Potential Role of Mesenchymal Stromal Cells in Pediatric Hematopoietic SCT
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Bone Marrow Transplantation (2008) 42, S60–S66 & 2008 Macmillan Publishers Limited All rights reserved 0268-3369/08 $32.00 www.nature.com/bmt REVIEW Potential role of mesenchymal stromal cells in pediatric hematopoietic SCT LM Ball1, ME Bernardo2, F Locatelli2 and RM Egeler1 1Department of Paediatric Immunology, Hematology, Oncology, Bone Marrow Transplantation, and Auto-immune Diseases, Leiden University Medical Center, Leiden, The Netherlands and 2Department of Pediatric Hematology/Oncology, Fondazione Istituti di Ricovero e Cura a Catattere Scientifico (IRCCS) Policlinico San Matteo, University of Pavia, Pavia, Italy Mesenchymal stromal cells (MSCs) can be isolated from restricted developmental potential, including fibroblasts, several human tissues and expanded for clinical use. osteoblasts, adipocytes and chondrocyte progenitors.1 MSCs are identified by phenotypic and functional Marrow stromal cells comprise a heterogeneous popula- characteristics, and are poor Ag-presenting cells not tion of cells, including reticular endothelial cells, fibro- expressing MHC class II or co-stimulatory molecules. blasts, adipocytes and osteogenic precursor cells that MSCs have potent immune-modulatory effects and provide growth factors, cell to cell interactions and matrix in vitro induce a more anti-inflammatory or tolerant proteins that play a role in the regulation of hematopoiesis.1 phenotype. Clinical studies have exploited both the Friedenstein2 originally described a population of immune-modulatory properties of MSCs as well as their adherent cells from the BM that were non-phagocytic and hematopoietic supportive role. MSCs have been safely exhibited a fibroblast-like appearance. Following ectopic administered for the treatment of severe steroid refractory transplantation under the kidney capsule, these cells GVHD. A phase I/II multicenter study included 25 (CFU-F) gave rise to a broad spectrum of differentiated children in whom 80% responded to either one or two connective tissues including bone, cartilage, adipose infusions of MSCs derived mainly from third party tissue and myelosupportive stroma.2,3 On the basis of donors. Twenty children have undergone co-transplanta- these observations it was proposed that these tissues were tion of haploidentical MSCs with PBSC in a phase I/II derived from a common precursor cell residing in the BM, study, which has overcome the problems of graft failure in termed the stromal stem cell, the BM stromal stem cell, the HLA-disparate grafts. Similarly, co-transplantation of MSC or the skeletal stem cell.4 MSCs secrete cytokines MSCs and cord blood stem cells is under investigation. important for hematopoiesis and promote engraftment of MSCs may have important future potential for the hematopoietic stem cells in experimental animal models, treatment of pediatric autoimmune disease as well as especially when the dose of hematopoietic stem cells is inborn errors such as osteogenesis imperfecta. Currently, limiting.5–7 much needed randomized studies under the auspices of the In light of the controversy as to whether MSCs at the EBMT are ongoing to determine the optimal use of these single cell level truly fulfill the criteria of self-renewal and exciting new modalities of treatment. multilineage differentiation capacity, it was recently pro- Bone Marrow Transplantation (2008) 42, S60–S66; posed to use the term multipotent mesenchymal stromal doi:10.1038/bmt.2008.286 cells (with the acronym MSCs) to describe fibroblast-like Keywords: mesenchymal stromal cells; hematopoietic plastic adherent cells.8 SCT; pediatrics Although the BM serves as the primary reservoir for MSCs, their presence has been reported in a variety of other tissues. These include periosteum and muscle connective tissue,9,10 fetal BM, liver and blood.11 There is still an ongoing debate about their presence in steady state Introduction peripheral blood, their frequency quite likely being extremely low. MSCs have been identified in cytokine The BM serves as a reservoir for different classes of stem (G-CSF) mobilized peripheral blood by some investigators,12 cells. In addition to hematopoietic stem cells, the BM although other studies were negative.13 MSCs have been comprises a population of marrow stromal cells or MSCs. identified in fetal blood15 and also in umbilical cord Stromal stem cells exhibit multilineage differentiation blood.14 However, other laboratories have been unable to capacity, and are able to generate progenitors with grow MSCs from umbilical cord blood.15 The frequency of MSCs in these sources is very low. In fetal blood, the frequency has been reported to decline with gesta- tional age, from about 1/106 mononuclear cells in first- trimester fetal blood to 0.3/106 mononuclear cells in term Correspondence: Dr LM Ball, Leiden University Medical Center, Depart- 11 ment of Pediatrics, PO Box 9600, 2300 RC Leiden, The Netherlands. cord blood. Recently, MSCs have been successfully E-mail: [email protected] isolated from human amniotic fluid.16 The phenotype of MSC and pediatric HSCT LM Ball et al S61 the culture-expanded amniotic fluid-derived cells was Mesenchymal stromal cells are poor Ag-presenting cells similar to that reported for MSCs derived from second- and do not express MHC class II or co-stimulatory trimester fetal tissues and adult BM. molecules. Human BM stromal cells suppress T-lympho- At present, no unique phenotype has been identified that cyte proliferation induced by cellular or nonspecific allows the reproducible isolation of MSC precursors with mitogenic stimuli20 and inhibit the response of naive and predictable developmental potential. The isolation and memory Ag-specific T-cells to their cognate peptide.20 characterization of stromal cell function therefore still rely Accordingly, expanded MSCs do not stimulate T-cell primarily on their ability to adhere to plastic and their proliferation in MLR and are able to downregulate expansion potential. The capacity of ex vivo expanded alloreactive T cell responses when added to MLCs.20,21 MSCs to differentiate into multiple mesenchymal lineages, However, in an immunocompetent host, MSCs have been including bone, fat and cartilage, is presently used as a shown to elicit an immune response (in the context of a functional criterion to define MSCs.1 murine model of reduced-intensity allogeneic SCT).24 Currently, no specific marker or combination of markers Human MSCs altered the cytokine secretion profile of are available to identify MSCs. Phenotypically, ex vivo DCs, naive and effector T cells (T helper 1 (T(H)1) and expanded MSCs express a number of nonspecific markers, T(H)2), and natural killer (NK) cells to induce a more anti- including CD105 (SH2 or endoglin), CD73 (SH3 or SH4), inflammatory or tolerant phenotype.21–23 CD90, CD166, CD44 and CD29.1 MSCs are devoid of Interaction of MSCs with the tryptophan catabolizing hematopoietic and endothelial markers such as CD11b, enzyme, indoleamine 2, 3-dioxygenase, has been described.26 CD14, CD31 and CD45.1 Recently, Buhring et al.17 However, the role of indoleamine 2, 3-dioxygenase in developed new antibodies, including W8B2 and frizzled-9 MSC-mediated immune suppression is not clear. Various (FZD9), for the isolation of human MSCs from BM and interactions mediating the suppression of T cell prolifera- placenta, respectively. tion have been proposed, but as yet the mechanisms remain Whether culture-expanded MSCs differ from their in vivo unclear. progeny is uncertain, as proliferation on plastic surfaces What is evident is that most studies demonstrate that could induce both phenotypic and functional changes.1,18 soluble factors are involved, as the separation of MSC and Recently, techniques have become available to isolate PBMC by a transwell permeable membrane does not and grow mesenchymal progenitors and to manipulate prevent the inhibition of proliferation.20,27 their growth under defined in vitro culture conditions. As a Mesenchymal stromal cells upregulate CD4 þ CD25 þ result, MSCs can be rapidly expanded to numbers that are Fox P3 þ cells (T regulatory immunophenotype),21 required for clinical application. This has allowed the although their depletion has no effect on the inhibition of clinical testing of culture-expanded MSCs in the context of T cell proliferation by MSC.22 hematopoietic SCT (HSCT). At low NK to MSC ratios, MSCs alter the phenotype of Standard conditions for expansion of MSCs include the NK cells and suppress proliferation, cytokine secretion and presence of serum, in most instances FBS, although cytotoxicity against HLA-class I expressing targets.23 Some platelet-rich plasma is currently being evaluated as an of these effects require cell-to-cell contact, whereas others alternative by some centers.19 Cell density is a critical factor are mediated by soluble factors, including transforming affecting the growth of cells. Culture attempts are usually growth factor-b1 and prostaglandin E2, suggesting the unsuccessful below a critical cell density. The cells can be existence of diverse mechanisms for MSC-mediated NK grown directly that is, unmanipulated, following collection cell suppression. On the other hand, MSCs are susceptible or after density gradient separation. The EBMT consor- to lysis by activated NK cells. tium has developed common expansion protocols for The pertinent in vitro immune modulatory properties of MSCs isolated from BM to be used in its ongoing and MSCs have recently been summarized.25 future clinical studies. Preclinical results Immunomodulatory properties of MSCs Almeida-Porada et