851: Regulation of Pd-L1 Expression on Exosomes by Mir-497-5p Promotes Foxp3t Cell Upregulation in Sepsis

Introduction: Sepsis is a life-threatening condition marked by profound immune suppression and multiple organ dysfunction. Accumulating evidence suggests that immune checkpoint pathways, particularly the PD-L1/PD-1 axis, play a critical role in driving sepsis-induced immunosuppression. However, the upstream regulatory mechanisms and the role of exosomal PD-L1 in modulating T cell function remain poorly understood. Methods: We analyzed serum-derived exosomes from patients with septic shock to evaluate miR-497-5p expression and its correlation with PD-L1 levels. Functional assays were performed in lung epithelial cells and CD4+ T cells to investigate the impact of miR-497-5p silencing on exosomal PD-L1 release and Treg differentiation. Luciferase reporter assays, Western blotting, and co-immunoprecipitation were used to validate PD-L1 as a direct target of miR-497-5p and to elucidate downstream signaling mechanisms. Results: miR-497-5p was significantly downregulated in exosomes from septic shock patients and exhibited an inverse correlation with PD-L1 expression. Silencing miR-497-5p in lung epithelial cells led to increased secretion of PD-L1-enriched exosomes, which promoted Foxp3+ Treg differentiation in CD4+ T cells. This effect was abolished by anti-PD-L1 antibody treatment. Mechanistically, miR-497-5p directly targeted PD-L1, suppressing its protein expression. Exosomal PD-L1 activated the SHP-1/STAT3/RORγt axis via PD-1 membrane engagement and was also internalized into CD4+ T cells, where it bound to PIAS3, inhibited its ubiquitination, and enhanced STAT3 SUMOylation. This dual mechanism resulted in RORγt downregulation and FOXP3 induction. Conclusions: Our study reveals a novel dual mechanism through which exosomal PD-L1 promotes Treg differentiation in sepsis and identifies miR-497-5p as a key upstream regulator of PD-L1 expression. Targeting exosomal PD-L1 or restoring miR-497-5p levels may represent promising strategies to reverse sepsis-induced immunosuppression.