Cell therapies have been used to regenerate the heart by direct myocardial delivery, by coronary infusion and by surface attached scaffolds. Multipotent mesenchymal stem cells (MSC) with capacity to differentiate into cardiomyocytes and other cell lines have been predominantly trialled in rodents. However, large animal models are increasingly needed to translate basic research into new, safe regenerative therapies. Understanding the mode of action of cell therapies in the mammalian heart has been limited by cell tracking capability. This study examined the ability to track the fate of allogeneic MSC in sheep using various fluorescent dyes.

MSC isolated from sheep bone marrow were grown in culture following extraction and flow cytometric characterisation. After labelling with fluorescent tracking dyes (e.g. CFSE and DiI) cells were tested for in vitro and in vivo signal up to six weeks. Labelling effect on cell division and differentiation was studied.

Several dyes lost fluorescence and slowed cell division. However, the thiol reactive dye CM-DiI showed detectable in vivo fluorescence in labelled MSC six weeks after injection into sheep skeletal muscle and two weeks after implantation of an MSC coated biomaterial scaffold. CM-DiI labelled MSC differentiated in vitro showed label retention over four weeks.

The fluorescent membrane dye CM-DiI tracks implanted sheep MSC and provides an alternative to traditional cell markers such as gene modified GFP.