MESENCHYMAL STEM CELLS
Friedenstein and coworkers were the first to investigate the characteristics of the colony forming fibroblastic cells, which were isolated from the bone marrow by their selective adherence to tissue culture plastics[1]. Several other groups extended the pioneering work of Friedenstein et al[1] showing that these plastic adherent human cells derived from bone marrow were able to differentiate into a number of mesenchymal cell types including osteoblasts, chondrocytes and adipocytes[2-4]. These cells were called “mesenchymal stem cells (MSCs)” in reference to their high self-renewing properties and ability to form cartilage and bone, and were suggested to be responsible for the normal turnover and maintenance of adult mesenchymal tissues[5]. Although the initial application of MSC was to form feeder layers for hematopoietic stem cells, hence the alternate name “marrow stromal cells”, MSC’s therapeutic potential to cure a plethora of debilitating diseases was soon discovered and has generated significant excitement in the field of regenerative medicine[6]. Over the last two decades, the field of MSC has progressed rapidly from the preclinical to the early clinical trial arena for a wide range of diseases.
MSCs IN CELL THERAPY
MSCs hold tremendous promise for a variety of clinical applications. Ongoing clinical trials using human mesenchymal stem cell (hMSC) include ischemic stroke, multiple sclerosis, acute leukemia, graft-versus-host disease, critical limb ischemia, articular cartilage and bone defects among others (for the clinical trials presently tested, please see: http://www.clinicaltrial.gov). At the time of writing this article, there were about 90 clinical trials involving hMSC at various stages world-wide. The progress in clinical trials with MSCs in various diseases has been reviewed extensively[7-12].
Although the concept of cell-based therapy is not new and bone marrow transplants have been the standard of care for years, MSC-based cell therapies represent a new generation of regenerative therapies that extend into other organ systems and meet pressing clinical needs for a broad range of diseases. MSCs are among the most widely used stem cell types in cell therapy owing to several favorable biological characteristics, including their convenient isolation from adult donors, ease of expansion in culture while maintaining genetic stability[13], lack of significant immunogenicity and feasibility for allogenic transplantation[14,15], and the homing capacity that facilitates intra-arterial/intravenous administration under minimally invasive conditions[16]. MSCs or MSC-like cells are now being isolated from blood[17], adipose tissue[18], trabecular bone[19], umbilical cord blood[20], and placenta[21] among other tissues. MSCs also have the remarkable property that they home to sites of tissue injury and institute repair, either by differentiating into tissue-specific cell phenotypes[22-25] or by creating a milieu that increases the capacity of the endogenous cells to repair tissue and modulates the immune response[26-28]. While the early studies have focused on cell differentiation, the recent results that demonstrate MSC’s ability to repair tissues without significant engraftment or differentiation have led to new concepts for hMSC therapeutic effects. Critical features of this new paradigm are MSC’s ability to not only secrete a rich mixture of soluble factors but also the ability to specifically respond to the immediate needs of the injured tissues. One specific example of the responsiveness of MSCs to microenvironment was the report that hMSCs injected into the hippocampus of mice following transient global ischemia decrease neuronal death by modulating inflammatory and immune responses. The transcriptomes of the hMSC changed with upregulation of 170 human genes that were largely involved in anti-inflammatory or anti-immune genes[29]. As another example, MSCs were activated by interferon-γ together with proinflammatory cytokines to express nitric oxide (NO) and several chemokines, suggesting that MSC-mediated immuno-suppression occurs through the concerted action of chemokines and NO[30]. MSC’s responsiveness to the microenvironment of injured tissues suggests that the MSCs can be injected locally to enhance tissue repair, which could be one of the most useful cell therapy strategies.
While the original focus of hMSC’s therapeutic potential was their ability to engraft and their plasticity, recent findings suggest that MSC’s primary function is to inhibit immune responses and to establish a favorable microenvironment for tissue repair through immune modulation, down-regulation of inflammatory responses and paracrine effects[31]. Thus, the defining properties for hMSC should include not only their multi-lineage potential but also their robustness to respond to biological cues and to modulate the microenvironment. It is also likely that the therapeutic benefits of hMSC are a combined result of multiple contributing factors, generating both short-term tissue responses and long-term tissue repair and regeneration. For this reason, basic science studies are important to elucidate the controlling factors and to gain mechanistic insights underpinning MSC therapies.
CLINICAL APPLICATIONS OF MSCs
The beneficial outcomes from an increasing number of clinical trials using hMSCs without any major side effects has been a major driving force behind interest in MSCs’ clinical application. As scientists learn more about MSC biology and tissue repair mechanisms, the encouraging clinical results, most notably in cardiac repair and bone disorders, have generated a growing enthusiasm.
Cardiac repair
A compelling clinical need exists in cardiovascular therapies to protect, restore and regenerate cardiomyocytes that are lost due to myocardial infarctions and heart failure. Bone marrow-derived cells, including both hematopoietic and MSCs, have shown remarkable clinical efficacy in terms of functional improvements including ejection fraction, ventricular volumes, infarct size and myocardial perfusion[32-34]. The functional improvement that occurred within 72 h was far earlier than would be expected for cell regeneration, leading to intense debate about repair mechanisms after cell transplantation[35]. The prevailing concept of stem cell efficacy has now shifted toward the cytokine-paracrine effects, which have been shown to modulate angiogenesis, inflammation, cytoprotection, metabolism and apoptosis. Despite the exciting possibilities that stem cell therapy have major beneficial effects on myocyte regeneration, inconsistent outcomes and, in some cases, poor engraftment and modest improvement have been reported in human trials[36-38]. These results highlight the need to understand the MSC tissue repair mechanisms and exact biology of stem cells in order to address the limitations such as the optimal cell type, mode of cell processing and delivery. The focus of improving and standardizing cell processing and delivery methods should be on enhancing cell engraftment while maintaining their therapeutic potency.
Bone disorders
MSCs have considerable potential for treatment of musculoskeletal disorders owing to their expansion capacity, immunosuppressive properties and ability to differentiate into bone and cartilage. Autologous bone marrow-derived MSCs have been used in fracture nonunion, osteogenesis imperfecta, and bone metabolic diseases, and demonstrated bone formation and limb function recovery in patients[39-42]. In addition, MSCs are also combined with scaffolds that are inductive or instructive to direct MSCs down specific lineage pathways and augment the therapeutic effect. Considerable in vitro and animal studies suggest MSCs have the potential for rapid bone regeneration and are the cell of choice in bone repair. However, in contrast with most studies in cardiovascular therapies, the numbers of patients studied in stem cell therapy for bone diseases and repair are relatively low and more long-term and sufficiently controlled clinical trials are needed to assess the therapeutic outcome. As MSCs are the progenitors responsible for the normal turnover of adult mesenchymal tissues and have high responsiveness to tissue injury, “intelligent” materials that are able to recruit endogenous MSCs in vivo and direct them down specific pathways will be a useful therapeutic avenue.
PROMISE AND OBSTACLES OF MSC THERAPY
The last few years have witnessed a growing enthusiasm for the clinical application of MSC-based therapy. Despite the significant potential, challenges in MSC’s clinical applications include low survival of transplanted cells, limited targeting capabilities, and low grafting efficiency and potency, which often requires use of a high number of cells to achieve therapeutic benefits. To date, clinical studies using stem cells have not been conclusive and are, in many cases, less impressive than what has been observed in preclinical models. A major obstacle limiting MSC clinical application is the lack of defining markers due to the inherent heterogeneity of MSC populations and variation associated with cell processing and expansion. The lack of standardization and variation in cell characterization and processing may help explain the discrepancies observed in some of the clinical studies[43]. Standardization is also critical for meaningful interpretation and comparison of experimental outcomes and understanding the mechanisms underlying the potential benefits of stem cells.
A hallmark of stem cells is their ability to expand in culture without phenotypic alternations. In the bone marrow obtained from human donors, hMSC’s are rare and in the range of approximately 1 in 105 nucleated cells. Because of the low occurrence of MSC in bone marrow, only culture-expanded MSCs are likely to meet the demand in clinical application. However, DNA replication is not a perfect process and in vitro cell processing and expansion could induce potential changes to the cell and increase risks in their therapeutic applications. In addition to the safety concerns, the impact of culture expansion and cell processing on hMSC therapeutic potency is largely unknown and requires further investigation. Recent studies have shown that sequential passaging of MSC using standard culture methods has been associated with a decrease in expression of adhesion molecules, the loss of chemokine receptors, enlargement of cell size and lack of chemotactic response to chemokines, thus compromising their therapeutic potency[44-46].
Several recent studies have illustrated the increasingly recognized importance of cell processing of MSC for specific clinical indications. Le Blanc’s group has recently shown that cryopreservation reduces the yield of ex vivo expanded MSC obtained from freshly harvested bone marrow mononuclear cells (MNC). In addition, MSC from fresh MNC were more potent in suppressing the lymphocyte responses in a mixed lymphocyte culture compared with MSC prepared from cryopreserved MNC[47]. In still another study, MSC pre-conditioned under hypoxic condition (0.5% O2 for 24 h) increased expression of pro-survival and pro-angiogenic factors and enhanced the capacity of MSC to repair infarcted myocardium, owing to reduced cell death and apoptosis of transplanted cells, increased angiogenesis, and paracrine effects[48]. While these studies confirmed the seemingly obvious notion that MSC properties and functional capacity vary depending on the processing protocols, they also represent the beginning of an important research arena that addresses a bottleneck in MSC therapy.
PROSPECTIVE
Stem cells produce all multi-cellular tissues in the body in tightly controlled microenvironments. As a result, they are particularly sensitive to their immediate environmental cues. A case in point is the importance of a seemingly pedestrian factor of oxygenation for stem cell fate. Low oxygen tension, traditionally termed “hypoxia”, is known to profoundly influence cellular events, cytokine physiology, and regenerative potential, and may in fact represent an “in situ” normoxia[49,50]. Although oxygen tension has been recognized as a developmentally important stimulus in vivo, it has not been adequately accounted for in in vitro cultures[51]. As the concept of MSC therapy shifted from the early proliferation-differentiation-engraftment assumption to the paracrine hypothesis, MSC therapeutic properties are now defined not only by their proliferative and multi-lineage potentials but also their ability to respond to and influence their immediate surrounding environments. To this end, basic and preclinical research will continue to play an important role in uncovering the dynamic interplay between stem cells and their microenvironments. Implementing these discoveries in clinical trials will be critical to advance MSC therapy from bench to a clinical reality.
Peer reviewer: Nadya L Lumelsky, PhD, NIH, NIDCR, 6701 Democracy Blvd, Room 618, Bethesda, MD 20892-4878, United States
S- Editor Li LF L- Editor Lutze M E- Editor Yang C
