Ischemia-Reperfusion Injury in Sickle Cell Disease: From Basics to Therapeutics

Abstract

Sickle cell disease (SCD) is one of the most common hereditary hemoglobinopathies worldwide, affecting almost 400,000 newborns globally each year. It is characterized by chronic hemolytic anemia and endothelial dysfunction, resulting in a constant state of disruption of the vascular system and leading to recurrent episodes of ischemia-reperfusion injury (I/RI) to multiple organ systems. I/RI is a fundamental vascular pathobiological paradigm and contributes to morbidity and mortality in a wide range of conditions, including myocardial infarction, stroke, acute kidney injury, and transplantation. I/RI is characterized by an initial restriction of blood supply to an organ, which can lead to ischemia, followed by the subsequent restoration of perfusion and concomitant reoxygenation. Recent advances in the pathophysiology of SCD have led to an understanding that many of the consequences of this disease can be explained by mechanisms associated with I/RI. The following review focuses on the evolving pathobiology of SCD, how various complications of SCD can be attributed to I/RI, and the role of timely therapeutic intervention(s) based on targeting mediators or pathways that influence I/R insult.

Figure 1

Proposed overview of the vascular immunopathology of sickle cell disease (SCD). The A6T mutation in the β-globin gene causes a G6V mutation in the globin polypeptide, leading to the deoxygenated hemoglobin S (HbS) variant. When red blood cells (RBCs) are deoxygenated, HbS polymerizes and the cells take on a sickle phenotype and express adhesion molecules, such as CD36, which mediate attachment with the endothelium. Hemolysis of the sickled cells produces heme microparticles and arginase into the plasma and scavenge nitric oxide and stimulates the release of adhesion molecules, such as intracellular adhesion molecule, vascular cellular adhesion molecule, P-selectin, and E-selectin. Tissue resident macrophages and dendritic cells secrete interleukin (IL)-23, causing T cells to release IL-17A, which stimulates the secretion of granulocyte colony-stimulating factor (G-CSF) to activate neutrophil production. The process of neutrophil maturation is under the control of mainly two transcription factors (PU.1 and CCAAT). Neutrophils adhere and roll along the endothelium through selectin and integrin interactions in the direction of blood flow and are activated by chemokines along the endothelial layer. DAMPs (eg, ATP, high mobility group 1) trigger Toll-like receptor (TLR)-4 and induce an inflammatory response with the secretion of cytokines and chemokines. They also send activation signals to neutrophils and modulate their phenotype (aged neutrophils) that promote sickle cell vaso-occlusion. Activated platelets cause neutrophils to release chromatin and granule proteins to form neutrophil extracellular traps (NETs) and capture sickle RBCs (sRBCs). After activation from glycolipids derived from the tissue damage, invariant natural killer T (iNKT) cells cause further neutrophil recruitment and inflammation through production of interferon-γ and CXCR3 chemokines. sRBCs have phosphatidylserine externalization and express β₂-adrenergic receptors, which activate their procoagulant nature and adhesion properties, respectively. The subsequent responses consist of continuous accumulation of leukocytes, platelets, and RBCs with the activation of the coagulation cascade. These interactions may be mediated by the production of various proinflammatory and prothrombotic mediators, such as cathepsin G, neutrophil elastase, NETs, histones (citH3), and coagulant factors (FV and FX). Neutrophil elastase can co-localize with tissue factor pathway inhibitor (TFPI) on NETs and facilitates TFPI degradation, resulting in an activated coagulation system. The presence of citH3 on NETs is known to induce platelet aggregation via involvement of TLR2 and TLR4. Cathepsin G can activate protease activated receptor (PAR)-4, resulting in further platelet activation. As more blood cells become further incorporated and recruited into growing thrombi, fibrin scaffolds are formed. Under homeostatic conditions, neutrophils generally produce annexin A1 (AnxA1), which counteracts proinflammatory responses and enables resolution. However, in SCD, AnxA1 levels are low. Dotted lines separate the different phases of SCD vascular immunopathology.

Figure 2

Schematic depiction of potential therapeutic targets in sickle cell disease (SCD). The depicted targets have been tested as potential therapeutics for ischemia reperfusion injury (I/RI) and its associated symptoms in SCD. A: Red blood cell (RBC) ion channel blockers (Gardos channel blockers, eg, senicapoc) can preserve sickle RBC (sRBC) hydration and improve survival. B: Hydroxyurea acts by inducing fetal hemoglobin (HbF), donating nitric oxide (NO), and decreasing circulating leukocytes and reticulocytes. C: Anticoagulants, such as direct thrombin inhibitors (eg, dabigatran), will reduce thromboinflammation. D: Antioxidants, such as arginine and l-glutamine, reduce the oxidant stress by improving NAD redox potential. E: Antiadhesion agents prevent cellular rolling and adhesion to the endothelium by decreasing the expression of intercellular adhesion molecule (ICAM), vascular cell adhesion molecule (VCAM), E-selectin, and P-selectin (eg, rivipansel and crizanlizumab). F: β-blockers and mitogen-activated protein/extracellular signal-regulated kinase (MEK) inhibitors reduce sRBC adhesion to the vascular endothelium and can be used for management of acute vaso-occlusive episodes. G: HbF-inducing factors, such as hydroxyurea, and other agents (eg, decitabine, pomalidomide, butyrates) can increase HbF, lessen the HbS load, and decrease the rate of hemolysis. H: Nitric oxide–related agents may act by vasodilation and inhibition of vascular remodeling. I: Anti-inflammatory agents include antiadhesion agents, adenosine A2A receptors, and β₂-adrenergic pathways, which will help manage SCD by reducing overall inflammation and ischemia-reperfusion–related injury. J: Antiplatelet agents may reduce inflammatory tone, decrease platelet activation and aggregation, and reduce expression of adhesion molecules and selectins. Green arrows indicate activation/stimulation; brown lines, inhibition.