Viral aggregation: mixed suspensions of poliovirus and reovirus

Viral Aggregation: The Knowns and Unknowns

Viral aggregation is a complex and pervasive phenomenon affecting many viral families. An increasing number of studies have indicated that it can modulate critical parameters surrounding viral infections, and yet its role in viral infectivity, pathogenesis, and evolution is just beginning to be appreciated. Aggregation likely promotes viral infection by increasing the cellular multiplicity of infection (MOI), which can help overcome stochastic failures of viral infection and genetic defects and subsequently modulate their fitness, virulence, and host responses. Conversely, aggregation can limit the dispersal of viral particles and hinder the early stages of establishing a successful infection. The cost–benefit of viral aggregation seems to vary not only depending on the viral species and aggregating factors but also on the spatiotemporal context of the viral life cycle. Here, we review the knowns of viral aggregation by focusing on studies with direct observations of viral aggregation and mechanistic studies of the aggregation process. Next, we chart the unknowns and discuss the biological implications of viral aggregation in their infection cycle. We conclude with a perspective on harnessing the therapeutic potential of this phenomenon and highlight several challenging questions that warrant further research for this field to advance.

Comparative study on the inactivation of MS2 and M13 bacteriophages using energetic femtosecond lasers

Femtosecond (fs) laser irradiation techniques are emerging tools for inactivating viruses that do not involve ionizing radiation. In this work, the inactivation of two bacteriophages representing protective capsids with different geometric constraints, that is, the near-spherical MS2 (with a diameter of 27 nm) and the filamentous M13 (with a length of 880 nm) is compared using energetic visible and near-infrared fs laser pulses with various energies, pulse durations, and exposure times. Intriguingly, the results show that inactivation using 400 nm lasers is substantially more efficient for MS2 compared to M13. In contrast, using 800 nm lasers, M13 was slightly more efficiently inactivated. For both viruses, the genome was exposed to a harmful environment upon fs-laser irradiation. However, in addition to the protection of the genome, the metastable capsids differ in many properties required for stepwise cell entry that may explain their dissimilar behavior after (partial) disassembly. For MS2, the dominant mechanism of fs-laser inactivation was the aggregation of the viral capsid proteins, whereas aggregation did not affect M13 inactivation, suggesting that the dominant mechanism of M13 inactivation was related to breaking of secondary protein links.

Nanoparticle Tracking of Adenovirus by Light Scattering and Fluorescence Detection

The detailed characterization of biological nanoparticles is of paramount importance for various industrial sectors, as for production of viral therapeutics. More recently, technologies that allow real-time quantification with simultaneous sizing and determination of surface potentials of virus particles in solution have been developed. In this study, nanoparticle tracking analysis (NTA) was applied to determine the size and the zeta potential of human adenovirus type 5 (AdV5), one the most frequently used therapeutic/oncolytic agents and viral vectors. Virus aggregation was detected, and the kinetics of the dissolution of virus aggregates were studied in real time. In addition, advanced fluorescence detection of AdV5 was performed enabling the measurements in matrices and discrimination of viral subpopulations. It was shown that NTA is an efficient approach for investigating infectious viruses in a live viewing mode. Consequently, NTA provides a promising methodology for virus particle detection and analysis in real time beyond assays requiring nucleic acids or infectivity.

Why viruses sometimes disperse in groups?†

Many organisms disperse in groups, yet this process is understudied in viruses. Recent work, however, has uncovered different types of collective infectious units, all of which lead to the joint delivery of multiple viral genome copies to target cells, favoring co-infections. Collective spread of viruses can occur through widely different mechanisms, including virion aggregation driven by specific extracellular components, cloaking inside lipid vesicles, encasement in protein matrices, or binding to cell surfaces. Cell-to-cell viral spread, which allows the transmission of individual virions in a confined environment, is yet another mode of clustered virus dissemination. Nevertheless, the selective advantages of dispersing in groups remain poorly understood in most cases. Collective dispersal might have emerged as a means of sharing efficacious viral transmission vehicles. Alternatively, increasing the cellular multiplicity of infection may confer certain short-term benefits to viruses, such as overwhelming antiviral responses, avoiding early stochastic loss of viral components required for initiating infection, or complementing genetic defects present in different viral genomes. However, increasing infection multiplicity may also entail long-term costs, such as mutation accumulation and the evolution of defective particles or other types of cheater viruses. These costs and benefits, in turn, should depend on the genetic relatedness among collective infectious unit members. Establishing the genetic basis of collective viral dispersal and performing controlled experiments to pinpoint fitness effects at different spatial and temporal scales should help us clarify the implications of these spread modes for viral fitness, pathogenicity, and evolution.

Multi-virion infectious units arise from free viral particles in an enveloped virus

Many animal viruses are enveloped in a lipid bilayer taken up from cellular membranes. Because viral surface proteins bind to these membranes to initiate infection, we hypothesized that free virions may also be capable of interacting with the envelopes of other virions extracellularly. Here, we demonstrate this hypothesis in the vesicular stomatitis virus (VSV), a prototypic negative-strand RNA virus composed of an internal ribonucleocapsid, a matrix protein and an external envelope1. Using microscopy, dynamic light scattering, differential centrifugation and flow cytometry, we show that free viral particles can spontaneously aggregate into multi-virion infectious units. We also show that, following establishment of these contacts, different viral genetic variants are co-transmitted to the same target cell. Furthermore, virion-virion binding can determine key aspects of viral fitness such as antibody escape. In purified virions, this process is driven by protein-lipid interactions probably involving the VSV surface glycoprotein and phosphatidylserine. Whereas we found that multi-virion complexes occurred unfrequently in standard cell cultures, they were abundant in other fluids such as saliva, a natural VSV shedding route2. Our findings contrast with the commonly accepted perception of virions as passive propagules and show the ability of enveloped viruses to establish collective infectious units, which could in turn facilitate the evolution of virus-virus interactions and of social-like traits3.

Specific ion effects on the particle size distributions of cell culture-derived influenza A virus particles within the Hofmeister series

Virus particle (VP) aggregation can have serious implications on clinical safety and efficacy of virus-based therapeutics. Typically, VP are suspended in buffers to establish defined product properties. Salts used to achieve these properties show specific effects in chemical and biological systems in a reoccurring trend known as Hofmeister series (HS). Hofmeister series effects are ubiquitous and can affect colloidal particle systems. In this study, influences of different ions (anions: SO4 2-, HPO4 2-, Cl-, Br-, NO3 -, I-; cations: K+, Na+, Li+, Mg2+, Ca2+) on particle size distributions of cell culture-derived influenza VP were investigated. For the experimental setup, influenza virus A/Puerto Rico/8/34 (H1N1) VP produced in adherent and suspension Madin Darby canine kidney cells were used. Inactivated and concentrated virus harvests were dialyzed against buffers containing the ions of interest, followed by differential centrifugal sedimentation to measure particle size distributions. VP from both cell lines showed no aggregation over a wide range of buffers containing different salts in concentrations ≥60 mM. However, when dialyzed to low salt or Ca2+ buffers, VP produced in adherent cells showed increased aggregation compared to VP produced in suspension cells. Additionally, changes in VP diameters depending on specific ion concentrations were observed that partially reflected the HS trend.

Collective Infectious Units in Viruses

Increasing evidence indicates that viruses do not simply propagate as independent virions among cells, organs, and hosts. Instead, viral spread is often mediated by structures that simultaneously transport groups of viral genomes, such as polyploid virions, aggregates of virions, virion-containing proteinaceous structures, secreted lipid vesicles, and virus-induced cell-cell contacts. These structures increase the multiplicity of infection, independently of viral population density and transmission bottlenecks. Collective infectious units may contribute to the maintenance of viral genetic diversity, and could have implications for the evolution of social-like virus-virus interactions. These may include various forms of cooperation such as immunity evasion, genetic complementation, division of labor, and relaxation of fitness trade-offs, but also noncooperative interactions such as negative dominance and interference, potentially leading to conflict.

Rethinking the Significance of Reovirus in Water and Wastewater

The genus Orthoreovirus contains nonenveloped viruses with double-stranded gene segments encased in a double-layered icosahedral capsid shell. These features constitute major determinants of virion stability in the environment and virion resistance against physical and chemical agents. Reovirus (ReoV) is the general term most commonly used for all virus strains that infect humans and nonhuman animals. Several studies have demonstrated the frequent occurrence of ReoV in wastewaters and natural waters, including surface and ground waters from different geographical areas. Most of these studies have reported higher concentrations of ReoV than any other enteric virus analyzed. They are more commonly isolated in chlorine-disinfected wastewaters than other enteric viruses, and appear to survive longer in water. The ability of ReoV to form large aggregates, even with different types of enteric viruses (e.g., poliovirus) and their ability to undergo mechanisms of gene segment reassortment among different serotypes may also explain their greater stability. Different approaches have been applied for concentration of ReoV from water; however, the recovery efficiency of the filtration methods has not been fully evaluated. Recently, molecular methods for identification of ReoV strains and quantification of virus genome have been developed. Studies have shown that the overall detection sensitivity of ReoV RNA is enhanced through initial replication of infectious virions in cell culture. More studies are needed to specifically address unresolved issues about the fate and distribution of ReoV in the environment since this virus is not commonly included in virological investigations.

Quantification of Norwalk virus inocula: Comparison of endpoint titration and real-time reverse transcription-PCR methods

Human noroviruses (NoV) are the leading cause of epidemic acute gastroenteritis. In order to fully characterize features such as persistence and infectious dose, precise quantification of virus concentration is necessary. The purpose of this study was to compare two methods [endpoint titration RT-PCR and quantitative RT-PCR (RT-qPCR)] with respect to quantification of Norwalk virus (NV) in inocula made from purified stock suspensions of human fecal specimens. A full-length NV RNA transcript was developed to facilitate quantification using RT-qPCR and provided log linear detection in the range of 49-4.9 x 10(4) genome equivalent copies (GEC) per reaction. Endpoint titration RT-PCR was used to estimate PCR detection units, and RT-qPCR was used to estimate genome copies in two NV inocula (8fIIa and 8fIIb) used in previous human challenge studies. Overall, RT-qPCR was 1.1-1.6 log(10) more sensitive (lower detection limit) than endpoint titration RT-PCR when the same RNA release method, PCR primers and thermocycle program were used. These findings have important implications for many experimental interpretations, not the least of which is estimating the median infectious dose in human challenge studies.

Aggregation and surface properties of F-specific RNA phages: implication for membrane filtration processes

We report an experimental investigation of the electrokinetic properties and size variations of four F-specific bacteriophages of the types MS2, GA, Qbeta and SP (21-30 nm in diameter) over a broad range of pH values (1.5-7.5) and NaNO3 electrolyte concentrations (1-100 mM). The results obtained by dynamic light scattering show that the aggregation of SP and GA particles takes place over the whole range of pH and ionic strength conditions examined. For MS2 phages, the aggregation of MS2 particles is not observed for pH higher than the isoelectric point (pI) and large ionic strengths for which interparticular repulsive electrostatic interactions are however expected to be sufficiently screened. Aggregation of the MS2 phages, dispersed in 1 and 100 mM electrolyte concentration, occurs at pH 4, which basically corresponds to the pI as determined by electrophoresis measurements. The Qbeta particles suspended in solutions of low electrolyte concentrations aggregate at low pH values (pI approximately 3) and, unlike MS2, at large ionic strengths over the whole range of pH conditions considered in this study. These elements allow the determination of the hydrophobic sequence for the four phages SP approximately GA>Qbeta>MS2. Close inspection of the electrokinetic results reveals small to significant variations of the pI values-depending on the phage considered-with respect to the concentration of indifferent NaNO3 electrolyte. This indicates that features other than chemical and electrostatic in nature play a key role in determining the pI and more generally the electrophoretic mobility mu of viral particles. A qualitative interpretation is given and is based on the consideration of inner electro-osmotic flow within the isolated or aggregated particles. The impact of the flow properties within the particles is further in agreement with recent theoretical formalism developed for the electrokinetics of soft multiplayer particles, the phages analyzed here being some illustrative examples. The determination and qualitative interpretation of the surface properties of the viral particles as reported in the current study are commented within the context of water treatment especially concerning viral removal by membrane filtration processes.

Primary recovery and chromatographic purification of adeno-associated virus type 2 produced by baculovirus/insect cell system

Adeno-associated virus (AAV) is making its place in gene therapy applications; however, the industry is still facing obstacles in producing a large quantity of highly purified material for clinical studies. Insect cell technology can be used to produce AAV to meet the current demand. During the purification process it was observed that there was a reduced recovery of AAV produced in insect cells, Spodoptera frugiperda (Sf9). It was assumed that the formation of AAV agglomerates and the interaction of AAV with other cellular components were major contributors to this loss. After studying different systems of extraction a sequence of treatment for primary recovery of AAV from cell paste was developed. This sequence was necessary to reduce the AAV losses and to increase the recovery. The purification method avoided the use of ultracentrifugation and adopted chromatographic methods for the purification of AAV. Primary recovery, ion exchange chromatography and hydrophobic interaction chromatography gave an overall yield of 75% from the extracted AAV. The purification process was based on chromatographic methods; therefore, it can be scaled up. Although this method was developed for AAV type 2, it is believed that this method could be modified easily to purify other AAV serotypes.

A simple method for the evaluation of antiseptic and disinfectant virucidal activity

A simple method for evaluating the virucidal activity of water-soluble antiseptic and disinfectant products using poliovirus type 1 (Sabin strain) is described. Using a commercial concentrator, kinetic studies of four products were carried out. It was shown that 2% glutaraldehyde and 5% povidone iodine are rapidly virucidal. 0.2% glutaraldehyde and 2.5% noxythiolin (37 degrees C) are considerably less effective. 0.05% chlorhexidine digluconate has no virucidal activity.

Diagnostic virology using electron microscopic techniques

This chapter illustrates the development of the use of electron microscopy in viral diagnosis. The field covered is confined to medical viral diagnosis, but parallel developments have taken place in both veterinary and botanical fields and techniques derived from both these sources are also included where relevant. It is reported that the scanning transmission mode of operation, which can induce image contrast changes electronically, may enhance studies with unstained sections and perhaps facilitate thin section immune electron microscopy (IEM). The application of negative stain IEM has been particularly useful for the study of the antigenic nature of some of the newly discovered noncultivable viruses. Viral antigens can also be detected in thin sections of infected cells by IEM with suitably labeled specific antibodies. Confirmation of viral infection by electron microscopy on tissues originally processed for light microscopy is also frequently useful.