Satoshi Yasuda

Viral safety is an extremely important issue for the clinical application of cell therapy products. The viral contamination of products can lead to serious adverse events in patients treated with them. Therefore, it is strongly recommended to evaluate the viral safety of products using the most advanced science and technology available at the time, based on scientific rationality. We are also obligated to respond to society’s strong interest in and social demand for public health preservation. Because various types of cell therapy products have been developed, some measures to ensure viral safety for each product are common, and others should be addressed on a case-by-case basis. This chapter outlines the basic approach for viral safety confirmation for cell therapy products and introduces our recent work on viral safety using the next-generation sequencer.

The presence of malignantly transformed cells in human cell-based therapeutic products (hCTPs) is a significant safety concern. Although such cellular impurities in hCTPs can be assessed by detecting anchorage-independent growth using conventional soft agar colony formation (SACF) assays, the sensitivity of these assays is often insufficient. To overcome this limitation, we previously developed a novel tumorigenicity-associated testing method, the digital SACF (D-SACF) assay, which combines a partitioned culture of test cells to concentrate target cells with colony detection via image analysis. However, conventional soft agar culture involves complicated operations, such as preparing multilayered culture media and temperature control, and further technical optimization is required for the widespread adoption of the D-SACF assay. In this study, we focused on a new culture system incorporating a three-dimensional (3D) culture method using a liquid medium containing the low-molecular-weight agar polymer LA717 in low-adhesion culture vessels. We initially confirmed conditions for the efficient high-density 3D culture of normal cells using LA717-supplemented medium in low-adhesion 96-well plates. Using human mesenchymal stem/stromal cells (MSCs) as a normal cell model and HeLa cells as a transformed cell model, we demonstrated that the new 3D culture system effectively maintained the dispersion of MSCs and prevented their aggregation, while transformed HeLa cells exhibited robust anchorage independence, thereby establishing the new liquid/low-molecular-weight agar colony formation (LACF) method as an alternative to SACF. Finally, by systematizing the digital analysis system for the LACF assay (D-LACF assay), which streamlines the overall workflow from the performance evaluation of the test method to product testing and result interpretation, the limitations of the conventional soft agar-based D-SACF assay were addressed and its practicality and utility were enhanced. This in vitro evaluation system is expected to provide a promising approach for improving the quality and safety of hCTPs.

The presence of residual undifferentiated pluripotent stem cells (PSCs) in PSC-derived cell therapy products (CTPs) is a major safety issue for their clinical application, due to the potential risk of PSC-derived tumor formation. An international multidisciplinary multisite study to evaluate a droplet digital PCR (ddPCR) approach to detect residual undifferentiated PSCs in PSC-derived CTPs was conducted as part of the Health and Environmental Sciences Institute Cell Therapy-TRAcking, Circulation & Safety Technical Committee. To evaluate the use of ddPCR in quantifying residual iPSCs in a cell sample, different quantities of induced pluripotent stem cells (iPSCs) were spiked into a background of iPSC-derived cardiomyocytes (CMs) to mimic different concentrations of residual iPSCs. A one step reverse transcription ddPCR (RT-ddPCR) was performed to measure mRNA levels of several iPSC-specific markers and to evaluate the assay performance (precision, sensitivity, and specificity) between and within laboratories. The RT-ddPCR assay variability was initially assessed by measuring the same RNA samples across all participating facilities. Subsequently, each facility independently conducted the entire process, incorporating the spiking step, to discern the parameters influencing potential variability. Our results show that a RT-ddPCR assay targeting ESRG, LINC00678, and LIN28A genes offers a highly sensitive and robust detection of impurities of iPSC-derived CMs and that the main contribution to variability between laboratories is the iPSC-spiking procedure, and not the RT-ddPCR. The RT-ddPCR assay would be generally applicable for tumorigenicity evaluation of PSC-derived CTPs with appropriate marker genes suitable for each CTP.