Mesenchymal Stem Cells – A Scientific Review

Introduction

Mesenchymal Stem Cells (MSCs) were first recognized in bone marrow as a non-hematopoietic stem cell by a German pathologist, Julius Cohnheim in 1867. They are generally defined as clonogenic, non-hematopoietic stem cells that are found mostly in the adult bone marrow. MSCs are distinguished by their natural ability to differentiate into multiple mesodermal lineages, such as chondrocytes, osteoblasts, adipose tissue, and endothelial cells.

MSCs can cross germ layer by forming neural cells, if cultured in the appropriate microenvironment to cells of neural lineages (Arnhold S, 2005). MSC research has received considerable recognition due to attributes such as ease of isolation, expansion potential, compatibility with multiple delivery methods, and immunosuppressive properties, all allowing for great therapeutic potential in cell-based medicine. (Maccario, 2005).

Isolation

The isolation and exploration on the differentiation potential of MSCs have been an issue of research for more than three decades (Freidsten 1976, 1991). MSCs are generally isolated from the mononuclear cell fraction of from bone marrow. The population of MSCs are selected/isolated based on the physical surface properties in which they are placed to adhere (Kassem, 2004). Although MSCs are found in the bone marrow, they can also be isolated from cyro-preserved umbilical cord blood, placental tissue, and exfoliated teeth (Le Blanc, 2005). The inevitable problem with isolating MSCs by fractionation and adherence is contamination from hematopoetic cells and other bone marrow resident cells. This is circumvented by isolating cells negative for hematopoietic surface markers, such as CD34, CD45, and CD14, and positive for CD105, CD166, CD54, CD55, CD13 and CD44.

Stro-1 has been suggested as a marker of mesenchymal progenitors (Granthos 1994). However, despite the identification of cell surface molecules, the studies should be complemented with functions. Examples of functional studies are the ability of MSCs to generate bone, adipocytes and bone marrow stromal cells. Once the cells are isolated, they go through an initial lag phage within the culture media. After treatment with either human or fetal calf sera (≈10%), the cells undergo rapid cell division with doubling times between 12-24 hrs. MSCs can be expanded 500-fold within a 3-week period (Alhadlaq, 2004).

Expansion of MSCs and Immune Properties

MSCs that were transplanted into irradiated mice have been shown to engraft into bone, cartilage, tendon, and adipose tissue (Re Percra, 1998). Bone morphogenic proteins have been show to induce MSCs towards osteoblastic cells. In other induction protocols, similar cell fate has been demonstrated with ascorbic acid, β-glycerol phosphate and dexamethasone (D.C. Colter, Friedenstein). Mechanistic studies have been done to understand the intracellular pathways involved in chondrocyte differentiation when MSCs are stimulated with TGF- (A. M. Mackey (1998). While Wnt is activated during chondrogenic differentiation, it is suppressed during adipogenic formation. Adipocytes are generated by a cocktail of dexamethasone, isobutyl methyl xanthine and iodomethacin, which is a phosphodiesterase inhibitor (I.Sckiva, 2001). MSCs have been reported to generate functional neurons (Arnhold S, 2005).

The expansion potential of MSCs is relevant due to their engraftment capability for clinical purpose. It appears that MSCs persist for up to 13 months after implantation. The fact that the MSCs show long-term survival in human, this suggests that they have special property that prevents them from rejections by the allogeneic host (KW Liechty 2000, LeBlanc 2005). Angello, et al (2005) have recently reported that MSCs produce soluble factors that suppress B-cell proliferation, which might explain why the MSCs show a low level of rejection when injected into an allogeneic subject. These studies are supported by the demonstration of veto properties by MSCs (Potian JA, 2003). In support of the immunosuppressive properties of MSCs is their ability to inhibit CD4+ T-Helper cells (Maccario, 2005). Not surprising, Djouad (2005) reported a facilitating effect of the microenvironement, through the production of TNF- in the immunosuppressive functions of MSCs. While much is being uncovered on the immune suppressive properties of MSCs, further research studies are required to untangle the complex functions involved in this property of MSCs. This area of investigation is important for the therapeutic application of MSCs.

Therapeutic Potential of MSCs

The therapeutic potential of MSCs can be divided into four major categories: 1. Localized implantation, 2. Systemic implantation, 3. Gene therapy, 4. Tissue engineering. Direct implantation of MSCs appears to be useful for various conditions. Direct injection of MSCs has been shown to improve the prognosis of patients suffering form osteopenia, a congenital disease where osteoblasts are impaired (Horwitz 1999). Similar models have also been reported for tendon regeneration (Pittenget, 2005). MSCs implanted in cardiac tissue after myocardial infarction induced cardiac regeneration and angiogenesis (Fazal, 2005). Implantation of MSCs has also been shown to improve the repair of chronic non-healing skin wounds (Badiavas, 2003).

Another use of MSCs is their utility as third party cells for hematopoietic stem cell transplantation. The premise is that MSCs would reduce graft vs. host response and reduced the use of immune suppressive drugs. In addition, the thoughs are that MSCs could facilitate efficient transplantation of the hematopoietic stem cells during bone marrow transplantation (Koc, 2002)

Along with their immunosuppressive properties, MSCs are believed to be vehicle for gene therapy. Hamada et al (2005) have demonstrated that MSCs transduced with a BMP4-expression vector induce bone growth in vivo. The same group also showed that MSCs transfected with IL-2, implanted intracranially, migrated and reduced the tumor size of glioma in rats. Gene therapy has been coupled with tissue engineering, allowing researchers to use the patients’ own stem cells. These cell lines are then expanded into a bio-safe scaffold, allowing for tissue repair. The scaffolds are biologically derived polymers from, intracellular matrix, fibronectin, or synthetic polymers such as tri-calcium phosphate ceramics. Research studies with bioengineering techniques have illustrated in vivo regeneration of cartilage, growth plate, bone, and tendon (Hui 2005).

Conclusion

Mesenchymal Stem Cells expansion potential, ease of isolation, and most importantly their veto immune properties have generated great expectations in cell-based therapeutics. Although there is much promise in this field, further research is needed in the understanding of MSC differentiation pathways, homing cues, expansion techniques, and plastic growth surfaces for effective therapeutic treatment. While in vitro experiments have proven promise, behavior of MSCs in vivo must be refined for predictable implantation.

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Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey: George Lambrinos, Rob Margolin, Laila Menon, Julio Ortega, Miral Parikh
(in alphabetical order).

Teaching Assistant: Steven Greco

The review was edited by two stem cell biologists.