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

## TL;DR

This study shows that PD-L1 on endothelial cells worsens lung injury in sepsis, while PD-L1 on neutrophils may protect against it.

## Contribution

The study reveals distinct roles of PD-L1 on endothelial cells versus neutrophils in sepsis-induced lung injury using lineage-specific knockout mice.

## Key findings

- Endothelial cell PD-L1 deficiency reduced lung vascular permeability and 14-day mortality in sepsis.
- Neutrophil PD-L1 deficiency increased mortality and altered cytokine levels in sepsis.
- Endothelial PD-L1 appears harmful, while neutrophil PD-L1 may be protective in sepsis.

## Abstract

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.

Here we leverage the construction of Cre-lox mouse models to produce mice constitutively lacking either PD-L1 gene expression on endothelial cells (ecPD-L1−/−) or neutrophils (pmnPD-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.

Adult male C57BL/6 (WT), ecPD-L1−/−, pmnPD-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.

14-day mortality in the ecPD-L1−/− mice was lower than in the Hem/CLP Control group, while the mortality rate was increased in the pmnPD-L1−/− vs. Controls. Lung vascular permeability was also markedly decreased in the ecPD-L1−/− Hem/CLP mice but no such decline was seen in the lungs of pmnPD-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 ecPD-L1−/− vs. Control mice. Alternatively, the lung levels of Angiopoietin-2 and BALF MIP-2 and IL-6 concentrations significantly increased in Hem/CLP pmnPD-L1−/− animals.

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.