THE EQUINE MESENCHYMAL STROMAL CELL SECRETOME AS A THERAPY FOR HORSE CUTANEOUS WOUNDS Rebecca M. Harman, Ph. D Cornell University 2018 Horses suffer from chronic skin wounds such as exuberant granulation tissue (EGT), also called “proud flesh”, and allergic dermatitis (AD) in part because there are no adequate treatments for these pathologies. Equine mesenchymal stromal cells (MSC) are adult multipotent progenitor cells that can be isolated from a variety of horse tissues and expanded in culture. These cells have been used for decades by clinicians as a biologic treatment for orthopedic injuries of horses, but the mechanisms by which MSC improve healing in vivo have not been well-defined. Based on work carried out in other species, we hypothesized that equine MSC can be used as a therapy to promote the healing of horse cutaneous wounds because factors secreted by MSC, collectively called the MSC secretome, act on the wound environment, in ways that stimulate healing. We carried out a series of in vitro studies to (i) characterize the secretome of MSC isolated from horse peripheral blood and (ii) determine if the MSC secretome acts on target cell types found in the skin, in ways that suggest it will promote wound healing in vivo. First, I present a description of MSC and a history of equine MSC research and therapy, to provide a context for our experimental work. Then, I review the utility of the horse as a model for cutaneous wound healing, which provides additional justification for our studies. We used various experimental methods to evaluate the composition of the MSC secretome and performed series of in vitro assays to determine that specific factors secreted by MSC stimulate equine dermal fibroblast (DF) migration, a necessary aspect of skin wound healing. We carried out an additional set of experiments showing that MSC secreted factors act on dysregulated equine DF, pushing them toward a more normal phenotype that may be better suited to contribute to wound closure. Finally, we demonstrated that the MSC secretome inhibits the growth of bacterial species found as contaminants in chronic horse skin wounds. These results show that factors secreted by equine MSC do act on cell types found in horse skin wounds in ways that may lead to improved wound healing in vivo and warrant future studies to test the efficacy of the MSC secretome as a treatment for horse skin wound in vivo.