Effect of temperature and oxygen on cell growth and recombinant protein production in insect cell cultures

Adaptive Laboratory Evolution to Improve Recombinant Protein Production Using Insect Cells

Adaptive laboratory evolution (ALE) is a powerful tool for enhancing the fitness of cell lines in specific applications, including recombinant protein production. Through adaptation to nonstandard culture conditions, cells can develop specific traits that make them high producers. Despite being widely used for microorganisms and, to lesser extent, for mammalian cells, ALE has been poorly leveraged for insect cells. Here, we describe a method for adapting insect High Five and Sf9 cells to nonstandard culture conditions via an ALE approach. Aiming to demonstrate the potential of ALE to improve productivity of insect cells, two case studies are demonstrated. In the first, we adapted insect High Five cells from their standard pH (6.2) to neutral pH (7.0); this adaptation allowed to improve production of influenza virus-like particles (VLPs) by threefold, using the transient baculovirus expression vector system. In the second, we adapted insect Sf9 cells from their standard culture temperature (27 °C) to hypothermic growth (22 °C); this adaptation allowed to improve production of influenza VLPs by sixfold, using stable cell lines. These examples demonstrate the potential of ALE for enhancing productivity within distinct insect cell hosts and expression systems by manipulating different culture conditions.

Identification of optimal flow rate for culture media, cell density, and oxygen toward maximization of virus production in a fed-batch baculovirus-insect cell system

In recent times, it has been realized that novel vaccines are required to combat emerging disease outbreaks, and faster optimization is required to respond to global vaccine demands. Although, fed-batch operations offer better productivity, experiment-based optimization of a new fed-batch process remains expensive and time-consuming. In this context, we propose a novel computational framework that can be used for process optimization and control of a fed-batch baculovirus-insect cell system. Since the baculovirus expression vector system (BEVS) is known to be widely used platforms for recombinant protein/vaccine production, we chose this system to demonstrate the identification of optimal profile. Toward this, first, we constructed a mathematical model that captures the time course of cell and virus growth in a baculovirus-insect cell system. Second, the proposed model was used for numerical analysis to determine the optimal operating profiles of control variables such as culture media, cell density, and oxygen based on a multiobjective optimal control formulation. Third, a detailed comparison between batch and fed-batch culture was perfromed along with a comparison between various alternatives of fed-batch operation. Finally, we demonstrate that a model-based quantification of controlled feed addition in fed-batch culture is capable of providing better productivity as compared to a batch culture. The proposed framework can be utilized for the estimation of optimal operating regions of different control variables to achieve maximum infected cell density and virus yield while minimizing the substrate/media, uninfected cell, and oxygen consumption.

Asexual Blood-Stage Malaria Vaccine Candidate PfRipr5: Enhanced Production in Insect Cells

The malaria asexual blood-stage antigen PfRipr and its most immunogenic fragment PfRipr5 have recently risen as promising vaccine candidates against this infectious disease. Continued development of high-yielding, scalable production platforms is essential to advance the malaria vaccine research. Insect cells have supplied the production of numerous vaccine antigens in a fast and cost-effective manner; improving this platform further could prove key to its wider use. In this study, insect (Sf9 and High Five) and human (HEK293) cell hosts as well as process-optimizing strategies (new baculovirus construct designs and a culture temperature shift to hypothermic conditions) were employed to improve the production of the malaria asexual blood-stage vaccine candidate PfRipr5. Protein expression was maximized using High Five cells at CCI of 2 × 10⁶ cell/mL and MOI of 0.1 pfu/cell (production yield = 0.49 mg/ml), with high-purity PfRipr5 binding to a conformational anti-PfRipr monoclonal antibody known to hold GIA activity and parasite PfRipr staining capacity. Further improvements in the PfRipr5 expression were achieved by designing novel expression vector sequences and performing a culture temperature shift to hypothermic culture conditions. Addition of one alanine (A) amino acid residue adjacent to the signal peptide cleavage site and a glycine-serine linker (GGSGG) between the PfRipr5 sequence and the purification tag (His₆) induced a 2.2-fold increase in the expression of secreted PfRipr5 over using the expression vector with none of these additions. Performing a culture temperature shift from the standard 27–22°C at the time of infection improved the PfRipr5 expression by up to 1.7 fold. Notably, a synergistic effect was attained when combining both strategies, enabling to increase production yield post-purification by 5.2 fold, with similar protein quality (i.e., purity and binding to anti-PfRipr monoclonal antibody). This work highlights the potential of insect cells to produce the PfRipr5 malaria vaccine candidate and the importance of optimizing the expression vector and culture conditions to boost the expression of secreted proteins.

Advancements in molecular design and bioprocessing of recombinant adeno-associated virus gene delivery vectors using the insect-cell baculovirus expression platform

Despite rapid progress in the field, scalable high-yield production of adeno-associated virus (AAV) is still one of the critical bottlenecks the manufacturing sector is facing. The insect cell-baculovirus expression vector system (IC-BEVS) has emerged as a mainstream platform for the scalable production of recombinant proteins with clinically approved products for human use. In this review, we provide a detailed overview of the advancements in IC-BEVS for rAAV production. Since the first report of baculovirus-induced production of rAAV vector in insect cells in 2002, this platform has undergone significant improvements, including enhanced stability of Bac-vector expression and a reduced number of baculovirus-coinfections. The latter streamlining strategy led to the eventual development of the Two-Bac, One-Bac, and Mono-Bac systems. The one baculovirus system consisting of an inducible packaging insect cell line was further improved to enhance the AAV vector quality and potency. In parallel, the implementation of advanced manufacturing approaches and control of critical processing parameters have demonstrated promising results with process validation in large-scale bioreactor runs. Moreover, optimization of the molecular design of vectors to enable higher cell-specific yields of functional AAV particles combined with bioprocess intensification strategies may also contribute to addressing current and future manufacturing challenges.

Improving Influenza HA-Vlps Production in Insect High Five Cells via Adaptive Laboratory Evolution

The use of non-standard culture conditions has proven efficient to increase cell performance and recombinant protein production in different cell hosts. However, the establishment of high-producing cell populations through adaptive laboratory evolution (ALE) has been poorly explored, in particular for insect cells. In this study, insect High Five cells were successfully adapted to grow at a neutral culture pH (7.0) through ALE for an improved production of influenza hemagglutinin (HA)-displaying virus-like particles (VLPs). A stepwise approach was used for the adaptation process, in which the culture pH gradually increased from standard 6.2 to 7.0 (ΔPh = 0.2–0.3), and cells were maintained at each pH value for 2–3 weeks until a constant growth rate and a cell viability over 95% were observed. These adapted cells enabled an increase in cell-specific HA productivity up to three-fold and volumetric HA titer of up to four-fold as compared to non-adapted cells. Of note, the adaptation process is the element driving increased specific HA productivity as a pH shift alone was inefficient at improving productivities. The production of HA-VLPs in adapted cells was successfully demonstrated at the bioreactor scale. The produced HA-VLPs show the typical size and morphology of influenza VLPs, thus confirming the null impact of the adaptation process and neutral culture pH on the quality of HA-VLPs produced. This work strengthens the potential of ALE as a bioprocess engineering strategy to improve the production of influenza HA-VLPs in insect High Five cells.

Efficient production of sTNFRII-gAD fusion protein in large quantity by use of the modified CHO-S cell expression system

TNFα is one of the initial and important mediators to activate downstream signaling pathways by binding to trimerized TNFα receptors (TNFR), and thus is an ideal drug target for cancer therapy. Taking advantage of intrinsic homotimerization of the globular domain of adiponectin (gAD), we have developed a novel TNFα antagonist, the trimerized fusion protein named sTNFRII-gAD. However, our previously-used CHO expression system yielded less than 10 mg/L of sTNFRII-gAD. To produce large quantities of sTNFRII-gAD efficiently, we used a modified CHO-S cell expression system, which is based on a pMH3 vector with non-coding GC-rich DNA fragments for high-level gene expression. We obtained stable clones that produced 75 mg/L of sTNFRII-gAD in the 96-well plate, adapted the clones to 40 ml suspension serum-free batch culture, then optimized the culturing conditions to scale up the fed-batch culture in a 3 L shake-flask and finally in a 5 L AP30 bioreactor. We achieved a final yield of 52 mg/L of sTNFRII-gAD. The trimerized sTNFRII-gAD exhibited the higher affinity to TNFα with a dissociation constant (Kd) of 5.63 nM than the dimerized sTNFRII-Fc with a Kd of 13.4 nM, and further displayed the higher TNFα-neutralizing activity than sTNFRII-Fc (p<0.05) in a L929 cytotoxicity assay. Therefore, the strategy employed in this study may provide an efficient avenue for large-scale production of other recombinant proteins by use of the modified CHO-S cell expression system.

Protein production using the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is widely used in producing recombinant proteins. This review is focused on the use of this expression system in developing bioprocesses for producing proteins of interest. The issues addressed include: the baculovirus biology and genetic manipulation to improve protein expression and quality; the suppression of proteolysis associated with the viral enzymes; the engineering of the insect cell lines for improved capability in glycosylation and folding of the expressed proteins; the impact of baculovirus on the host cell and its implications for protein production; the effects of the growth medium on metabolism of the host cell; the bioreactors and the associated operational aspects; and downstream processing of the product. All these factors strongly affect the production of recombinant proteins. The current state of knowledge is reviewed.

The effect of oscillating dissolved oxygen concentrations on the metabolism of a Spodoptera frugiperda IPLB-Sf21-AE clonal isolate

The effect of oscillating dissolved oxygen (DO) concentration on the metabolism of a clonal isolate of the Spodoptera frugiperda IPLB-Sf21-AE insect cell line was investigated. Specifically, the effect on cell growth, re- combinant protein synthesis, glucose and glutamine consumption, and lactate accumulation was determined. Prior to conducting the oscillating DO experiments, it was found that the DO concentration could be reduced to 15% air saturation without adversely affecting the growth rate. Under these conditions, glucose and glutamine became depleted as the maximum cell density was reached. The introduction of DO oscillations, that is, cycles consisting of 30 min at 15% DO followed by 30 min of anoxia, significantly altered cell metabolism, including inhibition of cell growth and recombinant protein synthesis. The effect of DO oscillations on glucose consumption was dependent on the experimental conditions. Glucose exhaustion occurred when the DO oscillations contained either an "apparent" anoxia period (nitrogen sparging discontinued upon reaching 0% DO) without pH control or a "true" anoxia period (nitrogen sparging continued throughout anoxia period) with pH control. Glucose consumption was significantly decreased, however, when the cells were exposed to a "true" anoxia period without pH control, that is, low pH inhibited glucose utilization. Glutamine uptake was not significantly affected by DO oscillations. Lactate only accumulated in the oscillating DO runs, a finding consistent with previous results demonstrating that significant lactate accumulation only occurs under DO-limited conditions. (c) 1995 John Wiley & Sons, Inc.

Rational scale-up of a baculovirus-insect cell batch process based on medium nutritional depth

We have developed a serum-free cell culture process utilizing a recombinant baculovirus (AcNPV) expression vector to infect Trichoplusia ni insect cells for the production of the human lysosomal enzyme, glucocerebrosidase. The enzyme, which is harvested as a secreted protein in this process, can serve as a replacement therapy for the genetic deficiency Gaucher disease. In the course of pilot scale-up of a batch glucocerebrosidase process from 25-mL working volume shaker flask units to 25-L working volume stirred bioreactor units, a semi-empirical model was developed for the rational determination of scaleable process parameters, including host cell density at infection, multiplicity of infection (MOI), and harvest time. A key assumption of the model is that maximum protein production is limited by the serum-free medium's nutritional capacity, which can, in turn, be determined from the growth of uninfected cells. For the host cell/medium combination used in this study, the nutritional limit was determined to be 1.3 x 10(7) to 1.7 x 10(7) viable-cell-days/mL. Based on this, the model predicts that optimal protein expression is consistent with a 4-day batch process where the host cell density at the time of infection is 1.5 x 10(6) to 2.0 x 10(6) cells/mL and the MOI is 0.09-0.3. These parameters were empirically confirmed to give the highest achievable batch product yield, first in shaker flasks and then at larger scales. The low MOI allows at least one population doubling to take place post viral addition, so that the effective infected cell density producing product generally exceeds 4 x 10(6) cells/mL. It was also interesting to note that this process consistently achieved the same level of maximum protein production at the 25-L bioreactor scale in 4 days compared to 5 days at the shaker flask scale. This may be attributable to better control of the culture environment in the bioreactor. Unlike some other lepidopteran insect cells, such as Sf-9, T. ni cells were found to produce significant levels of the inhibitory metabolites ammonia and lactate. Our results suggest that reduction and/or removal of inhibitory metabolites might be beneficial for infection of high-density cultures of these cells and might also facilitate application of more sophisticated culture strategies, including fed-batch. (c) 1996 John Wiley & Sons, Inc.

Effects of temperature shift on cell cycle, apoptosis and nucleotide pools in CHO cell batch cultues

Temperature reduction in CHO cell batch culture may be beneficial in the production of recombinant protein and in maintenance of viability. The effects on cell cycle, apoptosis and nucleotide pools were studied in cultures initiated at 37°C and temperature shifted to 30 °C after 48 hours. In control cultures maintained at 37 °C, viable cells continued to proliferate until the termination of the culture, however, temperature reduction caused a rapid decrease in the percent of cells in S phase and accumulation of cells in G-1. This was accompanied by a concurrent reduction in U ratio (UTO/UDP-GNAc), previously shown to be a sensitive indicator of growth rate. Culture viability was extended following temperature shift, as a result of delayed onset of apoptosis, however, once initiated, the rate and manner of cell death was similar to that observed at 37 °C. All nucleotide pools were similarly degraded at the time of apoptotic cell death. Temperature reduction to 30 °C did not decrease the energy charge of the cells, however, the overall rate of metabolism was reduced. The latter may be sufficient to extend culture viability via a reduction in toxic metabolites and/or limitation of nutrient deprivation. However, the possibility remains that the benefits of temperature reduction in terms of both viability and productivity are more directly associated with cultures spending extended time in G-1.

Chip-based size-selective sorting of biological cells using high frequency acoustic excitation

This work presents the size-selective sorting of single biological cells using the assembly process known as templated assembly by selective removal (TASR). We have demonstrated experimentally, for the first time, the selective placement and sorting of single SF9 cells (clonal isolate derived from Spodoptera frugiperda (Fall Armyworm) IPLB-Sf21-AE cells) into patterned hemispherical sites on rigid assembly templates using TASR. Nearly 100% of the assembly sites on the template were filled with matching cells (with assembly density as high as 900 sites per mm(2)) within short time spans of 3 minutes. 3-D reconstruction of cell profiles and volume analysis of cells trapped inside assembly sites demonstrates that only those cells that match the assembly site precisely (within 0.5 μm) in size are assembled on the template. The assembly conditions are also compatible with the extension of TASR to mammalian cells. TASR-based size-selective structuring and sorting of biological systems represents a valuable tool with potential for implementation in biological applications such as cell sorting for medical research or diagnostics, templating for artificial tissue replication, or isolation of single cells for the study of biological or mechanical behavior.

On-line monitoring of respiration in recombinant-baculovirus infected and uninfected insect cell bioreactor cultures

Respiration rates in Spodoptera frugiperda (Sf-9) cell bioreactor cultures were successfully measured on-line using two methods: The O(2) uptake rate (OUR) was determined using gas phase pO(2) values imposed by a dissolved oxygen controller and the CO(2) evolution rate (CER) was measured using an infrared detector. The measurement methods were accurate, reliable, and relatively inexpensive. The CER was routinely determined in bioreactor cultures used for the production of several recombinant proteins. Simple linear relationships between viable cell densities and both OUR and CER in exponentially growing cultures were used to predict viable cell density. Respiration measurements were also used to follow the progress of baculoviral infections in Sf-9 cultures. Infection led to increases in volumetric and per-cell respiration rates. The relationships between respiration and several other culture parameters, including viable cell density, cell protein, cell volume, glucose consumption, lactate production, viral titer, and recombinant beta-galactosidase accumulation, were examined. The extent of the increase in CER following infection and the time postinfection at which maximum CER was attained were negatively correlated with the multiplicity of infection (MOI) at multiplicities below the level required to infect all the cells in a culture. Delays in the respiration peak related to the MOI employed were correlated with delays in the peak in recombinant protein accumulation. DO levels in the range 5-100% did not exert any major effects on viable cell densities, CER, or product titer in cultures infected with a baculovirus expressing recombinant beta-galactosidase.

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.

Influence of culture medium supplementation of tobacco NT1 cell suspension cultures on the N-glycosylation of human secreted alkaline phosphatase

We report for the first time that culture conditions, specifically culture medium supplementation with nucleotide-sugar precursors, can alter significantly the N-linked glycosylation of a recombinant protein in plant cell culture. Human secreted alkaline phosphatase produced in tobacco NT1 cell suspension cultures was used as a model system. Plant cell cultures were supplemented with ammonia (30 mM), galactose (1 mM) and glucosamine (10 mM) to improve the extent of N-linked glycosylation. The highest levels of cell density and active extracellular SEAP in supplemented cultures were on average 260 g/L and 0.21 U/mL, respectively, compared to 340 g/L and 0.4 U/mL in unsupplemented cultures. The glycosylation profile of SEAP produced in supplemented cultures was determined via electrospray ionization mass spectrometry with precursor ion scanning and compared to that of SEAP produced in unsupplemented cultures. In supplemented and unsupplemented cultures, two biantennary complex-type structures terminated with one or two N-acetylglucosamines and one paucimannosidic glycan structure comprised about 85% of the SEAP glycan pool. These three structures contained plant-specific xylose and fucose residues and their relative abundances were affected by each supplement. High mannose structures (6-9 mannose residues) accounted for the remaining 15% glycans in all cases. The highest proportion (approximately 66%) of a single complex-type biantennary glycan structure terminated in both antennae by N- acetylglucosamine was obtained with glucosamine supplementation versus only 6% in unsupplemented medium. This structure is amenable for in vitro modification to yield a more human-like glycan and could serve as a route to plant cell culture produced therapeutic glycoproteins.

pH Condition in temperature shift cultivation enhances cell longevity and specific hMab productivity in CHO culture

Controlling cell proliferation during cell culturing is an effective way to improve the production yield in mammalian cell culture. We examined the effect of temperature shifts (TS) under pH control conditions in Chinese hamster ovary cells. When we shifted the culture temperature from 37 degrees C to 31 degrees C before a stationary phase at pH 6.8 (TS/pH 6.8), cell viability remained high, and the final human monoclonal antibody (hMab) concentration increased to 2.3 times that in the culture remaining at 37 degrees C. However, there were no significant effects on the cell viability or production yield with the same TS at pH 7.0 (TS/pH 7.0). The average specific hMab productivity and mRNA level of TS/pH 7.0 were the same as that of TS/pH 6.8. The control of cell growth by the TS or the addition of rapamycin was effective in the maintenance of cell viability, but there was no significant increase of the average specific hMab productivity in the culture where cell proliferation was controlled with rapamycin. The hMab mRNA concentration decreased to 55%-65% at a 37 degrees C culture with the addition of actinomycin D. In contrast, actinomycin D did not affect the mRNA level in the TS culture. This result suggested that the increase in the mRNA level in the TS condition was caused by an increase in mRNA stability. In this study, we show that TS can produce two unrelated effects: a prolongation of cell longevity and an improvement in mRNA stability.

Improving AAV vector yield in insect cells by modulating the temperature after infection

Vectors based on adeno-associated viruses (AAV) are sought for therapeutic gene delivery because of their ability to transduce a variety of tissues with no significant immunological response. Production using the baculovirus expression vector (BEV)/insect cell system has the potential to meet the needs for pre-clinical and clinical trials. In this co-infection system, three baculoviruses are used to produce the AAV vector. A strategy aimed at increasing encapsidation/maturation of the viral vector involved varying the temperature over the course of the process. Cultures were subjected to temperature changes at various times pre- and post-infection (up to 24 h post-infection). It was found that raising the culture temperature to 30 degrees C at the time of infection nearly tripled the infectious titer. In fact, increasing the temperature to 30 degrees C at any time in the process investigated resulted in an increase in titer. Also, raising the culture to 33 degrees C or lowering the temperature to 24 degrees or 21 degrees C resulted in lower titers. The rise in infectious titer was also confirmed by an increase in DNase resistant particles (DRPs). Varying the temperature, however, did not affect the total amount of capsids significantly. Therefore increasing the culture temperature resulted in better encapsidation as determined by the ratio of capsids to DRPs to infectious particles. It is believed that an increase in early proteins and possibly a quicker cascade of baculovirus infection events resulted in this increased packaging efficiency.

Higher expression of fab antibody fragments in a CHO cell line at reduced temperature

A chimeric Fab was expressed in Chinese hamster ovary cells under the control of the CMV promoter in a two-stage production process. Cells were first grown to 90% confluence at 37 degrees C in a proliferation phase, followed by a production phase at either 37 degrees C or 28 degrees C. Medium supplemented with serum and medium free from serum was tested in the production phase at both temperatures. Comparison of Fab expression revealed that reducing the temperature to 28 degrees C resulted in a 14-fold increase in product yield when cells were cultivated in serum-containing medium, and in a 38-fold increase in product yield when serum-free medium was applied.

The response of virally infected insect cells to dissolved oxygen concentration: recombinant protein production and oxidative damage

The effects of dissolved oxygen (DO) concentration on virally infected insect cells were investigated in 3-L bioreactor culture. Specifically, cultures of Spodoptera frugiperda Sf-9 (Sf-9) and Trichoplusia ni BTI-Tn-5B1-4 (Tn-5B1-4) were infected with Autographa californica multiple nucleopolyhedrovirus expressing secreted alkaline phosphatase (SEAP). Following infection at a DO concentration of 50% air saturation, the DO concentration was adjusted to a final value of either 190%, 50%, or 10% air saturation. Recombinant SEAP production, cell viability, protein carbonyl content, and thiobarbituric acid reactive substances (TBARS) content were monitored. The increases in protein carbonyl and TBARS contents are taken to be indicators of protein oxidation and lipid oxidation, respectively. DO concentration was found to have no noticeable effect on SEAP production or cell viability decline in the Sf-9 cell line. In the Tn-5B1-4 cell line, cells displayed an increased peak SEAP production rate for 190% air saturation and displayed an increased rate of viability decline at increased DO concentration. Protein carbonyl content showed no significant increase in the Sf-9 cell line by 72 h postinfection (pi) at any DO concentration but showed a twofold increase at 10% and 50% DO concentration and a threefold increase at 190% DO concentration by 72 h pi in Tn-5B1-4 cells. TBARS content was found to increase by approximately 50% in Sf-9 cells and by approximately twofold in Tn-5B1-4 cells by 72 h pi with no clear relationship to DO concentration. It is hypothesized that oxygen uptake changes due to the viral infection process may bear a relation to the observed increases in protein and lipid oxidation and that lipid oxidation may play an important role in the death of virally infected insect cells.

Integrated process optimization: lessons from retrovirus and virus-like particle production

The optimization of production and purification processes is usually approached by engineers from a strictly biotechnological point of view. The present paper envisages the definition and application of an optimization model that takes into account the impact of both biological and technological issues upon the optimization protocols and strategies. For this purpose, the optimization of three analogous but different systems comprising animal cell growth and bioparticle production is presented. These systems were: human immunodeficiency 1 (HIV-1) and porcine parvovirus (PPV) virus-like particles (VLPs) produced in insect cells and retrovirus produced in mammalian cells. For the systematization of the optimization process four levels of optimization were defined-product, technology, design and integration. In this paper, the limits of each of the optimization levels defined are discussed by applying the concept to the systems described. This analysis leads to decisions regarding the production of VLPs and retrovirus as well as on the points relevant for further process development. Finally, the definition of the objective function or performance index, the possible strategies and tools for bioprocess optimization are described. Although developed from the three described processes, this approach can, based on the recent literature evidence reviewed here, be applied more universally for the process development of complex biopharmaceuticals.

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.

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.

Performance of small-scale CHO perfusion cultures using an acoustic cell filtration device for cell retention: characterization of separation efficiency and impact of perfusion on product quality

Several small-scale Chinese hamster ovary (CHO) suspension cultures were grown in perfusion mode using a new acoustic filtration system. The separation performance was evaluated at different cell concentrations and perfusion rates for two different CHO cell lines. It was found that the separation performance depends inversely on the cell concentration and perfusion rate. High media flow rates as well as high cell concentrations resulted in a significant drop in the separation performance, which limited the maximal cell concentration achievable. However, packed cell volumes of 10% to 16% (corresponding to 3 to 6. 10(7) cells/mL) could be reached and were maintained without additional bleeding after shifting the temperature to 33 degrees C. Perfusion, up to 50 days, did not harm the cells and did not result in a loss of performance of the acoustic filter as often seen with other perfusion systems. Volumetric productivities in perfusion mode were 2- to 12-fold higher for two cell lines producing two different glycoproteins when compared to fed-batch or batch processes using the same cell lines. Product concentrations were in the range of 20% to 80% of batch or fed-batch culture, respectively. In addition, using the protease-sensitive product rhesus thrombopoietin, we could show that cultivation in perfusion mode drastically reduced proteolysis when compared to a batch culture without addition of protease inhibitors such as leupeptin.

Proteolytic activity in infected and noninfected insect cells: degradation of HIV-1 Pr55gag particles

In this work the proteolytic activity in the supernatant and inside insect cells in culture was evaluated for different multiplicities of infection (MOI) and times of infection (TOI). Several methods to detect proteolytic activity in insect cells were tested and that using fluorescein thiocyanite-casein as a substrate was chosen. It was observed that infection caused not only a reduction in the concentration of proteases by decreasing their synthesis but also an inhibition of the intracellular proteolytic activity by increasing the intracellular ATP level (measured by in vivo nuclear magnetic resonance, NMR). The maximum proteolytic activity in the supernatant was observed at 72 hpi except when the cells were infected in the late exponential growth phase or with very low MOI, yielding a nonsynchronous infection. The proteolytic degradation of Pr55gag particles was studied during culture and after harvest. In this particular case it was concluded that the supernatant should be stored at low temperature or quickly purified, since the degradation after 24 h is only 3% at 4 degrees C while at 27 degrees C this value rises to 23%. There is a complex relationship between MOI, TOI, proteolytic activity, and product titer and quality. Thus, the optimal conditions for each case will be a compromise between the final product titer, the desired product quality, and operational issues like process time and capacity, requiring proper integration between bioreaction and downstream processing.

Influence of low temperature on productivity, proteome and protein phosphorylation of CHO cells

Proliferation of mammalian cells can be controlled by low cultivation temperature. However, depending on cell type and expression system, varying effects of a temperature shift on heterologous protein production have been reported. Here, we characterize growth behavior and productivity of the Chinese hamster ovary (CHO) cell line XM111-10 engineered to synthesize the model-product-secreted alkaline phosphatase (SEAP). Shift of cultivation temperature from 37 degrees C to 30 degrees C caused a growth arrest mainly in the G1 phase of the cell cycle concomitant with an up to 1.7-fold increase of specific productivity. A low temperature cultivation provided 3.4 times higher overall product yield compared to a standard cultivation at 37 degrees C. The cellular and molecular mechanisms underlying the effects of low temperature on growth and productivity of mammalian cells are poorly understood. Separation of total protein extracts by two-dimensional gel electrophoresis showed altered expression levels of CHO-K1 proteins after decrease in cultivation temperature to 30 degrees C. These changes in the proteome suggest that mammalian cells respond actively to low temperature by synthesizing specific cold-inducible proteins. In addition, we provide the first evidence that the cold response of mammalian cells includes changes in postranslational protein modifications. Two CHO proteins were found to be phosphorylated at tyrosine residues following downshift of cultivation temperature to 30 degrees C. Elucidating cellular events during cold exposure is necessary for further optimization of host-cell lines and expression systems and can provide new strategies for metabolic engineering.

Effect of temperature oscillation on insect cell growth and baculovirus replication

Temperature oscillation can enhance cell viability of sf9 insect cells and baculovirus production of occlusion bodies (OB) and extracellular virus (ECV) compared with constant temperature in stationary culture and suspension culture. The optimal oscillation range was 24 to 28 degreesC. At this temperature oscillation, the viability of uninfected and infected sf9 cells can be maintained much longer than at 28 degreesC. Although the rate of virus infection was a little low at 24 to 28 degreesC, the final cell infectivity was similar to that at a constant temperature of 28 degreesC. The production of OB was increased from 13.4 to 17.4/cell in stationary culture and from 13.9/cell to 18.1/cell in suspension culture. The titer of ECV was increased from 87 to 114 PFU/cell in stationary culture and from 79 to 114 PFU/cell in suspension culture.

Multiplication of a granulosis virus isolated from the potato tuber moth in a new established cell line of Phthorimaea operculella

A newly established cell line was obtained from the culture of embryonic cells of the potato tuber moth Phthorimaea operculella in low temperature conditions (19 degrees C) using modified Grace's medium supplemented with 10% fetal bovine serum. The population doubling time was about 80 h when cells were cultivated at 19 degrees C and 38 h at 27 degrees C. The cell line had a relatively homogeneous population consisting of various sized spherical cells. The cells were cultivated for more than 25 passages. Their polypeptidic profile was different from profiles of other P. operculella cell lines we previously described and from other lepidopteran cells. The new cell line was designated ORS-Pop-95. The complete replication of the potato tuber moth granulosis virus (PTM GV) was obtained in vitro by both viral infection and DNA transfection. PTM GV multiplied at a significant level during several passages of the cell line that was maintained at 19 degrees C. As long as the cells were maintained at 19 degrees C, virus multiplication could also be obtained at the same rate at 27 degrees C. To compare PTM GV multiplied both in vivo and in vitro, we used morphological identification, serological, DNA probe diagnosis and endonuclease digest profile analysis and confirmed the identity of the virus.

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.

Large-scale insect and plant cell culture

Currently, insect and plant cell cultures are not widely used to make products of commercial interest, largely because the development of large-scale cultivation methods is still in its infancy. With the advances made over the past year, some of the limitations associated with scale-up of these two types of expression system have been addressed. Increasing the oxygen supply and the concentration of various nutrients supplied to insect cells after infection has enabled high specific protein production to be maintained to higher cell densities than ever before, improving overall volumetric yields. Detailed work has focused on the capacity of insect cells to carry out complex post-translational modifications; however, as yet, evidence is conflicting as to the extent of protein processing and complex glycosylation possible in infected cells. In plant cell culture, the accepted axioms concerning large-scale culture have been re-examined. Recent studies have assessed culture at high cell densities and the constraints in reactor design resulting from the 'shear sensitivity' of plant cells. Results show that, as cell densities increase, alterations occur in the pathways of secondary metabolism, leading to decreases in specific productivity. The use of nutrient supplements and a medium cycling strategy shows promise for increasing and sustaining product formation. Furthermore, the importance of dissolved gas composition has been clearly demonstrated by use of a gas recirculation reactor. Reports of taxol and vindoline production in vitro demonstrate the potential and the necessity for further research in scale-up of plant cell culture.