HbSS is unique amongst human diseases in its extraordinary complexity with a highly aberrant milieu (see box on The Complex Sickle Milieu). Remarkably, this state derives from only two proximal events that are interrelated and mutually promotive, vasoocclusion/ischemia-reperfusion (I/R) and hemolysis. Although it is not possible to identify their proportionate contributions to pathogenesis, our view is that ischemia/reperfusion and hemolysis likely exert co-dominant impacts. Indeed, HMGB1 (high mobility group box 1) from ischemia/reperfusion and free heme from hemolysis biologically converge at TLR4 inflammatory signaling (Fig. 1).
Fig1. BIOLOGICAL CONVERGENCE. Free heme from hemolysis and HMGB1 from ischemia/reperfusion biologically converge at sterile inflammatory signaling by Toll-like receptor 4 (TLR4). (Reproduced with permission from Hebbel RP, Belcher JD, Vercellotti GM. The multi-faceted role of ischemia/reperfusion in sickle cell anemia. J Clin Invest, 2020;130[3]:1062–1072.)
Ischemia-Reperfusion Physiology
Sickle transgenic mice reveal that proximal instigation of endothelial activation derives from I/R injury (Fig.2). The complicated biology of I/R can occur when vascular ischemia is followed by reperfusion that reintroduces oxygen to the formerly ischemic area. In the unique sickle context, the initiating occlusive events are likely to be microvascular and multifocal, and they occur recurrently, possibly incessantly. Consequently, sickle transgenic mice exhibit the reperfusion-dependent, classical, early I/R processes: localized and rapid onset of oxygen radical generation, complement activation, nuclear factor kappa-B (NF κ B) activation, the release of HMGB1, production of TNF- α , and local activation of leukocytes and mast cells. This results in local I/R injury, i.e., extension of tissue injury beyond the volume affected by the inciting ischemia itself. In HbSS, this most clearly participates in the genesis of stroke, damage to the kidney and liver, and post-priapism damage, for example.
Fig2. ISCHEMIA/REPERFUSION AS A CORE DRIVING FORCE IN SICKLE CELL ANEMIA. As the consequence of vasoocclusion, ischemia reperfusion comprises an incessant driving force causing systemic inflammation with microvascular dysfunction. The adhesion biology resulting from activated/ dysfunctional endothelial cells slows microvascular transit of sickle RBC and WBC, thereby establishing a positive feedback loop that slows microvascular transit, increasing the likelihood of polymer formation. (Reproduced with permission from Hebbel RP, Belcher JD, Vercellotti GM. The multi-faceted role of ischemia/ reperfusion in sickle cell anemia. J Clin Invest. 2020;130(3):1062–1072.)
Thereafter, I/R pathobiology can explosively arborize, seen in HbSS as the chronic, systemic inflammatory state that is unique in its cyclicity, complexity, instability, and perpetuity. Specifically, inflammation begets adhesion/occlusion begets inflammation (Fig.2). This robust systematization of inflammation promotes the multitude of vascular biologic abnormalities in HbSS, including endothelial dysfunction and arterial vasculopathy. In addition, remote organ injury can develop at a site remote from a triggering I/R injury site, the most common manifestation being an acute lung injury syn drome, such as the acute chest syndrome (ACS) of sickle disease.
Hemolysis
Hemolysis exerts plethoric adverse effects, both indirect and direct. The accompanying expanded erythropoiesis enhances the production of adhesive reticulocytes and augments the elaboration of placental growth factors. This mediator activates blood monocytes to promote the production of ET-1 that, in turn, can induce vasoconstriction, cause nociceptive hypersensitization, activate RBC NADPH oxidase activity, and prompt the release of inflammatory mediators. Released cell-free oxyHbS is rapidly converted to met HbS that easily loses its heme, which is then able to initiate inflammatory injury responses such as oxidization of blood and membrane lipids, injuring endothelial cells, activating blood monocytes to produce TNF- α and express tissue factor (TF), and triggering disgorgement of neutrophil extracellular traps, among other effects. Some pathogenic responses to heme are mediated by its binding to TLR4; for example, in sickle mice, this causes enhanced endothelial surface expression of P-selectin and triggers vascular stasis. Released RBC arginase lowers plasma argi nine somewhat, hypothesized—but unlikely—to impede endothelial nitric oxide synthase (eNOS). Released PS-positive RBC microparticles can exert an inflammatory signaling impact upon endothelial cells, accelerate thrombin generation, and promote pulmonary sequestration of WBCs.
Cell-free Hb derived from lysing sickle RBC can consume NO. This contributes to the NO biodeficiency of sickle disease, and this specific process can mimic some features of endothelial dysfunction; for example, it limits NO’s vasodilatory and superoxide buffering functions, and it weakens NO’s normal braking effects on platelet activation and inflammation. In the sickle context, any potential effects of liberated cell-free Hb/heme are substantially augmented due to the markedly low levels of scavenging proteins, haptoglobin and hemopexin.
Concept of Hyperhemolysis
The “hyperhemolysis” hypothesis argues that HbSS patients having the highest hemolytic rates develop a specific subset of complications (stroke, leg ulcer, priapism, and pulmonary hypertension) caused by a unique mechanism, the accompanying greater consumption of NO. However, support for this concept is limited to an association between these complications and the level of LDH. So, it is notably problematic that correlative association says nothing about causality and that the sole relevant study found that LDH level does not correlate with the measured hemolytic rate in sickle subjects. Therefore, in our view, the artifice linking higher hemolysis to NO biodeficiency to certain specific complications is, rather, more parsimoniously explained by the well-known, simple fact that greater hemolysis driven anemia lowers blood viscosity, thereby lessening the likelihood of vasoocclusion-instigated disease in high-hemolysis patients.
