Myofibroblast transcriptome indicates SFRP2+ fibroblast progenitors in systemic sclerosis skin

Skin and lung fibrosis in systemic sclerosis (SSc) is driven by myofibroblasts, alpha-smooth muscle actin (SMA) expressing cells that produce matrix as well as increase tension on surrounding tissues. Myofibroblast progenitors have been described to arise from a variety of cell types in murine fibrosis models, but relatively modest insight is available as to their source and differentiation in fibrotic human diseases. We utilize single cell RNA-sequencing to examine the transcriptome changes that occur in fibroblasts in SSc skin and during myofibroblast differentiation. We show that dermal myofibroblasts arise from an SFRP2/DPP4-expressing progenitor fibroblast population. In SSc skin, fibroblasts undergo global changes in gene expression, including SFRP2-expressing fibroblasts, which globally upregulate expression of markers, such as PRSS23 and THBS1. However, only a fraction of SSc fibroblasts differentiate into myofibroblasts, as shown by expression of additional markers, such as SFRP4 and FNDC1. These results indicate that myofibroblasts derive from a specific fibroblast subpopulation in SSc skin, that their differentiation proceeds in two stages and that other fibroblast populations also shift transcriptomes in SSc skin, suggesting a cytokine driven process. The myofibroblast transcriptome implicates upstream transcription factors that should help further unravel the drivers of myofibroblast differentiation. Overall design: Using droplet-based single cell RNA sequencing we analyzed 12 scleroderma (SSc) biopsy skin specimens against 10 healthy controls. Clustering, pseudotime, and transcription factor inference analyses were performed and select marker gene were examined for coexpression using immunofluorescence.