How is viral titer determined in VectorBuilder?
After harvesting viral particles, if the viral vector carries a fluorescent reporter gene, we usually first check the quality of virus by transducing the virus into some common cell lines (e.g. 293T or 293A) to observe the expression of fluorescent protein. Different methods are then used to quantify the titer of virus depending on viral type. Occasionally, if there is a major discrepancy between fluorescence observation and quantitative measurement, we will perform re-measurement or additional validation to ensure that viruses manufactured by VectorBuilder are of high quality.
We use p24 Elisa for measuring lentivirus titer. This method employs a sandwich immunoassay to measure the levels of the HIV-1 p24 core protein in lentiviral supernatants. The lentivirus samples are first added to a microtiter plate, the wells of which are coated with an anti-HIV-1 p24 capture antibody, to bind the p24 in the lentivirus samples. This is followed by the addition of a biotinylated anti-p24 secondary antibody, which in turn binds to the p24 captured by the first antibody on the plate. A streptavidin-HRP conjugate is then added for binding the biotinylated anti-p24 antibody due to the interaction between streptavidin and biotin. A substrate solution is ultimately added to the samples which produces color upon interaction with HRP. The intensity of the colored product is proportional to the amount of p24 present in each lentivirus sample, which is measured by the use of a spectrophotometer and is then precisely quantified by comparing against a recombinant HIV-1 p24 standard curve. The p24 value is then correlated with the viral titer of the corresponding lentivirus sample.
For adenovirus, we also measure the functional titer. After transducing serially-diluted adenovirus into 293A cells, we use an immunocytochemistry-based approach to count the number of cells being successfully transduced via the detection of adenovirus-specific hexon protein, and each immunostained cell is considered as one infectious unit. Cells are infected at very low multiplicity of infection (MOI) to ensure that most transduced cells are each infected by a single viral particle. This assay shows good correlation with conventional plaque assay. For ultra-purified adenovirus, we directly measure the optical density (using OD260) of the viral particles to estimate titer, because there is a tight correlation between the optical density of ultra-purified adenovirus and functional titer. Adenovirus has very good stability. In our preparation, the viral particles are essentially all alive and can remain functional at room temperature for many days.
Adeno-associated virus (AAV)
We measure the physical titer of AAV by directly extracting viral genome from lysed viral particles, and then using qPCR to accurately quantify the copy number of viral genome (using the copy number of ITR region as a proxy) in the stock. AAV particles are very stable. In our AAV preparation, viral particles are essentially all alive and can remain functional at room temperature for many days. As such, the physical titer, though not measured in a way involving the transduction of cells, is very close to the functional titer.