Viral titer is an essential assay for researchers studying infectious disease, pathogenesis, vaccine development, even cell and gene therapy. The main goal is to determine the appropriate viral concentrations to be utilized in downstream assays such as antibody neutralization, ELISA-based neutralization, antiviral therapeutic discovery, cytopathic effect (CPE), or simply lentiviral transduction for gene and cell therapy.
Specifically for virology research, infectious viral titers are critical for clinical research and testing. The traditional measurement of viral titers is extremely laborious, subjective and the data points are often limited due to manual experimentation.
What is an infectious viral titer assay?
There are multiple methods to measure the amount of virus in the sample, such as Real-Time (RT) PCR, Western Blot, ELISA, and flow cytometry. These methods utilize the amount of viral DNA, RNA, or proteins to quantify the virus. However, these methods do not measure the actual biological activities of the virus. In order to investigate the infectivity of the virus, in vitro assays are required to assess the neutralizing ability of antibody candidates as well as the effectiveness of antiviral small molecule drugs. An infectious viral titer assay is conducted to determine the strength of a virus against the host cells.
How to measure infectious viral titer?
Viral infectivity is a critical characteristic of an infectious virus, which shows the ability of the virus to infect and replicate in host cells. In order to determine viral infectivity, there are two categories of assays that can be performed. In the first category, are quantitative assays that include plaque formation assays (PFA) and focus formation assays (FFA), which can quantify viral particles. The second category contains end-point dilution assays that include 50% tissue culture infective dose (TCID50), lethal dose (LD50), and egg infective dose (EID50).
It is common practice to perform a serial dilution of the viral stock, infect host cells, and measure the infectivity and viral titer using a variety of methods. Traditionally, viral titers are quantified by the number of formed plaques, foci, or individually infected cells. The plaque formation assay (PFA) and focus formation assay (FFA) are typically counted with the naked eye or microscopy (light/fluorescence). Individual infected cells can be measured by a conventional flow cytometer.