Background: Cardiac immunity is critical for maintaining cardiac integrity at baseline and cardiac disease. The Programmed cell death protein 1 (PDL1) immune checkpoint and its ligand PDL1 play a crucial role in regulating T cell function and activity, thereby contributing to immune homeostasis. As observed in anti-PD1 immune checkpoint inhibitor therapy, a disruption of PD1/PDL1 signaling can lead to severe immune-related adverse events, including cardiovascular complications. PDL1 is densely expressed in the myocardium, hence rendering PD1/PDL1 signaling as a potential key mediator of cardiac immunity. Here we aim to assess the impact of PD1 deficiency on cardiac immunity at baseline and in reperfused myocardial infarction. 

Methods: The expression of PDL1 on individual cardiac cell subpopulations was assessed by flow cytometry recapitulated in immunofluorescence staining of mid-ventricular cross-sections from C57BL/6J Pdcd1^-/- and wild-type mice. Key factors of cardiac inflammation were examined in heart homogenates by western blot analysis. The response of PD1/PDL1 in reperfused acute myocardial infarction was analyzed using an in vivo ischemia-reperfusion model. In brief, mice were anesthetized and subjected to 45 min of ischemia by temporary ligation of the left coronary artery, followed by 1-3 days of reperfusion while being treated with anti-PD1 antibodies or control. Infarct sizes were determined by triphenyltetrazolium chloride (TTC) Evan’s blue staining. PDL1 expression was assessed as described above. Immune cell subpopulation infiltrating the myocardium were examined with a newly designed flow cytometry-based panel.

Results: A profound upregulation of pro-inflammatory cytokines was observed during baseline conditions in cardiac tissue of PD1-deficient mice, including interleukin (IL) 1ɑ, IL4, and phosphorylated extracellular-signal regulated kinase (ERK) 1/2 which is involved in proliferation, differentiation, and activation of immune cells. While PDL1 was broadly expressed on cardiac endothelial cells at baseline, a significant decline in endothelial PDL1 expression was observed in reperfused acute myocardial infarction, mainly attributed to steep decrease in the infarct zone. The infarct size following in-vivo I/R injury was not altered upon PD1-blockade as determined by TTC staining. However, flow cytometry determined increased numbers of CD8⁺ T cells in reperfused acute myocardial infarction when receiving anti-PD1 therapy compared to controls, thus further highlighting the role of PD1/PDL1 signalling in mediating T cell-related cardiac inflammatory processes.

Conclusions: PD1/PDL1 plays a key role in mediating cardiac immunity at baseline and in acute myocardial infarction. The results indicate that relevant effects are not limited to complications from cancer therapy as previously described but can be expected in various forms of cardiovascular disease, hence requiring further investigations.