Inflammatory and non-inflammatory inclusion body myositis. Characterization of the mononuclear cells and expression of the immunoreactive class I major histocompatibility complex product.

In ten patients with inclusion body myositis (IBM) five muscular biopsies showed profuse inflammatory exudates and three showed a few scattered inflammatory cells with partial invasion in some muscle fibers. No inflammatory cells were seen in two cases. In all patients, histopathological, histomorphometric and immunocytochemical studies were performed. Immunocytochemistry for the class I and class II major histocompatibility complex gene product (MHC) was performed in all cases and in ten control muscles including: normal muscles [3], dermatomyositis [3], polymyositis [3], scleroderma [1]. In the five cases of IBM with inflammatory exudates, subsets of lymphocytes were analyzed with a panel of monoclonal antibodies against B cells, T4 cells, T8 cells, K and natural killer cells and macrophages. Some muscle fibers expressed class I MHC antigens in the inflammatory cases of IBM. These fibers were near the inflammatory exudates and occasionally showed a partial invasion. No expression of class I MHC was found in normal muscles and in non-inflammatory cases of IBM. The antigen which triggers the mononuclear cells in the inflammatory forms of IBM is probably not the filamentous inclusions in rimmed vacuoles. In other inflammatory myopathies, expression of class I MHC was present on all fibers in polymyositis, only in the perifascicular area in dermatomyositis and in scleroderma. It could be suggested that the term "inclusion body muscle disease" be applied to cases with rimmed vacuoles and "IBM-like" filaments without inflammatory cells.