The impact of cell culture sensitivity on rapid viral diagnosis: a historical perspective

The contribution of cell culture systems in the diagnosis of viral infections has been well recognized over the years. Not only did such systems make possible the direct isolation and identification of viruses, but also the production of viral diagnostic reagents for rapid diagnosis, the evaluation of antiviral agents, and the production of vaccines for the control of viral diseases. Although many reagents for rapid detection of viral antigens/genomes are currently available, none will make possible discoveries of new viral agents. Thus sensitive cell culture systems are still essential for the rapid and accurate diagnosis of viral infections. Since, as yet, no single cell culture system is susceptible to all viruses, the constant search for additional sensitive cell culture systems for detecting those unknown and/or currently non-cultivable viral agents continues to be an open area of investigation in the field of diagnostic virology.     

A sensitive in vitro assay for the detection of residual viable rabies virus in inactivated rabies vaccines

Rabies is a viral disease transmitted through bites from rabid animals and can be prevented by vaccines. Clinically used rabies vaccines are prepared from inactivated rabies viruses grown in cell cultures or embryonated eggs. In Japan and across the world, tests that confirm complete inactivation, such as the in vivo suckling mouse assay, in which suckling mice are intracerebrally inoculated with vaccine products, are required for quality control. In this study, we developed a novel cell-based immunofluorescence assay that does not require mice for testing rabies vaccine inactivation for human use. The sensitivity of this cell-based in vitro assay was 5.7 times that of the in vivo suckling mouse assay, with a detection limit of one focus forming units per ml of test sample. This newly developed in vitro assay may replace the established in vivo suckling mouse assay for confirming viral vaccine inactivation.     

Testing the purity of cell cultures using clinical diagnostic PCR kits

Cell cultures of higher organisms, especially human cells, are increasingly used in medical, pharmaceutical, and scientific research. The main difficulty for cell cultures is hidden nonlethal contamination by mycoplasmas, viruses, and cross contamination with other cells. We suggest using PCR kits designed and officially employed in clinical diagnostics as an accessible and reliable method for monitoring the purity of cell cultures. We tested 50 human cell lines using commercial diagnostic systems for detection of papillomaviruses, herpes viruses, adenoviruses, Mycoplasma hominis, and total bacterial mass. No contamination was revealed in the cell lines tested. For the cell lines that contained integrated parts of viral genomes, the presence of the corresponding DNA sequences was confirmed. These diagnostic systems can be effectively used to test the purity of cell lines and for qualitative detection of possible contamination, as well as for quantitative evaluations using calculation of viral load, just as is practiced in clinical diagnostics.     

Virus-associated human tumors: cervical carcinomas and papilloma viruses

The latest experimental data on the role of viruses in the origin of human tumors are discussed. This group of viruses consists of T-cell leukemia virus type 1 (HTLV 1), herpes viruses (HHV 8 and Epstein-Barr virus), hepatitis B virus, and human papilloma viruses. The most typical feature of this group of viruses is a very long latent period from the initial infection to the development of the disease that varies between 10 and 40 years. The mechanism of malignant cell conversion is specific for each viral type but is mainly associated with a disruption of functions of cellular genes participating in the control of cell division and proliferation. It can be a direct inactivation of tumor suppressor genes by their interaction with viral gene products (papilloma viruses), or a trans-activation of cellular genes modulating cell proliferation by viral gene products (hepatitis B virus and HTLV 1). Viruses play an initiative role and additional genetic changes in the genome of infected cells are necessary for complete expression of the oncogenic potential of the viral genes. Only these cells will give rise to a monoclonal cell population with uncontrolled proliferation. New approaches for the creation of vaccines against cancers associated with hepatitis B virus and papilloma viruses (hepatocellular carcinomas and cervical tumors, respectively) are in progress. These vaccines have been found to be effective in prevention of the disease in the experimental models and are now beginning to be used for human vaccination.     

Dengue-2 and yellow fever 17DD viruses infect human dendritic cells, resulting in an induction of activation markers, cytokines and chemokines and secretion of different TNF-α and IFN-α profiles

Flaviviruses cause severe acute febrile and haemorrhagic infections, including dengue and yellow fever and the pathogenesis of these infections is caused by an exacerbated immune response. Dendritic cells (DCs) are targets for dengue virus (DENV) and yellow fever virus (YF) replication and are the first cell population to interact with these viruses during a natural infection, which leads to an induction of protective immunity in humans. We studied the infectivity of DENV2 (strain 16681), a YF vaccine (YF17DD) and a chimeric YF17D/DENV2 vaccine in monocyte-derived DCs in vitro with regard to cell maturation, activation and cytokine production. Higher viral antigen positive cell frequencies were observed for DENV2 when compared with both vaccine viruses. Flavivirus-infected cultures exhibited dendritic cell activation and maturation molecules. CD38 expression on DCs was enhanced for both DENV2 and YF17DD, whereas OX40L expression was decreased as compared to mock-stimulated cells, suggesting that a T helper 1 profile is favoured. Tumor necrosis factor (TNF)-α production in cell cultures was significantly higher in DENV2-infected cultures than in cultures infected with YF17DD or YF17D/DENV. In contrast, the vaccines induced higher IFN-α levels than DENV2. The differential cytokine production indicates that DENV2 results in TNF induction, which discriminates it from vaccine viruses that preferentially stimulate interferon expression. These differential response profiles may influence the pathogenic infection outcome.     

Use of a diploid cell line for detecting the toxic components in medical immunobiological preparations

Besides specific antigens medical immunobiological agents (MIBA) contain chemical compounds (formaldehyde, aluminium hydroxide and mercury salt, merthiolate) in permissible concentrations. Therefore, the investigation of MIBA and their components should involve methods studying the effect of chemical compounds on cells and their structural components. For this purpose WHO recommends to use cell cultures. The results obtained show that cell cultures (constant and diploid lines) allow the differentiation in the degree of toxicity of chemical compounds constituting MIBA. Merthiolate had the strongest irreversible lethal effect. The technique can prove useful for more accurate evaluation of toxicity in inactivated bacterial and viral vaccines as well as in serum preparations. Cell culture can be successfully used for the detection of toxic components in vaccines and serum drugs, with the final safety tested by their injection to animals.     

Comparative Study of Influenza Virus Replication in MDCK Cells and in Primary Cells Derived from Adenoids and Airway Epithelium

Although clinical trials with human subjects are essential for determination of safety, infectivity, and immunogenicity, it is desirable to know in advance the infectiousness of potential candidate live attenuated influenza vaccine strains for human use. We compared the replication kinetics of wild-type and live attenuated influenza viruses, including H1N1, H3N2, H9N2, and B strains, in Madin-Darby canine kidney (MDCK) cells, primary epithelial cells derived from human adenoids, and human bronchial epithelium (NHBE cells). Our data showed that despite the fact that all tissue culture models lack a functional adaptive immune system, differentiated cultures of human epithelium exhibited the greatest restriction for all H1N1, H3N2, and B vaccine viruses studied among three cell types tested and the best correlation with their levels of attenuation seen in clinical trials with humans. In contrast, the data obtained with MDCK cells were the least predictive of restricted viral replication of live attenuated vaccine viruses in humans. We were able to detect a statistically significant difference between the replication abilities of the U.S. (A/Ann Arbor/6/60) and Russian (A/Leningrad/134/17/57) cold-adapted vaccine donor strains in NHBE cultures. Since live attenuated pandemic influenza vaccines may potentially express a hemagglutinin and neuraminidase from a non-human influenza virus, we assessed which of the three cell cultures could be used to optimally evaluate the infectivity and cellular tropism of viruses derived from different hosts. Among the three cell types tested, NHBE cultures most adequately reflected the infectivity and cellular tropism of influenza virus strains with different receptor specificities. NHBE cultures could be considered for use as a screening step for evaluating the restricted replication of influenza vaccine candidates.     

Foreign protein production using plant cell and organ cultures: Advantages and limitations

Plants and plant tissue cultures are used as host systems for expression of foreign proteins including antibodies, vaccines and other therapeutic agents. Recombinant or stably transformed plants and plant cell cultures have been applied for foreign protein production for about 20 years. Because the product concentration achieved exerts a major influence on process economics, considerable efforts have been made by commercial and academic research groups to improve foreign protein expression levels. However, post-synthesis product losses due to protease activity within plant tissues and/or extracellular protein adsorption in plant cell cultures can negate the benefits of molecular or genetic enhancement of protein expression. Transient expression of foreign proteins using plant viral vectors is also a practical approach for producing foreign proteins in plants. Adaptation of this technology is required to allow infection and propagation of engineered viruses in plant tissue cultures for transient protein expression in vitro.     

Virus contaminations of cell cultures – A biotechnological view

In contrast to contamination by microbes and mycoplasma, which can be relatively easily detected, viral contamination present a serious threat because of the difficulty in detecting some viruses and the lack of effective methods of treating infected cell cultures. While some viruses are capable of causing morphological changes to infected cells (e.g. cytopathic effect) which are detectable by microscopy some viral contaminations result in the integration of the viral genome as provirus, this causes no visual evidence, by means of modification of the cellular morphology. Virus production from such cell lines, are potentially dangerous for other cell cultures (in research labs)by cross contaminations, or for operators and patients (in the case of the production of injectable biologicals) because of potential infection. The only way to keep cell cultures for research, development, and the biotech industry virus-free is the prevention of such contaminations. Cell cultures can become contaminated by the following means: firstly, they may already be contaminated as primary cultures (because the source of the cells was already infected), secondly, they were contaminated due to the use of contaminated raw materials, or thirdly, they were contaminated via an animal passage. This overview describes the problems and risks associated with viral contaminations in animal cell culture, describes the origins of these contaminations as well as the most important virsuses associated with viral contaminations in cell culture. In addition, ways to prevent viral contaminations as well as measures undertaken to avoid and assess risks for viral contaminations as performed in the biotech industry are briefly described. This revised version was published online in August 2006 with corrections to the Cover Date.     

A microcarrier-based cell culture process for the production of a bovine respiratory syncytial virus vaccine

Veterinary viral vaccines generally comprise either attenuated or chemically inactivated viruses which have been propagated on mammalian cell substrates or specific pathogen free (SPF) eggs. New generation vaccines include chemically inactivated virally-infected whole cell vaccines. The NM57 cell line is a bovine nasal turbinate persistently infected (non-lytic infection) with a strain of the respiratory syncytial virus (RSV). The potential of microcarrier technology for the cultivation in bioreactors of this anchorage dependent cell line for RSV vaccine production has been investigated. Both Cytodex 3 and Cultispher S microcarriers proved most suitable from a selection of microcarriers as growth substrates for this NM57 cell line. Maximum cell densities of 4.12×105 cells ml-1and 5.52×105 cells ml-1 respectively were obtained using Cytodex 3 (3 g l-1) and and Cultispher S (1 g l-1) in 5 l bioreactor cultures. The fact that cell growth was less sensitive to agitation rate when cultured on Cultispher S microcarriers, and that cells were efficiently harvested from this microcarrier by an enzymatic method, suggested Cultispher S is suitable for further evaluation at larger bioreactor scales (>5 l) than that described here. This revised version was published online in July 2006 with corrections to the Cover Date.     

Visualization of viral assembly in the infected cell.

The study of the virus life cycle in infected cells is a methodological challenge due to the small size and diversity of the viral components. Recent developments on preservation of fine structure and molecular localization have provided a group of powerful methods with wide applications in cell biology and virology. Among the different electron microscopy (EM) techniques available to visualize viral assembly at the intracellular level, we will focus on conventional ultrathin sections, cryosections, and freeze-substitution. For obtaining molecular information associated to ultrastructure we have now a group of methods to detect viral proteins (immunogold labeling), as well as the viral genome, through the different techniques for detection of nucleic acids (the enzyme-gold approach, in situ hybridization, and elemental mapping). We will illustrate the applications of these methods with examples of viruses that exhibit different levels of structural complexity. These new approaches help to detect and identify viruses in clinical samples and to characterize the virus life cycle and the cellular components involved, to obtain data that could help for a therapeutic intervention, and to characterize virus-like particles that can be the basis of new and safe vaccines.     

Construction of recombinant fowlpox viruses carrying multiple vaccine antigens and immunomodulatory molecules

Here we describe plasmid vectors and selection protocols developed to allow the construction of recombinant fowlpox viruses (rFPVs) with up to three insertions of foreign DNA in the viral genome. Transient dominant selection allows the construction of recombinant viruses that do not retain the selection markers and can therefore be used for the insertion of additional genes at other sites in the viral genome. A SYBR Green real-time PCR sequence detection assay was applied to the identification of recombinant viruses from individual plaques, eliminating the need for amplification and hybridization from the transient dominant protocol and resulting in significant savings in time at each round of plaque purification. Dominant selection techniques allow more rapid recombinant virus construction; however, as the markers are retained along with the gene of interest, they can only be used to generate the final recombinant. rFPV vaccines constructed using these techniques have reached preclinical nonhuman primate and phase I human clinical trials in prime/boost vaccination studies as human immunodeficiency virus (HIV) therapeutic andprophylactic vaccines.     

Endogenous agents in primary cell cultures with special reference to latent viruses

Summary A survey study of primary cell cultures prepared from primate and nonprimate tissues has shown that viruses are commonly found as endogenous agents in these cultures. Cultures prepared from monkey tissues yielded the greatest variety of virus isolates. Almost all strain 2 guinea pig cultures contained a Herpes-like virus, and both strain 2 and Hartley animals contained C-type virus. Bovine embryo and rabbit kidney cell cultures were rarely infected with viruses. Toxoplasmas and microfilariae were also detected. Most of the endogenous viral agents were obtained by holding the cultures for long-term incubation while testing for cytopathic effect, hemadsorption, and staining with hematoxylin and eosin for virus-induced cellular inclusions. Fluorescent antibody staining and electron microscopy were found to be efficient for detection of certain types of viruses. The screening of animals by testing for the presence of neutralizing antibody was not an effective procedure in selecting virus-free animals for cell culture purposes. This study was supported in part by the Food and Drug Administration, Department of Health, Education and Welfare, Contract NIH-DBS-72-2105, and Research Grant AI 08648 from the Institute of Allergy and Infectious Diseases, National Institutes of Health. Presented at the Session in Depth on Endogenous Viruses in Cell Culture at the Twenty-fifth Annual Meeting of the Tissue Culture Association, June 1974.     

Prospects and achievements of genetic engineering in development of antiviral vaccines

Proceeding from the known data various theoretical and experimental approaches to the construction of gene-engineering vaccines are considered. Gene-engineering subunit vaccines of the first generation are based on isolation of the genes coding for the synthesis of full length capsid proteins with the main antigenic determinants and their subsequent expression in suitable recipient cells. Initial idea of the microbiological synthesis as the main way for production of any antiviral vaccines was not confirmed by the later development. Now for this type of vaccines eucaryotic systems are widely employed using the animal virus vectors and the animal cell cultures. Gene-engineering subunit vaccine of the second generation appears to be a chimeric protein with built-in antigenic determinants of different viruses and maximal immunogenicity in monomeric form. The last point reopens the perspective to use a microbiological synthesis for the production of antiviral vaccines. Besides that the chemically synthesized polypeptide antiviral vaccine will be used widely. In gene-engineering subunit vaccines of the third generation it is possible to use not the natural antigenic determinants which often are characterized by high level of the primary structure changes but artificial (non-natural) antigens, that are the capsid protein conservative regions which under natural conditions of infection or immunization do not induce the protective antiviral antibodies. The recombinant DNA technology in addition to subunit type vaccine allows to construct living vaccines which represent a DNA-containing attenuated virus with build-in natural or synthetic gene of the capsid or chimeric protein with antigenic determinants of another viral species.     

Viral Nucleic Acids in Live-Attenuated Vaccines: Detection of Minority Variants and an Adventitious Virus

Metagenomics and a panmicrobial microarray were used to examine eight live-attenuated viral vaccines. Viral nucleic acids in trivalent oral poliovirus (OPV), rubella, measles, yellow fever, varicella-zoster, multivalent measles/mumps/rubella, and two rotavirus live vaccines were partially purified, randomly amplified, and pyrosequenced. Over half a million sequence reads were generated covering from 20 to 99% of the attenuated viral genomes at depths reaching up to 8,000 reads per nucleotides. Mutations and minority variants, relative to vaccine strains, not known to affect attenuation were detected in OPV, mumps virus, and varicella-zoster virus. The anticipated detection of endogenous retroviral sequences from the producer avian and primate cells was confirmed. Avian leukosis virus (ALV), previously shown to be noninfectious for humans, was present as RNA in viral particles, while simian retrovirus (SRV) was present as genetically defective DNA. Rotarix, an orally administered rotavirus vaccine, contained porcine circovirus-1 (PCV1), a highly prevalent nonpathogenic pig virus, which has not been shown to be infectious in humans. Hybridization of vaccine nucleic acids to a panmicrobial microarray confirmed the presence of endogenous retroviral and PCV1 nucleic acids. Deep sequencing and microarrays can therefore detect attenuated virus sequence changes, minority variants, and adventitious viruses and help maintain the current safety record of live-attenuated viral vaccines.Highly effective, safe, and relatively inexpensive, live-attenuated viruses protect against numerous human and animal viral infections. Attenuation is achieved by genetically adapting viruses for replication in a different host species or under nonphysiological conditions, such that viruses lose their pathogenic potential in their original host species while remaining sufficiently antigenic to induce lasting protective immunity. Live-attenuated vaccines are highly efficacious due to the physiologic presentation of native antigen to the host''s immune system and include the earliest human vaccine developed by serial passages of rabies virus in rabbits. In very rare instances, one attenuated viral vaccine, the oral poliovirus vaccine (OPV), can accumulate mutations as well as recombine with other coinfecting enteroviruses and revert to a pathogenic state (18, 24). Attenuated live vaccines also carry a potential risk of contamination with adventitious viruses introduced during the attenuation process, from the cell lines used, and/or from the animal sera or other biologics often used in cell cultures. Very early Theiler''s yellow fever attenuated virus was once “stabilized” with human plasma thought to contain hepatitis B virus, resulting in many cases of hepatitis (5, 28). Some early Sabin poliovirus vaccines were contaminated with the simian virus 40 (SV40) polyomavirus from the monkey cells used to amplify polioviruses. While carcinogenic in rodents, SV40 has no epidemiologic association with human cancers (10). Avian leukosis virus (ALV) and endogenous avian virus (AEV) have been reported in attenuated vaccines grown in chicken embryo fibroblasts (CEF), but extensive testing has also ruled out human infections (14, 15). Vaccine-associated ALV and AEV are thought to originate from endogenous retroviruses in the chicken germ line (14, 15, 17).Because the chemical inactivation used in the manufacture of killed-virus vaccines is also likely to inactivate adventitious viruses, we focused on eight live-attenuated viruses, OPV (Biopolio), rubella (Meruvax-II), measles (Attenuvax), yellow fever (YF-Vax), human herpesvirus 3 (HHV-3) (Varivax), rotavirus (Rotarix and Rotateq), and multivalent measles/mumps/rubella (MMR-II), to resequence the attenuated viruses and test for the presence of adventitious viruses after viral particle purification, massively parallel pyrosequencing, and viral sequence similarity searches. Vaccine nucleic acids were also analyzed using a panmicrobial microarray.     

Engineering attenuated virus vaccines by controlling replication fidelity

Long-lasting protection against viral infection is best achieved by vaccination with attenuated viruses. Obtaining stably attenuated vaccine strains has traditionally been an empirical process, which greatly restricts the number of effective vaccines for viral diseases. Here we describe a rational approach for engineering stably attenuated viruses that can serve as safe and effective vaccines. Our approach exploits the observation that restricting viral population diversity by increasing replication fidelity greatly reduces viral tissue tropism and pathogenicity. We show that poliovirus variants with reduced genetic diversity elicit a protective immune response in an animal model of infection. Indeed, these novel vaccine candidates are comparable in efficacy to the currently available Sabin type 1 vaccine strain, but have the added advantage of being more stable, as their increased replication fidelity prevents reversion to the pathogenic wild-type phenotype. We propose that restricting viral quasispecies diversity provides a general approach for the rational design of stable, attenuated vaccines for a wide variety of viruses.     

Optimization of vaccine production for animal health

Vaccines on the basis of mammalian cell cultures are of major importance for human and animal health. Therefore efforts are undertaken for the improved production of more effective vaccines. Of course, the main purpose of all these approaches is to save lives and improve the quality of life for human beings. However, there is also some remarkable effort in the food industry and the associated animal production, especially in the case of some Flaviviridal viruses (BVD), where>80% of all cattle herds are found to be infected. These viruses can cause tremendous economic losses of calfs and embryos (Ames, 1990). Because of these facts, there is a continuous endeavour for improving the manufacturing of therapeutics or preventing agents such as vaccines for the treatment of cattle. The competitive economic situation and the specific market demands still require effective and high yield production methods, especially in the case of one of the most widespread viral diseases in cattle like BVD (Ames, 1990).We have succeeded in establishing an improved method for the production of BVD on the basis of a continuous fermentation mode, that consist of modifications of the corresponding process and media improvements.