Abstract: Background At present, the analysis of myositis specific antibody (MSA) and clinical phenotype in dermatomyositis or juvenile dermatomyositis (JDM) is affected by some confounding factors, including simple or complex antibodies and treatment.Objective To observe the relationship between MSA and clinical phenotype in JDM.DesignCase series report.Methods The diagnosis of JDM was based on Bohan/Peter criteria or 2017 EULAR/ACR criteria. We enrolled JDM patients who were listed in the JDM follow-up system of the rheumatology departments of Children's Hospital of Fudan University since 2011 and were detected by MSAs and MAAs with Euroline Autoimmune Inflammatory Myopathies 16Ag kit from March 2017 to April 2020. Clinical characteristics including clinical data, onset age, course of disease at diagnosis, follow-up course, body temperature, skin, bone and joint system, lung, liver, kidney, laboratory examination, macrophage activation syndrome(MAS), overlapping SLE and CMAS scores were collected from the HIS system and JDM follow-up system.Main outcome measures Clinical phenotypic characteristics of children with simple and combined anti-MDA5 antibody.Results A total of 103 patients with JDM who met the inclusion criteria of this study were included in the analysis with 54 males(52.4%). Sixty patients were detected with specific antibody spectrum before treatment after the diagnosis of JDM. The median CMAS score was 33 (23.0,44.7). The main clinical manifestations were fever, skin ulcer, skin calcification, arthralgia or arthritis, interstitial pneumonia, severe pneumonia, dysphagia, hematuria and IgA nephropathy, and MAS. Among 103 cases of JDM, 68 cases (66.0%) were myositis antibody positive, 64 cases were MSA positive, 24 cases were MAA positive and 20 cases were MSA and MAA positive. There were significant differences in age of onset and course of disease at first diagnosis among anti-MDA5 antibody positive group, anti-NXP2 antibody positive group, anti-TIF-1γ antibody positive group and all antibody negative antibody group. Younger onset age and longer course of disease at first diagnosis were seen in anti-TIF-1γ antibody positive group. Skin ulcer, arthritis / arthralgia, ILD and fever were more likely to occur in patients with anti-MDA5 antibody. The CMAS score, ALT, AST, LDH, HBDH, CK and Fer were significantly different among the four groups. Anti-NXP2 antibody positive patients have the lowest CMAS score and the highest CK, LDH and HBDH level. Anti-MDA5 antibody positive patients have the highest ALT, AST, Fer, but the lowest CK. Especially, Anti-TIF-1γ antibody positive patients have the lowest ALT, AST, LDH and Fer. Antibody negative group did not show significant characteristics in ALT, AST, CK, LDH or HBDH. Furthermore,HBDH and Fer were normal in initial group of anti-MAD5 alone group (n=9) while increasing significantly in initial group of combined anti-MDA5 group (n=10).Conclusion Anti-MDA5、 anti-NXP2 and anti-TIF-1γ antibody positive JDM present identifiable clinical phenotypes and laboratory test results.The untreated patients only with anti-MDA5 positive and with other positive antibodies are of differential significance. MSAs and MAAs negative JDM have no obvious clinical phenotypes and laboratory test results.

Key words: Juvenile dermatomyositis, Myositis specific antibody, Myositis related antibody, Anti-MDA5 antibody