Sickle cell disease (SCD) is associated with chronic activation of coagulation and an increased risk of venous thromboembolism. Erythrocyte sickling, the primary pathologic event in SCD, results in dramatic morphological changes in red blood cells (RBCs) because of polymerization of the abnormal hemoglobin. We used a mouse model of SCD and blood samples from sickle patients to determine if these changes affect the structure, properties, and dynamics of sickle clot formation. Sickling of RBCs and a significant increase in fibrin deposition were observed in venous thrombi formed in sickle mice. During ex vivo clot contraction, the number of RBCs extruded from sickle whole blood clots was significantly reduced compared with the number released from sickle cell trait and nonsickle clots in both mice and humans. Entrapment of sickled RBCs was largely factor XIIIa–independent and entirely mediated by the platelet-free cellular fraction of sickle blood. Inhibition of phosphatidylserine, but not administration of antisickling compounds, increased the number of RBCs released from sickle clots. Interestingly, whole blood, but not plasma clots from SCD patients, was more resistant to fibrinolysis, indicating that the cellular fraction of blood mediates resistance to tissue plasminogen activator. Sickle trait whole blood clots demonstrated an intermediate phenotype in response to tissue plasminogen activator. RBC exchange in SCD patients had a long-lasting effect on normalizing whole blood clot contraction. Furthermore, RBC exchange transiently reversed resistance of whole blood sickle clots to fibrinolysis, in part by decreasing platelet-derived PAI-1. These properties of sickle clots may explain the increased risk of venous thromboembolism observed in SCD.
Recent studies have demonstrated a much higher incidence of venous thromboembolism (VTE) among Asian patients compared with previous studies. This study aims to determine dietary and behavioral factors that may have contributed to this increase. A case-control study was conducted. Cases were objectively confirmed VTE between 2006 and 2009 at King Chulalongkorn Memorial Hospital. Patients with underlying cancer, antiphospholipid syndrome and arterial thrombosis were excluded. Controls were age and sex-matched healthy volunteers. Food consumption was assessed using a food frequency questionnaire modified from the Thailand National Health Examination Survey III previously validated in the Thai population. There were 97 cases and 195 controls. The mean age was 54.6 years and 70% were women. VTE patients consumed significantly less vegetable, fish and spicy food compared with normal individuals with an odds ratio (OR) for venous thrombosis of 3.74 [95% confidence interval (CI) 2.24-6.26, P < 0.001], 2.05 (95% CI 1.24-3.41, P = 0.005) and 2.30 (95% CI 1.29-4.11, P = 0.01), respectively. Additionally, thrombosis was associated with overweight (OR 2.1, 95% CI 1.21-3.62, P = 0.002), obesity (OR 3.1, 95% CI 1.46-6.74, P = 0.001) and estrogen uses (OR 3.7, 95% CI 1.05-13.2, P = 0.02), but not with smoking or lack of exercise. A multivariate analysis showed that low vegetable consumption (OR 3.74, 95% CI 1.85-7.55, P < 0.001), female hormones (OR 5.80, 95% CI 1.51-22.22, P = 0.011) and body mass index (BMI, P = 0.048) were independently associated with VTE. Low vegetable intake, hormonal use and high BMI are the risk factors for noncancer-related VTE in Thai population.
In sickle cell disease (SCD), a mutation of the β-globin gene leads to abnormal polymerization of hemoglobin, resulting in formation of sickled red blood cells, hemolytic anemia, and vaso-occlusive crisis (VOC). These primary events produce clinical complications and trigger additional multiple pathologies driven by chronic oxidative stress, sterile inflammation, and activation of coagulation. Heparins, highly sulfated forms of heparan sulfate (HS), are a group of polysaccharide compounds with great variance in structure. In addition to their anticoagulant effect, heparins have anti-adhesive and anti-inflammatory properties. These are determined by both sulfation pattern and length of the polysaccharide chain, which influence the ability to bind HMGB-1, histones, and P-selectin (Psel). This heterogeneity together with the short half-life and dosing regimen based on anticoagulant activity, limit the use of heparins as anti-adhesive and anti-inflammatory agents. To overcome these limitations, we previously developed a chemoenzymatic approach to synthesize a structurally defined HS oligosaccharides and demonstrated their ability to reduce sterile inflammation in animal models by binding HMGB-1 and histones. In the present study, we investigated the compound's anti-Psel properties in vitro and in a mouse model of SCD. First, using a Psel inhibition ELISA assay, we determined that a heptadecasaccharide (17-mer) is the minimum polysaccharide chain length required for inhibition of Psel binding to Sialyl Lewis X polyacrylamide. Based on these data, we synthesized three 18-mer compounds with a different sulfation position on each monosaccharide ring (NS2S, NS6S and NS2S6S). To obtain 18-mers with no anticoagulant activity, we omitted 3-O-sulfation of glucosamine, which is important for binding antithrombin III (confirmed by anti-FXa activity assay). In the Psel inhibition ELISA assay, all compounds demonstrated dose dependent (0.1 - 1000 µg/mL) anti-Psel activity comparable to that observed for low molecular weight heparin (LMWH). Psel is a key molecule mediating VOC by promoting formation of multicellular aggregates. Therefore, we evaluated the effect of 18-mers on Psel-mediated platelet/leukocyte aggregates (PLA) formation ex vivo. Leukocytes and platelets isolated from healthy donors were stimulated with PMA (100 nM) for 1 hour or thrombin (5 µg/mL) for 30 minutes, respectively, then incubated with vehicle, 18-mer compounds, or LMWH (0.5, 5, 50 and 500 µg/mL) for 15 minutes. After incubation, cells were combined to allow PLA formation for 15 minutes and analyzed by flow cytometry. At the highest tested 18-mer concentration, all compounds attenuated PLA formation. However only NS2S6S, the most highly sulfated compound, showed significant inhibition at all concentrations. NS2S6S decreased PLA formation to 82.8% 90.8%, 76.3% and 68.3%, lowest to highest concentrations respectively (p<0.01 for all concentrations versus vehicle). LMWH demonstrated significant decreases only at the two highest concentrations (82.1% and 63.8%, p<0.001). The number of circulating PLA was increased in sickle Townes HbSS mice by 6.1-fold (p<0.01) compared to non-sickle Townes HbAA controls. In ex vivo experiments, addition of NS2S6S (1 mg/ml) to the HbSS blood decreased PLA formation to 66.4% (p=0.03) compared to untreated HbSS blood. Finally, we determined the effect of NS2S6S on heme-induced microvascular stasis in Townes HbSS mice. Sickle mice were implanted with a dorsal skinfold chamber to visualize dermal microvessels. PBS or NS2S6S (3 mg/kg, s.c.) were injected 15 min before infusion of heme (1.2 µmol/kg, iv). 0ne, 2, 3 and 4 hours after heme infusion, microvascular stasis was observed in 31.5, 20.5, 18.9 and 14.2% of preselected vessels in PBS treated sickle mice, and NS2S6S treatment reduced that numbers to 10.9, 6.2, 3.1 and 3.2%, respectively (p<0.01 for all time points). In summary, we showed that NS2S6S prevents Psel dependent formation of PLA ex vivo and reduces heme-induced stasis in sickle mice. Together with previously described anti-HMGB1 and anti-histone effects, this compound is a good candidate for multi-modal therapy to mitigate the pathophysiology of SCD. However, like LMWH, NS2S6S has a short half-life which makes prophylactic treatment of SCD patients impractical. Studies to extend the half-life of HS are currently ongoing in our group. Disclosures Xu: Glycan Therapeutics: Current Employment. Belcher: Mitobridge/Astellas: Consultancy, Research Funding; CSL Behring: Research Funding. Vercellotti: CSL Behring: Research Funding; Mitobridge, an Astellas Company: Consultancy, Research Funding. Liu: Glycan Therapeutics: Current Employment.
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