CD8/MHC-I complex is specific but not sensitive for the diagnosis of polymyositis.

This study aimed to investigate immunostaining patterns for major histocompatibility complex class I (MHC-I) in different types of myopathies and to assess the diagnostic value of CD8/MHC-I complex for definite polymyositis. The study included 20 cases of definite polymyositis, 20 cases of dermatomyositis and 10 cases of dystrophies with muscle inflammation (inflammatory muscular dystrophy). Immunohistochemical staining with MHC-I antibody demonstrated the presence of MHC-I along the sarcolemma of scattered or small groups of non-necrotic fibres in both polymyositis and inflammatory muscular dystrophy, whereas intense sarcolemmal immunostaining for MHC-I was diffusely present in almost all fibres in dermatomyositis. Double immunofluorescence labelling for CD8 and MHC-I detected the CD8/MHC-I complex in 20% of polymyositis cases with mononuclear cell infiltrates. No CD8/MHC-I complex was found in the dermatomyositis or inflammatory muscular dystrophy cases. The results suggest that MHC-I detection alone cannot be used as a reliable diagnostic test to differentiate polymyositis from dystrophies with secondary muscle inflammation. The CD8/MHC-I complex, although showing high specificity, is neither a sensitive nor an easy-to-handle diagnostic test for polymyositis.