[Aberrant expression of major histocompatibility complex class II antigen in inflammatory myopathies]

Although major histocompatibility complex (MHC) class I molecules have been consistently demonstrated on muscle fibers of patients with inflammatory myopathies (IM), results concerning the expression of MHC class II molecules have been conflicting and mechanism of the expression remains to be elucidated. Expression of MHC and neural cell adhesion molecules (N-CAM) was analyzed in 38 cases of IM and in 13 cases of normal and disease controls by the immunocytochemical method. The sarcolemmaL HLA-ABC molecules were seen in all IM cases, and were visualized on the every muscle fiber in 68% of the cases. In contrast, the HLA-DR, -DP, or -DQ expression was seen on scattered fibers in 37%, 11%, 5% of IM cases, respectively. The HLA-DR reactivity was often seen on muscle fibers in the vicinity of infiltrating cells or on the perifascicular fibers. The HLA-DR positive fibers did not express N-CAM, suggesting a degenerating/regenerating process of muscle fibers was not contributory to the aberrant expression of MHC class II molecules. The perifascicular HLA-DR expression was related to the perifascicular atrophy and was more frequent in polymyositis complicated with interstitial pneumonitis. Those results suggest that increased expression of MHC class I molecules precede the aberrant expression of MHC class II molecules, and the perifascicular atrophy might reflect immunopathological processes.