# Rapid and Economic Baculovirus Titer Determination Using a Novel Transgenic Sf9-QE Cell Line

## TL;DR

A new method using a modified Sf9-QE cell line allows for quick and cost-effective measurement of baculovirus titers, speeding up recombinant protein production.

## Contribution

A novel transgenic Sf9-QE cell line enables rapid baculovirus titer determination through early fluorescence expression.

## Key findings

- Sf9-QE cells express fluorescence early after infection due to multiple EGFP transgene copies.
- Viral titers can be reliably quantified 15-30 hours post-infection using Sf9-QE cells.
- The method is rapid, cost-effective, and comparable to conventional titration techniques.

## Abstract

This study introduces a novel direct titration method for rapidly and economically determining baculovirus titers using a specially engineered Sf9-QE cell line. Sf9-QE transgenic cells were used because they express fluorescence early after viral infection, providing a prompt phenotype; this rapid expression was confirmed to be due to the integration of at least seven copies of the transgene into the cell genome. In addition, the direct titration method was validated using Sf9-QE cells in the three days after subculturing, demonstrating that efficient viral titer quantification is possible between 15 and 30 h after viral infection. Compared to conventional titration methods, similar titer values were obtained, indicating that the time required for protein production using recombinant baculoviruses can be significantly reduced. This new virus quantification method will be very useful in the production of and research into various medically and pharmaceutically useful recombinant proteins using insect viruses.

A baculovirus expression system (BES) for the production of recombinant proteins requires rapid and easy virus titer determination. In this study, a novel direct titration method was developed using a novel Sf9-QE cell line to easily and economically determine virus titers in a short time. This direct titration method can determine virus titers by directly counting the initially infected cells. This method requires the rapid identification of the initial virus-infected cells. The genome of Sf9-QE cells, which fluoresce upon virus infection, was found to contain at least seven copy numbers of the enhanced green fluorescent protein (EGFP) transgene. This result suggests that Sf9-QE cells in the early stages of virus infection can be identified by the high expression of EGFP. It was also shown that for accurate virus titration using the direct titration method, Sf9-QE cells should be used within 3 d of subculturing. Additionally, counting fluorescent cells to establish virus infection should be performed within 15 to 30 h after virus infection for reliable virus titration. The direct titration method using Sf9-QE cells provides a rapid, reliable, and cost-effective alternative for determining baculovirus titers in BES research.