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i. Mesenchymal Stem Cells page 2

L. Mesenchymal stem cells: paradoxes of passaging. Javazon EH, Beggs KJ, Flake AW. Exp Hematol. 2004;32:414-25.

This is an extensive and critical (and at times iconoclastic) review of the field of mesenchymal stem cell (MSC) biology, particularly in respect to the current paradox between in vitro promise and in vivo efficacy. The authors observe that MSCs are easily manipulated in vitro to virtually perform on command, but have proven surprisingly resistant to in vivo manipulations. Further, expansion in culture may alter the fundamental nature of MSCs, thereby confounding understanding and analysis of their true potential.

The basic question of how to define MSCs remains a point of discussion and controversy, and whether they qualify as "stem cells" remains a legitimate question. The authors review current knowledge regarding the developmental origin of MSCs and their in vitro characteristics, concluding that morphologic or phenotypic criteria cannot be used for their specific identification. Rather, the capacity for induced in vitro differentiation of MSCs to bone, fat, and cartilage is perhaps the single critical requirement to identify putative MSC populations.

In contrast to in vitro characterization, there has been minimal information on the in vivo behavior of MSCs. It appears that site-directed administration of MSCs can result in successful engraftment and integration under specific circumstances, most commonly injury repair. However, in most cases it is more likely that the beneficial effect of MSCs has been related to the local production of growth factors rather than direct participation. There is surprisingly little positive data supporting efficacy of MSCs for any purpose when administered systemically.

A great deal of work suggests that MSCs can be therapeutically useful in specific settings, as in repair of skeletal tissues, myocardial repair, CNS or spinal injury, and for facilitation of BMT. However, no conclusive evidence of efficacy of MSC infusions for facilitation of BMT or reduction of GVHD has yet been presented. Future progress will depend on further characterization of MSCs in both their fresh and culture expanded states so that observations on comparable cell populations can be made and reproduced. We will need to understand what is required for favoring self-renewal over differentiation in MSC cultures and how to maintain homing and engraftment potential during MSC expansion. More rigorous analyses of the immunology of MSCs in allogeneic systems will be required to determine whether these barriers may be crossed.

M. Mesenchymal stem cells. Roberts I. Vox Sang. 2004;87(Suppl 2):38-41.

The author succinctly reviews the definition and characteristics of mesenchymal stem cells (MSC), sources of MSC, their functions and therapeutic uses. He indicates that there is within the bone marrow a population of MSC that are able to differentiate into a number of different mesenchymal tissues, including bone and cartilage. MSCs have been demonstrated in a variety of fetal and adult tissues, including bone marrow, fetal blood and liver, cord blood amniotic fluid and, in some circumstances, in adult peripheral blood. There are no markers which specifically identify MSC and therefore they are defined by their immunophenotypic profile, their characteristic morphology, and by their extensive capacity for self-renewal while retaining their ability to differentiate into a number of mesenchymal lineages. MSC can differentiate in vitro into adipocytes, chrondrocytes, osteocytes, myocyctes, glial cells and neural cells. The function of MSC remains to be determined, although they are highly likely to play a role in the establishment and maintenance of hemopoietic stem and progenitor cells. Co-culture of hemopoietic cells and MSC shows that's MSC can support the proliferation and differentiation of hemopoietic stem/progenitor cells in vitro and cotransplantation of adult MSC has been shown to enhance engraftment of hemopoietic stem cells in animal models. Studies in animal models of cerebral injury, myocardial ischemia/infarction, muscular dystrophy and bone fractures show that MSC injected intravenously or implanted locally are able to migrate to sites of injury and to diffefentiate into cells of the appropriate phenotype. In some cases this approach has been shown to improve the function oft the damaged tissue, e.g., healing of a bony defect, but in others, no beneficial effects on tissue or organ function are seen. However, there are still very few direct examples of the clinical value of MSC.

N. Mesenchymal stem cells from cryopreserved human umbilical cord blood. Lee MW, Choi J, Yang MS, Moon YJ, Park JS, Kim HC, Kim YJ. Biochem Biophys Res Commun. 2004;320:273-8.

The authors state that the presence of mesenchymal stem cells (MSCs) in UCB has been disputed and it remains to be validated. In this study, they examined the ability of cryopreserved UCB harvests to produce cells with characteristics of MSCs. They were able to obtain homogeneous plastic adherent cells from the mononuclear cell fractions of cryopreserved UCB using specific culture conditions. These adherent cell populations exhibited fibroblast-like morphology and typical mesenchymal-like immunophenotypes (CD73+, CD105+, and CD166+, etc.). These cells showed prolonged proliferative capacity without any morphological changes for more than 6 passages (over 3 months) and had a differentiation potential to mesenchymal derivatives including osteoblast, adipocyte, and chondrocyte. Further, they expressed mRNAs of multi-lineage genes including SDF-1, NeuroD, and VEGF-R1, suggesting that the obtained cells had the multi-differentiation capacity as bone marrow-derived MSCs. The authors conclude that their results indicate that cryopreserved human UCB fractions can be used as an alternative source of MSCs for experimental and therapeutic applications.

O. Mesenchymal stem/progenitor cells developed in cultures from UC blood. Yang S-E, Ha C-W, Jung MH, et al. Cytotherapy 2004;6:476-486.

The authors point out that mesenchymal stem/progenitor cells (MSPC) are the best candidates for tissue engineering and cellular therapy of orthopedic musculoskeletal tissues, and for promoting the engraftment of CD34⁺ cells in hematopoietic cell transplantation. They indicate that MSPC can be transplantable between HLA-incompatible individuals, because they do not elicit alloreactive lymphocyte proliferate responses and modulate immune responses.

They identified, expanded in culture, and characterized MSPC from umbilical cord blood (UCB) harvests on a large scale. Ninety-five out of 411 units (23.1%) generated MSPC-like cells during cultivation. Nine UCB units (2.2%) yielded MSPC with more than 1000-fold expansion capacity, which the authors state is enough in cell numbers to be an allogeneic source for cellular therapy.

P. Mesenchymal stem cells: isolation and therapeutics. Alhadlaq A, Mao JJ. Stem Cells Dev. 2004;13:436-48.

This rather extensive review not only outlines several approaches relevant to the isolation and therapeutic use of MSCs, but also presents several examples of phenotypic and functional characterization of isolated MSCs and their progeny.

MSCs are defined as self-renewable, multipotent progenitor cells with the capacity to differentiate into several distinct mesenchymal lineages. The review compares multiple methodological approaches to isolate MSCs with elaboration on new technical advances and their implications toward potential therapeutic applications. Specific examples of isolation and culturing protocols of BM-derived rat and human MSCs are provided with some phenotypic and functional characterization assays performed on isolated MSCs and their progeny.

Q. Cotransplantation of HLA-identical sibling culture-expanded mesenchymal stem cells and hematopoietic stem cells in hematologic malignancy patients. Lazarus HM, Koc ON, Devine SM, Curtin P, Maziarz RT, Holland HK, Shpall EJ, McCarthy P, Atkinson K, Cooper BW, Gerson SL, Laughlin MJ, Loberiza FR Jr, Moseley AB, Bacigalupo A. Biol Blood Marrow Transplant. 2005;11:389-98.

The authors point out that mesenchymal stem cells (MSCs) are found in a variety of tissues, including human bone marrow; secrete hematopoietic cytokines; support hematopoietic progenitors in vitro; and possess potent immunosuppressive properties. They hypothesized that cotransplantation of culture-expanded MSCs and hematopoietic stem cells (HSCs) from HLA-identical sibling donors after myeloablative therapy could facilitate engraftment and lessen graft-versus-host disease (GVHD). To assess the safety and feasibility of this approach, they conducted a multicenter trial in which culture-expanded MSCs were coadministered with HLA-identical sibling-matched HSCs in 46 adult patients with hematologic malignancies.

Their results indicated that cotransplantation of HLA-identical sibling culture-expanded MSCs given in the course of an HLA-identical HSC transplantation after myeloablative conditioning is safe. Additionally, it is feasible to provide a homogeneous population of donor MSCs at a dose of 2.5 x 10⁶ cells per kilogram without resulting in a delay in performing the transplantation; sufficient MSCs were available for infusion from 51 (91%) of 56 donors.

However, the authors were not able to detect an effect on engraftment of on the incidence and severity of acute or chronic GVHD. They state that a randomized phase III clinical trial using uniformity in the preparative regimen and stem cell donor source will be necessary to document whether MSC infusion enhances the time to neutrophil and platelet count recovery in the course of an allogeneic transplantation.

R. Immunobiology of human mesenchymal stem cells and future use in hematopoietic stem cell transplantation. Le Blanc K, Ringden O. Biol Blood Marrow Transplant. 2005;11:321-34.

Mesenchymal stem cells (MSCs) may be derived from adult bone marrow, fat, and several fetal tissues. In vitro, MSCs can be expanded and have the capacity to differentiate into several mesenchymal tissues, such as bone, cartilage, and fat. They escape the immune system in vitro, and this may make them candidates for cellular therapy in an allogeneic setting. They also have immunomodulatory effects, inhibit T-cell proliferation in mixed lymphocyte cultures, prolong skin allograft survival, and may decrease graft-versus-host disease (GVHD) when cotransplanted with hematopoietic stem cells.

The authors summarized the published clinical experience of MSCs in hematopoietic cell transplantation in a table:

Disease	No. Patients	Source of MSCs/SCT Setting	Outcome	Study
Hematologic malignancies	15	Autologous IV infusion	No adverse events of 1, 10, and 50 x 10⁶ cells	Lazarus et al.
Breast cancer	28	Autologous in autologous SCT	IV infusion was safe; autologous SCT recovery was rapid	Koc et al.
Inborn errors of metabolism	11	HLA-identical from SCT donor	No immune response against donor; improved nerve-conduction velocity in metachromatic leukodystrophy	Koc et al.
Osteogenesis imperfecta	5	HLA-identical from SCT donor	Gene-marked MSCs engrafted; new dense bone formation; few fractures	Horwitz et al.
Acute myeloid leukemia	1	HLA-haploidentical from SCT donor	SCT engraftment with no GVHD	Lee et al.
Leukemia	31	HLA-identical from SCT donor	Rapid platelet engraftment; low incidence of acute GVHD	Frassoni et al.
Severe aplastic anemia	1	Allogeneic MSC	Engraftment; improved stroma	Fouillard et al.
Severe acute GVHD	1	Haploidentical MSC; unrelated donor SCT	Clearance of grade IV acute GVHD, twice	Le Blanc et al.

The authors comment that, in HSC transplantation, MSCs may be important for several indications. Overall, in allogeneic hematopoietic cell transplantation, MSCs may enhance engraftment of hematopoietic cells. This may be particularly important in cord blood transplantation, in which the limited cell dose delays engraftment of the absolute neutrophil count and platelets and in which there is an increased risk of graft failure. Whether MSCs in this setting have immunomodulatory effects and prevent rejection remains to be proven. Furthermore, with nonmyeloablative conditioning, the risk of graft failure is increased compared with myeloablative conditioning. Whether MSCs enhance donor cell engraftment and prevent rejection may be worthwhile to explore.

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Page Updated
11 Feb 2008

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