Partial Reprogramming Exerts a Rejuvenating Effect on Human Mesenchymal Stem Cells That Underwent Replicative Senescence in Culture

Mesenchymal stem/stromal cells (MSCs) are becoming increasingly important for biomedical applications, such as cell therapy, disease modeling, and drug screening. At the same time, long-term cultivation, which is necessary to prepare a sufficient amount of cellular material for therapeutic and research purposes, is accompanied by the development of replicative senescence. Partial reprogramming emerged as a novel method that shows promising results in the rejuvenation of cells in vitro and in vivo; however, it has not yet been applied for human MSCs that have undergone replicative senescence in culture. In the present study, we subjected senescent human endometrial MSCs to partial reprogramming using Sendai virus vectors containing genes encoding Yamanaka transcription factors Oct4, Sox2, Klf4, and c-Myc. Characterization of the MSCs 5 days after transduction showed the loss of key markers of senescence: the youthful morphology was restored, the expression of senescent-associated β-galactosidase and the number of double-strand DNA breaks decreased, proliferation was activated, and the DNA damage response was enhanced. Further, using an in vitro wound-healing assay, we demonstrated that conditioned medium from partially reprogrammed MSCs showed higher therapeutic activity than that from senescent cells. However, a biosafety test revealed the presence of viral components in conditioned medium, which caused the agglutination of erythrocytes. Collectively, our data suggest that partial reprogramming is a potentially effective strategy for the rejuvenation of cultured MSCs in late passages but requires the use of virus-free protocols, such as chemical reprogramming.