Fibrosis in systemic sclerosis: common and unique pathobiology

Abstract

Fibrosis in systemic sclerosis (SSc), a complex polygenic disease associated with autoimmunity and proliferative/obliterative vasculopathy, shares pathobiologic features in common with other fibrosing illnesses, but also has distinguishing characteristics. Fibroblast activation induced by transforming growth factor-β (TGF-β), Wnts and innate immune receptors, along with oxidative stress and reactive oxygen species (ROS) are implicated in pathogenesis. On the other hand, the roles of endothelial-mesenchymal differentiation and bone marrow-derived fibrocytes remain to be established. Fibrotic responses are modulated by transcriptional activators and cofactors, epigenetic factors, and microRNAs that can amplify or inhibit ligand-induced signaling. The nuclear orphan receptor PPAR-γ appears to be important in governing the duration and intensity of fibroblast activation and mesenchymal progenitor cell differentiation, and defects in PPAR-γ expression or function in SSc may underlie the uncontrolled progression of fibrosis. Identifying the perturbations in signaling pathways and cellular differentiation programs responsible for tissue damage and fibrosis in SSc allows their selective targeting using novel compounds, or by innovative uses of already-approved drugs (drug repurposing).

Figure 1

Vicious cycle of FIBROSIS. Innate immune signaling alters self-limited repair into sustained fibrogenic process. Following injury, fibroblasts undergo a regulated activation. Once repair has been accomplished, tissue regeneration is complete. Prolonged injury leads to damage, causes activation of toll like receptors and sustained fibroblast activation culminating in excessive fibrogenesis.