ObjectiveReactive oxygen species (ROS) contribute to the pathogenesis of fibrosis in systemic sclerosis (SSc; scleroderma), and NADPH oxidase (NOX) is an important source of ROS. Since the role of single NOX isoforms has not been previously investigated in SSc, this study was undertaken to assess the expression of NOX in SSc fibroblasts compared to normal healthy cells and to analyze their role in cell activation.MethodsExpression of NOX isoforms in dermal fibroblasts from patients with SSc and healthy control subjects was analyzed by real‐time polymerase chain reaction, immunoblotting, and immunofluorescence. NOX isoforms were silenced using small interfering RNA. Production of ROS was measured by fluorometry and confocal microscopy.ResultsScleroderma fibroblasts showed up‐regulation of NOX‐2 and NOX‐4 protein and messenger RNA (mRNA) expression. Treatment of the cells with diphenyleneiodonium, a nonselective inhibitor of flavin‐containing enzymes, and silencing of NOX2 and NOX4 decreased the production of ROS as well as the expression of type I collagen and α‐smooth muscle actin in SSc fibroblasts. ROS generated by NOX‐2 and NOX‐4 were involved in DNA damage and activation of a DNA repair checkpoint. Incubation of healthy control fibroblasts with platelet‐derived growth factor (PDGF) or with IgG isolated from SSc patient serum enhanced the expression of NOX2 and NOX4 mRNA, via ROS, in a time‐dependent manner. Treatment with actinomycin D, a transcription inhibitor, reversed the effects of PDGF stimulation but not the effects of SSc IgG.ConclusionBoth NOX2 and NOX4 generate ROS in SSc fibroblasts and play a critical role in cell activation and DNA damage. Expression of NOX‐2 and NOX‐4 in SSc fibroblasts is maintained by a ROS‐mediated loop.