Sickle cell vasoocclusion: many issues and some answers

Oxidative stress activates red cell adhesion to laminin in sickle cell disease

Vaso-occlusive crises are the hallmark of sickle cell disease (SCD). They are believed to occur in two steps, starting with adhesion of deformable low-dense red blood cells (RBCs), or other blood cells such as neutrophils, to the wall of post-capillary venules, followed by trapping of the denser RBCs or leukocytes in the areas of adhesion because of reduced effective lumen-diameter. In SCD, RBCs are heterogeneous in terms of density, shape, deformability and surface proteins, which accounts for the differences observed in their adhesion and resistance to shear stress. Sickle RBCs exhibit abnormal adhesion to laminin mediated by Lu/BCAM protein at their surface. This adhesion is triggered by Lu/BCAM phosphorylation in reticulocytes but such phosphorylation does not occur in mature dense RBCs despite firm adhesion to laminin. In this study, we investigated the adhesive properties of sickle RBC subpopulations and addressed the molecular mechanism responsible for the increased adhesion of dense RBCs to laminin in the absence of Lu/BCAM phosphorylation. We provide evidence for the implication of oxidative stress in post-translational modifications of Lu/BCAM that impact its distribution and cis-interaction with glycophorin C at the cell surface activating its adhesive function in sickle dense RBCs.

Electrical Impedance Characterization of Erythrocyte Response to Cyclic Hypoxia in Sickle Cell Disease

Cell sickling is the process in which intracellular polymerization of deoxygenated sickle hemoglobin (HbS) leads to distorted, rigid cells, resulting in abnormal blood rheology and painful vaso-occlusion. Current methods for detection of this process mainly rely on optical microscopy of cellular morphology and measurements of cell deformability and blood rheology. As electrical impedance of cells is a sensitive indicator of changes in cellular structure and biophysical characteristics, it can be a promising marker for characterization of abnormal blood rheology and a means more convenient than optics to be integrated into point-of-care devices. In this work, a microfluidics-based electrical impedance sensor has been developed for characterizing the dynamic cell sickling-unsickling processes in sickle blood. The sensor is capable of measuring the continuous variation in the sickle cell suspension due to cyclic hypoxia-induced intracellular HbS polymerization and depolymerization. Simultaneous microscopic imaging of cell morphological change shows the reliability and repeatability of the electrical impedance-based measurements of cell sickling and unsickling processes. Strong correlation is found between the electrical impedance measurement and patients' hematological parameters such as levels of HbS and fetal hemoglobin. The combination of electrical impedance measurement and on-chip hypoxia control provides a promising method for rapid assessment of the dynamic processes of cell sickling and unsickling in patients with sickle cell disease.

Hemodynamic Functionality of Transfused Red Blood Cells in the Microcirculation of Blood Recipients

The primary goal of red blood cell (RBC) transfusion is to supply oxygen to tissues and organs. However, due to a growing number of studies that have reported negative transfusion outcomes, including reduced blood perfusion, there is rising concern about the risks in blood transfusion. RBC are characterized by unique flow-affecting properties, specifically adherence to blood vessel wall endothelium, cell deformability, and self-aggregability, which define their hemodynamic functionality (HF), namely their potential to affect blood circulation. The role of the HF of RBC in blood circulation, particularly the microcirculation, has been documented in numerous studies with animal models. These studies indicate that the HF of transfused RBC (TRBC) plays an important role in the transfusion outcome. However, studies with animal models must be interpreted with reservations, as animal physiology may not reflect human physiology. To test this concept in humans, we have directly examined the effect of the HF of TRBC, as expressed by their deformability and adherence to vascular endothelium, on the transfusion-induced effect on the skin blood flow and hemoglobin increment in β-thalassemia major patients. The results demonstrated, for the first time in humans, that the TRBC HF is a potent effector of the transfusion outcome, expressed by the transfusion-induced increase in the recipients' hemoglobin level, and the change in the skin blood flow, indicating a link between the microcirculation and the survival of TRBC in the recipients' vascular system. The implication of these findings for blood transfusion practice and to vascular function in blood recipients is discussed.

Myonecrosis in Sickle Cell Anemia: Case Study

Patient: Male, 27

Final Diagnosis: Myonecrosis of sickle cell anaemia

Symptoms: Pain • redness to feet • swelling foot

Medication: —

Clinical Procedure: MRI

Specialty: Podiatry

Rheology of red blood cells in patients with HbC disease

Patients with hemoglobin C disease (CC) usually do not develop severe complications in comparison with individuals with sickle cell anemia (SS) or with sickle cell hemoglobin C disease (SC). The present study compared the hematological, biochemical, hemorheological and clinical characteristics of CC patients to those of SS, SC and healthy individuals (AA). Blood viscosity was measured at 225 s(-1) with a cone plate viscometer. The hematocrit-to-blood viscosity ratio (HVR), i.e. an index of red blood cell (RBC) oxygen transport effectiveness, was calculated. RBC deformability was determined at 30 Pa by ektacytometry, and RBC aggregation properties by syllectometry. CC and SC had higher blood viscosity and lower HVR than AA. Nevertheless, HVR was higher in CC compared to SS and tended to be higher than in SC. The CC group exhibited very rigid hyperchromic RBC compared to the three other groups. RBC aggregation abnormalities were observed in CC: low RBC aggregation index and high RBC aggregates strength. Despite these hemorheological abnormalities, CC never had hospitalized painful vaso-occlusive crisis or acute chest syndrome. In contrast, all of them had splenomegaly. Of note, 2 out of 7 CC developed retinopathy or otologic disorders. Whether the blood hyperviscosity and decreased RBC deformability are responsible for these complications is unknown. The higher oxygen transport effectiveness (i.e., HVR) of CC compared to SS is probably at the origin of the very low risk of medical complication in this population.

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.

The role of rs1984112_G at CD36 gene in increasing reticulocyte level among sickle cell disease patients

AIMS AND BACKGROUND

Mediators of adhesion become a potential new target for pharmacological therapy to struggle the complications of sickle cell disease (SCD). Several mechanisms for increased adherence have been postulated and the well-studied are CD36 and VLA4 which encoded by ITGA4. Herein, we sought to determine whether one polymorphism of CD36 namely: rs1984112 and three exons of ITGA4 (4, 5, and 6) are implicated in hemolytic status and clinical events among SCD Tunisian patients.

MATERIAL AND METHODS

This study enrolled 99 unrelated Tunisian subjects (63SS and 36Sβ). All SCD patients are children (less than 16 years old). The rs1984112 and the ITGA4's exons 4, 5, and 6 were analyzed for all subjects by PCR/sequencing. The association of each genotype found with both clinical complications and hemolytic status was performed using t-test. Clinical events studied included vaso-occlusive crisis (VOC), osteonecrosis, stroke, frequent infection, priapism, and acute syndrome.

RESULTS

The results show that rs1984112_G allele at CD36 gene revealed to be associated with higher levels of reticulocyte count (p < 0.01). The statistical result show a near significance of homozygous mutant GG genotype with VOC (p = 0.051). No association between rs1984112_G allele and the clinical severity of SCD were found. Mutational screening of exon 4, 5, and 6 of ITGA4 gene revealed absence of mutated variant.

CONCLUSION

Our results are similar to those found in Portuguese population which reported the role of rs1984112_G in increasing reticulocyte level among SCD patients. Consequently, the rs1984112_G of CD36 could be considered as a reliable biomarker for predicting patients at high risk for vascular occlusions and thus, allows earlier and more effective therapeutic management.

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.

Predictors of abnormal bone mass density in adult patients with homozygous sickle-cell disease

BACKGROUND

Adult patients with sickle-cell disease (SCD) often have multiple bone compactions causing tissue hypoxia and osteonecrosis. The impact on bone abnormalities lesion detected by bone mass density is not well defined.

AIM

The study is a cross sectional, perspective was designed to assess the prevalence of abnormal BMD in adult Bahraini patients with SCD and to assess the predictive risk of different metabolic variables such as serum level of vitamin D3, testosterone, and parathyroid hormone in addition to lactate dehydrogenase (LDH), hemoglobin (Hb), and reticulocyte count for the development of abnormal bone density on dual X-ray absorptiometry (DXA) scan.

METHOD

The study was conducted over the period of 12 months from first of January 2012 to end of December 2012. All patients were evaluated clinically for severity of SCD and abnormal bone mass density (BMD) using DXA scan. Blood samples were withdrawn for measuring the serum level of vitamin D3, testosterone, and parathyroid hormone in addition to Hb, LDH, and reticulocyte count. Multiple logistic regression analysis was used to assess risk prediction of different variables for the development of abnormal BMD on DXA with T-score ≤-2.5 standard deviation (SD).

RESULTS

The study included Bahraini patients with SCD (n = 55, age 29.24 ± 9.47 years, male 60% and female 40%) compared with an age-matched healthy control group (n = 55, age 28.82 ± 8.64 years, with 62% male and 38% female). Of the 55 patients with SCD compared with the control group, there were 33 (58%) patients with low BMD and 2 (3%) in the control. Among the 33 patients with SCD and with low BMD, there were 20 (36%) with osteoporosis (T-score of ≤-2.5 SD) and 13 (24%) with osteopenia (T-score of <-1 to -2.5 SD). The most affected site of low BMD was lumbar spine (55%), followed by the radius (30%) and neck of the femur (15%). SCD patients with osteoporosis compared with the healthy subjects had significantly lower body surface area (BSA, m(2)) of 1.4 ± 0.3 vs. 1.63 ± 0.5 BMI, low level of vitamin D3 of 21.11 ± 6.95 ng/mL vs. 46.2 ± 15.19 (P < 0.001), lower testosterone level of 1.34 ± 0.54 vs. 2.18 ± 0.56 ng/mL (P < 0.001), higher reticulocyte count (P < 0.001), and higher LDH level (P < 0.001). The low serum level of vitamin D3 (<20 ng/mL) and low testosterone of <0.9 ng/mL had risk prediction (odds ratio) of 1.14 and 1.2, respectively, for abnormal BMD in SCD. In the risk prediction of other variables of parathormone (PTH), LDH, and reticulocyte, were not significant.

CONCLUSION

The prevalence of abnormal bone mass density (BMD) is high (60%) in Bahraini patients with SCD. There is significant low serum level of vitamin D3 and low testosterone hormone in those with very low bone mass density (BMD) (osteoporosis and T-score <-2.5). The low serum level of vitamin D3 (<20 ng/mL) and low testosterone of <0.9 ng/mL had risk prediction (odds ratio) of 1.14 and 1.2, respectively, for abnormal BMD in SCD.

Mild haemorheological changes induced by a moderate endurance exercise in patients with sickle cell anaemia

The levels and duration of physical activity that can be considered as completely safe in patients with sickle cell anaemia (SCA) is unknown. The present study compared the haemorheological and haematological profile, cell density distribution and basic biochemistry between a group of 17 patients with SCA and 21 healthy subjects before and after a 20 min duration submaximal cycling exercise at the same absolute workload. Blood was sampled at rest and 3 min after the end of exercise for measurement of biological parameters. Exercise did not affect the haematocrit and blood viscosity in the two groups. Plasma viscosity was not different between the two groups at rest and similarly increased with exercise. The proportion of intermediary dense cells (with density between 1·11 and 1·12 g/ml) decreased with exercise in the SCA group resulting in an increase in the proportion of red blood cells with a density >1·12 g/ml. No change was observed in the control group. The present study suggests that mild-moderate exercise is not very harmful for SCA patients. The haemorheological and haematological changes were very mild, except for the formation of dense cells but no clinically significant signs of medical complication were present in any of the patients.

Phenylhydrazine as a partial model for beta-thalassaemia red blood cell hemodynamic properties

beta-Thalassaemia is a congenital haemoglobinopathy, associated with red blood cells (RBC) anomalies, leading to impairment of their flow-affecting properties, namely, RBC deformability, self-aggregability, and adherence to endothelial cells (EC). Treatment of normal RBC with phenylhydrazine (PHZ) causes selective association of oxidized alpha-globin chains with the membrane skeleton, leading to reduced RBC deformability, characteristic of beta-thalassaemia. PHZ has thus been used to mimic phenotypes of beta-thalassaemia RBC. The present study was undertaken to further elucidate the suitability of PHZ-treated RBC as a model for beta-thalassemic RBC, by comparing the aggregability and adhesiveness of PHZ-treated RBC to those of RBC from thalassaemia intermedia (TI) patients, using image analysis of RBC under flow. In addition, the externalization of phosphatidylserine (PS), a mediator of RBC/EC interaction, was determined. It was found that PHZ caused enhanced RBC adhesiveness to extracellular matrix, similar to TI-RBC. Furthermore, in both conditions, the enhanced adhesiveness was mediated by PS translocated to the RBC surface. In contrast, PHZ treatment completely abolished RBC aggregability, while TI-RBC aggregability was slightly elevated. It is proposed that PHZ-treated RBC resemble beta-thalassaemia RBC in their deformability and adhesiveness, but not in their aggregability, and thus can be used as a limited model for beta-thalassaemia RBC phenotypes.

Bone mass density in adults with sickle cell disease

Sickle cell disease (SCD) leads to many complications including osteoporosis and osteopenia. We studied the prevalence and predisposing factors of low bone mass density (BMD) in adults with SCD. In this retrospective study, dual X-ray absorptiometry bone scans were used to determine BMD in the lumbar spine, femoral neck and ultra distal radius of 103 patients (73 females, 30 males, aged 15-80 years). Chart reviews and a patient questionnaire were used to collect patient characteristics, disease course and severity, and low BMD risk factors. The 79.6% of patients (mean age 36.5 +/- 12.5 years) had an abnormal BMD, with a predilection for the lumbar spine (P = 0.001). Analysis by 3 (low BMD versus very low BMD versus normal) or by 2 groups (abnormal versus normal) showed that abnormal BMD was associated with lower body mass index (BMI) (P = 0.003), lower Hb level (P = 0.001) and higher ferritin (P = 0.003). Low BMD patients were more likely to be SS, SC or Sbeta(0)thal than Sbeta(+)thal (P = 0.022). Abnormal BMD was not related to age, sex, menarche, SCD complications, number of crises, iron overload, treatment with hydroxycarbamide or desferal, renal disease, smoking or alcohol. Patients treated with hydroxycarbamide for at least 6 months were more likely to have an abnormal BMD. In this SCD population, abnormal BMD seemed to be independent of sex, age and menopause, whereas BMI, ferritin level, Hb type and level appeared to play a major role.

Effects of progressive and maximal exercise on plasma levels of adhesion molecules in athletes with sickle cell trait with or without α-thalassemia

The aim of the study was to examine the effects of exercise on soluble vascular cell adhesion molecule-1 (sVCAM-1) and intercellular adhesion molecule-1 (sICAM-1) in sickle cell trait (SCT) athletes with or without α-thalassemia. Six athletes with SCT, seven athletes with both SCT and α-thalassemia (SCTAT), and seven control athletes (Cont) performed an incremental and maximal test on cycloergometer. Levels of sICAM-1 and sVCAM-1 were assessed at rest, immediately after the end of exercise, and 1, 2, and 24 h after exercise. Although Cont and SCTAT groups exhibited similar basal plasma levels of inflammatory and adhesion molecules, the SCT group had higher sVCAM-1 basal concentrations. Incremental exercise resulted in a significant increase of sVCAM-1 in all subjects, which remained elevated only in the SCT group during the recovery period. In conclusion, as sVCAM-1 increased with exercise and during the recovery period, our findings support the concept that SCT athletes might be at risk for microcirculatory disturbances and adhesive phenomena developing at rest and several hours after exercise. α-Thalassemia might be considered protective among exercising SCT subjects.

Pediatric Perfusion MR Imaging Using Arterial Spin Labeling

Cerebral blood flow (CBF, cerebral perfusion) mirrors cerebral metabolic demand and neuronal function, and therefore, is a vital parameter in the evaluation of pediatric brain injury and recovery. Until recently, measurement of CBF involved intravenous bolus injection of contrast agents or nuclear medicine methods that were technically difficult or ethically problematic in pediatrics. The development of arterial spin label (ASL) perfusion MR imaging as a noninvasive method for measuring CBF allows for the increased ability to measure this vital physiologic parameter in any age group. This article presents the technical aspects of performing ASL perfusion MR imaging in pediatrics, and discusses its current use in clinical studies and its potential for influencing important management strategies in specific disease entities.

Hemorheology, Sickle Cell Trait, and α-Thalassemia in Athletes: Effects of Exercise

PURPOSE

This study investigated hemorheological parameters in response to exercise in sickle cell trait (SCT) athletes with or without alpha-thalassemia

METHODS

Six athletes with SCT (HbAS), 7 athletes with SCT and alpha-thalassemia (HbASAT), and 10 control athletes (HbAA) performed a progressive and maximal exercise test on cycloergometer. Blood viscosity (etab), plasma viscosity (etap), etab at corrected hematocrit (etab45), hematocrit (Hct), and red blood cell (RBC) rigidity were assessed at rest, at maximal exercise and 24 h after exercise

RESULTS

etab and etap were not different between the three groups at any time. Exercise induced changes in etab in HbAA and HbASAT groups but not in HbAS group. etab45 was higher in HbAS group compared with the other groups (P < 0.05), at rest and 24 h after exercise and increased only in HbAA group in response to exercise. HbAS group had lower Hct than HbAA group at any time. Hct and etap increased after exercise and declined under baseline values 24 h after exercise in all groups. RBC rigidity was higher in HbAS group compared with HbAA and HbASAT groups at any time, and was lower and higher at maximal exercise and 24 h after exercise, respectively, in all groups compared with resting values

CONCLUSIONS

These results demonstrate that HbAS group is prone to higher RBC rigidity, which might lead to hemorheological alterations that are thought to participate to microcirculation disorders. However, these alterations are limited by the coexistence of alpha-thalassemia. Moreover, hemorheological parameters were not further impaired in SCT athletes with or without alpha-thalassemia in response to exercise. Training status might be protective from physiological stresses usually leading to sickling process in SCT carriers.

Attention deficits in children with sickle cell disease

Previous research has suggested that children with sickle cell disease may exhibit cognitive deficits even in the absence of direct cerebrovascular involvement (stroke). This study was designed to assess specific attentional deficits in children with sickle cell disease. 12 children with sickle cell disease (Hb SS) with a prior history of stroke, 14 children with sickle cell disease (Hb SS) without evidence of stroke, and 13 similar aged siblings (Hb AA or Hb AS) were compared on measures of attention, intellectual functioning, achievement, and adaptive functioning. Significant differences were found between children with sickle cell disease (with or without stroke) and healthy controls on a timed test of visual scanning, the Coding subtest of the Wechsler Intelligence Scale for Children-Revised, and subtests of Reading, Arithmetic, and Spelling from the Wide Range Achievement Test-Revised. The differences between children with sickle cell disease and their healthy siblings appear to be the result of strokes rather than sickle cell disease itself as children with sickle cell disease without strokes did not significantly differ from controls. Implications for the effects of sickle cell disease and stroke on academic performance are discussed.

Differences in adhesion receptor expression between immature and older platelets and red blood cells of neonates and adults

PURPOSE

To examine the hypothesis that reticulated platelets and reticulocytes show elevated adhesion receptor expression compared with mature cells in both adult and neonatal cells.

METHODS

Flow cytometry was used to examine laminin, fibronectin (VLA-6), and thrombospondin (glycoprotein IV [GPIV]) expression in reticulated red cells, reticulated platelets, and older peripherally circulating mature red cells and mature platelets in seven newborn cord blood samples and blood samples from eight adult volunteers.

RESULTS

The difference in the neonatal reticulated platelet percentage of 9.2+/-14.8% was not statistically significant from the adult reticulated platelet percentage of 5.0+/-1.5% in this small population. There was a statistically significant difference between the reticulated cord blood red cell mean of 7.7+/-1.8% and the adult mean of 3.1+/-0.43%. Mean expression of VLA-6 was 96% in adult reticulated platelets, 79% in adult mature platelets, 81% in cord reticulated platelets and 65% in cord mature platelets. Mean expression of GPIV was similar, with corresponding values of 90%, 71%, 78%, and 57%. Reticulated red cells in adults averaged 44% VLA-4 and 46% GPIV; cord reticulocytes were 9% and 15%, respectively.

CONCLUSIONS

Reticulated cells newly released from the bone marrow express more adhesive receptors than mature cells in both groups. Cord blood samples showed hypoexpression of both receptor types in red blood cells and platelets.

Blood Polymorphonuclear Leukocytes From the Majority of Sickle Cell Patients in the Crisis Phase of the Disease Show Enhanced Adhesion to Vascular Endothelium and Increased Expression of CD64

There is increasing interest in the role of blood polymorphonuclear leukocytes (PMNs) in the pathogenesis of sickle cell crisis. We studied the adherence of PMNs from 18 sickle cell patients in crisis, 25 out of crisis, and 43 healthy subjects (controls) to monolayers of human umbilical cord endothelium that were either untreated or pretreated with tumor necrosis factor α (TNFα). Overall, the PMNs from patients in crisis were more adherent than control PMNs to untreated endothelial monolayers (mean 53% increase; P < .001) and TNFα-treated monolayers (mean 41% increase; P < .002). Increased adhesiveness was not associated with an abnormal expression of CD11a, CD11b, CD11c, CD18, CD62L, or CD15. There was an increase in the number of PMNs expressing CD64 in patients in crisis (median value, 44%) compared with patients out of crisis (median, 21%; P = .025) and controls (median, 6.5%; P < .001). Sera from patients in crisis had normal levels of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interferon-γ, TNFα, interleukin-1 (IL-1), IL-6, or IL-8 and did not modify the adherence of PMNs or their expression of CD64. Only IFN-γ induced CD64 expression on PMNs, but this effect was not associated with enhanced binding to endothelium. Because PMNs bound to endothelial monolayers were CD64+ and CD64-enriched PMNs were 7 times more adherent to endothelial monolayers than CD64-depleted PMNs, it is likely that CD64 is a marker of adherent PMNs. Two of the three anti-CD64 antibodies used in our antibody blocking studies (clones 32.2 and 197) partially inhibited the binding of sickle cell PMNs to untreated endothelium (mean inhibitions of 33% [P = .01] and 21% [P = .03], respectively), whereas only one (clone 197) inhibited binding to TNFα-treated endothelium (mean inhibition, 29%; P = .004). In some patients with sickle cell disease, an enhanced PMN adhesion to vascular endothelium could contribute to the vascular occlusion that characterizes the acute crisis of the disease.

Blood Polymorphonuclear Leukocytes From the Majority of Sickle Cell Patients in the Crisis Phase of the Disease Show Enhanced Adhesion to Vascular Endothelium and Increased Expression of CD64

There is increasing interest in the role of blood polymorphonuclear leukocytes (PMNs) in the pathogenesis of sickle cell crisis. We studied the adherence of PMNs from 18 sickle cell patients in crisis, 25 out of crisis, and 43 healthy subjects (controls) to monolayers of human umbilical cord endothelium that were either untreated or pretreated with tumor necrosis factor α (TNFα). Overall, the PMNs from patients in crisis were more adherent than control PMNs to untreated endothelial monolayers (mean 53% increase; P < .001) and TNFα-treated monolayers (mean 41% increase; P < .002). Increased adhesiveness was not associated with an abnormal expression of CD11a, CD11b, CD11c, CD18, CD62L, or CD15. There was an increase in the number of PMNs expressing CD64 in patients in crisis (median value, 44%) compared with patients out of crisis (median, 21%; P = .025) and controls (median, 6.5%; P < .001). Sera from patients in crisis had normal levels of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, interferon-γ, TNFα, interleukin-1 (IL-1), IL-6, or IL-8 and did not modify the adherence of PMNs or their expression of CD64. Only IFN-γ induced CD64 expression on PMNs, but this effect was not associated with enhanced binding to endothelium. Because PMNs bound to endothelial monolayers were CD64+ and CD64-enriched PMNs were 7 times more adherent to endothelial monolayers than CD64-depleted PMNs, it is likely that CD64 is a marker of adherent PMNs. Two of the three anti-CD64 antibodies used in our antibody blocking studies (clones 32.2 and 197) partially inhibited the binding of sickle cell PMNs to untreated endothelium (mean inhibitions of 33% [P = .01] and 21% [P = .03], respectively), whereas only one (clone 197) inhibited binding to TNFα-treated endothelium (mean inhibition, 29%; P = .004). In some patients with sickle cell disease, an enhanced PMN adhesion to vascular endothelium could contribute to the vascular occlusion that characterizes the acute crisis of the disease.

Band 3 Peptides Block the Adherence of Sickle Cells to Endothelial Cells In Vitro

Malaria-parasitized erythrocytes have increased endothelial adherence due to exposure of previously buried intramembranous sites of band 3. Because sickle erythrocytes also show increased adhesiveness and because the membrane portion of band 3 is aggregated in both types of cells, we examined the role of band 3 in sickle cell adhesiveness. Synthetic peptides derived from the second and third exofacial, interhelical regions of band 3 completely inhibited the abnormal adherence of sickle cells to an endothelial monolayer in a static assay. This effect was observed independently of plasma factors, required micromolar levels of peptide, was sequence-specific, and was found with both L- and D-isomers. The active peptides also inhibited the increased adherence induced by low-dose calcium loading of normal red blood cells. Finally, a monoclonal antibody against an active peptide specifically immunostained a fraction of sickle cells. These findings implicate a role for band 3 in at least one type of sickle cell adhesiveness via the exposure of normally cryptic membrane sites.

Decrease of Very Late Activation Antigen-4 and CD36 on Reticulocytes in Sickle Cell Patients Treated With Hydroxyurea

Sickle cell disease (SCD) is characterized by repeated vaso-occlusive events, which result in substantial morbidity. Abnormal adhesion of sickle red blood cells (RBC) to the vascular endothelium is postulated to play a role in the pathogenesis of vaso-occlusion. Two adhesion receptors, very late activation antigen-4 (VLA-4) and CD36, are found in unusually high numbers on sickle cell reticulocytes and do mediate adhesion of sickle RBC to endothelium. Hydroxyurea (HU) therapy results in fewer vaso-occlusive episodes, and we postulated that HU-related modulation of VLA-4 and CD36 receptors may contribute to its clinical benefit. Using flow cytometry, eight patients were followed from the onset of HU treatment through a mean treatment length of 200 ± 49 days. Mean corpuscular volume and percent fetal hemoglobin (Hb F ) increased from 87% ± 6% to 98% ± 9% and 6.6% ± 3.9% to 12.7% ± 5.6%, respectively. The percentage of reticulocytes expressing VLA-4 decreased from 29.0% ± 5.9% to 14.9% ± 2.3% (P = .0003). Two thirds of the total decrease in VLA-4 expression occurred after 10 weeks of HU and plateaued by 20 weeks. Changes in VLA-4 expression occurred before substantial increases in Hb F. The percentage of reticulocytes expressing CD36 decreased from 55.3% ± 6.4% to 42.6% (P = .0046). Changes in adhesion receptor expression were not caused by a decrease in reticulocytosis with HU therapy. This report is the first to associate a decrease in adhesion receptor expression with a therapy known to reduce the clinical severity of SCD.

Inhibition of plasma-mediated adherence of sickle erythrocytes to microvascular endothelium by conformationally constrained RGD-containing peptides

Adherence of sickle erythrocytes to vascular endothelium likely initiates or participates in microvascular occlusion, leading to ischemic tissue and organ damage characteristic of sickle-cell pain episodes. In vitro, sickle-cell adherence to endothelium involves adhesive plasma proteins and integrin and nonintegrin receptors on sickle cells and endothelial cells. The involvement of arginine-glycine-aspartic acid (RGD) sequences in adhesive plasma proteins and integrin receptors suggests that RGD-containing peptides may inhibit sickle-cell/endothelial-cell adherence. In the present study, inhibition of plasma-mediated sickle-erythrocyte adherence to endothelium using conformationally constrained RGD-containing peptides was quantified in vitro under continuous flow at a shear stress of 1.0 dyn/cm2. Two conformationally constrained RGD peptides were investigated: 6Z (which has high affinity for alpha5beta1, alpha(V)beta3, and alpha(IIIb)beta3 integrin receptors), and TP9201 (which preferentially binds to alpha(IIb)beta3). Peptide 6Z at 50 microM inhibited plasma-mediated sickle-cell adherence to microvascular endothelium 70% when incubated with sickle red cells, and 63% when incubated with endothelium. Under similar conditions, peptide TP9201 inhibited plasma-mediated sickle-cell adherence up to 85% at concentrations from 250 to 500 microM TP9201. The inhibition of plasma-mediated adherence by conformationally constrained RGD peptides, but not by linear or circular constructs, suggests that the tertiary structure of the peptide containing the binding sequence is important. Inhibition of plasma-mediated sickle-cell adhesion with these peptides in vitro suggests that such conformationally constrained RGD peptides could provide therapeutic interventions in the course of the disease by inhibiting receptor-ligand interactions.

Migraine and sickle-cell disorders: is there a cause for concern?

Migraine and sickle-cell disease are common diseases, and may coexist in the same individual. Such a chance association is potentially dangerous because the two disorders may act synergistically to cause major neurological complications which may be permanent and contribute to the well-known mortality of sickle-cell disease. In theory, it is possible to minimize the risk of adverse sequelae of this combination of common maladies.

In vivo demonstration of red cell-endothelial interaction, sickling and altered microvascular response to oxygen in the sickle transgenic mouse

Intravascular sickling, red cell-endothelium interaction, and altered microvascular responses have been suggested to contribute to the pathophysiology of human sickle cell disease, but have never been demonstrated under in vivo flow. To address this issue, we have examined a transgenic mouse line, alphaHbetaSbetaS-Antilles [betaMDD] which has a combined high (78%) expression of beta S and beta S-Antilles globins. In vivo microcirculatory studies using the cremaster muscle preparation showed adhesion of red cells, restricted to postcapillary venules, in transgenic mice but not in control mice. Electron microscopy revealed distinct contacts between the red cell membrane and the endothelium surface. Some red cells exhibiting sickling were regularly observed in the venular flow. Infusion of transgenic mouse red cells into the ex vivo mesocecum vasculature also showed adhesion of mouse red cells exclusively in venules. Under resting conditions (pO2, 15-20 mmHg), there were no differences in the cremaster microvascular diameters of control and transgenic mice; however, transgenic mice showed a drastic reduction in microvascular red cell velocities (Vrbc) with maximal Vrbc decrease (> 60%) occurring in venules, the sites of red cell adhesion and sickling. Local, transient hyperoxia (pO2, 150 mmHg) resulted in striking differences between control and transgenic mice. In controls, oxygen caused a 69% arteriolar constriction, accompanied by 75% reduction in Vrbc. In contrast, in transgenic mice, hyperoxia resulted in only 8% decrease in the arteriolar diameter and in 68% increase in VrBC; the latter is probably due to an improved flow behavior of red cells as a consequence of unsickling. In summary, the high expression of human sickle hemoglobin in the mouse results not only in intravascular sickling but also red cell-endothelium interaction. The altered microvascular response to oxygen could be secondary to blood rheological changes, although possible intrinsic differences in the endothelial cell/vascular smooth muscle function in the transgenic mouse may also contribute. These sickle transgenic mice could serve as a useful model to investigate vasoocclusive mechanisms, as well as to test potential therapies.