Genetic Stability of Mesenchymal Stromal Cells for Regenerative Medicine Applications: A Fundamental Biosafety Aspect

Mesenchymal stem/stromal cells (MSC) show widespread application for a variety of clinical conditions; therefore, their use necessitates continuous monitoring of their safety. The risk assessment of mesenchymal stem cell-based therapies cannot be separated from an accurate and deep knowledge of their biological properties and in vitro and in vivo behavior. One of the most relevant safety issues is represented by the genetic stability of MSCs, that can be altered during in vitro manipulation, frequently required before clinical application. MSC genetic stability has the potential to influence the transformation and the therapeutic effect of these cells. At present, karyotype evaluation represents the definitely prevailing assessment of MSC stability, but DNA alterations of smaller size should not be underestimated. This review will focus on current scientific knowledge about the genetic stability of mesenchymal stem cells. The techniques used and possible improvements together with regulatory aspects will also be discussed.

Platelet-derived growth factor receptor beta identifies mesenchymal stem cells with enhanced engraftment to tissue injury and pro-angiogenic property

Mesenchymal stem cells (MSCs) are heterogeneous likely consisting of subpopulations with various therapeutic potentials. Here we attempted to acquire a subset of MSCs with enhanced effect in wound healing. We found that human placental MSCs expressing platelet-derived growth factor (PDGF) receptor (PDGFR)-β exhibited greater proliferation rates and generated more colony-forming unit-fibroblast (CFU-F), compared to PDGFR-β- MSCs. Notably, PDGFR-β+ MSCs expressed higher levels of pro-angiogenic factors such as Ang1, Ang2, VEGF, bFGF and PDGF. When 106 GFP-expressing MSCs were topically applied into excisional wounds in mice, PDGFR-β+ MSCs actively incorporated into the wound tissue, resulting in enhanced engraftment (3.92 ± 0.31 × 105 remained in wound by 7 days) and accelerated wound closure; meanwhile, PDGFR-β- MSCs tended to remain on the top of the wound bed with significantly fewer cells (2.46 ± 0.26 × 105) engrafted into the wound, suggesting enhanced chemotactic migration and engraftment of PDGFR-β+ MSCs into the wound. Real-Time PCR and immunostain analyses revealed that the expression of PDGF-B was upregulated after wounding; transwell migration assay showed that PDGFR-β+ MSCs migrated eightfold more than PDGFR-β- MSCs toward PDGF-BB. Intriguingly, PDGFR-β+ MSC-treated wounds showed significantly enhanced angiogenesis compared to PDGFR-β- MSC- or vehicle-treated wounds. Thus, our results indicate that PDGFR-β identifies a subset of MSCs with enhanced chemotactic migration to wound injury and effect in promoting angiogenesis and wound healing, implying a greater therapeutic potential for certain diseases.

Downregulation of Melanoma Cell Adhesion Molecule (MCAM/CD146) Accelerates Cellular Senescence in Human Umbilical Cord Blood-Derived Mesenchymal Stem Cells

Therapeutic applications of mesenchymal stem cells (MSCs) for treating various diseases have increased in recent years. To ensure that treatment is effective, an adequate MSC dosage should be determined before these cells are used for therapeutic purposes. To obtain a sufficient number of cells for therapeutic applications, MSCs must be expanded in long-term cell culture, which inevitably triggers cellular senescence. In this study, we investigated the surface markers of human umbilical cord blood-derived MSCs (hUCB-MSCs) associated with cellular senescence using fluorescence-activated cell sorting analysis and 242 cell surface-marker antibodies. Among these surface proteins, we selected the melanoma cell adhesion molecule (MCAM/CD146) for further study with the aim of validating observed expression differences and investigating the associated implications in hUCB-MSCs during cellular senescence. We observed that CD146 expression markedly decreased in hUCB-MSCs following prolonged in vitro expansion. Using preparative sorting, we found that hUCB-MSCs with high CD146 expression displayed high growth rates, multilineage differentiation, expression of stemness markers, and telomerase activity, as well as significantly lower expression of the senescence markers p16, p21, p53, and senescence-associated β-galactosidase, compared with that observed in hUCB-MSCs with low-level CD146 expression. In contrast, CD146 downregulation with small interfering RNAs enhanced the senescence phenotype. In addition, CD146 suppression in hUCB-MSCs caused downregulation of other cellular senescence regulators, including Bmi-1, Id1, and Twist1. Collectively, our results suggest that CD146 regulates cellular senescence; thus, it could be used as a therapeutic marker to identify senescent hUCB-MSCs.

SIGNIFICANCE

One of the fundamental requirements for mesenchymal stem cell (MSC)-based therapies is the expansion of MSCs during long-term culture because a sufficient number of functional cells is required. However, long-term growth inevitably induces cellular senescence, which potentially causes poor clinical outcomes by inducing growth arrest and the loss of stem cell properties. Thus, the identification of markers for evaluating the status of MSC senescence during long-term culture may enhance the success of MSC-based therapy. This study provides strong evidence that CD146 is a novel and useful marker for predicting senescence in human umbilical cord blood-derived MSCs (hUCB-MSCs), and CD146 can potentially be applied in quality-control assessments of hUCB-MSC-based therapy.

Ultra-sensitive detection of tumorigenic cellular impurities in human cell-processed therapeutic products by digital analysis of soft agar colony formation

Contamination with tumorigenic cellular impurities is one of the most pressing concerns for human cell-processed therapeutic products (hCTPs). The soft agar colony formation (SACF) assay, which is a well-known in vitro assay for the detection of malignant transformed cells, is applicable for the quality assessment of hCTPs. Here we established an image-based screening system for the SACF assay using a high-content cell analyzer termed the digital SACF assay. Dual fluorescence staining of formed colonies and the dissolution of soft agar led to accurate detection of transformed cells with the imaging cytometer. Partitioning a cell sample into multiple wells of culture plates enabled digital readout of the presence of colonies and elevated the sensitivity for their detection. In practice, the digital SACF assay detected impurity levels as low as 0.00001% of the hCTPs, i.e. only one HeLa cell contained in 10,000,000 human mesenchymal stem cells, within 30 days. The digital SACF assay saves time, is more sensitive than in vivo tumorigenicity tests, and would be useful for the quality control of hCTPs in the manufacturing process.

GMP-grade human fetal liver-derived mesenchymal stem cells for clinical transplantation

Stem cell therapy seems a promising avenue in regenerative medicine. Within various stem cells, mesenchymal stem cells have progressively used for cellular therapy. Because of the age-related decreasing in the frequency and differentiating capacity of adult MSCs, fetal tissues such as fetal liver, lung, pancreas, spleen, etc. have been introduced as an alternative source of MSCs for cellular therapy. On the other hand, using stem cells as advanced therapy medicinal products, must be performed in compliance with cGMP as a quality assurance system to ensure the safety, quality, and identity of cell products during translation from the basic stem cell sciences into clinical cell transplantation. In this chapter the authors have demonstrated the manufacturing of GMP-grade human fetal liver-derived mesenchymal stem cells.

Therapeutic mesenchymal stromal cells: where we are headed

With a range of therapeutic uses, from diabetes and Crohn's disease to wound repair, interest in the function, characterization, and expansion of mesenchymal stromal cells (MSCs) is growing rapidly. When considering the therapeutic use of MSCs, one must take into account a multitude of options including the ideal source of MSCs, the ideal donor, and the best means of expansion. Here we discuss different sources of MSCs, including cord blood, bone marrow, and adipose tissue, the option of using autologous and allogeneic donors, and finally we discuss GMP-applicable expansion protocols aimed at expanding MSCs for clinical use.

Human mesenchymal stromal cells: identifying assays to predict potency for therapeutic selection

Multipotent mesenchymal stromal cells (MSCs) have the potential to repair and regenerate damaged tissues, making them attractive candidates for cell-based therapies. To maximize efficacy of MSCs, prediction of their therapeutic abilities must be made so that only the best cells will be used. Our goal was to identify feasible and reproducible in vitro assays to predict MSC potency. We generated cell lines from 10 normal human bone marrow samples and used the International Society for Cellular Therapy's minimal criteria to define them as MSCs: plastic adherence, appropriate surface marker expression, and trilineage differentiation. Each MSC line was further characterized by its growth, proliferation, and viability as determined by cell count, bromodeoxyuridine incorporation, and cellular ATP levels, respectively. To determine whether these tests reliably predict the therapeutic aptitude of the MSCs, several lines were implanted in vivo to examine their capacity to engraft and form granulation tissue in a well-established murine wound model using polyvinyl alcohol sponges. Long-term engraftment of MSCs in the sponges was quantified through the presence of the human-specific Alu gene in sponge sections. Sections were also stained for proliferating cells, vascularity, and granulation tissue formation to determine successful engraftment and repair. We found that high performance in a combination of the in vitro tests accurately predicted which lines functioned well in vivo. These findings suggest that reliable and reproducible in vitro assays may be used to measure the functional potential of MSCs for therapeutic use.

[Genetic safety of cellular therapy]

This paper presents the main results of the study on chromosome and genome variability of mesenchymal stem cell cultures from bone marrow and adipose tissue carried out in the Laboratory of Mutagenesis, Research Centre for Medical Genetics, over the last three years. Genome stability was assessed from DNA damage using the DNA comet assay, karyotyping and registration of aneuploidy by the FISH method. We found that DNA damage rate in MSC cultures from bone marrow was 3.9% and 3.8% at the early (2-5) passages and the late (10-15) passages respectively. The cultures were characterized by high dispersion of individual values. Karyotyping showed mosaicism in both types of MSC cultures at the early and late stages of cultivation. The fraction of abnormal cells in some cultures amounted to 80-90%. Evaluation of aneuploidy in interphase cells revealed 1.34% of aneuploid cells (on the average) per one "conventional" chromosome; their overall frequency in the genome amounted to 20-40%. The frequency of aneuploid cells was similar at the early and late passages. Cultures with clones of trisomic and monosomic cells were revealed. The probability of occurrence of abnormal cells may increase by virtue of de novo mutations in the culture and as a result of positive selection of the cells existing in the organism that exhibit a higher reproduction rate in culture. Based on the experimental data on mutational process, selection of mutant cells and clone formation, it is concluded that cytogenetic control of stem cells is necessary to ensure the safety of cellular therapy.

Clinical grade production of mesenchymal stem cells

Mesenchymal Stem Cells (MSCs) are multipotent adult stem cells having an immunosuppressive effect. These characteristics lead to an increasing use of MSC in graft process or for regenerative medicine. For the clinical uses of MSCs, standards are needed. The clinical grade production necessitates adhering to good manufacturing practices (GMP) to insure the delivery of a "cell drug" that is safe, reproducible and efficient. All parts of the process must be defined: the starting material (tissue origin, separation or enrichment procedures), cell density in culture, and medium (fetal calf serum (FCS) or human serum, cytokines with serum-free medium for target). But to reach the GMP goal, cells have to be cultured in as close to a closed system as possible. Analytical methods are needed to assay the active compound and impurities. At a minimum, quality control (QC) of cells must consider the phenotype, functional potential, microbiological safety, and ensure the cultured cells remain untransformed. Finally, quality assurance system (QA) procedures specific to the production of MSCs as a cell drug must be determined and implemented.

Protein aggregate-containing neuron-like cells are differentiated from bone marrow mesenchymal stem cells from mice with neurofilament light subunit gene deficiency

Autologous bone marrow mesenchymal stem cell (MSC) transplantation has great potential in cell therapy used for the treatment of neurodegenerative disorders. Since many genetic deficiencies have been reported in pathogenesis of the diseases, genetic backgrounds of donor stem cells should be concerned. In this study, effects of neurofilament light subunit (NFL) gene deficiency on proliferation and neuronal differentiation of MSCs were studied in vitro. Lower proliferation rate was observed in NFL-/- MSCs. When exposed to retinoic acid (RA), both NFL-/- and normal MSCs could express several markers of neuronal lineage, such as Nestin, MAP-2, NeuN, O4 and GFAP. However, the NFL expression at mRNA and protein levels was observed only in normal MSCs but absent in NFL-/- MSCs. Significant reductions in amount of neurofilament heavy subunit (NFH) protein and number of neuron-like cells were detected in differentiated NFL-/- MSCs. Interestingly, NFH positive protein accumulations were observed in the neuron-like cells derived from NFL-/- MSCs. These accumulations were perinuclear and morphologically similar to protein aggregations in motoneurons of the spinal cord in NFL-/- mice. The results suggest that NFL gene deficiency could retard MSCs proliferation and neuronal generation, even though the capability of neuronal lineage differentiation of MSCs may not be deterred. Moreover, the NFL-/- MSCs differentiated neuron-like cells carried on the genetic and pathologic deficiency, suggesting that the genetic quality of donor cells must not only be tested, but also modified before transplantation. This also points towards the possibility of creating a stem cell-derived cell model for pathogenesis study.

Technical Report: Assessment of Viability and Osteogenic Ability of Human Mesenchymal Stem Cells After Being Stored in Suspension for Clinical Transplantation

Human mesenchymal stem cells (MSCs) were suspended in phosphate-buffered saline (PBS) and stored up to 24 h at 4 degrees C, 24 degrees C, and 37 degrees C. More than 80% viability was maintained at any temperature for at least 1 h, then gradually decreased over time. After 24 h, the viabilities at 4 degrees C, 24 degrees C, and 37 degrees C were about 81%, 70%, and 62%, respectively. The MSCs suspended/stored in PBS at 4 degrees C for 24 h also exhibited in vitro osteogenic differentiation capability as evidenced by mineralized matrix formation as well as high alkaline phosphatase activity when cultured in an osteogenic medium. Furthermore, in vivo implantation experiments using the MSCs also demonstrated new bone formation. Because MSCs are known to possess multipotential stem cell characteristics, these data indicate that human MSCs stored in PBS at 4 degrees C could be delivered to distant medical facilities for the purpose of hard tissue and other types of tissue regeneration therapy.

A quantitative, randomized study evaluating three methods of mesenchymal stem cell delivery following myocardial infarction

AIMS

Mesenchymal stem cells (MSCs), rare bone marrow-derived stem cell precursors of non-haematopoietic tissues, have shown promise in potentially repairing infarcted myocardium. These and similar cell types are being tested clinically, but understanding of delivery and subsequent biodistribution is lacking. This study was designed to quantitatively compare MSC engraftment rates after intravenous (IV), intracoronary (IC), or endocardial (EC) delivery in a porcine myocardial infarction (MI) model.

METHODS AND RESULTS

Allogeneic, male MSCs were cultured from porcine bone marrow aspirates. Iridium nanoparticles were added during culturing and internalized by the MSCs. An MI was induced in female swine (27-40 kg in size) by prolonged balloon occlusion of the mid-left anterior descending artery. Animals (n = 6 per group) were randomized to one of three delivery methods. Cellular engraftment was determined 14+/-3 days post-delivery by measuring ex-vivo the iridium nanoparticle concentration in the infarct. Confirmation of cellular engraftment utilized both DiI and fluorescence in situ hybridization (FISH) labelling techniques. During MSC infusion, no adverse events were noted. However, following IC infusion, half of the pigs exhibited decreased blood flow distal to the infusion site. At 14 days, the mean number of engrafted cells within the infarct zone was significantly greater (P< or =0.01) following IC infusion than either EC injection or IV infusion and EC engraftment was greater than IV engraftment (P< or =0.01). There was less systemic delivery to the lungs following [EC vs. IV (P = 0.02), EC vs. IC (P = 0.06)]. Both DiI and FISH labelling demonstrated the presence of engrafted male MSCs within the female infarcted tissue.

CONCLUSION

IC and EC injection of MSCs post-MI resulted in increased engraftment within infarcted tissue when compared with IV infusion, and IC was more efficient than EC. However, IC delivery was also associated with a higher incidence of decreased coronary blood flow. EC delivery into acutely infarcted myocardial tissue was safe and well tolerated and was associated with decreased remote organ engraftment with compared with IC and IV deliveries.