Role of Neutrophil Extracellular Traps and Vesicles in Regulating Vascular Endothelial Permeability

Abstract

The microvascular endothelium serves as the major barrier that controls the transport of blood constituents across the vessel wall. Barrier leakage occurs during infection or sterile inflammation, allowing plasma fluid and cells to extravasate and accumulate in surrounding tissues, an important pathology underlying a variety of infectious diseases and immune disorders. The leak process is triggered and regulated by bidirectional communications between circulating cells and vascular cells at the blood-vessel interface. While the molecular mechanisms underlying this complex process remain incompletely understood, emerging evidence supports the roles of neutrophil-endothelium interaction and neutrophil-derived products, including neutrophil extracellular traps and vesicles, in the pathogenesis of vascular barrier injury. In this review, we summarize the current knowledge on neutrophil-induced changes in endothelial barrier structures, with a detailed presentation of recently characterized molecular pathways involved in the production and effects of neutrophil extracellular traps and extracellular vesicles. Additionally, we discuss the therapeutic implications of altering neutrophil interactions with the endothelial barrier in treating inflammatory diseases.

Keywords: cell-cell junction; endothelial barrier; extracellular vesicles; glycocalyx; inflammation; neutrophil extracellular traps; permeability.

Figure 1

Endothelial barrier structure. (A) The endothelial barrier of exchange microvessels is composed of endothelial cells connected to each other via junctions, with its luminal surface protected by glycocalyx and basolateral side anchored to the extracellular matrix in the basement membrane through focal adhesions. Endothelial cell-cell adhesion is mediated by two types of junction: the claudin-based tight junction which is linked to the actin cytoskeleton through zonula occludens (ZO), and the VE-cadherin-based adherens junction which binds actin through catenins. Some images of cells or organelles were obtained from Servier Medical Art (www.servier.com). (B) Glycocalyx in mouse lung capillary under transmission electron microscopy. EC, endothelial cells. Red arrows indicate glycocalyx. Scale bar = 1 μm. (C) Immunofluorescent staining of VE-cadherin on human umbilical vein endothelial cells. Green, VE-cadherin. Blue, DAPI. Scale bar = 20 μm.

Figure 2

Neutrophils regulate endothelial barrier function through adhesion-dependent and secretion-dependent mechanisms. Neutrophil adhesion to endothelial cells activates ICAM-1 signaling, which increases permeability through both para-endothelial and trans-endothelial routes. In addition, neutrophils can generate ROS, inflammatory mediators, granular contents, neutrophil extracellular traps (NETs), and extracellular vesicles (EVs), which in turn cause junction disruption, glycocalyx degradation, focal adhesion reorganization, and cytoskeletal contraction, leading to intercellular gap formation and increased para-endothelial permeability. Neutrophils also release barrier-protecting factors, including annexin 1. EC, endothelial cells. Blue dots, blood constituents. Images of cells were obtained from Servier Medical Art (www.servier.com).

Figure 3

Effects of specific NET constituents on endothelial barrier function. NETs are composed of decondensed chromatin (e.g., citrullinated histone 3) and granular enzymes (MMPs and serine proteinases). Citrullinated histone 3 induces actin stress fiber formation and VE-cadherin junction discontinuity; MMPs and serine proteinases cleave glycocalyx and other barrier molecules; both lead to increased para-endothelial permeability. Images of cells were obtained from Servier Medical Art (www.servier.com).

Figure 4

Effects of neutrophil-derived EVs on endothelial barrier function. Neutrophil-derived EVs display either positive or negative impact on endothelial permeability depending on their cargo contents. Barrier-disrupting cargo, such as S100A8, A9, MPO, and cathepsin G, are able to disrupt junction integrity and increase permeability. In contrast, barrier-protecting cargo, such as annexin 1, maintain junction integrity, and decrease permeability. Images of cells were obtained from Servier Medical Art (www.servier.com).