Input of Major Histocompatibility Complex class II immunostaining in idiopathic inflammatory myopathies

BackgroundIdiopathic inflammatory myopathies (IIMs) are a group of rare acquired muscular diseases. In healthy muscle, myofibers do not express major histocompatibility complex (MHC) class I and II. It was established that MHC-I positive immunostaining, although non-specific, is a marker for IIM diagnosis, while the significance of MHC-II immunostaining remains unclear. The present study investigates the expression of MCH-II in myofibers and capillaries of IIM muscles, taking into account the current IIM classification. Patients & MethodsA historical cohort was designed, including dermatomyositis (DM), inclusion body myositis (IBM), anti-synthetase syndrome (ASyS), immune-mediated necrotizing myopathy (IMNM), or overlap myositis (OM). MHC-II immunostaining was performed on patient muscle sections and was analyzed in a standardized and blind manner. ResultsMuscle sections from biopsies of 72 IIM patients were included: 23 DM, 17 IBM, 12 IMNM, 9 ASyS, and 11 OM. Overall, abnormal MHC-II immunostaining was found in myofibers and/or capillaries in 67 (93%) patients. Myofiber MHC-II immunostaining patterns differed according to the IIM subgroup: the immunostaining was diffuse in IBM (100%), negative in IMNM (75%), perifascicular in ASyS (67%), and either diffuse heterogeneous, clustered, or perifascicular in OM (27%, 27%, and 18%, respectively). MHC-II expression was found in 50% of DM (n=11/22). While all IIM subgroups presented quantitative and qualitative abnormalities of MHC-II immunostaining in capillaries, some subgroups displayed specificities. Most IBM and IMNM muscles presented frequent dilated capillaries (88% and 67%, respectively). DM, ASyS, and OM exhibited high frequencies of capillary lesions, including capillary dropout, leaky capillaries, and dilated capillaries. ConclusionWhile recent expert opinion (EURO-NMD pathology working group) recommended that MHC-II immunostaining of muscle biopsy remains optional, the present work demonstrates that the expression pattern of MHC-II allows to distinguish between several IIM subgroups. Our data argue for the inclusion of MHC-II immunostaining in the routine histological diagnosis for IIMs.