Application of a serum-free medium for the growth of Vero cells and the production of reovirus

Desmosomes undergo dynamic architectural changes during assembly and maturation

Desmosomes are macromolecular cell-cell junctions critical for maintaining adhesion and resisting mechanical stress in epithelial tissue. Desmosome assembly and the relationship between maturity and molecular architecture are not well understood. To address this, we employed a calcium switch assay to synchronize assembly followed by quantification of desmosome nanoscale organization using direct Stochastic Optical Reconstruction Microscopy (dSTORM). We found that the organization of the desmoplakin rod/C-terminal junction changed over the course of maturation, as indicated by a decrease in the plaque-to-plaque distance, while the plaque length increased. In contrast, the desmoplakin N-terminal domain and plakoglobin organization (plaque-to-plaque distance) were constant throughout maturation. This structural rearrangement of desmoplakin was concurrent with desmosome maturation measured by E-cadherin exclusion and increased adhesive strength. Using two-color dSTORM, we showed that while the number of individual E-cadherin containing junctions went down with the increasing time in high Ca²⁺, they maintained a wider desmoplakin rod/C-terminal plaque-to-plaque distance. This indicates that the maturation state of individual desmosomes can be identified by their architectural organization. We confirmed these architectural changes in another model of desmosome assembly, cell migration. Desmosomes in migrating cells, closest to the scratch where they are assembling, were shorter, E-cadherin enriched, and had wider desmoplakin rod/C-terminal plaque-to-plaque distances compared to desmosomes away from the wound edge. Key results were demonstrated in three cell lines representing simple, transitional, and stratified epithelia. Together, these data suggest that there is a set of architectural programs for desmosome maturation, and we hypothesize that desmoplakin architecture may be a contributing mechanism to regulating adhesive strength.

Production- and Purification-Relevant Properties of Human and Murine Cytomegalovirus

There is a large unmet need for a prophylactic vaccine against human cytomegalovirus (HCMV) to combat the ubiquitous infection that is ongoing with this pathogen. A vaccination against HCMV could protect immunocompromised patients and prevent birth defects caused by congenital HCMV infections. Moreover, cytomegalovirus (CMV) has a number of features that make it a very interesting vector platform for gene therapy. In both cases, preparation of a highly purified virus is a prerequisite for safe and effective application. Murine CMV (MCMV) is by far the most studied model for HCMV infections with regard to the principles that govern the immune surveillance of CMVs. Knowledge transfer from MCMV and mice to HCMV and humans could be facilitated by better understanding and characterization of the biological and biophysical properties of both viruses. We carried out a detailed investigation of HCMV and MCMV growth kinetics as well as stability under the influence of clarification and different storage conditions. Further, we investigated the possibilities to concentrate and purify both viruses by ultracentrifugation and ion-exchange chromatography. Defective enveloped particles were not separately analyzed; however, the behavior of exosomes was examined during all experiments. The effectiveness of procedures was monitored using CCID₅₀ assay, Nanoparticle tracking analysis, ELISA for host cell proteins, and quantitative PCR for host cell DNA. MCMV generally proved to be more robust in handling. Despite its greater sensitivity, HCMV was efficiently (100% recovery) purified and concentrated by anion-exchange chromatography using QA monolithic support. The majority of the host genomic DNA as well as most of the host cell proteins were removed by this procedure.

Optimization of Vero Cells Grown on a Polymer Fiber Carrier in a Disposable Bioreactor for Inactivated Coxsackievirus A16 Vaccine Development

At present, there are no vaccines available for hand, foot, and mouth disease, which is caused by Coxsackie virus A16 (CVA16) infection. In the present study, we isolated epidemic strains of CVA16 and optimized the production of the virus in Vero cells. The system comprised growing the infected cells on polymer fiber paper carriers in a serum-free medium containing 0.5% (w/v) lactalbumin hydrolysate a mini bioreactor. Disposable Bioflo310 and AmProtein Current perfusion bioreactors were used to monitor virus infection and Vero cell culture. The total number of cells increased from 1.5 × 10⁹ to 3.0 × 10¹⁰. In our optimized culture process, the virus titer reached 7.8 × 10⁷ TCID₅₀/mL at three days after infection. The inactivated CVA16 prepared from our optimized culture procedure elicited a slightly higher neutralizing antibody titer compared with that derived from routine culture procedures. These results will promote the large-scale production of inactivated CVA16 vaccines using nonwoven polymer fiber paper cell cultures.

Vero cell upstream bioprocess development for the production of viral vectors and vaccines

The Vero cell line is considered the most used continuous cell line for the production of viral vectors and vaccines. Historically, it is the first cell line that was approved by the WHO for the production of human vaccines. Comprehensive experimental data on the production of many viruses using the Vero cell line can be found in the literature. However, the vast majority of these processes is relying on the microcarrier technology. While this system is established for the large-scale manufacturing of viral vaccine, it is still quite complex and labor intensive. Moreover, scale-up remains difficult and is limited by the surface area given by the carriers. To overcome these and other drawbacks and to establish more efficient manufacturing processes, it is a priority to further develop the Vero cell platform by applying novel bioprocess technologies. Especially in times like the current COVID-19 pandemic, advanced and scalable platform technologies could provide more efficient and cost-effective solutions to meet the global vaccine demand. Herein, we review the prevailing literature on Vero cell bioprocess development for the production of viral vectors and vaccines with the aim to assess the recent advances in bioprocess development. We critically underline the need for further research activities and describe bottlenecks to improve the Vero cell platform by taking advantage of recent developments in the cell culture engineering field.

Process optimization for the rapid production of Enterovirus 71

Enterovirus 71 (EV71) infection can cause hand-foot-and-mouth disease (HFMD). Inactivated EV71 vaccine was effective to prevent EV71 derived HFMD. A highly efficient and economical process for producing EV71 is needed. In our study, the epidemic strain of EV71 (EV71-2013ZJHFMD) was obtained and purified. The Vero cells were cultured for production of EV71. The mini-bioreactor vessel (Amprotein Inc., China) packed with a 0.6 g polymer fiber carrier was used to determine the best seeding cell density, multiplicity of infection (MOI) and temperature. Then the optimized procedure was further applied in a 10 L disposable perfusion bioreactor ACPB (AmProtein Current Perfusion Bioreactor). The Vero cell culture and viral titer were monitored. The seeding density of 1.5 × 10⁷ cells per 0.6 g disk was considered to be the most appropriate for the culture. The best MOI was 0.1 and the temperature was 32 °C. The total cell number increased from 1.5 × 10⁹ to 3.0 × 10¹⁰. The maximum viral titers reached 1.0 × 10⁸/mL 3 days post-infection in our optimized special culture procedure (serum-free during the harvest period, supplemented with 0.25% Lactalbumin Hydrolysate). The total volume of the harvested supernatant was 25 L and the total virus yield was 1.93 × 10¹². The procedure using Vero cells grown on polymer fiber paper carriers was effective for the large-scale production of EV71.

Process intensification for Peste des Petites Ruminants Virus vaccine production

Process intensification for Peste des Petites Ruminants Virus (PPRV) vaccine production in anchorage dependent Vero cells is challenging, involving substantial amount of bioprocess development. In this study, we describe the implementation of a new, scalable bioprocess for PPRV vaccine production in Vero cells using serum-free medium (SFM), microcarrier technology in stirred-tank bioreactors (STB), in-situ cell detachment from microcarriers and perfusion. Vero cells were successfully adapted to ProVero™-1 SFM, reaching growth rates similar to serum-containing cultures (0.030 1/h vs 0.026 1/h, respectively). An in-situ cell detachment method was successfully implemented, with efficiencies above 85%. Up to 2.5-fold increase in maximum cell concentration was obtained using perfusion when compared to batch culture. Combining perfusion with the in-situ cell detachment method enabled the scale-up to 20 L STB directly from a 2 L STB, surpassing the need for a mid-scale platform (i.e. 5 L STB) and thus reducing seed train duration. Head-to-head comparison of cell growth and PPRV production in the 2 L and 20 L STB was performed, and no significant differences could be observed. Estimated infectious PPRV titers in Tissue Culture Infection Dose (TCID₅₀) (TCID₅₀/mL = 5 × 10⁶ and TCID₅₀/cell = 5) are within the log-range reported in literature for PPRV production in STB and SFM by Silva et al. (2008), thus confirming the feasibility and scalability of the seed train designed [1]. The novel and scalable vaccine production process herein proposed has the potential to assist the upcoming Peste des Petites Ruminants (PPR) Global Eradication Program (targeted by FAAO for 2030) by providing African local and/or regional manufacturers with a platform capable of generating over 25,000 doses of Nigeria 75/1 strain in just 19 days using a 20 L STB.

NKp46 Recognizes the Sigma1 Protein of Reovirus: Implications for Reovirus-Based Cancer Therapy

The recent approval of oncolytic virus for therapy of melanoma patients has increased the need for precise evaluation of the mechanisms by which oncolytic viruses affect tumor growth. Here we show that the human NK cell-activating receptor NKp46 and the orthologous mouse protein NCR1 recognize the reovirus sigma1 protein in a sialic-acid-dependent manner. We identify sites of NKp46/NCR1 binding to sigma1 and show that sigma1 binding by NKp46/NCR1 leads to NK cell activation in vitro Finally, we demonstrate that NCR1 activation is essential for reovirus-based therapy in vivo Collectively, we have identified sigma1 as a novel ligand for NKp46/NCR1 and demonstrated that NKp46/NCR1 is needed both for clearance of reovirus infection and for reovirus-based tumor therapy.IMPORTANCE Reovirus infects much of the population during childhood, causing mild disease, and hence is considered to be efficiently controlled by the immune system. Reovirus also specifically infects tumor cells, leading to tumor death, and is currently being tested in human clinical trials for cancer therapy. The mechanisms by which our immune system controls reovirus infection and tumor killing are not well understood. We report here that natural killer (NK) cells recognize a viral protein named sigma1 through the NK cell-activating receptor NKp46. Using several mouse tumor models, we demonstrate the importance of NK cells in protection from reovirus infection and in reovirus killing of tumors in vivo Collectively, we identify a new ligand for the NKp46 receptor and provide evidence for the importance of NKp46 in the control of reovirus infections and in reovirus-based cancer therapy.

Transformation and Tumorigenicity Testing of Simian Cell Lines and Evaluation of Poliovirus Replication

The key role of cell cultures in different scientific fields is worldwide recognized, both as in vitro research models alternative to laboratory animals and substrates for biological production. However, many safety concerns rise from the use of animal/human cell lines that may be tumorigenic, leading to potential adverse contaminations in cell-derived biologicals. In order to evaluate the suitability of 13 different cell lines for Poliovirus vaccine production, safety and quality, in vitro/in vivo tumorigenicity and Poliovirus propagation properties were evaluated. Our results revealed that non-human primate cell lines CYNOM-K1, FRhK-4, 4MBr-5 and 4647 are free of tumorigenic features and represent highly susceptible substrates for attenuated Sabin Poliovirus strains. In particular, FRhK-4 and 4647 cell lines are characterized by a higher in vitro replication, resulting indicated for the use in large-scale production field.

Serum-Free Suspension Culture of MDCK Cells for Production of Influenza H1N1 Vaccines

Development of serum-free suspension cell culture processes is very important for influenza vaccine production. Previously, we developed a MDCK suspension cell line in a serum-free medium. In the present study, the growth kinetics of suspension MDCK cells and influenza virus production in the serum-free medium were investigated, in comparison with those of adherent MDCK cells in both serum-containing and serum-free medium. It was found that the serum-free medium supported the stable subculture and growth of both adherent and suspension cells. In batch culture, for both cell lines, the growth kinetics in the serum-free medium was comparable with those in the serum-containing medium and a commercialized serum-free medium. In the serum-free medium, peak viable cell density (VCD), haemagglutinin (HA) and median tissue culture infective dose (TCID₅₀) titers of the two cell lines reached 4.51×10⁶ cells/mL, 2.94Log₁₀(HAU/50 μL) and 8.49Log₁₀(virions/mL), and 5.97×10⁶ cells/mL, 3.88Log₁₀(HAU/50 μL), and 10.34Log₁₀(virions/mL), respectively. While virus yield of adherent cells in the serum-free medium was similar to that in the serum-containing medium, suspension culture in the serum-free medium showed a higher virus yield than adherent cells in the serum-containing medium and suspension cells in the commercialized serum-free medium. However, the percentage of infectious viruses was lower for suspension culture in the serum-free medium. These results demonstrate the great potential of this suspension MDCK cell line in serum-free medium for influenza vaccine production and further improvements are warranted.

Inactivated Enterovirus 71 Vaccine Produced by 200-L Scale Serum-Free Microcarrier Bioreactor System Provides Cross-Protective Efficacy in Human SCARB2 Transgenic Mouse

Epidemics and outbreaks caused by infections of several subgenotypes of EV71 and other serotypes of coxsackie A viruses have raised serious public health concerns in the Asia-Pacific region. These concerns highlight the urgent need to develop a scalable manufacturing platform for producing an effective and sufficient quantity of vaccines against deadly enteroviruses. In this report, we present a platform for the large-scale production of a vaccine based on the inactivated EV71(E59-B4) virus. The viruses were produced in Vero cells in a 200 L bioreactor with serum-free medium, and the viral titer reached 10(7) TCID50/mL 10 days after infection when using an MOI of 10(-4). The EV71 virus particles were harvested and purified by sucrose density gradient centrifugation. Fractions containing viral particles were pooled based on ELISA and SDS-PAGE. TEM was used to characterize the morphologies of the viral particles. To evaluate the cross-protective efficacy of the EV71 vaccine, the pooled antigens were combined with squalene-based adjuvant (AddaVAX) or aluminum phosphate (AlPO4) and tested in human SCARB2 transgenic (Tg) mice. The Tg mice immunized with either the AddaVAX- or AlPO4-adjuvanted EV71 vaccine were fully protected from challenges by the subgenotype C2 and C4 viruses, and surviving animals did not show any degree of neurological paralysis symptoms or muscle damage. Vaccine treatments significantly reduced virus antigen presented in the central nervous system of Tg mice and alleviated the virus-associated inflammatory response. These results strongly suggest that this preparation results in an efficacious vaccine and that the microcarrier/bioreactor platform offers a superior alternative to the previously described roller-bottle system.

The impact of microcarrier culture optimization on the glycosylation profile of a monoclonal antibody

Microcarriers are widely used for the large-scale culture of attachment-dependent cells with increased cell densities and, ultimately, higher product yield. In these processes, the specific culture conditions can affect the quality of the product, which is closely related to its glycosylation pattern. Furthermore, the lack of studies in the area reinforces the need to better understand the effects of microcarrier culture in product glycosylation. Consequently, in this work, the glycosylation profile of a monoclonal antibody (mAb) produced by adherent CHO-K1 cells grown in Cytodex 3 was evaluated under different conditions, and compared to that obtained of typical adherent cultures. It was found that microcarrier cultures result in a glycosylation profile with different characteristics from T-flask cultures, with a general increase in galactosylation and decrease in fucosylation levels, both with a potentially positive impact on mAb activity. Sialylation also varied but without a general tendency. This study then showed that the specific culture conditions used in microcarrier culture influence the mAb glycan profile, and each functional element (galactose, core fucose, sialic acid) is independently affected by these conditions. In particular, great reductions of fucosylation (from 79 to 55%) were obtained when using half volume at inoculation, and notable decreases in sialylation (from 23 to 2%) and glycoform heterogeneity (from 20 to 11 glycoforms) were observed for shake flask culture, potentially associated with the improved cell densities achieved in these culture vessels.

Assessment of long-term effects of nanoparticles in a microcarrier cell culture system

Nano-sized materials could find multiple applications in medical diagnosis and therapy. One main concern is that engineered nanoparticles, similar to combustion-derived nanoparticles, may cause adverse effects on human health by accumulation of entire particles or their degradation products. Chronic cytotoxicity must therefore be evaluated. In order to perform chronic cytotoxicity testing of plain polystyrene nanoparticles on the endothelial cell line EAhy 926, we established a microcarrier cell culture system for anchorage-dependent cells (BioLevitator(TM)). Cells were cultured for four weeks and exposed to doses, which were not cytotoxic upon 24 hours of exposure. For comparison, these particles were also studied in regularly sub-cultured cells, a method that has traditionally been used to assess chronic cellular effects. Culturing on basal membrane coated microcarriers produced very high cell densities. Fluorescent particles were mainly localized in the lysosomes of the exposed cells. After four weeks of exposure, the number of cells exposed to 20 nm polystyrene particles decreased by 60% as compared to untreated controls. When tested in sub-cultured cells, the same particles decreased cell numbers to 80% of the untreated controls. Dose-dependent decreases in cell numbers were also noted after exposure of microcarrier cultured cells to 50 nm short multi-walled carbon nanotubes. Our findings support that necrosis, but not apoptosis, contributed to cell death of the exposed cells in the microcarrier culture system. In conclusion, the established microcarrier model appears to be more sensitive for the identification of cellular effects upon prolonged and repeated exposure to nanoparticles than traditional sub-culturing.

Dye free automated cell counting and analysis

We have developed an automated cell counting method that uses images obtained at multiple focal heights to enumerate cells in confluent culture. By taking the derivative of image intensity with respect to focal height using two complementary images, we are able to count high-density monolayers of cells over a large image area. Our method resists errors arising from variability in the focal plane caused by flatness or tilt non-uniformities with a minimal amount of focal plane alignment, allowing the automated collection of images across a large area.

Production of EV71 vaccine candidates

Enterovirus 71 (EV71) is now recognized as an emerging neurotropic virus in Asia and with Coxsackie virus (CV) it is the other major causative agent of hand-foot-mouth diseases (HFMD). Effective medications and/or prophylactic vaccines against HFMD are urgently needed. From a scientific (the feasibility of bioprocess, immunological responses and potency in animal challenge model) and business development (cost of goods) points of view, we in this review address and discuss the pros and cons of different EV71 vaccine candidates that have been produced and evaluated in animal models. Epitope-based synthetic peptide vaccine candidates containing residues 211–225 of VP1 formulated with Freund’s adjuvant (CFA/IFA) elicited low EV71 virus neutralizing antibody responses, but were protective in the suckling mouse challenge model. Among recombinant EV71 subunits (rVP1, rVP2 and rVP3) expressed in E. coli, purified and formulated with CFA/IFA, only VP1 elicited mouse antibody responses with measurable EV71-specific virus neutralization titers. Immunization of mice with either a DNA plasmid containing VP1 gene or VP1 expressed in Salmonella typhimurium also generated neutralizing antibody responses and protected animals against a live EV71 challenge. Recombinant EV71 virus-like particles (rVLP) produced from baculovirus formulated either with CFA/IFA or alum elicited good virus neutralization titers in both mice and non-human primates, and were found to be protective in the suckling mouse EV71 challenge model. Synthetic peptides or recombinant EV71 subunit vaccines (rVP1 and rVLP) formulated in alum were found to be poorly immunogenic in rabbits. Only formalin-inactivated (FI) EV71 virions formulated in alum elicited cross-neutralizing antibodies against different EV71 genotypes in mice, rabbits and non-human primates but induced weak neutralizing responses against CAV16. From a regulatory, economic and market acceptability standpoint, FI-EV71 virion vaccines are the most promising candidates and are currently being evaluated in human clinical trials. We further describe and analyze some new bioprocesses technologies that have great potential applications in EV71 vaccine development. This review also demonstrates the opportunities and challenges that the Asian vaccine industry faces today.

Real-time monitoring of adherent Vero cell density and apoptosis in bioreactor processes

This study proposes an easy to use in situ device, based on multi-frequency permittivity measurements, to monitor the growth and death of attached Vero cells cultivated on microporous microcarriers, without any cell sampling. Vero cell densities were on-line quantified up to 10(6) cell mL(-1). Some parameters which could potentially impact Vero cell morphological and physiological states were assessed through different culture operating conditions, such as media formulation or medium feed-harvest during cell growth phase. A new method of in situ cell death detection with dielectric spectroscopy was also successfully implemented. Thus, through permittivity frequency scanning, major rises of the apoptotic cell population in bioreactor cultures were detected by monitoring the characteristic frequency of the cell population, f(c), which is one of the culture dielectric parameters. Both cell density quantification and cell apoptosis detection are strategic information in cell-based production processes as they are involved in major events of the process, such as scale-up or choice of the viral infection conditions. This new application of dielectric spectroscopy to adherent cell culture processes makes it a very promising tool for risk-mitigation strategy in industrial processes. Therefore, our results contribute to the development of Process Analytical Technology in cell-based industrial processes.

Selection and characterization of vaccine strain for Enterovirus 71 vaccine development

Enterovirus 71 (EV71) has recently emerged as an important neurotropic virus in Asia because effective medications and prophylactic vaccine against EV71 infection are not available. Based on the success of inactivated poliovirus vaccine, the Vero cell-based chemically inactivated EV71 vaccine candidate could be developed. Identification of EV71 vaccine strain which can grow to high titer in Vero cell and induce cross-genotype virus neutralizing antibody responses represents the first step in vaccine development. In this report we describe the characterization and validation of a clinical isolate E59 belonging to B4 sub-genotype based on VP1 genetic analysis. Before selected as the vaccine strain, the genetic stability of E59 in passage had been analyzed based on the nucleotide sequences obtained from the Master Virus Seed, Working Seed banks and the virus harvested from the production lots, and found to be identical to those found in the original isolate. These results indicate that E59 vaccine strain has strong genetic stability in passage. Using this vaccine strain the prototype EV71 vaccine candidate was produced from 20L of Vero cell grown in serum-containing medium. The production processes were investigated, characterized and quantified to establish the potential vaccine manufacturing process including the time for virus harvest, the membrane for diafiltration and concentration, the gel-filtration chromatography for the down-stream virus purification, and the methods for viral inactivation. Finally, the inactivated virion vaccine candidate containing sub-microgram of viral proteins formulated with alum adjuvant was found to induce strong virus neutralizing antibody responses in mice and rabbits. Therefore, these results provide valuable information for cell-based EV71 vaccine development.

Serum-free microcarrier based production of replication deficient influenza vaccine candidate virus lacking NS1 using Vero cells

BACKGROUND

Influenza virus is a major health concern that has huge impacts on the human society, and vaccination remains as one of the most effective ways to mitigate this disease. Comparing the two types of commercially available Influenza vaccine, the live attenuated virus vaccine is more cross-reactive and easier to administer than the traditional inactivated vaccines. One promising live attenuated Influenza vaccine that has completed Phase I clinical trial is deltaFLU, a deletion mutant lacking the viral Nonstructural Protein 1 (NS1) gene. As a consequence of this gene deletion, this mutant virus can only propagate effectively in cells with a deficient interferon-mediated antiviral response. To demonstrate the manufacturability of this vaccine candidate, a batch bioreactor production process using adherent Vero cells on microcarriers in commercially available animal-component free, serum-free media is described.

RESULTS

Five commercially available animal-component free, serum-free media (SFM) were evaluated for growth of Vero cells in agitated Cytodex 1 spinner flask microcarrier cultures. EX-CELL Vero SFM achieved the highest cell concentration of 2.6 × 10^6 cells/ml, whereas other SFM achieved about 1.2 × 10^6 cells/ml. Time points for infection between the late exponential and stationary phases of cell growth had no significant effect in the final virus titres. A virus yield of 7.6 Log10 TCID50/ml was achieved using trypsin concentration of 10 μg/ml and MOI of 0.001. The Influenza vaccine production process was scaled up to a 3 liter controlled stirred tank bioreactor to achieve a cell density of 2.7 × 10^6 cells/ml and virus titre of 8.3 Log10 TCID50/ml. Finally, the bioreactor system was tested for the production of the corresponding wild type H1N1 Influenza virus, which is conventionally used in the production of inactivated vaccine. High virus titres of up to 10 Log10 TCID50/ml were achieved.

CONCLUSIONS

We describe for the first time the production of Influenza viruses using Vero cells in commercially available animal-component free, serum-free medium. This work can be used as a basis for efficient production of attenuated as well as wild type Influenza virus for research and vaccine production.

Proteomics for development of vaccine

The success of genome projects has provided us with a vast amount of information on genes of many pathogenic species and has raised hopes for rapid progress in combating infectious diseases, both by construction of new effective vaccines and by creating a new generation of therapeutic drugs. Proteomics, a strategy complementary to the genomic-based approach, when combined with immunomics (looking for immunogenic proteins) and vaccinomics (characterization of host response to immunization), delivers valuable information on pathogen-host cell interaction. It also speeds the identification and detailed characterization of new antigens, which are potential candidates for vaccine development. This review begins with an overview of the global status of vaccinology based on WHO data. The main part of this review describes the impact of proteomic strategies on advancements in constructing effective antibacterial, antiviral and anticancer vaccines. Diverse aspects of disease mechanisms and disease preventions have been investigated by proteomics.

Production of inactivated influenza H5N1 vaccines from MDCK cells in serum-free medium

BACKGROUND

Highly pathogenic influenza viruses pose a constant threat which could lead to a global pandemic. Vaccination remains the principal measure to reduce morbidity and mortality from such pandemics. The availability and surging demand for pandemic vaccines needs to be addressed in the preparedness plans. This study presents an improved high-yield manufacturing process for the inactivated influenza H5N1 vaccines using Madin-Darby canine kidney (MDCK) cells grown in a serum-free (SF) medium microcarrier cell culture system.

PRINCIPAL FINDING

The current study has evaluated the performance of cell adaptation switched from serum-containing (SC) medium to several commercial SF media. The selected SF medium was further evaluated in various bioreactor culture systems for process scale-up evaluation. No significant difference was found in the cell growth in different sizes of bioreactors studied. In the 7.5 L bioreactor runs, the cell concentration reached to 2.3 × 10(6) cells/mL after 5 days. The maximum virus titers of 1024 Hemagglutinin (HA) units/50 µL and 7.1 ± 0.3 × 10(8) pfu/mL were obtained after 3 days infection. The concentration of HA antigen as determined by SRID was found to be 14.1 µg/mL which was higher than those obtained from the SC medium. A mouse immunogenicity study showed that the formalin-inactivated purified SF vaccine candidate formulated with alum adjuvant could induce protective level of virus neutralization titers similar to those obtained from the SC medium. In addition, the H5N1 viruses produced from either SC or SF media showed the same antigenic reactivity with the NIBRG14 standard antisera.

CONCLUSIONS

The advantages of this SF cell-based manufacturing process could reduce the animal serum contamination, the cost and lot-to-lot variation of SC medium production. This study provides useful information to manufacturers that are planning to use SF medium for cell-based influenza vaccine production.

Kinetic characterization of vero cell metabolism in a serum-free batch culture process

A global kinetic study of the central metabolism of Vero cells cultivated in a serum-free medium is proposed in the present work. Central metabolism including glycolysis, glutaminolysis, and tricarboxylic acid cycle (TCA) was demonstrated to be saturated by high flow rates of consumption of the two major substrates, glucose, and glutamine. Saturation was reavealed by an accumulation of metabolic intermediates and amino acids, by a high production of lactate needed to balance the redox pathway, and by a low participation of the carbon flow to the TCA cycle supply. Different culture conditions were set up to reduce the central metabolism saturation and to better balance the metabolic flow rates between lactate production and energetic pathways. From these culture conditions, substitutions of glutamine by other carbon sources, which have lower transport rates such as asparagine, or pyruvate in order to shunt the glycolysis pathway, were successful to better balance the central metabolism. As a result, an increase of the cell growth with a concomitant decrease of cell death and a better distribution of the carbon flow between TCA cycle and lactate production occurred. We also demonstrated that glutamine was a major carbon source to supply the TCA cycle in Vero cells and that a reduction of lactate production did not necessary improve the efficiency of the Vero cell metabolism. Thus, to adapt the formulation of the medium to the Vero cell needs, it is important to provide carbon substrates inducing a regulated supply of carbon in the TCA cycle either through the glycolysis or through other pathways such as glutaminolysis. Finally, this study allowed to better understand the Vero cell behavior in serum-free medium which is a valuable help for the implementation of this cell line in serum-free industrial production processes.

Development and strategies of cell-culture technology for influenza vaccine

Influenza is a pandemic contagious disease and causes human deaths and huge economic destruction of poultry in the world. In order to control and prevent influenza, mainly type A, influenza vaccine for human and poultry were available since the 1940s and 1920s, respectively. In the development of vaccine production, influenza viruses were cultured originally from chicken embryos to anchorage-dependent cell lines, such as MDCK and Vero. The anchorage-independent lines have also been used to produce influenza virus, such as PER.C6 and engineering modified MDCK and Vero. During the process of influenza vaccine production, the common problem faced by all producers is how to improve the titer of influenza virus. This paper focuses on the developments of cell culture for influenza virus vaccine production, limitations of cell culture, and relative strategies for improvement virus yields in cell-culture systems.

Production of yellow fever virus in microcarrier-based Vero cell cultures

In this work, the propagation of the 17DD yellow fever virus in Vero cells grown on Cytodex-1 microcarriers was evaluated. After verifying that upon infection the virus adsorption step could be performed under continuous agitation, experiments were carried out in spinners and sparged lab-scale stirred-tank bioreactor to evaluate the use of a commercial serum-free medium (VP-SFM) and to investigate the effects of multiplicity of infection (MOI) and time of infection (TOI) on virus production. Virus titers as high as 8.4 x 10(8)pfu/mL were obtained upon infection with MOI of 0.02 and TOI of 3 days, using the serum-free medium in the sparged bioreactor.

A novel animal-component-free medium for rabies virus production in Vero cells grown on Cytodex 1 microcarriers in a stirred bioreactor

Vero cells growth and rabies production in IPT-AF medium, a property animal-component-free medium are described in this work. Kinetics of cell growth and rabies virus (strain LP 2061) production were first conducted in spinner flasks. Over eight independent experiments, Vero cell growth in IPT-AF medium, on 2 g/l Cytodex 1 was consistent. An average Cd (cell division number) of 3.3+/-0.4 and a specific growth rate micro of 0.017+/-0.006 h(-1) were achieved. Such performances were comparable to those obtained in serum-containing medium (MEM+10% FCS). Rabies virus production on Vero cells in IPT-AF medium was also optimised in spinner flasks. The effects of multiplicity of infection (MOI), regulation of glucose level at 1 g/l and cell washing step, were investigated. The highest virus titer was achieved when the cells were infected at an MOI of 0.1; this level was equal to 10(7) FFU/ml. The step of medium exchange before cell infection can be omitted; nevertheless in this case glucose level should be maintained at 1 g/l to avoid a decrease of specific virus productivity. Process optimisation in a 2-l stirred bioreactor pointed out that the aeration mode was the prominent parameter that affected cell growth in IPT-AF medium and on Cytodex 1 microcarriers. An acceptable level of cell density (cell density level of 1.5x10(6) cells/ml) was achieved when cells were grown in batch mode and using headspace aeration. Nevertheless, this aeration mode is not optimal for large-scale culture. The addition of Pluronic F68 at 0.1% at 24 h post inoculation as well as the switch from surface aeration mode to the sparged mode, 2 days after the start of the culture, had markedly improved cell growth performance. A cell density level of 5.5x10(6) cells/ml was reached when cells were grown in a 2-l bioreactor, on 3 g/l Cytodex 1 in IPT-AF medium and using the recirculation culture mode. Cell infection at an MOI of 0.1 and using perfused culture, resulted in a maximal virus titer of 3.5x10(7) FFU/ml. The activity of the pooled inactivated rabies virus harvests showed a protective activity that meets WHO requirements.

Assessment of immunogenic potential of Vero adapted formalin inactivated vaccine derived from novel ECSA genotype of Chikungunya virus

The recent resurgence of Chikungunya virus (CHIKV) in India and Indian Ocean Islands with unusual clinical severity is a matter of great public health concern. Despite the fact that CHIKV resurgence is associated with epidemic of unprecedented magnitude, no approved licensed vaccine is currently available. In the present study, a Vero cell adapted purified formalin inactivated prototype vaccine candidate was prepared using a current Indian strain implicated with the explosive epidemic during 2006. The bulk preparation of the vaccine candidate was undertaken in microcarrier based spinner culture using cytodex-1 in virus production serum free medium. The inactivation of the virus was accomplished through standard formalin inactivation protocol. The mice were immunized subcutaneously with alhydrogel gel formulation of inactivated virus preparation. The assessment of both humoral and cell-mediated immune response was accomplished through ELISA, plaque reduction neutralization test (PRNT), microcytotoxicity assay and cytokine production assay. The results revealed that formalin inactivated vaccine candidate induced both high titered ELISA (1:51,200) and plaque reduction neutralizing antibodies (1:6400) with peak antibody titer being observed during 6 -- 8 weeks of post-vaccination. In the absence of suitable murine challenge model, the protective efficacy was established by both in vitro and in vivo neutralization tests. Further assessment of cellular immunity through in vitro stimulation of spleenocytes from immunized mice revealed augmentation of high levels of both pro- and anti-inflammatory cytokines, indicating a mixed balance of Th1 and Th2 response. These findings suggest that the formalin inactivated Chikungunya vaccine candidate reported in this study has very good immunogenic potential to neutralize the virus infectivity by augmenting both humoral and cell-mediated immune response.

Synthetic Low-Density Lipoprotein, a Novel Biomimetic Lipid Supplement for Serum-Free Tissue Culture

Lipid supplementation in serum-free tissue culture employs solubilization techniques to permit the addition of lipids, but these systems are potentially cytotoxic and do not present lipid in a natural form. In this research a simplified preparation method for synthetic low-density lipoprotein (sLDL) has been developed that involves microfluidization of a solvent lipid solution in a simple aqueous solution. This produces material with size and zeta potential characteristics similar to those of native LDL. sLDL supplementation in tissue culture media provides cholesterol concentrations higher than those achieved by 10% serum supplementation and existing chemically defined lipid supplements. sLDL stimulates NS0 and U937 cellular proliferation in completely serum-free media, the former in a lipid concentration dependent manner that is also related to both the receptor peptide structure employed and its concentration on the particle. The greatest NS0 cellular proliferation was obtained at the highest cholesterol concentration tested (0.5 mg/mL), which was 10 times higher than the cholesterol concentration achieved by standard 10% serum supplementation. U937 cellular proliferation was influenced by variation of sLDL's fatty acid constituents with a natural mixture producing maximal effect. Cell uptake studies in NS0 with fluorescently labeled sLDL indicated that assimilation is reduced by competition from native LDL. The planktonic nature of NS0 cell growth meant that cell binding and uptake experiments were difficult to conduct because of cellular aggregation. However, sLDL-induced U937 proliferation is ablated by the presence of an anti-LDL receptor antibody. The results indicate that sLDL uptake is via the LDL receptor and that sLDL can function as a lipid supplement for serum-free media capable of supplementation to cholesterol concentrations up to 0.5 mg/mL. Cellular uptake studies also suggest that sLDL will be useful for the targeting and delivery of materials to cells. sLDL therefore represents a new and promising synthetic biomimetic alternative to native LDL with multiple applications.

Long-term stability of Vero cell-derived inactivated Japanese encephalitis vaccine prepared using serum-free medium

We established a method of producing a Vero cell-derived Japanese encephalitis vaccine using serum-free medium, and tested its stability using various stabilizers during the inactivation process and storage at 4 degrees C and 28 degrees C. Similar to previously reported results of cell culture in serum-containing medium, Vero cells were cultured in a serum-free medium multiplied well, and the viral yield was successfully increased to about 10(9)PFU/ml. Following formalin-inactivation and purification via ethanol precipitation and sucrose density ultracentrifugation of the virus solution, the vaccine had the same quality as, and higher immunogenicity, the mouse brain-derived vaccine in current use. Testing of several stabilizers showed that the addition of 0.5% glycine during the virus inactivation process facilitated the maintenance of immunogenicity for a long period of time. Furthermore, the addition of 0.5% glycine and 1.0% sorbitol as vaccine stabilizers after purification led to the maintenance of immunogenicity for 1 year, not dependent on the storage temperature (4 degrees C or 28 degrees C). These results indicate that, in contrast to the current mouse brain-derived vaccine, the Vero cell-derived vaccine can be prepared using serum-free medium containing no animal-derived components, and that the vaccine can be stored at room temperature by adding stabilizers, suggesting the possibility of producing room temperature-stable vaccines.

Scalable culture systems using different cell lines for the production of Peste des Petits ruminants vaccine

Peste des Petits ruminants (PPR) is considered as one of the major constraints to the productivity of small ruminants in Africa and Asian countries. Currently PPR control is done by vaccination with an attenuated PPR strain (Nigeria 75/1) produced in monolayers of Vero cells grown in roller bottles or static flasks. This work focuses on the production of a PPR vaccine strain using stirred conditions as an advanced option for process scale-up. Non-porous microcarriers (Cytodex-1) were used to support Vero cell growth in suspension cultures. The use of Ex-Cell medium could improve cell specific productivities obtained with standard serum containing medium, independently of the type of system used, i.e. static as well as suspension stirred cultures. As an alternative, several cell lines adapted to grow as single cells in suspension (CHO-K1, BHK-21A and 293) and another anchorage-dependent (MRC-5) were evaluated in their capacity to produce a PPR vaccine. BHK-21A and 293 cells grown as single-cell suspension in serum free medium were both suited to produce PPR vaccine with productivities similar to Vero cells, namely 10(6)TCID(50)/mL. However, for the 293 cells, these results were only obtained 2-3 days later. CHO-K1 and MRC-5 cells have shown not to be suitable to adequately produce this virus. These results provide further insights into the feasibility of applying microcarrier cell culture technology to produce PPR vaccine in Vero cells as well as in the alternative use of single-cell suspension cultures of BHK-21A, significantly simplifying the existing production process.

High genetic stability of dengue virus propagated in MRC-5 cells as compared to the virus propagated in vero cells

This work investigated the replication kinetics of the four dengue virus serotypes (DEN-1 to DEN-4), including dengue virus type 4 (DEN-4) recovered from an infectious cDNA clone, in Vero cells and in MRC-5 cells grown on Cytodex 1 microcarriers. DEN-1 strain Hawaii, DEN-2 strain NGC, DEN-3 strain H-87, and DEN-4 strain H-241 , and DEN-4 strain 814669 derived from cloned DNA, were used to infect Vero cells and MRC-5 cells grown in serum-free or serum-containing microcarrier cultures. Serum-free and serum-containing cultures were found to yield comparable titers of these viruses. The cloned DNA-derived DEN-4 started genetically more homogeneous was used to investigate the genetic stability of the virus propagated in Vero cells and MRC-5 cells. Sequence analysis revealed that the DEN-4 propagated in MRC-5 cells maintained a high genetic stability, compared to the virus propagated in Vero cells. Amino acid substitutions of Gly₁₀₄Cys and Phe₁₀₈Ile were detected at 70%, 60%, respectively, in the envelope (E) protein of DEN-4 propagated in Vero cells, whereas a single mutation of Glu₃₄₅Lys was detected at 50% in E of the virus propagated in MRC-5 cells. Sequencing of multiple clones of three separate DNA fragments spanning 40% of the genome also indicated that DEN-4 propagated in Vero cells contained a higher number of mutations than the virus growing in MRC-5 cells. Although Vero cells yielded a peak virus titer approximately 1 to 17 folds higher than MRC-5 cells, cloned DEN-4 from MRC-5 cells maintained a greater stability than the virus from Vero cells. Serum-free microcarrier cultures of MRC-5 cells offer a potentially valuable system for the large-scale production of live-attenuated DEN vaccines.

A microcarrier cell culture process for propagating rabies virus in Vero cells grown in a stirred bioreactor under fully animal component free conditions

Rabies virus strain production in Vero cells grown on Cytodex 1 in a 2 L stirred tank bioreactor and in a medium free of components of human or animal origin (VP-SFM) is described. Cell banking procedure in VP-SFM supplemented with an animal components free mixture (10%DMSO+0.1%methylcellulose) was reported and cell growth after revitalization was assessed. Vero cells exhibited growth performances (specific growth rate and cell division number) similar to that obtained in serum containing medium. To design a scalable process that is totally free of animal-derived substances, two proteases: TrypLE Select and Accutase, were assessed as an alternative to trypsin which is routinely used for cell passage. Growth performance of Vero cells grown in VP-SFM and MEM+10% fetal calf serum (FCS) over four passages and subcultivated with either TrypLE Select or Accutase was evaluated. TrypLE Select showed the best performance in terms of specific growth rate and cell division number. Kinetics of cell growth and rabies virus production (LP2061/Vero strain) were investigated in spinner flask and in a 2 L bioreactor. In spinner flask, a maximal cell density level of 1.85x10(6) cells/mL was achieved when the cells were grown in VP-SFM on 2 g/L Cytodex 1. Cell infection experiments conducted at an MOI of 0.3 and without the medium exchange step, typically needed for serum containing rabies virus production, resulted in a maximal virus titer equal to 2x10(7) (Fluorescent Focus Unit) FFU/mL. In stirred tank bioreactor, Vero cell growth in VP-SFM on 3 g/L Cytodex 1 was shown to be sensitive to the aeration mode. Sparging the culture was detrimental for cell growth, whereas cell density level was greatly enhanced when only headspace aeration was used. A cell density level of 2.6x10(6) cells/mL was obtained when the cells were grown on 3g/L Cytodex 1 and in batch culture mode. Cell infection at an MOI of 0.1 without any medium exchange, yielded a maximal rabies virus titer of 2.4x10(7) FFU/mL. Furthermore, Vero cell growth in a 2 L bioreactor using recirculation culture mode during cell proliferation step and perfusion for virus multiplication phase was investigated. In comparison to batch culture, a higher cell density level that was equal to 5x10(6) cells/mL was reached. Cell infection under conditions similar to batch culture, resulted in a maximal virus titer equal to 1.38x10(8) FFU/mL. The potency of the pooled inactivated virus harvests showed an activity of 2.58 IU/mL which was comparable to that obtained in serum supplemented medium.

Bioreactor production of recombinant herpes simplex virus vectors

Serotypical application of herpes simplex virus (HSV) vectors to gene therapy (type 1) and prophylactic vaccines (types 1 and 2) has garnered substantial clinical interest recently. HSV vectors and amplicons have also been employed as helper virus constructs for manufacture of the dependovirus adeno-associated virus (AAV). Large quantities of infectious HSV stocks are requisite for these therapeutic applications, requiring a scalable vector manufacturing and processing platform comprised of unit operations which accommodate the fragility of HSV. In this study, production of a replication deficient rHSV-1 vector bearing the rep and cap genes of AAV-2 (denoted rHSV-rep2/cap2) was investigated. Adaptation of rHSV production from T225 flasks to a packed bed, fed-batch bioreactor permitted an 1100-fold increment in total vector production without a decrease in specific vector yield (pfu/cell). The fed-batch bioreactor system afforded a rHSV-rep2/cap2 vector recovery of 2.8 x 10(12) pfu. The recovered vector was concentrated by tangential flow filtration (TFF), permitting vector stocks to be formulated at greater than 1.5 x 10(9) pfu/mL.

High immunogenic enterovirus 71 strain and its production using serum-free microcarrier Vero cell culture

Developing an effective vaccine against enterovirus 71 (EV71) infection provides the best means to control the disease. We have previously reported that large-scale preparation of a low immunogenic EV71 strain can be achieved using serum free microcarrier Vero cell culture in a 2-l bioreactor [Wu SC, Liu CC, Lian WC. Optimization of microcarrier cell culture process for the inactivated enterovirus type 71 vaccine development. Vaccine 2004;22:3858-64]. This present work further investigated the virus growth and the immunogenicity of two high immunogenic strains (EV71-075 and EV71-117) prepared in serum-free microcarrier cell cultures. Our results showed that serum free culture increased cell death rate after infection, reduced the virus specific productivity, but resulted in elicitation of higher neutralizing titers in immunized mice as compared to that parallel obtained in serum-containing cultures. Therefore, serum-free microcarrier culture is a valuable process for developing inactivated EV71 vaccines.

Intracellular ATP and total adenylate concentrations are critical predictors of reovirus productivity from Vero cells

The productivity of reovirus type-3 Dearing was studied in cultures of Vero cells in serum-free media. Viral productivity was dependent upon the metabolic state of the cells rather than the phase of growth at which the cells were infected. Cells at different energy states were established by 24-h incubation in nutrient-depleted media. This resulted in variable intracellular nucleotide concentrations but high cellular viability was maintained. Of the nucleotides analyzed at the time of infection only the intracellular [ATP] and total adenylate nucleotides were positively correlated with viral productivity. The correlated data followed a sigmoidal plot with an equation defined by polynomial regression analysis. Apparent threshold values of 3.2 fmol/cell and 3.3 fmol/cell were established for ATP and total adenylate, respectively, at which the viral production was 50% the maximal value. Cultures with lower ATP and total adenylate levels at the time of infection resulted in as much as a 95% reduction in overall viral titer compared to the control. The adenylate energy charge (AEC) showed a negative correlation with viral production with an AEC value >0.97 resulting in low virus productivity. Intracellular ATP or total adenylate concentration at the point of infection may be used as a predictor of viral yield in bioprocesses designed for virus/vaccine production.

Production and glycosylation of recombinant beta-interferon in suspension and cytopore microcarrier cultures of CHO cells

Microcarriers are suitable for high-density cultures of cells requiring surface attachment and also offer the advantage of easy media removal for product recovery. We have used the macroporous microcarriers Cytopore 1 and 2 for the growth of CHO cells producing recombinant human beta-interferon (beta-IFN) in stirred batch cultures. Although these cells may grow in suspension, in the presence of Cytopore microcarriers they become entrapped in the inner bead matrix where they can be maintained at high densities. Cell growth rates were reduced in microcarrier cultures compared to suspension cultures. However, the beta-IFN yield was up to 3-fold greater as a result of an almost 5-fold higher specific productivity. Maximum productivity was found in cultures containing 1.0 mg/mL of Cytopore 1 or 0.5 mg/mL of Cytopore 2 with a cell/bead ratio of 1029 and 822, respectively. Beta-IFN molecules aggregated in the later stages of all cultures, causing a decrease in response by ELISA. However, the degree of aggregation was significantly less in the microcarrier cultures. The N-linked glycans from beta-IFN were isolated and analyzed by normal phase HPLC. There was no apparent difference in the profile of glycans obtained from each of the suspension and Cytopore culture systems. This suggests that Cytopore microcarriers may be useful in bioprocess development for enhanced recombinant glycoprotein production without affecting the glycosylation profile of the protein.

Inhibition of reovirus by mycophenolic acid is associated with the M1 genome segment

Mycophenolic acid (MPA), an inhibitor of IMP dehydrogenase, inhibits reovirus replication and viral RNA and protein production. In mouse L929 cells, antiviral effects were greatest at 30 microg of MPA/ml. At this dosage, MPA inhibited replication of reovirus strain T3D more than 1,000-fold and inhibited replication of reovirus strain T1L nearly 100-fold, compared to non-drug-treated controls. Genetic reassortant analysis indicated the primary determinant of strain-specific differences in sensitivity to MPA mapped to the viral M1 genome segment, which encodes the minor core protein mu2. MPA also inhibited replication of both strains of reovirus in a variety of other cell lines, including Vero monkey kidney and U373 human astrocytoma cells. Addition of exogenous guanosine to MPA-treated reovirus-infected cells restored viral replicative capacity to nearly normal levels. These results suggest the mu2 protein is involved in the uptake and processing of GTP in viral transcription in infected cells and strengthens the evidence that the mu2 protein can function as an NTPase and is likely a transcriptase cofactor.

Optimization of reovirus production from mouse L-929 cells in suspension culture

Reovirus serotype 3 Dearing (T3D) has shown potential as a novel cancer therapy. To support the increasing demand for reovirus, a two-stage perfusion mode scheme is proposed for cell growth and reovirus production. Mouse L-929 cells were used as the host for reovirus infection due to their ability to grow well in suspension culture. Several L-929 cell growth and reovirus infection characteristics were investigated and optimized in spinner flask batch cultures. For the growth of L-929 cells, a balanced nutrient-fortification of SMEM medium increased the maximum cell density by 30%, compared to normal SMEM; however, ammonia and lactate accumulations were found to inhibit further cell growth. For the production of reovirus, approximately 90% increase in viral yield resulted when the infection temperature was reduced from 37 to 33 degrees C. Infectious reovirus particles were shown to be stable in conditioned medium at 37 and 33 degrees C. The final virus titer was dependent on the multiplicity of infection (MOI) and the host cell density at the time of infection. A combination of an MOI of 0.1 pfu/cell and an initial host cell density of 1.0 x 10(6) cells/mL in fortified medium resulted in a maximum virus titer of (4.59 +/- 0.16) x 10(9) pfu/mL and a specific yield of (2.34 +/- 0.08) x 10(3) pfu/cell. At an optimal harvest time of the infection process, 99% of the virus was associated with the cellular debris. Finally, the presence of 5.0 mM ammonia in the culture medium was shown to seriously inhibit the reovirus yield, whereas lactate concentrations up to 20 mM had no effect.

Metabolic active-high density VERO cell cultures on microcarriers following apoptosis prevention by galactose/glutamine feeding

The control of cell death occurring in high density cultures performed in bioreactors is an important factor in production processes. In this work, medium nutrient removal or feeding was used to determine at which extension apoptosis could be, respectively, involved or prevented in VERO cell cultures on microcarriers. Glutamine and galactose present in the VERO cell culture medium was consumed after, respectively, 6 and 12 days of culture. Kinetics studies showed that fresh medium replacement and, to some extent, galactose or glutamine depleted-fresh medium replacement provided a nutritional environment, allowing the VERO cell cultures to attain high densities. Galactose was shown to be a more critical nutrient when cultures reached a high density. In agreement with that, VERO cell cultures supplemented with galactose and/or glutamine were shown to confirm previous findings and, again at high densities, galactose was shown to be a critical nutrient for VERO cell growth. These observations also indicated that in VERO cell cultures, for feeding purposes, the glutamine could be replaced by galactose. The inverse was not true and led, at high densities, to a decrease of cell viability. In the absence of glutamine and galactose, apoptosis was observed in VERO cell cultures by cytofluorometry, Acridine orange staining or light and electron microscopy, reaching high levels when compared to cultures performed with complete medium. VERO cells apoptosis process could be prevented by the galactose and/or glutamine feeding and, at high densities, galactose was more efficient in protecting the cultures. These cultures, prevented from apoptosis, were shown to synthesize high levels of measles virus following infection. Our data show that apoptosis prevention by glutamine/galactose feeding, led to high productive and metabolic active VERO cell cultures, as indicated by the high cell density obtained and the virus multiplication leading to higher virus titers.

Higher production of rabies virus in serum-free medium cell cultures on microcarriers

Rabies virus suspensions were obtained from VERO cells cultivated on solid microcarriers in a bioreactor after infection with the Pasteur rabies virus strain (PV). Virus production-serum free medium (VP-SFM) or Leibovitz 15 (L15) medium supplemented or not with fetal calf serum (FCS) were used to cultivate the VERO cells, before and after virus infection. The cell growth was shown to reach higher densities (1.6 x 10(6) cellsmol(-l)), when VP-SFM supplemented with 1% of FCS was used during the cell growth phase of culture, and then replaced by VP-SFM alone for the virus multiplication phase. In the cultures performed from the beginning with VP-SFM, lower densities accompanied by an altered cell morphology and detachment from the microcarriers were always observed. In rabies virus infected cultures, kinetic studies showed that higher virus yields (10(4.7) FFD(50) per 0.05 ml) were always obtained in cultures performed initially on VP-SFM supplemented with 1% FCS and after infection on VP-SFM alone. In agreement with that, rabies virus production, as measured by the average of virus titers in harvests obtained at different times after infection were shown to be 5.5 times higher in the cell cultures using initially VP-SFM+1%FCS and, following infection, VP-SFM alone. Besides the advantages of using media with a well-controlled composition, these data indicate the usefulness of serum free media also in terms of virus productivity.