Human leukocyte antigen class I in polymyositis: Leukocyte infiltrates, regeneration, and impulse block

In polymyositis (PM), T‐cell mediated myocytotoxicity is directed against strongly human leukocyte antigen class I positive (HLA‐I+) muscle fibers. Fiber regeneration probably is partly responsible for this HLA‐I up‐regulation. We have evaluated regeneration, denervation/impulse blockade, and focal leukocyte infiltrates as possible HLA‐I inducing factors in PM. Distinctive patterns of HLA‐I, nerve cell adhesion molecule (NCAM), and vimentin expression accompany denervation and regeneration. Regenerating fibers also have centralized nuclei. Using semiquantitative methods, we examined strongly HLA‐I+ fibers in PM muscle biopsies for these markers. Sarcoplasmic HLA‐I levels were related to the presence of leukocyte infiltrates and invasion of fibers. Strongly HLA‐I+ fibers were frequently invaded, and regeneration‐associated changes were usually observed at sites of fiber damage. Sarcoplasmic HLA‐I levels were stable along intact fibers, also adjacent to leukocyte infiltrates. A majority of the strongly HLA‐I+ fibers were nonregenerating (NCAM+ only). Though other mechanisms cannot be excluded, this suggests that impulse blockade or denervation may contribute to extra HLA‐I up‐regulation in these fibers. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1534–1540, 1997