Gene Therapy (2008) 15, 730–738 & 2008 Nature Publishing Group All rights reserved 0969-7128/08 $30.00 www.nature.com/gt REVIEW Inflammation and tumor microenvironments: defining the migratory itinerary of mesenchymal stem cells E Spaeth, A Klopp, J Dembinski, M Andreeff and F Marini Molecular Hematology and Therapy, Department of Stem Cell Transplantation and Cellular Therapy, UT-M.D. Anderson Cancer Center, Houston, TX, USA Mesenchymal stem cells (MSC) exhibit tropism for sites of ulating MSC migration to tumors is essential to ultimately tissue damage as well as the tumor microenvironment. Many develop novel clinical strategies aimed at using MSC as of the same inflammatory mediators that are secreted by vehicles to deliver antitumor proteins or suppress MSC wounds are found in the tumor microenvironment and are migration to reduce tumor growth. For example, radiation thought to be involved in attracting MSC to these sites. Cell enhances inflammatory signaling in the tumor microenviron- migration is dependent on a multitude of signals ranging from ment and may be used to potentiate site-specific MSC growth factors to chemokines secreted by injured cells and/or migration. Alternatively, restricting the migration of the MSC respondent immune cells. MSC are likely to have chemo- to the tumor microenvironment may prevent competent tactic properties similar to other immune cells that respond to tumor-stroma formation, thereby hindering the growth of injury and sites of inflammation. Thus, the well-described the tumor. In this review, we will discuss the role of model of leukocyte migration can serve as a reasonable inflammatory signaling in attracting MSC to tumors. example to facilitate the identification of factors involved in Gene Therapy (2008) 15, 730–738; doi:10.1038/gt.2008.39; MSC migration.Understanding the factors involved in reg- published online 10 April 2008 Keywords: mesenchymal stem cell; migration; tumor microenvironment; tumor stroma; inflammatory chemoattractants Brief description of the mesenchymal The use of bone marrow-derived MSC have been employed in support and engraftment of the transplan- stem cell tation of hematopoietic stem cells (HSCs) following Mesenchymal stem cells (MSC) are non-hematopoietic high-dose chemotherapy in an effort to replenish the adult stem cells with multilineage potential. MSC are destroyed bone marrow cell population.3 Additionally, defined by plastic adherence, differentiation potential pre-clinical studies have explored the use of MSC in the and cell surface marker expression.1 MSC, or MSC-like reduction of graft-versus-host disease, for tissue repair; cells, have been isolated from nearly every organ or including cerebral injury,4 bone fracture,5 myocardial tissue in the body, making it challenging to characterize ischemia/infarction,6 muscular dystrophy,7 as well as the MSC as a completely homogenous population. MSC tumor homing. contribute to the maintenance and regeneration of Migration of MSC to tumors is thought to be due to connective tissues and have the capacity to differentiate inflammatory signaling in a tumor resembling that of an within osteoblasts, adipocytes, chondrocytes, myocytes unresolved wound.8 The innate tropism of MSC for and cardiomyocytes. MSC express markers including tumors can be exploited for the delivery of antitumor CD29, CD44, CD51, CD73 (SH3/4), CD105 (SH2), CD166 agents to the tumor microenvironment. Gene-modified (ALCAM) and Stro-1, but the expression of specific MSC expressing interferon-b have been used to signifi- combinations of markers appear microenvironment- cantly reduce tumor burden and in some cases extend dependent, suggesting a strong influence of tissue survival in murine models of melanoma,9 lung,10 breast context on MSC phenotypes. In general, MSC appear to cancer11 and glioma.12 However, the mechanism and be a non-immunogenic population of cells; however, a factors responsible for the targeted tropism of MSC to few studies demonstrate immune-repressive functions of these wounded microenvironments remain to be fully MSC through the induction of peripheral tolerance elucidated. MSC are likely to have chemotactic proper- evident in autoimmune disorders such as multiple ties similar to other immune cells that respond to injury sclerosis.2 and sites of inflammation. Thus, the well-described model of leukocyte migration can serve as a reasonable Correspondence: Dr F Marini, Molecular Hematology and Therapy, example to facilitate the identification of factors involved Department of Stem Cell Transplantation, UT-M.D. Anderson in MSC migration. Following a discussion of alterations Cancer Center, Box081, 1515 Holcombe Boulevard, Houston, in the ‘wounded’/tumor microenvironment that are TX 77030, USA. E-mail: [email protected] shown to enhance MSC tumor-specific migration, we Received 14 February 2008; accepted 18 February 2008; published will introduce alternative, injury-induced cell migration online 10 April 2008 models based on literature reviewing leukocytes and Inflammation-driven mesenchymal stem cell migration E Spaeth et al 731 their progenitor cell line, the HSC. These alternative known to have an important role in leukocyte/macro- migration systems will provide rational for the tumor- phage infiltration into tumors.14 Based on this evidence, specific MSC migration and will lead us into an overview one can speculate that inflammation-induced chemo- of the current literature concerning MSC migration kines participate in the directed migration of stem cells, specifically. Ultimately, we will conclude with a discus- such as MSC, to tumors and inflamed microenviron- sion of the potential clinical applications of tumor- ments. Previous studies have shown the importance of directed MSC migration. inflammation to the successful homing of systematically infused stem cells (HSC) to cardiac tissue,15 thus reinfor- cing the notion of inflammation and chemokine production Inflammation-targeted homing in the in migration of MSC. In addition to the secreted tumor microenvironment chemotactic molecules secreted by the tumor and its surrounding stroma, the tumor cells themselves retain a Inflammation is a cellular response that takes place chemotactic disparity amongst its cellular components. under conditions of cellular injury and in sites of tissue When fractionated, the tumor cell membrane possesses a wounding. Over two decades ago, Dvorak and co- superior chemotaxis-induction potential compared with workers described the tumor as an unhealed wound the other cellular components such as cytosolic fractions, that produces a continuous source of inflammatory including organelles such as the nuclei, mitochondria, mediators (cytokines, chemokines and other potential lysosomes, microsomes and ribosomes (Figure 1). chemoattractant molecules). Cancer progression has been correlated with an increase in inflammatory mediator gene expression, and this is thought to occur Hypoxia contribution to MSC homing via disruption, damage and cellular turnover occurring in the tumor microenvironment. This constant produc- Many tumors exhibit hypoxia, a state of reduced oxygen tion of inflammatory mediators perpetuates the main- that often parallels and perpetuates inflammation. A tenance and progression of the tumor environment and common feature between inflammation and hypoxic becomes a target for the MSC. Tumor-generated inflam- environments is the expression of pro-angiogenic mole- matory mediators have a role in determining the cules. The hypoxia-induced transcription factor HIF-1a conditions of the tumor microenvironment, as they activates the transcription of genes including vascular regulate invasion, motility, extracellular matrix interac- endothelial growth factor (VEGF), macrophage migra- tion through autocrine effects as well as coordinating cell tion inhibitory factor, tumor necrosis factors (TNF-a), movement through paracrine signaling.13 Tumor-pro- numerous proinflammatory cytokines and the activation duced and tumor-induced inflammatory chemokines are of the transcription factor nuclear factor kB.16,17 Nuclear Matrigel Migration Assay 350 100 300 24 Hour 48 Hour 250 72 Hour 200 50 150 % of Migrated MSC 100 # of GFP Positive Cells 50 0 VEGF PDGF bFGF IGF TGFB FBS Live 293s Live MDA231 Live A375sm bFGF Live HEY Live SKOV3 Cell Membrane - 293s Cell Membrane - MDA23 Cell Membrane - A375sm Cell Membrane - OVAR3 Cell Membrane - HEY Cell Membrane - SKOV3 bFGF & IGF VEGF & PDGF PDGF & TGFB VEGF & TGFB bFGF & Serum 1 Cytokines Tumor cells Cell membrane Mixtures Figure 1 Cellular membrane components and mixtures of cytokines are potent mesenchymal stem cell (MSC) attractants. In an in vitro matrigel assay, the migratory capacity of human MSC was analyzed. MSC preferentially migrated toward selected chemoattractive regions of the matrigel matrix. The more effective migratory inducers include the growth factors, platelet-derived growth factor and FBS when alone. In combination, growth factors were more potent than when used as a single chemoattractant. The attractiveness of the tumor cell membrane is evident in comparison to other chemoattractants tested. The most potent cell membrane fraction was from the SKOV3 tumor cell lines, followed by the MDA-231 tumor cell lines. Gene Therapy Inflammation-driven mesenchymal stem cell migration E Spaeth et al 732 Table 1 Cell surface markers and receptors associated with cell migration that are known to be expressed on MSC Cell surface receptors Ligands Present on other cell types found on MSC Growth hormone EGFR (ErbB) EGF DC, neutrophil