Search

Mesenchymal stromal cells (MSCs) from both humans and horses, which represent a clinically relevant translation animal model for human cutaneous wound healing, were recently found to possess antimicrobial properties against planktonic bacteria, and in the case of equine MSCs, also against biofilms. This, together with previous findings that human and equine MSCs promote angiogenesis and wound healing, makes these cells an attractive approach to treat infected cutaneous wounds in both species. The anti‐biofilm activities of equine MSC, via secretion of cysteine proteases, have only been demonstrated in vitro, thus lacking information about in vivo relevance. Moreover, the effects of the equine MSC secretome on resident skin cells have not yet been explored. The goals of this study were to (a) test the efficacy of the MSC secretome in a physiologically relevant ex vivo equine skin biofilm explant model and (b) explore the impact of the MSC secretome on the antimicrobial defense mechanisms of resident skin cells. Our salient findings were that secreted factors from equine MSCs significantly decreased viability of methicillin‐resistant Staphylococcus aureusbacteria in mature biofilms in this novel skin biofilm explant model. Moreover, we demonstrated that equine MSCs secrete CCL2 that increases the antimicrobial activity of equine keratinocytes by stimulating expression of antimicrobial peptides. Collectively, these data contribute to our understanding of the MSC secretome's antimicrobial properties, both directly by killing bacteria and indirectly by stimulating immune responses of surrounding resident skin cells, thus further supporting the value of MSC secretome‐based treatments for infected wounds. CCL2 secreted by mesenchymal stromal cells promotes cutaneous antibacterial defense mechanisms through increased antimicrobial peptide expression in keratinocytes. The secretome of cultured equine mesenchymal stromal cells (MSCs) was collected as conditioned medium (CM) and applied to methicillin‐susceptible Staphylococcus aureus(MSSA) and methicillin‐resistant Staphylococcus aureus(MRSA) biofilms in an ex vivo cutaneous wound explant model, where it effectively reduced bacteria viability. Primary equine keratinocytes, in two‐dimensional cultures, significantly increased expression of the antimicrobial peptides (AMP) ß‐defensin and cathelicidin after stimulation with the MSC secretome. ß‐defensin expression was mediated by CCL2, a cytokine secreted by MSCs.

Mesenchymal stromal cells (MSCs) from various species, such as humans, mice, and horses, were recently found to effectively inhibit the growth of various bacteria associated with chronic infections, such as nonhealing cutaneous wounds, via secretion of antimicrobial peptides. These MSC antimicrobial properties have primarily been studied in the context of the planktonic phenotype, and thus, information on the effects on bacteria in biofilms is largely lacking. The objectives of this study were to evaluate the in vitro efficacy of the MSC secretome against various biofilm‐forming wound pathogens, including the methicillin‐resistant Staphylococcus aureus(MRSA), and to explore the mechanisms that affect bacterial biofilms. To this end, we used equine MSCs, because the horse represents a physiologically relevant model for human wound healing and offers a readily translatable model for MSC therapies in humans. Our salient findings were that the equine MSC secretome inhibits biofilm formation and mature biofilms of various bacteria, such as Pseudomonas aeruginosa, S. aureus, and Staphylococcus epidermidis. Furthermore, we demonstrated that equine MSC secrete cysteine proteases that destabilize MRSAbiofilms, thereby increasing the efficacy of antibiotics that were previously tolerated by the biofilms. In light of the rise of antibiotic‐resistant bacterial strains as an increasing global health threat, our results provide the rationale for using the MSC secretome as a complementary treatment for bacterial skin infections in both humans and horses. This study demonstrates that MSC are effective against bacteria in biofilms, including methicillin‐resistant Staphylococcus aureus, via secretion of active proteases that destabilize biofilms by protein degradation, resulting in increased antibiotic effectiveness. In light of antibiotic‐resistant bacterial strains as an increasing global health threat, our results provide the rationale for using the MSC secretome as a complementary treatment for bacterial infections.