Anti-mitochondrial antibodies, the hallmark of primary biliary cirrhosis, have been detected in many patients with idiopathic inflammatory myopathies and these anti-mitochondrial-antibody-associated idiopathic inflammatory myopathies frequently show unique characteristics. We detected anti-mitochondrial antibodies in Chinese idiopathic inflammatory myopathy and summarized the clinical findings of these anti-mitochondrial-antibody-positive patients. Of 136 patients, seven (5.15%) were found to be anti-mitochondrial-antibody-positive. Primary biliary cirrhosis was present in 2 of these 7 patients, chronic disease duration in 2 patients and asymmetrical muscle weakness in 4 patients. The mean disease course was 8.58 months, and the mean creatine kinase level was 2256.53 U/L. Myositis-specific antibodies were found in 3 patients. According to clinical features and muscle histopathological findings, 3 patients were classified as dermatomyositis, 2 as possible polymyositis and 2 as necrotizing autoimmune myopathy. Of the 6 anti-mitochondrial-antibody-positive patients receiving follow-ups of 12-83 months, they all showed marked clinical improvement. Our study indicates that anti-mitochondrial antibodies are relatively rare in Chinese idiopathic inflammatory myopathy patients. These patients generally show various clinical features and have favorable treatment outcomes. Anti-mitochondrial antibody testing may be helpful to confirm the diagnosis of idiopathic inflammatory myopathy, especially in patients with atypical manifestations.

Background: The immunopathologic mechanism underlying dermatomyositis (DM) and polymyositis (PM) remains poorly understood. Many cytokines play a pathogenic role in DM and PM. Interleukin 21 (IL-21) has a pleiotropic effect on inflammation regulation. This study aimed to detect the serum IL-21 level and investigate the expression of IL-21 and IL-21 receptor (IL-21R) in muscle tissues of patients with DM and PM. Methods: Biopsied muscle samples were obtained from 11 patients with DM, 12 with PM, and six controls; mRNA levels of IL-21 and IL-21R were analyzed by real-time quantitative reverse transcription-polymerase chain reaction; and immunohistochemical staining was used to evaluate the protein expression of IL-21 and IL-21R. Serum samples were obtained from 36 patients with DM, 19 with PM, and 20 healthy controls. The serum IL-21 level was detected by enzyme-linked immunosorbent assay. Results: The expression of IL-21 was upregulated in patients with DM and PM. The IL-21 mRNA level was significantly increased in muscle tissues of patients with DM and PM (DM vs. control, P = 0.001; PM vs. control, P = 0.001), whereas IL-21R mRNA level in patients with DM/PM was not statistically different from that of healthy controls. Immunohistochemical staining showed both IL-21 and IL-21R were significantly expressed in the inflammatory cells in muscle tissues of patients with DM and PM. The serum IL-21 level was also significantly higher in patients with DM/PM than in controls (DM vs. control, 49.12 [45.28, 60.07] pg/ml vs. 42.54 [38.69, 48.85] pg/ml, P = 0.001; PM vs. control, 50.77 [44.19, 60.62] pg/ml vs. 42.54 [38.69, 48.85] pg/ml, P = 0.005). Conclusions: IL-21 expression is upregulated in patients with DM and PM in both muscle tissue and serum. In addition, IL-21R protein is highly expressed in affected muscle tissues of patients with DM and PM. IL-21 may play a pathogenic role through IL-21R in patients with DM and PM.