Temporal alterations of immunohistochemical findings in polymyositis

To investigate the pathological mechanisms of polymyositis, we performed immunohistochemical analyses with biopsied muscles. Comparative studies using specimens obtained from the same patient enabled us to analyze the pathological alterations at different sequential clinical phases without taking into account each immunogenetic difference. Expression of MHC (major histocompatibility complex) antigens, especially ectopic MHC-class-II antigens, in muscle fibers and infiltrating T-cells were shown to increase concomitantly with the clinically observed exacerbation. Moreover, other observations seem to support the possibilities that 1) administration of steroids lowers the number of invading CD8-positive cells, 2) induction of MHC-class-I antigens in muscle fibers precedes the inflammatory cell infiltration and that 3) remaining MHC-class-I antigens in muscle fibers may explain the recurrence of myositis often observed in the follow up period.