Utilizing the virus-induced blocking of apoptosis in an easy baculovirus titration method

Rapid titration of recombinant baculoviruses based on NanoLuc secretion in early infection

The baculovirus expression vector system has become a powerful tool for recombinant protein production and gene delivery. However, existing titration methods for baculovirus are not economical in terms of test time and cost. A titration method based on NanoLuc secretion that allows for titration of recombinant baculoviruses at 4 h post infection (hpi) is described. In the assay, the envelop protein GP64 signal peptide-guided NanoLuc was secreted into the culture medium in proportion to the virus amount during early infection under combined control by the homologous region 5 (hr5) enhancer and the promoter of immediate early gene 1 (ie1) plus L21. Two timepoint standard curves of luciferase activity to virus titers of 5-9 logs were established with excellent linearity and correlation coefficients (slope = 1.050, R² ≥ 0.9969) using a secretory Nanoluc (secrNluc) - inserted standard baculovirus. Through the assay, the titers of three recombinant viruses prepared independently were calculated by directly measuring luciferase activity in the supernatant at 4 and 6 hpi, with greater accuracy compared to the endpoint dilution assay. These results show the efficacy of this proposed method as a streamlined assay for rapidly titrating recombinant baculoviruses.

Simple and rapid plaque assay for recombinant baculoviruses expressing influenza hemagglutinin

Recombinant baculoviruses (rBVs) have been extensively used to generate virus-like particles, and baculoviruses expressing antigenic proteins have become efficient tools for inducing protective immunity. However, current methods for generating baculoviruses are costly and inefficient. Thus, the development of a simple, rapid, and accurate method of baculovirus titration is critically important. We established a method of plaque assay using an immunostaining method by which plaques can be easily visualized in Sf9 cells under a light microscope. Sf9 cells were infected with recombinant baculoviruses expressing influenza hemagglutinin surface proteins from H1N1 (A/California/04/09) or rH5N1 (A/Vietnam/1203/04). The infected cells were incubated with anti-HA antibody and the plaques were visualized using the chromogen 3'3-diaminobenzidine (DAB). Plaques were observed from days 1 to 6 post-infection, and differences in Sf9 cell seeding densities resulted in variations in the final plaque quantification. Sf9 cells seeded at a concentration of 5.5 × 104 cells/well or 7.5 × 104 cells/well showed the higher plaque titers at days 3, 4, and 5 post-infection than those found at days 1, 2, and 6 post-infection. With 5.5 × 104 cells/well or 7.5 × 104 cells/well of cell concentrations, recombinant baculovirus for rBV-HA (H1N1) showed 6 × 107 pfu/ml of titer and rBVs for rBV-HA (rH5N1) showed 5.4 × 107 pfu/ml of titer. Three days of baculovirus incubation with a certain concentration of Sf9 cells seeded are required for a rapid, simple, and accurate plaque assay, which could significantly contribute to all baculovirus-related studies.

Image-based cell-size estimation for baculovirus quantification

Measurement of virus concentration is essential for effective virus-based transfection technologies. Here, we describe a user-friendly, image-based cell-size estimation (ICSE) assay for baculovirus quantification that relies on automated determination of cell diameters from bright-field microscopy images. In the ICSE assay, microplate-based imaging systems and our custom ICSE-Tools software enable measurement of cell morphological parameters over time. Results from the ICSE assay were in agreement with virus concentration measurements obtained using the traditional plaque assay as well as the Coulter principle–based cell-size measurement assay. ICSE-Tools is designed for data organization and image analysis from microplate-based imaging systems, and is freely available at www.gpcr.ut.ee/software.html .