Systemic sclerosis (SSc) is manifested by fibrosis, vasculopathy and immune dysregulation. So far, a unifying hypothesis underpinning these pathological events remains unknown. Given that SSc is a multifactorial disease caused by both genetic and environmental factors, we focus on the two transcription factors, which modulate the fibrotic reaction and are epigenetically suppressed in SSc dermal fibroblasts, Friend leukaemia integration 1 (Fli1) and Krüppel-like factor 5 (KLF5). In addition to the Fli1 silencing-dependent collagen induction, the simultaneous knockdown of Fli1 and KLF5 synergistically enhances expression of connective tissue growth factor. Notably, mice with double heterozygous deficiency of Klf5 and Fli1 mimicking the epigenetic phenotype of SSc skin spontaneously recapitulate all the three features of SSc, including fibrosis and vasculopathy of the skin and lung, B-cell activation and autoantibody production. These studies implicate the epigenetic downregulation of Fli1 and KLF5 as a central event triggering the pathogenic triad of SSc.

Systemic sclerosis (SSc), or scleroderma, is a multisystem autoimmune disorder characterized by vasculopathy and fibrosis in the skin and internal organs, most frequently in the esophagus and lungs. Hitherto, studies on SSc pathogenesis centered on immune cells, vascular cells, and fibroblasts. Although dysregulated keratinocytes in SSc have been recently reported, the contribution of epithelial cells to pathogenesis remains unexplored. In this study, we demonstrated the induction of SSc-like molecular phenotype in keratinocytes by gene silencing of transcription factor Friend leukemia virus integration 1 (Fli1), the deficiency of which is implicated in SSc pathogenesis. Keratin 14–expressing epithelial cell–specific Fli1 knockout mice spontaneously developed dermal and esophageal fibrosis with epithelial activation. Furthermore, they developed remarkable autoimmunity with interstitial lung disease derived from thymic defects with down-regulation of autoimmune regulator (Aire). Importantly, Fli1 directly regulated Aire expression in epithelial cells. Collectively, epithelial Fli1 deficiency might be involved in the systemic autoimmunity and selective organ fibrosis in SSc. This study uncovers unidentified roles of dysregulated epithelial cells in SSc pathogenesis.

Objective: To determine the function and serum levels of soluble forms of programmed death 1 (sPD‐1) and one of its ligands, soluble PD ligand 2 (sPD‐L2), in patients with systemic sclerosis (SSc) and in a mouse model of topoisomerase I (topo I)–induced SSc.Methods: Serum levels of sPD‐1 and sPD‐L2 in 91 patients with SSc were examined by enzyme‐linked immunosorbent assay (ELISA). Expression of PD‐1 and PD‐L2 on T cells, B cells, and macrophages was quantified by flow cytometry. The effects of blockade of PD‐1 and PD‐L2 were analyzed by microfluidic ELISA (micro‐ELISA), a technique that can measure very low amounts of cytokines. In addition, the effects of sPD‐1 and sPD‐L2 on disease progression were assessed in mice with topo I–induced SSc.Results: Serum levels of sPD‐1 and sPD‐L2 were elevated in patients with SSc and correlated with the extent of fibrosis and immunologic abnormalities. Expression levels of PD‐1 and PD‐L2 were significantly elevated on SSc T cells, B cells, and macrophages. Micro‐ELISA analysis of serum samples from patients with SSc showed that PD‐L2^high B cells had higher levels of interleukin‐10 (IL‐10) production compared with PD‐L2^low B cells, indicating that PD‐L2 acts as a regulator of T cell cytokine production via cognate interactions with T cells and B cells. In mice with topo I–induced SSc, production of IL‐10 by topo I–specific B cells in cultures with T cells and topo I protein was significantly higher than that by conventional B cells, and intraperitoneal injection of recombinant chimeric PD‐1‐Fc and PD‐L2‐Fc canceled these enhanced effects.Conclusion: These results suggest that sPD‐1 and sPD‐L2 contribute to disease development in SSc via the regulation of cognate interactions with T cells and B cells.

Significance: The present study is, to our knowledge, the first demonstration of the molecular mechanisms underlying the association of IFN regulatory factor 5 (IRF5) expression with milder clinical manifestations of systemic sclerosis (SSc). It is speculated that endogenous ligands induce Toll-like receptor 4 signaling and promote IRF5 transcriptional regulation of its target gene promoters, which may be required for the development of SSc. Our present study supports this notion. Symptoms associated with SSc in humans were suppressed in mice deficient in the Irf5 gene. As such, this study offers previously unidentified insight into the complexity of SSc pathology, giving impetus to further clinical studies for the treatment of the disease.