[Effects of PD-L1 on immunosuppression of bacterial sepsis and its relevant mechanism]

Objective: To investigate the expression of programmed death ligand-1 (PD-L1) in dendritic cells (DCS) and its related signaling pathway in lipopolysaccharide (LPS)-induced immunosuppression of bacterial sepsis.Methods: Stimulating with bacterial LPS, bone marrow-derived dendritic cells could induce T lymphocyte immunosuppression imitating bacterial sepsis model. The experiments were divided into 5 groups: control group, LPS group, 2-(4-morpholinyl)-8-phenyl-4H-1- benzopyran-4-one (LY294002)+LPS group, pyrrolidinedithiocarbamate(PDTC)+LPS group and LPS+anti-PD-L1 group with 6 multiple wells in each group. After mice bone marrow source monocytes were cultured with rmGM-CSF (10 ng/ml) and rmIL-4 (1 ng/ml) in 10% fetal bovine serum 1640 for 4 days, DCs cells were treated with with 10 ng/ml LPS for 12 h to obtain immunosuppressive cells with high expression of PD-L1. Pathway-inhibitors LY294002 (10 μmol/L) and PDTC (20 μmol/L) were used to block PI3K and NF-κB signals. Flow cytometry and confocal laser scanning microscopy were used to detect the PD-L1 expression and phosphatidylinositol 3 kinase/protein kinase B (PI3K/AKT) signal activation on DCs. BrdU cell proliferation assay and γ-interferon enzyme-linked immunospot assay were used to detect ovalbumin specific T lymphocyte proliferation response and cytotoxic T cell response, respectively. Results: Compared with the control group, the percentage of PD-L1 positive cells and PD-L1 red fluorescence intensity of DCs were all increased(P＜0.01), while DCs- mediated T cell proliferation and γ-interferon spot-forming cell number were decreased (P＜0.01).PI3K inhibitor LY294002, NF-κB inhibitor PDTC and PD-L1 blocking antibody could significantly reverse the inhibition of DCs mediated T lymphocytes immunosuppression above (P＜0.01). Conclusion: PD-L1 was a key molecule that mediates immunosuppression in lipopolysaccharide induced bacterial sepsis. PI3K Signal and NF- κB signal were also involved in this immunosuppressive process.