Activated endothelial cells limit inflammatory response, but increase chemoattractant potential and bacterial clearance by human monocytes

Abstract

Inflammation is the normal immune response of vascularized tissues to damage and bacterial products, for which leukocyte transendothelial migration (TEM) is critical. The effects of cell-to-cell contact seen in both leukocyte and endothelial cells include cytoskeleton rearrangement, and dynamic expression of adhesion molecules and metalloproteinases. TEM induces expression of anti-apoptotic molecules, costimulatory molecules associated with antigen presentation, and pattern recognition receptors (PRR), such as TLR-4, in monocytes. However, little is known about how TLR-4 increment operates in monocytes during an inflammatory response. To understand it better, we used an in vitro model in which monocytes crossed a layer of IL-1β stimulated Human Umbilical Vein Endothelial Cells (HUVEC). After TEM, monocytes were tested for the secretion of inflammatory cytokines and chemokines, their phenotype (CD14, CD16, TLR-4 expression), and TLR-4 canonical [Nuclear Factor kappa B, (NF-κB) pathway] and non-canonical [p38, extracellular signal-regulated kinases (ERK) 1/2 pathway] signal transduction induced by lipopolysaccharide (LPS). Phagocytosis and bacterial clearance were also measured. There was diminished secretion of LPS-induced inflammatory cytokines (IL-1β, IL-6, and TNF-α) and higher secretion of chemokines (CXCL8/IL-8 and CCL2/MCP-1) in supernatant of TEM monocytes. These changes were accompanied by increases in TLR-4, CD14 (surfaces expression), p38, and ERK1/2 phosphorylated cytoplasmic forms, without affecting NF-κB activation. It also increased bacterial clearance after TEM by an O2 -independent mechanism. The data suggest that interaction between endothelial cells and monocytes fine-tunes the inflammatory response and promotes bacterial elimination.

Polystyrene-Divinylbenzene-Based Adsorbents Reduce Endothelial Activation and Monocyte Adhesion Under Septic Conditions in a Pore Size-Dependent Manner

“…We stimulated HUVEC for 4 h under flow with conditioned medium containing plasma from LPS-stimulated whole blood to reflect the complexity of a septic environment. Monocytes were chosen to study leukocyte adhesion, as they act as sentinels in the earliest stages of sepsis and guide neutrophils towards inflammatory foci [ 24 , 25 ]. Conditioned medium from LPS-stimulated whole blood induced reproducible endothelial activation and monocyte adhesion with little inter-assay variability in the flow model.…”

Mechanisms of Endothelial Activation in Sepsis and Cell Culture Models to Study the Heterogeneous Host Response

“…Next to their adhesion, flow conditions influence the transendothelial migration of activated leukocytes, as evidenced by the finding that the stimulation of human monocytes with lipopolysaccharide resulted in increased adhesion, but strongly reduced transendothelial migration at low flow rates, while monocyte adhesion and transmigration remained unaffected by lipopolysaccharide under physiological flow (67). As transendothelial migration is associated with a shift of monocytes towards an increased chemoattractant potential and bacterial clearance, the interaction between endothelial cells and monocytes may fine-tune the inflammatory response and promote bacterial elimination (68). Beside monocytes, platelets enhance neutrophil adherence to the endothelium with fibrinogen acting as a ligand between CD11b/CD18 on neutrophils and glycoprotein IIb/IIIa on platelets.…”

Smart mechanosensing machineries enable migration of vascular smooth muscle cells in atherosclerosis‐relevant 3D matrices

“…Stimulation of injury results in a higher expression of leukocyte adhesion receptors, including vascular cell‐adhesion molecule 1 (VCAM1) intercellular adhesion molecule 1 (ICAM1), and cytokines like MCP1 (Hahn and Schwartz, ). Consequently, a growing number of leukocytes (mainly monocytes) migrate into the endothelium to bind ECs (Hahn and Schwartz, ; Mancilla‐Herrera et al, ). Intimal monocytes further differentiate into macrophages in reaction to chemokines and oxidized low density lipoproteins (oxLDLs).…”