br Conclusion While our in vivo data

Conclusion While our in vivo data show that Nilotinib blocks fibrogenesis in the context of skeletal muscle regeneration, our in vitro model provides evidence that mesenchymal progenitor mg115 prompt the expansion of tissue-specific stem cells, strengthening the notion that fibrogenic cell activity following acute damage is important for effective regeneration. This work was supported by a grant from the Heart and Stroke Foundation (HSF) of Canada and by Canadian Institute for Health Research grant MOP 97856 to FMV Rossi.
Disclosure of potential conflict of interest
Author contributions
Acknowledgements
Introduction Adult mesenchymal stem cell therapy has been proposed as a promising therapy for regenerative tissue repair, for example to prevent heart failure development after acute myocardial infarction (AMI) (Shah and Shalia, 2011; Wollert et al., 2004). However, one of the major problems of stem cell therapy is a lack of engraftment of sufficient stem cells at the site of injury (van Dijk et al., 2011; Berardi et al., 2011). We hypothesized that when retention and engraftment of stem cells are increased, the therapeutic effect of stem cells will improve. Therefore, we designed a novel targeting technique to direct adipose-derived stromal/stem cells (ASCs) specifically to the activated endothelium of blood vessels within the infarct area in the heart by coating them with dual-targeted microbubbles. We used ASCs, because adipose tissue is a rich source of mesenchymal stem cells, which can be harvested easily, show high proliferation rates in culture and have the capacity to differentiate into several cell types amongst which cardiomyocytes (Oedayrajsingh-Varma et al., 2006; van Dijk et al., 2008; Carvalho et al., 2013; Rangappa et al., 2003). Furthermore, it has been shown that ASCs have a beneficial effect on cardiac function post-AMI in several pre-clinical studies (van Dijk et al., 2011; Berardi et al., 2011; Yamada et al., 2006). Early clinical trials using ASC therapy post-AMI, however, show that there is a room for improvement for ASC therapy (Houtgraaf et al., 2012; Janssens et al., 2006; Jeevanantham et al., 2012; Assmus et al., 2014). To be able to direct the ASCs to the infarct area we employed the so-called microbubbles, which are small (2–4μm) gas-filled bubbles originally developed as contrast agents for echocardiography (Dijkmans et al., 2004). Nowadays, microbubbles can also be designed as targeting agents by conjugating antibodies, ligands or peptides to the microbubble shell (Klibanov et al., 2004; Kokhuis et al., 2013). We have constructed stem cell-microbubble complexes, named ‘StemBells’, by coating ASCs with microbubbles using a CD90 antibody via biotin-streptavidin bridging (Fig. 1A). Additionally, an antibody against ICAM-1, an adhesion molecule expressed on activated endothelium of blood vessels within the infarct area (Benson et al., 2007), was simultaneously conjugated to the microbubble shell to create a bridge and improve the attachment of the StemBells specifically in the infarct area. Application of the microbubbles has several benefits. First, it allows coupling of a targeting antibody to the ASCs without modifying the stem cell itself. Second, the microbubbles cause buoyancy and susceptibility of the ASCs to the acoustic radiation force exerted by diagnostic ultrasound, as we previously showed in an chicken embryo using intravital microscopy (Kokhuis et al., 2014). StemBells can thus be pushed from the center of the blood stream to the vessel wall by ultrasound, further enhancing the effect of targeting.
Methods
Results
Discussion In this study we described the development of a novel technique to target stem cells specifically to the infarct area by the assembly of stem cell-microbubble complexes, named StemBells. These StemBells were first in vitro characterized, showing high viability after assembly and ultrasound exposure, as well as susceptibility to acoustic radiation force. In vivo this new stem cell delivery technique proved to be safe for intravenous injection with StemBells reaching the infarct area of the heart. In addition, StemBells significantly improved cardiac function, independent of an effect on infarct size or inflammatory macrophages five weeks after injection.