To investigate the prevalence of ectopic lymphoid structures (ELSs) and association between ELSs and myositis autoantibodies in the muscles of patients with adult-onset idiopathic inflammatory myopathies (IIMs).

ELSs are follicles of the lymphocytes in the non-lymphoid target tissues of various autoimmune diseases, including rheumatoid arthritis (RA), Sjögren syndrome (SjS), and multiple sclerosis. ELSs play a role in the pathogenesis of these diseases and indicate disease severity. However, few studies have documented ELSs in IIMs. The prevalence and significance of ELS formation in overall IIMs remain unclear.

Myositis-specific antibodies (MSAs) were measured in the sera of 415 consecutive patients with IIMs using immunoprecipitation and enzyme-linked immunosorbent assay. Hematoxylin and eosin staining of the patients’ muscle biopsies was performed to review and screen for ELSs. The candidate samples of ELSs were immunostained for CD20. ELSs were defined as the presence of nodular aggregation of more than 100 mononuclear cells and 50 CD20-positive B-cells in one follicle.

ELSs were observed in 28 patients (5.9%). The most common MSA profile of the patients with ELSs was MSA-negative (n = 14), followed by anti-tRNA synthetase (n = 6), anti-Mi-2 (n = 3), anti-TIF1-γ (n = 2), anti-HMGCR (n = 2), and anti-MDA5 (n = 1). Concomitant connective tissue diseases (RA, SjS, and systemic scleroderma) were observed in 5 of the 14 MSA-negative patients. Among 12 anti-Mi-2-positive IIM patients, 3 with and 9 without ELSs, ELS formation was significantly associated with younger onset age (26.0 ± 6.2 vs. 54.6 ± 16.3 years, p < 0.05) and more than 20 necrotic and regenerating muscle fibers in one fascicle (3/3 vs. 3/9, p < 0.05).

ELSs are rare pathological findings of IIMs, found mainly in MSA-negative IIMs and some MSA-positive IIMs (anti-tRNA synthetase syndrome and dermatomyositis). ELSs may serve as markers of clinicopathological severity in IIMs.