Factors influencing recombinant protein yields in an insect cell-bacuiovirus expression system: multiplicity of infection and intracellular protein degradation

Critical parameters on Zika virus-like particles' generation

The Zika virus became a global threat in 2015 due to its association with microcephaly. Preventing its spread depends on developing vaccines, with virus-like particles (VLP) being a promising approach, especially because of their safety profile and high immunogenicity. This study focused on the production of Zika VLP using Sf9 cells and the baculovirus expression system, evaluating cell growth kinetics, nutrient consumption, and metabolite production in Sf-900™ III medium. As a methodology, this study includes bioreactor experiments, cell density and viability quantification, nutrient and metabolite analysis, Dot Blot, Western Blot, and transmission electron microscopy. Among the critical conditions tested are culture medium supplementation with 0.028 mM cholesterol/ 6 nM bovine serum albumin, multiplicity of infection (MOI= 0.2 or 2), and dissolved oxygen tension (DOT= 5 or 30 % air saturation). As a result, in the growth phase, Sf9 cells achieved rapid exponential growth, with doubling times ranging from 22.8 to 35.4 hours and standard nutrient consumption and metabolite generation profiles for this cell line. The infection phase recorded cell death rates between 8200 and 12600 cells mL⁻¹ h⁻¹ , with higher VLP production under low MOI (0.2) and low DOT (5 %). These conditions also reduced protein degradation and nutrient consumption. The produced VLP ranged from 32 to 73 nm in size, with smaller sizes observed under low MOI conditions. Finally, controlling the DOT at 5 % air saturation without cholesterol/albumin supplementation increased VLP production without the need to raise the viral load, highlighting the importance of choosing the appropriate combination of critical parameters (MOI, DOT, and medium supplementation) as key factors in optimizing the upstream process. This finding impacts substantially upstream stage efficiency and economy, which could be useful for future scaling up to the commercial manufacturing scale.

Expression of bluetongue virus full-length VP7 protein in insect cells and its diagnostic utility for detection of antibodies to the virus infection

Bluetongue (BT) is a vector-borne viral disease of multiple domestic and wild ruminants across the globe. The VP7 protein of bluetongue virus (BTV) is the major immune-dominant structural protein that is conserved across the BTV serotypes and therefore, targeted for the development of immuno-diagnostics for BT. In this study, full-length recombinant VP7 protein (rVP7) of BTV-1 was expressed in Trochoplusia ni derived insect cells (Tn5) using codon-optimized synthetic gene construct through baculovirus expression system. The seed stock of recombinant baculovirus was amplified to a high titre (>1 × 108 pfu/ml) at P2 and P3 in sf9 cells. The rVP7 was successfully produced and purified from infected Tn5 culture lysate for evaluation of the immuno-reactivity and its diagnostic potential. The purified protein showed strong reactivity in western blot analysis with the polyclonal immune serum produced against BTV core antigen in guinea pigs. An indirect ELISA (iELISA) was optimized by using the purified rVP7 for the detection of the group-specific antibodies to BTV in sheep and goats. The iELISA was found to be highly sensitive (98.9 %), specific (98.1 %), and reproducible (CV < 10 %) for detection of the antibodies to BTV in sheep and goat serum. The iELISA could detect the specific antibodies in naturally infected goat serum containing type-specific neutralizing antibodies to different BTV serotypes indicating the potential of the rVP7 for the development of the group-specific sero-diagnostics for BT.

Robust and resource-efficient production process suitable for large-scale production of baculovirus through high cell density seed train and optimized infection strategy

The aim of this study was the development of a scalable production process for high titer (108 pfu/mL and above) recombinant baculovirus stocks with low cell line-derived impurities for the production of virus-like particles (VLP). To achieve this, we developed a high cell density (HCD) culture for low footprint cell proliferation, compared different infection strategies at multiplicity of infection (MOI) 0.05 and 0.005, different infection strategies and validated generally applicable harvest criteria of cell viability ≤ 80%. We also investigated online measurable parameters to observe the baculovirus production. The infection strategy employing a very low virus inoculum of MOI 0.005 and a 1:2 dilution with fresh medium one day after infection proved to be the most resource efficient. There, we achieved higher cell-specific titers and lower host cell protein concentrations at harvest than other tested infection strategies with the same MOI, while saving half of the virus stock for infecting the culture compared to other tested infection strategies. HCD culture by daily medium exchange was confirmed as suitable for seed train propagation, infection, and baculovirus production, equally efficient as the conventionally propagated seed train. Online measurable parameters for cell concentration and average cell diameter were found to be effective in monitoring the production process. The study concluded that a more efficient VLP production process in large scale can be achieved using this virus stock production strategy, which could also be extended to produce other proteins or extracellular vesicles with the baculovirus expression system.

Production of Rabies VLPs in Insect Cells by Two Monocistronic Baculoviruses Approach

Rabies is an ancient zoonotic disease that still causes the death of over 59,000 people worldwide each year. The rabies lyssavirus encodes five proteins, including the envelope glycoprotein and the matrix protein. RVGP is the only protein exposed on the surface of viral particle, and it can induce immune response with neutralizing antibody formation. RVM has the ability to assist with production process of virus-like particles. VLPs were produced in recombinant baculovirus system. In this work, two recombinant baculoviruses carrying the RVGP and RVM genes were constructed. From the infection and coinfection assays, we standardized the best multiplicity of infection and the best harvest time. Cell supernatants were collected, concentrated, and purified by sucrose gradient. Each step was used for protein detection through immunoassays. Sucrose gradient analysis enabled to verify the separation of VLPs from rBV. Through the negative contrast technique, we visualized structures resembling rabies VLPs produced in insect cells and rBV in the different fractions of the sucrose gradient. Using ELISA to measure total RVGP, the recovery efficiency of VLPs at each stage of the purification process was verified. Thus, these results encourage further studies to confirm whether rabies VLPs are a promising candidate for a veterinary rabies vaccine.

Production and Easy One-Step Purification of Bluetongue Recombinant VP7 from Infected Sf9 Supernatant for an Immunoenzymatic Assay (ELISA)

Bluetongue (BT) is non-contagious, vector-borne viral disease of domestic and wild ruminants, transmitted by midges (Culicoides spp.) and is caused by Bluetongue virus (BTV). BTV is the type species of the Orbivirus genus within the Reoviridae family and possesses a genome consisting of 10 double-stranded RNA segments encoding 7 structural and 4 nonstructural proteins. Viral Protein 7 (VP7) is the major sera group-specific protein and is a good antigen candidate for immunoenzymatic assays for the BT diagnosis. In our work, BTV-2 recombinant VP7 (BTV-2 recVP7), expressed in Spodoptera frugiperda (Sf9) cells using a baculovirus system, was produced and purified by affinity chromatography from the supernatant of infected cell culture. The use of the supernatant allowed us to obtain a high quantity of recombinant protein with high purity level by an easy one-step procedure, rather than the multistep purification from the pellet. RecVP7-BTV2 was detected using a MAb anti-BTV in Western blot and it was used to develop an immunoenzymatic assay.

Bioreactor-Based Antigen Production Process Using the Baculovirus Expression Vector System

Several vaccines are already produced using the baculovirus expression vector system (BEVS). This chapter describes methods for generating recombinant baculoviral DNA (also called bacmid) for cultivating Spodoptera frugiperda Sf-9 cells and producing a baculovirus stock from the recombinant bacmid and for producing a protein-based vaccine with the BEVS in a stirred tank reactor.

Construction of polycistronic baculovirus surface display vectors to express the PCV2 Cap(d41) protein and analysis of its immunogenicity in mice and swine

To increase expression levels of the PCV2 Cap(d41) protein, novel baculovirus surface display vectors with multiple expression cassettes were constructed to create recombinant baculoviruses BacSC-Cap(d41), BacDD-2Cap(d41), BacDD-3Cap(d41), and BacDD-4Cap(d41). Our results reveal that the recombinant baculovirus BacDD-4Cap(d41) was able to express the highest levels of Cap(d41) protein. Optimum conditions for expressing the PCV2 Cap(d41) protein were determined, and our results show that 107 of Sf-9 infected with the recombinant baculovirus BacDD-4Cap(d41) at an MOI of 5 for 3 days showed the highest level of protein expression. Mice immunized with the 4Cap(d41) vaccine which was prepared from the recombinant baculovirus-infected cells (107) elicited higher ELISA titers compared to the Cap (d41) vaccine. The 4Cap(d41) vaccine could elicit anti-PCV2 neutralizing antibodies and IFN-γ in mice, as confirmed by virus neutralization test and IFN-γ ELISA. Moreover, the swine lymphocyte proliferative responses indicated that the 4Cap(d41) vaccine was able to induce a clear cellular immune response. Flow cytometry analysis showed that the percentage of CD4+ T cells and CD4+/CD8+ ratio was increased significantly in SPF pigs immunized with the 4Cap(d41) vaccine. Importantly, the 4Cap(d41) vaccine induced an IFN-γ response, further confirming that its effect is through cellular immunity in SPF pigs. An in vivo challenge study revealed that the 4Cap(d41) and the commercial vaccine groups significantly reduce the viral load of vaccinated pigs as compared with the CE negative control group. Taken together, we have successfully developed a 4Cap(d41) vaccine that may be a potential subunit vaccine for preventing the disease associated with PCV2 infections.

Expression of Cloned Genes Using the Baculovirus Expression System

The general strategy of the baculovirus expression system is to infect insect cells with a virus that expresses a foreign protein at a very late stage of infection. Almost all baculovirus expression systems use the procedures for insect cell transfection, baculovirus production, and protein expression given in the main portion of this protocol. This protocol also includes a method that uses molecular biology techniques to produce recombinant baculovirus DNA in E. coli before transfection of insect cells. It is important to quantify the viral titer to achieve optimal and reproducible expression of target proteins. Accordingly, the viral plaque assay is also described here.

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.

Infection of neuroblastoma cells by rabies virus is modulated by the virus titer

Rabies is a lethal viral infection that can affect almost all mammals, including humans. To better understand the replication of Rabies lyssavirus, we investigated if the viral load in brains naturally infected with rabies influences viral internalization and viral growth kinetics in neuroblastoma cells, and if the viral load affects mortality in mice after intradermal infection. We noted that high initial viral loads in brains (group II) were unfavourable for increasing viral titers during serial passages in neuroblastoma cells when compared to low initial viral loads in brains (group I). In addition, group I strains showed higher viral growth and enhanced internalization efficiency in neuroblastoma cells than group II strains. However, we observed that the dominant virus subpopulation in group II promoted efficient viral infection in the central nervous system in the new host, providing a selective advantage to the virus. Our data indicate that rabies infection in animal models depends on not only the virus strain but also the amount of virus. This study may serve as a basis for understanding the biologic proprieties of Rabies lyssavirus strains with respect to the effects on viral replication and the impact on pathogenesis, improving virus yields for use in vaccine development.

Functional analysis of a survivin-like gene in Bombyx mori

The survivin (svv) gene is a newly discovered member of the inhibitors of apoptosis gene family. In recent years, svv has been confirmed to have an anti-apoptosis function and to play a critical role in cell division. We identified a survivin-like gene in the silkworm, Bombyx mori (Bm-svv). In this study, to gain insight into its function, a baculovirus expression system was used to express the Bm-svv gene in insect cell lines. The recombinant viruses were then used as a vector to transform insect cells, and cell activity was determined using the Cell Counting Kit-8 (CCK-8), which is usually employed for detecting mammalian cell number. The results indicated that the Bm-svv gene plays a role in the cell growth arrest or apoptosis induced by viruses. Furthermore, the CCK-8 kit is effective in determining the activity of insect cells.

Low multiplicity infection of insect cells with a recombinant baculovirus: The cell yield concept

In vitro infection of insect cells with baculoviruses is increasingly considered a viable means for the production of biopesticides, recombinant veterinary vaccines, and other recombinant products. Batch fermentation processes traditionally employ intermediate to high multiplicities of infection necessitating two parallel scale-up processes-one for cells and one for virus. In this study, we consider the use of multiplicities of infection as low as 0.0001 plaque-forming units per cell, a virus level low enough to enable infection of even large reactors (e.g., 10 m(3)) directly from a frozen stock. Using low multiplicities in the Sf9/beta-gal-AcNPV system, recombinant protein titers comparable with the maximum titer observed in high multiplicity infections were achieved. Cultures yielding the maximum titer were characterized by reaching a maximum cell density between 3 and 4 x 10(9) cell L(-1). This optimal cell yield did not depend on the multiplicity of infection, supporting the existing view that batch cultures are limited by availability of substrate. Up to a certain cell density, product titer will increase almost linearly with availability of biocatalyst, that is, cells. Beyond this point any further cell formation comes at the expense of final product titer. Low multiplicity infections were found not to cause any significant dispersion of the protein production process. Hence, product stability is not a major issue of concern using low multiplicities of infection. The sensitivity to initial conditions and disturbances, however, remains an issue of concern for the commercial use of low multiplicity infections. (c) 1996 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.

Production of parvovirus B19 vaccine in insect cells co-infected with double baculoviruses

Recombinant human parvovirus B19 virus-like particles (VLPs), a candidate vaccine, were produced using the insect cell (Sf-9)-baculovirus (AcNPV) expression system. The synthesis and assembly of the particles in Sf-9 cells are directed by double infections with one recombinant virus (bacVP1) expressing the parvovirus minor viral protein VP1 and a second virus (bacVP2) expressing the major viral protein VP2. Previous animal studies demonstrated that the polypeptide composition of the VLPs strongly affects the elicitation of virus neutralizing antibodies. The key factor controlling the production of an immunologically potent product in bioreactors was identified to be the multiplicity of infection (MOI) of bacVP1 and bacVP2 used for infection. A probabilistic model, which correlates well with the experimental results, was employed to facilitate the selection of MOIs and to provide a better understanding of the baculovirus co-infection process. A novel production process based on secondary infections was developed to ensure product consistency and to simplify large-scale logistics. The effects of other critical process parameters, such as temperature, dissolved oxygen concentration, lactate concentration, cell concentration at infection, and harvest time, were also investigated. (c) 1996 John Wiley & Sons, Inc.

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.

Optimisation of protein expression and establishment of the Wave Bioreactor for Baculovirus/insect cell culture

As the interest of research is beginning to shift from genomicsto proteomics the number of proteins to be expressed is rapidlyincreasing. To do so, well-established, high-level expressionsystems and rapid, cost-effective production means are needed. For addressing the latter, a novel cultivation system for recombinant cells, the Wave Bioreactortrade mark has recently becomeavailable. We describe the set-up and the optimisation of parameters essential for successful operation and growth of insect cells to high cell densities in the Wave Bioreactor. According to our experience, the Cellbagtrade mark system comparesvery favorably to conventional cultivation vessels such as bioreactors and roller cultures with respect to simplicity ofoperation and cost. Additionally, we developed a rapid and simple protocol for assessing expression and production conditions for the Baculovirus/insect cell system applicable to many different genes/proteins. Important parameters like MOI,TOI, peak cell density (PCD) and expression levels are determinedin pre-experiments on small scale to achieve optimal expressionof a given protein. These conditions are subsequently transformedand applied to large scale cultures grown in nutrient-supplemented medium in the Wave Bioreactor.

Production of recombinant proteins in high-density insect cell cultures

The effect of the growth phase of Spodoptera frugiperda (Sf9) cells on the production of recombinant proteins (beta-galactosidase and glucocerebrosidase) was investigated. Cells infected with the recombinant Autographa californica nuclear polyhedrosis virus at the late exponential and stationary phases yielded low quantities of expressed protein. Highest enzyme yields were obtained using Sf9 cells from the early exponential phase (0.9 mg beta-galactosidase/10(6) cells and 1.7 microg glucocerebrosidase/10(6) cells). Infection of resuspension of cells collected from various phases of growth in fresh medium resulted in 75% restoration of maximal expression levels. This finding suggested either nutrient limitation or waste product accumulation as the cause of the decrease in productivity at the latter phases of growth. Further experiments revealed that the highest productivity levels could be obtained with cultures of Sf9 cells grown in a fermentor to a cell concentration of 4 x 10(6) mL(-1). The medium needed to be replaced prior to infection with the recombinant virus and supplemented with a mixture of glucose, L-glutamine, and yeastolate ultrafiltrate.

Modeling the population dynamics of baculovirus-infected insect cells: Optimizing infection strategies for enhanced recombinant protein yields

The insect cell-baculovirus model presented here is capable of simulating cell population dynamics, extracellular virion densities, and heterologous product titers in reasonable agreement with experimental data for a wide rang of multiplicities of infection (MOI) and times of infection. The model accounts for the infection of a single cell by multiple virions and the consequences on the time course of infection. The probability of infection by more than one virion was approximated using the Poisson distribution, which proved to be a refinement over second-order kinetics. The model tracks initiation and duration of important events in the progression of infected cell development (virus replication, recombinant protein synthesis, and cell lysis) for subpopulations delineated by the time and extent of their initial infection. The model suggests infection strategies, weighing the importance of MOI and infection time. Maximum product titers result from infection in the early exponential growth phase with low MOI.

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.

Designing experiments for high-throughput protein expression

The advent of high-throughput protein production and the vast amount of data it is capable of generating has created both new opportunities and problems. Automation and miniaturization allow experimentation to be performed more efficiently, justifying the cost involved in establishing a high-throughput platform. These changes have also magnified the need for effective statistical methods to identify trends and relationships in the data. The application of quantitative management tools to this process provides the means of ensuring maximum efficiency and productivity.

Recombinant protein production by the baculovirus-insect cell system in Basal media without serum supplementation

The production of beta-galactosidase by Sf9 cells infected with recombinant Autographa californica nucleopolyhedrovirus (AcNPV) was investigated in shake-flask culture using two serum-free basal media: Grace's medium and TNM-FH (Grace's medium supplemented with lactalbumin hydrolysate and yeast extract). At the time of infection, cells grown in serum-supplemented TNM-FH were transferred into fresh basal media without adaptation. The absence of serum depressed the beta-galactosidase yield considerably in Grace's medium, but to a much lesser extent in TNM-FH, where it reached around 2/3 of the level obtained in TNM-FH supplemented with 10% fetal bovine serum (FBS). While both lactalbumin hydrolysate and yeast extract promoted beta-galactosidase production, their removal by medium replacement on post-infection day 1 gave a beta-galactosidase yield nearly equal to that obtained in their continuous presence. Supplementation of basal media with phosphatidic acid (PA) from egg yolk lecithin, which has been shown to enhance cell growth and recombinant protein production in serum-free culture of Chinese hamster ovary (CHO) cells, was also effective in increasing beta-galactosidase yield. Elevating the multiplicity of infection (MOI) from 2 to 10 plaque-forming units per cell (pfu/cell) also resulted in an increase in product yield. These results provide information important to the development of cost-effective serum-free culture technology for use in large-scale production of recombinant proteins by the baculovirus-insect cell system.

Modeling rotavirus-like particles production in a baculovirus expression vector system: Infection kinetics, baculovirus DNA replication, mRNA synthesis and protein production

Rotavirus is the most common cause of severe diarrhoea in children worldwide, responsible for more than half a million deaths in children per year. Rotavirus-like particles (Rota VLPs) are excellent vaccine candidates against rotavirus infection, since they are non-infectious, highly immunogenic, amenable to large-scale production and safer to produce than those based on attenuated viruses. This work focuses on the analysis and modeling of the major events taking place inside Spodoptera frugiperda (Sf-9) cells infected by recombinant baculovirus that may be critical for the expression of rotavirus viral proteins (VPs). For model validation, experiments were performed adopting either a co-infection strategy, using three monocistronic recombinant baculovirus each one coding for viral proteins VP(2), VP(6) and VP(7), or single-infection strategies using a multigene baculovirus coding for the three proteins of interest. A characteristic viral DNA (vDNA) replication rate of 0.19+/-0.01 h(-1) was obtained irrespective of the monocistronic or multigene vector employed, and synthesis of progeny virus was found to be negligible in comparison to intracellular vDNA concentrations. The timeframe for vDNA, mRNA and VP synthesis tends to decrease with increasing multiplicity of infection (MOI) due to the metabolic burden effect. The protein synthesis rates could be ranked according to the gene size in the multigene experiments but not in the co-infection experiments. The model exhibits acceptable prediction power of the dynamics of intracellular vDNA replication, mRNA synthesis and VP production for the three proteins involved. This model is intended to be the basis for future Rota VLPs process optimisation and also a means to evaluating different baculovirus constructs for Rota VLPs production.

A homologue of cathepsin L identified in conditioned medium from Sf9 insect cells

Gelatin zymography revealed the presence of proteolytic activity in conditioned medium (CM) from a serum-free, non-infected Spodoptera frugiperda, Sf9 insect cell culture. Two peptidase bands at about 49 and 39 kDa were detected and found to be proform and active form of the same enzyme. The 49-kDa form was visible on zymogram gels in samples of CM taken on days 4 and 5 of an Sf9 culture, while the 39-kDa form was seen on days 6 and 7. On basis of the inhibitor profile and substrate range, the enzyme was identified as an Sf9 homologue of cathepsin L, a papain-like cysteine peptidase. After lowering the pH of Sf9 CM to 3.5, an additional peptidase band at 22 kDa appeared. This peptidase showed the same inhibitor profile, substrate range and optimum pH (5.0) as the 39-kDa form, indicating that Sf9 cathepsin L has two active forms, at 39 and 22 kDa. Addition of the cysteine peptidase inhibitor E-64c to an Sf9 culture inhibited all proteolytic activities of Sf9 cathepsin L but did not influence the proliferation of Sf9 cells.

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.

Evaluation of production parameters with the vaccinia virus expression system using microcarrier attached HeLa cells

Parameters that affect production of the recombinant reporter protein, EGFP, in the T7 promoter based VOTE vaccinia virus-HeLa cell expression system were examined. Length of infection phase, inducer concentration, and timing of its addition relative to infection were evaluated in 6-well plate monolayer cultures. One hour infection with 1.0 mM IPTG added at the time of infection provided a robust process. For larger scale experiments, anchorage-dependent HeLa cells were grown on 5 g/L Cytodex 3 microcarriers. The change to this dynamic culture environment, with cell-covered microcarriers suspended in culture medium in spinner flasks, suggested a re-examination of the multiplicity of infection (MOI) for this culture type that indicated a need for an increase in the number of virus particles per cell to 5.0, higher than that needed for complete infection in monolayer tissue flask culture. Additionally, dissolved oxygen level and temperature during the protein production phase were evaluated for their effect on EGFP expression in microcarrier spinner flask culture. Both increased dissolved oxygen, based on surface area to volume (SA/V) adjustments, and decreased temperature from 37 to 31 degrees C showed increases in EGFP production over the course of the production phase. The level of production achieved with this system reached approximately 17 microg EGFP/10(6) infected cells.

A model of the dynamics of insect cell infection at low multiplicity of infection

In the present paper, we offer a preliminary mathematical model that describes the dynamic process of cell infection with baculovirus at low multiplicity of infection (MOI). The model accounts for the chain of events that follow the infection of insect cells, namely the eclipse period, the budding of viral particles from those cells, their attachment to non-infected cells and the initiation of a new infection cycle. These cycles appear as fluctuations in the viral concentration of actual cell culture media. The potential of the present approach in simulating the in vitro production of biological insecticides is demonstrated. The influence of the shape of the virus-budding function is shown, and parameter sensitivity analysis is carried out. The model provides a quantitative tool for the analysis of this complex dynamic system.

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.

Production of recombinant proteins by vaccinia virus in a microcarrier based mammalian cell perfusion bioreactor

The HeLa cell-vaccinia virus expression system was evaluated for the production of recombinant proteins (enhanced green fluorescent protein (EGFP) and HIV envelope coat protein, gp120) using microcarriers in 1.5 L perfused bioreactor cultures. Perfusion was achieved by use of an alternating tangential flow device (ATF), increasing the length of the exponential phase by 50 h compared to batch culture and increasing the maximum cell density from 1.5x10(6) to 4.4x10(6) cell/mL. A seed train expansion method using cells harvested from microcarrier culture and reseeding onto fresh carriers was developed. EGFP was first used as a model protein to study process parameters affecting protein yield, specifically dissolved oxygen (DO) and temperature during the production phase. The highest level of EGFP, 12+/-1.5 microg/10(6) infected cells, was obtained at 50% DO and 31 degrees C. These setpoints were then used to produce glycoprotein, gp120, which was purified and deglycosylated, revealing a significant amount of N-linked glycosylation. Also, biological activity was assayed, resulting in an ID50 of 3.1 microg/mL, which is comparable to previous reports.

Strategies for manipulating the relative concentration of recombinant rotavirus structural proteins during simultaneous production by insect cells

Adequate production strategies of virus-like particles are among the challenges that must be addressed before such complex multimeric structures find practical applications as vaccines. Attainment of the correct stoichiometric relation between proteins that constitute virus-like particles should result in an increased productivity by maximizing the concentration of assembled proteins and preventing the accumulation of waste monomers. In this work, strategies for manipulating the relative concentration between two of the structural proteins that constitute rotavirus-like particles (VP2 and VP6) were explored using the insect cell baculovirus expression vector system. It was shown that multiplicity of infection is a useful tool for manipulating protein production rates and maximum concentrations in cultures expressing one or two recombinant proteins. Thus, multiplicity of infection can be employed for improving production of rotavirus-like particles. VP2 and VP6 production rates obtained during individual infections remained unchanged when both were simultaneously produced, indicating that such rates can be utilized for estimating protein concentrations during coexpression. Manipulation of the time of infection between the two recombinant baculoviruses, proposed here for the first time, also proved to be effective for controlling the relative protein concentrations. The use of such sequential infections constituted an effective production alternative that does not require high amounts of virus stocks and is easy to implement. In addition to VP2 and VP6, kinetic parameters for the individual production of the other two proteins (VP4 and VP7) that constitute rotavirus-like particles were also obtained.

Increased production of active human beta(2)-adrenergic/G(alphas) fusion receptor in Sf-9 cells using nutrient limiting conditions

Using the baculovirus/insect-cell expression vector system, we succeeded in obtaining a high yield of active human beta(2)-adrenergic receptor/G(alphas) fusion protein. This was achieved following high cell density production under nutrient-limiting conditions using a very low multiplicity of infection (MOI). This approach was found to significantly reduce inactive protein accumulation that occurred when production was done using conventional high MOI procedures. The maximum specific and volumetric yields of active receptor using this strategy increased by factors of two- and sixfold, respectively. Our results suggest that the increase in the ratio of active/total protein produced results from production under nutrient limitation. Since low multiplicity of infection offers many advantages for large-scale applications, we suggest that this simple production method should be considered when optimizing expression of G-protein-coupled receptors and other complex proteins.

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.

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.

An immunological assay for determination of baculovirus titers in 48 hours

Th baculovirus expression system is a system of choice for expressing eukaryotic proteins. Large amounts of biologically active material can be generated using this system by infecting insect cells with a baculovirus expressing the target protein. At several stages during the production of a baculovirus stock, it is necessary to titer the virus. Current methods have long time lines and are either technically difficult or are limited to viruses expressing a reporter gene. The new assay described here yields titers in 48 h, is easy to perform using 96-well plates, and is applicable to any Autographa californica nucleopolyhedrovirus-based recombinant baculovirus. This assay uses an antibody to a viral envelope glycoprotein to detect infected cells via immunostaining. The titer is determined by counting foci of infection under a light microscope. The required incubation period is shortened considerably because infected cells express viral antigens long before the macroscopic signs of infection scored in other assays become apparent. Titers determined using this immunological assay are comparable, both in value and variability, to those obtained using a traditional method, provided that the stocks have titers above 10(4) pfu/ml.

Synthesis of rotavirus-like particles in insect cells: comparative and quantitative analysis

When the three major structural proteins, VP2, VP6, and VP7, of rotavirus are co-expressed in insect cells infected with recombinant baculoviruses, they self-assemble into triple-layered virus-like particles (VLPs) that are similar in morphology to native rotavirus. In order to establish the most favorable conditions for the synthesis of rotavirus VLPs, we have compared the kinetics of 2/6/7-VLP synthesis in two different insect cell lines: High Five cells propagated in Excell 405 medium and Spodoptera frugiperda 9 cells in Excell 400 medium. The majority of VLPs produced in both cell lines were released into the culture medium, and these released VLPs were predominantly triple-layered and were found to be stable for the period of six or seven days examined. The optimal synthesis of VLPs depended upon the cell line and the culture medium used as well as the time of harvesting infected cell cultures. The highest yield of VLPs was obtained from High Five cultures in the late phase of infection when the yield was at least 5-fold higher than that from S. frugiperda 9 cultures on a per cell basis. Our results demonstrate the usefulness of High Five cells for the production of VLPs as potential rotavirus subunit vaccines.

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.

The indirect effects of multiplicity of infection on baculovirus expressed proteins in insect cells: secreted and non-secreted products

The baculovirus expression vector system was employed to produce human apolipoprotein E and β-galactosidase in order to study the effect of multiplicity of infection on secreted and non-secreted recombinant protein production. Prior knowledge of the influence of other cell culture and infection parameters, such as the cell density at time of infection and the time of harvest, allowed determination of the direct and indirect influences of multiplicity of infection on recombinant protein synthesis and degradation in insect cells. Under non-limited, controlled conditions, the direct effect of multiplicity of infection (10(-1)-10 pfu/cell) on specific recombinant product yields of non-secreted β-galactosidase was found to be insignificant. Instead, the observed increased in accumulated product was directly correlated to the total number of infected cells during the production period and therefore ultimately dependent on an adequate supply of nutrients. Only the timing of recombinant virus and protein production was influenced by, and dependent on the multiplicity of infection. Evidence is presented in this study that indicates the extremely limited predictability of post-infection cell growth at very low multiplicities of infection of less than 0.1 pfu/cell. Due to the inaccuracy of the current virus quantification techniques, combined with the sensitivity of post-infection cell growth at low MOI, the possibility of excessive post-infection cell growth and subsequent nutrient limitation was found to be significantly increased. Finally, as an example, the degree of product stability and cellular and viral protein contamination at low multiplicity of infection is investigated for a secreted recombinant form of human apolipoprotein E. Comparison of human apolipoprotein E production and secretion at multiplicities of infection of 10(-4)-10 pfu/cell revealed increased product degradation and contamination with intracellular proteins at low multiplicities of infection.