To obtain a good animal model for
polymyositis, we previously induced
experimental autoimmune myositis (EAM) in Lewis rats by immunization with partially purified skeletal
myosin. However, the nature of EAM-inducing
antigen(s) in the partially purified
myosin preparation remains unclear because it may contain several myositogenic
antigens. In the present study, we further purified
myosin and C-
protein from partially purified
myosin preparations and examined their EAM-inducing ability. It was revealed that immunization with both C-
protein and purified
myosin elicited EAM, which was essentially the same as that induced by partially purified
myosin. However, their myositogenic ability was quite different. C-
protein induced severe EAM of high histological grade and lesion frequency, whereas purified
myosin induced only mild EAM. Immunohistochemical staining of C-
protein-induced lesions demonstrated that muscle fiber-infiltrating cells were CD8beta+ T cells and macrophages and that CD4+ cells were mainly located in the endomysium and interfiber connective tissue. Collectively, these findings suggest that C-
protein in the skeletal muscle is the major myositogenic
antigen and induces inflammatory lesions mimicking those of human
polymyositis.