Objective To explore the effect of recombinant human tumor necrosis factor-α receptor Ⅱ:IgG Fc fusion protein injection (rhTNFR:FC) on the expression of cartilage oligomeric matrix protein (COMP) in the synovial fluid and peripheral blood of juvenile idiopathic arthritis (JIA); and to explore the clinical significance of COMP for JIA and the relationship between rhTNFR:FC and COMP in JIA.Methods Thirty-five patients with JIA (JIA group),30 patients with traumatic arthritis (trauma group) and 30 patients with indirect inguinal hernia hernioplasty (normal group) were included.Peripheral blood from all enrolled patients and synovial fluid from 15 JIA and 10 trauma arthritis were obtained for COMP detection before the treatment.Fifteen JIA (group A) patients were treated with combined rhTNFR:FC,diseasemodifying antirheumatic drugs (DMARDs) and non-steroid anti-inflammatory drugs (NSAIDs),20 JIA (group B) were treated with combined DMARDs and NSAIDs.After three to six months' treatment and when the disease were in remission,peripheral blood from group A and B were drawn for COMP detection.In group A,the synovial fluid from 5 patients were obtained for COMP detection after treatment.At the same time,such as tender joint count (TJC),swollen joint count (SJC),time for morning stiffness,blood routine,erythrocyte sedimentation rate (ESR),and C-reactive protein (CRP) and other parameters before and after treatment were measured.The level of COMP was tested by double antibody sandwich enzyme-linked immunosorbent assay.The measurement data were tested for variance and independent sample t-test; and the enumeration data were tested by chi-squared or Fisher's exact test.Pearson's correlation analysis was adopted to analyze the association among the variables.Results ① The blood COMP level before treatment was (0.77±0.29) ng/ml in the JIA group,(1.00±0.28) ng/ml in the traumatic arthritis group,and (1.33±0.37) ng/ml in the normal control group.The level in the former two groups was obviously lower than that in the normal control group.The variation was statistically significant (F=25.345,P＜0.05).The comparison between any two groups was statistically significant (P＜0.05).② The COMP level in the synovial fluid before treatment were (14.8±1.6) ng/ml in the JIA group,(15.1±1.0) ng/ml in the traumatic arthritis group.The variation was not stati-stically significant (t=0.523,P=0.606).③ The serum COMP level of the systemic JIA group was obviously lower than that of the oligoarticular JIA patients,and patients with enthesitis-related arthritis and polyarticular JIA (0.26± 0.03 vs.0.87±0.17,0.89±0.22 and 0.70±0.35 ng/ml,respectively; F=9.244,P＜0.05).④ The serum COMP level of JIA at the acute phase was negatively correlated with white blood cells count (WBC),CRP and ESR (r=-0.556,-0.582 and-0.684,respectively; P all＜0.05).By contrast,no correlation was detected between the serum COMP level and joint tenderness index,joint swelling index,morning stiffness duration,hemoglobin level and platelet count(r=0.06,-0.206,-0.107,0.15 and-0.185,respectively; P all ＞0.05).⑤ The serum COMP level was obviously lower in the JIA with joint destruction than that without joint destruction (0.52±0.22 vs.0.92±0.22 ng/ml; t=5.207,P＜0.05).⑥After treatment,the blood COMP level in group A was (1.33±0.21) ng/ml and (0.96±0.22) ng/ml in group B,which was obviously higher than that in the JIA group before treatment (0.77±0.29) ng/ml.In addition,the level in group A was higher than that in group B.The variation was statistically significant (F=24.681,P＜0.05).⑦ After treatment,the COMP level in the synovial fluid (18.4± 1.1) ng/ml (n=5) was higher than that before the treatment was (14.8± 1.6) ng/ml (n =15).The variation was of statistical significant (t=4.565,P＜0.05).Conclusion The COMP level in blood and synovial fluid declines before treatment and increases after treatment.The increase is more obvious after combined with rhTNFR:FC treatment.The serum COMP level is remarkably decreased in JIA at the acute phase,systemic JIA,and the JIA with destruction of joint,and showes a negative correlation with WBC,CRP and ESR.Serum COMP may be a useful marker of active disease,destruction of joint and growth inhibition for patients with JIA.rhTNFR:FC treatment for JIA can facilitate the recovery of COMP.