Myosin-induced autoimmune polymyositis in the rat

Experimental autoimmune myositis (EAM) was induced in Lewis rats by immunization with partially purified and purified skeletal myosin. Although clinical signs such as muscle weakness were very mild, multiple inflammatory lesions in the skeletal muscle, but not in the heart, were found by histological examination. Immunohistochemical staining revealed that muscle fiber-infiltrating cells were CD8+ and CD11b+ cells and that CD4+, TCR alphabeta+, B and NK cells were mainly located in the endomysium and interfiber connective tissue. These findings were in contrast to those obtained in experimental autoimmune encephalomyelitis lesions in which CD4+ cells predominate over CD8+ cells. T cells and sera isolated from myosin-immunized animals responded vigorously to myosin. However, neither sensitized lymphoid cells mainly comprising CD4+ cells nor purified anti-myosin immunoglobulin G mediated the disease into naive rats, suggesting that T cells other than CD4+ cells such as CD8+ cells may be the final effector. Taken together, EAM induced in Lewis rats is similar to human polymyositis (PM). EAM can serve as a good model for human PM and give insight into the pathogenesis of the disease.