Skeletal muscle expression of class II histocompatibility antigens (HLA-DR) in polymyositis and other muscle disorders with an inflammatory infiltrate.

Muscle biopsy specimens with a pronounced inflammatory component were evaluated for myocyte expression of class II histocompatibility antigens (HLA-DR) by immunohistochemical techniques. All 15 cases of polymyositis were positive; six cases of muscular dystrophy (two Duchenne, four facio-scapulo-humeral), and one case of acute denervation (motor neurone disease) were negative, despite having a comparably intense mononuclear infiltrate. Twelve entirely normal biopsy specimens were also negative for HLA-DR expression. Expression of this membrane glycoprotein may have a clinically important aetiological role in polymyositis, and demonstration of its presence may prove useful as a marker for this disorder in selected problematic biopsy specimens.