IntroductionBone marrow (BM) stromal cells were first identified by Friedenstein, who described an adherent fibroblast-like population able to differentiate into bone that he referred to as osteogenic precursor cells. 1 Subsequent studies demonstrated that these cells have the ability to differentiate into various other mesodermal cell lineages, including chondrocytes, tenocytes, and myoblasts (reviewed in Prockop 2 ). Based on this multilineage differentiation capacity, Caplan introduced the term mesenchymal stem cells (MSCs), 3 although many other terms have been introduced to describe a nonhomogenous population of multipotent cells. Although MSCs at a population level fulfill stem-cell criteria (ie, self renewal and multilineage differentiation capacity), it remains questionable whether the qualification "stem cell" is legitimate for MSCs at the single cell level. It was therefore recently proposed to use the term multipotent mesenchymal stromal cells (with the acronym MSCs) to describe fibroblast-like plastic-adherent cells. 4 Recently, Bonnet et al demonstrated that single cell-derived populations of murine BM-derived MSCs characterized by stage-specific embryonic antigen-1 expression, were capable of differentiation in vivo, 5 thus showing their true stem-cell properties. In this review, we will refer to the multipotent mesenchymal stromal cells with the acronym MSCs.Although MSCs originally were isolated from BM, 6 similar populations have been isolated from other tissues, including adipose tissue, 7 placenta, 8 amniotic fluid, 9 and fetal tissues such as fetal lung and blood. 10 In addition, umbilical cord blood (UCB) has been identified as a source of MSCs. 11,12 Probably as a result of their low frequency in UCB, conflicting reports initially have been published on the presence of MSCs in UCB. It has now become clear that the volume and storage time of the cord blood are important parameters for successful isolation of MSCs from UCB. 11 At present no specific marker or combination of markers has been identified that specifically defines MSCs. Phenotypically, ex vivo expanded MSCs express a number of nonspecific markers, including CD105 (SH2 or endoglin), CD73 (SH3 or SH4), CD90, CD166, CD44, and CD29. 6,13 MSCs are devoid of hematopoietic and endothelial markers, such as CD11b, CD14, CD31, and CD45. 6 The capacity to differentiate into multiple mesenchymal lineages, including bone, fat, and cartilage, is being used as a functional criterion to define MSCs. 2 21 and recently in amniotic fluid. 22 These primitive cell types require specific and stringent culture conditions, including embryonic stem cell-specific fetal calf serum (FCS), coated culture dishes (a.o. fibronectin), medium with specific growth factor requirements, specific type or culture dish, and prolonged culture duration at low cell density. Culturing these cells at higher cell density promotes differentiation toward a mesenchymal progenitor cell with restricted differentiation potential. 23 It has not been possible to prospectively isolat...

IntroductionHuman bone marrow stromal cells, also referred to as mesenchymal stem cells (MSCs), are able to differentiate along multiple lineages such as chondrocytes, osteoblasts, adipocytes, myocytes, and astrocytes. 1 MSCs, rare residents in the bone marrow, can be rapidly expanded ex vivo without loss of their multilineage differentiation potential. Because of their ability to migrate to sites of tissue injury, 2,3 MSCs have emerged as a promising therapeutic modality for tissue regeneration and repair. Several studies in animal models have demonstrated that MSCs are capable of long-term engraftment and in vivo differentiation, and encouraging results have been reported in clinical use. [4][5][6] MSCs are known to secrete a number of cytokines and regulatory molecules implicated in different aspects of hematopoiesis. 7 These characteristics have generated clinical interest to use MSCs to enhance hematopoietic stem cell engraftment. Although animal models provide experimental evidence that MSCs facilitate engraftment, 8,9 no conclusive evidence has yet been presented in humans. 6 In addition to providing critical growth factors, MSCs display immunosuppressive properties that might facilitate engraftment. In vitro studies with human, baboon, and murine MSCs demonstrated that MSCs suppress the proliferation of T cells induced by alloantigens or mitogens. 10-12 Furthermore, MSCs have been reported to induce T-cell division arrest, 13 to inhibit the differentiation and maturation of dendritic cells, 14,33 and to decrease the production of inflammatory cytokines by various immune cell populations. 15 Controversy exists regarding their effect on cytotoxic T cells and NK cells. 16,17 Animal studies indicate that, in line with their immunosuppressive capacities in vitro, MSCs also display immunosuppressive capacities in vivo; allogeneic MSCs may prolong skin allograft survival in immunocompetent baboons 10 and may prevent the rejection of allogeneic tumor cells in immunocompetent mice. 18 The mechanisms underlying these effects of MSCs have not been clearly identified. Although conflicting results have been reported, most studies agree that soluble factors are involved. 11,[18][19][20] The therapeutic application of the immunosuppressive properties of MSCs has already been exploited in the clinical setting for the treatment of acute graftversus-host disease after allogeneic stem cell transplantation. 21 The immunophenotype of MSCs, the low expression of human leukocyte antigen (HLA) major histocompatibility complex (MHC) class I, and the absence of costimulatory molecules, together with the observation that MSCs do not elicit a proliferative response from allogeneic lymphocytes, suggest that MSCs are of inherently low immunogenicity. 11,20 These properties might open attractive possibilities to use universal donor MSCs for different therapeutic applications.The aim of this study was to examine whether MSCs display immunosuppressive properties in vivo in murine allogeneic bone marrow transplantation models. The transp...

To study the biodistribution of MSCs, we labeled adult murine C57BL/6 MSCs with firefly luciferase and DsRed2 fluorescent protein using nonviral Sleeping Beauty transposons and coinfused labeled MSCs with bone marrow into irradiated allogeneic recipients. Using in vivo whole-body imaging, luciferase signals were shown to be increased between weeks 3 and 12. Unexpectedly, some mice with the highest luciferase signals died and all surviving mice developed foci of sarcoma in their lungs. Two mice also developed sarcomas in their extremities. Common cytogenetic abnormalities were identified in tumor cells isolated from different animals. Original MSC cultures not labeled with transposons, as well as independently isolated cultured MSCs, were found to be cytogenetically abnormal. Moreover, primary MSCs derived from the bone marrow of both BALB/c and C57BL/6 mice showed cytogenetic aberrations after several passages in vitro, showing that transformation was not a strain-specific nor rare event. Clonal evolution was observed in vivo, suggesting that the critical transformation event(s) occurred before infusion. Mapping of the transposition insertion sites did not identify an obvious transposonrelated genetic abnormality, and p53 was not overexpressed. Infusion of MSC-derived sarcoma cells resulted in malignant lesions in secondary recipients. This new sarcoma cell line, S1, is unique in having a cytogenetic profile similar to human sarcoma and contains bioluminescent and fluorescent genes, making it useful for investigations of cellular biodistribution and tumor response to therapy in vivo. More importantly, our study indicates that sarcoma can evolve from MSC cultures. STEM CELLS 2007;25:371-379

Mesenchymal stem cells (MSCs) are not only able to evade the immune system, but they have also been demonstrated to exert profound immunosuppressive properties on T cell proliferation. However, their effect on the initiators of the immune response, the dendritic cells (DCs), are relatively unknown. In the present study, the effects of human MSCs on the differentiation and function of both CD34+-derived DCs and monocyte-derived DCs were investigated. The presence of MSCs during differentiation blocked the differentiation of CD14+CD1a− precursors into dermal/interstitial DCs, without affecting the generation of CD1a+ Langerhans cells. In line with these observations, MSCs also completely prevented the generation of immature DCs from monocytes. The inhibitory effect of MSCs on DC differentiation was dose dependent and resulted in both phenotypical and functional modifications, as demonstrated by a reduced expression of costimulatory molecules and hampered capacity to stimulate naive T cell proliferation. The inhibitory effect of MSCs was mediated via soluble factors. Taken together, these data demonstrate that MSCs, next to the antiproliferative effect on T cells, have a profound inhibitory effect on the generation and function of both CD34+-derived and monocyte-derived DCs, indicating that MSCs are able to modulate immune responses at multiple levels.

Pentraxin 3 (PTX3) is a recently characterized member of the pentraxin family of acute-phase proteins produced during inflammation. Classical short pentraxins, C-reactive protein, and serum amyloid P component can bind to C1q and thereby activate the classical complement pathway. Since PTX3 can also bind C1q, the present study was designed to define the interaction between PTX3 and C1q and to examine the functional consequences of this interaction. A dose-dependent binding of both C1q and the C1 complex to PTX3 was observed. Experiments with recombinant globular head domains of human C1q A, B, and C chains indicated that C1q interacts with PTX3 via its globular head region. Binding of C1q to immobilized PTX3 induced activation of the classical complement pathway as assessed by C4 deposition. Furthermore, PTX3 enhanced C1q binding and complement activation on apoptotic cells. However, in the fluid-phase, pre-incubation of PTX3 with C1q resulted in inhibition of complement activation by blocking the interaction of C1q with immunoglobulins. These results indicate that PTX3 can both inhibit and activate the classical complement pathway by binding C1q, depending on the way it is presented. PTX3 may therefore be involved in the regulation of the innate immune response.