Cell aggregate suspension culture for large-scale production of biomolecules

Mucin-coating technologies for protection and reduced aggregation of cellular production systems

Optimization of host-cell production systems with improved yield and production reliability is desired to meet the increasing demand for biologics with complex posttranslational modifications. Aggregation of suspension-adapted mammalian cells remains a significant problem that can limit the cellular density and per volume yield of bioreactors. Here, we propose a genetically encoded technology that directs the synthesis of antiadhesive and protective coatings on the cellular surface. Inspired by the natural ability of mucin glycoproteins to resist cellular adhesion and hydrate and protect cell and tissue surfaces, we genetically encode new cell-surface coatings through the fusion of engineered mucin domains to synthetic transmembrane anchors. Combined with appropriate expression systems, the mucin-coating technology directs the assembly of thick, highly hydrated barriers to strongly mitigate cell aggregation and protect cells in suspension against fluid shear stresses. The coating technology is demonstrated on suspension-adapted human 293-F cells, which resist clumping even in media formulations that otherwise would induce extreme cell aggregation and show improved performance over a commercially available anticlumping agent. The stable biopolymer coatings do not show deleterious effects on cell proliferation rate, efficiency of transient transfection with complementary DNAs, or recombinant protein expression. Overall, our mucin-coating technology and engineered cell lines have the potential to improve the single-cell growth and viability of suspended cells in bioreactors.

Single-Cell Tracking of Breast Cancer Cells Enables Prediction of Sphere Formation from Early Cell Divisions

The mammosphere assay has become widely employed to quantify stem-like cells in a population. However, the problem is there is no standard protocol employed by the field. Cell seeding densities of 1,000 to 100,000 cells/mL have been reported. These high densities lead to cellular aggregation. To address this, we have individually tracked 1,127 single MCF-7 and 696 single T47D human breast tumor cells by eye over the course of 14 days. This tracking has given us detailed information for the commonly used endpoints of 5, 7, and 14 days that is unclouded by cellular aggregation. This includes mean sphere sizes, sphere-forming efficiencies, and a well-defined minimum size for both lines. Importantly, we have correlated early cell division with eventual sphere formation. At 24 hr post seeding, we can predict the total spheres on day 14 with 98% accuracy in both lines. This approach removes cell aggregation and potentially shortens a 5- to 14-day assay to a 24 hours.

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Single-cell tracking removes confounding aggregation from the mammosphere assay

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Tracking reveals sphere-forming efficiencies much higher than commonly reported

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True clonal spheres are smaller than commonly reported

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At 24 hours, tracking can predict total day 14 spheres with 98% accuracy

Fabrication of Adipose-Derived Mesenchymal Stem Cell Aggregates using Biodegradable Porous Microspheres for Injectable Adipose Tissue Regeneration

Injectable mesenchymal stem cell aggregates were formed using hyaluronic acid (HA)-immobilized porous biodegradable microspheres for adipose tissue regeneration. Adipose tissue-derived mesenchymal stem cells (AMSCs) were aggregated in a controlled manner and differentiated into adipocytes by cultivating in a stirred suspension bioreactor. The resultant cellular aggregates were approx. 1700 μm in diameter and exhibited fully differentiated adipocytes, as shown by immunocytochemistry and RT-PCR. The cultured aggregates could be smoothly injected into the subcutaneous region of mice through a syringe needle due to their soft elasticity and deformability. The in vivo regenerated adipose tissue maintained a proper dimension and shape, showing natural adipose tissue characteristics, as demonstrated by various histological staining procedures. HA-immobilized microspheres significantly enhanced cell differentiation during 3D cultivation, and tissue regeneration when implanted in vivo, compared to unmodified porous microspheres. This study showed that AMSC cellular aggregates prepared by using porous microspheres could be delivered in an injectable manner into the body and could have great therapeutic potential for soft tissue augmentation and reconstruction.

Large-scale mammalian cell culture: Design and use of an economical batch suspension system

Large-scale mammalian cell culture in the absence of antibiotics requires stringent conditions of sterility for all vessels, procedure, and systems used. Application of existing fermentation technology suffers from the differences between mammalian and bacterial cultures. Relatively simple and inexpensive 100-L vessels have been designed specifically for medium storage and antibiotic-free mammalian cell culture. These vessels are portable and sterilized in a 2 x 3 x 5 ft conventional or VACUMATIC autoclave. They consist of 30-gal 316 stainless-steel sanitary process drums whose heads have been modified to meet the rapid pressure changes that occur during autoclaving. The vessels incorporate systems for aseptic introduction and removal of both liquids and gases required for inoculation, growth, and harvesting of cell suspensions. A two-disk vibromixer is used for agitation with inoculation at a laminar flow hood and incubation in a warm room. These vessels have been used for culture of one rat and eight human tumor lines for over 2 x 10(5) L of suspension.

Injectable cellular aggregates prepared from biodegradable porous microspheres for adipose tissue engineering

Cellular aggregates were prepared using biodegradable porous microspheres for injectable reconstruction of soft tissues in vivo. Biodegradable porous microspheres with sizes of approximately 50 microm were prepared by a porogen leaching-phase separation process in an oil-in-water single-emulsion method using poly(D,L-lactide-co-glycolide). 3T3 L1 mouse preadipocyte cells were transformed into cellular aggregates by suspension cultivation in a spinner flask using the porous microspheres as effective buoyant and cell-adhesive microcarriers. Spherically shaped aggregates were readily formed with sizes of up to 1000 microm with a higher number of viable cells compared to the nonporous microspheres. The resultant cellular aggregates exhibited far better extent of differentiation into adipocytes than the cell aggregates prepared from nonporous microspheres or monolayer cultured cells. The cellular aggregates could also be injected into nude mice using a syringe needle for adipose tissue regeneration in vivo. Immunohistochemical studies showed that the aggregate-derived tissues with porous microspheres displayed histological characteristics similar to adipose tissues 4 weeks after injection.

Cultivation of recombinant Chinese hamster ovary cells grown as suspended aggregates in stirred vessels

Recombinant Chinese hamster ovary (rCHO) cells capable of producing a prourokinase mutant (mPro-uk) grown as suspended aggregates in stirred vessels were described and characterized. The addition of chitosan to a mixture of DMEM and Ham's F12 (D-MEM/F-12) medium promoted cell aggregation and spheroid formation efficiently. Multicellular aggregates formed immediately after the rCHO cells were inoculated into the chitosan-added medium, and the mean diameter of the cell aggregates reflecting the aggregate size increased with culture time, shifting from 65 to 163 mum after 2 and 9 d of culture in spinner flasks. No significant difference in the metabolism performance of the rCHO cells was observed between suspended aggregates and anchored monolayers. However, the cells cultured as suspended aggregates showed a marked decrease in growth rate as evaluated from specific growth rate (mu). Replacing D-MEM/F-12 medium with CD 293 medium caused compact spherical cell aggregates to dissociate into small irregular aggregates and single cells without apparent effects on cell performance in subcultures. The perfusion culture of the rCHO cells grown as suspended aggregates in a 2-l stirred tank bioreactor for 15 d resulted in a maximum viable cell density of 5.6 x 10(6) cells ml(-1) and an mPro-uk concentration of about 2.6 x 10(3) IU ml(-1), and cell viability was remained at roughly 90% during the entire run.

Suspended aggregates as an immobilization mode for high-density perfusion culture of HEK 293 cells in a stirred tank bioreactor

Cells of the human embryonic kidney cell line (HEK 293) grown in repeated suspension and perfusion systems were characterized and described. Cell aggregates that formed immediately after the HEK 293 cells were inoculated in stirred vessels in serum-containing Dulbecco's modified Eagle's medium (D-MEM)/F-12 medium. The mean diameter of the cell aggregates reflecting the aggregate size increased with culture time, shifting from 63 to 239 mum after 1 and 8 days of culture in spinner flasks, respectively. No significant differences in cell performance were observed between HEK 293 cell populations grown as suspended aggregates and those grown as anchored monolayers. Replacing the D-MEM/F-12 with CD 293 medium caused the compact spherical cell aggregates to dissociate into single cells and small irregular aggregates without any apparent effect on cell performance. Moreover, the spherical cell aggregates could reform from individual cells and small aggregates when exposed to the serum-containing D-MEM/F-12 dominant medium. Perfusion culture of HEK 293 cells grown as suspended aggregates in a 7.5-l stirred tank bioreactor for 17 days resulted in a maximum viable cell density of 1.2 x 10(7) cells ml(-1). These results demonstrate the feasibility and proof-of-concept for using aggregates as an immobilization system in large-scale stirred bioreactors because a small-scale culture can be used as easily as the inoculum for larger bioreactors.

Aggregate formation of rCHO cells and its maintenance in repeated batch culture in the absence of cell adhesion materials

Aggregate formation of recombinant Chinese hamster ovary (rCHO) cells capable of producing granulocyte colony-stimulating factor (G-CSF), using medium lacking cell adhesion materials in a repeated batch culture, was examined together with cell growth, cell viability and G-CSF production. The rCHO culture was conducted in a rotary shaker and the medium was changed every five days. The formation of stable cell aggregates with high reproducibility was observed after the first medium change. The size of the cell aggregates (consisting of several 10s to 40,000 cells) formed during the repeated batch culture ranged from 30 to 600 microm. The cell density of the aggregates reached as high as 2 x 10(6) cells/ml and the viability was maintained at more than 80% for 19 d. Changing the medium to avoid glucose exhaustion effectively maintained the cell density, cell viability and G-CSF productivity at high levels.

Large-scale transient expression in mammalian cells for recombinant protein production

Large-scale transient expression from mammalian cells is a new technology. Breakthroughs have been achieved for non-viral delivery methods: transfections can now be done at the 1-10 L scale with mammalian cells grown in suspension. Production of 1-20 mg/L of recombinant protein have been obtained in stirred bioreactors. Modified alphaviruses have provided a fast and efficient expression technology based on viral vectors.

Studies of baby hamster kidney natural cell aggregation in suspended batch cultures

Microcarrier cultures of animal cells of industrial relevance are known to shed aggregates into the suspension phase. For a BHK cell line, which is known to be prone to aggregate naturally, microcarrier and aggregate forms of culture are compared in spinner culture. In microcarrier cultures, it is shown that increasing initial microcarrier concentration yields decreasing concentration of smaller aggregates in suspension; roughly equivalent concentrations of total cells and single cells in suspension are obtained. In the absence of Cytodex 3, aggregate final size is hydrodynamically controlled in batch and semicontinuous suspension culture. Rate of agitation is the main variable controlling aggregate size in batch cultures. The range of agitation rates studied (20 to 70 rpm in 250 mL spinner flasks) produced aggregates with maximum sizes of 200 microns. Necrotic centers were not observed; this was confirmed by Trypan blue viability measurements after mechanical dissociation of aggregates and also by the constant productivity obtained from different aggregate sizes. Comparing aggregate and microcarrier culture conditions, it is shown that at 100 rpm maximum total cell concentration is larger in the absence of microcarriers; dead cell concentrations, most of which exist in suspension, are slightly larger in microcarrier culture. Total viable cell concentrations in aggregate, hydrodynamically controlled culture, are almost one order of magnitude higher than in microcarrier cultures. These results suggest that there might be advantages in using aggregate cultures under hydrodynamic control of aggregate size in lieu of microcarrier cultures for naturally aggregating cell lines.

Changes in animal cell natural aggregates in suspended batch cultures

Some anchorage-dependent animal cells can form natural aggregates in stirred tanks. Baby hamster kidney (BHK) natural aggregates are described and characterized. Total cell concentration and viability could be obtained after aggregate mechanical dissociation, with negligible cell lysis and no change in cell membrane permeability. During a normal batch run, aggregates were formed immediately after inoculation, a few spherical aggregates increasing in size during the initial growth phase. At the end of the growth phase, an increase in aggregate concentration was observed, without a considerable increase in aggregate diameter. At the end of the batch run, 160 h after inoculation, aggregates disintegrated into smaller, non-spherical units, following a sharp viability decrease. Cell concentrations of 1.2 x 10(6) cells/ml were obtained, with 60% of the total cells being in aggregates; the cell concentration in aggregates achieved 5 x 10(8) cells/ml, with a porosity of 55%. Viability was consistently in the range 85-90%, both for aggregate and suspended cells.

Production of a membrane-bound protein, the human gamma-glutamyl transferase, by CHO cells cultivated on microcarriers, in aggregates and in suspension

Recombinant Chinese Hamster Ovary (CHO) cells, engineered for the production of human gamma-glutamyl transferase (GGT), have been grown on Cytodex 1 microcarriers, as aggregates, or as single cells in suspension after adaptation. GGT is a membrane bound enzyme which was not secreted during the culture period. The maximal enzyme activity was found to be directly related to the achieved maximal cell density. Culture of CHO on microcarriers yielded the fastest growth, with a specific growth rate of 0.04 h-1, the highest cell density (near 1.3 x 10(6) cells ml-1), and the highest enzyme activity around 300 mU ml-1, which corresponded to a specific cellular level of 20 mU 10(-5) cells. GGT could also be produced by growing CHO cells in suspension as single cells or as aggregates. Under these conditions, however, the specific CHO growth rate was significantly slower and the GGT level per cell was divided by a factor 6. Growing CHO cells without microcarriers also resulted in differences in cell metabolism, with a higher conversion yield of glutamine into ammonia, and a higher cell lysis. The catalytic kinetic constants of the enzyme were found identical for the three culture systems.

Repeated-batch cultures of baby hamster kidney cell aggregates in stirred vessels

Natural aggregates of Baby Hamster Kidney cells were grown in stirred vessels operated as repeated-batch cultures during more than 600 hours. Different protocols were applied to passaging different fractions of the initial culture: single cells, large size distributed aggregates and large aggregates. When single cells or aggregates with the same size distribution found in culture are used as inoculum, it is possible to maintain semi-continuous cultures during more than 600 hours while keeping cell growth and viability. These results suggest that aggregate culture in large scale might be feasible, since a small scale culture can easily be used as inoculum for larger vessels without noticeable modification of the aggregate characteristics. However, when only the large aggregates are used as inoculum, it was shown that much lower cell concentrations are obtained, cell viability in aggregates dropping to less than 60%. Under this 'selection' procedure, aggregates maintain a constant size, larger than under batch experiments, up to approximately 400 hours; after this time, aggregate size increases to almost twice the size expected from batch cultures.

The growth of Vero cells in suspension as cell-aggregates in serum-free media

Vero cell lines, usually considered anchorage-dependent, could be grown as cell-aggregates in suspension culture with serum-free media. Several different combinations of base media gave growth results above 10(6) cells/ml (NCTC 135:SFRE 199-1; NCTC 135:Waymouth MB 752/1; NCTC 135:RPMI 1640). Insulin was not essential for growth and Bovine Serum Albumin could be diluted out of the media if linoleic acid was present. The size and density of the aggregates formed varied depending on the media used.

Evaluation of anchorage-dependent cell propagation systems for production of human acetylcholinesterase by recombinant 293 cells

Production of recombinant human acetylcholinesterase (AChE) by a high producer human embryonic kidney cell line (293) was evaluated by three main cell propagation systems; surface propagator, fixed-bed reactor and stirred microcarrier cultures. The recombinant cell line expresses AChE levels as high as 10-20 mg/l/day. System productivities in either the surface propagator (multitray system), or in the fixed-bed reactor (polyurethane macroporous sponges) were 4-8 mg AChE/l/day during a production period of 8 days. Similar productive rates, yet longer production periods (up to 22 days), were obtained in microcarrier (MC) cultures using either polystyrene beads (Biosilon); collagen-coated dextran beads (Cytodex-3); or gelatin macroporous beads (Cultispher-G). Best results were obtained in an aggregate culture using cellulose beads charged with diethylaminoethyl (DEAE) groups, (Servacel), as carriers. In this culture, a system productivity of 6-10 mg/l/day was maintained for 28 days.

An assay measuring the stimulation of several types of bovine endothelial cells by growth factor(s) derived from cultured human tumor cells

Endothelial cell growth factor(s) from several previously untested human tumor cell lines (i.e., SK-HEP-1, MG63, A375, TE671-C1, RD) were detected using a low cell inoculum growth assay. The final cell density in the 2-cm2 wells was determined by a highly sensitive DNA content measurement performed directly in the tissue culture plates. The sensitivity of the assay to human tumor cell growth factors depended critically on the low cell inocula, 2,000 to 5,000 cells/well. Most of the bovine endothelial cells used were cloned from primary cultures; all the cell lines obtained from various fetal and nonfetal sources responded to the growth factor(s) (up to a 16x stimulation) as well as to endothelial cell growth supplement. Dose response curves showing the cell specific response of bovine endothelial cells were obtained. The growth stimulatory activity and the in vivo chick embryo chorioallantoic membrane assay responses correlated sufficiently to imply that the assay is detecting tumor angiogenesis factor or some closely related activity. This in vitro assay should prove useful in the identification and purification of tumor-derived factors and in the elucidation of the role of these factors in the events comprising angiogenesis.