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 ICI-associated myocarditis in vivo, we generated two conditional PD-L1 knockout mouse lines, with inducible deletion of PD-L1 either in cardiomyocytes (under the control of the mer-Cre-mer; CM-PD-L1- cKO) or endothelial cells (under control of Cadh5-Cre; EN-PD-L1-cKO). These were then challenged in an ICI protocol by pharmacological treatment with the ICI CTLA-4 antibody.
Results: EN-PD-L1-cKO showed only a mild infiltration with immune cells, whereas CM-PD-L1-cKO displayed a significantly higher immune cell infiltration. This comprised observable histological differences, such as a significantly increased number of detectable cells per area and a higher fibrotic percentage in the treated CM-PD-L1-cKO animals compared to controls. These differences were not present in EN-PD-L1-cKO animals. CM-PD-L1-cKO showed significantly decreased fractional shortening (FS 31.71%±1.11 vs. 21.13%±0.845 WT controls vs. CM-PD-L1-cKO; p<0.01) as well as ejection fractions (EF 60.92 %±1.73 vs. 43.74 %±1.52 WT controls vs.CM-PD-L1-cKO; p<0.01). Animals with the endothelial-specific deletion (EN-PD-L1-cKO) were functionally unchanged (FS 30.72%±1.93 vs. 28.57%±0.67 WT controls vs. CM-PD-L1-cKO; p>0.05) and showed no significant changes in ejection fraction (EF 58.66%±2.93 vs. 55.57%±0.96 WT controls vs. EN-PD-L1-cKO; p>0.05). Our findings highlight the critical role of PD-L1 expression on cardiomyocytes in the induction of immune checkpoint inhibitor–mediated myocarditis (ICIM). The myocarditis observed in CM-PD-L1-cKO compared to EN-PD-L1-cKO, together with the minimal inflammation seen in other cell type–specific knockouts, suggests that cardiomyocytes are the primary drivers of the inflammatory process, while normal PD-L1 expression serves a protective, anti-inflammatory function.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.
