Expression of immunoreactive major histocompatibility complex products in human skeletal muscles.

Immunoreactive class 1 and class 2 major histocompatibility complex gene products (MHCP) and beta 2 microglobulin (beta 2 MG) were demonstrated by microscopic immunocytochemistry in cryostat sections of skeletal muscle biopsies of 67 patients with various neuromuscular diseases. Diagnoses included normal muscle, chronic partial denervation, Duchenne dystrophy, polymyositis, dermatomyositis, inclusion body myositis, and miscellaneous neuromuscular diseases. Normal mature muscle fibers did not express MHCP, but blood vessels showed both class 1 and 2 MHCP and beta 2 MG. Regenerating muscle fibers showed consistent sarcolemmal class 1 MHCP expression irrespective of the disease. In polymyositis, the majority of extrafusal muscle fibers of most patients showed strong sarcolemmal class 1 MHCP expression. In dermatomyositis, muscle fibers situated either in perifascicular or in randomly clustered distribution revealed strong class 1 MHCP reactivity. In inclusion body myositis, scattered small clusters of muscle fibers were positive for class 1 MHCP. In polymyositis and inclusion body myositis, particularly strong class 1 MHCP expression was invariably seen in nonnecrotic muscle fibers partially invaded by lymphocytes whose cytotoxic effects are believed to be class 1 MHCP restricted. Factors or agents that trigger class 1 MHCP expression are presumed also to sensitize lymphocytes to muscle fibers in these diseases, but their identity remains obscure at this time. In dermatomyositis, the expression of MHCP in perifascicular muscle fibers and in areas of capillary loss may represent the triggering of MHCP expression by a nonspecific cellular stress reaction, in this case probably low-grade ischemia.