A. Marc

A new Chinese hamster ovary cell line expressing α2,6-sialyltransferase used as universal host for the production of human-like sialylated recombinant glycoproteins

Chinese hamster ovary (CHO) cells are widely employed to produce glycosylated recombinant proteins. Our group as well as others have demonstrated that the sialylation defect of CHO cells can be corrected by transfecting the alpha2,6-sialyltransferase (alpha2,6-ST) cDNA. Glycoproteins produced by such CHO cells display both alpha2,6- and alpha2,3-linked terminal sialic acid residues, similar to human glycoproteins. Here, we have established a CHO cell line stably expressing alpha2,6-ST, providing a universal host for further transfections of human genes. Several relevant parameters of the universal host cell line were studied, demonstrating that the alpha2,6-ST transgene was stably integrated into the CHO cell genome, that transgene expression was stable in the absence of selective pressure, that the recombinant sialyltransferase was correctly localized in the Golgi and, finally, that the bioreactor growth parameters of the universal host were comparable to those of the parental cell line. A second step consisted in the stable transfection into the universal host of cDNAs for human glycoproteins of therapeutic interest, i.e. interferon-gamma and the tissue inhibitor of metalloproteinases-1. Interferon-gamma purified from the universal host carried 40.4% alpha2,6- and 59.6% alpha2,3-sialic acid residues and showed improved pharmacokinetics in clearance studies when compared to interferon-gamma produced by normal CHO cells.

Determination of cell lysis and death kinetics in continuous hybridoma cultures from the measurement of lactate dehydrogenase release

The death of the hybridoma VO 208 in a continuous culture at pH 7 and 6.8 was investigated by measuring both the appearance of visible dead cells which do not exclude the trypan blue dye and the release of lactate dehydrogenase (LDH) in the culture medium. The intracellular LDH was found to be completely released either when live cells lysed or when they were transformed into visible dead cells. No significant lysis of blue dead cells could be observed at the two different pH. Using a LDH balance over the culture system, cell lysis was found negligible at pH 7, but accounted for 20% of the total cell death at pH 6.8. A methodology is proposed to evaluate the rate constants of hybridoma lysis and total death. For the investigated cell line in continuous culture, the calculated total cell death rate constant was found to increase from 0.002 h−1 to 0.01 h−1 when decreasing the pH from 7 to 6.8.

Influence of intracellular nucleotide and nucleotide sugar contents on recombinant interferon-γ glycosylation during batch and fed-batch cultures of CHO cells

Both the macroheterogeneity of recombinant human IFN‐γ produced by CHO cells and intracellular levels of nucleotides and sugar nucleotides, have been characterized during batch and fed‐batch cultures carried out in different media. Whereas PF‐BDM medium was capable to maintain a high percentage of the doubly‐ glycosylated glycoforms all over the process, mono‐glycosylated and non‐glycosylated forms increased during the batch culture using SF‐RPMI medium. Intracellular level of UTP was higher in PF‐BDM all over the batch culture compared to the SF‐RPMI process. UDP‐Gal accumulated only during the culture performed in PF‐BDM medium, probably as a consequence of the reduced UDP‐Glc synthesis flux in SF‐RPMI medium. When the recombinant CHO cells were cultivated in fed‐batch mode, the UTP level remained at a relatively high value in serum‐containing RPMI and its titer increased during the fed‐phase indicating an excess of biosynthesis. Besides, an accumulation of UDP‐Gal occurred as well. Those results all together indicate that UTP and UDP‐Glc syntheses in CHO cells cultivated in SF‐RPMI medium in batch process, could be limiting during the glycosylation processes of the recombinant IFN‐γ. At last, the determination of the energetic status of the cells over the three studied processes suggested that a relationship between the adenylate energy charge and the glycosylation macroheterogeneity of the recombinant IFN‐γ may exist. Biotechnol. Bioeng. 2008;100: 721–733.

Influence of amino acids on hybridoma cell viability and antibody secretion

It is generally accepted that the phase of cell decline observed in batch culture of mammalian cells is related to exhaustion of medium nutrients (principally glucose and glutamine) and/or to waste products accumulation. In the present paper, we have studied the influence of glutamine on the proliferation of mouse hybridoma cells. We showed that repeated addition of glutamine prolonged the life span of the culture and significantly increased the secretion of monoclonal antibody. Flow cytometry analysis suggests that this effect of glutamine is related to a delay in cell death rather than to a stimulation of proliferation.Addition of glutamine and glucose failed however to prevent the death of the culture. Determinations of amino acid consumption in glutamine-supplemented samples and experiments carried out with complementary sources of amino acids (e.g. tryptose phosphate) strongly suggest that amino acid supply is a critical factor governing cell growth and productivity.

Limiting cell aggregation during mesenchymal stem cell expansion on microcarriers.

Mesenchymal stem cells (MSC) are known to be a valuable cell source for tissue engineering and regenerative medicine. However, one of the main limiting steps in their clinical use is the amplification step. MSC expansion on microcarriers has emerged during the last few years, fulfilling the lack of classical T‐flasks expansion. Even if the therapeutic potential of MSC as aggregates has been recently highlighted, cell aggregation during expansion has to be avoided. Thus, MSC culture on microcarriers has still to be improved, notably concerning cell aggregation prevention. The aim of this study was to limit cell aggregation during MSC expansion on Cytodex‐1®, by evaluating the impact of several culture parameters. First, MSC cultures were performed at different agitation rates (0, 25, and 75 rpm) and different initial cell densities (25 and 50 × 106 cell g−1 Cytodex‐1®). Then, the MSC aggregates were put into contact with additional available surfaces (T‐flask, fresh and used Cytodex‐1®) at different times (before and after cell aggregation). The results showed that cell aggregation was partly induced by agitation and prevented in static cultures. Moreover, cell aggregation was dependent on cell density and correlated with a decrease in the total cell number. It was however shown that the aggregated organization could be dissociated when in contact with additional surfaces such as T‐flasks or fresh Cytodex‐1® carriers. Finally, cell aggregation could be successfully limited in spinner flask by adding fresh Cytodex‐1® carriers before its onset. Those results indicated that MSC expansion on agitated Cytodex‐1® microcarriers could be performed without cell aggregation, avoiding a decrease in total cell number.

Genetic engineering of α2,6-sialyltransferase in recombinant CHO cells and its effects on the sialylation of recombinant interferon-γ.

The CHO cell line has achieved considerable commercial importance as a vehicle for the production of human therapeutic proteins, but is known to lack a functional copy of the gene coding for α2,6-sialyltransferase (EC 2.4.99.1). The cDNA for rat α2,6-ST was expressed in a recombinant CHO cell line making interferon-γ, using a novel in vitro amplification vector. The enzyme was expressed efficiently, and resulted in up to 60% of the total sialic acids on interferon-γ being linked in the α2,6-conformation. This sialic acid linkage distribution was more akin to that seen in natural human glycoproteins. In the most successful cell clones, expression of α2,6-sialyltransferase improved the overall level of sialylation by up to 56%, and had no adverse effects on cell growth, IFN-γ productivity or other aspects of IFN-γ glycosylation. These experiments demonstrate how the glycosylation machinery of rodent cells can be genetically manipulated to replicate human tissues.

Stimulation of monoclonal antibody production of hybridoma cells by butyrate: evaluation of a feeding strategy and characterization of cell behaviour

Experiments, earlier performed in our laboratory, showedthe stimulating effect of butyric acid on monoclonalantibody production by hybridoma cells. Itssimulaneous inhibitory effect on cell growth canhowever compensate for this, so that no increase ofmonoclonal antibody titer might be obtained. We showin this article an experiment with addition of butyricacid in the middle of the growth phase of a batchculture, as a strategy to take real profit of such anaddition by a significant increase of final monoclonalantibody concentration. Indeed, in this way asignificant cell density could be obtained before theaddition of butyric acid, while the remaining culturetime was still sufficiently long for its action,resulting in a two fold increase of final monoclonalantibody titer. The experiment was carried out in a 2 L bioreactor, showing the real practical interest ofsuch an addition for the large scale production ofproteins. Furthermore, analysis of the produced IgG bySDS-PAGE and Western blot did not reveal structuralchanges after stimulation by butyric acid. An originalpoint of our study is the characterization of the cellbehaviour, by flow cytometry and other relatedtechniques, leading to a better insight in the effectof the butyric acid addition on cell growth andmonoclonal antibody production. Although there existsa lot of knowledge about the effects of butyrate oncells in the field of molecular biology, our article isat our knowledge one of the first to show some of itseffects on cell behaviour in bioreactor culture,carried out under perfectly defined and controlledconditions, and with the aim to stimulate monoclonalantibody production.

Na-butyrate increases the production and α2,6-sialylation of recombinant interferon-γ expressed by α2,6- sialyltransferase engineered CHO cells

A non-human like glycosylation pattern in human recombinant glycoproteins expressed by animal cells may compromise their use as therapeutic drugs. In order to correct the CHO glycosylation machinery, a CHO cell line producing recombinant human interferon- γ (IFN) was transformed to replace the endogenous pseudogene with a functional copy of the enzyme α2,6-sialyltransferase (α2,6-ST). Both the parental and the modified CHO cell line were propagated in serum-free batch culture with or without 1 mM sodium butyrate. Although Na-butyrate inhibited cell growth, IFN concentration was increased twofold. The IFN sialylation status was determined using linkage specific sialidases and HPLC. Under non- induced conditions, IFN expressed by α2,6-engineered cells contained 68% of the total sialic acids in the α2,6- conformation and the overall molar ratio of sialic acids to IFN was 2.3. Sodium butyrate addition increased twofold the molar ratio of total sialic acids to IFN and 82% of total sialic acids on IFN were in the α2,6-conformation. In contrast, no effect of the sodium butyrate was noticed on the sialylation of the IFN secreted by the α2,6-ST deficient parental cell line. This study deals for the first time with the effect of Na-butyrate on CHO cells engineered to produce human like sialylation.

Kinetics and metabolic specificities of Vero cells in bioreactor cultures with serum-free medium.

The aim of this study was to understand the metabolism kinetics of Vero cells grown on microcarriers in bioreactors in serum-free medium (SFM). We sought to determine what nutrients are essential for Vero cells and how they are consumed. Contrary to glucose and to most of the amino acids, glutamine and serine were very quickly depleted in this medium and can be supposed to be responsible for cell apoptosis. Lactate and ammonium ions did not reach toxic levels for Vero cells. We payed more attention to the lactate metabolism. Usually we observed that after about 2 days lactate was consumed in serum-containing media, but its concentration plateaud in SFM. Moreover, the addition of serum in SFM provoked lactate consumption and the rate of glucose and glutamine consumption was twice as high as in the SFM not supplemented with serum. The depletion of glutamine and serine and the metabolic deviations leading to a shortage of intermediate products required for other metabolic pathways probably contribute to the lower cell yield and higher cell death rate in SFM.

HPCE monitoring of the N-glycosylation pattern and sialylation of murine erythropoietin produced by CHO cells in batch processes.

Using capillary electrophoresis coupled to laser‐induced fluorescence (HPCE‐LIF), it was possible to profile N‐linked oligosaccharides from EPO, including species containing sialic acid, during the course of batch cultures performed either in serum‐free or serum‐containing medium. Although an unusual high heterogeneity of the N‐linked oligosaccharides was observed by both SDS‐PAGE and HPCE analysis, the patterns of mEPO glycans after desialylation by mild acid hydrolysis were found to be quite constant over the course of the cultures either with or without serum supplementation. In contrast, when the protein was analyzed by HPCE without acidic desialylation, fingerprints of N‐linked oligosaccharides changed with time in serum‐containing conditions. This phenomenon appeared to be mainly due to the desialylation of mEPO as a result of a sialidase activity released upon cell lysis. These results demonstrate that though a higher EPO titer was obtained in serum supplemented conditions, sialylation of EPO was severely affected by the presence of serum in the culture medium.