Opinion on adventitious agents testing for vaccines: why do we worry so much about adventitious agents in vaccines?

Concentration of viruses and electron microscopy

Nearly all lethal viral outbreaks in the past two decades were caused by newly emerging viruses. Viruses are often studied by electron microscopy (EM), which provides new high-resolution data on the structure of viral particles relevant to both fundamental virology and practical pharmaceutical nanobiotechnology. Electron microscopy is also applied to ecological studies to detect viruses in the environment, to analysis of technological processes in the production of vaccines and other biotechnological components, and to diagnostics. Despite the advances in more sensitive methods, electron microscopy is still in active use for diagnostics. The main advantage of EM is the lack of specificity to any group of viruses, which allows working with unknown materials. However, the main limitation of the method is the relatively high detection limit (107 particles/mL), requiring viral material to be concentrated. There is no most effective universal method to concentrate viruses. Various combinations of methods and approaches are used depending on the virus and the goal. A modern virus concentration protocol involves precipitation, centrifugation, filtration, and chromatography. Here we describe the main concentrating techniques exemplified for different viruses. Effective elution techniques are required to disrupt the bonds between filter media and viruses in order to increase recovery. The paper reviews studies on unique traps, magnetic beads, and composite polyaniline and carbon nanotubes, including those of changeable size to concentrate viral particles. It also describes centrifugal concentrators to concentrate viruses on a polyethersulfone membrane. Our review suggests that the method to concentrate viruses and other nanoparticles should be chosen with regard to objectives of the study and the equipment status of the laboratory.

Use of tangential flow filtration for improving detection of viral adventitious agents in cell substrates

In this study, we assessed the feasibility of tangential flow filtration (TFF) for primary concentration of viral adventitious agents (AAs) from large volumes of cell substrate-derived samples, such as cell-free Chinese hamster ovary (CHO) culture supernatants (500 mL) and CHO cell lysates (50 mL), prior to virus detection in them by nucleic acid-based methods (i.e., qPCR and massively parallel sequencing (MPS). The study was conducted using the samples spiked with four model DNA viruses (bovine herpesvirus type 4, human adenovirus type 5, simian polyomavirus SV-40, and bovine parvovirus). The results showed that the combined TFF/MPS approach enables reliable detection of as low as 1000 genome equivalents (GE) of each of the four viruses spiked into the cell substrate samples. The final achieved sensitivities of 2 GE/mL for cell culture supernatant and 20 GE/mL for cell lysate make this approach more sensitive than virus-specific PCR and qPCR assays. The study results allowed us to propose that TFF might be useful and valuable method for simple and rapid concentration of potential AAs in cell substrate samples prior to AAs detection by conventional in vivo, in vitro, or molecular methods.

Systematic evaluation of in vitro and in vivo adventitious virus assays for the detection of viral contamination of cell banks and biological products

Viral vaccines and the cell substrates used to manufacture them are subjected to tests for adventitious agents, including viruses, contaminate. Some of the compendial methods (in vivo and in vitro in cell culture) were established in the mid-20th century. These methods have not been subjected to current assay validation, as new methods would need to be. This study was undertaken to provide insight into the breadth (selectivity) and sensitivity (limit of detection) of the routine methods, two such validation parameters. Sixteen viral stocks were prepared and characterized. These stocks were tested in serial dilutions by the routine methods to establish which viruses were detected by which methods and above what limit of detection. Sixteen out of sixteen viruses were detected in vitro, though one (bovine viral diarrhea virus) required special conditions to detect and another (rubella virus) was detected with low sensitivity. Many were detected at levels below 1 TCID50 or PFU (titers were established on the production cell line in most cases). In contrast, in vivo, only 6/11 viruses were detected, and 4 of these were detected only at amounts one or more logs above 1 TCID50 or PFU. Only influenza virus and vesicular stomatitis virus were detected at lower amounts in vivo than in vitro. Given the call to reduce, refine, or replace (3Rs) the use of animals in product safety testing and the emergence of new technologies for the detection of viruses, a re-examination of the current adventitious virus testing strategies seems warranted. Suggested pathways forward are offered.