Previous studies have provided evidence for the existence of adult human bone marrow stromal stem cells (BMSSCs) or mesenchymal stem cells. Using a combination of cell separation techniques, we have isolated an almost homogeneous population of BMSSCs from adult human bone marrow. Lacking phenotypic characteristics of leukocytes and mature stromal elements, BMSSCs are non-cycling and constitutively express telomerase activity in vivo. This mesenchymal stem cell population demonstrates extensive proliferation and retains the capacity for differentiation into bone, cartilage and adipose tissue in vitro. In addition, clonal analysis demonstrated that individual BMSSC colonies exhibit a differential capacity to form new bone in vivo. These data are consistent with the existence of a second population of bone marrow stem cells in addition to those for the hematopoietic system. Our novel selection protocol provides a means to generate purified populations of BMSSCs for use in a range of different tissue engineering and gene therapy strategies.

The maintenance of bone marrow stromal stem cells (BMSSCs) is tightly controlled by the local microenvironment and by autocrine regulatory factors secreted by BMSSCs. To identify such factors, a cDNA subtraction library was generated from purified BMSSCs, based on their high expression of the STRO-1 antigen. Stromal-derived factor-1 (SDF-1) was one differentially expressed gene highly expressed by purified BMSSCs prior to culture. In vitro, immature preosteogenic cells expressed greater levels of SDF-1 when compared with mature cell types representative of osteoblasts and osteocytes/bone lining cells. Furthermore, SDF-1 expression was rapidly down-regulated when BMSSCs were cultured under osteoinductive conditions. BMSSCs were also shown to express functional cell surface SDF-1 receptors (CXCR4). Transduced BMSSC lines, secreting high SDF-1 levels, displayed an enhanced ability to form ectopic bone in vivo, in comparison with control BMSSC lines. Moreover, high SDF-1-expressing BMSSCs displayed an increased capacity for cellular growth and protection against interleukin-4-induced apoptosis. Similarly, fibroblast colonyforming units (CFU-Fs) also displayed increased growth and resistance to ␣-interferon-2a-induced apoptosis, in synergy with platelet-derived growth factor BB (PDGF-BB) and SDF-1 in vitro. These studies indicate that the chemokine, SDF-1, may play a role in the maintenance, survival, and osteogenic capacity of immature BMSSC populations. IntroductionPostnatal human bone marrow stromal stem cells (BMSSCs) or mesenchymal stem cells (MSCs) have the capacity to regenerate a hematopoietic-supportive bone marrow organ and associated bone trabecular, when transplanted into immunocompromised mice. [1][2][3][4][5] Recent studies have also reported that BMSSCs are more plastic than first realized, by virtue of their ability to develop into diverse cell lineages such as myelosupportive stroma, osteoblasts, chondrocytes, adipocytes, myoblasts, hepatocytes, cardiomyocytes, and neural cells. [6][7][8] These developments have prompted investigations into the possible use of ex vivo-expanded BMSSC populations for a wide range of tissue engineering and gene therapy applications. 9,10 Thus far, encouraging preliminary results have been reported for different human clinical trials. [11][12][13][14][15] However, the progress of these studies has largely been restrained because of a lack of understanding of the critical factors that regulate the growth and differentiation of human multipotential BMSSCs.Postnatal stem cells have been identified in various tissues residing in specialized microenvironments or stem cell niches, endowed with the capacity to mediate stem cell proliferation, migration, and differentiation. 4,16,17 These unique attributes involve a complex array of both paracrine and autocrine signaling molecules, specific cell-cell and cell-extracellular matrix interactions, and physiochemical and mechanical stimuli. We have recently reported that BMSSCs reside in a perivascular niche, based on a composite pr...

Multiple myeloma (MM) is an incurable plasma cell (PC) malignancy able to mediate massive destruction of the axial and craniofacial skeleton. The aim of this study was to investigate the role of the potent chemokine, stromalderived factor-1A (SDF-1A) in the recruitment of osteoclast precursors to the bone marrow. Our studies show that MM PC produce significant levels of SDF-1A protein and exhibit elevated plasma levels of SDF-1A when compared with normal, age-matched subjects. The level of SDF-1A positively correlated with the presence of multiple radiological bone lesions in individuals with MM, suggesting a potential role for SDF-1A in osteoclast precursor recruitment and activation. To examine this further, peripheral blood-derived CD14 + + + + + osteoclast precursors were cultured in an in vitro osteoclast-potentiating culture system in the presence of recombinant human SDF-1A. Although failing to stimulate an increase in TRAP + , multinucleated osteoclast formation, our studies show that SDF-1A mediated a dramatic increase in both the number and the size of the resorption lacunae formed.