PURPOSE: To investigate whether c-Jun and c-Fos contribute to the pathologic activation of fibroblasts in systemic sclerosis (SSc) and evaluate the anti-fibrotic potential of selective AP-1 inhibition. METHODS: Expression of c-Jun and c-Fos was determined by real-time PCR and immunohistochemistry. Fibroblasts were stimulated with TGF-β and incubated with T-5224, a small-molecule inhibitor of AP-1 or transfected with small interfering RNA duplexes against c-Jun and c-Fos. Collagen synthesis was quantified by real-time PCR and hydroxyproline assay. Differentiation of resting fibroblasts into myofibroblasts was assessed by staining for α-smooth muscle actin and stress fibers. The anti-fibrotic potential of T-5224 was evaluated in mouse models of dermal fibrosis induced by bleomycin or by adenoviral overexpression of a constitutively active TGF-ß receptor type I. RESULTS: Upregulation of c-Jun and c-Fos was detected in animal models of SSc and the skin and dermal fibroblasts of SSc patients. Stimulation of healthy fibroblasts with TGFβ induced the expression of c-Jun and c-Fos. T-5224 or nucleofection with SiRNA directed against c-Jun and c-Fos abrogated the profibrotic effects of TGF-β. T-5224 decreased the release of collagen selectively in SSc fibroblasts. T-5224 was well tolerated and prevented dermal fibrosis induced by bleomycin or adenoviral activation of TGF-β signaling. CONCLUSION: AP-1 specify is upregulated in a TGF-β dependent manner in SSc. The selective AP-1 inhibitor T-5224 reduced collagen synthesis selectively in SSc fibroblasts and efficiently prevented the development of experimental dermal fibrosis. Thus, AP-1 might be a promising new molecular target for the treatment of SSc.