Review of Mesenchymal Stem Cells and Tumors: Executioner or Coconspirator?

Abstract

Mesenchymal stem cells (MSCs) are a versatile group of nonhematopoietic stem cells with high potency of proliferation and pluripotency of differentiation. Endogenous MSCs circulate in the blood until being called to sites of inflammation or tumors, where they could differentiate into bone, cartilage, fat, or muscle cells to repair or replace damaged tissue. Ectogenous MSCs also demonstrate satisfactory tropism toward primary tumor sites and metastatic foci with low immunogenicity and thus can be exploited as a cell-mediated gene therapy to counteract tumor growth and development. However, tumor-promoting, and even carcinogenetic, effects of MSCs are also observed both in vitro and in vivo, raising potential risks in clinical applications. Recent advances of MSCs in tumor-targeted biotherapy were summarized. New findings of the interaction between MSCs and tumor cells in the tumor microenvironment were expounded.

Introduction

Mesenchymal stem cells (MSCs) are a versatile group of nonhematopoietic stem cells with high potency of proliferation and pluripotency of differentiation. They derive from mesodermal progenitors and can be found in several adult and fetal tissues, as well as umbilical cord blood. MSCs have generated a great deal of interest for their potential use in regenerative medicine and tissue engineering due to the ease of their isolation, the extensive expansion rate, and their differentiation potential. Further, they can be easily transduced with viral vectors to be employed as a novel cellular vehicle in gene-therapy protocols.

In cancer-therapy approaches, MSCs have been used for the targeted delivery of immunostimulatory cytokines and chemokines (interferon [IFN]-α, IFN-β, interleukin [IL]-2, IL-12, and CX3CL1), suicide genes (thymidine kinase and cytosine deaminase), growth-factor antagonists (NK4), and oncolytic viruses after systemic administration, taking advantage of their homing capacities to the primary tumor site and, eventually, the metastatic locations, or by directly intratumoral injection. However, recent evidence suggests that MSCs participate in tumor growth and metastasis, partially due to their immunosuppressive and proangiogenic properties. They are also involved in the early stages of carcinogenesis through spontaneous transformation or transformation triggered by the introduction of oncogenes, which implies a potential risk in their application as tumor-targeted biotherapies. This review highlights recent works in this field.

General Properties of MSCs

An MSC is defined in the following aspects: 1) the property of adhering to plastic; 2) the phenotype of CD14⁻ or CD11b⁻, CD19⁻ or CD79α⁻, CD34⁻, CD45⁻, HLA-DR⁻, CD73⁺, CD90⁺, CD105⁺; and 3) the capacity of differentiating toward three lineages: chondrocyte, osteoblast, and adipocyte.¹

Isolation and Culture

Although MSCs could be isolated from adipose tissue, peripheral blood, fetal liver, lung, amniotic fluid, chorionic villi of the placenta, and umbilical cord blood, the bone-marrow–derived MSCs are the most prevalently used. Density gradient centrifugation and the adherence sieve method are common ways to isolate the cells because MSCs are morphologically symmetrical fibroblastoid-type cells with the property of adherence to plastic. When cultured in vitro, MSCs keep a consistent karyotype and telomerase activity within 12 generations and an adequate reproductive activity within 20–30 generations.²

Immunophenotype

MSCs derived from multitissue and organs have some phenotype in common.³ For example, they express CD44, CD29, CD73, CD105, and STRO-1 markers, but lack CD34, CD45, and T-lymphocyte–costimulating molecules, such as B7-1 (CD80), B7-2 (CD86), CD40, and CD40L. They also express a lot of adhesion molecules, including vascular cell molecule (VCAM, CD106), intercellular adhesion molecular (ICAM, CD54), and leukocyte-function–associated antigen-3 (LFA-3). MSCs have interleukin-receptors (IL-R-1, 3, 4, 6, and 7), γ-interferon-receptor (IFNγ-R), and transforming growth factor-β-receptor (TGF-β-R) on the cell surface. They express intermediate levels of MHC-I molecules, but are negative for MHC-II molecules. So far, no specific cell marker of MSCs is known.

Pluripotency of Differentiation

MSCs are pluripotent stem cells with “plasticity.” They could differentiate into various types of cells under appropriate stimulations, including osteoblasts, adipocytes, chondrocytes, myoblasts, astrocytes, liver cells, pancreatic islet β-cells, and endothelial precursor cells.⁴ This pluripotency maintains after continuous passage culture and cryopreservation. There are mainly two ways to achieve the directed differentiation of MSCs: One is to make mild reversible cell lesions via chemical drugs, and the other is to use chemokines to change the cell's phenotype.

Targeted Migration

MSCs reside predominantly in the bone marrow, while they are also distributed throughout a variety of connective tissues, such as adipose, cartilage, and muscle, serving as local sources of dormant stem cells for tissue maintenance and regeneration. They are called into action with a priority following tissue damages, such as injury or chronic inflammation. These conditions are typically accompanied by the release of specific endocrinal signals from the damaged tissues that are then transmitted to the bone marrow, leading to the mobilization and subsequent recruitment of MSCs to the destination. The way MSCs cross the vascular endothelium and basilar membrane resembles leukocyte chemotaxis, both sharing lots of chemokine and adhesion molecule receptors.⁵ Solid tumor cells infiltrate and destroy normal tissues, forming a tumor microenvironment with large amounts of inflammatory cells.⁶ This tumor microenvironment preferentially promotes the engraftment of MSCs, as compared with other tissues, via various soluble factors secreted by the inflammatory, as well as tumor, cells, including epidermal growth factor (EGF), vascular endothelial growth factor-A (VEGF-A), fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), stromal cell-derived factor-1α (SDF-1α), IL-8, IL-6, granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), monocyte chemoattractant protein-1(MCP-1), hematopoietic growth factor (HGF), transforming growth factor-β1 (TGF-β1), urokinase-type plasminogen activator (uPA),⁷ and so on. MSCs are known to functionally express chemokine receptors as CCR1, CCR4, CCR7, CCR9, CCR10, CXCR4, CXCR5, CXCR6, CX3CR1, and c-met, which might be responsible for their tumor-homing process.

During the migration of MSCs toward tumors, the route of administration, the nature of the tumor cells, the location of the primary tumor, and the type of MSCs injected appear to be the key determinants. Intravenous (i.v.), subcutaneous (s.c.), and intraperitoneal (i.p.) injections have all been proven efficient routes of administration. Menon et al. have demonstrated that the SDF-1 expression of rat MSCs differs when exposed to conditioned medium from tumor cells and bone marrow cells, suggesting a differential gene regulation in MSCs exposed to different microenvironments and the subsequent influence on cell functions, such as chemotaxis.⁸

Although in vivo studies suggest that MSCs could migrate toward both primary and metastatic tumor sites, the process is not completely specific. In most cases, MSCs are also distributed to a wide range of organs not affected by the disease, such as lung, kidney, liver, and spleen. There is no quantification to assess the tumor-homing efficiency or the possible side-effects on other healthy organs as yet.

MSCs in Tumor Progression and Metastasis

Tumor cells interact with the tumor microenvironment and the whole internal environment to fulfill their development, invasion, and metastasis. As important components of the tumor microenvironment, MSCs could modulate tumor growth and development in many ways. The results arise from both in vitro and in vivo studies, either by injection of MSCs in the same site of tumor cells or at a distance from the tumor cells.

Inhibitory Effects

MSCs inhibit tumor growth directly

Khakoo et al. have proven that MSCs could decrease the activity of Akt protein kinase in the PI3K/Akt signaling pathway of tumor cells via direct contact, which could be inversed by E-cadherin neutralizing antibody. This antiproliferative action has nothing to do with systemic immune responses and is dependent on the dose of MSCs.⁹ Indeed, different doses of MSCs lead to different effects on tumor growth: A 10-fold enhancement of a tumor-suppresive MSCs dose might be tumor promoting, on the contrary.¹⁰

MSCs could upregulate the mRNA expression of cell-cycle negative regulator p21 and apoptosis-associated protease caspase-3, resulting in a G₀/G₁ phase arrest and apoptotic cell death of tumor cells.¹¹ They also secrete Dickkopf-1 (DKK-1) to suppress the Wnt/β-catenin signaling pathway, attenuating the malignant phenotype of tumor cells.¹²

MSCs serve as antigen-presenting cells

Recent studies have demonstrated that MSCs could behave as potent antigen-presenting cells (APCs) under appropriate stimulations, amplifying antigen-specific immune responses. With proper microenvironmental concentration of IFN-γ, MSCs could process and present human leukocyte antigen (HLA) class II–restricted antigens to helper T-cells (TH).¹³ They can also process and present HLA class I–restricted viral or tumor antigens to specific cytotoxic T-lymphocytes (CTLs) with a limited efficiency, likely because of some defects in antigen-processing machinery (APM) components, and induce tumor-suppressive cytokine release, such as IFN-γ and granzyme B (GrB). MSCs themselves are protected from CTL-mediated lysis through a mechanism that is partly soluble human leukocyte antigen-G (sHLA-G) dependent.¹⁴

Exosomes are 30–100-nm vesicles secreted by a wide range of mammalian cell types after fusion of the multivesicular body (MVB) and the cell membrane.¹⁵ A recent study in vitro has demonstrated that when IFN-γ–stimulated bone marrow MSCs are pulsed with tumor-derived exosomes, the immune response against hepatocellular carcinoma becomes significantly enhanced. MSCs uptake a large amount of tumor-derived heat shock proteins (HSPs) and tumor-associated antigens on the surface of exosomes via endocytosis, leading to a subtle change in their own phenotype, displaying antitumor activities as effector cells with great enhancement. Meanwhile, MSCs are also serving as APCs, inducing significant CD8⁺ T-lymphocyte-mediated antitumor immune response.¹⁶

MSCs serve as cellular vehicles

Genetically modified MSCs could migrate specifically to and produce biologic agents locally at tumor sites, with their proliferation and differentiation capabilities retained. Besides, their negative phenotype of HLA class II–restricted antigens and T-lymphocyte costimulating factors makes it less possible to trigger the host-immune rejection. With all the advantages above, MSCs are considered a promising new tool in cell-based antitumor therapies to transport antitumor drugs, activate immune responces, and suppress angiopoiesis.

At the beginning, MSCs were modified to express IFN-β and displayed an inhibitory effect on the growth of A375SM melanoma cells in vivo.¹⁷ MSCs expressing IFN-γ inhibited leukemic cells proliferation and induced apoptosis.¹⁸ In the study of Chen et al., MSCs were adenovirally engineered to secrete IL-12 and evaluated for their anticarcinogenesis efficacy against three kinds of unestablished tumor models, including B16 melanoma, Lewis lung cancer, and hepatocellular carcinoma. As a novel approach, AdIL-12-MSCs have revealed expected preventive effects on carcinogenesis (p < 0.01) with low-toxic, broad-spectrum, and long-range superiorities. This suggests that AdIL-12-MSCs possess the potential for targeting preclinical tumor lesions and depriving surviving or hibernating tumor cells, which have escaped from conventional treatments.¹⁹

A suicide gene, in genetics, will cause a cell to kill itself through apoptosis. Kucerova et al. reported that the suicide gene of cytosine deaminase (CD)-introduced human adipose tissue-derived mesenchymal stem cells (AT-MSCs) could produce a tumor-specific prodrug-converting effect. CD-AT-MSCs, in combination with prodrug 5-fluorocytosine (5-FC), could augment the “bystander effect” and selective cytotoxicity against tumor target cells when directly cocultured in vitro. Significant inhibition of s.c. tumor xenograft growth by s.c.- or i.v.-administered CD-AT-MSCs was also observed in immunocompromised mice treated with 5-FC in vivo.²⁰ This therapy may be particularly suitable for treating diffuse cancers, such as glioblastoma multiforme.

Except for primary tumor cells, gene-engineered MSCs also demonstrate a targeted inhibitory effect on tumor metastasis as well as tumor angiogenesis and lymphangiogenesis. Hong et al. infected mouse MSCs with adenoviruses expressing CX3CL1 (fractalkine) and administered them systemically to the mice bearing lung metastases of C26 and B16F10. CX3CL1-MSCs strongly inhibited the development of metastasis and prolonged survival of the tumor-bearing mice. This antitumor effect was dependent on both innate and adaptive immunity.²¹ Ren et al. preestablished metastases in three kinds of advanced cancer models, including B16 melanoma, 4T1 breast tumor, and hepatoma carcinoma. They found that the progression of metastasis into multistep lymph nodes (LNs) and internal organs was markedly impeded in the midway stage and reversed in the ultimate stage following repeated i.v. injections of IL-12 gene-engineered MSCs. This antimetastasis effect was due to activation of natural killer (NK) cells and CD8⁺ T-lymphocytes,²² as well as the downregulation of activated VEGF-D. An increased tumor apoptosis index was observed at the same time.²³ It is well known that NK4 is an antagonist of HGF and an inhibitor of angiogenesis. MSCs expressing NK4 strongly inhibited the development of lung metastases in the C-26 lung metastasis model after systemic administration and significantly prolonged survival of the tumor-bearing animals. With an inhibited signaling pathway of HGF-c-Met, NK4-MSCs inhibited tumor-associated angiogenesis and lymphangiogenesis and induced apoptosis of the tumor cells.²⁴ Adenovirus type 5 early-region 1A (Ad5.E1A) gene has recently been demonstrated to have antitumor effects. I.v. injection of conditional replicative adenovirus (CRAds)-loaded human MSCs into mice with established MDA-MB-231 pulmonary metastatic disease homed to the tumor site, began a CXCR4-promoted E1A gene expression, and successfully led to extended mice survival.²⁵

Promoting Effects

MSCs contribute to tumor formation

Mesodermal cells, especially endothelial cells and pericytes, are meant to help with tissue repair and stroma formation during tumor growth. Both circulating or exogenously administered MSCs can be recruited in large numbers to the tumor site. This process could be related to high local concentrations of paracrine growth factors, such as VEGF-A, IL-8, TGF-β1, epidermal growth factor (EGF), PDGF, and other mediators, within the tumor microenvironment.²⁶ Once placed into the tumor microenvironment, MSCs integrate into the tumour-associated stroma and compete with local mesenchymal precursors in their ability to proliferate. They become incorporated into the tumor architecture and form a fibrous capsule at the tumor periphery.¹⁷

MSCs enhance tumor growth directly

The tumor microenvironment facilitates tumor development and metastatic spread by eliciting reversible changes in the phenotype of tumor cells. Karnoub et al. reported that, in vitro, weakly metastatic human breast cancer cells stimulated MSCs to oversecrete the chemokine, CCL5 (RANTES), via a cell-to-cell contact, which then acted in a paracrine fashion on the cancer cells and enhanced their motility, invasion, and metastasis. This enhancement was reversible and dependent on CCL5 signaling through the chemokine receptor, CCR5.²⁷ MSCs could also facilitate low-invasive breast cancer cells (MCF7 and T47D) to entry across the blood vessels into bone marrow in an early period when the tumor burden was low, partly through Tac1-mediated regulation of CXCL12 (SDF-1α) and its receptor, CXCR4.²⁸

MSCs suppress immune response

It has been accepted that malignant cell proliferation is caused by the “immunoparalysis” of the host. MSCs could inhibit or restrict inflammatory responses and promote the mitigating and anti-inflammatory pathways, participating in building an “immunosuppressive microenvironment” via their weak immunophenotype and secreteion of anti-inflammatory molecules, which favors tumor growth and metastasis.²⁹

MSCs have a reversibly inhibitory effect on the differentiation of dendritic cells (DCs) from CD14⁺ monocytes.³⁰ They inhibit DCs maturation by downregulating APC-related molecules, such as CD1a, CD40, CD80, CD83, CD86, and HLA-DR.²⁹ They also alter the cytokine secretion profile of DCs, such as TNF-α.

MSCs suppress CD3⁺, CD4⁺, and CD8⁺ T-lymphocyte proliferative response to allogenic or xenogenic antigens³¹ via soluble factors, such as TGF-β, HGF, IL-10, IL-4, and prostaglandin E2 (PGE2). It is reported that human MSCs could decrease IFN-γ secretion of TH1 cells, but increase the IL-4 secretion of TH2 cells. They also lead to an increase in the proportion of regulatory T-cells (TRegs) present. MSCs are not sensitive to CD8⁺ CTL-mediated lysis and are able to inhibit CTL cytotoxicity in a time- and dose-dependent manner when present at CTL priming.³²

NK cells possess both activating receptors and inhibitory receptors on their surface. MSCs could inhibit NK-cell activation via an augmentation during interaction with the inhibitory receptors. They inhibit IFN-γ production of IL-2- activated NK cells,³⁰ with themselves lysed at the same time.³³

The inhibitory effect of MSCs on B-lymphocytes was recently demonstrated to occur through an arrest in the G₀/G₁ phase of the cell cycle, but not the induction of apoptosis.³⁴ MSCs secrete chemokine receptor CCL2, displaying an inhibitory effect on immunoglobulin production with the presence of matrix metalloproteinase.³⁵

MSCs induce angiogenesis

For most solid tumors, angiogenesis is the key point in tumor growth and metastasis. MSCs express proangiogenic factors, including angiopoietin-1 (Ang-1) and VEGF, and growth factors, such as PDGF, FGF-2, and FGF-7, but also cytokines (IL-6 and TNF-α), as well as plasminogen activator. All these molecules act synergistically on endothelial cells to promote vasculogenesis and angiogenesis.³⁶ Among them, VEGF-A, IL-8, and TGF-β1 are key mediators in this complex process. This indicates that the same pathways are involved in both the induction of tumor angiogenesis and MSC recruitment by tumors. Besides, MSCs could induce the expression of junction proteins, such as occludin, and an increase in microvascular integrity.³⁷ MSCs also participate in the angiogenesis process directly as endothelial cell, pericytes, or smooth muscle cell.²⁶

MSCs in Carcinogenesis

MSCs have high potency of proliferation and multidirectional differentiation. They display genomic instability during long-time ex vivo cultures.³⁸ The mutation brings distinct genotype and phenotype, often with an enhancement in telomerase activity.³⁹ In vivo, MSCs are involved in the early stages of carcinogenesis through spontaneous transformation or transformation triggered by the introduction of oncogenes. Houghton et al. were the first to report that in a model of gastric cancer induced by helicobacter, the transplantation of MSCs led to their engraftment into gastric glands, suggesting MSCs may be one source of tumor stem cells.⁴⁰

Both in vitro and in vivo studies have demonstrated that sarcomas could be derived from MSCs.⁴¹ Rubio et al. have characterized tumors generated by spontaneously transformed human mesenchymal cells (TMCs) previously obtained in their laboratory. Immunohistopathologic analyses identified these tumors as poorly differentiated carcinomas, suggesting that a mesenchymal-epithelial transition was involved in the generation of TMCs.⁴²

Conclusions

Tumor-targeted biotherapy is a promising new field among the general antitumor treatments. Gene technology ensures the selective expression of antitumor agents, while stem-cell technology makes it possible to bring these therapeutic agents selectively to the tumor sites. MSCs appear to hold great advantage for reasons that include easier propagation in culture, possible genetic modification to express therapeutic factors, and preferential homing ability to sites of tumor growth upon in vivo transfer. However, the safety of using MSCs is to be questioned, as they exhibit immunomodulatory and proangiogenic effects and are involved in the early stages of carcinogenesis through transformation in vivo. For this reason, Marta et al. have recently raised a design of synthetic extracellular matrix scaffold to confine genetically engineered MSCs within a determined location distant from the primary tumor site. An effective antitumor response and tumor regression were observed through the release of functional therapeutic agents into the bloodstream.⁴³ Overall, MSCs represent great hope for antitumor therapies, but the precise biologic characteristics of them and a thorough evaluation of their potential risks still need to be explored.

Footnotes

Disclosure Statement

No competing financial interests exist.

References

Dominici

, Le

, Mueller

et al. Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy, 2006; 8:315.

Pittenger

, Mackay

, Beck

et al. Multilineage potential of adult human mesenchymal stem cells. Science, 1999; 284:143.

Majumdar

, Keane-Moore

, Buyaner

et al. Characterization and functionality of cell surface molecules on human mesenchymal stem cells. Biomed Sci, 2003; 10:228.

Jiang

, Jahagirdar

, Reinhardt

et al. Pluripotency of mesenchymal stem cells derived from adult marrow. Nature, 2002; 418:41.

Henschler

, Deak

, Seifried

. Homing of mesenchymal stem cells. Transfus Med Hemother, 2008; 35:306.

Coussens

, Werb

. Inflammation and cancer. Nature, 2002; 420:860.

Honczarenko

, Lea

, Swierkowski

et al. Human bone marrow stromal cells express a distinct set of biologically functional chemokine receptors. Stem Cells, 2006; 24:1030.

Menon

, Picinich

, Koneru

et al. Differential gene expression associated with migration of mesenchymal stem cells to conditioned medium from tumor cells or bone marrow cells. Stem Cells, 2007; 25:520.

Khakoo

, Pati

, Anderson

et al. Human mesenchymal stem cells exert potent antitumorigenic effects in a model of Kaposi's sarcoma. Exp Med, 2006; 203:1235.

10.

Djouad

, Bony

, Apparailly

et al. Earlier onset of syngeneic tumors in the presence of mesenchymal stem cells. Transplantation, 2006; 82:1060.

11.

, Yuan

, Wang

et al. The growth inhibitory effect of mesenchymal stem cells on tumor cells in vitro and in vivo. Cancer Biol Ther, 2008; 7:245.

12.

Etheridge

, Spencer

, Heath

et al. Expression profiling and functional analysis of wnt signaling mechanisms in mesenchymal stem cells. Stem Cells, 2004; 22:849.

13.

Romieu

, Francois

, Boivin

et al. Regulation of MHC class II expression and antigen processing in murine and human mesenchymal stromal cells by IFN-gamma, TGF-beta, and cell density. Immunol, 2007; 179:1549.

14.

Morandi

, Raffaghello

, Bianchi

et al. Immunogenicity of human mesenchymal stem cells in HLA-class I-restricted T-cell responses against viral or tumor-associated antigens. Stem Cells, 2008; 26:1275.

15.

Wolfers

, Lozier

, Raposo

et al. Tumor-derived exosomes are a source of shared tumor rejection antigens for CTL cross-priming. Nat Med, 2001; 7:297.

16.

, Ren

, Jiang

et al. Antitumor activities against hepatocellular carcinoma induced by bone marrow mesenchymal stem cells pulsed with tumor-derived exosomes. Beijing Da Xue Xue Bao, 2008; 40:494.

17.

Studeny

, Marini

, Champlin

et al. Bone marrow derived mesenchymal stem cells as vehicles for interferon-beta delivery into tumors. Cancer Res, 2002; 62:3603.

18.

, Lu

, Huang

et al. In vitro effect of adenovirus-mediated human gamma-interferon gene transfer into human mesenchymal stem cells for chronic myelogenous leukemia. Hematol Oncol, 2006; 24:151.

19.

Chen

, Wang

, Zhao

et al. Prophylaxis against carcinogenesis in three kinds of unestablished tumor models via IL12-gene-engineered MSCs. Carcinogenesis, 2006; 27:2434.

20.

Kucerova

, Altanerova

, Matuskova

et al. Adipose tissue-derived human mesenchymal stem cells mediated prodrug cancer gene therapy. Cancer Res, 2007; 67:6304.

21.

Hong

, Masahiko

, Hiroyuki

et al. Targeted delivery of CX3CL1 to multiple lung tumors by mesenchymal stem cells. Stem Cells, 2007; 25:1618.

22.

Ren

, Kumar

, Chanda

. Cancer gene therapy using mesenchymal stem cells expressing interferon-beta in a mouse prostate cancer lung metastasis model. Gene Ther, 2008; 15:1446.

23.

Chen

, Lin

, Zhao

et al. A tumor-selective biotherapy with prolonged impact on established metastases based on cytokine gene-engineered MSCs. Mol Ther, 2008; 16:749.

24.

Kanehira

, Xin

, Hoshino

et al. Targeted delivery of NK4 to multiple lung tumors by bone marrow-derived mesenchymal stem cells. Cancer Gene Ther, 2007; 14:894.

25.

Stoff-Khalili

, Rivera

, Mathis

et al. Mesenchymal stem cells as a vehicle for targeted delivery of CRAds to lung metastases of breast carcinoma. Breast Cancer Res Treat, 2007; 105:157.

26.

Birnbaum

, Roider

, Schankin

et al. Malignant gliomas actively recruit bone marrow stromal cells by secreting angiogenic cytokines. Neurooncology, 2007; 83:241.

27.

Karnoub

, Dash

, Vo

et al. Mesenchymal stem cells within tumour stroma promote breast cancer metastasis. Nature, 2007; 449:557.

28.

Corcoran

, Trzaska

, Fernandes

et al. Mesenchymal stem cells in early entry of breast cancer into bone marrow. PLoS ONE, 2008; 3:e2563.

29.

Djouad

, Plence

, Bony

et al. Immunosuppressive effect of mesenchymal stem cells favors tumor growth in allogeneic animals. Blood, 2003; 102:3837.

30.

Aggarwal

, Pittenger

. Human mesenchymal stem cells modulate allogeneic immune cell responses. Blood, 2005; 105:1815.

31.

Glennie

, Soeiro

, Dyson

et al. Bone marrow mesenchymal stem cells induce division arrest anergy of activated T cells. Blood, 2005; 105:2821.

32.

Angoulvant

, Clerc

, Benchalal

et al. Human mesenchymal stem cells suppress induction of cytotoxic response to alloantigens. Biorheology, 2004; 41:469.

33.

Spaggiari

, Capobianco

, Becchetti

et al. Mesenchymal stem cell-natural killer cell interactions: Evidence that activated NK cells are capable of killing MSCs, whereas MSCs can inhibit IL-2-induced NK-cell proliferation. Blood, 2006; 107:1484.

34.

Corcione

, Benvenuto

, Ferretti

et al. Human mesenchymal stem cells modulate B-cell functions. Blood, 2006; 107:367.

35.

Rafei

, Hsieh

, Fortier

et al. Mesenchymal stromal cell-derived CCL2 suppresses plasma cell immunoglobulin production via STAT3 inactivation and PAX5 induction. Blood, 2008; 12:4991.

36.

Kinnaird

, Stabile

, Burnett

et al. Marrow-derived stromal cells express genes encoding a broad spectrum of arteriogenic cytokines and promote in vitro and in vivo arteriogenesis through paracrine mechanisms. Circ Res, 2004; 94:678.

37.

Zacharek

, Chen

, Cui

et al. Angiopoietin1/Tie2 and VEGF/Flk1 induced by MSCS treatment amplifies angiogenesis and vascular stabilization after stroke. J Cereb Blood Flow Metab, 2007; 27:1684.

38.

Rubio

, Garcia-Castro

, Martin

et al. Spontaneous human adult stem cell transformation. Cancer Res, 2005; 65:3035.

39.

Serakinci

, Guldberg

, Burns

et al. Adult human mesenchymal stem cell as a target for neoplastic transformation. Oncogene, 2004; 23:5095.

40.

Houghton

, Stoicov

, Nomura

et al. Gastric cancer originating from bone marrow–derived cells. Science, 2004; 306:1568.

41.

Tolar

, Nauta

, Osborn

et al. Sarcoma derived from cultured mesenchymal stem cells. Stem Cells, 2007; 25:371.

42.

Rubio

, Garcia

, De la

Cueva T

et al. Human mesenchymal stem cell transformation is associated with a mesenchymal-epithelial transition. Expe Cell Res, 2008; 314:691.

43.

Marta

, Ángel

, David

et al. Tumor immunotherapy using gene-modified human mesenchymal stem cells loaded into synthetic extracellular matrix scaffold. Stem Cells, 2009; 27:753.