Cytokine Priming Enhances the Antifibrotic Effects of Mesenchymal Stromal Cells Conditioned Medium Via Fibrogenic Myofibroblasts Elimination and Extracellular Matrix Degradation.

Background: Fibrosis is a pathological scarring process characterized by persistent myofibroblasts activation with excessive accumulation of extracellular matrix (ECM). Fibrotic disorders represent an increasing burden of disease-associated morbidity and mortality worldwide for which there are limited therapeutic options. Reversing fibrosis requires the elimination of myofibroblasts, remodeling of the ECM, and regeneration of functional tissue. Multipotent mesenchymal stromal cells (MSC) have antifibrotic properties mediated by secreted factors present in their conditioned medium (MSC-CM). However, there are no standardized in vitro assays to predict the antifibrotic effects of human MSC, and, as a consequence, we lack evidence on the effect of cytokine priming on MSC’s antifibrotic effects. We hypothesize that the MSC-CM promotes fibrosis resolution in vitro and that this effect is enhanced following MSC cytokine priming. Methods: We tested the antifibrotic effects of resting and interferon gamma (IFN-γ) and tumor necrosis factor alpha (TNF-α) primed MSC-CM in three in vitro assays: prevention of fibroblast activation, myofibroblasts deactivation and ECM degradation. Furthermore, we performed transcriptomic analysis of myofibroblasts treated or not with resting- or primed-MSC-CM and proteomic characterization of resting- and primed-MSC-CM. Results: We report that MSC-CM treatment prevented TGF-β induced fibroblast activation and reduced fibrogenic myofibroblasts (i.e. transcriptomic upregulation of apoptosis, senescence, and inflammatory pathways). These effects were higher when primed rather than resting MSC-CM was used. Priming increased the ability of MSC-CM to remodel the extracellular matrix reducing its content of collagen I and fibronectin. Priming increased the following antifibrotic proteins in MSC-CM: DKK1, MMP-1, MMP-3, follistatin and cathepsin S. DKK1 inhibition reduced the anti-fibrotic effects of MSC-CM. Thus, cytokine priming increases antifibrotic factors in the MSC-CM which in turn amplify the anti-fibrotic effects of MSC-CM. Conclusions: In a pro-fibrotic in vitro environment MSC-CM promote fibrosis resolution, an effect enhanced following MSC cytokine priming. Specifically, MSC-CM reduces fibrogenic myofibroblasts through apoptosis, senescence, and inflammatory signals, as well as by enhancing ECM degradation. Future studies will establish the in vivo relevance of MSC priming to fibrosis resolution