LAUSR

OBJECTIVE Diagnostic muscle biopsies are routinely immunostained for major histocompatibility complex class I (MHC-I) protein. In this study, we analyzed the prevalence and patterns of MHC-I immunostaining in biopsies from patients with different types of myopathies and neurogenic disorders. METHODS All 357 diagnostic muscle biopsies processed at the Johns Hopkins Neuromuscular Pathology Laboratory from August 2013 to January 2017 were immunostained for MHC-I. The prevalence and patterns of MHC-I immunostaining were compared between patients with histologically normal muscle biopsies (n = 31), idiopathic inflammatory myopathies (IIMs; n = 170), non-inflammatory myopathies (n = 60), and neurogenic disorders (n = 96). RESULTS MHC-I immunostaining was abnormal in most patients with dermatomyositis (DM; 98%), sporadic inclusion body myositis (sIBM;100%), immune-mediated necrotizing myopathy (IMNM;100%), and polymyositis (77%). In contrast, MHC-I immunostaining was less frequently present in non-inflammatory myopathies (32%) or neurogenic disorders (30%). Overall, abnormal MHC-I immunostaining had a sensitivity of 0.95 and a specificity of 0.82 for diagnosing IIMs. A focal MHC-I staining pattern was associated with IMNM, whereas a global pattern was more prevalent in sIBM, and a perifascicular pattern was significantly more common in dermatomyositis. Among 18 DM biopsies without perifascicular atrophy, 50% had a perifascicular MHC-I staining pattern. Sarcoplasmic upregulation staining was more common than sarcolemmal staining across all groups. CONCLUSIONS MHC-I immunostaining was useful to distinguish IIMs from non-inflammatory myopathies or neurogenic disorders. Of note, a perifascicular MHC-I staining pattern was present only in those with DM, including half of those without perifascicular atrophy; many of these biopsies may not otherwise have been diagnostic for DM.