Automated, label-free TCID₅₀ assay to determine the infectious titer of virus-based therapeutics

A robust and precise infectivity assay is a prerequisite for the development and market supply of virus-based biologics. Like other cell-based assays, traditional infectivity assays suffer from high variability and require extensive hands-on time. Therefore, a faster and more robust method to measure infectivity is needed to fulfill the requirements of a higher sample throughput and speed in drug development. We developed a label-free tissue culture infectious dose 50 (TCID₅₀) assay using automated image analysis that determines the cell confluence to discriminate between cytopathic effect-positive and -negative wells. In addition, we implemented semi-automated bench-top pipetting robots for the required pipetting steps to further shorten the hands-on time of the assay. The automated image analysis categorized >99 % of the wells similar as operators did via visual evaluation and there was a close correlation between the titers that were determined by using either the automated image analysis or visual evaluation (r² = 0.99). Thus, here we present a label-free TCID₅₀ method with a stable automated image analysis that is ∼3.6x faster and more standardized compared to the classical TCID₅₀ assay.