Detection of Baculovirus Titer: The "Activity Scale" and Key Quality Control in Biological Product Production

I. Introduction

II. Technical Background of Baculovirus Titer Assay

(I) Necessity of Titer Assay

Recombinant Virus Screening: Verify the activity of purified recombinant viruses to eliminate defective viruses;

Virus Amplification: Monitor the proliferation efficiency of BVs to ensure the titer reaches 10⁶–10⁸ PFU/mL, meeting the needs of large-scale infection;

Cell Infection: Calculate the multiplicity of infection (MOI) based on titer—an excessively high MOI causes premature cell lysis, while an excessively low MOI reduces protein expression efficiency. Precise control of this ratio is critical for process stability.

(II) Correlation Between Host Cells and Assay

Sf9 Cells: Highly sensitive to AcNPV, with regular morphology during adherent growth, making them the first choice for virus amplification and titer assay;

Sf21 Cells: Larger in diameter than Sf9 cells, with more observable plaques, suitable for refined detection;

Hi5 Cells: More suitable for the expression of secretory proteins, but less sensitive to baculoviruses, so they are rarely used in titer assays.

III. Common Methods for Baculovirus Titer Assay

(I) Plaque Assay: The "Gold Standard"

Seed logarithmic-phase Sf9 cells (2×10⁵ cells/mL) into plates and incubate for 4–6 hours to form a monolayer;

Perform 10-fold serial dilution of the virus solution (10⁻²–10⁻⁸), with 3 replicate wells per dilution. Add 100–200 μL of the diluted virus solution to each well and incubate at 37°C for 1 hour;

Aspirate the virus solution, add medium containing low-melting-point agarose (at 40°C), and incubate at 27°C for 5–7 days after solidification;

Count the dilution with 10–100 plaques and calculate the titer using the formula: "Titer = Number of plaques × Dilution factor / Infection volume."

(II) End-Point Dilution Assay: A Semi-Quantitative Alternative

Seed Sf21 cells into 96-well plates (1×10⁴ cells/well) and incubate for 24 hours to allow adherence;

Perform 2-fold or 10-fold serial dilution of the virus solution (10⁻¹–10⁻¹²), with 8–12 replicate wells per dilution. Add 100 μL of the diluted virus solution to each well;

Incubate at 27°C for 7–10 days and record the number of wells with CPE;

Calculate TCID₅₀ using the Reed-Muench method and convert it using the relationship "1 TCID₅₀ ≈ 0.7 PFU."

(III) Rapid Detection Technologies: Efficiency-Upgraded Solutions

Immunofluorescence Assay (IFA): At 24–48 hours post-infection, use antibodies against viral structural proteins (e.g., anti-Polh antibodies) combined with fluorescent secondary antibodies to count positive cells and infer titer. The cycle is shortened to 2–3 days with high sensitivity, but it requires specific antibodies and has high costs;

Quantitative Real-Time PCR (qPCR): Detect the copy number of conserved viral genes (e.g., Polh, p10) and obtain the infectious titer using a conversion factor (1×10⁶ copies ≈ 10³–10⁴ PFU). It can be completed within 6 hours with high throughput, but it cannot distinguish between infectious and non-infectious viruses and needs verification with traditional methods.

IV. Application of Titer Assay in Biopharmaceutical Quality Control

(I) Production Process Control

rAAV Vector Production: The BV titer needs to reach above 10⁷ PFU/mL to achieve efficient packaging in Sf9 cells (yield: 10¹⁴ VG/L);

Subunit Vaccine Production: For example, in the production of FluBlok®, MOI (5–10) is controlled based on titer to ensure efficient expression of hemagglutinin (HA) protein in Sf9 cells;

Recombinant Protein Production: Adjust the infection time according to titer to avoid premature cell lysis affecting protein secretion.

(II) Host Residue Risk Management

V. Summary and Outlook