Introduction
Mesenchymal stem cells (MSCs) are one of the most studied and applied types of stem cells to date. These cells were first described by Friedenstein et al. as a cell population similar to fibroblasts [
1], which can differentiate into multiple cell types such as osteoblasts, adipocytes, and chondrocytes [
2]. MSCs have been isolated from many tissues including bone marrow [
3,
4], adipose tissue [
5‐
7], peripheral blood, umbilical cord blood (UCB) [
8‐
10], banked UCB [
11‐
14], umbilical cords [
15,
16], placenta [
17], amniotic fluid [
18], dental pulp [
19], and menstrual blood [
20].
Compared with other stem cell sources, UCB-MSCs have advantages such as non-invasive recovery, the abundance of MSCs, and well-known characteristics. In both pre-clinical and clinical settings, MSCs have been studied to treat a various diseases. Pre-clinically, UCB-MSCs have been used to treat neonatal brain injury [
21], fibrocartilaginous embolic myelopathy [
22], spinal cord injury [
23,
24], diabetic renal injury [
25,
26], bone loss [
27], ischemia [
28,
29], hearing loss [
30], damaged corneal endothelium [
31], Alzheimer’s disease [
32], graft-versus-host disease (GVHD) [
33], acute hepatic necrosis [
34], diabetes mellitus [
35], and liver cirrhosis [
36]. Clinically, UCB-MSCs have been transplanted for treatment of autism [
37], hereditary spinocerebellar ataxia [
38], foot disease in patients with type 2 diabetes mellitus [
39], and basilar artery dissection [
40]. Clinical trials (retrieved from clinicaltrial.gov) include mesenchymal stem cell transplantation for engraftment of unrelated hematopoietic stem cell transplantation (NCT00823316), treatment of steroid-refractory acute or GVHD (NCT01549665), articular cartilage defect treatment (NCT01733186), and hematologic malignancy treatment (NCT01854567).
The main concern in UCB-MSC applications is
in vitro expansion that is mostly affected by the culture medium. For production protocols of UCB-MSCs under clinical conditions, it is essential to include sterility controls, analysis for viral markers, and genetic testing such as karyotyping. Currently, UCB-MSCs can be produced at a GMP (good manufacturing practice) grade by automated processing protocols and some novel protocols. Procedures have been developed to isolate mononuclear cells (MNCs) in closed systems such as the SEPAX device [
41,
42]. Other systems can also be used to expand MSCs such as the Cell Stack System [
43]. However, almost all of these methods require fetal bovine serum (FBS) for culture. FBS-based medium has some limitations associated with clinical application, especially prion and viral transmission or adverse immunological reactions against xenogenic components.
Some novel methods use human serum for MSC culture, especially platelet-rich plasma (PRP). Recent studies have used PRP from peripheral blood [
44‐
48] and UCB [
49‐
52], which showed that PRP from peripheral blood or UCB significantly stimulates the proliferation of MSC from bone marrow [
45,
50], UCB [
49,
53], or adipose tissue [
44,
54]. More importantly, MSCs cultured in medium supplemented with PRP exhibit a normal phenotype and characteristics [
49‐
52], and maintain their multipotency for differentiation into adipocytes, osteoblasts, and chondrocytes. Taken together, these studies show that PRP can replace FBS for
in vitro MSC expansion.
All of these previous protocols have used allogeneic PRP. The use of PRP allows MSCs to avoid xenogenic immunological reactions, and prion and viral transmission, but MSCs may encounter human viral transmission and immunological reactions induced by allogeneic components. According to the European Medicines Agency and regulation No. [EC] 1394/2007 of the European Commission, MSC are considered as medicinal products [
55] and must be produced in compliance with GMP. The GMP standards ensure that cells are produced with the highest standards of sterility, quality control, and documentation following a standard operating procedure. Therefore, in this study, we aimed to establish an UCB-MSC isolation protocol using autologous PRP from the same umbilical blood sample. This protocol is GMP compliant and can be used for clinical applications.
Materials and methods
UCB collection and sample selection for study
UCB was collected from the umbilical cord vein with informed consent of the mother. The collection was performed in accordance with the ethical standards of the local ethics committee. To eliminate differences between UCB samples, the stem cell quantity was enumerated based on the number of hematopoietic stem cells (HSCs) using an Enumeration Pro-Count Kit (BD Bioscience) following the manufacturer’s guidelines. Only samples with ≥1 × 106 HSCs/ml were used in experiments.
MNC isolation and activated PRP preparation
First, blood samples were centrifuged at 2000 rpm for 15 min. The cell pellet was kept to isolate MNCs and the plasma was collected and centrifuged at 3500 rpm for 10 min. To prepare activated PRP (aPRP), a third of the plasma volume and the platelet pellet was collected and resuspended, and then 100 μL CaCl2 per 1 mL of PRP was added to activate growth factor release. The samples were then incubated at 37°C for 30 min or until the occurrence of clotting. The centrifuged blood cells were diluted at a ratio of 1:1 with phosphate buffered solution (PBS) and then applied to density centrifugation using Ficoll Hypaque (1.077 g/mL; Sigma-Aldrich, St Louis, MO). The collected MNCs were washed twice with PBS and then applied to experiments.
Primary culture
Twenty UCB samples were used for primary culture. MNCs were cultured in Iscove modified Dulbecco medium (IMDM) containing 1% antibiotic-mycotic (Sigma-Aldrich, Louis St, MO), 10 ng/mL epidermal growth factor (EGF), 10 ng/mL basic fibroblast growth factor (bFGF), and various concentrations of aPRP (2, 5, 7, or 10%) or 10% fetal bovine serum (FBS) for the control. The cells were plated at 5 × 104cells/mL in T-75 flasks (Corning) and incubated at 37°C with 5% CO2. After 3 days of incubation, 6 mL of fresh media were added to each flask. After 7 days, the media were replaced with fresh media. Then, the media was replaced every 4 days until the cells reached 70–80% confluence. The efficiency of the media was evaluated by the time required for adherent cells to appear and then reach 70–80% confluence for the first subculture.
Secondary culture
After successful primary culture, the samples were subcultured to evaluate the effects of the various media. The proliferation rate was evaluated by the eXCELLIgence system (Roche Applied Science, Indianapolis, IN). A total of 1 × 103 cells were seeded into each well of a 96-well E-plate in triplicate. The culture plates were placed into the eXCELLIgence system and incubated at 37°C with 5% CO2. Cell proliferation was monitored for 300 h with fresh medium changes every third day. Both the cell doubling time and slope value were determined by the software of the eXCELLIgence system.
Flow cytometry
Cell markers were analyzed following a previously published protocol [
11]. Briefly, cells were washed twice in PBS containing 1% bovine serum albumin (Sigma-Aldrich). The cells were then stained with anti-CD13-FITC, anti-CD14-FITC, anti-CD34-FITC, anti-CD44-PE, anti-CD45-FITC, anti-CD73-FITC, anti-CD90-PE, anti-CD105-FITC, anti-CD106-PE, anti-CD166-PE, or anti-HLA-DR-FITC antibodies (all purchased from BD Biosciences, San Jose, CA). Stained cells were analyzed by a FACSCalibur flow cytometer (BD Biosciences). Isotype controls were used in all analyses.
In vitro differentiation
For differentiation into adipogenic cells, UCB-MSCs were differentiated as described previously [
9]. Briefly, passage 5 cells were plated at 1 × 10
4 cells/well in 24-well plates. At 70% confluence, the cells were cultured for 21 days in IMDM containing 0.5 mmol/L 3-isobutyl-1-methyl-xanthine, 1 nmol/L dexamethasone, 0.1 mmol/L indo-methacin, and 10% FBS (all purchased from Sigma-Aldrich). Adipogenic differentiation was evaluated by observing lipid droplets in cells under a microscope.
For differentiation into osteogenic cells, UCB-MSCs were plated at 1 × 10
4 cells/well in 24-well plates. At 70% confluence, the cells were cultured for 21 days in IMDM containing 10% FBS, 10
-7 mol/L dexamethasone, 50 μmol/L ascorbic acid-2 phosphate, and 10 mmol/L β-glycerol phosphate (all purchased from Sigma-Aldrich) [
9]. Osteogenic differentiation was confirmed by Alizarin red staining.
For differentiation into chondrogenic cells, UCB-MSCs were induced to differentiate by a commercial medium for chondrogenesis (StemPro Chondrogenesis Differentiation Kit, A10071-01; Life Technologies). UCB-MSCs were differentiated in pellet form according to the manufacturer’s guidelines. After 21 days, the cell pellets were stained with an anti-aggrecan monoclonal antibody (BD Biosciences).
Tumorigenicity assay
The tumorigenicity of UCB-MSCs was examined in athymic nude mice. All manipulations of mice were approved by the Local Ethics Committee of Stem Cell Research and Application, University of Science (Ho Chi Minh city, Vietnam). Each mouse was injected subcutaneously with 5 × 106 cells (three mice per group). As a positive control, the mice were also injected with breast cancer cells at a different site. Tumor formation in mice was followed up for 3 months.
Statistical analysis
The significance of differences between mean values was assessed by t-tests and analysis of variance. A P-value of less than 0.05 was considered to be significant. All data were analyzed by Prism 6 software.
Discussion
The aim of this study was to establish a GMP-compliant protocol for isolation of UCB-MSC for clinical application. Therefore, we eliminated xenogenic and allogeneic components that can cause immunological reactions and viral transmission. Our approach replaced FBS in the medium with autologous aPRP that was isolated from the same UCB sample used to isolate MNCs.
Because the source of autologous aPRP was limited and the necessary number of MSCs for clinical application is high, we evaluated four concentrations of aPRP in complete medium, including 2, 5, 7, and 10%, and 10% FBS as a control. The effects of aPRP on MSC proliferation was evaluated in primary and secondary cultures. In primary culture, medium containing 10% aPRP significantly stimulated MSC proliferation compared with that of the other aPRP concentrations and 10% FBS. In medium containing 10% aPRP, MSCs adhered quickly and proliferated rapidly. These observations demonstrated that aPRP contains all the essential components similar to those in FBS for support of cell attachment and proliferation. In fact, aPRP contains high amounts of attachment proteins such as fibrin, fibronectin, vitronectin, and thrombospondin [
49,
56]. In addition, aPRP contains several growth factors that stimulate cell proliferation, such as EGF, acidic fibroblast growth factor, keratinocyte growth factor, vascular endothelial growth factor, platelet-derived growth factor, hepatocyte growth factor, and bFGF [
49,
52,
57]. Compared with bovine growth factors in FBS, aPRP can be obtained from humans, allowing better interactions between growth factors and cell receptors. In primary culture, the time needed to reach confluency in 10% aPRP indicated that this concentration of aPRP induced stronger MSC proliferation than that of FBS.
In expansion culture, the effects of the four concentrations of aPRP were also evaluated alongside the 10% FBS control. The results showed that there were no differences between 5, 7, or 10% aPRP supplementation compared with 10% FBS, but these aPRP concentrations showed significantly different effects than those of 2% aPRP. In fact, we confirmed these effects by the proliferation curve, doubling time, and slope value for proliferation. Based on proliferation curves, we could easily recognize differences in the proliferation rates between 2% aPRP and the other aPRP concentrations. However, the proliferation rates in 5, 7, or 10% aPRP or 10% FBS were not increased significantly. These data indicated the differences in the growth factor concentrations at 5, 7 and 10% aPRP did not cause any significant difference in the proliferation of UCB-MSCs.
Other important properties that we evaluated were the effects of aPRP-containing medium on surface marker expression and multilineage differentiation of UCB-MSCs. We used the marker profile of positive and negative markers suggested by Domicini et al. [
58]. The results showed that UCB-MSCs maintained their marker expression in aPRP-containing media compared with that in medium supplemented with FBS. UCB-MSCs cultured in aPRP- and FBS-containing media did not express hematopoietic markers, such as CD14, CD34 and CD45, or HLA-DR, while they expressed stromal cell markers such as CD13, CD44, CD73, CD90, CD105, and CD106. These results completely agreed with other studies of UCB-MSCs cultured in FBS-supplemented medium [
8,
9,
59‐
61], human peripheral blood-derived PRP [
62], and UCB-derived PRP [
49‐
51]. UCB-MSCs cultured in the various media also exhibited multilineage differentiation to adipocytes, osteoblasts, and chondrocytes. These results were similar to those of UCB-MSCs isolated in serum-supplemented medium [
8,
9,
59‐
61].
Some previous studies have shown that PRP has some effects on MSCs. In addition to PRP strongly stimulating MSC proliferation, PRP also triggers differentiation. However, these effects of PRP are different between the various types of MSCs. PRP induces UCB-MSCs and bone marrow-derived MSCs to differentiate into osteoblasts [
46], [
53,
63] and adipose-derived stem cells to differentiate into chondrocytes [
7]. In this study, we did not evaluate the effects of PRP on UCB-MSC differentiation. However, we found that MSCs cultured in medium containing 2, 5, 7, or 10% aPRP maintained their potential for differentiation into adipocytes, osteoblasts, and chondrocytes. This result indicated that aPRP cultured UCB-MSCs had not become mature cells such as osteoblasts or chondrocytes. In fact, in previous studies, although MSCs have been proposed to differentiate into osteoblasts and chondrocytes, they also maintain their differentiation capacity for adipocytes, osteoblasts, and chondrocytes [
62,
64]. In our final analysis, UCB-MSCs cultured in the various media were examined for tumorigenicity in athymic nude mice. The results showed that all mice injected with UCB-MSCs cultured in the various media showed no tumor formation at the injection site, while cancer cells caused tumor formation in all mice at their injection site.
In summary, we successfully established a protocol for isolation of GMP-compliant UCB-MSCs. For primary culture, IMDM plus 10% aPRP is appropriate. For expansion, culture medium plus 5% aPRP is suitable. This protocol complies with GMP because of its xenogenic- and allogeneic-free medium components.
Conclusion
UCB is a rich source of MSCs. UCB-MSCs can be isolated with xenogenic and allogeneic component-free medium. In this study, we successfully established a GMP-compliant UCB-MSC isolation protocol. Autologous aPRP can be used to replace FBS. Both aPRP and MNCs can be isolated from the same blood sample. In primary culture, MNCs should be cultured in IMDM plus 10% aPRP and 1% antibiotic-mycotic. However, in expansion culture, MSCs should be cultured in IMDM plus 5% aPRP and 1% antibiotic-mycotic. MSCs isolated by this protocol proliferate similarly as those in 10% FBS, maintain MSC phenotypes such as expression of CD13, CD44, CD73, CD90, CD105, CD106, and CD166, and do not express CD14, CD34, CD45, or HLA-DR. They also maintain their multilineage differentiation potential for adipocytes, osteoblast, and chondrocytes. In particular, the isolated MSCs do not form tumors at a high dose in athymic nude mice. This promising protocol is suitable for clinical applications of UCB-MSCs in the near future.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
PVP, NBV conceived the study, performed PRP preparation, evaluated the effects of PRP on mesenchymal stem cell proliferation. VMP, NHT primarily cultured mesenchymal stem cells from mononuclear cells; TLBP, TTN collected umbilical cord blood, isolated mononuclear cells from umbilical cord blood; LTTD, ANTB carried out the differentiation assays; NKP evaluated the tumorigenecity of MSCs in mice model. All authors read and approved the final manuscript.