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Sickle cell disease nephropathy (SCDN), a common sickle cell disease (SCD) complication, is strongly associated with mortality. Polygenic risk scores calculated from recent trans-ethnic meta-analyses of urinary albumin-to-creatinine ratio (UACR) and estimated glomerular filtration rate (eGFR) trended toward association with proteinuria and eGFR in SCD but the model fit was poor (R2

To address the global burden of sickle cell disease and the need for novel therapies, the American Society of Hematology partnered with the US Food and Drug Administration to engage the work of 7 panels of clinicians, investigators, and patients to develop consensus recommendations for clinical trial end points. The panels conducted their work through literature reviews, assessment of available evidence, and expert judgment focusing on end points related to patient-reported outcome, pain (non–patient-reported outcomes), the brain, end-organ considerations, biomarkers, measurement of cure, and low-resource settings. This article presents the findings and recommendations of the end-organ considerations, measurement of cure, and low-resource settings panels as well as relevant findings and recommendations from the biomarkers panel.

Altered mitochondrial function occurs in sickle cell disease (SCD), due in part to low nitric oxide (NO) bioavailability. Arginine, the substrate for NO production, becomes acutely deficient in SCD patients with vaso-occlusive pain episodes (VOE). To determine if arginine improves mitochondrial function, 12 children with SCD-VOE (13.6 ± 3 years; 67% male; 75% hemoglobin-SS) were randomized to 1 of 3 arginine doses: (1) 100 mg/kg IV 3 times/day (TID); (2) loading dose (200 mg/kg) then 100 mg/kg TID; or (3) loading dose (200 mg/kg) followed by continuous infusion (300 mg/kg per day) until discharge. Platelet-rich plasma mitochondrial activity, protein expression, and protein-carbonyls were measured from emergency department (ED) presentation vs discharge. All VOE subjects at ED presentation had significantly decreased complex-V activity compared to a steady-state cohort. Notably, complex-V activity was increased at discharge in subjects from all 3 arginine-dosing schemes; greatest increase occurred with a loading dose (P < .001). Although complex-IV and citrate synthase activities were similar in VOE platelets vs steady state, enzyme activities were significantly increased in VOE subjects after arginine-loading dose treatment. Arginine also decreased protein-carbonyl levels across all treatment doses (P < .01), suggesting a decrease in oxidative stress. Arginine therapy increases mitochondrial activity and reduces oxidative stress in children with SCD/VOE. This trial was registered at www.clinicaltrials.gov as #NCT02536170.

Cold storage of blood for 5 to 6 weeks has been shown to impair endothelial function after transfusion and has been associated with measures of end-organ dysfunction. Although the products of hemolysis, such as cell-free plasma hemoglobin, arginase, heme, and iron, in part mediate these effects, a complete analysis of transfused metabolites that may affect organ function has not been evaluated to date. Blood stored for either 5 or 42 days was collected from 18 healthy autologous volunteers, prior to and after autologous transfusion into the forearm circulation, followed by metabolomics analyses. Significant metabolic changes were observed in the plasma levels of hemolytic markers, oxidized purines, plasticizers, and oxidized lipids in recipients of blood stored for 42 days, compared with 5 days. Notably, transfusion of day 42 red blood cells (RBCs) increased circulating levels of plasticizers (diethylhexyl phthalate and derivatives) by up to 18-fold. Similarly, transfusion of day 42 blood significantly increased circulating levels of proinflammatory oxylipins, including prostaglandins, hydroxyeicosatrienoic acids (HETEs), and dihydroxyoctadecenoic acids. Oxylipins were the most significantly increasing metabolites (for 9-HETE: up to ∼41-fold, P = 3.7e-06) in day 42 supernatants. Measurements of arginine metabolism confirmed an increase in arginase activity at the expense of nitric oxide synthesis capacity in the bloodstream of recipients of day 42 blood, which correlated with measurements of hemodynamics. Metabolic changes in stored RBC supernatants impact the plasma metabolome of healthy transfusion recipients, with observed increases in plasticizers, as well as vasoactive, pro-oxidative, proinflammatory, and immunomodulatory metabolites after 42 days of storage.

Pulmonary and systemic vasculopathies are significant risk factors for early morbidity and death in patients with sickle cell disease (SCD). An underlying mechanism of SCD vasculopathy is vascular smooth muscle (VSM) nitric oxide (NO) resistance, which is mediated by NO scavenging reactions with plasma hemoglobin (Hb) and reactive oxygen species that can oxidize soluble guanylyl cyclase (sGC), the NO receptor. Prior studies show that cytochrome b5 reductase 3 (CYB5R3), known as methemoglobin reductase in erythrocytes, functions in VSM as an sGC heme iron reductase critical for reducing and sensitizing sGC to NO and generating cyclic guanosine monophosphate for vasodilation. Therefore, we hypothesized that VSM CYB5R3 deficiency accelerates development of pulmonary hypertension (PH) in SCD. Bone marrow transplant was used to create SCD chimeric mice with background smooth muscle cell (SMC)–specific tamoxifen-inducible Cyb5r3 knockout (SMC R3 KO) and wild-type (WT) control. Three weeks after completing tamoxifen treatment, we observed 60% knockdown of pulmonary arterial SMC CYB5R3, 5 to 6 mm Hg elevated right-ventricular (RV) maximum systolic pressure (RVmaxSP) and biventricular hypertrophy in SS chimeras with SMC R3 KO (SS/R3(KD)) relative to WT (SS/R3(WT)). RV contractility, heart rate, hematological parameters, and cell-free Hb were similar between groups. When identically generated SS/R3 chimeras were studied 12 weeks after completing tamoxifen treatment, RVmaxSP in SS/R3(KD) had not increased further, but RV hypertrophy relative to SS/R3(WT) persisted. These are the first studies to establish involvement of SMC CYB5R3 in SCD-associated development of PH, which can exist in mice by 5 weeks of SMC CYB5R3 protein deficiency.

Patient case: An 18-year-old male patient with homozygous hemoglobin SS disease was evaluated for progressive dyspnea and elevated tricuspid regurgitant jet velocity (TRV) on echocardiography. The patient’s case is described in detail in Lancet.1 He had been treated with regular transfusions since childhood for stroke, had rare episodes of vaso-occlusive pain episodes, and did not take narcotic pain medications. He presented with progressive severe dyspnea on exertion and lower extremity edema. His laboratory tests were notable for a total hemoglobin level of 11.8 g/dL and hemoglobin S levels 140 mm Hg (4 times the TRV squared = 4V2). Additional images in Figure 1D show a dilated right ventricle and right atrium with a compressed left ventricle. The patient’s right heart catheterization revealed a pulmonary artery systolic pressure of 147 mm Hg and diastolic pressure of 49 mm Hg; note that the normal values are ∼25/10 mm Hg.

Sickle cell disease (SCD) patients have an increased risk of venous thromboembolism (VTE). Population-based studies demonstrated that VTE has a cumulative incidence rate of approximately 25% in adult SCD patients and is associated with higher risk of mortality. VTE in SCD is most commonly manifested as deep vein thrombosis (DVT) with associated pulmonary embolism (PE). Although autopsy studies have regularly discovered pulmonary thromboembolic lesions in SCD patients, the pathophysiology of VTE in SCD remains largely unknown mostly due to the lack of relevant animal VTE model. Understanding the mechanisms that promote VTE in SCD is imperative to identify its prevention and treatment measures. To elucidate the cellular, molecular and biophysical mechanisms of VTE in SCD we developed an innovative two stage intravital imaging analysis experimental model in SCD mice. DVT in SCD mice was induced by surgical ligation of femoral vein. Venous thrombus formation was visualized using intravital multi-photon-excitation (MPE) microscopy. Venous thrombus took the form of a large mass of elliptical shape which extended in the long-axis direction of the femoral vein. It was composed of fibrin, erythrocytes and sparse platelets. Over time, thrombus was infiltrated by migrating neutrophils. To trigger acute PE, femoral vein ligation was removed and the venous thrombus was observed to spontaneously detach and travel to the SCD mouse lung. This method allowed real time visualization of acute PE in vivo using MPE microscopy of intact lung in live breathing mice. Acute PE involved embolization of the pulmonary arterioles and the arteriolar bottle-necks located at the junction of pulmonary arterioles and capillaries. The embolization of arteriolar circulation led to loss of blood flow in the arterioles and the down-stream capillaries. Herein we introduce an intravital microscopy approach to probe VTE in SCD live mouse. Our model has potential application in investigating the molecular determinants of VTE associated with SCD as well as evaluating efficacy of new antithrombotic drugs. Disclosures Sundd: Bayer: Research Funding; CSL Behring Inc: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Acute systemic painful vaso-occlusive episode (VOE) often serves as an antecedent to acute chest syndrome (ACS), which is a type of acute lung injury and the leading cause of mortality among sickle cell disease (SCD) patients. Based on clinical epidemiology, ACS is often preceded by thrombocytopenia and involves massive thrombosis across pulmonary artery branches in 10-20% of ACS patients. Although, released during hemolysis, adenosine diphosphate (ADP) is known to activate platelets by stimulating their P2Y1 and P2Y12 purinergic receptors, antagonists of P2Y12 have not shown any benefit in ACS therapy, justifying the need for better understanding of purinergic signaling in SCD. Ecto-nucleoside-tri-phosphate-diphosphohydrolase-1 (E-NTPDase1; CD39) maintains ADP homeostasis by degrading excessive ADP. Though CD39 inhibits ADP-dependent platelet activation and vascular thrombosis, its role in ASC is still unidentified. Here, we use intravital lung microscopy in transgenic humanized SCD mice to show that intravascular (IV) administration of ADP triggered pulmonary thrombosis in control mice but failed to trigger pulmonary thrombosis in SCD mice. Identical to intravital findings, IV ADP administration also evoked transient thrombocytopenia in control but not SCD mice, while, IV collagen led to comparable drop in platelet count in both SCD and control mice. In vitro turbidimetric aggregation study yet again demonstrated impaired SCD mouse platelet response to ADP, which was significantly augmented by CD39 inhibitor (sodium metatungstate, POM-1). Indeed, we found significantly higher plasma levels and activity of CD39 in SCD mice compared to control mice using ELISA and malachite green assays, respectively. Hemin, a major host-derived damage associated molecular pattern (DAMP) in SCD, was incubated with Human Lung MicroVascular Endothelial Cell (HMVEC-L) to assess CD39 expression using western blotting. Hemin (10 -100 µM) in a dose dependent manner decreased CD39 levels in HMVEC-L. Our current findings suggest that elevated CD39 plasma levels and activity possibly prevents ADP-mediated platelet aggregation and pulmonary thrombosis in SCD. We demonstrated that SCD milieu promotes loss of endothelial CD39, which may be directly associated with increased CD39 plasma levels and activity. Current study explains why P2Y12 blockers are not effective in SCD therapy and warrant the need for further studies to understand the role of purinergic signaling in pathogenesis of ACS. Disclosures Sundd: Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL Behring Inc: Research Funding; Bayer: Research Funding.

Reticulocytosis in sickle cell disease (SCD) is driven by tissue hypoxia from hemolytic anemia and vascular occlusion. Gene expression changes caused by hypoxia and other factors during reticulocytosis may impact SCD outcomes. We detected 1226 differentially expressed genes in SCD reticulocyte transcriptome compared to normal Black controls. To assess the role of hypoxia-mediating HIFs from other regulation of changes of the SCD reticulocyte transcriptome, we compared differential expression in SCD to that in Chuvash erythrocytosis (CE), a disorder characterized by constitutive upregulation of HIFs in normoxia. Of the SCD differentially expressed genes, 28% were shared between CE and SCD and thus classified as HIF-mediated. The HIF-mediated changes were generally in genes promoting erythroid maturation. We found that genes encoding the response to endoplasmic reticulum stress generally lacked HIF mediation. We then investigated the clinical correlation of erythroid gene expression for the 1226 differentially expressed genes detected in SCD reticulocytes, using clinical measures and gene expression data previously profiled in peripheral blood mononuclear cells (PBMCs) of 157 SCD patients at the University of Illinois at Chicago (UIC). Normal PBMCs contain only a small number of erythroid progenitors, but in SCD or CE PBMCs the erythroid transcriptome is enriched due to elevated circulating erythroid progenitors from heightened erythropoiesis (PMID: 32399971). We applied deconvolution analysis to assess the clinical correlation of erythroid gene expression, using a 16-gene expression signature of erythroid progenitors previously identified in SCD PBMCs. Deconvolution analysis uses the proportion of cell/tissue or specific marker genes (here the erythroid specific 16-gene signature) to dissect gene expression variation in biological samples with cell/tissue type heterogeneity. We correlated, in the 157 UIC patients, erythroid gene expression with i) degree of anemia as indicated by hemoglobin concentration, ii) vaso-occlusive severe pain episodes per year, and iii) degree of hemolysis measured by a hemolysis index. The analysis identified 231 genes associated with at least one of the complications. Increased expression of 40 erythroid specific genes, including 15 HIF-mediated genes, was associated with all three complications. These 40 genes are all upregulated in SCD reticulocytes and correlated with low hemoglobin concentration, frequent severe pain episodes, and high hemolysis index, suggesting that these manifestations may share a relationship to stress erythropoiesis-driven transcriptional activity. Expression quantitative trait loci (eQTL) contain genetic polymorphisms that associate with gene expression level, which can be viewed as a natural experiment to investigate the causal relations between gene expression change and phenotypic outcomes. To assess the causal effect of erythroid gene expression, we tested association between erythroid eQTL and the clinical manifestations in 906 SCD patients from the Walk-PHaSST and PUSH cohorts. We first mapped erythroid eQTL in the 157 UIC patients, who were previously genotyped by array, applying deconvolution algorithm on the same PBMC data for the 1226 differential genes in SCD reticulocytes, and detected 54 distinct eQTL for 30 genes at 5% false discovery rate. After adjusting for multiple comparisons, we found that the C allele of rs16911905, located in the β-globin cluster and associated with increased erythroid expression of HBD (encodes δ-globin of hemoglobin A 2), significantly correlated with lower hemoglobin concentration (β=-0.064, 95% CI -0.092 - -0.036, P=6.7×10 -6). The C allele was also associated with higher hemolytic rate (P=0.031), less frequent pain episodes (P=0.045), and increased erythroid expression of HBB here encoding sickle β-globin (P=5.1x10 -5). The association of the C allele with lower hemoglobin concentration was then validated in 242 patients from the UIC cohort (β=-0.071, 95% CI -0.13 - -0.011, P=0.023), as was the trend of association with higher hemolytic rate (P=0.0031) and less pain episodes (P=0.034). Our findings reveal HIF- and non-HIF-mediated genes in SCD stress erythropoiesis, and identify novel clinical associations for a HBD eQTL. Our study highlights the correlation of altered erythroid gene expression with SCD hemolytic and vaso-occlusive manifestations. Disclosures Saraf: Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding. Gordeuk: Modus Therapeutics: Consultancy; Novartis: Research Funding; Incyte: Research Funding; Emmaus: Consultancy, Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; CSL Behring: Consultancy.

Introduction. Stroke and silent infarcts are serious complications of sickle cell disease (SCD), occurring frequently in children between 3 and 14 years old. A vast amount of clinical and experimental evidence has concluded that decreased nitric oxide (NO) bioavailability and/or NO responsiveness, as is seen in SCD, is a major contributing factor in the pathogenesis of neurovascular disease. NO responsiveness, which occurs via NO-induced activation of soluble guanylate cyclase (sGC), requires reduced heme iron (Fe2+) in the sGC active site. We recently identified cytochrome b5 reductase 3 (Cyb5R3) as an sGC heme iron reductase in vascular smooth muscle (VSM), where it reverses the oxidized heme iron of sGC (Fe3+ --> Fe2+) to preserve NO sensing/signaling under conditions of oxidative stress. In a mouse model of SCD we have shown that knockdown of Cyb5R3 in VSM accelerates the development of pulmonary hypertension and cardiac remodeling. A missense variant of Cyb5R3 (T117S) that results in loss-of-function methemoglobin reductase activity occurs at a high frequency in persons of African ancestry (0.23 minor allele frequency). Unpublished baseline data from the Walk-PHaSSt trial (NCT00492531) reveals that persons with SCD who carry the T117S variant are at increased risk of ischemic stroke; these individuals self-reported almost 50% more (74 vs 51 cases per 1000 individuals) ischemic stroke than those with wild-type (WT) Cyb5R3. Hypothesis. We hypothesized that impaired reductase function of Cyb5R3 T117S leads to sustained sGC heme oxidation, which drives cerebral vascular dysfunction and exacerbates brain damage after ischemic stroke in SCD. Methods. Bone marrow transplant was used to create SCD mice with global expression of WT or T117S Cyb5R3, hereafter referred to as SS/WT or SS/T117S, respectively. All mice were male, C57Bl/6 background, and >85% engrafted with SS Hb for 12 weeks. Ischemic stroke was induced using transient middle cerebral artery occlusion (MCAO: 55 min occlusion, 48 hr reperfusion), after which brains were stained with 2,3,5-triphenyltetrazolium chloride (TTC,1%) to determine infarct volume. Blood was sampled before and after MCAO to assess effects of brain infarct on hematological parameters. Student's t-test was used for analysis of 2 groups and Pearson's R used for correlation analyses of brain infarct volume with hematology changes [(post-pre/pre) * 100]. Results. Global expression of T117S Cyb5R3 in SCD caused increased cerebral infarct volume (62.9 vs 26.7 cm3, P=0.003) and mortality (3/6 vs 0/6) relative to WT Cyb5R3. WT and T117S Cyb5R3 mice with SCD were similar in that both showed declining red blood cells (RBC), hemoglobin (Hgb) and hematocrit (Hct) as infarct volumes increased. In the SS/T117S group, the anemia was more severe in keeping with larger infarct volumes. There were different signatures to the hematologic changes that occurred with cerebral infarct in SCD. When compared to WT Cyb5R3, T117S caused the erythroid compartment to contract (RBC: -12.97% vs 13.41%, P=0.01; Hct: -19.75% vs 0.31%, P=0.025; Hgb: -17.93% vs 2.78%, P=0.017). In SS/WT mice platelet numbers increased more relative to SS/T117S (17.5 vs 9.7 * 103 cells/uL); and MPV, a measure of platelet activation, inversely correlated with brain infarct volume (r = -0.94, P=0.006), the opposite of what was seen in SS/T117S (r = 0.87, P=0.056). Monocytes seem to play an important role in the volume of brain infarct in SS/T117S as their numbers increased in parallel with infarct volume (r = 0.73, P=0.16), but followed the opposite trajectory in SS/WT mice (r = -0.75, P=0.14). Conclusion. These results indicate that Cyb5R3 is an important modifying factor in the evolution and outcome of ischemic brain injury in SCD. Our findings also raise questions on just how cerebral infarct modifies the anemia of SCD, as well as the role played by Cyb5R3 in the dynamics of that relationship. To what extent is the sGC-cGMP-PKG pathway involved at the cerebrovascular and erythropoietic levels? Does Cyb5R3 contribute resilience to ischemic stroke in SCD? The development and application of targeted therapies for effectively preventing and treating cerebrovascular disease in SCD rely on finding the answers to these questions. Disclosures No relevant conflicts of interest to declare.

The six-minute walk test (6MWT) is a well-established assessment of the cardiopulmonary function of sickle cell disease (SCD) patients. The test is used for people with SCD who are suspected of having hypoxia or an elevated estimated pulmonary arterial systolic pressure, which increases the risk for early death. Low 6MWT distances are associated with cardiopulmonary compromise. Six-minute walk distance examination has been proposed as a screening test, used in conjunction with plasma levels of brain natriuretic peptide and Doppler-echocardiography, through which to identify high-risk patients. Post-exercise tachycardia or lack of heart rate recovery following a 6MWT has been shown to be a predictor of pulmonary hypertension and mortality in people with pulmonary fibrosis. A rise in blood pressure after exercise is associated with impaired capacity for vasodilation in ischemic heart disease. The prognostic significance of patterns of vital sign change in adult SCD patients completing the 6MWT is currently unknown. In this study, we aimed to assess the distribution and predictors of vital sign change during 6MWT in adult SCD patients and test the association of these changes with patient survival. Data from a multinational observational study of SCD patients (Walk-PhassT), was used to calculate the change in vital signs (heart rate, O2 saturation, systolic blood pressure, diastolic blood pressure, pulse pressure) normalized for walk distance, after a 6MWT. Bivariate and LASSO regression analyses were performed to ascertain the significant predictors of change in each vital sign, in addition to Cox proportional hazard analysis to assess the impact of vital sign change and time to death. The median age of the 630 adult SCD patients was 37 years. 47% were male, 77% had the HbSS phenotype, and 22 (3.7% of 592 with follow-up data) died during a median time of follow up of 29 months. The most frequent changes in vital signs identified were increases in the heart rate (90%) followed by increases in systolic blood pressure (77%, Table 1b). Bivariate analysis revealed significant but weak positive correlations between tricuspid regurgitation velocity (TRV) and increases in both heart rate (r= 0.08; p These findings support links between changes of vital signs during the 6MWT and established markers of hemolysis and cardiovascular dysfunction in SCD patients. Evidence of a protective effect of increased systolic pressure is a novel finding. This might indicate that the ability to increase systolic pressure during submaximal exercise relates to cardiac output and conveys a physiological advantage for SCD patients. These findings could be used as the basis for future mechanistic studies of exercise effects on cardiovascular function in SCD patients. Disclosures Gordeuk: Imara: Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; Novartis: Consultancy; CSL Behring: Consultancy, Research Funding; Ironwood: Research Funding. Gibbs:Pfizer: Consultancy; United Therapeutics: Consultancy; MSD: Consultancy; GSK: Consultancy; Complexa: Consultancy; Bayer: Consultancy; Actelion: Consultancy; Acceleron: Consultancy. Little:BioChip Labs: Patents & Royalties: SCD Biochip (patent, no royalties); GBT: Membership on an entity's Board of Directors or advisory committees; NHLBI: Research Funding; Hemex Health, Inc.: Patents & Royalties: Microfluidic electropheresis (patent, no royalties); Bluebird Bio: Research Funding; GBT: Research Funding.

Hepatic crisis is an emergent complication affecting sickle cell disease (SCD) patients, however, the molecular mechanism of sickle cell hepatobiliary crisis remains poorly understood. We have used intravital imaging to show that sinusoidal ischemia occurs in the liver of transgenic-humanized SCD mice under basal condition. SCD mice manifested progressive hepatomegaly, liver injury and hyperbilirubinemia. RNA-sequence analysis identified dysregulation of genes encoding proteins responsible for fibrosis, bile acid synthesis, and bile transport in the SCD mice liver. Live injury was the result of NF-κB-dependent inhibition of FXR signaling and its downstream targets in hepatocytes, leading to loss of canalicular bile transport and bile metabolism. Blocking NF-κB activation rescued FXR-signaling, ameliorated liver injury and resolved sinusoidal ischemia in SCD mice. Moreover, administration of FXR agonist (GW4064) also ameliorated liver injury seen in SCD mice. These findings are the first to identify that FXR-dependent impaired bile secretion promotes intrahepatic bile accumulation, which contributes to hepatobiliary injury of SCD. Improved understanding of these pathways could potentially benefit the development of new therapies to treat sickle cell hepatic crisis. Disclosures No relevant conflicts of interest to declare.

Pulmonary thrombosis is a major complication associated with high morbidity. Despite advances in diagnosis and treatment, the pathophysiology of pulmonary thrombosis remains incompletely understood. New clinical evidence suggests that in situ platelet activation resulting in enhanced procoagulant activity may promote pulmonary thrombosis. Improved understanding of the etiological mechanism would enable the development of new therapies for pulmonary thrombosis. Collagen and thromboplastin (TF) were administered intravascularly (IV) to C57BL/6 (WT) mice and the pulmonary microcirculation was visualized using quantitative fluorescence intravital fluorescence lung microscopy (qFILM). Fluorochrome-conjugated anti-mouse CD49b Ab and dextran was administered IV for in vivo staining of circulating platelets and visualization of blood vessels, respectively. Pulmonary thrombosis was defined as occlusion of blood vessels with platelet aggregates leading to pulmonary ischemia. Additionally, quantitative microfluidic fluorescence microscopy (qMFM) was used to study the effect of platelet αIIbβ3 inhibition on platelet procoagulant activity in human blood under vascular mimetic flow conditions. Collagen and TF triggered dose-dependent pulmonary thrombosis in mice in vivo, which involved development of platelet-rich thrombi in the pulmonary arteriolar bottlenecks (junction of pulmonary arteriole and capillaries), resulting in a transient ischemia in the arteriole and the down-stream capillary tree. The pulmonary arteriole thrombosis triggered by IV collagen or TF was protracted, lethal and completely abrogated following IV administration of αIIbβ3 receptor inhibitor (eptifibatide). Inhibition of platelet αIIbβ3 also significantly reduced platelet procoagulant activity, fibrin formation and thrombus formation in human blood flowing through microfluidic channels ex vivo. Our current findings suggest that αIIbβ3-dependent platelet procoagulant activity promotes pulmonary thrombosis. Both our models have potential application in investigating the molecular determinants of pulmonary thrombosis in diverse pulmonary disorders as well as evaluating efficacy of new antithrombotic drugs. Disclosures No relevant conflicts of interest to declare.

Sickle cell disease (SCD) is an autosomal-recessive-genetic disorder that affects millions of people worldwide. Although hepatic crisis affects 10-40% of hospitalized SCD patients and can progress to fatal liver failure, the current treatment is primarily supportive and the molecular pathophysiology remains largely unknown. We found that transgenic, humanized SCD mice developed liver injury with age, manifested by increased inflammation, necrosis and hepatic iron accumulation. The presence of iron particles in SCD liver was confirmed by transmission electron micrograph (TEM) analysis and prussian blue staining which revealed increased iron accumulation in the central and midzonal region of the SCD liver tissue. An increase in aggregates of iron pigment reminiscent of hemosiderin-laden macrophages was also observed in SCD liver tissue. Interestingly, the SCD mice also showed significant enrichment of both hepatic (p=0.02) and serum iron (p=0.04) compared to control AS mice. We determined the expression level of genes commonly involved in iron homeostasis by RT-PCR. Interestingly, a significantly lower expression level of hepcidin transcripts was observed in the hepatocytes of SCD mice compared to control mice (AS) (p=0.01). In order to define the pathways controllinghepcidintranscription in SCD, we performed an RNA-seq analysis in the liver of SCD mice. Remarkably, our data showed significant misexpression of BMP signaling pathways. Further analysis revealed a significant misexpression in BMP2 and 6 levels in the liver of SCD mice by western blot. Reduced levels of hepcidin were also confirmed in serum samples from SCD patients compared to controls. Work is currently underway to understand how BMP2/6 might hypothetically regulate hepcidin expression in the liver. In summary, our results reveal a significant defect in iron homeostasis in the liver of SCD mice, suggesting that impaired iron homeostasis may contribute to hepatobiliary injury in SCD independent of blood transfusions. Our study also highlights the importance of hepcidin as potential therapeutic target in regulation of hepatic injury in SCD. Disclosures No relevant conflicts of interest to declare.

Introduction: Transfusional iron (Fe) overload is a significant problem among patients with chronic, transfusion-dependent anemias. Iron overload is an important problem in pediatric sickle cell disease (SCD) patients on chronic transfusion regimens predominantly for primary and secondary prevention of stroke. Coexistent hereditary iron overload conditions contribute to the iron overload phenotype in SCD. For example, the Q248H mutation (rs11568350) in SLC40A1, which encodes ferroportin (FPN), is associated with a mild tendency to increase serum ferritin in the general population and with increased ferritin levels in SCD patients. Nevertheless, the molecular mechanisms underlying the progression to iron overload in SCD patients are poorly understood and more sensitive markers for outcome prediction that can be applied at early clinical stages are lacking. We hypothesize that genetic variation modifies the risk for iron overload in SCD patients and seek to validate previously identified mutation and identify novel genetic markers of iron overload among participants from TOPMed SCD cohorts by performing whole genome sequencing (WGS) association analyses. Methods: The WGS was performed by several national sequencing centers sponsored by NHLBI's TOPMed program at an average depth of 30× using DNA from SCD patient blood samples. Variant calling was performed jointly across TOPMed studies for all samples using the GotCloud pipeline by the TOPMed Informatics Research Center. The TOPMed data Coordinating Center performed quality control for sample identity. The data across the following studies were shared through the database of Genotype and Phenotype (dbGaP) exchange area: Howard PUSH SCD (N=370), OMG SCD (N=636), Walk PHaSST SCD (N=381) and REDS-III Brazil SCD (N=2620) with a total sample size of 4007. The study was approved by the appropriate institutional review boards (IRB) and informed consent was obtained from all participants. Genome Wide Association Analysis of iron overload was carried out using the University of Michigan ENCORE server. We performed single variant tests to test the association of log-transformed serum ferritin levels with single nucleotide variants (SNVs) while adjusting for sex, age, self-reported race, numbers of lifetime red blood transfusions and genetic substructure (PC's 1-10). We used a significance threshold of p Results: We first included PUSH SCD, OMG SCD and Walk PHaSST SCD cohorts with 840 serum ferritin samples in the WGS association analyses, which revealed at the genome-wide level a new rare variant rs137929759 (chr7:49538810 (GRCh38.p12), MAF=0.0043532, p=2.25×10−8). A few variants such as rs80097634 in gene AL163195.3 and RNASE11 (Chr14:20579417. MAF=0.050373, p=3.1×10-7) were close to the genome-wide significance level. We confirmed previously identified associations in SLC40A1 for ferritin (rs11568350, Chr2:189565370, MAF = 0.16853, p = 5.2×10−4). We also found several variants in AC105411.1, TJP1 and DCC that were close to genome-wide significance level. Further analysis will be carried out on the cloud-based platform provided by NHLBI BioData Catalyst using data from all the four cohorts to validate the previous analysis and expand to related phenotype such as transferrin, iron-overload status. Discussion: In this study we identified common and rare variants that associate with serum ferritin concentration. The results from this pilot study point to novel gene variants that might contribute to iron overload in SCD patients and serve as new biomarkers. Future analysis is needed to determine whether the identified variants can also help with therapeutics and outcome prediction for early stages of SCD-associated iron overload. Our findings will be useful for the future treatment of SCD patients and design of novel SCD therapeutics. ACKNOWLEDGMENTS: This work was supported by NIH Research Grants (1P50HL118006, and 1R01HL125005). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Disclosures Gordeuk: Imara: Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; CSL Behring: Consultancy, Research Funding; Ironwood: Research Funding; Novartis: Consultancy. Telen:CSL Behring: Membership on an entity's Board of Directors or advisory committees, Research Funding; Forma Therapeutics: Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; GlycoMimetics Inc.: Consultancy.

Hemarthrosis is a chronic arthropathy responsible for irreversible joint damage, disability, and acute joint pain in hemophilia patients. Hemarthrosis is caused by spontaneous or traumatic bleeding into joints, which when recurrent, leads to synovial inflammation and cartilage degeneration. Although release of erythrocyte-derived damage associated molecular pattern molecules (eDAMPs) is believed to promote sterile inflammation in the synovium, the innate immune mechanism of hemarthrosis remains poorly understood and the current therapy is limited to factor replacement and pain management. Here, we use factor 8 total knock-out (F8TKO) hemophilia A mice (C57BL/6J background) that manifest a complete deletion of the F8 coding region, expressing no detectable F8 mRNA and exhibiting a severe hemophilia phenotype. Right knee joint capsules of F8TKO mice were punctured with a 30-g needle below the patella between the anterior portions of the femur and tibia, followed by assessment of bleeding severity score and histological analysis one-week post injury. Intravital multiphoton excitation fluorescence microscopy of injured synovium was performed to assess the role of thrombo-inflammatory events in promoting hemarthrosis. Neutrophil, platelets, and blood vessels were visualized by intravenous administration of fluorescent anti-Ly6G mAb, anti-CD49b mAb, and dextran, respectively. Protein and mRNA levels of proinflammatory cytokines were measured in plasma, joint, synovium, and cartilage tissue using Real-Time PCR and ELISA, respectively. F8TKO but not wild-type (WT) control (C57BL/6J) mice manifested unresolved joint bleeding, cartilage degeneration and synovitis leading to impaired mobility and high bleeding severity scores. Significantly more recruitment of neutrophils and neutrophil-platelet aggregates as well as elevated IL-1b levels were observed in the synovium of F8TKO compared to WT mice. These results are the first to suggest that sterile inflammation contributing to hemarthrosis in hemophilia involves enhanced neutrophil-platelet recruitment to the synovium. Currently, experiments are underway to identify the role of eDAMPs (heme and hemoglobin) mediated activation of TLR4 and inflammasome pathway in promoting IL-1b generation, neutrophil-platelet aggregation, and progression of joint injury in F8TKO mice Disclosures Ragni: OPKO: Research Funding; Sangamo: Research Funding; Shire/Takeda: Other: Advisory Board, Research Funding; Spark Therapeutics: Other: Advisory Board, Research Funding; Alnylam/Sanofi: Other: Advisory Board, Research Funding; BioMarin: Other: Advisory Board, Research Funding; Bioverativ/Sanofi: Other: Advisory Board, Research Funding. Gladwin:Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning; Bayer Pharmaceuticals: Other: Co-investigator; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases .

Sickle cell disease (SCD) is an autosomal-recessive-genetic disorder that affects millions of people worldwide. Although hepatic crisis affects 10-40% of hospitalized SCD patients and can progress to fatal liver failure, the current treatment is primarily supportive and the molecular pathophysiology remains largely unknown. We found that transgenic, humanized SCD mice developed liver injury with age, manifested by increased inflammation, necrosis and hepatic iron accumulation. The presence of iron particles in SCD liver was confirmed by transmission electron micrograph (TEM) analysis and prussian blue staining which revealed increased iron accumulation in the central and midzonal region of the SCD liver tissue. An increase in aggregates of iron pigment reminiscent of hemosiderin-laden macrophages was also observed in SCD liver tissue. Interestingly, the SCD mice also showed significant enrichment of both hepatic (p=0.02) and serum iron (p=0.04) compared to control AS mice. We determined the expression level of genes commonly involved in iron homeostasis by RT-PCR. Interestingly, a significantly lower expression level of hepcidin transcripts was observed in the liver of SCD mice compared to control mice (AS) (p=0.01). In order to define the pathways controlling hepcidin transcription in SCD, we performed an RNA-seq analysis in the liver of SCD mice. Remarkably, our data showed significant misexpression of hypoxia signaling pathways. Further analysis revealed a significant increase in hypoxia-inducible factor (HIF)-2α levels in the liver of SCD mice by western blot. Reduced levels of Hepcidin were also confirmed in serum samples from SCD patients compared to controls. Work is currently underway to understand how HIF2α might hypothetically regulate hepcidin expression in the liver. This is particularly relevant because HIF2a translation is known to be regulated by iron through an iron response element at the 5' end of its transcript. In summary, our results reveal a significant defect in iron homeostasis in the liver of SCD mice, suggesting that impaired iron homeostasis may contribute to hepatobiliary injury in SCD independent of blood transfusions. Our study also highlights the importance of hepcidin as potential therapeutic target in regulation of hepatic injury in SCD. Disclosures Gladwin: Bayer Pharmaceuticals: Other: Co-investigator; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases ; Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning. Kato:Novartis, Global Blood Therapeutics: Consultancy, Research Funding; Bayer: Research Funding.

Hepatic crisis is an emergent complication affecting sickle cell disease (SCD) patients, however, the molecular mechanism of sickle cell hepatobiliary crisis remains poorly understood. We examined the liver pathophysiology of SCD using a humanized mouse model (townes SCD mice; homozygous for Hbatm1(HBA)Tow, homozygous for Hbbtm2(HBG1,HBB*)Tow). These mice have the major features (irreversibly sickled red cells, splenomegaly, anemia, multiorgan pathology) found in humans with SCD and, as such, represent a useful in vivo system to study hepatobiliary changes in SCD disease. SCD mice manifested progressive hepatomegaly, liver injury and hyperbilirubinemia. RNA-sequence analysis of total RNA from SCD mouse liver compared to control (AS) identified dysregulation of genes encoding proteins responsible for fibrosis, bile acid synthesis, bile transport and cholesterol metabolism. Immunohistochemical analysis confirmed inflammation, fibrosis and increased ductular reaction in SCD mice. To mechanistically address the cholestatic phenotype in SCD mice, we used our recently developed multi-photon-excitation (MPE) enabled in vivo real-time fluorescence microscopy of intact liver in live mice. We used Texas-Red (TXR) dextran to visualize the blood flow through liver sinusoids and carboxyfluorescein (used as a surrogate of bile flow) to visualize bile flow through biliary canaliculi. Real time imaging show that sinusoidal ischemia occurs in the liver of transgenic-humanized SCD mice under basal condition. Intravital imaging also revealed impaired bile secretion into the bile canaliculi, which was associated with loss of apical bile acid transporters and bile acid biosynthetic enzymes, hepatic bile accumulation, and activation of Farnesoid X receptor (FXR) and small heterodimer partner (Shp) in the liver of SCD mice. These findings are the first to identify that impaired bile acid synthesis and misexpression of bile transporters promote intrahepatic bile accumulation and impaired bile secretion, leading to hepatobiliary injury of SCD. Improved understanding of these processes could potentially benefit the development of new therapies to treat sickle cell hepatic crisis. Disclosures Gladwin: Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning; Bayer Pharmaceuticals: Other: Co-investigator; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases . Kato:Novartis, Global Blood Therapeutics: Consultancy, Research Funding; Bayer: Research Funding.

Introduction: Acute chest syndrome (ACS) is a type of acute lung injury and among the primary reasons for mortality and morbidity among Sickle Cell Disease (SCD) patients. Although epidemiologic evidence suggests that vaso-occlusion in the lung may serve as an antecedent to ACS, the cellular, molecular and biophysical mechanism of ACS is incompletely understood. Our recent findings revealed that the lung vaso-occlusion is enabled by the entrapment of embolic neutrophil-platelet aggregates in the pulmonary arterioles of transgenic humanized SCD mice. Recent evidence also suggests a role for neutrophil extracellular traps (NETs) in ACS. NETs are web-like structures of decondensed nuclear DNA decorated with citrullinated-histones (H3-cit) and neutrophil granule proteins. Interestingly, circulating nucleosomes and NETs fragments are elevated in SCD patient blood and the levels correlate with onset of ACS, however, the molecular mechanism that promotes generation of circulating NETs and the role of circulating NETs in promoting ACS remains poorly understood. Materials and Methods: Townes knock-in humanized SS (hα/hα:βS/βS) and AS (hα/hα:βA/βS) mice were used as SCD and control mice, respectively. SS and AS mice were intravenously (IV) administered 10 µmole/kg Oxy-Hb followed by Sytox orange, FITC-dextran or fluorescent anti-mouse mAbs against Ly6G, CD49b, H3cit, and neutrophil elastase for in vivo visualization of extracellular DNA, blood vessels, neutrophils, platelets and NETs, respectively. Pulmonary microcirculation was monitored using multi-photon-excitation enabled quantitative fluorescence intravital lung microscopy (qFILM). Results and Discussion: IV Oxy-Hb triggered the occlusion of pulmonary arterioles by neutrophil-platelet aggregates leading to loss of pulmonary blood flow in SCD but not control mice. Surprisingly, pulmonary vaso-occlusion in SCD mice was accompanied by the arrival of circulating cell free DNA (CFD) and NETs fragments into the pulmonary circulation. The cell free DNA (CFD) and NETs fragments entered the lung through the arterial circulation suggesting that they originated outside of lung. These cell free DNA (CFD) and NETs fragments contributed to lung vaso-occlusion and injury by promoting neutrophil-platelet aggregation in the lung arterioles. Conclusion: These findings reveal for the first time that circulating cell free DNA (CFD) and NETs fragments originating outside of lung contribute to pathogenesis of ACS. Currently, experiments are underway to identify the innate immune pathways that promote circulating NETs dependent lung injury in SCD. Disclosures Gladwin: Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases ; Bayer Pharmaceuticals: Other: Co-investigator.

Pulmonary and systemic vasculopathy are significant risk factors for early morbidity and death in patients with sickle cell disease (SCD). An underlying mechanism of SCD vasculopathy is vascular smooth muscle (VSM) nitric oxide (NO) resistance, which is mediated by NO scavenging reactions with plasma hemoglobin and reactive oxygen species that can oxidize soluble guanylyl cyclase (sGC), the NO receptor. Prior studies show that cytochrome b5 reductase 3 (CYB5R3), known as methemoglobin reductase in erythrocytes, functions in VSM as an sGC heme iron reductase critical for reducing and sensitizing sGC to NO and generating cGMP for vasodilation. Therefore, we hypothesized that VSM CYB5R3 deficiency accelerates development of pulmonary hypertension (PH) in SCD. Bone marrow transplant was used to create SCD chimeric mice with background smooth muscle cell (SMC) specific tamoxifen-inducible Cyb5r3 knockout (SMC R3 KO) and wild-type (WT) control.Three weeks after completing tamoxifen treatment, we observed 60% knockdown of pulmonary arterial SMC CYB5R3, 5-6 mmHg elevated right ventricular maximum systolic pressure (RVmaxSP) and biventricular hypertrophy in SS chimeras with SMC R3 KO (SS/R3KD) relative to wild-type (SS/R3WT). Right ventricular (RV) contractility, heart rate, hematological parameters and cell-free hemoglobin were similar between groups. When identically generated SS/R3 chimeras were studied 12-weeks after completing tamoxifen treatment, RVmaxSP in SS/R3KD had not increased further, but RV hypertrophy relative to SS/R3WT persisted. These are the first studies to establish involvement of SMC CYB5R3 in SCD-associated development of PH, which can exist in mice by 5 weeks of SMC CYB5R3 protein deficiency. Disclosures Vitturi: Complexa Inc: Consultancy. Ofori-Acquah:Shire Human Genetic Therapies Inc: Other: Financial Relationship. Gladwin:Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning; Bayer Pharmaceuticals: Other: Co-investigator; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases . Straub:Bayer Pharmaceuticals: Research Funding.

Sickle cells disease (SCD) is an inherited red blood disorder that leads to acute and chronic organ complications. The kidneys are among the most commonly affected organ systems and chronic kidney disease (CKD) predicts increased morbidity and early mortality in SCD (PMID 1892333). Risk factors for CKD have been primarily investigated in patients with homozygous inheritance of the hemoglobin (Hb) S mutation but are not as well understood in other SCD variants, such as in those with Hb SC or Hb SB+-thalassemia. We investigated clinical, laboratory, and genetic predictors (α-thalassemia and the APOL1 G1 and G2 kidney disease risk variants) for CKD in a combined analysis of non-Hb SS patients from the University of Illinois at Chicago and the multi-center Walk-PHaSST cohort. Clinical and laboratory variables were obtained at the time of enrolment. Estimated glomerular filtration (eGFR) rate was calculated using the CKD-EPI formula and CKD stage was defined according to the KDIGO guidelines. Univariate analysis for the association of variables with eGFR, albuminuria, and CKD stage were conducted using the linear trend test and Cochran's test for linear trend. Variables with a P < 0.1 were entered into linear and ordinal logistic regression models adjusting for age and sex. Median and interquartile ranges (IQR) are provided. Two hundred and forty-seven patients were included in this analysis. The median age of this cohort was 39 years (IQR, 24 - 50 years), 62% were female, 16% were on hydroxyurea, and 72% were Hb SC genotype. Albuminuria data was available in 174 patients; microalbuminuria (albuminuria 30 - 300 mg/g creatinine) was present in 24% and macroalbuminuria (albuminuria > 300 mg/g creatinine) in 5% of patients. Patients with higher urine albumin concentration were older, had higher systolic blood pressures and LDH concentrations, and lower hemoglobin concentrations (P ≤ 0.03). On multivariate analysis, the urine albumin concentration was independently associated with a higher systolic blood pressure (log transformed β +3.08 ± 1.09; P = 0.005) and a lower hemoglobin concentration (β -0.17 ± 0.09; P = 0.05). Patients with lower eGFR were older, had higher body mass indexes and systolic blood pressures, lower hemoglobin concentrations, and were more frequently female (P ≤ 0.04). No variables were independently associated with eGFR on multivariate analysis after adjusting for age and sex. When applying the KDIGO CKD staging criteria, 67% were classified as not having CKD (urine albumin < 30mg/g creatinine and eGFR > 60 mL/min/1.73m2), 23% had stage 1 or 2 CKD (urine albumin ≥ 30mg/g creatinine and eGFR > 60 mL/min/1.73m2), and 10% had stage 3 or greater CKD (eGFR < 60 mL/min/1.73m2). Patients with increasing CKD stage were older, had higher systolic blood pressures and LDH concentrations, and lower hemoglobin concentrations (Table 1) (P ≤ 0.005). On multivariate analysis, incremental CKD stage was independently associated with lower hemoglobin (OR 0.91, 95%CI: 0.88 - 0.94; P = 0.002) and higher LDH (natural log OR 1.43, 95% CI: 1.27 - 1.61; P = 0.004) concentrations. We did not observe an association between coinheritance of α-thalassemia or the APOL1 G1/G2 risk variants with urine albumin concentration, eGFR, or CKD stage. In conclusion, we demonstrate that CKD is a common comorbidity that is observed in approximately one third of SCD patients with the Hb SC or Hb Sβ+-thalassemia genotypes. A lower hemoglobin concentration and higher systolic blood pressure were independently associated with greater urine albumin concentration. This may reflect anemia causing reduced oxygen delivery to the renal cortex and hypertension-mediated vascular and glomerular damage. The association of worsening CKD stage with lower hemoglobin and higher LDH concentrations, consistent with increased hemolysis, is similar to what has been observed in the Hb SS genotypes. The potential effects of hemolysis on kidney function in the Hb SC and Sβ+-thalassemia genotypes will need to be explored further. Disclosures Gladwin: Bayer Pharmaceuticals: Other: Co-investigator; United Therapeutics: Patents & Royalties: Co-inventor on an NIH government patent for the use of nitrite salts in cardiovascular diseases ; Globin Solutions, Inc: Patents & Royalties: Provisional patents for the use of recombinant neuroglobin and heme-based molecules as antidotes for CO poisoning. Gordeuk:Novartis: Consultancy, Honoraria, Research Funding; Global Blood Therapeutics: Consultancy, Honoraria, Research Funding; Emmaus: Consultancy, Honoraria; Modus Therapeutics: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Pfizer: Research Funding; Modus Therapeutics: Consultancy, Honoraria; Pfizer: Research Funding; Inctye: Research Funding; Inctye: Research Funding; CSL Behring: Consultancy, Honoraria, Research Funding; CSL Behring: Consultancy, Honoraria, Research Funding; Ironwood: Research Funding; Ironwood: Research Funding; Imara: Research Funding; Imara: Research Funding. Saraf:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Research Funding.

Two-year-old purpose-bred beagles (n = 24) infected with Staphylococcus aureus pneumonia were randomized in a blinded fashion for exchange transfusion with either 7- or 42-day-old canine universal donor blood (80 mL/kg in 4 divided doses). Older blood increased mortality (P = .0005), the arterial alveolar oxygen gradient (24-48 hours after infection; P ≤ .01), systemic and pulmonary pressures during transfusion (4-16 hours) and pulmonary pressures for ~ 10 hours afterward (all P ≤ .02). Further, older blood caused more severe lung damage, evidenced by increased necrosis, hemorrhage, and thrombosis (P = .03) noted at the infection site postmortem. Plasma cell–free hemoglobin and nitric oxide (NO) consumption capability were elevated and haptoglobin levels were decreased with older blood during and for 32 hours after transfusion (all P ≤ .03). The low haptoglobin (r = 0.61; P = .003) and high NO consumption levels at 24 hours (r = −0.76; P < .0001) were associated with poor survival. Plasma nontransferrin-bound and labile iron were significantly elevated only during transfusion (both P = .03) and not associated with survival (P = NS). These data from canines indicate that older blood after transfusion has a propensity to hemolyze in vivo, releases vasoconstrictive cell-free hemoglobin over days, worsens pulmonary hypertension, gas exchange, and ischemic vascular damage in the infected lung, and thereby increases the risk of death from transfusion.

In adults with sickle cell disease (SCD), an increased tricuspid regurgitation velocity (TRV) by Doppler echocardiography is associated with increased morbidity and mortality. Although sildenafil has been shown to improve exercise capacity in patients with pulmonary arterial hypertension, it has not been evaluated in SCD. We therefore sought to determine whether sildenafil could improve exercise capacity in SCD patients with increased TRV and a low exercise capacity. A TRV ≥ 2.7 m/s and a 6-minute walk distance (6MWD) between 150 and 500 m were required for enrollment in this 16-week, double-blind, placebo-controlled sildenafil trial. After 74 of the screened subjects were randomized, the study was stopped early due to a higher percentage of subjects experiencing serious adverse events in the sildenafil arm (45% of sildenafil, 22% of placebo, P = .022). Subject hospitalization for pain was the predominant cause for this difference: 35% with sildenafil compared with 14% with placebo (P = .029). There was no evidence of a treatment effect on 6MWD (placebo-corrected effect −9 m; 95% confidence interval [95% CI] −56-38; P = .703), TRV (P = .503), or N-terminal pro-brain natriuretic peptide (P = .410). Sildenafil appeared to increase hospitalization rates for pain in patients with SCD. This study is registered at www.clinicaltrials.gov as NCT00492531.

Introduction: Thirty-day readmission risk is widely accepted as an indicator of quality of care. Sickle cell disease (SCD) has one the highest hospital readmission risk with a wide variation between different studies. In recent studies, older age, insurance status, and systemic complications including sepsis, renal and liver disease increased the risk of readmission whereas blood transfusion reduced the risk. During hospital stay, patients experience a variety of changes in their symptoms and laboratory measures. Evidence on the role of these changes on readmission risk is limited. In the current study, we aimed to assess the clinical and laboratory predictors of 30-day readmission risk in SCD adult patients in a tertiary health care system. Methods: Medical record discharge abstract files which cover visits for the SCD patients at the University of Pittsburgh Medical Center (UPMC) were extracted from electronic health records. Laboratory test results were obtained for each admission and were linked to discharge data. For each admission ICD 9/10 codes were used to identify the comorbidities. Blood transfusion information was recorded during each the admission. Natural language processing was used to extract medical concepts from chest X-ray and CT scan reports during patient's admission. Acute chest syndrome/pneumonia were identified from a combination of ICD codes and radiologic reports. For each laboratory value, a single rate of change (trajectory) was calculated with a random coefficient model from any measures during the hospital stay. Rate of changes were categorized to negative and positive trajectory. We used Generalized Estimating Equations models to assess predictors of 30-day readmission risk including the relationship between negative trajectory of any laboratory values duration the admission. Results: During January 2010 to May 2016, data for 2,108 hospital admissions in 173 SCD unique adult patients (median age of 32, 57% female and 59% SS genotype) were extracted. Risk of 30-day readmission was 41.2%. Older age (P Conclusions: These results support that cardiopulmonary comorbidities are unexpectedly common in adult SCD patients. Blood transfusion in younger SCD patient reduced the readmission risk. Renal complications and leukocytosis in these patients contributed to health care utilization. Using advanced predictive models can help us to define patients who are at higher risk of readmission and generate strategies to reduce hospital readmission. Disclosures No relevant conflicts of interest to declare.

In sickle cell disease (SCD) polymerization of hemoglobin S under deoxygenated conditions causes vaso-occlusion, which can manifest as acute pain crisis and progressive bone/organ damage. Molecular studies have attributed vaso-occlusion to elevated vascular adhesion and inflammatory responses, whereas the genetic regulation has only recently been assessed. Genomic DNA isolated from peripheral blood mononuclear cells (PBMCs) was hybridized to Illumina Human 610-Quad SNP array for the PUSH and Walk-PHaSST cohorts and to Affrymetrix SNP 6.0 array for the Howard SCD expression cohort. Single nucleotide polymorphisms (SNPs) for 381 PUSH, 525 Walk-PHaSST, and 55 Howard patients were imputed to 1000 genomes project phase 3 data. Messenger RNA from PBMCs was profiled using Affymetrix Human Exon 1.0 ST Array for the Howard expression cohort and Affymetrix Human gene 2.0 ST array for the UIC expression cohort. Patients within the PUSH and Walk-PHaSST cohorts were classified to four groups according to a cumulative pain score, calculated based on pain frequency and questionnaire description of pain intensity. Pain grouping was examined for correlation with other SCD complications using Cochran Armitage test. History of acute chest syndrome (ACS, PUSH P=3.8×10-9, Walk-PHaSST P=2.4×10-5) and avascular necrosis (AVN, PUSH P=4.1×10-4, Walk-PHaSST P=3.7×10-5) were the most significant clinical manifestations that consistently associated with pain in the two cohorts. To investigate the genetic control of vaso-occlusive manifestations with appropriate power, we leveraged genetic association of pain, ACS, and AVN with genetic regulation of disease-specific gene expression. We mapped expression quantitative trait loci (eQTL) in the Howard expression cohort for SNPs The six eQTL were assessed for association with pain, ACS, and AVN, using the Walk-PHaSST cohort for discovery and the PUSH cohort for validation. At a significance of Bonferroni corrected P=0.05 (nominal P=0.0083), an eQTL of S100B (rs2154586) significantly associated with AVN in the Walk-PHaSST cohort (OR=1.8, P=0.00061) and the association was replicated in the PUSH cohort (OR=2.7, P=0.0052). The A allele of the eQTL (frequency=0.18) associated with increased risk for AVN and increased expression level of S100B in the Howard expression cohort (β=0.40, P=1.6 ×10-6). In an additional 64 sickle cell anemia patients without hydroxyurea treatment from the UIC expression cohort, expression levels of S100B were significantly elevated in the individuals with AVN (β=0.28, P=0.029). The 24 SNPs in linkage disequilibrium with the eQTL (r2 >0.7) constituted the third most significant peak in a meta-analysis of genome-wide association of AVN in the PUSH and Walk-PHaSST cohorts. To test the hypothesis that genes involved in vaso-occlusion in SCD may affect thrombotic risk in non SCD individuals, we examined the association of the locus with venous thromboembolism (VTE) in the ARIC, JHS and CHS cohorts from dbGaP. The locus was imputed in African Americans and VTE was defined as being told by a doctor to have a blood clot in the leg or lung as answered in questionnaires during medical exams. The SNPs were associated with VTE using logistic linear regression adjusting for age, gender, enrollment site, and the first 15 principal components per cohort. The risk allele of the leading SNP for AVN consistently associated with increased risk of VTE across the cohorts, with a combined P=0.0041 and OR=1.4. S100B encodes a calcium sensor that appears to intervene in a variety of biological functions. S100B can mediate the inflammatory effects of damage-associated molecular pattern molecules (DAMPs) produced by erythrocyte hemolysis [2, 3]. Serum concentration of S100B correlates with LDH and with TCD-determined peak velocity of the left middle cerebral artery in thalassemia patients[4]. Polymorphisms of S100B that lead to increased serum levels are associated with increased risk of ischemic stroke in the Chinese population[5]. Disclosures Nekhai: NIMHD, NIH: Research Funding; NHLBI, NIH: Research Funding; NIAID, NIH: Research Funding.

Sickle cell anemia results from a point mutation in both alleles of the β-globin gene. This homozygous mutation ultimately leads to a structural alteration of the hemoglobin protein that promotes polymerization of the mutant sickle hemoglobin tetramer (HbS) upon deoxygenation. HbS polymerization results in rigid, sickle-shaped RBCs with increased cell-to-cell adhesion properties. These sticky and rigid RBCs are prone to become trapped in small capillary networks leading to ischemia-reperfusion injury, endothelial damage and the hallmark pain crisis of sickle cell anemia. Neonates are protected from deoxy-HbS polymerization by high levels of fetal hemoglobin (HbF). HbF is composed of two α-globin and two γ-globin subunits(α2γ2), The γ-globin molecule cannot interact with deoxy-βS-globin polymers, which makes HbF an effective inhibitor of deoxy-HbS polymerization. The level of HbF required to reduce the symptoms of sickle cell anemia is 20-25%, but levels as low as 9% can prolong red cell survival. Treatment with hydroxyurea (HU) induces HbF and reduces the hematologic and clinical consequences of sickle cell anemia. Basal and inducible HbF levels are important in predicting the severity of sickle cell anemia and are highly phenotypically variable among patients, leading to varied responses to treatment. Patients are also variably susceptible to HU-induced cytopenias, which limits the use of HU in certain patients. HU is currently the only FDA-approved HbF-inducer for the treatment of sickle cell anemia and there is clearly a need for alternative HbF-inducers. Understanding the signaling pathways that regulate HbF induction will lead to novel therapeutic targets for sickle cell anemia. The soluble guanylate cyclase/cyclic guanosine monophosphate-dependent protein kinase (sGC/PKG) signaling pathway potentially links HU to the induction of HbF expression. In this study we investigated the direct role of sGC in HbF induction using novel pharmacologic modulators of sGC. Nitric oxide (NO) activates sGC by binding to the ferrous iron (Fe2+) in the active site heme moiety. Once activated, sGC converts GTP to cGMP, which in-turn activates PKG. Reactive oxygen species (ROS) oxidize the active site heme of sGC leading to NO-insensitivity. We tested the ability of a novel sGC activator, BAY 58-2667, to induce γ-globinin primary and immortalized (HUDEP-2) human erythroid progenitor cells. BAY 54-6544 binds to heme-free inactivated sGC to restore its guanylyl cyclase activity independent of NO. We also tested the ability of the sGC stimulator, BAY 41-2272, to induce γ-globinin primary and HUDEP-2 human erythroid progenitor cells. BAY 41-2272, binds to the ferrous iron at the active site of non-oxidized sGC to stimulate guanylyl cyclase activity in a synergistic manner with NO. We compared g-globin mRNA and protein expressionin the primary and immortalized human erythroid progenitors after treatment with different concentrations and combinations of BAY 54-6544, BAY 41-2667 and HU. We also evaluated g-globin induction in cellstreated with the pan-phosphodiesterase inhibitor IBMX and a synthetic cGMP analog. Although we see robust induction of cGMP and activation of PKG with all treatments, we only see significant induction of g-globin expression in the HU treated cells. This data suggests that the induction of HbF occurs through a non-sGC/PKG-dependent signaling pathway. These data demonstrate a very limited induction of γ-globin by BAY 54-6544 and BAY 41-2667 that appears to be disproportionate to, and independent of, cGMP/PKG signaling. These data also demonstrate, for the first time, that HU treatment of the immortalized HUDEP-2 cell line induces γ-globin expression more consistently than in primary erythroid progenitors. Disclosures No relevant conflicts of interest to declare.

Pulmonary hypertension (PH) is a chronic pulmonary complication of sickle cell disease (SCD) and a leading cause of death in adults. The exact molecular causes of SCD-associated PH are unknown although pathologic hemolysis related to chronic nitric oxide depletion and oxidative stress are recognized as contributing factors. We have reported that the protein thrombospondin-1 (TSP1) is elevated in the plasma of patients with SCD and by interacting with its receptor CD47 limits vasodilation of distal pulmonary arteries ex vivo . We hypothesized that the TSP1-CD47 interaction may promote PH in the BERK sickle mouse model. To test this hypothesis, we inquired whether TSP1 and CD47 are upregulated in the lungs of BERK mice and patients with SCD-associated PH. We found that lungs of age matched male 13-14 months old BERK sickle mice overexpressed TSP1 and CD47 as compared to both C57BL/6J and Hemi BERK mice and also confirmed our prior findings that sickle mice develop PH as they age. Immunohistochemistry of lung sections from 6 patients with SCD-associated PH revealed increased levels of CD47 as compared to sections from patients without PH or overt lung disease. We interrogated the TSP1-CD47 axis in vivo by generating chimeric animals with a sickle erythropoiesis on a CD47KO background by transplanting BERK sickle bone marrow into C57BL/6J (n=24) and CD47 knockout (CD47KO, n=27) mice. Fully engrafted chimeric mice underwent pulmonary hemodynamic assessment after 6 months. Right ventricular (RV) pressures were lower in Sickle-to-CD47KO chimeras as compared to Sickle-to-C57BL/6J chimeras as shown by the reduced maximum pressure of the RV (22.1 ± 3.6 vs. 28.1 ± 8.8 mmHg, p=0.013) and mean pulmonary artery pressure (15.3 ± 2.6 vs. 18.8 ± 5.7 mmHg, p=0.020). The afterload of the Sickle-to-CD47KO chimeras was lower compared to Sickle-to-C57BL/6J chimeras as shown by the diminished pulmonary vascular resistance (1.2 ± 0.7 vs. 2.4 ± 2.2 Wood units, p=0.024) and RV effective arterial elastance (1.5 ± 0.9 vs. 2.7 ± 2.6 mmHg/µL, p=0.052). The RV diastolic function as measured by the RV dP/dtmin also showed a trend towards improvement as compared to Sickle-to-C57BL/6J chimeras (-1297.0 ± 318.2 vs. -1604.0 ± 668.2 mmHg/s, p=0.059). Finally, the Sickle-to-CD47KO chimeras had a tendency for less RV hypertrophy as evidenced by lower RV free wall weight as compared to Sickle-to-C57BL/6J chimeras (21.45 ± 5.0 vs. 24.63 ± 6.0 mg, p=0.056). Interestingly, plasma levels of soluble TSP1 were reduced in Sickle-to-CD47KO chimeras (61.45 ± 10.11 vs. 76.89 ± 10.84 pg/mL, p=0.012). On myography, aortic segments from Sickle-to-CD47KO chimeras had improved relaxation to the endothelial activator acetylcholine. We hypothesized that in SCD TSP1-CD47 signaling promotes PH, in part, by increasing ROS generation. Treatment with exogenous TSP1 (2.2x10-9 M, a concentration found in the plasma of SCD patients) stimulated increased levels of superoxide and hydrogen peroxide in human pulmonary artery endothelial cells by cytochrome C and Amplex Red assays, respectively. Furthermore, lungs of CD47KO chimeras had decreased oxidative damage as measured by reduced 4-hydroxynonenal and 3-nitrotyrosine staining vs. Sickle-to-C57BL/6J chimeras. Finally, suppression of CD47 with a morpholino (10 mg/kg body weight i.p. weekly) for 8 weeks (n=8 animals) also reduced pulmonary pressures in BERK mice. Our results show that genetic deletion or suppression of CD47 ameliorates SCD-associated PH, which may be due, in part, to decreased ROS levels. Targeting TSP1-CD47 may provide a new molecular approach to the treatment of SCD-associated PH. Disclosures Isenberg: Tioma Therapeutics: Equity Ownership; Radiation Control Technologies, Inc.: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Increased platelet activation is recognized in patients with sickle cell disease (SCD), but its pathogenesis and clinical relevance remain uncertain. Pulmonary arterial hypertension (PAH), an important complication of SCD, is characterized by a proliferative pulmonary vasculopathy, in situ thrombosis, and vascular dysfunction related to scavenging of nitric oxide (NO) by hemoglobin released into blood plasma during intravascular hemolysis. We investigated links between platelet activation, PAH and NO scavenging in patients with SCD. Platelet activation marked by activated fibrinogen receptor correlated to the severity of PAH (r = 0.58, P < .001) and to laboratory markers of intravascular hemolysis, such as reticulocyte count (r = 0.44, P = .02). In vitro exposure of platelets to pathologically relevant concentrations of cell-free hemoglobin promoted basal- and agonist-stimulated activation and blocked the inhibitory effects on platelet activation by an NO donor. In patients with SCD, administration of sildenafil, a phosphodiesterase-5 inhibitor that potentiates NO-dependent signaling, reduced platelet activation (P = .01). These findings suggest a possible interaction between hemolysis, decreased NO bioavailability, and pathologic platelet activation that might contribute to thrombosis and pulmonary hypertension in SCD, and potentially other disorders of intravascular hemolysis. This supports a role for NO-based therapeutics for SCD vasculopathy. This trial was registered at www.clinicaltrials.gov as no. NCT00352430.

Pulmonary hypertension is a highly prevalent complication of sickle cell disease and is a strong risk factor for early mortality. However, the pathophysiological mechanisms by which sickle hemoglobin leads to pulmonary vasculopathy remain unclear. Transgenic mice provide opportunities for mechanistic studies of vascular pathophysiology in an animal model of severe sickle cell disease. Using micro-cardiac catheterization we found that all mice expressing exclusively human sickle hemoglobin develop pulmonary hypertension. Recognizing that the NO pathways can have complex abnormalities in other conditions of pulmonary hypertension, the NO axis in sickle mice was assessed by multiple methods. From a mechanistic standpoint the mice exhibit profound pulmonary and systemic endothelial dysfunction and vascular instability characterized by diminished responses to authentic nitric oxide (NO), NO donors and endothelium-dependent pulmonary vasodilators, and enhanced responses to vaso-constrictors. However, endothelium-independent vasodilation in the sickle mice was normal. Mechanisms of vasculopathy in sickle mice involve global dysregulation of the NO-axis: impaired constitutive nitric oxide synthase activity with loss of eNOS coupling (dimerization), increased NO scavenging by plasma hemoglobin and superoxide, increased arginase activity, and depleted intravascular nitrite reserves. Consistent with a functional rather than structural defect, light microscopy and computed tomography of the lungs revealed no plexogenic arterial remodeling, thrombi/emboli, or inflammation. Transplanting sickle marrow into wild-type mice conferred the same phenotype. Similar pathobiology was observed in a non-sickle mouse model of acute alloimmune hemolysis, supporting a major role of hemolysis as a mechanism for this dysregulation of NO and vasculopathy. In this study, alloimmune hemolytic mice were chosen for comparison in order to generalize beyond hemoglobinopathies. Future analogous studies with thalassemic mice may be useful to model pulmonary hypertension in human thalassemia intermedia. In conclusion, this animal model extends the evidence for global impairment in NO responsiveness and NO production in sickle cell disease, and suggests that hemolytic anemia is associated with endothelial dysfunction and pulmonary hypertension.

Plasma levels of nitrite ions have been used as an index of nitric oxide synthase (NOS) activity in vivo. Recent data suggest that nitrite is a potential intravascular repository for nitric oxide (NO), bioactivated by a nitrite reductase activity of deoxyhemoglobin. The precise levels and compartmentalization of nitrite within blood and erythrocytes have not been determined. Nitrite levels in whole blood and erythrocytes were determined using reductive chemiluminescence in conjunction with a ferricyanide-based hemoglobin oxidation assay to prevent nitrite destruction. This method yields sensitive and linear measurements of whole blood nitrite over 24 hours at room temperature. Nitrite levels measured in plasma, erythrocytes, and whole blood from 15 healthy volunteers were 121 plus or minus 9, 288 plus or minus 47, and 176 plus or minus 17 nM, indicating a surprisingly high concentration of nitrite within erythrocytes. The majority of nitrite in erythrocytes is located in the cytosol unbound to proteins. In humans, we found a significant artery-to-vein gradient of nitrite in whole blood and erythrocytes. Shear stress and acetylcholine-mediated stimulation of endothelial NOS significantly increased venous nitrite levels. These studies suggest a dynamic intravascular NO metabolism in which endothelial NOS-derived NO is stabilized as nitrite, transported by erythrocytes, and consumed during arterial-to-venous transit. (Blood. 2005;106:734-739)

In sickle cell disease, deoxygenation of intra-erythrocytic hemoglobin S leads to hemoglobin polymerization, erythrocyte rigidity, hemolysis, and microvascular occlusion. Ischemia-reperfusion injury, plasma hemoglobin-mediated nitric oxide consumption, and free radical generation activate systemic inflammatory responses. To characterize the role of circulating leukocytes in sickle cell pathogenesis we performed global transcriptional analysis of blood mononuclear cells from 27 patients in steady-state sickle cell disease (10 patients treated and 17 patients untreated with hydroxyurea) compared with 13 control subjects. We used gender-specific gene expression to validate human microarray experiments. Patients with sickle cell disease demonstrated differential gene expression of 112 genes involved in heme metabolism, cell-cycle regulation, antioxidant and stress responses, inflammation, and angiogenesis. Inducible heme oxygenase-1 and downstream proteins biliverdin reductase and p21, a cyclin-dependent kinase, were up-regulated, potentially contributing to phenotypic heterogeneity and absence of atherosclerosis in patients with sickle cell disease despite endothelial dysfunction and vascular inflammation. Hydroxyurea therapy did not significantly affect leukocyte gene expression, suggesting that such therapy has limited direct anti-inflammatory activity beyond leukoreduction. Global transcriptional analysis of circulating leukocytes highlights the intense oxidant and inflammatory nature of steady-state sickle cell disease and provides insight into the broad compensatory responses to vascular injury.

Two common genetic modifiers, α-thalassemia and the BCL11A rs1427407 T allele, are observed in approximately one-third of patients with sickle cell anemia (SCA) and are associated with reduced hemolysis and higher hemoglobin F (HbF) levels, respectively. We investigated the laboratory and clinical effects of α-thalassemia and the BCL11A rs1427407 T allele in the University of Ibadan cohort of SCA patients and replicated our findings in two independent SCA cohorts, University of Illinois at Chicago (UIC) and Walk-Treatment of Pulmonary Hypertension and Sickle cell disease with Sildenafil Therapy (Walk-PHaSST). Alpha-thalassemia status was determined by PCR in all 3 cohorts while the BCL11A rs1427407 genotype was determined by PCR in the Ibadan and UIC cohorts and imputed in the Walk-PHaSST cohort. Comparisons according to genotype were performed using the linear trend test for continuous variables and Cochran's test of linear trend for categorical variables. Alpha-thalassemia was observed in 43% of 257 SCA patients from Ibadan and was associated with higher body mass index and lower white blood cell count (Table 1). The BCL11A rs1427407 T allele was observed in 46% of SCA patients from Ibadan and was associated with higher hemoglobin concentration. HbF levels by HPLC were available in 25 patients (12 with the BCL11A rs1427407 T allele) enrolled in a study of low-dose hydroxyurea; these levels were higher in patients with at least one rs1427407 T allele at baseline and progressively during therapy with hydroxyurea 10 mg/kg/day (repeated measures P=0.01). We defined a high risk genetic group as the absence of α-thalassemia in combination with absence of the BCL11A rs1427407 T allele. This high risk group was observed in 31% of SCA patients from the Ibadan cohort and was associated with a higher reticulocyte percentage (15.0% vs. 7.8%, P=0.08) and a higher prevalence for a history of stroke (6% vs. 1%, P=0.02). The association with stroke history persisted on logistic regression analysis after adjusting for age, gender, and hydroxyurea therapy (OR 9.4, 95%CI: 1.2-72.8; P=0.03). We then replicated the association of this high risk group with markers of hemolysis and with history of stroke in the UIC and Walk-PHaSST SCA cohorts. In the UIC cohort, the high risk group was observed in 34% (92/271) and was also associated with higher reticulocyte counts (13.5% vs. 11.9%, P=0.10) and higher prevalence for stroke history (33% vs. 22%; age, gender, HU-adjusted OR 1.7, 95%CI: 1.0-3.0; P=0.066). In the Walk-PHaSST cohort, this high risk profile was observed in 38% (149/394) and was associated with a higher reticulocyte percentage (9.7% vs. 8.4%, P=0.0005), lower hemoglobin concentration (8.4 vs. 8.8 g/dL; P=0.017), and higher prevalence for stroke history (15% vs. 6%; age, gender, HU-adjusted OR 2.6, 95%CI: 1.3-5.3; P=0.007). In conclusion, a high risk group of SCA patients, defined by the lack of the protective α-thalassemia and the BCL11A rs1427407 variants, is associated with a higher degree of hemolysis and a higher prevalence of stroke history on cross sectional analysis in three independent cohorts. This high-risk profile may help identify patients to prioritize for hydroxyurea therapy and for closer monitoring strategies for stroke. Disclosures Hsu: Sancilio: Research Funding; Astra Zeneca: Consultancy, Research Funding; Purdue Pharma: Research Funding; Gerson Lehman Group: Consultancy; Eli Lilly: Research Funding; Centers for Medicare and Medicaid Innovation: Research Funding; Hilton Publishing: Consultancy, Research Funding; Mast Therapeutics: Research Funding; EMMI Solutions: Consultancy; Pfizer: Consultancy, Research Funding.

BACKGROUND: Sickle cell disease nephropathy (SCDN) is a common complication of sickle cell disease (SCD) associated with risk for early mortality (Platt et al., 1994; Elmariah et al, 2014). To identify potential genetic risk factors for SCDN, we performed genome-wide association studies (GWAS) for glomerular filtration rate (GFR) in three well-characterized SCD cohorts and performed in vivo functional analysis of one of the candidate genes in zebrafish. METHODS: Three previously described SCD cohorts were utilized in this analysis: Outcome Modifying Genes in Sickle Cell Disease (OMG-SCD) (Elmariah et al, 2014), Pulmonary Hypertension and Sickle Cell Disease with Sildenafil Therapy (Walk-PHaSST) (Machado et al, 2011) and Pulmonary Hypertension and the Hypoxic Response in Sickle Cell Disease (PUSH) (Minniti et al, 2009). Patients less than 16 years old were excluded from PUSH. GFR was estimated using the 'Modification of Diet in Renal Disease' (MDRD) study definition (Levey et al, 1999) and, in the OMG-SCD and Walk-PHaSST cohorts, dichotomized at the clinically relevant threshold of 90 ml/min/1.73m2. 1064 patients with complete data were included in the analysis (Table 1). Genotyping was performed using the Illumina Human610-Quad BeadChip (Illumina, San Diego, CA). Linear regression was utilized to test for association between each SNP and GFR, controlling for genome-wide principle components using PLINK (Purcell et al, 2008). Logistic regression was utilized for the analysis of GFR RESULTS: For the meta-analysis of continuous GFR, the SNP with the most evidence for association was rs7553158 in the TNNI3K-FPGT locus (p= 7.1E-7). For the meta-analysis of GFR DISCUSSION:The absence of mechanistic knowledge about SCDN has led to limited therapeutic options for patients. The present study seeks to identify genetic risk factors for SCDN and represents the largest GWAS of GFR in SCD patients to date. The GWAS was likely still underpowered, however, as none of the findings met FDR significance. Nonetheless, the most significant genes identified are excellent biologic candidates for kidney function. Fucose-1-Phosphate Guanylyltransferase (FPGT) converts GTP and β-l-fucose-1-phosphate to GDP-l-fucose, a process essential in the kidney to reutilize L-fucose from the turnover of glycolipids. It is also highly expressed in the porcine and human kidney (Pastuszak et al, 1998; GTEx, Broad Institute). Xylosyltransferase 1 (XYLT1, also known as XT-1) encodes the enzyme responsible for biosynthesis of heparin sulfate proteoglycans, which affect permeability of the glomerular basement membrane. Furthermore, the c.343G>T polymorphism in XYLT1 has been implicated in diabetic nephropathy (Schön et al, 2006). Here, we have identified putative novel nephropathy loci associated with kidney decline in the largest combined cohort of SCD patients to date. These results further our understanding of the underlying pathology of SCDN and have uncovered genetic markers that could be used for identification of at-risk SCD patients prior to the onset of kidney dysfunction. Disclosures No relevant conflicts of interest to declare.

Background: Vaso-occlusive crisis (VOC) is the primary reason for emergency medical care by sickle cell disease (SCD) patients. SCD patients hospitalized with VOC often develop acute chest syndrome (ACS), a form of acute lung injury, suggesting a role for pulmonary vaso-occlusion in the onset of ACS. However, the cellular, molecular and biophysical mechanism of pulmonary vaso-occlusion is unknown. Methods: SCD transgenic or non-sickle control mice were intravenously (IV) challenged with 2 to 3 ng of bacterial lipopolysaccharide (LPS). Fluorescent anti-mouse Ly-6G and CD49b mAbs were administered IV for in vivo staining of circulating neutrophils and platelets, respectively. Multiphoton excitation enabled quantitative fluorescence intravital lung microscopy (qFILM) was used to determine the molecular mechanism of pulmonary vaso-occlusion in live mice. Function-blocking anti-mouse P-selectin mAb (Fab fragments) was administered IV to assess the role of platelet P-selectin in promoting pulmonary vaso-occlusion. Results: A nanogram dose of IV LPS selectively triggered pulmonary vaso-occlusion in SCD but not control mice. Remarkably, pulmonary vaso-occlusion involved occlusion of the pre-capillary pulmonary arteriole bottle-necks (junction of an arteriole and capillaries) by large neutrophil-platelet embolic aggregates. IV administration of Fab fragments of function blocking anti-P-selectin mAb led to the resolution of pulmonary vaso-occlusion, which was primarily mediated by the attenuation of large neutrophil-platelet aggregates into smaller aggregates that are not stopped by the arteriolar bottle-necks. Conclusion: These results establish the relevance of neutrophil-platelet aggregation in pulmonary arterioles in promoting pulmonary vaso-occlusion in SCD and also highlight the therapeutic potential of inhibiting platelet P-selectin to prevent ACS in SCD patients hospitalized with VOC. Acknowledgments: This study was supported by 1R01HL128297-01 (P.S.), AHA 11SDG7340005 (P.S.), VMI startup funds (P.S.). M.F.B. was supported by NIH-NHLBI training grant T32HL110849 and NIH-NHLBI F32 NRSA 1F32HL131216-01. Disclosures No relevant conflicts of interest to declare.

Introduction: Acute vaso-occlusive crisis (VOC) is the hallmark clinical complication of sickle cell disease (SCD). VOC involves several mechanisms, including rigid erythrocytes, adhesive blood cells, activated coagulation, activated and dysregulated endothelium, inflammation, and ischemia-reperfusion injury. Surprisingly few data document clinical and laboratory markers that change from steady state to VOC and during the hospital stay, and that predict subsequent rehospitalization. We characterized such markers in this study. Methods: Data were collected as part of a clinical trial of nitric oxide as potential treatment for VOC (DeNOVO). Participants, age ≥ 10 years requiring hospitalization for VOC, were recruited from 11 academic medical centers at the time of VOC. Length of hospital stay (LOS), change in visual analogue scale (VAS) pain score, and hospital readmission within 30 days were study outcomes. Steady state laboratory values were available in a subgroup of patients for comparison. Differences between admission and steady state laboratory measures and changes in clinical and laboratory parameters from hospital admission to discharge were analyzed by Wilcoxon signed-rank test. Multivariate linear and logistic regression analyses were used to identify the independent predictors of LOS and 30-day readmission risk. Results: 150 recruited patients had a median (interquartile range; IQR) age of 24 (17-33) years; 50% were female, and 91% had HbSS genotype. The median VAS score at time of admission was 7.7 cm which subsided to 3.3 cm at time of discharge (P Conclusion: VOC is associated with neutrophilia and increased LDH, consistent with inflammation and hemolysis and/or tissue injury, although of insufficient magnitude to be used as diagnostic criteria. The magnitude of pain and marker of apparent bone involvement predict longer LOS. SCD patients with VOC carry a high burden of readmission risk, which is characterized by higher opioid utilization and earlier discharge from the index hospitalization. These concepts can shape models, especially to reduce readmission rate, to be evaluated in prospective clinical trials. *First two authors contributed equally to this work. Figure Figure. Table Table. Disclosures Lanzkron: NKT: Research Funding; Selexys: Research Funding; Prolong: Research Funding; PCORI: Research Funding; NHLBI: Research Funding; HRSA: Research Funding; GBT: Consultancy; Pfizer: Research Funding.

To the editor: We are honored that our work merits the attention of Dr Bunn and colleagues[1][1] but disagree with some of their conclusions. Traditional risk factors for vaso-occlusive pain crisis, such as leukocytosis and high hemoglobin levels, incompletely predict vasculopathic events and

In this issue of Blood, 2 articles test fundamental hypotheses relating intravascular hemolysis to sickle cell disease (SCD) pathogenesis. Detterich et al confirm a role for hemolysis and cell-free plasma hemoglobin (Hb) in pulmonary and systemic endothelial dysfunction in humans. Almeida et al show in mice that hemolysis induces inflammation that is caused by nitric oxide (NO) scavenging and ameliorated by NO donors and the NO-donor properties of hydroxyurea (HU).

Introduction: Sickle cell disease (SCD) is an autosomal recessive genetic disorder that affects ~100,000 Americans and millions of people worldwide. The acute chest syndrome (ACS), a form of acute lung injury, is a major cause of morbidity among SCD patients. The current treatment for ACS is primarily supportive and the molecular mechanism remains largely unknown. SCD patients hospitalized with vaso-occlusive pain crisis (VOC) often develop ACS within the ensuing days, suggesting a role for pulmonary vaso-occlusion in the onset of ACS. However, the cellular and molecular mechanism and the anatomical site of pulmonary vaso-occlusion are still elusive. Materials and Methods: Intravenous (IV) bacterial lipopolysaccharide (LPS) was used to induce VOC in SCD mice. Intravital multiphoton excitation (MPE) fluorescence microscopy was used to study the blood cell trafficking within the pulmonary microcirculation of live SCD or control mice. Fluorochrome-conjugated anti-mouse Ly-6G, Ter-119, and CD49b antibodies were administered IV for in vivo staining of circulating neutrophils, red blood cells and platelets, respectively. Cellular trafficking was recorded at baseline and 2 hours after IV challenge with LPS. Image sequences were analyzed to identify vaso-occlusion, which was defined as cellular aggregation and stasis of blood flow within the pulmonary blood vessels. Results and Discussion: Preliminary data using MPE imaging in transgenic SCD mice revealed that vaso-occlusion was absent at baseline in unchallenged SCD mice and the cellular trafficking within the pulmonary microcirculation was comparable in SCD and control mice. Doses of IV LPS (0.01-5 mg/kg of body weight), which were innocuous to control mice were found to be lethal to SCD mice. Remarkably, MPE imaging of the lung microcirculation revealed that IV LPS led to microembolism of the pre-capillary pulmonary arterioles by platelet-neutrophil aggregates in SCD but not control mice. The microembolism involved either entrapment of circulating platelet-neutrophil aggregates or in situ aggregation through sequential steps of neutrophil arrest on the arteriolar endothelium, followed by platelet nucleation on arrested neutrophils and microthrombus formation. Conclusions: Initial findings demonstrate that pulmonary vaso-occlusion in SCD mice involves microembolism of the pre-capillary pulmonary arterioles by platelet-neutrophil aggregates. Future studies will determine the molecular interactions responsible for pulmonary arteriolar microembolism. Acknowledgments: This study is supported by the 11SDG7340005 from the American Heart Association (P.S.) and the VMI startup account (P.S.). M.F.B is supported by the NIH NHLBI VMI T32 training grant T32HL110849. Disclosures No relevant conflicts of interest to declare.

Background The level of distorted erythrocytes due to polymerization of hemoglobin S in sickle cell disease (SCD) (Science 1949;110:543) is a major determinant of the severity of hemolysis and microvascular occlusion (Lancet 2010;376:2018). Erythropoietin (EPO) is elevated in SCD due to hemolytic anemia and a related increase in hypoxia-inducible factors (HIFs) (Eur J Haematol 2007;78:183). Hydroxyurea (HU) is widely used in the treatment of SCD. HU inhibits ribonucleotide reductase (Semin Oncol 1992;19(3 Suppl 9):1-10) and promotes γ globin synthesis thereby increasing HbF-containing erythrocytes (F cells) while suppressing sickle β hemoglobin production (J Clin Invest 1984;74:652 and 2003;111:231). Increased level of F cells reduces hemolysis and ameliorates clinical complications in SCD. We and others have observed an increase in serum EPO level with HU treatment in SCD despite an increase in the hemoglobin concentration, and we hypothesized that this may be due to the known increased affinity of hemoglobin F for oxygen and related tissue hypoxia (Blood 2009;114:4639). Methods Messenger RNA from peripheral blood mononuclear cells (PBMCs) was profiled using Affymetrix Human Exon 1.0 ST Array. Hypoxic transcriptional alteration was defined in 15 Chuvash polycythemia (CP) patients vs. 17 control individuals. CP leads to constitutive up-regulation of HIFs in the absence of anemia or hypoxia. Transcriptional alteration in SCD was determined in 13 HbSS subjects without HU treatment vs. 16 control individuals, and that induced by HU treatment was determined in 19 HbSS subjects with vs. 13 without HU treatment. For meta-analysis on serum EPO concentration, genomic DNA isolated from PBMCs was hybridized to the Illumina Human 610-Quad SNP array. Genotypes were imputed to 1000 genomes project phase 1 data. A linear regression model was applied adjusting for age, gender, hemoglobin concentration, and HU treatment. Results Gene expression changes by HbSS highly correlated with those associated with homozygous VHLR200W (Pearson's r=0.79, Figure 1A). At 5% false discovery rate (FDR), expression levels of 377 genes were altered in both VHLR200W homozygotes and HbSS by >1.2 fold. For these hypoxic genes, the correlation of expression changes between HbSS and homozygous VHLR200W reached r=0.97 (Figure 1B). In contrast to our hypothesis, HU treatment in general suppressed expression changes induced by HbSS (r=-0.85, Figure 1C), especially for the hypoxic genes (r=-0.95, Figure 1D). In VHLR200W homozygotes, 62 of the hypoxic genes correlated with plasma EPO levels (adjusted P Discussion Our study demonstrates a prominent release from hypoxic transcriptional responses by HU treatment in SCD despite an increase in serum EPO, a defining characteristic of an up-regulated hypoxic response. Our study hypothesizes that hypoxia-independent signals trigger EPO production in the setting of HU therapy and it identifies a potential genetic determinant in this alternative pathway. Figure 1. Figure 1. Disclosures No relevant conflicts of interest to declare.

Sickle cell disease (SCD) is characterized by recurring episodes of vascular occlusion in which neutrophil activation plays a major role. The disease is associated with chronic hemolysis with elevated cell-free hemoglobin and heme. The ensuing depletion of heme scavenger proteins leads to nonspecific heme uptake and heme-catalyzed generation of reactive oxygen species. Here, we have identified a novel role for heme in the induction of neutrophil extracellular trap (NET) formation in SCD. NETs are decondensed chromatin decorated by granular enzymes and are released by activated neutrophils. In humanized SCD mice, we have detected NETs in the lungs and soluble NET components in plasma. The presence of NETs was associated with hypothermia and death of these mice, which could be prevented and delayed, respectively, by dismantling NETs with DNase I treatment. We have identified heme as the plasma factor that stimulates neutrophils to release NETs in vitro and in vivo. Increasing or decreasing plasma heme concentrations can induce or prevent, respectively, in vivo NET formation, indicating that heme plays a crucial role in stimulating NET release in SCD. Our results thus suggest that NETs significantly contribute to SCD pathogenesis and can serve as a therapeutic target for treating SCD.

Hemoglobin and myoglobin are among the most extensively studied proteins, and nitrite is one of the most studied small molecules. Recently, multiple physiologic studies have surprisingly revealed that nitrite represents a biologic reservoir of NO that can regulate hypoxic vasodilation, cellular respiration, and signaling. These studies suggest a vital role for deoxyhemoglobin- and deoxymyoglobin-dependent nitrite reduction. Biophysical and chemical analysis of the nitrite-deoxyhemoglobin reaction has revealed unexpected chemistries between nitrite and deoxyhemoglobin that may contribute to and facilitate hypoxic NO generation and signaling. The first is that hemoglobin is an allosterically regulated nitrite reductase, such that oxygen binding increases the rate of nitrite conversion to NO, a process termed R-state catalysis. The second chemical property is oxidative denitrosylation, a process by which the NO formed in the deoxyhemoglobin-nitrite reaction that binds to other deoxyhemes can be released due to heme oxidation, releasing free NO. Third, the reaction undergoes a nitrite reductase/anhydrase redox cycle that catalyzes the anaerobic conversion of 2 molecules of nitrite into dinitrogen trioxide (N2O3), an uncharged molecule that may be exported from the erythrocyte. We will review these reactions in the biologic framework of hypoxic signaling in blood and the heart.

Modeling the complexity of sickle cell disease pathophysiology and severity is difficult. Using data from 3380 patients accounting for all common genotypes of sickle cell disease, Bayesian network modeling of 25 clinical events and laboratory tests was used to estimate sickle cell disease severity, which was represented as a score predicting the risk of death within 5 years. The reliability of the model was supported by analysis of 2 independent patient groups. In 1 group, the severity score was related to disease severity based on the opinion of expert clinicians. In the other group, the severity score was related to the presence and severity of pulmonary hypertension and the risk of death. Along with previously known risk factors for mortality, like renal insufficiency and leukocytosis, the network identified laboratory markers of the severity of hemolytic anemia and its associated clinical events as contributing risk factors. This model can be used to compute a personalized disease severity score allowing therapeutic decisions to be made according to the prognosis. The severity score could serve as an estimate of overall disease severity in genotype-phenotype association studies, and the model provides an additional method to study the complex pathophysiology of sickle cell disease.

Background : Acute chest syndrome (ACS) is a major cause of morbidity and mortality in patients with sickle cell disease (SCD). The elucidation of genetic modifiers and mechanisms responsible for the phenotypic variability in ACS and SCD would have important clinical implications such as the identification of individuals at high risk for ACS complications and the potential for use of individualized preventive and therapeutic strategies that could ultimately improve outcomes for patients with SCD. We hypothesize that the development of ACS is significantly influenced by ancestry. Methods : Autosomal SNPs were genotyped with the Affymetrix PanAFR array in a University of Illinois cohort and with the Illumina 6.0 platform in the NIH Walk-PHaSST cohort. We merged the two GWAS datasets and there were 100,079 SNPs available after removing SNPs with > 2 alleles and Hardy-Weinberg Equilibrium (HWE) P 0.2, the total number of SNPs available for ancestry estimates was 354. Genetic ancestry was estimated using HapMap YRI and CEU genotypes as ancestral population ( K =2) using STRUCTUR 2.1. We ran STRUCTURE under the admixture model using prior population information and assuming allele frequencies among populations, with a burn-in length of 30,000 for 70,000 repetitions. Logistic regression was used to examine the association between European ancestry (EA) and Acute Chest Syndrome. Age, gender, hemoglobin genotype (HbSS/HbSβ0 vs. others), and use of hydroxyurea were additionally considered during modeling. Results: Seven hundred and thirty patients (UIC= 233, Walk-PHaSST = 497) were included in the analysis (Table 1). In binary logistic regression analysis adjusting for age, gender, Hb genotypes (severe vs. mild) and hydroxyurea use, EA was associated with and increase in ACS risk in both cohorts and when the cohorts where combined (UIC P=0.02, Walk-PHaSST P=0.001, combined P

Introduction: Sickle Cell Disease (SCD) is an autosomal-recessive-hemolytic disorder caused by a single point mutation in the β-globin gene that leads to sickling of RBCs under deoxygenated condition. Sickle RBCs (sRBCs) are not only rigid but also express adhesion molecules, which are not normally expressed on RBCs. The sticky and rigid sRBCs are believed to get trapped in blood vessels along with leukocytes to cause vaso-occlusion, which is the predominant pathophysiology underlying acute pain crisis in SCD patients. The process of sickling and vaso-occlusion leads to sRBC hemolysis, which releases hemoglobin, ADP and other RBC contents into the blood giving rise to a pro-inflammatory and pro-coagulant state, characterized by activated leukocytes, platelets, endothelial cells (ECs), tissue factor (TF) and enhanced adhesion of these cells to each other. Leukocyte–endothelium adhesion starts with leukocyte rolling mediated by P-selectin-glycoprotein-ligand (PSGL)-1 on leukocytes binding to P-selectin on endothelium. Rolling is followed by firm arrest, which is mediated by activated β­2-integrins (LFA-1 and Mac-1) on the leukocytes binding to inter-cellular-adhesion-molecule (ICAM)-1 on endothelium. Although neutrophils have been shown to play a role in the onset of vaso-occlusion by interacting with sRBCs and platelets in cremaster venules of SCD mice; the cellular, molecular and biophysical mechanisms that enable vaso-occlusion in SCD patients are not known. Materials and Methods: Freshly collected heparinized blood from SCD patients and race matched control subjects was perfused through a polydimethylsiloxane (PDMS) based microfluidic flow chamber with a glass bottom coated with either human micro-vascular endothelial cells or a cocktail of recombinant human P-selection, ICAM-1 and IL-8 at a venular/arteriolar wall shear stress. Fluorochrome conjugated Abs against CD16, CD235a and CD49b were added to the blood to stain neutrophils, sRBCs and platelets, respectively, and cellular interactions were recorded using multi-color Quantitative Dynamic Footprinting (qDF; Sundd et al Nature Methods 2010) or epifluorescence microscopy. Specificity of cellular interactions was tested using function blocking Abs against human Mac-1, LFA-1, P-selectin and PSGL-1. Results: SCD patients had much higher number of circulating neutrophils than control patients. Neutrophils rolled, arrested and then captured free flowing platelets in both SCD and control blood. However, significantly larger number of neutrophils rolled and arrested in SCD blood than control blood. As a result, much higher number of platelets was captured by arrested neutrophils in SCD blood than control blood, which led to the formation of neutrophil-platelet micro-emboli. The micro-emboli formation was mediated by a unique biophysical mechanism, which involved PSGL-1 and Mac-1 on neutrophils binding to P-selectin and GPIbα on platelets, respectively. Conclusion: Vaso-occlusion involves a cascade of adhesive events. First, neutrophils roll and arrest at the site of vaso-occlusion. Second, arrested neutrophils capture free flowing platelets and RBCs to form micro-emboli. Third, eventually these micro-emboli give rise to micro-thrombi, which cause stasis of blood flow. Acknowledgments: This study is supported by 11SDG7340005 from the American Heart Association (P.S.), VMI start-up funds (P.S.) and CBTP-T32 fellowship HL076124 (M.J). Disclosures No relevant conflicts of interest to declare.

We recently reported that hemoglobinuria is associated with chronic kidney disease (CKD) stage and progression in sickle cell disease (SCD) (Saraf et al. BJH 2014). To further investigate a potential role of cell-free hemoglobin in SCD nephropathy, we measured urinary concentrations of kidney injury molecule-1 (KIM-1), a biomarker of tubular injury, and nephrin, a marker of glomerular injury, in 32 University of Illinois at Chicago (UIC) patients. Urine KIM-1 concentration directly correlated with increasing urine cell-free hemoglobin concentration (P = 0.003) (Figure 1) but urine nephrin concentration did not. To determine biological responses of tubular cells to cell-free hemoglobin, we added lyophilized hemoglobin to cultured human kidney-2 (HK2) tubular cells. Supernatant KIM-1 concentrations increased progressively with increasing cell-free hemoglobin exposure (Figure 2) while total cell number and cell viability were stable. Using a fluorescein-labeled hemoglobin assay, we found that cell-free hemoglobin bound to HK2 cells in a competitive manner. To determine whether enzymes for metabolizing hemoglobin and protecting from reactive oxygen species are affected by exposure of HK2 cells to cell-free hemoglobin, we evaluated candidate gene expression using rt-PCR. The relative expression of heme oxygenase-1 (HMOX1) (Figure 3) progressively increased with increasing cell-free hemoglobin dose while expression of superoxide dismutase 1 and 2, catalase, glutathione reductase, and glutathione synthetase did not change. We then focused on variants of HMOX1 in a cohort of 247 UIC SCD patients. Genotyping was carried out using Affymetrix Axiom genome-wide Pan-African array in DNA isolated from peripheral blood mononuclear cells. Examination of 11 tag SNPs within +/-10 kb of HMOX1 identified a SNP in the promoter region of HMOX1 (rs743811, minor allele frequency 0.14) that had a significant association with CKD stage (β = -1.0, P = 0.00032) and non-significant associations of the same direction with end stage renal disease (ESRD) (β = -0.9, P = 0.2) and hemoglobinuria (β = -0.2, P = 0.6). Validation studies were conducted in 517 SCD patients from the Walk-Treatment of Pulmonary Hypertension and Sickle Cell Disease with Sildenafil Therapy (Walk-PHaSST) cohort. Genotyping was performed using the Illumina Human 610-Quad SNP array and imputed using the HapMap II reference panels for HMOX1 tag SNPs. HMOX1 rs743811 had a significant associated with ESRD (β = -1.6, P = 0.014) and non-significant associations of a similar direction with CKD stage (β = -0.2, P = 0.3) and hemoglobinuria (β = -0.3, P = 0.3). Homozygosity or compound heterozygosity for the G1 and G2 variants of APOL1, encoding apolipoprotein 1, have been implicated in CKD in African Americans with and without SCD (Ashley-Koch et al. BJH 2011), and we therefore examined their association with hemoglobinuria-associated CKD in the UIC cohort. The S342G and I384M substitutions are in almost complete linkage disequilibrium and are termed G1; the deletion of two amino acids, N388 and Y389, is termed G2. The G2 variant was further imputed using reference panels of the 1000 Genomes Project. Homozygosity or compound heterozygosity of the G1/G2 variant was associated with hemoglobinuria (β = 2.0, P = 0.00018), CKD stage (β = 1.0, P = 0.022), and ESRD (β = 1.9, P = 0.036). For validation studies in Walk-PHaSST, the G1 and G2 variants of APOL1 were imputed using the 1000 Genomes Project. Homozygosity or compound heterozygosity of G1/G2 in the Walk-PHaSST cohort was associated with hemoglobinuria (β = 0.7, P = 0.021), but associations of a similar direction with CKD stage (β = 0.2, P = 0.6) and ESRD (β = 1.1, P = 0.3) were not statistically significant. Our findings are consistent with the possibility that cell-free hemoglobin contributes to sickle cell nephropathy through tubular injury. A SNP in the promoter region of HMOX1 is associated with CKD stage in the UIC cohort and ESRD in the Walk-PHaSST cohort, raising the possibility that altered HMOX1 expression can have a role in SCD-associated CKD. Our results also point to a novel association of the G1/G2 variants of APOL1 with cell-free hemoglobin-mediated CKD in SCD subjects. Future studies to explore the potential roles of HMOX1 and APOL1 in cell-free hemoglobin-associated sickle cell nephropathy are warranted. Figure 1: Figure 1:. Figure 2: Figure 2:. Figure 3: Figure 3:. Disclosures No relevant conflicts of interest to declare.

Introduction: Sickle cell disease (SCD) affects ~100,000 Americans and millions worldwide. SCD is the most commonly inherited hemolytic disorder that leads to red blood cell (RBC) sickling, hemolysis and presentation of adhesion molecules on sickle RBCs. Chronic hemolysis renders SCD as a hyper-inflammatory and hypercoagulable state, which is characterized by constitutive activation of circulating leukocytes, platelets and endothelial cells, and thrombin generation. Sepsis in SCD patients is a risk factor for acute chest syndrome (ACS), which is a form of lung injury and one of the leading causes of morbidity and mortality among SCD patients. Autopsies of ACS patients reveal the presence of lung infarction and congestion, which supports a role for pulmonary vaso-occlusion in ACS. However, the cellular and molecular events that enable pulmonary vaso-occlusion are not known. Methods: Intravital two-photon excitation (TPE) video microscopy was used to study myeloid cell trafficking within the pulmonary microcirculation of live BERK SCD or non-sickle control mice. Mice were anesthetized and mechanically ventilated, and a thoracotomy was performed to expose the left lung. The lower zone of the left lung was stabilized using a vacuum-enabled imaging window and observations were made using TPE microscopy. Arterial blood pressure, heart rate, and blood oxygen saturation were continuously monitored. Fluorchrome-conjugated anti-mouse Ly-6G, Ter-119, and CD49b antibodies were administered intravascularly (iv) to label circulating neutrophils, RBCs and platelets, respectively. Cellular trafficking was first recorded at baseline and then at different time points following iv challenge with bacterial lipopolysaccharide (LPS). Vaso-occlusion was defined as cellular aggregation and stasis of blood flow within the lumen of pulmonary capillaries and arterioles. Results: At baseline, vaso-occlusion was absent and cellular trafficking within the pulmonary microcirculation was comparable between the BERK SCD and control mice. RBC trafficking through pulmonary arterioles and capillaries was rapid. In contrast, neutrophil transit was a combination of fast moving cells that hopped from one capillary junction to the next and slowly transiting neutrophils. Remarkably, our initial findings revealed that iv LPS challenge induced nucleation of neutrophil-platelet-RBC micro-emboli in the pulmonary arteriole of BERK SCD mice, which eventually led to formation of micro-thrombi, complete occlusion of the pulmonary arteriole and death. Conclusions: Initial findings indicate that neutrophil-platelet-RBC interactions are crucial in initiating pulmonary vaso-occlusion and ACS. Future studies will be aimed at determining the cellular and molecular events responsible for pulmonary arteriolar micro-embolism. This study is supported by the 11SDG7340005 from the American Heart Association (P.S.) and the VMI startup account (P.S.). M.F.B is supported by the NIH Pulmonary T32 training grant 2T32HL007563-26. Disclosures No relevant conflicts of interest to declare.

To the editor: Fetal hemoglobin (HbF) protects against many but not all of the hematologic and clinical complications of sickle cell anemia.[1][1],[2][2] This protection is dependent on the ability of HbF to hinder deoxyHbS polymerization. HbF level is variable and highly heritable. Previous