Quantitative Measurement of Varicella-Zoster Virus Infection by Semiautomated Flow Cytometry

Abstract

Varicella-zoster virus (VZV; human herpesvirus 3) is the etiological cause of chickenpox and, upon reactivation from latency, zoster. Currently, vaccines are available to prevent both diseases effectively. A critical requirement for the manufacturing of safe and potent vaccines is the measurement of the biological activity to ensure proper dosing and efficacy, while minimizing potentially harmful secondary effects induced by immunization. In the case of live virus-containing vaccines, such as VZV-containing vaccines, biological activity is determined using an infectivity assay in a susceptible cellular host in vitro. Infectivity measurements generally rely on the enumeration of plaques by visual inspection of an infected cell monolayer. These plaque assays are generally very tedious and labor intensive and have modest throughput and high associated variability. In this study, we have developed a flow cytometry assay to measure the infectivity of the attenuated vaccine strain (vOka/Merck) of VZV in MRC-5 cells with improved throughput. The assay is performed in 96-well tissue culture microtiter plates and is based on the detection and quantification of infected cells expressing VZV glycoproteins on their surfaces. Multiple assay parameters have been investigated, including specificity, limit of detection, limit of quantification, range of linear response, signal-to-noise ratio, and precision. This novel assay appears to be in good concordance with the classical plaque assay results and therefore provides a viable, higher-throughput alternative to the plaque assay.Varicella-zoster virus (VZV; human herpesvirus 3) is a member of the Alphaherpesvirinae family. It is the etiological cause of two distinct and common diseases in humans: chickenpox and zoster. Exposure of immunologically naïve individuals to VZV results in chickenpox, a condition typically occurring during the first two decades of life. Chickenpox is usually a mild disease, although severe complications have been reported, especially in immune-compromised individuals or patients suffering from hematopoietic malignancies (29, 31). Resolution of the primary infection does not result in complete elimination of the virus, which subsists in a latent stage in sensory neural ganglia, despite sustained cellular and humoral immunity (1). This latent stage can be maintained for the remainder of the individual's life span. VZV reactivation from latency causes the symptoms of zoster which can be associated with severe and debilitating pain. A significant fraction of patients (up to 20%) will eventually suffer from long-term chronic neuralgia (postherpetic neuralgia) due to permanent nerve damage. The causes of reactivation are not fully understood, but a combination of fatigue, stress, and a declining level of cell-mediated immunity seems to be implicated. Indeed, there is a strong link between the rate of clinical reactivation and the increase in age of the affected patients (8).Several pediatric live attenuated vaccine formulations, which have proven very efficac...

Rapid Titration of Measles and Other Viruses: Optimization with Determination of Replication Cycle Length

“…Furthermore, it could be easily automated [31]. Titers obtained through this technique correlate significantly with those determined by the classical TCID50 method but with higher accuracy and a 3-fold lower range of confidence limit.…”

Approach to a highly-virulent emerging viral epidemic: A thought experiment and literature review

“…One may also predict vaccine candidates by their chemical properties; good vaccine candidates are typically large, complex peptides that are highly dissimilar to known human antigens [17] . A more direct approach would be to use in-vitro and in-vivo virulence screening assays; delete or mutate various viral components and introduce modified virus to cultured cells and quantify viral infectivity using a plaque assay or flow cytometry [82] . The same principle can be applied in-vivo by introduction to animal models.…”

Clinical Flow Cytometry - Emerging Applications

“…Recently Gates et al (2009) introduced a semi-automated flow cytometry protocol to quantitative measurement of Varicella-Zoster Virus Infection. They describe an alternate infectivity assay for the attenuated VZV strain, based on the enumeration of infected cells 24 to 72 h post-infection by semi-automated capillary flow cytometry.…”

“…Measurement of VZV infection using flow cytometry was made considering FSC (x axis) and red fluorescence intensity channel (RED; y axis) correlation dot plot showing selective gating on 7-AAD-negative events (live cells and non nucleated debris) and tested monoclonal antibodies which are directed at both immediate early/early genes (IE62) and late, structural proteins (VZV gE, gI, gB, and gH and MCP) and were titrated to measure mean fluorescence intensities at saturation (Gates, 2009). Lemos et al (2007) developed a flow cytometry-based methodology to detect anti-Leishmania (Leishmania) chagasi immunoglobulin G as a reliable serological approach to monitor posttherapeutical cure in patients affected by Visceral Leishmaniasis.…”