Sickle erythrocyte adherence to vascular endothelium. Morphologic correlates and the requirement for divalent cations and collagen-binding plasma proteins

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).

Emerging functional microfluidic assays for the study of thromboinflammation in sickle cell disease

PURPOSE OF REVIEW

This review briefly summarizes the significant impact of thromboinflammation in sickle cell disease in relation to recent advances in biomarkers that are used in functional microfluidic assays.

RECENT FINDINGS

Sickle cell disease (SCD) is an inherited hemoglobinopathy that affects 100 000 Americans and millions worldwide. Patients with SCD exhibit chronic haemolysis, chronic inflammation and thrombosis, and vaso-occlusion, triggering various clinical complications, including organ damage and increased mortality and morbidity. Recent advances in functional microfluidic assays provide direct biomarkers of disease, including abnormal white blood cell and red blood cell adhesion, cell aggregation, endothelial degradation and contraction, and thrombus formation.

SUMMARY

Novel and emerging functional microfluidic assays are a promising and feasible strategy to comprehensively characterize thromboinflammatory reactions in SCD, which can be used for personalized risk assessment and tailored therapeutic decisions.

Altered Ca2+ Homeostasis in Red Blood Cells of Polycythemia Vera Patients Following Disturbed Organelle Sorting during Terminal Erythropoiesis

Over 95% of Polycythemia Vera (PV) patients carry the V617F mutation in the tyrosine kinase Janus kinase 2 (JAK2), resulting in uncontrolled erythroid proliferation and a high risk of thrombosis. Using mass spectrometry, we analyzed the RBC membrane proteome and showed elevated levels of multiple Ca2+ binding proteins as well as endoplasmic-reticulum-residing proteins in PV RBC membranes compared with RBC membranes from healthy individuals. In this study, we investigated the impact of JAK2V617F on (1) calcium homeostasis and RBC ion channel activity and (2) protein expression and sorting during terminal erythroid differentiation. Our data from automated patch-clamp show modified calcium homeostasis in PV RBCs and cell lines expressing JAK2V617F, with a functional impact on the activity of the Gárdos channel that could contribute to cellular dehydration. We show that JAK2V617F could play a role in organelle retention during the enucleation step of erythroid differentiation, resulting in modified whole cell proteome in reticulocytes and RBCs in PV patients. Given the central role that calcium plays in the regulation of signaling pathways, our study opens new perspectives to exploring the relationship between JAK2V617F, calcium homeostasis, and cellular abnormalities in myeloproliferative neoplasms, including cellular interactions in the bloodstream in relation to thrombotic events.

Determination of red blood cell adhesion to vascular endothelial cells: A critical role for blood plasma

Red blood cell (RBC) adhesion to vascular endothelial cells (EC) is considered a potent effector of circulatory disorders, and its enhancement is implicated in the pathophysiology of numerous conditions, mainly hemoglobinopathies. The actual RBC/EC interaction is determined by both cellular and plasmatic factors, and the differentiation between them is essential for understanding its physiological implications. Yet, RBC/EC adhesion has been studied predominantly in protein-free media. To explore the plasma contribution to RBC/EC adhesion, we examined the adhesion of human RBC to human vascular endothelial cells in the presence of fresh frozen plasma (FFP) and compared it to that in a protein-free phosphate-buffered saline (PBS). RBC from blood samples freshly-collected from five healthy donors and from fifteen units of packed RBC units were used. The same FFP sample was used in all measurements. In FFP, the RBC form strongly adherent aggregates, which are dispersed as the shear stress (τ) increases to 3.0 Pa, and even at 5.0 Pa a large portion of the RBC are still adherent. In PBS, the RBC are singly dispersed and their adhesion becomes insignificant already at τ = 0.5 Pa. No cross-correlation was found between the adhesion in PBS vs. that in FFP at the same τ. However, in both media, under conditions that form singly dispersed adherent RBC, an inverse correlation between RBC/EC adhesion in PBS vs. that in FFP was observed. This study clearly implies that for understanding the physiological relevance of RBC/EC adhesion it should be determined in plasma.

The Role of RBC Oxidative Stress in Sickle Cell Disease: From the Molecular Basis to Pathologic Implications

Sickle cell disease (SCD) is an inherited monogenic disorder and the most common severe hemoglobinopathy in the world. SCD is characterized by a point mutation in the β-globin gene, which results in hemoglobin (Hb) S production, leading to a variety of mechanistic and phenotypic changes within the sickle red blood cell (RBC). In SCD, the sickle RBCs are the root cause of the disease and they are a primary source of oxidative stress since sickle RBC redox state is compromised due to an imbalance between prooxidants and antioxidants. This imbalance in redox state is a result of a continuous production of reactive oxygen species (ROS) within the sickle RBC caused by the constant endogenous Hb autoxidation and NADPH oxidase activation, as well as by a deficiency in the antioxidant defense system. Accumulation of non-neutralized ROS within the sickle RBCs affects RBC membrane structure and function, leading to membrane integrity deficiency, low deformability, phosphatidylserine exposure, and release of micro-vesicles. These oxidative stress-associated RBC phenotypic modifications consequently evoke a myriad of physiological changes involved in multi-system manifestations. Thus, RBC oxidative stress in SCD can ultimately instigate major processes involved in organ damage. The critical role of the sickle RBC ROS production and its regulation in SCD pathophysiology are discussed here.

Individual red blood cell nitric oxide production in sickle cell anemia: Nitric oxide production is increased and sickle shaped cells have unique morphologic change compared to discoid cells

Sickle cell anemia (SCA) is characterized by decreased red blood cell (RBC) deformability due to polymerization of deoxygenated hemoglobin, leading to abnormal mechanical properties of RBC, increased cellular adhesion, and microcirculatory obstruction. Prior work has demonstrated that NO• influences RBC hydration and deformability and is produced at a basal rate that increases under shear stress in normal RBC. Nevertheless, the origin and physiological relevance of nitric oxide (NO•) production and scavenging in RBC remains unclear. We aimed to assess the basal and shear-mediated production of NO• in RBC from SCA patients and control (CTRL) subjects. RBCs loaded with a fluorescent NO• detector, DAF-FM (4-Amino-5-methylamino- 2',7'-difluorofluorescein diacetate), were imaged in microflow channels over 30-min without shear stress, followed by a 30-min period under 0.5Pa shear stress. We utilized non-specific nitric oxide synthase (NOS) blockade and carbon monoxide (CO) saturation of hemoglobin to assess the contribution of NOS and hemoglobin, respectively, to NO• production. Quantification of DAF-FM fluorescence intensity in individual RBC showed an increase in NO• in SCA RBC at the start of the basal period; however, both SCA and CTRL RBC increased NO• by a similar quantity under shear. A subpopulation of sickle-shaped RBC exhibited lower basal NO• production compared to discoid RBC from SCA group, and under shear became more circular in the direction of shear when compared to discoid RBC from SCA and CTRL, which elongated. Both CO and NOS inhibition caused a decrease in basal NO• production. Shear-mediated NO• production was decreased by CO in all RBC, but was decreased by NOS blockade only in SCA. In conclusion, total NO• production is increased and shear-mediated NO• production is preserved in SCA RBC in a NOS-dependent manner. Sickle shaped RBC with inclusions have higher NO• production and they become more circular rather than elongated with shear.

Automated Oxygen Gradient Ektacytometry: A Novel Biomarker in Sickle Cell Anemia

Sickle cell anemia (SCA) is a hereditary hemoglobinopathy with a variable phenotype. There is no single biomarker that adequately predicts disease severity and can be used to monitor treatment response in patients in clinical trials and clinical care. The use of clinical outcomes, such as vaso-occlusive crises (VOC), requires long and expensive studies, sometimes with inconclusive results. To address these limitations, there are several biomarkers under study to improve the ability to predict complications and assess treatment response in both clinical and research settings. Oxygen gradient ektacytometry, also called as oxygenscan, is an assay that measures the effects of deoxygenation and reoxygenation on red blood cell (RBC) deformability and is gaining popularity in SCA research, because it captures the dynamic sickling capacity of a patient’s RBCs as they are subjected to an oxygen gradient under steady shear stress. We describe here the oxygenscan methodology and evaluate the correlation between oxygenscan parameters and more well-known biomarkers of SCA such as fetal hemoglobin (HbF), F-cells, and dense red blood cells (DRBCs). Our data indicate that the oxygenscan curve is affected by all these parameters and the result incorporates the effects of %HbF, %F-cells, RBC hydration, and RBC membrane deformability.

MEK1/2 as a Therapeutic Target in Sickle Cell Disease

Identification of novel therapeutic targets has improved diagnostics and treatment of many diseases. Many innovative treatment strategies have been developed based on the newly identified biomarkers and key molecules. Most of the research focused on ways to manipulate signaling pathways by activating or suppressing them, validate new therapeutic targets for treatment, and epigenetic treatment of diseases. With the identification of aberrations in multiple growth pathways, the focus then shifted to the small molecules involved in these pathways for targeted therapy. In this communication/short review, we highlight the importance of identification of abnormal activation of the mitogen-activated protein kinase (MAPK), ERK1/2, and its upstream mediator MEK1/2, in erythrocytes in patients with sickle cell disease (SCD) critical for the adhesive interactions of these cells with the endothelium, and leukocytes promoting circulatory obstruction leading to tissue ischemia and infraction. We also discuss how targeting this signaling cascade with MEK1/2 inhibitors can reverse acute vasoocclusive crises in SCD.

Intima-media thickness of the common carotid arteries as a marker of retinopathy and nephropathy in sickle cell disease

Purpose

This study was conducted to test the hypothesis that the carotid intima-media thickness (CIMT) is higher in patients with sickle cell disease (SCD) than in the normal population, and to determine the relationships of the CIMT with central retinal artery (CRA) and renal artery Doppler indices.

Methods

Forty-four confirmed steady-state SCD patients aged 16 years and above were recruited consecutively. The Doppler velocimetric indices of their right renal artery and both CRAs were obtained. The CIMT was also measured on each side via B-mode ultrasonography. The subjects were categorized by age and sex. Mean and median values for each group were determined. The Spearman correlation test was used to quantify the relationships between CIMT and the Doppler parameters.

Results

The participants had a median age of 24.50 years (interquartile range, 12.50 to 36.50 years). Twenty-three were men (52.3%) and 21 were women (47.7%). The median CIMT was 0.70 mm (IQR, 0.50 to 0.90 mm). Significant correlations with the CIMT were found for the CRA peak systolic velocity (r=0.312, P=0.003), the renal artery resistivity index (RI) (r=0.284, P=0.007), and the renal artery pulsatility index (PI) (r=0.273, P=0.010). There was no significant relationship between the CIMT and the CRA end-diastolic velocity, CRA RI, or CRA PI.

Conclusion

CIMT in SCD patients was higher than in the previously reported age groups of the reference populations. In addition, the CIMT was significantly correlated with the CRA peak systolic velocity, the renal artery RI, and the renal artery PI.

Cytoskeleton Remodeling Induces Membrane Stiffness and Stability Changes of Maturing Reticulocytes

Reticulocytes, the precursors of erythrocytes, undergo drastic alterations in cell size, shape, and deformability during maturation. Experimental evidence suggests that young reticulocytes are stiffer and less stable than their mature counterparts; however, the underlying mechanism is yet to be fully understood. Here, we develop a coarse-grained molecular-dynamics reticulocyte membrane model to elucidate how the membrane structure of reticulocytes contributes to their particular biomechanical properties and pathogenesis in blood diseases. First, we show that the extended cytoskeleton in the reticulocyte membrane is responsible for its increased shear modulus. Subsequently, we quantify the effect of weakened cytoskeleton on the stiffness and stability of reticulocytes, via which we demonstrate that the extended cytoskeleton along with reduced cytoskeleton connectivity leads to the seeming paradox that reticulocytes are stiffer and less stable than the mature erythrocytes. Our simulation results also suggest that membrane budding and the consequent vesiculation of reticulocytes can occur independently of the endocytosis-exocytosis pathway, and thus, it may serve as an additional means of removing unwanted membrane proteins from reticulocytes. Finally, we find that membrane budding is exacerbated when the cohesion between the lipid bilayer and the cytoskeleton is compromised, which is in accord with the clinical observations that erythrocytes start shedding membrane surface at the reticulocyte stage in hereditary spherocytosis. Taken together, our results quantify the stiffness and stability change of reticulocytes during their maturation and provide, to our knowledge, new insights into the pathogenesis of hereditary spherocytosis and malaria.

Sickle cell microvascular paradox-oxygen supply-demand mismatch

We have previously demonstrated that sickle cell disease (SCD) patients maintain normal global systemic and cerebral oxygen delivery by increasing cardiac output. However, ischemic end-organ injury remains common suggesting that tissue oxygen delivery may be impaired by microvascular dysregulation or damage. To test this hypothesis, we performed fingertip laser Doppler flowmetry measurements at the base of the nailbed and regional oxygen saturation (rSO₂ ) on the dorsal surface of the same hand. This was done during flow mediated dilation (FMD) studies in 26 chronically transfused SCD, 75 non-transfused SCD, and 18 control subjects. Chronically transfused SCD patients were studied prior to and following a single transfusion and there was no acute change in rSO₂ or perfusion. Laser Doppler estimates of resting perfusion were 76% higher in non-transfused and 110% higher in transfused SCD patients, compared to control subjects. In contrast, rSO₂ was 12 saturation points lower in non-transfused SCD patients, but normal in the transfused SCD patients. During cuff occlusion, rSO₂ declined at the same rate in all subjects suggesting similar intrinsic oxygen consumption rates. Upon cuff release, laser doppler post occlusive hyperemia was blunted in SCD patients in proportion to their resting perfusion values. Transfusion therapy did not improve the hyperemia response. FMD was impaired in SCD subjects but partially ameliorated in transfused SCD subjects. Taken together, non-transfused SCD subjects demonstrate impaired conduit artery FMD, impaired microcirculatory post-occlusive hyperemia, and resting hypoxia in the hand despite compensated oxygen delivery, suggesting impaired oxygen supply-demand matching. Transfusion improves FMD and oxygen supply-demand matching but not microcirculation hyperemic response.

Patrolling monocytes scavenge endothelial-adherent sickle RBCs: a novel mechanism of inhibition of vaso-occlusion in SCD

Painful vaso-occlusive crisis (VOC) is the most common complication of sickle cell disease (SCD). Increasing evidence suggests that vaso-occlusion is initiated by increased adherence of sickle red blood cells (RBCs) to the vascular endothelium. Thus, the mechanisms that remove endothelial-attached sickle RBCs from the microvasculature are expected to be critical for optimal blood flow and prevention of VOC in SCD. We hypothesized that patrolling monocytes (PMos), which protect against vascular damage by scavenging cellular debris, could remove endothelial-adherent sickle RBCs and ameliorate VOC in SCD. We detected RBC (GPA⁺)-engulfed material in circulating PMos of patients with SCD, and their frequency was further increased during acute crisis. RBC uptake by PMos was specific to endothelial-attached sickle, but not control, RBCs and occurred mostly through ICAM-1, CD11a, and CD18. Heme oxygenase 1 induction, by counteracting the cytotoxic effects of engulfed RBC breakdown products, increased PMo viability. In addition, transfusions, by lowering sickle RBC uptake, improved PMo survival. Selective depletion of PMos in Townes sickle mice exacerbated vascular stasis and tissue damage, whereas treatment with muramyl dipeptide (NOD2 ligand), which increases PMo mass, reduced stasis and SCD associated organ damage. Altogether, these data demonstrate a novel mechanism for removal of endothelial attached sickle RBCs mediated by PMos that can protect against VOC pathogenesis, further supporting PMos as a promising therapeutic target in SCD VOC.

Rapid and reproducible characterization of sickling during automated deoxygenation in sickle cell disease patients

In sickle cell disease (SCD), sickle hemoglobin (HbS) polymerizes upon deoxygenation, resulting in sickling of red blood cells (RBCs). These sickled RBCs have strongly reduced deformability, leading to vaso‐occlusive crises and chronic hemolytic anemia. To date, there are no reliable laboratory parameters or assays capable of predicting disease severity or monitoring treatment effects. We here report on the oxygenscan, a newly developed method to measure RBC deformability (expressed as Elongation Index ‐ EI) as a function of pO₂. Upon a standardized, 22 minute, automated cycle of deoxygenation (pO₂ median 16 mmHg ± 0.17) and reoxygenation, a number of clinically relevant parameters are produced in a highly reproducible manner (coefficients of variation <5%). In particular, physiological modulators of oxygen affinity, such as, pH and 2,3‐diphosphoglycerate showed a significant correlation (respectively R = ‑0.993 and R = 0.980) with Point of Sickling (PoS_5%), which is defined as the pO₂ where a 5% decrease in EI is observed during deoxygenation. Furthermore, in vitro treatment with antisickling agents, including GBT440, which alter the oxygen affinity of hemoglobin, caused a reproducible left‐shift of the PoS, indicating improved deformability at lower oxygen tensions. When RBCs from 21 SCD patients were analyzed, we observed a significantly higher PoS in untreated homozygous SCD patients compared to treated patients and other genotypes. We conclude that the oxygenscan is a state‐of‐the‐art technique that allows for rapid analysis of sickling behavior in SCD patients. The method is promising for personalized treatment, development of new treatment strategies and could have potential in prediction of complications.

Disrupting the vicious cycle created by NOX activation in sickle erythrocytes exposed to hypoxia/reoxygenation prevents adhesion and vasoocclusion

In sickle cell disease (SCD), recurrent painful vasoocclusive crisis are likely caused by repeated episodes of hypoxia and reoxygenation. The sickle erythrocyte (SSRBC) adhesion plays an active role in vasoocclusion. However, the effect of prolonged reoxygenation after hypoxic stress on the molecular mechanisms in SSRBCs involved in onset of episodic vasoocclusion remain unclear. Exposure of human SSRBCs to hypoxia followed by 2 h reoxygenation, increased reactive oxygen species (ROS) production. Using specific pharmacological inhibitors, we show that excess ROS production in both reticulocytes and mature SSRBCs is regulated by NADPH oxidases (NOXs), the mitogen-activated protein kinase (ERK1/2), and G-protein coupled-receptor kinase 2 (GRK2). Consequently, SSRBC ROS create an intracellular positive feedback loop with ERK1/2 and GRK2 to mediate SSRBC adhesion to endothelium in vitro, and vasoocclusion in a mouse model of vasoocclusion in vivo. Importantly, reducing ROS levels in SSRBCs with redox-active manganese (Mn) porphyrins, commonly known as mimics of superoxide dismutase (SOD), disrupted the cycle created by ROS by affecting NOX and GRK2 activities and ERK1/2 phosphorylation, thus abrogating RBC-endothelial interactions. Inhibition adhesion assays show that LW (ICAM-4, CD242) blood group glycoprotein and CD44 are the RBC adhesion molecules mediating endothelial binding. Conversely, hypoxia/reoxygenation of normal RBCs failed to activate this feedback loop, and adhesion. These findings provide novel insights into the pathophysiological significance of the deleterious cycle created by NOX-dependent ROS, GRK2 and ERK1/2 within SSRBCs activated by hypoxia/reoxygenation, and involved in SSRBC adhesion and vasoocclusion. Thus, this loop in SSRBCs, which can be disrupted by Mn porphyrins, likely drives the profound SCD vasculopathy, and may point to new therapeutic targets to prevent chronic vasoocclusive events.

Intima-media thickness of the common femoral artery as a marker of leg ulceration in sickle cell disease patients

Leg ulceration is a debilitating chronic complication of sickle cell disease (SCD) the pathogenesis of which is yet to be fully elucidated. We hypothesized that SCD patients with histories of previous leg ulcers would have intima hyperplasia of the common femoral artery (CFA). We enrolled 44 SCD patients and 33 age-matched and sex-matched controls with hemoglobin AA. Anthropometric measurements, biochemical parameters, and sonographic intima-media thickness (IMT) of the CFA were determined. The median CFA IMT in SCD limbs with history of leg ulcers (SWLU) was 1.0 mm, whereas it was 0.7 mm in SCD limbs with no history of leg ulcer (SNLU) and 0.60 mm in controls (P < .001). Among the SNLU, 70.3% had CFA IMT <0.9 mm, whereas only 29.7% had CFA IMT ≥0.9 mm. Conversely, only 20.8% of SWLU had CFA IMT <0.9 mm, whereas the remaining 79.2% had CFA IMT ≥0.9 mm. All the controls had CFA IMT <0.9 mm. Binary logistic regression to determine the odds of having leg ulcer among SCD limbs with CFA IMT of ≥0.9 mm yielded an odds ratio of 9, indicating that SCD limbs with CFA IMT ≥0.9 mm had a 9 times greater risk of having leg ulcer compared with those with CFA IMT <0.9 mm. There is a significant increase in the CFA IMT of SCD limbs with ulcer compared with controls and SCD limbs without ulcer, suggesting that arterial vasculopathy plays a major role in the formation of these ulcers.

Simultaneous polymerization and adhesion under hypoxia in sickle cell disease

Patients with sickle cell disease (SCD) suffer from painful vasoocclusive crises. Polymerization of sickle hemoglobin (HbS) in RBCs is generally considered a major contributor to such crisis events. Here, we present the simultaneous and synergistic coupling of adhesion and HbS polymerization. We show that the age of RBCs in circulation plays an important role in mediating this synergistic effect on blood rheology and clinical symptoms. In particular, the youngest RBCs exhibit unique adhesion dynamics, whereby polymerized HbS fiber bundles grow from cell surfaces to serve as sites of cytoadherence. Our molecular-level simulations show how the attachment and dissociation of molecular bonds influence adhesion dynamics. These results provide a framework that could elucidate the mechanistic basis of SCD vasoocclusive pain crises.

Polymerization and adhesion, dynamic processes that are hallmarks of sickle cell disease (SCD), have thus far been studied in vitro only separately. Here, we present quantitative results of the simultaneous and synergistic effects of adhesion and polymerization of deoxygenated sickle hemoglobin (HbS) in the human red blood cell (RBC) on the mechanisms underlying vasoocclusive pain crisis. For this purpose, we employ a specially developed hypoxic microfluidic platform, which is capable of inducing sickling and unsickling of RBCs in vitro, to test blood samples from eight patients with SCD. We supplemented these experimental results with detailed molecular-level computational simulations of cytoadherence and biorheology using dissipative particle dynamics. By recourse to image analysis techniques, we characterize sickle RBC maturation stages in the following order of the degree of adhesion susceptibility under hypoxia: sickle reticulocytes in circulation (SRs) → sickle mature erythrocytes (SMEs) → irreversibly sickled cells (ISCs). We show that (i) hypoxia significantly enhances sickle RBC adherence; (ii) HbS polymerization enhances sickle cell adherence in SRs and SMEs, but not in ISCs; (iii) SRs exhibit unique adhesion dynamics where HbS fiber projections growing outward from the cell surface create multiple sites of adhesion; and (iv) polymerization stimulates adhesion and vice versa, thereby establishing the bidirectional coupling between the two processes. These findings offer insights into possible mechanistic pathways leading to vasoocclusion crisis. They also elucidate the processes underlying the onset of occlusion that may involve circulating reticulocytes, which are more abundant in hemolytic anemias due to robust compensatory erythropoiesis.

Red Blood Cell Adhesion to Heme-Activated Endothelial Cells Reflects Clinical Phenotype in Sickle Cell Disease

In sickle cell disease (SCD), 'disease severity' associates with increased RBC adhesion to quiescent endothelium, but the impact on activated endothelium is not known. Increased concentrations of free heme result from intravascular hemolysis in SCD. Heme is essential for aerobic metabolism, and plays an important role in numerous biological processes. Excess free heme induces reactive oxygen species generation and endothelial activation, which are associated with cardiovascular disorders including atherosclerosis, hypertension, and thrombosis. Here, we utilized an endothelialized microfluidic platform (Endothelium-on-a-chip) to assess adhesion of sickle hemoglobin-containing red blood cells (HbS RBCs), from adults with homozygous SCD, to heme-activated human endothelial cells (EC) in vitro. Confluent EC monolayers in microchannels were treated with pathophysiologically relevant levels of heme in order to simulate the highly hemolytic intravascular milieu seen in SCD. RBC adhesion to heme-activated ECs varied from subject to subject, and was associated with plasma markers of hemolysis (LDH) and reticulocytosis, thereby linking those RBCs that are most likely to adhere with those that are most likely to hemolyze. These results re-emphasize the critical contribution made by heterogeneous adhesive HbS RBCs to the pathophysiology of SCD. We found that adhesion of HbS RBCs to heme-activated ECs varied amongst individuals in the study population, and associated with biomarkers of hemolysis and inflammation, age, and a recent history of transfusion. Importantly, the microfluidic approach described herein holds promise as a clinically feasible Endothelium-on-a-chip platform with which to study complex heterocellular adhesive interactions in SCD. This article is protected by copyright. All rights reserved.

Extracellular fluid tonicity impacts sickle red blood cell deformability and adhesion

Abnormal sickle red blood cell (sRBC) biomechanics, including pathological deformability and adhesion, correlate with clinical severity in sickle cell disease (SCD). Clinical intravenous fluids (IVFs) of various tonicities are often used during treatment of vaso-occlusive pain episodes (VOE), the major cause of morbidity in SCD. However, evidence-based guidelines are lacking, and there is no consensus regarding which IVFs to use during VOE. Further, it is unknown how altering extracellular fluid tonicity with IVFs affects sRBC biomechanics in the microcirculation, where vaso-occlusion takes place. Here, we report how altering extracellular fluid tonicity with admixtures of clinical IVFs affects sRBC biomechanical properties by leveraging novel in vitro microfluidic models of the microcirculation, including 1 capable of deoxygenating the sRBC environment to monitor changes in microchannel occlusion risk and an "endothelialized" microvascular model that measures alterations in sRBC/endothelium adhesion under postcapillary venular conditions. Admixtures with higher tonicities (sodium = 141 mEq/L) affected sRBC biomechanics by decreasing sRBC deformability, increasing sRBC occlusion under normoxic and hypoxic conditions, and increasing sRBC adhesion in our microfluidic human microvasculature models. Admixtures with excessive hypotonicity (sodium = 103 mEq/L), in contrast, decreased sRBC adhesion, but overswelling prolonged sRBC transit times in capillary-sized microchannels. Admixtures with intermediate tonicities (sodium = 111-122 mEq/L) resulted in optimal changes in sRBC biomechanics, thereby reducing the risk for vaso-occlusion in our models. These results have significant translational implications for patients with SCD and warrant a large-scale prospective clinical study addressing optimal IVF management during VOE in SCD.

Measurements of red cell deformability and hydration reflect HbF and HbA2 in blood from patients with sickle cell anemia

Decreased erythrocyte deformability, as measured by ektacytometry, may be associated with disease severity in sickle cell anemia (SCA). Heterogeneous populations of rigid and deformable cells in SCA blood result in distortions of diffraction pattern measurements that correlate with the concentration of hemoglobin S (HbS) and the percentage of irreversibly sickled cells. We hypothesize that red cell heterogeneity, as well as deformability, will also be influenced by the concentration of alternative hemoglobins such as fetal hemoglobin (HbF) and the adult variant, HbA₂. To test this hypothesis, we investigate the relationship between diffraction pattern distortion, osmotic gradient ektacytometry parameters, and the hemoglobin composition of SCA blood. We observe a correlation between the extent of diffraction pattern distortions and percentage of HbF and HbA₂. Osmotic gradient ektacytometry data indicate that minimum elongation in the hypotonic region is positively correlated with HbF, as is the osmolality at which it occurs. The osmolality at both minimum and maximum elongation is inversely correlated with HbS and HbA₂. These data suggest that HbF may effectively improve surface-to-volume ratio and osmotic fragility in SCA erythrocytes. HbA₂ may be relatively ineffective in improving these characteristics or cellular hydration at the levels found in this patient cohort.

Emerging point-of-care technologies for sickle cell disease screening and monitoring

INTRODUCTION

Sickle Cell Disease (SCD) affects 100,000 Americans and more than 14 million people globally, mostly in economically disadvantaged populations, and requires early diagnosis after birth and constant monitoring throughout the life-span of the patient. Areas covered: Early diagnosis of SCD still remains a challenge in preventing childhood mortality in the developing world due to requirements of skilled personnel and high-cost of currently available modalities. On the other hand, SCD monitoring presents insurmountable challenges due to heterogeneities among patient populations, as well as in the same individual longitudinally. Here, we describe emerging point-of-care micro/nano platform technologies for SCD screening and monitoring, and critically discuss current state of the art, potential challenges associated with these technologies, and future directions. Expert commentary: Recently developed microtechnologies offer simple, rapid, and affordable screening of SCD and have the potential to facilitate universal screening in resource-limited settings and developing countries. On the other hand, monitoring of SCD is more complicated compared to diagnosis and requires comprehensive validation of efficacy. Early use of novel microdevices for patient monitoring might come in especially handy in new clinical trial designs of emerging therapies.

Sickle cell disease biochip: a functional red blood cell adhesion assay for monitoring sickle cell disease

Sickle cell disease (SCD) afflicts millions of people worldwide and is associated with considerable morbidity and mortality. Chronic and acute vaso-occlusion are the clinical hallmarks of SCD and can result in pain crisis, widespread organ damage, and early movtality. Even though the molecular underpinnings of SCD were identified more than 60 years ago, there are no molecular or biophysical markers of disease severity that are feasibly measured in the clinic. Abnormal cellular adhesion to vascular endothelium is at the root of vaso-occlusion. However, cellular adhesion is not currently evaluated clinically. Here, we present a clinically applicable microfluidic device (SCD biochip) that allows serial quantitative evaluation of red blood cell (RBC) adhesion to endothelium-associated protein-immobilized microchannels, in a closed and preprocessing-free system. With the SCD biochip, we have analyzed blood samples from more than 100 subjects and have shown associations between the measured RBC adhesion to endothelium-associated proteins (fibronectin and laminin) and individual RBC characteristics, including hemoglobin content, fetal hemoglobin concentration, plasma lactate dehydrogenase level, and reticulocyte count. The SCD biochip is a functional adhesion assay, reflecting quantitative evaluation of RBC adhesion, which could be used at baseline, during crises, relative to various long-term complications, and before and after therapeutic interventions.

VLA-4 blockade by natalizumab inhibits sickle reticulocyte and leucocyte adhesion during simulated blood flow

Very Late Antigen-4 (VLA-4, α4β1-integrin, ITGA4) orchestrates cell-cell and cell-endothelium adhesion. Given the proposed role of VLA-4 in sickle cell disease (SCD) pathophysiology, we evaluated the ability of the VLA-4 blocking antibody natalizumab to inhibit SCD blood cell adhesion. Natalizumab recognized surface VLA-4 on leucocytes and reticulocytes in whole blood from SCD subjects. SCD reticulocytes were positive for VLA-4, while VLA-4 staining of non-SCD reticulocytes was undetectable. Titrations with natalizumab revealed the presence of saturable levels of VLA-4 on both SCD reticulocytes and leucocytes similar to healthy subject leucocytes. Under physiological flow conditions, the adhesion of SCD whole blood cells and isolated SCD leucocytes to immobilized vascular cell adhesion molecule 1 (VCAM-1) was blocked by natalizumab in a dose-dependent manner, which correlated with cell surface receptor binding. Natalizumab also inhibited >50% of whole blood cell binding to TNF-α activated human umbilical vein endothelial cell monolayers under physiological flow at clinically relevant concentrations (10 to 100 μg/ml). This indicates that VLA-4 is the dominant receptor that drives SCD reticulocyte and mononuclear cell adhesion to VCAM-1 and that the VLA-4 adhesion to VCAM-1 is a significant contributor to SCD blood cell adhesion to endothelium. Thus, VLA-4 blockade may be beneficial in sickle cell disease.

A Review of Cell Adhesion Studies for Biomedical and Biological Applications

Cell adhesion is essential in cell communication and regulation, and is of fundamental importance in the development and maintenance of tissues. The mechanical interactions between a cell and its extracellular matrix (ECM) can influence and control cell behavior and function. The essential function of cell adhesion has created tremendous interests in developing methods for measuring and studying cell adhesion properties. The study of cell adhesion could be categorized into cell adhesion attachment and detachment events. The study of cell adhesion has been widely explored via both events for many important purposes in cellular biology, biomedical, and engineering fields. Cell adhesion attachment and detachment events could be further grouped into the cell population and single cell approach. Various techniques to measure cell adhesion have been applied to many fields of study in order to gain understanding of cell signaling pathways, biomaterial studies for implantable sensors, artificial bone and tooth replacement, the development of tissue-on-a-chip and organ-on-a-chip in tissue engineering, the effects of biochemical treatments and environmental stimuli to the cell adhesion, the potential of drug treatments, cancer metastasis study, and the determination of the adhesion properties of normal and cancerous cells. This review discussed the overview of the available methods to study cell adhesion through attachment and detachment events.

Gαs proteins activate p72(Syk) and p60-c-Src tyrosine kinases to mediate sickle red blood cell adhesion to endothelium via LW-αvβ3 and CD44-CD44 interactions

G protein-coupled receptors (GPCRs) have been suggested as new drug targets to treat a variety of diseases. In sickle cell disease (SCD), the LW erythrocyte adhesion receptor can be activated by stimulation of β2 adrenergic receptors (β2ARs), to mediate sickle erythrocyte (SSRBC) adhesion to endothelium. However, the involvement of tyrosine protein kinases in β2AR signaling to activate SSRBC adhesion to endothelium has not been thoroughly elucidated. Either direct activation with Cholera toxin of Gαs protein, which acts downstream of β2ARs, or inhibition with Pertussis toxin of Gαi, mediating suppression of adenylyl cyclase, increased SSRBC adhesion to endothelium over baseline adhesion. This effect involved the non-receptor tyrosine kinases, p72(Syk) and p60-c-Src, which were more abundant in SSRBCs than in normal erythrocytes. In contrast, Pertussis toxin and Cholera toxin failed to increase adhesion of normal erythrocytes. SSRBC Gαi inhibition also increased phosphorylation of p72(Syk) and p60-c-Src. Further, we investigated the relevance of activation of p72(Syk) and p60-c-Src, and identified LW (ICAM-4, CD242) and CD44 as the erythroid adhesion molecules both physically interacting with activated p60-c-Src. As a result, SSRBC LW underwent increased tyrosine phosphorylation, leading to SSRBC LW and CD44 binding to endothelial αvβ3 integrin and CD44, respectively. These data provide in vitro mechanistic evidence that p60-c-Src, which could act downstream of Gαs/p72(Syk), associates with LW and CD44 on SSRBCs leading to their interactions with endothelial αvβ3 and CD44, respectively. Thus, increased activation of these signaling mechanisms in SSRBCs could initiate or exacerbate vascular occlusion, the hallmark of SCD.

Heterogeneous red blood cell adhesion and deformability in sickle cell disease

We present a microfluidic approach that allows simultaneous interrogation of RBC properties in physiological flow conditions at a single cell level. With this method, we studied healthy hemoglobin A (HbA) and homozygous sickle hemoglobin (HbS) containing RBCs using whole blood samples from twelve subjects. We report that HbS-containing RBCs are heterogeneous in terms of adhesion and deformability in flow.

Red blood cells of sickle cell disease patients exhibit abnormally high abundance of N-methyl D-aspartate receptors mediating excessive calcium uptake

Recently we showed that N-methyl D-aspartate receptors (NMDARs) are expressed in erythroid precursors (EPCs) and present in the circulating red blood cells (RBCs) of healthy humans, regulating intracellular Ca(2+) in these cells. This study focuses on investigating the possible role of NMDARs in abnormally high Ca(2+) permeability in the RBCs of patients with sickle cell disease (SCD). Protein levels of the NMDAR subunits in the EPCs of SCD patients did not differ from those in EPCs of healthy humans. However, the number and activity of the NMDARs in circulating SCD-RBCs was substantially up-regulated, being particularly high during haemolytic crises. The number of active NMDARs correlated negatively with haematocrit and haemoglobin levels in the blood of SCD patients. Calcium uptake via these non-selective cation channels was induced by RBC treatment with glycine, glutamate and homocysteine and was facilitated by de-oxygenation of SCD-RBCs. Oxidative stress and RBC dehydration followed receptor stimulation and Ca(2+) uptake. Inhibition of the NMDARs with an antagonist memantine caused re-hydration and largely prevented hypoxia-induced sickling. The EPCs of SCD patients showed higher tolerance to memantine than those of healthy subjects. Consequently, NMDARs in the RBCs of SCD patients appear to be an attractive target for pharmacological intervention.

Sickle cell disease with double stroke in a Moroccan family

The sickle-cell disease is a group of chronic hemolytic diseases which associates three types of injuries: severe anemia, severe infections, and ischemic vaso-occlusive crisis that are secondary to conflicts between small vessels and red blood cells too deformable. Thus, organic various complications may arise. Its prevalence in Europe is estimated to be about 1/150 and reaches 15 % in the Mediterranean areas. Clinical manifestations vary widely from one person to another and from one moment to another. In addition to anemia and bacterial infections, vaso-occlusive crisis may manifest by focal ischemia. In the long term, the VOC may compromise the function of a particular tissue or organ. The transmission is autosomal recessive. The sickle-cell diseases are determined by combinations of two abnormal alleles of beta globin gene including at least one which carries the mutation beta 6 glu-val (Hb S). We report the case of a girl aged 11 years, who presented two strokes in the interval of 8 months, which manifested by a complete right hemiplegia and aphasia confirmed by head CT scan; the electrophoresis of the hemoglobin and the molecular test had confirmed the diagnosis of sickle-cell disease, and we were allowed to spread better reflection on the prevention of stroke, which remains a frequent and serious complication of sickle-cell disease.

In vitro modeling of the microvascular occlusion and thrombosis that occur in hematologic diseases using microfluidic technology

In hematologic diseases, such as sickle cell disease (SCD) and hemolytic uremic syndrome (HUS), pathological biophysical interactions among blood cells, endothelial cells, and soluble factors lead to microvascular occlusion and thrombosis. Here, we report an in vitro "endothelialized" microfluidic microvasculature model that recapitulates and integrates this ensemble of pathophysiological processes. Under controlled flow conditions, the model enabled quantitative investigation of how biophysical alterations in hematologic disease collectively lead to microvascular occlusion and thrombosis. Using blood samples from patients with SCD, we investigated how the drug hydroxyurea quantitatively affects microvascular obstruction in SCD, an unresolved issue pivotal to understanding its clinical efficacy in such patients. In addition, we demonstrated that our microsystem can function as an in vitro model of HUS and showed that shear stress influences microvascular thrombosis/obstruction and the efficacy of the drug eptifibatide, which decreases platelet aggregation, in the context of HUS. These experiments establish the versatility and clinical relevance of our microvasculature-on-a-chip model as a biophysical assay of hematologic pathophysiology as well as a drug discovery platform.

Erythrocyte plasma membrane-bound ERK1/2 activation promotes ICAM-4-mediated sickle red cell adhesion to endothelium

The core pathology of sickle cell disease (SCD) starts with the erythrocyte (RBC). Aberration in MAPK/ERK1/2 signaling, which can regulate cell adhesion, occurs in diverse pathologies. Because RBCs contain abundant ERK1/2, we predicted that ERK1/2 is functional in sickle (SS) RBCs and promotes adherence, a hallmark of SCD. ERK1/2 remained active in SS but not normal RBCs. β(2)-adrenergic receptor stimulation by epinephrine can enhance ERK1/2 activity only in SS RBCs via PKA- and tyrosine kinase p72(syk)-dependent pathways. ERK signaling is implicated in RBC ICAM-4 phosphorylation, promoting SS RBC adhesion to the endothelium. SS RBC adhesion and phosphorylation of both ERK and ICAM-4 all decreased with continued cell exposure to epinephrine, implying that activation of ICAM-4-mediated SS RBC adhesion is temporally associated with ERK1/2 activation. Furthermore, recombinant ERK2 phosphorylated α- and β-adducins and dematin at the ERK consensus motif. Cytoskeletal protein 4.1 also showed dynamic phosphorylation but not at the ERK consensus motif. These results demonstrate that ERK activation induces phosphorylation of cytoskeletal proteins and the adhesion molecule ICAM-4, promoting SS RBC adhesion to the endothelium. Thus, blocking RBC ERK1/2 activation, such as that promoted by catecholamine stress hormones, could ameliorate SCD pathophysiology.

The rate of hemolysis in sickle cell disease correlates with the quantity of active von Willebrand factor in the plasma

Vaso-occlusion, hemolysis, and oxidative stress are hallmarks of sickle cell disease (SCD). This pathology is accompanied by systemic endothelial activation, rendering the endothelium more adhesive for blood cells, including sickle erythrocytes. Activated endothelial cells display or secrete several adhesive molecules, including von Willebrand factor (VWF). We assessed several VWF parameters in SCD patients at baseline: multimer pattern, antigen concentration (VWF:Ag), activation factor (VWF:AF), and total active VWF (VWF:TA). VWF:AF was determined using a llama nanobody (AU/VWFa-11) that detects a platelet-binding conformation of the A1 domain; VWF:TA was calculated by multiplying VWF:Ag by VWF:AF. SCD plasma contained elevated VWF:Ag and ultralarge VWF multimers. VWF:TA, a measure of total VWF reactivity, correlated closely with hemolysis, as determined by serum lactate dehydrogenase. ADAMTS13 activity and antigen were normal in all patients. These findings suggest an important role for hyperreactive VWF in SCD pathology and connect SCD to other microangiopathies, particularly thrombotic thrombocytopenic purpura.

Role of extracellular hemoglobin in thrombosis and vascular occlusion in patients with sickle cell anemia

Sickle cell anemia (SCA) is a common hemolytic disorder caused by a gene mutation in the β-globin subunit of hemoglobin (Hb) and affects millions of people. The intravascular hemolysis releases excessive amount of extracellular hemoglobin (ECHb) into plasma that causes many cellular dysfunctions in patients with SCA. ECHb scavenges NO which promotes crisis events such as vasoconstriction, thrombosis and hypercoagulation. ECHb and its degradation product, heme, are known to cause oxidative damage to the vessel wall and stimulate the expression of adhesive protein ligands on vascular endothelium. Our study shows that ECHb binds potently to VWF-largest multimeric glycoprotein in circulation-through the A2-domain, and significantly inhibits its cleavage by the metalloprotease ADAMTS13. Furthermore, a subpopulation of VWF multimers bound to ECHb exists in significant amount, accounting for about 14% of total plasma VWF, in SCD patients. The Hb-bound VWF multimers are resistant to ADAMTS13, and are hyperactive in aggregating platelets. Thus, the data suggest that Hb-bound VWF multimers are ultralarge and hyperactive because they are resistant to the protease. The Hb-bound VWF multimers are elevated parallely with the level of ECHb in patients' plasma, and is associated with the pathogenesis of thrombosis and vascular occlusion in SCA.

Sickle red cell-endothelium interactions

Periodic recurrence of painful vaso-occlusive crisis is the defining feature of sickle cell disease. Among multiple pathologies associated with this disease, sickle red cell-endothelium interaction has been implicated as a potential initiating mechanism in vaso-occlusive events. This review focuses on various interrelated mechanisms involved in human sickle red cell adhesion. We discuss in vitro and microcirculatory findings on sickle red cell adhesion, its potential role in vaso-occlusion, and the current understanding of receptor-ligand interactions involved in this pathological phenomenon. In addition, we discuss the contribution of other cellular interactions (leukocytes recruitment and leukocyte-red cell interaction) to vaso-occlusion, as observed in transgenic sickle mouse models. Emphasis is given to recently discovered adhesion molecules that play a predominant role in mediating human sickle red cell adhesion. Finally, we analyze various therapeutic approaches for inhibiting sickle red cell adhesion by targeting adhesion molecules and also consider therapeutic strategies that target stimuli involved in endothelial activation and initiation of adhesion.

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.

Intravenous immunoglobulins reverse acute vaso-occlusive crises in sickle cell mice through rapid inhibition of neutrophil adhesion

Previous studies using intravital microscopy in a sickle cell disease (SCD) mouse model suggest that adherent white blood cells (WBCs) play a key role in vaso-occlusion by capturing circulating red blood cells (RBCs) in venules. Commercial intravenous immunoglobulin (IVIG) given before the inflammatory stimuli increased microcirculatory blood flow and survival. To mimic the clinical situation in which SCD patients seek medical attention after the onset of symptoms, we developed an in vivo model in which the therapeutic intervention (eg, IVIG) was administered after in the inflammatory challenge. In this setting, IVIG rapidly (<10 minutes) reduced adherent leukocyte numbers and dramatically inhibited interactions between RBCs and WBCs, resulting in improved microcirculatory blood flow and survival of sickle cell "Berkeley" mice. Longer survival correlated positively with blood flow (P=.001) and negatively with the number of adherent leukocytes (P=.001) and RBC-WBC interactions (P=.002). Using multichannel digital fluorescence videomicroscopy, we found that IVIG affected specifically the recruitment of neutrophils. Moreover, further analyses of leukocyte behavior revealed that IVIG significantly increased rolling velocities, indicating that it alters adhesion pathways involved in slow rolling. These data suggest that the potential therapeutic benefits of IVIG in SCD crises should be evaluated in a clinical trial.

Epinephrine-induced activation of LW-mediated sickle cell adhesion and vaso-occlusion in vivo

Sickle red cell (SS RBC) adhesion is believed to contribute to the process of vaso-occlusion in sickle cell disease (SCD). We previously found that the LW RBC adhesion receptor can be activated by epinephrine to mediate SS RBC adhesion to endothelial alphavbeta3 integrin. To determine the contribution of LW activation to vaso-occlusive events in vivo, we investigated whether in vitro treatment of SS RBCs by epinephrine resulted in vaso-occlusion in intact microvasculature after RBC infusion into nude mice. Epinephrine enhanced human SS but not normal RBC adhesion to murine endothelial cells in vitro and to endothelium in vivo, promoting vaso-occlusion and RBC organ sequestration. Murine sickle RBCs also responded to epinephrine with increased adhesion to postcapillary endothelium in nude mice. Epinephrine-induced SS RBC adhesion, vaso-occlusion, and RBC organ trapping could be prevented by the beta-adrenergic receptor (beta-AR) antagonist, propranolol. Infusion of soluble recombinant LW also significantly reduced adhesion and vaso-occlusion. In addition, epinephrine-treated SS RBCs induced activation of murine leukocyte adhesion to endothelium as well. We conclude that LW activation by epinephrine via beta-AR stimulation can promote both SS RBC and leukocyte adhesion as well as vaso-occlusion, suggesting that both epinephrine and LW play potentially pathophysiological roles in SCD.

Biologically active CD40 ligand is elevated in sickle cell anemia: potential role for platelet-mediated inflammation

OBJECTIVE

After activation, platelets expose CD40 ligand (CD40L) on their surface, then subsequently release the inflammatory mediator as a soluble fragment (sCD40L). Because sickle cell anemia (SCA) is noted for both platelet activation and chronic inflammation, we asked whether platelet-released CD40L potentially plays a role in SCA.

METHODS AND RESULTS

ELISAs demonstrate that SCA patient plasma contains 30-fold more sCD40L than control plasma. Correspondingly, platelets from these patients contain less than half the CD40L found in control platelets. Platelets from patients in painful crises are further depleted of CD40L, with even higher plasma levels, suggesting a correlation to the patient's clinical state. In addition, elevated sCD40L correlates with increased tissue factor in SCA plasma. Blockage of the CD40L receptor CD40 reduces SCA plasma-induced production of tissue factor and endothelial intercellular adhesion molecule-1 (ICAM-1). Finally, sCD40L activity in SCA plasma is confirmed by its induction of B-cell proliferation.

CONCLUSIONS

Platelet-derived sCD40L is elevated in SCA, further elevated in crises, and biologically active. The participation of sCD40L in SCA plasma-induced production of B cells, tissue factor, and ICAM-1 suggests that CD40L may contribute to the chronic inflammation and increased thrombotic activity known to occur in SCA.

Hyperhemolysis during the evolution of uncomplicated acute painful episodes in patients with sickle cell anemia

BACKGROUND

Some patients with sickle cell anemia (SS) experience significant decrease in their hemoglobin (Hb) level and significant increase in reticulocyte count during the progression of uncomplicated acute painful episodes. These changes have been attributed to, but not proven, hyperhemolysis. This study provides evidence of hyperhemolysis in this group of patients.

STUDY DESIGN AND METHODS

Hospitalized patients who met the above criteria were identified and followed prospectively, and the measures of red blood cell (RBC) survival were determined. The latter included 1) laboratory measures of hemolysis: Hb level, reticulocyte count, nucleated RBC count, RBC distribution width (RDW), total bilirubin, lactate dehydrogenase (LDH), and aspartate transaminase (AST); 2) determination of RBC survival in vivo by the (51)Cr method in selected patients; and 3) the determination of the RBC Hb:reticulocyte Hb ratio. Baseline values of these measures were also obtained and compared to the crisis data.

RESULTS

During the evolution of the painful episode in the selected patients, there was a significant increase in total bilirubin, LDH, AST, nucleated RBC count, and RDW that are indicative of hyperhemolysis. There was a significant decrease in RBC Hb:reticulocyte Hb and (51)Cr RBC survival, which is diagnostic of hyperhemolysis.

CONCLUSION

Together, the data indicate that hyperhemolysis occurs during uncomplicated acute painful episodes in some patients with SS. The presence of free plasma Hb, consequent to hyperhemolysis, reduces nitric oxide bioavailability, promotes endothelial dysfunction, and contributes to the development of pulmonary hypertension and vasoocclusion.

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.

Johnson-Kendall-Roberts theory applied to living cells

Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion energies of soft slightly deformable material. Little is known about the validity of this theory on complex systems such as living cells. We have addressed this problem using a depletion controlled cell adhesion and measured the force necessary to separate the cells with a micropipette technique. We show that the cytoskeleton can provide the cells with a 3D structure that is sufficiently elastic and has a sufficiently low deformability for JKR theory to be valid. When the cytoskeleton is disrupted, JKR theory is no longer applicable.

Epinephrine acts through erythroid signaling pathways to activate sickle cell adhesion to endothelium via LW-alphavbeta3 interactions

The possible role of physiologic stress hormones in enhancing adhesion of sickle erythrocytes (SS RBCs) to endothelial cells (ECs) in sickle cell disease (SCD) has not been previously explored. We have now found that up-regulation of intracellular cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) by epinephrine significantly increased sickle but not normal erythrocyte adhesion to both primary and immortalized ECs. Inhibition of serine/threonine phosphatases also enhanced sickle erythrocyte adhesion at least partially through a PKA-dependent mechanism. Adhesion was mediated through LW (intercellular adhesion molecule-4 [ICAM-4], CD242) blood group glycoprotein, and immunoprecipitation studies showed that LW on sickle but not on normal erythrocytes undergoes increased PKA-dependent serine phosphorylation as a result of activation. The major counter receptor for LW was identified as the alphavbeta3 integrin on ECs. These data suggest that adrenergic hormones such as epinephrine may initiate or exacerbate vaso-occlusion and thus contribute to the association of vaso-occlusive events with physiologic stress.

B-CAM/LU expression and the role of B-CAM/LU activation in binding of low- and high-density red cells to laminin in sickle cell disease

Red blood cells from patients with sickle cell disease (SS RBC) adhere to laminin and over-express the high-affinity laminin receptor basal cell adhesion molecule/Lutheran protein (B-CAM/LU). This receptor has recently been shown to undergo activation in vitro through a protein kinase A-dependent mechanism. Low-density SS RBC express two-thirds more B-CAM/LU than high-density SS RBC. However, high-density SS RBC have been identified as most adherent to laminin under flow conditions. We investigated the ability of low- and high-density SS RBC to interact with laminin under various conditions and explored factors that might be responsible for the differences in B-CAM/LU-laminin interaction between high- and low-density SS RBC. We confirmed that high-density SS RBC adhere to laminin more strongly than low-density SS RBC under flow conditions. However, low-density SS RBC bind soluble laminin most strongly and are the most adherent to laminin under static conditions. Soluble recombinant Lutheran extracellular domain protein completely blocked SS RBC adhesion to laminin under both static and flow conditions. The protein kinase A inhibitor 14-22 amide inhibited adhesion to laminin during flow by high-density SS RBC from patients with strongly adherent cells but had no effect on adhesion observed after a static phase. Deletion of the cytoplasmic domain of B-CAM as well as mutation of the juxtamembranous tyrosine residue failed to reduce B-CAM-mediated adhesion to laminin by transfected MEL cells. These studies confirm that B-CAM/LU is the most critical receptor mediating adhesion to laminin under both static and flow conditions. Dense SS RBC are most adherent to laminin despite bearing fewer laminin receptors, apparently due to a reversible protein kinase A-dependent process that is unlikely to involve direct phosphorylation of B-CAM/LU. Our results also suggest that the nature of the interaction of B-CAM/LU with laminin may be different under static and flow conditions.

Inflammatory mediators promote strong sickle cell adherence to endothelium under venular flow conditions

Adherence of sickle erythrocytes to endothelium in venules is thought to initiate or propagate vaso-occlusive episodes. Because of blood shear forces with normal microvascular flow, adherence in post-capillary venules requires binding via high-affinity receptor-mediated pathways. Microvascular flow in sickle patients is episodic, even in asymptomatic patients, so adherence may also occur at low shear not requiring high-affinity binding. Sickle cell binding to endothelium was quantified under flow or static incubation with unusually large vWF, thrombospondin, alpha(4)beta(1)/VCAM-1 or alpha(4)beta(1)/fibronectin (FN). Adherence under flow at 0.5 dyne/cm(2) shear stress leads to the greatest number of adherent sickle cells. Adherence under flow at 1.0 dyne/cm(2) leads to the strongest adherence. Static incubation conditions promote weak adherence of low numbers of sickle cells to endothelium. Following attachment at 1.0 dyne/cm(2), adherence strength was 2.5 +/- 0.1 or 2.6 +/- 0.2 dynes/cm(2) for alpha(4)beta(1)/VCAM-1 or alpha(4)beta(1)/FN pathways, a level 50% greater than adherence strength mediated by thrombospondin or ULvWF (1.7 +/- 0.08 or 1.6 +/- 0.07 dynes/cm(2), respectively). Sickle cell adhesion promoted by simultaneous activation of alpha(4)beta(1)/VCAM-1 and alpha(4)beta(1)/FN pathways is the strongest at 6.2 +/- 0.2 dynes/cm(2) and adherent red cells resist detachment shear stresses up to 10 dynes/cm(2). These data demonstrate that sickle cell adhesion to endothelium is regulated both by receptor/ligand affinity and flow conditions. Thus, both microvascular flow conditions and receptor-ligand interactions may regulate sickle cell adherence in vivo.

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.

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.

Primary role for adherent leukocytes in sickle cell vascular occlusion: a new paradigm

Vascular occlusion is the major cause of morbidity and mortality in sickle cell disease but its mechanisms are poorly understood. We demonstrate by using intravital microscopy in mice expressing human sickle hemoglobin (SS) that SS red blood cells (RBCs) bind to adherent leukocytes in inflamed venules, producing vasoocclusion of cremasteric venules. SS mice deficient in P- and E-selectins, which display defective leukocyte recruitment to the vessel wall, are protected from vasoocclusion. These data uncover a previously unsuspected paradigm for the pathogenesis of sickle cell vasoocclusion in which adherent leukocytes play a direct role and suggest that drugs targeting SS RBC-leukocyte or leukocyte-endothelial interactions may prevent or treat the vascular complications of this debilitating disease.

Sickle cell vaso-occlusion: multistep and multicellular paradigm

Sickle cell disease is characterized by recurrent, painful episodes and organ damage resulting from microvascular occlusion. Seminal studies performed 20 years ago revealed increased adherence of sickle erythrocytes to vascular endothelial cells. Subsequent work showed that these interactions were mediated by multiple adhesion pathways, but the relevance of these interactions has not been evaluated in vivo. Clinical data suggest that leukocytes may play a role, because leukocytosis correlates with clinical severity and early death, and administration of myeloid growth factors to patients can precipitate sickle cell crises. In addition, recent experimental data using intravital microscopy indicate that sickle erythrocytes can interact with adherent leukocytes in inflamed postcapillary and collecting venules. A novel multistep model for sickle cell vaso-occlusion is proposed in which endothelial activation is induced by sickle cells or secondary inflammatory stimuli and leads to the recruitment of adherent leukocytes. The resulting adherent leukocytes interact with circulating sickle erythrocytes, and this interaction impedes microvascular blood flow. Finally, irregularly shaped sickle cells become nonspecifically trapped, resulting in vaso-occlusion. The molecular mechanisms and requirements for the heterotypic interactions between erythrocytes and leukocytes are currently unknown and may involve further activation of adherent leukocytes or circulating erythrocytes. This model offers exciting new opportunities for therapeutic intervention and suggests a critical participation of adherent leukocytes in sickle cell vaso-occlusion.