MHC Class I overexpression on muscles in early juvenile dermatomyositis.

OBJECTIVE To assess muscle expression of MHC Class I complexes (heavy chain and beta2-microglobulin) and to analyze the composition of infiltrating mononuclear cells, specifically cells that bear receptors for class I MHC molecules, in the muscles of children with early juvenile dermatomyositis (JDM). METHODS Light microscopic and immunohistochemical analysis of muscle biopsies from 10 patients with JDM and 3 controls. The mean duration from initial weakness was 2.8 months. At the time of biopsy, 9 patients had not received steroid treatment or immunomodulatory drugs. RESULTS MHC Class I over-expression was evident on muscle fibers in all 10 JDM samples, even in a biopsy reported as normal by conventional histology. MHC class I heavy chain and beta2-microglobulin were over-expressed in an identical distribution. Variable infiltration of T cells and macrophages was seen in the JDM biopsies, with minimal lymphocytic and monocytic infiltration in 4 cases, and none in one. Only very occasional natural killer lymphocytes were identified. Neuronal cell adhesion molecule (NCAM, CD56) staining of regenerating muscle fibers was seen in all samples and these cells were confirmed as being of muscle origin by co-staining for dystrophin. CONCLUSION MHC Class I over-expression is an early event in JDM, and may occur in the absence of lymphocytic infiltration and muscle damage. Immunostaining for MHC Class I could be used routinely in the assessment of muscle histology in juvenile dermatomyositis.