Quantitative measurement of varicella-zoster virus infection by semiautomated flow cytometry

Viral Infectivity Quantification and Neutralization Assays Using Laser Force Cytology

Despite the widespread need to assess cell-based viral infectivity during vaccine development and production, as well as viral clearance monitoring and adventitious agent testing for viral safety, traditional methods, including the end-point dilution assay (TCID50) and viral plaque assay, are slow, labor-intensive, and can vary depending upon the skill and experience of the user. LumaCyte's Radiance^® instrument uses Laser Force Cytology^TM (LFC), a combination of advanced optics and microfluidics, to rapidly analyze the viral infectivity of cell populations in a quantitative fashion. LFC applies optical and fluidic forces to single cells in order to measure their intrinsic biophysical and biochemical properties without the use of stains, antibodies or fluorescent labels. These properties, including refractive index, change with a wide variety of biological phenomena, including viral infection, cell differentiation, activation, size, and cytoskeletal stiffness. Here, we present the experimental design and methods to use LFC data to facilitate rapid and robust infectivity measurements for a variety of applications including initial titer measurement (TCID50 replacement), in-process infectivity (e.g., bioreactor monitoring), and viral neutralization (PRNT replacement).

Rapid quantification of vesicular stomatitis virus in Vero cells using Laser Force Cytology

The ability to rapidly and accurately determine viral infectivity can help improve the speed of vaccine product development and manufacturing. Current methods to determine infectious viral titers, such as the end-point dilution (50% tissue culture infective dose, TCID50) and plaque assays are slow, labor intensive, and often subjective. In order to accelerate virus quantification, Laser Force Cytology (LFC) was used to monitor vesicular stomatitis virus (VSV) infection in Vero (African green monkey kidney) cells. LFC uses a combination of optical and fluidic forces to interrogate single cells without the use of labels or antibodies. Using a combination of variables measured by the Radiance™ LFC instrument (LumaCyte), an infection metric was developed that correlates well with the viral titer as measured by TCID50 and shortens the timeframe from infection to titer determination from 3 days to 16 h (a 4.5 fold reduction). A correlation was also developed between in-process cellular measurements and the viral titer of collected supernatant, demonstrating the potential for real-time infectivity measurements. Overall, these results demonstrate the utility of LFC as a tool for rapid infectivity measurements throughout the vaccine development process.

Titration of cell-associated varicella-zoster virus with the MV9G reporter cell line for antiviral studies

Titration of the cell-associated virus (CAV) of varicella-zoster virus (VZV) is essential for antiviral studies. A VZV reporter cell line, MV9G, generated in our previous study expresses firefly luciferase upon CAV infection in a dose-dependent manner, suggesting that use of the cell line for titration is feasible. In this study, MeWo cells infected with VZV vaccine Oka (vOka) strain or with clinical isolates obtained from patients with varicella or zoster were used as CAV. A co-culture of MV9G cells with the virus-infected MeWo cells were set up and optimized for titration of CAV. Luciferase activities of MV9G cells measured as relative light units (RLUs) of chemiluminescence correlated well (r > 0.9, p < 0.05) both with quantities of viral DNAs measured by TaqMan PCR and with numbers of viral foci detected by immunostaining with a monoclonal antibody against VZV IE62. In addition, the usefulness of MV9G for antiviral studies was exemplified by treatment of the VZV-infected cells with various concentrations of acyclovir. Thus, the reporter cell-based titration of CAV by measuring the induced RLUs may be a reliable way to estimate viral foci and viral DNAs.

Identification of a varicella-zoster virus replication inhibitor that blocks capsid assembly by interacting with the floor domain of the major capsid protein

A novel anti-varicella-zoster virus compound, a derivative of pyrazolo[1,5-c]1,3,5-triazin-4-one (coded as 35B2), was identified from a library of 9,600 random compounds. This compound inhibited both acyclovir (ACV)-resistant and -sensitive strains. In a plaque reduction assay under conditions in which the 50% effective concentration of ACV against the vaccine Oka strain (V-Oka) in human fibroblasts was 4.25 μM, the 50% effective concentration of 35B2 was 0.75 μM. The selective index of the compound was more than 200. Treatment with 35B2 inhibited neither immediate-early gene expression nor viral DNA synthesis. Twenty-four virus clones resistant to 35B2 were isolated, all of which had a mutation(s) in the amino acid sequence of open reading frame 40 (ORF40), which encodes the major capsid protein (MCP). Most of the mutations were located in the regions corresponding to the "floor" domain of the MCP of herpes simplex virus 1. Treatment with 35B2 changed the localization of MCP in the fibroblasts infected with V-Oka but not in the fibroblasts infected with the resistant clones, although it did not affect steady-state levels of MCP. Overexpression of the scaffold proteins restored the normal MCP localization in the 35B2-treated infected cells. The compound did not inhibit the scaffold protein-mediated translocation of MCP from the cytoplasm to the nucleus. Electron microscopic analysis demonstrated the lack of capsid formation in the 35B2-treated infected cells. These data indicate the feasibility of developing a new class of antivirals that target the herpesvirus MCPs and inhibit normal capsid formation by a mechanism that differs from those of the known protease and encapsidation inhibitors. Further biochemical studies are required to clarify the precise antiviral mechanism.

Rapid and accurate determination of the potency of varicella vaccine by quantitative polymerase chain reaction

The potency of varicella vaccines is currently determined by a plaque assay technique, which usually takes seven days and is laborious and has considerable inter- and intra-assay variability. Here, we report a new potency assay for varicella vaccine based on quantitative polymerase chain reaction in conjunction with a much more efficient virus infection step. Potency results can be obtained within 24h of infection and demonstrates acceptable accuracy and reproducibility when compared with the plaque assay, which relies on manual counting of plaques formed one week after viral infection. Using multiple vaccine lots from 7 manufacturers, we found no significant difference in infectivity determined between the new assay and plaque assay. The optimized conditions for viral infection and polymerase chain reaction are of significant value for the potency determination of the vaccine due to its rapidity, accuracy and the high throughput capacity of the assay.

Rapid titration of measles and other viruses: optimization with determination of replication cycle length

Background

Measles virus (MV) is a member of the Paramyxoviridae family and an important human pathogen causing strong immunosuppression in affected individuals and a considerable number of deaths worldwide. Currently, measles is a re-emerging disease in developed countries. MV is usually quantified in infectious units as determined by limiting dilution and counting of plaque forming unit either directly (PFU method) or indirectly from random distribution in microwells (TCID50 method). Both methods are time-consuming (up to several days), cumbersome and, in the case of the PFU assay, possibly operator dependent.

Methods/Findings

A rapid, optimized, accurate, and reliable technique for titration of measles virus was developed based on the detection of virus infected cells by flow cytometry, single round of infection and titer calculation according to the Poisson's law. The kinetics follow up of the number of infected cells after infection with serial dilutions of a virus allowed estimation of the duration of the replication cycle, and consequently, the optimal infection time. The assay was set up to quantify measles virus, vesicular stomatitis virus (VSV), and human immunodeficiency virus type 1 (HIV-1) using antibody labeling of viral glycoprotein, virus encoded fluorescent reporter protein and an inducible fluorescent-reporter cell line, respectively.

Conclusion

Overall, performing the assay takes only 24–30 hours for MV strains, 12 hours for VSV, and 52 hours for HIV-1. The step-by-step procedure we have set up can be, in principle, applicable to accurately quantify any virus including lentiviral vectors, provided that a virus encoded gene product can be detected by flow cytometry.

FlowFP: A Bioconductor Package for Fingerprinting Flow Cytometric Data

A new software package called flowFP for the analysis of flow cytometry data is introduced. The package, which is tightly integrated with other Bioconductor software for analysis of flow cytometry, provides tools to transform raw flow cytometry data into a form suitable for direct input into conventional statistical analysis and empirical modeling software tools. The approach of flowFP is to generate a description of the multivariate probability distribution function of flow cytometry data in the form of a “fingerprint.” As such, it is independent of a presumptive functional form for the distribution, in contrast with model-based methods such as Gaussian Mixture Modeling. FlowFP is computationally efficient and able to handle extremely large flow cytometry data sets of arbitrary dimensionality. Algorithms and software implementation of the package are described. Use of the software is exemplified with applications to data quality control and to the automated classification of Acute Myeloid Leukemia.