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Background:The immunopathologic mechanism underlying dermatomyositis (DM) and polymyositis (PM) remains poorly understood.Many cytokines play a pathogenic role in DM and PM.lnterleukin 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-21 R) 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-21 R 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 I L-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 =0.00l;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.