Clonal CD8 T cells orchestrate arrhythmogenic inflammation in immune checkpoint inhibitor-associated myocarditis

Objective: Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy but are frequently associated with immune-related adverse events, particularly in combination regimens. ICI-associated myocarditis (ICI-M) is rare but highly lethal, with a mortality rate approaching 50% and a notable risk of fatal ventricular arrhythmias. Prevention and effective management of ICI-M remain significant clinical challenges. This study aims to elucidate the immune landscape and spatial organization of immune cells in ICI-M using single-cell multi-omics in a preclinical mouse model. Methods: We utilized Ctla4+/–Pdcd1–/– mice, modeling human ICI-induced myocarditis. Single-cell RNA (scRNAseq) and T cell receptor (TCR) sequencing were performed on heart-infiltrating immune cells and peripheral blood mononuclear cells (PBMCs). Spatial transcriptomics was applied to map immune infiltration in cardiac tissue. Results: Approximately half of Ctla4+/–Pdcd1–/– mice died after 100 days, while Ctla4+/+ mice showed normal survival. ECG monitoring identified ventricular arrhythmias in 15% of Ctla4+/–Pdcd1–/– mice. Histology revealed severe myocardial inflammation and fibrosis. Cardiac scRNAseq data indicated that T cells were the predominant immune population in ICI-M hearts. TCR sequencing demonstrated substantial overlap in TCR clonotypes between cardiac-infiltrating and circulating CD8+ T cells. These TCR-shared CD8+ T cells exhibited an activated phenotype and were enriched in pathways associated with allograft rejection and inflammation. Spatial mapping revealed localization near the AV node and subendocardium, implicating potential CD8+ T cell–mediated conduction disruption. Conclusion: Clonal, activated CD8+ T cells contribute to arrhythmogenic myocarditis in ICI-M. Targeting specific TCR-peptide-MHC interactions may enable myocarditis prevention without impairing antitumor efficacy.