Autoantibodies Recognizing Specificity Protein 4 Co‐occur With Anti–Transcription Intermediary Factor 1 and Are Associated With Distinct Clinical Features and Immunogenetic Risk Factors in Juvenile Myositis

Objective Autoantibodies recognizing specificity protein 4 (Sp4) were recently discovered in adults with idiopathic inflammatory myopathies (IIM). Anti‐Sp4 autoantibodies co‐occurred in patients with anti–transcription intermediary factor 1 (anti‐TIF1) autoantibody‐positive dermatomyositis (DM) and were associated with a reduced risk of cancer. In the present study, the prevalence and clinical features associated with anti‐Sp4 autoantibodies in juvenile‐onset IIM were investigated. Methods Serum samples from 336 patients with juvenile myositis in a cross‐sectional cohort and 91 healthy controls were screened for anti‐Sp4 autoantibodies using enzyme‐linked immunosorbent assay. Clinical characteristics, outcomes, and HLA alleles of those with and those without anti‐Sp4 autoantibodies were compared. Results Anti‐Sp4 autoantibodies were present in 23 patients (7%) with juvenile myositis and were not present in any of the controls. Anti‐Sp4 autoantibodies were found among each clinical myositis subgroup. The frequency of TIF1 autoantibody positivity was significantly higher among those with anti‐Sp4 autoantibodies (21 [91%] versus 92 [30%], P