The secretome of irradiated peripheral mononuclear cells attenuates hypertrophic skin scarring

Background Hypertrophic scars can cause pain, movement restrictions, and reduction of quality of life. Despite numerous options to tackle hypertrophic scarring, efficient therapies are still scarce, and cellular mechanisms are not well understood. Secreted factors from peripheral blood mononuclear cells (PBMCsec) were previously described for their beneficial effects in tissue regeneration. Here, we investigated the effects of PBMCsec on skin scarring in mouse models and human scar explant cultures at single cell resolution (scRNAseq). Methods Mouse wounds and scars were treated with PBMCsec either intradermally or topically. Human mature scars were treated with PBMCsec ex vivo in explant cultures. All experimental settings were analyzed by single cell RNA sequencing (scRNAseq). A variety of bioinformatics approaches were used to decipher gene regulation in the scRNAseq data sets. Components of the extracellular matrix (ECM) were investigated in situ by immunofluorescence. The effect of PBMCsec on myofibroblast differentiation and elastin expression was investigated by stimulating human primary fibroblasts with TGFβ. Findings Topical and intradermal application of PBMCsec regulated the expression of a variety of genes involved in pro-fibrotic processes and tissue remodeling. Our bioinformatics approach identified elastin as a common linchpin of antifibrotic action in both, the mouse and human experimental setting. In vitro, we found that PBMCsec prevents TGFβ-mediated myofibroblast-differentiation and attenuates abundant elastin expression through non-canonical signaling inhibition. Furthermore, TGFβ-induced breakdown of elastic fibers was strongly inhibited by addition of PBMCsec. Interpretation Together, we showed anti-fibrotic effect of PBMCsec on cutaneous scars in mouse and human experimental settings, suggesting PBMCsec as a novel therapeutic option to treat skin scarring. Research in context Evidence before this study Paracrine factors secreted from irradiated peripheral mononuclear cells (PBMCsec) show strong tissue regenerative properties in a variety of organs and are shown to enhance cutaneous wound healing. Whether PBMCsec shows anti-fibrotic properties on scar formation has not been investigated so far. Added value of this study In the present study, we were able to demonstrate that PBMCsec improves quality of developing and mature scars in mouse and human scar tissue. We found that PBMCsec is able to attenuate the expression of various genes, promoting scar formation and inhibit TGFβ-induced myofibroblast differentiation. Elastin and TXNIP were identified as a common linchpin of its anti-fibrotic action. Implications of all the available evidence Using in vivo, ex vivo, and in vitro models and analyses on a single-cell level, our study paves the way for clinical studies evaluating the use of PBMCsec for the treatment of human cutaneous scars.