Meta-analysis of the Mesenchymal Stem Cells Immortalization Protocols: A Guideline for Regenerative Medicine

BACKGROUND

This systematic review describes the most common methodologies for immortalizing human and animal mesenchymal stem cells (MSCs). This study follows the rules of PRISMA and is registered in the Institutional Review Board of PROSPERO International of systematic reviews, numbered protocol code: CRD42020202465.

METHOD

The data search systematization was based on the words "mesenchymal stem cell" AND "immortalization." The search period for publications was between 2000 and 2022, and the databases used were SCOPUS, PUBMED, and SCIENCE DIRECT. The search strategies identified 384 articles: 229 in the SCOPUS database, 84 in PUBMED, and 71 in SCIENCE DIRECT. After screening by titles and abstracts, 285 articles remained. This review included thirty-nine articles according to the inclusion and exclusion criteria.

RESULT

In 28 articles, MSCs were immortalized from humans and 11 animals. The most used immortalization methodology was viral transfection. The most common immortalized cell type was the MSC from bone marrow, and the most used gene for immortalizing human and animal MSCs was hTERT (39.3%) and SV40T (54.5%), respectively.

CONCLUSION

Also, it was observed that although less than half of the studies performed tumorigenicity assays to validate the immortalized MSCs, other assays, such as qRT-PCR, colony formation in soft agar, karyotype, FISH, and cell proliferation, were performed in most studies on distinct MSC cell passages.

Characterization of in vivo tumorigenicity tests using severe immunodeficient NOD/Shi-scid IL2Rγnull mice for detection of tumorigenic cellular impurities in human cell-processed therapeutic products

The contamination of human cell-processed therapeutic products (hCTPs) with tumorigenic cells is one of the major concerns in the manufacturing and quality control of hCTPs. However, no quantitative method for detecting the tumorigenic cellular impurities is currently standardized. NOD/Shi-scid IL2Rγnull (NOG) mice have shown high xeno-engraftment potential compared with other well-known immunodeficient strains, e.g. nude mice. Hypothesizing that tumorigenicity test using NOG mice could be a sensitive and quantitative method to detect a small amount of tumorigenic cells in hCTPs, we examined tumor formation after subcutaneous transplantation of HeLa cells, as a model of tumorigenic cells, in NOG mice and nude mice. Sixteen weeks after inoculation, the 50% tumor-producing dose (TPD50) values of HeLa cells were stable at 1.3 × 104 and 4.0 × 105 cells in NOG and nude mice, respectively, indicating a 30-fold higher sensitivity of NOG mice compared to that of nude mice. Transplanting HeLa cells embedded with Matrigel in NOG mice further decreased the TPD50 value to 7.9 × 10 cells, leading to a 5000-fold higher sensitivity, compared with that of nude mice. Additionally, when HeLa cells were mixed with 106 or 107 human mesenchymal stem cells as well as Matrigel, the TPD50 values in NOG mice were comparable to those of HeLa cells alone with Matrigel. These results suggest that the in vivo tumorigenicity test using NOG mice with Matrigel is a highly sensitive and quantitative method to detect a trace amount of tumorigenic cellular impurities in human somatic cells, which can be useful in the quality assessment of hCTPs.