Interleukin-6-deficient mice resist development of autoimmune myocarditis associated with impaired upregulation of complement C3

Background-Interleukin (IL)-6 regulates various aspects of the immune response. In the context of heart diseases, it has been recognized as a prognostic factor for dilated cardiomyopathy, which often results from myocarditis. Methods and Results-Using IL-6-deficient mice, we studied the role of IL-6 in a model of autoimmune myocarditis resulting from immunization with a peptide derived from cardiac a-myosin. Prevalence and severity of myocarditis were markedly reduced in the absence of IL-6. CD4(+) T cells from immunized IL-6-deficient mice proliferated poorly on restimulation with specific antigen in vitro and did not mediate disease on adoptive transfer into IL-6-competent RAG-2-deficient mice, which otherwise lack B cells and T cells. Production of complement C3, a crucial factor for the development of myocarditis, was strongly upregulated in IL-6(+/+) but not in IL-6-deficient mice after immunization. Conclusions-Our results demonstrate that IL-6 is required for the expansion of autoimmune CD4(+) T cells and the pathogenesis of autoimmune myocarditis, possibly by upregulation of complement C3.