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BackgroundNearly half of all U.S. oncology patients meet FDA eligibility criteria to receive treatment with an immune checkpoint inhibitor (ICI). With increasing use of ICIs, preventing, diagnosing and treating immune-related adverse events (irAEs) are urgent clinical challenges. Myocarditis is an uncommon irAE, affecting < 1% of ICI-treated patients, but is highly fatal, with a mortality rate of nearly 50%. Genetically altered Pdcd1-/-Ctla4± mice die prematurely and specifically due to myocarditis. This model recapitulates the clinical and pathological features of ICI-myocarditis, including abundant cardiac infiltrating CD8+ T cells. The potential autoantigen(s) involved in ICI-myocarditis are unknown for both human disease and our murine model.MethodsWe used Pdcd1-/-Ctla4± mice on the C57BL6 background as a model of ICI-myocarditis. Single cell RNA and T cell receptor (TCR) sequencing was performed on sorted CD45+ cardiac immune cells from four affected Pdcd1-/-Ctla4± mice compared to six healthy wild type mice. The most three clonal TCRs (TCR-A, B, C), derived from two independent Pdcd1-/-Ctla4± mice, were reconstructed using stiTChR and transduced into reporter T cell lines for antigen discovery. Alpha-myosin was selected as a candidate autoantigen due to lack of presentation in the thymus. Reporter TCR-A, B, and C cells were screened using a library of overlapping 20 amino acid peptides derived from alpha-myosin in co-culture with bone marrow derived dendritic cells.ResultsTreatment with anti-CD8, but not anti-CD4, depleting antibodies rescues survival of Pdcd1-/-Ctla4± mice. Furthermore, adoptive transfer of splenocytes from Pdcd1-/-Ctla4± mice with myocarditis to Rag1-/- recipient mice was sufficient to induce fatal myocarditis. Single cell RNA/TCR sequencing on the cardiac immune infiltrate of Pdcd1-/-Ctla4± mice identified highly activated, clonal CD8+ T cells as the dominant cell population. The TCR-A cell line, the most clonal TCR identified in single cell TCR sequencing, activates NFAT, NFkB, and AP-1 reporters in response to the alpha-myosin epitope VIQYFASI. The TCR-B and TCR-C cell lines activate their reporters in response to the alpha myosin peptide DALLVIQWNIRAFMGVKNWP, indicating that alpha-myosin is an autoantigen in this mouse model of ICI-myocarditis.ConclusionsClonal, activated CD8+ T cells are critical for the development of ICI-myocarditis. Alpha-myosin is an autoantigen recognized by the most clonal cardiac CD8+ T cells. Efforts are currently underway to determine whether human TCRs derived from ICI-myocarditis samples recognize similar antigens. These studies are the first to identify a candidate autoantigen in ICI-myocarditis and may yield new insights into irAE pathogenesis.Ethics ApprovalAll animal experiments were in accordance with the VUMC Institutional Animal Care and Use Committee (IACUC), protocol # M2000067