Immune Responses in Checkpoint Myocarditis Across Heart, Blood, and Tumor

Immune checkpoint inhibitors (ICIs) are widely used anti-cancer therapies that can cause morbid and potentially fatal immune-related adverse events (irAEs). ICI-related myocarditis (irMyocarditis) is uncommon but has the highest mortality of any irAE. The pathogenesis of irMyocarditis and its relationship to anti-tumor immunity remain poorly understood. We sought to define immune responses in heart, tumor, and blood during irMyocarditis and identify biomarkers of clinical severity by leveraging single-cell (sc)RNA-seq coupled with T cell receptor (TCR) sequencing, microscopy, and proteomics analysis of 28 irMyocarditis patients and 23 controls. Our analysis of 284,360 cells from heart and blood specimens identified cytotoxic T cells, inflammatory macrophages, conventional dendritic cells (cDCs), and fibroblasts enriched in irMyocarditis heart tissue. Additionally, potentially targetable, pro-inflammatory transcriptional programs were upregulated across multiple cell types. TCR clones enriched in heart and paired tumor tissue were largely non-overlapping, suggesting distinct T cell responses within these tissues. We also identify the presence of cardiac-expanded TCRs in a circulating, cycling CD8 T cell population as a novel peripheral biomarker of fatality. Collectively, these findings highlight critical biology driving irMyocarditis and putative biomarkers for therapeutic intervention. ### Competing Interest Statement S.M.B has been a paid consultant to Two River Consulting and Third Rock Ventures. He has equity positions in Kronos Bio, 76Bio, and Allogene Therapeutics. D.A.Z. has been a paid consultant to Bristol Myers Squibb, Freeline Therapeutics, and Intrinsic Imaging. L.Z. has received consulting fees from Bristol Myers Squibb and Merck. R.J.S has been a paid consultant to Bristol Myers Squibb, Merck, Pfizer, Marengo Therapeutics, Novartis, Eisai, Iovance, OncoSec, and AstraZeneca and has received research funding from Merck. T.G.N has been a paid consultant to Bristol Myers Squibb, Genentech, CRC Oncology, Roche, Sanofi and Parexel Imaging Pharmaceuticals and has received grant funding from Astra Zeneca and Bristol Myers Squibb related to the cardiac effects of immune checkpoint inhibitors. K.L.R has served as an advisory board to SAGA Diagnostics and received speakers fees from CMEOutfitters and Medscape as well as research funding from Bristol Myers Squibb. A.C.V. has been a paid consultant to Bristol Myers Squibb.