Acquisition of self-reactive effector CD4+ T cells is a major component of the autoimmune response that can occur during myocarditis, an inflammatory form of cardiomyopathy. Although the processes by which self-reactive T cells gain effector function have received considerable attention, how these T cells contribute to effector organ inflammation and damage is less clear. Here, we identified an IL-3–dependent amplification loop that exacerbates autoimmune inflammation. In experimental myocarditis, we show that effector organ–accumulating autoreactive IL-3+ CD4+ T cells stimulate IL-3R+ tissue macrophages to produce monocyte-attracting chemokines. The newly recruited monocytes differentiate into antigen-presenting cells that stimulate local IL-3+ CD4+ T cell proliferation, thereby amplifying organ inflammation. Consequently, Il3−/− mice resist developing robust autoimmune inflammation and myocardial dysfunction, whereas therapeutic IL-3 targeting ameliorates disease. This study defines a mechanism that orchestrates inflammation in myocarditis, describes a previously unknown function for IL-3, and identifies IL-3 as a potential therapeutic target in patients with myocarditis.
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