Cardiomyocytes upregulated PD-L1 expression to alleviate cardiac injury induced by irradiation combined with anti-PD-1 antibody: an in vitro and in vivo study

Objective: Preclinical studies suggest that irradiation combined with anti-PD-1 antibody (iRT) exacerbates cardiac injury in mice, partly through CD8 T-cell-mediated mechanisms. We investigated radiation-induced PD-L1 expression in cardiomyocytes and explored its potential role in iRT-associated cardiac injury. Methods: AC16 cells were exposed to different doses of irradiation. PD-L1 expression was assessed at different time points by flow cytometry, qRT-PCR, western blotting, and immunofluorescence. Cell viability, apoptosis, and cytokine levels were evaluated. PD-L1 was knocked down using siRNA to assess its function. PBMCs or CD8 T cells were co-cultured with irradiated or non-irradiated AC16 cells at different E:T ratios, with or without PD-1 blockade. A C57BL/6J mouse model of iRT-induced myocardial injury was established, and myocardial PD-L1 expression and T-cell infiltration were evaluated. Results: Irradiation upregulated PD-L1 expression in AC16 cells, with increased membrane and cytoplasmic signals at 48 h and prominent nuclear-associated fluorescence at 72 h. Irradiation increased IL-6, MCP-1, CCL5, and CXCL10 release, decreased IL-10, reduced cell viability, and increased apoptosis. PD-L1 knockdown further increased inflammatory cytokine and chemokine release but did not significantly alter apoptosis or further reduce viability in irradiated AC16 cells. In the presence of a high density of activated CD8 T cells, disruption of PD-1/PD-L1 signaling was associated with a further reduction in AC16 cell viability. In vivo, myocardial PD-L1 expression was increased, whereas CD8 T-cell infiltration remained limited and focal. Conclusion: PD-L1 upregulation in cardiomyocytes may exert an adaptive protective and immunoregulatory role during iRT-associated injury, potentially by limiting inflammatory mediator release and modulating susceptibility to activated CD8 T-cell mediated injury. CD8 T-cell density and activation are the key issues.