COping with acute sickle cell hemolysis

In this issue of Blood, Nguyen et al¹ use in vitro and in vivo models of hyperhemolysis in sickle cell disease (SCD) to study some of the disease’s most life-threatening complications for which treatment options continue to remain largely supportive in nature. Using this approach, they demonstrate that microparticles from red blood cells (RBCs) lead to the same type of endothelial injury observed in severe complications of SCD. By leveraging these models, they also discover the novel ability of carbon monoxide (CO)-releasing molecule CORM-401 to reduce hemolysis-induced endothelial injury and organ damage, providing a possible treatment strategy for hyperhemolysis-associated SCD complications.

SCD continues to be an enduring global health concern affecting millions of people. The first genetic disease described, SCD results from the hereditary transmission of a mutation in the β subunit of hemoglobin (Hb). The abnormal sickle hemoglobin polymerizes upon deoxygenation, causing RBCs to dehydrate and become less deformable and abnormally adhesive, triggering vascular occlusion and greatly shortened RBC lifespan. Among acute complications in SCD that can be potentially fatal, hyperhemolysis can result in a precipitous drop in Hb and primarily occurs during severe vaso-occlusive episodes, acute chest syndrome, or delayed hemolytic transfusion reaction (DHTR).² Increases in hemolysis may overwhelm the ability of haptoglobin to scavenge free Hb. Increased levels of free heme may further overwhelm hemopexin (scavenger of free heme). This is turn induces proinflammatory responses, directly activating the endothelium and promoting endothelial dysfunction.

Several lines of evidence suggest various mechanistic underpinnings of SCD complications, including heme-mediated Toll-like receptor 4 signaling, P-selectin-dependent processes, macrophage inflammatory skewing, nitric oxide bioavailability, and complement activation.^3-6 Release of heme can also engage heme oxygenase 1 (HO-1), a stress-inducible enzyme that catalyzes the breakdown of free heme to CO. CO can in turn upregulate HO-1 as a positive feedback loop that increases expression of HO-1 controlled by the transcription factor, nuclear factor erythroid 2-related factor 2 (Nrf2).⁷ These results suggest that CO may provide a useful strategy to treat hemolysis-associated complications. Consistent with this, prior studies suggested that CO can ameliorate inflammation in SCD mice.⁸ In this study, Nguyen et al demonstrated that CORM-401, a compound that can supply stable levels of CO, can protect endothelial cells from hemolysis-induced endothelial injury using in vitro and SCD mouse models.

Using a fluidics model seeded with human umbilical vein endothelial cells, Nguyen and colleagues demonstrated that hemolysate containing RBC membrane-derived particles induced the upregulation of inflammatory signaling pathways, including adhesion molecules NF-κB and HMOX1. Endothelial activation was also accompanied by RBC and platelet adhesion, in addition to platelet aggregation and activation. These findings suggest that RBC-derived particles generated during acute hyperhemolysis play a crucial role in the formation of microthrombi on damaged endothelium. Furthermore, these findings also indicate that the risk of thrombosis is high even in the very early stages of hemolysis before Hb is oxidized to free heme. Treatment with CORM-401 abrogated the observed increased endothelial activation and attendant consequences following hemolysate exposure.

Although the treatment of SCD mice with CORM-401 did not result in a significant impact on chronic hemolysis, CORM-401 did reduce the deleterious outcomes associated with acute hyperhemolysis. These results demonstrate that CORM-401 treatment can protect against acute hemolysis-associated complications that contribute to significant SCD morbidity and mortality. These protective effects coincided with suppression of NF-κB, Nfr2-dependent HO-1 expression, and vascular cell adhesion molecule 1 and P-selectin expression (see figure). It should be noted that although increased hemolysis is linked to vasculopathy in both humans and mice with SCD, the response is not consistent across all the organs (such as liver), indicating that hemolysis-related mouse and in vitro models can have variable response depending on the type of model, duration of exposure, and heterogeneity seen in SCD models.

Although previous studies have tested other CORM molecules in mouse models of SCD, this study is unique in that it examines not only the harmful consequences of acute exacerbations of hemolysis on endothelial dysfunction, but also the ability of CORM-401 to protect cells from hemolysis-induced injury. Understanding the role of hemolysis is crucial in many SCD complications, including DHTRs, where rapid hyperhemolysis is linked to significant risk of illness and death; the high mortality associated with these conditions is in part due to limited treatment options. A key limitation that the authors acknowledge is that although an immediate blockade of free Hb during hemolysis like in DHTR is in need, CORM-401 might not be effective in mitigating all the downstream effects of free heme on endothelial and organ dysfunction. Similarly, CO-mediated therapeutic strategies may not be effective at improving the consequences of the chronic hemolysis. Although currently approved medications for SCD such as hydroxyurea, l-glutamine, voxelotor, and crizanlizumab can reduce the frequency of some SCD complications, there is an urgent need for approaches that can be used to treat acute complications in SCD when they arise. These approaches could also be beneficial in managing drug-induced hemolysis, acute chest syndrome, and additional complications associated with alloimmunization. As high risk for hemolysis can also prevent patients with SCD from receiving needed surgeries, bone marrow transplantation, or gene therapy, developing strategies to more effectively manage hemolysis-associated complications will likely improve care beyond the direct impact of hemolysis itself. As a result, this study holds great promise in providing an approach to treat and possibly prevent acute complications in patients with SCD.

Conflict-of-interest disclosure: The authors declare no competing financial interests.