Scaleup of Insect Cell Cultures: Protective Effects of Pluronic F-68

Perspective on the influence of suspension manufacturing unit operations on bioburden viability and selection of sampling points at the pilot scale

The suspension wet media milling manufacturing process is a complex multi-unit operation, resulting in drug substance comminution to a target particle size. As a result of this complexity, microbial contamination is of paramount concern, particularly for suspensions dosed for parenteral use. This perspective sought to review the influence of (4) critical manufacturing unit operations using a quality risk management approach to better identify and articulate impact of each unit operation on bioburden viability. The manufacturing unit operations in scope included slurry compounding, deaeration, milling, and filling. Bow tie risk analysis was used as a visual gap analysis tool to evaluate if conventional controls were appropriate to detect and mitigate potential for microbial contamination. A deep dive into these unit operations clarified that mechanisms such as turbohypobiosis, cavitation during deaeration, high energy milling, and inert overlay may have an appreciable influence on bioburden viability and proliferation. The resultant analysis also explicated that endotoxin oversight must be closely monitored through barriers (input material controls, water quality controls) to minimize impact to the product and patient. The identified manufacturing unit operations were not appropriate as mitigating controls for endotoxin. The output of this article relates risk intersections for microbial contamination during wet media milling and offers insights in critical areas for intervention.

Quantitative Analysis of Poloxamer 188 in Biotherapeutic Process Streams Using Liquid Chromatography-Triple-Quadrupole Mass Spectrometry

Poloxamer 188, also known as Pluronic F-68, is an excipient added to the biotherapeutic protein-manufacturing process. Poloxamer 188 (P188) is a nonionic triblock copolymer surfactant that can be used as a shear protective excipient in bioreactors. In the current study, a method for the process clearance monitoring of poloxamer 188 during downstream processing of biotherapeutics using liquid chromatography-triple-quadrupole mass spectrometry was developed and validated. Chromatographic separation of P188 was achieved using a Phenomenex, Luna 3 μm phenyl-hexyl, 150 × 2 mm column, and quantitation was achieved using a triple-quadrupole mass spectrometer operated in selected reaction monitoring mode. Linearity was assessed over a working range of 250-10,000 ng/mL. Precision and accuracy were within 15% of the theoretical spike levels assessed across the three different concentration levels. For this study, two different IgG1 antibodies were used for the method validation activities. Analyte specificity and selectivity were deemed acceptable based on no extraneous peaks present. System suitability was evaluated throughout this study in anticipation of the introduction of this method into the quality control environment. This method was successfully validated and used to monitor the clearance of poloxamer 188 in a tangential flow filtration purification step during biotherapeutic downstream processing. In addition, the capability of the method to successfully support poloxamer 188 mixing studies is presented in this work.

Titer estimation for quality control (TEQC) method: A practical approach for optimal production of protein complexes using the baculovirus expression vector system

The baculovirus expression vector system (BEVS) is becoming the method of choice for expression of many eukaryotic proteins and protein complexes for biochemical, structural and pharmaceutical studies. Significant technological advancement has made generation of recombinant baculoviruses easy, efficient and user-friendly. However, there is a tremendous variability in the amount of proteins made using the BEVS, including different batches of virus made to express the same proteins. Yet, what influences the overall production of proteins or protein complexes remains largely unclear. Many downstream applications, particularly protein structure determination, require purification of large quantities of proteins in a repetitive manner, calling for a reliable experimental set-up to obtain proteins or protein complexes of interest consistently. During our investigation of optimizing the expression of the Mediator Head module, we discovered that the ‘initial infectivity’ was an excellent indicator of overall production of protein complexes. Further, we show that this initial infectivity can be mathematically described as a function of multiplicity of infection (MOI), correlating recombinant protein yield and virus titer. All these findings led us to develop the Titer Estimation for Quality Control (TEQC) method, which enables researchers to estimate initial infectivity, titer/MOI values in a simple and affordable way, and to use these values to quantitatively optimize protein expressions utilizing BEVS in a highly reproducible fashion.

Impact of Pluronic® F68 on hollow fiber filter-based perfusion culture performance

High cell density is an important factor in achieving high bioreactor productivity. To meet the oxygen demand with density at >100 × 10⁶ cells/mL, a frit sparger is often used. In this study, the impact of Pluronic^® F68 on a perfusion process using a frit sparger was studied. The perfusion process was developed using an alternating tangential flow device with a 0.2 µm PES hollow fiber filter. Pluronic^® F68 at 2 g/L was sufficient in preventing cell damage at gas flow rate of ~0.20 vvm from a drilled hole sparger (0.5 mm) but inadequate at ~0.025 vvm from a frit sparger (20 µm). Increase of Pluronic^® F68 concentration to 5 g/L prevented cell death at up to ~0.10 vvm from the frit sparger and was able to maintain high cell density at high viability in the range of 60-80 × 10⁶ cells/mL. Such positive effect was demonstrated in both 3- and 200-L bioreactors. Supplementing additional Pluronic^® F68 was also effective in restoring cell growth/viability from low viability cultures. Increased Pluronic^® F68 concentration had no adverse impact on target antibody, HCP, and Pluronic^® F68 transmissions.

Transforming Lepidopteran Insect Cells for Continuous Recombinant Protein Expression

The baculovirus expression vector system (BEVS) is widely used to produce large quantities of recombinant proteins. However, the yields of extracellular and membrane-bound proteins obtained with this system are often very low, possibly due to the adverse effects of baculovirus infection on the host insect cell secretory pathway. An alternative approach to producing poorly expressed proteins is to transform lepidopteran insect cells with the gene of interest under the control of promoters that are constitutively active in uninfected cells, thereby making cell lines that continuously express recombinant protein. This chapter provides an overview of the methods and considerations for making stably transformed lepidopteran insect cells. Techniques for the insertion of genes into continuous expression vectors, transfection of cells, and the selection and isolation of stably transformed Sf-9 clones by either colony formation or end-point dilution are described in detail.

Microfluidic bubbler facilitates near complete mass transfer for sustainable multiphase and microbial processing

A microfluidic device (channels <70 μm) was utilized to create micro-scale bubbles to significantly increase mass transfer efficiency at low flow rates. The convergence of one gas and two liquid channels at a Y-junction generates bubbles via cyclic changes in pressure. At low flow rates, the bubbles had an average diameter of 110 μm, corresponding to a volumetric mass transfer KL a of 1.43 h(-1) . Values of KL a normalized per flow rate showed that the microbubbler had a 100-fold increased transfer efficiency compared to four other commonly used bubblers. The calculated percentage of oxygen transferred was approximately 90%, which was consistent with a separate off-gas analysis. The improved mass transfer was also tested in an algae bioreactor in which the microbubbler absorbed approximately 90% of the CO2 feed compared to 2% in the culture with an alternative needle bubbling method. The microbubbler yielded a cell density 82% of the cell density for the alternative needle tip with an 800-fold lower flow rate (0.5 mL/min versus 400 mL/min) and a 700-fold higher ratio of biomass to fed carbon dioxide. The application of microfluidics may transform interfacial processing in order to increase mass transfer efficiencies, minimize gas feeding, and provide for more sustainable multiphase processes. Biotechnol. Bioeng. 2016;113: 1924-1933. © 2016 Wiley Periodicals, Inc.

Utilization of pyrolytic substrate by microalga Chlamydomonas reinhardtii: cell membrane property change as a response of the substrate toxicity

Acetic acid derived from fast pyrolysis of lignocellulosic biomass is a promising substrate for microalgae fermentation for producing lipid-rich biomass. However, crude pyrolytic acetic acid solution contains various toxic compounds inhibiting algal growth. It was hypothesized that such an inhibition was mainly due to the cell membrane damage. In this work, the cell membrane property of algal cells was evaluated at various conditions to elucidate the mechanisms of inhibition caused by the pyrolytic substrate solution. It was found that acetic acid itself served a carbon source for boosting algal cell growth but also caused cell membrane leakage. The acetic acid concentration for highest cell density was 4 g/L. Over-liming treatment of crude pyrolytic acetic acid increased the algal growth with a concurrent reduction of cell membrane leakage. Directed evolution of algal strain enhanced cell membrane integrity and thus increased its tolerance to the toxicity of the crude substrate. Statistical analysis shows that there was a significant correlation between the cell growth performance and the cell membrane integrity (leakage) but not membrane fluidity. The addition of cyto-protectants such as Pluronic F68 and Pluronic F127 enhanced the cell membrane integrity and thus, resulted in enhanced cell growth. The transmission electron microscopy (TEM) of algal cells visually confirmed the cell membrane damage as the mechanism of the pyrolytic substrate inhibition. Collectively, this work indicates that the cell membrane is one major reason for the toxicity of pyrolytic acetic acid when being used for algal culture. To better use this pyrolytic substrate, cell membrane of the microorganism needs to be strengthened through either strain improvement or addition of membrane protectant reagents.

Mean-field coarse-grained model for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer systems

The microscopic modeling of surfactant systems is of the utmost importance in understanding the mechanisms related to the micellization process because it allows for prediction and comparison with experimental data of diverse equilibrium system properties. In this work, we present a coarse-grained model for Pluronics, a trademarked type of triblock copolymer, from simulations based on a single-chain mean-field theory (SCMF). This microscopic model is used to quantify the micellization process of these nonionic surfactants at 37 °C and has been shown to be able to quantitatively reproduce experimental data of the critical micelle concentration (CMC) along with other equilibrium properties. In particular, these results correctly capture the experimental behavior with respect to the lengths of the hydrophobic and hydrophilic moieties of the surfactants for low and medium hydrophobicities. However, for the more highly hydrophobic systems with low CMCs, a deviation is found which has been previously attributed to nonequilibrium effects in the experimental data (Garcı́a Daza, F. A.; Mackie, A. D. Low Critical Micelle Concentration Discrepancy between Theory and Experiment. J. Phys. Chem. Lett. 2014, 5, 2027-2032).

Development of quenching and washing protocols for quantitative intracellular metabolite analysis of uninfected and baculovirus-infected insect cells

Metabolomics refer to the global analysis of small molecule metabolites in a biological system, and can be a powerful tool to elucidate and optimize cellular processes, particularly when integrated into a systems biology framework. Determining the endometabolome in cultured animal cells is especially challenging, due to the conflicting demands for rapid quenching of metabolism and retention of membrane integrity, while cells are separated from the complex medium. The challenge is magnified in virus infected cells due to increased membrane fragility. This paper describes an effective methodology for quantitative intracellular metabolite analysis of the baculovirus-insect cell expression system, an important platform for the production of heterologous proteins and baculovirus-based biopesticides. These two applications were represented by Spodoptera frugiperda (Sf9) and Helicoverpa zea (HzAM1) cells infected with recombinant Autographa californica and wild-type Helicoverpa armigera nucleopolyhedroviruses (AcMNPV and HaSNPV), respectively. Specifically, an ice-cold quenching solution comprising 1.1% w/v NaCl and 0.2% w/v Pluronic® F-68 (NaCl+P) was found to be efficacious in preserving cell viability and minimizing cell leakage during quenching and centrifugation-based washing procedures (prior to extraction using cold 50% v/v acetonitrile). Good recoveries of intracellular adenosine triphosphate, total adenosine phosphates and amino acids were obtained after just one wash step, for both uninfected and infected insect cells. The ability to implement wash steps is critical, as insect cell media are metabolites-rich, while infected insect cells are much more fragile than their uninfected counterparts. Hence, a promising methodology has been developed to facilitate endometabolomic analysis of insect cell-baculovirus systems for bioprocess optimization.

Production of the Anticarsia gemmatalis multiple nucleopolyhedrovirus in serum-free suspension cultures of the saUFL-AG-286 cell line in stirred reactor and airlift reactor

The velvetbean caterpillar, Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae), is one of the main plagues for soybean crops. Velvetbean caterpillar larvae are susceptible to be infected by occlusion bodies of the baculovirus Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV), a biological insecticide. The insect cell line saUFL-AG-286 produces very high yields of occlusion bodies of AgMNPV in suspension cultures done in the low-cost serum-free medium UNL-10 in shake-flasks. However, its ability to adapt to conditions of industrial production in bioreactors was unknown. The aim of this study was to characterize the growth of saUFL-AG-286 cell cultures in UNL-10 medium, as well as its capability to replicate AgMNPV in two different bio-reactors at laboratory scale. The cell line was able to adapt to conditions that can be used at industrial scale, both in an airlift reactor and a stirred reactor, although the former was better than the last to support the cell growth. The infection with AgMNPV in the airlift reactor produced a high yield of occlusion bodies, with very low production of budded virus, the progeny used as inoculums. On the other hand, infection in the stirred reactor yielded high titers of budded virus. These results suggest that a feasible strategy for scaling-up the production of AgMNPV might involve the use of airlift reactors for the scaling-up of cell suspension cultures and the final production of occlusion bodies, while the scaling-up of the viral inoculums being carried out under conditions as those existing in stirred reactors.

Virus-like particles in vaccine development

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome, potentially yielding safer and cheaper vaccine candidates. A handful of prophylactic VLP-based vaccines is currently commercialized worldwide: GlaxoSmithKline's Engerix (hepatitis B virus) and Cervarix (human papillomavirus), and Merck and Co., Inc.'s Recombivax HB (hepatitis B virus) and Gardasil (human papillomavirus) are some examples. Other VLP-based vaccine candidates are in clinical trials or undergoing preclinical evaluation, such as, influenza virus, parvovirus, Norwalk and various chimeric VLPs. Many others are still restricted to small-scale fundamental research, despite their success in preclinical tests. This article focuses on the essential role of VLP technology in new-generation vaccines against prevalent and emergent diseases. The implications of large-scale VLP production are discussed in the context of process control, monitorization and optimization. The main up- and down-stream technical challenges are identified and discussed accordingly. Successful VLP-based vaccine blockbusters are briefly presented concomitantly with the latest results from clinical trials and the recent developments in chimeric VLP-based technology for either therapeutic or prophylactic vaccination.

Viruses and Virus-Like Particles in Biotechnology

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this article, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitorization, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

Effect of surfactant pluronic F-68 on CHO cell growth, metabolism, production, and glycosylation of human recombinant IFN-γ in mild operating conditions

The control of glycosylation to satisfy regulatory requirements and quality consistency of recombinant proteins produced by different processes has become an important issue. With two N-glycosylation sites, γ-interferon (IFN-γ) can be seen as a prototype of a recombinant therapeutic glycoprotein for this purpose. The effect of the nonionic surfactant Pluronic F-68 (PF-68) on cell growth and death was investigated, as well as production and glycosylation of recombinant IFN-γ produced by a CHO cell line that was maintained in a rich protein-free medium in the absence or presence of low agitation. Under these conditions, a dose-dependent effect of PF-68 (0-0.1%) was shown not only to significantly enhance growth but also to reduce cell lysis. Interestingly, supplementing the culture medium with PF-68 led to increased IFN-γ production as a result of both higher cell densities and a higher specific production rate of IFN-γ. If cells were grown with agitation, lack of PF-68 in the culture medium decreased the fraction of the fully glycosylated IFN-γ glycoform (2N) from 80% to 65-70% during the initial period. This effect appeared to be due to a lag phase in cell growth observed during this period. Finally, a global kinetic study of CHO cell metabolism indicated higher efficiency in the utilization of the two major carbon substrates when cultures were supplemented with PF-68. Therefore, these results highlight the importance of understanding how media surfactant can affect cell growth as well as cell death and the product quality of a recombinant glycoprotein expressed in CHO cell cultures.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Transforming lepidopteran insect cells for continuous recombinant protein expression

The baculovirus expression vector system is widely used to produce large quantities of recombinant proteins. However, yields of extracellular and membrane-bound proteins obtained with this system often are very low, possibly because of the adverse effects of baculovirus infection on the host insect cell secretory pathway. An alternative approach to producing poorly expressed proteins is to transform insect cells with the gene of interest under the control of promoters that are constitutively active in uninfected cells, thereby making cell lines that continuously express recombinant protein. This chapter provides an overview of the methods and considerations for making stably transformed lepidopteran cells. Techniques for the insertion of genes into continuous expression vectors, transfection of cells, and the selection and isolation of stably transformed Sf-9 clones by either colony formation or end-point dilution are described in detail.

Molecular cloning and functional characterization of beta-N-acetylglucosaminidase genes from Sf9 cells

Sf9, a cell line derived from the lepidopteran insect, Spodoptera frugiperda, is widely used as a host for recombinant glycoprotein expression and purification by baculovirus vectors. Previous studies have shown that this cell line has one or more beta-N-acetylglucosaminidase activities that may be involved in the degradation and/or processing of N-glycoprotein glycans. However, these enzymes and their functions remain poorly characterized. Therefore, the goal of this study was to isolate beta-N-acetylglucosaminidase genes from Sf9 cells, over-express the gene products, and characterize their enzymatic activities. A degenerate PCR approach yielded three Sf9 cDNAs, which appeared to encode two distinct beta-N-acetylglucosaminidases, according to bioinformatic analyses. Baculovirus-mediated expression of these two cDNA products induced membrane-associated beta-N-acetylglucosaminidase activities in Sf9 cells, which cleaved terminal N-acetylglucosamine residues from the alpha-3 and -6 branches of a biantennary N-glycan substrate with acidic pH optima and completely hydrolyzed chitotriose to its constituent N-acetylglucosamine monomers. GFP-tagged forms of both enzymes exhibited punctate cytoplasmic fluorescence, which did not overlap with either lysosomal or Golgi-specific dyes. Together, these results indicated that the two new Sf9 genes identified in this study encode broad-spectrum beta-N-acetylglucosaminidases that appear to have unusual intracellular distributions. Their relative lack of substrate specificity and acidic pH optima are consistent with a functional role for these enzymes in glycoprotein glycan and chitin degradation, but not with a role in N-glycoprotein glycan processing.

Visualization of a recombinant gene protein in the baculovirus expression vector system using confocal scanning laser microscopy

Expression of the recombinant protein beta-galactosidase in the Spodoptera frugiperda Sf-9 insect cell line infected by the Autographa californica nuclear polyhedrosis virus expressing beta-galactosidase (AcNPV-betagal) was visualized using confocal scanning laser microscopy with fluorescent staining of both the recombinant protein and the cell nucleus. The average size of the insect cells and the intracellular DNA concentration both increased markedly, respectively reading 3.8- and 2.3-fold the values before infection. The average beta-galactosidase activity began to increase at 20-24 h post infection and finally reached 1.9 x 10(4) units/ml. As the post infection time increased, the stained nucleus images expanded and spread broadly. Beta-galactosidase was first identified by fluorescent staining at 12 h post-infection, filled the cell at 27 h, began to be released at 36 h, and finally spread out of the cell. The locations of the nucleus and expressed beta-galactosidase were identified from computerized tomograms and 3-dimensional images.

A physiological product‐release model for baculovirus infected insect cells

Existing models describe the product release from baculovirus infected insect cells as an unspecific protein leakage occurring in parallel with protein production. The model presented here shows that the observed product release of normally non-secreted proteins can be described through cell death alone. This model avoids the implicit non-physiological assumption of previous models that cells permeable to recombinant protein as well as trypan blue continue to produce protein.

Cloning of Tetrahymena cells in a chemically defined medium is possible in the presence of surfactants or at reduced temperature

When Tetrahymena cells are exposed to physical or chemical stress they may die. The effect of a given stress depends on the culture medium, the temperature, and the manipulation of the cells. Cells in broth-medium or buffer solution are more resistant than cells in chemically defined medium (CDM). A type of physical stress is caused by the hydrodynamic properties at the constriction of the pipette tip. This type of stress may be reduced/abolished by use of tips with maximal area and smoothness at the constrictions, underwater delivery of cell suspensions combined with gentle mixing, by use of reduced temperatures, by avoidance of medium-air interfaces or by addition of surfactants. By adjustment of these parameters it is possible to clone single cells of different species of Tetrahymena in CDM. In the presence of surfactants, cells can be cloned even under harsh manipulation. In absence of surfactants, cells can be cloned at 15 degrees C using mild manipulations. Tetrahymena cells are independent of unspecific growth factors and they do not exert autocrine growth control. Pluronic does not bind to the cells with significant affinity. Chemical stress cannot be counteracted by surfactants. Pre-stress (heat) protects the cells from subsequent lethal heat stress.

Scale-up of virus-like particles production: effects of sparging, agitation and bioreactor scale on cell growth, infection kinetics and productivity

The baculovirus-insect cells expression system was used for the production of self-forming Porcine parvovirus (PPV) like particles (virus-like particles, VLPs) in serum-free medium. At 2l bioreactor scale an efficient production was achieved by infecting the culture at a concentration of 1.5 x 10(6)cells/ml using a low multiplicity of infection of 0.05 pfu per cell. In a continuous bioreactor, it was shown that the uninfected insect cells were not sensitive to local shear stress values up to 2.25 N/m2 at high Reynolds numbers (1.5 x 10(4)) in sparging conditions. Uninfected insect cells can be grown at scaled-up bioreactor at high agitation and sparging rates as long as vortex formation is avoided and bubble entrapment is minimized. An efficient process scale-up to 25 l bioreactor was made using constant shear stress criteria for scale-up. The kinetics of baculovirus infection at low multiplicity of infection, either at different cell concentration or at different scales, are very reproducible, despite the different turbulence conditions present in the bioreactor milieu. The results suggest that the infection kinetics is controlled by the rate of baculovirus-cell receptor attachment and is independent of the bioreactor hydrodynamic conditions. Furthermore, the achieved specific and volumetric productivities were higher at the 25 l scale when compared to the smaller scale bioreactor. Different rates of cell lysis after infection were observed and seem to fully explain both the shift in optimal harvest time and the increase in cell specific productivity. The results emphasize the importance of integrated strategies and engineering concepts in process development at bioreactor stage with the baculovirus insect cell system.

Production of core and virus-like particles with baculovirus infected insect cells

In this paper the fundamental aspects of process development for the production of core and virus-like particles with baculovirus infected insect cells are reviewed. The issues addressed include: particle formation and monomer composition, chemical and physical conditions for optimal cell growth, baculovirus replication and product expression, multiplicity of infection strategy, and scale-up of the process. Study of the differences in the metabolic requirements of infected and non-infected cells is necessary for high cell density processes. In the bioreactor, the specific oxygen uptake rate (OURsp) plays a central role in process scale-up, leading to the specification of the bioreactor operational parameters. Shear stress can also be an important variable for bioreactor operation due to its influence on cell growth and product expression. The determination of the critical variables in process development is discussed, showing the relevance of the mathematical models that have been developed for the insect cells/baculovirus system in process implementation and control.

Biosynthesis and subcellular localization of a lepidopteran insect alpha 1,2-mannosidase

Like lower and higher eucaryotes, insects have alpha 1,2-mannosidases which function in the processing of N-glycans. We previously cloned and characterized an insect alpha 1,2-mannosidase cDNA and demonstrated that it encodes a member of a family of N-glycan processing alpha 1,2-mannosidases (Kawar, Z., Herscovics, A., Jarvis, D.L., 1997. Isolation and characterisation of an alpha 1,2-mannosidase cDNA from the lepidopteran insect cell line Sf9. Glycobiology 7, 433-443). These enzymes have similar protein sequences, require calcium for their activities, and are sensitive to 1-deoxymannojirimycin, but can have different substrate specificities and intracellular distributions. We recently determined the substrate specificity of the insect alpha 1,2-mannosidase, SfManI (Kawar, Z., Romero, P., Herscovics, A., Jarvis, D.L., 2000. N-glycan processing by a lepidopteran insect and 1,2-mannosidase. Glycobiology 10, 347-355). Now, we have examined the biosynthesis and subcellular localization of SfManI. We found that SfManI is partially N-glycosylated and that N-glycosylation is dramatically enhanced if the wild type sequon is changed to one that is highly utilized in a mammalian system. We also found that an SfManI-GFP fusion protein had a punctate cytoplasmic distribution in insect cells. Colocalization studies indicated that this fusion protein is localized in the Golgi apparatus, not in the endoplasmic reticulum or lysosomes. Finally, N-glycosylation had no influence over the substrate specificity or subcellular localization of SfManI.

Perfluorocarbon-mediated aeration applied to recombinant protein production by virus-infected insect cells

Perfluorocarbon (PFC) was used as an oxygen carrier in the cultures of insect cells and virus-infected insect cells. The cell suspensions were placed on a planar layer of PFC, which was re-oxygenated in an outer aeration unit and continuously recirculated, and were agitated by two sets of impeller blades, lower one of which was set in such a way that the ridge of the blade touched the PFC layer. The maximum cell density attained in the PFC-mediated aeration culture was higher than that in surface aeration culture. On viral infection, a recombinant protein yield was significantly high in the PFC-mediated aeration culture as compared with that in the surface aeration culture, though the production was largely decreased by setting apart the lower set of the blade from the PFC-medium interface. These results showed that the PFC-mediated aeration would be a useful technique for insect cell/baculovirus expression system. Overall mass-transfer coefficient K(L) for oxygen was examined in both the PFC-mediated aeration and surface aeration systems, by using a flask whose dimensions were identical to those of spinner flasks used for the cultures. The K(L) value in the PFC-mediated system was 2.60x10(-3)cms(-1), 1.6 times higher than that in the surface aeration system, when impeller blades were positioned at PFC-medium and medium-air interfaces, respectively. However, the K(L) values in both the PFC-mediated and surface aeration systems were decreased and their differences were brought so close, as the blade was set apart from the interfaces. DO behavior in the cultures was well explained by the model calculation using the determined K(L) values and oxygen-consumption rates of viable cells. This calculation further suggested that crucial DO, under which recombinant protein productions were unsuccessful, was 0.24-0.5ppm (3-7%) in the insect cell/baculovirus expression system.

Animal-cell damage in sparged bioreactors

The gas sparging of culture broth causes damage to suspended animal cells. However, despite this, sparged bioreactors remain the preferred means of cell culture because sparging is a robust method of supplying oxygen, especially on a large scale. This article examines the underlying mechanisms involved in bubble-associated cell damage and the methods available for controlling such damage.

Evidence of Pluronic F-68 direct interaction with insect cells: impact on shear protection, recombinant protein, and baculovirus production*

Pluronic F-68 has been widely used to protect animal cells from hydrodynamic stress, but its mechanism of action is still debatable. Published evidence indicates that Pluronic F-68 interacts with cells, yet scarce information exists of its effect on recombinant protein and virus production by insect cells. In this work, the effect of Pluronic F-68 on production of recombinant baculovirus and rotavirus protein VP7 was determined. Evidence of Pluronic F-68 direct interaction with Sf-9 insect cells also was obtained. Maximum recombinant VP7 concentration and yield increased 10x, whereas virus production decreased by 20x, in spinner flask cultures with 0.05% (w/v) Pluronic F-68 compared to controls lacking the additive. No differences were observed in media rheology, nor kinetics of growth and infection (as inferred from cell size) between both cultures. Hence, Pluronic F-68 influenced cell physiology independently of its shear protective effect. Cells subjected to a laminar shear rate of 3000 s(-1) for 15 min, without gas/liquid interfaces, were protected by Pluronic F-68 even after its removal from culture medium. Furthermore, the protective action was immediate in vortexed cells. The results shown here indicate that Pluronic F-68 physically interacts with cells in a direct, strong, and stable mode, not only protecting them from hydrodynamic damage, but also modifying their capacity for recombinant protein and virus production.

Yield, biological activity, and field performance of a wild-type Helicoverpa nucleopolyhedrovirus produced in H. zea cell cultures

This paper reports studies of the in vitro production of a virus from Helicoverpa armigera (HaSNPV) and its possible use as a specific Helicoverpa/Heliothis larvicide. Growth kinetics of Helicoverpa zea (H. zea) cells and virus occlusion body yields were compared in three SF900II-based media, namely, SF900II (serum-free), SF900II + 1% serum, or SF900II + 10% serum. Viable cell densities were usually higher in the media supplemented with serum than in the serum-free medium; however, in the serum-free medium, cell diameters were 1.7 times greater (i.e., individual cell volumes were five times larger). Both volumetric production of virus occlusion bodies and production per cell were higher in the serum-free medium than in the media supplemented with serum. However, the infectivity of the occlusion bodies from the serum-free medium was less than that with those from the medium supplemented with 10% serum, when compared in bioassays employing newly hatched larvae. The infectivity of the in vitro produced occlusion bodies was also less than that of in vivo produced occlusion bodies in a commercially available virus product, GemStar. High levels of infection of H. armigera larvae obtained in a preliminary field assessment on preflowering tomatoes using the in vitro produced occlusion bodies indicated the suitability of the in vitro process for biopesticide production.

Shear sensitivity of animal cells from a culture-medium perspective

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

Baculovirus-mediated expression of chicken growth hormone

A full-length chicken growth hormone (cGH) cDNA was placed downstream from the Autograph californica nuclear polyhedron virus, AcNPV, polyhedron gene promoter and expressed in Sf9 insect cells. Secreted recombinant cGH levels averaged 2-10 micrograms/ml from day 5-10 postinfection. The recombinant cGH analyzed by SDS-PAGE gels and Western blotting consisted of a doublet with M(r) of 26.5 and 23.5 kDa. Analysis by 2-D electrophoresis of partially-purified recombinant cGH and purified native cGH revealed similar immunoreactive charge isoforms and M(r) variants. The recombinant hormone was biologically active in a homologous radioreceptor assay. The results show that cGH expressed in insect cells is biologically and immunologically active, and that a variety of isoforms are secreted which exhibit size and charge properties similar to those of pituitary-derived cGH.

Mechanisms of animal cell damage associated with gas bubbles and cell protection by medium additives

Animal cell damage arising from gas sparging is considered to be a major barrier to large-scale production of recombinant biologicals in animal cell culture. Understanding sparging cell damage is therefore of significance to the application of animal cell culture. The paper reviews the hydrodynamics of bubble rupture, mechanisms of cell-bubble interaction, mathematical modelling and quantification of the sparging damage. Another interesting topic addressed in the paper is the protective effects of various medium additives against fluid mechanical cell damage, especially those surface-active polymers such as pluronic polyols, methylcellulose and polyethylene glycol. Experimental results obtained recently by the author and other researchers were examined to reveal the mechanisms of additive protection. The interactions of additives with air-liquid interfaces and the animal cells were analyzed with respect to their physical properties and chemical structure.

Optimization of the growth conditions of Sf21 insect cells for high-density perfusion culture in stirred-tank bioreactors

Spodoptera frugiperda insect cells (IPLB-Sf21-AE) (Sf21), infected with baculovirus expression vectors during their exponential growth phase, are commonly used to produce a variety of heterologous recombinant proteins. In the present study the culture conditions of these insect cells were studied to establish high-density suspension cultures with prolonged exponential growth phases. The Sf21 cells were grown in 125-ml spinner flasks using five different culture media supplemented with 5% fetal calf serum and four protein-free or low-protein culture media. The best results were achieved in EX-CELL 401 (protein-free media) and in IPL-41 modified with 2.5 g l-1 tryptose phosphate broth (serum-supplemented media), respectively. The latter was used for further batch and continuous cultivation of Sf21 cells in a perfused 1.4-l stirred-tank bioreactor with special attention to the oxygen requirement of these cells. Optimal growth was found at an oxygen concentration of 70% air saturation, resulting in a prolonged exponential growth phase that could be maintained for more than 16 days. A maximum cell density of 5.5 x 10(7) viable cells ml-1 was achieved.

Optimization of the physiochemical parameters for the culture of Bombyx mori insect cells used in recombinant protein production

Silkworm (Bombyx mori, Bm5) cell growth at different environmental conditions was systematically studied in controlled bioreactors. The physiochemical parameters, which affect the growth of Bm5 cells, were defined and optimized. The growth rate was maximal at a medium osmolality of about 370 mosm kg-1; and more than 90% of the maximum growth rate was achieved with osmolality between 350 and 385 mosm kg-1. The optimum pH for Bm5 cell growth was from 6.10 to 6.30. Moreover, the effect of dissolved oxygen concentrations (higher than 20% air saturation) on growth was minimal. However, the maximum cell density decreased markedly with dissolved oxygen tensions less than 20%. The shear protection additive Pluronic F-68 had a very important effect on the growth of Bm5 cells in bioreactors with an optimum concentration of 0.2% (w/v). Compared to the original culture condition, the optimized conditions resulted in a twofold increase in cell yield and a reduced population doubling time from 48 to 36 h. Furthermore, the yield of a recombinant protein (CAT) was increased substantially in the optimized cultures.