Thrombin generation and cell-dependent hypercoagulability in sickle cell disease

Evaluating thromboprophylaxis in the sickle cell disease population: Navigating the evidence gap

Sickle cell disease (SCD) arises from beta-globin gene mutations, with global estimates indicating around 500 000 affected neonates in 2021. In the United States, it is considered rare, impacting fewer than 200 000 individuals. The key pathogenic flaw lies in mutant haemoglobin S, prone to polymerization under low oxygen conditions, causing erythrocytes to adopt a sickled shape. This leads to complications like vascular occlusion, haemolytic anaemia, inflammation and organ damage. Beyond erythrocyte abnormalities however, there is a body of literature highlighting the hypercoagulable state that is likely a contributor to many of the complications we see in SCD. The persistent activation of the coagulation cascade results in thromboembolic events, notably venous thromboembolism (VTE) which is independently associated with increased mortality in both adults and children with SCD. While the increased risk of VTE in the SCD population seems well established, there is a lack of guidelines for thromboprophylaxis in this population. This Wider Perspective will describe the hypercoagulable state and increased thrombosis risk in the SCD population, as well as advocate for the development of evidence-based guidelines to aid in the prevention of VTE in SCD.

Constitutive hypercoagulability in pediatric sickle cell disease patients with hemoglobin SS genotype

Background

Constitutive inflammation and hemostatic activation have been identified as key contributors to the pathophysiology of sickle cell disease (SCD), leading to clinical consequences such as vaso-occlusive crises and stroke. Patients with hemoglobin SS (HbSS) and hemoglobin SC (HbSC) genotypes are reported to have different symptoms, as do patients in steady-state and crisis situations. Differences among these groups remain unclear in pediatric patients.

Objectives

To compare hemostatic activity in HbSS and HbSC pediatric patients during steady state, in crisis, and in clinical follow-up and compare HbSS and HbSC patients with normal healthy children.

Methods

Whole-blood coagulation assay thromboelastography (TEG) was used to assess hemostatic activity. In parallel, flow cytometry was used to assess procoagulant surface expression of platelets and red blood cells.

Results

TEG results indicated no significant differences in clotting onset (R time), clot maximum amplitude, or maximum rate of thrombus generation among steady-state, crisis, and follow-up subgroups of HbSS and HbSC patients. TEG parameters did not differ significantly between HbSC patients and healthy children, while HbSS patients showed significantly shorter R time and greater maximum amplitude and maximum rate of thrombus generation, all indicative of a constitutive hypercoagulable state. Flow cytometry results did not detect increased platelet integrin αIIbβ3 activation or red blood cell procoagulant surface expression in SCD patients compared with unaffected children.

Conclusion

Our results indicate that pediatric SCD patients with the HbSS genotype have constitutively activated hemostasis relative to HbSC patients and healthy children. It remains to be determined how treatments that improve clinical outcomes in SCD patients affect this constitutively hypercoagulable state.

VWF-ADAMTS13 axis dysfunction in children with sickle cell disease treated with hydroxycarbamide vs blood transfusion

Previous studies have reported elevated von Willebrand factor (VWF) levels in patients with sickle cell disease (SCD) and demonstrated a key role for the VWF-ADAMTS13 axis in the pathobiology of SCD vaso-occlusion. Although blood transfusion is the gold standard for stroke prevention in SCD, the biological mechanisms underpinning its improved efficacy compared with hydroxycarbamide are not fully understood. We hypothesized that the improved efficacy of blood transfusion might relate to differences in VWF-ADAMTS13 axis dysfunction. In total, 180 children with a confirmed diagnosis of SCD (hemoglobin SS) on hydroxycarbamide (n = 96) or blood transfusion (n = 84) were included. Despite disease-modifying treatment, plasma VWF and VWF propeptide were elevated in a significant proportion of children with SCD (33% and 47%, respectively). Crucially, all VWF parameters were significantly higher in the hydroxycarbamide compared with the blood transfusion cohort (P < .05). Additionally, increased levels of other Weibel-Palade body-stored proteins, including factor VIII (FVIII), angiopoietin-2, and osteoprotegerin were observed, indicated ongoing endothelial cell activation. Children treated with hydroxycarbamide also had higher FVIII activity and enhanced thrombin generation compared with those in the blood transfusion cohort (P < .001). Finally, hemolysis markers strongly correlated with VWF levels (P < .001) and were significantly reduced in the blood transfusion cohort (P < .001). Cumulatively, to our knowledge, our findings demonstrate for the first time that despite treatment, ongoing dysfunction of the VWF-ADAMTS13 axis is present in a significant subgroup of pediatric patients with SCD, especially those treated with hydroxycarbamide.

Crucial roles of red blood cells and platelets in whole blood thrombin generation

Red blood cells (RBCs) and platelets contribute to the coagulation capacity in bleeding and thrombotic disorders. The thrombin generation (TG) process is considered to reflect the interactions between plasma coagulation and the various blood cells. Using a new high-throughput method capturing the complete TG curve, we were able to compare TG in whole blood and autologous platelet-rich and platelet-poor plasma to redefine the blood cell contributions to the clotting process. We report a faster and initially higher generation of thrombin and shorter coagulation time in whole blood than in platelet-rich plasma upon low concentrations of coagulant triggers, including tissue factor, Russell viper venom factor X, factor Xa, factor XIa, and thrombin. The TG was accelerated with increased hematocrit and delayed after prior treatment of RBC with phosphatidylserine-blocking annexin A5. RBC treatment with ionomycin increased phosphatidylserine exposure, confirmed by flow cytometry, and increased the TG process. In reconstituted blood samples, the prior selective blockage of phosphatidylserine on RBC with annexin A5 enhanced glycoprotein VI-induced platelet procoagulant activity. For patients with anemia or erythrocytosis, cluster analysis revealed high or low whole-blood TG profiles in specific cases of anemia. The TG profiles lowered upon annexin A5 addition in the presence of RBCs and thus were determined by the extent of phosphatidylserine exposure of blood cells. Profiles for patients with polycythemia vera undergoing treatment were similar to that of control subjects. We concluded that RBC and platelets, in a phosphatidylserine-dependent way, contribute to the TG process. Determination of the whole-blood hypo- or hyper-coagulant activity may help to characterize a bleeding or thrombosis risk.

Association of Circulating Procoagulant Microvesicles with Painful Vaso-Occlusive Crisis in Sickle Cell Disease

Introduction

Thrombotic complication is one of the features of sickle cell disease (SCD), characterized by appearance of phosphatidylserine on the outer membrane of sickle-shaped red blood cells and most abundantly on membrane protrusions called microvesicles (MVs). However, the exact mechanism by which MVs may enhance coagulant activity in SCD patients has not been fully addressed. The aim of this study was to further investigate the procoagulant activity of circulating MVs in sickle cell crises.

Materials and Methods

Subjects included in this cross-sectional study were 47 patients with SCD and 25 normal subjects with written informed consent obtained from all the participants. MV analysis was conducted by using CD61, CD235α, and Annexin-V monoclonal antibodies. The coagulant activity of MVs was determined by an ELISA-based procoagulant activity assay.

Results

The majority of MVs were originated from platelets (CD61+) and erythrocytes (CD235+). These MVs demonstrated significantly enhanced levels during the painful crisis when compared with the steady-state period (p < 0.001) and controls (p < 0.001). Also, the procoagulant activity of MVs was significantly higher in crisis compared to those of steady state (p < 0.001) and positively correlated with the number of Annexin-V+ MVs (p < 0.001). Significant correlations were found between erythrocyte-derived MVs with hemolysis marker (r = 0.51, p < 0.001) and the hemoglobin level (r = -0.63, p < 0.001).

Conclusion

The numbers of platelet- and erythrocyte-derived MVs are related to painful crisis, and their quantification in SCD may be helpful for identifying cases at increased risk of thrombotic complications.

The APC-EPCR-PAR1 axis in sickle cell disease

Sickle Cell Disease (SCD) is a group of inherited hemoglobinopathies. Sickle cell anemia (SCA) is caused by a homozygous mutation in the β-globin generating sickle hemoglobin (HbS). Deoxygenation leads to pathologic polymerization of HbS and sickling of erythrocytes. The two predominant pathologies of SCD are hemolytic anemia and vaso-occlusive episodes (VOE), along with sequelae of complications including acute chest syndrome, hepatopathy, nephropathy, pulmonary hypertension, venous thromboembolism, and stroke. SCD is associated with endothelial activation due to the release of danger-associated molecular patterns (DAMPs) such as heme, recurrent ischemia-reperfusion injury, and chronic thrombin generation and inflammation. Endothelial cell activation is mediated, in part, by thrombin-dependent activation of protease-activated receptor 1 (PAR1), a G protein coupled receptor that plays a role in platelet activation, endothelial permeability, inflammation, and cytotoxicity. PAR1 can also be activated by activated protein C (APC), which promotes endothelial barrier protection and cytoprotective signaling. Notably, the APC system is dysregulated in SCD. This mini-review will discuss activation of PAR1 by APC and thrombin, the APC-EPCR-PAR1 axis, and their potential roles in SCD.

COVID-19 and Sickle Cell Disease: Two Independent Risk Factors for Venous Thromboembolism

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been widely documented as a multi-systemic illness and associated with an increased incidence of thromboses. Likewise, sickle cell disease (SCD) is a hematologic disease responsible for widespread effects on the vasculature and is also associated with elevated thrombotic risk. In this review, we examine the incidence rates of venous thromboembolism (VTE) in SCD and COVID-19 independently and review the mechanisms of coagulopathy associated with both diseases. We describe the possible associations and commonalities between VTE mechanisms, as both diseases cause widespread inflammation that influences each tenet of Virchow's triad. We also discuss current anticoagulation guideline recommendations for the prevention of VTE events in each of these diseases. We report on current literature to date describing rates of VTE in SCD-COVID-19 patients and outline prospective areas of research to further understand the possible synergistic influence of coagulopathy in these patients. The association between SCD and COVID-19 remains a largely under-researched area of coagulopathy in current hematology and thrombotic literature, and our report lays out potential future prospects in the field.

Epidemiology and prevention of venous thromboembolism

Venous thromboembolism, that consists of the interrelated conditions deep-vein thrombosis and pulmonary embolism, is an under-appreciated vascular disease. In Western regions, approximately 1 in 12 individuals will be diagnosed with venous thromboembolism in their lifetime. Rates of venous thromboembolism are lower in Asia, but data from other regions are sparse. Numerous risk factors for venous thromboembolism have been identified, which can be classified as acute or subacute triggers (provoking factors that increase the risk of venous thromboembolism) and basal or acquired risk factors (which can be modifiable or static). Approximately 20% of individuals who have a venous thromboembolism event die within 1 year (although often from the provoking condition), and complications are common among survivors. Fortunately, opportunities exist for primordial prevention (prevention of the development of underlying risk factors), primary prevention (management of risk factors among individuals at high risk of the condition) and secondary prevention (prevention of recurrent events) of venous thromboembolism. In this Review, we describe the epidemiology of venous thromboembolism, including the incidence, risk factors, outcomes and opportunities for prevention. Meaningful health disparities exist in both the incidence and outcomes of venous thromboembolism. We also discuss these disparities as well as opportunities to reduce them.

The Role of Inflammation in The Cellular and Molecular Mechanisms of Cardiopulmonary Complications of Sickle Cell Disease

Cardiopulmonary complications remain the major cause of mortality despite newer therapies and improvements in the lifespan of patients with sickle cell disease (SCD). Inflammation has been identified as a major risk modifier in the pathogenesis of SCD-associated cardiopulmonary complications in recent mechanistic and observational studies. In this review, we discuss recent cellular and molecular mechanisms of cardiopulmonary complications in SCD and summarize the most recent evidence from clinical and laboratory studies. We emphasize the role of inflammation in the onset and progression of these complications to better understand the underlying pathobiological processes. We also discuss future basic and translational research in addressing questions about the complex role of inflammation in the development of SCD cardiopulmonary complications, which may lead to promising therapies and reduce morbidity and mortality in this vulnerable population.

Inflammatory status in pediatric sickle cell disease: Unravelling the role of immune cell subsets

Introduction: The mutation of the beta-globin gene that causes sickle cell disease (SCD) results in pleiotropic effects, such as hemolysis and vaso-occlusive crisis that can induce inflammatory mechanisms with deleterious consequences on the organism. Moreover, SCD patients display an increased susceptibility to infections. Few studies are currently available that evaluate a wide immunological profile in a pediatric population. This study proposes an evaluation of the immune profile in subjects with SCD in a pediatric population through a detailed analysis by flow cytometry. Methods and Materials: Peripheral blood samples from 53 pediatric patients with SCD (mean age 9.8 years, interquartile range 9 years) were obtained and then analyzed by flow cytometry, in order to evaluate changes in the immune populations compared to 40 healthy donors (mean age 7.3 years, interquartile range 9.5 years). Results: Our data showed an increase in neutrophils (with a reduction in the CD62L + subpopulation) and monocytes (with a decrease in HLA-DRlow monocytes) with normal values of lymphocytes in SCD patients. In the lymphocyte subpopulations analysis we observed lower values of CD4⁺ T cells (with higher number of memory and central memory T lymphocytes) with increased frequency of CD8⁺ T cells (with a predominant naive pattern). Moreover, we observed higher values of CD39⁺ Tregs and lower HLA-DR+ and CD39^- T cells with an increased Th17, Th1-17 and Th2 response. Conclusion: We observed immunological alterations typical of an inflammatory status (increase in activated neutrophils and monocytes) associated with a peculiar Treg pattern (probably linked to a body attempt to minimize inflammation intrinsic to SCD). Furthermore, we highlighted a T helper pathway associated with inflammation in line with other studies. Our data showed that immunological markers may have an important role in the understanding the pathophysiology of SCD and in optimizing targeted therapeutic strategies for each patient.

Thrombin generation assay: the present and the future

The need for a more precise test that replicates the in vivo hemostatic conditions is increasingly being recognized. Up to now, the thrombin generation assay (TGA) has become the most reliable approach to evaluate the status of coagulation activation. The clinical potential for the TGA is most promising in the prediction of venous thromboembolism recurrence. However, there is currently an urgent need for a standardized global test that can reliably detect, predict and monitor coagulation disorders in both clinical and experimental studies. We have recently modified the TGA to analyze not only tissue factor-driven coagulation, but the intrinsic coagulation pathway as well. In the present review, we discuss different TG tests, emphasizing the requirement for a better understanding of the evaluation of distinct coagulation pathways using this technique, as well as the standardization and clinical validation.

Antithrombin-III mitigates thrombin-mediated endothelial cell contraction and sickle red blood cell adhesion in microscale flow

Individuals with sickle cell disease (SCD) have persistently elevated thrombin generation that results in a state of systemic hypercoagulability. Antithrombin‐III (ATIII), an endogenous serine protease inhibitor, inhibits several enzymes in the coagulation cascade, including thrombin. Here, we utilize a biomimetic microfluidic device to model the morphology and adhesive properties of endothelial cells (ECs) activated by thrombin and examine the efficacy of ATIII in mitigating the adhesion of SCD patient‐derived red blood cells (RBCs) and EC retraction. Microfluidic devices were fabricated, seeded with ECs, and incubated under physiological shear stress. Cells were then activated with thrombin with or without an ATIII pretreatment. Blood samples from subjects with normal haemoglobin (HbAA) and subjects with homozygous SCD (HbSS) were used to examine RBC adhesion to ECs. Endothelial cell surface adhesion molecule expression and confluency in response to thrombin and ATIII treatments were also evaluated. We found that ATIII pretreatment of ECs reduced HbSS RBC adhesion to thrombin‐activated endothelium. Furthermore, ATIII mitigated cellular contraction and reduced surface expression of von Willebrand factor and vascular cell adhesion molecule‐1 (VCAM‐1) mediated by thrombin. Our findings suggest that, by attenuating thrombin‐mediated EC damage and RBC adhesion to endothelium, ATIII may alleviate the thromboinflammatory manifestations of SCD.

Rapid degradation of protein tyrosine phosphatase 1B in sickle cells: Possible contribution to sickle cell membrane weakening

Although both protein tyrosine phosphatases and kinases are constitutively active in healthy human red blood cells (RBCs), the preponderance of phosphatase activities maintains the membrane proteins in a predominantly unphosphorylated state. We report here that unlike healthy RBCs, proteins in sickle cells are heavily tyrosine phosphorylated, raising the question regarding the mechanism underpinning this tyrosine phosphorylation. Upon investigating possible causes, we observe that protein tyrosine phosphatase 1B (PTP1B), the major erythrocyte tyrosine phosphatase, is largely digested to a lower molecular weight fragment in sickle cells. We further find that the resulting truncated form of PTP1B is significantly less active than its intact counterpart, probably accounting for the intense tyrosine phosphorylation of Band 3 in sickle erythrocytes. Because this tyrosine phosphorylation of Band 3 promotes erythrocyte membrane weakening that causes release of both membrane vesicles and cell free hemoglobin that in turn initiates vaso-occlusive events, we conclude that cleavage of PTP1B could contribute to the symptoms of sickle cell disease. We further posit that methods to inhibit proteolysis of PTP1B could mitigate symptoms of the disease.

Pathophysiology of deep vein thrombosis

Deep venous thrombosis is a frequent, multifactorial disease and a leading cause of morbidity and mortality. Most of the time deep venous thrombosis is triggered by the interaction between acquired risk factors, such as hip fracture, pregnancy, and immobility, and hereditary risk factors such as thrombophilias. The mechanisms underlying deep venous thrombosis are not fully elucidated; however, in recent years, important advances have shed light on the role of venous flow, endothelium, platelets, leukocytes, and the interaction between inflammation and hemostasis. It has been described that the alteration of venous blood flow produces endothelial activation, favoring the adhesion of platelets and leukocytes, which, through tissue factor expression and neutrophil extracellular traps formation, contribute to the activation of coagulation, trapping more cells, such as red blood cells. Thus, the concerted interaction of these phenomena allows the formation and growth of the thrombus. In this work, the main mechanisms involved in the pathophysiology of deep vein thrombosis will be described.

Reduction in Prevalence of Thrombotic Events in Sickle Cell Disease after Allogeneic Hematopoietic Transplantation

Thrombosis is a recognized complication in sickle cell disease (SCD). Allogeneic hematopoietic cell transplantation (allo-HCT) remains the only curative options for patients with severe SCD phenotypes. There is limited data describing the effects of allo-HCT on recurrent thrombotic events (venous and arterial events). We evaluated 31 patients with SCD who have undergone allo-HCT with a median follow up of 34.5 months (range:13-115) post-transplant. No patient continued anticoagulation or anti-platelets after allo-HCT. There was an absolute difference of 32% [95% CI=12.3-32.2, p=0.002] in the prevalence of venous thromboembolic (VTE) events before and after allo-HSCT. In addition, there was an absolute 38.5% [95% CI=10.63-45.96, p=0.006] difference in the number of ischemic cerebrovascular accidents (CVA) before and after allo-HSCT. Patients with severe SCD who undergo allo-HCT are less likely to develop recurrent thrombotic events OR 0.22 [95% CI=0.058 - 0.83, p=0.025] when compared to a control cohort of patients matched for age and genotype. Following curative therapy with allo-HCT, there is a reduction in recurrent arterial and venous thrombosis in patients with severe SCD phenotypes.

HSF1 Alleviates Microthrombosis and Multiple Organ Dysfunction in Mice with Sepsis by Upregulating the Transcription of Tissue-Type Plasminogen Activator

Sepsis is a life-threatening complication of infection closely associated with coagulation abnormalities. Heat shock factor 1 (HSF1) is an important transcription factor involved in many biological processes, but its regulatory role in blood coagulation remained unclear. We generated a sepsis model in HSF1-knockout mice to evaluate the role of HSF1 in microthrombosis and multiple organ dysfunction. Compared with septic wild-type mice, septic HSF1-knockout mice exhibited a greater degree of lung, liver, and kidney tissue damage, increased fibrin/: fibrinogen deposition in the lungs and kidneys, and increased coagulation activity. RNA-seq analysis revealed that tissue-type plasminogen activator (t-PA) was upregulated in the lung tissues of septic mice, and the level of t-PA was significantly lower in HSF1-knockout mice than in wild-type mice in sepsis. The effects of HSF1 on t-PA expression were further validated in HSF1-knockout mice with sepsis and in vitro in mouse brain microvascular endothelial cells using HSF1 RNA interference or overexpression under lipopolysaccharide stimulation. Bioinformatics analysis, combined with electromobility shift and luciferase reporter assays, indicated that HSF1 directly upregulated t-PA at the transcriptional level. Our results reveal, for the first time, that HSF1 suppresses coagulation activity and microthrombosis by directly upregulating t-PA, thereby exerting protective effects against multiple organ dysfunction in sepsis.

Comparison of the clinical course of COVID-19 infection in sickle cell disease patients with healthcare professionals

It is highly expected that COVID-19 infection will have devastating consequences in sickle cell disease (SCD) patients due to endothelial activation and decreased tissue and organ reserve as a result of microvascular ischemia and continuous inflammation. In this study, we aimed to compare the clinical course of COVID-19 in adult SCD patients under the organ injury mitigation and clinical care improvement program (BASCARE) with healthcare professionals without significant comorbid conditions. The study was planned as a retrospective, multicenter and cross-sectional study. Thirty-nine SCD patients, ages 18 to 64 years, and 121 healthcare professionals, ages 21 to 53, were included in the study. The data were collected from the Electronic Health Recording System of PRANA, where SCD patients under the BASCARE program had been registered. The data of other patients were collected from the Electronic Hospital Data Recording System and patient files. In the SCD group, the crude incidence of COVID-19 was 9%, while in healthcare professionals at the same period was 23%. Among the symptoms, besides fever, loss of smell and taste were more prominent in the SCD group than in healthcare professionals. There was a significant difference between the two groups in terms of development of pneumonia, hospitalization, and need for intubation (43 vs 5%, P < 0.00001; 26 vs 7%, P = 0.002; and 10 vs 1%, P = 0.002, respectively). Prophylactic low molecular weight heparin and salicylate were used more in the SCD group than in healthcare professionals group (41 vs 9% and 28 vs 1%; P < 0.0001 for both). The 3-month mortality rate was demonstrated as 5% in the SCD group, while 0 in the healthcare professionals group. One patient in the SCD group became continously dependent on respiratory support. The cause of death was acute chest syndrome in the first case, hepatic necrosis and multi-organ failure in the second case. In conclusion, these observations supported the expectation that the course of COVID-19 in SCD patients will get worse. The BASCARE program applied in SCD patients could not change the poor outcome.

Added Value of Blood Cells in Thrombin Generation Testing

The capacity of blood to form thrombin is a critical determinant of coagulability. Plasma thrombin generation (TG), a test that probes the capacity of plasma to form thrombin, has improved our knowledge of the coagulation system and shows promising utility in coagulation management. Although plasma TG gives comprehensive insights into the function of pro- and anticoagulation drivers, it does not measure the role of blood cells in TG. In this literature review, we discuss currently available continuous TG tests that can reflect the involvement of blood cells in coagulation, in particular the fluorogenic assays that allow continuous measurement in platelet-rich plasma and whole blood. We also provide an overview about the influence of blood cells on blood coagulation, with emphasis on the direct influence of blood cells on TG. Platelets accelerate the initiation and velocity of TG by phosphatidylserine exposure, granule content release and surface receptor interaction with coagulation proteins. Erythrocytes are also major providers of phosphatidylserine, and erythrocyte membranes trigger contact activation. Furthermore, leukocytes and cancer cells may be important players in cell-mediated coagulation because, under certain conditions, they express tissue factor, release procoagulant components and can induce platelet activation. We argue that testing TG in the presence of blood cells may be useful to distinguish blood cell-related coagulation disorders. However, it should also be noted that these blood cell-dependent TG assays are not clinically validated. Further standardization and validation studies are needed to explore their clinical usefulness.

HSF1 Alleviates Microthrombosis and Multiple Organ Dysfunction in Mice with Sepsis by Upregulating the Transcription of Tissue-Type Plasminogen Activator

Sepsis is a life-threatening complication of infection closely associated with coagulation abnormalities. Heat shock factor 1 (HSF1) is an important transcription factor involved in many biological processes, but its regulatory role in blood coagulation remained unclear. We generated a sepsis model in HSF1-knockout mice to evaluate the role of HSF1 in microthrombosis and multiple organ dysfunction. Compared with septic wild-type mice, septic HSF1-knockout mice exhibited a greater degree of lung, liver, and kidney tissue damage, increased fibrin/: fibrinogen deposition in the lungs and kidneys, and increased coagulation activity. RNA-seq analysis revealed that tissue-type plasminogen activator (t-PA) was upregulated in the lung tissues of septic mice, and the level of t-PA was significantly lower in HSF1-knockout mice than in wild-type mice in sepsis. The effects of HSF1 on t-PA expression were further validated in HSF1-knockout mice with sepsis and in vitro in mouse brain microvascular endothelial cells using HSF1 RNA interference or overexpression under lipopolysaccharide stimulation. Bioinformatics analysis, combined with electromobility shift and luciferase reporter assays, indicated that HSF1 directly upregulated t-PA at the transcriptional level. Our results reveal, for the first time, that HSF1 suppresses coagulation activity and microthrombosis by directly upregulating t-PA, thereby exerting protective effects against multiple organ dysfunction in sepsis.

Red Blood Cell Contribution to Hemostasis

Red Blood Cells (RBCs) have been increasingly recognized to play important roles in hemostasis and the mechanisms by which they do so continue to be elucidated. First and foremost, RBC biomechanics are the principal determinant of viscosity and flow dynamics of blood, which strongly influence all features of hemostasis. Of note, morphologic pathology, such as that found in sickle cell disease, leads to increased risk of thrombotic disease. RBC surface interactions govern signaling between platelets and RBCs and also aid in the conversion of prothrombin to thrombin. Additionally, RBCs generate microparticles which have been shown to reduce clotting time. Finally, blood clot structure and maturation are dependent on the inclusion of RBCs in forming thrombi. Here, we review the above mechanisms of RBC contribution to hemostasis.

COVID-19 Infection and Acute Pulmonary Embolism in an Adolescent Female With Sickle Cell Disease

A previously healthy 20-year-old female presented to the emergency room in April 2020 with complaints of shortness of breath, chest pain, and cough. She was diagnosed with coronavirus disease 2019 (COVID-19) infection and pulmonary embolism (PE). Workup for anemia led to the diagnosis of sickle cell disease (SCD). Patients diagnosed with COVID-19 are at an increased risk for the development of PE and venous thromboembolism (VTE). Anticoagulation prophylaxis and escalation to treatment dosing are recommended in patients admitted with moderate to severe symptoms of COVID-19. PE and VTE are relatively uncommon in the pediatric and adolescent population. Most commonly, patients are diagnosed with thrombophilia or have an underlying hypercoagulable state such as with SCD. Also, symptoms of COVID-19 infection, acute chest syndrome (ACS), and PE can have overlapping features. In this report, we present a case of a late adolescent female with SCD, who was diagnosed with COVID-19, and whose condition was complicated with PE.

Enhanced Hypercoagulability in Sickle Cell Anaemia Patients with Chronic Leg Ulcers

Sickle Cell Anaemia (SCA) is associated with a hypercoagulable state resulting in a predisposition to venous thromboembolism. With improvements in the quality of care, more patients with SCA survive into adulthood with an associated increase in the frequency of end-organ damage and chronic complications such as chronic leg ulcers (CLUs). These ulcers rarely occur in the first decade of life and are recurrent, painful, and slow-to-heal. This study tested the hypothesis that coagulation is enhanced in SCA patients with CLU. 145 participants (50 SCA with CLU, 50 SCA without CLU, and 45 with haemoglobin AA) were assessed to determine their coagulation profile using selected tests of coagulation. The SCA with the CLU group had the lowest mean haemoglobin (Hb) concentration. SCA patients with and without CLUs had elevated mean platelet counts, shorter mean aPTT, and marginally prolonged mean PT  compared to HbAA patients. SCA with CLUs patients had a significantly shortened aPTT than those without CLUs (p = 0.035) and HbAA (p = 0.009). There were significant differences in the mean PT between SCA with CLUs patients and HbAA (p = 0.017); SCA without CLU and HbAA (p = 0.014). SCA with and without CLUs patients had higher mean D-dimer levels compared to HbAA. There was a negative correlation between Hb concentration and duration of CLU (r = -0.331, p = 0.021). In conclusion, our study demonstrates a heightened hypercoagulability in SCA patients with CLUs. We did not test for platelet activation, and it is not clear what role, if any, the enhanced hypercoagulability plays in the pathogenesis of CLUs in SCA. It will be useful to ascertain if antiplatelet agents or/and anticoagulants quicken the healing of CLUs in SCA patients.

Thrombin generation in vivo and ex vivo in sickle cell disease patients

Activation of coagulation is an important hallmark of sickle cell disease (SCD) and it is believed that hypercoagulability plays a role to the disease pathophysiology. Studies have sought to identify how hemostatic biomarkers are expressed in SCD, however, the results are inconclusive. In this context, our objective was to evaluate the thrombin generation in vivo and ex vivo in SCD patients and the association between these biomarkers and the use of HU. This cross-sectional study was carried out with patients diagnosed with SCD, users or not of Hydroxyurea (HU), and healthy individuals as controls. D dimer (D-Di) was evaluated by ELISA and (TGT) thrombin generation test by CAT method. D-Di plasma levels were significantly higher in SCD patients when compared to the controls. TGT parameters such as peak, ETP and normalized ETP at low TF concentration and time-to-peak, peak, ETP and normalized ETP values at high TF concentration were lower in SCD patients than in controls. In contrast, the normalized activated protein C sensitivity ratio (nAPCsr) was higher in patients compared to controls, indicating resistance to the action of this natural anticoagulant. Regarding the use of HU, comparing users and non-users of this drug, no difference was observed in D-Di levels and in most TGT parameters. Our data analyzed together allow us to conclude that patients with SCD present a state of hypercoagulability in vivo due to the higher levels of D-Di and resistance to APC assessed ex vivo which is consistent with the coagulation imbalance described in SCD patients.

The molecular basis for the prothrombotic state in sickle cell disease

The genetic and molecular basis of sickle cell disease (SCD) has long since been characterized but the pathophysiological basis is not entirely defined. How a red cell hemolytic disorder initiates inflammation, endothelial dysfunction, coagulation activation and eventually leads to vascular thrombosis, is yet to be elucidated. Recent evidence has demonstrated a high frequency of unprovoked/recurrent venous thromboembolism (VTE) in SCD, with an increased risk of mortality among patients with a history of VTE. Here, we thoroughly review the molecular basis for the prothrombotic state in SCD, specifically highlighting emerging evidence for activation of overlapping inflammation and coagulation pathways, that predispose to venous thromboembolism. We share perspectives in managing venous thrombosis in SCD, highlighting innovative therapies with the potential to influence the clinical course of disease and reduce thrombotic risk, while maintaining an acceptable safety profile.

High molecular weight kininogen contributes to early mortality and kidney dysfunction in a mouse model of sickle cell disease

BACKGROUND

Sickle cell disease (SCD) is characterized by chronic hemolytic anemia, vaso-occlusive crises, chronic inflammation, and activation of coagulation. The clinical complications such as painful crisis, stroke, pulmonary hypertension, nephropathy and venous thromboembolism lead to cumulative organ damage and premature death. High molecular weight kininogen (HK) is a central cofactor for the kallikrein-kinin and intrinsic coagulation pathways, which contributes to both coagulation and inflammation.

OBJECTIVE

We hypothesize that HK contributes to the hypercoagulable and pro-inflammatory state that causes end-organ damage and early mortality in sickle mice.

METHODS

We evaluated the role of HK in the Townes mouse model of SCD.

RESULTS/CONCLUSIONS

We found elevated plasma levels of cleaved HK in sickle patients compared to healthy controls, suggesting ongoing HK activation in SCD. We used bone marrow transplantation to generate wild type and sickle cell mice on a HK-deficient background. We found that short-term HK deficiency attenuated thrombin generation and inflammation in sickle mice at steady state, which was independent of bradykinin signaling. Moreover, long-term HK deficiency attenuates kidney injury, reduces chronic inflammation, and ultimately improves survival of sickle mice.

Innate immune receptors in platelets and platelet-leukocyte interactions

Platelets are chief cells in hemostasis. Apart from their hemostatic roles, platelets are major inflammatory effector cells that can influence both innate and adaptive immune responses. Activated platelets have thromboinflammatory functions linking hemostatic and immune responses in several physiological and pathological conditions. Among many ways in which platelets exert these functions, platelet expression of pattern recognition receptors (PRRs), including TLR, Nod-like receptor, and C-type lectin receptor families, plays major roles in sensing and responding to pathogen-associated or damage-associated molecular patterns (PAMPs and DAMPs, respectively). In this review, an increasing body of evidence is compiled showing the participation of platelet innate immune receptors, including PRRs, in infectious diseases, sterile inflammation, and cancer. How platelet recognition of endogenous DAMPs participates in sterile inflammatory diseases and thrombosis is discussed. In addition, platelet recognition of both PAMPs and DAMPs initiates platelet-mediated inflammation and vascular thrombosis in infectious diseases, including viral, bacterial, and parasite infections. The study also focuses on the involvement of innate immune receptors in platelet activation during cancer, and their contribution to tumor microenvironment development and metastasis. Finally, how innate immune receptors participate in platelet communication with leukocytes, modulating leukocyte-mediated inflammation and immune functions, is highlighted. These cell communication processes, including platelet-induced release of neutrophil extracellular traps, platelet Ag presentation to T-cells and platelet modulation of monocyte cytokine secretion are discussed in the context of infectious and sterile diseases of major concern in human health, including cardiovascular diseases, dengue, HIV infection, sepsis, and cancer.

Thrombin generation in children with sickle cell Anemia is Higher in the presence of platelets⋆ and ⋆

Cellular and plasma interactions underlie hypercoagulability in sickle cell anemia (SCA). In healthy adults, thrombin generation (TG), a biomarker of hypercoagulability, is similar in plasma with and without platelets. Studies investigating TG in SCA using platelet-poor plasma (PPP) show conflicting results. There are no studies in SCA simultaneously comparing TG using platelet rich plasma (PRP) and PPP. This prospective study compares TG in children with SCA, at steady state, in PPP versus PRP and investigates the association of predefined clinical variables with the difference between PRP and PPP. Our secondary aim was to investigate derangements in the protein C and S pathway measuring TG with and without thrombomodulin (TM). In forty-three paired samples from SCA patients, aged 2-15 years, TG in the presence of platelets was 5.9 % higher [1239 nmol/(min*L) (SD: 224.1) vs. 1151 nmol/(min*L) (SD 223.3); p = 0.026]. The difference was highest in the 6-10 year age group (9.5 %; SD 14.1) followed by the 2-5 year age group (5.4 %; SD 21.4). In a multiple linear regression model, age, gender, current use of hydroxyurea, degree of hemolysis and severity of pain crises were not predictive of the difference between PRP and PPP. In PPP, TG reduction after TM addition was 7.4 % (SD 16.8), signifying activated protein C resistance. In conclusion, TG in children with SCA aged 2-10 years is higher in the presence of platelets. TG using PRP along with TM addition may be a useful biomarker of hypercoagulability in this population.

Reduction of thrombotic and inflammatory complications of polystyrene-block-polyisoprene-block-polystyrene (SIS) with one-step electrospinning

Polystyrene-block-polyisoprene-block-polystyrene (SIS) has been used as biomaterials due to its soft and stable properties under physiological conditions. However, the thrombotic and inflammatory complications caused by SIS restrain its application as blood-contacting implant. To overcome this problem, the hydrophilic core-shell structured SIS-based microfiber with antioxidant encapsulation is fabricated with one-step reactive electrospinning. We demonstrate that the phase separation of SIS and acylated Pluronic F127 (F127-DA) components and crosslinking during electrospinning renders the microfiber blood compatible and stable under physiological condition; the encapsulation of 2-O-d-glucopyranosyl-l-ascorbic acid (AA-2G) in microfiber and subsequent release of AA-2G detoxifies the excess reactive oxygen species (ROS). The microfibers are nontoxic to cells and promote the fast growth and proliferation of human umbilical vein endothelial cells (HUVECs) in the presence of ROS; the thrombotic and inflammatory complications are effectively reduced with implant evaluation in vivo. Therefore, our work paves a new way to improve the biocompatibility of SIS, making it a promising candidate for blood contact materials.

Hemolysis Derived Products Toxicity and Endothelium: Model of the Second Hit

Vascular diseases are multifactorial, often requiring multiple challenges, or ‘hits’, for their initiation. Intra-vascular hemolysis illustrates well the multiple-hit theory where a first event lyses red blood cells, releasing hemolysis-derived products, in particular cell-free heme which is highly toxic for the endothelium. Physiologically, hemolysis derived-products are rapidly neutralized by numerous defense systems, including haptoglobin and hemopexin which scavenge hemoglobin and heme, respectively. Likewise, cellular defense mechanisms are involved, including heme-oxygenase 1 upregulation which metabolizes heme. However, in cases of intra-vascular hemolysis, those systems are overwhelmed. Heme exerts toxic effects by acting as a damage-associated molecular pattern and promoting, together with hemoglobin, nitric oxide scavenging and ROS production. In addition, it activates the complement and the coagulation systems. Together, these processes lead to endothelial cell injury which triggers pro-thrombotic and pro-inflammatory phenotypes. Moreover, among endothelial cells, glomerular ones display a particular susceptibility explained by a weaker capacity to counteract hemolysis injury. In this review, we illustrate the ‘multiple-hit’ theory through the example of intra-vascular hemolysis, with a particular focus on cell-free heme, and we advance hypotheses explaining the glomerular susceptibility observed in hemolytic diseases. Finally, we describe therapeutic options for reducing endothelial injury in hemolytic diseases.

Is There Any Improvement of the Coagulation Imbalance in Sickle Cell Disease after Hematopoietic Stem Cell Transplantation?

Several components of the clotting system are modified towards hypercoagulability in sickle cell disease (SCD). To date, hematopoietic stem cell transplantation (HSCT) is the only validated curative treatment of SCD. Here, we investigated the changes in the hemostatic potential of SCD children who've received a successful HSCT. Seventeen children with severe SCD were enrolled in the study. Thrombin generation (TG) was performed on citrated platelet-poor plasma, obtained before and 3, 6, 9, 12 and 15 months after HSCT. TG was triggered using 1 pM tissue factor and 4 µM phospholipids with or without thrombomodulin (TM). Before the HSCT, SCD children showed a higher endogenous thrombin potential (ETP), higher peak, higher velocity and shorter time-to-peak of TG than the normal controls (NC). ETP did not significantly change following the HSCT. However, the peak, velocity and time-to-peak of TG reversed to normal ranges from 3 months post-HSCT and remained so up to 15 months post-HSCT. The reduction of ETP after the addition of thrombomodulin (RETP) was dramatically reduced in SCD children before HSCT as compared with the NC. A partial reversal of RETP was observed from 3 months through 15 months post-HSCT. No statistical difference was observed for patient age or donor hemoglobinopathy status. In summary, successful HSCT improves the kinetics of TG but not the total thrombin capacity in SCD children.

Vascular Instability and Neurological Morbidity in Sickle Cell Disease: An Integrative Framework

It is well-established that patients with sickle cell disease (SCD) are at substantial risk of neurological complications, including overt and silent stroke, microstructural injury, and cognitive difficulties. Yet the underlying mechanisms remain poorly understood, partly because findings have largely been considered in isolation. Here, we review mechanistic pathways for which there is accumulating evidence and propose an integrative systems-biology framework for understanding neurological risk. Drawing upon work from other vascular beds in SCD, as well as the wider stroke literature, we propose that macro-circulatory hyper-perfusion, regions of relative micro-circulatory hypo-perfusion, and an exhaustion of cerebral reserve mechanisms, together lead to a state of cerebral vascular instability. We suggest that in this state, tissue oxygen supply is fragile and easily perturbed by changes in clinical condition, with the potential for stroke and/or microstructural injury if metabolic demand exceeds tissue oxygenation. This framework brings together recent developments in the field, highlights outstanding questions, and offers a first step toward a linking pathophysiological explanation of neurological risk that may help inform future screening and treatment strategies.

Red blood cells modulate structure and dynamics of venous clot formation in sickle cell disease

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.

Overcoming challenges of venous thromboembolism in sickle cell disease treatment

INTRODUCTION

Venous thromboembolism (VTE) is a common comorbid condition found in sickle cell disease (SCD) and is associated with increased mortality for adults with SCD. The pathophysiology that leads to the thrombophilic state in SCD has been previously reviewed; however, evidence-based guidelines to aid in diagnosis, prevention, and management of VTE are lacking. Areas covered: This review article will cover the pathophysiology underlying the hypercoagulable state, the epidemiology of VTE, and management strategies of VTE in SCD. Expert opinion: Providers should have a high suspicion for diagnosing VTE to help reduce morbidity and mortality in the SCD population. Unlike other thrombophilias, the risk of life-threatening anemia while being treated with anticoagulation is compounded with the potential complications surrounding red blood cell transfusions in this population (i.e. alloimmunization, hyperhemolysis) and this provides another complexity to managing VTE in this population. Clinical trials evaluating the risk and benefit of treatment and treatment duration are needed.

Sickle cell disease: Hemostatic and inflammatory changes, and their interrelation

Sickle cell disease, the most common genetic blood disorder in the world, has high clinical variability, negatively impacts quality of life and contributes to early mortality. Sickled erythrocytes cause blood flow obstruction, hemolysis, and several hemostatic changes that promote coagulation. These events, in turn, induce chronic inflammation, characterized by elevated plasma levels of pro-inflammatory markers, which aggravates the already unfavorable state of the circulatory system. Empirical evidence indicates that the hemostatic and inflammatory systems continuously interact with each other and thereby further propagate the hypercoagulability and inflammatory conditions. In this review article, we discuss the pathophysiological aspects of sickle cell disease and the hemostatic and inflammatory changes that underlie its pathogenesis.

Therapeutic strategies for sickle cell disease: towards a multi-agent approach

For over 100 years, clinicians and scientists have been unravelling the consequences of the A to T substitution in the β-globin gene that produces haemoglobin S, which leads to the systemic manifestations of sickle cell disease (SCD), including vaso-occlusion, anaemia, haemolysis, organ injury and pain. However, despite growing understanding of the mechanisms of haemoglobin S polymerization and its effects on red blood cells, only two therapies for SCD - hydroxyurea and L-glutamine - are approved by the US Food and Drug Administration. Moreover, these treatment options do not fully address the manifestations of SCD, which arise from a complex network of interdependent pathophysiological processes. In this article, we review efforts to develop new drugs targeting these processes, including agents that reactivate fetal haemoglobin, anti-sickling agents, anti-adhesion agents, modulators of ischaemia-reperfusion and oxidative stress, agents that counteract free haemoglobin and haem, anti-inflammatory agents, anti-thrombotic agents and anti-platelet agents. We also discuss gene therapy, which holds promise of a cure, although its widespread application is currently limited by technical challenges and the expense of treatment. We thus propose that developing systems-oriented multi-agent strategies on the basis of SCD pathophysiology is needed to improve the quality of life and survival of people with SCD.

Red blood cells: the forgotten player in hemostasis and thrombosis

New evidence has stirred up a long-standing but undeservedly forgotten interest in the role of erythrocytes, or red blood cells (RBCs), in blood clotting and its disorders. This review summarizes the most recent research that describes the involvement of RBCs in hemostasis and thrombosis. There are both quantitative and qualitative changes in RBCs that affect bleeding and thrombosis, as well as interactions of RBCs with cellular and molecular components of the hemostatic system. The changes in RBCs that affect hemostasis and thrombosis include RBC counts or hematocrit (modulating blood rheology through viscosity) and qualitative changes, such as deformability, aggregation, expression of adhesive proteins and phosphatidylserine, release of extracellular microvesicles, and hemolysis. The pathogenic mechanisms implicated in thrombotic and hemorrhagic risk include variable adherence of RBCs to the vessel wall, which depends on the functional state of RBCs and/or endothelium, modulation of platelet reactivity and platelet margination, alterations of fibrin structure and reduced susceptibility to fibrinolysis, modulation of nitric oxide availability, and the levels of von Willebrand factor and factor VIII in blood related to the ABO blood group system. RBCs are involved in platelet-driven contraction of clots and thrombi that results in formation of a tightly packed array of polyhedral erythrocytes, or polyhedrocytes, which comprises a nearly impermeable barrier that is important for hemostasis and wound healing. The revisited notion of the importance of RBCs is largely based on clinical and experimental associations between RBCs and thrombosis or bleeding, implying that RBCs are a prospective therapeutic target in hemostatic and thrombotic disorders.

The Platelet Count and its Implications in Sickle Cell Disease Patients Admitted for Intensive Care

Background and Aims

In sickle cell disease (SCD) patients admitted for intensive care, evaluation of platelet counts in different types of sickle cell complications and its prognostic relevance are not well-studied. Illuminating these aspects were the objectives of this study.

Materials and Methods

A chart review of 136 adult SCD patients consecutively admitted to our Intensive Care Unit (ICU) was done. The prognosis on day 1 was assessed by Acute Physiology and Chronic Health Evaluation (APACHE II) and multiple organ dysfunction scores (MODS). Receiver operating characteristic (ROC) curves evaluated the ability of platelet counts, MODS, and APACHE II scores to predict survival.

Results

The most common types of crises were severe pain (n = 53), acute chest syndrome (n = 40), and infection (n = 18); 17 patients were nonsurvivors. Platelet counts varied widely (range, 19-838 × 10⁹/L) with thrombocytopenia (n = 30) and thrombocytosis (n = 11). Counts correlated directly with leukocytes and reticulocytes; inversely with lactate dehydrogenase, APACHE, and MODS scores. Areas under ROC curve for platelets, MODS, and APACHE scores to predict survival were 0.73, 0.85, and 0.93, respectively.

Conclusions

In severe sickle cell crisis thrombocytopenia is more common than thrombocytosis. In the ICU, day 1 platelet counts correlate inversely with prognostic scores and are significantly reduced in multi-organ failure and nonsurvivors. A platelet count above 175 × 10⁹/L predicts patient survival with high specificity and positive predictive value but lacks sensitivity.

Human cathepsins K, L, and S: Related proteases, but unique fibrinolytic activity

BACKGROUND

Fibrin formation and dissolution are attributed to cascades of protease activation concluding with thrombin activation, and plasmin proteolysis for fibrin breakdown. Cysteine cathepsins are powerful proteases secreted by endothelial cells and others during cardiovascular disease and diabetes. Their fibrinolytic activity and putative role in hemostasis has not been well described.

METHODS

Fibrin gels were polymerized and incubated with recombinant human cathepsins (cat) K, L, or S, or plasmin, for dose-dependent and time-dependent studies. Dissolution of fibrin gels was imaged. SDS-PAGE was used to resolve cleaved fragments released from fibrin gels and remnant insoluble fibrin gel that was solubilized prior to electrophoresis to assess fibrin α, β, and γ polypeptide hydrolysis by cathepsins. Multiplex cathepsin zymography determined active amounts of cathepsins remaining.

RESULTS

There was significant loss of α and β fibrin polypeptides after incubation with cathepsins, with catS completely dissolving fibrin gel by 24 h. Binding to fibrin stabilized catL active time; it associated with cleaved fibrin fragments of multiple sizes. This was not observed for catK or S. CatS also remained active for longer times during fibrin incubation, but its association/binding did not withstand SDS-PAGE preparation.

CONCLUSIONS

Human cathepsins K, L, and S are fibrinolytic, and specifically can degrade the α and β fibrin polypeptide chains, generating fragments unique from plasmin.

GENERAL SIGNIFICANCE

Demonstration of cathepsins K, L, and S fibrinolytic activity leads to further investigation of contributory roles in disrupting vascular hemostasis, or breakdown of fibrin-based engineered vascular constructs where non-plasmin mediated fibrinolysis must be considered.

Inflammation in sickle cell disease

The primary β-globin gene mutation that causes sickle cell disease (SCD) has significant pathophysiological consequences that result in hemolytic events and the induction of the inflammatory processes that ultimately lead to vaso-occlusion. In addition to their role in the initiation of the acute painful vaso-occlusive episodes that are characteristic of SCD, inflammatory processes are also key components of many of the complications of the disease including autosplenectomy, acute chest syndrome, pulmonary hypertension, leg ulcers, nephropathy and stroke. We, herein, discuss the events that trigger inflammation in the disease, as well as the mechanisms, inflammatory molecules and cells that propagate these inflammatory processes. Given the central role that inflammation plays in SCD pathophysiology, many of the therapeutic approaches currently under pre-clinical and clinical development for the treatment of SCD endeavor to counter aspects or specific molecules of these inflammatory processes and it is possible that, in the future, we will see anti-inflammatory drugs being used either together with, or in place of, hydroxyurea in those SCD patients for whom hematopoietic stem cell transplants and evolving gene therapies are not a viable option.

Detection of procoagulant imbalance

Each individual possesses his/her own endogenous-thrombin-potential (ETP) (i. e. the ability to generate thrombin) which depends on the relative strength of the pro- and anticoagulant drivers operating in plasma. This ability depends in turn on the clinical conditions in which the balance between the two drivers is variably affected. One of the major determinants of this balance is the factor (F)VIII-protein C(PC) axis and its effect can be conveniently explored by the thrombin generation procedures with results expressed as ETP ratio with/without thrombomodulin (TM) (ETP-TM ratio). Furthermore, owing to the many feedback mechanisms mediated by thrombin (e. g. activation of PC, FXI, FV, FVIII, platelets etc.) it is also possible that any perturbation of the balance between pro- and anticoagulants that may occur in plasma even outside the FVIII-PC axis could result in an increased ETPTM ratio and therefore may suggest a procoagulant imbalance. Indeed, other non-coagulation moieties (e. g. microparticles, neutrophil extracellular traps, pro-inflammatory cytokines and others) circulating in blood of patients with various clinical conditions may also contribute to the procoagulant imbalance even when FVIII and/or PC are apparently normal. It can be postulated that dual ETP measurements performed in the presence and absence of TM with results expressed as their ratio may be the candidate procedure to detect subtle procoagulant imbalance in many clinical conditions characterised by an increased risk of thromboembolism. This article aimed at reviewing the clinical conditions in which evidence for the value of the ETP-TM ratio has been provided.

D-Dimer in African Americans: Whole Genome Sequence Analysis and Relationship to Cardiovascular Disease Risk in the Jackson Heart Study

OBJECTIVE

Plasma levels of the fibrinogen degradation product D-dimer are higher among African Americans (AAs) compared with those of European ancestry and higher among women compared with men. Among AAs, little is known of the genetic architecture of D-dimer or the relationship of D-dimer to incident cardiovascular disease.

APPROACH AND RESULTS

We measured baseline D-dimer in 4163 AAs aged 21 to 93 years from the prospective JHS (Jackson Heart Study) cohort and assessed association with incident cardiovascular disease events. In participants with whole genome sequencing data (n=2980), we evaluated common and rare genetic variants for association with D-dimer. Each standard deviation higher baseline D-dimer was associated with a 20% to 30% increased hazard for incident coronary heart disease, stroke, and all-cause mortality. Genetic variation near F3 was associated with higher D-dimer (rs2022030, β=0.284, P=3.24×10^-11). The rs2022030 effect size was nearly 3× larger among women (β=0.373, P=9.06×10^-13) than among men (β=0.135, P=0.06; P interaction =0.009). The sex by rs2022030 interaction was replicated in an independent sample of 10 808 multiethnic men and women (P interaction =0.001). Finally, the African ancestral sickle cell variant (HBB rs334) was significantly associated with higher D-dimer in JHS (β=0.507, P=1.41×10^-14), and this association was successfully replicated in 1933 AAs (P=2.3×10^-5).

CONCLUSIONS

These results highlight D-dimer as an important predictor of cardiovascular disease risk in AAs and suggest that sex-specific and African ancestral genetic effects of the F3 and HBB loci contribute to the higher levels of D-dimer among women and AAs.

Red blood cells in thrombosis

Red blood cells (RBCs) have historically been considered passive bystanders in thrombosis. However, clinical and epidemiological studies have associated quantitative and qualitative abnormalities in RBCs, including altered hematocrit, sickle cell disease, thalassemia, hemolytic anemias, and malaria, with both arterial and venous thrombosis. A growing body of mechanistic studies suggests that RBCs can promote thrombus formation and enhance thrombus stability. These findings suggest that RBCs may contribute to thrombosis pathophysiology and reveal potential strategies for therapeutically targeting RBCs to reduce thrombosis.

Prothrombotic aspects of sickle cell disease

Sickle cell disease (SCD) is a hematologic disorder caused by a well-characterized point mutation in the β-globin gene. Abnormal polymerization of hemoglobin tetramers results in the formation of sickle red blood cells that leads to vascular occlusions, hemolytic anemia, vascular inflammation and cumulative, multiple organ damage. Ongoing activation of coagulation is another hallmark of SCD. Recent studies strongly suggested that hypercoagulation in SCD is not just a secondary event but contributes directly to the disease pathophysiology. In this article we summarize mechanisms leading to the activation of coagulation, review data indicating direct contribution of coagulation to the pathology of SCD and, we discuss the anticoagulation as a possible treatment strategy to attenuate the disease progression.

Tissue factor-dependent coagulation activation by heme: A thromboelastometry study

Heme has been characterized as potent trigger of inflammation. In hemostasis, although heme has been shown to both induce and inhibit different compartments of hemostasis, its net effect on the hemostatic balance, and the biological relevance of these effects remain to be determined. Herein we evaluated the effect of heme on hemostasis using a global assay able to generate clinically relevant data in several other complex hemostatic diseases. Citrated whole blood samples from healthy participants were stimulated by heme or vehicle and incubated for 4h at 37°C. Rotational thromboelastometry was immediately performed. The participation of tissue factor in coagulation activation was evaluated using inhibitory antibody. Heme was able of inducing ex vivo coagulation activation in whole blood, affecting predominantly parameters associated with the initial phases of clot formation. This activation effect was at least partially dependent on hematopoietic tissue factor, since the effects of heme were partially abrogated by the inhibition of human tissue factor. In conclusion, using a global hemostasis assay, our study confirmed that heme is able to activate coagulation in whole blood, in a tissue factor-dependent way. These findings could explain the disturbance in hemostatic balance observed in conditions associated with the release of heme such as sickle cell disease.

Venous Thromboembolism in Children with Cancer and Blood Disorders

Venous thromboembolism (VTE) in children is multifactorial and most often related to a combination of inherited and acquired thrombophilias. Children with cancer and blood disorders are often at risk for VTE due to disease-related factors such as inflammation and abnormal blood flow and treatment-related factors such as central venous catheters and surgery. We will review risk factors for VTE in children with leukemia, lymphoma, and solid tumors. We will also review risk factors for VTE in children with blood disorders with specific focus on sickle cell anemia and hemophilia. We will present the available evidence and clinical guidelines for prevention and treatment of VTE in these populations.