have different productivity. Developing a mechanistic understanding of the occurrence of steady state multiplicity and devising a strategy to steer the culture toward the desired steady state is critical. We establish a multi-scale kinetic model that integrates a mechanistic intracellular metabolic model and cell growth model in a continuous bioreactor. We show that steady state multiplicity exists in a range of dilution rate in continuous culture as a result of the bistable behavior in glycolysis. The insights from the model were used to devise strategies to guide the culture to the desired steady state in the multiple steady state region. The model provides a guideline principle in the design of continuous culture processes of mammalian cells.

A quantitative approach for understanding small-scale human mesenchymal stem cell culture - implications for large-scale bioprocess development

Human mesenchymal stem cell (hMSC) therapies have the potential to revolutionise the healthcare industry and replicate the success of the therapeutic protein industry; however, for this to be achieved there is a need to apply key bioprocessing engineering principles and adopt a quantitative approach for large-scale reproducible hMSC bioprocess development. Here we provide a quantitative analysis of the changes in concentration of glucose, lactate and ammonium with time during hMSC monolayer culture over 4 passages, under 100% and 20% dissolved oxgen (dO2 ), where either a 100%, 50% or 0% growth medium exchange was performed after 72h in culture. Yield coefficients, specific growth rates (h(-1) ) and doubling times (h) were calculated for all cases. The 100% dO2 flasks outperformed the 20% dO2 flasks with respect to cumulative cell number, with the latter consuming more glucose and producing more lactate and ammonium. Furthermore, the 100% and 50% medium exchange conditions resulted in similar cumulative cell numbers, whilst the 0% conditions were significantly lower. Cell immunophenotype and multipotency were not affected by the experimental culture conditions. This study demonstrates the importance of determining optimal culture conditions for hMSC expansion and highlights a potential cost savings from only making a 50% medium exchange, which may prove significant for large-scale bioprocessing.

Noninvasive oxygen measurements and mass transfer considerations in tissue culture flasks

Murine hybridomas were cultivated in tissue culture flasks. Dissolved oxygen tensions in the gas and liquid phases during cell growth were monitored. Oxygen levels were measured noninvasively by interrogating an oxygen-sensitive patch mounted on the interior surface of the tissue culture flask with an optrode from outside the tissue culture flask. Readings were made in tissue culture flasks with caps both cracked open and completely closed. Although the oxygen in the gas phase remained near atmospheric oxygen levels in both flasks, over time the liquid-phase oxygen tension at the bottom of the flasks reached zero during cell growth in both the open and closed tissue culture flasks. These results suggest that the widespread practice of cracking open tissue culture flask caps during cell growth with a view to supplying adequate oxygen to cells is ineffective and probably unnecessary.The mass transfer characteristics of the tissue culture flask were also studied. The dominant resistance to oxygen mass transfer to the sensor and the cells was through the liquid media. The mass transfer rates through the liquid layer under standard laboratory conditions were found to be greater than those predicted by diffusion alone. This suggests that mixing at a microscale occurs. Volumetric and specific oxygen consumption rates were also calculated from the sensor data. These consumption rates were comparable with values published elsewhere. (c) 1996 John Wiley & Sons, Inc.

The transient responses of hybridoma cells to nutrient additions in continuous culture: II. Glutamine pulse and step changes

The transient and steady-state responses of hybridoma growth and metabolism to glutamine pulse and step changes have been examined. Metabolic quotients are reported for oxygen, glucose, lactate, ammonia, glutamine, alanine, and other amino acids. The specific glutamine consumption rate increased rapidly after all glutamine additions, but the responses of the glucose and oxygen consumption rates and the cell concentration were found to depend on the intial feed glutamine concentration. The glucose consumption rate was 1.4-10.9 times that of glutamine, and serine and branched-chain amino acids were consumed in larger amounts at the higher glucose: glutamine uptake ratios. It was estimated that maintenance accounted for ca. 60% of the cellular ATP requirements at specific growth rates ranging from 0.57 to 0.68 day(-1).

Heterogeneous conditions in dissolved oxygen affect N-glycosylation but not productivity of a monoclonal antibody in hybridoma cultures

It is known that heterogeneous conditions exist in large-scale animal cell cultures. However, little is known about how heterogeneities affect cells, productivities, and product quality. To study the effect of non-constant dissolved oxygen tension (DOT), hybridomas were subjected to sinusoidal DOT oscillations in a one-compartment scale-down simulator. Oscillations were forced by manipulating the inlet oxygen partial pressure through a feedback control algorithm in a 220-mL bioreactor maintained at a constant agitation. Such temporal DOT oscillations simulate spatial DOT gradients that can occur in large scales. Different oscillation periods, in the range of 800 to 12,800 s (axis of 7% (air saturation) and amplitude of 7%), were tested and compared to constant DOT (10%) control cultures. Oscillating DOT decreased maximum cell concentrations, cell growth rates, and viability indexes. Cultures at oscillating DOT had an increased glycolytic metabolism that was evidenced by a decrease in yield of cells on glucose and an increase in lactate yield. DOT gradients, even several orders of magnitude higher than those expected under practical large-scale conditions, did not significantly affect the maximum concentration of an IgG(1) monoclonal antibody (MAb). The glycosylation profile of the MAb produced at a constant DOT of 10% was similar to that reported in the literature. However, MAb produced under oscillating culture conditions had a higher amount of triantennary and sialylated glycans, which can interfere with effector functions of the antibody. It was shown that transient excursions of hybridomas to limiting DOT, as occurs in deficiently mixed large-scale bioreactors, is important to culture performance as the oscillation period, and thus the time cells spent at low DOT, affected cell growth, metabolism, and the glycosylation pattern of MAb. Such results underline the importance of monitoring protein characteristics for the development of large-scale processes.

Accelerating perfusion process optimization by scanning non-steady-state responses

Perfusion processes provide consistent culture conditions, high productivity and low product residence times. However, process development can be slow due to the 1 week or more required to reach each steady state. The objective of this work was to accelerate process development in perfusion cultures by scanning non-steady-state transient responses to qualitatively predict steady-state performance. The method was tested using a shift in temperature every 3 days, scanned down by steps of 2 degrees C from 37 degrees C to 31 degrees C, then scanned up to 37 degrees C. Higher t-PA concentrations were predicted at lower temperatures, confirmed by subsequent pseudo-steady-state results. In most cases, transient values on the 3rd day were in close concordance with pseudo-steady-state values. To further accelerate process development, transient scanning was applied to small-scale, non-instrumented cultures. Similar results were obtained, although quantitative t-PA values were 15-30 times lower than in high cell density perfusion cultures. The method was further explored by investigating 1 day transient shifts in temperature where more variability was observed, suggesting that the cells were still adapting to the new environment. Nonetheless, the overall response again qualitatively predicted the pseudo-steady-state temperature response. Use of transient scanning in conjunction with pseudo-steady-state verification and refinement of optimal results could reduce process development time to a third or less of comparable steady-state-based optimization.

A new method for on-line measurement of the volumetric oxygen uptake rate in membrane aerated animal cell cultures

Oxygen is a key substrate in animal cell metabolism and its consumption is thus a parameter of great interest for bioprocess monitoring and control. A system for measuring it based on an oxygen balance on the liquid phase was developed. The use of a gas-permeable membrane offered the possibility to provide the required quantity of oxygen into the culture, while avoiding problems of foaming or shear stress generally linked to sparging. This aeration system allowed moreover to keep a known and constant k(L)a value through cultures up to 400 h. Oxygen uptake rate (OUR) was measured on-line with a very good accuracy of +/-5%, and the specific OUR for a CHO cell line was determined during batch (growth phase) and continuous culture as, respectively, equal to 2. 85x10(-13) and 2.54x10(-13) mol O(2) cell(-1) h(-1). It was also shown that OUR continuous monitoring gives actually more information about the metabolic state of the culture than the cell concentration itself, especially during transition phases like the end of the growth phase in a batch culture.

Measurement of volumetric (OUR) and determination of specific (qO2) oxygen uptake rates in animal cell cultures

Oxygen is a key substrate in animal cell metabolism. It has been reported that the oxygen uptake rate (OUR) is a good indicator of cellular activity, and even under some conditions, a good indicator of the number of viable cells. The measurement of OUR is difficult due to many different reasons. In particular, the very low specific consumption rate (0.2 x 10(-12) mol cell h-1), the sensitivity of the cells to variations in dissolved oxygen concentration and the difficulty to provide oxygen without damaging the cells are problems which must be taken into account for the development of OUR measurement methods. Different solutions based on an oxygen balance on either the liquid phase or around the entire reactor, and with a variable or stable concentration of dissolved oxygen have been reported. The accuracy of the OUR measurements and the required analytical devices are very different from method to method.

Fluxes and enzyme activities in central metabolism of myeloma cells grown in chemostat culture

Activities of enzymes in glycolysis, the pentose phosphate pathway, the tricarboxylic acid cycle, and glutaminolysis have been determined in the mouse myeloma SP2/0.Ag14. Cells were grown on IMDM medium with 5% serum in steady-state chemostat culture at a fixed dilution rate of 0.03 h-1. Three culture conditions, which differed in supply of glucose and oxygen, were chosen so as to change catabolic fluxes in the central metabolism, while keeping anabolic fluxes constant. In the three steady-state situations, the ratio between specific rates of glucose and glutamine consumption differed by more than twentyfold. The specific rates of glucose consumption and lactate production were highest at low oxygen supply, whereas the specific rate of glutamine consumption was highest in the culture fed with low amounts of glucose. Under low oxygen conditions, the specific production of ammonia increased and the consumption pattern of amino acids showed large changes compared with the other two cultures. For the three steady states, activities of key enzymes in glycolysis, the pentose phosphate pathway, glutaminolysis, and the TCA cycle were measured. The differences in the in vivo fluxes were only partially reflected in changes in enzyme levels. The largest differences were observed in the levels of glycolytic enzymes, which were elevated under conditions of low oxygen supply. High activities of phosphoenolpyruvate carboxykinase (E.C. 4.1.1.32) in all cultures suggest an important role for this enzyme as a link between glutaminolysis and glycolysis. For all enzymes, in vitro activities were found that could accommodate the estimated maximum in vivo fluxes. These results show that the regulation of fluxes in central metabolism of mammalian cells occurs mainly through modulation of enzyme activity and, to a much lesser extent, by enzyme synthesis.

Noninvasive oxygen measurements and mass transfer considerations in tissue culture flasks

Murine hybridomas were cultivated in tissue culture flasks. Dissolved oxygen tensions in the gas and liquid phases during cell growth were monitored. Oxygen levels were measured noninvasively by interrogating an oxygen-sensitive patch mounted on the interior surface of the tissue culture flask with an optrode from outside the tissue culture flask. Readings were made in tissue culture flasks with caps both cracked open and completely closed. Although the oxygen in the gas phase remained near atmospheric oxygen levels in both flasks, over time the liquid-phase oxygen tension at the bottom of the flasks reached zero during cell growth in both the open and closed tissue culture flasks. These results suggest that the widespread practice of cracking open tissue culture flask caps during cell growth with a view to supplying adequate oxygen to cells is ineffective and probably unnecessary.The mass transfer characteristics of the tissue culture flask were also studied. The dominant resistance to oxygen mass transfer to the sensor and the cells was through the liquid media. The mass transfer rates through the liquid layer under standard laboratory conditions were found to be greater than those predicted by diffusion alone. This suggests that mixing at a microscale occurs. Volumetric and specific oxygen consumption rates were also calculated from the sensor data. These consumption rates were comparable with values published elsewhere. (c) 1996 John Wiley & Sons, Inc.

The influence of dissolved oxygen tension on the metabolic activity of an immobilized hybridoma population

In the study presented here a laboratory scale (150 ml) fluidized bed bioreactor was used as a tool for making kinetic measurements on the production of monoclonal antibodies (MAb) with a hybridoma cell line. We determined the influence of dissolved oxygen tension (DOT) on the metabolic activity of a hybridoma population immobilized in macroporous collagen microspheres. The data obtained showed a reduction of the metabolic activity of the immobilized population at reduced DOT, the total number of immobilized cells, however, remained constant. At decreasing DOT an increasing lactate yield from glucose at reduced glutamine consumption was noticed, indicating a shift in the pattern of substrate usage. A mathematical description of maintenance metabolism was formulated and the parameters of growth and maintenance requirements were calculated. A growth associated MAb production was determined under the conditions applied leading to space time yields of 225 mg MAb per liter of total reactor volume and day.

Kinetic analysis of cellular nucleotides correlated to growth and tPA production in perfusion cultures of human fibroblast cells

A maximum cell density of 8 x 10(6) viable human fibroblast cells/ml was obtained on microcarriers at a perfusion rate of 0.6 mL/min-a rate which maintained a quasi-steady state. The maximum tPA production was approximately 1.2 micrograms/ml and obtained when the cell density was relatively constant during the later periods of cultivation. Specific UTP and tPA production rates had a correlation factor of 0.85, and cell growth to oxygen consumption had an 0.92 correlation factor. ATP generation was strongly correlated to glutamine consumption, with a lower correlation to oxygen utilization.

A nuclear magnetic resonance technique for determining hybridoma cell concentration in hollow fiber bioreactors

We have developed a technique for determining cell concentration in a hollow fiber bioreactor based on 23Na nuclear magnetic resonance (NMR) spectroscopy. Cell concentrations determined with this method agreed closely with concentrations calculated from 31P NMR nucleoside triphosphate (NTP) measurements and oxygen consumption rate measurements. Oxygen transfer limitations, which can complicate cell mass determinations based on oxygen consumption rates, were shown to be negligible for the bioreactor used. Specific antibody production rates in hollow fiber culture, calculated from these cell number estimates, were similar to those found in suspension culture for this cell line.

Studies on monoclonal antibody production by a hybridoma cell line (C1E3) immobilised in a fixed bed, porosphere culture system

The aim of this study was to investigate the potential of fixed beds of macroporous glass spheres as a production process for animal cell products. The growth, metabolism and monoclonal antibody expression of a mouse-mouse hybridoma cell line was investigated in order to both test the potential of and to optimise the system. After the initial growth phase, the culture went into a steady-state phase brought on by glutamine limitation. An event occurred after 120-160 h of steady-state operation which destabilised the culture, causing a decline in productivity, after which the culture recovered. This event was analysed in detail to determine its cause, and whether a major switch in metabolic function had occurred. The parameter which correlated most closely to antibody production rate was oxygen, but as this was kept constant in the void medium of the bed it has to be concluded that oxygen diffusion into the spheres was the regulatory factor. A comparison of the fixed bed and a flask culture identified interesting differences in glucose metabolism between the two systems. The data gave strong indications as to how the productivity of the fixed bed system can be further improved. This includes optimisation of the glutamine concentration and modifying the porous structure of the spheres to improve diffusion characteristics.

Methods for determination of growth-rate-dependent changes in hybridoma volume, shape and surface structure during continuous recycle

Hybridoma volume and surface membrane structure were found to vary as a function of specific growth rate using a method of cell recycle with continuous medium perfusion to vary growth rate. Mean hybridoma volume determined at constant osmolality by both electronic particle counting and scanning electron microscopic (SEM) methods indicated that rapidly growing cells are significantly larger than very slowly growing cells. We have previously determined that during both rapid and slow growth over a range of L-glutamine provision rates (Gln PR) that specific monoclonal antibody (MoAb) secretion rate was not changed. In this study a constant MoAb secretion rate per unit of membrane area was found which may indicate that changing membrane area is not a rate-determining factor in MoAb secretion. SEM methods were of limited use for accurate determination of cell volume due to cell shrinkage and large coefficients of variations. In spite of this limitation, SEM stereology methods were useful in confirming that cells remained spherical over a wide range of specific growth rates and that hybridoma cells were not circular. Sequential SEM observations also revealed that surface membrane structure of the 9.2.27 murine hybridoma investigated was correlated with growth rate. Under conditions of very slow growth, hybridoma surface microvilli density appeared to be significantly reduced.

Serum can act as a shear protecting agent in agitated hybridoma cell cultures

Hybridoma cells, S3H5/gamma 2bA2, were grown in spinner flasks containing RPMI 1640 media at different levels of serum. Cells in low serum media (1% fetal bovine serum) were more sensitive to shear induced by mechanical agitation than cells in high serum media (10%), indicating that serum can protect cells from shear induced by mechanical agitation. The data suggest that serum alters the physiological properties of the cells to make them more shear-resistant, rather than by changing the physico-chemical properties of medium.