Disturbance of plasma and platelet thrombospondin levels in sickle cell disease

The molecular mechanism responsible for HbSC retinopathy may depend on the action of the angiogenesis-related genes ROBO1 and SLC38A5

HbSC disease, a less severe form of sickle cell disease, affects the retina more frequently and patients have higher rates of proliferative retinopathy that can progress to vision loss. This study aimed to identify differences in the expression of endothelial cell-derived molecules associated with the pathophysiology of proliferative sickle cell retinopathy (PSCR). RNAseq was used to compare the gene expression profile of circulating endothelial colony-forming cells from patients with SC hemoglobinopathy and proliferative retinopathy (n = 5), versus SC patients without retinopathy (n = 3). Real-time polymerase chain reaction (qRT-PCR) was used to validate the RNAseq results. A total of 134 differentially expressed genes (DEGs) were found. DEGs were mainly associated with vasodilatation, type I interferon signaling, innate immunity and angiogenesis. Among the DEGs identified, we highlight the most up-regulated genes ROBO1 (log2FoldChange = 4.32, FDR = 1.35E-11) and SLC38A5 (log2FoldChange = 3.36 FDR = 1.59E-07). ROBO1, an axon-guided receptor, promotes endothelial cell migration and contributes to the development of retinal angiogenesis and pathological ocular neovascularization. Endothelial SLC38A5, an amino acid (AA) transporter, regulates developmental and pathological retinal angiogenesis by controlling the uptake of AA nutrient, which may serve as metabolic fuel for the proliferation of endothelial cells (ECs) and consequent promotion of angiogenesis. Our data provide an important step towards elucidating the molecular pathophysiology of PSCR that may explain the differences in ocular manifestations between individuals with hemoglobinopathies and afford insights for new alternative strategies to inhibit pathological angiogenesis.

Contributions of von Willebrand factor to clinical severity of sickle cell disease: a systematic review and metanalysis

AIM

To evaluate the contributions of VWF to the clinical manifestation and severity of SCD.

DESIGN

A systematic review of peer-reviewed articles published in English. The review was carried out in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist.

METHODS

The data sources for the review included MEDLINE, PubMed, CINAHL, and Academic Search Complete. Articles that applied a quantitative approach to the investigation of the relationship of vWF with clinical manifestations and severity indices were included. The risk of bias assessment was carried out with a mixed-method appraisal tool. We computed I ² to estimate the degree of heterogeneity.

RESULT

There was a significantly higher level of VWF in SCD than in the control (d = 2.7, Z = 4.865, P < 0.001, I ² = 96.41%). Significant positive correlations were obtained for the relationship of VWF with vasoocclusive crisis (r= 0.277, Z= 5.077, P < 0.001, 1 ² =15.62), rate of hemolysis (r=0.441; Z= 4.440, I ² = <1%), extracellular haemoglobin (r=-0.397, Z=-4.155, I ² =<1%) and CRP (r = 0.331, Z = 4.566, P < 0.001, I ² < 1%).The VWF is important in determining the clinical severity of sickle cell disease, which constitutes a putative therapeutic target. More work is required to understand the causal direction underlying the association between VWF levels and the clinical severity of sickle cell disease and the potential role that VWF plays in the clinical manifestations of sickle cell disease.

PROTOCOL REGISTRATION

The protocol was registered with PROSPERO (CRD42021262625).

Heme stimulates platelet mitochondrial oxidant production to induce targeted granule secretion

Hemolysis, a pathological component of many diseases, is associated with thrombosis and vascular dysfunction. Hemolytic products, including cell-free hemoglobin and free heme directly activate platelets. However, the effect of hemolysis on platelet degranulation, a central process in not only thrombosis, but also inflammatory and mitogenic signaling, remains less clear. Our group showed that hemoglobin-induced platelet activation involved the production of mitochondrial reactive oxygen species (mtROS). However, the molecular mechanism by which extracellular hemolysis induces platelet mtROS production, and whether these mtROS regulate platelet degranulation remains unknown. Here, we demonstrate using isolated human platelets that cell free heme is a more potent agonist for platelet activation than hemoglobin, and stimulates the release of a specific set of molecules, including the glycoprotein thrombospondin-1 (TSP-1), from the α-granule of platelets. We uncover the mechanism of heme-mediated platelet mtROS production which is dependent on the activation of platelet toll-like receptor 4 (TLR4) signaling and leads to the downstream phosphorylation and inhibition of complex-V by the serine kinase Akt. Notably, inhibition of platelet TLR4 or Akt, or scavenging of mtROS prevents heme-induced granule release in vitro. Further, heme-dependent granule release is significantly attenuated in vivo in mice lacking TLR4 or those treated with the mtROS scavenger MitoTEMPO. These data elucidate a novel mechanism of TLR4-mediated mitochondrial regulation, establish the mechanistic link between hemolysis and platelet degranulation, and begin to define the heme and mtROS-dependent platelet secretome. These data have implications for hemolysis-induced thrombo-inflammatory signaling and for the consideration of platelet mitochondria as a therapeutic target in hemolytic disorders.

Which adults with sickle cell disease need an evaluation for pulmonary embolism?

Individuals living with sickle cell disease (SCD) are at an increased risk of venous thrombo-embolism (VTE) including pulmonary embolisms (PEs). There is a high mortality associated with PE in individuals with SCD. It can be difficult to diagnose PE since presenting symptoms of PE often mimic those of other forms of vaso-occlusive crisis in SCD. Currently, there are no validated models for predicting PEs in patients with sickle cell disease, which often leads to frequent CT scans and exposure to harmful radiation and intravenous contrast. The aim of this study was to evaluate different host variables and potential clinical biomarkers of patients with SCD including those used in the Wells score to assess predictability for PE in order to create a more accurate diagnostic algorithm to predict PE. A retrospective chart review was performed on 349 patients with SCD who underwent testing for a PE with a CT scan of the chest. Forward and backward stepwise model selection was performed to obtain a parsimonious model of the predictors of PEs. The incidence of PE in this population was 9·7%. Of the factors evaluated for this study, the Wells score was the only one with clinical significance. A Wells score greater than 4 had a sensitivity and specificity of 72·5% and 70·1%, respectively, and a score greater than 6 had a sensitivity and specificity of 50% and 87%, respectively. The Wells score is an acceptable clinical tool which may prove useful in individuals with SCD to predict who is most likely to have a PE and therefore should undergo a CT scan. A prospective study is needed to further confirm these findings.

Vascular TSP1-CD47 signaling promotes sickle cell-associated arterial vasculopathy and pulmonary hypertension in mice

Pulmonary hypertension (PH) is a leading cause of death in sickle cell disease (SCD) patients. Hemolysis and oxidative stress contribute to SCD-associated PH. 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 SCD. We found that TSP1 and CD47 are upregulated in the lungs of Berkeley (BERK) sickling (Sickle) mice and patients with SCD-associated PH. We then generated chimeric animals by transplanting BERK bone marrow into C57BL/6J (n = 24) and CD47 knockout (CD47KO, n = 27) mice. Right ventricular (RV) pressure was lower in fully engrafted Sickle-to-CD47KO than Sickle-to-C57BL/6J chimeras, as shown by the reduced maximum RV pressure (P = 0.013) and mean pulmonary artery pressure (P = 0.020). The afterload of the sickle-to-CD47KO chimeras was also lower, as shown by the diminished pulmonary vascular resistance (P = 0.024) and RV effective arterial elastance (P = 0.052). On myography, aortic segments from Sickle-to-CD47KO chimeras showed improved relaxation to acetylcholine. We hypothesized that, in SCD, TSP1-CD47 signaling promotes PH, in part, by increasing reactive oxygen species (ROS) generation. In human pulmonary artery endothelial cells, treatment with TSP1 stimulated ROS generation, which was abrogated by CD47 blockade. Explanted lungs of CD47KO chimeras had less vascular congestion and a smaller oxidative footprint. Our results show that genetic absence of CD47 ameliorates SCD-associated PH, which may be due to decreased ROS levels. Modulation of TSP1-CD47 may provide a new molecular approach to the treatment of SCD-associated PH.

Morphine promotes neovascularizing retinopathy in sickle transgeneic mice

Neovascularizing retinopathy is a significant complication of sickle cell disease (SCD), occurring more frequently in HbSC than HbSS disease. This risk difference is concordant with a divergence of angiogenesis risk, as identified by levels of pro- vs anti-angiogenic factors in the sickle patient's blood. Because our prior studies documented that morphine promotes angiogenesis in both malignancy and wound healing, we tested whether chronic opioid treatment would promote retinopathy in NY1DD sickle transgenic mice. After 10 to 15 months of treatment, sickle mice treated with morphine developed neovascularizing retinopathy to a far greater extent than either of the controls (sickle mice treated with saline and wild-type mice treated identically with morphine). Our dissection of the mechanistic linkage between morphine and retinopathy revealed a complex interplay among morphine engagement with its μ opioid receptor (MOR) on retinal endothelial cells (RECs); morphine-induced production of tumor necrosis factor α and interleukin-6 (IL-6), causing increased expression of both MOR and vascular endothelial growth factor receptor 2 (VEGFR2) on RECs; morphine/MOR engagement transactivating VEGFR2; and convergence of MOR, VEGFR2, and IL-6 activation on JAK/STAT3-dependent REC proliferation and angiogenesis. In the NY1DD mice, the result was increased angiogenesis, seen as neovascularizing retinopathy, similar to the retinal pathology occurring in humans with SCD. Therefore, we conclude that chronic opioid exposure, superimposed on the already angiogenic sickle milieu, might enhance risk for retinopathy. These results provide an additional reason for development and application of opioid alternatives for pain control in SCD.

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.

Measuring success: utility of biomarkers in sickle cell disease clinical trials and care

Progress in the care of sickle cell disease (SCD) has been hampered by the extreme complexity of the SCD phenotype despite its monogenic inheritance. While epidemiological studies have identified clinical biomarkers of disease severity, with a few exceptions, these have not been routinely incorporated in clinical care algorithms. Furthermore, existing biomarkers have been poorly apt at providing objective parameters to diagnose sickle cell crisis, the hallmark, acute complication of SCD. The repercussions of these diagnostic limitations are reflected in suboptimal care and scarcity of adequate outcome measures for clinical research. Recent progress in molecular and imaging diagnostics has heralded a new era of personalized medicine in SCD. Precision medicine strategies are particularly timely, since molecular therapeutics are finally on the horizon. This chapter will summarize the existing evidence and promising data on biomarkers for clinical care and research in SCD.

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.

Human expression patterns: qualitative and quantitative analysis of thrombospondin-1 under physiological and pathological conditions

Thrombospondin-1 (TSP-1), a matricellular protein and one of the first endogenous anti-angiogenic molecules identified, has long been considered a potent modulator of human diseases. While the therapeutic effect of TSP-1 to suppress cancer was investigated in both research and clinical settings, the mechanisms of how TSP-1 is regulated in cancer remain elusive, and the scientific answers to the question of whether TSP-1 expressions can be utilized as diagnostic or prognostic marker for patients with cancer are largely inconsistent. Moreover, TSP-1 plays crucial functions in angiogenesis, inflammation and tissue remodelling, which are essential biological processes in the progression of many cardiovascular diseases, and therefore, its dysregulated expressions in such conditions may have therapeutic significance. Herein, we critically analysed the literature pertaining to TSP-1 expression in circulating blood and pathological tissues in various types of cancer as well as cardiovascular and inflammation-related diseases in humans. We compare the secretion rates of TSP-1 by different cancer and non-cancer cells and discuss the potential connection between the expression changes of TSP-1 and vascular endothelial growth factor (VEGF) observed in patients with cancer. Moreover, the pattern and emerging significance of TSP-1 profiles in cardiovascular disease, such as peripheral arterial disease, diabetes and other related non-cancer disorders, are highlighted. The analysis of published TSP-1 data presented in this review may have implications for the future exploration of novel TSP-1-based treatment strategies for cancer and cardiovascular-related diseases.

Shift in the Balance between Circulating Thrombospondin-1 and Vascular Endothelial Growth Factor in Cancer Patients: Relationship to Platelet α-Granule Content and Primary Activation

Tumoral angiogenesis is regulated by the balance between factors that activate and inhibit angiogenesis. Elevated levels of activators have been associated with a poor prognosis in cancer patients, but little is known about the net balance between circulating activators and inhibitors in these patients. This study was designed to determine whether the balance between circulating concentrations of the angiogenesis inhibitor TSP-1 and the activator VEGF differs from that in healthy persons, and to shed light on the possible role of platelets in this balance. Twenty-five cancer patients and 18 healthy subjects were included. Serum and plasma concentrations of VEGF, TSP-1 and PF4 were measured by ELISA. Our results showed that in healthy subjects the balance between the TSP-1 and VEGF concentrations in serum and in serum minus plasma was twice to three times as high as in cancer patients (p<0.05). The theoretical TSP-1 content per platelet was greater in healthy subjects than in patients (94 vs. 53.6 ng/mL, p<0.05), and platelet activation (determined indirectly as the plasma concentration of PF4) was greater in cancer patients (129 vs. 48 IU/mL, p<0.01). Platelet activation correlated significantly with serum concentration of TSP-1 (r=0.470, p=0.018) and showed a tendency toward correlation with plasma concentration of TSP-1 (r=0.382, p=0.059). Our findings show that the circulating TSP-1/VEGF balance is diminished in cancer patients. Platelet activation may play an important role in this decrease and may ultimately lead to increased angiogenic activity in these patients.

Prasugrel hydrochloride for the treatment of sickle cell disease

INTRODUCTION

Therapeutic options for sickle cell disease (SCD) are limited and, currently, only one drug (hydroxyurea) has FDA approval for the treatment of adult SCD. While this genetic disease is caused by hemoglobin polymerization, subsequent downstream events trigger platelet activation, vaso-occlusion and the disease's complex pathophysiology. Areas covered: The oral thienopyridine, prasugrel hydrochloride, irreversibly inhibits the P2Y12 receptors, inhibiting ADP-dependent platelet activation. We discuss recent clinical trials evaluating the pharmokinetics of prasugrel and its potential for use in SCD. Expert opinion: Prasugrel administration in SCD appears to be well tolerated and safe. However, although this drug modestly inhibits platelet activity in these patients, administration of prasugrel to a large group of children and adolescents for up to 24 months failed to convincingly reduce vaso-occlusive complications. Speculatively, prasugrel may be of occasional use for off-license purposes in patients unable or unwilling to take hydroxyurea (particularly in 12-17-year olds). Although there is currently no prospect of prasugrel being licensed for use in SCD, the success of on-going trials of other antiplatelet agents in SCD might lead to further trials of prasugrel in SCD.

Increased Vasoocclusive Crises in "O" Blood Group Sickle Cell Disease Patients: Association with Underlying Thrombospondin Levels

Objectives

To explore the incidence of vaso-occlusive crisis (VOC) in Blood Group “O” sickle cell disease (SCD) patients, and correlate it with the blood group and thrombospondin (TSP) levels.

Methods

In 89 consecutive SCD patients, blood samples were obtained for von Williebrand factor (vWF:Ag) antigen, collagen binding activity (CBA), ristocetin binding activity (RCo), blood group typing, C-reactive protein (CRP), high performance liquid chromatography (HPLC), Serum TSP 1 and TSP 2 levels, complete blood counts (CBC), lactic dehydrogenase (LDH) levels, liver function (LFT) and renal function tests (RFT) during VOC episodes and in steady state conditions.

Results

In steady state SCD patients (n=72), “O” blood group patients (n=37) showed a significantly higher median serum TSP 1 and TSP 2 levels as compared to non-O blood group patients [n=35] [p <0.05, Mann-Whitney test]; with an inverse relation between vWF:Ag, Factor VIII:C and TSP levels. Furthermore, the serum TSP 1 and TSP 2 levels were significantly higher in patients presenting with acute VOC [n=17], as well as in those with repeated VOC’s (group 1, n=16), especially amongst blood group “O” patients [p, <0.05, Mann-Whitney test].

Conclusions

The study demonstrates an inverse relation between TSP and vWF levels, in blood group “O” SCD patients, with an upregulation of the TSP levels. Expectedly, during active VOC crisis, the TSP 1 and TSP 2 levels were significantly elevated.

Therapeutic strategies in Sickle Cell Anemia: The past present and future

Sickle Cell Anemia (SCA) was one of the first hemoglobinopathies to be discovered. It is distinguished by the mutation-induced expression of a sickle cell variant of hemoglobin (HbS) that triggers erythrocytes to take a characteristic sickled conformation. The complex physiopathology of the disease and its associated clinical complications has initiated multi-disciplinary research within its field. This review attempts to lay emphasis on the evolution, current standpoint and future scope of therapeutic strategies in SCA.

Microparticles in sickle cell anaemia: promise and pitfalls

Blood from patients with sickle cell disease contains microparticles (MP) derived from multiple cell sources, including red cells, platelets, monocytes and endothelial cells. MPs are of great interest because of their disease associations, their status as promising biomarkers, and the intercellular communications they mediate. To illustrate the likelihood of their relevance in sickle cell disease, we discuss the nature of MP, their profiling in sickle disease, some caveats relevant to their detection, their roles in supporting coagulation and the disparate influences they may exert upon the pathobiology of sickle cell disease.

Pulmonary platelet thrombi and vascular pathology in acute chest syndrome in patients with sickle cell disease

A growing body of evidence suggests a role for platelets in sickle cell disease (SCD). Despite the proinflammatory, occlusive nature of platelets, a role for platelets in acute chest syndrome (ACS), however, remains understudied. To provide evidence and potentially describe contributory factors for a putative link between ACS and platelets, we performed an autopsy study of 20 SCD cases—10 of whom died from ACS and 10 whose deaths were not ACS‐related. Pulmonary histopathology and case history were collected. We discovered that disseminated pulmonary platelet thrombi were present in 3 out of 10 of cases with ACS, but none of the matched cases without ACS. Those cases with detected thrombi were associated with significant deposition of endothelial vWF and detection of large vWF aggregates adhered to endothelium. Potential clinical risk factors were younger age and higher platelet count at presentation. However, we also noted a sharp and significant decline in platelet count prior to death in each case with platelet thrombi in the lungs. In this study, neither hydroxyurea use nor perimortem transfusion was associated with platelet thrombi. Surprisingly, in all cases, there was profound pulmonary artery remodeling with both thrombotic and proliferative pulmonary plexiform lesions. The severity of remodeling was not associated with a severe history of ACS, or hydroxyurea use, but was inversely correlated with age. We thus provide evidence of undocumented presence of platelet thrombi in cases of fatal ACS and describe clinical correlates. We also provide novel correlates of pulmonary remodeling in SCD. Am. J. Hematol. 91:173–178, 2016. © 2015 The Authors. American Journal of Hematology Published by Wiley Periodicals, Inc.

Coagulation abnormalities of sickle cell disease: Relationship with clinical outcomes and the effect of disease modifying therapies

Sickle cell disease (SCD) is a hypercoagulable state. Patients exhibit increased platelet activation, high plasma levels of markers of thrombin generation, depletion of natural anticoagulant proteins, abnormal activation of the fibrinolytic system, and increased tissue factor expression, even in the non-crisis "steady state." Furthermore, SCD is characterized by an increased risk of thrombotic complications. The pathogenesis of coagulation activation in SCD appears to be multi-factorial, with contributions from ischemia-reperfusion injury and inflammation, hemolysis and nitric oxide deficiency, and increased sickle RBC phosphatidylserine expression. Recent studies in animal models suggest that activation of coagulation may contribute to the pathogenesis of SCD, but the data on the contribution of coagulation and platelet activation to SCD-related complications in humans are limited. Clinical trials of new generations of anticoagulants and antiplatelet agents, using a variety of clinical endpoints are warranted.

Thrombospondin-1 (TSP1) contributes to the development of vascular inflammation by regulating monocytic cell motility in mouse models of abdominal aortic aneurysm

RATIONALE

Histological examination of abdominal aortic aneurysm (AAA) tissues demonstrates extracellular matrix destruction and infiltration of inflammatory cells. Previous work with mouse models of AAA has shown that anti-inflammatory strategies can effectively attenuate aneurysm formation. Thrombospondin-1 is a matricellular protein involved in the maintenance of vascular structure and homeostasis through the regulation of biological functions, such as cell proliferation, apoptosis, and adhesion. Expression levels of thrombospondin-1 correlate with vascular disease conditions.

OBJECTIVE

To use thrombospondin-1-deficient (Thbs1(-/-)) mice to test the hypothesis that thrombospondin-1 contributes to pathogenesis of AAAs.

METHODS AND RESULTS

Mouse experimental AAA was induced through perivascular treatment with calcium phosphate, intraluminal perfusion with porcine elastase, or systemic administration of angiotensin II. Induction of AAA increased thrombospondin-1 expression in aortas of C57BL/6 or apoE-/- mice. Compared with Thbs1(+/+) mice, Thbs1(-/-) mice developed significantly smaller aortic expansion when subjected to AAA inductions, which was associated with diminished infiltration of macrophages. Thbs1(-/-) monocytic cells had reduced adhesion and migratory capacity in vitro compared with wild-type counterparts. Adoptive transfer of Thbs1(+/+) monocytic cells or bone marrow reconstitution rescued aneurysm development in Thbs1(-/-) mice.

CONCLUSIONS

Thrombospondin-1 expression plays a significant role in regulation of migration and adhesion of mononuclear cells, contributing to vascular inflammation during AAA development.

Autologous Platelet Gel

Leg ulcers represent a particularly disabling complication in patients with sickle cell disease (SCD). Platelet gel (PG) is a novel therapeutic strategy used for accelerating wound healing of a wide range of tissues through the continuous release of platelet growth factors. Here, we describe the use of PG preparation according to Anitua’s PRGF (preparations rich in growth factors) protocol for treating chronic nonhealing ulcers in patients with SCD. A positive response occurred in 3 patients with an area reduction of 85.7% to 100%, which occurred within 7 to 10 weeks, and a 35.2% and 20.5% of area reduction in 2 other patients, who however, had large ulcers. After calcium chloride addition, the platelet-rich plasmas demonstrated enhanced platelet-derived growth factors–BB ( P < .001), transforming growth factor-β1 ( P = .015), vascular endothelial growth factors ( P = .03), and hepatocyte growth factors (nonsignificant) secretion. Furthermore, calcium chloride addition induced a significant decrease in platelet number ( P = .0134) and there was no leukocyte detection in the PG product. These results demonstrate that PG treatment might impact the healing of leg ulcers in sickle cell disease, especially in patients with small ulcers.

Endothelial activation by platelets from sickle cell anemia patients

Sickle cell anemia (SCA) is associated with a hypercoagulable state. Increased platelet activation is reported in SCA and SCA platelets may present augmented adhesion to the vascular endothelium, potentially contributing to the vaso-occlusive process. We sought to observe the effects of platelets (PLTs) from healthy control (CON) individuals and SCA individuals on endothelial activation, in vitro. Human umbilical vein endothelial cells (HUVEC) were cultured, in the presence, or not, of washed PLTs from CON or steady-state SCA individuals. Supernatants were reserved for cytokine quantification, and endothelial adhesion molecules (EAM) were analyzed by flow cytometry; gene expressions of ICAM1 and genes of the NF-κB pathway were analyzed by qPCR. SCA PLTs were found to be more inflammatory, displaying increased adhesive properties, an increased production of IL-1β and a tendency towards elevated expressions of P-selectin and activated αIIbβ3. Following culture in the presence of SCA PLTs, HUVEC presented significant augmentations in the expressions of the EAM, ICAM-1 and E-selectin, as well as increased IL-8 production and increased ICAM1 and NFKB1 (encodes p50 subunit of NF-κB) gene expressions. Interestingly, transwell inserts abolished the effects of SCA PLTs on EAM expression. Furthermore, an inhibitor of the NF-κB pathway, BAY 11-7082, also prevented the induction of EAM expression on the HUVEC surface by SCA PLTs. In conclusion, we find further evidence to indicate that platelets circulate in an activated state in sickle cell disease and are capable of stimulating endothelial cell activation. This effect appears to be mediated by direct contact, or even adhesion, between the platelets and endothelial cells and via NFκB-dependent signaling. As such, activated platelets in SCD may contribute to endothelial activation and, therefore, to the vaso-occlusive process. Results provide further evidence to support the use of anti-platelet approaches in association with other therapies for SCD.

Role of the hemostatic system on sickle cell disease pathophysiology and potential therapeutics

Recent studies suggest that sickle cell disease (SCD) is a hypercoagulable state contributing to vaso-occlusive events in the microcirculation, resulting in acute and chronic sickle cell-related organ damage. In this article, we review the existing evidence for contribution of hemostatic system perturbation to SCD pathophysiology. We also review the data showing increased risk of thromboembolic events, particularly newer information on the incidence of venous thromboembolism. Finally, the potential role of platelet inhibitors and anticoagulants in SCD is briefly reviewed.

Stroke with intracranial stenosis is associated with increased platelet activation in sickle cell anemia

BACKGROUND

Overt stroke in sickle cell anemia (SCA) is associated with intracranial stenosis and thrombus formation. Platelet activation is critical for thrombus formation.

PROCEDURE

Platelet activation studies were performed in 50 subjects: 18 SCA patients with history of stroke or abnormal transcranial Doppler (TCD) and intracranial stenosis seen by magnetic resonance angiogram (MRA), 7 SCA patients with history of stroke or abnormal TCD but no intracranial stenosis, 13 SCA patients with no history of stroke or abnormal TCD, and 12 healthy African-Americans.

RESULTS

Of the 18 patients with intracranial stenosis, 11 (61%) had evidence of the moyo-moya phenomenon on MRA. SCA children with intracranial stenosis had a significantly greater total white cell count compared to both healthy African-American controls and SCA patients in the steady-state (P < 0.001). In addition, SCA patients with history of stroke or abnormal TCD had a significantly higher platelet count compared to healthy African-American controls (P < 0.002). The percentage of platelet surface P-selectin expression was significantly greater in patients with intracranial stenosis compared to the other groups (P < 0.05), particularly in individuals that did not have the moya-moya phenomenon seen on MRA.

CONCLUSION

Stroke with intracranial stenosis is associated with increased platelet activation in sickle cell anemia, and further investigation is needed on the role of anti-platelet agents in this high-risk population.

Expression of regulatory platelet microRNAs in patients with sickle cell disease

Background

Increased platelet activation in sickle cell disease (SCD) contributes to a state of hypercoagulability and confers a risk of thromboembolic complications. The role for post-transcriptional regulation of the platelet transcriptome by microRNAs (miRNAs) in SCD has not been previously explored. This is the first study to determine whether platelets from SCD exhibit an altered miRNA expression profile.

Methods and Findings

We analyzed the expression of miRNAs isolated from platelets from a primary cohort (SCD = 19, controls = 10) and a validation cohort (SCD = 7, controls = 7) by hybridizing to the Agilent miRNA microarrays. A dramatic difference in miRNA expression profiles between patients and controls was noted in both cohorts separately. A total of 40 differentially expressed platelet miRNAs were identified as common in both cohorts (p-value 0.05, fold change>2) with 24 miRNAs downregulated. Interestingly, 14 of the 24 downregulated miRNAs were members of three families - miR-329, miR-376 and miR-154 - which localized to the epigenetically regulated, maternally imprinted chromosome 14q32 region. We validated the downregulated miRNAs, miR-376a and miR-409-3p, and an upregulated miR-1225-3p using qRT-PCR. Over-expression of the miR-1225-3p in the Meg01 cells was followed by mRNA expression profiling to identify mRNA targets. This resulted in significant transcriptional repression of 1605 transcripts. A combinatorial approach using Meg01 mRNA expression profiles following miR-1225-3p overexpression, a computational prediction analysis of miRNA target sequences and a previously published set of differentially expressed platelet transcripts from SCD patients, identified three novel platelet mRNA targets: PBXIP1, PLAGL2 and PHF20L1.

Conclusions

We have identified significant differences in functionally active platelet miRNAs in patients with SCD as compared to controls. These data provide an important inventory of differentially expressed miRNAs in SCD patients and an experimental framework for future studies of miRNAs as regulators of biological pathways in platelets.

A double-blind, randomized, multicenter phase 2 study of prasugrel versus placebo in adult patients with sickle cell disease

Background

Platelet activation has been implicated in the pathogenesis of sickle cell disease (SCD) suggesting antiplatelet agents may be therapeutic. To evaluate the safety of prasugrel, a thienopyridine antiplatelet agent, in adult patients with SCD, we conducted a double-blind, randomized, placebo-controlled study.

Methods

The primary endpoint, safety, was measured by hemorrhagic events requiring medical intervention. Patients were randomized to prasugrel 5 mg daily (n = 41) or placebo (n = 21) for 30 days. Platelet function by VerifyNow® P2Y12 and vasodilator-stimulated phosphoprotein assays at days 10 and 30 were significantly inhibited in prasugrel- compared with placebo-treated SCD patients.

Results

There were no hemorrhagic events requiring medical intervention in either study arm. Mean pain rate (percentage of days with pain) and intensity in the prasugrel arm were decreased compared with placebo. However, these decreases did not reach statistical significance. Platelet surface P-selectin and plasma soluble P-selectin, biomarkers of in vivo platelet activation, were significantly reduced in SCD patients receiving prasugrel compared with placebo. In sum, prasugrel was well tolerated and not associated with serious hemorrhagic events.

Conclusions

Despite the small size and short duration of this study, there was a decrease in platelet activation biomarkers and a trend toward decreased pain.

Erythrocyte microparticles can induce kidney vaso-occlusions in a murine model of sickle cell disease

Patients with sickle cell disease suffer from painful crises associated with disseminated vaso-occlusions, increased circulating erythrocyte microparticles (MPs), and thrombospondin-1 (TSP1). MPs are submicron membrane vesicles shed by compromised or activated cells. We hypothesized that TSP1 mediates MP shedding and participates in vaso-occlusions. We injected TSP1 to transgenic SAD mice with sickle cell disease and characterized circulating phosphatidylserine+ MPs by FACS. TSP1 stimulated MPs in plasma and initiated vaso-occlusions within minutes. In vitro, TSP1 triggered rapid erythrocyte conversion into spicule-covered echinocytes, followed by MP shedding. MP shedding was recapitulated by peptides derived from the TSP1 carboxyterminus. We purified MPs shed by erythrocytes in vitro and administered them back to SAD mice. MPs triggered immediate renal vaso-occlusions. In vitro, MPs triggered the production of radical oxygen species by endothelial monolayers, favored erythrocyte adhesion, and induced endothelial apoptosis. MPs also compromised vasodilation in perfused microvessels. These effects were inhibited by saturating MP phosphatidylserine with annexin-V, or with inhibitors of endothelial ROS production. We conclude that TSP1 triggers erythrocyte MP shedding. These MPs induce endothelial injury and facilitate acute vaso-occlusive events in transgenic SAD mice. This work supports a novel concept that toxic erythrocyte MPs may connect sickle cell anemia to vascular disease.

Plasma thrombospondin-1 is increased during acute sickle cell vaso-occlusive events and associated with acute chest syndrome, hydroxyurea therapy, and lower hemolytic rates

Platelets are activated in sickle cell disease (SCD), and particularly during vaso-occlusive episodes (VOE). Thrombospondin-1 (TSP1), a major secretory product of activated platelets, is increased in the circulation in VOE and binds to sickle red blood cells (RBC) promoting vascular adhesion. Thus, we hypothesized that TSP1 may represent a plasma biomarker of disease severity in SCD. We tested the plasma collected from patients in steady state (n = 27) and VOE (n = 14), as well as healthy controls (n = 17) at the University of Pittsburgh Medical Center (UPMC), and from patients in steady state enrolled in the walk-PHaSST clinical trial (n = 483). We found that TSP1 levels were increased in VOE in the UPMC cohort. Among steady-state patients at UPMC, TSP1 values correlated positively with lifetime history of acute chest syndrome (r = 0.72, P < 0.0001) and hemoglobin concentration (r = 0.49, P = 0.01), and negatively with markers of hemolysis, such as LDH (r = -0.50, P = 0.009). Analysis of the walk-PHaSST cohort also showed a positive association between TSP1 levels and hydroxyurea use (r = 0.14, P = 0.003), and confirmed the negative associations with the severity of hemolysis. Our results suggest that TSP1 levels are associated with more VOE, hydroxyurea use and lower rates of hemolysis. High TSP1 concentrations may indicate higher risk of the viscosity/vaso-occlusion phenotype of SCD.

Platelet activation, P-selectin, and eosinophil β1-integrin activation in asthma

RATIONALE

Eosinophil β1-integrin activation correlates inversely with FEV1 and directly with eosinophil-bound P-selectin in subjects with nonsevere allergic asthma.

OBJECTIVES

Determine the relationships between β1-integrin activation and pulmonary function or eosinophil-bound P-selectin in subjects with asthma of varying severity and discern the source of eosinophil-bound P-selectin.

METHODS

Blood was assayed by flow cytometry for P-selectin and activated β1-integrin on eosinophils and platelets. Plasma was analyzed with ELISA for soluble P-selectin, platelet factor 4, and thrombospondin-1.

MEASUREMENTS AND MAIN RESULTS

Activated β1-integrin correlated with eosinophil-bound P-selectin among all subjects with asthma even though activated β1-integrin was higher in subjects with nonsevere asthma than severe asthma. Activated β1-integrin correlated inversely with FEV1 corrected for FVC only in younger subjects with nonsevere asthma. Paradoxically, platelet surface P-selectin, a platelet activation marker, was low in subjects with severe asthma, whereas plasma platelet factor 4, a second platelet activation marker, was high. Correlations indicated that P-selectin-positive platelets complexed to eosinophils are the major source of the eosinophil-bound P-selectin associated with β1-integrin activation. After whole-lung antigen challenge of subjects with nonsevere asthma, a model of asthma exacerbation known to cause platelet activation, circulating eosinophils bearing P-selectin and activated β1-integrin disappeared.

CONCLUSIONS

The relationship between eosinophil β1-integrin activation and pulmonary function was replicated only for younger subjects with nonsevere asthma. However, we infer that platelet activation and binding of activated platelets to eosinophils followed by P-selectin-mediated eosinophil β1-integrin activation occur in both nonsevere and severe asthma with rapid movement of platelet-eosinophil complexes into the lung in more severe disease.

Reconstructing sickle cell disease: a data-based analysis of the "hyperhemolysis paradigm" for pulmonary hypertension from the perspective of evidence-based medicine

The "hyperhemolytic paradigm" (HHP) posits that hemolysis in sickle disease sequentially and causally establishes increased cell-free plasma Hb, consumption of NO, a state of NO biodeficiency, endothelial dysfunction, and a high prevalence of pulmonary hypertension. The basic science underpinning this concept has added an important facet to the complexity of vascular pathobiology in sickle disease, and clinical research has identified worrisome clinical issues. However, this critique identifies and explains a number of significant concerns about the various HHP component tenets. In addressing these issues, this report presents: a very brief history of the HHP, an integrated synthesis of mechanisms underlying sickle hemolysis, a review of the evidentiary value of hemolysis biomarkers, an examination of evidence bearing on existence of a hyperhemolytic subgroup, and a series of questions that should naturally be applied to the HHP if it is examined using critical thinking skills, the fundamental basis of evidence-based medicine. The veracity of different HHP tenets is found to vary from true, to weakly supported, to demonstrably false. The thesis is developed that the HHP has misidentified the mechanism and clinical significance of its findings. The extant research questions identified by these analyses are delineated, and a conservative, evidence-based approach is suggested for application in clinical medicine.

Differential modulation of adhesion molecule expression by hydroxycarbamide in human endothelial cells from the micro- and macrocirculation: potential implications in sickle cell disease vasoocclusive events

BACKGROUND

All the cellular partners of the vascular system and especially endothelial cells are involved in the pathophysiology of the vasoocclusive crises associated with sickle cell disease. In sickle cell disease, circulating cells adhere abnormally to endothelial cells in a chronic pro-inflammatory context. Hydroxycarbamide is the only drug with demonstrated efficacy to reduce the frequency of vasoocclusive crises. Here, we investigated the effects of hydroxycarbamide and/or cytokines on the expression of genes related to adhesion events in endothelial cells from three different vascular sites.

DESIGN AND METHODS

Endothelial cells representative of the macro- (HUVEC) or microcirculation (TrHBMEC and HPMEC) were grown in the presence or absence of hydroxycarbamide and/or cytokines (TNFα and IFNγ). Expression of genes encoding adhesion proteins was analyzed by RQ-PCR, ELISA, flow cytometry, in situ ELISA for extracellular matrix proteins, and Western blot.

RESULTS

In cells from the microcirculation, expression of TSP-1, vWF, and PECAM-1 genes was decreased by hydroxycarbamide and/or cytokine treatment at the mRNA level. In the macro-circulation their expression was unaffected or increased. Hydroxycarbamide significantly decreased vWF incorporated in the TrHBMEC extracellular matrix. CD36 mRNA was strongly down-regulated by cytokines in HPMEC, the only cell type in which it is expressed. Hydroxycarbamide decreased soluble PECAM-1 in HUVEC supernatants.

CONCLUSIONS

Our results highlight the heterogeneity of vascular endothelial cell responses to hydroxycarbamide and/or cytokines depending upon their origin. They also suggest that hydroxycarbamide has an anti-adhesogenic effect on endothelial cells, but by mechanisms which could vary according to their macro- or microcirculation and organ origin.

Erythroid adhesion molecules in sickle cell disease: effect of hydroxyurea

In sickle cell disease, the complex scenario of vaso-occlusive crisis (VOC) typical of this disease is clearly multifactorial and not fully understood. Cell-cell and cell-cell matrix interactions mediated by adhesive molecules present on blood cells and endothelial cells (ECs) are thought to play an important role. Early studies have shown that sickle red blood cells (RBCs) are abnormally adherent to ECs and some of the molecules involved in these interactions have been identified, such as the alpha4beta1 integrin and CD36, exclusively present on stress reticulocytes, and CD47 on mature RBCs. More recently, attention focused on Lu/BCAM, the unique RBC receptor for laminin, and on ICAM-4, a red cell-specific adhesion receptor, which is a ligand for a large repertoire of integrins (alphaLbeta2, alphaMbeta2, alphaxbeta2, alphaVbeta3). The counter-receptors on ECs and the role of plasma proteins forming bridges between blood cells and ECs have been clarified in part. It has also been shown that reticulocytes from SCD patients express higher levels of alpha4beta1 integrin and CD36, and that under hydroxyurea (HU) therapy, both cell adhesion to ECs or extracellular matrix proteins and the levels of these adhesion molecules are reduced. These findings are consistent with the view that enhanced adhesion of blood cells to ECs is largely determined by the membrane expression level of adhesion molecules and could be a crucial factor for triggering or aggravating vaso-occlusion. In SCD patients, membrane expression of Lu/BCAM (and perhaps ICAM-4) is enhanced on RBCs whose adherence to laminin or ECs is also increased. Interestingly, Lu/BCAM- and ICAM-4-mediated adhesion are enhanced by the stress mediator epinephrine through a PKA-dependent pathway initiated by a rise in intracellular cAMP and leading to receptor activation by phosphorylation according to the same signaling pathway. More recently, studies based on quantitative expression analysis of adhesion molecules on RBCs and during erythroid differentiation in patients undergoing HU therapy, surprisingly revealed that Lu/BCAM level was enhanced, although alpha4beta1, CD36 and ICAM-4 (to a lower extent) levels were indeed reduced. CD47 and CD147 expression were also enhanced in HU-treated patients. Based on these findings we suggest that the signalization cascade leading to receptor activation rather than the expression level only of adhesion molecules may be the critical factor regulating cell adhesion, although both mechanisms are not mutually exclusive.

Morphological and functional platelet abnormalities in Berkeley sickle cell mice

Berkeley sickle cell mice are used as animal models of human sickle cell disease but there are no reports of platelet studies in this model. Since humans with sickle cell disease have platelet abnormalities, we studied platelet morphology and function in Berkeley mice (SS). We observed elevated mean platelet forward angle light scatter (FSC) values (an indirect measure of platelet volume) in SS compared to wild type (WT) (37+/-3.2 vs. 27+/-1.4, mean+/-SD; p<0.001), in association with moderate thrombocytopenia (505+/-49 x 10(3)/microl vs. 1151+/-162 x 10(3)/microl; p<0.001). Despite having marked splenomegaly, SS mice had elevated levels of Howell-Jolly bodies and "pocked" erythrocytes (p<0.001 for both) suggesting splenic dysfunction. SS mice also had elevated numbers of thiazole orange positive platelets (5+/-1% vs. 1+/-1%; p<0.001), normal to low plasma thrombopoietin levels, normal plasma glycocalicin levels, normal levels of platelet recovery, and near normal platelet life spans. Platelets from SS mice bound more fibrinogen and antibody to P-selectin following activation with a threshold concentration of a protease activated receptor (PAR)-4 peptide compared to WT mice. Enlarged platelets are associated with a predisposition to arterial thrombosis in humans and some humans with SCD have been reported to have large platelets. Thus, additional studies are needed to assess whether large platelets contribute either to pulmonary hypertension or the large vessel arterial occlusion that produces stroke in some children with sickle cell disease.

Cytokines and plasma factors in sickle cell disease

PURPOSE OF REVIEW

Plasma cytokines and related factors represent a burgeoning area of inquiry related to the pathogenesis in sickle cell disease. Cytokines derived from platelets, white blood cells and endothelial cells have all been implicated in the development of several sequelae of this disease. In this review, we seek to provide an overview of the noted and potentially novel roles for several key plasma factors in sickle cell disease. We also consider the putative role for those cytokines implicated by genetic analysis in sickle cell disease, but where the pathogenic, or ameliorative, role has yet to be determined.

RECENT FINDINGS

New roles for the platelet as a key mediator in the release of cytokines in sickle cell disease have recently been demonstrated. Angiogenic and inflammatory factors are also being explored in this illness. Members of the vascular endothelial growth factor and transforming growth factor-beta superfamilies have been suggested to contribute to several key events in pathogenesis of sickle cell disease, but with the promise of nitrous oxide therapy in this disorder, these cytokines merit a fresh perspective in the context of sickle cell disease.

SUMMARY

Increased understanding of the origin and pathology of cytokine levels in sickle cell disease may provide novel therapeutic approaches in the management of the disease.

Redox-dependent impairment of vascular function in sickle cell disease

The vascular pathophysiology of sickle cell disease (SCD) is influenced by many factors, including adhesiveness of red and white blood cells to endothelium, increased coagulation, and homeostatic perturbation. The vascular endothelium is central to disease pathogenesis because it displays adhesion molecules for blood cells, balances procoagulant and anticoagulant properties of the vessel wall, and regulates vascular homeostasis by synthesizing vasoconstricting and vasodilating substances. The occurrence of intermittent vascular occlusion in SCD leads to reperfusion injury associated with granulocyte accumulation and enhanced production of reactive oxygen species. The participation of nitric oxide (NO) in oxidative reactions causes a reduction in NO bioavailability and contributes to vascular dysfunction in SCD. Therapeutic strategies designed to counteract endothelial, inflammatory, and oxidative abnormalities may reduce the frequency of hospitalization and blood transfusion, the incidence of pain, and the occurrence of acute chest syndrome and pulmonary hypertension in patients with SCD.

Beta-thalassaemia and sickle cell anaemia as paradigms of hypercoagulability

Thalassaemia and sickle cell disease (SCD) represent the most common forms of hereditary haemolytic anaemia and result from a partial or complete lack of synthesis of one of the major alpha- or beta-globin chains of haemoglobin A or from a single amino acid mutation (beta(6Glu-->Val)) of the beta-globin chain respectively. Although they have different pathophysiologies, patients with these conditions manifest both biochemical and clinical evidence of hypercoagulability. While the frequency of various thrombotic complications may vary in beta-thalassaemia and homozygous SCD [sickle cell anaemia (SCA)], patients with both diseases manifest decreased levels of natural anticoagulant proteins, as well as increased markers of thrombin generation and platelet activation. The abnormal phospholipid membrane assymetry present in the red blood cells of beta-thalassaemia and SCA patients, with resultant phosphatidylserine exposure appears to play a significant role in the aetiology of the observed hypercoagulable state. This review presents the available data on the aetiology and clinical manifestations of the coagulation and platelet activation that exist in both beta-thalassaemia and SCA, as well as the potential therapeutic implications resulting from this hypercoagulability.

Sickle Cell Vaso-Occlusion

The vaso-occlusion model has evolved impressively over the past several decades from polymerization-based concepts to a complex, wide-ranging schema that involves multistep, heterogeneous, and interdependent interactions among sickle erythrocytes (SSRBCs), adherent leukocytes, endothelial cells, plasma proteins, and other factors. Endothelial activation, induced directly or indirectly by the proinflammatory behavior of SSRBCs, is the most likely initiating step toward vaso-occlusion. Given the complexity and dynamic relationships of the potential mechanisms leading to vaso-occlusion, further in vivo studies in relevant sickle cell animal models will most likely yield the greatest advances and promote the development of novel, more effective therapeutic strategies.

Platelet P-selectin and platelet mass, volume and component in sickle cell disease: relationship to genotype

BACKGROUND AND PURPOSE

Excess platelet activation (e.g. increased soluble P selectin [sPsel] and beta thromboglobulin [beta-TG]) is well established in sickle cell disease (SCD) and may contribute to the prothrombotic/hypercoagulable state and vascular occlusion characteristic of the disease. We hypothesised altered whole platelet P-selectin (pPsel), and morphological platelet indices mass, volume and component in SCD and two of its major genotypes.

METHODS

We recruited 35 SCD patients [mean age 31 years, 54% men]. Of these, 16 had homozygous sickle cell (HbSS) disease and 19 had sickle-haemoglobin-C (HbSC) disease. Patients were compared with 29 subjects with normal haemoglobin (HbAA) matched for age and ethnicity. Platelet mass, volume and component were measured by flow cytometry, pPsel in platelet lysate, sP-sel and beta-TG by ELISA.

RESULTS

SCD patients had lower pP-sel and mean platelet volume (MPV) but elevated platelet component (MPC), and, as expected, elevated platelet count, and sP-sel (all p<0.05) compared to HbAA subjects. In both groups, pPsel correlated with MPV, and MPV correlated positively with mean platelet mass (MPM) and negatively with MPC. sPsel correlated with platelet count only in SCD, not in the controls. Platelet count alone was different (higher) in HbSS compared to HbSC, and sPsel correlated with platelet count only in HbSC disease, not in HbSS disease.

CONCLUSION

Patients with SCD have various abnormalities in their platelets regardless of genotype: there are more numerous platelets, which are smaller, contain less P selectin per cell, but have a higher concentration of granules than those of HbAA subjects. These differences may mark and/or promote the prothrombotic state in SCD.

Erythrocyte adhesion receptors: blood group antigens and related molecules

During the second half of the 20th century, blood bankers quickly expanded our knowledge of human erythrocyte blood group antigens. By the dawn of the 21st century, several hundred blood group antigen polymorphisms had been identified. Hot on the heels of the serologists, membrane biochemists and molecular geneticists defined both the biochemical and genetic bases of most of these antigens. Perhaps to their surprise, this work has led to the discovery of functionally diverse and important membrane proteins expressed on the surface of red cells, including numerous adhesion molecules. Red cells express an unexpected number of such adhesion receptors, some of which contribute to human disease, as well as to normal red cell development. And perhaps most interestingly, study of these molecules has elucidated ways in which even mature red cells respond to external stimuli, such as adrenergic hormones.

Sickle cell adhesion depends on hemodynamics and endothelial activation

Venular microvascular circulation in patients with sickle cell anemia exhibits reduced and episodic blood flow. Sickle erythrocyte adhesion to postcapillary venule endothelium is postulated to initiate and propagate episodes of vasoocclusive pain. Hemodynamics likely mediate the adherence of sickle cells to endothelium, controlling delivery of potentially adherent erythrocytes and removal of loosely adherent erythrocytes on the endothelium. This study found a high dependence on shear stress of sickle erythrocyte adhesion to vascular cell adhesion molecule-1 (VCAM-1) on endothelium stimulated by tumor necrosis factor-alpha (TNF-alpha). Shear stress varied from 1.0 dyne/cm 2 (microvascular venular flow), in which VCAM-1 ligand interactions induced by TNF-alpha primarily controlled adherence, to 0.1 dyne/cm 2 (low flow), in which stimulation had little effect on adherence. At shear stresses analogous to in vivo velocities from laser Doppler ultrasound studies (0.8 and 0.6 dyne/cm 2 ), TNF-alpha promoted 1.9- and 2.7-fold increases in adhesion compared with unstimulated (baseline) adherence. These findings suggest a dynamic vasoocclusive process that depends on both receptor expression and shear stress. These results indicate that, in the microvasculature, slightly reduced inflow rate, increased receptor expression, or both may result in large increases in sickle erythrocyte adhesion.

Mechanism of CD47-induced α4β1 Integrin Activation and Adhesion in Sickle Reticulocytes

We recently reported that CD47 (integrin-associated protein) on sickle red blood cells (SS RBCs) activates G-protein-dependent signaling, which promotes cell adhesion to immobilized thrombospondin (TSP) under relevant shear stress. These data suggested that signal transduction in SS RBCs may contribute to the vaso-occlusive pathology observed in sickle cell disease. However, the CD47-activated SS RBC adhesion receptor(s) that mediated adhesion to immobilized TSP remained unknown. Here we demonstrate that the alpha4beta1 integrin (VLA-4) is the receptor that mediates CD47-stimulated SS RBC adhesion to immobilized TSP. This adhesion requires both the N-terminal heparin-binding domain and the RGD site of TSP. CD47 signaling induces an "inside-out" activation of alpha4beta1 on SS RBCs as indicated by an RGD-dependent interaction of this integrin with soluble, plasma fibronectin. However, CD47 engagement also induces an alpha4beta1-mediated, RGD-independent adhesion of SS RBCs to immobilized vascular cell adhesion molecule-1 (VCAM-1). CD47 signaling in SS RBCs appears to be independent of large scale changes in cAMP formation but nonetheless promotes alpha4beta1-mediated adhesion via a protein kinase A-dependent, serine phosphorylation of the alpha4 cytoplasmic domain. CD47-activated SS RBC adhesion absolutely requires the Src family tyrosine kinases and is also enhanced by treatment of SS RBCs with low concentrations of cytochalasin D, which may release alpha4beta1 from cytoskeletal restraints. In addition, CD47 co-immunoprecipitates with alpha4beta1 in a sickle reticulocyte-enriched fraction of SS RBCs. These studies therefore identify the alpha4beta1 integrin on SS RBCs as a CD47-activated receptor for TSP, VCAM-1, and plasma fibronectin, revealing novel binding characteristics of this integrin.

Cytokines in sickle cell disease

Sickle red cells express adhesion molecules including integrin alpha4beta1, CD36, band 3 protein, sulfated glycolipid, Lutheran protein, phosphatidylserine and integrin-associated protein. The proadhesive sickle cells may bind to endothelial cell P-selectin, E-selectin, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), CD36 and integrins leading to its activation. Monocytes also activate endothelium by releasing proinflammatory cytokines like tumor necrosis factor alpha (TNF-alpha) and interleukin 1beta (IL-1beta). Sickle monocytes also express increased surface CD11b and cytoplasmic cytokines TNFalpha and IL-1beta indicating activated state. Polymorphonuclear leukocytes (PMNs) are also activated with reduced L-selectin expression, enhanced CD64 expression and elevated levels of sL-selectin, sCD16 and elastase resulting in increased adhesiveness to the endothelium. Platelets are also activated and secrete thrombospondin (TSP) and cytokine IL-1. They also form platelet- monocytes aggregates causing endothelial cell P-selectin expression. Endothelial cell activation by these multiple mechanisms leads to a loss of vascular integrity, expression of leukocyte adhesion molecules, change in the surface phenotype from antithrombotic to prothrombotic, excessive cytokine production and upregulation of HLA molecules. Furthermore, contraction of these activated endothelial cells leads to exposure of extracellular matrix proteins, such as TSP, laminin, and fibronectin and their participation in adhesive interactions with bridging molecules from the plasma such as von Willebrand factor (vWf) released from endothelial cells, ultimately culminating in vasoocclusion and local tissue ischemia, the pathognomonic basis of vasoocclusive crisis.

Hypercoagulability in sickle cell disease: a curious paradox

There is evidence of activation of both blood coagulation and platelets in sickle cell disease. For example, plasma samples obtained in the steady state and during painful crisis demonstrate high levels of thrombin generation, depletion of anticoagulant proteins, and abnormal activation of the fibrinolytic system. Similarly, exposure of surface markers such as CD62P and CD40L, along with increased circulating levels of thrombospondin, signal platelet activation. In addition to its effects on the cleavage of fibrinogen and its ability to activate platelets, the increase in circulating thrombin levels, with its wide-ranging effects on endothelial cells and blood vessels, may be important in the pathophysiology of sickle cell disease. Therefore, treatments that could decrease thrombin generation or platelet activation may be beneficial in both the treatment of sickle cell disease and the prevention of complications that characterize this genetic disorder. This review discusses hypercoagulability in the various forms of sickle cell disease, including homozygous sickle cell anemia, hemoglobin SC disease, hemoglobin SD disease, and sickle cell-beta-thalassemia.

Single-chain antibody fragments derived from a human synthetic phage-display library bind thrombospondin and inhibit sickle cell adhesion

The enhanced adhesion of sickle red blood cells (RBCs) to the vascular endothelium and subendothelial matrix likely plays a significant role in the pathogenesis of vaso-occlusion in sickle cell disease. Sickle RBCs have enhanced adhesion to the plasma and extracellular matrix protein thrombospondin-1 (TSP) under conditions of flow in vitro. In this study, we sought to develop antibodies that bind TSP from a highly diverse library of human single-chain Fv fragments (scFvs) displayed on filamentous phage. Following 3 rounds of phage selection of increasing stringency 6 unique scFvs that bound purified TSP by enzyme-linked immunosorbent assay were isolated. Using an in vitro flow adhesion assay, 3 of the 6 isolated scFvs inhibited the adhesion of sickle RBCs to immobilized TSP by more than 40% compared with control scFvs (P <.001). Furthermore, scFv TSP-A10 partially inhibited sickle RBC adhesion to activated endothelial cells (P <.005). Using TSP proteolytic fragments to map the binding site, we showed that 2 of the inhibitory scFvs bound an epitope in the calcium-binding domain or proximal cell-binding domain of TSP, providing evidence for the role of these domains in the adhesion of sickle RBCs to TSP. In summary, we have isolated a panel of scFvs that specifically bind to TSP and differentially inhibit sickle RBC adhesion to surface-bound TSP under flow conditions. These scFvs will be useful reagents for investigating the role of the calcium and cell-binding domains of TSP in sickle RBC adhesion.

Inhibition of platelet aggregation by rat globin

This study was undertaken to identify and characterize the anti-aggregatory protein factor present in rat polymorphonuclear leukocytes (PMNs) supernatant. Since the purified protein exhibited sequence homology to beta globin, globin was also isolated from rat blood by acid-acetone precipitation and was purified on Superdex-75 column in FPLC. Elution of rat globin on the gel filtration column yielded two peaks of approximately 60 and 30 kDa as observed in the PMNs supernatant. Purity of globin and eluted fractions was further evaluated by SDS-PAGE. Platelet aggregation induced by agonists viz. adenosine-5'-diphosphate (ADP; 2-5 microM), arachidonic acid (AA; 10 microM), A23187 (2.50 microg/ml) was inhibited by globin and the purified fractions. ADP-induced rise in intracellular calcium levels and expression of CD62 on the platelets were reduced by both globin and active fraction of PMNs supernatant. Results obtained suggest that globin or globin-related protein present in the PMNs supernatant inhibits platelet aggregation response.

Sickle erythrocyte adherence to endothelium at low shear: role of shear stress in propagation of vaso-occlusion

Under venular flow conditions, sickle cell adherence to endothelium is mediated by cell adhesion molecules and adhesive proteins associated with inflammation, coagulation, and endothelial perturbation. Periodic and reduced blood flow are observed in sickle microcirculation during hematologic steady state, suggesting that blood flow is compromised in sickle microcirculation. We tested the hypothesis that low blood flow enhances adherence by quantifying sickle cell adhesion to endothelium under venular flow (1.0 dyne/cm(2) shear stress) and low flow (0.1 dyne/cm(2) shear stress), with and without addition of adhesion promoting agonists. Under low flow, sickle cell adherence to endothelium increases with contact time in the absence of endothelial activation or adhesive protein addition. In contrast, at venular shear stress, sickle cell adherence only occurs following endothelial activation with TNF-alpha or addition of thrombospondin. Analysis of these data with a mathematical model reveals that at low flow adherence is "transport-controlled," meaning that contact time between sickle cells and endothelium is a more important determinant of adherence than high-affinity receptor-ligand interactions. Low-affinity interactions are sufficient for adhesion at low flow. In contrast, at venular flow (1 dyne/cm(2) shear stress) adherence is "affinity-controlled," meaning that adherence requires induction of specific high-affinity receptor-ligand interactions. These findings demonstrate that in addition to activating factors and adherence proteins, microvascular shear stress is an important determinant of sickle cell adhesion to endothelium. This suggests that in vivo, erythrostasis is an important determinant of adhesion that can act either independently or concurrently with ongoing acute events to induce adhesive interactions and vaso-occlusion.