Immunologic drivers of immune checkpoint inhibitor therapy myocarditis, fatality and anti-tumor response in cancer patients

Immune checkpoint inhibitors (ICIs) can offer remarkable benefits to patients with cancer, but these agents can cause morbid and potentially fatal immune-related adverse events (irAEs), the most lethal of which is ICI-related myocarditis (irMyocarditis). The pathogenesis of irMyocarditis and its relationship to anti-tumor immunity remain poorly understood. We sought to define how immune cell trafficking between heart, tumor and blood contributes to irMyocarditis pathogenesis and fatality 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 the heart and blood specimens identified enrichment of cytotoxic T-cell subsets, inflammatory macrophage subsets, conventional dendritic cells (cDCs), and fibroblasts in the heart that accompany the upregulation of potentially targetable, pro-inflammatory transcriptional programs. Expanded TCR clones enriched in irMyocarditis are largely distinct from those in paired tumors, suggesting distinct mechanisms between anti-tumor immune response and irMyocarditis. We also identified circulating biomarkers of fatality, including decreased circulating CD4+ T cells, decreased cDCs, and the presence of cardiac-expanded TCRs in a circulating, cycling CD8+ T cell population. Collectively, these findings highlight critical biology driving disease pathogenesis and nominate putative biomarkers and tailored therapeutic interventions for patients with irMyocarditis.