Influence of red blood cell-derived microparticles upon vasoregulation

Biopreservation and Reversal of Oxidative Injury During Blood Storage by a Novel Curcumin-based Gel Formulation

Blood storage lesion induces cytosolic and membrane changes driven in part by hemoglobin (Hb) oxidation reactions within red blood cells (RBCs). A novel gel formulation containing the antioxidant curcuminoids in a biocompatible solvent system was used to deliver curcumin into RBCs. Incubation of peroxide treated RBCs stored in PBS with curcumin gel led to a reduction in prooxidant ferrylHb and recovery in ATP. Curcumin treatment prevented band 3 tyrosine (Y359 and Y21) phosphorylation. RBCs stored in AS-3 solutions for 28, 35, 42 and 49 days, following a single-dose of 100μM curcuminoids at each time points, caused reduction in protein carbonylation and considerable recovery in ATP levels. Proteomic analysis revealed minimal changes in the proteomic landscape in 35 days. However, a downregulation in fibrinogen was observed in the treated samples which may reduce RBC aggregation. Additionally, we used a guinea pig model where the circulation of infused aged RBCs can be extended (approximately 10%) when treated with curcumin gel at the start of storage. Our data therefore provide mechanistic insights and supportive animal data into benefits of treating stored RBCs with a novel curcuminoid formulation based on the biopreservation of RBC membrane integrity, redox balance, and increased longevity in circulation.

Extracellular vesicles in venous thromboembolism and pulmonary hypertension

Venous thromboembolism (VTE) is a multifactorial disease, and pulmonary hypertension (PH) is a serious condition characterized by pulmonary vascular remodeling leading with increased pulmonary vascular resistance, ultimately leading to right heart failure and death. Although VTE and PH have distinct primary etiologies, they share some pathophysiologic similarities such as dysfunctional vasculature and thrombosis. In both conditions there is solid evidence that EVs derived from a variety of cell types including platelets, monocytes, endothelial cells and smooth muscle cells contribute to vascular endothelial dysfunction, inflammation, thrombosis, cellular activation and communications. However, the roles and importance of EVs substantially differ between studies depending on experimental conditions and parent cell origins of EVs that modify the nature of their cargo. Numerous studies have confirmed that EVs contribute to the pathophysiology of VTE and PH and increased levels of various EVs in relation with the severity of VTE and PH, confirming its potential pathophysiological role and its utility as a biomarker of disease severity and as potential therapeutic targets.

Extracellular vesicles: effectors of transfusion-related acute lung injury

Respiratory transfusion reactions represent some of the most severe adverse reactions related to receiving blood products. Of those, transfusion-related acute lung injury (TRALI) is associated with elevated morbidity and mortality. TRALI is characterized by severe lung injury associated with inflammation, pulmonary neutrophil infiltration, lung barrier leak, and increased interstitial and airspace edema that cause respiratory failure. Presently, there are few means of detecting TRALI beyond clinical definitions based on physical examination and vital signs or preventing/treating TRALI beyond supportive care with oxygen and positive pressure ventilation. Mechanistically, TRALI is thought to be mediated by the culmination of two successive proinflammatory hits, which typically comprise a recipient factor (1st hit-e.g., systemic inflammatory conditions) and a donor factor (2nd hit-e.g., blood products containing pathogenic antibodies or bioactive lipids). An emerging concept in TRALI research is the contribution of extracellular vesicles (EVs) in mediating the first and/or second hit in TRALI. EVs are small, subcellular, membrane-bound vesicles that circulate in donor and recipient blood. Injurious EVs may be released by immune or vascular cells during inflammation, by infectious bacteria, or in blood products during storage, and can target the lung upon systemic dissemination. This review assesses emerging concepts such as how EVs: 1) mediate TRALI, 2) represent targets for therapeutic intervention to prevent or treat TRALI, and 3) serve as biochemical biomarkers facilitating TRALI diagnosis and detection in at-risk patients.

Exploration of cerebral vasospasm from the perspective of microparticles

Cerebral vasospasm is a frequently encountered clinical problem, especially in patients with traumatic brain injury and subarachnoid hemorrhage. Continued cerebral vasospasm can cause cerebral ischemia, even infarction and delayed ischemic neurologic deficits. It significantly affects the course of the disease and the outcome of the patient. However, the underlying mechanism of cerebral vasospasm is still unclear. Recently, increasing studies focus on the pathogenic mechanism of microparticles. It has been found that microparticles have a non-negligible role in promoting vasospasm. This research aims to summarize the dynamics of microparticles in vivo and identify a causal role of microparticles in the occurrence and development of cerebral vasospasm. We found that these various microparticles showed dynamic characteristics in body fluids and directly or indirectly affect the cerebral vasospasm or prompt it. Due to the different materials carried by microparticles from different cells, there are also differences in the mechanisms that lead to abnormal vasomotor. We suggest that microparticle scavengers might be a promising therapeutic target against microparticles associated complications.

Blood Cell-Derived Microvesicles in Hematological Diseases and beyond

Microvesicles or ectosomes represent a major type of extracellular vesicles that are formed by outward budding of the plasma membrane. Typically, they are bigger than exosomes but smaller than apoptotic vesicles, although they may overlap with both in size and content. Their release by cells is a means to dispose redundant, damaged, or dangerous material; to repair membrane lesions; and, primarily, to mediate intercellular communication. By participating in these vital activities, microvesicles may impact a wide array of cell processes and, consequently, changes in their concentration or components have been associated with several pathologies. Of note, microvesicles released by leukocytes, red blood cells, and platelets, which constitute the vast majority of plasma microvesicles, change under a plethora of diseases affecting not only the hematological, but also the nervous, cardiovascular, and urinary systems, among others. In fact, there is evidence that microvesicles released by blood cells are significant contributors towards pathophysiological states, having inflammatory and/or coagulation and/or immunomodulatory arms, by either promoting or inhibiting the relative disease phenotypes. Consequently, even though microvesicles are typically considered to have adverse links with disease prognosis, progression, or outcomes, not infrequently, they exert protective roles in the affected cells. Based on these functional relations, microvesicles might represent promising disease biomarkers with diagnostic, monitoring, and therapeutic applications, equally to the more thoroughly studied exosomes. In the current review, we provide a summary of the features of microvesicles released by blood cells and their potential implication in hematological and non-hematological diseases.

Red Blood Cell Extracellular Vesicle-Based Drug Delivery: Challenges and Opportunities

Recently, red blood cell-derived extracellular vesicles (RBCEVs) have attracted attention for clinical applications because of their safety and biocompatibility. RBCEVs can escape macrophages through the binding of CD47 to inhibitory receptor signal regulatory protein α. Furthermore, genetic materials such as siRNA, miRNA, mRNA, or single-stranded RNA can be encapsulated within RBCEVs and then released into target cells for precise treatment. However, their side effects, half-lives, target cell specificity, and limited large-scale production under good manufacturing practice remain challenging. In this review, we summarized the biogenesis and composition of RBCEVs, discussed the advantages and disadvantages of RBCEVs for drug delivery compared with synthetic nanovesicles and non-red blood cell-derived EVs, and provided perspectives for overcoming current limitations to the use of RBCEVs for clinical applications.

COVID-19 Infection and Circulating Microparticles-Reviewing Evidence as Microthrombogenic Risk Factor for Cerebral Small Vessel Disease

Severe acute respiratory syndrome corona virus-2 (SARS-CoV-2) due to novel coronavirus disease 2019 (COVID-19) has affected the global society in numerous unprecedented ways, with considerable morbidity and mortality. Both direct and indirect consequences from COVID-19 infection are recognized to give rise to cardio- and cerebrovascular complications. Despite current limited knowledge on COVID-19 pathogenesis, inflammation, endothelial dysfunction, and coagulopathy appear to play critical roles in COVID-19-associated cerebrovascular disease (CVD). One of the major subtypes of CVD is cerebral small vessel disease (CSVD) which represents a spectrum of pathological processes of various etiologies affecting the brain microcirculation that can trigger subsequent neuroinflammation and neurodegeneration. Prevalent with aging, CSVD is a recognized risk factor for stroke, vascular dementia, and Alzheimer's disease. In the background of COVID-19 infection, the heightened cellular activations from inflammations and oxidative stress may result in elevated levels of microthrombogenic extracellular-derived circulating microparticles (MPs). Consequently, MPs could act as pro-coagulant risk factor that may serve as microthrombi for the vulnerable microcirculation in the brain leading to CSVD manifestations. This review aims to appraise the accumulating body of evidence on the plausible impact of COVID-19 infection on the formation of microthrombogenic MPs that could lead to microthrombosis in CSVD manifestations, including occult CSVD which may last well beyond the pandemic era.

Diets and Cellular-Derived Microparticles: Weighing a Plausible Link With Cerebral Small Vessel Disease

Cerebral small vessel disease (CSVD) represents a spectrum of pathological processes of various etiologies affecting the brain microcirculation that can trigger neuroinflammation and the subsequent neurodegenerative cascade. Prevalent with aging, CSVD is a recognized risk factor for stroke, vascular dementia, Alzheimer disease, and Parkinson disease. Despite being the most common neurodegenerative condition with cerebrocardiovascular axis, understanding about it remains poor. Interestingly, modifiable risk factors such as unhealthy diet including high intake of processed food, high-fat foods, and animal by-products are known to influence the non-neural peripheral events, such as in the gastrointestinal tract and cardiovascular stress through cellular inflammation and oxidation. One key outcome from such events, among others, includes the cellular activations that lead to elevated levels of endogenous cellular-derived circulating microparticles (MPs). MPs can be produced from various cellular origins including leukocytes, platelets, endothelial cells, microbiota, and microglia. MPs could act as microthrombogenic procoagulant that served as a plausible culprit for the vulnerable end-artery microcirculation in the brain as the end-organ leading to CSVD manifestations. However, little attention has been paid on the potential role of MPs in the onset and progression of CSVD spectrum. Corroboratively, the formation of MPs is known to be influenced by diet-induced cellular stress. Thus, this review aims to appraise the body of evidence on the dietary-related impacts on circulating MPs from non-neural peripheral origins that could serve as a plausible microthrombosis in CSVD manifestation as a precursor of neurodegeneration. Here, we elaborate on the pathomechanical features of MPs in health and disease states; relevance of dietary patterns on MP release; preclinical studies pertaining to diet-based MPs contribution to disease; MP level as putative surrogates for early disease biomarkers; and lastly, the potential of MPs manipulation with diet-based approach as a novel preventive measure for CSVD in an aging society worldwide.

Erythrocyte-derived microvesicles induce arterial spasms in JAK2V617F myeloproliferative neoplasm

Arterial cardiovascular events are the leading cause of death in patients with JAK2V617F myeloproliferative neoplasms (MPNs). However, their mechanisms are poorly understood. The high prevalence of myocardial infarction without significant coronary stenosis or atherosclerosis in patients with MPNs suggests that vascular function is altered. The consequences of JAK2V617F mutation on vascular reactivity are unknown. We observe here increased responses to vasoconstrictors in arteries from Jak2V617F mice resulting from a disturbed endothelial NO pathway and increased endothelial oxidative stress. This response was reproduced in WT mice by circulating microvesicles isolated from patients carrying JAK2V617F and by erythrocyte-derived microvesicles from transgenic mice. Microvesicles of other cellular origins had no effect. This effect was observed ex vivo on isolated aortas, but also in vivo on femoral arteries. Proteomic analysis of microvesicles derived from JAK2V617F erythrocytes identified increased expression of myeloperoxidase as the likely mechanism accounting for their effect. Myeloperoxidase inhibition in microvesicles derived from JAK2V617F erythrocytes suppressed their effect on oxidative stress. Antioxidants such as simvastatin and N-acetyl cysteine improved arterial dysfunction in Jak2V617F mice. In conclusion, JAK2V617F MPNs are characterized by exacerbated vasoconstrictor responses resulting from increased endothelial oxidative stress caused by circulating erythrocyte-derived microvesicles. Simvastatin appears to be a promising therapeutic strategy in this setting.

Thrombospondin-1/CD47 signaling modulates transmembrane cation conductance, survival, and deformability of human red blood cells

BACKGROUND

Thrombospondin-1 (TSP-1), a Ca²⁺-binding trimeric glycoprotein secreted by multiple cell types, has been implicated in the pathophysiology of several clinical conditions. Signaling involving TSP-1, through its cognate receptor CD47, orchestrates a wide array of cellular functions including cytoskeletal organization, migration, cell-cell interaction, cell proliferation, autophagy, and apoptosis. In the present study, we investigated the impact of TSP-1/CD47 signaling on Ca²⁺ dynamics, survival, and deformability of human red blood cells (RBCs).

METHODS

Whole-cell patch-clamp was employed to examine transmembrane cation conductance. RBC intracellular Ca²⁺ levels and multiple indices of RBC cell death were determined using cytofluorometry analysis. RBC morphology and microvesiculation were examined using imaging flow cytometry. RBC deformability was measured using laser-assisted optical rotational cell analyzer.

RESULTS

Exposure of RBCs to recombinant human TSP-1 significantly increased RBC intracellular Ca²⁺ levels. As judged by electrophysiology experiments, TSP-1 treatment elicited an amiloride-sensitive inward current alluding to a possible Ca²⁺ influx via non-selective cation channels. Exogenous TSP-1 promoted microparticle shedding as well as enhancing Ca²⁺- and nitric oxide-mediated RBC cell death. Monoclonal (mouse IgG1) antibody-mediated CD47 ligation using 1F7 recapitulated the cell death-inducing effects of TSP-1. Furthermore, TSP-1 treatment altered RBC cell shape and stiffness (maximum elongation index).

CONCLUSIONS

Taken together, our data unravel a new role for TSP-1/CD47 signaling in mediating Ca²⁺ influx into RBCs, a mechanism potentially contributing to their dysfunction in a variety of systemic diseases. Video abstract.

Circulating Extracellular Vesicles and Endothelial Damage in Sickle Cell Disease

Endothelial damage is central to the pathogenesis of many of the complications of sickle cell disease. Circulating extracellular vesicles (EVs) have been implicated in modulating endothelial behavior in a variety of different, diseases with vascular pathologies. As seen in other hemolytic diseases, the plasma of sickle cell patients contains EVs of different sizes and cellular sources. The medium-sized vesicles (microparticles) primarily derive from mature red blood cells and platelets; some of these EVs have procoagulant properties, while others stimulate inflammation or endothelial adhesiveness. Most of the small EVs (including exosomes) derive from erythrocytes and erythrocyte precursors, but some also originate from platelets, white blood cells, and endothelial cells. These small EVs may alter the behavior of target cells by delivering cargo including proteins and nucleic acids. Studies in model systems implicate small EVs in promoting vaso-occlusion and disruption of endothelial integrity. Thus, both medium and small EVs may contribute to the increased endothelial damage in sickle cell disease. Development of a detailed understanding of the composition and roles of circulating EVs represents a promising approach toward novel predictive diagnostics and therapeutic approaches in sickle cell disease.

Interplay Between Plasma Membrane Lipid Alteration, Oxidative Stress and Calcium-Based Mechanism for Extracellular Vesicle Biogenesis From Erythrocytes During Blood Storage

The shedding of extracellular vesicles (EVs) from the red blood cell (RBC) surface is observed during senescence in vivo and RBC storage in vitro. Two main models for EV shedding, respectively based on calcium rise and oxidative stress, have been proposed in the literature but the role of the plasma membrane lipid composition and properties is not understood. Using blood in K⁺/EDTA tubes stored for up to 4 weeks at 4°C as a relevant RBC vesiculation model, we showed here that the RBC plasma membrane lipid composition, organization in domains and biophysical properties were progressively modified during storage and contributed to the RBC vesiculation. First, the membrane content in cholesterol and linoleic acid decreased whereas lipid peroxidation and spectrin:membrane occupancy increased, all compatible with higher membrane rigidity. Second, phosphatidylserine surface exposure showed a first rapid rise due to membrane cholesterol decrease, followed by a second calcium-dependent increase. Third, lipid domains mainly enriched in GM1 or sphingomyelin strongly increased from the 1st week while those mainly enriched in cholesterol or ceramide decreased during the 1st and 4th week, respectively. Fourth, the plasmatic acid sphingomyelinase activity considerably increased upon storage following the sphingomyelin-enriched domain rise and potentially inducing the loss of ceramide-enriched domains. Fifth, in support of the shedding of cholesterol- and ceramide-enriched domains from the RBC surface, the number of cholesterol-enriched domains lost and the abundance of EVs released during the 1st week perfectly matched. Moreover, RBC-derived EVs were enriched in ceramide at the 4th week but depleted in sphingomyelin. Then, using K⁺/EDTA tubes supplemented with glucose to longer preserve the ATP content, we better defined the sequence of events. Altogether, we showed that EV shedding from lipid domains only represents part of the global vesiculation mechanistics, for which we propose four successive events (cholesterol domain decrease, oxidative stress, sphingomyelin/sphingomyelinase/ceramide/calcium alteration and phosphatidylserine exposure).

Intra-operative blood transfusion significantly increases the risk of post-operative pulmonary embolism

Background

Pulmonary embolism (PE) is a serious and preventable complication after surgery. Blood transfusion is a common event during surgery. The aim of the present study was to identify whether intra-operative blood product transfusions increase the risk of symptomatic post-operative PE.

Methods

A retrospective, single-center case-control study at Sun Yat-Sen Memorial Hospital, Sun Yat-Sen University between 1 January 2013 and 31 December 2016 was performed. Adult hospitalized patients with confirmed symptomatic PE after surgery were defined as case group. Each patient was matched with two adult patients without symptomatic PE who underwent the same procedure on the same day or within one week. Perioperative data especially detailed blood transfusion was collected. Forward stepwise logistic regression analysis was performed.

Results

During the study period, altogether 188,512 procedures were included, and postoperative PE were confirmed in 40 (0.02%) patients. Twelve (30.0%) case patients and 16 (20.0%) control patients received intra-operative blood transfusion. Intra-operative blood transfusion was revealed as a significant predictor for symptomatic post-operative PE (OR 80.669, 95% CI: 3.312-1,964.641, P=0.007), along with female sex (OR 86.921, 95% CI: 4.242-1,781.124, P=0.004), older age (OR 1.078, 95% CI: 1.005-1.156, P=0.035), longer length of stay before surgery (OR 1.124, 95% CI: 1.040-1.214, P=0.003) and longer operation time (OR 1.013, 95% CI: 1.002-1.025, P=0.024).

Conclusions

Intra-operative blood transfusion was associated with an increased risk of symptomatic post-operative PE. PE prophylaxis and selective screening should be considered in patients who had received intra-operative blood transfusion, especially in female patients.

The role of microvesicles containing microRNAs in vascular endothelial dysfunction

Many studies have shown that endothelial dysfunction is associated with a variety of cardiovascular diseases. The endothelium is one of the primary targets of circulating microvesicles. Besides, microRNAs emerge as important regulators of endothelial cell function. As a delivery system of microRNAs, microvesicles play an active and important role in regulating vascular endothelial function. In recent years, some studies have shown that microvesicles containing microRNAs regulate the pathophysiological changes in vascular endothelium, such as cell apoptosis, proliferation, migration and inflammation. These studies have provided some clues for the possible roles of microvesicles and microRNAs in vascular endothelial dysfunction‐associated diseases, and opened the door towards discovering potential novel therapeutic targets. In this review, we provide an overview of the main characteristics of microvesicles and microRNAs, summarizing their potential role and mechanism in endothelial dysfunction, and discussing the clinical application and existing problems of microvesicles for better translational applications.

Transfusion in children with acute respiratory distress syndrome

Transfusion is a frequent treatment in pediatric patients with acute respiratory distress syndrome (PARDS) although evidence to support transfusion decision-making is lacking. The purpose of this review is to review the current state of knowledge on the issue of transfusion in children with PARDS and to detail the possible beneficial effects and potential deleterious impacts of transfusion in this patient population. Based on the current literature and recent guidelines, a restrictive red blood cell (RBC) transfusion strategy (avoidance of transfusion when the haemoglobin level is above 7 g/dL) is indicated in stable patients without severe PARDS, as these were excluded from the large trials. In children with severe PARDS, further research is needed to determine if factors other than the haemoglobin level might guide RBC transfusion decision-making by better characterizing the presence of low oxygen delivery (DO₂). Additionally, appropriate indications for prophylactic transfusion of hemostatic products (plasma or platelets) in children with PARDS are lacking.

Total plasma heme concentration increases after red blood cell transfusion and predicts mortality in critically ill medical patients

BACKGROUND

Relationships between red blood cell (RBC) transfusion, circulating cell-free heme, and clinical outcomes in critically ill transfusion recipients are incompletely understood. The goal of this study was to determine whether total plasma heme increases after RBC transfusion and predicts mortality in critically ill patients.

STUDY DESIGN AND METHODS

This was a prospective cohort study of 111 consecutive medical intensive care patients requiring RBC transfusion. Cell-free heme was measured in RBC units before transfusion and in the patients' plasma before and after transfusion.

RESULTS

Total plasma heme levels increased in response to transfusion, from a median (interquartile range [IQR]) of 35 (26-76) μmol/L to 47 (35-73) μmol/L (p < 0.001). Posttransfusion total plasma heme was higher in nonsurvivors (54 [35-136] μmol/L) versus survivors (44 [31-65] μmol/L, p = 0.03). Posttransfusion total plasma heme predicted hospital mortality (odds ratio [95% confidence interval] per quartile increase in posttransfusion plasma heme, 1.76 [1.17-2.66]; p = 0.007). Posttransfusion total plasma heme was not correlated with RBC unit storage duration and weakly correlated with RBC unit cell-free heme concentration.

CONCLUSIONS

Total plasma heme concentration increases in critically ill patients after RBC transfusion and is independently associated with mortality. This transfusion-associated increase in total plasma heme is not fully explained by RBC unit storage age or cell-free heme content. Additional studies are warranted to define mechanisms of transfusion-related plasma heme accumulation and test prevention strategies.

Red blood cell storage lesion: causes and potential clinical consequences

Red blood cells (RBCs) are a specialised organ that enabled the evolution of multicellular organisms by supplying a sufficient quantity of oxygen to cells that cannot obtain oxygen directly from ambient air via diffusion, thereby fueling oxidative phosphorylation for highly efficient energy production. RBCs have evolved to optimally serve this purpose by packing high concentrations of haemoglobin in their cytosol and shedding nuclei and other organelles. During their circulatory lifetimes in humans of approximately 120 days, RBCs are poised to transport oxygen by metabolic/redox enzymes until they accumulate damage and are promptly removed by the reticuloendothelial system. These elaborate evolutionary adaptions, however, are no longer effective when RBCs are removed from the circulation and stored hypothermically in blood banks, where they develop storage-induced damages ("storage lesions") that accumulate over the shelf life of stored RBCs. This review attempts to provide a comprehensive view of the literature on the subject of RBC storage lesions and their purported clinical consequences by incorporating the recent exponential growth in available data obtained from "omics" technologies in addition to that published in more traditional literature. To summarise this vast amount of information, the subject is organised in figures with four panels: i) root causes; ii) RBC storage lesions; iii) physiological effects; and iv) reported outcomes. The driving forces for the development of the storage lesions can be roughly classified into two root causes: i) metabolite accumulation/depletion, the target of various interventions (additive solutions) developed since the inception of blood banking; and ii) oxidative damages, which have been reported for decades but not addressed systemically until recently. Downstream physiological consequences of these storage lesions, derived mainly by in vitro studies, are described, and further potential links to clinical consequences are discussed. Interventions to postpone the onset and mitigate the extent of the storage lesion development are briefly reviewed. In addition, we briefly discuss the results from recent randomised controlled trials on the age of stored blood and clinical outcomes of transfusion.

Hemoglobin oxidation-dependent reactions promote interactions with band 3 and oxidative changes in sickle cell-derived microparticles

The contribution of intracellular hemoglobin (Hb) oxidation to RBC-derived microparticle (MP) formation is poorly defined in sickle cell disease (SCD). Here we report that sickle Hb (HbS) oxidation, coupled with changes in cytosolic antioxidative proteins, is associated with membrane alterations and MP formation in homozygous Townes-sickle cell (Townes-SS) mice. Photometric and proteomic analyses confirmed the presence of high levels of Hb oxidation intermediates (ferric/ferryl) and consequent β-globin posttranslational modifications, including the irreversible oxidation of βCys93 and the ubiquitination of βLys96 and βLys145. This is the first report to our knowledge to link the UPS (via ubiquitinated Hb and other proteins) to oxidative stress. Ferryl Hb also induced complex formation with band 3 and RBC membrane proteins. Incubation of Townes-SS MPs with human endothelial cells caused greater loss of monolayer integrity, apoptotic activation, heme oxygenase-1 induction, and concomitant bioenergetic imbalance compared with control Townes-AA MPs. MPs obtained from Townes-SS mice treated with hydroxyurea produced fewer posttranslational Hb modifications. In vitro, hydroxyurea reduced the levels of ferryl Hb and shielded its target residue, βCys93, by a process of S-nitrosylation. These mechanistic analyses suggest potential antioxidative therapeutic modalities that may interrupt MP heme-mediated pathophysiology in SCD patients.