Glycosylation of CHO-derived recombinant tPA produced under elevated pCO2

Cell-culture process optimization via model-based predictions of metabolism and protein glycosylation

In order to meet the rising demand for biologics and become competitive on the developing biosimilar market, there is a need for process intensification of biomanufacturing processes. Process development of biologics has historically relied on extensive experimentation to develop and optimize biopharmaceutical manufacturing. Experimentation to optimize media formulations, feeding schedules, bioreactor operations and bioreactor scale up is expensive, labor intensive and time consuming. Mathematical modeling frameworks have the potential to enable process intensification while reducing the experimental burden. This review focuses on mathematical modeling of cellular metabolism and N-linked glycosylation as applied to upstream manufacturing of biologics. We review developments in the field of modeling cellular metabolism of mammalian cells using kinetic and stoichiometric modeling frameworks along with their applications to simulate, optimize and improve mechanistic understanding of the process. Interest in modeling N-linked glycosylation has led to the creation of various types of parametric and non-parametric models. Most published studies on mammalian cell metabolism have performed experiments in shake flasks where the pH and dissolved oxygen cannot be controlled. Efforts to understand and model the effect of bioreactor-specific parameters such as pH, dissolved oxygen, temperature, and bioreactor heterogeneity are critically reviewed. Most modeling efforts have focused on the Chinese Hamster Ovary (CHO) cells, which are most commonly used to produce monoclonal antibodies (mAbs). However, these modeling approaches can be generalized and applied to any mammalian cell-based manufacturing platform. Current and potential future applications of these models for Vero cell-based vaccine manufacturing, CAR-T cell therapies, and viral vector manufacturing are also discussed. We offer specific recommendations for improving the applicability of these models to industrially relevant processes.

Factors affecting the quality of therapeutic proteins in recombinant Chinese hamster ovary cell culture

Chinese hamster ovary (CHO) cells are the most widely used mammalian host cells for the commercial production of therapeutic proteins. Fed-batch culture is widely used to produce therapeutic proteins, including monoclonal antibodies, because of its operational simplicity and high product titer. Despite technical advances in the development of culture media and cell cultures, it is still challenging to maintain high productivity in fed-batch cultures while also ensuring good product quality. In this review, factors that affect the quality attributes of therapeutic proteins in recombinant CHO (rCHO) cell culture, such as glycosylation, charge variation, aggregation, and degradation, are summarized and categorized into three groups: culture environments, chemical additives, and host cell proteins accumulated in culture supernatants. Understanding the factors that influence the therapeutic protein quality in rCHO cell culture will facilitate the development of large-scale, high-yield fed-batch culture processes for the production of high-quality therapeutic proteins.

Osmolality as a lever to modulate the N-glycolylneuraminicacid (Neu5Gc) level of a recombinant glycoprotein produced in Chinese hamster ovary cells

Glycoproteins could be highly sialylated, and controlling the sialic acid levels for some therapeutic proteins is critical to ensure product consistency and efficacy. N-acetylneuraminic acid (Neu5Ac, or NANA) and N-glycolylneuraminic acid (Neu5Gc, or NGNA) are the two most common forms of sialic acids produced in mammalian cells. As Neu5Gc is not produced in humans and can elicit immune responses, minimizing Neu5Gc formation is important in controlling this quality attribute for complex glycoproteins. In this study, a sialylated glycoprotein was used as the model molecule to study the effect of culture osmolality on Neu5Gc. A 14-day fed-batch process with osmolality maintained at physiological levels produced high levels of Neu5Gc. Increase of culture osmolality reduced the Neu5Gc level up to 70-80%, and the effect was proportional to the osmolality level. Through evaluating different osmolality conditions (300-450 mOsm/kg) under low or high pCO₂ , we demonstrated that osmolality could be an effective process lever to modulate the Neu5Gc level. Potential mechanism of osmolality impact on Neu5Gc is discussed and is hypothesized to be cytosol NADH availability related. Compared with cell line engineering efforts, this simple process lever provides the opportunity to readily modulate the Neu5Gc level in a cell culture environment.

Development of a scale-up strategy for Chinese hamster ovary cell culture processes using the kL a ratio as a direct indicator of gas stripping conditions

Herein, we described a scale-up strategy focused on the dissolved carbon dioxide concentration (dCO₂ ) during fed-batch cultivation of Chinese hamster ovary cells. A fed-batch culture process for a 2000-L scale stainless steel (SS) bioreactor was scaled-up from similarly shaped 200-L scale bioreactors based on power input per unit volume (P/V). However, during the 2000-L fed-batch culture, the dCO₂ was higher compared with the 200-L scale bioreactor. Therefore, we developed an alternative approach by evaluating the k_L a values of O₂ (k_L a[O₂ ]) and CO₂ [k_L a(CO₂ )] in the SS bioreactors as a scale-up factor for dCO₂ reduction. The k_L a ratios [k_L a(CO₂ )/k_L a(O₂ )] were different between the 200-L and 2000-L bioreactors under the same P/V condition. When the agitation conditions were changed, the k_L a ratio of the 2000-L scale bioreactor became similar and the P/V value become smaller compared with those of the 200-L SS bioreactor. The dCO₂ trends in fed-batch cultures performed in 2000-L scale bioreactors under the modified agitation conditions were similar to the control. This k_L a ratio method was used for process development in single-use bioreactors (SUBs) with shapes different from those of the SS bioreactor. The k_L a ratios for the SUBs were evaluated and conditions that provided k_L a ratios similar to the 200-L scale SS bioreactors were determined. The cell culture performance and product quality at the end of the cultivation process were comparable for all tested SUBs. Therefore, we concluded that the k_L a ratio is a powerful scale-up factor useful to control dCO₂ during fed-batch cultures.

Global transcriptomic analysis suggests carbon dioxide as an environmental stressor in spaceflight: A systems biology GeneLab case study

Spaceflight introduces a combination of environmental stressors, including microgravity, ionizing radiation, changes in diet and altered atmospheric gas composition. In order to understand the impact of each environmental component on astronauts it is important to investigate potential influences in isolation. Rodent spaceflight experiments involve both standard vivarium cages and animal enclosure modules (AEMs), which are cages used to house rodents in spaceflight. Ground control AEMs are engineered to match the spaceflight environment. There are limited studies examining the biological response invariably due to the configuration of AEM and vivarium housing. To investigate the innate global transcriptomic patterns of rodents housed in spaceflight-matched AEM compared to standard vivarium cages we utilized publicly available data from the NASA GeneLab repository. Using a systems biology approach, we observed that AEM housing was associated with significant transcriptomic differences, including reduced metabolism, altered immune responses, and activation of possible tumorigenic pathways. Although we did not perform any functional studies, our findings revealed a mild hypoxic phenotype in AEM, possibly due to atmospheric carbon dioxide that was increased to match conditions in spaceflight. Our investigation illustrates the process of generating new hypotheses and informing future experimental research by repurposing multiple space-flown datasets.

Understanding of decreased sialylation of Fc-fusion protein in hyperosmotic recombinant Chinese hamster ovary cell culture: N-glycosylation gene expression and N-linked glycan antennary profile

To understand the effects of hyperosmolality on protein glycosylation, recombinant Chinese hamster ovary (rCHO) cells producing the Fc-fusion protein were cultivated in hyperosmolar medium resulting from adding NaCl (415 mOsm/kg). The hyperosmotic culture showed increased specific Fc-fusion protein productivity (q_Fc ) but a decreased proportion of acidic isoforms and sialic acid content of the Fc-fusion protein. The intracellular and extracellular sialidase activities in the hyperosmotic cultures were similar to those in the control culture (314 mOsm/kg), indicating that reduced sialylation of Fc-fusion protein at hyperosmolality was not due to elevated sialidase activity. Expression of 52 N-glycosylation-related genes was assessed by the NanoString nCounter system, which provides a direct digital readout using custom-designed color-coded probes. After 3 days of hyperosmotic culture, nine genes (ugp, slc35a3, slc35d2, gcs1, manea, mgat2, mgat5b, b4galt3, and b4galt4) were differentially expressed over 1.5-fold of the control, and all these genes were down-regulated. N-linked glycan analysis by anion exchange and hydrophilic interaction HPLC showed that the proportion of highly sialylated (di-, tri-, tetra-) and tetra-antennary N-linked glycans was significantly decreased upon hyperosmotic culture. Addition of betaine, an osmoprotectant, to the hyperosmotic culture significantly increased the proportion of highly sialylated and tetra-antennary N-linked glycans (P ≤ 0.05), while it increased the expression of the N-glycan branching/antennary genes (mgat2 and mgat4b). Thus, decreased expression of the genes with roles in the N-glycan biosynthesis pathway correlated with reduced sialic acid content of Fc-fusion protein caused by hyperosmolar conditions. Taken together, the results obtained in this study provide a better understanding of the detrimental effects of hyperosmolality on N-glycosylation, especially sialylation, in rCHO cells. Biotechnol. Bioeng. 2017;114: 1721-1732. © 2017 Wiley Periodicals, Inc.

Engineer Medium and Feed for Modulating N-Glycosylation of Recombinant Protein Production in CHO Cell Culture

Chinese hamster ovary (CHO) cells have become the primary expression system for the production of complex recombinant proteins due to their long-term success in industrial scale production and generating appropriate protein N-glycans similar to that of humans. Control and optimization of protein N-glycosylation is crucial, as the structure of N-glycans can largely influence both biological and physicochemical properties of recombinant proteins. Protein N-glycosylation in CHO cell culture can be controlled and tuned by engineering medium, feed, culture process, as well as genetic elements of the cell. In this chapter, we will focus on how to carry out experiments for N-glycosylation modulation through medium and feed optimization. The workflow and typical methods involved in the experiment process will be presented.

Investigation of the interactions of critical scale-up parameters (pH, pO2 and pCO2) on CHO batch performance and critical quality attributes

Understanding process parameter interactions and their effects on mammalian cell cultivations is an essential requirement for robust process scale-up. Furthermore, knowledge of the relationship between the process parameters and the product critical quality attributes (CQAs) is necessary to satisfy quality by design guidelines. So far, mainly the effect of single parameters on CQAs was investigated. Here, we present a comprehensive study to investigate the interactions of scale-up relevant parameters as pH, pO₂ and pCO₂ on CHO cell physiology, process performance and CQAs, which was based on design of experiments and extended product quality analytics. The study used a novel control strategy in which process parameters were decoupled from each other, and thus allowed their individual control at defined set points. Besides having identified the impact of single parameters on process performance and product quality, further significant interaction effects of process parameters on specific cell growth, specific productivity and amino acid metabolism could be derived using this method. Concerning single parameter effects, several monoclonal antibody (mAb) charge variants were affected by process pCO₂ and pH. N-glycosylation analysis showed positive correlations between mAb sialylation and high pH values as well as a relationship between high mannose variants and process pH. This study additionally revealed several interaction effects as process pH and pCO₂ interactions on mAb charge variants and N-glycosylation pattern. Hence, through our process control strategy and multivariate investigation, novel significant process parameter interactions and single effects were identified which have to be taken into account especially for process scale-up.

A comparison of orbitally-shaken and stirred-tank bioreactors: pH modulation and bioreactor type affect CHO cell growth and protein glycosylation

Orbitally shaken bioreactors (OSRs) support the suspension cultivation of animal cells at volumetric scales up to 200 L and are a potential alternative to stirred-tank bioreactors (STRs) due to their rapid and homogeneous mixing and high oxygen transfer rate. In this study, a Chinese hamster ovary cell line producing a recombinant antibody was cultivated in a 5 L OSR and a 3 L STR, both operated with or without pH control. Effects of bioreactor type and pH control on cell growth and metabolism and on recombinant protein production and glycosylation were determined. In pH-controlled bioreactors, the glucose consumption and lactate production rates were higher relative to cultures grown in bioreactors without pH control. The cell density and viability were higher in the OSRs than in the STRs, either with or without pH control. Volumetric recombinant antibody yields were not affected by the process conditions, and a glycan analysis of the antibody by mass spectrometry did not reveal major process-dependent differences in the galactosylation index. The results demonstrated that OSRs are suitable for recombinant protein production from suspension-adapted animal cells. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1174-1180, 2016.

Strategies for Engineering Protein N-Glycosylation Pathways in Mammalian Cells

Complexity and heterogeneity of oligosaccharides present a considerable challenge to the biopharmaceutical industry to manufacture biotherapeutics with reproducible and consistent glycoform profiles. Mammalian cells, especially Chinese hamster ovary cells, are the most widely used platform for the production of biotherapeutics. The glycans produced are predominantly of the complex type, with some differences between human and nonhuman mammalian glycosylation existing. This review briefly summarizes metabolic glyco-engineering strategies used in mammalian cells in order to alter the glycosylation patterns attached to proteins applied for diverse biotechnology applications.

Glycosylation: impact, control and improvement during therapeutic protein production

The emergence of the biopharmaceutical industry represented a major revolution for modern medicine, through the development of recombinant therapeutic proteins that brought new hope for many patients with previously untreatable diseases. There is a ever-growing demand for these therapeutics that forces a constant technological evolution to increase product yields while simultaneously reducing costs. However, the process changes made for this purpose may also affect the quality of the product, a factor that was initially overlooked but which is now a major focus of concern. Of the many properties determining product quality, glycosylation is regarded as one of the most important, influencing, for example, the biological activity, serum half-life and immunogenicity of the protein. Consequently, monitoring and control of glycosylation is now critical in biopharmaceutical manufacturing and a requirement of regulatory agencies. A rapid evolution is being observed in this context, concerning the influence of glycosylation in the efficacy of different therapeutic proteins, the impact on glycosylation of a diversity of parameters/processes involved in therapeutic protein production, the analytical methodologies employed for glycosylation monitoring and control, as well as strategies that are being explored to use this property to improve therapeutic protein efficacy (glycoengineering). This work reviews the main findings on these subjects, providing an up-to-date source of information to support further studies.

Application of quality by design principles to the development and technology transfer of a major process improvement for the manufacture of a recombinant protein

This study describes the application of quality by design (QbD) principles to the development and implementation of a major manufacturing process improvement for a commercially distributed therapeutic protein produced in Chinese hamster ovary cell culture. The intent of this article is to focus on QbD concepts, and provide guidance and understanding on how the various components combine together to deliver a robust process in keeping with the principles of QbD. A fed-batch production culture and a virus inactivation step are described as representative examples of upstream and downstream unit operations that were characterized. A systematic approach incorporating QbD principles was applied to both unit operations, involving risk assessment of potential process failure points, small-scale model qualification, design and execution of experiments, definition of operating parameter ranges and process validation acceptance criteria followed by manufacturing-scale implementation and process validation. Statistical experimental designs were applied to the execution of process characterization studies evaluating the impact of operating parameters on product quality attributes and process performance parameters. Data from process characterization experiments were used to define the proven acceptable range and classification of operating parameters for each unit operation. Analysis of variance and Monte Carlo simulation methods were used to assess the appropriateness of process design spaces. Successful implementation and validation of the process in the manufacturing facility and the subsequent manufacture of hundreds of batches of this therapeutic protein verifies the approaches taken as a suitable model for the development, scale-up and operation of any biopharmaceutical manufacturing process.

Fucose content of monoclonal antibodies can be controlled by culture medium osmolality for high antibody-dependent cellular cytotoxicity

Antibody-dependent cellular cytotoxicity (ADCC) is dependent on the fucose content of oligosaccharides bound to monoclonal antibodies (MAbs). As MAbs with a low fucose content exhibit high ADCC activity, it is important to control the defucosylation levels (deFuc%) of MAbs and to analyze the factors that affect deFuc%. In this study, we observed that the deFuc% was inversely related to culture medium osmolality for MAbs produced in the rat hybridoma cell line YB2/0, with r (2) values as high as 0.92. Moreover, deFuc% exhibited the same correlation irrespective of the type of compound used for regulating osmolality (NaCl, KCl, fucose, fructose, creatine, or mannitol) at a culture scale ranging from 1 to 400 L. We succeeded in controlling MAb deFuc% by maintaining a constant medium osmolality in both perfusion and fed-batch cultures. In agreement with these observations, reverse transcription PCR analyses revealed decreased transcription of genes involved in glycolysis, GDP-fucose supply, and fucose transfer under hypoosmotic conditions.

Cell culture processes for monoclonal antibody production

Animal cell culture technology has advanced significantly over the last few decades and is now generally considered a reliable, robust and relatively mature technology. A range of biotherapeutics are currently synthesized using cell culture methods in large scale manufacturing facilities that produce products for both commercial use and clinical studies. The robust implementation of this technology requires optimization of a number of variables, including 1) cell lines capable of synthesizing the required molecules at high productivities that ensure low operating cost; 2) culture media and bioreactor culture conditions that achieve both the requisite productivity and meet product quality specifications; 3) appropriate on-line and off-line sensors capable of providing information that enhances process knowledge; and 4) good understanding of culture performance at different scales to ensure smooth scale-up. Successful implementation also requires appropriate strategies for process development, scale-up and process characterization and validation that enable robust operation that is compliant with current regulations. This review provides an overview of the state-of-the art technology in key aspects of cell culture, e.g., engineering of highly productive cell lines and optimization of cell culture process conditions. We also summarize the current thinking on appropriate process development strategies and process advances that might affect process development.

Enhancement of recombinant erythropoietin production in CHO cells in an incubator without CO(2) addition

The effect of low levels of carbon dioxide (CO(2)) in the gas phase on the production of recombinant human erythropoietin (EPO)in CHO cells was explored. A T-flask culture in an incubator without CO(2) addition showed a slow cell growth initially followed by the cessation of growth, while other cultures incubated under 0.5-5% CO(2) concentrations grew normally at the same rate during the entire period of cultivation. Interestingly, the production of EPO in the culture incubated under no CO(2) supply was highest among the tested cultures. The cell specific secretion rate of EPO (q(EPO)) of the culture under no CO(2) supply was about 3 times higher than that of the culture under 5% CO(2) supply. Western blot analysis and in vivo bioassay of EPO showed no apparent changes in EPO quality between the two cases of different CO(2) environments (air vs. 5% CO(2)), suggesting robust glycosylation of EPO by CHO cells even under very reduced CO(2) environment. Various combinations of the two extreme cases, with 5% CO(2) supply (suitable for cell growth) and no CO(2) addition (better for EPO production), were made in order to maximize the volumetric productivity of EPO secretion (P(V)) in CHO cells. The P(V) of the cultures programmed with initial incubation under 5% CO(2) followed by no CO(2) supply was about 2 times superior to that of the culture incubated only under no CO(2) supply. The P(V) of the culture under no CO(2) supply was slightly lower than that of culture grown under 5% CO(2). However, the q(EPO) of the no CO(2) supply case was more than 5 times higher than that of the culture under 5% CO(2) supply. In conclusion, we have demonstrated that a simple programming of CO(2) supply to an incubator can enhance the production of EPO in CHO cells remarkably, without any apparent change of the EPO quality.

Living with heterogeneities in bioreactors: understanding the effects of environmental gradients on cells

The presence of spatial gradients in fundamental culture parameters, such as dissolved gases, pH, concentration of substrates, and shear rate, among others, is an important problem that frequently occurs in large-scale bioreactors. This problem is caused by a deficient mixing that results from limitations inherent to traditional scale-up methods and practical constraints during large-scale bioreactor design and operation. When cultured in a heterogeneous environment, cells are continuously exposed to fluctuating conditions as they travel through the various zones of a bioreactor. Such fluctuations can affect cell metabolism, yields, and quality of the products of interest. In this review, the theoretical analyses that predict the existence of environmental gradients in bioreactors and their experimental confirmation are reviewed. The origins of gradients in common culture parameters and their effects on various organisms of biotechnological importance are discussed. In particular, studies based on the scale-down methodology, a convenient tool for assessing the effect of environmental heterogeneities, are surveyed.

Decreased pCO2 accumulation by eliminating bicarbonate addition to high cell-density cultures

High-density perfusion cultivation of mammalian cells can result in elevated bioreactor CO(2) partial pressure (pCO(2)), a condition that can negatively influence growth, metabolism, productivity, and protein glycosylation. For BHK cells in a perfusion culture at 20 x 10(6) cells/mL, the bioreactor pCO(2) exceeded 225 mm Hg with approximate contributions of 25% from cellular respiration, 35% from medium NaHCO(3), and 40% from NaHCO(3) added for pH control. Recognizing the limitations to the practicality of gas sparging for CO(2) removal in perfusion systems, a strategy based on CO(2) reduction at the source was investigated. The NaHCO(3) in the medium was replaced with a MOPS-Histidine buffer, while Na(2)CO(3) replaced NaHCO(3) for pH control. These changes resulted in 63-70% pCO(2) reductions in multiple 15 L perfusion bioreactors, and were reproducible at the manufacturing-scale. Bioreactor pCO(2) values after these modifications were in the 68-85 mm Hg range, pCO(2) reductions consistent with those theoretically expected. Low bioreactor pCO(2) was accompanied by both 68-123% increased growth rates and 58-92% increased specific productivity. Bioreactor pCO(2) reduction and the resulting positive implications for cell growth and productivity were brought about by process changes that were readily implemented and robust. This philosophy of pCO(2) reduction at the source through medium and base modification should be readily applicable to large-scale fed-batch cultivation of mammalian cells.

Dependence on glucose limitation of thepCO2 influences on CHO cell growth, metabolism and IgG production

The culture levels of glucose and CO(2) have been reported to independently have important influences on mammalian cell processes. In this work the combined effects of glucose limitation and CO(2) partial pressure (pCO(2)) on monoclonal antibody (IgG) producing Chinese Hamster Ovary cells were investigated in a perfusion reactor operated with controlled cell specific medium feed rate, pH and osmolality. Under high glucose conditions (14.3 +/- 0.8 mM), the apparent growth rate decreased (from 0.021 to 0.009 h(-1)) as the pCO(2) increased to approximately 220 mmHg, while the cell specific IgG productivity was almost unchanged. The lactate yield from glucose was not affected by pCO(2) up to approximately 220 mmHg and glucose was mainly converted to lactate. A feed medium modification from high (33 mM) to low (6 mM) glucose resulted in <0.1 mM glucose in the culture. As a result of apparently shifting metabolism towards the conversion of pyruvate to CO(2), both the ratio of lactate to glucose and the alanine production rate were lowered (1.51-1.14 and 17.7-0.56 nmol/10(6) cells h, respectively). Interestingly, when the pCO(2) was increased to approximately 140 mmHg, limiting glucose resulted in 1.7-fold higher growth rates, compared to high glucose conditions. However, at approximately 220 mmHg pCO(2) this beneficial effect of glucose limitation on these CHO cells was lost as the growth rate dropped dramatically to 0.008 h(-1) and the IgG productivity was lowered by 15% (P < 0.01) relative to the high glucose condition. The IgG galactosylation increased under glucose- limited compared to high-glucose conditions.

Reduction of CMP-N-acetylneuraminic acid hydroxylase activity in engineered Chinese hamster ovary cells using an antisense-RNA strategy

Rodent cells, widely used for the industrial production of recombinant human glycoproteins, possess CMP-N-acetylneuraminic acid hydroxylase (CMP-Neu5Ac hydroxylase; EC 1.14.13.45) which is the key enzyme in the formation of the sialic acid, N-glycolylneuraminic acid (Neu5Gc). This enzyme is not expressed in an active form in man and evidence suggests that the presence of Neu5Gc in recombinant therapeutic glycoproteins may elicit an immune response. The aim of this work was, therefore, to reduce CMP-Neu5Ac hydroxylase activity in a Chinese Hamster Ovary (CHO) cell line, and thus the Neu5Gc content of the resulting glycoconjugates, using a rational antisense RNA approach. For this purpose, the cDNA of the hamster hydroxylase was partially cloned and sequenced. Based on the sequence of the mouse and hamster cDNAs, optimal antisense RNA fragments were selected from preliminary in vitro translation tests. Compared to the parental cell line, the new strain (CHO-AsUH2), which was transfected with a 199-bp antisense fragment derived from the mouse CMP-Neu5Ac hydroxylase cDNA, showed an 80% reduction in hydroxylase activity. An analysis of the sialic acids present in the cells' own glycoconjugates revealed a decrease in the percentage of Neu5Gc residues from 4% in the parental cells to less than 1% in the CHO-AsUH2 cell line.

Determination of dissolved CO2 concentration and CO2 production rate of mammalian cell suspension culture based on off-gas measurement

The determination of dissolved CO(2) and HCO(3)(-) concentrations as well as the carbon dioxide production rate in mammalian cell suspension culture is attracting more and more attention since the effects on major cell properties, such as cell growth rate, product quality/production rate, intracellular pH and apoptosis, have been revealed. But the determination of these parameters by gas analysis is complicated by the solution/dissolution of carbon dioxide in the culture medium. This means that the carbon dioxide transfer rate (CTR; which can easily be calculated from off-gas measurement) is not necessarily equal to carbon dioxide production rate (CPR). In this paper, a mathematical method to utilize off-gas measurement and culture pH for cell suspension culture is presented. The method takes pH changes, buffer and medium characteristics that effect CO(2) mass transfer into account. These calculations, based on a profound set of equations, allow the determination of the respiratory activity of the cells, as well as the determination of dissolved CO(2), HCO(3)(-) and total dissolved carbonate. The method is illustrated by application to experimental data. The calculated dissolved CO(2) concentrations are compared with measurements from an electrochemical CO(2) probe.

Effects of osmoprotectant compounds on NCAM polysialylation under hyperosmotic stress and elevated pCO(2)

Elevated osmolality and pCO(2) have been shown to alter sialylation in a protein-specific manner. In Chinese hamster ovary (CHO)MT2-l-8 cells, tPA sialylation changed only slightly from 40 to 250 mm Hg pCO(2), whereas neural cell adhesion molecule polysialic acid (NCAM PSA) content decreased by up to 70% at 250 mm Hg pCO(2), pH 7.2. NCAM PSA content also decreased with increasing NaCl or NH(4)Cl concentration. This suggests that PSA content is a sensitive indicator of conditions that may alter glycosylation. Amino acids and their derivatives have been used to protect hybridoma and CHO cell growth under hyperosmotic stress. We examined the impact of osmoprotectants on NCAM PSA content in CHO MT2-1-8 cells under hyperosmolality (up to 545 mOsm/kg) and at 195 and 250 mm Hg pCO(2). NCAM PSA content at 545 mOsm/kg was at least two-fold greater in the presence of glycine betaine or L-proline compared to that without osmoprotectant. Surprisingly, in the presence of 20 mM glycine betaine, PSA levels were 50-60% of the control level for osmolalities ranging from 320 to 545 mOsm/kg. Thus, glycine betaine inhibits NCAM polysialylation at osmolalities below 435 mOsm/kg and is beneficial at higher osmolalities. In contrast to glycine betaine, L-proline increased PSA content by 25-120% relative to the unprotected culture at < or =545 mOsm/kg. The decrease in NCAM PSA levels of CHO MT2-1-8 cells cultured at 195 mm Hg pCO(2)-435 mOsm/kg was not mitigated by the presence of 25 mM glycine betaine, glycine, or L-threonine, even though all of these compounds enhanced cell growth. At 250 mm Hg pCO(2), all osmoprotectants tested (20 mM L-threonine, L-proline, glycine, or glycine betaine) increased NCAM polysialylation, with 20 mM glycine betaine restoring NCAM PSA to near control levels. Thus, osmoprotectants may (partially) offset changes in glycosylation, as well as the inhibition of growth, in cells under environmental stress. Supernatant beta-galactosidase levels, which increase upon alkalization of acidic organelles, did not differ significantly under elevated pCO(2) and hyperosmolality from that at control conditions.

Considerations for osmolality measurement under elevated pCO(2): comparison of vapor pressure and freezing point osmometry

Osmolality increases with pCO(2) in bioreactors with pH control, and it has been shown that osmolality compensation by decreasing the basal NaCl concentration partially mitigates the adverse effects of elevated pCO(2) on animal cell growth, protein production, and glycosylation. Thus, measurement of osmolality is important for a complete characterization of the culture environment under elevated pCO(2). However, osmolality measurement may be compromised by CO(2) evolution. Freezing point depression and vapor pressure depression osmometry were directly compared for the measurement of osmolality in samples at elevated pCO(2) (up to 250 mmHg) and at a variety of pH values (6.7-7.5). More extensive degassing may be expected with the vapor pressure osmometer due to the smaller sample volume and larger surface area employed. However, both types of osmometer yielded similar results for all pCO(2) and pH values studied. Moreover, the measured values agreed with osmolality values calculated using a semi-empirical model. Further analysis showed that, while sample degassing may result in a large decrease in pCO(2), there is little associated decrease in osmolality. The great majority of total CO(2) in solution is present as bicarbonate (HCO(3)(-)). Although a small amount of HCO(3)(-) is converted to CO(2) to compensate for CO(2) evolution, further depletion of HCO(3)(-) is inhibited by the associated increase in medium pH and by the need for HCO(3)(-) to maintain charge neutrality in solution. This explanation is consistent with the observed similarity in osmolality values for the two types of osmometer. It was also observed that osmolality did not change in samples that were frozen at -20 degrees C for up to 1 year.

Bicarbonate concentration and osmolality are key determinants in the inhibition of CHO cell polysialylation under elevated pCO(2) or pH

Accumulation of CO(2) in animal cell cultures can be a significant problem during scale-up and production of recombinant glycoprotein biopharmaceuticals. By examining the cell-surface polysialic acid (PSA) content, we show that elevated CO(2) partial pressure (pCO(2)) can alter protein glycosylation. PSA is a high-molecular-weight polymer attached to several complex N-linked oligosaccharides on the neural cell adhesion molecule (NCAM), so that small changes in either core glycosylation or in polysialylation are amplified and easily measured. Flow-cytometric analysis revealed that PSA levels on Chinese hamster ovary (CHO) cells decrease with increasing pCO(2) in a dose-dependent manner, independent of any change in NCAM content. The results are highly pH-dependent, with a greater decrease in PSA at higher pH. By manipulating medium pH and pCO(2), we showed that decreases in PSA correlate well with bicarbonate concentration ([HCO(3)(-)]). In fact, it was possible to offset a 60% decrease in PSA content at 120 mm Hg pCO(2) by decreasing the pH from 7.3 to 6.9, such that [HCO(3)(-)] was lowered to that of control (38 mm Hg pCO(2)). When the increase in osmolality associated with elevated [HCO(3)(-)] was offset by decreasing the basal medium [NaCl], elevated [HCO(3)(-)] still caused a decrease in PSA, although less extensive than without osmolality control. By increasing [NaCl], we show that hyperosmolality alone decreases PSA content, but to a lesser extent than for the same osmolality increase due to elevated [NaHCO(3)]. In conclusion, we demonstrate the importance of pH and pCO(2) interactions, and show that [HCO(3)(-)] and osmolality can account for the observed changes in PSA content over a wide range of pH and pCO(2) values.

Enhancing therapeutic glycoprotein production in Chinese hamster ovary cells by metabolic engineering endogenous gene control with antisense DNA and gene targeting

Recombinant glycoprotein therapeutics have proven to be invaluable pharmaceuticals for the treatment of chronic and life-threatening diseases. Although these molecules are extraordinarily efficacious, many diseases have high dosage requirements of several hundred milligrams of protein for each administration. Multiple doses at this level are often required for treatment. One of the major challenges currently facing the biotechnology industry is the development of large-scale, cost-effective production and manufacturing processes of these biologically synthesized molecules. Metabolic engineering of animal cell expression hosts promises to address this challenge by substantially enhancing recombinant protein quality, productivity, and biological activity. In this report, we describe a novel approach to metabolic engineering in Chinese hamster ovary cells by control of endogenous gene expression. Analysis of the advantages and limitations of using antisense DNA and gene targeting as a means of control are discussed and several gene candidates for regulation with these techniques are identified. Practical considerations for using these technologies to reduce the levels of the CHO cell sialidase (Warner et al., Glycobiology, 3, 455-463, 1993) as a model gene system for regulation are also presented.