Cytotoxic T cells specific for alpha-myosin drive immunotherapy related myocarditis

Immune-related adverse events, particularly severe toxicities such as myocarditis, are major challenges to immune checkpoint inhibitor (ICI) utility in anti-cancer therapy1. The pathogenesis of ICI-myocarditis is poorly understood. Pdcd1-/-Ctla4+/- mice recapitulate clinicopathologic features of ICI-myocarditis, including myocardial T cell infiltration2. Single cell RNA/T cell receptor (TCR) sequencing on the cardiac immune infiltrate of Pdcd1-/-Ctla4+/- mice identified activated, clonal CD8+ T cells as the dominant cell population. Treatment with anti-CD8, but not anti-CD4, depleting antibodies rescued survival of Pdcd1-/-Ctla4+/- mice. Adoptive transfer of immune cells from mice with myocarditis induced fatal myocarditis in recipients which required CD8+ T cells. Alpha-myosin, a cardiac specific protein absent from the thymus3,4, was identified as the cognate antigen source for three MHC-I restricted TCRs derived from mice with fulminant myocarditis. Peripheral blood T cells from two patients with ICI-myocarditis were expanded by alpha-myosin peptides, and these alpha-myosin expanded T cells shared TCR clonotypes with diseased heart and skeletal muscles, indicating that alpha-myosin may be a clinically important autoantigen in ICI-myocarditis. These studies underscore the critical role for cytotoxic CD8+ T cells, are the first to identify a candidate autoantigen in ICI-myocarditis and yield new insights into ICI toxicity pathogenesis.