Effects of proximate cholesterol precursors and steroid hormones on mouse myeloma growth in serum-free medium

Characterization of lipoprotein supplement and influence of its oxidized lipid content on cell culture performance and monoclonal antibody production by a SP2/0 hybridoma cell line

A challenging aspect with the use of the Sp2/0 hybridoma cell line in commercial manufacturing processes of recombinant therapeutic proteins is their exogenous lipids requirement for cell proliferation and optimal protein secretion. Lipids are commonly provided to the culture using serum or serum-derivatives, such as lipoprotein supplement. The batch-to-batch variability of these non-chemically defined raw-materials is known to impact cell culture process performance. Lipoprotein supplement variability and its impact on fed-batch production of a recombinant monoclonal antibody (mAb) expressed in Sp2/0 cells were studied using 36 batches from the same vendor. Several batches were associated with early viability drops leading to low process performance during fed-batch production. Increased caspase-3 activity (an indicator of apoptosis) was correlated to viability drops when low-performing batches were used. Addition of an antioxidant to the culture limited the increase in caspase-3 activity. Physicochemical characterization of batches confirmed that lipoproteins are mainly composed of lipids and proteins; no clear correlation between low-performing batches and lipoprotein supplement composition was observed. Controlled lipoprotein oxidation leads to lipoprotein solution browning, increasing absorbance at 276 nm and results in poor process performance. Because low-performing batches absorb more at 276 nm than other batches, oxidized lipids were suspected to be the root cause of low-performing batches. This study increased the understanding of lipoprotein supplement composition, its sensitivity to oxidation and its impact on process performance.

PD-1 agonism by anti-CD80 inhibits T cell activation and alleviates autoimmunity

Targeted blockade of the checkpoint molecule programmed cell death 1 (PD-1) can activate tumor-specific T cells to destroy tumors, whereas targeted potentiation of PD-1 is expected to suppress autoreactive T cells and alleviate autoimmune diseases. However, the development of methods to potentiate PD-1 remains challenging. Here we succeeded in eliciting PD-1 function by targeting the cis-PD-L1-CD80 duplex, formed by binding of CD80 to the PD-1 ligand PD-L1, that attenuates PD-L1-PD-1 binding and abrogates PD-1 function. By generating anti-CD80 antibodies that detach CD80 from the cis-PD-L1-CD80 duplex and enable PD-L1 to engage PD-1 in the presence of CD80, we demonstrate that the targeted dissociation of cis-PD-L1-CD80 duplex elicits PD-1 function in the condition where PD-1 function is otherwise restricted. We demonstrate using murine models that the removal of PD-1 restriction is effective in alleviating autoimmune disease symptoms. Our findings establish a method to potentiate PD-1 function and propose the removal of restraining mechanisms as an efficient strategy to potentiate the function of inhibitory molecules.

Multiple myeloma patients have a specific serum metabolomic profile that changes after achieving complete remission

PURPOSE

Multiple myeloma remains an incurable disease. New approaches to develop better tools for improving patient prognostication and monitoring treatment efficacy are very much needed. In this study, we aimed to evaluate the potential of metabolomics by (1)H-NMR to provide information on metabolic profiles that could be useful in the management of multiple myeloma.

EXPERIMENTAL DESIGN

Serum samples were collected from multiple myeloma patients at the time of diagnosis and after achieving complete remission. A matched control set of samples was also included in the study. The (1)H-NMR measurements used to obtain the metabolic profile for each patient were followed by the application of univariate and multivariate statistical analyses to determine significant differences.

RESULTS

Metabolic profiles of multiple myeloma patients at diagnosis exhibited higher levels of isoleucine, arginine, acetate, phenylalanine, and tyrosine, and decreased levels of 3-hydroxybutyrate, lysine, glutamine, and some lipids compared with the control set. A similar analysis conducted in multiple myeloma patients after achieving complete remission indicated that some of the metabolic changes (i.e., glutamine, cholesterol, lysine) observed at diagnosis displayed a variation in the opposite direction upon responding to treatment, thus contributing to multiple myeloma patients having a closer metabolic profile to those of healthy individuals after the disappearance of major disease manifestations.

CONCLUSIONS

The results highlight the potential of metabolic profiles obtained by 1H-NMR in identifying multiple myeloma biomarkers that may be useful to objectively discriminate individuals with and without multiple myeloma, and monitor response to treatment.

Adaptation of cholesterol-requiring NS0 mouse myeloma cells to high density growth in a fully defined protein-free and cholesterol-free culture medium

NS0 has been used as a fusion partner for the production of hybridomas and has more recently been engineered to produce recombinant protein. A protein-free culture medium, designated W38 medium, has previously been developed which supported high density growth of rat myeloma and hybridoma cell lines. NS0 cells failed to grow in W38 medium and in a number of protein-free culture media which support the growth of other myeloma cell lines. NS0 cells are derived from the NS-1 cell line, which is known to require exogencus cholesterol. It was found that NS0 cells grew in W38 medium supplemented with phosphatidylcholine, cholesterol, and albumin and that NS0 were auxotrophic for cholesterol. Protein-free growth of NS0 cells was achieved by using β-cyclodextrin to replace albumin as a lipid carrier. The maximal cell density reached in this protein-free medium was in excess of 1.5×10(6) cell ml(-1). The lipid supplements in the medium precipitated after a few days storage at +4°C. In order to overcome this problem a protocol was developed which allowed NS0 cells to be adapted to cholesterol-independent growth in W38 medium. NS0.CF (cholesterol-independent NS0 cells) were cultured continuously in W38 medium for several months. In shake flask culture a cell density of 2.4×10(6) cells ml(-1) was achieved in W38 medium compared with 1.41×10(6) cells ml(-1) in RPMI 1640 medium containing 10% foetal bovine serum. NS0.CF cells readily grew in a 1 litre stirred bioreactor using W38 medium supplemented with Pluronic F68 reaching a density of 3.24×10(6) cells ml(-1). NS0.CF were cloned protein-free by limiting dilution in W38 medium, giving colonies in wells that were seeded at an average density of 0.32 cells per 200 μl. This study has demonstrated for the first time the growth of a cholesterol-requiring mouse myeloma cell line in a completely defined protein-free medium and its subsequent adaptation to cholesterol-independence.

Principles of cancer cell culture

The basics of cell culture are now relatively common, though it was not always so. The pioneers of cell culture would envy our simple access to manufactured plastics, media and equipment for such studies. The prerequisites for cell culture are a well lit and suitably ventilated laboratory with a laminar flow hood (Class II), CO(2) incubator, benchtop centrifuge, microscope, plasticware (flasks and plates) and a supply of media with or without serum supplements. Not only can all of this be ordered easily over the internet, but large numbers of well-characterised cell lines are available from libraries maintained to a very high standard allowing the researcher to commence experiments rapidly and economically. Attention to safety and disposal is important, and maintenance of equipment remains essential. This chapter should enable researchers with little prior knowledge to set up a suitable laboratory to do basic cell culture, but there is still no substitute for experience within an existing well-run laboratory.

Derivation and characterization of cholesterol-independent non-GS NS0 cell lines for production of recombinant antibodies

Presented is an antibody production platform based on the fed-batch culture of recombinant NS0-derived cell lines. NS0 host cells, obtained from the European Collection of Cell Cultures (ECACC, Salisbury, UK, Part No. 85110503), were first adapted to grow in a protein-free, cholesterol-free medium. The resulting host cell line was designated NS0-PFCF (protein-free, cholesterol-free). The five production cell lines presented here were generated using a common protocol consisting of transfection by electroporation and subcloning. The NS0-PFCF host cell line was transfected using a single expression vector containing the Escherichia coli xanthine-guanine phosphoribosyl transferase gene (gpt), and the antibody heavy and light chain genes driven by the CMV promoter. The five cell lines were chosen after one to three rounds of iterative subcloning, which resulted in a 19-64% increase in antibody productivity when four mother-daughter cell pairs were cultured in a fed-batch bioreactor process. The production cell lines were genetically characterized to determine antibody gene integrity, nucleotide sequences, copy number, and the number of insertion sites in the NS0 cell genome. Genetic characterization data indicate that each of the five production cell lines has a single stably integrated copy of the antibody expression vector, and that the antibody genes are correctly expressed. Stability of antibody production was evaluated for three of the five cell lines by comparing the early stage seed bank with the Working Cell Bank (WCB). Antibody productivity was shown to be stable in two of three cell lines evaluated, while one of the cell lines exhibited a 20% drop in productivity after passaging for approximately 4 weeks. These five NS0-derived production cell lines were successfully cultured to produce antibodies with acceptable product quality attributes in a standardized fed-batch bioreactor process, consistently achieving an average specific productivity of 20-60 pg/cell-day, and a volumetric productivity exceeding 120 mg/L-day (Burky et al., 2006). In contrast to the commonly available NS0 host cell line, which requires serum and cholesterol for growth, and the commonly used expression vector system, which uses a proprietary glutamine synthetase selection marker (GS-NS0), these NS0 cells are cholesterol-independent, grow well in a protein-free medium, use a non-proprietary selection marker, and do not require gene amplification for productivity improvement. These characteristics are advantageous for use of this NS0 cell line platform for manufacturing therapeutic antibodies.

Cholesterol delivery to NS0 cells: Challenges and solutions in disposable linear low-density polyethylene-based bioreactors

We report the successful cultivation of cholesterol dependent NS0 cells in linear low-density polyethylene (LLDPE) Wave Bioreactors when employing a low ratio of cyclodextrin to cholesterol additive mixture. While cultivation of NS0 cells in Wave Bioreactors was successful when using a culture medium supplemented with fetal bovine serum (FBS), cultivation with the same culture medium supplemented with cholesterol-lipid concentrate (CLC), which contains lipids and synthetic cholesterol coupled with the carrier methyl-beta-cyclodextrin (mbetaCD), proved to be problematic. However, it was possible to cultivate NS0 cells in the medium supplemented with CLC when using conventional cultivation vessels such as disposable polycarbonate shake-flasks and glass bioreactors. A series of experiments investigating the effect of the physical conditions in Wave Bioreactors (e.g., rocking rate/angle, gas delivery mode) ruled out their likely influence, while the exposure of the cells to small squares of Wave Bioreactor film resulted in a lack of growth as in the Wave Bioreactor, suggesting an interaction between the cells, the CLC, and the LLDPE contact surface. Further experiments with both cholesterol-independent and cholesterol-dependent NS0 cells established that the concurrent presence of mbetaCD in the culture medium and the LLDPE film was sufficient to inhibit growth for both cell types. By reducing the excess mbetaCD added to the culture medium, it was possible to successfully cultivate cholesterol-dependent NS0 cells in Wave Bioreactors using a cholesterol-mbetaCD complex as the sole source of exogenous cholesterol. We propose that the mechanism of growth inhibition involves the extraction of cholesterol from cell membranes by the excess mbetaCD in the medium, followed with the irreversible adsorption or entrapment of the cholesterol-mbetaCD complexes to the LLDPE surface of the Wave Bioreactor. Controlling and mitigating these negative interactions enabled the routine utilization of disposable bioreactors for the cultivation of cholesterol-dependent NS0 cell lines in conjunction with an animal component-free cultivation medium.

17β-Hydroxysteroid dehydrogenase type 7 (Hsd17b7) reverts cholesterol auxotrophy in NS0 cells

NS0 is a host cell line widely used for the production of recombinant therapeutic proteins. In this work, we investigated the cholesterol-dependent phenotype of NS0 cells. Growth response to different precursors and comparative transcript analyses pointed to deficiency of 17beta-hydroxysteroid dehydrogenase type 7 (Hsd17b7) in NS0 cells. Hsd17b7 was previously shown to encode for an enzyme involved in estrogenic steroid biosynthesis. Its recent cloning into a yeast mutant deficient in ERG27 led to its functional characterization as the 3-ketoreductase of the cholesterol biosynthesis pathway. To ascertain that its cholesterol biosynthesis is blocked at the reduction reaction catalyzed by Hsd17b7, we genetically engineered NS0 cells to over express Hsd17b7. The stable transfectants of Hsd17b7 were able to grow independent of cholesterol. The results affirm the role of Hsd17b7 in the cholesterol biosynthesis pathway in mammals. Further, the findings allow for rational engineering of this industrially important cell line to alleviate their cholesterol dependence.

Development of Animal-free, Protein-Free and Chemically-Defined Media for NS0 Cell Culture

There has been a recent boom of monoclonal antibodies on the market, and a significant portion of them were produced by NS0 cell lines. As regulations become more stringent in ensuring production processes are free of potential contamination by adventitious agents, it is highly desirable to further develop serum-free media into ones that do not contain any components of animal origin, or 'animal-free media'. Using a shake-flask batch culture system, recombinant proteins (human albumin and human insulin) and synthetic compounds (tropolone and ferric ammonium citrate) were identified to be capable of replacing the animal-sourced proteins commonly found in serum-free media for NS0 cell culture, namely bovine albumin, insulin and transferrin. The cholesterol requirement of NS0 cells was satisfied by the use of a commercially available non-proteinaceous, non-animal sourced cholesterol/fatty acid mix in place of bovine lipoproteins, which in effect also eliminated the need for recombinant albumin. In the animal-free medium thus formulated, NS0 cell lines, either the host or recombinant constructs, were all able to grow in batch culture to 1~ 3x10(6) viable cells/ml for multiple passages, with no requirement for gradual adaptation even when seeded from 10% serum-containing cultures. It was surprising to observe that the recombinant insulin was essentially ineffective as sodium salt compared to its zinc salt. Studies showed that the zinc deficiency in the former resulted in a rapid decline of cell viabilities. Supplementation of zinc ions greatly improved growth, and even led to the total replacement of recombinant insulin and hence the formulation of a protein-free medium. When the cell lines were adapted to cholesterol-independent growth which eliminated the need for any lipid source, a completely chemically-defined animal-free medium was formulated. In all cases, antibody production by various GS-NS0 constructs in animal-free media was stable for multiple passages and at least similar to the original serum-free medium containing the animal-sourced proteins. The medium also served well for cryopreservation of NS0 cells in the absence of serum.

Large-scale gene expression analysis of cholesterol dependence in NS0 cells

NS0, a nonsecreting mouse myeloma cell, is a major host line used for recombinant antibody production. These cells have a cholesterol-dependent phenotype and rely on an exogenous supply of cholesterol for their survival and growth. To better understand the physiology underlying cholesterol dependence, we compared NS0 cells, cultivated under standard cholesterol-dependent growth conditions (NS0), to cells adapted to cholesterol-independent conditions (NS0 revertant, NS0_r). Large-scale transcriptional analyses were done using the Affymetrix GeneChip array, MG-U74Av2. The transcripts expressed differentially across the two cell lines were identified. Additionally, proteomic tools were employed to analyze cell lysates from these two cell lines. Cellular proteins from both NS0 and NS0_r were subjected to 2D gel electrophoresis. MALDI-TOF mass spectrometry was performed to determine the identity of the differentially expressed spots. We examined the expression level of mouse genes directly involved in cholesterol biosynthesis, lipid metabolism, and central energy metabolism. Most of these genes were downregulated in the revertant cell type, NS0_r, compared to NS0. Overall, a large number of genes are expressed differentially, indicating that the reversal of cholesterol dependency has a profound effect on cell physiology. It is probable that a single gene mutation, activation, or inactivation is responsible for cholesterol auxotrophy. However, the wide-ranging changes in gene expression point to the distinct possibility of a regulatory event affecting the reversibility of auxotrophy, either directly or indirectly.

Long-term culture of Xenopus presumptive ectoderm in a nutrient-supplemented culture medium

Animal cap assay is a useful experimental model for investigating the activity of inducers in amphibian development. This assay has revealed that activin A is a potent mesoderm-inducing factor. However, it has been very difficult to induce highly differentiated tissues such as cartilage in a 3-4 day culture period. It was recently reported that jaw cartilage was induced in vitro in an animal cap that had been cultured for 14 days in Steinberg's solution using the sandwich culture method and activin A. Under these conditions, necrosis was occasionally observed in the explants. In this study, we have achieved long-term animal cap cultures in a nutrient-supplemented culture medium designated RDX. This medium was made by modifying the saline concentration of the RD medium previously developed as a basal medium for the serum-free culture of various kinds of mammalian cells. The explants cultured in RDX grew more vigorously compared with those in Steinberg's solution. RDX medium promoted a wider variety of tissue induction and gene expression in the animal caps than Steinberg's solution, and also increased the frequency of cartilage induction. Therefore, the supplemental nutrients may support and promote the differentiation of cartilage. This long-term culture method using RDX medium is useful for studying the differentiation of tissues or organs such as cartilage in vitro.

Phosphate feeding improves high-cell-concentration NS0 myeloma culture performance for monoclonal antibody production

Phosphorus depletion was identified in high-cell-concentration fed-batch NS0 myeloma cell cultures producing a humanized monoclonal antibody (MAb). In these cultures, the maximum viable and total cell concentration was generally ca. 5 x 10(9) and 7 x 10(9) cells/L, respectively, without phosphate feeding. Depletion of essential amino acids, such as lysine, was initially thought to cause the onset of cell death. However, further improvement of cell growth was not achieved by feeding a stoichiometrically balanced amino acid solution, which eliminated depletion of amino acids. Even though a higher cell viability was maintained for a longer period, no increase in total cell concentration was observed. Afterwards, phosphorus was found to be depleted in these cultures. By also feeding a phosphate solution to eliminate phosphorus depletion, the cell growth phase was prolonged significantly, resulting in a total cell concentration of ca. 17 x 10(9) cells/L, which is much greater than ca. 7 x 10(9) cells/L without phosphate feeding. The maximum viable cell concentration reached about 10 x 10(9) cells/L, twice as high as that without phosphate feeding. Apoptosis was also delayed and suppressed with phosphate feeding. A nonapoptotic viable cell population of 6.5 x 10(9) cells/L, as compared with 3 x 10(9) cells/L without phosphate feeding, was obtained and successfully maintained for about 70 h. These results are consistent with the knowledge that phosphorus is an essential part of many cell components, including phospholipids, DNA, and RNA. As a result of phosphate feeding, a much higher integral of viable cell concentration over time was achieved, resulting in a correspondingly higher MAb titer of ca. 1.3 g/L. It was also noted that phosphate feeding delayed the cell metabolism shift from lactate production to lactate consumption typically observed in recombinant NS0 cultures. The results highlight the importance of phosphate feeding in high-cell-concentration NS0 cultures.

Selecting and designing cell lines for improved physiological characteristics

We have developed several approaches to create cell lines with improved characteristics in cell culture. In some cases it has been possible to isolate natural variants with useful properties. Cholesterol independent variants of the mouse NSO myeloma cell line were isolated by cloning in a selective medium. A glutamine independent variant of a hyridoma was isolated by continuous (chemostat) culture under glutamine limited conditions in the presence of glutamate. Choline independent cells were isolated from a choline limited chemostat. In an alternative approach to modifying cell behaviour, we have used recombinant DNA techniques to introduce the glutamine synthetase (GS) gene to a hybridoma. This resulted in glutamine independence and increased productivity.

Serum-free cell culture

Cell culture is one important tool when studying cellular functions and molecular biology. It is also a basic method in most virological investigations. Serum has been an obligatory component in most cell culture media. During the last decades serum-free, chemically defined media have been developed, that are supplemented with a number of substances with specific cellular activities. The main developments of defined media are presented. Examples are given of investigations with different cell types.

Growth and protein production kinetics of a murine myeloma cell line transfected with the human growth hormone gene

A model mammalian cell system for the production of recombinant proteins was investigated. Murine myeloma cells which had lost the ability to produce both heavy and light chain immunoglobulin molecules were transfected with a vector containing the immunoglobulin heavy chain promoter and enhancer elements linked to the human growth hormone gene. The growth kinetics of G32, a clonal isolate, were found to be similar to both the parent myeloma and hybridomas. However, production of hGH by G32 was growth associated, rather than as a secondary metabolite as is the case for hybridomas. In addition, G32 produced hGH at molar levels greater than most hybridomas.

An alternative method for the isolation of NS-1 hybridomas using cholesterol auxotrophy of NS-1 mouse myeloma cells

We have used the cholesterol auxotrophy of NS-1 mouse myeloma cells as the basis for selecting NS-1 hybridomas. The outgrowth of nascent NS-1 hybridomas in cholesterol-free serum-free medium was 3- to 9-fold more efficient than that in HAT medium and resulted in 3- to 13-times as many antigen-reactive hybridoma wells. This method of hybridoma selection can be applied with any sterol-dependent parent cell line. Hybridomas established under serum-free culture conditions were growth inhibited by fetal calf serum.