Endothelial dysfunction and immunothrombosis in sepsis

Abstract

Sepsis is a life-threatening clinical syndrome characterized by multiorgan dysfunction caused by a dysregulated or over-reactive host response to infection. During sepsis, the coagulation cascade is triggered by activated cells of the innate immune system, such as neutrophils and monocytes, resulting in clot formation mainly in the microcirculation, a process known as immunothrombosis. Although this process aims to protect the host through inhibition of the pathogen's dissemination and survival, endothelial dysfunction and microthrombotic complications can rapidly lead to multiple organ dysfunction. The development of treatments targeting endothelial innate immune responses and immunothrombosis could be of great significance for reducing morbidity and mortality in patients with sepsis. Medications modifying cell-specific immune responses or inhibiting platelet-endothelial interaction or platelet activation have been proposed. Herein, we discuss the underlying mechanisms of organ-specific endothelial dysfunction and immunothrombosis in sepsis and its complications, while highlighting the recent advances in the development of new therapeutic approaches aiming at improving the short- or long-term prognosis in sepsis.

Keywords: endothelial dysfunction; immunothrombosis; neutrophil extracellular traps; sepsis; tissue factor.

Figure 1

Endothelial dysfunction in sepsis. (A) PAMPs, TNF-α, IL-1, IL-6, Ang2, and NETs activate endothelial cells during sepsis. This results in intracellular signaling activation, leading to transcription factors activation, and mainly NF-κB. (B) Endothelial glycocalyx damage during sepsis is mediated through heparanase and ADAM15, which cleave heparan sulfate and CD44, respectively. Glycocalyx deconstruction results in increased inflammation, thrombosis, and hyperpermeability in sepsis. Furthermore, sepsis leads to endothelial cell junctions’ disruption, which subsequently leads to increased endothelial permeability. (C) Sepsis is characterized by dysregulated nitric oxide synthase activation, leading to the production of reactive nitrogen species. Moreover, infection triggers endothelial cells to produce ROS, which in turn destroy the endothelium. (D) Endothelium activation during sepsis results in the expression of E-selectin, P-selectin, VCAM-1, and ICAM-1 on the endothelial cell surface and subsequent leukocyte rolling, adhesion, and transmigration through endothelial cells. Leukocytes express specific ligands for binding with endothelial receptors such as PSGL-1 and ESL-1 for leukocyte rolling and αLβ2 and α4β1 integrins for leucocyte adhesion. (E) Activated endothelial cells during sepsis express molecules such as vWF and P-selectin on their surface, which mediate platelet aggregation. This leads to the formation of platelet-neutrophil aggregates and subsequent neutrophil activation. Weibel–Palade bodies contain procoagulant factors such as VIIIF, vWF, and P-selectin that can undergo exocytosis under sepsis conditions. The procoagulant phenotype of the endothelium is further characterized by increased production of TF and subsequent extrinsic coagulation pathway activation, decreased production of anticoagulant proteins (TFPI, EPCR, TM, PC), and increased production of PAI-1, which inhibits fibrinolysis. Ang2, angiopoietin 2; ADAM-15; disintegrin and metalloproteinase 15; AJ, adherens junctions; AP-1, activation of activator protein-1; eNOS, endothelial nitric oxide synthase; ESL-1, E-selectin ligand; GJ, gap junctions; gp130, glycoprotein 130; H₂O₂, hydrogen peroxide; HO·, hydroxyl radical; ICAM-1, intercellular adhesion molecule 1; IL, interleukin; IL-1R, interleukin-1 receptor; iNOS, inducible nitric oxide synthase; IRF3, interferon regulatory transcription factor-3; NETs, neutrophil extracellular traps; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; ·NO, nitric oxide; ·NO₂, nitrogen dioxide; N₂O₃, dinitrogen trioxide; O₂·–, superoxide anion; ONOO−, peroxynitrite; PAI-1, plasminogen activator inhibitor-1; PAMPs, pathogen associated molecular patterns; PC, protein C; PRR, pattern recognition receptor; PSGL-1, P-selectin glycoprotein ligand-1; RNS, reactive nitrogen species; ROS, reactive oxygen species; sIL-6R: soluble interleukin-6 receptor; STAT3, signal transducer and activator of transcription 3; TF, tissue factor; TFPI, tissue factor pathway inhibitor; TJ, tight junctions; TNF-α, tumor necrosis factor alpha; TNF-α-R, TNF-α receptor; VIIIF, VIII factor; VCAM-1, vascular cell adhesion molecule 1; vWF, von Willebrand factor; WPBs, Weibel–Palade bodies.

Figure 2

Cell-specific immunothrombosis mechanisms. Neutrophils and monocytes can induce thrombosis through specific mechanisms, while pathogens can directly activate platelets, leading to thrombosis. NETs, neutrophil extracellular traps. ↑ (arrow pointing up) means increased. ↓ (arrow pointing down) means decreased.

Figure 3

Drugs targeting sepsis-related immunothrombosis in clinical trials. Both FDA-approved and unapproved drugs have been studied in clinical trials for their effect on sepsis-related immunothrombosis. The drugs depicted in the figure showed conflicting results regarding their effect on mortality rates and coagulation parameters in sepsis patients. DNase is the only one with no clinical results yet, as the phase 1 study (NCT05453695) using DNase intervention has recently started. CD14, cluster of differentiation 14; DNase, deoxyribonuclease; GPIIb/IIIa, glycoprotein IIb/IIIa; IL, interleukin; mAb, monoclonal antibody; PAR-1, protease-activated receptor-1.

Figure 4

Potential therapeutic targets under preclinical investigation in sepsis. Inhibitors of innate immune components, like neutrophil extracellular traps and complement components C5 and C3, drugs interfering with tissue factor formation and procoagulant activity as well as molecules with multiple actions, have shown promising results in reducing immunothrombosis in preclinical sepsis models. However, the safety, tolerability, and effectiveness of these drugs in abrogating the deleterious effects of immunothrombosis in patients with sepsis are still unknown. C5, complement 5; C3, complement 3; CD14, cluster of differentiation 14; DNase, deoxyribonuclease; HMGB1, high mobility group box 1; IFN, interferon; mAb, monocloncal antibody; NETs, neutrophil extracellular traps; NLRP3, NLR family pyrin domain containing 3; PAD4, protein arginine deiminase; SQSTM1, sequestosome 1; TF, tissue factor.