The Red Blood Cell-Inflammation Vicious Circle in Sickle Cell Disease

Abstract

Sickle cell disease (SCD) is a genetic disease caused by a single mutation in the β-globin gene, leading to the production of an abnormal hemoglobin called hemoglobin S (HbS), which polymerizes under deoxygenation, and induces the sickling of red blood cells (RBCs). Sickled RBCs are very fragile and rigid, and patients consequently become anemic and develop frequent and recurrent vaso-occlusive crises. However, it is now evident that SCD is not only a RBC rheological disease. Accumulating evidence shows that SCD is also characterized by the presence of chronic inflammation and oxidative stress, participating in the development of chronic vasculopathy and several chronic complications. The accumulation of hemoglobin and heme in the plasma, as a consequence of enhanced intravascular hemolysis, decreases nitric oxide bioavailability and enhances the production of reactive oxygen species (ROS). Heme and hemoglobin also represent erythrocytic danger-associated molecular pattern molecules (eDAMPs), which may activate endothelial inflammation through TLR-4 signaling and promote the development of complications, such as acute chest syndrome. It is also suspected that heme may activate the innate immune complement system and stimulate neutrophils to release neutrophil extracellular traps. A large amount of microparticles (MPs) from various cellular origins (platelets, RBCs, white blood cells, endothelial cells) is also released into the plasma of SCD patients and participate in the inflammation and oxidative stress in SCD. In turn, this pro-inflammatory and oxidative stress environment further alters the RBC properties. Increased pro-inflammatory cytokine concentrations promote the activation of RBC NADPH oxidase and, thus, raise the production of intra-erythrocyte ROS. Such enhanced oxidative stress causes deleterious damage to the RBC membrane and further alters the deformability of the cells, modifying their aggregation properties. These RBC rheological alterations have been shown to be associated to specific SCD complications, such as leg ulcers, priapism, and glomerulopathy. Moreover, RBCs positive for the Duffy antigen receptor for chemokines may be very sensitive to various inflammatory molecules that promote RBC dehydration and increase RBC adhesiveness to the vascular wall. In summary, SCD is characterized by a vicious circle between abnormal RBC rheology and inflammation, which modulates the clinical severity of patients.

Keywords: heme; inflammation; oxidative stress; red blood cell; sickle cell disease.

Figure 1

The red blood cell—inflammation vicious circle in sickle cell disease. From RBC alterations to oxidative stress, inflammation and endothelial dysfunction: Sickled RBC are very fragile and prone to hemolyze. Hemolysis leads to the release of heme, iron, Hb and arginase into the plasma, which interfere with the metabolism/bioavailability of NO: (I) free arginase may hydrolyze the NO precursor Arginine; (II) free Hb scavenges NO at a rate of 1,000-fold faster than Hb encapsulated in the RBCs; (III) heme and iron increase ROS generation, which lead to the production of peroxynitrite. ROS production is also enhanced by Xanthine Oxidase activation, caused by the repetition of ischemic/reperfusion events. Decreased NO bioavailability and increased ROS activate endothelial cells, which in turn express adhesion molecules of both the CAM and Selectin families, promoting cell-cell interactions. Free heme is able to activate endothelial TLR4, which promotes inflammasome activation and cytokines production through NF-κB activation. Heme may also activate neutrophils, which would release NETs that can also affect endothelial cells and act as a scaffold for platelets and RBCs. Recent evidence also showed that free heme could stimulate the complement pathway with potential consequences at the endothelial cell level. From inflammation and oxidative stress to RBC alterations: This pro-inflammatory and pro-oxidative environment, resulting from sickle RBCs alterations, also impacts on RBC rheology and physiology. Increased ROS production may lower RBC deformability and increase RBC aggregation. Decreased NO bioavailability could also participate in the decrease of RBC deformability and promote eryptosis. NETs could also promote RBC eryptosis. Circulating inflammatory molecules, such as ET-1 and TGF-β, may activate RBC NADPH Oxidase, which in turn would produce ROS and further alter RBC. ROS and ET-1 are known to activate the RBC Gárdos channel, which could favor RBC dehydration and further promote HbS polymerization. The enhanced release of MP by sickled RBCs could further exacerbate inflammation and oxidative stress. Increased RBC phosphatidylserine exposure may favor the binding of complement proteins at the surface of RBCs, which can induce their lysis. RBCs also act as a reservoir and/or a sink for pro-inflammatory cytokines/chemokines. IL-8, TNF-α, and RANTES promote RBC dehydration through Gárdos channel activation in RBCs expressing DARC. IL-8 and RANTES can also lead to the activation of α4β1 integrin in sickle reticulocytes expressing DARC, contributing to the adhesion of these cells to the endothelium.