Effect of dilution rate on growth, productivity, cell cycle and size, and shear sensitivity of a hybridoma cell in a continuous culture

Osmolality Effects on CHO Cell Growth, Cell Volume, Antibody Productivity and Glycosylation

The addition of nutrients and accumulation of metabolites in a fed-batch culture of Chinese hamster ovary (CHO) cells leads to an increase in extracellular osmolality in late stage culture. Herein, we explore the effect of osmolality on CHO cell growth, specific monoclonal antibody (mAb) productivity and glycosylation achieved with the addition of NaCl or the supplementation of a commercial feed. Although both methods lead to an increase in specific antibody productivity, they have different effects on cell growth and antibody production. Osmolality modulation using NaCl up to 470 mOsm kg^-1 had a consistently positive effect on specific antibody productivity and titre. The addition of the commercial feed achieved variable results: specific mAb productivity was increased, yet cell growth rate was significantly compromised at high osmolality values. As a result, Feed C addition to 410 mOsm kg^-1 was the only condition that achieved a significantly higher mAb titre compared to the control. Additionally, Feed C supplementation resulted in a significant reduction in galactosylated antibody structures. Cell volume was found to be positively correlated to osmolality; however, osmolality alone could not account for observed changes in average cell diameter without considering cell cycle variations. These results help delineate the overall effect of osmolality on titre and highlight the potentially negative effect of overfeeding on cell growth.

Osmolytes in vaccine production, flocculation and storage: a critical review

Small molecule osmolytes, responsible for protecting stresses have long been known to rescue proteins and enzymes from loss of function. In addition to protecting macromolecules integrity, many osmolytes also act as potential antioxidant and also help to prevent protein aggregation, amyloid formation or misfolding, and therefore are considered promising molecules for neurodegenerative and many other genetic diseases. Osmolytes are also known to be involved in the regulation of several key immunological processes. In the present review we discuss in detail the effect of these compounds on important aspects of vaccines i.e., increasing the efficiency, production and purification steps. The present review therefore will help researchers to make a better strategy in vaccine production to formulation by incorporating specific and appropriate osmolytes in the processes.

High glucose and low specific cell growth but not mild hypothermia improve specific r-protein productivity in chemostat culture of CHO cells

In the biopharmaceutical sector, Chinese hamster ovary (CHO) cells have become the host of choice to produce recombinant proteins (r-proteins) due to their capacity for correct protein folding, assembly, and posttranslational modification. However, the production of therapeutic r-proteins in CHO cells is expensive and presents insufficient production yields for certain proteins. Effective culture strategies to increase productivity (q_p) include a high glucose concentration in the medium and mild hypothermia (28–34 °C), but these changes lead to a reduced specific growth rate. To study the individual and combined impacts of glucose concentration, specific growth rate and mild hypothermia on culture performance and cell metabolism, we analyzed chemostat cultures of recombinant human tissue plasminogen activator (rh-tPA)-producing CHO cell lines fed with three glucose concentrations in feeding media (20, 30 and 40 mM), at two dilution rates (0.01 and 0.018 1/h) and two temperatures (33 and 37 °C). The results indicated significant changes in cell growth, cell cycle distribution, metabolism, and rh-tPA productivity in response to the varying environmental culture conditions. High glucose feed led to constrained cell growth, increased specific rh-tPA productivity and a higher number of cells in the G2/M phase. Low specific growth rate and temperature (33 °C) reduced glucose consumption and lactate production rates. Our findings indicated that a reduced specific growth rate coupled with high feed glucose significantly improves r-protein productivity in CHO cells. We also observed that low temperature significantly reduced q_p, but not cell growth when dilution rate was manipulated, regardless of the glucose concentration or dilution rate. In contrast, we determined that feed glucose concentration and consumption rate were the dominant aspects of the growth and productivity in CHO cells by using multivariate analysis.

Understanding central carbon metabolism of rapidly proliferating mammalian cells based on analysis of key enzymatic activities in GS-CHO cell lines

The central carbon metabolism (glycolysis, the pentose phosphate pathway [PPP], and the tricarboxylic acid [TCA] cycle) plays an essential role in the supply of biosynthetic precursors and energy. How the central carbon metabolism changes with the varying growth rates in the in vitro cultivation of rapidly proliferating mammalian cells, such as cancer cells and continuous cell lines for recombinant protein production, remains elusive. Based on relationships between the growth rate and the activity of seven key enzymes from six cell clones, this work reports finding an important metabolic characteristic in rapidly proliferating glutamine synthetase-Chinese hamster ovary cells. The key enzymatic activity involved in the TCA cycle that is responsible for the supply of energy became elevated as the growth rate exhibited increases, while the activity of key enzymes in metabolic pathways (glycolysis and the PPP), responsible for the supply of biosynthetic precursors, tended to decrease-suggesting that rapidly proliferating cells still depended predominantly on the TCA cycle rather than on aerobic glycolysis for their energetic demands. Meanwhile, the growth-limiting resource was most likely biosynthetic substrates rather than energy provision. In addition, the multifaceted role of glucose-6-phosphate isomerase (PGI) was confirmed, based on a significant correlation between PGI activity and the percentage of G2/M-phase cells.

Death rate in a small air-lift loop reactor of vero cells grown on solid microcarriers and in macroporous microcarriers

The death rate of Vero cells grown on Cytodex-3 microcarrierswas studied as a function of the gas flow rate in a smallair-lift loop reactor. The death rate may be described byfirst-order death-rate kinetics. The first-order death-rateconstant as calculated from the decrease in viable cells, theincrease in dead cells and the increase in LDH activity islinear proportional to the gas flow rate, with a specifichypothetical killing volume in which all cells are killed ofabout 2.10(-3)m(3) liquid per m(3) of air bubbles.In addition, an experiment was conducted in the sameair-lift reactor with Vero cells grown inside porous Asahimicrocarriers. The specific hypothetical killing volumecalculated from this experiment has a value of 3.10(-4)m(3) liquid per m(3) of air bubbles, which shows thatthe porous microcarriers were at least in part able to protectthe cells against the detrimental hydrodynamic forcesgenerated by the bubbles.

Scale up aspects of sparged insect-cell bioreactors

CONCLUSION

In this chapter we have attempted to evaluate the most important parameters which can be useful for the pur-pose of design and scale up. Insect cells and animal cells in general can be grown well in large vessels. However, none of the theories and parameters discussed in this chapter have been validated on a larger scale than laboratory and small pilot reactors. Selection of the most suitable design and scale-up method there-fore needs in particular studies in larger vessels. The Kolmogorov theory and the killing-volume model are in this respect the most promising approaches for the optimal design of large-scale animal-cell bioreactors.

The effect of the catalytic topoisomerase II inhibitor dexrazoxane (ICRF-187) on CC9C10 hybridoma viability and productivity

The effect of dexrazoxane on monoclonal antibody (Mab) production by CC9C10 hybridoma cells was investigated. Dexrazoxane is a catalytic inhibitor of DNA topoisomerase II. DNA topoisomerase II has a critical role in DNA metabolism and its inhibition by dexrazoxane can prevent completion of cytokinesis. Incubation of hybridomas with dexrazoxane was found to increase specific monoclonal antibody production by up to four-fold. However, due to the growth inhibitory effects of dexrazoxane the total Mab yield decreased by 40%. Under high density culture conditions(defined here as 10(6) cells ml(-1)) specific monoclonal antibody production increased by up to 37%, which was, however, accompanied by up to a 48% decrease in Mab yield. Hybridomasthat were incubated with dexrazoxane significantly increased in size due to the inhibition of cytokinesis. Dexrazoxane was also observed to induce a delayed apoptosis in the hybridomas. The caspase inhibitor Z-VAD-fmk slightly decreased the apoptotic effects of dexrazoxane. Preincubation with the caspase inhibitorZ-Asp-CH2-DCB had no effect on dexrazoxane-treated hybridomas, but it did have antiapoptotic effects on the untreated hybridomas which normally undergo a significant basal level of apoptosis. In conclusion, dexrazoxane-induced growth inhibition (which results in higher specific antibody production) and apoptosis inhibition (which results in prolonged viability) has the potential to significantly enhance the productivity of hybridoma cell cultures.

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.

A simple apparatus for measuring cell settling velocity

Accurate cell settling velocity determination is critical for perfusion culture using a gravity settler for cell retention. We have developed a simple apparatus (a "settling column") for measuring settling velocity and have validated the procedure with 15-μm polystyrene particles with known physical properties. The measured settling velocity of the polystyrene particles is within 4% of the value obtained using the traditional Stokes' law approach. The settling velocities of three hybridoma cell lines were measured, resulting in up to twofold variation among cell lines, and the values decreased as the cell culture aged. The settling velocities of the nonviable cells were 33-50% less than the corresponding viable cells. The significant variation of settling velocities among cell populations and growth phases confirms the necessity of routine measurement of this property during long-term perfusion culture.

Growth and production modeling in hybridoma continuous cultures

Several experimental data on continuous cultures of hybridoma cells show that monoclonal antibody productivity is a decreasing function of dilution rate. It has been suggested that this unusual behavior may be due to the arrest of a fraction of cycling cells at a critical point of Phase G(1). Although this hypothesis has been recently investigated by using population balance models, mathematical analysis has been performed without accounting for the dynamics of the arrested cells properly. In this article, a more general and accurate approach is presented and new specific assumptions are introduced to characterize the arrest and the later progress through the cycle. Two different models (stochastic and deterministic) and two different critical points for the arrest (at the beginning and at the end of G(1)) are considered. The cell cycle parameters are estimated so that data predicted by the model fit those reported in the literature. In particular, the fraction of arrested cells, the cell arrest probability, and the mean cell generation time are computed as functions of the dilution rate. Results so far obtained predict that there is an optimal value of dilution rate for maximizing specific production rate of monoclonal antibody.

Cell cycle kinetics and monoclonal antibody productivity of hybridoma cells during perfusion culture

The flow-cytometric (FCM) analysis of bivariate DNA/lgG distributions has been conducted to study the cell cycle kinetics and monoclonal antibody (MAb) production during perfusion culture of hybridoma cells. Three different perfusion rates were employed to demonstrate the dependency of MAb synthesis and secretion on cell cycle and growth rate. The results showed that, during the rapid growth period of perfusion culture, the level of intracellular igG contents of hybridoma cells changed significantly at each perfusion rate, while the DNA histograms showing cell cycle phases were almost constant. Meanwhile, during the reduced growth period of perfusion culture, the fraction of cells in the S phase decreased, and the fraction cells in the G1/G0 phase increased with decreasing growth rate. The fraction of cells in the G2/M phase was relatively constant during the whole period of perfusion culture. Positive correlation was found between mean intracellular IgG contents and the specific MAb production rate, suggesting that the deletion of intracellular IgG contents by a flow cytometer could be used as a good indicator for the prediction of changes in specific MAb productivity following manipulation of the culture condition.

Kinetic simulation of a centrifugal bioreactor for high population density hybridoma culture

Demand for increasingly complex post-translationally modified proteins, such as monoclonal antibodies (mAbs), necessitates the use of mammalian hosts for production. The focus of this article is a continuous centrifugal bioreactor (CCBR) capable of increasing volumetric productivity for mAb production through high density hybridoma culture, exceeding 10(8) cells/mL. At these extreme densities, environmental conditions such as substrate and inhibitor concentrations rapidly change dramatically affecting the growth rate. The development of a kinetic model predicting glucose, mAb, lactate, and ammonium concentrations based on dilution rate and cell density is shown in this article. Additionally, it is found that pH affects both growth rate and viability, and a range of 6.9-7.4 is needed to maintain growth rate above 90% of the maximum. Modeling shows that operating an 11.4 mL CCBR inoculated with 2.0 x 10(7) cells/mL at a dilution rate of 1.3 h(-1), results in a predicted growth rate 82% of the maximum value. At the same dilution rate increasing density to 6.0 x 10(7) cells/mL decreases the predicted growth rate to 60% of the maximum; however, by increasing dilution rate to 6.1 h(-1) the growth rate can be increased to 86% of the maximum. Using the kinetic model developed in this research, the concentration of glucose, mAb, lactate, and ammonium are all predicted within 13% of experimental results. This model and an understanding of how RPM impacts cell retention serve as valuable tools for maintaining high density CCBR cultures, ensuring maximum growth associated mAb production rates.

Noninvasive estimation of cell cycle phase and proliferation rate of human mesenchymal stem cells by phase-shifting laser microscopy

The simultaneous determination of the cell cycle phase of individual adherent mesenchymal stem cells (MSCs) using a fluorescence microscope after staining with 4',6-diamidine-2'-phenylindole dihydrochloride and bromodeoxyuridine and the laser phase shift by phase-shifting laser microscopy (PLM) revealed that the laser phase shift of cells in the G(2)/M phase was markedly higher than that of cells in the G(0)/G(1) phase. Even in the synchronous cultures to G(0)/G(1) and G(2)/M cell cycle phases, the laser phase shift of the cells in the G(2)/M phase was markedly higher than that of the cells in the G(0)/G(1) phase. The analysis of the cultures of MSCs from different donors with the addition of FGF2 at different concentrations revealed that there was a marked negative correlation between the average phase shift and mean generation time. In conclusion, it is possible to estimate noninvasively the proliferation activity of MSCs population by measuring the phase shift using PLM.

Application of a continuous bioreactor cascade to study the effect of linoleic acid on hybridoma cell physiology

The aim of the present study is to demonstrate the use of controlled bioreactors for toxicological studies. As a model system the effect of linoleic acid on hybridoma cells is studied in two well-controlled continuously operated bioreactors placed in series. In the first reactor the effect on rapid proliferating cells can be studied, while in the second reactor a special steady state is created, which allows studying the effect on apoptotic cells. Experiments are done at 0, 25, and 50 microM linoleic acid. At the end of the experiment with 50 microM linoleic acid, the concentration of linoleic acid is increased stepwise to determine the cytotoxic level. For rapid proliferating cells exposed to 25 and 50 microM stimulation of growth was observed. At 50 microM there was at the same time an increase in cell death through apoptosis. For stressed apoptotic cells linoleic acid caused partial growth inhibition at 25 and 50 microM and arrest of cell proliferation in the G(2)/M phase at 50 microM. For both, rapid proliferating cells and stressed apoptotic cells, complete growth inhibition occurred at 85 microM, with cells being arrested in the G(2)/M phase and dying mainly through necrosis. Cells in the bioreactor system appeared to be more sensitive towards linoleic acid than cells grown in multi-well plates. (IC(50) = 300 microM; IC(100) = 400 microM). Altogether the results of the present study reveal that the biostat experiments allow detailed analysis of the effect of a bioactive ingredient on cell physiology and behavior.

Effect of Bcl-2 overexpression on cell cycle and antibody productivity in chemostat cultures of myeloma NS0 cells

Chemostat cultures of NS0 cell lines were carried out at dilution rates ranging from 0.8 d(-1) to 0.2 d(-1). Compared with the control, the viable cell density of the Bcl-2 cell line was approximately 10% higher at 0.8 d(-1) and increased to 55% when the dilution rate was reduced to 0.2 d(-1). As the dilution rate was reduced, the viability of the two cultures diverged reaching a difference of 43% at 0.2 d(-1). The specific growth rate of the control cells was the same as the dilution rate down to a value of 0.6 d(-1). By contrast, the specific growth rate of Bcl-2 cells was parallel to the dilution rate down to a value as low as 0.3 d(-1). For both NS0 cell lines, the G1 cell population decreased, while the S and G2/M cell populations increased as the dilution rate was reduced. The antibody titer of the control cells increased from 7 to 21 microg.ml(-1) as the dilution rate was reduced from 0.8 to 0.2 d(-1). With an initial increase from 2 to 15 microg.ml(-1) as the dilution rate was reduced from 0.8 to 0.4 d(-1), the antibody titer of the Bcl-2 cells remained constant as the dilution rate was further reduced to 0.2 d(-1). A good correlation between specific antibody production rate and the percentage of G2/M cells was observed.

Effects of three-dimensional culturing in a fibrous matrix on cell cycle, apoptosis, and MAb production by hybridoma cells

The effects of culturing hybridoma cells in a three-dimensional (3-D) poly(ethylene terephthalate) (PET) fibrous matrix on cell cycle, apoptosis, metabolism, and monoclonal antibody (MAb) production were evaluated by comparing with two-dimensional (2-D) culturing on microcarrier and multiwell plate surfaces. The percentage of cells in the G1/G0 phase increased during the long-term culturing period of approximately 4 weeks. Compared to the 2-D culture systems, cells grown in 3-D matrices had higher MAb productivity for long-term culture. Decreasing serum content in the culture medium increased both MAb productivity and apoptosis. However, the 3-D culture had a greater increase in MAb productivity and a much lower apoptotic rate than the 2-D culture, especially at 0% serum. Most cells in the 3-D fibrous matrix formed large aggregates and were smaller than cells grown on a 2-D surface or in suspension. The smaller cell size allowed cells to survive better in the high-cell-density environment. The fibrous matrix also selectively retained healthy, nonapoptotic cells. These results suggested that the 3-D fibrous matrix contributed to growth arrest, protected cells to better resist low-serum environments, and reduced apoptosis, all of which contributed to the high viable cell density and volumetric MAb productivity in the long-term 3-D culture.

Effect of feed and bleed rate on hybridoma cells in an acoustic perfusion bioreactor: Part I. Cell density, viability, and cell-cycle distribution

For the development of optimal perfusion processes the effect of the feed and bleed rate on cell growth in a perfusion bioreactor was studied. The viable-cell density, viability, growth, death, and lysis rate and cell-cycle distribution of a hybridoma cell line producing an IgG1 were studied over a range of specific feed and bleed rates. It was found that the feed and bleed rates applied in the different cultures could be divided into two regions based on the viable-cell density and cell-cycle distribution. The cultures in the first region, low feed rates (0.5 and 1.0 d(-1)) combined with low bleed rates (0.05 and 0.10 d(-1)), were nutrient-limited, as an increase in the feed rate resulted in an increase in the viable-cell density. The cultures in the second region, high feed and bleed rates, were nonnutrient-limited. In this region the viable-cell density decreased more or less linearly with an increase in the bleed rate and was independent of the feed rate. This suggests that the cells were limited by a cell-related factor. Comparison of Trypan-blue dye-exclusion measurements and lactate-dehydrogenase activity measurements revealed that cell lysis was not negligible in this bioreactor set-up. Therefore, lactate-dehydrogenase activity measurements were essential to measure the death rate accurately. The specific growth rate was nearly constant for all tested conditions. The viability increased with an increase of the bleed rate and was independent of the feed rate. Furthermore, the specific productivity of monoclonal antibody was constant under all tested conditions. For the optimal design of a perfusion process it should first be established whether viability is an important parameter. If not, a bleed rate as low as possible should be chosen. If low viabilities are to be avoided, the bleed rate chosen should be higher, with the value depending on the desired viability. Next, the feed rate should be set at such a rate that the cells are just in the nonnutrient-limited region.

A fibrous-bed bioreactor for continuous production of developmental endothelial locus-1 by osteosarcoma cells

Genetically engineered human osteosarcoma cells containing developmental endothelial locus-1 (del-1) gene were studied for production of Del-1, a protein that has the properties of an extracellular matrix protein and can regulate vascular morphogenesis and remodeling. Del-1 has been studied as a potential anti-angiogenesis drug targeting solid tumors. In this study, osteosarcoma cells were cultured in a fibrous-bed bioreactor (FBB) to continuously produce Del-1. The FBB was constructed by packing a polyester fibrous matrix into a 1.5-l spinner flask. The effects of media composition, including the serum content in the medium, and dilution rate on cell growth, metabolism, and Del-1 production were studied. A gradual reduction of serum content from 10% (v/v) to 0.5% (v/v) caused no loss in Del-1 production. However, the production of Del-1 decreased significantly in a serum-free medium, suggesting some nutrients present in the serum were important to culture viability and Del-1 production. The continuous FBB culture was stable for long-term production of Del-1, with a higher Del-1 titer than that normally obtained in T-flask cultures and overall productivity similar to the total production from 300 25-cm(2) T-flasks. Reducing geneticin in the medium from 250 microg ml(-1) to zero at later culturing stages had no significant effect on Del-1 production. The FBB was operated for a period of more than 4 months without any notable degeneration, and reached a final cell density of 3 x 10(8) cells ml(-1) of packing volume with >90% cell viability. The good reactor performance can be attributed to the three-dimensional environment provided by the fibrous matrix that allows for efficient mass transfer and cell immobilization and growth. Scanning electron microscopic and confocal scanning laser microscopic studies of the cell-matrix showed that cells formed large aggregates in the fibrous matrix and cell density was relatively uniform in the matrix.

Cell cycle-dependent protein secretion by Saccharomyces cerevisiae

Synchronized Saccharomyces cerevisiae cell populations were used to examine secretion rates of a heterologous protein as a function of cell cycle position. The synchronization procedure had a profound effect on the type and quality of data obtained. When cell synchrony was induced by cell cycle-arresting drugs, a significant physiological perturbation of cells was observed that obscured representative secretion data. In contrast, synchronization with centrifugal elutriation resulted in synchronized first-generation daughter cells with undetectable perturbation of the physiological state. The synchronized cells did not secrete significant amounts of protein until they reached cell division, suggesting that the secretion process in these cells is strongly cell cycle dependent. However, the maximum secretion rate of the synchronized culture (7-14 molecules/cell/second) was significantly lower than that of an asynchronous culture (29-51 molecules/cell/second). This result indicates that young daughter cells isolated in the synchronization process exhibit different protein secretion behavior than older mother cells that are absent in the synchronized cell population but present in the asynchronous culture.

Rapamycin reduces hybridoma cell death and enhances monoclonal antibody production

Rapamycin was used as a medium additive to slow the progression of CRL 1606 hybridomas through the cell cycle, under the hypothesis that such a modulation might reduce cell death. Cell cycle distributions for CRL hybridomas in the G1 phase of the cell cycle ranged from 20% to 35% during batch, fed-batch, and continuous culture experiments, independent of culture time, dilution rate, growth rates, or death rates. Rapamycin, an mTOR signaling inhibitor, immunosuppressant, and G1-phase arresting agent, was identified and tested for efficacy in restraining cell cycle progression in CRL 1606 hybridoma cultures. However, in the presence of 100 nM rapamycin, the percentage of cells in the G1 phase of the cell cycle during fed-batch cultures was only increased from 28% to 31% in control cultures to 37% to 48% for those with rapamycin. Accordingly, rapamycin only slightly reduced culture growth rate. Instead, the use of rapamycin more notably kept viability higher than that of control cultures by delaying cell death for 48 h, thereby enabling viable proliferation to higher maximum viable cell densities. Furthermore, rapamycin enhanced specific monoclonal antibody production by up to 100% during high-viability growth. Thus, over the course of 6-day fed-batch cultivations, the beneficial effects of rapamycin on viable cell density and specific productivity resulted in an increase in final monoclonal antibody titer from 0.25 to 0.56 g/L (124%). As rapamycin is reported to influence a much broader range of cellular functions than cell cycle alone, these findings are more illustrative of the influence that signal transduction pathways related to mTOR can have on overall cell physiology and culture productivity.

Induction of a mitosis delay and cell lysis by high-level secretion of mouse alpha-amylase from Saccharomyces cerevisiae

Some foreign proteins are produced in yeast in a cell cycle-dependent manner, but the cause of the cell cycle dependency is unknown. In this study, we found that Saccharomyces cerevisiae cells secreting high levels of mouse alpha-amylase have elongated buds and are delayed in cell cycle completion in mitosis. The delayed cell mitosis suggests that critical events during exit from mitosis might be disturbed. We found that the activities of PP2A (protein phosphatase 2A) and MPF (maturation-promoting factor) were reduced in alpha-amylase-oversecreting cells and that these cells showed a reduced level of assembly checkpoint protein Cdc55, compared to the accumulation in wild-type cells. MPF inactivation is due to inhibitory phosphorylation on Cdc28, as a cdc28 mutant which lacks an inhibitory phosphorylation site on Cdc28 prevents MPF inactivation and prevents the defective bud morphology induced by overproduction of alpha-amylase. Our data also suggest that high levels of alpha-amylase may downregulate PPH22, leading to cell lysis. In conclusion, overproduction of heterologous alpha-amylase in S. cerevisiae results in a negative regulation of PP2A, which causes mitotic delay and leads to cell lysis.

Anti-TNP monoclonal antibodies as reagents for enzyme immunoassay (ELISA)

The aim of this study was to produce anti-TNP monoclonal antibodies (MAbs) that could be conjugated and used for the detection of antigen-antibody reactions, in which the antigen specific-antibody had been previously bound to trinitrophenyl (TNP). For hybridoma production, SP2/0-Ag14 cells were fused with spleen cells from mice previously immunized with TNP-ovalbumin (TNP-OVA). After 10 days, enzyme-linked immunoadsorbent assay (ELISA) was used to detect anti-TNP antibodies in the supernatants, and five cultures were found to be strictly positive for TNP. Three of these were subsequently cloned by limiting dilution, and 15 clones were chosen for expansion based on the criterion of high reactivity against TNP. Anti-TNP MAbs produced by those clones were isotyped as IgG1, and purified by Sepharose-protein G affinity cromatography from ascites developed in BALB/c mice. Two purified MAbs (1B2.1B6 and 1B2.1E12) were coupled to horseradish peroxidase (HRPO). The resulting conjugates were evaluated in ELISA tests for interferon-gamma and interleukin-4 detection, in which the secondary anti-cytokine antibodies were coupled either to TNP or biotin. The performance of anti-TNP conjugates in these assays were compared with a biotin-streptavidin/peroxidase system. Both types of conjugates were similarly able to detect cytokines with r2 (linear correlation coefficient) close to unity value. Growth studies of one of those hybridomas (1B2.1B6) yielded a specific growth rate of 0.042 h(-1) and a doubling time of 16.5 h. Data discussed here show that at least two MAbs against TNP raised in this work can be used as a reagent for enzyme immunoassays.

Modeling and optimization of alpha-amylase production in a recombinant yeast fed-batch culture taking account of the cell cycle population distribution

A simple mathematical model describing the cell cycle dependency of rice alpha-amylase production by a recombinant yeast was constructed to investigate the efficiency of cell cycle population control. First, the effects of the glucose concentration and cultivation temperature on the specific growth rate, the specific production rate of rice alpha-amylase, and the distribution of the cell cycle population were studied under balanced growth conditions. On the basis of the results, parameter values for the mathematical model were then estimated. The proposed model was shown to be applicable for unbalanced as well as balanced growth phases. The optimal control strategy in respect of temperature and glucose concentration for maximum rice alpha-amylase production, taking into account the cell cycle population, was determined and the result was compared with that obtained by a simple mathematical model in which cell cycle distribution was not considered. Finally, the effect of the initial population of each cell cycle phase on the final amount of the product under optimal operational conditions was investigated. The simulation and experimental data coincided well with each other, and the model was used to optimize the control strategy for maximum alpha-amylase production.

Shear sensitivity of animal cells from a culture-medium perspective

Recently, several groups have published data on the shear sensitivity of suspended animal cells and the protective effect of certain polymers. These findings did not, at the time, seem to have great practical application because shear sensitivity did not cause great problems for large-scale applications in sparged and stirred-tank reactors using the then-current culture media and fermentation procedures. However, two recent developments might require renewed attention in sparged animal-cell cultures--protein-free media and new fermentation techniques.

Descriptive parameter evaluation in mammalian cell culture

Several methods exist for assessing population growth and protein productivity in mammalian cell culture. These methods were critically examined here, based on experiments with two hybridoma cell lines. It is shown that mammalian cell culture parameters must be evaluated on the same basis. In batch culture mode most data is obtained on a cumulative basis (protein product titre, substrate concentration, metabolic byproduct concentration). A simple numerical integration technique can be employed to convert cell concentration data to a cumulative basis (cell-hours). The hybridoma lines used in this study included a nutritionally non-fastidious line producing low levels of MAb and a nutritionally fastidious hybridoma with high productivity. In both cases the cell-hour approach was the most appropriate means of expressing the relationship between protein productivity and cell population dynamics. The cell-hour approach could be used as the basis for all metabolic population parameter evaluations. This method has the potential to be used successfully for both prediction and optimization purposes.

Modelling hybridoma cell growth and metabolism--a comparison of selected models and data

Unstructured models for cell growth (cell specific growth and death rate) and metabolism (cell specific substrate uptake and metabolite production rates) of hybridoma cell lines were compared with special respect to significance, analytical error and range of validity. The diversity of the unstructured models cited reveals their mostly descriptive character compared to structured models. Bearing in mind this limited knowledge, empirical models can still serve as a valuable tool for process design. For understanding of the cell metabolism itself they might have been overemphasized in the past. For proper model design, care has to be taken to cover the whole range of process conditions. In particular if a process is to be run at very low substrate and high metabolite concentrations, chemostat cultures which have mostly been used for the model formulations, are not sufficient and have to be completed by, for example, fed-batch cultures.

Improvement of productivity of yeast cell with a novel airlift loop reactor

Two different strains of baker's yeast are cultivated using a fed-batch process with a novel airlift loop reactor. The reactor can be operated not only under steady-state conditions as the traditional airlift loop reactor, but also under forced periodically operational conditions in which the direction of liquid circulating flow is alternatively changed. Compared with the traditional steady-state operation, both the growth rate and yield of cells are much higher in the forced periodic operation.

Optimizing antibody production in batch hybridoma cell culture

Optimizing productivity by hybridoma cells relies partly on developing suitable methods for screening and selection of high producing cultures and on understanding regulation of antibody production. In this study, the behavior of hybridoma cells in batch culture was investigated using flow cytometry, and a simple model for antibody production was used to explain production data obtained from these cultures. Surface antibody fluorescence values were found to closely follow the decreasing trend of specific antibody secretion rate over the course of several batch cultures. Therefore, for the hybridoma cell lines studied here (ATCC HB124 and TIB138), surface immunofluorescence levels can be used to select high producing cells as well as to monitor culture productivity. Surface and intracellular antibody fluorescence values were also found to be correlated for cells exhibiting a bimodal distribution with respect to intracellular antibody content. The population of cells containing a bimodal distribution with respect to intracellular antibody content. The population of cells containing lower levels of intracellular antibody was determined to secrete significantly less antibody than the population possessing high intracellular antibody concentrations. Factors which influence antibody production rates and possible strategies for optimizing monoclonal antibody yield are discussed.

Hyper-stimulation of monoclonal antibody production by high osmolarity stress in eRDF medium

An earlier study (Chua et al., 1994) showed that hybridoma 2HG11 cultivated in a basal medium called eRDF, which is enriched in amino acids, enabled higher immunoglobulin (Ig) production with and without serum, when compared to two other traditional media RPMI and DMEM/F12. A further enhancement of Ig productivity was achieved when the osmolarity of the culture medium was increased from 300 mOsm to 350 mOsm (Oh et al., 1993). To determine whether the eRDF media was indeed better, three other cell lines, two IgG producers (TB/C3 and I13/17) and an IgM producer (B10), were tested. The results showed that maximum viable cell densities in eRDF medium were up to 3-times higher than in RPMI and maximum Ig titres were 2-8-times higher than in DMEM/F12 and RPMI. The three cell lines were similarly subjected to osmotic increases from 300 mOsm to 350 and 400 mOsm by addition of NaCl. There was an increase in Ig titres of between 30% to 100% compared to the control medium, although cell growth was reduced. Thus, hyper-stimulation by osmolarity stress was found to be generally effective in eliciting higher Ig production; the extent of enhancement being more pronounced for certain cell lines. Other osmolytes such as sucrose and KCl demonstrated similar effects of increasing Ig productivity. Study on the mechanism of action of osmotic stress on hybridoma 2HG11 revealed that hyper-stimulation of Ig productivity was fundamentally related to a greater availability of amino acids to cells as the cells actively accumulated more salt and amino acids to compensate for the higher medium osmolarity. Uptake of the amino acid analogues 14C-aminoisobutyric acid and 3H-methylaminoisobutyric acid into cells increased to 2.34 x 10(3) cpm per cell per min and 6.35 x 10(3) cpm per cell per min, respectively, under osmotic stress. This corresponds to an 85% increase in uptake via the Na(+)-dependent symport and a 50% increase in uptake via the Na(+)-independent and Na(+)-dependent symports. In the 350 mOsm medium, hybridomas also demonstrated an increase in metabolic activities of 5-10% compared to the control. This, together with the reduced specific growth rate in cells under osmotic stress, suggests that more energy was channelled into the biosynthetic pathway of Ig production.

Effect of lactic acid on the kinetics of growth and antibody production in a murine hybridoma: secretion patterns during the cell cycle

The effects of elevated lactic acid concentration on the cell cycle kinetics of hybridoma cell growth and antibody production in batch culture were studied using conventional methods based on population-average data analysis and using flow cytometry based on single-cell data analysis. When 33 mM lactic acid was initially present, the true specific growth rate was reduced by 37% and the cell specific antibody production rate increased by a factor of 2.6 relative to a control culture with no additional lactic acid. DNA content distribution measured during balanced exponential growth were not affected by lactic acid concentration indicating lactic acid has a uniform effect on cell growth throughout the cell cycle. There was little or no effect on single-cell distributions of intracellular antibody content measured for the total population and for each cell cycle phase. The net rate of total antibody synthesis was found to be independent of specific growth rate. This implies that the balance of the total amount of antibody synthesized is shifted from cellular accumulation towards secretion when specific growth rate decreases. Our data predict that a maximum specific secretion rate of 2.7 pg per cell per h could be achieved if the specific growth rate was reduced to zero. The rates of secretion in the G1 and S phases increased with decreasing specific growth rate, while the rate of secretion in the G2+M phase remained relatively constant. Under the assumptions that (a) at the fastest growth rate, secretion in the G1 phase is negligible and (b) the rate of synthesis increases exponentially as cells proceed from the S phase to the G2+M phase, our data predict that for the slowest growth rate, the rate of secretion in G2+M is approx. 3-times that in the G1 phase and 5-times that in the S phase.

Large-scale mammalian cell culture

Over the past year, progress has been made toward a better understanding of the physiological and biochemical responses of cell cultures to various environmental changes. Much of the theoretical and experimental work that has been reported will help improve expression in mammalian cell culture, which is one of the key steps in biopharmaceutical manufacturing.

Metabolic and kinetic studies of hybridomas in exponentially fed-batch cultures using T-flasks

Exponentially fed-batch cultures (EFBC) of a murine hybridoma in T-flasks were explored as a simple alternative experimental tool to chemostats for the study of metabolism, growth and monoclonal antibody (MAb) production kinetics. EFBC were operated in the variable volume mode using an exponentially increasing and predetermined stepwise feeding profile of fresh complete medium. The dynamic and steady-state behaviors of the EFBC coincided with those reported for chemostats at dilution rates below the maximum growth rate. In particular, steady-state for growth rate and concentration of viable cells, glucose, and lactate was attained at different dilution rates between 0.005 and 0.05 h-1. For such a range, the glucose and lactate metabolic quotients and the steady-state glucose concentration increased, whereas total MAb, volumetric, and specific MAb production rates decreased 65-, 6-, and 3-fold, respectively, with increasing dilution rates. The lactate from glucose yield remained relatively constant for dilution rates up to 0.03 h-1, where it started to decrease. In contrast, viability remained above 80% at high dilution rates but rapidly decreased at dilution rates below 0.02 h-1. No true washout occurred during operation above the maximum growth, as concluded from the constant viable cell number. However, growth rate decreased to as low as 0.01 h-1, suggesting the requirement of a minimum cell density, and concomitant autocrine growth factors, for growth. Chemostat operation drawbacks were avoided by EFBC in T-flasks. Namely, simple and stable operation was obtained at dilution rates ranging from very low to above the maximum growth rate. Furthermore, simultaneous operation of multiple experiments in reduced size was possible, minimizing start-up time, media and equipment costs.

Effect of endogenous proteins on growth and antibody productivity in hybridoma batch cultures

It has been shown that some B-cell hybridomas secrete autocrine factors in vitro which can influence cell metabolic processes. Rather than screen specifically for suspected cytokines, that may or may not affect our cell line, we have examined the lumped effects of intracellular and secreted factors on cell proliferation and monoclonal productivity in hybridoma batch cultures. Firstly, supplements of total soluble intracellular proteins combined with other intracellular metabolites were found to both decrease the specific growth rate and increase the antibody production rate at higher concentrations in batch culture. This is an important consideration in high cell density cultures, such as perfusion systems, where a reduction of growth by the presence of intracellular factors may be compensated by an increase in MAb production. In addition, flow cytometry data revealed that the average cell cycle G1 phase fraction was unaffected by the variation in the maximum specific growth rates during the exponential growth phase, caused by the addition of intracellular factors; this suggests that higher MAb productivity at lower growth rates are not a result of cell arrest in the G1 phase. Secondly, secreted extracellular proteins larger than 10,000 Daltons, which were concentrated from spent culture supernatant, were shown to have no significant effect on growth and specific MAb productivity when supplemented to batch culture at levels twice that encountered late in normal batch culture. This indicates that endogenous secreted cytokines, if at all present, do not play a major autocrine role for this cell line.