Differential Effects of Endothelial Cell- as opposed to Neutrophil-Lineage Restricted PD-L1 Gene Expression on Experimental Murine Shock/ Sepsis-Induced Lung Injury

Introduction : Our laboratory and others have shown that Programmed cell death receptor-Ligand 1 (PD-L1), contributes to the development of shock/ sepsis induced morbidity/ mortality, but its role appears to vary across organ/cell type. Objective : Here we leverage the construction of Cre-lox mouse models to produce mice constitutively lacking either PD-L1 gene expression on endothelial cells ( ec PD-L1 ^-/- ) or neutrophils ( pmn PD-L1 ^-/- ), respectively, to test the hypothesis that endothelial cell as opposed to neutrophil deficiency PD-L1 differentially contributes to shock/ sepsis induced lung injury/ death. Methods : Adult male C57BL/6 (WT), ec PD-L1 ^-/- , pmn PD-L1 ^-/- and/or mixed ^flox -no cre (Control) mice were subjected to either hemorrhagic (Hem) shock followed 24 hrs by cecal ligation & puncture (CLP) (Hem/CLP) or sham Hem and sham CLP (Sham). Survival studies were done. A separate set of animals were taken at 24 hrs post-procedure for peripheral blood, broncho-alveolar lavage fluid (BALF), lung tissues were harvested, processed/ stained for flow cytometry, cytokine/ chemokine/ angiopoietin ELISAs and indices of organ injury assays. A subset of animals was also examined for changes in lung permeability using Evan’s Blue dye exclusion. Results : 14-day mortality in the ec PD-L1 ^-/- mice was lower than in the Hem/CLP Control group, while the mortality rate was increased in the pmn PD-L1 ^-/- vs. Controls. Lung vascular permeability was also markedly decreased in the ec PD-L1 ^-/- Hem/CLP mice but no such decline was seen in the lungs of pmn PD-L1 ^-/- mice. While Hem/CLP increased the lung tissue, BALF and blood levels of several cytokine/ chemokine/ angiopoietin levels, the concentrations of lung tissue, BALF MCP-1 and blood BUN markedly declined in the ec PD-L1 ^-/- vs. Control mice. Alternatively, the lung levels of Angiopoietin-2 and BALF MIP-2 and IL-6 concentrations significantly increased in Hem/CLP pmn PD-L1 ^-/- animals. Conclusions : Taken together, these results support the hypothesis we have previously proffered that expression of PD-L1 on endothelial cells has a morbid impact. However, surprisingly, we have also uncovered a potential immune protective role of PD-L1 expression on neutrophils.