Relevance of anti-Ku antibodies in Ku-positive myositis with a necrotizing myopathy-like phenotype: a pediatric case report.

Muscle-specific antibodies against signal recognition particle (SRP) and 3‑hydroxy-3-methylglutaryl-CoA reductase (HMGCR) are established pathogenic factors in immune-mediated necrotizing myopathy (IMNM), and their titers are generally considered to correlate with disease activity. In contrast, the relevance of anti-Ku antibodies, which are myositis-associated autoantibodies typically detected in overlap syndromes and also found in a subset of IMNM patients lacking anti-SRP and anti-HMGCR antibodies, remains unclear. We report a 13-year-old girl with steroid-resistant Ku-positive myositis with necrotizing myopathy-like pathology who was exclusively positive for anti-Ku antibodies. Rituximab was introduced after failure of multiple immunosuppressive therapies and resulted in marked clinical improvement, accompanied by normalization of serum creatine kinase levels, improvement on muscle magnetic resonance imaging, and recovery of Childhood Myositis Assessment Scale scores. Immunostaining showed relatively enhanced nuclear Ku expression in the sarcolemmal region of the patient's muscle fibers compared with control muscle. However, anti-Ku antibody titers remained persistently elevated despite sustained clinical remission. These findings suggest that anti-Ku antibody titers did not correlate with disease activity in this patient, indicating that anti-Ku-associated myositis may differ from anti-SRP- or anti-HMGCR-associated myopathies.