br Experimental Procedures br Author Contributions br Acknow

Experimental Procedures
Author Contributions
Acknowledgments We thank Dr. Yao Zhi for performing the calcium studies. This study was supported by MRC CoEN grants MR/J009660/1 and MR/L501499/1, and MRC Experimental Medicine grant MR/M006646/1, the Kattan Trust, and Javon Trust. A.H.V.S. is an NIHR Senior Investigator and is supported by NIHRRCF103/AS/2014, and the NIHR BRC grant to UCLH.
Introduction Mesenchymal stem xpo 1 (MSCs) are self-renewing, multipotent progenitor cells with the potential to differentiate into different cell types such as osteoblasts, chondrocytes, neurons, cardiomyocytes, hepatocytes, and epithelial cells (Uccelli et al., 2008). Due to their accessibility, multipotency, and convenient expansion protocols, MSCs have been recognized as promising candidates for cellular therapy in regenerative medicine and tissue repair (Caplan, 2007; Uccelli et al., 2008). In fact, MSC transplantation is considered safe and has been widely applied in clinical trials of cardiovascular (Ripa et al., 2007), neurological (Lee et al., 2008), and immunological diseases (Kim et al., 2013; Lazarus et al., 2005) with encouraging results. On the other hand, due to their inherent tumor-homing behavior, MSCs are considered advantageous for delivering anti-tumor agents as an adjuvant therapy for different types of cancer (Hong et al., 2009; Kim et al., 2008; Kosaka et al., 2012; Nakamizo et al., 2005; Nakamura et al., 2004; Ryu et al., 2012). Accumulating evidence has shown that MSCs can deliver various therapeutic agents, such as interleukin-2 (IL-2) (Nakamura et al., 2004), IL-12 (Hong et al., 2009), tumor necrosis factor (TNF)-related and apoptosis-inducing ligand (Kim et al., 2008), and suicide genes (Kosaka et al., 2012; Ryu et al., 2012) selectively to tumor loci, and that such delivery elicits a significant anti-tumor effect in animal models. However, it should be noted that the success of such treatment strategies largely depends on the migratory and homing ability of MSCs. Thus, a complete understanding of the molecular mechanisms underlying migration of MSCs toward target tissues is essential for improving the therapeutic application of MSCs. Homing of transplanted MSCs into injured tissue or tumors is regulated by multiple processes that include cell attachment and rolling in the vessel lumen, adhesion and extravasation across the vascular endothelium, and migration through the tissue stroma (Eseonu and De Bari, 2015). Although the molecular mechanisms that govern directed migration of MSCs are not fully understood, there is an accumulating body of evidence showing that chemotactants, such as chemokines, cytokines, and growth factors, play a crucial role in the homing ability of circulating MSCs to injured tissues or tumors (Eggenhofer et al., 2014; Marquez-Curtis et al., 2014; Vanden Berg-Foels, 2014). Interaction between chemotactants and their receptors leads to multiple intracellular events that allow extravasation of cells from circulation and directional migration toward the area with the highest chemotactic gradient (Alexeev et al., 2013). Chemokine (C-C motif) ligand 5 (CCL5, also known as RANTES [regulated upon activation, normal T cell-expressed and secreted]), is secreted by various cell types including platelets, immune cells, fibroblasts, and endothelial and epithelial cells (Lin et al., 2013), and has been originally identified as an inducer that can recruit leukocytes to sites of inflammation (Donlon et al., 1990). After binding to its receptors, namely CCR1, CCR3, and CCR5, CCL5 induces phosphorylation of mitogen-activated protein kinase and other signaling pathways involved in the regulation of various cellular functions, such as proliferation, migration, and differentiation (Aldinucci and Colombatti, 2014; Marques et al., 2013). Interestingly, it has been reported that the CCL5/CCR1 axis is critical for maintaining MSC identity and multipotency (Kauts et al., 2013). In addition, more recent studies have demonstrated that the CCL5/CCR1 signal is pivotal for the recruitment of MSCs toward inflammatory tissues and subsequent therapeutic effects (Lu et al., 2015; Wise et al., 2014). These findings raise fundamental questions as to how the CCL5/CCR1 axis is regulated, and how this axis controls MSC access to the target sites.