Tissue Factor Procoagulant Activity and Coagulation Activation In Sickle Cell Disease: Relationship with Biomarkers of Hemolysis and Inflammation

Abstract 4210

Several lines of evidence suggest that hemostasis is activated in patients with chronic hemolytic anemias, including sickle cell disease (SCD), with documented laboratory evidence for increased thrombin generation, fibrin formation, and elevated circulating levels of tissue factor (TF). TF, a cell surface receptor for factor-VII/VIIa, is the physiologic cellular activator of hemostasis. Previous studies have identified a circulating pool of TF that is associated with both hematopoietic cells and microparticles. We investigated: (i) whether cell-associated or microparticle-associated TF contributes to thrombin generation in SCD; and (ii) whether any of the SCD-related pathobiologic processes such as hemolysis, inflammation and/or endothelial activation contribute to hemostatic perturbations. The study cohort included 40 subjects with SCD in steady state (ages 2 to 21 years), 25 with SS and 15 with the SC genotype. Whole blood TF (WBTF) and microparticle-associated TF (MPTF) were measured using a two-stage functional clotting assay. Besides the early stage markers (WBTF and MPTF), we also assessed end-stage markers of thrombin generation and fibrin formation [thrombin-antithrombin (TAT) complexes, and D-dimer]. Surrogate biomarkers of hemolysis [lactate dehydrogenase (LDH), reticulocyte count, hemoglobin, and % phosphatidylserine-positive-RBCs]; inflammation [high sensitivity C-reactive protein (hs-CRP), white blood cell (WBC), neutrophil and monocyte counts]; and endothelial activation (soluble VCAM-1, E-selectin, and P-selectin) were assessed. Data were analyzed using Spearman correlation and multiple regression models. While correlations were observed in univariate analyses between WBTF and markers of hemolysis, inflammation, and endothelial activation (p<0.01 to <0.00001), the hemolytic marker LDH was the only biomarker that stayed in the regression models (significant contribution of 42%). In similar regression analysis with TAT as the dependent variable, the only biomarkers that stayed in the models were WBC and hs-CRP contributing 29% and 9%, respectively to the overall association. Using D-dimer as the dependent variable, reticulocyte count was the only independent variable that stayed in the regression models (43% contribution). Significant correlations were noted between early and late stage hemostatic markers, i.e. between WBTF and TAT (p<0.05), between WBTF and D-dimer (p=0.005) and between TAT and D-dimer (p=0.006). There were no correlations between MPTF and the other markers of hemostatic activation or with the markers of hemolysis, inflammation or endothelial activation. In addition, we noted that fetal hemoglobin did not provide any protection against hemostatic activation. A previous study from our laboratory has demonstrated that heme, a product of intravascular hemolysis, upregulated endothelial expression of TF mRNA, protein and procoagulant activity in vitro (J Thromb Haemost 2008; 6:2202). Our present findings provide further evidence that intravascular hemolysis (LDH and reticulocyte count), and inflammation (WBC and hs-CRP) play a crucial role in SCD-related hemostatic activation. In addition, our results also suggest that cell-associated TF, rather than microparticle-associated TF, contributes to laboratory evidence of hemostatic activation in subjects with SCD.