A new murine model to define the critical pathologic and therapeutic mediators of polymyositis

Objective To establish a new murine model of polymyositis (PM) for the understanding of its pathologic mechanisms and the development of new treatment strategies. Methods C protein–induced myositis (CIM) was induced by a single immunization of recombinant human skeletal C protein in C57BL/6 mice, as well as in CD4-depleted, CD8-depleted, and mutant mice as controls. Some mice were treated with high-dose intravenous immunoglobulin (IVIG) after disease induction. Muscle tissues were examined histologically. Results In mice with CIM, inflammation was confined to the skeletal muscles. Histologic examination revealed a common pathologic feature of CIM and PM, involving abundant infiltration of CD8 and perforin-expressing cells in the endomysial site of the injured muscle. Suppression of myositis was achieved by depletion of both CD4 and CD8 T cells. Despite the development of serum anti–C protein antibodies in wild-type mice, severe myositis was induced in mice deficient in B cells. Induction of myositis was suppressed in interleukin-1α/β (IL-1α/β)–null mutant mice, but not in tumor necrosis factor α (TNFα)–null mutant mice. Use of IVIG, a treatment with proven efficacy in PM, suppressed CIM in the subgroup of treated mice. Conclusion CIM mimics PM pathologically and clinically. Infiltration of CD8 T cells is the most likely mechanism of muscle injury, and IL-1, but not B cells or TNFα, is crucial in the development of CIM. IVIG has therapeutic effects in CIM, suggesting that the effects of IVIG are not mediated by suppression of antibody-mediated tissue injury. This murine model provides a useful tool for understanding the pathologic mechanisms of PM and for developing new treatment strategies.