Hydrophobic complementarity-determining region 3 (CDR3) sequences elucidate the cardiotoxic effects of immune checkpoint inhibitors.

Immune checkpoint inhibitors (ICIs) have significantly changed cancer treatment, demonstrating strong efficacy across multiple cancers. However, their use also carries the risk of serious immune-related side effects (irAEs), especially cardiotoxicity. To understand how these adverse effects occur, we studied peripheral blood mononuclear cells and T cells taken from the heart tissue of cancer patients who experienced cardiotoxicity during ICI therapy. Using spectral flow cytometry, single-cell RNA sequencing, and T cell receptor (TCR) sequencing, we found key differences in the immune profiles of affected patients. Those with cardiotoxicity had a noticeable increase in circulating CD4 + FOXP3 + and CD8 + PRF1 + T cells at disease onset. Our results also show that effector CD8 T cells are present in the heart tissue and pericardial fluid of patients with myocarditis, highlighting their role in starting the disease. TCR sequencing revealed expansions of CD8 GZMK + GZMA + and CD8 PRF1 + GZMA + T cells in myocarditis patients, along with increased activation markers CD69 and KLRG1, supporting the idea that specific cytotoxic CD8 T cell groups promote inflammation. Notably, we also found that T cells from patients with irAE myocarditis have shorter TCR CDR3 sequences, with a higher proportion of hydrophobic residues. This discovery suggests a new mechanism for TCR involvement in irAE myocarditis, focusing on T cell activation through the TCR's functional promiscuity, which relies more on TCR-MHC interactions than on specific peptide features. Overall, this research provides a foundation for new strategies targeting TCR physical properties to reduce risks and develop more precise therapies for vulnerable patients.