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

A flow cytometric study of hydrodynamic damage to mammalian cells

Flow cytometry has been used to study the mechanisms of damage to mammalian cells by hydrodynamic forces. Cell damage resulted from the stresses created both by bubble entrainment and by bubble bursting caused by vortex formation in highly agitated culture. Damage to the antigen molecules on the cell surface together with increasing leakage and passive transport were observed. Pluronic F-68 reduced the efflux of fluorescein out of cells suggesting the possibility of plugging damaged parts of the membrane or change in membrane molecular organisation. Surface-associated immunoglobulin molecules were also protected by Pluronic F-68. The loss of plasma membrane integrity was followed by the destruction of cytoplasmic matrix. The nuclei were last to be destroyed. The fraction of metabolically active cells was drastically decreased by the intense hydrodynamic forces. However the mitochondrial transmembrane potential, which is linked to the ATP requirements of cells was well as the mean cell size of the live cell population, remained unaffected by the stressful hydrodynamic conditions. F-Actin content peaked during the early exponential phase and declined during the mid and late exponential and death phases of batch culture. The increase of actin during exponential phase was faster in stationary culture than in agitated culture. The increase was associated with the proliferative rate. Furthermore, the response to hydrodynamic forces was not related either to cell size difference or to the stage in the cell cycle.

Fed-batch culture of insect cells: a method to increase the yield of recombinant human nerve growth factor (rhNGF) in the baculovirus expression system

A fed-batch method was developed which increased the density of insect cells (Spodoptera frugiperda, Sf-9 cells) in suspension culture and the feeding of nutrients improved the yield of a recombinant protein produced by a baculovirus expression system. Analysis of spent medium samples indicated that depletions of glucose and glutamine correlated with the retardation of cell growth. Feeding of a mixture of nutrients consisting of glucose, glutamine, yeastolate and lipids solution restored the growth rate. In fed-batch culture, cell density was increased from 3 x 10(6) cells per ml to 1.2 x 10(7) cells per ml and the increased cell density enhanced the yield of the desired recombinant product, in this case, human nerve growth factor (rhNGF). The optimal conditions for the production of rhNGF were also defined by selecting the appropriate viral multiplicity of infection (MOI). At a cell density of 5 x 10(6) ml-1, a MOI of 0.05 (plaque forming units per cell) gave the highest yield of rhNGF in culture fluid 3 d post-infection. The yield of rhNGF was 20 mg l-1. The fed-batch method was scaled up to 12 l stirred bioreactor.

Intravenous Pluronic F-127 in early burn wound treatment in rats

A dramatic improvement in full skin thickness burn wounds in rats treated intravenously with the non-ionic surfactant Pluronic F-127 (F-127) has been demonstrated. In this study the F-127 was given 30 min postburn to simulate conditions encountered in a clinical setting. Anaesthetized male rats (300-320 g) received full skin thickness burns by immersion of the anterior chest wall (8 per cent body surface area in a 70 degrees C water-bath for 12 s). Burn wound area was measured immediately and after 48 h. Thirty minutes after the burn, half the animals received equal volumes (8 ml/kg body wt) of either saline or F-127 (12 mM/l concentration) via the tail vein. The animals autopsied at 48 h showed a significant (P < 0.05) reduction in the degree of wound contraction and the wound appeared grossly less damaged in the F-127-treated animals. Histologically, skin biopsies showed less of the microscopic damage usually associated with full skin thickness burns in the F-127-treated animals than in the saline controls. We also used thermography to measure skin temperature of the burn area at 90 min and 48 h postinjury demonstrating alterations in the F-127-treated animals (P < 0.05). In animals followed for 30 days postinjury, there was a significant (P < 0.01) improvement in the wound closure rates in the F-127-treated animals. These observations show a positive therapeutic effect of F-127 on the inflammatory process in the area of a burn that may improve wound healing.

A review of the effects of shear and interfacial phenomena on cell viability

The shear sensitivity of animal and plant cells is a problem often encountered in large-scale cell culture. Such sensitivity varies with different cell lines and the severity of cellular damage may depend on both the magnitude and the duration of the shear stress. In a bioreactor, the shear susceptibility of cells depends on their response to hydrodynamic forces arising from fluid motions of particular scale. Cell damage may be induced by forces in the bulk liquid phase, but fluid motions associated with the gas-liquid interface are especially energetic. The detrimental effects of hydrodynamic forces are abated by the addition of some polymers, such as Pluronic F-68, methylcellulose, or serum; the exact mechanisms of protection are the subject of current research.

Phosphorylatable and epitope-tagged human erythropoietins: utility and purification of native baculovirus-derived forms

The hematopoietic glycopeptide erythropoietin (EPO) is a prime regulator of red cell production in mammals, yet the precise nature of its interaction with specific cell surface receptors is poorly understood. Towards defining domains of EPO that are involved in receptor activation, we have developed (i) conditions for the expression of recombinant human EPO (rhEPO) at high levels in SF9 cells using modified 2- and 5-liter stirred reactors, (ii) a two-step procedure for the purification of this EPO without denaturation, and (iii) forms of EPO tagged with either a hemagglutinin influenza virus epitope or a consensus sequence for in vitro phosphorylation. Compared to EPO expressed in mammalian cells, rhEPO from SF9 cells in N-glycosylated with simple, neutral oligosaccharides of limited size, yet as purified presently using nondenaturing procedures, possesses exceptionally high in vitro activity (> or = 500,000 U/mg). Thus, this form of EPO should prove advantageous for direct physicochemical analyses. Regarding epitope-tagged and phosphorylatable EPOs, forms modified at the amino terminus (Ala1) fully retained receptor binding and in vitro biological activities. In contrast, forms modified at the carboxy terminus (Cys161) were inactive and did not compete for receptor binding, indicating that integrity of this domain is essential for receptor recognition. For active amino-terminal-modified forms, the specific binding of MAb 12CA5 to native HAI-EPO and the utility of 32P-labeled PHOS-EPO in receptor binding and internalization studies also were demonstrated. The development of these unique, highly active forms of human EPO should advance studies of essential interactions between this cytokine and its cell surface receptor.

Effect of Pluronic F-68 on the mechanical properties of mammalian cells

The mechanical properties of TB/C3 hybridoma cells taken from a continuous culture were measured by micromanipulation. The culture conditions were constant except for the presence or absence of Pluronic F-68 in the medium. It was found that the mean bursting membrane tension and the mean elastic area compressibility modulus of the cells were significantly greater (60% and 120%, respectively) in a medium with 0.05% (w/v) Pluronic F-68 compared to that without Pluronic. Pluronic F-68 therefore affected the strength of the membranes when the cells were exposed to it for a long period of time, i.e., in culture. The short-term effect of Pluronic F-68 on cell strength was also tested by its addition at various levels up to 0.2% (w/v) immediately before the mechanical property measurements. The resulting cell strength depended on the Pluronic concentration, but a significant short-term effect could only be detected above a threshold of 0.1% (w/v). Previous reports on the effect of Pluronic F-68 on animal cell culture are evaluated in the light of these observations.

Effects of oxygen on recombinant protein production by suspension cultures of Spodoptera frugiperda (Sf-9) insect cells

Spodoptera frugiperda (Sf-9) insect cells have been grown in serum-free medium in 250-ml spinner flasks. The maximum cell density obtained in these cultures was dependent on the aeration rate of the culture. Similar yields of uninfected cells were obtained when cultures were stirred in spinner flasks at 80 rev min-1 and in a 4-1 stirred-tank bioreactor and the dissolved oxygen in the bioreactor was controlled at 20% of air saturation. Cells were infected with a recombinant baculovirus at different multiplicities of infection: the timing and maximum level of expression of the recombinant protein were dependent on the multiplicity of infection, the cell density at infection, and on the aeration rate of the culture. Oxygen-limited growth resulted in undetectable levels of recombinant protein (< 6 ng recombinant protein 10(-7) cells). Compared with the maximum yields observed in spinner flask cultures, higher levels of recombinant protein were produced when cells were grown and infected in the bioreactor. The level of dissolved oxygen in the bioreactor was controlled at 50% of air saturation.

Characterization of rabies virus glycoprotein expressed by recombinant baculovirus

A cDNA of the glycoprotein (G protein) gene of rabies virus Nishigahara strain was cloned and inserted into a baculovirus genome under the control of the polyhedrin promoter. Infection of Spodoptera frugiperda cells with this recombinant virus produced a large quantity of new protein instead of the parental polyhedrin protein. By immunofluorescent and immunoblotting analyses, the recombinant protein was antigenically similar to the authentic G protein. Its molecular mass estimated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, however, was slightly smaller than that of the authentic one, and this observation was suggested to be due to the difference in glycosylation level between the two G proteins. The recombinant G protein expressed on the cell surface of the insect cells showed a fusion activity at low pH. The fusion activity was inhibited by antiserum against either whole virions or G protein of rabies virus.

Large-scale insect cell culture

Significant advances have been made over the past year in our understanding of the protective mechanisms, both fluid-mechanical and biological, of media additives on suspended animal cells. The degree of protection offered by different additives, such as pluronic polyol, appears to be cell-type dependent, varying quite dramatically not only between insect and mammalian cells, but also between different insect cell lines themselves.

Continuous β-galactosidase production with a recombinant baculovirus insect-cell system in bioreactors

Insect cells were exploited to produce bacterial beta-galactosidase by infecting them with a recombinant nuclear polyhedrosis virus (baculovirus) of Autographa californica. The insect cells were cultured in a continuous stirred tank reactor (CSTR) and led to a second CSTR where they were infected with a recombinant virus in which the lacZ gene from Escherichia coli was inserted. In the effluent of the production reactor, maximum activities of 15 units beta-galactosidase per 10(6) cells were measured. For about 25 d beta-galactosidase production remained constant, but then rapidly declined. This drop was due to a decrease in production of active beta-galactosidase rather than to inactivation of this enzyme. It was concluded that the reduced production was due to reduced polyhedrin promoter-driven synthesis.

Requirements for nuclear localization and supramolecular assembly of a baculovirus polyhedrin protein

This study defines the requirements for the nuclear localization, stable nuclear association, and supramolecular assembly of a baculovirus polyhedrin protein in lepidopteran insect cells. Fragments of the polyhedrin protein were genetically fused to two different nonnuclear reporter proteins and the intracellular distribution of the fusion proteins was analyzed in infected insect cells. Analysis by indirect immunofluorescence showed that the domain between amino acids 30 and 57 could mediate nuclear localization of polyhedrin. However, biochemical fractionation experiments showed that this domain was not sufficient for a detergent-stable association of polyhedrin with the nucleus. This required a slightly larger domain, between amino acids 30 and 110. Differential interference-contrast microscopy showed that the supramolecular assembly of polyhedrin into nuclear occlusion-like particles required the domain between amino acids 19 and 110. The most likely candidate for a minimal nuclear localization signal was the sequence KRKK, located between amino acids 32 and 35. Therefore, oligonucleotide-directed mutagenesis was used to change this sequence to NGNN and the intracellular distribution of the mutant protein was analyzed. The results showed that the mutant protein was predominantly localized in the cytoplasm of infected cells, where it assembled into large, cubic, occlusion-like particles. Thus, the KRKK sequence is necessary for the nuclear localization of polyhedrin, but nuclear localization is not required for its supramolecular assembly into occlusion-like particles.

Use of a stationary bed reactor and serum-free medium for the production of recombinant proteins in insect cells

Insect cells (Spodoptera frugiperda) have been cultured in a stationary bed reactor, packed with a fibrous polyester carrier. When the bioreactor was perfused with serum-supplemented medium, a cell density of 6 x 10(6) cells ml-1 packed carrier was reached. Scanning electron microscopy investigations have shown that the insect cells grew along the three-dimensionally oriented fibers of the Fibra-cel carrier. After infection of the logarithmically growing cells with a recombinant baculovirus (Autographa californica) containing the gene coding for beta-galactosidase, the medium in the bioreactor was changed to serum-free medium. At day 13 postinfection (p.i.), a beta-galactosidase level of 320 microgram ml-1 and, at day 17 p.i., a virus titer of 2.1 x 10(8) TCID50 units ml-1 (day 17 p.i.) were reached. In another bioreactor, operated in a similar way but with serum-containing medium, a beta-galactosidase concentration of 360 microgram ml-1 and a virus titer of 2.3 x 10(8) TCID50 units ml-1 were obtained. These results indicate the potential use of this production system for the production of recombinant protein and baculovirus in insect cells.

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.

Examination of serum and bovine serum albumin as shear protective agents in agitated cultures of hybridoma cells

A murine hybridoma cell line (167.4G5.3) was cultured in batch mode using IMDM containing different serum concentrations and bovine serum albumin (BSA). Cell growth and death, metabolism and antibody production were studied in these cultures. The cells were more susceptible to shear in the stationary and in the decline phase of growth as evidenced by higher death rates. Cell growth was best at high serum concentrations with high specific growth and low specific death rates. When BSA was used instead of serum in IMDM, no protective effect was observed. Cell metabolism and monoclonal antibody production rates were not influenced by the level of serum or by BSA. The use of serum in commercial serum-free media (OPTI-MEM) also resulted in no change in both growth and death rates.

Enhanced secretion from insect cells of a foreign protein fused to the honeybee melittin signal peptide

The baculovirus/insect cell system has been remarkably successful in yielding high levels of synthesis of many proteins which have been difficult to synthesize in other host/vector systems. The system is also capable of secreting heterologous proteins, but with generally low efficiency. We have increased the efficiency of secretion of the system by using signal peptides of insect origin to direct the secretion of a foreign protein. The precursor of the plant cysteine protease papain (propapain) has been used as a report enzyme to compare secretion efficiency. Insect cells infected with a baculovirus recombined with the gene encoding propapain fused to the sequence encoding the honeybee melittin signal peptide secreted over five times more papain precursor than the wild-type prepropapain which used the plant signal peptide. Based on these results, we have assembled pVT-Bac, an Autographa californica nuclear polyhedrosis virus transfer vector that may enhance secretion of other foreign proteins from insect cells. The vector incorporates a number of features: phage f1 ori to facilitate site-directed mutagenesis, the strong polyhedrin promoter upstream from the melittin signal peptide-encoding sequence, and eight unique restriction sites to facilitate fusion of heterologous genes.

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.

The protective effect of serum against hydrodynamic damage of hybridoma cells in agitated and surface-aerated bioreactors

The effect of serum on the growth rate and metabolism of CRL-8018 hybridoma cells in an agitated, surface-aerated bioreactor was examined. In the employed well-controlled bioreactors at high agitation rates, hybridoma cells in medium containing 1% fetal bovine serum rapidly die in the presence of a vortex with accompanying gas-bubble entrainment, whereas non-agitated control cultures grow normally in medium containing 1% serum. Serum levels greater than 5% counteract the detrimental hydrodynamic effects due to agitation and bubble entrainment. The protective effect is present after short-term (less than 1 h) exposure to 10% serum concentration, suggesting a protection mechanism which is, at least in part, of a physical nature. The apparent cell yields on glucose, lactate, and glutamine decreased with decreasing growth rate due to low serum concentrations. The results are incorporated into a simple model in which the apparent growth rate is the sum of an invariable growth rate and a changing death rate.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.