Growth kinetics of free and immobilized insect cell cultures

The MultiBac BEVS: Basics, applications, performance and recent developments

The baculovirus expression vector system (BEVS) delivers high yield heterologous protein expression and is widely used in academic and industrial R&D. The proteins produced enable many applications including structure/function analysis, drug screening and manufacture of protein therapeutics. Vital cellular functions are controlled by multi-protein complexes, MultiBac, a BEVS specifically designed for heterologous multigene delivery and expression, has unlocked many of these machines to atomic resolution studies. Baculovirus can accommodate very large foreign DNA cargo for faithful delivery into a target host cell, tissue or organism. Engineered MultiBac variants exploit this valuable feature for delivery of customized multifunctional DNA circuitry in mammalian cells and for production of virus-like particles for vaccines manufacture. Here, latest developments and applications of the MultiBac system are reviewed.

Infection, growth and maintenance of Wolbachia pipientis in clonal and non-clonal Aedes albopictus cell cultures

Insect cell lines provide useful in vitro models for studying biological systems, including interactions between mosquitoes and obligate intracellular endosymbionts such as Wolbachia pipientis. The Aedes albopictus Aa23 cell line was the first cell line developed to allow examination of Wolbachia infections. However, Wolbachia studies using Aa23 can be complicated by the presence of different cell types in the cell line and the substantial temporal variation in infection level. Two approaches were examined to ameliorate infection variability. In the first approach, multiple Aa23 passaging regimes were tested for an effect on infection variability. Fluorescence in situ hybridization (FISH) staining was used to characterize Wolbachia infection level over time. The results demonstrate an impact of passaging method on Wolbachia infection level, with some methods resulting in loss of infection. None of the passaging methods succeeded in effectively mitigating infection level variation. In a second approach, the clonal C7-10 A. albopictus cell line was infected with Wolbachia from Aa23 cells and Drosophila simulans (Riverside), resulting in cell lines designated C7-10B and C7-10R, respectively. Characterization via FISH staining showed greater stability and uniformity of Wolbachia infection in C7-10R relative to the infection in C7-10B. Characterization of the Aa23, C7-10B and C7-10R lines is discussed as a tool for the study of Wolbachia-host cell interactions.

Antihypertensive effect of insect cells: in vitro and in vivo evaluation

In this study, we investigated the in vitro ACE inhibitory and in vivo antihypertensive effect of insect cell extracts. The IC(50) of three insect cell lines from different type and insect species origin: S2 (embryo, Drosophila melanogaster), Sf21 (ovary, Spodoptera frugiperda) and Bm5 (ovary, Bombyx mori), were evaluated. Most interesting results were that the IC(50) values ranged between 0.4 and 0.9 mg/ml, and that an extra hydrolysis with gastrointestinal enzymes did not increase the ACE inhibitory activity conspicuously. Finally, a single oral administration with a gavage of 150 mg cell extract/kg BW to spontaneous hypertensive rats (SHR) significantly decreased (p<0.05) their systolic blood pressure (SBP) with 5-6% (9-12 mmHg) compared to the controls at 6 h post-administration. Here the undigested and digested insect S2 cell extracts were equal in activity to lower the SBP. To the best of our knowledge, this is the first report of in vivo antihypertensive activity of insect cell extracts and this without an extra digestion requirement.

Utilization of glucose and amino acids in insect cell cultures: Quantifying the metabolic flows within the primary pathways and medium development

The current understanding of insect cell metabolism is very limited. In order to gain some insight into the growth and metabolism of insect cells Spodoptera frugiperda (Sf9), a comprehensive characterization of culture conditions for cells grown in the IPL-41 medium was made by measuring the amino acid composition of the growth medium and the cell extract, the macromolecular composition of the cells (DNA, RNA, and protein), medium concentrations of various metabolites and sugars, and the evolved CO(2). Since in the IPL-41-based serum-free medium all of the amino acids except cysteine are in great excess of what is needed by the cells for energy and protein production, a medium formulation with an osmolarity similar to the IPL-41 but with a lower amino acid content than IPL-41 was also developed. The new medium also lacks maltose and sucrose (contains only glucose), supported cell growth to a high cell density of 8 x 10(6) cells/mL. The cellular and energetic yields indicated that a tight coupling between the biosynthetic and energetic reactions was attained for cells grown in the new medium. Moreover, it was found that the intermittent feeding of glucose may not be required as the cell yield and growth rate were comparable whether the same total amount of glucose was provided intermittently or was included initially in the medium. The eventual cessation of growth in the new medium is believed to be due to the amino acid limitation because concentrations of both glutamine and glutamate were very low at the end of the growth phase. Thus, further optimization, which may include higher initial glutamine in the medium or its intermittent feeding, could lead to a further increase in the cell density. Finally, a stoichiometrically based analysis of metabolic reactions confirmed the operation of the key pathways and was used to quantify the distribution of metabolites among primary metabolic reactions. The quantitative flow values were used to highlight some key aspects of insect cell metabolism.

Heterologous expression and purification of the infectious salmon anemia virus hemagglutinin esterase

This study presents the heterologous production and purification of a soluble and functional form of the hemagglutinin esterase (HE) of the infectious salmon anemia virus (ISAV) isolate 4 (Glesvaer/2/90). The HE possesses receptor binding and receptor destroying enzyme (RDE) activity and is probably involved in the infection process. The recombinant HE protein (recHE 4) was expressed in insect cells (Sf9) using the baculovirus expression vector system. Both the transmembrane region and the cytoplasmic tail were deleted, and a C-terminal His(6)-tag was attached to facilitate identification and purification of the recHE 4 protein. As determined by Western analysis the recHE 4 was secreted at 20 degrees C and not at 28 degrees C. By testing three HE constructs differing in their promoter and secretion signal sequences it was clear that the HE's own secretion signal sequence is more important than the promoter with respect to the amount of secreted recHE 4 obtained under the conditions used. A one-step purification by nickel-affinity chromatography resulted in a highly purified recHE 4, identified by matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis. Also, the recHE 4 is glycosylated and contains disulfide bridges within the molecule. Functional studies including the verification of the receptor destroying enzyme (RDE) activity as well as the binding to Atlantic salmon erythrocytes (hemagglutination) indicate that the recHE 4 has similar functions as its native counterpart. In conclusion, insect cells secrete a functional form of the ISAV 4 HE. This is suitable for further analyses on its function and immunogenicity.

Insect cells for human food

There is a need for novel protein sources. Insects are a possible interesting source of protein. They are nutritious in terms of protein (40-75 g/100g dry weight) and minerals. Insect protein is of high quality and has a high digestibility (77-98%) and concentration of essential amino acids (46-96% of the nutritional profile). Also insect cells may be a promising novel source of protein. Choice of cell line, growth conditions and use of the baculovirus expression system opens up possibilities to engineer the nutritional value of the biomass. The technological limits as well as consumer acceptance of insect cell based food remains to be investigated.

Stabilized baculovirus vector expressing a heterologous gene and GP64 from a single bicistronic transcript

The efficient scale-up of recombinant protein production in insect-cell bioreactors using baculovirus expression vectors is hampered by reductions in yield with increasing viral passage, the so-called passage effect. This phenomenon is characterized by the generation and subsequent accumulation of defective interfering baculoviruses (DIs), which interfere with the replication of genomically intact virus. A novel baculovirus expression vector is presented equipped with a bicistronic expression cassette that allows the simultaneous expression of the recombinant gene (GFP, first cistron) and an essential baculovirus gene (GP64, second cistron) from a single messenger RNA (mRNA). The translation of GP64 is mediated by an internal ribosome entry site (IRES) element from Rhopalosiphum padi virus (RhPV) while the native GP64 gene is deleted. In this way, a dominant selection pressure is placed on the entire bicistronic mRNA and hence on the maintenance of the foreign gene. The bicistronic expression vector was superior to the control baculovirus vector in that GFP expression remained at much higher levels upon continued virus passage. The versatility of this stabilized vector was demonstrated by its ability to propagate in a number of cell lines including Sf21, Sf9 and High Five cells. This novel baculovirus vector is especially valuable for large-scale recombinant protein production in insect-cell bioreactors where the number of viral passages is high.

Use and engineering aspects of immobilized cells in biotechnology

A short review of the research in the past two years (1990-1991) on immobilized whole cells, such as microbial, plant, and animal cells, is presented including a discussion from an engineering point of view. Recent works concerning the intraparticle mass transfer effect on immobilized microbial cells by the authors and their co-workers are also introduced. Finally, future prospects of the immobilized cell system will be discussed.

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.

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.

Large-scale insect cell culture: methods, applications and products

The primary development in large-scale insect cell culture over the past year has been the continuing accumulation of documented evidence (fundamental and applied) that conventional aerated stirred-tank and air-lift bioreactors may be employed for insect cell cultivation and recombinant protein production, provided that air sparging, agitation, and the addition to the medium of Pluronic F-68 and methyl cellulose polymers are carefully controlled.

Production scale insect cell culture

Insect cells in culture are currently commanding great interest as superior hosts for the efficient production of biologicals with applications in health care and in agriculture. Insect cell culture is ripe for scale-up technologies, in order to meet future projected production requirements of (a) insect viruses used as bioinsecticides and (b) recombinant proteins of therapeutic potential for humans and animals. The single most prominent system used in research-based and in commercial insect cell culture today involves lepidopteran cells transfected with baculovirus expression vectors for abundant formation of recombinant biologicals. However, dipteran insect cell lines also are beginning to emerge as useful tools in biotechnology. Current practices in bioprocess development using insect cell culture, advances in media formulation and in insect cell bioreactor design, and emerging trends are presented and critically evaluated.