Clinicopathological features and treatment outcome of juvenile idiopathic inflammatory myopathies with anti-melanoma differentiation associated gene 5 antibodies: A case series study.

To characterize the clinicopathological features and treatment outcomes of juvenile idiopathic inflammatory myopathies (JIIM) with anti-melanoma differentiation associated gene 5 (MDA5) antibodies in a Chinese cohort. Anti-MDA5 antibody was detected by immunodot assay and indirect immunofluorescence assay on HEK293 cells in a series of Chinese JIIM cohort between 2005 and 2022. The clinical features, histological findings, and treatment outcomes of these anti-MDA5-antibody-positive patients were summarized. Of 59 JIIM patients, 3 (5.08%) were found to be anti-MDA5-antibody-positive. The frequency of anti-MDA5 antibody did not show significant difference between adult idiopathic inflammatory myopathies and JIIM cohorts (P = .720). The disease duration in patients with anti-MDA5 antibody was 2.83 ± 1.04 months. All 3 patients had typical skin lesions including Gottron sign and heliotrope rash, while interstitial lung disease and arthritis was only found in 1 patient. All 3 patients showed normal creatine kinase levels. On muscle biopsy, diffuse major histocompatibility complex class-I expression was seen in 3 patients and myxovirus-resistance protein A expression was found in 2 patients. All patients received long-term follow-up (6.42 ± 4.01 years). They were all drug-free and showed favorable treatment outcome with prednisone and additional immunosuppressant. Our study indicates that anti-MDA5 antibodies may not be common in Chinese JIIM. Anti-MDA5-positive JIIMs are characterized by typical skin lesions of dermatomyositis, normal CK levels, and increased major histocompatibility complex class-I expression. JIIMs with anti-MDA5 generally have good response to immunotherapies.