The role of immunosuppression of mesenchymal stem cells in tissue repair and tumor growth

Bone marrow is the most important source of adult stem cells, which contains heterogeneous populations of cells including hematopoietic stem cells, erythrocytes, fibroblasts, adipocytes and etc. Mesenchymal stem cells (MSCs) is a subset of nonhematopoietic stem cells existed in bone marrow originating from the mesodermal germ layer[1,2]. MSCs were first identified by Friedenstein and were described as an adherent, fibroblast-like population in the in vitro culture of bone marrow which were also found to be able to differentiate into bone in vivo[3]. MSCs also existed in other tissues, including adipose, umbilical cord, fetal liver, muscle and lung [4-8].MSCs have the ability to differentiate into multiple lineages such as chondrocytes, osteocytes, adipocytes, myocytes, and astrocytes, so MSCs could be considered as a potential source of stem cells for cellular and genetic therapy [5,9]. The phenotype of MSCs is identified by the absence of the CD34 and CD45 hematopoietic cell markers and is positive for CD29, CD90 and CD105[10]. MSCs express the major histocompatibility complex (MHC) class I but do not express MHC class II, B7-1, B7-2, CD40 and CD40L molecules. MSCs can be expanded more than 104-fold in culture without loss of their multilineage differentiation potential. Some studies have established that bone-marrow-derived MSCs can engraft injured tissues, such as those of the lung, liver, heart, and brain, and recover their function. In past ten years, many studies have confirmed that the use of stem cell transplantation is an important tool in the treatment of several types of malignancies[11-13]. For these reasons[14], such cells are currently being tested for potential use in cell and gene therapy for tumors[15,16]. In contrast, newer studies have proposed that stem cells may be the direct cellular targets of the genetic alterations that lead to tumor formation and important contributors to the maintenance of human cancers[17,18]. Emerging evidence suggests t