While the association of MDSCs in a wide
While the association of MDSCs in a wide range of cancers has been known for some time, the mechanisms involved in expansion of these immunomodulatory p-Cresyl sulfate are just beginning to be unraveled. IL-6 has been reported to mediate the expansion of MDSCs, and this cytokine is known to be secreted by both MSCs and stromal cells (Hung et al., 2007; Nemunaitis et al., 1989). However, in this work, we did not find IL-6 to be involved. This may be due to the lower levels of this cytokine and the masking of its effects by the much higher levels of HGF in our system (see Figure 3B). Moreover, the strong association of MDSCs with cancer suggests that multiple and redundant pathways are likely involved (Ostrand-Rosenberg and Sinha, 2009; Gabrilovich and Nagaraj, 2009). There is consensus, however, that STAT3 may be the final transcription factor involved in the expansion of MDSCs. Our finding that STAT3 activation is involved in HGF-mediated MDSC expansion further supports the importance of this molecule in inflammation and cancer (Yu et al., 2009).
Introduction Mesenchymal stem/stromal cells (MSCs) are defined as nonhematopoietic, plastic-adherent, self-renewing cells that are capable of in vitro trilineage differentiation into fat, bone, and cartilage (Pittenger et al., 1999). Additional plasticity of MSCs has been suggested by experiments demonstrating their in vitro differentiation into myocytes, neuron-like cells, and hepatocytes (Drost et al., 2009; Galvin and Jones, 2002; Tao et al., 2009). Despite these data, the term “MSCs” has been controversial, as a definitive demonstration of their “stemness” by single-cell isolation and in vivo serial transplantation experiments has been lacking (Bianco et al., 2013). These multipotent cells are found in various fetal and adult human tissues, including bone marrow (BM), umbilical cord blood (UCB), liver, and term placenta (Battula et al., 2007; Erices et al., 2000; Yen et al., 2005; Zvaifler et al., 2000). MSCs are multipotent and have low immunogenicity, and therefore are considered as potential candidates for a variety of clinical applications (Jung et al., 2012; Stappenbeck and Miyoshi, 2009), including cartilage reconstitution and the treatment of rheumatoid arthritis, acute osteochondral fractures, spinal disk injuries, and inherited diseases such as osteogenesis imperfecta (Guillot et al., 2008). However, to date, these cells have been poorly characterized, which raises significant concerns because human trials using MSCs are currently under way. MSCs can be retrospectively identified based on their ability to form colony-forming unit fibroblasts (CFU-Fs) in vitro (Friedenstein et al., 1974). Traditionally, the isolation of MSCs from unfractionated whole BM (WBM) has relied on their adherence to plastic dishes. This technique gives rise to heterogeneous cell populations that frequently are contaminated with osteoblasts and/or osteoprogenitor cells, fat cells, reticular cells, macrophages, endothelial cells, and hematopoietic cells (Pittenger et al., 1999). Prolonged culture is often required to remove these contaminants and obtain a reasonably pure population of MSCs. However, during this process, the differentiation, proliferation, and migration potency of the MSCs gradually diminishes as the cells acquire a more mature phenotype (Kim et al., 2009; Rombouts and Ploemacher, 2003). In an effort to overcome these problems, investigators have made an intense effort to identify reliable MSC surface markers that could facilitate the prospective isolation of colony-initiating cells. Various surface markers, including CD49a, CD73, CD105, CD106 (VCAM-1), CD140b, CD146, CD271 (LNGFR), MSCA-1, and STRO-1, have been used alone or in combination to isolate human MSCs (hMSCs) (Aslan et al., 2006; Battula et al., 2009; Boiret et al., 2005; Bühring et al., 2007; Gronthos et al., 2003; Quirici et al., 2002; Sacchetti et al., 2007). CD49a, CD73, CD140b, and CD146 are widely expressed in stromal cells (e.g., pericytes and reticular cells) and thus are not unique to MSCs. STRO-1 is a popular MSC marker and is often used in combination with VCAM-1 for MSC isolation. However, these markers are also found on some hematopoietic cells, and additional markers, including CD45 and Glycophorin A (GPA), are required to exclude contaminating cells (Gronthos et al., 2003; Simmons and Torok-Storb, 1991). Therefore, the identification of a combination of cell surface markers specific to hMSCs has remained an important prerequisite for the repeated isolation of purified multipotent MSC fractions.