In a study by Nishimori et al. (2025), researchers investigated whether myositis with anti-mitochondrial M2 antibody (AM2A) is a distinct subtype of autoimmune myositis (AIM). They analyzed muscle biopsy samples from 201 patients and compared their clinicopathological features with other myositis subtypes. AM2A-positive patients had a notably long pre-biopsy disease duration (48.7 months on average) and a high frequency of arrhythmia (51.1%). Their muscle fibers showed similarities to immune-mediated necrotizing myopathy (IMNM), with 93.5% having necrotic or regenerating fibers and 43.3% with membrane attack complex deposits.
Interestingly, patients with a shorter disease duration before biopsy had more CD8-positive lymphocyte infiltrates and higher serum creatine kinase levels, indicating more active muscle inflammation. In contrast, longer disease duration correlated with more arrhythmias. Using clinical and pathological variables, the researchers achieved high accuracy in distinguishing AM2A-positive myositis from IMNM. These findings suggest that AM2A-positive myositis could indeed be a separate subtype of AIM, characterized by chronic inflammation, IMNM-like muscle pathology, and significant cardiac involvement.