Thromboinflammatory mechanisms in sickle cell disease - challenging the hemostatic balance

Abstract

Sickle cell disease (SCD) is an inherited hemoglobinopathy that is caused by the presence of abnormal hemoglobin S (HbS) in red blood cells, leading to alterations in red cell properties and shape, as the result of HbS dexoygenation and subsequent polymerization. SCD pathophysiology is characterized by chronic inflammatory processes, triggered by hemolytic and vaso-occlusive events, which lead to the varied complications, organ damage and elevated mortality seen in individuals with the disease. In association with activation of the endothelium and leukocytes, hemostatic alterations and thrombotic events are well-documented in SCD. Here we discuss the role for inflammatory pathways in modulating coagulation and inducing platelet activation in SCD, due to tissue factor activation, adhesion molecule expression, inflammatory mediator production and the induction of innate immune responses, amongst other mechanisms. Thromboinflammatory pathways may play a significant role in some of the major complications of SCD, such as stroke, venous thromboembolism and possibly acute chest syndrome, besides exacerbating the chronic inflammation and cellular interactions that trigger vaso-occlusion, ischemia-reperfusion processes, and eventually organ damage.

Figure 1.

Thromboinflammatory pathways in sickle cell disease. Hemolytic and ischemia-reperfusion events (due to vaso-occlusion) drive inflammation and hemostatic alterations in sickle cell disease (SCD). Hemolysis leads to the release of cell-free hemoglobin, heme and other red blood cell contents into the vascular environment, causing nitric oxide depletion and contributing to platelet activation. Non-protein bound heme activates the endothelium (with some participation of tolllike receptor 4), induces tissue factor (TF) expression and activation in monocytes and endothelial cells, neutrophil extracellular trap formation, leukocyte and complement activation. Endothelial activation by platelets, leukocytes, ischemia-reperfusion events and heme/heme-laden microparticles results in the expression of adhesion molecules, including P-selectin and integrin α_iibβ₃, which recruit leukocytes and, in turn, red cells to the blood vessel walls. Activated endothelial cells, leukocytes and platelets also produce mature interleukin-1β, which further activates platelets and clot formation, in addition to having major inflammatory effects. Plateletderived proteins, such as CD40L and thrombospondin-1, activate the endothelium and may trigger red cell microparticle release. TF expression by monocytes leads to thrombin generation via the extrinsic (TF/FVIIa/FXa) pathway, while TF expression in activated endothelium signals through PAR-2 receptors (TF/FVIIa/FXa) triggering pro-inflammatory responses (interleukin-6 expression and leukocyte recruitment). Thrombin also mediates pro-inflammatory pathways through PAR-1 receptors, triggering von Willebrand factor release and P-selectin expression, which contribute to platelet adhesive mechanisms. Thrombin generation is further increased by phosphatidylserine-expressing red blood cells and microparticles, platelet polyPs and cell-free DNA derived from NET, via the intrinsic pathway of coagulation. Thrombin generation ultimately results in fibrin production and clotting, but also induces red cell adhesion to the vascular wall, in addition to inflammation through PAR-1 signaling. Oxidative stress contributes to NET formation and endothelial activation. These cellular responses are part of thromboinflammation, illustrated in the inset of figure 1. Leukocyte, platelet and red blood cell recruitment to the vascular wall, together with clotting processes, neutrophil extracellular trap components and the formation of heterocellular aggregates between platelets, leukocytes and red blood cells, with subsequent red cell trapping, results in the vaso-occlusive processes that characterize SCD. ADP: adenosine diphosphate; ICAM: intercellular adhesion molecule; IL: interleukin; NET: neutrophil extracellular trap; NO: nitric oxide; PAR: protease-activated receptor; TF: tissue factor; TLR4: toll-like receptor 4; TSP: thrombospondin; VWF: von Willebrand factor. Figure created with BioRender.com.

Figure 2.

Complications of sickle cell disease and thromboinflammation. Thromboinflammation, or immunothrombosis, can be viewed as a physiological response to pathogens or tissue damage in which hemostasis activation and localized activation of inflammatory pathways at the microvascular level (i.e. vascular inflammation) act in concert to facilitate pathogen removal or tissue repair. However, loss of localization or deregulated activation of these pathways underlies the pathogenesis of a multitude of immune-mediated diseases in which the relative contribution of hemostasis and/or vascular inflammation varies, determining disease presentation, as proposed by Jackson et al. Sickle cell disease (SCD) is a condition in which thromboinflammatory mechanisms have long been recognized as critical to its pathogenesis. However, the complex interplay between hemostasis and innate immunity activation makes it difficult to define precisely the relative contribution of each of these two processes to the pathogenesis of different complications, possibly explaining the absence of a straightforward association between classical biomarkers of hemostasis activation and the risk or the severity of some of these clinical manifestations. Accordingly, although it is likely that venous thromboembolism and avascular necrosis of the hip are complications in which hemostasis activation plays a dominant role (in orange), while pulmonary hypertension, priapism and leg ulcers can be more adequately placed in the vascular inflammation (in red) side of this thromboinflammatory spectrum, for most other complications, there is yet not enough evidence to precisely identify their localization in this cycle/spectrum, which could facilitate the identification of therapeutic targets, as well as biomarkers for each SCD complication.

Graphical Abstract