Abstract 4360121: PD-1-deficient Dendritic Cells Show Reduced Lysosomal Activity in a Model of Immune Checkpoint Inhibitor-associated Myocarditis

Introduction: Although immune checkpoint inhibitors (ICIs) have transformed cancer therapy, they can cause immune-related adverse effects, including ICI myocarditis, a potentially fatal form of inflammation-induced cardiac injury. What triggers autoreactive T cell activation in patients treated with ICIs remains unclear. Dendritic cells (DCs), which take up and present antigens to T cells for activation, could play a key role in the development of ICI myocarditis. Hypothesis: We hypothesize that reduced DC lysosomal activity in ICI myocarditis leads to reduced degradation of internalized antigens, preserving antigen epitopes and enhancing antigen presentation, leading to increased T cell activation. Methods: We have developed a mouse model of ICI myocarditis wherein three priming doses (100 mg/kg) of isoproterenol (ISO) followed by a high dose (300 mg/kg) of ISO provokes persistent autoreactive CD8+ T cell-driven myocarditis in mice deficient in the immune checkpoint inhibitor PD-1 (PD-1 KO). PD-1 KO mice have increased mortality and reduced cardiac function compared to wild-type mice. We studied DC lysosomes in this model in vivo (heart, mediastinal lymph nodes) and in vitro (bone marrow-derived DCs) using flow cytometry, biochemical tools, and mass spectrometry. Results: Using our ISO model, we observed that PD-1 KO DCs in the heart and mediastinal lymph nodes showed significantly reduced lysosomal acidification than wild-type in response to ISO priming. PD-1 KO bone marrow-derived DCs had significantly reduced cathepsin B activity and antigen processing capabilities than wild-type. Proteomic analyses of lysosomes isolated from antigen presenting cells in lymph nodes also showed increased cardiac peptide abundance in PD-1 KO DCs versus wild-type. Conclusions: Our findings demonstrate altered lysosomal degradation in PD-1 KO DCs. Future work will investigate how this affects antigen presentation, CD8+ T cell expansion, and cardiac outcomes in ICI myocarditis.