Dr Alejandro Erices, Unidad de Biología Celular, INTA, Universidad de Chile, Casilla 138, Santiago 11, Chile. E-mail: firstname.lastname@example.org
Haemopoiesis is sustained by two main cellular components, the haematopoietic cells (HSCs) and the mesenchymal progenitor cells (MPCs). MPCs are multipotent and are the precursors for marrow stroma, bone, cartilage, muscle and connective tissues. Although the presence of HSCs in umbilical cord blood (UCB) is well known, that of MPCs has been not fully evaluated. In this study, we examined the ability of UCB harvests to generate in culture cells with characteristics of MPCs. Results showed that UCB-derived mononuclear cells, when set in culture, gave rise to adherent cells, which exhibited either an osteoclast- or a mesenchymal-like phenotype. Cells with the osteoclast phenotype were multinucleated, expressed TRAP activity and antigens CD45 and CD51/CD61. In turn, cells with the mesenchymal phenotype displayed a fibroblast-like morphology and expressed several MPC-related antigens (SH2, SH3, SH4, ASMA, MAB 1470, CD13, CD29 and CD49e). Our results suggest that preterm, as compared with term, cord blood is richer in mesenchymal progenitors, similar to haematopoietic progenitors.
The circulatory capacity of the haematopoietic stem cell is well documented in adults by transplantation studies and supported by the observation that these cells circulate in umbilical cord blood (Rubinstein et al, 1995; Wyrsch et al, 1999) and, under certain circumstances, in peripheral blood (Siena et al, 1989). In turn, although direct evidence for the presence of MPCs in the blood does not exist, the observation that after transplantation MPCs functionally engraft in the marrow (Horwitz et al, 1999) has been considered as proof of their circulatory and homing capacity. In addition, mesenchymal progenitors have been shown to circulate in cytokine-mobilized blood of cancer patients (Fernández et al, 1997).
As there is no information on whether mesenchymal progenitor cells circulate in umbilical cord blood (UCB), in this study we evaluated the ability of human UCB harvests to give rise in vitro to a population of cells exhibiting the characteristics of mesenchymal progenitors.
Materials and methods
Cord blood harvest and preparation of adherent cells Cord blood harvests were obtained, with the mother's consent, from preterm and term deliveries at the time of birth from Hospitales Barros Luco-Trudeau and Del Salvador, Santiago. After removal of the placenta, blood was allowed to drain from the severed end of the cord into glass bottles containing 10 ml of M-199 culture medium with 250 U/ml of preservative-free heparin. In all cases, blood samples were processed ≤ 24 h after harvest. From each blood harvest, aliquots were set apart for routine haematological analysis (Cell-Dyn 3500 System, Abbott) and for immunophenotyping of haematopoietic progenitors.
Cord blood cells were separated into a low-density fraction (Hystopaque-1077; Sigma, St. Louis, USA) and mononuclear cells were washed, suspended in culture medium [α-MEM containing 20% fetal bovine serum (FBS) and 80 µg/ml gentamicin sulphate; Gibco BRL, NY, USA] and seeded (T-25 flasks and 35-mm dishes) at a concentration of 1 × 106 cells/cm2. Cultures were maintained at 37°C in a humidified atmosphere containing 5% CO2, with a change of culture medium every 7 d. Cells in the developing adherent layer were used for the experiments described. All determinations hereafter described were performed in the respective cell cultures originating from at least five cord blood harvests.
Cytochemical characterization Cells in situ were analysed for the following cytochemical markers: acid phosphatase, alkaline phosphatase, periodic acid–Schiff, sudan black B, naphthol AS-D chloroacetate esterase and α-naphthyl acetate esterase. In all cases, analyses as well as the selection of positive and negative controls were performed according to the manufacturer's guidelines (Sigma Diagnostics Kits).
Immunophenotyping of UCB or cultured adherent cells To detect surface antigens, aliquots of fresh UCB or cultured adherent cells after detachment with 0·25% EDTA were washed with phosphate-buffered saline (PBS) containing 2% FBS. To detect intracellular antigens, cultured adherent cells were detached with 0·25% trypsin, washed with PBS and permeabilized with 70% ethanol (10 min at 4°C). For direct assays, cells were immunolabelled with the following anti-human antibodies: CD13–PE, CD31–FITC, CD54–PE, CD90–FITC, CD51/CD61–FITC (Pharmingen, Los Angeles, CA, USA), CD14–PE, CD38–FITC, CD34–PE (Dako, Glostrup, Denmark), CD29–FITC, CD45–PerCP, CD49d–PE, CD49e–FITC, CD64–FITC (Becton Dickinson, San José, CA, USA) and/or CD106–FITC (R & D Systems, Abingdon, UK). As controls, mouse IgG1–PE, IgG1–FITC, IgG1–perCP or IgG2a–PE (Becton Dickinson) were used. For indirect assays, cells were immunolabelled with the following anti-human antibodies: SH2, SH3, SH4 (Osiris Therapeutics, Baltimore, MS, USA), von Willebrand factor (Pharmingen), α-smooth muscle actin, ASMA (Sigma) or MAB1470 (Chemicon, Temecula, CA, USA). As secondary antibodies, anti-mouse IgGwm–FITC or –PE (Sigma) were used. Labelled cells were analysed either by epifluorescence microscopy or by flow cytometry. In the latter case, 10 000 events were acquired and analysed in a FACScan flow cytometer (Becton Dickinson) using the cellquest software. Note that antibodies against the antigens SH2, SH3, SH4, ASMA and MAB 1470 have been widely utilized to characterize marrow-derived mesenchymal progenitor cells (Conget & Minguell, 1999; Pittenger et al, 1999).
Proliferation studies Adherent cells, after detachment with trypsin, were seeded (20 × 103 cells/ml) in culture medium. At selected time intervals, the total number of viable cells was counted and expressed as fold increase over the starting cell number, which was set to 1.
Cell cycle analysis and quantification of quiescent cells For cell cycle analysis, cells were permeabilized and labelled with 10 µg/ml propidium iodine (Sigma), followed by treatment with 100 µg/ml RNAse A (Sigma) (Conget & Minguell, 1999). DNA content was analysed in a FACScan flow cytometer by using the modfit software. For assessment of quiescent cells, adherent cells were trypsinized, permeabilized and their RNA and DNA content evaluated by flow cytometry after staining with acridine orange. Under these conditions, quiescent cells (G0) are characterized by minimal RNA content and uniform DNA content (Juan & Darzynkiewicz, 1998; Conget & Minguell, 1999).
Differentiation of mesenchymal-like cells into osteoblasts and adipocytes For these studies, cells were cultured either in an osteogenic (0·1 µm dexamethasone, 10 mmβ-glycerol phosphate and 50 µm ascorbate) (Jaiswal et al, 1997) or adipogenic (1 µm dexamethasone, 5 µg/ml insulin, 0·5 mm isobutylmethylxanthine and 60 µm indomethacin) (Dennis et al, 1999) medium. The onset of osteoblasts was evaluated by the expression of alkaline phosphatase (Pittenger et al, 1999) or by calcium accumulation (von Kossa staining) (Jaiswal et al, 1997). The presence of adipocytes was assessed by the cellular accumulation of neutral lipid vacuoles that stained with Oil red O (Conget & Minguell, 1999).
Characteristics of cord blood harvests used for the in vitro growth of adherent cells.
Thirty-one cord blood harvests were entered and analysed in this study. As seen in Table I, the median values for the haematological parameters analysed were in close agreement with those reported for UCB collections for banking purposes (Beguin et al, 1995; Denning-Kendall et al, 1996). UCB from donors with a wide range of gestational ages (32–41 weeks) were included to establish whether gestational age correlates with the potential of UCB to develop adherent cells in vitro.
Morphological characteristics of primary cultures of UCB-derived adherent cells.
UCB-derived mononuclear cells were set in culture and the onset of an adherent layer was monitored continuously. By day 15 of culture, 29 out of 31 cord blood harvests had produced an adherent layer, which remained as such even after regular changes of the medium.
From 76% of the cord blood specimens analysed, the evolved adherent layer was formed by a heterogeneous population of cells. Based on the morphology of the most abundant phenotype present (Fig 1A and C), cells in this group of cultures were termed osteoclast-like cells (OLCs). In turn, in 24% of the UCB samples evaluated, the adherent layer contained a homogeneous population of cells showing a fibroblastoid morphology (Fig 1B and D). Cells in this group of cultures were termed mesenchymal-like cells (MLCs).
Characteristics of osteoclast-like cells
The morphology of OLCs was heterogeneous. Microscopic examination revealed cells with an elongated or oval/round shape with smooth borders, showing in certain cases cytoplasmic extensions. Usually, the cells were in contact with each other, however the most remarkable feature was the presence of multinucleated cells with nuclei congregated around a central area. After 3 weeks in culture, the multinucleated cells tended to predominate over the elongated and small rounded cells (Fig 1C) and reached a semiconfluent condition after 4 weeks. By subcultivation, these cells gave rise again to multinucleated cells, however their proliferation capacity was limited.
As seen in Table II, more than 90% of OLCs were strongly positive for tartrate-resistant acid phosphatase (Fig 1E), acid phosphatase and α-naphthyl acetate esterase activity, but negative for periodic acid–Schiff, sudan black B, alkaline phosphatase and naphthol AS-D chloroacetate esterase activities. As detected by flow cytometry (Fig 2), although all OLCs expressed the common leucocyte antigen CD45, only a subpopulation expressed the monocyte–macrophage antigen CD14. Together, OLCs expressed the osteoclast-related antigen CD51/CD61 (vitronectin receptor), but did not express the macrophage–polykaryon (CD64)-, the mesenchymal (SH2)- or the endothelial (CD31)-related antigens (Fig 2).
Characteristics of mesenchymal-like cells
Primary cultures of MLC mainly consisted of colonies of bipolar fibroblastoid cells (Fig 1D) which, after subcultivation, proliferated with a population-doubling time of 48 h and reached a confluent growth-arrested condition (Fig 3A). In primary cultures, the MLC frequency varied from one harvest to another, however in each UCB sample the average number of MLC colonies (day 15) for 25 × 106 UCB-derived mononuclear cells was 7 ± 3.
Cell cycle analysis (at log phase of growth) indicated that ≥85% of the cells were in the G0/G1 phases (Fig 3B). Analysis to detect quiescent cells revealed that 5% of all MLCs displayed a pattern of RNA and DNA staining which is distinctive for G0 cells (Fig 3C).
Cytochemical analysis (Table II) revealed that MLCs were strongly positive for α-naphthyl acetate esterase and periodic acid–Schiff (Fig 1F), but were negative for all the other cytochemical markers assessed. The immunophenotype of these cells (Fig 4), as studied by flow cytometry, disclosed the homogeneous expression of the mesenchymal-related antigens SH2, SH3 and SH4. Together, MLCs were positive for an antigen (detected by MAB 1470) which although expressed by endothelial cells was also expressed by mesenchymal cells (Conget & Minguell, 1999). MLCs expressed antigens CD13, CD29, CD49e, CD54 and CD90 and α-smooth muscle actin (ASMA), but did not express antigens CD14, CD34, CD45, CD49d and CD106. In addition, MLCs did not express the endothelial-related antigens CD31 and von Willebrand factor.
Further characterization studies performed on MLCs revealed a potential to differentiate into osteoblasts and adipocytes. Thus, after switching MLCs from the regular culture medium into an osteogenic medium, cells with an osteoblast-like phenotype evolved, as judged by the expression of alkaline phosphatase (Fig 5A) and by the deposition of a mineralized matrix (Fig 5B). Moreover, when MLCs were exposed to an adipogenic medium, cells differentiated into adipocytes which displayed a perinuclear accumulation of lipid vacuoles, as detected by phase-contrast microscopy or after staining with Oil red O (Fig 5C). The above features were not observed in MLC cultures grown in regular culture medium.
Our results show that UCB-derived mononuclear cells, when cultured in a medium containing only FBS, were able to generate adherent cells. However, we found that the nature of the adherent cells was not the same in all cases, but exhibited either an osteoclast- or a mesenchymal-like phenotype, each one characterized by the following features. First, approximately 75% of the cord blood collections gave rise to cultures of adherent cells (OLCs), which displayed the morphology and characteristics of multinucleated osteoclasts. OLCs expressed several markers of osteoclasts, such as a strong tartrate-resistant acid phosphatase activity and expression of antigens CD45 and CD51/CD61 (Udagawa et al, 1990; Suda et al, 1992). Together, these cells did not express antigens related to the macrophage–polykaryon (Quinn et al, 1995), mesenchymal (Conget & Minguell, 1999) or endothelial (Schwachula et al, 1994) lineages. In primary cultures of OLCs, we found within CD45+ cells a subset that also expressed the monocyte–macrophage antigen CD14. However, the percentage of these cells decreased with cultivation time (53% and 26% at weeks 3 and 5 respectively; data not shown). The decrease in the number of CD45+CD14+ cells was coincident with the microscopic observation of a time-dependent enrichment in multinucleated cells (Fig 1C). Thus, the above characteristics strongly suggested that cells in the osteoblast-like cultures correspond to osteoclasts and/or their progenitors.
Second, almost 25% of cord blood harvests gave rise to an adherent layer, initially formed by individual cells or colonies of a few cells, which rapidly gave rise to a well-established layer of fibroblastoid (MLC) cells. The rapid growth of these cells seemed to be sustained by a population of (self-renewing?) quiescent (G0) cells. MLCs expressed several mesenchymal progenitor-related antigens, such as SH2, SH3, SH4, ASMA, MAB 1470, CD13, CD29, CD49e and CD54 (Bendall et al, 1993; Conget & Minguell, 1999; Pittenger et al, 1999). In contrast, they did not express myeloid or endothelial antigens. In addition, MLCs upon proper stimulation could be differentiated into other mesenchymal lineages, such as osteoblasts and adipocytes. Thus, in the presence of dexamethasone, β-glycerol phosphate and ascorbate, MLCs expressed bone cell markers such as alkaline phosphatase and formed mineralizing colonies (Jaiswal et al, 1997; Conget & Minguell, 1999). Moreover, MLCs exposed to a defined adipogenic medium for 15 d gave rise to foci of cells containing large lipid droplets that stained positively with Oil red O. Thus, the immunophenotype and functional properties displayed by cord blood-derived MLCs resembled very closely the characteristics assigned to bone marrow-derived mesenchymal progenitor cells (Galmiche et al, 1993; Conget & Minguell, 1999; Pittenger et al, 1999).
Our results showing that mature osteoclasts or their progenitors circulate in umbilical cord blood are not without precedent (Roux et al, 1996). However, to our knowledge, this is the first report documenting that in umbilical cord blood the presence of cells can generate MLCs upon culture. The presence of mesenchymal progenitor cells in cord blood is justified because it can be hypothesized that both haematopoietic and mesenchymal progenitors are travelling, via cord blood, from early fetal haematopoietic sites to the newly formed bone marrow (Tavassoli, 1991; Péault, 1996).
Despite the relatively low number of UCB harvests that developed into MLCs in this study, our results suggest that preterm, as compared with term, cord blood is richer in mesenchymal progenitors, as occurs with haematopoietic progenitors (Shields & Andrews, 1998; Wyrsch et al, 1999). The above seems to be supported by the observed correlation between the characteristics of cord blood (gestational age and CD34 content) and the frequency of emergence of MLC-like cultures (Table III).
Based on their large ex vivo expansion capacity as well as on their differentiation potential, cord blood-derived mesenchymal progenitor cells can be visualized as attractive targets for cellular or gene transfer therapeutic options.
A.E. thanks Diego F. Erices Contreras, his first son, for donating the most beloved UCB harvest. We thank Drs A. Pereira, J. Lira and J. Palma (Hospitales Barros-Luco Trudeau and Del Salvador, Santiago, Chile) for help with cord blood collections. We also thank Mrs. V. Simon (Clínica Las Condes, Santiago) for flow cytometry support. This work was supported by grants FONDECYT-Chile (2980017 and 8970028) and ICGEB-Italy (97/009). A.E. and P.C. are doctoral fellows from CONICYT (Chile) and Fundación Andes (Chile) respectively.