Cardiomyocyte but not endothelial expression of PD-L1 is required to protect from Immune-Checkpoint-Inhibitor-Associated Myocarditis (ICIM)

Abstract Background Immune checkpoint inhibitors (ICIs) targeting ‘programmed cell death protein 1’ (PD-1) or its ligand PD-L1 have revolutionized cancer therapy. However, ICI-associated myocarditis (ICIM) is a potentially fatal immune-related adverse event. While PD-1 knockout mice have been extensively used to study ICIM, the predominant immunogenic cell type, driving the misguided T-cell response, remains unclear. Objective To investigate the role of specific cell types in ICIM, we generated conditional PD-L1 knockout lines, with inducible deletion of PD-L1 either in cardiomyocytes (under the control of the aMHC-MerCreMer; CM-PD-L1-cKO) or endothelial cells (under control of CDH5-Cre; EN-PD-L1-cKO). These lines were then challenged by an ICI protocol by adding CTLA-4 antibody. Results EN-PD-L1-cKO showed very mild infiltration with immune cells while CM-PD-L1- cKO showed a severe phenotype. It also included significantly decreased fractional shortening (FS 28.14%±1.15 vs. 21.17%±0.96 WT controlsvs. CM-PD-L1-cKO; p0.01) as well as ejection fractions (EF 55.39%±1.8 vs. 43.84%±1.73 WT controls vs. CM-PD-L1-cKO; p0.01). Animals with the endothelial-specific deletion (EN-PD-L1-cKO) were functionally unchanged (FS 28.35%±1.1 vs. 28.57%±0.67 WT controls vs. CM-PD-L1-cKO; p0.05) and showed no significant changes in ejection fraction (EF 55.39%±1.6 vs. 55.57%±0.96 WT controls vs. EN-PD-L1-cKO; p0.05). Our findings highlight the importance of the PD-L1 on cardiomyocytes to protect from ICIM. The severe myocarditis observed in CM-PD-1L-cKO compared to EN-PD-1L-cKO, combined with the minimal inflammation in other cell-specific knockouts, suggests that cardiomyocytes are the driver of the inflammatory process while normal expression of PD-L1 counteracts ICIM. Implications Understanding the specific roles of various cell types in ICIM could lead to more precise therapeutic interventions that protect the heart without compromising the efficacy of ICI on cancer progression.