Novel Role for PD-1:PD-L1 as Mediator of Pulmonary Vascular Endothelial Cell Functions in Pathogenesis of Indirect ARDS in Mice

Abstract

Deficiency of the co-inhibitory receptor, Programmed cell death receptor (PD)-1, provides a survival benefit in our murine shock/sepsis model for the development of indirect acute respiratory distress syndrome (iARDS). Further, of clinical significance, patients that develop ARDS express increased PD-1 on their blood leukocytes. While PD-1 expression and its regulatory role have been associated with mainly T-cell responses, the contribution of its primary ligand, PD-L1, broadly expressed on non-immune cells such as lung endothelial cells (ECs) as well as immune cells, is less well-understood. Here we show that a “priming insult” for iARDS, such as non-lethal hemorrhagic shock alone, produced a marked increase in lung EC PD-L1 as well as blood leukocyte PD-1 expression, and when combined with a subsequent “trigger event” (polymicrobial sepsis), not only induced marked iARDS but significant mortality. These sequelae were both attenuated in the absence of PD-L1. Interestingly, we found that gene deficiency of both PD-1 and PD-L1 improved EC barrier function, as measured by decreased bronchoalveolar lavage fluid protein (i.e., lung leak). However, PD-L1 deficiency, unlike PD-1, significantly decreased EC activation through the Angiopoietin/Tie2 pathway in our iARDS mice. Additionally, while PD-1 gene deficiency was associated with decreased neutrophil influx in our iARDS mice, EC monolayers derived from PD-L1 deficient mice showed increased expression of EC junction proteins in response to ex vivo TNF-α stimulation. Together, these data suggest that ligation of PD-1:PD-L1 may play a novel role(s) in the maintenance of pulmonary EC barrier regulation, beyond that of the classic regulation of the leukocyte tolerogenic immune response, which may account for its pathogenic actions in iARDS.

Delicate Role of PD-L1/PD-1 Axis in Blood Vessel Inflammatory Diseases: Current Insight and Future Significance

“…Indeed, the expression of ZO-1 is highly regulated by undisturbed VE cadherin molecules [98,99]. In line with this, a loss of VE cadherin expression and ZO-1 occurred on the ECs monolayers with PD-L1 sufficiency [89], whereas the levels of the junctional molecules were shown to be intact even with TNF-α stimulation under PD-L1-deficient conditions, implying an uprooting quality of PD-L1 on EC-associated vascular barrier integrity.…”

“…Though the nuclear accumulation of FoxO1 is needed for the sustained expression of PD-1 and for the survival of PD-1 high CD8 + -exhausted T cells [108], the relationship between FoxO1 and PD-L1 remains obscure and is not yet reported. Surprisingly, the endothelial cell release of ang2 was significantly reduced in thrombin and TNF-α-stimulated PD-L1 -/-EC monolayers [89], signifying a novel and indirect link of the endothelial cell regulation of PD-L1 by ang2 at the gene level, which requires further investigations. This could be well corroborated with another cell type, where excessive serum-circulating ang2 fosters the upregulation of PD-L1 on M2-type macrophages [109].…”

“…A similar phenomenon was noticed on PD-L1 high cells that equally tend to overexpress the notch 3 molecules, where down-modulation of notch 3 proteins hampered PD-L1 expression [83], indicating a strong link between optimal levels of notch signaling and PD-L1 expression. Although heightened expression of PD-L1 attenuates T cell-mediated cytolytic insults on ECs, the natural barrier functions of ECs are indeed disrupted by PD-L1 high ECs, where PD-L1 is thought to contribute to the downstream reprogramming of EC activation, affecting the vascular integrity [89]. One of the major endothelial cell-specific tight junctional proteins is an adhesion molecule, such as VE cadherin (CD144), which is indispensable for maintaining the inter-ECs' cell contact stability and the molecular permeability of high-molecular weight plasma components and, therefore, it controls vascular integrity [97].…”

Overexpression of TGFβ1 in murine mesenchymal stem cells improves lung inflammation by impacting the Th17/Treg balance in LPS-induced ARDS mice

“…(1) Overexpressing TGFβ1 inhibited the proliferation of mMSCs during days 5-7 but had no effect on their differentiation or migration. 2 As an important part of adaptive immunity, T cell immunity is involved in the development of ARDS 21,22 .…”

“…Several studies have shown that speci c regulation of T cell differentiation could improve the immunity and in ammation state of ARDS in animal models, resulting in prognosis improvement 21,23,24 . It was also found in our study that the regulation of T cell differentiation in patients with community-acquired pneumonia was directly related to their prognosis 25 .…”