Major histocompatibility complex class I expression can be used as a diagnostic tool to differentiate idiopathic inflammatory myopathies from dystrophies.

AIM Utility of major histocompatibility complex (MHC) Class I antigen immunostaining was studied to differentiate idiopathic inflammatory myopathies from dystrophies. MATERIALS AND METHODS Forty muscle biopsies including seven dermatomyositis (DM), six polymyositis (PM), two sporadic inclusion body myositis (sIBM), 20 dystrophies (one Duchenne, three Becker's, four alpha, one gamma sarcoglycanopathy, nine limb girdle, one myotonic and one fascioscapulohumeral muscular dystrophy) and five controls were stained with antibody for MHC Class I antigen (Novocastra clone W6/32 HL 1:100 dilution). RESULTS Polymyositis and sIBM showed MHC class I antigen positivity along sarcolemma of single and small groups of muscle fibers. The regenerating fibers in the perifascicular area in DM showed intense cytoplasmic positivity of MHC class I antigen. Muscle fibers in all dystrophies except regenerating fibers and control normal muscle were negative for MHC. Capillaries and lymphocytes were positive controls. There were no false positives in the study. CONCLUSION MHC Class I immunostaining can be used as a complementary diagnostic tool for the diagnosis of idiopathic inflammatory myopathies.