Coexistence of TDP-43 and C5b-9 staining of muscle in a patient with inclusion body myositis

While sporadic inclusion body myositis (sIBM) is the most commonly acquired inflammatory myopathy above 50 years of age, its refractory response to conventional immunosuppressive treatments raises questions about its perplexing pathogenesis. Muscle biopsy typically reveals major histocompatibility complex I antigens and CD8+ T cell endomysial infiltrates invading non-necrotic muscle fibres early in the disease course with rimmed vacuoles, protein aggregates and amyloid inclusions later in the disease. Transactive response DNA-binding protein-43 (TDP-43), a protein implicated in transcriptional repression in neurodegenerative diseases, is also found in sIBM. C5b-9 membrane attack complex, an effector protein involved in the complement cascade of the immune response, is commonly found in dermatomyositis, but has rarely been reported in IBM. We describe a novel case of IBM with simultaneous C5b-9 and TDP-43 staining on quadriceps biopsy, raising the question of a possibility of concurrent immune-mediated inflammatory and myodegenerative pathogenesis.