Introduction: We sought to study the intensity and pattern of major histocompatibility complex (MHC) I and II expression in muscle from patients with biopsy-proven idiopathic inflammatory myositis (IIM) including the subgroups, polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM).
Methods: A total of 120 muscle biopsies (61 PM, 14 DM, and 45 IBM) were immunostained for MHC I and II. Staining was graded as follows. 0: no staining, 1+: ≤10% fibers, 2+: 10% to 25%, 3+: 25% to 50%, 4+: 50% to 99%, and 5+ 100%.
Results: All IIM biopsies showed MHC I positivity; 93% showed MHC II positivity. The proportion of patients with MHC II score ≥3+ was higher in IBM than DM or PM. In DM, MHC I expression showed a perifascicular pattern. All IBM biopsies were immunopositive for MHC I and II; 30/45 were scored 5+.
Discussion: Immunostaining for MHC I and II is a useful adjunctive test in diagnosis and subclassification of IIM.