The baculovirus-based protein expression system has gained increased prominence as a method for expressing recombinant proteins that are used in a wide range of biomedical applications. An important step in the use of this system is the ability to determine the virus infectious titer, i.e., the number of active baculovirus particles produced during an infection of the insect host cell. The current “gold standard” methods used for determining baculovirus titers, such as the plaque and end point dilution assays, can be costly, take a long time to complete (up to 7-8 days), and are sometimes difficult to interpret as they involve observing the cytopathic effects (CPE) that baculovirus infection has on the infected insect host cell.
To solve these problems, researchers at the National Cancer Institute (NCI) have developed a modified insect cell line, Sf9-ET, to genetically express the green fluorescent protein (GFP) when infected with baculovirus. In these cells, the gene for GFP is placed under the control of a baculovirus promoter so that the cells express GFP when they are infected with the virus. The baculovirus titer can then be quantitated from the level of GFP expression in the insect host cell. The results are obtained within 3 days compared to the 7-8 day period typical of the traditional CPE-based methods.
The GFP-based system is capable of replacing the traditional methods as it is faster, more accurate, and may be less expensive than the currently used systems. This proprietary technology can become an indispensable tool for the quantitation of baculovirus titers; a step that is important in the production of recombinant proteins and vaccine like particles (VLPs) for academic and commercial purposes.